Unit Name: Grosmont Formation
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Frasnian (385.3 - 374.5 ma)
Originator: Belyea, H.R., 1952.
Imperial Grosmont No. 1 well, in 13-17-67-23W4M, central Alberta, between 876 and 1,045 m (2,875 and 3,430 ft).
Covers some 100,000 km2 (39,062 mi2) of central and northern Alberta from Twp. 61 to the Alberta-Northwest Territories border. Principally occurs in subsurface, although a single outcropping is known along the Peace River at Vermilion Chutes, at 58 deg 22'N, 114 deg 55'W. The western and southern limits are defined by a facies change from carbonate to shale of the Ireton Formation. To the north and east the Grosmont Formation is absent due to pre-Cretaceous erosion. Thickness varies from about 100 to 230 m (328 to 655 ft) generally thickening near the southern and western depositional limits.
Fossiliferous limestones and dolomites of medium to light grey and brown color are dominant in the lower portions and along the western and southern margins of the upper portions. To the east, toward the erosional subcrop nonfossiliferous, well-laminated, coarse- to cryptocrystalline dolomites predominate, with minor amounts of argillaceous dolomite, limestone, siltstone and shale. Anhydrite and anhydritic dolomite occur in the upper Grosmont as the Hondo Member behind the western and southern margins. Near the eastern subcrop edge these evaporitic sediments are absent and the dolomite is often brecciated. Northward, toward the Peace River, the formation becomes increasingly calcareous, and is dominantly a limestone even farther north.
The Grosmont overlies the Duvernay and Ireton formations; it is laterally equivalent to the Ireton Formation to the south and west, is overlain either by a thin Ireton Formation or directly by the Nisku Formation, and encases and/or overlies Leduc reefs that extend north-northeastward from the Rimbey-Meadowbrook trend. The Mikkwa Formation in northern Alberta is equivalent to lower portions of the Grosmont Formation in Central Alberta. At least the lower portions of the Twin Falls Formation (Alexandra Member), and part of the Hay River Formation of the Hay River area, District of MacKenzie are equivalent to the Grosmont Formation of Alberta. The Hondo occurs within the upper portions of the Grosmont, and because of the diachronous nature of its lower contact the authors propose to reduce it in rank to a member of the Grosmont. The Grosmont Formation is overlain by Lower Cretaceous clastics at the subcrop edge at its northeastern limit.
Belyea, 1952, 1956; Cutler, 1983; Norris, 1963; Stoakes, 1980; Williams, 1977.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: L.S. Eliuk; W.G. Cutler; P.A. Monahan
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Apr 2003