Structural features of coal measures of the Kootenay formation, southeastern Canadian Rocky Mountains

Coal measures of the Late Jurassic-Early Cretaceous Kootenay Formation are complexly deformed in the southeastern Canadian Rocky Mountains. The structural style and associated features of the coal measures are in part characteristic of the ‘Foothills Family’ of structures. In addition, by virtue of the major contrast in competency between the coal seams and adjacent strata, the structural features of the coal measures display considerable variation which, to some extent, can be correlated with the regional and local structural setting., The variation in the structural features of the coal measures have a marked influence on the mineability of the coal and both directly and indirectly on coal quality. During deformation the coal seams were the loci of interstratal slip, thrust faulting and detachment during folding.. The coal seams vary markedly in thickness; in some areas coal seams have been thickened as much as an order of magnitude in response to thrust faulting, normal faulting and folding, whereas in other adjacent areas, the seams may be completely pinched-off or faulted out. Structural thickening of the coal seams has been facilitated by cataclastic flow of the finely sheared coal along a myriad of discrete shear surfaces. The mesoscopic and microscopic fabric of the coal is cataclastic with the exception of local areas of apparently high strain where the vitrain and clarain components have behaved plastically. Shearing of the coal and adjacent strata has resulted in the introduction and dissemination of formerly discrete rock partings which in turn have produced abnormally high ash contents and poor washability characteristics and has made the coal more susceptible to oxidation. Measurement of vitrinite reflectance of coal in some major shear zones suggests, by comparisipn with samples heated in the laboratory for short durations, that frictional heating during shearing may have resulted in temperatures of up to 450°C. . Adjacent to and within other shear zones there is no evidence for frictional heating. The presence or absence of frictional heating may be the result respectively of stick-slip and stable sliding conditions during shear, which in turn may be a product of variable pore pressures. In underground mines the structural features of the roof rock and the coal seams have a pronounced effect on roof stability.. In the Vicary Creek mine, located in the hanging wall of the Coleman Fault, the Number 2 seam and some of the roof rock were pervasively sheared as a result of interstratal slip during flexure of the coal measures and possibly as a result of drag from overriding thrust faults. In such areas the coal pillars have low bearing strength and the cohesion between successive beds in the immediate roof rock has been destroyed, resulting in poor roof conditions. Slickenside striae on bedding surfaces, joints in the roof strata and some extension faults which cut the seam, define a kinematic and dynamic pattern which is consistent with the regional structure. In the Balmer North, Five Panel and Six Panel mines, located in the northern part of the Fernie synclinorium, the coal measures are only mildly deformed. A cleat system is present at all sample localities but no consistent pattern exists which can be related to the overall structure or to joints in the roof and floor.. In the Balmer North mine, young, gently west dipping, shear surfaces are present throughout which, in conjunction with slickensided bedding surfaces, have promoted roof and coal rib failure along north to northwesterly trends. In the Five Panel mine roof and coal rib failure have been facilitated by steep easterly dipping fractures. The absence of a consistent joint or cleat pattern in the Balmer North, Five Panel or Six Panel mines may be the result of mechanical anisotropy of the strata or of multiple episodes of deformation. Striated structures, many of which are conical in form, are common mesoscopic elements on fracture surfaces in the deformed coal. . Such structures, although rarely reported previously in the literature, occur at many localities in the study area. The structures are planar, conical and pyramidal in form, and are characterized by striae which radiate from a common apex and 'horsetail' to form subsidiary structures on the master surface. All three types of striated structures are considered the products of dynamic, brittle shear fracture which was possibly facilitated at failure by high inter- and/or intra-particle pore pressure.

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