WHAT DO WE KNOW ABOUT MARINE SAND WAVES? A REVIEW OF THEIR OCCURRENCE, MORPHOLOGY AND STRUCTURE

A review of the published literature on marine sand waves from thirty-nine localities around the world reveals that these flow transverse bedforms are abundant in high-energy nearshore and shallow marine tidal environments. They display a range of morphologies from straight-crested forms to sinuous or branching ridges to barchan-like dunes. Most consist of quartz sand but they may also contain abundant biogenic material and/or gravel. Reported crest heights range from about 1 to 15 m, although some are even larger. Wavelengths typically vary between 100 and 500 m. Sand waves may develop asymmetrical profiles and actively migrate in localities where the flood tide is stronger than the ebb tide, or vice versa, or where residual currents dominate. Lee slope angles are typically <20^o but have been reported to reach >30^o. The primary internal architecture, predicted by theoretical models and confirmed by limited field data, consists of various forms of cross-stratification. Sand waves are often one component of a hierarchy of superimposed bedforms, with ripples on megaripples on sand waves on sand ridges. The most important factors promoting the formation of sand waves appear to be an abundant supply of sand and strong unidirectional or tidal currents. They most often develop where the prevalent sediment grain size ranges from 0.25 to 0.5 mm (2.0-1.0 phi), and are absent where mud or silt comprises more than about 10-15% of the bottom sediment. Most sand waves occur in water less than 100 m deep, such as those distributed over large areas of the Southern Bight of the North Sea, although much greater water depths are occasionally recorded. Sand waves provide a useful model for understanding the origin of many cross-bedded sandstone sequences in the stratigraphic record. Nevertheless, given the abundance of sand waves on modern shallow marine shelves and in other tidally dominated environments, it is puzzling that so few sand wave complexes have been identified in ancient rocks. This raises the possibility that some sand wave deposits have been misidentified as the products of other sedimentary environments.