Annot Sandstone Research Articles

Supercritical‐Flow Deposits: A New Quantitative Tool to Characterise Deep‐Marine Depositional Environments (Oligocene Annot Sandstones, SE France)?

ABSTRACTDespite a growing interest, few studies integrate quantitative supercritical‐flow deposit (SFD) distribution into paleoenvironmental reconstructions in deep‐marine settings. We present a re‐evaluation of the Oligocene Annot Sandstones (SE France) in terms of flow criticality, by examining the spatial distribution of SFDs. Proximal settings exhibit SFD proportion varying from ~90% to 40% with a common occurrence of cyclic steps, whereas distal environments show <40% SFDs with antidunes and upper‐plane beds prevalent. Comparing SFD spatial distribution to other proxies in the Annot Sandstones and published proxies from the Eocene Ainsa‐Jaca basins (Spanish Pyrenees), SFD trends appear more consistent than those in sandstone percentages, or the distribution of the Bouma sequence divisions. Our approach achieves two goals: (1) establishing the reliability of quantitatively analysing SFDs as a proxy for paleoenvironment reconstructions, characterising the proximal‐to‐basin floor transition at ~40% SFDs and (2) providing fundamental insights into the formation processes compared to other proxies.

Geology of the Colle di Tenda – Monte Marguareis area (Ligurian Alps, NW Italy)

ABSTRACTThe 1:25,000 geological map of the Colle di Tenda – Monte Marguareis area covers an area of about 130 km2 in the Italian Ligurian Alps, between the Vermenagna and Tanaro valleys. It is a detailed geological map of a sector of the Ligurian Alps of renewed scientific interest, and represents the eastern continuation of a recently published geological map of the Entracque-Colle di Tenda area. In addition to the increased detail and scale, the more relevant new contents of this map are represented by:a map of all the tectonic elements making up the Limone-Viozene Zone and the Refrey Unit, which represent the south-eastern portion of a major regional transfer zone developed at the southern termination of the Western Alps arc;the representation of km-scale Cretaceous palaeoescarpments previously overlooked or interpreted as Alpine faults;a new interpretation of some dark shales with interbedded sandstones, which were previously mapped as Helminthoides Flysch tectonic remnants, as belonging to the Annot Sandstone unit, the uppermost term of the Alpine Foreland Basin succession; anda map legend designed following the same criteria of the 1:250,000 Map of the Piemonte Region.

Annot Sandstone in the Peïra Cava basin: An example of an asymmetric facies distribution in a confined turbidite system (SE France)

High-resolution physical stratigraphy and detailed facies analysis have been carried out in the foredeep turbidites of Annot Sandstone in the Peïra Cava basin (French Maritime Alps) in order to characterize the relationship between facies and basin morphology. Detailed correlation patterns are evidence of a distinction between a southern bypass-dominated region, coincident with a channel-lobe transition and a north-eastern depositional zone, represented by sheet-like basin plain. These depositional elements are characterized by three main groups of beds related to the downcurrent evolution of bipartite flows. These facies groups are: 1) pebbly coarse-grained massive sandstones with rip-up mudstone clasts and impact mudstone breccias (Type I and II beds) deposited by basal dense flows, 2) coarse-grained massive sandstone overlain by tractive structures (Type III and IV beds) indicating the bypass of overlying turbulent flows and 3) massive medium-grained and fine-grained laminated sandstones related to the deposition of high and low density turbidity currents (Type V and VI beds). Ponding and reflection processes, affecting the upper turbulent flows, can characterize all type beds, but especially the beds of the third group. As described in other confined basins of the northern Apennines (Italy), the lateral and vertical distribution of these type of beds, together with other important sedimentary characteristics, - such as the sandstone/mudstone ratio, bed thicknesses, amalgamation surfaces and paleocurrents - reveal that the deposition of the Annot Sandstone in the Peïra Cava basin was controlled by an asymmetric basin with a steep western margin. This margin favored, on the one hand, basal dense flow decelerations and impacts, as well as bypass and deflection of the upper turbulent flows towards the north east.

