|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|
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.
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|
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.