ABSTRACT: Basement Structure, In-Plane Stresses, and the Stratigraphic Evolution of Cratonic Ramps in Foreland Basins

ROSS, G. M., Geological Survey of Canada, Calgary, Alberta, Canada

The stratigraphic record of the Western Canada sedimentary basin (WCSB) provides evidence for two main periods of anomalous sedimentation patterns that appear to have been controlled by basement anisotropies. The curvilinear reef trends of the Middle to Late Devonian of the Alberta basin, such as the Rimbey-Meadowbrook trend, are some of the most enigmatic structures in the basin, in addition to hosting prolific hydrocarbon reservoirs. The reason why the Alberta basin underwent rapid evolution from a nearly featureless carbonate shelf to a mosaic of reefs and basinal sediments remains unknown. In addition, Cretaceous sedimentation patterns, while dominated by flexural subsidence in front of the advancing thrust sheets, also display subtle basement controls of strandline positions and ransverse arches. The recently documented anisotropic nature of the basement suggests that some basement weaknesses may have been susceptible to reactivation. Basement control of some sedimentation patterns has been a long-held hypothesis, although a neglected element of the "basement-control" model is the recognition of a mechanical rationale for the linkage between basement and cover.

In-plane stresses are gaining wide recognition for their potential significance in controlling sea level fluctuations and contributing to subsidence patterns in basins. A variety of processes may contribute to in-plane stress levels including short-wavelength contributions from tectonic activity at plate margins and long wavelength contributions from the position of lithospheric plates with respect to geoid anomalies. The Late Devonian was a time when the North American continent was influenced by unusually high in-plane compressive stresses related to the presence of orogenic belts along the margin of the continent and its position in a pre-Pangean geoid low. It is proposed here that anomalously high in-plane compressive stresses during the Middle to Late Devonian may have exceeded t e yield stress of basement structures, leading to reactivation of basement weaknesses and thereby fragmenting the Alberta shelf into a complex of reefs and interreef areas. In-plane compressive stresses during Laramide convergence also appear to have locally reactivated basement structures. The lack of basement control during periods of tensile stress suggests that basement reactivation may be restricted to periods of compressive stress, in support of rheologic models that indicate higher yield strength for continental crust under compression rather than extension.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)