Asymmetrical cross-current turbidite facies tract in a structurally-confined mini-basin (Priabonian-Rupelian, Ranzano Sandstone, northern Apennines, Italy)

This work discusses the stratigraphy and facies analysis of the Ranzano Sandstone, in the northern Apennines (Italy), a confined low-efficiency turbidite system deposited in a series of small piggy-back basins, which show strong analogies with intraslope minibasins commonly observed in divergent margins. The detailed physical stratigraphy of these deposits, which are the counterpart of the Annot Sandstone of the classic “Trilogie Priabonienne” cropping out in the western Alps, shows a cross-current asymmetrical distribution of the facies related to basin morphology. In an E-W oriented transect, roughly perpendicular to the general paleocurrents directed towards SSW, coarse-grained amalgamated massive sandstones that onlap against the eastern basin margin, grade towards the west, over very short distances, into a stratigraphic succession dominated by coarse-grained sandstones reworked in megaripples. This westward lateral facies change is also associated with: a) progressive deviation of the megaripple paleocurrents towards the west, b) progressive eastward increase in the sandstone/mudstone ratio, impact flow structures and beds with mudstone clasts, and c) progressive westward decrease in bed thickness characterizing massive facies. This cross-current transition is interpreted as related to bipartite turbidity currents experiencing deceleration against a relatively steeper eastern margin, producing flow decoupling and consequent bypass of upper turbulent flows that can spread out towards the less steep western bounding slope. The lateral spreading of turbulent flows causes reworking of the coarse-grained massive facies, producing tractive structures that indicate westward flow divergence. The lack of medium, fine-grained sands and mud testifies that these grain sizes must have been transported by turbulent flows into another adjacent basin, allowing the described deposits to be interpreted as recording a “flow stripping” phase as found in the ponded intraslope basins of the Gulf of Mexico and other confined settings, such as wedge-top basins and inner foredeeps.

Submarine fans: A critical retrospective (1950–2015)

When we look back the contributions on submarine fans during the past 65 years (1950–2015), the empirical data on 21 modern submarine fans and 10 ancient deep-water systems, published by the results of the First COMFAN (Committee on FANs) Meeting (Bouma et al., 1985a), have remained the single most significant compilation of data on submarine fans. The 1970s were the “heyday” of submarine fan models. In the 21st century, the general focus has shifted from submarine fans to submarine mass movements, internal waves and tides, and contourites. The purpose of this review is to illustrate the complexity of issues surrounding the origin and classification of submarine fans. The principal elements of submarine fans, composed of canyons, channels, and lobes, are discussed using nine modern case studies from the Mediterranean Sea, the Equatorial Atlantic, the Gulf of Mexico, the North Pacific, the NE Indian Ocean (Bay of Bengal), and the East Sea (Korea). The Annot Sandstone (Eocene–Oligocene), exposed at Peira-Cava area, SE France, which served as the type locality for the “Bouma Sequence”, was reexamined. The field details are documented in questioning the validity of the model, which was the basis for the turbidite-fan link. The 29 fan-related models that are of conceptual significance, developed during the period 1970–2015, are discussed using modern and ancient systems. They are: (1) the classic submarine fan model with attached lobes, (2) the detached-lobe model, (3) the channel-levee complex without lobes, (4) the delta-fed ramp model, (5) the gully-lobe model, (6) the suprafan lobe model, (7) the depositional lobe model, (8) the fan lobe model, (9) the ponded lobe model, (10) the nine models based on grain size and sediment source, (11) the four fan models based on tectonic settings, (12) the Jackfork debrite model, (13) the basin-floor fan model, (14) supercritical and subcritical fans, and (15) the three types of fan reservoirs. Each model is unique, and the long-standing belief that submarine fans are composed of turbidites, in particular, of gravelly and sandy high-density turbidites, is a myth. This is because there are no empirical data to validate the existence of gravelly and sandy high-density turbidity currents in the modern marine environments. Also, there are no experimental documentation of true turbidity currents that can transport gravels and coarse sands in turbulent suspension. Mass-transport processes, which include slides, slumps, and debris flows (but not turbidity currenrs), are the most viable mechanisms for transporting gravels and sands into the deep sea. The prevailing notion that submarine fans develop during periods of sea-level lowstands is also a myth. The geologic reality is that frequent short-term events that last for only a few minutes to several hours or days (e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more important in controlling deposition of deep-water sands than sporadic long-term events that last for thousands to millions of years (e.g., lowstand systems tract). Submarine fans are still in a stage of muddled turbidite paradigm because the concept of high-density turbidity currents is incommensurable.

