Geology and geomorphology--Offshore of San Francisco Map Area, California

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What does this data set describe?

Geology and geomorphology--Offshore of San Francisco Map Area, California
This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of San Francisco map area, California. The polygon shapefile is included in "," which is accessible from <>.

The Offshore of San Francisco map area includes the Golden Gate inlet which connects the Pacific Ocean and San Francisco Bay. San Francisco Bay, the largest estuary on the U.S. west coast, is located at the mouth of the Sacramento and San Joaquin rivers and drains over 40 percent of the state of California. The large surface area of the bay and diurnal tidal range of 1.78 m creates an enormous tidal prism (about 2 billion cu m) and strong tidal currents, commonly exceeding 2.5 m/s (Barnard and others, 2006a, 2006b, 2007). Acceleration of these currents through the constricted inlet has led to scouring of a bedrock channel that has a maximum depth of 113 m. Large fields of sand waves (Barnard and others, 2007) (unit Qmsw) have formed both west and east of this channel as flow expands and tidal currents decelerate. Active tidally influenced map units inside San Francisco Bay also include sand-dominated deposits (unit Qbs) and more coarse-grained sand, gravel, and pebble deposits (unit Qbsc). Sand wave fields resulting from tidal flow are also present in the nearshore along the Pacific Coast, both north and south of the Golden Gate inlet. The sand wave fields appear to be variably mobilized by both ebb and flood tides, but the presence of a large (~150 sq km) ebb-tidal delta at the mouth of the bay west of the inlet indicates net sediment transport has been to the west. The ebb-tidal delta west of the Golden Gate inlet is mapped as two units. The inner part of the delta (unit Qmst) comprises a semi-circular, inward-sloping (i.e., toward the Golden Gate inlet), sandy seafloor at water depths of about 12 to 24 m. This inner delta has a notably smooth surface, indicating sediment transport and deposition under different flow regimes (defined by tidal current strength and depth) than those in which the sand waves formed and are maintained.

Further deceleration of tidal currents beyond the inner delta has led to development of a large, shoaling (about 8 to 12 m water depth), horse-shoe shaped, delta-mouth bar (unit Qmsb). This feature (the "San Francisco Bar") surrounds the inner delta, and its central crest is cut by a dredged shipping channel that separates the nothern and southern parts of the bar, the "North Bar" and "South Bar," respectively. The San Francisco Bar is shaped by both tidal currents and waves, which regularly exceed 6 m in height on the continental shelf during major winter storms (Barnard and others, 2007). This mix of tidal and wave influence results in a variably hummocky, mottled, and rilled seafloor, and this surface texture is used as a primary criteria for mapping the unit and defining its boundaries. Outside the San Francisco Bar to the limits of the map area, the notably flat shelf (less than 0.2 degrees) and the nearshore are wave-dominated and characterized by sandy marine sediment (unit Qms). Local zones of wave-winnowed (?) coarser sediment (unit Qmsc) indicated by high backscatter occur along the coast offshore Ocean Beach. Unit Qmsc is also mapped inside and at the mouth of the Golden Gate inlet where it presumably results from winnowing by strong tidal currents. Coarser sediment also occurs as winnowed lags in rippled scour depressions (unit Qmss), recognized on the basis of high-resolution bathymetry and backscatter. These depressions are typically a few tens of centimeters deep and are bounded by mobile sand sheets (for example, Cacchione and others, 1984). This unit occurs primarily in the nearshore south of the Golden Gate inlet offshore of Ocean Beach (water depth less than 13 m) and north of the inlet offshore Muir Beach (water depth less than 17 m).

Artificial seafloor (unit af) has several distinct map occurrences: (1) sites of active sand mining inside San Francisco Bay; (2) the dredged shipping channel at the central crest of the San Francisco Bar; (3) the sewage outfall pipe, associated rip rap, and surrounding scour channel offshore Ocean Beach; and (4) the location of a former waste disposal site about 2.5 km offshore Point Lobos.

Although the map shows the areas in which several active sedimentary units (Qmsw, Qmst, Qmsb, Qms, Qmsc, Qmss, Qbsm, Qbsc) presently occur, it is important to note that map units and contacts are dynamic and ephemeral, likely to change during large storms, and on seasonal to decadal scales based on changing external forces such as weather, climate, sea level, and sediment supply. Dallas and Barnard (2011) have noted, for example, that the ebb-tidal delta has dramatically shrunk since 1873 when the first bathymetric survey of the area was undertaken. They document an approximate 1 km landward migration of the crest of the San Francisco Bar, which they attribute to a reduction in the tidal prism of San Francisco Bay and a decrease in coastal sediment.

Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data. The bathymetry and backscatter data were collected between 2006 and 2010.

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.

Barnard, P.L., Hanes, D.M., Kvitek, R.G., and Iampietro, P.J., 2006a, Sand waves at the mouth of San Francisco Bay, California: U.S. Geological Survey Scientific Investigations Map 2944, 5 sheets.

Barnard, P.L., Hanes, D.M., Rubin, D.M., and Kvitek, R.G., 2006b, Giant sand waves at the mouth of San Francisco Bay: EOS, V. 87, p. 285, 289.

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.

Dallas, K.L., and Barnard, P.L., 2011, Anthropogenic influences on shoreline and nearshore evolution in the San Francisco coastal system: Estuarine Coastal and Shelf Science, v. 92, p. 195-204.


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 Bathymetry Hillshade--Offshore San Francisco, California, DS 781, for more information).
  1. How should this data set be cited?

    Greene, H.G., Johnson, S.Y., Manson, M.W., Hartwell, S.R., Endris, C.A., and Bruns, T.R., 2014, Geology and geomorphology--Offshore of San Francisco Map Area, California:.

    This is part of the following larger work.

    Cochrane, Guy R., Johnson, Samuel Y., Dartnell, Peter, Greene, H. Gary, Erdey, Mercedes D., Golden, Nadine E., Hartwell, Stephen R., Endris, Charles A., Mansion, Michael W., Sliter, Ray W., Kvitek, Rikk G., Watt, Janet T., Ross, Stephanie L., and Bruns, Terry R., 2015, California State Waters Map Series--Offshore of San Francisco Map Area, California: Open-File Report OFR 2015-1068, U.S. Geological Survey, Reston, VA.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -122.62
    East_Bounding_Coordinate: -122.42
    North_Bounding_Coordinate: 37.86
    South_Bounding_Coordinate: 37.70

  3. What does it look like?

    <> (JPEG)
    Geology offshore San Francisco.

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 2006
    Ending_Date: 2010
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: vector digital data

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Vector data set. It contains the following vector data types (SDTS terminology):

      • GT-polygon composed of chains (594)

    2. What coordinate system is used to represent geographic features?

      The map projection used is WGS 1984 UTM Zone 10N.

      Projection parameters:
      Scale_Factor_at_Central_Meridian: 0.9996
      Longitude_of_Central_Meridian: -123.0
      Latitude_of_Projection_Origin: 0.0
      False_Easting: 500000.0
      False_Northing: 0.0

      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest 0.0001
      Ordinates (y-coordinates) are specified to the nearest 0.0001
      Planar coordinates are specified in Meter

      The horizontal datum used is D WGS 1984.
      The ellipsoid used is WGS 1984.
      The semi-major axis of the ellipsoid used is 6378137.0.
      The flattening of the ellipsoid used is 1/298.257223563.

  7. How does the data set describe geographic features?

    Polygons representing geologic / geomorphic map units (Source: This report)

    Internal feature number. (Source: ESRI)

    Sequential unique whole numbers that are automatically generated.

    Feature geometry. (Source: ESRI)

    Coordinates defining the features.

    Map Unit abbreviation (Source: This report)

    afArtificial fill and anthropogenic material
    QmsMarine nearshore and shelf deposits
    QmssMarine shelf scour depressions
    QmscCoarse-grained marine nearshore and shelf deposits
    QmswMarine sediment wave deposits
    QmstMarine ebb-tidal channel deposits
    QmsbMarine ebb-tidal bar deposits
    QbsSan Francisco Bay deposits
    QbscSan Francisco Bay deposits, coarse grained
    KJfFranciscan Complex, undivided

    short description of map unit (Source: This report)

    text description of map unit

    Length of feature in internal units. (Source: ESRI)

    Positive real numbers that are automatically generated.

    Area of feature in internal units squared. (Source: ESRI)

    Positive real numbers that are automatically generated.

