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 http://dx.doi.org/10.3133/ofr20151088 or http://pubs.usgs.gov/of/2015/1088/

Suggested Citation: Johnson, S.Y., Hartwell, S.R., Watt, J.T., and Sliter, R.W., 2015, Local (Offshore of Tomales Point map area) and 

regional (offshore from Salt Point to Drakes Bay) shallow-subsurface geology and structure, California, sheet 9 in Johnson, S.Y., 

Dartnell, P., Golden, N.E., Hartwell, S.R., Greene, H.G., Erdey, M.D., Cochrane, G.R., Watt, J.T., Kvitek, R.G., Manson, M.W., Endris, C.A., 

Dieter, B.E., Krigsman, L.M., Sliter, R.W., Lowe, E.N., and Chin, J.L. (S.Y. Johnson and S.A. Cochran, eds.), California State Waters Map 

Series—Offshore of Tomales Point, California: U.S. Geological Survey Open-File Report 2015–1088, pamphlet 38 p., 10 sheets, scale 

1:24,000, http://dx.doi.org/10.3133/ofr20151088.

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151088

Figure 1. USGS high-resolution minisparker profile PR–82 (collected in 2009 on survey S–8–09–NC) extending from western part of San Andreas Fault across rugose outcrops of Cretaceous granitic rocks to gently dipping continental shelf underlain by 

folded Neogene sedimentary rocks (see sheet 10). Dashed red lines show faults. On San Andreas Fault, circle with dot indicates strike-slip motion toward reader, and circle with “x” indicates motion away from reader. Pink symbols show fold axes 

(diverging arrows, anticline; converging arrows, syncline). Blue shading shows thin, inferred uppermost Pleistocene and Holocene strata, deposited since the last sea-level lowstand about 21,000 years ago. Dashed green lines highlight some continuous 

reflections that illustrate structure (not distinctive stratigraphic markers). Dashed yellow line is seafloor multiple (echo of seafloor reflector). Purple triangle shows location of outer boundary of California’s State Waters (yellow line on Map A).

NORTHEAST

160

0 

Approximate depth, in meters

80

240

Two-way travel time, in seconds

0

0.1

0.2

SOUTHWEST

0.3

1 km

VERTICAL EXAGGERATION ~ 12.5

western part of San 

Andreas Fault Zone seafloor outcrops of 

Cretaceous granitic rocks

DISCUSSION

This sheet includes maps that show the thickness and the depth to base of uppermost Pleistocene and Holocene deposits for the Offshore of Tomales Point area (Maps A, B), as well as for 

a larger area that extends about 115 km along the coast from Salt Point to Drakes Bay (Maps C, D) to establish a regional context. The uppermost stratigraphic unit (blue shading on the 

seismic-reflection profiles on sheet 8 of this report) is inferred to have been deposited during the post-Last Glacial Maximum (LGM) sea-level rise in the last 21,000 years (see, for example, 

Peltier and Fairbanks, 2006). The unit is commonly characterized either by “acoustic transparency” or by parallel, low-amplitude, low- to high-frequency, continuous to moderately continuous 

reflections (terminology from Mitchum and others, 1977). The acoustic transparency can be caused by extensive wave winnowing, which results in uniform sediment grain size and consequent 

lack of acoustic-impedance contrasts needed to produce seismic reflections. On the continental shelf, the contact with underlying units is a transgressive surface of erosion commonly marked 

by angularity, channeling, or a distinct upward change to lower amplitude, more diffuse reflections.

In the northern part of this region (Offshore of Salt Point map area), the sequence of uppermost Pleistocene and Holocene deposits, which lies above a prominent unconformity, includes a 

lower, older stratigraphic unit, made up of a downlapping sediment wedge that formed along the southwest flank of nearshore bedrock outcrops. Stratigraphic position, depth of occurrence, and 

reflection geometry suggest that this lower unit formed during the latter stages of the pre-LGM sea-level fall (about 30,000 to 21,000 years ago). Because this unit and overlying post-LGM 

deposits each comprise unconsolidated sediments and together overlie a prominent unconformity, our regional thickness and depth-to-base maps (Maps C, D) combine these units in this 

northern part of the Salt Point to Drakes Bay region. 

