numerical simulation for groundwater distribution after mining in zhuanlongwan mining area based on visual modflow with large scale coal resources being discovered and mined in mid west china latent environment problems gradually come into view in arid and semi arid region environmental impact assessment of mining on groundwater distribution is a vital mission before mining works in zhuanlongwan mining area zma data of surface elevation were performed by remote sensing information and arcgis the study area was divided into clayey fine sand aeolian loose sand weathered fracture and low permeability zones according to hydrologic conductivities and borehole data by simplifying as a single layer system of phreatic groundwater and optimizing with geological parameters in each unit zone a reasonable model of the groundwater distribution in steady state was then found and calibrated combined with local climate variable groundwater behavior was discussed in relation to different precipitations which reduced the loss of the drain in degrees the study results may give benefit and safety to management in mining as well as being refined in the future

subaqueous delta of the ganges brahmaputra river system abstract the ganges brahmaputra is among the world s three largest river systems in terms of sediment load but until now no high resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers mouths the other two amazon huanghe discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy high resolution seismic reflection profiles of the bengal shelf reveal similar stratigraphy topset beds dip gently 0 036 and diverge offshore more steeply dipping foreset beds 0 190 converge farther seaward and relatively thin gently dipping bottomset beds 0 022 extend across the outer shelf overlying an erosional surface presumed to be of late pleistocene age sediment accumulation rates are highest in the foreset region 5 cm year and reduced in the bottomset region

short term biogenic particle flux under late spring sea ice in the western weddell sea abstract in the framework of the ice station polarstern ispol expedition in the western weddell sea two sediment traps were deployed at 10 and 70 m water depth under a drifting ice floe in december 2004 the amount and composition of the vertical particle flux under sea ice were determined during a period of 30 days in order to investigate the influence of biological processes in sea ice and on its underside on the flux the total mass flux was dominated by diatoms faecal material and aggregates and ranged from 95 28 to 197 67 mg m 2 d 1 at 10 m depth and from 51 54 to 55 34 mg m 2 d 1 at 70 m depth a strong increase with time of the flux of chlorophyll equivalents biogenic silica and faecal material was recorded during the observation period coincident with the increase in the concentration of chlorophyll a in the bottom ice layer above the trap array the latter suggests a concomitant increase in the amount of food available for grazers such as krill in the bottom ice layer and on the underside of the ice floe resulting in an increased downward transport of ice algal material into the water column the sinking faecal material was dominated by krill faecal strings and contained large amounts of diatom frustule debris as well as intact diatom frustules mainly of the species fragilariopsis curta and f cylindrus single pronounced flux events of phaeocystis antarctica and aggregates were also observed early in the study period low poc pon and biogenic silica poc ratios of the sinking particulate matter suggest that the material collected in the traps was relatively fresh

geostrophic circulation or shallow marine turbidity currents the dilemma of paleoflow patterns in storm influenced prograding shoreline systems abstract in ancient storm influenced prograding shoreline sequences sole marks mainly tool marks from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin asymmetrical tool marks typically indicate flows directed offshore several workers have attributed their formation to storm generated shallow marine turbidity currents this interpretation conflicts with observations from modern shelves where storm driven circulation generally is geostrophically balanced and time averaged bottom currents approximately parallel bathymetric contours and the local shoreline the resolution of these apparently conflicting observations may lie in the realization that tool marks and many other small paleoflow indicators form almost instantly as the result of instantaneous flow conditions very near the bed beneath storm generated flows in the shallow ocean instantaneous and time averaged characteristics of the bottom boundary layer generally exhibit little similarity storm generated tool marks are formed by the movement of large tools within the thin less than 1 m inner boundary layer resulting from the superimposition of waves and currents the orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave orbital motions which typically are normal to shore the magnitude of stress is great y increased in the offshore direction and decreased in the onshore direction by superimposition of a steady current with an offshore component of flow but the direction of stress is only slightly affected in ancient storm influenced sequences therefore shore normal tool marks generally were not formed by turbidity currents rather their orientation is best attributed to shoaling waves approaching the coast at a very high angle asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component tool marks do not reflect the time averaged bottom flow direction in fact they provide almost no information concerning steady bottom currents in contrast high angle cross beds formed in coarser sediment by the migration of dunes and sandwaves although relatively rare in offshore storm deposits generally reveal approximately shore parallel flows in ancient systems cross beds closely reflect the time averaged flow direction in the outer boundary layer of a geostrophic current for three reasons 1 the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time averaged shear stress 2 large ripples disrupt the thin inner boundary layer and 3 long time intervals relative to wave induced velocity oscillations are required to form large ripples

the submarine delta of the ganges brahmaputra cyclone dominated sedimentation patterns the ganges brahmaputra belongs to the world s largest rivers in terms of sediment discharge but little is known yet about the processes controlling sediment distribution on the shelf and the documentation of these processes in large and small scale sedimentary structures for this study the submarine delta of the river system was investigated by high resolution seismic reflection profiles and piston and gravity coring logged physical properties of the cores were combined with visual core description grain size analysis and radiography sedimentation rates were estimated by 210pb and 137cs gamma spectrometry and cross checked with evaluation of seismic profiles the data reveal that the broad delta topset 20 m waterdepth is formed by a thick layer of sand silt highest sedimentation rates about 10 cm a 1 occur at the centre of clinoform delta foreset beds where about 20 of the total riverine sediment load is deposited and thus cause an annual progradation of the subaqueous delta front by about 15 m the foreset beds consist of graded sand silt layers which are thought to be deposited by sediment laden flows generated during the surge of tropical cyclones tempestites interbedded with silty clay layers the head of a canyon the swatch of no ground that deeply incises into the shelf forms a sediment trap along the westward cyclonic transport path where sedimentation rate increases to about 50 cm a 1 episodically earthquakes or storms destabilize especially the rapidly accumulated foreset beds in the eastern delta creating up to 8 m thick mass flows on the outer shelf in water depth beyond 80 m no holocene sediment is deposited

