The Chebistuan Formation was established by Caty (1977) after mapping the eastern-half portion of the Richardson Township (NTS sheet 32J01), in the Waconichi Lake area. The sedimentary unit recognized on both sides of North Chebistuan and South Chebistuan lakes (unofficial names) was first interpreted as belonging to the Stella Formation (Caty, 1975). Caty (1977) continued this unit eastward and renamed it the Chebistuan Formation. Caty (1978) then attached it to the Opémisca Group. It was later considered equivalent to the Stella and Haüy Formations (Allard et al., 1979).

The Opemisca Group’s Chebistuan Formation is a stratigraphic equivalent to the Stella and Haüy formations. It is composed of polymictic conglomerate, arkosic wacke and mudrock (unit nAch1). It also includes a unit dominated by polymictic conglomerate (unit nAch2) and a mudrock unit (unit nAch3). 

Unit nAch1 consists of interstratified polymictic conglomerate beds (see description of unit nAch2 below), orange-beige to greenish beige lithic arenite and medium to dark grey mudrock. These three lithologies form beds ranging in thickness from a few centimetres to more than one decimetre. This sequence, characterized by sorting, synsedimentary faults, parallel laminations and channels, appears to represent ta-td-type turbidites (Caty, 1978). In the vicinity of the Eau Noire stock (sheet 32J03-200-0202), the finer-grained and more aluminous portion of these turbidites contains andalusite porphyroblasts due to contact metamorphism associated with this intrusion.

In the Eau Noire Lake area (sheet 32J03-200-0102), two types of conglomerates were observed by Leclerc and Daoudene (2021). The first, directly located at the contact with the Eau Noire stock, consists of joint polymictic clasts of mafic to felsic volcanic rocks, gabbro and granodiorite and syenite from erosion of adjacent intrusions. Mafic volcanic rock clasts are amphibolitized and transformed into hornblende-biotite schists due to contact metamorphism. Granodiorite and syenite clasts have a slightly flattened elongated cigar form. This morphology shows constrictive-type deformation prevailing at the immediate edge of the Eau Noire stock. The second type of polymictic conglomerate, which is more widespread, consists of floating clasts in a coarse-grained, grey to dark-green lithic arenite matrix containing disseminated sulphides, giving it a typical rusty patina. Clasts consist of dark grey mudrock, volcanic rock containing millimetric plagioclase phenocrystals, and lithic arenite that can originate from the underlying stratigraphic pile (i.e. Blondeau Formation). These conglomerates are interstratified with sorted lithic arenite and mudrock beds and have oblique and cross-bedded laminations.