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The interaction of sedimentary systems with oceanographic processes in deep-water environments is not well understood yet, despite its importance for palaeoenvironmental reconstructions, and for a full understanding of source-to-sink sediment transport. The aim of this study is to improve the understanding of how contourite moats, elongated depressions formed by bottom currents associated with contourite drifts, develop and of the link between moat-drift system morphology and bottom current dynamics. This study provides a systematic comparison of 185 cross-sections of moat-drift systems distributed at 39 different locations worldwide, and a detailed analysis of the morphology of six moats that cover a wide range of typical geological and hydrodynamic settings. Additionally, in situ measured current data were analysed to better link hydrodynamics to moat morphology. The median of all profiles across all moat-drift systems reveals a 50 m relief, a width of 2.3 km, a relief to width ratio of 0.022, a slope angle of 6°, a drift angle of 3° and a concave-up shaped morphology. Moats can be over 100 km long. Some moats are driven by sediment erosion while others are depositional and primarily exist due to differential sedimentation inside the moat compared to the drift alongside the moat. A new sub-classification of moat-drift systems based on their stratigraphy is proposed. This classification distinguishes moats depending on the degree of erosion versus deposition. No relation is found between latitude and moat-drift morphology or stratigraphy in the analysed examples. The combined data indicate that a steeper slope focuses the current more than a gentle slope, resulting in an increase of the relief-width ratio and drift angle. Thus, this study provides new insides into the interaction of ocean currents with sedimentary morphology, which thereby affects the evolution of a poorly understood deepwater sedimentary system.

Downstream variation in grain size associated with changes in river pattern is a topic that interests multiple disciplines. How grain size varies between adjacent reaches with strongly contrasting river pattern is an outstanding question. This study presents a combined field and numerical modelling investigation of a river with a downstream planform change from single channel to anabranching, where the planform is controlled by a change in underlying lithology. This approach enabled exploration of the controls on sedimentology in a river for which there is very limited opportunity to collect flow and sediment transport data. This study shows that the surficial grain size decreases as a result of the downstream change in planform. This is because of a decrease in flow velocity and shear stress associated with a decrease in channel depth related to the planform change. Channel geometries in both the field and modelling data fit into distinct groups based on channel depth, the deepest being the single-channel reach and the shallowest being the anabranching. This downstream reduction in channel dimension (depth) is caused because the total discharge is split from one channel into multiple channels. The coarsest grain sizes (cobble) are deposited at the terminus of the single channel and in the distributary channels; anabranching channels contain sand-size sediments. This study shows that in a transition from single channel to anabranching, the channel dimensions decrease as the number of channels increases, resulting in a decrease in bed shear stress and the fining of bed material downstream. This article is protected by copyright. All rights reserved.

The formative conditions for bedform spurs and their roles in bedform dynamics and associated sediment transport are described herein. Bedform spurs are formed by helical vortices that trail from the lee surface of oblique segments of bedform crestlines. Trailing helical vortices quickly route sediment away from the lee surface of their parent bedform, scouring troughs and placing this bed material into the body of the spur. The geometric configuration of bedform spurs to their parent bedform crests is predicted by a cross-stream Strouhal number. When present, spur-bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms. Field measurements collected at the North Loup River, Nebraska and flume experiments described by Venditti et al. (2005a, 2005b, 2005c) demonstrate that this trailing helical vortex mediated sediment transport is a mechanism for bedform deformation, interactions, and transitions between two-dimensional and three-dimensional bedforms. This article is protected by copyright. All rights reserved.

Past geomorphological models assume that erosion of sediments from old mountain belts occurred at a relatively constant rate, based on comparatively uniform isostatic adjustment caused by unloading. Late Miocene strata of the south-eastern United States provide an example of pulsed tectonism resulting in a surge in siliciclastic sediment production and transport. Regional tectonism (uplift of the southern Appalachian Mountains) and climatic conditions during the Late Miocene resulted in the long-distance (up to 1000 km) fluvial transport of coarse siliciclastic sediments onto a stable carbonate platform in southern Florida. The sediments are unusual in that they are significantly coarser than marine-transported sands in southern Florida, with discoidal quartz and quartzite clasts up to 40 mm in diameter locally present, and have relatively high potassium feldspar contents (up to 16% in some sample fractions), whereas feldspar is rare in modern Florida beach sands. It is suggested that previously documented rejuvenation of the southern Appalachian Mountains during the Middle to Late Miocene time, coupled with the Messenian sea-level low, generated the increased rate of sediment production and necessary hydraulic gradient to allow rapid transport of coarse sediments. Tectonic influence on the river pathway in Florida, as well as in the southern Appalachian Mountains, may have maintained the river on the narrow carbonate platform. The Florida Platform during the Late Miocene must also have had a sufficiently wet climate to cause episodic transport of the coarse sediments. Siliciclastic sediment transport on the Florida Platform during the Late Miocene greatly differed from Pleistocene to modern conditions, which are dominated by the transport of fine-grained sands by longshore marine processes.