wind driven circulation in a shelf valley part i mechanism of the asymmetrical response to along shelf winds in opposite directions abstractmotivated by observations in hudson shelf valley showing stronger onshore than offshore flows this study investigates wind driven flows in idealized shallow shelf valleys this first part of a two part sequence focuses on the mechanism of the asymmetrical flow response in a valley to along shelf winds of opposite directions model simulations show that i when the wind is in the opposite direction to coastal trapped wave ctw phase propagation the shelf flow turns onshore in the valley and generates strong up valley transport and a standing meander on the upstream side in the sense of ctw phase propagation of the valley and ii when the wind is in the same direction as ctw phase propagation the flow forms a symmetric onshore detour pattern over the valley with negligible down valley transport comparison of the modeled upstream meanders in the first scenario with ctw characteristics confirms that the up valley flow results from ctws being arrested by the wind driven shelf flow establishin

contributions of a strengthened early holocene monsoon and sediment loading to present day subsidence of the ganges brahmaputra delta the contribution of subsidence to relative sea level rise in the ganges brahmaputra delta gbd is largely unknown and may considerably enhance exposure of the bengal basin populations to sea level rise and storm surges this paper focuses on estimating the present day subsidence induced by holocene sediment in the bengal basin and by oceanic loading due to eustatic sea level rise over the past 18 kyr using a viscoelastic earth model and sediment deposition history based on in situ measurements results suggest that massive sediment influx initiated in the early holocene under a strengthened south asian monsoon may have contributed significantly to the present day subsidence of the gbd we estimate that the holocene loading generates up to 1 6 mm yr of the present day subsidence along the gbd coast depending on the rheological model of the earth this rate is close to the twentieth century global mean sea level rise 1 1 1 7 mm yr thus past climate change by way of enhanced sedimentation is impacting vulnerability of the gbd populations

the height of fractured water conducting zone in undermined rock strata the height of fractured water conducting zone is of significant importance for the safety of underground mining because of the conspicuous discrepancy of the height of fractured water conducting zone in bulianta coal mine as predicted by the traditional method with those observed in practice a systematic research has been carried out the study indicates that two aspects should be taken into account firstly the traditionally proposed empirical formula in prediction of the height of fractured zone was based on the conditions of single slice mining where the mining height was less than 3 m and the total slice mining height was limited within 15 m in such a case it would not be suitable to predict the maximum fractured zone height for the conditions where a single mining height is larger than 3 m secondly the stratum structure such as the thick and strong rock layer termed as key stratum plays an important role in controlling of the height of movement in the overburden physical model tests by making use of a similar material illustrate that if the vertical distance from the primary key stratum to the coal seam is shorter than a certain value then the developing fracture zone will extend above the key stratum into the top of bedrock resulting in a larger height of water conducting zone than is calculated by the empirical method the outcome of the work presented will be helpful in practice to prevent the overburden aquifer inrush and to avoid mine water hazard

height of water conducting fractured zone in a coal seam overlain by thin bedrock and thick clay layer a case study from the sanyuan coal mine in north china the accurate determination of the height of the water conducting fractured zone hwcfz in a coal seam covered by thin bedrock and thick clay layer is crucial for aquifer protection mine safety and eco environmental conservation using the sanyuan coal mine in north china as a case study theoretical analysis numerical simulation and field measurements were used to analyze the regularity of the hwcfz in this area per mechanical theory a geometrical model for calculating rock bending and subsidence was established two arcs of equal curvature and lengths and opposite curvature direction were used to fit the curve section of intermediate rock layer after bending the theoretical method for solving the hwcfz was given by calculating the rock layer s tensile ratio in addition rock failure process analysis rfpa software was used to simulate the development of the water conducting fractured zone wcfz during the mining process and the hwcfz was measured by the drilling fluid loss measurement both which verified the feasibility of the theoretical calculation to determine the primary controlling factors affecting the hwcfz the seepage characteristics and plastic properties of the quaternary clay layer were also analyzed using laboratory tests results show that the quaternary clay with a depth greater than 130 m has a permeability coefficient of e 10 m s a liquidity index less than 0 25 and a plasticity index more than 17 indicating the clay layer is impermeable and of great importance to inhibiting the development of wcfz the research can provide significant theoretical insights for effectively preventing water hazards on mine roofs

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

reappraisal of sea ice distribution in atlantic and pacific sectors of the southern ocean at 18 000 yr bp the question of the distribution of winter and summer sea ice cover during the last glacial maximum 18 000 yr bp is important in climatic modelling of the glacial world we have now compared satellite derived 5 yr monthly averages of per cent summer sea ice cover in the atlantic and pacific sectors of the southern ocean with the distribution of reported sedimentological indicators of sea ice cover at the sediment water interface these data strongly suggest that sediment type cannot be used to identify summer sea ice limits neither in recent sediments nor in sediments of the last glacial maximum rather the data seem to indicate that sediment type can be used to identify spring sea ice limits for these two time intervals

floodplain processes in the bengal basin and the storage of ganges brahmaputra river sediment an accretion study using 137cs and 210pb geochronology floodplain deposits are recognized as potentially large sinks for fluvial sediments yet spatial and temporal patterns of accumulation are not known for many river systems in the tectonically active bengal basin the ganges and brahmaputra rivers coalesce and have formed a large floodplain delta complex where widespread overbank flooding may deposit a significant portion of the river s estimated sediment discharge to investigate the magnitude and distribution of this process accumulation rates were determined by 137cs and 210pb radioisotope geochronology at 60 sites located in three regions of the bengal basin differing in age physiography and river influence these geochronological methods have been applied to floodplain environments in only a few recent studies and the present research offers a revised approach for heterogeneous systems patterns of sediment deposition in the bengal basin reveal three dominant controls on floodplain accretion channel processes overbank flooding and surface runoff accretion is most rapid in the river braidbelt and adjacent floodplain decreasing rapidly with distance from the main channel accumulation rates increase again in low lying distal basins where several meters of annual precipitation and associated runoff rework surrounding floodplain surfaces and transport remobilized sediments to local catchments the extent and magnitude of this process indicate its comparable importance with overbank deposition for floodplain accumulation a sediment budget for the study area reveals that at least 15 of the estimated 1 billion tonnes of fluvial sediment discharge is stored annually and not reaching the oceans as previously assumed extrapolated to the remaining 60 000 km2 of floodplain not covered in this project the sequestered load could rise to 30 of annual sediment discharge

