The Falkirk Mining Company (Falkirk), a wholly-owned subsidiary of North American Coal (NACoal), which is a wholly-owned subsidiary of NACCO, owns and operates the Falkirk Mine.
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Summary:
The Falkirk Mine is located in the Sentinel Butte Formation of the Fort Union Group which is one of the most prolific lignite-bearing stratum in the state.
The Bullion Creek Formation (Paleocene) underlies much of the reserve area. The Sentinel Butte Formation (Paleocene) conformably overlies the Bullion Creek Formation. Lithologically, the two formations are very similar. Interbedded silts and clays that occur in beds that range in thickness from less than one (1) foot to tens of feet make up about sixty) to eighty percent of the sediment of the Bullion Creek and Sentinel Butte Formations. From fifteen to thirty-five percent of the sediment making up these formations consists of silty, fine-grained to medium-grained sand in beds that range in thickness from one-half (½) to one hundred feet. Lignite is a minor constituent, generally comprising less than five percent of the formations. The lignite occurs in beds ranging in thickness from less than one-tenth) foot to about fifteen feet locally. The Coleharbor Formation (Pleistocene) unconformably overlies the Sentinel Butte and Bullion Creek Formations. It includes all of the unconsolidated sediments resulting from deposition during glacial and interglacial periods. Lithologic types include gravel, sand, silt, clay, and till. The Oahe Formation (Holocene) occurs as a thin veneer of eolian silt sized sediment that blankets upland surfaces in the area. The modified glacial channels that delineate the reserve limits are in-filled with sediments of the Coleharbor Formation. The channel fill systems contain a complex of interbedded glaciofluvial gravels, sands, silts, and clays overlain by till. The coarser gravel and sand beds are generally limited to near the bottom of the channel fill.
The general stratigraphic sequence in the upland portions of the reserve area (Sentinel Butte Formation) consists of till, silty sands and clayey silts, main Hagel (Hagel A) lignite bed, silty clay, lower lignite of the Hagel lignite interval (Hagel B), and silty clays. Both the Hagel A Bed and the Hagel B Bed are split by clay partings in portions of the reserve; although the two beds are not split in the same areas. Where the beds have partings, the splits are refered to as Hagel A1, Hagel A2, Hagel B1, and Hagel B2. There are thinner beds of lignite above the Hagel bed in some areas. These thin lignite beds are part of the Kinneman Creek seam. The Kinneman Creek in most areas is thin, very weathered, and very high in ash. In areas where it is mineable it can reach up to 3.5 feet thick and is lower in ash. Where the Kinneman Creek is parted, it is refered to as the Upper Kinneman Creek and Lower Kinneman Creek.
The primary coal bed is the “Hagel” which is divided into the Upper Hagel (A-1) and Lower Hagel (A-2). These two seams are coincident throughout most of the permit, but are sometimes separated by a clay parting. The Hagel coal bed is overlain in a portion of the permit area by the Kinneman Creek (KNC) seam. This coal seam averages 2.7’ in thickness and ranges from 2.0’-4.3’ within the estimated economically minable area. The KNC seam lies an average of 65’ above the Hagel seam, but varies greatly, from less than 25’ to over 88’ above the Upper Hagel seam. Below the Lower Hagel is a layer of interburden and the final minable “B” seam. The Upper Hagel seam averages 8.5’, and ranges from 3’ - 9’ in thickness. The parting varies from 0’ - 3’ in thickness, and averages less than 1’. Thickness on the Lower Hagel seam ranges from 0’ - 5’ and averages less than 1’ .Throughout most of the present permit area, the parting is absent and the Hagel or A seam averages 8.5’ in thickness. Interburden thickness ranges from 5’ - 25’ and averages 13.5’. The B seam ranges from 1.5’ - 4.5’ and averages 3.5’. Portions have B seam separated into two seams by a parting layer which renders this seam uneconomical to mine due to quality impacts.
These economically minable coal seams are underlain by numerous other seams. The C bed of the Hagel lignite interval is the stratigraphically lowest seam in the Sentinel Butte Formation. The Tavis Creek, Coal Lake Coulee, and Weller Slough beds all occur in the upper portion of the Bullion Creek Formation. These lower coal seams are not economically recoverable by surface mining methods because of prohibitive overburden thickness. However, in portions of the Riverdale 4th Addition, the Tavis Creek and Coal Lake Coulee beds are economically recoverable and are proposed to be mined.