CENOZOIC TOPOGRAPHY OF THE WESTERN UNITED STATES

The oxygen and hydrogen isotope composition of authigenic minerals in terrestrial chronosequences can be used to reconstruct past topographic histories of mountain belts. We undertook a study of the western United States in an effort to understand the topographic history of the Rocky Mountains, the Great Basin, and the Sierra Nevada Mountains. Our work concentrated on the oxygen isotope composition of smectite in altered felsic airfall ashes present throughout this region. Cenozoic airfall ashes are ubiquitous throughout the western United States and authigenic smectite occurs in many of these ash deposits.

Our isotopic study shows that topography of the much of this region was established in the early Tertiary. The central and northern portion of the Sierra Nevada Mountains attained its topography prior to 16 Ma. In contrast, the southern Sierra may have undergone a reduction of topography between 14 and 7 Ma. Our study of the eastern Rocky Mountains shows that the present topographic gradient has existed in this region since Eocene times. The unusual climatic regime caused by interaction of three air masses and the steep topography of the eastern Rocky Mountains has been in place since the Eocene.