Geological Society of America 3300 Penrose Place P.O. Box 9140 Boulder, CO 80301 (303) 447-2020 • fax 303-357-1073 www.geosociety.org Chapter 10: “Geochronology of clinker and implications for evolution of the Powder River Basin landscape, Wyoming and Montana” (Heffern et al.), in Stracher, G.B., ed., Geology of Coal Fires: Case Studies from Around the World: Geological Society of America Reviews in Engineering Geology, v. XVIII. This PDF file is subject to the following conditions and restrictions: Copyright © 2007, The Geological Society of America, Inc. (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. 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Box 1726, Bodega Bay, California 94923, USA ABSTRACT In the Powder River Basin of southeast Montana and northeast Wyoming, coal beds exposed by regional erosion have burned naturally from as early as the Pliocene to the present. Layers of reddish clinker, formed by baking, welding, and melting of sediments above burned coal beds, cover over 4000 km2 and cap ridges and escarp- ments throughout the dissected landscape of the Powder River Basin. Fission-track (ZFT) and (U-Th)/He (ZHe) ages of zircon grains from baked sandstones in clinker provide new insights about rates of regional erosion as well as episodic advance of coal fi res into hillsides. Older, resistant clinker layers up to 60 m thick, formed by the burning of thick coal beds, cap summits and broad benches. Younger clinker rims, from thinner coals, form ledges on valley sides. ZHe ages of clinker, mainly from the Wyodak-Anderson coal zone of the Fort Union Formation in the Rochelle Hills east of Wright, Wyoming, and from the Wyodak-Anderson and Knobloch coal zones in the Tongue River valley near Ashland and Birney, Montana, range from 1.1 Ma to 10 ka. These dates generally agree with ZFT ages of clinker analyzed in the early 1980s, but they are a more precise record of ancient coal fi res in the region. Our data indicate 0.2–0.4 km of vertical erosion in the past 1 m.y. Spatial-temporal patterns of clinker ages may prove to be useful in deciphering the patterns of fl uvial incision and basin excavation in the Powder River Basin during the late Cenozoic and in weighing the relative importance of uplift, variations in climate, and base-level change. Keywords: coal-bed fi res, clinker, geochronology, geomorphology, Powder River Basin. *[email protected] Heffern, E.L., Reiners, P.W., Naeser, C.W., and Coates, D.A., 2007, Geochronology of clinker and implications for evolution of the Powder River Basin landscape, Wyoming and Montana, in Stracher, G.B., ed., Geology of Coal Fires: Case Studies from Around the World: Geological Society of America Reviews in Engineering Geology, v. XVIII, p. 155–175, doi: 10.1130/2007.4118(10). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved. 155 156 Heffern et al. INTRODUCTION and Coates, 2000). It is literally a landscape formed by fi re—albeit many separate fi res in many different places over time scales of In the western United States, coal beds have burned natu- 103–106 yr. Most parts of the southern Powder River Basin are rally in several basins in Wyoming, Montana, North Dakota, characterized by broad rolling hills, and fl at-topped buttes capped Colorado, Utah, New Mexico, and Arizona (Sigsby, 1966; Hoff- by clinker, with relief typically less than 100 m. However, in the man, 1996). The most extensive burning has been in the Powder Rochelle Hills (Fig. 1), natural burning of the Wyodak-Anderson River Basin of northeast Wyoming and southeast Montana (Rog- coal zone has left an eastward-facing escarpment 100–200 m high ers, 1918; Coates and Heffern, 2000). The Powder River Basin that is capped by a 20–50-m-thick layer of reddish clinker (Fig. 2). covers ~56,000 km2, an area about four times larger than the state In the central and northern parts of the basin, the major drainages of Connecticut. About 4100 km2 (7%) of the Powder River Basin of the Powder River, Tongue River, and Rosebud Creek, and their is covered by outcrops of clinker—baked, welded, and melted dominantly ephemeral tributaries, form an intricate trellis pattern. rocks formed by the natural burning of coal beds. These outcrops Here, generally fl at-lying clinker beds create fl ights of terraces that record the natural burning of tens of billions of tons of coal in the ascend from stream level to clinker-capped plateaus or ridges, with geologic past (Heffern and Coates, 2004). a total relief of ~200–400 m. More than one-third of U.S. coal comes from the Powder The semiarid climate, low-rank coal rich in volatile matter, River Basin. Seventeen coal mines, including some of the largest common range fi res, and regional erosion in the Powder River on Earth, produced 430 million short tons (390 million metric Basin provide ideal conditions for natural coal-bed fi res. Sev- tons) of coal there in 2005 using surface-mining methods (Energy eral coal beds 20 m or more thick, as well as many thinner beds, Information Administration, 2006). Beginning in the 1990s, bio- have burned over large areas of the Powder River Basin, fi ring genic methane originating in the coal beds has been extracted the overlying rock to a brick-like hardness and melting the rock at increasing rates. The coal is subbituminous and is found in in places. As erosion lowers the general land surface, coal is the Upper Paleocene Tongue River Member of the Fort Union exposed in stream beds, gullies, and hillsides. In some places, Formation and Lower Eocene Wasatch Formation. the coal is degassed and oxidized by the time it is exposed to The heat generated from in-situ burning of coal beds alters the air, and it does not combust spontaneously. However, where the strata overlying the coal, much as fi ring alters bricks. Clinker, fresh coal is exposed to the air by rapid erosion or removal of hardened by heating, forms distinctive erosion-resistant reddish overburden along stream banks and gullies, in the headwalls of layers that cap plateaus, hilltops, and escarpments. The highly landslides, in roadcuts, or in mines, spontaneous combustion is fractured nature of the clinker allows rainfall and snowmelt to common, especially when heat of oxidation is abetted by heat infi ltrate rather than run off the surface and erode the outcrop of wetting. The openpit coal mines, for example, experience (Coates, 1991). Clinker generally erodes more slowly than sur- the greatest numbers of spontaneous fi res in the spring when rounding unbaked rocks in this environment, leaving the resistant humidity increases due to melting snow and seasonal precipita- clinker standing in relief. Clinker fragments are abundant in land- tion. When Lewis and Clark explored this region two centuries slides and talus deposits below clinker cap rock. ago, they reported numerous coal-bed fi res and correctly noted Clinker-controlled topography dominates the exposed areas the relation between clinker and coal beds in the “burnt hills” of the Upper Paleocene Tongue River Member of the Fort Union of the northern Great Plains (Thwaites, 1969). Clark named one Formation in the northern, eastern, and western parts of the Pow- river the Redstone River because of the many reddish cobbles der River Basin (Fig. 1). This topography extends north to the of clinker in its streambed where it joined the Yellowstone Yellowstone River near Miles City, Montana (Heffern et al., River. That river was later renamed the Powder River because 1993), and south along the west fl ank of the Powder River Basin the smell of the ever-burning coal fi res evoked the smell of from the Wolf Mountains of Montana past the towns of Sheridan burning gunpowder. and Buffalo, Wyoming. It extends south along the east fl ank of The numerous thick coal beds that originally accumulated in the basin past Broadus, Montana, and into northeast Wyoming. vast peat swamps of the Paleocene intermontane foreland basin Clinker in the Tongue River Member covers over 400 km2 in the have provided the setting for a natural geomorphic experiment Rochelle Hills east of the towns of Gillette and Wright, Wyoming with the ability to reveal a detailed record of fl uvial incision and (Heffern and Coates, 2004). Clinker also caps hills and buttes in landscape evolution in the Powder River Basin.