ABSTRACT: Middle Devonian Thrustbelt and Foreland Basin Development, Southeastern British Columbia

ROOT, KEVIN G., Shell Canada Limited, Calgary, Alberta, Canada

Middle Devonian foreland strata in the southern Canadian Cordillera comprise the Eifelian Mount Forster Sequence and the overlying Eifelian Harrogate Formation. These successions are restricted to the western part of the Rocky Mountain Belt and are bounded to the east by the Western Alberta Ridge, considered to be a peripheral bulge associated with Middle Devonian thrusting. The Mount Forster Sequence includes the Cedared Formation, Burnais Formation, and Mount Forster Formation, and consists of carbonate, gypsum, and fine-grained clastics in eastern exposures. The sequence thickens to the west and contains a progressively greater proportion of texturally and mineralogically immature coarse-grained clastics that were derived from Upper Proterozoic and Lower Paleozoic strata. At the we ternmost exposure, at Delphine Creek in the Purcell Mountains, the Mount Forster Sequence is up to 540 m thick and comprises sandstone, pebble, and boulder conglomerate, slate, and volcanic flows. The Harrogate Formation exhibits little lateral variation and comprises carbonate and mudstone.

Direct evidence of Middle Devonian contractional deformation is found at Delphine Creek, where folds developed (1) during Mount Forster deposition, resulting in significant lateral changes in thicknesses and lithologies of lithostratigraphic members, (2) after Mount Forster deposition but before Harrogate deposition, resulting in an angular unconformity with pre-Harrogate dips locally up to about 70 degrees, and (3) during Harrogate deposition, resulting in lateral changes in thickness and facies. A thrust fault with several hundred meters of displacement cuts the Mount Forster Sequence and is truncated by the sub-Harrogate unconformity.

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