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Holocene sedimentological parameters of the outer California Continental Borderland

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Holocene sedimentological parameters of the outer California Continental Borderland

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Content HOLOCENE SEDIMENTOLOGICAL PARAMETERS
OF THE OUTER
CALIFORNIA CONTINENTAL BORDERLAND
by
Michaele Lyn Bergan
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(Geological Sciences)
May 19 8 9
Copyright 1989 Michaele Lyn Bergan
UMI Number: EP58802
All rights reserved
INFORMATION TO ALL USERS
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a note will indicate the deletion.
Dissertation Publishing
UMI EP58802
Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author.
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789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, Ml 48106 - 1346
UNIVERSITY OF SOUTHERN CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES. CALIFORNIA 90 0 0 7
This thesis, written by
.,Michaele..Lyn..Bergan
under the direction of her.....Thesis Committee,
and approved by a ll its members, has been p re
sented to and accepted by the D ean of The
Graduate School, in p artial fu lfillm en t of the
requirements fo r the degree of
Master of Science
Dean
D a te Januar. y 17, 1989
•>
DEDICATION
to my parents, Tyrone, and
M & M's Neehi,
all of whom helped me grow
' )
ACKNOWLEDGEMENTS
A great number of people have contributed to the
completion of my degree. I am deeply indebted to those
who offered their technical expertise, advice,
encouragement and financial support as I labored through
the labyrinth of higher education.
Foremost among those I wish to thank is Dr. Donn S.
Gorsline. His knowledge and enthusiasm for geology guided
me to a greater appreciation and understanding of the
processes that shape our world. Gratitude is also
extended to Drs. D.J. Bottjer and B. Pipkin for critically
reveiwing the manuscript. In addition, the completion of
this thesis would have been much maligned without the
drafting assistance of Mary-Alice DeBoer.
Dr. Suzanne Reynolds deserves special recognition for
her contribution to my research. Without her innovative
pioneering in sediment analysis this project would have
suffered immeasurably. Along with her ground-breaking
efforts, I would like to thank her for introducing me to
the wealth of information and beauty to be found in clay
floccules and fish poop.
Along with the aforementioned people, I would like to
thank the following people for the scientific dialogue,
advice and encouragement they offered throughout my
i i i
endeavor: Drs. B. Edwards, M. Field, D. Drake, and M.
Torresan of the U.S. Geological Survey in Menlo Park; Drs.
T. Dickey and R. Osborne of the University of Southern
California; Dr. C. Savrda of Auburn University; and Dr. R.
Kolpack. Also, I would like to give special
acknowledgement to Dr. Freyd Rad of Unocal International
Oil and Gas for giving me insight into the world of oil
exploration and for supporting me professionally and
academically.
Numerous graduate students at the University of
Southern California contributed to my education in one way
or another. In particular, I would like to recognize
those who shared the trials and tribulations of the
incoming class of 1986: Chris Lerch, Margaret Dahlen,
Reese Barrick, Chia-Chen Yeh and Kyung Cho. Eric Bender
should be recognized for supplying the friendship and
comic relief necessary for surviving in the staid
environment of academia.
Finally, I would like to thank my parents (Martin and
Margie), my sisters (Mona, Mindy and Mary-Alice), and
Tyrone Howard. Their unfailing encouragement and support
is greatly appreciated and fondly regarded as the basis of
my success. I am deeply indebted to Tyrone for putting up
with me throughout the long ordeal and offering
inspiration whenever I needed it.
i v
TABLE OF CONTENTS
DEDICATION....................................................ii
ACKNOWLEDGEMENTS............................................iii
ABSTRACT............... X
INTRODUCTION...................................................1
General Statement and Purpose......................... 1
Previous Work............................... 6
Geologic Setting............................... 7
Topographic Definitions.......... 14
Bank Tops......................................... 14
Basin Slopes......................................15
Basin Floors......................................16
Physical Dimensions....................................16
Methods..................................................18
OCEANOGRAPHY 2 2
SEDIMENTOLOGY...................................... 28
Grain Size Analysis....................................39
Sand Fraction.... 7 0
Calcium Carbonate and Organic Carbon................77
Sedimentation Rates............... 93
ANIMAL-SEDIMENT RELATIONSHIPS.................. 102
CONCLUSIONS. ............................................... 132
v
REFERENCES............ .....................................13 4
APPENDICES.................................................141
A. Box core locations and bottom depths...........141
B. True moment measurements....................... 14 4
C. Percentages of grain size components.... 147
D. Biologic and organic component percentages... 150
vi
LIST OF FIGURES
Figures
1: Physiograpy Map.......................................... 2
2: Basin Filling Phases................................... 4
3: Location Map Of Outer Borderland...................... 9
4: Structural Framework Of The Borderland............... 11
5: Box Core Station Location Map........................19
6: California Current Circulation...................... 23
7: Basin Filling Cross-Sections.........................29
8: Sediment Accumulation Rates..........................37
9: Mean Grain Size Distribution.........................41
10: Bottom Depth Versus Mean Grain Size.................44
11: True Moment Grain Size Measurements: Mean Phi
Versus Standard Deviation And Skewness.............47
12: True Moment Grain Size Measurements: Skewness
Versus Standard Deviation And Kurtosis............. 49
13: True Moment Grain Size Measurements: Kurtosis
Versus Mean Phi And Standard Deviation............. 51
14: Grain Size Distribution Curves For Stations
30999 And 25989............................ 54
15: Grain Size Distribution Curve For Station 30650...56
16: Ternary Diagram Of Sand, Silt, And Clay Percent...59
17: Silt Percent Versus Clay Percent....................61
18: Silt Percent Distribution............................63
19: Clay Percent Distribution............................66
vi i
~2 ~0~. Sand Percent Distribution................... .........68
21: Sand Fraction Composition Station Locations....... 71
22: Percent Carbonate And Organic Carbon Distribution.78
23: Percent Carbonate Distribution....................... 81
24: Percent Organic Carbon Distribution................. 85
25: Organic Carbon Content Versus Mean Grain Size......87
26: Organic Carbon Content Versus Clay Percent......... 89
27: Organic Carbon Versus Carbonate Content.............94
28: X-Ray Radiograph Station Locations................104
29: Homogeneous Hemipelagic Sediment Profile,
Station 30638.........................................107
30: Homogeneous Hemipelagic Sediment Profile,
Station 30132.........................................109
31: Vertical Distribution Of Biogenic Traces.........112
32: Scolicia, Station 30644.............................114
33: Enigmatic Pellet-like Features, Station 25993....117
34: Turbidite Layer Preserved At Station 30123...... 119
35: Fine-grained Turbidite Layers, Station 26000.... 122
36: Swirl Signature, Station 31008..................... 125
37: Bottom Photograph Locations........................ 127
38: Bottom Photographs...................................12 9
vi i i
LIST OF TABLES
Tables
1: Annual Sediment Contributionsi....................... 3 2
2: Summary Of Accumulation Rates................ 99
ix
ABSTRACT
The northern outer California Borderland basins are
currently dominated by low rates of continuous hemipelagic
sedimentation. Coarse residual sands dominate on outer
borderland bank tops due to winnowing of fine-grained
sediment by bottom currents. Reworked sediment is
important on basin slopes and bank tops where Miocene and
Pliocene sediments outcrop.
The Santa Rosa-Cortes Ridge is a major barrier to
terrigeneous sediment distribution and therefore, the
relative importance of biologic, aeolian, ocean-current
transported and insular sediment contributions is greater
in the outer borderland than in the inner and central
basins.
The California Current dominates the oceanographic
circulation in the borderland and partially controls
biologic production and distribution of sediment. In
addition, the current may serve as a significant source of
well-sorted, fine-grained sediment from northern
provenances.
The distribution of carbonate and organic carbon is
controlled by dilution by terrigeneous sediment, winnowing
[by bottom currents, biogenic production, as well as the
California Current. Bottom currents play a much larger
role in the disrtribution of sediment in the outer
borderland basins where sill to basin floor depths are
much shallower than in the inner and central basins.
Radiocarbon dates of box core samples from the center
of each basin reveal that Tanner Basin has the fastest
sedimentation rate of 9 mg/cm2/yr. San Miguel Gap and
Patton Basin have intermediate rates of 6 and 5 mg/cm2/yr,
respectively. San Miguel Basin has the lowest rate of 3
mg/cm2yr.
Tanner Basin traps the little suspended sediment that
reaches the outer basins and is adjacent to a relatively
large insular source area. In addition, Tanner Basin
underlies a zone of upwelling that contributes a high
percentage of carbonate material. San Miguel Basin is
significantly affected by bottom currents that winnow
finer-grained sediment away.
The outer borderland basins are characterized by a
low diversity biogenic trace association typical of deep
sea sediments. As a result of low sedimentation rates and
relatively high rates of bioturbation the sediment is
homogenized and few primary sedimentary structures are
preserved.
xi
INTRODUCTION
General Statement and Purpose
On the continental margin off the coast of Southern
California, the continental shelf is separated from the
true continental slope by three, roughly parallel series
of fault-controlled basins and sills (Figure 1). Shepard
and Emery (1941) designated this a Continental Borderland
type margin. The California Continental Borderland
provides a natural laboratory for the study of marine
depositional environments, in as much as progressively
seaward basins are characterized by different depositional
rates and dominant processes of sedimentation. The inner
borderland basins are characterized by high sedimentation
rates and abundant submarine fans (Malouta and others,
1981; Nardin and others, 1979). Central basins are
characterized by lower rates of hemipelagic sedimentation
(Barnes, 1970; Reynolds, 1984; Schwalbach and Gorsline,
1985). The purpose of this study is to conduct a detailed
study of Holocene sedimentation in the northern outer
basins, which are dominated by even lower rates of
continuous hemipelagic sedimentation. The outer
borderland probably represents the first stages of outer
basin-filling outlined in the model of Gorsline and Emery
(1959)(Figure 2). The study of sedimentation
1
Figure 1 California Continental Borderland physiography
and location map (after Gorsline and others,
19 68). Study area is outlined by bold line.
1 20° 00' ll**00‘ IIS*00
34*00*
38*00
Continental Borderland Physiography
depths in meters
RA BASIN
LOS A N SELES
SAN JUAN
SEAMOUNT
PACIFIC OCEA
SCALE
1 0 20
nautical mllaa
84*00'
33*00*
GO
Figure 2: Phases of filling of basins by sediments,
showing relative roles of deposition from
general suspension and from turbidity currents.
Note that latter deposits fill nearshore basins
before they can spill over into adjacent basins
(from Gorsline and Emery, 1959).
4
m a i n l a n d
se al e v e l
J /b a n k
PHASE 1
INNER BASIN FILLIN G RAPIDLY WITH
TLJRBIDITY CURRENT AND SLUMP
SED IM EN TS. OUTER B A S IN S FILLING
Sl o w l y w it h o r g a n ic a n d f in e
SUSPENDED LOAD SEDIMENT. BANK
TO P SEDIMENT R E S ID U A L, Fin e s
WINNOWED OUT.
PHASE 2
INNER
OUTER
BASIN
B a S in
FILLED
AS
BASIN BEGINNING
BEFORE. MIDDLE
PHASE 1.
cn
PHASE 3
INNER BA S IN SURFACE GRADED AND
BYPASSED. MIDDLE BASIN IN PHASE 1 .
OUTER BASIN AND BANKS AS BEFORE.
PHASE < 4 -
INNER BASIN FILLING W iTH SHALLO W
MARINE AND A L L U V IA L SEDIMENTS
MIDDLE BASIN IN PHASE 3 . OUTER
BASIN IN F>HAS£ I. BANKS AS BEFORE .
characteristics of the outer borderland is necessary to
obtain a more complete understanding of the spectrum of
conditions that have affected the sediment input to the
entire borderland.
Previous Work
Few studies have focused on the outer borderland
basins (Gorsline and others, 1968? Reynolds, 1984). There
have been numerous studies that are regional in scope and
stressed the sedimentation and topography of the
borderland as a whole (e.g. Emery, 1960; Teng, 1985? Teng
and Gorsline, 1988)? or general tectonics of the
borderland (Moore, 1969? Vedder and others, 1974? Crouch,
1978; Howell, 1976). Studies of a more local nature have
examined the shallower areas, particularly Santa Rosa-
Cortes Ridge (e.g. Field and Richmond, 1980? Uchupi,
1954), Tanner-Cortes Banks (e.g. Holzman, 1950? Barton,
1976? Limerick, 1978), and Rodriguez Sea Mount (Palmer,
1964). Continued interest in Santa Rosa-Cortes Ridge was
generated by OCS Lease Sales 36 and 48 (e.g. Clarke and
others, 1980). The Patton Escarpment has been the subject
of regional continental slope studies by Uchupi (19 62) and
studies associated with DSDP Leg #63 (Yeats, Haq, and
Pisciotto, 1981). Regional oceanographic studies have
been conducted by Sverdrup and Fleming (1941), Emery
6
(1960) , and Hickey (1979).
Sedimentation rates for Tanner Basin have been
determined using piston core data (Gorsline and others,
1968) and in connection with borderland-wide sedimentation
rate studies (Emery, 1960? Schwalbach and Gorsline, 1985).
Sedimentological characteristics of San Miguel Gap were
studied by Wright (1967) using piston core data. Reynolds
(1984) and Teng (1985) used seismic profiles to delineate
Neogene stratigraphy and structure of Tanner Basin and
Tanner and Patton Basins, respectively.
Geologic Setting
The California Continental Borderland is a composite
of elongate northwest-trending blocks separated by major
faults along which large lateral motion has occurred (Teng
and Gorsline, 1988). Deep marginal basin sedimentation
dominates in the present depositional centers (Teng,
1985), which are topographic expressions of the tectonic
forces that have operated in this area since the Late
Miocene (Vedder and others, 1974? Teng and Gorsline,
i
1988). Secondary east-west structures are superimposed on
the dominate NW-SE trend (Emery, 1960).
Regionally, the margin is underlain by Franciscan and
Great Valley-type basement complexes which occur in two
subparallel belts offshore (Crouch, 1979). In the outer
7
__________________________________________________________i
borderland area the Franciscan-Great Valley-type sequence
is repeated and is juxtaposed against the inner borderland
sequences by major right-lateral faults. The Franciscan
and Great Valley-type belts are viewed as linear
lithologic entities, which prior to 30 mybp, were
longitudinally continuous from Los Angeles to the southern
tip of Baja California. The repetition of the Franciscan-
Great Valley-type belts in the outer borderland
corresponds well with lithologic and structural data and
explains the atypical width of this active continental
margin.
The study area is a rough parallelogram, designated
here as the outer borderland (Figure 3). It is bounded by
the Santa Rosa-Cortes Ridge, the base of the Patton
Escarpment, 32°30/N Lat, and 34°00/N Lat. In this area
the Franciscan-type basement is covered by thin Neogene
sedimentary sequences. The region is mainly a topographic
high with several basins developed as a result of Neogene
tectonics (Teng, 1985).
In the outer borderland NW-SE trending faults
dominate the structural grain (Teng, 1985)(Figure 4).
Most of the faults are strike-slip with some vertical
displacement. Structurally, the outer borderland consists
of, from east to west: Santa Rosa-Cortes Ridge, a
compressional anticlinorium bordered by the Santa Rosa-
8
Figure 3 Location map of outer borderland. Study area
is outlined by bold line.
9

Figure 4: Structural Framework of the Continental
Borderland, southern California (from Teng,
1985).
First-order structures:
a. Santa Barbara Basin Synclinorium
b. Northern Channel Islands Anticlinorium
c. North Patton Ridge Anticlinorium
d. South Patton Ridge Anticlinorium
e. Santa Rosa-San Nicolas Ridge Anticlinorium
f. Nidever-Tanner Ridge Anticlinorium
g. Cortes Bank Anticlinorium
h. Santa Cruz Basin Synclinorium
i. San Nicolas Basin Synclinorium
j. Santa Cruz-Catalina Ridge Anticlinorium
k. San Clemente Ridge Anticlinorium
1. Catalina Ridge Anticlinorium
Basins:
1. Santa Barbara Basin
2. Patton Basin
3. Tanner Basin
4. Santa Cruz Basin
5. San Nicolas Basin
6. Catalina Basin
7. Santa Monica Basin
8. San Pedro Basin
9. San Diego Trough
11
STRUCTURAL FRAMEWORK
CONTINENTAL BORDERLAND
CALIFORNIA
N
Los .
Angelos
0 50Km
arpara Low
Os \ v r X,. v
< ' \ \ °S\
v % \ V
\ \ %
San
lego
Cortes Ridge Fault Zone? Patton-Tanner low, a synclinorium
in which down-warped basins are situated; Patton Ridge, a
structural high bounded on the east by another major fault
zone? and Patton Escarpment, the fault-controlled
continental slope. San Miguel Gap marks the northern
boundary of the outer borderland. The gap offsets the
Patton Escarpment and may be an extension of the Murray
Fault Zone (Crouch, 1981).
In Tanner and Patton Basin early to middle Miocene
sequences unconformably overlie the Franciscan-type
basement (Teng, 1985). The sequences consist of
diatomaceous shales, volcanoclastics, and turbidites.
They are widespread over the entire area, not confined to
basins (Crouch, 1981; Teng, 1985). Middle to late Miocene
strata are lithologically similar, but unconformably
overlay the earlier sequences. These strata have a more
restricted distribution, marking the beginning of basin
development (Teng, 1985). Overlying the Miocene strata
are Pliocene to Recent clastic deposits. The late Neogene
sediments are hemipelagic and fine-grained mass-flow
deposits confined to basins. Lower fan and basin plain
facies dominate in both Tanner and Patton Basin with local
ridges serving as the major source terrain. Teng (1985)
noted a decline of sediment accumulation in the offshore
basins during the late Neogene. He attributed the decline
13
to the continual subsidence of the borderland area which
reduced the insular source area.
Topographic Definitions
Several major topographic forms have been recognized
in the borderland (Emery, 1960). In the outer borderland
the important forms include bank tops, basin slopes, and
basin floors. The surficial sediments represent the
response of depositional processes to the existing
environment.
Bank T o p s
The winnowing of fine-grained sediment and organic
material off of the bank tops by storm waves and currents
creates lag concentrations of residual, relict, authigenic
and organic sediment (Holzman, 1950? Uchupi, 1954? Emery,
1960? Field and Richmond, 1980). The tops of banks are
characterized by very coarse sands with mean particle size
values decreasing downslope. There is little or no
detrital sedimentation due to the lack of source area
(Emery, 1960).
On Tanner and Cortes Banks benthic organisms in shoal
areas produce coarse calcareous sands, below the euphotic
zone the sediment is predominantly pelagic foraminifera
sands (Holzman, 1950). Field and Richmond (198 0) report
14
that the margins of Santa Rosa-Cortes Ridge are clastic
sands, and benthic foraminifera sands are restricted
almost entirely to the center of the ridge. The crest of
the Patton Escarpment is dominated by detrital sand and
gravel, Globiaerina ooze and volcanic ash (Uchupi, 1962).
The detrital sediments are probably reworked from Tertiary
outcrops.
Basin Slopes
Dredging and continuous seismic profile studies
suggest that the sediment cover is very thin or absent on
the steep basin slopes (Moore, 1969). The steepest linear
slopes are probably fault-scarps, but some of the more
gentle declivities are interpreted as sedimentary dip
slopes (Gorsline and others, 1968).
The basin slopes and continental rise are blanketed
by red silts and clays indicative of unusually slow
sedimentation (Uchupi, 1962). There is a downslope
transport of fines off the bank tops, but most of this
material bypasses the slopes and settles on the basin
floors. Slumps and gullies are common (Field and
Richmond, 1980), but there are few submarine canyons. The
existing submarine canyons are probably relicts from lower
sea stands when there was a greater input of detrital
material from insular sources.
15
Basin Floors
Basin floor sediments are typically light olive-gray
(5Y5/2) silty clay (Gorsline and others, 1968). The fine
grained nature of the sediment is similar to other deep
basin hemipelagic sediments deposited a great distance
from their source, and probably reflects the effect of
flocculation, bioaggregation, and sorting in transit.
Physical Dimensions
Tanner Basin is here defined by the 1000 m contour.
It is 90 km long and 40 km at its widest. The basin has
approximately 1300 km2 total area which is predominantly
basin apron (Emery, 1960). It can be divided into two
zones: a shallower, broader Western zone with a mean
depth of 1150 m; and a narrower, deeper Eastern zone with
mean depths of 1420- 1480 m. Bathymetric contours
indicate that canyons enter the basin from the north,
south and southeast.
The basin is bounded on the east by the Santa Rosa-
Cortes Ridge. The ridge extends to within 100 m of the
surface. The eastern slope of the basin is steep and
gullied. Tanner and Cortes Bank borders on the southeast
reaching minimum depths of 50 m. To the south and
southwest Garrett Ridge forms a sill with depths ranging
16
from 1000 m up to 500 m. To the northwest an unnamed
knoll (550 m) divides Tanner Basin from Patton Basin.
Patton Basin is elongate in the northwest-southeast
direction approximately 90 km. Maximum width is 25 km.
Below 1000 m the basin has a total area of approximately
1000 km . Mean depth of the basin is 1200 m, reaching a
maximum of 1400 m in the north where a low sill (1100 m)
separates Patton Basin from San Miguel Gap. There are
four small shallow depressions in the southern end of the
basin.
Irregular contours suggest gullying on the slopes,
but there are no well-developed canyons. The slopes are
steepest along the western edge marked by the Patton
Ridge. To the north, Patton Ridge reaches minimum depths
of 450 m. The eastern slope of the basin is a relatively
gentle slope to the saddle of the Santa Rosa-Cortes Ridge.
In the north a narrow gap opens Patton Basin to San Miguel
Gap at 1150 m.
At 33°30' N latitude Patton Ridge is separated from
the Patton Escarpment by a flat, relatively shallow
depression referred to here as San Miguel Basin. It can
be defined irregularly by the 1000 m contour. The basin
is 50 km long and 30 km wide with a total area of 850 km2.
It opens on the Patton Escarpment in the west and to San
Miguel Gap in the north.
17
The Patton Escarpment is separated from the outer
basins by a ridge 8 to 20 km wide and 400 to 1600 m deep
(Uchupi, 1962). From the crest of this ridge the slope
drops abruptly to depths of 3400 to 3800 m. The gradient
of the slope ranges from 10° to 17°, averaging about 12°.
There are no prominent submarine canyons, but there are
gullies and tectonic offsets. On the north end of the
study area the escarpment is offset to the east by San
Miguel Gap.
The study area also includes San Miguel Gap below
1500 m depth. The floor of the gap slopes gently to the
southwest with declivities ranging from 2° to 10°.
Submarine canyons, many of which appear to be fault-
controlled, enter from the north, east and south. The gap
is slightly elongate in the east-west direction with
length and width of 35 km and 3 0 km respectively. It
breaches the Patton Escarpment at 2500 m where the gap
opens to the Pacific Basin. The gap is bounded by
Rodriguez Seamount to the north, the Insular Ridge and
Santa Rosa-Cortes Ridge to the east, and Patton Ridge to
the south.
Methods
Sixty-eight box cores and two hydroplastic cores were
available for this study (Figure 5). The cores were
18
Figure 5: Outer borderland box core station location
19
BOX CORE LOCATIONS
T"'- \
*4
\
vVY t J - > . ' H. . . ' N
b \
collected on R/V Velero IV cruises 1394-95, 1432, 1533,
1539, and 1551 between 1977 and 1983. The cores are
archived at the University of Southern California campus
and available from the Marine Geology Laboratory.
Cores were obtained using a modified Reineck box
corer. A Benthos underwater camera system was mounted on
the frame to take bottom photographs prior to impact.
Cores were subsampled on deck acccording to the method
described by Edwards (1979). Vertically oriented slabs
were x-ray radiographed following the techniques of
Edwards (1979).
Surface sediment analyses were performed following
standard laboratory procedures used at the sedimentology
laboratory at the University of Southern California.
Textural analysis was accomplished by Coulter Counter
(Coulter Electronic, 1975) and seiving techniques
(Krumbein and Pettijohn, 1938). All textural data was
reduced to true moment measures. Calcium carbonate and
organic carbon content were determined by LECO gasometric
and combustion apparatus as described by Kolpack and Bell
(1968). The composition of surficial coarse fractions was
estimated using point count methods. Clay and silt
mineralogy was determined by x-ray diffraction methods
described by Fleischer (1970). Radiocarbon dating
analyses were performed by Beta Analytic Inc. using
21
carbonate carbon.
Isopleth maps were contoured by hand because of the
paucity of data on bank tops, ridges, and knolls.
Topography is an important control on sediment
distribution and should be recognized as such on sediment
characteristic maps.
OCEANOGRAPHY
Oceanographic circulation in the California
Continental Borderland is dominated by the California
Current System. The California Current System is
comprised of the southeastward-flowing California Current
and the northward-flowing Southern California
Countercurrent (Hickey, 1979). The California coastal
oceanographic conditions vary through three seasons:
oceanic (July-November), Davidson (December-February), and
upwelling (March-June)(Brandsma and others, 1985)(Figure
6). Nearsurface current flow is generally southward
except during the Davidson season.
The California Current is the eastern boundary
current of the North Pacific gyre. It is a relatively
slow current (25 cm/sec) and is characterized by low
temperatures and salinities and high oxygen and phosphate
contents (Sverdrup and others, 1942). It is about 700 km
22
Figure 6: California Current circulation during the three
oceanographic seasons: Davidson (Dec.-Feb.),
upwelling (March-June), and oceanic (July-
Nov.)(from Brandsma and others, 1985).
i
23
JAN MAY
SEPT
1 1 5 *
40°
CONTOUR INTERVAL 0.04 DYNAMIC METERS
.2 0 ° I AT
( 3 0 ° LA Y
A O * LAT. S A N
F R A N C IS C O
AO 30 C M /U C j
!N T S P C IO
P O IN T
C O N C E P T IO N
O IE G O
wide and extends up to 1000 km offshore (Emery, 1960?
Reid, 1965). At Point Conception the California Current
departs from the coastline and continues to the southeast.
Off northern Baja California and the central borderland it
branches into a continuing southward segment and the
California Countercurrent.
The California Countercurrent moves shoreward of the
Channel Islands and it marks the eastern limb of the
Southern California Eddy System. The eddy system is a
permanent circulation feature of the California
Continental Borderland. It is a slow, counterclockwise
eddy centered over the Santa Rosa-Cortes Ridge (Pao,
1977). The eddy is strongest during the late fall and
winter (Davidson season) when the North Pacific
atmospheric high lessens, allowing the Davidson Current to
surface along the coast (Pirie and others, 1975). The
Davidson Current is the warmer, more saline, nutrient-
rich, and oxygen-poor current (Reid and others, 1958)
derived from Equatorial-Pacific Waters and probably North
Pacific Intermediate Waters (Pao, 1977). The eddy has
temporally variable flow patterns with smaller temporary
eddies generated by wind stresses (Reynolds, 1984).
During spring and early summer (upwelling season),
north-northwest winds drive the California Current
shoreward, causing the Davidson Current to flow 2 00 m
25
below the surface (Pirie and others, 1975). This induces
upwelling, usually over topographic highs (Emery, 1960;
Jones, 1971? Pao, 1977). Upwelling usually involves water
less than 200 i deep (Pao, 1977).
Sverdrup and Fleming (1941) proposed that the area of
cold water, and its related eddy, centered just west of
the Santa Rosa-Cortes Ridge, are products of upwelling
caused by winds from the northwest which blow more
strongly during spring and summer. Emery (1960) proposed
an alternate concept, the entrainment of nearshore water
by the California Current. When the California Current
passes Point Conception it encounters a large body of
water to its left. The Santa Rosa-Cortes Ridge rises to
depths of less than 200 m, therefore only water shallower
than the ridge's crest Can be entrained. The entrained
water is replaced by cold subsurface water producing the
same surface effect as upwelling without requiring the
presence of high wind velocities. Both processes probably
work together to produce the characteristic surface
circulation pattern off Southern California (Emery, 1960).
Wright (1967) concluded that the principal mixing
mechanism in the vicinity of San Miguel Gap is upwelling
as proposed by Sverdrup and Fleming (1941). The gap is a
bathymetric break in the continental slope and may cause
particularly comprehensive vertical mixing. Phytoplankton
26
productivity in the borderland is especially high toward
the eastern flank of San Miguel Gap.
Surface circulation shows a marked seasonal variation
over the eastern two-thirds of the gap region (Wright,
1967). Rarely does the seasonal influence extend over the
entire area. The western one-third has oceanic conditions
throughout the year. A definite gradient of oceanic
influence exists from east to west across San Miguel Gap.
Intermediate waters are derived from the south
(Sverdrup and Fleming, 1941). The flow is to the north
with characteristically more saline, oxygen-poor waters.
The oxygen is progressively depleted as it moves north by
the oxidation of infalling organic matter (Reid, 1965).
In Tanner and Patton Basin intermediate waters enter the
basins over their southern sills. The basinal waters in
San Miguel Basin and San Miguel Gap probably enter from
the west where there is communication with the open sea.
The individual basin bottom water masses are more or less
uniform from sill depth -to bottom and almost equal to
values found in the open sea at corresponding depths
(Emery, 1960).
Tanner, and probably Patton and San Miguel Basins,
have bottom waters within the dysaerobic zone. Emery
(1960) recorded dissolved oxygen values of 0.6 ml/liter
for Tanner Basin. The level of oxygen is not depleted
27
enough in any of the basins to discourage colonization by
benthic organisms, as is evident by the high level of
bioturbation observed in x-ray radiographs and the variety
of macrofuana present in bottom photographs from Tanner
Basin.
SEDIMENTOLOGY
Emery (I960) suggested that deposition of sediments
in the borderland can be characterized by three main modes
of basin infilling (Figure 7). Pelagic deposition
uniformly blankets the area with a rain of material
through the water column. Organic blooms due to upwelling
cause seasonal variations in pelagic sedimentation rates.
A symmetrically draped basin cross-section results when
the pelagic mode dominates deposition.
The deposition of clastic material from suspension
creates progressively finer-grained and thinner
accummulations with increasing distance from source areas.
Due to the wet/dry seasonality of the adjacent mainland
area, there are distinct variations of sediment input
during the year and over longer multi-year cycles. Much
of the suspended elastics are deposited as physical
floccules or fecal pellets before reaching the outer
basins.
28
Figure 7: Cross sections of basins showing sediment fill
consisting of: A, organic debris and chemical
precipitates from near the water surface: B,
detrital sediments diffused from mainland by
volume suspension: C, detrital sediments
contributed by turbidity currents from near
mainland (from Emery, 1960).
29
M A IN L A N D B A N K
%
30
The outer basins have symmetrically draped cross-
sections indicating that one or both of the afore
mentioned depositional modes are active. This is in
contrast with inner basins which can have highly
unsymmetrical cross-sections characteristic of the third
mode of infilling, turbidity currents. Turbidite
deposition in the outer basins is rare and areally
restricted.
Five types of unconsolidated sediments are recognized
in the borderland: 1) recent detrital, 2) relict, 3)
residual, 4) organic, and 5) authigenic (Emery, 1952? Pao,
1977). Uchupi (1962), working on the Patton Escarpment,
also recognized unconsolidated volcanic debris. Several
workers (e.g. Emery, 1960; Schwalbach and Gorsline, 1985?
Teng and Gorsline, 1988) have estimated the annual input
from the various sources (Table 1).
Teng (1985) showed that the classic model of
Gorsline and Emery (1959) applies to the full history of
sediment accumulation since the initiation of the present
borderland morphology. The total sediment input rate has
remained at the present order of magnitude since the late
Miocene.
Emery (1960) used radiocarbon dating methods to
determine sedimentation rates from piston cores. He was
the first to define the gross balance between sediment
31
I
i
Table 1: Annual Input from the various sediment sources.
32
TABLE 1
Sediment sources (from Schwalbach and Gorsline, 1985)
Yearly Contribution Percent Contribution
to Borderland (tons/yr) to Borderland
Entire_______Outer Entire Outer
Fluvial
Biogenic
Ocean Currents
Aeolian
5-10X10
2-3X10'
<106
<106
<0.5X10'
0. 2X101
105
105
75-80%
20-25%
5-10%
<10%
50-70%
25-50%
up to 10%
up to 10%
33
influx and sedimentation in the Borderland. Schwalbach
and Gorsline (1985) refined this first sediment balance by
incorporating data from carbonate stratigraphy (e.g.
Gorsline and others, 1968), radioisotope dating methods
(e.g. Malouta and others, 1981), sediment volumes measured
from seismic profiles (Moore, 1969), sediment traps
(Dunbar and Berger, 1981), and counting of annual varves
(e.g. Thornton, 1981).
Schwalbach and Gorsline (1985) identified three
primary sediment sources: fluvial, biogenic and aeolian.
In addition to these three sources, ocean currents and
insular contributions are important in outer borderland
basin sediment accumulation.
Terrigenous sediment is introduced into the
borderland from major seasonal rivers that drain the
northern and central mainland interiors of Southern
California (Gorsline, 1980? Norton 1985). The major
discharge is from the Santa Clara River which drains the
Transverse Ranges (Teng -and Gorsline, 1988). The largest
terrigeneous contribution occurs during exceptional flood
events when discharge from streams can be one to two
orders of magnitude higher than in normal years
(Schwalbach and Gorsline, 1985). The fine sediment
discharge is predominantly silt (Fleischer, 1970).
Fleischer (1970) conducted a regional study of the
34
mineral composition in the borderland. The following
compositional information is from that study unless
otherwise noted.
The clay fraction is dominated by illite,
montmorilIonite, and smectite with minor kaolinite,
vermiculite, and chlorite. This suite is typical of arid
to semiarid weathering. Fleischer (1970) used subtle
differences in clay mineral suites to identify clays
contributed by ocean current transport from northern
provenances. He estimated that clays from the north may
contribute as much as 10 % of the sediment load.
Detrital sands are quartz-feldspar. The feldspar
compositions parallel the heterogeneous source rock types.
A general decrease in quantity of detrital sands offshore
implies current transportation from the mainland. Quartz-
feldspar ratios increase offshore, in part a result of
decreasing grain size, but also related to high quartz-
feldspar ratios on the Santa Rosa-Cortes Ridge. In
addition, fine-grained quartz increases offshore and
reflects an enrichment by aeolian processes. Due to the
decrease in the total terrigeneous sedimentation rate in
the outer borderland, aeolian-derived, silt-sized quartz
becomes an important component of the sediment (as much as
10 %). Emery (19 60) noted that dust often collects on
ships offshore during Santa Ana wind conditions. He
35
concluded that North America is the dominant source of
aeolian sediment to the East Pacific. A broad suite of
ferromagnesian minerals is present and reflects incomplete
weathering in arid to semiarid climates. Much of the sand
is recycled sediment, but is still relatively immature.
The biological sediment contribution varies with the
strong seasonal upwelling that supplies nutrients for
production. The biological fraction includes carbonate
(predominantly foraminifera tests), opaline silica, and
other organic matter (Emery, 1960). The biological
material comes from a large reservoir in the surface
waters of the coastal zone that is subject to extensive
recycling. In the outer basins biological contributions
can be as much as 50% by weight. The present coarse
fraction has been suggested as a measure of surface
productivity at the time of deposition (Ericson and
others, 1956? Prensky, 1973). The sediment is coarse due
to the high percentage of accumulated tests, spicules and
capsules.
Terrigeneous and biological sedimentation rates each
have a characteristic regional pattern (Schwalbach, 1982;
Teng, 1985)(Figure 8). In the outer borderland
terrigeneous input is obstructed by banks, therefore
biological sediment is not diluted as in the inner basins.
This is evident by the higher carbonate and organic carbon
36
Figure 8: Terrigeneous sediment, calcium carbonate and
organic carbon accumulation rates in the inner
and central borderland basins (mg/cm2/yr)(from
Schwalbach, 1982).
37
TERRIGENOUS SE3IMENT
ACCUMULATION RATES
'^C/'ca^/jrr)
CALCIUM CARBONATE
ACCUMULATION RATES
(ng/cn2/yr)
ORCANIC ClRBON
ACC EMULATION RATES
' * (mg/ca2/y r )
38
percentages found in the outer borderland.
Present day insular sources are minor contributors
and of limited areal extent (Teng, 1985). At low sea
stands the insular areas were much larger and were
important detrital sources (Schwalbach and Gorsline,
1985).
Authigenic sediments, particularly glauconite, can be
major constituents of the sediment. Cloud (1955) reported
that glauconite typically forms under the following
conditions: normal salinity, reducing conditions,
available source material, and low or absent
sedimentation. Glauconite can either be replacement or
primary from precipitation. Foraminifera infilling and
fecal pellet replacement is common.
Pratt (1962) studied glauconite occurrence in the
borderland. He concluded that maximum concentrations are
found in areas of slow or absent deposition such as banks,
ridges, and upper slopes. Mature forms of glauconite have
been recognized as reworked from Pliocene and probably
Miocene submarine outcrops. Glauconite is a useful tracer
downslope of source outcrops.
Grain Size Analysis
The physical characteristics of the uppermost 2 cm of
the outer borderland deposits is described below.
39
Although the surficial 0-2 cm are not a discernible
physical layer, it represents most closely the response of
sediments to the existing environment (Gorsline and
others, 1968). All evidence points to continuous
sedimentation throughout the Holocene in this area.
Figure 9 illustrates the mean grain size distribution
in the outer borderland. The highest mean phi values
(smallest grain size) are found on basin floors. The
grain sizes on basin floors are relatively uniform within
the fine silt range, increasing in size toward slope
areas. North Tanner Basin, east-central Patton Basin, and
central San Miguel Gap have the highest recorded values of
7.5 phi. Bank tops have the lowest phi values, generally
