Changes in Structural Style Controlled by Lithofacies Contrast Across Transverse Carbonate Bank Margins—Canadian Rocky Mountains and Scaled Physical Models

Abstract

Profound variations in fold and fault geometries are mapped across a transverse lithofacies boundary in the Upper Devonian (Frasnian) Fairholme Group between hydrocarbon-bearing dolostones of the Southesk and Cairn Formations (Leduc and Nisku reefs) and off-reef shales of the Mount Hawk, Perdrix, and Flume Formations (Cline Channel) in the North Saskatchewan River area. These lithofacies constitute only 10% of the thickness of the stratigraphic package, but, during compression subparallel to the bank margin, they affected the deformational style of the entire supercrustal wedge, including units below them as well as above them. This area is ideal for studying lateral changes in structural style because the transition occurs over a short distance (1.5 km) within individual thrust sheets, where the overlying lithologies, percent shortening, burial depth and thermal history remain constant along strike. The excellent exposures are at the same stratigraphic and structural level, and the Palliser Formation contains peloidal strain markers, making it possible to quantify the changes in structural geometry in cross sections and also to determine the mechanisms of deformation in outcrops and in thin sections. Upper Devonian (Famennian) and Mississippian platform carbonates overlying the reef facies are concentrically folded, and thrust faulting is the primary mechanism of shortening. Fault-bend folding broadly warps strata and the rare folds with wavelengths shorter than 500 m are complexly faulted in their cores. Where the platform carbonates overlie the off-reef shale facies, they are tightly folded above detachment faults. Pressure-solution cleavage is strongly developed and fractures are filled with calcite, thereby significantly reducing the porosity and permeability of reservoir rocks that overlie the incompetent shale facies. Scaled physical models deformed by the centrifuge technique reproduce the variations in structural style seen across transverse lithofacies boundaries and illustrate how a fault-propagation fold can convert into a detachment fold along strike. The position and geometry of the underlying reef margin are evident in plan view at early stages of deformation, but structure contour mapping and detailed structural analysis are required to locate the underlying reef margin at later stages of deformation in the models and real-world examples.