sediment dynamics over sand ridges on a tideless mid outer continental shelf abstract a sand ridge field with crests oriented ne sw 52 located between 55 and 85 m water depth on the valencia continental shelf spain was mapped with multibeam swath bathymetry and characterized with seismic profiling and sediment sampling boundary layer hydrodynamic measurements conducted over a sand ridge at 66 m depth show evidence of wave and current sediment resuspension during a major storm event with a three year recurrence period from the measured intrawave sediment concentration profiles it is shown that the suspended sediment time evolution is related not only to mean currents but also to the wave field the near bottom shear stress generated during storm conditions reached around 1 n m2 and was able to resuspend and transport sandy and muddy sediments for more than 32 h the total suspended sediment transport in a layer of 1 m above the bottom was around 4000 kg per meter width and directed 160 it is argued that these sand ridges were developed in a shallow environment and there is no evidence of present day ridge migration or morphological degradation it is hypothesized that sediment dynamics affecting sand ridges during storms favors the maintenance of their morphology

radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions 1 the investigations carried out between 2002 and 2004 during six field experiments within the operational radar and optical mapping in monitoring hydrodynamic morphodynamic and environmental parameters for coastal management oroma project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters the coastal monitoring radar of the gkss research center geesthacht germany is based on a kelvin hughes rsr 1000 x band 9 42 ghz vertical vv polarized river radar and was mounted on board the research vessel ludwig prandtl during the experiments in the lister tief a tidal inlet of the german bight in the north sea the important progress realized in this investigation is the availability of calibrated x band radar data another central point of the study is to demonstrate the applicability of the quasi specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed radar data have been taken at very low grazing angles 2 6 of flood and ebb tide oriented sand wave signatures at the sea surface during ebb tidal current phases current speeds perpendicular to the sand wave crest 0 6 m s 1 have been measured at wind speeds 4 5 m s 1 and water depths 25 m the difference between the maximum measured and simulated normalized radar cross section nrcs modulation of the ebb tide oriented sand wave is 27 for the flood tide oriented sand wave a difference of 21 has been calculated the difference between the minimum measured and simulated nrcs modulation of the ebb tide oriented sand wave is 10 and for the flood tide oriented sand wave a value of 43 has been derived phases of measured and simulated nrcs modulations correspond to asymmetric sand wave slopes the results of the simulated nrcs modulation show the qualitative trend but do not always quantitatively match the measured nrcs modulation profiles because the quasi specular scattering theory at very low grazing angle is a first order theory

pattern formation in a thin layer of sediment abstract a stability analysis is performed to investigate the process which leads to the formation of bottom forms when a thin layer of sediment covers a rigid substratum subject to the oscillatory flow induced by a surface gravity wave the amplitude of the bottom perturbation is assumed to be so small to linearize the hydrodynamic problem and the flow field is determined by analytical means however nonlinear effects are significant and play a fundamental role into the morphodynamic problem when the rigid substratum is bared by the growth of the most unstable mode hence the development of the bottom profile is evaluated by a numerical approach the model results show that the wavelength of the morphological patterns which are generated when the rigid substratum is bared is longer than the wavelength of the bottom forms which appear when the rigid substratum is always covered by the sediment the results of the theoretical model are supported by some experimental observations made in our laboratory

morphodynamic response of sand waves in the taiwan shoal to a passing tropical storm abstract data from three repeated multi beam echo sounder surveys of pre storm post storm and one year after the storm were used to study the response of sand waves in the taiwan shoal to the tropical storm talim the measurements showed significant decreases in wave height of giant sand waves between post storm and pre storm and moderate increases in one year period after the storm the maximum decrease in wave height is more than 2 m and the averaged decrease is 1 2 m along the measured profile the linear regression analysis shows an increasing trend of wave height changes from northwest to southeast along the profile the uneven evolution pattern was possibly caused by the asymmetry of the currents induced by the cyclone winds the analysis based on the morlet wavelet function showed that the storm smeared out small scale sand waves and eroded the giant sand wave crests and recovery of bedforms after the storm two dimensional features of storm effects were analyzed using the imagery of a digital terrain model processed in a 2d survey area for pre storm post storm and one year after the storm the analysis revealed sand wave migration processes during the storm and the growing back feature of the eroded wave crests after the storm

the morphodynamics of tidal sand waves a model overview this paper reviews recent theoretical studies of sand waves which are rhythmic large scale bedforms observed in the continental shelf far from the near shore region emphasis is given to the investigations carried out in the framework of the eu research project humor first the results of linear morphodynamic stability analyses are described which allow to understand the initial behavior of the sand waves hence indications on the physical processes controlling the appearance and development of sand waves are obtained along with quantitative predictions of the wavelength of sand waves and of their migration speed then nonlinear models are described which are used to predict the equilibrium profile of sand waves and their interaction with human interventions like sand extraction or the construction of pipelines finally we discuss an analytical model which describes how the sand wave instability behaves when it is triggered locally this leads to the generation growth and expansion of a so called sand wave packet

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

applications of sediment pick up function two applications are presented of a recently proposed empirical sediment pick up function the first application deals with bed load transport which is computed as the product of the pick up rate and the saltation length comparison with measured bed load transport rates shows encouraging results the second application deals with suspended load transport the classical problem of sediment concentration profiles developing in a steady uniform flow which is initially free of sediment is studied

measurements and analysis of setting velocities of natural quartz sand grains abstract settling velocities in water were measured for natural quartz sand grains in the sieve size range 0 75 to 1 50 0 35 1 68 mm obtained from beach sand samples the grains were selected so as to provide a wide range of roundness from very angular to very well rounded r 0 6 5 1 in a first set of experiments only two axial diameters were measured under a microscope the longest axial diameter d1 and the intermediate diameter di in a second data set the smallest axial diameter ds was measured as well settling velocities were measured in a 6 meter long settling tube it is found that the intermediate grain diameter is on average equal to the nominal diameter computed as dn dsdid1 1 3 this permits a simpler analysis of grain settling through microscopic measurements of di alone but does not allow for corrections of grain shape effects the measured settling velocity wm is then compared with the settling velocity of a sphere ws calculated using di as the sphere diameter and the relationship wm 0 977w0 913s is found to be consistent with the data an alternative approach is provided where ws is calculated directly from the sieve diameter and then corrected to the actual settling velocity wm for that mean grain size the results thus provide graphs and empirical relationships which permit the evaluation of actual grain settling rates rather than settling velocities of spheres from the sieve diameter or from microscopic measurements of di for individual grains these procedures provide a mean settling velocity that takes into consideration the average degree of grain irregularities found in natural sands a more detailed analysis is then conducted to examine the effects of roundness and sphericity on the settling of individual grains it is found that grain roundness has no measurable effect on the settling rate the settling velocity is shown to be a function of grain sphericity either expressed as the corey shape factor or e shape factor grain asymmetries ust also be important accounting for much of the scatter in the results

morphology of submarine barchans and sediment transport in barchans fields off the dongfang coast in beibu gulf submarine barchans in three new regions were described using new high resolution multibeam bathymetric data off the dongfang coast in beibu gulf currents seismic profiles of horns surface sediments and comparative surveys in 2007 and 2009 were obtained to study barchan mobility and sand transport in the barchan fields the submarine barchans occur at depths of 15 25 m reaching up to 551 9 m in horn length and 8 6 m in height most barchans have asymmetrical horns and display wider plane shapes than subaerial barchans for the same length and height the morphology of barchans exhibits regional variations that are mainly controlled by finite water depth daily reverse currents sand supply changes and interaction of dunes barchan asymmetries internal bedding inclinations and grain size changes are in agreement with the dominant current directions results show that submarine barchans migrated 5 43 m in two years 2007 2009 through comparison of bathymetric data because of the marginally cohesive fine sediment composition and long distance from contemporary sediment sources it is believed that barchans in region c are not of modern origin however these bedforms can be also affected by high energy flow regimes collisions limit barchan widths and provide efficient transport paths between barchans on their horns large sand supply through the horns increases the barchan heights and also changes their shapes some small dunes are identified that are moving away from the barchans field in 2007 and have departed from the host barchans in 2009 these small barchans were probably caused by the separation of the faster moving horn tails from the host barchans when extreme situations happen the mechanism gives a new explanation for sand loss and size determination of submarine barchans c 2014 elsevier b v all rights reserved

spatially varying environmental properties controlling observed sand wave morphology sand wave morphology and dynamics on continental shelves vary substantially and we hypothesize that these spatial variations depend on local bed properties and hydrodynamic characteristics to date process based modeling studies have not been able to simulate realistic equilibrium sand wave heights and empirical studies are mostly limited to case studies in order to explain the spatial variation in the morphology of equilibrium sand waves on continental shelves with processes and local bed conditions a large scale investigation is required in this paper we use high resolution multibeam echo soundings hydrodynamic models and databases and sedimentary data for the analysis of respectively sand wave shape characteristics and the comparison to hydrodynamic and sedimentary characteristics for the netherlands continental shelf the results are quantified lengths heights and asymmetry of all sand waves in the dutch part of the north sea furthermore we show that the mode of sediment transport bed load or suspended is a dominant factor in explaining sand wavelength height and asymmetry full results of shape characteristics of all sand waves on the netherlands continental shelf together with the tidal velocity water depth surface wave height and median grain size are provided in a repository with this paper http doi org 10 4121 uuid 0d7e016d 2182 46ea bc19 cdfda5c20308 these results are highly valuable for applied offshore engineering projects and to modelers for validating their morphodynamic model results

