LATE PALEOGENE PALEOELEVATION OF THE NORTHERN CORDILLERAN OROGENIC FRONT BASED ON CARBONATE OXYGEN ISOTOPES

Delineating the timing and magnitude of the North American Cordilleran orogen is essential to determining processes and mechanisms that caused the growth of the mountains. This study uses oxygen isotope compositions (δ¹⁸O) of late Paleogene molluscan fossils and paleosol nodules in the Kishenehn basin in northwestern Montana and southern British Columbia to reconstruct the paleorelief and climate during early stage extensional collapse of the mountain belt. Mollusk samples include both aquatic and terrestrial taxa, and contain tropical wet, arid subtropical, and humid temperate sympatric groups based on the habitats of their modern living relatives (Pierce and Constenius, 2001). The δ¹⁸O values of the mollusk shells and paleosol nodules vary between -19.8‰ and -6.5‰, and -16.2‰ and -7.7‰ (VPDB), respectively. Reconstructed surface water δ¹⁸O values from mollusks are between -21.2‰ and -7.4‰, and reconstructed values from paleosol nodules range from -12.9‰ to -4.4‰ (VSMOW). The high δ¹⁸O values reconstructed from paleosol nodules represent δ¹⁸O values of lowland precipitation. The wide range of reconstructed surface water δ¹⁸O values (-21.1‰ to -6.3‰ VSMOW) from the aquatic mollusks suggest they lived in rivers with variable proportional mixing between highland (with low δ¹⁸O) and lowland (with high δ¹⁸O) precipitation. The range of reconstructed surface water δ¹⁸O values from terrestrial snails (-15.7‰ to -7.2‰ VSMOW) suggest they lived at various elevations with a varying degree of ambient water evaporation. High-resolution sampling along shell growth bands reflects up to 1.8‰ of seasonal variation in river water δ¹⁸O values. By using both the Rayleigh condensation model and an isotope-enabled global climate model for the Paleogene, we estimate the relief between the Kishenehn basin floor and its surrounding mountains in the Cordilleran orogenic front to be about 4 km, and the basin floor elevation to be less than 2.5 km during the late Paleogene. These results suggest that the Cordilleran orogenic front was at high elevations during the late Paleogene.