Convolute laminations and load structures in turbidites as indicators of flow reflections and decelerations against bounding slopes. Examples from the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France)

This work discusses the significance of particular types of soft-sediment deformations very common within turbidite deposits, namely convolute laminations and load structures.Detailed facies analyses of the foredeep turbidites in the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France) show that these deformational structures tend to increase near morphological obstacles, concomitantly with contained-reflected beds. The lateral and vertical distribution of convolute laminae and load structures, as well as their geometry, has a well-defined depositional logic related to flow decelerations and reflections against bounding slopes. This evidence suggests an interaction between fine-grained sediment and the presence of morphologic relief, and impulsive and cyclic-wave loadings, which are produced by flow impacts or reflected bores and internal waves related to impinging bipartite turbidity currents.

Evidence of multi-stage faulting by clay mineral analysis: Example in a normal fault zone affecting arkosic sandstones (Annot sandstones)

Fault affecting silicoclastic sediments are commonly enriched in clay minerals. Clays are sensitive to fluid–rock interactions and deformation mechanisms; in this paper, they are used as proxy for fault activity and behavior. The present study focuses on clay mineral assemblages from the Point Vert normal fault zone located in the Annot sandstones, a Priabonian-Rupelian turbidite succession of the Alpine foredeep in SE France. In this area, the Annot sandstones were buried around 6–8 km below the front of Alpine nappes soon after their deposition and exhumed during the middle-late Miocene. The fault affects arkosic sandstone beds alternating with pelitic layers, and displays throw of about thirty meters. The fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick. The foliated sandstones display clay concentration along S–C structures characterized by dissolution of K-feldspar and their replacement by mica, associated with quartz pressure solution, intense microfracturation and quartz vein precipitation. The gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites. However, a detailed petrographical investigation suggests complex polyphase deformation processes. Optical and SEM observations show that the clay minerals fraction of all studied rocks (pelites and sandstones from the damage and core zones of the fault) is dominated by white micas and chlorite. These minerals have two different origins: detrital and newly-formed. Detrital micas are identified by their larger shape and their chemical composition with a lower Fe–Mg content than the newly-formed white micas. In the foliated sandstones, newly-formed white micas are concentrated along S–C structures or replace K-feldspar. Both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions. They have the following structural formulas: Na0.05 K0.86 (Al 1.77 Fe0.08 Mg0.15) (Si3.22 Al0.78) O10 (OH)2. They are enriched in Fe and Mg compared to the detrital micas. Newly-formed chlorites are associated with micas along the shear planes. According to microprobe analyses, they present the following structural formula: (Al1,48 Fe2,50 Mg1,84) (Si2,82 Al1,18) O10 (OH)8. All these data suggest that these clay minerals are synkinematic and registered the fault activity. In the gouge samples, illite and chlorite are the major clay minerals; smectite is locally present in some samples.In the foliated sandstones, Kubler Index (KI) ((001) XRD peak width at half height) data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 °C (diagenesis to anchizone boundary). KI measured on pelites and sandstones from the hanging and footwall, display similar values coherent with the maximal burial temperature of the Annot sandstones in this area. The gouge samples have a higher KI index, which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the Annot sandstones formation.

Characteristics of axially-sourced turbidite sedimentation on an active wedge-top basin (Annot Sandstone, SE France)

In order to develop viable depositional models for wedgetop basins, the control exerted by active structural relief on turbidite depositional patterns should be well understood at different scales (from limb slope to interconnected synclinal troughs). This is particularly the case for systems with axial sediment supply, for which little data are available. This paper presents a detailed field study of two depocentres in the Upper Eocene – Lower Oligocene Annot Sandstone of the alpine foreland basin of SE France, which was fed axially from the Corsica–Sardinia Massif to the south. The depocentres are partially preserved in a series of outliers. The Annot outlier preserves turbidites deposited on the gently dipping limb of an asymmetrical syn-depositional syncline, while, to the north, the NE Grand Coyer outlier preserves highly confined turbidites deposited on a steep and complex synclinal limb. Structural, stratigraphic and sedimentological data demonstrate that these turbidite depocentres were controlled by active folding and faulting, including oblique structures. Structural controls were more complex on the steep eastern synclinal limbs than on shallowly dipping western limbs. Integration of palaeocurrent data allow feeder pathways and their evolving interconnections to be traced between successive downstream depocentres in space and time. A 3D depositional model for axially supplied active wedgetop depocentres is proposed and compared to transversely fed wedgetop systems, particularly in terms of facies distributions and variations in reservoir quality. Axially supplied systems are marked by a higher lateral confinement and, as a consequence, are more sensitive to relief created by oblique structures. As a result facies distributions are more strongly controlled by (active or inactive) substratum relief than by intrinsic flow properties, leading to a higher potential for stratigraphic traps.

