Seismicity of the Lower Wabash Valley: Fact Sheet John R. Hill Associate Director and Geologist Indiana Geological Survey Bloomington, Indiana 47405

The Lower Wabash Valley lies within the Illinois Basin, which is a classic sag basin characterized by Paleozoic aged rock layers that dip gently toward the depositional center from the crests of six arches (Figure 1)*1. More than 12,000 feet of sediments accumulated in extreme southwestern Indiana during Paleozoic time and the subsequent rock formations dip more or less southwesterward from 30 to 70 feet per mile.

Figure 1. Basic structural features of the Illinois Basin. From Zuppann, 1988

*2 - Most geologists familiar with the Illinois Basin would agree that the southern extreme of the basin is perhaps the most structurally complex region in the Midcontinent. The Reelfoot Rift and Rough Creek Graben are elements of a rift system that formed during late Precambrian to Middle Cambrian time and these structures flank the southernmost boundary of the Illinois Basin. Plate tectonic forces are credited with being the primary agents in the formation of the Illinois Basin and the complex faulting systems within and adjacent to the structure. The basin began as an element of a failed rift that started to open with the breakup of a super continent that existed in Late Precambrian through Middle Cambrian time. Subsequent to rifting, basin development was more typical of a classic sag basin that gradually subsided as it filled with sediment varying from normal marine carbonates in deeper water to mud, sand, and silt near the basin margins. As continental masses accreted then broke apart through geologic time, the Illinois Basin was subject to alternate periods of compression and tension. Profound high angle normal faulting, for example, cuts the entire Paleozoic section in southwestern Indiana. The stresses responsible for these faults were primarily tensional and may have coincided with the breakup of Pangea (yet another super continent) that existed some time after post-Early Permean (Figure 2).*2

Figure 2. Major structural features of the southern Illinois Basin (from Kolata, 1997 *2)

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