synkinematic microscopy of transparent polycrystals abstract deformation of weak polycrystalline materials in thin section on the stage of a petrographic microscope offers insight into the origin of optical microstructures similar to microstructures in naturally deformed rocks sample materials used to date include several soft minerals ice carnallite bischofite nitratite and various organic plastic materials they can be simple sheared or pure sheared at temperatures to 300 c at a variety of strain rates using relatively simple and inexpensive apparatus the complete microstructural evolution can be recorded photographically along with the complete kinematic history research applications have focussed on dynamic recrystallization and progress has been made towards understanding how the average grain size is maintained constant in steady state recrystallization flow records have been obtained of the development of various crystal plastic microstructures and of lattice reorientation trajectories during flow teaching applications have begun to be developed and several videotapes are available future technical developments are expected to permit stress measurement or control during experiments and synkinematic recording of lattice reorientation numerous research areas invite attention these include investigation of the microstructural developments that precede faulting the behaviour of deforming crystal melt systems and the behaviour of dynamically reactive multi phase systems

a model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with gloria sidescan sonar where the seafloor consists of a highly backscattering surface covered with a drape of fine grained sediment and where the sediment does not contribute significantly to the backscattered signal the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer a first order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments data collected with geological long range inclined azdic gloria are used to develop and test this model at an acoustic frequency of 6 5 khz the range of sediment thickness that the sonar can potentially recover is 0 20 m a range that is often below the resolution of vertical profilers in the first example the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer the surface is a lava flow north of hawaii covered by 1 2 m of fine grained sediments clague et al 1990 and gloria data analyzed with the model suggest an attenuation coefficient of 0 2 0 4 db m a value consistent with results from laboratory and field measurements of sediment attenuation in the second example a rocky surface is buried by varying amounts of sediment which is analyzed at almost constant grazing angle the surface is the volcanic basement of the southeast indian ridge and gloria data reveal the thickening of the sediment cover with distance away from the spreading center this is used to derive an average sedimentation rate of 6 mm ka which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions furthermore the model is used to calculate from image amplitude distributions the sediment thickness distribution which represents the accumulation of sediments in ponds and the exposure of abyssal hills these sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age i explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes the first order model assumes a uniform thickness at the subfootprint scale

using multi radiotracer techniques to better understand sedimentary dynamics of reworked muds in the changjiang river estuary and inner shelf of east china sea abstract reworked muds including mobile muds resuspended sediments and bio mixed muds are ephemeral layers in the benthic boundary of large river delta front estuaries ldes in this study multiple radiotracers 210pb 137cs 234th 7be 235u 228th 228ra 226ra and 40k were combined to evaluate the sedimentary dynamics of the mud belt on the inner shelf of the east china sea ecs adjacent to the changjiang river estuary we first categorized eight types of typical 210pb profiles in marine sediments and specified which ones could be used as possible evidence of reworked muds the types of 210pb profiles observed in previous studies and this study were then used to examine the depth distribution of reworked mud layers of the benthic boundary of ecs we found that reworked mud depth in the along shore mud belt of the inner shelf of ecs was deeper than those in the outer shelf and slope area reaching up to 150 cm deep in the changjiang river lde and up to 50 cm deep in the zhejiang fujian coast mud area sediment core tracer profiles also showed that the reworked mud layer could be further divided into several sub layers with distinct redox conditions and microbe sulfate reducing bacterial abundance srb abundance intensively fluctuated with changing redox environment and larger abundance values were usually observed in anoxic conditions based on 7be and 234th the top layer ca 2 4 cm thick was the most modern and active zone showing evidence of reworking on seasonal timescales the deeper sections of the reworked mud layer were generally modified over longer timescale years which were examined using the more long lived tracers e g 137cs 228thex and 228ra 226ra ratio we found for the first time in this region that age of the reworked layers was usually much younger than underlying accumulating burial layers we posit that this multi radionuclide approach is critical for evaluating the complex biogeochemical sediment record on delta front estuaries of large rivers subject to changing climate conditions and human impacts over the last several centuries

