The type locality of the Chakonipau Formation is located at latitude 56°17′N and longitude 68°25′W, near the eastern shore of Chakonipau Lake (Table 1; Dimroth, 1978). Other good sections are located near Chakonipau Lake and Prospecteurs Lake (Dimroth, 1978), as well as near Cramolet Lake (sheet 24O13), Musset Lake (sheet 23O11), Lerreau (Pickup) Lake (sheets 23O06, 23O11), Morambert Lake (sheet 23O14) and Wade Lake (sheet 23I05) (Table 2; Baragar, 1967; Wardle, 1979).

Table 1: Characteristics of the Type Locality

Table 2: Characteristics of Representative Localities

According to Baragar (1967) and Dimroth (1978), the detrital material constituting the Chakonipau Formation is thought to have been derived mostly from Archean bedrock of the Superior Province, west of the Labrador Trough. The detritus would have been initially transported by streams flowing eastwards. The mineralogical composition of sediments reflects the composition of the source. Some of the detritus may also have come from the erosion of basalts contemporaneous with sediments. Crossbedding measurements (indicating the direction of palaeocurrents) indicate that the sedimentary detritus was redistributed by NW-SE oriented rivers parallel to the axis of the Trough (Baragar, 1967; Wardle, 1979). In the Wakuach Lake and Snelgrove Lake Arch areas, palaeocurrent measurements indicate predominantly NW-directed flow (Baragar, 1967; Wardle, 1979).

Dimroth (1968, 1978) proposed that boulder conglomerates in the Chakonipau lake area were derived from an E-W oriented palaeoscarp extending between present-day Otelnuk and Cambrien lakes. These conglomerates would therefore be of local origin. At Cambrien Lake (west of Chakonipau Lake), the Chakonipau Formation unconformably overlies Archean bedrock (autochthonous relationship) and is much thinner than the allochthonous Chakonipau Formation (Clark, 1984).

Much of the sediments of the Chakonipau Formation would have been deposited rapidly by seasonal torrents, probably in an environment of foothills cones and anastomosing streams (Dimroth, 1978). The colour of red beds is due to hematite that coats silicate grains and is disseminated in the interstitial material (Baragar, 1967). This colour suggests strongly oxidizing transport and depositional conditions in a subaerial environment. Thus, 2.2 billion years ago, the Earth’s atmosphere was already sufficiently oxidizing to allow the deposition of red beds.

Volcanic rocks of the Chakonipau Formation are very different from other volcanic rock sequences of the Labrador Trough. They are distinguished by: 1) their geology (subaerial volcanic rocks associated with red sedimentary rocks versus marine volcanic rocks associated with shales and greywackes), and 2) their chemical composition (high alkali contents and high Fe₂O₃/FeO ratios in Chakonipau volcanic rocks versus very low K₂O contents and low Fe₂O₃/FeO ratios in marine tholeiitic volcanic rocks).

The deposition of the Chakonipau Formation is thought to have marked the initiation of an intracratonic rift involving the Archean continent ~2.2 billion years ago (Hoffman, 1988; Wardle et al., 2002). The palaeorift now marks the NE edge of the Superior Province. Most of the normal faults associated with the rift were parallel to the rift axis; however, some faults may have been oriented at a steep angle to this axis. The absence of the Chakonipau Formation north of latitude 57°N suggests that rifting did not progress beyond this latitude at this time. In this regard, Skulski et al. (1993) proposed the presence of a « crustal promontory » north of latitude 57° N. The mafic and slightly alkaline volcanism of the Chakonipau Formation was probably associated with the development of the rift (Wardle and Bailey, 1981). The presence of mafic volcanic rocks indicates that rift faults reached the lower part of the crust or mantle. Most of the Chakonipau Formation outcrops within overthrusted slices where the degree of deformation is highly variable depending on the context and proximity of faults. Rocks of the formation are slightly metamorphosed to the greenschist facies or lower.

Economically, very little syndiagenetic to diagenetic copper mineralization was found to date in oxidized sandstones and conglomerates deposited in the Chakonipau Formation during early rifting of the Archean continent. However, an example of minor copper mineralization is located in basalts, SE of Mistamisk Lake (see Mineralized Zone #194 in Clark and Wares, 2004). A decametre-wide alkaline-trending gabbro dyke near Musset Lake contains low-grade copper and silver vein mineralization (Brassard, 1984). Some radioactivity associated with minor shear zones in the dyke has been noted locally. This dyke cuts oxidized red sandstone and may be contemporaneous with basalts. Apart from these examples of mineralization, the metallogenic importance of the Chakonipau Formation seems to imply that unstable minerals in sediments may have acted as a source of metals for copper mineralization hosted in reducing units higher in the sequence (Kupferschiefer or Zechstein style; Clark and Wares, 2004). Metals would have been remobilized by diagenetic and epigenetic fluids and subsequently deposited in reducing environments.

Unit pPcp2 includes trachybasalts and basalts locally associated with mafic or felsic tuffs. The unit was identified in the Musset Lake (sheet 23O11) (Baragar, 1967; Brassard, 1984) and Mistamisk Lake (sheet 24C08) areas (Clark, 1986). They consist of massive, grey to greenish grey aphanitic lavas, commonly with a reddish patina, pyroclastics and green- or red-spotted volcaniclastic rocks. Stratification was not recognized in basalts. In volcaniclastics, fragments of red sandstone, red siltstone and volcanic rocks up to 15 cm in diameter are observed. Volcanic rocks are highly altered and formed of tiny feldspar crystals, chlorite, hornblende and a considerable amount of fine iron oxides. The feldspars’ composition is difficult to determine because of their small size, but probably alkaline. In amygdaloidal basalt from Mistamisk Lake, plagioclase rods are present, as well as magnetite, pyrite and chalcopyrite (Clark, 1986). Chemical analyses of four massive volcanic rock samples indicate that they consist of trachybasalt (Dimroth, 1978; Brassard, 1984). These lavas are highly oxidized and enriched in alkalis (Dimroth, 1978, see Table 23, analyzes 281 and 282). At Mistamisk Lake, intensely fractured and hydrothermally altered basalt is enriched in Na₂O (Clark, 1986). At Musset Lake, both volcanic rock samples are also fractured and altered (Brassard, 1984, analyzes 1982003206, éch. 82-BB-158, and 1982003207, éch. 82-BB-160c).

Chakonipau Formation 3 (pPcp3): Pink or Locally White, Grey or Green Arkose and Subarkose