Ground-water resources and geology of northern and central Johnson County, Wyoming
Northern and central Johnson County, Wyo., is an area of about 2,600 square miles that lies principally in the western part of the Powder River structural basin but also includes the east flank of the Bighorn Mountains. Sedimentary rocks exposed range in age from Cambrian to Recent and have an average total thickness of about 16,000 feet. Igneous and metamorphic rocks of Precambrian age crop out in the Bighorn Mountains. Rocks of pre-Tertiary age, exposed on the flanks and in the foothills of the Bighorns, dip steeply eastward and lie at great depth in the Powder River basin. The rest of the project area is underlain by a thick sequence of interbedded sandstone, siltstone, and shale of Paleocene and Eocene age. Owing to the regional structure, most aquifers in Johnson County contain water under artesian pressure. The Madison Limestone had not been tapped for water in Johnson County at the time of the present investigation (1963), but several wells in eastern Big Horn and Washakie Counties, on the west flank of the Bighorn Mountains, reportedly have flows ranging from 1,100 to 2,800 gallons per minute. Comparable yields can probably be obtained from the Madison in Johnson County in those areas where the limestone is fractured or cavernous. The Tensleep Sandstone reportedly yields 600 gallons per minute to a pumped irrigation well near its outcrop in the southwestern part of the project area. Several flowing wells tap the formation on the west flank of the Bighorn Mountains. The Madison Limestone and the Tensleep Sandstone have limited potential as sources of water because they can be developed economically only in a narrow band paralleling the Bighorn Mountain front in the southwestern part of the project area. Overlying the Tensleep Sandstone is about 6,000 feet of shale, siltstone, and fine-grained sandstone that, with a few exceptions, normally yields only small quantities of water to wells. The Cloverly Formation and the Newcastle Sandstone may yield moderate quantities of water to wells; but, in some areas, properly constructed wells tapping both formations might yield large quantities of water. The Shannon Sandstone Member of the Cody Shale will probably yield only small quantities of water to Wells, but it is the best potential source of ground water in the stratigraphic interval between the Newcastle and Parkman Sandstones. The Parkman Sandstone and the Lance Formation yield water to relatively shallow wells principally in the southwestern part of the project area. The Fort Union Formation yields adequate supplies of water for stock and domestic use from relatively shallow wells near its outcrop almost everywhere in the county. A few deep wells tap the Fort Union along the Powder River valley in the northeastern part of Johnson County. Some of these wells flow, but their flows rarely exceed 10 gallons per minute; larger yields could be undoubtedly be obtained by pumping. The Wasatch Formation is the principal source of ground water in Johnson County. It yields adequate supplies to many relatively shallow stock and domestic wells, some of which flow, but much larger yields probably would require pumping lifts that are prohibitive for most purposes. The Kingsbury Conglomerate and Moncrief Members of the Wasatch Formation, though, may yield moderate quantities of water in some places. Alluvial deposits underlying the valleys of the Powder River and Crazy Woman, Clear, and Piney Creeks are potential sources of moderate to large supplies of water in the Powder River drainage basin. The permeability of these deposits decreases with distance from the Bighorn Mountain front, so that largest yields can probably be obtained along the upper reaches of these streams. Most ground water utilized in the project area is for domestic and stock supplies and is obtained from drilled wells and from springs. Water for irrigation is obtained almost entirely by diverting flows of perennial streams. The discharge of wel