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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />7.0 HYDROGEOLOGY <br /> <br />7.1 STRUCTURAL GEOLOGY <br />In general, the prevailing easterly slope of the topography in the Basin reflects the <br />dominant geologic structure in the study area. Minor tectonic influences include the Las <br />Animas Arch to the northeast and the Apishapa Uplift to the southwest, which both <br />created the east-southeasterly dipping strata toward the Hugoton embayment. The local <br />structure is complicated at Two Buttes, which is a Tertiary volcanic intrusion that formed <br />prominent resistant stocks and uplifted the overlying sediments. Thus, at Two Buttes <br />Reservoir, erosion has exposed the underlying Triassic Dockum and Jurassic Morrison <br />formations, which locally dip in all directions, away from the intruded volcanics. <br /> <br />7.2 STRATIGRAPHY <br />7.2.1 General <br />The subsurface stratigraphy of the Basin is variable and complicated by inconsistent <br />nomenclature. The generalized geologic section is shown in Table 7-1. Generally, the <br />rocks that outcrop west of the study area have slightly different terminology than the <br />deeper strata of southeastern Colorado, which also differs from Kansas subsurface <br />terminology. Alternating verbage between stratigraphic and hydrostratigraphic units <br />further complicates this regional study. Table 7-2 correlates the rock stratigraphic units <br />of Colorado and Kansas with the hydrostratigraphic units of western Kansas. <br /> <br />This study refers to the Kiowa Shale and Cheyenne Sandstone as separate formations <br />where they are distinguishable most commonly in outcrop, and as the Purgatoire <br />Formation where they are indistinguishable more common to the subsurface. Some <br />experts may also refer to the collective unit of the Kiowa Shale and Cheyenne <br />Sandstone as the Purgatoire Formation. The lower Cretaceous-aged Dakota, Kiowa <br />and Cheyenne formations each contain gray, interbedded sandstones, siltstones and <br />shales, which are difficult to distinguish in cuttings for a driller and/or geologist. These <br />vague lithologic descriptions further complicate the subsurface information available for <br />this study. The result is generalized geologic interpretations that are somewhat inferred. <br />A description of the stratigraphy in the Basin follows in ascending order of strata <br />occurrence. <br /> <br />7.2.2 Permian and Older Rocks Undifferentiated <br />Permian-age rocks probably underlie all of the Basin. The upper part of the Taloga <br />Formation (or "Big Basin" in Kansas) may outcrop along Two Buttes Creek near the <br />prowers and Baca county line. These red beds are thin interbedded shale, siltstone and <br />sandstone. They generally yield small quantities of poor-quality water in Kansas and <br /> <br />VII-1 <br />