THE CENTRAL COLORADO ICEFIELD AND MIS 12-11 (~480-370 KA): A MECHANISM FOR PROTRACTED FLUVIAL INTEGRATION OF FRAGMENTED WATERSHEDS, AND ~250-300 METERS OF LANDSCAPE RELIEF, NORTHERN RIO GRANDE RIFT-SOUTHERN ROCKY MOUNTAINS, COLORADO AND NEW MEXICO, U.S.A

The physiography of the southern Rocky Mountains-Rio Grande rift region is marked by high-elevation plateaus >2300 m asl, mountain summits >3200 m asl, active faults, and regionally-correlative Pliocene?-Pleistocene erosional and depositional surfaces. The region forms headwaters for the Rio Grande and Arkansas and Platte River systems, which flow through a diverse range of tectonic environments. A regional unconformity exists between finer-grained basin fill deposits, eroded bedrock surfaces and straths, and overlying, coarse boulder-cobble gravel deposits that, based on complex geomorphic relationships, has been interpreted to be a Neogene phenomenon. Boulder deposits above this unconformity are primarily limited to high elevations >2200 meters, mountain-range perimeters and interfluves, and depositional plains grading to parks, passes, and glaciofluvial surfaces within ranges. Their distribution suggests they can be produced by connectivity of broad icefields and permanent snowfields developed prior to fluvial capture and topographic development. ¹⁰Be/²⁶Al cosmogenic burial ages for three locales indicate <630 ka integration and incision created profound geomorphic change resulting in this unconformity. Along the Arkansas River, the pre-unconformity Malta Gravel has a burial age of~1.8-2.0 Ma, while downstream, middle Pleistocene glacial deposits overlie low-energy, fine-grained basin fill deposits containing the Lava Creek B ash (~630 ka) and underlying sediments with a burial age of ~670 ka. On the South Platte River system, the Central City Gravel has basal sediment ages of ~1.5 Ma, and <400 ka incision across the adjacent piedmont. Combined with existing evidence from studies along the Rio Grande, La Jara Creek, and the Jemez River, we demonstrate coeval capture and incision along fluvial systems occurred in response to middle Pleistocene glacial intensification. The overall physiography indicates Neogene transtensional tectonism, at rates of 0.1-0.3 mm/yr, created broad, fragmented, or closed alluvial/fluvial systems punctuated by <500 ka glaciofluvial capture and incision at rates >0.5 mm/yr.