U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004–5183

Late Quaternary Deposition in the Inner Basins of the 

California Continental Borderland—

Part A. Santa Monica
Scientific Investigations Report 2004–5183

U.S. Department of the Interior

U.S. Geological Survey 

Late Quaternary Deposition in the Inner Basins of the 

California Continental Borderland—

Part A. Santa Monica Basin 

By William R. Normark and Mary McGann
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2004

This publication is available only online at:

http://pubs.usgs.gov/sir/2004/5183/

Text edited by James W. Hendley II

Production and design by Stephen L. Scott

Manuscript approved for publication, September 9, 2004

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 
Contents

Abstract ------------------------------------------------------------------------------------------------------ 1

Introduction -------------------------------------------------------------------------------------------------- 1

Earthquake hazard studies --------------------------------------------------------------------------- 1

Sediment accumulation rates -----------------------------------------------------------------------3

Methods ------------------------------------------------------------------------------------------------------ 3

Deep-tow boomer system ---------------------------------------------------------------------------- 3

Selection of coring sites ---------------------------------------------------------------------------- 10

AMS radiocarbon dating ---------------------------------------------------------------------------- 10

Data -------------------------------------------------------------------------------------------------------10

ODP Site1015 ----------------------------------------------------------------------------------------- 10

Core site 361P ---------------------------------------------------------------------------------------- 17

Core site 362P ---------------------------------------------------------------------------------------- 17

Core site 363P ---------------------------------------------------------------------------------------- 17

Core site 364P ---------------------------------------------------------------------------------------- 17

Core site 365P ---------------------------------------------------------------------------------------- 18

Core site 366P ---------------------------------------------------------------------------------------- 18

Discussion -------------------------------------------------------------------------------------------------- 18

Conclusions ------------------------------------------------------------------------------------------------- 19

Acknowledgments ----------------------------------------------------------------------------------------- 19

References -------------------------------------------------------------------------------------------------- 19

Figures

1. Bathymetric map of Santa Monica Basin piston core sites and ODP Site 

 1015. Seismic-reflection profiles of this report are shown ---------------------------------------- 2

2. Structural interpretation of Santa Monica Basin (modified from Fisher and 

 others (2003) and multibeam shaded relief image of Santa Monica Bay ------------------------ 4

3. Single-channel seismic-reflection profiles relative to ODP Site 1015 ---------------------------- 5 

4. Deep-tow Huntec boomer profiles showing key reflectors for the upper 

 part of the Santa Monica Basin fill and the correlation to ODP Site 1015 ----------------------- 6

5. Sediment log for ODP Site 1015 ----------------------------------------------------------------------- 7

6. Sediment log, core photographic, geologic setting, and geophysical log 

 properties for piston core site 361P ----------------------------------------------------------------- 11

7. Sediment log, core photographic, geologic setting, and geophysical log 

 properties for piston core site 362P ----------------------------------------------------------------- 12

8. Sediment log, core photographic, geologic setting, and geophysical log 

 properties for piston core site 363P ----------------------------------------------------------------- 13

9. Sediment log, core photographic, geologic setting, and geophysical log 

 properties for piston core site 364P ----------------------------------------------------------------- 14

10. Sediment log, core photographic, geologic setting, and geophysical log 

 properties for piston core site 365P ----------------------------------------------------------------- 15

11. Sediment log, core photographic, geologic setting, and geophysical log 

 properties for piston core site 366P ----------------------------------------------------------------- 16

12. Examples of dated fault and landslide movement ------------------------------------------------- 20

Table

Radiocarbon ages from piston cores and ODP 1015 ---------------------------------------------------- 8

III
1

Abstract

Radiocarbon dating of sediment core samples from Santa 

Monica Basin document Holocene (younger than approximately 11 ka) landslides and fault offsets along the basin 

margin. The new dates include 17 from six piston cores on 

the continental slope and 11 from Ocean Drilling Program 

Site 1015 on the basin floor. The dates, which are based 

on data from pelagic and benthic foraminifera in addition 

to several dates from mollusk shells, are used to provide 

chronostratigraphic control for a previously determined basinwide seismic stratigraphy. The geologic setting at the core 

sites and a sediment log for each core are shown. In addition, 

each sediment log is accompanied by a color core photograph 

as well as P-wave velocity and gamma-ray density profiles. 

The primary purpose of the report is to make the radiocarbon 

dates available for other studies in the Santa Monica Basin. 

A comparison of sediment accumulation rates between the 

late Pleistocene and Holocene provides insight to the effects 

of sea-level change on sediment input to the basin. In addition, the results can be used to evaluate the effectiveness of 

wire-line piston coring in providing age control for earthquake 

hazard and sedimentologic studies.

Introduction

In recent years, the Santa Monica Bay has been the focus 

of intensive sampling programs in support of studies that 

dealt with sediment sources, sediment distribution patterns, 

and transport processes, particularly in regard to pollutant 

issues (see for example, Lee and Weisberg, 2003). Beginning 

in 1998, a series of sediment coring efforts beginning in the 

Santa Monica Basin focused on the deeper water areas of the 

inner basins of the California Borderland. The primary intent 

of the deeper water work was to determine sediment accumulation rates along the basin margins in order to provide 

age control for active faults and recent submarine landslides. 

Radiocarbon dating of U.S. Geological Survey (USGS) piston 

cores was combined with dating of the core recovered at 

ODP Site 1015 (Shipboard Scientific Party, 1997). The sedimentation rate data show limited effects of sea level change 

on sediment supply to the basin, compared to the effect on 

sedimentation rates on the adjacent shelf and slope of Santa 

Monica Bay reported by and Lee (2004).

Earthquake hazard studies

The focus of the southern California earthquake-hazard 

task is to identify the nature of threats from landslides, earthquakes, and tsunamis in the offshore southern California 

coastal region. To meet this objective, it is necessary to determine the style and timing of active (Holocene) deformation 

within the basins and slope areas adjacent to the highly populated urban corridor from Santa Barbara to San Diego. Highresolution seismic-refection profiles are used to determine the 

sites for sediment-core samples that can be used to date fault 

offsets, active folding, and submarine landslides. This report 

is the first in a series that will describe the geological setting 

of the sample localities, the sedimentological and geophysical 

log properties of the core samples, the radiocarbon ages of the 

sediment recovered, and the resulting implications for dating 

of tectonic activity.

