Abstract: Depositional Environment and Paleogeography of Some Sandstone Lenses in the Upper Cozy Dell Formation. Upper Sespe Creek, Ventura County, California

A. Eugene Fritsche

The Cozy Dell Formation, exposed along upper Sespe Creek, Ventura County, California, is mainly a regressive sequence of middle and upper Eocene slope and shelf mudstone deposits that are overlain conformably by and interfinger with the marginal-marine sandstone deposits of the Coldwater Formation. Within the uppermost part of the Cozy Dell Formation are several distinct and separate sandstone lenses that interfinger with the mudstone portions of the formation. The dominant lithology in these lenses is a resistant, yellowish gray, very fine- to fine-grained, well-sorted sandstone that is structureless due to extensive bioturbation. Sandstone of medium to coarse grain size occurs in a few places. Rare sedimentary structures include parallel lamination, rip-up clasts, ripple marks, clim ing-ripple lamination, large-scale wavy bedding, parting lineation, convolute lamination, and dish structures. Even though bioturbation is abundant, body fossils, including bivalves, gastropods, and wood fragments, are rare. The geometry of the sandstone lenses indicates that they were deposited as sand lobes on top of and adjacent to the dominant shelf and slope mud of the Cozy Dell Formation. Deposition was from traction currents and/or mass flows that may have been generated during storms. Provenance was felsic plutonic rocks and some reworked sedimentary rocks. After back rotation of about 90° is done to compensate for Miocene clockwise rotation of the Transverse Ranges, paleocurrent data from ripple marks and climbing-ripple lamination indicate that the traction currents were c ming from the northeast. Inauguration of sand-lobe deposition could have been due to either lateral migration of a terrestrial river source or a lowering of sea level.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994