Detrital zircon U-Pb and (U-Th)/He geo-thermochronometry and submarine turbidite fan development in the Mio-Pliocene Gulf of California, Fish Creek-Vallecito Basin, southern California

The Fish Creek-Vallecito Basin exposes an archive of sediment related to early rifting of the Gulf of California beginning at 8.0 Ma followed by Colorado River delta progradation from 5.3-3.0 Ma. Mio-Pliocene deposits from the Fish Creek-Vallecito Basin of southern California and a sample from the modern Colorado River delta were analyzed through detrital zircon U-Pb (n=1996) and (U-Th)/He (n=280) double-dating in order to better constrain sediment provenance, hinterland exhumation, and Colorado River evolution. Coupling this dataset with outcrop study of the first Colorado River-sourced turbidites into the basin at 5.3 Ma, allows for evolution of the Colorado River system to be viewed from a source-to-sink perspective. Detrital zircon U-Pb and (U-Th)/He (ZHe) ages obtained in this study suggest earliest derivation of sediment was from the Peninsular Ranges followed by more distant sediment sourcing from the Colorado River. Initial Colorado River-sourced deposits show Yavapai-Mazatzal U-Pb ages with Laramide ZHe ages suggesting that the river was sourcing from Laramide basement cored uplifts at the onset of deposition into the Gulf of California, supporting a top-down model of river evolution. An increased percentage of Grenville U-Pb age grains as well as a wider range of ZHe ages associated with western US basement-derived zircon from a modern Colorado River delta sample indicate erosion into older stratigraphic units through time which is consistent with deep erosion on the Colorado Plateau since ~6 Ma. Vertically measured sedimentology logs through the Wind Caves Member, the first Colorado River-sourced unit deposited, were used to determine slope and basin floor architecture as the Colorado River and delta dispersed subaqueous sediment gravity flows into the marine Gulf. Measured sections arrayed along depositional strike show a 4.5 km wide pod of sand-rich turbidites that were delivered through a broad Fish Creek exit point from the paleo-Colorado shelf. The vertical sedimentation trend is one showing thick bedded, amalgamated channelized and sheet-like sandstones initially, shifting to thinnerbedded sheets and more isolated channels higher in the increasingly muddy section. The facies variability up section is interpreted as a change from a submarine basin floor fan to a lower slope environment as the Colorado River prograded its delta into the Gulf.