Summary

The timing of surface uplift of two major mountain ranges of the western United states, the Sierra Nevada and Rocky Mountains, is not well constrained. One group of studies of the Sierra Nevada indicates surface uplift occurred within the past ~10 Ma, while another group of studies supports older surface uplift. Similarly in the Rocky Mountains, one group argues for surface uplift since ~20 Ma, while another group infers that high elevations existed as long ago as 50 Ma. Many techniques have been used to address surface uplift and paleoelevations of these two mountain ranges. This webpage summarizes the application of one of these methods, oxygen isotopes, to estimating paleoelevations of the Sierra Nevada and the Rocky Mountains. Oxygen isotope data collected on the eastern side of the Sierra Nevada indicates that the Sierra were as high as or higher than present day elevations 14 to 16 million years ago (Poage and Chamberlain, 2002). Data collected in the Green River basin of southern Wyoming (west side of the Rocky Mountains) is less definitive. The oxygen isotope signal of carbonate samples show episodes of depleted oxygen isotopes, which may indicate an influx of snowmelt from high peaks to the Green River basin (Norris et al., 1996), or isotope alteration during diagenesis (Morrill and Koch, 2002). Although many assumptions are made in oxygen isotope analyses, when used with other geologic information, oxygen isotopes are a valuable tool for understanding paleoelevation.

References

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