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

long term sediment mobilization at a sandy inner shelf site leo 15 1 nearly 2 years of wave and current observations are used to drive a calibrated bottom boundary layer model to examine sediment transport at the long term ecosystem observatory leo 15 located off of the southern coast of new jersey the multiyear record is of sufficient resolution to categorize sediment mobilization characteristics such as storm duration frequency of occurrence seasonal trends and modes of transport at this shallow water sandy site a total of 51 sediment transport events are identified within the 2 year timeframe thirty two are categorized as winter events and wintertime sediment transport constitutes 63 of the total for the 2 year period in nearly all cases bed load transport dominates the suspended load the majority of the bed load transport is onshore due to wave asymmetries that lead to higher orbital velocities during the forward half of the wave cycle for the remainder of events a bimodal wave spectrum is suspected to reverse the orbital velocity skewness leading to offshore transport suspended sediment transport is primarily directed alongshore toward the southwest consistent with current patterns during northeasters by including bed load transport the net sediment transport for the 2 year period is directed primarily onshore and slightly alongshore toward the southwest

glider observations of the dotson ice shelf outflow abstract the amundsen sea is one of the most productive polynyas in the antarctic per unit area and is undergoing rapid changes including a reduction in sea ice duration thinning ice sheets retreat of glaciers and the potential collapse of the thwaites glacier in pine island bay a growing body of research has indicated that these changes are altering the water mass properties and associated biogeochemistry within the polynya unfortunately difficulties in accessing the remote location have greatly limited the amount of in situ data that has been collected in this study data from a teledyne webb slocum glider was used to supplement ship based sampling along the dotson ice shelf dis this autonomous underwater vehicle revealed a detailed view of a meltwater laden outflow from below the western flank of the dis circumpolar deep water intruding onto the shelf drives glacial melt and the supply of macronutrients that along with ample light supports the large phytoplankton blooms in the amundsen sea polynya less well understood is the source of micronutrients such as iron necessary to support this bloom to the central polynya where chlorophyll concentrations are highest this outflow region showed decreasing optical backscatter with proximity to the bed indicating that particulate matter was sourced from the overlying glacier rather than resuspended sediment this result suggests that particulate iron and potentially phytoplankton primary productivity is intrinsically linked to the magnitude and duration of sub glacial melt from circumpolar deep water intrusions onto the shelf

accelerated relative sea level rise and rapid coastal erosion testing a causal relationship for the louisiana barrier islands abstract the role of relative sea level rise as a cause for the rapid erosion of louisiana s barrier island coast is investigated through a numerical implementation of a modified bruun rule that accounts for the low percentage of sand sized sediment in the eroding louisiana shoreface shore normal profiles from 150 km of coastline west of the mississippi delta are derived from bathymetric surveys conducted during the 1880s 1930s and 1980s an rms difference criterion is employed to test whether an equilibrium profile form is maintained between survey years only about half the studied profiles meet the equilibrium criterion this represents a significant limitation on the potential applicability of the bruun rule the profiles meeting the equilibrium criterion along with measured rates of relative sea level rise are used to hindcast shoreline retreat rates at 37 locations within the study area modeled and observed shoreline retreat rates show no significant correlation thus in terms of the bruun approach relative sea level rise has no power for hindcasting and presumably forecasting rates of coastal erosion for the louisiana barrier islands

late cretaceous sediment sample from the amirante passage western indian ocean erosion associated with deep thermohaline flow through the amirante passage western indian ocean has caused upper cretaceous sediments to become exposed at the sea floor the age of a core of these sediments is the strongest direct evidence to date with which to support interpretations of the sea floor spreading history of the area based on geophysical data

evidence for the influence of form drag on bottom boundary layer flow posed of two intersecting segments smith 1977 and smith and mclean 1977 demonstrate that multiple roughness scales can generate velocity profiles with more than two segments thus when form drag is important the constantstress assumption will not be valid in an earlier paper caldwell and chriss 1979 we demonstrated the existence of the viscous sublayer in the bottom boundary layer and found a logarithmic layer above it in examining additional data from the experiment segmented profiles were found in the logarithmic region figures 1 and 2 as expected if form drag influences the flow when the original data set was reanalyzed using thinner averaging intervals in the upper portion of the profile and also incorporating both upward and downward traverses it too shows two distinct logarithmic slopes in the original study only downward traverses were used although deviation from a single logarithmic form is not necessarily large the slope of the logarithmic regression is significantly different in the two regions implying that the turbulent stress above is significantly larger than it is nearer the bed

motion between the indian antarctic and african plates in the early cenozoic we used a three plate best fit algorithm to calculate four sets of euler rotations for motion between the india capricorn africa somali and antarctic plates for 14 time intervals in the early cenozoic each set of rotations had a different combination of data constraints the first set of rotations used a basic set of magnetic anomaly picks on the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir and fracture zone constraints on the cir and seir but did not incorporate data from the carlsberg ridge and did not use fracture zones on the swir the second set added fracture zone constraints from the region of the bain fracture zone on the swir which were dated with synthetic flowlines based on the first data set the third set of rotations used the basic constraints from the first rotation set and added data from the carlsberg ridge the fourth set of rotations combined both the swir fracture zone constraints and the carlsberg ridge constraints data on the indian plate side of the carlsberg ridge arabian basin were rotated to the capricorn plate before being included in the constraints plate trajectories and spreading rate histories for the cir and swir based on the new rotations document the major early cenozoic changes in plate motion on the cir and seir there was a large but gradual slowdown starting around chron 23o 51 9 ma and continuing until chron 21y 45 3 ma followed 2 or 3 myr later by an abrupt change in spreading azimuth which started around chron 20o 42 8 ma and which was completed by chron 20y 41 5 ma no change in spreading rate accompanied the abrupt change in spreading direction on the swir there was a continuous increase in spreading rates between chrons 23o and 20o and large changes in azimuth around chrons 24 and 23 and again at chron 21 unexpectedly we found that the two sets of rotations constrained by the carlsberg ridge data diverged from the other two sets of rotations prior to anomaly 22o when compared to rotations for the cir that are simultaneously constrained by data from all three branches of the indian ocean triple junction there is a progressively larger separation of anomalies on the carlsberg ridge with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y these data require that there was previously unrecognized convergence somewhere in the plate circuit linking the indian capricorn and somali plates prior to chron 22o we quantify this motion by summing our new capricorn somalia rotations with previously published rotations for neogene india capricorn motion and for early cenozoic somali india motion based solely on carlsberg ridge data the most likely possibility is that there was motion within the somalia plate due to a distinct seychelles microplate as young as chron 22o the sense of the misfit on the carlsberg ridge is consistent with roughly 100 150 km of convergence across a boundary passing through the amirante trench and extending north to the carlsberg ridge axis between anomalies 26y and 22o alternatively there may have been convergence within the indian plate either along the western margin of indian or east of the cir in the region of the current capricorn indian diffuse plate boundary our work sharpens the dating of the two major eocene changes in plate motion recognized in the indian ocean

