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Marine Geology Of The Andaman Basin, Northeastern Indian Ocean

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Marine Geology Of The Andaman Basin, Northeastern Indian Ocean

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Content This dissertation has basn
microfilmed sxactly as received 68-1201
RODOLFO, Kelvin Schmidt, 1936-
MARINE GEOLOGY OF THE ANDAMAN BASIN,
NORTHEASTERN INDIAN OCEAN.
University of Southern California, Ph.D., 1967
Geology
University Microfilms, Inc., Ann Arbor, Michigan
MARINE G EO LO G Y OP THE A N D A M A N
BASIN, NORTHEASTERN INDIAN OCEAN
by
K elvin Schm idt R od o lfo
A D is s e r ta tio n P re s e n te d to th e
FACULTY OP THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In P a r t i a l F u lf illm e n t of th e
R equirem ents f o r th e Degree
D O C T O R O F PHILOSOPHY
(Geology)
Septem ber, 1967
UNIVERSITY O F SO U TH ERN CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES, CALIFORNIA 9 0 0 0 7
This dissertation, written by
under the direction of h^*...Dissertation Com
mittee, and approved by all its members, has
been presented to and accepted by the Graduate
School, in partial fulfillment of requirements
for the degree of
D O C T O R OF P H I L O S O P H Y
.............
Dtmn
Date September..2* .1967
HSSERTATION COMMITTEE
ChairituiH
|«Ua.
CO NTENTS
Page >
ABSTRACT.................................................................................................... 1
INTRODUCTION........................................................................................... 4
L o c a tio n and g ro s s c o n f ig u r a tio n .................................. 4
P urpose o f s tu d y ............................................................... 8
S o u rces o f in fo rm a tio n .......................................................... 8
P re v io u s i n v e s t i g a t i o n s .................................................... 19
Acknowledgm ents ....................................................................... 22
PHYSIOGRAPHIC A N D GEOLOGIC SETTING...................................... 24
I
R e g io n al z o n a tio n .................................................................. 24
Ja v a T r e n o h ................................................................................ 27
A rakan-M entaw al Range .................................................... 31
P h y s io g r a p h y ......................................................................... 31
S tr a tig r a p h y ............................................................................ 32
S t r u c t u r e ................................................................................ 37
Burma-Men ta w a i T r o u g h ......................................................... 38
P h y sio g rap h y ............................................................................ 38
S tr a tig r a p h y ........................................................................... 40
i
V o l o a n l c s ................................................................................ 4 4 1
S t r u c t u r e ................................................................................ 47
E a s te rn M a ssif ........................................................................... 49
P h y sio g rap h y ............................................................................ 49
S tr a tig r a p h y ............................................................................ 50
S t r u o t u r e ......................................................................... 51
i l l
Page
A N D A M A N BASIN PHYSIOGRAPHY............................................... 54
I n tr o d u c tio n ....................................................................... 54
Irraw addy D e l t a ............................................................. 58
C o a s t ................................................................................. 58
S h e l f ................................................................................. 59
S l o p e ........................................ 68
M artaban C a n y o n ....................... 68
M alay c o n t i n e n t a l s h e l f ........................................... 69
G en eral s ta te m e n t .................................................... 69
In n e r c o n t i n e n t a l s h e l f and th e M ergul
A rc h ip e la g o ............................................... 69
M ergul T e rra c e ............................................................. 71
Sum atra S h e lf -B a s ln ............................................... 76
i
C o n tin e n ta l s lo p e .................................................... 79
A ndam an-N lcobar R idge .......................................... 83
G en e ra l s ta te m e n t .................................................... 83
I n s u la r c o a s ts ............................................................. 83
I n s u la r s h e lv e s ......................................................... 84
S i l l s ................................................................................. 85
E a s te rn s lo p e ............................................................. 87
W estern s lo p e ............................................................. 91
B asin F l o o r ....................................................................... 92
C e n tra l Andaman T r o u g h .......................................... 92
R i f t v a l l e y s ............................ 97
N arcondam -B arren B a s i n ................................................... 106
Page
A lcock Seam ount ...................................... 106
S e w e ll Seam ount ........................................................ 107
Narcondam I s la n d • • • • • • • • • • • • • 107
B a rren I s la n d ............................................................. 112
S m a lle r s e a h lg h s ........................................................ 114
LITHOLOGY OP THE A N D A M A N B A SIN ...................................... 120
G e n e ra l s ta te m e n t ........................................................ 120
A ndam an-N icobar Ridge ............................................... 120
V o lo an ic ro c k s ............................................... 124
L ith o lo g ic z o n a tlo n .................................................... 128
REGIONAL GEOPHYSICAL TRENDS .......................................... 129
V o l c a n l B m ........................... 129
S e i s m i c i t y ........................................................................... 131
G r a v i t y .......................................... 132
M a g n e t i s m ........................................................................... 138
H eat f l o w ........................................................................... 141
A N D A M A N BASIN STRUCTURE.................................................... 143
G e n e ra l s ta te m e n t ........................................................ 143
C o m p ressio n al f e a t u r e s ................... • • • • • • 146
T e n s io n a l o r te n B io n - tr a n s o u r re n t f e a t u r e s • 149
G e n e ra l tr e n d s ............................................................. 149
F a u lts o f th e c o n tin e n ta l m a r g i n ................... 151
R i f t s ................................................................................ 154
H idden f a u l t s ............................................................. 158
R e g io n a l l m p l i o a t l o n s ............................................... 164
Page
CLIMATE AND OCEANOGRAPHY................................................... 166
R e g io n a l c lim a tic s e t t in g ..................................... 166
I n t r o d u c t i o n ................................................................. 166
Ja n u a ry and F eb ru ary ................................. 167
M arch to m i d - J u n e ................................................... 168
M id-June to m id - S e p te m b e r ................................. 168
M id-Septem ber to D e c e m b e r ................................. 169
P r e c i p i t a t i o n and ru n o ff • • • • • • • • • • 170
B u r m a ............................................................................... 170
P e n in s u la r T h ailan d and M a la y s i a .................... 175
S u m a t r a .......................................................................... 175
C u r r e n t s .................................................. 176
I n tro d u c tio n • • • • • • • • • • • • • • • 176
November to J a n u a r y ............................................... 176
F eb ru ary to A p r i l ................................................... 185
May to A u g u s t ............................................................ 186
S eptem ber and O c t o b e r .......................................... 187
C h a r a c t e r i s t i c s of th e d eeper w aters • • . • 188
W ater m a s s e s ................................................................. 188
O x y g e n .................................................. 195
SED IM EN TS................................................................................... 198
G e n e ra l s ta te m e n t ........................................................ 198
A n a ly tic a l methods .......................................... 200
L ogging and sample p r e p a r a t i o n ........................ 200
C lay a n a l y s i s ............................................................ 203
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Page j
T e x tu ra l a n a ly s is . . . ...................................... 207
C alcium c a r b o n a t e ..................................................... 209
O rg an ic carbon .............................................................. 210
O rg an ic n itr o g e n ......................................................... 210
S edim ent p ro v ln o es ......................................................... 211
G u lf o f M artaban p r o v i n c e ................................. 211
O u ter d e l t a p r o v i n c e ..................................... 214
M alay c o n tin e n ta l m argin p ro v in c e • • • • 217
C e n tr a l B asin p r o v i n c e ........................................... 218
An daman -N io o b ar R idge p ro v in ce .................... 223
C o lo r ................................................................................ 223
T e x t u r e ................................................................................ 223
Calcium c a rb o n a te 231 ;
O rg an ic m a t t e r .................................................................. 242
C lay m in eralo g y • •• • •• • • • • • • • • 233
S o u r o e s ........................... 260
T ra n s p o rt and d e p o s itio n ........................................... 263
R a te s o f d e p o s i t i o n ..................................................... 267
SU M M A R Y OF GEOLOGIC H IST O R Y ........................................... 276
P re-O retaceo u B .................................................................. 276
L a te C r e t a c e o u s .............................................................. 276
E a rly and m iddle T e r t i a r y ...................................... 283
L a te r C e n o z o lo ................................................................. 284
R e g io n a l i m p l i c a t i o n s ................................................ 289
▼ 11
Page
CONCLUSIONS............................................................................... 298
REFERENCES..................................................................................... 304
FIGURES
F ig u re Page
1 . L o catio n c h a r t o f th e Andaman Sea • • • . • 5
2 . L o catio n o f PIONEER PD R p r o f i l e s
employed a s I l l u s t r a t i o n s ...................................... 12
3 . Index c h a rt o f sed im en t sam ples ......................... 15
4 . L o catio n s o f H ydrographic S ta tio n s • • • • 17
5* R eg io n al p h y sio g ra p h ic s e t t i n g o f th e
Andaman B a s i n .................................................................. 25
6 . R e c o n stru c te d b a th y m e tric p r o f i l e from
PIONEER tr a v e rs e a c ro s s Andam an-Nicobar
R id g e, showing e a s t fla n k o f E a s t In d ia n
Ridge and th e low a re a betw een th e r id g e s • 28
7* D iagram m atic lo n g itu d in a l s e c tio n
th ro u g h th e Burmese T e r tia r y B asin • • • • 42
8 . D is tr ib u tio n o f T e r tia r y v o lc a n lc s
In B u r m a ........................................................................... 45
9* G en e ra liz e d p h y sio g rap h ic c h a r t o f th e
Andaman B asin ..................................... 55
10. S u rfac e o f th e Irraw addy D e lta and G ulf
o f M artaban, aooordlng to R oyal In d ia n
M arine Survey o f 1854-1859 62
11. N et s h o a lin g on th e Irraw addy D e lta
s h e l f from 1854-1859 to 1960-1964 ................... 66
12. B ath y m etric p r o f i l e s a c ro s s th e Malay
c o n tin e n ta l m a r g i n ........................... 72
13* PDR p r o f il e a c ro s s In n er s h e l f and
In n e r s h e l f - b r e a k ..................................... 74
i
14. W est-n o rth w est - e a s t- s o u th e a s t PDR
p r o f il e a o ro ss th e Malay c o n tin e n ta l
m argin from 10° to 10*5° N ................................. 77
ix
I
Figure Page1
15* E a st-w e st PER p r o f i l e a c ro s s th e Malay
c o n tin e n ta l slo p e and deep te r r a o e , and
the n o rth e rn end o f th e C e n tra l Andaman
Trough, along 13° N ............................................. 81
16. P re c is io n Depth Reoord p r o f i l e s a c ro s s
e a s te rn slo p e o f Andaman-Nicobar R idge,
I n v is ib le Bank, and In te rv e n in g tro u g h • • 88
17* W e st-east p r o f il e th ro u g h P re p a rls
Channel a c ro s s c o n tin e n ta l slo p e and
Irraw addy D e lta s h e lf ............................................... 93
1 8 . E a s t-n o rth e a s t - w e st-so u th w e st PDR
p r o f ile thro u g h G re at P assag e ............................ 95
1 9 . N o rth -n o rth e a s t - so u th -so u th w e st PDR
p r o f ile a c ro s s C e n tra l Andaman R i f t
V alley in th e v i c i n i t y o f 10° N, 94° E . • 98
2 0 . W est-northw est - e a s t- s o u th e a s t
PDR p r o f ile a c ro s s C e n tra l Andaman
R i f t V alley a t 10045' N l a t i t u d e . 101
2 1 . W e st-ea st PDR p r o f il e a lo n g 10° N
a c ro ss th e C e n tra l Andaman R i f t V alley • • 103
2 2 . W e st-east PDR p r o f i l e a c ro s s th e
w estern slo p e o f Aloook Seamount and a
peak o f the B arren I s la n d seam ount
o o m p le z ......................................................... 108
2 3 . W e st-east PDR p r o f i l e a lo n g 10° N
a c ro s s S ew ell Seamount ............................ . . . 110
2 4 . W e st-ea st PDR p r o f i l e a lo n g 10° N
ao ro ss N lcobar R i f t V a lle y and
a d ja c e n t rid g e to th e e a s t ........... 118
2 5 . L o catio n s o f ro ck sam ples from th e
Andaman B asin 121 '
2 6 . V o lo a n ic ity and s e is m ic ity o f th e
Andaman B asin and i t s su rro u n d in g s . . . . 125 ;
2 7 . G ra v ity anom alies in th e Andaman
Basin and s u r r o u n d i n g s ..................... 133
Figure
2 8 . M agnetic an o m alies and h e a t- f lo w d a ta
in th e Andaman B asin ...........................................
2 9 . S t r u c t u r a l tr e n d s in th e Andaman
B asin re g io n ..............................................................
3 0 . S ub-bottom p r o f i l e a c ro s s th e
n o rth e rn A ndam an-N icobar R idge . • . • •
31* S ub -b o tto m p r o f i l e a o ro s s S e w e ll
Seam ount ........................................................................
3 2 . S ub-bottom p r o f i l e s a c r o s s th e
N ic o b a r R i f t V a lle y ................................................
33* S ub -b o tto m p r o f i l e a c r o s s th e
w e ste rn p o r tio n o f Weh B a n k ........................ ....
3 4 . S ub-bottom p r o f i l e a c r o s s th e
Irraw ad d y D e lta s h e l f ............................................
35* D rain ag e b a s in o f th e Irraw ad d y R iv e r • •
3 6 . A verage s u r fa c e o c e a n o g ra p h ic c o n d i
ti o n s in th e Andaman S e a , November to
Ja n u a ry .............................................................................
37* A verage s u r fa c e o c e a n o g ra p h ic c o n d itio n s
in th e Andaman S e a , F e b ru a ry to A p r il • •
3 8 . A verage s u r f a c e o c e a n o g ra p h ic c o n d itio n s
in th e Andaman S e a , May to A ugust « • • •
39* A verage s u r fa c e o c e a n o g ra p h ic c o n d itio n s
in th e Andaman S e a , S ep tem b er and O cto b er
4 0 . P r o f i l e s show ing v e r t i c a l d i s t r i b u t i o n s
o f te m p e ra tu re , s a l i n i t y and d is s o lv e d
oxygen c o n te n ts a lo n g an e a s t- w e s t
tr a v e r s e a c ro s s th e Andaman S ea a t 10° N
l a t i t u d e • • • • .....................................................
4 1 . In s i t u and p o t e n t i a l te m p e ra tu re s o f
th e d ee p er w a te rs o f th e Andaman S ea • .
4 2 . T e m p e r a tu r e - s a lin ity d iag ram o f w a te r
m asses in th e n o r th e a s te r n In d ia n Ooean •
4 3 . Flow o h a r t o f se d im e n t a n a ly s e s • • . • •
Page j
139
144
147
3.52
156
159
162
172
177
179
181
183
189
193
196 ;
201
F igure Page
4 4 . S edim ent p ro v in c e s and g e n e ra l
d i s t r i b u t i o n o f t e x t u r a l ty p e s in th e
Andaman B asin ........................................................................... 212
4 5 . SERRANO C ores 25 and 232, i l l u s t r a t i n g
th e r e l a t i o n s h i p o f G u lf o f M artaban
and o u te r d e l t a se d im e n ts .............................................. 215
4 6 . C oarse c a l c a r e n i t l c la y e r s in PIONEER
Core 24 from th e b ase o f I n v i s i b l e Bank . • 220
4 7 . V a r ia tio n o f mean d ia m e te r w ith
w a te r d e p t h ................................................................... 226
4 8 . V a r ia tio n s o f s o r t i n g and skew ness
w ith mean d i a m e t e r ............................................................. 229
49. V a r ia tio n o f f o r a m in lf e r a l number w ith
t o t a l c a lc iu m c a rb o n a te c o n te n t ................................ 232
50. C h a rt o f f o r a m in if e r a num bers in s u rfa c e
sed im en ts o f th e Andaman B a s i n ........................ 234
51* W eight p e rc e n ta g e s o f c a lciu m c a rb o n a te
in s u r fa c e se d im e n ts o f th e Andaman B asin • 236
5 2 . V a r ia tio n o f c a lc iu m c a rb o n a te c o n te n ts
w ith d ep th o f w a te r in th e Andaman B asin • 240
53* V a r ia tio n o f o rg a n ic n itr o g e n c o n te n t
w ith c la y c o n t e n t ......................................................... 243
5 4 . A re a l d i s t r i b u t i o n o f o rg a n io carb o n in
s u r fa c e se d im e n ts o f th e Andaman B asin • • 245
55* A re a l d i s t r i b u t i o n o f o rg a n ic n itr o g e n
in s u rfa c e se d im e n ts o f th e Andaman B asin • 247
56. V a r ia tio n o f o rg a n ic carb o n w ith
o rg a n ic n itr o g e n ......................................................... 251
57* P e rc e n ta g e s o f com bined m o n tm o rlllo n ite
and m ix e d -la y e r c la y s in s u r fa c e se d im en ts
o f th e Andaman B a s i n ............................................... 254
5 8 . D is tr ib u ti o n o f K a o lln lte in s u r fa c e
se d im e n ts o f th e Andaman B a s i n ........................ 257
Z li • !
I
Figure Page
5 9 • L e p o s ltio n a l r a t e s in th e Andaman B asin ,
and s e is m io a lly d eterm in ed t o t a l
sedim ent th ic k n e s s e s ............................................... 269
60. D iagram m atic r e p r e s e n ta tio n o f th e
p o s tu la te d h i s t o r y o f th e Andaman B asin • • 281
61. P o s tu la te d c r u s t a l s e c tio n th ro u g h th e
Andaman B asin ................................................................. 287
62. S u g g ested r e l a t i o n s h i p o f c r u s t a l
movmment to m a rg in a l s t r u c t u r e o f
s o u th e a s t A s i a ............................................................. 293
i
!
TABLES
Table Page
1 . C o rre c tio n s f o r th e v e lo c ity o f sound
In th e Andaman Sea w ater c o lu m n ....................... 10
2 . Summary o f Is la n d s alo n g th e Andaman,
N ico b ar and M entawal fe s to o n s ............................ 33
3* Summary o f r e g io n a l T e r tia ry s tr a tig r a p h y
In Burma and th e Andaman B asin a re a • • • • 33
4 . R ate o f southw ard movement o f s e le c te d
Is o b a th s on th e s u rfa o e o f th e Irraw addy
D e lta and G ulf o f M a r t a b a n ................................. 61
5* Summary o f C hannels a c ro s s th e Andaman-
N lcobar Ridge ................................................................. 86
6 . Summary o f Andaman B asin se a h lg h s .................... 113
7 . D e s c rip tio n o f ro ck s dredged from th e
Andaman B a s i n ......................................... 123
8 . Average m onthly p r e c i p i t a t io n a t s e le c te d
s t a t io n s in B u rm a ....................................................... 171
9 . P ro ced u re f o r th e c a lc u la tio n o f
r e l a t i v e am ounts o f th e c la y m in e ra ls . . . 206
PLATES
P la te Page
I . B athym etry o f th e Andaman B asin • • • • In p o ck et
ABSTRACT
The Andaman B a s in , th e n o r th e a s te r n c o rn e r o f th e
o
In d ia n O cean, c o v e rs 8 0 0 ,0 0 0 km , i s 1200 km lo n g from
Burma to S u m atra, and 650 km w ide from th e M alay P e n in
s u la to th e A ndam an-H icobar R id g e . The n o r th - s o u th
p h y s io g ra p h ic g r a in o f th e b a s in i s dom inated by
s t r u c t u r e s p ro d u ced by e z te n s io n a l and t r a n s c u r r e n t
t e c t o n i c s . R e g io n a l s tu d i e s s u g g e s t t h a t th e b a s in began
to form d u rin g l a t e M iocene tim e a s a c r u s t a l t e a r cau sed
by s o u th -s o u th e a s tw a rd movement o f s o u th e a s t A s ia .
T e c to n ic s and s t r u c t u r e o f Burma, S u m atra, Ja v a and th e
P h ilip p in e s a p p a r e n tly conform to t h i s h y p o th e s is . Move
m ents r e i n i t i a t e d d u rin g l a t e P lio c e n e o o n tin u e to th e
p r e s e n t, and r e s u l t e d in th e developm ent o f th e H ic o b a r
R i f t V a lle y a lo n g th e e a s te r n f la n k o f th e Andaman-
H ic o b a r R idge and th e C e n tr a l Andaman R i f t V a lle y . M axi
mum d e p th s o f n e a r l y 4400 m o c c u r in th e r i f t v a l l e y s ,
and v o lc a n o e s , in c lu d in g Harcondam and B a rre n i s l a n d s ,
a r e l o o a l i s e d a lo n g r i f t s .
The A ndam an-N ioobar R idge s e p a r a te s th e Andaman
B a sin from th e Bay o f B e n g a l. O ld e s t ro c k s o f th e r id g e
a r e l a t e C re ta c e o u s ( ? ) s e r p e n t i n i t e s , o p h i o l i t e s and
o h e r t s . These ro c k s a r e o v e r la in by th i c k grayw aokes and
s h a le s r e p r e s e n tin g d e t r i t u s sh ed from an e a s te r n h ig h -
s ta n d in g a r e a and d e p o s ite d in a P ale o g en e m a rg in a l
2
tro u g h . I t i s p ro b a b le t h a t th e h ig h a re a was b ev eled
to form th e M alay c o n tin e n ta l s h e l f , w hich was su b
s e q u e n tly r i f t e d from th e Andam an-Hicobar R id g e.
Below 1600 m d e p th s th e Andam an-Nlcobar Ridge
e f f e c t i v e l y c o n ta in s th e Is o th e rm a l and ls o h a lln e Andaman
Sea w a te rs , w hich a r e re p le n is h e d by movement o f Bay o f
B engal w a te rs o v er th e s i l l s . The Irraw ad d y R iv e r I s th e
p r in c ip a l sed im en t so u rc e f o r th e Andaman B a sin . Annual
Irraw addy sed im en t lo a d , fo rm e rly e s tim a te d a t 265 m illio n
to n s , may be a s la r g e a s 400 m illio n to n s . E ffe c tiv e n e s s
o f th e Andaman B asin as a sed im en t t r a p I s enhanced by
w e s te rly c u r r e n ts w hich sweep suspended sedim ent eastw ard
from th e r i v e r m ouths d u rin g th e so u th w e st m onsoonal
seaso n o f maximum r u n o f f and sedim ent lo a d . F a u lt d i s
t o r t i o n o f th e s o u th e a s te r n d e l t a - s h e l f may p ro v id e p re
f e r r e d d e p o s ltlo n a l s i t e s , a s se d im e n ta tio n s t r i v e s to
r e - e s t a b l i s h a low , s t a b l e d e l t a - s h e l f g r a d ie n t.
G r a ln - s lz e , o a rb o n a te and o rg a n ic c o n te n t, and c la y
m in era lo g y o f Andaman B asin sed im en ts e x h ib it p a tte r n s In
co n fo rm ity w ith p ro v en an ce, ooean c u r r e n ts , b io lo g ic
a o t i v i t y , p h y sio g rap h y and s e d im e n ta tio n r a t e s . The
o u te r p o rtio n o f th e d e l t a - s h e l f , th e M alay c o n tin e n ta l
s h e l f and th e i n s u l a r Andaman and H ic o b a r s h e lv e s , have
m eager ac cu m u latio n s o f se d im e n t. In th e s e a re a s s e d i
m ents a re c o a rs e , r e f l e o t l n g th e winnowing o f f in e s e d i
m en ts. O arbonate o o n te n ts a re la rg e lnasm uoh a s b io lo g ic
a c t i v i t y i s fa v o re d by low t u r b i d i t y and s h e l l d e b ris I s
u n d ilu te d by te r r ig e n o u s se d im e n t. O rganic m a tte r i s
low due to th e la o k o f a d s o rb e n t c la y s and b ecau se slow
b u r i a l allo w s tim e f o r o x id a tio n a t th e s u r fa o e . In con
t r a s t , sed im en ts o f th e in n e r Irraw addy D e lta s h e l f a re
f in e - g r a in e d , r e f l e c t i n g th e d a y and s i l t lo a d o f th e
Irraw ad d y , r a p id a c c u m u la tio n , and la c k o f w innow ing.
D ilu tio n by te rr ig e n o u s sed im en t ca u ses low o arb o n a te and
o rg a n lc -m a tte r o o n te n ts .
A pproxim ately 10 p e r c e n t o f th e Irra w a d d y -su p p lie d
sed im en t re a c h e s th e d ee p er p o r tio n s o f th e b a s in beyond
th e d e l t a . T his f in e - g r a in e d sed im en t i s augm ented by
f o r a m ln lf e r a l t e s t s , w hich com prise up to 15 p e r o en t o f
tro u g h se d im e n ts. S eah ig h s e d im e n ts , im p o v erish ed in
te rr ig e n o u s m a te r ia l and e n ric h e d in f o ra m ln if e r a , i s
c l a s s i f i e d a s g lo b ig e r in a o o ze . D e p o s ltlo n a l r a t e s on
th e d e l t a - s h e l f a re betw een 100 and 200 cm/1 0 0 0 y r s ;
r a t e s in th e d eeper b a s in a re o n ly 15 om/1 0 0 0 y r s .
The h is t o r y o f th e Burmese T e r tia r y B asin n o rth o f
th e Andaman Sea i s one o f lo n g it u d in a l i n f i l l i n g by
so u th w ard -en cro a ch in g d e l ta s o f r i v e r s a n c e s t r a l to th e
Irraw ad d y . The Andaman B a sin i s th e s o u th e rn , more
r e c e n t e x te n sio n o f th e Burmese T e r tia r y B a s in . D ep o sits
o f b o th b a s in s th u s r e p r e s e n t a c o n tin u o u s O enozoic s e d i
m e n ta tio n s ! ev en t w hich m ig ra te d so u th w ard .
INTRODUCTION
L o c a tio n and C ro ss C o n fig u ra tio n
The Andaman S e a , in th e n o r th e a s te r n In d ia n Ooean,
o
i s in a p h y s io g ra p h ic b a s in w ith an a r e a o f 8 0 0 ,0 0 0 km
( F ig . 1 ; P I . I ) . From th e Irraw ad d y D e lta c o a s t o f
Burma a t 1 6 °-1 7 ° N l a t i t u d e s th e Andaman S ea e x te n d s
1200 km so u th w ard to th e n o r th e r n S um atra c o a s t and
M alacca S t r a i t betw een 5° and 6° N l a t i t u d e s . The s e a
h a s a maximum w id th o f 650 km from th e w e st c o a s t o f th e
M alay P e n in s u la (9 8 ° E lo n g itu d e ) to th e w estern m o st i s
la n d s o f th e A ndam an-N icobar o h aln (9 1 .5 ° E lo n g it u d e ) .
T h is c h a in d e f in e s th e w e s te rn m argin o f th e s e a and
s e p a r a te s i t from th e Bay o f B e n g a l. The Andaman and
N ic o b a r is l a n d s a r e th e s u b a e r i a l e x p re s s io n s o f a
c o n tin u o u s r id g e w hich o o n n e o ts th e A rakan Toma M ountain
Range o f w e ste rn Burma to th e fe s to o n o f i s l a n d s s o u th
and w e st o f S u m atra . T o p o g ra p h ic a lly th e r id g e i s e x
c e e d in g ly com plex, f a l l i n g s te e p ly and I r r e g u l a r l y on
b o th s id e s to th e f l o o r s o f th e Bay o f B engal and th e
Andaman B a s in .
W aters o f th e Andaman S ea and Bay o f B en g al oon-
n e o t th ro u g h th e A ndam an-N ioobar r id g e by 11 c h a n n e ls
( F ig . 1 ) o f w hloh th e m a jo r on es a r e , from n o r th to
s o u th : P r e p a r ls C h an n el, o v e r 400 m d eep ; Ten D egree
F ig u re 1 . L o ca tio n o h a rt o f th e
Andaman S ea.
INDEX CHART
OP THE
ANDAMAN SEA
C hannel, named f o r th e p a r a l l e l s e p a r a tin g th e Andaman
and N lco b ar is la n d s and a p p ro x im a te ly 1200 m deep; and
G re at P a ssa g e , betw een G re a t N io o b ar and S u m atra, where
th e rid g e has i t s g r e a t e s t s i l l d e p th s o f 1400 to 1600 m.
The n o rth e rn and e a s te r n o n e - th ir d o f th e Andaman
B asin i s com prised o f th e sh a llo w Irraw ad d y D e lta s h e lf
and M alay s h e l f , r e s p e c tiv e ly 200 and 170 km b ro ad from
th e c o a s t to th e s h e lf - b r e a k . Along t h i s te r r a c e th e
bottom drops o f f r a p id ly from th e s h e lf - b r e a k to b a s ln -
f lo o r d ep th s betw een 1300 m and 2600 m.
A o e n tr a l tro u g h 2200 m deep a t th e n o r th e a s te r n
c o rn e r o f th e b a s in e x te n d s s in u o u s ly southw estw ard and
deepens to 4100 m a t i t s te rm in u s o n ly 3 km e a s t o f th e
Andaman-Nioobar Ridge in th e v i c i n i t y o f T illa n c h o n g
I s la n d . T his tro u g h s e p a r a te s th e Andaman B asin in to two
a re a e o f c o n tr a s tin g to p o g ra p h ic s t y l e s . West o f th e
tro u g h i s a com plex m osaic o f s te e p , e lo n g a te r i f t s and
seam ounts, in c lu d in g th e v o lc a n ic Naroondam and B arren
i s l a n d s . Topography s o u th and e a s t o f th e tro u g h i s le s s
co m p licated and in c lu d e s th e M alay C o n tin e n ta l S lo p e ,
th e c i r c u l a r s h e lf - b a a in o f f S um atra and th e M alacca
S t r a i t , and a s in g le e lo n g a te seam ount 200 km lo n g .
O v e ra ll av erag e d ep th o f th e Andaman B asin i s 870 m, and
th e g r e a t e s t d ep th o f 4390 m was sounded in a r i f t v a lle y
130 km e a s t o f Oar N io o b ar.
8
Purpose o f Study
I
Kuenen (1957) c ite d th e Andaman B asin as an
example o f a lo n g itu d in a l tro u g h b e in g f i l l e d from one end
r a th e r than tr a n s v e r s e ly . Study o f th e T e r tia r y geology
o f Burma I n d ic a te s th a t th e p ro o ess o f lo n g itu d in a l
f i l l i n g evidenced by p re s e n t-d a y p a tte r n s o f ru n o ff and
sedim ent supply began In th e e a r ly T e r t i a r y . A ccurate
d e p ic tio n o f th e b a s i n 's subm arine p h y sio g rap h y ; u n d er
s ta n d in g o f th e te o to n lc p ro c e s s e s w hich shaped and
a l t e r e d the d e p o s ltlo n a l b a s in ; and d e p ic tio n o f th e
manner In w hich th e b a s in I s b e in g f i l l e d a re th e main
o b je c tiv e s o f t h i s s tu d y .
S ources o f In fo rm a tio n
The p r in c ip a l d a ta em ployed In co m p ilin g th e
b a s i n 's morphology a re 42,900 km o f co n tin u o u s a c o u s tic -
sounding tr a c k s made by s h ip s p a r ti c i p a t in g In th e I n t e r
n a tio n a l In d ia n Ocean E x p e d itio n betw een I960 and 1964
( I n s e r t , P I. I ) . This f ig u r e In c lu d e s 6700 km from ------
O ru lses 33, 35 and 36 o f th e S o v ie t R esearch V essel
VITTAZ between I960 and 1963; 18,000 km from th e 1961 and
1963 o ru ls e s o f th e USS SERRANO; 3700 km from th e 1962.
o ru ls e o f th e R esearoh V essel ARGO; 3300 km from C ru ise 1
o f th e A N T O N B R IT O N In 1963; and 10,600 km from th e 1964
o ru ls e o f th e USC&GSS PIONEER. The U. S . Navy H ydro-
g ra p h ic O ffic e (now U. S . Navy O cean o g rap h ic O f f ic e ) h as
co m p lied a d d i tio n a l b a th y m e tric In fo rm a tio n from v a r io u s
s o u r c e s , p r i n c i p a l l y from so u n d in g s by th e R o y al In d ia n
M arine S urvey and th e B r i t i s h A d m ira lty p r i o r to W orld
War I I .
A ll so n lo so u n d in g s w ere c o r r e c te d f o r th e v e l o c i t y
o f sound In th e Andaman Sea w a te r colum n, u t i l i z i n g a
t a b l e o f c o r r e c t io n s com puted from h y d ro g ra p h ic d a ta and
em ployed on th e PIONEER d u rin g h e r c r u i s e (T a b le I ) .
P r i n c i p a l c o n tr o l on th e g r id o f so u n d in g tr a o k s was th e
PIONEER and VITYAZ d a t a , w hioh a r e a lr e a d y c o r r e c te d f o r
sound v e l o c it y and w hich a p p e a r to have e n jo y e d optimum
n a v i g a ti o n a l c o n d itio n s , ju d g in g from th e p r e c is io n o f
m ost a s tro n o m ic a l f i x e s and th e m u tu a l ag ree m en t betw een
P IONEER and VITYAZ so u n d in g s w h erev e r th e two s h i p s '
itn v w * .
tr a o k s c r o s s e d .
Only th e VITYAZ d a ta a r e r e p o r te d In m e te rs ; th e
d a ta from a l l o th e r s h ip s w ere c o n v e rte d to th e m e trlo
sy stem from th e o o rre o te d d e p th s In fa th o m s . W herever
n e o e s s a ry , c o p ie s o f th e o r i g i n a l p r e c i s io n d e p th re o o rd s
from th e SERRANO and PIONEER c r u i s e s w ere c o n s u lte d .
P r e o ls lo n d e p th r e c o rd s o f th e PIONEER t r a c k a r e o f e x
c e l l e n t q u a l i t y f o r p h o to g ra p h in g , and a r e u sed to
I l l u s t r a t e s a l i e n t f e a t u r e s o f p h y s io g ra p h y . P lg u re 2 I s
a c h a r t show ing th e lo c a ti o n o f th e s e p r o f i l e s .
A t o t a l o f 228 b o tto m sam p les w ere a n a ly z e d , ln o lu d -
10
TABLE 1 . C o rre c tio n s f o r th e v e lo c ity o f sound in th e
Andaman Sea w a te r colum n. A ll c o r r e c tio n s to
be added to o r ig in a l so u n d in g .
D epth D epth
I n t e r v a l , C o rre c tio n , I n t e r v a l , C o rre c tio n ,
Fathoms_________FathomB___________ Fathoms__________ Fathoms
0- 15 0 1353-1430 34
16- 35 3
1431-1508 36
36- 62 4 1509-1581 38
63- 92 5 1582-1651
40
9 3 - 124 6 1652-1712 42
125- 161 7 1713-1775
44
162- 202 8
1776-1835
46
203- 242 9
1836-1892 48
243- 283 10 1893-1950 50
284- 324 11 1951-2000 52
325- 372
12 2001-2050 54
373- 415 13
2051-2100 56
416- 461 14 2101-2150 58
462- 518
15
2151-2196 60
519- 566
16 2197-2242 62
567- 616 17
2243-2290 64
617- 708 18
2291-2335
66
709- 809
20 2336-2379
68
810- 908 22 2380-2420 70
909-1004 24 2421-2461 72
1005-1098 26 2461-2502 74
11
TABLE 1 (C o n tin u ed )
D epth
I n t e r v a l ,
1099-1185
1186-1267
1268-1552
C o rre c tio n ,
Pathoms
28
30
32
Depth
I n t e r v a l ,
Fathome
2503*2543
2544-2583
2584-2623
C o rre c tio n ,
Fathoms
76
78
80
F ig u re 2 . L ocation o f PIONEER PER
p r o f ile s employed as i l l u s t r a t i o n s
PIONEER
PDR PROFILE
LOCATIONS
EXPLANATION
TEXT-FIGURE
NUMBER
14
ln g : 80 s u r f a c e sam ples and 44 g r a v ity c o r e s ta k e n by
th e SERRANO In 1961; 14 s u rfa c e sam ples and 42 s h o r t
K u lle n b e rg p is to n c o re s from th e 1963 SERRANO o r u ls e ; 15
s u r f a c e sam ples from O ru lse 1 o f th e ANTON BRUUN; and 6
s u r f a c e sa m p le s, 10 la r g e p is to n c o r e s , 7 th erm o p ro b e
c o re s and 10 dredge h a u ls tak en d u rin g th e PIONEER c r u is e
( ? l g . 3 ) . L im ited su p p lem en tary co v erag e was o b ta in e d
from 255 sh ip b o a rd d e s c r ip tio n s o f se d im e n t sam pled by
th e R oyal In d ia n M arine Survey d u rin g th e 1 9 2 0 's and
1 9 3 0 's . D e s c rip tio n s o f 31 o o re s ta k e n by th e YITTAZ
p ro v id e d a d d i tio n a l in fo rm a tio n o f lim ite d v a lu e . An
a d d i ti o n a l 22 sam ples ta k e n by th e SERRANO in th e s o u th e rn
end o f th e s e a were a n a ly z e d by s t a f f members o f th e
O cean o g rap h ic O f f ic e , who c o o p e ra tiv e ly s u p p lie d th e r e
s u l t s o f t h e i r w ork.
The r e s u l t s o f a l l sed im en t a n a ly s e s co nduoted
d u rin g t h i s in v e s tig a tio n a re f i l e d w ith th e N a tio n a l
O cean o g rap h ic D ata C e n te r and w ith th e U n ite d S ta te s
N av al O oeanographic O ffic e in W ashington, D. 0 . C opies
o f th e d e t a i l e d d a ta summary a re a v a ila b le from th o s e
o f f i c e s upon r e q u e s t.
H y d ro g rap h ic d a ta in th e a r e a I n c lu d e : 34
s t a t i o n s o cc u p ied by th e VITYAZ d u rin g h e r 3 3 rd and 3 5 th
c r u i s e s ; 63 s t a t i o n s o ccu p ied by th e SERRANO in 1961 and
1963; and 5 s t a t i o n s o ocupied by th e PIONEER ( P ig . 4 ) .
The R u ssia n d a ta a re on f i l e a t W orld D ata C e n te r A and
F ig u re 3# In d ex c h a r t o f sed im en t sam ples
t? M n •i • 4 M H ft
ANDAMAN BASIN
BOTTOM-SAMPLE
LOCATIONS
OF
B E N G A L
MWfWM, M ill
• m m m m m r a r v r t m i
2 P
F ig u re 4 . L o catio n s o f H ydrographic S ta tio n s *
18
HYDROGRAPHIC
STATIONS
2890
V M M
m
• v SSSf'
4 9 7 2
AV4984
W 2792
m
3$
J V8230 224
1 0
9 V 4 9 7 8
•1-25 \l
3*1-24
3*3-4
1 0 0 0
q S * 3 - 2
O tmIm I t tw»lt 4tpM » it Mow till! #
O rttlttl ttmplt dtfth it *b«M t*fc O H ! '2
P -------------- PIONEER, 19*4
8*1------------ SCMMNO, 19*1
8 * 3 ----------- SCMUN0.I9U
V4000 ttritt-VITVAZ, 19*1
VSOOO ttfitt-VITVA2,19*2
V8300
S -
19 :
i
th e o th e r d a ta a re a v a ila b le in th e a r o h lv e s o f th e
N a tio n a l O cean o g rap h ic D ata C e n te r In W a sh in g to n .
P re v io u s I n v e s ti g a ti o n s
G eo lo g ic s t u d i e s In th e re g io n o f th e Andaman
B a sin d a te from th e m id -n in e te e n th c e n tu r y . The D an ish
c o r v e t t e GALATHEA v i s i t e d th e N lco b a r I s la n d s In 1846,
and R ink (1847) d e s c rib e d t h e i r s u r f l c l a l g e o lo g y . The
v i s i t by th e A u s tria n f r i g a t e N O V A RA In 1860 was th e
b a s i s o f p u b lic a tio n s by H o c h s te tte r (1866) and Schw ager
(1 8 6 6 ) on th e p h y s ic a l g eo lo g y and f o s s i l m ic ro fa u n a ,
r e s p e c t i v e l y , o f th e N lo o b a rs . I s la n d s o f th e Andaman-
N lc o b a r c h a in w ere e x p lo re d by th e B r i t i s h g e o l o g i s t s
B a ll (1 8 7 0 ), Oldham (1885) and T ip p e r (1911)# and have
been mapped In some d e t a i l by g e o lo g is ts o f th e G e o lo g ic a l
S u rv ey o f I n d ia (G ee, 1926; R ao, 1942; Ghose and M a ltr a ,
1947; Ja co b and S a s t r l , 1951; Ja c o b , 1954; J a i n , 1958;
R a ln a , I9 6 0 ; O h a tt e r je e , 1961; C h a t t e r j l , 1964;
K a ru n a k a ra n , Pawde, R aln a and Ray, 1964; K a ru n a k a ra n , Ray
and S a h a , 1 9 6 4 a, b ) . The v o lc a n ic Narcondam and B a rre n
I s la n d s w ere f i r s t d e s c rib e d by Hobday and M a lle t In
1885 and l a t e r by W ashington (1 9 2 4 ).
I n v e s ti g a t i o n s o f th e O enozolo h i s t o r y o f Burma
a r e h ig h ly r e l e v a n t to th e g eo lo g y o f th e Andaman B a s in ,
an d p u b lic a t io n s by Stamp (1 9 2 2 , 1925, 1927> 1 9 4 0 ),
O h h lb b er (1 9 3 4 ), Evans (1941) and T a ln sh (1 9 5 0 ) have
20
p ro v id e d im p o rta n t background m a te r ia l f o r th e p re s e n t
s tu d y . The g r a v ity an o m alies o f Burma and t h e i r g e o lo g io
s ig n if ic a n c e have been s tu d ie d by Evans and Crompton
(1 9 * 6 ).
B r i t i s h work in th e Malay P e n in s u la o v er th e l a s t
c e n tu ry h as been sum m arized by S o riv e n o r (1931) and more
r e c e n tly by A lex an d er (1 9 6 2 ). Of th e many g e o lo g is ts who
have e tu d le d th e In d o n esia n p e rip h e ry o f th e Andaman S ea,
Umbgrove (1947, 1 9 5 0 ), van Bemmelen (1 9 4 8 ), Klompe (1957,
1 9 6 2 ), and V ening M einesz (1934, 1964) have w r itte n
im p o rta n t sum m aries o f th e r e g io n 's g e o lo g ic h i s t o r y .
B a th y m e tric s tu d ie s o f th e Andaman Sea w ere
i n i t i a t e d in 1826 o v er th e sh a llo w Irraw addy D e lta s h e l f
by s h ip s o f th e B r i t i s h A d m iralty . S ubsequent su rv e y s in
1854, 1859, and 1911-1912 a ls o were concerned p r im a r ily
w ith th e Irraw ad d y D e lta s h e lf and th e e s tu a r ln e ap
p ro ach es to Burmese p o r t s . Deep soundings w ere f i r s t
c o l le c te d from th e R oyal In d ia n M arine Survey S h ip
INVESTIGATOR in 1910, and th e f i r s t c o n to u re d b a th y m e tric
c h a r t o f th e b a s in was p u b lis h e d by S ew ell in 1925.
S ew ell a ls o d e s c rib e d dredged m a te r ia ls from th e Andaman
S ea (1 9 2 5 b ), th e te m p e ra tu re and s a l i n i t y o f Andaman Sea
and Bay o f B engal w a te rs (1928, 1929, 1 9 3 2 ), and th e
d i s t r i b u t i o n o f c o r a l r e e f s in In d ia n w a te rs (1935)*
A ll o o ean o g rap h lc d a ta o o lle o te d in th e re g io n
p r i o r to I960 w ere sum m arized by th e U. S . Navy O ceano-
g ra p h ic O ffic e ( I9 6 0 ) . T his summary I s th e p r i n c i p a l
s o u rc e o f th e r e g io n a l s u rfa c e oceanography d e s c rib e d In
t h i s p a p e r.
I n t e r n a t i o n a l In d ia n Ocean E x p e d itio n c r u is e s o f
th e e a r ly 1 9 6 0 's r e s u l t e d In s e v e r a l p e r t i n e n t p u b lic a
t i o n s on th e oceanography and m arine g eo lo g y o f th e
n o r th e a s te r n In d ia n O cean. The VITYAZ c r u i s e s y ie ld e d
s h o r t p a p e rs on b ath y m etry (B ezrukov and K ayanev, 1963;
Z a to n sk y , 1 9 6 4 ), c u r r e n ts and w a te r m asses (F o m ltch ev ;
Iv an o v ; P a n f llo v a ; S a b In In ; T areev ; a l l 1 9 6 4 ), ch em ical
o cean o g rap h y (Iv an en k o v ; Iv an en k o v , V ln to v k ln and
S h atz k o v ; K abanova; Rozanov; Rozanov and Bykova; a l l
1 9 6 4 ), sed lm en to lo g y (B ezrukov; G ordeev; I s a e v a ;
S h is h k in a and Z h elezn o v a; Z heleznova and S h is h k in a ; a l l
1 9 6 4 ), and g e o p h y s lo a l m easurem ents o f se d im en t t h i c k
n e s s e s (N eprochftov, 1 9 6 4 ). At p r e s e n t, B ezrukov and
L l z l t s l n o f th e Akademlya Nauk USSR a r e p re p a rin g a
t r e a t i s e on th e oceanography o f th e In d ia n Ocean embody
in g m ost o f th e enum erated w ork.
The c r u is e o f th e PIONEER h as r e s u l t e d In th e
p u b lic a tio n o f re c o n n a is s a n c e g e o p h y s lo a l p a p e rs by
B urns (1 9 6 4 ); W eeks, H arblson and P e te r (1 9 6 5 ); and P e te r ,
Weeks and B urns (1 9 6 6 ). A se d lm e n to lo g lo s tu d y o f th e
M alacca S t r a i t In v o lv in g SERRANO and PIONEER d a ta h as
been com pleted by K e lle r and R ic h a rd s (1 9 6 7 ). F re rlo h s
(1967) h a s w r i tte n a m le ro fa u n a l a n a ly s is o f th e se d im en t
22
sam ples used in th e p re s e n t s tu d y .
Acknowledgments
The w r i te r w ishes to ex p re ss h is g r a tit u d e to th e
E n v iro n m en tal S c ie n ces S e rv ic e A d m in is tra tio n and C oast
and G eo d etic Survey o f th e U n ited S ta te s D epartm ent o f
Commerce f o r th e o p p o rtu n ity to g a th e r d i s s e r t a t i o n d a ta
w h ile p a r t i c ip a t i n g In th e o ru ls e o f th e USC&GSS PIONEER
to th e In d ia n Ocean; and in p a r t i o u l a r , to D r. H. B.
S te w a rt, J r . o f th e I n s t i t u t e f o r O ceanography, f o r h i s
s u s ta in e d I n t e r e s t and o o o p e ra tio n . C ap t. E . B. Brown
and th e e n t i r e wardroom and crew o f th e PIONEER were un
s t i n t i n g o f t h e i r h e lp d u rin g th e c r u is e .
The U n ited S ta te s Navy O ceanographic O ffio e
p ro v id ed sedim ent sam p les, b ath y m etry , and h y d ro g ra p h ic
d a ta . M essrs. G. H. K e lle r , R. H. S tew art and 0 . B.
E ch o ff o f t h a t o rg a n iz a tio n were generous w ith t h e i r
tim e and e f f o r t to p ro v id e th e s e m a te r ia ls .
A rohived o cean o g rap h ic d a ta were f r e e l y o b ta in e d
from th e N a tio n a l O ceanographic D ata C e n te r, th ro u g h th e
k in d o f f lo e s o f M rs. S . B ersh ad . S o v ie t b a th y m e tric and
h y d ro g rap h lo d a ta in th e f i l e s o f th e World D ata C e n te r A
p ro v id ed v ery u s e f u l In th e s tu d y , and Mr. B. Richmond i s
th an k ed f o r th e e f f ic ie n c y and speed w ith whioh he s e n t
t h i s in fo rm a tio n .
L ab o ra to ry f a c i l i t i e s and o f f io e space were
p ro v id e d by th e A llan Hancock F o u n d a tio n . M r. P . B arn es
a s s i s t e d in v a rio u s la b o r a to r y a n a ly s e s . M r. W. E.
F r e r io h s p erform ed m ic ro p a le o n to lo g ic a n a ly s e s on many
o f th e sam ples as a d i s s e r t a t i o n p roblem , and s h a re d h i s
r e s u l t s w ith th e a u th o r . Mr. P . F le is c h e r a s s i s t e d in
p e tro g r a p h ic d e s c r ip tio n s o f dredged r o c k s .
C r i t i c a l re a d in g o f th e m a n u sc rip t by D rs . D. S.
G o r s lin e , G. A. D a v is, J . W. R e lth and R. 0 . S to n e o f
th e U n iv e r s ity o f S o u th ern C a lif o r n ia i s m ost a p p r e c ia te d .
D r. 0 . L. Bandy p ro v id ed many h e l p f u l s u g g e s tio n s and
oomments and a rra n g e d f i n a n c i a l a s s is ta n c e f o r th e s tu d y .
T h is stu d y was su p p o rte d by G ra n ts N os. GP-2530
and GA-730 o f th e N a tio n a l S c ie n c e F o u n d a tio n .
PHYSIOGRAPHIC A N D G EO LO G IC SETTING
Rep* jn a l Z o n atio n
The Andaman B asin l i e s in a re g io n r e p r e s e n te d by a
s e r i e s o f to p o g ra p h ic b e l t s whioh tre n d sou th w ard from as
f a r n o r th as th e e a s te r n H im alayas and cu rv e e a stw a rd
from Sum atra tow ard Ja v a (F ig . 5)* From e a s t to w est
th e s e l i n e a r p h y sio g ra p h ic zones a r e :
1 . The Ja v a T ren ch , a lth o u g h i t i s n o t d e a r l y
re p r e s e n te d in th e l a t i t u d e s o f th e Andaman B a s in .
2 . A zone o f h ig h r e l i e f w hich in c lu d e s th e
A rakan Yoma m ountain ran g e o f w estern Burma; th e Andaman-
N lco b a r R id g e; and th e is la n d c h a in s o u th and w e st o f
S um atra in c lu d in g S im elu e, N ia s , th e B atu gro u p and th e
M entaw al I s la n d s . F o r b r e v ity t h i s zone i s r e f e r r e d to
a s th e A rakan-M entaw al Range.
