Slivers of gneisses and amphibolites (termed the Sainte-Marguerite Complex) having a minimum isotopic age of 900 Ma are interpreted to represent thrust slices of Grenvillian-type basement within the western margin of die internal domain of the Appalachian orogen. These slices constitute the basement to a series of spilitized volcanic rocks and interbedded shallow-water sandstones (termed the Montagne de Saint-Anselme Formation) which are interpreted to be Precambrian to Cambrian(?) rift-related precursors to the filling of the Appalachian mobile belt.

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