Pascal Bertran
email: pascal.bertran@inrap.fr

Eric Andrieux

Mark D Bateman

Markus Fuchs

Michael Klinge

Fabrice Marembert

Mapping and chronology of coversands and dunes from the Aquitaine basin, southwest France

Keywords: Pleistocene coversands, Younger Dryas inland dunes, Holocene dunefield, OSL, GPR profiles

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Introduction

The aeolian system of the Aquitaine Basin (southwest France) is one of the mid-latitude coastal to inland systems that were intermittently active during the Mid and Late Quaternary [START_REF] Sun | Dune fields: Mid-Latitudes[END_REF], in a region that is today characterised by temperate (mean annual air temperature of 12.2 to 13°C) and humid (mean annual precipitation of 900 to 1200 mm) climate. The Pleistocene coversands of the Plateau Landais, which is located in the seaward part of the Aquitaine basin, have been the subject of various studies in recent decades with respect to sedimentology, geochemistry and chronology [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF][START_REF] Sitzia | The quaternary coversands of southwest France[END_REF], 2017;[START_REF] Andrieux | The chronology of Late Pleistocene thermal contraction cracking derived from sand wedge dating in central and southern France[END_REF][START_REF] Bosq | Stratigraphy and chronology of Pleistocene coastal deposits in northern Aquitaine, France: a reinvestigation[END_REF]. These studies showed that the coversands form a vast transgressive unit reaching about 13,000 km 2 , surrounded by a wreath of loess on higher ground (fig. 1). The Plateau Landais is thought to have turned into an aeolian system following the entrenchment during the Quaternary of the surrounding rivers, the Garonne and the Adour, and the subsequent disconnection of the plateau from the hydrographic network. The coversands, which derived from Lower Pleistocene fluvial formations outcropping on the continental shelf exposed during glacial low sea levels, are mainly composed of quartz and other weathering-resistant minerals. They form sheets showing barchanoid and transverse megaripples without avalanche face and with an amplitude of less than one metre. The first chronological data obtained [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF][START_REF] Sitzia | The quaternary coversands of southwest France[END_REF] indicate that their accumulation has occurred for at least 400 ka during the Middle and Late Pleistocene glacial periods.

The coversands are overlain by dunes, divided into inland and coastal dunes, the latter forming an approximately continuous dunefield along the Atlantic coast. Some inland dunes have been dated by optical stimulated luminescence (OSL) and yielded either a Younger Dryas (YD) or historical age [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF][START_REF] Sitzia | The quaternary coversands of southwest France[END_REF]. The coastal dunefield has also been dated by OSL on quartz by [START_REF] Smith | Optical dating of sediments: initial quartz results from Oxford[END_REF] and infrared stimulated luminescence (IRSL) on feldspars by [START_REF] Clarke | Evidence for the timing of dune development on the Aquitaine Coast, southwest France[END_REF][START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF]. The studies have demonstrated the Late Holocene age of the whole dunefield and have highlighted the lack of a clear relationship between age and morphology (parabolic dunes, barchans and barchanoid ridges).

Since then, many other ages have been measured in the course of archaeological works at various locations in the coversands and the coastal dunefield, allowing for more detailed understanding of the chronology of aeolian formations. The recent improvement of available digital elevation models (DEMs), in particular the release of the 5 m DEM from the Institut Géographique National (IGN) (RGE ALTI® version 2.0, 5 m resolution) derived from Light Detection And Ranging (LiDAR), has also made a precise determination of dune morphologies possible. At the same time, new stratigraphic data from cross-sections and ground penetrating radar (GPR) profiles were collected. The purpose of this article is to review the recent improvements of the chronostratigraphy of aeolian formations in the Aquitaine Basin, which make it possible to better determine the main depositional phases and their potential controlling factors. To help understand the distribution of the different generations of aeolian accumulations, a map of coversands and dunes was produced using complementary GISbased approaches.

Methods

During archaeological work, trenches were made in the aeolian deposits using an excavator. After manual cleaning of the trenches, the stratigraphy was recorded in detail and samples were taken for dating. In the Castets area, undisturbed profiles of the subsoil structure were also obtained using a GPR (SIR-4000 from Geophysical Survey Systems Inc., GSSI) combined with a 200 MHz antenna (Model 5106) and a survey wheel for distance tracking. Paths were cleaned from wood and ground vegetation between the trees to create an even ground surface ensuring permanent contact between the device and the sediment. The correct transect positions were determined by GPS (Garmin Montana 680T). The data editing and radargram creation was performed using the RADAN 7 software from GSSI. The RGE ALTI® 5 m was used to assign elevation data for surface normalization to the radargrams. The recording parameters were: 50 scans per metre, 512 samples per scan, dielectric constant 4.0, maximum depth approx. 16 m.

To complete the stratigraphic data obtained from the trenches, logs from boreholes stored in the Banque du Sous-Sol (BSS) of the Bureau des Recherches Géologiques et Minières (BRGM) (http://infoterre.brgm.fr/) were consulted. Logs with detailed lithological description were then selected to document the stratigraphy of the Quaternary formations along two representative profiles of the Plateau Landais. All field data were georeferenced and processed in a Geographic Information System (GIS) using the QGIS software (version 3.4).

For mapping the coversands, we used data from [START_REF] Bertran | A map of Pleistocene aeolian deposits in Western Europe, with special emphasis on France[END_REF] with some modifications. The map was created using the soil texture map developed by [START_REF] Ballabio | Mapping topsoil physical properties at European scale using the LUCAS database[END_REF] based on an interpolation of the database from the European project Land Use and Cover Area frame Statistical survey (LUCAS) (Toth et al., 2013). Pixels (500 m resolution) satisfying the texture range of aeolian sands measured by particle size analysis on a panel of samples were extracted. To remove isolated pixels and generate more continuous surfaces, additional treatment was then applied. The value of each pixel (1: satisfies the aeolian sand texture, 0: does not satisfy the target texture) was recalculated by using the average of the pixel value plus those of the 8 surrounding pixels. Values below the threshold of 0.45 were eliminated. Comparison with the available maps showed that the distribution obtained is in good agreement with that proposed by [START_REF] Legigan | L'élaboration de la formation du Sables des Landes. Dépôt résiduel de l'environnement sédimentaire Pliocène-Pléistocène centre aquitain[END_REF] and suggests a greater extension of coversands than that indicated on the BRGM 1:50 000 geological map (http://infoterre.brgm.fr/) [START_REF] Bertran | A map of Pleistocene aeolian deposits in Western Europe, with special emphasis on France[END_REF]. The main reason for this extension is the inclusion of areas where the sands are thin (< 2 m) and, therefore, are not recognized in standard geological maps.

Dune mapping was carried out using the RGE ALTI® 5 m. Extraction of the dune envelope surfaces was carried out according to the following procedure proposed by F. Atilio (https://www.sigterritoires.fr/index.php/outils-et-methodes/hydrologie/): (1) construction of a virtual raster with the selected DEM tiles, (2) relief inversion, (3) filling in hollows using the SAGA "fill sinks" tool, (4) relief re-inversion, (5) subtraction of the result from the initial DEM; the reliefs which subsist correspond to terrain higher than the average surface, (6) extraction of the 1 m contours, (7) elimination of contours smaller than a threshold and contours not linked to dunes, (8) assembling the different areas into a single file for the whole area under study, and (9) transforming the contours (1D) into polygons (2D). In areas where the dunes are superimposed on older reliefs, the 2 m or 3 m contours were selected to eliminate parasitic features. The polygons obtained in that way illustrate the distribution of dunes over the whole basin. The maximum and average slopes and the maximum relative elevation were then extracted from the DEM for each polygon. This procedure relying solely on relief had the advantage of simplicity in comparison with more sophisticated tools for automated landform extraction using slope gradient and curvature (e.g. [START_REF] Drǎguţ | Automated classification of landform elements using object-based image analysis[END_REF][START_REF] Iwahashi | Automated classifications of topography from DEMs by an unsupervised nested-means algorithm and a three-part geometric signature[END_REF] and allowed mapping of units of geological interest, i.e. those corresponding to the deposition of substantial sand thicknesses.

The chronological data for the period 0-100 ka used in this study is a compilation of previously published luminescence ages [START_REF] Smith | Optical dating of sediments: initial quartz results from Oxford[END_REF][START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF][START_REF] Sitzia | The quaternary coversands of southwest France[END_REF][START_REF] Noppradit | OSL dating study from the Dune du Pilat in France[END_REF] and new luminescence dating from samples collected during recent archaeological excavations. The new samples were prepared and measured in the luminescence laboratories of the University of Sheffield, Royal Holloway (University of London), Durham University and Giessen University, using the methods outlined in Supplementary Information. In order to limit possible biases linked with the heterogeneity of dating methods, the ages from the literature were only considered when obtained using OSL on quartz. Further to this initial filtering, additional efforts were undertaken to re-calculate, when possible, previous luminescence dating using updated parameters and attenuation factors. This reduces the sources of variation between the different datasets and enables comparisons. IRSL ages from feldspars calculated by [START_REF] Clarke | Evidence for the timing of dune development on the Aquitaine Coast, southwest France[END_REF][START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF] were used for comparison since they are the only data available for the coastal dunefield, with the exception of the Dune du Pilat ages published by [START_REF] Smith | Optical dating of sediments: initial quartz results from Oxford[END_REF]. For older periods (> 100 ka), infraredradiofluorescence (IR-RF) ages on feldspars and electron spin resonance (ESR) ages on quartz [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF][START_REF] Kreutzer | Deciphering coastal dynamics using IR-RF and ESR dating: a case study from Médoc, south-west France[END_REF][START_REF] Bosq | Stratigraphy and chronology of Pleistocene coastal deposits in northern Aquitaine, France: a reinvestigation[END_REF] were also considered. A total of 98 dates were compiled into the database, out of which 22 were new (Tables 1,2).