hydrodynamics and sediment dynamics of north sea sand waves and sand banks and discussion seabed drag coefficients have been measured at a site within the norfolk banks and at a site within the sand wave field in the southern north sea using pressure sensors and moored current meters at the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used the results are generally in agreement with values used in numerical models at the sand banks site the drag coefficient increases with wind conditions but at the sand waves site there is a reduction during the highest wave conditions attributed to the drag reduction caused by sand resuspension from the bed this result suggests that sediment effects must be considered if wave current interaction is included in numerical models of the region the drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face suggesting a small form drag component studies of sand movement used bedform mapping fluorescent sand tracing and photography of migrating ripples there is evidence for the early stages of formation of a new bank between broken bank and well bank at the sand waves site ripple migration is found to be a useful estimator of bedload transport under conditions of negligible suspension over the sand banks a definite correlation of surface water properties notably the turbidity with the topography of the banks was observed the mechanism for this surface effect is not known but it is consistent with the observation of features seen in satellite visible light images which outline the shapes of the banks

on the calculation of suspended load for noncohesive sediments calculation of suspended sediment transport rates requires integration of the product of sediment concentration times velocity over the depth of the flow hence accurate profiles of these quantities are required especially near the sediment bed although this would appear to be a straightforward task many effects can cause significant deviation of these quantities from those values expected from simple known relationships for example density stratification by suspended particulates can dampen turbulence fluctuations and significantly reduce the effective viscosity and diffusivity of the flow the momentum and suspended sediment equations are solved herein including the effects of stratification by suspended sediment and the effects due to size distribution and bedforms the results show that stratification and bedforms generally reduce the transport capacity of the flow whereas treating the sediment as an assemblage of different sizes rather than a single size d50 increases the expected transport

a suspended sediment stratification correction for combined wave and current flows a simple model for the near bottom combined wave and current flow over a moveable sediment bed is presented the model is an extension of the grant and madsen 1979 combined wave and current model with moveable bed effects on the physical bottom roughness included as by grant and madsen 1982 the unsteady conservation of fluid momentum and sediment mass equations which are coupled through an eddy diffusivity closure scheme are solved for the wave and current velocity profiles along with the suspended sediment concentration and transport profiles sample runs are presented to illustrate the effect of varying wave conditions on the current the mean sediment concentration and the mean sediment transport results show that for fine to medium sands self stratification of the flow can be important during storms owing to the large amount of sediment suspended by the waves and the enhanced turbulent mixing associated with the wave current interaction the theory easily can be generalized for inclusion in a full ekman layer model

modeling bottom roughness in the presence of wave generated ripples 1 near bottom wave current and microtopography measurements have been obtained at the leo 15 site off the southern coast of new jersey the measurements are used in conjunction with a recently calibrated continental shelf bottom boundary layer model to refine estimates of bottom roughness for a rippled bed under combined wave and current flows in the presence of nearly two dimensional wave generated ripples application of the grant and madsen 1979 type bottom boundary layer model produces estimates of the bottom roughness felt by the current that are several times greater than established formulations for rippled beds under waves modifying one of the primary length scales in the eddy viscosity profiles returns estimates that are more consistent with the wave roughness results application of the roughness model in conjunction with a refined empirical ripple geometry model produces highly accurate estimates of measured shear velocities and apparent roughnesses during a moderate storm the demonstrated accuracy of this simple combined roughness and ripple geometry formulation reveals its utility when considering applications of coupled bottom boundary layer shelf circulation models

a graptolite rich ordovician silurian boundary section in the south central pyrenees spain stratigraphical and palaeobiogeographical significance an ordovician silurian boundary section marked by an uninterrupted relatively high rate of black shale sedimentation and abundant diverse graptolites is described from the south central pyrenees the structurally simple estana section comprises the uppermost part of the quartzite dominated bar formation and overlying black shales of late hirnantian and early rhuddanian age which have been dated by graptolites to the upper metabolograptus persculptus and lower middle akidograptus ascensus parakidograptus acuminatus biozones due to the absence of m persculptus a metabolograptus parvulus biozone correlative with the upper part of the persculptus biozone is recognized below the lowest occurrence of akidograptids which indicate the base of the silurian system the graptolite fauna comprise 27 species including normalograptus minor n lubricus n rhizinus hirsutograptus korenograptus bifurcus k bicaudatus k lanpherei and nd shanchongensis most of which were formerly considered to be endemic to the low latitude palaeobiogeographical province of china siberia and northern north america two new species n baridaensis and n ednae are described the succession of graptolite assemblages in the estana section and occurrence of several cosmopolitan taxa in its parvulus and lower ascensus acuminatus biozones that are unknown elsewhere in peri gondwanan europe suggest that strata immediately surrounding the ordovician silurian boundary may be absent highly condensed or oxic and barren of graptolites in other sections of northwestern peri gondwana common graptolite synrhabdosomes and abnormal rhabdosomes may indicate some environmental stress in the parvulus biozone although the rather uniform black shale lithology total organic carbon content and 13 c org values suggest uninterrupted sedimentation under stable anoxic conditions

the tectonic setting of the seychelles mascarene and amirante plateaus in the western equatorial indian ocean abstract a granitic crustal fragment occurs within a system of the marine plateaus in the western equatorial indian ocean the plateaus form an arcuate series of wide shallow banks with small islands in places in spite of the geographical proximity and the morphological similarity the geological structure and origin of the seychelles bank amirante arc and mascarene plateau contrast sharply the seychelles bank is founded on precambrian granite basement the mascarene plateau from saya de malha bank to mauritius island is founded on paleocene basalts and upper cretaceous basalts prevail in the amirante arc geological and geophysical data suggest that the precambrian granite of the seychelles bank is a fragment separated from the indian continental plate during the paleocene this break up was associated with the volcanic activity that built the mascarene plateau the occurrence of middle eocene syenite diorite and microgranite in the western part of the seychelles bank could indicate that crustal accretion across the northwestern indian ocean ridge apparently drove the seychelles block southwestwards causing it to collide with the northeastern edge of the mascarene basin this collision could have led to an early tertiary subduction and subsequently to the development of the amirante trough the absence of post eocene igneous suites in the seychelles could indicate that a change in the spreading pattern of the northwestern indian ocean during the late eocene probably terminated the amirante subduction

