ABSTRACT

Late Devonian-early Mississippian stratigraphic sections representing similar sequences of depositional environments occur at various places around the continental margins and within cratonic basins of North America. The deposition of Devonian-Mississippian black, organic-rich shale was followed by local sediment-starvation during the early Mississippian. A large scale, perhaps world wide, event or condition caused oxygen stratification to expand from the ocean basins, across the continental shelves, and into cratonic basins such as the Il­linois and Williston Basins. Black, organic-rich shale was deposited under anaerobic conditions, greenish-gray shale under dysaerobic conditions, and carbonate production was restricted to aerobic conditions in near-shore areas. As the basins continued to subside, carbonate production was restricted and, because there were few clastic sources, the Illinois and Williston Basins became sediment starved. Sediment-starvation occurs when sediment which is produced within the basin, or reaches the basin from outside, is only deposited around the basin margin and little reaches the deep water in the basin center.

The oxygen stratification event ended in the early Mississippian and carbonate production resumed in shallow shelf areas. The basin centers, however, subsided below the photic zone, where carbonate-producing organisms could not survive. Carbonate sedimentation rapidly expanded beyond the shelves by filling in the deep-water, sediment-starved basins along delta­like clinoform slopes or ramps. The shelf carbonates are predominantly light colored, coarse skeletal and oolitic grainstones and packstones. Although the sediment largely originated on the shelf, the basinal carbonates are mostly dark coloured, fine-grained wackestones and mudstones. The sediment-starved basins filled with carbonates during the early Mississippian and shelf conditions conducive to in situ carbonate production prevailed across the basin.