Unit Name: Alexo Formation
Unit Type: Lithostratigraphic
Usage: Partly superseded
Age Interval: Famennian (374.5 - 359.2 ma)
Originator: DeWit and McLaren, 1950
North Saskatchewan River Gap, north side, where the river cuts through the Brazeau Range. Located 15 km (9 mi) southeast of Nordegg, Alberta. Lat. 52 deg 26'N, Long. 115 deg 54'W.
The Alexo Formation is essentially a basinal unit which onlaps and thinly covers the Fairholme Group carbonate buildups in the Rocky Mountains. It is fully developed only far from such buildups, where it may reach thicknesses of 100 m (328 ft), i.e., south of the North Saskatchewan River. It is 73 m (240 ft) thick at the type section, with the lower and upper members being 50 and 23 m (163 and 77 ft) thick respectively (McLaren, 1955). To the north the upper member of the Alexo Formation, now termed the Sassenach Formation thickens to approximately 200 m (656 ft) in basinal sections in the Jasper area.
The Alexo Formation is informally divided into two members (McLaren, 1955). The lower member consists of interbedded dolomite and silty and argillaceous dolomites, grading up through laminated siltstones and silty dolomites to thick-bedded, vuggy grey dolomite. The basal part of this member weathers thin-bedded and somewhat recessive. The upper member is composed of laminated, thin- to medium-bedded grey and green-grey argillaceous siltstones and silty dolomites. Small penecontemporaneous slump structures are often present in this interval.
The Alexo Formation overlies the Southesk and Mount Hawk formations of the Fairholme Group. The contact is unconformable on the carbonate buildups (i.e., overlying Southesk Formation, Arcs Member) and conformable or paraconformable in the basins (i.e., overlying the Mount Hawk Formation). The lower contact is popularly taken as marking the Famennian-Frasnian boundary. This determination is based on a sparse brachiopod fauna which occurs within the Alexo Formation, although the basal part of the Alexo is very poorly fossiliferous and the boundary may occur within the Alexo Formation (e.g., McLaren, 1955, 1959). The Alexo is conformably and gradationally overlain by the Palliser Formation (McLaren, 1955). In the mountains north of the type section McLaren and Mountjoy (1962) showed that the lower member of the Alexo grades into the Southesk Formation and designated it the Ronde Member of the Southesk Formation; the upper member was re-designated the Sassenach Formation. Price (1964) tentatively recognized the Sassenach Formation in the Crowsnest Pass area, but its relationship with the Alexo Formation of southern Banff National Park is not fully understood. In the subsurface the Alexo Formation is homotaxial with the Crowfoot Formation of southeastern Alberta and the Calmar and Graminia formations of the central Alberta subsurface.
The formation, named after the village of Alexo, Alberta was erected by deWit and McLaren (1950) to include all the silty carbonate beds between the top of the Southesk and Mount Hawk Formations and the base of the Palliser Formation. A minor thickness revision was made by McLaren (1955), who also divided the formation informally into upper and lower members. Further paleontologic (McLaren, 1959) and stratigraphic studies in the Jasper region led McLaren and Mountjoy (1962) to revise the formation nomenclature for that area. They designated the lower Alexo as the Ronde Member of the Southesk Formation and the upper Alexo as the Sassenach Formation. The term Alexo Formation is therefore no longer applied in the mountains north of the type section, but is used to the south where the stratigraphy of this interval has not been fully elucidated and is possibly due for revision.
Belyea, 1958; de Wit and McLaren, 1950; McLaren, 1955, 1959; McLaren and Mountjoy, 1962; Price 1964.
DeWit, R.W., and McLaren, D.J. 1950. Devonian Sections in the Rocky Mountains between Crowsnest Pass and Jasper, Alberta. Geological Survey of Canada, Paper 50-23.
McLaren, D.J. and Mountjoy, E.W., 1962. Alexo equivalents in the Jasper region, Alberta (Report, 4 figures, appendix); Geological Survey of Canada, Paper 62-23, 36 p.
McLaren, D.J., 1955. Devonian formations in the Alberta Rocky Mountains between Bow and Athabasca rivers; Geological Survey of Canada, Bulletin 35.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: E.W. Mountjoy; M.P Coppold
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 17 Feb 2009