LATE-QUATERNARY GLACIAL HISTORY OF THE TAYLOR RIVER BASIN, SOUTHERN SAWATCH RANGE AND ELK MOUNTAINS, COLORADO: PREVIEW OF A POST-MEETING FIELD TRIP

The late-Pleistocene in the Taylor River drainage was characterized by extensive glaciation. Glacial systems during this time existed as small cirque glaciers, valley glaciers and piedmont lobes, and glacier complexes consisting of both large valley glaciers and upland ice fields. Moraines of two glaciation are found within the basin and can be distinguished on the basis of relative-dating criteria. Moraines of the older Cow Creek glaciation are correlated morphostratigraphically to those of Bull Lake age in the Wind River Range, Wyoming. Recent age determination on the latter thus suggest the Cow Creek glaciation occurred ~150 ka, coinciding with marine isotope stage (MIS) 6. The younger Taylor River glaciation is correlated to the Pinedale glaciation in the Wind River Range and elsewhere and culminated during the last glacial maximum (LGM). Cosmogenic radionuclide surface exposure (CRSE) ages obtained from boulders on Taylor River-aged terminal moraines suggest the local LGM occurred between 22.2 and 16.3 ka, coincident with MIS 2. During this time, equilibrium-line altitudes (ELAs) of glaciers were at least ~900 m below an estimate of modern regional snowline. Based on degree-day modeling of ELA depression and an analysis of steady-state mass balances required to maintain twelve reconstructed glaciers, LGM climate is inferred to have been ~6.5 to 8 °C cooler than present. Thereafter, ice retreat in the main valleys was somewhat continuous but punctuated by several minor stillstands (or readvances) as evidenced by multiple recessional moraines. Upland ice most probably disintegrated prior to the onset of major ice recession in the valleys. A CRSE age of 11.8 ka on one of the recessional moraines suggest a local readvance during the Younger Dryas cold interval, ca.12.8-11.6 ka. By ~10 ka many cirques were ice free. Episodes of Holocene cooling apparently did not result in the re-formation of glaciers, but were instead characterized by extensive periglacial activity. Three discrete intervals of talus production/rock glacier activity are dated lichenometrically at ~3100, 2100, and 1200 cal yr B.P. These correspond to other periods of climatic deterioration documented in the region.