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Analyse haute résolution de la transgression holocène dans un secteur subsident du littoral français : l’enregistrement du bassin-marais de St Omer, Pas de Calais, France

Résumé : This study is part of a multidisciplinary palaeo-ecological program (sediment, pollen, chironomids, molluscs and coleoptera) which aims at reconstructing the palaeo-environment and palaeoclimate of the Saint-Omer basin (northwestern France), from the end of the Late Glacial period to the Late Holocene. The present paper focuses on sediment analyses and radiocarbon dating of 22 boreholes drilled through the basin. The dynamics of the Flandrian (Holocene) sea-level rise have been studied: at the end of the late glacial period, the basin functioned as a transition system from a braided (upstream) to a meandering (downstream) river system. During the Preboreal, Boreal and Subboreal periods, the drainage system reverted to a main fluvial channel meandering through a vast, marshy region. At the Boreal-to-Atlantic transition, the beginning of the Calaisian transgression seems to have been slightly earlier than is regionally known. During the Atlantic period, the sea progressively invaded the basin, sometimes interrupted by a calmer phase or a slight regression during which intercalated peat or charophytic sedimentation developed. The marine flooding stopped at the end of the Atlantic and during the Subboreal period. The compaction of sediment and the presence of gypsum in the underlying sediments have shown that a lowering of the water table and the regional sea level probably occurred during the Subboreal. The Subboreal peat layer is overlain by marine sediments of the Dunkerque flooding. Calais I, II (~7700-6000 BP) and Dunkerque 0-I (~3500-2500 BP) are only attributed to a sea-level rise, but the Calais III and Dunkerque II are probably due to storm-flood events. Dunkerque III is poorly represented in sediments, which may be related to the increasing drainage of the marsh by human since the 8 th Century. Neotectonic processes at Saint-Omer probably influenced the Flandrian transgression. The massive incision of Pleistocene sediments; the precocity of the Calaisian at Saint-Omer; and the over-representation of the Dunkerque 0-I, may have been induced by subsidence of the basin. Moreover, a recent reactivation of the flexural complex has probably controlled the location of the main tidal channel through the basin.
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Emmanuel Gandouin, Brigitte van Vliet-Lanoë, Evelyne Franquet, Valérie Andrieu-Ponel, David Keen, et al.. Analyse haute résolution de la transgression holocène dans un secteur subsident du littoral français : l’enregistrement du bassin-marais de St Omer, Pas de Calais, France. Géologie de la France, 2007. ⟨hal-02959270⟩

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