STABLE ISOTOPE EVIDENCE (δ18O AND δD) FOR LATE EOCENE–LATE MIOCENE TOPOGRAPHY AND CLIMATE IN THE NORTHERN ROCKY MOUNTAINS, SOUTHWESTERN MONTANA

The northern Rocky Mountains in southwestern Montana contain intermontane basins with a relief contrast of 1–1.5 km. Late Eocene–late Miocene deposits in these basins include continental strata of the Renova and Sixmile Creek formations. Previous studies suggested that the late Eocene–early Miocene Renova Formation was deposited in a low-elevation, low-relief setting, whereas others advocated a similar-to-present topography since the middle Eocene. Most agree on significant topographic relief existing during mid–late Miocene Sixmile Creek Formation time. This study reconstructs the late Eocene–late Miocene paleoelevation and paleoclimate in the northern Rocky Mountains by studying paleo-meteoric water isotopic compositions reconstructed from carbonate oxygen and volcanic glass hydrogen isotopes sampled along a transect from southwestern Montana to western North Dakota.

The stable isotope values of modern river water and precipitation along the transect decrease from east to west. Single particle back trajectory analysis indicates that moisture in mountainous western Montana is nearly exclusively from the west, whereas the lowlands in eastern Montana and western North Dakota have sources from both the west and north. The differences in moisture sources and meteoric water isotopic values may be caused by: 1) the northern Rocky Mountains blocking westward transport of northern moisture, and 2) enhanced surface water recycling in the lowlands. These observations indicate that, in the northern Rocky Mountains, Rayleigh fractionation-based stable isotope paleoaltimetry is not applicable for paleoelevation estimations.

The δD values of 14 hydrated late Eocene–late Miocene volcanic glass samples from southwestern Montana range between -143 to -181‰. One glass sample has a δD value of -103‰, possibly a result of evaporation. The reconstructed δD values of paleo-meteoric waters are between -113 to -153‰. These values have identical or greater variation magnitude as modern meteoric water δD values in the same area, which are between -120 to -145‰, suggesting that southwestern Montana may have obtained similar-to-present topography since the late Eocene. Paleo-meteoric water δ¹⁸O values reconstructed from carbonate samples will be used to further test the hypothesis and reconstruct paleoclimate.