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| Figure 16. Generalized aquifers in semiconsolidated and consolidated rocks located at or near the land surface, Yellowstone River Basin. (Click on image for a larger version, 119 kb) |
The hydrologic and water-quality characteristics of ground water in the study unit differ with location, elevation, and geologic unit. The physical and geochemical properties of the rock units comprising the aquifers to a large extent determine the quantity and quality of ground water available for use. The primary aquifers in the YRB are unconsolidated Quaternary deposits, lower Tertiary rocks, and rocks of Mesozoic and Paleozoic age. Aquifers are located throughout most of the study unit (fig. 16). The unconsolidated Quaternary deposits and lower Tertiary rocks are found primarily in the structural basins, whereas the Mesozoic and Paleozoic units are at or near the surface chiefly in the uplifted areas of the YRB. The lower Tertiary aquifers have the greatest surface extent in the study unit. The quality of ground water in shallow aquifers may be influenced by the quality of nearby surface water, such as lakes and streams. The areal distribution of the uppermost principal aquifers in semiconsolidated and consolidated rocks of the study unit is shown in figure 16.
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| Figure 16. Generalized aquifers in semiconsolidated and consolidated rocks located at or near the land surface, Yellowstone River Basin. (Click on image for a larger version, 119 kb) |
The four major structural basins of the YRB--the Wind River, Bighorn, Powder River, and Williston Basins (fig. 5)--each contain substantial quantities of ground water and will be the focus of this section. The remainder of the study unit is composed of small basins, structural uplifts, and mountain ranges that contain smaller quantities of ground water. Ground-water resources in these regions will not be discussed in this report.
The Wind River Basin contains both confined and water-table aquifers. The Wind River Formation of Eocene age and older sedimentary rocks often yield water under confined conditions. In alluvial and eolian sand deposits of Quaternary age, unconfined (water-table) conditions often prevail (Whitcomb and Lowry, 1968). The physical and water-bearing characteristics of the geologic formations in the Wind River Basin are summarized in table 16 (at end of report). The following text briefly summarizes the hydrologic characteristics of the rock units within the Wind River Basin. For a more complete description the reader is referred to Whitcomb and Lowry (1968), Richter (1981), Plafcan and others (1995), and Daddow (1996).
Whitcomb and Lowry (1968) report that coarse sand and gravel beds in the valleys of perennial streams whose headwaters are in the Wind River and Owl Creek Mountains, are capable of yielding moderate to large quantities of water. Generally, the concentration of dissolved solids in the upper reaches of floodplain deposits of perennial streams is less than that in deposits further downstream. For example, a well drilled in the alluvium of the central Wind River valley yielded a calcium bicarbonate water that contained 272 ppm (parts per million) of dissolved solids, whereas a well about 16 miles downstream yielded a sodium sulfate water that contained 1,230 ppm of dissolved solids (Whitcomb and Lowry, 1968). Large-scale development of this water resource has not occurred because surface-water supplies generally are adequate for present needs.
According to Whitcomb and Lowry (1968), alluvial deposits located along ephemeral and intermittent streams that originate within the Wind River Basin generally consist of fine to coarse sand intermixed with silt and clay. Wells in these deposits normally only yield enough water for stock and domestic use. Recharge to these aquifers is mainly from infiltration of precipitation on the drainage area, and the amount of ground water available to wells fluctuates accordingly.
Terrace deposits can yield adequate quantities of water for stock or domestic use in areas where surface water has been applied for irrigation. Yields fluctuate in response to irrigation, and some of the shallower wells go dry during the winter (Whitcomb and Lowry, 1968).
In the northeastern part of the basin, Whitcomb and Lowry (1968) reported that deposits of eolian sand are an important source for stock and domestic supplies of ground water. The water in these deposits is generally derived from local infiltration of precipitation.
According to Whitcomb and Lowry (1968), the major Tertiary aquifers in the Wind River Basin are the Split Rock Formation of Oligocene and Miocene age and the Eocene Wind River Formation. The Oligocene White River Formation has similar hydrologic characteristics as the Split Rock Formation but has not been widely developed as a ground-water source because shallower aquifers usually provide adequate water for stock and domestic uses. The Tepee Trail, Aycross, and Indian Meadows Formations of Eocene age, together with undifferentiated rocks of Tertiary age, may yield moderate to large supplies of water, but these rocks have not been tested as a potential source of ground water.
Cretaceous aquifers generally are not used for ground-water supplies throughout the basin for several reasons. According to Whitcomb and Lowry (1968), although the water in these formations is usually under artesian pressure and piezometric surfaces are often near the land surface, the formations themselves lie at great depths except around the margins of the basin and in areas of local uplift. The formations principally consist of shale containing minor amounts of fine-grained silty sandstone and generally have low permeabilities. The Upper Cretaceous Lance Formation and the Lower Cretaceous Cloverly Formation have more suitable aquifer characteristics, but areas in which the formations lie within economical drilling depths are small. Water from the Mesaverde Formation, Cody Shale, and Frontier Formation is generally unsuitable for domestic use, and in some wells it is so unpalatable that stock will not drink it (Whitcomb and Lowry, 1968).
Pre-Cretaceous formations are used to a limited extent as supplies of ground water in the basin. Whitcomb and Lowry (1968) reported that many of the formations have suitable aquifer characteristics but occur below economical drilling depths in most of the Wind River Basin. The formations form narrow outcrops on the flanks of the Wind River, Owl Creek, and Bighorn Mountains and in some of the major anticlinal structures within the basin. The Jurassic Morrison and Sundance Formations, the Triassic Chugwater Formation, the Permian Phosphoria Formation and related rocks, the Pennsylvanian Tensleep Sandstone, and Mississippian Madison Limestone are used in small areas of the basin as ground-water supplies. Pre-Mississippian rocks in the basin are unused because of great burial depths and inaccessibility in outcrop areas.
The Bighorn Basin contains both artesian and water-table aquifers. Aquifers in Quaternary unconsolidated deposits are generally unconfined, and the water table is generally at shallow depths. Water in bedrock aquifers occurs under either confined or unconfined conditions (Plafcan and others, 1993). The physical and water-bearing characteristics of the geologic formations in the Bighorn Basin are summarized in table 16 (at end of report). The hydrologic characteristics of the rock units within the basin are briefly summarized in the following two sections. For a more complete description the reader is referred to Lowry and others (1976), Libra and others (1981), Plafcan and others (1993), Susong and others (1993), and Plafcan and Ogle (1994).
Holocene and Pleistocene unconsolidated deposits in the Bighorn Basin include floodplain alluvium, terrace deposits, pediments, minor colluvium, and alluvial fan deposits. The floodplain alluvium is generally less than 10 m thick. Alluvial and colluvial deposits located along the larger streams, such as the Bighorn and Shoshone Rivers, are among the most productive and predictable sources of ground water in the basin (Plafcan and others, 1993; Susong and others, 1993).
Lower Tertiary aquifers in the Bighorn Basin are used primarily as a drinking-water source for residents without access to a public drinking-water supply (Libra and others, 1981). The areally extensive Willwood Formation of Eocene age is the principal Tertiary aquifer in the basin. The sandstone content of the formation ranges from 3 to 88 percent with the average being 25 percent. Ground water occurs in sandstone lenses that are often small, making it difficult to locate productive wells for stock or domestic uses (Plafcan and Ogle, 1994; Plafcan and others, 1993; Susong and others, 1993).
The Paleocene Fort Union Formation is a minor aquifer in parts of the basin. The average sandstone content is 25 percent, but sandstone lenses are seldom continuous for more than a few hundred yards. The formation is used as an aquifer primarily where it crops out west of the Bighorn River (Plafcan and Ogle, 1994; Plafcan and others, 1993; Susong and others, 1993).
Cretaceous aquifers have not been widely developed throughout the basin, although the Upper Cretaceous, Lance, Mesaverde, and Frontier Formations have potential for more extensive development (Plafcan and others, 1993). Jurassic and Triassic sandstones are locally used as aquifers. The Gypsum Spring Formation of Jurassic age consists of interbedded red shale, dolomite, and gypsum (table 16, at end of report). Solution of gypsum in this formation may provide enough secondary porosity to provide high yields to wells in some areas (Lowry and others, 1976). Mesozoic rocks generally do not occur at economical drilling depths except at the margins of the basin and along areas of structural uplift.
The major Paleozoic aquifers in the basin include the Pennsylvanian Tensleep Sandstone, the Mississippian Madison Limestone, the Ordovician Bighorn Dolomite, and the Cambrian Flathead Sandstone (Plafcan and Ogle, 1994; Plafcan and others, 1993; Susong and others, 1993). These aquifers are confined by overlying Mesozoic shales as well as by impermeable Paleozoic strata such as the Goose Egg Formation of Triassic and Permian age, the Amsden Formation of Pennsylvanian and Mississippian age, the Ordovician Gallatin Limestone, and the Cambrian Gros Ventre Formation (Plafcan and others, 1993). Paleozoic aquifers in the Bighorn Basin are recharged primarily in the mountains on the basin margins. Except near basin margins and other areas of structural uplift, Paleozoic rocks do not occur at economical drilling depths (Plafcan and Ogle, 1994; Plafcan and others, 1993; Susong and others, 1993).
The physical and hydrologic characteristics of the geologic formations in the Powder River Basin are summarized in tables 16 and 17. The following two sections briefly summarize the hydrologic characteristics of the aquifers within the basin. For a more complete description the reader is referred to Crist and Lowry (1972), Hodson and others (1973), Lewis and Hotchkiss (1981), and Slagle and others (1984).
According to Lewis and Hotchkiss (1981), the shallow aquifer system of the Powder River Basin is composed of five mappable hydrogeologic units located stratigraphically above the regionally persistent and essentially impermeable Upper Cretaceous Bearpaw Shale. The uppermost hydrogeologic unit in the shallow aquifer system is the Wasatch-Tongue River aquifer (table 17) (Lewis and Hotchkiss, 1981). The aquifer is extensive, thick (up to 1,190 m thick), and is exposed at the land surface in most of the basin. The depositional environments of the geologic units included in the aquifer are generally terrestrial. The average sand content of this unit is 54 percent, indicating it could be an aquifer over most of the area.
The Lebo confining layer (up to 920 m thick) extends over most of the basin and underlies the Wasatch-Tongue River aquifer (table 17). The unit generally correlates with the Lebo Shale Member of the Fort Union Formation. The sand content of the unit has a mean value of 31 percent, indicating that it generally acts as a confining layer (Lewis and Hotchkiss, 1981).
The Tullock aquifer (up to 600 m thick) underlies the Lebo confining layer except near outcrop areas. With an average sand content of 53 percent the unit is considered an aquifer in most of the basin (Lewis and Hotchkiss, 1981).
The Cretaceous Upper Hell Creek confining layer (table 17) underlies the Tullock aquifer (Lewis and Hotchkiss, 1981). The unit is a major confining layer throughout the basin, is up to 610 m thick, and has a mean sand content of 35 percent (Lewis and Hotchkiss, 1981).
The Upper Cretaceous Fox Hills-Lower Hell Creek aquifer (table 17) underlies the Upper Hell Creek confining layer except at outcrop areas (Lewis and Hotchkiss, 1981). The aquifer is up to 780 m thick, has a mean sand content of 50 percent, and yields water to wells in most areas. The base of the aquifer is the top of the Bearpaw Shale (equivalent to Lewis Shale).
Several formations of the Lower Cretaceous Series in the basin consist of shales that are not usable as aquifers. However, the Cloverly, Fall River, and Lakota Formations contain sandstone aquifers, but are too deeply buried to be useful, except at the basin margins.
According to Hodson and others (1973), several Paleozoic formations, including the Minnelusa Formation and Tensleep Sandstone of Permian and Pennsylvanian age, and the Madison Limestone of Mississippian age, have great potential for ground-water development. The Bighorn Dolomite of Ordovician age and the Flathead Sandstone of Cambrian age also have potential. Although these Paleozoic formations would likely produce relatively high yields of ground water, they are largely unused because, except at the basin margins, they are deeply buried.
Only a small, southwestern part of the Williston Basin is included in the study unit. The shallow hydrogeologic units of the Williston Basin included in the study unit are similar to the shallow hydrogeologic units of the Powder River Basin.
Stoner and Lewis (1980) mapped the shallow hydrogeologic units in southeastern Montana, including the southwestern part of the Williston Basin. The shallow aquifer system was divided into mappable units (table 18) that are similar to those mapped by Lewis and Hotchkiss (1981) in the Powder River Basin. The most notable difference between the aquifer units defined in the Williston Basin and those in the Powder River Basin occurs in the upper part of the shallow aquifer system. The aquifers lying above the Lebo confining layer in the Williston Basin are divided into four separate mappable units, whereas in the Powder River Basin they are grouped as a single unit. For a more complete description of the shallow aquifer system in the Williston Basin the reader is referred to Stoner and Lewis (1980), and Slagle and others (1984).
Return to Environmental Setting of the Yellowstone River Basin, WRIR 98-4269
