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://pubs.usgs.gov/sim/3306/

Suggested Citation: Watt, J.T., Hartwell, S.R., Johnson, S.Y., Sliter, R.W., Phillips, E.L., Ross, S.L., and Chin, J.L., 2014, Local (Offshore 

of San Gregorio map area) and regional (offshore from Bolinas to Pescadero) shallow-subsurface geology and structure, California, 

sheet 9 in Cochrane, G.R., Dartnell, P., Greene, H.G., Watt, J.T., Golden, N.E., Endris, C.A., Phillips, E.L., Hartwell, S.R., Johnson, S.Y., 

Kvitek, R.G., Erdey, M.D., Bretz, C.K., Manson, M.W., Sliter, R.W., Ross, S.L., Dieter, B.E., and Chin, J.L. (G.R. Cochrane and S.A. 

Cochran, eds.), California State Waters Map Series—Offshore of San Gregorio, California: U.S. Geological Survey Scientific 

Investigations Map 3306, pamphlet 38 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/sim3306.

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3306

Kilometers 0 5 10

0 5 10Nautical Miles

122°40' 122°20'

38°

37°40'

37°20'

San

Francisco

Bay

San

Francisco

Area of

Map

Limit of California’s

State Waters

CALIF.

MAP LOCATION

Figure 2. Schematic map showing structural blocks within Offshore of San Gregorio 

map area. Pigeon Point block and Pigeon Point high (McCulloch, 1987) are separated 

from Santa Cruz Mountains block by Pomponio block, which is bounded by Frijoles 

Fault (to west) and Coastways Fault (to east), two strands of San Gregorio Fault Zone 

(SGFZ). Blue line shows coastline. Modified from Weber (1990).

0 1 2

Frijoles Fault (west strand of SGFZ)

SANTA CRUZ

MOUNTAINS

BLOCK

San Gregorio

State Beach

Pescadero

State Beach

axis of Pigeon Point high

San Gregorio Fault Zone

3 KILOMETERS

PIGEON POINT

BLOCK

POMPONIO

BLOCK

Coastways Fault (east strand of SGFZ)

0

0.1

0.2

0.3

0

80

160

240

Approximate depth, in meters

SOUTHWEST NORTHEAST

Two-way travel time, in seconds

1 km

VERTICAL EXAGGERATION ~ 8

Figure 1. USGS high-resolution minisparker seismic-reflection profile P10–15 (collected in 2010 on survey S–15–10–NC), which crosses west flank of Pigeon Point high (see fig. 2) on shelf west of 

Pescadero Point, roughly perpendicular to shoreline (see Map A for location). The Upper Cretaceous Pigeon Point Formation is imaged as shallow, reflection-free basement rock near northeast 

end of profile. To southwest, these basement rocks are unconformably overlain by west-dipping strata inferred to be of Neogene and Pleistocene age that are, in turn, unconformably overlain by 

2- to 20-m-thick layer of shelf strata (blue shading), inferred to be of latest Pleistocene and Holocene age (that is, deposited since last sea-level lowstand about 21,000 years ago). This uppermost 

unit thins to northeast and to southwest, suggesting deposition within shallow basin (see Map B). Dashed green lines highlight continuous, southwest-dipping reflections on west flank of Pigeon 

Point high. Dashed blue line shows erosional unconformity. Dashed yellow line is seafloor multiple (echo of seafloor reflector). Purple triangle shows location of California’s State Waters limit 

(yellow line on Maps A, B, C, D, E).

 Pigeon Point

Formation bedrock Pigeon Point high

DISCUSSION

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

uppermost Pleistocene and Holocene deposits in California’s State Waters for the Offshore of 

San Gregorio map area (Maps A, B), as well as for a larger area that extends about 91 km 

along the coast from Bolinas to Pescadero (Maps C, D) to establish a regional context. This 

uppermost stratigraphic unit (blue shading in seismic-reflection profile of fig. 1; see also, 

figs. 6, 7, 8, 9, 11 on sheet 8) is inferred to have been deposited during the post–Last Glacial 

Maximum (LGM) sea-level rise in the last about 21,000 years (see, for example, Peltier and 

Fairbanks, 2006; Stanford and others, 2011). The unit commonly is characterized either by 

“acoustic transparency” or by parallel, low-amplitude, low- to high-frequency, continuous to 

moderately continuous, diffuse reflections (terminology from Mitchum and others, 1977). 

The acoustic transparency can be caused by extensive wave winnowing, which results in a 

uniform sediment grain size and the 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. 

To make these maps, water bottom and depth to base of the post-LGM horizons were 

mapped from seismic-reflection profiles (fig. 1; see also, sheet 8 of this report). 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 post-LGM 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 zero-thickness bedrock 

outcrops (sheet 10 of this report), and contoured, following the methodology of 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, minor manual editing of the 

preliminary thickness contours was undertaken. Contour modifications and regridding were 

repeated several times to produce the final sediment-thickness maps. Information for the 

depth to base of the post-LGM unit (Maps A, C) was generated by adding the sedimentthickness data to water depths determined by multibeam bathymetry (sheet 1 of this report). 

The thickness of the post-LGM unit in the Offshore of San Gregorio map area ranges 

from 0 to 23 m (Map B), and the depth to base of the unit ranges from less than 10 to 76 m 

(Map A). The relatively thin sediment cover over most of the area suggests a lack of 

sediment “accommodation space” (Catuneanu, 2006), consistent with regional uplift 

expressed by the young, high topography of the Santa Cruz Mountains and Coast Ranges, as 

well as nearby uplifted marine terraces (Weber, 1990; Lajoie and others, 1991; Brabb and 

others, 1998). The uplift raises and, therefore, exposes much of the shallow shelf to the high 

wave energy that is characteristic of this region (Barnard and others, 2007), so that sediments 

are efficiently reworked and then transported off the inner shelf and midshelf areas to deeper 

water. This uplift has been attributed to the northwest-transpressional bend in the San 

Andreas Fault about 15 to 25 km northeast of the map area (see, for example, Zoback and 

others, 1999). The San Gregorio Fault Zone, which cuts through the Offshore of San Gregorio map area, consists of an east strand (“Coastways Fault” or “Seal Cove Fault”), a west 

strand (“Frijoles Fault”), and related splay faults (fig. 2). McCulloch (1987) and Weber 

(1990) suggested that these faults divide the area into three separate blocks, distinguished by 

different basement rocks (fig. 2): the Santa Cruz Mountains block (east of the Coastways 

Fault), the Pomponio block (between the Frijoles and Coastways Faults), and the Pigeon 

Point block (west of the Frijoles Fault). Minimal amounts of sediment are present on the 

Santa Cruz Mountains and Pomponio blocks, whereas the thickest accumulation of sediment 

in the map area is on the west-dipping, wave-cut surface of the Pigeon Point block.

Five different “domains” of sediment thickness are recognized on the regional 

sediment-thickness map (Map D): (1) the Bolinas shelf, located west of the east strand of the 

San Gregorio Fault Zone, in the northwestern part of the regional map (Map D); (2) the San 

Andreas graben, located between the San Gregorio Fault Zone and the Golden Gate Fault, 

east-southeast of the Bolinas shelf and both southwest and southeast of the Marin shelf; (3) 

the Marin shelf, located both northeast and northwest of the San Andreas graben and north of 

the San Francisco ebb-tidal delta paleovalley; (4) the northeast-trending San Francisco 

ebb-tidal delta paleovalley, located outside the Golden Gate at the mouth of San Francisco 

Bay, between the Marin shelf and San Andreas graben on the north and the PacificaPescadero shelf on the south; and (5) the Pacifica-Pescadero shelf, which is located south of 

the San Francisco ebb-tidal delta paleovalley and which extends south all the way to 

Pescadero Point (including all of the Offshore of San Gregorio map area). 

The five sediment-thickness domains have distinct geologic controls. The Bolinas and 

Pacific-Pescadero shelves are uplifting and are relatively sediment poor (mean sediment 

thicknesses of 0.8 and 3.6 m, respectively). Thicker sediment accumulations (as much as 20 

m) on the western margins of the Pacifica-Pescadero shelf (within California’s State Waters) 

are associated with west-side-down slip on the west strand of the San Gregorio Fault Zone 

and with deposition on the outboard, west-dipping Pigeon Point block (McCulloch, 1987; see 

also, fig. 2) farther south, offshore of Pescadero Point. The San Andreas graben is a rapidly 

subsiding, fault-controlled sedimentary basin that has sediment thicknesses of as much as 57 

m; the Marin shelf forms the uplifted northeastern and northwestern margins of this basin. 

The San Francisco ebb-tidal delta is filling a paleovalley that formed during the last sea-level 

lowstand, with sediment thicknesses of as much as 32 m along the trough axis. Although the 

southern part of the San Andreas graben may extend into the paleovalley, the north flank of 

the paleovalley is used here as the boundary when calculating sediment volumes for the five 

sediment-thickness domains (see table 7–1 in accompanying pamphlet). Subsidence in the 

San Francisco ebb-tidal delta paleovalley and the San Andreas graben can be partly attributed to the northward change in strike of both the San Andreas and San Gregorio Fault Zones 

offshore of San Francisco, which has resulted in the local change from contractional deformation to extensional deformation (Zoback and others, 1999).