infilling stratigraphy of macrotidal tide dominated estuaries controlling mechanisms sea level fluctuations bedrock morphology sediment supply and climate changes the examples of the seine estuary and the mont saint michel bay english channel nw france this article presents a synthesis of recent works performed on two macrotidal tide dominated estuaries located along the southern coast of the english channel the seine estuary and the mont saint michel bay nw france on the basis of very high resolution seismic data combined with sediment cores and ams dating these works allow reconstructing the holocene in ll of such singular estuaries yet rarely documented in the literature rate in sea level rise bedrock morphology sediment supply are the main controlling factors of the general in ll pattern both in lls are composed of two units the transgressive systems tract tst and highstand systems tract hst the maximum ooding surface mfs is dated around 6500 cal bp when the holocene sea level rise slowed down and corresponds to the main architectural change the tst is poorly preserved compared with the hst that constitutes the main depositional unit making this incised valley ll deviating from the classical models the hst is composed above the axis of the incised valley of a highly wandering estuarine channel belt characterized by deep tidal reworking throughout its building on the margin of this high energy estuarine system the hst is made of wave dominated barriers tidal ats and offshore tidal banks they constitute dominantly aggradational systems the aggradation rate of which is in accordance with the rate of sea level rise from 6500 cal bp onwards consequently the impacts of the rapid climate changesthat characterized mid to late holocene times can be preserved in these marginal successions various sedimentary and geometrical signatures indicate that periodic enhanced storminess episodes probably related to the bond cold events 1500 years periodicity are responsible for the most drastic environmental changes in these macrotidal tide dominated settings the role of long term tidal cycles as well as the impact of human activities are also considered

giant dune morphologies and dynamics in a deep continental shelf environment example of the banc du four western brittany france the aim of this paper is to improve the knowledge of spatio temporal variability of very large to giant dunes in deep tide dominated environment their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations this motivated the realization of three recent swath bathymetry surveys across the wide dune field of the banc du four located offshore the western brittany the banc du four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2d to 3d shapes and reaching the largest sizes of such sedimentary structure ever described wavelength over 1000 m and height over 30 m the analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters our results show a good height spacing power law correlation but do not strike the usual statistical regression observed in previous studies the steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime interacting with bed load transport and sediment supply no clear relationship has been observed between asymmetry and size parameters water depth can not be considered as a major factor controlling the size of dunes otherwise the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the banc du four were found to range from 3 to 20 m yr 1 such velocities were only mentioned before by van landeghem et al 2012 on deep continental shelves 70 m deep and attest of the still present morphodynamical equilibrium of the large dunes furthermore as was previously found for similar dunes in the irish sea no power law relationship between dune migration rates and dune size can be deduced similarly no strong correlation is established between migration rates and the degree of asymmetry nevertheless the dune asymmetry can be used to predict the migration direction as for dimension parameters the evolution asymmetry depends on the study area and migration rates more precisely when the crest lines of dunes form straight lines and their migration rates remain below 8 10 m yr 1 the degree of dune asymmetry increases with the migration rate this trend is reversed for the dunes with higher migration velocities and crests which are sinuous these results relativize the migratory predictions based on punctual geometrical observations

a facies based depositional model for ancient and modern tectonically confined tidal straits modern and ancient tidal straits are the least well understood of all tide dominated depositional systems to provide an increased understanding of these systems a facies based depositional model is assessed by comparing multibeam surveys of three present day tidally dominated seaways with a number of superbly exposed neogene to quaternary strait fill successions of calabria south italy the model points out the existence of four depositional zones laterally adjacent from the narrowest strait centre to its terminations distributed along symmetrical or asymmetrical seaways these zones whose signature is recorded in four facies associations in the calabrian tidal straits are as follows i the strait centre zone associated with the tidal current maxima and where sediments are scarce or absent ii the dune bedded zone where sediments form dune complexes due to tidal flow expansion iii the strait end zone where currents decelerate accumulating thinly bedded fine grained deposits and iv the strait margin zone where sediment massflows descend tectonically active steep margins towards the strait axis in ancient tectonically confined narrow seaways these facies generate a distinctive deepening upward vertical succession where tidal currents are the dominant process in the sediment distribution

history and evolution of the north betic strait prebetic zone betic cordillera a narrow early tortonian tidal dominated atlantic mediterranean marine passage abstract the north betic strait located in the prebetic zone outermost part of the betic cordillera was a narrow seaway connecting the atlantic ocean and the mediterranean sea through the betic cordillera during the early late miocene strait facies consist of heterozoan bioclastic carbonates and mixed siliciclastics carbonates local fan delta deposits conglomerates also occur the north betic strait deposits exhibit huge trough cross bedded structures single troughs are up to 5 m high and point to the east in easternmost outcrops in central outcrops they reach up to 15 m in height and some tens of metres in length and point both to the east and to the west the westernmost outcrops show the largest cross bedded structures up to 20 m in height and some hundred of metres in length all pointing to the west the large scale cross bedding was generated by the migration of very large dunes moved by tides the largest structures are inferred to have formed at depths of 90 m depth estimations indicate that the north betic strait shallowed to the east and deepened to the west the miocene evolution towards the formation of the north betic strait can be divided into a series of steps the palaeogeography evolved from a south facing platform marginal to a northeastern relief to a wide open marine passage limited by a southern platform and finally to a tidal dominated strait the closing of the north betic strait is recorded by the presence of lagoonal silty sediments covered by a stromatolite layer and crowned by a red soil in the westernmost outcrops

use of swath bathymetry in the investigation of sand dune geometry and migration around a near shore banner tidal sandbank banner tidal sandbanks in the bristol channel have been repeatedly surveyed with a multibeam sonar to study the geometry and migration patterns of superimposed dunes the data presented in this paper constitute one of the first studies concerned with sediment transport around a banner sandbank helwick sands in the bristol channel using repeated swathbathymetry the data reveal that the dunes maintain their shapes over a period of 11 months and that they migrate in opposite directions on the alternate sides of the bank curiously dunes connect over the crest of the bank despite opposing sediment transport directions on the flanks dune height increases with water depth as found in similar environments we suggest how the morphology of the dunes results from the complex interaction between surface waves and tidal currents that occurs within the proximity of the headland 1995 furthermore repeated full bathymetric coverage of sub tidal sedimentary structures using swath sonar technology will provide information of their dynamics and threedimensional characteristics which are of interest for the understanding of coastal and shelf sedimentary transport this paper describes the morphology and dynamical characteristics of sand dunes at the eastern end of the helwick sandbank lying near a headland in the bristol channel fig 1 the analysis is based on two sets of swathbathymetry data collected on 26 september 2001 and 20 august 2002 the main objective of this paper is to demonstrate the potential for tracking dune migration and dune geometry changes from repeated bathymetric multibeam surveys this paper is therefore intended to a describe the morphological characteristics of the dunes and their spatiotemporal evolution b estimate dune migration rate and from that infer sediment transport flux and c relate these data to the work elsewhere of sediment transport around sandbanks and their maintenance in describing the sand dunes in this paper we follow the terminology developed by ashley 1990

internal architecture of mixed tide and storm influenced deposits an example from the alcântara formation northern brazil abstract the uppermost portion of the itapecuru group is exposed in the eastern margin of the sao luis basin northern brazil where it consists of two units the alcantara formation cenomanian and the cujupe formation late cretaceous early tertiary rossetti d f truckenbrodt w 1997 revisao estratigrdfica para os depositos do albiano terciario inferior na bacia de sao luis ma norte do brasil bol mus paraense emilio goeldi ser cienc terra in press the alc ntara formation which contains the large scale structures discussed in this paper consists of deposits attributed to mid to upper shoreface foreshore tidal channel and lagoon washover environments attributed to a regressive barred shoreline several types of large scale cross bedding i e simple foreset compound mixed undulatory and intricately bounded were recognized in the shoreface facies association these structures are interpreted to record the interaction of storm and tidal processes the storm influence is suggested by a combination of factors mostly including a the genetic association with other storm generated sedimentary structures i e swaley cross stratification and undulating parallel lamination with internal truncations b the deposition on prominent surfaces formed by storm erosion which are defined by large scale either symmetrical or asymmetrical scours arranged in a regular repeating pattern c the sedimentary features formed under combined unidirectional and oscillatory flow processes e g compound mixed bedding with superimposed either swaley cross sets or complexly truncating cross sets with highly undulating boundaries large scale undulatory and intricately bounded cross beddings and d the lateral change in structural styles within short distances which records frequent modification from asymmetrical to symmetrical nearly symmetrical bedform profiles more likely to occur under storm generated combined flows the tidal signature is locally recognized by regularly spaced thick thin sandstone bundles defined by reactivation surfaces and or mud drapes which are attributed to tidal ebb flood cycles the analysis of paleocurrent distribution suggests that vigorous southwestward oriented storm flows interacted with local tidal currents on the shoreface to promote the landward transport of significant volumes of sand which resulted in the large scale cross stratification described in this paper in addition a secondary southeastward directed oblique to shore parallel combined flow would have periodically interacted with the main flow the origin of this oblique to shore parallel flow is attributed to either the refraction of the main storm waves as they approached the paleocoastline or the interference of a separate storm episode which competed with the major landward moving one this complex storm flow pattern can be related to past penecontemporaneous seismic activity i e tsunamis as suggested by the paleogeography of the study area during the cenomanian combined with the structural history of the sao luis basin and the sedimentary features recognized in the alcantara formation

