Ripple marks in intertidal Lower Bhander Sandstone (late Proterozoic), Central India: A morphological analysis

The late Proterozoic, intertidal Lower Bhander Sandstone (Bhander Group, Vindhyan Supergroup) developed around Maihar, central India, is characterized by alternations of sandstone and shale in different scales and shows profuse ripple marks of widely varying morphology. Visual examination of their external morphology led to the identification of wave ripple, current ripple and others of intermediate character.Standard deviation and average of ripple spacing and height of symmetrical and assymmetrical ripples show genetically significant differences analogous to those obtained by Harms (1969) for wave- and current-generated ripples. Different dimensionless parameters, e.g., R.I., R.S.I., S.I., etc., processed separately for the two types of ripples, show a wide variation in their range which encompasses the total spectrum of values stipulated for wave and current ripples. However, the frequency of any particular genetic type of ripple differs widely when analysed in terms of different dimensionless parameters. Several scatter plots, prepared after Tanner (1967) also indicate the presence of various genetic types of ripples, but there are ripples for which results remain inconclusive. Furthermore, scatter plots involving the vertical form index (ripple length/ripple height) and median grain size of a few asymmetrical ripples, following Reineck and Wunderlich (1968a), led to the discrimination between current ripple and wave ripple and the distinction is grossly consistent with the results obtained by other means.Ripple spacing, ripple index and grain-size data of a few representative samples of ripples of possible wave origin, analysed after Tanner (1971) and Allen (1979) indicate that they were generated in a shallow basin with restricted fetch.Internally, the ripples, irrespective of their symmetry, are often characterized by unidirectional bundles of foresets consisting of rhythmically alternating sand and mud laminae. The sets of cross-laminae may be complexly organized with planar or curved erosional boundaries separating them. In many instances internal structures typical of wave ripples are also noted.Inconsistencies, however, exist between the results obtained by application of different criteria in interpretation of these ripple marks. The limitations in applicability of     

Wave-generated structures in the Devonian lacustrine sediments of south-east Shetland and ancient wave conditions

The most common wave-generated structures in the nearshore lacustrine sediments of the south-east Shetland basin are cosets of undulatory and unidirectional ripple cross-lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post-vortex) ripples, and the unidirectional cross-sets were formed by the migration of vortex (orbital) ripples at lower strengths. Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand-starved substrate. Some lenses were reworked during periods of more prolonged wave activity. The Inman-Komar plot of near-bottom orbital diameter versus ripple spacing (λ= 0.80d₀ for small d₀, or λ= 0.65d₀ as modified by Miller & Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimum d₀ conditions) is utilized in this study since wave periods in lakes are small. The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the order of 20 km wide.     

Effect of velocity hiatuses in oscillatory flow on migration and geometry of ripples: wave‐flume experiments

A series of wave‐flume experiments was conducted to closely look at characteristics of geometry and migration of wave‐generated ripples, with particular reference to the effect of velocity ‘hiatuses’ during which the near‐bed flow velocity becomes much smaller than the threshold of sediment movement. Three types of wave patterns were generated: two types for simulating waves with intervening velocity hiatuses; and regular waves for comparison purposes. In the former two types, two different wavelengths of water waves were generated alternately in the course of a wave test: the wave with a longer wavelength was set large enough to mobilize the bottom sediment, whereas the wave with a shorter wavelength was set too small to mobilize the sediment. The former two types were designed to be different in sequence of convexity and concavity of wave patterns. The sequence with the convex–concave longer wave and successive convex–concave shorter wave was described as a ‘zero‐up‐crossing’ wave pattern, and the inverse sequence was described as a ‘zero‐down‐crossing’ wave pattern. The ripples developed under oscillatory flow with intervening hiatuses manifested the following characteristics in geometry and migration. (i) The morphological characteristics of ripples, namely wavelength, height and the ripple steepness, are unaffected by the intervening hiatuses of velocity. (ii) The directions of ripple migration under the zero‐up‐crossing and zero‐down‐crossing wave patterns corresponded well with the directions of the flow immediately before onset of the hiatuses. (iii) The observation of sand particle movement on the ripple surface indicated that, under the zero‐up‐crossing waves, the velocity hiatus prevents the entrained sediment cloud from being thrown onshore, and thus the sediment grains thrown onshore are fewer than those thrown offshore. As a result of the sediment movement over one wave‐cycle, the net sediment transport is directed offshore under the zero‐up‐crossing wave pattern. (iv) The velocity of ripple migration was highly correlated with acceleration skewness. Under most of the zero‐up‐crossing (zero‐down‐crossing) wave patterns, flow acceleration skewed negative (positive) and ripples migrated offshore (onshore).     

