SHALLOW SEISMIC REFLECTION IMAGING OF THE PENITENTIARY FAULT AND ITS ASSOCIATION WITH THE COMMERCE GEOPHYSICAL LINEAMENT, TAMMS, SOUTHERN ILLINOIS

The northeast-trending Commerce Geophysical Lineament (CGL), a series of gravity and magnetic anomalies extending from northeast Arkansas to central Indiana, is associated with Quaternary deformation northwest of the New Madrid seismic zone. In southern Illinois, a previous study imaged Quaternary sediments deformed along the Penitentiary Fault (PF), a northeast-striking fault coincident with the CGL. In southeastern Missouri, about 25 km southwest of the PF, geophysical and paleoseismological investigations also document late Quaternary faulting along the CGL. The PF is coincident with a linear northeast-trending, east-facing bluff line that separates late Quaternary Cache River Valley (CRV) deposits on the east from Paleozoic upland rocks on the west. A 1-km-long shear-wave seismic reflection profile, intersecting the PF and CGL, was acquired near Tamms, Illinois, using a 12-channel landstreamer and a sledgehammer/I-beam seismic energy source. The seismic profile records coherent reflectors at both the bedrock-CRV unconformable contact and within the Quaternary deposits. Depth to bedrock was interpreted as approximately 30-50 m deep in the CRV and approximately 10 m deep west of the bluff line. Coincident with the bluff line and the PF is a complex zone of high-angle faults that may project into overlying Quaternary alluvium. In the eastern part of the section, transpressive (?) faulting is interpreted as projecting upsection across several reflectors within the Quaternary CRV sediments. These eastern faults may be associated with a previously unidentified fault zone east of the PF. Quaternary geologic and geomorphic mapping, coupled with acquisition of geophysical data, is ongoing near Tamms, Illinois, to assess the northeastern limit of late Quaternary deformation along the PF and CGL.