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

Fluvial systems of the Rocky Mountain-Colorado Plateau region are a product of temporo-spatial migration of Neogene tectonism and episodic, glacial hydrologic fluxes. Structural geology, sedimentology, paleontology, tephrachronology, geomorphology, geodetics, and historic/paleoseismic inventories indicate: 1) tectonic activity rates have been ~0.1-0.3 mm/yr; and 2) major incision occurred across the region <640 ka (MIS 16), and globally within the northern hemisphere <500 ka (MIS 12?), coeval to appearance of the largest Laurentide and Fennoscandinavian ice sheets. Based on well-constrained evidence from the Rio Grande and Arkansas fluvial systems and existing evidence from the Colorado River system, we present a conceptual model for landscape evolution of major drainages of the Rocky Mountains, providing a testable hypothesis and platform for future work. Herein, Pliocene to early Pleistocene tectonism, volcanism, and climate aridity disconnected basins, fragmented drainages, and parched watersheds. Elevated, broad regions (>3200m asl) provided areas for ice development and runoff during onset of major glacial episodes. Middle Pleistocene (<MIS 16 ~676-621ka)(MIS 12?) glaciation of the Rocky Mountain-Colorado Plateau region initiates protracted tributary integration and incision across preexisting structures and basins. Erosion and incision of these elevated regions resulted in deeply incised valleys containing trunk glacier systems fed by fragmented and diminished icefields. For the Henry Mountain region, existing data suggests similar timing and climate forcing for landscape and topographic development. MIS 12 glaciation of the Boulder Mountains and Fremont River watershed initiates SE-directed, glaciofluvial incision of relict, underfit canyons of Capitol Reef National Park and vicinity. Protracted watershed is captured into Escalante and Fremont River canyons ~200ka, as headward incision of the Colorado migrates to the northern side of the Henry Mountains. Similar relationships are demonstrated in upstream tributary watersheds, contributing to the overall erosive capabilities of the middle Pleistocene (≤400 ka) Colorado River system.