URANIUM-SERIES AGES OF TRAVERTINES AND TIMING OF THE LAST GLACIATION IN THE NORTHERN YELLOWSTONE AREA, WYOMING-MONTANA

Uranium-series age determinations by mass spectrometric methods were d one for travertines and associated carbonate veins related to clastic deposits of the last glaciation (Pinedale) in the northern Yellowstone area. Dramatic variations in the hydrologic head are inferred from va riations in the elevation of travertine deposition with time and are c onsistent with the expected hydrologic effects of glaciation. We deter mine the following chronology of the Pinedale Glaciation, with the key assumption that travertine deposits (and associated carbonate veins) perched high above present thermal springs were deposited when glacier s filled the valley below these perched deposits: (1) the early Pineda le outlet glacier advanced well downvalley between 47,000 and 34,000 y r B.P.; (2) the outlet glacier receded to an interstadial position bet ween 34,000 and 30,000 yr B.P.; (3) an extensive Pinedale ice advance occurred between 30,000 and 22,500 yr B.P.; (4) a major recession occu rred between 22,500 and 19,500 yr B.P.; (5) a minor readvance (Deckard Flats) culminated after 19,500 yr B.P.; and (6) recession from the De ckard Flats position was completed before 15,500 yr B.P. This chronolo gy is consistent with the general trend of climatic changes in the nor thern hemisphere as revealed by recent high-resolution ice-core record s from the Greenland ice sheet. (C) 1994 University of Washington.