Article

Mise en évidence de dépôts à caractère fluvio-marin parmi les nappes alluviales de la Basse Somme

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Abstract

EnglishOne of the gravel sheets of the alluvial complex of the "Basse Somme" shows some characteristics which are considered as witness of a marine influence. This gravel sheet, named Croix l'Abbe Formation was deposited about a height of 25 m during a cold climatic episode of the Lower Pleistocene ; relationships with the terraces of the Somme between Pinchefalise and Saigneville are discussed here. One of the writters (De H.) describes the associated Palaeolithic artefacts. francaisUne des nappes du complexe alluvial de la Basse Somme presente des caracteristiques qui sont interpretees comme temoignant d'une influence marine. Cette nappe, decrite ici sous le nom de Formation de Croix l'Abbe, s'est mise en place vers 25 m d'altitude au cours d'un episode froid du Pleistocene inferieur ; ses relations avec les nappes alluviales de la Somme entre Pinchefalise et Saigneville sont discutees. L'un des auteurs (De H.) caracterise l'industrie paleolithique associee.

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... Geophysical records [11,12] show evidence of seismic activity on the Somme southern flexure and a normal fault, located on the right bank of the lower Somme [4]. An analysis of horizontal gravity gradients [8,9,19] also defines the position of the main fault, that lies at depth, south below the main flexure of the left side of the valley (figures 1B and 1C). ...
... La fin du Crétacé voit une première surrection du Bray [22,27,34] et probablement du Boulonnais [3,9,14,20]. Cette phase induit un raccourcissement nordsud [3], provoquant l'inversion du Weald-Boulonnais ou du Bray [8] ( figure 1A). Elle amène, de ce fait, une flexure des strates crétacées, ici, à l'aplomb de la faille de Somme, flexure recoupée par une surface d'érosion maëstrichtienne [23]. ...
... Cette flexure amène un affaissement de plus de 30 m depuis le Thanétien [5], probablement 40 m depuis 3 Ma, 5 m depuis 110 ka [33], et semble toujours fonctionnelle dans l'Holocène des Bas-Champs [5,11]. La « faille » d'Abbeville commande aussi une pente plus forte des paléotalwegs « Somme » vers l'estuaire [8]. La faille normale nord délimite un couloir en affaissement jusqu'en baie de Somme ( figure 1B), avec un rejet à regard sud au nord de l'estuaire [11]. ...
... Geophysical records [11,12] show evidence of seismic activity on the Somme southern flexure and a normal fault, located on the right bank of the lower Somme [4]. An analysis of horizontal gravity gradients [8,9,19] also defines the position of the main fault, that lies at depth, south below the main flexure of the left side of the valley (figures 1B and 1C). ...
... La fin du Crétacé voit une première surrection du Bray [22,27,34] et probablement du Boulonnais [3,9,14,20]. Cette phase induit un raccourcissement nordsud [3], provoquant l'inversion du Weald-Boulonnais ou du Bray [8] ( figure 1A). Elle amène, de ce fait, une flexure des strates crétacées, ici, à l'aplomb de la faille de Somme, flexure recoupée par une surface d'érosion maëstrichtienne [23]. ...
... Cette flexure amène un affaissement de plus de 30 m depuis le Thanétien [5], probablement 40 m depuis 3 Ma, 5 m depuis 110 ka [33], et semble toujours fonctionnelle dans l'Holocène des Bas-Champs [5,11]. La « faille » d'Abbeville commande aussi une pente plus forte des paléotalwegs « Somme » vers l'estuaire [8]. La faille normale nord délimite un couloir en affaissement jusqu'en baie de Somme ( figure 1B), avec un rejet à regard sud au nord de l'estuaire [11]. ...
