A NEW AGE MODEL FOR THE GLACIAL REVERSAL, ASSEMBLY, AND INCISION OF THE ALLEGHENY RIVER, NORTHWESTERN PENNSYLVANIA

A new terrestrial cosmogenic nuclide (TCN) and optically-stimulated luminescence (OSL)-based age model for eight fluvial terraces in the middle Allegheny River and upstream correlative glacial deposits has been constructed from a USGS EDMAP-funded surficial geologic map of the Parker and Emlenton 7.5 min quadrangles. The map, age model, and existing data for Glacial Lake Monongahela (GLM) test long-held views for when and where low divides were breached in the assembly of the modern Allegheny River and the respective roles of upstream glacial margin or down stream base level change in driving post-glacial river incision. The age model is anchored by a ~20 m thick paired fill terrace containing abundant rock-types exotic to the Allegheny watershed (Qt3), with a strath ~60 m above the modern channel (AMC). A TCN burial age in Qt3 of 1.1+0.4/-0.3 Ma indicates a south-flowing Allegheny River connected to the glacial margin in the early Pleistocene, and a long term rate of incision of ~60m/Ma. In contrast, above Qt3 are few, scattered strath terraces (Qt1 and Qt2) that lack exotic clasts, and have opposing north and south gradients astride a now breached low divide upstream of the Clarion-Allegheny rivers confluence. Inset 5 m below Qt3 lies an extensive, paired, low-relief strath terrace (Qt4-the Parker Strath), followed by scattered, unpaired, and poorly preserved strath terraces (Qt5) that decorate the steep bedrock valley walls and extend down to within ~ 20 m AMC. At least three thick, paired fill terraces containing abundant exotic material (Qt6, Qt7, and Qt8), the bases of which are not exposed, are inset into the inner Allegheny valley. The tread of Qt6 lies ~15 m AMC, the underlying alluvium has a TCN burial age of 0.513+0.15/-0.17 Ma, and it is subsequently capped by thick colluvial deposits with a TCN burial age of 0.24+0.071/-0.06 Ma. Qt7 is a late Pleistocene terrace with an OSL age of 0.017+/-0.002 Ma. These middle and late Pleistocene terraces and colluvia have similar ages to two tills exposed ~45 km to the north at Franklin, PA dated to 0.4+0.31/-0.18 Ma and 0.14+/-0.19 Ma using burial TCN and OSL respectively. These data indicate reversal and assembly of the Allegheny River during a very early glacial advance, perhaps the same one that was responsible for the formation of the Ohio River via spillover of GLM >> 1 Ma.