Systematic Jointing in the Cardium Sandstone Along the Bow River, Alberta, Canada

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Four steeply dipping joint sets are present in folded and thrusted Cretaceous Cardium sandstone beds where they crop out along the Bow River in the Rocky Mountain foothills of Alberta, Canada. All four sets are infilled by quartz and calcite. The relative ages have been determined by cross-cutting relations of the fillings at joint intersections. Some of the joint fillings in three of the sets show slickenside striations parallel to the trace of bedding and in two of these sets the fillings are also scored by thrust-fault slickensides. Thus these three sets must have formed, filled, and accommodated lateral displacements before folding. Steeply pitching slickenside striations score the surfaces of many of the fourth set, indicating that this set probably formed before or at the time of regional normal faulting which post-dates folding and thrusting.

Joint-surface structures indicate that all four sets formed by incremental crack growth outward from near the center of the sandstone beds.

Joint spacing increases with bedding thickness and each joint set present within a bed has its own spacing. Quartz-filled microjoint sets in the same orientation as the macro-joint sets are present in each bed, even in beds where the corresponding macro-joint set is absent. The macro-joint spacing and the presence of micro-joint sets suggest that each sandstone bed jointed independently in response to a common stress field.

The orientation of each joint set is interpreted to be perpendicular to a horizontal component of a tectonic stress field that was either the maximum principal tension or least principal compression and which changed azimuthal direction with time. This interpretation is based on an analysis of the orientation of cracks propagated in marble plates loaded in uniaxial tension which suggests that cracks propagate perpendicular to the maximum-circumferential-tensile-stress component of the stress field in the region of the crack tip.

The energy consumed by the formation of a joint of typical area (6 m('2)) is calculated to be between 624 and 1038 Joules based on laboratory measurement of the fracture-energy of the Cardium sandstone.

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