THE CENTRAL COLORADO ICEFIELD, A MECHANISM FOR INCEPTION OF REGIONAL EXHUMATION AND DEEP-CANYON INCISION DURING THE PLEISTOCENE?

The geographic extent and age of alpine glacation at the Colorado Last Glacial Maximum (LGM) is well defined, and coincides with the global LGM (~21± 2 ka) during marine oxygen isotope stage (MIS) 2. Recent work within the Sawatch Range indicates LGM ice crossed many of the present divides previously thought to be topographic barriers, creating a continuous Central Colorado Icefield connecting separate tectonically-controlled hydrographic basins. Locally outlying, higher geomorphic surfaces in Sawatch Range valleys indicate at least three pre-LGM glacial episodes. Both the San Luis Basin (SLB) and Upper Arkansas Valley (UAV), which share a drainage divide at Poncha Pass, show evidence of major Pleistocene incision and topographic development. This incision post-dates maximum aggradation surfaces in the SLB (~420 ka, MIS 12), which grade across Poncha Pass and can be tentatively correlated to the oldest glacial deposits in the southern Sawatch. Sediments in the SLB indicate a <640-ka age for the onset of middle Pleistocene glacial episodes. Profiles along the Arkansas River corridor extending downstream from these maximum aggradational surfaces grade to a network of broad erosional channels perched above the present steep-canyon topography and are truncated at the head of deeply incised canyons along the eastern margin of the Front Range. Similar relationships can be observed in other Front Range drainages. Based on the geomorphic position of the oldest till deposits on basin divides, and the geomorphic relationships between oldest till at Poncha Pass and maximum aggradation surfaces in the SLB, dated to ~400 ka (MIS 12), we tentatively assign MIS 12 (~480–420 ka) as the major interval when the UAV integrated into through-going fluvial systems, and induced deep-canyon incision down it’s corridor. Similar relationships between oldest glacial deposits and broad erosional channels occur across the Front Range. Regional Pleistocene tectonic uplift rates have been steady at 0.1–0.3 mm/yr. Therefore, we tentatively attribute the high elevations and thick basin sediment sequences in the SLB and UAV to record continuous tectonic uplift, whereas the onset of glacial episodes ≤ MIS 16 (~680–620 ka), drove the dramatic change in landscape incision rates in the Front Range region.