The Precambrian Shield and the Lower Paleozoic Shelf: The Unstable Provenance of the Lower Paleozoic Flysch Sandstones and Conglomerates of the Appalachians between Beaumont and Bic, Quebec

Abstract

Three sections of lower Paleozoic flysch in the Quebec Appalachians were sampled at Beaumont, Bic, and Trois Pistoles in order to determine the position, composition, and relief of the source area. At Beaumont, the mean flow direction of sediment transport is east-southeasterly; at Trois Pistoles and Bic it is south-southeasterly. Locally there is strong dispersion of the data but no northerly directions have been observed.Albite is the common plagioclase in all Cambrian sandstones; grains generally show polysynthetic twins, but in a few beds only untwinned albite is present. Oligoclase and andesine are the dominant plagioclases in Lower Ordovician rocks. The accessory suite has few diagnostic species; pink garnet is present in all sections, but absent in the basal unit at Bic. Diopside, sphalerite, and barite are found only in the Lower Ordovician rocks at Beaumont. At the three localities, the feldspar content, grain size, and sand/shale ratio vary up-section.The original source area for the sandstones and conglomerates consisted of an early Paleozoic shelf and a Precambrian land-mass. The oldest sands were derived from Precambrian metasediments, Paleozoic shelf sediments, Precambrian sodic plutons, and to a minor degree from gneisses. In Late Cambrian time the major contributors were sedimentary rocks of the shelf, sodic plutons, and gneisses. By Early Ordovician time most of the sands were derived from Grenvillian gneisses and shelf sedimentary rocks.The relief of the Grenvillian Orogenic Belt was high and denudation rapid from Early Cambrian to Late Cambrian, suggesting continuous, but irregular uplift. Uplift of the shelf began early in Cambrian, with major movements occurring in Late Cambrian when the Grenvillian source was rejuvenated. The source area was stabilized by Early Ordovician time.