Portland State University. Department of Earth Sciences
Michael L. Cummings
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
Coal -- Geology -- Wyoming -- Converse County, Sedimentation and deposition -- Wyoming -- Converse County
1 online resource (3, ix, 85 pages)
The coal-bearing sediments of the Antelope coal field in the southcentral Powder River Basin, Wyoming were deposited in paludal and tributary subsystems of the fluvial system that existed in the basin during the early Tertiary. A depositional model for the Antelope coal field was constructed from data collected from approximately 500 drill holes that penetrated the upper 90 meters (300 feet) of the Fort Union Formation. The depositional environments were interpreted from lithologic descriptions and guidelines established in the literature.
The two main coal seams at the Antelope coal field are the Anderson and stratigraphically lower Canyon coal seams. They represent poorly-drained swamp depositional environments. Each of the coal seams exhibit splits into multiple and thinner coal seams to the southwest. The parting rocks that lie between these splits, sedimentary structures, and isopach maps of the partings indicate that crevasse splaying with lacustrine and small channel development caused the observed splits in the coal seams. Distal overbank deposits occur at the top of the Canyon seam and at the base of the Anderson seam; well-drained swamp deposits and crevasse splay, lacustrine, lacustrine delta, and small channel-fill deposits occur in between the coal seams. The rocks 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 channel development. The area once again digressed to a poorly-drained swamp which was the beginning of the Canyon coal swamp. The rocks overlying the Anderson seam represent a combination of the environments mentioned above with deposits from lacustrine and well-drained swamp environments dominating.
The observed splits in the Anderson and Canyon coal seams to the southwest at the Antelope coal field suggest that a change in the fluvial system and/or tectonic stability of the Powder River Basin occurred and affected deposition in the southcentral portion of the basin. A combination of 1) relative basin subsidence, 2) a prograding and aggrading trunk stream with a thick levee deposit, and 3) peat accumulation that kept pace with relative basin subsidence are proposed mechanisms for the formation of the thick, continuous coal seams present in the basin and a disturbance or change in any of these processes could produce the splits observed in the Anderson and Canyon coal seams at the Antelope coal field.
Syn- and post-depositional processes that have affected the coal quality and reserves at the Antelope coal field include compaction, erosion and deposition from modern stream action, and burning and oxidation of the coal seams. The position of the paleowater table during stream downcutting and erosion of the coal seams controlled the occurrence and extent of oxidation and burning.
Exploration and development of the Antelope coal deposit can be executed in a more efficient manner by using the depositional model. Future exploration drilling programs, design of the mine site, mining and marketing the coal, and later reclamation of the mined area are all affected by the depositional model.
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Budai, Christine M., "Depositional Model of the Antelope Coal Field, Wyoming" (1983). Dissertations and Theses. Paper 57.