Bulletin of Canadian Petroleum Geology, Vol.44, No.2, 299-312, 1996
Three-dimensional geometry of the structural front between Berland River and Smoky River, central Alberta Foothills
The geometry of the triangle zone and its variation along strike in the Alberta Foothills between 53 degrees 25＇N and 54 degrees 15＇N latitude were examined in detail by drawing closely spaced (1 to 3 km), balanced structural cross-sections constrained by seismic data interpretation and well-log analysis. The structural style of the triangle zone in this area is substantially different from that of the typical triangle zone which usually contains a series of foreland-verging thrust sheets which merge upward with a hinterland-verging backthrust sheet, The study area can be divided into northwest and southeast subregions. The stratigraphic sequence (Devonian to Tertiary in the northwest or Mississippian to Tertiary in the southeast) in the study area was divided into three structural packages by three detachments. In the northwest, the lower detachment Lies in the Lower Devonian Woodbend Group, the middle detachment in the Shaftesbury Formation and the upper detachment in the upper Kaskapau Formation. Tn the southeast, the lower detachment lies in the Exshaw Formation, the middle detachment in the Shaftesbury Formation and the upper detachment in the Wapiabi Formation. In the northwest, both the upper and middle detachments, along with the upper and middle structural packages, are folded into a syncline and anticline pair. The internal deformation of the rocks between the two detachments (middle structural package) is not clearly defined because no good seismic marker exists, but the existence of these two detachments is implied by the differential shortening among the three structural packages. Flexural folding of the middle and upper structural packages has resulted in local reversals of slip direction along the upper and middle detachments; initially there is one sense of slip along a horizontal detachment but during flexural folding the slip vectors point and decrease in magnitude toward the hinges of the syncline and anticline. In the northwest part of the study area the upper detachment is sharply kinked adjacent to the Plains and the lower structural package is thrusted into a pop-up structure. In the southeast, both the upper and middle detachments flatten smoothly toward the Plains; the middle detachment, however, acted as a floor thrust to the middle structural package as well as a roof thrust to the lower structural package. Each of these detachments combines with the foreland-verging thrust sheets beneath them forming a triangle zone structure. The fact that the lower structural package deformed the middle structural package implies that the higher triangle zone formed earlier than the lower triangle zone.