Paper No. 12
Presentation Time: 11:20 AM
REEVALUATION OF THE BUTANO-POINT OF ROCKS CROSS-FAULT TIE USING DETRITAL ZIRCON GEOCHRONOLOGY: IMPLICATIONS FOR EOCENE PALEOGEOGRAPHY AND EARLY DISPLACEMENT ON THE SAN ANDREAS FAULT
SHARMAN, Glenn R., Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford University, Stanford, CA 9305 and GRAHAM, Stephan A., Department of Geological Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, CA 94305-2115, gsharman@stanford.edu
Seven Eocene sandstone samples in central California have been analyzed for U-Pb ages of detrital zircons (688 age dates) using laser-ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). The samples are divided into two groups based on similarity of their detrital zircon age distributions and Kolmogorov-Smirnov statistics. One group includes one sample from the Whiskey Hill Sandstone (San Francisco Bay block) and three samples from the Point of Rocks Sandstone Member of the Kreyenhagen Formation (southern San Joaquin basin). This group is dominated by Cretaceous (67-80%) zircons with lower abundances of Eocene (0-9%), Jurassic (10-20%) and Proterozoic (1-12%) zircons. The other group includes two samples from the Butano Sandstone and one sample from the Pilarcitos Formation (Santa Cruz Mountains) and is characterized by subequal or higher abundances of Jurassic (36-61%) than Cretaceous (23-44%) zircons with lesser abundances of Permo-Triassic (0-8%) and Proterozoic grains (8-13%).
Each group is hypothesized to have a distinct provenance. The Point of Rocks Sandstone and Whiskey Hill Sandstone have affinities to the western Sierra Nevada, a conclusion supported by comparison with detrital zircon signatures from Eocene fluvial drainages to the north. While speculative, the Butano Sandstone may have had a source from the southern Sierra Nevada and/or northwestern Mojave block. This interpretation calls into question paleogeographic reconstructions which interpret the Butano Sandstone and Point of Rocks Sandstone to be part of the same submarine fan system, a cross-fault tie which has widely been used to constrain middle Eocene to early Miocene displacement on the San Andreas fault. The results of this study have implications for understanding regional Eocene provenance and sediment dispersal patterns in central California and the early displacement history of the San Andreas fault.
