Tidal Dunes Research Articles

Channel, dune and sand sheet architectures of a strait-adjacent delta, Rifian Corridor, Morocco

Abstract Sea straits are elongated zones where geographical constriction can amplify marine currents. Sediment deposits that formed in sea straits are relatively well studied and robust models of tidal strait depositional systems have been developed. However, models of strait-adjacent systems remain uncommon. In this study, we present such a model which is based on detailed outcrop descriptions of Miocene deposits that formed adjacent to the Rifian Corridor, an ancient sea strait that existed in what is now northern Morocco. The kilometre-scale outcrops at the Ben Allou locality (30 km north of Fez, Morocco) contain a succession of claystones, siltstones and sandstones that formed under a sea strait-adjacent process regime characterized by marine current constriction, tidal amplification and reduced wave activity. Detailed descriptions of the sandstone's sedimentary architectures reflect a development from mud drape-bearing sand sheets in the proximal regions of the system to stacked cross-strata and channel forms nearer to the basin depocentre. In the deep, distal reaches of this system, sediment grain sizes are generally fine (silt and clay) and are sparsely interbedded with sandy turbidites. These results confirm existing models which show that proximal strait-adjacent deposits tend to be tide dominated, and wave and fluvial influence can be overprinted by tidal processes. Some insights into the formation of tidal dunes are discussed. For example, it is shown that compound tidal dunes may not be preserved differently than simple tidal dunes, which is relevant for predicting sedimentary architectures and fluid flow properties of ancient tide-dominated successions.

Progressive opening of a shallow-marine bay (Oliete Subbasin, Spain) and the record of possible eustatic fall events near the Barremian-Aptian boundary

In this paper, we analyse the upper Barremian-lowermost Aptian Alacón Formation (Oliete subbasin, Maestrazgo Basin) of Spain to understand the progressive opening of a shallow-marine bay found on the northeastern margin of Iberia. The succession consists of four transgressive-regressive sequences (S1S4) bounded by widespread unconformities caused by rapid base-level fall. The transgressive interval of each sequence is characterized by relatively high detrital input and fresh-water inflow that caused the demise of shallow-marine benthic communities. The regressive interval of each sequence triggered progressive regeneration of the benthic carbonate producers. Sequences 1 and 2 consist of lacustrine to restricted shallow-marine facies, followed by accumulations of the gryphaeid oyster Ceratostreon , with progressive colonization of the bay by marine benthic communities subjected to frequent salinity oscillations. Sequences 3 and 4 represent the stabilization of open-marine conditions in the bay with widespread development of high-energy bioclastic tidal dunes. The long-term transgression around the Barremian-Aptian boundary, which gave rise to the progressive flooding of the Oliete bay, was episodically interrupted by shorter-term episodes of abrupt base-level fall, with the eventual subaerial exposure of the entire bay at the end of the deposition of the Alacón Formation. Our data suggest that the observed sequence architecture was mostly controlled by climatic and eustatic sea-level oscillations, with local synsedimentary tectonics having a minor impact. Two major sea-level fall events responsible for the discontinuities found on top of sequences 3 and 4 are possibly correlated to the cooling episodes close to the Barremian-Aptian boundary and during the earliest Aptian (predating the OAE 1a) described in many other worldwide basins. • The progressive opening of a shallow marine bay in Iberia (W Tethys) is documented. • Four sequences register progressive paleogeographical and paleoecological changes. • Sequence boundaries are tentatively linked to major cooling events KBa5 and KAp1.

Hydrodynamics and sedimentary processes in the modern Rion strait (Greece): Interplay between tidal currents and internal tides

