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Summary and Conclusions
Logan County is divisible into three parts on the basis of geology and ground-water conditions: (1) an upland plain composing about 35 percent of the county, (2) a small area of flood plain along Smoky Hill River and its tributaries, and (3) rolling, dissected "pediment" slopes. The streams have gradients between 14 and 24 feet per mile and drain eastward and southeastward across the county. Lakes in the county are intermittent and are of four types: (1) large, deep depressions originating by subsidence of Cretaceous rocks, (2) small, shallow depressions enlarged by deflation from smaller undrained areas in broad, shallow upland valleys, (3) ponds along intermittent and ephemeral watercourses, and (4) artificial livestock ponds.
The Pierre shale and the shaly chalk of the Niobrara formation, of Gulfian age, exposed along the major valleys are intricately faulted. Features of the jointing and folding associated with the faults aided in the field mapping, but no definite conclusion as to origin of the faults was reached.
The Ogallala formation underlies the upland plain and was deposited on an erosional surface developed in pre-Pliocene time on the Gulfian (Cretaceous) rocks. The surface seems to be influenced in part by regional structure, of which the chief feature is the gentle northeast dip of the east flank of the Las Animas Arch. The drainage on the pre-Ogallala surface trends north and northeast in the northern upland and east and southeast in the southern upland. The principal valleys contain the thickest sediments of the Ogallala formation, which is chiefly calcareous silty sandstone and thin persistent shale layers and which includes much sand and gravel, particularly in the deeper valleys in the bedrock. The upland plains and "pediment" slopes are mantled by Pleistocene deposits, which are chiefly loess in the uplands, underlain by coarse quartzose sand and gravel at the edge of the "pediment".
One of the most important mineral resources of Logan County is ground water. The Ogallala formation beneath the large northern upland plain yields moderate to abundant supplies of hard calcium magnesium bicarbonate water to domestic, livestock, public, and irrigation wells. The large wells for public supply and irrigation are mainly in the troughs of the pre-Pliocene valleys where saturated water-bearing materials are thickest. The permeability of the formation as determined by four aquifer tests ranged from 230 to 1,150 gpd per square foot. Contours drawn to represent the upper surface of the ground water in the Ogallala formation reveal general eastward movement of the water at an average gradient of about 10 feet per mile. The configuration of the water table is in part dependent upon the configuration of the underlying pre-Pliocene surface, which is relatively impervious and confines the ground water to the permeable beds of the Ogallala. The influence of soluble materials in pre-Pliocene rocks is reflected in the chemical character of the ground water, which shows an inverse relation between amount of dissolved solids and saturated thickness.
The area of rolling dissected "pediment" slopes from which the Ogallala formation has been stripped by erosion is not underlain by a continuous sheet of water-bearing material at shallow depths. The wells obtain hard and mostly unpalatable water from Pleistocene sand and gravel in small disconnected areas of valley fill and in "pediment" deposits. Where saturated Pleistocene deposits are thin or absent, a few wells are drilled into the Niobrara formation to obtain livestock water where the shaly chalk is sufficiently fractured to yield water to wells. Wells drilled into the fractured zone adjacent to faults in the Niobrara formation or into the Dakota formation probably will yield water of satisfactory quality for livestock in those localities to which water is now being hauled from Winona and Oakley. Alluvium and Pleistocene deposits beneath flood plains and low terraces yield small to abundant amounts of hard and in many places strongly mineralized water to domestic, livestock, and irrigation wells.
The chemistry of the ground waters in Logan County is related to geologic source. Five chemical types of water occur in the area: (1) water from the Ogallala formation, having a high percentage of Ca and Mg and bicarbonate, (2) water from the Niobrara formation, high in Ca and Mg and low in CO3 and HCO3, (3) water from the Pierre shale, having about 50 percent Ca and Mg and 50 percent CO3 and HCO3, (4) water from the Codell sandstone zone of the Carlile shale, having a low percentage of Ca and Mg and CO3 and HCO3, and (5) soft water from the Dakota formation, low in Ca and Mg and high in CO3 and HCO3. The water from the Ogallala is low in sodium and other dissolved solids and is satisfactory for irrigation unless admixed with large quantities of water of other type. Analyses of water from the base flow of streams show a close relation of water quality to geologic source.
The principal development of ground water in Logan County at the present time is for livestock and domestic use because wheat farming, cattle-raising, and general farming rather than irrigation are the dominant economic activities. The development of ground water for irrigation on the upland probably exceeds 600 acre-feet pumped by five wells irrigating a total of about 1,000 acres annually. At least 60,000 acres of irrigable land is underlain by water-bearing material, and the utilization of the ground-water for irrigation probably will increase greatly in the future.
The amount of future development will be limited by the quantity of water that can be pumped from storage without imperiling the ground-water supply of livestock and domestic water wells and by the average annual rate of recharge to the Ogallala formation. In the northern upland area the annual recharge available for irrigation from deep wells is approximately the annual volume of flow passing eastward through the aquifers near the east edge of the county. The permeability of the Ogallala was estimated as 440 gpd per square foot, the average gradient of the water table shown on cross section A-A' was 10 feet per mile, and the cross-sectional area of the water-bearing materials was about 2,750,000 square feet. The volume of annual recharge available for irrigation, as computed from these data, was somewhat less than 3,000 acre-feet.
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Kansas Geological Survey, Geology
Placed on web Dec. 14, 2007; originally published May, 1958.
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