Cleavage in the Mississippian Banff Formation at Pigeon Mountain in the Front Ranges of the Rocky Mountains, Alberta, makes an average angle to bedding of 25 °with the pole to the cleavage approximately parallel to the transport direction of the McConnell Thrust. Detailed examination of the microfabrics that define the cleavage indicates that mechanisms of formation include dissolution, local precipitation in strain shadows, and rigid body rotations. Dissolution includes pressure solution of host rock at the immediate contact with thin, parallel, and anastomosing clay seams and passive solution in a zone up to 100 μm wide adjacent to the seams. Lens shaped pods of carbonate in all stages of dissolution along the clay seams appear to form part of a sequence of progressive coalescence of dissolution zones to form well defined tight cleavage with characteristically strong preferred orientation of clay platelets. The cleavage probably formed at a temperature of 150–280 °C and under a lithostatic pressure of less than 2 kbar (2 × 105 kPa).

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