Straits are crossed by marine currents that are amplified due to constrictions. These nearshore high-velocity flows are problematic for offshore infrastructures (bridge pillars, cables, pipelines, etc), but constitute an interesting carbon-free energy source. Many modern straits are dominated by tidal currents which flow axially, with reversal directions and phase difference between the two interlinked basins. These tidal currents interplay with: sediment sources (including in situ carbonate production and deltas), tectonic activity, and inherited lowstand features, all shaping the seafloor into complex geomorphologies. Previous studies have highlighted a common tidal strait depositional model with a strait-center zone in erosion and on each side a dune-bedded strait zone with 3D and 2D tidal dunes and tidal ripples. Even if the internal waves associated with and generated by the straits are widely documented, the effects of the internal waves on the seafloor need to be further investigated. The aim of this study is to unravel the combined effects of the tidal currents and the internal tides on current pattern and on the morphosedimentary features.We present a strait example based on an interdisciplinary approach using high-resolution geophysical and oceanographical data to better constrain the hydrodynamics and the processes acting on the seafloor. We focus on the Rion Strait in Greece which controls the connection between the Corinth Gulf and the Mediterranean Sea. Based on high-resolution multibeam bathymetry (MBES) over an area of 211 km2, we identify and quantify different morphologies by extracting bathymetric swath profiles. These results are integrated with currents data (ADCP) and CTD profiles. In addition, we use high-resolution chirp sub-bottom profiles and sparker seismic reflection profiles to document the morphology and internal architectures of the sedimentary deposits and the erosional features in the strait bottom. To complete this dataset, we analyzed Sea Surface Temperatures (SST) from satellite sensors.The Rion Strait displays complex bathymetric features without tidal dunes. At the excepted depositional location of tidal dunes, we identify an erosive area with a pool and crest morphology. This new example completes the tidal strait depositional model by a re-localization of erosion, bypass and deposition in an asymmetric strait swept by baroclinic currents. This example illustrates the key role of internal tides in straits located between a confined deep basin and an open sea.

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

AbstractThe bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large‐scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large‐scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood‐ and ebb‐oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune‐induced directional bed roughness in numerical models of estuarine and tidal environments.

Integrated outcrop and subsurface geomodeling of the Turonian Wall Creek Member of the Frontier Formation, Powder River Basin, Wyoming, USA

Inter-well heterogeneities influencing fluid migration in deltaic reservoirs are controlled by lateral lithofacies changes and vertical complexities such as low permeability thin-beds (i.e. mudstones). Subsurface tools cannot adequately predict the spatial and stratigraphic organization of these architectural elements, nor their influence on effective reservoir properties and connectivity. Our outcrop-based geomodeling study of the Turonian Wall Creek Member of the Frontier Formation in the Powder River Basin, Wyoming, USA, integrates subsurface production and flow simulation data to quantify the impact of multi-scale stratigraphic heterogeneity on analogous reservoir behavior and horizontal well design. The upscaled representation of thin-bed complexity in our 500 m × 715 m x 15 m geomodel is derived through flow simulation of internally nested, facies specific, centimeter-scale models (basic depositional element models or BDEMs). These BDEMs are populated with quantitative data of mudstone thin-bed geometries directly derived from digital outcrop measurements and measurements in the field, and aided by subsurface (core) petrophysical properties. Reservoir simulations are performed using various landing zone and completion configurations, and under both single-phase and multi-phase flow conditions history matched to subsurface production data.Results from our workflow demonstrate that, in heterolithic settings, the permeability of mudstone thin-beds impart the most significant impacts toward upscaled reservoir behavior, and deliver unique Kv/Kh flow anisotropies dependent on the governing depositional environment. Thin, laterally continuous and planar mudstone beds in the wave-and river-dominated distal delta front setting have the greatest negative effect on Kv and a minimal impact on Kh, promoting laterally continuous and vertically baffled flow units. Conversely, the more curvilinear and intersecting architectures of mudstone beds in the tidal dune facies maintain better vertical connectivity but decreased Kh compared to the wave-influenced facies. Reservoir flow is only sensitive to changes in engineering design (landing zone, well orientation, completion size) where thin-beds act as complete barriers rather than baffles to flow. Layer cake subsurface models failing to account for high-resolution stratigraphic architectures are likely to overestimate reservoir production where thin-beds act as barriers capable of appreciable reservoir compartmentalization, which in our models is more characteristic of the distal delta front setting. Synthesized results of this study are immediately applicable toward development planning in similar tight sandstone reservoirs.

