|Mineralogical composition and origin of marine clays in the Poitou coastal plain (France)|
Nijs, R. (1975). Mineralogical composition and origin of marine clays in the Poitou coastal plain (France). Pedologie 25(2): 126-133
In: Pedologie: Bulletin van de Belgische Bodemkundige Vereniging = Pédologie: Bulletin de la Société belge de pédologie. Belgische Bodemkundige Vereniging = Société belge de Pédologie: Gent. ISSN 0079-0419, more
Marine geology; Mineral composition; France, Charente-Maritime, Marais Poitevin; Marine
The mineralogical composition of a well-chosen set of Holocene marine clay samples from the Marais Poitevin, tbe most important coastal marsh of France, consists of well-crystallized illite and kaolinite, and an important amount of beidellite. Chlorite, vermiculite and other interstratified clay minerals only occur in small amount.There is a clear qualitative parenthood between this composition and that of the recent alluvium of the local rivers, which makes a far origin of the marine clay superfluous. Quantitatively however, the alluvium shows a lower illite content and a dominance of (mostly interstratified) illite weathering products. As a matter of fact, the mineralogical composition of the alluvium has been inherited from the soils of the region (developed in Jurassic marls), characterized by pedogenetical degradation of illite into open illite, vermiculite, beidellite ánd a large range of interstratified minerals. In marine environment agradation into illite of the alluvial minerals, in which the illite framework was still rather well preserved, probably took place; hence the marine clay shows a lower content of interstratified minerals and a better crystallinity . Finally, an important part of the enormous “bri”-masses must consist of remobilized sediments stored close to the shore on he continental shelf and accumulated there as a detritical product of Jurassic marls during the pleistocene periods with a low sea-level and characterized by strong erosion on the land.Consequently, the origin of the bulk of the marine clay or “bri” in the Poitou coastal plain may be considered as (merely undirectly) continental and local.