The Upper Ordovician Tremblay Formation was introduced by Sinclair (1953) in the Lake Saint-Jean area. This formation belongs to the St. Lawrence Platform, which forms a slice in the Grenville Province. All Ordovician sedimentary rocks in this region were assigned to the Trenton Group by several authors from a chronostratigraphic perspective (e.g., Dresser and Denis, 1946; Sinclair, 1953). Other authors have done so on lithostratigraphic bases (Benoit and Valiquette, 1971; Harland et al., 1985, 1987). Instead, Desbiens and Lespérence (1989) proposed the creation of a new group for these rocks, but did not. Although spatial correlation between these Ordovician rocks in the Lake Saint-Jean area and those of the Trenton Group of the St. Lawrence Platform is not formally established, the Tremblay Formation is maintained in this group.

The Tremblay Formation consists of conglomerate, sandstone and greyish limestone, bluish in altered surface. The base of the formation consists of ~1 m of weakly sorted conglomerate with no important stratification. Conglomerate is composed of subangular syenitic blocks. The block size can reach 80 cm, but does not exceed 20 cm on average. The conglomerate base is overlain by a 1.9 m thick horizon of coarse-grained calcareous sandstone, partly composed of low-cemented conglomeratic beds, approximately 15 cm thick. Approximately 1.2 m thick, the sommital part is similar, but cement is limestone and includes calcilutite lenses forming about 10% of the total (Desbiens and Lespérence, 1989). At the Niobec Mine in the Saint-Honoré area, the Tremblay Formation consists of regoliths with carbonatite blocks. For Lavoie and Asselin (1998), this formation is a siliciclastic unit composed of conglomerate, sandstone and siltsone. It represents a deposit with a dominant detritic fraction (regolith of the Grenvillian bedrock) combined with near-shore marine action such as that associated with tides (Harland et al., 1985).

In the Lake Saint-Jean area, the Tremblay Formation and the three other related formations (Galets, Shipshaw and Simard) cannot be mapped at the 1:50 000 scale because they are generally not more than 50 m thick and subhorizontal (<4° dip). On the geological map of the Saguenay–Lac-Saint-Jean region (sheets 32A08 and 22D), the Trenton Group is hence shown instead of these four related formations.