A new shallow-marine, high-latitude record of the trace fossil Macaronichnus in Miocene, reworked delta-front clinoforms, Punta Basílica, Tierra Del Fuego, Argentina

A new high-latitude record of the trace fossil Macaronichnus segregatis degiberti is documented from the lower Miocene Punta Basílica beds, Tierra del Fuego, which were previously considered to be deep-marine channel and levee deposits. Based on a combined ichnological and sedimentological study, however, these beds are herein re-interpreted as shallow-marine, delta-front clinoform deposits. Macaronichnus-bearing beds in the stratigraphically older clinoforms are characterized by: (1) sandstones with wave ripples and wave-ripple cross-lamination; (2) cross-stratified sandstones forming large compound dunes and tidal bars; and (3) intervening heterolithic mudstone-sandstone couplets with current and wave ripples. The sedimentary facies preserving M. s. degiberti were deposited during the onset of transgression, indicated by wave and tide reworking of underlying steeply dipping, delta-front clinoforms deposits. The master bedding surfaces of compound tidal bars are characterized by a dense, monospecific fabric of M. s. degiberti. Compared with other Macaronichnus ichnofabrics, reported elsewhere as characterizing short colonization windows on foresets of tidal dunes or bars, the Punta Basílica ichnofabric represents a longer colonization window, associated with episodic reworking and redeposition of loose, nutrient-rich sand grains just above the master bedding surfaces. The record of M. s. degiberti in Tierra del Fuego supports the previous interpretations that restrain its producer to high-latitude, shallow-marine cold waters.

Depositional controls on a hypertidal barrier‐spit system architecture and evolution, Pointe du Banc spit, north‐western France

AbstractPhysical stratigraphy, architecture and evolution of barrier systems in hypertidal environments (tidal range above 6 m) are understudied, and depositional controls are poorly understood compared with wave‐dominated barrier systems and barrier spits in microtidal, mesotidal and macrotidal settings. Based on vibracores, ground‐penetrating radar, radiocarbon and optically stimulated luminescence age data, the formation and evolution of the hypertidal barrier system of Pointe du Banc, north‐western France has been reconstructed. The study shows that the barrier spit has a complex composite sedimentary architecture consisting of wave‐dominated and tide‐dominated sedimentary bodies. A morpho‐sedimentary model is presented that demonstrates how barrier‐spit progradation resulting from littoral drift was the main manner of spit elongation, whereas sediment convergence caused by landward migration of swash bars and seaward migration of tidal dunes caused the spit terminus to grow in height and width. These results suggest that long‐term accretion rates varied considerably in response to changes in sediment supply. Variations in storminess together with the large‐scale topography of the coast controlled the sediment supply and thus the evolution of the barrier system during latest Holocene. Despite architectural complexity, hypertidal barrier systems preserve records of past climate changes.

Architecture of growth basins in a tidally influenced, prodelta to delta‐front setting: The Triassic succession of Kvalpynten, East Svalbard

AbstractWorld‐class examples of fault‐controlled growth basins with associated syn‐kinematic sedimentary fill are developed in Upper Triassic prodelta to delta‐front deposits exposed at Kvalpynten, SW Edgeøya in East Svalbard. They are interpreted to have interacted with north‐westerly progradation of a regional delta system. The syn‐kinematic successions consist of 4 to 5 coarsening‐upward units spanning from offshore mudstones to subtidal heterolithic bars and compound tidal dunes, which were blanketed by regional, post‐kinematic sandstone sheets deposited as laterally continuous, subaqueous tidal dune fields. The rate of growth faulting is reflected in the distribution of accommodation, which governs sedimentary architecture and stacking patterns within the coarsening‐upward units. Fully compartmentalized basins (12, 200–800 m wide and c. 150 m high grabens and half grabens) are characterized by syn‐kinematic sedimentary infill. These grabens and half‐grabens are separated by 60–150 m high horsts composed of pro‐delta to distal delta‐front mudstones. Grabens host tabular tidal dunes (sandwaves), whereas half‐grabens bound by listric faults (mainly south‐dipping) consist of wedge‐shaped, rotated strata with erosive boundaries proximal to the uplifted fault block crests. Heterolithic tidal bars (sand ridges) occur in narrow half‐grabens, showing migration oblique to the faults, up the dipslope. Structureless sandstone wedges and localized subaqueous slumps that formed in response to collapse of the block crests were only documented in half‐grabens. Late‐kinematic deposition during the final stages of faulting occurred in partly compartmentalized basins, filled with variably thick sets of continuous sandstone belts (compound tidal dunes).

Tidal strait to embayment: Seismic stratigraphy and evolution of a rock-bounded embayment in the context of Holocene sea level change

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system.New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped.The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region.A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.

Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician–Silurian boundary and Early Silurian deposits from Argentina

The La Chilca Formation is exposed at the Poblete Norte and Ancha sections, Talacasto, Central Precordillera. Detailed facies analysis on field outcrops suggests a tide‐dominated tidal plain model on the Silurian siliciclastic ramp, integrated by supratidal to subtidal plain deposits. The lower part of the La Chilca Formation (upper supratidal plain) is herein attributed to reworked glaciomarine deposits that occurred during interglacial Hirnantian age, under conditions of sea rise and the action of tidal amplification. The intertidal–supratidal depositional environments are represented by tidal channels, scour and fill, tidal dunes, and hummocky cross‐stratified sandstones. The estimated tidal conditions for the tidal plain were mesotidal to macrotidal, under action of combined flows, attributed to the interaction of tidal currents and orbital low‐velocity waves. The identification of microbially induced sedimentary structures (MISS) in these deposits, such as petee polygonal structures, pitted texture, imploded sand volcanoes, Astropolithon sp., Manchuriophycus sp., domal build‐ups, biofilms, elephant skin, and trace fossils such as Chondrites sp., Zoophycos sp., Undichnia sp., Planolites sp., Monomorphichnus sp., and gastropod traces, may have played a protection and stabilization role on the clastic sediment surfaces against erosion of the La Chilca Formation deposits. The prolific growth of microbial mat on the intertidal to supratidal zone occurs explicitly at the turning points of transgression–regression cycles in shallow‐water environments under stable base‐level conditions. The middle‐upper section of the La Chilca Formation shows non‐cyclic rhythmites that are replaced by the cyclic rhythmites, suggesting an open tidal plain subject to an orbital/climatic rather than a tectonic control of cyclicity (Milankovitch cycles). The aggradation in this section indicates a shallowing towards the top of the formation. The La Chilca Formation records two third‐order transgressive events and a shallowing event. The transgressive events are found in the base and top of the formation and are marked by the presence of Fe‐oolites, while the shallowing event is determined by the presence of MISS.

On the formation of periodic sandy mounds

Le Bot and Trentesaux (Marine Geology 211, 2004) surveyed the periodic morphological patterns which are present in the English Channel close to the strait of Calais-Dover, where the shortage of sand does not allow the formation of typical sand waves (tidal dunes). The field observations show that, for similar hydrodynamic and morphodynamic conditions, the crest-to-crest distance of the observed sandy mounds is larger than the wavelength of the sand waves which form where sand is abundant.The present contribution describes an idealized model able to predict the hydrodynamics and the morphodynamics of the interaction of tidal currents with large scale bedforms such as sand waves and sandy mounds in sand-starved environments. Indeed, when the availability of sand is limited, classical morphodynamic stability analyses cannot be applied for two main reasons. First, part of the rigid substratum becomes bared when bedforms appear and the bed profile is no longer sinusoidal. Second, the formulae commonly used to quantify sand transport are no longer valid when sandy mounds alternate with a rigid substratum.In accordance with the field observations, the analysis shows that the bedforms which appear when the rigid substratum is bared (sandy mounds) are longer than those which form in a rich sand environment (sand waves).

Morphologic and hydrodynamic controls on the occurrence of tidal bundles in an open-coast macrotidal environment, northern Gyeonggi Bay, west coast of Korea

