PRELIMINARY DETRITAL ZIRCON PROVENANCE ANALYSIS OF MIDDLE EOCENE THROUGH LOWER MIOCENE SANDSTONES OF THE SALINIAN BLOCK AND WESTERN SAN JOAQUIN BASIN, CENTRAL CALIFORNIA

Twelve sandstone samples have been collected from central California for U-Pb age determinations (1170 age dates) using laser-ablation inductively-coupled mass spectrometry (LA-ICPMS). These samples range in age from middle Eocene to late Oligocene-early Miocene and were collected in the Santa Cruz Mountains (La Honda basin) and Temblor Range (San Joaquin basin), central California. Eocene formations sampled include the Butano Sandstone, Whiskey Hill Sandstone, Pilarcitos Formation, and Kreyenhagen Formation (Point of Rocks Sandstone Member). Oligocene to Miocene formations sampled include the Vaqueros Sandstone (Castle Rock Member) and Temblor Formation (Carneros Sandstone Member). The majority of the samples collected were deposited onto or adjacent to the northern Salinian block, a displaced composite terrane predominately composed of Cretaceous plutonic rocks. Post-early Miocene movement along the San Andreas fault has separated the once contiguous La Honda and southern San Joaquin basins and complicated provenance relationships and paleogeographic reconstructions in this region.

Preliminary results show that most samples have dominant Cretaceous (~85-125 Ma) and Jurassic (~135-175 Ma) signatures with lesser abundances of Paleozoic and Proterozoic grains. These ages are consistent with local derivation from the Salinian block or recycling from older Sierran arc-derived sedimentary rocks, among other possibilities. Five samples contain a population of Eocene to Paleocene grains (43-55 Ma) and three samples contain Oligocene to early Miocene grains (18-33 Ma). These post-Cretaceous grain populations represent sediment contributions from currently unrecognized source terranes to central California. Future work will focus on the implications of this initial dataset on sedimentary provenance, regional paleogeography, and cross-San Andreas fault correlations.