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Marine Geology Of The Baja California Continental Borderland, Mexico

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Marine Geology Of The Baja California Continental Borderland, Mexico

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Content MARTNE GEOLOGY OF THE BAJA CAT TFORNTA
CONTINENTAL BORDERLAND, MEXICO
by
Larry James Doyle
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CATTFORNIA
Tn Partial Fulfillment of the
Requirements for the Decree
DOCTOR OF PHILOSOPHY
(Geological Sciences)
June 1973
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Xerox University Microfilms
300 North Zoob Rood
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! i
;l
73-31,339
DOYLE, Larry James, 1943-
MARINE GEOLOGY OF THE BAJA CALIFORNIA
CONTINENTAL BORDERLAND, MEXICO.
University of Southern California, Ph.D.,
1973
Geology
University Microfilms, A X E R O X Company, Ann Arbor, Michigan
THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED,
UNIVERSITY O F SO U T H E R N CA LIFO RN IA
THE GRADUATE SCH OOL
UNIVERSITY PARK
LOS A N GELES, C ALIFO RNIA SOOOT
This dissertation, written by
IARRY JAMBS DOYLE
under the direction of hX&... Dissertation Com
mittee, and approved by all its members, has
been presented to and accepted by The Gradu
ate School, in partial fulfillment of require
ments of the degree of
D O C T O R OF P H I L O S O P H Y
Dtm*
! cj ' 1 3
DISSERTATION. COMM ITTEE.
Chmtrmm w
CONTENTS
Page
ABSTRACT............................................. ix
ACKNOWLEDGMENTS.................................... xii
INTRODUCTION ......................................... I
General statement • 1
Purpose of the investigation .................. 6
Methods of investigation ....................... 6
Continuous reflection seismic profiling • 6
Dredging and piston coring................ 10
Navigation.................................. 11
PREVIOUS W O R K ....................................... 18
General statement.................... 18
Bala California ............. 18
California Continental Borderland ........... 26
Bala California seamount Province •••••■ 27
Baja California Borderland ..*.*••••• 27
LTTHOLOGY........................................... 34
General statement ........................ 34
Sedimentary rocks •••••.•• ......... • 39
General statement •• .................... 39
Sandstones and mudrocks.................. 40
ii
Page
Conglomerates 48
Discussion................................ 50
Other sedimentary rocks 56
Metamorphic rocks . . . ....................... 57
Igneous rocks .................................. 58
Volcanic rocks . . ........................... 58
Plutonic rocks .... .. 60
PHYSIOGRAPHY AND STRUCTURE .................. 62
Physiography...........................* . . . 62
Additional bathymetric data .............. 62
Discussion of regional physiography . . . 62
Structure . . . . . . . . . 73
General statement . . . .................. 73
Transition from deep sea to Ba1a
California Borderland .................. 76
The continental escarpments
Popcorn Ridge
The g a p s .........* ............... 110
General statement
Ba la Gap
Santo Tomas Gap
Santo Tomas Fault Zone •«.•••.«. 128
Interior of the borderland . . . . . . . . . 142
ill
Pn^e
TECTONICS............................. 154
SUMMARY AND CONCLUSIONS............... 158
General statement • • . • .............. 158
Baja California Borderland summary .... 158
General model for borderland development • 165
REFERENCES............................... 176
APPENDIX................................. 186
iv
ILLUSTRATIONS
Figure Page
1, The Bala California Borderland as studied
in thl s paper . 3
2, Locations of the seismic profiles referred
to in this studv.................., , • , . 7
3, Profile line 21B from the Colnert Basin
to the continental shelf.................. 11
4, One of the composites of high contrast
pictures of the original seismic records . 13
5, Locations of dredge hauls collected during
this study............... 15
6, Northern part of the Baja California
Peninsula adjacent to the study area . . . 19
7, Portion of a bathymetric chart of the Baja
California Borderland * ■ * . , ......... 29
8, Approximate location of the Santo Tomas
F a u l t ...................................... 32
9, Location of Scripps Institution of
Oceanography dredges (SOB) and Allan Han
cock Foundation dredges 35
10, Lithologic summary of dredge hauls from
the Baja California Borderland........... 37
11a, Large diatomite concretion with manganese
coating dredged at station 14937 on Sole-
dad Ridge from a depth of 900 m • , • • , 43
11b, Cross section of the diatomite concretion 45
12, Circulation pattern for Viscaino Bay
illustrating southerly nearshore flow of
California Current ......................... 52
v
F1 gure Page
13, Bathymetric chart for the Baja California
Borderland.................................. 63
14, Depths of bank tops and basin floors in
the Southern California and Baja Cali
fornia Borderlands......................... 66
15, Major structures recognized in the Baja
California Borderland .................... 74
16, Profile 19C from Santo Tomas Gap to the
escarpment south of Velero Basin ......... 77
17, Profile 31B from the base of the con
tinental escarpment across the head of
Santo Tomas Gap to the high area northwest
of Animal B a s i n ............. 79
18, Profile 18C from the deep sea across the
middle of Santo Tomas Gap to the escarp
ment south of Velero Basin................ 81
19, Profile 13 parts (A) and ( B ) .............. 83
20, Profile 17C from the base of the con
tinental escarpment across the Santo Tomas
Fault Zone to the high area west of Animal
B a s i n .................. 85
21, Profile 20B from the deep sea over the
escarpment.................................. 87
22, Profile 19B which crosses the entire Baja
California Borderland .................... 89
23, Profile 12A which depicts the transition
from the deep sea to the escarpment north
of Soledad Ridge........................... 91
24, Transition from the ocean basin to Soledad
Ridge is shown in profile 1 6 B ........... 93
25, Profile 10A showing the transition from
deep sea to Soledad R i d g e ................ 95
vi
Figure Page
26. Profile 7B from the deep sea over Soledad
R i d g e ....................................... 97
27. Profile 13B traverses part of Baja Gap , , 99
28. Profile 15C depicts the structure from
Soledad Ridge across Soledad Basin and
down Into the trough at the northern end
of San Quint in Basin........... 101
29. Profile 14C reveals transition from the
deep sea to the borderland via the southern
flank of Ba 1a G a p .......................* 103
30. Profile 12B runs from the base of Ferre1
Sea Mount across the trough south of San
Quintin Basin .............................. 105
31. Profile IB runs from Cedros Deep to San
Quintin Basin and then to the San Quintin
s h e l f ....................................... Ill
32. Profile 3B shows the fault at the base of
Popcorn Ridge and sediments on top of the
r i d g e .................. 113
33. Profile IOC depicts the Popcorn Ridge
fault, sediments within the Cedros Deep,
and the sedimentary section on Popcorn
Ridge east of Ferrel Seamount........... 115
34. Profiles 4B and 5B are presented.......... 117
35. Profile 11C from the deep sea across Pop
corn Ridge.................................. 119
36. Profile 12C shows Popcorn Ridge and its
associated fault and the Baja Gap at its
northwest e n d ...................... • . . 121
37. Section along traverse 9B crossing Baja
G a p ............. 123
38. Profile 13C is a short section crossing
the outermost part of Baja G a p ........... 125
vl 1
Figure Page
39. Profile 16C Is along and .lust east of the
continental escarpment of the Baja Cali
fornia Borderland . ..... 129
40. Profile 30B transects Animal Basin and the
Santo Tomas Fault Zone ......... 131
41. Profile 28B, on which the Santo Tomas
Fault Zone Is shown ......................... 133
42. Profile 26B shows the Santo Tomas Fault at
Its southeast end and part of the Descanso
Plain northwest of Punta Santo Tomas . . . 135
43. Profile 12B depicts Animal Basin and
Santo Tomas Faults and the ridge between
Animal Basin and Descanso Plain at the
northern limit of the Baja California
Borderland • 137
44. Profile 24B on which the Santo Tomas
Fault Zone Is clearly evident............... 139
45. Profile 22B shows the Santo Tomas Faults
Just to the southeast of Punta Santo Tomas 143
46. Profile 15B crosses the Southern Border
land, showing Soledad Basin, San Quintin
Basin, and Colnett Basin and related
structures...................................... 145
47. Profile 7A, which shows possible folding
west of San Quintin Basin and the two-
tiered nature of San Quintin Basin are
apparent.............................................147
48. Transition from Cedros Deep to the margin
south of Popcorn Ridge is shown in Profile
1 0 B ............................................. 149
vill
ABSTRACT
3200 km of continuous reflection se1*mlc profiling!
dredge hauls and new bathymetric data have been gathered
from the continental borderland west or Baja California.
The Baja California Borderland has a typical profile from
Its seaward edge of ridge-trough-ridge-trough-sheIf•
Within the large troughs smaller Individual basins have
developed. Physiographic data from new soundings show
many modifications are necessary In the older bathymetric
charts and. in addition* yield valuable information on the
general regional structure. Basin configurations have been
redefined by the new data. Fault control is clearly seen
In some instances. Parts of the borderland exhibit pro
files that resemble the burled ridge-trough pattern that
underlies the present Atlantic continental margin of the
southern United States.
A distinct bathymetric and structural break occurs
between the deep-sea and the Baja California Borderland
over the entire length of the study area. Two gaps. com
parable to the five described by Uchupi and Emery (1963)
in the northern borderland, are present in the southern
escarpment. The northern gap of the two* Santo XfiOAft SfiRt
is Interpreted to be structurally controlled by the Santo
ix
Tomas Fault, and has significant sedimentary fill. The
southern gap, Ba1a Gap, also doubtless is fault-
controlled and contains appreciable sedimentary fill.
A major left-lateral strike-slip fault with over 90
kms of offset is Interpreted to lie at the base of Popcorn
Ridge, and may have a deep-sea expression in offset mag
netic anomalies (Theberge, 1971).
The Santo Tomas Fault Zone crosses the entire
borderland and its relationship to the Santo Tomas Gap
has been clearly defined. Sense of offset is not entirely
resolved although the fault is essentially of the strike-
slip type. No major regional breaks in basin bottom depths
or ridge top depths occur across the zone as has been
earlier postulated.
Positive areas of the fiaja California Borderland
have a varied lithology as represented in dredge hauls.
These consist mainly of sedimentary rocks ranging in age
from Paleocene to Holocene. Sonoran source areas are
suggested by clasts found in the dredge samples that have
identical composition to Poway-type conglomerates of
Eocene age. These may be reworked clasts. In addition,
metagraywackes are also found. Basalts are common and
representative specimens have been dated from three samples
spanning the borderland. The resulting K-Ar dates indl-
x
cate a late period of volcanism superimposed on the
older rocks. Dates of from one to four million years
before present have been determined*
Late Miocene (Mohnian) sediments on the outermost
escarpment of the Baja California Borderland and the pre
sence of Eocene or younger conglomerates within the area
limit the time of origin of the province to after their
deposition. Advent of tectonlsm responsible for border
land formation after deposition of the uppermost Miocene
elastics is placed at no earlier than four million years
before present. This is penecontemporaneous with the
opening of the Gulf of California and northwestward move
ment of Baja California. This is much later than is
theorized in earlier hypotheses of borderland formation.
xi
ACKNOWLEDGMENTS
Acknowledgment is due to the many people without
whose assistance this study could not have been completed.
Marina Doyle, my wife, provided moral support In the final
stages and helped with the typing. Dr. D. S. Gorsllne was
Instrumental In providing support for the research and
gave time and advice. Dr. R. H. Merriam aided with the
petrology and offered his Poway collection for study. Dr.
0. L. Bandy analyzed the foramlnlfera. Jeffery Coleman
of the Pacific Support Group, Naval Oceanographic Office,
provided bathymetric data used In this Investigation. Dr.
D. G. Moore of the U. S. Navy Underwater Research Center
gave some seismic profiles from his large collection, and
J. A. Forman and the Mobil Oil Corporation provided K-Ar
dates and advice. Helpful criticism and comments were
given by Drs. G. A. Davis, T. L. Henyey, R. L. Kolpack and
R. 0. Stone, and by Mr. J. S. Booth.
This research has been supported under National
Science Foundation grants GA-25233, GB-8628, GA-34145 and
by Office of Naval Research grant N00014-67-A-0269-009C.
The R.V. VELERO IV Is supported by National Science
Foundation grant GN-27255.
xii
INTRODUCTION
General Statement
The continental margin off southern California and
northern Baja California is a complex system of basins and
ridges (see Shepard and Emery* 1941), and Is defined as a
continental borderland. Forming a province with its long
axis oriented northwest-southeast, the California Border
land roughly parallels the structural grain of the ad
jacent land areas. The Transverse Ranges form the northern
boundary) and the Peninsular Ranges are the eastern
boundary. Shepard and Emery (1941) vaguely defined the
southern limit at about 28° 30* N. Krause (1964, 1965)
and Moore (1969) arbitrarily placed the southern limit at
the Vlscalno Peninsula and Isla Cedros, respectively. The
true limit is most probably the southern boundary of the
broad Vlscalno Plain and its encompassing ranges.
That part of the continental borderland adjacent to
southern California has been the subject of extensive
geologic surveys, chiefly by groups from the University of
Southern California and Scripps Institution of Oceano
graphy. As a result, the Southern California Borderland
is one of the most Intensely studied continental margins
I
2
in the world. Much of what is known of this area is sum
marized by Emery (1960) and Moore (1969).
In contrast to the northern portion, that segment
of the continental borderland south of about 32° N is
relatively unknown (Fig. 1). Reconnaissance studies by
Krause (1964, 1965) and Moore (1969) suggested that the
southern part of the borderland might be physlographlcally
and structurally distinct from the northern portion.
Foremost among its several distinguishing characteristics
is an atypical continental margin in which the transition
from deep-sea to shore occurs over a broad zone rather
than a more usual sharply defined continental slope.
Based upon basalt dredged by Krause (1964, 1965)
and upon oversimplified bathymetry which emphasized an
impression that basin floors in the Baja California
Borderland were commonly more than 2000 m deep, Suppe
(1970) suggested that the southern continental borderland
is actually a rhombochasm which opened during early move
ment on the San Andreas Fault System. He postulated that
oceanic crust formed in the widening gap. Offset pro
duced by the rhombochasm when added to the offset on the
San Andreas Fault System owing to a later opening of the
Gulf of California thus yielded the total offset suggested
by Hill and Dibblee (1953). Suppe further observed that a
reconstruction of the southwestern margin of North
America based upon the assumed combined offsets would re-
Figure 1, The Baja California Borderland as
studied in this paper. Actual
southern limit is probably at about
27° N latitude.
3
4
< ! *
[ft
suit in a reconstruction whereby the Salinian Block would
assume a position producing continuity of parallel belts
of Franciscan and bathollthlc rocks. Paired belts of
this kind are shown by Suppe (1970) to be common around
the Pacific margin and are considered to indicate old
subduction zones.
While total offsets based upon the rhombochasm
hypothesis approach those required to meet the total off
set on the San Andreas Fault System proposed by Hill and
Dibblee (1953), timing of the various phases of movement
remains a problem. Geologic relationships associated
with the San Andreas Fault System enumerated by Suppe
(1970) constrain the formation of the proposed rhombochasm
to the time interval between the end of the Cretaceous and
the end of the Oltgocene.
Atwater (1970) perceived a similar role for the
southern continental borderland in her treatment of the
plate tectonic history of western North America. She sug
gested that a zone of northwest-southeast crustal exten
sion existed in the region about 10 m.y. B.P. and that all
of the basins, ridges, and rifts were initiated at that
time. Deformation was complete 5 m.y. B.P,, prior to the
opening of the Gulf of California.
Purpose of the Investigation
6
Little is known concerning the detailed geology of
the Baja California Borderland) although prior reconnais
sance studies indicated that the area has some unusual
geologic features. Considering the paucity of data,
there has been considerable speculation concerning the
geology and tectonic historv of the region. Understand
ing the area appears to provide a key to the interpreta
tion of the post-Cretaceous geologic history of the lead
ing edge of the North American Plate in this zone encom
passing Baja California, the Gulf of California and most
of California south of the Transverse Ranges,
Specific objectives of the project were (1) to as
certain the nature of the transition from deep-sea to
shore in the regioni (2) to better define the physiography,
its boundaries, llthology and structure% and, (3) to
provide a definitive interpretation of the geologic
history of the borderland.
Methods of Investigation
Continuous Reflection Seismic Profiling
More than 3200 km of continuous reflection seismic
profiles (Fig. 2) were recorded during two cruises total
ling 24 days aboard the R.V. VELCRO IV and 13 days aboard
the A.G.O.R. de STETGUER. All cruises were accomplished
Figure 2 Locations of the seismic profiles
referred to In this study.
7
ar
8
J*'
«
K
9
between Ortober 1970 and November 1971, Profiling was
accomplished using a Bolt air gun system interfaced with
a Gifft wet paper recorder aboard the R*V. VELERO IV. Al-
3
though a chamber of 10 in was occasionally used* most air
3
gun records were obtained using a 20 in chamber. A
30,000 Joule sparker system with a dry paper Raytheon re
corder w a s used in the profiling work done aboard the
A.G.O.R. De STE1GUER. Although both systems produced
interpretable records* those obtained with the sparker
system were superior.
Standard reflection seismic profiling techniques
and procedures were used (Hersey* 1963i Moore* 1969).
Seismic records were reduced to a workable size using a
procedure recommended by E. C. Bufflington of the U. S.
Naval Underwater Research Center, Records were reduced
photographically using a Polaroid MP-3 camera system with
high-contrast Polaroid film (Type 51 P/N), Settings were
held constant for all photographs of the records and
individual photographs were overlapped with those on each
side so that the central portions free of aberration were
available for constructing the final complete reduced
section. The trimmed and butted sections were mounted on
stiff cardboard sheets in proper depth orientation thus
constructing a continuous profile at reduced scale.
Transparent overlays were placed on the photographed pro
files and interpretive cross-sections produced by tracing
10
first echoes* Geologic detail was added as interpretation
proceeded together with appropriate dimensional scales*
An example is shown in Figure 3* For comparison a photo
graphed field record is shown in Figure 4.
Dredging and Piston Coring
Dredge hauls were collected from both vessels dur
ing four of the six cruises* Dredges were constructed
from meter-wide sections of oil well casing approximately
1.5 m long. Chain bridles were attached and a grid of
rods welded to the bottom end to catch only the larger
rock fragments and allow free washing of the muds. A
conventional piston corer was used with a 6 m barrel.
The piston cores were collected for a related study*
Locations of dredge and piston core stations are shown in
Figure 5. Also included in the same figure are the loca
tions of Krause's dredge hauls which were examined for
this study courtesy of the Scripps Institution of Oceano
graphy. Thin sections were made of representative rock
clasts from each dredge haul and several volcanic and
plutonic rock specimens were dated using the K-Ar method.
When foraminlferal assemblages were available in the sedi
mentary clasts* these sedimentary rocks were dated pale
ontologically (see Doyle and Bandy, 1972).
Figure 3. Profile line 21B from the Colnett
Basin to the continental shelf*
Fauls which border the east side
of the basin are apparent.
11
03S
12
21 B
NE
10 km. 20 km
X35
sw
Figure 4. One of the composites of high con
trast pictures of the original
seismic records* Overlays were
made from these*
13
m m m ,
w *
Figure 5 Locations of dredge hauls collected
during this study. Scripps samples
are SOB designations.
15

