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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.

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The Geological Society of America Reviews in Engineering Geology, Volume XVIII 2007

Geochronology of clinker and implications for evolution of the Powder River Basin landscape, Wyoming and Montana

Edward L. Heffern* U.S. Bureau of Land Management, 5353 Yellowstone Road, Cheyenne, Wyoming 82009, USA

Peter W. Reiners Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

Charles W. Naeser U.S. Geological Survey, MS 926A, Sunrise Valley Drive, Reston, Virginia 20192-0002, USA

Donald A. Coates Consultant, P.O. 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. This record has the overlying Eocene Wasatch Formation, from east of Sheridan the potential to constrain the timing, rate, and ultimate tectonic and Buffalo to Gillette and Wright (Heffern and Coates, 1997). and/or climatic driving forces behind late Cenozoic exhumation The landscapes of the northern and southern parts of the Pow- of the Powder River Basin and other Rocky Mountain foreland der River Basin are different in several ways. Where the Tongue basins in the region. As coals in the basin are exhumed by fl u- River Member is exposed in the northern part of the basin, thick vial incision and scarp retreat and burn naturally, they leave sandstone beds form cliffs and ledges. The less-consolidated clinker as a record. The age of the clinker can be dated with exposures of the Wasatch Formation in the southern part of the roughly 10% or better precision using zircon (U-Th)/He (ZHe) basin have gentler topography. However, relief throughout the chronometry, and to a lesser precision using zircon fi ssion-track basin is largely controlled by the distribution of clinker (Heffern (ZFT) dating. In several cases, Pliocene through Recent ZHe Figure 1. Relief map of Powder River Basin in Wyoming and Montana (after Heffern and Coates, 2004). 158 Heffern et al. and ZFT clinker ages show reproducible and systematic spa- Second, the fi re may penetrate beneath a slope so far that tial-temporal patterns with respect to fl uvial networks and other collapse of overburden deprives it of air. The fi re goes out natu- topographic features across the Powder River Basin. rally where the overburden becomes so thick that fractures from the collapse fail to reach the surface to draw in more air. The dis- COAL-BED FIRES tance that the fi re penetrates depends on the thickness of the coal bed and the lithology and fracture patterns in the overburden that Coal-bed fi res are typically ignited by either spontaneous com- affect the ability of the collapsed rock to allow ventilation. Burn- bustion or wildfi res. Spontaneous combustion of Powder River ing of a thicker coal bed causes the overburden to collapse to a Basin coals is largely due to exothermic reactions associated with greater degree, which allows the fi re to burn farther and deeper oxidation of organic matter, and it is aided by the high volatile con- into the hillside. Other factors, such as surface runoff fi lling frac- tent of the low-grade coal (Lyman and Volkmer, 2001; Coates and tures with clay and sealing the fi re from its source of oxygen, Heffern, 2000; Heffern and Coates, 2004). In dry years, lightning- as well as splitting, faulting, and truncation of coal beds in the and human-induced wildfi res can ignite hundreds of coal fi res by subsurface, may also extinguish a coal fi re. direct burning of coal outcrops or of trees and bushes rooted in fresh Over time, regional erosion lowers the ground surface and coal seams (Heffern and Coates, 2004). Aggressive control mea- water table relative to coal beds and dries out the coal near the sures have extinguished many coal-bed fi res in the Powder River surface, which is then susceptible to burning as it is exposed Basin, but in the prehistoric past, wildfi res probably were a continu- to air. Present-day burn fronts are, at the deepest, 60 m below ous source of natural ignition. the surface, and in most cases, less than 30 m below the sur- Hundreds of natural coal-bed fi res are currently burning in face. Systematic age trends across exposed clinker-dominated the Powder River Basin (Fig. 3). The coal beds in the Powder landscapes (e.g., Coates and Naeser, 1984; Reiners and Hef- River Basin are nearly horizontal, and they commonly dip less fern, 2002) strongly suggest that coal burns within a relatively than one degree except for the western and southern edges of the restricted range of distances, horizontally and vertically, from basin, where dips are steeper. When a fi re starts, it fi rst spreads exposure in any given location at a given time. Because coal along the horizontal outcrop because this is the path with the most burning and clinker formation are restricted to the uppermost oxygen. This pattern of advance occurs in coal fi res in active tens of meters in the subsurface, and because coal in the Powder mines as well as natural outcrops, and it is documented by the River Basin does not reside within this zone for very long, in a many shallow bands of clinker marking the position of coal beds geologic sense, before burning, spatial patterns of clinker ages along hillsides. With time, the fi re burns deeper into the hillside. provide estimates of the timing and rates of fl uvial incision and A 10-m-thick coal bed may burn down to form an ash layer less scarp retreat throughout the basin. They can be used to map the than 1 m thick, causing the overlying rocks to subside into the larger-scale exhumation of the basin from at least Pliocene to burned-out void. The resulting fracturing allows air to enter and Holocene time. gas to escape, feeding the fi re. The removal of the coal bed dis- rupts and fractures the overlying rock enough that air access and CLINKER venting allow the fi re to burn laterally back into the hill beneath a considerable thickness of overburden—commonly as much as The term “clinker” encompasses all the rocks that have been two to three times the original thickness of the coal bed (Heffern altered by heating from a coal-bed fi re. Thus, it includes a vari- and Coates, 2004). The fi re may smolder for long periods but ety of thermally metamorphosed or melted lithologies resulting sometimes erupts into a roaring inferno that melts rock. from the burning of coal (Sarnecki, 1991; Papp, 1998; Coates Two major controls determine the depth to which a coal fi re and Heffern, 2000). These variations are partly due to differ- can burn beneath overburden and the distance it can penetrate ent types of source rocks in the Upper Cretaceous and Lower beneath a slope. First, the fi re may encounter groundwater and be Tertiary strata of the American West but also to variations in the unable to progress. Ultimately, an underground coal fi re burns all dynamics of burning, ventilation, and proximity of the source the coal that it can before running out of air or fuel. As a coal bed rocks to the coal fi re. The most distinguishing feature of clinker burns back into a hillside and the fi re encounters water-saturated in the landscape is its red color. In hand specimen, clinker may coal, further spread is constrained, and the fi re dies for lack of air display little obvious alteration besides reddening (especially for and fuel. Along the contact between clinker and unburned coal in sandstones), may have a pronounced ceramic-like texture from the subsurface (the “burn front” or “burn line”), the top part of more intense heating of shale or siltstone (porcellanite), or may a coal bed may be burned away and replaced by clinker that has be melted to form black paralava and appear nearly identical subsided into the void, while the lower, saturated part of the coal to pahoehoe. Such melting requires temperatures greater than bed may still be present below the water table. These patterns 1300 °C (Cosca et al., 1989). Some portions of clinker, espe- can be seen in many cross sections drawn across the coal-clinker cially baked shale, may be dark gray, black, or green, because contact in mine plans on public fi le with the state agencies in of heating under reducing conditions. Porcellanite was worked Wyoming and Montana delegated to enforce the Surface Mining by early Native Americans for tools and blades, and it is found Control and Reclamation Act of 1977. in archaeological sites throughout the Powder River Basin and Figure 2. Clinker of Wyodak-Anderson coal zone capping Rochelle Hills escarpment, Wyoming.

Figure 3. Active coal-bed fi re along bank of Tongue River north of Sheridan, Wyoming. 160 Heffern et al. beyond. Porcellanite Folsom and Goshen points as old as 11 ka that has been shattered by dehydration and shrinkage of clays have been found (Clark, 1985; Fredlund, 1976). during heating but otherwise exhibits little deformation. Most of The basal part of a clinker exposure is characterized by a the heat is transferred upward by convection, leaving strata that thin zone, usually less than 1 m thick, of gray or tan ash and are more than 0.5–1 m below the original base of the coal unal- greenish glass from burned coal. Vesicular and shattered para- tered. The tension cracks are generally fi lled with welded breccia lava may lie directly above the ash. This is overlain by shattered and paralava, indicating that they were vents where burning of layers of porcellanite and baked shale, as well as lenses or thick coal gas created intense heating. These areas, which are harder sections of sandstone that show little metamorphism other than than the surrounding clinker, are left by erosion to stand as resis- reddening. In most cases, collapse of overlying strata has formed tant chimneys on the land surface, recording the location of fi s- a chaotic jumble of intensely fractured and faulted blocks. Chim- sures where the temperature was high enough to weld and melt neys that allowed ventilation during burning are typically found the rock. every few tens of meters laterally in a clinker unit. Rocks in these Published studies (Heffern et al., 1993; Heffern and Coates, chimneys usually have evidence of intense heating and melting, 1997; Coates and Heffern, 2000) show that the Powder River Basin including fl ow structures along voids and fractures. The trace of in Wyoming and Montana contains ~4100 km2 of clinker outcrops: the end of clinker exposures on a plateau or hillside—the burn ~2800 km2 in Montana and 1300 km2 in Wyoming. These clinker line—typically marks the beginning of unburned coal that is too outcrops are only remnants of a much larger volume of clinker that far, horizontally and vertically, from the surface to have burned. has been removed by erosion. As erosion deepened and widened The burn front on the eastern margin of the Powder River Basin valleys and lowered the water table, successively lower coal beds in Wyoming marks the eastern boundary of some of the world’s were dewatered, exposed, and burned back into the hillsides. The largest surface coal mines. clinker that was formed has been eroded and carried downstream The structure of a typical clinker hillside (Fig. 4) shows into the tributaries of the Missouri River, along with other rock and progressive collapse of overburden as the coal-bed fi re advances soil eroded during dissection of the landscape. Fragments of clin- back into the hill. As the support of underlying coal is removed, ker are found in the gravel terraces and streambed alluvium of this the overburden subsides. Some subsidence takes the form of major river system. The clinker remaining in its original position large, highly fractured slump blocks separated by tension cracks appears to be only a small fraction of, perhaps an order of magni- that allow air to fl ow into the burning area and direct combustion tude less than, the total volume of clinker that has been produced gases upward from the burn zone. Other rock subsides as a mass over time (Heffern and Coates, 2004).

