Laserfiche WebLink
SediGn - 2.04.6 <br />underlying Mancos Shale and is relatively continuous throughout the area, serving as a common <br />• marker bed. <br />Mancos Shale (Cretaceous) - The Mancos Shale is a regionally extensive bed of marine shales <br />ranging up to 4,000 feet in thickness. In the mine permit and immediately adjacent areas, it underlies <br />the exposed geologic sequence. West of the Town of Somerset, however, the North Fork of the <br />Gunnison River has cut through the upper portion of the Mancos Shale, exposing the gray marine <br />shales and associated thin inter-bedded sandstones and resulting in steep, highly erosive slopes that <br />are capped by the overlying Rollins Sandstone. <br />Igneous Intrwives - A diorite plug approximately 1,000 feet across is exposed in the Hubbard Creek <br />drainage and sills of similar dioritic material were encountered in the Bowie Member during <br />exploration drilling near Terror Creek These intrusive remnants indicate a volcanic flow feeder <br />system and additional intrusive remnants may be encountered in other areas. <br />Structure <br />Structural contours developed from exploration drilling of the main A, B, C, D, and E coal seams <br />indicate an average strike in the sedimentary units in this area of approximately N 66° W and a dip of <br />approximately 3.5 degrees to the northeast. While minor offsets have been encountered in the coal <br />seams during mining operations, surficial indications, drilling information, and mining experience <br />indicate no significant faults or other major structural features in the mine permit and adjacent areas. <br />Slumps and surface landslides are relatively common in the area and have been noted to occur <br />primarily where the D Coal Seam outcrops in the ste7 tributary drainage valleys as indicated by the <br />Regional Geology/Hydrology Map, (Map 2.04-W). This occurrence is believed to be due to a <br />combination of steep terrain, wetting along potential failure planes where ground water within the <br />• coal seam migrates to the surface, and overlying low-strength rock units and surficial materials. <br />Characterization of Overburden. Ground Water Occurrence. Surficial Materials. Roof and <br />Floor Materials and Coal Seams <br />As previously described in the discussion of stratigraphy, the overburden above and interburden <br />between the mineable coal seams consists of inter-bedded sandstones, shales and siltstones varying in <br />thickness from 0 feet at the coal seam outcrops to over 2,500 feet at the northern mining limits. <br />Ground water occurrence is limited by the rugged terrain, deeply incised drainages, and stratigraphy to <br />the alluviu m/coUuvium associated with the North Fork of the Gunnison River and lower portions of <br />the larger tributary drainages; isolated perched ground water occurrences; stored mine water, and local <br />or regional water table (unconfined or partially confined) occurrences associated with the higher <br />permeability coal seams and Rollins Sandstone unit. Additional detail on ground water occurrence <br />and characteristics is provided in Section 2.04.7, Hydrology Information. As an underground mining <br />operation, OMI's current and future surface activities will be limited to coal handling, equipment <br />maintenance, and support, with minirnal associated surface disturbance. <br />Surficial Materials - Given practical constraints imposed by the rugged topography, thine-related <br />surface facilities are generally limited to low-lying valley and terrace areas. With a long history of <br />mining operations in the area, many of the low-lying areas have been previously disturbed by mining, <br />trine support, or related transportation and residential development. In many cases the exposed <br />surficial materials in these areas consist of a mixture of soil, unconsolidated alluvial/colluvial <br />materials, and coal refuse up to 50 feet in thickness, overlying shale or sandstone bedrock The <br />following describes general surficial material characteristics for the principal mine disturbance areas: <br />. Elk Creek Facilities Area - The Elk Creek Facilities area was previously disturbed by surface support <br />activities associated with the Somerset Mme. Surficial materials in this area consist of a mixture of <br />disturbed sods, alluvial/colluvial materials, and coal refuse up to 20 feet thick over relatively thick <br />unconsolidated alluvial/colluvial materials. Both disturbed and undisturbed surficial materials in the <br />PR04 2.04-11 Revised August 2000