EFFECT OF REGIONAL TOPOGRAPHY AND HYDROLOGY ON THE LACUSTRINE ISOTOPIC RECORD OF MIOCENE PALEOCLIMATE IN THE ROCKY-MOUNTAINS

Oxygen isotopic compositions of upper Miocene (9.2 m.y.) lacustrine limestone from northwestern Wyoming range from -21.5 to -35.2 parts per thousand. (PDB) and are the lightest yet reported for a non-marine carbonate sequence. These values require that lake water was greatly depleted in O-18 relative to modem meteoric precipitation and indicate that most inflow was sourced as meltwater from glaciers in the adjacent Gros Ventre range. Assessment of climatic factors influencing the isotopic composition of global meteoric water indicates either that nearly all of Miocene precipitation in the southern Rocky Mountains was derived from large lake systems in the Snake River Plain and northern Great Basin to the west, or that Gros Ventre catchment elevations were up to 2300 m higher than at present. Because erosion rates in modem alpine regions suggest that subaerial denudation could only account for up to 1200 m of post-Miocene elevation reduction, any additional lowering must reflect the influence of post-Laramide epeirogeny during Basin and Range extensional tectonism. Lacustrine isotopic data therefore provide boundary conditions on the timing and magnitude of changes in late Cenozoic paleoclimate, topography, hydrology, and tectonism in the western Wyoming portion of the Rocky Mountains.