Distribution and coalification patterns in Canadian bituminous and anthracite coals

Abstract

Canadian resources of bituminous and anthracite coal occur in six of ten provinces and in the Yukon and Northwest Territories. These higher-rank coals are found in 12 of the 16 coal-bearing basins within Canada. For some of these basins, knowledge of rank and coalification patterns is limited; in others, it is extensive. Coalification patterns were examined in detail for two areas: the southern end of the Canadian Rocky Mountain coal belt which is underlain by the coal-bearing Kootenay Group and the Maritimes Basin which is underlain by several coal-bearing formations of Pennsylvanian (Late Carboniferous) age. In the southern Rockies, reflectances on coal range from 0.61 to 2.65% R 0 max, which represent high-volatile B bituminous coal to semianthracite. In general, there is an increase in rank from SE to NW and from NE to SW. Superimposed on this pattern are several anomalies. At several locations, ranks change rapidly over short distances. An example is in the Mount Allan syncline, where the reflectances in the Kootenay section on the SW vertical limb range from 0.68 to 0.94% R 0 max in contrast to a range of 1.30–2.49% R 0 max for a comparable section on the NE upright limb. This variation occurs over a distance of about 3 km. Multiple intersections of Kootenay strata in oil wells at the south end of the area show low reflectances (0.76% R o max) at depths of about 3800 m with no change in the shallower Kootenay repeats, whereas wells farther north show higher reflectances and increase with depth. Different burial depths, the influence of thick thrust plates, different paleogeothermal gradients and ground-water movements, all likely have played a part in generating these coalification patterns. A significant part of the coalification in the Kootenay Group appears to have been postdeformational. The rank of the coals in the various subbasins of the Maritimes Basin is nearly all bituminous. Evidence indicates that most of the coalification is postdeformational. Rank within a given coal bed generally increases with increase in depth of present-day burial and the pattern seems to be generally true for most of the coal fields studied. The average postdeformational component of total coalification has been calculated to be 72%. The most important coal field in the Maritimes Basin is the Sydney coal field. Here ranks have been shown to increase from NW to SE and from SW to NE, the latter direction following the downdip structure of the field. Coalification gradients for the Sydney field range from 0.052 to 0.088 R 0 max/100 m, reflecting different paleogeothermal gradients. In New Brunswick, local rank increases to anthracite are believed to be related to igneous activities. Reflectance measurements have been used to interpret stratigraphy and structure in several fields and even to trace the origin of coaly fragments obtained from deep-sea drilling near Bermuda.