FACIES RECONSTRUCTION: THE KEY TO RECONSTRUCTING THE PLEISTOCENE TRANSFORMATION OF THE PITTSBURGH RIVER SYSTEM AND LAKE MONONGAHELA INTO THE UPPER OHIO RIVER WATERSHED

Recurring Laurentide glaciation rerouted the southernmost St. Lawrence River drainage into the greater Mississippi River watershed, thereby greatly enlarging the Ohio River system and altering freshwater inputs into the North Atlantic and Gulf of Mexico. High-terrace deposits and stream profiles indicate this rerouting led to > 40 m of valley aggradation, followed by > 60 m of incision. Paleomagnetically reversed lake sediments in the Monongahela River basin indicate drainage began to change in the early Pleistocene (Jacobson et al 1988), but a preponderance of normal remnant magnetism in high Allegheny River valley terrace deposits (Marine 1997) suggests the transformation continued into the Middle Pleistocene.

High-level deposits of the Carmichaels Formation of Pennsylvania and Calcutta Silt of Ohio have been ascribed generally to a “preglacial” Pittsburgh River and “glacial” Lake Monongahela. However, to apply overly simplified interpretations to a variety of deposits formed in widely differing facies in these two stratigraphic units risks clouding the interpretation of regional Quaternary drainage history. Clouded misperception is exemplified by several 22 to 39 ka radiocarbon dates once used to assign a Late Wisconsin age for Lake Monongahela, despite fossil plants, clay mineralogy, and correlation with other sites that show the Late Wisconsin deposits formed in different conditions than typical Lake Monongahela rhythmite facies.

Widespread evidence shows neither Lake Monongahela nor Pittsburgh River were simple systems. Morgan (1994) recognized seven facies in high terrace deposits, but only three are firmly associated with Lake Monongahela: distinctly laminated silty clay, deltaic deposits, and subaqueous debris flows. Morgan’s colluvium, weakly stratified silty clay, silty alluvium, and sandy alluvium facies should be treated independently, as should a widespread, newly identified basal cobble gravel facies, tentatively assigned to the Pittsburgh River. Formal subdivision of the unconsolidated facies within the Monongahela Valley on the basis of age and mode of origin may not be justified for geologic mapping purposes, but detailed stratigraphic analysis, coupled with high-resolution DEM analysis will be key to understanding the sequence and timing of this complex drainage history.