Shoreface sandstone deposits within the Early Carnian part of the Snadd Formation of the Norwegian Barents Sea can be traced for hundreds of kilometres in the depositional strike direction and for tens of kilometres in the depositional-dip direction. This study uses three-dimensional seismic attribute mapping and two-dimensional regional seismic profiles to visualize the seismic facies of these shoreface deposits and to map their internal stratigraphic architecture at a regional scale. The shoreface deposits are generally elongate but show variable width from north-east to south-west, which corresponds to a sediment source in the northern part of the basin and a southward decrease in longshore sediment transport. The Snadd Formation presents an example of how large-scale progradational shoreface deposits develop. The linear nature of its shoreface deposits contrasts with more irregular, cuspate wave-dominated deltaic shorelines that contain river outlets, and instead implies longshore drift as the main sediment source. In map view, discrete sets of linear features bounded by truncation surfaces scale directly to beach ridge sets in modern counterparts. The shoreface deposits studied here are characteristic in terms of scale and basin-wide continuity, and offer insight into the contrast between shallow marine deposition under stable Triassic Greenhouse and fluctuating Holocene Icehouse climates. Findings presented herein are also important for hydrocarbon exploration in the Barents Sea, because they describe a hitherto poorly understood reservoir play in the Triassic interval, wherein the most prominent reservoir plays have so far been considered to be found in channelized deposits in net-progradational delta-plain strata that form the topsets to shelf-edge clinoforms. The documented presence of widespread wave-dominated shoreface deposits also has implications for how the relative importance of different sedimentary processes is considered within the basin during this period.

Aeolian sand transport results from interactions between the ground surface and airflow. Previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity, but the influence of air density, which strongly constrains airflow characteristics and the resulting sand flow, has not been widely considered. In the present study, entrainment, saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from −154 m below sea-level (Aiding Lake) to 5076 m above sea-level (Tanggula Mountain pass on the Qinghai–Tibetan plateau). At each site, a portable wind tunnel and high-speed camera system were set up, and the friction wind velocity, threshold friction velocity and sand flow structure were observed systematically. For a given volumetric airflow, lower air density increases the wind velocity. Low air density also creates a high threshold friction velocity. The Bagnold wind erosion threshold model remains valid, but the value of empirical parameter A decreased with decreasing air density and ranged from 0·10 to 0·07, the smallest values reported in the literature. For a given wind velocity, increased altitude reduced total sand transport and creeping, but the saltation rate and saltation height increased. The present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude (for example, alpine deserts and low-lying lake basins) or on other planets, including Mars. These results also provide theoretical support for improved sand-control engineering measures. The data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind-blown sand.

To achieve a more robust interpretation of sediment conditions and transport based on grain size, this study combines the perspectives of two different interpretation techniques that have been developed separately and applied in the literature: (i) ‘grain size trend analysis’ interprets changes between interrelated sampling sites assuming that sediment in the direction of transport should become either coarser, better sorted and more positively skewed, or finer, better sorted and more negatively skewed; and (ii) the ‘population anomalies’ method, which estimates the balance between erosion and accumulation processes at each sampling site based on individual site deviations compared with average values for grain-size parameters in the area of interest. The simultaneous use of both approaches enables sedimentological conditions and transport alternatives to be distinguished. This improvement is necessary because site-specific changes and general textural trends provide information that complements the strengths and weaknesses of each perspective. The methods are implemented here using two-dimensional Geographic Information System tools and illustrated for the Lithuanian coast. Grain-size characteristics of 712 surface samples of sandy sediments are used to interpret sediment transport pathways and predominant sedimentological conditions in the Lithuanian coastal zone. In general, coast-parallel sediment transport dominates the entire investigation area, although wave-induced movement perpendicular to the coastline is inferred in the shallow nearshore zone. The deepest areas offshore are characterized by sea floor erosion in the north, whereas an accumulation zone occurs in the relatively deep central part of the study area.

The development of bedforms under unidirectional, oscillatory and combined-flows results from temporal changes in sediment transport, flow and morphological response. In such flows, the bedform characteristics (for example, height, wavelength and shape) change over time, from their initiation to equilibrium with the imposed conditions, even if the flow conditions remain unchanged. These variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record. Hence, understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record. This article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional, purely oscillatory and combined-flow conditions, which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples. The present results yield a unified model for bedform development and equilibrium under unidirectional, oscillatory and combined-flows. The experimental results show that the processes of bedform genesis and growth are common to all types of flows, and can be characterized into four stages: (i) incipient bedforms; (ii) growing bedforms; (iii) stabilizing bedforms; and (iv) fully developed bedforms. Furthermore, the development path of bedform; growth exhibits the same general trend for different flow types (for example, unidirectional, oscillatory and combined-flows), bedform size (for example, small versus large ripples), bedform shape (for example, symmetrical or rounded), bedform planform geometry (for example, two-dimensional versus three-dimensional), flow velocities and sediment grain sizes. The equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition.

Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well-mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face-face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre-scale, and four microfacies are recognized: Well-sorted siltstone forms millimetre-scale combined-flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt-streaked claystone comprises parallel, sub-millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave-supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay-rich mudstone has little silt and may represent late-stage settling of fluid mud, or settling from wave-dissipated fluid mud. It is difficult or impossible to correlate millimetre-scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined-flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope-flowing, wave-supported fluid mud. In the upper part of the studied section, centimetre-scale interbeds of very fine to fine-grained sandstone show wave ripple crests trending shore normal, whereas combined-flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north-east drove shore-oblique geostrophic sand transport whereas simultaneously, wave-supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small-scale bioturbation of mud beds co-occurs with interbedded sandstone but stratigraphically lower, sand-free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom-water oxygen content collectively inhibited benthic fauna in the distal prodelta.

