No abstract

The Beauceville Formation of the Magog Group is mainly composed of graptolitebearing pelagic mudstone and volcaniclastic rocks, which were deposited in a fore-arc basin during the Ordovician. The volcaniclastic rocks of the Beauceville area, their transport and depositional mechanisms are not well known.On the basis of sedimentary structures, composition, grain size and delicate textures of debris, four distinct lithofacies were identified. The massive lapilli tuff-tuff faciès A is composed of: i) 2-5m-thick, fines-poor, massive lapilli tuffs (MLT) that are either massive or graded and ii) 1-1 lm-thick, fines-poor, massive crystal-rich tuffs (MCT) with normal or inverse grading. Faciès B, represented by the stratified and laminated lapilli tuff-tuff faciès, is divided in two subfacies: i) 0,5-lm-thick, fines-poor, stratified lapilli-tuffs (SLT) are composed of 5-40cm-thick, poorly graded layers and ii) l-10m-thick, fines-poor, stratified and laminated crystal-rich tuffs (SLCT) show 5-10cm-thick parallel stratified layers with normal grading and parallel laminae. Faciès C is characterized by the massive finer-grained tuff faciès (MFT) composed of 0,4-5m-thick beds rich en shards. Faciès D, a laminated finer-grained tuff faciès (LFT) is l-7m-thick shard-rich unit composed of 2 mm-6 cm-thick laminae and beds.The four lithofacies are organized into two distinct faciès associations that each display a well defined doubly graded sequence. Faciès association 1 with the assemblage MCT->SLCT->MFT->LFT is 28m-thick, whereas faciès association 2 characterized by the assemblage MLT->SLT->MCT->SLCT->LFT is 24m-thick. Both faciès associations rest on pelagic mudstone suggesting that they could be either lateral or downslope equivalents. Based on the grain size distribution, faciès association 1 is probably the downslope or lateral (distal ?) equivalent of faciès association 2.Abundance of delicate shards and pumice in these two faciès associations, the good sorting of the framework constituents and the absence of pelagic sedimentary interbeds as well as the doubly graded sequence favour direct sedimentation from an explosive phreatomagmatic or phreatoplinian eruption that was probably subaqueous due to the bounding pelagic sedimentary faciès. Faciès A is interpreted as partially laminar flow deposits, whereas faciès B reflects a more turbulent flow deposition. The faciès C and faciès D are either subaqueous fallout deposits related to waning eruptive stages or ashturbidites produced by elutriating fine ash from initialy high-concentration flows.

Nous voudrions remercier notre directeur, M. K. Fall, pour ses conseils avisés et le support qu'il nous a prodigué tout au long de notre travail. Nous aimerions profiter de l'occasion pour remercier M. Mario Bergeron pour son aide technique, ainsi que Mme Sylvie Fortin, qui a assuré la présentation matérielle de ce mémoire. Leur travail et leur efficacité ont été grandement appréciés.