Distribution and chronology of Pleistocene permafrost features in France: Database and first results

Abstract

International audienceNumerous periglacial features (polygons, nets, soil stripes, ice-wedge pseudomorphs and sand-wedge casts, involutions) have been recorded in France by examining bibliographical sources and aerial photographs. These data show that a large part of France was affected by permafrost during the Pleistocene and only the southern Aquitaine Basin and Languedoc seem to have been beyond its maximum extent. The first OSL ages obtained from the aeolian infill of wedge structures indicate that at least two phases of thermal contraction cracking occurred in southwestern France between ∼25 and 36 ka. Chronostratigraphical data from loess in northern France indicate that these episodes correspond to the formation of ice-wedge networks associated with tundra gleys. In the latter region, two additional permafrost episodes probably occurred during the Last Glacial, the older one corresponding to the end of Marine Isotope Stage (MIS) 4 around 60 ka and the more recent one to MIS 2 around 19-16 ka. Although stratigraphical data indicate that these episodes were relatively short (about one millennium), relict permafrost may have existed for longer periods in northern France

The Quaternary coversands of southwest France

“…In our study area, two mechanisms can be reasonably assumed: (1) processes associated with freezing and thawing of the sediments, such as differential frost heave (Sharp, 1942;Dylikowa, 1961;Pissart, 1976;Corte, 1977;Washburn, 1979;Van Vliet-Lano€ e, 1985 and periglacial load casting (Vandenberghe and Van Broek, 1982;Murton and French, 1993); (2) earthquakeinduced processes such as liquefaction and fluidization (Reineck and Singh, 1975;Obermeier, 1996;Van Vliet-Lano€ e et al, 2004;Moretti, 2008, 2011;Plaziat et al, 2009). A periglacial origin for most of the deformations observed seems relevant insofar as periglacial conditions are well-documented in southwest France during the Last Glacial (Bertran et al, 2014). Comparable conditions certainly prevailed during earlier glacial cycles, also characterized by roughly similar climatic deterioration events (Masson-Delmotte et al, 2010).…”

“…Abundant frost cracks have been found in the periphery of the coversands, and have been recently studied by Lenoble et al (2012) and Bertran et al (2014). Three main types of cracks were recognized: epigenetic sand wedges, composite wedges, and small cracks without identifiable infill.…”

“…No evidence for sand accumulation between 40 and 25 ka has been found, except in sand wedges (Bertran et al, 2014;Sitzia, 2014). The main period for Weichselian coversand emplacement ranges between 24 and 14 ka, which yielded 19 dates from six sites (Table 1).…”

“…Indeed, data from Bertran et al (2014) indicated that southwest France was affected by deep seasonal frozen ground and discontinuous permafrost conditions during the coldest Weichselian stadials. The effect of permafrost and the associated poorly drained active layer on sand availability has already been considered as a the key explanatory factor for the absence of significant coversand prior to ca.…”

“…The drainage of meltwater from snow patches was limited by residual permafrost lenses and this promoted waterlogging and the development of reducing conditions in low-lying areas. Available data in southwest France indicate the possibility of discontinuous permafrost episodes until about 21 ka (Bertran et al, 2014).…”