The Cordero Rojo Mine extracts thermal coal from the Wyodak Seam, which ranges from approximately 55 to 70 feet in thickness.
General Geology and Coal Resources
Stratigraphic units include, in descending order recent (Quaternary age) alluvial and eolian deposits, the Eocene age Wasatch Formation (the overburden), and the Paleocene age Fort Union Formation (which contains the target coal seam).
Surficial deposits in the general analysis area include alluvial and eolian deposits and weathered Wasatch Formation. Alluvial deposits occupy the Belle Fourche River valley and the lower portions of tributary draws where they join the river.
The Eocene Wasatch Formation forms most of the overburden in the general analysis area. The boundary between the Wasatch Formation and the underlying Paleocene Fort Union Formation is not distinct. From a practical standpoint, the top of the mineable coal zone is considered as the contact between the two formations. The regional dip in this area is to the west; as a result, the overburden thickness is generally thinner to the east and increases to the west.
The Fort Union Formation is divided into three members: the Tongue River, the Lebo, and the Tullock, in descending order.
The mineable coal seams in the Powder River Basin are part of the Tongue River Member of the Fort Union. At the Cordero Rojo Mine there is one mineable coal seam. Locally, this coal zone is referred to as eith ........