cold front driven storm erosion and overwash in the central part of the isles dernieres a louisiana barrier island arc abstract tropical and extratropical storms produce significant erosion on the barrier islands of louisiana over the past 100 years such storms have produced at least 2 km of northward beach face retreat and the loss of 63 of the surface area of the isles dernieres a low lying barrier island arc along the central louisiana coast elevations on the islands within the arc are typically less than 2 m above mean sea level the islands typically have a washover flat topography with occasional poorly developed dune terrace topography consisting of low lying and broken dunes the central part of the arc consists of salt marsh deposits overlain by washover sands along the gulf of mexico shoreline sand thicknesses range from zero behind the beach to less than 2 m under the berm crest and back to zero in the first nearshore trough the sand veneer is sufficiently thin that storms can strip all the sand from the beach face exposing the underlying marsh deposits the geomorphic changes produced by cold fronts a type of extratropical storm that commonly affect the isles dernieres between late fall and early spring are described between august 1986 and september 1987 repeated surveys along eleven shore normal transects that covered 400 m of shoreline revealed the timing and extent of cold front produced beach change along a typical section of the central isles dernieres during the study period the beach face retreated approximately 20 m during the cold front season but did not rebuild during the subsequent summer because the volume of sand deposited on the backshore 5600 m 3 was less than the volume of material lost from the beach face 19 200 m 3 approximately 13 600 m 3 of material disappeared assuming that underlying marsh deposits decrease in volume in direct proportion to the amount of beach face retreat an estimate of the mud loss during the study period is 14 000 m 3 thus the decrease in volume along the profiles can be accounted for without removing any sand from the area suggesting that a major effect of cold fronts is first to strip the sand from the beach face and then to erode the underlying marsh deposits after being eroded the mud is lost from the islands because currents transport it away from the islands

spatially explicit bathymetric reconstruction from lead line depth soundings of the late 17th century abstract sequences of repeated bathymetric surveys through time have been successfully used to document the morphodynamical evolution of coastal environments within historical times most chronobathymetric sequences have a restricted temporal coverage as surveys rarely date back earlier than the 19th century the aim of this study is to explore the potential of older lead line depth measurements as a reliable source of bathymetric data a late 17th century marine chart of the pertuis charentais france was used as a test case high resolution scans of the chart sheets were retrieved from a public online digital library a rare manuscript contemporary to the chart was obtained from the french national archives in order to constrain the methodology used to acquire depth measurements a spatially explicit bathymetry covering 1500 km2 was reconstructed from the interpolation of about 4600 georeferenced depth soundings vertical error was estimated by assuming the equivalence of depth values between the late 17th century and the recent 1960 bathymetries above subtidal rocks outcropping on the seafloor documentary data indicated that the chart author did not acquire depth measurements on purpose but instead collected data taken by local sailors and coasting pilots over an unknown period of time probably ranging from a few years to a few decades the large vertical error estimated to 4 5 m mainly resulted from the lack of tide correction bathymetric difference maps could not be computed but despite these weaknesses the large scale sediment infilling of the area and the long term evolution of an estuarine sandbank could be documented in agreement with previous works beyond its utility for local studies this article provides insights into the processing of depth measurements acquired long before modern hydrography surveys and demonstrates that such old marine charts deserve consideration provided conditions are met for their critical assessment

the extent of continental crust beneath the seychelles abstract the granitic islands of the seychelles plateau have long been recognised to overlie continental crust isolated from madagascar and india during the formation of the indian ocean however to date the extent of continental crust beneath the seychelles region remains unknown this is particularly true beneath the mascarene basin between the seychelles plateau and madagascar and beneath the amirante arc constraining the size and shape of the seychelles continental fragment is needed for accurate plate reconstructions of the breakup of gondwana and has implications for the processes of continental breakup in general here we present new estimates of crustal thickness and v p v s from h stacking of receiver functions from a year long deployment of seismic stations across the seychelles covering the topographic plateau the amirante ridge and the northern mascarene basin these results combined with gravity modelling of historical ship track data confirm that continental crust is present beneath the seychelles plateau this is 30 33 km thick but with a relatively high velocity lower crustal layer this layer thins southwards from 10 km to 1 km over a distance of 50 km which is consistent with the seychelles being at the edge of the deccan plume prior to its separation from india in contrast the majority of the seychelles islands away from the topographic plateau show no direct evidence for continental crust the exception to this is the island of desroche on the northern amirante ridge where thicker low density crust consistent with a block of continental material is present we suggest that the northern amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup

long range sonar observations of sedimentary facies patterns in and around the amirante trench western indian ocean long range side scan sonographs are used to map two distinct acoustic facies in the amirante trench western indian ocean a structural barrier formed by a seamount within the trench separates the two facies types the acoustic facies developed s of the seamount is characterized by weak reflectivity returned from sediments deposited by the flow of the deep western boundary undercurrent through the channels of the amirante passage into the southern part of the amirante trench n of the seamount the acoustic facies is characterized by high reflectivity and is confined to the trench axis it arises from sediments episodically deposited by turbidity currents or debris flows possibly originated by slumping of reef talus from the atolls surmounting the adjacent amirante ridge

a synchronously coupled tide wave surge model of the yellow sea abstract a coupled wave tide surge model has been established in this study in order to investigate the effect of tides storm surges and wind waves interactions during a winter monsoon on november 1983 in the yellow sea the coupled model is based on the synchronous dynamic coupling of a third generation wave model wam cycle 4 and the two dimensional tide surge model the surface stress generated by interactions between wind and waves is calculated using the wam cycle 4 directly based on an analytical approximation of the results obtained from the quasi linear theory of wave generation the changes of bottom friction factor generated by waves and current interactions are calculated by using simplified bottom boundary layer model the model simulations showed that bottom velocity and effective bottom drag coefficient induced by combination of wave and current were increased in shallow waters of up to 50 m in the yellow sea during the wintertime strong storm conditions

