2012-01-06_PERMIT FILE - C1982057 (16)

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Geology. A detailed discussion of the Seneca II Mine geology is presented in Tab G of this PAP. Appropriate sections are summarized in the following text. • The study area structurally lies in the southeastern extension of the Tow Creek Arctir:line, a south-southwest trending geologic structure with steeply dipping flanks (Bass et al., 19551 The coal reserves of the study area occur in the Middle Coal group between the Trout Creek sandstone member of the Cretaceous Iles formation and the Twentymile sandstone member of the Williams Fork formation. The Iles formation contains a thick sequence of ledge-forming sandstones interbedded with sanity shale, shale, and coal beds, with a prominent ledge- forming sandstone named the Trout Creek sandstone member. Prominent outcrops of the ledge-forming Trout Creek sandstone member are exposed just inside the eastern boundary of the study area. The Williams Fork formation is similar in characteristics and bedding attitude to the Iles formation with a persistent cliff-forming white sandstone named the Twentymile sandstone member. The Twentymile sandstone member outcrops immediately west oT Hubberson Gulch about one-half rnile west of the study boundary. The resistant sandstone beds consist chiefly of well sorted, fine grain quartz minerals with admixtures of fine feldspathic minerals and some clay. Together the Williams Fork and Iles formations comprise the Mesaverde Group. The Mesaverde Group forms steep slopes and cliffs with bare ledges of sandstone. Long dip slopes are formed in many areas by interbedded thin soft sandstones and sandy shales. The steeper slopes are generally covered by thick deposits of fine textured colluvium. Areas of soft • shale generally form gently rounded grass-covered ridges and knobs. The Cretaceous Lewis shale outcrops northwest of the mine study area and occurs in the Tie- Across Haul Road area where it forms broad undulating ridge and slope topography. In the survey area, the Lewis shale is composed of tan [o brown clay shales. There are few exposures as much of the terrain has been covered by Pleistocene loess. The lower Cretaceous Mancos shale outcrops in one small area along [he northeast survey boundary. It occurs as a steep, nearly barren clay shale slope. Geomorphology The study area is in an area of high relief eroded in Cretaceous sedimentary rocks (Bass et al., 19551 The center of the study area (the mine site) is located about eight miles south of Hayden, Colorado. The study area is drained by two principal stream systems, Hubberson Gulch and Dry Creek on the west, and Sage Creek along the eastern border. Both are tributaries to the Yampa River and join the river about eight miles north of the mine permit area. Hubberson Gulch has laid down recent stratified alluvial deposits in a narrow floodplain between 100 and 400 feet wide. • 4 Revised 9/98