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Mapping and Chronology of Coversands and Dunes from the Aquitaine Basin, Southwest France

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Mapping and Chronology of Coversands and Dunes from the Aquitaine Basin, Southwest France Mapping and chronology of coversands and dunes from the Aquitaine basin, southwest France. Pascal Bertran, Eric Andrieux, Mark Bateman, Markus Fuchs, Michael Klinge, Fabrice Marembert To cite this version: Pascal Bertran, Eric Andrieux, Mark Bateman, Markus Fuchs, Michael Klinge, et al.. Mapping and chronology of coversands and dunes from the Aquitaine basin, southwest France.. Aeolian Research, Elsevier, 2020, 47, pp.100628. 10.1016/j.aeolia.2020.100628. hal-03136502 HAL Id: hal-03136502 https://hal.archives-ouvertes.fr/hal-03136502 Submitted on 10 Feb 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Manuscript (without Author Details) 1 Mapping and chronology of coversands and dunes from the Aquitaine Basin, southwest 1 France 2 3 4 5 Pascal Bertran1,2*, Eric Andrieux3,4, Mark D. Bateman5, Markus Fuchs6, Michael Klinge7, Fabrice 6 7 Marembert1 8 9 *Corresponding author 10 11 E-mail address: [email protected] (P. Bertran) 12 13 14 15 Abstract 16 17 New stratigraphic (including GPR profiles) and chronological data provide a detailed understanding of 18 19 the emplacement of aeolian sands in the Aquitaine basin (SW France). The main phase of coversand 20 deposition ranges from 25 ka to 14 ka. The ages cluster around two periods that match Heinrich 21 events (He-2 and He-1) and reflect increased sedimentation in response to aridification and high 22 23 mean wind speeds. The age distribution is skewed toward recent phases, which likely results from 24 continuous sand recycling. The dunes currently visible in the landscape were formed during the 25 Younger Dryas/very early Holocene and the historical period. The Younger Dryas dunes are inland 26 27 low parabolic dunes that have been strongly degraded. Only the last phases of the progression of the 28 coastal dunefield are preserved because of coastal erosion. Two depositional phases have been 29 identified, the Dark Ages Cold Period and the Little Ice Age, which correspond to phases of increased 30 31 storminess already known from the coastal record in Europe. The Dark Ages Cold Period dunes are 32 high parabolic dunes, whereas the Little Ice Age dunes are predominantly barchanoid. The impact of 33 34 trampling of the dunes by cattle during the latter period, resulting in reduced vegetation cover, and 35 their artificial stabilization by pine forests seems to be the plausible causes of this morphological 36 difference. Overall, the Aquitaine aeolian system is a good example of a mid-latitude coastal to 37 38 inland system that was intermittently active during the Quaternary, and where the reconfiguration of 39 the coast following sea level fluctuations played an important role in the chronological distribution of 40 sediments. 41 42 Keywords: Pleistocene coversands, Younger Dryas inland dunes, Holocene dunefield, OSL, GPR 43 44 profiles 45 46 47 48 1. Introduction 49 50 The aeolian system of the Aquitaine Basin (southwest France) is one of the mid-latitude coastal to 51 inland systems that were intermittently active during the Mid and Late Quaternary (Sun and Muhs, 52 53 2013), in a region that is today characterised by temperate (mean annual air temperature of 12.2 to 54 13°C) and humid (mean annual precipitation of 900 to 1200 mm) climate. The Pleistocene coversands 55 of the Plateau Landais, which is located in the seaward part of the Aquitaine basin, have been the 56 57 subject of various studies in recent decades with respect to sedimentology, geochemistry and 58 chronology (Bertran et al., 2011, Sitzia et al., 2015, 2017; Andrieux et al., 2018; Bosq et al., 2019). 59 2 60 These studies showed that the coversands form a vast transgressive unit reaching about 13,000 km , 61 62 63 64 65 2 surrounded by a wreath of loess on higher ground (fig. 1). The Plateau Landais is thought to have 1 turned into an aeolian system following the entrenchment during the Quaternary of the surrounding 2 rivers, the Garonne and the Adour, and the subsequent disconnection of the plateau from the 3 4 hydrographic network. The coversands, which derived from Lower Pleistocene fluvial formations 5 outcropping on the continental shelf exposed during glacial low sea levels, are mainly composed of 6 7 quartz and other weathering-resistant minerals. They form sheets showing barchanoid and 8 transverse megaripples without avalanche face and with an amplitude of less than one metre. The 9 first chronological data obtained (Bertran et al., 2011, Sitzia et al., 2015) indicate that their 10 11 accumulation has occurred for at least 400 ka during the Middle and Late Pleistocene glacial periods. 