Dataset Identification:
Resource Abstract:
- description: This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Half Moon Bay map area,
California. The vector data file is included in "Geology_OffshoreHalfMoonBay.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreHalfMoonBay/data_catalog_OffshoreHalfMoonBay.html.
The continental shelf within California's State waters in the Half Moon Bay area is shallow (0 to ~55 m) and flat with
a very gentle (less than 0.5 degrees) offshore dip. The morphology and geology of this shelf result from the interplay between
local tectonics, sea-level rise, sedimentary processes, and oceanography. Tectonic influences are related to local faulting
and uplift (see below). Sea level has risen about 125 to 130 m over the last ~21,000 years (for example, Lambeck and Chappel,
2001; Gornitz, 2009), leading to progressive eastward migration (a few tens of km) of the shoreline and wave-cut platform
and associated transgressive erosion and deposition (for example, Catuneanu, 2006). The Offshore of Half Moon Bay map area
is now an open-ocean shelf that is subjected to full, and sometimes severe, wave energy and strong currents. Given the relatively
shallow depths and high energy, modern shelf deposits are mostly sand (unit Qms). More coarse-grained sands and gravels (units
Qmss and Qmsc) are primarily recognized on the basis of bathymetry and high backscatter (Bathymetry; Backscatter A [8101];
and Backscatter B [7125]--Offshore Half Moon Bay, California, DS 781). Unit Qmsc occurs only as a nearshore bar (~ 10 m water
depth) just south of the Pillar Point Harbor jetty. Unit Qmss forms erosional lags in rippled scour depressions (see, for
example, Cacchione and others, 1984) and is more extensive and distributed, with the largest concentrations occurring at water
depths of 30 to 55 m offshore Pillar Point, and in the nearshore (depths of 5 to 15 m) south of Pillar Point Harbor and north-northwest
of Pillar Point. Such rippled-scour depressions 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 sediment supply from rivers and
to significant sediment erosion and offshore transport during large winter storms. Although the general areas in which both
unit Qmss scour depressions and unit Qmsc bars occur are not likely to change substantially, the boundaries of the unit(s)
are likely ephemeral, changing seasonally and during significant storm events. Areas where shelf sediments form thin (<
2.5 m or less) veneers over low relief Purisima Formation (upper Miocene and Pliocene) or undifferentiated Cretaceous and
(or) Tertiary bedrock are mapped as units Qms/Tp and Qms/TKu. These areas are recognized based on the combination of flat
relief, continuity with moderate to high relief bedrock outcrops, high-resolution seismic-reflection data (see field activity
S-15-10-NC), and in some cases moderate to high backscatter. These units are regarded as ephemeral and dynamic sediment layers
that may or may not be present at a specific location based on storms, seasonal/annual patterns of sediment movement, or longer-term
climate cycles. In a nearby similarly high-energy setting, Storlazzi and others (2011) have described seasonal burial and
exhumation of submerged bedrock in northern Monterey Bay. Offshore bedrock outcrops are mapped as the upper Miocene and Pliocene
Purisima Formation (unit Tp), the Cretaceous granitic rocks of Montara Mountain (unit Kgr), and undivided sedimentary rocks
of Cretaceous and (or) Tertiary age (unit TKu). These units are delineated through extending outcrops and trends from mapped
onshore geology and from their distinctive surface textures as revealed by high-resolution bathymetry (Bathymetry--Offshore
Half Moon Bay, California, DS 781). Purisima Formation outcrops form distinctive straight to curved "ribs," caused
by differential erosion of more- and less-resistant lithologies (for example, sandstone and mudstone). In contrast, granitic
rocks have a densely cross-fractured surface texture. 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 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. Catuneanu, O., 2006,
Principles of Sequence Stratigraphy: Amsterdam, Elsevier, 375 p. Gornitz, V., 2009, Sea level change, post-glacial, in Gornitz,
V., ed., Encyclopedia of Paleoclimatology and Ancient Environments: Encyclopedia of Earth Sciences Series. Springer, pp. 887-893.
Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679-686.; abstract:
This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Half Moon Bay map area, California.
The vector data file is included in "Geology_OffshoreHalfMoonBay.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreHalfMoonBay/data_catalog_OffshoreHalfMoonBay.html.
The continental shelf within California's State waters in the Half Moon Bay area is shallow (0 to ~55 m) and flat with
a very gentle (less than 0.5 degrees) offshore dip. The morphology and geology of this shelf result from the interplay between
local tectonics, sea-level rise, sedimentary processes, and oceanography. Tectonic influences are related to local faulting
and uplift (see below). Sea level has risen about 125 to 130 m over the last ~21,000 years (for example, Lambeck and Chappel,
2001; Gornitz, 2009), leading to progressive eastward migration (a few tens of km) of the shoreline and wave-cut platform
and associated transgressive erosion and deposition (for example, Catuneanu, 2006). The Offshore of Half Moon Bay map area
is now an open-ocean shelf that is subjected to full, and sometimes severe, wave energy and strong currents. Given the relatively
shallow depths and high energy, modern shelf deposits are mostly sand (unit Qms). More coarse-grained sands and gravels (units
Qmss and Qmsc) are primarily recognized on the basis of bathymetry and high backscatter (Bathymetry; Backscatter A [8101];
and Backscatter B [7125]--Offshore Half Moon Bay, California, DS 781). Unit Qmsc occurs only as a nearshore bar (~ 10 m water
depth) just south of the Pillar Point Harbor jetty. Unit Qmss forms erosional lags in rippled scour depressions (see, for
example, Cacchione and others, 1984) and is more extensive and distributed, with the largest concentrations occurring at water
depths of 30 to 55 m offshore Pillar Point, and in the nearshore (depths of 5 to 15 m) south of Pillar Point Harbor and north-northwest
of Pillar Point. Such rippled-scour depressions 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 sediment supply from rivers and
to significant sediment erosion and offshore transport during large winter storms. Although the general areas in which both
unit Qmss scour depressions and unit Qmsc bars occur are not likely to change substantially, the boundaries of the unit(s)
are likely ephemeral, changing seasonally and during significant storm events. Areas where shelf sediments form thin (<
2.5 m or less) veneers over low relief Purisima Formation (upper Miocene and Pliocene) or undifferentiated Cretaceous and
(or) Tertiary bedrock are mapped as units Qms/Tp and Qms/TKu. These areas are recognized based on the combination of flat
relief, continuity with moderate to high relief bedrock outcrops, high-resolution seismic-reflection data (see field activity
S-15-10-NC), and in some cases moderate to high backscatter. These units are regarded as ephemeral and dynamic sediment layers
that may or may not be present at a specific location based on storms, seasonal/annual patterns of sediment movement, or longer-term
climate cycles. In a nearby similarly high-energy setting, Storlazzi and others (2011) have described seasonal burial and
exhumation of submerged bedrock in northern Monterey Bay. Offshore bedrock outcrops are mapped as the upper Miocene and Pliocene
Purisima Formation (unit Tp), the Cretaceous granitic rocks of Montara Mountain (unit Kgr), and undivided sedimentary rocks
of Cretaceous and (or) Tertiary age (unit TKu). These units are delineated through extending outcrops and trends from mapped
onshore geology and from their distinctive surface textures as revealed by high-resolution bathymetry (Bathymetry--Offshore
Half Moon Bay, California, DS 781). Purisima Formation outcrops form distinctive straight to curved "ribs," caused
by differential erosion of more- and less-resistant lithologies (for example, sandstone and mudstone). In contrast, granitic
rocks have a densely cross-fractured surface texture. 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 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. Catuneanu, O., 2006,
Principles of Sequence Stratigraphy: Amsterdam, Elsevier, 375 p. Gornitz, V., 2009, Sea level change, post-glacial, in Gornitz,
V., ed., Encyclopedia of Paleoclimatology and Ancient Environments: Encyclopedia of Earth Sciences Series. Springer, pp. 887-893.
Lambeck, K., and Chappell, J., 2001, Sea level change through the last glacial cycle: Science, v. 292, p. 679-686.
Citation
- Title Geology and geomorphology--Offshore of Half Moon Bay Map Area, California.
-
- creation Date
2018-05-21T12:31:56.272199
Resource language:
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- Linkage for online resource
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- name Dublin Core references URL
- URL: https://pubs.usgs.gov/ds/781/
- protocol WWW:LINK-1.0-http--link
- 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
- link function information
- Description URL provided in Dublin Core references element.
Metadata data stamp:
2018-08-06T23:04:08Z
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 2018-08-06T23:04:08Z
Metadata contact
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pointOfContact
- organisation Name
CINERGI Metadata catalog
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- Address
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- electronic Mail Address cinergi@sdsc.edu
Metadata language
eng
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Metadata standard for this record:
ISO 19139 Geographic Information - Metadata - Implementation Specification
standard version:
2007
Metadata record identifier:
urn:dciso:metadataabout:6da6cb51-fdca-4b61-92bc-071f11228808
Metadata record format is ISO19139 XML (MD_Metadata)