ABSTRACT

The Rocky Mountains are an extensive area of high elevation in the U.S. continental interior. Today, elevations in the central Rockies reach up to 4 kilometers above sea level, while basin floors mostly lie at about 1.5 kilometers. But 80 million years ago the entire region was near sea level. It is unlear to what extent present high elevations were generated during the last mountain-building event (the Laramide Orogeny, ending about 50 million years ago) or were formed during more recent surface uplift. Contrasting data sets have led to two schools of thought regarding high plateau development in this region: tectonic interpretations primarily focus on Neogene uplift due to processes active in the Earth?s upper mantle, while climatic interpretations suggest only a small amount of uplift has taken place, but instead the region has undergone a significant amount of erosion caused by climate change during the last few million years. This project provides new constraints on these interpretations by combining sedimentology and stable isotope geochemistry of upper Cenozoic sedimentary rocks in the central Rockies and adjacent Great Plains. These data clarify when the basins formed and were integrated through time, as well as allow reconstruction of changes in elevation and climate during the last few million years. These data constrain (1) the occurrence and timing of a change from river-laid to wind-laid deposits in the basins, possibly related to regional changes in climate; and (2) the timing of surface elevation changes by reconstructing the hydrogen and oxygen isotope ratios of ancient water from volcanic glass and soil and lake carbonates and ancient air temperature from clumped isotopes of these same carbonates. These results can be used to evaluate and refine proposed mechanisms of formation of the high central Rockies and help determine whether the Rocky Mountains have recently gained elevation or are a long-dead mountain chain.

Broader impacts of this study include providing early career support for a female assistant professor. The study also establishes an innovative methodology by integrating three sets of stable isotope data with basin sedimentology to provide a means of distinguishing climate change from surface uplift. In addition, this study integrates with other similar, NSF-supported, research initiatives that cover other areas of the western U.S. to paint a broader picture of recent vertical movements of the land surface across the region. Lastly results of this study are to be incorporated into an electronic presentation that will form part of an installation at the Geology Museum on the University of Wyoming campus. The museum is a frequent destination for middle and high school classes from across the state, as well as a stop for other visitors. This presentation will provide insight into the origins of high elevation across the state and region.