Edward L. Heffern

Gas emissions, minerals, and tars associated with three coal fires, Powder River Basin, USA.

Ground-based surveys of three coal fires and airborne surveys of two of the fires were conducted near Sheridan, Wyoming. The fires occur in natural outcrops and in abandoned mines, all containing Paleocene-age subbituminous coals. Diffuse (carbon dioxide (CO(2)) only) and vent (CO(2), carbon monoxide (CO), methane, hydrogen sulfide (H(2)S), and elemental mercury) emission estimates were made for each of the fires. Additionally, gas samples were collected for volatile organic compound (VOC) analysis and showed a large range in variation between vents. The fires produce locally dangerous levels of CO, CO(2), H(2)S, and benzene, among other gases. At one fire in an abandoned coal mine, trends in gas and tar composition followed a change in topography. Total CO(2) fluxes for the fires from airborne, ground-based, and rate of fire advancement estimates ranged from 0.9 to 780mg/s/m(2) and are comparable to other coal fires worldwide. Samples of tar and coal-fire minerals collected from the mouth of vents provided insight into the behavior and formation of the coal fires.

Sr isotope tracing of aquifer interactions in an area of accelerating coal-bed methane production, Powder River Basin, Wyoming

Sr isotope data on groundwater samples from coal and overlying sandstone aquifers in the eastern Powder River Basin, Wyoming, demonstrate that the Sr isotope ratio effectively identifies groundwater from different aquifers where major ion geochemistry and O and H stable isotope data fail. Groundwaters from sandstone aquifers have a uniform 87Sr/86Sr ratio of 0.7126–0.7127. Waters from coal seams vary from 87Sr/86Sr ratio = 0.7127 near the recharge area to 0.7151 farther into the basin. The distinct Sr isotope signatures of sandstone and coal aquifers may reflect different sources of Sr in these two rock types: Sr in sandstones is held primarily in carbonate cement, whereas coals contain more radiogenic Sr in organic matter. The Sr isotope ratio is useful in identifying wells that contain mixed waters, whether due to well construction or to incomplete aquifer isolation. Measurement and continued monitoring of the Sr isotope ratio in groundwaters should provide a powerful tool for characterizing the impact of the burgeoning coal-bed methane industry on the hydrology of the Powder River Basin.

Clinker geochronology, the first glacial maximum, and landscape evolution in the northern Rockies

Late Cenozoic erosion in the Powder River Basin of northern Wyoming and southern Montana has exhumed numerous coal beds to shallow depths where they burn naturally, forming erosion-resistant metamorphic rocks called clinker. Because most clinker forms tens of meters from the surface, its formation age records the timing and rate of exhumation through this depth, which can be used to constrain incision and lateral backwasting rates and the evolution of topographic relief. Zircon (U-Th)/He ages from ~100 distinct clinker units provide several insights into the geomorphic evolution of the region. Ages of in-situ clinker range from as old as 1.1 Ma to as young as 10 ka, but most formed in one of the last three interglacial periods, reflecting either changes in fluvial incision caused by glacial-interglacial cycles or other climatic effects on rates of natural coal burning. Most clinker older than ca. 200 ka is either detrital or >~200 m above local base level. Detrital clinker atop a broad strath terrace in the northern part of the basin provides a maximum age of 2.6 ± 0.2 Ma for terrace formation. This corresponds to the onset of major Northern Hemisphere glaciation interpreted from marine records, suggesting that the terrace formed by lateral erosion of the landscape as rivers were overwhelmed with sediment during the earliest PlioPleistocene glacial episode. The overall correlation of in-situ clinker ages with elevation above local base level can be interpreted with a simple model for shallow exhumation ages that requires increasing incision and topographic relief over at least the past ~1 Myr at rates of ~0.1–0.3 km/Myr, assuming typical clinker formation depths of 20–40 m. INTRODUCTION Landscapes lacking large spatial gradients in rock uplift rates are typically dominated by erosional landforms with relief of tens to hundreds of meters. The evolution of these features reflects changes in regional drainage patterns that in turn reflect climatic and tectonic forcing over large areas. Conventional low-temperature thermochronologic approaches are not well suited to understanding the evolution of erosional landforms at these scales because even low-temperature systems have closure depths much greater than the scales of the features themselves and therefore constrain denudation rates over much larger length scales and time scales. Conversely, cosmogenic nuclide approaches generally constrain denudation rates through depths at approximately meter-length scales, much smaller than those of the landforms. Quantitative understanding of how landscape features in the range of tens to hundreds of meters in scale evolve requires an approach that provides estimates of ages and rates of exhumation through commensurate depths. In this paper, we summarize insights on landscape evolution of the Powder River Basin derived from formation ages of both in-situ and detrital clinker—metamorphic rock produced by the near-surface natural burning of coal. The approaches we use to interpret shallow exhumation ages provide several conclusions about the evolution of the region and highlight how similar types of constraints may be used to reveal patterns of relief change in erosional landscapes. THE POWDER RIVER BASIN AND CLINKER The Powder River Basin (Fig. 1) covers ~60,000 km of northeastern Wyoming and southeastern Montana near the northeast margin of the Rocky Mountain plateau, a region characterized by alternating mountain ranges with elevations up to 4.2 km above sea level and sedimentary basins with up to 11 km of Cenozoic structural depth. The Powder River Basin is both a Laramide syncline filled by Cretaceous and Paleogene sedimentary rocks and a modern drainage basin occupied by the Powder and Tongue Rivers draining to the north and the Belle Fourche and Cheyenne Rivers to the east. Most exposed rocks are fluvial sandstones and shales of the Paleocene Fort Union and Eocene Wasatch Formations, with coal beds up to ~60 m thick (Flores and Bader, 1999). Some of the thickest and most laterally continuous coals are associated with the WyodakAnderson coal zone, which, together with a few other Fort Union beds, made up ~42% of the 1.2 billion tons of coal mined in the U.S. in 2008 (U.S. Dept. of Energy, 2009). Powder River Basin coals are relatively low-grade and volatile-rich, causing them to burn naturally when ignited by spontaneous combustion or wildfires. Coal beds only burn when exhumed to depths less than a few tens of meters from the surface, where they are adequately ventilated and above the water table. Burning results in locally intense heating of adjacent rock units (primarily those overlying the coal, due to advection of heat by escaping gases), producing a variety of baked and melted rock types collectively called clinker (Rogers, 1918; Cosca et al., 1989; Heffern and Coates, 2004; Heffern et al., 2007), which covers ~3700 km of the Powder River Basin. Clinker tends to form erosion-resistant units that create escarpments and mesas, so its distribution dominates topography over much of the basin. Most of the southern Powder River Basin has broad rolling hills and flat-topped buttes capped by clinker, with relief typically <~200 m. However, large (~200 m) clinker-capped escarpments are present in some areas, such as the Rochelle Hills on the eastern side of the basin, formed by GSA Today, v. 21, no. 7, doi: 10.1130/G107A.1