GEOLOGIC EVOLUTION OF THE PRE-MT. SIMON BELOW WEST-CENTRAL INDIANA: IMPLICATIONS FROM SEISMIC REFLECTION

Much speculation still surrounds the geologic evolution of the Precambrian eastern U.S. midcontinent, due in large part to a lack of outcrop and well control. Here, we couple regional 2-D seismic reflection and potential field data to interpret the emplacement, depositional, and deformational history of pre-Mt. Simon strata below west-central Indiana, along the eastern flank of the Illinois Basin. Two distinct pre-Mt. Simon seismic sequences are discussed here. The upper sequence, termed the Wilbur, is poorly reflective and thins to the west. Low-amplitude reflectivity is consistent with the seismic facies of the pre-Mt. Simon Middle Run Formation of Ohio and Kentucky. Weak internal reflections appear concordant with the lower sequence, the Quincy, which exhibits internal high-amplitude, discontinuous reflectivity. COCORP imaged and interpreted a similar reflective pre-Mt. Simon sequence, the Centralia, below and across southern Illinois and Indiana which was interpreted to be volcanic and/or sedimentary rocks associated with the Eastern Granite-Rhyolite Province (EGRP). Apparent four-way structural closure of the Wilbur and Quincy is evident below northeast Owen County, Ind.

The pattern of low-amplitude (Wilbur) over high-amplitude, discontinuous (Quincy) reflectivity fosters two potential regional interpretations. Poor reflectivity of the Wilbur and Middle Run may suggest a regional correlation of the two units. A westward extension of the Middle Run would support previous interpretations of the Middle Run being deposited in the Grenville foreland basin and much more extensive. This scenario requires the domal deformation of the Wilbur and underlying Quincy at this location would be of Grenvillian (~0.99 Ga) age. Alternatively, the Wilbur and Quincy sequences could pre-date the Middle Run and Grenville Orogeny, and be older volcanic and/or sedimentary components of the EGRP. These sequences could then be EGRP affinity with the domal deformation present attributable to late-stage EGRP magmatism, Southern Granite-Rhyolite Province magmatism (1.40-1.34 Ga), and/or 1.27 Ga intrusions into the midcontinent upper crust (Bickford et al., 2015).