Abstract

Only a few examples of ancient tide-dominated shelf sand bodies are known in the geological record. One of the reasons for this is that these deposits are rarely preserved. Sand bodies primarily formed by tidal currents generally take the form of a thin veneer on the shelf and are modified either by wave action or by the prograding shoreline system, in the case of a regressive megasequence. The best circumstances for the preservation of tidal sand bodies on the shelf is sedimentation during a progressive rise in sea level or constant basin subsidence. In addition, most of the ancient tidal sand bodies can be found within the inner shelf; the outer shelf usually consists mainly of fine-grained elastics or highly modified sand bodies. An example from the Lower Tertiary Roda Sandstone in Spain shows the change in sand body geometry and the sequential build up from the estuary mouth to the inner shelf. Evidence of tidal action can mostly be found within the estuary mouth and the adjacent ebb tidal delta. Sand bodies on the inner shelf, however, show less evidence of tidal processes. Products derived from modifying processes, such as wave action, are dominant.

The occurrences of tidal sand banks and sand waves on present day tide-dominated shelves have only been supported by a few examples from ancient analogues. After a detailed facies analysis, most of the ancient examples were found to have been formed in more nearshore or even inshore environments. A hypothetical geometrical and sequential model constructed from data of recent offshore sand waves and ancient analogues will be presented.