A GEOCHRONOLOGIC FRAMEWORK FOR THE BUCK TANK DRAW FOSSIL SITE, BIG WATER, UTAH

The Buck Tank Draw site was recently discovered as a concentration of disarticulated bones within olive-green, clay-rich silt site near the town of Big Water in southernmost Utah. The fossil remains and sediments were exposed by erosion along Buck Tank Draw, an ephemeral stream and tributary of Wahweap Creek and the Colorado River. This site has not yet been surveyed or excavated, however, preliminary reconnaissance identified three partial mammoth specimens, comprising exposed bone, molar and tusk fragments. Other taxa at the site include horse, a bovid (possibly bison or shrub ox) and a small coyote-sized carnivore. Floral remains include sedge seeds, suggesting a wetland habitat.

At the time of discovery, the age of the deposit was not known. While the Big Water site is near latest Pleistocene (< 15ka) mammoth sites in the region, the geomorphology of the area suggests that it may be much older. For example, the deposits are capped with with a well-developed carbonate soil profile and eolian sands and are associated with terrace gravel deposits, suggesting a middle to late Pleistocene age. Correlations to nearby Colorado River terraces suggest that these terrace deposits are >90ka. Geomorphic and stratigraphic relationships suggests that the marshy conditions associated with the fossil-bearing silt may have formed in response to to alluvial damming the mouth Buck Tank Draw.

Additional information about the Buck Tank Draw site is needed prior to excavation. Research methods include geomorphic mapping of the site and stratigraphic description of the fossil-bearing silt deposit. To establish a geochronologic framework, five samples were collected in a stratigraphic profile for optically stimulated luminescence (OSL) dating, which provides an age estimate for the last time sediments were exposed to light. OSL dating was chosen over radiocarbon dating due to the expected antiquity of the site and its versatility in dating sediment deposition. Final OSL ages are still forthcoming, but preliminary results suggest the silty deposit and faunal remains at the Buck Tank Draw site date to 100-120 ka. These results help place the faunal assemblage into a regional context.