Abstract

Segments of the study are part of an ongoing mapping initiative involving the Illinois State Geological Survey (ISGS), United States Geological Survey (USGS) and Southern Illinois University at Carbondale (SIU-C). Financial support for parts of this work was provided by the Cooperative Geologic Mapping Program (COGEOMAP) of the USGS and matching funds from the ISGS. Inkind contributions have been provided by both ISGS and SIU-C.

Deposition of Middle Devonian carbonate rocks within the Illinois Basin lasted approximately 5 million years. These limestones reach a maximum thickness of about 122 m (400 ft) within the study area; they are primarily transgressive from the southern part of the basin. However, a structurally closed Illinois Basin did not exist during Middle Devonian time (Eifelian and Givetian). The Middle Devonian sea inundated the Sparta Peninsula (new name), Sparta Shelf and the northern arches through the Reelfoot Inlet (new name). Tectonic activity by the end of early Middle Devonian (late Eifelian) time produced movements along two large fault zones; the Ste. Genevieve to the west, and the Rough Creek to the east. Movements on the faults resulted in erosion of lower Middle Devonian and Silurian rocks to the north of these faults. As a result, the Sparta Shelf was truncated to the south along the Ste. Genevieve Fault Zone, and McDaniels Island (new name) was formed to the north of the Rough Creek Fault Zone.

With the rise of the Pascola Arch during the Mesozoic, these Middle Devonian rocks were eroded farther south and truncated by the unconsolidated Cretaceous sediments of the Mississippi Embayment.