Tectonic Subsidence History of Illinois Basin

Janis D. Treworgy, Michael L. Sargent, Dennis R. Kolata

Decompaction and backstripping calculations were performed on several deep wells to model quantitatively tectonic subsidence of the Illinois basin during the Paleozoic. Results indicate that tectonic subsidence was controlled by three mechanisms: (1) rifting, (2) thermal subsidence, and (3) an isostatically uncompensated mass in the lower crust.

The most rapid subsidence occurred during latest Precambrian to St. Croixan (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 the Late Cambrian and continuing to the end of Blackriveran (Middle Ordovician) time, a period of approximately 65 m.y. For the remainder of the Paleozoic, tectonic subsidence was slower and apparently was driven primarily by an isostatically uncompensated mass in the lower crust beneath the Reelfoot rift.

This isostatically uncompensated mass was emplaced during the initial rifting phase and may have been a secondary cause of subsidence during the subsequent thermal subsidence phase. During the Middle Ordovician to mid-Mississippian, the mass apparently was supported by the strength of the lithosphere and subsidence was relatively slow. Compressional stress associated with the Ouachita and Alleghenian orogenies may have led to a decrease in viscosity of the lithosphere, allowing the uncompensated load to subside more rapidly during the latest Mississippian through Permian.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.