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Layer: Geology_OffshoreHalfMoonBay (ID: 3)

Name: Geology_OffshoreHalfMoonBay

Display Field: MapUnitAbb

Type: Feature Layer

Geometry Type: esriGeometryPolygon

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.

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