Abstract

The Nisku Formation of the south Alberta and northwest Montana plains and the Birdbear Formation of the U.S. and Canadian Williston Basin are broadly contiguous and correlative, but possess divergent depositional histories. The Birdbear is dominated by level-bottom marine shelf, stromatoporoid biostrome, and Amphipora shoal facies, displaying peritidal and salina facies only near the end of its deposition. While the lower third of the Nisku likewise consists of marine shelf facies, the remainder of the formation is dominated by highly saline lagoon and tidal flat facies formed south of an arcuate barrier bank extending nearly 340 km (210 mi) across southern Alberta. Dolomite, argillaceous dolomite, and anhydrite forming the Nisku thicken northward from 15 to 50 meters (50-165 ft) across northwest Montana and south Alberta.

The lower Nisku marine shelf facies, which generally thickens to the northwest and supports a diverse marine fossil assemblage, is interrupted by a broad arcuate basin filled by pyritic, argillaceous dolomite bearing a dysaerobic fossil assemblage. This elongate, silled basin was an area of accelerated subsidence that roughly coincides with the crustally weak basement of the Vulcan Low. This arcuate area was subsequently occupied by barrier banks initiated after lower Nisku sedimentation. Vertical and lagoonward accretion of peloid grainstones comprising the banks was maintained until the end of Nisku deposition, forming an imperfect barrier to a vast lagoon to the south, broken only by a single prominent pass and intermittently by lesser passes. Northwest of the barrier marine shelf sedimentation continued unabated. The northwest barrier face was colonized by stromatoporoids and corals, the windward crest by Amphipora and serpulids, and the leeward slope exclusively by stromatolites.

Carbonate sediment manufactured on the barrier was carried leeward into the lagoon as calcarenite and calcisiltite washover aprons, concurrent with cyclical lagoon fill by carbonate mud and silt, punctuated by episodes of lagoon-wide marine to lacustrine deposition and by gypsum-precipitating salinas. Carbonate mud, silt and sand, produced in situ but constantly reworked, formed extensive tidal flats and local Renalcis-serpulid hardgrounds in frequently exposed coastal regions of northwest Montana. With subaerial exposure of much of the barrier arc during upper Nisku deposition, carbonate accretion shifted to the lagoon, resulting in centripetally thickening tidal flat deposits, supplemented by southeastward accretion of calcarenite aprons from the still-active eastern segment of the barrier. Nisku sediments were exposed repeatedly, allowing widespread establishment of vadose, meteoric phreatic, and brine phreatic zones above hydroseals, leading to extensive leaching, karst development, and massive sabkha-style gypsum precipitation. Brine seepage from thick Duperow-Peechee evaporites below and Potlatch-Stettler evaporites above may have been responsible for some part of the pervasive Nisku dolomitization.

Princess Field, discovered in 1939, marked the first Devonian oil discovered in Alberta. Princess and Jenner fields are structural closures over basement highs and currently produce gas from Nisku washover apron facies. East Kevin and Nine Mile fields are stratigraphic traps on the east flank of Kevin-Sunburst Dome in Montana. Updip gas production was discovered in 1975, and the downdip oil leg in 1984. Reservoir dimensions are controlled by lateral permeability loss in middle Nisku island margin and tidal flat facies. Nearly 30 oil pools have been discovered in the Hays-Enchant area of south Alberta since 1985, all producing from Nisku washover apron, lagoon fill and tidal flat facies, and all representing structural traps formed along prominent northwest-southeast structural highs created by multi-stage removal of the underlying Beaverhill Lake salt.