wind driven circulation in a shelf valley part ii dynamics of the along valley velocity and transport abstractthe dynamics controlling the along valley cross shelf flow in idealized shallow shelf valleys with small to moderate burger number are investigated and analytical scales of the along val

non steady state 210pb flux and the use of 228ra 226ra as a geochronometer on the amazon continental shelf the recovery of quasi cyclic 210pb profiles from the foreset region of the amazon subaqueous delta has raised questions concerning the non steady state flux of 210pb to this region of the shelf radioisotopic profiles 210pb 234th 228ra 226ra 137cs from seven kasten cores along with the analysis of grain size organic carbon and clay sized mineralogy are used to investigate the time scale and the controlling mechanisms of the 210pb cyclicity a common assumption used in 210pb geochronology is that the initial specific activity is constant with time an assumption that is invalid in this region variations of particle affinity for 210pb have been shown to affect the initial specific activity reaching the seabed in various other studies however investigations of grain size organic carbon and mineralogy have revealed no relationship with the fluctuating 210pb activity in this area a new geochronometer 228ra 226ra is used to determine sediment accumulation rates over a 30 year time scale which range from 10 to 60 cm yr in the study area spectral analysis of the 210pb cyclicity has revealed that the major component of 210pb cyclicity has a frequency of approximately 2 3 years in addition components of the 210pb cyclicity occur on annual and semi annual frequencies the controlling mechanism for the 210pb cyclicity on these time scales appears to be the seaward transport and accumulation of low activity fluid muds from the topset region

water inrush and environmental impact of shallow seam mining shallow seam mining is very common in china for enhancing coal recovery however this may cause a series of severe problems in environment and mining safety in some cases because the alluvium aquifers are directly above the shallow coal seams some typical water and sand inrush incidents resulting from shallow mining in china are analyzed based on in situ observation the mining induced strata failures are investigated the height of the fractured zone decreases as the size of the outcrop pillar or mining depth decreases the spatial distributions of the failure zones in the shallow mining have specific features compared to the deep mining these provide an advantageous condition for the shallow mining under alluvial aquifers optimal designs for outcrop pillar sizes are also given in different hydrogeological conditions for environment friendly and production safe mining at shallow depths according to in situ mining experiments in several coalfields technical measures for shallow mining are presented these primarily include the lift mining for thick seams and reduction of the thickness of extraction in the first lift short wall mining and detection of faults and other potential hazards prior to mining

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

factors controlling frequency of turbidites in the bengal fan during the last 248 kyr cal bp clues from a presently inactive channel abstract the seafloor of the bay of bengal is covered by thick sediment deposits that constitute the largest turbiditic system in the world this system is fed primarily by the ganges and brahmaputra rivers which drain the high himalayan ranges sediment transfers from the delta to the deep sea fan take place as turbidity currents in channel levee systems previous studies have shown that during high sea level stand periods the sediments were being mainly stored in the ganges brahmaputra delta and turbiditic transfer was occurring through active channels most of these channels are now inactive and sealed by hemipelagic deposits however the evolution of the inactive channels during the last sea level variations has never been described in detail sedimentation in the currently active channel the active valley was particularly important during the last sea level rise which suggests a very good connection between the fluvial systems and the deep turbidite system at this time during the monopol cruise 2012 we retrieved a giant piston core md12 3412 near the currently inactive e4 channel previous studies have hypothesized that this channel is connected to the swatch of no ground canyon on the upper fan the upper part of the core covers the last 250 kyr it reveals that contrary to what is known about the active valley the turbidite activity in e4 took place mainly during low sea level phases glacial stages and stopped around 11 8 kyr cal bp this different mode of activity suggests that i e4 was not abandoned but served as a secondary channel and ii that the supply of turbidite material at the site of core md12 3412 was not related to past changes in summer monsoon strength periods of activation of the e4 channel observed on core md12 3412 were previously identified on the shelf area by hubscher and spiess 2005 as thick forced regression system tracts frst after a displacement of deltaic edifices high turbidites record on the deep basin are mainly synchronous with sea level fall and rise conditions but mostly during low sea level periods this could be explained by a residual connection between the coastal system and the e4 channel during sea level low stands

importance of flood plain sedimentation for river sediment budgets and terrigenous input to the oceans insights from the brahmaputra jamuna river recent authors have suggested that a significant proportion of the worldwide terrigenous sediment budget is trapped landward of the river ocean boundary in high load tectonically active basins to test this idea modern flood plain sediment accumulation rates were determined along a 110 km reach of the brahmaputra locally jamuna river using 137cs geochronology of sediment cores and geographic information system gis extrapolation to adjacent areas 137cs accumulation rates decrease exponentially away from the channel from 4 cm yr on the natural levees to 1 cm yr within a few tens of kilometers basinward important controls on sedimentation in this area include proximity to distributary channels local topography and interannual variability of the flood pulse model results indicate that an average of 23 m t yr of sediment are sequestered in this section of the flood plain extrapolated basinwide as much as 39 71 of the river sediment budget may be trapped landward of the ganges brahmaputra mouth

source to sink sedimentary systems and global carbon burial a river runs through it abstract source to sink sedimentary systems are important settings of carbon cycling serving as sites of carbon transfer between terrestrial and marine reservoirs and as the primary locations for organic carbon burial on earth the age and character of the carbon that is buried at the terminal ends of these systems reflects the sources and transformations of the organic carbon oc throughout their linked terrestrial and marine segments profound differences are observed between large passive and small active margin systems large passive margin systems are characterized by large floodplains and relatively broad shelves where oc has protracted exposure to oxidants rapid burial in prograding subaqueous deltaic clinoforms or bypass to submarine fans however leads to high burial efficiency of terrestrial biospheric oc in some passive margin settings the oc in small active margin systems in contrast follows relatively short pathways from headwaters to seabed this rapid transit facilitated by the important role of storm driven transport in such settings can lead to high oc burial efficiencies the study of oc sources and transformations in contemporaneous source to sink sedimentary systems informs interpretations about the systems in which oc was buried in the geologic past their stratigraphic records of environmental change and their potential to produce petroleum resources

