PROVENANCE AND PALEOGEOGRAPHY OF UPPER OLIGOCENE TO MIDDLE MIOCENE SANDSTONES OF CENTRAL CALIFORNIA: IMPLICATIONS FROM DETRITAL ZIRCON GEOCHRONOLOGY

The structural emplacement of the Salinian block and Cenozoic development of the modern San Andreas fault system had a profound effect on the configuration of the central Californian margin with respect to basin sedimentation. A preliminary dataset of 42 outcrop and subsurface sandstone samples were collected from the Oligocene–Miocene Temblor Formation and equivalent units to investigate trends in sediment dispersal patterns throughout the San Joaquin and adjacent basins. Detrital zircon (DZ) U-Pb age distributions obtained using laser-ablation inductively-coupled mass spectrometry (LA-ICPMS) are used to infer changes in local and extra-regional source, and are compared to previous interpretations based on sandstone petrography in order to reevaluate late Cenozoic paleogeography.

Preliminary results show that most samples are dominated by Cretaceous (~80–100 Ma) and Jurassic (~140–175 Ma) age signatures with lesser components of Paleozoic and Proterozoic ages, and are consistent with a primary Sierra Nevada batholith source. However, Temblor sandstone units from the Maricopa and eastern extent of the Carneros submarine fan systems are dominated by 16–27 Ma zircon (21–70%). These contemporaneous Oligocene–Miocene grains were likely derived from either volcanism associated with the northward migration of the Mendocino Triple Junction or renewed volcanism in the Sierran arc, and provide leverage for delineating transport pathways and sediment mixing throughout the San Joaquin basin. Most samples have minor (<2%) components of extra-regional Eocene ages (43–55 Ma) that were likely originally derived from the Idaho batholith and subsequently recycled from the underlying Eocene stratigraphy. Comparison of sandstones from the Vaqueros Formation of the La Honda basin with the equivalent Temblor Formation of the western San Joaquin basin show a discrepancy in DZ age distributions, particularly Jurassic (~145–200 Ma) populations, and warrant a reassessment of previously accepted San Andreas cross-fault ties (e.g., Castle Rock-Recruit Pass submarine fan system). These provenance changes may have important implications for the displacement history of the San Andreas fault, emergence of the Coast Ranges, and persistence of the southern Sierra Nevada as a drainage divide throughout the Miocene.