HOLOCENE EVOLUTION OF RELATIVE SEA-LEVEL AND LOCAL MEAN HIGH WATER SPRING TIDES IN BELGIUM - A FIRST ASSESSMENT

Holocene age-depth data from the Belgian coastal plain, basal peat data particularly, are assessed in terms of local water and tide levels. Basal peat formation was generally induced by sea-level rise; effects of local seepage were limited and significant river-gradient effects were absent. No relation was found between the timing of basal peat inception and substrate permeability. In general, edaphic (moisture) conditions were unfavourable for organic accumulation prior to sea-level related rise of the groundwater table. Error envelopes for the minimum level of local mean high water spring tides and for upper and extreme lower relative mean sea-level limits during the last 9400 solar years (8400 yrs B.P.) are presented. These indicate that the rate of relative sea-level rise showed marked retardations at ca. 7500-7000 yrs cal B.P. (+/-6650-6100 yrs B.P.) and ca. 5500-5000 yrs cal B.P. (4400 yrs B.P.). A eustatic explanation appears to be plausible, particularly for the former. Prior to ca. 4000 yrs cal B.P. (ca. 3600 yrs B.P.), the relative sea-level rise in Belgium differs noticeably from that established for the western Netherlands, with both diverging progressively back in time. This implies that considerable differential crustal movement occurred between the two areas in the early Holocene.