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which form the Twentymile Park basin. They cause the strata to dip in excess of 15 percent near the
<br />periphery of the basin and form the outer boundaries of the mine area. The coal measures are also disturbed
<br />• by northwest striking normal faults. Detailed structure and Wadge Seam outcrop are shown on Map 6,
<br />Wadge Seam Structure. Additional structural information, as well as lithologic information are shown on
<br />Maps 7, 8, 9, 9A, and Figure IA Geologic Cross-Sections A-A', B-B', C-C', D-D', and E-E'.
<br />Overburden depths in the planned permit area range from "outcrop" near the portal area, to approximately
<br />1,700 feet in the central portion of the basin. This information is presented on Map 10, Wadge Seam
<br />Overburden.
<br />Stratigranhy & Physical Characteristics of the Coal Seam and Other Strata in the Area to be Mined
<br />Generalized stratigraphy of the area is shown on Figure 2, General Stratigraphic column. Detailed site-
<br />specific stratigraphy is shown on Map 6, Map 7, Geologic Cross-Sections A-A', Map 8, Geologic Cross-
<br />Sections B-B', Map 9, Geologic Cross-Sections C-C', Map 9A, Geologic Cross-Sections E-E', and Figure
<br />I A, Geologic Cross-Sections D-D' respectively. It should be noted that the faults shown on the maps are of
<br />a regional character and are outside of the PR 99-OS mine plan azea. Map 23, Mine Plan, presents the faults
<br />that have been encountered in the mine. The coal to be mined by the planned underground operations is [he
<br />Wadge Seam. The Wadge Seam lies within a sequence of sedimentary rocks characterized by shales,
<br />claystones, mudstones, siltstones, sandstones, and minor coal stringers in the Upper Cretaceous Mesaverde
<br />Group. This sedimentary sequence is about 75 million years old and is composed of terrigenous elastics
<br />deposited in offshore, shallow, and near-marine environments at the western edge of the epieric seaway.
<br />This seaway was located in interior western North America during the later part of the Early Cretaceous,
<br />and most of the Late Cretaceous, time.
<br />The sandstones were formed in beach environments and are generally fine to fine-grained, of low porosity,
<br />• and are well-cemented. The very thick shale formations, such as the 650-foot shale member below the
<br />Twentymile Sandstone, are offshore marine deposits and are characterized by their lateral continuity,
<br />extremely fine-grained nature, and extremely low horizontal and vertical permeability.
<br />The youngest rocks exposed in the planned mine area belong to the massive marine-deposited Lewis Shale
<br />Formation, which varies from zero to 700 feet in thickness. Near the base of the Lewis Shale is the Fish
<br />Creek Seam, approximately four feet thick. Approximately 200 feet of interbedded shale, siltstone, and fine-
<br />grained sandstone separate the Fish Creek Seam from the underlying massive Twentymile Sandstone. This
<br />interval is locally referred to as the Holderness Member.
<br />The Twentymile Sandstone is a key mapping unit and can be identified and traced throughout most of the
<br />Yampa Coal Field. The rock immediately below the Twentymile Sandstoneis another massive marine shale
<br />member: a "tongue" of [he Lewis Shale (Fig. 2j. The thickness averages approximately 650 fee[.
<br />The Fish Creek Seam, Holderness Member, the Twentymile Sandstone, and the [hick marine shale member
<br />make up the majority of the Williams Fork Formation. The lower portion of the Williams Fork Formation
<br />consists of the coal-bearing sequence. This sequence is the "Middle Coal Group" of the Mesaverde Group.
<br />The "Middle Coal Group" contains the Lennox Seam, Wadge Seam, and the Wolf Creek Seams.
<br />The Lennox Seam is located 10 to I S feet below the marine shale member, and ranges in thickness from zero
<br />to four feet. The low seam height, poor quality, and poor lateral continuity of the Lennox seam render it
<br />unminable. In the proposed mine area, the strata below the Lennox Seam consists mainly of stacked deltaic
<br />sequences. Four distinct sequences have been mapped through the mine area. They are characterized by a
<br />rnarsrniog of .mrao gfaio _cizr ilnwarils within rash srgyenca. The JithoJggios csmsist of mlidstsuaes and
<br />• claystones, siltstones, and very fine-grained interbedded sandstones. The combined thickness of these
<br />sequences range from sixty to ninety feet. These rocks directly overlie the Wadge Seam, which is the target
<br />of the planned underground operation. ~,{,,,,,~,( ~ a ) /d P / ae
<br />APPR®lJED JUN 2 8 2000 ~`~" `~ ~M
<br />PR 99-OS 2.04-14 5/3/00
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