Characteristics of axially-sourced turbidite sedimentation on an active wedge-top basin (Annot sandstone, SE France).

Characteristics of axially-sourced turbidite sedimentation on an active wedge-top basin (Annot sandstone, SE France).

Characteristics of axially-sourced turbidite sedimentation on an active wedge-top basin (Annot sandstone, SE France).

Characteristics of axially-sourced turbidite sedimentation on an active wedge-top basin (Annot sandstone, SE France).

Forward modelling as a method for predicting the distribution of deep-marine sands: an example from the Peïra Cava Sub-basin

Abstract The aerial extent, thickness and internal distribution of grain-size are key, bed-scale controls on turbidite reservoir performance. Process-based modelling provides a method for predicting both the sandbody thickness and the grain-size distribution within the bed. The goal of the current study is to test such an approach on the Annot Sandstone in the Peïra Cava Sub-basin in SE France, and determine whether the process-based model can reliably recreate the bed characteristics observed in the outcrop. Turbidity currents were modelled using a commercially available software solution called MassFlow3D that is based on computational fluid dynamics. The goal of the current study was to accurately replicate a single bed (termed MU5). To achieve this goal, the base of the basin was structurally restored and then a simplified simulation of the beds below MU5 was generated using four flow events in order to recreate the bathymetry on to which MU5 was simulated. Several versions of MU5 were simulated with different input parameters for the flow, and the results were compared with the observed thickness and grain-size distribution from the outcrop. The study suggests that process-based modelling has the potential to be a useful tool in reservoir modelling. Supplementary material: Multi-phase flow modelling of transport, deposition and erosion of sediments is available at http://www.geolsoc.org.uk/SUP18719 .

Discovery of an outcropping Mass Transport Deposit (MTD) in the Palaeogene subalpine synclines of southeastern France: Implications on flexural sub-basin deformation

We document the sedimentary facies of a large Mass Transport Deposit (MTD) within the sand-rich sediment gravity flow-dominated deposits of the Eocene–Oligocene south-western Alpine forelands (Annot Sandstone system). The MTD with an approximated volume of several hundreds of cubic kilometres fills a sub-basin located in the Mont-Tournairet confined sub-basin. Autochthonous facies are very typical sediment gravity-flow deposits (thin-bedded classical turbidites and thick-bedded hyperconcentrated to concentrated flow deposits) that stratigraphically belong to the Annot Sandstone infill. Slumps and internal metre-large fold axes of the deformed stratigraphic intervals indicate a main transport direction towards the northwest. The seafloor instability that led to the mass-flow events within the Mont-Tournairet sub-basin could have been favoured by high sedimentation rates in a small, confined and tectonically active sub-basin, possibly enhanced by local structural deformation associated with the Triassic evaporites on the eastern side of the Mont-Tournairet confined basin. The presence of the MTD suggests that a period of increased flexural subsidence rate and basin deformation occurred in this portion of the subalpine foreland basin. Therefore, the MTD forms a stratigraphic marker of a period of tectonic activity.

Formation of phyllosilicates in a fault zone affecting deeply buried arkosic sandstones: their influence on petrophysic properties (Annot sandstones, French external Alps)

The Restefond fault, located in the Late Eocene-Early Oligocene Alpine foreland basin, affects the well lithified and low porosity Grès d’Annot. The fault core zone is characterized by the occurrence of highly deformed sandstone lenses. Deformation inside the lenses corresponds to mm to sub-mm-spaced cleavage planes rich in phyllosilicates and up to cm-thick and dm-long quartz-calcite pure extensional veins. The cleavages are mostly composed of newly-formed synkinematic white mica and chlorite. By using thermodynamic thermometers based on the chemical composition of chlorite, a temperature of 200 ± 20 °C of fault activity was computed. This temperature shows that the Restefond fault was active at burial conditions comprised between 6.5 and 8 km, assuming a mean geothermal gradient between 25 and 30 °Ckm−1. The petrophysic properties of sandstones from the core zone and in the hanging and foot wall of the fault were determined on drilled plugs following three spacial directions. The permeability of the highly deformed sandstone from the core zone is about one order of magnitude higher than in the host rock. This increase in permeability occurs in the direction parallel to the S–C structures and is explained by the occurrence of well-connected micropores localized between platy phyllosilicates. This study shows that the fault petrophysic properties are mostly controlled by the precipitation of synkinematic phyllosilicates under deep burial conditions.