On 21 April 2007, a Mw 6.2 earthquake struck Aysen fjord (Chilean Patagonia) and caused onshore and offshore mass movements which triggered tsunamis and density flows in the fjord. To better understand the facies successions in, and the intercalation of, the density-flow deposits, a study was made of the 2007 deposits in 22 short sediment cores taken in the inner Aysen fjord. By combining grain-size analysis with X-ray computed tomography scanning, it was possible to demonstrate that the encountered facies correspond to classical divisions of debrites and turbidites. The single-event deposits consist of a succession of several sub-deposits deposited under different flow directions and can be interpreted as stacked turbidites. Orientations of: (i) folds; (ii) imbricated mud clasts; iii) backsets and foresets of climbing ripples; and (iv) asymmetrical convolute lamination, were used to determine relative flow directions at the location of the cores. By assigning the basal flow of the stacked debrites and turbidites to the closest principal mass-flow, the absolute flow directions of the sub-deposits were determined which, in combination with multibeam basin-floor morphology, allowed reconstruction of the 2007 density-flow successions in Aysen fjord. Furthermore, alternating flow directions provide evidence for a seiche induced by the density flows. It was concluded that X-ray computed tomography scans provide crucial information for reconstructing palaeoflows and can be a useful tool in marine and lacustrine sedimentology and palaeoseismology. The multidirectionality of sub-deposits in turbidites is, next to differences in mineralogy, a new criterion to identify stacked turbidites. These multidirectional, stacked turbidites are an indication of simultaneous triggering of density flows and can therefore, in most cases, be attributed to earthquakes, ruling out other triggers, such as floods, storms or other sediment failures. This article is protected by copyright. All rights reserved.

Modern mixed alluvial-bedrock channels in mountainous areas provide natural laboratories for understanding the time scales at which coarse-grained material has been entrained and transported from their sources to the adjacent sedimentary sink, where these deposits are preserved as conglomerates. This article assesses the shear stress conditions needed for the entrainment of the coarse-bed particles in the Glogn River that drains the 400 km2 Val Lumnezia basin, eastern Swiss Alps. In addition, quantitative data are presented on sediment transport patterns in this stream. The longitudinal stream profile of this river is characterized by three ca 500 m long knickzones where channel gradients range from 0·02 to 0·2 m m−1, and where the valley bottom confined into a

Although the stratigraphy of sedimentary basins depends on the balance between the magnitude and grain-size characteristics of the sediment supply (Qs) and the spatial distribution of tectonic subsidence generating accommodation σ(x), Qs is problematical to measure in present-day sediment routing systems and formidably difficult to predict in their ancient counterparts. This challenge was tackled by treating the sediment discharge from the outlet of mountain catchments as the result of incision by a drainage network with a bulk diffusivity based on the length over which the mean annual rainfall is concentrated. The size, relief and slope of palaeo-catchments acting as feeders for sediment routing systems are used to run simulations of sediment discharge and bulk diffusivity for a range of annual precipitation values. A wide range of observable geological phenomena can be used to converge on the most likely solutions for Qs, including depositional volumes in the basin, and bedrock thermochronology and detrital cosmogenic nuclide dating to constrain catchment erosion rates. Modelled sediment discharges can be checked with estimates derived from global regressions. The sediment efflux of mountain catchments serves as a boundary condition for down-system sediment transport and deposition. Variations in the volumetric ratio of sediment supply to available accommodation, Qs/σ(x), determines patterns of transverse versus longitudinal (axial) sediment dispersal. The volumetric ratio may change as a result of variations in climatic parameters, tectonic uplift rate and catchment expansion. An abrupt climate change to higher precipitation values promotes higher Qs/σ(x), but transient landscape response causes a return to values close to the baseline, generating a distinctive down-system extension of a gravel ‘spike’. Catchment expansion has a similar, but more prolonged, effect on gravel progradation. In contrast, a change in tectonic forcing, such as an increase in slip rate on a border fault, causes little change in Qs/σ(x), because increased subsidence compensates for the increased sediment supply. Studies of mid Eocene–Oligocene sediment routing systems in the south-central Pyrenees allow the discrimination of different types of proximal wedge-top sedimentary systems on the basis of the volumetric ratio of Qs to accommodation σ(x): (i) small, steep, local fan systems in tectonically ponded, underfilled basins, supplied by low sediment discharges; (ii) tectonically guided, long-range, axial systems fed by large sediment discharges from widely spaced palaeovalleys; and (iii) large, shallow-sloping transverse megafans burying underlying defunct or active tectonic structures, supplied by high to very high sediment discharges. Understanding the role of variations in Qs helps to explain the syntectonic evolution of proximal foreland basin systems. The Oligocene–Miocene North Alpine Foreland Basin, Switzerland, is qualitatively identified as a high-Qs example, the Miocene–Recent northern Apennines of Italy as a low-Qs example and the Eocene-Oligocene southern Pyrenees of Spain as intermediate in character.

Cross-shore grading of sediment has been observed on the surface of estuarine beaches but the swash zone processes responsible for this grading have not been measured. This study was conducted to provide an explanation for the cross-shore grading of sediment on a predominantly sandy estuarine foreshore. Data on wave and swash characteristics and sediment trapped in the uprush and backwash during 25 swash events were gathered from mid-rising to mid-falling tide on a small transgressive barrier in Delaware Bay, New Jersey, USA. Sediment is predominantly quartz and feldspar, medium to coarse sands with a gravel fraction of granules and pebbles. Wave energies increased with tidal rise. The percentage of gravel in transport in the uprush and backwash is similar (11% and 13%) during the rising tide when the swash zone is at mid-foreshore, decreases in the uprush (9%) and increases in the backwash (18%) when the swash zone is on the upper foreshore. When the swash zone is at mid-foreshore on the falling tide, the quantity of gravel in the backwash (30%) is greater than in the uprush (24%). The low proportion of gravel within the foreshore prior to trapping, and the increase in the percentage of gravel when the waves and swash are on the upper foreshore, suggests that the step is the primary source of gravel high on the foreshore. The size of the step increases as wave heights increase with tidal rise. The rate of delivery of gravel into the swash is enhanced by sediment entrained during wave breaking and interaction of the uprush with the previous backwash. The lag in the rate of step migration relative to breaker migration during the falling tide increases the likelihood of mining gravel from the step and subsequent transport in the uprush and backwash. These findings are important for low energy estuarine beaches sensitive to small changes in tidal range and wave energy that cause sedimentological change across the foreshore.

