|Type area:||Menihek and Howell lakes, Newfoundland and Labrador (NTS sheet 23J07)|
|Geological province:||Churchill Province|
|Geological subdivision:||New Quebec Orogen (Labrador Trough) / Bérard, Hurst, Mélèzes, Payne, Schefferville and Tamarack lithotectonic zones|
|Lithology:||Clastic sedimentary rocks and, in lesser proportions, chemical or volcanic sedimentary rocks|
The name Menihek Slates was first proposed by geologists at the Labrador Mining and Exploration Company in 1949 (Frarey and Duffel, 1964). The name Menihek Formation became formal with Harrison (1952). Several works describing the Menihek Formation took place in the New Quebec Orogen (Labrador Trough). In the northern part of the orogen, the Menihek Formation was primarily described by Dressler (1973, 1975, 1979) in the Patu Lake area and around Fort Mackenzie, and by Clark (1977, 1979) in the Forbes Lake and Napier Lake areas. Hardy (1976) mapped the Menihek Formation rocks in the Roberts and Des Chefs lakes area, north of the village of Kangirsuk, in the Labrador Trough’s northern end. Rocks of the Menihek Formation in the southern part of the orogen were mapped in Quebec by Baragar (1967), Fahrig (1967) and Dimroth (1978), and on the Newfoundland and Labrador side by Frarey (1961), Evans (1978) and Wardle (1979, 1982).
In the northern part of the orogen, Bergeron (1954, 1956) proposed the name Larch River Formation for a sandstone, siltstone and mudstone sequence along the Mélèzes River. The name Larch River Formation had previously been given to these rocks by Fenimore Iron Mines geologists. In mapping the area straddling the Mélèzes River, Bérard (1965) identified the unit as the Formation de Rivière Larch. Sauvé and Bergeron (1965) used the same name for the unit west of Gerido and Léopard lakes. Clark and Wares (2004) recommended abandoning the name Larch River Formation as they consider the unit to be partially equivalent to the senior member of the Baby Formation and the Menihek Formation.
The name Dragon Formation was proposed by Bérard (1965) for a series of detrital sediments (rhythmites) located between the Sokoman Formation (known as the Fenimore Formation by Bérard, 1965) and the structurally overlying Chioak Formation in the northern part of the Trough. The unit consists of thin, greenish grey siltstone beds alternating with thin, black shale beds. These have lithostratigraphic similarities with black shale at or near the base of the Menihek Formation in the Schefferville area (Clark et al., 2008), the apparent base black shale of the Menihek Formation at the Mélèzes River (latitude 57°35′ N) and black shale of the Dragon Formation in the northern Labrador Trough. The unit was considered a sandstone member of the Menihek Formation by Clark and Wares (2004), and these authors recommended abandoning the name Dragon Formation.
The Menihek Formation is the transgressive sequence’s upper unit of platform sediments belonging to the Ferriman Group, which deposited in the foreshore of the New Quebec Orogen during the second cycle of sedimentation and volcanism (Clark and Wares, 2004). The Menihek Formation consists primarily of rhythmites (turbidites) that are indicative of a deep-water deposit. These detrital sedimentary rocks consist of sandstone, siltstone, mudstone, graphitic mudstone (locally pyritic), grauwacke, wacke, arenite and, in smaller proportions, quartz-biotite-muscovite-chlorite-garnet schist, dolomite, phyllite, conglomerate and iron formations. Mafic volcanic activity began near the end of the Menihek deposit, mainly in the allochtonous Hurst Zone (Baragar, 1967; Clark and Wares, 2004) in the southern orogen. Volcanic rocks consist of massive and pillow tholeitic basaltic flows with a small proportion of mafic pyroclastic rocks (Baragar, 1967; Dimroth, 1978).
