A flysch-like mudrock and greywacke sequence was originally described by Dimroth (1965) to the NW of Otelnuk Lake. The name Otelnuc Formation was introduced by Dimroth (1968) to designate this sequence. The rocks belonging to the Otelnuc Formation were mainly mapped by Dimroth (1965, 1972, 1978).

The grey greywacke (denoted « quartzitic wacke » by Dimroth, 1978) forms thick beds (generally >30 cm) containing numerous fragments of slate, mudrock and chert up to 2 cm long (Dimroth, 1965, 1978). The presence of these fragments gives a spotted salt-and-pepper appearance to the rock. The greywacke is usually homogeneous and massive. Many beds lack laminations or any other internal structure. The rock and mineral fragments are not oriented parallel to the stratification plane. Fragments make up 60 to 70% of the rock and consist of quartz, plagioclase, several types of graphitic mudrock, siltstone, dolomitic siltstone and chert. The matrix is composed of fine green chlorite and sericite. Rhombohedral crystals of authigenic dolomite are present in the matrix, in mudrock fragments and, more rarely, in chert fragments. Authigenic plagioclase rods and muscovite flakes are common, while authigenic magnetite is rare (Dimroth, 1978).

Fine-grained greywacke (more common than coarse-grained greywacke) and siltstone are olive-green, brownish green or brownish grey, finely bedded and do not contain rock fragments visible to the naked eye. These rocks are characterized by well-developed crossbedding, oscillation ripples, subsidence marks, load marks and synsedimentary folds (Dimroth, 1965, 1978). Fissility is well developed parallel to the stratification plane. Flow and penecontemporaneous deformation phenomena, as well as irregular marks of uncertain origin and convolute laminations are also reported by Dimroth (1965, 1978).

Dimroth (1978) points out the presence of relatively pure, loosely structured sandstone at the base of the Otelnuc Formation. The rock is composed of rounded quartz grains (1 mm in diameter) in a matrix of fine quartz fragments. The matrix contains very little clay.

Unit pPon1 consists mainly of laminated grey mudrock interbedded with some greywacke (Dimroth, 1965, 1978).

Otelnuc Formation 2 (pPon2): Greywacke (« Quartzitic Wacke »); Lower Amount of Laminated Grey Mudrock

Thickness and distribution

The Otelnuc Formation belongs to the allochthonous Schefferville Lithotectonic Zone, as defined by Clark and Wares (2004). It only outcrops in the Otelnuk Lake area, with representative outcrops on the western shore and NW end of the lake (Dimroth, 1965, 1969, 1978). Rocks of the formation are also present south of Otelnuk Lake. The formation is >900 m thick (Dimroth, 1978).

Dating

None.