LATE QUATERNARY HISTORY OF THE WACO MAMMOTH SITE: ENVIRONMENTAL RECONSTRUCTION AND INTERPRETING THE CAUSE OF DEATH

The Waco Mammoth site (WMS), located in central Texas, has been interpreted as the largest single-herd, non-human-related death site in the world. Previous work, however, has resulted in: a 45,000 year discrepancy in the age of the site (i.e. standard radiocarbon of bone ~ 29 ka B.P. vs. Ur/Th tooth enamel ~ 72-73 ka B.P.), uncertainty in the cause of death, and conflicting interpretations as to whether the animals comprise one or more herds from different time periods. To address these concerns we re-examined the site by employing a new dating technique (OSL) and conducting a detailed stratigraphic assessment. Stratigraphic and buried soil descriptions were written from excavation exposures and cores with samples taken for particle-size analysis, organic/inorganic carbon content, thin section analysis, and stable carbon and oxygen isotopic analysis. OSL samples were obtained from 8 sandy beds associated with, and bracketing, the stratigraphic position of the mammoths. The site is buried in alluvium of an upper terrace within the modern confluence of the Brazos and Bosque Rivers. Core and backhoe trench data suggests that the WMS is situated within an abandoned channel, which cuts into the highest alluvial terrace of a paired three-tier terrace system generated by the Bosque River (i.e. grassland Mollisols) to the south and the Brazos River (i.e. hardwood savannah Alfisols) to the north. The stratigraphic sequence at the site consists of basal fluvial gravels resting on an incised Cretaceous carbonate bedrock surface and overlain by multiple semi-continuous to continuous laminated sand layers serving as correlative markers for relative positioning of the mammoths. New OSL ages range from 65-73 ka, overlapping with previous Ur/Th ages, and suggesting that the mammoths were living during a period of cold/dry climate coinciding with a major sea-level lowstand associated with marine oxygen isotope stage 4. Preliminary stratigraphic evidence indicates more than one mammoth death event, possibly related to periodic entrapment and drowning within the low-order tributary channel near the paleo-Bosque River.