MAPPING TOPOGRAPHIC RELIEF AND ELEVATION OF THE CENTRAL ROCKY MOUNTAIN REGION DURING THE LATEST EOCENE USING STABLE ISOTOPE DATA FROM MAMMALIAN TOOTH ENAMEL

Geoscientists have debated for several decades whether the Rocky Mountains eroded after the Laramide orogeny (ca. 65 mya) and subsequently underwent a third orogeny or whether they simply remained high throughout the Cenozoic. Workers have used various methods to approach these problems of paleotopography and paleoelevation, most notably leaf-margin analysis of fossil floras, stable isotope analysis of sediments and fossil remains, and geomorphic analysis of sediments. This study relies on stable isotope analyses of fossil remains, particularly of mammalian tooth enamel, but compares isotope data from a broad geographic region in order to make isotopic maps. These maps can provide insight into topographic relief and elevation that are difficult to obtain by studying individual localities alone.

Samples of tooth enamel were taken from five genera of mammals that lived during the latest Eocene (Chadronian LMA). These remains are from seven fossil localities in Colorado, Wyoming, and Nebraska. Samples from eastern Colorado and western Nebraska have oxygen and carbon isotope ratios of ~-6.1 +/- 1.4 per mil and ~-10.1 +/- 0.9 per mil, respectively. At and to the west of the eastern edge of the present-day Front Range, samples from Castle Rock and Florissant have similar oxygen and carbon isotope ratios to those of eastern Colorado and western Nebraska. In contrast to these other localities, samples from the flanks of the present-day Laramie Mountains in Wyoming have oxygen and carbon isotope ratios of ~-9.7 +/- 1.3 per mil and ~-7.7 +/- 1.9 per mil, respectively.

At present, these sites in eastern and central Colorado and western Nebraska range in elevation from 1165 to 2445 meters, and have oxygen isotope ratios for river water ranging from -13 to -18 per mil. Because our Eocene samples have both lower oxygen isotope ratios and a smaller range in values, we interpret them to reflect low relief and low elevation at that time when compared to the present day. In contrast, lower oxygen isotope ratios and higher carbon isotope ratios from sites along the flanks of the Laramie Mountains during the late Eocene are consistent with localized relief in this area of ~1 km. The latter inference is consistent with previous sedimentological studies of the late Eocene-early Oligiocene erosional surface in this region.