Background

Description

Unsubdivided Menihek Formation (pPme):Locally Pyritic Mudstone, Siltstone and Sandstone (Turbidites and Rhythmites); Minor Proportions of Conglomerate, Micaceous Schist, Dolomite, Quartzite and, Locally, Mafic Volcanic Rocks

This unsubdivided unit is composed of various Menihek Formation lithologies. In the south-central part of the Labrador Trough (sheet 23O01), these rocks consist mainly of a sequence of rhythmites formed of locally pyritic black mudstone, grey siltstone and sandstone (turbidites) (Dimroth, 1978; Wardle, 1982).They include a minor proportion of conglomerate, micaceous schist, dolomite and quartzite (‘quartz sandstone’ according to Dimroth, 1978) (Frarey, 1967; Wardle, 1982).In places, these sedimentary rocks may include mafic volcanic rocks: mafic flows, tuffaceous rocks, flow breccia (Frarey, 1967).In the central part of the Labrador Trough, in the area east of Canichico Lake (sheet 24C10), the unit consists mainly of a sequence of sandstone, siltstone and mudstone (Dressler, 1979).Further north, in the Feuilles Lake area (sheets 24K13 and 24L16), the rocks in this unit consist mainly of micaceous schist with a lesser proportion of dolomite and quartzite (Bérard, 1965).

Menihek Formation 1 (pPme1): Wacke, Siltstone and Phyllite ± AreniteMudstone, Siltstone and Phyllite; Minor Proportion of Wacke

The pPme1 unit overlies the Sokoman Formation.It consists mainly of mudstone, siltstone and phyllite, with a minor proportion of wacke (Baragar, 1967; Dimroth, 1978; Clark, 1977, 1979; Dressler, 1979).These different lithologies are generally interbedded and show a rhythmic structure (turbidites) (Clark, 1979).The mudstone and phyllite are dark grey to black and very fine-grained to aphanitic.Siltstone is grey, greenish grey or dark grey, very fine-grained and commonly fissile.These rocks are usually laminated and form beds millimetric to centimetric in thickness, locally reaching >1m.In places, they are slightly graphitic and pyritic (Clark, 1977, 1979; Dimroth, 1978).

The wacke is generally paler in colour than the siltstone and is typically fine-grained and massive (Clark, 1979).It is interbedded in mudstone and siltstone and is 0.1 to 2 m thick, and locally up to 6 to 8 m (Sauvé and Bergeron, 1965; Clark, 1979).The rock consists of angular grains (0.1 to 0.5 mm) of quartz, feldspar, muscovite, sphene and chert, along with a few fragments of volcanic and sedimentary rock.The matrix is composed of chlorite, sericite, epidote, carbonate and pyrite (Sauvé and Bergeron, 1965).In the area south of Bérard Lake (sheets 24L01 and 24L08), the rock locally contains stilpnomelane (Bérard, 1965).

Sedimentary structures most commonly observed in the upper part of wacke beds include contortions, sliding, sorting, erosion channels, cross bedding and load prints (Clark, 1977, 1979). Carbonate concretions (a few centimetres to 0.6 m in diameter) are common locally (Sauvé and Bergeron, 1965; Clark, 1977; Dimroth, 1978).

Menihek Formation 1a (pPme1a): Schistose Wacke; Minor Proportion of Schistose Siltstone, Phyllite and Arenite

Menihek Formation 2 (pPme2): Grauwacke, Quartz Wacke, Sandstone, Quartz Sandstone and Siltstone and Mudstone BedsWacke, Quartz Wacke, Sandstone, Quartz Sandstone; Interbedded Siltstone and Mudstone

This unitUnit pPme2 formis the upper part of the Menihek Formation in the Labrador Trough’s northern partcentral-northern and southern parts of the Labrador Trough(sheets 23J NE, 23O, 24C, 24F NW, 24E NE and 24L SE). It consists mainly of centimetric to metric beds of grauwacke, quartz wacke, sandstone and quartz sandstone interstratified with millimetric to centimetric beds of siltstone and mudstone(Baragar, 1967; Dimroth, 1978; Clark, 1979; Dressler, 1979).Wacke and quartz wacke are grey, greenish grey or green and locally have a grey to yellowish-brown alteration patina. They are fine grained and are usually laminated or massive. These rocks routinely show millimetre-scale laminations, although outcrops located to the NW of Hématite Lake (sheet 24C10) rarely show evidence of stratification (Dressler, 1979).In the Otelnuk Lake area (sheet 24C01), Dimroth (1978) points out that stratifications, invisible in outcrop, can be observed under the microscope.In thin sections, wacke and quartz wacke show angular fragments (0.01 to 0.20 mm) of quartz, plagioclase and microcline in a matrix of green to light green-brown chlorite, minor proportions of sericite and iron oxides, and traces of carbonate.Biotite and opaque minerals are sometimes observed in places (Dressler 1979).

