A Depositional Model of the Antelope Coal Field, Powder River Basin, Wyoming

Abstract

ABSTRACT The Anderson and Canyon coal seams are the main seams at the Antelope coal field in the south-central Powder River Basin. Each seam splits into multiple and thinner coal seams to the southwest in the study area. The strata underlying the Canyon coal seam suggest that the area of the Antelope coal field was a poorly drained swamp that developed into a well-drained swamp with minor small channels. The area further changed to a poorly drained swamp which initiated peat accumulation in the Canyon coal swamp. The lithologies, sedimentary structures, and isopach maps of the Canyon seam partings indicate that crevasse splaying, lacustrine conditions, and small crevasse-channel development produced the observed splits in the Canyon coal seam. Distal overbank deposits occur at the top of the C nyon seam and at the base of the Anderson seam; well-drained swamp deposits and crevasse splay, lake, lacustrine delta, and small channel-fill deposits occur between the coal seams. The Anderson coal seam represents another development of a poorly drained swamp where peat accumulation was significant. The rocks overlying the Anderson seam represent a combination of the environments mentioned above, with deposits from lakes and well-drained swamp environments dominating. The coal-beating sediments were deposited in paludal and tributary subsystems along the northern margin of the Wright delta. Deposition of the Canyon and Anderson coal seams and associated parting lithologies may have occurred when a river system or parts of that system underwent anastomosis. The coal swamp in which the Anderson seam was deposited was destroyed when the anastomosed character of the river system ended or the anastomosed river system shifted westward. This may reflect a relative change in the rate of subsidence within the basin.