inferring maximum geostrophic current velocities in the norwegian greenland sea from settling velocity measurements of sediment surface samples methods application and results abstract critical shear velocities and geostrophic current velocities were calculated from measured grain sizes and settling velocities using a shields type threshold curve of particle movement and assuming bedload transport for particles that were found to be transported by currents because this reconstruction of the current velocities is based on the coarsest transported component that could be recognized in the sediment the resulting current velocities represent a maximum flow speed that was effective during deposition of sediment the shape of the settling velocity distributions identifies the dominant type of deposition that controlled the accumulation of sediment residual well sorted or low energy depositional the resulting pattern of reconstructed maximum geostrophic current velocities in the norwegian greenland sea reflects the known features of present day thermohaline circulation which is influenced by the bathymetry at many places reconstructed current velocities are higher in areas where the cross section of meridional flows is restricted by topographic barriers the distribution of the depositional characters shows that residual and well sorted sediment types are generally associated with higher current velocities in areas of strong geostrophic currents e g fram strait and east greenland continental slope the low energy depositional sediment type is widespread in the basins of the eastern norwegian greenland sea where lower reconstructed maximum current velocities occur the high reconstructed velocities on the spitsbergen bank can be attributed to the disturbance from storm waves in shallower water depths

detecting temporal shoreline changes and erosion accretion rates using remote sensing and their associated sediment characteristics along the coast of north sinai egypt the variation during 15 years in the shoreline along the north sinai coast has been determined by analysing tm and etm true colour landsat images from 1986 to 2001 the analyses identified erosion and accretion patterns along the coast the shoreline has advanced west of el bardawil inlet1 el bardawil inlet2 and el arish harbour where the wave induced littoral transport has been halted by jetty construction and beach growth rates are 20 681 69 855 and 20 160 m2 year respectively on the downdrift side of the constructed jetties to the east the shoreline is retreating and beaches erode at rates of 71 710 69 968 and 11 760 m2 year respectively sedimentological analyses of beach sediment samples have indicated selective transport of heavy minerals according to their densities and grain sizes a general correspondence has been found between variation in grain size sorting and heavy mineral content of beach sand and the patterns of shoreline changes

spatial and temporal variability of titanomagnetite placer deposits on a predominantly black sand beach abstract titanomagnetite forms rich placer deposits along the northwest coast of new zealand these deposits were sampled along 3 shore normal transects spaced over the southern 2 km of a dissipative high energy beach on the west coast in 5 field campaigns covering one year the percentage of opaque minerals mainly titanomagnetite was extremely high in the upper 30 m of the beach face extending seaward where these opaque minerals were gradually replaced with variable amounts of lighter augite hornblende and plagioclase the pattern appeared to be divided into two regions a lower seaward and an upper landward region separated by a point where either marine dominated over aeolian processes or where swash dominated over breaking processes in the seaward region the percentage of opaques increased and particle size fined landward as undertow removed the lighter larger particles seaward in the landward region the percentage of opaques and particle size were more constant or even showed the reverse pattern as wind transported the lighter material shoreward or swash asymmetry transported the heavier material seaward the similarity of settling velocities over the whole beach face suggests that sorting by size rather than weight plays a dominant role in separating the mineral assemblages considerable variations existed between transects this could be explained by the spatial changes in surfzone waves and currents that were associated with proximity to the southern headland and various rip current channels that characterised this dissipative site surprisingly the percentage of opaques decreased when the wave conditions of the day of sampling were more energetic in contrast to many other placer deposits these deposits are abundant on the beach face forming an armouring layer during lower wave energy conditions during higher wave conditions the surface layer erodes allowing lighter augite plagioclase and hornblende to be released from the sediments below

