Geological Atlas of the Western Canada Sedimentary Basin - Chapter 33

Chapter 33
Coal Resources of the Werstern Canada Sedimentary Basin

Authors:
G.G. Smith - Geological Survey of Canada, Calgary
A.R. Cameron - Geological Survey of Canada, Calgary
R.M. Bustin - University of British Columbia, Vancouver

Introduction

Canada has vast coal resources, most of which are found in the Western Canada Sedimentary Basin, where they are being mined and used extensively for the generation of electricity. Considerable tonnage is shipped abroad for use mainly in the production of metallurgical coke.

At various times during the Late Jurassic to Paleocene, tectonic, sedimentological and ecological factors combined to provide favourable coal-forming environments in the foreland basin along the eastern flank of the Cordillera, resulting in major deposits of thermal and metallurgical coals. These coals are widespread and have diverse characteristics with regard to their composition, physical properties, maturity (rank), and stratigraphic and structural framework.

The term "coal" is used for a rock that comprises mainly plant-derived carbonaceous material. The term is generic and is applied to rocks having significantly different properties. These differences have profound implications on the potential utility of a coal. Most coals are consumed either by combustion to raise steam for electric power generation, or by carbonization to produce metallurgical coke. Coals that are used to fuel electric power generating plants are referred to as thermal coals. Coals that are suitable for the production of metallurgical coke are referred to as metallurgical coals.

In addition to their commercial applications, coals are useful indicators of environments of deposition within sedimentary basins, and of the thermal histories of the basins. The present distribution and character of coals in the Western Canada Sedimentary Basin reflect mainly regional variations in environments of deposition and post-depositional development of the foreland basin during the Columbian and Laramide orogenies.

The composition of coals in the basin was controlled mainly by depositional environment and climate. These factors influenced the types and proliferation of coal-forming flora, and conditions of early diagenesis of accumulated plant debris. Post-depositional tectonic and thermal history of the basin, mineralization within the fractures and pores of coal beds, and oxidation have modified the composition and properties of the coals.

Coal maturation is characterized by a progressive loss of volatile matter and increase in carbon content, an increase in latent heat value, and a decrease in porosity and inherent moisture content. Increasing maturation, which changes the basic properties of coal, is commonly expressed in terms of coal rank in the continuous series that ranges from lignite through subbituminous, high volatile, medium volatile and low volatile bituminous ranks, to anthracite and meta-anthracite (Fig. 33.1). Metallurgical coals in the Western Canada Sedimentary Basin range in rank from high volatile A bituminous to low volatile bituminous. Coals of all other ranks are classed as thermal coals.

The variation of coal rank in the Western Canada Sedimentary Basin closely reflects the maximum depth of burial of the coal measures, which was related to burial beneath a thick Tertiary molasse, much of it subsequently eroded during the latter stages of the Laramide Orogeny (Nurkowski, 1984), and/or tectonic burial below stacked thrust sheets (Hughes and Cameron, 1985; England and Bustin, 1986; Bustin and England, 1989). Variations in paleogeothermal conditions also have left an imprint on coal rank patterns.

Variations in the patterns of subsidence, orogenesis and stratigraphic fill in the Cordilleran foreland basin resulted in the development of geologically, geographically and physiographically distinct regions between which coal properties and deposits differ fundamentally. These differences are a major consideration in coal exploration, evaluation, development and resource management.

Within the southern Canadian Rocky Mountains major coal deposits occur in the Front Ranges, inner foothills and outer foothills (MacKay, 1947; Smith, 1989a; Bustin and Smith, in press). Middle and upper Paleozoic carbonates and Mesozoic clastics of the Front Ranges are characterized by major east-verging thrust faults with up to tens of kilometres displacement and coeval folds. Deposits of high to low volatile bituminous metallurgical coals and rare semianthracite occur in the Jurassic-Cretaceous strata of the Mist Mountain Formation (Fig. 33.2a). The Rocky Mountain Foothills, which lie between the Front Ranges and the western Interior Plains, comprise mainly deformed Mesozoic and Cenozoic clastic rocks. The inner foothills, immediately east of the Front Ranges, embrace a high-relief area of mainly Lower Cretaceous coal-bearing strata. In the inner foothills of northeastern British Columbia and west-central Alberta, significant resources of medium to low volatile bituminous metallurgical coals occur in the Lower Cretaceous Gething (Aptian to Lower Albian) and Gates (Albian) formations (Fig. 33.2b). The topography of the outer foothills is more subdued and underlain by more recessive-weathering Upper Cretaceous and Tertiary coal-bearing strata. In the outer foothills of western Alberta, resources of high-volatile bituminous thermal coals occur mainly in the Coalspur Formation (Maastrichtian-Paleocene), with minor amounts in the Belly River (Campanian) and upper Brazeau (Maastrichtian) formations (Fig. 33.2a,c).

East of the Cordilleran deformed belt, relatively undeformed Upper Cretaceous and Paleocene coal-bearing strata occur near the surface within the region of the Interior Plains. In the Interior Plains, resources of thermal, mainly subbituminous and lignite coals occur in: 1) Upper Cretaceous strata of the Foremost, Oldman and Horseshoe Canyon formations, and the Wapiti Group; 2) Upper Cretaceous-Paleocene strata of the Scollard Formation; and 3) Paleocene strata of the Paskapoo and Ravenscrag formations (Fig. 33.2d,e). Coal beds also occur in the Lower Cretaceous Mannville Group and correlative strata (e.g., Swan River Formation) that underlie the Interior Plains. With the exception of the lignitic to subbituminous coals in the Firebag and Wapawekka coalfields (Fig. 33.2d,e), these Lower Cretaceous coal deposits occur at depths beyond current conventional coal mining capabilities.

In the Front Ranges and inner foothills, tectonic deformation of coal measures is commonly the major factor that controls the present areal extent, thickness variability, lateral continuity, and geometry of coal beds. Both sedimentological factors and tectonic deformation have left a significant impression on the present distribution and geometry of coal beds in the outer foothills. In the Interior Plains, sedimentological factors have been the major control on the present distribution of coal beds. Variable coal accumulation in areas within each region, in addition to the truncation of coal deposits by erosion and/or faulting, has resulted in the formation of discrete coalfields (Fig. 33.2).