ABSTRACT

Hydrocarbon accumulations in the "foothills" at the eastern margin of the Canadian Rocky Mountains are structurally trapped. Exploration for them entails predicting the deep geometric configuration of potential reservoir beds in imperfectly understood areas. This prediction is commonly derived from analogies with the most appropriate of the region's typical structures, a pragmatic approach which is effective because the "foothills" contain a limited suite of relatively simple structural types:

1. Concentric folds (with their attendant decollement)
2. Low-angle thrust faults (commonly folded)
3. Tear faults (usually transverse)
4. Late normal faults (commonly listric)

The assemblage in a particular area is also a function of the degree of deformation and of the lithology of the deformed rocks. Intensity of deformation increases from east to west. Regional stratigraphic changes alter the major lithologic units, while local isopach or facies changes alter the distribution of incompetent rocks within units.

The structural styles are all "thin-skinned," as the underlying Hudsonian basement is not involved.

This review article summarizes current knowledge of the geometry of "foothills" structures, their mode of occurrence, and some of the empirical rules for their interpretation. Application of this knowledge is illustrated by attempts to provide geometric solutions for the structural conundrums posed by the Brazeau Range, the Flathead Fault and the Rocky Mountain Trench.