DEEPENING-UPWARD SEQUENCES IN OLIGOCENE AND LOWER MIOCENE FAN-DELTA DEPOSITS, WESTERN SANTA-YNEZ MOUNTAINS, CALIFORNIA

Coarse, matrix-supported conglomerates, an abundant invertebrate fauna l assemblage, and large vertical burrows record high-energy nearshore deposition in the Vaqueros Formation of the westernmost Santa Ynez Mou ntains. These strata document the effect of Late Oligocene-Early Mioce ne eustatic sea-level rise on fan-delta deposits of the tectonically a ctive southern California continental margin. The Vaqueros fan-delta s uccession can be divided into six facies deposited in three fan-delta facies assemblages: fan-delta plain, fan-delta front, and fan-delta sl ope. The fan-delta-plain facies assemblage consists of inversely to no rmally graded, matrix-supported, boulder to pebble conglomerates. Thes e conglomerates are angular, nonfossiliferous, and laterally persisten t. The fan-delta front facies assemblage is characterized by nongraded to normally graded, sandy, cobble to pebble conglomerates and conglom eratic sandstones organized in both channelized and nonchannelized bed ding units. Pectens, oysters, and shallow-water gastropods are abundan t in this facies, and individual beds have sharp bases and burrowed to ps. Small-scale lenticular bedding and reworked lenses of fossil mater ial are common throughout. Locally, the nonchannelized units show subh orizontal bedding. Fan-delta-slope strata are dominated by extensively bioturbated, locally cross-bedded, pebbly, coarse sandstone overlain by sandstone/fossil conglomerate couplets. Pebble imbrication througho ut the sequence, along with channel orientations and cross-bedding in the fan-delta-front units, record paleoflows to the east and the south east. These facies assemblages represent deposition in a deepening-upw ard fan-delta system on the southern flank of the then-emergent Santa Maria basin (the paleotrench-slope break). Fan-delta-plain conglomerat es record braided fluvial and intertidal deposition; fan-delta-front c onglomerates and sandstones record channel-mouth and bar/spit depositi on; 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 e ustatic sea-level rise.