    Representation rule identifier (Source: This report)

    This field contains the representation rule in the ArcGIS file geodatabase which applies a solid color fill of a specified CMYK value to each polygon. Representation rules have the same name as the map unit abbreviation.

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    USGS Pacific Coastal and Marine Science Center
    Attn: Stephen Hartwell
    400 Natural Bridges Drive
    Santa Cruz, CA 95060

    (831) 460-7814 (voice)
    (831) 427-4748 (FAX)

Why was the data set created?

To expand geologic mapping to the seafloor within the California's State Waters, to update coastal geologic mapping, and to contribute to a uniform regional geologic database, which can be used geographic information systems. Additionally, to provide a geologic map for the public and geoscience community to aid in assessments and mitigation of geologic hazards in the San Gregorio coastal region and to provide sufficient geologic information for land-use and land-management decisions both onshore and offshore. This information is not intended for navigational purposes.

How was the data set created?

  1. From what previous works were the data drawn?

    DS 781 (source 1 of 3)
    Dartnell, Peter, Kvitek, Rikk G., and Bretz, Carrie K., 2014, Bathymetry--Offshore of San Francisco Map Area, California: Data Series DS 781, U.S. Geological Survey, Reston, VA.

    Online Links:

    See metadata for bathymetry ("Bathymetry_OffshoreSanFrancisco_metadata.txt") in DS 781 for source data and postprocessing/reprocessing information.
    Type_of_Source_Media: digital file of gridded bathymetry data (ArcInfo GRID)
    Source_Contribution: Gridded bathymetry data (2-meter resolution).

    DS 781 (source 2 of 3)
    Dartnell, Peter, Erdey, Mercedes D., Kvitek, Rikk G., and Bretz, Carrie K., 2014, Bathymetry--Offshore of San Francisco Map Area, California: Scientific Investigations Map DS 781, U.S. Geological Survey, Reston, VA.

    Online Links:

    See metadata for backscatter data ("BackscatterA_8101_2004--Offshore San Francisco_metadata.txt", "BackscatterB_8101_2007--Offshore San Francisco_metadata.txt", "BackscatterC_8101_2008--Offshore San Francisco_metadata.txt", and "BackscatterD_7125_2006--Offshore San Francisco_metadata.txt") in DS 781 for amplitude source data and postprocessing/reprocessing information.
    Type_of_Source_Media: digital file of gridded amplitude data (ArcInfo GRID)
    Source_Contribution: Gridded amplitude data (2-meter resolution).

    S-15-10-NC (source 3 of 3)
    U.S. Geological Survey (USGS), Coastal and Marine Geology Program (CMGP), 2013, Seismic-reflection data acquisition data of field activity S-15-10-NC in offshore Pescadero from 08/02/2010 to 08/04/2010: U.S. Geological Survey (USGS), Coastal and Marine Geology (CMG), Menlo Park, CA.

    Online Links:

    Type_of_Source_Media: ASCII lat/long shot point files
    Digital seismic data used to interpret subsurface geologic structure

  2. How were the data generated, processed, and modified?

    Date: 2012 (process 1 of 2)
    Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data. Derivatives such as slope and curvature were generated from source rasters. Interpreted rasters include amplitude, hillshaded bathymetry (using various illumination angles and vertical exaggeration), slope, and curvature. Curvature was decomposed into profile and plan curvature for analysis purposes.

    Date: 2011 (process 2 of 2)
    The mapped area was extended to the shoreline by using digital orthophotos to interpret the region between the inner edge of the multibeam bathymetry and the approximate shoreline. The approximate shoreline was generated at the NAVD88 +1.46 m contour, defined as the operational MHW shoreline by Weber and others (2005). The resulting boundary was transformed to WGS 84 UTM Zone 10 North in ArcGIS 10 using the NAD83 to WGS84 (ITRF00) transformation algorithm. This boundary was then used to extend and trim both onshore and offshore geology in the print and PDF product. The transformed boundary is contained within the WGS84 "contours" feature class and identified as a water boundary in the associated representation rules.

    Only data for offshore map units are released digitally in this publication. For onshore geology see Bonilla (1961, 1998), Knudsen and others (1997), Brabb and others, (1998), Blake and others (2000), and Witter and others (2006).

    References Cited:

    Blake, M.C., Jr., Graymer, R.W., and Jones, D.L., 2000, Geologic map and map database of parts of Marin, San Francisco, Alameda, Contra Costa, and Sonoma counties, California: U.S. Geological Survey Miscellaneous Field Studies Map 2337, scale 1:62,500.

    Bonilla, M.G., 1961, City College fault, San Francisco, California: U.S. Geological Survey Professional Paper 424-C, pp. 190-192.

    Bonilla, M.G., 1998, Preliminary geologic map of the San Francisco South 7.5-minute quadrangle and part of the Hunters Point 7.5-minute quadrangle, California: a digital database: U.S. Geological Survey Open-File Report 98-354; scale 1:24,000.

    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.

    Knudsen, K.L., Noller, J.S., Sowers, J.M., Lettis, W.R., Graham, S.E., Randolph, C.E., and May, T.E., 1997, Quaternary geology and liquefaction susceptibility, San Francisco, California 1:100,000 quadrangle: a digital database: U.S. Geological Survey Open-File Report 97-715, scale 1:100,000.

    Weber, K.M., List, J.H., and Morgan, K.L., 2005, An operational Mean High Water datum for determination of shoreline position from topographic lidar data: U.S. Geological Survey Open-File Report 2005-1027, available at <>.

    Witter, R.C., Knudsen, K.L., Sowers, J.M., Wentworth, C.M., Koehler, R.D., Randolph, C.E., Brooks, S.K., and Gans, K.D., 2006, Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California, U.S. Geological Survey Open-File Report 06-1037, scale 1:24,000.

  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

    Polygons were primarily mapped by one of the following methods: (1) interpretation of 2-meter-resolution hillshaded bathymetry data from bathymetric lidar and sonar surveys (see Bathymetry Hillshade--Offshore San Francisco, California, DS 781, for more information); (2) interpretation of 2-meter-resolution amplitude (backscatter) data from bathymetric sonar surveys (see BackscatterA_8101_2004; BackscatterB_8101_2007; BackscatterC_8101_2008; and BackscatterD_7125_2006--Offshore San Francisco, California, DS 781, for more information); (3) interpretation of 2-meter interpretation of seismic-reflection-profile data (see field activity S-15-10-NC).
    Map Unit contact locations were interpreted typically at a scale of between 1:1,000 and 1:2,000 using the above base data. Bathymetric sonar and LiDAR data have a horizontal accuracy greater than the resolution of the base data.
    Map unit contacts were digitized by heads-up screen digitization of line data on 2-meter-resolution DEMs described above. Horizontal accuracy is estimated to be between 2 and 5 meters depending on how clearly contacts can be resolved.
    Most digitized positions on the map are estimated to have better than 5 m horizontal accuracy. There is no elevation data in the database.

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    Data are complete: no offshore features that could be accurately identified and represented at the compilation scale of 1:24,000 were eliminated or generalized. The smallest area represented is approximately 100 square meters. All geospatial database elements are attributed.

  5. How consistent are the relationships among the observations, including topology?

    Map elements were visually checked for overshoots, undershoots, duplicate features, polygon closure, and other errors by the lead authors and by the GIS technician(s) who created the digital database. Review drafts of the map were reviewed internally by at least two other geologists for consistency with basic geologic principles and general conformity to USGS mapping standards.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

If physical samples or materials are available, constraints on their on-site access are described in "WR CMG Sample Distribution Policy" at URL: <>
This information is not intended for navigational purposes.
Read and fully comprehend the metadata prior to data use. Uses of these data should not violate the spatial resolution of the data. Where these data are used in combination with other data of different resolution, the resolution of the combined output will be limited by the lowest resolution of all the data.
Acknowledge the U.S. Geological Survey in products derived from these data. Share data products developed using these data with the U.S. Geological Survey.
This database has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use.
Although this Federal Geographic Data Committee-compliant metadata file is intended to document these data in nonproprietary form, as well as in ArcInfo format, this metadata file may include some ArcInfo-specific terminology.

Who wrote the metadata?

Last modified: 2013
Last Reviewed: 02-Sep-2014
Metadata author:
U.S. Geological Survey, Coastal and Marine Geology Program
Attn: Stephen R. Hartwell
400 Natural Bridges Drive
Santa Cruz, CA 95060-5792

831-460-7814 (voice)
831-427-4748 (FAX)

Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

Generated by mp version 2.9.16 on Mon May 18 16:09:09 2015