Abstract: The Cozy Dell Formation--Delta Progradation and Turbidite Deposition During Sealevel Highstand

Michael S. Clark

Strata of the middle Eocene Cozy Dell Formation, Topatopa Mountains, California were deposited during a eustatic highstand in the siliciclastic shelf-to-basin transition of a tectonically active depocenter. The formation thins from 1030 m (3380 ft) near the type section in the western Topatopa Mountains eastward to <82 m (270 ft) in the Sespe Oil Fields. Further eastward, these strata are truncated beneath the non-marine Sespe Formation and/or grade into non-marine facies. If clockwise, Mio-Pliocene tectonic rotation is assumed, north-to-south sediment transport is indicated.

A basal, organic-rich (3% TOC) shale represents a condensed interval. Landward-stepping parasequences below this shale represent a transgressive systems tract. Westward-prograding, seismic-scale (amplitudes >100 m) clinoforms above represent a highstand systems tract deposited in a small, confined basin by river-dominated, turbidite-bearing deltas. The clinoform toplap surface correlates to an incised, sequence-bounding unconformity with deeper-water facies above the unconformity than below. Formation of this surface is attributed to sediment bypass during delta progradation with termination of bypass and subsequent flooding during subsidence-induced deepening of the basin. The deltas are replaced basinward, on the downthrown sides of fault blocks, by turbidite depositional lobes that are not attached to channelized facies nor associated with submarine canyons. Apparently, these lobes were sourced by numerous submarine channels and gullies incised onto Cozy Dell delta fronts contemporaneous with delta progradation. These relations indicate that sediment flux and differential subsidence represented the primary controls on Cozy Dell deposition.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California