Bulletin 056

Bulletin 056

BUL 056

Deformed Lower Cretaceous coal-bearing strata of the Grande Cache area, Alberta

Deformed coal-bearing strata of the Grand Cache area were studied to obtain techniques that could be used in exploration and development of the Smoky River coal field and other coal fields of Alberta''s foothills and mountains. The strongly folded and thrusted, exposed ocks of the Grande Cache area can be divided into three thrust heets: the Syncline Hills, Mason, and Muskeg thrust sheets. The oldest rocks are marine and non marine strata of the Jurassic/Cretaceous Nikanassin Formation. The Lower Cretaceous Luscar Group includes a thin basal conglomerate (Cadomin Formation), a predominantly non marine sandstone and shale unit that locally contains coal (Gladstone Formation), a shallow marine shale and sandstone unit (Moosebar Formation), and an upper non-marine sandstone and sand unit that contains thick commercial coal seams (Gates Formation). The thick coals of the Gates Formation were robably deposited on a coastal or delta plain. That formation is overlain by marine shales of the Albian/Cenomanian Shaftesbury Formation, largely marine sandstones and sands of the Dunvegan Formation, and marine shales of the late Cenomanian Kaskapau Formation. These strata are complexly deformed by the Laramide rogeny, between early Campanian and late Eocene. The deformation proceeded from southwest to northeast it is estimated that the main eformation in the Grand Cache area took place at the end of the Paleocene.

Shortening of these rocks, estimated from balanced downplunge cross sections, averages 31 percent. This shortening is accomplished by folding and faulting. Folds in the area are generally of the chevron variety, caused by shortening of a multilayered sequence of lternating competent and incompetent strata, where the thickness of the competent layers is fairly constant. The folds are cylindrical and maintain their shapes over distances of up to 2 km along the trend. However, at their tapering ends, they are conical. The majority of faults are southwest-dipping thrusts, displaying ramps that cut up stratigraphic section and flats that are parallel to bedding. Close connection between thrusting and folding is shown by ault-to-fold displacement transfer. The deformation model for the Grande Cache area is a series of fold-thrust structures with cylindrical chevron folds, which become conical at their tapering ends.

Incompetent material, such as coal, has flowed into dilation zones, which develop in the hinges of chevron folds. Another process of structural thickening of coal is duplex thrusting, where the roof thrust is the top and the floor thrust the bottom of the coal seam. Both structural situations are important coal exploration targets in the Grande Cache area. The rank of the coals, as determined by measuring mean maximum vitrinite reflectances, ranges from medium to low volatile bituminous. The rank data indicate that the degree of oalification was largely established during burial from Albian to late Paleocene times, before folding and faulting started.

Most joints are extensional with preferred orientations that conform o regional patterns of foothills and plains. They probably result from erosional unloading and uplift. Other joints show effects of compressive stress across the fracture plane, as indicated by slickensides. Some of these can be related to the regional movement picture of the fold and thrust belt; others are probably a result of gravitational sliding of blocks into valleys after erosional unloading and resulting uplift.

Langenberg, C.W., Kalkreuth, W.D. and Wrightson, C.B. (1987): Deformed Lower Cretaceous coal-bearing strata of the Grande Cache area, Alberta; Alberta Research Council, ARC/AGS Bulletin 56, 62 p.