Article
Within the context of the western Paris Basin, considered to be a region experiencing slight uplift during Neogene and Quaternary, the ESR dating of Quaternary deposits, together with those of Neogene age, the morphology and available geophysical data suggest that the Somme valley is an actively subsiding sector (tectonic shortening between two active inversions). The dissymmetry of the valley tributaries and of the deposits preservation is controlled by a flexuring above the hidden Somme fault. The record of neotectonic events and the geophysical data demonstrate its activity and elucidate the recent history of the Paris Basin.
... In thé lower Somme area, three high alluvial sheets hâve been distinguished (Agache, 1963;Dupuis, 1977) lithic artefacts and can be placed in a précise chronostratigraphic scale. ...
... Des phénomènes tectoniques ont affecté ce secteur et, de ce fait, l'inclinaison des nappes est plus prononcée que la nappe de fond de vallée. Les études effectuées ont permis de distinguer trois hautes terrasses dans la zone autour de Boismont-Pinchefalise (tableau) (Agache, 1963;Dupuis, 1977) : la nappe du Purgatoire montre, à son point d'affleurement, des graviers à matrice sableuse reposant sur le substratum crayeux; la nappe du Bois de Prêle est formée de graviers à passées sableuses reposant sur des poches de sables thanétiens; la nappe du Mont Pillart possède une série constituée d'argile surmontée d'un niveau de sable, d'un niveau de graviers à matrice sableuse, d'un niveau de limon à passées sableuses et d'un limon à nombreux cailloux, le tout reposant sur une couche de sable thanétien. ...
Article
Full-text available
L'étude géochronologique des terrasses fluviatiles quaternaires du bassin de la Somme a permis de confirmer la chronostratigraphie précédemment établie sur les basses et moyennes terrasses et de placer les hautes terrasses dans la chronologie du Quaternaire.
... On the other hand, all the fault systems observed up to now in the fluvial and slope sequences of this area result from dissolution features in the underlying chalk bedrock and are related to weathering and pedogenesis during interglacial and early glacial phases (Antoine, 1990). However, in the lower Somme the occurrence of tectonic deformation is likely to be related with faults (Dupuis et al., 1977). Moreover, in the Somme valley the strong difference between the continental area, with a stepped terrace system, and the submarine extension with overlapping alluvial formations, has been interpreted as the result of a difference in the absolute values of uplift between the present-day continental chalk plateau and the Channel area (Fig. 1B) (Antoine et al., 2000). ...
... d'une ancienne ballastière ouverte dans la nappe du Bois Cassin ; celle-ci appartient à un ensemble de dépôts fluviatiles de la Somme échelonnés entre les graviers des plateaux et la haute terrasse (C. Dupuis, P. Haesaerts et J. de Heinzelin, 1978). ...
... On the other hand, all the fault systems observed up to now in the fluvial and slope sequences of this area result from dissolution features in the underlying chalk bedrock and are related to weathering and pedogenesis during interglacial and early glacial phases (Antoine, 1990). However, in the lower Somme the occurrence of tectonic deformation is likely to be related with faults (Dupuis et al., 1977). Moreover, in the Somme valley the strong difference between the continental area, with a stepped terrace system, and the submarine extension with overlapping alluvial formations, has been interpreted as the result of a difference in the absolute values of uplift between the present-day continental chalk plateau and the Channel area (Fig. 1B) (Antoine et al., 2000). ...
