Sedimentary Processes on Continental Shelf Off Southern California: ABSTRACT

Abstract

Depositional processes characterizing San Pedro continental shelf have been extensively investigated by the University of Southern California (USC) and the U.S. Geological Survey (USGS). Data collected during 1973-76 by USC document the sedimentologic framework of San Pedro shelf. Textural patterns can be explained as responses to long-term oceanic processes; however, no unique transport directions can be inferred from the data. Almost all bed forms observed have been small-scale, wave-generated ripples, the distribution of which reflects increasing biogenic activity with decreasing energy levels seaward. Owing to biological reworking, no stratigraphic evidence of these ripples was preserved in cores. Mesoscale current lineations resolved on sonographs indicate sediment transport transverse to isobaths. Forty-day records taken in April to June 1978, with two USGS Geoprobe tripods and moored transmissometers, show several significant features: (1) near-bottom transport is seaward with mean speeds of about 1 cm/s in shallow water (22 m) and 3 cm/s near the shelf break (68 m); (2) although mean speeds are low, instantaneous currents caused by tides and surface waves exceed threshold velocities at the shallow site; and (3) currents from internal waves can increase ambient nearbottom energy levels sufficiently to entrain sediment. Integration of data from USC and USGS investigations provides information necessary to understand the fair-weather dynamic processes that affect sediment transport and deposits on a narrow southern California shelf. End_of_Article - Last_Page 731------------