When mapping the St. Lawrence Lowlands, Clark (1952, unpublished manuscript; in Globensky, 1993) gave the name of Deschambault to a limestone unit of the Lower Trenton Group in reference to the village of Deschambault, in the Portneuf area (sheet 21L12). Husain (1955) studied the main microfossils and Clark (1959) officially introduced the term Deschambault Formation.

The Deschambault Formation is described in detail in several regions of the St. Lawrence Lowlands (Clark, 1972; Clark and Globensky, 1973a, b 1975, 1976a, b, c 1977; Brun, 1974, 1975; Globensky, 1983, 1986, 1987; Lacoste and Hébert, 1999; MNRF, 2009a-i; Bilodeau and Brun, 2001; Hébert et al., 2012; Lacoste et al., 2012); Harland and Pickerill (1982) and Globensky (1993) summarized it well: “The formation consists typically of a bioclastic crystalline limestone, thick-bedded, cross-bedded, containing several coquina and fragmented shell beds. Limestone is light grey to dark, slightly brown, and varies from very fine to very coarse grained. Bedding is irregular and the numerous basins and tabular cross-bedding are accentuated by the presence of argillaceous laminae. Otherwise, shale beds are sparse or non-existent, although more present at the top of the formation.”

The formation is rich in fossils and contains a very diverse marine fauna, largely composed of polished and water-rounded crinoid plates and stems prior to sedimentation. The formation is dotted with chert nodules and lamellae, which can be up to 30 cm in diameter, but are usually only a few centimetres long. The Deschambault Formation is primarily a high-energy deposit that has accumulated in a complex environment of subtidal shoals, bars, and large moving wrinkles (Harland and Pickerill, 1982). Argillaceous laminae were deposited in calm periods reflecting pauses in the movement of these bottom floor sedimentary figures.

For more than a century, thick upper beds of the Deschambault Formation have been exploited for construction stone and lime-making in Saint-Marc-des-Carrières.

From five sections (Miron Ltd. Company Quarry, Montreal; St-Vincent de Paul Quarry, St-Vincent-de-Paul; Sintra Quarry, St-Jacques; Ciments Indépendants Inc. Quarry, Joliette; Turnbull Construction Inc. Quarry, St-Thomas), Brun (1974, 1975) defined boundary stratotypes between the Mile End and Deschambault formations, as well as between the Deschambault Formation and the Saint-Michel Member of the Montréal Formation: “Mile End-Deschambault boundary: Sharp transition from a very large ‘wakestone’ limestone bed with numerous internal discontinuous undulations (Mile End) to a succession of small ‘packstone’ or ‘wakestone’ limestones arranged in large undulations with shallow oblique laminations (Deschambault 1 [lower package]). Deschambault-St-Michel boundary: Transition from a large ‘grainstone’ or ‘packstone’ limestone bed with steep-slope oblique laminations (Deschambault 3 [upper package]) to small regular beds of dark grey ‘mudstone’ limestone (St-Michel).”

In the Beaupré area (sheet 21M02), Deschambault Formation limestone is described as a fossilliferous micrite. In addition, Globensky (1983) describes a 1.3 m section of the Deschambault Formation in an abandoned quarry of the Laurentian region, 2.5 km NE of Sainte-Anne-des-Plaines (sheet 31H13), the base of which consists of “a grey, micritic, very fossiliferous limestone; bryozoa form real mats there […] bedding is thin (2-10 cm) […]”

Thickness and distribution

The Deschambault Formation is present throughout the St. Lawrence Lowlands between Montreal (sheet 31H) and Saint-Siméon (sheet 21N13), where the Trenton Group is observed. The thickness of the Deschambault Formation is generally 24-28 m, but decreases to 3-5 m in the Montreal area and reaches 64 m in the Trois-Rivières area (Harland and Pickerill, 1982; Globensky, 1993). The Deschambault Formation forms spectacular gorges where it is cut by important rivers (Clark and Globensky, 1973, 1975, 1976a, b, c, 1977; Globensky, 1987).

Dating

None.