Geological Record of Littoral Sedimentary Processes at Short Time Scales

Abstract

Location of the shoreline, nature and structure of sedimentary units and other changes in the coastal zone are often attributed to absolute or relative sea-level variations. Tectonics and sediment supply provide secondary factors explaining different systems tracts, at a 10⁶ yr time span and a 10-10² km scale. Recent changes similarly organized sedimentary units at shorter time (10¹-10³ yr) and space (1-10 km) scales as demonstrated by various prodeltas and series of beach barriers of the Rhone delta (France). In the Saloum Delta (Senegal), a succession of beach-ridge sequences also occurred resulting from alternation between erosional processes and lateral construction of sand barriers. In both regions, greater importance must be given to fluctuations in the volume of sediment input and the hydrodynamical and hydrological conditions of both marine and fluvial environments. Examples given by these study areas allow discussion of four points: (1) the possibility for the short-term clastic depositional sequences to be described by sequence stratigraphic terminology; (2) the degree of accuracy which could be expected to define sequence changes at short time scales (0-200 yr); (3) the possibility for rapid changes of sedimentary conditions to be recorded in geological sequences; and (4) the part in recent coastal changes which can be attributed to relative eustatic variations as opposed to sediment-budget changes.