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Pistolet Group
Stratigraphic label: [ppro]pi
Map symbol: pPpi
 

First published: 7 December 2020
Last modified:

 

 

 

  DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail. 

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
 
pPpi15 Arkose and conglomerate, with interbedded very fine-grained sandstone and red and green siltstone and mudrock
pPpi14 Quartzitic arenite, with interbedded very fine-grained sandstone and red and green siltstone and mudrock
pPpi13 Vacant
pPpi12 Laminated or thinly bedded, pink or grey dolomite, commonly brecciated (syn-sedimentary breccia); pink stromatolitic dolomite
pPpi11 Vacant
pPpi10 Vacant
pPpi9 Vacant
pPpi8 Vacant
pPpi7 Vacant
pPpi6 Massive, locally brecciated dark grey dolomite and sandy dolomite with a brown or beige patina; interbedded dolomitic sandstone locally
pPpi5 Laminated grey mudstone and siltstone, commonly calcareous and locally graphitic; interbedded brown patina dolomite and locally fine-grained grey sandstone or dark grey slate
pPpi4 Grey dolomitic sandstone with a brown patina
pPpi3 Red, green or grey siltstone, mudstone and fine-grained sandstone; interbedded purple sandstone and white quartzitic arenite (« quartzite »)
pPpi2 White patina quartzitic arenite, locally dolomitic
pPpi1 Pink arkosic conglomerate
 
Author(s): Dimroth, 1968
Age: Paleoproterozoic
Stratotype: None
Type area: Pistolet Lake area, in the vicinity of Lace Lake (NTS sheet 24C02), where the Lace Lake, Alder and Uvé formations, forming the Pistolet Group, outcrop.
Geological province: Churchill Province
Geological subdivision: New Quebec Orogen (Labrador Trough) / Cambrien, Howse, Romanet (formerly Wheeler) and Schefferville lithotectonic zones
Lithology: Arkose, conglomerate, quartzitic arenite, siltstone, mudstone, sandstone, calcarenite, dolomite
Category: Lithostratigraphic
Rank: Group
Status: Formal
Use: Active

Background

The name « Pistolet Subgroup » was introduced by Dimroth (1968) to designate a stratigraphically very complex succession of mudstone, siltstone, quartzitic arenite and dolomite overlying rocks of the « Seward Subgroup ». The Pistolet Subgroup was divided by Dimroth (1968, 1969) into three formations named, from base to top: Lace Lake Formation, Alder Formation and Uvé Formation. The name of the unit comes from Pistolet Lake where all three formations outcrop (Dimroth, 1969, 1972, 1978). Clark and Wares (2004) subsequently reclassified all of Dimroth’s subgroups into groups, including the Pistolet Group. Rocks of the Pistolet Group have also been mapped by Dressler (1973, 1979), Clark (1984, 1986), Girard (1988) and Brouillette (1989).

 

Description

The Pistolet Group is an autochthonous/parochthonous to allochthonous sedimentary unit belonging to the first cycle of the Labrador Trough (Clark and Wares, 2004). It is lithologically very heterogeneous and consists mainly of sedimentary rocks forming a passive continental margin platform (Dimroth, 1969, 1970, 1972, 1978; Clark and Wares, 2004). The Pistolet Group is distinguished from the underlying Seward Group by its distinctly marine character, the abundance of mudstone and the general absence of red rocks. The presence of a few dolomite beds in all Pistolet Group formations and most of their informal units distinguish it from the overlying Swampy Bay Group (Dimroth, 1969).

