Incision of the Youghiogheny River Through the Laurel Highlands
Incision of the Youghiogheny River through the Laurel Highlands determined by a new river terrace stratigraphic age model, Ohiopyle State Park, southwestern Pennsylvania 5 Kurak, E.1, Pazzaglia, F. J.1, Li, C. X.2, Patching, A.1, Shaulis, J.3, Corbett, L.4, Bierman, P.4, Nelson, M.5, and Rittenour, T.5 1Earth and Environmental Sciences, Lehigh University 2Earth, Ocean, and Environment, University of Delaware 3Pennsylvania Geological Survey 10 4Community Cosmogenic Lab, University of Vermont 5Luminescense Lab, Utah State University Abstract New surficial mapping and dating of alluvial deposits along the Youghiogheny River in 15 southwestern Pennsylvania has generated a new terrace stratigraphic model linking well-known deposits of the Carmichaels Formation with terraces further upstream through Ohiopyle State Park. Flights of four to six terraces are found in three distinct zones with gradients that are subparallel to the channel, including a steep convex reach of the river. Numeric ages obtained from 25 terrestrial cosmogenic nuclide (TCN) samples and one optically stimulated 20 luminescence (OSL) sample constrains the timing of terrace genesis on the Youghiogheny River, over the past 1.2 Ma, with terrace deposition coinciding with glacial climates. TCN burial and isochron ages of ~610 ka and ~300-350 ka are used to construct long-term incision rates ranging from ~20 m/Myrs upstream of Ohiopyle where the channel gradient and subparallel terrace profiles are gentle to ~50 m/Myrs downstream of Ohiopyle where the river profile is steeper in a 25 broad convex knickzone. There were at least two base level falls totaling ~81 m conflated in the knickzone between Ohiopyle and Connellsville, the top of which includes Ohiopyle Falls and is retreating at a rate of ~1 cm/yr.
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