less than 3 phi. The lower phi values emphasize the
importance of lag concentration of coarser sediments on
bank tops as finer-grained particles are winnowed out and
deposited downslope.
In general, the mean phi isopleths follow bathymetric
contours and outline bas-in floors. There are some notable
exceptions, the two incursions of coarser sediment that
extend off local ridges onto the floor of western Tanner
Basin and the lobe in western San Miguel Basin probably
represent recent turbidites of limited areal extent.
Another prominent coarse-grained lobe is found in a canyon
in the southeast corner of Tanner Basin. Medium sand-
40
Figure 9: Mean grain size distribution of surficial
sediments of the outer borderland.
41
GRAIN s iz e d is t r ib u t io n of
\ SURFICIAL SEDIMENTS
sized sediment, is restricted to the canyon axis. There is
no lobe of progressively finer sediment extending onto the
basin floor. This could imply that finer material had
been winnowed out before the coarser sediment was
deposited as a slump off the bank.
San Miguel Gap is unique in the outer borderland
because it is elongate in the east-west direction and
open-ended to the west. The Channel Islands provide a
local insular source area for sediments, but below 1500 m
the gap has sediment characteristics similar to the other
basin floors. The island shelf area east of San Miguel
Gap has low phi values, demonstrating the transport of
finer-grained sediments off bank or shelf areas onto basin
floors.
San Miguel Basin is distinct from Tanner and Patton
Basins. Although it is also dominated by basin
sedimentation, San Miguel Basin is really just a shallow-
silled depository. Since there is not a large difference
between sill depth and floor depth in San Miguel Basin it
is still affected by bottom currents that transport finer-
grained sediment. This is evidenced by the slightly
lower mean phi values compared with the other basin
floors.
Two trends can be distinguished on a bottom depth
versus mean grain size plot (Figure 10); a lower linear
43
Figure 10: Bottom depth (in meters) versus mean grain
size (in phi). Basin floor sediments are
clustered, the basin slope sediments are
typical of continental slope sedimentation
(R=0.69).
44
©rain Siza in phi
Bottom Depth vs Mean Grain Size
8
7
6
5
4
3
2
0.9 1.1 1.3 1.5 1.7 19 2.1 2.3 2 5 2.7
- P *
cn
(Thousands)
Depth in meters
trend encompasses samples from slope areas and an upper
cluster that includes basin floor samples. The basin
trend illustrates the uniformity of basin floor sediments
within the fine silt range. The few lower outliers are
from San Miguel Basin further emphasizing the slightly
coarser nature of those sediments. The slope trend shows
a positive relationship (R=0.69) between bottom depth and
mean phi. Increasing mean phi values with incresed bottom
depth is typical of continental slope sedimentation
(Emery, 1960).
The entire region is texturally similar to other deep
hemipelagic sediments (Gorsline and others, 1968). They
are poorly sorted, and sorting becomes better with
decreasing grain size. There is a change from slightly
positive skewness to slightly negative skewness with
decreasing grain size. Kurtosis increases logarithmically
with decreasing grain size. All moment measurements are
shown in Figures 11-13. Skewness appears to be
independent of standard-deviation and kurtosis. This
textural relationship is typical of fine silt-clay
deposited a great distance from source, and probably
reflects the effect of flocculation, bioaggregation, and
sorting in transit as the suspended load moves from the
coast seaward (Gorsline and others, 1968). Much of the
terrigeneous signature, however, is masked by a relatively
46
Figure 11: True moment, grain size measurements: mean phi
versus standard deviation and skewness.
47
n phi vs std. ^eviatio
j.2 -|
1.9 H
I
1.3-
1 .6 -
1 •+ “
1 . 3 -
1 .1 -
1 -
0.9
3 a
□
□
a
c
□
□
a
B
<4? 3 ^
a uS
□
1 3 5
mean phi
skewness vs mean phi
□ □
CO
0.6
7 5 3
mean phi
Figure 12: True moment grain size measurements: skewness
versus standard deviation and kurtosis.
49
skewne:
skewness vs std. deviation
1.6
1.4
1.2
1
O.S
0.6
0.4
0.2
0
- 0.2
- 0 . 4
- 0.6
— 0, B
-1
- 1.2
0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3
standard deviation
skewness vs kurtosis
1,6
1.4
1.2
1
0.B
0.6
0.4
0.2
0
- 0.2
- 0 .4
- 0.6
“ 0.B
-1
- 1.2
0 4 B 12 16 20 24 23
kurtosis
cP
a
a n q - ,
° a 0 □
CP
□ □
a
i □ c P
%
Figure 13: True moment grain size measurements: kurtosis
versus mean phi and standard deviation.
51
kurtosis kudos
mean 3hi vs I - urtosis
26
20
J P □
1 0
aa
4 -
$0 □
8 3 6 5
mean phi
std. deviation vs kurtosis
26
24 -
20
4 -
10
□cP
4 -
CD
□ S3 ■ f t
acr
2.2
9
1 . 8 1.6 1 , 2 1.4
std. deviation
52
high contribution of biogenic material from the overlying
water column. The California Current may also be
contributing an appreciable amount of sediment from
northern sources. Emery (1960) noted an increasing grain
size trend in the outer basins and attributed the increase
to planktonic foraminifera tests.
Figure 14 illustrates the grain size distribution
curves of Station 30999 and 25989. The positive skewness,
relatively low kurtosis, and high standard deviation at
Station 3 0999 is typical of slope areas in the outer
borderland. The pronounced dip in the curve at 4.5 phi is
primarily an artifact of overlapping textural analyses
techniques and probably does not represent distinct
bimodal sedimentation.
The curve for Station 25989 is an example of basin
floor sediments. There is a slight negative skewness,
lower standard deviation and higher kurtosis than at
30999. Since there is less than 10 percent of the sample
greater than 63 micronsr only Coulter Counter analyses was
performed, eliminating the artificial dip
Station 30650 (Figure 15) is unique in that it is
relatively coarse-grained and extremely well-sorted. The
sand fraction at 30650 is almost exclusively Uvigerina
tests, all fine-grained material has been winnowed out.
The basin and slope grain size trends are further
53
Figure 14: Grain size distribution curves for stations
30999 and 25989. Station 30999 is typical of
the basin slope environment. Station 25989 is
typical of basin floors.
54
lumc percent
Grqin
26 1
24 -
22 -
20 ~
18 -
16 -
14 -
12 -
Size Distribution
30999/259B9
Figure 15: Grain size distribution curve for station
3 0650 on Patton Ridge. Sediment accumulation
at station 30650 is almost entirely Uviaerina
tests.
56
v ' o i urne oer
rain Size Distributior
30BS0
cn
4
grain size in phi
emphasized by a ternary plot of sand, silt and clay
percents (Figure 16). The lower cluster is the basin
sediments dominated by silt and clay, with minor sand
input. The slope sediments are characterized by
increasing amounts of sand, with very little clay.
Silt/clay ratios for each trend show two distinct
relationships (Figure 17). The basin sediments show an
inverse linear relationship starting at 50% silt and clay
(R=0.75). This simple relationship is expected for
sediments with little sand; as silt increases, clay
decreases.
The positive linear relationship (R=0.64) extending
up from 0% silt is interesting because it shows a constant
ratio between silt and clay. The trend, therefore, is a
function of increasing sand. This relationship implies
that there is a sediment source with a constant silt/clay
ratio. This source could be the California Current
transporting finer-grained sediment from northern
provenances, with the sand component originating from
local sources.
Silt percent isopleths (Figure 18) show a
distribution similar to mean grain size, emphasizing the
dominance of silt in outer borderland sediments. The silt
content outlines basin floors where silts compose up to 75
percent of the basin floor sediments. Contours in western
58
Figure 16: Ternary diagram of sand, silt and clay
percent. Basin floor sediments are clustered
between silt and clay. Increasing sand
content marks basin slope samples.
59
t
C T t
O
clay
Figure 17: Silt percent versus clay percent.
O
BASIN FLOOR
□□
n
BASIN SLOPE
□ a
Figure 18: Silt percent distribution in the outer
borderland.
PERCENT SILT IN
SURFIC^L SEDIMENTS
Tanner Basin outline the lobes of increased silt content
from the local slopes. Again, these lobes are areally
restricted and represent fine-grained turbidites. The
majority of the silt fraction is planktonic foraminifera
and fine-grained quartz. Planktonic foraminifera dominate
due to the proximity of the upwelling zone centered on
Santa Rosa-Cortes Ridge and to the lack of terrigeneous
material available to dilute the biologic component. Much
of the fine-grained quartz is probably derived from desert
areas of Southern California and transported by aeolian
processes during Santa Ana offshore wind conditions.
Distribution of clay-sized particles in the outer
borderland is characterized by an absence of such material
on bank tops where energies are high (Figure 19). The
presence of clays increases downslope and clays are
concentrated on basin floors. Schwalbach (1982) described
a similar distribution in the nearshore basins.
Deposition of clay is probably controlled, in large part,
by filtering and aggregating mechanisms in the water
column. Fleischer (1970) found that illite,
montmorilIonite, and smectite dominated the clay suites.
In examining sand distributions (Figure 2 0), it can
be seen that sands are concentrated on bank tops (>3 0%
sand) with sand percentages decreasing downslope. Basin
floors rarely exhibit more than 10 percent sand. The
65
Figure 19: Clay percent distribution in the outer
borderland.
J O
percent clay in
SURFICIAL SEDIMENTS
Figure 20: Sand percent distribution in the outer
borderland.
PERCENT SAND IN
SURFICIAL SEDIMENTS
<10%
10-30%
> 30%
distribution of sand components primarily corresponds with
local sources of reworked material or biological
production.
Sand Fraction
The sand fraction of samples with more than 15% sand
(Figure 21) was split into 63-250 micron and greater than
250 micron fractions. Compositional components were
designated as dominant (>50%), abundant (30-50%), common
(10-30%), or rare (<10%) based on estimates from point
counts. Categories included glauconite, foraminifera
tests, reworked fragments, aggregates, biological debris,
detrital grains, and fecal pellets. Organic debris,
except tar aggregates, is rare or nonexistent in the sand
fraction of the outer borderland.
Glauconite occurs in a broad range from fully mature,
dark green relict grains to incipient replacement of fecal
pellets and foraminifera fillings. The fully mature
specimens dominate and are probably reworked from Pliocene
outcrops on bank tops and slopes. It is improbable that
there is much modern authigenic production of glauconite
in the California Borderland area (D.S. Gorsline, pers.
comm.).
Glauconite is dominant in many of the greater-than-
250 micron splits. Abundant glauconite in the 63-250
70
Figure 21: Sand fraction composition station locations.
71
SAND COMPOSITION
LOCATIONS
micron splits is only found at stations dominated by
glauconite in the greater-than-250 micron split.
Glauconite is particularly associated with upper Patton
Escarpment and isolated Tanner Basin stations. This
distribution probably reflects the outcrop pattern of
glauconite-rich Pliocene sedimentary rocks. Glauconite is
conspicuously rare in San Miguel Gap and central basin
cores•
Foraminifera tests are an important component in all
samples. They can be divided into benthic and planktonic
forms. Planktonic foraminifera dominate the 63-250 micron
split in all but three stations. In the three stations
they are abundant, but make up less than 50% of the
sample. Benthic foraminifera are much more important in
the greater-than-250 micron split where they make up at
least 20% of the sample. They dominate where glauconite
is rare in San Miguel Gap and at station 25988 in Tanner
Basin. Arenaceous foraminifera are common in many of the
samples•
Benthic forams, almost exclusively Uvigerina,
dominate at station 30650. Uviaerina are large benthic
foraminifera that live in the surface sediment and "bloom"
in areas of high organic carbon (Faye Woodruff, pers.
comm.). Station 30650 is unique in that it is 92% sand,
of which 75% is Uvigerina tests. The station is
73
relatively shallow, 990 m, and has a low organic carbon
content. The lack of organic carbon is undoubtedly a
result of winnowing of fine organic material and thus, the
station represents a lag concentration of coarse sand
sized Uviaerina tests and glauconite.
Detrital sediments are defined as individual quartz
and feldspar grains. Sand-sized micas are noticeably
absent from the outer borderland area. Detritals dominate
the smaller end of the 63-250 micron splits in most
samples. Detritals are particularly important in both
splits at the base of Santa Rosa-Cortes Ridge and in San
Miguel Gap. This is probably due to being closer to
terrigeneous source areas on San Nicolas Island or the
Channel Islands. In addition, San Miguel Gap may be
receiving some sediment from Santa Barbara Basin to the
northeast.
Fleischer (1970) estimated the aeolian contribution
to be about 10^ tons/yr for the California Borderland.
The silt-sized quartz and feldspar in the outer borderland
are, at least partially, aeolian derived. There is an
increase in relative importance of fine-grained quartz and
feldspar offshore probably due to the decrease in
terrigeneous dilution. The relative importance of
detritals at the base of Santa Rosa-Cortes Ridge and in
San Miguel Gap may also be due to a decrease in dilution
74
of any of the other components as a result of limited
outcrop sources.
Reworked sediments are rock fragments, primarily
light gray siltstone which is presumably eroded from
Pliocene outcrops on bank tops and slopes. Patton
Escarpment has abundant reworked sediments suggesting
local outcrop sources. Stations located at the base of
Santa Rosa-Cortes Ridge in Tanner Basin have very low
percentages of reworked sediments.
Aggregates of primarily silt-sized debris are
cemented by black bituminous matter. Many of the tar
aggregates appear to be selective for small planktonic
foraminifera. The aggregates are found predominantly in
the greater-than-250 micron splits.
Gorsline and others (1968) documented high organic
carbon contents in some Tanner Basin piston cores. They
attributed the higher values to small bits of bituminous
matter which probably originated from petroleum seeps on
the adjacent shallow bank tops. The distribution of tar
aggregates does not necessarily show a positive
relationship with total organic carbon contents determined
in this study. This is probably the result of consumption
or winnowing of finer-grained organic matter. Presence of
tar and bituminous aggregates prohibited the use of
organic carbon for C14 dating. Thus, CaC03 carbon was
75
used.
Fecal pellets can be a locally important component of
the sand fraction. They are only recognized from 8
stations, but are abundant in 5 of those. Fecal pellets
are abundant at stations on basin floors which apparently
do not have local sources for the other compoents. The
lack of wide spread fecal pellets may be because they are
easily destroyed if not well consolidated. Those that are
preserved may have unrecognized incipient replacement of
glauconite.
Biological debris, other than foraminifera tests, is
found in many of the samples. Fish vertebrae, sponge
spicules, diatoms and radiolarians are recognized as well
as unidentified shell debris. Diatoms and radiolarians
are often important contributors to the 63-250 micron
splits.
One interesting observation of the greater-than-63
micron fraction of the outer borderland basins is the
virtual lack of micas. -Although many of the local insular
islands have large proportions of metamorphic terrains
(Vedder and Howell, 1976), they do not appear to
contribute micas to the outer basins. In contrast,
Schwalbach (1982) illustrated the usefulness of micas as
indicators of depositional environments in inner
borderland basins. Up to 40 percent of the greater-than-
76
63 micron fraction in Santa Monica Basin consists of
micas. In the inner basins, mica percents may outline
preferentaial transport corridors as described by Karl
(1976). The lack of mica adds evidence for using Santa
Rosa-Cortes Ridge as a major sediment divider.
Calcium Carbonate and Organic Carbon
Biogenic production is source of a significant amount
' of calcium carbonate and organic carbon in the borderland
‘ (Schwalbach, 1982). Emery (1960) and Schwalbach (1982)
i described the general increase in carbonate and organic
carbon percentages as one progresses further from shore
. and major sources of terrigeneous material (Figure
22)(Schwalbach, 1982). Highest carbonate and lowest
1
organic carbon percentages are typically found on shelf
areas and bank tops (Emery, 1960). Compared with mainland
shelf sediments, island shelves tend to exhibit both
higher carbonate and higher organic carbon percentages due
I
I
i to the decreased effect of terrigeneous input. (Gorsline,
1981).
The carbonate content of progressively further
offshore basin floors also tends to increase. Values in
Santa Barbara Basin generally are less than 10 percent,
Santa Nicolas Basin values range to over 2 5 percent
(Schwalbach, 1982), while in the outer borderland basins
77
I
I
I
I
I
j Figure 22: Percent carbonate and organic carbon
I
distribution in the inner borderland (from
Schwalbach, 1982).
i
78
PERCENT ORGANIC CARBON
IN SURFACE SEDIMENTS
values are generally over 3 0 percent. On Patton
Escarpment values range up to 60 percent. These values,
however, should not be mistaken for estimates of actual
1 rates of carbonate deposition.
Dilution of carbonate and organic carbon by
terrigeneous input appears to be a major factor in
; determining the percentage distribution of these materials
in the inner borderland basins (Gorsline, 1981). In the
inner borderland, high carbonate sediments are often
thought of as highlighting areas of little or no
Iterrigeneous deposition. In the outer borderland,
:evidence suggests that dilution, winnowing of fine-grained
i
jsediment by bottom currents, increased biogenic production
'due to upwelling over the Santa Rosa-Cortes Ridge, and
I
!California Current effects all contribute to the
.distribution of carbonate and organic carbon.
^ Each basin within the study area exhibits unique
;carbonate distribution patterns (Figure 23), therefore,
i
|each basin is discussed individually in regards to
i
'processes controlling carbonate sedimentation.
I
| The distribution of carbonate percentages in Tanner
iBasin is typical of inner borderland basins. Highest
;carbonate values are found on bank tops and decrease
i
downslope to minimums on basin floors. Carbonate values
reach a minimum of 25% on the basin floor, while on Cortes
|
80
Figure 23: Percent carbonate distribution in the outer
borderland.
81
ERCENT CARBONATE IN
URFICIAL SEDIMENTS
Bank values exceed 80%. Bank tops are probably enriched
in carbonate due to increased productivity of benthic
foraminifera and winnowing of fine-grained clastic
material.
The pattern within Patton Basin is slightly modified.
Carbonate percentages increase slightly off Santa Rosa-
Cortes Ridge on to the broad, shallow basin floor and
increase more dramatically up Patton Ridge. The widely
spaced contours that increase away from Santa Rosa-Cortes
Ridge may be the result of declining terrigeneous dilution
and the diversion of pelagic sediments seaward as the
Southern California Counter-current breaches the ridge
through its saddle.
San Miguel Basin carbonate distribution shows an
inverse relationship with adjacent banks in contrast to
other borderland basins. Carbonate percentages are
generally more than 40 percent on the basin floor, higher
than on the adjacent banks. It is unlikely that this
increase in carbonate percent is a reflection of absolute
carbonate production, but rather a result of dilution by
other components on Patton Ridge and the dissolution of
carbonate down the slope of Patton Escarpment. At the
base of Patton Escarpment (3900 m), values are less than
2 0%, probably because water depths are approaching the
Carbonate Compensation Depth (CCD).
i Carbonate percentages in San Miguel Gap are extremely
low, ranging from 19% to 26%. Several processes may
: contribute to the low values: 1) terrigeneous sediments
j from the Channel Islands and Santa Barbara Basin dilute
! the carbonate, 2) depths are increasing toward the open
| sea and the carbonate is dissolving, or 3) the California
j Current influences carbonate deposition by transporting
; much of the surface water productivity southward to San
i Miguel Basin.
1 In general, organic carbon percentages increase
: offshore, mimicking the borderland-wide carbonate trend
! (Figure 24). Again, this is probably controlled primarily
I
■ by terrigeneous sedimentation diluting the nearshore basin
; sediments. As in the rest of the borderland, organic
carbon values are lowest on island shelves and bank tops
1 (less than 1 percent). Values increase rapidly downslope
with highest values found on basin floors (up to 6.6
percent). Organic carbon tends to behave
sedimentologically like fine-grained clay and silt and is
probably absorbed on clay surfaces. The distribution of
organic carbon is, therefore, primarily a reflection of
decreasing grain size (R=.58) (Figure 25). Expectedly,
organic carbon also tends to increase with increasing clay
percent (R=.66)(Figure 26). Since sedimentation rates are
slow enough for complete bioturbation in all outer
84
Figure 24: Percent organic carbon distribution in the
outer borderland.
85
ENT ORGANIC CARBON IN
1IRFICIAL SEDIMENTS "
I
Figure 25: Organic carbon content versus mean grain size
(R=0.58).
I
l
I
i
87
Mean Grain Size vs. Organic Carbon
4 - -
J 5
□□
C O
C O
Mean Grain Size in phi
Figure 26: Organic carbon content versus clay percent
(R=0.63).
89
> rg a n i e C o m o n 0 o n te n 1 : .
i_ i
Q
cr
n
n
o a
borderland basins, carbon content is probably not a
function of exposure to bio- or inorganic oxidation.
On a regional scale, organic carbon values tend to
increase to the south. San Miguel Gap has the lowest
basinal organic carbon contents (maximum 3.5%), while
Tanner Basin has the highest, with values up to 6.6 %.
This is possibly due to increased terrigeneous input from
the Channel Islands and Santa Barbara Basin into San
Miguel Gap or enrichment by California Current-transported
sediment. Subtle winnowing of finer-grained sediment,
including organic carbon, by bottom currents may be
important in both San Miguel Basin and San Miguel Gap.
These basinal areas are somewhat open to normal marine
processes and are not sheltered by relatively high sills
as in the true basins of the borderland.
Organic carbon values in Patton Basin are offset
from the basin center toward the eastern flank of Patton
Ridge. Current flow through the saddle of Santa Rosa-
Cortes Ridge may influence the distribution or tar seepage
from the faulted slope of Patton Ridge may be enriching
the pelagic organic carbon. Gorsline and others (19 68)
noted that in zones of highest carbon content in Tanner
Basin, the coarse fractions contain small bits of
bituminous matter which probably originated from petroleum
seeps on the adjacent shallow bank tops. The distribution
of surface organic carbon in Tanner Basin based on values
from this study also shows slightly higher values near the
base of Santa Rosa-Cortes Ridge, but the organic carbon
high also corresponds to the deepest part of the basin
I
j where the finest sediments accumulate.
J Correlation between tar aggregates in the coarse
| fraction observed in this study and high organic carbon
content is not readily apparent. This could be because
only samples with appreciable coarse fractions were
studied for sand composition and the majority of organic
carbon is associated with finer sediment. If this is the
I
case, the observed higher organic carbon values are, j
i
therefore, probably more closely related to grain size and j
the added contribution from petroleum seeps on the slopes |
I
is relatively minor.
i
Organic carbon contents on Patton Escarpment are j
