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ABSTRACT: Distribution and Control of Bed-Forms on the Continental Shelf, Gulf of the Farallones, California

H. A. Karl, J. L. Chin, D. M. Rubin, W. C. Schwab, D. C. Twichell

A side-scan mosaic was constructed from an 800 km2 bed-form field located in the Gulf of the Farallones on the central shelf between Point Reyes and the Golden Gate. Sediment samples were collected at 1-km grid intervals within a small area (130 km2) of the field. Large-scale bed-formed, broad, shallow (<= 2 m relief) depressions, floored by medium and coarse sand, are arranged regionally in a digitate pattern. Where fully developed, the palms of these features are as wide as 2 km with northerly striking finger-like projections as long as 8 km that narrow to 150 m. Small-scale bed forms, ripples that have a wavelength of about 1 m and that strike north-northeast, cover the digitate bedforms. No ripples were resolved with the 100 and 120 kHz side-s an systems on the intervening areas of fine and very fine sand. The digitate bed forms are most distinct in the southwestern part of the field where there is an abrupt transition between the coarse sediment of the depressions and the nonrippled areas of finer grained material. Digitate bed forms gradually disappear toward the southeast where they are partly covered with a veneer of finer sediment and the boundaries between depressions and intervening areas become less distinct. Bathymetry exercises major control on the characteristics of the bed-form field by influencing water circulation. The sharp arcuate northwestern boundary of the field mimics the arcuate 80-m isobath, which is perpendicular to the shelf break. Water depths gradually shoal to 50 m toward the southeast and the isobat s translate into shelf-parallel bands. The area of translation coincides with the diffusion of the field boundaries and gradual disappearance of the digitate bed forms. The bed forms are erosional features that reflect the intensity of water flow over the area.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990