Rocks of the formation are usually metamorphosed to the greenschist facies. However, metamorphic facies range from greenschist to amphibolite in the Labrador Trough’s northern end, north of Aupaluk. On the western flank of the Roberts Synclinal as well as on the north side of the lake of the same name, mineralogical assemblages are representative of the greenschist facies. On the eastern flank of the Roberts Synclinal and in the Brochant River area south of the Arnaud River, they are instead representative of the amphibolite facies, evidenced by the Menihek 5 unit (pPme5) (Bilodeau and Caron-Côté, 2018). Primary structures, such as cross-bedding, are common in the northern part of the Labrador Trough (Clark, 1979; Dimroth, 1978; Dressler, 1979), but are very local in the north end (Hardy, 1976). However, primary structures generally appear to be obliterated by tectonometamorphic secondary structures, such as schsitosity and banding, generated during the building of the New Quebec Orogen (Bilodeau and Caron-Côté, in preparation). The Menihek Formation is prospective for polymetalic mineralization of exhalative massive sulphides in graphic and pyritic mudstones (Clark and Wares, 2004).
This unit overlies the Sokoman Formation. In the Trough’s northern end, it consists mainly of wacke and, in smaller proportions, siltstone, phyllite and arenite (Bilodeau and Caron-Côté, 2018). Further south, the unit is described as alternating millimetric to centimetric bands of mudstone and grey to black siltstone, finely laminated, slightly graphitic and pyritic in places (Baragar, 1967; Dimroth, 1978; Dressler, 1979).
Wacke has a brown or beige alteration patina and is light to dark medium grey in fresh exposure. Outcrops usually have millimetric to decimetric veinlets and veins, boudinaged and folded, composed of medium-grained quartz. Wacke is composed of quartz, muscovite, biotite and locally plagioclase. Minor and accessory mineral phases are plagioclase, carbonate, chlorite and pyrite. The rock is fine grained. It is generally schistose and usually contains discontinuous millimetric quartz laminae. Schistosity planes are marked by the orientation of mica. The rock is usually deformed and exhibits isoclinal folds and crenulation cleavages (Bilodeau and Caron-Côté, 2018).
Siltstone has a medium brown to dark grey and locally rusty patina. It is dark grey to blackish in fresh exposure. It is very fine grained to aphanitic and is usually laminated or schistose. This rock is composed of biotite, muscovite and quartz. Minor and accessory mineral phases are generally graphite and sulphides (Bilodeau and Caron-Côté, 2018).
Phyllite is a metasiltstone or metamudstone with more penetrative schistosity. In macroscopic samples, the surface of schistosity planes has a glossy appearance, likely caused by the sericite content. Phyllite has the same mineralogical composition as siltstone, but appears to be distinguished locally by a higher graphite content, ranging from 1 to 5%. Within the Roberts Synclinal, phyllite occurs locally in different stratigraphic levels of the Menihek Formation. However, it is observed at several locations in direct contact with the underlying unit of the Sokoman Formation (Bilodeau and Caron-Côté, 2018).
The majority of arenite is composed of quartz and contains accessory minerals such as plagioclase, biotite, muscovite, epidote, K-feldspar, carbonate and magnetite. It is fine to medium grained size and generally has a massive, more locally laminated or schistose texture. This rock is light grey to dark grey in fresh exposure and altered surface. Cross-beds were observed locally (Hardy, 1976). Arenites observed in the Hardy (1976) and Bilodeau and Caron-Côté (2018) work are found mainly in the northern part of the Roberts Synclinal, north of the lake of the same name. They are also found more locally along the eastern synclinal contact.
This unit is the upper part of the Menihek Formation in the Labrador Trough’s northern part. 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.
Grauwacke 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 (Dressler, 1979). However, Dressler (1979) indicates that outcrops localized to the northwest of the Hématite Lake (NTS sheet 24C10) rarely show evidence of stratification or lamination. In the Otelnuk Lake area (sheet 24C01), Dimroth (1978) points out that laminations, invisible in outcrop, can be observed under the microscope. In thin sections, grauwacke 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 amounts of sericite and iron oxides, and traces of carbonate. Biotite and opaque minerals are sometimes observed (Dressler 1979).
