Geology and occurrence of ground water in Lyon County, Minnesota

Abstract

Lyon County is in southwestern Minnesota, mostly within the drainage basin of the Minnesota River. The basement rocks in the area consist largely of Precambrian granite and quartzite. These are overlain locally by flat-lying Upper Cretaceous strata composed of thick sections of soft dark-bluish-gray shale and some thin beds of loosely consolidated sandstone. The Cretaceous strata are more than 500 feet thick near the center of the county but gradually pinch out toward the northeast and southwest against the highs of the Precambrian bedrock surface. Glacial drift overlies the Precambrian and Cretaceous rocks and forms the surface of the area. The drift consists largely of till and ranges in thickness from about 10 feet in the north and northeast to approximately 550 feet in the southwest. The most prominent surflcial glacial deposits are five southeast-trending end moraines, two of which are associated with, and parallel to, relatively extensive belts of outwash. Recent deposits averaging less than 20 feet in thickness overlie the glacial drift in stream valleys. The principal aquifers in Lyon County are glacial-melt-water deposits of sand and gravel, and sandstone of Cretaceous age. The underlying Precambrian rocks and the Recent alluvium are of only local importance as water sources. Melt-water deposits composed of stratified clay and silt as well as sand and gravel occur in channels having surficial expression, in buried channels having no direct surface expression, and as small isolated bodies within the till. Well logs of test holes show that the buried melt-water channels are generally parallel to the surflcial channels. However, melt-water deposits are not necessarily confined to the area beneath the surflcial channel but may extend laterally 1 mile or more beyond its limits. Sand and gravel are commonly interbedded with other melt-water materials. They range in thickness from 10 to 75 feet, are usually less than 1 mile in width, and may be as much as 8 miles in length. Although these aquifers are extensive, they underlie less than 10 percent of the county area. In most places, water from the drift is obtained from small isolated bodies of sand and gravel within the till. Water in the glacial drift is usually very hard (more than 500 parts per million) and low in chlorides (less than 50 parts per million). Drift wells generally yield from 2 to 30 gallons per minute; however, in areas where wells tap melt-water-channel deposits, sustained yields of as much as 500 gallons per minute are obtained. The sandstone beds of Cretaceous age occur between the Precambrian bedrock surface and an altitude of 825 feet as a basal sandstone, between altitudes of 890 and 1.020 feet, and between altitudes of 1,050 and 1,160 feet. Water-well data, supplemented by test drilling, show that each of these stratigraphic intervals is developed only in the county. The sandstone beds are of low permeability and are usually less than 2 feet thick, but they may be more than 20 feet thick in places. Water from these aquifers range in quality from soft (less than 60 parts per million) to very hard (more than 500 parts per million) and may contain excessive amounts of chloride (500 to 2,000 parts per million). Wells tapping rocks of Cretaceous age usually yield 2 to 7 gallons per minute, but in areas where the sandstone beds are thicker or hydrologically interrelated with aquifers of other geologic units, yields of as much as 75 gallons per minute have been obtained. Large quantities of ground water are available from melt-water channels in the county. Moderate quantities, adequate for domestic and small industrial needs, are available from many of the small isolated deposits of sand and gravel in the till. Small quantities of ground water, adequate only for domestic supply, generally can be obtained from Cretaceous sandstone.