ABSTRACT: The Record of Sea Level Rise by Tidal Sand Bodies of the English Channel

Serge Berne, Gilles Lericolais, Francois Lafont

Improvements of very high resolution seismic reflection provide new information about internal structures of modern sand bodies. This allows us to reconstruct their recent history, which is related to the Holocene sea level rise. A major distinction is found between inner shelf sand bodies, dominated by autocyclic processes, and outer shelf sand bodies, where allocyclic processes are invoked to explain the apparent contradiction between internal structures and present-day dynamics. On the inner shelf, evidence of the migration of tidal dunes (sand waves) has been obtained by repeated surveys using accurate positioning systems. Major bounding surfaces are thought to result from the action of tidal current and/or from episodic storms. A rough estimation of the age of these and bodies can be proposed. On the outer shelf, some dunes of the English Channel exhibit cross-beds indicative of a past net bed-load transport at the opposite of present days dynamics, inherited from different tidal conditions when sea level was between 20 and 40 m lower. Some large tidal sand banks (e.g., the Sark Bank, near the Channel Islands) display a more complicated pattern. The upper part fo the sand bank is the result of the migration of very large dunes climbing at positive angles, whereas the lower part shows major erosional surfaces, attributed to the action of storms during lower sea levels.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990