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., Sliter, R.W., Hartwell, S.R., and Chin, J.L., 2015, Seismic-reflection profiles, Offshore of Tomales Point 

map area, California, sheet 8 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

DISCUSSION

This map sheet shows seismic-reflection profiles from two different surveys of the Offshore of Tomales 

Point map area, providing imagery of the subsurface geology. The northeast part of the map area is cut by the San 

Andreas Fault (pamphlet, fig. 1–1), which forms a right-lateral transform boundary between the North American 

and Pacific tectonic plates. Fault slip along the San Andreas Fault has uplifted Cretaceous granitic rock and 

overlying Neogene, folded, marine sedimentary rock forming Tomales Point and much of the rugged, rocky, 

continental shelf on its west flank. These two different rock types are mapped (sheet 10) on the seafloor based on 

their distinctive morphology and seismic character. Granitic seafloor outcrops are characterized by a massive 

hackly texture and are cut by dense networks of crosscutting fractures and small faults. In contrast, folded 

Neogene sedimentary rocks are characterized by a “ribbed” seafloor morphology which results from dipping 

beds (lithologies) having variable resistance to erosion. In the southern part of the map area offshore of Abbots 

Lagoon, the sedimentary rocks are less “ribbed,” owing to shallower dips and more uniform sandstone lithology 

(sheet 10). Seafloor underlain by granitic rocks typically slopes more than 1°, whereas seafloor underlain by 

sedimentary rocks slopes about 0.5°. 

On seismic-reflection data, granitic rocks are notably “reflection-free,” (see, for example, figs. 1, 2, 3, 5) 

whereas sedimentary rocks are characterized by continuous, parallel to subparallel, variable amplitude, highfrequency reflections (terminology after Mitchum and others, 1977). Within the sedimentary rocks, intrastratal 

low-angle unconformities and broad (as wide as about 1 km) channels are common (figs. 3, 4, 6). Lenses of 

chaotic reflections (figs. 3, 4) are inferred to represent smaller-scale channels and (locally) zones of prograding 

clinoforms. 

Surficial and shallow sediments in the Offshore of Tomales Point map area were deposited on the bedrock in 

the last about 21,000 years during the sea-level rise that followed the Last Glacial Maximum (LGM) and the last 

major lowstand (Lambeck and Chappell, 2001). Sea level was about 125 m lower during the LGM, at which time 

the map area was emergent and the shoreline was about 30 km west of its present location. Post-LGM sea-level 

rise was rapid (about 9 to 11 m per thousand years) until about 7,000 years ago, when it slowed considerably to 

about 1 m per thousand years (Stanford and others, 2011). Sea-level rise lead to broadening of the continental 

shelf, progressive eastward migration of the shoreline and wave-cut platform, and associated transgressive 

erosion and deposition. 

Sediments deposited during post-LGM sea-level rise (the rapid transgression and highstand after slowing at 

about 7 ka) are shaded blue in the high-resolution seismic-reflection profiles (figs. 1, 2, 3, 4, 6); their thickness is 

shown on Sheet 9 of this report (Maps B, D). The contact between underlying granitic and sedimentary rocks and 

post-LGM sediment is an abrupt transgressive surface of erosion commonly marked by minor channeling, and a 

common upward change to lower amplitude, more diffuse reflections.

 Faults in the offshore part of the map area are identified on seismic-reflection profiles on the basis of 

abrupt truncation or warping of reflections and (or) the juxtaposition of reflection panels that have differing 

seismic parameters, such as reflection presence, amplitude, frequency, geometry, continuity, and vertical 

sequence. Only the western part of the San Andreas Fault is imaged on one seismic-reflection profile in the map 

area (fig. 1); the profile shows a down-dropped sedimentary basin, bounded to the west by uplifted granitic 

rocks. Nearby, onland geologic studies along this section of the San Andreas Fault suggest a slip rate of 17 to 30 

mm/yr (Bryant and Lundberg, 2002; Grove and Niemi, 2005). 

