Pliocene Seaknoll at South Mountain in Ventura Basin, California: ABSTRACT

Abstract

Extensive drilling in the southern Ventura basin in the last decade has provided useful data about Pliocene basin floor topography. The pre-basinal Miocene Modelo Formation, mainly siliceous shale, limestone, and organic shale with a bathyal microfauna, underwent extensive undersea tilting and faulting, resulting in a surface of high relief upon which the deep-sea Plio-Pleistocene Pico Formation was deposited. Among the land forms of this surface was a seaknoll at South Mountain. Seaknoll features preserved include relatively uneroded fault scarps exceeding 40° in slope, a thin veneer of glauconite sandstone locally containing a talus of marine-laid Modelo limestone fragments from the scarps, and, near the seaknoll summit, a small biostrome in a shelly glauconite sandstone matrix. The steep fault scarps at South Mountain contrast with bevelled fault scarps at nearby Berylwood anticline and the Oxnard plain, on the site of a pre-Pico submarine slope eroded in the Modelo Formation. The Pico Formation has onlapped and buried the seaknoll and the submarine slope. The Pico contains graded sandstones interbedded with siltstones containing indigenous bathyal and displaced neritic microfaunas. The sandstones shale out toward the seaknoll, suggesting that the seaknoll was mildly positive during sandstone deposition. The submarine slope was scoured by bottom currents laden with sediment from subaerially eroded highlands toward the south and east. The seaknoll was unaffected by such currents because it was separated from highlands by deep-sea channels; hence its scarps were relatively unscoured by sediment-laden currents. Both submarine slope and seaknoll remained below sea level until buried. Erosional submarine unconformities of the Berylwood and Oxnard plain submarine slope type are believed to be relatively common in basins where sedimentation and deformation have occurred simultaneously. Regarding these unconformities, the overlying sediments are of deep-sea rather than shallow-water origin, and are transgressive. The argument for submarine origin of the Mio-Pliocene unconformity in the southern Ventura basin, even where extensively eroded, is strengthened by the presence of the seaknoll, locally preserved from turbidity current scour, on which fault scarps and a glauconite-rich veneer have been preserved. End_of_Article - Last_Page 1092------------