Abstract

On modern non-tidal shelves, where there is active fluvial sediment supply, the sand-dominated zone is generally only a few tens of kilometres wide (e.g. shelves north of the Columbia River and west of the Rhone). Wider, sandy shelves have negligible fluvial supply (e.g. eastern U.S.A. shelf, Spanish Saharan shelf). If there is sediment supply and a stable shoreline position, a narrow sand accumulation is expected, and if there is negligible supply, reworking of previously deposited sands results. The emplacement of sand on wide modern shelves beyond the present coastal zone appears to have been at times of lower sea level. Absence of mudstone from broad tracts of sandstone requires fluctuations of shoreline position in this scenario. (Either the mudstone was deposited but was then removed by reworking as sea level fell, or it was not deposited because rate of sediment input was low. In the latter case, formation of the sand accumulation required migration of the coastal zone where sand is deposited.) Sand deposits are formed in the coastal (including braided stream plain) and shallow marine (<40 m) zones which migrate in response to relative changes of sea level. The bed forms created in this zone are often ridges oblique to the shoreline. In areas where waves and wind-driven currents are opposed (but tidal currents are weak) shoreface-connected ridges are formed and left behind by a rise in sea level (e.g. eastern U.S.A. shelf, Argentine shelf). On tidal shelves where currents are strong (> 1 m/s) oblique ridges form part of systems of parabolic sand banks, which give place to parallel linear sand banks farther offshore. It is proposed that the parallel linear sand banks grow from the parabolic banks which are left offshore during a rise in sea level. Thus the formation of some large parallel sand banks is seen as a consequence of a rise in sea level. (There are other parallel sand banks formed in embayments and on shelf edges where different mechanisms prevail.) Tidal shelves with weaker currents (0.6 to 0.9 m/s) display sand waves which, in the best documented case (the southern North Sea) are reworking a sand sheet emplaced at low sea level.