Results

Mapping and dune morphologies

The map of coversands and dunes is shown in Figure 1. Coversands form a large triangle between the Atlantic Ocean to the west, the Garonne to the north-east and the Adour to the south-east, and cover an area of 11,413 km 2 , which is slightly less than the value proposed by [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF] probably because of more accurate delineation. The map also shows previously reported sand patches on the plateaus on the right bank of the Garonne north of Bordeaux, which cover approximately 490 km 2 .

Comparison between the dunes identified by the method used in this study and those shown on the BRGM 1:50,000 homogenized geological map shows overall good agreement (fig. 2A). However, more dunes appear on the new map. The low-relief arms of parabolic dunes, which are clearly visible from the DEM, are poorly captured by the map and sometimes form a series of aligned patches. The dunes are mainly present along the Atlantic coast (coastal dunefield) and south of the parallel 44.7°N passing through the Arcachon lagoon. As previously indicated by [START_REF] Sitzia | Dynamics and sources of last glacial aeolian deposition in southwest France derived from dune patterns, grain-size gradients and geochemistry, and reconstruction of efficient wind directions[END_REF], inland dunes are concentrated along west-east oriented corridors and on both banks of rivers which are oriented north-south (fig. 1, 2A-C).

As a general rule, the coastal dunes show strong morphological variations and are structured as follows due to the complex interplay between sand availability, wind strength, vegetation and groundwater table, from the coast to inland (fig. 3): (1) narrow coastal dune fringe, affected by blowouts from which small parabolic dunes start, (2) isolated to coalescent domed dunes (without avalanche face) and "fat" barchans (see [START_REF] Parteli | Minimal size of a barkhan dune[END_REF] alternating with parabolic dunes, (3) high, often overlapping, barchanoid ridges (which can exceed 30 m in height), (4) isolated barchans, and (5) shallow sand blanket spreading in a zigzag pattern over the Pleistocene surface. Locally, high parabolic dune fields extend ahead of the dunefield (fig. 2B). These dunes are partially overlapped by the high barchanoid ridges.

Inland dunes are typically elongated, low parabolic dunes (modal height ~4 m), exceptionally exceeding 10 m (fig. 4), often coalescing. Unlike coastal dunes, the maximum slope is low (mode around 10°) and much lower than the maximum stability slope (32-35°). However, a few dunes are approaching these values. Some of these, close to rivers, are juxtaposed to blowouts that are still clearly visible in topography. This is particularly the case at Rion-des-Landes (fig. 2D).

Stratigraphy

Borehole data show that the thickness of the aeolian formations is strongly variable across the basin (fig. 5). In general, the thickness decreases from the shoreline to inland. In the northern part of the basin, which is bounded to the south by the Leyre fault, the thickness occasionally reaches 10 m, except in the coastal dunefield. Trenches excavated at Salaunes showed that the sand thickness can vary rapidly, from less than 0.5 m on the plateau to more than 2 m in filled gullies. At Mios, located close to the fault, the dunes rest directly on Lower Pleistocene fluvial gravel.

To the south, the homogeneous sands found in boreholes, which are ascribed either to the Sable des Landes Formation (aeolian) or to the Castets-Marcheprime Formation in the BSS, can reach up to 30 m in thickness. According to the arguments developed by [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF] and [START_REF] Bosq | Stratigraphy and chronology of Pleistocene coastal deposits in northern Aquitaine, France: a reinvestigation[END_REF], the Castets-Marcheprime Formation, initially assumed to be fluvial by [START_REF] Dubreuilh | Dynamique d'un comblement continental néogène et quaternaire: l'exemple du bassin d'Aquitaine[END_REF], does not correspond to a homogeneous chronostratigraphic unit and is of aeolian origin. It is nevertheless possible that the transitional few metres between fluvial gravel at the base of the boreholes (Belin or Onesse-et-Beliet Formations) and purely aeolian sand may correspond to finingupward fluvial sequences or mixed fluvio-eolian deposits. The lack of accurate description of the sedimentary structures, which would make it possible to determine the depositional environment, does not allow clear conclusions to be drawn.

The schematic stratigraphy of the new sections studied in trenches is shown in Figure 6 and supplements the data provided by [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF] and [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF]. As indicated by these authors, subhorizontally bedded facies predominate in coversands (sandsheet facies) (fig. 7A,B). Units showing crinkly bedding (adhesion ripples) are sometimes interbedded in sands, as well as palaeosols of Histosol, Arenosol and Gleysol type, which are generally cryoturbed. Dune facies include subhorizontal to gently sloping bedded units (aeolian ripple ramps) alternating with steeply sloping cross-bedded units (avalanche faces) (fig. 7C,D).

The GPR profiles carried out at Castets allow determining the structure of the deposits over a thickness of more than 15 m (fig. 8). They show, above a fluvial substrate typified by well-marked reflectors, a unit about 7 m thick with sub-horizontal bedding. According to the borehole BSS002DYHV stored in the BSS, this unit consists of homogeneous sand. It is interpreted as coversands. The coversands are overlain by a parabolic dune up to 8 m high, characterized by predominantly oblique reflectors. The slope of the reflectors locally exceeds 30° and reflects avalanche face stratification. Both profiles show a discontinuity in the stratification, interpreted as a reactivation surface. In the lee side of the dune, the gradient of the strata decreases and becomes progressively sub-parallel to the surface, whose slope does not exceed 15°. The structure thus appears fairly simple and reflects the progression of a single parabolic dune over the Pleistocene coversands, then its degradation.

Because of their height, stratigraphic observations on coastal dunes are more limited. Coastal cliffs are the best available outcrops. The stratigraphy recorded near Soulac (fig. 6) in the northern part of the basin and at the Dune du Pilat (Froidefont and [START_REF] Froidefond | La grande dune du Pilat et la progression des dunes sur le littoral aquitain[END_REF][START_REF] Bressolier | Chronology of coastal dunes in the south-west of France[END_REF]) typically shows (1) at the base, massive, slightly clayey, bioturbed sands of infra-metric thickness sometimes showing preserved horizontal bedding (fig. 7E), resting on Pleistocene coversands or estuarine formations, (2) cross-stratified sands with variable slope, up to 35°. Units with convolute beds, often enriched in organic matter, are also visible (fig. 7F). Lower units are interpreted as shallow sand blanket spreading in front of the dunes, while upper units correspond to the dunes per se. According to the observations of [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF], convolute bedding resulted from cattle trampling, with footprints sometimes preserved on the bed surface. Successive units are separated by increasingly poorly developed Histosols/Humic Gleysols and Arenosols towards the top of the sequence.

Chronology

The geographical distribution of dated sites is shown in Figure 1 and the ages are listed in Table 2. A total of 98 dates were included in our analysis. Among the ages obtained, 45 fall into the Holocene, 39 into the Late Pleistocene-YD and 14 into the Middle Pleistocene. As already noted by [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF], the distribution is not spatially homogeneous and the northern part of the basin mainly yielded Middle Pleistocene ages, while the Late Pleistocene and YD ages come from the southern part (fig. 9). The new ages support this pattern. According to [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF], the main reason is that the sand sources were mostly coastal during the glacials as they are today, while the contribution of aeolian sand previously deposited inland was minor owing to armouring by coarsegrained material, vegetation (although scarce) and/or groundwater table close to the surface. Widening of the continental shelf to the north and the subsequent remoteness of sand sources during periods of low sea level from the currently emerged areas best explains why these were not reached by coversands during the Last Glacial Maximum (LGM).

For the period < 100 ka, the chronological distribution of ages was analysed using Kernel Density Estimation (KDE) and Cumulated Probability Function (CPF) methods [START_REF] Vermeesch | On the visualisation of detrital age distributions[END_REF]. The first method does not consider the uncertainties associated with the ages but allows a visualisation of how they cluster. The second method takes into account the distribution of age probability densities for each age. As a result, CPF tends to disadvantage older ages and those for which uncertainty is high. Although some pitfalls may affect such a chronological analysis (e.g., Thomas and Bailey, 2019, and other references herein), we assume here that there is no substantial sampling bias affecting the age distribution, which thus provides a good approximation of the periods of sand deposition. The resulting diagram (fig. 10) highlights the following points:

(1) The age distribution is not uniform over time, but phases of high density are distinctly identifiable. These phases correspond mainly to the Late Pleniglacial and the YD/very early Holocene falling between ca. 25 ka and 10.5 ka as well as to the Late Holocene. For the older periods, the age density is low and is probably not representative of the true frequency and timing of aeolian sedimentation. Three ages around 55 ka come from a single well-dated site (Barp-Pot-aux-Pins, [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF].

