THE CENTRAL CITY GRAVEL-ROCKY FLATS-COLORADO ICEFIELD CONNECTION AND MIDDLE PLEISTOCENE EROSION OF THE FRONT RANGE-DENVER BASIN REGION AND FORMATION OF THE SOUTH PLATTE RIVER, COLORADO, USA

The physiography of the High Plains-Rocky Mountain front is marked by broad, alluvial surfaces abruptly truncated at the Front Range with little to no conclusive data on correlative surfaces or deposits within the mountain range. Within both intermountain and High Plains regions, a major regional unconformity exists between generally localized, finer-grained, sandy pebble-cobble basin fill deposits, or scoured bedrock, and overlying, coarse, subrounded to rounded boulder-cobble fluvial gravel. These boulder deposits are primarily limited to high elevations (>2,200 meters asl), mountain range perimeters, abandoned mountain flank interfluves, and vast depositional plains grading to older glaciofluvial surfaces within the mountain ranges. In most places, this relationship has been interpreted to be an Oligocene to Miocene phenomena based on the high geomorphic position and/or complicated relationships to the modern fluvial system. We correlate new 10Be/26Al cosmogenic burial isochron chronologies for the Central City Gravel, basal sediment isochron age of 2.04 +/- 0.15 Ma, with classic Rocky Flats-equivalent deposits (>2 Ma-400 ka) to demonstrate probable timing and breadth of glaciofluvial erosion of the Front Range and Denver Basin and formation of the South Platte River. In this model, connectivity of much broader icefields and permanent snowfields developed at elevations >2,200 meters asl eroded underfit canyons and deposited anomalous debris, with major <500 ka glaciofluvial flooding, protracted drainage integration, and coalesced incision along tributaries to the South Platte River. In addition, geomorphic relationships of these early-middle Pleistocene glacial deposits allude to a neotectonic signature along northwest-trending faults presumed to be Cretaceous-Paleocene in age. The overall physiography from 1,600-4,200 meters asl can be explained by slow Neogene tectonism (0.1-0.3 mm/yr) with broad fluves forming into surrounding terrain and up to 350 meters of vertical canyon incision being produced in the last ~500 ka at rates >0.5 mm/yr.