ALPINE GLACIATION IN THE LARAMIDE MOUNTAINS DURING THE LATE CRETACEOUS AND EARLY PALEOCENE

Well-preserved freshwater mollusks from the Late Cretaceous Upper Fox Hills and the Paleocene Sentinel Butte Fms. of North Dakota provide detailed d¹⁸O and d¹³C values for multiple years of growth. These isotope records indicate that meltwaters from high-altitude alpine glaciers in the Laramide Mtns. flowed eastward into the most distal portions of a large fluvial-deltaic system before and after the K-T boundary.  Cretaceous d¹⁸O values as low as -22.3 ‰ (PDB) and Paleocene d¹⁸O values as low as -24.5 ‰ (PDB) exceed previously reported values. High resolution micro-sampling of shell aragonite yields d¹³C value time series markedly different than those collected from extant riverine mollusks (same genus). Rather than having high d¹³C values from spring-summer growth intervals as in modern shells (indicative of elevated ambient productivity), Cretaceous and Paleocene shells have high d¹³C values during winter growth intervals.  Such time series indicate stream productivity increases during the winter -- behavior found in modern pro-glacial streams (high suspended loads, bed instability, and variable nutrient supply).  Shell d¹⁸O values vary from -22 to -13 ‰ in a sigmoidal pattern due to seasonal variation of meltwater discharge from high-elevation alpine glaciers in concert with background low-altitude precipitation (spring thaws inc. discharge and turbidity and fall freezes dec. discharge and turbidity). Comparison of low altitude abiotic carbonates precipitated from local, low-altitude groundwaters and riverine biogenic carbonates precipitated from high-elevation waters (d¹⁸O values of –8 ‰ and < -20 ‰ SMOW, respectively) suggests that the elevation of the Laramide mountains was 4-6 km.  Exceptionally low d¹⁸O values of Hell Creek river water are maintained over the entire length of this river system (~750 km) and appear to be relatively undiluted by local precipitation - suggesting that the amount of local precipitation and/or runoff was low in eastern Montana and the Dakotas.  Modeling of latitudinal gradients for meteoric water d¹⁸O values conflict with this finding. Occurrence of warm rainforest flora from the Denver Basin during the early Paleocene suggests that the source of high-elevation waters was not from the western side of the Laramides, but instead was from the Western Interior Seaway.