Multiple scale characterization of sand-rich distal lobe deposit variability: Examples from the Annot Sandstones Formation, Eocene–Oligocene, SE France

The present paper investigates the outcrop-scale architecture of submarine gravity flow deposits in both moderately to highly confined settings. A significant dataset was gathered on the sand-rich, confined, siliciclastic turbidite system of the Annot Sandstone Formation (SE France). We focused on thick sedimentary bodies formed by depletive gravity flows, in relatively distal settings (sand-rich lobes). The purpose was to investigate the architectural variability of their small scale components (beds and bedsets) and consequently to evaluate the associated heterogeneities (facies changes, grain size, sedimentary features, sediment thickness, etc.). Therefore, we provide a set of detailed descriptions on well-exposed outcrops, through very high-resolution correlation panels built thanks to multiple vertical logs. These turbidite system end-members have rarely been described in such details and have often been considered as laterally continuous and homogeneous. On the basis of the hierarchical subdivisions of turbidite lobes, results show a high complexity in internal architecture from lobe bed to lobe scale whereas large scale (scenery-seismic scale) is characterized by apparently homogeneous tabular and very extensive sandbodies (thus corresponding to the classical definition of “sheet-sands”). This complexity implies heterogeneities not only in terms of sediment thickness changes (abrupt pinch-outs and variable scale compensation features), but also in terms of facies and associated sedimentary features distributions (tractive structures, internal scours with associated by-pass, freezed mudclast-rich layers, etc.). This facies variability is controlled by the interplay between internal erosive bedforms (i.e. large scours) and constructive bedforms, such as dome-shaped or laterally stacked bedsets that are related to local topographic controls and compensation processes. Variability then appears both laterally and vertically. Besides, results also showed small scale evidences of interactions between turbidite flows and basin floor, only expressed at lobe bed to lobe element scale, especially on the basin sides where paleoslope is subject to erosion and remobilisation and where flows can undergo significant changes. Concerning both seismic and reservoir implications, although those deposits are characterized by a sheet-like geometry, this high internal variability necessarily implies heterogeneities in terms of petrophysical characteristics (porosity, permeability) that may have a significant impact on fluid circulation. These elements lead us to reconsider both sedimentary processes involved in their building and also reservoir models that can be established on field analogues.

Proximal to distal turbiditic sheet-sand heterogeneities: Characteristics of associated internal channels. Examples from the Trois Evêchés area, Eocene-Oligocene Annot Sandstones (Grès d'Annot), SE France

The present study brings new insights on the small-scale characteristics of relatively distal turbidite deposits (classically defined as depositional lobes or sheets) from the deep-water clastic slope system of the Annot Sandstones (Grès d'Annot), in the Trois Evêchés area. This “seismic scale” outcrop has been studied in detail to characterize sedimentary geometries and facies associations within and between sheet-like sandstone-bodies. In terms of facies, their internal architecture is dominated by numerous erosional features, variable scale cross-bedding and dewatering structures. Facies range from gravelly sandstones to fine-grained sandstones and their spatial distribution inside sandstone-bodies is provided, allowing us to discuss the spatial changes in flow behaviour. A detailed dataset, made of very high-resolution correlations in thick to thin-bedded sheet-like sandstone-bodies, enables to identify five main types of architectural elements, composed of both channelised and unchannelised depositional bodies. Gravelly-filled channel-elements, laterally stacked channel-elements (interpreted as being related to sinuous channel migrations) and wing-like channel-elements constitute the channelised sheet systems whereas tabular sandstones (sheets sensu stricto), prograding and/or dome-shaped units constitute layered sheet systems. Sinuous channels are represented by the association of erosive-based, laterally stacked bedsets and very coarse-grained, low-angle cross-bedded facies. These latter facies are interpreted as the result of flow deconfinement and overbanks above channel margins. The stratigraphic relationships between these architectural elements permit establishing a conceptual model of the distribution and organisation of the components. This distribution scheme is based on the longitudinal differentiation of gravity processes, which are directly linked with architectural element types. This model is composed of proximal confined units where channelization processes, amalgamation and bypass dominate, to more distal and less confined units where compensation, constructive structures and high deposition rates control sedimentary architectures. The Trois Evêchés sub-basin may be considered as a good example of a channelised lobe to unchannelised lobe transitional environment, which explains the lateral and vertical high variability during its infill. The widespread apparition of residual facies, such as cross-stratified sandstones as well as basal lags, suggests significant bypass. The appearance of both channelised and unchannelised units is consistent with this transitional environment. The provided detailed dataset in this paper displays the associated wide range of architectural elements in these settings, and highlights the significant heterogeneities of sand-rich confined turbidite sheet systems. These heterogeneities are under the seismic resolution and may influence fluid migration in sand-rich deep-water reservoirs.

