Les séries fluvio-éoliennes du "Cutler Group" :: Permien (Utah, USA). Modèle architectural et expression des unités génétiques

National audienceL'essor qu'a connu la stratigraphie séquentielle dans les environnements continentaux a permis de définir des modèles architecturaux pour des séries fluvio-lacustres. Plus récemment, les modèles proposés ont intégré les dépôts éoliens dans les systèmes continentaux côtiers ou lacustres (préservation de l'éolien vs variations de la table d'eau, e.g. Carr-Crabaugh & Kocurek, 1998) ou dans les systèmes continentaux endoréiques (préservation de l'éolien vs climat (e.g. Clemmensen et al., 1998) ou préservation de l'éolien vs accommodation et flux de sédiments, e.g. Bourquin et al., 2009). Cependant, peu de modèles discutent les variations de préservation des dépôts éoliens par rapport aux environnements continentaux associés (fluvial, lac, playa) afin de prédire un modèle architectural des dépôts éoliens (du réservoir au bassin) en fonction du contexte géodynamique. L'objectif de cet exposé est de contraindre l'expression des unités génétiques et de proposer un modèle architectural montrant les variations de préservation des dépôts éoliens en fonction du contexte tectonique, climatique et eustatique. L'étude des séries du Permien inférieur, " Cutler Group ", du " Paradox Basin " (SE Utah), caractérisées par des dépôts fluvio-éoliens (e.g. Condon, 1997), a été réalisée à partir de 5 sections sédimentologiques. Elles sont localisées sur un profil proximal représenté par des dépôts dominés fluvial jusqu'à des dépôts dominés éolien dans le domaine distal (e.g. Mountney & Jagger, 2004). Une étude sédimentologique et stratigraphique détaillée permet de proposer une évolution spatiale et temporelle des environnements de dépôt. Nous avons ainsi mis en évidence (1) les variations d'expression des unités génétiques d'un environnement proximal à un environnement distal en climat semi-aride, (2) plusieurs modèles architecturaux montrant l'évolution verticale depuis un contexte de bassin côtier, à dunes éoliennes faiblement préservées, vers des environnements plus continentaux dominés éolien puis vers des systèmes fluvio-lacustres à faible préservation des dépôts éoliens

Bridging the gap between giant Gilbert-type bottomset and associated mixed turbiditic-contouritic systems in high sediment supply setting

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Bridging the gap between Gilbert-type bottomset and associated turbidic systems : insignts for depositional profiles in high sediment supply setting.

International audienceGravity-driven processes produce turbiditic systems with an overall downslope direction. Slope, nature of the sea floor and bottom-currents are key parameters constraining the sediment transfer from the delta to the deep-sea fan. They are extensively described in the literature mainly from seismic and in less proportion from outcrop and modern data. Involved internal processes are also intensively studied by numerical modeling. To bridge the gap between the marginal coarse-grained delta and the basin axis depocenter in highsediment supply, we propose to study outcrops of the Ilias Gilbert-type pro-delta along the southern margin of the gulf of Corinth in Greece. An entire sedimentological profile is documented from source-to-Sink based on standard field observations enhanced by 3D photogrammetric models (UAV acquisition), and with a focus on the facies distributions, the facies associations, the internal architectures and the morphologies. The Gilbert-type delta bottomset reveals four bottomset-dynamics under specific processes integrated within a stratigraphic frame. During highstand normal regression, the gravelly bottomset mainly develops under subcritical flow. The supercritical flow undergoes a stationary hydraulic jump in the toeset due to the slope break and becomes subcritical. As a result, a " low-relief channel-levees " system is formed in the bottomset. The channels are reworked by backstepping conglomeratic lenses interbedded with silty concave-up and concave-down levees. During normal regression, the foreset beds are steeper and scoured in the upper part. In the bottomset, significant erosion recording sediment bypass downstream toward the prodelta. During lowstand normal regression, starved fine silt to shale bottomset onlaps onto the major erosional surface. During the transgressive stage, the topset and foreset are eroded by high-density turbidity currents and massive coarse-grained sandy turbidites are deposited in the bottomset, which onlap onto the foreset beds and form a slope apron geometry in the delta toe. Downstream of the Gilbert-type delta bottomsets, the sedimentary system is dominated by conglomeratic channels with an axis of 60° from the delta axis (fault controlled. The channels are limited by both external and internal levee with specific facies and architecture. They are commonly characterized by low sinuosity geometries. The external levee could locally include sediment waves. This study provides the key elements to recognize the specific facies and architectures of the different bottomset typologies in relation with the associated turbiditic system. These sedimentary and stratigraphic models improve the prediction for the sand and conglomerate distribution and their connectivity within the various parts of the delta toe in a clastic depositional profile induces by a high-sediment discharge

