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

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary between electronic plotters 

and between X and Y directions on the same plotter, and paper may change size due to atmospheric conditions; therefore, scale and 

proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS

Digital files available at https://doi.org/10.3133/ofr20181024

Suggested Citation: Johnson, S.Y., and Hartwell, S.R., 2018, Local (Offshore of Point Conception map area) and regional (offshore 

from Point Conception to Hueneme Canyon) shallow-subsurface geology and structure, Santa Barbara Channel, California, sheet 8 in

Johnson, S.Y., Dartnell, P., Cochrane, G.R., Hartwell, S.R., Golden, N.E., Kvitek, R.G., and Davenport, C.W. (S.Y. Johnson and S.A. 

Cochran, eds.), California State Waters Map Series—Offshore of Point Conception, California: U.S. Geological Survey Open-File 

Report 2018–1024, pamphlet 36 p., 9 sheets, scale 1:24,000, https://doi.org/10.3133/ofr20181024.

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20181024

DISCUSSION

This sheet includes maps that show the thickness and the depth to base of uppermost 

Pleistocene and Holocene (in other words, post–Late Glacial Maximum) deposits for the 

Offshore of Point Conception map area (Maps A, B), as well as for a larger area that extends 

about 130 km along the coast from Hueneme Canyon to Point Conception (Maps D, E) to 

establish a regional context. To make these maps, water bottom and depth to base of the 

uppermost Pleistocene and Holocene sediment layer were mapped from seismic-reflection 

profiles (sheet 7). The difference between the two horizons was exported for every shot point as 

XY coordinates (UTM zone 10) and two-way travel time (TWT). The thickness of the uppermost Pleistocene and Holocene unit (Maps B, E) was determined by applying a sound velocity 

of 1,600 m/sec to the TWT. The thickness points were interpolated to a preliminary continuous 

surface, overlaid with zero-thickness bedrock outcrops (see sheet 9), and contoured, following 

the methodology of Wong and others (2012). Data within Hueneme Canyon were excluded 

from the contouring because the seismic-reflection data are too sparse to adequately image the 

highly variable changes in sediment thickness that characterize the canyon (Maps D, E).

Several factors required manual editing of the preliminary sediment-thickness maps to 

make the final products. The Red Mountain Fault Zone (RMFZ), Pitas Point Fault (PPF), and 

Oak Ridge Fault (ORF) disrupt the sediment sequence in the region (Maps D, E). The data 

points also are dense along tracklines (about 1 m apart) and sparse between tracklines (1–2 km 

apart), resulting in contouring artifacts. To incorporate the effect of the faults, to remove 

irregularities from interpolation, and to reflect other geologic information and complexity, the 

resulting interpolated contours were modified. Contour modifications and regridding were 

repeated several times to produce the final regional sediment-thickness map (Wong and others, 

2012). Data for the depth to base of the uppermost Pleistocene and Holocene unit (Maps A, D) 

was generated by adding the thickness data to water depths determined by multibeam bathymetry (see sheet 1).

South and east of Point Conception, most of the shelf is underlain by bedrock that has thin 

(less than 2.5 m) post-LGM sediment cover (Map B). West and northwest of Point Conception, 

thicker sediment (as much as 13 m) is present in the midshelf area, along the axes of the 

regionally extensive (22-km-long) Government Point Syncline and an unnamed syncline 

farther north. The post-LGM sediment unit has its maximum thickness (18 m) in the midshelf 

area next to a large asphalt mound (see fig. 8 on sheet 7) about 1.75 km west of Point Conception. Mean sediment thickness on the shelf in the map area is 2.6 m, and the total sediment 

volume on the shelf is 1,248×106

 m3. Small coastal watersheds that drain the western Santa 

Ynez Mountains are the inferred primary sediment source. The shelf in the map area also 

possibly receives sediment derived from the Santa Ynez River (fig. 1); however, this would 

require about 25 km of sediment transport southward around Point Arguello. 

Five different “domains” of sediment thickness, which are bounded either by faults or by 

Hueneme Canyon, are recognized on the regional maps (Maps D, E): (1) north of the south 

strand of the Red Mountain Fault Zone; (2) between the south strand of the Red Mountain Fault 

Zone and the Pitas Point Fault; (3) between the Pitas Point and Oak Ridge Faults; (4) between 

the Oak Ridge Fault and Hueneme Canyon; and (5) southeast of Hueneme Canyon. Table 6–1 

(in pamphlet) shows the area of these five domains, along with estimates of their mean 

sediment thicknesses and total sediment volumes. These data highlight the contrast among 

three general zones of sediment thickness: (1) the uplifted, sediment-poor Santa Barbara and 

Point Conception shelf (domain 1; mean sediment thickness of 4.2 m); (2) a transitional zone 

(domain 2; mean sediment thickness of 18.0 m); and (3) the subsiding, sediment-rich delta and 

shelf offshore of the Ventura and Santa Clara Rivers and Calleguas Creek (domains 3, 4, and 5; 

mean sediment thicknesses of 39.2, 38.9, and 28.3 m, respectively). 