Figure 4. Block diagram of clinker-dominated landscape, showing collapse features, breccia zones (chimneys), and ash zones in clinker (after Heffern and Coates, 1997). Geochronology of clinker and implications for evolution of the Powder River Basin 161

Most of the clinker in the Fort Union Formation was dating of detrital zircon grains in baked sandstones. A third formed by burning of three regionally extensive coal zones method—paleomagnetic orientations of magnetite, hematite, and in the Tongue River Member—the Wyodak-Anderson zone goethite in paralava and baked sediments—provides additional in both Wyoming and Montana and the Knobloch and Rose- indirect age constraints. These dates can lead to a better under- bud-Robinson zones in Montana (Heffern et al., 1993; Flores standing of landscape evolution and incision of fl uvial networks and Bader, 1999). Other less-extensive coal zones, such as the into the sedimentary fi ll of the Powder River Basin, and they can Wall, Pawnee, Terret, and Dominy (Bass, 1932; Matson and constrain the patterns and rates by which coal beds were exposed Blumer, 1973), have locally produced large areas of clinker and burned. in Montana. In the overlying Wasatch Formation, mostly in Wyoming, extensive clinker was produced by the burning of Zircon Fission-Track (ZFT) Dating the Lake De Smet (Healy-Ucross) and Ulm coal zones in the western part of the basin near Sheridan and Buffalo, as well as Zircon (ZrSiO4) grains are widespread in sandstones of the the Felix coal zone near Gillette and Wright. The stratigraphic Fort Union and Wasatch Formations, and they contain ppm-level column in Figure 5 (from Flores, 2004) shows the relative posi- uranium. Over time, the radioactive uranium decays in two dif- tion of these coal zones. ferent modes, alpha decay and spontaneous fi ssion, and each mode has a different half-life. During spontaneous fi ssion, parti- METHODS FOR DATING CLINKER cles smash through the lattice of the zircon crystal, leaving a track of weakened and disturbed bonds between atoms in the lattice. Clinker has been dated by two means: (1) fi ssion-track dating These fi ssion tracks can be revealed by etching and counted under of detrital zircon grains in baked sandstones; and (2) (U-Th)/He a microscope. The ratio between the density of fi ssion tracks and

Figure 5. Stratigraphic column of coal zones in Fort Union and Wasatch Formation assess- ment units (AU), Powder River Basin (from Flores, 2004). 162 Heffern et al.

uranium concentration in a crystal provides an estimate of how formation, events. The accuracy of ZHe ages determined by long the tracks have been accumulating (Naeser, 1979). When conventional methods can be affected by strong and systematic that crystal is heated to a suffi ciently high temperature, fi ssion intracrystalline zonation of U-Th in single zircons (Farley et al., tracks anneal and disappear, resetting the clock. Nearly all of 1996; Reiners et al., 2004; Hourigan et al., 2005). the sandstones that have been baked into clinker contain a Because these samples are detrital and single clinker sam- small proportion of detrital zircon. Because these zircon grains ples likely contain grains from a wide range of sources, mul- are heated and annealed during the formation of clinker, it is tiple single-grain analyses should not yield consistently inac- possible to determine when the underlying coal bed burned. curate ages, though individual fl iers in each sample may be due Once the fi re is over, the fi ssion-track clock starts again. On to U-Th zonation. For this reason, we measured multiple single- time scales characteristic of coal fi res in the shallow subsur- grain ages on most samples. Laser-ablation depth profi ling to face (e.g., 101–102 yr), zircons will completely anneal fi ssion check core-to-rim U-Th gradients in about ten grains was also tracks, fully resetting the ZFT age, at temperatures of ~375– performed on zircons from one sample (CLK5) because of sys- 450 °C (Figure 14 in Reiners, 2005). A limiting factor in deter- tematic age discrepancies with ZFT ages from this sample. No mining fi ssion-track ages of Powder River Basin clinker signifi cant or systematic zonation was found in these grains. younger than ~0.1 Ma is the low uranium content in most zir- cons, which results in very low fi ssion-track densities. Conse- Paleomagnetic Orientations of Clinker Outcrops quently, the statistical uncertainty of the determined age is high, whereas in older clinker, ages are better constrained Clinker is more magnetic than unbaked overburden (Has- because more fi ssion tracks are present. Another limitation on brouck and Hadsell, 1978). Changes in mineralogy in the bulk both ZFT and ZHe age measurements is that the zircon crys- of the clinker that result in increased magnetism are not yet well tals in a sample may not be big or numerous enough to date. understood. In the chimneys, however, the reason for increased Some clinker samples collected from the Tongue River Mem- magnetism is evident. Paralava, common in chimneys, is ber in Montana did not have enough zircon crystals to date, but marked by the enrichment of iron, much of it in the form of almost all samples collected from the uppermost Tongue River magnetite and ilmenite (Cosca et al., 1989). Formation of these Member and the Wasatch Formation in Wyoming had a suffi - magnetic minerals, which contain iron in the ferrous (Fe2+) cient number. oxidation state, apparently results from the presence of car- bon monoxide driven off the burning coal face below (Rogers, U-Th/He (ZHe) Dating of Zircons 1918) and a restricted oxygen supply in the chimney. As these newly formed minerals cool through the Curie temperature, Uranium (U) and thorium (Th) in detrital zircons experi- they become ferromagnetic, imparting a strong and coherent ence alpha decay, producing helium (4He). As in the case of magnetic fi eld to the rock that refl ects the orientation of Earth’s zircon fi ssion-track dating, zircon (U-Th)/He dating is a ther- magnetism at that time. This can be used to constrain the age of mochronometer because He diffuses out of crystals at elevated clinker. Reversed polarity in clinker indicates ages older than temperatures. The diffusivity of He in zircon has been studied by ca. 778 ka (Fullerton et al., 2004a, 2004b), and more-precise step-heating diffusion experiments and comparisons of zircon constraints are possible if magnetic orientation is combined (U-Th)/He ages with ages of other thermochronometers with with other geologic constraints. known thermal sensitivities (e.g., Reiners et al., 2004; Reiners, 2005). For normal crystal sizes and under slow-cooling condi- RESULTS tions (dT/dt ~ 10 °C/m.y.) typical of many exhuming orogens, ZHe has an effective closure temperature (Dodson, 1973) of Overview ~170–190 °C. On time scales characteristic of coal fi res in the shallow subsurface (e.g., 101–102 yr), zircons will completely The natural burning of coal beds, and therefore the forma- degas all He, fully resetting the ZHe age, at temperatures of tion of clinker, is a process that has been ongoing since the ~375–450 °C (Reiners, 2005). Zircon He ages typically have a sedimentary fi ll of the Powder River Basin began to erode in precision of ~9% (two standard deviations of replicate analy- the late Cenozoic. Radioisotopic ages of clinker outcrops range ses on typical zircons such as those in the frequently used geo- from 10 ka to as old as 2.9 Ma, showing a history of progressive chronologic standard of the Fish Canyon Tuff from southwest burning through geologic time. Stream gravels on high terraces Colorado). contain clinker boulders dated as old as 3.8 Ma, indicating even While fi res in coal beds more than ~30 cm thick easily earlier burning in the region. Relative ages of some clinker have reset clinker zircon, there is little to no concern that surface been constrained paleomagnetically (Jones et al., 1984), but wildfi res reset zircons more than ~1 cm below the rock surface numerical dating precise enough for geomorphic applications (Mitchell and Reiners, 2003). Although clinker ages are typi- requires a technique such as thermochronometry that records cally quite young (10 ka to 4 Ma), no corrections for U-series the thermal effects of coal fi res and is insensitive to protolith disequilibria are required because ages represent resetting, not or detrital ages. Geochronology of clinker and implications for evolution of the Powder River Basin 163