In this study, the distribution of channel-bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high-intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio-deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach-scale distribution of channel-bed facies is related to river planform. From a broader perspective, the distribution of channel-bed facies is related to channel sinuosity — higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45 km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock-alluvial channel. The analyses demonstrate that a mixed bedrock-alluvial channel boundary can be associated with low-gradient and sand-bed rivers near their marine outlet.

Turbidity currents in the ocean are driven by suspended sediment. Yet results from surveys of the modern sea floor and turbidite outcrops indicate that they are capable of transporting as bedload and depositing particles as coarse as cobble sizes. While bedload cannot drive turbidity currents, it can strongly influence the nature of the deposits they emplace. This paper reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud which is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream-migrating antidunes and downstream-migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for open-channel flows and, in fact, is fitted well by an existing relation determined for open-channel flows. In the case of dunes and downstream-migrating antidunes, for which flow separation on the lee sides was observed, form drag falls in a range that is similar to that due to dunes in sand-bed rivers. This form drag can be removed from the total bed shear stress using an existing relation developed for rivers. Once this form drag is subtracted, the bedload data for these cases collapse to follow the same relation as for plane beds and upstream-migrating antidunes, for which no flow separation was observed. A relation for flow resistance developed for open-channel flows agrees well with the data when adapted to density underflows. Comparison of the data with a regime diagram for field-scale sand-bed rivers at bankfull flow and field-scale measurements of turbidity currents at Monterey Submarine Canyon, together with Shields number and densimetric Froude number similarity analyses, provide strong evidence that the experimental relations apply at field scale as well.

Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse-grained deltaic sandstones passing distally into fine-grained offshore sediments. Dune-scale cross-beds superimposed on delta-front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse-grained sand across the entire depth range of the clinoforms (ca 35 m). Bi-directional, alongshore-oriented, trough cross-set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally-influenced deposits in a setting where co-oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain-size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present-day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi-directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide-driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record.

A sedimentary model for hooked spit depositional systems based on ground-penetrating radar and sedimentological data is presented. The recurved main spit of Sylt Island (southern North Sea) is dominated by migrating sand dunes; the investigated hooked spit exhibits a system of foredune ridges, oriented perpendicular to the dunes of the recurved spit. The development of the hooked spit is related closely to the presence of an adjacent tidal inlet, where strong tidal currents and a steep bathymetry prevent a further northward progradation of the main spit and trigger a deflection from northerly-directed to easterly-directed net sediment transport. Ground-penetrating radar data and shallow sediment cores reveal the sedimentary architecture of the hooked spit in high resolution and allow the proposition of a genetic stratigraphic model. It is shown that the growth of the hooked spit is controlled by the interplay of alongshore migrating foreshore beach drifts under fair-weather conditions and strong erosional events, interpreted as the result of rare severe storms. These storms may excavate scarps in the backshore, which play an important role in the development of foredune ridges. Accelerator mass spectrometry 14C ages indicate an absolute age of at least 1300 years for the hooked spit, which possibly correlates with strengthened erosion of the main spit. In contrast to the main spit, where the sediment budget is negative nowadays, growth of the hooked spit beach accelerated significantly during the last decades. This effect can probably be attributed to enhanced beach-nourishments updrift along the main spit and makes the investigated hooked spit a natural laboratory to study the influence of increasing sediment supply into a system developing under the conditions of sea-level rise. The study shows that the same external forces lead to distinct progradational processes along one barrier-spit system.

The links between large-scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Rio Parana, Argentina, time series of three-dimensional velocity, and at-a-point suspended sediment concentration and particle-size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee-side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune-related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.

Hodge, R.A., Brasington, J., Richards, K.S. (2009). Analysing laser-scanned digital terrain models of gravel bed surfaces: linking morphology to sediment transport processes and hydraulics. Sedimentology, 56(7), 2024-2043. Sponsorship: NERC

Spit systems are seldom recognized in the pre-Quaternary sedimentary record compared to their common occurrence along present-day coasts and in Quaternary successions. This lack of recognition may partly be due to the lack of widely accepted depositional models describing the facies characteristics of spit systems and their subaqueous platforms in particular. The Skagen spit system is a large active system that began to form 7150 yr bp and from 5500 bp to Recent times it has prograded 4 m year−1 and accumulated 3·5 × 109 m3 of sand. The spit system provides a unique opportunity for establishing a well-constrained depositional model because uplift and erosion have made large windows into the preserved facies, while active spit-forming processes can be examined at the young prograding end of the same system. The depositional model presented here thus builds on excellent outcrops, surface morphology, a well-defined palaeogeography and detailed C14 age control supplemented with observations from continuous well cores and profiles obtained by ground-penetrating radar and transient electromagnetic surveys. The factors that have governed the development of the spit system, such as relative sea-level change, wave and current climate, tidal range, sediment transport and depositional rates are also well-understood. The sedimentary facies of the spit system are grouped into four principal units consisting from below of thick storm sand beds, dune and bar-trough deposits, beach deposits and peat beds. These four units form a coarsening and shallowing upward sand-dominated succession, up to 32 m thick, which overlies offshore silt with a transition zone and is topped by a diastem overlain by young aeolian dune sand. The sedimentary structures and depositional processes are described in detail and integrated into a depositional model, which is compared to other spit systems and linear shoreface models.