shelf sedimentation off the ganges brahmaputra river system evidence for sediment bypassing to the bengal fan the nature of shelf sedimentation seaward of the world9s largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples sediments from the ganges brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m forming a clinoform like deposit similar to subaqueous deltas found off other major river systems the highest sediment accumulation rates on the shelf occur near the head of the swatch of no ground a major submarine canyon that indents the shelf west of the present river mouths this observation together with textural data suggests that river sediments are transported seaward and westward and that the swatch of no ground is currently a major conduit for the transport of sediments from the bengal shelf

high frequency environmental changes and deposition processes in a 2 kyr long sedimentological record from the cap breton canyon bay of biscay the core md03 2693 43 39 258 n 01 39 805 w 431 m water depth was collected on an abandoned meander of the capbreton canyon se bay of biscay filled over the last millennia by very high sedimentation rates mean sedimentation rate of 1 2 cm yr linked to its specific environmental location and fine grained clayed sediment decantation from the proximal canyon axis this archive thus permits to undertake the study of late holocene regional climatic patterns at a decadal temporal resolution in the present work we use data derived from planktonic foraminifera assemblages coupled to a multiproxy approach that associates grain size measurements x ray fluorescence xrf elemental analysis and stable oxygen isotope on globigerina bulloides shells to infer sea surface temperature sst and sea surface salinity sss changes over the last two millennia signals reconstructed in the bay of biscay show significant oscillations that are consistent with well known temperature anomalies such as the little ice a

holocene and modern sediment budgets for the ganges brahmaputra river system evidence for highstand dispersal to flood plain shelf and deep sea depocenters the partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development however few quantitative sediment budgets that encompass entire dispersal systems exist for the ganges brahmaputra river system sediment discharge is estimated to be 10 9 t yr at gauging stations 300 km inland of the coast but little has been known of the downstream fate of this material geochronological geophysical and stratigraphic investigations of the lowland flood plain delta plain and shelf help to delineate the extent of holocene fill and allow calculation of a first order sediment budget results reveal that 1500 10 9 m 3 of sediment fill has been sequestered within the flood plain and delta plain since ca 7000 yr b p or about one third of the annual discharge the remaining load appears to be apportioned between the prograding subaqueous delta 1970 10 9 m 3 and transport to the deep sea bengal fan via a nearshore canyon modern 100 yr budget estimates based on short term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters the roughly equal partitioning of sediment among flood plain shelf and deep sea settings reflects the respective influence of an inland tectonic basin a wide shelf and a deeply incised canyon system the findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems furthermore results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations

discussion ʻchanging river courses in the western part of the ganga brahmaputra delta by kalyan rudra 2014 geomorphology 227 87 100 abstract rudra 2014 geomorphology 227 87 100 presented a review of geomorphic changes of the indian part of the ganga brahmaputra delta gbd during the last 240 years emphasising deterioration of distributary channels effects of discharge augmentation from the farakka barrage project into the bhagirathi hugli river and the patterns of coastal erosion the work contained many inconsistencies improper understanding and misrepresentation of facts the principal of these involved 1 usage of nomenclature like gbd bengal basin and their delineation 2 discussions made on evolution and current status of the deltaic distributaries and 3 generation and presentation of data on discharge tides and sea level besides this most figures of the work contained positional inaccuracies and or errors in survey years we systematically addressed all these issues in the present discussion and offered corrected and up to date information wherever applicable

suspended sediment movement in the estuary of the ganges brahmaputra meghna river system abstract topographical flow volume and suspended sediment concentration ssc measurements are presented in the large area of a turbidity maximum in the estuary of the ganges brahmaputra meghna river system in bangladesh the zone of turbidity maximum is indicated by the satellite imagery taken during the low river discharge period topographical and flow volume measurements the aspect ratio and the flow ratio indicate that the system is divided into flood and ebb dominated channels suspended sediment movement in the estuary appears to be associated with the horizontal stratification of flow and with the residual movement induced by the asymmetry of the tidal wave the flow ratio at different sections indicates that net import of suspended sediment takes place through flood channels while the net export occurs through ebb channels despite this difference in the direction of net movement of sediments measurements show that the ssc both in the flood channel and in the ebb channel is dependent on the tidal range the spring tidal ssc is about twice that of the neap tide and locations with higher tidal range show higher magnitudes of ssc the response of sediments to the varying energy environment of tidal range and tidal current however is not instantaneous the ssc shows a time lag in its variation with the varying energy condition the amplification and asymmetry of the tidal wave in the flood channels and the slack water asymmetry measured in a flood dominant location indicate that a possible landward residual transport of suspended sediment due to this factor is more pronounced through flood channels

sea ice diatom contributions to holocene nutrient utilization in east antarctica combined high resolution holocene 30sidiat and 13cdiat paleorecords are presented from theseasonal ice zone east antarctica both data sets re ect periods of increased nutrient utilization by diatomsduring the hypsithermal period circa 7800 to 3500 calendar years cal years b p coincident with a higherabundance of open water diatom species fragilariopsis kerguelensis increased biogenic silica productivity bsi and higher regional summer temperatures the neoglacial period after circa 3500 cal years b p isre ected by an increase in sea ice indicative species fragilariopsis curta and fragilariopsis cylindrus upto50 along with a decrease in bsi and 13cdiat 18 to 23 however over this period 30sidiatdata show an increasing trend to some of the highest values in the holocene record average of 0 43 competing hypotheses are discussed to account for the decoupling trend in utilization proxies including ironfertilization species dependent fractionation effects and diatom habitats based on mass balance calculations we highlight that diatom species derived from the semi enclosed sea ice environment may have a confoundingeffect upon 30sidowncorecompositions of the seasonal sea ice zone a diatom composition with approximately28 of biogenic silica derived from the sea ice environment diat si can account for the increased averagecompo sition of 30sidiatduring the neoglacial these data highlight the signi cant role sea ice diatoms can playwith relation to their export in sediment records which has implications on productivity reconstructions fromthe seasonal ice zone

