1999-10-15_GENERAL DOCUMENTS - C1994082

• Permit No: C1994082
• IBM Index Class Name: General Documents
• Doc Date: 10/15/1999
• Doc Name: PROPOSED DECISION AND FINDINGS OF COMPLIANCE FOR ‎(PR1)‎
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

characterized by broad west and northeast facing moderately sloping benches and ridges that are
deeply incised, and barren, brush covered, southwestward facing cliffs where the more resistant
sandstone outcrops. The long, sloping landforms of these areas roughly coincide with the strike and
dip of the underlying bedrock.
The proposed permit area is dissected by several small, ephemeral drainages which flow into two
principal stream systems; Sage Creek on the southwest, Grassy Creek on the southeast. A detailed
discussion of the Yoast surface water system is presented in Tabs 7 and 16 of the permit application
package.
The Yoast permit area is in the Williams Fork Mountains, within the southeastern synclinal portion
of the Sand Wash structural Basin of northern Colorado and central Wyoming. The stratigraphy of
the permit area is illustrated in Figure 2.
The coal seams to be mined are the Wadge and the Wolf Creek. These seams are contained within
the Middle Coal Group, an interval of interbedded sandstones, sandy shales, shales and coal within
the Williams Fork Formation. The Middle Coal Group also contains the Lennox Coal seam which
is non-commercially mined in the Yoast mine. Above the Middle Coal Group is the Twentymile
Sandstone, a 100-200 foot thick fine-grained, massive sandstone. The upper unit of the Williams
Fork Formation, which contains the Upper Coal Group of the Mesaverde group, consists of layers
of sandstone, sandy shale, shale, and coal. The thickness of this unit varies from approximately 200
to 850 feet throughout the region. The Williams Fork comprises the upper 200 to 850 feet of the
Cretaceous Mesaverde Group.
Underlying the Williams Fork Formation is the Iles Formation which is the lower member of the
Mesaverde and the oldest unit to out crop in the permit area. The Iles Formation is approximately
1500 feet thick. The basal unit of the Iles Formation is the Tow Creek Sandstone, a 35-125 foot
thick massive, fine-grained,well-sorted sandstone. Overlying the Tow Creek Sandstone is a 1200
foot thick interval of interbedded shales, sandstones and coals known as the lower coal group. The
Iles Formation is crowned by the Trout Creek Sandstone,which is a massive fine-grained sandstone
with a thickness of approximately 100 feet.
The Late Cretaceous Mancos Shale underlies the Iles Formation and outcrops southwest of the
permit boundary along Sage Creek and the Sage Creek Anticline. The Mancos Shale is a dark grey
marine shale with interbedded sandstones near the top and interbedded sandstone and limestone near
the bottom.
Overlying the Mesaverde Group is the Lewis Shale, a homogeneous marine shale with a thickness
of between 1000 and 1500 feet. The Lewis Shale is overlain by the Cretaceous Lance Formation and
the Tertiary Fort Union Formation. The Wasatch Formation of Eocene age unconformably overlies
the Fort Union Formation. It is comprised primarily of fresh water stream-laid sandstone and shale
deposits. Unconsolidated alluvial, terrace, and surficial deposits of Quaternary age overlie the
exposed geologic formations in some areas, particularly along larger streams.
Yoast Mine 9 October 19, 1999