ABSTRACT: Interaction of Tectonic Subsidence, Eustasy, and Epeirogeny in the Illinois Basin During the Paleozoic

TREWORGY, JANIS D., DENNIS R. KOLATA, and MICHAEL L. SARGENT, Illinois State Geological Survey, Champaign, IL

Depth-time curves derived from several deep wells in the Illinois basin indicate that tectonic subsidence was controlled successively by three mechanisms: rifting, thermal subsidence, and an isostatically uncompensated mass in the lower crust. The most rapid subsidence occurred during latest Precambrian to Late Cambrian time when the Reelfoot rift and Rough Creek graben formed by brittle faulting of the upper lithosphere. This period of rifting was followed by thermal subsidence beginning in Late Cambrian time and continuing to the end of Middle Ordovician Blackriverian time, a period of approximately 65 m.y. For the remainder of the Paleozoic, tectonic subsidence was slower and apparently driven primarily by an isostatically uncompensated mass in the lower crust beneath the Reelfoot ift. The effect of tectonic subsidence extended far beyond the immediate area of the rift, but with reduced magnitude, because of the flexural rigidity of the crust.

Superimposed on this history of tectonic subsidence in the Illinois basin are the phenomena of eustasy and epeirogeny. During the episode of rifting, sedimentation was restricted to the rift. Subsequently, when thermal subsidence was the major factor creating accommodation space in the southern part of the Illinois basin, concurrent eustatic sea-level rise and/or epeirogenic lowering of the North American craton resulted in deposition of siliciclastics and carbonates throughout the basin-and over much of the mid-continent. Tectonic subsidence continued to create accommodation space in the southern part of the basin during the subsequent craton-wide post-Sauk lowstand. From the end of Blackriverian through Pennsylvanian time, rates of tectonic subsidence in the basin were relatively lo , and deposition towards and over the flanks of the basin was probably controlled by eustatic and epeirogenic factors. Deposition of each of the sequences-Tippecanoe, Kaskaskia, and Absaroka-was terminated by a lowstand period caused by a eustatic sea level fall and/or epeirogenic uplift of the craton that resulted in continent-wide unconformities. Biostratigraphic evidence suggests, however, that there was nearly continuous deposition in the southern part of the basin during these Paleozoic lowstands, indicating that tectonic subsidence, driven by the uncompensated mass, continued to create accommodation space in that region.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)