Tectonics and alluvial sedimentation of the upper Oligocene/lower Miocene Vasquez Formation, Soledad basin, southern California

Abstract

The upper Oligocene/lower Miocene Vasquez Formation marks the earliest sedimentation in the Soledad basin, central Transverse Ranges, southern California. The Vasquez consists primarily of alluvial sediments and presently appears as outcrops in three geographically restricted, fault-bounded sub-basins. The two southern sub-basins (Vasquez Rocks and Texas Canyon) shared similar tectonic and depositional histories; the northernmost sub-basin (Charlie Canyon) appears to have had a distinct history. The Soledad basin originated as a predominantly orthogonal rift during the latest Oligocene. Incipient subsidence was concentrated in the southeastern region of the basin, as debris-flow deposits accumulated as small, thick, alluvial fans draining local source areas adjacent to Vasquez Rocks sub-basin. Intense rifting, volcanism, and rapid subsidence produced the half-graben geometry of this sub-basin, as a source area rose to the south/southeast across the active Soledad fault. Coeval displacements along the Vasquez Canyon and Pelona faults led to the asymmetric-graben geometry of Texas Canyon sub-basin, as abundant detritus derived from an eastern/southeastern source interfingered with small, debris-flow-dominated fans along the Pelona fault margin. Periodic faulting and source-area uplift, followed by tectonic quiescence and erosion, produced thick, upward-fining alluvial megacycles in both sub-basins. Major tectonic uplift in the ancestral San Gabriel area led to drainage-system enlargement, increased water discharge into the depositional systems, and deposition by hyperconcentrated-flood and braided-fluvial processes in both sub-basins. Erosional dissection of Mint Canyon Ridge allowed physical interconnection of the two depocenters at this time. Charlie Canyon sub-basin, the northernmost Vasquez depocenter, displays no clast suites or alluvial megacyclicity to suggest close affinities with the other two depocenters. A sequence of easterly derived braid-plain sediments low in the section is overlain by a 1,600-m, upward-coarsening, alluvial sequence. This sequence reflects probable inception of the San Francisquito fault, uplift of a marine-sedimentary/quartz-monzonite/quartz-diorite source terrane, and northward progradation of an alluvial-fan system. Post-Oligocene clockwise rotation of the Soledad basin by as much as 40 degrees is indicated by paleomagnetic data of other workers. Restoration of the Soledad basin to its Oligocene orientation indicates that southeast-northwest extension caused rifting and heralded Vasquez deposition in small, rapidly subsiding basins. Neither compressional deformation nor strike-slip deformation seems to have been a significant factor during Vasquez sedimentation. The sedimentary and tectonic history of the Soledad basin is consistent with a plate-tectonic model involving extension in the North American plate north of the unstable Mendocino triple junction.