Sixty-eight kilometers of 12-fold seismic reflection data were collected in the Wabash River Valley of southwestern Indiana and southeastern Illinois to investigate the configuration of a basement structure inferred from regional gravity and magnetic anomaly data. The seismic profiles were also positioned to cross faults of the Wabash Valley fault system in a number of locations. Interpretation of the seismic reflection profiles and detailed gravity and magnetic profile data provides evidence for a series of northeasterly trending grabens in the basement. These grabens are filled with pre-Mt. Simon layered rocks and are overlain by Paleozoic sedimentary rocks of the Illinois basin. Beneath the Wabash River near Grayville, Illinois, an interpreted graben (the Grayville graben) is approximately 15 km wide and contains about 3 km of fill. Individual boundary faults for the graben cut prominent reflectors within pre-Mt. Simon rocks and display offsets of up to 500 m. The interpreted configuration of basement faults and thickness of pre-Mt. Simon layered rocks provide evidence of a late Precambrian rift inferred to be one arm of the New Madrid rift complex. Post-Pennsylvanian faulting of the Wabash Valley fault system is visible on the seismic reflection record sections as small offsets (less than 100 m) on steeply dipping normal faults. The downward projection of these faults intersects the older large-offset faults within the pre-Mt. Simon rocks suggesting that the Wabash Valley faults represent a post-Pennsylvanian reactivation of the rift system.

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