Opémisca Group
Stratigraphic label: [narc]op
Map symbol: nAop

First published: 27 March 2018
Last modified:
















Translation of original French



Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
Author: Norman, 1937
Age: Neoarchean
Reference section: None
Type area: The best outcropping areas are located in the western part of sheet 32G15-200-0102 (Stella Formation) and in the central part of sheet 32G14-200-0102 (Haüy Formation)
Geological province: Superior Province
Geological subdivision: Abitibi Subprovince
Lithology: Sedimentary rocks and potassic basalt
Type: Lithostratigraphic
Rank: Group
Status: Formal
Use: Active


Related unit(s)
  • Opémisca Group
    • Chebistuan Formation
    • Haüy Formation
      • Dolomieu Member
      • Pantoufle Member
      • Gribouille Member
      • Christian Member
      • Vanina Member
    • Daubrée Formation
    • La Trêve Formation
    • Stella Formation




The Opemisca Group was originally designated by Norman (1937, 1941a,b) as the “Opemisca Series”. Rejecting the idea of a discordance in Neoarchean rocks, Duquette (1970) grouped all sedimentary rocks in the region into the Blondeau Formation. During mapping of the Queylus Township, Cimon (1976) reintroduced the concept of a discordance separating units from the Roy and Opémisca groups.


The Opemisca Group consists of conglomerate, arenite, wacke and mudrock interstratified with lenses of andesitic and trachytic volcanic rocks (Picard and Piboule, 1986b). It includes, from base to top: (a) the Stella Formation; (b) the La Trêve Formation, located west of Chapais, which is the lateral stratigraphic equivalent of the coarser part of the Stella Formation (Charbonneau et al., 1991); (c) the Daubrée Formation, also west of Chapais, which is the lateral stratigraphic equivalent of the finest part of the Stella Formation (Charbonneau et al., 1991); (d) the Haüy Formation; and (e) the Chebistuan Formation, which is a stratigraphic equivalent of the Stella and Haüy formations in the Waconichi Syncline basin north of the Chibougamau Anticline. 

Thickness and Distribution

The Opémisca Group is located along the Chapais and Waconichi synclines (Daigneault and Allard, 1990). Its estimated total thickness in the Chapais Syncline is between 3 and 4 km (Mueller and Donaldson, 1992). Northwest of Chapais, rocks of the Opémisca Group also appear in a synclinal structural basin cut by the NE-SW reverse-senestral Lamarck Shear Zone.


A sample of polymictic conglomerate collected east of Laura Lake in the Chapais area (NTS sheet 32G10) determined a maximum sedimentation age for the Stella Formation at <2704 ±2 Ma (Leclerc et al., 2012). A sample of polymictic conglomerate collected south of Chapais determined the maximum sedimentation age for the Haüy Formation’s Gribouille Member at <2691.7 ±2.9 Ma (David et al., 2007). 

Isotopic System Mineral Crystallization Age (Ma) (+) (-) Reference(s)
U-Pb Zircon <2704 2 2 Leclerc et al., 2012
U-Pb Zircon <2691.7 2.9 2.9 David et al., 2007

Stratigraphic Relationship(s)

The contact at the base of the Opémisca Group sedimentary rocks is generally characterized by an angular discordance with underlying volcanic rocks of the Roy Group (Cimon, 1976; Caty, 1978; Simoneau, 1986). However, sedimentary rocks of the Bordeleau Formation in the Waconichi Syncline are locally a transitional unit between the two groups (Caty, 1979; Dimroth et al., 1983). The sommital portion of the Opémisca Group is cut by the Kapunapotagen Shear Zone (steep dip, reverse movement from south to north) at the axial trace of the Chapais Syncline, which contrasts with the northward polarity of Roy Group rocks and the southward polarity of Opémisca Group rocks. This structural relationship (Charbonneau et al., 1991, Figure 34) is also recognized in the Chapais area (Morin, 1994; Leclerc et al., 2010) and in the Chibougamau area up to the Grenville Front (Daigneault and Allard, 1990, Figure 160).


Does not apply.


Author(s) Title Year of Publication Hyperlink (EXAMINE or Other)
CATY, J.L. Canton de Richardson (comté d’Abitibi-Est) – Rapport intérimaire. Ministère des Richesses naturelles, Québec; DP 606, 37 pages, 1 plan. 1978 DP 606
CATY, J.L. Demie-ouest du canton de Bignell. Ministère de l’Énergie et des Ressources, Québec; DPV 678, 22 pages, 1 plan. 1979 DPV 678
CHARBONNEAU, J.-M. – PICARD, C. – DUPUIS-HÉBERT, L. Synthèse géologique de la région de Chapais-Branssat (Abitibi). Ministère de l’Énergie et des Ressources, Québec; MM 88-01, 200 pages, 13 plans. 1991 MM 88-01
CIMON, J. Géologie du canton de Queylus (NE), Abitibi-Est. Ministère des Richesses naturelles du Québec; DPV 439, 38 pages, 1 plan. 1976 DPV 439
DAIGNEAULT, R. – ALLARD, G.O. Le Complexe du Lac Doré et son environnement géologique. Ministère de l’Énergie et des Ressources, Québec; MM 89-03, 375 pages. 1990 MM 89-03
DAVID, J.  – DAVIS, D.W.  – DION, C.  – GOUTIER, J. – LEGAULT, M. – ROY, P. Datations U-Pb effectuées dans la Sous-province de l’Abitibi en 2005-2006. Ministère des Ressources naturelles et de la Faune, Québec; RP 2007-01, 17 pages. Géologie du canton de Queylus (NE), Abitibi-Est. Ministère des Richesses naturelles du Québec; DPV 439, 38 pages, 1 plan. 2007 RP 2007-01
DIMROTH, E. – MUELLER, W.  – ROCHELEAU, M. – ARCHER, P. – JUTRAS, M. – PICHÉ, M. – SIMONEAU, P. – CARIGNAN, J.  – CHOWN, E.H. – GUHA, J. – GOULET, N. – ALLARD, G.O. – FRANCONI, A. – GOBEIL, A. Stratigraphie et évolution du bassin de transition entre les groupes de Roy et d’Opémisca, région de Chibougamau-Chapais. Dans : Stratigraphie des ensembles volcano-sédimentaires archéens de l’Abitibi : état des connaissances. Ministère de l’Énergie et des Ressources, Québec; DV 83-11, pages 21-33. 1983 DV 83-11
LECLERC, F. – HARRIS, L.B. – BÉDARD, J.H. – VAN BREEMEN, O. – GOULET, N. Structural and stratigraphic controls on magmatic, volcanogenic and syn-tectonic mineralization in the Chapais-Chibougamau mining camp, northeastern Abitibi, Canada. Economic Geology; volume 107, pages 963–989. 2012 Source
MUELLER, W. – DONALDSON, J.A. Development of sedimentary basins in the Archean Abitibi belt, Canada: an overview. Canadian Journal of Earth Sciences; volume 29, pages 2249–2265. 1992 Source
NORMAN, G.W.H. East half Opemiska map area, Quebec. Geological Survey of Canada; Studies 37-11, 27 pages, 1 plan. 1937 Source
NORMAN, G.W.H. Opemiska, West Half, Abitibi Territory, Québec. Geological Survey of Canada; Map 602A, 1:63 360 scale. 1941a Source
NORMAN, G.W.H. Opemiska, East Half, Abitibi Territory. Geological Survey of Canada; Map 401A, 1:63 360 scale. 1941b Source
SIMONEAU, P. Pétrologie, sédimentologie et analyse des faciès de la Formation de Daubrée, Chapais, Québec. Université du Québec à Chicoutimi; master’s thesis, 486 pages, 1 plan. 1986 Source



12 septembre 2019