The sedimentary sequence of the Pistolet Group begins with mudstone, siltstone, fine-grained sandstone and dolomite, generally greenish to greyish, of the Lace Lake Formation. These rocks are thought to have been deposited in coastal or subcostal environments (Dimroth, 1972, 1978). In places, such as near Castignon and Romanet lakes (Dimroth, 1978; Chevé, 1985; Clark, 1986), rocks are dark green, dark grey or black and locally contain graphite (e.g. in the « central basin facies » of Dimroth, 1978), indicating a significant change in redox conditions early in the deposition of the group in these areas (Clark and Wares, 2004). Lace Lake Formation rocks are overlain by beds of stromatolitic dolomite, locally reefy, dolomitic sandstone and white quartzitic arenite of the Alder Formation (Dimroth, 1972, 1978). These red, green or white rocks were deposited in a shallow marine environment and exhibit characteristics indicative of transient oxidizing to reducing chemical conditions similar to those of the marginal facies of the Lace Lake Formation. The upper part of the Pistolet Group consists of a massive, non-stromatolitic, brown patina grey dolomite of the Uvé Formation. These rocks are thought to have been deposited largely in relatively still, deep water where conditions were locally reducing (Dimroth, 1972, 1978).

Formations of the Pistolet Group were divided by Dimroth (1969, 1972, 1978) into numerous informal units to allow detailed correlation between the various localities where this group was mapped. Dimroth (1972, 1978) also points out that the Alder and Uvé formations contain facies that are difficult to distinguish from those of the Lace Lake Formation. According to Dimroth (1972, 1978), the typical subdivision of the Pistolet Group into three formations can be applied to the Castignon Lake and Wheeler River areas and largely to the Granite Falls area (sheet 23N16). In the Luché and Girafe lakes area (Table 1; sheet 24C07), NW of Castignon Lake, Dimroth (1972, 1978) recognized particular facies of the group, which he described as a succession of white quartzitic arenite and dolomitic sandstone layers typical of the Alder Formation, and brown patina dolomite characteristic of the Uvé Formation. Dimroth (1972, 1978) mentions, however, that this succession is relatively unknown because of the limited number of outcrops.

Table 1: Characteristics of the Pistolet Group in the Luché and Girafe Lakes Area 

Thickness (m) Description
150 Grey mudstone and siltstone; locally interbedded (30 cm thick) brown patina dolomite
0-60 Massive, dark grey dolomite having a brown patina, locally brecciated
30 Grey mudstone and siltstone; interbedded brown patina dolomite
150 Red and green siltstone and mudstone; interbedded (3-15 m thick) medium-grained purple sandstone and white quartzitic arenite
30 White patina quartzitic arenite
60 Medium-grained pink arkosic conglomerate
   
Reference: Dimroth (1978)

The Pistolet Group is the result of the development of a marine platform on the passive margin of the Archean continent (Dimroth, 1970, 1972, 1978). The source of the detrital material is thought to have been derived from terrain to the west and east of the Labrador Trough. Metamorphism is low in rocks of the Pistolet Group and increases from west to east, from sub-greenschist to greenschist facies (Dimroth and Dressler, 1978).

The Pistol Group contains nine informal units, numbered 1 to 15; numbers 7 to 11 and 13 are not used.

 

Pistolet Group, Undivided (pPpi): Siltstone, Mudstone, Sandstone, Dolomite, Sandy Dolomite

This undivided unit consists of siltstone, mudstone, sandstone, dolomite and sandy dolomite.

 

Pistolet Group 1 (pPpi1): Pink Arkosic Conglomerate

Unit pPpi1 outcrops SE of Luché Lake (sheet 24C07), where it is estimated to be 60 m thick. It consists of medium to very coarse-grained (>1 cm) pink arkosic conglomerate formed of quartz, plagioclase and microcline fragments cemented by some sericite. The rock is highly compacted and intensely folded, and usually has tectonic cleavage (Dimroth, 1972, 1978).

 

Pistolet Group 2 (pPpi2): White Patina Quartzitic Arenite, Locally Dolomitic

Unit pPpi2 is located between Luché Lake and Giraffe Lake, and west of Patu Lake (Dimroth, 1969, 1972, 1978; Dressler, 1973, 1979; Clark, unpublished). Dimroth (1972, 1978) estimates its thickness at 30 m. The unit consists of grey quartzitic, locally dolomitic arenite having a white alteration patina. The rock is massive and fine, medium or coarse grained (Dressler, 1979). According to Dimroth (1972, 1978), quartzitic arenite of unit pPpi2 is slightly tectonized and is indistinguishable from that of the Alder Formation. Arenite is in gradual contact with pink arkosic conglomerate (pPpi1) (Dimroth, 1972, 1978). In thin sections, arenite contains minor amounts of dolomite and feldspar (Dressler, 1979).

