Unit Name: Bow Island Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: middle Albian - late Albian (108.8 - 99.6 ma)
Province/Territory: Alberta; Saskatchewan; Montana

Originator: Not known.

Type Locality:
Name of subsurface unit taken from the Bow Island No. 1 well, in Lsd. 6-15-11-11W4M, drilled by the Canadian Pacific Railroad, northwest of Bow Island, Alberta.

The unit occurs in southeastern Alberta and southwestern Saskatchewan; Bow Island terminology is also widely applied in north-central Montana. The stratigraphic interval between the base of the Fish Scale Sandstone and the top of the Mannville Group is some 175 m (574 ft) thick in the Bow Island area, where the aggregate sandstone thickness is 24.4 m (80 ft). A maximum aggregate sandstone thickness of some 42.7 m (140 ft) is attained in the Lethbridge area. Farther north and east the unit is replaced by the Viking Formation. Individual sandstone bodies are up to 15 m (49 ft) thick.

Locality Data:
WELL 100061501111W400; C.W.N.G.BOW ISLAND NO.1. Interval(m): From 557.2, To 584.

Relatively well-washed and variably shaly, fine- to coarse-grained sandstone, with interbedded siltstone and mudstone and with generally subordinate conglomerate and pebbly sandstone. The well-washed sandstones characteristically exhibit tabular cross-laminae, but ripple drift and trough types and horizontal laminae also occur. The shaly sandstones include bioturbated deposits several metres thick, with variable proportions of mudstone as discontinuous partings and sequences, made up of thin, graded sandstones and siltstones, regularly alternating in vertical succession with thin mudstones and shales. Varicolored chert and reworked-relict, nodular phosphorite are noteworthy coarse components of the conglomerates. Coalified plant fragments are locally abundant. The mudstones and shales are dark grey and noncalcareous. Bentonites and concretionary layers of siderite are also present. The principal lithologies occur as up to three composite sandstone bodies, termed First, Second and Third Bow Island Sands in order of increasing age, separated in vertical succession by sequences composed mainly of mudstones and shales. Each of these sandstone bodies and the lithologically similar Cessford Sand at the base of the Colorado Group is largely made up of coarsening upward sandy sequences, occurring in either solitary or multistorey arrangements and characterized by upward decrease in the proportion of intercalated shale. Locally, individual Bow Island sandstones are further subdivided into discrete "sands", alphabetically designated A through D in order of increasing age. The sandstone bodies undergo progressive northward and eastward decrease in thickness and concomitant grain size diminution. Near the International boundary, where it crosses the Sweetgrass Arch the unit incorporates bentonitic clays and clayey sandstones with distinctive orange-red specks of clinoptilolite in the interval between the First and Second Bow Island Sands.

The Bow Island sandstone bodies pass northwards and eastwards into shales and mudstones referable to the Upper Albian part of the Big River Formation and to the Joli Fou Formation respectively, which are separated in central Alberta and much of southern Saskatchewan by the eastward thinning sandstones of the Viking Formation. In the southern foothills region the Bow Island Formation is overlain, apparently with unconformity by the Fish Scale Sandstone. However, in the foothills the boundary with the underlying Mannville Group is hard to determine, since the Bow Island Formation appears to grade vertically downwards and laterally westwards into Blairmore or Mannville sandstones. To the north and east of the foothills the lower boundary of the unit with the Blairmore is relatively sharply defined. The Bow Island Formation is equivalent to the lower part of the Blackleaf Formation of northwestern Montana; and red-speckled strata near the International Boundary belong to the Vaughn Member of this formation and are known as the Red Speck Zone.

Other Citations:
Gammell, 1955; Rudkin, 1964; Simpson, 1975; Teague, 1975; Tizzard and Lerbekmo, 1975; Simpson, 1979a, Simpson and O'Connell, 1979.

Gammell, H.C., 1955. The Viking Member in central Alberta; J. Alberta Soc. Petrol. Geol., vol. 8, pp. 141-153.
Rudkin, R.A., 1964. Lower Cretaceous; in, Geological history of western Canada; McCrossan, R.G and Glaister, R.P (Eds.); Alberta Soc. Petrol. Geol.
Simpson, F. and O'Connell, S., 1979. Low-permeability gas reservoirs in marine, Cretaceous sandstones of Saskatchewan. 2. Lower Colorado (middle Albian to Cenomanian) strata of southern Saskatchewan. In: Saskatchewan Geol. Surv., Summ. of Invest.; Christopher, J.E. and Macdonald, R. (Eds.). Saskatchewan Min. Res., Misc. Rept. 79-10, p. 181-185.
Simpson, F., 1975. Marine lithofacies and biofacies of the Colorado Croup (middle Albian to Santonian) in Saskatchewan. In: The Cretaceous System in the western interior of North America; Caldwell, W.G E. (Ed.), p. 553-537. Geol. Assoc. Can., Spec. Paper 13.
Simpson, F., 1979a. Low-permeability gas reservoirs in marine, Cretaceous sandstones of Saskatchewan. 1. Project outline and rationale. In: Saskatchewan Geol. Surv., Summ. of Invest., Christopher, J.E. and Macdonald, R. (Eds.). Saskatchewan Min. Res., Misc. Rept. 79-10, p. 174-180.
Teague, K., 1975. South Devon, a typical Bow Island gas field, Toole County, Montana. Montana Geol. Soc., 22nd Ann. Pub., Energy Res. of Montana, p. 27-29
Tizzard, P.C. and Lerbekmo, J.F., 1975. Depositional history of the Viking Formation, Suffield area, Alberta, Canada. Bull. Can. Petrol. Geol., v. 23, p. 715-752.

Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: F. Simpson
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 26 Jun 2009