3 . A zone o f to p o g ra p h ic lows w hich in c lu d e s th e
Irraw ad d y V a lle y o f Burma; th e deep w e ste rn p o r tio n o f
th e Andaman B a sin ; th e tro u g h w hich tr e n d s n o r th e a s t-
s o u th e a s t s e p a r a tin g th e N lco b ars from S u m atra; and th e
M entawal Trough betw een th e M entawal I s la n d s and
S u m atra, h e r e i n a f t e r r e f e r r e d to as th e Burma-M entawal
T rough.
A. A p e r ip h e r a l e a s te rn zo n e, th e E a s te rn M a s s if,
o f r e l a t i v e l y h ig h -s ta n d in g a re a s in o lu d ln g th e Shan
F ig u re 5* R eg io n al p h y sio g ra p h ic s e t t i n g
o f th e Andaman B asin
26
» a y
or
BENGAL
e u tr
27
P la te a u o f e a s te r n Burma; th e M alay P e n in s u la and I t s
w e ste rn s h e l f ; th e M alacca S t r a i t , and Sum atra*
Ja v a T renoh
The Ja v a T rench e x te n d s f o r 4000 km so u th w e st o f
Sum atra and s o u th o f Ja v a and th e L e s s e r Sunda Is la n d s *
In I t s e a s te r n h a l f w hloh i s w e ll- d e f in e d and n arro w ,
maximum d e p th s o f 7500 m a r e p r e s e n t s o u th o f J a v a ,
w here th e to p o f th e seaw ard s lo p e o f th e tre n o h s ta n d s
3000 m above i t s a x is* As th e tr e n c h o u rv es n o rth w e s t
w ard a lo n g Sum atra i t i s p o o rly d e fin e d and c o n s id e ra b ly
sh a llo w e r* The tr e n c h can be tr a c e d w ith c e r t a i n t y o n ly
to a few d e g re e s n o r th o f th e E q u a to r to n e a r th e
n o r th e r n t i p o f Sum atra* H ere th e g r e a t e s t d e p th s a r e
o n ly s l i g h t l y more th a n 5000 m (H eezen and T h arp , 1 9 6 4 a ),
and th e tre n o h a x is l i e s l e s s th a n 500 m below th e s e a -
f l o o r to th e s o u th .
E a s t-w e s t p r o f i l e s a o ro s s th e A ndam an-N loobar
R idge show a b ro ad d e p re s s io n e a s t o f th e is la n d s whioh
S ew ell (1925) r e f e r r e d to a s th e I n v e s t i g a t o r B eep, and
w hich su b se q u e n t w o rk ers (B ezrukov and K ayanev, 1963)
assum ed to be th e n o r th e r n c o n tin u a tio n o f th e Ja v a
Trench* More r e o e n t in v e s t ig a t i o n s r e v e a l t h a t t h i s i s
n o t th e oase* An e a s t- w e s t p r o f i l e a lo n g th e 5° N
p a r a l l e l from a 1964 PIONEER t r a v e r s e shows th e tre n o h to
be m is s in g (? lg * 6 ) . Along t h i s p r o f i l e , th e w e ste rn
F ig u re 6 . R e c o n stru c te d b a th y m e tric p r o f i l e
from PIONEER tr a v e r s e a c ro s s
A ndam an-N icobar R id g e, show ing
e a s t f la n k o f E a s t In d ia n Ridge
and th e low a r e a betw een th e
r id g e s .
K X L .
N
90
PROFILE
T * WEST-EAST BATHYMETRIC PROFILE ACROSS-
EAST INOIAN AND ANDAMAN-NICOBAR RIDGES
94*E
VERTICAL EXAGGERATION • S4.3X 3 0 0
" r i --------
200 1 0 0
HORIZONTAL SCALE (KILOMETERS)
“I
0
ro
vo
slo p e o f th e Andam an-Nicobar Ridge m eets th e f l a t f lo o r
o f th e Bay o f B engal a t a d ep th o f 4400 m, th en r i s e s
v ery g e n tly to th e w e st, s h o a lin g o n ly 400 m in 100 km
o f t r a v e r s e . The bottom o o n tln u e s to r i s e g e n tly f o r
a n o th e r 40 km, to 3800 m. S h o a lin g th en beoomes s l i g h t l y
more a b ru p t, and a peak 2900 m deep i s reaohed a t 91° E
lo n g itu d e , 200 km w est o f th e Andaman-Nicobar R idge.
T h is prom inence i s a p o rtio n o f a l i n e a r to p o g rap h ic
f e a tu r e w hich h as been v a r ia b ly c a lle d th e E a st In d ia n
Ridge (B ezrukov and K ayanev, 1963)* th e C a rp e n te r Ridge
(S e w e ll, 1925) and th e B engal Ridge (S to c k s, I9 6 0 ).
Bezrukov and Kayanev (1963) p o in t o u t th a t th e l a t t e r
name i s in a p p r o p r ia te , in view o f i t s p e r s is te n c e f a r
s o u th o f th e Bay o f B en g al. Heezen and Tharp (1964b)
r e f e r to t h i s same f e a tu r e a s N in ety E a st R idge, named
f o r th e m e rid ian p a r a l l e l to much o f i t s le n g th . The
r id g e l i e s betw een E lo n g itu d e s 90° and 8 8 °, tre n d in g
s l i g h t l y w est o f so u th from 10° N to 34° S, a d is ta n o e
o f ap p ro x im a tely 3000 km.
D epths o f th e Ganges A byssal Fan w est o f th e E a st
In d ia n Ridge a re oom parable to th e 4400 m d ep th o f th e
f l a t p la in betw een th e E a st In d ia n and Andaman-Nioobar
r id g e s . Thus, i t would a p p e ar t h a t S e w e ll's " I n v e s t i
g a to r Deep" m erely m arks th e low betw een th e two r id g e s ,
and t h a t th e Jav a Trenoh e i t h e r does n o t ex ten d to th e
l a t i t u d e s o f th e Andaman B a sin , o r may be f i l l e d by
sed im en t.
A rakan-M entaw al Range
P h y sio g rap h y
The 1100 km -long A rakan Yoma Range o f w e s te rn Burma
I s a s o u th w a rd -tre n d in g b ra n c h o f th e e a s te r n H im alay as.
T h is s in u o u s ran g e I s oonoave tow ard th e e a s t alo n g I t s
n o r th e r n h a l f and concave tow ard th e Bay o f B engal In
I t s s o u th e rn p o r tio n . To th e n o r th I t m erges w ith th e
P a tk o l, L u s h a l, N aga, M anipur and Ohln h i l l s , fo rm in g a
oom plez o f s u b p a r a l l e l n o r th - n o r th e a s t- tr e n d ln g r id g e s
some 240 km w id e. Along i t s e n t i r e le n g th th e ra n g e
d is p la y s a m arked assym m etry, w ith c r e s t s lo c a te d w e ll
to th e e a s t o f th e r a n g e 's a x i s . B e st known o f I t s peaks
I s M t. V i c t o r i a , w hich s ta n d s ap p ro x im a te ly 3055 m above
s e a l e v e l midway a lo n g th e ra n g e , ab o u t 550 km s o u th o f
S a r a m a tl. To th e s o u th o f M t. V lo to r ia th e ra n g e c r e s t
m a in ta in s e l e v a t io n s In e x c e s s o f 1000 m f o r 400 km,
b e fo re th e r id g e d is a p p e a rs u n d e r th e a l l u v i a l f l a t s o f
th e Irraw ad d y D e lta .
The same s t r u c t u r a l tre n d m a n ife s ts I t s e l f a g a in
s u b a e r l a l l y 135 1 c m so u th o f th e Irraw ad d y ooaBt In
P r e p a r ls I s l a n d , n o rth e rn m o st o f th e I s la n d sum m its o f
th e A ndam an-N loobar R id g e . A summary o f th e more
p ro m in e n t r id g e I s l a n d s , w ith t h e i r s iz e s and h ig h e s t
32
e le v a tio n s , i s p re se n te d In T able 2 .
The assym m etrlo d i s t r i b u t i o n o f maximum e le v a tio n s
in th e Arakan Yoma i s c o n tin u ed a lo n g th e Andaman and
N ico b ar is l a n d s , a lth o u g h th e assym m etry i s b lu r re d by th e
s u b a e r ia l d is c o n tin u ity o f t h i s p o rtio n o f th e A rakan-
M entawal Range. The m a jo rity o f th e is la n d s p o sse ss
s te e p e a s te r n s lo p e s and i r r e g u l a r e a s te rn s h o r e lin e s
and r e l a t i v e l y g e n tle w estern s lo p e s and r e g u la r w estern
c o a s ts .
S outh o f G re at N icobar d e ta ile d b ath y m etry re v e a ls
a lig n e d subm arine h ig h s o r seam ounts w hich tre n d s o u th
e astw ard alo n g th e so u th w est c o a s t o f Sum atra u n t i l th e
A rakan-M entaw al Range i s again ex p ressed s u b a e r la ll y , in
a ch ain o f is la n d s se p a ra te d from Sum atra by th e M entawal
T rough. The m ajor is la n d s o f th e M entawal fe s to o n a re
enum erated and t h e i r dim ensions d e fin e d in T able 2 .
S tra tig ra p h y
The co re o f th e Arakan Yoma c o n s is ts m ain ly o f
s tr o n g ly fo ld e d and c o n to rte d O retaoeous sh a llo w -w a te r
band8to n e s , con g lo m erates and s h a le s , w ith some lim e
s to n e s , la v a s , t u f f s and ag g lo m erates (T ab le 3)« L ate
O re ta o e o u s(? ) s e r p e n tln it e s in tr u d e th e s e sed im en ts
a lo n g th e Arakan Yoma's c o re , e a s t o f th e r a n g e 's a x i s .
The s e r p e n tln it e s a re more r e s i s t a n t to e ro s io n th an th e
su rro u n d in g ro o k s , and oause th e to p o g ra p h io assym m etry o f
NICOBAR ISLANDS A N D A M A N ISLANDS
TABLE 2 . Summary o f is la n d s a lo n g th e Andaman, N icobar
and M entawai fe s to o n s (from n o r th to so u th )
Maximum
E le v a
tio n (m)
L ength
(km)
W idth
?km)
Appro:
Aro&
K m 2
P rep ariB 81 6
1 .5
9
G re at Coco 112
9
2 18
G re at Andaman Grouo
N orth Andaman 732 79 25
1280
M iddle Andaman
511
96 30
1775
S outh Andaman 463 85
28 1620
N orth S e n tin e l 122 8 8 60
R u tlan d 438 19
11 165
L i t t l e Andaman 190 43
24 970
Car N ioobar
89 15
12
165
B a tti Malv 46
1 .5
1 1
Chawra 104
3 1 .5 3
T illan ch o n g 323
16 2
25
T eressa
273
20
5
78
Bompoka
197 3
2
5
Camorta 224 26 6 140
T rin k a t low 10
1 .5
10
K a tc h a ll 254 20
13 156
Nancowry
163 13 7 52
L i t t l e N ioobar 438
23
11 138
G re at N ioobar 824
53
26 865
M E N T A W A I FESTOON
34
TABLE 2 (C ontinued)
Maximum A pprox.
E le v a - Length W idth A rea, !
tio n (m) (km) (km) K m 2
Sim elue 567
100 26
1905
N ias 886 130 40 4020
B atu Group 270 - - -
S ib e r u t 406 110 40 3400
S ip o ra 313
50 22 880
N orth and South
P agai 372
- - -
Mega low r e e f 4 .5 1 .5 5
Enggano 28 36 12
345
I
l
IM A M M T VAUCT
(MU le le s ^ im )
(IIM M MA AICMM Hi)
( U N I , I t M i C M tlA PjA A
-------------- i t t A i — ...............
m r
I tL M M i t I T Af tM M T M
i M M l M , ( M t )
K tClflT A M
AVA.ICCMT
I r r M i M i A M t m i * | A l I l K
U H aA CAPAl P M f * l P ltA f
l l l l f l m AM N l f k M m * H i .
( M i l « M AAfAl I ( M l I AAAI l
CA PA IIH A M A M «AAM.
tlllA A CAPAl fA A fl AAA AAPA M A l
Af AAM f ((M ltA A A .
CAAflAM PAlAA. f r l t l , fAlAA*
aaA A a A aaaAa * I i N H I A A U
d i l l .
AAA| c IIU A A A A . CAUAPAAAA
aaA f A r n a l M M ca a c paI I a m i
aAaM aa! i H l d f l N M A A i
• m a U a i rM A lA I.
I l C A B t U l l f f lA V lA lllA .
• v t r MAA • l A l c l .
I a U pAa M aA A A M H tU aaA
AAAAHU l l f f l AAA lAAAA
(A I A# A t. N f A r t f l H .
‘ OAAfAA a H apA A IIA A I*.
• A f U l f A ltA fA A ttA A M lA
•AAAl AAA C l A f l | AftAA
f
raA aA i A lA fi ffM A A A tly
A a I a a I M .
A K W i U U I .
( I l l l l l t AAlCAtAM A•
N A U lV A l fflA A lA IA
• A l I . I M i r A l M .
M aI I av a a pI M fAAAA.
« i it i» m • u n i.
* f* M A C lA fl AAA IAAAHAAAI.
CAACPAttAM Pf I fP A f tA
I 1 N | (A C A lIf i n i l f i r N I I
AAVAPAl U lA IA M K A M I .
M aM aaI i Aa I I aa AAPlAA
n m • u n i. fAAAI. M l
H H l A t | I A | IAAAIIAAAI.
SM tlee m p Iaa>
A -IM A • u n i.
a a m m i c I a j t i i i a i I a i « iU i
• • A a p A Ia a Ia i a a M I A a a i i
m p I a a i a * U M M a k l j l k
I l M l l l A I I AAAI (AAAI | IA a IIA *
i
i:
ijis
]
*
a;
(R a p I m m M i I a m a «AA i AaI a i
APAAm A fPAA AAIIAPA f(AAA
aaA aaaM aAa Af I N A aAaaaa*
■ U aAa p IIA m )
lAtAPVlAA 11 A l l (AAA K l t A -
K IaIIAV m p I aa fAIAA.
(P flllA C A A A l l(A |A .
I l t l f l t fAAAA.
JkrAAACAAAt I t l t A « IIA A ffA f
AAlCAPAAlU AAA l l r t fP A M U
AAAfAA I A f I aA U K aA.
•PAfVAClA ItA fA *
M f A l l l 1 1fAPAAlI AAAIPAll
(A P A U IIA ItPA C tA PA II
I a i i i a I fAAAA. M a p
— a n i c k .
l l l l A l CAPAl P A A fl| A A lllV A IA A «|
fAPAAlAlfAPAl I N CAPAl 11A A
llA A ltA A A l AAA A A Plf llA A ItAAA.
M A ( A i d .
A A Plf IA M IIA A A I A A A llA A ltA A A l.
• P A d llfe (A fP A lh -M tA P fAAAA.
I M A ( A i d .
CAACPAllAAAPf lA A iK A A A l, (A C A lIf
I l f A l t l C .
iP A C k lll (A fPAIfc«AA(AP fAAAA.
1 M A U U I .
T i f f l l (A fflCA A A l A A M ltA N I*
• I llllA A A l A N IkA lA A .
IkA llA A AAPlAA AAi lP A C l1 |h 1a
(A M P lA tllA A . AAA(AAfA« lAAAPC
(AA A f A C ll. I M - f M A U U I .
A pA I I aU a I a a pI i .
th A llA A AAPlAA.
N tA IM A A U U I .
•AAAl CA A flAAAPlUt AM AAM -
I t AAAI | lA A ll U lP M lA A l Af
A A PA A A tH U M I f p M f lA A l
A A fU AM A llPA A A fU PACl.
I A M A U U I .
IAA A lA lA A l Ik A llA A AAPlAA
A U U I .
• r l t • ! • « •
N . W I . H ( M . U I M I
CAAAIAAAPAIA A lA fA '
U a IIAA AAlAP fAAAA. V l t l M lA M A t
lA A A flN AlAlAA
fAAAAfA CAAfICAAPAtAA
U A llA A AAPlAA 11 AAA (A N
lllll.
M i l U tlA C I M I
UPAIAA TAM A«
M 4A A A A »IU M A P
I I A f t )
C -T IR T IM T *
( U I aMA N i l
Af M m ( p a )
I a p I a a a M i a m a a M u a I a a
(* M f P A U M P lA A " ) I 1A(AP*
f e t M d U I a H n i I a a a i a m
a A a M a ( • • a - * M p I a a * ) i
a a pa a a a ia u m f A i m i u n
(A lPA A lA M a i a m U A M a I U
I a p a a A M I c f f i ( 'A a U I
M p I a a * ) . __________________
M p m a I I a U M M f lM I U A A .
M a m U I M a I U a m m i h a a .
f l l l w I a v a i a m p M H I a pI aa
cA A P lI.
•fP A » fA P ((A P f* fkflinu A U U t l
AM C A M aIaM B a M A A A P tlllA II
A M A IM IH a a a I U f M P d PA lA I.
TABLE 3 . Summary o f r e g io n a l T e r tia ry e tr a tig r a p h y
In Burma and th e Andaman B asin a re a
3 6
th e A rskan Toma. C h h lb b er (1934) d e s c rib e d th e s e u l t r a -
m a fic I n tr u e I v e s and n o te d la v a s and t u f f s a s s o o la te d
w ith them . P le ld r e l a t i o n s h i p s le d C legg (1941) to I n f e r
a l a t e C re ta c e o u s to e a r l y Eocene (F a le o c e n e ) tim e o f
u ltr a m a f lc em placem ent.
The s e r p e n t i n i t e b e l t c o n tin u e s s o u th o f th e A rakan
Yoma, fo rm in g th e e a s te r n c o re s o f th e m ajo r Andaman and
E lc o b a r I s l a n d s . In c o n t r a s t to th e s h a llo w -w a te r n a tu r e
o f th e se d im e n ts in tr u d e d by th e A rakan Yoma u ltr a m a f lo s ,
th e s e r p e n t i n i t e 8 o f th e Andaman and N lc o b a r is l a n d s a p
p e a r to In tru d e r a d i o l a r i a n c h e r ts b e lie v e d to have been
d e p o s ite d in an a b y s s a l en v iro n m en t (J a c o b , 1954) and a r e
a s s o c ia te d w ith o p h i o l i t l o v o lc a n ic s (K a ru n a k aran , Ray
and S ah a, 1 9 6 4 a ,b ; K aru n ak a ran , Pawde, R ain a and Ray,
1 9 6 4 ). !
R e cen t s t r a t i g r a p h l c and m ic ro p a le o n to lo g ic work
in th e Andamans ( C h a t t e r j l and P a n t, 1964; C h a t t e r j i ,
1964) in d i c a t e s t h a t a c o n tin u o u s sequence o f P ale o o en e
th ro u g h u p p e r M iocene ro c k s a t l e a s t 3300 m t h i c k was
d e p o s ite d o v e r th e s e r p e n tln iz e d p e r i d o t i t e s u n d er
i
f l u c t u a t i n g sh a llo w to deep w a te r c o n d itio n s . The t h l o k - ;
e a t seq u en ce c o n s i s t s o f o v e r 3000 m o f l a t e Eocene and I
O lig o c en e g ray w ack es.
The g eo lo g y o f th e fe s to o n o f is l a n d s so u th w e st o f
S um atra i s p o o rly u n d e rs to o d , and th e l i t e r a t u r e o o n s is ts
!
o f r e f e r e n c e s to u n p u b lis h e d re o o n n a issa n o e r e p o r t s o f th e
37
1 9 3 0 'b (van Bemmelen, 1 9 ^ 9 ). P r e - T e r tia r y m etam orphosed
sed im en ts oom prlse the o ld e s t fo rm a tio n s o f S lp o ra , N ias
and S im alu r is l a n d s . Lower T e r tia r y b a s a l co n g lo m erates
and s a n d s to n e s , In tru d e d on S ip o ra and N ias by s e r p e n tln -
lz e d m afic and u ltr a m a f lc ro c k s , unconform ably o v e r lie
th e m etam orphosed basem ent. S h allo w -m arin e and b ra c k is h -
w a te r m a rls , lim e s to n e s , sa n d sto n e s and s h a le s uncon
form ably succeed th e P aleogene se d im e n ts. Lower to
m iddle Miooene t u f f s and tu ffa o e o u s e l a s t i c s a re found
in m ost o f th e Mentawal c h a in .
S tru c tu r e
The Arakan-M entawal Range i s a com plex a n t l -
c lln o rlu m w hich i s s tro n g ly fo ld e d and r e v e r s e - f a u lt e d
w estw ard in th e Arakan Yoma and in th e Andamans, and
southw estw ard in N ia s . L i t t l e i s known re g a rd in g th e
s tr u c t u r e o f th e Arakan Yoma, a s id e from th e ex trem ely
fo ld e d and f a u lte d n a tu re o f th e C retao eo u s se d im e n ts, and
t h e i r in tr u s io n by upper C retaceo u s s e r p e n t l n l t e s . Along
th e e a s te r n T e r tia r y f o o t h i l l s o f th e A rakan Yoma f a u l t
in g o f th e P aleooene fo rm a tio n s i s common and h as been
d e s c rib e d by Clegg (1941, p . 87) a s " u n d e rth r u s tin g from
th e w est" w ith "a r e s u l t a n t r e p e t i t i o n o f th e o ld e r ro o k s
exposed in th e e a s te r ly lim b a o ro ss th e f a u l t s to th e
w est • • . N.
C retaceo u s s e r p e n tln lt e s and P a le o o e n e -to -lo w e r-
38
i
M iocene s e d im e n ta ry ro o k s in th e Andaman and N lo o b ar i s -
la n d s a p p e a r to be s tr o n g ly fo ld e d an d r e v e r s e - f a u l t e d
to w ard th e w e s t. P l a t - l y i n g U pperm ost M io o e n e -to -R e o e n t
e l a s t i c s , s h a llo w -w a te r r e e f lim e s to n e s and r e e f - d e r iv e d
e l a s t i c s o v e r lie th e C re ta c e o u s b asem en t and lo w er
T e r t i a r y se d im e n ts ( C h a t t e r j i , 1964; K a ru n a k a ra n , Pawde,
R a in a and Ray, 1 9 6 4 ). On N ia s , P a le o o e n e se d im e n ts and
s e r p e n t i n i t e i n t r u s i v e 8 a r e i n t e n s i v e l y f o ld e d , r e v e r s e -
f a u l t e d to w ard th e s o u th w e s t, and u n co n fo rm ab ly o v e r la in
i
by g e n t ly fo ld e d and f l a t - l y i n g P lio c e n e and p o s t-
P lio c e n e d e p o s its (van Bemmelen, 1949)*
Burm a-M entawal Trough
P h y sio g rap h y
The s t r u c t u r a l l y low B urm a-M entaw ai Trough in c lu d e s
th e c e n t r a l v a l l e y o r C e n tra l B e lt o f Burma (O h h ib b e r,
1 9 3 4 ), th e deep w e s te rn h a l f o f th e Andaman B a s in , th e
tro u g h s e p a r a tin g th e N lo o b ar I s l a n d s from S u m atra, and
th e d is c o n tin u o u s tro u g h s w hich l i e b etw een th e M entaw ai
o h a ln o f i s l a n d s and S u m atra. The C e n tr a l B e l t , n o r th e r n
h a l f o f th e Burm a-M entawai T rough, l i e s b etw een th e A rakan
Toma and th e Shan P la te a u . I t i s a p p ro x im a te ly 1500 km
lo n g and 270 km w id e . T h is 400,000-km 2 a r e a i s d ra in e d
by th e Irraw ad d y R iv e r and i t s m a jo r w e s te rn t r i b u t a r y ,
th e Ohlndwln R iv e r . M ost o f th e C e n tr a l B e lt I s f l u v i a l
p la in a lth o u g h low f o ld m o u n tain s and v o lo a n lo h i l l s s p l i t
39
i
!
i t l o n g i t u d i n a l l y i n t o th e E a s te r n and W estern Burmese
tro u g h s (Um bgrove, 1 9 4 6 ). P ro m in en t among th e f o ld h ig h s
d iv id in g th e B e lt i s th e Pegu Yoma, w hich e x te n d s f o r 470
km a lo n g i t s s o u th e rn en d , w ith maximum e l e v a t io n s o f
700 m. The b e s t known o f th e v o lc a n ic h ig h s , M t. P opa,
s ta n d s 1300 m above th e a l l u v i a l f l a t s .
The w edge-shaped s u b a e r i a l Irraw ad d y D e lta i n
c lu d e s 3 5 ,0 0 0 km2 o f a l l u v i a l f l a t s w hloh e x te n d 280 km
so u th w ard from th e d e l t a ap e x to a 300 km a r c u a te c o a s t
l i n e . W aters from th e Irraw ad d y e n t e r th e s e a th ro u g h
n in e d i s t r i b u t a r i e s , a lth o u g h th e e n t i r e lo b a te lo w er
d e l t a i s a maze o f t i d a l c re e k s w hich d iv id e and
a n a sto m o se . The d e l t a i s e x tre m e ly f l a t and lo w . More
O
th a n 5000 km l i e below h ig h s p r in g t i d e l e v e l s and an
e q u a l a r e a s ta n d s o n ly 0 .3 m h ig h e r (S tam p, 1 9 4 0 ).
The Burm a-M entawai T rough i s r e p r e s e n te d to th e
s o u th by th e w e s te rn h a l f o f th e Andaman B a sin and i s
bounded by th e e a s te r n s lo p e s o f th e A ndam an-H loobar
R idge and th e c o n t in e n t a l s lo p e o f f th e M alay P e n in s u la .
The tro u g h r e p r e s e n te d by th e n o r th e r n p a r t o f th e B asin
i s a p p ro x im a te ly 400 km w id e ; so u th w ard i t t a p e r s to a
n a rro w s t r u o t u r a l low betw een G re a t H lo o b ar and S u m atra.
H e re , th e tro u g h i s r e p r e s e n te d by a n a rro w , f l a t - f l o o r e d
v a l l e y 2600 m deep and 50 km w id e . To th e so u th w e st th e
s t r u c t u r a l l y low zone i s d e f in e d by a s t r i n g o f n a rro w ,
d is c o n tin u o u s tro u g h s betw een th e M entaw al i s l a n d o h aln
4° j
and Sum atra w ith d e p th s o f 700 to 2400 m (Heezen and
T h arp , 1 9 6 4 b ).
U m bgrove's (1948) fo rm a liz e d d iv is io n o f th e
C e n tra l B e lt o r T e r tia r y B a sin in to E a s te rn and W estern
Troughs was ex ten d ed by him so u th w ard s th ro u g h th e Andaman
B asin to S u m atra. The W estern Burmese Trough would be
e q u iv a le n t to th e M entaw al T rough, and th e E a s te rn
Burmese Trough would have a s o u th e rn e q u iv a le n t in th e
t
N o rth Sum atra T e r tia r y B a s in . W hereas t h i s d iv is io n i s
p h y s lo g ra p h lo a lly and g e n e t i c a l l y v a l i d in Burma, r e l a t i o n
s h ip s a re m\lch more u n c e r ta in in S u m atra. The N o rth
Sum atra T e r tia r y B a sin i s n o t a p h y s lo g ra p h lo a lly d i s c r e t e
u n i t , b e in g m erely th e n o r th e a s te r n s lo p e o f S u m atra.
S tr a tig r a p h y
The B tra tig r a p h y o f th e Burmese T e r t i a r y B asin and
th e A rakan Yoma i s sum m arized in T ab le 3 . A g r e a t th lo k -
n e s s o f T e r tia r y se d im e n ts was d e p o s ite d in th e re g io n
fo llo w in g th e r i s e o f th e a n o e s t r a l A rakan Yoma above se a
l e v e l . Maximum th ic k n e s s e s in th e w e s te rn s u b -tro u g h w ere
m easured e a s t o f th e A rakan Yoma by C o tte r (1918) a s b e in g
o v e r 1 2 ,0 0 0 m. The se d im e n ts a r e now here a s th ic k e a s t o f !
th e Pegu Yoma, w here Eooene d e p o s its a re la c k in g .
T ain sh (1950) drew an a r b i t r a r y e a s t- w e s t l i n e
a o ro s s th e T e r tia r y B a sin a lo n g th e 22° N p a r a l l e l .
G e n e ra lly th e E o o en e-to -M id d le-M io o en e ro o k s s o u th o f th e
l i n e a r e s h a llo w m a rin e d e p o s its w hich p ass i n t o p r e
d o m in an tly f l u v i a t i l e and d e l t a i c d e p o s its n o r th o f th e
l i n e . O o tte r (1918) p o s tu la te d i n f i l l i n g o f th e s u b s id
in g T e r tia r y Burmese G u lf by th e southw ard encroaohm ent
o f d e l t a i c d e p o s its s u p p lie d by r i v e r s a n c e s t r a l to th e
Irraw ad d y R iv e r , much a s th e p r e s e n t Irraw ad d y D e lta i s
e n c ro a c h in g upon th e n o r th e r n Andaman B a s in . He assum ed
t h i s i n f i l l i n g to be n e a r ly c o n tin u o u s , a lth o u g h th e
d e p o s itio n a l h i s t o r y o f th e g u l f was m arked by s e v e r a l
tem p o rary m arin e tr a n s g r e s s io n s d u rin g th e T e r t i a r y .
Stamp (1927) d ev e lo p ed th e id e a and h i s d iag ram m atic
lo n g it u d in a l s e c tio n o f th e T e r tia r y B asin i s rep ro d u c ed
in F ig u re 7* T ain sh (1950) f u r t h e r m o d ifie d t h i s re c o n
s t r u c t i o n o f T e r t i a r y d e p o s itio n a l h i s t o r y , r e s t r i c t i n g
a l l Eocene s e d im e n ta tio n to th e tro u g h w est o f th e Pegu
Toma and v o lc a n ic l i n e , and th e re b y e x p la in e d th e la c k o f
Eooene se d im e n ts in th e e a s te r n p o r tio n o f th e B a s in .
C o t t e r 's o r i g i n a l co n o ep t o f s e d im e n ta tio n in th e e n t i r e
g u l f a p p e a rs v a l i d f o r p o st-E o ce n e s e d im e n ta tio n .
U nconform ably o v e rly in g O ligo-M locene ro c k s o f th e
Pegu System i s th e 300 m th io k f l u v i a t i l e Irraw ad d y System
o f M io -P lio o en e a g e . These beds a r e c h a r a c te r iz e d by
ab u n d an t s i l i o e o u s , o a lo a re o u s and f e rru g in o u s o o n o r e tio n s ,
B l l l o i f i e d wood, and r e p t i l i a n and mammalian f o s s i l s . In
th e v i c i n i t y o f M t. Popa th e upperm ost Irraw ad d y d e p o s its
a re in te rb e d d e d w ith a n d e s ite s and a s s o c ia te d t u f f s .
F igure 7
D iagram m atic lo n g itu d in a l s e c tio n
th ro u g h th e Burmese T e r tia r y B a sin
43
SOUTH
I8*N
NORTH
23* N
Morin* ond *stuarin«
b*dt, mainly sandy.
Marin* and brackish-
watar bads, mainly
clayay.
Irrawaddian frtth w a ttr
b*ds, mainly fluvial
c o a r t* tanas.
Fr*shwat*r b*dt, similar
to th* Irrawaddian,
but P*guan and old*r.
DIAGRAMMATIC LONGITUDINAL SECTION THROUGH
THE WESTERN BURMESE TERTIARY TROUGH
(MODIFIED FROM STAMP, 1927)
44
No m arine P le is to c e n e se d im en ts have heen found in
the.B urm ese Basin* P le is to c e n e and su b -R ecen t r i v e r
te r r a c e g ra v e ls and R ecen t d e l t a i c d e p o s its o f th e I r r a
waddy B iv e r com prise th e y o u n g est sed im en ts o f th e C e n tra l
B elt* H ost o f th e Burmese r i v e r s have formed as many as
s i x te r r a c e s d u rin g th e P le is to c e n e , some o f w hich a re as
much as 50 m above p r e s e n t stre a m l e v e ls (De T e rra , 1943)*
The o o a rse n ess o f m ost o f th e te r r a c e g ra v e ls c o n tr a s ts
s tro n g ly w ith th e f i n e r modern r i v e r sedim ents* B ecause
o f t h i s C o tte r (1918) concluded t h a t Burmese strea m
g r a d ie n ts were s te e p e r d u rin g th e P le is to c e n e th a n d u rin g
th e p re se n t*
Y o lcan io s
Cenozoic volcanism in Burma was d is t r i b u t e d
th ro u g h o u t a n o rth -s o u th zone 150 km wide (P ig u re 8 ) .
O hhibber (1934) p re se n te d a case f o r d i s t r i b u t i o n o f
B urm a's C enozoic v o lc a n lo s a lo n g two m ajor p a r a l l e l tr e n d s .
The w esternm ost i s th e zone whloh d iv id e s th e C e n tra l
B e lt in to W estern and E a s te rn tro u g h s and ln o lu d e s ,
among o th e r s , th e v o lc a n lc s o f Mt* Popa and th e d o le r lt e s
in tru d e d in to sed im en ts o f th e Pegu Toma h i l l ra n g e .
V olcanio a c t i v i t y in n o rth e rn s i t e s , w hich was sub-aqueous
in e a rly s ta g e s , oommenced in th e Eocene and co n tin u ed
i n te r m it t e n t l y in to th e P le is to c e n e and Sub-B eoent
(C h h lb b er, 1 9 3 4 ). F a rth e r so u th a l l v o lc a n lc s ap p ear to
F ig u re 8
D is tr ib u ti o n o f T e r tia r y v o lc a n lc s
in Burma
-25°N
95°E
JADE MINES
Basalts, Jf
Andesites
- 20 °
WUNTHu/*
Basalts,
Granodiorites
LOWER . .
CHfNDWINy*
Basalts, *
Andesites
MT. POPAt
Andesites,
Rhyolites,
Basalts
THAYETMYO-
Dolerites
ZIGON
\
THARRAWADDY
Dolerites
■KABWET
1 Dolerites
'MANDALAY
•THATON
Rhyolites
500K m
be o f Miooene to R ecen t age and a r e d o m in an tly P lio c e n e
to R e ce n t. L lth o lo g lc s u i t e s In b o th n o r th and so u th a re
m afic to In te rm e d ia te In co m p o sitio n and o a lc a lk a lin e In
p e tro lo g lo ty p e (C h h lb b e r, 1 9 3 4 ).
C h h lb b e r's second and e a s te r n C enozoic v o lc a n ic
tre n d c o in c id e s w ith th e b ase o f th e Shan S carp and In
c lu d e s P lio c e n e p illo w b a s a l t s , P le is to c e n e I n tr u s iv e
d o l e r i t e s and R e ce n t p la te a u b a s a l ts In th e v i c i n i t y o f
Kabwet and M andalay, and P le is to c e n e r h y o lite s and t u f f s
o f th e Thaton D i s t r i c t a t th e head o f th e G ulf o f M artaban.
The b a s a l t s o f th e Kabwet a re a form a p la te a u a p p ro x i
m ately 60 km2 In a r e a , o v e r lie fo ld e d and eroded P lio c e n e
se d im e n ts, and cau sed a change In th e Irraw addy R iv e r 's
c h a n n e l. The "rem a rk a b ly f r e s h , u n a lte r e d ap p earan ce" o f
th e p l a t e a u 's ro p y and v e s i c u l a r s u rfa c e flow s le d
C hhlbber (1934, p . 454) to oonclude t h a t th e e ru p tio n
o cc u rred d u rin g th e R ecen t epoch, th ro u g h f i s s u r e s alo n g
th e b ase o f th e Shan S c a rp . The Thaton r h y o l i t e s l i k e
w ise ap p e ar to have em anated from f a u l t s a lo n g th e e s
carpm ent and a r e a ls o b e lie v e d to be v ery young.
S tr u c tu r e
F o ld in g In th e Burmese T e r tia r y B asin a f f e c te d
ro ck s as young a s P lio c e n e , and p e rio d s o f fo ld in g d u rin g
th e Eooene, O lig o o en e and l a t e M iocene a re docum ented by
T ainsh (1 9 5 0 ). M ost f o ld s a re a lig n e d ro u g h ly n o r th -
46 ,
s o u th a lth o u g h a m ajo r u p l i f t tr e n d in g e a s t - n o r t h e a s t
t r a n s e c t s th e B asin n o r th o f th e 22° N p a r a l l e l (E v an s,
1 9 4 1 ). The Pegu Yoma i s a s e t o f c lo s e f o ld s w hloh b ra n c h
o f f from one a n o th e r n o rth w estw a rd to th e w e ste rn m argin
o f th e C e n tr a l B a sin (T a ln s h , 1950)*
S t r u c t u r a l r e l a t i o n s h i p s betw een th e A rakan Yoma
and th e w e s te rn m argin o f th e T e r tia r y B asin a re n o t w e ll
u n d e rs to o d . N o rth -s o u th f a u l t s m ark t h i s boundary to th e
!
n o r th . C legg (1941) and T aln sh (1950) have i n f e r r e d
l a r g e - s c a l e w estw ard r e v e r s e - f a u l t i n g on th e b a s is o f
f a c i e s c o n t r a s t s betw een lo w er Eocene ro c k s on b o th s id e s
o f th e f a u l t s .
R e la tio n s h ip s betw een th e Shan M a ssif and th e
e a s te r n m arg in o f th e T e r tia r y B asin a r e o b scu red by e x
te n s iv e a l l u v i a l c o v e r in th e a r e a o f c o n ta c t betw een th e
T e r tia r y and p r e - T e r t l a r y ro o k . A lo n g , s te e p escarp m en t
more th a n 600 m h ig h m arks th e d iv id e betw een th e Shan
P la te a u and th e T e r tia r y B a s in , and i s in t e r p r e t e d by m ost
i n v e s t i g a to r s a s a w e s t-d ip p in g n o r m a l- f a u lt s u r f a c e .
S u b o rd in a te s t e p - f a u l t s ooour a lo n g th e s o a rp , and th e
S itta n g R iv e r i s p o s tu la te d a s ly in g in a graben betw een
two o f them (T a ln s h , 1 9 5 0 ). K rish n an (1953) c i te d un
named w o rk e rs a s r e p o r tin g w estw ard th r u s ti n g o f th e Shan
M a ssif o v e r th e T e r tia r y B a s in , b u t t h i s r e l a t i o n s h i p i s
n o t docum ented. The l i n e o f T e r tia r y v o lc a n lc s a lo n g th e
s o a rp f a v o r a te n s l o n a l i n t e r p r e t a t i o n .
Eastern M assif
P h y s io g ra p h y
E a s te rn Burma i s dom inated by th e Shan P la te a u ,
th e s o u th e rn c o n tin u a tio n o f th e T lb e tia n and Yunnan
p l a te a u s . Long, rounded n o r th -s o u th f o ld ra n g e s r i s e
g e n tly from th e g e n e ra l p la te a u s u rfa c e w hich i s betw een
900 and 1200 m h ig h . The h ig h e s t o f th e s e ra n g e s i s th e
L ol L in g , o u lm in a tln g 2673 m above se a l e v e l .
S o u th e a s t o f th e G ulf o f M artaban th e Shan P la te a u
n arro w s and m erges w ith th e T enasserim YomaYRange w hich
co m p rises th e n o rth e rn backbone o f th e Malay P e n in s u la ,
a lth o u g h th e E a s te rn M assif i s h e re a ls o re p r e s e n te d by
th e subm erged w e ste rn s h e lf o f th e Malay P e n in s u la . The
T en asserim Yoma Range i s com prised o f a s e r i e s o f n o r th -
s o u th rid g e s w ith deep in te rv e n in g v a l le y s . The h ig h e s t
p o in t on th e T en asserim Yoma i s 2070 m, in th e Tavoy D is
t r i c t a t 13*5° N l a t i t u d e . S outh o f t h i s p o in t e le v a tio n s
d e c re a s e to 150 m above sea le v e l in th e v i c i n i t y o f th e
Pakchan R iv e r.
The webte r n s id e o f th e Malay P e n in s u la i s b o rd e re d
by 200 km o f f l a t c o n tin e n ta l m argin, s e p a ra te d from th e
deep f l o o r o f th e Andaman B asin by th e Malay S c a rp , an
escarp m en t a s lo n g a s th e Shan S carp , o f th e same g e n e ra l
tr e n d , and ab o u t th r e e tim es a s h ig h (1800 m ). The M alay
P e n in s u la i s s e p a ra te d from Sum atra by th e 300 km-wide
50
M alacca S t r a i t , an e x te n sio n o f th e c o n tin e n ta l s h e l f
w est o f th e P e n in s u la .
The most prom inent p h y slo g rap h io f e a tu r e o f
S um atra, a ls o in c lu d e d w ith in th e E a s te rn M a ssif, i s th e
B a ris a n R ange, w hich i s 1650 km lo n g and a p p ro x im a tely 100
km w id e. In n o rth e rn Sum atra th e ran g e o c c u p ie s a median
p o s itio n alo n g th e is la n d a x is ; southw ard i t d e p a rts from
th e is la n d a x is and hugs th e so u th w e ste rn c o a s t. The
h ig h e s t e le v a tio n s alo n g th e B a risa n Range a re young
v o lc a n o e s , w ith maximum e le v a tio n s in n o rth e rn and o e n tr a l
Sum atra o f 2855 m and 3800 m, r e s p e c tiv e ly . A m edian
d e p re s s io n o r s e t o f d e p re s s io n s , in te r r u p te d by v o lo a n o e s,
p a sse s alo n g th e a x is o f th e ra n g e . Van Bemmelen (1949)
named t h i s f e a tu r e th e Semangko R i f t Zone, a f t e r th e
Semangko V alley o f so u th e rn 3 u m atra. T h is r i f t p a sse s
from th e n o rth e rn t i p o f Sum atra betw een Weh and Breweh
is la n d s in to th e Andaman B a sin .
S tra tig r a p h y
The Shan P la te a u and Malay P e n in s u la a re composed
o f Cambrian to T r la s s ic c l a s t i c and o a lc a re o u s sed im en tary
rookB w hich were s tro n g ly fo ld e d and e x te n s iv e ly in tru d e d
by g r a n ite s d u rin g th e M esozoic e ra (C h h lb b er, 1934;
A lex an d er, 1962; Klompe, 196 2 ). The B a risa n Range o f
Sum atra p o s s e s s e s a oore o f c a lc a re o u s se d im en tary rooks
o f P erm o-C arboniferous age oom parable to th e younger
51
P a le o z o ic rooks o f th e Malay P e n in s u la and s im ila r ly in
tru d e d by M esozolo ( p o s t- T r ia s s io ) g r a n ite s ( K a t l l l ,
1962).
P o st-C re ta c e o u s sed im en ts on th e Shan P la te a u and
Malay P e n in su la a re lim ite d in e x te n t and th ic k n e s s , and
were d e p o s ite d g e n e ra lly u n d er c o n tin e n ta l c o n d itio n s . In
S u m atra, s e v e r a l hundred m e ters o f sh allo w -m arin e and
b rac k iB h -w a te r c l a s t i c se d im en ts were d e p o s ite d d u rin g th e
Eocene and O llgocene p r i o r to th e fo rm a tio n o f th e N orth
Sum atra T e r tia r y B asin in whioh accum ulated much th lo k e r
d e p o s its o f Neogene se d im e n ts.
T e r tia r y v o lcan ism o c c u rs a t s e v e r a l l o c a l i t i e s
w ith in th e E a ste rn M a ssif, p a r t i c u l a r l y alo n g th e B arisan
Range o f S um atra, w hich h as ex p e rie n c e d e x p lo siv e volcanism
from O llgocene tim e to th e p r e s e n t (Van Bemmelen, 1949)*
D a c ltio , r h y o d a o itic , and a n d e s iti c la v a s and t u f f s a re
p r e s e n t. T e r tia ry v o lcan ism i s much le s s im p o rta n t in
th e Malay P e n in su la and Shan P la te a u , a lth o u g h l a t e
T e r tia r y a n d e s ite s and o liv in e b a s a l t s a re reco g n iz ed a t
s c a tte r e d l o c a l i t i e s on th e Shan P la te a u and a t one
l o c a l i t y in th e M ergui A rch ip elag o (C h h lb b er, 1 9 3 4 ).
S tr u c tu r e
One o f th e m ost obvious s t r u c t u r a l l y c o n tro lle d
f e a tu r e s o f th e E a s te rn M a ssif i s th e Shan S o arp , o f p re
sumed norm al d isp la c e m e n t, w hioh was d iso u s se d e a r l i e r .
52
D uring th e M esozoic o ro g en y th e P a le o z o io and lo w e r Meso
z o ic se d im e n ta ry ro c k s o f th e Shan P la te a u and Malay
P e n in s u la e a s t o f th e S c a rp w ere s tr o n g ly f o ld e d . Axes
o f th e r e s u l t i n g s t r u c t u r e s tr e n d g e n e r a lly n o r th - s o u th
f o r h u n d red s o f k ilo m e te r s (C h h lb b e r, 1934; A lexander*
1 9 6 2 ). B u rto n (1965) h a s re c o g n iz e d two com plem entary
f a u l t s e t s w ith t r a n s c u r r e n t d is p la c e m e n t in M alaya and
s o u th e rn T h a ila n d . The m a jo r s e t form s a narrow * a ro u a te
n o r th w e s t- tr e n d in g zone o f f a u l t e w hioh b i s e c t s th e
s o u th e rn h a l f o f th e M alay P e n in s u la f o r o v e r 900 km ( P ig .
2 9 ). Com plem entary t o t h i s s e t i s a m in o r s e t o f s h o r t e r
f a u l t s tr e n d in g n o r t h e a s t . B u rto n p o s tu la te d 51 to. 58 km
o f s i n l s t r a l d is p la c e m e n t a lo n g th e m ajo r f a u l t , r e ly in g
l a r g e ly on r e c o n s t r u c t i o n o f d i s t i n c t i v e fo rm a tio n s o f f
s e t by th e z o n e . P a u l t s o f th e m inor* n o r t h - e a s t - t r e n d i n g
s e t d is p la y d e x t r a l d is p la c e m e n t. B u rto n o b se rv e d t h a t
th e se n s e s o f d is p la c e m e n t a lo n g th e two f a u l t s e t s a re
o p p o s ite to th o s e Im p lie d by th e d i s p o s i t i o n o f th e
f a u l t s ( e . g . o f . A n d erso n , 1951)> and co n c lu d ed t h a t th e
s h e a rs may have been p ro d u o ed by an i n i t i a l phase o f
n o r th - s o u th co m p re ssio n d u rin g p r e - J u r a s s l c tim e , and
w ere r e a c t i v a t e d by J u r a s s i o e a s t- w e s t o o m p re ssio n a l
f o r c e s w hich pro d u o ed th e n o r th - s o u th - tr e n d in g f o ld s o f
th e p e n in s u la .
In S u m atra th e Semangko R i f t Zone a lo n g th e B a ris a n
Range ( F ig . 29) h a s b een i n t e r p r e t e d by Van Bemmelen
(1949) a s a oomplex g rab en r e s u l t i n g from te n s lo n a l
s t r e s s e s r e l a t e d to a rc h in g o f th e ra n g e . Durham (1940)
s tu d ie d th e O oloe A er f a u l t w ith in th e r i f t zone in
n o rth e rn and c e n t r a l Sum atra and in f e r r e d d e x tr a l t r a n s
c u r r e n t movement a lo n g i t , in a d d itio n to v e r t i c a l d i s
p lacem en t. H eiskanen and V ening M einesz (1958) hypo
th e s iz e d t h a t f a u l t i n g in S um atra i s d o m in an tly d e x tr a l
s t r l k e - s l l p , and c i t e d t r i a n g u l a t i o n ev id e n ce by M u ller
(I8 9 5 )f who m easured 2 m o f d e x t r a l d isp la c e m e n t o f
t r ia n g u l a ti o n p o in ts a lo n g a f a u l t zone a lig n e d p a r a l l e l
to S u m a tra 's a x is a f t e r an e a rth q u a k e o f May, 1892. F i r s t -
m otion s tu d i e s o f 14 sh a llo w and 9 in te rm e d ia te e a r t h
quakes in S um atra c o rro b o ra te t h i s se n se o f movement
(Veldkamp, 1 9 5 7 ).
A N D A M A N BASIN PHYSIOGRAPHY
I n tr o d u c tio n
The r e g io n a l s e t t i n g o f th e Andaman B a sin I s one
o f c o n s id e ra b le s t r u c t u r a l and p h y s io g ra p h ic c o m p le x ity .
D e ta ile d b ath y m etry o f th e Andaman B a sin r e v e a ls com par
a b le to p o g ra p h ic d i v e r s i t y , s c a r c e ly m o d ifie d by ero B lu .
Many o f th e s a l i e n t p h y s io g ra p h ic f e a t u r e s o f th e
Andaman B a sin a re unnamed, and f o r th e sak e o f co n v en ien ce
s e v e r a l names a re h e r e in p ro p o sed . W herever p o s s ib le
f e a t u r e s have been named, fo llo w in g I n t e r n a t i o n a l Nomen
c l a t u r e Com mittee usage (Wiseman and O vey, 1953, 1954,
1 9 5 5 )t a f t e r g e o g ra p h ic a l names o f p ro x im a te s u b a e r ia l
l o c a l i t i e s . A s c a r c ity o f g e o g ra p h ic l o c a l i t i e s n e c e s
s i t a t e d th e nam ing o f two f e a tu r e s a f t e r p e rs o n s .
A f e a t u r e o f th e Andaman B a sin seco n d In s iz e o n ly
to th e c o n tin e n ta l s h e l f i s th e s im o u s tro u g h w hioh
d iv id e s th e b a s in in to h a lv e s o f c o n t r a s t in g to p o g ra p h ic
s t y l e ( P ig . 9)« T his f e a tu r e i s r e f e r r e d to a s th e
C e n tr a l Andaman Trougfi'. A V -shaped v a l l e y w hich b i s e c t s
th e C e n tra l Andaman Trough a c ro s s i t s w id e s t ex p an se o f
f l a t f l o o r i s c a lle d th e C e n tra l Andaman R i f t V a lle y .
