THE ARAPAHOE CONGLOMERATE IN THE DENVER BASIN, COLORADO

The Arapahoe Conglomerate is a polymict unit of coarse sandstone and gravel that was dispersed across an erosion surface in the Denver Basin during the latest Cretaceous. This unit was described over a century ago and has often been cited as a harbinger of the Laramide Orogeny, yet its regional distribution and thickness trends remained enigmatic. Surface outcrops are from 1 to 10 meters in thickness and are concentrated on the western margin of the Denver Basin. These include the type locality at Willow Creek first described by Emmons, Cross and Eldridge in a 1896 USGS Monograph. Over much of the remainder of the basin, outcrop exposures are inadequate to clearly identify the facies offset that defines this basal conglomerate. In the subsurface, electric log correlations allow the conglomerate to be confidently mapped over about 5000 square kilometers. Detailed log correlations suggest several tens of meters of relief on the unconformity and conglomerate deposition on an easterly flowing drainage system. Conglomerate deposition marks the transition from erosion to deposition in this area. This unit occurs immediately above the unconformity marking the top of the Laramie Formation and the base of the D1 Sequence of synorogenic strata in the Denver Basin. Because of the asymmetric nature of the basin, the amount of time not represented by rock is likely to be greater to the mountain ward side (west) and the bounding units may gradually become conformable to the east. Clasts in the conglomerate are dominated by resistant materials including chert, aphanitic volcanic rocks, vein quartz and fossilized wood. Paleogeographic reconstructions suggest the constituents of the conglomerate represent a residuum resulting from erosion of Mesozoic and Paleozoic sedimentary deposits from the crest of the uplifting Front Range. High porosity and permeability in the coarse strata are interpreted to be important in developing the Arapahoe Aquifer in Douglas County. To the south the unit is correlative to the Raton Conglomerate in the Raton Basin; to the north in the Cheyenne Basin and to the west in South Park comparable facies have not been found. This may suggest a difference in style of basin development in these areas.