Navigation
17
Precise navigation within the study area was dif
ficult due to the lack of good reference stations and
beacons. Only one Loran station (San Diego) is receivable}
none exist in Baja California, Longitude was established
with reasonable accuracy from the one Loran bearing but
latitude was difficult to determine with the same pre
cision.
A number of other techniques were used in this
study for positioning. When within range of the Baja
California coastline, navigation was accomplished by radar.
Celestial navigation and dead reckoning were used when out
of radar range of the coast. Longitudes were checked
using the one Loran station. Bathymetry was also used to
check general position, especially when dredging and cor
ing. These methods were applied aboard both R.V, VELERO
IV and A.G.O.R. De STEIGUER.
Since navigation in the area was not precise,
plotted positions out of radar range of the coast are
probably accurate to only 3 or 4 km. Bathymetric crests
were established by crossing the ridges at right angles to
the ridge trend. Dredge hauls were taken at or near the
crests.
PREVIOUS WORK
General Statement
Several geologically diverse areas surround the
Baja California Continental Borderland. To the east and
southeast Is the peninsula of Baja California. The re
mainder of the continental borderland lies to the north-
northwest, and the California Seamount Province of the
Pacific Basin is to the west.
These surrounding areas are geologically related
to the southern portion of the entire continental border
land, and a synoptic view of their geology establishes an
overall setting for the study area.
Baja California
Baja California is a mountainous peninsula which
extends south from the California border for over 1100 km
(Fig, *) and forms a malor portion of the Peninsular Range
Province. General geologic descriptions of a reconnais
sance nature were presented by Beal (l948), Wlsser (1954),
Jahns (1954), and United Nations staff (1969). Several
Investigations of more limited scope have been conducted
18
Figure 6, Northern part of the Baja Cali
fornia Peninsula adjacent to the
study area. Major mountain ranges
are shown along with the Agua
Blanca and Santo Tomas Faults.
19
C A L I F O R N I A
O f
OU L F
#*r* Marti t
Nj
O
>*
21
(Minch, 1967, 1970, 1971, 1972* Minch and others, 1970i
Flynn, 1970). Along with Minch, the group at California
State University at San Diego under Gastll and Allison
have been conducting widespread surveys of northern and
central Baja California adjacent to the Baja California
Borderland. Preliminary geologic maps which they have
prepared show a widespread system of faults which trend
northwest-southeast, parallel to the major structural
grain of Baja California. Basin and ridge physiography
which characterizes the borderland does not appear to ex
tend into the Peninsular Ranges of Baja California. Ad
jacent to the study area are a few small faults which
trend northeast-southwest toward the coast* Data from the
California State University at San Diego maps show them
dying out before they reach the coastline, however. The
major exception to this is the Santo Tomas Fault.
The northern portion of Baja California adjacent to
the study area is composed of a high westward-tilted block
separated from the Gulf of California trough and the
Colorado Desert on the east by imposing scarps between
1800 m and 2000 m high (Beal, 1948) Jahns, 1954). The
core of the tilted block is part of the Southern Cali
fornia Batholith of Upper Cretaceous age (Larson, 1941,
1945, 1948i Jahns, 1954). Exposed on both flanks of the
batholithic complex, and in places on the batholith it
self, are older metamorphic and plutonlc rocks (Beal,
22
1948). These rocks nay be Paleozoic to Mesozoic In age
(Jahns, 1954). Lower Cretaceous and early Upper Cretaceous
Irregularly netanorphosed sedlnents, pyroclastlc and other
volcanic rocks overlie the basement complex on the west
flank (Sant11lan and Barrera, 1930j Beal, 1948% Jahns,
1954), Resting unconformably upon the Intruded meta-
morphlc rocks on the west flank is a series of unmeta-
morphosed marine sediments of Cretaceous and Tertiary
ages (Beal, 1948* Flynn, 1970| Jahns, I954f Minch, 1967|
Minch and others, 1970).
Marine sedimentation on the Baja California main
land ceased at the time of the mld-Flelstocene orogeny
which led to a period of emergence. Non-marine sedimenta
tion has occurred since that time In many parts of the
peninsula.
Northern Baja California has been an area of
volcanlsm throughout Its known geologic history. Cenozolc
volcanic rocks are reported from many areas along the west
coast of the peninsula. Miocene tholelltlc basalt
(14.3 - 2.6 m.y. B.P.) was reported In the Tljuana-
Ensenada region (Minch, 1967t Hawkins, 1970), basalt
dated at 4.3 - 2.0 m.y. from Punta Canoas, and basalt with
an age of 2.5 * 0,05 m.y. Is described from east of
Rosarlto (Krumnenscher, 1969, unpublished). Basai.t and
andesite of Pleistocene age have been reported near Santa
Rosalia by Wilson (1948) and tholelltlc basalt of Pleisto-
23
cene age is reported by Woodford (1928) from the San
Quintin area.
Highest elevations and the most rugged topography
in northern Baja California are on the eastern side of the
Sierra Juarez and the Sierra San Pedro Martir which com
prise the westward-tilted block. The western flank slopes
toward the Pacific and has a more subdued topography.
Several old erosion surfaces occur south of the inter
national border (Millert 1935t Beal, 1948i Jahns, 1954i
Minch, 1971, 1972i others). Scattered over several of
these old erosion surfaces Minch (1972) reported out
croppings of what appears to be Poway Conglomerate, a non-
marine pebble to boulder conglomerate. Irregularly inter
calated with marine sandstone (Ellis and Lee, 1919i Jahns,
1954).
Northern Baja California, like the remainder of the
Peninsular Ranges and the borderland offshore, is dominated
by a complex northwest-trending, steeply dipping fault
system. Transverse to this dominant series in northern
Baja California is the Agua Blanca Fault, a roughly east-
west trending structure at least 120 km longi it has
right-lateral displacement along its entire length (Allen
and others, I960). According to Hamilton (1971), the fault
ends in a structurally complicated area in eastern Baja
California before it reaches the Gulf of California. The
Agua Blanca Fault branches as it approaches the Pacific
24
coast of the peninsula. The northern branch, showing
recent activity, extends into the sea at Punta Banda (Fig,
1) and may connect with the San Clemente Fault in the
northern portion of the continental borderland (Allen and
others, 1960| Gastil and Allison, unpublished). The
southern portion appears to be inactive in the area of
exposure 16 km inland from the coast. It enters the sea
Just north of Punta Santo Tomas in the Bahia Soledad
(Allen and others, 1960t Gastil and Allison, unpublished)
(Fig, 1), The seaward extension, called the Santo Tomas
Fault, has been postulated by Krause (1964, 1965) and
Moore (1969) to be a major left-lateral fault separating
the northern and southern portions of the continental
borderland,
A broad, gentle syncline deforms the Cenozoic
rocks of the Pacific margin of mainland Baja California
(Beal, 1948), This syncline follows the major structural
grain of the peninsula, northwest. The axis is at the
coastline and part was postulated by Beal (1948) to lie
offshore.
South-southeast of the study area lies the western
cape region of Baja California (Beal, 1948), Plutonic
rocks of the Southern California Batholith constitute the
basement in this area. An important part of the plutonlc
basement rocks are Franciscan-type graywacke, chert,
greenschlst, blueschist, serpentine, amphlbolite, and
25
hornblende gneiss which are exposed on Isla Cedros, Viz
caino Peninsula, Santa Margarita, Islas Benitos, Santa
Magdalena (Beal, 1948i Cohen and others, 1963\ Yeats and
others, 1971). Normark (1969) discovered a series of
"basement ridges" offshore south of the Isla Cedros area.
He postulated that at least some of these ridges are com
posed of rocks comparable to the Franciscan assemblage--
that the ridges probablv occur gn echelon--and that they
generally trend toward the southern portion of the con-
rinental borderland* Sedimentary rocks of Cretaceous and
Tertiary age unconformably overlie the basement complex.
Volcanic rocks occur in the Franciscan equivalent rocks
and in the Tertiary sequences.
The peninsula of Baja California formed as the re
sult of the opening of the Gulf of California by sea floor
spreading from short sections of the East Pacific Rise
separated by transform faults (Larson and others, 1968i
Moore and Buffington, 1968). As in California, the San
Andreas Fault, west of the Baja California Peninsula, is
considered to be bounding a part of the Pacific Plate which
is moving to the northwest. Initiation of the northwest
ward movement of Baja California began about 5 million
years B.P. (Atwater, 1970).
California Continental Borderland
26
The California Continental Borderland from about
34° 30' N latitude to 31° 31* N latitude is among the most
studied continental margins of the world* It is character
ized by a basin-and-range physiography which structurally
has been divided into three broad zonesi an outer zone
characterized by block faulting* a central zone warped
into broad folds* and an inner zone characterized by block
faulting. Several of the tops of the upthrown blocks and
anticlinal folds are above sea level* whereas others form
flat-topped banks which are relatively shallow. General
ly, bank tops and basin floors deepen toward the south. A
northwest-trending fault system is dominant in the northern
borderland as well as In adjacent areas.
Rocks of the California Borderland range from
Jurassic to Plio-Pleistocene in age. Miocene shales,
cherts and limestones are the most common rocks dredged
from this area. Volcanic rocks of presumed Miocene age
also are common. Hawkins and Allison (1970) and Hawkins
(1970) reported alkaline basalt from Northeast Bank* and
note that Pliocene fossils are incorporated In the vol
canic material. This is the youngest dated volcanic
activity reported in the borderland.
Baja California Seamount Province
27
The Pacific BaaIn seaward of the continental
borderland area, both north and south, is characterized by
a large number of submarine volcanoes of Late Tertiary or
Quaternary age (Menard, 1955, 1964), One volcano, Isla
Guadalupe, breaches the surface of the sea southwest of
the study area, Isla Guadalupe is composed of calcic
andesite and basalt (Johnson, 1953). Fossils included
within cemented tuff are of Late Tertiary or Quaternary
age.
Krause (1961) named the sea floor east of Isla
Guadalupe the Arruaado■ It is a smooth rolling area of
abyssal hills covered by a thin layer of soft sediment.
Average depth is about 3500 m. Popcorn Ridge forms the
northern border of the Arrugado (Fig. 1), and south of
Popcorn Ridge in the Arrugado Is a basin with depths of
about 4300 m. To the east is Cedros Deep, a flat-floored
trough which abuts the Baja California Borderland from
the south (Fig. 1).
Baja California Borderland
The bathymetry of the southern portion of the con
tinental borderland was first studied by Shepard and
Emery (1941). Additional soundings reported by Shepard
(1950) and by Emery (1960) added to the basic data.
28
Krause (1964, 1965) gathered more bathymetric information*
compiled unpublished information from Menard* Fisher* and
Moriarty of Scripps Institution of Oceanography and pre
sented a chart based upon all available data. Moore
(1969) republished Krause's basic chart with minor modi
fications and added detail (Fig. 7),
Krause (1965) listed several problems basic to
construction of the chart. Irregularly spaced tracks com
bined with the naturally rugged character of the bottom
led to difficulty in interpolation. Celestial navigation
and shore fixes were used for positioning* and as a re
sult* positioning was not precise introducing additional
contouring errors. Despite these limitations the chart
in Figure 7 represents the most detailed bathymetry of the
borderland off Baja California available until the present
study.
Earlier investigators recognized that the basic
basin-ridge character of the Southern California Border
land continued in the Baja California offshore area. In
all* about 20 closed basins within the Southern California
Borderland have been recognized and described (Fig. 7).
Despite the general physiographic similarity* Krause
(1964* 1965) and Moore (1969) suggested several Important
characteristics of the continental borderland off Baja
California which d! *tinp;n1 nh it from the southern Cali
fornia portion. They recognized the Santo Tomas Fault
Figure 7, Portion of a bathymetric chart of
the Baja California Borderland,
after Moore (1969). Contours are
In fathoms. Latitude and longi
tude lines are 30' apart or about
30 nautical miles.
29
119°H 118°tf U7°W 1X6 °W
(?
C M
r>
*
m
o
ro
30
rsi
£
31
(Fig. 8) as a major structural feature* They suggested
that in addition to about 15 km of left-lateral offset*
the regional bathymetry dropped by about 450 m on the
southern side of the fault and Implied that the border
land south of the fault was downthrown as a block relative
to the northern side. Thus* a profound difference between
the two areas was postulated, Krause (1964* 1965} em
phasized the importance of basalt found in some of his
dredge hauls implying that significant llthologic dif
ferences existed between the two portions of the border
land as well. Even though Krause (1964* 1965) described
these rocks as "fresh** and probably young, other authors
(Moore* 1969 and Suppe* 1970) inferred that these basalts
are samples of the basement of the Baja California Border
land. These earlier workers noted that ridge tops off
Baja California lack the flat top characteristic of banks
off southern California, Finally and perhaps most signi
ficantly, they postulated that the transition from the
Pacific Basin to the continent between 29° N and 31° 31* N
occurs over a broad zone rather than over a steep, con
tinuous, continental slope more typical of most continental
margins. These reconnaissance studies therefore pinpointed
several objectives deserving more detailed study.
Figure 8. Approximate location of the Santo
Tomas Fftult» after Moore (1969).
32
STUDY AREA
60
N. M l,
iPN RlQGE
LITHOLOGY
General Statement
Bathymetric highs were sampled in order to deter
mine the types of rock which crop out in the study area.
Additional dredge hauls collected by Krause (1964, 1965)
were sampled through the courtesy of Scripps Institution
of Oceanography. Locations of dredge hauls discussed
here are shown in Figure 9,
The lithology of the Baja California Borderland is
summarized in Figure 10 and at each dredge location major
constituent rock types are listed.
Dredge haul contents show that the southern border
land is an area of lithologlc diversity similar to the
northern portion. Conglomerates, sandstones, mudrocks,
diatomites, calcareous rocks, metavolcanics, metasedi
ments, acid plutonics, basalts, pyroclastics, and authlgen-
ic rocks commonly are present. Manganese oxide coats
rocks on many of the ridges, and is present as well in
the form of micro-manganese nodules in unconsolidated
sediment from some of the bathymetric highs, Lithologles
have continental rather than oceanic affinities.
Dredge hauls usually consisted of several rock
34
Figure 9
Location of Scrlpps Institution of
Oceanography dredges (SOB) and
Allan Hancock Foundation dredges.
35

