Transport of Conglomerate into Deep Water: A Study of the Cambro-Ordovician Cap Enragé Conglomerate at St. Simon de Rimouski, Québec

Abstract

The Cambro-Ordovician sequence at St. Simon, Québec, was divided informally into ten horizons by Mathey (1970). The most prominent lithologies within the horizons are pelites, feldspathic sandstones and petromict conglomerates. One horizon 50 metres thick of feldspathic sandstones and conglomerates, described in this study, consists of three large fining upward sequences·. The fining upward sequences are defined by the occurrence of five facies. These facies are: poorly sorted coarse conglomerates, well sorted coarse conglomerates, medium conglomerates with scattered pebbles and boulders, fine conglomerate with scattered pebbles and boulders and coarse sandstones. The base of each sequence is characterized by the occurrence of coarse conglomerates; the top is characterized by the occurrence of fine conglomerates and coarse sandstones. Rarely do the coarse conglomerates grade into thick developments of medium and fine conglomerates. The fine conglomerates grade in places into coarse sandstones, although generally the coarse sandstones have sharp bases. The conglomerates display sharp bases, normal and inverse grading, grain imbrication and orientation. The long axes of the grains, which define the orientation are parallel to, and not transverse to the flow direction suggested by the imbrication. To produce these features it is suggested that turbulence and dispersive pressures were operative within the flow. If the pebbles had moved as bed load material, they would have come to rest with their long axes transverse and not parallel to the flow direction suggested by the imbrication. The term "fluxoturbidite" (Dzulynski et al., 1959) has been applied to some coarse grained deposits in geosynclinal sequences. The characteristic features of "fluxoturbidites" are their unusually coarse grain size, thick irregular bedding with associated slump structures and poorly developed grading. The differences between the conglomerates described in this study and the typical "fluxoturbidite" preclude the use of this term to describe the deposits described by the author. It is suggested that the conglomerates were deposited upon a submarine fan complex by currents which flowed parallel to the present tectonic axes.