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Abstract: Catastrophic Storm Deposit in a Sandstone Lens of the Eocene Cozy Dell Shale along Sespe Creek, Ventura County, California


An unusual, fine-grained sandstone lens in the middle Eocene Cozy Dell Shale is exposed for a distance of approximately 100 m along a cliff on the north bank of Sespe Creek, east of Beaver Camp, Ventura County, California The lens is about 10 m in total thickness and is arranged in pinch-and-swell units that vary from 5 to 8 m in thickness and have a wave length of severe tens of meters. Within these pinch-and-swell units most of the sandstone is parallel laminated, but in places the parallel lamination changes laterally into climbing-ripple lamination that shows deposition on both the stoss and lee sides of the ripples. Moderately straight-crested ripple marks with wave lengths of about 7 cm are preserved on some separation planes. A few layers of convolute lamination, up to 7 cm in thickness, occur between climbing-ripple sequences. Near the top of the lens the parallel- and climbing-ripple laminated layers are eroded along a wavy surface that has a wavelength of about 2 m; this surface is overlain by hummocky cross-stratified sandstone layers that are convoluted in places. The parallel- and climbing-ripple-laminated layers point toward rapid deposition near the boundary between the high- and low-flow regimes. The hummocky cross stratification indicates storm deposition. The shale that engulfs the sandstone lens was deposited in a relatively quiet shelf environment. I propose that this entire sandstone lens was deposited during a short period of time, several hours to a few days in length, in an area on the shelf that was well below fairweather wave base, but above maximum storm wave base, and that the depositing current was the channelized, offshore flow from a major storm surge, with the pinch-and-swell nature of the deposit resulting from standing waves in the current.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California