RECONSTRUCTING EARLY EOCENE ELEVATIONS OF EASTERN IDAHO AND SOUTHWEST MONTANA USING HYDRATED VOLCANIC GLASS
Eocene paleovalleys found on the reentrant of the Cordilleran fold-and-thrust belt in southwest Montana indicate a possible paleoriver that flowed southwest towards the foreland, which transported sediment into Eocene basins in Montana and western Wyoming, suggesting relatively higher elevations to the east of the thrust front. δD values of ancient hydration water within silicate volcanic glasses document the δD value of precipitation water very soon after the glass is deposited. Eocene volcanic tuffs sourced from the Challis volcanic province were collected from eastern Idaho to southwest Montana. Samples were separated to ~99% isotropic glass shards from the tuffs, and analyses were performed on a TC/EA-MAT 253 IRMS. δDglass values decrease from west (-104‰ to -121‰) to east (-152‰ to -161‰) across the proposed hinterland plateau, likely reflecting a paleotopographic gradient with higher elevations in the east. These results reflect regions of higher than modern elevations in the Eocene in eastern Idaho and southwest Montana along the continental divide, as well as regions where mixing of two isotopically distinctive air masses are contributing to meteoric water isotopic compositions.