The n o r th e r n end o f th e C e n tr a l Andaman Trough i s
s e p a r a te d from a s m a lle r b a s in betw een th e Andaman,
Naroondam and B a rre n is la n d s w hioh i s th e B a reo n d an -
F ig u re 9* G e n e ra liz e d p h y sio g ra p h ic c h a r t
o f th e Andaman B a sin .
GENERALIZED
PHYSIOGRAPHY
OF THC
ANDAM AN
BASIN
HHS#jS3
G AU G ES
A B Y S S A L
F A N
IR RAW AD D Y D E L T A -S H E L F —
\ / -
9
I
E
1 ~
l
KEY TO NUMBERED FEATURES
M ARTABAN CANYON
NARCONDAM-BARREN B A S IN
B A R R E N SEAMOUNT COMPLEX
IN V IS IB L E B A N K
CENTRAL ANDAM AN R IF T VALLEY
N IC O B A R R I F T V A LLE Y
B arren B a s in .
The la rg e seam ount lo c a te d betw een th e C e n tra l
Andaman Trough and N arcondam -Barren B asin I s named Aloook
Seam ount. In th e absenoe o f proxim ate g eo g rap h lo f e a t u r e s .
A lfre d W illiam Aloook (1859-1933) was th e B r i t i s h m arine
b i o lo g i s t who f i r s t dredged in th e Andaman B asin and
s tu d ie d i t s sed im en ts and fau n a a t th e tu r n o f th e
c e n tu ry . Aloook i s b e s t known f o r h i s book A N a tu r a l i s t
in In d ia n S eas (1902) in w hich he d e s c rib e d th e re c o n n a is
sance oceanography and b io lo g y o f th e Bay o f B engal and
Andaman S ea.
Southward a c ro s s th e C e n tra l Andaman Trough from
A lcook Seamount i s a second seam ount o f com parable s iz e
and r e l i e f . A gain, la c k o f a n earb y g eo g rap h lo f e a tu r e
n e c e s s ita te d naming th e seam ount a f t e r a p e rs o n . In
an alo g y to A lcock Seamount th e name S ew ell Seamount i s
p ro p o sed . P r io r to le a d in g th e John H urray E x p e d itio n
to th e Red Sea in 1935$ B. B. Seymour S e w e ll, in h is
c a p a c ity a s Surgeon N a tu r a l is t w ith th e M arine S u rrey o f
I n d ia from 1913 to 1935* oonduoted th e f i r s t s y s te m a tic
in v e s tig a tio n o f th e Andaman S e a 's w a te rs , b ath y m etry and
bottom m a te r ia l s . The many S ew ell p u b lic a tio n s on th e
oceanography o f th e Andaman Sea c ite d in t h i s stu d y l n -
olude one (1925a) w hich d e s c rib e s rook drsdged from th e
v i c i n i t y o f S ew ell Seam ount.
Numerous sm a ll se ah lg h s alo n g th e w e ste rn m argin o f
58
th e Andaman B a sin a r e in a d e q u a te ly su rv ey ed and a r e r e
f e r r e d to in d iv i d u a ll y by th e code proposed by Wiseman and
Ovey (1955t p . 2 6 1 -2 6 2 ):
• • • Each f e a t u r e s h a l l be g iv e n a number con
s i s t i n g o f th e mean g e o g ra p h ic a l c o o r d in a te s , p r e
f ix e d by an a b b r e v ia tio n in d ic a t in g th e q u a d ra n t. I f
s u f f i c i e n t in fo rm a tio n i s a v a ila b le to ln d lo a te th e
p o s s ib le c h a r a c te r o f th e f e a tu r e th e n a s u f f i x may
be a tta c h e d to th e oode num ber. The fo llo w in g a re
s u g g e s te d : (m) f o r seam ount, ( t ) f o r ta b le m o u n t,
(b ) f o r o c e a n ic b an k , (p ) f o r seap eak and ( r ) f o r
r id g e o r r i s e .
F o r exam ple S W 5930-6857 (p ) = S eapeak a t
5 9 °3 0 'S 6 8 057'W . . .
The name M ergul T e rra o e i s p roposed f o r th e 200-
500 m deep m a rg in a l te r r a c e o f f th e H erg u i A rc h ip e la g o .
S o u th w ard , M ergul T e rra c e m erges in to Weh Bank, a 70-km
w ide f e a t u r e w ith a f a i r l y f l a t to p ab o u t 600 m below Bea
l e v e l . The s h e lf - b a s in im m ed iately s o u th o f M ergul
T e rra o e , e a s t o f Weh Bank and n o r th o f S um atra i s h e r e in
d e s ig n a te d th e S um atra S h e lf B a s in .
Irraw ad d y D e lta
C o ast
The lo b a te Irraw ad d y D e lta o o a s t i s a p p ro x im a te ly
300 km lo n g . Along i t s w e ste rn h a l f i t tr e n d s s l i g h t l y
s o u th o f e a s t and i s in te r r u p te d by seven o f th e n in e
m a jo r r i v e r m ouths, o f whioh th e p r in o ip a l i s th e Eya
e s tu a r y ( P la te I ) . The e a s te r n h a l f o f th e d e l ta o o a s t
a ls o o o n s t i t u t e s th e w e ste rn sh o re o f th e G u lf o f M artab an ,
and tre n d s n o r t h e a s t . C h h lb b er (1934) d e s c rib e d th e
d e l t a o o a s t a s one o f th in s t r i p s o f sandy b each w ith
num erous s h o a ls and m angrove swamp f r i n g e s . He e s tim a te d
t h a t th e s h o r e lin e In t h i s a r e a I s ad v an cin g a p p ro x im a te ly
5 km ev e ry hundred y e a r s . H is c a lc u l a ti o n s w ere b ased on
changes betw een th e su rv e y s o f 1860-1870 and 1909-1910.
More r e c e n t c h a r ts I n d ic a te o n ly l o c a l grow th o f a s much
as 5 tan, b u t an av erag e r a t e o f grow th o f 2 .5 tam/100
y e a r s . The manner o f d e l t a g ro w th I s a s fo llo w s : sa n d
banks form a s h o r t d is ta n c e o f f s h o r e an d , w h ile s t i l l
u n d er h ig h -w a te r l e v e l s , a r e f ix e d by g r a s s and m angroves.
N ext r a p id s i l t i n g d e v e lo p s th e banks I n to low I s la n d s
s e p a ra te d from th e m a in la n d by n arro w o h a n n e ls .
E v e n tu a lly th e c h a n n e ls a l s o s i l t up (Stam p, 1 9 4 0 ).
S h e lf
The d e l t a form s a b ro a d s h e l f o u t to th e 200 m
I s o b a th , and I s d iv id e d I n to w e s te rn and e a s te r n h a lv e s
by c o n tr a s tin g to p o g rap h y ( P l a t e I ) . The w e ste rn h a l f ,
o f f th e d e l t a 's main lo b e , i s 170 km b ro a d , and th e
j
e a s te r n h a l f , Bouth o f th e G u lf o f M artab an , I s alm o st
tw io e as w id e. The g u l f i s e n t i r e l y s h o a le r th a n 30 m,
and th e o u te r p o r tio n o f th e e a s te r n d e l t a s lo p e s 1 .2 m/km,
o r an a n g le o f 0° 4 ' from th e h o r i s o n t a l . In c o n t r a s t ,
th e d e l t a o f f th e m ain w e s te rn s u b a e r ia l lo b e h as no f l a t
n e a rs h o re p o rtio n and s lo p e s u n in te r r u p te d ly a t an a n g le
o f 0° 2 ' from c o a s t to s h e l f b re a k .
The d e l t a s h e l f i s in c is e d by a d e n d r iti o system
o f p o o rly d e fin e d c h a n n e ls , m arking th e b oundary o f th e
two s h e l f h a lv e s . On th e s h e l f th e o h a n n e ls a r e b ro ad
and s h a llo w , a s w ide a s 20 km and seldom d e e p e r th a n 20
m. The e a s te r n c h a n n e ls m erge so u th w ard to p a ss in to th e
deep M artaban subm arine canyon w hich i s in o ls e d in to th e
d e l t a s lo p e and tr e n d s southw ard in to th e c e n t r a l b a s in ,
p a r a l l e l i n g th e M alay c o n t in e n t a l s lo p e .
1 com parison betw een th e b a th y m e try o f th e s h e l f
a s m easured in 1860-1870 and 1909-1910 by th e M arine
S urvey o f I n d ia was made by C h h ib b er in 1934 who e x t r a
p o la te d h i s f in d in g s to o b ta in r a t e s o f s h o a lin g o v e r a
hundred y e a rs (T ab le 4 ) . H is d a ta may be d i s t o r t e d b e
ca u se s u c c e s s iv e su rv e y s In c o rp o ra te d so u n d in g s a o q u ire d
many y e a rs p re v io u s ly . C opies o f th e o r i g i n a l c h a r t s
w ere o b ta in e d by th e w r i t e r from th e B r i t i s h A d m iralty
and w ere exam ined in d e t a i l . Only th e c h a r t s o f th e
1854-1859 su rv e y s oan be r e l i e d upon n o t to in c lu d e th e
so u n d in g s o f p re v io u s s u rv e y s , a f t e r e a s i l y d e te o te d
p a tc h e s o f so u n d in g s made in 1826 a re rem oved. A d e t a i l e d
com parison was made betw een th e 1854-1859 s u rv e y ( F ig . 10)
and th e d a ta a o q u ire d d u rin g th e y e a rs 1960-1964 ( P la te I ) .
The d eg ree o f s h o a lin g in d ic a te d by th e com parison i s
s u r p r i s i n g l y l a r g e . As s h a l l be s e e n , th e su p p ly o f s e d i
m ent a v a ila b le f o r d e p o s itio n on th e d e l t a - s h e l f i s f a r
61
TABLE 4 . R ate o f southw ard movement o f s e le c te d Is o b a th s
on th e s u rfa c e o f th e Irraw addy D e lta and G ulf
o f M artaban, I n f e r r e d from com parison o f s u rv e y s .
A. O h h ib b er, 1934; su rv ey s o f 1854-1859 and 1903-1910
Is o b a th
RATE OF A D V A N CE
9 4 °4 5 ' - 95°30'
E Io n a .
95 30* - 97 0 0 '
E Io n a .
Mouths o f th e
Irraw addy
G u lf o f
M artaban
Fm M ml km ml km
3 5 .5 3 .5
5 .6 ? ?
5 9 .1
4 .0 6 .4 5 .0 8 .0
10
1 8 .3
1 .0 1 .6 1 4 .0
2 2 .5
15
2 7 .4
1 .2 5
2 .0 2 8 .0 4 5 .0
20 3 6 .6 3 .0 4 .8 3 5 .0
5 6 .3
B. Com parison betw een 1854-1859 d a ta and 1961-1964 d a ta
RATE OF ADVANCE, Km.
I s o
b a th . m
W estern D e lta E a ste rn D elta
Maximum A veraae Maximum A veraae
10 10 0 0 0
20 10 0 42 24
30 22 0 70 48
40 40 20 60 38
50 40 20
55 31
60 no d a ta
67 40
70 no d a ta
63 38
80 61
35
F ig u re 1 0 . S u rfa c e o f th e Irraw addy D e lta
and G u lf o f M artaban* a c c o rd in g
to R oyal In d ia n M arine Survey o f
1854-1859* (C ontoured to 10 m
I s o b a th i n t e r v a l . )
63
0 2 0 4 0 6.0 8.0 IQO Km
DEPTH IN METERS
DELTA-SURFACE BATHYMETRY
1854-1859
64 ;
to o sm a ll to acco u n t f o r sh o a lin g o f th e m agnitude
i
In d ic a te d by th e b a th y m e tric com parison. A lthough low
n a v ig a tio n a l ao cu racy nay have c o n trib u te d to th e ap
p a re n t changes In d e p th , I t I s p o s s ib le t h a t s h o a lin g may
be a t t r i b u t e d In la rg e p a r t to a re g io n a l drop In se a
l e v e l . M o rris (1935) rec o g n iz e d th e e x is te n c e o f fiv e
Burmese r i v e r te r r a c e s 5 to 106 m above p re s e n t stream
l e v e ls and su g g e sted t h a t each te rr a o e was graded to
s u c c e s s iv e ly low er Q u atern ary s e a - s ta n d s , and t h a t a
s i x t h te r r a c e I s In th e p ro cess o f fo rm a tio n . Ih e e n t ir e
m argin o f th e Andaman B asin e x h ib its ev id en ce o f su b -
R ecent to R ecent em ergence. Ih e w estern c o a s t o f th e
Malay P e n in s u la has emerged 17 m (Rama Rao, 1930;
S o rlv e n o r, 194 6 ), and th e Andamans and N lco b ars e x h ib it
numerous r a is e d beaches and dead c o r a l r e e f s w hich were
e le v a te d s e v e r a l m eters ( O h a tte rje e , 196 1 ).
B oth C h h lb b e r's c a lc u la tio n s and th e p re s e n t stu d y
re v e a le d th a t g r e a t e s t s h o a lin g o ccu rred In th e o u te r
p o rtio n o f th e e a s te rn d e l t a , p a r t i c u l a r l y a t d ep th s o f
30 m and d e e p e r. A ll Is o b a th s a t 10 m I n te r v a ls betw een
30 and 80 m ex p e rien ce d maximum southw ard advance o f b e
tween 55 and 70 km, and o v e r a ll average advanoe o f 32 km
o v er th e p a s t 100 y e a r s . In c o n t r a s t, th e s h e lf so u th o f
th e r i v e r mouths ex p e rie n c e d v i r t u a l l y no n e t advanoe a t
d ep th s s h o a le r th an 40 m, lnasmuoh as lo o a l a re a s o f
e ro s io n n e g a te lo o a l advances o f more th an 22 km. The 50
65 ;
and 60 m is o b a th s e x p e rie n c e d maximum advances o f 40 km
and an av e ra g e southw ard advance o f 20 km.
These d a ta a r e d e p ic te d more c l e a r l y by a c h a r t o f
is o p le th 8 o f b a th y m e tric ohanges (P ig . 11) d eterm in ed by
su p erim p o sin g th e so u n d in g s o f th e 1 9 6 0 's on th e c h a rt o f
th e 1 8 5 0 '8 and d e te rm in in g th e d if f e r e n c e s in d ep th a t
in d iv id u a l p o in t s . The r e s u l t d e lin e a te s an a re a o f
2
a p p ro x im a te ly 1 2 ,0 0 0 km w hich a p p a re n tly h as sh o a led by
2
o v e r 5 m, o f w hich an a r e a o v e r 200 km a p p e a rs to have
sh o a le d a s much a s 60 m. As d e fin e d by th e s e ls o p l e t h s ,
th e a r e a o f maximum s h o a lin g i s th e s o u th e rn and e a s te rn
d e l t a s h e l f , so u th o f th e G u lf o f M artaban and s o u th e a s t
o f th e r i v e r m ouths. U n fo rtu n a te ly , th e e a r ly so u n d in g s
do n o t ex ten d so u th o f 15° N l a t i t u d e , and th e sh o a lin g
o f f th e main lo b e i s n o t a s w e ll c o v e re d ; how ever, some
lo s s o f d ep th oan be seen to have o c o u rre d on th e o u te r
w e ste rn s h e l f . The n e a rs h o re p o r tio n o f th e w estern
d e l t a a p p e a rs to be one o f s c o u r by waves and c u r r e n ts , as
ln d lo a te d by a zone o f e ro s io n o f as much as 10 m. To th e
e a s t , th e a r e a o f c u t i s th e a r e a o f o h an n el h ead s in th e
M artaban Canyon sy ste m . A m inor tro u g h tre n d in g w e st-
so u th w e st from th e w e ste rn d e l t a s h e l f p a s t P re p a r is
I s la n d and in to th e Bay o f B engal in d ic a t e s t h a t some o f
th e p ro d u c ts o f wave and c u r r e n t e ro s io n may make t h e i r
way in to th e Bay o f B engal a s w e ll a s i n to th e o e n tr a l
tro u g h o f th e Andaman B a s in .
F ig u re 11. N et sh o a lin g on th e Irraw addy
D e lta s h e lf from 1854-1859 to
1960-1964.
67
BATHYMETRIC CHANGE BETWEEN
1854-1859 AND 1961-1964
S lo p e
68
The s u r f a o e o f th e su b m arin e d e l t a ohanges slo p e
r a d i c a l l y a t th e 200 m I s o b a th , d ro p p in g o f f to 2000 m
d e p th s a lo n g a p r o f i l e w hich I s s l i g h t l y concave upw ards
and whloh a v e ra g e s 1 .5 ° . O ff th e G u lf o f M artaban th e
slo p e I s r e g u l a r , a p a r t from th e I n c is io n o f th e
M artaban C anyon. To th e w e s t, how ever, th e d e l t a slo p e
I s I n te r r u p t e d by th e subm arine r id g e a s s o c ia te d w ith
Narcondam I s l a n d , and a lo n g th e s l o p e 's w e ste rn e x tre m ity
w est o f th e Narcondam Ridge i t p a s s e s I n to 1600 m d e p th s
o f th e n o r th e r n f l o o r o f th e N arcondam -B arren B a s in .
M artaban Canyon
The M artab an Canyon m arks th e e a s te r n end o f th e
d e l t a s lo p e ; so u th o f th e canyon I s th e M ergul S lo p e , o f f
th e w e stern s h e l f o f th e M alay P e n in s u la . Along I t s
s o u th w a rd -tre n d in g th alw eg th e canyon I s 140 km lo n g ,
w ith an a lm o s t l i n e a r g r a d ie n t o f 1° betw een th e depthB
o f 150 m and 2200 m. The oanyon I s 10 to 20 km w id e, w ith
a s te e p V -shaped c r o s s - s e c ti o n e x h i b itin g a maximum r e l i e f
o f 500 m from l i p to a x i s . T h is f e a t u r e I s b e s t ex
p re sse d betw een th e d ep th s o f 200 m and 1600 m. At I t s
head th e oanyon p a s s e s In to th e vague sy stem o f d e l t a -
s h e lf o h an n elB , and betw een 1600 m and 2200 m I t d e-
bouohes o n to th e g e n t le s lo p e m arking th e n r th e rn end o f
th e Andaman B a s in 's c e n t r a l tr o u g h .
The q u a l i t y o f to p o g ra p h ic c o v e ra g e o v e r b o th th e
d e l t a c h a n n e l sy stem and M artaban Canyon i s n o t v e ry
r e l i a b l e . In a d d itio n to th e p a u c ity o f oanyon c r o s s
i n g s , th e r a p i d l y ch an g in g d e p th s on th e s h e l f ca u se
s u rv e y s to become o b s o le te a f t e r one o r two d e c a d e s ;
c o n s e q u e n tly , r e l i a b l e d a ta a r e r e s t r i c t e d to th e few ,
m ost r e c e n t d a ta .
M alay C o n tin e n ta l M argin
G e n e ra l S ta te m e n t
The c o n t in e n t a l m argin o f f th e M alay P e n in s u la i s
d iv id e d i n t o f o u r p h y s io g ra p h ic s u b - p ro v in c e s :
a . I n n e r c o n t in e n t a l s h e l f and th e M ergui
A rc h ip e la g o .
b . M ergui T e r r a c e •
c . S um atra S h e lf B a s in .
d . C o n tin e n ta l s lo p e .
I n n e r C o n tin e n ta l S h e lf
and th e M ergui A rc h ip e la g o
The e a s te r n c o a s t o f th e G u lf o f M artaban i s
s i m i l a r to t h a t o f th e Irraw ad d y D e lta , b e in g p r e
d o m in a n tly m udflatB and m an g ro v es. To th e s o u th o f th e
S ltt a n g B iv e r m ouths th e c o a s t l i n e i s f r in g e d w ith o o r a l
r e e f s (O h h ib b er, 1 9 3 4 ). S outh o f th e Tavoy R iv e r m ouths
70
a t 13° N l a t i t u d e th e o o a e t i s b ro k en I n to th e 800 h i l l y
and f o r e s te d i s l a n d s o f th e M alay A rc h ip e la g o . These
is la n d s a r e composed o f th e more r e s i s t a n t F a le o z o lo
I
m e ta se d lm e n ts o f th e M ergui S e r i e s , and th e J u r a s s i c
g r a n i t e s w hich I n tr u d e them (Rama R ao, 1 9 3 0 ), and ra n g e
p
in s iz e from K ing I s la n d , w ith an a r e a o f 440 km > to
s m a ll hummocks. The is l a n d s a r e c o n tin u a lly b e in g
c a rv e d o u t o f th e m ain lan d and s u b d iv id e d by g a le tB . th e
Burm ese te rm f o r n arro w s t r a i t s w hich c o n n e c t two i n l e t s
o r w hich d iv id e i s l a n d s . A g a l e t o r i g i n a t e s a s two
stre a m s d r a in in g o p p o s ite s id e s o f an is l a n d o r p e n in s u la r
s a d d le . Combined s tre a m , wave and c u r r e n t a c t i v i t y b a s e -
l e v e l b o th s tr e a m s , and th e new ly form ed c h a n n e l i s
deepened and w idened by waves and t i d a l c u r r e n t s . The
w e s te rn T h ai c o a s t s h a re s m ost o f th e c h a r a c t e r i s t i c s o f
th e M ergui c o a s t , e x c e p t t h a t th e r e a re fe w e r i s l a n d s .
S h e lte r e d c o a s t l i n e s o f th e m ain lan d and th e
l a r g e r i s l a n d s a r e th e lo c a ti o n s f o r e x te n s iv e m u d fla ts
w hich l i e betw een low and h ig h t i d e l e v e l s and w hich
m erge in to swamps and m a rsh e s. Commonly th e ex p o sed
c o a s ts a r e w av e-o u t s te e p and roo k y s lo p e s . C o a s ts o f
th e o u te r i s l a n d s g e n e r a lly have sandy b e a c h e s . Rama Rao
(1930) o l t e d e v id e n c e o f r a i s e d b each es and f i l l e d
e s t u a r i e s i n d i c a t i n g su b -R eo en t em ergency o f th e M ergui
o o a s t r e s u l t i n g from a 17 n change in s e a l e v e l .
The w id th o f th e in n e r s h e l f from n o r th to s o u th
71
ra n g e s betw een 130 and 35 km m easured from th e m ain lan d
o r a r c h ip e la g lo c o a s ts to an in n e r s h e lf - b r e a k a t d e p th s
ra n g in g from 75 to 150 m ( F ig . 1 2 ) . E x ce ed in g ly f l a t
s u r f a c e s c h a r a c te r i z e t h i s s h e l f ; s lo p e s ra n g e betw een
0° 3 ' and 0 ° 8* and a v e ra g e 0° 5 '• At i t s n o rth e rn end
th e in n e r s h e l f e x te n d s w ith o u t I n t e r r u p t i o n to th e
o u te r s h e lf - b r e a k 130 km o f f th e o u te rm o st is la n d s o f th e
M ergui A rc h ip e la g o . Southw ard th e w id th o f th e in n e r
s h e l f d e c re a s e s a s more o f th e c o n t in e n t a l m argin i s
oco u p ied by th e d e e p e r M ergui T e rra o e . O ff Go P huket
I s la n d th e s h e l f i s o n ly 35 km w id e , b u t to th e s o u th and
e a s t in to th e M alacca S t r a i t th e s h e l f a g a in w id en s.
At i t s o u te r l i m i t s th e in n e r s h e l f i s te rm in a te d
by a m inor s h e lf - b r e a k and s lo p e o r s e r i e s o f b re a k s and
s lo p e s s tr o n g ly s u g g e s tiv e o f s te p f a u l t i n g . The o o n to u r
i n t e r v a l em ployed in P l a t e I I s to o la r g e to show
d e f i n i t i o n o f th e s e f e a t u r e s , and F ig u re 12 i s a ls o
drawn to to o s m a ll a s c a l e . P r e c is io n d e p th re c o r d s
a c ro s s th e s h e l f show th e f e a t u r e s in e x c e l l e n t d e t a i l
( F ig . 1 3 ). A verage r e l i e f o f th e so a rp o r s c a r p - s e t i s
250 m, and th e a v e ra g e w id th i s 35 km. Where b e s t
d e fin e d th e s o a rp h a s a 0° 4 5 ' s lo p e .
M ergui T e rra c e
The M ergui T e rra o e b e g in s j u s t n o r th o f 12° W
l a t i t u d e and w id en s from 85 km a t 12° V sou th w ard a lo n g
F ig u re 12. B a th y m e tric p r o f i l e s a c ro s s
th e M alay c o n tin e n ta l m argin
NATURAL. SCALE
SEA LEVEL
INNER SHELF C
f y MALAY PENINSULA
MALACCA STRAIT
VERTICAL EXAGGERATION M X
TOO Km
Figure 13
• FDR p r o f i l e a c ro s s in n e r s h e lf
and in n e r s h e lf - b r e a k .
EAST
0 10 20
I
! I . 1. . l u r i .
INNER SH ELF
WEST
MERGUI TERRACE
VERTICAL EXAGGERATION = 2 3.8X
V J I
76
th e le n g th o f th e M ergui ir o h ip e la g o to 140 km a t 9° N
l a t i t u d e . Southw ard th e te r r a o e a le o f l a t t e n s and d e e p e n s,
i t 12° 5 I t I s 200 to 400 m deep and s lo p e s 0° 4 '; a t 9°
N l a t i t u d e I t B lopes a mere 0 ° 2 . 5 ', from 360 m d ep th s a t
th e to e o f th e In n e r slo p e to 460 m a t th e tr u e s h e l f -
b re a k .
Ih e s t e p - f a u l t i n g w hich p ro d u ces th e In n e r s h e l f -
b re a k a p p a re n tly a f f e c t s th e M ergui T e rra c e a c ro s s m ost o f
I t s w id th , c a u sin g l o c a l downward d isp la c e m e n ts o f ab o u t
30 m. At 10° N l a t i t u d e , a s m a ll p in n a c le w ith a r e l i e f
o f a b o u t 120 m m arks one o f th e 30 m d isp la c e m e n ts (F ig .
1 4 ).
S outh o f 8° N l a t i t u d e th e w e ste rn te r r a o e I s
s e p a ra te d from th e In n e r s h e l f by th e Sum atra S h e lf B asin
and becomes Weh Bank, a 70 km b ro a d , 220 km lo n g f l a t -
topped bank w hich c r e s t s w ith o c c a s io n a l p ln n a o le s a t
1000 m d e p th s ( F ig . 1 2 ) . Weh Bank m erges southw ard In to
th e f l o o r o f th e Sum atra S h e lf B asin a t 6° N l a t i t u d e .
Sum atra S h e lf «Basln
To th e so u th o f 9° N l a t i t u d e th e M ergui T erra o e
g ra d u a lly becomes concave upw ard, d ev elo p s a s o u th -
tre n d in g 0° 1 3 ' s lo p e and p a s s e s I n to th e se m l-en o lo se d
Sum atra S h e lf B a s in . Along I t s e a s te r n p e rip h e ry th e
b a s in I s bounded by th e n o r th - s o u th a lig n e d u p p er slo p e
whloh te rm in a te s th e M alaooa S t r a i t and o u rv es w estw ard to
F ig u re 1 4 . W est-n o rth w est - e a s t- s o u th e a s t
FDR p r o f il e a c ro s s th e Malay
c o n tin e n ta l m argin from 10° to
1 0 .5 ° N.
WEST
DEEP TERRACE
150
_ j ___
200
J I I L i _ J L
EAST
2 5 0 Km
MERGUI TERRACE
i" i fr-r
INNER
SHELF
VER TIC A L EXAGGERATION 30.8X
79 I
become th e subm arine e x te n s io n o f th e n o r th e r n S um atra !
slo p e* T h is b a s in s lo p e i s s l i g h t l y oonoave upw ards and
i s tw ic e a s s te e p o f f S um atra a s o f f th e M alacca S t r a i t ,
s lo p in g 2° 2 0 ' and 1° 1 0 ' a t th e s e l o c a l i t i e s , r e s p e c t i v e - :
ly* Along th e n o r th e r n h a l f o f i t s w e s te rn s id e th e b a s in
i s c o n ta in e d by Weh Bank* S o u th o f Weh Bank th e b a s in
f l o o r i s open to th e m ain c o n tin e n t a l slo p e *
The s h e l f b a s i n 's f l o o r i s g e n tly u n d u la tin g and
a p p e a rs to be b ro k en by f a u l t i n g . Only in th e so u th e rn
p a r t o f th e b a s in does th e f l o o r e x h i b i t th e f l a t n e s s o f
u n d is tu rb e d ponded sedim ent*
C o n tin e n ta l S lope
The m ost p e r s i s t e n t p h y s io g ra p h ic f e a t u r e w ith in
th e Andaman B a sin i s th e 830 km lo n g c o n t in e n t a l s lo p e
o f f th e M alay P e n in su la * From th e n o r th e r n end e a s t o f
M artaban Canyon th e s lo p e tr e n d s a p p ro x im a te ly 10° e a s t
o f s o u th to 12° N l a t i t u d e , a d is ta n c e o f 180 km. S o u th
o f 12° N l a t i t u d e th e s lo p e bends s l i g h t l y to tre n d 10°
w e st o f s o u th f o r 650 km, to th e n o r th e r n t i p o f S u m atra.
As th e M ergui T e rra c e d e v e lo p s , b ro ad en s and
deepens southw ard th e d e e p e s t s h e lf - b r e a k s a t th e to p o f
th e o o n tln e n ta l s lo p e o c o u r a t in c r e a s in g d e p th , from
150 m a t 13° N l a t i t u d e , w here no in n e r s h e lf - b r e a k
e x i s t s , to 400 m a t 11° N l a t i t u d e , and 500 m a t 7 ° V
l a t i t u d e . The o u te r deep s h e lf - b r e a k i s m arked in p la o e s
80
by p ln n a o le s o f le s s th a n 200 m r e l i e f . These h ig h s a re
m ost p ro m in en t to th e s o u th , p a r t i c u l a r l y where th e s h e l f -
b re a k c o n s t i t u t e s th e w estern edge o f Weh Bank.
In g e n e ra l a s p e c t th e c o n tin e n ta l s lo p e i s oonvex
upw ards w ith g r a d ie n ts g ra d u a lly increasiT ag tow ard th e
b ase o f th e s lo p e . G r a d u n ts , rem ark ab ly p e r s i s t e n t and
g e n e r a lly s te e p e r In th e n o r th , ran g e betw een 1 .2 ° and
2 .2 ° and av erag e 1 .8 ° .
An u n u su a l f e a tu r e i s o b serv ed a t th e f o o t o f th e !
c o n tin e n ta l s lo p e betw een th e n o rth end o f S ew ell S ea
m ount and th e mouth o f M artaban Canyon ( F ig . 1 4 ) . At
1 0 .5 ° H l a t i t u d e th e s lo p e ends a b ru p tly in a te r r a o e a t
2435 m. T his deep t e r r a c e th en s lo p e s g e n tly f o r 62 km
to a d e p th o f 2670, b e fo re th e s u rfa c e a g a in d ro p s s h a rp
ly a n o th e r 380 m to th e f l o o r o f th e C e n tra l Andaman
Trough w hich i s a t a d ep th o f 3035 n . The deep te r r a c e
p o s s e s s e s th e same slo p e a s th e f l o o r o f th e C e n tra l
Andaman T rough. The e n t i r e p r o f i l e o f s h e l f and slo p e
in F ig u re 14 i s s tr o n g ly s u g g e s tiv e o f norm al f a u l t i n g .
The deep te r r a c e i s 250 km lo n g and e x te n d s to th e
v i o l n i t y o f th e mouth o f M artaban Canyon. At 13° N
l a t i t u d e th e te r r a o e i s a t B llg h tly o v er 2000 m d e p th ,
i s 33 km wide and a p p e ars in te r r u p te d midway a c ro s s i t s
w id th by e i t h e r a ch an n el o r f r a c t u r e ( F ig . 15)* S o u th
ward from 1 0 .5 ° N l a t i t u d e th e te r r a o e p a s s e s in to th e
B llg h tly oonoave s u rfa o e betw een th e c o n tin e n ta l slo p e
(
F ig u re 15* E a st-w e s t FDR p r o f i l e a c ro s s th e
Malay c o n tin e n ta l slo p e and deep
t e r r a c e , and th e n o rth e rn end o f
th e C e n tra l Andaman T rough, alo n g
13° N.
WEST EAST
5 0 100 150 Km
ALCOCK
SEAMOUNT
DEEP TERRAC
CENTRAL ANDAMAN
TROUGH
Km VERTICAL EXAGGERATION = 28.2X
oo
ro
and S ew e ll Seam ount and 1b s l i g h t l y c o n c a v e .
83
A ndam an-N icobar R idge
i
G e n e ra l S ta te m e n t
The A ndam an-N icobar R idge tr e n d s southw ard from
th e Irraw ad d y D e lta s lo p e (1 5 ° N l a t i t u d e ) f o r a d is ta n c e
o f 900 1cm . I t i s s l i g h t l y convex w estw ard s and m erges to
th e B outh in to th e H en taw ai c h a in o f i s l a n d s o f f S um atra
( F ig . 5 ) . The p h y sio g ra p h y o f th e r id g e i s com plex and
on b o th f la n k s th e r id g e d ro p s o f f s t e e p l y and i r r e g u l a r l y .
I n s u l a r C o a sts
C o a s tlin e s o f th e Andaman and N io o b a r i s l a n d s r e
f l e c t th e to p o g ra p h ic assym m etry o f th e i s l a n d s . W estern
c o a s ts a r e f a i r l y s t r a i g h t and g e n t l e , and c o a s ta l p la in s
o c c u r o n ly alo n g w e s te rn s h o r e s . The e a s te r n o o a s ts a re
s tr o n g ly in d e n te d and s t e e p , and in many p la c e s e x h i b it
c o r a l r e e f s and b e a o h e s r a i s e d from s e v e r a l m e te rs to a s
h ig h a s 20 m above s e a l e v e l (S e w e ll, 1925; C h a tte r j e e ,
1961).
N arrow o h a n n e ls s i m i l a r to th e g a l e t s o f th e
M ergui A rc h ip e la g o d i s s e c t th e Andaman I s l a n d s . N o rth ,
M iddle and S outh Andaman i s l a n d s o o l l e o t i v e l y have been
r e f e r r e d to a s G re a t Andaman, b ec au se th e g a l e t s w hioh
s e p a r a te th e is l a n d s a r e n o t c o n s p ic u o u s ly w id e , and do
n o t seem to s e p a r a te th e i s l a n d s when view ed from
84
i
s h ip b o a rd •
I n s u la r S h elv es
M ajor ch an n els w hich t r a n s e c t th e Andaman-Nicobar
Ridge d iv id e th e is la n d s in to fo u r g ro u p s, from n o rth to
s o u th : (1) th e Andaman A rc h ip e la g o , (2 ) Car N ico b ar and
B a t t i M all, (3) Chowra, T e re sa , T illa n c h o n g , Bompoka,
C am orta, C a to h a ll and T rin k a t, and (4 ) L i t t l e and G reat
N ic o b a r. Each group i s frin g e d by c o r a l r e e f s w hich form
in s u l a r sh e lv e s 10 to 50 km w ide w est o f th e is la n d s b u t
le s s than 10 km wide e a s t o f th e i n s u l a r c o a s ts . The
Andaman group i s p a r a lle le d 22 km o f f th e w e ste rn o o a s ts
by a s e r ie s o f c o r a l banks co m p risin g a d is c o n tin u o u s
b a r r i e r r e e f (A lcock, 1 9 0 2 ). W ater d ep th s betw een th e
b a r r i e r r e e f and th e is la n d s a r e a s g r e a t a s 90 m. The
p resen ce o f t h i s w estern b a r r i e r , ooupled w ith th e r a is e d
o o ra l r e e f s , s te e p e r subm arine s lo p e s and n arro w er
I n s u la r sh e lv e s o f th e e a s te rn s id e le d S ew ell (1925a,
1935) to conclude t h a t r e c e n t te c to n is m t i l t e d th e is la n d
group w estw ard, w ith g ra d u a l subm ergence o f th e w estern
o o a st r e s u lt in g in th e fo rm a tio n o f th e b a r r i e r r e e f .
S ew ell assumed th a t contem poraneous em ergence o f th e
e a s te rn c o a s t e le v a te d th e e a s te r n f r in g in g r e e f s above
se a l e v e l . Much o f t h i s c o n tr a s t in s h e lf- w ld th and
c o a s ta l c o n fig u ra tio n may a o tu a liy be due to d if f e r e n c e s
in lith o lo g y and r e s is ta n c e to c o a s ta l e r o s io n , ooupled
8 5,
w ith g r e a t e r wave e n e rg ie s expended on th e w e ste rn s id e s
o f th e i s l a n d s .
S i l l s
T able 5 l i s t s th e names and d e p th s o f th e v a r io u s
c h a n n e ls w hich d i s s e c t th e A ndam an-N icobar R id g e. The
d e e p e s t and m ost p ro m in en t i s G re a t P a s s a g e , w hich i s
170 km w ide betw een G re a t N lc o b a r and P u lau B reueh o f f
th e n o th e rn t i p o f S u m atra, and i s a p p ro x im a te ly 1600 m
d e e p . Topography in t h i s a r e a i s e x c e e d in g ly com plex, and
s i l l d ep th i s d e te rm in e d w ith more a c c u ra c y by e x a m in a tio n
o f w a te r c h a r a c t e r i s t i c s r a t h e r th a n by d e p th s o u n d in g s .
W ater te m p e ra tu re s and s a l i n i t i e s a r e I d e n t i c a l to
d e p th s o f 1300 m in th e Andaman Sea and Bay o f B engal on
b o th s id e s o f G re a t P a s s a g e . Below 1300 m in th e Andaman
S ea th e w a te rs d e c re a s e s l i g h t l y in te m p e ra tu re and
s a l i n i t y to a d e p th o f 1600 m. Between t h i s d ep th and
th e bottom th e Andaman Sea w a te rs a r e e s s e n t i a l l y i s o
th e rm a l and l s o h a l i n e . E v id e n tly , w a te rs can com m unicate
a c ro s s th e r id g e e f f e c t i v e l y o n ly to d e p th s o f 1300 m.
Between 1300 and 1600 m m inor am ounts o f w a te r can e n t e r
th e Andaman Sea from th e Bay o f B engal th ro u g h s m a ll,
r e s t r i c t e d c h a n n e ls . W ater c h a r a c t e r i s t i c s a r e d is o u s s e d
in more d e t a i l e lse w h e re in t h i s r e p o r t .
S u b o rd in a te o n ly to G re a t P assag e in d e p th and
m ag n itu d e i s Ten D egree C h an n el, betw een C ar N io o b ar and
TABLE 5 . Summary o f C hannels a c ro s s th e Andaman-
N ic o b a r R id g e .
A pproxim ate
C hannel D epth (m)
N o rth F r e p a r is C hannel 70
S o u th F r e p a r is C hannel 300
E lev en D egree C hannel 80?
Ten D egree C hannel 1200
Car N io o b a r-T illa n c h o n g C hannel 300
L i t t l e N ic o b a r C hannel 300
G re a t F assa g e 1600
L i t t l e Andaman. The w id th o f t h i s ch a n n el I s 145 km.
A c u rio u s f e a tu r e i s found a lo n g th e r id g e a x is in Ten
Degree C h an n el. As d e fin e d by th e 1200 m Is o b a th a sm a ll
b a s in i s n e s te d in th e rid g e c r e s t . The f l a t f l o o r o f
t h i s b a s in i s 1290 m below s e a l e v e l and h as an a p p ro x i-
o
mate a re a o f 500 km . To th e w e st and n o r th e a s t th e
b a s in i s open th ro u g h narrow c h a n n e ls 1000 to 1200 m
deep to th e Bay o f B engal and Andaman S e a . The extrem e
f l a t n e s s o f th e b a s in f l o o r s u g g e s ts ponded se d im e n t, a
c o n d itio n su b se q u e n tly borne o u t by se d im en t sam p lin g .
E a s te rn S lo n e
The Andaman-Nicobar R idge e x h ib its an e a s te rn slo p e
w ith a sim p le c o n fig u ra tio n to th e n o r th , becom ing i n
c r e a s in g ly complex so u th w ard . O ff th e m a jo r Andaman
Is la n d s th e rid g e s lo p e s 2 .5 ° d i r e c t l y to th e f l o o r o f th e
N arcondam -B arren B a s in , 2000 m below s e a l e v e l . H ere th e
slo p e i s convex upw ards. Southw ard th e s lo p e i s re n d e re d
more and more complex by a s e r i e s o f n o r th - s o u th a lig n e d
v a lle y s w hich i n t e r c e p t th e r id g e a t a s l i g h t a n g le and
become d e e p e r and n a rro w e r to th e s o u th . The n o r th e r n
m ost o f th e s e v a lle y s i s ju x ta p o se d betw een th e so u th e rn
Andamans and I n v is i b le Bank ( F ig . 1 6 ) . T h is v a lle y haB
sy m m etrical 3 .3 ° w e ste rn and e a s te r n s lo p e s , i s l e s s th a n
1400 m deep a t i t s n o rth e rn en d , and i s 2400 m below se a
le v e l a t i t s d e e p e st p o in t.
F ig u re 16* P r e c is io n D epth R ecord p r o f i l e s
a c ro s s e a s te r n s lo p e o f Andaman-
N ic o b a r R id g e, I n v i s i b l e Bank,
and in te r v e n in g tr o u g h .
WEST
ANDAMAN
NICOBAR
RIDGE
2 J Km
- I —
100 Km
1
INVISIBLE
BANK
NICOBAR
RIFT
VALLEY
VERTICAL EXAGGERATION = 30.4X
oo
V O
P re v io u s w o rkers (S e w e ll, 1925; C h h ib b er, 1934;
Weeks and o th e r s , 1965; P e te r and o th e r s , 1966) assumed
t h a t I n v i s i b l e Bank I s th e so u th e rn e x te n s io n o f th e
N arcondam -B arren v o lc a n ic tr e n d . C lo s e r ex am in atio n o f
th e b a n k 's b ath y m etry and lith o lo g y le a d s t h i s w r i t e r to
a d i f f e r e n t c o n c lu s io n . In c o n tr a s t to Narcondam and
B a rren i s l a n d s , w hich a r e s te e p , ro u g h ly e q u ld lm e n slo n a l
a r e a l l y and no more th a n 25 km a c r o s s , I n v i s i b l e Bank i s
170 km lo n g , a p p ro x im a te ly 50 km w ide, and p o s s e s s e s
sm o o th er, more c o n s is te n t s lo p e s . Along m ost o f i t s
le n g th th e bank i s s h o a le r th an 200 m. M iocene ro o k s
w ere dredged from two s i t e s a t 1000 m d ep th s on th e
e a s te r n slo p e o f th e b an k . P la t Rock, th e s h o a le s t
p o r tio n o f th e bank, i s a sm a ll a r e a a t 11° 08* N l a t i
tu d e , 93° 2 9 .5 ' E lo n g itu d e . At h ig h t id e F l a t Rock i s
aw ash, and a t low t i d e i t i s em erg en t. D iv ers from th e
PIONEER I n v e s tig a te d th e bank and re c o v e re d ro c k s im ila r
to th e M iocene m a te r ia l dredged from d ep th s o f 1000 m.
H igh g r a v ity an o m alies re p o rte d o v er th e bank ( P e te r and
o th e r s , 1966) m ig h t be a t t r i b u t e d to Upper O retaceo u s
s e r p e n t l n l t e s u n d e rly in g i t . S e r p e n tln ite s do oom prlse
m ajo r e a s te r n p o r tio n s o f th e Andamans.
A nother narrow tro u g h p a r a l l e l s Oar N lo o b ar 30 km
e a s t o f th e is la n d w ith d e p th s g r e a t e r th an 2000 m. T his
v a lle y tre n d s southw ard to form a broad embayment bounded
by T e re s a , Gamorta and T illan o h o n g is l a n d s .
The tr u e b a se o f th e A ndam an-N ioobar R idge e a s t
o f th e N lc o b a rs i s th e f l o o r o f th e d e e p e s t, m ost
s p e c ta c u la r r i f t in th e Andaman B a s in . T h is r i f t ,
r e f e r r e d to i n t h i s p a p e r a s th e N io o b ar R i f t , i s d l s -
cu8sed e ls e w h e re . N o rth and e a s t o f T illa n c h o n g , w here
i t i s b e s t e x p r e s s e d , th e s lo p e common to th e Andaman-
N ic o b a r R idge and th e N ic o b a r R i f t h a s a 17° g r a d ie n t
from th e 200 m I n s u l a r s h e lf - b r e a k to d e p th s in e x c e s s
o f 4000 m.
S o u th o f n o r th e r n G re a t N ic o b a r th e s lo p e i s muoh
g e n t l e r , and maximum d e p th s o f 1600 m a r e a t t a i n e d 18 km
e a s t o f th e i s l a n d . The N ic o b a r R i f t a g a in d eep en s a s i t
p a s s e s o u t i n t o th e Bay o f B e n g a l, and th e e a s te r n s lo p e s
o f th e A ndam an-N lcobar R idge a g a in beoome s t e e p e r , a t
t a i n i n g 6 ° o f f th e s o u th e rn t i p o f G re a t N ic o b a r.
W estern S lo n e
As a v a i l a b l e b a th y m e tric co v e ra g e f o r th e w e ste rn
s lo p e i s In a d e q u a te f o r th e com plex to p o g rap h y o f t h a t
a r e a , is o b a th s d e f in in g th e w e s te rn s lo p e o f th e Andaman-
N io o b ar R idge in P l a t e I a r e h ig h ly g e n e r a liz e d . Only
in th e v i c i n i t y o f Ten D egree C hannel i s th e d e n s ity o f
so u n d in g s s u f f i o l e n t to p e rm it s e m i- d e ta ile d c o n to u rin g .
In g e n e r a l, th e A ndam an-N lcobar R idge in c r e a s e s in
to p o g ra p h lo c o m p le x ity so u th w a rd . A c r o s s in g th ro u g h
F r e p a r is C hannel r e v e a le d a sim p le s te e p s lo p e from th e
d e l t a s h e l f to th e f l o o r o f th e Bay o f B engal ( F ig . 1 7 ).
The w e ste rn s lo p e o f th e rid g e o f f Ten D egree C hannel i s
a m osaic o f e lo n g a te rid g e s and in te rv e n in g v a l le y s .
These to p o g ra p h ic f e a tu r e s p r e s e n t l o c a l r e l i e f o f up to
1100 m, a r e betw een 5 and 10 km w id e, and r a r e l y a re
lo n g e r th a n 50 km. T h e ir c h a r a c t e r i s t i c e lo n g a tio n
p a r a l l e l to th e i s l a n d s ' alig n m e n t s u g g e s ts t h a t th ey a re
th e to p o g ra p h ic e x p re s s io n o f t i l t e d and f a u lte d ro c k
s t r a t a . In th e v i c i n i t y o f G re a t P assage r e l i e f and
to p o g ra p h io co m p lex ity i s even more extrem e and i s
c h a r a c te r iz e d by s te e p , sh a rp to p o g ra p h ic h ig h s w ith
r e l i e f in e x c ess o f 800 m ( F ig . 1 8 ).
B asin F lo o r
C e n tra l Andaman Trough
The C e n tra l Andaman Trough (F ig . 9) i s 750 km
lo n g and e x te n d s southw estw ard from th e mouth o f M artaban
Canyon a t 13*5° N l a t i t u d e to 6 .5 ° N l a t i t u d e , w here i t
ends a g a in s t th e Andam an-Nlcobar Ridge and N io o b ar R i f t
V a lle y . To th e n o rth w e st th e tro u g h i s bounded by A loock
Seam ount, b u t i t s w e ste rn s id e i s l o s t in a m osaic o f
r i f t v a lle y s and in te rv e n in g se a h ig h s n o r th e a s t and e a s t
o f th e N lc o b a rs . Along i t s e a s te r n p e rip h e ry th e tro u g h
1 b bounded by th e M alay s lo p e . The s o u th e rn h a l f o f th e
tro u g h i s d iv id e d by S ew ell Seam ount, w hich p a r a l l e l s th e
M alay S lope to i s o l a t e a 90 km wide band o f g e n tly oonoave
F ig u re 17* W e s t-e a s t p r o f i l e th ro u g h P re p a rle
C hannel a c ro s s c o n t in e n t a l s lo p e
and Irraw ad d y D e lta s h e l f .
-2
4 - * -
C/C
SE
80 Km
VERTICAL EXAGGERATION = 8.9 X
V O
- ( s -
F ig u re 1 8 . E a s t- n o r th e a s t - w e st-so u th w e st
PDR p r o f i l e th ro u g h G re a t
Passage*
ENE
0
0
i i I L
5.° ,
100
J I » i i
150
J I L
j I
2 9 O
J I L
w s w
^59 Km
ANDAMAN SEA BAY OF BENGAL
VERTICAL EXAGGERATION = 34.IX
V O
C T \
97
upward tro u g h f l o o r from th e t r o u g h 's m ain b o d y . I t
w i l l be r e c a l l e d t h a t t h i s s u r f a c e I s th e s o u th e rn e x
te n s io n o f th e deep t e r r a c e a t th e f o o t o f th e M alay
S lo p e ( F ig . 1 4 ) . The maximum w id th o f th e tr o u g h , o v e r
200 km, I s a t ta i n e d n o r th o f S e w e ll S eam ount. G r e a te s t
d e p th s o f th e f l a t tro u g h f l o o r s I s 3000 m a lo n g th e
tro u g h a x is betw een i l c o o k and S e w ell se a m o u n ts. Betw een
th e M alay S lo p e and S e w e ll Seam ount th e d e p th s a r e l e s s ,
p
on th e o r d e r o f 2600 m. M ost o f th e t r o u g h 's 1 1 0 ,0 0 0 km
a r e a l a com prised o f f l a t s a t d e p th s b etw een 2000 and
3000 m.