(2) Peaks in the distribution emerge within these phases. They correspond approximately to cold and arid periods in western Europe associated with the YD and Heinrich events He-2 and He-1 identified in marine records [START_REF] Bond | Iceberg discharges into the North Atlantic on millenial time scales during the last glaciation[END_REF][START_REF] Sanchez Goñi | Millennial-scale climate variability and vegetation changes during the Last Glacial: Concepts and terminology[END_REF].

Figure 11 focuses on the Lateglacial and the Holocene (14-0 ka). This diagram shows the following:

(1) Ages between the YD/very early Holocene and the beginning of the historical period are virtually absent. [START_REF] Smith | Optical dating of sediments: initial quartz results from Oxford[END_REF] obtained an age of 5.4 ka on a shallow unit of bioturbed sands sampled at the base of the Dune du Pilat. No age earlier than the historical period was provided by crossstratified dune sand.

(2) Two peaks occur during the historical period using the KDE method for a bandwidth of 0.3 ka, one at 1.6 ka, the other at 0.4 ka. These peaks correspond to the Little Ice Age (LIA) and the Dark Ages Cold Period (DACP, [START_REF] Helama | Dark Ages Cold Period: A literature review and directions for future research[END_REF] Early Medieval Dark Ages, EMDA, [START_REF] Kobashi | Volcanic influence on centennial to millennial Holocene Greenland temperature change[END_REF]. Cumulated age probability densities suggest that the LIA peak could be multi-phased and marked by at least three distinct sedimentation episodes at 0.82, 0.43 and 0.25 ka respectively. The multiphased nature of the LIA sedimentation is supported by the presence of weakly developed palaeosols preserved in some dunes (Dune du Pilat: Froidefont and [START_REF] Froidefond | La grande dune du Pilat et la progression des dunes sur le littoral aquitain[END_REF]Soulac: Stéphan et al., 2019). Whether these palaeosols developed only locally due to specific conditions or whether they are widespread features remains unknown, however.

(3) The peaks showed by the distribution of IRSL ages obtained by [START_REF] Clarke | Evidence for the timing of dune development on the Aquitaine Coast, southwest France[END_REF][START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF] are shifted towards the present with respect to those provided by OSL. Such rejuvenation is probably the result of a methodological issue, caused by differences in the measurement protocol and the lack of correction for fading. Therefore, the peak at 1.12 ka, contemporary to the Medieval Warm Period (MWP), is probably an artifact.

Discussion

This study provides new results regarding the mapping and chronology of the sandy aeolian formations of the Plateau Landais. Overall, the coversands are much thinner in the north of the basin than in the south. The Leyre Fault forms approximately the boundary between the two coversand areas. Concomitantly, the coversands 1-2 m depth are Middle Pleistocene in the north and final Late Pleistocene in the south. Plausible explanations for this pattern are:

(1) The southern area was affected by subsidence. According to [START_REF] Klingebiel | Cadre géologique et structure du bassin de la Leyre[END_REF], subsidence is still active with an estimated rate up to 0.5 mm.yr -1 in the Parentis-Biscarosse area. Even though these data should be updated, [START_REF] Serpelloni | Vertical GPS ground motion rates in the Euro-Mediterranean region: New evidence of velocity gradients at different spatial scales along the Nubia-Eurasia plate boundary[END_REF] suggest significant current downward vertical movements using GPS geodetic measurements. Because of subsidence, the southern area acted as a trap for sand accumulation which through continued to have accommodation space opening up.

(2) As already indicated by [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF], lower sea level during the LGM caused the coast to migrate further on the more shallow off-shore platform to the north. Thus aeolian sand would have had to be transported further from sources for it to have been preserved on the presently exposed land-surface. The impacts of sea-level driven coastal reconfiguration on the timing and extent of dune accumulation is well documented from elsewhere, e.g. South Africa [START_REF] Bateman | The evolution of coastal barrier systems: A case study of the Middle-Late Pleistocene Wilderness barriers, South Africa[END_REF][START_REF] Cawthra | Understanding Late Quaternary change at the land-ocean interface: A synthesis of the evolution of the Wilderness coastline, South Africa[END_REF]). As a result, the sands located at shallow depths under the Holocene soil yield old ages, corresponding to depositional phases associated with a higher sea level than during the LGM. This is in particular obvious for MIS 3 sands, which outcrop to the east of Bordeaux (e.g., Cestas-Pins-de-Jarry, Barp-Pot-aux-Pins). This pattern implies that in the southern part of the basin, Middle Pleistocene sands should be preserved under the younger sediment cover, but have not been observed in outcrops. In agreement with this assumption, a sample taken from the Pontonx quarry yielded an age of 157 ± 16 ka, i.e. Penultimate Glacial, corresponding to a period when sea level drop was close to that of the LGM. The ages obtained by [START_REF] Andrieux | The chronology of Late Pleistocene thermal contraction cracking derived from sand wedge dating in central and southern France[END_REF] on the aeolian sand fill of ground thermal contraction cracks (sand wedges) at the periphery of coversands indicate, however, that some sand transport was active during the LGM. Sand availability was likely low and did not allow significant accumulation outside of local traps. This pattern was conducive to the formation of ventifact pavements, which have been reported in many locations north of Bordeaux [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF].

The ages obtained on the coversands south of the Leyre fault, from the coast to the eastern limit, show that most of the sediments sampled were deposited between about 25 ka and 14 ka. This phase, corresponding to the maximum coversand extension, overlaps the Greenland stadials GSI-3 and GS-2 [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy[END_REF], and matches the main period of aeolian (loess) sedimentation in other European regions [START_REF] Antoine | Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany[END_REF]Mezner et al., 2013;[START_REF] Lehmkuhl | Loess-paleosol sequences at the northern European loess belt in Germany: Distribution, geomorphology and stratigraphy[END_REF][START_REF] Újvári | Coupled European and Greenland last glacial dust activity driven by North Atlantic climate[END_REF]Bosq et al., in press). As previously pointed out by [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF], coversand emplacement is older than in the North European Sand Belt, where it did not start until the very end of MIS 2 [START_REF] Bateman | Thermoluminescence dating of British coversand deposits[END_REF][START_REF] Kasse | Cold-climate aeolian sand-sheet formation in north-western Europe (c. 14-12.4 ka); a response to permafrost degradation and increased aridity[END_REF][START_REF] Frechen | The deposition history of coversands along the Bree Fault escarpment, NE Belgium[END_REF][START_REF] Kolstrup | Luminescence and radiocarbon ages from laminated Lateglacial aeolian sediments in western Jutland, Denmark[END_REF][START_REF] Vandenberghe | Late Weichselian (fluvio-)aeolian sediments and Holocene drift-sands of the classic type locality in Twente (E Netherlands): a high-resolution dating study using optically stimulated luminescence[END_REF][START_REF] Kasse | A complete Late Weichselian and Holocene record of aeolian coversands, drift sands and soils forced by climate change and human impact, Ossendrecht, the Netherlands[END_REF]. In these regions, sedimentation was essentially fluvio-eolian before ca. 16 ka (Schokker and [START_REF] Schokke | Sedimentology and facies distribution of Pleistocene cold-climate aeolian and fluvial deposits in the Roer Valley graben (southeastern Netherlands)[END_REF]Zielinski et al., 2015). The main reason suggested was the existence of permafrost in northern Europe, which limited soil drainage and particle entrainment by deflation [START_REF] Schwan | Sedimentologic characteristics of fluvial to aeolian succession in Weichselian Talsand in the Emsland (F.R.G.)[END_REF][START_REF] Kasse | Cold-climate aeolian sand-sheet formation in north-western Europe (c. 14-12.4 ka); a response to permafrost degradation and increased aridity[END_REF][START_REF] Sitzia | The quaternary coversands of southwest France[END_REF]. The release of considerable amounts of meltwater by the Fennoscandian Ice Sheet during the summers equally played an important role in the areas close to the ice margin (see [START_REF] Patton | Deglaciation of the Eurasian ice sheet complex[END_REF], for the reconstruction of glacial drainage).

The increased number of available ages in the present study compared to previous ones highlights well marked peaks in the age distribution. The peaks, which correspond approximately to He-2 and He-1, indicate that aeolian dynamics was mostly active during the driest and coldest periods of the Late Pleniglacial because of both vegetation reduction due to aridity and increased wind transport capacity [START_REF] Sima | Modeling dust emission response to North Atlantic millennial-scale climate variations from the perspective of East European MIS 3 loess deposits[END_REF][START_REF] Sitzia | Dynamics and sources of last glacial aeolian deposition in southwest France derived from dune patterns, grain-size gradients and geochemistry, and reconstruction of efficient wind directions[END_REF][START_REF] Pinto | Extratropical cyclones over the North Atlantic and western Europe during the Last Glacial Maximum and implications for proxy interpretation[END_REF].

