ABSTRACT

During the Pennsylvanian and Early Permian, block-faulted tectonism created mountain ranges (Ancestral Rocky Mountains) as much as 5,000-10,000 ft in elevation in Colorado and adjacent areas. These mountain ranges and associated basins profoundly affected sedimentation; thick sequences (up to 20,000 ft) of Pennsylvanian and Permian strata, with abrupt facies changes and thickness variations, were deposited adjacent to the uplifted mountain blocks.

Early Pennsylvanian tectonic activity developed the general outlines of the north- to northwest-trending Front Range, Apishapa, Uncompahgre, Pathfinder, and Sawatch uplifts. Up to 2,000 ft of nonmarine, alluvial and coastal-plain deposits (Kerber, Sharpsdale, Flechado, and Fountain Formations) accumulated locally in narrow facies bands adjacent to these uplifts, but generally marine shale and carbonate deposition (Morgan, Belden, Casper, and Minnelusa Formations) prevailed.

During the Desmoinesian, 5,000- to 9,000-ft displacements occurred on many faults that bounded the mountain-block uplifts. Narrow bands of thick, coarse-grained arkosic detritus (Minturn, Maroon, Fountain, Sangre de Cristo, Alamitos, Cutler) surround all of the major uplifts; these arid-climate alluvial-fan and coastal-plain deposits change facies abruptly to marine carbonates (Hermosa, Morgan), evaporites (Paradox, Eagle Valley), and shales within short distances from the mountain uplifts.

During the Late Pennsylvanian and Early Permian, the area of alluvial sedimentation expanded onto the mountainous areas and laterally from them, and filled the adjacent basins with up to 3,000 ft of red, arkosic sandstone and shale (Fountain, Maroon, Sangre de Cristo, and Cutler). Local relief of the Ancestral Rocky Mountains became subdued by Guadalupian time, and erosion of low-lying land area or marine deposition (Lykins, Park City, Kaibab) prevailed.

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