A hierarchical typology for the channels and bars within aggradational wandering gravel-bed rivers is developed from an examination of a 50 km reach of lower Fraser River, British Columbia, Canada. Unit bars, built by stacking of gravelly bedload sheets, are the key dynamic element of the sediment transfer system, linking sediment transport during individual freshets to the creation, development and remoulding of compound bar platforms that have either a lateral or medial style. Primary and secondary unit bars are identified, respectively, as those that deliver sediment to compound bars from the principal channel and those that redistribute sediment across the compound bar via seasonal anabranches and smaller channels. The record of bar accretion evident in ground penetrating radar sequences is consistent with the long-term development of bar complexes derived from historical aerial photographs. For two compound bars, inter-annual changes associated with individual sediment transport episodes are measured using detailed topographic surveys and longer-term changes are quantified using sediment budgets derived for individual bars from periodic channel surveys. Annual sediment turnover on the bars is comparable with the bed material transfer rate along the channel, indicating that relatively little bed material bypasses the bars. Bar construction and change are mainly accomplished by lateral accretion since the river has limited capacity to raise bed load onto higher surfaces. Styles of accretion and erosion and therefore the major bar-form morphologies on Fraser River are familiar and consistent with those in gravelly braided channels, but the wandering style does exhibit some distinctive features. For example, 65-year histories reveal the potential for long sequences of uninterrupted accretion in relatively stable wandering rivers that are unlikely in braided rivers.

Evolution of sedimentary systems at large temporal and spatial scales cannot be scaled down to laboratory dimensions by conventional hydraulic Froude scaling. Therefore, many researchers question the validity of experiments aiming to simulate this evolution. Yet, it has been shown that laboratory experiments yield stratigraphic responses to allocyclic forcing that are remarkably similar to those in real-world prototypes, hinting at scale independency with strong dependence on boundary conditions but weak dependence on the actual sediment transport dynamics. This paper addresses the dilemma by contrasting sediment transport rules that apply in the laboratory with those that apply in real-world geological systems. It is demonstrated that the generation of two-dimensional stratigraphy in a flume can be simulated numerically by the non-linear diffusion equation. Sediment transport theory is used to demonstrate that only suspension-dominated meandering rivers should be simulated with linear diffusion. With increasing grain-size (coarse sand to gravel) and shallowness of river systems, the prediction of long-term transport must be simulated by non-linear, slope-dependent diffusion to allow for increasing transport rates and thus change in stratigraphic style. To point out these differences in stratigraphic style, three stages in infill of accommodation have been defined here: (i) a start-up stage, when the system is prograding to base level (e.g. the shelf edge) with no sediment flux beyond the base-level point; (ii) a fill-up stage, when the system is further aggrading while progressively more sediment is bypassing base level with the progression of the infill; and (iii) a keep-up stage, when more than 90% of the input is bypassing the base level and less than 10% is used for filling the accommodation. By plotting the rate of change in flux for various degrees of non-linearity (varying the exponent in the diffusion equation) it was found that the error between model and real-world prototype is largest for the suspension-dominated prototypes, although never more than 30% and only at the beginning of the fill-up stage. The error reduces to only 10% for the non-linear sandy-gravelly and gravelly systems. These results are very encouraging and open up ways to calibrate numerical models of sedimentary system evolution by such experiments.

Experimental modelling of an aggrading braided river has allowed investigation of the relationship between the frequency of channel avulsion (A(f)), the duration of time that the braidplain is occupied by flow, the spatial pattern of braidplain sedimentation and how these respond to a change in sediment supply (S-s). Model results demonstrate a strong, positive relationship between S-s and A(f) and that there is no downstream change in A(f) over the short braidplain distances (ca 100 m) modelled herein. Although A(f) is strongly dependent on S-s, the degree of channel switching does not influence the rate, or spatial pattern, of braidplain sedimentation. All experiments used a single, central input for water and sediment, and the channels occupied the centre of the alluvial basin for a longer period of time than the margins for all sediment supply rates and distances downstream. Despite this spatio-temporal pattern in flow occupancy, braidplain sedimentation rates were nearly uniform both downstream and across the basin, and increased at approximately the same rate as increases in S-s. As a consequence, less frequent, and possibly short-lived, flows at the margins of the braidplain deposited and preserved more sediment per unit time in comparison with the centre of the basin where flow occupancy was higher. An approximate order of magnitude change in sediment supply resulted in only a factor of two change in bed slope, probably due to both an increase in channelization and adjustment of the channel form that maintained sediment transport through the basin. This result suggests that linear diffusion models are unlikely to be applicable in landscape evolution models that possess aggrading multi-thread rivers, which are capable of self-adjustment in channel number and form.

Open-framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during a specific flow stage and sediment transport rate (either high, falling or low); (3) separation of sand from gravel in bedforms superimposed on the backs of larger bedforms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment size distribution (sandy gravel with a mean diameter of 1·5 mm) was kept constant, but flow depth, flow velocity and aggradation rate were varied. Bedforms produced under these flow conditions were bedload sheets, dunes and unit bars. The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation at the crest of bedforms, and further segregation of grain sizes during avalanching on the steep lee side. Sand in transport near the bed is deposited in the trough of the bedform, whereas bed-load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the height of the bedform increases. Infiltration of sand into the gravel framework is of minor importance in these experiments, and occurs mainly in bedform troughs. The geometry and proportion of OFG in fluvial deposits are influenced by variation in height of bedforms as they migrate, superposition of small bedforms on the backs of larger bedforms, aggradation rate, and changes in sediment supply. If the height of a bedform increases as it migrates downstream, so does the amount of OFG. Changes in the character of OFG on the lee-side of unit bars depend on grain-size sorting in the superimposed bedforms (dunes and bedload sheets). Thick deposits of cross-stratified OFG require high bedforms (dunes, unit bars) and large amounts of aggradation. These conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound-bar tails and downstream of confluence scours. Increase in the amount of sand supplied relative to gravel reduces the development of OFG. Such increases in sand supply may be related to falling flow stage and/or upstream erosion of sandy deposits.

