High elevation of low-relief surfaces in mountain belts: does it equate to post-orogenic surface uplift?

International audienceWe present experiments of upraising and relaxing topographies showing that peneplanation can occur above the ultimate base level (sea level). After active uplift, the erosion of a topography bounded by a piedmont generates a final smooth and highly elevated topography. Smoothing at high elevation is even possible during active uplift if the evolution of topography is disrupted by the deposition of the products of erosion on its piedmont which is the case at the transition from underfilled to overfilled conditions in foreland basins

Reply to comment by Yanni Gunnell and Marc Calvet on "Origin of the highly elevated Pyrenean peneplain"

4 p.International audienceGunnell and Calvet [2006] (hereinafter referred to asGC) challenge the recent model that we proposed for theorigin of the highly elevated Pyrenean peneplain by contest-ing our morphometric analysis of this chain and the relationwe made between the morphological evolution and thepiedmont sedimentation. Their reasoning is as follows: (1)According to Calvet [1996] (on which their comment islargely based) the high-elevation, low-relief surfaces in theEastern Pyrenees are remnants of a peneplain that devel-oped before the Pliocene from applanation near to sea level,and which was later uplifted by 2000 m during the Plio-Quaternary (in other words, GC belong to the ‘‘applanation’’school, whereas we woul d belong to the ‘‘altiplanation’’school); (2) high-elevation, low-relief surfaces do not existin the Central Pyrenees; (3) therefore the relationships wemade between the morphology of the Central Pyrenees andthe pattern of the detrital sedimentation in the adjacent Ebroforeland basin is meaningless; (4) contrary to the initialinterpretation of Calvet [1996], GC recognize that crustalthickening did not develop since the Pliocene in the EasternPyrenees, so they appeal to another geodynamical processsuch as extension or lithosphere delamination to explain thesupposed uplift

Loess in eastern equatorial Pangea archives a dusty atmosphere and possible upland glaciation

Carboniferous−Permian strata in basins within the Central Pangean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (Pangea assembly, ice-age collapse, megamonsoon inception). The voluminous (∼1.5 km) succession of exclusively fine-grained red beds that comprises the Permian Salagou Formation (Lodève Basin, France) has long been interpreted to record either lacustrine or fluvial deposition, primarily based on a local emphasis of subaqueous features in the upper ∼25% of the section. In contrast, data presented here indicate that the lower-middle Salagou Formation is dominated by up to 15-m-thick beds of internally massive red mudstone with abundant pedogenic features (microscale) and no evidence of channeling. Up-section, limited occurrences of ripple and hummocky cross-stratification, and mudcracks record the intermittent influence of shallow water, but with no channeling nor units with grain sizes exceeding coarse silt. These data suggest that the most parsimonious interpretation for the Salagou Formation involves eolian transport of the sediment and ultimate deposition as loess in shallow, ephemeral lacustrine environments. Provenance analyses of the Salagou Formation indicate coarse-grained protoliths and, together with geochemical proxies (chemical index of alteration [CIA] and τNa) that correspond respectively to a low degree of chemical weathering and a mean annual temperature of ∼4 °C, suggest that silt generation in this case is most consistent with cold-weathering (glacial and associated periglacial) processes in the Variscan highlands. Together with previous studies that detailed voluminous Permian loess in western equatorial Pangea, this work shows a globally unique distribution of dust at low latitudes that can be linked either directly to glaciated alpine terranes or to reworked and deflated deposits of other types (e.g., fluvial outwash) where fine-grained material was originally generated from glacial grinding in alpine systems. These results further support a revised model for early Permian climate, in which extratropical ice sheets coexisted with a semiarid tropics that may have hosted significant ice at moderate elevation.This work was supported by the National Science Foundation (NSF) under International Research Ex-periences for Students (IRES) grant OISE-1658614 (principal investigators G.S. Soreghan, M.J. Soreghan) and also EAR-1338331 (Sedimentary Geology and Paleobiology Program; to G.S. Soreghan). Financial support for partially offsetting publication costs was provided from the Office of the Vice President for Research and the Office of the Provost, University of Oklahoma

Origin of the high elevated Pyrenean peneplain

Peneplanation of mountain ranges is generally considered the result of long-term erosional processes that smooth relief and lower elevation near sea level. Therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift. Such an interpretation has been proposed for the Pyrenees where high-elevation, low-relief erosional surfaces rose up to more than 2000 m. Because the Pyrenean foreland basins are filled with very thick continental deposits, which have buried the early jagged landscape, we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering. We propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range, therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief. Such a process allows a high-elevation, low-relief erosional surface to develop at the scale of the range. In the Pyrenees, occurrence of high-elevation, low-relief erosional surface remnants does not imply a posttectonic uplift, but is instead due to the dissection of the initial Miocene high-elevation, low-relief surface by the recent drainage system, the erosive activity of which has been enhanced by global climate change from the late Pliocene onward

