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CHRONOSTRATIGRAPHIE ET DISTRIBUTION SPATIALE DES DÉPÔTS ÉOLIENS QUATERNAIRES DU BASSIN AQUITAIN

Abstract : Quaternary aeolian deposits in the Aquitaine Basin (SW France) provide rare evidence of fossil periglacial aeolian systems located some distance from ice-sheet margins. Studying such systems sheds light on aeolian dynamics in a region which, during glacial periods, lay between the northern European zone with continuous permafrost and the permafrost-free semi-arid/arid belt of southern Europe. Understanding the evolution of Quaternary aeolian deposits in southwestern France also adds further detail to reconstructing landscapes exploited by the region's Upper Paleolithic groups. Here, a chronostratigraphic synthesis of Pleistocene and Holocene aeolian sand deposits in the Landes region is presented alongside a cartographic analysis of the Aquitaine Basin's main aeolian facies. The chronostratigraphic framework presented here is based on a detailed stratigraphic analysis (sedimentary facies, palaeopedology) of numerous unpublished profiles and 75 new radiometric dates (OSL, ESR, 14C). Sand dune morphology, as well as the spatial distribution and origin of the different facies, are addressed using a combination of (1) a cartographic study of dunes with the help of GIS; 2) a geostatistical analysis of dune thickness variability and granulometric gradients; 3) a geochemical study of loess deposits and their potential sources. The above updates and adds new elements to our understanding of the Landes Erg, which developed as early as the Early Pleistocene. Middle Pleistocene aeolian sandsheets were also documented, which fits well with the regional loess record. A detailed Upper Pleistocene chronostratigraphy was also established, whose earliest phase (56-50 ka) corresponds to wet aeolian sandsheets that accumulated in relatively wet conditions and were influenced by changes in the level of the water-table. This facies corresponds to the development of large fields of transverse, low-amplitude barkhanoids ridges (zibar). During this phase, the development of a peaty gley demonstrates a reduction in aeolian sand sedimentation (probably GI 14) and the emergence of steppic vegetation composed of grasses and sagebrush. There is relatively little evidence for sandy accumulations between 50 and 25 ka and it remains to be seen whether this is tied to a reduction in aeolian activity or a lack of relevant stratigraphic profiles. From 25 to 14 ka, large sandsheets formed in an increasingly dry context, probably coincident with the maximum expansion of the Landes desert. During the Lateglacial, the spread of vegetation is slowed by sandy aeolian landscapes, which remained unstable throughout the Bølling and into the first part of Allerød. Boreal forests develop in the Landes at the end of the Allerød, a period marked by the development of an albic arenosol. During the Younger-Dryas climatic deterioration, parabolic dunes fields form in the region; however, such dune formations may have developed as early as the cold intra-Allerød cold event. The main factors controlling the distribution of aeolian facies and the sedimentary balance over time have been also defined. In terms of paleogeography, we highlight the spatial complexity of the Aquitaine's Erg surface that appears to be composed of several generations of aeolian deposits: primarily Middle Pleistocene in the Medoc region, Middle Pleniglacial south of the Gironde plain, and Upper Pleniglacial in southern part of the Landes region. Comparison with the European record (Netherlands, England, France, Spain, Portugal) demonstrates that the Aquitaine Basin shares many chrono-stratigraphical similarities with southwestern Europe. Spatial modeling of aeolian granulometric gradients combined with geochemical analysis provide insights concerning genetic relationships between aeolian facies. Loess deposits primarily result from aeolian abrasion, with only those from the Entre-deux-Mers region incorporating a fluvial component. Prevailing wind directions, as demonstrated by granulometric gradients and aeolian dune morphology, appear to have changed over time; blowing from the west during the Middle Pleniglacial, the north-west during the Upper Pleniglacial, and from south-southwest throughout the Younger Dryas. Here, we connect changing wind directions to seasonal variations in aeolian transport. Comparing annual azimuthal distribution of current wind regimes with wind regime models for LGM suggests that during the Upper Pleniglacial the dominant aeolian transport season shifted towards the summer, i.e. after seasonally frozen ground thawed and snow cover melted. Conversely, Middle Pleniglacial and Younger Dryas wind regimes reflect a general deflation during the winter.
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Submitted on : Wednesday, June 18, 2014 - 11:52:41 AM
Last modification on : Monday, January 13, 2020 - 10:26:03 PM
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Luca Sitzia. CHRONOSTRATIGRAPHIE ET DISTRIBUTION SPATIALE DES DÉPÔTS ÉOLIENS QUATERNAIRES DU BASSIN AQUITAIN. Géomorphologie. Université Sciences et Technologies - Bordeaux I, 2014. Français. ⟨tel-01009617⟩

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