Geophysical investigation of subsurface structure of the Pennsylvanian and younger strata associated with the Inman East Fault, Gallatin County, Illinois

Seismic reflection and relative gravity techniques are used to interpret faulting in the Pennsylvanian and younger strata in Gallatin County, Illinois. The previously mapped Inman East Fault (IEF) and the Inman Fault cross the study area trending north-northeast as part of the Wabash Valley Fault Zone located in Southeastern Illinois and Southwestern Indiana. A high-resolution seismic reflection survey with over 14 miles of 2D data was conducted to evaluate faulting of the Springfield Coal seam for mining purposes. The Springfield Coal dips to the north-northwest as imaged on the seismic profiles and ranges in depth from 150 feet to 850 feet below the surface in the study area. Several flexural or small-offset faults are interpreted throughout the seismic survey. These faults cause significant dip change of the Springfield Coal reflection and have offsets of up to approximately 20 feet. Fifteen faults including the IEF are correlated between seismic profiles. The IEF is interpreted to significantly bifurcate in the Pennsylvanian strata, and the maximum interpreted vertical offset of the Springfield Coal across a single bifurcation of the IEF is 32 feet. In the northern part of the study area, the coal seam is down-dropped approximately 50 feet within a small graben structure associated with the IEF.;A gravity survey was conducted to further explore overall subsurface structure associated with the Inman East Fault and Inman Fault in the northwestern part of the study area, specifically targeting possible faulting west of the seismic survey. Based on gravity modeling, the Inman East Fault is interpreted to bifurcate in the Pennsylvanian strata to form an eastern branch as imaged on the northernmost seismic profile and a primary western branch with an approximate vertical offset of 325 feet. All interpreted bifurcations of the Inman East Fault are modeled to converge in the Mississippian strata. Last movements of the IEF and Inman Fault are interpreted to be post-Pennsylvanian and pre-Pleistocene in age. Quaternary age faulting is modeled between the Inman Fault and the IEF. Additionally, a bedrock paleochannel with Quaternary sediment fill is interpreted between the eastern and western branches of the Inman East Fault. The interpreted paleochannel does not affect the Springfield Coal member, which is approximately 600 feet below the modeled channel location.