Abstract

The Lithoprobe Southern Alberta Lithospheric Transect (SALT 1995) data offer an excellent opportunity to study the structural styles of the Western Canada Sedimentary Basin (WCSB) as it extends from the triangle zone in the Foothills to the Plains of southeastern Alberta. The SALT data in southernmost Alberta contain five significant Late Cretaceous extensional faults, which are spaced regularly ( approximately 11 to 15 km apart), and occur as far as 140 km to the east of the triangle zone in a westerly downflexed basin. Two distinct fault geometries are imaged by the seismic reflection data: (1) a break in near-basement through Devonian reflectors and folding of overlying reflectors (up into the Upper Cretaceous strata) with no apparent evidence for thickness change across the extensional faults; and (2) a break in near-basement up into lowermost Upper Cretaceous reflectors (Base of Fish Scales), folding of Upper Cretaceous reflectors (up into Milk River Group strata), and horizontal undeformed uppermost Upper Cretaceous reflectors (Belly River Group strata and younger). These geometries are interpreted to be the product of extensional forced folding developed above master extensional faults, cutting Lower Paleozoic strata (Cambrian and Devonian) with distributed strain in overlying strata (Mississippian, Jurassic and Cretaceous). Horizontal undeformed uppermost Upper Cretaceous (Belly River Group and younger) strata constrain the age of these faults to pre-Belly River (Campanian) age. The mechanism for Late Cretaceous extensional faulting in the WCSB of southern Alberta is interpreted to be the flexural subsidence of the foreland plate, generated by thrust loading along the continental margin of western North America during Laramide Orogeny. The westernmost fault on the SALT data, located about 52 km east of the triangle zone may have inverted shortly after its down-to-the-west extensional development (at present a compressional fault dipping to the west), as the compressional Laramide stresses produced far field effects. The regional strike of the extensional faults appears to be northwest-southeast, which agrees with basement fabric trends inferred from magnetic signature of the Archean Medicine Hat Block. This alignment suggests that basement structure may have influenced the positioning of Late Cretaceous extensional faults within the WCSB in southern Alberta.

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