GMEG - Geology and Geophysics
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TASK 2 - Geologic framework of the Lake Mead National Recreation Area and surrounding areasTask leader: L. Sue Beard
Statement of Problem: The Lake Mead National Recreation Area lies within the eastern Great Basin, where three of America's four desert ecosystems--the Mojave, the Great Basin, and the Sonoran Deserts—meet. It includes two major reservoirs on the Lower Colorado River, Lakes Mead and Mohave, that provide recreation for millions of visitors each year. In addition, the NRA faces increasing public pressure from rapid urban growth both in the Las Vegas area at its northern end, Kingman and Mohave County to the east, and Laughlin/Bullhead City at its southern end. The geology of the NRA is truly spectacular, displaying Proterozoic basement overlain by Paleozoic, Mesozoic, and Cenozoic strata in very scenic landscapes. These landscapes were formed by Mesozoic thrusting and uplift, followed by Tertiary extension that broke the area into multiple tilted fault blocks. The Colorado River has carved a series of rugged canyons through the landscape since about 5 Ma. Hoover Dam, which separates Lake Mead to the north from Lake Mohave, also marks a strong contrast in the geology of the area. Unlike the Lake Mead region, which includes colorful stratified Paleozoic and Mesozoic strata, the area to the south contains complexly extended Tertiary intrusive, volcanic, and sedimentary strata, directly overlying Proterozoic basement terrane.
|Geologic mapping for that part of Lake Mead NRA north of Hoover Dam was completed under the Las Vegas Urban
Corridor project. This task focuses on the part of the NRA south of Hoover Dam. Bedrock mapping under this task will
complement and extend local detailed studies by J. Faulds and others (Nevada Bureau of Mines and Geology) that have
elucidated the complex relations between volcanism and extension. The geologic mapping provides baseline geologic
information for some of the pressing issues facing the NPS in its management of the recreation area. These include
habitats of rare and threatened species and their possible dependence on soils and geology, inventory of unique or
sensitive geologic features, abandoned mines, vulnerability of groundwater resources and effect of groundwater
withdrawal on thermal springs.
Boulder Dam as seen from downstream in Black Canyon
Objectives: The objective of this project is to provide geologic framework studies in support of both resource management issues and outreach efforts of the NPS. The lower Lake Mead NRA includes the Colorado River corridor and adjoining uplands. The NPS has requested geologic mapping by the USGS of this stretch of the recreation area, as well as the valley areas to the east and west. The USGS, Nevada Bureau of Mines, and Arizona Geological Survey have been coordinating mapping efforts and have several ongoing joint mapping efforts on 7.5-minute quadrangles. The intent of this task is to complete the mapping for the Lake Mead NRA, which includes the west half of two 1:100,000-scale quadrangles, Boulder City and Davis Dam. Time and money permitting, the task will then extend the mapping eastward to complete the two 100k quadrangles. We will also provide the NPS a geologic map of the entire Lake Mead National Recreation Area at 1:100,000-scale.
In addition to the 1:100,000 scale maps, the NPS is specifically interested in more detailed mapping (1:24,000) of four to six quads in the northern part of the Boulder City quadrangle. Most of these quads have some level of pre-existing mapping, and so will involve both compilation and new field mapping. These quads cover areas of the NRA where the NPS is concerned that proposed groundwater pumping in nearby valleys outside of the park will adversely affect springs and seeps.
The NPS and USGS (Water Discipline, Nevada District, and Geology Discipline) were funded to study unique thermal springs in Black Canyon and their geohydrology by the Southern Nevada Public Land Management Act (SNPLMA). Our portion is about $140,000 over three years (see PLMA: Data Gaps - Source water (NV-010; project no. 9705CYG; the study began the spring of 2008. The maps, kinematic studies and accompanying cross sections will provide geologic framework for the spring studies. Mapping products are at two scales: a regional map at 1:250,000-scale (in review) that will be used to construct the cross-sections, and detailed maps and structural studies around the springs. In addition, USGS Water Discipline in Arizona began a high-profile study of alluvial basins in NW Arizona in 2006 (NW Arizona Alluvial Basins Study); in 2007 they requested geologic expertise and a regional geologic map at 1:250,000 for use in their studies; we received about $25k (used to purchase aerial photography and support a student) midway in FY08 (Margot Truini, Water Discipline, project chief)and ~$40K in FY09. The map extends the regional map described above southward and includes the Boulder City and Davis Dam 30' x 60' quadrangles and surrounding areas. The 1:250,000- scale map is a derivative of the 1:100,000-scale mapping objective of this task.
Outreach—Lake Mead NRA includes some of the best exposed and most spectacular geology of the southwest. It rivals
Death Valley in terms of geologic complexity and is a popular field trip for academic groups. The USGS has been providing geologic expertise to the NPS interpretation staff through field trips with NPS Interpretative and Resource staff, and geologic information for various issues. Task 7 was designed to take that geologic expertise and put it into useable format for both the NPS interpretation staff and for the public.
|Highlights and Key Findings: Ongoing mapping has revealed previously unknown outcrops and geologic relations pertinent to our understanding of the
young tectonic history of the area and the ancestral Colorado River system. Young faulting along the east side of Wilson
Ridge has cut and tilted thick deposits of river gravels and probably had a profound influence on modifying the course of
the ancestral Colorado River. Further south in Detrital Valley, newly discovered ancient and active spring mounds that are
fault controlled provide baseline information on changing groundwater levels. We have discovered that young volcanic
flows originally interpreted to be faulted have probably intruded up along some of the faults. These flows erupted along the flanks of the Black Mountains and flowed down toward the ancestral Colorado River; they now lie at varying levels above
the modern river gradient. Dating of these flows indicates that sediment derived from the ranges was aggrading until at
least 4.8 Ma, suggesting a large aggradational pulse that has been documented for the Colorado River from about 3.5 to 4
Ma may actually have started before 4.8 Ma. We have also made significant progress in identifying mapable units in the
older volcanic rocks in the Boulder City quadrangle, and in deciphering their complex syn-tectonic volcanic history.
Lake Mohave along the Arizona-Nevada border below Black Canyon of the Colorado River. Quaternary and older river terrace and alluvial fan deposits crop out on the left (Nevada). Late Tertiary volcanics and sedimentary rocks crop out on the Arizona side in the Black Canyon area.
Our kinematic study of faults related to hot and cold springs below Hoover Dam suggest a complex structural and lithologic control to the springs. NNW striking near vertical faults and moderately tilted volcanic contacts provide pathways for fluids. The puzzle of immediately adjacent springs with hot and cold waters is now known to be a result of differential near surface cooling of waters from the same water source. Recent mapping indicates that some of the mapped faults are intruded by dacite dikes and plugs, further complicating the flow path analysis. Preliminary results from the study indicate that there is a significant component of Lake Mead water influencing the spring chemistry, most likely due to the hydrostatic pressure of the lake pushing water down through the major faults and fractures in the vicinity of the dam. However, because the hot springs existed prior to the dam, a separate source of water is needed, most likely local groundwater that is circulating to some depth along major north-south faults.
The collaborative study with the Arizona Water Science Center on the geohydrology of three large alluvial valleys in NW Arizona has resulted in a 1:250,000-scale geologic map (in review) of a very large area that had no mapping more detailed than 1:1,000,000-scale. The geologic map, depth-to-bedrock geophysical models, and interpretation of about 40 TEM geophysical lines across the three basins have been used for 3D interpretation. The results provide significantly enhanced understanding of the structural framework and alluvial/bedrock architecture of the three basins.
Task Products and Publications
Beard, L. S., Anderson, R. E., Block, D. L., Bohannon, R. G., Brady, R. J., Castor, S. B., Duebendorfer, E. M., Faulds, J. E., Felger, T. J., Howard, K. A., Kuntz, M. A., and Williams, V. S., 2007, Preliminary geologic map of the Lake Mead 30' X 60' quadrangle, Clark County, Nevada, and Mohave County, Arizona: U.S. Geological Survey Open-File Report 2007-1010, 109 p., 3 plates, scale 1:100,000. http://pubs.usgs.gov/of/2007/1010/.
Presentations, Delivered: Beard, L. S., Umhoefer, P. J., Martin, K. L., and Blythe, N., 2007, From detachment to transtensional faulting: A model for the Lake Mead extensional domain based on new ages and correlation of sub basins: EOS Trans. AGU, v. 88 (52) Fall Meet. Suppl.
Presentations, Delivered: Howard, Keith A., Kuntz, Mel A., Beard, L. S., Sarna-Wojcicki, Andrei M., Kunk, Michael J., Lucchitta, Ivo, and Perkins, Michael E., 2008, Late Miocene erosion of tilted fault blocks in the Gold Butte area and deposition in half-graben basins, Basin and Range Province: Geological Society of America combined Rocky Mountain and Cordilleran Meeting, Las Vegas Nevada, v. 40, no. 1, p. 84.
Posters, Delivered: Paces, J. and Beard, L. S., 2008, Hydrogeology of perennial hot and cold springs in Black Canyon below Hoover Dam, Nevada and Arizona Abstracts with Programs - Geological Society of America, v. 40, no. 1, pp.80.
