THE HARMON AND HANSON COAL BEDS IN WESTERN NORTH DAKOTA

The Harmon and Hanson lignite beds occur in the lower Bullion Creek Formation (Paleocene) of the Fort Union Group in western North Dakota. The Bullion Creek Formation consists of alternating beds of sandstone, siltstone, claystone, mudstone, and lignite deposited in a nonmarine setting. Hares (1928) named the lower coal the Hansen bed, an apparent misspelling because the historical land records for this area only list Hansons. For this reason, it is herein proposed that the spelling of the bed be changed to Hanson. The Harmon bed generally overlies the Hanson by 5 to 15 m, but only a few meters separate the beds in north-central Slope County. The coal pair appears to be the most extensive lignites in the North Dakota portion of the Williston Basin. They extend over an area of approximately 34,000 square kilometers, roughly one-third the total area underlain by coal- bearing strata in the state. The Harmon and Hanson beds were correlated using electrical logs from more than 6,900 coal and uranium test holes, water wells, and oil wells. The Harmon bed averages about 6 m thick, compared to a range of 2 to 7 m for the underlying Hanson bed. The Harmon reaches a maximum thickness of 16 m in south-central McKenzie County, the thickest known occurrence of lignite in North Dakota. It is several hundred meters beneath the surface in McKenzie County, too deep for conventional surface mining, but of interest as a potential source of coalbed methane. Although, in recent years, it has been strip mined only near Gascoyne in eastern Bowman County, the Harmon bed is economically mineable in an area of 640 square kilometers in parts of five counties. The Harmon bed has a maximum mineable thickness of 11 m near Amidon in Slope County and 10 m at Beach in Golden Valley County. The Harmon bed accounts for approximately one-fourth (5.8 billion tons) of North Dakota's 25.1 billion tons of strippable reserves. Subsurface control is not adequate to differentiate between the Harmon and Hanson beds in eastern Golden Valley County or portions of Billings, Stark, Dunn, and McKenzie counties. A thick coal that is present throughout much of this area at the stratigraphic position of the Harmon and Hanson coal pair, is assumed to be the Harmon bed because it is the dominant of the two coals in Bowman and Adams counties.