Abstract

This contribution describes evidence for Middle Devonian contractional deformation and foreland basin development in the Purcell and Rocky mountains of southern Canada. These are interpreted as early manifestations of the Antler Orogeny, which is well documented in the western United States. Middle Devonian foreland basin strata include the Mount Forster Formation and correlative strata, which change laterally from > 500 m of lithic sandstones, conglomerates and volcanic flows in westernmost exposures to predominantly shale and gypsum in exposures to the east, and the Harrogate Formation, which comprises shallow water carbonates and minor mudstone. Significant local lateral thickness and facies changes, and angular unconformities that truncate folds, were created within the Middle Devonian foreland basin strata in the Delphine Creek region of southeastern British Columbia in response to syndepositional folding within an interpreted wedge-top basin. A normal fault, formed during Neoproterozoic rifting, was inverted during the Devonian contraction. Middle Devonian foreland basin strata pinch out to the northeast against the West Alberta Ridge, which was a Middle Devonian paleohigh interpreted to have been a peripheral bulge.

Integration of the interpretations of Middle Devonian fold and thrust belt and foreland basin development with published structural and stratigraphic evidence from elsewhere in western Canada leads to the conclusion that Antler fold and thrust belt and foreland basin development may have occurred along the length of the Canadian portion of the ancient Cordilleran continental margin during the Devonian and Mississippian. The Western Canada Sedimentary Basin is interpreted to have been a distal foreland basin during the Devonian and Mississippian. Important changes in subsidence and uplift patterns within the basin, and an increase in the general rate of subsidence, may have been influenced by a combination of flexural loading by thrust sheets and foreland basin strata and horizontal in-plane stresses associated with Antler convergent tectonism. Faults that influenced deposition of mid-Paleozoic sediments, and created permeable pathways for the flow of hot fluids, may have formed as the result of horizontal in-plane stresses. Hot fluids interpreted to have significantly influenced diagenetic processes in mid-Paleozoic carbonate reservoir rocks may have been tectonically expelled in response to Antler thrust belt development.

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