PALEOHYDROLOGIC CONDITIONS ACROSS A LATE CRETACEOUS FORELAND BASIN INFERRED FROM OXYGEN ISOTOPE RECORDS, MONTANA, U.S.A

Oxygen isotope (d¹⁸O) data from unionid bivalves and lacustrine carbonates allow us to reconstruct terrestrial paleohydrologic conditions for a 77.5 to 74.3 Ma time interval across a 200+ km transect in the Late Cretaceous foreland basin of Montana. Samples are derived from the Two Medicine and Judith River Formations that represent alluvial uplands and coastal lowlands, respectively. Lacustrine carbonates from a widespread (>15 km²) lake interval in the Two Medicine Formation have an average d¹⁸O of -11.01‰ (VPDB). Unionid bivalves from the Two Medicine Formation have an average d¹⁸O of -8.40‰. Judith River Formation unionids recovered from fluvial sandstones have an average d¹⁸O of -15.96‰. Unionid bivalves recovered from ponded, paralic sediments in the Judith River Formation have an average d¹⁸O of -15.74‰ with a few samples having values ~ -8.30‰. The isotopic variation among unionids and lacustrine carbonates likely stems from mixing between local precipitation across the foreland basin and high altitude precipitation in the Sevier highlands to the west. Estimated d¹⁸O of water in equilibrium with the more positive unionid d¹⁸O values in the ponded settings is -6.8‰ (SMOW), and likely represents a local precipitation signal. In contrast, estimated d¹⁸O of water in equilibrium with fluvial sandstone unionids is -15.2‰, and are too negative to represent local precipitation alone (because of the proximity of the Western Interior Seaway). Thus water in these fluvial systems was likely sourced from higher altitudes in the Sevier highlands. Lacustrine carbonate d¹⁸O values are intermediate and suggest that lake water was sourced from both local precipitation and fluvial/high elevation input. Our results indicate that Sevier highlands played an important role as a barrier to air mass movement, and suggest the extremely low d¹⁸O may be caused by high altitude snow-melt and/or possible monsoonal precipitation patterns. Our interpretations are consistent with previous climatic reconstructions based upon paleofloras and sedimentologic features suggesting a semi-arid uplands and humid lowlands. Lastly, a comparison of these data with isotopic data from younger invertebrates from the same region indicates a significant shift in the paleohydrologic conditions between the Campanian and Maastrichtian stages.