Cap des Rosiers Formation at Grosses Roches, Quebec—deposits of the mid-fan region on an Ordovician submarine fan

Abstract

Resedimented deposits in the Ordovician Cap des Rosiers Formation near Grosses Roches, Quebec, are arranged in fining-upwards sequences, each with conglomerate at the base, and argillaceous sandstones, or interbedded sandstones and shales at the top. The conglomerates are primarily cobble-grade, and are poorly sorted; graded bedding and clast imbrication are poorly developed, and internal stratification is uncommon. They pass upwards and laterally to pebbly sandstones and sandstones, most of which are massive, but which contain cross-stratification, parallel stratification, or dish structure in places. The sandstones are overlain by finer-grained sandstones with many load-deformation and water-escape structures and at the top of some sequences there are sandstone and siltstone turbidites. The presence of large sediment slabs and soft clasts in the conglomerates and sandstones, and of the abundant shale chips in the argillaceous sandstones, provides evidence of penecontemporaneous erosion. Load casts, flame structures, dish structures, and other water-escape structures indicate relatively high rates of deposition. At the base of the section, 10–12 m of conglomerate and sandstone beds in the lowermost sequence onlap against the eroded pelites of a channel margin. The exposed bases of four other fining-upwards sequences are erosional, but no onlapping relationships are observable, and the eroded sediments are coarser grained than the pelites at the very base of the section.The sequences are considered to represent the back-filling of a channel-mouth in the mid-fan region of a submarine fan, and subsequent accumulation of fining-upwards lobes beyond the front of a principal channel which continued to supply sediment. The superposition of conglomerate-bearing fining-upwards sequences is interpreted as a result of fluctuations in supply from a tectonically active source.