CHARACTER OF FAULTING BENEATH THE WESTERN EDGE OF THE LASALLE ANTICLINORIUM, CENTRAL ILLINOIS BASIN: INSIGHT FROM A NEW SEISMIC-REFLECTION PROFILE AND A STRUCTURE-CONTOUR MAP

The LaSalle Anticlinorium (or LaSalle Deformation Belt), is an overall N-S- to NE-SW-trending, 320 km by 130 km, zone of tectonic deformation along the axis of the Illinois Basin. Where exposed in northern Illinois, the western margin is a west-facing monocline. A new 15-km long E-W-trending seismic-reflection profile, that crosses the county line between Champaign and McLean Counties in east-central Illinois, provides insight into the configuration of underlying faults. This seismic line extends below the Great Unconformity (the Cambrian/Precambrian contact). It reveals a fault that dips steeply to the east, and penetrates Precambrian basement. The Great Unconformity has been uplifted by about 300 m on the hanging wall (eastern) side of the fault. A steeply dipping antithetic fault splays from the main fault just below the Great Unconformity. This splay also displays a reverse-sense component of displacement, but it dies out below the Late Cambrian - Early Ordovician Knox Dolomite. Two new structure-contour maps, one on the base of the Devonian-age New Albany Shale, and one on the top of the Ordovician-age Galena Dolomite, delineates the 3-D shape of the LaSalle Anticlinorium in the central Illinois Basin. Notably, an overall west-dipping slope, trending N25°W, defines the western edge of the La Salle Anticlinorium. Across this boundary, the base of the New Albany drops from an elevation of +120 m (+400') -600 m (-2000') over a horizontal distance of 9 km (30,000'). The uplift appears to consist of several N-S-trending en echelon splays. The study area is underlain by a fault that cuts across and displaces the basement-cover contact, splays up-dip into a positive flower structure in the sedimentary cover, and dies out up-dip into monoclinal folds. It may represent inversion of Proterozoic normal faults. The presence of a flower structure, and of en echelon second-order folds, may indicate a component of strike-slip displacement along the overall belt (i.e., the LaSalle Anticlinorium transpressional). Movement on the faults in the LaSalle Anticlinorium occurred during distinct events in the Paleozoic.