Sediment transport in sand-bedded alluvial channels is strongly conditioned by bedforms, the planimetric morphology of which can be either two- or threedimensional. Experiments were undertaken to examine the processes that transform the bed configuration from two-dimensional (2D) dunes to threedimensional (3D) dunes. A narrowly graded, 500 lm size sand was subjected to a 0AE15 m deep, non-varying mean flow ranging from 0AE30 to 0AE55 m sec )1 in a 1 m wide flume. Changes in the planimetric configuration of the bed were monitored using a high-resolution video camera that produced a series of 10 sec time-lapsed digital images. Image analysis was used to define a critical value of the non-dimensional span (sinuosity) of the bedform crestlines that divides 2D forms from 3D forms. Significant variation in the non-dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random. Images also reveal that, once 2D bedforms are established, minor, transient excesses or deficiencies of sand are passed from one bedform to another. The bedform field appears capable of absorbing a small number of such defects but, as the number grows with time, the resulting morphological perturbations produce a transition in bed state to 3D forms that continue to evolve, but are pattern-stable. The 3D pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment, which leads to bifurcation. The experiments demonstrate that 2D bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator.

In settings where the transport of sand is partially or fully supply limited, changes in the upstream supply of sand are coupled to changes in the grain size of sand on the bed. In this manner, the transport of sand under the supplylimited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in 1963, the downstream reach of the Colorado River in Marble and Grand Canyons has exhibited evidence of sand-supply limitation. Sand transport in the river is now approximately equally regulated by changes in the discharge of water and changes in the grain sizes of sand on the channel bed and eddy sandbars. Previous work has shown that changes in the grain size of sand on the bed of the channel (driven by changes in the upstream supply of sand owing to both tributary floods and high dam releases) are important in regulating sand transport over timescales of days to months. In this study, suspended-sand data are analysed in conjunction with bed grain-size data to determine whether changes in the grain size of sand on the bed of the channel or changes in the grain size of sand on the surface of eddy sandbars have been more important in regulating sand transport in the post-dam Colorado River over longer, multi-year timescales. The results of this study show that this combined theory- and field-based approach can be used to deduce which environments in a complicated setting are the most important environments for regulating sediment transport. In the case of the regulated Colorado River in Marble and Upper Grand Canyons, suspended-sand transport has been regulated mostly by changes in the surface grain size of eddy sandbars.

The Barataria barrier coast formed between two major distributaries of the Mississippi River delta: the Plaquemines deltaic headland to the east and the Lafourche deltaic headland to the west. Rapid relative sea-level rise (1AE03 cm year )1 ) and other erosional processes within Barataria Bay have led to substantial increases in the area of open water (> 775 km 2 since 1956) and the attendant bay tidal prism. Historically, the increase in tidal discharge at inlets has produced larger channel cross-sections and prograding ebb-tidal deltas. For example, the ebb delta at Barataria Pass has built seaward >2AE 2k m since the 1880s. Shoreline erosion and an increasing bay tidal prism also facilitated the formation of new inlets. Four major lithofacies characterize the Barataria coast ebb-tidal deltas and associated sedimentary environments. These include a proximal delta facies composed of massive to laminated, fine grey-brown to pale yellow sand and a distal delta facies consisting of thinly laminated, grey to pale yellow sand and silty sand with mud layers. The higher energy proximal delta deposits contain a greater percentage of sand (75‐100%) compared with the distal delta sediments (60‐80%). Associated sedimentary units include a nearshore facies consisting of horizontally laminated, fine to very fine grey sand with mud layers and an offshore facies that is composed of grey to dark grey, laminated sandy silt to silty clay. All facies coarsen upwards except the offshore facies, which fines upwards. An evolutionary model is presented for the stratigraphic development of the ebbtidal deltas in a regime of increasing tidal energy resulting from coastal land loss and tidal prism growth. Ebb-tidal delta facies prograde over nearshore sediments, which interfinger with offshore facies. The seaward decrease in tidal current velocity of the ebb discharge produces a gradational contact between proximal and distal tidal delta facies. As the tidal discharge increases and the inlet grows in dimensions, the proximal and distal tidal delta facies prograde seawards. Owing to the relatively low gradient of the inner continental shelf, the ebb-tidal delta lithosome is presently no more than 5 m thick and is generally only 2‐3 m in thickness. The ebb delta sediment is sourced from deepening of the inlet and the associated channels and from the longshore sediment transport system. The final stage in the model envisages erosion and segmentation of the barrier chain, leading to a decrease in tidal discharge through the former major inlets. This process ultimately results in fine-grained sedimentation seaward of the inlets and the encasement of the ebb-tidal delta lithosome in mud. The ebb-tidal deltas along the Barataria coast are distinguished from most other ebb deltas along

This paper describes the formation, migration and sedimentology of sandwaves along the distal end of Long Point, a 40 km long spit in Lake Erie. Some seven to nine sandwaves occur in a zone over the last 14 km of the spit. They are characteristically 50–100 m wide at the downdrift end, range in length from 350 m to > 1500 m and migrate alongshore at rates that are typically 100–300 m year )1 . Measurements over a 7-year period show two forms of alongshore sandwave migration: (1) a migratory jump; and (2) downdrift accretion. The migratory jump is commonly 200–500 m year )1 and results from the onshore migration and welding of an inner nearshore bar to the downdrift end of the sandwave. This in turn leads to emergence of the bar over a distance of several hundred metres downdrift of the sandwave and isolation of the trough landward of the bar. Infilling of the trough abstracts large volumes of sediment from the local sediment transport system and may affect movement of the sandwave in the following year or movement of the next sandwave downdrift. Downdrift accretion commonly results in migration of 50–150 m year )1 and results from the refraction of waves around the distal end of the sandwave and episodic accretion of small swash bars. This mechanism occurs less frequently and appears to reflect a local condition of lower sediment abundance, often triggered by a large migratory jump in the previous year. The process of bar emergence and infilling produces a distinct suite of sedimentary structures associated with the infilling of the landward trough and building of the sandwave berm. The initial shoreline perturbation that generates the sandwave results from onshore migration and welding of inner nearshore bars, and the development and growth of the sandwaves is promoted by refraction of highly oblique waves.

