Large regions of the North American mid-continent are underlain by Precambrian layered rocks buried beneath Phanerozoic sedimentary strata. South of the Wichita Mountains, published seismic reflection profiles show a Precambrian layered assemblage extending for at least 40 km in both the north-south and east-west directions, and industry data show that it may continue 150 km to the southeast. Seismic reflection data in the Illinois region show a Precambrian layered assemblage extending 320 km in an east-west direction and 200 km in a north-south direction. In both cases, the layered rocks are as much as 12 km thick. Apparent sequence boundaries (onlap, downlap) within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strata due to faulting. The layered sequences correlate with regions of relatively long-wavelength and low-amplitude magnetic anomalies; the extent of this magnetic signature suggests that about 200,000 km2 of Illinois, Indiana, and western Ohio, about 50,000 km2 of southernmost Oklahoma and north-central Texas, and about 32,000 km2 of southern Missouri and northern Arkansas may be underlain by similar Precambrian strata.
Drill holes indicate that the top of the mid-continent Precambrian “basement” is composed largely of silicic igneous rocks. Such material may comprise a large part of the layered sequences. Alternatively, these igneous rocks could be intermixed with, or underlain by, nonvolcanic (meta?)sedimentary strata. The strong reflectivity of some layers suggest that minor mafic flows and/or sills may also be present. Analysis of U/Pb and Nd/Sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1.7-2.0 Ga and predate or are contemporaneous with the 1.3-1.5 Ga crystallization ages of the granites and rhyolites. Though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed, the possibility that they contain Precambrian hydrocarbons that escaped heating is as yet untested.