GEOLOGIC SETTING AND STRATIGRAPHY OF THE ZIEGLER RESERVOIR FOSSIL SITE, SNOWMASS VILLAGE, COLORADO

The geologic setting of the Ziegler Reservoir fossil site is unusual – the sediments containing the Pleistocene fossils were deposited in a lake that was located on top of a ridge. The lake basin was formed when a glacier flowing down Snowmass Creek Valley became thick enough to overtop a low point in the eastern valley wall and entered the head of Brush Creek Valley near the town of Snowmass Village, Colorado. When the glacier retreated at the end of the marine isotope stage (MIS) 6, ~155-130 ka (thousands of years before present), the Brush Creek Valley sublobe left an encircling, broad-crested moraine that impounded a ~14.2 ha drainage basin, forming a small alpine lake that was initially ~10 m deep. The lake was highly productive during most of its existence, based on the abundant and exquisitely preserved organic material present in the sediments, and slowly filled with mostly eolian sediment over time. By ~85 ka, the lake had become more of a marsh or wetland than a true lake, but open lake conditions returned between ~75 and 55 ka before the impoundment was finally breached to the east, establishing ties with the Brush Creek drainage system and creating an alpine meadow. In this presentation, we will describe the sedimentology and stratigraphy at Ziegler Reservoir, including both lake-center and near-shore sediments that were initially exposed as part of a reservoir expansion project and later during excavation of one of the most prolific Pleistocene fossil sites in North America. We will also use particle size and mineralogical data, as well as field observations, to establish relative lake levels and infer depositional settings of the various units in an effort to reconstruct the site history independent of other proxy methods. In addition to the spectacular nature of the Pleistocene fossils, the Ziegler Reservoir fossil site offers a rare view into paleoenvironmental conditions high in the Rocky Mountains of Colorado through all five substages (a-e) of MIS 5, MIS 4, and the earliest part of MIS 3.