Processes and typology in Gilbert-type delta bottomset deposits based on outcrop examples in the Corinth Rift

International audienceMiddle Pleistocene Gilbert-type delta in the Gulf of Corinth, Greece, has been investigated combining field methods and photo acquisition by drone to generate a high-resolution 3D model. This study case can be used to document four different dynamics in Gilbert-type bottomset deposits, each one of which is characterized by a specific range of facies, facies associations and geometries: (1) the sandy-gravelly bottomset, (2) the erosional-bypass stage, (3) the fine-grained bottomset and (4) the massive-sandy bottomset. By comparing the typologies of the bottomset, we propose a conceptual model that predicts the occurrence of these four different bottomset stage dynamics depending on the stratigraphic context.During highstand normal regression, the gravelly bottomset develops under subcritical flow. The supercritical flow undergoes a stationary hydraulic jump in the toeset due to the slope break. As a result, a low-relief channel-levees system is formed in the bottomset. The channels are filled/reworked by backstepping conglomeratic lenses interbedded with silty concave-up and concave-down levees. During normal regression, the foreset beds are steeper than during previous stage and scoured in the upper part. In the bottomset, significant erosion recording sediment bypass downstream toward the pro-delta can be observed. During lowstand normal regression, a starved fine silt to shale bottomset onlaps onto the major erosional surface. The bulk of the coarse-grained sediments is stored in the delta topset and foreset. During the transgressive to highstand stage, the former topset and foreset are eroded by high-density turbidity currents and massive-sandy turbidites are deposited in the bottomset, which onlap onto the foreset beds and form a slope apron geometry in the delta toe.The stratigraphic model improves the prediction for the sand distribution within the various parts of the bottomset. This approach is particularly relevant for clastic depositional systems with high sediment discharge and a high accommodation rate

Sequence stratigraphy architecture and variations of aeolian deposits preservation in a coastal environment (Permian Cutler Group, SE Utah, USA).

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New sedimentological and stratigraphical data on the Permian and Triassic in northeastern Utah, USA

International audienceWe present the Permian to Triassic sedimentological evolution in the Vernal area, northeastern Utah, todetermine: (1) the magnitude of the Permian-Triassic (P-T) unconformity, (2) the depositional environmentsabove the unconformity, as well as (3) their evolution in space and time. Within this area, Permian depositsconsists of the Weber Sandstone, which represents aeolian deposits, overlain by the Park City Formation,composed of silty and sandy marine limestones, dolomites and phosphatic shales. In some places, anunconformity is observed at the top of the Weber Sandstone, which is directly overlain by the MoenkopiGroup. This formation is mostly composed of siltstone, with gypsiferous levels and ripple marks, depositedin nearshore continental to marine environments. Within the studied area, this formation is assumed to beEarly Triassic. A second unconformity at the base of the Chinle Group truncates the Moenkopi Group. Thebase of the Chinle Group is characterized by the Gartra Formation composed of coarse-grained and conglomeraticsandstones that are stream and river deposits overlain by red beds that are stream (primarilyfloodplain) and lake deposits.The P-T succession in northeastern Utah encompasses a profound unconformity and is not a completesuccession that could be used to analyze P-T boundary extinctions. Detailed sedimentological analyses allowthe characterization of the depositional environments across the P-T unconformity. The upper Permiandeposits preserved below the unconformity show erosion or non-deposition in the Vernal area. Above theunconformity, sandstone facies including coiled and uncoiled nautiloids, as well as stromatolite mounds,which suggest an open marine environment and attributed to the Permian based on its faunal content. Thesemarine deposits are abruptly overlain, above a breccia surface, by more classical Moenkopi facies attributedhere to delta and turbidite facies overlain by sabkha deposits with gypsum and microbialites. The unconformityabove the Moenkopi Group is overlain by either fluvial deposits of the Gartra Formation or red bedfacies of the Chinle Group. This study also helps to define the relationships between the Gartra Formation,the red beds and the paleosols within the Petrified Forest Formation of the Chinle Group

