Carboniferous of the Eastern Interior Basin

The Carboniferous rocks of the Eastern Interior basin reach a maximum thickness of 5,700 ft., and the Mississippian and Pennsylvanian subdivisions are separated by a major widespread erosional unconformity. The major subdivisions of the Mississippian are the Kinderhook, Osage, Meramec, and Chester Series, and those of the Pennsylvanian are the McCormick, Kewanee, and McLeansboro Groups. These series and groups constitute the basic stratigraphic slices treated herein. More than 3,200 ft. of Mississippian sediments were deposited in the Eastern Interior basin while thinner deposits accumulated on adjacent arches and uplifts. Biologically and chemically derived sediments dominate the Meramecian rocks of the region, whereas the volume of terrigenous detritus is significant in Kinderhookian and Osagian rocks and constitutes the bulk of Chesterian rocks. The distal marine parts of southward-prograding deltas pulsed into the area during early Kinderhook time. As they waned, carbonate deposition spread from the west and southwest to the east, covering extensive areas by the end of Kinderhook time. The Borden Delta complex prograded into the basin and adjacent eastern areas during Osage time. Siliceous, cherty Osagian rocks accumulated in the southwestern part of the region. Carbonate sedimentation, which was initially restricted to the western areas, again spread eastward and northward as the deltas waned, and reached its maximum extent during the Meramec. Evaporite deposition during mid-Meramec time marked a widespread episode of restricted circulation. The carbonate environment retreated as major delta deposits prograded southward and dominated the central part of the Eastern Interior basin during Chester time. Although generally restricted to southern parts of the region, carbonate accumulation periodically extended over large areas in and beyond the basin during this time. Depositional environments throughout the Mississippian were mostly shallow marine over broad areas, but deeper marine environments were present in the southern parts of the region during the Osage and early Meramec. These shallow seas opened and deepened southward across a broad shelf and connected with the deepening Ouachita Trough. The terrigenous deltaic sediments were transported into the region by the large Michigan River system, which drained eastern parts of the Canadian Shield and northern extensions of the Appalachian Mountain belt. Broad regional uplift marked the close of Mississippian time when the littoral zone retreated southward out of the area. By Early Pennsylvanian time the region was a southwest-inclined coastal plain with a well-developed linear drainage pattern and river valleys as deep as 200 ft. With renewal of subsidence during Early Pennsylvanian, the littoral zone transgressed north toward the Eastern Interior basin resulting in the accumulation of thick sequences of alluvial sands and muds in the pre-existing valleys and in the eventual burial of the unconformity under an apron of alluvial and upper delta-plain sediments. Shallow-marine environments moved into the area, and the long period of southwestward progradation, abandonment, and progradation of a series of deltas began. This Pennsylvanian deltaic sedimentation resulted in deposition of as much as 2,500 ft. of dominantly terrigenous, clastic sediments on a slowly subsiding shallow-water cratonic platform. The Michigan River system passed through the Michigan basin region, depositing mostly fluviatile and upper delta-plain sediments there. The river system with its delta plain and widespread coal-swamp deposits repeatedly prograded into the shallow-marine environments of the Eastern Interior basin, where thin but widespread bioclastic carbonates were accumulating. The cyclothemic character of the Pennsylvanian rocks here is the result of this repeated southwestward regressive progradation of at least 51 delta and subdelta sequences. Erosion of the latest Pennsylvanian sediments prevents us from deducing precisely when Carboniferous deposition ceased. A gradual shift took place from deposition of orthoquartzite sandstones in Late Mississippian–Early Pennsylvanian rocks to subgraywacke sandstones in the Middle and Late Pennsylvanian rocks. The terrigenous sediments of the Eastern Interior basin were derived largely from the tectonic borderlands of the northern part of the Appalachian basin and were transported west and southwestward by the Michigan River system. Throughout Carboniferous time, the Eastern Interior basin was connected across the shallow-marine cratonic platform with the Appalachian geosyncline to the east, the midcontinent basin to the west, and the deepening Ouachita geosyncline to the south.