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other Cretaceous strata to the southeast and varies from 40 to 100 feet across the rest <br />of the study area. It may be thinner or discontinuous in the subsurface. <br />6.2.7 Dakota Sandstone <br />The Dakota Sandstone is a series of sandstones and sandy shales present in all except <br />the southeastern part of the Basin and adjacent to the Two Butte intrusive, where it was <br />uplifted and removed by erosion. It outcrops north and west of the Basin and in the <br />deeper incised drainages. It is estimated to be between 85 and 220 feet thick, with cliff - <br />forming ledges up to 50 feet thick exposed in the southwestern canyons. The Dakota <br />bedding character varies from thin -bedded to cross -bedded, to massive and from fine- <br />grained quartzites to coarse-grained sand/gravel lenses. <br />The color is usually buff to dark brown, but can be dark red -brown with ironstone <br />concretions, and dark -gray to dark -brown where lignitic or coaly beds are present. The <br />Dakota, while thinner, generally has more shaley or silty interbeds, and has more <br />gravels resulting in an overall higher hydraulic conductivity than the Cheyenne <br />Sandstone. <br />6.2.8 Graneros Shale <br />The Graneros Shale is the lowest member of the Colorado Group. The Graneros and <br />Greenhorn are the only significant members of the Colorado Group present in the Basin. <br />Some Carlile Shale may be present in the Basin, but this has not been confirmed. The <br />overlying Niobrara Chalk and Pierre Shale formations are not present in the Basin. As <br />shown in Table 6-1, the Colorado Group includes, in ascending order, the Graneros <br />Shale and Greenhorn Limestone. North of the Basin, it also includes the Carlile Shale, <br />Niobrara Chalk and Pierre Shale. The Graneros is present only in the northern part of <br />the Basin where it may be up to 150 feet thick. It is shown on the northern part of cross- <br />sections B -B', C -C', D -D' and E -E', and pinches out to the southeast. The Graneros is <br />exposed in the Bear Creek tributaries west of Springfield and in northwest Baca County, <br />just northwest of the Basin boundaries (i.e., where the drainages are tributary to the <br />Arkansas River Basin). <br />6.2.9 Greenhorn Limestone <br />The Greenhorn Limestone is mapped above the Graneros in the Bear Creek and Plum <br />Creek drainages in the northwestern part of the Basin. The Greenhorn is a light- to dark - <br />gray, thin -bedded, chalky limestone with shale interbeds. Both the Greenhorn and <br />Graneros pinch out to the southeast (i.e., the beds were eroded prior to the Tertiary <br />deposition of the Ogallala Formation). The Greenhorn was not distinguished or shown in <br />cross-sections as it is thin and shallow in this area. <br />VI 4 <br />99-028.003\Phase 2 Report\Hydrogeology <br />