PROTEROZOIC LAYERED SEQUENCE OF THE EASTERN MIDCONTINENT: PRECAMBRIAN SEDIMENTATION AND TECTONISM CONCEALED

Large scale deformation of the Proterozoic sequence in the eastern Midcontinent (Illinois, Indiana, Ohio, and Kentucky) is largely concealed beneath the relatively undeformed overlying Paleozoic sedimentary section. Borehole information, potential field data, and seismic profiles illuminate a layered sequence divisible into four units underlying the Cambrian pre-Knox rocks. Below this sequence, reflectors are dominantly chaotic and inferred to represent highly deformed middle-to-lower crust. Descending stratigraphically, the units display increasing deformation, the uppermost units being essentially concordant with overlying Paleozoic strata while the lowermost units are significantly folded and faulted. The dominant tectonic fabric is expressed as a series of west-verging fold and thrust structures interpreted as Grenville (1,240-1,025 Ma) in age. Two later periods of transpression (600 Ma and 435 Ma (?)) created a series of wrench structures. The age of the sequence is inferred to be Proterozoic although the uppermost portion of the sequence may be Cambrian. Fission track data, K/Ar dates, and structural relationships indicate that the lower portion of the sequence may be as older than 1,500 Ma, the middle section 1,025 to 1,500 Ma, and the upper portion of the sequence 1,025 to 600 Ma. The entire sequence of Proterozoic rocks appears to thicken to the east, reaching a maximum thickness at the Grenville Front and thinning to the west where it pinches out against the basal Paleozoic strata of the Illinois Basin. This thickness trend is disrupted by a series of rift like structures that abruptly thicken the sequence in the Rough Creek Graben of western Kentucky and the English Graben of southern Indiana. Additionally, the thickness of the sequence is substantially modified by a regional unconformity that possibly represents the Proterozoic/Phanerozoic boundary. This unconformity appears to have eroded the sequence more deeply to the west. The very limited borehole penetrations of this sequence demonstrate that the sequence is composed of lithic arenites and other clastics interlayered with volcanic rocks (e.g. basalts). The relationship between the granites and rhyolites as defined in the ~1,500 Ma Eastern Granite Rhyolite Province (Transcontinental Proterozoic Province) and this sequence is unclear.