Middle Pennsylvanian Rocks of Eastern Great Basin

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The Atokan and Desmoinesian Series in the eastern Great Basin consist mostly of nine major carbonate rock types ranging from carbonate mudstone through micritic allochemical limestone to grainstone. On the east, in the region of the Oquirrh Formation, orthoquartzites are important, and in some local sections they constitute nearly half the sedimentary sequence. Stratigraphic and petrographic data were combined to define lithofacies which were linked in a paleogeographic pattern and model of sedimentation. Areas demarked by the main lithofacies correspond closely to distribution of lithostratigraphic units recognized in the region: (1) the upper Ely Group, particularly the Hogan Formation, comprises a basin assemblage deposited mostly in relatively deep water; (2) the Bird Spring Group and Callville Limestone represent a transition between a shallow shelf on the east and southeast, and the basin in east-central Nevada and west-central Utah; (3) the Oquirrh Formation is a transitional slope assemblage between the basin and Weber shelf on the east. During the Atokan, the basin was not large, and the central Great Basin region was dominated by shifting conditions resulting from sea level changes superimposed on differentially subsiding areas. Conditions were more regular in the basin during the Desmoinesian Epoch, when the basinal environment expanded southward and eastward. However, in the area of the Oquirrh Formation, periodic subsidence coupled with rapid sea level fluctuations resulted in alternation of orthoquartzite and quiet water calcareous sedimen s. Orthoquartzites in the Oquirrh Formation are polygenetic, but most are considered to have been built westward by shore progradation following pulses of subsidence. Transport down a paleoslope by density currents following subsidence probably was another important means of deposition. Maximum thickening of Middle Pennsylvanian rocks was not in the basin, but in the region encompassed by the Oquirrh Formation as a result of greater subsidence. Determination of the original thickness of the Desmoinesian Series in the central Great Basin is made difficult by probable erosion of all or part of the Desmoinesian record at several key localities. Coupled with this is the difficulty in evaluating amounts of compaction in the predominantly fine-grained sediments of the basin. The Middle Pennsylvanian part of the Bird Spring Group does not show significant thickening. Previous attempts to correlate areas of maximum thickening with environmental centers of deposition have resulted in difficulties in environmental interpretation. Post-Paleozoic structural deformation was not evaluated in this study, although the effects probably were great.