INTRODUCTION
The Colorado Plateaus aquifers underlie an area of approximately 110,000 square miles in western Colorado, northwestern New Mexico, northeastern Arizona, and eastern Utah (fig. 107). This area is approximately coincident with the Colorado Plateaus Physiographic Province. The distribution of aquifers in the Colorado Plateaus is controlled in part by the structural deformation and erosion that has occurred since deposition of the sediments that compose the aquifers. The principal aquifers in younger rocks are present only in basins such as the Uinta, Piceance, and San Juan Basins (fig. 108). In uplifted areas, such as the Monument and Defiance Uplifts and the Coconino Plateau, younger rocks have been eroded away, and aquifers are present in older rocks that underlie more extensive parts of the Colorado Plateaus area. Although the quantity and chemical quality of water in the Colorado Plateaus aquifers are extremely variable, much of the land in this sparsely populated region is underlain by rocks that contain aquifers capable of yielding usable quantities of water of a quality suitable for most agricultural or domestic use.
In general, the aquifers in the Colorado Plateaus area are composed of permeable, moderately to well-consolidated sedimentary rocks. These rocks range in age from Permian to Tertiary and vary greatly in thickness, lithology, and hydraulic characteristics. The stratigraphic relations of the rocks are complicated in places, and the stratigraphic nomenclature consequently is diverse. Many water-yielding units have been identified in these rocks, and most publications that pertain to the hydrogeology of the area describe only a few of the units or pertain to only part of the Colorado Plateaus. In this Chapter, the many water-yielding units in the area have been grouped into four principal aquifers for purposes of discussion. The principal aquifers are the Uinta-Animas aquifer, the Mesaverde aquifer, the Dakota-Glen Canyon aquifer system, and the Coconino-De Chelly aquifer (fig. 107). Most widespread and productive water-yielding units are included in these aquifers; however, some locally productive water-yielding units have been excluded.
Water-yielding units excluded from the principal aquifers can form aquifers of local importance, but these units either are not extensive enough or not productive enough to be considered as principal aquifers for the purposes of this Atlas. In general, these rocks are considered to be confining units containing minor water-yielding units.
Relatively impermeable confining units separate each of the four principal aquifers in the Colorado Plateaus. The two thickest units are the Mancos confining unit, which immediately underlies the Mesaverde aquifer, and the Chinle-Moenkopi confining unit, which immediately underlies the Dakota-Glen Canyon aquifer system. Thinner and less extensive confining units separate some water-yielding zones within the principal aquifers; however, these units generally form less effective barriers to ground-water movement than the confining units between the principal aquifers. Where the intra-aquifer confining units are thin or absent, water can move between adjacent water-yielding zones within an aquifer.
UINTA-ANIMAS AQUIFER
The Uinta-Animas aquifer primarily is composed of Lower Tertiary rocks in the Uinta Basin of northeastern Utah, the Piceance Basin of northwestern Colorado, and the San Juan Basin of northwestern New Mexico (fig. 108). Aquifers in each basin are present in different parts of the stratigraphic section (fig. 109). Some formations are considered to be an aquifer in more than one basin; however, some formations vary so much in their hydraulic characteristics that they are considered to be an aquifer in one basin and a confining unit in another.
Hydrogeologic Units
The Uinta-Animas aquifer in the Uinta Basin is present in water-yielding beds of sandstone, conglomerate, and siltstone of the Duchesne River and Uinta Formations, the Renegade Tongue of the Wasatch Formation, and the Douglas Creek Member of the Green River Formation (fig. 109). The Duchesne River Formation consists mostly of permeable fluvial sandstone and conglomerate. Grain size of these sediments decreases with distance from the Uinta Uplift, and relatively impermeable shale is common in the center of the basin. The Uinta Formation consists of permeable, poorly sorted, fine to coarse sandstone with some siltstone and mudstone. These rocks become more coarse-grained and permeable toward the top of the formation. Coarse-grained rocks adjacent to the Uinta Uplift and the Wasatch Plateau grade into finer-grained sediments away from the uplifted areas. The Renegade Tongue of the Wasatch Formation and the Douglas Creek Member of the Green River Formation contain an aquifer along the southern and eastern margins of the basin where the rocks primarily consist of fluvial, massive, irregularly bedded sandstone and siltstone. Water-yielding units in the Uinta-Animas aquifer in the Uinta Basin commonly are separated from each other and from the underlying Mesaverde aquifer by units of low permeability composed of claystone, shale, marlstone, or limestone.