Map E shows the regional pattern of major faults and of earthquakes occurring between 

1967 and April 2014 that have inferred or measured magnitudes of 2.0 and greater. Fault 

locations, which have been simplified, are compiled from our mapping within California’s 

State Waters (see sheet 10 of this report) and from the U.S. Geological Survey’s Quaternary 

fault and fold database (U.S. Geological Survey and California Geological Survey, 2010). 

Earthquake epicenters are from the Northern California Earthquake Data Center (2014), 

which is maintained by the U.S. Geological Survey and the University of California, 

Berkeley, Seismological Laboratory. Map E also shows the inferred location of the devastating great 1906 California earthquake (M7.8, 4/18/1906), thought to have nucleated on the 

San Andreas Fault offshore of San Francisco (see, for example, Bolt, 1968; Lomax, 2005). 

Map E clearly shows that the largest number of earthquakes in the region occur within the 

broad San Andreas Fault Zone between Pacifica and Bolinas; events west of the east strand 

of the San Gregorio Fault Zone and east of the Golden Gate Fault are much less common. 

Epicentral data indicate that recent seismicity in the Offshore of San Gregorio map area has 

been notably minimal.

REFERENCES CITED

Barnard, P.L., Eshelman, J., Erikson, L., and Hanes, D.M., 2007, Coastal processes study at 

Ocean Beach, San Francisco, CA—Summary of data collection 2004–2006: U.S. 

Geological Survey Open-File Report 2007–1217, 165 p., available at: http://pubs.usgs.gov/

of/2007/1217/. 

Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological 

Society of America, v. 58, p. 457–471.

Brabb, E.E., Graymer, R.W., and Jones, D.L., 1998, Geology of the onshore part of San 

Mateo County, California—A digital database: U.S. Geological Survey Open-File 

Report 98–137, scale 1:62,500, available at http://pubs.usgs.gov/of/1998/of98-137/.

Catuneanu, O., 2006, Principles of sequence stratigraphy: Amsterdam, Elsevier, 375 p.

Lajoie, K.R., Ponti, D.J., Powell, C.L., II, Mathieson, S.A., and Sarna-Wojcicki, A.M., 1991, 

Emergent marine strandlines and associated sediments, coastal California—A record of 

Quaternary sea-level fluctuations, vertical tectonic movements, climatic changes, and 

coastal processes, in Morrison, R.B., ed., The Geology of North America, Quaternary 

nonglacial geology—Conterminous U.S.: Geological Society of America, Decade of 

North American Geology, v. K–2, p. 190–213. 

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.

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: 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, 2014, Northern California earthquake catalog: 

Northern California Earthquake Data Center database, accessed April 5, 2014, 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.

Stanford, J.D., Hemingway, R., Rohling, E.J., Challenor, P.G., Medina-Elizalde, M., and 

Lester, A.J., 2011, Sea-level probability for the last deglaciation—A statistical analysis 

of far-field records: Global and Planetary Change, v. 79, p. 193–203.

U.S. Geological Survey and California Geological Survey, 2010, Quaternary fault and fold 

database for the United States: U.S. Geological Survey database, accessed April 5, 

2014, at http://earthquake.usgs.gov/hazards/qfaults/.

Weber, G.E., 1990, Late Pleistocene slip rates on the San Gregorio fault zone at Point Año 

Nuevo, San Mateo County, California, in Greene, H.G., Weber, G.E., Wright, T.L., and 

Garrison, R.E., eds., Geology and tectonics of the central California coast region—San 

Francisco to Monterey: American Association of Petroleum Geologists, Pacific Section, 

volume and guidebook, v. 67, p. 193–204.

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/.

Zoback, M.L., Jachens, R.C., and Olson, J.A., 1999, Abrupt along-strike change in tectonic 

style—San Andreas Fault zone, San Francisco Peninsula: Journal of Geophysical 

Research, v. 104 (B5), p. 10,719–10,742.

Local (Offshore of San Gregorio Map Area) and Regional (Offshore from Bolinas to Pescadero) Shallow-Subsurface Geology and Structure, California

By

Janet T. Watt, Stephen R. Hartwell, Samuel Y. Johnson, Ray W. Sliter, Eleyne L. Phillips, Stephanie L. Ross, and John L. Chin

2014

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

10 5 0 10 KILOMETERS

40000 30000 20000 10000 0 40000 FEET

10 5 0 10 MILES

14°

SCALE 1:200 000

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC 

 NORTH

TRUE NORTH

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC NORTH

13 / ° 1 2

TRUE NORTH

Depth and thickness mapped by Janet T. Watt and 

Stephen R. Hartwell, 2011-2012

GIS database and digital cartography by Stephen R. 

