PALEOCLIMATIC AND PALEOTECTONIC EVOLUTION OF WESTERN PANGAEA INFERRED FROM UPPER PALEOZOIC LOESSITE DEPOSITS

Upper Paleozoic loessite deposits are widespread throughout the area of western Pangaea. These deposits appear to have responded sensitively to changes in climatic and paleogeographic conditions on various temporal and spatial scales. Understanding the provenance of these deposits is an important tool for documenting these changes. Our present dataset of detrital zircon geochronology includes 35 loessite, reworked loessite, paleosol and eolian sandstone samples ranging in age from earliest Pennsylvanian to latest Permian and spanning across western Pangaea from the western margin to the mid-continent. These data indicate that, during the early to mid Pennsylvanian, silt was derived from regional sources, primarily the Ancestral Rocky Mountain (ARM) uplifts (Yavapai-Mazatzal basement, 1800-1600 Ma). By earliest Permian time, the silt provenance shows increased mixing, with a large influx of Grenvillian (1100-900 Ma) and Neoproterozoic basement (740-570 Ma) grains, likely signaling input of sediment from the Appalachian orogen. The presence of this Appalachian signal, however, is tempered spatially by the development of the western Pangaean monsoon by earliest Permian time such that the ARM provenance signal is actually stronger in the eastern parts of western Pangaea. By mid to late Permian time, the ARM signal is overwhelmed by Grenville-age grains, particularly in the mid-continent region, indicating a fundamental shift in the paleogeographic and climatic regime. Documented mid to late Permian burial of ARM uplifts coinciding with sedimentary evidence for increased aridity suggests that the transport distance of silt grains likely increased. Whether the Grenville-aged detrital grains in these mid-Permian units derived directly from eastern sources or from western Pangaea is not yet resolved. Whole-rock geochemistry of the loessite units support these basic interpretations. On shorter timescales, a number of loessite-paleosol couplets exhibit contrasting detrital-zircon age spectra, indicating atmospheric circulation changes accompanied the facies (climate?) changes; these inferences are also supported by whole-rock geochemistry and sedimentology of the facies pairs.