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The AAPG/Datapages Combined Publications Database

North Dakota Geological Society



Symposium on the Geology of Rocky Mountain Coal, October 2-4, 1984

Pages 200 - 201


Donald A. Coates, U.S. Geological Survey, Box 25046, M.S. 972 Federal Center, Denver, Colorado 80225


Natural coal fires in the western coal basins of the United States have consumed large amounts of coal during late Tertiary and Quaternary time. The baked and fused rocks, known collectively as clinker, are conspicuous and in many areas dominate the landscape because they are more resistant than unbaked rocks. Ongoing studies are revealing much about the structure, age, geomorphic relationships, mineralogy and petrology, and paleomagnetism of clinker. Some features of this natural clinker rocks may be similar to aspects of rocks altered during underground gasification of coal.

Although the broken and deformed rocks of many clinker outcrops give the impression of being chaotic rubble, statistical studies of orientation of bed fragments, faults, slickensides, and folds show that collapse was generally systematic in most places. Collapse is commonly produced by progressive block slumping during burning back from the free face where combustion originates; thus, structural study can show the direction of fire propagation. Because the deformation is initiated at a free face and progresses beneath overburden with resultant surface collapse, the style of deformation and the structures produced probably are significantly different from those produced in an underground controlled burn.

Fission-track dating of detrital zircons in the baked sediments reveals the age of burning and can be used to study the directional history of burning and the development of associated landforms. In the Rochelle Hills in Campbell County, Wyoming, ages range systematically from about 0.7 m.y. at the end of a headland supported by clinker to less than 0.02 m.y. 8 km away where the headland joins other hills. These dates show the progress of burning and the developmental history of the headland. South-southwest of Forsyth, Montana, a clinker clast in a terrace gravel has been dated at 4.0 m.y. This date shows the maximum age of the gravel and is a tool in determining the downcutting history of the Yellowstone River valley.

Where flammable gasses given off by heated coal vent toward the surface, they burn upon reaching a level where sufficient oxygen is available. This combustion reaches high enough temperatures to fuse part of the rock to paralava. Such paralavas resemble volcanic lavas; they contain vesicles, microphenocrysts, flow features, engulfed rock fragments, and commonly a glass phase. But the chemistry and phenocryst mineralogy of these paralavas differ substantially from those of igneous lavas. Glasses of paralavas are generally calcic, probably from preferential incorporation of calcite cement in the sediments; they also have a low Na/K ratio, which may be due to differential volitilization of alkali components.

Paleomagnetic studies of clinker in Wyoming and Montana have revealed reversed magnetism in localities where fission-track ages are greater than 0.7 m.y., and when compared to the known reversal chronology they confirm and add precision to the clinker ages. In addition, magnetic studies reveal widespread occurrence of magnetite- rich spinel in clinker, indicating that reducing conditions are more widespread than the generally red and yellow colors of the bulk of the clinker might suggest.

Continued study of natural clinker and the processes that lead to its formation may also be important to understanding the processes, the products, and the short- and long-term effects of underground coal gasification.