Abstract
An ash bed has been recognized as a chronostratigraphic horizon in the Ventura basin, California, at a level near the Pliocene-Pleistocene contact. Suites of samples just above and below the ash bed were taken for facies analyses from seven outcrop sections and one well.
The rock types recognized on the basis of petrographic and microfaunal analyses are (1) tuff, (2) laminated shales, (3) mudstones, (4) cobbly mudstones, (5) sandstones, with graded bedding, (6) sandstones and siltstones, without graded bedding and commonly cross-laminated, and (7) conglomerates with mud matrix. These sediments are believed to be the products of four different kinds of sedimentary agents: (1) catastrophic sediment-gravity flows which deposited muddy conglomerates near the source, (2) turbidity currents which deposited graded sands distant from the source, (3) bottom currents which sorted and reworked turbidites and deposited cross-laminated silty sediments, and (4) surface currents which deposited laminated shales in the anoxic zone, and mudstones where bioturbation was active.
The sediment types are present in four characteristic associations: (1) the conglomerate facies characterized by an interbedded sequence of conglomerates, cobbly mudstones, mudstones, and sandstones; (2) the graded-sand facies characterized by sequences of turbidite sands with graded bedding, separated by laminated-shale intercalations; (3) the thin-bedded sand facies characterized by an interbedded sequence of thin-bedded, laminated, or cross-laminated sandstones, mudstones, and shales; and (4) the mudstone facies characterized by monotonous sequences of mudstones, with rare intercalations of laminated shales and cross-laminated siltstones.
The paleogeography, paleobathymetry, paleooceanography, and geometry and trends of the sediment bodies led to the identification of those four associations, or facies, as sediments of (1) submarine canyons and fans, (2) basin trough, (3) basin flanks or continental rise, and (4) basin slopes or escarpments respectively of the Pliocene-Pleistocene Ventura basin. The Ventura facies model emphasizes longitudinal transport and deposition of turbidite sands as potential reservoirs in a basin-trough setting, and is thus different from the submarine-fan model which postulates deposition of fine sediments there.
