PALEOCENE-EOCENE MEGAFAN SEDIMENTATION IN THE DENVER BASIN, COLORADO

Syn-orogenic sediments associated with the Laramide Orogeny offer insight into the tectonic and climate conditions in central Colorado in the Late Cretaceous to Eocene. Material in the Denver Basin, derived from the Rocky Mountains to the west, is divided into two sequences (referred to as D1 and D2) based on lithological differences (Reynolds, 2002). The older D1 sequence (~68-64 Ma) is a fluvial package of largely andesitic material that preserves fine-grained overbank deposits. The overlying D2 sequence (~55-38 Ma) rests unconformably on D1 and is an arkosic pebble conglomerate about 200m thick proximally. The package outcrops as buttes and escarpments throughout the Denver Basin. Based on sedimentary facies, paleocurrent data, and isopach mapping, we interpret the D2 sequence as a fluvial megafan. A lack of abundant preservation of overbank facies and domination of stacked and overlapping channel deposits in the D2 indicates a rapidly avulsing fluvial system. Paleocurrent data indicate a radial distribution system, with the apex of the megafan near the Wildcat Mountain Fan paleovalley. Isopach data combined with depositional slope calculations indicate a > 4,000 m² area fan shaped lobe of sediment. The identification of a fluvial megafan has significant implications for Paleocene-Eocene climate in North America; fluvial megafans are typically found in monsoonal or seasonal climates where rivers experience a high variance in seasonal discharge (Leier et al, 2005). The existence of a megafan in the earliest Eocene implies a shift to a more seasonal climate in the Denver basin that may be associated with the Paleocene-Eocene thermal maximum.