Depositional Characteristics of Middle to Upper Miocene Point Fermin Submarine Fan, Palos Verdes, California

Perry W. Russell

The Point Fermin submarine fan lies within the upper Monterey Formation of the Palos Verdes Peninsula, California. Exposures of sediment gravity flow deposits extend from the upper portion of the tuffaceous lithofacies of the Altamira shale member to the middle of the Valmonte Diatomite. Outstanding sea-cliff exposures display a three-dimensional view of a large (approximtely 1,000 ft across) backfilled channel, scoured into thin-bedded shale and sandstone deposits.

Channel backfill deposits primarily consist of Catalina schist-bearing sandstone and breccia, and lenses of disorganized intraformational breccia and conglomerate. The exposures exhibit an overall thinning and fining-upward sequence. The coarse-grained basal portion of the sequence displays sedimentary structures and bedding characteristics which are indicative of mass deposition by extremely concentrated, viscous dispersions. Channelized breccia beds contain schist clasts up to 9 ft in diameter, and intraformational breccia lenses contain rip-up clasts of Monterey Shale up to 30 ft long. Inferred flow mechanisms are a combination of grain flow, high-density turbulent flow, and sandy debris flow. The coarseness of these deposits is indicative of a proximal source. Higher in the sequen e, sedimentary structures in upper portions of individual beds are characteristic of low-density turbulent flows. This upward change in sedimentary structures within individual beds is indicative of the passage of a high-density gravity flow and the subsequent waning, low-density turbulent flow conditions which follow.

Directional features indicate a mean southeasterly flow direction. These results (which contradict data from previous published reports), in conjunction with the overall coarseness of these deposits, suggest the source to be the Palos Verde uplift.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.