Zircon Fission-Track (ZFT) Ages Wyoming In addition to the 19 ZFT analyses conducted by Naeser In the late 1970s and early 1980s, Naeser measured zir- from clinker samples in the Wyoming part of the Powder River con fi ssion-track ages of 39 baked sandstone samples from the Basin in the late 1970s and early 1980s, Reiners and Heffern Powder River Basin and northeast Montana, focusing mostly collected clinker samples for ZHe analysis from 15 locations on the Rochelle Hills on the eastern side of the basin in Wyo- in the Wyoming Powder River Basin during 2002. Most of the ming, and on the region around Ashland, Montana, in the north samples were collected from the Rochelle Hills, with a few in (Fig. 1). The samples were collected by Donald Coates and Ed the Felix clinker near Wright, and a few in the Lake De Smet Heffern, with assistance from Jason Whiteman of the North- area near Buffalo. Figure 6 shows both ZFT and ZHe ages for ern Cheyenne Tribe. Most of the results were published in a clinker in the Rochelle Hills. Viewing this fi gure, the reader can series of abstracts and two U.S. Geological Survey maps and easily compare clinker formation ages generated by each method were discussed in subsequent papers (Coates and Naeser, 1984; in nearby locations. Heffern et al., 1983, 1993; Coates and Heffern, 2000; Heffern and Coates, 2004). Although these fi ssion-track ages have large Rochelle Hills uncertainties (typical 2σ of 30%–60%), in many cases they The Rochelle Hills, a major east-facing escarpment on the show remarkably systematic trends with respect to geomor- eastern fl ank of the Powder River Basin in Wyoming (Fig. 2), phic features. For this report, we have recalculated the original are capped by a 20- to 50-m-thick layer of clinker that marks the age determinations to the latest zeta calibration value (Hurford prehistoric burning of the Wyodak-Anderson coal zone. This is and Green, 1983) and rounded the ages to the nearest 0.1 Ma the same thick coal zone that is being mined immediately west (Tables 1 and 2). The ZFT ages in this article are recalculated of the band of clinker (Fig. 6). The beds dip westward 10–15 m/ numbers, but the references are to previous publications with km, and the landscape drains to the east. Consequently, the pla- the original age determinations. Table 1 provides age and loca- teaus and ridges of clinker that extend eastward from the burn tion data for the 39 outcrops used in this study; Table 2 pro- line rise higher toward the east above the streams and above the vides supporting data used to calculate the ZFT ages. In the local water table. subsequent tables, “Wyodak” denotes the Wyodak-Anderson Fission-track ages of zircon grains in clinker from 14 sites coal zone in Wyoming, and “Anderson” denotes the Wyodak- on the ridge north of Little Thunder Creek in the Rochelle Hills Anderson coal zone in Montana. and two sites on the ridge south of Little Thunder Creek were determined by Naeser (Coates and Naeser, 1984). These ZFT (U-Th)/He (ZHe) Ages ages (Fig. 6) decrease from 0.8 ± 0.5 Ma on the eastern edge of the escarpment to less than 0.09 Ma near the burn front on the Over the past fi ve years, we collected clinker samples from west, with average uncertainties of 55%. Along the burn front 27 locations in eastern Wyoming and Montana and analyzed zir- north of Little Thunder Creek, the name Burning Coal Draw indi- con grains from the baked sandstones to obtain their ZHe ages cates an area where an active coal fi re was mentioned by military (Tables 3 and 4). All sites except one were from the Powder River expeditions in the 1870s, which continued to burn until the U.S. Basin. Some clinker samples collected from additional locations Bureau of Mines extinguished it in 1951 (Russell and Smith, had zircon grains that were too small to date, especially in the 1951). West of the burn front, coal is currently mined from the northern part of the basin. Table 3 provides age and location data Wyodak-Anderson zone at the Jacobs Ranch, Black Thunder– for the 27 ZHe sites; Table 4 provides supporting data used to North Rochelle, and North Antelope–Rochelle operations. The calculate the ZHe ages, including individual analyses where a age progression shows variations, due to (1) different areas burn- weighted mean of several measurements was used to derive a ing at different times in different tributaries of Little Thunder single age for a given location. Some ZHe samples were col- Creek, and (2) a wide margin of error in age determinations; lected at or near where samples were collected for ZFT analysis; however, the overall pattern suggests a progressive westward in all such cases, these ZHe ages are within the error range of migration of the escarpment with time. ZFT ages from the same locations (Reiners and Heffern, 2002) For the ZHe study described in this article, the authors (Rein- but are generally more precise than the ZFT ages. ers and Heffern) collected and analyzed samples of the Wyodak- Anderson clinker from 12 locations in the Rochelle Hills. Here, DISCUSSION ZHe ages ranged from 0.615 ± 0.035 to 0.010 ± 0.001 Ma, sys- tematically decreasing from east to west toward the present-day Interpretation of Results burn front (Fig. 6). These ZHe and ZFT ages suggest progression of the burn front to the west at a rate of ~10–14 mm/yr and, refer- The clinker ages determined in Wyoming and Montana are enced to the elevation of the base of the clinker unit where these best understood in the context of the geologic and topographic ages are found, vertical denudation at a rate of ~0.2–0.3 mm/yr setting of the region. This section interprets the sample ages from (Fig. 7). For a given elevation, the ZFT ages tend to be slightly the tables in the results section. older. If the escarpment has maintained the same width and relief 164 Heffern et al.

TABLE 1. ZIRCON FISSION-TRACK AGES OF CLINKER IN THE POWDER RIVER BASIN Sample Age Coal zone Legal description Latitude* Longitude* Elevation Comments number (Ma) (°N) (°W) (m) Wyoming Rochelle Hills 77C-1 0.4 ± 0.2 Wyodak SENW Sec. 22, T43N, R69W 43.6878 105.1329 1488 N of Little Thunder Creek 77C-18 0.7 ± 0.3 Wyodak NESW Sec. 13, T43N, R69W 43.6996 105.0923 1530 N of Little Thunder Creek 77C-19 0.7 ± 0.3 Wyodak SWNW Sec. 13, T43N, R69W 43.7050 105.0965 1516 N of Little Thunder Creek 77C-20 0.8 ± 0.5 Wyodak SWSW Sec. 13, T43N, R69W 43.6964 105.1003 1524 N of Little Thunder Creek 77C-21 0.3 ± 0.2 Wyodak NENW Sec. 14, T43N, R69W 43.7073 105.1159 1522 N of Little Thunder Creek 77C-22 0.4 ± 0.2 Wyodak NENW Sec. 15, T43N, R69W 43.7082 105.1323 1515 N of Little Thunder Creek 77C-23 0.3 ± 0.3 Wyodak NWNW Sec. 16, T43N, R69W 43.7064 105.1598 1506 N of Little Thunder Creek 77C-24 0.3 ± 0.2 Wyodak NENW Sec. 29, T43N, R69W 43.6797 105.1751 1458 N of Little Thunder Creek 77C-25 <0.09 Wyodak SESW Sec. 7, T43N, R69W 43.7115 105.1930 1476 N of Little Thunder Creek 77C-26 0.5 ± 0.3 Wyodak NWNE Sec. 28, T43N, R69W 43.6792 105.1466 1493 N of Little Thunder Creek 77C-27 0.5 ± 0.3 Wyodak NENW Sec. 23, T43N, R69W 43.6923 105.1152 1515 N of Little Thunder Creek 81G-10 <0.5 Wyodak NESW Sec. 11, T43N, R69W 43.7146 105.1131 1524 S of HA Creek 81G-11 0.5 ± 0.4 Wyodak NWNW Sec. 14, T43N, R69W 43.7074 105.1207 1518 N of Little Thunder Creek 81G-14 <0.6 Wyodak NENE Sec. 8, T43N, R69W 43.7203 105.1648 1482 S of HA Creek 81G-15A <0.5 Wyodak SWNW Sec. 18, T42N, R68W 43.6169 105.0784 1555 S of Little Thunder Creek CPSE-3, 0.7 ± 0.4 Wyodak NESE Sec. 6, T41N, R68W 43.5568 105.0647 1595 N of Keyton Creek 4b, 5, 6, 9† Felix Coal Zone 79F-2 0.9 ± 0.6 Felix SESE Sec. 10, T44N, R71W 43.7962 105.3674 1546 W of Hilight Road 79F-A <0.3 Felix NENE Sec. 28, T45N, R72W 43.8524 105.5068 1485 NW of Wright 79F-C <0.3 Felix NWSW Sec. 1, T44N, R72W 43.8182 105.4591 1500 N of Wright Montana Little Wolf Mountains CP-32 2.2 ± 0.8 Anderson? NESE Sec. 8, T1N, R39E 45.8492 106.8980 1430 Summit—Little Wolf Mts. CP-33 2.9 ± 0.9 Anderson? SENW Sec. 17, T1N, R39E 45.8422 106.9115 1448 Summit—Little Wolf Mts. Gravel Terrace 80M-2 3.8 ± 1.2 Unknown SESE Sec. 6, T3N, R39E 46.0384 106.9189 1143 Gravel terrace SW of Forsyth Tongue River Valley CP-1 0.8 ± 0.7 Anderson NWSE Sec. 32, T5S, R42E 45.3559 106.5706 1235 NW of Birney CP-2 0.8 ± 0.4 Anderson SENW Sec. 32, T5S, R42E 45.3604 106.5767 1226 NW of Birney CP-3 1.0 ± 0.4 Anderson NWNW Sec. 29, T4S, R43E 45.4634 106.4570 1220 N of Birney Day School CP-4 1.5 ± 0.5 Anderson NENE Sec. 25, T4S, R42E 45.4697 106.4830 1210 NW of Birney Day School CP-5 1.0 ± 0.4 Anderson SESW Sec. 13, T4S, R42E 45.4862 106.4936 1226 NW of Birney Day School CP-6 <0.2 Knobloch SESW Sec. 33, T4S, R43E 45.4422 106.4336 951 N of Birney Day School CP-8 1.1 ± 0.4 Anderson SESW Sec. 29, T5S, R43E 45.3656 106.4508 1241 Summit—Browns Mtn. CPE-30 0.6 ± 0.6 Knobloch SESE Sec. 22, T2S, R44E 45.6420 106.2752 966 N of Ashland 83M-1 0.8 ± 0.2 Anderson NENE Sec. 29, T2S, R43E 45.6385 106.4431 1300 N of Stebbins Creek 83M-2 1.1 ± 0.3 Anderson NWNW Sec. 9, T2S, R43E 45.6849 106.4362 1293 Summit—Garfield Peak 83M-3 1.0 ± 0.3 Anderson NWSE Sec. 4, T3S, R43E 45.6039 106.4255 1280 S of Stebbins Creek 83M-4 0.9 ± 0.3 Anderson NESW Sec. 34, T3S, R43E 45.5309 106.4096 1268 N of Kelty Creek 83M-5 1.3 ± 0.3 Anderson NENW Sec. 16, T4S, R43E 45.4953 106.4315 1250 S of Kelty Creek 83M-6 0.4 ± 0.3 Knobloch SWNE Sec. 9, T3S, R44E 45.5936 106.3036 939 Roadcut W of Ashland 83M-7 0.2 ± 0.1 Knobloch NWSE Sec. 8, T2S, R44E 45.6757 106.3207 957 S of Reservation Creek 83M-8 <0.4 Knobloch NWSW Sec. 28, T1S, R44E 45.7221 106.3130 960 N of Lay Creek Northeast Montana 79M-11 <0.06 Unknown NE Sec. 8, T27N, R56E 48.1100 104.5100 600 Gorge S of Missouri River, W of Highway 16 bridge *Latitude and longitude in North American Datum 27. †Average of five samples from same location.

