Abstract

Paleobathymetric and paleoenvironmental analyses of upper Cretaceous and Cenozoic sediments of the Santa Ynez Mountains and adjacent Ventura Basin illustrate that Paleogene, Neogene, and Quaternary depositional cycles have produced the widespread stratigraphic patterns present in the northern portion of the continental borderland of southern California. Cretaceous through Eocene sedimentation was centered in an east-west structural trough termed the Santa Barbara Embayment. This feature was filled from east-to-west as rates of sedimentation exceeded rates of subsidence in Paleogene time. Abyssal basin plain deposits were sequentially buried by prograding fan, slope, and shelf deposits terminating in widespread non-marine deposition and erosion in Oligocene time as documented by the Sespe Formation. Upper Eocene and Oligocene alluvial deposits of this latter unit neatly partition Paleogene and Neogene depositional packages in this area.

Rapid subsidence and marine transgression occurred over wide areas of the California margin in latest Oligocene and early Miocene time initiated by collision of the Pacific and North American plates with subsequent birth of the San Andreas fault zone, translational margin tectonics, and creation of a complex series of bathyal Neogene basins. Continued subsidence of these basins and eustatic drowning of the adjacent Miocene strandline created a series of mid Miocene silled basins starved of terrigenous debris allowing relatively undiluted deposition of diatomites during a period of climatic deterioration and consequent intensified diatom productivity in the California Current system. A well developed oxygen minimum layer allowed preservation of laminated diatomites and dia-tomaceous muds in these basins ultimately forming the characteristic lithologies of the mid and upper Miocene Monterey Formation.

Further tectonic reorganization of the continental borderland occurred in Pliocene time with uplift of the Santa Ynez Mountains and other highs and simultaneous acceleration of subsidence in adjacent synclinal areas including the Ventura Basin. These events led to the rapid progradation of Plio-Pleistocene terrigenous debris into the Ventura Basin, capping underlying diatomaceous basin muds, and filling the basin just prior to a late Pleistocene tectonic event which uplifted portions of these deposits to their present position above sea level initiating the present phase of borderland deposition.