The regional pattern of faults (Minor and others, 2009; Jennings and Bryant, 2010; 

Johnson and others, 2017b) and earthquakes occurring between 1932 and June 14, 2016, that 

have inferred or measured magnitudes greater than 2.0 are shown on Map C. Although 

locations have been provided by the CalTech network since 1932, significantly greater precision began in 1969 with installation of a USGS seismographic network (see, for example, Lee 

and Vedder, 1973; Sylvester, 2001; U.S. Geological Survey, 2016). Epicentral data indicate that 

seismicity in the western and central Santa Barbara Channel is characterized by earthquake 

swarms, relatively frequent minor earthquakes, and infrequent major earthquakes.

Three significant earthquakes affected the Santa Barbara Channel area prior to 1932 (the 

year of the earliest earthquake in the U.S. Geological Survey [2016] catalog): in 1812, 1857 

(the Fort Tejon earthquake on the San Andreas Fault), and 1925 (Stover and Coffman, 1993). 

Sylvester and others (1970) reported a location in the northern Santa Barbara Channel for the 

1925 event (M6.3), as well as for a 1941 earthquake (~M5.5, 7/1/1941) about 7 km offshore 

and 15 km southeast of Santa Barbara, the largest earthquake shown on Map C. In addition, 

Sylvester and others (1970) documented a swarm of 62 earthquakes (M2.5–M5.2) that occurred 

between 6/26/1968 and 8/3/1968, which also were located 10 to 15 km south (offshore) of 

Santa Barbara. The largest recorded event in the Offshore of Point Conception map area 

(~M3.7) occurred on 9/10/2009, near the shoreline in the northwestern part of the map area 

(U.S. Geological Survey, 2016).

REFERENCES CITED

Jennings, C.W., and Bryant, W.A., 2010, Fault activity map of California: California 

Geological Survey Geologic Data Map no. 6, scale 1:750,000, available at 

http://www.conservation.ca.gov/cgs/cgs_history/Pages/2010_faultmap.aspx.

Johnson, S.Y., Hartwell, S.R., Sorlien, C.C., Dartnell, P., and Ritchie, A.R., 2017b, Shelf 

evolution along a transpressive transform margin, Santa Barbara Channel, California: 

Geosphere, v. 13, p. 2,041–2,077.

Lee, W.H.K., and Vedder, J.G., 1973, Recent earthquake activity in the Santa Barbara Channel 

region: Bulletin of the Seismological Society of America, v. 63, p. 1,757–1,773.

Minor, S.A., Kellogg, K.S., Stanley, R.G., Gurrola, L.D., Keller, E.A., and Brandt, T.R., 2009, 

Geologic map of the Santa Barbara coastal plain area, Santa Barbara County, California: 

U.S. Geological Survey Scientific Investigations Map 3001, scale 1:25,000, 1 sheet, 

pamphlet 38 p., available at http://pubs.usgs.gov/sim/3001/.

Stover, C.W. and Coffman, J.L., 1993, Seismicity of the United States, 1568-1989 (revised): 

U.S. Government Printing Office, 418 p.

Sylvester, A.G., 2001, Catalog of Santa Barbara earthquakes—1800 to 1960: Santa Barbara, 

University of California, Santa Barbara database, accessed March 2016 at 

http://projects.crustal.ucsb.edu/sb_eqs/SBEQCatlog/SBEQCATINTRO.html.

Sylvester, A.G., Smith, S.S., and Scholz, C.H., 1970, Earthquake swarm in the Santa Barbara 

Channel, California, 1968: Bulletin of the Seismological Society of America, v. 60, p. 

1,047–1,060.

U.S. Geological Survey, 2016, Earthquake catalog: U.S. Geological Survey database, accessed 

June 14, 2016, at https://earthquake.usgs.gov/earthquakes/search/.

Wong, F.L., Phillips, E.L., Johnson, S.Y, and Sliter, R.W., 2012, Modeling of depth to base of 

Last Glacial Maximum and seafloor sediment thickness for the California State Waters 

Map Series, eastern Santa Barbara Channel, California: U.S. Geological Survey Open-File 

Report 2012–1161, 16 p., available at http://pubs.usgs.gov/of/2012/1161/.

120°30' 120° 119°30'

34°

34°30'

Limit of California’s

State Waters

Kilometers 0 5 10

Santa Barbara Channel

Area of

Map

Santa Barbara CALIF.

