Subaqueous Pyroclastic Deposits in an Ordovician Fore-Arc Basin: An Example from the Saint-Victor Formation, Quebec Appalachians, Canada

ABSTRACT The Saint-Victor Formation is mainly composed of graptolite-bearing, nonvolcanic turbidite sequences, 7 km thick, deposited in a fore-arc basin during the Ordovician Taconian orogeny. The bedded tuff and lapilli tuff (BTL) facies, 10-30 m thick, is the principal volcaniclastic facies in the Saint-Victor Formation. The BTL, which is repeated at several stratigraphic levels within the formation, contains two divisions. The lower division is a well sorted, fines-poor, mostly massive, lapilli tuff representing 50% of the BTL. It was emplaced as a single, subaqueous volcanic debris flow. The upper division is made up of upward-thinning and-fining tuff beds emplaced by frequently recurring subaqueous ash turbidites. The abundance of pyroclast (shards and pumice) in the BTL, good sorting of the framework constituents, and the absence of nonvolcanic background sediment interbeds favor direct sedimentation from one subaqueous explosion. A bedded lapilli tuff (BL) or bedded tuff (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 processes. Similarly, the BT is similar to the upper division of the BTL and is interpreted to have formed as ash turbidites. Either they resulted from flow decoupling 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.