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Frouin M.,Bordeaux Montaigne University | Ploquin F.,CNRS Poitiers Institute of Chemistry: Materials and Natural Resources | Soressi M.,Inrap Institute National Of Recherches Archeologiques Preventives | Soressi M.,Max Planck Institute for Evolutionary Anthropology | And 5 more authors.
Quaternary International | Year: 2013

The crystal-chemical properties of clay minerals are closely dependent on the physicochemical conditions prevailing at the time when they formed in soils and weathered zones. The degree of alteration of primary minerals and the composition of the secondary products are clearly linked to climate and plant cover. In particular, changes are rapid under temperate conditions which favor the development of thick soils, but slow in cold contexts favoring thin and fragile soils. In this paper, the mineral composition of two Pleistocene clay-rich deposits is investigated in order to contribute to reconstructing paleoenvironmental changes. Jonzac (Charentes, SW France) and Les Cottés (Vienne, SW France) are two rock-shelters with long archaeological sequences which include faunal remains and were recently radiometrically dated. The decomposition of XRD patterns at each site allows identification of the principal mineral phases and semi-quantitative estimation of their relative proportions. The variation of these proportions is analyzed through indexes based on the relative intensities of the decomposed bands. The clay fraction of both sites mostly results from the transformation of the minerals forming the sediments in the surrounding area of the rock-shelter terrace, including contributions from the hillside. Mineralogical clay data are compared with the faunal spectrum identified at both sites in order to discuss paleoenvironmental implications. At Jonzac, during the Quina Mousterian period dated to MIS4, the environment was cold and dry. The dominant mineralogical phases are inherited from the rocks (weak pedogenesis). This is consistent with the faunal analysis data typical of the MIS4. During the Mousterian of Acheulean Tradition period, dated to MIS3, the mineralogical composition of the clay deposits changed significantly, suggesting a less wet environment. The Aurignacian deposits dated to the end of MIS3/early MIS2 are rich in mixed layer minerals (MLMs) which are produced by the hydrolysis processes in soils. The climatic conditions probably became milder. However, because no faunal remains were found, this evolution is uncertain. The Les Cottés deposits, dated between MIS3 and early MIS2, are mineralogically typical of a cold environment where soil reactions are not very active. However, some weak variations of the indexes suggest temporary temperate conditions during the Les Cottés interstadial and in the Aurignacian. The parallelism between mineral signature and paleontological evidence in recording relatively rapid variations between contrasted climates suggests that clay mineral assemblages from rock-shelter deposits can be used to assess paleoclimatic and paleoenvironmental dynamics at a local scale. © 2012 Elsevier Ltd and INQUA.

The Scarpe plain is a depression in the Eocene marine sands between the Pevele and Ostrevant regions. A "high plain" is distinguished northwards from a "low plain" southwards. The infilling is mainly sandy (with a peaty part in the low plain). Numerous micro-topographies are scattered throughout the plain. The small water courses which drain the plain are not proportionate to its size. The laying of a gas pipeline across the plain was preceded by archaeological borings. We present the stratigraphy as a long cross-section and propose an evolution scenario for the plain since the Weichselian Upper Pleniglacial. The sedimentation consists mainly of sandy beds alternating with loamy laminae. This alluvial sheet, present everywhere in the plain, was deposited under strongly contrasted hydrological conditions during the Weichselian Upper Pleniglacial (OSL dates about 34,21 and 21 ka). It may be correlated to the northern sand belt Older coversand I. Repeated avulsions of wide yet shallow channels have left deflection marks in the micro-topography. The high plain is comparable to flat and broad alluvial fans emanating from the Pévèle.At the end of the Weichselian Upper Pleniglacial, under a dryer climate, small elongated dunes (sand or sand alternating with loess) formed in the center of the plain or alongside the Pévèle border (OSL dates about 19 and 15 ka). This phase is at least partly contemporary with the Beuningen gravel bed. An ultimate deflation phase is imputed to the Younger Dryas. This last aeolian deposit is even fainter and looks like a thin yet continuous sheet restricted to a large southern part of the plain. Slightly scoured Late Glacial and Holocene streams can be found in the last state of Pleniglacial channels (C 14 dates since 12 345 BP). The former main Pleniglacial channels may thus constitute the overbank channel of the faint Late Glacial and Holocene channels. During the Holocene, the water table rose and consecutively the peat spread in the lower areas of the plain. The widely spread yet often thin peat gradually takes over and blurs the Pleistocene topography. We have crossed three Late Glacial-Holocene channel systems (two Scarpe minor channels and an affluent). Two of them are detailed. The Vred meander bears traces of archaeological remains (a Bronze Age bridge-like structure from around 1000-800 AD) and of anthropic piracy in the upstream "Satis" during the Early Middle Ages (calcareous tufa loam flood deposits).

The Deule River has scoured the chalk substratum in two places in the vicinity of Lille. The older scouring is under the lower slopes of the Weppes country. It occurred prior to the Weichselian. It was still active in the Early Glacial. A more recent scouring, predating the Upper Pleniglacial, appeared one kilometre eastward. The Hegel street excavations allow direct observation of a portion of the in-filled channel; a 12,5 m thick sandy alluvium deposited in small channels associated with indications of cold. It dated to about the end of the Pleniglacial (17960 ± 90 BP, i.e. 21 639 ± 361 cal. BP). These deposits gave way to -apparently continuous- fluvio-aeolian formations, bedded and clayey at first, becoming loess-like in the upper strata. According to the C'4 date and to two OSL dates (21.7 ± 1.9 and 23.7 ± 2,0 ka), this silty, primarily aeolian formation seems to have been rapidly deposited (about 21.7 ka?). These first dates of loess upper valley infilling from northern France should be considered with caution. They appear to indicate the simultaneity of aeolian deposit on slope and valley bottoms at the end of the Pleniglacial, earlier than in the sandy valleys of Belgium and Netherlands.

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