Tidal dunes with well-defined rhythmic tidal bundles are documented from the lower intertidal zone of an open-coast macrotidal environment in Gyeonggi Bay, Korea. Based on combined morphologic, sedimentologic and hydrodynamic datasets, this study aims to characterize the factors that govern the temporal and spatial variability of tidal bundles in a non-barred, unconfined macrotidal environment. The tidal dunes are flood-asymmetric and of longer wavelength (10–20m) with small ebb caps on the upper bank, and symmetric to slightly ebb-asymmetric and of shorter wavelength (5–10m) with larger ebb caps on the lower bank. The upper-bank dunes are characterized by more steeply dipping flood-directed planar cross-beds and thinner mud drapes than the lower-bank dunes. Each tidal bundle consists of a single mud drape that is stratified to cross-stratified, rich in silt and very fine sand. It overlies ebb-directed ripples and represents dynamic mud deposition during the ebb tidal phase. The presence of strong rotary currents (up to 0.25m/s) and low suspended-sediment concentration of flood currents prevent deposition of mud drapes during the high-tide slack-water period. The distinct asymmetry in the water elevation at which the velocity peaks during the ebb and flood phases results in the preferential preservation of flood-directed cross-beds in the lower intertidal zone, where the ebb current – although stronger than the flood currents – is of shorter duration and hence unable to reverse the dune profile. The pronounced time–velocity asymmetry at the higher elevation combined with the distinct velocity peak asymmetry leads to a better preservation of hierarchical tidal cycles in the upper-bank dunes. The present study suggests that the persistent occurrence of single, stratified to cross-stratified mud drapes, which reflect dynamic mud deposition during the ebb phase, and the dominance of flood-directed cross-beds are diagnostic features of tidal bundles in the intertidal zone of unbarred, open-coast macrotidal environments. A proposed model for mud drape deposition provides a new perspective on the origin of tidal bundles together with useful criteria for reconstructing the paleo-depositional setting.

A model to predict the migration of sand waves in shallow tidal seas

A model is presented to determine the wavelength, the height and, more importantly, the migration speed of sand waves (also named tidal dunes) in shallow tidal seas. The model is based on a linear stability analysis and predicts the growth and migration of the fastest growing mode of a random perturbation of the flat sea bed forced by tidal current and wind waves. The growth of the fastest growing mode is assumed to generate the sand waves and to have the same geometrical and kinematical characteristics of these bottom forms. The characteristics of the sand waves predicted by the stability analysis are found to be in fair agreement with field observations, even though the number of accurate quantitative and simultaneous measurements of sand waves and forcing tidal currents is small and does not allow an exhaustive comparison between theoretical results and field data.

Three‐dimensional to two‐dimensional cross‐strata transition in the lower Pleistocene Catanzaro tidal strait transgressive succession (southern Italy)

AbstractSandstone tidal cross‐strata are the predominant sedimentary feature of strait‐fill stratigraphic successions. However, although widely described in numerous studies, tidal strait‐fill two‐dimensional and three‐dimensional cross‐strata have rarely been reported to occur in discrete intervals which are laterally adjacent or vertically stacked, and the meaning of this stratigraphic architecture has not yet been fully investigated. Understanding of the processes responsible for changes in the internal features of modern and ancient tidal bedforms is essential in order to predict lateral and vertical heterogeneities in analogous reservoir strata. This facies‐based study aims to interpret the three‐dimensional to two‐dimensional cross‐strata transition observed in the lower Pleistocene mixed siliciclastic/bioclastic sandstone filling the Catanzaro Strait, in southern Italy, during a continuous phase of tectonically driven marine transgression. Tidal cross‐strata disappear in the uppermost interval of the studied succession, where mudstone strata prevail. This stratigraphic trend is interpreted as the evidence of an important change in the tidal strait hydrodynamics due to a phase of relative sea‐level rise. At the beginning of the transgression, three‐dimensional tidal dunes migrated throughout the ca 3 to 4 km wide and ca 30 km long, WNW–ESE‐oriented Catanzaro Strait, due to strong tidal currents amplified through the seaway and flowing in semi‐diurnal phase opposition. As the intermediate phase of transgression enlarged the seaway width, the tidal current strength decreased as tidal water exchange occurred over a larger cross‐sectional area. The progressive reduction of the bed shear stress modified three‐dimensional tidal dunes into an extensive two‐dimensional bedform field. At the end of the transgression, the further widening of the Catanzaro Strait into a ca 10 to 12 km wide marine passageway changed the tidally dominated strait into a non‐tidal open shelf. The results of this research suggest the presence of a ‘critical cross‐sectional area’ in the narrowest strait‐centre zone which controls the activation and deactivation of tidal current amplification along a marine seaway.