Sandstone and quartz sandstone hasve a chocolate brown, greenish grey or grey patina (Clark, 1979). It isThey are fine to very fine grained and is locally fissile (Clark, 1979). In thin section, sandstone and quartz sandstone isare composed of angular to saubangular grains (0.03 to 0.30 mm) of quartz, microcline and plagioclase, as well as minor amountsproportions of carbonate and small fragments of highly altered pyroxene (Dressler, 1979). The matrix is composed of chlorite, some sericite and very fine hematite dust. The latter gives the rock a chocolate-brown colour (Dressler, 1979).

GrauwWacke, quartz wacke and sandstone usually show cross-bedding, convolute bedding and carbonate (dolomitic) concretions (Clark, 1979; Dimroth, 1978; Dressler, 1979). Dolomitic concretions range in size from a few centimetres to 0.6 m (Clark, 1977).

Siltstone has a brown chocolate, green, grey-green or grey alteration patina (Clark, 1979; Dressler, 1979). It is very fine grained, laminated, and sometimeslocally fissile (Clark, 1979; Dressler, 1979). Mudstone has a reddish-brown alteration patina and is fissile (Clark, 1979). According to Clark (1979), the reddish colour of some beds may be associated with alteration because it cuts bedding. In some places, rare beds of grey chert, very coarse-grained sandstone and conglomerate with siltstone, quartz sandstone and microconglomerate and quartz sandstone pebbles are also reported (Clark, 1979, 1984; Dressler, 1979).

Menihek Formation 3 (pPme3): Quartz Sandstone, Subarkosic Sandstone, Wacke and Wacke; Locally Interbedded Chert-Pebble Conglomerate Beds

This unit consists of quartz sandstone, subarkosic sandstone and fine to medium-grained wacke with a medium grey, dark grey or black wackealteration patina (Clark, 1977). In the Forbes Lake area (sheet 24F05), the unit is located primarily at the base of the Menihek Formation and is estimated to be more than 100 m thick. Rocks are massive or locally show pit locally cross-bedding. Channels are also observed in places. Conglomerate beds 0.01 to 1 m thick are interstratifed locally in the unit. Conglomerate is commonly composed of grey chert pebbles, but white quartzite pebbles and rare jasper pebbles are also present(Ciesielski, 1975; Clark, 1977). The pebbles are subrounded to elongated and subangular to well rounded and are contained in a quartz sandstone matrix (Clark, 1977).

Menihek Formation 4 (pPme4): Dolomite and Sandy Dolomite

This unit consists of dolomite, sandy dolomite or dolomitic sandstone. However, it is not widespread. It is located on the Mélèze River’s north side (sheet 24E09), where it occurs as 2 to 3 m thick grey dolomite beds and 30 to 60 cm laminated sandy dolomite beds, interstratified with centimetre-thick mudstone fissile beds (Clark, 1979). In the Kyak Bay area, northSW of MargeryRoberts Lake (sheet 25D08), small dolomite outcrops is present inside the pPme1 unit in small outcrops rarely more than 15 m long by 3 m wide (Hardy, 1976) are found locally inside subunit pPme1a (Hardy, 1976). The rock is beige to brown in altered surface and light grey in fresh exposure. It is cut by many quartz veins, giving it a crosshatched appearance (Dimroth et al., 1970; Hardy, 1976). The most important veins are parallel to regional schistosity, while other veins are perpendicular to the first. The rock is composed of 87 to 97% dolomite with minor amounts of quartz (1-7%), biotite (up to ≤3%) and tremolite (up to ≤1%). Traces of talc and opaque minerals are also present (Hardy, 1976).

