Pacific Section AAPG Convention

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Turbidite Stratigraphy And Distribution Of Lithofacies Associations In A Structurally Complex Depocenter: Upper Modelo Formation In Eastern Ventura Basin, California


Outcrops of the Miocene upper Modelo Formation in the eastern Ventura basin, California, which will be visited in the associated field trip on May 6, provide 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.