Abstract

The hypothesis that eustatic sea level changes produced Upper Pennsylvanian cyclothems in Midcontinent North America has been supported by recent documentation of many episodes of Mississippian through Permian glaciation in Gondwanaland (Crowell, 1978). Changes in Midcontinent paleogeography and sedimentary facies during a single eustatic advance and retreat of the sea are described in 6 phases:

1. At maximum transgression, deep water promoted development of a thermocline, quasi-estuarine circulation cell, upwelling, and anoxic bottom conditions, all leading to widespread deposition across the Mid-continent of phosphatic black shale, which graded in shallower peripheral areas to gray marine shale and carbonates.

2. Shallowing during early regression destroyed the thermocline, which restored bottom oxygenation and changed deposition from black to gray shale, then to skeletal calcilutite as benthic algal carbonate production became established across the Midcontinent. Deltas began prograding from Oklahoma and the Appalachians, and shoreline carbonates began prograding southward from the Dakotas.

3. During late regression, shoal-water calcarenites developed over most of Kansas, carbonate shoreline facies prograded into southern Nebraska and Iowa, and deltas of Appalachian origin prograded across Illinois.

4. At maximum regression, the sea became nearly confined to the deep basins of west Texas and Oklahoma, while the exposed carbonate terrain to the north underwent formation of karst, caliche and residuum, and the extensive deltaic deposits to the east underwent channeling, alluviation, and soil and coal-swamp formation.

5. Expansion of the sea during early transgression restored shoal-water calcarenite deposition across western Kansas, caused gray shale deposition in embayments and lagoons along the inundated deltaic terrain to the east, and impounded Appalachian-derived streams flowing westward across the immense alluvial plain to form widespread coal swamps in Illinois.

6. Deepening during late transgression restored skeletal calcilutite deposition across the Midcontinent, caused marine shell accumulations over coals in Illinois, and shifted transgressive coal-swamp formation eastward into the Appalachian region.

In Texas the same sea level changes resulted in much less lateral shifting of shoreline and associated facies because of greater depositional relief, narrower shelves, and closer detrital sources. Thus individual units there are less widespread, and eustatic effects are less noticeable than in the Midcontinent.