Geometry and grain-size sorting of ripples on low-energy sandy beaches: field observations and model predictions

ABSTRACT There are very few field measurements of nearshore bedforms and grain‐size distribution on low‐energy microtidal beaches that experience low‐amplitude, long‐period waves. Field observations are needed to determine grain‐size distribution over nearshore bedforms, which may be important for understanding the mechanisms responsible for ripple development and migration. Additional nearshore field observations of ripple geometry are needed to test predictive models of ripple geometry. Ripple height, length and sediment composition were measured in the nearshore of several low‐energy beaches with concurrent measurements of incident waves. The distribution of sediment sizes over individual ripples was investigated, and the performance of several models of ripple geometry prediction was tested both spatially and temporally. Sediment samples were collected from the crest and trough of 164 ripples. The sand‐sized sediment was separated from the small amount (generally <3%) of coarser material (>2 mm) that was present. Within the sand‐sized fraction, the ripple crests were found to be significantly coarser, better sorted and more positively skewed than the troughs. Overall, the troughs were finer than the crests but contained a greater proportion of the small fraction of sediment larger than 2 mm. The field model of Nielsen (1981 ) and the model of Wiberg & Harris (1994 ) were found to be the most accurate models for predicting the wavelength of parallel ripples in the nearshore of the low‐energy microtidal environments surveyed. The Wiberg & Harris (1994 ) model was also the most accurate model for predicting ripple height. Temporal changes in ripple wavelength appear to be dependent on the morphological history of the bed.     

Sediment suspension under waves and currents: time scales and vertical structure

Field measurements of the vertical structure of near-bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time-dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind-wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near-bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near-bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.     

Sedimentology,ichnology and hydrodynamics of strait‐margin,sand and gravel beaches and shorefaces: Juan de Fuca Strait,British Columbia,Canada

On the south‐west coast of Vancouver Island, Canada, sedimentological and ichnological analysis of three beach–shoreface complexes developed along a strait margin was undertaken to quantify process–response relations in straits and to develop a model for strait‐margin beaches. For all three beaches, evidence of tidal processes are expressed best in the lower shoreface and offshore and, to a lesser extent, in the middle shoreface. Tidal currents are dominant offshore, below 18 m water depth (relative to the mean spring high tide), whereas wave processes dominate sediment deposition in the nearshore (intertidal zone to 5 m water depth). From 18 to 5 m water depth, tidal processes decrease in importance relative to wave processes. The relatively high tidal energy in the offshore and lower shoreface is manifest sedimentologically by the dominance of sand, of a similar grain size to the upper shoreface/intertidal zone and, by the prevalence of current‐generated structures (current ripples) oriented parallel to the shoreline. In addition, the offshore and lower shoreface of strait‐bound beach–shoreface complexes are recognized ichnologically by traces typical of the Skolithos Ichnofacies. This situation contrasts to the dominantly horizontal feeding traces characteristic of the Cruziana Ichnofacies that are prevalent in the lower shoreface and offshore of open‐coast (wave‐dominated) beach–shorefaces. These sedimentological and ichnological characteristics reflect tidal influence on sediment deposition; consequently, the term ‘tide‐influenced shoreface’ most accurately describes these depositional environments.     

