Abstract

Tidal shelf deposits of the Early Triassic, Western Alps, France record the final stages of an Early Triassic marine transgression. The shelf sequences are predominantly composed of cross-bedded quartz-arenitc with laterally persistent planar erosion surfaces and top surface pebble or granule lags. The cross-bedded facies, forming the major component of the sequence (>90%), is dominated by small- to large-scale (0.15 to 1.47 m) sets of planar cross-stratification with normally graded foresees. The grain size and thickness of the graded foresets is proportional to the scale of the cross-bedding. Small- to medium-scale (0.15 to 0.35 m) sets display 0.2 to 0.7 cm thick, graded foresets composed of medium- to fine-grained sandstone. In contrast, the graded foresets of the larger scale sets (>0.50 m) are significantly thicker (1 to 7 cm thick) and composed of granular to medium-grained sandstone. Sets of planar cross-bedding display reactivation surfaces spaced laterally at intervals of 0.20 to 20 m. The graded foresets increase in thickness, grain size and angle of dip away from the reactivation surface. This systematic variation in internal structure is reversed toward the next reactivation surface in the inferred downcurrent direction.

The graded foresets are interpreted as reflecting the migration of superimposed megaripples on sandwaves (large-scale sets) and ripples superimposed on megaripples (small- to medium-scale sets). The variation in the internal structure of the cross-bedding is interpreted as being the product of the passive, active and static stages of tidal bed form development, while 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 present day tidal shelf shear velocities, it is suggested that bed form migration and the development of reactivation surfaces occurred as a long-term response (greater than months) to fluctuations in tidal current velocities enhanced by storm processes.