A Study of Relations between Rock Fabric and Joints in Sandstone

Abstract

Significant relations are visible between properties of sedimentary rock fabric determined by microscopic and experimental study of sandstone and the pattern of jointing in these rocks where observed on structures in the field. Fine-grained sandstones in the Cardium Formation were studied in fold and fault structures along South Ram River in the central Alberta Foothills.X-ray diffraction determination of lattice-spacing in the quartz component of samples from the outcrop indicates that a state of residual elastic strain resides in grains comprising the rock fabric. The direction of relative compression in grains is parallel to regional trend of structure. Results of uniaxial compression experiments at low levels of strain, and of point-loading tests on discs cored from Cardium sandstone suggest a preferential orientation of microfractures parallel to this regional trend. These and other experimentally measured parameters of the rock fabric bear a direct geometrical relation to the pattern of joints observed and measured at various sites.These relations have importance for interpretation of jointing because they suggest how joint sets, though formed primarily during later stages of a tectonic cycle—during uplift and unloading of strata—have their directions influenced by earlier stages in that cycle, namely basin subsidence, sediment diagenesis, and initial tectonic loading. The relations observed in the present study also have importance wherever an understanding of joint patterns is a matter of economic importance. Two relevant examples are the interpretation of in situ stress measurements and the evaluation of fracture permeability in rock masses.