geological interpretation of a low backscatter anomaly found on the new jersey continental margin abstract an enigmatic low backscatter acoustic anomaly occurs on the new jersey continental margin between hudson and wilmington canyon channels the presence of the low backscatter anomaly as seen with 6 5 and 12 khz data indicates a change in the physical properties of the seafloor or near sub surface analyses of seafloor and sub surface acoustic data with previously collected sediment cores suggest the low backscatter feature corresponds to an outcrop of older strata uncovered by erosion and non deposition by the western boundary undercurrent wbuc the decrease in backscatter strength is enhanced by the presence of gas in the sub surface sediments found in the buried chesapeake drift

marine sediment acoustics new field and laboratory data indicate that the assumption of a constant q or equivalently an attenuation coefficient that depends on the first power of frequency is unacceptable in the case of nearly all marine sediments the fluid component of marine sediments causes various forms of viscous damping that are always frequency dependent in coarser sediments overall motion of the fluid field relative to the skeletal frame causes a strong frequency dependence that depends on permeability and the tortuosity of the intergranular pores in the finer sediments local viscous losses occur near the intergranular contacts producing a viscoelastic response similar to the squeeze film phenomenon that is well known in lubrication theory both kinds of viscous damping as well as frictional losses at the grain contacts can be included in the generalized biot model which has been used to describe the acoustical response of marine sediments in this paper we summarize recent field data which illustrate the wide spread of q values that are to be expected and present some new experimental data that demonstrate both kinds of viscous damping mentioned above finally we use the model to estimate the response of a p wave based on laboratory measurements of torsional vibrations a simple mathematical model is used to establish the connection between these two modes of motion

new high resolution acoustic data from the braided system of the orinoco deep sea fan abstract the interpretation of approximately 100 km of em12 multibeam bathymetry and backscatter data together with high resolution topas profiles obtained during a transit over the orinoco deep sea fan near the barbados accretionary prism shows that the braided drainage pattern seen on gloria images by belderson et al 1984 corresponds to wide 10 km low relief

faunal characteristics and sediment accumulation processes in the james river estuary virginia abstract this study was designed to relate fauna characteristics and sediment accumulation processes in the james river va and was conducted during june 1981 physical sedimentary and benthic biological parameters as well as sediment structure and radionuclide profiles were evaluated for 11 stations faunal distribution patterns reflected species response to salinity changes along the estuarine gradient but not to differences in sediment accumulation rates levels of bioturbation could not be predicted easily on the basis of faunal characteristics alone results suggest that the physical processes of erosion and deposition strongly influence the ability of macrobenthos to bioturbate sediments in this estuary areas of rapid deposition 3 cm y 1 exhibit little evidence of bioturbation as do areas where erosion or relatively constant physical reworking of sediments dominate areas with low sediment accumulation rates 0 5 3 cm y 1 exhibit the highest levels of mixing as evidenced in x radiographs estuarine organisms inhabiting soft bottoms are typically opportunistic shallow living and short lived species and the composition of their communities is not strongly influenced by rates of deposition physical reworking of sediments is most likely to occur near to the sediment water interface where reworking by shallow living organisms is most intense sediment mixing processes should be characterized using a range of approaches the phasing of interactions among erosion physical transport deposition and biological mixing must be resolved on the appropriate time scales if the mechanics of processes governing the formation of the sedimentary record are to be elucidated

sand wave fields beneath the loop current gulf of mexico reworking of fan sands extensive fields of large barchan like sand waves and longitudinal sand ribbons have been mapped by deep towed seamarc ia sidescan sonar on part of the middle and lower mississippi fan that lies in about 3200 m of water the area is beneath the strongly flowing loop current the bedforms have not been adequately sampled but probably consist of winnowed siliciclastic foraminiferal sands the size about 200 m from wingtip to wingtip and shape of the large barchans is consistent with a previously observed peak current speed of 30 cm s measured 25 m above the seabed the types of small scale bedforms and the scoured surfaces of chemical crusts seen on nearby bottom photographs indicate that near bed currents in excess of 30 cm s may sometimes occur at the time of the survey the sand transport direction was to the northwest in the opposite direction to the loop current but consistent with there being a deep boundary current along the foot of the florida escarpment some reworking of the underlying sandy turbidites and debris flow deposits is apparent on the sidescan sonar records reworking by deep sea currents resulting in erosion and in deposits characterised by coarsening upwards structures and cross bedding is a process that has been proposed for sand found in cores in shallower parts of the gulf of mexico this process is more widespread than hitherto supposed

