Impact of an irregular margin on the sedimentary evolution of a narrow shelf-slope environment (Quebec City area)





Impact of an irregular margin on the sedimentary evolution of a narrow shelf-slope environment (Quebec City area)

Pierre A Cousineau (UQAC) and Hugues Longuépée (Champion Iron Ltd)

Leaders and affiliations : Pierre A Cousineau (UQAC), Hugues Longuépée (Champion Iron Ltd)

Contact : [email protected]

Description : Irregularities in the geometry of continental margins, such as promontories and re-entrants, act as major controls on sediment distribution throughout a Wilson cycle. Major changes in both along-strike and across-strike stratigraphic variations allow identification of second- and third-order irregularities in geometry. This is the case for sediments in thrust sheets adjacent, West, and East of the Montmorency Promontory (i.e., Quebec City).

Within the Bacchus Nappe, the Cambro-Ordovican Ile d’Orléans Group is unique in the Quebec Appalachians in several aspects. In the lower Cambrian Anse Maranda Formation, trace fossils and glauconite textures enabled the reinterpretation of the sequence from deep-sea turbidites to shelf-edge deposits formed during the rift-to-drift tectonic phase. There is an abrupt transition to a more typical turbiditic environment of the overlying the Lauzon Formation, where irregular thick packages of sandstone/conglomerate within much finer-grained turbidites record instabilities (eustatic and tectonic) of an Atlantic-type passive margin. Muddy turbidites of the Pointe-de-la-Martinière Formation signal the onset of the Taconic Orogeny. Time equivalent rocks in adjacent thrust sheets, either more typical of deep-sea fan environment (i.e., Sillery Group) or of slope environment adjacent to a carbonate platform (i.e., Lévis Formation) will also be examined.

Sediments near Quebec City were thus deposited at the edge of a promontory with a narrow steep-sided, horst-and-graben shelf with local sediment input. To the Southwest, the platform widens and has a ramp geometry where a large fluvial system to the Southwest (e.g., Pinnacle Formation of the Oak Hill Group) develops. To the Northeast, major canyons cut through a narrow steep-sided shelf and feed directly deep-sea fans. Most outcrops are located along the shore of majestic Saint-Lawrence River, between Lévis and Saint-Michel-de-Bellechasse.

Number of participants (Min/Max) : 12 / 40

Duration : 1 day

Date : Post-Congress