Pre-Pennsylvanian stratigraphic and tectonic patterns of the Illinois Basin region are revealed through analysis and segregation of the Sauk (Late Cambrian-Early Ordovician), Creek (Middle and Late Ordovician), Tutelo (Silurian-Late Devonian), Piankasha (intra-Devonian), and Tamaroa (latest Devonian-Mississippian) sequences. To relate the Illinois Basin to its regional setting, these pre-Pennsylvanian (pre-Absaroka) unconformity-bounded successions are delineated from Wisconsin to Tennessee and from Ontario to Oklahoma. Abundant Absaroka strata of the Illinois and other major basins involved are excluded because the regional identity of an adequate number of intra-Pennsylvanian stratigraphic datum surfaces is uncertain.

Approximate basin limits are customarily outlined by the outcrop of the base of the Absaroka sequence in Illinois, western Indiana, and northwestern Kentucky. Original interregional continuity of most units comprising the five pre-Absaroka sequences, and the differential degradation patterns beneath each and beneath the Absaroka, deny inception of basin development until at least Pennsylvanian time. For example, the Creek sequence of the basin is neither depositionally nor preservationally thicker than that to the west in Iowa or to the north in southeastern Wisconsin; but it is both depositionally and preservationally thinner than that to the east and south in Indiana, Kentucky, and Tennessee. Except for areas of its later erosional removal, such as the Cincinnati Arch and the Nashvi le and Ozark Domes, the Tutelo sequence is at least as well preserved in areas adjacent to the Illinois Basin as it is within. The Piankasha and Tamaroa sequences thicken depositionally from north to south across the basin.

Although these patterns do not date the actual onset of basin formation, they nevertheless demonstrate that the long-prevailing hypothesis that the Illinois Basin was persistently negative during most of Paleozoic time is invalid, and that subsidence was post-depositional at least with respect to its pre-Absaroka content.

End_of_Article - Last_Page 363------------