Hummocky cross-stratification and megaripples; a geological double standard?

Abstract Hummocky cross-stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits. Megaripples of 2-5 m spacing are typical responses of the inner Atlantic Shelf of North America to storm flows. Side-scan sonar records have shown that megaripples on the inner shelf are commonly hummocky (oval to circular in plan view) and lack asymmetry. The megaripples are produced by combined-flow storm currents. These are alongcoast geostrophic flows with a mean flow component of 20-60 cm sec (super -1) . The bottom boundary layer of such flows is characterized by dense sediment suspensions (40-60 mgl 1 ) and is highly turbulent due to the interaction of the mean flow and wave orbital current components (eddy diffusivity up to 200 cm 2 sec (super -1) at 40 cm above the bed). Sand transport in this coastal boundary zone is obliquely offshore, but neither the density of the sediment suspension nor "storm-surge ebb" appear to be important driving forces. Combined-flow currents are the typical response of most modern shelves and epicontinental seas to storms and presumably were so in the past. Hummocky cross-stratified beds are generally too thick (20-80) cm) to be accounted for by in-situ resuspension; they are more likely to have been deposited by combined-flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm's duration. Evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime. Measurements on the Atlantic Shelf likewise suggest a transitionlike regime, whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes, but more intense turbulence and a higher suspended sand concentration. The lack of normal cross-bedding in both modern and ancient hummocky bedforms indicates that these features did not migrate. The characteristic may be a consequence of the complex trajectories followed by near-bottom water particles in response to combined-flow currents.--Modified journal abstract.