The study area for this report is the Santa Monica Basin 

west of Los Angeles (fig. 1). This northwest-southeast trending 

basin, which has a maximum water depth approaching 1,000 

m, subsided during the Pliocene and Quaternary in a predominantly strike-slip tectonic regime (Crouch and Suppe, 1993; 

Sorlien and others, 2003). Except to the southeast, where 

the Santa Monica Basin is separated from San Pedro Basin 

by bedrock of the Redondo Knoll, the basin is bounded by a 

complex arrangement of strike slip, reverse, and buried thrust 

faults (see for example, Nardin and Henyey, 1978; Sorlien, 

1999, Dolan and others, 2000; Sorlien and others, 2003; Fisher 

and others, 2003). During the latest Quaternary, the Santa 

Monica Basin has been filling with sediment at accumulation 

rates as much as 3 m/ka (Shipboard Scientific Party, 1997). 

Dume and Santa Monica canyons provide modest sediment 

to low-relief submarine fans at the base of the slope, but the 

dominant source of sediment is the Hueneme Canyon at the 

western end of the basin (fig. 1; Normark and others, 1998; 

Piper and others, 1999; Piper and Normark, 2001). Hueneme 

Canyon cuts across the shelf and receives much of the coarse 

sediment delivered to the coast from the Santa Clara River, 

from which hyperpycnal flows deliver sediment directly to 

the canyon and adjacent basin slope (Warrick and Milliman, 

2003).

Normark and Piper (1998) provided a preliminary assessment of Holocene deformation along the margins of the 

Santa Monica Basin. High-resolution seismic-reflection data 

collected by the Geological Survey of Canada (GSC; see 

Late Quaternary Deposition in the Inner Basins of the 

California Continental Borderland—

Part A. Santa Monica Basin

By William R. Normark and Mary McGann
Fig. 4

Fig. 3

119°30'W 119°00'W 118°30'W 

34°00'N

33°30'N

Fig. 12B,C

Fig. 12A

364P1

361P1

362P1

363P1

365P1

*ODP 1015

366P1

Area of fig. 2A

20

40

60 80

100

100

100

200

500

600

700

500

900

800

800

100

700

500

200

Santa Barbara

Channel 

Santa Monica

Basin

Hueneme

Canyon

Santa Monica

Canyon

San Pedro Basin

Santa Cruz - Catalina Ridge

Redondo

Knoll

L

Be

Pt

Dume

Redondo

Canyon

Los

Angeles

Anacapa I.

0 20

km

Piston core site

ODP Site 1015

*

NOT FOR NAVIGATION

Figure 1. Bathymetric map of Santa Monica Basin showing position of cores used in this study (ODP Site 1015 and piston cores 361P1 through 366P1). Thshow tracklines for Geological Survey of Canada (GSC) cruise 91-062. Thin dark gray lines show tracklines for deep-tow boomer profiles collected durincruises A-1-98-SC and O-1-99-SC in support of earthquake hazard assessment for offshore southern California. Locations of the seismic-reflection profilSite 1015 presented in figures 3 and 4 are shown as heavy solid lines in black and red, respectively. Locations of deep-tow boomer lines used to correlatstratigraphic framework (Normark and others, 1998; Piper and others, 1999) with ODP 1015 and piston-core site 364P1 are shown as thin, red dashed linethe piston core sites give length of boomer profiles illustrated in figures 6 through 11. Thick red lines near northwest and southeast ends of the basin shofiles of figure 12. Rectangle with thick lines shows location of figure 2

A. Bathymetric contours in meters with 20-m-interval on shelf and 100-m-interval fo

from NOAA, 1998).
3

tracklines in fig. 1) were interpreted using the depth to the 

Pleistocene-Holocene boundary from the initial results from 

Ocean Drilling Program (ODP) Site1015 (Shipboard Scientific 

Party, 1997). Preliminary results on the timing and recurrence 

of deformation in Santa Monica Basin showed that during 

the last 5 ka the western end of the basin is actively deforming with multiple deformational events (folding, tilting, fault 

offsets). Within the last 10 ka, large submarine slides have 

occurred, primarily south and west of Santa Monica Bay. 

During the last 2 ka, the northeast margin of the basin has 

many faults that offset the seafloor and that have been active 

(Normark and Piper, 1998).

The main effort of the ongoing evaluation of seismic 

hazard in Santa Monica Basin has been to map active structures that lie within 40 to 50 km of the shore (see USGS 

tracklines in fig. 1). Fisher and others (2003) demonstrated that 

the northeastern margin of the basin shows different degrees 

of deformation north and south of Santa Monica Canyon 

(fig. 2A). Northwest of this canyon, two shallow anticlines 

on the lower slope extend toward Point Dume and are cored 

by faults showing reverse or thrust separation. Southeast of 

Santa Monica Canyon, the lower-slope sediment is deformed 

by a complex arrangement of strike-slip, normal, and reverse 

faults along the San Pedro Escarpment that locally form flower 

structures (Normark and Piper, 1998; Fisher and others, 2003). 

These faults merge downward with the San Pedro Basin Fault 

Zone (fig. 2A), which is nearly vertical and strike-slip (Fisher 

and others, 2003).

Sediment accumulation rates

The high-resolution seismic-reflection data from the 

GSC provided a basis for establishing a detailed stratigraphic 

framework for sediment of late Quaternary age in the deep 

Santa Monica Basin (Normark and others, 1998; Piper and 

others, 1999; Piper and Normark, 2001). These studies identified about 15 key reflectors, many of which can be recognized 

across the basin floor and lower basin slopes. Some of these 

reflectors are shown in figures 3 and 4 where they have been 

further mapped using other reflection profiles collected by 

the USGS during 1998 (fig. 1). The GSC data did not directly 

intersect ODP Site 1015 (fig. 1), but the USGS data collected 

in 1998 did cross the drill site. Thus, the key reflectors can 

now be correlated with confidence to Site 1015 (figs. 4 and 5; 

Shipboard Scientific Party, 1997). 

Site 1015 was not studied in detail for the ODP Scientific Results volume for Leg 167 and, to date, only a few 

unpublished radiocarbon ages have been available to evaluate 

deformation in Santa Monica Basin. The unpublished dates 

confirmed the shipboard evaluation that the Holocene deposits 

are nearly 30 meters thick, which is much greater than previous estimates (Shipboard Scientific Party, 1997). The Santa 

Monica Basin is a closed basin and makes an ideal subject 

for a source-to-sink evaluation of the effects of sea level on 

sediment supply to the deep basin floor. In addition, deeptow boomer data have been used to show that the uppermost 

key reflectors can be traced from ODP Site 1015 to about 40 

m above the basin floor. This suggests that turbidity currents 

from the Hueneme Canyon are thick enough to deposit sediment well above the basin floor (Piper and others, 2003).