The Uinta-Animas aquifer in the Piceance Basin consists of the Uinta Formation and the Parachute Creek Member of the Green River Formation. The Uinta Formation consists of silty sandstone, siltstone, and marlstone. Much of the intergranular space in these rocks has been filled by sodium and calcium bicarbonate cements, but fractures are numerous and produce substantial permeability. The Parachute Creek Member primarily consists of dolomitic marlstone. Kerogen, which is a waxlike hydrocarbon, is present in some parts of the member in the Piceance and Uinta Basins. Marlstone that contains large concentrations of kerogen is known as oil shale and generally is less fractured than marlstone that contains smaller concentrations of kerogen (lean marlstone). Fractures and dissolution openings along fractures in the lean marlstone form the principal pathways for water movement in the aquifer. Oil shale generally is less permeable and forms confining units. The Mahogany zone in the Piceance Basin is an example of one such confining unit (fig. 110). In the central part of the Piceance Basin, a saline zone in the marlstone contains the minerals nahcolite and halite, is not extensively fractured, and forms part of the relatively impermeable lower confining unit of the aquifer. The lower part of the Green River Formation and the Wasatch Formation form most of the lower confining unit of the aquifer.
The Uinta-Animas aquifer in the San Juan Basin consists of the San Jose Formation, the underlying Animas Formation and its lateral equivalent, the Nacimiento Formation, and the Ojo Alamo Sandstone. The San Jose Formation is the uppermost significant bedrock formation in the San Juan Basin and primarily consists of permeable, coarse, arkosic sandstone interlayered with mudstone. The Animas and Nacimiento Formations and the Ojo Alamo Sandstone primarily consist of permeable conglomerate and medium to very coarse sandstone interlayered with relatively impermeable shale and mudstone.
The thickness of the Uinta-Animas aquifer generally increases toward the central part of each basin. In the Uinta Basin, for example, the part of the aquifer in the Duchesne River and Uinta Formations ranges in thickness from 0 feet at the southern margin of the aquifer to as much as 9,000 feet in the north-central part of the aquifer. The part of the aquifer in the Renegade Tongue and Douglas Creek Member in the Uinta Basin is about 500 feet thick. In the Piceance Basin, the Uinta-Animas aquifer is as much as 2,000 feet thick in the central part of the basin. In the northeastern part of the San Juan Basin, the maximum thickness of the Uinta-Animas aquifer is about 3,500 feet.
Recharge and Discharge
Ground-water recharge to the Uinta-Animas aquifer generally occurs in the areas of higher altitude along the margins of each basin. Ground water is discharged mainly to streams, springs, and by transpiration from vegetation growing along stream valleys.
In the Uinta Basin, the part of the aquifer in the Duchesne River and Uinta Formations has about 200,000 acre-feet per year of recharge. The rate of ground-water withdrawal is small, and natural discharge is approximately equal to recharge. In the Renegade Tongue and Douglas Creek Member part of the aquifer, recharge and discharge also are approximately equal and total about 1,000 acre-feet per year. Recharge occurs near the southern margin of the aquifer, and discharge occurs near the White and Green Rivers.
The Uinta-Animas aquifer in the Piceance Basin receives about 24,000 acre-feet per year of recharge, primarily in the upland areas near the margins of the aquifer. Discharge is approximately equal to recharge and primarily occurs in the valleys of Piceance Creek and other tributaries to the White River or in the valley of the Colorado River and its tributaries.
In the San Juan Basin, water recharges the Uinta-Animas aquifer in the higher altitude areas that nearly encircle the basin. Ground water generally flows toward the San Juan River and its tributaries where it is discharged to streamflow, to the alluvium that locally is present in the valleys, or to evapotranspiration. During 1985, about 28,000 acre-feet of ground water was withdrawn from the aquifer in the San Juan Basin.
Water-Level Conditions
The potentiometric surface of the Uinta-Animas aquifer generally ranges from about 100 feet above land surface to about 500 feet below land surface; the surface generally is near or above land surface in valleys in areas of ground-water discharge. Large depths to water are more common in highland areas that are remote from streams or other sources of recharge.
