numerical modelling of storm and surge events on offshore sandbanks abstract this contribution uses a 3 dimensional coastal area numerical model dhi s mike3 to simulate the impact of storm and surge events on offshore sandbanks three offshore sandbanks in the bristol channel are considered due to the region s sensitivity to anthropogenic pressures and the gradients in wave and tidal forcing in the area two extreme storm and surge events are simulated one co incident with spring tide and the other with neap tide reference simulations of astronomical tidal forcing only are also presented it is shown that for the two sandbanks with greater wave exposure storm conditions can provide a mechanism for the maintenance of crest position for these cases bed level changes over the crest are in the opposite direction compared to astronomically forced change for the least wave exposed bank both wave and tide only cases exhibit similar patterns of bed level change volumetric changes under astronomical forcing are shown to vary with changing maximum tidal current accretion occurs over a neap tidal cycle for all three sandbanks and as maximum tidal current increases the amount of accretion increases however over a spring tidal cycle accretion is observed for the less tidally dominated site but increasing maximum tidal current leads to reduced accretion and then erosion for the most tidally dominated bank volumetric changes under storm conditions are related to sandbank morphology and setting rather than relative wave exposure the two single banks closely tied to headlands show similar magnitude of percentage volumetric change despite being at the two extremes of wave exposure and greater erosion occurs over the neap tide event the sandbank that has associated secondary banks shows lesser percentage change and greater erosion over the spring tide event

nearshore oblique sand bars 1 the coupling between hydrodynamics and the evolving topography in the surf zone has been theoretically examined for oblique wave incidence it is shown that positive feedback can lead to the initial growth of several types of rhythmic systems of sand bars the bars can be down current oriented or up current oriented which means that the offshore end of the bar is shifted down current or up current with respect to the shore attachment in the limit of strong current compared to wave orbital motion very oblique down current oriented b ars are obtained with a spacing of several times the surf zone width when wave orbital motions are dominant systems of up current oriented bars and crescentic down current oriented bars appear with spacings of the order of the surf zone width the latter feature consists of alternating shoals and troughs at both sides of the break line with the inner shoals being bar shaped and oblique to the coast the growth e folding time of the bars ranges from a few hours to a few days and it is favored by constant wave conditions the range of model parameters leading to growth corresponds to intermediate beach states in between the fully dissipative and the fully reflective situations preliminary comparison with field observations shows qualitative agreement

a new type of sea bed waves sandy beds of shallow tidal seas often exhibit a range of rhythmic patterns from small scale ripples a few metres long to large tidal sandbanks with a wavelength of kilometres for example on the access route to rotterdam harbour ships cross a field of sandwaves the crests of these sandwaves determine the effective navigation depth to warrant navigability the north sea directorate of the netherlands ministry of transport public works and water management continually monitors the bathymetry in the sandwave area originally using echo sounding our analysis of these data has revealed a new rhythmic pattern in addition to the well known sandwaves and tidal sandbanks the wavelength of this new pattern labelled here as long bedwaves is three times the one of sandwaves and the crest orientation is different interference of the three modes leads to the rather complex bathymetry revealed by echo soundings

remobilization of deep basin brine during exhumation of the illizi basin algeria abstract understanding the processes that drive fluid flow in sedimentary basins has important implications for models of metallic ore and petroleum migration and accumulation in the subsurface potential drive mechanisms include gravity driven flow compaction driven flow and density driven convection in this study we demonstrate that the brine hosted in ordovician sandstone in the illizi basin in algeria is genetically linked to triassic liassic evaporites deposited 400 km to the north in the berkine basin this observation confirms that long distance lateral brine migration has occurred within the basin in the past we assess the hydrogeologic record preserved in aqueous fluid inclusions within the ordovician sandstone document a marked increase in formation water salinity during cooling and exhumation and evaluate the drive mechanisms for late stage remobilization of deep brines within the basin it is hypothesized that the release of overpressure during exhumation of the illizi basin may have been a critical contributor to updip fluid flux this model could be applicable to other exhumed basins worldwide

diapiric growth within an early jurassic rift basin the tazoult salt wall central high atlas morocco the central high atlas morocco constitutes a diapiric province that hosts a complex array of elongated diapirs and minibasins that formed during the lower jurassic rift of the atlas basin this paper aims to study the structure and growth evolution of the tazoult diapiric wall located in the central high atlas by means of structural and sedimentological fieldwork integrated with remote sensing mapping the tazoult salt wall is a 20 km long x 3 km wide ne sw trending ridge that exposes upper triassic red beds and basalts along its core the succession flanking the salt wall ranges from hettangian to bajocian ages displaying spectacular sedimentary wedges in the se and nw flanks the hettangian early sinemurian carbonates mainly crop out as blocks embedded in the core rocks the 1 km thick pliensbachian platform carbonates display large subvertical flap structures along the flanks of the tazoult salt wall with unconformities bounding tapered composite halokinetic sequences in contrast the 2 5 km thick late pliensbachian aalenian mixed deposits form tabular composite halokinetic sequences displaying small scale hook halokinetic sequences passive diapirism resulted in the lateral extrusion of the evaporite bearing rocks to form an allochthonous salt sheet towards the adjacent se amezrai minibasin the bajocian platform carbonates partially fossilized the tazoult salt wall and thus constitute a key horizon to constrain the timing of diapir growth and discriminate diapirism from alpine shortening the pliensbachian carbonate platform evolved as a long flap structure during the early growth of the tazoult salt wall well before the onset of the alpine shortening

medium term 2dh coastal area modelling the state of the art in depth averaged mathematical modelling of 3 d coastal morphology is described for the medium term morphodynamic model type in which constituent models of waves currents and sediment transport based on first physical principles are linked together to describe the time evolution of the bed topography various aspects of the combined system of equations are discussed such as its mathematical character its inherent stability and its equilibrium state the results of an intercomparison of different models are shown for two test cases and the potentials and limitations of the model concept are discussed

early jurassic climatic trends in the south tethyan margin abstract the early jurassic period was characterized by extreme environmental changes as reflected by major global carbon isotope anomalies and abrupt changes in oxygen isotope and elemental records of marine organisms available data suggest an overall warm early jurassic climate interrupted by periods of severe cooling with a climatic optimum during the early toarcian available geochemical studies however have mainly focused on the northern margin of the tethys ocean so that the palaeogeographic extent of these environmental perturbations latitudinal palaeotemperature gradients and climate belt boundaries remain poorly constrained here we report the first stable isotope records of brachiopod shells 13c and 18o values from the upper sinemurian middle toarcian interval in the southern margin of the tethys ocean northwest algeria these data were used to better constrain the palaeoenvironmental evolution of the north gondwana margin during the early jurassic which likely played an important role on supra regional climate the diagenetic history of the analysed brachiopod shells was monitored using scanning electron microscopy and elemental manganese and strontium compositions the brachiopod 13c and 18o data show very similar trends as those reported for various tethyan regions and record negative carbon and oxygen isotope excursions near the sinemurian pliensbachian and pliensbachian toarcian transitions and during the toarcian oceanic anoxic event t oae despite these similarities the carbon and oxygen isotope records are systematically offset towards more positive 13c values average 0 5 and more negative 18o values 1 0 compared to those obtained from sites of higher palaeolatitudes in the northern tethyan margin these offsets suggest a spatial heterogeneity in the stable isotope composition of dissolved inorganic carbon in the early jurassic ocean and a marked latitudinal temperature gradient between the southern and northern margins of the tethys

sequence stratigraphy of carbonate evaporite basins models and application to the upper permian zechstein of northeast england and adjoining north sea the successions of carbonate evaporite basins which are periodically cut off from the world ocean are considered in terms of their sequence stratigraphy two basin types are distinguished those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation it is argued that following relative sea level falls sequences begin with the evaporites precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin centre halite fills in the second the evaporites pass up into carbonate sediments which are mainly deposited on shallow water platforms around the basin during transgressive and highstand systems tracts the concepts and models developed are applied to the zechstein upper permian strata of northeast england and adjoining north sea seven sequences are proposed with some containing thin parasequences the sequence approach to carbonate evaporite basins which considers both basin margin and basin centre facies gives a more encompassing perspective of sedimentation than the traditional cycle concept with its emphasis on the basin centre

