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Cossart E.,Paris-Sorbonne University | Fort M.,University Paris Diderot | Bourles D.,French National Center for Scientific Research | Carcaillet J.,Laboratoire Of Geodynamique Des Chaines Alpines | And 3 more authors.
Catena | Year: 2010

Recent advances in geochronology provide opportunities to identify the glacial and periglacial sequences in mountain area. In this paper, we focus on the Upper Durance catchment (Southern French Alps), in the Clarée valley, where no absolute chronological benchmark was defined. Glacial remnants and periglacial features were investigated, integrated within a chronological scenario thanks to relative (weathering rind thickness) and absolute (CRE technique) dates. We find evidence of the existence of a Late-Glacial valley glacier, that has never been reported before. Then the upper valleys became ice-free during a period corresponding to the Early-Holocene Climate Optimum, whereas two sequences of cirque glaciation are identified during the second half of the Holocene. We also show evidence of three main generations of rockglaciers, which also mainly developed during the second half of the Holocene, probably at the end of the Subboreal period. The location and the altitudes of rockglaciers are strongly dependent on the geomorphic setting (especially the location of the sediment sources), hence complicating their palaeo-climatic interpretations. © 2009 Elsevier B.V. All rights reserved.

Dubille M.,Laboratoire Of Geodynamique Des Chaines Alpines | Lave J.,French National Center for Scientific Research
Basin Research | Year: 2015

Radical grain size changes between two main units of a sedimentary megacycle in a foreland basin are commonly interpreted to result from changes in tectonic activity or climate in the adjacent mountain range. In central Nepal, the Cenozoic Siwalik molasse deposits exposed in the frontal Himalayan folds are characterized by such a radical grain size transition. Locally gravel deposits completely replace sands in vertical succession over approximately a hundred metres, the median grain size (D50) displaying a sharp increase by a factor of ca. 100. Such a rapid gravel-sand transition (GST) is also observed in present-day river channels about 8-20 km downstream from the outlet of the Siwalik Range. The passage from gravel-bed channel reaches (proximal alluvial fans) to sand-bed channel reaches (distal alluvial fans) occurs within a few kilometres on the Gangetic Plain in central Nepal, and the D50 ratio between the two types of channels equals ca. 100. We propose that the dramatic and remarkably similar increase in grain size observed in the Neogene Siwalik series and along modern rivers in the Gangetic foreland basin, results from a similar hydraulic process, i.e. a grain sorting process during the selective deposition of the sediment load. The sudden appearance of gravels in the upper Siwalik series would be related to the crossing of this sorting transition during progressive southward migration of the gravel front, in response to continuous Himalayan orogen construction. And as a consequence, the GST would be diachronous by nature. This study demonstrates that an abrupt change in grain size does not necessarily relate to a change in tectonic or climatic forcing, but can simply arise from internal adjustment of the piedmont rivers to the deposition and run out of coarse bedload. It illustrates, in addition, the genesis of quartz-rich conglomerates in the Himalayan foreland through gravel selective deposition associated with differential weathering, abrasion processes and sediment recycling during thrust wedge advance and shortening of the foreland basin. © 2014 The Authors.

Bonin A.,CNRS Biogeosciences Laboratory | Vennin E.,CNRS Biogeosciences Laboratory | Puceat E.,CNRS Biogeosciences Laboratory | Guiraud M.,CNRS Biogeosciences Laboratory | And 4 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2012