To make further use of the ODP core data for both stratigraphic studies and seismic hazard evaluation, this report 

documents a comprehensive suite of radiocarbon ages for 

sediment from cores in Santa Monica Basin that can be used 

to refine initial efforts to determine the deformation history 

within the basin (see for example, Normark and Piper, 1998; 

Fisher and others, 2003). This study is based on data from 

ODP Site 1015 in addition to site-specific piston cores along 

the basin margin (table 1 and fig. 2B). Chronostratigraphic 

control for the basin-wide, high-resolution seismic stratigraphy developed for Santa Monica Basin can be used to date 

faulting and mass-wasted deposits within the basin floor and 

base-of-slope areas. Short piston cores from the northeastern 

flank of the basin were intended to provide age control for 

folding and faulting on the mid-slope area. The dated sediment 

cores are also used to evaluate sedimentation rates during 

the change from glacial sea-level lowstand to the present 

highstand.

Methods

The Santa Monica Basin receives sediment from at least 

four submarine canyons that are fed directly from rivers 

or from sand being transported in the littoral zone (fig. 1; 

Normark and others, 1998). For this reason, deposits in the 

basin are relatively sand rich (see for example, fig. 5). To 

develop a fine-scale seismic stratigraphy and to image active 

faults that offset the sea floor or strata close to the sea floor, 

a deep-tow boomer system was chosen to obtain the necessary vertical resolution of bedding and yet still be capable of 

imaging sandy deposits that are tens of meters thick. We used 

boomer reflection profiles to select specific core sites adjacent 

to fault and fold structures and then used microfossil carbonate 

tests for radiocarbon dating to date recent displacements.

Deep-tow boomer system

The high-resolution Huntec DTS boomer system is well 

suited for reflection profiling studies in basins of the California 

Borderland. The DTS boomer source is towed between 50 m 

and 160 m below the sea surface (depending upon the water 

depth) and can image the upper few tens of milliseconds of 

strata with a resolution of better than 0.5 ms (0.4 m). Power 

output is 350 Joules, with a firing rate that is dependent on 

water depth, ranging from 0.75 sec to 1.25 sec with the longer 

interval over the deeper water parts of the basin. For the profiles obtained in support of the Santa Monica Basin study, 

the receiver was a 5-m-long Benthos 10-element hydrophone 

array towed from the same fish that houses the boomer source. 

 Methods
Figure 2. A, Summary of primary structural features (shown in black) along the northeastern margin of Santa Monica Basin as established by Fisher Features shown in red are schematic representation of other faults in the area (see references for figure 1 in Fisher and others, 2003). The extension Fault (PVF) remains conjectural north of Redondo Canyon and is shown as a dashed red line. SPA is shelf projection anticlinorium. See figure 1 for lorelief map of the shelf and basin slope east of Santa Monica Basin based on multibeam bathymetric data; view is from directly overhead with illuminnorthwest (Gardner and Dartnell, 2002). The fault and fold patterns are the same as for figure 2A. Piston core locations are shown together with tracreflection-profile segments of figures 6 thorugh 11 (see figure 1 for explanation). San Pedro Basin Fault Zone 118 ̊ 50' W 

364P1 361P1362P1 363P1 365P1 ODP 1015 366P1 SantaMonica Bay P

 

 

Redondo Canyon 100 m 10 k

Pt Dume San Pedro Escarpment 

? Dume Canyon SPA Santa Monica Cyn ? 118 ̊ 50' W 118 ̊ 20' W 33 ̊ 40' N 34 ̊ 00' N Fig. 2B SantaMonicaBay Redondo Canyon 100 m Lower-slope anticlines 10 km

Pt Dume

San Pedro Escarpment

San Pedro Escarpment

San Pedro Basin Fault Zone

800 m

500 m

Redondo

Knoll

Thrust or reverse fault

Strike-slip fault

Anticline

?

Dume

Canyon

SantaMonicaCanyonSPA

Palos

 Verdes

 Peninsula

900 m

900 m

100 m

800 m

B

Monica Fault

Dume Fault

Palos Verdes Fault

Santa

SantaMonica

A
1.0

2.0

Two-way travel time (sec)

0 5

km

O-

VAMP

J

D

C

ODP Line BA-4

PZ1 Line 06 ODP 1015

S 

Figure 3. Seismic-reflection profile showing basin-floor stratigraphy of Santa Monica Basin sediment at ODP Site 1015. Line 13 from U.S. Geological Survebasin-wide stratigraphic reflectors C, D, and J are shown in this profile, which was obtained with a single-channel sparker sound source; refer to figure 4 the seafloor and see text for explanation. Profile location is shown in figure 1. This profile shows evidence for gas accumulation in the sediment below abotime, which is just a few tens of meters deeper than the bottom of ODP Hole 1015A at 149.5 mbsf; see, for example, high-amplitude (bright) reflections loca(vamp) -like structure south of the ODP drill site (see Scholl and Cooper, 1978).
MBSF

1.12

1.15

1.20

1.25

Two-way travel time (sec) O

J

L

N

M

Intersection with

PZ1 line 002 Crossing ODP 1015

SW N

A-1-98-SC Line 03

Figure 4. High-resolution deep-tow boomer profile that passes through ODP Site 1015 and intersects Line 2 from Geological Survey of Canada (GSC) cruise location. Reflectors O to J are regional reflectors that can be correlated throughout much of the Santa Monica Basin using the GSC survey of the western others, 1999) and the U.S. Geological Survey data farther east (see fig. 1).
7

3.0

1.0

4.0

sed rates 2.1

in mm/yr

0 10 20 30 100

80

60

40

20

0

Age (yrBP)

(reservoir corrected C-14 years)

Core depth (m)

Planktonic foraminifera

 spp.

mixed benthic foraminifera

Uvigerina

mudsiltfine sandmed sand

0

10

20

mudsiltfine sandmed sand

0

10

20

30

40

50

60

70

80

90

100mcd

ODP 1015

1.68

2.10

2.97

3.78

5.19

6.64

8.33

Age

(yrBP x 10 )

Age

(yrBP x 10 )

10.10

12.53

14.20

17.93

19.55

21.75

27.95

31.75

3

3

All core depths in meters

Hemipelagic mud

Turbidite mud

Sand

O

N

M

L J

F

D

K

K

Figure 5. Graphical sediment log for ODP Site 1015 constructed from the Visual Core 

Descriptions (VCDs) obtained from the Ocean Drilling Program archive at Texas 

A&M University. The log is constructed combining the VCDs from Holes 1015A and 

1015B in order to avoid disturbed sections wherever possible; thus, the vertical 

scale (mcd = meters core depth) reflects the combined logs. Radiocarbon dates are 

shown in red to the left of the sediment logs. The upper 20 m of the log is expanded 

(at the right) to better illustrate the relation between the radiocarbon dates and the 

late Holocene interval of key reflector stratigraphy. The graph at lower right shows 

the sediment accumulation rate in mm/yr. Location of Site 1015 in figures 1 and 2B.

