Abstract

Tidal sand bank and sand sheet facies have distinct hydrodynamic settings. The former is associated with mean spring near-surface peak tidal currents of 90 cm/s or more, and the latter with near-surface peak tidal currents ranging between about 50 cm/s and 90 cm/s (corresponding to about 30 cm/s and 55 cm/s at 1 m above the bottom in water 30 m deep. In both instances, the field of existence and asymmetry of sand waves are influenced more by tidal, rather than storm-induced, currents. The bulk of the deposits of both facies are typified by pervasive “normal” cross-stratification. This contrasts with the hummocky cross-stratification or storm-graded bedding of storm-generated sand ridge and sheet facies. A morphological distinction can be drawn between offshore tidal sand banks and storm-generated sand ridges, which also develop in response to differing flow regimes. As deposits they should also be distinguishable, although some moribund tidal sand banks under a waning tidal regime retain a core of normally cross-stratified sands overlain by a reworked layer of variable thickness characterized by storm-influenced deposits. The following gradient of decreasing hydrodynamic intensity is evident in continental shelf sand facies: tidal sand bank → tidal sand sheet → storm-generated ridge → storm sand sheet → shelf mud sheet facies Lateral and vertical transitions between these facies are to be expected in abundance in the stratigraphic record.