Holocene depositional history and profile type mapping of the central part of the Ijzer paleovalley (Belgian coastal plain)
Bertrand, S. (2002). Holocene depositional history and profile type mapping of the central part of the Ijzer paleovalley (Belgian coastal plain). Geol. Belg. 5(1-2): 62 In: Geologica Belgica. Geologica Belgica: Brussels . ISSN 1374-8505; e-ISSN 2034-1954, more |  |
| Abstract | The aim of this study was to map Holocene deposits and to investigate the evolution of the sedimentary environments and the depositional history during the post-glacial transgression in a part of the Belgian coastal plain. The central part of the IJzer palaeovalley was selected and, during the summer of 2000, 78 new hand-augered gouge cores were made, completing the database of the Belgian Geological Survey. The method of profile type mapping worked out by Streif (1978) was applied. First of all a general profile type map was made, delineating the major differences in the Holocene sequence, such as: presence or absence of basal peat, areas where the sequence consists of an alteration of peat beds and mud, or entirely of clastic deposits. This map does not show yet the details of the different facies, but give a clear overview of the Holocene geological setting. The detailed or special profile type map shows the different facies of the subsoil, organised in 19 special profile types. Such a map allows a three dimensions view as well as the understanding of the re1ationship between the different sedimentological units. Because the profile type maps do not reveal any information about the thickness of the Holocene sequence, an isohypse map of the pre-Holocene deposits was made, based on numerous boreholes reaching the Holocene/ Pleistocene boundary. It clearly shows the existence of two palaeovalleys former to the Holocene transgression and a network of depressions caused by vertical erosion of the tidal channels during the Late Holocene. These three maps permit the understanding of the different sedimentary facies depending on the pre-Holocene morphology. Moreover, these maps, used here in a scientific aim, can have a practical use in geotechnics. Indeed, field stability in a coastal plain is directly in relation with the thickness and nature of soft surface sediments. Three selected boreholes have been sampled for mineralogical and grain size analyses. The results prove a constancy in the source of Holocene clay sediments and the impact of peat beds on the nature of the underlying sediments. They confirm the non-existence of different transgressive phases in the Late Holocene, a strongly discussed subject in the Belgian literature. Grain size analyses provide granulometric characterization of the sedimentary environments defined macroscopically. The last part deals with the Holocene depositional history of the studied area. It is based on the interpretation of the boreholes correlated in cross-sections as well as on the results from the mapping, together with radiocarbon datings. The depositional history shows that the environmental evolution is in relation with the rate of relative sea-level (RSL) rise (according to the Belgian RSL curve worked out by Denys and Baeteman, 1995), the morphology of the pre-Holocene deposits and the position of tidal channels. The rapid RSL rise prior to ca. 7800 cal. BP allowed the deposition of a thick clastic sediment body in the deepest part of the palaeovalleys on the basal peat. A first retardation in the rate of RSL rise at ca. 7800 cal. BP resulted in an emersion of the sedimentary environments, however, temporarily and not always at the same location, leading to the formation of intercalated peat beds in the mud. This typical alteration of peat beds and mud is not caused by regressive phases but is a function of the position of the tidal channels. The second retardation in the rate of RSL rise at ca. 6000 cal. BP caused the further infilling of the area, evolving into a freshwater swamp. The accumulation of peat lasted for about 3000 years resulting in a two meters thick peat bed. Finally as from ca. 2500 cal. BP, tidal conditions were again installed in the study area. During this time, the plain was characterized by the development of tidal channels eroding the underlying deposits and causing the deposition of a more than one meter thick mud layer allover the coastal plain while channels were tilled by sand. As theoretically discussed by Allen (2000), sediments auto compaction is briefly described. Finally, the impact of reclamation on the uppermost sediments leading to geochemical modifications is analysed. |
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