Geology and Geomorphology of the Youghiogheny River and Laurel Highlands
Frank J. Pazzaglia, PhD
Dept. of Earth and Environmental Sciences
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Who should attend: Anyone interested in geomorphology, Quaternary geology, landscape evolution, and tectonics in PA.
Webinar Overview: New surficial mapping and dating of alluvial deposits along the Youghiogheny River in 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 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 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. Of the total base level fall, ~45 m is likely attributed to the draining of Glacial Lake Monongahela and formation of the Ohio River now dated at ~1.8 Ma by TCN burial ages on type-Carmichaels lacustrine facies exposed along the river in the Pittsburgh low plateau. The other ~36 m is attributed to non-uniform uplift of the Laurel Highlands, with a hinge more or less at Connellsville, which may be ongoing.
About our Presenter: Frank Pazzaglia is a geologist and geomorphologist who studies active tectonics, using rivers and dated geomorphic markers as observables to measure and model river incision into uplifting landscapes. He earned his PhD at Penn State in 1993, completed a post-doc at Yale, was Assistant Professor at the University of New Mexico before moving to Lehigh in 1999 as an Associate and now Full Professor. The webinar is a follow up to work completed by Frank and his students at Lehigh over the past 5 years, in collaboration with the PA Geological Survey, and the recent focus of the 2021 Field Conference of PA Geologists.
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