--> --> Miocene paleogeography and stratigraphic evolution of the Ventura Basin

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Miocene paleogeography and stratigraphic evolution of the Ventura Basin

Abstract

Ventura Basin in southern California is an elongate, west-east-oriented modern physiographic feature. In some locations, Ventura Basin is less than three kilometers (~two miles) wide, as Quaternary thrust faults have narrowed the basin due to north-south convergence. This basin formed in the early-middle Miocene, partly as a result of extension and normal faulting. Ventura Basin was much broader in the Miocene than it is today, and is comprised of multiple depocenters containing successions of deepwater sandstones, siltstones, and organic and siliceous mudstones. In certain locations, Miocene stratigraphic successions are several thousand meters in thickness. Isopach and net-sand-thickness maps based on wells and outcrop exposures of the middle-late Miocene Monterey-Modelo Formation suggest that the Eastern Ventura Basin (EVB) might have been separated from the Western Ventura Basin depocenters by a sill, the ‘Fillmore High.’ The northeastern and southeastern margins of the EVB depocenter and submarine feeder systems were controlled by syndepositionally active normal faults adjacent to the San Gabriel fault. Mapping and well correlations suggest that fan-delta and turbidite feeder systems were commonly influenced by relay ramps and half-graben topography. Modelo strata in the EVB are dominated by clastic detritus shed into a borderland-style deepwater depocenter, due to proximity to the San Gabriel basement complex and highlands to the east at that time. West of the Fillmore High, north-south-oriented submarine gullies allowed for transverse sediment transport into sand-rich deepwater depocenters located along the basin's main west-east axis. Sand content within the Oak Ridge fault zone, widely interpreted as a syndepositionally active normal fault system near the southern margin of the basin, can provide evidence for locations of transverse feeder systems. Overall, however, sandstone is generally sparse along the northern and southern margins of Ventura Basin. More Monterey-Modelo-sourced oil has been found in the Western Ventura Basin than in the EVB; we interpret that this is due in part to extreme dilution of the EVB Modelo source rocks by clastics. The latest Miocene Towsley turbidite systems were deposited as ‘out-of-grade’ depositional systems locally near the San Gabriel fault, but the roughly time-equivalent silty-diatomaceous upper Sisquoc Formation and related mudstones were deposited more extensively throughout the basin. After deposition of the Monterey-Modelo and Towsley Formations, there was rapid clastic sedimentation and increased subsidence into the Pliocene. Paleobathymetry, and hence accommodation, was greatest in the early Pliocene. In the late Pliocene, the Ventura Basin entered a major regressive phase marked by more active thrust tectonics, uplift, and a seaward migration of the shoreline.