Subaqueous Pyroclastic Deposits in an Ordovician Fore-Arc Basin: An Example from the Saint-Victor Formation, Quebec Appalachians, Canada
Journal of Sedimentary Research
AmT~ACr: The Saint-Victor Formation is mainly composed of graptolitebearing, nonvoleanic turbidite sequences, 7 km thick, deposited in a forearc basin during the Ordavician Taconian orogeny. The bedded tuff and lapilli tuff(BTL) facies, 10-30 m thick, is the principal voleanielastic facies in the Saint-Victor Formatien. The filL, which is repeated at several stratigraphic levels within the formation, contains two divisions. The lower division is a well sorted, fines-poor, mostly massive, lapiHi
... tly massive, lapiHi tuff representing -< 50% of the BTL. It was emplacod as a single, snbaqueoes volcanic debris flow. The upper division is made up of upward-thinning and-fining tuff beds emplaced by frequently recurring snbaqueons ash turbidites. The abundance of pyroclasts (shards and pumice) in the BTL, good sorting of the framework constituents, and the absence of nonvoleanic background sediment interbeds favor direct sedimentation from one subaqucous explosion. A bedded lapilli tuff (BL) or bedded taft (BT) facies (1-5 m) is locally present below the BTL facies. The BL contains the same constituents and sedimentary structures as the lower division of the BTL and is interpreted to have formed from similar precesses. Similarly, the BT is similar to the upper division of the BTL and is interpreted to have formed as ash tur- bidites. Either they resulted from flow deconpling during emplacement of the overlying BTL or they represent dislodged debris from an eruptive event that occurred before the main eruptive event that led to deposition of the BTL.