MIS 12-11 (478-374 KA) ~ MIDDLE PLEISTOCENE GLACIATION OF THE SOUTHERN ROCKY MOUNTAIN-GREAT PLAIN REGION: LINKS TO LANDSCAPE EVOLUTION OF MAJOR CENTRAL NORTH AMERICAN RIVERS

Marine isotope stages (MIS) 12-11 (~478-374 ka) represent the first large isotopic change in the marine isotopic record associated with the shift to 100-kyr glacial cycles. Herein, we present geomorphic and chronologic constraints on Pleistocene terrestrial landscape development since this period in the southern Rocky Mountains-Great Plains region. We apply geologic mapping assisted with stratigraphic, structural, geomorphic, chronologic, paleontologic, paleoclimatic, paleoseismic, and geodetic data to-date. Ultimately, we provide a simplistic, but comprehensive, view of the timing, extent, and impact of Neogene tectonics and Pleistocene glaciations on Southern Rocky Mountain-Great Plains landscape development. Pliocene to early middle Pleistocene (ca. 5 Ma-600 ka) geomorphic environments were dominated by tectonic and volcanic processes, with local aggradation along major inter-basin drainages. Within the western Great Plains, dispersed Neogene tectonics (e.g., Cheraw and Meers faults and Anton Escarpment) created depressions permitting local aggradation and subtle dissection westward toward the Rocky Mountain front and Continental Divide, and developing pre-Rocky Flats abandonment (>400 ka) geomorphic relationships. Major canyon incision of Rocky Mountain drainages and exhumation of the adjacent piedmont began with the maximum extent of the Central Colorado Icefield (CCI) and western margin of the Laurentide Ice Sheet, which we tentatively place during MIS 12. With deglaciation during MIS 11, the Rio Grande, Arkansas, and Platte River headwaters, Front Range piedmont, and Denver Basin began to exhume. Within the Rio Grande rift (RGR), tectonics permitted sediments to aggrade above the ~631 ka Lava Creek B tephra, but the fluvial capacity of the MIS 12-11 geomorphic interval removed any tectonic or volcanic basin-closing feature, reintegrating many RGR drainages with the Gulf of Mexico.