Migration of a synclinal depocentre from turbidite growth strata: the Annot syncline, SE France

AbstractThe Annot Sandstone turbidites of the Alpine foreland basin in SE France (Eocene-Oligocene: 40-32 Ma), provide an excellent case-study of tectono-sedimentary relations in a deepwater compressional system. The Annot outlier is a synclinal remnant previously interpreted as a static depocentre. A multi-disciplinary approach including geometrical and kinematic analyses and modelling demonstrates instead that this was a tectonically active turbidite depocentre where gentle thrust related folding controlled turbidite architecture.Stratigraphic and new structural field data are integrated with previous sedimentological studies to build a 3D geometric model of the Annot depocentre. Derived thickness maps associated with paleocurrent measurements clearly illustrate three main phases in the evolution of depocentre topography. (1) Early turbidite flows were mainly trapped by oblique intrabasinal inherited structures. (2) Once these structures were buried, the NNW-SSE active syncline constituted the main topographic control. (3) Decreasing activity of this syncline is recorded by filling and flow bypass. The progressive stages of the accepted depositional model (flow ponding, flow stripping and flow bypass), for the Annot depocentre, may therefore have a tectonic origin.The kinematic evolution of the synclinal depocentre was defined at different scales. Stratigraphic architecture records a decrease in bedding dips up through the turbidite succession on the western synclinal limb. Comparison with idealized case studies of the interaction of sedimentation with an active syncline indicates that this geometrical pattern corresponds to progressive westward migration of the synclinal hinge and depocentre. This tends to promote lateral rather than vertical stacking of sand bodies during turbidite sedimentation. Trishear kinematic modelling was used to simulate (in 2D) the rolling synclinal hinge. Stratigraphic surface geometries and turbidite depocentre migration define thrust and fold geometries at depth. The synclinal depocentre developed between two alternating or coeval fault propagation anticlines that exploited two detachment levels (Triassic evaporites and Cenomanian marls) in the underlying succession.

High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)

The Lauzanier area represents the northernmost extension of the Annot Sandstone series and contains deposits between 650 and 900 m-thick. This basin was active from upper Bartonian or lower Priabonian to early Rupelian. It is composed of two superposed units separated by a major unconformity. The sediment supply is due to channelled flows coming from the south. Flow processes include mass flow to turbidity currents. The size of the particles and the absence of fine-grained sediment suggest a transport over a short distance. The Lower Unit is made of coarse-grained tabular beds interpreted as non-channelled lobe deposits. The Upper Unit is made of massive conglomerates interpreted as the channelled part of lobes. These lobe deposits settle in a tectonically confined basin according to topographic compensation that occurs from bed scale to unit scale. The abrupt progradation between the lower and the upper unit seems related to a major tectonic uplift in the area. This uplift is also suggested by a change in the petrographic nature of the source and an abrupt coarsening of the transported clasts. This field example allows providing high resolution analysis for depositional sedimentary sequences of terminal lobe deposits in a coarse-grained turbidite system. The outcrop analysis shows the lateral evolution of deposits and the system progradation allows a longitudinal analysis of facies evolution by superposing on the same outcrops the channelled lobe system and the non-channelled lobe system. These results of high-resolution outcrop analysis can be extrapolated to results obtained on sedimentary lobes in recent deep-sea turbidite system that are either restricted to cores, or with a lesser resolution (seismic).