Despite the large uncertainty associated with OSL ages, the hypothesis suggested by [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF] of a continuation of coversand deposition during the early Lateglacial up to the Allerød is not contradicted by the new age analysis. Samples from two sequences, St-Geours-de-Maremne and Pindères, show that sandsheet facies yielded Bølling or early Allerød ages, whereas no palaeosol previously dated by 14 C on charcoal provided an age earlier than the second half of the Allerød [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF][START_REF] Sitzia | The quaternary coversands of southwest France[END_REF]. As a result, sand stabilisation and colonization by vegetation was delayed at the beginning of the Lateglacial, probably due to the nutrient deficiency of soils and the difficulty of plants to resist sand blasting. A similar, albeit shorter delay is also likely at the very beginning of the Holocene, as indicated by the age obtained at Mios (10.4 ± 0.7 ka) in the easternmost (downwind) part of the dune.

In contrast, the paucity of ages older than 25 ka, especially contemporary to MIS 4 (a period of considerable dust increase in glacial records, [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy[END_REF], is puzzling. Several factors, or more likely a combination of factors, may be involved:

(1) Recycling by deflation of previously deposited sands. Some sections (e.g., Pontonx) show that significant gaps exist in the sediment record. Sand bodies of contrasting ages are separated by flat erosional surfaces, sometimes lined with small ventifacts, interpreted as deflation surfaces that were probably controlled by the groundwater table [START_REF] Fryberger | Stokes surfaces and the effect of near-surface groundwater-table on aeolian deposition[END_REF]. At Pontonx the sand bodies above and below the erosional surface were respectively dated to the Late Pleniglacial and the Penultimate glacial, which implies a gap of over 100 ka. The skewed Late Pleniglacial/Lateglacial age distribution of the coversands (the older phases are poorly represented in comparison to the younger) (fig. 10) is assumed to be an effect of recycling. However, although we can envisage that MIS 4 deposits had been largely recycled during the LGM, it remains difficult to imagine that all these deposits have been fully eroded where they existed.

(2) MIS 4 coversands were of limited extension over the currently emerged continent. The sea level reached about -90 m at the peak of MIS 4 [START_REF] Spratt | A Late Pleistocene sea level stack[END_REF] and the coastline underwent significant reconfiguration. The Middle Pleistocene coversands in the northern part of the basin were not covered during this period. In contrast, MIS 4 coversands were buried under LGM sediments in the south making them unaccessible by trenching. They are consequently absent from our dataset.

(3) Low aeolian activity occurred during a large part of MIS 4 This would imply that local environmental conditions (soil moisture, vegetation) during MIS 4 were unfavourable to deflation. However, this assumption seems unlikely for the coldest and driest phase of MIS 4, i.e. the GS-19 and GS-18 stadials.

Dating shows that dune formation occurred mainly during a limited number of relatively recent phases, the YD around 12.8 ka for inland dunes and the historical period (1.8-0.2 ka) for coastal and some inland dunes. The YD corresponds to the main phase of inland dune formation in western Europe in areas that were not glaciated [START_REF] Bateman | The timing and relation of aeolian sand deposition in central Spain to the aeolian sand record of NW Europe[END_REF][START_REF] Kasse | Sandy aeolian deposits and environments and their relation to climate during the Last Glacial Maximum and Lateglacial in northwest and central Europe[END_REF][START_REF] Kasse | A complete Late Weichselian and Holocene record of aeolian coversands, drift sands and soils forced by climate change and human impact, Ossendrecht, the Netherlands[END_REF]. The GPR data show that the YD dune of Castets is a simple structure. The avalanche face degraded after the cessation of aeolian dynamics to reach a low value, around 15°, typical of Lateglacial dunes. A discontinuity in the cross stratification suggests that the dune advance could have taken place in two distinct phases. Since no palaeosols were identified in the available sections, these two phases obviously followed one another rapidly.

The transition from the Pleniglacial sandsheet depositional style to the YD low parabolic dunes reflects the role of vegetation, which partially recolonised the landscape during the Allerød. It should be noted that in southern Europe, characterised by a milder climate and a denser LGM shrub vegetation cover [START_REF] Strandberg | High-resolution regional simulation of last glacial maximum climate in Europe[END_REF], sandsheet facies are absent and cross-stratified sand units formed during the Last Glacial [START_REF] Bosq | Last Glacial aeolian landforms and deposits in the Rhône Valley (SE France): spatial distribution and grain-size characterization[END_REF][START_REF] Wacha | Last Glacial Maximum giant sand dunes on the island of Vis, Croatia[END_REF]. The conditions specific to the glacial environments of northern Europe, including Aquitaine, i.e. rare vegetation, frozen ground in winter and wet ground during thaw, can, therefore, be identified as the main factor at the origin of the sandsheet facies. Grain-size, mentioned in warm deserts as a decisive factor (sandsheets are usually coarser-grained than dunes, [START_REF] Kocurek | Conditions favourable for the formation of warm climate aeolian sand sheets[END_REF], played no obvious role here since no difference was noted between the grain-size mode of coversands and that of inland dunes [START_REF] Sitzia | Dynamics and sources of last glacial aeolian deposition in southwest France derived from dune patterns, grain-size gradients and geochemistry, and reconstruction of efficient wind directions[END_REF].

The almost complete lack of steeply sloping cross-stratified units within Pleistocene coversands is noteworthy. The only example of a thick (0.9 m) set of oblique beds in between sandsheet facies comes from Fargues-sur-Ourbise in the easternmost part of the coversands [START_REF] Sitzia | The quaternary coversands of southwest France[END_REF] and has yielded an age of 14.8 ± 0.9 ka, i.e. GI-1e or the very end of GS-2 [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy[END_REF]. Whether this dune corresponds to a barchan or a parabolic dune is difficult to determine, however, and its environmental significance remains unknown. This suggests that dunes likely developed repeatedly on the Plateau Landais, but that that they have a low preservation potential in the geological record and were mostly destroyed during later glacial phases.

Two main age density peaks emerge during the historical period, one around 1.6 ka, the other around 0.4 ka. These two peaks, which correspond to the DACP and the LIA respectively, have already been well identified in the coastal aeolian record across Europe [START_REF] Björck | Aeolian sediment in raised bog deposits, Halland, SW Sweden: a new proxy record of Holocene winter storminess variation in southern Scandinavia?[END_REF][START_REF] Wilson | Mid-to late-Holocene coastal dune event stratigraphy for the north coast of Northern Ireland[END_REF][START_REF] Thomas | Optical Dating of Late Quaternary Coastal Deposits in Northwestern Portugal[END_REF][START_REF] Costas | Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events[END_REF][START_REF] Vliet-Lanoë | Late Holocene coastal dune activity along the Dover Strait, Northern France -Insights into Middle Ages and Little Ice Age coastal dynamics constrained by optically stimulated luminescence dating[END_REF][START_REF] Jackson | Large-scale transgressive coastal dune behavior in Europe during the Little Ice Age[END_REF]) and reflect periods of increased storm frequency [START_REF] Vliet-Lanoë | Middle-to late-Holocene storminess in Brittany (NW France): Part II -The chronology of events and climate forcing[END_REF][START_REF] Auger | years of North Atlantic-Arctic climate[END_REF]. The comparison between OSL and IRSL ages suggests that the first IRSL estimates by [START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF] showing a peak around 1.2 ka, i.e. MWP, should not be retained.

Age mapping indicates that the LIA corresponds to high barchanoid dune ridges as well as barchanoid and parabolic dunes located seaward, while the DACP ages come from the high parabolic dune fields located ahead of the former. The distinction between 'primary dunes' and 'modern dunes' locally proposed by different authors (Tastet and Pontee, 1998 and references herein) is thus validated by the new ages and indicates two distinct phases of historical advance of the coastal dunefield. The variability of dune morphologies, only parabolic during the DACP (at least in the area not covered by the LIA dunes) and mostly barchanoid during the LIA, would reflect a less vegetated environment during the second phase. Increased dune vegetation cover is, indeed, the main factor behind their evolution towards a parabolic form [START_REF] Anthonsen | Evolution of a dune from crescentic to parabolic form in response to short-term climatic changes: Rabjerg Mile, Skagen Odde, Denmark[END_REF][START_REF] Hanoch | Modeling the bistability of barkhan and parabolic dunes[END_REF]. The factor involved remains difficult to assess, insofar as climatic control can be excluded (the conditions that prevailed during the DACP were not much different from those of the LIA). Section observations indicate that convolute bedding, often showing organic-rich laminae, accounts for a large proportion of dune deposits. This suggests that intensive grazing during the LIA and trampling by cattle may have exerted a major control on vegetation density and aeolian dynamics. Dune trampling is well documented in historical texts (Buffault, 1942, pp. 118-124). Cattle were driven to graze in the interdune areas, locally called "lettes", and were free to wander over the dunes. At that time, the role of grazing as a factor of dune reactivation, which threatened coastal villages, was already strongly debated [START_REF] Buffault | Histoire des dunes maritimes de la Gascogne[END_REF]. An additional factor that may explain the morphological difference between DACP and LIA dunes lies in the much slower stabilization of the former due to natural colonization by vegetation, which promoted a transition from barchanoid to parabolic forms, whereas the latter have been "frozen" with their original shape preserved through artificial stabilization by planting of pine forests during the 19 th century [START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF]. Such an evolution has been well documented in the Mu Us dunefield (China) using satellite imagery by [START_REF] Xu | Vegetated dune morphodynamics during recent stabilization of the Mu Us dune field, north-central China[END_REF]. In this dunefield, the growth rate of vegetation, natural or to the contrary controlled by man, had a profound impact on sand transport and was among the main factors of dune evolution.

