Coal deposits in the front ranges and foothills of the Canadian Rocky Mountains, southern Canadian Cordillera

Abstract

Canada has the fifth largest inventory of coal resources in the World. More than one-third of Canada's resources currently favourable for exploitation occur in the southern Canadian Rocky Mountains, with major deposits in the Front Ranges, Inner Foothills and Outer Foothills physiographic provinces. In the Front Ranges, major deposits of metallurgical and thermal, high-volatile bituminous to semi-anthracite coal occur in the Late Jurassic-Early Cretaceous Mist Mountain Formation. In the Inner Foothills of northeastern British Columbia and west-central Alberta, significant resources of metallurgical and thermal, medium- and low-volatile bituminous coal occur in Early Cretaceous strata of the Gething (Aptian) and Gates (Albian) Formations. In the Outer Foothills resources of thermal, high-volatile bituminous coal occur in Late Cretaceous and Paleocene strata of the Belly River (Campanian), Coalspur (Maastrichtian-Paleocene) and Paskapoo (Paleocene) Formations. The major coal deposits of the Front Ranges and Inner Foothills accumulated within the coastal plains of the Fernie and Moosebar-Clearwater seas, respectively. Shorelines prograded to the north and northeast into a foreland basin that evolved in advance of, and in response to, the evolving Columbian Orogen. In the Outer Foothills, coal deposits of the Belly River Formation originated within prograding coastal plains during the withdrawal of the Pakowki Sea. The major coal deposits of the Coalspur and Paskapoo Formations accumulated in the alluvial plain environments of the foreland basin during the Laramide orogeny. Coal measures of the southern Canadian Rocky Mountains were variably folded and faulted during the Laramide orogeny (Late Cretaceous-Eocene). Structural deformation has significantly affected the mineability and quality of the coals. Many of the coal seams are sheared, structurally thickened or thinned and repeated by thrust faults or truncated by normal faults. Shearing of the seams in adjacent hanging and footwall strata has led locally to pronounced oxidation, resulting in weak coal pillars and roofs in underground mines. Faulting and folding has partitioned many coal deposits into discrete structural domains of varying style and complexity, each of which can present a unique challenge in designing optimum coal extraction methods. The variation in coal rank in the southern Canadian Rocky Mountains mainly reflects the preorogenic (pre-Laramide) depth of burial. In some areas, however, there is evidence for syn- and post-orogenic coalification, resulting from the emplacement of thick overthrust sheets and/or deposition of thick Tertiary molasse, most of which has been subsequently eroded. With few exceptions, coals of the southern Canadian Rocky Mountains are characterized by low sulphur contents (< 1%) and ash yields that range from 10% to 30%. The maceral composition of the coals is highly variable; Late Jurassic and Early Cretaceous coals are known for high percentages of semi-fusinite (ca. 30%) and Late Cretaceous and Paleocene coals for high vitrinite contents compared to most coals. All of the coal-bearing stratigraphic successions include multiple, thick seams and all existing mining operations are designed to extract and, where necessary, blend multiple seams.