Type Locality:

White Man Gap, at the southeast end of Mount Rundle 6.5 km (4.0 mi) west of Canmore, Alberta on the Spray Lakes reservoir road. Section measured on the north side of the pass. (51 deg 04'N, 115 deg 25'W, between 1,708 and 1,720 m (5,606 and 5,643 ft).

Distribution:

The Grotto Member varies from 6 to 67 m (20 to 220 ft) in thickness, with 49 m (160 ft) recorded at the type section. At the Ancient Wall and Miette buildups the Grotto is less than 22 m (72 ft) thick. It is 25 to 43 m (82 to 141 ft) thick in the Banff area, the thickest development being at Cripple Creek in the Fairholme buildup, where 67 m (220 ft) were measured. In the subsurface the Grotto is between 11 and 43 m (36 and 141 ft) thick. The member is recognized in the carbonate buildups of the Fairholme Group in the Rocky Mountains from the Ancient Wall to the Flathead area of southeastern British Columbia, in the subsurface southern Alberta carbonate shelf south of Twp. 30 and also east of a line between Drumheller and Vermilion.

Lithology:

The member generally consists of recessive weathering, thin- to thick-bedded, dark brown to grey dolomites, variably argillaceous, and often having a fetid odor. The Grotto Member may be sparsely to abundantly fossiliferous and contains a distinctive suite of fossils including disphyllid, thamnoporid and alveolitid corals, and Amphipora. At the Ancient Wall buildup the Grotto Member contains fewer fossils and a larger clastic component (thin silty zones). There the Grotto consists of red and green dolomites and light grey limestones.

Relationship:

The Grotto Member paraconformably (and possibly unconformably) overlies the Peechee Member of the Southesk Formation and is overlain by the Arcs Member of the Southesk Formation of the carbonate buildups of the Fairholme Group. Rarely it is overlain instead by the Ronde Member (Southesk-Cairn buildup; Mackenzie, 1969). In some buildups (e.g., Fairholme-Southesk-Cairn) the Grotto is best developed at the margin and tends to thicken toward the basin and thin into the buildup, being compensated for by lateral change to the Arcs Member into the buildups. In other buildups (e.g., Ancient Wall /Miette), the Grotto Member is more uniform and occurs as a thin horizon right across the buildups. These two different apparent relationships may in part reflect the approaches used by different authors in mapping at buildup margins. Further basinward, in almost all cases the Grotto Member undergoes a rapid facies change into shale of the upper Mount Hawk Formation, and mapping of this facies change is critical in determining the lateral relationships of the Grotto Member. More stratigraphic study of the Grotto Member is required before its relationships to the basin formations can be well established. The Grotto-Peechee boundary marks the end of buildup growth and the start of a new sedimentary cycle. This is a very important break, which is obscured by grouping the various depositional units (members) into a single formation, the Southesk (Mountjoy and Mackenzie, 1973; Coppold, 1976; Harrison and Jackson, 1978, Mountjoy, 1980). In the subsurface of southern Alberta the Grotto Member is often referred to as the Nisku Formation and/or the Camrose Tongue of the Ireton Formation and is probably the approximate equivalent of these units in central Alberta, although the exact relationship is not well established. The Grotto is probably equivalent to part of the Birdbear Formation of Saskatchewan and part of the Jefferson Formation of Montana.

History:

Named after Grotto Mountain, east of Canmore in the Fairholme Range. Belyea and McLaren (1975a) formally named three members of the Southesk Formation described in a surface section in 1956. The Grotto Member replaces the informal "Coral bed member" previously used by these authors. Belyea and McLaren (1957a) also extended the Grotto Member into the southern Alberta subsurface, where it is equivalent to the Camrose "Tongue".

Other Citations:

Belyea, 1958; Belyea and McLaren, 1956, 1957a; Coppold, 1976; Dooge, 1966; Hargreaves, 1959; Harrison and Jackson, 1978; Mackenzie, 1969; Mountjoy, 1965, 1980; Mountjoy and Jull, 1979; Mountjoy and Mackenzie 1973; Price, 1964.