EVIDENCE OF CRUSTAL-SCALE THERMAL SUBSIDENCE SURROUNDING CAMBRIAN INTRACRATONIC RIFT STRUCTURES IN THE SOUTHERN ILLINOIS BASIN

Models of sedimentation across active normal fault systems inadequately describe the depositional pattern of the Knox Supergroup surrounding the Rough Creek and Mississippi Valley grabens in the southern Illinois basin. Syndepositional fault movement produces units with abrupt changes in thickness along the active fault trace. The thickness of the Late Cambrian to Early Ordovician Knox Supergroup increases dramatically over the axes of these grabens; however, these thickened regions extend well beyond the graben-bounding fault systems.

Lithology-specific decompaction and subsidence calculation equations from published sources were performed for a four-layer stratigraphic model across parts of Kentucky, Indiana, Illinois, Missouri, Arkansas, and Tennessee. Stratigraphic thicknesses and lithologies from 35 locations (33 from wells drilled to Precambrian basement and 2 from compilations of seismic profile interpretations) across the region were used in these computations. The model layers were defined along stratigraphic horizons that were both regionally extensive and resolvable on both seismic profile data and geophysical well logs. The five mapped surfaces that define these units are the base of the Upper Ordovician Maquoketa Shale, the top of the Late Cambrian-Early Ordovician Knox Supergroup, the top of the Middle-Late Cambrian Eau Claire Formation, the top of the Early Cambrian Reelfoot Arkose, and the top of Precambrian basement rocks. By calculating and removing the effects of water and sediment load (tectonic subsidence) and sediment compaction (porosity loss), the magnitude of thermal subsidence was determined from the overall subsidence curves.

Subsidence calculations of these stratigraphic packages indicate that the period of fault-dominated active rifting within these grabens ended around 502 Ma, with post-rift thermal subsidence extending until around 485 Ma. Despite the lack of syndepositional fault growth, accelerated thermal subsidence during the deposition of the Knox Supergroup resulted in the anomalous thickening over these graben structures. Furthermore, the rate of subsidence surrounding the grabens during this time steadily increased towards the graben axes from all directions.