Sedimentary Response to Subsidence History of Illinois Basin

D. T. Heidlauf, A. T. Hsui, G. deV. Klein

The Illinois basin formed through initial isostatic subsidence and subsequent thermal subsidence in response to mantle intrusion that formed a three-arm rift 535-555 Ma. Tectonic subsidence curves calculated from three wells show that, when extended across the Sauk-Tippecanoe, Ordovician-Silurian, and Tippecanoe-Kaskaskia unconformities, they fit a post-rift thermal contraction model. These unconformities formed because, as suggested by some investigators for other areas, the rate of eustatic sea level fall exceeded the rate of thermal basement subsidence.

Residual tectonic subsidence curves were calculated by subtracting post-rift thermal contraction curves from calculated tectonic subsidence curves. These residual curves indicate misassigned depositional times, periods of non-equilibrium sedimentation and subsidence, and the start of new subsidence episodes.

Middle to Late Ordovician negative residual curves were corrected to null values by revising depositional times. Accordingly, the duration of the Blackriveran expands to 17 m.y., and the Trentonian through middle Maysvillian contracts to 2.5 m.y. Silurian and Late Devonian through Early Mississippian positive residual curves correspond to formation and filling of 300-m-deep, sediment-starved basins. A strong negative residual curve, beginning in the middle Mississippian, corresponds to a second tectonic subsidence event.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.