Rank Studies of Coals in the Rocky Mountains and Inner Foothills Belt Canada
P. A. Hacquebard, J. R. Donaldson, 1974. "Rank Studies of Coals in the Rocky Mountains and Inner Foothills Belt Canada", Carbonaceous Materials as Indicators of Metamorphism, Kussell R. Dutcher, Peter A. Haequebard, James M. Schopf, Jack A. Simon
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Discussed in this study are regional and stratigraphic variations in the rank of coals in the Rocky Mountain region, as determined from vitrinite reflectance measurements. For the regional changes, the Kootenay coals of the Crowsnest Pass area have been examined; they show a progressive westward increase in rank. The variations in rank with stratigraphic position are illustrated in ten coal-bearing sections of Jurassic-Cretaceous age, situated between the Crowsnest field in the south and the Peace River canyon in the north. Both studies indicate preorogenic coalification, because the rank increases regularly with stratigraphic position, but not with geologic age, depth of mining, or degree of tectonic disturbance.
For each of the ten curves plotted, the coalification gradient is calculated in terms of percent reflectance (Ro) change per 100 m increase in depth. By relating this gradient to that of a known curve (the Peel curve of the Netherlands) a reference for comparison is obtained, which is expressed as the Peel rank ratio. Different ratios were obtained, which probably are related to variations in the temperature gradients. The lowest ratio was found in the Peace River canyon area, and the highest occurs in the Canmore coalfield.
The coalification gradient affects the availability of coking coals of most favorable rank, that is, the medium-volatile coals. With a low gradient (and corresponding steep curve), medium-volatile coals occur over a greater stratigraphic interval, with the possibility of a larger number of seams, than with a high gradient. Within limited areas of the same coalfield, the rank as determined from vitrinite reflectance can be used for correlating coal seams, provided a high coalification gradient is present. This method has been employed successfully in the Canmore coalfield on seams that lie not less than 120 ft apart stratigraphically.