THE NORTHERN RIO GRANDE RIFT: NEW STUDIES IN THE UPPER ARKANSAS RIVER VALLEY, COLORADO

The upper Arkansas River valley, between Salida and Leadville, Colorado, is controlled by a graben that forms part of the prominent northern Rio Grande Rift. We are currently undertaking a fresh geological study of this extensional feature using detailed mapping of more poorly understood areas and reinterpretation of previously studied areas, aided by LiDAR data covering the entire region. A 1:50,000-scale geologic map of the region nears completion and extends into the Proterozoic metamorphic and intrusive rocks on either side of the valley. The graben is filled with thick basin-fill deposits (Dry Union Formation and older sediments) which occupy two sub-basins, and has undergone deep erosion since the late Tertiary. Along the Arkansas River there are at least 5 major alluvial terraces. Numerous new isotopic ages (U-Pb zircon ages in the intrusive Proterozoic and Tertiary rocks adjacent to the valley and ⁴⁰Ar/³⁹Ar ages in the Late Cretaceous to Oligocene intrusive and extrusive rocks) better constrain the timing of both Proterozoic and Late Cretaceous to early Tertiary intrusive events. The U-Pb ages document widespread ~1,440 Ma granitic plutonism north of Buena Vista that produced batholiths that intruded an older bimodal suite of ~1,760-Ma metamorphic rocks and ~1,700 metaplutonic rocks. New detailed mapping, coupled with ¹⁰Be cosmogenic surface-exposure ages, have revealed the timing and rates of late Pleistocene deglaciation. Many previously unrecognized Neogene and Quaternary faults, some of which possibly have Holocene displacement, have been identified. LiDAR data have also permitted more accurate mapping of glacial, fluvial, and mass-movement deposits and aid in the determination of their relative ages. Glacial dams that impounded the Arkansas River at Clear Creek and Pine Creek failed at least 3 times during the Pleistocene, resulting in catastrophic floods and deposition of enormous boulders downstream; at least two failures occurred during the late Pleistocene Pinedale glaciation.