Paleoecologic and Biostratigraphic Significance of Trace Fossils From Shallow- to Marginal-Marine Environments From the Middle Cambrian (Stage 5) of Jordan

The Hanneh Member (Cambrian Stage 5) of the Burj Formation and the Umm Ishrin Formation of Jordan represent a transgressive-regressive succession that contains twenty-eight ichnotaxa, including vertical burrows (Arenicolitesisp.,Diplocraterionisp.,Gyrolithes polonicus,Rosseliaisp.,Skolithos linearis, escape trace fossils), horizontal simple burrows and trails (Archaeonassa fossulata,Gordia marina,Helminthoidichnites tenuis,Palaeophycus tubularis,Planolites beverleyensis,P. montanus), plug-shaped burrows (Bergaueria sucta), horizontal branched burrows (Asterosomaisp.,Phycodesisp.,Treptichnuscf.T. pedum), bilobate structures (various ichnospecies ofCruzianaandRusophycus), and trackways and scratch marks (Diplichnitesisp.,Dimorphichnuscf.D. obliquus,Monomorphichnusisp.). Eleven trace-fossil assemblages are identified. TheArenicolitesisp. andDiplocraterionisp. assemblages occur in transgressive tidal dunes and bars whereas theRosseliaisp. assemblage characterizes areas between tidal dunes. TheCruziana salomonisassemblage reflects a wide variety of environmental settings including channels within tidal-bar complexes, bottomsets of tidal dunes, and interdune areas. TheGordia marinaassemblage is present between dune patches. TheGyrolithes polonicusassemblage penetrates into firmground mudstone below the maximum flooding surface. TheBergaueria sucta,Archaeonassa fossulata,Rusophycus aegypticusandCruziana problematicaassemblages occur in different subenvironments of the progradational delta.Cruziana salomonisandRusophycus burjensis, originally considered indicative of an early Cambrian age, are actually middle Cambrian in their type locality. Occurrences ofCruziana jordanicaandRusophycus aegypticusprovide evidence that these ichnospecies are of the same age in Jordan and may co-exist in terms of stratigraphic distribution withC. salomonisandR. burjensis.

The Miocene Sommières basin, SE France: Bioclastic carbonates in a tide-dominated depositional system

The Miocene Sommières Basin in SE France is a semi-enclosed depression that was connected to the Mediterranean Sea by a flooded paleo-incised valley and then filled by a suite of sediments comprising carbonate grains coming from temperate factories that were largely deposited in tidal-dominated paleoenvironments. The strata are partitioned into two sequences that reflect repeated flooding of the incised valley system, one of several similar situations in this region of France. The carbonate grains are mostly bioclasts, namely from barnacles, bryozoans, coralline algae (encrusting, branching, and rhodoliths), echinoids, and benthic foraminifers (large and small) with ostracods, sponge spicules and planktic foraminifers prominent in muddy facies. Particles were produced by shallow water carbonate factories on hard substrates (valley walls in particular), associated with subaqueous dunes, and in deeper water basinal settings. Each depositional sequence is underlain by an eroded and bored hard surface that is progressively overlain by TST subaqueous tidal dunes or storm deposits that grade up, in one case, into HST marls (the HST of the upper sequence has been removed by erosion). The lower sequence is ebb tide dominated whereas the upper sequence is flood tide dominated. The succession is interpreted to represent a TST whose tidal currents were focused by the narrow valley and a HST that reflected flooding of the overbanks. This stratigraphic and depositional motif is comparable to that in other spatially separated Neogene paleovalleys that are filled with tide-dominated clastic carbonates in the region. Together with other recently documented similar systems, these limestones constitute an important new group of carbonate sand bodies in the carbonate depositional realm.

Tidal dunes versus tidal bars: The sedimentological and architectural characteristics of compound dunes in a tidal seaway, the lower Baronia Sandstone (Lower Eocene), Ager Basin, Spain

The Lower Eocene Baronia Formation in the Ager Basin is interpreted as a series of stacked compound dunes confined within a tectonically generated embayment or tidal seaway. This differs from the previous interpretation of lower Baronia sand bodies as tidal bars in the front of a delta. The key architectural building block of the succession, the deposit of a single compound dune, forms a 1–3m-thick, upward coarsening succession that begins with highly bioturbated, muddy, very fine to fine grained sandstone that contains an open-marine Cruziana ichnofacies. This is overlain gradationally by ripple-laminated sandstone that is commonly bioturbated and contains mud drapes. The succession is capped by fine- to coarse-grained sandstones that contain both planar and trough cross-strata with unidirectional or bi-directional paleocurrent directions and occasional thin mud drapes on the foresets. The base of a compound dune is gradational where it migrated over muddy sandstone deposited between adjacent dunes, but is sharp and erosional where it migrated over the stoss side of a previous compound dune. The cross strata that formed by simple superimposed dunes dip in the same direction as the inclined master bedding planes within the compound dune, forming a forward-accretion architecture. This configuration is the fundamental reason why these sandbodies are interpreted as compound tidal dunes rather than as tidal bars, which, in contrast, generate lateral-accretion architecture. In the Baronia, fields of compound dunes generated tabular sandbodies 100s to 1000s of meters in extent parallel to the paleocurrent direction and up to 6m thick that alternate vertically with highly bioturbated muddy sandstones (up to 10m thick) that represent the low-energy fringes of the dune fields or periods of high sea level when current speeds decreased. Each cross-stratified sandstone sheet (compound-dune complexes) contains overlapping lenticular “shingles” formed by individual compound dunes, separated by 10–30cm of bioturbated muddy sandstone, which migrated over each other in an offlapping, progradational fashion. Each compound-dune complex (the best reservoir rock) thins as it downlaps, at average rates of 3–4m/km in a dip direction. These reservoir units can be comprised of discrete compartments, each formed by a single compound dune, that extend for 500–1000m in the direction of the current, and are at least 350–600m wide in a flow-transverse direction. Distinguishing between tidal bars and tidal dunes in an ancient tidal succession can be difficult because both can contain similar cross-bedded facies and have overlapping thicknesses; however, the internal architecture and sandbody orientations are different. Tidal bars have their long axis almost parallel both to the tidal current direction and to the strike of the lateral-accretion master surfaces. In inshore areas, they are bounded by channels and fine upward. Large compound tidal dunes, in contrast, have their crest oriented approximately normal to the tidal currents and contain a forward-accretion architecture. Coeval channels are uncommon within large, sub-tidal dune fields. The above distinctions are very important to reservoir description and modeling, because the long axis of the intra-reservoir compartments in the two cases will be 90° apart.

Sediment mixtures, coastal bedforms and grain sorting phenomena: An overview of the theoretical analyses

Recent studies, which investigate the processes leading to the formation of rhythmic morphological features in the coastal region, are reviewed. Emphasis is given to morphodynamic stability analyses which consider the presence of a heterogeneous sediment. The selective transport of the different fractions of a sediment mixture leads to grain sorting phenomena, which significantly affect the dynamics of the bottom forms. Small scale bedforms (ripples), intermediate bedforms (sorted bedforms and tidal dunes) and large scale bedforms (shoreface-connected ridges and sand banks) are considered. References are given to papers describing the new aspects of the stability analyses arising from the simultaneous presence of a time-dependent forcing flow and a sediment mixture.

The formation of tidal sand waves: Fully three-dimensional versus shallow water approaches

Tidal sand waves, also named tidal dunes, are large scale bedforms generated by the growth of perturbations of the sea bottom driven by tidal currents. Indeed, the interaction of an oscillatory tidal current with a bottom waviness gives rise to steady recirculating cells which tend to drag the sediment from the troughs towards the crests of the bottom perturbation. The net motion of the sediment towards the crests is opposed by gravity force and the growth of the perturbation is controlled by a balance between these two effects. In the literature, to determine the conditions which lead to the formation of sand waves and to determine the characteristics of the bedforms generated by this instability mechanism, both fully three-dimensional and shallow water approaches are employed. The shallow water approach is computationally less expensive than the fully three-dimensional one but, in many cases, it might be less accurate. This paper compares the quantitative predictions obtained by means of the two approaches and quantifies the range of the parameters such that the shallow water approximation provides reliable predictions.

A parameterization of the wavelength of tidal dunes

The idealized model of Besio et al. (On the formation of sand waves and sand banks. Journal of Fluid Mechanics 2006; 557: 1–17) is used to predict the wavelength of tidal dunes (sand waves) generated by tidal currents in estuaries and shallow seas. The predictions are then analysed and a formula is proposed to estimate the wavelength of tidal dunes as a function of the parameters of the problem. The wavelength of the dunes is found to increase when the water depth is increased and/or the strength of the tidal current is decreased. On the other hand, the size of the bottom material (if medium sand is considered) and the tidal ellipticity are found to have a relatively small influence on the length of the bottom forms. The formula proposed provides results which are consistent with field observations of different authors. Copyright © 2011 John Wiley & Sons, Ltd.