DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail.
|Stratotype:||The type locality is at Milamar Lake (approximate UTM NAD83, zone 19 coordinates: 592000 m E, 6235300 m N)|
|Type area:||Milamar Lake area (NTS sheet 24B05)|
|Geological province:||Churchill Province|
|Geological subdivision:||New Quebec Orogen (Labrador Trough) / Romanet (formerly Wheeler) Lithotectonic Zone|
|Lithology:||Arkose, conglomerate, quartzitic arenite, sandstone, dolomite, marble, schist|
A sequence of metamorphosed arkose, conglomerate and quartzite was first recognized by Dimroth (1964) along the Wheeler River (sheet 24B05). Dimroth (1968) tentatively introduced the name Milamar Formation to refer to this sequence and later formalized it (Dimroth, 1972, 1978). The unit takes its name from Milamar Lake (sheet 24B05) where it was defined. Dimroth (1972, 1978) subsequently provided a more detailed description of this metamorphosed unit of arkose, quartzitic arenite, conglomerate and interbedded dolomite, representing the upper part of the « Seward Subgroup » in the eastern Labrador Trough. Clark and Wares (2004) later reclassified it as the Seward Group, thus abandoning the name « Seward Subgroup ».
The Milamar Formation represents the eastern marginal facies of the upper part of the Seward Group in the Labrador Trough (Dimroth, 1964, 1970, 1972, 1978; Clark and Wares, 2004). It is the eastern equivalent of the Dunphy Formation. It consists of arkose, polymictic pebble conglomerate, quartzitic arenite, dolomitic sandstone, dolomite, marble and schist (Dimroth, 1964, 1970, 1978). The presence of dolomitic layers distinguishes this formation from the underlying Chakonipau Formation (Dimroth, 1972, 1978). The rocks of the Milamar Formation were metamorphosed to the greenschist facies and deformed during the Hudsonian Orogeny (Dimroth and Dressler, 1978). Metamorphism is evidenced by recrystallization of coarser sedimentary rocks and sericitization of arkose and conglomerate matrix. Finer-grained sedimentary rocks consist of quartz-sericite schist (Dimroth, 1972, 1978; Kish and Tremblay-Clark, 1979). Primary sedimentary structures are lost. Rocks generally display folded schistosity (Dimroth, 1972, 1978).
According to Dimroth (1972, 1978), the detrital material constituting the Milamar Formation is the result of the erosion of Archean rocks of the Wheeler Complex which formed a positive relief dome on the eastern margin of a shallow marine basin. Sedimentary rocks of the formation were deposited in a coastal environment, bordering an elevated terrain represented by the Wheeler Dome (Dimroth, 1972, 1978). According to Dimroth (1972, 1978), dolomites at the top of the Milamar Formation were deposited in a marine environment and reflect minor marine transgressions. Sedimentary rocks (Milamar Formation) and Archean bedrock (Wheeler Complex) on which they rest were subsequently transported onto volcano-sedimentary rocks of the Laporte Supersuite (Clark and Wares, 2004).
Milamar Formation 1 (pPmm1): Interbedded Arkose, Polymictic Conglomerate and Quartzitic Arenite; Locally Interbedded Dolomite
At the type locality, unit pPmm1 consists of arkose, polymictic conglomerate and quartzitic arenite deposited unconformably (erosion) on Archean gneiss of the Wheeler Complex (Dimroth, 1972, 1978; Kish and Tremblay-Clark, 1979). The unit locally contains interbedded dolomite. The arkose is composed of 50% subrounded cataclastic grains of quartz and feldspar in a fine-grained sericite-quartz-albite matrix. The conglomerate consists of gneiss pebbles 2-10 cm in diameter, rarely up to 30 cm, in a mylonitic albite-sericite-quartz matrix. The quartzitic arenite occurs as lenses or thick banks interbedded with arkose and conglomerate. The rock is white and mainly composed of quartz. It is locally feldspathic. Coarse-grained metamorphosed equivalents of unit pPmm1 are reported to the south of the type locality (Dimroth, 1972, 1978).