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

understanding heavy mineral enrichment using a three dimensional numerical model layered deposits of relatively light and heavy minerals can be found in many aquatic environments quantification of the physical processes which lead to the fine scale layering of these deposits is often limited with flumes or in situ field experiments therefore the following research questions were addressed i how can selective grain entrainment be numerically simulated and quantified ii how does a mixed bed turn into a fully layered bed and iii is there any relation between heavy mineral content and bed stability herein a three dimensional numerical model was used as an alternative measurement to study the fine scale process of density segregation during transport the three dimensional model simulates particle transport in water by combining a turbulence resolving large eddy simulation with a discrete element model prescribing the motion of individual grains the granular bed of 0 004 m in height consisted of 200 000 spherical particles d50 500 m five suites of experiments were designed in which the concentration ratio of heavy 5000 kg m to light particles i e 2560 kg m was increased from 6 15 35 60 to 80 all beds were tested for 10 s at a predefined flow speed of 0 3 m s analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time the latter accumulated at the bottom of the domain forming a layer whereas the lighter particles were transported over the layer forming sweeps particles below the heavy particle layers indicated that the layer was able to armour the particles below consequentially enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart which enhances current understanding of heavy mineral placer formation this article is protected by copyright all rights reserved

studies on settling traction and entrainment of larger benthic foraminiferal tests implications for accumulation in shallow marine sediments settling and traction velocities were measured on optimally preserved tests of larger foraminifera using a settling tube and flume tank within larger foraminifera with porcelaneous tests the peneroplids peneroplis antillarum p planatus p pertusus and dendritina cf d zhengae are distinguished by low test densities ca 1 2 that do not change with growth buoyancy is high because of low reynolds numbers and increases in large individuals because of the allometric change of test shape the fusiform alveolinella quoyi with test densities ca 1 6 is characterized by high reynolds numbers inducing the weakest buoyancy within porcelaneous larger foraminifera the highest buoyancy was recorded for the three soritids parasorites orbitolitoides sorites orbiculus and amphisorus hemprichii because of their low test densities ca 1 25 and the extremely flat biconcave plate like shape flat tests however reduce traction and entrainment from smooth surfaces within hyaline larger foraminiferat the amphisteginids show thick lenticular amphistegina lobifera a radiata to thin lenticular tests a bicirculata a papillosa influencing buoyancy here high test densities ca 1 8 decrease with growth in a lobifera a lessonii and a bicirculata and remain constant in a radiata and a papillosa minimum velocities required for entrainment are lower for thick lenticular tests and higher for thin lenticular tests test densities remain constant with growth in the calcarinid baculogypsina sphaerulata 1 78 and decrease slightly in calcarina gaudichaudii and neorotalia calcar starting at 1 85 all living under extreme hydrodynamic conditions density decreases the most in baculogypsinoides spinosus starting at 1 8 resulting in higher buoyancy through low reynolds numbers traction is promoted in spherical tests of baculogypsina and baculogypsinoides within nummulitids the thick lenticular palaeonummulites venosus test density decreasing with size starting at 1 78 is less buoyant expressed in high reynolds numbers but easily entrained thick lenticular juveniles and extremely flat adults distinguish operculinella cumingii heterostegina depressa and the giant cycloclypeus carpenteri test densities increase during growth starting from ca 1 6 and attaining a maximum of 1 8 buoyancy is low in small tests and high in large tests while entrainment velocities are reduced as the tests flatten high buoyancy is also a characteristic of the entirely flat tests in operculina ammonoides from deeper regions and planostegina operculinoides which is expressed in the lowest reynolds numbers within larger foraminifera

magnetic mineral transport and sorting in the swash zone northern lake erie canada a combined field and laboratory study in northern lake erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non magnetic sediment work was conducted on the cross shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement in the field of samples of magnetic magnetite and non magnetic quartz and calcite grains respectively laboratory experiments examined how the burial of small dense magnetic minerals is affected by the grain size of the non magnetic host material and how grain burial affects magnetic susceptibility measurements at the surface the field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser non magnetic grains under low to moderate wave conditions and subsequently were unable to move in the longshore or cross shore directions the laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals and that magnetic grain burial was most effective beneath coarser rather than finer non magnetic sand and for the latter sediments under less rather than more energetic conditions the results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions and subsequent settling exposure and concentration in the upper swash zone during more energetic periods when the non magnetic grains are eroded it is probably during these erosional periods when the magnetic minerals are exposed in fairly homogeneous deposits that longshore and cross shore transport takes place

