Abstract: The East Continent Rift Basin: Its Age and Genesis

DRAHOVZAL, JAMES A., and DAVID C. HARRIS
Kentucky Geological Survey, Lexington, KY

The East Continent Rift Basin (ECRB) is defined in central Kentucky, southeastern Indiana, and western Ohio. Extensions to the west and north have been proposed and are likely, but remain unproven. Its age and genesis are in dispute, but strong evidence supports the ECRB as a Middle Proterozoic rift basin that was subsequently incorporated as part of the Grenville foreland fold and thrust belt and finally faulted by Late Proterozoic extensional tectonics. The rocks of the ECRB consist of at least two sequences. The upper sequence is dominantly a lithic arenite (Middle Run Formation) with interbeds of mafic volcanic rocks. Below is a volcanic rock sequence that may be partly equivalent to the silicic igneous rocks of the Granite-Rhyolite Province. The two sequences are faulted and folded. Reflection-seismic data indicate that the two sequences were overthrusted by the Grenville allochthon, making them pre-Grenville compression (~0.975 - 0.88 Ga) in age. Associated mafic and silicic igneous rocks indicate that the upper sequence is -1 Ga and is separated from the underlying, older (1.325 —> 1.5 Ga) volcanic sequence by an unconformity. Together the two sequences are more than 6 km in thickness near the Grenville Front and either represent a temporal and tectonic equivalent of the Midcontinent Rift System or its southeastern extension. Assignment of a Late Proterozoic age to the ECRB is the result of confusion concerning the correlation of seismic reflectors near the Grenville Front in southern Ohio. Thin, Late Proterozoic remnants have been interpreted farther west, but appear to be absent near the Grenville Front.

AAPG Search and Discovery Article #90930©1998 AAPG Eastern Section, Columbus, Ohio