A three-dimensional numerical model of sediment transport and deposition in coarse-grained deltas is used to investigate the controls on depositional sequence variability in marine half-graben extensional basins subject to eustatic sea-level change. Using rates of sea-level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall-sourced deltas in high-subsidence locations near the centre of a lault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope Different es in the three-dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall-sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault-tip and hangingwall dip-slope deltas mean that they are subject to relative sea-level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault-tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet like delta lobes characterize the hangingwall dip-slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. I urthermore, the results highlight pitfalls in sequence-stratigraphic interpretation and problems in interpreting controlling processes from the preserved stiatigraphic product.

A new theoretical scheme is presented to model the shape of a sand dune at equilibrium that does not require iterative calculations of the interaction between the wind flow and topography. The model is constructed by incorporating theory based on aerodynamics into a grain-scale model that estimates the shear velocity at the dune crest through the calculation of the sand trapping efficiency of the slipface in the lee of the dune. Published field data, collected in southern Peru, California and southern Morocco, show that as a dune becomes higher the windward slope becomes steeper. For the model proposed, the wind flow over a dune was first assumed to be similar to that over a Gaussian hill. By further assuming a fixed shear velocity on the level surface, the windward slope angle and migration speed of dunes in southern Peru can be explained. To comply with all available data, some aspects are still open to further investigation. However the theoretical insight presented herein implies that the upper limit of dune height may be greater in windier environments.

The trigonometric relationship between slope inclination, the horizontallyacting time-averaged traction force and the vertical depth of transport allowsthe estimation of one factor, when both others are known. Depth–transportfunctions can be deduced by comparing the depth distributions of livingorganisms and their skeletal remains, and this paper simplifies this comparisonusing foraminifera in which a single test represents an individual. Differencesin distribution parameters between living individuals and empty tests allowdepth–transport functions to be determined; these functions differ betweenspecies at a single transect according to the varying buoyancies of the tests.Within a single species, differences in depth–transport functions betweenlocations are based on either slope inclination or traction intensities. Afterestablishing a mean depth–transport function by averaging species-characteristic functions, the time-averaged traction force acting on thestudied transect can be calculated. Transport intensities are also estimatedusing an erosion–deposition diagram that combines the relative frequencydistributions of living individuals and empty tests. The proportion of ‘eroded’,‘parautochthonous’ and ‘allochthonous’ tests mirrors the influence of bothslope inclination and traction force for the deposition of empty tests. To test themodel, six species of symbiont-bearing benthic foraminifers were investigatedat two transects in front of a NW Pacific coral reef. One transect is distinguishedby a strong slope flattening below the steep reef slope (30 m), whereas furthersteepening characterizes the equivalent part in the other transect. Thesedifferences are mirrored in the depth–transport functions as well as in theerosion–deposition diagrams of all species. The time-averaged traction forcesdiffer in intensities between transects, because of the position of the reef frontwith respect to the predominant wind direction. However, the form of thefunctions is identical and distinguished by an increase from the surface to 35 mdepth, followed by a decrease down to 105 m. This can be explained bysuccessive onshore and offshore forces acting on the shallow slope, such as thetropical cyclones that cross the region every summer.Keywords Carbonate sands, coral reef slopes, foraminifera, sediment trans-port, traction force, tropical cyclones.INTRODUCTIONSediment transport on the slopes and forereefareas greatly influences the topography of coralreefs. As carbonate produced by reef-buildingorganisms is rapidly reduced to sediment,especially at the reef front (e.g. Hubbard et al.,1990), the overwhelming of reef growth bySedimentology (2001) 48, 1025–1046

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply-limited reach of the River Rhine. During daylong surveys, suspended sediment concentration, bedload transport rate, water depth, flow velocity, turbulence intensity, near-bed shear stress and water temperature were recorded over individual isolated dunes. This paper considers the hydrodynamic environment and sediment transport over the dunes. A companion paper details the sedimentology and morphology of the dunes. Flow over the flat gravel lag upstream of large dunes is more uniform than that over dunes, and gravels are rarely entrained by in-bank discharges. Unsteady and non-logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes, and the near-bed flow demonstrates evidence of large-scale, coherent low-frequency flow structures; these may reflect stacked sequences of separated boundary layers generated by secondary dunes. However, the low-amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides. Bed roughness and near-bed shear stress commonly increase steadily over the stoss of dunes, but may decrease near the crest, especially where a crestal platform exists that is devoid of secondary bedforms. Bed roughness scales with the physical size of bed roughness elements. However, variability in roughness lengths is large, owing to the composite nature of bed roughness. Bedload transport over stoss slopes is spatially variable, but often shows an increase with increasing bed shear stress over the stoss. Well-formed wakes only develop downstream of lee slopes, which are close to the angle of repose; otherwise, separation is weak, and suspension and settling of fine sediments is of little consequence to dune evolution. Wake flow is characterized by turbulence production one order of magnitude greater than over the stoss side, which may be related to vortex shedding from the dune. However, wake current speeds are extremely low and variable in direction; reverse flow is not sustained, and no retrogressive bedforms occur in the lee of large dunes. Near-bed shear stress and bed roughness are usually low within the wake, reflecting low current speeds. Weak wake-flow reattachment occurs at a variable distance downstream, up to several times the duneform height. A two-dimensional numerical model for flow over dune topography, calibrated using average parameter values, provided a reasonable description of the flow upstream, and over the backs of large dunes as far as the crestal region. Wake flow could not be modelled. However, the temporal and spatial complexity of natural three-dimensional flow over the dunes resulted in variance in parameter estimates; this variance precluded modelling of flow and bedload transport. Consequently, it was not possible to model dune evolution in a deterministic sense.

