Abstract: Depositional Environments and Paleogeography of the Eocene Coldwater Formation, Upper Sespe Creek, Ventura County, California

Zun S. Jiao, Eugene A. Fritsche

The Coldwater Formation, exposed along upper Sespe Creek, Ventura County, California, is a regressive sequence of upper Eocene marginal-marine deposits that are underlain conformably by and interfinger with the marine cozy Dell Shale and are overlain conformably by and interfinger with the nonmarine Sespe Formation. During deposition of the Coldwater Formation, a wide and gently sloping tide-dominated shoreline existed in the southwest part of the area and a narrower and more moderately sloping wave-dominated shoreline occurred toward the northeast.

The wave-dominated sequence coarsens upward from fine-grained shelf facies into facies of the shoreface, foreshore, and backshore. The sequence is dominated by moderately to well-sorted, fine- to medium-grained, structureless, laminated, and cross-bedded sandstone that was deposited in a low wave-energy environment.

The tide-dominated sequence fines upward, reflecting a transition from subtidal and lower intertidal sand flats upward into higher intertidal sand flats and supratidal mud flats. Rocks range from coarse-grained sandstone to claystone and contain bidirectional cross-bedding, ripple marks, mud cracks, flaser and lenticular bedding, and mud-draped foresets.

Provenance was mainly felsic igneous rocks (abundant granitoid rock fragments) and less abundant metamorphic (quartzite and slate) and sedimentary (gray chert) rocks. After Miocene clockwise rotation of the Transverse Ranges is backrotated about 90°, structures in both shoreline environments indicate southeastward progradation. Paleocurrent data from cross-bedding indicate that the offshore direction was southeast and the longshore direction was southwest. Sediment was eroded from a highland west of the Pine Mountain area and transported southeastward across an upper delta-plain environment of marshes and small, leveed channels to the wave- and tide-dominated shoreline.

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