cyclones and tides as feeders of a submarine canyon off bangladesh extremely high annual sedimentation rates of about 50 cm yr at the head of the submarine canyon swatch of no ground which connects the submarine delta of the ganges brahmaputra to the bengal deep sea fan were estimated by 210 pb and 137 cs gamma spectrometry the sediment in the canyon at 228 m water depth consists of graded sand and silt layers and laminated mud the sand and silt layers are assumed to have been deposited by tropical cyclones whereas the intercalated laminated mud is thought to have been deposited by daily variations of tidal currents the graded layers correlate well with the historical record of cyclones and allow an annual fine tuning the rapid infill of the canyon head is caused by trapping of suspended sediment that has been mobilized by storms and tides on the delta topset the sediment is stored only temporarily in the canyon head slides slumps and earthquakes occasionally remove and transfer it by turbidity currents to the deep sea fan the focusing effect of the canyon and the subsequent stepwise export of sediment probably combined with erosion of the canyon floor explain the long term regional stability of this canyon and probably other shelf canyons

parasound echosounding comparison of analogue and digital echosounder records and physical properties of sediments from the equatorial south atlantic abstract the new parasound echosounding system krupp atlas elektronik gmbh kae recently installed on the german research vessel meteor proved to be a very useful tool in high frequency sediment echography during two recent cruises in the equatorial atlantic conventional echosounder records provide only a qualitative picture of the structure of sedimentary sequences in contrast the main advantages of the parasound system are its ability to change frequency and signal length of the incident signal and its access to seismogram data via an interface for digital data acquisition first we present some examples of typical echosounder records from the african continental margin off angola which illustrate the capabilities of the new instrument these records facilitate lateral correlation of even small scale sediment structures secondly we compare synthetic seismograms calculated from measured physical properties of sediment cores with analogue echosounder records this shows that strong changes in lithology due to turbidites or sand layers as well as climatically controlled variations in carbonate content can be correlated directly both with whole core physical property logs and with seismic reflections in the parasound echograms finally we focus on digitized echosounder records obtained at coring stations and compare these data with synthetic seismograms derived from the core logs this procedure enables us to evaluate the relative importance of interference phenomena vis a vis lithological variations in the generation of reflection horizons in deep sea sediment sequences depending on the wavelength of the source signal small layers either with sharp or gradational changes in lithology and physical properties may be undetectable by seismic methods owing to destructive interference in contrast often even very weak reflections in echosounder records can be related to small variations in lithology and thus to physical properties

a data driven model of the global calcite lysocline gridded maps of sediment calcium carbonate calcite concentration and overlying water saturation state archer 1996 are combined with maps of benthic oxygen fluxes and sediment accumulation rates from jahnke 1996 and cweink 1986 to drive a diagenetic model of calcium carbonate preservation in deep sea sediments the only model input for which we cannot draw a detailed map is the rain rate of calcite to the seafloor so i use the model to calculate the calcite rain rate required to simulate the observed distribution of calcite concentration on the seafloor the predictive power of the model can be checked by searching for input parameters to which the model is sensitive and comparing the model requirements with independent data the model is sensitive to the ratio of organic carbon to calcite rain rates the rain ratio and within the constraint of pelagic sediment trap rain ratio data is unable to reproduce the calcite field without respiratory dissolution the promotion of calcium carbonate dissolution by the oxic degradation of organic carbon however relative to variability in sediment trap data the required model rain ratio is insensitive to the extent of anoxic respiration the stoichiometric ratio of o c during respiration the bioturbation rate the dissolution rate constant the effect of borate chemistry and small offsets in the saturation state for caco3 the model is sensitive to the accumulation rate of non caco3 material and predicts an atlantic pacific difference in non caco3 rain rate which is consistent with observations the model predicts that the dissolution flux of caco3 from sediments is 25 40 1012 mol yr 1 roughly half of the total deep sea caco3 dissolution rate estimated from the water column alkalinity distribution broecker and peng 1987 archer and maier reimer 1994 wollast 1994 the model also predicts that only 20 30 of the flux of caco3 to the seafloor globally escapes dissolution

early summer thermohaline characteristics and mixing in the western weddell sea abstract the ice station polarstern ispol cruise revisited the western weddell sea in late 2004 and obtained a comprehensive set of conductivity temperature depth ctd data this study describes the thermohaline structure and diapycnal mixing environment observed in 2004 and compares them with conditions observed more than a decade earlier hydrographic conditions on the central western weddell sea continental slope off larsen c ice shelf in late winter early spring of 2004 2005 can be described as a well stratified environment with upper layers evidencing relict structures from intense winter near surface vertical fluxes an intermediate depth temperature maximum and a cold near bottom layer marked by patchy property distributions a well developed surface mixed layer isolated from the underlying warm deep water wdw by a pronounced pycnocline and characterized by lack of warming and by minimal sea ice basal melting supports the assumption that upper ocean winter conditions persisted during most of the ispol experiment much of the western weddell sea water column has remained essentially unchanged since 1992 however significant differences were observed in two of the regional water masses the first modified weddell deep water mwdw comprises the permanent pycnocline and was less saline than a decade earlier whereas weddell sea bottom water wsbw was horizontally patchier and colder near bottom temperatures observed in 2004 were the coldest on record for the western weddell sea over the continental slope minimum temperatures were 0 4 and 0 3 c colder than during 1992 1993 respectively the 2004 near bottom temperature salinity characteristics revealed the presence of two different wsbw types whereby a warm fresh layer overlays a colder saltier layer both formed in the western weddell sea the deeper layer may have formed locally as high salinity shelf water hssw that flowed intermittently down the continental slope which is consistent with the observed horizontal patchiness the latter can be associated with the near bottom variability found in powell basin with consequences for the deep water outflow from the weddell sea

suspended sediment distribution and residual transport in the coastal ocean off the ganges brahmaputra river mouth abstract an examination of suspended sediment dispersal in the coastal ocean off the ganges brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world s largest sediment dispersal system during the low discharge period suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population satellite and sea bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20 isobath during the low discharge period a simplistic estimate of residual transport and its separation into tidal and non tidal components indicate that non tidal transport contributes about 47 of the total transport and has transport directions mostly southwestward during the northeast monsoon the tidal residual transport which contributes about 53 of the total transport however does not show any particular direction for most of the stations although some landward stations show shoreward transport although few water column measurements were made during the high discharge period data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year

