SUBAQUEOUS PYROCLASTIC DEPOSITS IN AN ORDOVICIAN FORE-ARC BASIN - AN EXAMPLE FROM THE SAINT-VICTOR FORMATION, QUEBEC APPALACHIANS, CANADA

The Saint-Victor Formation is mainly composed of graptolite-bearing, n onvolcanic turbidite sequences, 7 km thick, deposited in a forearc bas in during the Ordovician Taconian orogeny. The bedded tuff and lapilli tuff(BTL) facies, 10-30 m thick, is the principal volcaniclastic faci es in the Saint-Victor Formation. The BTL, which is repeated at severa l stratigraphic levels within the formation, contains two divisions. T he lower division is a well sorted, fines-poor, mostly massive, lapill i tuff representing less than or equal to 50% of the BTL. It was empla ced as a single, subaqueous volcanic debris flow. The upper division i s made up of upward-thinning and-fining tuff beds emplaced by frequent ly recurring subaqueous ash turbidites. The abundance of pyroclasts (s hards and pumice) in the BTL, goad sorting of the framework constituen ts, 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 th e BTL facies. The BL contains the same constituents and sedimentary st ructures as the lower division of the BTL and is interpreted to have f ormed from similar processes. Similarly, the BT is similar to the uppe r division of the BTL and is interpreted to have formed as ash turbidi tes. Either they resulted from flow decoupling during emplacement of t he overlying BTL or they represent dislodged debris from an eruptive e vent that occurred before the main eruptive event that led to depositi on of the BTL.