The control of the smectite–illite transition on passive-roof duplex formation: Canadian Rockies Foothills, Alberta

The roof décollement to the passive-roof duplex and triangle zone in the central Foothills, Rocky Mountain thrust belt is located within a sequence of siliciclastic sediments dominated by shale/mudstone. It is unclear why the décollement is found at a certain level in the stratigraphy, when the whole package appears to have similar lithilogical and hence mechanical properties. Investigation of the mineralogy of the mudstones reveals that the clay fraction preserves a smectite-to-illite transition, which formed during maximum burial conditions. The top of the transition is associated with the top of the roof décollement in the region. In the Gulf of Mexico, the transition correlates with both a zone of overpressure, favorable for detachment, and a corresponding drop in density (rise in porosity). In the Foothills, the transition also records a drop in density and a concomitant drop in sonic velocity. By analogy with the Gulf of Mexico, the drop in density may record a paleo-overpressured horizon that was favorable for detachment and formation of a roof décollement in the Foothills.