Posters, Delivered: Beard, L. S., and Campagna, D. J., 2008, Kinematics of the eastern Lake Mead fault system, Nevada and Arizona: Geological Society of America combined Rocky Mountain and Cordilleran Meeting, Las Vegas Nevada, v. 40, no. 1, p. 84.
Posters, Delivered: Lustet, L., Wilson, J.. W., Beard, S., and Paces, J. B., 2009, Hydrogeology of selected springs in Black Canyon, Nevada and Arizona: Nevada Water Resources Association, 2009 Conference, Reno, Nevada; p. 14.
Presentations, Delivered: Paces, J. and Beard, S., 2008, Hydrogeology of perennial hot and cold springs in Black Canyon below Hoover Dam, Nevada and Arizona: Geological Society of America combined Rocky Mountain and Cordilleran Meeting, Las Vegas Nevada, v. 40, no. 1, vol. 40, p. 80.
Presentations, Delivered: Umhoefer, P. J., Duebendorfer, E. M., Blythe, N., Swaney, Z., Beard, L. S., and McIntosh, W., 2008, Development of Gregg Basin and the southwestern Grand Wash Trough during late stage faulting in eastern Lake Mead: Geological Society of America combined Rocky Mountain and Cordilleran Meeting, Las Vegas Nevada, v. 40, no. 1 p. 85.
Beard, L. S. and Campagna, D. J. , 2009, Geologic Map of the Devils Throat 7.5-minute Quadrangle, Nevada Bureau of Mines (in press).
Anderson, R. E. and Beard, L. S., 2010, Geology of the Lake Mead region, an overview: in Umhoefer, Lamb, and Beard,(editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463, p. 1-28.
Beard, L. S., and Campagna, D. J., 2010, Geometry and Kinematics of the eastern Lake Mead fault system in the Virgin Mountains, Nevada and Arizona; in Umhoefer, Lamb, and Beard, (editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463, 243-274.
Howard, K., Kuntz, M., Beard, L. S., Sarna-Wojcicki, A.M., and Kunk, M., 2010, Erosion of tilted fault blocks and deposition of coarse sediments in half-graben basins during late stages of extension: Late Miocene evolution of the Gold Butte area, Basin and Range province: in Umhoefer, Lamb, and Beard, (editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463, p. 147-170.
Langenheim, V., Beard, L. S., and Faulds, J. E, 2010, Implications of geophysical analysis on basin geometry and fault offsets in the northern Colorado River extensional corridor and adjoining Lake Mead region, Nevada and Arizona: in Umhoefer, Lamb, and Beard, (editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463, p. 39-60.
Umhoefer, P. J., Beard, L. S., Martin, K. L., and Blythe, N., 2010, From detachment to transtensional faulting: A model for the Lake Mead extensional domain based on new ages and correlation of sub basins: in Umhoefer, Lamb, and Beard, (editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463, p. 371-394.
Umhoefer, P. J., Duebendorfer, E. M, Blythe, N., Swaney, Z., Beard, L. S., and McIntosh, W., 2010, Development of Gregg basin and the western side of the southern Grand Wash trough during late stage faulting in eastern Lake Mead: A relay ramp in a normal fault system in the hanging wall of the Grand Wash fault: in Umhoefer, Beard, and Lamb, (editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463.
Felger, T., and Beard, L. S., 2010, Regional Geologic Map of Lake Mead and surrounding Regions, Southern Nevada, Southwestern Utah, and Northwestern Arizona: in in Umhoefer, Lamb, and Beard, (editors): Miocene Tectonics of the Lake Mead Region: GSA Special Paper 463, p. 29-38.
Map, Planned: Amoroso, L., and others, 2011, Surficial Geologic Map of the Boulder City 30'x60' quadrangle, USGS, Open-File Report.
Map, Planned: Beard, L. S., Felger, T., and Fleck, W., 2011, Geologic Map of the Ringbolt Rapid 7.5-minute Quadrangle, USGS, Open-File Report.
Map, Planned: Howard, K., and others, 2011, 1:50,000 Geologic Map of Garnet Mountain NW, Senator Mountain NE, and Senator Mountain NW 7.5-minute Quadrangles, USGS, Open-File Report.
Map, Planned: Beard, L. S. and others, 2011, Geologic map of the west half of the Boulder City 30' x 60' Quadrangle, USGS, Open-File Report.
Report, Planned: Beard L. S. and others, 2012, Geologic Map of the Lake Mead National Recreation Area, USGS, Scientific Investigations Map.