TAPHONOMY OF MEGAFAUNAL ACCUMULATIONS FROM CLASTIC, WEDGE-SHAPED, LAKE MARGIN DEPOSITS AT THE SNOWMASS SITE, PITKIN COUNTY COLORADO

The Snowmass Site is a stratified late Pleistocene (ca. 125-70ka) lacustrine locality at approximately 2728 m (9000 ft) elevation in the central Rocky Mountains of Colorado. Multiple clastic, wedge-shaped deposits have formed at the margin of the lake, along a terminal moraine. The different stratigraphic wedges may be related to various environmental and climatic fluctuations during MIS 5. These deposits are poorly sorted with a heterogeneous mixture of variable grain sizes ranging from boulders to sands, silts and clays. Some wedges are grain supported while others are matrix supported. All of the wedges are extremely fossiliferous but almost completely composed of the bones and teeth of the American mastodon, Mammut americanum. To better understand the origin of these deposits and their included fossils, more than 1500 bone specimens from three different stratigraphic units were examined for 50 different taphonomic attributes (e.g., weathering stage, breakage, cracking, scratches, striations, pits, etc.). An electronic database was used to score the attributes for each bone. The results of the analysis of the data suggest that the bones were derived from time-averaged accumulations along the edge of the lake (weathering, carnivore activity, dry bone fractures, etc.) and they were then moved down slope by, and in, sediments (polish, striations, fracture patterns, etc.) that were derived from gravitational processes. The various taphonomic pathways resulted in a density-mediated accumulation of bone (hence the abundance in mastodon bones and teeth at the expense of other taxa) without any articulation of bones. This study refutes previously proposed models of entrapment of mastodons in thixotropic sediments and catastrophic death that have been proposed for the site.