PROVENANCE AND DEPOSITIONAL AGES OF LATE PALEOGENE FLUVIAL SEDIMENTARY ROCKS IN THE CENTRAL ROCKY MOUNTAINS, USA

The erosion and basin-filling histories of the central Rocky Mountains (Rockies) are critical to the understanding of landscape evolution in the interior of western U.S.A. Ages and sources of fluvial deposits formed during late Paleogene allow the reconstruction of paleogeography within an improved chronostratigraphic framework. Here we use detrital-zircon and single-grain-zircon U-Pb geochronology to constrain the provenance and depositional ages of upper Paleogene fluvial deposits in the central Rockies. A total of 885 concordant detrital-zircon dates from ten fluvial sandstone samples show major age populations of 24-44 Ma, 920-1326 Ma, 1334-1825 Ma, and 1830-3314 Ma, and minor age populations of 45-218 Ma and 227-708 Ma. The youngest population was derived from distal ignimbrite eruptions in western and southwestern North America. The populations older than 45 Ma were derived directly from local Precambrian basement-cored uplifts and recycled from Paleozoic-lower Cenozoic strata. The maximum depositional ages based on the weighted mean U-Pb ages of the youngest clusters of detrital zircons are generally consistent with the available ash radiometric ages for samples of latest Eocene-Oligocene, a time interval with high ignimbrite eruption flux in western and southwestern North America, suggesting that detrital-zircon U-Pb ages can be used to tightly constrain the depositional ages of these strata when pure ash beds are not present and when the strata contain abundant concurrent magmatic zircons. Sandstone samples of latest Eocene age have more Archean zircons (> 2500 Ma) than samples of Oligocene age in the central Rockies, suggesting that the Archean cores of the Laramide ranges were largely exposed until the Oligocene. By combining these results with previously published detrital-zircon provenance results of late Paleogene eolian sandstones in the same region, we suggest that the gradual burial of the Archean cores during the Oligocene resulted from widespread eolian deposition initiated during the latest Eocene-early Oligocene. Generally eastward-flowing fluvial systems and eolian depositional processes supplied unlithified source sediments for each other and promoted sediment recycling during the Oligocene. Furthermore, the abundance of zircons with 37-25 Ma ages in the late Paleogene deposits shows that the ignimbrite eruptions in western and southwestern North America were most active during this time interval.