The character and origin of thick base-of-slope sandstone units of the Peïra Cava outlier, SE France

Abstract Many hydrocarbon reservoirs occur within confined turbidite systems in which the depositional pattern of turbidity currents has been strongly influenced by basin-floor topography. In certain settings basin-floor topography may cause the development of anomalously thick (tens of metres) sandstones that are potentially excellent reservoir units. Southern exposures of the Peïra Cava outlier (Eocene-Oligocene; Annot Sandstones) provide well-exposed outcrops of such decametre-thick sandstone bodies. These units are located close to basin margins and downstream from an inferred topographic break-in-slope. Several base-of-slope sandstone bodies are examined that illustrate a common sedimentary theme of a complex basal unit, comprising laterally pinching or inter-fingering debrite and turbidite, abruptly overlain by a single, thick normally graded turbidite deposit. One of these sandstone bodies pinches out laterally over less than several hundred metres and sits within a deep (>20 m) ‘spoon shaped’ erosional scour. The scour is similar to morphological features observed in modern base-of-slope settings recently imaged using high-resolution submarine bathymetric surveys. Several different process interpretations may explain the occurrence of such sandstone bodies including remobilization of newly deposited sediment off basinmargins and enhanced deposition due to flow across a break-in-slope. A submarine channel interpretation is not consistent with the field observations. However, these units do share a number of similar features to channels that could lead to the misinterpretation of reservoir geometry.

Deep-marine tidal bottom currents and their reworked sands in modern and ancient submarine canyons

Submarine canyons provide a unique setting for tidal processes to operate from shallow-marine to deep-marine environments. In modern canyons, current-meter measurements at varying water depths (46–4200 m) show a close correlation between the timing of up- and down-canyon currents and the timing of semi-diurnal tides. These tidal bottom currents in submarine canyons commonly attain maximum velocities of 25–50 cm/s. Based on core and outcrop studies of modern and ancient deep-marine deposits, it is proposed here that sand-mud rhythmites, double mud layers, climbing ripples, mud-draped ripples, alternation of parallel and cross-laminae, sigmoidal cross-bedding with mud drapes, internal erosional surfaces, lenticular bedding, and flaser bedding can be used to interpret deposits of deep-marine tidal currents. This approach is an alternative to the conventional approach in which most deep-water traction structures (e.g. climbing ripples and cross-bedding) would be attributed to deposition from turbidity currents. Underwater photographs show active mass flows (i.e. slides, slumps, grain flows, and debris flows) in modern canyons. Box cores taken from modern submarine canyons (e.g. La Jolla, California) and conventional cores and outcrops of ancient canyon-fill facies (Oua Iboe, Pliocene, Nigeria and the Annot Sandstone, Eocene–Oligocene, SE France) contain deposits of both tidal processes and mass flows. This facies association in the rock record can be used as a criterion for recognizing submarine canyon settings. In a channel-mouth environment, deep-marine tidal deposits are likely to develop elongate bars that are aligned parallel to the channel axis within the channel, whereas turbidites are more likely to develop depositional lobes that are aligned perpendicular to channel axis. Turbidite depositional lobes are much larger than the channel width, whereas tidal sand bars are much smaller than the channel width. Therefore, the wrong use of a turbidite lobe model with sheet geometry in lieu of a tidal bar model with bar geometry will result in an unrealistic overestimation of sandstone reservoirs in deep-water exploration.

Basin‐Floor Topography and the Scaling of Turbidites

Abstract Turbidite bed thickness distributions from deep‐water settings result from the interaction of an initial magnitude/frequency of source‐derived flow volumes, modified within the receiving basin by (1) variable flow rheologies, (2) environmental controls such as channels and lobes, and (3) basin‐floor topography. This study attempts to isolate the effects of basin‐floor topography on turbidite bed thickness distributions using outcrop examples where the ancient sea‐floor topography has been reconstructed and is thought to dominate the signal. The Eocene and Oligocene Taveyannaz and Annot Sandstones of eastern Switzerland and France were deposited in confined intraslope basin and base‐of‐slope settings. The deposits of the confined basin record flow ponding and flow stripping; the base‐of‐slope deposits record the amalgamation of turbidites. Bed thickness data for both the confined basin and proximal base‐of‐slope settings are best approximated by an exponential distribution; the data from the more ...