PROGRESSIVE MIO-PLIOCENE TO HOLOCENE FLUVIAL DENUDATION OF THE UPPER COLORADO RIVER CATCHMENT RECORDED BY DETRITAL ZIRCON AGE DISTRIBUTIONS

Detrital zircon (DZ) U-Pb ages from the Colorado River show that the DZ age spectra for the river has been remarkably uniform over the past ~6 million years consistent with a top-down mode of river integration. The river’s DZ provenance signature is established far upstream in the modern Colorado River system by the Green, “Grand” and San Juan tributaries consistent with sediment transport records that demonstrate easily erodible Mesozoic-Cenozoic strata in the upper reaches of the basin are the dominant sediment-contributing portion of the catchment. Recent geologic and thermochronologic studies further demonstrate the upper catchment is coincident with a broad region of deep fluvial erosion that provided the bulk of Colorado River sediment throughout its history. Although the river DZ signature is consistent through time, comparison of aggregated DZ age distributions from the Holocene delta versus the Mio-Pliocene early river reveal subtle yet robust differences that record progressively deeper fluvial erosion following river integration at ~6 Ma. Simple binary and ternary mixing models utilizing previously reported DZ results from Colorado Plateau strata confirm this pattern of progressively deeper erosion as recorded by the river DZ age distributions. The best fit to the Early Pliocene Colorado River involves 73% Early Cenozoic and 27% Mesozoic strata while the best fit to the Modern river involves 48% Early Cenozoic and 52% Late Jurassic-Early Cretaceous strata. A third component represented by the Browns Park Formation is necessary to produce a good fit to the Mio-Pliocene early river which contains a distinct component of Late Eocene-Oligocene grains. Overall, the results demonstrate that the enrichment of the Early Pliocene River in Oligocene and Middle Proterozoic zircon is consistent with top down erosion in the eastern and northern regions of the catchment where Cenozoic strata are present.