rapid dune changes associated with overwash processes on the deltaic coast of south louisiana abstract the caminada moreau barrier headland of south louisiana is a low profile beach and dune coastline that is transgressing rapidly over the surfaces of the abandoned lafourche delta complex with the passage of cold fronts 10 30 times per year and hurricanes once every 4 yrs overwash events occur year round with varying degrees of frequency intensity and geomorphological modification this coastline consists of washover and dune surfaces that respond rapidly to overwash impact the fine sand stored in washover deposits is easily reworked by aeolian processes into a variety of dune forms in the shore zone that are vegetated rapidly geomorphological changes vary according to the position ground elevation and surface stability of the dunes independent factors are overwash surge elevations beach gradient and the presence of pre existing landforms for ten years detailed surveys supplemented by aerial photographs and videotape surveys have recorded these changes the analysis of these information sources provides insight into both the general evolution of this distinctive coastline and also the geomorphological interaction of dunes beaches and washover deposits in different physiographic settings

massive sediment bypassing on the lower shoreface offshore of a wide tidal inlet cat island pass louisiana abstract analysis of a series of historical bathymetric and shoreline surveys along the louisiana coast west of the mississippi river mouth detected a large area of deposition in water depths of 2 0 8 5 m offshore of a 9 km wide tidal inlet the cat island pass wine island pass system a 59 9 106 m3 sandy deposit formed from the 1930s 1980s spanning 27 km in the alongshore direction delineating the transport pathway for sediment bypassing offshore of the inlet on the shoreface bypassing connected the shorefaces of two barrier island systems the isles dernieres and the bayou lafourche the processes responsible for formation of this deposit are not well understood but sediment transport modeling suggests that sediment is transported primarily by wind driven coastal currents during large storms and hurricanes deposition appears to be related to changes in shoreline orientation closing of transport pathways into a large bay to the east and the presence of tidal inlets this newly documented type of bypassing an offshore bypassing of the inlet system naturally nourished the immediate downdrift area the eastern isles dernieres where shoreface and shoreline erosion rates are about half of pre bypassing rates erosion rates remained the same farther downdrift where bypassing has not yet reached as this offshore bypassing continues the destruction of the isles dernieres will be slowed

the origin and preservation of a major hurricane event bed in the northern gulf of mexico hurricane camille 1969 cores collected from mississippi sound in the gulf of mexico were studied using 210 pbxs and 137 cs geochronology x radiography granulometry and a multi sensor core logger our results indicate the presence of a widespread sandy event layer that we attribute to hurricane camille 1969 an initial thickness of more than 10 cm is estimated from the cores which is large compared to the time averaged apparent accumulation rate of 0 29 0 47 cm y 31 physical and biological post depositional processes have reworked the sandy layer producing a regional discontinuity and localized truncation and resulting in an imperfect and biased record of sedimentary processes during the storm the oceanographic and sedimentological processes that would have produced an event bed during hurricane camille are simulated using a series of numerical models i e 1 a parametric cyclone wind model 2 the swan third generation wave model 3 the adcirc 2d finite element hydrodynamic model and 4 a wave current bottom boundary layer model that is coupled to transport and bed conservation equations trans98 the simulated bed ranges from 5 cm to over 100 cm within a tidal channel near the barrier islands seaward of the islands the bed is more than 10 cm in thickness with local variability the magnitude and local variability of the storm bed thickness are consistent with the observed stratigraphy and geochronology on both the landward and seaward sides of the barriers 2002 elsevier science b v all rights reserved

morphology sediments and structure of the amirante trench western indian ocean implications for trench origin abstract the amirante trench is a 600 km long and up to 5200 m deep topographic depression whose origin has remained enigmatic the trench contains 1 km of undisturbed ponded turbidites and mass transport deposits that have been periodically modified by northward flowing contour currents of the deep western boundary current dwbc morphologically the trench and adjacent amirante ridge resemble an arcuate oceanic arc trench system and some investigators have suggested that this complex represents some type of subduction zone however our analysis of bathymetric and structural data indicate that the trench is not a single continuous arcuate feature but is actually a compound feature composed of three discrete essentially linear segments of differing structural orientations and probable sea floor spreading origins the northern segment 4 20 6 20 s trends ne 030 and apparently represents a fracture zone or transform fault trend related to sea floor spreading in the mascarene basin during the late cretaceous the central segment 6 20 8 40 s trends nnw 350 and appears to represent a fracture zone lineament that is parallel to fracture zones and lineaments created by the opening of the somali basin during the late jurassic and early cretaceous however somali basin crust apparently does not extend eastward as far as this portion of the trench so the origin of this segment remains uncertain the southern segment 8 40 10 s trends nw 310 and may represent a tensional rift related to sea floor spreading within the mascarene basin during the early tertiary further our data show no convincing evidence that any portion of this feature was a subduction zone at any time during its development the data also essentially rule out an extraterrestrial impact origin for this region at the end of the cretaceous

re examination of geophysical data off northwest india implications to the late cretaceous plate tectonics between india and africa abstract the gop and laxmi basins lying off northwest india have been assigned ambiguous crustal types and evolution mechanisms the chagos laccadive ridge clr complex lying along the southwest coast of india has been attributed to different evolutionary processes late cretaceous seafloor spreading between india and africa formed the mascarene basin and the plate reconstruction models depict unequal crustal accretion in this basin re interpretation of magnetic data in the gop and laxmi basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 ma at slow half spreading rates 0 6 to 1 5 cm yr separating the seychelles laxmi ridge complex from india the spreading ridge became extinct at 71 ma in the central region between 19 20 n and 65 5 67 e and at 68 7 ma in the gop basin extinction progressed southwards with time until 64 1 ma at 14 5 n in the laxmi basin this spreading probably limited the seafloor spreading in the northern mascarene basin while normal to fast spreading continued in the south the fracture zone fz2 acted as a major boundary between the northern and southern mascarene basin and may have also restricted the southward propagation of the laxmi basin spreading ridge plate reconstruction models generated in this study accommodate several microplates between madagascar and india and trace their movements over time within the india africa spreading corridor furthermore these models suggest the formation of the seychelles laxmi ridge microplate by 79 ma in a complex three plate system this microplate may have existed up to 64 1 ma and experienced an anticlockwise rotation of about 50 under the influence of the deccan hotspot activity the laxmi basin spreading ridge jumped southwestward splitting the seychelles laxmi ridge complex and initiating the carlsberg ridge the presence of about 160 km of oceanic crust in the gop and laxmi basins partially reduces the space problem faced in the earlier plate reconstruction models

