NEW MAP OF CLINKER IN THE POWDER RIVER BASIN - A RECORD OF ANCIENT COAL FIRES

Coal from the Powder River Basin (PRB) in northeast Wyoming and southeast Montana is not only burned today by power plants. Tens to hundreds of billions of tons of coal have ignited in innumerable natural fires since erosion first began to dissect the coal-bearing sediments of the PRB, at least as far back as five million years ago. Clinker – reddish baked rock created by the natural burning of these coal beds – covers 3810 square kilometers of the PRB. Resistant clinker-capped plateaus and benches affect development of the regional landscape. The Montana Bureau of Mines and Geology has recently published Geologic Map 64 – our effort to help the public better understand the distribution and age of clinker in the PRB. The bulk of the clinker (2830 sq. km.) lies in Montana, especially in the Tongue River drainage. Wyoming clinker (980 sq. km.) is most common in the Rochelle Hills east of Gillette and Wright and around Lake De Smet north of Buffalo. In addition, the map shows the location and ZHe (U-Th/He zircon dating) ages of 92 clinker samples we collected and analyzed, as well as locations of 38 ZFT (zircon fission-track dating) ages and 11 paleomagnetic orientations of clinker samples collected by previous researchers. Locations of 81 historic and/or active coal fires in the PRB are also noted. An associated spreadsheet includes data about each fire. The map is intended to serve as a companion piece to the PRB clinker articles in GSA Today by Reiners et al. (2011), in Geology by Riihimaki et al. (2009), and in GSA Reviews in Engineering Geology by Heffern et al. (2007).