Abstract

Clinoforms are classical geological features. Examples in the rock record or in seismic reflection data that originally formed in shallow water can potentially help to locate paleo–sea level, but their precise links with sea level need to be established using modern examples before such inferences can be made accurately. Some near-shore areas of high wave energy and steep coasts host sand bodies with upwards-convex surfaces. These clinoforms have been suggested to have been created from sand exported from the upper shoreface during stormy conditions, and deposited at depths where wave action diminishes below the threshold of motion (Field and Roy 1984; Hernandez-Molina et al. 2000). We have compiled the shapes of these bodies in a variety of wave environments in order to test this idea. The interpreted depths at which the sand surface steepens (i.e., the clinoform rollover) vary from 21 m off southern California to deeper than 50 m off SE Australia. To assess the link with wave action, rollover depths were compared with adjacent deep-water wave properties over the period 1992–2001 in the ERA-40 reanalysis dataset. Rollover bed shear stresses were computed from properties representing average wave conditions and from properties associated with upper–percentile conditions. Stresses were found to reach 0.1–0.5 Pa during upper 5-percentile wave conditions, exceeding the sediment threshold of motion estimated from reported grain-size data. Sediment to below the rollover is therefore mobile during extreme wave conditions.