extremely low. The relatively coarse grain size found on !
i
the escarpment would appear to exclude much organic carbon |
accumulation. In addition, most of the sediments on j
I
Patton Escarpment appear to be primarily derived from i
outcrops exposed on the slope, the low values may suggest !
that the horizons contributing organic matter to the j
sediment on Patton Ridge and Santa Rosa-Cortes Ridge do
not outcrop on Patton Escarpment. This is supported by
the presence of basaltic material in the majority of
92
dredge samples from Patton Escarpment (Uchupi, 1962) and
the lack of tar aggregates in the sand fraction.
( There does not appear to be any correlation between
i
! carbonate and organic carbon (Figure 27). This
I
| relationship could illustrate that there are a variety of
processes contributing to the distribution of sediments in ■
1 i
ithe outer borderland.
i
Sedimentation rates
!
Sedimentation rates have been determined for Tanner j
i
Basin using piston core data (Gorsline and others, 1968)
and in connection with borderland-wide sedimentation rate
studies (Emery, 1960? Schwalbach and Gorsline, 1985). j
Schwalbach (1982) determined sedimentation rates for the j
central and inner borderland basins. A comparison between |
inner, central and outer basins reveals an exponential
j
decrease in sedimentation rates with each successive jump !
I
offshore. Santa Barbara Basin has the highest rates of
90-130 mg/cm2/yr due to its proximity to the major point j
source of terrigeneous material in the borderland, Santa ,
i
Clara and Ventura Rivers. As you move west from the
I
mainland, sedimentation rates decrease to 32 mg/cm2/yr in
, o , I
Santa Cruz Basin and 13 mg/cm^/yr m San Nicolas Basin. j
In addition, there is a northern emphasis on the total j
regional sediment budget, evident by the southern decrease i
Figure 27: Organic carbon versus carbonate content
94
n
n
n
ljo.
lJ
n
n
□
n
in sedimentation rates.
While terrigeneous sedimentation dominates the
1 present inner borderland environment, 58-78% of the
i terrigeneous material is trapped by the inner borderland
i
; basins (Schwalbach and Gorsline, 1985). This trapping of
I
terrigeneous sediments leads to a relative increase in
hemipelagic sedimentation offshore. As a result, the
| annual sediment supply rate drops and hemipelagic drapes
l
; typify the outer "starved" basins (Teng and Gorsline,
; 1988). Slow sedimentation leads to greater slope
. . . . . i
stability with fewer slope failures. Rates of tectonic i
i
uplift and subsidence are equal to or greater than the
accumulation in the outer borderland basins, and
penecontemporaneous deformation of the draped sediments is
seen in Tanner and Patton Basin (Teng and Gorsline, 1988).
t
I
Control of suspended-load sediment argues for a 1
! strong lateral component in the movement of hemipelagic j
materials, and suggests that a slow circulation occurs in
the deep basin areas (Gorsline and others, 1968). The
suspended load tends to be carried at the surface, along
strong thermoclines, and along the bottom, and is strongly
influenced by current patterns in these zones (Rodolfo,
i I
j 1964; Wildharber, 1966). Bottom currents along bank tops !
j I
' and across basin floors greatly affect the ultimate
distribution of the sediment in the outer borderland
96
basins.
Gorsline and others (1968) noted a decrease in
sedimentation rate for Tanner Basin since 12,000 yrs bp.
i
i
I They concluded that the decrease was a function of large-
i
; scale factors of climatic and oceanographic change. The
i
i late Pleistocene was a period of lower sea level that
resulted in the greater exposure of land area, increased
stream gradients, and probably greater rainfall which
would supply greater loads of sediment prior to 12,000 yrs
bp.
For Tanner Basin, the exposure of parts of the |
presently submerged Santa Rosa-Cortes Ridge would provide
subaerial sources for sediment which could be transported
into the basin (Gorsline and others, 1968). Downcore
composition in piston cores show increased contribution of
coarse layers, particularly in the eastern part of the
basin.
i
For this study, samples from the bottom of box cores
were chosen from the center of each basin for radiocarbon
i
analysis. Each core is 3 0-3 4 cm in length. Radiocarbon j
i
t
dates were obtained using carbonate carbon due to the i
residual accumulation of organic carbon which would result j
in anomalously old dates. Although there is a problem !
i
with residual carbonate and bioturbation mixing sediments
i
of different ages, radiocarbon dating has the advantage of
97
covering relatively long time periods resulting in a long
term average. Carbonate and micropaleontology
stratigraphy is not useful on such short cores with slow
i sedimentation rates.
I
: Deposition in the outer borderland primarily results
i
from continuous hemipelagic sedimentation. The highest
sedimentation rates are found in Tanner Basin (10
l
o
mg/cm /yr). San Miguel Gap and Patton Basin have
• 0
intermediate values of 6 and 5 mg/cm /yr, respectively.
San Miguel Basin has the lowest rate of 3 mg/cm2/yr. Bank
tops, shelves and upper slopes accumulate little or no
sediment, as discussed earlier. The sedimentation rate
determined for Tanner Basin in this study is comparable to
I
j those found in previous studies (Emery, 1960? Gorsline and
others, 1968? Schwalbach and Gorsline, 1985) (Table 2).
There are a number of factors that may contribute to |
t
the higher accumulation rate determined for Tanner Basin, j
Tanner Basin is one of the innermost of the outer basins
and will trap more of the suspended load sediment that
reaches the outer basins. In addition, Tanner Basin is
adjacent to San Nicolas Island, an insular source area.
The high carbonate rate may be attributed to the zone of
upwelling located southwest of San Nicolas Island. The
steep basin slopes on either side of Tanner Basin prevents
the accumulation of sediment on the slopes, therefore
__________ 98
Table 2: Sedimentation rates for the outer borderland
basins determined in this study. Rates
determined in previous studies and for other
basins are included for comparison.
99
TABLE 2
p
Sedimentation rates (mg/cm /yr)
Total Detrital Carbonate
This study:
Tanner Basin 10 6 3.5
Patton Basin 5 3 2
San Miguel Gap 6 4 1.5
San Miguel Basin 3 1.5 1.0
• • ,
Tanner Basin from previous studies :
1. 11.2 5.4 3.1
2. 10 6 4
3. 5-20 3-10 2-8
Comparison with other Borderland basins (3.)
Santa Barbara 90-130
San Pedro 35-50
Santa Catalina 28-16
San Nicolas 13
1. Emery (1960)
2. Gorsline and others (1968)
3. Schwalbach and Gorsline (1985)
Organic
0.5
<0.5
<0.5
<0.5
1.0
<1.0
0.5-2
100
slope failures that contribute sediment to the basin floor
are more common than in the other outer basins. Tanner
Basin also has the greatest sill to floor relief and,
I
I
i therefore, is probably less affected by bottom currents
i
J
I that may modify the distribution of sediments after
deposition.
San Miguel Gap has two possible source areas
I
j unavailable to the other basins. There is probably some
J spillover from Santa Barbara Basin down San Miguel Canyon,
j and the Channel Islands provide a relatively large insular
source area. However, the Channel Islands also provide an
effective barrier to suspended sediment from the mainland,
reducing the amount of terrigeneous sediment available to
San Miguel Gap. Since San Miguel Gap is open to the
Pacific to the west, it is not an effective sediment trap
and much of the sediment entering the gap area may be
ultimately lost to the open Pacific. Based on the
i
distribution of carbonate, it appears that the California
Current transports much of the biologic production from
the upwelling zone south of the Channel Islands to the
south, depleting the percent of carbonate being deposited
in San Miguel Gap.
Some of the carbonate removed from the San Miguel Gap !
i
area is deposited in San Miguel Basin, but due to its low
sill to floor relief San Miguel Basin is still affected by
j 101
bottom currents that transport sediment after initial
deposition. In addition, since it is furthest from
^ terrigeneous source areas, both mainland and insular, and
isolated by Patton Ridge, the sediment available for
deposition in San Miguel Basin is limited to biologic,
i
; aeolian or current-transported material.
| Patton Basin has the advantage of being located under
I
I . . . . . 1
a major zone of upwelling and Patton Ridge is an effective ■
block for suspended sediment. However, the saddle of
Santa Rosa-Cortes Ridge appears to be a conduit for the
Southern California Countercurrent returning to the main
California Current. This results in a "current shadow"
over most of Patton Basin. The current is strong enough
to maintain most of the suspended load, transporting it
I
south into Tanner Basin or northwest into San Miguel Gap.
1
The floor of Patton basin is probably affected by bottom
currents due to the relatively low gradient of the eastern
basin slope.
ANIMAL-SEDIMENT RELATIONSHIPS
Biological activity can greatly modify surface j
sediments (Edwards, 1979; Savrda, 1983). With continuous I
sedimentation continuous bioturbation processes also take
place (Wetzel, 1984). Analysis of X-ray radiographs from
102
the outer borderland basins (Figure 28) shows a generally
nondescript, mottled texture with a low diversity biogenic
trace association typical of deep sea sediments.
There are two types of biogenic structures (Wetzel,
1984): biogenic traces with definite shapes and sharp
j contours; and biodeformational structures that destroy
i
| preexisting structures without leaving sharp contacts.
With the exception of some discrete biogenic traces, rare
turbidite layers, and mass flow slope deposits, the
sediment is dominanted by biodeformational structures.
The sediment is, therefore,homogeneous due to mixing by
infaunal organisms.
In unconsolidated sediments, contrasts in absorbance
on x-ray radiographs (XR) are largely a function of
composition, grain size, and fabric (Edwards, 1979).
Edwards (1979) described three XR facies which will be
used here: 1) homogeneous hemipelagic muds, 2) turbidite
interbeds, and 3) mass flow slope deposits distinguished
by their characteristic swirl signature. The mottled
homogeneous texture can be difficult to distinguish from
the swirl signature because bioturbation and debris flows
have a similar disruptive effect on sedimentary structures
(Suzanne Reynolds, pers. comm.). Depositional laminations
typical of anoxic basins, such as described by Malouta
(1978) for Santa Monica Basin, are not observed in any of
103
Figure 28: X-ray radiograph station locations.
105 I
3 06 3 6
X-RAY RADIOGRAPH
LOCATIONS
'v, ^ - 4
a ad11\
•I 6 66 3
26072
the outer borderland x-ray radiographs.
Reynolds (1984) studied x-ray radiographs from Tanner
Basin. She described a homogeneous or bioturbate fabric
for the basin sediments. The fabric is interpreted as
: being a result of low sedimentation rates combined with
\
; high rates of bioturbation. Thin debris flows and other
!
| mass movements occur on slope and base-of-slope areas.
The basin floors are characterized by predominately
homogeneous hemipelagic sediments. In many cores there is
a transition downcore in bioturbate fabric. Station 3 0638
from San Miguel Gap (Figure 29) and 30132 from Patton
Basin (Figure 30) are typical of basin floor cores. The
top 10 to 15 cm is dominated by discrete burrows and small |
I
i
! burrow networks. The burrows are often lined and range in |
size from 0.2 to 0.5 cm in diameter. There is a rather I
abrupt change in texture to large burrow mottles that
extends from 10 cm up to 3 5 cm. The sediment in this zone
is almost completely homogenized, but density contrasts
delineate individual burrows that can be up to 4 cm in
diameter. Below the zone of large burrows there is an
increased fading of contrast. The sediment becomes
totally homogenized due to compaction and dewatering.
There are few small burrow remnants that are preserved
below 15 cm due to the constant reworking by the organisms
that produce the large burrows.
Figure 29: Homogeneous hemipelagic sediment profile,
\
station 30638.
107

