ABSTRACT: Depositional Sequence Stratigraphy of Middle to Upper Eocene Units, Santa Ynex Mountains, California

Peter R. Thompson, Roger M. Slatt

A depositional sequence stratigraphic framework has been developed for the uppermost Juncal, Matilija, Cozy Dell, and lowermost Coldwater formations (Ulatisian to Narizian Stages; middle to upper Eocene) in the Santa Ynez Mountains of California. The deposits are composed of organic-poor, gray-brown siltstone and mudstone with several intervals of arkosic sandstone. The sandstones vary greatly in thickness and in total number if counted vertically across major regional strike-slip faults. Previous correlations, based mostly on lithology and inadequate biostratigraphic control, have been difficult to establish.

Micropaleontology, sedimentology, clay mineralogy, and organic geochemistry data from closely spaced samples from well-known sections were integrated using sequence stratigraphy concepts. Pelagic units with planktonic foraminifera suitable for dating and correlation were found at all the sections. Benthonic foraminiferal paleobathymetry of the siltstones and mudstones indicates a gradual deepening from outer neritic (shelf) on the east to bathyal (slope) on the west. Interpreting the pelagic units as condensed intervals (reduced sediment influx during peak transgression) and the arkoses as lowstand deposits (maximum basinward sedimentation during regression), three depositional sequences can be defined: (1) lower Matilija Sandstone through Matilija Shale; (2) upper Matilija Sandstone hrough lower Cozy Dell Shale; and (3) Cozy Dell Sandstone through upper Cozy Dell Shale.

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