hydrochemistry and isotopic composition of na hco3 rich groundwaters from the ballimore region central new south wales australia abstract groundwaters flowing under artesian pressure from the ballimore region central new south wales australia contain up to 95 na hco3 carbon dioxide effervesces from these groundwaters as they discharge from boreholes these groundwaters flow from a confined aquifer situated within fractured permian to jurassic sediments regional geological data and k ar dating of outcropping volcanic units indicate that magmatism occurred in this area between 12 and 14 ma ago radiometric and magnetic data supports the interpreted presence of intrusive bodies x ray fluorescence xrf data shows that the miocene magmatic units are sodium rich with the available samples being classified as either syenitic or trachytic hydrochemical and isotopic data indicate that these groundwaters are of meteoric origin and that this system is open with respect to co2 there is an external source of co2 feeding this groundwater system causing dic total alk the salinity of the na hco3 rich groundwaters range from 3000 to 8000 mg l and co2 aq concentrations are up to 1558 mg l log pco2 of 0 6 the ph of these samples range from 6 1 to 6 9 supporting the conclusion that the h2co3 hco3 couple control ph buffering the carbon 13 values 13c pdb of 5 to 3 indicate that the carbon source for these groundwaters is of inorganic origin it is inferred that this carbon source has a primary 13c similar to that typically quoted for mantle derived carbon 5 9 varying degrees of completeness of equilibrium isotopic fractionation resulting from the hydration dissociation of co2 into the groundwater forming hco3 co2 h2co3 hco3 leads to the observed range of carbon 13 data oxygen 18 18o smow and deuterium d smow isotopic data show a shift to the left of the meteoric water line supporting an interpretation of co2 exsolution gas exchange and fractionation resulting from incongruent silicate weathering reactions dissolved inorganic carbon carbon 13 and regional geological data suggest that the most plausible explanation for the high concentration of co2 in these effervescent sodium bicarbonate rich groundwaters is the mantle with this gas initially being delivered to the fractured strata by magmatic activity

eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank celtic sea the architecture of a tidal sand bank in the south eastern celtic sea was examined using very high resolution seismic surveys the bank comprises four depositional units the lowest unit 1 is characterized by gently dipping 1 8 strata that strike parallel to the length of the bank unit 1 is erosionally overlain by unit 2 which forms the bulk of the bank this unit consists of stacked sets of downcurrent dipping 7 12 master bedding formed by climbing sinuouscrested tidal dunes that are up to 20 m high these deposits are locally incised by an anastomosed channel network unit 3 that may represent a buried swatchway system the upper part of the bank comprises wave related deposits that are mainly preserved on the bank flanks unit 4 the outer bank surface is erosional the bank is believed to have formed during the last postglacial sea level rise the facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy mainly characterized by the thickening of dune cross bed sets in unit 2 the majority of bank growth is inferred to have occurred in water depths of the order of 60 m this evolution was controlled by relative sea level rise which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep cross cutting channels of unit 3 the transition to wavedominated sedimentation in unit 4 is related to the decay of resonance with continued sea level rise

lithofacies and cyclicity of mohilla evaporite basins on the rifted margin of the levant in the late triassic makhtesh ramon southern israel marine connected basins with evaporites occur beneath most extensional continental margins that originated at low latitudes and often are of major economic significance cyclicity in the evaporite lithofacies reflects the degree of restriction of the basin overprinted by sea level changes and caused by structural movements in the barrier region whether by fault block rotation footwall uplift or hanging wall subsidence in both extensional and compressional basins the upper triassic evaporites of the ramon section in southern israel model cyclic sedimentation in such environments the mohilla formation is a carbonate evaporate siliciclastic succession of carnian age that fills a chain of basins extending along the levant margin from southern israel to jordan and syria the basins developed in half grabens adjacent to normal faults that formed during a period of regional extension evaporites of this formation are well exposed in outcrops at makhtesh ramon the southernmost of these basins the m2 member of the mohilla formation is composed of 42 sub metre cycles of alternating dolostone gypsum and calcareous shales field and microfacies analysis showed these cycles to conform mostly to restricted shallow and marginal marine environments spatially limited by the uplifted shoulders of the half graben systems a total of 10 facies types belonging to six depositional environments have been identified from stacking patterns and analysis of bed to bed change cycles can be categorized into three groupings i low frequency exposure to exposure cycles that developed under eustatic or climate control ii high frequency deepening shallowing upward cycles characterized by gradual transitions due to short term sea level or runoff event oscillations possibly referable to orbital forcing and iii high frequency shallowing upward cycles characterized by abrupt transitions attributable to sporadic tectonic events affecting accommodation space or barrier effectiveness the way facies and cycling of the sedimentary environments was deciphered in the mohilla evaporite basin can be used to unravel the genesis of many other evaporite basins with barriers of tectonic origin

tidal and residual currents around a norfolk sandbank currents on either side of well bank were recorded for 41 days in may june 1981 in order to distinguish locally tidally generated asymmetries pertinent to maintenance of the bank from larger scale and wind driven flows semi diurnal currents were dominant nearly rectilinear and slightly inclined to the bank axis fourth diurnal currents were consistent with previously observed and computed fourth diurnal tides for the area overall amplitudes varied as the square of the semi diurnal amplitude hence they appear to be generated by tides but only a small part locally residual currents average over a tidal cycle were always in the sense of clock wise circulation around the bank consistent with simple models and giving strong evidence of local generation by tides this circulation combines with the semidiurnal currents to maintain the bank the slow increase of the residuals with the semi diurnal amplitude is interpreted as an effect of strong semi diurnal currents a persistent component onto the bank 7 m above the bottom at one position may indicate distortion of the frictional layer by the sloping bottom or by bank curvature

morphodynamic behavior and seismostratigraphy of a sandbank bahia blanca estuary argentina abstract sediment dynamics in bahia blanca estuary argentina permit the formation of longitudinal banks one of which is la lista bank located at the mouth of the la lista tidal channel the purposes of the present research were to study the processes and mechanisms involved in the formation and maintenance of this bank and to identify seismic sequences which could be indicative of its origin and current morpho evolutionary trend to this end different methods and techniques such as side scan sonar 3 5 khz profiler and bottom sediment sampling were followed the analysis of seismic acoustic records revealed bottom features sand dunes sand ribbons current marks escarpment which based on our analysis of current measurements and our estimations of transport as bedload have contributed to tracing a sediment circulation pattern in the study area taken together these findings allowed us to identify the areas dominated by erosion sediment transport or depositional processes bedload material was found to be exported to the outer sector and held on the flanks of the bank particularly in its southern part inference on the morpho evolutionary trend in the study area was based on evidence related to migration patterns and changes observed on the flanks and inner structure of the bank the morphological evolution of la lista bank is associated to a residual sediment transport pattern with trajectories opposite on its two flanks which results from the action of ebb tides in the northern flank and flood tides on the southern flank this process induces a gradual increase in the bank height and longitudinal growth the la lista bank evolution agrees with the motion sequence and development pattern proposed by caston 1972 for a linear bank except that this model is repeated as the bank grows the seismostratigraphic data presented here indicates that the origin of this bank is associated with the holocene marine regression process

potential gain and loss of sand by some sand banks in the southern bight of the north sea abstract a detailed echo sounding and side scan sonar survey has shown that sand banks in the southern bight of the north sea have rounded heads in the approximate upstream direction and tapered tails in the approximate downstream direction of net regional sand transport thus the plan view of sand banks can be used as an indicator of the approximate direction of net sand transport sand wave aprons occur in front of the heads and downstream of the tails of the sand banks the asymmetry of the sand waves implies that sand is being added to sand wave aprons at the heads and to the upstream flanks of these sand banks but is shed from the downstream sand wave aprons there is no evidence for a closed sand circulation on the banks

the cross sectional shape of tidal sandbanks modeling and observations to improve our understanding of tidal sandbank dynamics we have developed a nonlinear morphodynamic model a crucial property of the model is that it fully resolves the dynamics on the fast tidal timescale allowing for asymmetric tidal flow with an m0 m2 and m4 component this approach extending earlier research on the formation of tidal sandbanks leads to equilibrium profiles their heights 60 90 of the water depth and shapes are controlled by the mode of sediment transport and the hydrodynamic conditions bed load transport under symmetrical tidal conditions leads to high spiky banks several mechanisms tend to lower and smooth these profiles such as the relaxation of suspended sediment wind wave stirring and tidal asymmetry this last causes the profiles to be asymmetric as well the morphodynamic equilibrium expresses a tidally averaged balance between a destabilizing flux due to fluid drag and the downslope transport induced by both tidal flow and wind wave stirring the modeled profiles are in fair agreement with observations from the north sea

