Abstract

Although hummocky cross-stratification (HCS) is one of the most common and widely recognized structures in ancient storm-dominated successions, the stratigraphic variability and environmental significance of HCS wavelength (λ) are still not widely appreciated. New evidence from an open-coast intertidal flat where HCS might not have been expected to occur shows that the HCS becomes smaller in a landward direction because of a decrease of wave size. This confirms previous suggestions that the bedform responsible for HCS is a type of orbital ripple. A review of new and previously published data indicate that HCS wavelength is controlled by the bottom orbital diameter (d₀) according to the relationship λ 0.75 d₀. These observations imply that the maximum size of HCS should increase with decreasing water depth from the shelf to the surf zone (breaking point) but may then decrease landward of this point because wave size is depth limited. This suggests that it may be possible to use HCS size in paleo-environmental reconstructions to a greater degree than previously.