R i f t V a lle y s
The C e n tr a l Andaman R i f t V a lle y d i s s e c t s th e m ain
f l o o r o f th e C e n tra l Andaman T rough In a w e s t-s o u th w e s t -
e a s t - n o r t h e a s t d i r e c t i o n betw een 10° and 11°• T h is
v a l le y a b r u p tly changes d i r e c t i o n th r e e tim e s a lo n g th e
w e ste rn edge o f th e C e n tr a l T ro u g h , tr e n d in g f i r s t s o u th ,
th e n so u th w e st and f i n a l l y so u th w ard a g a in , a t th e b ase
o f S eah lg h NE1002-9358(m) • S o u th o f t h i s seam ount th e
C e n tra l Andaman R i f t V a lle y I s p a r t i a l l y s e p a r a te d from
th e N ico b ar R i f t V a lle y by two e lo n g a te r id g e s and jo in s
th e N ico b ar R i f t V a lle y th ro u g h a gap betw een th e
r id g e s ( F ig . 19)* The s o u th e rn p o r tio n o f th e C e n tr a l
Andaman R i f t V a lle y p a r a l l e l s th e N ic o b a r R i f t V a lle y a t
a d ls ta n o e o f 30 to 35 km f o r a lm o st 200 km, b e fo re
F ig u re 19* N o rth -n o rth e a s t - s o u th -s o u th w e s t
PDR p r o f i l e a c ro s s C e n tra l Andaman
R i f t V a lle y in th e f i o i n l t y o f 10°
N, 94° E.
I I
100 Km
CENTRAL
ANDAMAN
x RIFT
VALLEY
NICOBAR
RIFT
VALLEY
VERTICAL EXAGGERATION * 3 4,2 X$ |
V O
te rm in a tin g a t 8 ,5 ° N l a t i t u d e . T o p o g rap h ic o o n tr o l i s
la c k in g a lo n g th e C e n tr a l Andaman R i f t V a ll e y 's n o r th e r n
end* and th e c o n to u rs w hich d e l i n e a te th e v a lle y in P la te
I w ere drawn to conform w ith th e g e n e ra l c o n f ig u r a tio n o f
th e tro u g h ; t h i s i n t e r p r e t a t i o n may he e rro n e o u s . The
r i f t v a lle y i s n o t p r e s e n t on a b a th y m e tric p r o f i l e a lo n g
1 1 .5 ° N l a t i t u d e ( F ig . 15)* 60 km n o r th o f th e n o r th e r n
m ost tr a v e r s e aoroBS th e v a l l e y . T h is ab sen ce o f th e
r i f t v a lle y in th e n o r th e r n end o f th e C e n tr a l Andaman
T rough, th e r i f t ' s V -shape and s h a rp p in n a o le s ( F ig s . 20
and 21) and i t s tre n d in to th e N ic o b a r R i f t V a lle y fa v o r
a te o to n ic , r a t h e r th a n an e r o s io n a l o r ig in f o r th e
f e a t u r e •
I t h as been n o te d p re v io u s ly t h a t th e e a s te r n
s lo p e o f th e Andam an-N ioobar R idge i s te rm in a te d in r i f t
v a lle y s w hich tre n d s e v e r a l d e g re e s w est o r e a s t o f
n o r th - s o u th . Most p ro m in en t o f th e s e f e a t u r e s i s th e
N ico b ar R i f t V a lle y , w hich i s th e w e s te rn b ran o h o f th e
b a s in -w id e r i f t sy ste m .
The e a s te r n b ran ch o f th e r i f t v a lle y sy stem i s
th e V -shaped C e n tr a l Andaman R i f t V a lle y n o te d p r e v io u s ly . ;
The N io o b ar R i f t V a lle y can be tr a c e d c l e a r l y f o r 700 km i
from th e so u th w e ste rn tro u g h -te rm ln u s o f th e Naroondam -
B a rren B a sin to G re a t P assag e a t 5° N l a t i t u d e . S o u th o f
t h i s l a t i t u d e th e r i f t v a lle y m erges w ith a com plex o f
s e a h ig h s , a lth o u g h i t p ro b a b ly c o n tin u e s so uthw ard f o r
F ig u re 20. W est-n o rth w est - e a s t- s o u th e a s t
PDR p r o f ile a c ro s s C e n tra l Andaman
R i f t V alley a t 10o4 5 ' N l a t i t u d e .
2 - i r~
0
WNW
~ ~ T ...
20 4 0
— i—
6 0 80Km
ESE
1 I
t *
f ■*
1
i »•>.
j
w t
|v
rlfrjBKi
t |
!
4-» Km
VERTICAL EXAGGERATION = 30.5X
102
F ig u re 2 1 . W e st-e a st FDR p r o f i l e alo n g 10° N
a c ro ss th e C e n tra l Andaman R if t
V a lle y .
120 K m
WEST
4 J Km
VERTICAL EXAGGERATION * 30.9X
105 ,
i
l
a n o th e r 150 km as th e 80 km w ide Breuoh Trough w est o f
S um atra.
V alley s in th e r i f t v a lle y system a re seldom w id er
i
th a n 30 km. Between 8 .5 ° and 1 0 .5 ° N l a t i t u d e th e r i f t
v a lle y f lo o r s a re f l a t and a p p ro x im a te ly 10 km wide
(F ig . 1 9 ), and owe t h e i r f l a t n e s s to ponded se d im en t. I t
i s in t h i s g e n e ra l a r e a t h a t th e r i f t v a lle y system and
th e Andaman B asin as a whole a re d e e p e st) w ith m easured
d ep th s between 4200 and 4390 m below s e a l e v e l . The
C e n tra l Andaman R i f t V alley i s d i s t i n c t l y V -shaped, as
i s th e so u th e rn p o rtio n o f th e N ico b ar R i f t .
Three m inor w estern b ran ch e s o f th e N ioobar R i f t
V a lle y , th e v a lle y betw een I n v i s i b l e Bank and th e Andaman
Is la n d s (F ig . 16) and two v a lle y s e a s t o f Oar N io o b ar,
in te r o e p t th e Andaman-Nioobar Ridge and p a r t i a l l y I s o la t e
lo c a l rid g e s from th e main mass o f th e r id g e . These sub
s id ia r y r i f t v a lle y s a re 100 to 200 km lo n g , a p p ro x i
m ately 20 km w ide, and g e n e ra lly e x h i b it f l a t , sed im en t-
f i l l e d f lo o r s a t d e p th s betw een 2000 and 2400 m below
se a l e v e l .
A 170 km lo n g s e a v a lle y was d e lin e a te d so u th o f
S ew ell Seam ount, b u t a v a ila b le b a th y m e tric d a ta are i n
s u f f i c i e n t to d e fin e p r e o ls e ly i t s n o rth e rn en d . The
n o rth e rn h a l f o f th e v a lle y i s a lig n e d n o rth -s o u th ; th e
so u th e rn h a l f tre n d s so u th e a stw a rd to end a g a in s t th e
seamount N E06l4-9659(m ), n o r th o f P ulau Weh.
N aroondam -B arren B a sin
106
An i r r e g u l a r l y shaped b a s in w ith a maximum le n g th
I
and w id th o f 180 and 80 km r e s p e c tiv e ly l i e s s o u th o f th e
w e ste rn Irraw ad d y D e lta s lo p e , e a s t o f th e Andaman I s
la n d s , and w est o f Narcondam I s la n d and A lcock Seam ount.
T his b a s in , th e N arcondam -B arren B a s in , i s s l i g h t l y
d ee p er th an 1700 m a t i t s c i r c u l a r n o r th e r n end and
deepens southw ard to 2300 m midway betw een Narcondam and
B a rren i s l a n d s . A t i t s s o u th e rn end th e b a s in i s d iv id e d
in to two c o n s tr i c te d tro u g h s by B a rren I s la n d and a s
s o c ia te d s e a h ig h s . The w e ste rn tro u g h i s betw een I n v i s i b l e
Bank and B a rren I s la n d and i s a p p ro x im a te ly 12 km w ide and
70 km lo n g . The e a s te r n tro u g h h as a com parable w id th
and le n g th and c o n s t i t u t e s th e low a r e a w hich s e p a r a te s
th e B a rren I s la n d com plex o f h ig h s from A lcook Seam ount.
A pproxim ate a r e a o f th e N arcondam -B arren B a sin i s 1 6 ,5 0 0
km2 .
A lcock Seamount
E a s t o f th e N aroondam -B arren B a sin and th e n o r th e r n
end o f th e C e n tr a l Trough i s A lcock Seam ount, 220 km lo n g
and 100 km w id e . The seam ount l i e s midway betw een th e
A ndam an-N ioobar R idge and th e M alay S lope and i s ro u g h ly
p a r a l l e l to b o th f e a t u r e s , tr e n d in g 15° w est o f s o u th .
S e v e ra l peaks o o cu r a t in c r e a s in g d e p th s southw ard a lo n g
107
th e se a m o u n t's c r e s t , from 55 m to 220 m below s e a l e v e l .
The seam o u n t*8 g r e a t e s t r e l i e f I s a p p ro x im a te ly 2400 m,
m easured from th e n o r th e r n c r e s t to th e f l o o r o f th e
C e n tr a l T rough. S lo p e s o f th e seam ount a r e s l i g h t l y
more th a n 3 ° . The w e s te rn s lo p e o f A lcock Seamount
m erges w ith seam ounts a s s o c ia te d w ith B a rre n I s la n d ( F ig .
2 2 ) . An e a s t- w e s t a lig n e d s a d d le w ith a m easured d e p th
o f 1260 m d iv id e s th e seam ount, w ith th e n o r th e r n h a l f
somewhat lo n g e r th a n th e s o u th e rn .
S ew ell Seam ount
S o u th o f A lcook Seam ount and a c ro s s th e C e n tr a l
Andaman Trough I s S e w e ll Seam ount. T h is f e a t u r e ( F ig .
23) h as th e same ap p ro x im a te tr e n d , s i z e , r e l i e f end s lo p e
a s A lcock Seam ount. I t tr e n d s 10° w est o f s o u th , I s 220
km lo n g and 100 km w id e . S h o a le s t d e p th m easured on th e
seam ount I s 480 m, midway a lo n g I t s le n g th and on th e
e a s te r n p o r tio n o f I t s c r e s t . S ew ell Seam ount I s l e s s
r e g u la r th a n I n d ic a te d by P la te I , and a d d i t io n a l b a th y
m e tr ic co v erag e w i l l show I t s form to be more com plex
( F ig . 2 3 ) . Maximum r e l i e f o f th e seam o u n t, m easured from
th e h ig h e s t e a s te r n peak to th e b ase o f i t s w e ste rn s lo p e ,
I s a p p ro x im a te ly 2700 m.
Naroondam I s la n d
Narcondam I s l a n d , an e x t l n o t v o lc an o (W ash in g to n ,
F igure 22. W e st-e a st PD R p r o f il e a c ro s s the
w estern slo p e o f Alcock Seamount
and a peak o f th e B arren Is la n d
seam ount comt>lex.
60 K m
40 2 0
EAST
WEST
WESTERN
PORTION OF
ALCOCK SEAMOUNT
Km
VERTICAL EXAGGERATION = 23.6 X
601
P ig u re 23. W e st-e a st PDR p r o f i l e alo n g 10° N
a c ro s s S ew ell Seam ount.
I l l
WEST
VERTICAL EXAGGERATION = 31.1 X
112
1 9 2 4 ), l i e s 70 km so u th o f th e Irraw ad d y D e l t a 's s h e l f -
b re a k and i s Jo in ed to th e d e l t a by a broad rid g e w ith a
maximum d e p th o f 730 m. S u b a e r ia lly , Naroondam h as an
a r e a o f 7.25 km2 and a maximum le n g th , m easured from
n o r th e a s t to so u th w e st, o f 4 km. The is la n d summit i s
c r a t e r l e s s and s ta n d s 710 m above se a l e v e l . R avines a re
d e e p ly in c is e d in to th e is la n d s i d e s , and th e c o a s ts have
been w av e-cu t to form s e a c l i f f s o v er 100 m h ig h . Below
th e s e a s u rfa c e th e is la n d s lo p e s s te e p ly to a ro u g h ly
c i r c u l a r base 2000 m deep and 40 km w id e.
The is la n d i s com prised o f s i l i c i c la v a s w ith a
v e ry u n ifo rm co m p o sitio n w hich have been d e s c rib e d by
W ashington (1924) as h o rn b len d e d a c i t e . The homogeneous
ll t h o lo g y o f th e i s l a n d , to g e th e r w ith i t s la c k o f a
c r a t e r and a s h d e p o s its , le d W ashington to b e lie v e th a t
th e is la n d i s th e p ro d u c t o f a s in g le domal e ru p tio n d a te d
t e n t a t i v e l y a s l a t e P llo o e n e to P le is to c e n e (C h h ib b er,
1 9 3 4 ).
B arren Is la n d
B arren I s la n d , 135 km s o u th -s o u th w e s t o f Narcondam
I s la n d , i s th e o n ly v o lcan o in th e Andaman B asin w hich h as
been a c tiv e in h i s t o r i c tim e s . The is la n d i s a deeply
c r a te r e d cone alm o st c i r c u l a r in p la n , w ith a d ia m e te r o f
a b o u t 3 km and an a re a o f n e a rly 8 k m . E le v a tio n s o f th e
i s l a n d 's sum m its a lo n g th e o r a to r rim ran g e from 354 to
113
600 m. The w estern sid e o f th e is la n d I s deeply breao h ed ;
h e re th e c r a t e r f lo o r i s 25 m above se a l e v e l . A sym
m e tric a l c e n tr a l cone o ccu p ies th e m iddle o f th e o v al
c r a t e r f lo o r . T his in n e r c o n e 's s id e s r i s e w ith alm ost
p e r f e c t symmetry along 22° slo p e s to th e c o n e 's apex,
310 m above se a l e v e l . The c e n tr a l cone i s tru n c a te d by
an in n e r c r a t e r 20 m in d ia m eter w ith a f lo o r 28 m below
th e in n e r c r a t e r rim (O hhibber, 1934)* B arren Is la n d
e ru p te d l a s t in th e l a t t e r p a rt o f th e p a s t c e n tu ry
(W ashington, 1924); w isps o f smoke em anating from th e
c e n tr a l cone a re s t i l l d is c e r n ib le .
Below se a l e v e l, B arren Is la n d form s a subm arine
f e a tu re w ith a base 20 to 25 km broad a t d ep th s o f 2000 m
and com parable in s iz e and shape to subm arine Narcondam.
Three seah ig h s im m ediately a d ja c e n t to th e B arren h ig h
a re b e lie v e d to be o f r e la te d v o lc a n ic o r ig in .
The a n c ie n t cone o f B arren I s la n d was f i r s t s tu d ie d
by M a lle t (1895)» who d e s c rib e d i t as b ein g made up
la r g e ly o f b a s a lt " ... more o r l e s s v e s ic u la r , w ith
sm all c r y s ta ls o f w hite tr a n s lu c e n t f e ld s p a r and sm all
g ra n u le s and c r y s ta ls o f b o ttle - g r e e n , tr a n s lu o e n t a u g lte ,
d issem in ated th ro u g h a d a rk -g ra y groundm ass. B row nish-
y ello w o liv in e i s fre q u e n tly p re s e n t a ls o • • • Most o f
th e specim ens • • • may be d e sig n a te d d o l e r i t l c b a s a lt
• • •" (O hhibber, 1934, p . 4 4 1 ). Fragm entary m a te ria l i s
i n t e r s t r a t i f i e d w ith th i s la v a in s e v e r a l c o a s ta l s i t e s .
114
The c e n tr a l cone c o n s is ts o f a u g ite - a n d e s ite w ith some
b a s a l t w hich may c o n ta in o liv in e o r be o liv in e - f r e e *
W ashington (1924) b e lie v e d t h a t chem ical a n a ly se s and
norms o f th e ro o k s o f Narcondam and B arren is la n d s p o in t
c o n c lu s iv e ly to d e r iv a tio n from a common magma*
S m a lle r S eahighs
The d im en sio n s, r e l i e f and p ro b ab le n a tu re o f 22
se a h ig h s su rv ey ed w ith in th e Andaman B asin a re summ arized
on T able 6* A ll o f th e s e f e a tu re s a re in tim a te ly a s
s o c ia te d w ith th e r i f t v a lle y system and f a l l in to two
c la s s e s o f c o n tr a s tin g to p o g ra p h ic s t y l e .
The f i r s t c a te g o ry o f se a h ig h s In c lu d e s th o se
w hich a re g e n e ra lly betw een 10 and 50 km a c ro s s and a re
o v a te o r ro u g h ly c i r c u l a r in plan* G en e rally th e s e
f e a tu r e s a re e a s t o f th e N icobar R i f t V alley and are
lo o a te d w ith in a narrow zone a lig n e d ro u g h ly n o rth -s o u th
w hich in c lu d e s th e v o lc a n ic Narcondam and B arren is l a n d s .
S lo p es o f th e s e f e a tu r e s a re ex trem ely s te e p and i r
r e g u la r (F ig . 28)* T h e ir r e l i e f v a r ie s c o n s id e ra b ly as
e x e m p lifie d by th e c o n tr a s t between th e low s e a k n o ll w hich
i n t e r r u p ts th e f l o o r o f N arcondam -Barren B asin and S ea-
h ig h No* NE1002-9358(m) w hich s ta n d s 3600 m above the
f lo o r o f th e N ico b ar R i f t V a lle y .
Only one o f th e se a h ig h s was sampled* S eahigh No*
NE0940-9355(m) y ie ld e d f r e s h ly f ra c tu r e d b a s a lt to a
TABLE 6 . Summary o f Andaman B asin Seahighs
Peak A pprox.
S eah ig h D epth R e lie f S iz e P ro b a b le N atu re As I n f e r r e d from
Code No._____________ia J ____ £®i_______(km)____ B athym etry__________________________
NE 1300-940O m 1300 700 20x30 Seam ount o f N arcondam -B arren v o lc a n ic tr e n d
NE 1248-94l2m 700 1500 20x20 Seam ount o f N arcondam -B arren v o lc a n ic tr e n d
NE 1243-9345k 1750 250 15x20 S e a k n o ll o f N arcondam -B arren v o lc a n ic tre n d
NE 1205-9358m 380 1600
10x15 Seam ounts o f N arcondam -B arren tr e n d ;
co m p rise th e B a rren I s la n d com plex o f
NE 1153-9356m 760 1600 15x30 v o lc a n ic h ig h s
NE ll4 4 -9 3 3 2 b 110 1900
30x55 N o rth e rn e x te n s io n o f I n v i s i b l e Bank ( T e r t
ia r y se d im en ts and C re ta c e o u s (? )
s e r p e n t l n i t e 8 ( ?)
NE 1124-9350m 1100 900 25x30 Seam ount o f N arcondam -B arren v o lc a n ic tr e n d
NE 1101-9353m 1100 1300
10x35
Seam ount o f N arcondam -B arren v o lc a n ic tr e n d
NE 1045-9307r 1300 900 20x30 T o g e th e r p ro b a b ly com prise a T e r tia r y s e d i
m ent and C re ta c e o u s (? ; s e r p e n t l n i t e r id g e
NE 1 0 l8 -9 3 0 2 r 1100 2700 30x80 o f th e Andaman-N1cobar R idge com plex
NE 1041-9408m 2810 600
20x35 Seam ount o f N arcondam -B arren v o lc a n ic tr e n d
NE 1002-9358m 800 3600 35x50 Seam ount o f N arcondam -B arren v o lc a n ic tre n d
TABLE 6 . Summary o f Andaman B asin Seahighs (C ontinued)
Peak A pprox.
S eah lg h D epth R e lie f S iz e
Code Ho._____________(mj____ (m)_______ (km)
NE 0956-9335r 1300 3100 30x60
NE 0940-9355m 2480 1750 10x50
NE 0920-9A08m 2600 400 10x30
NE 0859-9333m
179
4000
20x65
NE 0 8 l6 -9 3 5 7 r 1178 1800 30xl0<
NE 0 7 3 2 -9 4 l2 r 676 1300
NE 0719-9407r 567 1400 15x20
NE 0721-9420m 1770 600 20x50
NE 0706-9442m 1780 800 25x60
NC 06l4-9659m
155
1650 30x30
P ro b a b le N atu re As I n f e r r e d from
B athym etry______________________________________
P ro b le m a tic . May b elo n g to th e Narcondam -
B a rre n com plex, o r , to g e th e r w ith NE 0859-
9 333r may c o n s t i t u t e a n o n -v o lc a n ic r id g e
an a lo g o u s to I n v i s i b l e Bank
Seam ount o f N arcondam -B arren v o lc a n ic tre n d
Seam ount o f N arcondam -B arren v o lc a n ic tre n d
N o n -v o lcan lc seam ount o r s o u th e rn p o r tio n
o f an o f fs h o o t from th e A ndam an-N icobar
Ridge
N o n -v o lc a n lc r id g e an alo g o u s to I n v is i b l e
Bank
T o g e th e r may c o n s t i t u t e a r id g e b e lo n g in g
to th e A ndam an-N lcobar com plex
V o lca n ic seam ount
V o lca n ic seam ount
V o lca n ic seam ount o f B a ris a n l i n e
117
p is to n c o r e r from a s i t e on i t s w e ste rn f la n k . A v o lc a n ic
o r i g i n i s presum ed f o r th e f i r s t group o f 14 se a h ig h s in
th e n arro w zone w hich in c lu d e s t h i s sam pled seafclgh and
th e v o lc a n ic i s l a n d s . H e a t-flo w , g r a v ity and m ag n etic
d a ta w hich a r e d is c u s s e d l a t e r s u p p o rt t h i s a ssu m p tio n .
The second c a te g o ry o f se a h ig h s in c lu d e s th e n o r th -
s o u th e lo n g a te f e a t u r e s w hich a r e s e p a ra te d from th e
A ndam an-N icobar R idge by th e N lco b a r R i f t V a lle y and i t s
w e s te rn s u b s i d i a r i e s . These a lig n e d r id g e s p o sse ss th e
same g e n e r a l c o n f ig u r a tio n a s I n v i s i b l e Bank, a lth o u g h
th e y la c k th e b a n k 's f l a t to p . The lo n g r id g e w hich i s
betw een th e N ic o b a r R i f t V a lle y and th e e a s te r n b ran ch
o f th e r i f t v a lle y sy stem a t 10° N l a t i t u d e ( F ig . 24)
e x e m p lifie s th e p h y s io g ra p h ic n a tu re o f th e r i d g e s . In
t h i s l o c a l i t y th e r id g e d is p la y s a h ig h d eg ree o f sym
m e try , w ith o p p o sin g sm ooth s id e s t h a t s lo p e 8 .5 °» and
a sm o o th ly rounded to p . T h is c o n f ig u r a tio n a g re e s w e ll
w ith th o s e o f th e r id g e s composed o f ro o k s co m p risin g th e
A ndam an-N icobar R id g e .
F ig u re 24. W e s t-e a s t PDR p r o f i l e alo n g 10° N
a c ro s s N ic o b a r R i f t V alley and ad
ja c e n t r id g e to th e e a s t .
WEST EAST
2 0 30 4 0 Km
NICOBAR
RIFT
VALLEY
Km
VERTICAL EXAGGERATION = I0.4X
LITH O LO G Y O P T H E A N D A M A N BASIN
G en eral S tatem en t
Twelve s t a t i o n s In th e Andaman B asin y ie ld e d f o s s i l
sed im en ts o r ig n eo u s rook to v a rio u s ty p e s o f sam plers
d u rin g th e c r u is e s o f th e SERRANO and PIONEER. In a d d i
t i o n , a sam ple o f b a s a l t was dredged by th e INVESTIGATOR
in 1925 and was d e s c rib e d by S ew ell (1925) and Wiseman
and S ew ell (1937)* The lo c a tio n s o f th e s e sam ples a re
shown in F ig u rs 25 and th e ro c k s d e s c rib e d in T able 7*
Andam an-Nicobar Ridge
Three ro ck sam ples from th e w e ste rn a re a o f th e
Andaman B asin a re l l t h o l o g i c a l l y s im ila r to ro ck s o f th e
Andaman and N ioobar i s l a n d s . PIONEER Dredge 8 , a t a s i t e
50 km n o r th o f T illa n c h o n g I s la n d , y ie ld e d an a lte r e d
e x tru s iv e ro c k in a d d itio n to Upper M iocene sed im en tary
ro c k s . The a l t e r e d v o lc a n ic rook c o n s is ts o f an a l t e r e d
groundm ass su rro u n d in g la b r a d o r lte p h e n o c ry sts and r e
p la ced phenoorystB w ith th e o u tlin e s o f h y p e rsth e n e and
sphene and p o s s ib ly a u g ite and o liv i n e . Replacem ent
m in e ra ls a re a l b i t e , c o a rs e -g ra in e d c a l c i t e and c h l o r i t e ,
and f i n e r m a g n e tite and e p id o te . T his ro ck i s a m afic
la v a o f p o s s ib le Upper O retaoeous age suoh as th o se a s
s o c ia te d w ith s e r p e n t l n i t e s and r a d io la r ia n c h e rts on th e
Andaman I s la n d s . Dredge 8 a ls o sam pled s o f t , f i s s i l e
F ig u re 25• L o c a tio n s o f ro ck sam ples from th e
Andaman B a s in .
122
LOCATIONS OF
ROCK SAMPLES
FROM THE
NDAM AN BASIN
• PD II
PD 9 *
LKCNO
PIONEER CORE (1964)
PIONEER DREDGE (1964)
SERRANO CORE (I9 6 0
INVESTIGATOR DREDGE (1925)
Hoovy lint t a il of fno Andaman-Nicobar
Ridgt indicoltt tha probabla lifhologic
boundorybotw«on_Mda«_an£_b^
123
TA BLE 7
PD 13
PD 12
PD 8
S 221
S 220
PD 4
PD 5
PD 10
PD 14
PD 11
PD 8
INV
PD 3
. D e s c rip tio n o f ro ck s dredged from th e Andaman
B asin
SEDIMENTARY RO CK
PIONEER DREDGE 13* C layey sa n d sto n e ; f e l s p a th ic ;
c a lc a re o u s c la y m a trix ; upper M iocene.
PIONEER DREDGE 1 2 . C a lca re o u s s h a le and
f r i a b l e sa n d sto n e ; low er M iocene.
PIONEER DREDGE 8 . S o f t, f i s s i l e r a d l o l a r l t e
s h a le ; upper M iocene. O ccurs w ith o p h lo li te .
SERRANO CORE 221. P le is to c e n e s h e lly and
f o r a m in if e r a l q u a rtz san d s w ith r e l i c t upper
M iocene f o ra m ln if e r a .
SERRANO CORE 220. Abundant rew orked upper
M iocene fo ra m ln ife ra w ith c o a rse rounded q u a rtz
and c h e r t g r a i n s .
PIONEER DREDGE 4 . T ab u lar c a lc a re o u s san d sto n e
c o n c re tio n s w ith numerous r o O t-c a s t c o n c re tio n s
adm ixed. P lio c e n e ?
PIONEER DREDGE 5* T ubular r o o t - c a s t c o n c re tio n s ;
c a lc a re o u s . S e v e ra l p ie c e s o f sa n d sto n e w ith
c a lc a re o u s cem ent, c o n ta in in g P lio c e n e f o r a -
m in if e r a .
S o ft g ray m udstone, in d u ra te d f o ra m in if e r a l
sa n d , c a lc a re o u s s h a le . P le is to c e n e .
IGNEOUS ROCK
PIONEER DREDGE 1 4 . I n te r g r a n u la r a u g ite b a s a l t .
Sampled in la r g e ta b u la r chunks.
V e s ic u la r b a s a l t , f r e s h ly f r a c tu r e d .
A lte re d o liv i n e ( ? ) h y p e rsth e n e b a s a l t . P e rro -
m agneslan m in e ra ls re p la c e d by c a l c i t e ,
c h l o r i t e , e p ld o te and m a g n e tite .
O ccurrence o f u n a lte r e d b a s a lt d e s c rib e d by
S ew ell (1925) and an a ly ze d by Wiseman (1 9 3 7 ).
H y a lo p h ltic h o rn b le n d e -h y p e rsth e n e b a s a lt and
a l t e r e d h y p e rs th e n e -a u g ite b a s a l t , as d e te r
mined from two sm a ll c h ip s .
124
r a d l o l a r l t e s h a le w hich c o n ta in e d an U pper M iocene f o r a
m in i f e r a l fa u n a (W. E. F r e r lo h s , p e rs o n a l co m m u n icatio n ).
Sam ples o f ro o k from I n v i s i b l e Bank I n d ic a te t h a t
th e bank i s s t r a t l g r a p h i c a l l y and l l t h o l o g l c a l l y as w e ll
a s p h y 8 lo g ra p h lc a lly p a r t o f th e A ndam an-N icobar R id g e .
PIONEER D redge 12 c o n ta in e d la r g e fra g m en ts o f c a lc a re o u s
s h a le and f r i a b l e sa n d sto n e o f Lower M iocene a g e , and
ro o k s from PIONEER D redge 13 w ere f e l s p a t h l c o la y ey sa n d
s to n e s c o n ta in in g an U pper M iocene m ic ro fa u n a (W. E .
F r e r ic h e , p e rs o n a l co m m u n icatio n ).
V o lcan ic Rooks
A lth o u g h few ro c k sam ples have been o b ta in e d from
th e Andaman B a s in , v o lc a n ic ro c k s a p p e a r to dom inate th e
lit h o lo g y o f th e b a s in from th e b ase o f th e Andaman-
N ic o b a r R idge to th e M alay s h e lf - b r e a k . The v a r io u s s e a
m ounts w hloh have e i t h e r y ie ld e d v o lc a n ic ro c k s to d re d g
in g o r w hich have c o n f ig u r a tio n s and p h y sio g ra p h ic p o s i
ti o n s c o m p a tib le w ith v o lo a n ic o r ig in a re p lo tt e d in
P ig u re 2 6 . Two, and p o s s ib ly t h r e e , p h y s io g ra p h lo a lly
d i s t i n c t v o lo a n ic zo n es a r e p r e s e n t in th e Andaman B a s in .
The f i r s t and m ost o b v io u s in c lu d e s B arren and Narcondam
i s l a n d s , to g e th e r w ith 14 s e a h ig h s w hich l i s in a n o r th -
s o u th tr e n d in g band no w id er th a n 30 km and w hloh a r e
in tim a te ly a s s o c ia te d w ith th e r i f t v a lle y sy stem . The
seoond zone in c lu d e s Aloook and S ew ell seam o u n ts. The
F ig u re 2 6 . V o lc a n ic ity and s e is m ic ity o f
th e Andaman B asin and i t s
su rro u n d in g s
YQLCANtSM
•H BURMESE TERTIARY
VOLCANIC SITE
VOLCANIC SEAMOUNT
ALCOCK AND SEWELL
SEAMOUNTS A
SOLFATARA OR FUMAROLE
FIELD
VOLCANO IN SOLFATARA
OR FUMAROLE STAGE
VOLCANO ACTIVE DURING
HISTORIC TIME
SEISMICITY
s h a l l o w ; M<7
s h a l l o w ; m>7
i n t e r m e d i a t e ; M<7
O i n t e r m e d i a t e ; m>7
DEEP, M<7
SOURCES
BEMMELEN, 1949
CHHIBBER, 1934
I
GUTENBERG AND RICHTER,
1949
STOVER, 1966
5 0 0 Km
V. BORNEO '
« i
-IO#S
1 10* a
127
t h i r d zone, a lth o u g h sam pled a t o n ly one l o c a l i t y , may he
presumed to in c lu d e th e peaked se a h ig h s w hich oocur along
th e Malay s h e I f - b r e a k .
F ive Bampies o f subm arine b a s a lt have been re c o v e r
ed from th e Andaman B asin (F ig . 26; T able 7)* PIONEER
Dredge 14, a t th e n o rth e rn end o f A lcook Seam ount, y ie ld e d
la r g e ta b u la r s la b s o f m a ssiv e , u n a lte r e d ln te r g r a n u la r
a u g lte b a s a l t s . F a r th e r s o u th , PIONEER Core 19 p e n e tra te d f
i 238 cm o f u n c o n s o lid a te d sed im en t b e fo re e n c o u n te rin g |
s o lid ro c k . C hips o f f r e s h ly f r a c tu r e d b a s a lt were r e
covered from th e o o r e - c u t t e r . A pproxim ately 100 km so u th
o f PIONEER Core 19 i s th e s i t e sam pled by th e INVESTIGATOR.
Wiseman and S ew ell (1937) found th e rec o v e re d ro ck chem i
c a lly s im ila r to o c e a n ic b a s a l t s from th e C a rlsb e rg
R id g e. U n fo rtu n a te ly , no p e tro g ra p h ic d e s c r ip tio n o f t h is
ro ck e x i s ts in th e l i t e r a t u r e , alth o u g h S ew ell (1925)
r e f e r r e d to i t as " o liv in e b a s a lt" and s ta te d th a t th e
b a B alt i s s im ila r to th e b a s a l t s o f B arren I s la n d .
Samples o f a l t e r e d v o lc a n ic ro ck were dredged from
a seam ount ap p ro x im a te ly 60 km n o rth w e st o f P ulao Weh
(PIONEER Dredge 3)* One sm a ll p leo e o f rook proved to be
an a lte r e d h y p e rs th e n e -a u g ite b a s a l t w ith a d e v l t r i f i e d
groundm ass. A nother ch ip i s i d e n t i f i e d t e n t a t i v e ly as a
h y a lo p h itic h o rn b le n d e -h y p e rsth e n e b a s a l t . T his rook is
h e a v ily v ein ed w ith o a l c i t e r i c h in f o s s i l fra g m e n ts, and
th e e n t i r e rook may be com prised o f fra g m e n ta l e je o ta -
128
m enta cem ented to g e th e r In th e m arine en v iro n m en t. A
sh a rp p in n a c le a t th e Malay s h e lf r b r e a k a t 10° N l a t i t u d e
y ie ld e d f r e s h ly f r a c tu r e d , u n a lte re d v e s i c u l a r b a s a l t w ith
a su g g e stio n o f pahoehoe s u rfa c e (PIONEER Dredge 1 1 ).
L lth o lo g ic Z o n atlo n
A lthough th e Andaman B a s in 's llth o lo g y has n o t been
sam pled I n te n s iv e ly , th e few ro c k s s tu d ie d , supplem ented
by th e geology o f th e su rro u n d in g la n d s d e s c rib e d In th e
l i t e r a t u r e and by p h y sio g rap h ic c lu e s , p ro v id e th e b a s is
f o r d iv id in g th e Andaman B asin In to th r e e main l l t h o l o g i c
u n i t s . The f i r s t o f th e s e I s th e n o n -v o lc a n lc Andaman-
N ico b ar R idge, oomposed o f a L ate C reta ce o u s (? ) s e r p e n l-
n l t e - r a d i o l a r l a n c h e r t- o p h io lite co re o v e r la in by g ra y -
wackes and sh a le s o f Eocene to M iocene a g e . Second o f th e
l l t h o l o g i c u n its I s the v o lc a n ic f l o o r o f th e Andaman
B a sin , la r g e ly b la n k e te d by u n c o n s o lid a te d se d im e n ts.
The t h i r d u n it Is th e Malay C o n tin e n ta l m a rg in , composed
o f P a le o z o ic and M esozoic sed im en tary ro c k s and M esozoic
v o lc a n lc s o v e rla in by a th in v e n e e r o f T e r tia r y se d im e n ts.
REGIONAL GEOPHYSICAL TRENDS
Volcanl8m
The v o lo a n io phenomena in Burma, th e Andaman B asin
and In d o n e sia (F ig . 26) have been c o l l e c t i v e l y r e f e r r e d
to as a v o lc a n ic l i n e (van Bemmelen, 1949)> a b e l t o f
v o lc a n ic ro ck s and vo lcan o es (Umbgrove, 1 9 5 0 ), and an
in n e r igneous a rc ( P e te r , Weeks and B u m s, 1 9 6 6 ). These
term s a re v a lid f o r In d o n e sia and, in h ig h ly g e n e ra liz e d
a s p e c t, to th e re g io n as a w hole. However, d e ta ile d
ex am in atio n o f th e Andaman B asin and Burmese C e n tra l B e lt
n e g a te s th e co n tin u o u s l i n e a r i t y im p lied by th e term s
l i n e , b e l t and a r c . The n o n - lin e a r n a tu re o f v olcanism
in th e Andaman B asin and Burma i s n o t a sim p le sem an tic
d i s t i n c t i o n , as th e assum ption o f l i n e a r i t y h as f a r -
re a c h in g te c to n ic im p lic a tio n s . F o r tu n a te ly , r e c e n t
b ath y m etry and d red g in g w ith in th e Andaman B a sin have
p ro v id ed th e means f o r a more a c c u ra te u n d e rs ta n d in g o f
th e a r e a 's v o lc an ism . For exam ple, in th e ab sen ce o f
good p h y sio g ra p h ic d e p ic tio n o f th e Andaman B a s in , many
w orkers presum ed t h a t th e s u b a e r ia l e x p re s s io n o f
I n v is ib le Bank a t F la t Rock i s an "o b v io u s" so u th e rn ex
te n s io n o f a v o lc a n ic li n e th ro u g h Narcondam and B arren
i s la n d s . B athym etry h a s , how ever, re v e a le d a p e r s i s t e n t
tro u g h betw een B arren Is la n d and I n v i s i b l e B ank, and
130
dred g in g has e s ta b li s h e d th e n o n -v o lc a n lc n a tu re o f
I n v i s i b l e Bank.
No volcanism i s o c c u rrin g in Burma. As d is c u s s e d
p r e v io u s ly , Cenozoic volcaniBm in Burma o cc u rred alo n g
two tre n d s (F ig . 8 ) . The only p r e s e n tly a c tiv e volcano
in th e Andaman B asin i s B arren I s la n d , which to g e th e r
w ith Narcondam Is la n d may be a s o u th e rly c o n tin u a tio n o f
th e w esternm ost Burmese v o lo a n ic tr e n d . The tre n d may be
co n tin u ed so u th by th e seamounts a s s o c ia te d w ith th e
N icobar R i f t .
Andaman B asin e q u iv a le n ts o f th e ea ste rn m o st
Burmese v o lc a n ic tre n d may In c lu d e th e s h e lf - b r e a k p in
n a c le dredged by th e PIONEER. The seamount j u s t n o r th o f
Sum atra may belong to th e s h e lf - b r e a k tre n d o r to th e
v o lc a n ic B a risa n Range o f Sum atra. Alcock and S ew ell
seam ounts o ccu r between th e Narcondam-Barren and s h e l f -
b re a k tr e n d s , and i t i s n o t Im probable th a t th e e n t i r e
b a s in f l o o r between th e Andaman-Nicobar Ridge and con
t i n e n t a l slo p e 1b b a s a l t b en eath the sedim ent v e n e e r.
Van Bemmelen (1949) l i s t s 46 v olcanoes which have
been a c tiv e d u rin g h i s t o r i c tim es in Sum atra, Java and
th e L e s s e r Sunda I s la n d s , an a d d itio n a l 33 v olcanoes
which a r e in fum arole o r s o l f a t a r l c s ta g e s , and 34 s o l f a -
t a r a and fum arole f i e l d s . Of theB e, th e 10 a c tiv e
v o lc a n o e s, 11 s o l f a t a r a and fum arole voloanoes and 10
s o l f a t a r a and fum arole f i e l d s on Sum atra are p lo tt e d in
131
F igure 26. P ro x im ity o f s e v e r a l o f th e f e a tu r e s caused
some lo s s o f d e t a i l . The l i n e a r i t y o f t h i s v o lc a n ic
tre n d i s s t r i k i n g . Along Sum atra th e band i s r a r e l y
w ider th a n 30 km. The B a risa n Range o f Sum atra, along
whloh t h i s l i n e o f modern v o lc an o es l i e s , has ex p erien ced
some T e r tia r y v o lc an ism s in c e th e end o f th e Paleogene
(O llg o c e n e ).
S e is m ic ity
G utenberg and R ic h te r (1949) s t a t e d th a t th e
seism ic d a ta in th e Burmese a r e a a re i n s u f f i c i e n t to r e
so lv e th e q u e s tio n of co n n e ctio n between th e A lpine and
c irc u m -P a c ific se ism ic b e l t s . Most reOorded Burmese
ea rth q u ak es have o c c u rre d a t sh allo w d epths (F ig . 2 6 ).
The Arakan Yoma ap p e a rs to be a s e is m lc , a lth o u g h i t
merges to th e n o r th in to th e Himalayan a r c which i s th e
most a c tiv e s e is m ic a r c in A sia .
The C e n tr a l B e lt o f Burma e x p e rie n c e s shallow
s e is m ic ity a t i t s n o r th e rn end which G utenberg and R ic h te r
(1949) b e lie v e d to be a s s o c ia te d w ith th e Cenozoic
volcanism o f th e Popa (w e ste rn ) tr e n d . Most Burmese
ea rth q u ak es o cc u r to th e e a s t o f t h i s tre n d and ap p ear
s it u a te d a t a d i f f u s e tr e n d a lo n g th e Shan S carp .
S eism ic e p i c e n t e r s so u th o f Burma a re c o n c e n tra te d
p rim a rily alo n g th e Andaman-Nioobar R idge, w ith a tendency
to f a l l e a i t o f th e r id g e in a n o r th - s o u th - tre n d in g zone
132
which p a r a l l e l s th e rid g e and N icobar R if t V a lle y . Three
quakes a t in te rm e d ia te depth have been reco rd ed ; these
f a l l in th e Naroondam-Barren tre n d o f volcanoes and se a
mounts and e a s t o f i t .
South of th e Andaman B asin , se ism ic e p ic e n te rs o f
shallow and in te rm e d ia te focus occur in a broad band im
m ediately southw est o f Sum atra, over th e Hentawai festo o n
o f is la n d s and the Hentawai Trough. No fo c i a t depths
g r e a te r than 200 km have been recorded from the Burma,
Andaman Sea and Sumatra r e g io n . The w esternm ost deep fo c i
quakes in th e Indonesian re g io n occu rred a t 110° E lo n g i
tu d e , n o rth o f Jav a.
G ravity
The c l a s s i c a l study o f the e a r t h 's g r a v ity f i e l d
in the Indonesian A rchipelago by Venlng Heinesz (1934)
re c e n tly has been expanded n o rth in to th e Andaman Basin by
th e PIONEER ex p e d itio n (P e te r and o th e r s , 1966). I t i s
u n fo rtu n a te th a t th e PIONEER survey stopped s h o rt a t 6°
N l a t i t u d e , and th a t a gap approxim ately 250 km wide
e x is ts between c h a rts o f th e PIONEER and Vening Meinesz
coverage. G ravity anomaly tre n d s a p p a re n tly s h i f t marked
ly in tre n d somewhere w ith in the h ia tu s o f ooverage, from
SE-NW along Sumatra to S-N in th e Andaman B asin (F ig . 27)*
A d e ta ile d survey o f th e u n stu d ied a re a would c o n trib u te
muoh toward the u n d ersta n d in g o f the r e la tio n s h i p between
Figure 27. G rav ity an om alies in the Andaman
Basin and su rro u n d in g s.
134
85*E
25*N-
2 0 -
15-
10-
I0*S
105 E
GRAVITY ANOMALIES
IN THE
ANDAMAN BASIN
AND SURROUNDINGS
BURMA
Isostatic anomalies
from Evans and
Crompton, 1946
POSITIVE
ANOMALY
NEGATIVE
ANOMALY
ANDAMAN BASIN
Free-air anomalies
from Peter and
others, 1966 0* .
/
INDONESIA
Free-air anomalies
from Woollard and Strange
1962 (modified
of Vening Meinesz
135
th e Andaman-Kicobar Ridge and Andaman B asin on th e one
hand, and th e Nlas-M entawal is la n d fe s to o n and Sumatra on
th e o th e r . U nderstanding o f th e topography o f the Andaman
Basin has improved sin c e th e p u b lic a tio n o f P e te r , Weeks
and B urns, and t h e i r i n t e r p r e t a t i o n o f th e g r a v ity p a tte r n
in th e b a s in may change as a r e s u l t .
Only th e f r e e - a i r anom alies in the Andaman B asin
have been p u b lish ed in c h a rt form ( P e te r and o th e r s , 1966).
As in In d o n esia (W oollard and S tra n g e , 1962), a stro n g ,
c o r r e la tio n i s e v id e n t between f r e e - a i r g r a v ity anom alies
and topography. West o f th e An daman-Nico b a r Ridge i s a
zone o f g e n e ra lly weak, p o s itiv e an o m alies, which may
c o r r e la te w ith th e zone o f p o s itiv e anom alies southw est
o f Sumatra and which i s s it u a te d over so u th e rn Ja v a . A
b e l t o f n e g a tiv e anom alies c o in c id e s w ith th e N ioobar I s
la n d s . I t d e v ia te s from th e is la n d chain by c o n tin u in g
i t s n o r th e r ly tre n d to p a r a l l e l th e Andaman Is la n d s be
tween t h e i r e a s t c o a s ts and I n v is ib le Bank.
The n o rth e rn end o f th e N icobar R i f t V alley co
in c id e s w ith a n o r th -s o u th -o rle n te d n e g a tiv e f r e e - a i r
anomaly. B ast o f T illan ch o n g Is la n d th e r i f t v a lle y i s
i n s u f f i c i e n t l y covered by g r a v ity d a ta to confirm g e n e ra l
c o r r e la tio n between topography and n e g a tiv e anomaly along
th e le n g th o f the v a l le y .
Most f r e e - a i r g r a v ity anom alies w ith in th e b a sin
d is p la y a n o r th - n o r th e a s t tr e n d . Among th e s e a re th e
136
lo c a liz e d p o s itiv e anom alies which probably in d ic a te
v o lc a n ic rock composing Narcondam I s la n d , the B arren I s
la n d complex o f topo g rap h ic h ig h s , o th e r seamounts in th e
Naroondam-Barren tre n d , and Alcock and Sew ell seam ounts.
A p e r s i s t e n t p o s itiv e anomaly tre n d s n o r th - n o r th e a s t along
th e Malay s h e lf- b r e a k , as d e te c te d by f iv e PIONEER
tr a v e r s e s a c ro s s th e s h e l f . This g ra v ity f e a tu r e may be
a s s o c ia te d w ith c r y s t a l l i n e rooks suoh as those which
c h a r a c te r iz e s h e lf- b r e a k s in o th e r a re a s (Drake and
o th e r s , 1959; P e te r and o th e r s , 1966). The v o lc a n ic p in
n a c le on the s h e lf- b r e a k a t 10° N l a t i t u d e from whloh
f r e s h ly f r a c tu r e d v e s ic u la r b a s a lt was dredged s tre n g th e n s
t h i s assum ption.
The Malay c o n tin e n ta l m argin, Malacca S t r a i t and
Sumatra a l l show p o s itiv e f r e e - a i r anom alies, w ith th e
e x c e p tio n o f th e N orth Sumatra S h e lf B a sin , which i s ex
p re sse d as an e lo n g a te , n o r th - n o r th e a s t- tr e n d in g a r e a o f
no o r s l i g h t l y n e g a tiv e anomaly. This p a tte r n r e f l e c t s
e i t h e r dow nfaulting o f th e blook u n d e rly in g th e s h e l f -
b a s in (P e te r and o th e r s , 1966) o r downbowlng o f the b a s in
by s y n c lin a l f o ld in g .
An e s p e c ia lly w ell-m arked, l o c a l , h ig h ly p o s itiv e
f r e e - a i r anomaly marks I n v is ib le Bank. The e lo n g a te
r id g e which s e p a ra te s two branches o f the N lcobar R i f t
V alley e a s t o f Car N loobar shows a s l i g h t p o s itiv e
anomaly and may be l i t h o l o g i c a l l y and g e n e tic a lly r e l a t e d
137
to I n v is ib le Bank.
P e te r and o th e rs (1966) c a lc u la te d a c r u s t a l
s e c tio n f o r th e Andaman Basin along an e a s t-w e s t tr a v e r s e
which c ro sse s th e Andaman-Nlcobar Ridge south o f G reat
N lco b a r. They assumed a d e n sity o f 2.84 f o r Andaman B asin
c r u s t , a d e n s ity o f 3*3 fo r the m antle, and a c r u s ta l
th ic k n e s s o f 30 km f o r th e Malay s h e lf . On th e b a s is o f
t h i s model, P e te r and o th e rs re p o rte d th a t an assumed
c r u s t a l th ic k n e ss o f 40 km fo r th e Andaman-Nlcobar Ridge
provided the b e s t f i t between c a lc u la te d and observed
Bouger ano m alies, and th a t th e c r u s t i s th in n e s t under the
C e n tra l Andaman Trough, where i t measuree only 15 km.
Because t h i s model was co n stru c te d s o le ly from g ra v ity
d a t a , each o f th e employed assum ptions a re open to
q u e s tio n . For example, P e te r and o th e rs (1966) them selves
re p o rte d th a t Bouger anom alies over I n v is ib le Bank i n d i
c a te th a t i t may be comprised o f m a te ria l o f h ig h e r
d e n s i t i e s than the assumed 2.84 c r u s ta l d e n s ity . This
h lg h -d e n s ity ro ck may w e ll be p e r ld o ti te such as th a t
w hich forms th e e a s te rn oores o f th e Andaman and N lcobar
i s l a n d s . Lower Miocene rocks were dredged from I n v is ib le
Bank, and volcanism w ith in the Andaman Basin and th e
C e n tra l B e lt o f Burma was probably n o t im p o rtan t u n t i l th e
P lio c e n e . The Andaman-Nioobar Ridge appears to c o n s is t
e n t i r e l y o f T e r tia r y sedim entary rooks ly in g on se rp e n -
t l n i z e d p e r l d o t i t e basement; th u s i t i s q u ite p o ssib le
t h a t no "o c ean ic c r u s t" o f 2.84 d e n s ity e x i s ts under the
r i d g e . In any o a se , c a lc u la tio n s o f c r u s ta l s tr u c t u r e are
o f l i t t l e v alu e w ith o u t c o rro b o ra tiv e seism ic in fo rm atio n
and w ith o u t exam ination o f the l i t h o l o g l c inform ation
a v a ila b le in th e l i t e r a t u r e .