Depositional model in subglacial cavities, Killiney Bay, Ireland. Interactions between sedimentation, deformation and glacial dynamics.

23 pagesInternational audienceSubglacial meltwater drainage and sedimentary processes play a major role in ice-sheet dynamic but there is a lack of study of subglacial environment because modern ice-sheet beds remain inaccessible. Previous authors already intended to provide diagnostic criterion and recent investigations suggest that fluid pressure variations are a key factor in subglacial environment. This paper investigated the late Devensian sedimentary record in order to describe subglacial sedimentological facies associations and deformation features related to fluid overpressures. We used an integrated approach, based on stratigraphy, sedimentology and deformations styles to demonstrate a subglacial depositional model. The studied interval is composed of stratified gravel and sand interbedded with diamicton and boulder pavement, deposited in depressions formed by irregularity of the upper surface of diamicton. Deformation structures include convolutes, dykes and normal micro-faulting. Dykes show different dip directions from vertical, oblique to subhorizontal from which both directions of shortening and extension can be determined. Vertical dykes are formed under pure shear strain related to ice weight only. Oblique dykes imply both ice-bed coupling and simple shear strain between ice and substrate induced by flowing ice related to progressively increasing meltwater drainage intensity. Horizontal dykes are formed when minimum strain is vertical and therefore the overpressure exceeds the weight of overburden. They are associated with high meltwater drainage intensity and ice-bed uncoupling and refer to hydrofracturing. Overall, depositional and deformation sequence also illustrates the increasing intensity of meltwater drainage in small cavity as high energy channelised deposits, and in large cavities where subaqueous fan are deposited under hydraulic jump conditions. Moreover, large cavities represent lee-side cavities formed by fast-flowing ice over an obstacle. Hydrofracturing is likely to occur when a dense fluid, potentially associated with catastrophic drainage of an upstream cavity enters the low-pressure confined environment of a downstream cavity and is injected under pressure in the soft substrate. The studied glacial sequence represents a regional pattern probably related to an allocyclic control on sedimentation linked to long-term glacial dynamics rather than local depositional conditions. Based on these results, we provided a synthetic model linking depositional and deformation processes during ice-sheet growth and decay, but also valid at different timescales from annual to seasonal scale

Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia).

28 pagesInternational audienceThe deep siliciclastic reservoir (>3500 m) of the Upper Cape Hay Formation of the Bonaparte Basin (Petrel gas field, Petrel sub-basin, Permian) exhibits wide heterogeneity in porosity (2–26%) and permeability (0.001–2500 mD). To investigate this variability, 42 samples were taken from five wells drilled through this formation. Six facies were identified from core descriptions and microscopic study of the sandstones. These facies are typical of a tide-dominated estuary, and include (1) mud flat, (2) sand flat, (3) top of tidal sand bar, (4) middle of tidal sand bar, (5) bottom of tidal sand bar, and (6) outer estuary facies. The paragenetic sequence comprises the emplacement of early aggregates of ferrous clay mineral precursors, mechanical compaction, recrystallization of those ferrous clay mineral precursors to Fe-rich chlorite and crystallization of Fe-rich chlorite forming coatings around detrital grains, chemical compaction, development of quartz overgrowth, feldspar alteration, crystallization of dickite and illite-rich illite/smectite (I-S) mixed layers, and ferrous calcite cementation. The middle and top of the tidal bars generally exhibit the highest porosity (Φ > 10%) and permeability values (k > 1 mD). Feldspar alteration released silica and aluminium into the reservoir promoting the development of dickite and illite-rich I/S mixed layers, which tended to destroy porosity and permeability, as calcite cements and quartz overgrowths. Diagenetic chlorite coatings around detrital grains are restricted to the sand bar facies deposited at the end of the last third-order transgressive systems tract of the Cape Hay Formation. The formation and conservation of ferrous clay precursors seems to be possible in an estuarine environment where seawater and fresh water are mixed and tidal sand bars are formed. These ferrous clay precursors recrystallized to Fe-rich chlorite coating after mechanical compaction. These coatings inhibited quartz cementation and prove to be the key parameter behind good reservoir qualities