through this time, then the lateral retreat of the escarpment would the ZFT and ZHe ages was ~150 k.y. (~25%) on the eastern end occur at the same rate as the progression of the burn front. of the ridge north of Little Thunder Creek. Multiple replicate Three of the ZHe measurements (Reiners and Heffern, analyses from several places on the east edge of the Rochelle 2002) resampled three of Coates and Naeser’s (1984) locali- Hills escarpment, detailed screening for U-Th zonation in ties along the ridge north of Little Thunder Creek and obtained these samples by laser-ablation–inductively coupled plasma– more accurate ZHe dates, all of which fell within the uncer- mass spectrometry (LA-ICP-MS), and careful extraction of tainties of the earlier ZFT dates. These results corroborated the crystals from interior slabs of rock to avoid potential wildfi re fi ssion-track dating and more precisely confi rmed the trend of effects suggest that our ca. 0.54 Ma ZHe age for the eastern younger ages toward the west. The largest discrepancy between edge is accurate. In addition, Jones et al. (1984) reported nor- Geochronology of clinker and implications for evolution of the Powder River Basin 165

TABLE 2. ZIRCON FISSION-TRACK DATA AND AGES FROM CLINKER IN THE POWDER RIVER BASIN Sample No. of Fission-track Fission Induced-track Induced F 2 Dosimeter Dosimeter Age number grains density tracks density tracks density tracks (Ma ± 2V (× 105 t/cm2) counted (× 106 t/cm2) counted (× 105 t/cm2) counted CPSE-3 6 1.54 20 11.4 736 P 2.03 2140 0.9 ± 0.4 CPSE-4B 6 1.44 20 12.7 883 P 2.01 2140 0.7 ± 0.3 CPSE-5 6 0.81 12 8.34 618 P 1.97 2140 0.6 ± 0.4 CPSE-6 6 1.34 18 11.3 757 P 1.95 2140 0.7 ± 0.4 CPSE-9 6 1.80 30 15.7 1310 P 1.92 2140 0.7 ± 0.3 77C-1 6 1.33 14 21.1 1115 P 2.13 3242 0.4 ± 0.2 77C-18 6 1.86 19 17.7 905 P 2.13 3242 0.7 ± 0.3 77C-19 6 1.47 23 13.8 1082 P 2.11 3242 0.7 ± 0.3 77C-20 6 1.59 13 13.3 541 P 2.09 3242 0.8 ± 0.5 77C-21 6 0.678 9 13.1 872 P 2.07 3242 0.3 ± 0.2 77C-22 6 1.14 12 21.1 1110 P 2.05 3242 0.4 ± 0.2 77C-23 6 1.14 7 22.0 672 P 2.03 3242 0.3 ± 0.3 77C-24 4 0.639 5 13.9 661 P 2.03 3242 0.3 ± 0.2 77C-25 3 <0.20 0 26.2 1113 F 2.01 3242 <0.09 77C-26 7 1.01 12 13.7 816 P 1.99 3242 0.5 ± 0.3 77C-27 6 1.51 17 18.0 1009 P 1.97 3242 0.5 ± 0.3 CP-1 2 2.59 6 19.5 226 P 1.98 2226 0.8 ± 0.7 CP-2 6 1.32 19 10.7 771 P 1.98 2226 0.8 ± 0.4 CP-3 6 2.19 26 14.4 857 P 1.98 2226 1.0 ± 0.4 CP-4 6 3.46 40 14.9 864 P 1.98 2226 1.5 ± 0.5 CP-5 6 2.95 30 18.8 957 P 1.98 2226 1.0 ± 0.4 CP-6 6 1.57 4 14.9 1893 P 1.98 2226 <0.2 CP-8 6 1.96 39 11.3 1121 P 1.98 2226 1.1 ± 0.4 79F-2 7 1.40 11 10.2 400 P 2.05 2204 0.9 ± 0.6 79F-A 6 0.12 1 7.15 298 P 2.03 2204 <0.3 79F-C 6 0.240 2 11.9 494 P 2.03 2204 <0.3 79G-2 6 0.524 8 9.60 733 P 1.96 2206 0.3 ± 0.2 79M-7 6 <0.100 0 10.3 551 P 1.96 2206 <0.2 79M-11 6 <0.04 0 13.1 1607 P 1.96 2206 <0.06 80M-2 5 0.882 49 12.1 336 P 1.61 2180 3.8 ± 1.2 81G-10 7 0.270 2 7.86 291 P 1.91 2143 <0.5 81G-11 6 0.488 7 6.44 462 P 1.91 2143 0.5 ± 0.4 81G-14 6 0.568 5 11.6 508 P 1.91 2143 <0.6 81G-15A 6 0.309 3 8.62 419 P 1.91 2143 <0.5 CPE-30 8 0.609 11 6.91 624 P 1.94 2179 0.6 ± 0.6 CP-32 6 4.32 30 12.2 425 P 1.94 2179 2.2 ± 0.8 CP-33 6 4.41 51 9.31 539 P 1.94 2179 2.9 ± 0.9 81-1F 6 0.060 1 4.85 404 P 1.94 2179 <0.3 81-1C 6 0.080 1 10.4 648 P 1.94 2179 <0.2 83M-1 9 2.08 55 16.2 526 P 1.92 2694 0.8 ± 0.2 83M-2 9 2.28 58 13.0 1657 P 1.92 2694 1.1 ± 0.3 83M-4 10 2.03 31 14.3 1091 P 1.92 2694 0.9 ± 0.3 83M-5 9 2.92 67 13.6 1555 P 1.92 2694 1.3 ± 0.3 83M-6 10 0.563 6 8.49 452 P 1.92 2694 0.4 ± 0.3 83M-7 10 0.531 15 13.0 1830 P 1.92 2694 0.2 ± 0.1 83M-8 7 0.756 7 18.2 842 P 1.92 2694 <0.4 Note: Zeta: (SRM 962) = 319.6 (zircon external detector); DF = laboratory number; F2 = pass (P) or fail (F) chi-square test at 5% (external detector runs). Central age was used if sample failed chi-square test (Green et al., 1989; Galbraith and Laslett, 1993).

