CHARACTERIZATION OF TURBIDITES AND DETRITAL ZIRCON PROVENANCE OF SEDIMENTS DEPOSITED IN THE MIOCENE TO PLIOCENE GULF OF CALIFORNIA EXPOSED WITHIN SPLIT MOUNTAIN GORGE, SOUTHERN CALIFORNIA

Split Mountain Gorge within the Fish Creek-Vallecito Basin of southern California exposes a record of sedimentation associated with the opening of the Gulf of California during the late Miocene to Pliocene.This study will focus on the deposition of the Wind Caves turbidites and also on detrital zircon provenance of the Miocene to Pliocene sedimentary units exposed at Split Mountain Gorge. The Wind Caves turbidites (a member of the Miocene to Pliocene Latrania formation) have previously been thought to have been sourced from local fluvial systems in the basal section with a shift in sediment source reflecting input from the Colorado River later in the section (Winker 1987). Field study involving measurement of ~600 meters of section has shown that depositional style changes from graded, thick beds lower in the section to thin, more laterally extensive, finer grained beds toward the top of the section. It is hypothesized that this facies change upsection is a result of the burial of chaotic topography previously generated by emplacement of a mass transport deposit. Also possibly influencing depositional style is the integration of a larger fluvial system at some point during Wind Caves deposition. Detrital zircon U-Pb data from six sandstone samples processed thus far show a dramatic change in age populations occurring during Wind Caves deposition. Samples from early in the section (Miocene Red Rock formation fluvial and Lycium member of the Latrania formation turbidite deposits) have prominent U-Pb age peaks around 75 and 95 Ma, likely having been shed by the Peninsular Range batholith and transported to the early gulf by local streams. Upper Wind Caves and later deposits display a more diverse suite of zircon ages ranging from Proterozoic to Miocene and are consistant with the hypothesis that the later sedimentary input was brought in by a river with a larger drainage basin (such as the Colorado River) after having been eroded from diverse terranes across the Western US. Further dating and analysis is necessary to better constrain what this new drainage is and when it begins draining into the gulf.