The timing of coalification in relation to structural events in the Grande Cache area, Alberta, Canada

Abstract

Lower Cretaceous coals from the inner Foothills of the Rocky Mountains were sampled for a study of the relationships between coal rank and folding and faulting. The coals were collected from laterally continuous seams in a highly folded and faulted area near Grande Cache, Alberta. Rank of the coals as determined by measuring mean maximum vitrinite reflectances ranges from medium- to low-volatile bituminous.The results indicate that the degree of coalification was established during burial from Albian to Late Paleocene times, before folding and faulting started. "Isoreflectance" lines run parallel to the bedding of the folded strata, and there are no significant changes in reflectances in the same seam from one thrust sheet to the other. However, in the footwall of the Mason Thrust, the mean maximum reflectances and bireflectances are locally increased above regional values.Regionally there appears to be a trend to slightly lower reflectances in the southeast of the study area. It is suggested that the lower reflectances are related to a somewhat lesser former depth of burial of the coals, probably of the order of 50 m.