EROSIONAL STRIPPING OF THE SOUTHEASTERN SAN JOAQUIN BASIN (SJB) MARGIN OFF OF THE SOUTHERN SIERRA NEVADA BASEMENT UPLIFT

Facies patterns of Neogene strata lying along the intensely faulted southern Sierra Nevada range front indicate that Neogene marine conditions extended for a non-trivial distance across the currently exposed southern Sierra basement. Emergence of the basement is controlled by active W-side-up normal faulting along the Kern Canyon-Breckenridge system, and active NE-side-up normal faulting along the Kern range front-Pond-Poso system. Together these systems produce the regional wedge shaped Breckenridge-Greenhorn horst. The lower slopes of the horst constitute the Kern Arch, a pervasively faulted homocline across which the remnants of the SE SJB Neogene section are undergoing erosion. Neogene basin slope, shelfal, shoreline and paralic facies strata occur continuously along the range front, both faulted against basement and as nonconformable tongues preserved along relay ramps in the fault system. Geologic relations in conjunction with He apatite thermochronometry suggest Neogene growth faulting and transfer motion along the Breckenridge fault, which bounded a system of NW-striking normal faults to the east. Together these faults produced the volcanic (Walker) graben in which the western flank of the 21-16 Ma Cache Peak volcanic center ponded. The southwest corner of the Walker graben was breached by normal faults of the Edison graben that formed a structural trough into the SJB. Currently it is unclear if marine conditions extended all the way into the Walker graben, either through an Edison graben channel, or by overtopping the growing Breckenridge fault. Regional U/Pb zircon basement geochronology and detrital zircon ages from upper Neogene strata, in conjunction with other provenance and stratigraphic data indicate that the Bena Gravel and “Kern River” Formation represent a prograding delta-fluvial plain system that flooded into the SJB, primarily through the Edison graben, as the southern Sierra began regional ascent in the Late Miocene. Continued regional uplift has resulted in the redistribution of most of the Walker graben fill into the SE SJB. In late Pliocene-Quaternary time the Breckenridge-Greenhorn horst began forming with the progressive exhumation of overlying SJB margin strata, and the incision of the lower Kern gorge as a superimposed drainage.