Regional outcrop correlation of the Niobrara formation northwestern Colorado

Abstract

The purpose of this research was to measure, describe, and correlate the Niobrara Formation in northwestern Colorado using four outcrop measured sections: New Castle, Wolcott, Kremmling, and Delaney Buttes. All four outcrops are located on the margins of their respective basins (Piceance, Eagle, Middle Park, and North Park basins) creating reference sections with nearby well logs. The study interval is the Juana Lopez calcarenite to the basal Pierre Shale (Mancos Shale). Once all the data was collected several major observations were made. There is a distinct lithology change in the Juana Lopez calcarenite from south to north across the study area. The chalks and marls of the Smoky Hill Member of the Niobrara Formation thicken from east to west from the Denver-Julesburg Basin to the Piceance Basin. The changes in thickness and lithology of the study interval are interpreted to have been caused by the Transcontinental Arch. Disappearance of ammonites and foraminiferal biozones are evidence of an unconformity exists at the base of the Juana Lopez. Thinning of the Storm King Mountain Shale from New Castle to Kremmling is evidence of a subtle unconformity at the base for the Fort Hays Limestone. The topographic profile of the Transcontinental Arch is not well understood. The changes in thickness and lithology of the study interval indicate movement along the Arch during time of deposition. The thinning observed in the Storm King Mountain Shale indicates a vertical relief of up to 200 feet for the Arch. The vertical relief of the Arch could have restricted clastic sediment to the west side of the Arch.