Hartwell, Eleyne L. Phillips, and Florence L. Wong

Edited by Taryn A. Lindquist

Manuscript approved for publication June 30, 2014

Depth and thickness mapped by Janet T. 

Watt and Stephen R. Hartwell, 2011-2012

GIS database and digital cartography by 

Janet T. Watt , Stephen R. Hartwell, 

Eleyne L. Phillips, and Florence L. Wong

Onshore elevation data collected by Photoscience in 

2005 for U.S. Geological Survey and County of San 

Mateo. California's State Waters limit from NOAA Office 

of Coast Survey

Universal Transverse Mercator projection, Zone 10N

Onshore elevation data collected by Photoscience in 

2005 for U.S. Geological Survey and County of San 

Mateo. California's State Waters limit from NOAA Office 

of Coast Survey

Universal Transverse Mercator projection, Zone 10N

122°20'

122°20'

122°40'

122°40'

37°40' 37°40'

37°20' 37°20'

San Francisco Bay

Pacific Ocean

 Marin 

Headlands

Bolinas

Pescadero

 Point

San Gregorio

Half 

Moon 

Bay

Pacifica

San Francisco

Golden Gate

SAF

SAF

GGF

PF

SGE

SGE

SGW

PPF

SGW

Map E

SANTA CRUZ MOUNTAINS

Pillar

Point

3-nautical-mile limit of 

California’s State Waters

Earthquake magnitude

7.8

Great 1906 California 

earthquake

Regional faults and 

earthquake epicenters, 

1967 - 2014

EXPLANATION

2.0 - 2.5

2.5 - 3.0

3.0 - 3.5

3.5 - 4.0

4.0 - 5.5

Fault—Dashed where

location is inferred. 

Abbreviations: GGF, 

Golden Gate Fault; PF, 

Pilarcitos Fault; PPF, 

Potato Patch Fault; 

SAF, San Andreas Fault 

Zone; SGE, San 

Gregorio Fault Zone, 

east strand; SGW, San 

Gregorio Fault Zone, 

west strand

Offshore of

San Gregorio

map area and

area of Maps A, B

122°20'

122°20'

122°40'

122°40'

37°40' 37°40'

37°20' 37°20'

Pacific Ocean

San Francisco Bay

Half 

Moon 

Bay

Pescadero

Pacifica

San Francisco

Golden Gate

 Marin 

Headlands

Bolinas

PACFICA-PESCADERO

SHELF DOMAIN

SAN FRANCISCO

PALEOVALLEY

DOMAIN

SAN ANDREAS

GRABEN DOMAIN

BOLINAS

SHELF

DOMAIN

MARIN SHELF

 DOMAIN

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

Map D

 San 

Gregorio

Offshore of

San Gregorio

map area and

area of Maps A, B

37°40' 37°40'

37°20' 37°20'

122°20'

122°20'

122°40'

122°40'

San Francisco Bay

Pacific Ocean

Pescadero

 San 

Gregorio

Half 

Moon 

Bay

Pacifica

San Francisco

Golden Gate

 Marin 

Headlands

Bolinas

-10 – -5

-5 – 0

-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

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

Map C

Offshore of

San Gregorio

map area and

area of Maps A, B

122°25'

122°25'

122°30'

122°30'

37°20'

37°20'

37°15'

37°15'

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—Solid where location is certain, 

dashed where location is inferred or 

concealed

Folds—Solid where location is certain, 

dashed where location is inferred or 

concealed

Anticline

Syncline

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

 PACIFICAPESCADERO

 SHELF

Map B

Pescadero

 San 

Gregorio

Frijoles Fault (west strand, San Gregorio Fault Zone)

Pescadero

 Point

Coastways Fault (east strand,

San Gregorio Fault Zone)

Map A

122°25'

122°25'

122°30'

122°30'

37°20'

37°20'

37°15'

37°15'

-10 – -5

-5 – 0

-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

EXPLANATION

Depth to base of uppermost Pleistocene and 

Holocene sediment, in meters

Depth-to-base contours

Index (10-m intervals)

Intermediate

Fault—Solid where location is certain, 

dashed where location is inferred or 

concealed

Folds—Solid where location is certain, 

dashed where location is inferred or 

concealed

Anticline

Syncline

Trackline of seismic-reflection profile 

shown in figure 1

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

Figure 1

Pescadero

 San 

Gregorio

Frijoles Fault (west strand, San Gregorio Fault Zone)

Coastways Fault (east strand,

San Gregorio Fault Zone)

 PACIFICAPESCADERO

 SHELF

Pescadero

 Point

Scientific Investigations Map 3306

Sheet 9 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey