Diatom assemblages along a former intertidal gradient: A palaeoecological study of a subboreal clay layer (Western coastal plain,Belgium)

Lateral and vertical variations in diatom taphocoenoses were studied in a clayey intercalation in the upper peat bed at Booitshoeke (western coastal plain, Belgium). The palaeoecological evolution, linked to sedimentary facies changes along the ecotone from fen to tidal flat, apparently involved two successive periods of increased local tidal activity. This result could be inferred from a weakly developed salt-marsh horizon within the clay, traceable by diatom analysis over a distance of a few hundred meters. Although influenced by taphonomic processes, the composition of the fossil assemblages appeared to reflect the relative position of the palaeoenvironment along the tidal gradient fairly accurately. Taphonomic groups of taxa could be distinguished, characterizing respectively the transition from fen to salt-marsh, the high salt-marsh, the low salt-marsh and high mud-flat, and the more polyhaline mud-flat. Sediments of drier salt-marsh conditions showed a very poor taphocoenosis, of mainly allochthonous origin, dominated by marine tychoplanktonic species with durable frustules. Some possible causes for the slight increase in marine activity during the mid-Subboreal, which resulted in the formation of the deposits considered here, are discussed.