Bergeron (1957) defined the Otish Mountains Group as the NE extension of the Mistassini Basin. He recognized two sedimentary sequences (the lower and upper sequences) and a gabbro unit. Chown (1971) refers to the lower sequence as the Lac Indicator Formation, the gabbros as the Otish Mountains Gabbros, while the Otish Mountains Group becomes the Otish Group. Later, Chown and Caty (1973) renamed the sedimentary units “Otish Group” and “Indicator Formation”, and assigned the upper sequence to the Péribonca Formation. In its synthesis of the Otish Basin, Genest (1989) subsequently subdivided and elevated the Indicator and Péribonca formations to group status, leading to the redefinition of the Otish Group as a supergroup. Genest (1989) also proposed to integrate all units of the Mistassini Group into the Otish Supergroup. This proposal is not accepted here. Correlations between the different nomenclature used in previous work are summarized in the table below, as well as in the summary table of Genest (1989, p. 21). Compilation maps of the Ministère (MRNF, 2010a-c) are essentially based on descriptions from Genest (1989) which are summarized in the following section.

The Otish Supergroup consists of two distinct lithostratigraphic units, the Indicator and Péribonca groups. The Indicator Group consists mainly of sandstone and oligomictic quartz pebble conglomerate. If overlying Archean volcano-sedimentary units, basal conglomerate is polymictic in a thin layer. The Péribonca Group is characterized by two sedimentary cycles, each consisting of a polymictic conglomerate unit at the base that gradually changes to sandstone and red siltstones. Although no major unconformity was observed between the Péribonca and Indicator groups, some field evidence suggests an erosional discordance. Indeed, angular fragments of quartzitic sandstone in the polymictic conglomerate of the Laparre Formation at the base of the Péribonca Group are believed to have originated from the underlying Shikapio Formation.

Thickness and distribution

The Otish Supergroup occupies almost the entire area of the Otish Basin, which is elongated along a general NE-SW axis, following the trace of a regional synform, and is approximately 150 km by 15-40 km. Its thickness is estimated to be 2400 m (Genest, 1989). Discontinuous strips up to 20 km long and up to 5 km wide are also located SE and SW of the basin, in the same general direction. Paleoproterozoic units of the Otish Basin generally have shallow dips. The distribution of the different sedimentary units is largely controlled by the underlying Archean basement paleotopography, fault zones and topography of the area.

Dating

The Otish Supergroup is cut by gabbro dykes and sills associated with the Otish Gabbro dated to 2169 Ma (Hamilton and Buchan, 2016; Miligragovic et al., 2016), giving it a minimum sedimentation age.