The potentiometric surfaces in the three basins containing the Uinta-Animas aquifer are similar in that the surfaces are higher near the margins of the basins and lower near one or two principal streams draining the basins. In the Uinta Basin, the potentiometric surface ranges in altitude from about 5,000 to 8,000 feet, and ground water primarily flows toward the discharge area along the Strawberry River (fig. 111). In the Piceance Basin, the potentiometric surface ranges in altitude from about 6,000 to 8,500 feet, and ground water primarily flows toward the discharge areas along Piceance and Yellow Creeks (fig. 112). In the San Juan Basin, the potentiometric surface is incompletely known but ranges in altitude from about 5,500 to 7,000 feet in the southern part of the basin (fig. 113). The valley of the San Juan River forms the principal area of ground-water discharge in this basin.
Ground-Water Quality
Dissolved-solids concentrations in water in the Uinta-Animas aquifer in the Uinta Basin generally range from 500 to 3,000 milligrams per liter; concentrations can exceed 10,000 milligrams per liter in some of the deeper parts of the Uinta Formation. Smaller dissolved-solids concentrations are prevalent near recharge areas where the water usually is a calcium or magnesium bicarbonate type. Larger dissolved-solids concentrations are more common near discharge areas where the water generally is a sodium bicarbonate or sulfate type. Dissolved-solids concentrations in water from the upper part of the aquifer in the Piceance Basin generally range from about 500 to more than 1,000 milligrams per liter (fig. 114). Concentrations in the lower part of the aquifer exceed 10,000 milligrams per liter (fig. 115) where extensive fracturing of the saline zone that underlies the aquifer has enabled upward movement of brine. The Uinta-Animas aquifer in the San Juan Basin contains fresh to moderately saline water. Dissolved-solids concentrations generally increase along the groundwater flow path from less than 1,000 milligrams per liter near recharge areas to about 4,000 milligrams per liter near the discharge area along the valley of the San Juan River.
MESAVERDE AQUIFER
The Mesaverde aquifer comprises water-yielding units in the Upper Cretaceous Mesaverde Group, its equivalents, and some adjacent Tertiary and Upper Cretaceous formations. The Mesaverde aquifer is at or near land surface in extensive areas of the Colorado Plateaus and underlies the Uinta-Animas aquifer. The aquifer is of regional importance in the Piceance, Uinta, Kaiparowits, Black Mesa, and San Juan Basins and is of lesser importance in the Wasatch Plateau and High Plateaus areas (fig. 116). Some of the rocks that form the Mesaverde aquifer contain coal beds, some of which have been mined for at least a century. The hydrologic effects of mining have been of increasing concern in the areas underlain by the aquifer.
Hydrogeologic Units
In the Piceance, Black Mesa, and San Juan Basins, the Mesaverde aquifer is present in rocks of the Mesaverde Group. In the western part of the Uinta Basin and in parts of the Wasatch Plateau, the Tertiary and Cretaceous North Horn Formation overlies the Mesaverde Group and also is considered part of the aquifer (fig. 117). In the Kaiparowits Basin, the aquifer is in the Cretaceous Straight Cliffs and Wahweap Sandstones, and the Kaiparowits Formation, which together are approximate equivalents of the Mesaverde Group, and the overlying Tertiary and Cretaceous Canaan Peak Formation. The Cretaceous Mancos Shale and its equivalent in the Kaiparowits Basin, the Tropic Shale, generally do not yield water. However, in the Uinta Basin, the water-yielding Frontier Sandstone Member is at the top of the Mancos Shale and is considered to be part of the Mesaverde aquifer. The non-water-yielding strata of the Mancos Shale and the Tropic Shale compose the Mancos confining unit, which underlies the Mesaverde aquifer everywhere the aquifer is present (fig. 117).
The rocks that compose the Mesaverde aquifer are conglomerate, sandstone, siltstone, mudstone, claystone, carbonaceous shale, limestone, and coal. Because these rocks primarily were deposited in environments that changed as sea level changed during the Late Cretaceous, lithology varies vertically and laterally, and intertonguing is common among the various formations and strata that make up the aquifer.
In the Piceance and Uinta Basins, the Mesaverde Group predominantly consists of sandstone with interbedded shale and coal. The North Horn Formation, which forms part of the aquifer in the Uinta Basin and Wasatch Plateau, consists of shale interbedded with sandstone and minor amounts of fresh-water limestone and conglomerate. In the Kaiparowits Basin, the upper part of the Mesaverde aquifer is in the Canaan Peak Formation, which mainly consists of conglomerate and conglomeratic sandstone with minor amounts of mudstone. The Kaiparowits Formation and the Wahweap and Straight Cliffs Sandstones predominantly consist of fine to coarse sandstone interbedded with shale, mudstone, and coal beds. In the Black Mesa Basin, the upper part of the Mesaverde Group consists of sandstone; the lower part consists of sandstone or silty sandstone interbedded with siltstone and coal. In most of the Black Mesa area, the upper part of the Mesaverde Group has been removed by erosion, so the interbedded sequence of the lower part of the group forms the Mesaverde aquifer. Although rocks of the Mesaverde Group are present on the High Plateaus, information concerning these rocks is sparse. The lithology of the rocks probably is similar to that of equivalent rocks in the Wasatch Plateau and the Kaiparowits Basin. In the San Juan Basin, the Mesaverde aquifer consists of sandstone, coal, siltstone, and shale of the Mesaverde Group. The formations of the Mesaverde Group intertongue extensively with the Mancos Shale and, to a lesser extent, with the Lewis Shale. The Point Lookout Sandstone is the most areally extensive of the Mesaverde Group formations in the San Juan Basin.
The Mancos confining unit generally comprises the Mancos Shale or its equivalent in the Kaiparowits Basin, the Tropic Shale. The thickness of the confining unit typically ranges from 1,000 to 6,000 feet. The rocks that compose the Mancos confining unit predominantly are marine shale, mudstone, and claystone; interbedded minor sandstone, siltstone, and limestone also are common. Some of the sandstone strata locally are water-yielding. However, in general, the Mancos confining unit is a thick barrier to vertical and lateral groundwater flow.
The altitude of the top of the Mesaverde aquifer has been mapped in parts of the Uinta, Piceance, and San Juan Basins. In the Uinta Basin, the altitude of the top of the aquifer ranges from about 10,000 feet below sea level in the north-central and deepest part of the basin to about 5,000 feet above sea level near the margins of the basin. In the Piceance Basin, the top of the aquifer ranges in altitude from about sea level in the central part of the basin to between 5,000 and 7,500 feet above sea level near the margins of the basin. In the San Juan Basin, the top of the aquifer is about 2,500 to 5,000 feet above sea level. In the Piceance and Uinta Basins, the thickness of the Mesaverde aquifer generally is between 2,000 and 4,000 feet. However, the thickness exceeds 7,000 feet locally in the eastern part of the Piceance Basin and is less than 1,000 feet near the margins of the basins. In the San Juan Basin, the Mesaverde aquifer has a maximum thickness of about 4,500 feet in the southern part of the basin.
Recharge and Discharge
Water generally recharges the Mesaverde aquifer in upland areas that receive more precipitation than lower altitude areas. In the Piceance Basin, recharge occurs on the northern flanks of the West Elk Mountains, in the area near Grand Mesa, and along the Roan Plateau. Ground water in the Uinta Basin is recharged near the basin margins. Interbasin flow from the Piceance Basin contributes water to the Uinta Basin. Ground-water flow directions in much of the west-central part of the Uinta Basin are poorly defined by available data. The available data in the San Juan Basin indicate recharge in the area of the Zuni Uplift, Chuska Mountains, and in northern Sandoval County, N. Mex.
Ground water discharges from the aquifer directly to streams, springs, and seeps, by upward movement through confining layers and into overlying aquifers, or by withdrawal from wells. The natural discharge areas generally are along streams and rivers, such as the Colorado River and the North Fork of the Gunnison River in the Piceance Basin; the Strawberry, Duchesne, and Green Rivers in the Uinta Basin; the Colorado River and its tributaries in the Kaiparowits Basin; and the San Juan River and the Chaco River and its tributaries in the San Juan Basin.
Water-Level Conditions
In most areas of the Mesaverde aquifer, ground-water withdrawals have been small. Consequently, water-level declines have been limited to localized areas; elsewhere, the potentiometric surface generally represents predevelopment conditions. Water-level measurements and reports of measurements made during the period of development of the aquifer and during oil and gas test-well drilling were combined to generate a generalized potentiometric-surface map (fig. 118).
Ground water in the Uinta, Piceance, and San Juan Basins generally flows from recharge areas near the margins of the basins to discharge areas near principal stream valleys. The altitude of the potentiometric surface in these basins generally ranges from about 5,000 to 8,000 feet. In the Kaiparowits Basin, ground-water flow generally is toward the southeast. In the Black Mesa Basin, ground-water flow is localized because of the shallow canyons cut by tributaries of the Little Colorado River into the rocks that form the Mesaverde aquifer. In other areas of the Mesaverde aquifer, data are insufficient to define the potentiometric surface and ground-water flow directions.
Aquifer Characteristics
Transmissivity of the Mesaverde aquifer is less than 50 feet squared per day in large areas of the Colorado Plateaus but exceeds 2,000 feet squared per day locally in the western part of the Uinta Basin and the eastern part of the Wasatch Plateau. Fracturing of rocks that form the Mesaverde aquifer locally increases the secondary permeability; as a result, the transmissivity also is increased locally to values as much as 100 times greater than those for the unfractured rock. In areas where the aquifer is deeply buried, such as in the Piceance Basin, overburden pressure, compaction, and cementation have caused hydraulic conductivity to be small. As a result, although the thickness of the aquifer generally is large in these areas, transmissivity is small.
Ground-Water Quality
The quality of the water in the Mesaverde aquifer is extremely variable. The dissolved-solids concentration of water from the aquifer is less than 1,000 milligrams per liter in many of the basin-margin areas but locally can be very large (more than 35,000 milligrams per liter in the central part of the Uinta Basin, and more than 10,000 milligrams per liter in the central part of the Piceance Basin) (fig. 119). In general, areas of the aquifer that are recharged by infiltration from precipitation or surface-water sources contain relatively fresh water. Sparse data indicate that the dissolved-solids concentration ranges from about 1,000 to 4,000 milligrams per liter in parts of the Kaiparowits and San Juan Basins and the High and Wasatch Plateaus.
DAKOTA-GLEN CANYON AQUIFER SYSTEM
Water-yielding rocks ranging in age from late Cretaceous to Triassic underlie most of the Colorado Plateaus area. These rocks contain a series of aquifers and confining units, which, for the purposes of this chapter, are referred to as the Dakota-Glen Canyon aquifer system. In much of the area underlain by the aquifer system (fig. 120), the great depth to the aquifers or poor water quality make the aquifers unsuitable for development. However, in areas where an aquifer is near land surface, the aquifer may be an important source of water.
Rocks that compose the Dakota-Glen Canyon aquifer system are older than the Mancos and Tropic Shales, which form the overlying Mancos confining unit; and are younger than the Chinle, Ankareh, or Moenkopi Formations, which form the underlying Chinle-Moenkopi confining unit. In general, both confining units are thick, low-permeability zones that severely restrict vertical flow between the Dakota-Glen Canyon aquifer system and overlying and underlying aquifers.
The Dakota-Glen Canyon aquifer system includes four permeable zones that herein are referred to as the Dakota aquifer, the Morrison aquifer, the Entrada aquifer, and the Glen Canyon aquifer. The units that form the bulk of these aquifers are, respectively: (1) The Dakota Sandstone and adjacent water-yielding rocks; (2) water-yielding rocks generally of the lower part of the Morrison Formation; (3) the Entrada Sandstone and its equivalent in the western part of the Uinta Basin, the Preuss Sandstone; and (4) the Glen Canyon Sandstone or Group and its equivalent in the western part of the Uinta Basin, the Nugget Sandstone. These rocks are at land surface or at reasonable drilling depths below land surface primarily on the flanks of the San Rafael Swell, White River, and Circle Cliffs Uplifts, in the Henry Mountains Basin, in parts of the Paradox Basin, Uncompahgre Uplift, and Four Corners Platform, in the Black Mesa Basin, and in the Acoma Sag (fig. 120). The stratigraphic relations among the formations that contain these aquifers and the adjacent confining units are shown in figure 121.
Sandstone, conglomerate, and conglomeratic sandstone are the major water-yielding materials in this series of aquifers. The aquifers commonly also contain interbedded siltstone. Mudstone, claystone, siltstone, shale, and limestone generally form the confining units that separate these aquifers (table 1).
The aquifers described in this section are grouped together as an aquifer system because they are separated everywhere from overlying and underlying aquifers by thick confining units and because some hydraulic connection exists between each of the aquifers in the system at some point in the Colorado Plateaus area. For example, in the Black Mesa Basin, the Morrison and Curtis-Stump confining units are missing; as a result, the Dakota, Morrison, and Entrada aquifers are in direct contact (fig. 122). This contact likely allows interaquifer flow among these three aquifers, although the rate of interaquifer flow may be limited by low-permeability zones within the aquifers. The confining units in the aquifer system generally are not as thick as the more substantial Mancos and Chinle-Moenkopi confining units, and interaquifer flow is more likely among the aquifers of the Dakota-Glen Canyon aquifer system than between these aquifers and those that overlie or underlie the aquifer system.
In a regional context, recharge areas, discharge areas, ground-water flow directions, and water quality are similar among the four aquifers. The uppermost aquifer (the Dakota) and the lowermost aquifer (the Glen Canyon) are best defined by data, and these two aquifers are discussed here as examples of the hydrogeology near the top and bottom of the aquifer system.
Hydrogeologic Units
The Dakota aquifer is in the Upper Cretaceous Dakota Sandstone and underlying Lower Cretaceous Burro Canyon and Cedar Mountain Formations (fig. 121). The lithology of the Dakota Sandstone varies widely and includes conglomerate, sandstone, siltstone, mudstone, carbonaceous shale, and coal. Three units can be recognized over a large area: a basal conglomeratic sandstone; a middle sequence of interbedded carbonaceous shale, impure coal, and lenticular sandstone and siltstone; and an upper, massive, fine to medium sandstone. Sandstone, which is commonly interbedded with thin mudstone beds, constitutes about one-half of the total thickness of the Burro Canyon Formation; in some places, the sandstone forms a single, thick bed. Minor chert and limestone beds also are present in the formation. The lithology of the Cedar Mountain Formation is similar to that of the Burro Canyon Formation, except that sandstone generally composes less than 30 percent of the thickness of the Cedar Mountain Formation. In some places, the Cedar Mountain Formation includes a basal conglomeratic sandstone unit. The Dakota aquifer is present in the Piceance and Uinta Basins, along the Wasatch and High Plateaus, in the Kaiparowits, Henry Mountains, Black Mesa, and San Juan Basins, in the eastern part of the Four Corners Platform, and in parts of the Paradox Basin and Uncompahgre Uplift (fig. 120). The depth to the top of the aquifer is less than 2,000 feet in many areas but exceeds 12,000 feet in parts of the Piceance and Uinta Basins (fig. 123).
The Upper Jurassic Morrison Formation underlies the Dakota aquifer in the Colorado Plateaus (fig. 121). In most parts of the area, the Morrison Formation includes an upper, non-water-yielding unit called the Brushy Basin Member, which forms the Morrison confining unit. This member mainly consists of relatively impermeable siltstone, mudstone, and claystone. The member is absent in the Black Mesa Basin.
The middle and lower parts of the Morrison Formation consist of interbedded fine to medium sandstone, siltstone, and mudstone. This sequence is called the Morrison aquifer, although only the coarser-grained strata generally can be expected to yield water. In the Four Corners Platform and San Juan and Black Mesa Basins, the Morrison aquifer includes two underlying water-yielding sandstone units, the Middle Jurassic Cow Springs and Junction Creek Sandstones.
In most places in the Colorado Plateaus, the Morrison aquifer is underlain by non-water-yielding Middle Jurassic rocks that form the Curtis-Stump confining unit. The formations that make up the Curtis-Stump confining unit are the Curtis, Summerville, Stump, and Wanakah Formations. These formations predominantly consist of siltstone with interbedded shale and sandstone. Minor amounts of limestone and gypsum also are present.
The Middle Jurassic rocks that form the Entrada aquifer underlie either the Curtis-Stump confining unit or the Morrison aquifer. The Entrada aquifer mainly is in the Entrada Sandstone: in the western part of the Uinta Basin, the Preuss Sandstone, which is an equivalent of the Entrada, forms the aquifer. In the Kaiparowits Basin, the Romana Sandstone overlies the Entrada Sandstone, and the aquifer includes both formations. The lithology of the formations that make up the Entrada aquifer generally is very fine to fine sandstone, which is commonly of eolian origin. In some places, the sandstone is interbedded with siltstone. The sandstone and siltstone locally are clayey. The degree of cementation of the Entrada Sandstone varies considerably.
In parts of Utah and northeastern Arizona, the Entrada aquifer is underlain by either the Middle Jurassic Carmel Formation or, in the western Uinta Basin, the Middle Jurassic Twin Creek Limestone (fig. 121). These two formations form the Carmel-Twin Creek confining unit. The Carmel Formation mainly consists of siltstone and shale interbedded with smaller amounts of limestone, sandstone, and gypsum; west of the San Rafael Swell, evaporites, including halite, are common. The Twin Creek Limestone consists of sandy to shaly limestone interbedded with siltstone and some sandstone. In part of the Colorado Plateaus, however, the Carmel-Twin Creek confining unit is absent, and the Entrada aquifer directly overlies the Glen Canyon aquifer.
Rocks of the Lower Jurassic Glen Canyon Group and its equivalents compose the Glen Canyon aquifer. In most areas, the Glen Canyon Group is divided into three formations; at the base is the Wingate Sandstone; above the Wingate Sandstone lies the Kayenta Formation; the uppermost formation is the Navajo Sandstone (fig. 121). In some areas of the Black Mesa Basin, the Glen Canyon Group includes a fourth formation, the Moenave Formation, which overlies the Wingate Sandstone. In northwestern Colorado and the eastern part of the Uinta Basin, the stratigraphic equivalent of the Glen Canyon Group is the Glen Canyon Sandstone, and, in the western Uinta Basin, the equivalent is the Nugget Sandstone. From the San Rafael Swell to the Black Mesa Basin, the Glen Canyon aquifer includes the Middle Jurassic Page Sandstone, which unconformably overlies the Glen Canyon Group. The Page, Navajo, Nugget, Glen Canyon, and Wingate units consist of sandstone that is for the most part of eolian origin; the Wingate Sandstone also contains some siltstone. The eolian sandstones vary in their degree of cementation. The variability of the cementation is visible where the erosive action of water and wind removes the less well-cemented parts of a rock outcrop and creates arches and other unusual features (fig. 124). The Kayenta Formation consists of sandstone, siltstone, mudstone, claystone, and minor amounts of limestone. The Moenave Formation comprises interbedded lenticular sandstone, siltstone, claystone, and minor amounts of limestone.
The depth to the top of the Glen Canyon aquifer is less than 2,000 feet in a large area, but the depth exceeds 12,000 feet in substantial parts of the Piceance and Uinta Basins (fig. 125). The Glen Canyon is the thickest of the aquifers of the Dakota-Glen Canyon aquifer system (table 1), and the water-yielding materials in the aquifer commonly are well sorted, permeable, and fractured in some areas. These factors produce relatively high transmissivity values for much of the aquifer.
The Dakota-Glen Canyon aquifer system is underlain by the Chinle-Moenkopi confining unit (fig. 121). The Triassic Chinle and Moenkopi Formations are the two main formations that compose the confining unit. In the western Uinta Basin, the Ankareh Formation is the equivalent of the Chinle Formation and forms the upper part of the confining unit. In the eastern end of the Four Corners Platform, the Triassic Dolores Formation composes the entire confining unit. In eastern Utah and northeastern Arizona, the Kaibab Limestone and Toroweap Formation of Permian age underlie the Moenkopi Formation and compose the lower part of the confining unit. The thickness of the Chinle-Moenkopi confining unit typically is 1,000 to 2,000 feet. Shale and sandy shale are the most prevalent rock types in the confining unit; limestone, claystone, mudstone, siltstone, and shaly sandstone also are common. Conglomerate, sandstone, and conglomeratic sandstone locally are present. In some parts of northern Arizona, sandstone in the lowermost member of the Chinle Formation or the Kaibab Limestone yields small amounts of water to wells. Elsewhere, the formations generally do not yield water. Overall, the Chinle-Moenkopi confining unit is an effective barrier to interaquifer ground-water flow and forms the base of the Dakota-Glen Canyon aquifer system.
Recharge and Discharge
Water-level data for the Dakota aquifer are sparse, and as a result, the potentiometric surface can be defined only in the northeastern part of the aquifer (fig. 126). Major recharge areas indicated by the potentiometric surface are in the southeastern end of the Uncompahgre Uplift, the northern margin of the Uinta Basin, and the eastern side of the Piceance Basin. From these recharge areas, water in the Dakota aquifer flows toward discharge areas along the White, Colorado, and Gunnison Rivers.
The potentiometric surface for the Glen Canyon aquifer has been defined for much of the northern part of the aquifer (fig. 127). Ground-water flow directions inferred from the potentiometric surface indicate major recharge areas along the western margins of the San Rafael Swell and Circle Cliffs Uplift, in the northern part of the Four Corners Platform, in the southeastern parts of the Uncompahgre Uplift and Paradox Basin, at the eastern margin of the Piceance Basin, and at the northeastern margin of the Uinta Basin. Ground-water flow in the Glen Canyon aquifer is toward major discharge areas along the Green, Colorado, Dolores, and San Juan Rivers.
Aquifer Characteristics
The transmissivity of the Dakota aquifer is poorly defined but probably ranges from less than 10 to about 100 feet squared per day in the northeastern part of the Colorado Plateaus. The large thickness of permeable rocks in the Glen Canyon aquifer produces transmissivities that generally range from about 100 to 1,000 feet squared per day; fractures form the principal pathways for water movement in the well-consolidated materials.
