Three main geological units are distinguished in the Mount McGerrigle area of north-central Gaspé, each different in lithology, structural style, and age: a Cambro-Ordovician, complexly deformed assemblage of eugeosynclinal rocks is discordantly intruded by a composite pluton, and is unconformably overlain by mildly deformed Silurian–Devonian platform deposits. Three phases of deformation (F1, F2, F3) have affected the Quebec and Schickshock Groups of Cambro-Ordovidian age. The F1, deformation was accompanied by lowgrade regional metamorphism resulting in a penetrative planar fabric. The F2 deformation deformed pre-existing structures and produced the prevalent northeasterly trending and shallowly plunging folds. The third phase was least penetrative, resulting in broad and upright east-northeasterly striking F3 folds that modify the orientation of F2 structures. The forcefully injected Mount McGerrigle pluton, a late intrusion of the Acadian orogeny, truncates and deforms the F1 and F2 fabric elements of the Cambro-Ordovician rock. Multiphase deformation thus preceded the Acadian orogeny in the area. Silurian–Devonian rocks, on the other hand, have been affected only by the F3 deformation during the Acadian orogeny.

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