On modern and ancient continental shelves, sediments are composed mostly of silts and clays carried in suspension long distances from the shoreline. Sand occurs only where it is being supplied by a specific bottom-transport shelf process, or where it remains as palimpsest material deposited under earlier sedimentary regimes. For modern shelves, sands distributed and being moved, even at the outer shelf edges at depths of 600 feet, were originally placed on the shelf by fluvial and nearshore processes operating over the shelf during low sea level stands of the Pleistocene (Tillman and Siemers, 1984). A premise is thus established for the discussion of shelf sedimentation and the distribution of shelf sands: that the distinction must be made between how the sand was originally placed on the shelf and how it was last reworked in the shelf environment. Making this distinction involves recognizing the influence of both intrabasin tectonics and regional and global sea level fluctuations. The combination of structurally controlled sea floor topography and fluctuating sea levels has a major effect on depositional systems, particularly in cratonic basins. Shelf sequences are a complex record of such interplay through time.

Shelf sands may, or may not, be shoreline connected. Generally, they are deposited beyond the influence of shoreline zone processes, and, therefore, in the ancient record are enclosed in marine shales. Although the facies of shelf sequences have many similarities to shoreface or delta front sequences of the shoreline zone, a separate terminology such as the one used in this section (Fig. 7.1) is required to emphasize the absence of shoreline influence. Size, geometry, bathymetry and orientation of discreet shelf sandstone deposits are dependent upon the specific shelf processes that formed the deposit, such as tidal currents (e.g. North Sea sand ridges), and storm-generated currents (e.g. storm bars of mid-Atlantic shelf).