Chester A. Ruleman

Data release of Geologic Map of the Upper Arkansas River Valley Region, North-Central, Colorado

This 1:50,000-scale geologic map represents a compilation of the most recent geologic studies of the upper Arkansas River valley, between Leadville and Salida, Colorado. The valley is structurally controlled by an extensional fault system that forms part of the prominent northern Rio Grande rift, an intra-continental region of crustal extension. This work also incorporates new detailed geologic mapping of poorly understood areas within the map area and reinterprets previously studied areas, aided by lidar data that covers 59 percent of the map area. The mapped region extends into the Proterozoic metamorphic and intrusive rocks in the Sawatch Range west of the valley and the Mosquito Range to the east. Paleozoic rocks are preserved along the crest of the Mosquito Range, but most of them have been eroded from the Sawatch Range. Numerous new isotopic ages (U-Pb zircon ages for the intrusive Proterozoic and some Tertiary rocks adjacent to the valley and 40Ar/39Ar ages for 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 suite of ~1,760-Ma metamorphic rocks and ~1,700-Ma plutonic rocks. As a result of extension during the Neogene and possibly latest Paleogene, the graben underlying the valley is filled with thick basin-fill deposits (Dry Union Formation and older sediments), which occupy two sub-basins, separated by a bedrock high near the small town of Granite. The Dry Union Formation has undergone deep erosion since the late Miocene or early Pliocene. During the Pleistocene, ongoing steam incision by the Arkansas River and its major tributaries has been interrupted by periodic aggradation. From Leadville south to Salida as many as 7 mapped alluvial depositional units, which range in age from early to late Pleistocene, record periodic aggradational events along these streams that are commonly associated with deposition of glacial outwash or bouldery glacial-flood deposits. Many previously unrecognized Neogene and Quaternary faults, some of the latter with possible Holocene displacement, have been identified on lidar imagery. This imagery has also permitted more accurate remapping of glacial, fluvial, and mass-movement deposits and has aided in the determination of their relative ages. Recently published 10Be cosmogenic surface-exposure ages, coupled with new geologic mapping, have revealed the timing and rates of late Pleistocene deglaciation. Glacial dams that impounded the Arkansas River at Clear Creek and possibly at Pine Creek failed at least 3 times during the middle and late Pleistocene, resulting in catastrophic floods and deposition of enormous boulders and bouldery alluvium downstream; at least two failures occurred during the late Pleistocene during the Pinedale glaciation.