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

dynamics diffusion and geomorphological significance of tidal residual eddies many shallow tidal areas show the occurrence of ebb and flood surplusses organized in cell like structures or tidal residual eddies they seem to arise from a nonlinear transfer of vorticity from the oscillating tidal field to the mean field the irregular structure of the bottom topography or the coastline acting as a catalyst to produce the necessary vorticity gradients the dynamical characteristics of the eddies assist our understanding of the geomorphology of the sea bed

evolution of a classic sand ridge field maryland sector north american inner shelf the ridge and swale topography of the middle atlantic bight is best developed on the delaware maryland inner shelf here sand ridges can be seen in all stages of formation several aspects of the ridge field are pertinent to the problem of ridge genesis the first is ridge morphology there is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges which reflects the changing hydraulic regime as successively more seaward ridges are examined maximum side slope decreases the ratio of maximum seaward slope to maximum landward slope decreases and the cross sectional area increases these changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the holocene transgression a second aspect is the change in bottom sediment characteristics that accompanies these large scale morphologic changes megaripples sand waves and mud lenses appear in the troughs between nearshore and offshore ridges these changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector and that the role of high frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea floor a third aspect is the systematic relationship of grain size to topography grain size is 90 out of phase with topography so that the coarsest sand lies between the axis of the landward trough and the ridge crest while the finest sand lies between the ridge crest and the axis of the seaward trough this relationship is characteristic of large scale bedforms finally flow was measured and transport calculated on the same ridge during a one month period november 1976 threshold was exceeded only during storm events mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline these flow measurements are in conformity with the pattern of smaller bedforms a 43 year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion seaward flank deposition and seaward crest migration sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress the relationship between grain size and topography supports this model but does not account directly for the oblique angle of the ridge with respect to the coastline this feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge and the relationship between this migration rate and the rate of shoreface retreat

understanding the salinisation process for groundwater in an area of south eastern australia using hydrochemical and isotopic evidence abstract increases in groundwater salinity down gradient are a common occurrence in catchments throughout australia exacerbating the effects of dryland salinisation in discharge areas groundwater hydrochemical and isotopic data from the willaura catchment in south eastern australia which is affected by both primary natural and secondary human induced salinity allowed the factors that cause the down gradient trend in salinisation within this catchment to be determined the overall salinity of the groundwater 0 8 70 ms cm is controlled by evapotranspiration which concentrates cyclic salts in the unsaturated zone greatly increasing the salinity of the soil water to 3700 6400 mg l cl the progressive addition of this saline soil water to fresher groundwaters recharged on the catchment margins causes a gradual increase in salinity down gradient the groundwater stable isotope compositions show that most recharge occurs in winter and spring groundwater salinities increase around the saline lakes of the discharge zone due to direct evaporation from shallow watertables and minor dissolution of halite as indicated by cl br ratios mineral water interactions within the aquifers cause a slight overall reduction in salinity conversion of kaolinite to smectite and illite and cation exchange of na for ca2 on smectites cause na cl mg2 cl k cl si cl and hco3 cl ratios to progressively decline and ph to gradually increase down gradient oxidation of organic matter causes a strong decrease in the redox potential of the groundwaters down gradient the groundwater ages increase from modern containing tritium on the catchment margins to 8000 years based on 14c dating at the end of the 25 km flow path the older groundwaters are isotopically heavier than the younger waters probably due to recharge during a wetter climate in the early holocene the groundwater ages show clearly that the progressive addition of saline soil water causing groundwaters to become more saline down gradient has occurred throughout the holocene and may explain the occurrence of primary salinisation within many australian catchments

a reassessment of sand transport paths in the bristol channel and their regional significance abstract data on bedform morphology sediment parameters heavy mineral abundance current measurements sand transport rates numerical models of shear stress sea bed drifters and wave activity are all considered it is concluded that the dominant transport of sand and gravel is westwards from the transverse bristol channel bed load parting the notion of easterly transport of sand apart from wave induced transport on the beach and in the nearshore zone by the flood tidal current in continuous zones along the northern and southern margins of the bristol channel is largely discounted however there are some small disconnected areas where easterly transport of sand is locally induced by coastal irregularities by sand bank relief and by a local deep the ground for up to 2 km north of scarweather sands provides a rare example of the net sand transport direction being determined by the weaker but substantially longer flowing flood tidal current a facies model of the deposits is outlined

long term changes in morphology and textural sediment characteristics in response to energy variation on shoreface connected ridges off the east frisian barrier island coast southern north sea abstract patterns of surficial sediment distribution across shoreface connected sand ridges located along the barrier island coast of the southern north sea generally show a strong affinity between textural parameters and ridge morphology comparable to that observed in other parts of the world thus the sediment of the upper shoreface and the inner ridge crest consists of fine sand water depths 12 m in order to assess the mobility of the sediments and the consistency of the observed textural patterns two high resolution multi beam bathymetric surveys were carried out along selected corridors across a ridge and trough system off spiekeroog island located along the southern north sea coast of germany one in 2003 the other in 2007 the bathymetric data of the two surveys show only small scale topographic changes these were produced mainly by local changes in the bedform patterns on the inner ridge the bathymetric surveys therefore document a remarkable morphological stability of the ridges over time periods of several years spatial and temporal variations in the distribution of standard textural parameters were assessed on the basis of 95 sediment samples recovered on the same grid in 1989 and 2005 these data show that for example the distribution of coarse sand 0 1 phi became enhanced along the axis of the outer trough while expanding onto the lower ridge flanks current measurements in fair weather demonstrate that the coarse sediments cannot be moved by the local tidal currents the observed changes in the distribution of coarse sand and in particular its expansion onto the adjacent ridge flanks must therefore be associated with wave action probably in combination with tidal currents this is supported by an assessment of the entrainment potential of coarse sand d 1 mm by typical waves during storms t 8 s h 2 5 m and fair weather t 4 s h 0 7 m the results demonstrate that the effective wave base for coarse sand during fair weather conditions is limited to a water depth of 4 8 m whereas during storms it exceeds a depth of 26 4 m the mobility of the coarse sands in the outer trough at water depths 16 m and with it the morphodynamics of the ridges as a whole is thus clearly controlled by the superimposition of storm waves on the tidal current regime