In many European regions, the ages obtained in coastal dunefields show that other periods were accompanied by the progression of dunes, particularly the late Middle and Late Holocene (see [START_REF] Walker | Subdividing the Holocene Series/Epoch: formalization of stages/ages and subseries/subepochs, and designation of GSSPs and auxiliary stratotypes[END_REF] for the subdivisions of the Holocene Epoch) around 5.5 ka, 4.2 ka and 3.2-2.4 ka [START_REF] Wilson | Mid-to late-Holocene coastal dune event stratigraphy for the north coast of Northern Ireland[END_REF][START_REF] Thomas | Optical Dating of Late Quaternary Coastal Deposits in Northwestern Portugal[END_REF][START_REF] Costas | Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events[END_REF][START_REF] Goslin | Decadal variability of north-eastern Atlantic storminess at the mid-Holocene: New inferences from a record of wind-blown sand, western Denmark[END_REF]. In Aquitaine, [START_REF] Smith | Optical dating of sediments: initial quartz results from Oxford[END_REF] obtained a single age of 5.4 ka on a thin layer of bioturbed sand at the base of the Dune du Pilat. This sand layer does not correspond to dune deposits per se, but to a sand blanket ahead of a dune according to the pattern currently visible (fig. 3). At Soulac, bioturbed sand with partially preserved horizontal bedding, located underneath cross-stratified dune sand, yielded an age of 1.9 ka. These facies are, therefore, time-transgressive and precede the arrival of dunes in the area under consideration. In the context of considerable retreat of the sandy coast since at least the Late Holocene [START_REF] Pontee | Morpho-sedimentary evidence of Holocene coastal changes near the mouth of the Gironde and on the Medoc Peninsula, SW France[END_REF][START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system[END_REF], documented by measurements since the beginning of the 20th century [START_REF] Buffault | Histoire des dunes maritimes de la Gascogne[END_REF][START_REF] Aubié | Caractérisation de l'aléa érosion (2020-2040) de la côte aquitaine dans le cadre de l'étude stratégique de gestion du trait de côte[END_REF], the near absence of ages prior to the historical period must be interpreted as the result of coastal erosion and the subsequent loss of oldest dunes. Only historic dunes currently remain in the landscape. Some inland dunes have also yielded historical ages (i.e. Rion-des-Landes, Retjons), mostly LIA. These dunes are all located in the immediate vicinity of rivers and to have steep slopes, much steeper than those measured for YD dunes (fig. 12). At Rion-des-Landes, a visual inspection of the DEM also shows that the dunes are connected to blowouts that are still well identifiable (fig. 2D). They reflect the resumption of aeolian activity during the LIA, a feature already identified in other inland sandy areas elsewhere in Europe [START_REF] Tolksdorf | Holocene aeolian dynamics in the European sand-belt as indicated by geochronological data[END_REF][START_REF] Pierik | Controls on late-Holocene drift-sand dynamics: The dominant role of human pressure in the Netherlands[END_REF][START_REF] De Keyzer | Late Holocene landscape instability in the Breckland (England) drift sands[END_REF]. As in these areas, it is likely that anthropogenic activity played an important role in dune remobilisation. The blowouts may reflect areas where the protective vegetation cover have been stripped away due to anthropogenic activities. In addition, convolute bedding related to trampling by cattle is abundant at Rion-des-Landes as well as at Retjons [START_REF] Bertran | Inland aeolian deposits of southwest France: facies, stratigraphy and chronology[END_REF]. Similar facies have also been described in the Netherlands [START_REF] Koster | Genesis and sedimentary structures of late Holocene aeolian drift sands in northwestern Europe[END_REF].

Conclusion

The cartographic and chronological study of the aeolian sands of Aquitaine allows us to draw the following conclusions:

(1) Two areas separated by the Leyre Fault and with different depositional histories can be distinguished. The coversands are thin and old (Middle Pleistocene, MIS 3) in the northern area. They were deposited during glacial phases associated with a moderate fall in sea level. Owing to subsidence, the thickness of sand accumulations is greater in the south and the age of subsurface deposits is much younger (Late Pleniglacial, YD). This area is likely to have preserved a detailed record of sedimentation phases during the Middle and Late Pleistocene and should be the focus of future research efforts.

(2) The main phase of coversand emplacement in the south ranges from 25 ka to 14 ka (GS-3 and GS-2). Sedimentation occurred mainly during He-2 and He-1 in response to aridification and high mean wind speeds. The age distribution is skewed toward recent phases because of continuous recycling of the deposits.

(3) The dunes currently visible in the landscape were formed during the YD/very early Holocene (13-10.5 ka) and the historical period. The YD dunes are mainly elongated, low parabolic dunes, which have been strongly degraded.

(4) Only the last phases of progression of the coastal dunefield are preserved because of coastal erosion. Two phases already known elsewhere in Europe have been identified, the DACP and the LIA, which correspond to phases of increased storminess. The MWP phase proposed by [START_REF] Clarke | Evidence for the timing of dune development on the Aquitaine Coast, southwest France[END_REF][START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF] is probably an artifact due to methodological issues.

(5) The currently exposed DACP dunes are high parabolic dunes, whereas the LIA dunes are predominantly barchanoid. The impact of grazing and trampling of the dunes by cattle during the LIA, resulting in reduced vegetation cover, seems to be a plausible cause of this morphological difference. Artificial stabilization of the LIA dunes by planting of pine forests during the 19 th century may also be involved in the preservation of their original barkhanoid shape.

(6) Some inland dunes also developed near rivers during the LIA. They are characterized by steeper slopes and are connected to blowouts still visible in topography.

Section observations allowed the identification of a wide range of palaeosols within the sands. The palaeosols are only preserved locally due to subsequent deflation phases. Therefore, the available data are still too sparse to provide a clear picture of the succession of environments during the Last Glacial. These features merit further investigations.

Shapefiles of the coversand and dune distribution are available on request from the first author (P. Bertran). Figure 7: Main facies. A -horizontally-bedded sandsheet (ca. 220 ka), with a thin syngenetic sand wedge (Anse-du-Gurp); B -from base to top, horizontally-bedded sandsheet (12.7 ± 0.8 ka) with slightly humic beds, massive sand overlain by a bioturbated humic horizon (Arenosol), then massive dune sand (11.6 ± 0.8 ka, Younger Dryas), St-Geours-de-Maremne; C -massive, bioturbated sand with a buried humic horizon overlying low angle cross-bedded sand (1.86 ± 0.13 ka), parabolic dune at Ondres; D -LIA cross-bedded sands, Dune du Pilat; E -bioturbated massive to horizontallybedded sand and incipient palaeosols at the base of the dune, Amélie; F -slightly humic convolute beds (0.49 ± 40 ka), Soulac. [START_REF] Vermeesch | On the visualisation of detrital age distributions[END_REF]. The NGRIP 18 O curve is taken from [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy[END_REF]. The Heinrich Events (Hes) are from Sanchez [START_REF] Sanchez Goñi | Millennial-scale climate variability and vegetation changes during the Last Glacial: Concepts and terminology[END_REF].

Figure captions

Figure 11: A -Chronological distribution of OSL ages between 14 ka and 0 ka; KDE: Kernel Density Estimation, CPF -Cumulated Probability Function [START_REF] Vermeesch | On the visualisation of detrital age distributions[END_REF]; B -idem, IRSL ages, from [START_REF] Clarke | Evidence for the timing of dune development on the Aquitaine Coast, southwest France[END_REF][START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF]. The DACP and LIA are from [START_REF] Helama | Dark Ages Cold Period: A literature review and directions for future research[END_REF] and [START_REF] Kobashi | Volcanic influence on centennial to millennial Holocene Greenland temperature change[END_REF]. Supplementary Information: Dating methodology (including recalculated ages).

Figure 1 :

 1 Figure1: Distribution of the coversands, dunes and loess of Aquitaine and location of the sites mentioned in the text. The faults are taken from[START_REF] Bourbon | Gestion des eaux souterraines en Région Aquitaine[END_REF]. Profiles P1 and P2 (in red) refer to figure5. A -Location of the study area, B -Enlargement of the framed area. The Plateau Landais corresponds to the area of low relief covered with sand.

Figure 2 :

 2 Figure 2: Comparison between the dunes mapped by BRGM (blue line) and according to the method used in this paper (in pink). The DEM is the RGE ALTI® 5 m of IGN. A -Morphology of dunes in the Castets area, B -idem, Biscarosse area, C -idem, Sabres area, D -idem, Rion-des-Landes area. The sites indicated correspond to sites dated by luminescence. BO: blowout, BR: barchanoid ridge, HBR: high barchanoid ridge, IB: isolated barchan, PD: parabolic dune, LPD: low parabolic dune.

Figure 3 :

 3 Figure 3: Morphology of the coastal dunefield in the Hourtin area. A -Shaded relief and altitude (RGE ALTI® 5 m), B -Satellite view of the framed area (Google Earth, photo 2009). BR: barchanoid ridge, DD: dome dune; IB: isolated barchan, PD: parabolic dune, FSB: foredune sand blanket, PS: Pleistocene surface, MR: megaripples. The dotted line indicates the limit between the coastal dunefield and the Pleistocene surface.