Menihek Formation 5 (pPme5): Quartz-Biotite-Muscovite-Chlorite ± Garnet Schist and Phyllite

This unit is located in the Roberts Synclinal’s eastern flank and south of the Arnaud River in the Brochant River area.Unit pPme5 is found in the Nagvaraaluk Lake area (sheets 25D10 and 25D15), in the eastern flank of the Roberts Syncline (sheets 25C04 and 25D01) and south of the Arnaud River, in the Brochant River area (sheet 24N13). Schist is derived from wacke and mudstoneIt consists essentially of quartz-biotite-muscovite-chlorite ± garnet schist derived from wacke and mudstone that hasve undergone higher metamorphism than other metasedimentaryunits(e.g., unit pPme1a) (Bilodeau and Caron-Côté, 2018). Phyllite has also been observed. The rock is generally fine to medium grained, schistose, banded and folded. The rock has varying proportions of quartz, biotite, muscovite and chlorite, a smaller amounts proportion of garnet locally and contains plagioclase in places.Garnet or biotite porphyroblasts are found locally. Tectonometamorphic banding is marked by millimetre-thick, and more locally centimetre-thick bands, displaying variations in mineral proportions between quartz and mica. Quartz is mostly granoblastic and usually occurs as monomineralic bands.

Mica is mostly fine grained, but biotite is also found in a few places in medium-grained porphyroblasts(Bilodeau and Caron-Côté, 2018). Biotite commonly contains zircon inclusions. Mica emphasizes the main lepidoblastic foliation which is generally parallel to banding. Microscopic observation of samples collected in the Roberts Synclinale shows that mica also adopts, in smaller proportions, of other orientations that may correspond to other generations of less penetrative schistosity. Mica also marks CS structuresfabrics in shear zones. Garnet is usuallycommonly porphyroblastic and poikilitic, locally helicitic. Inclusions in garnet are mostly quartz and more locally biotite. Accessory mineral phases are epidote, titanite and very locally tourmaline. This subunits’ mineralogical assemblage of this subunit suggests metamorphism at the amphibolite facies (Bilodeau and Caron-Côté, 2018).

Menihek Formation 6 (pPme6): Massive and Pillow Basalt; Minor Proportions of Tuff Locally

This uUnit pPme6 consists of massive and pillow flows of tholeiitic basalt which mayand minor proportions include of tuff in places(Baragar, 1967; Dimroth, 1978).These rocks are mainly located in the south-central part of the Labrador Trough (sheets 23O E and 23J16) where they are at the top of the formation. In the Labrador Trough’s southern part, massive basalt flows are widespread in the Menihek Formation, while pillow basalt flows are scarce (Baragar, 1967; Dimroth, 1978). Flows are generally between 5 and 20 m thick.A breccia of hyaloclastic material a few centimetres thick is usually present at the base or top of the flows.  Massive flows are commonly divided into columns. Pillows are sometimes packed together and are generally surrounded by a black crust of hyaloclastic material (Dimroth, 1978). In the Walsh Lake area (sheet 23O01), poorly preserved variolitic crusts were observed around pillows by Baragar (1967).Pillows commonly contain parallel, tabular cavities filled with quartz (Dimroth, 1978). Basalt is light grey in fresh exposure and has a light greenish grey to buff alteration patina (Baragar, 1967; Frarey, 1967; Dimroth, 1978). It is fine grained to aphanitic, with grain sizes ranging from less than 0.1 mm at the edge of flows and in the pillow areas, to about 1 mm in the massive zones (Dimroth, 1978). In thin section, basalt contains plagioclase microliths 0.2 mm long, generally forked and empty, in a very fine-grained brownish matrix composed of pyroxene (augite), plagioclase, chlorite and titanite. Plagioclase phenocrystals 2 cm long and 0.5 mm thick are also observed (Baragar, 1967; Dimroth, 1978).

Pillow flows were observed by Baragar (1967) west of Irène Lake (sheet 23O10) and by Dimroth (1978) in the Aubin Lake area (sheet 23O07). According to Gebert (1991), no pillows were observed in the Frederickson Lake area (sheet 23O01). Flows are 5 to 20 m thick. They are massive in the centre and pillowed at the top, sometimes at the base. Pillows usually contain parallel, tabular cavities filled with quartz (Dimroth, 1978).