MAP LOCATION

Anacapa Is.

Santa Cruz Is.

Santa Rosa Is.

San Miguel Is.

Local (Offshore of Point Conception Map Area) and Regional (Offshore from Point Conception to Hueneme Canyon)

Shallow-Subsurface Geology and Structure, Santa Barbara Channel, California

By

Samuel Y. Johnson and Stephen R. Hartwell

2018

ONE MILE = 0.869 NAUTICAL MILES

1 0.5 0 1 KILOMETER

7000 3500 0 7000 FEET

2 1 0 2 MILES

SCALE 1:50 000

ONE MILE = 0.869 NAUTICAL MILES

20 10 0 20 KILOMETERS

80000 60000 40000 20000 0 80000 FEET

20 10 0 20 MILES

SCALE 1:300 000

ONE MILE = 0.869 NAUTICAL MILES

10 5 0 10 KILOMETERS

40000 30000 20000 10000 0 40000 FEET

10 5 0 10 MILES

SCALE 1:200 000

APPROXIMATE MEAN

DECLINATION, 2018

MAGNETIC NORTH

12 / ° 1 2

TRUE NORTH

APPROXIMATE MEAN

DECLINATION, 2018

MAGNETIC NORTH

12 / ° 1 2

TRUE NORTH

APPROXIMATE MEAN

DECLINATION, 2018 MAGNETIC NORTH

12 / ° 1 2

TRUE NORTH

Onshore elevation data from National Oceanic and 

Atmospheric Administration (NOAA) Office for Coastal 

Management’s Digital Coast (available at 

http://www.csc.noaa.gov/digitalcoast/data/coastallidar) 

and from U.S. Geological Survey’s National Elevation 

Dataset (available at http://ned.usgs.gov/). Offshore 

shaded-relief bathymetry from map on sheet 2, this 

report. California's State Waters limit from NOAA Office 

of Coast Survey

Universal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

Depth and thickness mapped by Samuel Y. 

Johnson and Stephen R. Hartwell, 2016

GIS database and digital cartography by Stephen 

R. Hartwell

Compiled by Stephen R. Hartwell and Samuel 

Y. Johnson, 2016

Onshore elevation data from National Oceanic and Atmospheric Administration (NOAA) Office for Coastal 

Management’s Digital Coast (available at http://www.csc.noaa.gov/digitalcoast/data/coastallidar) and from U.S. 

Geological Survey’s National Elevation Dataset (available at http://ned.usgs.gov/). Offshore shaded-relief 

bathymetry from data collected by Monterey Bay Aquarium Research Institute in 2002, by U.S. Geological Survey in 

2006, and by California State University, Monterey Bay, Seafloor Mapping Lab in 2006–2007. California's State 

Waters limit from NOAA Office of Coast Survey

Universal Transverse Mercator projection, Zone 11N

NOT INTENDED FOR NAVIGATIONAL USE

Depth and thickness mapped by Samuel Y. Johnson, Eleyne L. Phillips, and Stephen 

R. Hartwell, 2010–2016

GIS database and digital cartography by Andrew C. Ritchie, Eleyne L. Phillips, 

Florence L. Wong, and Stephen R. Hartwell

Manuscript approved for publication February 13, 2018

Onshore elevation data from National Oceanic and 

Atmospheric Administration (NOAA) Office for Coastal 

Management’s Digital Coast (available at 

http://www.csc.noaa.gov/digitalcoast/data/coastallidar) 

and from U.S. Geological Survey’s National Elevation 

Dataset (available at http://ned.usgs.gov/). California's 

State Waters limit from NOAA Office of Coast Survey

Universal Transverse Mercator projection, Zone 11N

119°

119°

119°10'

119°10'

119°20'

119°20'

119°30'

119°30'

119°40'

119°40'

119°50'

119°50'

120°

120°

120°10'

120°10'

120°20'

120°20'

120°30'

120°30'

34°30'

34°40'

34°10'

34°

34°40'

34°30'

34°20'

34°20'

34°

34°10'

0 – 0.1

0.1 – 2.5

2.5 – 5

5 – 7.5

7.5 – 10

10 – 12.5

12.5 – 15

15 – 17.5

17.5 – 20

20 – 22.5

22.5 – 25

25 – 27.5

27.5 – 30

30 – 32.5

32.5 – 35

35 – 37.5

37.5 – 40

40 – 42.5

42.5 – 45

45 – 47.5

47.5 – 50

50 – 52.5

52.5 – 55

55 – 57.5

EXPLANATION

Thickness of uppermost Pleistocene

and Holocene sediment, in meters

Thickness contours

Index (10-m intervals)

Intermediate

Fault—ORF, Oak Ridge Fault; PPF, 

Pitas Point Fault; RMFZ, Red 

Mountain Fault Zone

Approximate shoreline

3-nautical-mile limit of California’s 

State Waters

Map E

RMFZ

PPF

ORF

Hueneme 

Canyon

Ventura

Oxnard

Santa 

Barbara

Goleta

 Point

Conception

Offshore of 

Point Conception

map area and area

of Maps A, B

119°

119°

119°10'

119°10'

119°20'

119°20'

119°30'

119°30'

119°40'

119°40'

119°50'

119°50'

120°

120°

120°10'

120°10'

120°20'

120°20'

120°30'

120°30'

34°30'

34°40'

34°10'

34°

34°40'

34°30'

34°20'

34°20'

34°10'

34°

-190 – -185

-185 – -180

-180 – -175

-175 – -170

-170 – -165

-165 – -160

-160 – -155

-155 – -150

-150 – -145

-145 – -140

-140 – -135

-135 – -130

-130 – -125

-125 – -120

-120 – -115

-115 – -110

-110 – -105

-105 – -100

-100 – -95

-95 – -90

-90 – -85

-85 – -80

-80 – -75

-75 – -70

-70 – -65

-65 – -60

-60 – -55

-55 – -50

-50 – -45

-45 – -40

-40 – -35

-35 – -30

-30 – -25

-25 – -20

-20 – -15

-15 – -10

-10 – -5

EXPLANATION

Depth to base of uppermost Pleistocene

and Holocene sediment, in meters

Depth-to-base contours

Index (20-m intervals)

Intermediate

Fault—ORF, Oak Ridge Fault; PPF, 

Pitas Point Fault; RMFZ, Red 

Mountain Fault Zone

Approximate shoreline

3-nautical-mile limit of California’s 

State Waters

Offshore of 

Point Conception

map area and area

of Maps A, B

RMFZ

PPF

ORF

Ventura

Oxnard

Santa 

Barbara

Goleta

 Point

Conception

Hueneme 

Canyon

Map D

120°20' 119°10'

119°10'

119°20'

119°20'

119°30'

119°30'

119°40'

119°40'

119°50'

119°50'

120°

120°10' 120°

120°20'

120°20'

120°30'

120°30'

120°40'

120°40'

34°30'

34°30'

34°20'

34°20'

34°10'

34°10'

34°

34°

33°50'

3-nautical-mile 

limit of California’s 

State Waters

Fault

EXPLANATION

Regional faults and 

earthquake epicenters, 

1932 – 2016

MAGNITUDE

2.0 - 2.5

2.5 - 3.0

3.0 - 3.5

3.5 - 4.0

4.0 - 5.5

Map C

Offshore of 

Point Conception

map area and area

of Maps A, B

120°10'

Approximate area 

of Maps D, E

33°50'

Map B

Point Conception 

 Anticline

 Point

Conception

120°25'

120°25'

120°30'

120°30'

34°30'

34°30'

34°25'

34°25'

0 – 0.1

0.1 – 2.5

2.5 – 5

5 – 7.5

7.5 – 10

10 – 12.5

12.5 – 15

15 – 17.5

17.5 – 20

20 – 22.5

22.5 – 25

25 – 27.5

EXPLANATION

Thickness of uppermost Pleistocene 

and Holocene sediment, in meters

Thickness contours

Index (10-m intervals)

Intermediate

Fault

Anticline—Dashed where location is 

concealed

Syncline—Dashed where location is 

concealed

Approximate shoreline

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

Government Point Syncline

Point Conception 

 Anticline

Government Point Syncline

Map A

-120 – -115

 Point

Conception

120°25'

120°25'

120°30'

120°30'

34°30'

34°30'

34°25'

34°25'

EXPLANATION

Depth to base of uppermost Pleistocene and 

Holocene sediment, in meters

Depth-to-base contours

Index (20-m intervals)

Intermediate

Fault

Anticline—Dashed where location is concealed

Syncline—Dashed where location is concealed

Approximate shoreline

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

-65 – -60

-60 – -55

-55 – -50

-50 – -45

-45 – -40

-40 – -35

-35 – -30

-30 – -25

-25 – -20

-20 – -15

-15 – -10

-100 – -95

-95 – -90

-90 – -85

-85 – -80

-80 – -75

-75 – -70

-70 – -65

-110 – -105

-105 – -100

-115 – -110

-125 – -120-125 – -120

-10 – -5

-130 – -125

-125 – -120

-140 – -135

-135 – -130

-145 – -140

-150 – -145

-155 – -150

Open-File Report 2018–1024

Sheet 8 of 9

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey