COASTAL CHANGES WITHOUT SEA-LEVEL FLUCTUATIONS. EXAMPLES FROM A TIDE-DOMINATED COASTAL LOWLAND (BELGIUM)

Facies changes registered in the coastal Holocene sedimentary sequence are far too often attributed solely to sea-level fluctuations. Detailed stratigraphic and sedimentological investigation of cores and outcrops from the western Belgian coastal plain, supported by numerous radiocarbon assays, demonstrate that other controlling factors such as sediment supply and accommodation space can play a dominant role in the development of the vertical sedimentary succession.

The particular characteristics of sedimentation processes in which tidal channels play a major role, should also be examined in order to detect the origin of successive coastal changes which might be recorded as distinct facies changes in the vertical sedimentary sequence. It is also essential to relate the sites studied on a local and regional scale in the same tidal basin (or estuary) before one can be conclusive about the relative importance of the controlling factor(s) that determined the change in the pattern of the sedimentary environments at a particular time.

A comparison of sites in different locations and showing a different stratigraphy, but formed in the same time span, will be discussed in relation to the sea-level curve. An example will be shown whereby in a period of decelerated relative sea-level rise, but characterized by a considerable supply of sediment, rapid facies changes registered as frequent alternations of mud, peat and gyttja are produced. These facies changes are determined by a sedimentological control related to the tidal channel and creek network, and not by sea-level fluctuations.

Another example will show the relative importance of accommodation space in Late Holocene deposits. No accommodation space resulted in deep vertical incisions of the tidal channels, while an increase in accommodation space forced the tidal channels to migrate laterally. These sedimentary processes resulted in distinct facies changes in the sedimentary sequence, but do not reflect a particular sea-level fluctuation.