External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba).

45 pagesInternational audienceActive, carbonate-mineralizing microbial mats flourish in a tropical, highly evaporative, marine-fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea-level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O2 and HS− profiles. High rates of O2 production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate-driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water-level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind-driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in-depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings

Processus et typologies du bottomset ausystème turbiditique des Gilbert-deltas(exemple du Golfe de Corinthe, Grèce).Implications stratigraphique et paramètres deforçage

National audienceUne étude sédimentologique, complétée par l’acquisition demodèles 3D photogrammétriques, depuis un Gilbert-delta jusqu’aulobes turbiditiques associés (Golfe de Corinthe, Grèce) a mis enévidence trois typologies de bottomset et la présence de complexesde chenaux/levées turbiditiques.Les bottomsets sont principalement formés par trois associationsde facies : (1) des dépôts finement laminés correspondant principalementà des silts, (2) des turbidites de haute densité (DeepWater Massive Sandstone, DWMS ) et (3) une alternance de siltset de conglomérats avec une architecture en ”chutes and pools”.La première association de facies, les silts fins, montre peu de ridesde courant, les flux sont faibles et l’essentiel des dépôts est stockéen amont. La deuxième typologie (DWMS) forme un cône enpied du delta qui rétrograde sur les pentes des foresets. Les faciesde DWMS : bancs métriques, riches en sables, faiblement triés etclassés sont interprétés dans la littérature comme étant déposéspar un écoulement turbulent. L’essentiel des sédiments est stockéen pied de delta, seuls les facies les plus fins sont exportés dansle bassin. La troisième typologie de bottomset, silt-conglomérat,montre de très nombreuses évidences transits (by-pass) et une architecturechenalisante. Un flux important associé à un ressauthydraulique stationnaire à la rupture de pente est nécessaire à laformation ce cette typologie. Une partie des sédiments avec desgranulométries allant des sables à l’argile, est exportée dans lebassin.Dans la partie aval du système, le pro-delta, l’interprétation dumodèle d’affleurement 3D et l’analyse de la cartographie réaliséeont permis de caractériser l’architecture des complexes chenauxlevéesturbiditiques. En amont, ils forment des ” ceintures ”de chenalisation avec des axes d’écoulements rectilignes, puis lesécoulements se concentrent pour former des complexes de plusgrandes tailles et aggradants. Les chenaux sont comblés par desconglomérats (galets, granules et sables grossiers à moyens) impliquantun export important du delta vers le bassin d’élémentsgrossiers.Les modèles existants dans la bibliographie ne permettent pasd’expliquer les observations, notamment l’export des sédimentsles plus grossiers. L’étude détaillée des associations de faciès etdes éléments architecturaux, nous permet de présenter un nouveaumodèle de dépôt générique ainsi qu’un modèle stratigraphique afinde discuter les différents paramètres de forçage

FROM ALLUVIAL FAN TO DISTAL TURBIDITIC SYSTEMS:PRELIMINARY RESULTS ON DEEP-WATER BYPASS PROCESSES BASED ONGILBERT-TYPE FAN DELTA OUTCROPS OF THE CORINTH RIFT (GREECE).

International audienceSource-to-sink (S2S) can be seen as the integration with other earth science disciplines;from geomorphology, sedimentary geology to basin geodynamic analysis. A major limitationto go further is the access to the entire basin geometry (proximal to distal setting, “closed” box,deep crustal and lithospheric geometries) as well as age constrains.We present the preliminary results of a sedimentological and sequence stratigraphyanalysis of outcropping Gilbert-type deltas from the Corinth rift (Plio-Pleistocene). Drone acquisitionhas also been carried out to propagate the correlations in 3D along steep cliffs. Thestudy case provides an unique opportunity to describe the sedimentary facies and correlatedthem though space and time from the proximal to the most distal setting, in a very wellconstrained basin geometry and timing. In this communication, we will mainly focus on deepwatersediment bypass in terms of flow processes and associated stratigraphic expression, as itis a key issue to address sedimentary budget analysis in a S2S perspective.A new bottomset typology has been established including 4 end-members: (1) Gravelybottomset, (2) Major erosion – by pass, (3) Fine-grained starved bottomset, and (4) Deep WaterMassive Sand (DWMS) bottomset. The relationship between the bottomset end-members andtheir associated turbiditic system has also been characterized. (1) During Gravely bottomsetdeposition, unconfined to low relief channel-levee complex occurs at the very end of the bottomset.(2) The major erosion in bottomset occurs when the foresets present a maximal progradationrate, the bottomset is eroded and bypassed. Coarse- to fine-grained sediments areexported in the turbiditic system to form large channel-levee complex. (3) During Fine-grainedstarved bottomset deposition, limited amount of sediment feed the turbiditic system from thedelta. Indeed, the major part of the sediments is stored in topset and foreset areas. (4) Deepwater massive sand bottomset forms coarse-grained slope apron geometry on the delta toe.Coarse-grained material is massively stored in the bottomset, and only fine-grained sedimentsare exported to the turbiditic system to form lobe complex. Superimposed onto this dynamic,evidences of contouritic events have also been identified

SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY ANALYSIS OFFLUVIAL AND AEOLIAN SYSTEMS WITHIN TERRESTRIAL ANDNEARSHORE ENVIRONMENTS: NOVEL INSIGHT AND CLIMATEIMPLICATION

International audienceStratigraphic correlations are often considered as impossible in thick accumulations of fluvial depositswhere arid conditions are often underestimated. In fact, most of the time, aeolian deposits in arid environmentsdominated by ephemeral alluvial fan or braided river are poorly preserved and particular attention should bepaid to any sedimentological marker of aridity, such as wind-worn stones (ventifacts), sand-drift surfaces andaeolian sheets. Only detailed study of alluvial plain facies and ephemeral streams provide indications ofclimatic conditions within the sedimentary basin, whereas fluvial system only reflects climatic conditions ofthe upstream erosional catchment. Moreover, in such context the deposition of conglomerate of alluvial fansystem could correspond to a peak of aridity without an increase in tectonic activity.Conversely, the sedimentary record of aeolian deposits, often considered as preserved in arid desertbasin, can occur under semi-arid and even humid climates, and from near the Equator to the Arctic Circle.Ventifacts are climate-sensitive sedimentary features that provide evidence for long periods without anyvegetation in terrestrial environment under hot or cold climate. Therefore, they can allow recognizing desertconditions, even where no aeolian dune deposits are preserved. Moreover, the well-preserved widespreadpaleoregs seem to mark major stratigraphic discontinuity above a geomorphic surface remained a long timewithout vegetation as a result of a very dry climate, hot or cold.Detailed sedimentological study and the recognition of bounding surfaces of regional extend (sanddriftsurface, palaeosols, flooding surface...) allow for the identification of stratigraphic cycles, permit correlationwithin time and space and precise the climate conditions. Moreover, these detailed sedimentological andstratigraphic analyses highlight the influence of climate or tectonic change on sediment facies and depositionalenvironments.Applied to various Permian and Triassic continental sections in Europe and USA and Jurassic–Cretaceous sections in China, this methodology allows to define fluvial and aeolian interaction processesthrough time and space, to discuss the allogenic vs autogenic controls and the preserved sediment record withinterrestrial and nearshore environments. The recognition of sand-drift surface and maximum flooding episodeat the scale of genetic unit, provide a valuable tool for stratigraphic correlation within continental environmentsdevoid of any biostratigraphic markers. Moreover, such results allow to discuss the climate condition of thebasin

Enhanced development of lacustrine microbialites on gravity flow deposits, Great Salt Lake, Utah, USA.

12 pagesInternational audienceThe Great Salt Lake, Utah, USA is a shallow, hypersaline, intracontinental lake hosting extensive microbial deposits. At a large spatial scale, the distribution of these deposits is driven by environmental and geodynamical factors (i.e. water-level fluctuations and a fault-related framework). A detailed mapping of the Buffalo Point area, in the north-western part of Antelope Island, indicates the presence of an anomalous concentration of microbial deposits dated ca. 5.8 ka BP and distributed along a lobe-shaped geometry. This uncommon microbial deposit geometry results from an extensive colonization of a conglomerate substrate exhibiting an accumulation of m-sized rounded Cambrian quartzite boulders. We suggest that this conglomerate substrate provides a stable nucleation point that promotes the development and preservation of the lobe-shaped microbial deposits. Microbial deposits may also have protected the conglomerate substrate from erosional processes and thereby increased the preservation potential of the lobe-shaped structure. Based on the characteristics of the conglomerate (e.g. grain size, texture) and its location (i.e. 200 m beyond the average shoreline), this lobe-shaped structure likely results from subaqueous debris or a hyperconcentrated density flow that transports sedimentary material from the Buffalo Point slopes downward to the shore. We estimate the age of the conglomerate deposition to be between 21 and 12 ka BP. The initiation of the flow may have been triggered by various mechanisms, but the existence of a major active normal fault in the vicinity of these deposits suggests that an earthquake could have destabilized the accumulated sediments and resulted in conglomerate emplacement. The catastrophic 15 ka BP Bonneville Flood, which led to a drop in the lake level (approximately 110 m), may also provide an explanation for the initiation of the flow