Basin Analysis, Facies Architecture, and Tectonic Implications of Oligocene-Miocene Vasquez Formation, Soledad Basin, California

Eric D. Hendrix

The Vasquez Formation in the central Transverse Ranges is a thick nonmarine stratigraphic unit representing the earliest sedimentation within the Oligocene-Miocene Soledad basin. Although three separate depocenters of Vasquez sedimentation have been recognized, only two (Vasquez Rocks and Texas Canyon subbasins) contain evidence of similar tectonic and sedimentary evolution. Sedimentologic differences in the third depocenter (Charlie Canyon subbasin) suggest that it may have been juxtaposed with the others subsequent to Vasquez sedimentation, during faulting coeval with middle Miocene clockwise rotations of the Transverse Ranges block.

Regional crustal extension at 24-25 Ma produced the small (4 to 8-km wide) half graben and asymmetric graben of Vasquez Rocks and Texas Canyon subbasins, respectively. Rapid vertical and lateral lithofacies variations, basinwide extent of thick allocyclic alluvial megasequences, and sedimentation rates exceeding 1.0 m/1,000 years imply rapid vertical aggradation and subsidence in both subbasins. First-order Markov analysis reveals significantly nonrandom facies architecture, with sedimentation dominated by low-frequency mass-flow events and higher frequency sheetflood events, and/or upper flow-regime fluvial sedimentation in channels with large width/depth ratios and minimal bedform migration. These data, coupled with conglomerate petrology, suggest early Vasquez sedimentation on smal thick fans derived from local basement sources within small drainage basins, in an environment where both subsidence and sedimentation rates exceed erosion rate.

Continued extension of the Soledad basin led to drainage-basin enlargement, decreased sediment/water ratios of individual depositional events, and hyperconcentrated flood flow and lower energy fluvial sedimentation in both subbasins. Sedimentologic data from upper Vasquez sections in both subbasins suggest broad fan geometries and erosion rate exceeding sedimentation rate, and probably subsidence rate, during later basin evolution.

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