deformation of two phase systems with contrasting rheologies abstract rheological model experiments are used as analogues for foliation development in quartz mica rocks in a regime of pure shear in situ observations involving ice containing varying proportions and distributions of naphthalene and camphor illustrated two contrasting types of behaviour a matrix controlled versus a particle controlled deformation the matrix controlled deformation accommodates shortening predominantly by intracrystalline slip in the ice accompanied by some recrystallization while the more rigid naphthalene plates undergo rotation and mechanical alignment where shortening is normal to an initial platelet preferred orientation the foliation is enhanced and the matrix material aggregates between the rigid plates whereas a conjugate foliation is produced during shortening parallel to an initial platelet preferred orientation during particle controlled deformation strain is localized in particle concentrates the matrix areas and particles isolated in the matrix show little deformation at shortenings 10 dissolution of selected particles is achieved through an artificial fluid transport mechanism to create voids further shortening does not create closure of the fluid filled holes where shape is supported by the rigid matrix

the eigenvalues of steady flow in mohr space abstract the eigenvalues of the velocity gradients tensor and of the position gradients tensor describing instantaneous flow or incremental deformations are easily determined in mohr space as the points of intersection between a mohr circle and the horizontal mohr axis intersections occur only if the tensor has real eigenvalues eigenvectors associated with the eigenvalues are directions of no rotation a line of material particles under an eigenvector does not rotate during steady flow although the line is instantaneously stretching at a rate equal to its corresponding eigenvalue for non spinning steady sub simple shearing there will always be two eigenvectors the angle between the eigenvectors is dependent on the ratio of pure to simple shearing rates this angle decreases as the pure shearing component decreases relative to the simple shearing component superimposed increments of pure shear and simple shear produce an equivalent set of non rotated material lines but the deformation histories of these lines are rotational the influence of eigendirections on geological structures is constrained by the rheology of a rock during deformation in homogenous steady flow no influence may be obvious spatially periodic material softening along eigenvectors during bulk hardening may result in a complex fabric of intersecting shear zones the geometry of the fabric elements provides important information on the bulk deformation of the rock particularly on the bulk shear sense

spatial and temporal variation in cohesive sediment erodibility in the york river estuary eastern usa a biologically influenced equilibrium modified by seasonal deposition abstract sediment erodibility was measured at three sites in the york river a sub estuary of the chesapeake bay monthly to bimonthly from april 2006 through october 2007 erodibility at the three sites was similar during the summer and fall a site near the estuary mouth maintained this level of erodibility greater than 90 of the time while two sites in the more physically dominated mid estuary region exhibited a consistent and pronounced increase in erodibility in the late winter and spring weak to non existent correlations between bed erodibility solids volume fraction and surficial concentrations of organic matter colloidal carbohydrate and extracellular polymeric substances were not sufficient to explain the observed seasonal pattern in bed erodibility digital x radiographs revealed thick sequences 10 20 cm of laminated sediments at the surface in the middle estuary coincident with the period of highest erodibility and more biologically reworked sediment during the rest of the year suggesting that periodic rapid deposition introduced new sediment that was seasonally easy to erode the finding that seasonal deposition influenced erodibility at the mid estuary sites is consistent with previous results indicating the occasional presence of a secondary turbidity maximum comparison of the biologically reworked but still low erodibility condition in the york to other published chesapeake bay erodibility data revealed a consistent critical shear stress range and profile suggesting that this equilibrium critical stress profile may be representative of other similar estuarine environments in the absence of rapid deposition at relatively low stresses and in the absence of rapid deposition we speculate that burrowing and or pelletization may play a role in maintaining high equilibrium bulk water content without reducing the strength of the surface of the seabed