 Methods
OS-40035 A2-98-SC 361P1, sec. 1, 40-44 cm 0.40-0.44 (benthic) -1.01 14,800±75 1,750 1

OS-36191 A2-98-SC 361P1, sec. 2, 36-37 cm 0.80-0.81 Mollusk 0.76 15,150±70 1,750 1

OS-40036 A2-98-SC 361P1, sec. 2, 75-77 cm 1.19-1.21 sp. (benthic) -0.41 17,950±95 1,750 1

OS-40037 A2-98-SC 361P1, sec. 2, 140-142 cm 1.84-1.86 sp. (benthic) -0.39 19,150±110 1,750 1

OS-40038 A2-98-SC 362P1, sec. 2, 80-82 cm 1.77-1.79 spp. (benthic) -1.41 13,050±55 1,750 1

OS-39100 A2-98-SC 362P1, sec. 2, 133-135 cm 2.30-2.32 Mixed benthics -0.98 13,000±65 1,750 1OS-40039 A2-98-SC 363P1, sec. 2, 60-62 cm 1.12-1.14 spp. (benthic) -1.23 13,650±65 1,750 1OS-40066 A2-98-SC 363P1, sec. 3, 104-106 cm 3.05-3.07 Mixed benthics -1.75 25,400±210 1,750 2OS-40067 A2-98-SC 363P1, sec. 4, 30-32 cm 3.65-3.67 (benthic) -1.09 28,200±280 1,750 2

OS-35616 A2-99-SC 363P1, sec 4, 123-125 cm 4.58-4.60 Mixed planktics -1.82 29,200±150 1,100 2

OS-40068 A2-98-SC 364P1, sec 1, 105-107 cm 1.05-1.07 sp. (benthic) -0.37 4,640±35 1,750 

OS-40069 A2-98-SC 364P1, sec. 3 , 28-30 cm 3.04-3.06 Mixed benthics -1.8 9,840±55 1,750 OS-35615 A2-98-SC-364P1, sec 3, 143-145 cm 4.19-4.21 0.12 10,600±75 800 OS-40070 A2-98-SC 365P1, sec. 2, 80-82 cm 1.39-1.41 spp. (benthic) -3.09 > 52,000 OS-40071 A2-98-SC 365P1, sec. 2, 100-102 cm 1.59-1.61 (benthic) -0.8 Too old to date OS-39101 A2-98-SC 365P1, sec. 2, 140-142 cm 1.99-2.01 Mixed benthics -2.38 > 54,000 OS-40072 A2-98-SC 366P1, sec. 1, 20-22 cm 0.20-0.22 Mixed benthics -1.01 7,990±40 1,750 

Table 1. Radiocarbon ages from piston cores and ODP 1015 from Santa Monica Basin.
OS-35811 ODP 1015B, 002H 02W, 55-57 cm 4.35 Mixed benthics 3,430±45 1,750 

OS-35835 ODP 1015B, 002H 03W, 144-146 cm 6.74 sp. 3,850±40 1,750 

OS-35836 ODP 1015B, 002H 04W, 101-103 cm 7.81 sp. 4,720±30 1,750 

OS-35837 ODP 1015B, 002H 06W, 12-14 cm 9.92 sp. 5,530±35 1,750 

OS-39187 ODP 1015B, 002H, 07W, 55-57cm ~11.68 Mixed benthics 0.65 6,940±70 1,750 

OS-39188 ODP 1015B, 003H, 02W,119-121cm 14.50 Mixed benthics 1.2 8,390±45 1,750 

OS-35838 ODP 1015B, 003H 04W, 55-57 cm 16.85 9,130±50 800 

22.483 Planktic foraminifera 1

32.243 Planktic foraminifera 1

OS-35839 ODP 1015B, 006H 03W, 112-114 cm 44.36 sp. 15,950±60 1,750 1

56.493 Planktic foraminifera 11

OS-35840 ODP 1015B, 008H 03W, 105-107 cm 63.35 sp. 21,300±95 1,750 1

73.653 Planktic foraminifera 2

OS-39189 ODP 1015B, 011H, 01W, 14-16cm 87.95 Mixed benthics -0.92 29,700±150 1,750 2

OS-39190 ODP 1015B, 012H, CCW, 28-30cm 97.59 Mixed benthics -1.15 33,500±220 1,750 3

Table 1. Radiocarbon ages from piston cores and ODP 1015 from Santa Monica Basin.—Continued

1 Ages were calibrated using a reservoir age of 1,750 years for the benthic assemblages as well as for mollusks and 800 and 1,100 years for the planktic foraminiferal samp

of less than, and older than, 12,000 radiocarbon years, respectively (following Mix and others, 1999; Southon and others, 1990; Kienast and McKay, 2001).

2 Calendar ages were calculated using Radiocarbon Calibration Program Revision 4.3 (CALIB 4.3) from Quaternary Isotope Lab, University of Washington based on Stuiver a

Reimer (1998).

3 Unpublished ODP radiocarbon ages (from Behl, oral communication, 1999).
10 Late Quaternary Deposition in the Inner Basins of the California Continental Borderland—Part A. Santa Monica Basin

Signals were filtered at 800 to 6,000 Hz and recorded at a 

0.25-sec sweep. The data were recorded on magneto-optical 

disc for later processing as well as on a stable medium using 

an EPC recorder. Survey speeds of about 7 km/hr provide a 

variable shot spacing between 1.5 and 2.5 m for the deep-tow 

boomer profiles.

Selection of coring sites

The coring program conducted in 1998 was part of a 

major effort to study sediment accumulation and the effects 

of anthropogenic sediment sources (for example, pollutants) 

in Santa Monica Bay (Alexander and Venherm, 2003). USGS 

cruise A-2-98-SC over the basin slope was devoted to box 

coring in upper slope and shelf areas of Santa Monica Bay 

(see station information at http://walrus.wr.usgs.gov/infobank/

a/a298sc/html/a-2-98-sc.meta.html). As a result, there was 

only limited ship time for piston-core operations. Six sites on 

the mid-slope area of the basin margin were adjacent to fold 

and fault structures that appeared to have recent movement, on 

the basis of boomer seismic-reflection profiles (fig. 2B). This 

study is based on radiocarbon dating of sediment from ODP 

Site 1015 and the six piston cores obtained in 1998.