Figure 30: Homogeneous hemipelagic sediment profile,
station 30132.
109

In continuously accumulating, oxygenated deep-sea
sediments from NW Africa, a tiering within the bioturbated
zone is observed, and several bioturbational levels can be
distinguished (Figure 31a)(Wetzel, 1984). With extremely
slow sedimentation rates typical of the outer borderland,
individual levels may be condensed, forming a surface
layer and a deeper mottled level (Figure 31b).
Under steady-state conditions, bioturbated zones move
upward corresponding to the sediment buildup. Shallow
burrows are destroyed by deeper-penetrating ones.
Consequently, the chance for a biogenic trace to be
preserved depends on the burrowing rate within deeper
levels and destruction of burrows by compaction or
diagenesis.
There is no direct evidence of the makers of the
small or large burrows. The discrete small burrows are
almost all lined, probably with a mucus-like substance.
The large burrows are unlined, therefore they are most
likely feeding or locomotive traces instead of being
domiciles (D.J. Bottjer, pers. comm.).
Station 30644 (Figure 32) shows a well preserved
swirling trace composed of pinnate lamellae. It is almost
identical to the one described by Edwards (1979) as an
irregular echinoid feeding trace. Burrows with this
typical internal structure have been classified as
111
Figure 31: Schematic diagrams of vertical distribution of
biogenic traces within the burrowed zone.
A. Water depths of 2000-3500 m off NW Africa
(from Wetzel, 1984)
B. Condensed profile typical of the outer
California Borderland.
112
I. top layer
II. Scolicia
III. Planolites
IV.
V.
Helminthopsis
Chondrites
Lophoctenium
[oophycus
ii^
s
cm
A. Off coast of NW Africa. (from Wetzel, 1984)
III. mottled
B. Outer California Borderland condensed profile
113
Figure 32: Scolicia, station 30644
114
115
Scolicia. Scolicia is a commonly-occuring biogenic trace
in slope and rise sediments off NW Africa (Wetzel, 1984).
Enigmatic pellet-like features first described by
Savrda (1983) from Santa Monica cores are also present in
several Tanner Basin cores (ex. 2599 3)(Figure 33) and one
San Miguel Basin core. These pellet-like features are
spherical with high density "haloes" around them. They
are relatively uniform in size, approximately 0.5 cm in
diameter. Their origin is problematic since they have not
been isolated for individual examination. The features
fade below 10 cm which suggests that they are not
mineralogic and are eliminated diagenetically by
compaction, dewatering or reworking.
There is a possibility that the features are biologic
and represent spherical fecal pellets of an epifaunal
organism. C.E. Savrda (per. comm.) noted a possible
relationship between the abundance of ophiuroids in bottom
photographs and X-ray radiographs exhibiting the haloed
features, however, ophiuroids do not have a developed
digestive tract and are unlikely to produce consolidated
fecal pellets (Russell Zimmer, pers. comm.).
Non-bioturbated turbidite layers are rare and areally
restricted. Stations 3 012 3 (Figure 34) and 3 012 5 have a
layer that may represent a single turbidite event near the
canyon in the center of San Miguel Gap. The layer has
116
Figure 33: Enigmatic pellet-like features, station 25993
117

Figure 34: Turbidite layer preserved at station 30123.
Note cross-cutting relationship with biogenic
traces.
119
120
similar features in both cores and is approximately the
same thickness. It occurs at 13 cm and 11 cm downcore,
respectively. 3 012 3 has 2 cm more cover, probably a
result of being nearer to the insular source area. Both
cores show biological overprinting of the turbidite layer.
The homogeneity of the remainder of 3 012 3 and 3 012 5
may be the result of sedimentation by debris flow or
bioturbation. There is no direct evidence for debris flow
sedimentation, but the general chaotic fabric is
suggestive of mass movement. The presence of discrete
burrows, particularly vertical ones, lead to the inference
that bioturbation is not impeded by the relatively steep
slope and, at least occasional, mass movement.
There is no continuity between the other cores
exhibiting discrete turbidite layers. Stations 2 6019,
26017 and 26972 have relatively coarse sediments and are
located at the base of slopes. Any turbidite layers seen
in these cores are not present in stations nearby and
therefore probably do not extend much further toward the
center of the basin. Station 26000 (Figure 35) is not
directly adjacent to the slope areas, but at the northern
end of the central depression in Tanner Basin. The
discrete layers are probably the result of localized fine
grained turbidites which are restricted to a small canyon
entering the basin floor from the north.
121
Figure 35: Fine-grained turbidite layers, station 26000.
122

Cores which have been interpreted as exhibiting the
'swirl' signature are restricted to steep slope regions.
Station 31008 (Figure 36) is a good example of the coarse
grained nature and swirl signature of slope debris flow
sedimentation. In many of the "swirl" cores bioturbation
is still a prominent sedimentary structure modifier.
Bottom photographs from Tanner Basin show a variety
of benthic organisms on the basin floor (Figure 37).
Ophiuroids, holothurians, pennatulids, a demersal fish and
a small gastropod can be observed in the photographs
(Figure 38). Furrowed traces, worm tubes, and large and
small burrows suggest other macrofauna may be living on or
in the sediment.
Ophiuroids are present in every photograph, ranging
in abundance from 5 to >100 specimens/2 m (e.g.
26001)(a). Holothurians are seen in four of the
photographs (e.g. 25995) (b) , but elongate feces produced
by holothurians (R. Zimmer, pers. comm.) are present in
several others. Tight planispiral feces are present in
various conditions in most of the bottom photographs.
Many of the spirals appear to be associated with small
holes and may be produced by infaunal organisms (such as
polychaete worms) that eject their feces onto the sea
floor from their burrows.
Burrow openings are common in most of the photographs
124
Figure 36: Swirl signature characteristic of slope debris
flows, station 31008.
125
126
Figure 37: Bottom photograph locations in Tanner Basin.
127
V--V— c—rc
4: : j wu
\
BOTTOM PHOTOGRAPH
LOCATIONS
w
(si ^ \ ■
v - \
j 1
r ,s! va 44 4 « \
< ^ V A V % 1.- a " x
'T--
o
i \ \
Figure 38: Bottom photographs from Tanner Basin:
a. station 26001
b. station 25995
c. station 25988
d. station 25991
e. station 25993
f. station 25989
129

