The Gaschet Formation was established by Genest (1989) and refers to a lake in the centre of the Otish Basin (sheet 23D03). This unit corresponds to the upper part of the type section of Member A and the lower part of the type section of Member B of the Péribonca Formation described by Chown and Caty (1973), which was subsequently elevated to the rank of group (Péribonca Group) by Genest (1989). Correlations between the different nomenclatures used in previous work is summarized in the Otish Supergroup record and in the summary table of Genest (1989, p. 21). The Minstère’s compilation maps (MRNF, 2010) are essentially based on descriptions from Genest (1989) which are presented in the following section.

The Gaschet Formation is characterized by a sequence of carbonates and terrigenous sediments rich in pyrolusite and hematite. Genest (1989) divided the formation into four separate members (members A to D). These divisions are not used here.

The lower lithofacies of the Gaschet Formation consists essentially of carbonates. In the Indicateur Lake area, carbonates are composed of magnesite and dolomite. Magnesite forms synsedimentary breccias with well-sorted and sorted elements. Breccia elements are micritic, irregularly shaped, but have a spheroidal appearance in thin sections; they are bound by a secondary cement of magnesite slightly altered to brucite. Dolomite is well laminated and characterized by the presence of three types of breccia: synsedimentary slump breccia, intraformational breccia without apparent fragment displacement, and early diagenetic breccia reminiscent of certain enterolithic structures encountered in evaporative deposits. Effects of tectonic stresses, hydrothermal inputs and karst and/or palekarst phenomena are superimposed on these brecciated structures, resulting in the carbonate deposits’ chaotic appearance. Elsewhere in the area, magnesite is not present and dolomite is even more altered than that observed at Indicateur Lake. In the Magneron Lake area, the lithofacies’ base is well preserved and consists of alternating dolomite, calcite and sandstone laminae, some of which are enriched in pyrolusite.

The next lithofacies consists of sandstone, silty sandstone and argillaceous arkosic siltstone with dolomitic cement. This sequence has significant lateral surface variations that are directly related to the presence and thickness of underlying carbonates. Thus, the greater the carbonate unit, the finer the grain size of overlying terrigenous rocks. The sequence is characterized by its composition, including pyrolusite enrichment and the presence of plagioclase (albite and oligoclase), gypsum and volcanic fragments. Sandstone forms large oblique beds characterized by sets of parallel laminae that are associated with wind dunes. Cement leaching is responsible for significant secondary porosity.

Sandstone quickly changes to silty sandstone, which defines a transitional lithofacies. Siltstone displays a negative sequence marked at the base by red mudstone beds that change to argillaceous siltstone, then to siltstone and, at the top, to silty sandstone. All rocks in this silty unit are characterized by a red colour, beige reduction spots, dissolution cavities, poikilitic dolomite rhombohedrons, brecciated calcite veinlets and 25% dolomitic cement. Sandstone contains sandy dolomite beds and dolomite beds. Dolomite beds <50 cm thick appear at the top of the sequence. They alternate irregularly with sandstone beds to define composite beds that can reach 2 m in places.

The next lithofacies consists of polymictic conglomerate and arkosic sandstone, which generally exhibit channel structures. Conglomerate contains a heterogeneous allochtonous fraction (clasts of quartz, feldspar, pegmatite and other granitoids) and a large proportion of dolomite and red siltstone fragments of local origin. The presence of dolomite fragments makes it a good stratigraphic marker.

Conglomerate is overlain by a unit of laminated arkosic sandstone with dolomitic cement, analogous to lower sandstone, and then by a unit of siltstone and red mudstone with reduction spots alternating with sandstone beds. At the top, red mudstone beds have desiccation cracks. Beds of polymictic conglomerate that are genetically related to underlying lithofacies were observed in places.

The Gaschet Formation stratotype is described in the type sections of the Péribonca Formation (now Péribonca Group) of Chown and Caty (1973). The section of Member A (division not used here) is roughly oriented N-S and corresponds to the trace of a small creek valley on the north side of the Otish Mountains main range; that of Member B (division not used here), oriented E-W, begins east of a small lake and continues on the western slope of the Otish Mountains main range:

Thickness and distribution

The Gaschet Formation outcrops along a strip generally oriented NE-SW, following the trace of a regional synform, approximately 80 km by 2-20 km in the SE part of the Péribonca Group. The basal carbonate lithofacies is weakly exposed (up to 50 m thick) NE of Indicateur Lake and in the Marie-Victorin Mountains area (sheet 23D04). Polymictic conglomerate does not exceed 16 m thick, while sandy and silty sequences reach 279 m and 170 m respectively (Genest, 1989).

Dating

The Gaschet Formation 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.