EVIDENCE FOR MODERATE TO HIGH ELEVATION OF THE LATE CRETACEOUS ‘NEVADAPLANO' IN THE NORTHERN BASIN AND RANGE PROVINCE FROM CARBONATE CLUMPED ISOTOPE (Δ47) THERMOMETRY

From the middle Mesozoic to the present, the topography of the Basin and Range province (BRP) of the western Cordillera of North America has evolved in response to diverse tectonic forcings. The details of the topographic evolution of this region are much debated however and few studies have attempted to quantify the elevation of the pre-existing topography that developed in response to sustained convergence off the western coast of North America during much of the Mesozoic. Some workers have argued that the region was a high elevation plateau, the ‘Nevadaplano,’ analogous to the South American Altiplano. To test this idea, we generated clumped isotope (Δ₄₇) temperature estimates from Late Cretaceous-aged lacustrine and paleosol carbonates from the Sheep Pass Formation in east-central Nevada and the North Horn Formation in central Utah.

Age constraints for our samples suggest that Member B of the Sheep Pass Formation (NV) and the lower red beds of the North Horn Formation (UT) were deposited within a 15 million year window from ~74 – 60 Ma. The textural characteristics, isotopic values and Δ₄₇ temperature estimates from co-occurring secondary carbonate suggest that, despite moderate burial, primary carbonate samples have undergone little diagenetic alteration. Average temperatures from samples closest in age between the NV and UT sections (~66.5 and 72 Ma. respectively) suggest that the NV site was ~15 °C cooler than the UT site during the Late Cretaceous. This thermal gradient implies an elevation difference between the two sites of ~2.5 km, given certain assumptions: 1) there was little global or regional climate change during the ~5 million years between formation of these samples, 2) the lacustrine and paleosol carbonates likely formed during summer, and 3) the global-average thermal lapse rate of 6°C/km applies. The use of alternate viable thermal lapse rates increases this elevation difference to ~3.5 km. In contrast, possible corrections to the average Δ₄₇ temperatures due to bias in the timing of lacustrine and paleosol carbonate precipitation or climate change decreases the elevation difference by up to ~1 km. These results support the inference that a moderate to high elevation ‘Nevadaplano’ existed during the Late Cretaceous in east-central Nevada.