Abstract The Powder River Basin of northeast Wyoming and southeast Montana contains coal resources larger than those of any other basin of comparable size in the United States. These coal resources are developed in 16 strip mines for coal-fired power plants and for coalbed methane in more than 20,000 wells for distribution throughout the country. During this field trip, strip mining methodology and technology at the Wyodak coal mine, and operations at the nearby Wyodak power plant and in coalbed methane fields will be observed and discussed. The power plant utilizes feed coal from the mine and supplies electricity for the City of Gillette and surrounding areas. Coalbed methane development in the basin has coexisted with coal mining since the early 1980s. The same coal beds that are being mined yield gas in the subsurface, which is produced, collected, and moved to pipelines to access the interstate pipeline grid to serve the Rocky Mountains, Midwest, and California customers. Keywords: Powder River Basin, Wyoming, coal, coalbed methane, gas.

Abstract This two-day excursion will travel to the Yampa coal field located in parts of Moffat, Rio Blanco, and Routt Counties, northwestern Colorado. The excursion will visit classic regression/transgression successions in the Upper Cretaceous coal-bearing Mesaverde Group, which was deposited along the western edge of the Cretaceous seaway. It includes visits to inactive and active mine sites where past and current mining practices will be discussed. This guide summarizes the stratigraphy, sedimentology, and coal geology of the Yampa coal field. The trip will emphasize the depositional setting, sedimentology, and quality of the Upper Cretaceous coals and coal-bearing strata of the Mesaverde Group.

δ 13 C and δ 34 S were determined for 47 coal samples from the Williams Fork Formation—31 samples from the Wadge coal bed and 16 samples from the Lennox coal bed. δ 13 C ranges from −23.4 to −27.2‰). Organic sulfur δ 34 S ranges from +5.3 to +13.5‰ for the Wadge bed and from +13.7 to +20.1‰ for the Lennox bed. The organic sulfur content of the coal samples ranges from 0.23 to 0.71 percent for the Wadge bed and from 0.65 to 2.72 percent for the Lennox coal bed. The ash content of both beds is low, averaging 8.5 percent for the Wadge bed and 6.4 percent for the Lennox bed. The carbon isotopic homogeneity of the Wadge and Lennox beds indicates that the plants in each mire were similar with respect to the carbon fixation processes and carbon source. Previous sulfur isotopic studies of coral and the coal-forming processes have shown that δ 34 S is determined by the aquatic composition of sulfur in the peat-forming environment. In a freshwater mire, the δ 34 S of aquatic sulfate fluctuates about a mean of 5 ± 3‰, whereas in the marine environment, δ 34 S of aquatic sulfate clusters around +20‰. In peat-forming mire that is inundated by marine water, much of the sulfate is reduced by sulfate-reducing bacteria. As a result of this microbiologic activity, the active sulfur, which is assimilated into the decaying organic substrate, is depleted in 34 S. However, if the sulfate-reducing bacteria are absent, the peat possess only sulfur with the isotopic composition of the growth environment. In a coastal mire, this sulfur could be similar to that of the marine water. The low sulfur content and the isotopic composition in the lower part of the Wadge bed are consistent with sulfur assimilation in a freshwater growth environment. The increasing sulfur content and the increasing abundance of heavier isotopes toward the top of the bed suggest that, during the later stage of development of the coal mire, the peat-forming plants were increasingly influenced by a marine source and that marine sulfur was assimilated. The moderately high sulfur content and the 34 S enrichment in the Lennox coal samples suggest that this mire was clearly influenced by a marine source.

The Cretaceous coal-bearing facies of North and West Africa are directly related to the breakup of Africa and North and South America, global sea-level fluctuations, and the resulting marine transgressive and regressive cycles. In the Cretaceous, the coals of North Africa formed near the equator in a warm and humid climate, in contrast to the North American temperate coals. Lower Cretaceous coal-bearing facies have been reported in North Africa in Algeria, Egypt, Ethiopia, Libya, Mauritania, and Senegal. Upper Cretaceous coal has been reported in Benin, Egypt, Mali, Niger, Nigeria, Senegal, and Sudan. A better understanding of Cretaceous facies relationships and shoreline trends should lead to the discovery of more coal deposits within North Africa. Major marine transgressions and regressions associated with rifting, the breakup of Gondwana and Laurasia, and eustatic sea level cycles developed during Cretaceous time in North and West Africa. During the Mesozoic Era, most of Africa was above sea level and marine deposition took place only in the marginal basins associated with the rifting of the continents. The opening of the South Atlantic and the breakup of Africa and south America occurred along a north-south rift system that began in the south during the latest Jurassic and migrated northward in the Early Cretaceous, reaching Nigeria by mid-Cretaceous time. The first marine connection between the North and South Atlantic occurred during late Albian time. Final separation of the continents occurred during the Santonian, and the permanent seaway developed in early Turanian time. The Tethys sea advanced across the Saharan Platform during the late Cenomanian-Turonian, Coniacian, late Campanian-Maastrichtian, and the Paleocene. Concurrently, the South Atlantic transgressed through the Benue Trough and Nupe Basin and linked with the Tethys sea in the Chad and lullemmeden Basins, forming the four trans-Saharan epeiric seaways.