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St. John's 2001
Technical Programme

SS16:  The Architecture, Origin and Evolution of the Paleozoic Continental Margin of Laurentia 
Organizers / Organisateurs:  Dennis Lavoie
Room / Salle:  A1046

Date:  30/05/2001
Time:  8:20 AM
Presenter:  Hugues Longuépée

Depositional environments for the Ile d’Orleans Group near Québec City:  Influence of the Montmorency Promontory

LONGUÉPÉE, H. and COUSINEAU, P.A., Université du Québec à Chicoutimi, 555, Boul. de l’Université, Chicoutimi, QC, G7H 7J5, hlonguep@sympatico.ca

The Cambro-Ordovician Ile d’Orléans Group (IOG) is located within the external part of the Appalachian Humber zone.  All rocks of the group have been interpreted as deep water turbidites, mostly on the basis of litho- and biostratigraphic correlations with other better studied formations.  However, recent sedimentology data and proposed paleogeographic reconstructions suggest shallower water depth for the depositional setting of the IOG, such as the outer shelf / higher slope.

The lowermost Anse Maranda Formation (Lower – Middle Cambrian ?) is composed of glauconitic sandstones and mudrocks, and show a general coarsening upward trend.  Intense, previously unrecognized, bioturbation has left few primary sedimentary structures.  Traces fossils are mostly Planolites, Helminptosis hieroglyphica and an unidentified trace.

The Lauzon Formation (Tremadocian) overlies the Anse Maranda Formation.  The base of the Lauzon Formation is the thick Ville Guay limestone conglomerate which sits on an apparent 10 Ma hiatus.  Rocks include arkosic sandstone, grey to green mudrocks and calcareous turbidites that show facies typical of fine-grained turbidite fan models.  Traces fossils are few compared to the previous formation and dominated by Skolithos type, supporting mean deposition on the outer shelf.

The uppermost Pointe de La Martinière Formation begins with the first red mudrock beds.  This mudstone-rich formation has recently been interpreted as a turbidite sequence on the upper continental slope.  However, presence of solitary ripples and apparent current inversion suggest the influence of combined-flow, such as storm-induced or contour currents.

Thus, the IOG is tentatively interpreted as a sediment-starved shelf located adjacent to the Montmorency Promontory which is presumed to be a topographic high.  Current work on acritarches, total organic carbon and glauconite maturity, as well as a facies analysis of the Anse Maranda Formation is expected to provide further, more precise information about this formation.