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Layer: Geology_OffshoreSaltPoint (ID: 3)

Name: Geology_OffshoreSaltPoint

Display Field: MapUnitAbb

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Description: This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Salt Point map area, California. The vector data file is included in "Geology_OffshoreSaltPoint.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSaltPoint/data_catalog_OffshoreSaltPoint.html. The morphology and the geology of the Offshore of Salt Point map area result from the interplay between local sea-level rise, sedimentary processes, oceanography, and tectonics. The offshore part of the map area extends from the shoreline to water depths of about 90 to 100 m on the mid-continental shelf; the shelfbreak occurs about 20 km farther offshore at water depths of about 200 m. The nearshore and inner shelf (to water depths of about 50 to 60 m) typically dips seaward about 1.0 to 1.5 degrees; the mid to outer shelf dips more gently, generally less than 0.5 degrees. Sea level has risen about 125 to 130 m over about the last 21,000 years, leading to broadening of the continental shelf, progressive eastward migration of the shoreline and wave-cut platform, and associated transgressive erosion and deposition. Land-derived sediment was carried into this dynamic setting, then subjected to full Pacific Ocean wave energy and strong currents before deposition or offshore transport. Tectonic influences impacting shelf morphology and geology are related to local faulting, folding, uplift, and subsidence. Bedrock of the Eocene and Paleocene German Rancho Formation (unit Tgr) underlies much of the inner shelf, extending to water depths of as much as 60 m. Although onshore coastal outcrops of this unit are well bedded, seafloor outcrops imaged on high-resolution bathymetry have a hackly surface texture and abundant fractures. Embayments in the outer margin of the seafloor bedrock outcrops are commonly paired with the mouths of coastal watersheds and are inferred to have formed by fluvial erosion during the last sealevel lowstand. One of the more prominent embayments occurs about one kilometer north of Salt Point at the mouth of Miller Creek. These coastal watersheds are relatively small and steep, extending to a drainage divide just 2 to 3 km east of the shoreline, and are inferred sources of coarse-grained sediments. Immediately east of this onshore topographic divide, drainage along this part of the coast is captured by the northwest-flowing South Fork of the Gualala River, which runs parallel to the coast along the trace of the San Andreas fault. Given relatively shallow water depths (0 to about 50 m) and exposure to high wave energy, modern nearshore to mid-shelf sediments are mostly sand (unit Qms) and a mix of sand, gravel, and cobbles (units Qmsc and Qmsd). The more coarse-grained sands and gravels (units Qmsc and Qmsd) are primarily recognized on the basis of bathymetry and high backscatter. Both Qmsc and Qmsd typically have abrupt landward contacts with bedrock (unit Tgr) and form irregular to lenticular exposures that are commonly elongate in the shore-normal direction. Contacts between units Qmsc and Qms are typically gradational. Unit Qmsd forms erosional lags in scoured depressions that are bounded by relatively sharp and less commonly diffuse contacts with unit Qms horizontal sand sheets. These depressions are typically a few tens of centimeters deep and range in size from a few 10's of sq m to more than one sq km. Similar unit Qmsd scour depressions are common along this stretch of the California coast where surficial offshore sandy sediment is relatively thin (thus unable to fill the depressions) due to both lack of sediment supply and to erosion and transport of sediment during large northwest winter swells. Such features have been referred to as "rippled-scour depressions" or "sorted bedforms". Although the general areas in which both unit Qmsd scour depressions and surrounding mobile sand sheets occur are not likely to change substantially, the boundaries of the individual Qmsd depressions are likely ephemeral, changing seasonally and during significant storm events. The offshore decrease in slope at mid-shelf water depths (about 60 m) approximately coincides with a transition to more fine-grained marine sediments (unit Qmsf), which extends to the outer (3-nautical-mile) limit of California's State Waters. Unit Qmsf consists primarily of mud and muddy sand and is commonly extensively bioturbated. These fine-grained sediments are inferred to have been derived from from the Russian River, which has its mouth about 15 km south of the map area. Both Drake and Cacchione (1985) and Sherwood and others (1994) have documented seasonal, mid-shelf, northwest-directed, bottom currens capable of transporting fine-grained, suspended sediment from the Russian River to the Offshore of Salt Point map area. Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see Bathymetry--Offshore Salt Point, California and Backscattter--Offshore Salt Point, California, DS 781, for more information). The bathymetry and backscatter data were collected between 2006 and 2010.

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