 

Pistolet Group 3 (pPpi3): Red, Green or Grey Siltstone, Mudstone and Fine-Grained Sandstone; Interbedded Purple Sandstone and White Quartzitic Arenite (« Quartzite »)

Unit pPpi3 is located between Luché Lake and Giraffe Lake. Dimroth (1972, 1978) estimates its thickness at 150 m. The unit consists of red, green or grey siltstone, mudstone and fine-grained sandstone with interbedded purple sandstone and quartzitic arenite having a white alteration patina (Dimroth, 1969, 1972, 1978; Dressler, 1973, 1979). Purple sandstone forms beds 3 to 15 m thick. It is medium grained, laminated and composed of quartzitic sand grains and a minor amount of feldspar grains covered with hematite. Sandstone is highly compacted and not very porous. It changes laterally to white patina quartzitic arenite. Intermediate arenite varieties are light purple or pink in fresh surface. According to Dimroth (1972, 1978), quartzitic arenite is indistinguishable from that of the Alder Formation. Locally, white quartzitic arenite changes to dolomitic sandstone similar to that of the Alder Formation. Mudstone and siltstone are similar to those of the Lace Lake Formation (Dimroth, 1972, 1978).

 

Pistolet Group 4 (pPpi4): Grey Dolomitic Sandstone with a Brown Patina

Unit pPpi4 is located south of Giraffe Lake (sheet 24C07). It consists mainly of grey dolomitic sandstone having a brown alteration patina (Dimroth, 1969, 1972, 1978). These rocks contain lenses (up to 30 cm thick) of dolomite having a light grey alteration patina, with wavy stromatolitic beds, similar to those found in calcarenitic layers of the Alder Formation (Dimroth, 1972, 1978).

 

Pistolet Group 5 (pPpi5): Laminated Grey Mudstone and Siltstone, Commonly Calcareous and Locally Graphitic; Interbedded Brown Patina Dolomite and Locally Fine-Grained Grey Sandstone or Dark Grey Slate

Unit pPpi5 is located between Luché Lake and Giraffe Lake, and west of Patu Lake (Dimroth, 1969, 1972, 1978; Dressler, 1973, 1979; Clark, unpublished). It consists of laminated, grey, commonly calcareous and locally graphitic mudstone and siltstone, with interbedded dolomite having a brown alteration patina as well as locally fine-grained grey sandstone or dark grey slate. In the Luché and Girafe lakes area, its thickness is estimated at 30 m. It gradually changes to unit pPpi3 (Dimroth, 1972, 1978).

 

Pistolet Group 6 (pPpi6): Massive, Locally Brecciated Dark Grey Dolomite and Sandy Dolomite with a Brown or Beige Patina; Interbedded Dolomitic Sandstone Locally

Unit pPpi6 outcrops locally between Luché Lake and Giraffe Lake, and west of Patu Lake (Dressler, 1973, 1979; Dimroth, 1972, 1978; Clark, unpublished). It consists mainly of dark grey dolomite and sandy dolomite having a brown or beige alteration patina. These rocks are massive and locally brecciated. According to Dimroth (1972, 1978), unit pPpi6 dolomite is similar to that of the Uvé Formation. Dolomite occurs in both recrystallized and unrecrystallized varieties. Recrystallized dolomite is light grey in fresh surface, beige in altered patina and fine to medium grained. In places, it contains lenses, strips or rounded clusters of black chert. Unrecrystallized dolomite is dark grey in fresh surface, brown in altered patina and very fine grained (Dressler, 1973, 1979). Grey dolomitic sandstone having a brown alteration patina is interbedded (10-15 cm thick) locally (Dimroth, 1969, 1972, 1978; Dressler, 1973).