fine scale stratigraphy in proximal and distal deposits of sediment dispersal systems in the east china sea abstract the east china sea contains the proximal deposit of the changjiang yangtze dispersal system and the distal deposit of the huangho yellow dispersal system despite similarity of sediment characteristics and oceanic setting the two deposits show a significant difference in fine scale stratigraphy the proximal changjiang deposit is characterized by physically stratified mud and the distal huangho deposit is characterized by homogeneous mud the difference in fine scale stratigraphy is explained by the ratio of mixing rate to accumulation rate which is much larger for the distal huangho deposit 17 than for the proximal changjiang deposit 1 a larger ratio allows biological mixing to destroy physical stratification with increasing distance away from the mouth of a major river generally mixing rate increases and accumulation rate decreases along the sediment dispersal system therefore the ratio of mixing rate to accumulation rate should increase and sedimentary structure should change from physically stratified to homogeneous at the leading edge extreme distal portion of a prograding mud deposit the relatively rapid mixing and slow accumulation can cause a transitional boundary between the mud and the ambient sediment which is transgressive sand in the east china sea for a prograding mud deposit of a major dispersal system sedimentary sequences should reveal the following upward changes a transitional boundary with underlying sediment homogeneous sediment representing distal deposits successively more stratified sediment representing proximal deposits

a scanning electron microscope technique for identifying the mineralogy of dust in ice cores dust particles in an ice core from east rongbuk glacier on the northern slope of qomolangma mount everest 28 01 n 86 58 e 6518 m a s l central himalaya have been identified as mica using a combination of scanning electron microscope based techniques and energy dispersive x ray spectroscopy to identify the elements present and electron backscatter diffraction to identify the crystal type this technique for identifying individual crystalline dust particles in samples of glacial ice could be especially useful in the future for identifying water soluble crystals in ice for studying the strain history glaciotectonics of basal ice or in studies of ice mica composites used as analogs of quartz mica rocks

foliation and strain development in ice mica models abstract the behaviour during experimental deformation of composite foliated rock models composed of ice and ice mica is described in order to replicate deformation in quartz mica rocks the deformation involves pure shear and a component of simple shear oblique to the shortening axis a foliation is developed which is defined by an alignment of micas and a preferred orientation of airbubbles that closely parallels the principal plane of the local finite strain ellipsoid the degree of mica re alignment is not purely dependent on a parameter such as strain but is also controlled by the nature of the initial mica fabric and the orientation of layering relative to the shortening direction where there is some obliquity of a primary layering to the bulk shortening direction the deformation is markedly inhomogeneous with non uniform extension parallel to the flow direction over short distances this results in varying local strain and mica distributions values of strain obtained from such mica orientation distributions are always less than the observed finite strain and are therefore incompatible with a march model however deformation of an initial layering parallel to the shortening direction that contains a uniform mica distribution results in a homogeneous finite strain distribution and the mica distribution strengthens parallel to the principal plane of the local strain ellipsoid this mica distribution produces values of strain that correspond to the predictions of the march model in contrast deformation of initial bimodal fabrics or fabrics asymmetric to the shortening direction will develop strong preferred orientations quicker than an initial uniform distribution and characteristics of the initial fabric pattern always persist within the deformed mica preferred orientation shortening perpendicular to a strong initial preferred orientation only strengthens the mica fabric intracrystalline slip in the ductile ice matrix is the main means of accommodating the deformation and also produces a strong c axis preferred orientation grain boundary mobility in the ice is retarded by the dispersed mica phase however grain growth during and post dating deformation does not effect mica orientation distributions these features resemble and are compared with identical structures observed in many quartz mica rocks

time for anisotropy the significance of mechanical anisotropy for the development of deformation structures hr acknowledges financial support by the china scholarship council csc grant nr 201506400014 egr acknowledges the support of the beatriu de pinos programme of the government of catalonia s secretariat for universities and research of the department of economy and knowledge 2016 bp 00208 we thank bruce hobbs and an anonymous reviewer for their suggestions to improve this article

