1988-04-11_PERMIT FILE - C1981008A (5)

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The principal coal seam being mined is the #2 or lower Dakota coal seam. One localized area of upper Dakota coal (#1 seam), which includes Drill Hole Location 871E, may be mined in a subsequent permit term. These seams lie within the "middle" Dakota lithologic unit described by Young (1973), and are characterized by carbonaceous shale and impure coal with lessor amounts of interbedded sandstone and siltstone. Basically, these coals are high BTU, low sulfur bituminous coals that show good lateral continuity. Each seam shows a well-defined system of vertical to near vertical joints (coal "cleats") at the outcrop. These joints are less evident in core samples indicating a lower porosity than would be expected by outcrop examination. Fractures of this sort are highly anisotropic. The upper Dakota (#1) seam generally ranges from 0.6 to 2.1 feet (partings omitted) in thickness with an average of about 1.2 feet and lies approximately 99 feet from the base of the Dakota. The lower Dakota (#2) seam generally ranges from 4.2 to 6.9 feet (partings omitted) in thickness with an average of about 5.5 feet and lies approximately 86 feet from the base of the Dakota. There is approximately 12.1 feet of interburden separating these seams within the Nucla East permit area. A 0.4 foot laterally discontinuous "rider" coal seam is found about 7.2 feet above the upper Dakota coal seam in some parts of the permit area. In the underburden, two additional coal seams, the Nucla (#3) seam and an unnamed seam, are observable in the Nucla East study area. The Nucla seam typically ranges from 1 .5 to 3.6 feet (partings omitted) in thickness with an average of about 2.5 feet and lies approximately 56 feet from the base of the Dakota. The laterally discontinuous unnamed coal seam generally ranges from 0.5 to 1.6 feet in thickness with an average of about 1.0 feet and lies approximately 24 feet from the base of the Dakota. Sandstone and sandy shale are the predominant lithologies in overburden material within the Nucla East study area. The sandstones are generally fine to medium grained, of low porosity, well cemented, lenticular, discordant, and crossbedded. Most sandstones show a well-defined system of iron stained joints and fractures. Continuous horizontal gypsum lenses occur between bedding planes and are prevalent throughout the study area in the 20 foot thick sandstone strata which lies about 40 to 60 feet above the base of the lower Dakota coal seam (Figure 6-1-1 and Appendix 6-1). The gypsiferous sandstone is situated within a transitional zone between oxidized sulfate-bearing horizons and unoxidized sulfide-bearing, calcareous strata. The "upper" sandstone unit of the Dakota (Young, 1973) is absent within the study area. Two thin, white clay (bentonitic) beds are widely traceable in the western half of the study area. These clays average 0.5 to 2.5 feet in thickness and are found approximately 60 to 70 feet above the base of the lower Dakota coal seam (Figure 6-1-1 and Appendix 6-1). 6-1-8 Revised 04/11/88