2010-12-17_REVISION - C1981019 (153)

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Geologic setting 9 In general, the site stratigraphy consists of: • Alluvium, colluvium, and landslide deposits in the valley bottoms (hereafter referred to as "valley fill "); • a thin veneer of soil overlying the bedrock formations; up to 1,000 ft of thin, interbedded deposits of sandstone, coal, and shale/ mudstone of the Williams Fork Formation; and • the Trout Creek Sandstone Member of the Iles Formation (Trout Creek Sandstone). Valley fill in the Collom study area is restricted to the incised creek drainages. The thickness of the valley fill is variable but appears to increase with distance down drainage. Thickness ranged from 40 to greater than 70 ft in those areas where valley fill wells were drilled. The valley fill is Quaternary in age and consists of a heterogeneous mixture of silty clay with sand and gravel, and sandy /silty gravel. Other Quaternary deposits include fan and landslide deposits. The fan deposits consist of clay- to boulder - sized alluvial and colluvial sediments located where tributary drainages of steeper gradient join lower gradient streams. The streams also contain occasional landslide deposits composed of heterogeneous mixtures of angular rock fragments generated by small slumps off the valley walls (Reheis, 1981). Various lithologic units of the Williams Fork Formation outcrop at the site, except in the stream beds, where they are covered with valley fill. In many areas, these units have weathered to form a soil layer with a maximum thickness of about 3 ft. At the Collom site, the Williams Fork Formation contains 18 different coal groups (A through O, TC, X, and Y) but includes up to 77 individual seams. Coal beds are separated by discontinuous sandstone units, which are prefixed with the letter S followed by the adjacent underlying coal seam (i.e., SE2: sandstone above the E2 Coal Seam). Correlation of the coal seams was facilitated by the presence of two distinct, laterally persistent, "marker beds ": the KM volcanic ash bed and the Trout Creek Sandstone. As shown in Figure 2.1, the KM marker bed is well below the proposed bottom of the Collom Mine pit. In addition to its use as an excellent datum for correlation study work, the KM bed has potential hydrogeologic impacts. Within the Collom study area, it was easily distinguishable in most drill holes and ranges up to 4 ft thick. Mineralogic and petrographic analysis of three samples indicates that the KM is composed almost exclusively of smectite clay with minor amounts of kaolinite. This material was originally deposited as intermediate to felsic volcanic ash. Laboratory reports for the mineralogic and petrographic analysis are provided in Appendix 2.A. Permeability analyses were also performed on samples of the KM (Section 5.2.1). The most important finding of the analyses is that the KM bed has a very low permeability and will function as an aquitard. Natural burning of the coal seams is evident in many localities within the Collom study area. The locations and extent of the burned horizons were identified during the drilling program performed by the Collom Geologic Team. Hydrogeological characterization of the burn zones was not conducted for the pre - feasibility study. 2572 -R2 Colowyo Coal Company Water Management Consultants