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

ground truthing 6 5 khz side scan sonographs what are we really imaging a 1000 km2 area on the distal lobe of monterey fan shows a digitate pattern of juxtaposed high and low backscatter on gloria side scan sonographs this area was investigated using stereo photography high resolution seismic profiles and measurements of physical properties of cores to quantitatively evaluate the causes of backscatter from the 6 5 khz side scan sonar stereo photography and bottom video were used to determine that the sediment water interface typically has a bed roughness less than 10 cm over the entire ground truth area consequently bed roughness is not a significant contributor to the sonar backscatter vertical incidence 3 5 khz profiles reveal that high backscatter areas allow less penetration and have slightly more relief than low backscatter areas closely spaced measurements of p wave velocity density and grain size made on transponder navigated cores are used to investigate the geoacoustic properties of the sediment with the aid of a numerical model the model results demonstrate that the sediment water interface is in most cases acoustically transparent to the sonar energy and that most or all of the energy is refracted into the sediment to depths of at least a few meters rather than scattered from the surface the variability of the dominant lithofacies sands with minor silty clays or silty clay with minor sands accounts for a regional correlation with backscatter when viewed over hundreds to thousands of square kilometers but at the detailed scale of hundreds to a few square meters we have not yet been able to identify a correlation of backscatter intensity with the angle of incidence surface lithology measured acoustic impedance or attenuation however there is a qualitative but unexpected correlation with lithostratigraphic variability in this area thick up to 50 cm sand deposits with thin interbeds of silty clay correlate with lower backscatter than do silty clay deposits with thin interbeds of sand this suggests that volume inhomogeneities and complex constructive and destructive interferences caused by the subsurface volume inhomogeneities within the top few meters of the sediment ultimately modulate the intensity of backscatter the ground truthing effort was further complicated by trying to compare the very large sonar footprint of a pixel 70 000 m2 relative to the area of an individual core 0 006 m2 bottom photograph 5 m2 or seismic profile although 6 5 khz sonographs appear easy to interpret in a conventional and simplistic manner caution should be used when interpreting lithofacies from backscatter intensities

deep sea fan pinch out geometries and their relationship to fan architecture tanqua karoo basin south africa exceptional exposures of permian basin floor fans fans 3 4 and a slope fan fan 5 in the tanqua karoo foreland basin of south africa have enabled an investigation of the relation between the pinch out geometries and fan architecture the pinch out geometry of fan 3 is characterized by the down dip transition from thin to medium bedded sheet deposits to pinch out fingers which are overlain by younger prograding sheet deposits this geometry reflects the progradational stacking pattern of the fan in contrast the fan 4 pinch out fingers consist of stacked channel fills in the same conduit this pinch out configuration relates to the dominant aggradational style observed on the mid and distal parts of fan 4 fan 5 represents a slope fan comprising an axial channel conduit which branches down slope into three distributary channels the distal fan is characterized by larger channel fills which may represent bypass channels to other basin floor fans the very thick bedded nature of the youngest channel fill unit suggests early bypass followed by retrogradation as indicated by the presence of thinner bedded heterolithic channel fill deposits along the axial conduit although some of the massive pinch out channels exhibit basal scour their depositional morphology suggests that they mainly originated due to the infill of subtle topographic depressions by low concentration turbidity currents instead of describing these features as channel fills the use of the term pinch out fingers is preferred

mesotidal estuarine sequences a perspective from the georgia bight abstract although much has been written about estuaries and estuarine sediments very few facies models have been proposed to date and considerable ambiguity remains in coastal classification schemes current stratigraphic concepts fail to elucidate not only the differences between certain deltaic estuarine and lagoon fill sequences but also the pronounced similarities between many riverine and salt marsh estuarine sequences these distinctive latero vertical successions recent and ancient warrant increased scrutiny and conceptual refinement preliminary stratigraphic models and definitions presented here based largely upon modern georgia estuaries should provide a broad framework for further evaluation and clarification of the above facies relationships from that standpoint an estuarine sequence consists of complex intertidal to subtidal mostly channel form facies dominated to some extent by tidal effects typically displaying abrupt variations in sediment texture and composition and in physical and biogenic sedimentary structures subtle to pronounced salinity gradients may be discernible nevertheless the features most diagnostic of the overall sequence are 1 tidally influenced channel form deposits and 2 characteristic bioturbate textures and trace fossils

