Les formations superficielles entre la Loire et le Loir (sud-ouest du Bassin parisien) : les enseignements de l’analyse sédimentologique

L’analyse sédimentologique des formations détritiques tertiaires et quaternaires situées sur l’interfluve Loire-Loir, en Touraine, permet de caractériser plusieurs nappes détritiques. Datée du Pliocène supérieur par la présence de grains de quartz issus de ponces du Mont-Dore, la formation des Sables de Château-Renault et de Montreuil n’est constituée pour les autres dépôts que d’apports locaux, ce qui va à l’encontre de l’hypothèse d’un écoulement chenalisé de la Loire-amont vers l’Atlantique à cette époque. Issus du remaniement de dépôts marins locaux, les Sables de Mazières sont probablement un peu plus récents. Les premières alluvions quaternaires au nord de Nazelles, mieux classées, très riches en zircon, témoignent d’un transport fluviatile lointain effectué par une paléo-Loire chenalisée. Enfin, des sables soufflés, provenant des alluvions du Val de Loire, recouvrent localement les Sables de Mazières.In the Touraine area, the plateaux which form the Loire-Loir interfluve stand at rather low and equal altitudes and bear locally detrital deposits which are not easy to identify (fig. 1, 2). The sedimentological study of those deposits makes it possible to distinguish several formations (fig. 3, 4 and tabl. 1, 2): The Château-Renault sands and the Montreuil sands look very much the same because of their bad sizing, their strong weathering and their heavy minerals distribution: a great deal of staurolite and not any garnet. The presence of pumice quartz, coming from the Mont-Dore eruption, makes it likely to date the Montreuil sands back to the Upper Pliocene. The Mazières sands, located north-west of Tours, come from the erosion of local marine deposits as it is proved by the morphoscopy and the absence of garnet and zircon which are present in the Faluns and the Sologne sands. The alluvial deposits, located in the north of Nazelles, are better grain-sized and very rich in zircon and therefore may have been brought from upstream by the river Loire. The upper sands covering the Mazières sands are abundant in round and mat grains and heavy minerals of volcanic origin and have been brought by the wind from the alluvial deposits of the Val de Loire. If a part of the Montreuil sands certainly dates back to the Upper Pliocene, on the other hand their sedimentological characteristics invalidate the hypothesis of a flow of the upstream Loire towards the Atlantique in that period. The curving towards the Atlantique did not take place until the ground was slightly abrased before the accumulation of the alluvial deposits which are preserved north of Nazelles (fig. 5). Coming from local substratum, those detrital formations, which are preserved on very closed levels, prove the relative tectonic steadiness in the Touraine area

les formations superficielles entre la Loire et le Loir (sud-ouest du Bassin parisien) : les enseignements de l’analyse sédimentologique

L’analyse sédimentologique des formations détritiques tertiaires et quaternaires situées sur l’interfluve Loire-Loir, en Touraine, permet de caractériser plusieurs nappes détritiques. Datée du Pliocène supérieur par la présence de grains de quartz issus de ponces du Mont-Dore, la formation des Sables de Château-Renault et de Montreuil n’est constituée pour les autres dépôts que d’apports locaux, ce qui va à l’encontre de l’hypothèse d’un écoulement chenalisé de la Loire-amont vers l’Atlantique à cette époque. Issus du remaniement de dépôts marins locaux, les Sables de Mazières sont probablement un peu plus récents. Les premières alluvions quaternaires au nord de Nazelles, mieux classées, très riches en zircon, témoignent d’un transport fluviatile lointain effectué par une paléo-Loire chenalisée. Enfin, des sables soufflés, provenant des alluvions du Val de Loire, recouvrent localement les Sables de Mazières.In the Touraine area, the plateaux which form the Loire-Loir interfluve stand at rather low and equal altitudes and bear locally detrital deposits which are not easy to identify (fig. 1, 2). The sedimentological study of those deposits makes it possible to distinguish several formations (fig. 3, 4 and tabl. 1, 2): The Château-Renault sands and the Montreuil sands look very much the same because of their bad sizing, their strong weathering and their heavy minerals distribution: a great deal of staurolite and not any garnet. The presence of pumice quartz, coming from the Mont-Dore eruption, makes it likely to date the Montreuil sands back to the Upper Pliocene. The Mazières sands, located north-west of Tours, come from the erosion of local marine deposits as it is proved by the morphoscopy and the absence of garnet and zircon which are present in the Faluns and the Sologne sands. The alluvial deposits, located in the north of Nazelles, are better grain-sized and very rich in zircon and therefore may have been brought from upstream by the river Loire. The upper sands covering the Mazières sands are abundant in round and mat grains and heavy minerals of volcanic origin and have been brought by the wind from the alluvial deposits of the Val de Loire. If a part of the Montreuil sands certainly dates back to the Upper Pliocene, on the other hand their sedimentological characteristics invalidate the hypothesis of a flow of the upstream Loire towards the Atlantique in that period. The curving towards the Atlantique did not take place until the ground was slightly abrased before the accumulation of the alluvial deposits which are preserved north of Nazelles (fig. 5). Coming from local substratum, those detrital formations, which are preserved on very closed levels, prove the relative tectonic steadiness in the Touraine area

