In the Rocky Mountain Foothills, major oil and gas accumulations occur in the folded and faulted leading edges of thrust sheets involving Paleozoic carbonates. These structures underlie a complex of imbrications involving Mesozoic clastic rocks. In this area the integration of seismic and geologic data leads to the definition of prospects and also illustrates concepts fundamental to an understanding of mountain building.
Reflection data show that for its entire width of about 80 miles, the Rocky Mountain fold belt is underlain by the gently westward dipping extension of the crystalline Precambrian Shield. Shortening, exceeding 100 miles in Paleozoic beds, takes place along décollement zones and curved thrust faults which flatten at depth (listric thrust faults). Late Mesozoic and early Tertiary thrusting was followed by late Tertiary normal faulting. Reflection data suggest that these normal faults, which are steep at the surface, also flatten at depth (listric normal faults) and may merge with older thrust faults.
Reflection sections show that at depth the structural style on both sides of the Rocky Mountain Trench is similar and they suggest a continuation of the westward dipping basement beneath, and well to the west of the Trench. Therefore the Trench and the associated post-orogenic Tertiary basins are probably related to a system of shallow, listric, normal faults that are responsible for the location and direction of this morphologic feature.
A palinspastic reconstruction based on seismic and subsurface data is essential background for discussions concerning the relations between the Rocky Mountains and the igneous and metamorphic western half of the Cordillera. More generally, relations between continental drift and the formation of the Western Cordillera are placed in perspective using such reconstructions.
The seismic reflection data shown provide insight into the structure of the crust down to depths of ten kilometers, and effectively bridge the gap between surface geology and deep crustal refraction data.