Silurian Stratigraphy and Paleogeography of Matapediatemiscouata Region, Quebec

Abstract

The Silurian System in the Matapedia-Temiscouata region overlies the Cambro-Ordovician with angular unconformity. Field evidence suggests that the orogeny occurred in this area prior to the early Llandoverian and after the Middle Ordovician (Caradocian). In the Matapedia Valley and in the central part of the region, sedimentation was continuous from the Silurian into the Devonian; in the Temiscouata area, a disconformity apparently exists between Silurian and Devonian rocks. Five formations are of Llandoverian age. The Llandoverian Cabano Formation is overlain by late Llandoverian Pointe Aux Trembles volcanic conglomerate and arenite which intertongue on the northeast with the Lac Raymond arkosic wacke and lutite, and farther east with the Awantjish lutite. The Awantjish in the Matapedia Valley is overlain by the Val Brillant Formation which is of latest Llandoverian and earliest Wenlockian ages. The rocks of Wenlockian age consist of parts of the Val Brillant quartz arenite and the Sayabec fossiliferous limestone. Both intertongue toward the west with the Robitaille and Asselin Formations. In the southwestern part of the region the basal Ludlovian strata are the red siltstone and sandstone beds of the Robitaille Formation, overlain by the Mont Wissick Group (formerly the Mont Wissick Formation) of Ludlovian and Skalian ages. The Mont Wissick includes, in ascending order, the Sayabec limestone, the St. Leon siltstone and sandstone, and the Lac Croche limestone and dolomite. Farther east, the St. Leon siltstone grades upward into the Cap Bon Ami calcareous mudstone and argillaceous limestone. Helderbergian fossils are present in the Cap Bon Ami at about 500 ft above the base; below that the rocks may be Silurian (Skalian). The distribution of rocks of Skalian, Ludlovian, and Wenlockian ages suggests a transgression of the sea from the northeast to the southwest. The Early Silurian sea transgressed from northeast to southwest, over and around the partly eroded Taconic Mountains. The deposition of terrigenous sediments of sedimentary origin was followed by deposition of Pointe Aux Trembles-Lac Raymond (late Llandoverian) sediments of volcanic origin derived from the south-southeast. The major transgression of the Silurian began during Wenlockian and climaxed during Ludlovian time. The Llandoverian sediments are characteristic of a rapidly sinking trough, the Wenlockian sediments are indicative of a shallow-marine, more stable environment, whereas the Ludlovian and Skalian sediments suggest a more rapidly sinking trough.