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-reflection 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.

Abstract Minerals have been extracted from the Illinois-Kentucky Fluorite District for over 170 years. Theories concerning the inter-relationship between the fluorite mineralization, tectonism, and igneous activity will be discussed by several geologists during this field trip. The Columbia mine (vein deposit) will be visited in Kentucky, and the only mine currently producing fluorite in this district, the Hastie Limestone Quarry (strata-bound deposit), will be visited in Illinois. The mining history of this region will be explained at the American Fluorite Museum, where numerous mineral specimens can be examined. The Hicks Dome, a Permian crypto-volcanic feature (?) in Illinois, will also be discussed. The trip will conclude with a walking tour at the Garden of the Gods Recreational Area to view the Eagle Valley Syncline and Lower Pennsylvanian units.