identification of intense intermittent coherent motions under shoaling and breaking waves laboratory measurements of the instantaneous horizontal and vertical turbulent velocities u and w induced by regular waves spilling on a rough impermeable slope were analyzed to elucidate the nature of turbulence generated in the bottom boundary layer and by wave breaking the analyses focused on the instantaneous turbulent events such as the absolute shear stress u w and turbulent kinetic energy k u 2 w 2 2 below trough level inside the surf zone the horizontal and vertical velocity records showed intense intermittent turbulent events that did not occur with the passing of each wave and the instantaneous quantities of and k could not be explained in terms of the phase averaged quantities the intermittent turbulent events extended into the bottom boundary layer inside the surf zone and in this region the infrequent but intense turbulence generated by wave breaking was an order of magnitude larger than the turbulence generated locally at the boundary two techniques were used to analyze the turbulent motions a quadrant analysis technique showed that although the large turbulent motions did not occur with each wave they were phase dependent near trough level and less so near the bottom a conditional sampling technique quantified the intensity and duration of the turbulent motions indicating that coherent events intense events occurred for 10 2 of the record and accounted for approximately 50 20 of the turbulent motion it is likely that the exactitude of these statistics will differ depending on breaker type nevertheless these statistics indicated that large turbulent motions are infrequent but contribute significantly to the turbulence intensity and possibly the suspension of bottom sediments

observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves acoustic measurements of near bed sediment diffusivity profiles are reported the observations were made over two sandy rippled beds classified as medium and fine in terms of sand grain size under slightly asymmetric regular waves for the medium sand the ripples that formed had relatively steep slopes while for the fine sand the slopes were roughly half that of the medium sand in the medium sand case the form of the sediment diffusivity profiles was found to be constant with height above the bed to a height equal approximately to the equivalent roughness of the bed ks while above this the sediment diffusivity increased linearly with height for the case of the fine sand there was no constant region the sediment diffusivity simply increased linearly with height from the bed to understand the difference between the respective diffusivity profiles advantage has been taken of the high temporal spatial resolution available with acoustic systems using intra wave ensemble averaging detailed images have been built up of the variation in concentration with both the phase of the wave and also height above the bed these intra wave observations combined with measurements of the bed forms and concepts of convective and diffusive entrainment have been used to elucidate the mixing mechanisms that underlie the form of the diffusivity profiles observed over the two rippled beds these mechanisms centre on coherent vortex shedding in the case of steeply rippled beds and random turbulent processes above ripples of lower steepness

dynamics of a wave dominated tidal inlet and influence on adjacent beaches currumbin creek gold coast australia currumbin creek on the australian gold coast is a wave dominated tidal inlet which exhibits a particularly active morphology the recent history of currumbin creek entrance has seen rapid growth of the entrance for access to the ocean by fishermen as a world class surfing site and as a recreational area before the construction of two groynes in the 70 s currumbin creek entrance was highly variable in terms of inlet location and sand bar characteristics due to a cyclical behaviour of spit migration nowadays the entrance is stabilised however natural processes continue with the entrance infilling causing flood and navigation issues resulting in a regular dredging program to maintain an open entrance and for regular beach nourishment plans this paper investigates the behaviour of the entrance and adjacent beaches from aerial photographs and numerical modelling before groyne construction sand by passing was intense resulting in channel migration and sometimes the closure of the mouth after training works the longshore drift is diverted further north from the mouth leading to new circulation patterns behind the headland during fair weather conditions the sand transported by the longshore current is trapped by currumbin rock groyne resulting in a negative sediment budget in palm beach for high energy conditions the diverted longshore current splits in the southern palm beach resulting in a circulation cell the sediment of the southern beaches is stirred up and transported both northward and toward the inlet

rippled scour depressions on the inner continental shelf off central california abstract side scan sonar records taken during the recent coastal ocean dynamics experiment code show elongate shore normal rippled depressions of low relief on the inner continental shelf off central california between bodega bay and point arena these features extend up to 2 km from the coast into water depths of up to 65 m the proposed mechanism for their generation is storm generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine sand sediment and expose the coarser sand substrate in the depressions the zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features the large straight crested ripples within the depressions heights to 40 cm wavelengths to 1 7 m are probably formed by large amplitude long period surface waves generated by winter storms

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

a numerical study of sediment transport and event bed genesis during tropical storm delia some event beds tempestites are thought to be emplaced on shallow marine shelves by the combined action of strong coastal currents and high waves during fairly short lived storms to test this hypothesis a storm sedimentation system has been constructed from six numerical models describing a cyclonic wind field three dimensional coastal circulation wind waves generated over the continental shelf the combined effect of steady and oscillatory currents within the benthic boundary layer suspended and bed load transport of sediment and conservation of the seafloor this model system is used to hindcast winds currents waves and resulting sedimentation during tropical storm delia which passed over the texas louisiana shelf on september 3 4 1973 sensitivity to the initial substrate is investigated in four experiments using uniform silt uniform sand a mud line at the 20 m isobath and a simplified modern sediment distribution modeled coastal currents are vertically uniform and do not reveal the structure predicted by the mid latitude geostrophic storm circulation model because the predicted depth of the wind mixed layer is greater than the water depth over the shelf shelf currents in excess of 2 m s flow predominantly along the coast to the southwest during most of the storm driven by the wind stress and the trapped coastal wave which peaks at 180 cm near galveston significant wave heights reach 8 m on the outer shelf but are less than 4 m over the inner shelf these waves combine with steady currents to produce bed shear velocities which locally exceed 20 cm s the region of highest stresses always lies to the right of the storm track viewed down the path and moves across the shelf with the eye of the storm three general sediment transport paths are evident 1 onshore transport of finer sediment over the outer shelf to the right of the storm track 2 westward directed along shelf transport of predominantly fine sediment between approximately 40 m and 20 m water depths and 3 minor offshore transport of sand from the shoreface to depths less than 30m the resulting event bed has a ragged appearance with a maximum thickness of about 20 cm in region 1 and covers an area of approximately 3 104 km2 to the right of the storm track aside from local transport associated with finer sediments these results are relatively insensitive to initial sediment type comparison of model results to observed data from buccaneer platform shows that the different models performed adequately during the peak of the storm except for a significant underprediction of the significant wave height by the wind sea model the estimated uncertainty in the calculated combined shear stresses u based on errors produced by the individual model components is most dependent on the wave bottom orbital amplitude the total uncertainty in u is estimated to be approximately 7

examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions duck north carolina 1 a benthic boundary layer tripod supporting six current meters and three profiling acoustic backscatter sensors abs documented storm and swell conditions during the fall of 1996 at a depth of 13 m on the inner shelf off duck north carolina sediment concentration was higher in the wave boundary layer wbl during storm conditions but higher 40 cm above the bed cm ab during swell conditions to test the applicability of a diffusive balance during storm versus swell abs data were used to invert the vertical diffusion equation and solve for eddy diffusivity from 1 to 50 cm ab during the storm period diffusivity derived from the abs up to 40 cm ab agreed well with viscosity derived above the wbl from observed current profiles and from the grant madsen glenn gmg model during the swell period diffusivity derived from the abs up to 40 cm ab did not agree with observed mean current shear above this level nor with the gmg model diffusivity did agree with viscosity derived from shear stress due to waves within the wbl extrapolated to a height greater than the modeled wbl we speculate that during swell conditions shedding vortices enhanced mass and momentum exchange extending the eddy viscosity associated with waves above the predicted wbl during storm conditions strong currents prevented vortices from penetrating beyond the predicted wbl rouse diffusion models with two and three layered eddy diffusivity and combined diffusion advection models with one and three layer were applied to the observational data set during the storm the two and three layered rouse models including multiple grain sizes and bed armoring reproduced the observed concentration well during swell weak current conditions all the models considered underpredicted the observed concentration if applied with a standard wbl thickness to correct this enhanced vertical exchange was represented by a thickened wbl whenever mean currents were weak relative to the estimated jet velocity associated with wave induced vortex shedding the two layer rouse model then reproduced the concentrations observed during swell remarkably well this implies that mean sediment suspension dominated by wave induced advection may still be approximated by a diffusion like process under some circumstances

storm dominated bottom boundary layer dynamics on the northern california continental shelf measurements and predictions measurements of near bottom velocity profiles in 85 m water depth during a storm on the continental shelf off northern california using the geoprobe tripod in december 1979 provided estimates of shear velocities u and roughness lengths z0 when the near bottom velocity profiles were logarithmic these estimates agree within 90 confidence intervals with values computed from a simple near bottom combined wave current model that includes movable bed effects the reasonably good comparison between model and profile estimates of u suggests that such models can be used to predict bed shear stresses or shear velocities under combined flows of waves and currents typical of stormy conditions on continental shelves if stratification corrections to the velocity profile resulting from suspended sediment are small the repeated occurrences of storms of similar intensities and wind velocities during the winter months off northern california suggest that the resultant high bottom stresses due to the combined effects of waves and currents are major factors in controlling the distribution of surficial sediment on the central portion of the northern california shelf

high sea floor stress induced by extreme hurricane waves 1 strong surface waves and currents generated by major hurricanes can produce extreme forces at the seabed that scour the seafloor and cause massive underwater mudslides our understanding of these forces is poor due to lack of concurrent measurements of waves and currents under these storms using unique observations collected during the passage of a category 4 hurricane ivan bottom stress due to currents and waves over the outer continental shelf in the gulf of mexico was examined during the passage of ivan the bottom stress was highly correlated with the wind with a maximum of about 40 of the wind stress the bottom stress was dominated by the wave induced stresses and exceeded critical levels at depths as large as 90 m surprisingly the bottom damaging stress persisted after the passage of ivan for about a week and was modulated by near inertial waves

bottom stress estimates and their prediction on the northern california continental shelf during code 1 the importance of wave current interaction abstract high quality near bottom boundary layer measurements obtained at a midshelf location 90 m water depth in the code region off northern california are described bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions june 1981 during code 1 are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates during the time period described the mean near bottom 0 997 approximately 30 percent of the time effects induced by unsteadiness from internal waves result in some degradation of the profiles 0 96 r 0 997 the rest of the time mean stress profiles indicate the logarithmic layer is approximately a constant stress layer the near bottom flow field is composed of mean currents and oscillatory currents due to well typing mean u values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0 5 1 0 cm s 1 mean stress valu

reconstructing sediment age profiles from historical bathymetry changes in san pablo bay california abstract sediment age profiles reconstructed from a sequence of historical bathymetry changes are used to investigate the subsurface distribution of historical sediments in a subembayment of the san francisco estuary profiles are created in a grid based gis modeling program that stratifies historical deposition into temporal horizons the model s reconstructions are supported by comparisons to profiles of 137 cs and excess 210 pb at 12 core sites the predicted depth of the 1951 sediment horizon is positively correlated to the depth of the first occurrence of 137 cs at sites that have been depositional between recent surveys reconstructions at sites that have been erosional since the 1951 survey are supported by a lack of detectable 137 cs and excess 210 pb below the upper 6 16 cm of the core a new data set of predicted near surface sediment ages was created to illustrate an application of this approach results demonstrate other potential applications such as guiding the spatial positioning of future core sites for contaminant measurements

hydrodynamic and sedimentary responses to two contrasting winter storms on the inner shelf of the northern gulf of mexico abstract results are presented from the deployment of three bottom mounted instrumentation systems in water depths of 6 9 m on the sandy inner shelf of louisiana usa the 61 day deployment included nine cold front passages that were associated with large increases in wind speed two of the most energetic cold front passages were characterized by distinct meteorological hydrodynamic bottom boundary layer and sedimentary responses and may potentially be treated as end member types on a continuum of regional cold front passages arctic surges ac storms have a very weak pre frontal phase followed by a fairly powerful post frontal phase when northeasterly winds dominate migrating cyclones mc storms are dominated by a strong low pressure cell and have fairly strong southerly winds prior to the frontal passage followed by strong northwesterly winds on the basis of measurements taken during this study ac storms are expected to have a lower average significant wave height than mc storms and are dominated by short period southerly waves subsequent to the frontal passage currents are weak and northerly during the pre frontal phase but become very strong and southwesterly following the passage sediment transport rate during as storms was not as high as during mc storms and the mean and overall direction tended to be southwesterly to westerly with low frequency flows producing easterly transport and wind wave flows producing southeasterly transport mc storms had the most energetic waves of any storm type with peaks in significant wave height occurring during both the pre and post frontal phases the wave field during mc storms tended to be more complex than during as storms with an energetic northerly swell band gradually giving way to a southerly sea band as the post frontal phase progressed currents during mc storms were moderate and northerly during the pre frontal phase but became much stronger and southeasterly during the post frontal phase shear velocity was high during both the pre and post frontal phases of the storm although sediment transport was highest following the frontal passage mean and overall sediment transport was directed southeasterly during mc storms with low frequency and wind wave flows producing northerly transport in summary the data sets presented here are unique and offer insight into the morphosedimentary dynamics of mid latitude micro tidal coasts during extratropical storms

