Constraints on the late Miocene/Pliocene reorganization of the San Andreas fault and basin drainage systems near Tejon Pass, CA, based on stratigraphy, sedimentology, and provenance of the Hungry Valley Formation

The Pliocene Hungry Valley Formation (HVF) in southern California is wedged between the San Gabriel fault and the San Andreas fault near the Tejon Pass and makes up the uppermost part of the Ridge Basin Group. These strata preserve sedimentological evidence of waning slip on the San Gabriel fault and the onset of slip along the San Andreas fault. I have divided the 1,500 m thick HVF into three members based on changes in sedimentology, paleocurrent directions, and conglomerate clast counts. The lowest member is bright white medium-grained sandstone with 1 – 2 m channels and 0.1 – 0.5 m cross beds, a clast assemblage that contains dominantly granitic and metamorphic clasts considered to be basement from local sources such as the San Gabriel Mountains and western Transverse Ranges (wTR), and northwest and southeast paleoflow directions. This member represents a braided fluvial-deltaic system that deposited local material from the central Transverse Ranges while the San Gabriel fault was still active. The middle member is yellow-white coarse to very coarse sandstone with some channels that include northeast, east, south, and southwest paleoflow indicators and dominantly contain Mojave Desert clasts and San Bernardino block basement in addition to San Gabriel type clasts. The middle member is interpreted a braided stream system and defines the onset of dextral slip on the San Andreas fault when major drainage reorganizations allowed Mojave clasts to penetrate the HVF from the northeast in addition to contribution from the Transverse Ranges. The upper member is white to brown very coarse sandstone to pebble conglomerate that contains northeast to southwest paleocurrent indicators and is dominated by clasts of basalt, pink/purple volcanics, and tuffaceous sandstone sourced from the Mojave rather than the Transverse Ranges, as well as marble from an unknown source. During deposition of the alluvial upper member, rivers sourced in the Mojave Desert flowed southwest into the HVF and debris flows and fans sourced the nearby Frazier Mountain and an unknown marble source possibly in the Mojave Desert. Sedimentation likely terminated in the late Pliocene as the region was uplifted due to transpression. As the first appearance of pink/purple volcanic clasts occur in the middle member, and the lower member overlays the top of the San Gabriel fault, it is likely that the San Gabriel fault shut down during deposition of the lower member and the San Andreas fault initiated during deposition of the middle member. Geochemically, the pink/purple volcanic clasts range from trachytes (60-65 wt % SiO2) to rhyolites (69-77 wt % SiO2) to trachyandesites (57-63 wt % SiO2). Major and trace element concentration comparisons indicate these clasts are likely Mojave sourced, though they have been significantly hydrothermally altered. These results imply that fault reorganization has a profound influence on sedimentary basin architecture and landscape evolution.