ZIEGLER RESERVOIR AND THE SNOWMASTODON PROJECT: OVERVIEW AND GEOLOGIC SETTING OF A RECENTLY DISCOVERED SERIES OF HIGH-ELEVATION PLEISTOCENE ECOSYSTEMS NEAR SNOWMASS VILLAGE, COLORADO

On Oct. 14, 2010 a bulldozer operator discovered a young female mammoth while excavating Zeigler Reservoir on the divide between Snowmass and Brush creeks near Snowmass Village, Colorado. Subsequent investigation by the Denver Museum of Nature & Science between Oct. 29 and Nov. 16, 2010, revealed an amazingly well-preserved sequence of high-elevation Ice Age ecosystems. In 18 days, DMNS excavated more than 600 bones comprising parts of more than 20 skeletons (Mammuthus columbi, Mammut americanum, Bison latifrons, cf. Odocoileus, Megalonyx jeffersoni, and numerous microvertebrates) and exceptionally preserved plant, insect and aquatic invertebrate fossils. A second excavation (May 14-July 3) involved 231 people (2,870 person days), moved more than 8,000 tons of sediment, and retrieved more than 4,800 bones representing more than 20 additional vertebrate species. The deposit sits in a small ridgetop catchment with no inflowing streams suggesting that the Pleistocene lake filled slowly with input primarily from slope wash, debris flows, and aeolian sediment. Lower units of the ~10m section contain isolated bones of Mammut, Bison, cf. Odocoileus, and Megalonyx in subaqueous debris flow deposits. Upper units contain Mammuthus, Bison, Camelops, and cf. Odocoileus in a well-preserved sedge peat. As a whole, the site represents the first relatively complete sequence of Pleistocene ecosystems at high elevation in the Rocky Mountains. The Ziegler Reservoir site is significant for 5 reasons: 1) it is located at 8,874 feet above sea level and high elevation Pleistocene sites are rare; 2) it contains several fossil-bearing horizons, which will allow for the reconstruction of a series of ecosystems; 3) it contains a broad diversity of plant, vertebrate, and invertebrate fossils, which will allow for more complete ecosystem analyses; 4) the preservation of the fossils discovered at Ziegler Reservoir is exceptional ; and 5) radiocarbon dating indicates that all of the sediment in Ziegler Reservoir site is more than 45,000 years old and local glacial geology indicates that the lake-bounding Bull Lake moraine is 130,000 to 150,000 years old. This presents the possibility that as much as 100,000 years may be represented by the lake sediments.