Influences of structural setting on coal rank and thickness in the Grande Cache area, Alberta, Canada

Abstract

In addition to mapping areas of deformed coal-bearing strata, structural geological studies are used to explain coal rank variations, to predict the location of the thickened coal and to explain certain optical properties of coal. In the coalfields of the Canadian Rocky Mountains and Foothills, north of Grande Cache, Alberta, the level of coalification decreases progressively from the undeformed part of the Alberta basin towards the western edge of the Foothills. This decrease is due to a westward decrease in duration and depth of burial as a consequence of the timing of Laramide deformation across the area, indicating synorogenic coalification. Within the smaller Grande Cache area it can be shown that coalification ended after deformation and resulting uplift and erosion. The presence of optically biaxial vitrinite anisotropy may indicate the presence of a tectonic stress field during the later stages of burial and subsequent deformation. The maximum vitrinite reflectance axis is generally oriented parallel to fold axes in the area. Structurally thickened coal can be attributed to at least two structural positions: fold hinges and fold limbs. Dilation occurs at chevron fold hinges and incompetent material, such as coal, flows into these dilation zones. The resulting structure is similar in geometry to a saddle reef. Duplexes are present in fold limbs, where the roof thrust is the top and the floor thrust the bottom of the coal seam, resulting in tectonic thickening of the coal. These structurally thickened coals are important exploration targets. Prediction of structurally thickened coal by computer constructed down-plunge cross sections has proved useful in coal exploration in the Canadian Rocky Mountain Foothills.