This cover contains faults in the unconformity at the Cambrian-Precambrianboundary. These data were contributed byGlenn Bear of Indiana University as part of the work onhis doctoral thesis. The southern part of the map was produced byJames Drahovzal of the Kentucky Geological Survey and merged withBear's map of the rest of the map area and beyond.Public and proprietary seismic reflection and welldata were used to define the basement structure.A prominent east-west structure in the southern half of the maparea is the western end of the Rough Creek Graben. At its deepestpoint the unconformity is more than 30,000 ft(9,100 m) below sea level. Faulted strata at this depth mean thatthe Rough Creek Fault System penetrates to hypocentral depths.At the western edge of the map area, the graben bends to thesouthwest into the Reelfoot Rift and becomes more shallow. North ofthe Rough Creek Graben, and extending northward along the WabashRiver, is the Wabash Valley Fault System. This fault system cuts anelliptical low in the unconformity. Some of the structure contoursshow horizontal offsets at depth that are opposite to the offsetsexpected from the known normal slip on the faults. Some authorsinterpret this as evidence of strike slip motion on these faults.