Backset bedding in turbidites; a further example from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec

Abstract

Abstract "Backset bedding" was found in a thick (144 cm) turbidite bed in the Cloridorme Formation, Gaspe Peninsula, Quebec. The "backsets" occur in coarse-medium sand below a train of three trochoidal profiles (wavelength 30 cm, amplitude 4 cm) and above a basal division (9-13 cm thick) of parallel lamination. Flute casts on the sole of the bed, and ripple-drift cross-lamination--forming the preservation blanket covering the trochoidal profiles and "backsets"--allow paleocurrent direction measurements. The "backset bedding" is interpreted to result from the upstream migration of antidunes. A suitable analysis for the formation of antidunes by density currents uses an equation appropriate for internal waves at a density interface between two superimposed fluids which are moving parallel and in the same direction.