Figure 10. Lithologlc summary of dredge hauls
from Che Baja California Border
land. Abbreviations are as fol
lows! cgl * conglomerate, pyr ■
pyroclastics, ss ■ sandstone and
sandy siItstone, b ■ basalt, p ■
phosphorite, f7 ■ possible Fran
ciscan metagraywacke. Enough data
to make a viable geologic map have
not been accumulated.
37
38
A
A
39
types and since hauls were consistently taken either at or
near ridge crests, it is unlikely that this mixture repre
sents float. Many of the rocks sampled appeared to be
broken from outcrops, especially the sedimentary rocks.
In part, the diversity may be due to the effect of sampl
ing a conglomerate. The dredging technique Itself doubt
less explains much of the mixture in that a dredge often
remains in contact with the bottom for as long as 20 or 30
minutes, and thus a number of different rock types may be
encountered, especially if the haul is taken perpendicular
to the ridge crest.
Sedimentary Rocks
General Statement
Sedimentary rocks are the most common constituents
of dredge hauls from the Baja California Borderland. Sand
stone and mudrock are the most ubiquitous, followed closely
by conglomerates. Olatomite and limestone are less often
dredged. Sedimentary rocks range in age from Upper
Paleocene to Holocene and resemble rocks in the stratl-
graphlc sections of both the Southern California Border
land and the Los Angeles Basin.
Sandstones and Midrocks
40
Fine silty sandstone and sandy siItstone comprise
the bulk of the datable sedimentary rock. With the help
of 0. L. Bandy, age determinations from foraminifera in
cluded within the sedimentary rocks were made (Doyle and
Bandy, 1972). Float from the bottom of Animal Basin
SOB-5 (Krause, 1964) contains Upper Paleocene sandstone.
Middle Miocene (Rellzian) sandy si Itstone was identified
in sample SOB-37, Sample 14937 contains silty sandstone
and a diatomite concretion of Upper Miocene (Mohnian) age,
and samples 3A and 10 are preglacial or lnterglacial
Pleistocene sandy siltstone and unlithifled fine sediment,
respectively.
In addition to age determinations paleoecologlc
interpretations from the samples were made (Bandy, per
sonal communicationi Doyle and Bandy, 1972). Table I
summarizes fauna1 data on the aforementioned samples.
Depths at which hauls were made are listed. They repre
sent ridge crests. Fauna in the rocks are therefore not
displaced from their real depths. In addition to samples
for which age determinations based upon micro-fauna were
undertaken, sample 14700 contained datable vertebrate
fossils. Fragments of phosphatlzed bone in sample 14700
were tentatively identified by personnel at the Los
Angeles County Museum of Natural History as portions of
Table I.
Samole # Age Lm.y*)
40 i
see Ar Rad/gm
x 10-5
Percent
Ar*0 Rad
Percent
K
14701 3,6? 1 0.012 37 0.82
4.8* 1 0.016 62 0.82
15073 2.8? 1 0.0135 29 1.17
2.9* 1 0.0135
40
see Ar Rad/gm
x 10-5
26
Percent
1.15
Percent
Samle # Age (m.y.)
Ar36 Rad
K
14938 1.0 -9.18 100 1.22
1.0 -0.19 100 1.24
Sample #14938 argon numbers are Che result of the averages of two double runs*
One was done in a helium atmosphere* According to Teledyne Isotopes (personal communi
cation), the results are consistent and indicate that the basalt is extremely young*
The error indicated is a summation of all analytical error.
42
the pre-maxtilery bone of a Miocene whale.
Sandstones and raudrocks containing no datable
material also are present in many samples* SOB-1, SOB-10,
SOB-20, SOB-22* SOB-27, and SOB-30 (Krause, 1964i personal
observation this study), and samples 14698, 14938, 14939,
15072, 15074 all have appreciable amounts of clastic sedi
mentary rocks present which could not be dated. Many of
the sandstones are graywackes and subgraywackes, some of
which are altered and are more properly termed metagray-
wacke. Altered rocks are discussed in the section on
metamorphic rocks.
A large diatomlte concretion, shown in Figure 11a,
was recovered from Sample 14937. The concretion is about
1 ra long and 25 cm wide at its widest point, 20 cm in
diameter at the small end, and 23 cm at the large end. A
hole from 8.3 cm In diameter in the small end to 5.7 cm
in diameter in the large end extends the length of the con
cretion. Manganese dioxide up to 1 cm thick coats the
exterior. This coating is in two layers, each about 0.5
cm thick. The inside surface of the concretion also is
coated with a much thinner layer of manganese dioxide,
1-2 non thick. Filaments of manganese oxides have invaded
the Interior of the concretion from both outside and in
side surfaces for about I cm and display the multi-
bifurcations common to dendrites.
Figure lib shows a cross section of the concretion.
Figure 11a, Large dlatomlce concretion with
manganese coating dredged at
station 14937 on Soledad Ridge
from a depth of 900 m.
43
44
Figure lib. Cross section of the diatoraite
concretion.
45

47
The central portion Is white to off-white In color, and Is
composed of diatoms, radlolarlans, sponge spicules,
foraminifera, and small molluscs cemented in a calcareous
matrix, A few sand-sized quartz grains and other detrltal
minerals are also present. Material of the concretion
resembles in color, composition, and texture diatomaceous
layers within the Monterey Formation of southern Cali
fornia.
Niino (1933, 1952, 1955) reported finding concre
tions superficially similar to that present in 14937 from
the continental shelf of Japan and from Dume Canyon off
southern California, A concretion which he calls a Msand
pipe," found at the Dume location, is associated with lime
stone of probably Miocene age. The Dume Canyon specimen
differs from its southern borderland counterpart in
several important ways. It is smaller, tapered and closed
at one end. The Dume Canyon Msand pipe*' consists largely
of sand-sized quartz and biotite grains with a few in
cluded molluscs, arthropods, and benthic foraminifera. It
has been drilled by several boring organisms, Niino (1955)
suggested that the "sand pipe" represented a fossil worm
or crab burrow favoring the latter organism as being re
sponsible for its formation.
The southern borderland specimen was dredged from
the crest of Soledad Ridge in 900 m of water, although the
incorporated fauna Indicated deposition at about 2000 m.
Burrows of considerable size have been observed in bottom
photographs at similar depths In the present oceans. The
best explanation for its formation Is that it represents
the burrow of a Mohnlan bottom-dwelling animal of con
siderable size. Most burrows are vertical. Since its
formation in the Upper Miocene, the concretion has been
uplifted about 1000 m. Significant submarine weathering
and erosion is indicated in that the concretion was free
on the bottom at the time of dredging, as evidenced by a
complete coating of manganese oxide.
Many of the dredge hauls (14698, 14701, 14938,
15074, 3A, 6, SOB-1, SOB-5, SOB-20, SOB-27, and SOB-31)
contain rounded to well-rounded crystalline boulders and
large cobbles. Lithology of the clasts as determined from
hand specimen suggests that they are derived from the
Poway Conglomerate or its equivalent. The Eocene Poway
Conglomerate has been described by Bellemln and Merrlam
(1958) and Milow and &mls (1961), and a description of
the distinctive types of clasts within the formation may
be found in Woodford and others (1968). Hand specimens
were compared with Merrlam*s collection of Poway-type
clasts. Merrlam (personal communication) also confirmed
the thin section and hand specimen identification of the
of the Poway-type clasts.
49
Identification of the clasts is based upon several
distinctive characteristics listed by Woodford and others
(1968), Most of the clasts are siliceous metavolcanlc
rocks of which soda rhyolite tuff is the most common.
Clasts contain large grains of feldspar, quartz and
altered blotite in a groundmasa of altered glass, quartz
and feldspar. Textures as described by Woodford and
others (1968) and recognized in the dredged clasts include
feldspar crystals wrapped around collapsed, recrystallized
pumice and relict pumice lenses. Recrystallized and col
lapsed pumice lenses and shards are present in many of the
dredged Poway-type clasts. Piedmontite, epldote, and
chlorite are present in some clasts as blotite pseudo-
morphs* The piedmontlte is especially diagnostic of
Poway-type clasts. In one instance hematite pseudomorphs
were observed.
In addition to the siliceous metavolcanlc clasts,
well-rounded boulders and cobbles of other metavolcanlc
rocks, quartzites. plutonic rocks* and andesltes were
dredged with the Poway-type clasts. They formed minor
constituents of the dredge hauls. According to Bellemin
and Merrlam (1958). Poway Conglomerate contains similar
small but significant percentages of these clasts.
PjgPMSglPB
50
Krause (1964) conjectured that the rounded crystal
line rocks were deposited by kelp rafting. An alternative
explanation Is that these coarse elastics represent an
extensive conglomerate deposit.
Emery and Tschudy (1941), Shumway (1953), Emery
(1955, 1960, 1963) and Pratt (1970), among others, have
suggested that rafting by various types of plants and
animals can account for the transportation and deposition
of rock material on continental margins and in the deep
sea. Kelp rafting Is the only viable mechanism available
for the biologic transport of significant quantities of
coarse elastics In the seml-arld southern California and
Baja California region, but kelp Is limited In the size of
the clasts it can transport. PelaaoDhvcus. a genus common
to southern California and Baja California waters, can
grow to a length of over 30 m (Emery and Tschudy, 1941),
The long blade of the plant Is supported near the surface
by a float chamber whose volume may be as large as one
liter. While such a large float has considerable buoyancy,
most of it Is required to float the remainder of the plant.
When the kelp's rootlike holdfasts are tom loose they
often carry with them pebbles and small cobb.ws. Hold
fasts examined by Emery and Tschudy (1941) contained both
rounded and angular pebbles.
51
Material considered by Shuraway (1953) to be kelp-
rafted In the deep sea west of Cedros Deep consisted only
of pebble-sized clasts. Furthermore* Shuraway stated that
circulation of surface waters through much of the year
would move floating kelp along the coast of Baja Cali
fornia as far south as Cedros Island before It was carried
to sea (Gorsllne, 1956) (Fig. 12).
Crystalline boulders and large cobbles on the Baja
California Borderland are uniformly rounded* In many
cases they are polished like stream transported clasts and
may weigh up to 10 kg. The uniform degree of roundness*
the appearance of having been transported in a fluvial
environment, their widespread distribution and abundance,
the current patterns described by Shuraway (1953) off Baja
California, and especially the size of the clasts them
selves argue against their presence in the Baja California
Borderland being the result of kelp rafting.
2
Poway Conglomerate covers about 320 km in San
Diego County. Although variable, it averages at least 100
m in thickness over Its extent (Woodford and others, 1968).
Poway Conglomerate commonly lies unconformably on the
Cretaceous bathollthlc complex and pre-Cretaceous meta-
morphic rocks of the Peninsular Ranges. Most of the Poway
is a continental deposit but a small part is marine
(Bellemin and Merrlam, 1958i Cushman and Dusenbury, 1934).
Minch (1972) recognized the conglomerate which
Figure 12. Circulation pattern for Viscaino
Bay (after Gorslinei 1956) il
lustrating southerly nearshore
flow of California Current.
52
53
THE SURFACE TEMPERATURE DISTRIBUTION
SEBASTIAN VISCAINO BAY
DATA FROM SCRIPP’ S INST'N. OF OCEANOGRAPHY
MLR CRUISE 52, SEPT. 8-21, 1933
CONTOURED AT INTERVAL OF I' CENTIGRADE
GENERALI ZF-D
* CURRENT 01R
-A A A M convergence
\
54
drapes bathollthlc rocks In parts of northern Baja
California, Mexico (Beal, 1948) as Poway Conglomerate and
concluded that It represents major Middle to Late Eocene
river channel deposits in this area. He assigned a west
to west-southwest transport direction to the rivers. Until
the recognition of Powa-type clasts in the dredge hauls
from the Baja California Borderland, Minch's work marked
the southern limit of Poway-type material. Presence of
Poway clasts on the borderland extends the range 290 km to
the south (Sample 3A) and 240 km to the southwest (Sample
14938) on Soledad Ridge.
From the dredge hauls it is impossible to determine
whether the Poway-type clasts are derived from the original
Eocene Poway Conglomerate or were reworked and deposited
higher in the stratigraphlc section of the southern part
of the borderland. No fosslllferous material was recovered
that definitely could be related to the clasts*
Discovery of the siliceous tuff boulders and cobbles
in the southern borderland is relevant to the problem of
source of the Poway-type clasts. Several possible source
areas previously have been considered. DeLisle and others
(1965) dated zircons by lead-alpha methods that ranged in
age from 190 m.y. to 260 m.y, B.P. in Eocene Poway Con
glomerate clasts in San Diego County. This age range
limits the possible source to three areas. DeLisle and
others (1965) favored the Sidewinder and Hodge Volcanic
55
Series In the central Mojave Desert because they are with
in the age range of the Poway clasts and no other rocks
similar to the Poway clasts in the correct age range are
known within a reasonable distance of outcrops of the
southern California Poway Conglomerate. Woodford and
others (1968) showed that the Sidewinder and Hodge Vol
canic Series were distinct petrologlcally and chemically
from Poway rocks and they favored a source in a Late
Paleozoic igneous complex which is no longer exposed. No
evidence for the existence of such a complex is provided.
The third possibility Is that the source is the Irmiris
volcanic rocks which cron out in Sonn-ra, Mericn, sooth of
Nogales, Arizona (Merrlam, 1968, and personal communica
tion* and Minch, 1972), Imuris volcanic rocks, whose age
is unknown, are chemically and petrographlcally similar to
Poway clasts (Merrlam, 1968). Other authors have not
favored an Imuris source because of their great distance
from the northern deposltional area. If the San Andreas
Fault System is restored 480 km along strike and 72 km
of "cross strike" separation is closed (Hamilton, 1961),
total river transport of at least 220 km is necessary to
move the Poway clasts from an Imuris source to northern
deposltional sites. Woodford and others (1968) believed
that this distance is excessive for the boulders and large
cobble-sized clasts found.
For the Baja California Borderland clasts, any re-
56
construction of transportation routes from a source area
north of the Mexican border results In a greater distance
of transport than the 220 km objected to by Woodford and
others (1968), Sonoran volcanics are the most central
source for all the Powav-type clasts In their northern and
southern deposltional sites. Paleo-drainage patterns in
northern Baja California suggesting a west to southwest
transport direction (Minch, 1972) are compatible with a
Sonoran source, which best explains the presence of the
clasts on the Baja California Borderland.
Other Sedimentary Rocks
Limestone and possibly dolomite occur in a few
dredge hauls on the Baja California Borderland, Krause
(1964) reported dolomite to be common in SOB-10, SOB-17,
and SOB-37, Examination of the SOB hauls during this
study did not confirm the presence of dolomite. Limestone
is common In SOB-22, and rare in SOB-6, SOB-17, and SOB-30
(Krause, 1964* and this study).
Authlgenic rocks comprise a few percent of most
dredge hauls. Manganese crusts occur on many of the rocks
and as separate pieces In many hauls. About 15 percent of
dredge hauls SOB-1 and SOB-6 and about 30 percent of hauls
15074 and 14698 are phosphorite. In the latter two samples,
phosphorite Is present as massive crusts In which rounded
boulders and cobbles are imbedded. Pasho (1973) reported
57
that northern borderland phosphorites are of Upper Miocene
age.
Metamorphis Rocks
In addition to the siliceous metavolcanlc clasts
discussed in the previous section* two other Important
groups of metamorphic rocks are present in dredge hauls
from the southern borderland. The most Important con
stituent of Sample 14700 is a fine-grained metagraywacke.
Similar rocks constitute about 40 percent of haul 15072,
These metagraywackes are angular* appear to be freshly
broken from outcrop, and are petrographically similar in
thin section to Franciscan graywackes on Isla Cedros
(R. H. Merrlam, personal communication).
Although presence of metagraywackes alone Is not
sufficient evidence to positively establish the existence
of Franciscan basement rocks in the southern borderland,
they do suggest that possibility. Dredge hauls 14700 and
15072* shown grouped in Figure 9, are the first indication
of Franciscan basement from the southern borderland.
Samples 14698, 15073, and 15074 contain meta-tuff
and quartz-mica schist which are similar in thin section
to the metamorphic roof pendants of the Cretaceous Penin
sular Range Bathollth.
Igneous Rocks
58
Volcanic Rocks
Volcanic rocks are important constituents of
several dredge hauls from the Baja California Borderland*
Basalt composes over 50 percent of hauls 15073, SOB-6,
SOB-17, SOB-21A, SOB-25, and SOB-28A and between 20 per
cent and 50 percent of samples 6, 14938, 14701, SOB-1,
SOB-20, and SOB-30. Pyroclastic rocks in the form of ash
and tuffaceous sandstone compose less than 20 percent of
hauls from Soledad Ridge and samples SOB-5, SOB-13, SOB-22,
and SOB-33.
Whole rock K-Ar age determinations were made on
basalts from hauls 14701, 14938, and 15073 by Teledyne
Isotopes Incorporated, Westwood, N. J. Samples were taken
from angular boulders which were most likely to have been
dredged from a site close to their source areas. They
were chosen from three widely separated areas within the
Baja California Borderland. Replicate determinations were
performed on each sample*
The results, shown in Table I, represent the young
est ages reported for igneous rocks from the continental
borderland. Other young rocks of late Pliocene age based
upon Included fossils» from Northeast Bank in the northern
part of the borderland, were cited by Hawkins and Allison
(1970) and Hawkins (1970).
59
Dating submarine basalts has Its pitfalls. Dal-
rymple and Lanphere (1969) discussed the problems at
length. They argue that initial does not escape from
lavas extruded at great water depth and as a result, K-Ar
dates on deeply extruded submarine basalts tend to be too
old, Dalrymple and Moore (1968) reported that lavas
erupted during the last few thousand years gave ages as
great as 43 m.y. Dalrymple and Lanphere (1969) suggested
that results from shallow water and from coarser grained
interiors may be more reliable, though caution should be
used In accepting old dates. Anomalously low ages may
result from badly weathered samples, although Armstrong
(1966) stated that when weathered samples are dated and
compare with fresh material discrepancies are slight.
Only samples that were unweathered were used, since
submarine basalts tend to give ages too old if in error,
and since all three samples have very young ages. It ap
pears that these age dates are consistent with a late
period of volcanlsm having superimposed basaltic rocks
upon older strata in the southern borderland. A similar
superposition has been reported for the Baja California
Peninsula (Beal, 1948i Hawkins, 1970), Therefore, the
basalt dredged from the Baja California Borderland in con
junction with older sedimentary rocks represents a late
geologic event and does not constitute samples of basement.
The young dates suggest that the basalts were either ex
60
truded at shallow water depths or subaerially. If they
were extruded at oceanic depths anomalously old ages would
have resulted*
Plutonic Rocks
Several sub-rounded to well-rounded boulders of
plutonlc rocks were dredged from the Baja California
Borderland. Samples 14701, 14938, 15072, and 15074 con
tained boulders of quartz monzonite and sample 14698 con
tained a boulder of hornblende dlorlte. In addition, in
sample 15072 angular boulder-sized blocks of pure quartz
were noted, which may be from a vein or pegmatite. The
plutonlc rock is similar lithologically to rocks of the
Peninsular Range Batholith in Baja California. A quartz
monzonite boulder from Soledad Ridge (14938) was dated
using K-Ar techniques. An age of 91 m.y, B.P, - 2,7 m.y.
was obtained. This date corresponds to the age of the
bathollth and confirms their relationship to It, eliminat
ing the possible provenance of Jurassic plutonlc rocks.
In samples 14698, 14701, 14938, and 15074 they are
associated with metavolcanlc clasts of the Poway type and
probably should be included within the conglomerate repre
sented by Poway-type clasts. Such clasts comprise a small
but significant portion of the Poway Conglomerate in
southern California (Bellemln and Merrlam, 1958). Boulders
of plutonlc rocks must have been transported an appreciable
61
distance because they are well-rounded and because thev
are associated with the Poway-type clasts whose distance
of transport Is known to be large.
PHYSIOGRAPHY AND STRUCTURE
Physiography
Additional Bathymetric Data
Figure 13 is a portion of a bathymetric chart
which* to the knowledge of the author, is appearing in
print for the first time courtesy of the Pacific Support
Group of the Navy Oceanographic Office, This chart was
last revised in September 1968. Sounding lines upon
which the chart is based are less than 9.2 km apart
throughout the study area, which is the same density as
off southern California. Generally, the sounding-line
density is a measure of the reliability of the bottom
contours and in this case reliability is good. To
eliminate additional errors of interpolation, contours
shown in fathoms on the original chart were converted
directly to metric equivalents in Figure 13. Therefore,
this chart should be a better representation of the actual
bathymetry than that shown in Figure 7.
Discussion of Regional Physiography
There are important differences in bathymetry be
tween Figures 7 and 13. The northeast-southwest transverse
62
Figure 13. Bathymetric chart for the Baja
California Borderland. Contours
are in meters in direct conver
sion from fathoms on the original
B.C. 1206 Naval Oceanographic
Office chart.
63

