INTEGRATED GEOLOGICAL AND GEOPHYSICAL STUDY OF NEOGENE AND QUATERNARY-AGE DEFORMATION IN THE NORTHERN MISSISSIPPI EMBAYMENT

A program of shallow drilling, trenching, outcrop mapping, and seismic reflection acquisition in southern Illinois, just north of the New Madrid seismic zone, has produced the most comprehensive documentation to date of Quaternary-age faulting in the northern Mississippi embayment. Structural cross-sections over 5 NE-trending faults that continue from the Fluorspar Area fault complex southward into the embayment indicate narrow grabens into which latest Cretaceous, Paleocene, and Eocene sediments were dropped and protected from erosion. The bounding faults have been active during Neogene through middle Pleistocene time and in the deepest graben, Late Miocene-Pleistocene-age sediments are downthrown 150 m. On the other hand, definitive faulting of Wisconsinan loess or Holocene alluvium is not observed, which suggests that the faults have been inactive for at least 55 ka (basal loess ages) to 128 ka (youngest Illinoian age). Seismic profiles reveal that faults in Quaternary sediments penetrate Paleozoic bedrock, thus indicating that these are tectonic faults and not products of landsliding, solution collapse, or other merely near-surface processes. The faults exhibit vertical to steeply dipping normal and occasional reverse displacements that outline a variety of structures including series of narrow grabens. At one site, observed faulting is superimposed over a prominent bedrock anticline, which may itself be a result of the Quaternary deformation that created the steep normal and reverse faults. Our results indicate that shallow sediment and bedrock faults extend beyond the localized fault zones implying that Quaternary deformation of the northern Mississippi embayment is more pervasive than formerly believed. The trend of Quaternary-age structures in southern Illinois is in line with the northeast-trending New Madrid seismic zone and is parallel to other geologically active features in the region such as the Commerce geophysical lineament; however, the lack of observed Holocene movement suggests that the faults in our study area are not currently active and thus may not contribute to seismic hazard.