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

comparison between predicted and observed physical features of sandbanks a linear stability analysis has been carried out to investigate the influence of parameters like tidal current depth and coriolis force on sand bank formation the applied analytical model allowed the computation of the horizontal wave number corresponding to the fastest initial growth rate of seabed form perturbations sandbank wavelength and the angle between the crest of the sandbank and tidal current direction were also obtained the experiments show an exponential decay of this angle for increasing values of tidal currents at a particular depth the sand bank wavelength grows almost linearly with the velocity we compared these theoretical results with observations of sandbank features in several geographic areas persian gulf the gulf of korea gulf of california and the north sea the results show that all observed sand bank wavelengths and angles are in very close agreement with the calculated theoretical curves

wave control on the rhythmic development of a wide estuary mouth sandbank a process based modelling study abstract this study presents the numerical hindcast of the morphological and stratigraphic evolution of a wide estuary mouth sandbank located along the french atlantic coast marennes oleron bay and primarily investigates the respective contribution of tide and waves to its pluri decadal evolution firstly the morphodynamic hindcast appears strongly improved when tide and waves are considered in the forcing rather than only tide secondly the modelling results suggest that the strong seasonal variation of the wave climate in this area explains the rhythmic lateral accretion of the bank i e normal to the main currents direction observed in seismic reflection profiles the grain size variation in the area of sediment accretion is also characterized by a seasonal cycle finally repetitive bathymetric surveys and seismic profiles revealed that the lateral accretion of the bank is associated with about one seismic reflector per year which is in agreement with our modelling results when assuming that these seismic reflectors are due to the seasonal variation of the grain size these new results may be considered when studying the morphological evolution and the stratigraphy of other tidal sandbanks potentially affected by waves

triassic liassic western moroccan synrift basins in relation to the central atlantic opening abstract the upper triassic lower liassic sequence of the atlantic margin of morocco is primarily composed of fine grained detrital sedimentary rocks containing basaltic intercalations its age is carnian to hettangian well data outcrop observations and seismic sequence analysis suggest that rifting started earlier carnian in the eastern and southern parts of the margin and later proceeded towards the west and the north resulting in the formation of the central atlantic rift on the moroccan margin the rift was composed of i eastern basins relatively narrow and bounded by e dipping faults which are reactivated hercynian structures ii a central horst and iii western basins bounded by w dipping faults the 020 trending basins are separated by w dipping faults the 020 trending basins are separated by 070 striking transfer faults estimations of the amount of lithospheric extension along the moroccan external rift system show a thinning ratio decreasing northward and probably associated to a necking of the lithosphere rift evolution on the part of the moroccan margin shows good correlation to the conjugate north american margin

tidal hydrodynamics and fine grained sediment transport on the radial sand ridge system in the southern yellow sea abstract the large radial sand ridge rsr system located in the southern yellow sea off the jiangsu coast is characterized by a radial current field and high suspended sediment concentrations sscs significant rectilinear currents are present over the northern sand ridge area whereas rotary currents prevail over the southern area the transition between the two being dominated by a locally generated trapped wave observations undertaken in 2006 and 2007 indicate a seasonal pattern in the ssc and associated dynamic characteristics the results of a calibrated hydrodynamic model show that the m2 tide dominates the rsr area the maximum tidal amplitude in the core region close to land being approximately 2 m an amphidromic point is located some distance north of the rsr off the abandoned yellow river mouth 121 686 e 34 6 n specifically the eulerian and lagrangian residual currents flow southward in both summer and winter whereas the stokes drift is directed southward in the north and northward in the south moreover in deep water the lagrangian residual current is mainly determined by the eulerian residual current whereas it is strongly influenced by the stokes drift in shallow water the seasonal variation in the residual current in the southern yellow sea is a good indicator for the location of the yellow sea warm current yswc in both summer and winter measurements show that a ssc peak appears off the abandoned yellow river delta in response to the strong currents and plentiful supply of fine grained sediment from coastal erosion another ssc peak lying close to the north branch of the changjiang river mouth is related to the abundant suspended sediment supplied by the river seasonal variations in ssc are observed in the study area maximum average values and widest diffusion from land occurring in winter this being contrasted by minimum mean values in summer sensitivity experiments demonstrate that the main factor influencing the distribution of ssc is the tide while river discharge wind and wave effects are small and very localized to explain the seasonal variation in ssc the role of hydrodynamics initial conditions and temperature are tested in the model the results show that the hydrodynamics play a significant role whereas the initial conditions only have a minor regional influence temperature on the other hand is also shown to greatly influence the seasonal variation in ssc because of substantial changes in the viscosity of the water and the erosion shear stress the modeled results are used to calculate sediment fluxes which are shown to be consistent with the lagrangian residual current pattern while wind waves can significantly change the fluxes in winter bed elevations change substantially on a monthly time scale in general the deep channels are undergoing erosion while the shallow ridges and coastal tidal flats are accreting the net sediment transport being directed towards the coastal tidal flat and the changjiang yangtze river subaqueous delta

measurements of the reynolds stress components close to a marine sand bank abstract soulsby r l 1981 measurements of the reynolds stress components close to a marine sand bank mar geol 42 35 47 observations of turbulence have been made at a height of 65 cm above the sea bed in water 42 m deep on the edge of the skerries sand bank off southwest england the observations extended over a 4 h period from the time of maximum flood velocity to slack water electromagnetic current meters were used to measure the streamwise u cross stream v and vertical w velocity components while simultaneously a conventional current meter measured the speed and direction 5 m below the water surface ratios of u2 1 2 v2 1 2 and w2 1 2 to shear velocity agreed with accepted values for laboratory and atmospheric flows near to the surface the current was found to flow across the bank while near the bed it ran nearly parallel to the isobaths consequently the bottom current was directed 480 clockwise of the surface current the azimuth of the reynolds stress uw vw near the bed lay between these directions and was directed about 60 anticlockwise of the near bed current this balanced the cross stream as well as the streamwise shear a lateral shear stress uv associated with the bank was observed taken together with previous measurements of the velocity distribution over the bank this yielded a value for lateral eddy viscosity of 1 2 105 cm2 s 1 the scale over which this acts is of the order 1 km and the value is appreciably smaller than reported values deduced for larger scales the veering of the current acts to lessen the bottom current convergence associated with coriolis and curvature effects and may contribute to lateral sorting by preferentially carrying finer sediment toward the crest of the bank the lateral eddy viscosity coefficient may be useful in assessing the diffusion of sediment on or off the bank

form function and feedbacks in a tidally dominated ooid shoal bahamas although the general aspects of oolitic depositional systems are well documented seascape scale 10 3 10 6 m 2 patterns of oolitic shoals and the details of processes acting on them are not well understood or quantified to begin to fill this basic gap in understanding this paper describes the morphology and hydrodynamics of lily bank a modern tidally dominated bahamian ooid shoal in this study integrating remote sensing imagery with quantitative geo located bathymetrical hydrological and granulometric data in a geographic information system documents geomorphic and sedimentological patterns and facilitates interpreting these patterns in the context of the processes operating in this system the results of these analyses reveal that parabolic bars up to several kilometres in wavelength and several metres in height form a common morphologic motif although there is considerable variation on that general theme the seascape scale configuration of bars and superimposed sedimentary structures is closely linked to spatial patterns of tidal movements and includes the presence of mutually evasive flood and ebb channels sedimentologically bars are neither homogenous nor random bodies instead granulometric parameters such as sorting and percentage mud vary systematically as shaped by hydro geomorphic controls the best sorted coarsest ooids are on bar crests whereas the finest grains are found in the lower energy deeper interior and flanking regions in short results clearly document hydrodynamic bathymetrical influences on these ooid shoals and their granulometry linkages akin to siliciclastic analogues sedimentological hydrodynamic and geomorphic observations are consistent with a conceptual model for the formation of parabolic bars in which initial irregularities in nonparabolic bars are enhanced through their effect of focusing flow constricted flow leads to higher flow velocities tidal flow velocity asymmetries differential net sediment transport and growth of bathymetrical highs this bathymetrical divergence creates separate paths for flood and ebb tides facilitating emergence of better developed parabolic forms the resultant parabolic geometries and component bedforms appear to be either in dynamic equilibrium with both ebb and flood tide flows or evolving toward that state in exploring patterns and processes within carbonate shoals this study illustrates some of the first documented insights on quantitative details of morphology and dynamics and in the links between geomorphic framework and grain size and sorting in an oolitic carbonate system assuming a