12 13 The coversands are overlain by dunes, divided into inland and coastal dunes, the latter forming an 14 approximately continuous dunefield along the Atlantic coast. Some inland dunes have been dated by 15 optical stimulated luminescence (OSL) and yielded either a Younger Dryas (YD) or historical age 16 17 (Bertran et al., 2011; Sitzia et al., 2015). The coastal dunefield has also been dated by OSL on quartz 18 by Smith et al. (1990) and infrared stimulated luminescence (IRSL) on feldspars by Clarke et al. (1999, 19 20 2002). The studies have demonstrated the Late Holocene age of the whole dunefield and have 21 highlighted the lack of a clear relationship between age and morphology (parabolic dunes, barchans 22 and barchanoid ridges). 23 24 Since then, many other ages have been measured in the course of archaeological works at various 25 26 locations in the coversands and the coastal dunefield, allowing for more detailed understanding of 27 the chronology of aeolian formations. The recent improvement of available digital elevation models 28 (DEMs), in particular the release of the 5 m DEM from the Institut Géographique National (IGN) (RGE 29 30 ALTI® version 2.0, 5 m resolution) derived from Light Detection And Ranging (LiDAR), has also made a 31 precise determination of dune morphologies possible. At the same time, new stratigraphic data from 32 33 cross-sections and ground penetrating radar (GPR) profiles were collected. The purpose of this article 34 is to review the recent improvements of the chronostratigraphy of aeolian formations in the 35 Aquitaine Basin, which make it possible to better determine the main depositional phases and their 36 37 potential controlling factors. To help understand the distribution of the different generations of 38 aeolian accumulations, a map of coversands and dunes was produced using complementary GIS- 39 based approaches. 40 41 2. Methods 42 43 During archaeological work, trenches were made in the aeolian deposits using an excavator. After 44 45 manual cleaning of the trenches, the stratigraphy was recorded in detail and samples were taken for 46 dating. In the Castets area, undisturbed profiles of the subsoil structure were also obtained using a 47 48 GPR (SIR-4000 from Geophysical Survey Systems Inc., GSSI) combined with a 200 MHz antenna 49 (Model 5106) and a survey wheel for distance tracking. Paths were cleaned from wood and ground 50 vegetation between the trees to create an even ground surface ensuring permanent contact 51 52 between the device and the sediment. The correct transect positions were determined by GPS 53 (Garmin Montana 680T). The data editing and radargram creation was performed using the RADAN 7 54 software from GSSI. The RGE ALTI® 5 m was used to assign elevation data for surface normalization 55 56 to the radargrams. The recording parameters were: 50 scans per metre, 512 samples per scan, 57 dielectric constant 4.0, maximum depth approx. 16 m. 58 59 60 61 62 63 64 65 3 To complete the stratigraphic data obtained from the trenches, logs from boreholes stored in the 1 Banque du Sous-Sol (BSS) of the Bureau des Recherches Géologiques et Minières (BRGM) 2 (http://infoterre.brgm.fr/) were consulted. Logs with detailed lithological description were then 3 4 selected to document the stratigraphy of the Quaternary formations along two representative 5 profiles of the Plateau Landais. All field data were georeferenced and processed in a Geographic 6 7 Information System (GIS) using the QGIS software (version 3.4). 8 9 For mapping the coversands, we used data from Bertran et al. (2016) with some modifications. The 10 map was created using the soil texture map developed by Ballabio et al. (2016) based on an 11 interpolation of the database from the European project Land Use and Cover Area frame Statistical 12 13 survey (LUCAS) (Toth et al., 2013). Pixels (500 m resolution) satisfying the texture range of aeolian 14 sands measured by particle size analysis on a panel of samples were extracted. To remove isolated 15 pixels and generate more continuous surfaces, additional treatment was then applied. The value of 16 17 each pixel (1: satisfies the aeolian sand texture, 0: does not satisfy the target texture) was 18 recalculated by using the average of the pixel value plus those of the 8 surrounding pixels. Values 19 20 below the threshold of 0.45 were eliminated. Comparison with the available maps showed that the 21 distribution obtained is in good agreement with that proposed by Legigan (1979) and suggests a 22 greater extension of coversands than that indicated on the BRGM 1:50 000 geological map 23 24 (http://infoterre.brgm.fr/) (Bertran et al., 2016).
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