GLOBAL COOLING INDUCED DIACHRONOUS ARIDIFICATION IN THE ROCKY MOUNTAINS DURING THE LATEST EOCENE-EARLIEST OLIGOCENE

The transition to an eolian depositional environment in continental interiors documents aridification caused by changing regional atmosphere circulation patterns. One example is the middle Cenozoic aridification in continental Asia, whose causes may be attributed to global cooling during the Eocene-Oligocene transition (EOT), land-sea redistribution, or tectonism. Similar records from other continents may shed light on this debate. We document a similar transition in the central Rocky Mountains (Rockies) and adjacent central Great Plains during the middle Cenozoic. The transition is defined by relatively abrupt changes from poorly-sorted, stratified fluvial lithofacies to massive, well-sorted, fine-grained sandstone of eolian origin in four localities in Wyoming and western Nebraska. The grain size distribution of the fluvial lithofacies varies between unimodal and multimodal, and that of the eolian sandstone is bimodal, with a major peak at 30-130 mm and a minor peak at 2-10 mm. To varying degrees, rock magnetic parameters (bulk magnetic susceptibility, anhysteretic remanent magnetization intensity, and saturation isothermal remanent magnetization intensity) all show appreciable enhancement within the eolian sequence. Available geochronologic age data show that the eolian deposition began during the latest Eocene-earliest Oligocene and expanded eastward through time. This transition postdates most, if not all, crustal shortening of the central Rockies, and persisted across the global cooling event that occurred at the EOT. Regional climate model simulations based on late Eocene paleogeography suggest that uplift of the Cordilleran hinterland and central Rockies during the early Cenozoic, prior to the EOT, resulted in drying in the Cordilleran hinterland and immediate foreland but moistening farther to the east. Global cooling at the Eocene-Oligocene transition eventually drove the observed eastward-migration of continental aridification in the central Rockies.