Middle Devonian Series through Mississippian System (Kaskaskia Sequence) Joseph A. Devera, W. John Nelson, and Rodney D. Norby 9 Introduction incised to the north. In part of northeastern Illinois, the Kaskaskia sequence is entirely absent, and the rocks of the Rocks of the Kaskaskia sequence (Middle Devonian Absaroka (Pennsylvanian strata) directly overlie Silurian through Late Mississippian) underlie nearly all of Illinois and older rocks (Figure 9-2). south of Interstate 80 but are covered by Pennsylvanian Sloss (1988) divided the Kaskaskia sequence into two bedrock and Pleistocene glacial deposits through most of subsequences. The Kaskaskia I subsequence comprises the this area. The rocks are best exposed along the bluffs of the Middle Devonian Series through the Mississippian Kinder- Mississippi River, extending from Bettendorf, Iowa, south- hookian Series, including rocks of the Grand Tower Lime- ward to Grand Tower in Jackson County, Illinois. Many stone in southern Illinois through the Chouteau Limestone outcrops are also found in the Shawnee Hills region of (Figure 9-1). The Kaskaskia II subsequence contains the southern Illinois and along the Illinois River and its tribu- Valmeyeran Series through upper Chesterian Series, in- taries in western Illinois. Outcrops along the Mississippi cluding formations from the Meppen Limestone through River bluffs in Illinois contain part of the world standard the Kinkaid Limestone (Figure 9-1). section for the Mississippian System, now officially desig- nated as the Mississippian Subsystem of the Carboniferous System. Geological History Devonian and Mississippian rocks are of great eco- The Kaskaskia sequence was greatly influenced by two nomic importance in Illinois. More than 80% of all the oil mountain-building events, the Acadian Orogeny and the and gas produced in the Illinois Basin came from reservoir Alleghany Orogeny. Collision of the Laurentian (North rocks of this age, and the Upper Devonian New Albany America) and Baltican (Europe) paleocontinents caused Shale is the principal petroleum source rock in the Basin. upwarping in Illinois, regression of the sea, and erosion Mississippian rocks are also the leading source of limestone of topographic high points prior to deposition of the Kas- quarried in the state. kaskia sequence. This erosion produced the sub-Kaskaskia In Illinois, the Kaskaskia sequence ranges in age from unconformity (Figure 9-2), which cuts across older rocks, Middle Devonian through Late Mississippian, from about except in southernmost Illinois. With the ensuing sea-level 390 to 295 million years before present (Figure 9-1). The rise, carbonate deposition (Grand Tower Limestone) shows Kaskaskia sequence is dominated by carbonate rock but a deepening-upward cycle of sediment in the basal part of also contains siliciclastics—shale, siltstone, and sandstone. the Kaskaskia sequence. These rocks were deposited in the Illinois Basin over a pe- During the Middle Devonian, the Illinois Basin was riod of about 60 million years. The Kaskaskia sequence lies positioned 10 to 20° south of the equator (Droste and above the sub-Kaskaskia unconformity and below the sub- Shaver 1983). The continent continued to move northward Absaroka unconformity. toward the equator throughout deposition of the Kaskaskia Rocks of this sequence are exposed near the Kaskaskia sequence. By Mississippian time, the Illinois Basin was 5° River in southwestern Illinois (Sloss et al. 1949). The Kas- south of the equator. A warm, equatorial climate is indicat- kaskia sequence is thickest, about 3,800 feet (1,158 m), and ed by the dominance of warm-water, limestone-producing most complete in southern Illinois counties (Gallatin, Sa- fossil organisms in the rocks. line, eastern Williamson, northern Johnson, and Pope). In Carbonate production was interrupted numerous times this area, subsidence during deposition of the Kaskaskia during the deposition of the Kaskaskia sequence, begin- sequence was more rapid than elsewhere, resulting in near- ning in late Middle Devonian time, when mud reached Il- ly continuous sedimentation and maximum preservation linois from the Catskill Delta in the eastern United States of Middle Devonian through Upper Mississippian strata (Collinson et al. 1967). By Late Devonian time, the Illinois beneath the sub-Absaroka unconformity. Northward, in- Basin was covered with poorly oxygenated seas west of the dividual units of the Kaskaskia sequence become thin- Catskill Delta. The New Albany Shale, deposited during ner, and the gaps in the geological record become greater. this time, is part of a vast deposit of black, organic-rich Notably, the sub-Absaroka erosion surface is more deeply Late Devonian shale covering much of North America.

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