Abstract: Depositional Environments and Paleogeography of the Lower Miocene Vaqueros Formation, Santa Ana Mountains, California

Lori L. Daniel, A. Eugene Fritsche

The lower Miocene Vaqueros Formation, exposed on the western flank of the northern and central Santa Ana Mountains, California, varies in thickness from 200 m in the north to 40 m in southern exposures. The Vaqueros overlies conformably, and interfingers with, the Oligocene Sespe Formation and is overlain unconformably by the middle Miocene Topanga Formation. The dominant rock type is very fine- to medium-grained sandstone that is structureless, although large-scale cross-bedding, flaser lamination, and parallel lamination occur in some beds. Minor rock types include siltstone, coarse- to very coarse-grained sandstone, conglomerate, and limestone. Fossils include Arca santana, Turritella inezana, Kewia fairbanksi, and Clavocerithium santana.

Provenance was mainly granitoid, probably from the northern Peninsular Ranges, and some reworked sedimentary rocks. Small transgressive and regressive cycles are superimposed on an overall transgressive sequence in the lower half of the formation and a regressive sequence in the upper half. The Vaqueros Formation in the north consists of river-dominated delta front, interdistributary-bay, distributary-mouth-bar, levee, distributary channel, and marsh deposits in the lower and upper thirds of the formation. The middle third of the formation in the north and all of the formation toward the south are composed of wave-dominated shoreface, foreshore, and backshore deposits.

During the early Miocene, a major river, flowing southwest from the northern Peninsular Ranges, created deltaic environments in what are now the northwestern Santa Ana Mountains, while wave-dominated environments existed toward the south. During middle Vaqueros deposition, either river currents were reduced or wave energy was increased, and a wave-dominated shoreline was produced along the entire coast.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994