NEW MAPPING OF LATE PLEISTOCENE SLACKWATER DEPOSITS IN THE LOWER OHIO RIVER VALLEY, WESTERN KENTUCKY

The lower Ohio River valley and its tributaries are incised into relatively soft, flat-lying Paleozoic sedimentary bedrock and constitute an incised-valley system that was filled with up to 75 m of Pleistocene outwash and alluvium. Tributary valleys adjacent to the lower Ohio River valley can be kilometers wide and are filled with bedded, fine-grained silt, sand, and clay; the valley-bottom topographic surfaces are remarkably flat. The sedimentary fill of these terraces is interpreted to be the result of slackwater or lacustrine processes. The tributary fill is composed of fine sediments from glacial outwash that spilled into the valleys from the Ohio River mixed with sediment eroded from tributary upland areas; however, the extent to which each source area contributed to the fill is presently unknown and is the focus of ongoing research.

Although tributary topographic surfaces (lacustrine terraces) are flat, surfaces of adjacent tributaries do not lie at identical elevations and thus were not deposited by an impounded Ohio River. Lacustrine terrace elevations systematically decrease downstream, suggesting that tributary valleys were filled as the Ohio River locally aggraded and blocked the mouth of each tributary valley. Previous studies hypothesized regional impoundment of the Ohio River caused by Wabash River sedimentation or by a narrow Ohio River valley “chokepoint” near Caseyville, Kentucky; the current results do not support these hypotheses.

The lacustrine terraces are locally underlain by 2 to 3 meters of laminated silt, interpreted as loess deposited in standing water, that grades upward into massive Wisconsinan (Peoria) loess. Loess source areas were glacial outwash terraces and post-glacial alluvial terraces of the Ohio River, and the lower Wabash River valley to the west. Recent luminescence and radiocarbon ages from the Peoria Loess in western Kentucky indicate that most of the tributary fill is older than ~18,000 ka.

The lacustrine terraces lie several meters above associated Ohio River high outwash terraces. The high outwash terraces are within the main valley and may have been scoured as the Ohio River transitioned from an aggradational to a transportational regime, while aggradation continued on lacustrine terraces during flood events.