ABSTRACT

Coarse, matrix-supported conglomerates, an abundant invertebrate faunal assemblage, and large vertical burrows record high-energy nearshore deposition in the Vaqueros Formation of the westernmost Santa Ynez Mountains. These strata document the effect of Late Oligocene-Early Miocene eustatic sea-level rise on fan-delta deposits of the tectonically active southern California continental margin.

The Vaqueros fan-delta succession can be divided into six facies deposited in three fan-delta facies assemblages: fan-delta plain, fan-delta front, and fan-delta slope. The fan-delta-plain facies assemblage consists of inversely to normally graded, matrix-supported, boulder to pebble conglomerates. These conglomerates are angular, nonfossiliferous, and laterally persistent. The fan-delta front facies assemblage is characterized by non-graded to normally graded, sandy, cobble to pebble conglomerates and conglomeratic sandstones organized in both channelized and nonchannelized bedding units. Pectens, oysters, and shallow-water gastropods are abundant in this facies, and individual beds have sharp bases and burrowed tops. Small-scale lenticular bedding and reworked lenses of fossil mater al are common throughout. Locally, the nonchannelized units show subhorizontal bedding. Fan-delta-slope strata are dominated by extensively bioturbated, locally cross-bedded, pebbly, coarse sandstone overlain by sandstone/fossil conglomerate couplets. Pebble imbrication throughout the sequence, along with channel orientations and cross-bedding in the fan-delta-front units, record paleoflows to the east and the southeast.

These facies assemblages represent deposition in a deepening-upward fan-delta system on the southern flank of the then-emergent Santa Maria basin (the paleotrench-slope break). Fan-delta-plain conglomerates record braided fluvial and intertidal deposition; fan-delta-front conglomerates and sandstones record channel-mouth and bar/spit deposition; and the fan-delta-slope sandstones record deposition in shoreface to inner-shelf environments. The sequence documents local sedimentary response during a period of regional tectonic subsidence and general eustatic sea-level rise.