Abstract

Regional investigations of bed forms in relation to peak tidal current strength has allowed the distinction between two major offshore tidal sand facies — the relatively higher energy offshore tidal sandbank facies and the relatively lower energy offshore tidal sand sheet facies. Lack of available sand, however, will lead to the development of a gravel sheet facies in areas where the sandbank facies might otherwise exist. With the exception of the rippled outer margin of the offshore tidal sand sheet and some shallow sandbank crests, both offshore tidal sand facies are generally covered in sand waves. The asymmetry of these sand waves is normally governed by the tidal current maximum bottom stress vector, indicating that although storm-induced currents can exert an influence on their development, the tidal currents play the dominant role. The resulting deposits of both facies (in their active, as opposed to waning phase) are expected to be pervasively cross-stratified (unless the structures are destroyed by burrowing organisms). This could contrast with deposits of other largely storm-generated sand sheet and sand ridge facies. Because the offshore tidal sandbank facies is maintained by relatively stronger tidal currents than the offshore tidal sand sheet facies, it is possible to predict that, where the sandbank facies is identified in ancient deposits, then an associated sand sheet facies should be suspected, both laterally and in the vertical succession. In contrast, where an ancient offshore tidal sand sheet is identified, an association in space and time with the sandbank facies is not a necessary condition. The identification of ancient nearshore and/or tidal flat deposits need not necessarily imply extensive offshore tidal sand deposits, but these should at least be suspected.