The e x te n sio n o f g r a v ity tre n d s n o rth in to Burma
i s s t i l l u n c le a r . Evans and Orompton (1946) have
d e lin e a te d a p o s itiv e i s o s t a t l c anomaly b e l t which co
in c id e s w ith th e Mt. Popa tre n d o f Cenozoic v o lc a n ic s and
which a p p e a rs to be a s im p lif ie d n o r th e r ly ex ten sio n o f
th e zone o f lo c a l p o s itiv e f r e e - a i r anom alies which mark
th e v o lc a n ic h ig h s in th e Andaman B asin . The n o rth e rn
h a l f o f th e Arakan Yoma c o in c id e s w ith a band o f low
Bouger anom alies and may c o r r e la te w ith th e n e g a tiv e
Bouger anomaly which has been found to co in cid e w ith th e
Andaman-Nicobar Ridge so u th o f G reat N lcobar (P e te r and
o th e r s , 1 9 6 6 ).
Magnetism
M agnetic anom alies were d e te c te d and d e lin e a te d
w ith in th e Andaman B asin from d a ta o f the PIONEER (P e te r
and o th e r s , 1 9 6 6 ). The g ro ss p ic tu r e p rese n ted by f r e e -
a i r g r a v ity anom alies i s d u p lic a te d by m agnetlo anom alies
(Fig* 2 8 ) . W esternm ost o f th e se i s a tre n d whioh p asses
through Naroondam and B arren is la n d s and th e seahighs
a s s o c ia te d w ith th e N loobar R i f t V alley . Sew ell and
F ig u re 28. M agnetic an o m alies and h e a t-flo w
d a ta In th e Andaman B a sin .
9 5
15*
NARCONDAM I.
1.94
0.9i
2.38
5 . 2 7 ®
• 0-
MAGNETIC ANOMALY
TREND
^ HEAT PROBE STATION
^ /iC al/C m ^Scc
MODIFIED FROM PETER
AND OTHERS. 1966
1 4 1
A lcock seam ounts a re marked by m agnetic an o m alies which
tre n d n o r t h - n o r t b e a s t , and th e f r e e - a i r g r a v ity anomaly
a lo n g th e s h e lf - b r e a k i s d u p lic a te d by an e q u a lly
p e r s i s t e n t m agnetic anom aly. The e a s te r n slo p e o f th e
Sum atra S h e lf B asin i s marked by a m agnetic anomaly which
p a r a l l e l s th e s h e lf - b r e a k anom aly, tr e n d in g n o r th -
n o r th e a s t from n o r th e a s te r n Sum atra to Go Phuket I s l a n d .
H eat Flow
A program o f h e a t- flo w m easurem ents conduoted in
th e Andaman B asin by th e PIONEER y ie ld e d v a lu e s a t f o u r
s t a t i o n s e a s t o f th e Andaman Is la n d s (B u rn s, 1 9 6 4 ). The
A
h ig h e s t v a lu e o f 5*27 m ic ro c al/cm sec was o bserved in th e
f l o o r o f th e N icobar R i f t V alley a t a d e p th o f 4206 m
( F ig . 2 8 ). Two o th e r h e a t- f lo w s t a t i o n s in th e N lco b ar
A
R i f t tr e n d y ie ld e d v a lu e s o f 1 .9 4 and 2 .3 8 m ic ro c a l/c m se c
(B u lla rd , 1963)* A f o u r th m easurem ent, made in th e
s e d im e n t - f ill e d v a lle y w est o f th e n o r th e r n t i p o f
p
I n v i s i b l e Bank was o nly 0 .9 0 m ic ro c al/cm s e c . Burns
(1964) c o n je c tu re d t h a t th e h ig h h e a t flo w in th e N ico b ar
R i f t V a lle y may in d ic a t e a la c k o f se d im e n ta ry f i l l .
Taken to g e th e r , a l l fo u r v a lu e s a r e com parable to h e a t -
flow m easured in th e g u lf s o f C a lif o r n i a and Aden. The
abnorm ally h ig h h e a t flow in th e s e g u lf s i s I n te r p r e t e d
a s i n d i c a t i v e o f m antle co n v en tio n c u r r e n ts b e lie v e d to
be r e s p o n s ib le f o r the te n s io n a l fo rc e s whioh formed th e
142
g u lfs (Von H erzen , 1963)*
A N D A M A N BASIN STR U C TU RE
C eneral Statem ent
Most phyeiographic f e a tu re s in the Andaman Basin
a re s u rfa c e e x p re ssio n s o f un d erly in g s tr u c t u r e , r e f l e c t
ing the s tro n g tecto n ism experienced by the a re a during
the T e r tia r y . Topographic co n fig u ra tio n w ith in th e b a sin
i s th e m ajor b a s is f o r th e d e lin e a tio n of s t r u c t u r a l
f e a tu r e s in Figure 29. Data on Andaman B asin llth o lo g y ,
s u b a e r ia l geology, v o lc a n ic lty and geophysics augment
p h y sio g rap h ic ev id en ce. Five e le c tro s o n lc su b -su rfa ce
p r o f i l e s o f e r r a t i c q u a lity (Weeks and o th e rs , 1965) con
firm some o f th e s tr u c tu r e s and re v e a l o th e rs .
Tectonism i s expressed in two major c o n tra s tin g
groups o f s tr u c t u r e s in the Andaman B asin . S tru c tu re s o f
th e f i r s t group in clu d e fo ld in g and re v e rse f a u ltin g
w hich have deformed and u p lif te d the Andaman-Nicobar
Ridge and which are in te r p r e te d as oom presslonal in
o r i g i n . The seoond group o f s tr u c tu r e s found w ith in th e
b a s in p ro p er and along th e Andaman-Nicobar Ridge and th e
Malay C o n tin e n ta l Margin are in te rp r e te d as e i t h e r
te n s lo n a l in o r ig in o r as r e s u ltin g from te n sio n coupled
w ith s t r i k e - s l i p movements.
F ig u re 29* S t r u c t u r a l tre n d s in the
Andaman B asin re g io n
%
I05*E
1 _
STRUCTURAL TRENOS
IN THE
ANDAM AN BASIN REGION
OtUOU( W M U TO * M O K C T IM
wrmmiurt k a l c ( « « i
sources:
BURMA
CHHIBBER, 1934
MALAY PENINSULA
ALEXANDER, I9B2
BURTON. 1963
SUMATRA
BEMMELEN. 1949
DURHAM. 1940
LEGENO
SyncliM
Antic lino
Fault. pratumably with normal
or combined normal and
(Irlko-tlio OiaplacamonL at
Monad from morphology
and/or rub-bottom profinr
Dathod whom irand it
infarrad. Crouhotchdd
araat ora tubmorina rcorpt
CARY Catdral Andaman Rift Valay
NR.Y Nicobar R ift V O U a y
A tit of tlongott uamount
x Volcanic inland
A | lln m iitd f
n nwvuiiuwii
B Barron
w
i A par of imallor volcanic
(•amount
146
Com pressional F eatu re s
N o rth -so u th a lig n e d fo ld s and re v e rs e f a u l t s o f
Post-M iddle O ligocene age tre n d along the Andaman-Nioobar
R idge. The re v e rs e f a u l t s and fo ld a x ia l p la n es a re p re
dom inantly In c lin e d to th e e a s t (Karunakaran and o th e rs ,
1964). These a u th o rs a ls o noted th a t th e d efo rm atio n al
fo rc e s are thought to have o ccurred d uring l a t e Miocene
time as M io-Pllocene s t r a t a on th e Is la n d s a re g e n e ra lly
f l a t l y i n g and u n d istu rb e d .
Submerged fla n k s o f th e Andaman-Nicobar Ridge do
n o t show d e t a i l s of th e fo ld in g and f a u l t i n g . Suggestions
o f b u rled s tr u c tu r e s are seen in su b su rface p r o f i l e s . One
o f th ese p r o f il e s (F ig . 30) lo c a te d w est and so u th o f the
Irrawaddy D elta and between th e Arakan Yoma and the
Andaman Is la n d s showed fo ld ed la y e r s which appear to be
tru n c a te d and o v e rla in by a v en eer o f f l a t - l y i n g sedim ent.
Folding i s asym m etric, w ith s te e p e r e a s te rn lim b s. There
f
i s a lso a su g g estio n o f f a u lti n g along th e p r o f i l e , a l
though th e q u a lity o f the re c o rd makes th e d eterm in atio n
of r e l a t i v e displacem ent im p o ssib le . Weeks and o th e rs
(1965) in te r p r e te d the f a u l t s as re v e rs e w ith westward
movement.
Figure 30. Sub-bottom p r o f i l e a c ro s s th e
n o rth ern Andaman-Nicobar R idge.
Vertical e»ea*ereiion aaeroiimately 2.9*.
MMitiat Iraai Week* ant other*. IM S, 9. 112.
Nele: tHit arefiie token slant tame track o> bathymetry el
Te»t-Fi(ure IT For location tea Tait-Fi(ora 2.
14$
T en sio n al o r T e n sio n -T ra n sc u rre n t F e a tu re s
G eneral Trends
In c o n tr a s t to th e com presslonal s tr u c t u r e s o f the
Andaman-Nicobar Ridge which a re n o t expressed p h y sio -
g r a p h ic a lly , s tr u c t u r e s in te r p r e t e d as e i t h e r te n s io n a l
in o r ig in o r r e s u l t i n g from combined te n s io n and s t r l k e -
s l i p movement dom inate th e p h y sio g rap h ic g ra in o f th e
Andaman B asin . The m ajor f e a tu r e s are th e c o n tin e n ta l
slo p e and a s s o c ia te d s c a r p s , and th e r i f t v a lle y system of
which the N icobar and C e n tra l Andaman r i f t v a lle y s a re the
p r in c ip a l b ran ch e s. These f e a tu r e s are n o t r e s t r i c t e d to
th e deeper p o rtio n s o f th e b a s in ; they d i s s e c t th e
Andaman-Nicobar Ridge and c o n tin e n ta l s h e l f as w e ll.
Most o f th e te n s lc u a l o r te n s lo n - tr a n s c u r r e n t
s tr u c t u r e s belong to two s e t s (F ig . 2 9 ). The dominant
s e t in c lu d e s th o se which tre n d in a N15°E-S15°W d i r e c t i o n ,
In c lu d in g th e so u th e rn th r e e - f o u r t h s o f th e c o n tin e n ta l
slo p e o f f th e Malay P e n in su la and th e s u b s id ia ry sc a rp
which d iv id e s th e deep t e r r a c e a t th e fo o t of the slo p e
from th e f l o o r o f the C e n tra l Andaman Trough. This deep
sc arp tre n d s northw ard in to M artaban Canyon; southw ard i t
c o in c id e s w ith th e l i n e a r e a s te rn base o f Sew ell seam ount.
L e sse r members o f the dom inant s e t in c lu d e the c o n s tr ic te d
n o rth e rn end o f th e C e n tr a l Andaman Trough, the s o u th e a s t
tro u g h -term in u s o f Narcondam-Barren B a sin , and th e narrow
150
tro u g h which i s th e r i f t - v a l l e y branch between the
Andaman Is la n d s and I n v i s i b l e Bank. The l i n e a r axes o f
S ew ell and Alcock seamounts a ls o may be members o f the
s e t .
The second s e t o f tr a n s c u r r e n t a n d /o r d is t e n s io n a l
f a u l t s tr e n d s n o rth -s o u th o r a few degrees weBt o f n o r th .
H ost prom inent o f th e s e lin e a t i o n s i s the N icobar R i f t
V a lle y . O th er members In clu d e th e 200-km long n o rth e rn
end o f th e C o n tin e n ta l s lo p e , th e zone o f m inor s te p -
f a u l t s which form th e in n e r Malay s h e lf - b r e a k , and the
c e n t r a l segment and so u th e rn end o f th e C e n tra l Andaman
R i f t V a lle y . The Narcondam-Barren zone o f v o lc a n ic i s
la n d s and seam ounts may be a n o th e r e x p re ssio n o f t h i s
f a u l t - s e t .
Only one, o r p o s s ib ly two, prom inent l i n e a r
s t r u c t u r a l elem ents do n o t conform to e i t h e r f a u l t - s e t .
The 90-km lo n g segment o f th e C e n tra l Andaman R i f t V alley
which b i s e c t s th e b ro a d e s t expanse o f f l a t C e n tra l Andaman
Trough f lo o r i s a lig n e d N70°W-S70°E. A nother f e a tu r e
which may be te n s io n a l in o r ig in and i s n o t a lig n e d w ith
e i t h e r f a u l t - s e t i s th e deep b reach in Ten Degree C hannel.
The y o u n g est rockB o f th e Andaman Is la n d s a re M iddle Mio
cene; P lio c e n e exposures a re common on th e N lc o b a rs. A
f a u l t t r a n s e c ti n g th e Andaman-Nicobar Ridge in Ten Degree
Channel w ith a norm al component r a i s i n g th e AndamanB
r e l a t i v e to th e N lcobars would e x p la in th e s t r a t i g r a p h l c
151
d if f e r e n c e . Ten Degree Channel i s a ls o th e focus o f a
s h i f t in the r i d g e 's alig n m e n t, from n o r th - n o r th e a s t along
th e Andamans to n o rth -n o rth w e s t along th e N lco b ars.
F a u lts o f the C o n tin e n ta l Margin
The physiography o f th e f e a tu r e s which sh a rp ly
d iv id e th e c o n tin e n ta l margin in to d is c r e te b ath y m etric
zones has a lre a d y been d is c u s s e d . The p e r s i s t e n t le n g th
o f th e c o n tin e n ta l s lo p e , i t s upward co n v ex ity which sto p s
a b ru p tly a t th e 2200-m deep t e r r a c e , th e occurrence o f a t
l e a s t one v o lc a n ic p in n a c le a t th e s h e lf - b r e a k , and the
o ccurrence o f a m agnetic anomaly along th e s h e lf-b re a k a l l
In d ic a te f a u l t i n g alo n g which th e deep b a sin f lo o r has
been d is p la c e d downward r e l a t i v e to the c o n tin e n ta l m argin.
The sc a rp which te rm in a te s th e 2200-m te r r a c e and which
extends n o rth in to M artaban Oanyon and tre n d s south in to
th e a x is of S ew ell Seamount, downdropplng the b asin f lo o r
500 to 600 m, must be s u b s id ia r y to th e main f a u ltin g
along th e c o n tin e n ta l s lo p e . Sew ell Seamount appears to
mark th e so u th e rn c o n tin u a tio n o f t h i s f a u l t . The
s l i g h t l y concave f l o o r between th e c o n tin e n ta l slope and
Sew ell Seamount sta n d s 500 m h ig h e r than th e b a sin f lo o r
w est o f the seam ount. A sub-bottom p r o f i l e a c ro s s the
seamount (F ig . 31) shows a f r a c tu r e bounding th e w estern
slo p e o f th e seam ount, along which th e u n co n so lid ated
la y e r has been d is p la c e d downward. F a r th e r w est along th e
Figure 31* Sub-bottom p r o f ile a c ro ss Sew ell
Seamount.
WNW
8 ‘ 53.8‘N
9 4 ‘ 17.3'E
I
0
I_____
1 0
- I —
e #43.e'N
9 4 * 3 2 .8'E
ESE
20
_L_
I
3 0 Km
- J
0.8
/W E ST E R N
f SEWELL
SEAMOUNT
2.4
3.2-*Km
Vertical exaggeration approximately 4X
Modified from Weeks and others, 1965, p. 112.
154
same p r o f i l e an o th er f a u l t seen in su b su rfa c e so n ic p ro
f i l e s r e p e a ts the same r e l a t i v e sense o f d isp la c e m e n t.
Sew ell Seamount may be the product o f v olcanism along
f i s s u r e s o f th e c o n tin e n ta l slo p e f a u l t s . I f t h i s i n t e r
p r e ta tio n i s v a lid , a f is s u r e - e r u p tiv e o r ig in should hold
fo r Alcock Seamount as w e ll, inasmuch as th e s i z e s ,
tre n d s and c o n fig u ra tio n s o f th e two seamounts a re n e a rly
i d e n t i c a l . Abrupt te rm in a tio n s of th e c o n tin e n ta l margin
sc a rp s a g a in s t b asin f lo o r w ith no m o d ific a tio n s by
a b y s s a l fan s (F ig s . 14 and 15) in d ic a te t h a t th e l a t e s t
ep iso d es o f f a u ltin g occurred in very r e c e n t tim e s .
R i f ts
The d eep est fe a tu re s o f the Andaman B asin a r e th e
r i f t v a lle y s which a re b e s t ex p lain ed as e x p re s sio n s o f
te n s io n a l s t r e s s , o r combined s t r l k e - s l i p and te n s io n .
T eotonic o r ig in i s im p lic it in the r e l a t i o n s h i p s o f the
v a lle y s w ith t h e i r su rro u n d in g s. For exam ple, a t 10° N
l a t i t u d e th e N icobar R i f t V alley i s about 4200 m deep,
more than a k ilo m e te r deeper than th e a d ja c e n t C e n tra l
Andaman Trough. Even th e f lo o r of th e Bay o f Bengal i s
s e v e r a l hundred m eters sh o a le r a t th e same l a t i t u d e .
Thus, a la c k o f g ra d ie n t preclu d es an o r ig in by subm arine
e ro s io n . No te c to n ic o rig in w ith o u t a s i g n i f i c a n t com
ponent o f te n s io n oould produce the 17° sym m etric r i f t s
in th e m arine environm ent. T ensional s t r e s s a ls o e x p la in s
155
th e d i s t r i b u t i o n of v o lc a n ic Isla n d s and seamounts along
th e m ajor r i f t a re a and the c o n c e n tra tio n o f selsm io
e p ic e n te r s along th e zone (F ig . 2 6 ). F a u lts w ith normal
d isp lacem en ts are n o tic e a b le In sub-bottom p r o f il e s a c ro ss
th e N icobar R i f t V alley (F ig . 3 2 ).
At f i r s t glance th e n o rth e rn end of th e C e n tral
Andaman R i f t as expressed in P la te I m ight be in te r p r e te d
as a deep e ro s lo n a l channel carved by d e lta - s u p p lie d
t u r b i d i t y c u rre n ts en ro u te from the n o rth e rn end o f the
C e n tra l Andaman Trough to d e p o s ltlo n a l s i t e s in the deeper
p o rtio n s o f the r i f t system . S ev eral lin e s o f evidence
in d ic a te a te c to n ic o r ig in . C ro ss-se c tio n s o f th e v a lle y
in th is a re a a re V-shaped w ith hummocky w a lls ; one p r o f ile
shows a p in n acle w ith th e same r e l i e f as th e v a lle y occupy
ing the r i f t ' s a x i s . An e ro s lo n a l f e a tu re o f t h i s magni
tude would have to be carved by co n sid erab le q u a n t itie s o f
t u r b l d l t e m a te r ia l, y e t th e f lo o r s o f the deep v a lle y s
which would be expected to re c e iv e th is sedim ent show
l i t t l e ponding. In s p ite o f th e homogenous sedim ent and
extrem ely low g ra d ie n t o f the C e n tra l Andaman Trough f lo o r
th e r i f t v a lle y changes d ir e c tio n sh a rp ly s e v e ra l tim es
along i t s le n g th , c lo s e ly m ain tain in g b is e c tio n o f the
trough f l o o r in a manner which can h ard ly be c o in c id e n ta l.
Recent movement in the N icobar R i f t V alley i s
s tro n g ly suggested by the sharp in c is io n o f the C e n tral
Andaman R i f t in to the trough f l o o r . Even though th e trough
F igure 32. Sub-bottom p r o f i l e s a c ro ss th e
N lcobar R i f t V a lle y .
WNW , ESE
I2*00'N 11*92 N
93*30* E 93*34*E
I I
o 10 2 0 3 0 4 0 Km
■ -------------------------------I------------------------------- 1 - — - i
0.0
0.9
j ^ n o r t h 1 ^
K END OF
INVISIBLE BANI
1.6
NICOBAR RIFT VALLEY
2.4
Vertical exaggeration approximately 3.2X.
Modified from Weeks and others, 1965, p. 112.
l i e s in th e main avenue o f l a t e r a l sedim ent tr a n s p o r t,
v i r t u a l l y no sedim ent has accumulated In th e r i f t a x i s .
S p e c ta c u la r morphology o f th e N lcobar R i f t V alley
te n d s to obscure the Importance o f the C e n tra l Andaman
Trough and R i f t V alley . The l a r g e s t a re a o f deep b asin
f l o o r i s the C e n tra l Andaman Trough, which i s a ls o d i s
t i n c t l y deeper than the N lcobar R i f t in th e n o rth e rn h a l f
o f the Andaman B asin . I f the p re se n t c o n fig u ra tio n o f the
b a sin i s th e r e s u l t of d i l a t l o n a l s t r e s s e s , i t fo llo w s
t h a t the g r e a te s t lo c a l d i la tio n i s th a t which formed the
C e n tra l Andaman Trough, and the C e n tra l Andaman R i f t
V alley i s m erely the ex p ressio n of more r e c e n t movement.
Hidden F a u lts
E le c tro so n ic sub-bottom p r o f ile s re v e a le d s e v e ra l
f a u l t s in the b asin which a re not expressed to p o g ra p h ic a l
ly (Weeks and o th e rs , 1965)* Some o f th e se f a u l t s a f f e c t
th e C retaceous and Paleogene rocks o f the Andaman N icobar
Ridge and are hidden under Neogene sedim entary co v e r.
Normal f a u ltin g in the su rfa c e la y e rs o f sedim ent w ith in
th e b asin was fre q u e n tly observed.
An e x c e lle n t sub-bbttom in d ic a tio n o f f a u l t i n g was
cro ssed on Weh Bank, in 790 m o f w ater (F ig . 3 3 ). The
w estern sid e o f the bank was a c o u s tic a lly opaque and
probably re p re s e n ts c r y s ta lli n e o r metamorphlc basem ent.
Along th e top o f the bank a f a u l t te rm in a te s th e c r y s t a l -
F ig u re 33* Sub-bottom p r o f il e a c ro ss the
w estern p o rtio n o f Weh Bank.
160
sw
6 * 4 4 .2 'N
9 5 * 4 3 .5 ’E
I
0
I ____
NE
6*48.2'N
9S * 5 0 .8 'E
I
5
_ l _
1 0
_ l _
15 Km
0.8
1.6 J Km
■UftlED —
CHANNEL?
ACOUSTICALLY OPAQUE
BASEMENT OR
VOLCANIC ROCKS
V articol tx a g g tr o tio n ap p ro x im o ttly 7X .
Photograph of sub*bottom profit* co u rtesy of ESSA .
161
l i n e (?) basem ent, and ap proxim ately 1600 m o f s t r a t i f i e d
sedim ent appear to have been downthrown a g a in s t th e b a s e
ment ro o k s . An i r r e g u l a r r e f l e c t i n g h o riz o n which ap
p e a rs to be an a n c ie n t e r o s lo n a l s u r fa c e tr u n c a tin g both
basem ent and sedim ents i s ap p ro x im ately 100 m below th e
se d im en t-w ater i n t e r f a c e . A second su b -b o tto m p r o f i l i n g
tr a c e was run so u th o f t h i s f a u l t , on th e presum ption t h a t
th e f a u l t e i t h e r tre n d s along th e c o n tin e n ta l slo p e o r
ru n s in to Sum atra, and in the hope o f e s t a b l i s h i n g th e
f a u l t ' s e x a c t tr e n d . The f a u l t d id n o t ap p e ar in th e
second p r o f i l e , e i t h e r because th e p r o f i l e was uade a c ro s s
a th ic k e r Sum atra S h e lf Basin sedim ent s e c tio n w hich ex
p re s s e s f a u lti n g d i f f e r e n t l y , o r because th e sed im en ts
along th e s tr u c t u r e a re younger th an th e f a u l t . In any
c a s e , th e f a u l t on Weh Bank in d ic a te s t h a t th e Sum atra
S h e lf Basin was formed as a down thrown b lo ck o f th e Malay
S h e lf (P e te r and o th e r s , 1966). The b u rie d e r o s lo n a l
B urfaoe on Weh Bank i s a ls o h ig h ly s i g n i f i c a n t , in d ic a t in g
t h a t th e e n t ir e te r r a c e was cu t a t o r n e a r se a l e v e l and
h as s in c e been d is p la c e d downwards alo n g f a u l t s o f th e
in n e r s h e lf - b r e a k , w ith d isp lacem en ts in c r e a s in g southw ard
to a maximum a t Weh Bank.
A nother s e t o f hidden s tr u o t u r e s was d e te o te d by
th e sub-bottom p r o f i l e r in the sh a llo w Irraw addy D e lta
sedim ents (P ig . 3 4 ). A zone w ith in th e sed im en ts i s fo ld e d
and f a u lte d and marks th e ohange in to p o g ra p h io e x p re s sio n
F ig u re 34. Sub-bottom p r o f i l e a c ro ss th e
Irraw addy D e lta s h e l f .
40 30 SO Km 20
0.0-1
0.8-
Verticol exaggeration opproximotely 3 X
Modified from Weeks and others, 1965, p. 112.
164
between th e w estern and e a s te r n h a lv e s o f th e d e l t a - s h e l f
s u r f a c e . This zone may mark th e e x te n s io n o f f a u l t i n g
alo n g th e e a s te rn base o f A lcock Seam ount.
R egional I m p lic a tio n s
The Andaman B a s in 's p r e s e n t c o n f ig u r a tio n a p p e ars
to have been molded by two m ajo r C enozolc e p iso d e s o f
te c to n ls m . The f i r s t e p is o d e , p ro b ab ly l a t e Miocene in
age, was c h a ra c te riz e d by e a s t- w e s t com pression which
caused f o ld in g and re v e rs e f a u l t i n g o f t h i c k Paleogene
sedim ents and C retaceous basem ent ro c k s o f th e Andaman-
N lcobar R idge. Subsequent te c to n ls m a p p a re n tly s u b je c te d
th e e n t i r e Andaman B a sin , in c lu d in g th e Andaman-Nicobar
Ridge and th e c o n tin e n ta l m a rg in , to d l l a t l o n a l s t r e s s e s
a lig n e d in a n o rth -n o rth w e s t d i r e c t i o n .
The l a t e Miocene p e rio d o f f o ld in g and r e v e rs e
f a u l t i n g a p p a re n tly a f f e c te d th e a r e a s n o r th and so u th o f
th e Andaman B asin as w e ll. P aleogene sed im en ts o f th e
Burmese T e r tia r y B asin were a ls o fo ld e d and f a u l t e d .
Umbgrove (1948) concluded t h a t southw ard th r u s t i n g o f
c o n tin e n ta l c r u s t in th e v i c i n i t y o f Ja v a 1b r e s p o n s ib le
f o r the fo rm atio n o f the Ja v a T ren ch . He b e lie v e d th a t
t h i s t h r u 8tin g began d u rin g post-M ld-M locene tim e , as
in d ic a te d by the i n t e r s e c t i o n o f Miocene fo ld s t r u c t u r e s
on Ja v a , Sumatra and th e Mentawai I s la n d s by th e p re s e n t
c o a s t l i n e s .
The post-M iocene te cto n lsm which oaused d i l a t i o n in
the Andaman B asin appears to have a f f e c te d the a re a s o u t
sid e the b a sin in a d i f f e r e n t manner. P lio ce n e fo ld in g
1b w e ll documented in Burma. Movements along th e Semangko
R i f t o f Sumatra during th i s tim e were form erly in te r p r e te d
as being g en e ra ted by te n sio n (van Bemmelen, 1949) but
Heiskanen and Vening Meinesz (1958) have s in c e documented
tr a n s c u r r e n t movement w ith d e x t r a l d isp la c e m e n t. The
re g io n a l h is t o r y appears to have been more u n if ie d d u rin g
the l a t e s t P lio c e n e , when volcanism became in Burma, the
Andaman Basin and Sum atra. Any re g io n a l te c to n ic s y n th e s is
s h a l l have to ex p lain th e se d if f e r e n c e s in post-M iocene
te c to n lsm .
R e la tio n s h ip s between th e f a u l t s o f th e Andaman
Basin and the 900 km long Malayan f a u l t s tu d ie d by Burton
(1965) a re n o t a p p a re n t. As d e lin e a te d by Burton the
f a u l t i s a r c u a te , w ith a n o rth w est tre n d in so u th ern
Malaya becoming n o rth -n o rth w e s te rly a t 9° N l a t i t u d e (F ig .
2 9 ). This tre n d i s not d u p lic a te d in the Andaman B a sin .
Burton re p o rte d th a t th e Malayan f a u l t i s tr a n s o u r r e n t,
w ith n e t s i n l s t r a l d isplacem ent o f 50 to 60 km accom plished
d u rin g J u r a s s lo tim e. This sense o f movement appears con
t r a r y to the r e g io n a l p a tte r n o f T e r tia r y s h e a rin g , and
the Malay f a u l t i s probably an e x p re ssio n o f te cto n lsm
which an ted ated th e form ation o f th e Andaman B a sin .
CLIMATE AND OCEANOGRAPHY
Regional C lim atic S e ttin g
In tro d u c tio n
The monsoonal clim ate o f S o u th east A sia and th e
n o rth e a s te rn Indian Ocean is the dominant c o n tro l over
both sedim entation and oceanography in th e Andaman B a sin .
The m oist so u th w esterly winds o f the summer monsoon cause
a pronounced wet season which c o n tra s ts s tro n g ly w ith the
dry w in te r c o n tro lle d by the dry winds o f th e n o r th e a s t
monsoon. G reat q u a n titie s of r a in (680 m m to 2200 mm) f a l l
over Burma during th e southw est monsoon, causing e ro sio n
and tr a n s p o rt o f huge volumes o f sedim ent which a re
d ep o site d u ltim a te ly in the Andaman B asin . The d i s t i n c t
s e a s o n a lity o f monsoonal p r e c ip ita tio n enhances th e
e ro siv e and tr a n s p o r tiv e powers o f the ru n o ff w a te rs .
In a d d itio n to c o n tro llin g the supply o f t e r r i
genous sedim ent to the b a sin , the monsoon winds govern
th e Andaman Sea c u rre n ts and thereby c o n tro l th e d i s t r i b u
ti o n , d e p o sitio n and reworking o f the sed im en t. Winds
change d ire o tlo n d ia m e tric a lly w ith the monsoons, causing
d i s t i n c t changes in su rfa ce c u rre n ts and lo c a l se a so n a l
u p sw ellln g . S tro n g ly c o n tra s tin g ru n o ff between th e two
monsoonal seasons product d r a s ti c se aso n a l changes in
p a tte r n s of s a l i n i t y , r e f le c te d c lo s e ly by changes in
167
s u rfa c e -w a te r d e n sity d i s t r i b u t i o n s .
Thus, a b r i e f d e s c r ip tio n o f th e monsoonal regim en
i s in o rd e r. Kendrew (1942), whose lu c id d e s c r ip tio n o f
th e re g io n a l clim ate i s follow ed c lo s e ly h e r e , employed a
convenient se aso n a l d iv is io n ac co rd in g to monsoon:
I . The season o f the n o r th e a s t monsoon.
a . January to F ebruary, cold w eath er se aso n .
b . March to m id-June, h o t w eath er se a so n .
I I . The season o f the southw est monsoon.
a . Mid-June to m id-Septem ber, season o f
g en e ra l r a in s .
b . Mid-September to December, season o f th e
r e t r e a t i n g monsoon.
January and F ebruary
During the cold w eather season a c o l d - a i r o e l l o f
high p re ssu re i s s it u a te d over c e n tr a l A s ia . A southw est
g ra d ie n t o f d e c re a sin g p re ssu re extends from thlB h ig h
a c ro s s In d ia and th e Bay of B engal. Winds blow from the
n o r th e a s t toward th e " e q u a to ria l" tro u g h o f low p re ssu re
( s itu a te d so u th o f th e E quator in th e In d ia n O cean).
Over th e Andaman Sea, winds a re 15 to 20 km/hr and a re
n o r th e a s te r ly ; over the Bay o f B engal wind v e l o c i t i e s a r e
between 18 and 30 km/hr and s e t to th e e a s t- n o r t h e a s t
(Anonymous, I9 6 0 ). R e la tiv e h u m id ity i s a t i t s low est f o r
th e e n t i r e y e a r, averag in g 71 p e rc e n t in th e Bay o f Bengal
j
168
( Jagannathan and Ram asastry, 1964); th e lands surrounding
th e bay r e f l e c t th is com parative dryness by r e c e iv in g
l e s s than 30 m m o f ra in during the two m onths.
March to Mid-June
As th e sun moves n o rth the hot w eather begins to
d is s ip a te the w in te r h ig h -p re ssu re c e l l over I n d ia and
atm ospherlo p ressu res become s li g h tly low er o v er the
lan d than over the bay which Is occupied by a s l i g h t l y
marked a n tic y c lo n e . By A p ril the lo w -p re ssu re c e l l over
th e land I s c le a r ly defined and emanates weak p re ssu re
g ra d ie n ts which stren g th en to near-maximum In May. Winds
begin to blow from the southw est a t th e head o f the Bay o f
B engal.
Mid-June to Mld-Sentember
Even as e a rly as May a d e f in ite lo w -p re ssu re c e n te r
has formed over n o rth ern In d ia . The s o u th e a s t tra d e s of
th e southern Indian Ocean cross the E quator, become
southw est winds by g eo stro p h ic d e f le c tio n , and move a c ro ss
th e bay in to I n d ia . The sudden a r r i v a l o f th e r a in s over
la n d , o fte n termed the "b u rst" of the southw est monsoon,
I s ex plained by Kendrew (1942, p. 125-126) as fo llo w s:
• • • th e n o rth trough of low p re ssu re which i s
th e g o al o f the so u th e a st tra d e , and whloh l i e s some
what so u th o f the lin e in January, s t i l l rem ains in
an a tte n u a te d form near the Equator d u rin g th e h o t
season o f In d ia , and here th e s o u th e a st tr a d e ends.
169
N orth o f i t th e re 1b a b e l t o f s l i g h t l y h ig h e r
p re s s u re , s e p a ra tin g i t from th e lo w -p re ssu re system
which i s being developed over In d ia by th e h e a t . . .
As th e h o t season wears on p re ssu re co n tin u es to f a l l
over sou th ern A sia, and much o f the a i r which has
expanded and r is e n makes i t s way in the h ig h e r a t
mosphere in to the e q u a to ria l low p re s s u re s , which
become f i l l e d up in June. There i s th en a continuous
and f a i r l y uniform g ra d ie n t from th e sub t r o p ic a l
h ig h -p re ssu re b e l t o f the south Indian Ocean, r i g h t
a c ro s s th e E quator, in to the low p re s s u re s o f n o rth
I n d ia . . • • The so u th e a st tr a d e s fin d no o b s ta c le
to sto p them a t the E quator, and sweep on over In d ia
as th e southw est monsoon . . .
R e la tiv e hum idity has reached a maximum average o f
approxim ately 65 p e rc e n t by June, and the m o istu re -la d e n
a i r pours northw ard, causing heavy p r e c ip ita tio n over
S o u th east A sia. Winds o f the summer monsoon blow
s te a d il y a t r a te s more than tw ice those o f th e n o rth e a s t
monsoon, averaging 30 km/hr over th e open bay. Normally
th e southw est monsoon co n tin u es u n t i l m id-Septem ber, by
which time f a l l i n g tem peratures cause an in c re a s e in
atm ospheric p ressu re over n o rth e rn In d ia , g ra d u a lly weaken
ing th e monsoon and causing i t to r e t r e a t southward as the
n o r th e a s t winds move in . In September th e lo w -p re ssu re
system over In d ia has begun to f i l l up.
Mld-Sentember to December
By O ctober the low -pressure system over In d ia i s
no lo n g e r in e x iste n c e and p re s s u re s a re uniform over
p e n in s u la r In d ia and th e Bay o f B engal. As th e southw est
monsoon r e t r e a t s i t recu rv es over the c e n te r o f the bay.
170
D uring t h i s season o f th e r e t r e a t i n g monsoon th e weak
b a ro m e tric g r a d ie n t, weak and v a r ia b le w inds, e x c e s s iv e
h e a t and m o istu re in th e a i r combine to produce c y c lo n e s ,
which a re most common in November*
P r e c i p i t a t i o n and Runoff
Burma
The o ro g ra p h ic In flu e n c e o f th e Arakan Yoma and
Shan P la te a u a re r e f l e c t e d in the annual 5000 m m o f r a in
which f a l l s on th e w estern Arakan Yoma slo p e and th e 3700
m m o f y e a rly p r e c i p i t a t i o n along th e c o a s t o f p e n in s u la r
Burma (T able 8 ) . The m iddle o f th e Burmese C e n tra l B e lt
i s in th e o ro g ra p h ic shadow o f th e Arakan Yoma and i s
d ep riv e d o f southw est monsoonal r a i n f a l l , r e c e iv in g only
750 to 1000 m m o f r a in annually* An e s tim a te d 1900 m m i s
p r e c i p i t a t e d an n u a lly over th e e n t i r e C e n tra l B e lt n o rth
o f th e Irraw addy D e lta (Stamp, 1940); ap p ro x im ately 90
p e rc e n t o f t h i s r a in oomes w ith th e southw est monsoon.
The so u th e rn end o f th e C e n tra l B e lt, occupied by th e
Irraw addy R iv e r, r e c e iv e s an average 2460 m m every y e a r ,
based upon summarized lo n g -term re c o rd s o f th e Burmese
M in is try o f A g ric u ltu re and F o re s ts (Anonymous, 1 9 5 2 ).
Most o f Burm a'8 d rainage p asses in to th e Andaman
Sea by th e Irraw addy R iv er system , whloh h as an a r e a o f
410,000 km2 (F ig . 35)* Gordon (1885) measured th e I r r a -
TABLE 8:
Area
Average Monthly P r e c ip it a ti o n a t S e le c te d S ta tio n s in Burma
(Anonymous, 1952)
Sample
S ta tio n
9
h
p.
« * <
©
© 3
£
H b O
3 3
• "3 « * )
A
©
C Q
■ P
O
O
> O
o ©
5 2 5 R
a
0 ^
.
0 p. a
© « 0
am S
H 0 1
© x a S B
• P
3 m
O
E H
C O w
West Arakan
Coast
N o rth e a st
Andaman Bas
Coast
Dry B e lt,
C e n tra l
Irraw addy
V alley
Base o f
Shan P la te a u
Irraw addy
D e lta
Akyab
Mergui
Mandalay
Toungoo
Rangoon
3 5 13 53 356 1191 1392 1148 574 277 140 20 5171 4938 95.
23 51 79 135 429 747 803 742 671 307 94 18 4097 3698 90.
2 2 5 28 150 140 84 117 145 119 41 10 843 754 89.
2 8 5 48 196 358 447 485 307 188 46 13 2106 1981 94.
5 5 8 41 305 457 544 506 389 175 71 10 2515 2375 94.
Figure 35 • D rainage b a sin o f th e Irraw addy
R iv e r.
98
28*
3 0 0 Km .
:
ANDALAY
RANGOON
174
waddy E l v e r 's d isc h a rg e a t th e d e l t a head o ver s e v e r a l
y e a rs and e s tim a te d an annual d is c h a rg e o f 400 b i l l i o n m **.
Stamp (1940) e s tim a te d t h a t o nly 58 p e rc e n t o f th e r a i n
f a l l o v er th e a re a n o r th o f G ordon's m easuring s t a t i o n i s
d isc h a rg e d , and t h a t 42 p e rc e n t o f th e r a in i s l o s t to
e v a p o ra tio n . I f t h i s e v a p o ra tio n lo s s i s e x tra p o la te d to
th e 118.5 b i l l i o n j s ? o f r a in which f a l l s over th e 48,200
2 3
km o f d e l t a s u r f a c e , ap p ro x im a tely 70 b i l l i o n m must be
added to G ordon's o r i g i n a l f i g u r e , making th e t o t a l annual
d isc h a rg e o f th e Irraw addy R iv e r a t i t s mouth 470 b i l l i o n
m^. In com parison the M is s is s ip p i R iv e r 's average annual
3
d isc h a rg e i s ap p ro x im ately 540 b i l l i o n m (L eopold, 1962).
The annual c y c le o f Irraw addy R iv e r d ra in a g e c lo s e ly
fo llo w s t h a t o f p r e c i p i t a t i o n , w ith peak d isc h a rg e la g g in g
behind peak r a i n f a l l by a month.
The Salween R iv e r, alo n g th e e a s te r n m argin o f th e
Burmese C e n tra l B e lt, i s much l e s s Im p o rtan t than th e
2
Irraw addy R iv e r. I t s 137,300 km catohm ent b a s in r e c e iv e s
l e s s r a i n f a l l , and th e r i v e r d is c h a rg e s only 47.3 b i l l i o n
3
m a n n u a lly (L eopold, 1962), ap p ro x im ately 10 p e rc e n t o f
th e Irraw addy R i v e r 's volume.
P e n in s u la r Burma a ls o r e c e iv e s la rg e amounts o f
o ro g ra p h ic so u th w est monsoonal r a i n , which i s e s tim a te d
from e x i s tin g r a i n f a l l maps (Anonymous, 1952) a t an
o
average o f 4300 m m p e r y e a r . T his 44,000 km a re a o f
s te e p s lo p e s and narrow c o a s ta l p la in s d r a in s along i t s
175
le n g th d i r e c t l y In to th e n o r th e a s te r n Andaman S ea. I t
may be assumed th a t a very la rg e percentage o f p e n in s u la r
1
Burma18 r a i n f a l l , e stim a te d h ere a t 200 b i l l i o n m p er
y e a r, reac h es th e Andaman Sea.
P e n in s u la r T hailand and M alaysia
P r e c ip it a ti o n and ru n o ff d a ta are sc a rc e f o r T hai
lan d and M alay sia. The p o rtio n o f th e p e n in su la which
d r a in 8 In to th e Andaman Sea between Burma and 4° N
2
l a t i t u d e has an a re a o f approxim ately 50,000 km , and
re c e iv e s an average annual r a i n f a l l o f 2500 mm, an ap
proxim ate volume o f 120 b i l l i o n m^. This a re a I s much
f l a t t e r than p e n in s u la r Burma. In any e v e n t, I f only 50
p ero en t o f th e annual r a i n f a l l re a c h e s th e s e a , the
volume I s s t i l l s i g n i f i c a n t .
Sumatra
N orthw estern Sumatra a ls o ex p erien ces a d e p riv a tio n
o f monsoonal r a i n f a l l because o f th e In te rv e n in g B arlsan
Range, and re c e iv e s th e low est r a i n f a l l f o r a l l o f Sumatra
o f between 1500 and 2000 m m p er y e a r (Van Bemmelen, 1949).
This p r e c i p i t a t i o n i s r e l a t i v e l y unim portant w ith in th e
Andaman B asin because o u rre n ts c a rry I t out In to th e Bay
o f Bengal through G reat P assage.
176
C u rren ts
In tro d u c tio n
Wind s t r e s s e s g en e ra ted by th e n o r th e a s t and so u th
w est monsoons are th e dominant fo rc e s which govern the
se a so n a l su rfa c e c u r r e n ts o f th e Andaman Sea. Monsoonal
changes are ex p erien ced a t sea approxim ately one month
b efo re t h e i r e f f e c t s a re f e l t on la n d .
Some m o d ific a tio n o f th e se a so n a l s u rfa c e flow
p a tte r n s Is caused by an I n flu x o f South China Sea w ater
through the M alacca S t r a i t ( P ig s . 36a, 37a, 38a, 3 9 a )•
The P a c if ic n o r th e a s t and s o u th e a s t tra d e s d riv e P a c if ic
w aters w estw ard, b u ild in g up th e su rfa c e along th e o c e a n 's
w estern m argin. Passage o f some o f t h i s w ater through
the Malacca S t r a i t In to th e In d ian Ocean I s m aintained
throughout th e y e a r. C u rren ts In th e c o n s tr ic tio n o f the
M alacca S t r a i t In any given season range between 50 and
100 cm /sec (1 and 2 k n o ts) (U. S . Navy H ydrographic O ffic e ,
I9 6 0 ). Minor m o d ific a tio n s o f th e c u rr e n t regime are
oaused by tem porary su rfa o e w a te r-d e n s lty g ra d ie n ts which
form In response to se a so n a l p r e c i p i t a t i o n and r u n o ff .
November to January
The n o r th e a s te r ly and e a s t e r l y winds whloh p re v a il
over th e Andaman Sea and Bay o f Bengal d uring th e w in te r
oause s o u th - s o u th e a s te r ly o u rre n ts to flow In th e Andaman
F igure 36. Average su rfa c e oceanographic
c o n d itio n s in th e Andaman Sea,
November to Ja n u ary .
178
>00*1 ioo»c
AVERAGE SURFACE OCEANOGRAPHIC CONDITIONS
IN THE ANDAMAN SEA, NOVEMBER TO JANUARY
(M odified from U.S. Navy Hydrogrophic O ffica SP-53, I960)
Figure 37. Average su rfa c e oceanographic
c o n d itio n s In the Andaman Sea,
February to A p ril.
180
to* IOO*E
AVERAGE SURFACE OCEANOGRAPHIC CONDITIONS
IN THE ANDAMAN SEA. FEBRUARY TO APRIL
(Modified from US Navy Hydrographic O ffice SP-53. I9 6 0 )
F igure 38. Average su rfa ce oceanographic
c o n d itio n s in the Andaman Sea,
Hay to August.
too^c
AVERAGE SURFACE OCEANOGRAPHIC CONDITIONS
IN THE ANDAMAN SEA, MAY TO AUGUST
(M odified from U S Navy H ydrogrophic O ffic t SP-53, I9 6 0 )
F igure 39. Average s u rfa c e oceanographic
co n d itio n s In the Andaman Sea,
September and O ctober.
184
IOO*C
100’C
AVERAGE SURFACE OCEANOGRAPHIC CONDITIONS
IN THE ANDAMAN SEA, SEPTEMBER AND OCTOBER
(Modified from U.S. Navy Hydrographic O ffice SP-53. I9 6 0 )
185
Sea from November through January (Fig* 3 6 a ). C urrents
pass from th e sea in to the Bay o f Bengal over the passages
in th e Andaman-Nicobar Ridge a t v e l o c it ie s ranging be
tween 10 and 36 cm/sec (0 .2 and 0*7 k n ).
' The Andaman Sea su rfa c e c u rre n ts are re in fo rc e d by
a s u rfa c e -w a te r d e n s ity g ra d ie n t along the e a ste rn h a lf
o f the sea during t h i s season. A g e n tle northw ard de
c re a se in s e a -s u rfa c e tem perature acro ss the Bay o f
Bengal and Andaman Sea (F ig . 3 6 b ), coupled w ith a stro n g
northw ard in c re a se in s a l i n i t y in the Malacca S t r a i t
(F ig . 36c) r e s u l t in a westward in c re a se in d e n s ity , from
1.019 gm/cm^ in the s t r a i t to 1.030 gm/cm^ in the Andaman
Sea (F ig . 3 6 d ). The s a l i n i t y p a tte rn in the Malacca
S t r a i t i s a p p a re n tly caused by the en tran ce o f South
China Sea w ater which i s d ilu te d by northeast-m onsoonal
r a i n f a l l and ru n o ff from southern Malaya and southw estern
Sum atra. Upwelling occurs along the Malay c o n tin e n ta l
slo p e during t h i s season (W yrtki, 1961).
February to A p ril
The n o rth e a s t monsoon and the s l i g h t atm ospheric
a n tic y c lo n e which occupy the Bay of Bengal during the
sp rin g months d riv e a clockw ise g y ra l whioh expands in to
the w estern Andaman Sea during February and March (F ig .
3 7 a ). Southw ard-flow ing 15*25 cm/sec c u rre n ts flow
through the Andaman-Nloobar Ridge passages and along the
186
w estern margin o f th e s e a . The e a s te rn p o rtio n of the
sea co n tin u e s to ex p erien ce s o u th -s o u th e a s t c u rre n ts
which again w holly occupy th e Andaman Sea In A p ril, as
th e n o r th e a s t monsoon and th e Bay o f Bengal antloyolone
d im in ish and the g y ra l o o n tra c ts (P ig . 37b).
S u rfa c e -w a te r te m p eratu res In the Bay o f Bengal
and Andaman Sea a re q u ite uniform during the sp rin g ,
averaging 29° 0 (U. S. Navy H ydrographic O ffic e , I9 6 0 ).
S urface s a l i n i t i e s a ls o vary only s l i g h t l y , ranging be
tween 32 and 33^0 (P ig . 3 7 c).
May to August
The a r r i v a l o f th e southw est monsoon I s r e f le c te d
d ra m a tic a lly in a s h i f t o f g e n e ra l c u rre n t d ir e c tio n in
the Andaman Sea, from s o u th e a s te r ly to w e ste rly (P ig .
3 8 a). C urrent v e l o c i t i e s In th e sea a re between 10 and
20 cm/sec (0 .2 and 0 .4 k n ), and th e northw estw ard c u rre n t
in the Malacca S t r a i t i s r e s t r i c t e d to th e n o rth e rn c o a st
o f Sumatra as Andaman Sea w ater I s forced in to the s t r a i t .
The huge q u a n t itie s o f f r e s h w ater discharged by
the Irraw addy R iv e r, supplem ented by the ru n o ff from the
c o a st o f p e n in s u la r Burma, are r e f le c te d in a s tr ik in g
p a tte r n o f s u rfa c e s a l i n i t i e s , which in c re a se southward
from 20
v alu es o f 3 2 .5 / in th e so u th ern Andaman Sea (F ig . 38b).
oo
Eastward tr a n s p o r t o f th e Irraw addy R iv e r 's d ischarge by
I
a t the G ulf o f Martaban to more normal marine
o.
187
th e p re v a ilin g w esterly c u rren ts Is evidenced by the
eastw ard displacem ent o f lso h y ets from the Irrawaddy
mouths to a common c e n te r in the G ulf o f M artaban.
