Age Control for the Buck Tank Draw mammoth site, Big Water, Utah

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CONFERENCE: Research Week

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Millsap, Evan
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A site containing Ice Age mammoth, horse, bison and carnivore bones was recently discovered near the town of Big Water, in southernmost Utah. Nearby mammoth remains from Bechan Cave, about 45 km northeast of Big Water, have been dated to 11-13 thousand years old. However, terraces at Lee’s Ferry that are believed to correlate with the mammoth site are between 90-150 thousand years old. To establish age control for the fossil site, five samples were collected in a stratigraphic profile for Optically Stimulated Luminescence (OSL) dating. OSL dating provides an age estimate for the last time the sediment was exposed to light, because light resets a natural “luminescence signal” within mineral grains. Initial analysis of the luminescence signal from the samples collected at Buck Tank Draw demonstrate an age much older than 13 ka (thousand years), but final lab results are still pending. If these mammoth remains are shown to be older than 13 ka, they will fill a gap in the fossil record. Previous work by Pederson et. al on the terrace chronology of the Colorado watershed indicated characteristic cycles of aggradation and incision that correlate strongly with climate patterns. The OSL dates, which will be processed shortly, will show how the mammoth site fits into this greater picture. Sedge seeds within the deposits indicate a marshy habitat, perhaps during a period of wetter regional climate. As results from this study bring clarification to the exact age of the deposits, they can be correlated to the greater Colorado River story and will give insight into understanding the history of the Ice Age in southern Utah.