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/ofr20181023

Suggested Citation: Johnson, S.Y., and Hartwell, S.R., 2018, Local (Offshore of Gaviota 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 Gaviota, California: U.S. Geological Survey Open-File Report 2018–1023, pamphlet 41 p., 9 sheets, scale 1:24,000, https://doi.org/10.3133/ofr20181023.

ISSN 2331-1258 (online)https://doi.org/10.3133/ofr20181023

DISCUSSIONThis 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 Gaviota 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 in 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) were generated by adding the thickness data to water depths determined by multibeam bathym-etry (see sheet 1).

In the Offshore of Gaviota map area, the thickness of the uppermost Pleistocene and Holocene sediments on the continental shelf ranges from 0 to 36 m (Map B). Mean sediment thickness on the shelf in the map area is 9.9 m, and the total sediment volume on the shelf is 853×106 m3. The thickest sediment in the map area (about 36 m) is found within the Gaviota sediment bar. The primary sediment source, Cañada de la Gaviota, is the largest coastal watershed (about 52 km2; Warrick and Mertes, 2009) between the Ventura River and Point Conception. Sediment thins markedly and is locally absent on the shelf both southeast and northwest of the Gaviota sediment bar, commonly forming only a thin veneer over bedrock outcrops.

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) south 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 Concep-tion 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 and of earthquakes occurring between 1932 and 2016 that have inferred or measured magnitudes greater than 2.0 is shown on Map C. Fault locations are based on our mapping within California’s State Waters and on a generalized compilation that includes the mapping of Heck (1998), Minor and others (2009), and Jennings and Bryant (2010). Earthquake data are from the U.S. Geological Survey (2016). Although earthquake

locations have been determined by the CalTech network since 1932, significantly greater precision began in 1969 with installation of a U.S. Geological Survey (USGS) seismographic network (see, for example, Lee and Vedder, 1973; Sylvester, 2001; U.S. Geological Survey, 2016). Epicentral data indicate that seismicity in the Santa Barbara Channel region is character-ized 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 (sheet 8). 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 Gaviota map area (M3.1) occurred on 11/4/1987, about 7 km south of Gaviota (U.S. Geological Survey, 2016).

REFERENCES CITEDHeck, R.G., 1998, Santa Barbara Channel regional formline map, top Monterey Formation, in

Kunitomi, D.S., Hopps, T.E., and Galloway, J.M., eds., Structure and petroleum geology, Santa Barbara Channel, California: American Association of Petroleum Geologists, Pacific Section, and Coast Geological Society, Miscellaneous Publication 46, 1 plate.

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.

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/.

Warrick, J.A., and Mertes, L.A.K., 2009, Sediment yield from the tectonically active semiarid Western Transverse Ranges of California: Geological Society of America Bulletin, v. 121, p. 1,054–1,070.

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°120°30' 119°30'

34°

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Limit of California’sState Waters

Kilometers0 5 10

Santa Barbara Channel

Area ofMap

Santa Barbara CALIF.

MAP LOCATION

Anacapa Is.Santa Cruz Is.

Santa Rosa Is.San Miguel Is.

Local (Offshore of Gaviota Map Area) and Regional (Offshore from Point Conception to Hueneme Canyon)Shallow-Subsurface Geology and Structure, Santa Barbara Channel, California

BySamuel Y. Johnson and Stephen R. Hartwell

2018

ONE MILE = 0.869 NAUTICAL MILES

1 0 1 KILOMETER0.5

7000 0 7000 FEET3500

2 0 2 MILES1SCALE 1:50 000

ONE MILE = 0.869 NAUTICAL MILES

10 20 KILOMETERS20 0

80000 FEET80000 60000 40000 20000 0

1020 0 20 MILESSCALE 1:300 000

ONE MILE = 0.869 NAUTICAL MILES

5 10 KILOMETERS10 0

40000 FEET40000 2000030000 10000 0

510 0 10 MILESSCALE 1:200 000

APPROXIMATE MEANDECLINATION, 2018

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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/). California's State Waters limit from NOAA Office of Coast SurveyUniversal Transverse Mercator projection, Zone 11N

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 SurveyUniversal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

Depth and thickness mapped by Samuel Y. Johnson and Stephen R. Hartwell, 2016GIS database and digital cartography by Stephen R. Hartwell

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 SurveyUniversal 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, 2010–2016GIS database and digital cartography by Andrew C. Ritchie, Eleyne L. Phillips, Florence L. Wong, and Stephen R. HartwellManuscript approved for publication February 12, 2018

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EXPLANATIONThickness of uppermost Pleistoceneand Holocene sediment, in meters

Thickness contoursIndex (10-m intervals)

IntermediateFault—ORF, Oak Ridge Fault; PPF,

Pitas Point Fault; RMFZ, Red Mountain Fault Zone

Approximate shoreline3-nautical-mile limit of California’s

State Waters

Offshore of Gaviota map area and areaof Maps A, B

RMFZ

PPF

ORF

Ventura

Oxnard

Santa Barbara

Goleta

PointConception

Hueneme Canyon

Map E

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EXPLANATIONDepth to base of uppermost Pleistoceneand Holocene sediment, in meters

Depth-to-base contoursIndex (20-m intervals)

IntermediateFault—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 ofGaviota map area and areaof Maps A, B

RMFZ

PPF

ORF

Ventura

Oxnard

Santa Barbara

Goleta

PointConception

Hueneme Canyon

Map D

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3-nautical-mile limit of California’s State Waters

Fault

EXPLANATIONRegional faults and

earthquake epicenters, 1932 – 2016

MAGNITUDE2.0 - 2.5

2.5 - 3.0

3.0 - 3.5

3.5 - 4.0

4.0 - 5.5

Map C

Offshore of Gaviota maparea and areaof Maps A, B

120°10'

Approximate area of Maps D, E

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Map B

Molino Anticline

Government Point Syncline

south st

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0 – 0.1

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EXPLANATIONThickness of uppermost Pleistocene and Holocene sediment, in meters

Thickness contoursIndex (10-m intervals)Intermediate

Fault—Dashed where location is inferred; dotted where location is concealed

Anticline—Dotted where location is concealed

Syncline—Dotted 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

12.5 – 15

15 – 17.5

27.5 – 30

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35 – 37.5

GaviotaState Park

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Map A

Molino Anticline

Government Point Syncline

south st

rand of S

anta

Ynez Fau

lt

EXPLANATIONDepth to base of uppermost Pleistocene and

Holocene sediment, in meters

Depth-to-base contoursIndex (20-m intervals)Intermediate

Fault—Dashed where location is inferred; dotted where location is concealed

Anticline—Dotted where location is concealed

Syncline—Dotted 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

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GaviotaState Park

Open-File Report 2018–1023Sheet 8 of 9

Pamphlet accompanies map

U.S. Department of the InteriorU.S. Geological Survey