65
trend from Punta Santo Tomas to the continental slope is
more striking in its outer portions In the later chart.
No structures except the continental slope and the near
shore shelf cross it. While the Santo Tomas fault is
bathymetrically better defined, sense of offset is vague
within the borderland. The depth across the Santo Tomas
Fault Zone locally drops dramatically into Animal Basin.
To the east relief is less precipitousi at the coastal
shelf it is absent-.
Emery fl954) postulated that the entire borderland
was gently folded Into a broad downwarp transverse to its
northwest-southeast axis. He demonstrated that the
heights or depths of mountain and ridge tops, basin floors,
and basin sills decreased regularly from 34° 30' N, the
Transverse Ranges, to 31° 30* N, the southern limit of his
data. Emery's mountain and ridge top and basin data for
the northern borderland are combined with similar data
compiled in this study for the southern borderland (Fig,
14). The depths decrease regularly for both ridge tops
and basin floors to the southwest to about 29° 50* N, after
which the depths for both begin to rise. There is no
abrupt break in the slope of the curve across the
northeast-southwest trend of the Santo Tomas Fault at about
31° 30* N. Emery's contention that all of the borderland
is in the form of a broad downwarp appears to be upheld.
The southern portion does not seem to be downfaulted as a
Figure 14, Depths of bank tops and basin
floors in the Southern California
and Baja California Borderlands
(modified after Emery, 1954),
66
ftttTltf X 101
w
w
w
c
O
Ml
W
M
m
68
large block as suggested by Krause (1964, 1965) and Moore
(1969),
South of the southwest-northeast trend at about
31° 30' N, the Baja California Borderland Is dominated by
two troughs which deepen toward the center of the area
(Fig. 13). Each large trough may be subdivided Into two
or more basins separated by sills. In this aspect the
region Is similar to the borderland off southern Cali
fornia.
Several basins shown In Figure 13 are substantially
different In shape than In the earlier data of Figure 7.
Animal Basin Is a two-lobed basin, one trending northeast-
southeast, the other northwest-southeast, rather than the
long relatively narrow basin. Each lobe is a separate
sub-basin. A sill of about 1900 m depth separates the two
sections. From the new data, San Isidro Basin appears as
a minor perched impoundment with a channel which probably
passes sediment either to the deeper adjacent Colnett
Basin or to the trough leading to San Quintin Basin. San
Quintin and Colnett Basins are separated by a sill nearly
2100 m deep. The large trough west of Punta Canoas falls
away into San Quint in Basin (Fig. 13), a deep basin
separated from Soledad Basin to the west by a sill about
2300 m deep. Soledad Basin appears as a much larger basin
in Figure 13 than in Figure 7 and has two subdivisions.
North Soledad Basin and South Soledad Basin, separated by
69
a si 11 approximately 2500 m deep. Blanca Basin, North San
QuintIn Baain, and Outer Baain (Fig. 7) do not appear to
be separate, closed basins in Figure 13. Table 11 com
pares the physical parameters of the basins in the border
land off Baja California and off southern California.
The continental slope of the southern region ia
significantly different in the two bathymetric charts. The
Rampart in Figure 7 is a regular escarpment between 30° 50*
N and 31° 30* N, No expression of the major northeast-
southwest trend of the Santo Tomas Fault Zone is apparent
in the continental slope in Figure 7, On the other hand.
Figure 13 shows a large gap at 31° 15* N and 118° 18' W
for which the name Santo Tomas Gap is proposed. The slope
appears to be offset in a left-lateral sense about 13 km
across this gap. Santo Tomas Gap is aligned with the
projection of the Santo Tomas Fault and is discussed in
more detail in a subsequent section.
A second large gap for which the name Ba 1a Gap is
proposed ia shown in Figure 13 at 29° 50' N and 117° 20* W,
The sense of offset across this gap is less clear than
across Santo Tomas Gap.
Uchupl and Emery (1963) noted that several similar
gaps, all with left-lateral offsets of from 4 to 13 km
occur in the borderland off southern California. The gaps
have a northwest-southeast orientation and they are 10 to
40 km wide.
70
Table II. Parameters of borderland basins
Southern Borderland off Baja California
Basin
No.
of
Sills
Depth of
Lowest Depth of
Sill _ Bottom
Area at
Sill Depth
(meters) (meters)
West Animal 2 1920 2080 1280 sq • km.
East Animal 2 1920 2060 640 sq. km.
Colnett 1 2100 2310 530 sq. km.
North Soledad 1 2290 2500 262 sq. km.
South Soledad 2 2681 3200 421 sq. km.
San Quintin 2 2290 3150 3350 sq. km.
Northern Borderland off Southern California
modified after fernery (I960)
Santa Barbara 1 470 600 663 sq. km.
Santa Monica 1 730 925 1810 sq. km.
San Pedro 2 730 900 656 sq . km.
San Diego 1 1350 1350 0 sq • km.
Santa Cruz 1 1070 1930 1780 sq. km.
Santa Catalina 2 965 1335 2140 sq. km.
San Clemente 1
1780
2080 1480 sq. km.
San Nicolas 1 1090 1810 2405 sq. km.
East Cortes 1 1390 1945 1060 sq. km.
West Cortes 5 1340 1765 1010 sq. km.
71
Table 11. Parameters of borderland basins (continued)
Northern Borderland off Southern California
(continued)
Basin
No.
of
Si 11a
Depth of
Lowest
SilL
Depth of
Bottom
Area at
Sill Deoth
(meters) (meters)
Tanner 1 1150 1550 1265 sq. kra
No Name 2 1550 1885 306 sq. km
Long 2 1670 1910 830 sq. km
Velero 1 1870 2525 1320 sq. km,
72
In Figure 13 the continental slope appears to be a
distinct physiographic unit off Baja California which Is
as continuous as the Patton Escarpment off southern Cali
fornia* Also in Figure 13, a bathymetrically distinct
continental slope separates the Baja California Border
land from the deep sea* not a broad zone of transition.
By far the largest percentage of area In the south
is less than 2000 m deep, depths greater than 2800 m are
rare, and true oceanic depths of greater than 4000 m do
not occur. Depth differences may be explained by a
regional downwarp previously considered and by increasing
distance from provenance areas, primarily the Transverse
Ranges. Emery (1960) reported a trend of basin deepening
and more irregular basin floor bathymetry from northeast
to southwest and southeast, away from the active sediment
sources of the Transverse Ranges. The deep Los Angeles
and Ventura Basins are completely filled, as shown by the
seismic profiles of Moore (1969) and this study. As the
basins fill, broad flat floors are Increasingly better
developed. Therefore, basins progressively more removed
frora sediment source areas tend to have more Irregular
floors. This trend holds except for San Quintln Basin In
the Baja California Borderland, the southernmost basin in
Figure 13, which appears to have more fill and a broader
flat floor than basins adlacent to the north and west.
These characteristics suggest that San Quintln Basin is
73
receiving major sediment contribution from the Vizcaino
shelf to the southeast * and ultimately from the Sierra
Vizcaino southeast of Punta Canoas (Fig. 13).
Structure
General Statement
Structural relationships discussed In this
section are the result of analysis of over 3200 km of
continuous reflection seismic profiles. Krause (1964*
1965) and Moore (1969) recognized that the basin and
ridge bathymetry of the Baja California Borderland Is
governed by hlgh-angle normal faulting. Their studies
raised Important questions, already summarized, concerning
the nature of the boundaries of the area and the transi
tion from adjacent areas. As a consequence, this section
concentrates heavily upon the boundary areas of the Baja
California Borderland and an attempt is made to answer the
previously raised questions. Results are shown In Figure
15 In which the major structures Interpreted In this study
are summarized.
Seismic profiles have appreciable vertical exaggera
tion shown in each figure as X (times) the horizontal
scale. The heavy black lines which represent faults do
not depict the true orientation of the fault planei they
only indicate that a fault is interpreted as being pre-
Figure 15, Major structures recognized in
the Baja California Borderland.
74
75
r>
m
76
sent and show Its sense of vertical displacement. Many of
these faults doubtless have a strike-slip component which
In some Instances may be appreciable or even dominant.
The vertical scale is In two-way travel time. Based upon
the average speed of sound in seawater, 1 sec of two-way
travel time is equivalent to about 750 ra of water depth.
The direction or geographic orientation of each line is
noted in each figure. Location of the seismic lines is
shown in Figure 2.
Trans11ion from Deep Sea to
Ba 1a California Borderland
The Continental Escarpmenti
Figures 16 through 30 show the transition from the
deep sea to the western Baja California Borderland. There
is a distinct structural and bathymetric break between the
deep sea and the Baja California continental margin over
the entire length of the area. Figures 16, 18, 19, 21
through 26, 29, and 30 show a thin veneer of deformed
sediments which are in apparent fault contact with a dis
tinct continental escarpment which plunges away beneath
them. Dredging has shown that the escarpment itself Is
covered by a llthified sedimentary section distinct from
the soft Holocene sediments exposed to seaward. The crest
of the escarpment is between 700 and 1800 m deep wherever
Figure 16. Profile* 19C from S«nto Toma* Gap
to the escarpment south of Velero
Basin.
77
19 C
0
L
a
ui
V )
ESCARPMENT
I0km.20km.
_l I
X 23
Figure 17. Profile 31B from the base of the
continental escarpment across the
head of Santo Tomas Gap to the
high area northwest of Animal
Basin.
79
10 km 20 km.
I I I
X 3 5
I
r i
o
UJ
C O
00
o
Figure 18. Profile 18C from the deep sea
across the middle of Santo Tomas
Gap to the escarpment south of
Velero Basin.
81
03$
ESCARPMENT
Figure 19. Profile 13 parts (A) and (B).
Part A extends from the deep sea
to the continental escarpment.
Part (B) runs northeast across
Soledad Basin and Into Animal
Basin.
83
E
CONTINENTAL
ESCARPMENT
Figure 20. Profile 17C from the base of the
continental escarpment across the
Santo Tomas Fault Zone to the
high area west of Animal Basin.
85
SEC.
I7 C
D EEP SEA
10km. 20km
i
E SC A R P M E N T
00
O
Figure 21. Profile 20B from the deep sea
over the escarpment. A sediment*
filled trough I* present behind
the ridge.
87
20 B
ESCARPMENT
DEEP SEA
10 km. 20 km
X35
Figure 22. Profile 19B which crosses the en
tire Baja California Borderland.
Transition from deep sea to border
land, the trough behind the outer
ridge, little sediment fill In Col-
nett Basin and the San Isidro Im
poundment are shown.
89
SAN ISIDRO
COLNETT
Figure 23. Profile 12A which depicts the
transition from the deep sea to
the escarpment north of Soledad
Ridge. Numerous faults• which
may be small thrust faults, are
shown in the deep sea. High
angle normal faults are evident
at the base and up the seaward
side of the escarpment.
91
\D
to
»*
Figure 24. Transition from the ocean basin
to Soledad Ridge Is shown 1n pro
file 16B, Normal faults or slumps
are shown on the seaward side of
the ridge. Deformed sediment is
visible at the base.
93
94
I6B
SOLEDAD RIDGE
- \>
0 10 km. 20 km.
1 I I
X 35
SEC.
Figure 25. Profile 10A showing the transition
from deep sea to Soledad Ridge.
Outlier is probably volcanic.
95
3M
*1
Figure 26, Profile 7B from the deep sea over
Soledad Ridge, Considerable sedi
ment thickness at the base of the
escarpment is visible. Soledad
basin behind the ridge is shown.
97
SOLEDAD
RIDGE
7B
0
L_
Stan iQkm
"XlC— '
DEEP SEA
V0
a
SEC
Figure 27. Profile 13B traverses part of Baja
Gap. The fault which makes San
Quintin Basin a two-tiered basin
Is shown.
99
130
I
SOUTH SAN QUINTIN
BASIN
0 10km. 2 0 k m
I 1 I
X35
Figure 28, Profile 15C depicts the structure
from Soledad Ridge across Soledad
Basin and down into the trough at
the northern end of San QuintIn
Basin.
101
SEC
SOLEDAD RIDGE
W
10km. 20km.
-L I
X 2 3
S. SAN QUINTIN
BASIN
Figure 29. Profile 14C reveals transition
from the deep sea to the border
land via the southern flank of
Baja Gap.
103
SEC.
I4C
SOUTH SAN QUINTIN
BASIN
DEEP SEA
W
\ \ \ *
t
IOkm. 20km.
i 1
§
104
Figure 30, Profile 12B runs from the base
of Ferrel Sea Mount across the
trough south of San Qulntin
Basin, Folded sediments at the
base of the sea mount are evident.
105
901
107
crossed. The seaward slope of the escarpment is trans
ected in many places by differing numbers of either small
normal faults or slumps which step down toward the ocean
floor.
Seaward of the escarpment itself is a section of
what is interpreted to be unconsolidated sediment with a
thickness of between about 0.3 sec and 0.4 sec two-way
travel time. Below this section the seismic records show
no Interpretable structure. Lack of structures atL similar
depths in the deep sea off Baja California in unpublished
profiles taken by D. G. Moore with a large sparker system
and subsequently made available to the author suggest that
this is due to a lack of reflecting horizons. Within the
sedimentary section (Figs. 19, 23, 25, and 26) there is a
reflector more distinct than those above and below, at
about 0.2 sec two-way travel time below the sedlment-water
interface.
According to Atwater's (1970) model, the Baja
California continental boundary must have been a zone of
subduction prior to its incorporation into the Pacific
Plate, A trench must have existed along the continental
margin similar to that present today off the west coast of
South America, At the base of the escarpment, the sedi
ments thicken in an apparent slight downwarp to as much as
0.6 sec to 0.7 sec and appear to be more deformed than
those adjacent to seaward. This thickening of deformed
108
sediments at the base of the escarpment is the only sug
gestion of a paleo-trench apparent from this study.
Moore (1969) calculated and plotted the interval
velocities of sediments within the San Diego Trough, San
Qulntin Basin, and San Clemente Basin, using wide-angle
seismic reflection. Velocities which he calculated are
approximately that for sound in seawater, 1,5 km/sec, to
a depth of about 300 m within the sedimentary column.
Below 300 m, velocities increase rapidly. At 0.6 sec two-
way travel time, velocity is about 1.8 km/sec. Based upon
Moore's interval velocities multiplied by half of the two-
way travel time for each Interval, the unconsolidated
sediment thickness on the ocean basin floor off the Baja
California continental escarpment is about 300 m, and the
section thickness at the base of the escarpment is 450 to
500 m. This can be compared with about 750 m of fill in
the Cedros Deep (Shor and others, 1963), Moore (1969)
stated that the deep sea in this area has thin sedimentary
cover and Raitt (Krause, 1965) calculated a layer thick
ness of 100 to 300 m for unconsolidated sediment in the
Guadalupe Arrugado, south of the Baja California Border
land. It is therefore likely that the base of the sedi
mentary section in the deep sea shown in the figures under
discussion represents the top of layer 2.
The sea floor west of the escarpment has undergone
considerable deformation. Its surface Is Irregular and
109
Che uppermost sediments are deformed. Either the deforma
tion is still active or the sediments have been draped
over a recently deformed portion of the sea floor,
A number of faults are evident on the sea floor
away from the escarpment (Figs. 19, 23, 25, 26). Usually
the downthrown side of the fault is seaward, the faults
transect the uppermost sediment layers, and hence either
have been active recently or are still active. Inasmuch
as the vertical exaggeration makes the definition of the
orientation of the fault plane uncertain, these faults
may represent small, low-angle, thrust faults showing com
pression at some time between the deep sea and the Baja
California margin.
The two layers of sediment in the unconsolidated
section may represent pre-orogenlc and post-orogenlc sedi
ments similar to those which Moore (1969) was able to
recognize in the basins within the continental borderland.
The bottom layer appears slightly more deformed. Sedi
ments either may have been involved in the late tectonic
event which may be still in progress or, alternatively,
may be draped over deformed shallow basement.
£fiB£sm_Rld£$«
Popcorn Ridge forms the boundary between the Baja
California Borderland and the Guadalupe Arrugado and
Cedros Deep to the south. Profiles from the deep sea over
110
Popcorn Ridge are shown In Figures 31-38. At Its western
end, Popcorn Ridge is dominated by Ferrel Seamount which
could provide the source for the basalt dredged by Krause
<1964, 1965). Figures 31, 32. 33, 35, and 36 indicate
that there are layered rocks on top of Popcorn Ridge and
dredge hauls 3 and 3A at the eastern end of the ridge re
turned sedimentary rocks. These data suggest that Popcorn
Ridge is parr of the Baja California Borderland and is
contlnental.
The escarpment separating Popcorn Ridge from the
deep sea to the south Is extremely steep. In the western
portion (Figs. 34-36) an outlier 1s apparent, possibly of
volcanic origin. The escarpment-deep sea contact Is inter
preted as a major fault. Left-lateral offset is indi
cated in the borderland transverse to the predominant
northwest-southeast trend, and is over 90 km. Theberge
(1971) reported a major offset In the magnetic anomalies
east of Isla Guadalupe which corresponds with an extension
of the Popcorn Ridge Fault trend.
Xfae.Gapa
General statementi
In the southern part of the Baja California Border
land, Figures 29 and 34 through 38 cross Baja Gap which
appears to be about 18 km wide (Fig. 37), Two definite
layers of sedimentary fill are evident within r*e gap.
Figure 31. Profile IB runs from Cedros Deep
to San Quint In Basin and then to
the San QuintIn shelf. Sediments
on top of the escarpment north
of the deep are revealed.
Ill
112
SAN QUINTI
SHELF
S A N
QUINTIN
b a s in
0 10km. 20km.
1 ------L 1
X35
CEDROS DEEP
Figure 32. Profile 3B shows the fault at
the base of Popcorn Ridge and
sediments on top of the ridge.
113
114
3B
NORTH SAN
QUtNTIN BASIN
CEDROS
DEEP
0 I O k m 20 km
X35
SEC
Figure 33, Profile IOC depicts the Popcorn
Ridge faultt sediments within the
Cedros Deep, and the sedimentary
section on Popcorn Ridge east of
Ferrel Seamount. The northwest
end of the profile crosses Baja
Gap.
115
SEC.
LINE IOC
Figure 34, Profiles 4B and SB are presented.
Profile 4B crosses the ridge west
of San Quintin Basin, Section 5B
shows the steep escarpment of Pop
corn Ridge.
117
SEC.
118
POPCORN
RIDGE
0 10 km. 20 km.
5B
X35
03S
Figure 35. Profile 11C from the deep sea
across Popcorn Ridge. Baja Gap
is at the north end of the
traverse.
119
lie
POPCORN RIDGE
o
UJ
CO
0 I Okm. 2 0 km.
1 I _____I
X 2 3
D EEP SEA
GUADALUPE ARRUGADO
i
*
120
Figure 36, Profile 12C shows Popcorn Ridge
and Its associated fault and the
Baja Gap at Its northwest end.
121
3 .5
o
UJ
V)
I2C
NW
10 km. 20 km
J I
POPCORN RIDGE
DEEP SEA
UADALUPE ARRUGADO
Figure 37. Section along traverse 9B cross
ing Baja Gap.
123
1 ?4
9B
N
Okm. 10
20km
X 35
03S
Fipiurp 3ft. Profile 13C 1* a short section
crossing the outermost part of
Baja Gap,
125
X23
m
SEC
o j
o j
o
921
127
The uppermost layer Is about 0.4 sec or about 300 m
thick* the second layer Is about 0.2 sec or about 170 m
thick for a total of about 470 m. This thickness is
maintained towards the head of the gap (Ftp, 28).
Ba 1a Gap t
Bala Gap is considered to be fault-controlled be
cause profiles which cross the gap in a north-south
direction show a series of normal faults at the southern
and northern boundaries, A block in the escarpment with
hinge-line to the east may have been rotated downward to
the west forming the gap. Alternatively, the gap may be
related to an east-west trending fault system upon which
there has been differential strike-slip movement as com
pared to the major left-lateral fault interpreted at the
base of Popcorn Ridge.
Santo Tomas Gapi
In the northern part of the Baja California
Borderland, Figures 16, 17, and 18 are north-south cross
sections of Santo Tomas Gap. Figure 18 is farther to the
east or toward the head of the gap. The gap appears to
be bounded on both sides by faults. About 400 ra of sedi
mentary fill is present in the gap in the westernmost
profile (Fig. 16) which thins to approximately 225 m
nearer the head (Fig. 18). Figure 17 illustrates a
section farther to the east where it is apparent that even
128
with the increased vertical exaggeration the gap has
narrowed appreciably forming a canyon.
Santo Toraas Gap la the expression of the Santo Tomas
fault In the submerged continental escarpment off Ba ja
California, The trough-like nature of the gap, broad at
the seaward margin and narrowing to the east as shown in
Figures 16 and 18, and the trend of the Santo Tomas fault
Into the gap, suggest this relationship. Faults shown
with small components of vertical throw in Figures 16 and
17 are Interpreted to be strike-slip faults which are part
of the Santo Tomas Fault Zone.
Santo Tomas Fault Zone
Figures 16-20 and 39-44 cross the trace of the
Santo Tomas fault zone at right angles. They show that
the fault zone is a major transverse structure which may
easily be delineated to the continental escarpment where
It is expressed as the Santo Tomas Gap* The fault zone
is a broad area with many individual faults, some of which
have only a small component of vertical movement. Prin
cipal motion on the fault appears to be strlke-slip.
Figure 15 shows the location of this major structure,
whose position over its entire length and Its relation
ship to the continental escarpment is defined completely
for the first time.
The sense of offset on the fault is not absolutely
Figure 39, Profile 16C la along and Just east
of the continental escarpment of
the Baja California Borderland.
129
see.
0 to t o n 20 t o *
1 I ___ I
X 2 3
ISC
SOLEDAO RIDGE
SE
130
Figure 40, Profile 30B transects Animal Basin
and the Santo Tomas Fault Zone.
131
0
L
1 0 k m .
I
X35
20 k m .
Figure 41. Profile 28B# on which the Santo
Tomas Fault Zone is shown.
133
28 B
~v
10 km. 20km.
_ J I
X35
Figure 42. Profile 26B shows the Santo Tomas
Fault at Its southeast end and part
of the Descanso Plain northwest of
Punta Santo Tomas.
135
SEC.
136
26 B
%
i
DESCANSO
PLAIN
o
L
10km. 20km.
_J I
NW
X 35
Figure 43, Profile 12B depicts Animal Basin
and Santo Tomas Faults and the
ridge between Animal Basin and
Descanso Plain at the northern
limit of the Baja California
Borderland.
137
L I N E 12
PA R T B
O E S C A N S O P L A IN
Figure 44, Profile 24B on which the Santo
Tomas Fault Zone Is clearly
evident.
139
24 B
DESCANSO PLAIN
I
10 km. 20 km.
J I
X35
•fs
O
0 3 S
53
141
clear. Bathymetry tn Figure 13 may be Interpreted as
showing about 13 km offset In the continental escarpment
In a left-lateral sense, an Interpretation which depends
upon the two prominences southwest of the Santo Tomas Gap
entirely volcanic In nature. Unfortunately only one
dredge haul was made from these two highs. Krause (1965)
described basalt In place In sample SOB-25 from the high
nearest the gap. If these two high areas are not sea
mounts, an interpretation of right-lateral offset can be
made.
Similar ambiguity concerning sense of displacement
exists for the portion of the fault within the mainland
of Baja California. Krause (1964, 1965) and Moore (1969)
assumed a left-lateral offset of 15 km on the submarine
portion of the Santo Tomas Fault. Allen and others
(1960) who studied the Santo Tomas Fault on land as a
branch of the Aqua Blanca Fault found a left-lateral
separation of about 7,5 km based upon offset in basal
conglomerates of the Upper Cretaceous Rosario Formation.
Exposures of the fault near the coast are poor. Allen
and others (1960) suggested that the apparent left-
lateral separation of these beds may Instead be due to a
strong vertical component of displacement for which they
found ample evidence. Indeed, they favor a right-lateral
offset of about 17.5 kra based upon tenuous correlation of
Alblan age limestones across the fault. Evidence Is not
142
conclusive.
Right-lateral offset. In conjunction with the
major left-lateral movement of the Popcorn Ridge Fault,
would Indicate that the Baja California Borderland and
part of Baja California moved westward relative to ad
jacent blocks to the north and south. Much more detailed
bathymetry and more dredging on the escarpment near Santo
Tomas Gap are necessary to establish the correct sense of
displacement.
Uchupt and Emery (1963) proposed that the gaps
which they discovered In the continental escarpment of the
northern borderland may be fault-controlled. Santo Tomas
Gap and probably Baja Gap are controlled by large trans
verse strlke-sllp features. If the gaps in the northern
borderland are similarly controlled, these faults repre
sent a major pattern transverse to the predominant
northwest-southeast trend of the southwestern continental
margin of North America,
Interior of the Borderland
Although location of seismic profiles was concen
trated upon peripheral areas of the Baja California
Borderland, most lines were extended within Its boundaries
and several completely crossed the area. Figures 19, 22,
23, 27-30, 43, 45-48 reveal the transition from deep sea
to borderland, and also show the trough Immediately to the
Figure 45, Profile 22B shows the Santo Tomas
Faults Just to the southeast of
Punta Santo Tomas*
143
228
Figure 46. Profile 15B crosses the Southern
Borderlandt showing Soledad Basin,
San QuintIn Basin, and Colnett
Basin and related structures. A
suggestion of folding la apparent
in the central part.
145
sec.
-fi-
IS B
NE
SW
SOLEDAD
NORTH
SAN
OUINTII
0 »km 20km
1 --1__I
X35
R10BE
146
Figure 47, Profile 7A, which shows possible
folding west of San Qulntln Basin
and the two-tiered nature of San
Qulntln Basin are apparent.
147
BASIN
u
u
oo
148
Figure 48. Transition from Cedros Deep to
Che margin south of Popcorn Ridge
is shown In Profile 10B, The
southern part of the trough in
which San Qulntln Basin lies Is
evident and appears to be fault-
controlled.
149
150
IOB
0
CEDRO
DEEP
O 20km.
151
east of the crest of the continental escarpment (see
Bathymetric Section). In the deeper Soledad Basin, there
Is as much as 650 m of sediment within the trough. Both
pre-orogenlc and post-orogenic sediments as described by
Moore (1969) are distinguishable.
This trough which runs parallel to the dominant
northwest-southeast regional grain Is similar and may be
an extension of that formed by Tanner Basin and adjacent
low areas (West Cortes Basin, Long Basin, and Velero
Basin) to the northwest in the northern borderland (see
chart In Emery, 1960). The trough near the edge of the
continental margin strongly resembles the ridge and trough
system buried deep within the present continental shelf and
slope off the eastern United States and summarized by
llchupl and Emery (1961),
East of the outer long trough In the Baja Cali
fornia Borderland are two ridges which trend toward each
other (Figs. 13, 22, 27-30, and 45-48. While normal
faulting modified the ridges, seismic profiles which cross
them indicate that they may be anticlinal structures.
Sedimentary rocks have been dredged from the crests. These
two structures are In a central position In the Baja Cali
fornia Borderland. Thev may correspond to the central
folded zone of the northern borderland described by Moore
(1969) and may be a southward continuation of It. Al
though the seismic lines which cross the whole of the
152
southern borderland are suggestive of a folded area,
more extensive deep penetration profiling within the area
Is needed to confirm or disprove this hypothesis.
Figures 27 and 47 cross the southern part of San
Qulntln Basin, which was described by Moore (1969). Both
profiles show a ridge which raises the western part of the
basin about 150 km above the eastern part. A fault Is
Interpreted as being at the base of this ridge. The
tiered nature of the southern portion of San Qulntln Basin
has not been previously reported.
Figures 22, 27, 30, and 45-48 cross the whole of
the southern borderland to the continental shelf. The
eastern parts of these figures show the second, Innermost
trough present within the region. Basins here are heavily
faulted and are probably exclusively fault-controlled.
These profiles reveal the rough nature of the bottom of
the northern Colnett Basin, which is probably a condition
related to the small amount of sedimentary fill apparent
In Figures 20 and 45 and which has been shown by Krause
(1964, 1965) and by Moore (1969). Figures 27 and 47 show
the large volume of fill In San Qulntln Basin, and Figures
30 and 48 cross the southern end of the trough which rises
to the southeast toward the Vizcaino shelf. Total fill In
the basin including pre-orogenlc and post-orogenlc sedi
ment exceeds 1 km. Bathymetry and the large mass of
sedimentary fill In San Qulntln Basin suggest that the
153
Vizcaino shelf Is an ultimate source. Sediment probably
feeds Into the basin down the trough and through the can
yons on the eastern rim which Intersect the Ba 1a Cali
fornia shelf.
TECTONICS
The tectonic history of the Baja California Con
tinental Borderland has not been well understood. Paleo-
ecologlc aspects of faunas In the rocks analyzed and sum
marized In this study and In Doyle and Bandy (1972) sug
gest that there have been large-scale differential verti
cal movements within the Baja California Borderland. A
new understanding of the geologic time scale as compared
with the radiometric time scale for the Miocene, combined
with paleoecologv and with llthologlc data from the sedi
mentary rocks, make possible the most precise estimate
yet of the timing of the tectonic events responsible for
formation of the Baja California Borderland.
To the north of the Santo Tomas Fault Zone in East
Cortes and Long Basins, Bandy and Chlerlci (1966) des
cribed a Melonls compeloides fauna from core segments
greater than 11,000 years B.P,, which show that sill
depths of the two basins are 1000 ra and 700 m shallower,
respectively, than they were when the fossils were living.
Thus during the Holocene, uplift at a rate of 6 to 9 cm/yr
Is indicated for East Cortes and Long Basins.
Doyle and Bandy (1972) cited faunal evidence for
ISA
155
over 1000 in of uplift since the late Miocene (SOB-37, see
Fig, 9 for locations). In haul 10 they found that the
fauna Indicated little change in depth since the Pleisto
cene. Dredge haul 3A shows faunal evidence for about 500
m of subsidence since the Pleistocene. Haul 3 has a
Middle Miocene fauna which Indicates 700 m subsidence. In
contrast, sample 2 based on faunal evidence has risen
about 500 m of subsidence during the Holocene.
An Upper Mohnlan fauna comparable to that in con
tinental rise deposits of the present eastern Pacific
Ocean was identified In the sedimentary rocks of sample
14937 on Soledad Ridge. This suggests that the Soledad
Ridge portion of the continental escarpment attained Its
present high bathymetric expression after late Miocene
time.
Bandy (1971) was able to correlate sinlstral coil-
ing Globorotalia pachyderms fauna representing a cool
climatic cycle with Neogene zone 18 of the Upper Miocene
in the Mohole section (Fig. 15), Moreover, Bandy (1971),
Bandy and others (1971), and Ujile and Mlura (1971) re
cognized this zone to be within ,the uppermost Gilbert and
possibly lower Gauss magnetic epochs. This brings the
radiometric time scale into harmony with the blostrati-
graphic time scale. Latest Miocene then is placed by this
correlation at between 3 m.y. to 4 ra.y. B.P, Therefore,
Soledad Ridge began to be uplifted to its present position
156
not earlier than Neogene Zone 18 or not earlier than
about 4 m.y. ago (Doyle and Bandy, 1972),
Deposition of Poway-type clasts as suggested from
dredge hauls described earlier could have occurred no
earlier than Eocene time, the age of the Poway Formation.
Such clasts may have been deposited in the area of study
subsequent to the Eocene If they represent reworked
material. Sandy si Itstone and silty sandstone comprise
much of the Mohnlan ape sedimentary rocks on Soledad
Ridge. Depositions! sites for the clastic sediments
could not have been physically separated from provenance
areas to the east and north by basln-rldge bathymetry
prior to clastic deposition. Hence, block faulting which
developed the present bathymetric expression of the Baja
California Borderland must be post 4 m.y. Although Emery
(1960) and Moore (1969) suggested a Late Miocene age for
the borderland formation, recent adjustment of the geo
logic time scale means that onset of borderland deformation
is more recent in absolute time than previously considered,
and corresponds in time with the opening of the Gulf of
California and westward movement of the Pacific Plate as
described by Moore and Buffington (1969), Atwater (1971),
Larson (1972), and others. Northern borderland deforma
tion was placed at late Miocene by Emery (1960) and thus
is penecontemporaneous with that of the Baja California
Borderland. Borderland deformation is subsequent to and
157
does not show the progression In age which would be ex
pected if borderland deformation were initiated by south
ward migration of a ridge-trench transform triple Junc
tion as postulated by Atwater (1970).
Another aspect of the late deformation Is the pre
sence of young basalts superimposed on other rocks of the
Baja California Borderland. Volcanism may still be active
and may explain the presence of high heat flows reported
from the borderland by Von Herzen (1964).
Emery (i960) speculated that prior to borderland
formation, the continental margin seaward of southern
California and northern Baja California was a broad,
gently sloping continental shelf-slope system similar to
that off the present Atlantic coast of the United States.
The presence of Poway-type boulders and cobbles and of the
deep water fauna in the sedimentary rocks of the Baja
California Borderland suggest that here, at least, the
continental margin must have been quite different, perhaps
steeply sloping. These rocks neither resemble typical
open shelf nor slope deposits of the Atlantic continental
margin.
SUMMARY AND CONCLUSIONS
General Statement
I aha 11 first review the discussion of the Baja
California Borderland and the conclusions to be drawn
from that portion of the study and then conclude with a
general model for the origin of the entire borderland
structure off California and Baja California, It Is
likely that the final solution of time of Initiation and
pattern of development will not be forthcoming until deep
drilling is done in the borderland. This may be achieved
in the next three to four years if present plans are ap
proved for extension of the Deep Spa Drilling Program,
Baia California Borderland Summary
Objectives of this study were (1) to add to the
body of geologic data concerning the Baja California
Borderland, (2) to test conclusions of previous works,
and (3) to provide data to meet questions posed by earlier
investigations. Specific objectives were to ascertain the
nature of the transition from deep-sea to continent and to
more clearly define the physiography, boundaries, lith-
ology, structure and geologic history of the area.
158
159
New bathymetric and seismic reflection profiling
data reveal that the Baja California Borderland south of
about 31° 31* N dominated by a ridge-trough-r1dge-
trough-shelf pattern. Each large trough has within It
low areas or basins separated from each other by sills.
Basins appear to be predominantly fault-controlled.
Since the borderland formed relativeiv r^r'pntlv and
Is still or reoentlv wss rhetorically active, it Is a
voung continental margin. The r1dge-trough complex which
Is present within tho borderland Is similar In form to a
rldge-trough complex found at depth within the Atlantic
continental margin, and therefore may be a common physio
graphic stage which many continental margins evolve
through.
The entire borderland including the southern border
land is interpreted to be folded in a gentle downwarp
transverse to its northwest-southeast axis. Depths within
the troughs and depths of the ridge tops decrease regu
larly to the south reaching a low at about 29° 50’ N.
South of this latitude basin bottoms and ridge tops become
progressively shallower. The curve of ridge top and basin
bottom depths is a relatively smooth one. The Baja Cali
fornia Borderland does not thus appear to have been drop
ped across the Santo Tomas fault as a large block relative
to the borderland off southern California.
Interpretation of seismic profiles suggest that the
160
centra! ridges, which In the Interior of the borderland
separate the major troughs, may be anticlinal structures
modified by faulting. This rone Is correlative with the
central folded zone recognized by Moore (1969) in the
northern borderland.
New bathymetry has Increased the detail and altered
the shapes of several basins discussed and named by Krause
(1964, 1965) and subsequently by Moore (1969), Animal
Basin has been found to be a two-lobed basin, each lobe of
which 1s a separate sub-basin. One lobe of the basin is
unique in that it trends northeast-southwest across the
major structural grain of the southern borderland. This
transverse lobe may be related to the Santo Tomas fault
svstem which appears to pass through it. San Isidro Basin
now appears to be a minor impoundment rather than a com
pletely closed basin. Soledad Basin is much larger than
was previously recognized and it too is divided into two
separate sub-basins. Blanca Basin and (Xiter Basin do not
appear as separate closed basins in the newer bathymetric
data.
Bathymetry and continuous reflection seismic pro
filing reveal that there is a distinct bathymetric and
structural break between the deep sea and the Baja Cali
fornia Borderland over the entire length of the area and
that the escarpment is in fault contact with the ocean
basin floor. The leading edge of the escarpment exhibits
161
a number of small faults or slumps.
Two gaps In the southern continental escarpment
are comparable to the five Raps reported by Uchupi and
Emery (1963) in the seaward slope of the northern border
land. The two southern gaps contain significant sediment
fill. Of the two, the northern gap, named Santo Tomas
Gap, is an expression of the Santo Tomas Fault Zone in the
Baja California continental escarpment, and the more
southerly gap, called Baja Gap, also Is considered to be
fault-controlled.
Transition from Guadalupe Arrugado and Cedros Deep
to the Baja California Borderland is over a precipitous
escarpment, Popcorn Ridge. A major left-lateral strlke-
sllp fault is interpreted to form the face of this ridge
which offsets the continental escarpment by as much as 90
km to the west. Seismic profiles show that there are
layered rocks on the crest of Popcorn Ridge, and so It is
not entirely volcanic as previously renorted.
The location of the Santo Tomas fault, doubtless
strike-slip, is defined to the seaward edge of the Baja
California Borderland. Sense of offset is not clear and
solution to the problem of offset on this fault awaits
further work. The Santo Tomas Fault Zone is a major
transverse structure of the Baja California Borderland.
The Pacific basin floor seaward of the Baja Cali
fornia Borderland is covered by unconsolidated sediments
162
about 300 m thick laid down over what is Inferred to be
oceanic layer 2. At the base of the escarpment sediment
thickens to about 450 to 500 m. It defines a downwarped,
small trench filled with sediment at the escarpment base,
not a prograding prism which would be expected if the
thickening represented an incipient continental slope.
This small feature is the only suggestion of a paleo-
trench at the edge of the Baja California Borderland.
The sea floor 1s rugged and 1s broken by a number
of faulrs, which may be small thrust faults Indicative of
oast oempress1 on between sea floor and continental margin.
Deformation of the sea floor may be recent or even cur
rent lv active since uppermost sediments are deformed. An
alternative explanation 1s what the thin layer of uncon
solidated sediments is draped over deformed rocks,
Lithology of the Baja California Borderland is
varied. Rocks are continental and not oceanic in aspect*
Sedimentary rocks are the major type dredged from the
region, A sedimentary section is present on parts of the
escarpment as well as on the ridges within the body of the
Ba 1a California Borderland, Sedimentary rocks were re
covered in dredge hauls which range in age from Paleocene
to Holocene, with most common lithologies being silty-
sandstone or sandy-siltstone. Rounded crystalline cobbles
and boulders are recognized as being Poway-type clasts
from a conglomerate. They are not kelp-rafted erratics
163
as previously suggested. Recognition of Poway-type
clasts on the Ba/)a California Borderland extends their
known range over 290 km to the south and over 240 km to
the southwest, A Sonoran volcanic source for the clasts,
suggested by Merriam (1968), is most compatible with the
presence of the clasts on the Baja California Borderland,
The clasts either may represent Eocene Poway deposits or
may represent reworked clasts 1n formations higher In the
strati graphic record.
Fine-grained metagraywacke occurs in some dredge
hauls from the southern borderland and is petrologically
similar to Franciscan graywacke on Isla Cedros.
Potassium-argon dating of volcanic rocks (14701,
15073, 14938) selected from dredge hauls made during the
study yielded the youngest ages yet recorded from the
continental borderland, north or south. On the basis of
the extremely young ages (AHF 14938, less than 1,0 m.y.
B.P.j AHF 15073, 2,8 - I m.y. B.F.r AHF 14201, 4.1 - 1
m.y. B.P.) of basalt combined with presence of other rock
types in many dredge hauls it is supposed that the area
has undergone volcanlsm which superimposed young volcanic
rocks over older sedimentary rocks. Dredged basalts thus
do not represent oceanic basement. Volcanic activity
also may explain high heat flows recorded in the border
land.
Presence of Eocene or younger conglomerate and of
164
uppermost Miocene finer clastic sediments on the outer
most escarpment of the Baja California Borderland indi
cate that tectonic events responsible for the present
borderland configuration must have begun after these
rocks were deposited. A recent adjustment bringing the
biostratigraphic and radiometric time scales into harmony
places the advent of tectonic events resulting in Baja
California Borderland formation at no earlier than 4 m.y.
B.P. Borderland formation corresponds in time with the
opening of the Gulf of California and the westward raft
ing of Baja California. Deformation is later and does
not show the migration in time which would be expected
from Atwater's (1970) model.
The Baja California Borderland has more similar
ities with than differences to the northern borderland
and is part of the continental margin, not an oceanic
t
tectonic "hole" or rhombochasm as suggested by Suppe
(1970). In addition to its continental nature, the
tectonic events responsible for Baja California Borderland
formation are appreciably later than is acceptable to
support Suppe*s theory. The entire borderland should be
regarded as a single geologic unit.
r
*
165
General Model for Borderland Development
According to data gathered during this study,
previously held theories concerning the formation of the
Baja California Borderland are no longer tenable. Data
discussed below also place constraints upon formulation of
future theories of borderland formation. Furthermore*
this study shows that the borderlands off southern Cali
fornia and off Baja California are not fundamentally dif
ferent and oust be considered as one unit. Borderland
development must be considered In the tectonic setting of
southwestern North America.
Southwestern North America Is dominated by two
structural trends* both of which are manifested In the
borderland. The most Intensely studied of these trends is
the northwest-southeast trend represented by the San
Andreas fault system. Many theories dealing with the
tectonic history of the southwestern United States have
considered most events to be related to this major trend.
Wilson (1965)* Moore and Buffington (1968)* Larson and
others (1968, 1972)* Atwater (1970)* and Henyey and Bls-
choff (1973), among others, consider the Gulf of Cali
fornia to have opened as the result of northwest-southeast
166
sea floor spreading. The San Andreas fault* no older than
30 m.y., acts as a transform between the northernmost
spreading center In the Gulf and the Gorda-Juan de Fuca
ridge system. These authors have Implied that coastal
California, Baja California, and the continental border
land are part of the Pacific plate moving to the northwest
in response to this system.
The second major structural trend is roughly east-
west. Extension in an east-west direction within the
Basin and Range Province has been proposed by many authors,
among them Hamilton and others (1966), Profett (1971),
Scholz and others (1971), and Davis and Burchfiel (1973).
Davis and Burchfiel (1973) suggest that the left-lateral
Garlock fault is a transform feature at the southern
boundary of the Basin and Range Province. They show that
there is about 100 percent east-west extension in the
crustal block to the north of the fault. They further
suggest that the Santa Cruz-Malibu Coast-Santa Montea-
Raymond-Sierra Madre fault zone, which separates the
northern borderland and the Peninsular Ranges from the
Transverse Ranges, and the Pinto Mountain, Blue Cut, and
Santo Tomas faults may be east-west, left-lateral transform
features of Garlock type having unequal amounts of exten
sion in terranes north and south of each fault. Sumner
(1971) has shown that east-west extension exists across
faults in northern Sonora and in southern Arizona.
167
The borderland contains structures related to both
these major trends. The pattern of the majority of the
faults and the trend of most of the basins and ridges
follow the northwest-southeast trend. Normal faulting!
at basin margins, the left lateral Santa Cruz-Sierra
Madre fault zone at the northern edge of the borderland,
the left lateral Santo Tomas fault in the central part of
the borderland, and Popcorn Ridge at the southern margin,
represent the east-west extensional trend. Gaps similar
to the Santo Tomas Gap and the San Miguel Gap, in both
the southern California and Baja California portions of
the borderland, show left-lateral offset in the continental
margin and may be additional manifestations of the east-
west extensional system (Uchupi and Emery, 1963).
In addition to the major fault controlled struc
tural features, the borderland also has a folded zone,
recognized by Moore (1969) in the southern California
borderland and suggested as being present in the Baja
borderland by this study (Fig. 1). The folded zone is
sandwiched between outer and inner zones where horst and
graben structures predominate. The axes of the folds are
generally parallel to the northwest trending faults.
Borderland structures like the Los Angeles Basin continue
onshore into southern California. Northwest-trending
strike-slip faults are active within the borderland's
inner faulted zone and on shore in southern California.
168
Unpublished data by Gastil and Allison does not
show obvious borderland physiography extended into Baja
California adjacent to the Baja borderland. The north
west structural trend is present in strike-slip faults