iron and silicic acid concentrations regulate si uptake north and south of the polar frontal zone in the pacific sector of the southern ocean abstract we investigated the relative roles of fe and silicic acid availabilities in regulating si uptake rates across the polar frontal zone in the pacific sector of the southern ocean 59 68 s 170 w during the us jgofs antarctic environment southern ocean process study aesops meridional gradients in silicic acid concentration si oh 4 of about 0 25 0 56 m km 1 were observed in this area during austral spring and summer 1997 1998 with si oh 4 ranging from 32 si oh 4 and 15 no 3 uptake rates biogenic silica concentrations and si oh 4 no3 uptake ratios fe addition had little or no effect on si uptake rates in enrichments conducted in the low si waters north of the si gradient however fe addition increased si uptake rates 3 5 times over controls in enrichments conducted in the high si waters south of the gradient in both spring and summer fe addition decreased si oh 4 no3 uptake ratios by 2 5 times largely due to stimulation of no3 uptake rates zn addition had no effect on si oh 4 and no3 uptake rates short term 24 h si additions had varying effects on si uptake rates depending on season and location in spring additions of 40 m si to water from bottle enrichments conducted north of the si gradient in situ si oh 4 15 m did not increase si uptake rates initially but did increase uptake rates after 8 days in the summer enrichment north of the si gradient in situ si oh 4 5 m 50 m si additions doubled in situ si uptake rates in the initial water collected for the enrichment and increased si uptake rates as much as 16 fold during the experiment south of the si gradient where in situ si oh 4 was 40 m in both spring and summer si addition had no effect on in situ si uptake rates in the initial enrichment water nor on any si uptake rates measured during the experiment our results indicate that both fe and si availabilities regulate si uptake rates and silica production in the southern ocean along 170 w fe limitation appears to restrict si uptake rates south of the si gradient and plays a role in preventing si depletion south of the acc where ambient si oh 4 never fell below 40 m during 1997 1998 our experiments in the seasonal ice zone at 62 suggest that si uptake in this area switched from being fe limited in the spring when in situ si oh 4 was 40 m to si limited in the summer when si oh 4 was

assessment of rock mass characteristics and the excavation disturbed zone in the lingxin coal mine beneath the xitian river china safe production and induced hazard prevention in coal mines mainly concern problems in the excavation disturbed zone edz but coal mining can also make a significant impact on the environment comprehensive techniques using a borehole tv viewer system electrical logging and monitoring of drilling fluid leakage offer the ability to identify changing geological conditions and the location of any nearby abandoned mining workings the inner collapsed zone and the expanding height of the crack zone transmitting water plus their spatial distribution after excavating the superincumbent coal seam stratum are determined at no l3414 workings lingxin coal mine beneath the xitian river china an integrated system of gis based on a stochastic model which has been developed for the prediction of dynamic subsidence coupling the time function and probabilistic integral method is verified through practice at the lingxin coal mine this development has greatly expanded the ability of the mine operator to characterize previously inaccessible areas of the mine providing a reliable basis for safe mining and prevention of induced hazards to ensure high production mining underneath the river bed

height of the mining induced fractured zone above a coal face abstract the development height of a gas conducting fracture zone gfz in the gob overlying strata is crucial to the gas drainage and safe production of a coal mine in order to address the issues of excessive gas concentration and uncertain gfz height in no 7435 face overlying strata of kongzhuang coal mine china the caving characteristics of overlying strata were explored using both physical experiments on similar materials and numerical simulations of particle flow code pfc software and verified each other the relationship of cracks development to porosity changing characteristics was introduced to quantitatively determine the height of the local gfz the quantified gfz heights were compared with those measured using the in situ drilling flow method the results showed that 1 pfc software could accurately simulate the overlying strata caving behaviors thus saving manpower materials and financial resources needed for related physical experiments and 2 the temporospatial distribution characteristics of porosity could be used to forecast gfz height and are of significant importance for determination of gfz overall the conclusions are of engineering significance for accurate arrangement of boreholes for gas drainage and reduction of mine gas disasters

tidal influence on suspended sediment distribution and dispersal in the northern andaman sea and gulf of martaban abstract surface and water column profiles of suspended matter collected during april may 2002 and satellite images were used to study factors influencing suspended sediment concentrations sscs and dispersal in the northern andaman sea and gulf of martaban one of the largest highly turbid areas of the world s oceans perennial high ssc in the gulf of martaban is due to a combination of factors including resuspension of sediments by strong tidal currents shallow bathymetry and seasonal sediment influx from rivers from satellite images it was observed that in the central portion of the gulf of martaban the turbidity front oscillates about 150 km in phase with spring neap tidal cycles and the area covered by the turbid zone ssc 15 mg l 1 increases from less than 15 000 km2 during neap tide to more than 45 000 km2 during spring tide the sediment discharged by the ayeyarwady river is transported mainly eastward along the coast into the gulf of martaban occasionally during the winter monsoon period sediment plumes are seen heading westward into the bay of bengal turbidity profiles show that bottom nepheloid layers are actively transporting some of the sediments into the deep andaman sea via the martaban canyon