Sandstone has a chocolate brown, greenish grey or grey patina. It is fine to very fine grained and is locally fissile (Clark, 1979). In thin section, sandstone is composed of angular to aubangular grains (0.03 to 0.30 mm) of quartz, microcline and plagioclase, as well as minor amounts of carbonate and small fragments of highly altered pyroxene. The matrix is composed of chlorite, some sericite and very fine hematite dust. The latter gives the rock a chocolate-brown colour (Dressler, 1979). Grauwacke, 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 patina. It is very fine grained, laminated, and sometimes fissile (Clark, 1979; Dressler, 1979). Mudstone has a reddish-brown 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 and quartz sandstone pebbles are also reported (Clark, 1979; Dressler, 1979).
This unit consists of quartz sandstone, subarkosic sandstone and fine to medium-grained wacke with a medium grey, dark grey or black wacke. 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 show 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 jaspe pebbles are also present. The pebbles are subrounded to elongated and subangular to well rounded and are contained in a quartz sandstone matrix (Clark, 1977).
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, north of Margery Lake (sheet 25D08), dolomite is present inside the pPme1 unit in small outcrops rarely more than 15 m long by 3 m wide (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).
This unit is located in the Roberts Synclinal’s eastern flank and south of the Arnaud River in the Brochant River area. Schist is derived from wacke and mudstone that have undergone higher metamorphism than other units. The rock has varying proportions of quartz, biotite, muscovite and chlorite, smaller amounts of garnet and contains plagioclase in places. It varies from fine to medium-grained and may contain coarse-grained biotite or garnet porphyroblasts. It is usually schistose, banded and folded. Tectonometamorphic banding is marked by millimetre-thick, and more locally centimetre-thick bands, showing variations in mineral proportion 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. 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 Synclinal shows that mica also adopts smaller proportions of other orientations that may correspond to other generations of less penetrative schistosity. Mica also marks CS structures in shear zones. Garnet is usually 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 suggests metamorphism at the amphibolite facies (Bilodeau and Caron-Côté, 2018).
This unit consists of massive and pillow flows of tholeiitic basalt which may include tuff in places. 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). 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. 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 usually 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 usually 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; Dimroth, 1978; Frarey, 1967). 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 sphene. Plagioclase phenocrystals 2 cm long and 0.5 mm thick are also observed (Baragar, 1967; Dimroth, 1978).
Mafic pyroclastites include lapilli tuffs, agglomerates and greenish volcanic breccias (Baragar, 1958, 1967). These rocks are associated with the basaltic unit (pPme6) and form horizons 3 to 60 m thick (Dimroth, 1978) at the base of the volcanic pile (Baragar, 1967). To the south, they are interbedded in the Menihek Formation’s underlying sedimentary units (Baragar, 1967). Pyroclastics are composed of basaltic and gabbroic fragments in a sheared basaltic matrix of chlorite, actinolite, albite, calcite and sphene (Dimroth, 1978). Fragment sizes range from 1 to 7.5 cm and can reach more than 30 cm in diameter (Baragar, 1967; Dimroth, 1978). Locally, Dimroth (1978) notes the presence of dolomite and slate fragments.
This subdivision is located east of Martin Lake (sheet 23J16). It consists of a greenish grey grauwacke and tuffaceous silstone sequence interstratified with some grey and black mudrock (Wardle, 1982).
This subdivision is located in the Frederickson Lake area (sheet 23O01) and belongs to the upper half of the Menihek Formation (Baragar, 1967; Dimroth, 1978). The unit forms in few places horizons greater than 30 m thick that can be tracked over several kilometres (Baragar, 1967; Dimroth, 1978). Iron formations are laminated and show alternating bands of black mudstone and siltstone, and very finely interstratified bands of sulphide (pyrrhotite and pyrite) (Baragar, 1967; Gebert, 1991). Band thickness ranges from 1 to 10 mm. Gebert (1991) notes that bedding is, in few places, displaced by several millimetres due to intraformational fractures. The mudstone and siltstone bands are sorted and sulphides are usually found in the fine-grained fraction. Sulphides are fine grained (0.01 mm) and consist of pyrrhotite and pyrite, which are partially recrystallized. The polished thin sections’ petrographic study shows that pyrrhotite is the most abundant sulphide, and that pyrite accounts for less than 10% of sulphides. Pyrrhotite is recrystallized and in metamorphic intergrowth with silicate minerals. It has, in few places, developed along cleavage planes where deformation is present (Gebert, 1991).