giant sandwaves in the hola glacial trough off vesteralen north norway abstract seabed data acquired from the hola glacial trough off vesteralen north norway reveal the morphology of spectacular sandwave fields and large sandwaves in great detail the sandwave fields are among the deepest known occurring in water depths of 200 260 m multibeam bathymetry and backscatter shallow seismic sediment samples and video data collected by the mareano programme and bottom current modelling results have been used to interpret morphology transport and distribution of the seabed sediments sand supply to the hola trough was initially from wave and current reworking of glacial and glaciomarine deposits during the eustatic sea level lowstand and sea level rise after the last glaciation more recently deposited bioclastic sand dominates the upper part of the sand unit which locally reaches 10 m thickness sand predominantly accumulates behind ridges where current strength decreases and in topographic lows straight crested and barchan type sandwaves up to 3 km long and 7 m high are interpreted to form by tidal and geostrophic currents with bottom current speeds reaching 0 7 0 8 m s 1 a large cluster of coral reefs occur immediately east of the barchan type sandwaves where modelling shows bottom current velocities up to c 1 m s 1 sandwaves occur in four major sandwave fields strong variation in the nature of sandwaves over short distances e g migration direction between neighbouring sandwave fields point to important local variations in current regime and sedimentary processes the present work demonstrates how local topography and interaction of several current systems may produce complex depositional patterns on a local scale detailed multibeam mapping and video inspection of open shelf troughs in other narrow shelf settings worldwide may reveal similar sandwave fields

northern new guinea wrench fault system a manifestation of late cenozoic interactions between australian and pacific plates abstract cenozoic plate interactions between the australian and pacific plates have yielded a variety of convergent tectonic styles two major products of this convergence are the sorong fault zone of irian jaya and the ramu markham fault zone of papua the sorong fault system has been documented to extend westward more than 500 mi 800 km from teluk sarera to kep banggai as a left lateral strike slip fault the ramu markham fault system is of disputed displacement and has been previously interpreted to extend from over 310 mi 500 km just east of the sepik river into the huon gulf a consideration of australian pacific cenozoic plate kinematics holocene vectors earthquake focal mechanisms photogeologic lineation analyses and the assimilation of other pertinent geologic data suggests that the ramu markham fault zone is an extension of the sorong fault system and that together they represent significant left lateral strike slip motion in rigid crustal basement this motion has produced large scale en echelon surface expressed faults and folds in detached overlying neogene sediments furthermore this wrench fault system has led to the transcompressional development and deformation of the meervlakt piore sepik and ramu basins of northern new guinea an understanding of the tectonic evolution of these basins is tantamount to an assessment of their evolving hy rocarbon potential end of article last page 980

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

migration and internal architecture of marine dunes in the eastern english channel over 14 and 56 year intervals the influence of tides and decennial storms submarine dune dynamics are controlled by tidal currents and wind forces according to the relative in uence of these forces and the nature of dune sediment different bedform behaviors can be observed the footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes this paper is concerned with bedforms that compose the thick sediment wedge located in the eastern english channel off the bay of somme this sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages the dynamics of large submarine dunes which are organized in elds are studied thanks to bathymetric and seismic data over the periods 1937 1993 and 1993 2007 dune morphology presents low lee and stoss side slopes on average 8 and 3 respectively and dune migration rate is not very high dune movements are in the direction of residual tidal currents i e toward the east with mean migration rates around 0 8 to 5 0 25 m yr 1 and up to 6 6 0 7 m yr 1 respectively at multi decennial and decennial time scales the dune internal archi tecture is complex with superimposed eastward prograding units displaying locally opposite progradation second order discon tinuities dip of 0 5 4 perpendicular to dune crests constitute dune master bedding by counting the number of second order re ectors between 1937 1993 and 1993 2007 the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years these time intervals coincide with the long term tidal cyclicities and also with the inter annual to decennial variability of storm activity in northern europe two theories were made to interpret the dune internal structures the second order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods weak tidal and storm action or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast

sedimentology and stratigraphy of a tidal sand bank in the southern north sea abstract a set of 125 vibrocores collected on the basis of a dense grid of high and very high resolution seismic coverage over a tidal sand bank the middelkerke bank provides a unique opportunity to reconstruct the stratigraphy and sedimentary facies succession of such a large tidal sand body five sedimentary facies are distinguished they range from clay to pebble and display various types of bedding and lithologies corresponding to different depositional environments a succession of four late quaternary units are defined on the basis of the stratigraphic framework given by the seismic grid the first unit represents the infill of a palaeovalley system by tidally influenced sedimentary facies it is followed by extensive and thick lagoonal or sub tidal deposits the third unit reveals the development of a landward prograding storm dominated sand bar this unit is dominated by clean fine sand the last unit corresponds to the present day tidal sand bank it shows the typical offshore tidal sand bank facies cross bedded sand prograding in an offshore direction like other banks located in a more offshore position the middelkerke bank is a composite bank incorporating facies which do not occur in classic tidal sand banks

on bagnold s sediment transport equation in tidal marine environments and the practical definition of bedload bagnold s sediment transport equation has proved to be important in studying tidal marine environments this paper discusses three problems concerning bagnold s transport equation and its practical application 1bagnold s suspended load transport equation and the total load transport equation with are incorrect from the viewpoint of energy conservation in these equations the energy loss due to bedload transport has been counted twice the correct form should be for suspended load transport and for total load transport with 2the commonly used bagnold s transport coefficient k varies as a non linear function of the dimensionless excess shear stress which can be represented best by the power law where the coefficient a and exponent b depend on sediment grain size d the empirical values of a and b for fine to medium grained sands are determined using guy et al s 1966 flume experiment data 3the sediment transport rates predicted from this equation are compared with bedform migration measurements in the flume and the field this comparison shows that the sediment transport rates measured from bedform migrations are higher than the predicted bedload transport rates but comparable to the calculated total load bedload plus intermittent suspended load transport rates this indicates that bedform migration involves both bedload and intermittent suspended load transport as a logical conclusion bedform migration data should be compared with bagnold s total load transport equation rather than with his bedload transport equation in this respect the term bed material might be more appropriate than the term bedload for estimating sediment transport rate from bedform migration data the sediment transport rates predicted from this modified bagnold transport equation are in good agreement with field measurements of bedform migration rates in four individual tidal marine environments which cover a wide range of sediment grain size flow velocity and bedform conditions ranging from small ripples megaripples to sandwaves

large scale sediment bodies and superimposed bedforms on the continental shelf close to the strait of gibraltar interplay of complex oceanographic conditions and physiographic constraints a previously unknown field of large scale sedimentary bodies has been mapped and studied on the continental shelf off the cape trafalgar near the strait of gibraltar with particular emphasis on the relationship between large scale sediment bodies and the superimposed bedforms this study is based on a grid of 975 km of high resolution seismic profiles collected at water depths ranging between 15 and 60 m high variability of large scale sedimentary bodies is attributed to the complex interaction of hydrodynamic agents the most prominent sedimentary features are sand banks and ridges that indicate long term southwest and southward directed sediment transport patterns possibly due to the interplay of two dominant current systems flowing southward and westward these sediment bodies evolve laterally to distinct external geometries such as sand shoals in shallow water and sand sheets in the vicinity of larger sand banks that indicate moderate current velocities in addition pre existing physiography is considered to play a role in the generation of certain sediment bodies developed over inclined surfaces or confined laterally by elevations relationships between superimposed bedforms mostly very large dunes and underlying sediment bodies vary across the study area most superimposed bedforms occur over the complex mosaic of sediment banks and sheets suggesting the interaction of several high energy currents with different directions such as tidal and or wind driven currents copyright 2010 john wiley sons ltd

morphology internal structure and reversal of asymmetry of large subtidal dunes in the entrance to gironde estuary france abstract the gironde is a macrotidal estuary characterized by important changes in tidal current velocity and river discharge two surveys were made in the lower estuary in june 1987 and october 1989 in an area with large dunes the symmetry of these dunes 1 5 6 7 m high changes from flood dominated in upstream areas to ebb dominated in downstream areas a transition zone consists of symmetrical dunes with a very low vertical form index the internal structure of the dunes revealed by seismic records shows a hierarchy of bounding surfaces related to fluctuations in movement speed and asymmetry of the dunes the lowest part of symmetrical dunes consists of ebb oriented reflectors while the upper part has flood oriented reflectors the same profile lines run in 1989 show that each large dune can be identified despite an upstream shift of the transition zone of about 1000 m comparison of seismic records shows that some lower reflectors are preserved between the two surveys and that the dunes moved downstream a mean distance of 30 m in 1989 the internal structure of symmetrical dunes was reversed with flood oriented reflectors in the lower part overlain by ebb oriented reflectors both seasonal changes in river discharge and fortnightly oscillations of the tidal range control the magnitude and orientation of the net bed load transport in the study area because the response time of the large dunes is longer than that of the superimposed small dunes the reversal of asymmetry of the large dunes requires a long period process while fortnightly oscillation or even semi iurnal reversal of the tidal current during spring tides could cause reversal of small dunes

facies analysis in the lower greensand using ground penetrating radar this paper presents the results of a ground penetrating radar survey in the woburn sands formation lower greensand group near leighton buzzard a total of almost 4 km of survey has been acquired to investigate the internal structure of the sands the radar survey achieved a resolution on the order of tens of centimetres with penetration up to 10 m and revealed a variety of horizontal dipping and curved reflectors the reflections are interpreted using seismic interpretation methodology two scales of unconformity defined radar sequences are described first order radar sequence boundaries can be identified at the termination of primary reflectors second order radar sequence boundaries can be identified at the terminations of both primary reflectors and first order surfaces first order radar sequence boundaries are interpreted as reactivation surfaces within bedforms which are produced by changes in bedform geometry in response to a change in hydraulic regime or superimposed bedforms second order surfaces are interpreted as the product of bedform migration reflection character and geometry are described and form the basis of a ground penetrating radar facies analysis nine different radar facies have been identified these are interpreted as large sets of planar cross stratification sets of trough cross stratification cosets of cross stratification tidal bundles clay drapes diagenetic concretions and possibly bioturbated sands a reconstruction of the bedforms suggests that they were formed by very large curved crested dunes these interpretations are checked against outcrop as the sands are excavated in working sand pits