Milamar Formation 2 (pPmm2): Interbedded Quartz-Albite-Sericite Schist, Quartz-Sericite Schist and White Quartzite
Unit pPmm2 consists of quartz-sericite rocks derived partly from arkose and conglomerate. These rocks were more strongly metamorphosed and deformed during the Hudsonian Orogenesis (Dimroth and Dressler, 1978) and their primary structures were obliterated. The unit comprises grey and white quartz-albite-sericite schist, locally pink or green in altered surface, and interbedded quartz-sericite schist and white quartzite (Dimroth, 1964, 1972, 1978). These rocks are fine grained and generally schistose and folded. To the south of the type locality, quartz-sericite schist layers occupy the upper part of the formation and locally contain dolomite or dolomitic sandstone beds.
In thin sections, arkose is composed of quartz, feldspar (albite and minor amount of microcline) and muscovite. Quartz and feldspar display significant deformation and undulatory extinction. Quartz is recrystallized and partially granulated, and occurs as coarse grains or small polygonal grains. Feldspar is highly sericitized, especially when the rock is highly deformed. Plagioclase (oligoclase) porphyroblasts are hosted in a matrix of quartz granules. Well-developed schistosity is marked by a minor amount of sericite. Epidote, calcite and chlorite are rarely observed. Dolomitic sandstone is composed of calcite, epidote, plagioclase, tremolite and quartz (Dimroth, 1972, 1978).
Milamar Formation 3 (pPmm3): Metamorphosed Dolomite, Marble
Two lenses of metamorphosed dolomite or white marble were mapped by Dimroth (1964, 1972, 1978) near the Wheeler River, west of the Wheeler Complex. According to this author, these marbles belong to the upper part of the Milamar Formation. The rock is generally composed of calcosilicate minerals (tremolite, talc) and saccharoidal calcite.
Thickness and Distribution
The Milamar Formation belongs to the Romanet Lithotectonic Zone, previously defined as the Wheeler Zone by Clark and Wares (2004). It occupies an area generally oriented NW-SE along the Wheeler River between latitudes 56°N and 56°30’N (Dimroth, 1964, 1970, 1972, 1978). The rocks belonging to this formation were also mapped by Dimroth (1978) in areas south and east of the Wheeler Complex. Kish and Tremblay-Clark (1979) observed schists attributed to the Milamar Formation south of Duhamel Lake (sheet 24B06). Rocks of this formation are absent north of latitude 56°30’N (Dressler, 1979). The thickness of the Milamar Formation is poorly known: a thickness of ~300 m was suggested by Dimroth (1970, 1978, p. 67).
The deposition of the Milamar Formation occurred between 2166 ±4 and 2142 +4/-2 Ma, i.e. during the depositional period of first-cycle sedimentary rocks (T. Krogh and B. Dressler, unpublished data cited in Clark, 1984, page 4; Clark and Wares, 2004; Corrigan et al., 2019).
At the type locality, the Milamar Formation overlies Archean gneiss of the Wheeler Complex in non-faulted, erosional unconformity (Dimroth, 1970, 1972, 1978). The contact between these units is visible east of the Wheeler River at latitude 56°17’N, as well as further SE between latitudes 56°14’N and 56°15’N. Dimroth (1972, 1978) observed gneiss pebble and boulder conglomerate at several locations in the Milamar Formation at or near the Wheeler Complex contact (between latitudes 56°11’N and 56°15’N). In the Yroquet Lake area, the contact between Wheeler gneiss and sedimentary rocks of the Milamar Formation is unconformable (Kish and Tremblay-Clark, 1979).
According to Dimroth (1972, 1978), it is not possible to determine the summits in rocks of the Milamar Formation because sedimentary structures were lost during the Hudsonian Orogenesis (Dimroth and Dressler, 1978). The upper part of the Milamar Formation is thought to consist of marble according to Dimroth (1972, 1978). In the Yroquet Lake area, the upper part of the formation contains dolomite layers with characteristics similar to typical dolomites of the Dunphy Formation. The Milamar Formation is thus the eastern equivalent of the Dunphy Formation (Dimroth, 1970, 1972, 1978; Dimroth et al., 1970). These formations occupy similar stratigraphic positions, i.e. overlying the Chakonipau Formation (Dimroth, 1970, 1972, 1978; Clark and Wares, 2004). Dimroth (1970) points out that facies variation between the Dunphy and Milamar formations is visible over a short distance, suggesting rapid uplift of the source region further east.