AMS radiocarbon dating

Sediment ages for the six Santa Monica Bay cores were 

determined by accelerator mass spectrometry (AMS) 14C, 

using foraminifera and a mollusk (table 1). Because the highest degree of precision possible by AMS dating was not necessary to answer the questions posed in this study, we chose 

not to use mono-specific planktic foraminiferal samples for 

age-dating. Instead, the most abundant foraminiferal species 

in each sample were utilized. As a result, three radiocarbon 

dates were determined using the benthic species Uvigerina 

peregrina or Gyroidina altiformis, nine samples with the 

genera Uvigerina or Cibicides, and five samples using a 

mixed benthic foraminiferal assemblage. Two samples used 

planktic foraminifera, including one with Neogloboquadrina 

pachyderma and another with a mixed planktic foraminiferal 

assemblage. In addition, one gastropod was dated in core 

361P1. A reservoir age of 1,750 years was chosen for the benthic foraminiferal samples following Mix and others (1999), 

and similarly applied to the gastropod because it occupied 

the same substrate as the benthic foraminifera. An 800-year 

reservoir age was used for the planktic foraminiferal samples 

because the radiocarbon ages of these samples were younger 

than 12,000 years (Southon and others, 1990, and Kienast and 

McKay, 2001). The CALIB 4.3 program (Stuiver and others, 

1998) was used to calculate calendar ages from these radiocarbon ages.

Foraminifera were also used to date eleven samples from 

ODP Hole 1015B. Ten of these used benthic foraminifera, 

including five samples with Uvigerina, and five of a mixed 

benthic assemblage. One other sample is based on the planktic 

foraminifera N. pachyderma. As with the Santa Monica Bay 

cores, reservoir ages applied to these samples were 1,750 years 

for the benthic assemblages and 800 years for the planktics 

(following Mix and others, 1999; Southon and others, 1990; 

Kienast and McKay, 2001). Calendar years were again calculated by the CALIB 4.3 program (Stuiver and others, 1998).

We also report four previously unpublished radiocarbon 

ages on planktic foraminifera obtained by Per Bodén (Department of Geology and Geochemistry, Stockholm University, 

Sweden) as part of the original ODP investigations. Reservoir ages of 800 and 1,100 years were applied to the planktic 

foraminiferal samples of less than, and older than, 12,000 

radiocarbon years, respectively (Southon and others, 1990; 

Kienast and McKay, 2001). These radiocarbon dates were 

converted to calendar years using the CALIB 4.3 program 

(Stuiver and others, 1998).

Data

The results of the radiocarbon dating are presented in 

table 1. In addition, the stratigraphic relations of the dates are 

shown for each core in figures 6 through 11. The geologic setting for each core is discussed and, for the USGS piston cores, 

the physical properties logs and whole core photograph are 

also provided.

ODP Site 1015

The primary objective for radiocarbon dating the sediment sequence at Site 1015 was to provide age control for 

the high resolution sequence stratigraphy developed for 

Santa Monica Basin (Piper and others, 1999). ODP Site 1015 

is located on the flat-lying basin plain at the southeastern end 

of Santa Monica Basin in water just deeper than 910 m (figs. 

1 and 2). The basin plain is underlain by flat-lying turbidite 

deposits that onlap sediment on the flank of Redondo Knoll 

to the south (fig. 3). To the north, the stratigraphic section 

penetrated at Site 1015 pinches out and onlaps the northern 

margin of the basin. In the section below reflector C, which 

is the deepest key reflector reached at the drill site, evidence 

for gas accumulation is indicated by the VAMP-like structure 

about 2 km south of Site 1015 (fig. 3). Other seismic-reflection profiles in the vicinity of ODP 1015 show additional 

evidence for gas accumulation at shallow depth in the basin 

fill (see site survey data in Lyle, 1995). Further, 20 km northeast of the drill site, methane hydrate was recovered from the 

crest of a mud volcano (Normark and others, 2003). Monitoring of gas composition during drilling showed no evidence for thermogenic sources (Shipboard Scientific Party, 

1997).

High-resolution deep-tow boomer profiles across the 

ODP Site show that the upper 60 m of basin-plain deposits 

have very low relief locally and gradually thin northward 
0.76

0.780.800.820.84

Two-way travel time (sec)

A-1-98-SC Line 053

1.5 1.6 

Vp (km/se

0

50

150

100

200

13,050+/- 75

13,400+/- 70

16,200+/- 95

Centimeters below sea floor

A-1-98-SC Core 361P1

Missing

photo

17,400+/- 110

SW NE

500 m

10 mCore 361P1

VOID

0

50

150

100

200

Core depths in centimeters

Hemipelagic mudTurbidite mud

Sand

Boomer profile

Core Photography

(dates in yr B.P.) Phy

Sediment

log

Figure 6. Graphical sediment log for core 361P1 taken during cruise A-2-98-SC. To the left of the sediment log is the deep-tow boomer profile used for siting thphotograph with radiocarbon dates. Bold black line in boomer profile shows buried high-angle fault. The physical-properties logs, P-wave velocity (Vp) and gashown to the right. The target depth is shown by the orange-rimmed yellow rectangle.
500 m

10 m

0.70

0.72

0.740.760.780.80

Two-way travel time (sec)

1.5 1.6 

Vp (km/sec

0

50

150

250

100

200

0

50

150

250

100

200

11,300+/- 55

Centimeters below sea floor

A-1-98-SC Core 362P1

Soupy sand bed

11,250+/- 65

SW NE

Core

362P1

A-1-98-SC Line 51A

Core depths in centimeters

Hemipelagic mudTurbidite mudSand

Boomer profile

Core Photography

(dates in yr B.P.) Phys

Sediment

log

Figure 7. Graphical sediment log for core 362P1 taken during cruise A-2-98-SC. To the left of the sediment log is the deep-tow boomer profile used for sitinphotograph with radiocarbon dates. The physical-properties logs, P-wave velocity (Vp) and gamma-ray density, are shown to the right. Bold black line in bfault. The southwest-dipping dashed blue lines denote possible back-rotated slump blocks although their internal bedding is consistent with sediment-waoverflowing from Santa Monica Canyon (see fig. 2B). The target horizon is shown by the orange-rimmed yellow rectangle.
1.00

1.04

Two-way travel time (sec)

10 m

500 m

0.96

0.92

28.40 ka

23.65 ka

11.90 ka

1.5 1.6 1.7 

Vp (km/sec)

0

50

150

250

350

100

200

300

400

450

490

0

50

150

250

350

100

200

300

400

450

490

11,900+/- 65

23,650+/- 210

26,450+/- 280

Centimeters below sea floor

A-1-98-SC Core 363P1

28,400+/- 150

SW NE

A-1-98-SC Line 051A

Core

363P1

Core depths in centimeters

Hemipelagic mudTurbidite mudSand

Boomer profile

Core Photography

(dates in yr B.P.) Physica

Sediment

log

Figure 8. Graphical sediment log for core 363P1 taken during cruise A-2-98-SC. To the left of the sediment log is the deep-tow boomer profile used for siting thphotograph with radiocarbon dates. The physical-properties logs, P-wave velocity (Vp) and gamma-ray density, are shown to the right. The target depth is shoyellow rectangle.
1.16 1.20 1.24 Two-way travel time (sec)

