Late Paleocene high Laramide ranges in northeast Wyoming: Oxygen isotope study of ancient river water

Abstract

The distribution and initial timing of the establishment of high surface elevations in the Rocky Mountains during the Early Cenozoic remain controversial despite the importance of these data in testing tectonic models for this region. We track the timing and distribution of high elevation in the Rocky Mountains during the Late Cretaceous–Early Eocene by examining annual and seasonal δ 18O values of the ancient river water, which are calculated from the δ 18O values of well-preserved freshwater bivalve shells. In the Powder River basin of the eastern Laramide province, the δ 18O values of the ancient river water vary between − 23.0‰ and − 8.0‰ SMOW in both seasonal and annual records in the Late Paleocene–Early Eocene. The large variation suggests that the ancient rivers were fed yearly or seasonally by snowmelt from highlands of 4.5 ± 1.3 km. This can be explained by the existence of the Bighorn Mountains and Black Hills with a drainage pattern similar to the present in northeast Wyoming. The δ 18O values of ancient river water along the front of the Sevier thrust belt generally follow a trend from lower values in north, − 14.2 ± 1.4‰ in the Early Paleocene Crazy Mountains basin, to higher values in south, − 11.1 ± 0.8‰ in the Late Paleocene Bighorn basin, and − 7.1 ± 1.6‰ in the Early Eocene Washakie basin. The variations within each basin are relatively small. These rivers most likely rise in the Sevier thrust belt, and may reflect highland elevation of 1–2 km. The δ 18O values in the Alberta foreland and Williston basin are very low (− 20.5‰) in the Late Cretaceous, indicating the rivers were fed by snowmelt from the Canadian Rocky Mountains of 4.3 ± 1.0 km high. The attainment of high elevation in the eastern Laramide province prior to the western province could be explained by southwestward progression of back-thrusts soled into an earlier east-directed master detachment, which may be formed by the westward rollback of subducted shallow slab.