Pennsylvanian and Lower Permian of Northern Denver Basin, Colorado, Wyoming, and Nebraska

Abstract

Permo-Pennsylvanian sedimentation in the northern Denver basin was influenced by the positive areas of Frontrangia (ancestral Front Range) and the Cambridge arch. Near these sources, coarse clastics interfinger with shales and limestones of the basin. The Cambridge arch was first covered by marine Pennsylvanian sediments during Desmoinesian time. Frontrangia contributed coarse arkosic sediments from Morrowan into Leonardian time. A northwest trend of Frontrangia and the Cambridge arch is indicated by isopach and lithofacies data. The Transcontinental arch was a positive pre-Morrowan feature which also was very slightly positive during Desmoinesian and Missourian time and at the end of the Pennsylvanian Period. The Hartville, Wyoming, area was basinal until uplifted near the end of Wolfcampian time as indicated by truncated Wolfcampian sediments. lsopach maps indicate that the Las Animas arch was mildly positive during Morrowan, Atokan, and Desmoinesian time. On the east flank of the Laramie Mountains the Casper Formation includes sediments of Atokan through Wolfcampian Age. Sediments of questionable Morrowan Age may be present in the basal Casper or in underlying tongues of the Fountain. At the type locality strata as young as Leonardian are believed to be present in the upper Casper. The Ingleside Formation is the Colorado equivalent of the Casper and near the Wyoming-Colorado boundary line also includes sediments of Atokan through Wolfcampian Age. South of the Wyoming-Colorado boundary line basal Ingleside intertongues with the Fountain Formation and at the type locality the Ingleside is entirely Wolfcampian. The Hartville Formation is divided into seven members ranging in age from Atokan to Leonardian, inclusive. Wolfcampian fusulinids occur 70-80 feet below the subsurface shale marker in Cheyenne County, Nebraska. The Satanka Formation consists of the predominantly red sediments that lie between the Casper or Ingleside Formation below and the Opeche Shale or the Lyons Sandstone above. Lateral equivalents of the Satanka are the uppermost Casper of the type locality, the Cassa Member of the Hartville Formation, the middle part of the Lyons Sandstone at the type locality and rocks of Leonardian Age in northwestern Kansas. Lithofacies maps of Pennsylvanian and early Permian sediments indicate decreasing clastic ratios in progressively younger strata. Pre-Desmoinesian clastics decrease in particle-size toward the center of the Denver basin, whereas in sediments deposited following burial of the Cambridge arch, during Desmoinesian time, the clastic particles become smaller eastward across the basin. The present structure of the northern Denver basin is abruptly asymmetrical, much steeper on the west than on the east side, and slightly elongate in a northerly direction. The deepest part of the basin lies only 18-20 miles east of the Permo-Pennsylvanian outcrop. Isopach maps of the Pennsylvanian and Lower Permian Series show relatively greater subsidence in the center of the basin than on the flanks, and also indicate a general southward tilting. About one well per 400 square miles has penetrated the Lower Permian-Pennsylvanian section. These objectives in the northern Denver basin must be considered largely unexplored.