Integration of sedimentology and GPR for high-resolution imaging of a carbonate platform

Ground-Penetrating Radar (GPR) has not been widely applied to the recognition of ancient carbonate platform geometries. This paper reports the results of an integrated study performed on an Upper Jurassic outcrop from the southeast Paris basin, where coral bioherms laterally change into prograding depositional sequences. GPR profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A GPR profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the GPR (20 meters with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (some few meters) and seismic scale (several hundreds of meters). This study supports recent evidence of icehouse conditions and induced sea-level fluctuations controlling the Upper Jurassic carbonate production

Hydrodynamic behaviour of Nummulites: implications for depositional models

Large benthic foraminifers are considered to be good indicators of shallow marine carbonate environments in fossil series. Over the last 50 years, the palaeoenvironment of Tertiary Nummulites accumulations has been a matter of debate, particularly because of difficulties in interpreting these deposits, and in this way, the absence of analogues in present-day seas does not help. The aim of this paper is to insight the different ways Nummulites tests and clasts may accumulate according to their hydrodynamic behaviour. Based on experimental measurements and on SEM observations, it appears that the high primary skeletal porosity of Nummulites made them easily transportable. The calculated threshold shear velocities confirm that large-sized Nummulites can be moved by weak wave-driven currents. This peculiar hydrodynamic behaviour of Nummulites could explain the diversity of depositional models. Depending on local hydrodynamic conditions, autochthonous Nummulites deposits can be preserved as in situ winnowed bioaccumulations or be accumulated offshore, onshore or alongshore, away from the original biotop

Volcaniclastic sedimentation on the submarine slopes of a basaltic hotspot volcano: Piton de la Fournaise volcano (La Réunion Island, Indian Ocean)

International audienceVolcaniclastic successions are well-described in volcanic arc setting but rare in hotspot environments. The present work proposes a facies model of volcaniclastic sedimentation related to basaltic hotspot volcanoes as exemplified by the Piton de la Fournaise volcano (La Réunion Island). The facies model is based on a multi-scale approach combining high-resolution multibeam and backscatter data, deep-water photographs, side scan imagery and Kullenberg piston cores. Data show that a wide range of gravity flows and erosional features develop in the active volcaniclastic sedimentary system. Coastal and submarine instabilities are the main processes shaping the volcano's submarine morphology. Meanwhile, the evolution and dynamics of the sedimentary system are strongly linked with the morpho-structural evolution of the subaerial part of the volcano. The proposed model is characterized by a cyclic succession of stages: (1) a growing stage during which sedimentary activity is mainly restricted to proximal and mid-slope areas; (2) a collapse stage that entirely reshapes the morphology of the submarine slopes; and (3) an erosional stage related to a slow down of volcanic activity, enabling the development of large deep-sea fans

Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island

International audienceRecent oceanographic surveys revealed the existence of five volcaniclastic deep-sea fans off La Réunion Island. The Cilaos fan is a large volcaniclastic submarine fan, connected to rivers that episodically experience torrential floods through a narrow and steep shelf-slope system. New piston cores presented in this study together with echosounder profiles give new insight into the evolution of this extensive and sand-rich turbidite system. The Cilaos fan extends over 15,000 km2 on an abyssal plain and is compartmentalized by topographic highs. Located southwest of the island, the sedimentary system consists of a canyon area and a deep sea fan divided into a proximal and a distal fan. The proximal fan is characterized by its wide extent and coarse-grained turbidites. The distal fan is characterized by elongated structures and fine-grained turbidites. A detailed morphological study of the fan which includes the analysis of swath bathymetry, backscatter, echosounder, and piston core data shows that the Cilaos fan is a complex volcaniclastic deep-sea fan, highly influenced by preexisting seafloor irregularities. The canyons and the slope area show a complex and evolving sediment feeding system with a direct sediment input by the river and irregular sediment supply by submarine landslide. Three main construction stages are identified for this system: (1) an old incision phase of the channels forming wide turbidites extending over the entire distal fan; (2) a period of no or low activity characterized by a thick layer of hemipelagic mud; and (3) a local reactivation of the channel in the proximal fan. Each stage seems to be linked to a different sediment source with a progressively increasing contribution of hemipelagic sediment and mud in younger stages

Direct sediment transfer from land to deep-sea: Insights into shallow multibeam bathymetry at La Réunion Island

International audienceSubmarine canyon heads are key areas for understanding the triggering factors of gravity currents responsible for the transfer of detrital sediment to the deep basins. This contribution offers a detailed picture of canyon heads off La Réunion Island, with high-resolution multibeam bathymetry in the water depth range of 4-220 m. The present feeding of the Cilaos turbidite system, one of the largest modern volcaniclastic systems in the world, is deduced from morphological and sedimentological interpretations of newly acquired data. The study highlights small-scale sedimentary features indicating hydrodynamic and sedimentary processes. A direct connexion between the Saint-Etienne river mouth and submarine canyons is evidenced by the complete incision of the shelf and the presence of canyon heads connected to the modern deltaic bar. This direct connection, supplied by river torrential floods (cyclonic floods every two or three years), suggests the continuity of high-density fluvial flows to submarine gravity flows, forming hyperpycnal flows in the canyon. The initiation of secondary submarine gravity flows by storm waves (large austral waves and cyclonic waves) is also proposed for submarine canyons with large canyon heads developed in the surf zone from a sandy coastal bar. Bedforms in active canyon axis are considered as an indicator of the frequent activity of high-density turbidity currents. Moreover, a morphological record of last glacial and deglacial sea level variations is preserved, and particularly the Last Glacial Maximum sea level with the presence of small vertical cliffs, observed in this bathymetric data, which likely corresponds to a paleo-shoreline or paleo-reefs

Crise sismo-volcanique de Mayotte : réactivation et évolution en-direct d’une ride volcanique sur la pente est de l’île mise en lumière par la bathymétrie et la réflectivité multifaisceaux.

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Deltaic and Coastal Sediments as Recorders of Mediterranean Regional Climate and Human Impact Over the Past Three Millennia

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Integration of sedimentology and ground-penetrating radar for high-resolution imaging of a carbonate platform

International audienceGround-penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south-east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground-penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground-penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground-penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea-level fluctuations controlling the Upper Jurassic carbonate production

Facies Distribution and Geometry of El Gueria Formation in Concession 137N: Preliminary Results of a Field Analog Study from Kesra Plateau

As a field analog to "B" field in Concession 137N in Tarabulus Basin, Kesra Plateau (80 km NW of Kairouan city, Tunisia) and its late Ypresian-Lutetian carbonate units have been studied in detail. These nummulitic facies also present a good field case for studying the facies distribution and geometry in a nummulitic depositional system affected by submarine palaeohighs. Two main facies tracts have been recgnized: 1) nummulitic packstone and grainstone, well developed in the western part of the plateau; and 2) skeletal-poor dolomitised wackstones and packstones with abundant nummulithoclasts well represented in the northern, southeastern and eastern parts. Between these two end members, high-energy facies with oysters and large robust nummulites passing to forebank facies with adundant discocyclinids and flat nummulties can be observed. Storm-deposited events and a high degree of bioturbation have been observed within the low-energy deposits with common nummulitic debris. A sedimentological model has been established for the nummulitic facies of Kesra Plateau showing similar facies variations as the ones in "B" field in offshore Concession 137N. Sequence stratigraphic analysis allowed correlation of nummulitic and nummulithoclastic facies, which were probably interfingered during this period. Structural activity was a major control in both areas. The main difference observed between these two models is the lack of lagoonal facies (characterised by abundant miliolids) in Kesra Plateau, whereas they are present and well represented in B field.</p