Article
The Seine, Yonne and Somme are the main rivers draining northwestern France and flowing into the Channel. During Pleistocene cold stages they were tributaries of the “Fleuve Manche”. They are characterised by well-developed stepped terrace systems showing up to 10 incision steps for the last 1 Ma. After 15 years of research and the synthesis of stratigraphical, sedimentological, bioclimatical data and numerical dating, these terrace systems are interpreted as the response of the fluvial environments to climatic cyclicity, superimposed on a background of slow tectonic uplift. The comparison of these three terrace systems shows that the incision budget is similar within the studied area (about 55–65 m/1 Ma), implying relative homogeneity of uplift. In the main part of the area, beyond marine influence, the analysis of each stepped alluvial formation shows regular sediment balance generally composed of coarse periglacial gravels (pleniglacial), covered by fine-grained sediments locally overlain by calcareous tufa (interglacial). However, in the Lower Seine valley, close to the present coast, the low terrace includes two estuarine interglacial units interbedded within periglacial gravel units. Nevertheless, until recently evidence of full interglacial conditions remained elusive in fluvial contexts, especially in the Somme. To address these issues a new research programme has been launched on tufa deposits that are the best candidate to register the climatic optimum. Initial results from la Celle (Seine), Saint-Acheul and Caours (Somme) allow palaeoenvironmental reconstructions of MIS 11 and 5e, and thus confirm the importance of calcareous tufa to define a reference record of Pleistocene Interglacials for northern France. Moreover, it has been previously demonstrated that the main incision leading to terrace formation can be attributed to the transition between interglacial and glacial (during early glacial phases). The new results from interglacial records have important implications concerning the precise occurrence of the major incision phases within the climatic cycle as they provide the starting point immediately preceding downcutting.
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Résumé La révision des nappes alluviales de la Basse Somme intègre dans un même canevas deux séries de points d’observations complémentaires. La première série, proche des graviers de base regroupe les données de V. Commont et de A. Briquet. La seconde série de points appartient pour l'essentiel aux hautes et très hautes nappes, cartographiées par les auteurs dans les années 1980 . Cette distribution conduit à reconnaître un ensemble de 10 nappes principales et de 2 nappes complémentaires, s’inscrivant dans le prolongement des nappes du secteur d’Abbeville. La nappe supérieure étant rapportée à Jaramillo, on dispose donc d’un enregistrement régional complexe mais cohérent, réparti sur environ un million d’années, dont la signature paléoenvironnementale de haute résolution s’accorde avec celles de la séquence isotopique marine ODP 677 et des loess de Chine. Cet enregistrement intègre également l’assemblage laminaire dit de Croix-l’Abbé, situant celui-ci dans la partie médiane de SIM-8, vers 250 ka.
Article
Full-text available
This paper represents a synthetic overview of the main results of the CNRS Programme "Palaeoenvironments and Hominids". This project focused on the study of interactions between human occupation and environmental modification, between 500 and 10 kyrs BP, based on multidisciplinary and diachronic studies of key sites in the Somme Basin. The main analytic results concern ESR, TL-IRSL and 14C dates, which reinforce the chronostratigraphical interpretation of the various Palaeolithic sites. At the same time, bioclimatic analyses of fluvial sediments (palynology, malacology, analysis of Coleoptera), combined with the measurement of δ13C variations of organic carbon, and of the low-field magnetic susceptibility in loess and fossil soils, allowed the restitution of the various Pleistocene environments, and especially of the full Interglacials, to be refined. The main conclusions of the synthesis of archaeological, chronostratigraphical and environmental data are the following. 1) The earliest human occupations in the Somme Basin are no older than 500-450 kyrs BP and are straight away represented by well evolved Acheulean industries (beginning of MIS 12, according to the data from modern excavations). 2) During the last climatic cycle, human occupation of the Somme Basin was clearly discontinuous and strongly influenced by climatic and environmental modifications: maximum during the Early-Glacial, some occupations during the Lower and Middle Maximum Glaciations, rare occurrences around 23-24 kyrs (14C), total abandonment of the area between 23 and 13 kyrs, then reoccupation with the beginning of the Lateglacial climatic improvement. 3) Although the data are less numerous, the model of the last climatic cycle seems to apply during the Middle Pleistocene for the end of the Saalian (occupation at the end of MIS 7 or at the transition MIS7/MIS6, and desertion of the area during the cold maximum of MIS 6). 4) Finally, between 500/450 and 200 kyrs human settlements mainly occurred during periods of climatic transition (Early or Lateglacial). From a general point of view, the human occupation of the Somme Basin during the Middle Pleistocene was therefore discontinuous and strongly influenced by climatic and environmental factors.
