Abstract

Permo-Pennsylvanian sedimentation in the north Denver basin was influenced by the positive areas Frontrangia (ancestral Front Range) and the Cambridge arch. Near these sources, coarse clastics interfinger with shales and limestones of the basin. Cambridge arch was first covered by marine Pennsylvanian sediments during Des Moinesian time. Frontrangia contributed coarse arkosic sediments from Morrowan into Leonardian time. A NW. trend of Frontrangia and the Cambridge arch is indicate by isopach and lithofacies data. The Transcontinental arch was a positive pre-Morrowan feature which also was very slightly positive during Des Moinesian 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. Isopach maps indicate that the Las Animas arch was mildly positive during Morrowan, Atokan, and Des Moinesian time. On the E. 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 believe 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. S. 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 7 members ranging in age from Atokan to Leonardian, inclusive. Wolfcampian fusulinids occur 70-80 ft. below the subsurface "red 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 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-Des Moinesian clastics decrease in particle-size toward the center of the Denver basin, whereas in sediments deposited following burial of the Cambridge arch, during Des Moinesian 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 W. than on the E. side, and slightly elongate in a northerly direction. The deepest part of the basin lies only 18-20 mi. E. 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 1 well per 409 sq. mi. has penetrated the Lower Permian-Pennsylvanian section. These objectives in the northern Denver basin must be considered largely unexplored.

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