DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail.
|Type area:||Swampy Bay River area (NTS sheet 24C01)|
|Geological province:||Churchill Province|
|Geological subdivision:||New Quebec Orogen (Labrador Trough)|
|Lithology:||Mudrock, sandstone, greywacke|
The name « Swampy Bay Subgroup » was introduced by Dimroth (1968) to designate a pelitic-dominated unit, devoid of dolomite interstratification, overlying the « Pistolet Subgroup » and underlying the « Attikamagen Subgroup » between latitudes 56° and 56°30’N. The name is derived from the Swampy Bay River, whose bed is partly cut through rocks constituting the unit (Dimroth, 1969). In the western part of the Labrador Trough, Dimroth (1968) divided the « Swampy Bay Subgroup » into three formations named, from base to top, the Hautes-Chutes Formation, the Savigny Formation and the Otelnuc Formation. In the eastern part of the Trough, Dimroth (1970a, 1972, 1978) introduced and added the Du Chambon and Romanet formations to the « Swampy Bay Subgroup ».
The Bacchus and Le Fer formations were defined by Dimroth (1972, 1978) in the southern part of the Trough as forming the lower part of the « Attikamagen Subgroup ». However, according to Dimroth (1978), the Le Fer Formation is transitional and interdigitated with the adjacent Savigny and Bacchus formations. Consequently, Clark and Wares (2004), while reclassifying all of Dimroth’s subgroups as groups, including the Swampy Bay Group, proposed that the Le Fer and Bacchus formations be included in the Swampy Bay Group because of their natural affinities. Thus redefined, the Swampy Bay Group includes the Hautes-Chutes, Savigny, Romanet, Otelnuc, Le Fer, Du Chambon and Bacchus formations.
Clark and Wares (2004) proposed that the following formation names be abandoned: « Nachicapau Formation » (Dressler, 1979) and « Mistamisk Formation » (Dimroth, 1972, 1978) since these units are equivalent to parts of the Swampy Bay Group.
The Swampy Bay Group is an allochthonous volcano-sedimentary unit belonging to the first cycle of the Labrador Trough (Clark and Wares, 2004). It consists generally of flysch-type sedimentary rocks and basalt deposited in a marine basin. The Swampy Bay Group sequence generally begins with slate and black, commonly pyritic, chert of the Hautes-Chutes Formation. These rocks are rich in organic matter and poor in terrigenous debris. They are thought to have been deposited in a quiet, shallow environment (Dimroth, 1978). Rocks of the Hautes-Chutes Formation are overlain by thick flyschoid sequences of mudrock, siltstone, impure sandstone, greywacke and locally conglomerate, represented by the Savigny, Otelnuc, Du Chambon and Romanet formations. These rocks are grey, black or green and locally graphitic. Sorted beds are commonly observed (Dimroth, 1978; Clark and Wares, 2004). Rocks that make up the Savigny, Otelnuc, Du Chambon and Romanet formations have been interpreted as turbidites (Dimroth, 1978; Clark, 1984, 1986). The source of turbidites would be sedimentary and not metamorphic or volcanic.
According to the interpretation of Dimroth (1968, 1978), the uplift of source areas in the central part of the Trough caused partial erosion of earlier sedimentary rocks (mainly the Pistolet Group). This uplifted area was termed the « central geanticline » by Dimroth (1968). Debris resulting from this erosion were then transported and deposited by turbidity currents in marine basins to the east and west of the area of positive relief (Dimroth, 1968, 1970b, 1978; Dimroth et al., 1970; Clark and Wares, 2004). The upper part of the Swampy Bay Group consists of red, green and grey shales of the Le Fer Formation and shales, siltstones and tholeiitic basalts of the Bacchus Formation. Sedimentary rocks of the Le Fer Formation are transitional and interdigitated with turbidites and the Bacchus Formation. They are interpreted as prodeltaic deposits of still water from a spring in the west, transitioning eastward to deposits in a deeper basin (Dimroth, 1978). Sedimentary rocks and basalts of the Bacchus Formation were deposited in a marine basin, which Clark and Wares (2004) suggest was located east of the continental margin platform, probably close to the initial rifting axis.
In the Wakuach Lake area, Dimroth (1978) recognized a succession of rocks equivalent to the Swampy Bay Group. This sequence consists of graphitic slate and mudrock, with some interbedded greywacke (« quartzitic wacke » in the terminology of Dimroth, 1978; pPsw1). North and east of Wakuach Lake, unit pPsw1 is in contact with a unit of sandstone, impure granular sandstone and greywacke (« quartzitic wacke »; pPsw2). These rocks are loosely structured. They are composed of coarse and rounded quartz, a minor amount of feldspar and some fragments of chert, siltstone and mudrock, in a medium to coarse-grained matrix. The argillaceous fraction usually does not exceed 10%. The rock is largely cemented by chert (Dimroth, 1978).