Surface w ater tem peratures remain uniform during
the season o f g en e ra l r a in s , averaging 28° 0 . S urface
w ater d e n s itie s r e f l e c t the s a l i n i t y p a tte rn (F ig . 3 8 c).
Low values o f 1.011 gm/cm^ in the G ulf o f Martaban i n
crease southward and westward, to 1.021 gm/cm^ in the
v i c in ity of the N lcobar Isla n d s .
September and October
As e a rly as August the southwest monsoonal winds
have generated a counter-clockw ise g y ra l in th e Bay of
Bengal; by September the g y ral has expanded s u f f i c i e n t l y
to cause northw ard flow o f 10 to 20 cm/sec in th e n o rth e rn
Andaman Sea (F ig . 39A). The g y ral dominates the Andaman
Sea almost com pletely by October (F ig . 39b).
Temperature o f the surface w aters i s s t i l l uniform
during th is p e rio d , averaging 28° C. However, th e ru n
o f f cyole o f the Burmese r iv e r s lagB behind th e p r e c i p i t a
tio n oycle by a month (Stamp, 1940); consequently the
stro n g s a l i n i t y g ra d ie n t a t the n o rth e rn end o f the
Andaman Sea, although more c o n s tric te d than d u rin g th e
h e ig h t of th e monsoonal r a in s , is s t i l l q u ite d i s t i n c t
(F ig . 39c). At the Gulf of Martaban v alu es a re s t i l l as
o.
low as 20/ • S a lin ity in c re ase s ra p id ly southw ard, to
188
o .
2 9 / n o r th o f th e Andaman I s la n d s , and more g ra d u a lly to
00 Q
maxima o f 33«5£0 ln th e southw est c o m e r o f th e s e a . The
r e s u l t a n t d e n s ity g r a d ie n t (F ig . 39d) may w e ll enhance
th e northw estw ard flow o f n o rth e rn Andaman Sea w a te r in to
th e Bay o f B engal.
C h a r a c t e r i s t i c s o f the Deeper W aters
W ater Masses
The w a te r column e x h ib its a th r e e - f o ld s t r a t i f i c a
t i o n in th e Andaman Sea and a d ja c e n t Bay o f B engal (F ig .
4 0 ) . At th e s u rfa c e i s a 7 0 -to -1 0 0 -m -th ic k la y e r o f low
s a l i n i t y and h ig h te m p e ra tu re . S u rfa o e -la y e r s a l i n i t i e s
a r e v a r ia b le owing to in t e r m i t t e n t d ilu t io n by monsoonal
p r e c i p i t a t i o n and r u n o ff ; however, maximum annual
s a l i n i t i e s observed d u rin g the d r i e s t season (December to
A p ril) a re s t i l l r e l a t i v e l y low, ran g in g between 33 and
o
34^o> S u rfa c e te m p e ra tu re s a re q u ite uniform th ro u g h o u t
th e y e a r, ra n g in g between 27 and 30° C.
B eneath th e s u rfa c e la y e r s a l i n i t y in c r e a s e s
r a p i d l y , d e f in in g th e top o f a s u b -su rfa c e s a l i n i t y maximum
la y e r some 500 m t h i c k , w ith s a l i n i t i e s o f ap p ro x im ately
o
3 5 / . A s tr o n g ly d e fin e d , c h a r a c t e r i s t i c a l l y e q u a to r ia l
oo
th e rm o c lin e aooompanies th e in c re a s e in s a l i n i t y . Below
th e h i g h - s a l l n i t y la y e r both s a l i n i t y and te m p e ra tu re de
c r e a s e more g ra d u a lly toward th e bottom . In the Bay o f
F igure 40. P r o f ile s showing v e r t i c a l d i s t r i
b u tio n s o f te m p e ra tu re , s a l i n i t y
and d is s o lv e d oxygen c o n te n ts along
an e a s t-w e s t tr a v e r s e a c ro ss the
Andaman Sea a t 10° N l a t i t u d e .

191
B engal bottom s a l i n i t y and te m p eratu re a re 34.73? and
oo
1 .3 6 ° C r e s p e c t i v e l y , a t 3433-m d ep th .
The s a l i n i t y and tem p eratu re o f th e Andaman Sea
w a te rs below s i l l d ep th d i f f e r from v a lu e s in th e Bay o f
B engal, as m ight be e x p e c te d . The w ate r c h a r a c t e r i s t i c s
a re v i r t u a l l y i d e n t i c a l on b o th s id e s o f th e Andaman-
N icobar Ridge from th e s u rfa c e to 1300-m d e p th s . Below
t h i s d e p th , Andaman Sea s a l i n i t i e s and te m p e ra tu re s
rem ain v i r t u a l l y c o n s ta n t, changing only s l i g h t l y tow ard
th e bottom . Minimum s a l i n i t y in th e bottom waterB o f th e
Andaman B asin i s 3 4 . 8 6 ^ ; e q u iv a le n t v a lu e s in th e Bay o f
B engal a re encountered a t 1500 m. This d ep th c lo s e ly
approxim ates th e maximum s i l l depth in th e G reat P a ssa g e .
The s tro n g d iv erg en ce o f w a te r c h a r a c t e r i s t i c s a t 1300 m,
and th e more g ra d u a l d ec re ase 1A s a l i n i t y o f Andaman
w a te rs to th e 3 4 .8 6 ^o minimum in d ic a te th a t th e In flo w o f
Bay o f B engal w a te r over th e s i l l i s accom plished m ainly
a t 1300-m d e p th s, and th a t th e deep b reach es in th e
Andaman-Nicobar Ridge a re too narrow to perm it more th an
a t r i c k l e o f in flo w below 1300 m.
In th e Andaman B asin te m p e ra tu re s r i s e s l i g h t l y
tow ard th e bottom in w aters d ee p er than 2000 m, from a
minimum o f 4 .8 9 ° 0 to bottom v a lu e s o f 5.1 6 ° 0 . Burns
(1964, p. 4918) re p o rte d t h a t th e s e bottom waterB a re
M . • • warmer th an can be ex p la in e d by sim ple a d ia b a tio
h e a tin g • • a lth o u g h he d id n o t show the evidence f o r
192
such a therm al anom aly. In th e b e l i e f th a t the ab
norm ally h ig h w a te r tem p eratu res may be caused by v o lc an -
lsm w ith in th e b a s in , Burns I n i t i a t e d a program o f h e a t-
flow and b o tto m -w ater tem p eratu re measurements during the
PIONEER c r u is e .
The w r i t e r c o rre c te d tem perature d a ta from 73
hydrographic s ta t i o n s w ith in th e Andaman Basin ( ? lg . 4)
f o r th e e f f e c t s o f a d ia b a tic h e a tin g , u t i l i z i n g th e ta b le
o f c o r r e c tio n s o f S verdrup, Johnson and Fleming (1942,
p. 4 2 ). Of th e 23 s t a t i o n s which had samples below th e
approxim ate s i l l depth o f 1500 m, no s t a t i o n e x h ib ite d an
In c re a se o f p o te n tia l tem perature w ith depth (F ig . 4 1 ).
P o te n tia l te m p eratu re In the bottom w aters deeper than
1500 m ranged between 4.63 and 4.71° C, and 11 o f the
s ta tio n s had bottom -w ater tem peratures between 4.68 and
4.71° C. The l a r g e s t d e p a rtu re from th e normal b ath y -
therm al curve was found a t PIONEER S ta tio n 6 In th e
Narcondam-Barren B a sin . At th e same s t a t i o n a bottom
-6 .
h e a t-flo w v a lu e o f 1 .9 4 x 10 cal/om sec was measured
(B urns, 1964); t h i s v alu e I s low In comparison to o th e r
measurements In th e Andaman B asin . A fte r th e a d ia b a tic
c o r r e c tio n s , I t i s c l e a r th a t th e tem p eratu res below 2000
m a re iso th e rm a l w ith in th e range o f measurement e r r o r
(D efant, 1961) and t h a t th e r i s e o f In s i t u tem peratures
I s th e normal a d la b a tio e f f e c t which occu rs In closed
b a s in s .
Figure 41. In s i t u and p o t e n t i a l tem p eratu res
o f th e deeper w a te rs o f th e Andaman
S ea.
o-
TEMPERATURE
3 4
I __________ L.
5
- I -
6*C
_ l__
BAY OF BENGAL
e f f e c t iv e ^ s / l l d e p j h _
MAXIMUM S/LL DEPTH
ANDAMAN SEA
S -2 -6 2 « -4
S -2 -5 5 I-H ^ S - 2 - 5 7
S '2*38W V 497I
*1------------
H—»V4965
2 -
z
i-
o .
u
o
POTENTIAL TEMPERATURE\
PIONEER STA. 2
V4957
* 4 9 8 0
S-2-S2
3 -
Km
J
V 4 9 6 3
s /'2 5 * 7 = 4 -s 2 2 6
/ V 4 9 3 4
/ . ^ V 4 9 5 6
s-i-a
I •— -fe S -l-4 2
S-I-4IIT
V 4966l^= ^ . s -2 -4 8
1 ^™I 1 V 4 9 7 6
S -2 -4 3 *
S -l-4 4
POTENTIAL TEMPERATURE,
PIONEER STA. 5
IN SITU TEMPERATURE,
PIONEER STA. 2
LEGEND
IN SITU T E M R ^
POT. TEMP.
V 4 9 8 0 - SHIP a STA
P PIONEER 1964
S -l SERRANO 1961
S -2 SERRANO 1963
V VITYAZ 1961
* 4 9 6 9
V 4 9 7 4
S -l-4 3
IN SITU TEMPERATURE,
PIONEER STA. 5
'O
-P -
195
Maximum flow o f th is w ater in to the Andaman Basin
occurs during the n o rth e a s t monsoon. P re v a ilin g winds
d riv e su rfao e w aters out of th e Andaman Sea in to the Bay
o f Bengal, and replenishm ent o f Andaman w ater i s a c
com plished by inflow over the Ten Degree Channel and
G reat Channel s i l l s (Sew ell, 1932).
Waters below the su rfa c e in both the Bay o f Bengal
and the Andaman Sea e x h ib it an alm ost l i n e a r te m p eratu re-
a a l l n i t y r e la tio n s h i p , id e n tic a l w ith the E q u a to ria l Indian
w ater c h a r a c t e r i s t i c s defined by Sverdrup and o th e rs
(1942) (F ig . 4 2 ). E q u a to ria l Indian w ater i s thought to
form by the slow eastw ard mixing o f h i g h - s a l i n l t y Arabian
Sea and Red Sea w ater w ith A n ta rc tic Clrcum polar w ater
(Sverdrup and o th e rs , 1942; S a b in in , 1964).
Oxygen
In the Andaman Sea and a d ja c e n t p o rtio n s o f the Bay
o f Bengal the su rfa c e 50 to 7 5 m o f w ater c o n ta in ap
proxim ately 4 .6 to 4 .0 ml/L o f oxygen (F ig . 4 0 ). With
depth th ese v alu es decrease s h a rp ly , to minima o f 0 .4 to
0 .7 ml/L a t 100 to 250 m, and again in c re a se slow ly toward
the bottom . In the deeper Andaman w aters oxygen c o n te n ts
s t a b i l i z e a t approxim ately 1.8 to 1 .9 m l/L. E q u iv alen t
v alu es in the Bay o f Bengal a re encountered a t the e f
f e c tiv e s i l l depth o f 1500 m, in d ic a tin g th a t the oxygen
o f the Andaman w aters i s re p le n ish e d by Inflow o f Bay
w aters aoross the s i l l during the n o r th e a s t monsoon.
F ig u re 42. T e m p e ra tu r e -s a lin ity diagram o f
w a te r masses In th e n o rth e a s te rn
In d ian Ocean.
19?
18-
16-
14-
O
o
12-
4 1 9 3 * A ndam an Saa
(PIONEER '
o Bay of B tngal
(PIONEER
W atar-m at* diagram
from Svardrup and
L lI
q: io -
3
I-
<
c c .
UJ 8 -
Q .
2
ui
► -6-I
4 -
2 -
34.0
I
J # ‘
NEAR-SILL
WATER
3 4 3 3 h f I INDIAN OCEAN DEEP WATER
ANTARCTIC CtRCUMPOLAR WATER
T
2
T
.4
T
.6
T
8
T
3 3 0
T
2
T
.4
T
.6
SALINITY, %•
SEDIMENTS
O en eral S tate m e n t
Sedim ents o f th e Andaman B asin were s tu d ie d w ith
th e prim ary aim s o f d e te rm in in g t h e i r so u ro e s, modes and
avenues o f t r a n s p o r t , and s i t e s o f d e p o s itio n . A la rg e
number o f sam ples were a v a i l a b l e , th e m a jo rity o f whioh
were s u r fa c e sa m p le s. I t was d ecid ed t h a t a stu d y o f th e
s u rfa c e se d im e n ts would b e s t s e rv e th e purposes o f t h i s
i n v e s t i g a t i o n . A c c o rd in g ly , chem ical and t e x t u r a l
a n a ly s e s were l i m i t e d to th e s u rfa c e sam ples, core to p s ,
and to c o re s in a r e a s o f s p e c ia l i n t e r e s t . Most co res
were m erely in s p e c te d f o r t e x t u r a l v a r i a t i o n s , logged and
p h o to g rap h ed . S tu d ie s o f se d im e n to lo g ic v a r ia tio n s w ith
d ep th s in c o re s s h a l l be th e s u b je c t o f f u tu r e work.
W herever a s t a t i o n was re p r e s e n te d by b o th grab
sam ples and c o re s th e g rab sam ple was a n a ly z e d . The
re a so n f o r t h i s p re f e re n c e f o r g rab sam ples o v er oores
was t h a t more s in g le -s a m p le s t a t i o n s a r e re p re s e n te d by
s u rfa c e sam ples th a n by c o r e s .
T e x tu r a l a n a l y s i s , chem ical a n a ly se s f o r o rg a n ic
ca rb o n , n i t r o g e n , and carbon c o n te n t, and a n a ly s is o f
oalcium o a rb o n a te o o n te n t were s e le c te d as sta n d a rd
a n a ly s e s , b ased upon th e s e p a ra m e te rs ' proven u s e fu ln e s s
and ea se o f p erfo rm a n c e . A t o t a l o f 183 s u rfa c e samples
199
were analyzed f o r th e se p a ra m e te rs. Of th e se sam ples,
107 were grabs and 76 were core to p s . In a d d itio n , 130
su b -su rfa c e cores sam ples were analyzed f o r oalcium
c a rb o n a te , 60 were analyzed f o r o rg a n ic n itro g e n and
carbon c o n te n ts , and 52 were analyzed f o r te x tu r e .
For X-ray olay a n a ly se s 86 sam ples covering the
Andaman B asin in as even a p a tte r n as p o s s ib le were
s e le c te d . Clay a n a ly s is was lim ite d to th e s iz e f ra o tio n
f i n e r than 2 ^ i, in th e b e l i e f th a t t h i s s iz e f ra o tio n i s
most s e n s itiv e to h em ip elag ic movement and thereb y p ro
v id e s g e n e ra liz e d p a tte r n s o f source and d i s p e r s a l . Clay
d ata are re p o rte d in s e m i- q u a n tita tiv e fa s h io n , sin ce
q u a n tita tiv e a n a ly s is was n o t th e o b je c tiv e o f t h i s
stu d y . I t i s thought t h a t m agnitudes and r a t i o s o f th e
x -ray peak p aram ete rs, i f determ ined uniform ly fo r a l l
sam ples, provide i n s i g h t in to sedim ent supply and t r a n s
p o r t.
Only a cu rso ry study o f th e c o a r s e - f r a c tio n
m ineralogy was oonducted, a f t e r t e x t u r a l a n a ly se s rev ealed
th a t the sedim ents o f th e deeper p o rtio n s o f th e Andaman
Basin re c e iv e only i n s i g n i f i c a n t amounts o f sand from th e
s h e lv e s. A ccordingly, exam ination o f th e coarse f r a c tio n s
c o n siste d mainly o f d eterm in in g t h e i r te rr ig e n o u s ,
a u th lg e n ic and b io g e n ic components under a b in o c u la r
m iorosoope.
A n a ly tic a l Methods
200
Logging and Sample P re p a ra tio n
A ll su rfa c e samples and s e le c te d co res were
analyzed according to th e flow c h a rt given in Figure 43.
Cores were s p l i t lo n g itu d in a lly , e i t h e r by sawing through
t h e i r p l a s t i c l i n e r s and s p l i t t i n g th e contained sedim ent
w ith a sharp k n if e , o r by e x tru d in g th e core out o f i t s
l i n e r in to a trough and s p l i t t i n g i t w ith a s tre tc h e d
w ire . The s p l i t core was allow ed to dry s u f f i c i e n t l y fo r
s t r u c t u r a l d e t a i l to appear in b e s t c o n t r a s t. O ne-half
o f the core was then s to re d in s e c tio n s and th e o th e r h a l f
photographed w ith a P o laro id camera, u t i l i z i n g p o s itiv e -
n e g a tiv e film . The c o n ta c t-p o s itiv e photograph was
e n tered in to th e core logbook and th e n e g a tiv e film
washed and f i l e d . Next th e core h a l f was logged as to
v is u a l te x tu r e , s tr u c tu r e and M unsell c o lo r (G eological
S o ciety o f America, 1963)*
The h a lf - c o r e was again d iv id ed e q u a lly and one
q u a rte r-c o re given to W. E. F re rlo h s fo r m lcropaleon-
to lo g lc a n a ly s is . R e p re se n ta tiv e samples were cu t from
th e rem aining q u a rte r-c o re in v ary in g in t e r v a l s depending
on the se d im en t's degree o f homogeneity and s to re d a f t e r
a ir - d r y in g . Long homogenous i n t e r v a l s were sampled in 5
o r 10 om le n g th s .
Surface samples were d iv id ed in to sto ra g e and
F igure 43. Flow c h a rt o f sediment a n a ly s e s .
202
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SEDIMENT ANALYSIS
203
a n a l y t i c a l p o rtio n s a f t e r being logged according to Mun-
s e l l c o lo r and v is u a l tex tu re* From th is p o in t th e
a n a ly se s o f core and su rfa c e -g ra b m a te ria ls are I d e n t i c a l .
For the v a rio u s an aly ses as many as th re e s p l i t s
were removed from eaoh sam ple. One s p l i t , w eighing b e
tween 5 and 10 gm, was fo r te x tu r a l s tu d ie s . The second
B p llt weighed between 2 and 20 gm, depending on th e amount
o f c lay p re se n t and on a v a ila b le sample b u lk , and was used
f o r c la y a n a ly s e s . The th ir d s p l i t , weighing approxim ately
5 gm, was f o r chem ical a n a ly se s. This th ir d s p l i t was
o v en -d ried a t 60° C, ground In to a fin e powder In a
p o rc e la in m o rtar, placed In a g la ss storage v i a l , allow ed
to come to e q u ilib riu m w ith normal atm ospheric m o istu re ,
and capped according to the recommendations o f H llle b ra n d
and o th e rs (1953)* Employing oven-dried powder f o r
a n a ly se s In tro d u ces unnecessary e r r o r ; each time th e v i a l
1b uncapped to e x tr a c t m a te ria l fo r a n a ly sis the powder
re s o rb s more m o istu re .
Clay in a ly 8 l8
Each clay s p l i t was a g ita te d w ith 20 to 40 ml o f
d i s t i l l e d w ater In a p l a s t i c o en trlfu g e tu b e , c e n trifu g e d
u n t i l a l l sedim ent had s e t t l e d , and the su p e rn a ta n t w ate r
d is c a rd e d . Three o r fo u r such washings removed a l l s a l t
from the sedim ent and thoroughly d isaggregated th e olay
c o n te n t. The olean suspension was then a g ita te d and a l l
204
m a te r ia l c o a rs e r than 2 yu allow ed to s e t t l e o u t o f the top
7 cm in the tu b e, employing s e t t l i n g tim es in d ic a te d by
the nomogram o f Krumbeln and P e ttijo h n (1938). Olay
rem aining in suspension was then removed by p ip e tte and
s to re d in a clean g la s s v i a l .
D u p licate o rie n te d clay mounts were made by shak
ing the d a y suspension in each v i a l , w ithdraw ing ap
proxim ately 1 ml w ith a p ip e tte and placing approxim ately
0 .5 ml on each o f two g la s s s l i d e s . These mounts were
allow ed to dry under a g la s s d ish which p ro te c te d the
sedim ent from contam ination and prevented the w ater from
evaporated so r a p id ly th a t the mounts crack ed . One of
each p a ir o f r e p l i c a t e s lid e s was s to re d and the o th e r
an aly zed .
Each s lid e analyzed was su b je c te d to X-ray d l f -
f r a o tlo n u n tre a te d , a f t e r s a tu r a tio n w ith e th e le n e g ly c o l
vapor f o r 24 h o u rs, and a f t e r h e a tin g in a fu rn ace a t 550°
C fo r 12 h o u rs. A ll d iffra c to m e te r runs were made on a
P h ilip p s-N o re lc o d iffra c to m e te r w ith CuKa r a d ia tio n , a t a
scanning speed o f l°20/m ln, and w ith a time c o n sta n t of 2
seconds. Records o f each run were made w ith a sc a le
f a c t o r allow ing maximum peak h e ig h t w ith in th e reco rd
paper w idth of 10 in , and w ith a re c o rd e r speed of 0 .5
in /m in .
Only the g ly c o la te d and h e a t- tr e a te d runs were
u t i l i z e d in computing the olay-peak param eters. Back-
205
ground I n t e n s i t i e s were drawn on each re c o rd w ith a
f l e x i b l e r u l e . A reas o f th e 7% and lo2 g ly c o la te d peaks
and lo2 h e a te d peak were m easured w ith a p o la r p la n im e te r
m a in ta in e d a t a c o n v e n ie n t a r b i t r a r y s e t t i n g , and h e ig h ts
above background were m easured and reco rd ed f o r th e g ly -
c o la te d 72 and h e a te d 142 p e a k s.
C om putations were made f o r r e l a t i v e abundances o f
c h l o r i t e , k a o l l n i t e and i l l i t e by a scheme (Table 9)
m o d ifie d from S c h u ltz (1 9 6 4 ). An a tte m p t was made to
d i f f e r e n t i a t e m o n tm o rillo n ite from m ix e d -la y e r m ont-
m o r i l l o n l t e - l l l l t e by d e te rm in in g a form f a c t o r f o r mont
m o r ill o n ite as S c h u ltz d id f o r th e P ie r r e S h a le . A fte r
a n a ly s e s o f 12 B ureau o f S ta n d a rd s m o n tm o rillo n lte s
y ie ld e d form f a c t o r s ra n g in g between 1 and 4 th e a tte m p t
was abandoned. For th e p u rp o ses o f t h i s s tu d y , mont
m o r ill o n ite and th e m ix e d -la y e r c la y s a re n o t re p o rte d
s e p a r a t e l y . I n s te a d , th e y a r e re p o r te d c o l l e c t i v e l y as
th e p e rc e n ta g e rem a in in g a f t e r c h l o r i t e , k a o l l n i t e and
i l l i t e p e rc e n ta g e s a r e s u b tr a c te d from 100 p e rc e n t; f o r
b r e v ity t h i s c la y oomponent i s r e f e r r e d to sim ply as
" m o n tm o rillo n ite " .
I t m ust be em phasized h e re t h a t q u a n t i t a t i v e
d e te rm in a tio n s o f c la y p e rc e n ta g e s by X-ray te c h n iq u e s
a re n o t a c c u r a te . The " p e rc e n ta g e s" o f th e v a rio u s o lay
m in e ra ls r e p o r te d in t h i s stu d y only se rv e as sedim ent
s ig n a t u r e s , i n d i c a t i n g th e a r e a l v a r i a t i o n s in r e l a t i v e
206
TABLE 9 . P rocedure f o r th e C a lc u la tio n o f R e la tiv e
Amounts o f th e Clay M in erals
1. Measure 7% and l o i g ly c o liz e d and lo£ h e a te d
(550° C) peak a r e a s ; measure th e h e ig h ts o f
the 7a g ly c o liz e d and 14a 550° C p eak s.
2. C o rrected 7? peak a re a = 7% a re a
X •
3. K a o lln ite + c h l o r i t e (p e rc e n t) =
_______c o r r e c te d 7% peak a r e a _________ v
Q A ^ 100
CORRECTED 7A peak a r e a + 10A 550° C
peak a re a
4 . C h lo rite (p e rc e n t) =
( k a o l l n i t e + c h l o r i t e ) X 141 0 Peal1 h e lKh t
1 .5 X 7a peak h e ig h t
5* K a o lln ite ( p e rc e n t) = ( k a o l l n i t e + c h l o r i t e ) -
c h l o r i t e .
6 . I l l i t e (p e rc e n t) =
_________________ 10% g ly c o l peak a re a ________________ x
c o rre c te d 7% peak a r e a + 10% 550° C peak a re a
7 . M o n tm o rillo n ite + m ix e d -la y e r c la y (p e rc e n t) =
100 - ( k a o l l n i t e + c h l o r i t e + i l l i t e ) .
amounts o f th e m in e ra ls
207
T e x tu ra l A n a ly sis
Techniques m o d ified a f t e r th e sta n d a rd p ip e tte and
s ie v in g methods o f Krumbeln and P e ttiJ o h n (1938) were em
ployed f o r th e d e te rm in a tio n o f sedim ent s iz e p aram ete rs.
Each 5 to 10 gm te x tu r e sub-sam ple was soaked In 200 ml o f
d i s t i l l e d w a te r, to w hich was added 25 ml o f 30 p e rc e n t
hydrogen p e ro x id e . This re a g e n t serv ed a d u al fu n c tio n :
(1) to d is a g g re g a te th e c la y p a r t i c l e s d u rin g e ffe rv e sc e n c e
and (2) to o x id iz e th e o rg a n ic compounds which cause
f lo c c u la ti o n . Depending on th e p a r t i c l e s iz e and s t a t e
o f compaction o f th e sam ple, e ffe rv e s c e n c e would l a s t
from a few m inutes to s e v e r a l h o u rs .
F ollow ing d is a g g re g a tio n th e sample was c e n t r i
fuged In a 50 ml p l a s t i c tube u n t i l a l l th e sedim ent had
s e t t l e d . A ll th e s u p e rn a ta n t w ate r was th en d isc a rd e d and
th e washing re p e a te d s e v e r a l tim e s u n t i l th e sedim ent was
r i d o f a l l s a l t s and co n ta m in a n ts.
Next th e sample was w e t-sie v e d through a 62yu
sc re e n In to a 1 - l i t e r s o i l c y l in d e r . The volume o f s i l t -
c la y 8U8pensate was th en b ro u g h t to 500 ml and th e c y lin d e r
placed In a w a te r b a th . In g e n e r a l, 16 such su sp en sio n s
would c o n s t i t u t e a b a t t e r y o f p i p e t t e a n a ly se s to be con
ducted to g e th e r .
P ip e tte a n a ly s e s were conduoted w ith s i g n i f i c a n t
m o d if ic a tio n s . I t has been found th a t th e l a r g e s t pos
s i b i l i t y fo r e r r o r in th e sta n d a rd R itte n h o u s e (1933)
te ch n iq u e i s th e p ip e tte taken im m ediately a f t e r a g i t a
t i o n . This c r u c i a l a l iq u o t, which i s assumed to r e p r e
s e n t th e t o t a l w eight o f the m in u s-6 2 jn f r a c t i o n , was
found to vary by a s much as 7 p e rc e n t (R odolfo and B arn es,
in p r e p a r a tio n ) . A m o d ific a tio n employed d u rin g th e
p r e s e n t stu d y c o n s is te d o f d isp e n sin g e n t i r e l y w ith th e
"zero " p i p e t t e , and in s te a d d ry in g th e s u s p e n s a te rem ain
in g a f t e r a l l th e o th e r a liq u o ts had been rem oved. T o ta l
s i l t p lu s c la y was then re p o rte d a s th e cu m u lativ e w eig h ts
o f th e a liq u o ts p lu s th e w eight o f th e rem a in in g suspen
s a t e .
A nother m o d ific a tio n c o n s is te d o f c u t t i n g the
suB pensate volume in h a l f , from th e t r a d i t i o n a l 1000 ml
to 500 ml. No r e a l reason n e c e s s ita t e d th e u se o f a
l i t e r o f s u s p e n s a te , o th e r than t h a t th e f i r s t in n o v a to rs
o f th e p ip e tte method used t h i s volum e. U t i l i z a t i o n o f
500 ml allow s th e p ro d u ctio n o f a s u s p e n s a te o f s ta n d a rd
d e n s ity w ith only h a l f th e volume o f se d im e n t. When v ery
lim ite d amounts o f sedim ent a re a v a i l a b l e , su ch a s when
th e 8ample i s a 1 -in c h diam eter g r a v ity o o re , th e 500 ml
m o d ific a tio n r e p r e s e n ts a s i g n i f i c a n t sa v in g o f i r r e p l a c e
a b le m a te r ia l.
A fte r d rying th e coarse f r a c t i o n rem ain in g a f t e r
w e t-s ie v in g , i t was siev ed fo r 10 m inutes th ro u g h a
sta n d a rd Wentworth sequence o f 3 -in ch diam eter screen s
a g ita te d by an e le c tro m a g n e tic v ib r a to r (V ibrapad, manu
fa c tu re d by the Sepor C o rp o ra tio n ). Each Wentworth s iz e
f r a c tio n was weighed to the n e a r e s t m illig ram tn d the
e n t ir e sand f r a c tio n was r e c o n s titu te d and s to r e d .
W eights fo r each s iz e f r a c tio n from the combined
sie v e and p ip e tte an a ly se s o f each sample were summarized
on stan d ard forms and subm itted fo r s t a t i s t i c a l computer
a n a ly s is , u t i l i z i n g a program which r e p o r ts w elg h t-
p ercentages fo r each Wentworth u n i t , mean g ra in s i z e ,
sta n d ard d e v ia tio n , skewness, and k u r to s ls in phi n o ta
tio n •
Oalcium Carbonate
A gasom etric technique re c e n tly devised (Kolpack
and Rodolfo, in p re p a ra tio n ) was employed f o r th e d e te r
m ination o f oaloium carbonate c o n te n ts . The method i n
v olves the d ig e s tio n of 0 .5 to 1 .0 gm o f sample w ith 6N
h y d ro c h lo ric aoid in a closed system which i s h eated and
swept clean o f evolved carbon dioxide by b o ttle d oxygen.
The m ixture of carbon dioxide and oxygen i s d riv en in to a
LEGO Carbon A nalyzer b u r e tte and i s brought to sta n d ard
volume w ith more oxygen. Next th e m ixture o f gases i s
scrubbed clean o f carbon d io x id e by passage in to the
oarbon a n a ly z e r 's potassium hydroxide r e s e r v o i r . A fte r
sc ru b b in g , the gas i s re tu rn e d in to th e b u r e tte and th e
210
volume o f gas l o s t to th e c a u s tic s o lu tio n i s re a d a s a
fu n c tio n o f calcium c a rb o n a te -c a rb o n c o n te n t, w hich i s
computed a f t e r c o r r e c tio n s f o r te m p e ra tu re and a tm o sp h e ric
p r e s s u r e .
A ll a n a ly s e s were r e p l i c a t e d and were c o n s id e re d
s a t i s f a c t o r y when th e r e p l i c a t e calciu m c a rb o n a te -c a rb o n
p e rc e n ta g e s ag reed w ith in 1 p e r c e n t.
O rganic Oarbon
T o ta l-c a rb o n c o n te n ts were determ ined by com bust
in g r e p l i c a t e d powder sam ples in a LEGO in d u c tio n fu rn a c e
coupled to th e carbon a n a ly z e r . U su ally th e same sam ple
s iz e o f 0 .5 to 1 gm was em ployed. O rganic carbon was
computed a s th e d if f e r e n c e between to ta l- c a r b o n and
ca lciu m c a rb o n a te -c a rb o n v a lu e s .
O rganic N itro g en
R e p lic a te powdered sam ples w eighing between 0 .5
and 1 .0 gm were an a ly ze d f o r o rg a n ic A ltro g en c o n te n t
em ploying a m ic ro -K Je ld a h l te c h n iq u e and a p p a ra tu s d e v ise d
by th e Hengar Company. Agreement between r e p l i c a t e
r e s u l t s was c o n sid e re d a c c e p ta b le w ith in 5 p e r c e n t. In a
lo w -n ltro g e n sample t h i s would be ab o u t 0.005 p e r c e n t
n itr o g e n ; in a sample r io h in o rg an lo n itro g e n th e v a r i a
ti o n r e p r e s e n ts ab o u t 0 .0 1 p e rc e n t n itr o g e n .
Sedim ent P rovin ces
211
A ll sedim ent p aram ete rs in d ic a te f iv e g e n e ra l
p ro v in c e s o f sedim ent ty p e s in th e Andaman B asin (F ig .
4 4 ). These a r e :
1 . The G u lf o f M artaban P rovince o f n e a rs h o re
d e l t a i c sed im en t, in c lu d in g a 25 km wide band o f f th e
mouths o f th e Irraw addy R iv e r.
2. An O uter D e lta P ro v in c e .
3 . The Malay C o n tin e n ta l Margin P ro v in c e , which
merges w ith th e O uter D e lta P rovince and which in c lu d e s
th e a r e a o f th e Mergui T e rra c e .
4 . The C e n tra l B asin P ro v in c e , which com prises
th e f lo o r s o f th e Narcondam-Barren B a sin , th e C e n tra l
Andaman Trough and th e Sum atra S h e lf B a sin , as w e ll as
A lcock and S ew ell seam ounts and th e se ah lg h s a s s o c ia te d
w ith th e N icobar R i f t .
5* The Andaman-Nicobar P ro v in c e, in c lu d in g
I n v i s i b l e Bank.
G u lf o f M artaban Province
The G u lf o f M artaban P rovince i s c h a ra c te riz e d by
homogenous s i l t y c la y s which c o n ta in only f r a c tio n s o f a
p e rc e n t o f sa n d , g e n e ra lly te rr ig e n o u s m ica. R a re ly ,
th in layerB o f co a rse s h e l l fragm ents a re in co re s tak en
a lo n g th e Malay o o ast o f th e g u l f . Elsew here in th e
F igure 44. Sediment p rovinces and g en e ra l
d i s t r i b u t i o n o f t e x tu r a l types in
th e Andaman B asin .
213
SEDIMENTARY
PROVINCES
AND
TEXTURAL
TYPES
( I M M M M t U M t m
n T n S M ^ i a M t u i a u i
■ i Silty Clays
■
Silty Clays
Clayey silts nP
■
Laminated Basin ^
Cloys and Sifts
| m Foraminiferal Ooze
H
Coarse Coralline
Sediments
214
p rovince the cores a re homogenous th ro u g h o u t.
O uter D elta Province
Between w ater depths o f 50 and 70 m on th e I r r a
waddy D e lta th e su rfa c e sedim ents change c h a r a c te r , from
th e n e a rsh o re s i l t y c la y s o f the G ulf o f M artaban Province
to s i l t y and clayey sa n d s. The sands o f th e o u te r d e l ta
are la r g e ly te r r ig e n o u s , w ith a n g u la r to su b a n g u lar q u a rtz
i
and f e ld s p a r g ra in s com prising 40 and 30 p e rc e n t,
r e s p e c tiv e ly . M ollusc fragm ents and fo ra m ln lfe ra are
about e q u a lly abundant in th e w estern o u te r d e l t a . F ora-
m ln ife ra d im in ish in p ercentage ea stw ard .
Cores from th e o u te r d e l t a did n o t p e n e tra te more
than 30 cm and re v e a le d no s tr u o tu r e , being uniform s h e lly
te rrig e n o u s sands w ith a "p oured-in" a s p e c t. Two c o re s ,
S-232 and S-25» taken a t the boundary o f th e G ulf o f
M artaban and O uter D e lta p ro v in c e s, have s u rfa c e sedim ent
o f th e M artaban s i l t y c la y , r e s t i n g unevenly on O uter
D elta s h e lly clayey sand w ith obscure i n t e r c a l a t i o n s o f
sandy c la y (F ig . 4 3 ). The p ic tu re t h a t th e se cores
p re a e n ts i s one o f f r e s h d e l ta i c sedim ent encroaohing o u t
over O uter D e lta sedim ent. To th e e a s t th e O uter D elta
Province i s se p a ra te d from the Malay C o n tin e n ta l Margin
by th e M artaban Canyon.
F igure 45. SERRANO Cores 25 and 232* l l l u s t r a t
ing th e r e la tio n s h ip of G ulf o f
Martaban and o u te r d e lta sed im en ts.
216
SERRANO
CORE 25
2 9
4 3
7 5 Cm
Gulf of Martaban
clay; low earbonato
and organic contents
Mottled lumps of
cloy and fins sand
Outor Delta sand;
fins>grainsdt silty,
shelly
SERRANO
CORE 232
1 6
19
22
3 3
5 0
5 4 Cm
Gulf of Martaban
clay; sandy
Shelly, clayey sand
Cloyay sand
Shelly, clayey sand
Shelly, sandy clay
Obscurely bedded
sandy clay and
clayey sand
Outer Delta sand;
clayey
OUTER
DELTA
PR O V IN C E
GULF OF
MARTABAN
PROVINCE
S C n n A N O '
cons t u
one ts
Malay C o n tin en ta l Margin P rovince
217
S i l t y and clay ey sands w ith a g ro ss a s p e c t l i t t l e
d i f f e r e n t from th a t o f th e O uter D e lta sedim ents
c h a r a c te r i z e th e s u rfa c e o f th e Malay S h e lf and Mergul
T e rra c e . Ab in th e o u te r d e l t a c o a rs e , poorly s o rte d
muddy san d s w ith v a ry in g adm ixtures o f m olluscan fragm ents
a re dom inant and e x h i b it obscure bedding, i f any.
T e rrig e n o u s sand i s th e l a r g e s t component. In
o o n tr a s t to th e f e ld s p a th lc o u te r d e l t a sed im en ts, f e l d s
p a rs o cc u r o nly in tr a c e am ounts. S u b -an g u lar to rounded
q u a rtz g r a in s c o n tr ib u te 80 to 90 p e rc e n t o f the sedim ent.
Rock fra g m e n ts, n o ta b ly c h e rt and b a s a l t , a re p re se n t
l o c a l l y in q u a n t i t i e s o f as much as 10 p e rc e n t. F ra c tio n s
o f a p e rc e n t o f th e sands a re composed o f heavy m in e ra ls ,
dom inated by b i o t l t e , m a g n e tite , ilm e n ite and le u co x en e .
M olluscan frag m en ts and fo ra m in ife ra c o n s titu t e a p p ro x i
m ately 10 p e rc e n t o f th e sed im en t.
The te r r ig e n o u s sands a re w ell s o r te d , and i t i s
o nly th e adm ixture o f b io g e n ic m a te r ia l o f w idely v a ry in g
s iz e s t h a t makes th e sedim ent poorly s o r te d . The
m in e ra lo g y , s o r tin g and rounding of th e q u a rtz sands
i n d ic a t e g r a n i t i c provenanoe and rew orking o f th e s h e l f
sed im en ts o v er a long i n t e r v a l .
In th e so u th e rn p o rtio n o f the p ro v in ce, where th e
s h e l f becomes th e f l o o r o f th e n o rth e rn Malacca S t r a i t ,
218;
a se m i-p ro te c te d a re a between Go Phuket and Lankawi I s
lands i s c h a ra c te riz e d by s i l t y c la y s which d i f f e r in
g ro ss a s p e c t from th e M artaban sedim ents only in having a
s l i g h t l y more g re e n is h c a s t . This p o rtio n o f th e Andaman
Sea p ro p erly belongs in the M alacca S t r a i t a re a , the
sedim entology o f which h as been s tu d ie d by K e lle r (1966).
C e n tra l B asin Province
Homogenous s i l t y c lay dom inates th e c e n tr a l p o rtio n
o f th e Andaman B a sin . F o ra m in ife ra ( r a d lo la r la n s ) con
s t i t u t e alm ost 100 p e rc e n t o f th e co arse f r a c tio n s o f
th ese sedim ents and a re complemented by fin e -g ra in e d
te rrig e n o u s m ica. Samples from the to p s and fla n k s o f
seahlghs are g e n e ra lly r i c h e r in fo ra m in ife ra , being
r e l a t i v e l y u n d ilu te d by te rrig e n o u s sed im en t. These s e a -
high sedim ents may a t t a i n th e f o r a m ln if e r a l and o v e r a ll
carbonate c o n te n t o f g lo b lg e rin a ooze a s d efin ed by
P e ttiJo h n (1957).
Although most o f th e b a s i n - f l a t co res a re homo
genous s i l t y c la y s , some c o res e x h ib it two d if f e r e n t
types o f coarse la y e r s I n te r c a la te d w ith th e fin e s e d i
ment. The f i r s t type a re g e n e ra lly s e v e r a l m illim e te rs
th ic k , ungraded, and a re f o ra m ln if e r a l and pteropod t e s t s
and c o r a l lin e d e b r is which h as slumped down from a d ja c e n t
se a h lg h s. These la y e rs a re r a r e and, w ith th e ex cep tio n
o f PIONEER Core 24, c o n s t i t u t e only f r a c tio n s o f a p ero en t
219
o f th e sedim ent volume. PIONEER Oore 24 was o b tain ed a t
the base o f I n v is ib le Bank. B ank-top d e b ris c o n s is tin g
m ainly o f c o r a llin e and m olluscan fragm ents which has
slumped down to form coarse c a l c a r e n l t l c la y e rs as th ic k
as 35 m m (P ig . 46) c o n s titu te 23 p e rc e n t o f the sedim ent.
This a l te r n a ti o n o f th in la y e rs o f sh allo w -w ater b io g en ic
d e t r i t u s w ith th ic k hem lpelaglc la y e r s may occur e l s e
where along the unsampled b a s i n - f l a t a re a s a d ja c e n t to
th e Andaman-Nlcobar Ridge.
The second type o f la y e rin g In sedlmentB o f th e
C en tral Basin province I s the r a r e , th in lam inae o f
graded s i l t and fin e sand p re se n t In cores from the deep
e s t p o rtio n s o f the C en tral Trough. These lam inae a re
probably t u r b l d l t e s from d e n s ity flow s o r ig in a tin g In
Martaban Canyon. O utside t h i s r e s t r i c t e d a re a b asin f lo o r
sedim ents a re n o t lam inated and o c c a s io n a lly e x h ib it worm
burrow s. I t I s I n te r e s tin g to n o te th a t co res which
possess lam inae have low fo ra m in ife ra numbers, and th a t
th e few ca lca re o u s fo ra m in ife ra show the e f f e c t s of
severe s o lu tio n (W. E. P re rlo h s , p e rso n a l com m unication).
An exam ination o f the b e n th lc biomass In th e Andaman
Basin (Sokolova and P a ste rn a k , 1964) rev ea led th a t t h i s
a re a Is Impoverished In burrowing organism s; t e n t a t i v e l y ,
P re rlo h s concluded th a t the s o lu tio n o f fo ra m in ife ra In
t h i s a re a may be due to the p ro d u ctio n o f carbon d io x id e
In the upper la y e rs o f th e sedim ent as o rg an ic m a tte r
F ig u re 46. Coarse c a l c a r e n i t i c la y e r s in
PIONEER Core 24 from tn e base
o f I n v i s i b l e Bank.
221
175-
180-
185-
190-
195 J
179-192 Cm,
Coorse colcorenite,
homogeneous except
for obscure layering,
cleon bottom I cm,
ond mud lumps at
top of unit.
215 H
220
2 2 5 -
2 3 0 -
2 1 7 - 2 3 8 Cm.
Series of 5 graded
laminae in top 4 cm;
also obscurely bedded
at top of unit.
2 3 5
2 7 5 -
2 8 0 -
2 8 5 -
2 9 0 -
2 9 5 -
3 0 0 -
3 0 5 -
310
3 1 5
2 7 8 - 3 1 7 Cm.
Homogeneous except
for upward decrease
in grain size in top
5 cm of unit.
4 4 9 - 4 5 3 Cm.
Obscurely bedded.
4 4 8
4 4 9
4 5 0
451
4 5 2
4 5 3 -
PIONEER CORE 2 4
decays. By t h i s mechanism, s o lu tio n proceeds because th e
evolved carbon d io x id e i s n o t re le a s e d in to th e o v erly in g
w ater column by th e a c t i v i t y o f b u rro w ers. The homogen
eous b a s i n - f l a t sedim ents may a ls o have been comprised in
p a r t by t u r b i d i t e s which were d estro y ed by b io lo g ic
s t i r r i n g . T his e x p la n a tio n le a v e s much to be d e s ire d ;
p a r tic u la r ly th e manner by which carbon d io x id e i s evolved
w ithout b io lo g ic a c t i v i t y . A b e t t e r u n d erstan d in g of th e
s o lu tio n phenomenon r e q u ir e s geochem ical an a ly se s o f
bottom and i n t e r s t i t i a l w a te rs .
A core reco v e re d in th e immediate v i c i n i t y of
Barren Is la n d was homogeneous hem ipelagic s i l t y clay w ith
a s o l i t a r y 4 cm th ic k sand la y e r a t 86 to 90 cm depths in
the core. T his sand i s ungraded and i s composed mainly o f
b lack v o lc a n ic g la s s and p la g io c la s e frag m en ts, w ith an
adm ixture of f o r a m in if e r a . I t does n o t have th e a sp ect of
an a s h f a l l d e p o s it, and probably slumped down from the
beach of B arren I s la n d . The only la y e rs o f v o lc a n ic ash
found w ith in th e Andaman B asin were observed in SERRANO
Core 66, taken in th e N orth Sumatra S h e lf B a sin . In t h i s
core c o lo rle s s v o lc a n ic g la s s sh ard s o c c u rrin g e x c lu siv e ly
in the 250 to 62yU s iz e g rad es formed 84 p erce n t o f a
clayey sand la y e r 6 cm th i c k , 63 to 69 cm below th e core
to p . A s u rfa c e grab sample 100 km w est o f Core 66 also
y ield ed s im ila r g la s s s h a rd s . O ther sam ples in th e s h e lf
b asin did not c o n ta in g l a s s .
Andaman-Nlcobar Ridge Province
223
Sediments o f the Andaman-Nicobar Ridge vary w idely
from c o a rse , poorly so rte d c o r a llin e sands and g ra v e ls on
the In s u la r sh elv es to s i l t y c la y s I d e n tic a l In gross a s
pect w ith those o f the C e n tra l Basin P ro v in ce. Some o f
the s i l t y clay s e x h ib it th e slump la y e rin g d escrib ed In
the sedim ents of th e flo o rs o f th e C e n tra l B asin.
The sm all sem l-enclosed b asin n ested In th e c r e s t
o f the Andaman-Nlcobar Ridge a t 10° N l a t i t u d e Is flo o re d
w ith ponded sedim ent which Is I d e n tic a l In te x tu re and
appearance w ith the sedim ents o f th e C e n tra l Basin f lo o r .
One core was a homogeneous s i l t y c la y ; th e o th e r possessed
I n te r c a la tio n s o f ungraded fo ra m ln if e ra l sands no th ic k e r
than 10 cm, sep arate d by 15 to 25 cm o f homogeneous s i l t y
c la y .
C olor
The sedim ents o f the Andaman Basin are alm ost mono
chrom atic, w ith o liv e hues predom inant. Sediments o f the
M artaban, O uter D elta and C e n tra l Basin provinces are
o liv e gray , o f 5Yn/2, where n re p re s e n ts the M unsell code
co lo r value ( lig h tn e s s ) , In c re a s in g from a n o ta tio n o f 2
(dark) to 7 ( l i g h t ) (Geol. Soc. America, 1963). The
v a r ia tio n o f c o lo r lig h tn e s s was c o n tro lle d la rg e ly by the
degree of dryness o f the sedim ent when th e c o lo r was
examined and reco rd ed . In a la rg e number o f C e n tra l B asin
co res th e su rfa c e 5 to 10 cm o f sedim ent has a re d d is h
c o lo r component and the n o ta tio n ranges from 10YR6/2
("p a le y e llo w ish brown") to 10YR3/2 ("d a rk y e llo w ish
brow n"). T his re d d is h tin g e i s probably a t t r i b u t a b l e to
the o x id a tio n o f ir o n . W ithin the C e n tra l B asin th e re i s
a m ild tendency f o r seahigh sedim ents to be s l i g h t l y
p a le r due to g r e a te r adm ixtures of w hite f o ra m in ife r a .
Sedim ents o f the Andaman-Nlcobar Ridge and Malay
C o n tin e n ta l Margin provinces were s l i g h t l y more g re e n ish
than th e d e l t a i c and b a s ln - f lo o r sedim ents, t y p i c a l hues
being " o liv e " (10Y) and "yellow green" (5GY), w ith w idely
d if f e r i n g chromas (g ray n ess) and v alu es due in la r g e p a r t
to the d i f f e r e n t c o a r s e - f r a c tio n p erce n tag e s and com
p o n en ts.
Sew ell (1925) examined th e shipboard d e s c r ip tio n s
o f sedim ent smears from over 200 le a d - lin e soundings in
th e Andaman B asin and re p o rte d two a re a s o f c o n tr a s tin g
c o lo r . The d e l t a a r e a , c o n tin e n ta l m argin, and c e n tr a l
and s o u th e a s te rn p o rtio n s o f th e C e n tra l B asin he
re p o rte d as an a re a o f "brown mud" se p a ra te d sh a rp ly from
th e n o rth e rn p o rtio n o f th e Andaman Ridge P ro v in ce which
he re p o rte d as an a re a o f "green mud". Sew ell then con
cluded th a t th e "brown mud" in d ic a te s Irraw addy, S itta n g
and Salween provenance, and th a t th e "green mud" is
fo re ig n to th e b a s in , in f e r r i n g th a t i t s main sou rce i s
the Arakan c o a st ol‘ th e Bay o f B engal, which h i s o h a rt o f
225
sedim ent ty p es shows as a lso covered by "green mud". I t
now appears t h a t t h i s d is tin c tio n i s due in la rg e p a r t to
im p recisio n w ith which th e co lo rs were assig n ed by two
decades o f w orkers on the INVESTIGATOR.
