DETERMINING PALEOELEVATION OF THE ROCKY MOUNTAINS AND COLORADO PLATEAU: A CHALLENGE FOR PALEOGEOGRAPHY

There are a number of very different techniques now being employed as proxy measures of paleoelevation for the purpose of determining the timing and magnitudes of plateau uplift and other epeirogenic movements. Such approaches include arguments based on geomorphology, fission tracks, cosmogenic nuclides, stratigraphy, paleoflora, O-isotopes, vesicular basalts, and perhaps other novel techniques.

However, the different techniques often result in disagreement in timing or magnitude of epeirogeny, undermining their utility for tectonic reconstructions. This suggests that a closer look at the various methodologies, approaches, and assumptions seems warranted in order to explore the causes of any differences in tectonic interpretation. For example, the Colorado Plateau has undergone kilometers of uplift since the Cretaceous. The timing of this uplift is the subject of some debate, but bears on the mechanisms of mantle-crust interaction. Various mechanisms have been suggested, but there are few constraints. With a definitive uplift history, it would be possible to place constraints on the mechanisms of uplift and thereby shed light on mantle processes and lithospheric evolution.

Unfortunately, no single measurement technique appears adequate to definitively settle the issue. For example, sedimentary and faunal data are generally lacking from highland areas, some isotopic methods and floristics rely on paleoclimatology, fission tracks and cosmogenic nuclides are sensitive to unroofing rates, and vesicular basalts are only present around the plateau margins. However, by combining all techniques applied to the plateau for Cenozoic time, it may be possible to not only elucidate further details of plateau uplift, but also to explore the underlying assumptions and formulations of each method. This could then lead to refinements not otherwise possible. Furthermore, in some cases, the techniques could be inverted to shed light on other aspects of Rocky Mountain history such as erosion rates and paleoclimatic variability. Thus, differences between paleoelevation assessment techniques can be more revealing than agreements. This suggests a "call to arms" for the paleoelevation community to combine forces for an integrated approach to the tectonic history of the Rocky Mountains and Colorado Plateau.