mal paleomagnetic orientations of clinker on the east end of Anderson outcrop/burn front. The one ZHe clinker sample this ridge, suggesting that the clinker at the east edge of the from the Felix coal zone, southwest of Wright, had a very escarpment formed more recently than the magnetic reversal young age of 0.024 ± 0.001 Ma. at 0.778 Ma (Fullerton et al., 2004a). West Side of Powder River Basin Felix On the west side of the basin near Buffalo, Wyoming, clin- Three clinker samples from the Felix coal zone near ker beds from the Healy-Ucross (Lake De Smet) coal zone in the Wright, Wyoming, had ZFT ages of 0.9 ± 0.6 Ma or younger. Wasatch Formation at 1402 and 1439 m above sea level yield The Felix coal lies ~100 m above the Wyodak-Anderson coal, ZHe ages of 0.019 ± 0.001 Ma and 0.117 ± 0.007 Ma, respec- and its outcrop/burn front is 10–15 km west of the Wyodak- tively, suggesting denudation rates of ~0.4 mm/yr. TABLE 3. (U-Th)/He AGES OF CLINKER IN THE POWDER RIVER BASIN Sample Age* No. of Coal zone Legal description Latitude† Longitude† Elevation Comments number (Ma) samples (°N) (°W) (m) Wyoming Rochelle Hills SPL1zb, 0.205 ± 0.012 2 Wyodak NENW Sec. 35, 43.5792 105.1159 1518 Piney Canyon Road CLK1c T42N, R69W SPL2zA,B 0.482 ± 0.027 2 Wyodak NWNE Sec. 28, 43.6796 105.1502 1484 Ridge N of Little T43N, R69W Thunder Creek SPL3zA,B 0.198 ± 0.011 2 Wyodak NENW Sec. 29, 43.6797 105.1758 1460 Ridge N of Little T43N, R69W Thunder Creek CLK5a,c,d 0.536 ± 0.025 3 Wyodak NESW Sec. 13, 43.7002 105.0924 1527 Ridge N of Little T43N, R69W Thunder Creek CLK6b 0.127 ± 0.010 1 Wyodak NWNW Sec. 6, 43.6499 105.2008 1445 Quarry E of Black T42N, R69W Thunder Mine CLK9a,b 0.013 ± 0.002 2 Wyodak NESE Sec. 34, 44.0911 105.3652 1417 Quarry pit E of Belle T48N, R71W Ayr Mine PRB11zA,B 0.010 ± 0.001 2 Wyodak NWSE Sec. 32, 43.5684 105.1681 1497 S of Piney Canyon T42N, R69W Road PRB12zA,B 0.120 ± 0.008 2 Wyodak SWSE Sec. 34, 43.5667 105.1299 1538 100 m E of Red Spring T42N, R69W PRB15zA,B 0.216 ± 0.012 2 Wyodak NESE Sec. 12, 43.5397 105.0856 1563 Keyton Canyon T41N, R69W PRB17zA,B 0.615 ± 0.035 2 Wyodak SWNE Sec. 5, 43.5586 105.0502 1608 N of Keyton Creek T41N, R68W PRB18zA,B 0.550 ± 0.031 2 Wyodak SWNE Sec. 5, 43.5572 105.0504 1598 N of Keyton Creek T41N, R68W PRB19zA,B 0.502 ± 0.028 2 Wyodak SWNE Sec. 6, 43.5574 105.0681 1602 N of Keyton Creek T41N, R68W Felix coal zone PRB20zA,B 0.024 ± 0.001 2 Felix NWNE Sec. 27, 43.6777 105.4892 1540 SW of Wright T43N, R72W Lake De Smet CLK7c,d 0.019 ± 0.001 2 Lake De Smet SWSW Sec. 31, 44.5166 106.7879 1415 Road W of Lake T53N, R82W De Smet dam CLK8b,c 0.117 ± 0.007 2 Lake De Smet NENW Sec. 31, 44.3535 106.5319 1436 Hilltop NE of McNeese T51N, R80W Draw Montana Tongue River valley 03NPRB1z 0.013 ± 0.001 2 Anderson NESE Sec. 10, 45.0584 106.8087 1063 Railroad cut N of A,B T9S, R40E Decker mine 03NPRB3z 0.652 ± 0.026 4 Anderson SWSE Sec. 16, 45.6568 106.1819 1295 Butte N of Cook EH1,2,3,4 T2S, R45E Mountain 03NPRB4zB 0.110 ± 0.009 1 F NWNE Sec. 21, 45.6533 106.1799 1213 Butte N of Cook T2S, R45E Mountain 03NPRB5z 0.155 ± 0.009 2 E SENW Sec. 21, 45. 6480 106.1822 1122 Butte N of Cook A,B T2S, R45E Mountain 03NPRB6zA 0.287 ± 0.023 1 Sawyer SENW Sec. 8, 45.5894 106.2034 1000 Quarry pit SW of Cook T3S, R45E Mountain 03NPRB14zA, 0.056 ± 0.004 2 Knobloch NESE Sec. 28, 45.5445 106.2970 912 Where Bridge Creek B T3S, R44E joins Tongue River 03NPRB16zA 1.07 ± 0.086 1 Anderson SWNE Sec. 15, 45.6687 106.5317 1305 Headwaters of Miller T2S, R42E Creek 03NPRB18zA, 0.105 ± 0.005 3 Anderson SWSW Sec. 27, 45.6327 106.5403 1267 Highway 212 S of B,F T2S, R42E Badger Peak 03NPRB19zA, 0.981 ± 0.040 4 Anderson NENE Sec. 29, 45.6397 106.4442 1309 Ridge S of Garfield B,C,D T2S, R43E Peak 03NPRB22zA, 0.603 ± 0.028 3 Knobloch NENE Sec. 16, 45.7591 106.2971 960 N of Roe and Cooper C,D T1S, R44E Creek 03NPRB25zB 0.152 ± 0.012 1 Knobloch NWNW Sec. 45.5788 106.2283 928 Hwy 212 E of Ashland 18, T3S, R45E Northeast Montana 638-2z A,B 0.144 ± 0.014 2 Unknown NWSW Sec. 16, 48.177 104.498 655 Pit E of Hwy 16, N of T28N, R56E Culbertson *Weighted mean calculated for multiple measurements from same site; ± error is two standard deviations (internal errors only). †Latitude and longitude in North American Datum 27. TABLE 4. (U-Th)/He DATA AND AGES FOR ZIRCONS FROM POWDER RIVER BASIN CLINKER Sample number 4He U Th Ft* Mass Half-width U Th Age ± (fmol) (ng) (ng) (μg) (μm) (ppm) (ppm) (Ma) (2V) 03NPRB1zA 0.175 2.756 0.749 0.775 6.13 43 450 122 0.014 0.002 03NPRB1zB 0.624 8.219 3.371 1† 8.29 51 992 407 0.013 0.001 Weighted mean 0.013 0.001 03NPRB3z_EH1 0.756 0.307 0.149 0.669 1.98 28 155 75.5 0.612 0.049 03NPRB3z_EH2 2.84 1.066 0.332 0.715 2.75 34 388 121 0.644 0.052 03NPRB3z_EH3 1.29 0.460 0.167 0.705 2.04 35 225 82.0 0.679 0.054 03NPRB3z_EH4 1.28 0.483 0.100 0.685 1.71 32 283 58.6 0.682 0.055 Weighted mean 0.652 0.026

03NPRB4zB 0.188 0.466 0.078 0.652 1.49 30 313 52.6 0.110 0.009 03NPRB5zA 0.261 0.584 0.167 0.564 0.82 21 713 203 0.138 0.011 03NPRB5zB 0.079 0.144 0.048 0.509 0.50 19 288 95.9 0.186 0.015 Weighted mean 0.155 0.009

03NPRB6zA 0.613 0.607 0.158 0.616 1.22 25 498 130 0.287 0.023 03NPRB14zA 0.509 2.283 0.683 0.715 3.00 34 760 227 0.054 0.004 03NPRB14zB 0.055 0.195 0.124 0.673 1.59 31 122 78.8 0.068 0.010 Weighted mean 0.056 0.004

03NPRB16zA 1.35 0.248 0.299 0.732 3.09 40 80 96.8 1.07 0.086 03NPRB18zF 0.509 1.223 1.190 0.703 2.30 34 531 517 0.089 0.007 03NPRB18zA 0.577 1.115 0.663 0.717 3.30 36 338 201 0.117 0.009 03NPRB18zB 0.996 2.032 0.703 0.710 4.16 32 488 169 0.119 0.009 Weighted mean 0.105 0.005 03NPRB19zC 6.63 1.649 0.614 0.699 4.10 33 402 150 0.981 0.079 03NPRB19zD 7.63 1.962 0.529 0.660 3.15 27 623 168 1.03 0.082 03NPRB19zA 2.37 0.493 0.285 0.689 4.07 28 121 70.0 1.14 0.091 03NPRB19zB 1.93 0.559 0.148 0.704 3.03 33 185 48.8 0.857 0.069 Weighted mean 0.981 0.040 03NPRB22zA 4.24 1.615 0.599 0.754 5.76 40 280 104 0.595 0.048 03NPRB22zC 1.58 0.617 0.262 0.703 2.55 32 242 103 0.613 0.049 03NPRB22zD 1.37 0.582 0.169 0.678 1.65 30 352 102 0.602 0.048 Weighted mean 0.603 0.028

