Type Locality:

Type section is the B.A. Pyrcz No. 1 well, in 12-25-50-26W4, Alberta, between 1,496.3 and 1,548.8 m (4,909 and 5,065 ft).

Distribution:

The Nisku Formation is 47.5 m (156 ft) thick at the type section and generally ranges between 40 to 60 m (131 to 197 ft) thick throughout southern and central Alberta. The formation thins to 35 to 45 m (115 to 148 ft) where it overlies Leduc carbonate complexes. In eastern Alberta the Nisku Formation has been truncated by pre-Cretaceous erosion. In west-central Alberta thicknesses of greater than 100 m (328 ft) are encountered within the Winterburn Basin (Chevron, 1979).

Lithology:

At the type section the Nisku Formation is composed of light brown to light grey crystalline dolomite, with lesser amounts of brownish grey dolomitic siltstones, green shales and anhydrite. In south-central Alberta it is composed of two distinct units: a lower, open marine dolomitized carbonate; and an upper, anhydritic dolo-mudstone with shale interbeds. The lower carbonate unit is fossiliferous and comprises small stromatoporoid-coral bioherms, Amphipora lagoonal sediments and laminated tidal-flat dolo-mudstones through dolo-grainstones. The upper unit, which shows a reciprocal thickness variation with the lower unit is composed of laminated and bedded, unfossiliferous anhydrite and dolo-mudstones. West of the Rimbey-Meadowbrook chain of Leduc reefs the Nisku Formation is almost entirely composed of open marine carbonate sediments that pass laterally into more basinal sediments in the West Pembina area. This is accompanied by locally developed coral-stromatoporoid, downslope "pinnacle reefs" (Chevron, 1979; Watts, 1987). To the west and northwest in Alberta the Nisku Formation comprises a blanket of carbonate platform sediments that also shale out into equivalent basin sediments of the Besa River west of the Peace River Arch in northeastern British Columbia.

Relationship:

The Nisku Formation conformably overlies dolomitic and calcareous shales of the Ireton Formation (Woodbend Group) and is overlain conformably (locally unconformably) by dolomitic siltstones, green shales and anhydrites of the Calmar Formation (Winterburn Group). To the south and east the Nisku has equivalents in the Birdbear Formation in Saskatchewan, Montana, North Dakota and Manitoba (A.S.P.G., 1964). In Rocky Mountain outcrops the Nisku has been correlated with the Grotto and Arcs Members of the Southesk Formation (Belyea and McLaren, 1957). In the West Pembina area it is composed of a number of distinct members (Chevron, 1979) marking its transition to the Winterburn shale basin. In northern Alberta and northeastern British Columbia it equates to the Jean Marie Member and the upper member of the Redknife Formation.

History:

The term 'D-2' zone was first used to describe the first-discovered oil producing zone of the Leduc field Alberta (Layer et al., 1949) The term was superseded by the designation of the Nisku at the type well (Geological Staff, Imperial Oil, 1950). In this usage the Nisku was assigned member status within the then Winterburn Formation. Andrichuk and Wonfor (1954, p. 2505) recommended elevation of the Winterburn to group status and the term Nisku Formation came into general usage.

Other Citations:

A.S.P.G., 1964; Belyea, 1962; Belyea and McLaren, 1957; Exploration Staff, Chevron Standard Limited, 1979; Layer et al., 1950; Watts, 1987; Stoakes, 1987; Geological Staff, Imperial Oil Ltd., 1950; Andrichuk and Wonfor, 1954; Gilhooly, 1987.