TESTING THE ARIZONA RIVER HYPOTHESIS: DETRITAL ZIRCON SPECTRA FROM ORTHOQUARTZITE CLASTS IN THE MID-TERTIARY SESPE FORMATION OF COASTAL SOUTHERN CALIFORNIA

Thermochronological studies of the Upper Granite Gorge of Grand Canyon suggest that the erosion surface was within a few hundred meters of the modern one by mid-Tertiary time, following a pulse of erosional unroofing from 28-18 Ma (Flowers et al., 2008, GSAB). A broadly distributed mid-Tertiary conglomeratic sandstone interval in coastal southern California (Sespe Formation) appears to have a source, in part, in the Upper Granite Gorge, at least 400 km inland from the coast at the time of deposition, suggesting they were connected through an ancient "Arizona River." Quartzite cobbles in the Sespe Formation appear to be a good petrographic match for the ~1.1 Ga Shinumo Quartzite, exposed only in the Upper Granite Gorge of Grand Canyon, low within the Unkar Group. Three Sespe Formation orthoquartzite clasts (out of a sample population of 52), are indistinguishable from Shinumo Quartzite, based on the combined criteria of detrital modes, major element chemistry, paleomagnetic inclination, and detrital zircon ages. Similar densely cemented, hematitic orthoquartzites are exposed throughout the SW US and NW Mexico, and range in age from Paleoproterozoic to Cambrian. However, the very high-silica varieties (>98% SiO2) that dominate the Shinumo Quartzite occur only in relatively minor proportion in other successions. Of the relatively sparse but viable petrologic matches for these clasts, none are known to have both (1) moderate paleomagnetic inclination (ca. 45°) and (2) cosmopolitan detrital zircon profiles with modes near 1.2, 1.4 and 1.7 Ga, as observed for all three clasts and the Shinumo Quartzite. Kolmogorov-Smirnoff tests between age spectra from the three clasts and those of four samples from the middle and upper Shinumo Quartzite yield 8 comparisons with P-values greater than 0.05 (range 0.06 to 0.37). These values are comparable to those for tests between the spectra from the four samples of Shinumo Quartzite. Both clast and Shinumo spectra contain small but significant peaks that are less than 1100 Ma. Cumulative age profiles, for all ages in the clasts and all ages in the Shinumo that are less than 1100 Ma, are strikingly similar, varying by 10 Myr or less along most of the profile.