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Sedimentology of the Mist Mountain formation, in the Fording River area, southeastern Canadian Rocky Mountains

University of British Columbia

The Upper Jurassic-Lower Cretaceous Mist Mountain Formation at the Fording Mine in southeastern British Columbia is a 550 metre thick interval of non-marine, coal-bearing sediments. Three units can be distinguished in the stratigraphic sequence based on sandstone body geometry and coal seam characteristics. Unit I, the lowermost unit, represents prograding lower coastal plain deposits. Two major distributary channels trended northwest-southeast through the study area. These channels limited the lateral extent of coexisting peat swamps responsible for coal seams 1, 2 and 3. The dominantly herbaceous peats grew in hummock and depression topography in interchannel areas. Crevasse splay and overbank deposits flooded peat swamps causing cessation of growth and deposition of fine-grained muds and silts on peat surfaces. These mud and silt beds are the mudstone and siltstone partings that occur in the Mist Mountain coals. Unit II sediments accumulated in a fluvial-flood plain environment. Rivers trended northeast-southwest in a wide meander belt. Peat swamps supported herbaceous and arboraceous plants and were frequently disturbed by coexisting rivers. River and splay waters actively eroded peats and deposited fine- to medium-grained sediments on peat surfaces. The sediments of Unit III were deposited in a flood plain dominated environment on the upper coastal plain. Major rivers were concentrated outside of the study area allowing thick forest peat accumulations. Lake and flood waters covered peats causing mudstone splits in some coal seams and some argillaceous coals. Coals of the Mist Mountain Formation are low in sulfur (<0.8%) reflecting fresh water terrestrial origin and lack of marine influence. Ash content of the coals (5-33%) reflects a combination of biologically derived, authigenic and detrital ash. Authigenic ash is in the form of spherulitic siderite partings, whereas mudstone and siltstone partings constitute the detrital ash. Macerals in the Mist Mountain coals reflect the changing depositional environments in which the peats formed. Lower coals in Units I and II are high in inertinite and low in vitrinite. Higher in the stratigraphic section the coals are low in inertinite and high in vitrinite which reflects the peat composition and the degree of peat degradation. The depositional environments of the Mist Mountain Formation at Eagle Mountain have a marked influence on the mine-ability of the coal. Coal seam thickness and lateral continuity are modified by sandstone bodies. Draping of coal seams over channel sandstones and channel washouts of coal have resulted in rapid variations in interburden thickness. Prediction of sandstone geometry will allow more accurate definition of coal reserves and seam trends in initial exploration stages. Pit design and blasting can also benefit from a detailed knowledge of permeable sandstone bodies. By accurately defining sandstone trends, water seepage into pits can be minimized improving highwall stability and maximizing blast results.

Text

2010-05-11

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http://hdl.handle.net/2429/24578

Geological Sciences

University of British Columbia

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