Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Stratigraphy and Distribution of Lithofacies Associations in a Structurally Confined Turbidite System: The Upper Modelo Formation in Eastern Ventura Basin, California

Abstract

Outcrops of the Miocene upper Modelo Formation in the eastern Ventura basin, California, represent a longitudinal transect through a fault-controlled deepwater depositional system, from proximal structural terraces, through a submarine canyon, to the basin floor and basin margin. This study area is divided into three regions based on geographic location, stratigraphic character, and interpreted paleogeographic environment: Region 1 (Feeder System), Region 2 (Proximal Basin Floor), and Region 3 (Medial-Distal Basin Floor). Region 1 contains interpreted syndepositionally active normal faults (e.g., Devil Canyon fault) near the proximal basin margin; these faults are associated with abrupt changes in depositional environments, lithofacies associations, paleobathymetry, and stratigraphic thickness. The adjacent submarine canyon was an area of bypass, yet there are sand-rich submarine channels located within it. This area has a relatively low net-sandstone content overall. Isopach maps reveal that gross thickness is greatest near the interpreted canyon mouth. In Region 2, a proximal-basin-floor fairway located immediately basinward of the canyon contains the highest sandstone content in the basin. This fairway comprises a number of channel complexes, mostly characterized by amalgamated, medium-grained-to-pebbly sandstone lithofacies. In Region 3, gradual changes in proportions of lithofacies associations result from subtle variations in the gradient of the basin floor near the lateral/distal basin margin, as interpreted from the isopach map. In this area, channel complexes comprising structureless, amalgamated sandstone overlie thin-to-medium bedded, “dirty” sandstones deposited at the fringes of lobes. This upward architectural pattern is interpreted to result from an outwardly expanding depocenter through time.