The Milamar Formation gradually changes to the overlying Lace Lake Formation (Pistolet Group) (Dimroth, 1978; Kish and Tremblay-Clark, 1979). The contact is exposed on the west shore of Yroquet Lake, where it is marked by a 3 m thick transition zone (Dimroth, 1972, 1978). In the area east of the Wheeler Complex, in the Rachel-Laporte Shear Zone, the Milamar Formation may be partly equivalent to the Secondon Suite (Charette et al., 2016) consisting of meta-arkose, conglomeratic meta-arenite, metaconglomerate and schist.
No fossils reported.
Publications Available Through SIGÉOM Examine
CHARETTE, B., LAFRANCE, I., MATHIEU, G. 2016. Géologie de la région du lac Jeannin (SNRC 24B). Ministère de l’Énergie et des Ressources naturelles, Québec. BG 2015-01
CLARK, T. 1984. GEOLOGIE DE LA REGION DU LAC CAMBRIEN – TERRITOIRE DU NOUVEAU-QUEBEC. MRN. ET 83-02, 77 pages and 1 plan.
CLARK, T., WARES, R. 2004. SYNTHESE LITHOTECTONIQUE ET METALLOGENIQUE DE L’OROGENE DU NOUVEAU-QUEBEC (FOSSE DU LABRADOR). MRNFP. MM 2004-01, 182 pages and 1 plan.
CORRIGAN, D., SAPPIN, A.-A., HOULÉ, M.G., RAYNER, N., VAN ROOYEN, D. 2019. Corrugated Hills: les restes d’une grande province ignée d’environ 2,17 Ga dans la Fosse du Labrador. In: MERN. 2020. Résumés des conférences et des photoprésentations, Québec Mines+Énergie 2019. DV 2019-01, 78 pages.
DIMROTH, E. 1964. GEOLOGIE DE LA REGION DU LAC ROMANET, NOUVEAU-QUEBEC. MRN. RP 523, 20 pages ND 1 plan.
DIMROTH, E. 1964. PRELIMINARY REPORT, GEOLOGY OF ROMANET LAKE AREA, NEW QUEBEC. MRN. RP 523(A), 18 pages AND 1 plan.
DIMROTH, E. 1972. STRATIGRAPHY OF PART OF THE CENTRAL LABRADOR TROUGH. MRN. DP 154, 304 pages AND 6 plans.
DIMROTH, E. 1978. REGION DE LA FOSSE DU LABRADOR ENTRE LES LATITUDES 54° 30′ ET 56° 30′. MRN. RG 193, 417 pages AND 16 plans.
DRESSLER, B., CIESIELSKI, A. 1979. REGION DE LA FOSSE DU LABRADOR. MRN. RG 195, 136 pages AND 14 plans.
KISH, L., TREMBLAY-CLARK, P. 1979. GEOCHIMIE ET RADIOACTIVITE DANS LA FOSSE DU LABRADOR. MRN. DPV 666, 33 pages.
DIMROTH, E. 1970. Evolution of the Labrador Geosyncline. Geological Society of America Bulletin; volume 81, pages 2717–2742. doi.org/10.1130/0016-7606(1970)81[2717:EOTLG]2.0.CO;2
DIMROTH, E., BARAGAR, W.R.A., BERGERON, R., JACKSON, G.D. 1970. The filling of the Circum-Ungava geosyncline. In: Symposium on Basins and Geosynclines of the Canadian Shield (A.J. Baer, editor). Geological Survey of Canada; Paper 70-40, pages 45-142. doi.org/10.4095/124922
DIMROTH, E., DRESSLER, B. 1978. Metamorphism of the Labrador Trough. In: Metamorphism in the Canadian Shield. Geological Survey of Canada; Paper 78-10, pages 215-236. doi.org/10.4095/104534
Ministère de l’Énergie et des Ressources naturelles (MERN). Milamar Formation. Quebec Stratigraphic Lexicon. https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-churchill/formation-de-milamar_en [accessed on Day Month Year].
Charles St-Hilaire, GIT, M.Sc. firstname.lastname@example.org (redaction)
Mehdi A. Guemache, P. Geo., Ph.D. (redaction and coordination); Thomas Clark, P. Geo., Ph.D. (critical review); Simon Auclair, P. Geo., M.Sc. (editing); Céline Dupuis, P. Geo., Ph.D. (English version); André Tremblay (HTML editing).