Abstract

Thick sandstone beds from a late Cretaceous and Paleocene succession of interbedded sandstone and shale, which occur on the west side of the San Francisco Peninsula, show distinctive sedimentary structures. The sandstones show normal grading, some localized inverse grading, and contain both convolutions and dish-structures. Parallel lamination is developed throughout most beds, which in some cases exceed 30 feet in thickness, and cross-lamination and cross-bedding are very, rare. The thicker beds are composites of many small beds, contain lenses of coarse sediment and are locally channelled. The dish-structure, convolutions, and diapiric structures indicate that the bed was emplaced in a fluid state, and the transport mechanism is presumed to be either a high density turbidity current, a grain flow, or a slurry. Similarity was found between the sedimentary structures in the thick sandstones, and the observed features of experimental high density turbidity currents. The depositional environment is considered to have been a submarine fan, on the basis of internal sedimentary structures and the general nature of the sedimentation. The thick sandstone beds are thought to be channel deposits on the submarine fan. Paleocurrents indicate that the source of the sediments lay through a submarine canyon to the southeast, probably in the Northern Santa Cruz Mountains, where the same or a neighboring canyon may have supplied sediment to the central Santa Cruz Mountains.

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