Sedimentation of the Pico Formation in the Ventura Quadrangle, California

Abstract

The rocks of the Ventura quadrangle are sedimentary strata ranging from Cretaceous to Recent. The pre-Pliocene beds reach a thickness of 22,000 feet, and crop out in parallel, east-west belts. They consist of conglomerates, sandstones, and shales, with minor proportions of dolomite and limestone. The Pliocene beds comprise the Pico and Saugus formations, which crop out south of the older formations. The Pico formation is divided for convenience of discussion into a lower member 9,300 feet thick and an upper member 3,000 feet thick. The lower Pico consists of heavy conglomerates, sandstones, and clays containing many cross-laminated beds, ripple-marks, and cut-and-fill structures; while the upper Pico consists of monotonously even-bedded, homogeneous, silty clay. A comparison of the petrology of the older formations and of the Pico indicates that the Pico sediments were derived from these older rocks. During lower Pico time short, swift streams carried detritus from a youthful land-mass located just north of the present outcrop of the formation, to a marine basin. Deposition took place rapidly, and the bottom was recurrently depressed while the adjacent land-mass was recurrently uplifted. By upper Pico time the land-mass had been reduced to maturity or old age, and only the finer debris was carried to the basin.