Sequence Stratigraphy and Facies Architecture of a Middle Eocene Shallow Marine to Basin Transition, Sespe Wilderness Area, California

Abstract

Several unconformity bounded depositional sequences are present in the Middle Eocene Coldwater, Cozy Dell, and Matilija formations in the Sespe Wilderness Area of California. Five of the sequence-bounding unconformities can be correlated from the canyon walls of Lower Sespe Gorge to the Sespe oil fields 5 km to the east. Three of these unconformities are characterized by a basinward shift of the overlying facies. The Cozy Dell unconformity is a flooded sequence boundary which formed as a surface of bypass and erosion during base-level fall but which was subsequently flooded during subsidence-induced deepening of the basin. The middle Coldwater unconformity is incised 58 m into a depositional sequence recognized on well logs, and alluvial valleys incised into the same unconformity in outcrop contain 58 m of estuarine fill. The unconformity on the paleotopographic highs is a combined flooding surface/sequence boundary with deeper water facies above the unconformity than below. In contrast, the unconformity in the paleotopographic lows is a sequence boundary with shallower water facies above the unconformity than below. The Cozy Dell Formation in outcrop is equivalent to a highstand systems tract, whereas the interval identified by lithology as Cozy Dell in well logs is equivalent to the condensedmore » section at the base of the same systems tract. The systems tract is dominantly shale and contains interbedded lenticular turbidite sandstones on the downthrown, basinward side of an Eocene growth fault. Continuing landward, the turbidites are eventually replaced by shoaling upward parasequences.« less