DROUGHT’S SILVER LINING: MAPPING UNDER MILLERTON LAKE AND ITS BEARING ON TILT AND INCISION OF THE SIERRA NEVADA, CA.

Due to extreme drought, low water at Millerton Lake, CA has enabled us to map the onlapping contact of Pleistocene Turlock Lake Formation over Cretaceous Tonalite of Blue Canyon for the first time. The tonalite represents the exhumed Sierra Nevada batholith, while Turlock Lake Formation constitutes Great Valley fill pinching out along the Valley-Sierra margin. Previous workers have used migration of this fill-batholith margin as indicative of tectonic uplift or subsidence, though clearly, it is also influenced by sediment supply. Turlock Lake Formation at the study site includes basal matrix-supported conglomerate beneath planar-laminated siltstone. This basal conglomerate includes a wide variety of far-traveled cobble lithologies, leading us to conclude that the clasts were deposited fluvially. Consequently, we can calculate paleo-stream gradient between this site and other contemporaneous localities. Since Turlock Lake Formation at this locality includes probable Bishop Tuff (0.76 Ma) pumice clasts, we tentatively correlate to Bishop Tuff at nearby Friant Quarry, where it has been independently dated. We also quantify time-averaged incision. The ~9 Ma datum of intracanyon Trachyandesite of Kennedy Table exhibits a current paleo-stream gradient of ~24.7 m/km and an average incision rate of 39.6 m/million years since emplacement. However, incision was faster since Bishop-aged deposits of the Turlock Lake Formation, rising to 92.9 m/million years since 0.76 Ma, with present-day gradient of this Bishop-aged horizon of only 6.2 m/km. The present day San Joaquin gradient along this reach is 2.0 m/km. These gradients are consistent with progressive Neogene Sierra Nevada tilting, while the rapid Pleistocene incision could be a post-Bishop transient due to instantaneous volcanic aggradation. Increased Pleistocene incision measured here differs from slowing incision found for the adjacent Kings River system.