Figure 4 :

 4 Figure 4: Maximum height (A) and slope (B) of coastal (orange) and inland (blue) dunes. The values were extracted from the RGE ALTI® 5 m.

Figure 5 :

 5 Figure 5: Stratigraphy of the Miocene to Pleistocene formations of Aquitaine, based on BSS boreholes (http://infoterre.brgm.fr/). The location of the profiles is shown in Figure 1 (A: profile P1; B: profile P2). The names of the geological formations are taken from the BSS (cf. Dubreuilh et al., 1995, for the Pleistocene).

Figure 6 :

 6 Figure 6: Schematic stratigraphy of the new sections observed in trenches and position of dated samples. SW: sand wedge.

Figure 8 :

 8 Figure 8: Georadar profiles of a parabolic dune at Castets. Borehole BBS002DYHV is taken from the BSS.

Figure 9 :

 9 Figure 9: Geographical distribution of the age of sand deposits in Aquitaine. A -Middle Pleistocene (> 120 ka), B -Last Glacial (100-30 ka and 30-14 ka), C -Younger Dryas and Holocene.

Figure 10 :

 10 Figure 10: Chronological distribution of Last Glacial to Holocene OSL ages. KDE: Kernel Density Estimation, CPF -Cumulated Probability Function[START_REF] Vermeesch | On the visualisation of detrital age distributions[END_REF]. The NGRIP 18 O curve is taken from[START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy[END_REF]. The Heinrich Events (Hes) are from Sanchez[START_REF] Sanchez Goñi | Millennial-scale climate variability and vegetation changes during the Last Glacial: Concepts and terminology[END_REF].

Figure 12 :

 12 Figure 12: Maximum slope of the dunes dated by OSL. The slope values were taken from the RGE ALTI® 5 m.

Figure 12

 12 Figure 12

Table 1 :

 1 New OSL ages obtained in this study.

Table 2 :

 2 List of the ages of aeolian sands used in this study.

Manuscript (without Author Details)

Acknowledgements: This study benefited from the financial support of Inrap and the University of Bordeaux. We would like to thank the archaeologists who provided access to the sites, notably L. Bourguignon, J.F. Chopin, B. Ducournau, V. Elisagoyen, P. Fouéré and A. Pons-Méthois, as well as the assistant scientific and technical director L. Detrain. D. Sauer is also acknowledged for help in the field. The OSL dating work in this study was undertaken in different laboratories as outlined in the SI. We thank the institutions (University of Sheffield, Durham University and Royal Holloway, University of London, Justus-Liebig University of Giessen) and in particular S. Armitage for providing us access to the Luminescence equipment. S. Armitage also participated to the improvement of the text. The two anonymous reviewers are also greatly acknowledged for their constructive remarks on the manuscript.


PhD Pascal Bertran
email: pascal.bertran@inrap.fr

Ph.D.; Kevin Manchuel

Ph.D Deborah Sicilia

Discussion on '

Keywords: Palaeoseismology, Permafrost, Brittle deformation Palaeoseismology, Permafrost, Brittle deformation

    Discussion on 'Palaeoseismic structures in Quaternary sediments, related to an assumed fault zone north of the Permian Peissen-Gnutz salt structure (NW Germany) -Neotectonic activity and earthquakes from the Saalian to the Holocene' (Grube, 2019)

Introduction

Grube's (2019) article on brittle and ductile deformations affecting Quaternary sediments in the Peissen region (northwest Germany) provides interesting data on potential traces of palaeoearthquakes and is part of a recent effort to extend seismicity catalogues in Europe beyond the period for which instrumental data and historical texts are available. These deformation structures complement the many other structures already described in northern Europe, particularly by [START_REF] Hoffmann | Soft-sediment deformation of Late Pleistocene sediments along the southwestern coast of the Baltic Sea (NE Germany)[END_REF], [START_REF] Brandes | Soft-sediment deformation structures in NW Germany caused by Late Pleistocene seismicity[END_REF], [START_REF] Van Loon | Sedimentological evidence of Pleistocene earthquakes in NW Poland induced by glacio-isostatic rebound[END_REF] and [START_REF] Van Loon | Seismites resulting from high-frequency, high-magnitude earthquakes in Latvia caused by Late Glacial glacio-isostatic uplift[END_REF], which were also interpreted as earthquake-induced. However, the geographical proximity of all these structures does not constitute proof of a common origin. The main reasons for this relate to the complex local geomorphological context, marked by (1) the development of permafrost during the last glaciation, (2) the proximity of the Scandinavian ice sheet during the last Glacial Maximum, and (3) the presence of a salt diapir in the immediate vicinity of the study sites. These aspects will be developed in detail here. The outcome of this analysis is that, while the morphology of deformation generally makes it possible to determine the stress state to which the sediments have been subjected and the quantity of water available in the system at the time of deformation, the nature of the factors causing the stresses remains difficult to identify. Laboratory experiments and field observations indicate that similar brittle and/or ductile deformations may occur in relation to various geomorphological processes. Therefore, the multiplicity of potential factors that may have been involved in the sector studied by Grube (2019) does not make it possible to determine confidently the origin of the deformation structures.

Sand blows and sand wedges in a permafrost context

The wedge-shaped structures filled with sand described by Grube (2019) and interpreted as sand blows due to ground fluidization caused by seismic shaking are very similar to the wedges caused by thermal contraction of the ground in periglacial environments and filled with aeolian sand (sand wedges) or a mixture of sand and ice (composite wedge pseudomorphs) (e.g., [START_REF] Murton | Ice wedges and ice wedge casts[END_REF]Andrieux et al., 2016a, b). These wedges have a V shape, sand veins extending from the base (apophyses), a depth (up to 1.7 m) and an internal organization typical of periglacial structures.

Vertical lamination is preserved in one of the wedges illustrated in Fig. 11e of Grube (2019). The lamination, common in thermal contraction wedges, is related to repeated cracking of the ground during winters and the infiltration of aeolian sand into the crack (Fig. 1). When the filling is composed of sand and ice (composite wedges), melting of ice causes the lamination to vanish and the wedge to deform. Coarser material from the host sediment can fill the depression. Ghysels and Heyse (2006) and [START_REF] Buylaert | Optical dating of relict sand wedges and composite-wedge pseudomorphs in Flanders, Belgium[END_REF] have described similar examples in Belgium OSL-dated to the Weichselien and Saalien. Creep of fine-grained host material during melting or repeated thaw cycles can also lead to the formation of globular structures comparable to those shown in Grube's (2019) Fig. 11b (Fig. 1D).

One of the most relevant criteria for discriminating these structures from sand blows is their organization into large polygons, which reach 10 to 30 m in diameter. Unfortunately, Grube (2019) has not documented this aspect. However, some data indicate that the presence of sand wedges or composite wedge pseudomorphs is highly probable in the study area. These data are (1) polygons visible on satellite photographs accessible in Google Earth; one of the identified sites is located in the immediate vicinity of Pit 3, another of Pit 2 (Fig. 2); (2) the mention by [START_REF] Christensen | Waterstress conditions in cereals used in recognizing fossil ice-wedge polygonal patterns in Denmark and northern Germany[END_REF] of abundant fossil thermal contraction polygons visible in aerial photographs in Denmark and northern Germany; the geo-referencing of Christensen's map shows that the study sites are located in an area where more than 5% of agricultural land is affected by polygons; and (3) the concomitance of continuous permafrost during the last glaciation [START_REF] Vandenberghe | The Last permafrost Maximum (LPM) map of the Northern Hemisphere: permafrost extent and mean annual air temperatures, 25-17 ka[END_REF] and coversand deposition [START_REF] Kasse | Cold-climate Aeolian Sand-sheet formation in North-Western Europe (c. 14-12.4 ka); a response to permafrost degradation and increased Aridity[END_REF][START_REF] Zeeberg | The European sand belt in Eastern Europe and comparison of Late Glacial dune orientation with GCM simulation results[END_REF], which is a pattern highly favourable to the formation of sand wedges over large surfaces. Therefore, the interpretation of these structures as periglacial sand wedges rather than seismogenic sand blows seems most likely in the current state of the analysis and a seismic origin cannot therefore be retained without more supporting evidence.

Brittle deformation

Unconsolidated and well-drained sediments under stress deform in a brittle manner. The stress can be caused by wide range of geological processes. Many experiments using analog models under stress have been described in the literature. These models use cohesionless granular materials, most often sand, sometimes interlayered with ductile levels (cohesive wet clays, or purely viscous silicone paste). The results constitute a data set that can be used to understand the brittle deformation of non-lithified Quaternary sediments.