four storm event beds and the tropical cyclones that produced them a numerical hindcast a three dimensional numerical prediction system for storm sedimentation has been constructed to compute a cyclonic wind field coastal circulation storm waves generated over the continental shelf the combined effects of steady currents and waves on the benthic boundary layer both suspended and bed load transport of sediment and conservation of the sea floor it has been used to hindcast the oceanographic and sedimentological responses of the western gulf of mexico to four historical tropical cyclones in order to investigate the effects of coastal configuration and storm variability on event bed genesis the simulations reveal several common responses to these storms 1 onshore flow to the fight of the storm track generally transports fine sediment landward 2 off shore flow to the left transports coarser sediment seaward and 3 for an observer facing the coast right to left along shelf flow transports finer sediment in deep water and coarser sediment in shallow water the main source of sandy sediment on the inner shelf is the shoreface which is eroded by currents driven by wind stress geostrophy and offshore flows associated with collapse of the coastal setup coastal geometry is the dominant factor in determining sedimentation patterns along the coast in front of each storm the volume of sediment transported obliquely onshore or offshore is a function of shelf gradient and coastal configuration steeper gradients constrain flow to a more longshore pattern concave coastlines promote greater shoreface erosion because of increased coastal setup none of the simulations reveals the vertical current structure predicted by the geostrophic model for continental shelf circulation because the calculated depth of the wind mixed layer exceeded local water depth everywhere over the shelf thus in these experiments most of the continental shelf fell within the friction dominated zone in which the upper and lower boundary layers overlap resulting in vertically uniform along shelf flow

formation of fractured zones in overburden due to longwall mining the fractured zones caused by mining were studied in the overburden of the torezko snezhnyanskaya area ukraine through the change in natural gas emission from these zones during longwall coal excavation zones of interconnected fractures and separate horizontal fractures were studied with vertical wells drilled from the ground surface down to active underground workings the maximum heights of the zone of interconnected fractures and separate horizontal fractures may reach 19 41 and 53 92 times the thickness of the coal seam respectively it was found that the ratio between the maximum height of the zone of interconnected fractures and the thickness of the extracted coal seam increases with the increasing number of rock layer interfaces and decreases with the increasing stiffness of immediate roof it is shown that the growth of the zone of interconnected fractures occurred during 17 39 days at an average rate of 0 94 1 97 m day 1 and it was accompanied by increasing methane emission from overburden observation shows that the formation of separate horizontal fractures began only 11 49 5 days after the height of the zone of interconnected fractures reached its maximum value formation of separate horizontal fractures in overburden over the longwall excavation occurred as a stepped process from lower to upper sandstone sandy shale layer interfaces in the direction of the ground surface

sediment transport in the shelf canyon swatch of no ground bay of bengal the swatch of no ground is a shelf canyon that deeply incises the bengal shelf near the ganges brahmaputra river mouth cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and bengal fan sedimentation rates in the swatch of no ground are highest near the canyon head at 50 cm a 1 decreasing to 15 cm a 1 in 600 m water depth the canyon deposits consist of intercalated fine silt clay and coarse silt sand grained deposits in seismic profiles small scale sedimentary structures and parallel bedded layers reveal that sediment in the canyon is mainly deposited from suspension during fair weather conditions tidal currents are the dominant mechanism that transports plumes of suspended river load towards the canyon forming fine grained silt clay layers during storm conditions sediment is resuspended on the inner shelf and subaqueous delta east of the canyon storm generated bottom currents transport the resuspended sediment alongshelf to the canyon where the particles are trapped and form graded coarse silt sand layers channels and gullies in the swatch of no ground indicate active gravity driven currents with an erosional character in the upper canyon and non depositional character below 450 m suggesting that persistent or ephemeral currents presently export sediment to the bengal fan numerous slumps and slides observed in the canyon show small scale acoustically transparent layers that indicate sediment liquefaction presumably initiated along the steep canyon margin these mudflows move downslope and halt where the canyon gradient decreases one widespread acoustically transparent layer with an age of approximately 140 160 years bp can be traced throughout much of the canyon and is probably the result of large scale sediment remobilization triggered by a catastrophic event like an earthquake this may represent decennial to centennial scale events that remove large quantities of sediment and prevent the canyon from rapid infilling

particle fluxes to the interior of the southern ocean in the western pacific sector along 170 w abstract an array of five bottom tethered moorings with 19 parflux time series sediment trap at three depths 1 and 2 km below the surface and 0 7 km above the sea floor was deployed in the western pacific sector of the southern ocean along 170 w the five stations were selected to sample settling particles in the main hydrological zones of the southern ocean the sampling period spanned 425 days november 28 1996 january 23 1998 and was divided into 13 or 21 synchronized time intervals a total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass tmf organic carbon carbonate biogenic silica and lithogenic particles the fluxes of biogenic material were higher than anticipated challenging the notion that the southern ocean is a low productivity region organic carbon fluxes at 1 km depth within the polar frontal zone and the antarctic zone were relatively uniform 1 7 2 3 g m 2 yr 1 and about twice the estimated ocean wide average ca 1 g m 2 yr 1 carbonate fluxes were also high and uniform between the subantarctic front and ca 64 s 11 13 g m 2 yr 1 a large fraction of the carbonate flux in the antarctic zone was due to the presence of pteropod shells coccoliths were found only to the north of the polar front and calcium carbonate became the dominant phase in the subantarctic zone in contrast carbonate particles were nearly absent near 64 s latitudinal variations in biogenic silica fluxes were substantial the large opal flux 57 g m 2 yr 1 measured in the antarctic zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the polar front unlike biogenic material fluxes of lithogenic particles were among the lowest measured in the open ocean 0 12 0 05 g m 2 yr 1 reflecting a very low dust input

prediction of the height of destressed zone above the mined panel roof in longwall coal mining abstract longwall mining is one of the most widely used underground mining methods most suitable in relatively flat lying thick and uniform coalbeds due to extraction of the coal seam the panel roof strata above the mined zone will be destressed and then the roof loads will be redistributed and transferred to the front abutment and neighboring solid sections with higher load bearing capacity where the adjacent access tunnels and the corresponding barrier pillars are located the height of destressed zone hdz in this paper is taken as equivalent to the combined height of the caving and fracturing zones above the mined panel roof induced due to longwall mining the height of destressed zone plays a vital role in accurate determination of the amount of loads being transferred towards front abutment and panel sides the paper describes the mechanism of development of the height of this zone finally five new simple yet conclusive mathematical approaches to estimate the height of destressed zone are presented the results of the methods proposed are compared with each other and with the comparable methods the methods proposed are further compared and verified with in situ measurements reported in the literatures the comparative results confirm the agreement that exist among the methods and those with the in situ measurements as well finally the methods have shown that in short term the height of destressed zone ranges from 6 5 to 24 times the extraction coal seam height while in long term the height of destressed zone ranges from 11 5 to 46 5 times the extraction coal seam height therefore beyond this height the overburden pressure will be transferred towards the front abutment the adjacent access tunnels the intervening barrier pillars as well as the panel rib sides

