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low <br />2.04.6 GEOLOGIC DESCRIPTION <br />The area of mining activity is located in the northeast portion of the Denver Basin. <br />Consolidated and unconsolidated sediments up to 14,000 feet are found in the deepest <br />portions of Denver Basin. These deposits range in age from Cambrian to Holocene and <br />are underlain by Precambrian igneous and metamorphic rocks. With relation to the <br />existence of coal in the exploration area, only the Cretaceous Laramie Formation and <br />adjoining formations are described in more detail below. <br />The Laramie Formation typically consists of interbedded brackish water and fluvial beds of <br />shale, clays, clay -shales, claystone, coal and sandstone. In the Denver Basin, the Laramie <br />is often divided into two members. The lower part consists of shale, claystone, coal and <br />lenticular channel sandstones, approximately 280 feet thick in the proposed extension area. <br />The top of the lower member is defined as the top of the upper most coal bed. The upper <br />member of the Laramie Formation is 60 - 180 feet thick in the mine area and consists <br />primarily of shale, clay -shales, claystone, mudstone concretions and siltstone with minor <br />amounts of sandstone. Portions of this upper member are characteristically weathered and <br />overlain by 5-60 feet of unconsolidated eolian sand. Geologic cross-sections are <br />presented in Appendix F -2 - <br />The clays and clay -shales found in the Laramie Formation can be defined as uncemented <br />or weakly cemented, over consolidated clays generally exhibiting the following properties: <br />• 1. low strength. <br />2. has swelling and shrinking potential with increases and decreased in moisture content. <br />3. has a tendency to "slake" (decompose) when exposed to air and water. <br />4. has a high compressibility under intense loading. <br />5. is prone to differential weathering. <br />The properties of clay shales in the mining area can be largely attributed to their geologic <br />history. Clay mineral content, lithologic variations and bedding are primarily linked to <br />their depositional history, while secondary features such as jointing, slickened sides, <br />fissures and faulting are mainly a result of the stress history. Chemical and physical <br />alteration due to exposure and weathering tend to modify these primary and secondary <br />features, creating a zone underlying the eolian sand deposits that has variable engineering <br />properties. <br />From the analysis of core retrieved in the area, occasional zones (usually 2 in. - 6 in <br />thickness) of stained/altered clays were discovered at depth. These fire probably remnants <br />of materials that were, at one time, exposed at the surface. Such layers would be potential <br />45 5/96 <br />