the hydrogeology of the ballimore region central new south wales australia an integrated study the integrated use of geophysical geological hydrogeochemical and hydrogeological data has allowed the development of a plausible conceptual model for groundwater flow in the ballimore region a realistic model for this under explored system could not be derived solely by the use of hydrogeological data interpretation of the available datasets indicates that two groundwater systems are active a regional and a local system these are separated by a regionally extensive aquiclude groundwater flow in the regional groundwater system is controlled by the structural fabric of the palaeozoic basement rocks the local groundwater system is restricted to the permian to recent sequence of cover rocks the local groundwater system is subdivided into three cells the deep intermediate and shallow cells groundwater flow within the deep cell of the local groundwater system is controlled by fracture flow groundwaters from this aquifer are under artesian pressure and are effervescent co2 gas the intermediate cell is a leaky aquitard that acts as a mixing zone between the deep and shallow cells groundwater flow within the shallow cell is controlled by the influx of surface waters which migrate laterally through permeable beds

liquid carbon dioxide of magmatic origin and its role in volcanic eruptions natural liquid carbon dioxide is produced commercially from a 2 5 km deep well near the 4 500 yr old maar volcano mount gambier south australia the carbon dioxide has accumulated in a dome that is located on the extension of a linear chain of volcanic activity a magmatic origin for the fluid is suggested by the geological setting 13cpdb of 4 0 for the co2 where pdb represents the carbon isotope standard and a relatively high 3he component of the contained helium and high 3he c ratio 6 4 x 10 10 the 3he 4he and he ne ratios are 3 0 and 1 370 times those of air respectively the co2 as collected at the earth s surface at 29 5 c and 75 bar expands more than 300 fold to form a gas at 1 atm and 22 c we suggest that liquid co2 or high density co2 fluid the critical point is 31 1 c 73 9 bar of volcanic origin that expands explosively from shallow levels in the earth s crust may be a major contributor to phreatic volcanic eruptions and maar formation less violent release of magmatic co2 into crater lakes may cause gas bursts with equally disastrous consequences such as occurred at lake nyos cameroon in august 1986

sequence stratigraphy and sedimentology of the late triassic tag i blocks 401 402 berkine basin algeria abstract the berkine basin is an intra or pericratonic basin that developed during the middle to late triassic on the margin of the saharan platform the basin lies to the east of the north south trending hassi messaoud ridge which separates it from the oued mya basin to the west these algerian basins lie to the south and east of the network of rift basins that developed in the iberian peninsular and along the margins of the north atlantic seaboard the principal hydrocarbon reservoir is the upper triassic argilo greseux inferieur tag i which in blocks 401a and 402a is carnian to norian in age elsewhere in north africa especially to the south and east there are older triassic formations for example in zarzaitine in the extreme south east of the triassic algerian outcrop anisian vertebrates are documented the tag i sits unconformably on palaeozoic basement rocks and with the basal lower carbonate comprises a laterally and vertically variable sequence which has been sub divided into four depositional sequences sequence 1 an unconformity bounded ephemeral fluvial interval that fills palaeorelief on the hercynian unconformity sequence 2 an initially upward fining and subsequently upward coarsening package of perennial fluvial sandstones and floodbasin shales with thin crevasse splay elements and interfluve palaeosols sequence 3 an erosively based fluvio lacustrine section characterised by fluvial sandstones with associated crevasse sandstones and floodbasin lacustrine shales this sequence is the main hydrocarbon reservoir section and is divided into two main packages 3a and 3b the base of 3b is distinguished by basin wide fluvial incision and the widespread channel sand deposition sequence 4 lower carbonate is a coastal plain and shallow marine system comprising shales sabkha type evaporites and bay fill sandstones the four depositional sequences reflect differences in depositional style resulting from base level shifts tectonics and climate throughout tag i times the overall increase in relative sea level was interrupted by periods of incision which may relate to periods of rifting and erosion of the rift shoulders of the berkine basin the initial valley fill sequence 1 was deposited under relatively arid or semi arid conditions during sequences 2 and 3a perennial fluvial systems with anastomosed channels and floodplain lakes became dominant and the climate increasingly humid at this time a major longitudinal drainage divide developed due to intrabasinal rifting the basin was ultimately flooded by the transgressive systems tract of sequence 4

short term dynamics and maintenance processes of headland associated sandbanks shambles bank english channel uk abstract the short term over a spring tidal cycle dynamics of a headland associated sandbank shambles bank english channel are investigated by means of field measurements synchronous data using an acoustic doppler current profiler adcp and repeated side scan sonar imagery and two dimensional 2d hydrodynamic and sediment transport models the dynamics of the bank are described in terms of along and cross bank velocity components sand transport pathways and bedform asymmetries the results showed the occurrence of a net bedload convergent zone along the crest of the bank as a result of anti clockwise veering of sediment transport towards the crest this transport pattern is suggested to be the result of the dynamic interplay between two distinct tidally related processes acting over the sandbank during each phase of the tidal cycle a the formation and different stages of a transient tidal eddy that drives bedload movement during the flood phase of the tide and b the bottom friction induced by the presence of the sandbank that governs the bedload transport dynamics during the ebb phase of the tidal cycle

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

sand transport in the bristol channel bedload parting zone or mutually evasive transport pathways two different interpretations of sand transport pathways in a macrotidal estuarine system are assessed with respect to published numerical modelling results and current meter data the inclusion of residual currents in numerical models of m2 m4 tidal interactions yields ebb directed maximum shear stresses along the axis of the system with flood directed vectors located inshore it is demonstrated that there is no simple correlation between the directions of maximum bed stress and bedload transport particularly in nearshore zones large residuals may occur in such narrow nearshore zones which are estimated to be only 1 km in width in the bristol channel the available numerical model output field observations and sedimentological data sets from the bristol channel are considered to be explained in terms of the mutually evasive transport model ebb directed sand transport occurs along the central axis of the estuary whilst flood directed transport occurs within narrow inshore zones the bedload parting zone descriptive scheme which calls for ebb directed bedload flux in the absence of any flood directed flux inshore fails to provide a reasonable basis for the interpretation of holocene sand transport pathways and estuarine evolution transport paths around the u k defined previously as bedload parting zones may in many cases be interpreted more completely in terms of the mutually evasive transport model

tidal asymmetry and residual circulation over linear sandbanks and their implication on sediment transport a process oriented numerical study received 12 january 2007 revised 24 july 2007 accepted 17 september 2007 published 22 december 2007 1 a series of process oriented numerical simulations is carried out in order to evaluate the relative role of locally generated residual flow and overtides on net sediment transport over linear sandbanks the idealized bathymetry and forcing are similar to those present in the norfolk sandbanks north sea the importance of bottom drag parameterization and bank orientation with respect to the ambient flow is examined in terms of residual flow and overtide generation and subsequent sediment transport implications are discussed the results show that although the magnitudes of residual flow and overtides are sensitive to bottom roughness parameterization and bank orientation the magnitude of the generated residual flow is always larger than that of the locally generated overtides also net sediment transport is always dominated by the nonlinear interaction of the residual flow and the semidiurnal tidal currents although cross bank sediment transport can occur even in the absence of a cross shore residual flow on the other hand net sediment divergence convergence increases as the bottom drag decreases and as bank orientation increases the sediment erosion deposition is not symmetric about the crest of the bank suggesting that originally symmetric banks would have the tendency to become asymmetric

platform collapse and pelagic seamount facies jurassic development of central tunisia abstract in central tunisia the lower liassic facies consists of peritidal carbonates deposited on a wide platform which has been broken into irregular tilted blocks in the early pliensbachian black shale was deposited in some of the down faulted blocks during the early toarcian oceanic anoxic event the bajocian bathonian deposits are represented by ironstones and hemipelagic ammonite and zoophycos rich marl limestones the upper jurassic is composed by pelagic limestones and debris flow deposits the middle jurassic deep marine sedimentation was frequently associated with condensed sections submarine erosion leading to stratigraphic hiatuses the latter results from the interaction between global sea level fluctuations and regional tectonic activity generating an uplifted block in central tunisia in relation with early mesozoic rifting and associated distensional drifting of the north african tethyan margin this horst block has acted as a starved pelagic shoal on which sedimentation has been recorded especially during retrogradational and the aggradational phases

on the formation of sand banks of finite extent we consider a model where the fluid depth depends on both horizontal coordinates quasi steady depth uniform non divergent fluid flow is governed by inertial pressure and bottom frictional forces sand transport is proportional to the cube of the instantaneous current but augmented by a down slope component and by wind wave action and sand is conserved it is found that low parallel banks grow fastest so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate the inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump a small bump can evolve to an equilibrium bank typically after an initial rapid extension across the tidal currents provided that sand is sufficiently restricted and particularly if some wind wave action prevents growth up to the sea surface sand banks are likely to be in a late stage of evolution when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition the equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents or when sand is abundant

modeling the effect of nonuniform sediment on the dynamics of offshore tidal sandbanks tidal sandbanks are large scale bed features present in many shallow shelf seas here we investigate the effect of nonuniform sediment on their dynamics with a particular aim to explain observed surficial grain size variations over tidal sandbanks from a process based modeling perspective to this end we use a linear stability analysis that describes the positive feedback mechanism between hydrodynamics sediment and the seabed responsible for sandbank formation on a horizontal shelf in this model the sediment transport and bed evolution modules are extended by introducing an active layer and a bimodal sediment mixture we include a dynamic hiding exposure description of sediment transport enhancing the transport of coarse grains and inhibiting the transport of finer grains the model results show that for symmetrical tidal conditions coarse grains tend to accumulate at the bank crests moreover the growth rates of the perturbations increase compared to the case of uniform sediment while the preferred wavelength and bank orientation remain unchanged for asymmetrical tidal conditions we find a spatial phase shift between topography and the mean grain size fraction indicating an accumulation of coarse grains on the lee side of the bank the model results qualitatively agree with observations from banks on the belgian continental shelf

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

the relationship of sediment transport rates and paths to sandbanks in a tidally dominated area off the coast of east anglia u k sand transport measurements of bedload and suspended load in the sizewell dunwich banks area east anglia have shown that the suspended mode is dominant the depth integrated spring tidal residual is 5 66 g cm 1 sec 1 although the neap rate is only one fifth of this the calculated bedload transport rates also vary from 0 012 to 0 040 g cm 1 sec 1 correlating with changing meteorological conditions in order to predict the bedload sediment circulation pattern from midwater current meter measurements five sediment transport equations were calibrated using fluorescent dyed sand yalin s relationship gave the best estimates the bed shear stress was determined by extrapolating the velocity profile as a power law relationship with an exponent equal to 0 1 from midwater down to 2 m and as a lognormal profile from 2 m to the sea bed roughness length values appropriate to the substrate were used although bedload transport residuals are mainly to the south the banks trend northwards from the coast and have also elongated in this direction this is thought to be in response to the dominance of the suspended sediment transport it is suggested that a tidal residual eddy mechanism is responsible for the banks maintenance similar to the process operating in start bay devon the well documented westward movement of the banks is likely to be related to wave processes

saline groundwater seepage zones and their impact on soil and water resources in the spicers creek catchment central west new south wales australia saline seepage zone development and hence dryland salinity is a major environmental problem which many arid to semiarid landscapes in australia are experiencing due to the geological complexity of the regional aquifer system and the heterogeneous nature of the local groundwater system each groundwater seepage zone in the spicers creek catchment central west new south wales australia possesses different mechanisms which control its development saline seepage zones have formed adjacent to a fault zone and two experimental sites were established through these groundwater discharge zones to understand geochemical processes which have led to the development of soil sodicity gully erosion and the flushing of salts into the surface water systems seepage zone groundwaters contain a distinctive geochemical signature with elevated concentrations of na cl hco3 ca sr b as and li the mixing of deep saline groundwaters together with ion exchange processes lead to a distinctive seepage zone groundwater chemistry being developed altering the landscape features within this rural groundwater system has developed water toxicity for crops soil sodicity leading to land degradation and waterlogging problems

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

seafloor response to flow in a southern hemisphere sand ridge field argentine inner shelf abstract the inner continental shelf adjacent to buenos aires province argentina is characterized by a series of sand ridges averaging 4 7 m in relief and spaced about 2 7 km apart the ridges trend north south forming a 20 to 35 southward opening angle with the northeast southwest trending coast although the adjacent coast is of depositional strand plain origin it is presently undergoing erosional retreat and the formation of the ridges appears to be part of the retreat process seafloor grain size distributions and bedform arrays are responses to the same velocity field that has formed the ridges along transects normal to the ridges grain size variation is 90 out of phase with the topography the coarsest sediment is not at ridge crests or trough axes but instead occurs on the landward slope finest sands are on the seaward slope side scan sonar reveals bedforms aligned obliquely to both ridges and the coast these are inferred to be sand waves the grain size and bedform patterns suggest that the formative flows are northward currents with an offshore flow component near the bottom although flows observed during the study period were dominantly north to south the grain size gradients and bedform orientations are equivalent to those characteristic of the atlantic shelf of north america with a north south inversion the comparison suggests that inner shelf transport directions in response to wind events can be predicted from the regional geometry and climate of the shelf

co2 rich gases from lakes nyos and monoun cameroon laacher see germany dieng indonesia and mt gambier australia variations on a common theme abstract helium r a 3 0 to 5 6 and carbon 13 c from 7 2 to 3 4 isotopic compositions and relative co 2 ch 4 n 2 he and ar contents of co 2 rich gases from lakes nyos and monoun cameroon laacher see germany dieng volcanic plateau indonesia and a well at mt gambier australia point to a common essentially magmatic origin absorption of the original magmatic gases into deeply circulating groundwater and equilibration of the resulting solutions with crustal rock at temperatures of about 300 c fix co 2 and ch 4 contents on further rise the solutions start to boil separating gas rich vapors which on encountering an impermeable barrier may accumulate to form gas pockets with steadily increasing pressures in the case of sufficiently high gas contents the pressures may exceed lithostatic pressures leading to a blow out or a pneumatic eruption dieng otherwise gas may accumulate to form a stable pocket mt gambier minor leakage from such pockets may lead to surface discharges of co 2 rich gases as at laacher see re absorption into shallow groundwater to the formation of the low salinity co 2 charged waters encountered at lakes nyos and monoun the occurrence of these high co 2 low temperature systems is likely to be favored in tectonically active regions allowing deep possibly mantle gases to rise but with sufficiently low regional heat flows to prevent the establishment of large scale geothermal activity

flow induced by upwelling winds in an equatorward facing bay gulf of arauco chile 1 shipborne observations of hydrographic and flow velocity profiles were combined with wind velocity measurements to describe the characteristics of the wind induced flow in an equatorward facing bay of central chile in south america the measurements which were taken from two transects and one anchor station were made during late austral spring between 4 and 10 december 2000 most observations concentrated on boca grande a transect that crossed the deep northern equatorward entrance to the bay the other transect crossed the smaller and shallower westward entrance to the bay boca chica the anchor station was located inside the bay close to boca chica the period of observations was characterized by persistent upwelling winds of up to 17 m s the direction of the wind showed very little variability but its magnitude exhibited well defined diurnal fluctuations wind speeds peaked at around sunset 2100 local time which coincided with the deepest pycnocline location the shallowest pycnocline depth appeared a couple of hours after sunrise these pycnocline oscillations had amplitudes of 5 m and were believed to propagate poleward along the coast and dissipate at the shallow entrance in boca chica this mechanism could have mixed low oxygen nutrient rich waters into the photic zone and enhanced the primary productivity observed flows at the boca grande transect consisted of downwind flow over the shallow ends and upwind flow in the deepest part this wind induced pattern is believed to cause a pair of counterrotating gyres that allow near surface divergence and ventilation of upwelled subsurface waters at the southern end of the bay this is the region where the highest concentrations of chlorophyll are identified in color imagery

low sea level tidal origin of celtic sea sand banks evidence from numerical modelling of m2 tidal streams abstract numerical modelling of m 2 tidal streams in the celtic sea with a sea level lowered by 100 m indicates tidal currents about twice as strong as those at present such currents would have been sufficient to generate and maintain the linear tidal sand banks now found in that region as moribund bedforms furthermore an indicated clockwise rotation of the major axes of the tidal ellipses at a sea level lowered by 100 m with respect to the present day is in the correct sense to further the comparison with modern examples of actively maintained linear tidal sand banks

