Depositional environments of the Eocene through Oligocene Sespe Formation in the northern Simi Valley area Ventura County, Southern California

Abstract

The nonmarine Sespe Formation (early late Eocene through late Oligocene age) is well exposed in a northward-dipping, homoclinal sequence of strata north of the Simi anticline, northern Simi Valley, Ventura County, California. Three members and one transitional zone were recognized in the formation. The lower member and lower three-fourths of the upper member represent sandy braided-river deposits. Braided-bar deposits consist of parallel, low-angle, and trough cross bedded, medium to coarse lithic arkose and cross-bedded pebble conglomerate. Structureless and graded lithic arkose and structureless, silty claystone represent overbank-sheetflood and braided-river floodplain deposits. Migrating, primary channel-fill deposits consist of intraclast-bearing sandstone and clast supported, pebble-cobble conglomerate. Anabranch channellag deposits are represented by discontinuous, stratified pebble-cobble conglomerate beds. Climbing-ripple laminated, carbonaceous, mudstone indicates braided-river swamp (pond) deposits. The middle member consists predominantly of cyclical sandstone and claystone with lesser amounts of conglomerate and represents meandering-river floodbasin deposits. Structureless and graded, medium to coarse lithic arkose and mottled, structureless, claystone represent overbank-sheetflood and suspension deposits. Crevasse-splay deposits consist of cross-bedded, intraclast-bearing, medium to coarse lithic arkose and lenticular sandy claystone beds. Rare matrix-supported pebble-cobble conglomerate represents meandering channel-fill deposits. The upper one-fourth of the upper member (Sespe/Vaqueros transition) represents lower-delta-plain deposits that are gradational with overlying wave-dominated delta-front deposits of the Vaqueros Formation. Parallel-bedded, medium to coarse, intraclast-bearing lithic arkose represents distributary channel-fill deposits. Laminated well-sorted fine arkose occurs near the top of the formation and represents foreshore deposits. Mottled, rhizomorph-bearing claystone occurs near the top of the formation and represents salt-marsh deposits. The provenance appears to be primarily from reworked Paleogene formations which crop out in the Simi Hills area. Less dominant and more distant mafic and high-rank metamorphic source areas may have originated from the southern San Gabriel Mountains. In addition, the rare occurrence of anorthosite clasts in the upper member may indicate a northern San Gabriel Mountains source. The Sespe Formation north of Simi Valley represents a southeastward- transgressive fluvial to deltaic sequence, which is indicated by four depositional phases. The lower phase represents a braided-river floodplain which onlapped onto a pre-Sespe erosion surface in the Simi Valley area. The middle phase consists of meandering-river floodbasin deposits resulting from infilling of a stable depositional basin. The upper phase consists of northwesterly-flowing braided-river deposits which, based on stratigraphic position, were deposited in a upper-delta plain environment. The Sespe/Vaqueros transition phase represents lower-delta plain deposits which moved into the area by continued southeastward transgression of the "Vaqueros shoreline".