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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 43 (1959)

Issue: 5. (May)

First Page: 1101

Last Page: 1101

Title: Rocky Mountain Association of Geologists Symposium on Pennsylvanian Rocks of Colorado: ABSTRACT

Author(s): William Wyman Mallory

Article Type: Meeting abstract


The Rocky Mountain Association of Geologists, Denver, Colorado, published in 1958 a volume with the above title in conjunction with its field conference on Pennsylvanian rocks of the Maroon basin.

The principal results of these collected studies show that Pennsylvanian tectonism was dominated by vigorous growth of northwest-trending mountain ranges (Frontrangia and Uncompahgria) and their attendant depositional troughs. These obliterated the amoeboid patterns of gentle epeirogeny in Colorado established during early and middle Paleozoic time. Pennsylvanian depositional history began in Colorado with the accumulation of a red clay regolith, the Molas formation, upon the maturely dissected Mississippian Leadville limestone. Four Pennsylvanian depositional basins (or troughs), the Denver, Maroon, Paradox, and Raton, between and adjacent to Uncompahgria and Frontrangia, contain abundant thicknesses of all lithologic types common to cratonic sediments. Two of the basins in western C lorado, the Maroon and Paradox, exhibit extensive evaporites, the Paradox and Eagle sequences. All the basins contain large volumes of red, arkosic conglomerates and finer clastics (the Fountain, Maroon, Cutler and Sangre de Cristo formations) which grade laterally into marine limestones, shales and sandstones with or without passing through an evaporite facies.

Western Colorado marine sequences bear the names Hermosa group, Morgan formation and Weber sandstone. In the Raton basin the Sangre de Cristo and the Magdalena group, predominantly clastic, comprise the section. In the Denver basin, only the Fountain arkose facies outcrops (except for a trace of Glen Eyrie claystone near Colorado Springs); the series terms Morrow, Atoka, Des Moines, Missouri, and Virgil have been borrowed from the Mid-Continent region as faunal subdivisions.

The uplifts attained maximum development in the Des Moines epoch and continued tectonically active into middle Permian or Leonard time. Pennsylvanian deposition carried over without hiatus into the Wolfcamp epoch of the Permian period. The rejuvenated Uncompahgre and the Front range uplifts (now masked by the Laramide Front and Park Ranges) are part of the present tectonic pattern of Colorado.

The bulk of Colorado oil may be classified as follows: (1) Cretaceous sandstone lenses scattered across the Denver basin, (2) the Weber sandstone of the Rangely pool in the Maroon basin, (3) the Hermosa carbonate reservoirs in the Paradox basin. The last two, Permo-Pennsylvanian in age, contain outstandingly important additional reserves. The size and relative youth of Pennsylvanian carbonate pools indicate that additional large reserves can be anticipated in these rocks.

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