The name Denault formation appeared in the late 1940s in a series of unpublished exploration reports by Labrador Mining and Exploration. The unit was named after Denault Lake (sheet 23J15) located near Schefferville. These reports describe it as consisting mainly of dolomite, dolomitic breccia and chert. This unit is thought to be stratigraphically above shales of the Attikamagen Group and volcanic rocks designated as Nimish, and underlying chert breccia of the Fleming Formation (see Frarey and Duffell, 1964). Harrison (1952) formalized the name Denault Formation in his description of the geology of a NE-SW oriented strip located at Schefferville (« Burnt Creek strip« ). Subsequently, a homotaxial dolomite unit located further NE, between Marion Lake (sheet 23I13), in Labrador, and Gachet Lake (sheet 24B03), was assigned to the Denault Formation by Donaldson (1963, 1966) and Baragar (1967). However, Dimroth (1978, p. 173) questioned the physical continuity between the Denault Formation in its type area and this homotaxial dolomite unit, although he retained the name Denault for the latter unit (see Thickness and Distribution section).

Harrison (1952) gave a general overview of the Denault Formation in the Schefferville area. He mentioned that the formation contains mainly dolomite and chert, with interbedded slate and quartzitic sandstone. Dimroth (1971, 1978) recognized four lithological facies in the western part of the unit. Baragar (1967) described it between Attikamagen Lake to the south and Silver Lake to the north. Donaldson (1963, 1966) provided a detailed description of stromatolites that characterize this formation in the Marion Lake area. Dimroth (1978, pp. 198-200) located the Denault Formation below the Wishart Formation in a series of stratigraphic sections from the upper part of the first volcano-sedimentary cycle of the Labrador Trough.

The discovery of a major thrust fault (Garigue Fault) beneath the Abner Formation in the northern Labrador Trough (Clark, 1979; Budkewitsch, 1986; Goulet, 1986) led to a reinterpretation of the position of the latter unit in the stratigraphic pile (Clark and Wares, 2004). According to this new interpretation, rocks attributed to the Abner Formation were overthrusted over the Ferriman Group (including the Sokoman Formation), allowing the Abner Formation to be correlated with the Denault Formation. Consequently, the Abner Formation was abandoned. The Harveng (Sauvé and Bergeron, 1965) and Nachicapau (Dressler, 1979) formations have also been abandoned as dolomitic sedimentary rocks forming them were recognized equivalent to the Denault Formation (Clark and Wares, 2004).

Descriptions of units correlated with the Denault Formation were provided by Bérard (1965; Abner Formation), Sauvé and Bergeron (1965; Abner Formation, Harveng Formation), Ciesielski (1977; Abner Formation), Dressler (1979; Abner Formation, Nachicapau Formation, in part), Clark (1977, 1978, 1979, 1980; Abner Formation) and Bélanger (1982; Harveng Formation). Hoffman and Grotzinger (1989) interpreted the various lithofacies of the Abner/Denault Formation in terms of a reef complex. Rocks of the Denaut Formation have also been the subject of geological reconnaissance work in the Schefferville area (Clark et al., 2008).

Denault Formation, Undivided (pPde): Dolomite, Sandy Dolomite, Dolomitic Sandstone, Breccia and Intraformational Conglomerate; Stromatolitic Dolomite; Interstrtified Siltstone, Shale and Fine-Grained Sandstone

In the area south of latitude 56°N, Dimroth (1971, 1978) recognized four facies. Near the western margin of the Labrador Trough, the rock consists of fine-grained, grey, laminated dolomite having a brown alteration patina. It contains minor beds of medium to coarse-grained dolomitic sandstone and some red and green siltstone, shale and fine-grained sandstone strata of variable thickness. Rhythmic bedding consisting of tabular laminae millimetres to ~2-5 cm thick is common. Commonly observed sedimentary structures include crossbedding, contorted and convoluted laminations, and syn-sedimentary faults. Chert nodules are widespread in laminated dolomite. Eastward, laminated dolomite changes to intraformational breccias and conglomerates alternating with laminated dolomite beds and minor dolomitic sandstone beds. Some interbedded red and green, locally grey, siltstone and fine-grained sandstone are also present. Intraformational breccias and conglomerates gradually change eastward to fine fragment conglomerate alternating with laminated sandy dolomite and grey shale beds. East of the Ferrum River, the Denault Formation consists of lenses of light grey massive dolomite having a brown alteration patina. These different rocks are described in detail by Dimroth (1971).

Further east, between Oiseaux Lake and Argent Lake, three facies are recognized by Dimroth (1978) (from south to north): 1) fine-grained grey dolomite containing chert laminae and interbedded massive dolomite; 2) grey stromatolitic dolomite interbedded with intraclastic dolomite and dolomite containing chert laminae; and 3) facies similar to the previous one, but also including beds of coarse-grained dolomitic sandstone having a brown alteration patina and sandstone dolomite. In places, these rocks display strong synsedimentary deformation. A detailed description of the stromatolitic facies is given by Donaldson (1963, 1966) and Baragar (1967).

In the areas west of Hérodier Lake (sheets 24F03, 24F04 and 24F06), the unit consists of dolomite, sandy dolomite, dolomitic sandstone, mudstone, siltstone, conglomerate and undivided chert (Clark, 1978). In the area north of Feuilles Lake, it consists of generally fine-grained dolomite having a buff alteration patina and small folds. The rock is characterized by abundant quartz veins and contains interbedded chlorite-mica schist and talc schist. Marble occurs locally (Gold, 1962).

Unit pPde2 corresponds to the former Abner Formation (abandoned) and partly to the Nachicapau Formation (abandoned). It consists mainly of thick beds of massive dolomite, stromatolitic dolomite and laminated dolomite. Dolomite generally has a light grey or buff alteration patina (Berard, 1965; Clark, 1977, 1978, 1979; Dressler, 1979). In fresh surface, the dolomite’s colour is highly variable: light grey, dark grey, buff, pinkish, red, yellow, orange, green, brown or white (Bérard, 1965; Clark, 1977; Dressler, 1979). According to Bérard (1965), the latter is partly determined by the presence of accessory minerals such as graphite, magnetite, hematite and chlorite. Dolomite is usually fine grained or aphanitic and is locally recrystallized. Stromatolitic dolomite is common and occurs in centimetre to metre-thick beds. Stromatolites are commonly bulbous and range from 2 cm to 1 m in diameter (Bérard, 1965; Clark, 1977, 1978, 1979; Dressler, 1979). In the Bérard Lake area (sheets 24L08 and 24L09), dolomite is cut by an orthogonal network of quartz veins and veinlets giving the rock a characteristic gridded appearance in altered surface (Bérard, 1965; Sauvé and Bergeron, 1965).

Dolomite is commonly interbedded with centimetric to metric beds of grey, dark grey or black chert, sandy dolomite, dolomitic sandstone and intraformational dolomitic conglomerate (Bérard, 1965; Clark, 1977, 1978, 1979; Dressler, 1979). Low-amplitude (10 cm) crossbedding is common in sandy dolomite. Locally, dolomite contains terrigenous fragments, oolites or pellets as well as oolite and pellet intraclasts, laminated or sandy dolomite (Clark, 1979; Dressler, 1979). In the Laumont Lake area (sheet 24L01), dolomite contains a dolomitic intraformational breccia consisting of angular dolomite fragments in a sandy ferrodolomite matrix (Bérard, 1965).

In thin sections, dolomite is microcrystalline with grains ranging from 0.05 to 0.6 mm in diameter. It also contains detrital quartz (up to 20% locally) and minor amounts of microcline, apatite and zircon (Bérard, 1965; Sauvé and Bergeron, 1965).

Denault Formation 3 (pPde3): Dolomitic Marble, Dolomitic Schist, Calcosilicate Rocks, Calcarenite, Sandy Dolomite, Calcareous Mudstone

Thickness and Distribution

The Denault Formation belongs to the Gerido, Howse, Hurst, Mélèzes and Schefferville lithotectonic zones as defined by Clark and Wares (2004). It was mapped from the west coast of Ungava Bay to the Grenville Front, a distance of ~700 km. The unit continues SW into the Grenville Province as the Duley Formation to the area west of the Manicouagan Reservoir (Clarke, 1977; Moukhsil et al., 2013), an additional distance of 230 km.

Initially, the Denault Formation was mapped from the Grenville Front in Labrador to the southern end of Wakuach Lake in Quebec (sheet 23O06) (Baragar, 1967). A homotaxial dolomite unit was also recognized between Marion Lake (sheet 23I13), in Labrador, and Gachet Lake (sheet 24B03) (Donaldson, 1963, 1966; Frarey, 1967; Baragar, 1967; Dimroth, 1978). Further north, the former Abner Formation was mapped between Minguarutittalik Lake (sheet 24F03) and Feuilles Lake (sheet 24K13) (Bérard, 1965; Dressler, 1979; Clark, 1979). North of Feuilles Lake, as far as Hopes Advance Bay (sheet 24N05), some dolomitic metasedimentary rocks are probably equivalent to the Abner Formation further south (Bérard, 1965). These metasedimentary rocks have been assigned to the Denault Formation. Thus, the Denault Formation is the first formation of the first volcano-sedimentary cycle present along the entire length of the Labrador Trough.

Baragar (1967) mentions that the thickness of the Denault Formation varies greatly from one area to another. Harrison (1952) reported a thickness of 30 m SW of Schefferville, which increases towards the NE to reach 185 m near Schefferville. A thickness of ~120 m was estimated near Annabel Lake, NW of Schefferville (Baragar, 1967). Harrison et al. (1972) assumed that the formation thickened to the NE, from Schefferville, reaching >300 m thick west of the Ferrum River. This possibility was confirmed by observations made ~35 km NE of Schefferville (see Dimroth, 1978, Figure 23). Thus, in the area NE of Chassin Lake, the thickness of the formation has been estimated at 1280 m (Baragar, 1967). At Lake Marion, the Denault Formation reaches a maximum thickness of 850 m (Donaldson, 1966). In the northern part of the Labrador Trough, the thickness of the former Abner Formation was estimated at 150 m (for the dolomitic part) by Dressler (1979), at ≥75 m by Clark (1979) and between 30 and 120 m by Bérard (1965). Hoffman and Grotzinger (1989) interpreted the Denault Formation at Minguarutittalik Lake as representing a prograding carbonate reef having a total thickness of 345 m. The thickness of the former Harveng Formation was estimated at ~215 m by Sauvé and Bergeron (1965), but this does not take into account thickening by folding.

Dating

None.