Both the high-resolution bathymetry (sheets 1, 2) and the seismic-reflection data (sheet 8) reveal complex 

structure beneath the bedrock shelf west of Tomales Point. Numerous northwest-striking faults and folds cut both 

granitic and sedimentary rocks, and their depositional contact (figs. 1, 6; sheet 10). Folds axes trend west to 

northwest and folds have common wavelengths of 0.5 to 2 kilometers. The folding occurred during the late 

Pliocene or Pleistocene; it does not obviously involve post-LGM sediments, and the upper Pliocene to upper 

Miocene Purisima Formation is apparently the youngest folded unit (Clark and Brabb, 1997). Folding may be 

synchronous with activity on the northwest-trending Point Reyes Thrust Fault located about 16 km offshore of 

the map area (McCulloch, 1987; Heck and others, 1990).

All but one of the profiles displayed on this map sheet (figs. 1, 2, 3, 4, 6, 7) were collected in 2009 on U.S. 

Geological Survey (USGS) cruise S–8–09–NC using the SIG 2Mille minisparker system. This system used a 

500-J high-voltage electrical discharge fired 1 to 4 times per second, which, at normal survey speed of 4 to 4.5 

nautical miles per hour, gives a data trace every 0.5 to 2.0 meters. The data were digitally recorded in standard 

SEG-Y 32-bit floating-point format, using Triton Subbottom Logger (SBL) software that merges seismicreflection data with differential GPS-navigation data. After the survey, a short-window (20 ms) automatic gain 

control algorithm and a 160- to 1,200-Hz bandpass filter were applied to the minisparker data, followed by a 

heave correction that uses an automatic seafloor-detection window (averaged over 30 m of lateral distance 

covered). These data can resolve geologic features a few meters thick (hence considered “high-resolution”), 

down to subbottom depths of about 400 m.

Figure 5 shows a deep-penetration, migrated, multichannel seismic-reflection profile collected in 1976 by 

WesternGeco on cruise W–14–76–SF. This profile and other similar data were collected in many areas offshore 

of California in the 1970s and 1980s when these areas were considered a frontier for oil and gas exploration. 

Much of these data have been publicly released and are now archived at the USGS National Archive of Marine 

Seismic Surveys (U.S. Geological Survey, 2009). These data were acquired using a large-volume air-gun source 

that has a frequency range of 3 to 40 Hz and recorded using a multichannel hydrophone streamer about 2 km 

long; shot spacing was about 30 m. These data can resolve geologic features that are 20 to 30 m thick, down to 

subbottom depths of about 4 km.

REFERENCES CITED

Bryant, W.A., and Lundberg, M.M., compilers, 2002, Fault number 1b, San Andreas fault zone, North Coast 

section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, 

accessed 04/04/2013 at http://earthquakes.usgs.gov/hazards/qfaults.

Clark, J.C., and Brabb, E.E., 1997, Geology of Point Reyes National Seashore and vicinity, California—A digital 

database: U.S. Geological Survey Open-File Report 97–456, scale 1:48,000, available at 

http://pubs.usgs.gov/of/1997/of97-456/.

Grove, K., and Niemi, T.M., 2005, Late Quaternary deformation and slip rates in the northern San Andreas fault 

zone at Olema Valley, Marin County, California: Tectonophysics, v. 401, p. 231–250, doi:10.1016/

j.tecto.2005.03.014.

Heck, R.G., Edwards, E.B., Kronen, J.D., Jr., and Willingham, C.R., 1990, Petroleum potential of the offshore 

outer Santa Cruz and Bodega basins, California, in Garrison, R.E., Greene, H.G., Hicks, K.R., Weber, G.E., 

and Wright, T.L., eds. Geology and tectonics of the central California coastal region, San Francisco to 

Monterey: American Association of Petroleum Geologists, Pacific Section, Bulletin GB67, p. 143–164.

Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679–686, 

doi:10.1126/science.1059549.

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 Oceans—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.

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, 11 p. doi:10.1016/j.gloplacha.2010.11.002.