selective entrainment and transport of mixed size and density sands flume experiments simulating the formation of black sand placers a process based model for the selective entrainment of grains from sands of mixed sizes and densities has been tested by measuring threshold stresses for a range of relative grain sizes in a laboratory flume the model was found to yield good predictions for the selective entrainment of different grain size fractions it predicts that a minimum threshold stress exists for intermediate grain sizes of a mixed size sediment while higher threshold stresses are required for the movement of both smaller and coarser size fractions this prediction is in part confirmed by our flume experiments and it also agrees with results of experiments by other investigators we also conducted flume tests with settling equivalent sediment mixtures to examine grain sorting leading to the concentration of heavy minerals the experiments yielded systematic increases in grain sizes and decreasing heavy mineral concentrations in the transport direction sorting patterns which imitate those found in our parallel studies of placers on ocean beaches

influence of nearshore sediment dynamics on the distribution of heavy mineral placer deposits in sri lanka beach sediments in sri lanka contain industrial grade heavy mineral occurrences samples of both offshore and onshore sediments were collected to examine the provenance mineralogy and geochemical compositions of the heavy mineral occurrences coastal morphodynamic changes along the coastline of sri lanka were analyzed using the time series satellite images these coastal morphodynamic changes were used to identify the prominent directions of monsoon influenced longshore currents coastal sediment accretion and depositional trends and their relationships to the provenance of the heavy minerals results show the concentrations of detrital ilmenite zircon garnet monazite and rutile vary in the onshore and offshore sediments the heavy mineral potential of the northeastern coast is high average contents of about 45 50 in the verugal deposit 70 85 in the pulmoddai deposit and 3 5 5 0 in offshore samples stretching from nilaveli to kokkilai compared to sediments in southwest average content about 10 in onshore sediments and 2 in offshore sediments from the mouth of the gin river therefore no economic grade heavy mineral placers were identified in the offshore environments the high concentrations of heavy minerals in beach sediments and low concentrations in offshore sediments suggest operation of a panning system in the surf zone to form enriched placer deposits major and trace element compositions of beach sediments show marked enrichments of tio2 fe2o3 la ce zr cr nb th and v compared to average upper continental crust ucc values analysis of prominent coastal longshore transport patterns identifies bidirectional sediment transport in the northeast coast of sri lanka in the southwestern coast two transport directions occur with anti clockwise transport from galle to hambantota and clockwise transport from hikkaduwa to wadduwa the heavy minerals in the placers were mainly derived from precambrian metamorphic rocks and transported to the coast through the river systems of sri lanka

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

geological setting and age of the flemish cap granodiorite east of the grand banks of newfoundland flemish cap a large isolated submarine knoll 600 km east of newfoundland consists of a central core of hadrynian rocks and an onlapping sequence of undisturbed to disturbed mesozoic cenozoic sediments the central core of the cap sampled with an electric rock core drill comprises pink fine to medium grained granodiorite dacite and volcanic siltstone the granodiorite samples collected in six out of eight cores are remarkably similar lithologically despite a separation of up to 65 km and probably represent a single pluton the aphanitic dacite and laminated cherty volcanic siltstone have been metamorphosed to subgreenschist prehnite facies but show no evidence of contact metamorphism u pb analyses of coarse and fine zircon fractions from a granodiorite core yielded upper intercepts of 751 and 833 ma respectively although not precise these ages probably represent the age of intrusion sometime in the 750 830 ma range and are older than those reported for similar hadrynian granitic rocks of e

planform and mobility in the meaipe maimba embayed beach on the south east coast of brazil the meaipe maimba embayed beach mmeb on the south east coast of brazil has been subject to anthropogenic pressures since the 70 s in this study we discuss the adequacy and contribution of the parabolic planform model to determine the planform and variability of the mmeb taking into consideration variation in wave conditions the role of different controlling conditions on the planform variability is analyzed as well as the morphological and planform mobility mmeb exhibited a new configuration in response to the construction of a harbor which interrupted the longshore sediment transport after four decades three particular morphodynamic sectors have been recognized along the beach the central sector is more exposed to normal wave incidence and cross shore processes predominate the northern and southern sectors are influenced by wave diffraction processes around the headlands and port respectively in the northern sector the presence of secondary headlands and inner islands imposed a geomorphological control on beach morphology and coastal processes the use of the parabolic planform model provided useful insights for the assessment of potential planform mobility since the decadal shoreline evolution combined with beach profiles and sediment characteristics allowed understanding of the beach mobility processes and supported the interpretation of modeling results

distribution of sulfur aerosol precursors in the spcz released by continuous volcanic degassing at ambrym vanuatu abstract the melanesian volcanic arc mva emits about 12 kt d 1 of sulfur dioxide so2 to the atmosphere from continuous passive non explosive volcanic degassing which contributes 20 of the global so2 emission from volcanoes here we assess from up to date and long term observations the so2 emission of the ambrym volcano one of the dominant volcanoes in the mva and we investigate its role as sulfate precursor on the regional distribution of aerosols using both satellite observations and model results at 1 1 spatial resolution from wrf chem gocart without considering aerosol forcing on clouds our model parameterizations for convection vertical mixing and cloud properties provide a reliable chemical weather representation making possible a cross examination of model solution and observations this preliminary work enables the identification of biases and limitations affecting both the model missing sources and satellite sensors and algorithms for aerosol detection and classification and leads to the implementation of improved transport and aerosol processes in the modeling system on the one hand the model confirms a 50 underestimation of so2 emissions due to satellite swath sampling of the ozone monitoring instrument omi consistent with field studies the omi irregular sampling also produces a level of noise that impairs its monitoring capacity during short term volcanic events on the other hand the model reveals a large sensitivity on aerosol composition and aerosol optical depth aod due to choices of both the source function in wrf chem and size parameters for sea salt in flexaod the post processor used to compute offline the simulated aod we then proceed to diagnosing the role of so2 volcanic emission in the regional aerosol composition the model shows that both dynamics and cloud properties associated with the south pacific convergence zone spcz have a large influence on the oxidation of so2 and on the transport pathways of volcanic species across the south pacific atmosphere for example in the tropical cloudy air the sulfate production in the aqueous phase is very efficient resulting in the formation of a large cloud of highly scattering sulfate aerosols advected horizontally to eastern indonesia in agreement with the aod feature captured by modis aqua but missed in caliop calipso lidar products model sensitivity experiments indicate that aerosol re suspension due to evaporating droplets is a significant pathway for the supply of volcanic sulfur species in the remote marine boundary layer by strongly modulating the irreversible loss due to wet scavenging this aerosol process has a similar influence on the sulfur burden as natural emission from volcanoes or biogenic sources like dimethyl sulfate dms the results emphasize the importance of mva passive degassing and spcz dynamics on the aerosol background and raise questions about potential impacts on the local climate and marine ecosystems

experiments on the entrainment threshold of well sorted and poorly sorted carbonate sands few studies have examined the hydrodynamic behaviour of carbonate sediments the data presented here are the result of preliminary research on entrainment in well and poorly sorted carbonate sands experiments were performed using naturally occurring sediments in a tilting recirculating freshwater flume results indicate that when of similar size shape and density the transport threshold of carbonate sands is similar to that of quartz however owing to their lower density and often platy or irregular shape skeletal sands require a lower shear stress to initiate transport because the density of carbonate particles may increasingly vary with grain size the threshold of motion in coarse carbonate grains may differ more markedly from that of quartz in poorly sorted samples results show that the coarse grained constituents move before the finer grained components grain properties and boundary layer dynamics are believed to explain this phenomenon rollability of the larger grains combined with physical trapping and immersion within a low velocity sublayer are believed to prevent finer particles from moving given the appropriate sediments and flow conditions it may therefore be possible to deposit and preserve fine grained sediments in a flow regime typically thought to transport such materials

a currents of removal approach for interpreting carbonate sedimentary processes abstract this study develops a currents of removal methodology to examine and quantify the nature of physical transport processes affecting the formation of bioclastic deposits in the cocos keeling islands an indian ocean atoll this approach is based on the hydraulic settling and threshold of entrainment characteristics of bioclastic deposits and measured current energy potential mobility pm analysis of 157 samples quantifies the proportion of deposits able to be transported in each geomorphic zone results show that under mean energy conditions wave induced currents entrain and transport sediment within the atoll and show that transport of sediment is not solely reliant on storm energy conditions gradients of pm values are used to reconstruct sediment transport pathways from the reef flat 85 100 pm through shallow passages 75 100 pm to sand aprons 50 75 pm and shallow 0 20 pm and deep lagoon 0 pm comparison of settling velocity distributions of bed material and sediment retained in sediment trap experiments show that actual mobility levels correspond with pm estimates potential mobility analysis also identifies the immobile portions of deposits which increase lagoonward constituent analysis of immobile fractions and analysis of settling frequency distributions are used to differentiate the importance of physical and biological processes in the formation of deposits throughout cocos and identify the role of each geomorphic zone in the transport system the pattern of sediment is controlled by physical processes between the reef flat the primary sediment production zone with sediment transported through the shallow passage conduits to the sand aprons the formation of shallow and deep lagoon deposits is controlled by autochthonous sediment production and storm deposition potential mobility analysis is a powerful tool enabling physical transport processes within bioclastic sedimentary environments to be quantified the ability to examine an individual deposit s hydraulic behaviour and development also enables depositional processes to be examined at a much finer resolution than previously attempted using conventional textural approaches

