Hummocky cross-stratification is not produced purely under progressive gravity waves

Abstract

Hummocky cross-stratification is a type of bedding in sedimentary rocks caused by the draping of sand around low-steepness three-dimensional bedforms. This distinctive form of stratification has been thought to be diagnostic of the former action of storm waves and hence its occurrence has been used to reconstruct ancient palaeogeographies and marine processes. Here I examine theoretical and empirical aspects of hummocky cross-stratification pertaining to its origin and argue that the notion that it was produced purely under progressive storm waves is incompatible with observed hummock spacings and possible values of the ratio a/λ under waves (a is the amplitude of oscillation near the bed and λ is the bedform spacing). My view is supported by recent observations on naturally-occurring hummocky megaripples from the sea floor of the inner Atlantic Shelf of North America.