Granule ripples in the Kumtagh Desert,China: Morphology,grain size and influencing factors

Granule ripples are found mainly in four regions of the Kumtagh Desert in China; they are characterized by an asymmetrical shape, with gentle lower slopes on both sides and abrupt crests. The ripples tend to be oriented perpendicular to the prevailing winds, except when they form near obstacles such as yardangs. The wavelengths (λ) range between 0·31 m and 26 m and heights (h) range from 0·015 m to 1 m. The relationship between wavelength and height can be described by a simple linear function, and the mean ripple index (λ/h) is about 20·4 for the study sites. The sediments are poorly sorted, with negative to very negative skewness at lee and stoss slopes and between‐ripple troughs, which confirms the ‘poured in’ and ‘shadow’ appearance described by previous researchers. The bimodal or trimodal distributions of grains (with modes of ?1·16φ, ?0·5φ and 3·16φ) and the enrichment of coarse particles at the ripple surface (with coarse granule contents ranging between 5·2% and 62·1%) indicate that the underlying layer is the original sediment source and that the granule ripples resist erosional processes. Although the impact of saltating particles and, consequently, the creep and reptation of coarse grains are responsible for granule ripple initiation at a micro‐scale, however, the characteristics of local sediments, wind regimes and topographical obstacles, as well as the feedbacks among bedform and airflow, more strongly affect the development and alignment of granule ripples at a macro‐scale.     

Gravelly shoreface deposits: a comparison of modern and ancient facies sequences

The morphology and dynamics of modern gravel shorefaces are poorly documented. This hinders the interpretation of possible ancient counterparts. A comparative study of a modern (Chesil Beach, England) and an ancient (Baytree Member of the Cardium Formation, Alberta) gravel shoreface shows that the two systems are very similar close to and above sea-level, with a high (about 1 m) gravel plunge step lying below plane-bedded sands and gravels of the beachface. The shoreface at Chesil Beach is dominated by asymmetrical gravel wave ripples. These are oriented offshore near the toe of the shoreface, and onshore in shallower depths. This may reflect offshore movement during storms and landward reworking during fair weather. The Baytree Member is over 12 m thick and comprises over 80% conglomerate. Conglomerate is decimetre-bedded, massive or cross-bedded, with sets over 60 cm thick produced by gravel bedforms migrating alongshore. It is interbedded with discontinuous cm- to dm-bedded sandstones which may be cross-bedded. Pebble fabric and cross-bed orientation both indicate strong alongshore sediment transport. Near the base of the section, pebble orientations suggest that gravel wave-ripples developed below the zone of strong longshore flows. Differences between these two examples may be attributed to different directions of wave approach.     

Identification of Permian palaeowind direction from wave-dominated lacustrine sediments (Lodève Basin, France)

Lacustrine environments are an excellent indicator of continental palaeoclimate. In particular, the sedimentary record of waves in lakes may be used to constrain atmospheric palaeocirculation. Wave ripples have been identified in a Permian lacustrine basin (the Salagou Formation, 260–250 Ma, Lodève Basin) located in the southern French Massif Central, part of the western European Hercynian mountain chain. Wave ripple patterns are interpreted with regards to hydrodynamics and water palaeodepth. It is shown that, in the case of the Salagou Formation, wave ripple orientations were controlled by the direction of the prevailing palaeowind. The Late Permian wind blew from between north and 20° east of north, possibly over several millions of years and certainly throughout the period of deposition of about 2000 m of strata in the Lodève Basin. Permian lacustrine sedimentation is widespread and well preserved on the Earth's surface and so wave ripple data may help constrain numerical modelling of the Earth's past climates, especially with regards to Permian times outside of desert regions.     

Mud dispersal across a Cretaceous prodelta: Storm‐generated,wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

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.     

Sediment transport and bedforms in a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas

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.     

