Summary

The roof decollement 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 decollement is found at a certain level in the stratigraphy, when the whole package appears to have similar lithologic and hence mechanical properties. Investigation of the mineralogy of the mudstones reveals that the clays preserve a smectite-to-illite transition, which formed during maximum burial conditions. The top of the transition correlates with the top of the roof decollement 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 decollement in the Foothills.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas