First published: 31 March 2021
Last modified:
DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail.
| Author(s): | Harrison, 1952 |
| Age: | Paleoproterozoic |
| Stratotype: | Tyoical outcrops are located north and east of Murdoch Lake |
| Type area: | Murdoch Lake area (NTS sheet 23O10) |
| Geological province: | Churchill Province |
| Geological subdivision: | New Quebec Orogen (Labrador Trough) / Retty and Gerido lithotectonic zones |
| Lithology: | Basalt, mafic pyroclastic rocks, schist, sedimentary rocks |
| Category: | Lithostratigraphic |
| Rank: | Formation |
| Status: | Formal |
| Use: | Active |
Background
Labrador Mining and Exploration geologists (1949, in Frarey and Duffell, 1964) used the term « Murdoch Series » to refer to a sequence of lavas, pyroclastic rocks and chloritized sedimentary rocks in the eastern part of the southern Labrador Trough. Harrison (1952) renamed this sequence Murdoch Group. Subsequently, Baragar (1958, 1960) relegated the unit to the rank of informal formation. Following their revision of the stratigraphic nomenclature of the central part of the Trough, Frarey and Duffell (1964) formalized the name Murdoch Formation and included the unit in the Doublet Group.
Rocks of the Murdoch Formation have been described by Frarey (1961, 1967), Hashimoto (1964), Baragar (1967), Dimroth (1972, 1978), Clark (1978), Penrose (1978), Dressler (1979) and Kheang (1984), among others. Wardle (1979, 1982) mapped the Murdoch Formation on the Labrador side. Note that the name of the unit is spelled « Murdock » in some publications (Frarey, 1961; Dimroth, 1978; Wardle, 1979). This name refers to Murdoch Lake located in NTS sheet 23O10.
Description
The Murdoch Formation is an allochthonous volcano-sedimentary unit forming the base of the Doublet Group. It consists mainly of mafic pyroclastic rocks (tuff, lapilli tuff, block tuff, volcanic breccia), chloritic schist, massive and pillow basalt, and sedimentary rocks. The latter consist of mudstone (locally graphitic and pyritic), siltstone, wacke, phyllite, dolomite, conglomerate and quartzitic sandstone (Baragar, 1967; Frarey, 1967; Dimroth, 1978). The various volcano-stratigraphic units of the Murdoch Formation are closely interstratified and display little lateral continuity, which does not exceed a few hundred metres (Frarey, 1967; Dimroth, 1978; Dressler, 1979). Rocks of the Murdoch Formation are metamorphosed to the greenschist facies (Dimroth, 1978; Dimroth and Dressler, 1978). They locally reach the lower amphibolite facies near the eastern margin of the Labrador Trough. Rocks there consist of amphibolitized mafic pyroclastic rocks, mafic metatuff, mafic lapilli and block metatuff, amphibolitic gneiss and locally interstratified metasedimentary rocks (Penrose, 1978; Dressler, 1979). Rocks of the Murdoch Formation are generally schistose and primary structures have been obliterated by deformation. In places, the origin of some rocks cannot be determined due to the intensity of deformation (Dimroth, 1978).
The Murdoch Formation is formed of deposits derived from a series of mafic volcanic cones or volcanic fissures of explosive character (Baragar, 1967; Frarey, 1967; Dimroth, 1978). The centres of volcanic activity were possibly located in the Murdoch Lake area (Baragar, 1967; Wardle and Bailey, 1981) and further NW in the Aulneau Lake area (Dressler, 1979). The Murdoch Formation would have been deposited in a relatively shallow marine basin with reducing conditions locally (Clark and Wares, 2004).
Murdoch Formation 1 (pPmr1): Massive or Pillow Basalt Flows
Unit pPmr1 groups massive or pillow basalt flows (Frarey, 1967; Dressler, 1979). These flows are interstratified with mafic pyroclastic rocks of unit pPmr2 and are intruded by gabbro sills (Gerido Intrusive Suite). Massive basalt is green, fine to medium grained, uniform and monotonous in appearance, and displays little primary structure (Frarey, 1967; Dressler, 1979). According to Dressler (1979), ~20% of the flows are pillowed. Pillows vary in size from a few centimetres to 3 m long. Pillow margins are aphanitic or hyaloclastic in origin. Pillow breccias are present locally. The interstices between pillows can be up to 15 cm wide and are in places filled with volcanic material and, less frequently, with chert, mudstone or carbonate. In thin sections, the rock is composed of intergrown actinolite crystals in a matrix of chlorite, epidote, leucoxene, albite and minor amounts of stilpnomelane and quartz. Primary minerals, such as olivine or augite, are not observed. Aphanitic pillow margins commonly contain plagioclase and pyroxene.
Murdoch Formation 2 (pPmr2): Mafic Pyroclastic Rocks (Tuff, Volcanic Breccia), Locally Enriched in Magnetite; Chloritic Schist; Some Basalt, Felsic Tuff and Sedimentary Rocks
Unit pPmr2 consists mainly of mafic pyroclastic rocks (tuff, lapilli tuff, block tuff, volcanic breccia) and chlorite schist (Baragar, 1967; Frarey, 1967; Clark, 1978; Dimroth, 1978; Dressler, 1979). Mafic pyroclastic rocks are dark grey in fresh surface and light grey to greenish grey or greenish in altered surface. They are composed of light to dark grey, angular to rounded fragments hosted in a darker, fine-grained, schistose chloritic matrix. The fragments and matrix are generally basaltic in composition. Fragments are commonly amygdaloidal and generally range in size from <1 to 50 cm. Near Murdoch Lake, Baragar (1967) observed fragments up to 60 cm. Other large fragments have been observed in the Aulneau Lake area, west of Le Moyne Lake. Dressler (1979) noted that the fragment size decreased as they moved away from this locality towards the SE. Fragments are commonly deformed and flattened due to deformation (Baragar, 1967; Frarey, 1967; Dimroth, 1978; Dressler, 1979). Stratification is rarely observed in coarser pyroclastic rocks (Baragar, 1967). Finer-grained rocks (tuff) are finely stratified, having millimetric to centimetric beds (Baragar, 1967; Frarey, 1967; Clark, 1978; Dimroth, 1978; Dressler, 1979). Mafic pyroclastic rocks are locally enriched in magnetite (Frarey, 1967; Dressler, 1979). In thin sections, the fragments and matrix are composed of actinolite, albite, chlorite, epidote, leucoxene, sphene, biotite and magnetite. The matrix is usually schistose (Frarey, 1967; Dimroth, 1978; Dressler, 1979).
Chloritic schist is greenish, very fine grained and exhibits well-developed schistosity. The rock is generally folded and finely bedded at millimetre to centimetre scale (Frarey, 1967; Dimroth, 1978). In thin sections, chloritic schist is composed of actinolite, albite, epidote, chlorite and quartz. Accessory minerals are calcite, sphene, ilmenite and magnetite (Frarey, 1967; Dimroth, 1978).
Smaller amounts of basalt, laminated felsic tuff and sedimentary rocks (graphitic slate, pyritic slate, quartzite arenite, wacke, massive pyrite beds) are interstratified with pyroclastic rocks (Baragar, 1967; Dimroth, 1978).
Murdoch Formation 4 (pPmr4): Conglomerate, Quartzitic Sandstone, Mudstone
Unit pPmr4 consists of conglomerate, quartzitic sandstone and mudstone. In the Walsh Lake area (sheet 23O01) and south of Keating Lake (sheet 23O08), Frarey (1967) and Dimroth (1978) observed polymictic conglomerate containing pebbles of quartzitic sandstone, quartz, mudstone, grey chert, jasper and a few fragments of dioritic and felsic composition in a quartzitic matrix. North of Walsh Lake, Frarey (1967) noted outcrops of mudstone and highly altered or sheared quartzitic sandstone. According to Frarey (1967) and Dimroth (1978), this unit may form the base of the Murdoch Formation at Walsh Lake. West of Le Moyne Lake (sheet 24F01), Dressler (1979) mapped a small unit that he refers to as conglomerate.
Murdoch Formation 5 (pPmr5): Magnetite Dolomite, Magnetite Metadolomite
Unit pPmr5 consists of dolomite and metadolomite (Penrose, 1978; Dressler, 1979). These rocks are commonly grey with a light grey alteration patina. The strong magnetic signature suggests that these rocks contain magnetite.