Experiments by [START_REF] Komuro | Experiments on cauldron formation: a polygonal cauldron and ring fractures[END_REF] and [START_REF] Walter | Formation of caldera periphery faults: an experimental study[END_REF] reproduced the growth of a lenticular body (a putty ball or an inflated chamber) under a granular cover. The increase in volume leads to the formation of a dome and the development of subvertical radial cracks at the top (Fig. 3A,B). As the dome grows, cracks are formed that cut at right angles the radial cracks and are arranged in a more or less concentric pattern. At the top of the dome, the blocks bounded by the cracks collapse, leading to the formation of a polygonal central depression limited by normal faults. [START_REF] Walter | Formation of caldera periphery faults: an experimental study[END_REF] reproduced the succession of growth phases followed by collapse of the cover. The interaction between the fractures created during swelling and those related to collapse generates blocks of variable size and a high degree of material fragmentation (Fig. 3F). This type of structure accurately reflects the fracturing of the sediment or peat layer that covers intrusive ice mounds (pingos, frost blisters) or segregated ice mounds (lithalsas) in a periglacial context. Examples have been described in the Canadian Arctic by [START_REF] Mackay | Pingo collapse and paleoclimatic reconstruction[END_REF][START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula area, western Arctic coast, Canada: A long-term field study[END_REF] and [START_REF] Pollard | Seasonal frost-mounds[END_REF] (Fig. 4A). [START_REF] Davison | Overburden deformation patterns and mechanisms of salt diaper penetration in the Central Graben, North Sea[END_REF] and [START_REF] Marco | Radial clastic dykes formed by a salt diapir in the Dead Sea Rift, Israel[END_REF] also observed a system of radial and tangential cracks in the sedimentary cover at the periphery of salt diapirs (Fig. 4B). In the example detailed by [START_REF] Marco | Radial clastic dykes formed by a salt diapir in the Dead Sea Rift, Israel[END_REF], many structures correspond to clastic dykes and probably result from the filling of cracks opened from the surface, which were formed in connection with the rise of the diapir.

Experiments that reproduce the rise of a block of substratum, and those that simulate the collapse of an unconsolidated cover over a cavity, provide comparable results. In both cases, the faults are concentrated to a limited area at the edge of the raised block or the cavity. [START_REF] Sanford | Analytical and experimental study of simple geologic structures[END_REF] experiments consisted in lifting a rigid block limited by vertical edges under a cover of sand or sand and clay. Reverse curved faults propagate towards the surface and the subsided area (Fig. 3C). As pointed out by [START_REF] Sanford | Analytical and experimental study of simple geologic structures[END_REF], the reverse faults, which are subvertical at the base and evolve into thrusts towards the surface, are not related to horizontal compression in the model, but to vertical movements at depth. Similar results were also obtained in a series of experiments designed to reproduce the roof collapse over a cavity [START_REF] Roche | Sub-surface structures and collapse mechanisms of summit pit[END_REF][START_REF] Walter | Formation of caldera periphery faults: an experimental study[END_REF][START_REF] Geyer | Relationship between caldera collapse and magma chamber withdrawal: an experimental approach[END_REF][START_REF] Coumans | Caldera collapse at near-ridge seamounts: an experimental investigation[END_REF]. In these experiments, bell-shaped reverse fractures form above the cavity together with annular extension fractures starting from the surface at the periphery.

Progressive roof collapse occurs in connection with the propagation of the bell-shaped fractures up to the surface (Fig. 3D,E). [START_REF] Coumans | Caldera collapse at near-ridge seamounts: an experimental investigation[END_REF] reproduced the situation in which the thickness of the cover is not homogeneous. In this case, subsidence associated with reverse faulting occurs where the sediment thickness above the cavity is lowest, whereas normal faults develops at the opposite side (Fig. 3G). The final depression has an asymmetrical shape.

Many examples of curved reverse faults and associated normal faults have been described in natural environments. They comprise the cover of salt diapirs undergoing dissolution (Simon and [START_REF] Simón | Diapiric deformations in the Quaternary deposits of the central Ebro Basin, Spain[END_REF][START_REF] Davison | Deformation and sedimentation around active Miocene salt diapirs on the Tihama Plain, northwest Yemen[END_REF] (Fig. 4B), sediments affected by the collapse of a karstic cavity (Soriano et al., 2012;Simon et al., 2014;[START_REF] Luzón | Karst subsidence as a control on the accumulation and preservation of aeolian deposits: A Pleistocene example from a proglacial outwash setting, Ebro Basin, Spain[END_REF] (Fig. 4E), subglacial deposits (eskers) deformed by glacier melt-out [START_REF] Mcdonald | Interpretation of faults in glaciofluvial sediments[END_REF], and lahars or fluvioglacial deposits (jökulhlaup) deformed by melting of ice-blocks [START_REF] Branney | Ice-melt collapse pits and associated features in the 1991 lahar deposits of Volcan Hudson, Chile: criteria to distinguish eruption-induced glacier melt[END_REF][START_REF] Fay | Formation of kettle holes following a glacial outburst flood (jökulhlaup), Skeidararsandur, southern Iceland[END_REF]. [START_REF] Calmels | Development and decay of a lithalsa in Northern Quebec: a geomorphological history[END_REF] identified reverse ice-filled faults dipping 50 to 90° in a segregated ice mound (lithalsa) in northern Quebec. According to [START_REF] Calmels | Development and decay of a lithalsa in Northern Quebec: a geomorphological history[END_REF], these faults formed during the growth of ice lenses during permafrost build-up. Reverse faults have also been identified in ramparts created by the melting of Pleistocene ice mounds [START_REF] Payette | Les buttes minérales cryogènes dans les basses terres de la Rivièreaux-Feuilles, Nouveau Québec[END_REF][START_REF] Kasse | Weichselian Upper Pleniglacial aeolian and ice-cored morphology in the southern Netherlands (Noort-Brabant, Groote Peel)[END_REF][START_REF] Pissart | Remnants of lithalsas of the Hautes Fagnes, Belgium: a summary of present-day knowledge[END_REF][START_REF] Bertran | Features caused by ground ice growth and decay in Late Pleistocene fluvial deposits, Paris Basin, France[END_REF] and in thermokarst lake deposits [START_REF] Murton | Thermokarst-lake-basin sediments, Tuktoyaktuk Coastlands, western arctic Canada[END_REF][START_REF] Bertran | Features caused by ground ice growth and decay in Late Pleistocene fluvial deposits, Paris Basin, France[END_REF] (Fig. 4C).

Other experiments have reproduced deformations created by shortening [START_REF] Bonini | Modelling hanging wall accomodation above rigid thrust ramps[END_REF][START_REF] Bonini | Deformation patterns and structural vergence in brittle-ductile thrust wedges: An additional analogue modelling perspective[END_REF]. In these models, detachment occurs at the substratum -cover interface and reverse faults with low dip (thrusts) form in the cover to accommodate the shortening (Fig. 3E). Thrusting leads to the formation of an anticline at the top of the ramp and conjugate reverse faults delimiting pop-up structures develop. The frontal bulge can collapse along normal faults that form as shortening and bulging progress. This type of deformation is typically observed in the frontal bulge of landslides [START_REF] Coombs | The east Abbotsford, Dunedin, New Zealand, landslide of august 8, 1979, an interim report[END_REF][START_REF] Mccalpin | Bhuj-Kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, Gujarat, Western India[END_REF] (Fig. 4D) and at the front of emerging deep-seated reverse faults [START_REF] Philip | The Armenian earthquake of 1988 December 7: faulting and folding, neotectonics and palaeoseismicity[END_REF][START_REF] Mccalpin | Bhuj-Kachchh earthquake: surface faulting and its relation with neotectonics and regional structures, Gujarat, Western India[END_REF].

In summary, the available models show that it is possible to identify the stress state that caused brittle deformation from the fracture geometry. However, the factors underlying stress remain more difficult to determine and observations in natural environments indicate that many geological processes are able to generate similar deformations. Comparable conclusions were also drawn for soft-sediment deformations (e.g., [START_REF] Moretti | The environmental significance of soft-sediment deformation structures: key signatures for sedimentary and tectonic processes[END_REF]. Analysis of the fracture pattern can provide insight into the factors that may be involved. However, the usually limited extent (i.e., a few tens of metres) of the outcrops is one of the main limitations for documenting accurately the general fracture pattern. An exception is fracturing due to thermal contraction of the ground in a periglacial context, where the growth of ice or sand wedges generates easily identifiable polygonal patterns.

The site studied by Grube ( 2019) is located in a complex geological environment, where deformations created by the rise of a salt diapir, salt dissolution, the development of Pleistocene permafrost and possibly, by water circulation under pressure in the Scandinavian ice sheet margin (see [START_REF] Boulton | Deep circulation of groundwater in overpressured subglacial aquifers and its geological consequences[END_REF][START_REF] Murton | Ground-ice stratigraphy and formation at North Head, Tuktoyaktuk Coastlands, Western Arctic Canada: a product of glacier-permafrost interactions[END_REF][START_REF] Ravier | Clastic injection dynamics during ice front oscillations: a case example from Sólheimajökull (Iceland)[END_REF] have overlapped. Consequently, this site seems unfavourable to the detection of palaeoearthquakes, insofar as the structures observed at the scale of the outcrops do not allow the factors potentially involved to be discriminated against. The association of normal and reverse faults (Fig. 12 of Grube ( 2019)) may well reflect the deformation associated with the rise and dissolution of the underlying salt diapir, or that created by the growth and melting of ice bodies during the Weichselian, rather than earthquake-induced processes. Diapir uplift caused by loading of the surrounding land by the Weichselian ice sheet likely occurred as demonstrated by [START_REF] Lang | Response of salt structures to ice loading: implications for ice-marginal and subglacial processes[END_REF].

Conclusion and prospects

The arguments proposed by Grube (2019) for a seismic origin of the structures observed in the Peissen quarries are not convincing when considering the context. In such geological settings, the types of criteria that should be favoured in palaeoseismic research are (1) surface lineation identifiable by the relief or an offset in geological structures, which may reflect the emergence of deep-seated faults (although possibly non-seismogenic), and (2) fractured pebbles associated with a fault or in the associated damage zone, which provide evidence for the seismogenic nature of the fault.

The first criterion is traditionally used in palaeoseismology (e.g., [START_REF] Chardon | First paleoseismological constraints on the strongest earthquake in France (Provence) in the twentieth century[END_REF][START_REF] Camelbeeck | Relevance of active faulting and seismicity studies to assessments of long-term earthquake activity and maximum magnitude in intraplate northwest Europe, between the Lower Rhine Embayment and the North Sea[END_REF][START_REF] Baize | First assessment of recent tectonics and paleoearthquakes along the Irtysh fault (eastern Kazakhstan)[END_REF] and serves as a guide for trenching to study the fault's history. This approach ensures that the structures analysed in cross-section are effectively related to a deepseated fault. The persistence of relief, however, implies that the fault has been active recently.

Pebble fracturing along seismogenic faults is attested by some authors, particularly [START_REF] Kübler | Historical coseismic surface deformation of fluvial gravel deposits, Schafberg fault, Lower Rhine Graben, Germany[END_REF], in settings where the lithostatic stress is null (subsurface). Fragmentation results from the development of stress higher than pebble strength (especially in the case of poorly resistant lithologies such as argillites or sandstones) caused by shearing along the fault. This stress remains significantly lower than that required for quartz grain cataclasis (typically in deep fault gouges, e.g., [START_REF] Cashman | Cataclasis and deformation-band formation in unconsolidated marine terrace sand, Humbolt County, California[END_REF][START_REF] Torabi | Shear zones in porous sand: insights from ringshear experiments and naturally deformed sandstones[END_REF][START_REF] Mair | 3D numerical simulations of fault gouge evolution during shear: Grain size reduction and strain localization[END_REF][START_REF] Kristensen | The microstructure and internal architecture of shear bands in sand-clay sequences[END_REF] and requires the pebbles to be in contact with each other (clast-supported material). According to [START_REF] Radjai | Bimodal Character of Stress Transmission in Granular Packings[END_REF], the stress transmitted along the load-bearing network is much higher than the average vertical stress, allowing pressures higher than pebble strength to develop during shear. Seismic compression waves and waves released by the bursting of neighbouring pebbles are likely to be involved in fracturing [START_REF] Davies | Elastic strain energy release from fragmenting grains: effects on fault rupture[END_REF], as long as fracturing is not observed along nonseismogenic faults.

Cataclasis could thus provide a reliable indicator that can be used in palaeoseismological analysis, particularly when it affects many pebbles in the damage zone around a fault (Fig. 5A,B). Other factors that can cause fracturing of subsurface pebbles include gelifraction [START_REF] Matsuoka | Microgelivation versus macrogelivation: towards bridging the gap between laboratory and field frost weathering[END_REF][START_REF] Matsuoka | Frost weathering and rockwall erosion in the southeastern Swiss Alps: Long-term (1994-2006) observations[END_REF][START_REF] Jia | Path-dependent frost-wedging experiments in fractured, lowpermeability granite[END_REF] and mass flow of debris. Gelifraction is caused by crack expansion resulting from ice growth in fissured rocks and segregation ice growth due to water migration in weak and highly porous rocks such as chalk. Gelifraction is a common feature in cold environments and mainly affects limestones, shales and all kinds of fractured rocks (Fig. 5C,D). It remains ineffective on compact rocks such as most alluvial pebbles, for which the impacts caused by the fluvial transport have eliminated the least resistant and most fissured parts. Fractured pebbles are actually rare (but not totally absent) in Quaternary alluvial deposits and are scattered throughout the deposits. Cataclasis typifies the sedimentary flows involving a large volume of debris, i.e., rock avalanches [START_REF] Siebert | Large volcanic debris avalanches: characteristics of source areas, deposits, and associated eruptions[END_REF][START_REF] Yarnold | Rock-avalanche characteristics in dry climates and the effect of flow into lakes: insights from mid-Tertiary sedimentary breccias near Artillery Peak, Arizona[END_REF][START_REF] Bertran | The rock-avalanche of February 1995 at Claix (French Alps)[END_REF]. It develops within the whole flow and does not concentrate along identifiable faults. It gives rise to so-called "jigsaw" structures (Fig. 5E,F). In smaller flows (debris flows, snow avalanches), flaking of the transported blocks dominates. In most cases, sedimentological criteria and the non-localized nature of cataclasis make it possible to identify the factor involved.

Fractured pebbles associated with faults and embedded in unconsolidated or weakly cemented fluvial deposits were mentioned by several authors [START_REF] Jorda | La tectonique plio-quaternaire des Préalpes de Digne et ses prolongements récents. L'enseignement des hautes nappes alluviales[END_REF][START_REF] Carbon | Enregistrement d'un paléoséisme dans des sédiments du pléistocène supérieur dans la vallée du Rhône : quantification de la déformation[END_REF][START_REF] Baize | Inventaire des indices de rupture affectant le Quaternaire, en relation avec les grandes structures connues, en France métropolitaine et dans les régions limitrophes[END_REF][START_REF] Guignard | Géométrie et cinématique post-oligocène des failles d'Aix et de la moyenne Durance (Provence, France)[END_REF] in southeastern France, the most seismic region of the country during historical times, and provide reliable indices for palaeo-earthquakes. To our knowledge, similar such structures have not been reported in areas further north and should focus research. Fracturing of a sand cover following chamber inflation, from [START_REF] Walter | Formation of caldera periphery faults: an experimental study[END_REF]; (C) Formation of curved reverse faults in sand due to the lifting of a substratum block, from [START_REF] Sanford | Analytical and experimental study of simple geologic structures[END_REF]; (D) Bellshaped reverse faults above a cavity, from [START_REF] Geyer | Relationship between caldera collapse and magma chamber withdrawal: an experimental approach[END_REF]; (E) Annular fracture above a cavity, from [START_REF] Walter | Formation of caldera periphery faults: an experimental study[END_REF] (the chamber is indicated by the grey dashed line); (F) Deformation after swelling followed by emptying of a chamber, from [START_REF] Walter | Formation of caldera periphery faults: an experimental study[END_REF]; (G) Asymmetric collapse above a cavity in the presence of a relief, from [START_REF] Coumans | Caldera collapse at near-ridge seamounts: an experimental investigation[END_REF]; (H) Deformation of a cover as a result of horizontal shortening, from [START_REF] Ballard | Propagation des chevauchements au-dessus des zones de décollement : modèles expérimentaux[END_REF].

Figure captions

Figure 1 .

 1 Figure 1. (A) Pleistocene sand wedge, Salaunes (SW France), (B) Close up view of the vertical lamination in a sand wedge from La Louverie (Loire valley, France), (C) Sand wedge with apophyses, Saint-Amand-les-Eaux (N France), (D) Globular structure (DSW) caused by the deformation of a sand wedge, Saint-André-de-Cubzac (SW France); the sand wedge (SW) is visible on the bottom of the trench below the dotted line.

Figure 2 .

 2 Figure 2. (A) Ground thermal contraction polygons, Peissen (X = 9.5878°E, Y = 54.0414°N); (B) Polygons, Beldorf (X = 9.3540°E, Y = 54.1213°N) (Google Earth, photos 2009).

Figure 3 .

 3 Figure 3. Brittle deformation of sandy soil models. (A) Radial fractures and central polygonal depression formed by the rise of a ball under a granular cover, redrawn from Komuro (1987); (B)

Figure 4 .

 4 Figure 4. (A) Radial cracks on a pingo (ice-cored mound), Tuktoyaktuk, Canada (Google Earth); (B) Radial and tangential faults around a salt diapir, redrawn from Davison et al. (2000); (C) Reverse faults in Pleistocene thermokarst lake deposits, Gourgançon (Paris Basin, France); (D) Thrust planes at the toe of a landslide, Les Leches (SW France); (E) Bell-shaped reverse faults above a karstic cavity, Mérignac (SW France).

Figure 5 .

 5 Figure 5. (A, B) Fractured pebbles in the Lower Pleistocene Valensole II Formation, near Sisteron (SE France); the finer-grained material to the left of photo B is a fault gouge; the largest pebble is 10 cm long; (C) Gelifracted pebble in an active layer above permafrost, Tuktoyaktuk (Canada); (D) Gelifracted sandy limestones, French Pyrenees (France); the largest pebbles are 20 cm in diameter;(E) Jigsaw structure in the Mont Granier rock avalanche (French Alps); knife for scale; (F) Jigsaw structure in a volcanic rock avalanche, Guadeloupe Island (French West Indies); the section is 1.5 m high.

Figure

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Acknowledgements

We acknowledge O. Bellier for showing us sites with fractured pebbles in SE France. The funds have been provided by Electricité de France (EDF) and Institut de Recherches en Archéologie Préventive (Inrap) through the SISMOGEL project. M. Moretti and an anonymous reviewer are also thanked for their constructive remarks on the manuscript.