Abstract

The natural burning of Tertiary coal beds in the Powder River Basin (PRB) has consumed billions of tons of coal and has thermally altered the overlying bedrock. This metamorphosed rock, known as clinker, presently covers about 1600 mi² in the PRB—1100 mi₂ in Montana and 500 mi² in Wyoming. Much additional clinker has been eroded away. A wide variety of clinker rock types is produced, depending on lithology of the parent rock, temperature and duration of heating, and degree of oxidation. Clinker ranges from slightly yellowed and hardened strata through brick-like fired and sintered rock to black vesicular paralava. Coal bed fires are ignited primarily by range fires and spontaneous combustion on outcrops that are above water table. During regional lowering of the land surface by erosion, the water table also drops, making progressively lower coal susceptible to burning. Episodes of burning have occurred over the past several million years as erosion has exposed coal through downcutting and hillslope retreat. Being more erosion-resistant than unbaked bedrock, clinker forms caprock throughout the basin. The oldest clinker in place generally caps the highest buttes and plateaus. Detrital zircon fission-track ages as great as 2.8 +/- 0.6 my from clinker in place, and as great as 4.0 +/- 0.7 my from a clinker boulder in a stream terrace gravel, provide a tool to help decipher late Tertiary and Quaternary rates of downcutting by streams in the PRB. The ages indicate that much of the downcutting in the present day Tongue River valley occurred during Quaternary time.