Abstract
Offshore sand bodies are common features found on continental shelves today. Proposed origins for offshore sand bodies include drowned sandspits and deltas, tidal bedforms, hyperpycnal currents, and subaqueous clinoforms. Each model of formation has different implications for interpreting past sea levels, sedimentary processes operating on the shelf, and in some cases rates of tectonic motion. On the central California shelf, near Estero Bay, a SE striking offshore sand body, informally known as the Cross-Hosgri slope (CHS) is cross-cut by the Hosgri fault. Twenty-four vibracores and 20 km of seismic data were collected across and around the feature to determine its age and origin. Within the seismic profiles the CHS displays a progradational character with sigmoidal geometries in cross section view. Three facies were identified within the cores sampling the CHS. The lowest is a gastropod sand facies composed by fine sand and abundant gastropod shells. The second facies is an organic rich sand facies, thinning seaward along the CHS; both the gastropod and organic rich sand facies eventually pinch out. These facies are interpreted to represent the main body of the prograding feature. Overlying the two lower facies is a blanket of sediment interpreted to represent ambient sedimentation on the modern continental shelf, post-dating the time during which the main sediment body of the CHS was active. Two possible origins for the CHS are considered: a drowned spit and a subaqueous clinoform. Radiometric dating of articulated shells and plant fragments returned ages for the main CHS sand body spanning a period between 7,280 cal years B.P. and 1,279 cal years B.P. Based on these ages with a comparison to a local relative sea-level curve, the feature appears to have formed in water depths greater than 50 m, which, along with its relatively fine grain-size (e.g. 80-90 microns), renders the hypothesis of a drowned spit unlikely. For these reasons, we favor the subaqueous clinoform hypothesis as a mode of formation of the CHS. Our study of the CHS provides insight on the processes governing sediment transport on the California shelf.