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply-limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D50 of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well-sorted medium to coarse sand (D50 of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D50

Aeolian dune dimensions and migration rates are analysed along the Ceara coast, north-east Brazil. Dunes that are currently mobile along the Ceara coast are composed of barchans and sand sheets. The results show that barchans maintain an equilibrium form, which can be characterized by values of dimensionless shape parameters H/W and W/L, where H is the dune height, W is the wing-to-wing width and L is the dune length. Dunes are highly mobile, with average migration rates of 17·5 m year−1 for barchans and 10 m year−1 for sand sheets. The calculated migration rates were found to depend strongly on dune dimensions for both barchans and sand sheets, i.e. the larger the dune is, the lower the migration rate will be. This size dependence was associated with the existence of a representative common transport rate along the dune fields, which induces a different dune migration rate dependent on dune size. Finally, from the observed dune evolution, an aggregated scale aeolian sediment transport was inferred. This bulk transport rate, of the order of 90–100 m3 m−1 year−1, is only valid for a timescale of years to decades, which is the timescale used in dune evolution analysis.

The continental shelf of the State of Rio Grande do Norte, Brazil, is an open shelf area located 5∞S and 35∞W. It is influenced by strong oceanic and winddriven currents, fair weather, 1AE5-m-high waves and a mesotidal regime. This work focuses on the character and the controls on the development of suites of carbonate and siliciclastic bedforms, based on Landsat TM image analysis and extensive ground-truth (diving) investigations. Large-scale bedforms consist of: (i) bioclastic (mainly coralline algae and Halimeda) sand ribbons (5‐10 km long, 50‐600 m wide) parallel to the shoreline; and (ii) very large transverse siliciclastic dunes (3AE4 km long on average, 840 m spacing and 3‐8 m high), with troughs that grade rapidly into carbonate sands and gravels. Wave ripples are superposed on all large-scale bedforms, and indicate an onshore shelf sediment transport normal to the main sediment transport direction. The occurrence of these large-scale bedforms is primarily determined by the northwesterly flowing residual oceanic and tidal currents, resulting mainly in coastparallel transport. Models of shelf bedform formation predict sand ribbons to occur in higher energy settings rather than in large dunes. However, in the study area, sand ribbons occur in an area of coarse, low-density and easily transportable bioclastic sands and gravels compared with the very large transverse dunes in an offshore area that is composed of denser mediumgrained siliciclastic sands. It suggests that the availability of different sediment types is likely to exert an influence on the nature of the bedforms generated. The offshore sand supply is time limited and originates from sea floor erosion of sandstones of former sea-level lowstands. The trough areas of both sand ribbons and very large transverse dunes comprise coarse calcareous algal gravels that support benthic communities of variable maturity. Diverse mature communities result in sediment stabilization through branching algal growth and binding that is thought to modify the morphology of dunes and sand ribbons. The occurrence and the nature of the bedforms is controlled by their hydrodynamic setting, by grain composition that reflects the geological history of the area and by the carbonate-producing benthic marine communities that inhabit the trough areas.

An active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 105 m3 of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s−1. At least 300–400 m3 of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean-orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform-orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood-tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide. During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb-lee side and the trough is covered with fan systems, sinuous ripples and dunes. The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.

Cross-bedded, cool-water, bioclastic limestones of the Te Kuiti Group on the North Island of New Zealand are composed primarily of bryozoans, echinoderms, and benthic foraminifers. Their prominent, large-scale, unidirectional cross-stratification is interpreted as produced by migrating subaqueous dunes on the floor of a 50–100 km wide, north-east-trending seaway in water depths of 40–60 m. These dunes are thought to have developed in response to strong, seaway-parallel, tidal currents combined with a north-east-directed, set-up or oceanic current. Cross-stratification is organized into four hierarchical levels: (1) cross-lamination; (2) first-order sets; (3) second-order sets; and (4) cross-stratified successions. The levels are based on increasing degrees of internal complexity. Distinct attributes such as internal organization, cross-set thickness, foreset shape, and lower bounding-surface shape are used to describe and interpret the cross-stratification. All these attributes are here integrated in a new and expanded classification of unidirectional cross-stratification that emphasizes flow and bedform dynamics rather than overall set shape. Individual cross-stratified successions are interpreted to have formed by dunes with varying sinuosity, superposition, and flow history, under conditions of different current strength but constant sediment production. Horizontally bedded successions are the result of robust, active dune fields that grew during times of vigorous sediment transport. Formset successions were produced from large compound dunes and are the expression of languid and decaying dune fields that developed during times of decreasing sediment transport. These decaying dunes were gradually smothered by continuously and locally produced bioclastic sediment. Formset cross-stratified successions are most likely to develop in carbonates, where the sediment is produced in place, than in terrigenous clastics where the sediment is imported.

In this study the modelling of aeolian dunes is treated from the background of the physical processes that form them. The paper concentrates on two-dimensional modelling, which can be applied to transverse and barchan dunes. The basic assumptions and equations are discussed and three models are presented: (a) a kinematic model which is based on a linear variation of the sediment transport with the topographic height; (b) an analytical model based on a boundary-layer model of the wind velocity and Bagnold's linearized sediment transport formula; (c) a generalization of this analytical model in a computer program which includes a simple routine for simulating the redistribution of sediment through avalanching. The relative importance of the simplifications is considered and indications are given for the practical applications of these three models in field studies.