ABSTRACT

Hummocky cross-stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits. The geometry of the cross-strata sets implies a low-relief oval bedform of a few meters spacing; orientation is poorly defined or random. Hummocky cross-strata sets have been attributed to the action of strong storm-wave surges.

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. Box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure. The megaripples am produced by combined-flow storm currents. These are alongcoast geostrophic flows with a mean flow component of 20-60 cm sec^-1. A high-frequency wave orbital current of similar or weaker intensity is present, but its role in megaripple formation is subordinate to that of the mean flow. The high-velocity coastal margin of the flow is often jetlike and downwelling in nature, and is characterize by a down-current decrease in sand flux and rapid deposition of sand. 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 cm² sect 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.

It is difficult to account for either modern hummocky megaripples or ancient hummocky cross-stratification as responses to purely oscillatory flows. Combined-flow currents are the typical response of mostmodern 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 despoited 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 likewi e 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 bothmodern and ancient hummocky bedforms indicates that these features did not migrate. This characteristic may be a consequence of the complex trajectories followed by near-bottom water particles in response to combined-flow currents.