Petrographic Analyses of Knobloch Coal Seam (Paleocene), Powder River County, Southeastern Montana: ABSTRACT

Abstract

A single core of Knobloch coal from Powder River County, southeast Montana, was drilled to obtain samples for coal quality studies. The coal occurs in the lower Tongue River Member of the Paleocene Fort Union Formation. The Knobloch coal core (63 ft long) was divided into 1-ft increments and analyzed using chemical and petrographic methods. Definite variations in maceral content were seen. Preliminary studies show relationship between ash, gelinite, inertinite, and humodetrinite contents. A zone of low gelinite, low humodetrinite, and high inertinite, located in the lower quarter of the seam, implies a period of severe oxidation occurred, possibly as swamp fires. Four zones of high inertinite and high humodetrinite (three in the upper half and one in the lower half of the seam) indicate fluctuations in the water table, allowing moderate oxidation and weathering of plant material and subsequent mechanical reworking of humic grains. Near the center of the seam, a zone of high inertinite, high humodetrinite, and high ash content suggests water levels were high enough to allow significant sediment influx as well as reworking of the humic materials. These conclusions suggest the Knobloch coal is autochthonous and hypautochthonous in origin, a result of several water-table fluctuations and/ormore » climatic changes due to drought.« less