Counting marmots…
Coals of the eastern Piceance Basin occur in the Upper Cretaceous Iles and Williams Fork Formations that make up the Mesaverde Group, which outcrops along the Grand Hogback and related structures in Gunnison, Pitkin, Garfield, and Rio Blanco Co., Colorado. The Grand Hogback monocline dominates the area structurally and forms the eastern-southeastern boundary of the Piceance Basin. Other major folds include the Sulphur Creek syncline and the Coal Basin anticline. Faulting is minor except in the southern part of the area. Only the lithogenetically equivalent Trout Creek and Rollins Sandstones, the top members of the Iles Formation, were correlated with certainty throughout the study area, although tentative coal zone and associated basal sandstone correlations were made. The Iles and Williams Fork were deposited by a southeastward-prograding delta-deltaic plain complex. Intertonguing of marine and nonmarine deposits was caused on a local basis by compaction, shifting centers of deposition, variations in sediment supply, and possibly by growth faulting, and regionally by hinge-line faulting. Coal deposits in the Carbondale, Grand Hogback, and southern Danforth Hills coal fields range in rank from high volatile C bituminous to anthracite, and possibly graphite. Analyses of these coals by various techniques indicates that forest-swamp deposition in amore » subtropical to warm-temperate climate led to initial peat formation. Burial to depths of 11,700 ft or less produced coals as they are found today north of Four Mile Creek. South of Four Mile Creek, the coals were subjected to abnormally high geothermal gradients, the result of heat from the intrusions of the Elk and West Elk Mountains. The coking quality of the coals, while secondarily dependent upon the amount of woody material present in the original swamp and on certain other swamp conditions such as water chemistry and current velocity, is primarily a function of rank.« less
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