constraints on the motion of madagascar with respect to africa abstract three constraints on the motion of madagascar relative to africa are investigated in order to arrive at a model for madagascar s drift these are 1 fracture zone morphology and direction in the somali basin and mozambique channel 2 the relative positions of the carboniferous to jurassic palaeomagnetic poles for madagascar and africa and 3 the crustal structure along a geophysical traverse between lamu kenya and mahe seychelles the evidence for the bunce and molnar 1977 pole for the rotation of madagascar relative to africa defined by the davie dhow vlcc and ars fracture zones is critically examined and accepted it is used to rotate madagascar northwards back to a position that brings the carboniferous to jurassic palaeomagnetic poles for madagascar and africa into good agreement the rotation of approximately 16 places madagascar in the north of the somali basin with its northern tip at the southwestern end of the chain ridge it moves madagascar almost exactly the length of the extended davie fracture zone accepting that this position may have been the pre drift position of madagascar some interesting correlations with the lamu mahe crustal section emerge continental crust appears to extend 300 km offshore from lamu and thus abuts precisely the continental crust of rotated madagascar between 300 km and 1200 km from lamu the crust is probably oceanic of jurassic cretaceous age this would have been left in the wake of madagascar as it drifted southwards four fracture zones are seen in this crust a major one appears in the crustal section about 700 km from lamu the dhow fracture zone at 850 km may be associated with a large step up to the east in basement which has acted as an efficient dam to sediments from africa and the vlcc and ars are smaller fracture zones the easternmost part of oceanic crust between 1100 and 1200 km may have been formed in the wake of the farquhar islands drifting southwards with madagascar more likely however it could have formed as the seychelles bank area drifted away from madagascar having previously moved southwards with madagascar eastwards from 1200 km thicker possibly continental crust adjoins the seychelles bank this model assumes that the comores basin is oceanic in origin which may not be the case if it is continental a rotation of madagascar of only 9 or 10 can be accommodated moving the island almost exactly the length of the topographic expression of the davie fracture zone but providing a poor fit of the palaeomagnetic poles

validity of sea level indicators a comment on a new depositional model for the buried 4000 yr bp new orleans barrier implications for sea level fluctuations and onshore transport from a nearshore shelf source by f w stapor and g w stone marine geology 204 2004 215 234 the holocene history of the se louisiana mississippi coast and a recently revised regional sea level curve fail to support the concept of an abrupt gulf wide decline from 2 0 m to 1 5 m and regression with barrier aggradation between 4 1 and 3 9 14c ka bp triggered by massive onshore sediment transport a subsequent rapid rise followed by minor oscillations are equally unsubstantiated suggestions for an overwhelming role of onshore transport during the assumed regression or otherwise may not be justified a small hardcrust lens in the continuous subaqueous barrier sequence does not prove major fall in sea level regional regression and subaerial exposure elevation and lithofacies of florida shore scarps and radiocarbon dated beach ridges fail to support sea level fluctuations the presented paleogeographic and sea level models indicate continuous shoal and associated island formation between ca 5 2 and 4 0 14c ka bp uninterrupted by sea level decline rapid progradation of a mississippi river st bernard delta lobe terminated barrier development

the silurian of the pyrenees a detailed stratigraphical study of the silurian rocks of the pyrenees and analysis of their graptolite and conodont faunas has allowed the construction of composite lithostratigraphical successions and accurate correlations between them the lithostratigraphy reflects generally continuous deposition of pelitic and shale sediments across the whole of the pyrenean range from rhuddanian to homerian times there is however some variation eastwards where sandier intercalations appear in the rhuddanian also carbonate facies were deposited in the mouthoumet massif during the latest telychian and then from sheinwoodian times spread across the major part of the central and eastern pyrenees pelitic sedimentation persisted during the gorstian and ludfordian but with some differentiation into more clastic facies westwards and more carbonate rich facies to the east these features of sedimentation became more pronounced during pridoli times and continued until the early devonian the silurian lithofacies of the pyrenees are interpreted as epicontinental deposits formed in low energy reducing environments on the northern margin of the gondwanan continent

the amirante ridge trough complex response to rotational transform rift drift between seychelles and madagascar the amirante ridge trough complex developed along the late cretaceous transform boundary that separated the seychelles india and madagascar somali basin plates motion between these plates was complex comprising sinistral n s strike slip movement coupled and coeval with counter clockwise rotation induced when seafloor spreading developed in only the southern portion of the transform the overall morphology of the complex comprises a series of arcuate ridge and trough segments these segments were successively produced by tectonic and volcanic activity within the zone of migrating plate contact adjacent to the rotational pivot where compression was replaced by extension along the transform boundary in the extensional regime to the south of this contact zone the mascarene oceanic basin developed whilst in the compressional zone further north island arcs developed and the ophiolites of baluchistan were obducted from the somali basin onto the leading edge of the seychelles india plate

