Provenance of quartzite clasts in the Eocene-Oligocene Sespe Formation: Paleogeographic implications for southern California and the ancestral Colorado River

Quartzite clasts in fluvial Sespe Formation conglomerates of the Santa Moni ca Mountains were compared petrographically with potential source rocks in the Basin and Range and Transition Zone provinces to determine their proven ance. Modal analysis indicates that Sespe orthoquartzite clasts are mainly quartzofeldspathic and derived from source rocks with craton-interior prove nance, Such clasts were probably derived from Stirling Quartzite and Wood C anyon Formation sources in the northern Mojave Desert. The Mazatzal Peak Qu artzite of central Arizona is the probable source of quartzolithic orthoqua rtzite clasts containing abundant detrital jasper grains, Sespe metaquartzi te clasts are mainly from sources in the southern Mojave Desert, but some m ay be from the Yavapai terrane of central Arizona, The dual provenance of o rthoquartzite clasts suggests that the Sespe paleoriver was a bifurcating s ystem tapping both Mojave Desert and Sonora Desert sources. After compensat ing for Neogene displacement along the San Andreas fault, a paleogeographic reconstruction shows that there is a general spatial coincidence between S espe (and other Cenozoic) paleodelta deposits around Los Angeles that conta in exotic clasts, and the inferred location of the mouth of the Colorado Ri ver (assuming that it existed during the Eocene), Thus, the Colorado River may be much older than previously thought (as old as late Paleocene), and p erhaps was once part of an ancient fluvial connection with the Mojave Deser t interior.