Paleogeography of Western Transverse Range Province, California: New Evidence from Upper Oligocene-Lower Miocene Vaqueros Formation

Catherine A. Rigsby

Vaqueros Formation outcrops in the westernmost Transverse Ranges and Vaqueros reservoir strata in the adjacent Santa Barbara Channel reveal previously undescribed dual sediment sources and a complex late Oligocene-early Miocene shoreline geometry. This paleogeography strongly affects the distribution of Vaqueros reservoirs.

Shelf deposits in the central Santa Ynez Range separate a gently sloping, west- and southwest-facing mainland shoreline to the east (eastern Santa Ynez Range and Santa Monica Mountains) from a steep, east- and south-facing shoreline to the west (westernmost Santa Ynez Range). The western and central regions are dominated by lithic-rich arenites and by Franciscan-clast conglomerates (lithofacies 1) derived locally from an emergent portion of the early Tertiary trench-slope break. The eastern region is dominated by lithic-poor quartzofeldspathic sandstones (lithofacies 2) derived from older sandstones on the mainland to the east. The two lithofacies interfinger in the central region, and lithofacies 2 overlies lithofacies 1 in the west.

Sediment was shed onto the Vaqueros shelf from both the mainland to the east and the partially emergent trench-slope break to the west. Subsequently, transgression proceeded across the region, drowning the westernmost shoreline, filling incised valleys, and producing backstepping inner-shelf and shoreline sequences to the east.

These strata record deposition in an obliquely convergent forearc basin prior to and during a relative rise in sea level. They suggest that the Pacific-North American plate boundary maintained a forearc basin geometry throughout the Oligocene and that Vaqueros Formation reservoirs are actually several discreet, chronostratigraphic units.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.