large scale seabed dynamics in offshore morphology modeling human intervention we extend the class of simple offshore models that describe large scale bed evolution in shallow shelf seas in such seas shallow water flow interacts with the seabed through bed load and suspended load transport for arbitrary topographies of small amplitude we derive general bed evolution equations the initial topographic impulse response initial sedimentation and erosion patterns around an isolated feature on a flat seabed provides analytical expressions that provide insight into the inherent instability of the flat seabed the coriolis induced preference for cyclonically oriented features and bed load transport being a limiting case of suspended load transport the general evolution equation can be used to describe sandbank formation known as the result of self organization examples of human intervention at the seabed include applications to a dredged channel and an offshore sandpit an outlook toward future research is also presented

residual transport model in correlation with sedimentary dynamics over an elongate tidal sandbar in the gironde estuary southwestern france abstract this study qualitatively describes by a multidisciplinary approach hydrodynamic and residual sediment transport over a sandbar in a macrotidal environment the sandbar is elongated and attached to a headland that is located in the gironde estuary southwestern france bathymetric surveys side scan sonar sonograms grain size trends of surficial sediment and 2dh numerical simulations of current patterns were used to define a conceptual model for the dynamics of this sandbar the sandbar is oriented anticlockwise with respect to the main flow the channel oriented bar face is ebb dominated whereas the opposite face is flood dominated resulting in a clockwise residual transport the line of sediment flux convergence corresponds to the crest of the bar the residual flow over the bar appears to originate from the regional estuarine flow the dominant process responsible for the maintenance of the sandbar seems to be the depth averaged main tidal flow rather than secondary circulation due to headlands or the interaction between the tidal flow and the seabed to generalize this regional conceptual model a qualitative comparison is made with the theories of sandbar dynamics due to tidal currents pingree and maddock 1979 zimmerman 1981 huthnance 1982 and others the seabed stability approach developed by huthnance 1982 seems to be the most reliable theory to explain the saint georges bank dynamics even though many natural factors are not taken into account

morphology and internal structure of sand shoals and sandbanks off the dorset coast english channel the morphology and internal structure of sand shoals and sandbanks around a coastal headland portland bill southern uk are described on the basis of sidescan sonar and high resolution seismic data sets morphological and architectural evidence combined with the spatial distribution and nature of the bedrock surface indicates that the evolution of these deposits especially the sandbanks may not only be hydrodynamically controlled but also morphologically controlled the internal structure of the sand shoals reveals a simple pattern of clinoforms dipping in the same direction as their steeper profile in contrast the internal structure of the sandbanks reveals that their lower seismic units consist of a sedimentary core interpreted as the remains of a lowstand deposit or an early stage of the bank development under different prevailing hydrodynamic conditions the present morphology of the sandbanks is represented by upper seismic units characterized by large scale sandwave foresets dipping at 6 8 the nature and morphology of the bedrock surface may also influence the development of sandbanks in some cases for example to the west of portland bill the morphology of the bedrock surface basal reflector restricts the development of the portland bank this information may explain the asymmetric evolution of sandbanks around portland bill

architecture and long term evolution of a tidal sandbank the middelkerke bank southern north sea abstract the internal structure of the middelkerke bank one of the flemish banks located in the southern north sea off the coast of oostende belgium has been studied in the framework of the marine science and technology mast program co funded by the european community a dense grid of high and very high resolution seismic profiles has been used as well as several vibrocorings seven major seismic units can be identified in the quaternary sediments bounded by major discontinuities correlated across the whole study area the lower units clearly appear as being deposited during periods of relative low sea level channel infillings shoreface estuarine and or ebb tidal delta deposits the present shape of the bank results partly from recent erosional processes reworking the underlying deposits thus the lower part of the bank as a morphological feature does not consist of offshore tidal sands the master bedding of the upper part of the bank consists of inclined reflectors dipping at an angle of about 5 in the same direction as the bank s steep face these reflectors very similar to those described by houbolt 1968 are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms

sand transport under the combined influence of waves and tidal currents an assessment of available formulae abstract various formulae are used to predict the transport of sand under the combined influence of waves and currents these approaches include the use of the unidirectional formulae of einstein 1950 frijlink 1952 yalin 1963 engelund and hansen 1967 sternberg 1972 ackers and white 1973 and gadd et al 1978 modified for the presence of waves in accordance with the techniques described by bijker 1967 and swart 1976 and those of bagnold 1963 madsen and grant 1976 and vincent et al 1981 predicted rates are compared with measurements based upon fluorescent sand tracer studies in an area of high tidal current and wave energy northern bristol channel u k in general predicted rates are lower than those measured and only the formulae of madsen and grant 1976 and the modified formula of sternberg 1972 appear to provide realistic estimates of transport under wave and tidally induced currents in comparison with the field measurements

spatial variability of cave air carbon dioxide and methane concentrations and isotopic compositions in a semi arid karst environment there is insufficient information on the movement of air in karst environments to constrain the uncertainty associated with quantifying sources and sinks of methane ch4 and carbon dioxide co2 within caves for global carbon accounting we analysed cave air samples for their co2 and ch4 concentrations co2 and ch4 and carbon isotopic compositions from sampling campaigns in winter august 2014 and summer february 2015 at numerous heights and locations throughout gaden and cathedral caves in a semi arid environment wellington caves nsw australia ventilation is the dominant control on cave air co2 and ch4 with the highest cave air co2 concentrations co2 cave occurring in summer in association with the lowest cave air ch4 concentrations ch4 cave analyses show that the cave air co2 has both atmospheric and soil sources soil air and cave air in both caves undergo methanogenesis and methanotrophy but we identify cave specific differences in cave air ch4 and co2 ch4 cave in cathedral cave shows an inverse relationship to co2 cave particularly in areas separated from the main cave passage in contrast gaden cave has near atmospheric ch4 cave and isotopic ratios present at all locations sampled in winter where no ventilation is occurring in summer ch4 cave in gaden cave decreases but remains reasonably high compared to cathedral cave our research shows adjacent caves vary in their ability to act as a net sink for ch4 and highlights the need for further studies before global generalisations can be made about the carbon budget of karst environments

migration in relation to possible tectonic and regional controls in eastern australian volcanism abstract the wellman mcdougall model for southward migration of central volcano activity in eastern australia is extended to the basaltic lava provinces latitude age plots of volcanic episodes are related to trails initiated from regions of active volcanism at the commencement of northward drift of australia 53 m y b p from southern ocean spreading these trails intersect at least 75 and possibly up to 95 of basaltic episodes and suggest a migratory control the migration of central volcano felsic activity however consistently exceeds sea floor spreading rates with a relative southward motion of 4 10 mm yr the trails give a mean migration direction of 24 9 w of s before 29 30 m y but 8 12 w of s after 29 m y extrapolation of the central volcano activity back in time places it across the coral sea louisiade spreading system and new guinea collision zone at the beginning of drift this suggests that these structures may provide residual thermo tectonic anomalies in the mantle with potential to initiate volcanism under the moving australian plate kinematic reconstruction of australia s drift relative to these structures shows that the western felsic and k rich ultramafic activity falls on progressive overlaps of the coral sea louisiade anomalies the basaltic volcanism generally overlies the central regions of the anomalies the young basalt fields of victoria south australia correspond with positions of the coral sea louisiade anomalies over the last 5 m y the regions of youngest volcanism conductivity anomaly and high heat flow lie near the present projected position of the anomalies some volcanic episodes are not obviously related to migration trails the young basaltic activity in northern queensland may result from movement of the australian plate towards unstable regions of volcanism and uplift in papua new guinea central volcano province spacings appear to match expected lithospheric thickness spacings of basaltic episodes suggest that other factors such as zonal rotational flow in the asthenosphere may also be involved cessation of much volcanism over the eastern australian highlands about 10 m y ago is attributed to imposition of the present regional compressive stress field probably as a result of changes in the sea floor spreading patterns an apparent change in migration direction of volcanism 29 30 m y ago may be related to changes in east west spreading and continental motions adjacent to the australian plate differences between migration directions of volcanism and absolute motions determined for the australian plate may reflect effects of the earth s rotation and migration of spreading ridges the migratory model developed here predicts future australian volcanism mostly in or south of the present regions of younger volcanics and high heat flow it would be basaltic but may be accompanied by felsic activity in tasmania

