Abstract

The dimensions of low-steepness bed forms found in ancient marine sequences are used to test three fluid mechanical hypotheses. The first is that they are formed under purely oscillatory flows generated by the passage of progressive gravity waves. The second is that hummocky bed forms are similar to supercritical bed forms (such as three-dimensional antidunes) under a purely unidirectional flow. The third hypothesis is that hummocks arc produced by the combined action of oscillatory and unidirectional shear flows, such as gravity waves superimposed on a gradient current related to storm set-up. Preliminary analyses indicate that the first hypothesis is the least likely, based essentially on the incompatibility of observed hummock spacing and possible values of the ratio α/δ under waves (where a is the amplitude of oscillation near the bed and × is the bed form spacing). An origin beneath standing surface waves causing a spatially periodic near-bed mass transport is an alternative. For the second hypothesis, the origin of the density interface that acts as an internal wave is problematical, and the preservation of well defined laminae is inconsistent with an origin under antidunes. The third hypothesis of combined flows is the most probable. However, formation of hummocks under vigorous storm return flows is unlikely since the bed planation threshold would be over-reached even without the enhancement of bottom shear stress due to waves. If hummocky cross-stratification is formed in water depths of up to 80 m (Dott and Bourgeois, 1982), any superimposed waves are most likely to have been moderate in period, more characteristic of waning storm conditions combined with sluggish to moderate storm surge gradient currents. The implication is that the interpretation of hummocky bedding as formed on the shoreface solely by the action of progressive storm waves should be treated with considerable scepticism.