An empirical model for the development and equilibrium morphology of current ripples in fine sand

A flume study on the development and equilibrium morphology of current ripples in fine sand (D₅₀ = 0·238 mm) was performed to extend an empirical model for current ripple stability in 0·095 mm sand to larger grain sizes. The results of the flume experiments agree with the very fine sand model that current ripple development from a flat bed is largely independent of flow velocity. At all flow velocities, ripples evolve from incipient, through straight, sinuous and non-equilibrium linguoid, to equilibrium linguoid plan morphology. The time needed to achieve an equilibrium linguoid plan form is related to an inverse power of flow velocity and ranges from several minutes to more than hundreds of hours. Average equilibrium height and length are 17·0 mm and 141·1 mm respectively. These values are about 20% larger than in very fine sand. Equilibrium ripple height and length are proportional to flow velocity near the stability field of dunes. In the same velocity range, a characteristic grouping of ripples with smaller ripples migrating on the upstream face of larger ripples was observed. Bed-form development shows a conspicuous two-phase behaviour at flow velocities < 0·49 m s^?1. In the first phase of development, ripple height and length increase along an exponential path, similar to that at higher flow velocities, thus reaching intermediate equilibrium values of 14·8 mm and 124·5 mm respectively. After some time, however, a second phase commences, that involves a rapid increase in bed-form size to the typical equilibrium values for 0·238 mm sand. A comparison with literature data shows that the results obtained for 0·238 mm sand agree reasonably well with other flume studies at similar grain size. Yet considerable variability in the relationships between ripple dimensions and flow strength ensues from, among others, underestimation of equilibrium time, shallow flow depths and differences in sediment texture.     

Deposition of climbing-ripple beds: a flume simulation

Thirteen runs were made in a small recirculating flume to simulate the deposition of the climbing-ripple sequences commonly present in fine-grained facies of fluvial and deltaic deposits. These sequences consist of intergradational climbing-ripple cross laminae and draped laminae. The experiments were based on the assumption that stratification type depends mainly on near-bottom flow structure and uniform sediment fallout from an overloaded flow. Various combinations of curves of velocity versus time and of sediment feed versus time in runs lasting from 45 to 840 min were used in an exploratory program; conditions for each run were selected on the basis of experience in previous runs. The runs verified that Type A (erosional-stoss) climbing ripples are produced by aggradation rates that are small relative to ripple migration rate, and Type B (depositional-stoss) climbing ripples are produced by aggradation rates that are large relative to ripple migration rate. Draped lamination results from continued fallout of sediment from suspension after ripple migration ceases or almost ceases. Comparison of geometric details of the ripple stratification produced in the flume runs with that in natural sequences, supplemented by considerations on maximum and minimum migration rates of ripples, suggests times of no more than a few tens of hours for the deposition of the climbing-ripple portions of sequences 10-20 cm thick. Runs in which deposition of a 20 cm sequence took more than 10 h produced such atypical features of ripple geometry as sharp crests, planar lee-side laminae, and angular toeset-foreset contacts.     

Undular hydraulic jumps and the formation of backlash ripples on beaches

Field observations are made of the formation of backwash ripples on the beach face, formed by undular hydraulic jumps generated by backwash down the beach face colliding with wave bores. Measured ripple wavelengths range from set averages of 48 to 70 cm. Within a particular set of ripples the spacing tends to decrease in the offshore direction. These observations are compared with laboratory experiments where undular jumps are generated in a flume, and with a computer simulation model which calculates both the flow within an undular hydraulic jump and the resulting sediment transport which gives rise to the backwash ripples. The computer model involves a numerical solution of the Bousssinesq equations which govern the fluid flow, and sediment transport equations which relate the sand transport rate to the local mean flow velocity. The model permits a study of the detailed time-history of the undular jump development and the formation of the backwash ripples and shows good agreement with the field observations of backwash ripples, predicting an offshore decrease in their spacings. The laboratory experiments showed a similar result so long as the Froude number of the supercritical flow before the jump occurs is small (c. 1–4). Small differences between the computer model and experiments arose principally from the neglect of internal friction and surface tension in the model.     

