An Overview of the Hydrogeology
This section describes the ground water in the greater Mojave area and in the Preserve, reviewing the ground-water recharge processes, directions of ground-water flows, and characteristics of aquifers.
The geologic formations in the Preserve generally fall into two main groups: the consolidated rocks of the mountains and hills, and the unconsolidated deposits in the valleys. The mountains consists of the older rocks, essentially impermeable igneous and metamorphic rocks of the pre-Tertiary age. These hard rocks of the mountain areas separate the ground water in one basin from another. Volcanic rocks of Tertiary age also appear, and have slightly more permeability, but still yield only a little water, generally of poor quality.
The valleys’ unconsolidated deposits of Pleistocene age consist of gravel, sand, silt, and clay. These deposits are the principal ground-water reservoirs (CDWR, 1967a). (Note: ground water is defined as subsurface water in a zone of saturation, where water fills the pore spaces). Older fan deposits of Pleistocene age occur as isolated remnants which dip away from the mountains, which can store and yield ground water for wells (Dyer et al, 1963).
Mountain rains and snows recharge the upper levels of the ground-water aquifers (Thompson, 1929; Geoscience, 1995; Freiwald, 1984). The mountain springs and seeps appear along geologic fractures or fissures, and the springs’ flows fluctuate greatly, depending on how much precipitation has occurred (Hall, 1981 and rancher interviews, 1998). The springs occur mostly along in the main mountain range, where precipitation is highest. Section 3.2 provides details on springs.
The main source of ground-water recharge is the runoff from the higher mountain ranges. Ground water is recharged when streams from the mountains --from storms or melting snow-- flow across permeable alluvial fans. The aquifers are then recharged by the water, which percolates through the alluvium. A significant amount of the water in the area of the Preserve is lost to evaporation and transpiration. This alluvium is referred to by some authors as the alluvial aquifer, and is an important ground water reservoir. In wetter years, surface runoff therefore plays an important role in the recharge of aquifers in the Preserve (Thompson, 1929; Geoscience, 1995; Freiwald, 1984). Many shallow wells are found along these alluvial areas.
11 Larger valleys, for example the greater Fenner and Lanfair Basins, below the Providence and New York Mountains, contain hundreds of vertical feet of sediment washed in over millions of years by runoff, and these unconsolidated materials constitute the deeper aquifers (CDWR, 1993). Few drillers’ logs show the total thickness of these unconsolidated deposits; however, in places the deposits run deep. For example, the deposits in Lanfair Valley are over 500 feet thick, and in parts of Fenner Valley, about 10 miles south of the Preserve to Cadiz, over 1,100 feet (Freiwald, 1984). Information on the gradient of the ground-water is apparently limited to a few, scattered ground-water contours on the maps of Freiwald (1984). He shows a high mountain valley in the Providence Mountains (Gold Valley) with a 500 ft/mile drop in ground-water contours. A location near Fenner at the foothills of the Hackberry Mountains shows about 135 ft/mile gradient. A few map contours in the sloping reaches of the broader Fenner Valley, from Fenner to Cadiz (all outside the Preserve) shows a 16 to 36 ft/mile drop in the ground-water contour over a 20 mile reach.
The USGS has drilled over 2,000 feet in the Lanfair Basin (Freiwald, 1984), however, the water quality was very poor. Geoscience Inc (1995), which conducted the Cadiz study discussed in Section 4.2, believes the deposits to be perhaps a mile deep near Bristol Dry Lake, 12-15 miles south of the Preserve. Some deeper wells extract water from these valley areas.
Terminology for the aquifers varies. Some geologists refer to “upper and lower aquifers” or “younger and older aquifers” respectively --with a layer of less permeable materials separating the two (Moyle, 1972; Geoscience, 1995). The younger, upper aquifer is unconsolidated and highly permeable, supporting springs and shallow wells. The lower layer, of older Pleistocene age and sedimentary rocks of Tertiary age, store and yield water to deeper wells. Some geologists refer to a deeper “regional aquifer,” over broader areas, composed of Pleistocene and Tertiary materials. The “alluvial aquifer” near the Mojave River, consists of the relatively shallow deposits within about a mile of the river (Stamos and Predmore, 1992; Izbicki et al, 1995; Densmore and Londquist, 1997; Mendez and Christensen, 1997). Research has shown that considerable ground-water recharge occurs in the alluvium along the Mojave River. During years of big floods, the alluvium is recharged, yielding water to wells for months afterward (Buono and Lang, 1980; Hardt, 1969, 1971). Recharge concepts are important from a viewpoint of ground-water management, in order to understand when water is sustainably used, versus being mined. Geochemical studies can help identify whether water comes from older geologic deposits or from more recently recharged aquifers. Isotopic studies of ground water in the Mojave River area, west of the Preserve, indicates that the alluvial aquifer has younger water, decades old or less. However, the deeper regional aquifer there contains some water recharged 20,000 years or more ago, when the climate was colder and wetter (Izbicki et al, 1995). The USGS notes that more research is needed to determine the age of the climatic periods that recharged the ground water found in some wells and springs (Gleason et al, 1992).
Deeper layers were recharged in wetter climatic periods, but it is not clear just how much recharge of the deeper aquifers in the Preserve occurs at this time --since research information is lacking. Also, in some areas spreading (artificial recharge) may not work well to recharge the deeper aquifers, when an impermeable stratum (e.g., caliche) sits above the deeper aquifer (Izbicki et al, 1995).
4.1. MINING ISSUES RELATED TO WATER RESOURCES
Historically, mining has been a major land use in the Mojave Desert. Mineral resources within the Preserve include gold, silver, iron, cinders, limestone, and industrial rare earths (lanthanide elements), and a number of these minerals have been extracted commercially (Mojave NP, 1998).
Two larger mines, the Molycorp Inc operation at Mountain Pass, and the Viceroy Gold Corporation’s mines at Castle Mountain, are dominant in the area (Figures 4.1.a and 4.1.b). These major mining areas and some other nearby mines were excluded from the Preserve when drawing the boundary. Some smaller operations, such as the Morning Star Mine, lie within the Preserve boundary. Recently closed mines, such as the Colosseum Mines in the Preserve’s Clark Mountain area, still have the potential to affect ground water and other aspects of the environment, so the concern with closed mines is mainly for rehabilitation of the land and longer-term monitoring. Some long inactive mines, for example the Vulcan Mine worked in the 1940s, left large open pits and other scars behind, and restoration of the landscape could be a long-term objective of the Preserve for some of the older mines.
Both active and closed mining operations in and near the Preserve have the potential to harm the water resource and other aspects of the environment. Once pollution from mining enters an aquifer, the contamination can persist for decades or centuries, and can extend down valley over many miles (for example, mining from the 1800s still contaminates streams over great distances in some areas in the Appalachian and Rocky Mountains).
I n simplest terms, water resource impacts from mining in the Preserve area can cause:
• ground-water table drawdown;
• erosion and sedimentation problems on hillslopes and in arroyos; and
• ground-water contamination.
Eventually these impacts can reduce the flow of springs, contaminate potable wells and springs, lower ground-water tables in wells, and eventually pollute deeper aquifers, with pollution plumes extending for many miles. The Preserve managers are logically concerned about the potential effects of the mining. The following sections provide a brief overview of mining activities and discuss some of the related monitoring and water resource assessments now underway.
A number of other small mines or abandoned mines exists in the Preserve, and the Preserve Geologist is aware of their location and characteristics. Some of these other mines may be interesting from a water perspective. The Colosseum Mine, in the Clark Mountain area, was a gold mining operation that completed its work in the early 90s. Site reclamation work is basically completed, although a huge open pit contains about a 3 acre pool. Some old solution ponds remain (Figure 4.1.d) and the heap leach pads have been reshaped.
Some monitoring continues at the Colosseum Mine, in conjunction with the State’s Lahontan Water Quality Control Board. BLM indicates that the Colosseum operation has been basically satisfactory, and is a relatively lower level of concern from an environmental perspective.
A number of abandoned mines have old leach pads, dumps, or other features which could continue to seep, or need some reclamation work. The Preserve Natural Resource Specialist will want to continue to work with the Geologist to identify any sites of concern from a water perspective and recommend some water sampling at certain sites. Project Statement No. 6 proposes a reclamation survey.
Source Water Resources Division http://www.nature.nps.gov/water/completedwrsr.cfm
Playa lakes are among the flattest landforms in the world. They form under arid conditions in interior basins with no outlet to the sea. During wetter climatic conditions, the playa lake basins of the Basin-and-Range province were filled with perennial lakes.
Soda Lake lies at the terminus of the Mojave River. In wet years, the playa may contain standing water and flooding has occurred near the highway at Baker. In drier times, water lies very near the surface in parts of the playa. Capillary action draws the water upward where it evaporates, leaving a puffy “efflorescent” crust of evaporite minerals such as sodium carbonate and sodium bicarbonate.
At least twice, a long-lived lake (Lake Mojave I and II) existed in the region of Soda and Silver Lake playas, from about 18,000 to 16,000 years ago and from 13,700 to 11,400 years ago. The second lake developed when Afton Canyon was incised and a large lake near Barstow (Lake Manix) was drained.
Lake Mojave dried out by about 8,700 years ago. Although short-lived, lakes have periodically existed in the basin. Desiccated lakes can be major sources of eolian sediments, as the man-made desiccation of Owen Lake has shown recently. Thus, the complex history of Soda Lake is interwoven with the history of the dune fields at its southern margin.
Limestone: Seas, Minerals, and Caves
Limestone and dolomite testify to the presence of shallow seas in the Mojave region during the Paleozoic. Many forms of sea life make skeletons of calcium carbonate and calcium magnesium carbonate. Their shells accumulate on the seafloor, as does limey mud from the eroded remains of carbonate producers. When cemented, this material is called limestone, or if it has a high magnesium content and slightly different structure, dolomite. Limestone has played a very important role in the history of the Mojave because it is a host rock for many of the metallic mineral deposits in the area. When the region was intruded by magmas, hot fluids carrying dissolved metals flowed through the relatively soluble and permeable limestone, eventually precipitating out ore-grade material. The same properties of permeability and reactivity led to the dissolution of limestone along fracture and bedding surfaces and the creation of Mitchell Caverns.
Kelso Dunes National Natural Landmark
The Kelso Dunes are a magnificent dune complex with some of the highest dunes (160 m) in the region. They lie at the eastern end of a southeast-trending wind corridor where the corridor abuts the Granite Mountains and intersects the southwest-trending Kelso Wash corridor. Wind transported most of the sand at Kelso Dunes all the way from the Mojave River sink just east of Afton Canyon. Thus, Kelso Dunes represents only a small part of a much larger sand transport system which includes the area called the Devil's Playground.
Quartz (silicon dioxide) forms 50-80% of the sand grains in the dunes, but feldspar forms up to 50% of the sand as well. These minerals were probably eroded from granitic rocks in the San Bernadino Mountains and transported eastward by the Mojave River. Black grains of the heavy iron oxide mineral called magnetite often accumulate on ridge crests. As the name indicates, magnetite can be collected by trailing a magnet through the sand.
Wind blowing about 15 miles an hour is capable of transporting sand particles. They travel near the surface by bouncing or rolling along the surface by traction. Finer clay and silt-sized particles are winnowed away and may be carried high into the atmosphere. Larger gravel-sized particles are too heavy to be transported by normal wind systems. Thus, the sand in eolian fields does not show a large variation in grain size. This size-sorting is one way a geologist can recognize ancient eolian systems in buried strata.
Dunes are asymmetrical in cross section. Wind carries sand up the gently sloping side of the dune. It then avalanches down the steep lee side of the dune. Thus, the main wind direction responsible for forming dunes and surface ripples is easily determined. Surface ripples formed by the most recent winds may be in a completely different orientation from larger dunes formed by prevailing winds.
New sand is not arriving at Kelso Dunes today. In fact, active transport of sand is occurring only in the areas around the Mojave River sink and the western part of Devil's Playground. Further evidence for more active eolian activity in the past is provided by the presence of vegetation-stabilized dunes. The dunes exhibit complex patterns of shape, size, orientation, and spacing indicating that they formed under different wind regimes and that several generations of dunes are stacked or shingled. New dating methods show that the sand system developed in five main pulses over the last 25,000 years, partly over-lapping in time with lake highstands and the youngest volcanism in the region.
Granite Mountains – A discussion of surfaces
Some of the more striking rock formations in the Mojave National Preserve lie in the Granite Mountains. These granitic rocks have eroded into unusual rounded shapes that include spires, perched boulders, and curved cliff faces. The rocks are composed of interlocking mineral grains, which disaggregate into individual grains as the rock surface weathers. This characteristic, along with the massive, non-layered nature of granitoids and the lack of closely spaced fractures in these outcrops, resulted in the curved weathering surfaces in the mountains. Granitic rocks represent the roots of ancient continental-margin volcanic systems. Most of the granitic rock in the Mojave Desert is late Mesozoic in age (80 to 180 million years old). The Mojave National Preserve lies within a belt of late Mesozoic granites that parallels the western continental margin from Mexico to Canada. The granites formed at depth within a volcanically active mountain range comparable in geologic setting to the Andes Mountains chain in South America. The granitoids formed by the slow cooling and solidification of molten magma bodies that developed above sinking slabs of oceanic crust overridden by the edge of the continent.
At least 55 or 60 million years elapsed between the crystallization of the last Mesozoic magma bodies and deposition of the youngest-preserved overlying strata. The Mojave National Preserve probably formed a highlands during much of this period and erosion gradually stripped off Paleozoic and Mesozoic sedimentary rocks overlying the granites. In addition, faults may have helped to uplift rocks and make them more susceptible to erosion. For some reason the Granite Mountains did not erode as rapidly as surrounding granitic rocks. The western and southern range front rises abruptly from a beveled off bedrock surface called a pediment. The Granite Mountains pediment was already reduced to a low relief surface before deposition of the 20 million-year-old volcanic rocks that cap the Van Winkle Mountains to the east. Pediments of similar age developed on granitic and metamorphic rock surfaces over large areas of the Mojave National Preserve. Part of the process of pediment formation may have involved mountain retreat, weathering, burial of the pediment surface, and subsequent erosion of Piedmont. Many of the modern planar surfaces of the Mojave National Preserve are inherited from these older surfaces. The concave surface of Cima Dome to the north has been cited as a classic example of coalesced pediments, an end-stage of desert landform development.
About 17.8 million years ago, a powerful eruption blasted outward from a volcanic center in the Woods Mountains in the Eastern Mojave. Propelled by the force of rapidly rising and expanding superheated gases, a ground-hugging cloud of ash and rock fragments spread out at near-supersonic speed across the countryside. Hot, suffocating ash buried shallow lakes and stands of trees. The remains of birds, mammals, and plants are preserved as fossils in the sediments below the ash layer. The May 18, 1980 lateral blast from Mt. Saint Helens was somewhat analogous. The deposits from three closely spaced, violent eruptions comprise the rock unit called the Wild Horse Mesa Tuff which forms the cliffs of Hole-in-the-Wall.
A series of events led up to the volcanic explosions that so radically altered the landscape. Following millions of years of relative quiescence on the land surface, the continental crust beneath the Mojave began to extend in response to geologic process farther west at the continent's edge and deep beneath the Mojave. This extension, which was regional in scope, marked the beginning of basin-and-range development, associated faulting, and renewed volcanism. The oldest volcanic rocks preserved in this area, the Peach Springs Tuff, erupted 18.5 million years ago from a volcano near the southern tip of Nevada. The airfall deposit settled on pediment surfaces and fluvial deposits. The Peach Springs Tuff event approximately coincided with the beginning of basin formation in the region, which may explain why it is overlain in several places by shallow lake deposits. Local volcanism in the Woods Mountains area began soon thereafter with volcanic strata deposited in the Woods and Hackberry Mountains. About 17.8 million years ago, viscous siliceous magma approached the surface of the volcano. A plume of ash was spewed high into the atmosphere. Then the volcano exploded with devastating force. Abundant angular fragments of volcanic rocks in the Wild Horse Mesa Tuff at Hole-in-the-Wall must have been transported by a powerful blast. Nearer to the volcanic center, the erupted rocks contain boulder-sized fragments of basement rock ripped off by rising magma and hurtled out from the blast site. Two similar explosive cycles followed within less than 100,000 years. The resulting deposits formed a flat plateau extending from the Pinto Mountains to Blind Hills and from Wildhorse Mesa to the Hackberry Mountains. Such a large volume of Wild Horse Mesa Tuff was ejected from the volcano's magma chamber that overlying strata collapsed downward, forming a cone-shaped depression called a caldera. The Woods Mountains caldera, the most well preserved caldera in the Mojave, was 10 km wide and 4 km deep. It was largely in-filled with collapsed tuff and younger light-colored (rhyolite) flows.
Fluvial processes have dissected the once continuous plateau of volcanic rocks, leaving mesas capped by flat-lying tuffs and flows. Erosion and weathering also created the nooks, crannies, and narrow winding canyons of Hole-in-the-Wall. The cavernous weathering surfaces of the tuff were created by local variations in resistance to erosion resulting from minor differences in welding and crystallization during cooling, alteration, and early stage devitrification of glassy material.
The volcanic units in this area have been well dated using the K-Ar or 40 Ar/39 Ar methods (Nielsen and others, 1990; McCurry and others, 1997). By measuring progressive changes in the tilt (dip) of the flat layers, geologists have used these dates to calculate the rate of local tilting during the main interval of regional basin formation. They have also used them to integrate the geologic time scale with fossil zones because the lake deposits sandwiched between the Peach Spring and Wild Horse Mesa Tuffs contain scientifically important vertebrate fossils.
Cinder Cones National Natural Landmark
Looking south from Interstate Highway 15 between Baker and Halloran Summit, the skyline is interrupted by the conical outlines of volcanic cinder cones and flat lava flows surrounding the cones. This area, part of a 150 square kilometer field, has been designated a National Natural Landmark for its remarkably well preserved and abundant cinder cones.
Volcanic eruptions in the Cima field first began about 7.6 million years ago and continued until at least as recently as 10,000 years ago (based on the K-Ar dating method), near the end of the most recent ice age. The field is characterized by basalt, which is a black to dark gray volcanic rock formed from lava rich in magnesium and iron. The lava erupted onto a relatively smooth erosional surface, a pediment, developed on much older granitoids and metamorphic rocks. Rather than erupting violently, the basaltic lava fountained or flowed from multiple point sources. Each of the 40 cinder cones in the volcanic field represents one or more sites from which lava erupted.
Cinder cones form when lava erupts as liquid fountains. The force propelling the lava into the air comes from rapidly rising hot gases (mostly carbon dioxide and steam). Uncapping a gently shaken carbonated drink creates an analogous small-scale eruption. As droplets of lava are spewed through the air, they "freeze-dry", creating instant rock fragments that fall to the ground and accumulate. Larger fragments may preserve bubble-spaces created by the escaping gases. If an eruption of this type continues long enough from a single site or closely spaced sites, an accumulation of fragments, a cinder cone, develops. A cinder cone's steeply sloping sides (about 30 degrees) reflect the angle-of-repose of the loose pyroclastic fragments which avalanche downhill when the slopes become over-steepened with new material. Generally, the eruptive vent site lies beneath the central crater within the cone.
A cinder cone can develop rapidly. In a period of nine years, the nearly continuously erupting Paricutin cinder cone in Mexico built a cone 410 meters high and one kilometer wide. In contrast, the cinder cones in the Cima volcanic field range from 50 to 155 meters high and 400 to 915 meters wide. Some are remarkably well-preserved while others are deeply eroded. Many show a history of several distinct cone-building events. The internal "plumbing system" of some cones is preserved at some localities in the form of dikes, plugs and solidified lava tubes.
Several of the Cima cinder cones have been mined for cinder block and garden gravel. At the Aikens Mine, the interior structure of a cinder cone has been exposed by mining operations. The volcanic fragments vary in size and include boulder-sized volcanic bombs. Exotic rock fragments (xenoliths) from deeply underlying mantle and crust have been found in some cones and flows, and may occur here. They were ripped off and carried to the surface by the force of the erupting magma.
The end stage in the life cycle of a cinder cone is typically marked by the outpouring of lava. The lava may breach the cone and flow across the landscape. Overlapping flows eventually create a field of basalt. The relations between cinder cones and flows indicate that individual flows traveled several kilometers downhill to the south and west. The farther-traveled younger flows are typically 2-4 m thick with low surface relief. These flows may have ropy (pahoehoe) or blocky (aa) surfaces.
Kelbaker Road passes by the western edge of one of the youngest lava flows. At this locality, a cross-section of the flow and the land surface across which it flowed is well exposed in a wash turnout just north of Mojave Road. The lava flowed across light-colored, uncemented gravels, sands, and clays and then across irregular surfaces of resistant, dark brown Precambrian metamorphic rocks that are tilted at a steep angle.
The internal structure of the flow itself is not uniform. Boiling gases bubbling out of the base and top of the flow left a wake of ovoid holes (vesicles) along the flow margins. The vesicles formed when lava solidified around trapped gas pockets. Fractures in the flow surface resulted from cooling-related contraction. The rapid solidification of lava preserved not only trapped bubbles, but also a record of the earth's ancient magnetic field. The two small drill holes at this site represent samples taken for studies of the rock's magnetic record, which was used to help verify the age of the flow.
The General park map handed out at the visitor center is available on the park's map webpage.For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.
A geology photo album has not been prepared for this park.For information on other photo collections featuring National Park geology, please see the Image Sources page.
Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.
Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.
For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.
The NPS maintains a searchable data base of research needs that have been identified by parks.
A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.
NPS Geology and Soils PartnersAssociation of American State Geologists
Geological Society of America
Natural Resource Conservation Service - Soils
U.S. Geological Survey
General information about the park's education and intrepretive programs is available on the park's education webpage.For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.