Quaternary thermokarst and thermal erosion features in northern France: origin and palaeoenvironments |
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| Authors: | Brigitte Van Vliet‐Lanoë Jacques Brulhet Philippe Combes Cédric Duvail Frédéric Ego Stéphane Baize Isabelle Cojan |
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| Institution: | 1. Domaines Océaniques Laboratory, UMR 6538 UBO‐CNRS, IUEM, Plouzane, 29280, France;2. Geology & Surface Environments, Andra, Chatenay‐Malabry Cedex, France;3. Geology, Tectonic Environment & Risks, GEOTER SAS – FUGRO Group, Clapiers, France;4. Seismic Hazard Division, IRSN, Fontenay‐aux‐Roses Cedex, France;5. PSL Geosciences, MINES ParisTech, Fontainebleau, France |
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| Abstract: | A thermokarst is a collapse feature resulting from the thawing of ice‐rich permafrost or of massive ice of various origins. Little attention has been paid to the sedimentary fabric resulting from this type of collapse, except for glaciotectonic features. In western Europe, two palaeo‐forms are commonly studied: lithalsas and ice‐wedge casts. Collapsed pingos are much rarer. Very few papers have compiled present‐day and fossil data. Here, field data collected from quarries in the eastern Paris Basin were analysed, providing useful records of thermokarst collapses in alluvial calcareous silts, sands, and gravels. These forms have a circular shape when viewed on satellite images. Permafrost is attested regionally by the recurrent occurrence of meter‐sized pattern grounds at the surface of the chalk and of ice‐wedge casts. Traces of segregation and reticulate ice are common. These features are primarily connected to a major interstadial, c. 150 ka BP, orbitally forced and commonly associated with a major glacial retreat. They occur both in drained and waterlogged situations, resulting in a specific pattern of deformation. They are controlled by the brittle and plastic behaviour of sediments and resemble passive glaciotectonism. Normal and reverse faults, with the offset decreasing downward, are common, and those with local shear are reported. Lithalsas, seasonal frost blisters, spring frost blisters and perhaps pingos seem to have formed. Most of these deformations correspond to thermokarst sinkholes bordered by gravitational collapse faults. The offset of these faults increases towards the surface, and the faults have been recurrently confused with neotectonism triggered by palaeo‐earthquakes. However, there are no faults beneath the observed deformation features, and the region lacks recorded seismic activity over the last century. Our data may be helpful in interpreting similar structures elsewhere. |
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