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Clinker, rock that has been baked or fused by the burning of underlying coal beds, is abundant in the Tongue River Member of the Paleocene Fort Union Formation in the northern Powder River basin. Being more resistant than unbaked rocks above and below, the clinker commonly caps ridges and plateaus, and forms topographic benches and escarpments. This clinker is primarily red and orange baked sandstone and shale, but it includes gray sintered siltstones (porcellanite) and bodies of black, fused and welded breccia. Detrital zircons in the sandstones are annealed during baking and yield fission-track ages that show the time of cooling.
An inventory of clinker areas has been completed for the Montana part of the northern Powder River basin east of the Crow Indian Reservation. The study area lies within the Powder River resource area of the U.S. Bureau of Land Management. Data were compiled from existing literature, color infrared aerial photographs, and unpublished mapping provided by R. B. Colton, W. C. Culbertson, and S. J. Luft of the U.S. Geological Survey. Clinker covers approximately 2,700 km2 (1,050 mi2) or 20% of the Tongue River exposures in the study area. Assuming a 15 to 25 m (50 to 80 ft) average thickness, the volume of clinker would be 40 to 70 km3 (10 to 17 mi3).
The most extensive clinker layers locally exceed 60 m (200 ft) in thickness. They are produced by the thickest coal beds, some of which exceed 15 m (50 ft). Two major clinker layers form extensive topographic surfaces and escarpments. (1) Near Decker, Montana, the clinker produced by the Anderson-Dietz coal zone forms benches adjacent to the Tongue River. Because of the gentle southerly regional dip of the beds, this clinker zone rises to the north, where it caps large plateaus dividing the valley of the Tongue River from the valleys of Otter Creek and Rosebud Creek. These clinker plateaus stand up to 400 m (1,300 ft) above the Tongue River. (2) Near Ashland, Montana, the clinker produced by the Knobloch-Nance coal zone, which lies about 300 m (980 ft) stratigraphically below the Ande son-Dietz zone, forms broad benches bordering the Tongue River.
The distribution of fission-track ages shows that coal has burned to form clinker in the region at least since the late Pliocene. A clinker boulder from the base of a gravel deposit 365 m (1,200 ft) above the level of the Yellowstone River west of Forsyth has been dated at 4.0 ± 0.7 m.y. This age establishes a maximum age for the gravel. The oldest in-place clinker sample dated thus far comes from the summit of the Little Wolf Mountains west of Colstrip and is dated at 2.8 ± 0.6 m.y. Clinker from the Anderson-Dietz plateau that rims the Tongue River Valley west and south of Ashland ranges in age from 1.4 ± 0.4 m.y. to 0.7 ± 0.3 m.y. Ages from clinker of the Knobloch-Nance coal zone range from 0.5 ± 0.3 m.y. to < 0.06 m.y. The older ages are from topographic lly higher clinker layers. Inasmuch as a coal bed cannot start burning until it is exposed by erosion, these ages indicate the Tongue River cut its present valley primarily during the Pleistocene.
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