2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 17
Presentation Time: 4:26 PM


BOHANNON, Robert, Earth Surface Processes, United States Geol Survey, MS 980, Box 25046, Federal Center, Denver, CO 80225 and HOWARD, Keith, Earth Surface Processes, United States Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025, bbohannon@usgs.gov

Late Miocene, Pliocene, and Quaternary changes in base level that occurred in response to regional incision caused by the Colorado River drainage system have had a major impact on basin evolution and sedimentation patterns in large areas of the arid Southwest. The physiography of much of the Great Basin and Mojave and Sonoran Deserts downstream of the Grand Canyon has changed considerably since the introduction of the Colorado River in the southern Great Basin 5 m.y. ago. The physiographic changes were accompanied by a shift from aggradation to degradation and both surface drainage patterns and subsurface water-storage systems were strongly impacted. This resulted in new basin depositional styles, lower ground-water levels, and altered patterns of spring discharge. Available isotopic evidence indicates that pre-incision, deep-flow systems involved carbonate spring discharges around the edge of Colorado Plateau from karst flow systems. One such system, the Hualapai Limestone lake and spring system, was large and approximately matched the level of a paleowater table. This former water table is identified known from alteration of igneous rocks 0.5 km above the present river near Hoover Dam, which is west across the bedrock ridge of the Black Mountains.

Sites that offer promise for calibrating the timing of this lowering history include (a) dateable high-level, river-carved potholes stranded above Black Canyon, (b) a series of thick fluvial deposits that contain a Pleistocene mammoth site and interfinger with flights of alluvial-fan terraces near Golden Shores, Arizona, and (c) a flight of terraces around the Overton Arm of Lake Mead that contain the Bishop Tuff (~730 Ka). Another more localized system that underwent similar sedimentary and hydrologic effects is the Pahrump area where base level lowered as Death Valley subsided and widened. The base-level changes are likely to have affected ground-water flow paths and rates through basin-fill and through rock aquifers