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

kinematic and metamorphic aspects of cleavage formation in very low grade metamorphic slates abstract fabric studies on first cleavages in very low grade metamorphic slates from the rheinisches schiefergebirge and other very low grade metamorphic fold belts reveal two types of slaty cleavage 1 1 cleavage without syntectonic phyllosilicate recrystallization these are generally crenulation cleavages 2 2 cleavage with syntectonic phyllosilicate recrystallization with increasing syntectonic metamorphism the crenulation fabric is modified by solution of grains along cleavage planes and precipitation of material within the cleavage lamellae with further increase of metamorphism the crenulation fabric is gradually obliterated the finite fabric results from progressive solution crystallization and recrystallization superimposed with bending kinking and internal rotation achieved by slip parallel to the basal planes of phyllosilicates

ephemeral deposition seabed mixing and fine scale strata formation in the york river estuary chesapeake bay abstract a process oriented sedimentary facies model is developed for the york river estuary a sub estuary of the lower chesapeake bay this facies model was based on 210pb and grain size profiles as well as x radiographs taken from kasten cores and box cores collected in a series of across river transects throughout most of the energetic microtidal york river the seabed is characterized by physical mixing to depths of 25 200 cm a strong cross estuary gradient in processes is observed with one side including channel flank and shoal dominated by frequent deep erosion and redeposition physical mixing while physical mixing is reduced on the other side resulting in a greater preservation of biological mixing signatures within the physically dominated side of the river the mixed layer is characterized by stair stepped 210pb profiles with one or more segments 25 200 cm thick of nearly uniform excess activity x radiographs reveal that although a record of limited biogenic sediment modification is preserved sedimentary structures within the mixed layer are dominated by centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces this demonstrates that mixing results primarily from erosion resuspension and deposition reduced salinity limits the number of benthic species in the york river physical disturbance leads to an impoverishment of this community which is composed primarily of small opportunistic species with a paucity of larger macrofauna as a result mixing in the biologically dominated side of the river is generally on the order of a few centimeters but may be as deep as 40 cm and 210pb geochronology yields low biodiffusion rates 0 43 3 35 cm2 yr 1 x radiographs reveal the presence of some laminations which suggest that although the mixing is controlled by biological processes the mixing intensity is relatively low based on 210pb geochronologies residence time estimates for particles within the mixed layer are on the order of centuries residence time calculations based on the sediment mass in the physically mixed layer is equivalent to 70 yr of river sediment yield consistent with century scale residence times from core data the frequency and intensity of seabed mixing appears to differ between the lower and upper river the lower york river is wider and deeper and is more susceptible to large storms and sea surges which we suspect drives much of the recorded seabed mixing within the upper river longer term events storms may cause the deepest mixing but much of this record is destroyed by shorter term high frequency events which produce shallow to mid depth

morphology volcanism and mass wasting in crater lake oregon crater lake was surveyed nearly to its shoreline by high resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys the bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca 7700 calibrated yr b p caldera the andesitic wizard island and central platform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents wizard island eruptions ceased when the lake was 80 m lower than at present lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor the wizard island and central platform volcanoes andesitic merriam cone and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse whereas a small rhyodacite dome was emplaced on the flank of wizard island at ca 4800 cal yr b p bedrock outcrops on the submerged caldera walls are shown in detail and in some cases can be correlated with exposed geologic units of mount mazama fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer wider ridges downslope hummocky topography and scattered blocks up to 280 m long below many of the embayments in the caldera wall mark debris avalanche deposits that probably formed in single events and commonly are affected by secondary failures the flat floored deep basins contain relatively fine grained sediment transported from the debris aprons by sheet flow turbidity currents crater lake apparently filled rapidly ca 400 750 yr until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at 1845 m elevation thereafter a gradual climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave cut platform commonly 40 m wide where suitable material is present the new survey reveals landforms that result from intermediate composition volcanism in rising water delineates mass wasting and sediment transport into a restricted basin and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards

time lapse side scan sonar imaging of bleached coral reefs a case study from the seychelles abstract high resolution 675 khz side scan sonar surveys were collected six and thirty months after a major coral bleaching event in the seychelles the surveyed areas contain four different reef morphologies the distribution of which depends on water depth and distance from the shoreline these four reef types also have different coral densities and colony morphologies branching or mixed massive branching textural analysis based on first order statistics unsupervised cluster analysis and mann whitney u tests showed a correlation between recorded backscatter response and reef type that is attributed to the link between community composition and rugosity on a millimetre to tens of metre scale branching coral colonies are found to be relatively good scatterers of acoustic energy and result in a broad intermediate to high intensity response massive coral colonies and reefs with a hard carbonate pavement are found to be principally reflectors of the acoustic energy resulting in a narrow low intensity response comparison of the two surveys separated in time by two years showed seabed texture to change most significantly over the reef areas that contained the highest coral abundances and mortality rates in particular the disintegration of dense branching colonies that suffered almost 100 mortality during the bleaching event resulted in a characteristic loss of intermediate to high backscatter intensity the work demonstrates the contribution that side scan sonar could make in the assessment of loss of rugosity following a bleaching episode which has important implications to both the recovery of the reef itself and the abundance and distribution of associated reef organisms

sidescan sonar images of mud volcanoes from the mediterranean ridge possible causes of variations in backscatter intensity abstract a survey of a large field of mud diapirs on the mediterranean ridge south of crete was carried out using both okean 9 5 khz and mak 1m 30 khz sidescan sonar systems during leg 2 of the third training through research cruise of r v gelendzhik in june and july 1993 high backscatter intensities were recorded by both systems to different degrees over the mud diapirs and their immediate surroundings causes of variations in the backscatter intensity are considered to be the spatial distribution of mud breccia associated with the mud domes both the burial depth of the mud breccia beneath hemipelagic drape and its areal extent and its physical properties along with contributions from seafloor roughness and the topography of the mud domes significant sources of volume scattering of the sidescan sonar sound signal are considered to be mud breccia inhomogeneities represented by millimetric centimetric clasts of various lithologies and free gas bubbles distributed through the mud matrix a good general relationship is established between these sources and the observed backscatter variations through analysis of ground truth data from core samples with 5 khz deep tow subbottom profiler records and seafloor video observations as well as the sidescan sonar records but a fully quantitative assessment of this relationship would require acoustic modelling and additional ground truth data

preservation potential of sedimentary event layers preservation potential of sedimentary event layers e g ash layers or tempestites is a function of net sedimentation rate and biogenous mixing rate previous studies that have used an inverse peclet number to scale the problem in terms of advection or diffusion are in error due to the improper inclusion of the surface mixed layer thickness as the sole length scale herein focus is on time scales if the transit time i e time required to advect the signal through the biologically active zone is less than the dissipation time i e time required to destroy the signal then some fraction of the original signal is preserved a simple quantitative model based on the diffusion analogy of sediment mixing estimates the actual percentage of preserved signal under various combinations of sedimentation rate mixing rate and event layer thickness

