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Lithostratigraphy and Depositional Environments of the Vaqueros Formation in Hondo Field, Capitan Field, and the Western Santa Ynez Mountains, California

RIGSBY, CATHERINE A., California State University, Long Beach, CA, GREGORY A. MILES, Exxon Company, U.S.A., Thousand Oaks, CA, and DANIEL E. SCHWARTZ, Shell Western E&P, Inc., Bakersfield, CA

The boundary between the Santa Maria and Santa Barbara/Ventura basins is marked by the junction between the western Transverse Ranges and the southern Coast Ranges. The Santa Ynez Mountains and the adjacent offshore areas are situated on the northwest edge of the western Transverse Range province. Oligocene-lower Miocene strata in this province record complex interactions among local and regional eustatics and local and regional tectonics and shed light on the sedimentary and tectonic history of this poorly understood region.

Correlation of lithofacies units in Oligocene-lower Miocene Vaqueros Formation outcrops the Santa Ynez Mountains with Vaqueros cores from the Hondo and Capitan fields leads to the delineation of three major facies tracts within the offshore and coastal areas: fan-delta conglomerates, shoreline sandstones and conglomerates, and inner shelf sandstones. Vaqueros strata at both Hondo and Capitan fields are part of the inner shelf sandstone facies tract.

Outcrops of the inner shelf facies typically consist of amalgamated packets of locally burrowed, cross-bedded, gravelly sandstones. In outcrop, bioturbation is most common near the tops of the cross-bed sets. In Hondo field a thinner, more basinward portion of this facies is characterized by bioturbated medium- to coarse-grained sandstones. Both in outcrops and at Hondo the base of the Vaqueros is marked by a sharp grain-size change and by a change from interbedded fluvial/deltaic strata below (the Alegria/Sespe formations) to inner shelf strata above (the Vaqueros Formation).

The preservation of laminations and cross-bedding and the lack of significant amounts of fine-grained material incorporated in the sandstone units suggests that the Vaqueros at Capitan field occupies a more proximal position than that of the Vaqueros at Hondo field. Further, the intensity of bioturbation and the occurrence of fine sand and clay at the tops of some intervals suggests that the Vaqueros at Capitan is more distal than that exposed in outcrops in the Saint Ynez Mountains.

The inner shelf facies tract represented by these three localities fines, thins, and becomes more pervasively bioturbated in the present-day offshore direction suggesting a similar shoreline polarity during the deposition of these strata. When viewed regionally, however, the Oligocene-early Miocene paleogeography is more complex. Facies tracts present in outcrop in the western Santa Ynez Mountains become thinner and coarser in the present-day westerly direction and record deposition on a topographically and geomorphically complex shoreline. That complexity was, undoubtedly, the result of the interactions between sedimentation and tectonics in a tectonically active depocenter.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)