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

magnetic mineral and sediment porosity distribution on a storm dominated shelf investigated by benthic electromagnetic profiling bay of plenty new zealand abstract magnetic susceptibility and electric conductivity within the uppermost meter of the seafloor were measured with high resolution along 33 coast normal profiles in the bay of plenty new zealand using the recently developed electromagnetic benthic profiler marum neridis iii these parameters are used to determine magnetic mineral concentration and porosity of mostly volcanoclastic sediments between 2 and 35 m water depth to investigate distribution and formation of magnetic mineral enrichments on a storm dominated shelf in general magnetic mineral concentration susceptibility is inversely correlated to porosity conductivity along profiles susceptibility maxima and conductivity minima were commonly found on bathymetrical elevations such as dune and ripple crests cores and grab samples show the highest degree of enrichment in the uppermost 20 cm of the seafloor complementing petromagnetic analysis indicates a spectrum of feti oxides in the sediment ranging from magnetite and hematite over titanomagnetite tm40 and tm60 to titanohematite likely th80 and th95 three distinct zones of magnetic mineral enrichment could be identified a coast parallel structure with low porosity in recent fine sand between 5 and 20 m water depth a widespread structure with low porosity in older transgressionally reworked coarse sand in up to 30 m water depth and third structure coarse sand with high porosity located mostly below 30 m while the fine sand structure is interpreted as the result of an active process the other two appear to be relics sorting by grain size selective entrainment appears to be the major formation mechanism this study demonstrates how electromagnetic benthic profiling in combination with environmental magnetic laboratory analysis provides reliable highly interpretable data that allow insight into sorting processes within the boundaries of established lithofacies that are difficult to detect using a classical sedimentological approach

analytical modelling of wave refraction and convergence on coral reef platforms implications for island formation and stability abstract an analytical model was constructed to simulate the refraction of waves on coral reef platforms comprising an idealised bathymetry of uniform depth and vertical reef faces model simulations were conducted to investigate the influence of key parameters such as reef shape and depth as well as wave period on the propagation behaviour of incident waves results of the refraction analysis demonstrate that different reef shapes produce characteristic patterns of wave convergence on reef surfaces the location and stability of focal zones and hence wave convergence is largely controlled by the shape of platforms platform configuration further controls the distribution of wave energy across platform surfaces and determines the influence of incident wave forcing on different reef sections results have significant implications for sedimentation processes and hence the formation and stability of islands on reef platforms wave propagation patterns define sediment transport vectors and subsequently control the transport and deposition of different sized material platforms which promote marked wave convergence behaviour such as elliptical and circular reefs are more likely to retain sediment on reef surfaces whereas narrow linear structures have a higher potential for the off reef evacuation of sediment over leeward reef margins and the subsequent infill of deeper lagoonal areas the study provides a physical basis for future investigations of reef hydrodynamics and platform sedimentation processes

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

seismic crustal structure of galicia continental margin nw iberian peninsula the galicia margin is one of the most intensively surveyed continental margins however little is known about its deep structure between 11 and 9 w in this work we present the interpretation of a w e 425 km long survey section using near vertical seismic reflection coincident refraction wide angle seismic reflection and gravity data our results show that the crustal thickness varies from 30 km inland to 9 km near the ocean continent boundary with two areas of crustal thinning the main thinning of the crust is located below the deep galicia margin the crust under the galicia interior basin is also thinned reaching a total thickness of 19 km the galicia bank corresponds to a thicker area reaching 22 km