DEEP WEATHERING PRODUCTS AS APPALACHIAN LANDSCAPE MARKERS: DID NEOGENE UPLIFT OCCUR?

River profiles and offshore sediment records have been interpreted to suggest that the southern Appalachian mountains were uplifted by hundreds of meters in the Neogene (Gallen et al., 2013; Miller et al., 2013), triggering rapid river incision and mountain erosion. These hypotheses have been supported by geodynamic models and interpretation of seismic tomography (Wagner et al., 2012; Liu, 2014). Here, we address whether Neogene uplift is consistent with the preservation of deep weathering products on low-relief relict surfaces (i.e., the surfaces formerly known as peneplains) found throughout the southern Appalachians.

It was recognized early in the 20^th century that bauxite deposits near Chattanooga, Tennessee are the deep weathering products of the ancestral Tennessee River preserved in ancient sinkholes. These bauxites lie only 60-80 m above the modern river level (arl), and have been correlated with dated Paleocene-Eocene bauxites in Alabama and Georgia (Bridge, 1950). Karst-type bauxites are restricted to tropical environments today, which is consistent with the paleoclimatic conditions expected for the Paleocene-Eocene Thermal Maximum (PETM). Similar deposits at Elizabethton, Tennessee are found up to 160 m arl. In addition, sediments at the Gray Fossil Site, Tennessee are located 80-120 m arl and have been palynologically dated to the Paleocene-Eocene (Zobaa et al., 2011). The antiquity of the relict surface is supported by widespread manganese oxide deposits that also indicate prolonged weathering in a warmer climate. Together, these deep weathering deposits suggest that the Tennessee River and its upstream tributaries have not incised more than about 100 m over most of the Cenozoic, directly contradicting predictions of Neogene uplift and incision. Instead, cosmogenic nuclide dating of cave deposits suggests that river incision today is primarily in response to Plio-Pleistocene climate change and drainage reorganization associated with the Laurentide Ice Sheet.