Dataset Identification:
Resource Abstract:
- description: 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 "Geology_OffshoreSanFrancisco.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSanFrancisco/data_catalog_OffshoreSanFrancisco.html.
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.;
abstract: 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 "Geology_OffshoreSanFrancisco.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSanFrancisco/data_catalog_OffshoreSanFrancisco.html.
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.
Citation
- Title Geology and geomorphology--Offshore of San Francisco Map Area, California.
-
- creation Date
2018-05-20T04:25:30.251102
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- name Dublin Core references URL
- URL: https://pubs.usgs.gov/ds/781/
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- link function information
- Description URL provided in Dublin Core references element.
Linkage for online resource
- name Dublin Core references URL
- URL: http://pubs.usgs.gov/ds/781/
- protocol WWW:LINK-1.0-http--link
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- Description URL provided in Dublin Core references element.
Metadata data stamp:
2018-08-06T23:13:34Z
Resource Maintenance Information
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- notes: This metadata record was generated by an xslt transformation from a dc metadata record; Transform by Stephen M. Richard, based
on a transform by Damian Ulbricht. Run on 2018-08-06T23:13:34Z
Metadata contact
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pointOfContact
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CINERGI Metadata catalog
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2007
Metadata record identifier:
urn:dciso:metadataabout:1d6d4ac5-78b8-495b-8b40-7044b8715f11
Metadata record format is ISO19139 XML (MD_Metadata)