RECONSTRUCTING EARLY EOCENE ELEVATIONS OF EASTERN IDAHO AND SOUTHWEST MONTANA USING HYDRATED VOLCANIC GLASS

Topography, which can link atmospheric processes to the solid Earth, is the result of a combination of tectonic, climatic, and erosional processes. Since surface topography is transformed by plate interactions, topography can directly connect the processes of the Earth’s mantle and crust to the surface. Recent research in the North American Cordillera has helped aid in reconstructing the topography of the central Cordilleran hinterland during the Cenozoic. Yet absolute paleoelevations of the modern continental divide region in Idaho and Montana, from the hinterland across the Sevier thrust front, are not known. We determined the paleotopography of the region in order to constrain the timing of the onset of extension, the migration of the drainage divide, and the northward extent of the Cordilleran hinterland plateau.

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. δD_glass 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.