Middle to late Cenozoic cooling and high topography in the central Rocky Mountains: Constraints from clumped isotope geochemistry

The timing and mechanism of the formation of the high elevations of the central Rocky Mountains (Rockies) in the western interior of North America remain controversial. We examine the middle and late Cenozoic paleorelief between the Great Plains and the central Rockies using clumped isotope geothermometry of carbonate cements of fluvial and eolian sandstones. Our petrographic observations suggest that the studied carbonate cements were formed primarily during eodiagenesis in near-surface conditions and retain primary paleoclimate and paleoelevation signals. Carbonate clumped isotope temperatures decrease similar to 10 degrees C, and calculated water delta O-18 values decrease similar to 4 parts per thousand from similar to 35 Ma to similar to 5 Ma in both the central Rockies and the western Great Plains, following middle and late Cenozoic global cooling trend. A persistent longitudinal gradient in temperature and delta O-18 value existed between the central Rockies and the Great Plains during the Oligocene and Miocene, with temperatures 4-8 degrees C higher, and calculated water delta O-18 values 3-6 parts per thousand higher in the Great Plains. These observations suggest the mean elevation of the central Rockies was similar to 1 km higher than the western Great Plains during the middle and late Cenozoic, similar to the present-day regional relief. When placed in the context of other paleoaltimetry studies and geological observations, our findings support the hypothesis that the high mean topography of the Rockies was developed during the late Eocene, possibly related to isostatic adjustment, or dynamic uplift caused by foundering of lower mantle lithosphere or the Farallon slab, or both. (C) 2014 Elsevier B.V. All rights reserved.