DEPOSITIONAL AGES, SEDIMENT PROVENANCE, AND PALEOGEOGRAPHY IN THE CENTRAL GREAT PLAINS AND ADJACENT ROCKY MOUNTAINS DURING LATE PALEOGENE (Invited Presentation)

Ages and sediment provenance of Cenozoic sedimentary rocks in the Great Plains and adjacent Rocky Mountains are critical to the understanding of landscape evolution in the interior of western U.S.A. Late Paleogene sedimentary units in the region are tuffaceous and contain abundant magmatic zircons derived from widespread late Paleogene ignimbrite eruptions in western and southwestern North America. Recent sedimentologic observations to these units revealed a transition from fluvial to eolian deposition. The eolian sedimentary units are massive, and better sorted compared to the underlying fluvial units. Detrital zircon U-Pb geochronology study to these units shows that both the fluvial and eolain depositional units contain zircons of 24-3314 Ma, and the two lithofacies have similar zircon age distributions. Maximum depositional ages of these units, based on weighted mean U-Pb ages of the youngest clusters of detrital zircons, are generally consistent with the available ash radiometric ages and North American land mammal ages, suggesting that maximum detrital-zircon U-Pb ages can be used to tightly constrain the depositional ages of the Paleogene strata in the central Rocky Mountains and its adjacent Great Plains. Zircons of 24-44 Ma in the late Paleocene strata were derived from the distal ignimbrite eruptions, and zircons of 45-3314 Ma were derived directly from local Laramide basement-cored uplifts and recycled from Paleozoic-lower Cenozoic strata widely distributed in western North America. Maximum depositional ages of the oldest eolian deposition at four study sites along an east-west transect show that the eolian deposition initiated during the latest Eocene-early Oligocene and became younger eastward, suggesting eastward progressive drying during the late Paleogene. The diachronous drying may result from the combined effect of high topography of the Cordillera hinterland and central Rocky Mountains during the late Eocene and global cooling at the Eocene-Oligocene boundary. The similarity of zircon age distributions between the fluvial and eolain units suggests that eastward-flowing fluvial systems and eolian depositional processes supplied unlithified source sediments for each other and promoted sediment recycling during the late Paleogene.

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