Red, green, grey and black shales locally containing interbedded sandstone and dolomite were described by Harrison (1952), Frarey (1961), Frarey and Duffell (1964), Baragar (1967), Dimroth (1970a, 1971), Dimroth et al. (1970) and Harrison et al. (1972) in the southern part of the Labrador Trough, NW of Schefferville. These authors then incorporated these lithologies into the « Attikamagen Formation », which was later elevated to the rank of « subgroup » by Dimroth (1970b, 1972, 1978). Dimroth (1978) introduced the name Le Fer Formation to refer to a poorly stratified, red, green or grey, silty shale unit located west of Le Fer Lake (NTS sheet 23O06), from which it takes its name, and included it in the « Attikamagen Subgroup ». Following the change in status of the Attikamagen unit, Wardle (1982) replaced the term « Attikamagen Formation », then in use in the southern Labrador Trough, with the Le Fer Formation. Clark and Wares (2004) later redefined Dimroth’s « Attikamagen Subgroup » as the Attikamagen Group. According to Dimroth (1978), the Le Fer and Bacchus formations, which consist largely of flyschoid sedimentary rocks, form the lower part of the Attikamagen Group. Dimroth (1978) also pointed out that the Le Fer is transitional and interdigitated with adjacent formations including the Savigny and Bacchus formations. Because of the similarities between these formations, Clark and Wares (2004) proposed that the Le Fer and Bacchus formations be included in the Swampy Bay Group. In compiling sheet 24F11 (Lafrance et al., 2012), other mudrock, siltstone and sandstone sequences described by Clark (1978) south of the Koksoak River were assigned to the Le Fer Formation by virtue of their similar stratigraphic position to the sequence further south.

Le Fer Formation, Undivided (pPlf): Silty Shale; Interbedded Siltstone, Sandstone and Dolomite

This undivided unit corresponds to the Le Fer Formation as mapped NW of Schefferville by Dimroth (1978). In this area, the Le Fer Formation is a sedimentary unit typically consisting of red, green or grey, poorly laminated silty shale (Dimroth, 1978). The transition from one colour to another is irregular and occurs over short distances. Red and/or green silty shale generally predominates in the western part of the Labrador Trough and changes towards the east to a grey, finely laminated silty shale. According to Dimroth (1978), the Le Fer shale is characterized by a relatively coarse-grained silty component, which is invariably present. Laminations are therefore poor and result from small variations in composition. However, laminations are more prominent in mottled shales where red and green borders alternate. Millimetre to centimetre-thick tabular laminations are characteristic. Dimroth (1978) also observed rare clay pebbles. On outcrops, the detailed layering is hardly visible due to minute variations in the composition of laminations. A lesser amount of carbonate is invariably present in shale (Dimroth, 1978).

Interbedded red, green, yellow or grey fine-grained sandstone and siltstone, 5 to 20 cm thick, are commonly observed in silty shale. Some interbedded medium-grained sandstone is also observed. Beds or lenses of dolomitic mudrock, schistose dolomite or dolomite, up to 30 cm thick, are also found interbedded with silty shale. These dolomite beds or lenses are generally found in the upper part of the formation (Dimroth, 1978). Dolomitic mudrock and schistose dolomite are light grey, green or red. They display millimetric to centimetric tabular laminations. Dolomitic laminae are grey and altered to brown. Crossbedding is rarely observed. Dolomite is well stratified or massive and recrystallized with chert concretions and quartz veinlets (Dimroth, 1978).

Red and green shale is interpreted as a pro-deltaic, still-water deposit of sediments from a spring to the west, while grey shale is thought to have been deposited in a deeper basin to the east (Dimroth, 1971, 1978; Clark and Wares, 2004).

The rocks belonging to unit pPlf1 outcrop south of the Koksoak River (Clark, 1978) and were assigned to the Le Fer Formation during compilation of sheet 24F11 (Lafrance et al., 2012). The unit consists of grey, dark grey, black or green shale, siltstone and sandstone. Sandstone and siltstone are fissile, very fine-grained, and contain local beds of coarser-grained quartzitic sandstone up to 2 m thick. Shale contains some disseminated pyrite cubes near the contact with overlying grey dolomite of the Denault Formation (pPde2) (Clark, 1978).

Thickness and Distribution

The Le Fer Formation belongs to the Howse, Mélèzes and Schefferville lithotectonic zones, as defined by Clark and Wares (2004). South of latitude 55°30’N, it forms a strip oriented NW-SE, 105 km long and up to 19 km wide. A small occurrence also occurs northwest of Cramolet Lake (sheet 23O13). According to Wardle and Bailey (1981), the thickness of the formation is estimated to be between 500 and 1000 m. The Le Fer Formation also occurs south of the Koksoak River and 1 to 2 km east of the Caniapiscau River (sheet 24F11) where it forms a strip ~14 km long parallel to the Caniapiscau River. In this area, the exposed thickness of the formation varies from ~90 to 145 m (Clark, 1978).

Dating

None.