north of the Agua Blanca and Santo Tomas faults. Few
faults have been mapped on the western side of the Penin
sula south of these two faults, perhaps at least partly
because recognition of faults in the Igneous terrain is
difficult. A small number of essentially westward trend
ing faults have been mapped by Gastil and Allison (un
published) but they are not recognized as going through to
the borderland.
According to Beal (1948) and Hamilton (1971). the
western two-thirds of northern Baja California consists of
a westward-tilted fault block. The eastern margin of this
block is a northwest trending normal fault manifested in an
imposing scarp. The Agua Blanca fault divides this scarp
into the Sierra Juarez in the north and the Sierra San
Pedro Martlr to the south. The crest is lower and diffuse
Just north of the fault (Hamilton. 1971).
On the eastern side of the Peninsula. Gastil and
Allison (unpublished) show a large number of northwest
striking strike-slip faults which are parallel to the
transform faults of the Gulf of California. This north
west fault trend is Interrupted by a group of northeast
striking normal faults in the Valle Chico and Valle San
169
Felipe (Hamilton* 1971, and Gastil and Allison, un
published). According to Hamilton (1971), Valle San
Felipe and Valle Chico are areas of west-north west exten
sion. Extension Is taken up by the right-lateral offset
of the Agua Blanca fault which apparently ends in the
northern edge of the expanded zone (Hamilton, 1971). The
extensional zone is an area of horsts and grabens physio-
graphically similar to faulted parts of the borderland.
Hamilton (1971) has Interpreted some northwest trending
flexures to be present west of the crest of the Sierra
San Pedro Martir. These features may also be similar to
some of those in the borderland.
The most important conclusion of this study
relevant to borderland tectonics is the timing of the
initiation of formation of its present physiographic ex
pression at about 4 m.y. B.P., penecontemporaneously with
the opening of the Gulf of California. This timing is
based upon the fact that Late Miocene (Doyle and Bandy,
1972) clastic rocks could not have been separated from
their provenance by basin and ridge physiography until
after they were deposited. Supporting evidence for a
young age of borderland formation Includes the fact that
the top layers of sediment in the outermost borderland
basins appear to be deformed or to be draped upon deformed
sediments, Indicating a relatively recent period of de
formation has occurred and possibly still is occurring.
170
Young age of volcanic rocks dated in this study lends
more support. More radiometric dates are needed to test
whether or not young volcanic rocks are widespread. By
Atwater's (1970) model, the borderland formed as the re
sult of northwest-southeast crustal extension about 20
m.y. to 10 m.y. B.P, Deformation by her model was com
plete by 5 m.y. B.P., prior to the opening of the Gulf of
California. Her model does not explain the east-west
extension and her timing is opened to question by this
study.
Timing of plate tectonic events in the Baja Cali
fornia area has been a problem in other investigations.
The last recognizable anomaly west of Baja California, 5A,
was formed 10 m.y. to 12 m.y. B.P., according to the
interpretations of Pitman and others (1968). The last
recognizable anomaly off southern California formed 16 to
20 m.y. B.P. Atwater (1970), Chase and others (1970), and
Larson (1972) used these anomalies to mark the end of sea-
floor spreading and subductlon west of southern Cali
fornia and Baja California. In their plate tectonic
models of the region, a gap of at least 6 m.y. exists be
tween cessation of subductlon west of the tip of Baja
California and initiation of the present configuration of
northwest-southeast spreading at the mouth of the Gulf of
California. Motion between the Pacific and North American
plates during this gap has been assumed to have been strike
171
slipi parallel to the present strike of the San Andreas
fault system» which was taken up at the base of the con
tinental slope. No evidence for take up of plate motion
at the edge of the continental margin exists.
Another interpretation of timing of plate tectonic
events off California and Baja California is possible.
Atwater (1970) states that ages based upon the last
recognizable magnetic anomaly represent only upper limits
for cessation of subductlon and that it is entirely pos
sible that still younger anomalies once existed offshore
but were subducted beneath the continent, this pos
sibility is strengthened by several lines of evidence,
which includes as a major part, the timing of borderland
formation Itself.
Additional support is provided by the Deep Sea
Drilling Project operations off central California,
Paleontologically-dated Late Miocene abyssal fan sedi
ments rest unconformably upon basalts of magnetic
anomalies 10 and 13 which formed 32 m.y. B.P, to 35 m.y.
B.P. (McManus and Bums, 1969). The anomaly closest to
the continent in this area is anomaly nine which formed
about 30 m.y. B.P, A gap of at least 20 m.y. exists be
tween formation of anomaly nine and deposition of Late
Miocene sediments over anomalies 10 and 13, which lie to
the west. This hiatus in the deposltlonal record sug
gests that sediments were being trapped before reaching
172
the deep sea, and that the trench and spreading center Ln
this area may have been active for a significant period of
time after anomaly nine was formed. If this is correct,
other anomalies existed to the east in the Pacific plate
which were subsequently destroyed.
The thinness of the sediments recognized to be
present in the deep sea adjacent to 6aJa California and
the weak suggestion of thrust faulting there (as reported
herein) may be Indications that the continent was not ln
proximity to anomaly 5A when it was formed and that it has
overridden other anomalies once present east of 5A. Lack
of evidence for a Cenozolc subductlon zone along the part
of California now moving northwestward may be explained by
its having been overridden by the continent.
If an adjustment in timing is made, there need be
no gap between cessation of subductlon and borderland
formation and commencement of the tectonic opening of the
Gulf of California.
Any theory concerning formation of the borderland
must explain a complex set of structures within the time
frame provided by this study. It must explain the east-
west extension within the borderland, as well as the
northirest-southeast pattern of faults and fault controlled
structures, and the northwest trending folds which indi
cate compression. Borderland formation must also be
related to events ln Baja California and the opening of the
173
Gulf of California which have been going on simultaneously
and show some similarity ln styles of deformation. Some
structures are common to both the Peninsula and border
land. The fact that the Slerran uplands and scarps show
little dissection indicates a Quaternary or very(?) late
Tertiary uplift and deformation, according to Hamilton
(1971). Early to middle Miocene age volcanlcs wholly
antedate faulting and tilting in northern Baja California
(Gastil, 1969), limiting its formation to post-middle
Miocene, The Agua Blanca fault, which is a major
structure in Baja California, has been projected into the
borderland as the major active San Clemente fault (Krause,
1964, 1965, and Moore, 1969).
Portions of the continental borderland may be ex
plained by deformation in response to northwest-southeast
shear. Internal deformation within the relatively newly
attached portion of the Pacific plate may be caused by
spreading and transform faulting at the mouth of the Gulf
of California. Several lines of evidence point to
Internal deformation within the borderland and the newly
attached part of the Pacific plate besides the structural
trends and features already mentioned. Seismicity shows
that, while the borderland and Baja California are not as
active as the main transform system to the east, deforma
tion is occurring (Sykes, 1968, and Barazangl and Dorman,
1969). Moore (l969) has Interpreted some of this activity
174
as being along northwest striking faults showing at least
some strike slip. Clements and Emery (1947) and Sykes
(1968) show that the inner zone of the southern California
borderland, which contains a large number of northwest
trending faults and may be bordered by a continuation of
the Agua Blanca fault, is seismically more active than
outer and southern portions. Strangway and others (1971)
suggest that portions of northeastern Baja California have
undergone a clockwise rotation of about 30° since the
Pliocene.
Northwest trending folded zones which suggest com
pression and the extensional features are not explained by
the northwest trending shear. It is unlikely that com
press ional and extensional features formed at the same
time. Only the initiation of formation of the Basin and
Range physiography of the borderland is fixed in time by
this study. East-west extension occurred after release of
compression caused by the formally active subductlon zone.
Folding may have occurred earlier while the continental
margin was being subjected to compression from a Cenozolc
subductlon zone located along Its margin.
Adjustments in timing of the ending of subductlon
along the western margin of North America create additional
problems and leave others. If the continents subducted
magnetic anomalies west of the last position of the spread
ing center, then the time at which motion between the two
plates began to be taken up by strike-slip movement was
later than 30 m.y. B.P. If the San Andreas fault is
solely the result of the strike-slip motion between the
two plates, it must be even younger than the 30 m.y. age
ascribed to it by Atwater (1970) in her preferred model.
Finally, the mechanism by which the east-west
extensional features in the borderland formed is not yet
solved and needs to be addressed in future studies.
REFERENCES
176
REFERENCES
Allen, C. R., Silver, L. T., and Stehli, F. G., 1960,
Aqua Blanca fault - a major transverse structure of
northern Baja California, Mexicot Geol. Soc. America
Bull., v. 71, p. 457-482.
Anderson, D. L., 1971, The San Andreas Faulti Scientific
American, v. 225, no. 5, p. 52-68.
Armstrong, R. L., 1966, K-Ar dating of plutonic and vol
canic rocks in orogenic belts, iQ Schaffer, 0. A. and
Zahringer, J,, Editors. Potassium-Argon Datingi
Heidelberg, Springer-Verlag, 234 p.
Atwater, T., 1970, Implications of plate tectonics for the
Cenozoic tectonic evolution of western North Americai
Geol. Soc. America Bull., v. 81, p. 3513-3536.
, 1971, Evidence from plate tectonics for the age
of initiation of deformation on the San Andreas-Gulf
of California systemt Geol. Soc. America Abs. with
Programs, v. 3, p. 75-76.
Bandy, 0. L., 1960, The geologic significance of coiling
ratios in the foraminifer Globlaerina pachvderma
(Ehrenberg)t Jour. Paleontology, v. 34, p. 671-681.
, 1971, Recognition of Upper Miocene Neogene Zone
18, Experimental Mohole, Guadalupe Sitei Nature, v.
233, p. 476-478.
in press, Late Paleogene-Neogene planktonlc
oiostratigraphy and its geologic implications, Cali
fornia t Soc. Econ. Paleontologists and Mineralogists
Spec. Pub.
Bandy, 0. L., Casey, R. E., and Wright, R. C., 1971, Late
Neogene planktonlc zonation, magnetic reversals, and
radiometric dates, Antarctic to the tropicsi
Antarctic Research Series, v. 15, Antarctic Ocean
ology I, fid* J. L. Reid, p. 1-26,
Bandy, 0. L. and Chierlci, M. A,, 1966, Depth-temperature
evaluation of selected California and Mediterranean
177
178
bathyal foramintferat Marine Geology* v. 4, p. 259*
271.
Barazangi, M., and Dorman, J., 1969, World seismicity
maps compiled from ESSA, Coast and Geodetic survey,
epicenter data, 1961-1967s Seismol. Soc. America
Bull., v. 59, p. 369-380.
Beal, C. H., 1948, Reconnaissance of the geology and oil
possibilities of Baja California, Mexicoi Geol.
Soc. America Mem. 31, 138 p.
BelLemin, G, J,, and Merrlam, Richard, 1958, Petrology and
origin of the Poway Conglomerate, San Diego County,
Californiat Geol. Soc. America Bull., v. 69, p.
199-220.
Clements, T., and Emery, K. O., Seismic activity and
topography of the sea floor off southern Californiat
Seismol. Soc. America Bull*, v. 37, p. 307-313.
Cohen, L. H., Condie, K, C., Kuest, L. J., Jr., Mac
kenzie, C. S., Melster, F. H., Pushkar, Paul, and
Steuber, A. M., 1963, Geology of the San Benito Is
lands, Baja California, Mexico» Geol. Soc. America
Bull., v. 74, no. 11, p. 1355-1370.
Cushman, J. A. and Dusenbury, A. N., Jr., 1934, EScene
foramlnifera of the Poway Conglomerate of Californiai
Cushman Lab. Foram* Research Contr., v. 10, p. 51-65.
Dalrymple, G . B., and Moore, J. G •, 1968, Argon 40i Ex-
cess in submarine pillow basalts from Kilauea Vol
cano, Hawaiii Science, v. 161, p, 1132-1135.
Dalrymple, G, B., and Lanphere, M. A., 1969, Potassium-
Argon Datings San Francisco, W. H. Freeman & Co.,
258 p.
Davis, G. A., and Burchfiel, B. C., 1973, Garlock faults
An intracontinental transform structure, southern
Califomias Geol. Soc. America Bull., v. 84, p. 1407-
1422.
DeLlsle, Mark, Morgan, J. R,, Heldenbrand, Jay, and
Gastil, Gordon, 1965, Lead-alpha ages and possible
sources of metavolcanic rock clasts in the Poway
Conglomerate, southwest Californiai Geol. Soc.
America Bull., v. 76, p. 1069-1074.
179
Doyle, L. J.* and Bandy, 0. L., 1972, Southern continental
borderland, Baja Californiat Its tectonic and
environmental developmenti Geol. Soc. America Bull.,
v, 83, p. 3785-3794.
Ellis, A. J., and Lee, C. H., 1919, Geology and ground
waters of the western part of San Diego County,
Californiai U. S. Geol. Survey Bull., v. 309, 266 p.
Emery, K. 0., 1954, General geology of the offshore area,
southern California, Jahns, R. H.t Geology of
Southern Californiai California Div. Mines and
Geology Bull., v. 170, p. 107-111.
________, 1955, Transportation of rocks by driftwoodt
Jour. Sed. Petrology, v. 25, p. 51-57.
________, 1960, The sea off southern California, a modem
habitat of petroleum* New York, Wiley and Sons, 366 p.
________, 1963, Organic transportation of marine sediments,
in Hill, M, N., , The Sea, p. 776-793.
Emery, K.. 0., and Tschudy, R. H., 1941, Transportation of
rock by kelps Geol. Soc. America Bull,, v. 52, p.
431-478.
Flynn, C. J., 1970, Post-batholithic geology of the La
Gorla-Presa Rodriguez Area, Baja California, Mexicoi
Geol. Soc. America Bull., v. 81, p. 1789-1806.
Gastil, R. G,, 1969, Geologic evidence relating to the
origin of the northern half of the Gulf of California,
in Pan American symposium on the upper mantle, 1968i
Mexico, D. F., Mexico Univ. Nac. Autonoma Inst.
Geoftsica, v. 2, group 2, p. 1-5.
Hamilton, W., 1961, Origin of the Gulf of Californias
Geol. Soc. America Bull., v. 72, p. 1307-1318.
________, 1971, Recognition on space photographs of
structural elements of Baja Californiai U. S. Geol.
Survey Prof. Paper 718, 2o p.
Hamilton, W. and tyers, W. B., 1966, Cenozoic tectonics
of the western United Statest Rev. Geophysics, v. 4,
p. 509-549.
180
Hawkins, J* W., 1970, Petrology and possible tectonic
significance of late Cenozolc volcanic rocks, southern
California and Bala Californiai Geol. Soc. America
Bull., v. 81, p. 3323-3338.
Hawkins, J. W., and Allison, E. C., 1970, Volcanic
petrology and paleontology of Northeast Bank, southern
California borderland, is Abstracts with Programs for
1970, part 3i Geol. Soc. America, p. 99-100.
Henyey, T. L., and Blschoff, J. L., 1973, Tectonic
elements of the northern part of the Gulf of Cali
fornia i Geol. Soc. America Bull., v. 84, p. 315-330.
Hersey, J. B., 1963, Continuous reflection profiling, in
Hill, M. N,, eg., The Sea, Vol. 3i New York, Inter-
sclence, p. 47^72.
Hill, M, L., and Dlbblee, T. W., Jr., 1953, San Andreas,
Garlock, and the Big Pine faults, California! Geol.
Soc. America Bull., v. 64, p. 443-458.
Jahns, R. H,, 1954, Geology of the Peninsular Range
Province, southern California and Baja California, in
Jahns, R. H., fid*, Geology of Southern California!
California Dlv. Mines and Geology Bull., v. 170, p.
29-52.
Johnson, C. W., 1953, Notes on the geology of Guadalupe
Island, Mexlcoi Am. Jour. Scl., v. 251, p. 231-232.
Krause, D. C., 1961, Geology of the sea floor east of
Guadalupe Islandt Deep-Sea Research, v. 8, p. 28-
38.
1964, Llthology and sedimentation In the southern
Continental Borderland, p. 274-318, iQ Miller, R. L.,
ed.. Papers In marine geology, Shepard commemorative
volumei New York, Macmillan, 1964.
. 1965, Tectonics, bathymetry, and geomagnetism of
the southern Continental Borderland west of Baja
California, Mexlcoi Geol. Soc. America Bull., v. 76,
p. 617-650.
Krummenacher, D., and others, 1969, Summary of K-Ar dates
determined at San Diego and Berkeley! Unpublished.
Larsen, E. S., Jr., 1941', The bathollth of southern Cali
fornia! Science, v. 93, p. 442-443.
181
Larsen, E. S., Jr., 1945, Time required for the crystal
lization of the great bathollth of southern and lower
Californiai Amer. Jour. Scl., v. 243-A, p. 399-416.
________, 1948, Bathollth and associated rocks of Corona,
Elsinore, and San Luis Rey quadrangles, southern
Californiai Geol. Soc. America Mem. 29, 182 p.
Larson, P. A., Mudie, J* D., and Larson, R. L., 1972,
Magnetic anomaly profiles in the southern Gulf of
Califomlai Geol. Soc. America Bull., v. 83, p.
3361-3368.
Larson, R. L., Menard, H. W., and Smith, S. M., 1968,
Gulf of California! A result of ocean floor spread
ing and transform faulting! Science, v. 161, p.
781-784.
Larson, R, L., 1972, Bathymetry, magnetic anomalies, and
plate tectonic history of the mouth of the Gulf of
California! Geol. Soc. America Bull., v. 83, no.
11, p. 3345-3360.
McManus, 0. A., and Bums, R. E., 1969, Scientific report
on Deep Sea Drilling Project Leg Vi Ocean Industry,
p. 40-42.
Menard, H. W., 1955, Deformation of the northeastern
Pacific Basin and the west coast of North Americat
Geol. Soc. America Bull., v. 66, p. 1149-1198.
________, 1964, Marine geology of the Pacifici New York,
McGraw-Hill, 271 p.
Merrlam, R. H., 1968, Geologic reconnaissance of north
west Sonorai Abstract, in Proceedings of conference
on geologic problems of San Andreas Fault systemi
Stanford, Stanford Univ., p. 287.
________, 1972, Reconnaissance of geologic map of the
Sonoyta Quadrangle, northwest Sonora, Mexico! Geol.
Soc. America Bull., v. 83, p. 3533-3536.
Miller, W. J,, 1935, A geologic section across the
southern Peninsular Range of California! California
Jour. Mines and Geology, v. 31, p. 115-142.
182
Mllow* £• D., and Ennis, 0* B., 1961* Guide to geologic
field trip of southwestern San Diego County■ Field
Trip Guidebook, San Diego County-1961* San Diego
State College, San Diego* California, p. 23-43*
Minch, J. A.* 1967, Stratigraphy and structure of the
TiJuana-Rosarlto Beach area* northwestern Baja Cali
fornia, Mexlcoi Geol. Soc. America Bull.* v. 78, p.
1155-1178.
, 1970, Early Tertiary paleogeography of a portion
of the northern Peninsular Range, Ajj Allison, E. C.,
and others. Pacific slope geology of northern Baja
California and adjacent Alta California, p. 100-104.
________, 1971, Early Tertiary paleogeography of the
northern Peninsular Ranges, Baja California* Mexico,
in Abstracts with Programs for 1970t Geol. Soc.
America, v* 3* no. 2, p. 164.
________, 1972, The Late Mesozoic-Early Tertiary framework
of continental sedimentation, northern Peninsular
Ranges (Ph.D. dissertation)i Univ. of California,
Rivers ide, 164 p.
Minch, J. A., Schulte, K, C,, and Hofman, G., 1970, A
Middle Miocene age for the Rosarlto Beach Formation,
Baja California* Mexlcoi Geol. Soc. America Bull.,
v. 81, p. 3149-3154.
Moore, D. G., 1969, Reflection profiling studies of the
California Continental Borderlandt Structure and
Quaternary turbldlte basinsi Geol. Soc. America
Spec. Paper 107* 142 p.
Moore, D. G», and Buffington, E. C., 1968, Transform
faulting and Late Pliocene to Recent growth of the
Gulf of California! Science* v. 161, p. 1238-1241.
. 1972, Deep mantle convection plumes and plate
motionsi Am. Assoc. Petroleum Geologists Bull., v.
56, p. 203-213.
Nilno* H., 1933, On the sand pipe dredged from the sea
bottom around the Japanese Islands* Jour. Geographic
Association of Tokyo* v. 45, no. 528, p. 1-6.
183
Nllnoi H., 1952, The bottom character of the banks and
submarine valleys on and around the continental shelf
around the Japanese Islandi Jour* Tokyo University
of Fisheries* v. 38, no. 1, p. 353-410*
________, 1955, Sand pipe from the sea floor off Cali
fornia i Jour. Sed. Petrology, v. 25, no. 1, p. 41-44,
Noble, C. S., and Naughton, J. J,, 1968, Deep-ocean
basalts-Inert gas content and uncertainties in age
datingi Science, v. 162, p. 265-267.
Nomark, W. R., 1969, The geology of the Pacific con
tinental margin of the Peninsula of Baja California,
Mexico, irj Abstracts with Programs for 1969, part 7t
Geol. Soc. America Bull., p. 163*
Pasho, D. W,, 1973, Character and origin of marine
phosphorites! Unpublished Master's thesis, Univer-
sith of Southern California, 189 p.
Pitman, W. C., Ill, Herron, E. M., and Heirtzler, J. R.,
1968, Magnetic anomalies in the Pacific and sea floor
spreadingt Jour. Geophysical Research, v. 73, p.
2069-2087.
Pratt, R. M., 1970, Granitic rocks from the Blake Plateaui
Geol. Soc. America Bull., v* 83, p. 3117-3122.
Profett, J. M., Jr., 1971, Late Cenozolc structure in the
Yerington District, Nevada and the origin of the
Great Basini Geol. Soc. America abs. with Programs
(Cordllleran Sec.), v. 3, no. 2, p. 181.
Sant11lan, M,, and Barrera, T., 1930, Las posibllldades
petroliferas en la costa occidental de la Baja
California, entre los paralelos 30 y 32 de latldud
nortei Inst. Geol. Mexico, Ann,, v. 5, p. 1-37.
Scholz, E. H., Barazangi, M., and Shar, M, L., 1971, Late
Cenozolc evolution of the Great Basin, western
United States, as an ensiallc Interarc basini Geol.
Soc. America Bull., v. 82, p. 2979-2990.
Shepard, F. P., 1950, 1940 Scripps cruise to the Gulf of
California, part III* Geol. Soc. America Mem, 43,
Chart I.
184
Shepard, F. P., and Emery, K. 0*, 1941, Submarine topo
graphy off the California coasti Canyons and
Tectonic Interpretations* Geol* Soc* America Spec.
Paper 31, 171 p*
Shor, G. G., Jr., Moore, 0. G., and Huckabay, W. B.,
1963, Deep-sea tests of a new nonexplosive reflection
profileri Jour. Geophys. Research, v. 68, p* 1567-
1571.
Shumway, G. A., 1953, Rafted pebbles from the deep ocean
off Baja Californiat Jour. Sed* Petrology, v. 23,
p. 24-33.
Strangway, D. W., McMahon, B* E*, Walker, X. R., and
Larson, E. E., 1971, Anomalous Pliocene paleo-
magnetlc pole positions from Baja Californiat Earth
and Planetary Science Letters, v. 13, p. 161-166.
Sumner, J* R., 1972, Tectonic significance of gravity and
aeromagnetic investigations at the head of the Gulf
of Californiat Geol. Soc. America Bull., v. 83, p.
3103-3120.
Suppe, J,, 1970, Offset of late Mesozoic basement terrains
by the San Andreas Fault System* Geol. Soc. America
Bull., v. 81, p. 3253-3258.
Sykes, L. R., 1968, Seismologlcal evidence for transform
faults, sea-floor spreading, and continental drift,
ln Phlnney, R. A., £d., Proceedings of NASA
symposium. History of the earth's crust* Princeton,
Princeton Univ. Press, p. 120-150.
Theberge, A. E., Jr., 1971, Magnetic survey off southern
California and Baja California* United States Dept,
of Commerce, Operational Data Report NOS DR-12, 10 p.
Uchupi, E,, and Emery, K. 0., 1963, The continental slope
between San Francisco, California and Cedros Island,
Mexico* Deep-Sea Research, v. 10, no. 4, p. 397-448.
Ujlle, H., and Miura, M., 1971, Planktonlc foramlnlferal
analysis of a calcareous ooze core from the
Philippine Sea, in Farlnacci, A., II Planktonlc
Conference, Roma, 1970, Edlzlonl Tecnosclenza, Proc.,
p. 1231-1249.
United Nations, 1969, Estudlo de yaclmlentos de mlnerales
metallferos en Mexico* United Nations, New York,
92 p.
185
Von Herzen, R. P., 1964, Ocean-floor heat-flow measurements
west of the United States and Baja Californiai
Marine Geology, v. 1, p. 225-239,
Wilson, £. F,, 1948, Buried topography, initial structures,
and sedimentation in Santa Rosalia area, Baja Cali
fornia, Mexlcoi Am. Assoc. Petroleum Geologists
Bull., v. 32, p. 1762-1807.
Wilson, J, T., 1965, A new class of faults and their
bearing on continental drifti Nature, v. 207, July
24, p. 343-347.
Wlsser, £., 1954, Geology and ore deposits of Baja Cali
fornia, Mexlcoi Econ. Geology, v. 49, p. 44-76.
Woodford, A. O., 1928, The San Qulntln volcanic field,
Lower California! Am. Jour. Sci., 5th ser*, v. 15,
p. 337-345.
Woodford, A. 0., Weiday, E. E., and Merriam, R., 1968,
Siliceous tuff clasts in the Upper Paleogene of
southern Callfomiai Geol. Soc. America Bull., v.
79, p. 1461-1486.
Yeats, R. S., 1971, Paired basement terranes in Baja
California Sur, Mexlcoi IQ Abstracts with programs
for 1971i Geol. Soc. America, v. 3, no. 7, p. 760
APPENDIX
186
Table I. Location, depths, faunas, ages, and suggested environments of deposition
(by faunas) for dredged samples. See Figure 2 for locations. The samples
are arranged in order of increasing ages of the fossil assemblages.
Dredge
Haul Position Depth _£ml
Age (1), Bnvironment (2),
and Faima (3)_____________
23°23'N| 116°30'W 1540
(1)
( 2)
(3)
3A 29°31’N* 116°37*W 950
(1)
( 2)
(3)
Holocene, 11,000 yrs•
2000 m
Globigerinita glutinata (Egger)
Globorotalla pachyderms (Ehrenberg)
dextral
Bulimina striata mexicana Cushman
Cibicides spiralis Natland
Globocassidulina subglobosa (Brady)
Uvigerina hlsplda Schwager
Uvigerina sentlcosa Cushman
Pleistocene, preglacial or inter
glacial
500 m (upper bathyal facies)
Globigertna bulloides d*0rbigny
Globigerina qninqueloba Natland
Globigerinita glutinata (Egger)
Globigerinoides ruber (d’Qrbignv)
Globorotalia inflata (d'Orbigny)
Globorotalia pachvderroa (Ehrenberg)
50 percent dextral
Globorotalia septula (Brady) ^
oo
Table I. Locations, depths, faunas, ages, and suggested environments of deposition
(by faunas) for dredged samples (continued)
Dredge
Haul Position Depth (m)
(Age (1), Environment (2),
and Fauna (3)______________
3A (cont)
10
31°10'N| U7°35'N 1450-1650
(1)
( 2)
(3)
* Uvigerinids Indicate a depth of about 1000 m.
Globorotalia truncatulinoldes
(d'Orbigny) sinistral
Globorotaloides hexigonus (Natland)
Neogloboouadrina dutertrel sub-
cretacea (Lomnickl)
Orbulina universa d'Orbigny
Ciblcides mckannai Galloway and
Wissler
Ciblcides spiralis (?) Natland
Gyroidina gemma Bandy
Gyroidtna subtener (Galloway and
Wissler)
Hoeglundlna elegans (d'Orbigny)
Islandiella lomltensis (Galloway
and WissleF)
Uvigerina juncea Cushman and Todd
Uvigerina peregrina Cushman
Pleistocene, preglacial or inter
glacial
1200 m*
Globigerina bulloides d'Orbigny
Globorotalia inflata (d'Orbigny) M
00
00
Table I. Locations, depths, faunas, ages, and suggested environments of deposition
(by faunas) for dredged samples (continued)
Dredge
Haul Position Depth (m)
Age (l), Environment (2),
and Fauna (3)_____________
10 (cont)
AHF 14937 30o08'30"N|
ll7°35'oo"W
900
( 1)
(2)
(3)
Globorotalia pachyderms (Ehrenberg)
$0 percent dextral
Orbullna universa d'Orbigny
Buliwina rostrada Brady var.
Gvroldlna gemma Bandy
Hoeglundina elegans (d'Orbigny)
Laticarinina pauperata (Parker and
Jones)
Martinottiella sp*
Uvigerina peregrins dlrupta Todd
Uvigerina spinicostata Cushman and
Jarvis
Valvulineria araucana (d'Orbigny)
Upper Mohnlan, Upper Miocene
2000 m
Globlgerina bulloides d'Orbigny
Globlgerina bulloides subsp, duad-
rilatera Galloway and Wissler
Globlgerina concinna Reuss
Globorotalia obesa Bolli
Globorotalia pachyderma (Ehrenberg)
sinistral
Prunopyle titan Campbell and Clark
189
Table I. Locationsv depths, faunas, ages, and suggested environments of deoositlon
(by faunas) for dredged samples (continued)
Dredge
Haul Position Depth (m)
Age (1), Environment (2),
and Fauna (3 )_____________
AHE 14937
(cont)
AHF 14937
Outside
portion
of con
cretion
listed
above
Same location
as above
Same depth
as above
AHF 14937 Same location
as above
Same depth
as above
Radiolarian-diatom complex
Chilostomella oolina Schwager
Chilostomella ovoidea Reuss
(1) Mohnian, Upper Miocene
(2) 2000 m
(3) Globlgerina bulloides d'Orbigny
Globlgerina bulloides subsp. quad-
rilatera Galloway and Wissler
Globlgerina concinna Reuss
Giobigerinella siohonifera
" 7^%bigny) --------------------------
Prunopyle titan Campbell and Clark
Radlolarian complex
Chilostomella oolina Schwager
(1) Mohnian, Upper Miocene
(2) 2000 m
(3) Globlgerina bulloides d'Orbigny
Globlgerina bulloides subsp. quad-
rilatera Galloway and Wissler
Orbullna unlversa d'Orbigny
Prunopyle titan Campbell and Clark
Radiolarian-dlatom complex
so
Tflhip t, Locations, den^hs, faunas, aepg, and suggested environments of deposition
(by fannas) for dredPed sables (continued)
Dredge
Haul___________Position__________ Depth (m)
AHF 14937
(cant)
SOB 37 S^lS'Ni 116°50'W 750-650
3 29°18.5'N| 1225
116 29.9*W
Age (l), Fnvironment (2),
and Fauna (3)____________* _________________
Chilostomella oolina Schwager
Chilostomella ovoidea Reuss
(1) Mohnian, Upper Miocene
(2) 2000 m or possibly deeper (?)
(3) Prunopyle titan Campbell and Clark
Radiolarians
(1) Relizian, lower Middle Miocene
(2) 500 m (upper bathyal facies)
(3) Globlgerina coneInna Reuss
Globlgerina praobnlloides Reuss
Globorotaloides suteri subsp.
relizensls Bandv, Ingle and Wright
Bolivina advena striaella Cushman
Buliralnella~ curta Cushman
G1 obobulimina ovula d'Orbigny
Uvigerina auberiana d'Orbigny
Valvullnerta califomlca obesa
Cushman
Valvulineria depressa Cushman
Table T, Locations, depths, faunas, ages, and suggested environments of deposition
(bv faunas) for dredged samples (continued)
Dredge
Haul
SOB 5(1)
SOB 5(2)
Position
31 l7*N| 117 35*W-
31 21*N| 117°
40* W
Holocene fauna from
previous sample,
same location
Depth (m)
2100-2120
Same depth
Age (I), Environment (2),
and Fauna (3)_____________
(1) Upper Paleocene
(2) Probablv at least middle bathyal or
greater ( 500 m)
(3) Acarlnina cf. A, decepta (Martin)
Acarinlna nlcoTi (Martin)
Mororovella cf, M. acuta (Toulmin)
Moroaovella cf, M. aequa (Cushman
and Renz)
(1) Holocene
(2) 2000 m
(3) Globorotalia pachvderma (Ehrenberg)
dextral
Neogloboauadrina dutertrei sub-
cretacea (Lomnicki)
Bulimlna rostrata Brady
Uvigerina senticosa Cushman
Table I. Locations, depths, faunas, ages, and suggested environments of deposition
(by faunas) for dredged samples (continued)
Notei A deepening of about 500 m or more is Indicated for the Plelstocene-to-Holocene
by samples 3A and 10,
A deepening of about 700 m or more Is suggested for the Rellzian-to-Holocene
by sample 3,
Shoaling of about 1100 m Is shown for the Upper Mohnian to Holocene tof sample
14937 atop Soledad Ridge.
Colling characteristics for Globorotalia pachyderms (Ehrenberg) are from Bandy
(1960) and others (1971, p. 20, 23Ti
SOB samples are courtesy of Scripps Instutlon of Oceanography,
Table I after Doyle and Bandy (1972),

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Asset Metadata

Creator Doyle, Larry James (author)

Core Title Marine Geology Of The Baja California Continental Borderland, Mexico

Degree Doctor of Philosophy

Degree Program Geological Sciences

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

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Gorsline, Donn S. (committee chair), Davis, Gregory A. (committee member), Henyey, Thomas L. (committee member), Zimmer, Russel L. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-892366

Unique identifier UC11363386

Identifier 7331339.pdf (filename),usctheses-c18-892366 (legacy record id)

Legacy Identifier 7331339

Dmrecord 892366

Document Type Dissertation

Rights Doyle, Larry James

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA