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ABSTRACT: Coarse-Grained Deltaic Sedimentation in the Miocene Cuyama Strike-Slip Basin, California Coast Ranges

J. Alan Bartow

The Cuyama basin, located in the southern Coast Ranges southwest of the San Andreas fault, developed early in the history of the San Andreas transform system. The Miocene marine basin formed in a transtensional setting along a dextral strike- slip fault of the transform system, the San Juan--Chimineas fault, following Oligocene nonmarine basin formation in an extensional setting.

The lower and middle Miocene Vaqueros Formation in the northwestern part of the basin, which represents the first of two transgressive-regressive cycles, consists of eight facies making up two depositional systems. The 400-m-thick Soda Lake Shale Member constitutes a basinal system consisting of deep-basin and starved-basin facies. The overlying 2200-m-thick Painted Rock Sandstone Member consists mostly of coarse-grained, pebbly sandstone and constitutes a deltaic depositional system of prodelta, slope channel, delta front, tidal channel, interdistributary bay, and fluvial channel facies.

The basinal depositional system consists of turbidite sand and mud, and hemipelagic and pelagic sediments that were deposited in a rapidly subsiding basin. The deltaic depositional system prograded into the deep basin and had a steep prodelta slope that extended to bathyal depths. The delta is inferred to be a river-dominated fan delta, in which coarse sediment was transported down the prodelta slope into deep water by sediment gravity flows.

The overall basin history and geometry of the northwestern Cuyama basin are typical of strike-slip basins. The initial rapid subsidence to bathyal depths at rates of more than 500 m/m.y. in the early Miocene is interpreted to be a result of extension at the releasing bend of a dextral strike-slip fault.

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