This polygon shapefile contains geologic features within the offshore area of Coal Oil Point, California. The offshore part of the Offshore of Coal Oil Point map area largely consists of a gently offshore-dipping (less than 1 degree) shelf under
This polygon shapefile contains geologic features within the offshore area of Coal Oil Point, California. The offshore part of the Offshore of Coal Oil Point map area largely consists of a gently offshore-dipping (less than 1 degree) shelf underlain by sediments derived primarily from relatively small coastal watersheds that drain the Santa Ynez Mountains. Shelf deposits are primarily sand (Qms) at depths less than about 35 to 50 m, and they are finer grained sediment such as very fine sand, silt, and clay (Qmsf) from depths of 35 to 50 m southward to the shelf break at a depth of about 90 m. The boundary between units Qms and Qmsf is based on observations and extrapolation from sediment sampling (see, for example, Reid and others, 2006) and camera ground-truth surveying. It is important to note that the boundary between units Qms and Qmsf should be considered transitional and approximate and is expected to shift as a result of seasonal- to annual- to decadal-scale cycles in wave climate, sediment supply, and sediment transport. Fine-grained deposits that are similar to unit Qmsf also are mapped at water depths greater than 90 m, below the shelf break on the upper slope; however, here they are identified as a separate unit (unit Qmsl) because of their location below the distinct shelf-slope geomorphologic break. Coarser grained, marine deposits (coarse sand to boulders) of units Qmsc, Qmscl, and Qsc are recognized on the basis of their high acoustic backscatter, their ground-truth-survey imagery, and, in some cases, their moderate seafloor relief. This coarse-grained facies is linked either to the mouths of steep coastal watersheds or to adjacent seafloor bedrock outcrops, and the deposits generally represent wave-winnowed lags of deltaic sediment. Two distinct lobes of coarse-grained sediment (unit Qmscl), present in deeper water (about 50 m) near the west edge of the map area, may similarly represent winnowed deltaic deposits that formed at lower sea levels during the latest Pleistocene or early Holocene. An isolated patch of clast-supported cobbles (unit Qsc), which rests on bedrock south of Coal Oil Point at a water depth of 70 m, also may have been deposited at lower sea levels during the late Pleistocene. Offshore bedrock exposures are mapped as either the Miocene Monterey Formation (Tm, Tmu, Tmm), the late Miocene and early Pliocene Sis