According to Dimroth (1968, 1978), the Swampy Group Bay is largely the result of erosion of an uplifted area, the « central geanticline ». In contrast, according to Wardle and Bailey (1981) and Clark and Wares (2004), it results from rifting and collapse of a passive continental margin platform. Metamorphism is weak and increases from west to east, from sub-greenschist to greenschist facies (Dimroth and Dressler, 1978).
Swampy Bay Group 1 (pPsw1): Mudrock and Graphitic Mudrock; Some Greywacke
In the Wakuach Lake area, unit pPsw1 is overlying dolomite of the Uvé Formation. At the base it consists of black graphitic slate, which is equivalent to the Hautes-Chutes Formation. A well-bedded grey mudrock with some interbedded greywacke (« quartz wacke »), equivalent to the Savigny Formation, follows towards the top (Dimroth, 1978).
Swampy Bay Group 2 (pPsw2): Impure Sandstone and Greywacke
Unit pPsw2 was mapped by Baragar (1967) and Dimroth (1978) to the north and east of Wakuach Lake (sheet 23O12). It forms two NW-SE oriented strips, 19 and 11 km long respectively. The unit consists of thick, massive beds of grey, impure, loosely structured sandstone and granular sandstone, as well as greywacke (« quartzitic wacke ») and mudrock. Sorting is observed in this sequence (Baragar, 1967; Dimroth, 1978). According to Dimroth (1978), these rocks represent an equivalent of the Otelnuc Formation, the facies probably represented being closer to the source area than the Otelnuc Formation.
Thickness and Distribution
The Swampy Bay Group belongs to the Cambrien, Howse, Schefferville and Wheeler lithotectonic zones, as defined by Clark and Wares (2004). It extends ~320 km long in a NW-SE direction from the Canispiscau River area west of Hérodier Lake (sheet 24F06) to the NW end of Attikamagen Lake (sheet 23J15).
The Swampy Bay Group was deposited during the first volcano-sedimentary cycle of the Trough, between 2169 ±4 Ma and 2142 +4/-2 Ma. The latter age is derived from a rhyolite dyke cutting the top of the Bacchus Formation at Colombet Lake (T. Krogh and B. Dressler, unpublished data cited in Clark, 1984, page 4; Rohon et al., 1993; Clark and Wares, 2004).
Rocks of the Swampy Bay Group generally conformably overlie dolomite of the Uvé Formation (Pistolet Group; Dimroth, 1969, 1978; Dressler, 1979; Clark, 1984). In places in the central Labrador Trough, the Bacchus Formation lies directly on sandstones and conglomerates of the Seward Group (Chakonipau and Portage formations) or mudstones and siltstones of the Pistolet Group (Lace Lake Formation; Dimroth, 1978; Clark and Wares, 2004). According to Dimroth (1978), these contrasting relationships are due to erosional unconformity caused by the emergence of a zone of positive relief called the « central geanticline ». An alternative interpretation (Clark, unpublished) is that the Seward Group and Bacchus Formation were deposited east of sedimentary rocks of the passive margin (formed by the Pistolet and Swampy Bay groups) and these units were transported westward over the passive margin during regional tectonism.
The Swampy Bay Group is concordantly overlain by the Attikamagen Group in the central part of the orogen (Dimroth, 1971, 1978; Clark and Wares, 2004). In the northern part of the Trough, east of the Koksoak River (sheet 24F11), the Le Fer Formation of the Swampy bay Group is in sharp contact with the overlying Denault Formation (Attikamagen Group) (Clark, 1978). South of Wakuach Lake, it gradually changes eastward to the Denault Formation (Dimroth, 1978). According to Hoffman (1987), the Attikamagen Group represents a marine regression and the establishment of a carbonate platform, whereas the underlying formations of the Swampy Bay Group were deposited in a marine basin.
On the western side of the Trough, rocks of the Swampy Bay Group are unconformably overlain (erosional unconformity) by the Wishart Formation (Ferriman Group) (Dimroth, 1969, 1978; Dimroth et al., 1970; Dressler, 1979; Chevé, 1993). On the eastern side, in the Du Chambon and Romanet lakes area (sheets 24B05 and 24C08), the Swampy Bay Group (Du Chambon Formation) is in structural contact with the Seward Group (Chakonipau and Dunphy formations) via a thrust fault, the Du Chambon Fault (Dimroth, 1978). South of the Romanet River (sheet 24B05), rocks of the Dunphy Formation (Seward Group) and Lace Lake Formation (Pistolet Group) were overthrusted onto the Romanet Formation (Dimroth, 1978; Clark, 1986).
In the Du Chambon and Romanet lakes area, an albite breccia cluster of the Mistamisk Volcanic Complex is hosted by the Romanet Formation (Kearvell, 1985; Clark, 1986; Kearvell and Clark, 1988). Other albite breccia clusters of the Mistamisk Volcanic Complex are conformable and occur at the contact between dolomites of the Uvé Formation and black slates of the Du Chambon Formation (Kearvell, 1985; Clark, 1986; Kearvell and Clark, 1988; Clark and Wares, 2004). In the area west of Castignon Lake (sheet 24C07), Dimroth (1970c, 1978) described carbonatite diatremes that, in some places, were emplaced in the brecciated Savigny Formation. These diatremes are part of the Castignon Volcanic Complex (Dimroth, 1970c, 1978; Chevé, 1993). Dimroth also observed dykes of carbonatitic to meimechitic composition, generally oriented N-S, which cut the Savigny Formation. According to the observations of Chevé (1993), some of these dykes are in fact conformable with bedding of the host rock. Chevé (1993) also notes that slate fragments from the Savigny Formation are incorporated in a diatreme breccia, which suggested to him that regional deformation preceded the emplacement of the breccia. Finally, numerous gabbroic sills belonging to the Montagnais Supersuite cut the upper part of the Swampy Bay Group (Bacchus Formation) in the central part of the Trough (Dimroth, 1978; Wardle and Bailey, 1981).
No fossils reported.
Publications Available Through SIGÉOM Examine
CHEVE, S. 1993. CADRE GEOLOGIQUE DU COMPLEXE CARBONATIQUE DU LAC CASTIGNON – FOSSE DU LABRADOR. MRN. MB 93-64, 100 pages and 1 plan.
LAMARCHE, R Y., DUQUETTE, G., GOBEIL, A., LATULIPPE, M., MacINTOSH, J A. 1978. RAPPORT D’ACTIVITE DES GEOLOGUES RESIDENTS POUR L’ANNEE 1977. MRN. DPV 578, 137 pages.
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. 1986. GEOLOGIE ET MINERALISATIONS DE LA REGION DU LAC MISTAMISK ET DE LA RIVIERE ROMANET. MRN. ET 83-22, 56 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.
DIMROTH, E. 1969. GEOLOGIE DE LA REGION DU LAC CASTIGNON, TERRITOIRE DU NOUVEAU-QUEBEC. MRN. RP 571, 62 pages and 7 plans.
DIMROTH, E. 1970. CARTES GEOLOGIQUES DU LAC ROMANET ET DU LAC CRAMOLET (FOSSE DU LABRADOR). MRN. DP 068, 3 plans.
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.
KEARVELL, G. 1985. BRECHES ET ALTERATIONS ALBITIQUES DE LA RIVIERE ROMANET-FOSSE DU LABRADOR. MRN. DP-85-24, 2 plans.
KEARVELL, G., CLARK, T. 1987. ETUDE D’INDICES Au – U ET DE BRECHES POLYGENIQUES DANS LA VALLEE DES LACS MISTAMISK ET ROMANET – FOSSE DU LABRADOR -. MRN. DP-87-24, 32 pages and 1 plan.
BARAGAR, W R A., 1967. Wakuach Lake map-area, Quebec-Labrador (23O). Geological Survey of Canada; Memoir 344, 174 pages. http://doi.org/10.4095/123960
DIMROTH, E., 1970. Evolution of the Labrador Geosyncline. Geological Society of America Bulletin; volume 81, pages 2717-2742. http://doi.org/10.1130/0016-7606(1970)81[2717:EOTLG]2.0.CO;2
DIMROTH, E., 1970. Meimechites and carbonatites of the Castignon Lake Complex, New Quebec. Neues Jahrbuch für Mineralogie; volume 112, pages 239-278.
DIMROTH, E., 1971. The Attikamagen-Ferriman transition in part of the central Labrador Trough. Canadian Journal of Earth Sciences; volume 8, pages 1432-1454. http://doi.org/10.1139/e71-132
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. http://doi.org/10.4095/124922
DIMROTH, E, DRESSLER, B O., 1978. Metamorphism of the Labrador Trough. In: Metamorphism in the Canadian Shield. Geological Survey of Canada; Study 78-10, pages 215-236. http://doi.org/10.4095/104534
HOFFMAN, P. F., 1987. Early Proterozoic foredeeps, foredeep magmatism and Superior-type iron-formations of the Canadian shield. In: Proterozoic Lithospheric Evolution (A. Kroner, editor). American Geophysical Union, Geodynamics Series; volume 17, pages 85-98. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GD017p0085
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. http://doi.org/10.1139/e93-136
WARDLE, R J, BAILEY, D G., 1981. Early Proterozoic sequences in Labrador. In: Proterozoic Basins in Canada (F.H.A. Campbell, editor). Geological Survey of Canada; Study 81-10, pages 331-358. http://doi.org/10.4095/124192
Ministère de l’Énergie et des Ressources naturelles (MERN). Swampy Bay Group. Quebec Stratigraphic Lexicon. https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-churchill/groupe-de-swampy-bay_en [accessed on Day Month Year].
Charles St-Hilaire, GIT, M.Sc. email@example.com
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); Nathalie Bouchard (HTML editing).