10 m

500 m

SW NE

Core

364P1

A-1-98-SC Line 041

O

L

M N

1.3 1.4 1.5 

Vp (km/sec)

0

50

150

250

350

100

200

300

400

430

0

50

150

250

350

100

200

300

400

430

A-1-98-SC Core 364P1

2,890+/- 35

8,090+/- 35

9,800+/- 35

Centimeters below sea floor

Core depths in centimeters

Hemipelagic mudTurbidite mudSand

Boomer profile

Core Photography

(dates in yr B.P.) Phys

Sediment

log

Figure 9. Graphical sediment log for core 364P1 taken during cruise A-2-98-SC. To the left of the sediment log is the deep-tow boomer profile used for siting photograph with radiocarbon dates. The physical-properties logs, P-wave velocity (Vp) and gamma-ray density, are shown to the right. Bold black lines in boburied faults; sea floor offsets have been smoothed by draping of sediment starting just before key reflector L was deposited. The target depth is shown by trectangle.
0.70

0.780.800.82

Two-way travel time (sec)

0.760.740.72

500 m

10 m

>54 ka

>52 ka

1.3 1.4 1.5 1

Vp (km/sec

0

50

150

210

100

200

0

50

150

210

100

200

No date

possible (rock)

Too old to date

>52,000+/- 

Centimeters below sea floor

A-1-98-SC Core 365P1

>54,000+/-

SW NE

Core

365P1

No date

possible (rock)

A-1-98-SC Line 52

Core depths in centimeters

Hemipelagic mudTurbidite mud

SandSedimentary rock

Boomer profile

Core Photography

(dates in yr B.P.) Phys

Sediment

log

Figure 10. Graphical sediment log for core 365P1 taken during cruise A-2-98-SC. To the left of the sediment log is the deep-tow boomer profile used for siting photograph with radiocarbon dates. The physical-properties logs, P-wave velocity (Vp) and gamma-ray density, are shown to the right. The older age of the sthat the sediment is talus from the faulted anticlinal fold immediately southwest. Bold black line in boomer profile shows buried fault. The top of the north-diptarget horizon, is shown by the orange-rimmed yellow rectangle.
0.70 0.72 0.74 0.80 0.68

Two-way travel time (sec)

0.760.78

500 m

10 m

0

50

25

Centimeters below sea floor

A-1-98-SC Core 366P1

SW NE

Core

366P1

A-1-98-SC Line 50

Core d

H

Boomer profile

Core Photog

(dates in yr 

Figure 11. Graphical sediment log for core 366P1 taken during cruise A-2-98-SC. To the left of the sediment log is the deep-tow boomer profile used for siting tand a whole-core photograph with radiocarbon dates. The target horizon is shown by the orange-rimmed yellow rectangle.
17

toward the basin (fig. 4). The bulk of the sediment in the 

upper part of Santa Monica Basin was derived from the 

Hueneme Canyon area at the west end of the basin (Normark 

and others, 1998; Piper and others, 1999; Piper and Normark, 

2001). The acoustic character of the reflections in this area 

is typical of that for ponded basin turbidite facies (Piper and 

others, 1999).

Two holes were drilled at the ODP site in Santa Monica 

Basin; Hole 1015A reached 149.5 meters below the sea 

floor (mbsf) and Hole B bottomed at 97.8 mbsf (Shipboard 

Scientific Party, 1997). The sediment log (fig. 5) is based on 

the Visual Core Description (VCD) forms used by the Leg 

167 sedimentologists. Although core recovery exceeded 90 

percent at Hole 1015A, some intervals were disturbed sufficiently that the sediment log constructed for this site is a 

composite of both holes. The log intervals from 32 to 40 

mbsf, 56 to 63 mbsf, 74 to 79 mbsf, and 91 to 97 mbsf are 

based on Hole 1015A; otherwise, the VCDs for Hole 1015B 

are used. The sediment log depicts only the upper 100 m 

at this site because we were unable to find datable material deeper in 1015A. All samples obtained for radiocarbon 

dating under this study were taken from Hole 1015B.

Beds in the upper 100 m of sediment cored in the basin 

plain are dominantly silt or sand (~70 percent). Two muddier 

intervals with generally thinner sand beds occur between 7 

and 16 mbsf and 52 and 84 mbsf (fig. 5). The lower 50 m 

cored in Hole 1015A is mostly sand, which contributed to the 

problem of finding datable carbonate. The average sediment 

accumulation rate at the ODP site is about 3 mm/y, and this 

rate holds even after sea-level rise during the Holocene.

Core site 361P

Core station 361 is located at the northwestern end of a 

fold ridge on the mid-slope, adjacent to a narrow pass that 

leads to the deeper basin floor (figs. 2B and 6). The goal was 

to date the upper part of a sequence of well-bedded sediment 

that appears to be faulted and uplifted to form a bench on the 

slope (fig. 2B). The core is less than 2 m in length, however, 

and thus it failed to reach the primary objective, which was 

a dipping horizon at about 6 mbsf. During the short interval 

of the latest Pleistocene that was sampled, deposition of sand 

beds was common but only muddy sediment represents the 

Holocene. 

Four intervals from this core were dated (fig. 6 and table 

1). The shallowest dated interval is from a whole-round 

sample that was taken for geotechnical studies. The sediment 

remaining after the geotechnical evaluation was sieved for 

foraminifera for dating. Assuming that the core recovered 

the mudline (that is, the water/sediment interface), the Holocene accumulation rate is less than 0.03 mm/y, which is two 

orders of magnitude less than on the basin floor. The latest 

Pleistocene rate for the underlying sand-rich interval is 0.33 

mm/yr.

Core site 362P

Core station 362P is north of a fold ridge on the mid-slope 

where a layer of acoustically transparent sediment wedges out 

and overlies a northward-dipping section of sediment. The 

acoustic character of this wedge suggests turbidite deposits 

(figs. 2B and 7; see Piper and others, 1999). The turbidite section appears truncated and is unconformably overlain by the 

sediment drape. The objective for coring at this site was to 

sample the truncated and tilted turbidite sequence. The core 

sampled a graded sand bed at 2 m depth, confirming that the 

tilted turbidite section was reached by the core.

Both dated intervals in core 362P1 are basically from the 

Pleistocene-Holocene boundary and are effectively the same 

age, within the radiocarbon-age error limits (fig. 7 and table 

1). The two dates bracket a graded sand bed tending to confirm 

that turbidite deposition can be considered instantaneous with 

respect to geologic time. The Holocene accumulation rate for 

the mud above the sand bed is 0.16 mm/yr.

Core site 363P

Core station 363P is sited on the same deep-tow boomer 

profile as for 362P but is about 150-m deeper water depth than 

core site 362P (figs. 2B and 8). The site was selected to sample 

the top of a gently folded section of well-bedded sediment. 

The folded section is overlain by a horizontal sequence that 

is ponded on the northeast side of the fold but thins abruptly 

on the basinward (southwest) limb of the underlying fold. The 

core is nearly 5 m in length but might not have sampled the 

shallowest reflector that is associated with the fold structure. 

The well-bedded unit ponded behind the fold consists of 

thin sand and silt beds and turbidite mud. The coarser beds 

tend to thin and fine upwards, and the shallowest silt bed is 

just below the Pleistocene-Holocene boundary (fig. 8 and table 

1). Only mud is deposited above this silty bed, and the Holocene accumulation rate is about 0.095 mm/yr. The deposits 

with sand and silt beds was deposited during marine oxygen 

isotope Stage 2 (Martinson and others, 1987) at a rate of 0.21 

mm/yr. The age of the beds at the bottom of the core suggests 

that the fold was essentially in its present form by about 30 ka.

Core site 364P

Core station 364P is from the lower slope of the north 

side of Santa Monica Basin, about 40 m above the basin plain 

(figs. 2B and 9). This site was selected to sample the bedded 

sequence in which key reflectors from L to O can be traced 

continuously through deep-tow boomer profiles from ODP 

1015 (see tracklines in fig. 1). On the basin floor, the interval 

above reflector L consists of numerous thin- to thick-bedded 

sand units; hemipelagic mud beds are more common in the 

 Data
18 Late Quaternary Deposition in the Inner Basins of the California Continental Borderland—Part A. Santa Monica Basin

interval from 6 to 16 mbsf (fig. 5). Core 34P1 recovered more 

than four meters of sediment and confirmed that turbidity currents are able to deposit sediment well above the floor of the 

basin. Similar to the correlative sequence in ODP 1015, the 

middle part of the core consists of mostly hemipelagic mud; 

graded silty mud and fine sand beds are common in the section 

below 3 mbsf in core 364P1.

Three radiocarbon dates from this core show the entire 

core is Holocene in age (fig. 9 and table 1). The sediment 

accumulation rate is 0.43 mm/yr, which is the highest for any 

of our cores from the continental slope.

Core site 365P

Core station 365P is north of a folded and fault-bounded 

ridge on the mid-slope. Similar to the setting for core site 

362P, there is a layer of sediment that wedges out to the 

southwest (toward the ridge) and unconformably overlies 

a northeast-dipping section of relatively well-layered sediment (figs. 2B and 10). The northeast-dipping section under 

the unconformity appears truncated at or near the base of 

the uplifted ridge. The objective for coring at this site was to 

sample the truncated and tilted sequence.

The core sampled just over two meters of sediment with 

medium to thick-bedded sand in the upper 70 cm. Indurated 

sediment (rock) was penetrated at two levels, and our attempts 

to date the sequence only determined that the sediment is older 

than 50 ka (fig. 10 and table 1). The older dates and the presence of sedimentary rock fragments within the core suggest that 

much of the cored material may be debris from the fault scarp 

bounding the north side of the uplifted ridge. The core apparently did not penetrate deeply enough to sample the northeastdipping sedimentary section, the objective for this site.

Core site 366P

Core station 366P is on a mid-slope bench near the Santa 

Monica Canyon (figs. 2B and 11). The objective at this site 

was to penetrate the toe of an acoustically transparent sediment drape that pinches out southwestward across the margin 

of the mid-slope bench. The key-reflector sequence defined 

for the Santa Monica Basin floor indicates that most of the 

sediment filling the basin during the late Quaternary has come 

from the west end of the basin (Normark and others, 1998). 

Santa Monica Canyon does not appear to be a major pathway 

for sediment transport (Piper and others, 1999). The acoustically transparent drape is thought to be Holocene hemipelagic 

mud overlying well-layered sediment that might be overbank 

deposition from Santa Monica Canyon.

The core obtained was too short to address the objectives at this site, and no physical property logs were attempted 

because of its short length. The mud recovered is late Holocene (fig. 11 and table 1), but the Holocene accumulation rate 

is less than 0.04 mm/yr, assuming that the core recovered the 

water/sediment interface. Because the transparent layer is just 

over 2 m thick at the target site, the base of the layer would 

be nearly 60 ka, which is unlikely. It seems probable that the 

piston core did not sample the sediment-water interface at the 

sea floor and the radiocarbon date represents some interval 

deeper in the transparent layer.

Discussion

The high-resolution seismic-stratigraphy defined for the 

Santa Monica Basin floor and the base-of-slope areas can be 

combined with the radiocarbon chronology for ODP Site 1015 

to date movement on faults and landslides (figs. 5 and 12). In 

addition, these data can be used to help construct a sediment 

budget for the late Quaternary using the results of previous 

work in the Santa Monica Bay area (see for example, Alexander and Venherm, 2003; Sommerfield and Lee, 2003, 2004). 

The deep-tow boomer profiles in figure 12 are taken from 

data collected by the Geological Survey of Canada. These 

data highlight examples that are chosen to show the utility of a 

basin-wide chronostratigraphy for Santa Monica Basin, should 

new high-resolution reflection profiles be collected to extend 

the seismic-hazard analysis farther seaward. An example of 

a buried landslide on the northern flank of Redondo Knoll is 

from an area between boomer profiles collected by the USGS 

(figs. 1 and 12A). The small landslide came to rest on a surface that is about 3 m above reflector J (measured along the 

left-hand side of fig. 12A). The slide mass is onlapped by a 

reflector that is about three meters below reflector K, which in 

turn onlaps the highest part of the slide mass. Thus, the failure 

event probably occurred about 12 ka (fig. 5).

The second example is from the western margin of the 

basin that falls outside the work area of the USGS earthquake 

hazard task (figs. 1 and 12B and C). A major fault marks 

the southwestern margin of Santa Monica Basin at the base 

of the Santa Cruz-Catalina Ridge. The structure in this part 

of the basin remains poorly defined, however, and the fault 

shown in figure 12B lies within the basin sediment fill, east 

of the main fault trace along the Santa Cruz-Catalina Ridge. 

Locally, narrow depressions have formed over this fault suggesting transtension in this area (figs. 1 and 12B). The youngest deformed reflection surface lies just below key reflector 

O; sedimentation above reflector O has resulted in a gradual 

filling of the depression above the fault. As seen from figure 

5, the time of the most recent offset is approximately 2 ka. In 

contrast, the deeper penetration sleeve-gun profile in figure 

12C defines a steeply dipping fault that cuts the surface of 

a shallow anticlinal structure, thus suggesting transpression 

along the fault. The northeastern flank of the anticline is 

onlapped by key reflectors B to J, which show little evidence for penecontemporaneous folding. The growth of the 

anticlinal structure thus predates the oldest sediment dated 

in this study and might be as old as 90 ka (see key reflector 

ages in Normark and others, 1998). When the change from 

transpression to transtension occurred on this structure is not 
19

determined with available data except to be sometime post 

reflector J (fig. 12C).

Although the usefulness of the chronostratigraphic control provided by radiocarbon dating of samples from ODP 

Site 1015 is easy to demonstrate, the effectiveness of short, 

conventional piston cores is more problematic. In general, the 

limited depth of sampling as a result of coring malfunction or 

the core hitting hard (including unconsolidated sand) substrate 

limits the overall success rate. For example, stations 361P, 

363P, and 366P did not provide constraints on the timing of 

deformation (figs. 6, 8, and 11). Core 362P1 was successful in 

confirming that the sediment drape was Holocene in age but 

did not sample to a sufficiently great depth to provide an age 

for the tilted and truncated section, which may be pre-Holocene in age (fig. 7). Core 365P1 provided no age control for 

the folding or faulting, but the age of the sediment recovered 

(>50 ka) suggests that older sediment is exposed and being 

shed from the adjacent uplifted ridge (fig. 10).

Core 364P1 was the most successful of the wire-line 

piston cores attempted for this study. This core confirmed that 

the stratigraphy from the well-dated ODP Site 1015 could be 

locally extended onto the lower slope (fig. 9). Thus, it should 

be possible to date offsets on structures adjacent to the basin 

floor. It has also been thought that rates of uplift of basin sediment might be used to determine strain on Borderland faults 

(Normark and Piper, 1998; Fisher and others, 2003). Although 

this approach has some applicability, caution must be used 

because the significance of Core 364P1 is that it shows that 

silt and sand beds from turbidity currents are being deposited 

on the lower slopes of the basin a minimum of 40 m above the 

basin floor. Intervals of thick sands on the basin floor have correlative fine-grained turbidites on the basin margins, whereas 

intervals with only thin sands on the basin floor lack identifiable turbidites (compare the sediment logs in figs. 5 and 9). 

The thicker turbidity currents such as those that deposited sand 

40 m above the basin floor at 364P1 probably deposit silt and 

mud as much as 50 to 100 m above the basin floor. It is important to remember that the same key reflector beds observed to 

extend to core site 364P1 are also found on the overbank area 

of the Hueneme fan valley at comparable elevations above 

the channel floor. Apparently elevated or dipping sections of 

turbidite beds that are found above the basin floor are, therefore, not necessarily the result of tectonism.

Sommerfield and Lee (2004) showed that sediment-accumulation rates on the shelf and upper slope of Santa Monica 

Bay began to decrease between 12 and 9 ka after having been 

maximal ca. 15 to 10 ka, which they observed was after the 

interval of greatest sea-level lowering (ca. 18 ka). They argued 

that sea-level rise above the shelf break led to transgressive 

sedimentation on the shelf reducing sediment supply to the 

slope-basin area. The record documented by our new radiocarbon dates at ODP Site 1015, however, shows that there is limited correspondence between accumulation rates on the upper 

slope and shelf in Santa Monica Bay and that on the deep 

basin floor. For example, the maximum accumulation rate of 

about 4mm/yr on the basin floor lasted from 22 to 8 ka, thus 

encompassing both the last glacial maximum lowering and 

most of sea-level rise during the Holocene (fig. 5). Although 

the accumulation rate on the basin plain decreased to about 1 

mm/yr for about 4 millennia, by 4 ka, the accumulation rate 

was again high (~3 mm/yr). This indicates that sedimentation 

in the Santa Monica Bay area is not a reliable indicator of 

rates in the adjacent deep basin, in this case because the deltafed Hueneme Canyon (and not the much closer Santa Monica 

Canyon) is the dominant source for the deep basin. 

Conclusions

This study presents 28 new radiocarbon dates for sediment from the Santa Monica Basin. The new radiocarbon dates 

include 17 from six piston cores on the northeastern margin of 

the basin and 11 from ODP Site 1015 on the basin floor. These 

dates provide an improved understanding of the changes in 

sediment supply to the basin as a result of sea-level rise from 

the late Pleistocene to current highstand condition. The study 

confirms a high sediment accumulation rate for the Santa 

Monica Basin floor, averaging more than 3.1 mm/yr for the last 

32 ka. During the marine oxygen-isotope Stage 2 lowstand, 

the sediment accumulation rate reached 4 mm/yr. Although 

the rates dropped to about 1 mm/yr during the mid-Holocene, 

during the last few thousand years, the accumulation returned 

to about 3 mm/yr. The Hueneme Canyon at the west end of 

the basin remains the dominant sediment source even during 

highstand conditions. These new radiocarbon dates also provide chronologic control for a high-resolution basin-wide stratigraphic framework. Preliminary results using these dates show 

Holocene-age landslide and fault activity within the basin. 

Acknowledgments

We thank the scientific staff and crew of USGS cruise A2-98-SC, especially B.D. Edwards and H.J. Lee, the co-chief 

scientists. Some of the samples for radiocarbon dating from 

ODP Site 1015 were prepared by S. Weir and T. Hundert of the 

Geological Survey of Canada (GSC). D. Brushett prepared the 

sediment log for the ODP site under the guidance of D.J.W. 

Piper, also of GSC. R. Sliter processed the deep-tow boomer 

profiles used for the core figures and C.E. Gutmacher provided 

corrected plots of core positions. The radiocarbon dates were 

provided by the National Ocean Sciences Accelerator Mass 

Spectrometry Facilities (NOSAMS) at the Woods Hole Oceanographic Institution. H.F. Ryan and M.A. Fisher provided critical reviews that improved the manuscript.

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Alexander, C.R., and Venherm, C., 2003, Modern sedimentary processes in the Santa Monica, California continental 

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20 Late Quaternary Deposition in the Inner Basins of the California Continental Borderland—Part A. Santa Monica Basin

500

500

1000

1500

100

12B,C

12A ODP-1015

LA

Santa Monica Basin

1,000 m

100 m

V.E. x 4.8

W E

?

A

B

C D

F

J

W E 10 

m

500 m

M

M

L

?

Shallowest deformed surface

V. E. = 33

~O

from 91-062 Line 05

N S

Slide

L

10 

m

500 m

V.E. x 8.2

J

from 91-062 Line 05

K

A

B

C

Figure 12. Along the lower slopes and floor of Santa Monica Basin mass wasting and fault offsets 

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References