03NPRB25zB 0.172 0.376 0.218 0.491 0.48 18 784 454 0.152 0.012 638-2zA 0.094 0.204 0.070 0.576 0.65 23 316 108 0.138 0.015 638-2zB 0.051 0.081 0.028 0.549 0.48 22 169 58.6 0.198 0.048 Weighted mean 0.144 0.014 PRB20zA 0.132 1.34 0.71 0.69 2.73 31 491 261 0.0236 0.0019 PRB20zB 0.193 2.10 0.50 0.662 2.28 27 920 220 0.0245 0.0020 Weighted mean 0.0240 0.0010 PRB18zA 1.80 0.80 0.42 0.699 3.27 31 245 128 0.530 0.042 PRB18zB 1.75 0.75 0.39 0.679 2.12 32 352 183 0.572 0.046 Weighted mean 0.550 0.031 PRB17zA 5.75 2.52 0.39 0.709 3.33 33 757 118 0.576 0.046 PRB17zB 1.77 0.63 0.30 0.701 3.28 32 192 92.6 0.667 0.053 Weighted mean 0.615 0.035 PRB19zA 3.07 1.47 0.52 0.713 3.43 34 429 151 0.501 0.040 PRB19zB 1.90 1.01 0.37 0.643 1.82 26 553 204 0.502 0.040 Weighted mean 0.502 0.028 PRB11zA 0.455 8.50 1.25 0.796 1.25 46 682 101 0.0121 0.0010 PRB11zB 0.080 1.72 1.05 0.804 1.27 51 136 83.0 0.0094 0.0010 Weighted mean 0.010 0.0010 PRB12zA 0.976 1.77 0.62 0.763 6.97 40 254 89.4 0.124 0.010 PRB12zB 0.817 1.56 0.69 0.755 5.34 42 291 129 0.117 0.010 Weighted mean 0.120 0.0080

(continued ) 168 Heffern et al.

TABLE 4. (U-Th)/He DATA AND AGES FOR ZIRCONS FROM POWDER RIVER BASIN CLINKER (continued ) Sample number 4He U Th Ft* Mass Half-width U Th Age ± (fmol) (ng) (ng) (μg) (μm) (ppm) (ppm) (Ma) (2V) PRB15zA 0.828 0.93 0.22 0.753 5.32 40 174 40.6 0.209 0.017 PRB15zB 1.27 1.35 0.91 0.673 2.31 30 584 395 0.224 0.018 Weighted mean 0.216 0.012 SPL1zb 1.34 1.48 0.259 0.785 8.84 46 167 29.3 0.207 0.017 CLK1c 0.939 1.09 0.553 0.717 4.84 32 220 115 0.203 0.016 Weighted mean 0.205 0.012 CLK5a 0.692 0.318 0.212 0.681 2.45 31 130 86.6 0.513 0.041 CLK5c 4.86 2.19 0.956 0.703 3.42 32 629 287 0.539 0.043 CLK5d 2.31 0.996 0.540 0.703 2.80 35 346 189 0.559 0.045 Weighted mean 0.536 0.025 SPL2zA 3.08 1.46 0.695 0.726 3.93 39 371 177 0.486 0.039 SPL2zB 2.63 1.24 0.495 0.755 6.14 32 202 80.6 0.478 0.038 Weighted mean 0.482 0.027 SPL3zA 1.20 1.74 0.641 0.621 1.44 24 1210 445 0.191 0.015 SPL3zB 0.428 0.575 0.192 0.625 1.29 26 446 149 0.206 0.016 Weighted mean 0.198 0.011

CLK6b 3.10 6.24 1.47 0.686 15.3 30 401 142 0.127 0.010 CLK7c 0.221 2.14 0.761 0.782 8.84 45 236 178 0.021 0.002 CLK7d 0.182 2.37 0.771 0.781 8.71 45 265 125 0.017 0.001 Weighted mean 0.019 0.001 CLK8b 0.290 0.624 0.360 0.711 3.23 35 187 117 0.109 0.009 CLK8c 0.385 0.774 0.349 0.669 2.08 29 364 168 0.127 0.010 Weighted mean 0.117 0.007 CLK9a 0.060 0.690 0.460 0.731 4.31 37 154 108 0.020 0.004 CLK9b 0.215 4.28 0.178 0.786 7.51 49 557 41.0 0.012 0.002 Weighted mean 0.013 0.002 638-2zA 0.0945 0.204 0.070 0.576 0.646 23 316 108 0.138 0.015 638-2zB 0.0513 0.081 0.028 0.549 0.480 22 169 58.6 0.198 0.048 Weighted mean 0.144 0.014 Incompletely reset zircons from thin clinker units 638-1zA 2458 0.663 0.477 0.684 1.56 32 424 305 819 65 638-1zB 3328 1.326 0.153 0.743 3.15 41 421 48.6 586 47 PRB13zA 8.76 1.07 0.45 0.724 3.83 36 281 118 1.90 0.15 PRB13zB 1.75 1.77 0.54 0.682 2.77 29 637 194 0.252 0.020 PRB14zA 23.9 0.37 0.36 0.723 3.83 37 95.6 95.2 13.5 1.08 PRB14zB 2.22 0.44 0.20 0.695 2.56 33 174 78.5 1.21 0.097 Note: Uncertainties are based on internal errors only. *Ft is the alpha-ejection correction (see Farley et al., 1996; Reiners, 2005). †Ft of unity is used for this sample because crystal was completely coated in >17 μm thickness of clinker matrix.

Montana zones in the Tongue River valley. Figure 8 shows clinker dis- In the late 1970s and early 1980s, Naeser analyzed ZFT tribution and ZFT and ZHe ages in a part of the Tongue River ages of clinker samples collected from 20 locations in east- valley near Ashland, Montana. Reiners also analyzed one addi- ern Montana, 16 of which were in the Tongue River valley, a tional sample of clinkered glacial till from northeast Montana. dissected drainage system carved through the northern Pow- der River Basin. Eleven of the Tongue River valley samples Little Wolf Mountains were from clinker of the Wyodak-Anderson coal zone, and fi ve The oldest in-place clinker samples dated thus far, with were from clinker of the Knobloch coal zone. In addition, ZFT ZFT ages of 2.9 ± 0.9 Ma and 2.2 ± 0.8 Ma (Coates and Hef- ages were determined for two clinker samples from the Little fern, 2000; Heffern and Coates, 2004), come from the summits Wolf Mountains, one from detrital clinker in a gravel terrace, of the Little Wolf Mountains west of Colstrip, Montana (Fig. 1; and one from clinker in northeast Montana. Recently, Reiners Table 1) in the northern Powder River Basin. This narrow, iso- obtained ZHe ages from 11 clinker samples collected in 2003 lated range stands 300–400 m above the surrounding landscape from the Wyodak-Anderson, Knobloch, and intermediate coal and is capped by a layer of clinker as much as 30 m thick. Figure 6. Clinker outcrops, sample ages, and coal mines, Rochelle Hills, Wyoming (adapted from Coates and Naeser, 1984; Heffern and Coates, 2000, 2004). ZFT—zircon fi ssion-track age; ZHe—zircon (U-Th)/He age. 170 Heffern et al.

Figure 7. Zircon (U-Th)/He (ZHe) and zircon fi ssion-track (ZFT) ages versus sample elevation in Wyodak-Anderson clinker, Rochelle Hills, Wyoming (Fig. 6). Circles—ZHe; triangles—ZFT; black symbols—northern transect; gray symbols—southern transect. Error bars are 2σ. Solid lines are regressions through ZHe data for each transect, yielding apparent vertical erosion rates of 0.21 ± 0.01 km/m.y. and 0.25 ± 0.01 km/m.y. for the northern and southern transects, respectively (a.s.l.—above sea level).