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness

sensitivity of bottom stress and bottom roughness estimates to density stratification eckernforde bay southern baltic sea thermohaline density stratification may significantly alter the classic near bottom logarithmic velocity profile in many weak to moderately energetic partially mixed estuaries results from eckernforde bay suggest fits to log profiles which neglect thermohaline stratification may lead to overestimates of bottom stress and roughness of the order of 130 and 600 respectively measurements of velocity obtained at four heights within 1 m of the seabed are input to theoretical models for velocity shear derived via dimensional arguments for the overlap layer previous investigators applying dimensional arguments to thermohaline stratification in estuaries have assumed buoyancy flux to be independent of height within the overlap layer this may be a poor assumption since there is no significant source or sink of thermohaline buoyancy at the sediment water interface in this paper dimensional arguments which do not assume constant buoyancy flux are used to reduce estimates of the drag coefficient and bottom roughness to below the unreasonably high values predicted by simple log profiles formulations assuming very weak and relatively strong stratification are applied and estimates of buoyancy frequency derived from fits to velocity profiles are compared with independent estimates of stratification estimates of bottom stress and roughness derived from velocity profiles are also found to be sensitive to fluid acceleration uncertainties in instrument settling and limitations in current meter accuracy but these latter effects appear secondary in eckernforde bay to the impacts of thermohaline stratification

predicting ripple roughness and sand resuspension under combined flows in a shoreface environment ripple measurements and flow and sediment dynamical data obtained from the shoreface of the middle atlantic bight using instrumented tripods were analyzed to evaluate various predictors of ripple geometry and roughness ripple roughness controls on sand resuspension and suspended sediment concentration profiles under combined waves and currents were also evaluated the limited observation of sand ripples in the field indicates that the grant and madsen 1982 method overestimates ripple roughness while the nielsen 1981 method tends to under predict ripple roughness a modified ripple predictor is thus proposed based on the grant and madsen method and it is shown to give reasonable predictions under the present experiment conditions this modified ripple prediction along with wave current and suspended sediment concentration data recorded by the tripods were then brought into the combined flow bottom boundary layer model of grant and madsen 1986 and the modified rouse equation of glenn and grant 1987 to calculate sand resuspension coefficient 0 and to predict suspended sediment concentration profiles it was found that under low energy fair weather conditions sand ripples are in the equilibrium range and ripple roughness increases with the bed shear stress this causes strong vortex activity close to seabed and thus higher resuspension coefficient reference concentrations are moderate due to this high resuspension coefficient even though bed shear stresses are low under moderate storm conditions ripple break off occurs and ripple roughness will decrease with bed shear stress this reduces the vortex activity and hence sand resuspension coefficient 0 the combination of this moderately high bed shear stress and reduced but still moderate ripple roughness favours sand suspension and produces the highest reference concentration for the encountered experimental conditions as bed shear stress is further increased ripples are nearly washed out and sand resuspension coefficient is further decreased approaching the previously suggested constant value of 1 3 10 4 this corresponds with the lowest reference concentration despite the high bed shear stress suspended sediment concentrations predicted by the modified rouse equation using this time variable resuspension coefficient and properly calculated bottom boundary layer parameters are reasonable compared to the field measured concentration profiles

transgressive depositional systems of the mississippi delta plain a model for barrier shoreline and shelf sand development abstract depositional sequences generated in the mississippi river delta plain consist of a regressive and a transgressive component the transgressive component has been considerably less studied but accounts for the majority of the surface area on the lower mississippi river delta plain and up to 50 percent of the total sequence thickness in shallow water deltas the development and preservation of transgressive depositional systems in abandoned delta complexes follows the process of transgressive submergence in which the horizontal component of reworking occurs during shoreface retreat combined with a vertical component of submergence acting to preserve the sequence the evolution of transgressive depositional systems in each of the abandoned holocene mississippi river delta complex s can be summarized in a three stage model beginning with stage 1 an erosional headland and flanking barriers in this stage regressive sand deposits contained within abandoned deltaic headlands are reworked by the eroding shoreface and dispersed longshore into contiguous flanking barriers enclosing restricted interdistributary bays submergence of the delta plain during relative sea level rise generates an intradeltaic lagoon separating the former stage 1 sand body from the shoreline forming stage 2 a transgressive barrier island arc the landward migrating barrier island arc is unable to keep pace with relative sea level rise and the retreating mainland shoreline resulting in submergence and the formation of stage 3 an inner shelf shoal following submergence the former barrier island arc sand body continues to be reworked into a marine sand body on the inner continental shelf during stage 3 this sequence of coastal evolution provides direct evidence of barrier island formation with each stage producing a distinctive stratigraphic signature the current sea level rise models of shoreface retreat and in place drowning developed for the u s atlantic continental shelf do not adequately explain either the morphology or the stratigraphy of transgressive mississippi river delta sand bodies current models of mississippi deltaic stratigraphy emphasize the deep water artificially maintained balize delta which differs considerably from the shallow water shelf phase delta complexes that are the primary depositional constituents of he holocene mississippi river delta plain

gravity models of the amirante arc western indian ocean abstract a free air gravity anomaly chart has been compiled for the amirante arc amirante basin and the western margin of the seychelles plateau the new gravity data obtained during r r s discovery cruise 101 has tracks orthogonal to the amirante arc at approximately 40 km separation and detailed coverage of the south and west margins of the seychelles plateau that has been merged with earlier data the free air anomaly across the southern amirante arc has a wavelength close to 75 km and an average amplitude of 215 mgal three profiles are interpreted using two dimensional models with the moho at 14 km depth in the amirante and south somali basins and a significant variation in the depth to the mantle of between 11 and 25 km beneath the arc trough structure the density distributions show evidence of partial subduction analogous to that of the north spanish margin and high lithostatic stress reflecting the absence of local isostatic compensation

a modeling study of the impact of major storms on seabed shear stress and sediment transport on the grand banks of newfoundland waves current and sediment transport processes in major storms on the grand banks of newfoundland were simulated using integrated wave three dimensional tide and circulation and combined flow sediment transport models while the tidal and nontidal currents are generally low and cause little sediment transport storm induced waves and currents enhance bed shear velocity by more than 5 times and cause significant sediment transport over the entire grand banks the impact of storms on shear stress and transport strongly depends on water depths and the greatest impact occurs over the bathymetric highs on southeastern grand bank where the maximum shear velocity reaches 15 cm s 1 and the maximum transport rates are 5 kg m 1 s 1 the direction of sediment transport rotates clockwise progressively through nearly 360 during the passage of a storm although peak transport typically occurs on central and southeastern grand bank with a southeastward direction the magnitude direction and timing of peak transport show strong spatial and temporal variability the variability of the peak transport largely depends on the timing and relative intensity of the waves and the total bottom currents which in turn depends on the addition of the storm induced and tidal currents the calculation of the maximum transport potential suggests that sediments as coarse as small pebbles are mobile in water depths 80 m under 1 1 year storms and that medium sand is transported in water depths as deep as 200 m during major storms results of the sediment transport models corroborate the observed sediment erosion and accretion patterns

sediment connectivity a framework for analyzing coastal sediment transport pathways connectivity provides a framework for analyzing coastal sediment transport pathways building on conceptual advances in graph theory from other scientific disciplines connectivity schematizes sedi