Murdoch Formation 6 (pPmr6): Amphibolitized Mafic Pyroclastic Rocks, Mafic Metatuff, Amphibolitic Gneiss; Locally Interbedded with Metasedimentary Rocks
Unit pPmr6 consists of amphibolitized mafic pyroclastic rocks, mafic metatuff, mafic lapilli and block metatuff and amphibolitic gneiss (Penrose, 1978; Dressler, 1979). These rocks were originally included in the Laporte Group (now Laporte Supersuite) by Dressler (1979). According to Penrose (1978), the unit locally contains interstratified metasedimentary (mudstone, marble, wacke) and metavolcanic (basalt) rocks. Unit pPmr6 probably represents the metamorphosed equivalent of unit pPmr2.
Amphibolitized mafic pyroclastic rocks and mafic metatuff are dark grey-green to black in fresh surface, grey-green to brown in altered patina, fine to medium grained, and have strong schistosity. These rocks are composed of hornblende or actinolite, plagioclase, epidote and lesser amounts of biotite, quartz and carbonates (Penrose, 1978; Dressler, 1979). The mafic lapilli and block metatuff contains subangular to subrounded fragments of basaltic composition that may account for up to 90% of the rock. These fragments, ranging in size from 1 to 15 cm, have little or no deformation and are embedded in a fine, dark green, tuffaceous matrix of basaltic composition (Dressler, 1979).
Amphibolitic gneiss is dark grey-green to black in fresh surface, black and rusty in altered patina, medium to coarse grained and well foliated. According to Penrose (1978), amphibolitic gneiss is distinguished from metatuff by its coarse grain size (up to 2 cm) and by its metamorphic character. It is mainly composed of green hornblende (30-90%), plagioclase and opaque minerals. Accessory minerals are biotite, garnet, quartz, chlorite, calcite, allanite and apatite (Penrose, 1978; Dressler, 1979).
Murdoch Formation 7 (pPmr7): Mudstone, Siltstone, Wacke, Phyllite, Dolomite, Conglomerate, Biotite Paraschist
Unit pPmr7 consists of mudstone, siltstone, wacke, phyllite, dolomite and conglomerate (Hasmimoto, 1964; Clark, 1978; Dimroth, 1978; Penrose, 1978; Dressler, 1979). These rocks occur as thin beds interstratified with mafic pyroclastic rocks (pPmr2). Mudstone, siltstone, wacke and phyllite are generally dark grey to black, very fine grained and laminated. Wacke is composed of quartz and a small amount of dark rock fragments. Phyllite is usually graphitic. It locally exhibits rusty alteration due to the presence of pyrite (Hashimoto, 1964). Small amounts of dolomite and conglomerate are reported by Dressler (1979) NE of Erich-Dimroth Lake (sheet 24F02).
In the area SE of Nachicapau Lake (sheets 24B05 and 24B12), the unit consists of metamorphosed equivalents of sedimentary rocks already mentioned, namely biotite-muscovite schists, biotite-garnet ± graphite schists, biotite quartzites and biotite-amphibole ± graphite schists (Dimroth, 1978; Penrose, 1978). These rocks are similar to those of the Bacchus Formation (pPbc11) (Swampy Bay Group).
Murdoch Formation 8 (pPmr8): Pillow Basalt Flows and Flow Breccia
Unit pPmr8 occurs mainly west of Hérodier Lake (sheets 24F06 and 24F07) (Clark, 1978). It consists of pillow basalt flows and flow breccias. Pillow lavas are interstratified with mafic pyroclastic rocks of unit pPmr2. They are elongated to very slightly flattened. Their size varies from ~1 m long to 3 m in places. Pillows observed in pyroclastic sequences are highly vesicular. In places, top flow breccias, 1 to 2 m thick, are observed on outcrops.
Murdoch Formation 10 (pPmr10): Biotite-Carbonate Schist; May Include Some Marble and Biotite ± Garnet Schist or Gneiss
Unit pPmr10 is recognized in the Le Moyne Lake area (sheets 24C16, 24F01 and 24F02) where it consists essentially of biotite-carbonate schists. These rocks, initially included in the Laporte Group by Dressler (1979), are now largely assigned to the Freneuse Suite (formerly Laporte Group). Biotite-carbonate schist may locally include minor amounts of marble and biotite ± garnet schist or gneiss (Dressler, 1979). According to Dressler (1979), biotite-carbonate schist is grey, fine grained and has a brownish grey alteration patina. The rock is very easily altered and outcrops are of poor quality. Fine bedding and banding are commonly observed. In thin sections, the rock is composed mainly of biotite, carbonate, plagioclase, quartz and opaque minerals. The carbonate content reaches ~40% of the rock in places.
Murdoch Formation 11 (pPmr11): Quartzitic Sandstone; Some Mudstone and Arkose
Unit pPmr11 forms a small occurrence NW of Jogues Lake (sheet 24F02) (Hashimoto, 1964). It consists of a strip of white, medium-grained quartzitic sandstone interstratified with mafic pyroclastic rocks of unit pPmr2. The unit also includes minor amounts of mudstone and arkose.
Thickness and Distribution
The Murdoch Formation belongs to the Gerido and Retty lithotectonic zones as defined by Clark and Wares (2004). It extends ~340 km in a NW-SE direction between the Martin Lake area (sheet 23J16) in the southern Labrador Trough and the Herodian Lake area (sheet 24F06). According to Baragar (1967) and Dimroth (1978), the thickness of the formation is difficult to assess due to extensive deformation. Near Murdoch Lake, Baragar (1967) estimates a thickness of 2000 m. Frarey (1967) estimates a thickness of >1200 m north of latitude 56°N, <300 m NE of Low Lake (sheet 23O14), due to tectonically associated reduction, and 600 to 1500 m at Doublet Lake (sheet 23O01). Dressler (1979) estimates the thickness of the formation at ~2000 m.
Dating
None.
Stratigraphic Relationship(s)
Rocks of the Murdoch Formation were deposited between 1.88 and 1.87 Ga, during the second volcano-sedimentary cycle of the Labrador Trough (Clark and Wares, 2004). The Murdoch Formation forms the base of the Doublet Group east of the Lac Walsh Fault in the southern part of the Labrador Trough. It is conformably overlain by the Thompson Lake Formation (Baragar, 1967; Frarey, 1967). The lower contact of the Murdoch Formation is not exposed south of latitude 56°N. The formation is generally in fault contact (Lac Walsh Fault) with the Menihek Formation (Ferriman Group) (Frarey, 1967; Dimroth, 1978). The Murdoch and Menihek formations are presumably conformable in places (Baragar, 1967; Dimroth, 1978). However, the presence of the Lac Walsh Fault between the two formations makes interpretation of stratigraphic relationships difficult (see Baragar, 1967). The Murdoch Formation is correlated with the lower part of the Menihek Formation dated between 1877.8 ±1.3 and 1884 ±2 Ma (Findlay et al., 1995).
Further north, near latitude 57°N, pyroclastic rocks of the Murdoch Formation are interdigitated with sedimentary rocks of the Baby Formation (Koksoak Group). The whole is locally overlain by the Le Moyne Group as well as by a carbonatite intrusion and associated sedimentary rocks (Birkett and Clark, 1991; Machado et al., 1997; Clark and Wares, 2004). The Murdoch Formation is generally in fault contact with the Freneuse Suite (Laporte Supersuite) located just to the east. In the Le Moyne Lake area, some of the rocks of the Murdoch Formation may be equivalent to metasedimentary rocks of the Freneuse Suite.
The Murdoch Formation is in fault contact with Archean rocks of the Wheeler Complex (Dimroth, 1978). Many mafic sills of the Gerido Intrusive Suite intrude into rocks of the Murdoch Formation.
Paleontology
Does not apply.
References
Publications Available Through SIGÉOM Examine
CLARK, T., WARES, R., 2004. SYNTHESE LITHOTECTONIQUE ET METALLOGENIQUE DE L’OROGENE DU NOUVEAU-QUEBEC (FOSSE DU LABRADOR). MRNFP; MM 2004-01, 182 pages, 1 plan.
DIMROTH, E., 1972. STRATIGRAPHY OF PART OF THE CENTRAL LABRADOR TROUGH. MRN; DP 154, 304 pages, 6 plans.
DIMROTH, E., 1978. Région de la fosse du Labrador entre les latitudes 54° 30′ et 56° 30′. MRN; RG 193, 417 pages, 16 plans.
DRESSLER, B., CIESIELSKI, A., 1979. Région de la fosse du Labrador. MRN; RG 195, 136 pages, 14 plans.
HASHIMOTO, T., 1964. GEOLOGIE DE LA REGION DU LAC JOGUES, TERRITOIRE DU NOUVEAU-QUEBEC. MRN; RP 524, 13 pages, 1 plan.
KHEANG, L., 1984. ALTERATION DES RHYOLITES ET DES BASALTES DANS LA REGION DES LACS LA LANDE ET DOUAY. MRN; DP-84-33, 20 pages.
PENROSE, B., 1978. GEOLOGIE DE LA REGION DU LAC HORSESHOE (NOUVEAU-QUEBEC). MRN; DPV 573, 39 pages, 1 plan.
Other Publications
BARAGAR, W.R.A. 1958. Ahr Lake map-area, New Quebec (23O/10). Geological Survey of Canada; Paper 57-7, 9 pages. https://doi.org/10.4095/104037
BARAGAR, W.R.A. 1960. Petrology of basaltic rocks in part of the Labrador Trough. Bulletin of the Geological Society of America; volume 71, pages 1589-1643.
BARAGAR, W.R.A. 1967. Wakuach Lake map-area, Quebec-Labrador (23O). Geological Survey of Canada; Memoir 344, 174 pages. https://doi.org/10.4095/123960
BIRKETT, T.C., CLARK, T. 1991. Géologie et potentiel métallifère de la carbonatite protérozoïque du lac LeMoyne dans le nord du Québec. Geological Survey of Canada; Work in Progress Forum, Programme and Abstracts, page 20.
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. https://doi.org/10.4095/104534
FINDLAY, J.M., PARRISH, R.R., BIRKETT, T., WATANABE D.H. 1995. U-Pb ages from the Nimish Formation and Montagnais glomeroporphyritic gabbro of the central New Québec Orogen, Canada. Canadian Journal of Earth Sciences; volume 32, pages 1208-1220. https://doi.org/10.1139/e95-099
FRAREY, M.J. 1952. Preliminary map Willbob Lake, Quebec and Newfoundland. Geological Survey of Canada; Paper 52-16, 10 pages. https://doi.org/10.4095/124122
FRAREY, M.J. 1967. Willbob Lake and Thompson Lake map-areas, Quebec and Newfoundland (23 O/1 and 23 O/8). Geological Survey of Canada; Memoir 348, 73 pages. https://doi.org/10.4095/123896
FRAREY, M.J., DUFFELL, S. 1964. Revised stratigraphic nomenclature for the central part of the Labrador Trough. Geological Survey of Canada; Paper 64-25, 13 pages. https://doi.org/10.4095/123909
HARRISON, J.M. 1952. The Quebec-Labrador iron belt, Quebec and Newfoundland. Geological Survey of Canada; Paper 52-20, 21 pages. https://doi.org/10.4095/123923
MACHADO, N., CLARK, T., DAVID, J., GOULET, N. 1997. U-Pb ages for magmatism and deformation in the New Quebec Orogen. Canadian Journal of Earth Sciences; Volume 34, pages 716-723. https://doi.org/10.1139/e17-058
WARDLE, R.J. 1979. Geology of the eastern margin of the Labrador Trough. Department of Mines and Energy, Government of Newfoundland and Labrador; Report 78-9, 22 pages. https://gis.geosurv.gov.nl.ca/geofilePDFS/WBox040/LAB_0415.pdf
WARDLE, R.J. 1982. Geology of the south-central Labrador Trough. Government of Newfoundland and Labrador, Department of Mines and Energy, Mineral Development Division, Map 82-005.
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. https://doi.org/10.4095/124192
Suggested Citation
Ministère de l’Énergie et des Ressources naturelles (MERN). Murdoch Formation. Quebec Stratigraphic Lexicon. https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-churchill/formation-de-murdoch_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); Claude Dion, Eng., M.Sc. (critical review); Simon Auclair, P. Geo., M.Sc. (editing); Céline Dupuis, P. Geo., Ph.D. (English version); André Tremblay (HTML editing). |