Gravel Terrace the Wyodak-Anderson coal zone. Eleven ZFT samples of this A clinker boulder at the base of a gravel deposit on a strath clinker range in age from 1.5 ± 0.5 Ma to 0.8 ± 0.7 Ma. The terrace 360 m above the Yellowstone River, is even older, with stratigraphically lower clinker of the Knobloch coal zone is less a ZFT age of 3.8 ± 1.2 Ma (Coates and Heffern, 2000; Heffern than 100 m above present river level. Five ZFT samples of the and Coates, 2004). The site is 28 km southwest of Forsyth, Mon- Knobloch clinker range in age from 0.6 ± 0.6 Ma to less than tana, and 300 m below and 22 km north of the summits of the 0.2 Ma (Coates and Heffern, 2000; Heffern and Coates, 2004). Little Wolf Mountains (Fig. 1; Table 1). This age indicates that In 2003, we collected clinker samples for ZHe dating from coal beds were exposed and subjected to burning there as early 25 locations in the Tongue River valley. Samples from 11 of these as the Pliocene. Boulders of clinker were eroded from their place locations yielded zircon grains suitable for dating. Of these 11 of origin and deposited in a valley bottom in stream gravel. The locations, four were from the Wyodak-Anderson clinker, three high percentage of basaltic, porphyritic, and rhyolitic cobbles, in were from the Knobloch clinker, and three were from clinker addition to the locally derived clinker boulders, indicates that the of intermediate coal beds between the Knobloch and Wyodak- stream depositing these gravels originated to the southwest in the Anderson clinker, near Ashland, Montana; one was from the Absaroka Mountains of north-central Wyoming (Colton et al., Wyodak-Anderson clinker near the Montana-Wyoming state 1996; Fig. 1). Later, this gravel-fi lled valley bottom was isolated line. These ZHe ages ranged from 0.013 ± 0.001 Ma to 1.07 as a terrace when the stream cut down to a lower level. ± 0.086 Ma (Fig. 8). Strata in the Tongue River valley dip gently to the south, Tongue River Valley causing the clinker benches to descend southward to river level. In the northern Powder River Basin near Ashland, Montana, However, the Tongue River fl ows north, intersecting and eroding a layer of clinker as much as 60 m thick caps a plateau that forms stratigraphically lower coal beds. No coal has burned below the the rim of the Tongue River valley, 300–400 m above present level of the river, which controls the water table. The Knobloch river level (Fig. 8). This clinker results from ancient burning of coal is exposed in the bed of the Tongue River near Birney Day Figure 8. Clinker outcrops and sample ages, Tongue River valley, Montana (adapted from Bass, 1932; Coates and Heffern, 2000; Heffern et al., 1993; Heffern and Coates, 2004; Matson and Blumer, 1973; Vuke et al., 2001a, 2001b). ZFT—zircon fi ssion-track age; ZHe—zircon (U-Th)/He age. 172 Heffern et al.