(e.g. 25988) (c) . They range in size from 0.5 to 2 cm in
diameter. Many of the larger openings appear to be paired
and several are elevated above the substrate on conical
mounds.
Discrete traces can be described as either
locomotive, feeding, or resting. It is difficult using
bottom photographs to distinguish between locomotive and
feeding traces or to determine the maker unless the
organism is found in its tracks.
The substrate is relatively smooth at most of the
stations due to the "sweeping" locomotion of ophiuroids
(e.g. 26001). The stations where the most distinct traces
are observed correspond to those with the fewest number of
ophioroids (e.g. 25991)(d).
A small gastropod is found in its own furrowed trail
at station 25993 (e) . Other furrowed trails (25989) (f)
appear to be similar to those attributed to irregular
echinoids (Edwards, 1979). An enigmatic feature is found
at station 25989. It is a crater approximately 10 cm in
diameter. It can probably be ascribed to a relatively
large benthic organism which either temporarily rested
there or is burrowing for protection. Bright spots
described as possible organic floccules by Barnes (1970)
and Edwards (1979) from Santa Cruz Basin are also present
in bottom photographs from Tanner Basin.
131
CONCLUSIONS
Holocene sedimentation in the northern outer
borderland basins is dominated by low rates of hemipelagic
sedimentation. For the most part, the outer basins are
characterized by hemipelagic sediment drapes. Mass slope
failures and fine-grained turbidites are rare and areally
restricted. In western Tanner Basin there are several
notable exceptions of coarser-grained material extending
onto the basin floor from the local basin slopes.
Silty clay is the predominant grain size on basin
floors. Lag concentrations of sand dominate the bank tops
and sills due to the winnowing of fine-grained sediment by
bottom currents.
Two grain size trends are evident in the outer
borderland: 1. a relatively uniform basin floor sediment
cluster, and 2. a linear basin slope trend. The basin
slope trend is typical of continental slope sedimentation,
grain size decreases with increasing bottom depth.
Silt/clay ratios of slope sediments are uniform for
all grain sizes indicating that a uniformly graded
sediment source, possibly the California Current
transporting material from northern provenances, is
132
contributing sediment to the study area.
The Santa Rosa-Cortes Ridge acts as a major sediment
divider between the inner/central borderland and the outer
basins. The ridge effectively blocks most of the mainland
terrigeneous suspended-load and, therefore, biologic
production, aeolian, and California Current-transported
sediments contribute a relatively high proportion of
sediment to the outer borderland. Surface and bottom
currents play an important role in the distribution of
sediments in the outer borderland basins.
Biologic reworking of sediments is currently
modifying the surface sediments. A generally nondescript,
mottled texture with a low diversity biogenic trace
association typifies the outer borderland sediment column.
133
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140
Appendix A: Box core locations and bottom depths
Sample
Number
Latitude Longitude Depth in
Meters
Plla 33°25 03 N
Pllb 3 3°2 6 01 N
P12b 33°26 08 N
P14a 3 3°24 05 N
P14b 33°24 06 N
P15 33°22 08 N
P16a 33°24 03 N
P16b 33°24 03 N
P17a 33°26 03 N
11273 33°05 00 N
24383 32°3 2 00 N
24683 32°39 00 N
24949 32°47 00 N
25988 32°4 0 24 N
25989 32°43 20 N
25990 32°4 6 00 N
25991 32°46 00 N
25992 32°48 54 N
25993 32°48 54 N
25994 32°48 09 N
25995 32°51'40 N
120°30'03"W 1086
120°30'07"W 1088
12 0°3 8'02 "W 1455
12 0°45'03"W 2460
120°44'02 "W 2547
12 0°41'05"W 2018
12 0°4 4'02 "W 2296
120°42'02 "W 2322
120°46'01"W 2745
120°38'00"W 3925
119°19'00"W 90
119°17'00"W 240
119°17'00"W 485
119°27'54 "W 1261
119°28'00, , W 1336
119°25/10, , W 1298
119°31'10"W 1400
119°28'00"W 1167
119°34'00,,W 1428
119°39'06"W 1317
119°30'00,,W 1470
142
Sample Latitude
Number
25996 32°52
o
o
V
N
25997 32°54 10 N
25998 32°54 30 N
25999 32°57 06 N
26000 3 3°00 15 N
26001 3 3°02 50 N
26017 33°05 30 N
26019 3 3°04 42 N
26020 3 3°02 30 N
26021 3 3°00 00 N
26022 32°56 45 N
26023 32°52 48 N
26024 32°50 45 N
26025 32°49 30 N
30119 33°55 12 N
30125 33°51 50 N
30126 33°50 12 N
30127 33°37 42 N
30128 33°37 24 N
30130 33°30 20 N
30132 33°09 '18 N
Longitude Depth in
Meters
119°42 ' 3 6"W 1319
119°39 15 "W 1430
119°46 00"W 1373
119°43 24 'W 1503
119°45 45"W 1503
119°4 3
£
o
CM
1262
119°45 3 0 "W 1348
119°52 37 "W 1411
119°48 3 0"W 1484
119°52 15 "W 1229
119°48 10"W 1403
119°52 15"W 1128
119°49 24 'W 1184
119°47
£
o
o
1079
12 0°47 00"W 2012
12 0°3 3 36"W 1885
12 0°4 0 06"W 2012
120°09 54 'W 914
120°23 12 "W 1179
12 0°16 3 0"W 1230
119°58 30"W 1170
143
1050
1006
2160
2610
2226
1620
2100
1170
1090
990
1124
1829
1062
930
1143
1098
936
Latitude Longitude
3 3°08'54 HN 12 0°07 00HW
3 3°15 3 6"N 12 0°18 18 "W
3 3°58 00"N 120°56 48 "W
3 3°51 3 0 "N 121°04 00'W
3 3°51 12 "N 120°49 3 0 "W
3 3°44 45 "N 12 0°41 3 0"W
3 3°37 3 0 "N 12 0°50 00"W
33°37 30MN 12 0°32 3 0"W
3 3°3 0 12 "N 12 0°32 00"W
3 3°2 2 30"N 12 0°2 3 00HW
3 3°2 2 3 6MN 12 0°12 12 MW
3 3°2 9 45"N 12 0°40 54 "W
3 3°22 10MN 12 0°32 00"W
3 3°15 18 "N 12 0°21 54 "W
3 3°07 54 "N 120°28 54 "W
33°03'3 0"N 120°03 24"W
3 3°17'00"N 119°57 00"W
Appendix B: True moment measurements.
145
Sample
Number
Mean Phi Standard Skewness
Deviation
Kurtosis
Plla
Pllb
P12b
P14a
P14b
P15
P16a
P16b
P17a
23946
24928
25988
25989
25991
25992
25993
25994
25995
25996
25997
25998
25999
6.7
6.02
3.5
5. 93
6.53
4 .73
5.05
5.58
5.62
6.2
6.72
5.28
7 . 08
6.4
4 .64
6.85
6.43
5.97
6. 59
6.83
7 .11
7. 16
1. 63
1.66
1.93
1.71
1.6
2 .16
2 .02
2 . 02
1.2
1.48
1.71
2 . 0
1.42
1.78
2.08
1.59
1. 84
2 . 07
1.7
1.51
1.46
1.5
-0 . 12
-0. 14
1. 14
0.21
0. 16
0.43
0.44
0. 18
1.54
0.49
-0. 15
0.4
-0 .16
0.1
0.87
-0.18
0.02
- 0.22
- 0.01
-0 . 03
-0.18
-0.31
8 . 78
5. 04
5. 07
4.25
8.3
1.49
1.75
1.87
15.22
9 . 01
7 . 24
1.81
19.92
4.96
1.78
10.79
4 . 37
2 . 02
6.79
12 . 95
18 . 37
17.40
146
Sample
Number
Mean Phi Standard Skewness
Deviation
Kurtosis
26000
26001
26017
26019
26020
26021
26022
26023
26024
26025
30117
30119
30125
30126
30127
30128
30130
30132
30133
30134
30638
30639
6.47
7 . 48
5.07
3.31
7 . 08
6.82
6. 64
6.79
7 .09
4 .12
6.86
6.98
5.73
6.88
6.6
6.89
7.11
6.22
7.03
6.89
5. 77
6. 96
1.7
1.45
2 .13
1.8
1.58
1.74
1.72
1. 66
1.44
2 .26
1.57
1. 55
1.87
1.63
1.62
1.42
1.37
2.04
1.53
1.47
2. 22
1. 66
-0. 02
-0. 59
0. 38
1. 54
-0.51
-0.31
-0. 12
-0. 22
-0. 05
0.61
-0. 07
-0.2
0. 07
-0. 09
0.13
0. 09
0. 01
-0.31
-0.42
-0.01
0. 02
-0.25
6.27
25. 09
1. 42
7 . 63
13 .13
7 . 33
6. 74
8. 67
19. 09
1.73
11.44
13 . 17
2 .66
10. 12
8.28
17 . 54
23 . 68
2 . 54
14 .43
15.2
1.38
9.8
147
Sample
Number
Mean Phi Standard Skewness Kurtosis
Deviation
30642 7.13
30644 4.51
30645 6.21
30647 6.54
30649 6.48
30651 6.81
30999 4.41
31000 6. 61
31007 6.51
31008 5.15
31012 7.21
31013 7.53
.6 -0.42 12.93
.24 0.7 1.43
.03 -0.28 2.55
.5 0.16 17.75
.52 0.05 9.76
.54 -0.03 11.8
.13 0.74 1.86
.54 -0.03 10.15
.51 0.17 10.2
.03 0.33 1.71
.52 -0.28 16.87
.94 -1.12 8.02
1
2
2
1
1
1
2
1
1
2
1
1
148
Appendix C: Grain size component percentages
Sample Depth Mean Phi Sand Silt Clay
Number (meters) Percent Percent Percent
Plla 1086 6.70 9.02 69.33 22. 63
Pllb 1088 6. 02 13.80 73 .94 12 .26
P12b 1455 3 .50 73 . 61 19.43 3 . 97
P14a 2460 5.93 13 .23 72.14 14.41
P14b 2547 6.53 7.23 71.27 12 . 50
P15 2018 4.73 48.95 41.21 9.59
P16a 2296 5. 05 42 . 34 46. 62 10. 78
PI 6b 2322 5. 58 28.23 56.11 15. 08
P17a 2745 5. 62 2 .15 90.62 7 .23
24383 90 1.10 96. 00 3 . 00 1. 00
24683 240 4.80 41.00 49.00 10. 00
24949 485 3 . 60 78. 00 15. 00 7.00
25988 1261 5. 28 36. 64 49 . 29 13 . 93
25989 1336 7. 08 0. 80 63 .22 33.45
25990 1298 5.00 51.00 35.00 14 . 00
25991 1400 6.40 5.15 67.14 25.36
25992 1167 4 . 64 64 . 80 24 .32 10.43
25993 1428 6.85 6.52 67 . 04 26.45
25994 1317 6.43 15.67 60. 38 23 .94
25995 1470 5.97 21. 68 58 .72 19 .13
25996 1319 6.59 7 . 09 68.86 24 . 05
25997 1430 6.83 4 . 30 70.87 24.83
Sample Depth Mean Phi Sand Silt Clay
Number (meters) Percent Percent Percent
25998 1373 7.11 3 .20 67.10 29.70
25999 1503 7.16 2 .94 64. 39 32 . 66
26000 1503 6.47 8. 25 70. 29 21.46
26001 1262 7.48 3 . 60 57. 33 39 . 07
26017 1348 5. 07 47.81 41.12 11.82
26019 1411 3 . 31 80. 85 12. 36 3.86
26020 1484 7. 08 8 . 72 60. 42 30.76
26021 1229 6.82 9 . 75 61.91 28 . 34
26022 1403 6. 64 9 . 97 65. 14 24.89
26023 1128 6.79 10. 01 63 .95 26. 04
26024 1184 7 . 09 2 . 03 69.77 28 .20
26025 1079 4. 12 59 . 00 33 . 06 6.82
26279 650 3 . 20 89 . 00 11. 00 0. 00
26971 1085 7.70 0. 00 53 . 00 46. 00
26972 1020 4. 20 63 . 00 17 . 00 20. 00
26973 1134 7 . 90 0. 00 50. 00 50. 00
29664 666 2 . 30 96. 00 4. 00 0. 00
30119 2012 6.98 5. 27 66. 54 28 . 20
30125 1885 5. 73 29. 11 57.93 12 . 87
30126 2012 6. 88 4.98 66.49 28. 54
30127 914 6. 60 6. 01 70. 92 23 . 07
30128 1179 6.89 2.70 73 . 60 23 . 70
Sample Depth Mean Phi Sand Silt Clay
Number (meters) Percent Percent Percent
30130
30132
30133
30134
30638
30639
30642
30644
30645
30647
30649
30650
30651
30999
31000
31007
31008
31012
31013
1230
1170
1050
1006
2160
2610
2226
1620
2100
1170
1090
990
1124
1829
1062
930
1143
1098
936
7.11
6.22
7 . 03
6.89
5.77
6.96
7.13
4 .51
6.21
6.54
6.48
1.98
6.81
4 .41
6. 61
6. 51
5.15
7.21
7. 53
1.40
24.35
7.80
3.61
33.12
7.74
7.40
61.43
20.47
5 . 00
8 .30
92 . 00
3 . 67
60 .55
8.38
6.88
40.23
3.35
11.28
70. 65
54 .17
64 . 67
71. 33
46.83
61.44
60.14
27.31
57.11
76 . 08
74 .31
8 . 00
71.94
30.71
71.40
74.48
48.53
63.08
63. 19
27.95
21.33
27.64
25.06
19.93
30.82
32.46
10. 39
22 . 09
18.92
17. 39
0. 00
24 . 38
8.37
20.22
18 . 63
10.87
33.56
34 .42
152
Appendix D: Biologic and Organic Component Percentages
153
Sample Depth Carbonate Total Organic
Number (meters) Percent Carbon Carbon
Plla 1086 42 .61 8.61 3 . 49
Pllb 1088 43 . 76 9.61 4.36
P12b 1455 26.59 4.82 1.62
P14a 2460 37 .79 6.13 1. 60
P14b 2547 38 . 84 6.14 1. 48
P15 2018 28.21 4.32 0. 94
P16a 2296 31. 17 4.48 0.74
P16b 2322 34.79 5.99 1.81
P17a 2745 39. 34 5.88 1.15
11273 3925 13 . 81 2.84 1.18
24383 90 80. 40 0. 10
24683 240 53 . 90 2 .10
24949 485 37 . 30 0. 50
25988 1261 31. 47 7.10 3.33
25989 1336 38. 00 8.00 3.4
25990 1298 32 . 00 6. 00 2.2
25991 1400 33 . 75 7.73 3 . 68
25992 1167 35. 00 6.00 1.8
25993 1428 31. 92 9.58 5.75
25994 1317 34 . 00 9 . 00 4.9
25995 1470 39 . 00 7 . 00 2 . 3
25996 1319 33 . 00 7 . 00 3 . 0
154
Sample Depth Carbonate Total Orgai
Number (meters) Percent Carbon Carbon
25997 1430 28.00 10. 00 6.6
25998 1373 32 .95 8.37 4.42
25999 1503 28 .00 9.00 5.6
26000 1503 29 .59 9.57 6. 01
26001 1262 37.40 8.80 4 . 3
26017 1348 27.25 5.80 2 . 53
26019 1411 26 .50 4.20 1.0
26020 1484 37.41 8.95 4.46
26021 1229 37.00 8 .10 3.7
26022 1403 30.90 8 .10 4 . 39
26023 1128 39.60 8 . 00 3 . 2
26024 1184 34 .20 8.90 4 . 8
26025 1079 41.60 5.50 0.5
26279 650 22 .08 3 .33 0. 68
26971 1085 43 .00 9.20 4 . 0
26972 1020 30.66 5.82 2 . 14
26973 1134 37.22 8 .74 4.27
29664 666 21.42 3 .38 0.81
30119 2012 27.98 6.84 3 .48
30125 1885 19 .49 5.11 2.77
30126 2012 26.35 5.70 2.54
30127 914 25.88 6.75 3.65
155
Sample Depth Carbonate Total Orga:
Number (meters) Percent Carbon Carbon
30128 1179 34.70 6.79 2 . 63
30130 1230 28.79 9. 08 5. 62
30132 1170 33 . 39 7.48 3 .47
30133 1050 36.12 8. 81 4 . 48
30134 1006 30.40 7.96 4 .31
30638 2160 19. 45 3 .97 1. 64
30639 2610 21.40 4.28 1.71
30642 2226 18.70 4.89 2 . 64
30644 1620 21.75 4 . 06 1.45
30645 2100 36. 87 5. 61 1.18
30647 1170 33.30 7.82 3 .82
30649 1090 38.48 9. 00 4 . 38
30650 990 36.28 5.40 1. 05
30651 1124 29. 49 8.89 5.35
30999 1829 32 . 01 5.23 1.39
31000 1062 47.42 8. 66 2 .96
31007 930 42.85 10. 30 5.15
31008 1143 58. 51 8.09 1. 06
31012 1098 33.76 7. 11 3 . 06
31013 936 36. 62 8. 09 3 . 69
156

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Asset Metadata

Creator Bergan, Michaele Lyn (author)

Core Title Holocene sedimentological parameters of the outer California Continental Borderland

Contributor Digitized by ProQuest (provenance)

Degree Master of Science

Degree Program Geological Sciences

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

Tag Marine Geology,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c30-128176

Unique identifier UC11225656

Identifier usctheses-c30-128176 (legacy record id)

Legacy Identifier EP58802.pdf

Dmrecord 128176

Document Type Thesis

Rights Bergan, Michaele Lyn

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