U.S. Geological Survey, 2009, National Archive of Marine Seismic Surveys: U.S. Geological Survey database, 

available at http://walrus.wr.usgs.gov/NAMSS/.

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

Seismic-Reflection Profiles, Offshore of Tomales Point Map Area, California

By

Samuel Y. Johnson, Ray W. Sliter, Stephen R. Hartwell, and John L. Chin 

2015

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

Figure 1. USGS high-resolution minisparker seismic-reflection 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); see trackline map for location. 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 California’s State Waters limit (yellow line on trackline map).

western part of San 

Andreas Fault Zone 

 seafloor outcrops of 

Cretaceous granitic rocks

Fig. 4

NORTHEAST

160

0 

Approximate depth, in meters

80

240

Two-way travel time, in seconds

0

0.1

0.2

SOUTHWEST

0.3 Figure 2. USGS high-resolution minisparker seismic-reflection profile PR–76 (collected in 2009 on survey S–8–09–NC) showing rugged bedrock of Cretaceous granitic rocks to northeast, and 

folded Neogene strata to southwest; see trackline map for location. Dashed red lines show faults. Pink symbols show fold axes (diverging arrows, anticlines; converging arrows, synclines). 

Blue shading shows very thin, inferred uppermost Pleistocene and Holocene strata, deposited since 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 

California’s State Waters limit (yellow line on trackline map).

1 km

VERTICAL EXAGGERATION ~ 12.5

 seafloor outcrops of 

Cretaceous granitic rocks

seafloor outcrops of 

folded Neogene strata

Fig. 4

0

0.1

0.2

SOUTHWEST

0.3

NORTHEAST

160

0 

Approximate depth, in meters

80

240

Figure 3. USGS high-resolution minisparker seismic-reflection profile PR–65 (collected in 2009 on survey S–8–09–NC) showing rugged Cretaceous granitic bedrock to northeast, and folded Neogene strata beneath gently dipping continental shelf 

to southwest; see trackline map for location. Dashed red lines show faults. Pink symbols show fold axes (diverging arrows, anticlines; converging arrows, synclines). Blue shading shows thin, inferred uppermost Pleistocene and Holocene strata, 

deposited since last sea-level lowstand about 21,000 years ago. Dashed green lines highlight some continuous reflections that illustrate structure (not distinctive stratigraphic markers). Blue dashed lines show intrastratal unconformities and (or) 

boundaries of zones of chaotic reflections. Dashed yellow line is seafloor multiple (echo of seafloor reflector). Dashed green lines highlight some continuous reflections that illustrate structure (not distinctive stratigraphic markers). Blue dashed 

lines show intrastratal unconformities and (or) boundaries of zones of chaotic reflections. Dashed yellow line is seafloor multiple (echo of seafloor reflector). Purple triangle shows location of the outer boundary of California’s State Waters.

1 km

VERTICAL EXAGGERATION ~ 12.5

Two-way travel time, in seconds

 seafloor outcrops of 

Cretaceous granitic rocks

lense of chaotic

 reflections

channel fill

Fig. 4

thickened zone of 

chaotic reflections

Two-way travel time, in seconds

0

0.1

0.2

NORTHWEST

0.3

SOUTHEAST

160

0 

Approximate depth, in meters

80

240

1 km

VERTICAL EXAGGERATION ~ 12.5

1 km

VERTICAL EXAGGERATION ~ 12.5

Fig. 6

Fig. 7

Fig. 1

Fig. 3

Fig. 2

Figure 4. USGS high-resolution minisparker seismic-reflection profiles PR–71 (left) and 71A (right), (collected in 2009 on survey S–8–09–NC) showing folded and faulted Neogene strata beneath continental shelf; see trackline map for location. Dashed red lines show faults. Pink symbols show fold 

axes (diverging arrows, anticlines; converging arrows, synclines). Blue shading shows thin, inferred uppermost Pleistocene and Holocene strata, deposited since last sea-level lowstand about 21,000 years ago. Dashed green lines highlight some continuous reflections that illustrate structure (not 

distinctive stratigraphic markers). Blue dashed lines show intrastratal unconformities and boundaries of a lens comprising south(?)-dipping clinoforms and chaotic reflections. Dashed yellow line is seafloor multiple (echo of seafloor reflector).