To make these maps, water bottom and depth to base of the uppermost Pleistocene and Holocene horizons were mapped from seismic-reflection profiles (fig. 1; see also, sheet 8). 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, D) 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 zerothickness bedrock outcrops (see sheet 10), and contoured following the methods outlined in Wong and others (2012).

The thickness data points are dense along tracklines (about 1 m apart) and sparse between tracklines (1 km apart), resulting in minor contouring artifacts. To incorporate the effect of a few 

rapid thickness changes along 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 maps (Maps B, D). Information for the depth to base of the uppermost 

Pleistocene and Holocene unit (Maps A, C) was generated by adding the thickness data to water depths determined by multibeam bathymetry (see sheet 1). 

The thickness of the uppermost Pleistocene and Holocene sediments in the offshore of Tomales Point map area ranges from 0 to 11 meters (Map B), and the depth to base of the unit 

ranges from about 10 to 85 meters (Map A). Mean sediment thickness for the map area is 2.3 m and total sediment volume is 253×106 m3 (table 7–1 in pamphlet). The thickest sediment 

occurs along the northern map boundary, at the mouth of Tomales Bay and at water depths of about 60 to 75 m due west of Tomales Point. These two areas are connected and represent upper 

and lower reaches of a now-submerged channel system that provided an outlet for Tomales Bay and small coastal drainages north of the map area during sea-level lowstand (Maps B, D).

To the south, Cretaceous granitic and Tertiary sedimentary rock crop out on the seafloor or are buried by only a thin cover (mostly less than 2.5 m) of uppermost Pleistocene and Holocene 

sediment. This thin cover results from a combination of factors including active uplift of the Point Reyes Peninsula (Grove and others, 2010), high wave energy, and very limited sediment 

supply. Tomales Point blocks southward littoral drift, and west-facing coastal watersheds on this part of the Point Reyes Peninsula are very small; therefore, much of the relatively meager 

sediment that is present on the shelf could be derived from erosion of local coastal bluffs and sand dunes. 

Five different “domains” of sediment thickness are recognized on the regional sediment-thickness maps (table 7–1 in pamphlet; Map D), each with distinctive geologic controls: (1) The 

Salt Point shelf domain, located in the far northwestern part of the region has a mean sediment thickness of 11.7 m. The thickest sediment (20 to 25 m) is found where a pre-LGM, regressive, 

downlapping sediment wedge formed above a break in slope that is controlled by a contact between harder bedrock and softer, folded, Pleistocene strata. Sediment thinning in this domain in 

the outer portions of California’s State Waters is the result of a relative lack of sediment supply from local watersheds, as well as a more distal Russian River source. (2) The Russian River 

delta and mud belt domain, located offshore of the largest sediment source on this part of the coast, has the thickest uppermost Pleistocene and Holocene sediment in the region (mean thickness, 21.1 m). The northward extension into the midshelf “mud belt” results from northward shelf-bottom currents and sediment transport (Drake and Cacchione, 1985). This domain includes a 

section of the San Andreas Fault Zone (Map E), which here is characterized by several releasing, right-stepping strands that bound narrow, elongate, pull-apart basins; these sedimentary basin 

contain the greatest thicknesses of uppermost Pleistocene and Holocene sediment (about 56 m) in the region. (3) The Bodega Head–Tomales Point shelf domain, located between Bodega Head 

and the Point Reyes headland, contains the least amount of sediment in the region (mean thickness of 3.4 m). The lack of sediment results from decreased “accommodation space” (note 

shallower depth contours on Maps A and C) and limited sediment supply. (4) The Point Reyes bar domain, located west and south of the Point Reyes headland, is a local zone of increased 

sediment thickness (mean thickness of 14.3 m) created by bar deposition on the more protected south flank of the Point Reyes headland during rising sea level. (5) The Bolinas shelf domain, 

located east and southeast of the Point Reyes headland, has a thin sediment cover (mean thickness of 5.6 m) which likely results from limited sediment accommodation space caused by 

tectonic uplift (water depths in this domain within California’s State Waters are less than 45 m) as well as limited sediment supply and the high wave energy that is capable of reworking and 

transporting shelf sediment to deeper water.

Map E shows the regional pattern of major faults and earthquakes occurring between 1967 and March 2012 that have inferred or measured magnitudes of 2.0 and greater. Fault locations, 

which have been simplified, are compiled from interpretation of regional multi-channel industry seismic data, our mapping within California’s State Waters (for example, sheet 10), 

(McCulloch, 1987), and the Quaternary fault and fold database (U.S. Geological Survey and California Geological Survey, 2010). Earthquake epicenters are from the Northern California 

Earthquake Data Center (2012), which is maintained by the U.S. Geological Survey and the University of California, Berkeley, Seismological Laboratory; all events of magnitude 2.0 and 

greater for the time period 1967 through March 2012 are shown. The largest earthquake in the map area (M2.1, 11/18/1978), occurred about 5 km offshore of the Point Reyes Peninsula. There 

has been a notable lack of microseismicity on this section of the San Andreas Fault since the great 1906 California earthquake (M7.8, 4/18/1906), thought to have nucleated on the San Andreas 

Fault offshore of San Francisco (fig. 1–1 in pamphlet; Bolt, 1968; Lomax, 2005). 

REFERENCES CITED 

Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological Society of America, v. 58, p. 457–471.

Drake, D.E., and Cacchione, D.A., 1985, Seasonal variation in sediment transport on the Russian River shelf, California: Continental Shelf Research, v. 4, p. 495–514, 

doi:10.1016/0278-4343(85)90007-X.

Grove, K., Sklar, L.S., Scherer, A.M., Lee, G., and Davis, J., 2010, Accelerating and spatially-varying crustal uplift and its geomorphic expression, San Andreas Fault zone north of San 

Francisco, California: Tectonophysics, v. 495, p. 256–268, doi:10.1016/j.tecto.2010.09.034.

Lomax, A., 2005, A reanalysis of the hypocentral location and related observations for the Great 1906 California earthquake: Bulletin of the Seismological Society of America, v. 95, p. 

861–877, doi:10.1785/0120040141.

McCulloch, D.S., 1987, Regional geology and hydrocarbon potential of offshore central California, in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and Resource Potential of the 

Continental Margin of Western North America and Adjacent Ocean basins—Beaufort Sea to Baja California: Houston, Texas, Circum-Pacific Council for Energy and Mineral Resources, 

Earth Science Series, v. 6., p. 353–401.

Mitchum, R.M., Jr., Vail, P.R., and Sangree, J.B., 1977, Seismic stratigraphy and global changes of sea level, part 6—Stratigraphic interpretation of seismic reflection patterns in depositional 

sequences, in Payton, C.E., ed., Seismic stratigraphy—Applications to hydrocarbon exploration: Tulsa, Okla., American Association of Petroleum Geologists, p. 117–133.

Northern California Earthquake Data Center, 2012, Northern California earthquake catalog: Northern California Earthquake Data Center database, accessed April 5, 2012, at 

http://www.ncedc.org/ncsn/.

Peltier, W.R., and Fairbanks, R.G., 2006, Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record: Quaternary Science Reviews, v. 25, p. 

3,322–3,337, doi:10.1016/j.quascirev.2006.04.010.

U.S. Geological Survey and California Geological Survey, 2010, Quaternary fault and fold database of the United States: U.S. Geological Survey database, accessed April 5, 2012 at 

http://earthquake.usgs.gov/hazards/qfaults/.

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/.

123°20 123°

38°20'

38°

0 5 10 Kilometers

0 5 10 Nautical Miles

Area of

Map

Limit of California’s

State Waters

Point Reyes

Tomales Bay

Bodega

Bay

CALIF.

MAP LOCATION

Bodega

Harbor

Local (Offshore of Tomales Point Map Area) and Regional (Offshore from Salt Point to Drakes Bay) Shallow-Subsurface Geology and Structure, California

By

Samuel Y. Johnson, Stephen R. Hartwell, Janet T. Watt, and Ray W. Sliter

2015

ONE MILE = 0.869 NAUTICAL MILES

10 5 0 10 KILOMETERS

40 000 30 000 20 000 10 000 0 40 000 FEET

10 MILES 10 5 0

SCALE 1:250 000

ONE MILE = 0.869 NAUTICAL MILES

1 0.5 0 1 KILOMETER

7000 3500 0 7000 FEET

2 1 0 2 MILES

14°

SCALE 1:50 000

APPROXIMATE MEAN

DECLINATION, 2015

MAGNETIC 

 NORTH

TRUE NORTH

14°

APPROXIMATE MEAN

DECLINATION, 2015

MAGNETIC 

 NORTH

TRUE NORTH

Depth and thickness mapped by Samuel Y. Johnson 

and Stephen R. Hartwell, 2011–2012

GIS database and digital cartography by Stephen R. 

Hartwell

Manuscript approved for publication May 4. 2015

Onshore topography from the U.S. Geological Survey 

(2012) National Elevation dataset

Universal Transverse Mercator projection, Zone 10N

Depth and thickness mapped by Samuel Y. 

Johnson and Steven R. Hartwell, 2012-2013

GIS database and digital cartography by 

Stephen R. Hartwell

Onshore elevation data from California Coastal Conservancy, available at 

http://www.csc.noaa.gov/digitalcoast/data/coastallidar/, and from U.S. Geological 

Survey, 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 10N

NOT INTENDED FOR NAVIGATIONAL USE

Pacific Ocean Offshore of 

Tomales Point

map area and 

area of Maps A, B

Map E

Salt

Point

123°

123°

123°20'

123°20'

38°20' 38°20'

38° 38°

3-nautical-mile limit of 

CA State Waters

2.0 - 2.5

2.5 - 3.0

3.0 - 3.5

3.5 - 4.0

4.0 - 5.5

Earthquake magnitude

Regional Faults and 

Earthquake Epicenters 

1967 - 2012

Fault

GF - Gualala Fault

RCF - Rodgers Creek Fault

PRF - Point Reyes Fault

SAF - San Andreas Fault

WPRF - Western Point Reyes Fault

GF

SAF

SAF

PRF

WPRF

RCF

Point 

Reyes

Drakes 

 Bay

Tomales Bay

Bodega Bay

Fort 

Ross

Russian River

123°

123°

123°20'

123°20'

38°20' 38°20'

38° 38°

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

3-nautical-mile limit of California’s 

State Waters

Boundary of sediment-thickness domain

Area of “no data”—Areas where data are 

insufficient for contouring

Pacific Ocean

Offshore of 

Tomales Point

map area and 

area of Maps A, B

Map D

Salt

Point

Point 

Reyes

Tomales Bay

Fort 

Ross

Bodega

 Bay

Bolinas

shelf 

domain

Point Reyes

bar domain

Bodega HeadTomales Point

shelf domain

Russian River

delta and mud

belt domain

Salt Point

shelf domain

123°

123°

123°20'

123°20'

38°20' 38°20'

38° 38°

-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

-5 – 0

EXPLANATION

Depth to base of uppermost Pleistocene 

and Holocene sediment, in meters

Depth-to-base contours

Index (10-m intervals)

Intermediate

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

Pacific Ocean

Offshore of 

Tomales Point

map area and

area of Maps A, B

Salt

Point

Point 

Reyes

Tomales Bay

Fort 

Ross

Bodega

 Bay

Russian River

Map C

122°55'

122°55'

123°

123°

123°5'

123°5'

38°15' 38°15'

38°10' 38°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—Dashed where inferred; dotted 

where concealed

Folds—Solid where location is certain; 

dotted where location is concealed

Anticline

Syncline

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

San Andreas Fault

Tomales Bay

Bodega HeadTomales Point 

 shelf

Bodega

 Bay

Abbots 

Lagoon

Pacific Ocean

Figure 1

Map B

Dillon 

Beach

122°55'

122°55'

123°

123°

123°5'

123°5'

38°15' 38°15'

38°10' 38°10'

-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

-5 – 0

EXPLANATION

Depth to base of uppermost Pleistocene 

and Holocene sediment, in meters

Depth-to-base contours

Index (10-m intervals)

Intermediate

Fault—Dashed where inferred; dotted 

where concealed

Folds—Solid where location is certain; 

dotted where location is concealed

Anticline

Syncline

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

San Andreas Fault

Tomales Point

Tomales Bay

Bodega HeadTomales Point 

 shelf

Bodega

 Bay

Abbots 

Lagoon

Pacific Ocean

Map A

Figure 1

Dillon 

Beach

Open-File Report 2015–1088

Sheet 9 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey