SKIING IN THE EOCENE UINTA MOUNTAINS - ISOTOPIC EVIDENCE IN THE GREEN-RIVER FORMATION FOR SNOW MELT AND LARGE MOUNTAINS

Isotopic analysis of lacustrine carbonates from the Eocene Green River Formation suggests that lake waters were derived partly from snow mel t. This evidence for cool climates is in marked contrast to paleontolo gical and model evidence for mild temperatures in the continental inte rior. Oxygen isotope ratios of carbonates frequently reach -12 parts p er thousand to nearly -16 parts per thousand (Peedee belemnite), which suggests that lake waters probably had delta(18)O of <-13 parts per t housand (standard mean ocean water). Consideration of the evaporative O-18 enrichment that typically occurs in modern large saline lakes sug gests that the source waters to the Green River basin had a delta(18)O of <-18 parts per thousand. These ratios are consistent with snow mel t and are too negative to be easily accounted for by distillation in t he atmosphere during heavy rainfall. The Green River lakes formed in a closed basin encircled by large mountains; this suggests that the sno w melt was locally produced. The mountains surrounding the lake must h ave been high enough to occasionally supply significant melt water to the much lower lake. Lapse rate calculations suggest minimum altitudes of >3000 m for the mountains encircling the Green River basin.