lense of south(?)-dipping

clinoforms and chaotic reflections

area of rough seafloor,

unit Qsr (sheet 10)

SOUTHWEST NORTHEAST

1 km

VERTICAL EXAGGERATION ~ 1.7

2

0 

Approximate depth, kilometers

1

3

Figure 5. Deep-penetration industry, 2-D, multichannel air-gun seismic-reflection profile WSF–102 (collected in 1976 on survey W–14–76–SF) from USGS National Archive of Marine Seismic Surveys (USGS, 2009); see trackline map for location. Note that vertical scale and 

exaggeration are significantly different than for high-resolution profiles shown in figures 1, 2, 3, 4, 6, and 7. Dashed yellow line shows fault with arrows indicating sense of offset. Pink symbols show fold axes (diverging arrows, anticlines; converging arrows, synclines) 

beneath continental shelf west of Tomales Point. Purple triangle shows location of California’s State Waters limit (yellow line on trackline map).

Fig. 4

1 km

VERTICAL EXAGGERATION ~ 12.5

seafloor outcrops of 

Cretaceous granitic rocks

Figure 6. USGS high-resolution minisparker seismic-reflection profile PR–66 (collected in 2009 on survey S–8–09–NC) extending from rugose outcrops of Cretaceous granitic rocks to gently dipping continental shelf underlain by folded Neogene rocks 

(see sheet 10); see trackline map for location. Dashed red lines show faults. Pink symbols show fold axes (diverging arrows, anticlines; converging arrows, synclines). Blue shading (northeast end of profile) shows thin, inferred uppermost Pleistocene 

and Holocene strata, deposited since 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 California’s State Waters limit (yellow line on trackline map). Fig. 4

NORTHEAST

160

0 

Approximate depth, in meters

80

240

Two-way travel time, in seconds

0

0.1

0.2

SOUTHWEST

0.3

data gap

Two-way travel time, in seconds

0

0.1

0.2

SOUTHWEST

0.3

NORTHEAST

160

0 

Approximate depth, in meters

80

240

1 km

VERTICAL EXAGGERATION ~ 12.5

Fig. 4

Figure 7. USGS high-resolution minisparker seismic-reflection profile PR–68 (collected in 2009 on survey S–8–09–NC) extending beneath continental shelf offshore of Tomales Point; see trackline map for location. Dashed red line shows fault. Pink symbols show fold axes (diverging arrows, anticlines; 

converging arrows, synclines) in inferred Neogene strata (see sheet 10). Dashed green lines highlight some continuous reflections that illustrate structure (not distinctive stratigraphic markers). Dashed blue lines show intrastratal unconformities. Dashed yellow line is seafloor multiple (echo of 

seafloor reflector).

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

GIS database and digital cartography by Peter 

Dartnell and Stephen R. Hartwell 

Manuscript approved for publication May 4, 2015

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

123°5' 123° 122°55'

123°5' 123° 122°55'

38°10'

38°15'

38°10'

38°15'

Tomales Bay

Tomales Point

Abbots

 Lagoon

Bodega

 Bay

Dillon 

Beach Trackline Map

EXPLANATION

Tracklines of select seismic-reflection 

surveys—Thick lines show locations 

of profile lines depicted in figures; 

thin lines show full extent of surveys

Survey S–8–09–NC (USGS)

Survey W–14–76–SF (Western)

3-nautical-mile limit of California’s State 

Waters

Area of “no data”—Areas near shoreline 

not mapped owing to insufficient 

high-resolution seafloor mapping 

data; areas beyond 3-nautical-mile 

limit of California’s State Waters were 

not mapped as part of California 

Seafloor Mapping Program

Figure 5

Figure 1

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 4

Fig. 5

Fig. 7

Fig. 6

Open-File Report 2015–1088

Sheet 8 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey