FIRST KNOWN PERMIAN ROCKS IN ILLINOIS IMPROVE UNDERSTANDING OF REGIONAL TECTONISM

A continuously cored borehole 500 feet (152 m) deep recently drilled near Shawneetown, southeastern Illinois, recovered an anomalous section of steeply dipping and faulted microgranular dolomite, variegated mudstone, and sandstone. These rocks do not resemble any Pennsylvanian or older formation in the region, but are closely similar to the Mauzy Formation drilled 35 km east in Kentucky, from which Early Permian fusulinids have been identified. On this basis the cored section at Shawneetown is considered to be Mauzy Formation and Permian age, at least in part. As in Kentucky, the Mauzy Formation near Shawneetown occupies a graben along the northern margin of the Rough Creek fault system, a major regional structure that has undergone several episodes of displacement beginning in Cambrian time. The Shawneetown core exhibits both soft-sediment deformation (microfaulting and contorted lamination) and brittle deformation (brecciated, shearing, and vein filling), indicating that faulting took place both before and after the Mauzy sediments were lithified. 3D acoustic imaging provides azimuth and dip of fractures and bedding planes in the borehole and, together with examination of the core, allows detailed structural analysis.

Previous regional investigations showed that the Rough Creek fault system underwent regional compression, raising the southern block along high-angle reverse faults, followed by extension that lowered the southern block and created grabens. Results from the Shawneetown core indicate that the extensional phase was already underway by latest Pennsylvanian or Early Permian time. Thus, compressive uplift of the southern block is constrained to Late Pennsylvanian (Alleghenian orogeny), and extension followed immediately. The extensional phase thus is better regarded as relaxation of compression and collapse of the hanging wall, rather than a separate Mesozoic episode of regional extension as previously hypothesized.