Geometrical analysis of ripple-drift cross-lamination

Abstract

The geometrical parameters of ripple-drift cross-lamination (climbing ripples) have been measured in coastal exposures of the Cloridorme Formation (Middle Ordovician turbidites of northern Gaspé Peninsula, Québec), in order to determine the factors that control the angle at which the ripples climb, and to investigate the context of the ripple-drift relative to the other sedimentary features in the Cloridorme Formation. Purely geometrical analysis shows that the angle of the stoss side is more sensitive in controlling the angle of climb than the angle of the lee side, but the single most important factor is the ratio of thickness of the stoss side and lee side laminae. When the lee laminae are about twice as thick as the stoss laminae, the angle of climb is about 30–40 degrees. If the ratio is greater than about 10, the angle of climb is reduced to about 5 degrees.The lamina thicknesses are controlled by the rate of deposition from suspension relative to the rate of bed load movement. At high flow regimes, sediment is swept onto the lee side and the angle of climb is low. In the Cloridorme Formation, the turbidites associated with the ripple-drift are proximal, and hence a rapid flattening of the basin floor gradient is suggested to cause the rapid deposition leading to ripple-drift formation.