TIMING, PROVENANCE, AND PALEOCLIMATE IMPLICATIONS OF THE CENOZOIC EOLIAN DEPOSITION IN THE CENTRAL ROCKY MOUNTAINS

Cenozoic sedimentation in the Central Rocky Mountains has undergone a regional transition from fluvial to eolian deposition. However, the timing of the transition and source of the eolian deposits are not well understood. We apply detrital zircon U-Pb geochronology and sandstone petrography to nine tuffaceous, fine-grained sandstone samples collected in Wyoming, western Nebraska, and northern Colorado, in order to constrain the timing of the transition and the provenance of the eolian deposits. The four oldest eolian samples taken along a west to east transect have maximum depositional ages of 35.9±0.7 Ma, 35.4±2.0 Ma, 33±0.9 Ma, and 30±1.0 Ma, respectively, suggesting the eolian deposition initiated during the latest Eocene and earliest Oligocene and became younger eastward. A total of 766 detrital zircon ages show populations of 17-40 Ma, 40-60 Ma, 80-220 Ma, 300-750 Ma, 950 Ma-1.3 Ga, 1.3-1.5 Ga, 1.6-1.8 Ga, 1.8-2.2 Ga, and 2.5-3.3 Ga. Zircons of the 17-40 Ma population were derived from the volcanic activity related to the ignimbrite flareup in the western and southwestern USA. Zircon populations older than 40 Ma were derived from the local Laramide based-cored uplifts either by direct wind erosion or by recycling of older basin fill. These sandstone samples contain high amounts of feldspar (~26 %) and lithics (~27 %), supporting mixed contributions of recycled local orogenic provenance and distal magmatic arc provenance. Our results show that the Miocene samples have less abundant Archean zircons than the Oligocene samples, indicating that the increased volume of volcanic eruptions in the Great Basin area or intensified wind activity during the Miocene may have nearly covered the basement cores in the Laramide uplifts, and reduced the direct contribution of Archean zircons from the basement cores. We further propose that the transition to eolian deposition represents regional drying during latest Eocene and earliest Oligocene, which is a combined result of global cooling due to initiation of Antarctica glaciation and the development of rain shadow caused by the uplift of the Central Rocky Mountains.