The presence of a cover of Paleozoic rocks on this part of the Canadian Shield greatly facilitated the interpretation of the readjustment of the Charlevoix impact structure. On the other hand, the study of these rocks and the knowledge of the relative movements they have undergone on both side of the St. Lawrence rift fault allow us to establish a first marine advance which encroached to lap the present border of the Shield during the Early Ordovician, or at the beginning of the Middle Ordovician time. This was followed by further marine transgression which progressively covered all this part of the shield. This episode was followed, in Early Utica time, by the deepening of a miogeosyncline between the shield and the uplift of the Vermont–Quebec geanticline, inducing the slumping of the latest calcareous deposits into a narrow band bordering the shield, followed by accumulations of flysch-type deposits in the basin. This deposition ceased after a last subsidence along the bordering fault, and the closing of the basin when the gliding Appalachian nappes abutted against the resulting escarpment.

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