 

Pistolet Group 12 (pPpi12): Laminated or Thinly Bedded, Pink or Grey Dolomite, Commonly Brecciated (Syn-Sedimentary Breccia); Pink Stromatolitic Dolomite

Unit pPpi12 is located west of Cambrien Lake, where it forms a ~1 km E-W strip. This unit consists of pink or grey dolomite having a pink or grey alteration patina. The rock is laminated and commonly brecciated (syn-sedimentary breccia). Beds are characterized by detachments, concentric folds and truncations, which are due to sliding. In places, beds display creep and are fractured into subparallel plates. The unit also contains pink stromatolitic dolomite in its upper part. Algal structures are 2 to 3 cm in diameter (Clark, 1984).

 

Pistolet Group 14 (pPpi14): Quartzitic Arenite, with Interbedded Very Fine-Grained Sandstone and Red and Green Siltstone and Mudrock

Unit pPpi14 is located west of Patu Lake (Dressler, 1973, 1979; Clark, unpublished). It consists of pink to light grey, fine to coarse-grained, massive, quartzitic arenite having a white alteration patina, interbedded with very fine-grained sandstone and red and green siltstone and mudrock. Sandstone and siltstone are quartzitic or arkosic and have a hematitized and chloritic matrix. In thin sections, quartzitic arenite contains minor amounts of dolomite and feldspar (Dressler, 1979).

 

Pistolet Group 15 (pPpi15): Arkose and Conglomerate, with Interbedded Very Fine-Grained Sandstone and Red and Green Siltstone and Mudrock

Unit pPpi15 is located west of Patu Lake (Dressler, 1973, 1979). It consists of arkose and conglomerate, with interbedded very fine-grained sandstone and red and green siltstone and mudrock. Arkose is pink, medium to coarse grained and locally microconglomeratic. According to Dressler (1979), the latter is derived from granitic rocks around Patu Lake. Conglomerate is clast-supported or paste-supported. It is composed of angular to subrounded granitic fragments up to 30 cm in diameter in an arkosic to locally dolomitic matrix (Dressler, 1979). Sandstone and siltstone are quartzitic or arkosic and have a hematitized and chlorite matrix (Dressler, 1979).

 

Thickness and Distribution

The Pistolet Group belongs to the Cambrien, Howse, Romanet (formerly Wheeler) and Schefferville lithotectonic zones, as defined by Clark and Wares (2004). It extends >220 km in a NW-SE direction from the NW end of Colombet Lake (sheets 24F02 and 24F03) to the SE end of Wakuach Lake (sheet 23O06). According to Wardle and Bailey (1981), the fact that the Pistolet Group is restricted to the area between latitudes 55° 30′ N and 57° N suggests that the first-cycle platform was possibly of local extent along the passive continental margin.

 

Dating

None.

Stratigraphic Relationship(s)

Deposition of the Pistolet Group rocks occurred between 2169 ±2 and 2142 +4/-2 Ma, i.e. during the period of deposition of first-cycle sedimentary rocks (T. Krogh and B. Dressler, unpublished data cited by Clark, 1984, page 4; Rohon et al., 1993; Clark and Wares, 2004). Recently, U-Pb ages of 2166 ±4 Ma and 2171 ±2 Ma were obtained by Corrigan et al. (2020) for the beginning of the first cycle.

The Pistolet Group conformably overlies the Seward Group. The basal part of the group (Lace Lake Formation) is generally in gradual contact with the upper part of the Seward Group (Portage, Dunphy and Milamar formations) (Dimroth, 1970, 1972, 1978; Clark, 1986). In the Luché and Giraffe lakes area, undivided rocks of the Pistolet Group overlie rocks of the Chakonipau Formation (Seward Group) (SIGÉOM interactive map).