School (Fig. 8), and the coal rises above river level northward, to allow the intersection of the river and the coal zone to migrate where it has burned to produce clinker as the water table has southward at a rate of ~40–50 mm/yr. This is consistent with the dropped below the coal. pattern found for the Wyodak-Anderson coal zone. The ZHe and The ZFT and ZHe ages (Fig. 8) record downcutting of the ZFT ages of the Wyodak-Anderson clinker compared with the Tongue River during the Pleistocene. The clinker ages indicate Knobloch clinker 300 m below indicate fl uvial incision rates of that the Wyodak-Anderson coal was fi rst exposed by the pre- ~0.3 mm/yr. historic Tongue River and burned progressively downdip. As East of the Tongue River near Ashland, three clinker rims the Tongue River continued to cut down through the strata of crop out at intermediate elevations between the 30-m-thick Wyo- the Fort Union Formation, successively lower coal beds were dak-Anderson clinker on a butte north of Cook Mountain (dated exposed and have burned to produce clinker, while backwasting at 0.652 ± 0.026 Ma) and the thick Knobloch clinker to the west along valley sides and incision of side drainages have exposed (dated at 0.152 ± 0.012 Ma), 360 m below the base of the Wyo- more of the Wyodak-Anderson coal farther away from the river. dak-Anderson near present-day river level (Fig. 8). Clinker of the More recently, the Knobloch coal zone (Fig. 5), 300 m below the F coal (Bass, 1932), on a steep hillside 70 m below the base of Wyodak-Anderson clinker, was exposed and has burned progres- the Wyodak-Anderson clinker, was dated at 0.110 ± 0.009 Ma. sively downdip to produce broad benches of clinker near present Clinker of the E coal on the same slope, 160 m below the base river level. of the Wyodak-Anderson, was dated at 0.155 ± 0.009 Ma. These West of Ashland, Montana, two ZHe samples on the edge of young ages are from thinner coal beds that created narrow rims of the Wyodak-Anderson clinker plateau have ages of 1.07 ± 0.086 clinker on the hillside and were only able to burn a short distance and 0.981 ± 0.040 Ma, while one sample in the interior of the pla- beneath the slope before being extinguished. The clinker bodies teau has an age of 0.105 ± 0.005 Ma. The ZHe and ZFT ages in produced by these thinner coals are less massive and less hard- this area suggest that the plateau capped by the Wyodak-Ander- ened by heating and therefore less resistant to erosion. The thick son clinker ~300 m above and 15 km west of the Tongue River resistant clinker that caps the butte likely protected the underlying has an age of 0.8–1.5 Ma for a large distance around its perim- coal beds from rapid erosion and resulted in those beds burning eter, but that clinker at the heads of deep (~8 km) lateral incisions a little bit at a time. The clinker of the Sawyer coal, which forms into the plateau has ages at least as young as 0.1 Ma. This pattern a broader bench 280 m below the base of the Wyodak-Anderson, of ages would suggest a lateral scarp retreat rate of ~8–10 mm/yr, has a somewhat older age of 0.287 ± 0.023 Ma. slightly less than that observed in the Rochelle Hills to the south. The Wyodak-Anderson (Garfi eld) clinker that caps a butte north Northeast Montana of Cook Mountain on the east side of the Tongue River valley Naeser obtained a ZFT age of less than 0.06 Ma for a sample (Bass, 1932) is considerably younger (0.652 ± 0.026 Ma) than of clinker derived from glacial till and collected from the wall the Anderson clinker promontories west of the river; the reason is of a postglacial gorge of the Missouri River south of Culbert- unclear. The Wyodak-Anderson clinker that caps Browns Moun- son, Montana (Table 1). Reiners obtained a ZHe age of 0.144 tain east of the Tongue River near Birney Day School has a ZFT ± 0.014 Ma (Table 3) on a sample of clinkered glacial till col- age of 1.1 ± 0.4 Ma. The ZHe age of a Wyodak-Anderson clinker lected by Wayne Van Voast of the Montana Bureau of Mines and sample taken from an outcrop 30 km north of Sheridan is very Geology from a gravel pit north of Culbertson. This ZHe age young (ca. 0.013 ± 0.001 Ma). Its location, 70 km south of and indicates baking of Illinoian or pre-Illinoian till after the Illinoian 200 m lower in elevation than the other samples, is where the glacier (Fullerton et al., 2004a, 2004b) receded from the area. Wyodak-Anderson coal zone descends to the level of the Tongue River at the Decker mine just north of the Montana-Wyoming Regional Considerations state line. This relationship shows that the Tongue River has cut down to the elevation of the Wyodak-Anderson coal zone at that Regional patterns of clinker ages enable us to make geomor- location only recently and that the river exposed this coal zone phic interpretations and inferences. While an individual clinker much earlier farther north, which is both downstream and updip. age represents the burn front of a particular coal bed at a spe- Three-hundred meters below the Wyodak-Anderson clinker, cifi c point in time, the spatial patterns of clinker and clinker ages downstream (northward) ZHe age progressions in the Knobloch refl ect the longer-term evolution of the landscape and exhuma- clinker in the Tongue River valley show ages of 0.056 ± 0.004, tion of the Powder River Basin. Three possible controls on late 0.152 ± 0.012, and 0.603 ± 0.028 Ma, with distances between Cenozoic fl uvial erosion rates in the Powder River Basin region the fi rst and second and second and third samples of ~5.0 and are: (1) base-level change of the Yellowstone-Missouri and/or 20 km, respectively (Fig. 8). The ZFT age of the Knobloch clin- Mississippi River systems (e.g., Zaprowski et al., 2001); (2) ker near Birney Day School is relatively young, less than 0.2 Ma, long-wavelength tectonic uplift (e.g., Leonard, 2002; McMillan but north of Ashland, it is as old as 0.6 Ma because the river cut et al., 2002); and (3) climate change that enhanced fl uvial erosion down and exposed the Knobloch zone much earlier to the north. rates and infl uenced groundwater levels (Molnar and England, These dates show that the north-fl owing Tongue River cut down 1990; Dethier, 2001; Molnar, 2001; Zhang et al., 2001). Because through the south-dipping Knobloch coal zone rapidly enough clinker is found throughout the Powder River Basin at a range of Geochronology of clinker and implications for evolution of the Powder River Basin 173 elevations, ZHe and ZFT dating offer a promising means to dis- coal beds and more coal fi res. Coal beds were exposed in north- tinguish among several of these processes. We note that the limi- east Montana as a result of incision of meltwater streams and tation of burning to tens of meters below the surface does not limit catastrophic drainage of glacial lakes and later burned to produce the available age record of landscape changes to only the tens of clinker. Clinker ages can potentially provide bracketing ages for meters nearest the surface because the scale of topographic relief glacial events or deposits but cannot date the actual glacial event in the Powder River Basin is hundreds of meters. This relief pre- (David Fullerton, 2005, personal commun.). serves in situ clinker in cap rock as old as 2.9 Ma, where, locally, vertical erosion rates have been relatively slow (e.g., <0.01 mm/ CONCLUSIONS yr, for an ~30-m-thick clinker of this age), though regional-scale erosion rates, as determined from age variations over greater dis- The most important control on clinker age patterns across tances, may be signifi cantly faster. the landscape is the progress of erosion that brings coal above Large-scale, regional fl uctuations of water-table elevation the water table and allows air from the surface to ventilate the resulting from tectonic or climatic factors may play some role in coal. Each natural coal fi re starts at a surface exposure but may suppressing or enhancing coal fi res close to the surface, where continue to burn at depth if the coal is ventilated and not satu- the coal aquifer becomes unconfi ned. Water-table fl uctuations by rated. At a given site, the clinker closest to the burn front (that themselves are probably only a minor factor in large-scale age furthest back beneath the slope) is generally younger than the variations because adequate ventilation by near-surface oxygen clinker closest to the outcrop on the hillside. Regional clinker is the most direct control on coal burning and clinker formation age patterns and observations of modern coal fi res indicate that (Lyman and Volkmer, 2001; Heffern and Coates, 2004). most coal in the Powder River Basin burns, forming clinker, as it Climate changes that trigger continental or alpine glacial- is exhumed above depths shallower than several tens of meters. interglacial cycles may (a) cause variations in water or sediment Thus, clinker ages provide estimates of the timing of local exhu- fl ux through the basins, (b) change large-scale drainage patterns, mation, and, through regional age patterns, rates of regional ero- and (c) infl uence exposure of coal beds and clinker formation. sion—both vertical and lateral—due to fl uvial incision. Our preliminary data set of replicated ZHe ages from 26 sepa- The limited number of dates collected so far prevents us rate clinker outcrops over a large part of the Powder River Basin from making detailed analyses of erosion rates; however, some and one clinker outcrop from northeast Montana shows no clear general patterns have become evident. Coal beds in the Powder relation with either the oxygen isotope record (Shackleton and River Basin have been exposed and have burned naturally since Pisias, 1985; Shackleton et al., 1990, 1995), or glacial cycles in at least the early Pliocene. Burning progresses in a systematic the Northern Plains or nearby Laramide ranges (Phillips et al., pattern as erosion removes the sedimentary fi ll of the basin, 1997; Chadwick et al., 1997; Fullerton et al., 2004a, 2004b); exposing stratigraphically lower coal beds, and rivers carve val- however, our data set is too limited and the resolution too coarse leys upstream. The burning of a thick coal bed generally pro- to compare with oxygen isotope stages. duces higher temperatures over a longer time than burning of a Continental glaciations in the northern plains of Montana thin bed, so that the clinker produced is thicker, harder, and more occurred ca. 15–30 ka (late Wisconsin), and 130–190 ka (Illi- resistant. The sheets of clinker produced by major coal beds are noian), with interglacial intervals in between (Fullerton et al., more durable and extensive than clinker produced by thin coal 2004a, 2004b; David Fullerton, 2005, personal commun.). Rem- beds, and the oldest clinker preserved in place is the thick clinker nants of the Archer till suggest two other glacial intervals between capping the higher divides. Thin clinker rims along valley sides 640 and 660 ka, and again between 710 and 740 ka. Deposi- are generally young. In addition, detrital clinker in gravel terraces tion of the Archer till occurred during oxygen isotope stages 16 and alluvial fans, derived from clinker bodies that have eroded and 18, before emplacement of the Lava Creek B tephra from away in the distant past, can in some cases provide older dates of the Yellowstone caldera at 639 ± 2 ka (Fullerton et al., 2004a, burning than in situ clinker outcrops. In at least one case, where 2004b) but after the Matuyama-Brunhes paleomagnetic reversal the gently tilted exposure of the Wyodak-Anderson coal in the at 778 ka (Fullerton et al., 2004a, 2004b). An interglacial interval Rochelle Hills has produced a laterally eroding clinker plateau, of nearly 500 k.y. followed deposition of the Archer till, and a depth versus age relationships allow us to estimate a vertical ero- long interglacial interval preceded deposition of the Archer till. sion rate of ~0.2–0.3 mm/yr for that area. At times during the Wisconsin and Illinoian glaciations, the Yel- The 27 ZHe ages and 39 ZFT ages collected from clinker in lowstone River was dammed northeast of Glendive, Montana, by northeast Wyoming and eastern Montana provide a glimpse of the advance of the continental ice lobe (Fullerton et al., 2004a, the geologic history of the region over the past several million 2004b), which may have limited downcutting and exposure of years. In limited areas, these ages have allowed us to derive rough coal beds when glacial lakes were present. In contrast, one could estimates of the rates of vertical and lateral erosion by some of speculate that the lowering of sea level or base levels during gla- the rivers and streams that carved into the Paleogene strata of the cial stages could have increased erosion in drainages not dammed Powder River Basin. Many more clinker outcrops and detrital by glacial lobes—such as the Cheyenne River drainage in the boulders over great areas in the region have not been isotopically southern Powder River Basin—leading to greater exposure of dated. Even in the areas where we have derived preliminary rate 174 Heffern et al. estimates, a greater density of data points would refi ne our analy- 1989, Pyrometamorphic rocks associated with naturally burned coal ses. The new ZHe data, as well as the earlier ZFT data, serve seams, Powder River Basin, Wyoming: The American Mineralogist, v. 74, p. 85–100. mainly to show that radioisotopic dating provides reproducible Dethier, D.P., 2001, Pleistocene incision rates in the western United States ages of clinker formation. These ages reveal systematic spatial calibrated using Lava Creek B tephra: Geology, v. 29, p. 783–786, doi: patterns that make sense in terms of expectations about landform 10.1130/0091-7613(2001)029<0783:PIRITW>2.0.CO;2. Dodson, M.H., 1973, Closure temperature in cooling geochronological and evolution in a basin undergoing exhumation. petrological systems: Contributions to Mineralogy and Petrology, v. 40, p. 259–274, doi: 10.1007/BF00373790. ACKNOWLEDGMENTS Energy Information Administration, 2006, Annual Coal Report 2005: U.S. De- partment of Energy Report No. DOE/EIA-0584(2005), 78 p. Farley, K.A., Wolf, R.A., and Silver, L.T., 1996, The effects of long alpha-stop- We wish to thank William Reiners of the University of Wyo- ping distances on (U-Th)/He ages: Geochimica et Cosmochimica Acta, ming for helping us to collect clinker samples from the Rochelle v. 60, p. 4223–4229, doi: 10.1016/S0016-7037(96)00193-7. Flores, R.M., 2004, Coalbed methane in the Powder River Basin, Wyoming and Hills during November 2002, Jason Whiteman of the Northern Montana: An assessment of the Tertiary–Upper Cretaceous coalbed meth- Cheyenne Tribe and Paul F. Gore for helping us collect samples ane total petroleum system, in Powder River Basin Province Assessment from the Tongue River valley during August 2003, and the late Team, eds., Total Petroleum System and Assessment of Coalbed Gas in the Powder River Basin Province, Wyoming and Montana: U.S. Geologi- Wayne Van Voast of the Montana Bureau of Mines and Geol- cal Survey Digital Data Series DDS-69-C, Chapter 2, 62 p. ogy for providing the sample of clinkered till from northeast Flores, R.M., and Bader, L.R., 1999, Fort Union coal in the Powder River Basin, Montana. We appreciate the constructive reviews of our draft Wyoming and Montana: A synthesis, in Fort Union Coal Assessment Team, eds., 1999 Resource Assessment of Selected Tertiary Coal Beds and Zones manuscript by Gretchen Hoffman of the New Mexico Bureau in the Northern Rocky Mountains and Great Plains Region: U.S. Geological of Geology and Mineral Resources, John Garver of Union Survey Professional Paper 1625-A, Chap. PS, CD-ROM, 75 p. College, and Timothy Rohrbacher, Art Schultz, and Jack Mc- Fredlund, D.E., 1976, Fort Union porcellanite and fused glass: Distinctive lithic materials of coal burn origin on the northern Great Plains: Plains Anthro- Geehin of the U.S. Geological Survey. The input from Roger pologist, v. 21, p. 207–211. Colton and David Fullerton of the U.S. Geological Survey re- Fullerton, D.S., Colton, R.B., and Bush, C.A., 2004a, Limits of mountain and garding glaciation and landscape evolution in eastern Montana continental glaciations east of the Continental Divide in northern Mon- tana and north-western North Dakota, U.S.A., in Ehlers, J., and Gibbard, was valuable. Catherine Riihimaki of Bryn Mawr College also P.L., eds., Quaternary Glaciations—Extent and Chronology, Part II: Am- provided help in interpreting clinker ages in relation to geomor- sterdam, Netherlands, Elsevier, p. 131–150. phic evolution of the Powder River Basin. Thanks also go to Fullerton, D.S., Colton, R.B., Bush, C.A., and Straub, A.W., 2004b, Map Show- ing Spatial and Temporal Relations of Mountain and Continental Glacia- Ken and Pam Kania of Ashland, Montana, for their hospitality, tions on the Northern Plains, Primarily in Northern Montana and North- to Stefan Nicolescu for analytical assistance with ZHe dating, western North Dakota: U.S. Geological Survey Scientifi c Investigations and to Larry Neasloney and Mike Londe of the U.S. Bureau of Map 2843, scale 1:1,000,000, 36 p. brochure. Galbraith, R.F., and Laslett, G.M., 1993, Statistical models for mixed fi ssion track Land Management for explaining the mysteries of geographic ages: Nuclear Tracks and Radiation Measurements, v. 17, p. 197–206. information systems and global positioning systems. 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