PALEOGEOGRAPHIC MAPS ILLUSTRATING THE LATEST CRETACEOUS AND EARLY PALEOGENE EVOLUTION OF THE LARAMIDE DENVER BASIN

New paleogeographic reconstructions of the Cretaceous and Early Paleogene depositional environments in the Denver Basin have been made using outcrop observations and subsurface correlation of well logs. Denver Basin D1 sequence depositional environments consist of a vertical succession of alluvial and fluvial distributary fan systems developed in response to Laramide uplift of the Front Range along the western margin of the basin. Outcrops are restricted to the periphery of the basin and are of moderate quality and limited continuity. Geologic mapping has historically subdivided these rocks on the basis of facies and composition: some workers grouped the variable successions into mappable formations like the Denver Formation and the Dawson Arkose, other workers have subdivided the strata into facies-specific packages or mappable facies assemblages.

These mapping efforts establish a framework for our new maps. Using both a more explicit temporal framework and the subsurface well control, we are able to map a series of aggradational depositional elements that make up the basin-filling sediments. Through mapping the fluvial deposits and considering their composition, aspects of the paleo-geomorphology of the evolving Front Range mountains can be deduced as well as the character of their cover. We have inferred the existence of a series of paleo-canyons in the uplands linked to distributary fans feeding into the subsiding foreland basin. The most important fluvial system is defined by the Wildcat Mountain distributary fan that developed from a canyon west of Castle Rock during latest Cretaceous time. This arkosic distributary fan has a very limited outcrop extent, but in the subsurface it is the most important aquifer in the basin.

Inter-fan areas and distal low-relief floodplain environments are characterized by poorly organized fluvial systems with discontinuous sandstone beds that were deposited by meandering and avulsing rivers. These environments and their associated strata provide lower quality aquifers than the proximal time-equivalent deposits. These paleogeographic interpretations with their aquifer quality implications should influence water resource planning in the Denver Basin.