The summit of the Pistolet Group (Uvé Formation) is conformably overlain by rocks of the Swampy Bay Group (Dimroth, 1969, 1972, 1978; Dressler, 1979; Clark, 1984). Locally, in the central Labrador Trough, the upper part of the group (Alder and Uvé formations) and most of the Swampy Bay Group appear to be absent. The Bacchus Formation (Swampy Bay Group) directly overlies sandstone and conglomerate beds of the Seward Group or mudstone and siltstone beds of the Lace Lake Formation (Dimroth, 1972, 1978; Clark and Wares, 2004). According to Dimroth (1970, 1978), these contrasting relationships are due to erosional unconformity caused by the emergence of a zone of positive relief that he termed the « central geanticline ». Another interpretation (Clark, unpublished, 1986) is that the Seward Group and Bacchus Formation were deposited east of passive margin sedimentary rocks (formed by the Pistolet and Swampy Bay groups) and that these units were transported westward over the passive margin during regional tectonism.

In the area west and SW of Nona Lake (sheet 24C01), Dimroth (1969, 1978) indicates that the Pistolet Group overlies iron-bearing rocks of the Sokoman Formation (Ferriman Group) along a thrust fault (Maraude Fault), which was folded. South of the Romanet River (sheet 24B05), rocks of the Lace Lake and Dunphy formations (Seward Group) were overthrusted by the Romanet Formation (Swampy Bay Group) (Dimroth, 1978; Clark, 1986). In the Du Chambon and Romanet lakes area, albitic breccia clusters of the Mistamisk Complex are conformable and occur at the contact between dolomite of the Uvé Formation and black slate of the Du Chambon Formation (Swampy Bay Group) (Kearvell, 1985; Clark, 1986; Kearvell and Clark, 1988; Clark and Wares, 2004).

Paleontology

No fossils reported.

References

Publications Available Through SIGÉOM Examine

BROUILLETTE, P. 1989. GÉOLOGIE ET MÉTALLOGÉNIE DE LA RÉGION DES LACS MINOWEAN ET DU PORTAGE (FOSSE DU LABRADOR). MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; ET 88-06, 84 pages and 2 plans.

CHEVÉ, S.R. 1985. LES INDICES MINÉRALISÉS DU LAC ROMANET, FOSSE DU LABRADOR. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; ET 83-13, 62 pages and 2 plans.

CLARK, T. 1984. GÉOLOGIE DE LA RÉGION DU LAC CAMBRIEN – TERRITOIRE DU NOUVEAU-QUÉBEC. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; ET 83-02, 77 pages and 1 plan.

CLARK, T. 1986. GÉOLOGIE ET MINÉRALISATIONS DE LA RÉGION DU LAC MISTAMISK ET DE LA RIVIÈRE ROMANET. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; ET 83-22, 56 pages and 1 plan.

CLARK, T., WARES, R. 2004. SYNTHÈSE LITHOTECTONIQUE ET MÉTALLOGÉNIQUE DE L’OROGÈNE DU NOUVEAU-QUÉBEC (FOSSE DU LABRADOR). MINISTÈRE DES RESSOURCES NATURELLES, DE LA FAUNE ET DES PARCS, QUÉBEC; MM 2004-01, 182 pages and 1 plan.

CORRIGAN, D., SAPPIN, A.-A., HOULÉ, M. G., RAYNER, N., VAN ROOYEN, D. 2020. Corrugated Hills: les restes d’une grande province ignée d’environ 2,17 Ga dans la Fosse du Labrador. In Résumés des conférences et des photoprésentations, Québec Mines+Énergie 2019. MERN; DV 2019-01, 78 pages.

DIMROTH, E. 1969. GÉOLOGIE DE LA RÉGION DU LAC CASTIGNON, TERRITOIRE DU NOUVEAU-QUÉBEC. MINISTÈRE DES RICHESSES NATURELLES, QUÉBEC; RP 571, 62 pages and 7 plans.

DIMROTH, E. 1969. PRELIMINARY REPORT, GEOLOGY OF THE CASTIGNON LAKE AREA, NEW QUEBEC TERRITORY. MINISTÈRE DES RICHESSES NATURELLES, QUÉBEC; RP 571(A), 58 pages and 7 plans.

DIMROTH, E. 1972. STRATIGRAPHY OF PART OF THE CENTRAL LABRADOR TROUGH. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; DP 154, 304 pages and 6 plans.

DIMROTH, E. 1978. RÉGION DE LA FOSSE DU LABRADOR ENTRE LES LATITUDES 54°30′ ET 56°30′. MINISTÈRE DE RICHESSES NATURELLES, QUÉBEC; RG 193, 417 pages and 16 plans.

DRESSLER, B. 1973. GÉOLOGIE DE LA RÉGION DU LAC PATU, TERRITOIRE DU NOUVEAU-QUÉBEC. MINISTÈRE DES RICHESSES NATURELLES, QUÉBEC; RP-603, 26 pages and 1 plan.

DRESSLER, B. 1979. RÉGION DE LA FOSSE DU LABRADOR (56°30′ – 57°15′). MINISTÈRE DES RICHESSES NATURELLES, QUÉBEC; RG-195, 136 pages and 14 plans.

GIRARD, A. 1988. GÉOLOGIE ET MÉTALLOGÉNIE DES INDICES CUPRIFÈRES ET URANIFÈRES DE LA RÉGION DU LAC COLOMBET (WAPANIKSKAN) – FOSSE DU LABRADOR. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; MB 88-20, 85 pages and 2 plans.

KEARVELL, G. 1985. BRÈCHES ET ALTÉRATIONS ALBITIQUES DE LA RIVIÈRE ROMANET. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; DP-85-24, 2 plans.

KEARVELL, G., CLARK, T. 1988. ÉTUDE D’INDICES AU-U ET DE BRÈCHES POLYGÉNIQUES DANS LA VALLÉE DES LACS MISTAMISK ET ROMANET – FOSSE DU LABRADOR. MINISTÈRE DE L’ÉNERGIE ET DES RESSOURCES, QUÉBEC; DP-87-24, 1 plan.

 

Other Publications

DIMROTH, E. 1968. The evolution of the central segment of the Labrador geosyncline; part I: stratigraphy, facies and paleogeography; Neues Jahrbuch für Geologie und Paläontologie; volume 132, pages 22-54.

DIMROTH, E. 1970. Evolution of the Labrador Geosyncline. Geological Society of America Bulletin; volume 81, pages 2717-2742. https://doi.org/10.1130/0016-7606(1970)81[2717:EOTLG]2.0.CO;2

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. http://doi.org/10.4095/104534

ROHON, M.-L., VIALETTE, Y., CLARK, T., ROGER, G., OHNENSTETTER, D., VIDAL, P. 1993. Aphebian mafic-ultramafic magmatism in the Labrador Trough (New Quebec): its age and the nature of its mantle source. Canadian Journal of Earth Sciences; volume 30, pages 1582-1593. https://doi.org/10.1139/e93-136

WARDLE, R.J., BAILEY, D.G. 1981. Early Proterozoic sequences in Labrador. In Proterozoic basins of Canada (F.H.A. Campbell, editor). Geological Survey of Canada; Paper 81-10, pages 331-359. http://doi.org/10.4095/124192

 

 

Suggested Citation

Ministère de l’Énergie et des Ressources naturelles (MERN). Pistolet Group. Quebec Stratigraphic Lexicon. https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-churchill/groupe-de-pistolet_en/ [accessed on Day Month Year].

Contributors

First publication

Charles St-Hilaire, GIT, M.Sc. charles.st-hilaire@mern.gouv.qc.ca; Thomas Clark, P. Geo., Ph.D. (redaction)

Mehdi A. Guemache, P. Geo., Ph.D. (coordination); anonymous (critical review); Simon Auclair, P. Geo., M.Sc. (editing); Céline Dupuis, P. Geo., Ph.D. (English version); Ricardo Escobar Moran (HTML editing).

 
11 octobre 2022