COAL PALYNOLOGY: CLUES TO DEPOSITIONAL ENVIRONMENTS CONTROLLING QUALITY AND CHEMISTRY OF MINABLE COALS IN THE POWDER RIVER BASIN

Stratigraphic palynology of the 120-ft thick Wyodak-Anderson coal in the Powder River Basin, Wyoming, indicates floral assemblages dominated by sphagnum, fern, and heath vegetation in the lower part and abundant Taxodium (cypress) and pines in the middle-upper part. These floras are interpreted as representing “bottomland” and “bog” plants, respectively. Such vegetation resembles modern Indonesian rain-fed, domed peat mires that evolved from flat-lying (topogenous) swamps to ombrogenous bogs. Flat topography permitted river floodwater detritus to be transported beyond the swamp margin, but after formation of a domed mire only volcanic ash from the Elkhorn volcanic arc in southwest Idaho was deposited.

Coal quality and chemistry of the Wyodak-Anderson coal indicate low average concentrations of ash (3.64%), sulfur (0.27%), and trace elements of environmental concern (TEEC) (0.001-22.4 ppm). Mudstone and sandstone partings are in the lower part of the coal and altered volcanic ash partings are in the middle-upper part. Sulfur generally increases upward. TEEC, especially phosphorus (as much as 0.19%), strontium (as much as 550 ppm), and barium (as much as 1,400 ppm) are high and associated with authigenic crandallite group minerals, which are indicative of altered volcanic ash in coal. Arsenic, mercury, and selenium are also high (1.5-0.069 ppm) in the volcanic ash partings.

Thus, concentrations of ash, sulfur, and TEEC in coal partings partially survived strong leaching of acids from plant decomposition in the domed mire and subsequent coalification that formed authigenic minerals. More importantly, a study of the coal palynology provided evidence as to the origin of the TEEC.