Lahar-related impacts after the 2010 eruption of Merapi volcano (Java, Indonesia)

International audienc

Methods, spatial and temporal scales to appraise volcanic environments in geography

The second edition of the Encyclopedia of Volcanoes published by Haraldur Sigurdsson et al. (2015) makes an emphasis on the physical volcanology and on the characteristics of the eruption, but provides the reader with a fairly good overview of all the other issues related to volcanoes. The all span of the earth sciences, but also the social sciences have brought their own topics and methods and have widened the knowledge about the specificities of the volcanic environment. Thus, they have con..

Methods, spatial and temporal scales to appraise volcanic environments in geography

The second edition of the Encyclopedia of Volcanoes published by Haraldur Sigurdsson et al. (2015) makes an emphasis on the physical volcanology and on the characteristics of the eruption, but provides the reader with a fairly good overview of all the other issues related to volcanoes. The all span of the earth sciences, but also the social sciences have brought their own topics and methods and have widened the knowledge about the specificities of the volcanic environment. Thus, they have con..

Approche des environnements volcaniques en géographie : méthodes et échelles

Si la seconde édition de de l’Encyclopedia of Volcanoes, publiée chez Academic Press par Haraldur Sigurdsson et al., (2015) fait la part belle au volcanisme et aux phénomènes éruptifs, elle offre néanmoins un assez bon aperçu de l’élargissement thématique des études portant sur les volcans. Ils sont devenus des objets scientifiques et des terrains étudiés à travers les méthodes et problématiques d’autres sciences. Les recherches anthropologiques sur les représentations (Dove, 2007) ou les rel..

Quand l’aléa devient la ressource : l’activité d’extraction des matériaux volcaniques autour du volcan Merapi (Indonésie) dans la compréhension des risques locaux

The valleys of the Merapi volcano are regularly filled with thick pyroclastic and volcaniclastic deposits, characterised by a large grain-size distribution, from fine ash to boulders. An important but quite informal mining of these materials (sands and blocks) has developed, and has become widespread around the volcano. It is now an essential activity in the spatial organisation of the Merapi volcano, and seems to play a major role in the local socio-economic pattern. Volcanic hazards such as pyroclastic flows or lahar flows produce a substantial resource which endangers people who dig these deposits. But the important benefits related to this activity encourage people to work in hazard-prone areas. Moreover, at a broader scale, block and sand mining generates many interactions and retroactions which seem to be a real factor of vulnerability for Merapi’s surrounding communities. Thus, this activity has to be taken into account when conducting risk and vulnerability studies around the volcano. They also help to better understand how people live and work there, and why most of them choose to stay and work even during intense volcanic activity

Géographie de l'environnement

à veni

The 2007 eruption of Kelut volcano (East Java, Indonesia): Phenomenology, crisis management and social response

International audienc