POLYGONAL CRACKING ON LATE PLEISTOCENE TERRACES WITHIN THE TENNESSEE RIVER VALLEY, A STRATIGRAPHIC MARKER HORIZON AND ITS POTENTIAL PALEOCLIMATIC SIGNIFICANCE

Geoarchaeology trenches within the Tennessee River Valley encountered fossil polygonal cracks in alluvial terrace deposits along the French Broad River in the upper drainage basin and ~130 km downstream in the Chickamauga Reservoir. Polygons occur buried on T2 terraces at a depth of 1 m on the French Broad River and 1.5 m on the Tennessee River. Radiometric dating of organics in Tennessee River alluvium yielded an age of 16.7 ka BP at 175 cm and 19.8 ka BP at 265 cm indicating the age of the alluvium in which the polygonal cracks formed. This correlates with the end of the Late Glacial Maximum (LGM), a cold, dry period within the Tennessee River Valley.

The buried polygonal cracks extend vertically 1 m in profile and taper to a point at the base. The polygons are up to 50 cm in diameter and the top of some cracks appear to be wedge-shaped. The polygons occur on the elevated Holocene T2 terraces. These treads are interpreted as the Late Pleistocene T1 terraces, prior to incision of the Tennessee River into its present channel. Preservation across this surface varies because of root bioturbation. Terrace sediment is a fine sandy, silt (51-68% fines) and clay mineralogy is dominated by chlorite, vermiculite, illite and montmorillonite. There is little difference in the particle size distribution between the alluvium and crack fillings.

One of the principal roles of a geomorphologist is to determine how deep archaeologists need to test in order to reach culturally sterile deposits. These Late Pleistocene features form an important marker horizon as they pre-date (16ka BP) documented Native American occupation within the SE United States. Further work is underway to interpret formation processes to provide a paleoclimatic proxy for conditions within the Tennessee River Valley during the LGM.