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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Bottom
Currents And Sediment Transport on San Pedro Shelf, California
David E. Drake, David A. Cacchione, Herman A. Karl
ABSTRACT
GEOPROBE (Geological Processes Bottom
Environmental) tripods were used to measure
bottom
currents, pressure, and light transmission and scattering and to obtain time-series photographs of the sea floor at depths of 23 m and 67 m on San Pedro shelf between 18 April and 6 June 1978.
Winds were light (< 5 m/s) with a mean direction from the southwest throughout the measurement period. Hourly averaged currents 1 m above the bottom
never exceeded 21 cm/s; average speeds were about 5 cm/s at the 23-m site and 6.8 cm/s at 67 m, and the strongest currents were produced by the tides. The mean flow of
bottom
water was less than 3 cm/s at both GEOPROBES and was rather persistently southward (offshelf). Wave-generated
bottom
currents and
bottom
-pressure variations were sampled at hourly intervals; average wave period and wave height were 12.8 s and 0.44 m, respectively, at the 23-m site. Wave orbital velocities ranged from about 5 to 30 cm/s at 23 m and from 2 to 8 cm/s at 67 m.
Bottom
photographs at 67 m show that the relatively sluggish tide-generated and mean currents were below threshold velocity for the silty, very fine sand throughout the observational period. Threshold depth for wave rippling of very fine sand averaged about 28 m with a range from about 12 m to 50 m. Wave-generated currents were the only currents that exceeded threshold levels. The wave currents maintained relatively high concentrations of sediment in suspension near the
bottom
over the inner shelf (< 25 m), and this material (principally silt and clay) was transported offshore by the weak mean flow. Approximately 50% of this material was deposited as the
bottom
orbital velocities decreased to subthreshold values (
10-15 cm/s). The o
served movement of fine sediment across the inner shelf can account for a portion of the mud content of the modern silty sands on the central shelf and on the outer shelf. However, it is clear that the sand fractions, which constitute greater than 70% of the central shelf substrate, must be transported during high-energy winter storms.
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