Tidal crevasse splays as the cause of rapid changes in the rate of aggradation in the Holocene tidal deposits of the Belgian Coastal Plain

Detailed stratigraphic and sedimentologic work combined with high-resolutio n radiocarbon dating in an exposure of an upto 12 m thick Holocene successi on near the town of Veurne in West Belgium has shown that filling of the co astal plain at continuous, but decelerating sea-level rise, occurs less smo othly than is usually assumed. Three tidal sequences are distinguished, sep arated by peat layers, the second peat and the surface peat, respectively. The second peat developed at about 6800 cal BP, after the rate of sea-level rise dropped from 7 m/ka to about 2.5 m/ka, indicating that sediment suppl y outran relative sea-level rise. In the excavation, the second peat drowns about 100 yr after it started and is covered by a thin veneer of inter- an d supratidal beds, after which low intertidal to subtidal conditions prevai led for more than 1000 pr with no deposition. These 1000 yr stand for a gap of about 2 m sea-level rise. This gap is filled almost instantaneously by an ebb-dominated crevasse splay deposit, in which the daily rhythm of ebb a nd flood can be recognised in the sediments of the lower part and the fortn ightly spring and neap tide alternation in the upper part. Crevassing is al so considered important by the eventual flooding of the surface peat which is covered by about 1 m of inter- and supratidal deposits. Chaotic and cata strophic crevasse-splay channels in the surface peat are the precursors of the eventual flooding of the peat. Based on the observations in the excavat ion it is concluded that the current models of Holocene infill of coastal p lains based on cores: often promote a gradualness which in reality dots not exist. (C) 1999 INQUA/Elsevier Science Ltd. All rights reserved.