MOUNDS (LAFAYETTE) GRAVEL AND RELATED DEPOSITS OF LOWER OHIO AND TENNESSEE VALLEYS

The Mounds(Lafayette)Gravel(late Miocene-Early Pleistocene)occurs widely in the northern Mississippi Embayment. This brown chert gravel occupies several terrace levels. Older, higher terraces represent alluvial fans, as detailed by previous authors. The youngest Mounds was deposited in the ancestral Tennessee Valley: a flat-bottomed, steep-sided valley about 10 km wide and 10 to 20 m deep between Paducah, Ky. and Cairo, Ill. When this segment was diverted in the early Pleistocene, it then contained only small, sluggish, meandering creeks. Their deposits comprise the Metropolis Formation: highly mottled and burrowed pebbly silts and sands containing thick, multiple buried soils.

While the youngest Mounds was deposited, the ancestral Ohio may have been a small stream that joined the ancestral Cumberland River in the Cache Valley of southern Illinois. Early Pleistocene glacial diversion of rivers in Ohio, Indiana, and Illinois enlarged the Ohio River greatly. It now cut a valley 40-50 meters deep and captured the ancestral Tennessee by headward erosion. The Cache Valley became entrenched much more deeply that the abandoned segment of the lower Tennessee west of Paducah.

By Wisconsinan time, the Cache Valley was choked with glacial outwash. Slack-water lakes backed up the Ohio, Cumberland, and Tennessee valleys, rising until water topped the col west of Paducah in the abandoned lower Tennessee valley. As the col was eroded, water probably flowed through it intermittently until the early Holocene; when the Ohio abandoned the Cache Valley and assumed its present course. Today, the Ohio, Cumberland, and Tennessee rivers reoccupy the long-abandoned lower valley of the ancestral Tennessee. Tectonic activity related to the New Madrid Seismic Zone may have influenced drainage changes.