PALEOELEVATION OF THE NORTH AMERICAN CORDILLERA FROM THE LATE CRETACEOUS TO LATE EOCENE: AN INTEGRATED CLIMATE MODEL-OXYGEN ISOTOPE APPROACH

The elevation history of the western US is intimately tied to, and offers valuable insight for the interpretation of, both the tectonic history and landscape evolution of this region. Oxygen isotope ratios (δ¹⁸O) of ancient precipitation have the potential to provide information regarding the paleoelevation of orogenic regions because of the cooling of rising air masses and the subsequent rainout of moisture and preferential removal of ¹⁸O. The end result is that for an individual air mass moving up a topographic gradient, a pattern of decreasing δ¹⁸O values is observed, and an isotopic lapse rate can be used to infer paleoelevation.

Interpretations of patterns in δ¹⁸O , however, are not straightforward in regions such as the western US where precipitation in any given place may be sourced from a number of different air masses with different source areas, distances of transport and elevation rainout histories. In an effort to overcome these potential problems, we use an atmospheric global climate model with integrated oxygen isotope tracers (isoCAM3) that predicts isotope ratios of precipitation in different regions of the western US based on ‘first principles’. This model is run for a given time period using a range of possible elevations/topographies, and simulated oxygen isotope ratios are compared to those obtained from geologic materials (e.g. fossil bivalves, mammal teeth, authigenic minerals). Quantification of match between model and data-derived δ¹⁸O values, assessment of possible source mixing ratios, and identification of air mass trajectories are then interpreted to identify the best approximation of regional paleotopography.

Application of this integrated climate model-oxygen isotope approach to the study of paleoelevation in the western US for three different time slices is ongoing. During the late Cretaceous (~75 Ma) when the Western Interior Seaway covered much of the eastern part of the region, results indicate that Sevier highlands to the west were comparable in mean elevation to the modern Andes. Preliminary comparisons for the early (~55 Ma) and late (~35 Ma) Eocene suggest that, along the eastern margin of the Cordillera, modern elevations had not yet been obtained.