Ground-Water Quality
In general, where the Glen Canyon aquifer is less than 2,000 feet below land surface, the dissolved-solids concentration of water in the aquifer is less than 1,000 milligrams per liter (fig. 128). However, in large areas where the aquifer is deeply buried, such as in parts of the Piceance and Uinta Basins, the dissolved-solids concentration exceeds 35,000 milligrams per liter. In an area in extreme southeastern Utah where oil and gas exploration and production are concentrated, water in the Glen Canyon aquifer is highly mineralized. Analysis of the water chemistry indicates that the source of the mineralized water likely is deeper strata, which contain substantial deposits of evaporite minerals, particularly halite (rock salt). The water quality in the aquifer might have been caused by upward movement of saline water through unplugged or poorly plugged oil-test holes or leaking water-injection wells, which are used to dispose of saline water that is produced with oil and gas.
COCONINO-DE CHELLY AQUIFER
Water-yielding rocks of Early Permian age underlie the southern part of the Colorado Plateaus. In this chapter these rocks are referred to as the Coconino-De Chelly aquifer (fig. 129).
Hydrogeologic Units
The formations that comprise the Coconino-De Chelly aquifer are the Coconino, De Chelly, and Glorieta Sandstones; the San Andres Limestone; and the Yeso and Cutler Formations (fig. 130). The Coconino and De Chelly Sandstones generally consist of well-sorted quartz sandstone with thin interbeds of siltstone, mudstone, and carbonates. The Glorieta Sandstone consists of well-sorted, well-cemented, fine to medium quartz sandstone. The San Andres Limestone consists of dolostone, limestone, and fine-grained clastic rocks. The carbonate rocks in the San Andres Limestone are characterized by solution openings, which substantially increase the hydraulic conductivity of the formation. The Yeso Formation consists of interbedded sandstone, siltstone, limestone, anhydrite, and gypsum and forms a low-permeability zone in the aquifer. The Cutler Formation consists of shale, siltstone, sandstone, arkose, and arkosic conglomerate.
In most areas near the Grand Canyon, the Coconino Sandstone probably does not yield water because of the proximity to the canyon, where the formation has been truncated and drained (fig. 131). Fractures and associated solution openings in underlying rocks in the vicinity of the Grand Canyon allow water to discharge from the Coconino Sandstone. In much of the northern part of the Colorado Plateaus, rocks equivalent to those included in the aquifer are present, but the water in these rocks generally has dissolved-solids concentrations in excess of 10,000 milligrams per liter. The hydrogeology of the aquifer in this area is not described in this chapter because of the salinity of the water.
Recharge and Discharge
In the areas where the altitude of the potentiometric surface of the Coconino-De Chelly aquifer has been mapped, ground water generally flows from the structural uplifts toward the major surface-water drainages (fig. 132). The aquifer is recharged in the Uncompahgre Uplift, Paradox Basin, San Rafael Swell, Circle Cliffs Uplift, Defiance Uplift, Zuni Uplift, and Mogollon Slope (fig. 129). Discharge mainly is to the Colorado and Green Rivers. Water in the Coconino-De Chelly aquifer near the Black Mesa Basin generally flows northwestward toward a discharge area near the mouth of the Little Colorado River. In the Grand Canyon, a series of springs issuing from the Mississippian Redwall Limestone (fig. 133) discharges water derived in part from the Coconino-De Chelly aquifer. Fractures and solution channels in the Redwall Limestone and the rocks separating the Redwall Limestone from the Coconino Sandstone provide conduits for the ground water. Similar processes affect the ground-water flow system elsewhere in the vicinity of the Grand Canyon.
Ground-Water Quality
In Utah, the dissolved-solids concentration in water from the Coconino-De Chelly aquifer ranges from less than 1,000 milligrams per liter in the San Rafael Swell and Monument Uplift to 10,000 milligrams per liter along the margin of the Uinta Basin (fig. 134). In northeastern Arizona and west-central New Mexico, the dissolved-solids concentration of water in the aquifer generally is less than 1,000 milligrams per liter. However, in an area near the southeastern margin of the Black Mesa Basin, the dissolved-solids concentration exceeds 25,000 milligrams per liter. The northwestward regional movement of ground water near the Black Mesa Basin may have produced the elongated distribution of the more mineralized water in that area.
