A newly discovered sedimentary basin underlies the Mount Simon Sandstone (Upper Cambrian) below part of the Cincinnati Arch in southwestern Ohio. On the basis of preliminary examination of samples and geophysical data, the basin is tentatively traced for at least 160 km north-south and 48 km east-west in Ohio and adjacent Kentucky and Indiana. The basin is located in an area with sparse control in the deep subsurface, near the boundary of the Grenville and the Eastern granite-rhyolite provinces. Core recovered from the basin consists of a homogeneous, tightly compacted, grayish-red, lithic arenite. A seismic profile across the core site shows a layered sequence containing eastward-dipping seismic reflectors which extend to a depth of at least 5200 m. Further work is required to determine the basin's precise structural history.