MULTIPLE PLEISTOCENE CUT-AND-FILL TERRACE-FORMING EPISODES INDICATED BY SURFICIAL DEPOSITS ALONG THE MIDDLE ALLEGHENY RIVER, WESTERN PENNSYLVANIA

An ongoing EDMAP aims to help address the venerable problem of how continental drainages, like the Allegheny, were integrated by glaciation. Initial work is focused on mapping the surficial geology along a section of the Allegheny River between East Brady and Kennerdell, inclusive of the Parker and Emlenton Quadrangles. Five distinct surficial deposits consisting of variable amounts of clayey-silt, sand and gravel, with geomorphic expression as terraces, are identified within 92 m above the modern river level (AMR). The oldest deposit, designated the Jackson Road gravel, is found in scattered outcrops, rarely more than 3m thick, with a strath roughly 79 m AMR, and is interpreted to be correlative to the Carmichaels Fm. Inset into the Jackson Road gravel is a thick alluvial fill named the Perryville meander fm, with the base ~67 m AMR that parallels the modern river gradient. This unit is up to 20 m thick, of nearly uniform thickness through the map area, with a distinctive clay-rich layer at the base that buries a pre-existing landscape with several meters of local relief. The Parker strath lies ~5 m below the base of the Perryville Meander fm, on a terrace surface ~53 m AMR, mantled by a thin layer of well-rounded gravels of diverse origin. This strath is not well preserved or expressed upstream of the confluence with the Clarion River, although previous studies have traced it downstream past Pittsburgh. The next stratigraphically younger unit, informally designated the Lower Kennerdell, is a mix of sands and gravels with a base at ~27 m AMR, and a thickness of up to 15 meters near Kennerdell that notably thins downstream. Like many of the other terrace gravel units, this Lower Kennerdell unit is preferentially preserved only along the inside bends of meanders. Lastly, there is a set of 3-4 terraces located within 24 vertical meters of the modern river, none with exposed straths, which are upwards of 20 m thick and preserved along much of the length of the Allegheny River. The emerging surficial map, comparative long profile projections of the terraces and modern Allegheny profile, and inverse modeling of fluvial topography are used to apportion downstream base level and upstream glacial margin processes driving Allegheny River incision and integration over the past several million years.