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6.0 HYDROGEOLOGY 6.1 STRUCTURAL GEOLOGY In general, the prevailing easterly slope of the topography in the Basin reflects the dominant geologic structure in the study area. Minor tectonic influences include the Las Animas Arch to the northeast and the Apishapa Uplift to the southwest, which both created the east -southeasterly dipping strata. The local structure is complicated at Two Buttes, which is a Tertiary volcanic intrusion that formed prominent resistant stocks and uplifted the overlying sediments. Thus, at Two Buttes Reservoir, erosion has exposed the underlying Triassic Dockum and Jurassic Morrison formations, which locally dip in all directions, away from the intruded volcanics. 6.2 STRATIGRAPHY 6.2.1 General The subsurface stratigraphy of the Basin is variable and complicated by inconsistent nomenclature. The generalized geologic section is shown in Table 6-1. Generally, the rocks that outcrop west of the study area have slightly different terminology than the deeper strata of southeastern Colorado, which also differs from Kansas subsurface terminology. Inconsistent terminology between stratigraphic and hydrostratigraphic units further complicates this regional study. Table 6-2 correlates the rock stratigraphic units of Colorado and Kansas with the hydrostratigraphic units of western Kansas. This study refers to the Kiowa Shale and Cheyenne Sandstone as separate formations. Some experts refer to the collective unit of the Kiowa Shale and Cheyenne Sandstone as the Purgatoire Formation. The lower Cretaceous -aged Dakota, Kiowa and Cheyenne formations each contain gray, interbedded sandstones, siltstones and shales, which are difficult to distinguish in cuttings for a driller and/or geologist. The top of the Kiowa Shale may be obvious when under sandstones of the Dakota, but drill cuttings from the hard, thick black shale may prevent the driller from accurately picking the top of the Cheyenne. The typically vague lithologic descriptions available from the SEO further complicate the subsurface information available for this study. The result is generalized geologic interpretations that are somewhat inferred. A description of the stratigraphy in the Basin follows in ascending order of strata occurrence. 6.2.2 Permian and Older Rocks Undifferentiated Permian -age rocks probably underlie the entire Basin. The upper part of the Taloga Formation (or "Big Basin" in Kansas) may outcrop along Two Buttes Creek near the Prowers and Baca county line. These red beds are thin interbedded shale, siltstone and sandstone. They generally yield small quantities of poor -quality water in Kansas and VI 1 99-028.003\Phase 2 Report\Hydrogeology