Menihek Formation 7 (pPme7): Mafic PyroclasticsVolcaniclastic Rocks

Unit pPme7 consists of greenish mafic volcaniclastic rocks made up of lapilli tuff, agglomerate and volcanic breccia (Baragar, 1958, 1967).In the Murdoch River area (sheet 23O10) and further south in the Jean Lake area (sheet 23O07), tThese rocks are associated with the basaltic unit (pPme6) and form layers 3 to 60 m thick (Dimroth, 1978) at the base of the volcanic pile (Baragar, 1967). In places, they are interbedded in the underlying sedimentary rocks (Baragar, 1967). Volcaniclastic rocks are composed of basaltic and gabbroic fragments in a sheared basaltic matrix of chlorite, actinote, albite, calcite and titanite (Dimroth, 1978). Fragment sizes range from 1 to 7.5 cm and can reach more than 30 cm in diameter (Baragar, 1967; Dimroth, 1978).Mafic pyroclastites include lapilli tuffs, agglomerates and greenish volcanic breccias (Baragar, 1958, 1967).  Locally, Dimroth (1978) notes the presence of dolomite and slate fragments.

Menihek Formation 8 (pPme8): Grauwacke and Tuffaceous SiltstoneTuffaceous Wacke and Siltstone; Minor Proportion of Grey or Black Mudstone

Unit pPme8 is restricted to the area east of Martin Lake (sheet 23J16).This subdivision is located east of Martin Lake (sheet 23J16). It consists of a greenish grey grauwacke and tuffaceous wacke and silstone sequence interstratifiebedded with a minor proportion some of grey andor black mudrockstone (Wardle, 1982).

Menihek Formation 9 (pPme9): Sulphide Facies Iron Formation With Sulphides

This subdivision is located in the Frederickson Lake area (sheet 23O01)Unit pPme9 consists of layers of iron formation with sulphides and belongs to the upper half of the Menihek Formation (Baragar, 1967; Dimroth, 1978).The suplhides iron formation locally forms layers >30m thick that can be traced for several kilometres in the area west of lakes Connolly and Frederickson (sheet 23O01). The iron formation is laminated and shows very fine-grained alternating bands of black mudstone and siltstone, and sulphides-rich (Baragar, 1967; Gebert, 1991). Band thickness ranges from 1 to 10 mm. Gebert (1991) notes that bedding is locally displaced by several millimetres due to intraformational fractures. Mudstone and siltstone bands are sorted and sulphides are usually found in the fine-grained fraction. Sulphides (0.01 mm in diameter) consist of pyrrhotite and partially recrystallizedpyrite. Petrographic studies of polished thin sections show that pyrrhotite is more abundant than pyrite, which accounts for <10% of sulphides. Pyrrhotite is observed in metamorphic intergrowth with silicate minerals. In places, it has developed along cleavage planes where deformation is present (Gebert, 1991).study 

Menihek Formation 10 (pPme10): Grey-Chert-Pebble Conglomerate

This subdivisionUnit pPme10 is located at the upper contact with the carbonate facies of the Sokoman Formation (pPso3) at Bergeron Lake (sheet 24F05)(Tom Clark, personal communication, 2015). It consists of a bed of grey-chert-pebble conglomerates that are millimetres to decimetre-sized. The bed is discontinuous laterally. The overlying pebble conglomerate and sandstone (unit pPme3) are associated with small normal faults, suggesting a change in tectonic conditions at the end of the Sokoman Formation’s deposition.

Thickness and Distribution

The Menihek Formation spans outcrops over much of the Labrador Trough, stretching more than 850 km from the Grenville Front to Ungava Bay in a NNW-SSE axis. On the Quebec side, this formation occursoutcrops mainly in the northern and southern ends of the orogen, as well as in its central part. The original thickness of this formation is difficult to determine since it is intensely folded into a synclinorium (Dimroth, 1978). The Menihek Formation outcropsAt the northern end of the Labrador Trough, the Menihek Formation outcrops along a cartographic width ranging from 0.5 to 4 km in the flanks of Roberts Synclinal’se flanks, where the stratigraphy is heavily folded and subvertically inclined. This formation extends over larger widths where the thickness of the stratigraphy is lower, reaching up to 30 km wide in the central part and up to 10 km in the southern part of the Labrador Trough‘s southern part.