The Valanginian is marked by a major platform demise inducing a hiatus in the northern Tethyan neritic carbonate record from the top of the lower Valanginian to the lower Hauterivian. New biostratigraphic and chemostratigraphic data from the Ollioules section (Provence Platform, southern France) are presented here, demonstrating that a large part of the upper Valanginian is preserved in an inner platform environment. The thick, upper Valanginian, aggrading carbonate succession is observed in an aborted rift domain, implying relatively low subsidence. In this context, a relatively long-term sea-level rise was required to sustain a keep-up style of carbonate production. Like the Apulian Platform, the remarkable preservation of the Provence Platform may have been favored by its remoteness from terrigenous source areas, as suggested by the low clastic inputs and low P-accumulation rates. Two main biotic community replacements are observed in Ollioules. The first saw the development of abundant microbialites and algae at the onset of the late Valanginian. A Tubiphytes concentration occurred during the coolest climatic conditions and the transition towards arid conditions, whereas the subsequent Lithocodium-Bacinella and orbitolinids assemblages developed under low nutrient conditions during a warmer interval. Both assemblages may have been triggered by increased alkalinity. The second community replacement saw the installation of coral- and rudist-dominated communities during the latest Valanginian to early Hauterivian. They indicate a change to oligotrophic, open marine conditions. Six medium-scale sequences have been defined in Ollioules, indicating short-term transgressive-regressive trends superimposed on a long-term transgression. Low nutrient inputs and relatively low subsidence in an aggradational context may explain the survival of the isolated Provence Carbonate Platform during a time of widespread drowning episodes and platform demise in the northern Tethyan domain. © 2012 Elsevier B.V.

Cossart E.,University of Paris Pantheon Sorbonne | Bourles D.,Center Europeen Of Recherche Et Denseignement Des Geosciences Of Lenvironnement | Braucher R.,Center Europeen Of Recherche Et Denseignement Des Geosciences Of Lenvironnement | Carcaillet J.,Laboratoire Of Geodynamique Des Chaines Alpines | And 2 more authors.
Geomorphologie: Relief, Processus, Environnement | Year: 2011

This paper aims to provide an overview of the investigations led in the Upper-Durance catchment (upstream Guillestre), focusing on the disappearance of glaciers after the Last Glacial Maximum. It is based on extensive fieldwork, describing and mapping the geomorphic remnants of past-glaciations, and 35 Cosmic Ray Exposure ages. A four-stage chronological sequence summarises glacial variations during the Late-Glacial/Holocene transition. The main results provide both chronological data (two generations of moraines identified as Younger Dryas and Preboreal) and spatial data (asymmetry of glaciers development in the Upper-Durance catchment during the Late-Glacial), providing new findings that may be useful for palaeo-climatologists.

Sulpizio R.,CIRISIVU | Van Welden A.,Laboratoire Of Geodynamique Des Chaines Alpines
Journal of Quaternary Science | Year: 2010

Two cores were recovered in the southeastern part of Lake Shkodra (Montenegro and Albania) and sampled for identification of tephra layers. The first core (SK13, 7.8m long) was recovered from a water depth of 7m, while the second core (SK19, 5.8m long) was recovered close to the presentday shoreline (water depth of 2m). Magnetic susceptibility investigations show generally low values with some peaks that in some cases are related to tephra layers. Nakedeye inspection of the cores allowed the identification of four tephra layers in core SK13 and five tephra layers in core SK19. Major element analyses on glass shards and mineral phases allowed correlation of the tephra layers between the two cores, and their attribution to six different Holocene explosive eruptions of southern Italy volcanoes. Two tephra layers have undersaturated composition of glass shards (foiditic and phonolitic) and were correlated to the AD 472 and the Avellino (ca. 3.9cal.ka BP) eruptions of SommaVesuvius. One tephra layer has benmoreitic composition and was correlated to the FL eruption of Mount Etna (ca. 3.4cal.ka BP). The other three tephra layers have trachytic composition and were correlated to Astroni (ca. 4.2cal.ka BP), Agnano Monte Spina (ca. 4.5cal.ka BP) and Agnano Pomici Principali (ca. 12.3cal.ka BP) eruptions of Campi Flegrei. The ages of tephra layers are in broad agreement with eight 14C accelerator mass spectrometric measurements carried out on plant remains and charcoal from the lake sediments at different depths along the two cores. The recognition of distal tephra layers from Italian volcanoes allowed the physical link of the Holocene archive of Lake Shkodra to other archives located in the central Mediterranean area and the Balkans (i.e. Lake Ohrid). Five of the recognised tephra layers were recognised for the first time in the Balkans area, and this has relevance for volcanic hazard assessment and for ash dispersal forecasting in case of renewed explosive activity from some of the southern Italy volcanoes. © 2009 John Wiley & Sons, Ltd.

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