Ripple formation in combined transdirectional steady and oscillatory flow

Measurements are described of the geometry of ripples formed on beds of sand exposed to a steady current at right angles to an oscillatory flow. Four different sands were studied. The oscillation was produced by an oscillating tray set into the bed of a steady-flow flume. It was observed that straight-crested ripples formed by oscillatory flow would usually develop a ‘serpentine’ form when the superimposed steady current exceeded a certain limit. For amplitudes of the tray velocity U_∞ less than about 0.38 m s^-1 this limit corresponded to U_∞/ū_*c>31, where ū_*c is the shear velocity measured just upstream of the oscillating tray. It is suggested that the serpentine form is caused by the interaction of vortices carried back and forth between adjacent ripples. On this assumption, the wavelength of the serpentine form would be proportional to the product of period of oscillation and near-bed steady current velocity. The present measurements appear to support this hypothesis although there is also evidence that the wavelength is influenced by preferred spacing patterns between vortices. The measurements also show the ratio of the amplitude of the serpentine form to its wavelength to be approximately constant. Empirical relationships are derived relating ripple geometry to flow and sediment properties. It is observed that the influence of Reynolds number and sediment properties on the geometry is very weak. It is suggested that this is typical of ripples formed with relatively low sediment transport rates. It is also found that, under the present experimental conditions, the ripple spacing in the direction of oscillation is almost independent of the magnitude of the steady current and in close agreement with the wavelengths previously measured in an oscillating water tunnel. This suggests that the additional inertia effects associated with oscillating tray rigs were not sufficient to affect bed geometry under the present test conditions.     

Time as an independent variable for current ripples developing towards linguoid equilibrium morphology

Flume experiments show that current ripples on very fine sand surfaces always develop towards a linguoid shape with constant height and wavelength provided that sufficient time is allowed for their formation. Straight and sinuous current ripples only reflect intermediate stages in ripple development and may be regarded as non-equilibrium bedforms. The time period which current ripples require to reach linguoid equilibrium morphology is related to an inverse power of flow velocity. In the transitional stage from current ripples to upper stage plane bed (i.e. washed-out ripple stage) only the equilibrium wavelength remains constant, whereas equilibrium height rapidly decreases to zero. Our observations imply that bed-roughness parameters in sediment transport calculations can be simplified when equilibrium conditions are attained, and that inferences about flow energy from the dimensions of current ripples in very fine sand need to be regarded with caution.     

Visualization of wave-induced suspension patterns over two-dimensional bedforms

High spatial and temporal resolution measurements of suspended sand concentration ( c ) over vortex ripples were collected with a three-transducer acoustic backscatter sensor (ABS) array, under irregular `natural' waves in a multidirectional wave basin. These measurements permit two-dimensional visualization of the movement of sediment-laden vortices over an individual vortex ripple under a series of waves. Patterns of sediment motion were tracked through consecutive zero-crossings in the horizontal velocity ( U ) record measured at 0·05 m above the ripple crest elevation. It was possible to trace the advection of individual sediment-laden vortices at the zero-crossings. During 73% of these events, shedding and advection of coherent suspension events occurred before the flow reversal associated with the zero-crossing. This may be caused by the bedforms retarding the near-bed flow inducing the eddy shedding before the zero-crossing. While at maxima in U , secondary suspension events with low c were observed to pass over the ripple crest moving with U measured at 0·05 m. This pattern is attributed to vortex shedding from adjacent bedforms and/or antecedent suspension events. The most energetic events appeared to persist for several wave cycles and reached heights of ≈0·20 m. These suspension events appeared to be more persistent when smaller waves follow larger waves, possibly as a result of weaker reversals in vorticity. Although the events appeared to be vertically coherent in the time series from the individual transducers, it is apparent through visualization that these events are associated with the pairing of antecedent and developing vortices.     