The Late Paleozoic Ice Age in western equatorial Pangea: context for complex interactions among aeolian, alluvial, and shoreface sedimentary environments during the Late Pennsylvanian - early Permian

The evolution of depositional environments in the Late Pennsylvanian-early Permian of the Paradox Basin in Utah, USA, is investigated through detailed sedimentological and high-resolution sequence stratigraphic analyses, in order to define a model of landscape evolution, to discuss the stratigraphic model, and to evaluate the significance of the cyclicity in the paleoclimatic context. Forty high-resolution cycles integrated in 15 minor and two major cycles are observed for the first time throughout the Late Pennsylvanian-early Permian units. A-three steps landscape evolution is recognized. First, the lower Cutler beds, mainly corresponded to a marine environment, with longshore bar, subtidal, tidal, mouth-bar, and with locally fluvial deposits. The upper part of the lower Cutler beds also contains an aeolian dune. Second, the Cedar Mesa Sandstone, corresponded to broad erg deposits which are present across the entire study area, whereas longshore bar, subtidal, mouth-bar, and some fluvial deposits are mainly preserved in the northern part of the studied area. Third, the Organ Rock Formation records decreasing aeolian dune field preservation. To the south, the aeolian environments are interbedded with shoreface deposits, whereas to the north, fluvial deposits with some mouth-bars are more developed. Semi-arid climatic conditions persisted, as indicated by the presence of calcretes. Everywhere in these three steps, root traces within sandstone bars indicate that a soil was once present above the sand dunes. This new model of landscape evolution documents complex interactions between aeolian, fluvial and marine environments within the entire Permian succession of the Paradox Basin. High-resolution stratigraphic analyses allow to discuss the significance of the cyclicity in a scenario that take into consideration sea-level variation in the Late Paleozoic Ice Age paleoclimatic context. This Late Pennsylvanian to early Permian succession reflects both relative sea-level fluctuations and the variability of sediment supply

Subglacial to proglacial depositional environments in an Ordovician glacial tunnel valley, Alnif, Morocco.

18 pagesInternational audienceThis paper presents the sedimentary analysis of an exceptional Ordovician glacial tunnel valley in the eastern part of the Anti-Atlas. The valley infill comprises two major glacial erosion surfaces (striated pavements) each overlain by a fining-upward glacial unit. These units are composed of five distinct facies associations, recording the evolution fromsubglacial to proglacial environments, and an additional sixth facies association, overtopping the tunnel valley infill, and associatedwith post-glacial environments. The tunnel valley infill also records a transitional environment between the subglacial and proglacial settings, which is compared with the Antarctic ice-sheet margin. These three environments are defined by the position of the grounding line and the coupling line. The new proposed depositional model also differs from usual Ordovician depositional models in which the main tunnel valley infill is interpreted as essentially proglacial outwash deposits, in a range of glaciomarine to glaciofluvial environments. Overall, a substantial part of the valley infill (~50% of volume) was deposited in a subglacial setting. The sedimentary bodies could form potentially thick and laterally extended, although these were limited by the shape and extent of the subglacial accommodation space. Finally, the sedimentary record, when compared with regional analogues, also provides information for the palaeogeographic reconstruction of the Ordovician ice-sheet in this region