full waveform ultrasonic transmission seismograms a fast new method for the determination of physical and sedimentological parameters of marine sediment cores detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic p wave seismograms recorded automatically during full waveform core logging the physical parameter which predominantly affects the elastic wave propagation in water saturated sediments is the p wave attenuation coefficient the related sedimentological parameter is the grain size distribution a set of high resolution ultrasonic transmission seismograms 50 500 khz which indicate downcore variations in the grain size by their signal shape and frequency content are presented layers of coarse grained foraminiferal ooze can be identified by highly attenuated p waves whereas almost unattenuated waves are recorded in fine grained areas of nannofossil ooze color encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation a modified spectral difference method is introduced to determine the attenuation coefficient and its power law a kf applied to synthetic seismograms derived using a constant q model even low attenuation coefficients can be quantified a downcore analysis gives an attenuation log which ranges from 700 db m at 400 khz and a power of n 1 2 in coarse grained sands to few decibels per meter and n s 0 5 in fine grained clays a least squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient when it is used to predict the mean grain size an almost perfect coincidence with the values derived from sedimentological measurements is achieved

2000years of frequent turbidite activity in the capbreton canyon bay of biscay abstract capbreton canyon extends from 200 m water depth near the aquitanian coast to 3000 m water depth before abruptly turning northward and merging with the cap ferret systems the capbreton canyon has been disconnected from the adour river input since 1310 ad despite this situation modern sedimentary activity is evident in the first kilometres from the head present sedimentation in the capbreton canyon was studied by combined analysis of acoustic data and cores cores from the topographic highs document moderate hemipelagic sedimentation and record the global climatic signal of the bay of biscay within the canyon at 80 km from the head on terraces bordering the thalweg at 1600 m water depth cores attest to recent turbidite activity massive medium sand is restricted to the thalweg floor where sediment transfer dominates fine grained turbidites are preferentially accumulated on the shallower terraces near the thalweg the accumulation rate is 3 and 1 cm year at 75 m and 125 m above the thalweg respectively on a higher terrace 225 m turbidite sequences are absent or too thin to be preserved from bioturbation the sedimentation rate is lower approximately 0 17 cm year but still 10 times higher than the hemipelagic sedimentation outside the capbreton canyon we demonstrate that frequent turbidity currents 1 year are currently responsible for very high accumulation of dominantly fine grained sediments on the terraces that are the nearest from the thalweg we assume an impact of nepheloid layers on the sedimentation of the highest terraces looking for any impact on sediment supply of the disconnection of the capbreton canyon head from the adour river mouth it appears that the impact on the sedimentation affected the nature of the particles which are less carbonate rich rather than the quantity of sediments this result suggests that the longshore drift at the canyon head and hydrodynamic events are major factors in the sedimentation of capbreton canyon

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

shelf to canyon connections transport related morphology and mass balance at the shallow headed rapidly aggrading swatch of no ground bay of bengal abstract the swatch of no ground song canyon in the bay of bengal is a shelf incising submarine canyon that is actively aggrading in its upper reaches despite regular gravity driven transport and mass wasting although the canyon lies 150 km downdrift of its main sediment source the ganges brahmaputra meghna gbm river mouth high sedimentation rates 5 50 cm year 1 are sustained by both progradation of the subaqueous delta into the canyon head and the conveyance of shelf generated hyperpycnal flows to the canyon floor this rapid accretion appears to be largely balanced by mass failures triggered by regularly occurring storms and less frequently by major earthquakes here we use high resolution sub bottom sonar data to elucidate dominant sediment dispersal pathways and their transport related morphology at the canyon head these include 1 a laterally prograding clinoform that intersects the canyon head at water depths of 20 120 m 2 several shelf incising bypass gullies that originate in 50 m the clinoform deposits reflect westward progradation of the gbm subaqueous delta into the upper canyon where its axis normal orientation leaves it dissected by cross cutting gullies and mass failures the morphology and acoustic stratigraphy of the gullies coupled with strong bed shear and high suspended sediment concentrations on the inner shelf suggest that these features are sustained by the regular conveyance of gravity driven fluid muds that are formed in shallow water

geostrophic circulation or shallow marine turbidity currents the dilemma of paleoflow patterns in strom influenced prograding shoreline systems in ancient storm influenced prograding shoreline sequences sole marks mainly tool marks from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin asymmetrical tool marks typically indicate flows directed offshore several workers have attributed their formation to storm generated shallow marine turbidity currents this interpretation conflicts with observations from modern shelves where storm driven circulation generally is geostrophically balanced and time averaged bottom currents approximately parallel bathymetric contours and the local shoreline the resolution of these apparently conflicting observations may lie in the realization that tool marks and many other small paleoflow indicators form almost instantly as the result of instantaneous flow conditions very near the bed beneath storm generated flows in the shallow ocean instantaneous and time averaged characteristics of the bottom boundary layer generally exhibit little similarity storm generated tool marks are formed by the movement of large tools within the thin less than 1 m inner boundary layer resulting from the superimposition of waves and currents the orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave orbital motions which typically are normal to shore the magnitude of stress is greatly increased in the offshore direction and decreased in the onshore direction by superimposition of a steady current with an offshore component of flow but the direction of stress is only slightly affected in ancient storm influenced sequences therefore shore normal tool marks generally were not formed by turbidity currents rather their orientation is best attributed to shoaling waves approaching the coast at a very high angle asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component tool marks do not reflect the time averaged bottom flow direction in fact they provide almost no information concerning steady bottom currents in contrast high angle cross beds formed in coarser sediment by the migration of dunes and sandwaves although relatively rare in offshore storm deposits generally reveal approximately shore parallel flows in ancient systems cross beds closely reflect the time averaged flow direction in the outer boundary layer of a geostrophic current for three reasons 1 the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time averaged shear stress 2 large ripples disrupt the thin inner boundary layer and 3 long time intervals relative to wave induced velocity oscillations are required to form large ripples