Print this pageSeismic Delineation of a Complex Tectonic Wedge, Rocky Mountain Foothills, Alberta, Canada
Kelley L. Classen, Donald C. Lawton, and Deborah A. Spratt
Department of Geoscience, University of Calgary, Calgary, AB, Canada
Triangle zones define the foreland extent of deformation in the Rocky Mountain Fold-Thrust Belt from northern Montana to northwestern Alberta. Exploration interest in this structural play has increased recently because of gas accumulations in Cretaceous sandstones in the hanging walls of blind thrust faults.
The 3D geometry of a triangle zone in the central Alberta Foothills is interpreted from 155 km2 of high-quality 3D reflection seismic data constrained by wells. Numerous foreland-verging, blind thrust faults control the structural framework of this tectonic wedge. They merge into a highly deformed zone beneath a hinterland-verging thrust that forms the upper detachment of the wedge. Overlying Upper Cretaceous and younger strata have been passively uplifted. The staircase geometry of the lower detachment cuts up-section towards the foreland from a décollement in Lower Cretaceous coals that occur above carbonates of Mississippian age. Deformation within the tectonic wedge is contained entirely within Mesozoic siliciclastic rocks.
Within the seismic survey area, the tectonic wedge is divided into two distinct domains. Domain 1, in the west, is dominated by foreland-verging thrust faults separating horses that rapidly change in geometry along strike, although the overall shortening remains relatively constant. Domain 2, in the east, lies beneath the upper detachment. It contains both foreland- and hinterland-verging thrust sheets, creating pop-up structures that exhibit an en échelon geometry along strike that may cause reservoir compartmentalization. Notably, strain associated with the pop-up structures does not fold the rocks above the upper detachment; it is absorbed by the surrounding shales. East of Domain 2, conformable reflections mark the foreland edge of the tectonic wedge. Domains 1 and 2 are up to 7.5 km wide and have been shortened by approximately 20%.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas