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Title: Geology: Offshore of San Gregorio, California, 2010

• Ross, Stephanie L.
• Dieter, Bryan E.
• Phillips, Eleyne L.
• Greene, H. G.
• Watt, Janet T.
• Endris, Charles A.
• Hartwell, Stephen R.
• Golden, Nadine E.
• Moss Landing Marine Laboratories
• Pacific Coastal and Marine Science Center

Dates

• Issued: 2013
• Coverage: 2013

Publishers

• Geological Survey (U.S.)

Summary

This poygon shapefile depicts geologic features within the offshore area of San Gregorio, California. The continental shelf within California's State waters in the San Gregorio map area is shallow (0 to ~55 m) and flat with a very gentle (less than 0.5 degrees) offshore dip. Shelf morphology and evolution result from the interplay between local tectonics and sedimentation as sea level rose about 125 to 130 m over the last ~ 21,000 years (Lambeck and Chappel, 2001). Shelf deposits are almost exclusively sand (unit Qms) at depths less than 60 m and transition to more fine grained, muddy sediment (unit Qmsf) at greater depths in the southwestern most part of the map area. The boundary between units Qms and Qmsf was determined based on seafloor sediment samples (Reid and others, 2006) and video observations (sheet 6) from the Offshore of San Gregorio and adjacent map area. This boundary likely shifts seaward or landward based on seasonal to decadal changes in sediment supply, sediment transport, and wave climate. More coarse-grained sands and gravels (units Qmss and Qmsc) are primarily recognized on the basis of high backscatter (sheet 3). Unit Qmsc occurs as a nearshore, shore-parallel bar at typical water depths between 5 and 10 meters. Unit Qmss forms erosional lags in rippled scour depressions (for example, Cacchione and others, 1984) at water depths of about 25 to 35 m, in contact with offshore bedrock uplifts and unit Qms. Although the general areas in which unit Qmsc and unit Qmss occur are not likely to change substantially, the boundaries of the unit(s) are likely ephemeral, changing seasonally and during significant storm events. Unit Qmss deposits are common along this stretch of the California coast where offshore sandy sediment can be relatively thin (thus unable to fill the depressions) due to both lack of river input and to significant sediment erosion and offshore sediment transport during large northwest winter swells. Areas where shelf sediments form thin (< 2.5 m or less) veneers over low relief, undivided Cretaceous and (or) Tertiary bedrock are mapped as units Qms/TKu and Qms/Tp. These areas are recognized based on the combination of flat relief, continuity with moderate to high relief bedrock outcrops, high-resolution seismic-reflection data (sheet 8), and in some cases moderate backscatter. These units are regarded as ephemeral and dynamic sediment layers that may or may not be present based on storms, seasonal/annual patterns of sediment movement, or climate cycles. Tertiary deposits mapped in the offshore include two units of the Purisima Formation (units Tp and Tpt). The Purisima units are characterized by high backscatter (sheet 3) and distinct bedding recognized in multibeam imagery and/or seismic-reflection data (sheet 8). These Tertiary rocks are underlain by or in fault contact with Upper Cretaceous basement rocks, including sedimentary rocks of the Pigeon Point Formation (unit Kpp). The Pigeon Point Formation is mapped on the basis of high backscatter, massive and (or) rugged texture on multibeam imagery (sheets 1, 2), and reflection-free character on seismic-reflection data (sheet 8). Offshore outcrops of the Pigeon Point Formation form the offshore Pigeon Point high, a major structural feature that extends ~30 km to the northwest and represents the northeast boundary of the Outer Santa Cruz Basin (McCulloch, 1987). Areas where bedrock is exposed on the seafloor but there is less certainty regarding age are mapped as Cretaceous and Tertiary, undivided (unit TKu). Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see sheets 1 and 2, SIM 3306). The bathymetry and backscatter data were collected between 2006 and 2010. A map that shows these data is published in Scientific Investigations Map 3306, "California State Waters Map Series--Offshore of San Gregorio, California." This layer is part of USGS Data Series 781. In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP) to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats and geology within the 3-nautical-mile limit of California's State Waters. CSMP has divided coastal California into 110 map blocks, each to be published individually as United States Geological Survey Open-File Reports (OFRs) or Scientific Investigations Maps (SIMs) at a scale of 1:24,000. Maps display seafloor morphology and character, identify potential marine benthic habitats and illustrate both the seafloor geology and shallow (to about 100 m) subsurface geology. Data layers for bathymetry, bathymetric contours, acoustic backscatter, seafloor character, potential benthic habitat and offshore geology were created for each map block, as well as regional-scale data layers for sediment thickness, depth to transition, transgressive contours, isopachs, predicted distributions of benthic macro-invertebrates and visual observations of benthic habitat from video cruises over the entire state. This coverage can be used to to aid in assessments and mitigation of geologic hazards in the coastal region and to provide sufficient geologic information for land-use and land-management decisions both onshore and offshore. These data are intended for science researchers, students, policy makers, and the general public. This information is not intended for navigational purposes.The data can be used with geographic information systems (GIS) software to display geologic and oceanographic information. U.S. Geological Survey. (2013). Geology: Offshore of San Gregorio, California, 2010. California State Waters Map Series Data Catalog: U.S. Geological Survey Data Series 781. Available at: http://purl.stanford.edu/dr534cf4984. Map political location: San Mateo County, California Compilation scale: 1:24,000 Base maps used are hillshades generated from IfSAR, LiDAR, and multibeam mapping both onshore and offshore (see sheet 2, SIM 3306, for more information). References Cited Cacchione, D.A., Drake, D.E., Grant, W.D., and Tate, G.B., 1984. Rippled scour depressions of the inner continental shelf off central California: Journal of Sedimentary Petrology, v 54, p. 1280-1291. Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679â686. 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 basinsâBeaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, v. 6, p. 353â401. Reid, J.A., Reid, J.M., Jenkins, C.J., Zimmerman, M., Williams, S.J., and Field, M.E., 2006, usSEABED: Pacific Coast (California Oregon, Washington) offshore surficial-sediment data release: U.S. Geological Survey Data Series 182, http://pubs.usgs.gov/ds/2006/182/. This layer is presented in the WGS84 coordinate system for web display purposes. Downloadable data are provided in native coordinate system or projection.

Subjects

• Geology
• Oceans
• California
• San Mateo County (Calif.)
• Pacific Ocean
• Geomorphology
• Sediments (Geology)
• Marine sediments
• Ocean bottom
• Geoscientific Information
• Datasets

Geospatial coordinates

• Bounding Box: BBOX (-122.492163, -122.401585, 37.391508, 37.229776)
• Geometry: BBOX (-122.492163, -122.401585, 37.391508, 37.229776)

Provider

Stanford

Rights

• Access rights: Public

Citation

Ross, Stephanie L., Dieter, Bryan E., Phillips, Eleyne L., Greene, H. G., Watt, Janet T., Endris, Charles A., Hartwell, Stephen R., Golden, Nadine E., Moss Landing Marine Laboratories, Pacific Coastal and Marine Science Center. Geology: Offshore of San Gregorio, California, 2010. Geological Survey (U.S.). Polygon data. https://purl.stanford.edu/dr534cf4984

Format

Shapefile

Languages

• English