Abstract

Recent studies of intertidal and shallow subtidal bed forms have led to an increased understanding of the mechanics of bed form migration in tidal environments. These studies suggest that a careful examination of ancient tidal deposits in both a lateral and vertical sense may provide a better understanding of the variability of processes related to short (diurnal/semidiumal) and long term (neap/spring) tidal cycles. How might these studies be extended to the interpretation of tidal structures observed in ancient shallow marine sequences? Within the Lower Trias of the Western Alps, France, tidal shelf deposits record the final stages of an Early Triassic marine transgression. The shelf sequences are dominated by cosets of cross-bedded quartz-arenite sandstone, separated by planar erosion surfaces overlying top-surface lags. Individual cross-sets, 0.15 to 1.47 m thick, show avalanche foresets composed of normally graded units of medium- to granule-grade grains. Sets are separated by reactivation surfaces spaced at intervals of 0.2 to 20 m. Between reactivation surfaces, graded foresets show a cyclic variation in thickness, grain size and angle of repose. The graded foresets are considered to reflect the avalanching of sediment previously sorted by smaller bed forms (ripples or megaripples) superimposed on the stoss side of a major bed form structure (megaripples or sand waves). The variation in foreset inclination is interpreted as the passive, active and static stages of bed form development, whilst the cyclicity in thickness and grain size of individual graded foresets may reflect the changing scale of superimposed bed forms in response to long-term tidal flow unsteadiness. From calculations of sand wave migration rates, assuming shear velocities of present day tidal shelves, it appears that bed form migration within the Trias shelf system was a response to long term (greater than months) fluctuations in tidal current velocities, enhanced by storm processes.