long term evolution and internal architecture of a high energy banner ridge from seismic survey of banc du four western brittany france the recent completion of a coupled seismic and swath bathymetric survey conducted across the sand ridge system of the banc du four located on the atlantic continental shelf of brittany mer d iroise france provided new data for the study of the long term evolution of deep tidal sand ridges five seismic units are distinguished within the ridge separated by pronounced major bounding surfaces the basal unit is interpreted to be shoreface deposits forming the core of the ridge it is overlaid by a succession of marine sand dunes fields forming the upper units sandwave climbing which combines progradation and accretion is the major process controlling the growth of the ridge the elevation of the preserved dune foresets reaches values of about 20 30 m within the ridge the foresets indicate a combination of giant dunes characterized by numerous steep up to 20 clinoforms corresponding to a high energy depositional environment moreover the presence of scour pits linked to the 3d geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines all of the radiocarbon ages of the biogenic surficial deposits of the banc du four range from 10 036 to 2748 cal years b p and suggest the banc du four has grown during the last sea level rise the apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non conservation of recent deposits in contrast the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge moreover the long term evolution of the ridge appears strongly controlled by the morphology of the igneous basement the multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement therefore the banc du four should be thought of as a representative example of a large scale high energy banner bank

direct versus indirect climate controls on holocene diatom assemblages in a sub tropical deep alpine lake lugu hu yunnan sw china the reconstruction of holocene environmental changes in lakes on the plateau region of southwest china provides an understanding of how these ecosystems may respond to climate change fossil diatom assemblages were investigated from an 11 000 year lake sediment core from a deep alpine lake lugu hu in southwest china an area strongly influenced by the southwest or the indian summer monsoon changes in diatom assemblage composition notably the abundance of the two dominant planktonic species cyclotella rhomboideo elliptica and cyclostephanos dubius reflect the effects of climate variability on nutrient dynamics mediated via thermal stratification internal nutrient cycling and catchment vegetation processes statistical analyses of the climate diatom interactions highlight the strong effect of changing orbitally induced solar radiation during the holocene presumably via its effect on the lake s thermal budget in a partial redundancy analysis climate solar insolation and proxies reflecting catchment process pollen percentages c n ratio were the most important drivers of diatom ecological change showing the strong effects of climate catchment vegetation interactions on lake functioning this diatom record reflects long term ontogeny of the lake catchment ecosystem and suggests that climatic changes both temperature and precipitation impact lake ecology indirectly through shifts in thermal stratification and catchment nutrient exports

between scylla and charybdis part 2 the sedimentary dynamics of the ancient early pleistocene messina strait central mediterranean based on its modern analogue abstract the 3 km wide messina strait divides sicily from the italian peninsula representing one of the most impressive examples of tide dominated marine passageway of the central mediterranean sea the tidal hydrodynamics governing the strait results from a semi diurnal phase inversion between two interlinked wider basins i e the tyrrhenian sea to the north and the ionian sea to the south due to the presence of a nodal tidal point in the centre of the strait this condition associated with coastal narrowing causes continuous water exchanges and a local phenomenon of tidal amplification producing collinear tidal currents up to 3 m s 1 in velocity in the point of tidal maxima of the strait centre currents follow bi directional patterns across the strait decelerating towards either opposite peripheries because of the progressive enlargement of the cross sectional areas tides exert a relevant influence on the strait bottom shaping characteristic erosional features governing the main sediment transport routes and promoting the migration of distinctive bedforms the textural and dimensional attributes of these bottom features vary according to the current velocity trends and main directions indicating a specific partition of the strait in individual sedimentary zones in the early pleistocene analogous conditions established across the same area and huge quantities of siliciclastic and bioclastic deposits accumulated in the ancient messina strait under a similar tidal hydrodynamics the 250 m thick strait fill succession which is magnificently exposed across the two opposite margins i e scylla and charybdis of the modern passageway represents a crucial correlative element in supporting the reconstruction of the sedimentary dynamics of the past strait based on the observation of the modern system in this work which represents the second part of a first article focused on the modern messina strait previous stratigraphic data are reviewed and integrated with new sedimentological observations carried out in both onshore and offshore key areas in particular outcrop analysis allows distinguishing a number of facies associations which are described and interpreted by taking into account the correspondent modern depositional strait setting the results of this study highlight a mutual correlation between the depositional zones detected in the modern messina strait and the individual facies associations recognised in the onshore margins the strait centre zone represented a bedload parting sector in the ancient system this zone which is one of the most difficult environment to detect in ancient straits due to a condensed sedimentary record is suggested to coincide with a structural palaeo high exposed in the southern sector of the investigated onshore area and continuing offshore in the modern strait this tectonic horst symmetrically separated two opposite highly depositional zones to the north and to the south hosting gravel and sand size siliciclastic and bioclastic deposits exhibiting diffuse motifs of large scale tidal cross stratification and a specific vertical facies stacking these characteristic tidal deposits which are believed to record the migration of several generations of tidal dunes forming extensive bedform fields in the strait reflect a tidal asymmetry between flood and ebb currents tidal asymmetry is referable to the oceanographic setting observable nowadays in the messina strait and deriving from a marked density driven water mass stratification between the two opposite flows in the ancient system tidal dunes transited down current to highly bioturbated fines including sporadic small scale tidal cross strata representing the distal periphery of the early pleistocene strait here bedforms followed migration pathways not always reflecting mutually evasive net gross sediment transport routes due to local conditions of current rotation occurring during each tidal phase inversion during these stages similar to slack water periods currents were weak enough to allow fine sediments to be settled from suspension reproducing a signature typical of other tide dominated coastal systems e g estuaries tidal flats etc these sedimentary zones of the ancient messina strait were laterally bounded by tectonically active margins where fan deltas clastic aprons and carbonate wedges developed under the constant interplay between mass transport processes and tidal currents the resulting architectural and sedimentary features observable in outcrop reveal impressive similarities with the morpho bathymetric features nowadays documented in the correspondent strait margin modern environments these considerations allow deciphering a general palaeo environmental reconstruction of the ancient strait which although suffering of limitations due to a partial preservation of the whole rock record discloses sedimentary processes and depositional environments of the ancient messina strait the process based analysis of this field based case study is crucial in predicting long term environmental changes and facies migration pathways and improves the state of knowledge on tidal straits which are the less understood among the most common tidal depositional systems

