THE MIDDLE PLEISTOCENE UNCONFORMITY AND MIS 12-11 (~478-385 KA): GLACIOFLUVIAL PROCESSES OVERRIDE LATE MIOCENE-MIDDLE PLEISTOCENE (~6 MA-500 KA) TECTONO-MAGMATIC CAPTURE AND DEEP-CANYON INCISION, WESTERN GREAT PLAINS-SOUTHERN ROCKY MOUNTAINS-COLORADO PLATEAU REGION, NORTH AMERICA

Pleistocene geologic and geomorphic relationships from the Great Plains to the Colorado Plateau indicate that glaciations were a major process for fluvial capture, deep-canyon incision, and topographic development. In the last 40 years, absolute Pleistocene chronologies have brought forth geomorphic rates and relationships that complicate and challenge classic structural, stratigraphic, and geomorphic interpretations. These recent studies illuminate a previously identified unconformity between >631 ka closed-basin deposits and bedrock erosional surfaces with overlying <500 ka coarse fluvial, glacio-fluvial, glacial, and eolian deposits, after which incision rates increase to >0.5 mm/yr. In addition, paleoclimate investigations indicate regionally pervasive cold, semi-arid to arid, early-middle Pleistocene climatic conditions, recorded by highly oxidized and deeply weathered pedologic characteristics beneath the unconformity. We combine new investigations with previous work along the Rio Grande, Arkansas, and Platte River systems with previous work along the Green, Colorado, and San Juan River systems to demonstrate the regional extent of this middle Pleistocene stratigraphic, geomorphic, and chronologic relationship. Within this framework, Neogene ~6 Ma-500 ka extension (~0.1-0.3 mm/yr) and volcanism fragmented and disconnected watersheds within this region, inducing local aggradation and closed-basin sedimentation. Beginning <500 ka, the largest glacial ice extents and permanent snowfields formed across the Rocky Mountains and distal ranges and plateaus of the Colorado Plateau at elevations >3,000 m/9,800 ft and >2,200 m/7,200 ft asl, respectively. Deglaciation and resulting meltwaters began to incise at rates >0.5 mm/yr across slower-forming (0.1-0.3 mm/yr) neotectonic structures, accounting for >250 meters of incision in <500 ky.