STABLE ISOTOPE PALEOTOPOGRAPHY OF THE COLORADO PLATEAU/BASIN AND RANGE TRANSITION ZONE

The Colorado Plateau, with a mean elevation of 2.2 km and covering a ~300,000 km² region affects regional circulation patterns of western North America thus acting as a primary factor in determining large-scale climatic conditions. As such, our ability to constrain terrestrial paleoclimate in mid-latitude North America is contingent upon the development of robust paleotopographic records for the region. At present, there is no general consensus in the geologic literature as to when the Colorado Plateau emerged as a significant topographic feature. In an attempt to better constrain the paleotopographic evolution of the Colorado Plateau, we performed a stable isotope based paleotopographic investigation of Eocene through recent sedimentary rocks across the Great Basin/Colorado Plateau transition zone. Authigenic calcite δ¹⁸O values for all samples analyzed range from ~ -6‰ to -20‰ (PDB), whereas intraformational δ¹⁸O variability is on the order of 1‰ to 7‰. We have juxtaposed the isotopic data with measured stratigraphic sections for each formation investigated. Petrographic and trace element analysis were also completed in order to constrain the diagenetic conditions within each formation.

These petrologic and geochemical data provide insight into the topographic, hydrologic and climatic evolution of the region on a variety of spatial and temporal scales. The oxygen isotopic data suggest that both the Colorado Plateau and the Great Basin were topographically elevated from the late Eocene through late Oligocene/early Miocene, with subsequent downdrop of ~1 km since the early Miocene. Our results agree with published structural analyses of Hurricane Fault slip-rates which suggest the topographic step between the Colorado Plateau and the Great Basin was created since the late Miocene. Oxygen and carbon isotope data also suggest that the Eocene Claron formation in the Great Basin evolved from a hydrologically open fluvial system to a closed-basin lacustrine system and back to an open system. The stable isotope based paleotopographic interpretations presented here suggest future paleoclimate models should include the Colorado Plateau as a regional topographic feature since at least the Eocene.