Texture
The a r e a l d i s t r ib u t io n o f su rfao e-sed lm en t te x
tu r e s i s p resen ted in Pigure 44. In an a re a as p h y sio -
g ra p h ic a lly and se d lm e n to lo g ic a lly d iv e rse as the Andaman
B a sin , th e s t a t i s t i c a l valu es o f sedim ent s iz e y ie ld
com plicated p a tte r n s which are m eaningful only in a
p ro v in c ia l se n se. Thus, a p lo t showing the v a r ia tio n o f
mean diam eter w ith depth (F ig . 47) is a broad s c a t t e r o f
p o in ts which a re m eaningful only i f the p o in ts are
se g re g ate d in to t h e i r re s p e c tiv e sedim ent p ro v in ces.
The shallow sedim ents o f the G ulf o f Martaban
Province a re th e most c o n s is te n t, w ith mean diam eters b e
tween 2 and 4 Sediments o f th e O uter D elta and Malay
C o n tin e n ta l Margin provinces are th e most d iv e rs e ,
e x h ib itin g a mean d iam eter range from 3 to 500 yu. Basin
f lo o r and Bay o f Bengal sedim ents are g e n e ra lly f i n e
g ra in e d , and range in mean diam eter between 2 and 31yU,
becoming c o a rs e r w ith in c re a sin g fo ra m ln ife ra l c o n te n t.
The few samples from th e Andaman-Nlcobar Ridge Province
e x h ib it a wide range o f s iz e s w ith no ap p aren t c o r r e la
tio n w ith d ep th . Mean diam eters o f th ese sedim ents range
F igure 47. V a ria tio n o f mean diam eter w ith
w ater d ep th .
< /)
(T
MEAN DIAMETER
0 1 2 3 4 9
1000 900 290 129 62 31
• •
-JW tfc 4 . . .
I •
6 7 8 90
16 8 4 2 m
-56 f '
60.
o
a o o
♦ O q
V)
O
Z ■
<
( />
§>
C L
U J
a
DO
■ a
• CONTINENTAL M ARGIN (SHELF AND STRAIT)
a OUTER DELTA
* GULF OF M ARTABAN
6 SEAHIGH
O OUTER SLOPE, RIDGE
□ CENTRAL BASIN FLATS
■ B A Y OF BENGAL
228
between 2 and 100yu.
F igure 48 p re s e n ts s o r tin g and skewness p lo tte d
a g a in s t mean d ia m e te r. The b e s t as w e ll a s th e p o o re st
s o rte d sedim ents are the c o a rs e r sands o f th e Malay
C o n tin e n ta l Margin and O uter D e lta p ro v in c e s , w ith phi
sta n d a rd d e v ia tio n s ra n g in g between 0*5 and 3 .5 . The
f i n e r C o n tin e n ta l Margin and O uter D e lta sed im en ts are
more uniform ly s o r te d ; a l l sam ples w ith a mean in the
s i l t and clay s iz e s ra n g in g in s o r t i n g between 2 and 3 .4
phi sta n d a rd d e v ia tio n s .
Sedim ents o f the in n e r d e l t a p r e s e n t a narrow
range o f s o r tin g , between 1 .5 and 2 .1 phi sta n d a rd d e v ia
t i o n s . Most o f the homogeneous s i l t y c la y s o f th e C e n tra l
B asin Province are s im ila r ly w e ll s o r te d , a lth o u g h th o se
sam ples w ith se a h ig h -d e riv e d f o r a m ln if e r a l sands a re
p o o rer s o r te d , w ith phi s ta n d a rd d e v ia tio n s re a c h in g 3 .0 .
A much more uniform p ic tu r e i s p re se n te d by th e phi
skewness v ersu s phi mean d ia m e te r p 2 o t. W ith th e excep
tio n o f th e c o a rs e r Malay C o n tin e n ta l Margin and O uter
D e lta province sa n d s, th e sedim ents o f th e Andaman B asin
e x h ib it a w ell-m arked tre n d from n e g a tiv e ly -sk e w e d f in e
sedim ents to p o s itiv e ly skewed c o a rse se d im e n ts. The G ulf
o f M artaban Province s i l t y c la y s a re a g a in t i g h t l y grouped,
and a re a l l s l i g h t l y n e g a tiv e ly skewed. S i l t y c la y s o f
th e C e n tra l B asin a re more lo o s e ly grouped, and range in
phi skewness from - 1 .2 to 0 .1 . The sands o f th e O uter
Figure 48• V a ria tio n s o f s o rtin g and skewness
w ith mean d iam eter.
230
MEAN DIAMETER, u
62 3 1 S 0 0 250 125 1000
15 -
3 0
oo
o °o
o o
2 5 -
Oo o o
-20 tO 20 -
INNER DELTA
1 5 -
-L O 1 .0 -
0 5 0 5 -
A A (O
A A m A i ______________
CENTRAL BASIN F L A T S ^ ^ ' - - - ^
00
OF MARTABAN
- 1.0
- 25 -
- 3. 0
-15
MEAN DIAMETER. 0
231;
D elta Province fo llo w th e tr e n d , th e f i n e r sands w ith a
p o s itiv e skew o f 1 .0 , and the c o a rs e r sands a t ta i n in g a
!
skewness of 2.3* Malay C o n tin e n ta l Margin sedim ents w ith :'
mean diam eters f i n e r than 125 p are stru n g out along the
e n t ir e tre n d . The c o a rs e r sedim ents of t h i s province
are skewed very e r r a t i c a l l y , as I s c h a r a c t e r i s t i c o f a l
most any coarse sedim ent; v alu es span the e n t ir e phi
skewness spectrum , from -2 .8 to 3 .3 .
!
Calcium Carbonate
M olluscan and c o r a llin e fragm ents compete w ith
fo ra m in ife ra as the dominant carbonate component on the
Malay S h e lf. The o u te r D e lta , c e n tr a l b asin and Andaman-
Nlcobar Ridge show a g r e a te r dependence on fo ra m in ife ra
as a carbonate c o n s titu e n t, as i l l u s t r a t e d in a p lo t of
fo ra m in ife ra number a g a in s t t o t a l calcium carbonate (F ig.
49), and in an a r e a l c h a rt o f fo ra m in ife ra number per
gram (F ig . 50).
V eight p e rce n tag e s o f calcium carbonate in th e
su rfa ce sedim ents o f th e Andaman B asin are i l l u s t r a t e d in
F igure 51. T his param eter provides e x c e lle n t sig n a tu re
fo r the v ario u s sedim entary p ro v in c e s.
The f lo o r o f th e G ulf of Martaban e x h ib its the
i
lows8t p ercentages o f calcium ca rb o n a te, w ith minimum
v alu es o f 0.7 p e rc e n t in c re a s in g southward to approxim ately
5 percent a t th e O uter D elta boundary. Carbonate lm-
F igure 49. V a ria tio n o f F o ra m ln ife ra l number
w ith t o t a l calcium carbonate c o n te n t.
c> c
PE R
r
> c
CEN T C
a c
3 C
U.CIUM
x
3
CARBON
k c
3 C
ATE
n c
3 C
1 • •
3 C
X
3
••
•
•
•
O '
n
«*
•
►
1
: . •
1 0 1 0 0 0 IO j C
FORAMINIFERA/GRAM
*
» •
•
• •
•
•
, • •
• •
•
•
•
• 1
• • •
• • #
»• •
• •
• •<
• •
•
•
• •
• •
• •
i
•
I
•
•
o
o
o
Oj
i
• •
•
•
•
•
• • •
•
• .
t r
o
o
6
o
•
•
•
•
b
o
o
F ig u re 50* C hart o f F o ra m in ife ra numbers in
s u r fa c e sed im en ts o f th e Andaman
B a sin .
B A Y
BENGAL
I0»-
%
0-100
100-1000
E l 1000-10/500
Q 10,000-100,000
IOOjOOO-1,000,000
FORAMINIFERA PER GRAM
(Frorichs. 1967)
Figure 51* Weight percentages o f calcium
carb o n ate in su rfa ce sedim ents of
th e Andaman B asin.
CaCO., %
H r£ £ £
l l p l ,
'.vW wI'A
238
poverishm ent in t h i s a re a i s due to th e trem endous quan
t i t i e s o f c a rb o n a te -p o o r s i l t and d a y w hich pour o u t o f
th e Irraw addy D e lta , and because the s tr o n g s e a s o n a l
f lu c t u a t i o n o f s a l i n i t y , coupled w ith th e extreme
t u r b i d i t y o f th e w ate rs are e o o lo g lo a lly u n fa v o ra b le f o r
m o llu sc s.
C le a re r w ater o ver the w estern O u ter D e lta , coupled
w ith th e I n tro d u c tio n o f n u t r l e n t - r l o h d eep er Bay o f
Bengal w ater during th e southw est monsoon, a llo w s m o llu scs
and o o ra ls to f l o u r i s h . F o ram ln lfera and th e rem ains o f
m ollusos c o n trib u te h ig h p ercen tag es o f oalcium c a rb o n a te
to the a r e a . The sands o f th e O uter D e lta a t t a i n
c a rb o n a te p e rc e n ta g e s as h igh as 20 p e r o e n t.
South o f th e main a re a of d e lta -s e d lm e n t d e p o s i
t io n m olluscan a c t i v i t y in c re a s e s and I s r e f l e c t e d in
c a rb o n a te p e rc e n ta g e s whloh in c re a s e southw ard o v er the
M ergul T errace to a maximum in exoess o f 60 p e ro e n t over
Weh Bank. S easonal upw elllng occurs a t th e c o n tin e n ta l
m argin d u rin g th e n o r th e a s t monsoon (Deoember to F ebru
a ry ) ( tfy rtk l, 1961), causing su rfa o e w a te rs to beoome r lo h
in n u t r i e n t s . The oarbonate maximum a lo n g th e te r r a o e I s
enhanced by slow r a t e s o f d e p o s itio n , a s ev id en ced by a
P le is to c e n e fauna from the top o f SERRANO Core 221 (W. E.
F re r io h s , p e rso n a l communication; F ig . 25) and r e l i o t
o o r a ls (S e w e ll, 1925a). The v a lle y whloh c o n s t i t u t e s the
n o rth e rn end o f the Sumatra S h e lf Basin p ro b ab ly tr a p s
most te rr ig e n o u s m a te r ia l b efo re i t can reac h the so u th ern
end o f th e t e r r a c e .
Westward the h ig h -c a rb o n a te p a tte r n o f the so u th ern
c o n tin e n ta l margin merges w ith th e h ig h -c a rb o n a te
p e rc e n ta g e s in th e g lo b ig e rin a ooze b lan k eted Sew ell Sea
mount. S e p a ra tio n o f th e two a re a s is s t i l l in d ic a te d by
a tro u g h of somewhat low er carbonate v a lu e s between th e
S ew ell and Mergui T errace h ig h s .
The main avenue o f sedim ent tr a n s p o r t from the
d e l t a to th e C e n tra l Andaman Trough i s r e f l e c t e d in a
band o f low carb o n ate p ercen tag es emanating from th e G ulf
o f M artaban. In th e C e n tra l B a sin , th e re i s a tendency
f o r ca rb o n ate p erce n tag e s to decrease w ith in c re a s in g
dep th (F ig . 5 2 ). Samples taken on o r n e a r g lo b ig e rin a -
ooze covered seah ig h s do n o t conform to th e p a tte r n .
Lowest p e rce n tag e s in th e C e n tra l Basin a re found in
sam ples from th e zone o f lam inated sedim ent.
The ea stw ard -flo w in g c u r re n ts which p r e v a il during
th e southw est-m onsoonal season o f maximum Irrawaddy
se d lm en t-d i8 ch arg e d ep riv e th e w estern C e n tra l Basin a re a
o f te rrig e n o u s m a te r ia l. Consequently sedim ents o f th e
Narcondam-Barren Basin and i t s surroundings a re r e l a t i v e l y
en ric h e d in calcium carbonate and co n tain 10 to 20 p e r
c e n t. In t h i s a re a Alcock Seamount is a lo c a l oarbonate
maximum, i t s f o r a m in if e r a l o o n te n t com prising more than
50 p ero en t o f i t s sedim ent.
F igure 52. V a ria tio n o f calcium carbonate
c o n ten ts w ith depth of w ater in
th e Andaman B asin.
241
PERCENT CALCIUM CARBONATE
O 1 0 20 3 0 4 0 9 0
.ANDAMAN-NICOBAR
\ ______ - RIDGE
CENTRAL BASIN FLATS
WITHOUT SLUMP LAYERS.
CENTRAL BASIN FLATS
NEAR SEAHIGHS; SLUMP
□ LAYERS
O 2-
■ a
* - 3-
• CONTINENTAL MARGIN (SHELF AND STRAIT)
* OUTER OELTA
a GULF OF MARTABAN
♦ SEAHIGH
CORES WITH
LAMINATIONS
O OUTER SLOPE, RIDGE
o CENTRAL BASIN FLATS
■ BAY OF BENGAL
242
A com bination o f v ig o ro u s c o r a l grow th and lim ite d
te r r ig e n o u s so u rce s make th e Andaman-N1cobar Ridge and
I n v i s i b l e Bank a re a s o f h ig h calcium c a rb o n a te . I n s u l a r
s h e l f sedim ent g e n e ra lly exceeds 50 p e r c e n t and even th e
p a r t i a l l y s h e lte r e d slo p e e a s t o f th e Andaman I s la n d s i s
covered w ith sedim ent c o n ta in in g between 20 and 50 p e r
c e n t ca lciu m c a rb o n a te .
O rganic H a tte r
In g e n e ra l th e sedim ents o f th e Andaman B asin show
a p o s it iv e c o r r e la tio n between c la y c o n te n t and o rg a n ic
c o n te n t (F ig . 5 3 ). The a v a i l a b i l i t y o f c la y s u r f a c e s f o r
th e a d s o rp tio n o f o rg a n ic m a tte r i s o n ly one o f s e v e r a l
f a c t o r s which determ ine th e d i s t r i b u t i o n o f o rg a n ic
carbon and n itr o g e n in th e b a s in . E xam ination o f F ig u re
53 r e v e a ls a grouping o f v a lu e s f o r sam ples from th e
d i f f e r e n t p ro v in c e s , each group c o r r e l a t i n g in d i f f e r e n t
d e g re e s w ith c la y c o n te n t.
F ig u re s 54 and 53 p re s e n t th e a r e a l d i s t r i b u t i o n s
o f o rg a n ic carbon and n itr o g e n , r e s p e c t i v e l y . N earshore
d e l t a i c sedim ents o f th e G u lf o f M artaban P ro v in c e ap
p a r e n tly e n t e r th e se a w ith r e l a t i v e l y low am ounts o f
o rg a n ic m a tte r . O rganic carbon i s p r e s e n t in q u a n t i t i e s
under 0*5 p e ro e n t, and o rg a n ic n itr o g e n c o n ta in s only
0 .0 5 to 0 .1 0 p e ro e n t n itr o g e n . In th e so u th e rn p o rtio n o f
th e G u lf o f M artaban o rg a n ic c o n te n ts in o r e a s e , p robably
F igure 53* V a ria tio n o f organic A itrogen
o o n te n t w ith clay c o n te n t.
244
.30-
2 5-
Z
UJ
(9
O
O C
t 20-
<
o
O C
o
UJ
o
O C
U J
Q .
1 3 •
1 0 -
« A
•
.05 • •
» 04™ •
f t **
• CONTINENTAL M A R G IN
ft OUTER DELTA
ft GULF OF M ARTABAN
♦ SEAHIG H
O OUTER SLOPE, RIO G E
□ CENTRAL BASIN FLATS
■ B A Y OF BENGAL
EMBAYMENT BETWEEN
PHUKET AND LANGKAW I
GULF OF M ARTABAN
MALACCA STRAIT
SHELF BREAK
.00 f -■ r -i, " , » ' , | , - ,
0 10 20 30 40 50 6 0 70 80
PERCENT CLAY
90
«
F ig u re 54* A re a l d i s t r i b u t i o n o f o rg a n ic carbon
In s u rfa c e sedim ents o f th e Andaman
BaBln*
ORGANIC
CARBON. X
J . V ?
Figure 55. A real d is tr ib u tio n o f o rg an ic
n itro g en In surfaoe sedim ents o f
the Andaman B asin.
248
BENGAL
• •!»?#!
,y.v*v.v.
. ■ -
p . v :
L v .
itK jV ir. -
•*3-
ff-SW Sjsaii
<0.05
0.05- 0.10
W h 0.10 0 .2 0
>0.20
■ ■ i
• 4* §5*
PERCENT NITROGEN
249 i
I
in consequence o f abundant burrow ing organism s (Sokolova
and P a s te rn a k , 1964) as w e ll as accum ulation o f p la n k to n lc
m a te r ia l.
Sedim ents o f th e O uter D elta and Malay C o n tin e n ta l
Margin a re im poverished in o rg an ic m a tte r due to t h e i r
low c la y c o n te n ts and slow d e p o s ltlo n a l r a t e s . The
h ig h e s t p e rc e n ta g e s o f o rg an ic m a tte r a re encountered in
th e C e n tra l B asin P ro v in c e, where o rg an ic carbon c o n te n ts
exceed 1 .0 p e rc e n t and o rg an ic n itro g e n i s p re s e n t in
amounts h ig h e r than 0 .1 0 p e rc e n t. Alcock and Sew ell se a
mounts and th e h ig h s com prising the Narcondam-Barren
v o lc a n ic tre n d form lo c a l a re a s which a re poor in organic
m a tte r, because c lay c o n te n ts a re r e l a t i v e l y low and
accum ulation r a t e s a re slow .
W ithin th e c e n tr a l b a sin th e a re a o f g r e a t e s t a c
cum ulation o f o rg a n ic m a tte r i s th e Sumatra S h e lf B asin.
Samples from th e b a sin average about 2 .0 p erce n t o rg an ic
carbon and o ver 0 .2 p e rc e n t o rg a n ic n itro g e n . F acto rs
which a re re s p o n s ib le f o r th e se h ig h q u a n t itie s o f
o rg an ic m a te r ia l a re th e sem i-enclosed n a tu re o f the
s h e lf - b a s ln ; th e In te rm e d ia te depths o f the b a sin f lo o r ,
which a re optimum depths in o th e r a re a s (Van A ndal, 1964);
and th e h ig h c lay c o n te n t o f th e s h e l f - f l o o r sedim ents.
The r a t i o o f o rg a n ic carbon to o rg an ic n itro g e n
in th e Andaman B asin sedim ents averages 9» somewhat low er
than th e g e n e ra l average quoted by T rask (1939)* A ll but
th e most extrem e v a lu e s ran g e between 5 and 18. Examina
tio n o f a p lo t o f o rg a n ic carbon v e rs u s o rg a n ic n itro g e n
(F ig . 5 6 ) r e v e a ls a p r o v in c ia l g ro u p in g . S u r p r is in g l y ,
th e n e a rs h o re d e l t a i c se d im en ts have a low er th a n average
r a t i o , in d ic a t in g a l a r g e r p ro p o rtio n o f n itr o g e n th an th e
a v e ra g e . I t has g e n e r a lly been assumed t h a t o rg a n ic
m a tte r from te r r ig e n o u s so u rc e s i s r i c h e r in carbon
r e l a t i v e to n itr o g e n . Thus, humio a c id s from v a rio u s
s o i l s have 60 p e rc e n t carbon and o nly 5 p e rc e n t n itro g e n
(Degens, 1965); and v e g e ta l d e b r is s u p p lie d by r i v e r s and
c o a s ts may be assumed to have v ery h ig h carbon and very
low n itr o g e n c o n te n ts . One p o s s ib le answ er to th e ab
n orm ally low c a rb o n -n ltro g e n r a t i o s o f th e n e a rs h o re
d e l t a i c sedim ents may be a v e ry r a p id r a t e o f d e p o s itio n .
B a c te r i a l d ecom position o f o rg a n ic m a te r ia l i s known to
cause a p r e f e r e n t i a l l o s s o f n itr o g e n w ith d ep th in th e
sed im en t, r e s u l t i n g in a downward in c r e a s e in ca rb o n -
n itro g e n r a t i o (Van A ndal, 1 9 6 4 ). The g r e a t e s t in c re a s e
o ccu rs in th e upperm ost few c e n tim e te rs o f se d im en t. A
very r a p id r a t e o f ac cu m u latio n o f sedim ent may keep th e
p r e f e r e n t i a l lo s s o f n itr o g e n to b a c t e r i a l a c t i v i t y a t a
minimum. T his does n o t e x p la in th e I n i t i a l l y low
c a rb o n -n itro g e n r a t i o , how ever, and t h i s problem i s worthy
o f f u r t h e r i n v e s t i g a t i o n . P o s s ib ly a unique p o p u la tio n
o f n it r o g e n - f i x i n g b a o t e r i a l i v e s in th e nearB hore zone.
The a re a s c h a r a c te r iz e d by c o a rse sed im en ts and
F igure 56. V a ria tio n o f o rg a n ic carbon w ith
o rg an ic n itr o g e n .
0 .S 0 -
ats-
0 .15-
0.10
• CONTINENTAL MARGIN
* OUTER OELTA
o »
* GULF OF MARTABAN
0 .0 5 -
♦ SEAHIGH
O OUTER SLOPE, RIDGE
a CENTRAL BASIN FLATS
■ BAY OF BENGAL
0. 00-
0.0 05 10 15 2 0
PERCENT ORGANIC CARBON
2 .5
slow r a t e s o f d e p o s itio n such as the O uter D e lta s u r f a c e ,
Malay C o n tin e n ta l M argin, and seah ig h s a re c h a r a c te r iz e d
by h ig h o rg an io c a rb o n /n itro g e n r a t i o s . N itro g en i s
p ro b ab ly l o s t to b a c t e r i a l decay w h ile th e sedim ent r e
m ains a t th e s u rfa o e b efo re b u r i a l . The a r e a e a s t o f th e
Andaman I s la n d s has e s p e c i a lly h igh o a rb o n /n ltro g e n r a t i o s
w hich p o s s ib ly a re enhanced by a l o c a l so u rce o f v e g e ta l
m a t e r i a l •
C a rb o n /n itro g e n r a t i o s in sedim ents o f th e C e n tra l
B asin P ro v in ce a r e q u ite uniform , ra n g in g between 7 and 8 .
These g e n e r a lly low v a lu e s r e f l e c t a p la n k to n lc d e r iv a tio n
(Van Andal and Postm a, 1954)* In th e Sum atra S h e lf B a sin ,
r a t i o s a r e s l i g h t l y low er, and approach th e c a rb o n -
n itr o g e n com position o f plan k to n (T rask , 1939)*
Clay M ineralogy
As m ight be ex p e cte d , th e d i s t r i b u t i o n o f m inus-2
f r a c t i o n o f Andaman B asin sedim ents shows g r e a t e r independ
ence from physiography than o th e r sedim ent p a ra m e te rs.
D is tr ib u ti o n p a tte r n s o f th e v a rio u s c la y components c u t
ao ro ss th e sedim ent p ro v in ce s o u tlin e d in P ig u re 44.
M o n tm o rlllo n lte and m ix e d -lay er c la y s were s tr o n g ly
predom inant in most o f the sam ples. The h ig h e s t v a lu e s ,
a v e ra g in g 75 p e ro e n t, o ccu r in th e deep w estern and
o e n t r a l p o r tio n s o f th e b a sin (F ig . 57)• D e lta io s e d i
ments a r e f a i r l y uniform in m o n tm o rlllo n lte c o n te n t,
F ig u re 57. P e rc e n ta g e s o f combined m ontm orll-
l o n i t e and m ix e d -la y e r c la y s in
s u r fa c e sed im en ts o f th e Andaman
B a sin .
9 » » E
I9 » -
10* -
5*-
BAY
BENGAL
< 9 0 %
B S 0 - 6 0 %
E 2 6 0 - 7 0 %
7 0 - 8 0 %
> 6 0 %
•p e r c e n t" MONTMORILLONITE
averaging 60 p e rc e n t. Clay c o n te n t o f sedimentB o f the
Malay S h e lf i s more e r r a t i c , alth o u g h p e rc e n ta g e s are
g e n e ra lly low er in t h i s a re a th an in o th e r p a r ts o f the
b a s in . A tongue o f m o n tm o rillo n ite -p o o r sedim ent
emanates from the v i c i n i t y o f th e mouth o f th e Pakchan
R iv er, which i s th e only m ajor r i v e r d ra in in g th e Malay
P e n in su la . This tongue extends westward a c ro s s the s h e lf
to th e v i c i n i t y o f Sew ell Seamount. Samples from th e
Andaman-Nicobar Ridge were found to c o n ta in as much as 92
p erce n t m o n tm o rillo n ite . H ig h -m o n tm o rillo n ite sedim ent
o f th e Malacca S t r a i t s extends in to th e Sumatra S h e lf
B asin, where sedim ents c o n ta in an average o f 75 p erce n t
m o n tm o rillo n ite . A lo c a l zone w ith low er p ercen tag es o f
the m in e ra l i s s it u a te d over th e so u th ern end o f Mergul
T e rra c e .
K a o lln ite c o n te n ts complement m o n tm o rillo n ite
p e rc e n ta g e s, being low er where m o n tm o rillo n ite i s most
abundant ( ? ig . 5 8 ). Thus, th e sedim ents o f th e C e n tral
Basin P rovince c o n ta in between 8 and 12 p e rc e n t k a o l l n i t e ;
d e l t a i c sedim ents average approxim ately 20 p e rc e n t, and
the Malay S h e lf e x h ib its th e h ig h e s t k a o l ln ite c o n te n ts ,
which range between 20 and 40 p e rc e n t. A lo b e , which
probably r e f l e c t s th e g r a n i t i c provenance o f th e Malay
P e n in su la , extends from the Pakohan R iv e r, w ith over 40
pero en t k a o l ln ite a t th e r i v e r mouth d e c re a sin g to 20
p erce n t over Sew ell Seamount. Sedim ents o f th e Malacca
Figure 58. D is trib u tio n o f K a o lln ite in
su rfa c e sedim ents o f th e Andaman
B asin.
, 4 a /
BENGAL
%
0 < 10 %
B 1 0 - 1 9 %
0 19-20 %
E 3 2 0 - 5 0 %
H 5 0 - 4 0 %
PERCENT KAOLINITE
259
S t r a i t c o n ta in only 10 p e rc e n t k a o l l n i t e , and s im ila r
percentage's In th e Sum atra S h e lf B asin I n d ic a te a M alacca
S t r a i t p ro v en an ce. Weh Bank d is p la y s an a f f i n i t e to Malay
S h e lf se d im en t, c o n ta in in g 20 p e rc e n t k a o l l n i t e . The
Andaman-Nicobar Ridge I s poor In k a o l l n i t e , c o n ta in in g
l e s s th an 10 p e r c e n t.
The p a t te r n o f l l l l t e d i s t r i b u t i o n I s n o t as
d e f i n i t e as th o s e f o r m o n tm o rillo n ite and k a o l l n i t e .
P e rc e n ta g e s o f l l l l t e g e n e r a lly range between 10 and 20
p e rc e n t o v er th e e n t i r e b a s in . There i s a vague tendency
f o r p e rc e n ta g e s to be h ig h e s t In sedim ents o f th e d e l t a
and o f th e n o rth w e s te rn p o r tio n o f th e b a s in .
The d e te rm in a tio n o f l l l l t e was ren d ered u n c e rta in
by th e p o s s ib le a d m ix tu res o f g la u c o n ite . A ttem pts to
d i f f e r e n t i a t e between th e two m in e ra ls by c o lo r and by
th e ex am in atio n o f t h e i r 060 peaks a f t e r th e te c h n iq u e
reoommended by Warshaw and Roy (1961) were u n s u c c e s s fu l.
"G la u c o n ite " in c lu d e s such a wide d i v e r s i t y o f m in e ra ls
( P r a t t , 1962) t h a t d i f f e r e n t i a t i o n o f Andaman B asin
v a r i e t i e s may be im p o ssib le and im p r a c tic a l.
C h lo r ite i s th e s m a lle s t component o f Andaman
B asin se d im e n ts, as may be expected from an a r e a in low
l a t i t u d e s ( G r i f f i n and G oldberg, 1963; G oldberg, 1964).
In g e n e r a l, c h l o r i t e com prises a very m inor p e rc e n ta g e o f
th e se d im en ts in th e n o rth e rn and w estern p o rtio n s o f th e
a r e a and i s e i t h e r a b s e n t o r d e te o ta b le only in tr a c e
260
amounts on th e Malay S h e lf and s o u th e a s te rn p o r tio n s o f
th e Andaman B a s in . The Irraw addy and Salween r i v e r s a p
p e a r to be th e l a r g e s t so u rc e s o f c h l o r i t e , w hich makes
up between 4 and 6 p e ro e n t o f th e n e a rs h o re d e l t a i c s e d i
m en ts. T e r t i a r y sa n d sto n e s o f th e Burmese C e n tra l B e lt
a re r e p o rte d to c o n ta in 2 to 7 p e rc e n t o f c h l o r i t e
( S a r in , 1963)* W hether o r n o t c h l o r i t e in th e form o f
g l a c i a l f l o u r from th e T lb e tla n P la te a u i s c a r r ie d by
th e Salween R iv e r can n o t be d eterm ined from th e p re s e n t
d a ta .
L ocal h ig h p e rc e n ta g e s o f c h l o r i t e were noted
n e a r th e Andaman-Nicobar R idge, l o g i c a l provenance being
th e r i d g e 's s e r p e n t l n e - o p h l o l l t e c o re . L ocal q u a n t i t i e s
n o t ex ceeding 3 p e rc e n t alo n g th e Malay S h e lf may r e f l e c t
o u tc ro p s o f b a s a l t i c v o lc a n lc s .
S ources
By f a r th e l a r g e s t so u rce o f Andaman B asin s e d i
ment i s th e Irraw addy R iv e r. S e v e ra l f a c t o r s c o n tr ib u te
tow ard making t h i s r i v e r ' s sedim ent d isc h a rg e one o f th e
l a r g e s t in th e w o rld . The r i v e r 's catchm ent b a s in i s
u n d e rla in by f r i a b l e , e a s i l y eroded T e r tia r y se d im e n ts.
A pproxim ately 90 p e ro e n t o f th e Irraw addy B asin r a in
f a l l s d u rin g th e 4 months from June to Septem ber. T his
s e a s o n a lity enhances th e eroB ive powers o f th e ru n o ff ,
p a r t i c u l a r l y in th e d r i e r n o rth e rn p o rtio n s o f th e b a s in .
261
In t h i s a r e a , c a lle d th e Dry B e lt (C h h ib b e r, 1934), th e
b u r s t o f th e m onsoonal r a i n s c a u ses f la s h - f lo o d in g which
c a rv e s deep, s te e p w a te rc o u rs e s and e x te n s iv e b ad lan d s
topography in to s o f t P lio c e n e s e d 'v e n ts and P le is to c e n e
t e r r a c e d e p o s its (Stam p, 1 9 4 0 ). A pproxim ately 90 p e rc e n t
o f th e Irra w a d d y 's sed im en t lo a d i s d isc h a rg e d in th e 5
months from June th ro u g h O cto b er (Gordon, 1885)*
M easurements o f w a te r d is c h a rg e and sedim ent lo ad
were c a r r ie d o u t a t th e head o f th e d e l t a from 1869 to
1879 by Gordon (1885)» who e s tim a te d t h a t 265 m illio n
m e tric to n s o f sedim ent i s c a r r ie d to th e head o f th e
d e l t a a n n u a lly . To t h i s f ig u r e Stamp (1940) added an
e s tim a te d 50 m illio n to n s o f d is s o lv e d m a t e r i a l . G ordon's
te c h n iq u e f o r m easuring sedim ent lo ad i s n o t d e s c rib e d in
th e l i t e r a t u r e and may be in e r r o r by a la r g e amount.
The sedim ent b ro u g h t to th e d e l t a head does n o t
s u f f e r a n e t lo s s o v e r th e s u b a e r ia l d e l t a , which i s n o t
b u ild in g up i t s s u rfa c e (Stam p, 1 9 4 0 ). Most o f th e s e d i
ment i s d isc h a rg e d i n t o th e Andaman Sea th ro u g h th e 10
d e l t a m ouths, o f w hich th e Bya R iv e r i s th e p r i n c i p a l .
The Salween R iv e r, a lth o u g h com parable to the
Irraw addy in le n g th , h as a much s m a lle r catchm ent b a s in
o f o nly 137,300 km^ w hich c o n s i s t s m ainly o f deep g o rg es
cu t in to r e s i s t a n t g r a n i t e s , metamorphio ro ck and
in d u ra te d P a le o z o ic se d im en tary ro c k . Thus, th e an n u al
Salween R iv e r r u n o f f , which i s only o n e - te n th a s la r g e as
th a t o f the Irraw addy R iv e r, o o n trib u te s an even sm a lle r
percentage o f sedim ent compared to the Irra w a d d y 's lo a d .
The I n s i g n i f i c a n t s iz e o f th e Salween D elta r e f l e c t s t h i s
low sedim ent d is c h a rg e . The Malay P e n in su la I s a
r e l a t i v e l y minor sedim ent s o u rc e . Few t r i b u t a r i e s d ra in
the p e n in s u la , and most o f the ru n o ff i s to the G ulf o f
T hailand; alth o u g h th e a b s o lu te q u a n tity o f w ater from
th is a re a i s la r g e , i t probably c a r r i e s l i t t l e sedim ent.
Another a re a which c o n tr ib u te s only i n s i g n i f i c a n t
amounts of sedim ent to th e Andaman B asin i s the Andaman-
N icobar R idge. The rid g e is la n d s have sm all a re a s and
t h e i r d iv id e s a re a p p re c ia b ly to the e a s t of the i s l a n d 's
ax es. C onsequently most o f the sm all volume b f is l a n d -
A
d eriv e d sedim ent makes i t s way in to the Bay o f B engal.
The narrow in s u l a r s h e lv e s , to g e th e r w ith banks o f the
e a s te rn p o rtio n o f the r id g e , provide sm all q u a n t itie s o f
reworked b io g en ic d e t r i t u s to a d ja c e n t b a sin f l o o r s .
This la c k o f im portance o f th e rid g e as a sedim ent source
o o n tra s ts s tro n g ly w ith th e provenanoe s i t u a t i o n which
p re v a ile d in the Andaman-Nloobar a re a during th e Paleogene,
and th e c o n tr a s t has f a r - r e a c h in g im p lic a tio n s which
s h a ll be d isc u sse d l a t e r .
The Malacoa S t r a i t i s a souroe o f sedim ent f o r the
so u th ern Andaman B a sin . Sedim ent shed from Sumatra and
the Malay P en in su la i s swept in to the Sumatra S h e lf B asin
are a by the c u rre n ts o f v a ry in g s tr e n g th whioh pass from
263
the s t r a i t in to the Andaman Sea th ro u g h o u t the y e a r.
B iogenic a c t i v i t y i s su b o rd in a te only to te rrig e n o u s
sou rces in q u a n t itie s o f sedim ent su p p lie d to th e Andaman
i
B asin . The average calcium carb o n a te c o n te n t o f su rfa c e
sedim ents over the e n t i r e b a sin averages 25 p e rc e n t. This
fig u re does n o t mean t h a t o n e -fo u rth of the sedim ent
su p p lied to the b asin per u n i t time i s calcium c a rb o n a te ,
inasmuch as the h ig h e s t carb o n a te v a lu e s are observed in
a re a s o f slo w est d e p o s itio n .
V olcanic supply i s p r a c t i c a l l y n i l a t th e p re se n t
tim e. Only th re e samples from the e n t i r e b asin co n tain e d
v o lc an ic m a te r ia l, and then only in n e g lig ib le amounts.
A uthigenic m a te r ia ls a ls o were observed in n e g lig ib le
amounts.
T ra n sp o rt and D eposition
The enormous q u a n t i t i e s o f Irraw addy R iv e r sedim ent
which e n te r the Andaman Basin a re composed o f s i l t and
c la y w ith sm all amounts o f san d . As the sedim ent e n te rs
the sea the c o a rs e r m a te r ia l i s d e p o s ite d , form ing the
d e l t a 's sandy beacheB and sand b a r s , and c o n trib u tin g to
the coarse sedim ents o f the O uter D e lta . H ost o f the
suspended f in e sedim ent i s d is p la c e d eastw ard by the p re
v a ilin g c u rre n ts o f th e southw est monsoon se aso n . A fte r a
s h o rt p e rio d of tr a n s p o r t in the m arine environm ent the
sedim ents f lo c o u la te and a t the same time re a c h the
264
q u i e t e r w ate rs o f th e e a s te rn d e l t a s h e l f , where the
bulk o f the sedim ent i s d e p o s ite d .
The c o a rs e r f r a c t i o n o f th e Irraw addy sedim ent
which I s d e p o s ite d In the O uter D e lta a r e a undergoes
winnowing by winds and waves d riv e n by b o th monsoons.
During th e n o r th e a s t monsoon th e d e l t a s u rfa c e e x p e rie n c e s
th e s tr o n g e s t c u r r e n ts of the y e a r , w hich flow o u t In to
th e Bay o f B engal. I t Is d u rin g t h i s season t h a t th e
o nly I 08B o f Irraw addy sedim ent to th e Bay o f Bengal ta k e s
p la c e . The lo s s i s p r in c ip a lly o f sa n d s, f u r t h e r d e p riv
in g th e Andaman B asin o f c o a rs e r se d im en t.
The s u rfa c e o f the Irraw addy D e lta i s t e c t o n i c a l l y
u n s ta b le . Known se ism ic e v e n ts , th e p o s tu la te d n o r th - .
n o rth e a stw a rd e x te n sio n o f the Malay C o n tin e n ta l Slope
boundary f a u l t s through th e G ulf o f M artaban, th e d i f
f e r e n t to p o g rap h ic e x p re ssio n o f th e d e l t a s u rfa c e on
e i t h e r s id e o f th e M artaban Canyon, and th e p h y sio g ra p h ic
evidence o f f a u l t i n g in s u rfa c e sedim ents a t th e n o rth e rn
end o f th e C e n tra l Andaman Trough a l l in d ic a t e th a t
te o to n ism i s a f f e c t in g the d o lta . P r o f i l e s a c ro s s the
d e l t a su g g e st th a t the su rfa c e has undergone r e c e n t
ohanges, and t h a t a p r e fe r re d slo p e o f th e d e l t a su rfa c e
i s being r e - e s ta b lis h e d by d e p o s itio n . T e c to n lo a lly -
lnduced changes in the c o n fig u ra tio n o f th e d e l t a s u rfa o e
may be th e p r ln o lp a l o o n tro l which d eterm in es s i t e s o f
maximum d e p o s itio n . The p r e s e n tly fa v o re d s i t e i s the
265
the a re a so u th e a st o f the d e l t a 's s u b a e r ia l lobe and south
o f the G ulf of M artaban. Martaban Canyon may i t s e l f be
s t r u c t u r a l l y c o n tro lle d , Inasmuch as i t l i e s along the
tre n d of a major f a u l t zone.
F a u ltin g in the Martaban Canyon may be im portant
in p ro v id in g d e lta sedim ent to the C e n tra l Trough. Be
s id e s p ro v id in g slumped sedim ent blocks to the hummocky
n o rth e rn end o f the tro u g h , the p h y sio g ra p h ic a lly and
t e c to n l c a l ly u n sta b le a re a may be an im p o rtan t tu r b id it y
c u rre n t so u rc e . Although few laminae in d ic a tiv e o f
tu r b id it y c u rre n ts were observed on the b asin f l o o r , the
lam inae were always observed in cores from the C e n tra l
Trough. I t i s conceivable th a t the basinw ard tr a n s p o r t
i s accom plished in la rg e p a rt by l a t e r a l d e n s ity flow s,
and th a t the homogenous deeper sedim ents may be oomprised
o f b oth hem ipelaglc la y e rs and s i l t y clay t u r b i d i t e s .
Even where no lam inae were observed, d e h s ity -flo w d e p o s its
may s t i l l have been d ep o sited and subsequently mixed by
burrowing organism s•
L en sity -flo w d e p o s its occur most fre q u e n tly in
sedim ents of the n o rth e rn p o rtio n o f the Sumatra S h e lf
B asin . This abundance o f lam inae may be due in p a r t to
the la r g e r adm ixtures o f c o a rs e r m a te ria l d eriv ed from the
c o n tin e n ta l s h e lf . A pparently the lim ite d amounts o f fin e
sedim ent which reach the Malay S h e lf from the Irraw addy
D e lta , to g e th e r w ith the sedim ent d eriv ed from the Malay
266 :
I
P e n in su la w hich does n o t s e t t l e on th e in n e r s h e l f , a re
swept southw ard by c u r r e n ts and d e p o site d in th e l a t i t u d e
o f Go P h u k et. From h e re th e f in e sedim ents e v e n tu a lly
move to th e f l o o r o f th e Sum atra S h e lf B asin as d e n s ity
flo w s. Sedim ents o f th e M alacca S t r a i t a ls o appear to
e n te r th e Sum atra S h e lf B asin a s d e n s ity flo w s, judging
from the r e s t r i c t i o n o f t h i s sedim ent to th e to p o g ra p h i
c a l l y c o n ta in e d s h e l f b a s in .
H em ipelaglc sedim ent admixed w ith biogenous calcium j
ca rb o n a te com prises a maximum o f 65 p e rc e n t o f the s e d i
ment b la n k e tin g s e a h ig h s . I t may th e re fo r e be assumed
t h a t h em ip elag lc tr a n s p o r t i s v ery im p o rta n t in the d i s t a l
a re a s o f th e Andaman B a sin . The 65 p e rc e n t f ig u r e may
r e p r e s e n t a minimum f ig u r e f o r hem ipelaglc tr a n s p o r t, i f
f in e sedim ent i s winnowed o f f th e s e a h ig h s . This mode of
t r a n s p o r t i s p ro b ab ly th e p r in c ip a l means by which s e d i
ment i s s u p p lie d to th e so u th e rn C e n tra l Trough.
The s tro n g r e l i e f o f th e Andaman-Nicobar Ridge and
o f th e s e a h ig h s in the w estern p a r t o f the Andaman B asin
p erm its slum ping o f sh a llo w -w a te r b io g e n ic d e t r i t u s to
be l o c a l l y im p o rta n t. In th e a re a o f I n v is ib le Bank
sedim ent from th e bank top com prises 23 p e rc e n t o f th e
sedim ent on th e a d ja c e n t b a sin f l o o r . However, slum ping
from se a h ig h s i s p ro b ab ly n o t im p o rta n t f o r th e b asin as a
w hole•
R ates o f D ep o sitio n
267
The Malay C o n tin e n ta l S h e lf may be assumed to be
an a re a o f l i t t l e o r no d e p o s itio n , ju d g in g from the
P le is to c e n e sedim ents w hich occur on th e s h e l f and the
rocky a s p e c t 6 f the s u r f a c e . I n s u la r sh e lv e s o f the
Andaman-Nioobar Ridge a r e a ls o a re a s o f low d e p o s itio n o r
a c tiv e e r o s io n . The a r e a o f th e Andaman B asin beyond
the d e l t a which i s r e c e iv in g s i g n i f i c a n t amounts o f s e d l-
2
ment i s the 175*000 km a r e a between th e d e l t a , the
Andaman-Nicobar Ridge and th e Malay C ontii^ental M argin.
F re rlc h s (1 9 6 7 ), u t i l i z i n g m ic ro p a le o n to lo g ic
te c h n iq u e s, examined c o re s taken w ith in the Andaman Basin
and d efin e d a c o r r e l a t a b l e tim e -h o riz o n w ith in th e co res
which he i d e n t i f i e d as th e P le is to c e n e -R e c e n t boundary.
This h o rizo n i s marked by th r e e f a u n a l tr e n d s , moving up
ward in the c o r e s : " •*'
1 . R a d io la ria in c r e a s e from v ery low numbers to
abundances many o rd e rs o f m agnitude g r e a t e r .
2 . Abundanoe o f th e f o ra m in lf e r G lo b lg e rln a
rubesoens d e c re a s e s .
3* G lo b lg e rln a b u llo ld e s shows a g e n e ra l In c re a se
in abundance.
P o ra m in lfe ra were s u f f i c i e n t l y abundant in the
P le isto o e n e -R e c e n t h o riz o n w ith in one core to p erm it a
rad lo o arb o n d a te . The oore was PIONEER Core 19, taken a t
268
0 9 °2 1 .3 ' N l a t i t u d e , 093 °5 6 .2 ' B lo n g itu d e (P ig . 5 9 ).
Here the fa u n a l boundary o ccu rred between 200 and 380 cm
below the core to p . This s e c tio n o f core was washed
through a 62yu sc re e n , and th e co arse f o ra m in if e r a l
f r a c tio n subm itted f o r ra d io c a rb o n a n a ly s is . Only coarse
m a te r ia l was used because i t h as been found t h a t fin e
carbonate causes la rg e e r r o r s in ra d io c a rb o n d a te s
(E rik sso n and O lsson, 1963). A date o f 8775tl45 y e a rs
was o b ta in e d . U n fo rtu n a te ly , no m a te r ia l was a v a ila b le
f o r d a tin g the su rfa c e se d im en ts. However, F re rlc h s b e
lie v e d th a t t h i s d ate i s v a l id , and t h a t the oceanographic
changes which accompanied the end o f th e P le is to c e n e and
which caused the change in fau n a o ccu rred a t v a rio u s
tim e8 in d if f e r e n t p a r ts of th e w orld.
Based on t h i s s in g le d a te rounded o f f to 9000 y rs
B .P ., and on the depth o f the fa u n a l boundary in o o re s,
F re rlc h s determ ined the th ic k n e s s e s o f sedim ent d e p o site d
sin c e the P le is to c e n e a t 20 s i t e s w ith in the Andaman
Basin and a t two s i t e s in the Bay o f Bengal a d ja c e n t to
the Andaman-Nlcobar R idge. These f ig u re s have been used
to c a lc u la te r a t e s o f n e t accum ulation o f sedim ent to the
n e a r e s t 5 cm/1000 y rs (F ig . 5 9 ).
Accumulation r a t e s w ith in th e Andaman B asin
average 15 cm/1000 y r s , and sedim ent d e p o s itio n a l r a t e s in
th e Bay o f Bengal average 10 om/1000 y r s , r e f l e c t i n g the
entrapm ent of most Irraw addy sedim ent in the Andaman Basin
F igure 59* D ep o sltlo n al r a te s In th e Andaman
B asin, and s e is m ic a lly determ ined
t o t a l sedim ent th ic k n e s s e s .
270
BA Y
BENGAL
DELTA SHELF
1 0 0 - 2 0 0 C m /1000 Yr«
20+a,
%
15
V 4 9 7 8 ©
• 10+
SHELF
AREA OF
NON-DEPOSITION
O
RUSSIAN SEISMIC
s t a t io n ; total
SE0. TH. ■ I.S Km
NBB NARCONOAM-BARREN BASIN
SS SEWELL SEAMOUNT
RATES OF DEPOSITION (C m /1 0 0 0 Yrs)
(Bosin rotes colculoted -from the doto of Frerichs, 1967)
and the s m a lle r sedim ent so u rce s In th e Bay o f B en g al.
2
I f th e 175,000 km a re a o f th e Andaman B asin
a c t i v e l y accum ulating sedim ent has re c e iv e d an av erag e
15 cm th ic k n e s s o f sedim ent In th e l a s t thousand y e a r s ,
t h i s ' th ic k n e s s r e p r e s e n ts a volume o f 26 km^. Of t h i s
volum e, ap p ro x im a tely 20 km^ may be assumed to have been
s u p p lie d by the Irraw addy R iv e r. This f ig u r e was a r r iv e d
a t by s u b tr a c tin g 14 p e rc e n t calcium carb o n ate from the
t o t a l volum e.
Calcium c a rb o n a te c o n te n ts may be used f o r a rough
com parison o f r e l a t i v e d e p o s ltlo n a l r a t e s In th e b a s in
and on th e d e l t a . F o ra m ln ife ra l ca rb o n ate com prises
v i r t u a l l y a l l o f th e n o n -te rrlg e n o u s c o n te n t o f b o th th e
In n e r d e l t a and th e deep b a s in . I f a uniform r a t e o f
c a rb o n a te p ro d u c tio n I s assumed f o r th e e n t i r e Andaman
S e a , th e p ero en tag e o f carb o n ate a t any s i t e should be a
gauge o f the t o t a l q u a n tity o f sedim ent d e p o s ite d .
Sedim ents o f th e In n e r d e l t a a re a o f maximum
d e p o s itio n have an average carb o n ate c o n te n t o f 1 p e r c e n t.
S edim ents o f the b a s in beyond th e d e l ta have an average
v a lu e o f 14 p e rc e n t, ex c lu d in g the a re a s In which s o lu
tio n Is ta k in g p la c e .
I f th e r a t e o f d e p o s itio n In th e b a sin I s 15 cm/1000
y r s , and I f 14 p e ro e n t o f the d e p o s ite d sedim ent I s
c a rb o n a te , a ca rb o n a te la y e r 2 cm th ic k may be assumed to
have been d e p o s ite d In a thousand y e a r s . I f a la y e r o f
272
the same th ic k n e s s r e p r e s e n ts the amount of carb o n ate
d e p o site d on th e d e lta s u rfa c e over the same p e r io d , th e
one p e rc e n t calcium carb o n ate c o n te n t o f the d e l ta i c
sedim ents in d ic a te s a t o t a l d e p o s itio n a l r a te o f 200
cm/1000 y r s .
