2022-05-10_GENERAL DOCUMENTS - M1993041

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Dowe Flats Quarry Soils and Geology Report 3.2.1.1 Dakota Group The Dakota Group contains the upper beds on the cuestas surrounding Dowe Flats. It has an average thickness in this area of 330 feet and is subdivided into the lower Lytle and upper South Platte formations. The Lytle Formation consists of nonmarine fluvial deposits. The lowest part is a fine to coarse- grained massive brown sandstone intercalated with a basal conglomerate containing quarter to half-inch diameter chert pebbles and granite fragments mixed with finer materials in a secondary silica cement. This bed is approximately 40 feet thick. The upper portion of the Lytle Formation consists of a series of variegated red and yellow claystones that, on exposure, weather to a reddish surface soil. This deposit varies in thickness from 30 to 60 feet. The South Platte Formation deposits are marine and near-marine in origin. The Plainview Member is a platy, fine-grained, hematite-stained quartzose and sandstone. It varies in thickness from 20 to 30 feet through differential incising of the underlying claystones. The middle part of the South Platte Formation is a 125- to 175-foot-thick gray to black carbonaceous shale interbedded with thin, buff-colored siltstones and sandstones. The uppermost part of this formation is the Muddy member, a massive, ridge-forming tan quartzose sandstone 20 to 30 feet thick that is slightly cross-bedded and distinctly jointed. 3.2.1.2 Benton Formation The Benton Formation is a 500-foot-thick layer of fine-grained marine deposits. The lowest part is composed of dark gray to black fossiliferous sandy shales. The middle portion is a zone of light to dark gray argillaceous limestone and dark gray to black calcareous shales. The upper part is calcareous dark gray sandy shales. Numerous thin but laterally continuous bentonite layers are found throughout the formation. The lower, middle, and upper parts are often referred to as the Graneros Shale, the Greenhorn Limestone, and the Carlile Shale, respectively. The Codell Sandstone is the uppermost unit in the Benton Formation. In the Dowe Flats area, it has a 15-foot total thickness and can be divided into a 7-foot-thick gray siltstone beneath an 8- foot-thick silty sandstone. This silty sandstone directly underlies the Niobrara Formation. 3.2.1.3 Niobrara Formation The Niobrara Formation is traditionally separated into two units, the Fort Hays and Smoky Hill members. The Fort Hays Member is an extremely fine-grained, light gray limestone with thin interbedded shales. A section of Fort Hays Member at the south end of Dowe Flats measured 16.5 feet thick (Lowman 1977). Limestone accounted for 13.6 feet or 82% of the outcrop thickness. The limestone is distributed as blocks ranging in thickness from 0.5 to 3.1 feet and vertical joints spaced on 1 to 3-foot centers. The remaining 2.9 feet of material is distributed as 11 thin bentonite layers having an average thickness of 6 inches. Drilling in other areas within the Dowe Flats valley has indicated an average limestone thickness of 20 feet (Mallette 1962). The Fort Hays has been the primary ore mined at Dowe Flats to produce cement; it has averaged about 16 feet thick and dips with the area geology about 20 degrees to the east. The overlying Smoky Hill Member of the Niobrara Formation is generally described as a dark gray, calcareous, fossiliferous marine shale. The three "ridges" mined at Dowe Flats are within the Smoky Hill Member. These units consist of an upper higher alkali/lower lime unit, a middle lower alkali/higher lime unit, and a lower unit that is higher alkali/lower lime. These units along 5 May 2, 2022