This subdivision is located at the upper contact with the carbonate facies of the Sokoman Formation (pPso3) at Bergeron Lake (sheet 24F05). 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 are associated with small normal faults, suggesting a change in tectonic conditions at the end of the Sokoman Formation’s deposition.
The Menihek Formation spans 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 occurs 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 outcrops along a cartographic width ranging from 0.5 to 4 km in the Roberts Synclinal’s 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 Trough’s southern part.
Two U-Pb datings were performed in two different areas: 1) Dyke Lake area, 40 km southeast of Schefferville and in the western portion of the New Quebec Orogen; and 2) Howse Lake area, 40 km northeast of Schefferville and a more eastern part of the orogen (Findlay et al., 1995). In the Dyke Lake area, an age of 1877.8 ±1.3 Ma was obtained from a syenite clast in a conglomerate interstratified with the Sokoman Formation, underlying the Menihek Formation. This date provides a minimum age at sedimentation slightly prior to the Menihek Formation in this area. In the Howse Lake area, a crystallization age of 1884 ±2 Ma was obtained for one of the Montagnais sills cutting the Menihek Formation. This results in a maximum age of sedimentation for the Menihek Formation in this area. This geochronological data suggests the Menihek Formation’s rejuvenation to the southwest; a hypothesis that is consistent with a transgression in the same direction (Findlay et al., 1995). Fryer (1972) dated a Menihek sedimentary rock using the Rb/Sr isochron method of whole rock and obtained an age of 1855 ±74 Ma. However, he does not specify the geographic location of the sample.
The Menihek Formation overlies ferriferous rocks of the Sokoman Formation. The nature of the contact between the two seems to vary by sector. According to Dimroth (1978), contact is usually sharp and conformable, although locally unconformable. According to Baragar (1968, Iron Ore Company of Canada. In: Dimroth, 1978), contact can be of a discordant nature since he observed ferriferous rock conglomerates between the Sokoman and Menihek formations, suggesting an erosion period between the deposition of these two formations. Interfingered contacts between the Menihek and Sokoman formations in the southern end of the Labrador Trough were observed (Zajac, 1974). In the southern part of the Labrador Trough, the Menihek Formation is in faulted contact with the Murdoch Formation (Frarey, 1967; Wardle and Bailey, 1981). According to Baragar (1967), the presence of the lake Walsh Fault make it difficult to interpret their stratigraphic relationships. The lower, median and upper parts of the Menihek Formation were correlated, respectively, with rocks of the Murdoch, Thompson Lake and Willbob formations belonging to the Doublet Group (Findlay et al., 1995).
In the northern half of the orogen (north of the Mélèzes River), the Menihek Formation is structurally overlain by rocks of the Denault Formation (Attikamagen Group), which are followed by rocks of the Koksoak Group. This relationship is caused by the presence of the Garigue Fault. The discovery of the latter allowed the Menihek Formation to be correlated with the upper member of the Baby Formation (Clark, 1987). The Menihek and Chioak formations are homotaxial in that they are both overlying the Sokoman Formation (Clark, 1979, 1988). At the northern end of the orogen, the Menihek Formation is overlain by the Hellancourt Formation. In the Roberts Synclinal, Bilodeau and Caron-Côté (2018) observed alternating and conformable contact between the Menihek and Hellancourt formations. In the southern part of the orogen, the summital portion of the Menihek Formation is intruded by numerous glomeroporphyric gabbro sills belonging to the Montagnais Sills (Findlay et al., 1995). The Nault Formation, located south of the Grenville Front, is equivalent to the Menihek Formation (Dimroth et al., 1970; Rivers, 1980).
Dressler (1979) reported cone structures nested in the Menihek Formation east of Chocolat Lake (sheet 24C10).