ISOCHRON BURIAL DATING OF PLIO-PLEISTOCENE TERRACE DEPOSITS ALONG THE LOWER TENNESSEE RIVER

The Tennessee River features a major lithologic knickpoint at Muscle Shoals, Alabama that delineates a relict landscape upstream from an incised landscape downstream. Upstream of the knickpoint, the landscape is blanketed by a deep weathering residuum and shows little evidence of recent incision, while downstream there are abundant fluvial terraces that record numerous episodes of incision and aggradation.

Self (2003) mapped five levels of terraces downstream of Muscle Shoals near Pickwick, Tennessee that range from 15 to 100 meters above river level (m arl). The four lowest terraces have similar gravel compositions to the modern Tennessee River, while the higher terraces may represent an older course of the Tennessee River. Profile reconstructions reveal that the third level of terraces (57 m arl) corresponds with the compositionally and texturally similar Upland Complex/Lafayette gravels perched above the Mississippi River, which are likely Pliocene in age and have been correlated to aggradation events along the Cumberland and Green Rivers (Anthony and Granger, 2007; Van Arsdale et al., 2007).

To better constrain the history of incision and aggradation along the Tennessee River and compare it to other tributaries of the Ohio River, we dated two terraces at 27 m arl and 57 m arl near Pickwick, TN using ²⁶Al-¹⁰Be burial isochrons. The 57 m arl terrace was deposited at 3.94±0.23 Ma, in agreement with the Pliocene age for the Upland Complex/Lafayette gravels at similar levels downstream. The 27 m arl terrace was dated to 2.42±0.08 Ma, possibly corresponding to the first advance of the Laurentide Ice Sheet dated by Balco and Rovey (2010). These data suggest that incision of the Tennessee River began prior to 2.4 Ma and exceeded upstream erosion rates, supporting the hypothesis that the Muscle Shoals knickpoint has isolated the upper Tennessee River watershed from base level fluctuations along the Ohio River.