GENERAL34195

• Permit No: C1981071
• IBM Index Class Name: General Documents
• Doc Date: 10/26/1987
• Doc Name: PROPOSED DECISION & FINDINGS OF COMPLIANCE FOR RN1
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

The lower Iles Formation is appro:~imatel;~ 1,50 `eu*, t'~i~.'; ani lies
conformably on the Mancos Shale, It consists o. intaroedced light-Brown to
will te, massive, fine-grained, ledge-forming sands *_o ne s, brown to black
carbonaceous shale, sandy-shale and coal beds. The coal beds are part of the
Lower Coal Group (Figure 4) of the Mesaverde Group, end are distributed
throughout the middle and upper parts of the formation. Four persistent
sandstone beds occur within the Iles Formation, In order of decreasing age
they are: 1) the basal Tow Creek Sandstone Member; 2) a double ledge-forming
sandstone sequence approximately 400 feet above the base of the formation; 3)
a light-gray sandstone sequence of variable composition associated with the
No. 3 coals approximately 900 to 1,000 feet above the base of the formation;
and, 4) the Trout Creek Sandstone Member at the top of the formation
(Figure 4), The Trout Creek Sandstone is a 50 to 100 foot thick, light-brown
to light-gray, fine-grained, massive sandstone.
The upper unit of the Mesaverde Group is the 1,000 to 2,000 foot thick
Williams Fork Formation. The Williams Fork Formation lies conformably on the
Trout Creek Sandstone and grades into the overlying Lewis Shale. The
formation consists of a lower, a middle and an upper unit.
The approximately 1,000 foot thick lower unit consists of shales, thin
sandstone beds, sandy-shale and the coal beds of the ididdle Coal Group. The
Middle Coal Group contains the coal seams of economic importance in the
Twentymile Park mines. In ascending stratigraphic order, they are the Wolf
Creek, Wedge and Lennox coal seams. The middle unit of the Williams Fork
Formation includes the Twentymile Sandstone Member a massive, white,
cross-bedded, cliff-forming sandstone approximately 100 to 200 feet thick
(Figure 4), It is a key marker bed throughout the region. The upper unit of
the Williams Fork Formation consists of interbedded sandstones, sandy-shales,
shales, and several thin coal beds of the Upper Coal Group, including the Fish
Creek seam.
The Lewis Shale (Figure 4), is a 1,500 to 2,000 foot thick sequence of
dark-gray to bluish, homogenous marine shale containing thin interbedded
sandstones and calcareous concretions which conformably overlies the Mesaverde
Group.
Unconsolidated alluvial deposits of Quaternary age are the youngest rock units
in the area and are generally less than 30 feet thick. The maximum thickness
of these unconsolidated deposits is estimated to be less than 100 feet along
the Yampa River (Brodgen and Giles, 1981). Alluvial deposits are found most
extensively along the Yampa River, Trout, Middle, Foidel and Fish Creeks and
consist of clay, sand and lenticular, discontinuous gravel layers. The
alluvium is thin or absent in areas where streams cross the resistant
sandstones of the Mesaverde Group. The alluvial deposits are areally
extensive where the streams cross the less-resistant Lewis and Mancos Shales.
14inor colluvial deposits are present locally at the base of the cliff-forming
sandstones.
Mining in Mine 1 and 2 and Eckman Park has replaced consolidated sedimentary
strata with unconsolidated spoils material. Strata above the Fish Creek coal
seam has been disturbed at Mine 2 •while strata above the 4Jadge coal have been
?n