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ESR dating of fluvial, fluvio-marine and beach sediment has been tested using the Al centre in quartz grains from a basin and fossil beaches located around the Manche Channel. Dating was performed in conjunction with sedimentological, stratigraphical and neotectonical studies. The technical basis of the method is presented in the light of the study made on recent and fossil sediment in order to establish a procedure for the determination of the palaeodose. The application on a Neogene Basin samples allows to push back the dating ESR limits. Results show that ESR dating of sediment can provide chronozons which are necessary to the uderstanding of the history of a basin where classical chronological markers do not exist.
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The Seine and the Somme are the two main rivers flowing from northwestern France into the Channel. During the Pleistocene cold stages both rivers were tributaries of the River Manche which was exporting sediments into the central deeps of the Channel. The River Seine has a very well developed terrace system recording incision that began at around 1 Ma. The same age is proposed for the beginning of the main incision in the Somme Valley on the basis of morphostratigraphy, pedostratigraphy, palaeontology, palaeomagnetism and ESR datings. The uplift rate deduced from analysis of the Seine and Somme terrace systems is of 55 to 60 m/Ma since the end of the Lower Pleistocene. The response of the two rivers to climatic variations, uplift and sea-level changes is complex and variable in the different parts of the river courses. For example, the evolution of the lower Seine system is influenced by uplift and climate changes but dominated by sea-level changes. In the middle Seine the system is beyond the impact of sea-level variations and shows a very detailed response to climatic variations during the Middle and Upper Pleistocene in a context of uplift. The Somme Valley response appears to be more homogeneous, especially in the middle valley, where the terrace system shows a regular pattern in which incision occurs at the beginning of each glacial period against a general background of uplift. Nevertheless, the lower Somme Valley and the Palaeo-Somme in the Channel area indicate some strong differences compared with the middle valley: influence of sea-level variations and probably differences in rates of tectonic uplift between the Channel and the present continent. The differences in the responses of the two river valleys during the Pleistocene are related to differences in the size of the fluvial basins, to the local tectonic characteristics, to the geometry of the platform connected to the lower parts of the valleys and to the hydrodynamic characteristics of each river. Finally, it is shown from these examples that the multidisciplinary study of Pleistocene rivers is a very efficient tool for the investigation of neotectonic activity.
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The lower valley of the Somme river (northern France) contains a succession of interglacial deposits--fluviomarine sands, littoral bands, alteration zones, tuffs, periglacial deposits, loess, and coarse fluviatile alluvium--which can be correlated with the glaciations of the French Alps. The deposits range from pre-Guenz to post-Wuerm and are identified by their flora and fauna and the remains of prehistoric industries. Cross-sections illustrate the positions of the levels discussed.
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Devismes R., Dupuis Christian, Haesaerts P., De Heinzelin Jean. Fouilles de 1975 a la carriere est du chemin des Salines, Bois-mont, Somme, France. In: Cahiers archeologiques de Picardie, n°4, 1977. pp. 31-42.
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An outline of the palaeogeographic evolution, of the quaternary Sedimentary basin of The Netherlands is given, based on a discussion of 12 maps, figured in the enclosure. In the second part of the paper a documentation of the sources used is given.
nous avons extrait jusqu'à présent plus d'une centaine d'éléments provenant d'un nucleus laminaire à deux plans de frappe
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nous avons extrait jusqu'à présent plus d'une centaine d'éléments provenant d'un nucleus laminaire à deux plans de frappe, déjà partiellement remonté. Les objets sont tous issus d'un sable jaune qui pourrait être analogue aux "Sables Roux" de V
  • D'un Lieu En Cours De Fouille
D'un lieu en cours de fouille, nous avons extrait jusqu'à présent plus d'une centaine d'éléments provenant d'un nucleus laminaire à deux plans de frappe, déjà partiellement remonté. Les objets sont tous issus d'un sable jaune qui pourrait être analogue aux "Sables Roux" de V. Commont mais la stratigraphie reste à élucider.
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