Nearshore Conglomerates of Vaqueros Formation, Westernmost Santa Ynez Range, California

Catherine A. Rigsby

Coarse, matrix-supported conglomerates, an abundant "Vaqueros stage" invertebrate faunal assemblage, and large vertical burrows record high-energy nearshore deposition in the Vaqueros Formation of the westernmost Santa Ynez Range. These strata document the effect of late Oligocene-early Miocene eustatic sea level rise on the tectonically active southern California margin.

The Vaqueros sequence in this area can be divided into three lithofacies. The basal facies (facies A) consists of inversely graded, matrix-supported, boulder to pebble-conglomerates. These conglomerates are angular, nonfossiliferous, and laterally persistent. The middle facies (facies B) is characterized by nongraded to normally graded, sandy, cobble to pebble-conglomerate. Pectens, oysters, gastropods, and clams are abundant in this facies and individual beds have sharp bases and burrowed tops. Small-scale lenticular bedding and reworked lenses of fossil material are common throughout. Facies C (the upper facies) is dominated by extensively bioturbated, locally cross-bedded, pebbly coarse sandstone. Pebble imbrication throughout the sequence, channel orientations in facies B, and cro s-bedding in Facies C record paleo-flows to the east and the southeast. Pebble lithologies and sandstone composition indicate a dominant Franciscan-assemblage provenance for the entire sequence.

These facies represent deposition in proximal (facies A) and distal (facies B and C) fan-delta and braid-delta environments. Facies A conglomerates record braided fluvial deposition, facies B conglomerates record channel-mouth deposition, and the sandstones of facies C record deposition in shoreface to inner shelf environments.

This sequence of strata documents local sedimentary response to complexly intertwined tectonic and eustatic events. The deepening-upward sequence described here was deposited on the southern flank of the then-emergent Santa Maria basin (the paleotrench-slope break?) during a late Oligocene-early Miocene eustatic sea level rise and suggests an eastward dipping paleoslope for the western margin of the late Oligocene-early Miocene Western Transverse Range province.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.