dynamic sedimentation and geomorphologic evolution of the laizhou shoal bohai sea northern china abstract as a headland associated sandbank off an unconsolidated coast cape in a progression process the laizhou shoal is just located at the coalescent position of the two tidal residual current circumferences its formation and maintenance dynamics sedimentation and geomorphology are different from the models proposed up till the present moment our findings indicate that 1 the tidal residual tidal current circumferences are located along the flanks of the shoal itself instead of the flanks of the headland cape and the sediments under the two circumferences are quite different from those at their coalescent position 2 the sediments made up of the shoal have an ultimate resource of river sediment instead of an erosion recession cape or seafloor nearby and are transported directly from the coastal zone through the wave resulted coastal current rather than through the tidal residual tidal eddies 3 the shoal continually migrates from southwest to northeast extending into the laizhou bay northwest the cape in the past 50 years 4 because more than 100 reservoirs have been built across the rivers in the northwestern shandong peninsula and have greatly reduced the sediments especially gravel and sand grains transported into the coastal zone some locations of the shoal have evolved from deposition to erosion and the shoal itself has become narrower but has not evolved from deposition to erosion as a whole

mid triassic to early liassic clastic evaporitic deposits over the maghreb platform abstract the development of triassic to lower liassic clastic evaporitic series over the epicratonic maghreb platform is closely associated with the eastern opening of a tethyan marine domain between africa and europe west of the platform morocco became separated from north america in late triassic times by rifting along the axis of the proto atlantic ocean in addition ne sw and ene wsw trending atlasic half grabens formed essentially in morocco as part of a late triassic early liassic atlas rifting episode this is the tectonic context in which the red bed to evaporite sequences were deposited a first depositional pattern is illustrated by the areally extensive onlapping of upper triassic clastic evaporitic series over the algerian saharan platform from the carnian to the norian distal mudflats and evaporites graded westward i e landward into proximal fluvial facies seven stratigraphic cycles formed part of a general base level rise of a major retrogradational clastic to evaporitic cycle indicative of general tectonic subsidence of the algerian platform as a whole the evaporites are thus interpreted as markers of persistent onlapping occurring at the edge of the basin as clastic influxes gradually diminished a second depositional pattern is provided by ladinian to rhaetian clastic evaporitic carbonate transgressive sequences of the atlasic domain in tunisia these terminate characteristically in marine facies in this domain intense tectonic activity was consistently offset by a high rate of evaporitic or clastic sedimentation in shallow water environments a third depositional pattern is illustrated in the moroccan high atlas in the central part of this range carnian red conglomerates and sheet like sandstones of a distal braided fluvial plain were deposited in tectonic troughs that linked together as part of a rift system episodic connections occurred with the tethyan domain to the northeast in the western high atlas an upper carnian norian sequence several hundred meters thick displays a general trend from clastics to evaporites reflecting the development of arid paleoclimatic conditions the moroccan meseta provides examples of clastic evaporitic depositional patterns within intracratonic troughs clastic filling of rift induced troughs in the oujda mountains exhibits significant facies and thickness variations ranging from local proximal alluvial and distal anastomosed channel systems through to dominant mudflat systems in the 1000 m thick series of the 60 km long khemisset basin facies range from red beds located mainly along the basin margins to salt dominated deposits in the basin center increasing accommodation controlled the deposition of evaporites and the maximum flooding surface is to be found in typical halite deposits with interbedded bands of clays at that time greater tectonic subsidence rates allowed more marked episodic marine incursions clastic evaporitic series from the passive margin of the moroccan atlantic coast are known only from bore holes and onshore and offshore seismic data recent papers have reported interesting sections across strongly subsiding grabens or tilted blocks related to predominantly ssw nne faults

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

sedimentary processes and depositional history of a cape associated shoal cape lookout north carolina abstract the sedimentary processes of a cape associated shoal are an integral component of the sediment budget of the surrounding cuspate foreland shoreline the manner in which sediment is delivered to a shoal and the fate of this sediment once delivered have important implications to shoreline management and yet remain largely unstudied modern sedimentary processes and the recent depositional history of cape lookout shoal a large cape associated shoal in the mixed energy environment of the north carolina continental shelf were examined in a field intensive study through high resolution seismic profiles hydraulic probes near bottom current meters sediment grab samples and detailed bathymetric surveys our findings indicate that 1 the sediment budget of the up drift littoral cell is coupled directly to cape lookout shoal 2 the sedimentary processes of the shoal remain active down its entire length but at a diminishing level with distance from the shore and 3 the shoal serves as a long term sink for littoral zone sediment and limits sediment exchange between adjacent littoral cells and shelf regions we present evidence suggesting that the position and morphology of cape lookout shoal are not controlled by underlying erosion resistant strata and that the shoal appears to have developed in the late holocene after the shelf was scoured by the transgressing shoreface

water and sediment movement in the vicinity of linear sandbanks the norfolk banks southern north sea geophysical surveys current meter measurements and fluorescent sand tracer experiments were undertaken in the vicinity of the broken bank the data are used to establish localised and regional patterns of water and sediment movement the results obtained confirm mechanisms proposed previously for sandbank growth with particular reference to the north sea these include the clockwise circulation of water and sand with convergence at the crestline secondary helical water circulations do not appear to be a dominant process influencing sandbank maintenance however such circulation may still be important in the initial formation of linear sandbanks and or at some stages in the bank development on a regional basis residual currents bedform asymmetries and tracer dispersion indicate an offshore sediment transport component this observation supports the hypothesis of other investigators formed on the basis of geomorphological observations and numerical model predictions concerning the movement of sand under extreme conditions moreover these transport pathways may persist under low wave and tidally dominated conditions as opposed to being restricted to storm activity

comparisons of measured and predicted sediment transport rates in tidal currents abstract this paper describes bedload transport measurements in a large scale sedimentary system on the british continental shelf steady flow sediment transport theories are applied to oscillatory tidal currents and various equations are compared with measured transport rates from tracer dispersion experiments the effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data

a model for sand ridges on the shelf effect of tidal and steady currents the dynamics of sand ridges as observed on the innershelves and midshelves of various coastal seas are investigated within the framework of an idealized model these seabed undulations include shoreface connected sand ridges and tidal sand banks the new aspects in this study are the explicit incorporation of both steady and tidal currents a shelf with a sloping bottom and a statistical approach to describe the sediment transport during storms linear in the current and fair weather conditions cubic in the current the present model is used to gain understanding of the different ridge characteristics in different geographical regions this is done by analyzing the initial growth of small bottom perturbations which evolve on a basic state describing a longshore uniform flow over a reference topography it is found that basically five different bottom modes can be excited shoreface connected sand ridges coriolis ridges friction induced bars trapped tidal ridges and tidal sand banks their growth which is due to different physical mechanisms strongly depends on the weather climate and the geometrical characteristics of the shelf

sand waves and sediment transport around the end of a tidal sand bank two detailed surveys have been made of the north western end of the haisborough sand off the norfolk coast using echosounder 3 5 khz reflection profiler and side scan sonar asymmetrical sand waves indicate north westerly sand movement on the southern side and south easterly sand movement on the steeper northern side of the bank secondary superimposed megaripples which are probably better indicators of sediment movement give evidence of a cross bank component between the north westerly and south easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves these are usually taken to indicate zero net transport but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests this suggests the route by which sand travels around the end of the bank to form a roughly closed circulation sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing though that could allow more than 0 25 km shift to the north east in 100 years to pass undetected a box model is drawn up for the estimated sediment fluxes around the end of the bank and implications for residence times and circulation rates are drawn from it