headland associated banner banks generated during the last deglaciation near the strait of gibraltar gulf of cadiz sw spain the inception and evolution of large scale bedforms on the barbate platform gulf of cadiz sw iberian peninsula are investigated based on a detailed study of multibeam bathymetric data high resolution seismic profiles and tidal current simulations elongated rocky outcrops and several large scale bedforms classified as sand banks were identified on the platform barbate zahara el palmar el varadero and canos de meca banks the barbate and zahara banks exhibit the most complex internal stratigraphic architecture which comprises five seismic units the basal unit u1 is interpreted as incised valley infill and is overlain by nearshore bars u2 two high energy growth phases u3 and u4 that combine progradation and accretion form the present day external bank shape superimposed bedforms covered these banks under the modern hydrodynamic regime u5 the multiphase accretion and progradation of u3 and u4 in the zahara and barbate banks were closely linked to the presence of a palaeo headland termed here as palaeo cape trafalgar and strong tidal current eddies that resulted from the progressive flooding of the barbate platform during the last deglaciation this configuration was relatively stable while the rocky outcrops were exposed to sub aerial conditions forming a palaeo coast with coastal bluffs we suggest that during this period of coastal stabilization u3 and u4 grew as banner banks on either side of the palaeo cape trafalgar the present study proposes that the configuration of palaeo headland associated with palaeo banner banks zahara and barbate banks along each side of the palaeo cape trafalgar represents a transgressive analogue model of headland associated tidal bank systems developed in funnel shaped coastal systems terminating in narrow straits

types of internal structure and external morphology of submarine dunes under the influence of tide and wind driven processes dover strait northern france abstract the expression of tide and wind driven processes in the internal architecture and external morphology of very large submarine dunes is investigated at a dune field located in the central part of the dover strait southern north sea current measurements and sediment transport calculations enable quantification of the dominant tide and wind driven processes and very high resolution seismic recordings are used to analyse internal structure and external morphology of dunes in the study area moderate to storm winds induce temporary currents that can lead to the reversal of sediment transport direction and induced erosive processes on the seabed dune master bedding consists of extended erosive surfaces with 9 11 slopes that bound storm generated sets of cross beds 1 5 4 85 m thick 2 65 m on average three types of architectures are observed within the dune field and occasionally within single dunes these types which display a strong linkage between internal structure and external morphology consist of 1 asymmetric dunes with cosets which dip in the direction of the marked external asymmetry of the dunes 2 dunes with a complex external morphology composed of mega herringbone structures each displaying an opposing progradation direction and 3 dunes with a symmetric megaripple bedding unit at their summit the diversity of the internal architecture and external morphology results from the variable combinations of tide and wind driven processes occurring in the study area the asymmetry of tidal peak current velocity and the relative strength of wind driven currents are the parameters that have the most important influence on dune architecture asymmetric dunes with cosets result from a strong asymmetry in tidal peak current velocity which is reversed only slightly during storms dunes with mega herringbone structures and dunes with symmetric megaripple units at their summit are observed in areas where the asymmetry of the tidal peak current velocity is negligible allowing reversal of the sediment transport and dune migration directions each time moderate winds blow the latter architectural type is specific to sand rich areas the different dune architecture types display strong geometrical similarities to structures observed in dunes from other shelf environments where driving processes are different this implies that a study based only on a geometric description of internal structures will not enable the reconstitution of ancient depositional environments a better discriminant criteria seems to be the thickness of the second order sets of cross beds which is correlated with specific hydro sedimentary processes dune height also probably has an important influence on the thickness of second order sets of cross beds as dune height and set thickness display a clear relationship in the present study