Sedimentation en marge d'un promontoire cambro-ordovicien: Le groupe d'Ile d'Orleans, Appalaches du Quebec

The understanding of mountain chains is in a large part a study of the morphology and nature of the colliding elements. The distribution of Taconian flysch deposits along the Laurentian margin was controlled by the presence of promontories and reentrants related to the lapetan rift. The uneven distribution of those sediments is known at a regional scale, but is still poorly defined at a local scale, where second order feature can influence sediment dispersion. The Montmorency Promontory is such a second-order feature found in the Quebec Reentrant, and was defined on the basis of facies changes observed in Middle Ordovician platform and flysch deposits. No influence of this headland has been recognized in older deposits, therefore it is difficult to determine if the promontory is related to the Iapetan rift. However, sedimentary sequences in the Quebec City area were interpreted using obsolete models, which would explain the "absence" of the promontory from Late Proterozoic to Early Ordovician.; Stratigraphic evidence of the presence of the Montmorency Promontory during the Cambrian and Early Ordovician is likely to be found in stratas of the Ile d'Orleans Group, which is immediately adjacent to the proposed location of this headland. More so, the sedimentary sequence is located in a key area (Quebec City) where correlations between the sedimentary units on both side are difficult to define. The study of the Anse Maranda Formation showed that glauconite, an authigenic mineral indicative of low sediment accumulation rate, was formed in-situ. However, glauconite in this study contained more aluminium than iron, which is unusual. Related to the aluminium-iron substitution is a layer charge variation that influences the retention of potassium in the crystal structure. A loss of layer charge induced by the substitution of divalent iron (and magnesium) by trivalent aluminium leads to the expulsion and consequent reduction in potassium. Relations between glauconite composition and diagenesic features, such as cemented burrows, in the Anse Maranda Formation indicate that the aluminium-rich composition is due to the early diagenesis of an iron-rich glauconite.; The occurrence of autochthonous glauconite in the Anse Maranda Formation does not support previous interpretations that suggest alternating massive sandstone and mudstone were deposited in the proximal part of a deep-sea fan. Study of the sandstone revealed that their massive aspect is due to bioturbation and not to a primary sedimentary process. Trace fossil suites indicate that the Anse Maranda Formation was deposited on the distal part of the continental shelf where low sedimentation rate permitted glauconitisation and complete reworking of sediment by trace making organisms. The gradual transition from low energy ichnofossils such as Helminthopsis to more robust forms like Chondrites suggests a relative sea-level drop as bioturbation controlled early diagenesis, bioturbated sandstone having less calcite cement and more clay than non-bioturbated sandstone.; The Lauzon and Pointe-de-la-Martiniere formations occur respectively in the middles and upper part of the Ile d'Orleans Group, a hiatus separating the Anse Maranda and Lauzon formations. At the base of the Lauzon Formation, a limestone conglomerate (Ville Guay conglomerate) resulted from erosion of a thrombolite reef following a continental shelf collapse. The conglomerate is overlain by a mud/sand-rich fan was deposited on the upper part of the slope and formed two fining-upward sequences. The Pointe-de-la-Martiniere Formation is composed of mud-rich turbidites and hemipelagites to the east and distal shelf deposits to the west. These distal shelf deposits are composed of a black shale - green mudstone - dolomitic mudstone assemblage, named Logan cycles and, of alternating black shale and limestone. These two lithofacies are related to high organic productivity and early diagenesis related to a high concentration of CO2. B...