UPPER CRUSTAL SEISMIC SEQUENCES BELOW WEST-CENTRAL INDIANA: LOCAL INTERPRETATION AND REGIONAL SIGNIFICANCE

Geophysical techniques are vital in understanding the upper crust across the U.S. midcontinent. We interpret four 2-D seismic reflection surveys provided by CountryMark to analyze pre-Mt. Simon seismic stratigraphy below west-central Indiana. Analyzing these data hold implications for understanding of Precambrian to early Cambrian Laurentian crustal evolution and are discussed here in a geologic framework. Moreover, seismic stratigraphic sequences observed here may suggest regional correlations that contribute to the geologic model of the deeply buried midcontinent basement.

Three seismic sequences are observed down to at least 2.0 s TWT on these seismic lines. The shallowest sequence represents the well-stratified and reflective Paleozoic sequence which is underlain by the second sequence, a poorly-reflective unit that thins locally due to apparent pre-Paleozoic uplift and erosion. Basal reflections of the Paleozoic Mt. Simon Sandstone onlap the top of this sequence. This second seismic package resembles the seismic character of the Middle Run Formation (Shrake et al., 1990) defined by seismic line and borehole in southwestern Ohio. A third and deeper sequence of sub-continuous reflections underlies the poorly reflective unit on the seismic lines of this study. Complex patterns within this third reflection sequence suggest sedimentary or volcanic stratal geometries and may be correlative with the layered sequence of deep reflections observed in COCORP seismic profiles across parts of Illinois and Indiana. Pratt et al. (1992) first termed this unit the Centralia Sequence, initially interpreted as part of the Eastern Granite-Rhyolite Province. This reflective third sequence may also be correlative with a similar layered reflective sequence beneath the Middle Run of Ohio. Consequently, some broad regional seismic sequence patterns may be becoming evident across the eastern U.S. midcontinent which are of regional significance.