Laserfiche WebLink
Table 6-1 Generalized Geologic Section' <br />Geologic Age <br />Formation Name (Member) <br />Approx. Thickness <br />Quaternary <br />Unconsolidated alluvium & eolian deposits <br />0-75 feet <br />Tertiary <br />(1-47 million years) <br />Volcanics - Basaltic extrusives <br />0-110 feet <br />Ogallala Formation <br />0-310 feet <br />Volcanics — intrusive rocks <br />Unknown <br />Cretaceous <br />(1-220 million years) <br />Greenhorn Limestone <br />0-120 feet <br />Graneros Shale <br />0-150 feet <br />Dakota Sandstone <br />0-220 feet <br />Kiowa Shale <br />0-100 feet <br />Cheyenne Sandstone <br />0-250 feet <br />Jurassic <br />Entrada Sandstone and Morrison Formation <br />0-500 feet <br />Triassic <br />Dockum Group <br />300-570 feet <br />Permian <br />undifferentiated <br />unknown <br />Table 6-1 is a generalized geologic section with approximate thicknesses of formations <br />in the study area. Note that all Jurassic and younger aged rocks are not continuous <br />across this large area. This has resulted in a truncation, or pinch -out of strata, generally <br />toward the southeast. <br />6.3 HYDROSTRATIGRAPHIC UNITS <br />6.3.1 General <br />Previous studies (McLaughlin, 1954, Beck, 1967, and Richards, 1968) have described <br />three primary aquifers in the District being the High Plains (which includes the Ogallala <br />and Quaternary alluvium), Dakota and Cheyenne formations. Subsequent ground -water <br />development and monitoring of water levels in southeastern Colorado and western <br />Kansas indicate more vertical leakance between these formations (or hydrostratigraphic <br />units) than indicated in these prior studies. Map 3, in the Map Section, is a composite <br />potentiometric surface of the District. This map, in combination with the saturated <br />thickness maps and the cross-sections, clearly shows that the water table is not confined <br />to any specific aquifers. Map 8 shows the areal distribution of the water -bearing portions <br />of the Dakota and Cheyenne members of the Dakota Aquifer, and of the Dockum <br />Aquifer. Additionally, Map 8 shows the extent of the Kiowa Shale which functions as a <br />leaky aquitard between the Dakota and Cheyenne where present. Thus, there appears <br />to be one aquifer system in the area, but for practical administrative purposes we <br />recommend referencing three principal aquifers: High Plains, Dakota, and Dockum. <br />The Dakota Aquifer is herein described as having upper and lower members that <br />' Modified from Beck, 1967 <br />VI 6 <br />99-028.003\Phase 2 Report\Hydrogeology <br />