The term « Railroad slates » was used by Harrison (1952) to describe a thin slate unit overlying the Denault Formation along the railway west of Knob Lake (sheet 23J15). The terrigenous sedimentary rocks now assigned to the Dolly Formation were included in the Attikamagen Formation (later Attikamagen Group) by Harrison (1952), Wynne-Edwards (1960), Frarey (1961), Frarey and Duffell (1964), Baragar (1967), Dimroth (1970, 1971) and Dimroth et al. (1970). The name Dolly Formation was formally introduced by Harrison et al. (1972) to describe a unit of slate and siltstone of various colours that overlies the Denault Formation, NE of Dolly Lake. This lake, from which the unit takes its name, is located ~3.5 km east of Schefferville (sheet 23J15). Harrison et al. (1972) briefly described the general nature of the Dolly Formation at the type locality east of Schefferville. Subsequently, Dimroth (1978) mapped the Dolly Formation in the Schefferville area to include it in the upper part of the Attikamagen Subgroup, which was later reclassified as the Attikamagen Group by Clark and Wares (2004). The rocks of the Dolly Formation were followed towards the SE by Wardle (1979a, b; 1982) on the Labrador side. They have also been the subject of geological reconnaissance work in the Schefferville area (Clark et al., 2008).

The upper member of the Dolly Formation occurs in the area east of Schefferville. In the area east of Dolly Lake, where it is very well exposed, this member consists of well-stratified and finely bedded red and green fine-grained sandstone and siltstone. These rocks commonly display crossbedding and tabular stratification. Towards the NE, the upper member changes to red and green siltstone and mudstone with tabular stratification. These rocks are well exposed at the NW end of Attikamagen Lake (Dimroth, 1971, 1972, 1978). Dimroth (1971, 1978) indicates that the upper member changes rapidly to grey mudstone with interbedded sandstone towards the east and SE. The upper member of the Dolly Formation is similar to the Le Fer Formation (Swampy Bay Group), which underlies the Denault Formation, and reflects a prograde deltaic deposit that accumulated in a west-to-east basin (Dimroth, 1971, 1978). SE of the Schefferville area, rocks of the upper member change to mudstone and grey siltstone locally having turbidite characteristics. These rocks were deposited towards the central part of the basin in deeper water (Dimroth, 1971, 1978; Wardle, 1979a, b; Wardle and Bailey, 1981).

Sedimentary rocks of the Dolly Formation were deposited in a shallow marine basin developed on the carbonate platform of reef stromatolitic dolomite of the Denault Formation (Dimroth, 1972, 1978; Wardle and Bailey, 1981; Hoffman and Grotzinger, 1989; Birkett, 1991). Rocks of the formation are slightly metamorphosed and at most reach the greenschist facies (Dimroth and Dressler, 1978). From an economic point of view, phosphate-rich sedimentary rocks (phosphatic cherty mudstone) were reported by Harrison et al. (1972) and Birkett (1991) in the Dolly Formation (Clark et al., 2008).

Thickness and Distribution

The Dolly Formation belongs to the allochthonous Schefferville Lithotectonic Zone as defined by Clark and Wares (2004). It has only been identified in the western part of the Labrador Trough, south of latitude 55°15′N (Dimroth et al., 1970; Dimroth, 1971, 1972, 1978; Wardle, 1979b). In Quebec, the distribution of this formation is limited, forming two parallel strips, oriented NW-SE, extending ~35 km and 13 km NE of Schefferville, respectively. On the Labrador side, the Dolly Formation occupies much of the Petitsikapau Lake basin (sheet 23J09) and extends SE to latitude 54°N (Wardle, 1979b, 1982; Wardle and Bailey, 1981). West of Schefferville, a grey mudstone and siltstone unit has been identified east of Stakit Lake (sheet 23J10) (Wardle, 1979b).

According to Harrison et al. (1972), the Dolly Formation is very thin near the western edge of the Trough (west of Knob Lake) and thickens considerably towards the NE. The minimum thickness of the formation is estimated between 2 m and 4 m at the crest of Knob Lake (Clark et al., 2008). NE of Dolly Lake (type locality), Harrison et al. (1972) estimated the thickness of the formation at >300 m. In the same area, Dimroth (1971) reported a thickness of 230 m at Dolly Ridge, NE of Schefferville. On the Labrador side, the Dolly Formation is ~180 m thick east of Stakit Lake, gradually increasing eastward to ~500 m in the Petitsikapau Lake basin (Wardle, 1979b). To the SE, the Dolly Formation thickens eastward to a maximum thickness of 1000 m to 2500 m (Wardle, 1979a).

Dating

None.