Escarpement de faille synsédimentaire (perturbations des écoulements gravitaires sous-marins et détermination de la cinématique des failles)

Le mouvement des failles synsédimentaires peut induire la création d’escarpement pendant la sédimentation dont l'importance est analysée selon deux aspects: (1) Impact sur les ecoulements de densité sédimentaires sous-marins Des escarpements d'une hauteur décimétrique/métrique perturbent fortement les écoulements sous-marins en favorisant (a) les érosions, le transit sédimentaire, et le dépôt des levées turbiditiques sur l'escarpement, (b) la transformation d'un écoulement hyperconcentré en un écoulement turbulent avec la localisation de faciès spécifique, (c) une canalisation des écoulements de débris parallèlement aux failles. (2) Détermination de la cinématique des failles synsédimentaires avec des données sismique. L'intégration des escarpements permet (a) une meilleure détermination de l'influence respective des variations des apports sédimentaires et des variations du déplacement, sur les variations d'épaisseur des strates de croissances et (b) d’accéder à la lithologie des dépôts.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Impact of synsedimentary metre-scale normal fault scarps on sediment gravity flow dynamics: An example from the Grès d'Annot Formation, SE France

International audienceSynsedimentary faults result in the direct interaction between tectonic and sedimentary processes at similar spatio-temporal scales. Sedimentological analysis of sediment-laden gravity flows in the northern part of the Grès d'Annot Formation (Sanguinière sub-basin, Col de la Moutière/Tête Ronde) has revealed the presence of fault scarps of metre-scale height. These synsedimentary fault scarps were sufficient to disturb the sediment gravity flow dynamics resulting in (i) a strong variation of the erosive behaviour of a concentrated flow and (ii) the transformation of a strongly stratified, laminar hyperconcentrated flow into a turbulent flow, in short distance (less than 500 m). These disturbances develop without the flows being deviated by the fault scarps but produce great facies heterogeneity, the least homogeneous facies (mixing sand and clay) being localized on the upstream obstacle side, the most homogeneous (massive sand) downstream

Filling sequence in Late Paleozoic continental basins: A chimera of climate change? A new light shed given by the Graissessac-Lodève basin (SE France)

International audienceThe characteristics of sedimentary deposits in continental basins, especially those controlled by extension, are often used to infer global climatic changes. We question the relevance of such an approach because the stratigraphic pattern of those basins are mostly similar whatever their latitudes/paleolatitudes (i.e. their general climatic conditions), namely the transition from (1) an ancestral drainage network to (2) deep lake with a dominance of black and grey shale deposits, (3) shallow oxic lake with a dominance of red pelites deposits and to eventually (4) a new drainage network. Such transitions observed in Late Palaeozoic continental basins throughout Laurasia and Gondwana are generally interpreted as evidence of global change from a wet climate to an arid climate. From the sedimentological and structural analysis of a Late Palaeozoic basin, the Graissessac-Lodève basin, in the south of the Variscan French Massif Central, we proposed a new tectono-stratigraphic sequence of this basin from the Late Carboniferous to the Early Trias. We estimate the paleo-area (from 295-450 km 2 to 880-1150 km 2) of the basin and we deduce a subsidence curve from the Late Carboniferous to the Late Permian (from 600 ± 200 m/My to 100 ± 10 m/My). We conclude that the Graissessac-Lodève basin stratigraphic pattern corresponds to a classical continental rift sequence controlled by tectonics. In order to assess a possible control of the stratigraphic sequence by the climate, we have reconstructed the paleo-depth of the lake with (1) the morphometric analysis of the wave ripples and (2) with an estimation of the lacustrine slope considering the tectonic origin of the lake. We find that the maximum lacustrine depth has evolved from a range of 80-300 m during the Late Carboniferous to 40-45 m during the Late Permian. From our estimates of the different configurations of the basin, we show that the volume of the water in the lake has remained nearly constant between 10 and 30 km 3 , from the Late Carboniferous (300 My) to the Late Permian (250 My). Eventually, we conclude that the sedimentological indicators of shallowing and episodic drying observed from the Late Carboniferous to Late Permian in this basin, and probably in all the Pangea basins of the same type, are the consequence of the basin enlargement by tectonics and its sedimentary filling rather than climatic change