Cenozoic faulting in southernmost Illinois

Numerous faults in the Mississippi Embayment area of far southern Illinois have been active since Late Cretaceous time. They have been documented via geologic mapping, drilling and trenching studies, and high-resolution seismic-refl ection surveys. The majority lie within the Fluorspar area fault complex, directly in line with the New Madrid seismic zone. Other active elements include the Commerce fault zone and southern end of the Ste. Genevieve fault zone. Most Cenozoic faults strike northeast; a few strike north to north-northwest. Dips are generally 60° to vertical. Narrow grabens, often deeper than they are wide, are typical. Some grabens contain sedimentary units that are elsewhere eroded, or restricted to the grabens. Faults have undergone multiple episodes of movement since the Late Cretaceous. Offsets of tens of meters are common in the late Miocene to early Pleistocene Mounds Gravel. At one site, the Mounds Gravel is downthrown by 150 m within a graben. Younger Pleistocene sediments overlie the Mounds Gravel and are confined to the graben. Illinoian and younger displacements are less common and amount to a few meters, at the most. Two sites show possible Holocene faulting. The overall fault pattern implies transtensional, right-lateral wrenching that produced pull-apart grabens. Such a pattern is consistent with the contemporary stress regime, both in southernmost Illinois and in the New Madrid seismic zone.