Lagoon–tidal flat sedimentation in an epeiric sea: Proterozoic Bhander Group,Son Valley,India

The Bhander Group, the uppermost stratigraphic unit of the Proterozoic Vindhyan Supergroup in Son Valley, exhibits in its upper part a 550 m thick, muddy siliciclastic succession characterized by features indicative of deposition in a wave‐affected coastal, lagoon–tidal flat environment suffering repeated submergence and emergence. The basic architecture of the deposit is alternation of centimetre‐ to decimetre‐thick sheet‐like interbeds of coarser clastics (mainly sandstone) and decimetre‐thick mudstones. The coarser interlayers are dominated by a variety of ripple‐formed laminations. The preserved ripple forms on bed‐top surfaces and their internal lamination style suggest both oscillatory and combined flows for their formation. Interference, superimposed, ladder‐back and flat‐topped ripples are also common. Synsedimentary cracks, wrinkle marks, features resembling rain prints and adhesion structures occur in profusion on bed‐top surfaces. Salt pseudomorphs are also present at the bases of beds. The mudstone intervals represent suspension settlement and show partings with interfaces characterized by synsedimentary cracks. It is inferred that the sediments were deposited on a coastal plain characterized by a peritidal (supratidal–intertidal) flat and evaporative lagoon suffering repeated submergence and emergence due to storm‐induced coastal setup and setdown in addition to tidal fluctuations. The 550 m thick coastal flat succession is surprisingly devoid of any barrier bar deposits and also lacks shoreface and shelfal strata. The large areal extent of the coastal flat succession (c. 100,000 km²) and its great thickness indicate an extremely low‐gradient epeiric basin characterized by an extensive coastal flat sheltered from the deeper marine domain. It is inferred that the Bhander coastal flat was protected from the open sea by the Bundelkhand basement arch to the north of the Vindhyan basin, instead of barrier bars. Such a setting favoured accumulation of a high proportion of terrigenous mud in the coastal plain, in contrast to many described examples from the Proterozoic. Copyright © 2001 John Wiley & Sons, Ltd.     

A wave‐dominated,tide‐modulated model for the Lower Ordovician of the Anti‐Atlas,Morocco

Hybrid depositional systems are created by the interaction of two or more hydrodynamic processes that control facies distribution and their characteristics in terms of sedimentary structures and depositional geometry. The interaction of wave and tide both in the geological sedimentary record and modern environments has been rarely described in the literature. Mixed coastal environments are identified by the evidence of wave and tidal structures and are well identified in nearshore environments, while their recognition in lower shoreface–offshore environments lacks direct information from modern settings. Detailed field analyses of 10 stratigraphic sections of the Lower Ordovician succession (Fezouata and Zini formations; Anti‐Atlas, Morocco) have allowed the definition of 14 facies, all grouped in four facies zones belonging to a storm‐dominated, wave‐dominated sedimentary siliciclastic system characterized by symmetrical ripples of various scales. Peculiar sedimentary organization and sedimentary structures are observed: (i) cyclical changes in size of sedimentary structures under fair‐weather or storm‐weather conditions; (ii) decimetre‐deep erosional surfaces in swaley cross‐stratifications; (iii) deep internal erosion within storm deposits; (iv) discontinuous sandstone layers in most depositional environments, and common deposition of sandstones with a limited lateral extension, interpreted to indicate that deposition at all scales (metric to kilometric) is discontinuous; (v) combined flow–oscillation ripples showing aggrading–prograding internal structures alternating with purely aggrading wave ripples; and (vi) foreshore environments characterized by alternating phases of deposition of parallel stratifications, small‐scale and large‐scale ripples and tens of metres‐wide reactivation surfaces. These characteristics of deposition suggest that wave intensity during storm‐weather or fair‐weather conditions was continuously modulated by another controlling factor of the sedimentation: the tide. However, tidal structures are not recognized, because they were probably not preserved due to dominant action of storms and waves. A model of deposition is provided for this wave‐dominated, tide‐modulated sedimentary system recording proximal offshore to intertidal–foreshore environments, but lacking diagnostic tidal structures.