Periglacial morphogenesis in the Paris Basin: insight from geophysical survey and consequences for the fate of soil pollution.
Résumé
Geophysical survey by of the Pierrelaye-Bessancourt area revealed conductive polygon patterns of 20-30 m diameter detected between 0.5 and 1.7 m depth. The patterns are formed by greenish glauconite and carbonated sand hollows where clay-rich pedological horizons bend downward, forming narrow tongs extending up to 2-3 m depth. Such structures were interpreted as a buried polygonal ice-wedge network. Geometrical relationships between the lithological units allowed the identification of successive landscape events and a landscape chronology. The sequence started during the Saalian glaciation with (1) development of patterned grounds by thermokarstic cryoturbation; (2) consecutive deflation/erosion during post-permafrost aridity; (3) loess and eolian sand deposits; (4) weathering of the former deposits with development of pedogenic horizons during the Eemian interglacial; (5) recurrent cryoturbation and thermal cracking leading to infolding of the pedogenic horizons during the Pleniglacial optimum (Weichselian); (5) finally erosion that levelled the periglacial microreliefs, leading to the modern landscape. In this agricultural area, urban waste water has been spread and has led to high levels of metal pollution in the surface horizons of the soils. The polygonal cryogenic structures have major impacts on soil hydrology and dispersion/distribution of heavy metals toward the geological substrate.
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