A volume o f predom inantly te rrig e n o u s sedim ent
3 2
amounting to 180 km would c o l l e c t over the 90,000 km
a re a o f the d e l t a in a thousand y e a rs , using the 200 cm
a c c r e tio n f ig u r e . As n o ted e a r l i e r , approxim ately 20 km^
o f te rr ig e n o u s sedim ent c o l l e c t s in the basin beyond th e
d e l t a . A t o t a l th o u san d -y ear Irraw addy R iver sedim ent
• 3
supply o f 200 knr i s in d ic a te d , d f which 10 p e rc e n t i s
d e p o site d beyond the d e l t a . The fig u re fo r t o t a l I r r a
waddy supply i s about 50 p e rc e n t g r e a te r than the volume
in d ic a te d by G ordon's (1885) f ig u r e , which y ie ld s a f ig u re
o f ap p ro x im ately 100 m illio n m^ o f dry sedim ent per
hundred y e a r s . I f i t i s assumed th a t t h i s sedim ent i s
d e p o site d w ith 50 p e rc e n t o f w ater by w eight, one a r r iv e s
a t a volume o f wet sedim ent o f 150 kmVlOOO y r s . The
assum ption o f uniform ca rb o n a te production over the
e n t ir e se a n e g le c ts the e c o lo g ic a l f a c to r s which may r e
s u l t in low er pro d u ctio n over the d e lta , in which case
the e s tim a te d d e l t a i c d e p o s itio n a l r a t e s would be too
h ig h . N e v e rth e le s s the Gordon e stim a te and the caloium
c a rb o n a te -d e riv e d f ig u re s in d ic a te a supply r a t e o f b e
tween 150 and 200 kmVlOOO y rs o f wet Irraw addy sedim ent,
273
and d e p o s itio n a l r a t e s which a re between 100 and 200
cm/1000 y r s on th e d e lta * These rate B a re a t l e a s t an
o rd e r o f m agnitude g r e a t e r than th o se in th e deeper
p o r tio n s o f th e Andaman B a sin .
The r a t e o f se d im e n ta tio n in th e Andaman Basin
p ro v id e s im p o rta n t c lu e s as to th e age and te c to n ic
^ h i s t o r y o f th e b asin * Neprochnov (1964) conducted
s e i s m i c - r e f l e c t i o n s t u d i e s w ith in th e Andaman B a sin , and
d eterm in ed t h a t th e t o t a l th ic k n e s s o f th e sedim entary
la y e r a t two s t a t i o n s in th e so u th e rn p o rtio n o f the
b a s in i s 1*5 km (Fig* 59)* The sed im en tary column p ro
v id e d s e is m ic r e f l e c t i o n r e tu r n s from, a h o riz o n 500 m
below th e se d lm e n t-w a te r I n te r f a c e as w e ll a s from the
base o f th e sedim ent* The upperm ost 500 m was i n t e r
p r e te d by N eprochnov a s c o n s t i t u t i n g u n c o n s o lid a te d s e d i
m ent, and th e low er 1000 m as c o n s o lid a te d sedim ent* Sub
s u r fa c e p r o f i l e s from th e same a r e a show p o st-o ro g e n ic
s u rfa c e -s e d im e n t th lo k n e s s on th e o rd e r o f 500 m (Fig*
3 0 ).
The e s tim a te d r a t e o f se d im e n ta tio n o f uncon
s o li d a te d m a t e r i a l in th e so u th e rn Andaman B asin i s 15
cm/1000 y r s ( F ig . 59)* O r ig in a l u n c o n s o lid a te d th ic k
n e s s e s were c a lc u l a te d f o r th e two sed im en tary la y e rs
u t i l i z i n g an av e ra g e w a te r c o n te n t o f 50 p e rc e n t fo r un
c o n s o lid a te d Andaman B asin sedim ent as determ ined from
PIONEER Core 26, and th e av erag e p o r o s i t i e s o f m ud-shale
columns c a lc u la te d by W eller (1959)* W e lle r's c h a rts
in d ic a te an average p o ro s ity o f 37 p erce n t fo r a 300 m
i
8ediment column and 28 p e rc e n t f o r a 1500 m column.
These fig u re s in d ic a te o r ig in a l th ic k n e ss e s of sedim ent
w ith the co n siste n c y o f Andaman Basin s i l t y olay o f 550 m
and 1760 m, r e s p e c tiv e ly .
D ividing th e se th ic k n e sse s by the sedim entation
r a t e of 15 cm/1000 y rs d eriv e d from P r e r ic h 's work i n d i
c a te s th a t the tim e which elap sed sin c e the in c e p tio n o f
sed im en tatio n in the Andaman Basin i s about 12 X 106
y r s , and th a t th e horizon 500 m below the sedlm ent-w ater
c
In te r fa c e was a t the in te r f a c e 3 .7 X 10 years ago. There
f o r e , w ith the assum ption th a t modern sedim entation r a te s
re p r e s e n t the average r a t e sin c e the l a t e Miocene, the
probable age o f the Andaman Basin i s very l a t e Miocene,
and the l a i t (and co n tin u in g ) episode o f d iastro p h ism
began during v ery l a t e P lio cen e tim e. This e x tra p o la tio n
o f modern se d im en tatio n r a te s may be erro n eo u s, p a r t i
c u la r ly in view o f the o p in io n s o f some workers th a t
"• • • the mean sed im en tatio n o f the Holocene i s n o t . . .
normal f o r a l l o f g eo lo g ic h is to r y • • th a t " . ..
th e re i s . . . a c o n sid e ra b le w eight o f evidence su g g e st
ing th a t during the whole Q uaternary p e rio d , sedim enta
tio n r a t e s were g r e a tly in exoess o f normal and v a rie d
g r e a tly from g l a c i a l to s u b g la c ia l sta g e s . • •" ( P a ir -
b rid g e , 1966, p. 4 7 3 ). However, th e l a t e Miocene and
l a t e P lio cen e d ia s tr o p h ic ev e n ts in f e r r e d from the s e d i
ment th ic k n e s s e s in th e Andaman B asin correspond c lo s e ly
to tim es of tectoni8m determ ined by v a rio u s s tu d e n ts of
the e n t ir e s o u th e a s t Asian m argin.
S U M M A R Y O F G EO LO G IC HISTORY
P re-C retaceous
The e a r l i e s t tim e th a t the Burmese T e r tia r y Basin
may have been se p a ra te d from the open ocean by the r i s e o f
the Arakan Yoma was during the l a t e C re ta c e o u s. I t ap
p e a rs t h a t the Andaman Basin formed much l a t e r . During
the l a t e Mesozoic th e a re a s now occupied by th e Shan
P la te a u , Burmese C e n tra l B e lt and Arakan Yoma was a broad
tro u g h accum ulating sh allo w -w ater sedim ents (C legg, 1941).
The tro u g h a x is i s b e lie v e d to have been in th e a re a of
th e p re s e n t Arakan Yoma (T ainsh, 1950). The n a tu re o f the
a re a now covered by the Andaman Sea i s u n c e r ta in . Small
th ic k n e ss e s o f the p o st-C retaceo u s sedim ents in th e
Andaman B asin in d ic a te th a t the a re a was u n d e rla in by
c r u s t which r i f t e d a f t e r the C retaceous and w e ll in to the
T e r tia r y .
Late CretaceouB
Toward the c lo se o f the C retaceous the sh a llo w -
m arine sed im en tatio n in the a re a o f th e p r e s e n t Shan
P la te a u was te rm in ate d a b ru p tly . A C retaceo u s age has been
given f o r the in c e p tio n o f movements u p l i f t i n g the p la te a u
along the Shan esoarpm ent (C hhibber, 1934; C legg, 1941).
At the same time the a n o e s tra l Arakan Yoma was formed as a
range o f p e r i d o t i t i c in tr u s io n s and contemporaneous m afic
e ff u s iv e s (C legg, 194-1).
The contem poraneity o f the form ation o f th e Shan
S carp w ith the Arakan Yoma i s in tr ig u in g , p a r t i c u l a r l y in
view of the s u b - p a r a lle l tren d s of the Shan Scarp and
Malay c o n tin e n ta l slope w ith the e a s te rn slo p e s o f the
Arakan Yoma and Andaman I s la n d s , and in the l i g h t o f th e
abundant te n s io n a l or te n s io n - tra n s c u r r e n t f e a tu r e s in the
Andaman Sea to the so u th . The in te r p r e t a ti o n o f the
Burmese C e n tra l B e lt as a complex graben i s n o t a new one;
Stamp e n te r ta in e d the h y p o th esis f o r many y e a rs b efo re
d isc a rd in g i t (Stamp, 1940). His abandonment o f a
te n s io n a l o rig in fo r the Burmese C e n tral B e lt was m otivated
by h is i n a b i l i t y to re c o n c ile the presence o f T e rtia ry
f o ld s w ith in the b e l t . He concluded th a t th e fo ld s can
only be ex p lain ed by eaB t-w est com pression, and in f e r r e d
t h a t the e n tir e C e n tra l B e lt must a lso owe i t s e x is te n c e
to s im ila rly alig n ed s tr e s s e s . F uture d e ta ile d a n a ly s is
o f the T e rtia ry s tr u c tu r e of Burma may y e t prove Stam p's
r e c a n ta tio n unnecessary. T a ln sh 's (1950) d e s c rip tio n o f
th e C e n tral B e l t 's o il-b e a rin g s tr u c tu r e s in d ic a te th a t
some of th e se s tr u c tu re s are a lig n e d northw estw ard,
d e p a rtin g s ig n i f ic a n t ly from re g io n a l tr e n d . An examina
tio n of the s tr u c tu r e s as fo ld s su b o rd in ate to prim ary
wrench movements by the scheme o f Moody and H i l l (1956)
i s d e s ira b le as soon as more d e ta ile d maps o f Burmese
278 ;
C e n tr a l B e lt geology a re a v a i l a b l e . The p o s s i b i l i t y o f a
combined te n s io n a l and s t r i k e - s l i p o r ig in f o r the Burmese
T e r tia r y B asin has f a r - r e a c h in g im p lic a tio n s f o r the
h i s t o r y o f th e e n t i r e s o u th e a s t A slan b lo c k .
R e g a rd le s s o f th e g e o te c to n ic mechanism in v o lv e d ,
th e c o n f ig u r a tio n o f Burma a t th e c lo s e o f th e C retaceo u s
movements was p ro b ab ly as fo llo w s : The a n c e s t r a l Arakan
Yoma s to o d above su rro u n d in g sh a llo w seas as a p e n in s u la
o r s t r i n g o f is l a n d s shedding d e t r i t u s to tro u g h s b o rd e r
ing th e r id g e to th e e a s t and w e s t. The tro u g h to th e
e a s t was p ro b ab ly bounded along i t s e a s te rn m argin by a
c o a s t c o in c id e n t w ith th e p r e s e n t Mt. Popa-Pegu Yoma
v o lc a n ic l i n e (C legg, 1941; T aln sh , 1950). E a st o f t h i s
c o a s t la y a s t r i p o f low la n d s s t r e t c h i n g to the base o f
th e Shan S c a rp . The c o in c id e n c e o f th e Neogene v o lc a n ic s
w ith th e m argin o f th e Paleogene Burmese Trough s u g g e sts
t h a t t h i s c o a s t may have been c o n tr o lle d by a zone o f
f a u l t i n g b elo n g in g to th e same s e t as the Shan S carp .
South o f the Arakan Yoma th e s e rp e n tin iz e d p e r l -
d o t l t e s w hich com prise th e backbone o f the p r e s e n t
Andaman-Nlcobar Ridge were em placed in deep-w ater o p h io -
l i t e s and r a d l o l a r i a n c h e r t s . The p h y sio g rap h ic n a tu re
o f th e Andaman-Nlcobar R idge and Andaman B asin a re a im
m e d ia te ly fo llo w in g th e p e r i d o t i t e in tr u s io n s i s
p r o b le m a tic . Modern r e g io n a l tr e n d s (th e c o n tin u ity o f
the Arakan Yoma w ith th e Andaman-Nlcobar Ridge and th e
279 ;
c o n tin u ity o f th e Burmese C e n tra l B e lt w ith the Andaman
B asin ) le a d to th e presum ption t h a t th e Andaman-Nlcobar
Ridge and Andaman B asin a lre a d y e x is te d im m ediately f o l
low ing th e l a t e C retace o u s movements. Presum ably, the
Burmese T e r tia r y B asin and Andaman B asin were formed a t
the. same tim e a s a u n i t , w hich has s in c e been only h a l f
f i l l e d by se d im e n ts, in th e manner su g g e ste d by Kuenen
(1 9 5 7 ). However, s e v e r a l l i n e s o f ev id en ce p o in t to an
e n t i r e l y d i f f e r e n t p o s t-C re ta c e o u s c o n f ig u r a tio n :
1. The p o s t-C re ta c e o u s sed im en ts o f the Andaman
B asin a p p e ar to be too th in f o r th e b a s in to have been in
e x is te n c e e a r l i e r than th e l a t e M iocene.
2 . P aleogene se d im en ts o f th e Andaman and N lcobar
is l a n d s ^ re r e s t r i c t e d to th e w estern p o r tio n s o f th e i s
la n d s , w est o f th e s e r p e n t i n i t e s .
3. The P ale o cen e -to -lo w e r-M io c en e se d im en tary
ro c k s o f th e Andaman and N lco b ar is l a n d s have an e n t i r e l y
d i f f e r e n t c h a r a c te r th an th e sed im en ts now bein g d e p o s ite d
in th e w estern Andaman B a s in . The P aleogene sedim ents
have a la r g e th ic k n e s s , e s tim a te d a t o ver 3000 m
( C h a tte r;]i and P a n t, 1964; K arunakaran and o th e r s , 1964b);
t h i s th ic k n e s s may be complemented by a com parable t h i c k
n e s s u n d e rly in g th e submerged w estern f la n k o f th e r i d g e .
The maximum th ic k n e s s o f sed im en t in th e Andaman B asin i s
ab o u t 1500 m (Neprochnov, 1 9 6 4 ). S eco n d ly , the l it h o lo g y
o f th e Paleogene sed im en ts on th e i s l a n d r id g e h as been
2 8 0
d e sc rib e d as graded a l te r n a ti o n s o f co a rse and f i n e
g ra in e d graywackes and sh a le s (K arunakaran and o th e r s ,
1964b). Sole m arkings and o th e r penecontem poraneous
s tr u c t u r e s in the sedim ents in d ic a te p e r s i s t e n t supply
from a h ig h -sta n d in g a re a to the n o r th - n o r th e a s t and
n o r th e a s t. The sedim ents o f t h i s tro u g h appear to be
s t r a t i g r a p h l c a l l y and l i t h o l o g i c a l l y c o n s is te n t w ith th e
Paleogene sedim ents o f comparable th ic k n e s s now exposed
in Assam, w est o f the Arakan Yoma (Sengupta, 1966). As
n oted e a r l i e r , the p re se n t Andaman and N lcobar is la n d s a re
I n s ig n if ic a n t sedim ent provenances f o r th e Andaman B a sin .
Thus, the modern deep-w ater sedim ents e a s t o f th e rid g e
are predom inantly hem ipelaglc s i l t y c la y s o f d e l t a i c
o r ig in , and the is la n d -d e riv e d sedim ent c o n s is ts o f r a r e
c a lc a r e n ite s . C le a rly , sedim ent provenances o f the
Paleogene and o f the p re se n t a re r a d i c a l l y d i f f e r e n t .
These d a ta in d ic a te th a t th e p o st-C re ta c e o u s
Andaman-Nicobar Ridge a re a was n o t se p a ra te d from the
m ainland to the w est (F ig . 6 0 a ). I n s te a d , a to p o g rap h ic
high must have stood e a s t of th e p r e s e n t r id g e , shedding
la rg e q u a n titie s o f c l a s t i c sedim ents i n to a deep trough
im m ediately to th e w est. The b e v e lin g o f th e c o n tin e n ta l
s h e lf W 6 81 o f the Malay P en in su la must have re q u ire d th e
removal o f la rg e q u a n titie s of sed im en t, and th u s the
p re s e n t s h e lf a re a beoomes the l o g ic a l provenanoe fo r th e
Paleogene graywaokes of the Andamans and N ic o b a rs.
F igure 60. Diagrammatic r e p r e s e n ta tio n o f th e
p o stu la te d h is to r y o f th e Andaman
B asin .
A. PA LEO CEN E
r u t
MWOSITT ACTIMM.LT MAT H
CAMCD I T LATC TCATkAAT
• . EARLY AMO MIOOLE T E R T IA R Y C . LA TE M IOCENE D. P L IO C E N E
282
The p e r l d o t i t e s probably marked the c o n tin e n ta l
margin im m ediately fo llo w in g th e l a t e C retaceous orogeny.
I t has been su g g e sted ( D ie tz , 1964) th a t o ccu rren ces in
a lp in e m ountain b e l t s o f a s s o c ia te d s e r p e n t i n i t e s ,
o p h io lite s and r a d io la r i a n c h e r ts may r e s u l t from i n
fo ld in g o f p re g e o s y n c lin a l ocean f lo o r w ith o v e rly in g
g e o sy n c lin a l sed im en ts, and t h a t no i n i t i a l orogeny i s
re q u ire d fo r the emplacement o f the p e r l d o t i t e s . This
h y p o th e sis i n t e r p r e t s th e th ic k s e c tio n s o f fo ld e d s e d i
ments in a lp in e b e l t s as form er c o n t in e n t a l- r i s e prism s
which a re c o lla p s e d and molded to the c o n tin e n ta l m argin.
The f i e l d r e la tio n s h i p s o f th e Andaman and N lcobar
s e r p e n tin ite s to t h e i r a s s o c ia te d o p h io lite s and r a d io
la r ia n c h e rts are B t i l l too im p e rfe c tly understood to
allow i n t e r p r e t a t i o n o f the Paleogene sedim ents o f the
rid g e in the l i g h t o f D ie tz 1 h y p o th e s is .
E a rly and Middle T e rtia ry
Marine se d im en tatio n commenced w ith in the Western
Burmese- Trough im m ediately fo llo w in g i t s fo rm a tio n .
Large r i v e r s a n c e s tr a l to the Irraw addy poured sedimentB
in to the n o rth e rn end o f the tro u g h , d e p o s itin g f l u v i a t i l e
and d e l ta i c sedim ents (F ig . 6 0 b ). Subsidence o f the
trough k e p t pace w ith s e d im e n ta tio n , allow ing a g r e a t
th ic k n e ss of shallow -m arine sedim ents to c o l l e c t in the
southern p o rtio n s o f th e Burmese Trough. During Oligooene
284
tim e the tro u g h a p p a re n tly underw ent m inor t e c to n i c
a c t i v i t y which fo ld e d i t and caused l o c a l u n c o n fo rm itie s
n o r th o f 22° N l a t i t u d e , w h ile th e s o u th e rn p o rtio n con
tin u e d to r e c e iv e m arine se d im e n ts. M eanwhile, th e
w estern p a r t o f the Andaman-Nicobar a r e a was p ro b ab ly a
m arg in al tro u g h ac cu m u latin g se d im en ts s u p p lie d from th e
e a s t in to v a ry in g d ep th s o f w a te r.
L a te r Cenozoic
In l a t e O ligocene to e a r l y Miocene tim e a w ide
sp read tr a n s g r e s s io n o f th e s e a flo o d e d n o rth w ard along
th e W estern Burmese Trough and a ls o In u n d ate d th e E a ste rn
Trough. The se d im en tary h i s t o r y o f th e e n t i r e Burmese
T e r tia r y B asin from th e Arakan Yoma to th e Shan S carp
d u rin g th e Miocene was one o f southw ard enoroachm ent o f
d e l t a i c and f l u v i a t i l e sed im en ts and co n tin u o u s m arine
se d im e n ta tio n to th e s o u th . E u g e o sy n c lin a l sed im en ts
co n tin u ed to c o l l e c t in th e tro u g h now o ccu p ied by the
Andaman-Nicobar R idge.
Renewed upheaval d u rin g th e l a t e Miocene ( F ig .
60c) a f f e c te d th e e n t i r e zone. S ed im e n ta tio n was d i s
ru p te d on th e Arakan Yoma f la n k s , and th e P aleogene s e d i
ments were fo ld e d and r e v e r s e - f a u l t e d tow ard th e w e st.
C ontinuing l a t e Miocene te c to n is m caused f o ld in g and non
d e p o s itio n o ver a wide a r e a o f th e Burmese Trough w est o f
th e p re s e n t v o lc a n ic l i n e , w h ile c o n t in e n t a l sed im en ts
285 ;
c o n tin u e d to c o l l e c t else w h e re in th e tr o u g h . The
Andaman and N ioobar a r e a s were s i m i l a r l y fo ld e d and
r e v e r s e - f a u l t e d w estw ard, and th e Andaman I s la n d s s to o d
above s e a l e v e l f o r th e f i r s t tim e d u rin g th e l a t e M iocene.
F o llo w in g t h i s co m p ressio n , te c to n is m a p p e a rs to have
changed and to have been m a n ife s te d in d i l a t i o n a l move
m ents w hich opened up th e Andaman B a sin f o r th e f i r s t
tim e . I f th e r a t e s o f d e p o s itio n *are c o r r e c t l y d e t e r
m ined f o r th e Andaman B a s in , th e basem ent was exposed a t
th e b a s in f l o o r d u rin g t h i s tim e .
The M io -P lio cen e and P le is to c e n e h i s t o r y o f the
Andaman-Nicobar Ridge and th e M entawai a r e a a p p e a rs to be
one o f r e l a t i v e l y q u i e t , sh a llo w w a te r s e d im e n ta tio n ,
w ith la r g e a r e a s rem ain in g above s e a l e v e l and r e c e iv in g
l i t t l e o r no se d im en t. M io -P lio c e n e v o lc a n ism to th e
e a s t had l i t t l e a p p a re n t e f f e c t on th e r i d g e .
I n t e r m i t t e n t v o lcan ism had been e x p e rie n c e d d u r
in g th e O ligocene and Miocene in S u m atra, d u rin g th e
M iocene in Burma, and q u ite p ro b a b ly d u rin g th e same
tim e s in th e Andaman B asin a s w e l l . I t waB n o t u n t i l th e
P lio c e n e , how ever, t h a t C enozolc v o lc a n ism commenced in
e a r n e s t . V o lo an lcs began to in tr u d e and e x tru d e alo n g
th e M t. Popa tre n d as w e ll as a lo n g th e Shan S c a rp ,
c o n t r ib u t in g la v a s and t u f f s to th e u p p er Irra w a d d ia n
se d im e n ts ( F ig . 6 0 d ). By R ecent ( p r e h i s t o r i c ) tim es
v o lc a n ism had ce ase d in Burma, a lth o u g h i t c o n tin u e s in
286
the Andaman Basin and along the B arisan Range to the
p re s e n t.
L ate P liocene teotonism w ith in th e b asin i s r e
f le c te d in an unconform ity 500 m below the p re s e n t
8edlm ent-w ater in te r f a c e . This teo to n ism , which co n tin u es
to the p re s e n t, i s m anifested in the form ation o f d l l a -
tio n a l and p o ssib ly tra n s c u rr e n t r lf t B ex e m p lifie d by the
N lcobar and C e n tral Andaman r i f t v a lle y s .
The r e la tio n s h ip o f c o n tin e n ta l and o cean ic c r u s t
in the Andaman Basin in the l i g h t o f th e b a s in 's p o s tu l a t
ed te c to n ic h is to r y i s I n te r p re te d d iagram m atically in
F igure 61. W ithout c o r r e la tiv e seism ic d a ta , the c r u s ta l
s e c tio n s d ep icte d must be regarded as la r g e ly s p e c u la tiv e .
C o n tin e n ta l c r u s t i s assumed to u n d e rlie the Malay Con
t i n e n t a l m argin, in c lu d in g the deep t e r r a c e . C ru st under
ly in g th e baBin w est of th e c o n tin e n ta l margin i s p re
sumably b a s a l t ("oceanic" c r u s t ) , o v e rla in by a d i s
continuous veneer o f sedim ent up to 1 .5 km th ic k . The
bulk of th e Andaman-Nlcobar Ridge i s com prised of s e rp e n t-
i n i t e basem ent, co n to rte d Paleocene to Miocene and f l a t -
ly in g P lio cen e to Recent marine sedim ents, and "c o n tin e n
t a l " c r u s t i s la c k in g .
By Pllooene time f l u v l a t i l e and d e l ta i o sedim ents
had covered the e n t ir e a re a between the Arakan Yoma and
Shan S carp, and the T e rtia ry Burmese G ulf sin c e then has
n o t been flooded by marine w a te rs. The subsidence whloh
F ig u re 61 • P o s tu la te d c r u s t a l s e c tio n through
th e Andaman B asin .
2 8 8
i i
FOLDED AND FAULTED
LOWER AND MIDDLE
TERTIARY SEDIMENTS
UNCONSOLIDATE
LEOZOIC AND
ESOZOIC SEDIMEN
VOLCANICS AND
ILICIC INTRUSIV
/
SERPENTINITE
INTRUSIONS
n o t e: n o t d r a w n t o s c a l e
\
had k e p t pace w ith T e rtia ry se d im e n ta tio n had ceased .
Minor u p l i f t during the P le is to c e n e Is evidenced In as
many as s ix te rr a c e le v e ls along r i v e r s d ra in in g the
b a s in ; t h i s u p l i f t may he c o n tin u in g to the p r e s e n t.
The Irraw addy D elta I s p r e s e n tly b u ild in g I t s su b -
a e r i a l d e lta southward a t an e s tim a te d r a t e of 2 .5 km/100
y r s , and supplying huge q u a n t itie s o f sedim ent to sh a llo w -
w ater d e p o s ltlo n a l s i t e s In the n o r th e a s te r n Andaman B asin.
Approxim ately 10 p e rc e n t of the sedim ent su p p lie d by the
Irraw addy R iver reach es the Andaman Basin beyond the
d e l ta , and dom inates the deeper w ater d e p o s its . R e la tiv e
ly minor q u a n titie s o f bio g en ic m a te r ia l, p r in c ip a lly
fo ra m ln lfe ra , augment th ese hem lpelaglc s i l t y c la y s and
a re lo c a lly dominant on topo g rap h ic h ig h s .
R egional Im p lic a tio n s
In fe re n c e s from the g e o lo g ic l i t e r a t u r e and
r e c e n tly acq u ired m arine d a ta , In c lu d in g the p re s e n t stu d y
and the work of Krause ( 1966), ln d lo a te th a t th e e n t ir e
S o u th ea st Aslan c o n tin e n ta l block has re a c te d to te cto n lsm
as a u n it sin c e the l a t e C retace o u s. The g e o lo g ic h is t o r y
o f the Andaman B asin, which I s the w estern margin o f th e
c o n tin e n ta l block, should be examined In the l i g h t o f
re g io n a l h is t o r y , which may be summarized as fo llo w s:
1. Late Cretaoeous In c e p tio n : Along th e w est wing
o f the S o u th ea st Aslan block l a t e C retaceous orogeny
2 9 0
form ed ‘ the Shan P la te a u and a n c e s t r a l Arakan Yoma. The
o l d e s t ro c k s o f th e p r e s e n t Andaman-Nicobar Ridge were
em plaoed a t the* same tim e . In Sum atra th e B a risa n Range
was a ls o u p l i f t e d d u rin g t h i s tim e (Van Bemmelen, 1 9 4 9 ).
I n i t i a l emergence o f th e P h ilip p i n e s , e a s te r n wing o f th e
c o n t i n e n t a l b lo c k , a ls o o c c u rre d d u rin g l a t e C retaceo u s
tim e (K rau se, 1966). U ltra m a flc in tr u s i v e s c h a r a c te r iz e
th e e n t i r e upper C retaceo u s m argin o f th e b lo c k .
2 . L ate Miocene Orogeny: F ollow ing a p e rio d o f
s e d im e n ta tio n in tro u g h s and b a s in s formed by the l a t e
C re ta c e o u s movements, a second ep iso d e o f orogeny o c c u rre d ,
commencing and c u lm in a tin g d u rin g th e l a t e M iocene. The
i n i t i a l movements com pressed th e Arakan Yoma and Andaman-
N ico b ar Ridge in to th e c o n f ig u r a tio n s which p e r s i s t e d w ith
m inor e r o s io n a l and te c to n ic m o d ific a tio n s to the p r e s e n t.
S u b se q u e n tly , l a t e Miocene d i l a t i o n a l movements a p p e ar to
have produced th e Andaman B asin e a s t o f the Andaman-
N ico b a r R idge, ju d g in g from s e i s m i c - r e f l e c t i o n s tu d ie s
(N eprochnov, 1964) and m io ro fa u n a lly determ ined d e p o s i-
t i o n a l r a t e s ( P r e r lc h s , in p r o g r e s s ) . The l a t e Miocene i s
th o u g h t to have been th e tim e o f in c e p tio n o f p o s tu la te d
t h r u s t i n g movements r e s p o n s ib le f o r the fo rm atio n o f
th e Ja v a Trench (Umbgrove, 1948). M ajor orogeny o c c u rre d
d u rin g th e l a t e Miocene in th e P h ilip p in e s , where Cenozftlc
te c to n is m a p p e a rs dom inated by th e 1650-km long s i n i s t r a l
s t r i k e - s l i p P h ilip p in e R i f t ( W i l l i s , 1937; King and MoKee,
291
194-9; R a n n eft a rd o t h e r s , I9 6 0 ; A lle n , 1962).
3. L a te -P lio c e n e -R e c e n t Orogeny: A nother m ajor
ep iso d e o f orogeny began In l a t e P lio c e n e tim e and con
tin u e s to th e p r e s e n t. In Burma th e orogeny was marked by
v olcanism w ith in th e C e n tra l B e lt along th e H t. Popa-Pegu
Yoma tr e n d , and alo n g th e base o f th e Shan P la te a u . W ith
in th e C e n tra l B e lt t h i s v o lc a n ic a c t i v i t y was accompanied
by f o ld in g tr a n s v e r s e to r e g io n a l s t r u c t u r a l tre n d s (E vans,
1940; T ain sh , 1 9 5 0 ). In the Andaman B asin th e e a r l i e s t
l a t e P lio c e n e o ro g e n ic movements d i s t o r t e d th e p re -
o ro g e n lc Miocene and P lio c e n e se d im e n ts, as re c o rd e d In an
unconform ity d e te c te d by s e i s m l c - r e f l e c t l o n 500 m below
the p r e s e n t w a te r-se d im e n t I n te r f a c e (Neprochnov, 1964).
C ontinuing orogeny has produced r i f t s a lig n e d n o r th -
so u th and e a s t- n o r t h e a s t- w e s t- s o u th in th e Andaman B asin;
th e se r i f t s can o n ly be e i t h e r te n s lo n a l o r *'p u l l - a p a r t s ,f
combined w ith tr a n s o u r r e n t movement. Volcanism lo c a liz e d
along th e r i f t system formed Narcondam and B arren is la n d s
and numerous seam ounts. On Sum atra modern te c to n lsm i s
c h a r a c te r iz e d by v o lcan ism and d e x t r a l s t r i k e - s l i p
f a u l t i n g . On Ja v a v o lc an ism 1b th e dom inant te c to n ic e x
p re s s io n ; s o u th o f Ja v a t h r u s t i n g i s th o u g h t to be
dom inant and e x p re sse d p r i n c i p a l l y in th e fo rm a tio n o f th e
Jav a Trench Mechanisms In v o lv in g u n d e r th r u s tin g of the
c o n tin e n ta l c r u s t by th e ocean f l o o r ( F is h e r and H ess,
1963) o r o v e r r id in g o f the o c e a n ic c r u s t by th e c o n tin e n ta l
292 ;
block (Helskanen and Vening M einesz, 1958) have been p ro
posed f o r m arginal tre n c h o r i g i n . Orogeny which a ls o com
menced in the P h ilip p in e s in l a t e P lio c e n e time (K rause,
1966) appears c le a r ly c h a ra c te riz e d by m ajor s i n i s t r a l
s t r i k e - s l i p movement along the P h ilip p in e R i f t a c
companied by ex ten siv e volcanism .
Thus, th re e major te c to n ic e v e n ts may be d i s
tin g u ish e d f o r the S o u th east Asian b lo c k a s a w hole: in
l a t e C retaceo u s, l a t e Miocene and l a t e P lio ce n e-R e cen t
tim e. The sim p le st e x p la n a tio n f o r th e v a r i a t i o n of
Cenozolc te c to n ic ex p ressio n along th e m argin o f the block
would be th a t the e n tir e block has moved a s a u n i t . By
th i s scheme, n e t displacem ent has moved th e e n t i r e block
in a so u th -s o u th e a s t d ir e c tio n . Such a movement would
ex p lain the post-M iocene Andaman B asin as a " p u ll a p a rt"
fe a tu re a t th e w estern wing o f the b lo c k , the b a sin being
produced as a sc a r l e f t behind the moving block (F ig . 6 2 ).
South o f the Andaman B asin, where th e b lo c k margin curves
eastw ard in to Sumatra, th e movement would paB8 in to d e x tra l
s t r i k e - s l i p . Movement even f a r t h e r to th e so u th and e a s t
would pass in to o v e rrid in g o f th e ocean b a s in by the
c o n tin e n ta l c r u s t, m anifested most s tr o n g ly along the
f r o n ta l margin o f the block o f f Ja v a . Along the zone o f
disp lacem en t, movement would change from th r u s tin g a t Java
in to s i n i s t r a l s t r i k e - s l i p along the P h ilip p in e s .
By t h i s meohanlsm o f a S o u th e a st A sian block moving
F ig u re 62. Suggested r e la tio n s h i p o f c r u s t a l
movement to m arg in al s tr u c t u r e o f
s o u th e a s t A sia.
A TAIW AN
r FAULT
“ t Vnilippine
S RIFT
SHAN
VI PLATEAU
v a
v d \u
/
MINDANAO
1000 Km
PHILIPPINE
TPENCH
as a d i s c r e t e u n i t , s e v e r a l c o n t r a d i c t i o n s in s t r u c t u r e
may be e x p la in e d :
1. The f o ld s in th e Burmese C e n tra l B e lt would be
drag fo ld s a s s o c ia te d w ith d i l a t i o n and tr a n s c u r r e n t move
m ent.
2. The l a t e Miocene r e v e r s e - f a u l t i n g and f o ld in g
w hich a f f e c t e d th e Burmese Trough and molded th e tro u g h
in to the Andaman-Nicobar Ridge may have been caused by an
i n i t i a l w estward or southw estw ard movement o f th e b lo c k .
A su b seq u en t sm all s h i f t in d i r e c t i o n co u ld th en cause
fo rm a tio n o f th e Andaman B asin a s a s c a r in t o m c r u s t .
3. The no rth w estw ard lo s s o f i n t e g r i t y o f th e Jav a
Trench as a p h y sio g ra p h ic u n i t may be th e r e s u l t o f p a s
sage o f te c to n i c e x p re s sio n from t h r u s t i n g o r r e v e r s e -
f a u l t i n g in th e v i c i n i t y o f th e tro u g h to s i n i s t r a l s t r i k e -
s l i p along th e w e ste rn wing o f th e S o u th e a s t A sian b lo c k .
I f th e Trench owes i t s e x is te n c e to t h r u s t i n g , i t s n o r th
westward d im in u tio n and e v e n tu a l absence i s l o g i c a l .
I n t e r p r e t a t i o n o f th e Andaman B asin as a r i f t
produced by l a t e r a l s h i f t i n g o f a c r u s t a l b lo c k i s n o t new.
Carey (1958, p. 272-285) term ed th e b a s in th e "Mergui
Rhombochasm". As Carey ( p . 192) d e fin e d i t , a rhombochasm
i s a " . • . p a r a l l e l - s i d e d gap in th e s i a l i c c r u s t o c
cu p ied by s im a tic c r u s t , and i n t e r p r e t e d as a chasm • • •"
Carey v is u a liz e d the "rhombochasm" a s r e s u l t i n g from a
southw ard movement o f Sum atra s in c e th e e a r l y M esozoic.
According to t h i s model, the Andaman-Nicobar Ridge i s th e
"Andaman O ro ta th ," a range which was s tr e to h e d to double
i t s o r i g i n a l le n g th d uring th e e a rly M esozoic. " . . .
e x te n sio n o f d is ta n c e between Sumatra and Burma, which i s
th e lo g io a l r e s u l t of th e drag eastw ard o f th e Tethyan
Shear System as A u s tr a lia moved eastw ard . . . " Carey
r e l i e d too h e a v ily upon h is attem p ted m atching o f
r e g io n a l s h o r e lin e s . H is r e c o n s tru c tio n o f th e Andaman
Basin a r e a , f o r example, matohes th e Irraw addy D elta
w ith Sumatra w ith o u t re g a rd to s tr a t ig r a p h y , lith o lo g y o r
g eo lo g ic h i s t o r y . Thus, alth o u g h the Andaman B asin may be
term ed a rhomboohasm in harmony w ith both C a re y 's g e n e tic
term inology and the b a s in 's o r ig in as proposed in th e
p re s e n t p ap e r, th e d ir e c tio n o f movement proposed by Carey
appears erro n e o u s.
The Andaman Basin is n o t unique in i t s t ran fl
our r e n t and d i l a t i o n a l g e n e s is . O ther s t r u c t u r a l f e a tu r e s
which have been I n te r p r e te d as r i f t s r e s u l t i n g from com
bined te n s lo n a l and tr a n s o u r re n t movements in o lu d e th e
Dead Sea (Q uennell, 1958), th e tfu lf o f C a lif o r n ia (Rusnak
and F is h e r, 1964), and th e c e n tr a l segment o f th e Death
V alley in C a lif o rn ia (B u rc h fie l and S te w a rt, 1966).
The a v a ila b le d a ta do n o t allo w d e p io tio n o f an
u n d erly in g meohanism which oauses th e m o b ility o f the
S o u th ea st Aslan blook. Any suoh meohanism would r e q u ir e
a b e t t e r und erstan d in g o f the T lb e tla n P la te a u ; th ie
f e i t u r e 's p ro x im ity to the S o u th e a st Asian b lo ck can h a rd
ly be due to h a p p e n sta n c e , and th e w r i te r b e lie v e s t h a t
th e p la te a u and the fo rc e s r e s p o n s ib le f o r i t s developm ent
may have a fundam ental r e l a t i o n s h i p w ith th e S o u th e a st
Asian b lo c k . The "elbow'9 o f th e e a s te rn Himalayas and
the n o r th - s o u th a lig n e d Arakan Yoma Range must a lso be
examined in the l i g h t o f th e S o u th e a st Asian movements.
i
CONCLUSIONS
1 . The S o u th e a st Aslan c o n tin e n ta l block appears
to have a c te d as a te c to n ic u n i t sin c e th e end o f the l a t e
M esozoic. The block m argin has a common h is t o r y o f l a t e
C retaceous em ergence, p e r l d o t i t l e in tr u s io n s and penecon-
temporaneous m afic v o lc a n ic a c t i v i t y . L ate Miocene move
m ents, r e a c tiv a te d in th e l a t e P lio cen e and c o n tin u in g to
the p re se n t have r e s u l t e d in n e t so u th -so u th e a stw a rd
movement of th e b lo c k , ex p ressed in e x te n siv e volcanism ,
s t r i k e - s l i p f a u l t i n g along the w estern and e a s te rn s id e s
o f the block coupled w ith " p u ll- a p a r t" a t the w estern
margin (Andaman B a s in ), and th r u s tin g along the Javan f r o n t
o f the b lo ck .
L o g ica l changes in te c to n ic e x p re ssio n along the
s id e s and f r o n t o f the block to g e th e r w ith minor v a r ia
tio n s in d ir e c tio n o f th e b lo c k 's movement b e s t ex p lain
such seem ingly incongruous s t r u c t u r a l f e a tu r e s as the
T e r tia r y fo ld s o f the Burmese C e n tra l B e lt, the fo ld s and
r e v e r s e - f a u l t s o f the Andaman-Nicobar R idge, and the ap
p a re n t absence o f the Jav a Trench n o rth of Sumatra and
w est o f the Andaman-Nicobar R idge.
2 . The o ld e s t ro c k s o f the Andaman-Nicobar Ridge
a re the s e r p e n tin lt e s which com prise the e a s te rn p o rtio n s
o f the rid g e and which a re contemporaneous w ith the
s e rp e n tin iz e d p e r i d o t l t e s o f th e Arakan Yoma and o f the
299
e n t i r e S o u th e a s t A sian b lo c k m a rg in .
3 . The Andaman-Nicobar R id g e 's e a s te r n edge i s the
w e ste rn slo p e o f th e N ic o b a r R i f t V a lle y , w hich s h a rp ly
d e lim its th e C retace o u s s e r p e n t i n i t e s and P a le o o e n e - to -
L ate Miocene e u g e o s y n c lln a l sed im en ts o f th e r id g e from
th e L ate M iocene-R ecent Andaman B asin v o lc a n ic s and s e d i
m e n ts.
4 . P r i o r to th e e x is te n c e o f th e Andaman B asin
th e Andaman-Nicobar a r e a was p ro b ab ly a p o s t-C re ta c e o u s
m a rg in a l tro u g h w hich, d u rin g P aleo cen e to l a t e Miocene
tim e , accu m u lated o v er 3000 m o f e u g e o s y n c lln a l sed im en ts
d e riv e d from a h ig h - s ta n d in g a r e a to th e e a s t . T his h ig h
a r e a may have been th e p r e s e n t Malay C o n tin e n ta l S h e lf
a r e a b e fo re i t was b e v e le d and r i f t e d away. Movements
o f th e c o n t in e n t a l b lo c k w ith a w estw ard com ponent d u rin g
th e l a t e Miocene a r e b e lie v e d to have molded th e s e d i
m ents o f th e tro u g h in to th e w e ste rn p o r tio n s o f th e
Andaman-Nicobar R idge, im m ediately b e fo re th e in c e p tio n o f
th e Andaman B a s in .
5. The Andaman B asin p ro b ab ly form ed a s a " p u l l -
a p a r t " o c c a sio n e d by th e n e t s o u th -s o u th e a s tw a rd movement
o f th e S o u th e a s t A sian c o n t in e n t a l b lo c k , w hich caused
combined te n s io n and s t r i k e - s l i p movements in th e Andaman
B asin a r e a . S e is m ic a lly d eterm in ed se d im en t th ic k n e s s e s
and m lc ro fa u n a lly d eterm in ed d e p o s itio n a l r a t e s f o r the
Andaman B asin i n d i c a t e t h a t i n i t i a l d i l a t i o n o c c u rre d
300
d uring th e l a t e M iocene, subsequent to the molding of th e
Andaman-Nicobar R idge; and t h a t a m ajor re c u rre n c e o f d i s
te n s io n commenced in the l a t e P lio cen e and co n tin u es to
th e p r e s e n t.
6 . Thus, the h is to r y o f the Burmese T e rtia ry B asin
and Andaman B asin i s n o t as sim ple as cu rso ry exam ination
o f physiography m ight in d i c a t e . Late C retaceous In c e p tio n
o f the Burmese Trough i s w e ll documented. However, th e
Burmese Trough and Andaman Basin.may s t i l l be co n sid ered
a compound f e a tu r e which i s an ex p re ssio n o f the same
d i l a t i o n , a lth o u g h d is te n s io n o ccurred e a r l i e r in Burma.
7 . S i l l depths o f the Andaman-Nlcobar Ridge a re
between 1300 and 1600 m. Deeper w aters of the Andaman
B asin a re re p le n is h e d by movement of Bay o f Bengal w ater
over th e s i l l s . Below the maximum s i l l depth Andaman
w ater i s is o th e rm a l, is o h a lin e , and uniform in d iss o lv e d
oxygen c o n te n t. Abnormal h e a tin g of deep Andaman w ater
by v o lc a n ic a c t i v i t y as re p o rte d by Sew ell (1933) and
Burns (1964) does n o t appear to e x i s t , a t l e a s t during
the y e a rs 1961 to 1964. S u b - s i ll tem p eratu res do n o t i n
c re a se downward in excess o f the a d ia b a tic h e a tin g e f f e c t
which norm ally occurs in a c lo se d b a s in .
8 . The Andaman Basin c o n s titu t e s ah e f f i c i e n t
sedim ent tr a p f o r the sedim ents which pour out o f the
Irraw addy R iv e r. This e f f ic ie n c y i s w e ll dooumented by
th e f a o t th a t d e p o s itlo n a l r a t e s in th e Bay o f Bengal
3 0 1
Immediately w est o f th e Andaman-Nicobar Ridge are only
o n e -h a lf as ra p id as w ith in the b a s in .
9. The Andaman Basin i s a c llm a to lo g lc as w ell as
a physiographic tr a p f o r sedim ents which are su p p lied by
the Irrawaddy R iv e r. During the southw est monsoonal
season of heavy summer r a in s which r e s u l t In 90 p ercen t of
the Irraw ad d y 's r u n o f f , 90 p e rc e n t of the r i v e r 's sedim ent
load i s brought to the d e l ta mouths. E stim ated annual
sedim ent lo ad i s between 263 m illio n and 400 m illio n to n s.
P re v a ilin g southw est monsoonal winds during t h i s season
d riv e c u rre n ts a c ro s s th e d e lta su rfa c e to the e a s t, d i s
p la cin g the sed im en t-lad en r i v e r d isch arg e in th a t
d ir e c tio n . C onsequently, most o f the sedim ent reach es a
d e p o s ltio n a l s i t e so u th and e a s t o f the r i v e r mouths.
During the dry n o r th e a s t monsoon the c u rre n ts are westward,
out o f the Andaman Sea in to the Bay o f B engal. However,
the clay s and s i l t s which a re d ep o site d during the wet
monsoon a p p a re n tly are n o t eroded by th e se c u r r e n ts . I f
any m a te ria l can le a v e th e Andaman B asin, i t i s the sm all
q u a n tity o f dry-monsoon lo a d , coupled w ith minor amounts
o f sand from the w estern Irrawaddy d e lta s u rfa c e .
10. D eposition o f the d e l t a - s h e l f may be c o n tro lle d
by tectonism in two ways. F i r s t , f a u ltin g of the d e lta
a re a , which i s in d ic a te d by s e is m ic ity and by topographic
c o n tra s ts between the w estern and e a s te rn h alv es o f the
d e lta , may tr i g g e r downslope movements o f sedim ent in the
3 0 2
v i c i n i t y o f M artaban Canyon. Second, th e d i s t o r t i o n o f
th e d e l t a s u r fa c e may p ro v id e p r e f e r r e d a r e a s f o r
d e p o s itio n , as se d im e n ta tio n s t r i v e s to r e g a in and m ain
t a i n a low , s t a b l e d e l t a - s h e l f g r a d i e n t .
11. Comparison o f so undings tak en In 1854-1859 w ith
b a th y m e tric d a ta o b ta in e d d u rin g th e 1 9 6 0 's i n d i c a t e t h a t
2
a 12,000 km a r e a com prising th e e a s te r n p o r tio n o f the
Irraw addy d e l ta s h e l f has sh o a le d by a t l e a s t 5 m. A l
though la r g e n a v ig a tio n a l e r r o r s may be r e s p o n s ib le f o r
th e a p p a re n t s h o a lin g , th e b a th y m e tric changes a re
s y s te m a tic a lly d i s t r i b u t e d and a p p e a r r e a l . This change
in d epths i s f a r too la r g e to be acco u n ted f o r by s e d i
m e n tatio n a lo n e , inasmuch a s th e volume o f sedim ent
s u p p lie d by the Irraw addy i s an o rd e r o f m agnitude l e s s
th an th e volume r e p r e s e n te d by s h o a lin g . Depth changes
may be a t t r i b u t a b l e in la r g e p a r t to a n e t f a l l in
r e l a t i v e Bea l e v e l . The re g io n su rro u n d in g th e Andaman
B asin ap p e a rs to have e x p e rie n c e d su b -R e cen t to R ecent
emergence o f s e v e r a l m e te rs .
12. D e p o s itlo n a l r a t e s on the d e l t a - s h e l f a r e b e
tween 100 and 200 cm/1000 y r s ; in th e deep b a s in beyond
th e d e l t a th e ra te B a r e a s much a s an o r d e r o f m agnitude
lo w er, 15 cm/1000 y r s . The Malay C o n tin e n ta l S h e lf and
Andaman-Micobar i n s u l a r s h e lv e s a r e a r e a s o f non
d e p o s itio n and e r o s io n .
13* Sedim ent p aram ete rs su ch a s g r a i n - s i z e ,
303 ,
ca rb o n ate and o rg a n ic c o n te n t, and c lay m ineralogy o f
Andaman B asin sedim ents e x h ib it p a tte r n s In conform ity
w ith sedim ent provenance, ocean c u r r e n ts , b io lo g ic
a c t i v i t y , physiography and sed im en tatio n r a t e s . The o u te r
p o rtio n o f th e w estern d e lta s h e lf , the Malay C o n tin e n ta l
S h e lf and th e I n s u la r sh e lv es o f the Andamans and N lcobars
accum ulate l i t t l e sedim ent. Consequently, g r a in - s iz e Is
h ig h , r e f l e c t i n g the winnowing away o f fin e m a te r ia ls ;
carb o n a te c o n te n ts a re h igh because b io lo g ic a c t i v i t y I s
fav o red by low tu r b id it y and because the s h e ll rem ains
a re u n d ilu te d by te rrig e n o u s sedim ent; and o rg an ic m a tte r
Is low because slow b u r ia l allow s co n sid erab le tim e f o r
o x id a tio n a t the s u r fa c e . In c o n tr a s t, sedim ents of the
In n e r Irraw addy D elta S h e lf are fin e -g ra in e d , r e f l e c t i n g
th e c la y and s i l t lo ad of the Irraw addy, ra p id accum ula
t i o n , and la c k of winnowing. D ilu tio n by te rrig e n o u s
m a te r ia l causes low carbonate and o rg a n lc -m a tte r c o n te n ts .
14. The p re se n t r e la tio n s h ip between the Irraw addy
D e lta and the Andaman Basin Is an e x c e lle n t example o f a
b a s in th a t i s being lo n g itu d in a lly f i l l e d from one end In
th e manner which Kuenen (1956) has suggested Is Im p o rtan t
today and may have been s ig n i f i c a n t during the g e o lo g ic
p a s t.
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SUPPLEMENTARY SOURCE! MISCELLANEOUS COLLECTIONS, USNOO.
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Creator Rodolfo, Kelvin Schmidt (author)

Core Title Marine Geology Of The Andaman Basin, Northeastern Indian Ocean

Degree Doctor of Philosophy

Degree Program Geology

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

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Advisor Gorsline, Donn S. (committee chair), Davis, Gregory A. (committee member), Stone, Richard O. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-585539

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