Deciphering the evolution and origin of the lower Colorado River and uplift of the Colorado Plateau: Preliminary U-Pb carbonate dating results and future directions
Abstract:Long-standing debate has focused on the timing, mode, and direction of Colorado River integration, either down or up river. Miocene–Pliocene Colorado River deposits are well exposed in the lower Colorado River corridor (i.e. Bouse Fm and Bullhead Alluvium). There, improved geochronologic constraints have the potential to test integration models and improve our understanding of the inception of a continental-scale river system. In addition, timing and magnitude of regional uplift could be estimated if 5-6 Ma paleoelevation of any part of the system can be confidently identified (e.g. paleo sea level).
Present data indicate that downward integration occurred in part through a series of lake basins that filled and spilled as fluvial-deltaic sediments prograded towards the Gulf of California. However, a possible marine connection and hence a near sea-level elevation for the lower Blythe basin of the Bouse Fm remains controversial. Non-marine Sr signatures in lower Bouse Fm carbonates are permissibly consistent with mixing in an estuary and conversely, limited assemblages of marine fossils are permissively explained by avian transport to inland saline lakes. Improved geochronology has the potential to discriminate between a < 100 ka episode of downward integration of short-lived lakes from a > 1 Ma longer-lived estuarine system. Improved geochronology can also help resolve controversy about the timing of the arrival of Colorado River sediment and water in each basin and, in particular in the Salton Trough.
We present initial U/Pb results on Hualapai Limestone and Bouse Fm carbonates. Hualapai samples give linear 3D isochron ages of 9.3 ± 1.1 and 10.4 ± 3.1 Ma that are consistent with available stratigraphic controls. Preliminary attempts to date Bouse marl have so far yielded imprecise results. However generally high U/Pb ratios and U concentrations in Bouse samples are promising. Efforts are currently underway to date fine-grained banded Bouse tufas from lake margins.