the origin of kinks in polycrystalline ice abstract in situ observations of events leading to development of kink hands kbs and kink band boundaries kbbs and changes in their morphology are recorded during the deformation of layered polycrystalline ice at temperatures above 10 c slip on 0001 preceeded kb initiation and two different mechanisms a shear mode and a buckling mode are proposed for the development of the observed kbs the shear mode occurs with axial compression parallel to 0001 and is restricted to a band of material with sharply defined kbbs involved with an effective macroscopic simple shear sub perpendicular to the slip plane during deformation these bands progressively reduce their width while maintaining a stable orientation secondary kbs and microfractures are propagated within the kb as a result of increasing shear deformation in the band these phenomena are attributed to a limiting of the shear deformation to a band and the necessity to accommodate the dilatation changes in the buckling mode kb initiation is preceeded by the formation of deformation bands and this type of kb can initiate from inherent heterogeneities within the grain aggregate microstructural sensitivity is put forward to explain the apparent instability of this type of kb which is often curved and capable of migrating through a crystalline lattice the buckling mode occurs in grains where 0001 is oblique to the compression direction

the use of bomb spike calibration and high precision ams 14c analyses to date salt marsh sediments deposited during the past three centuries abstract a combination of bomb spike calibration and conventional calibration of ams 14c dating has been used to determine a detailed age depth model for a 1 m sediment section collected from a salt marsh in poole harbour southern england these data were compared with the chronology obtained from 210pb analysis and 137cs age markers we report post bomb values of over 1 46 f14c 146 modern 14c and both the rising and falling limbs of the atmospheric bomb spike are identified five pre bomb samples were analysed using multi target high precision 2 ams analysis and after the replicates were combined the one sigma uncertainty was as low as 9 14c yr on some ages these data and an additional three normal precision pre bomb 14c samples were calibrated using calib 5 0 and the chronology constrained using the prior knowledge of independent age markers obtained from the analysis of pollen and spheroidal carbonaceous particle scps no agreement was found between the 14c bomb spike dates and the crs 210pb chronology modelled for this sequence in addition poor agreement was found between the signal of the 1960s weapons test fallout indicated by the 14c bomb spike dates and the timing suggested by the 137cs data this disagreement is attributed to the influence of the local discharge of 137cs from the former ukaea site at winfrith we use our new chronology to confirm the existence of an acceleration in sedimentation rates in poole harbour during the last 100 yr previously reported for this site by long et al long a j scaife r g edwards r j 1999 pine pollen in intertidal sediments from poole harbour uk implications for late holocene sediment accretion rates and sea level rise quaternary international 55 3 16 and conclude that bomb spike 14c calibration dating may offer a more robust alternative to the use of 210pb chronologies for dating sediment deposition in salt marsh environments in addition we demonstrate how the use of high precision ams analysis has the potential for reducing some of the uncertainties involved in the high resolution dating of recent salt marsh sediments

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

lattice rotation in polycrystalline aggregates and single crystals with one slip system a numerical and experimental approach abstract in this study lattice rotations in polycrystals and single crystals with one slip system have been analysed for pure shear and simple shear by re visiting previously published data and by conducting new numerical models that are compared with results from experiments on polycrystalline ice the numerical models are based on the finite difference method and on the assumption that dislocation glide on one slip system is the dominant crystalline deformation mechanism and is controlled by the critical resolved shear stress law such a deformation scheme corresponds to the operation of glide on 0001 in polycrystalline ice used in the physical deformation experiments the results show that lattice rotation is primarily controlled by the bulk deformation kinematics in both the polycrystalline aggregates and the single crystals in the single crystals the lattice rotation is entirely consistent with the vorticity of the bulk deformation kinematics whereas in the polycrystalline aggregates extensive grain interactions significantly modify the local lattice rotations and may even lead to the lattice planes of individual grains rotating in an opposite sense to that of the bulk deformation these results can reasonably explain the development of crystallographic preferred orientations widely reported in the literature

hatteras transverse canyon hatteras outer ridge and environs of the u s atlantic margin a view from multibeam bathymetry and backscatter abstract previously unknown features in hatteras transverse canyon and environs were recently mapped during multibeam surveys of almost the entire eastern u s atlantic continental margin the newly identified features include 1 extensive landslide scarps on the walls of hatteras transverse and hatteras canyons 2 an area of multiple landslide deposits that block lower hatteras transverse canyon 3 a large depositional feature down canyon from the landslide deposits that rises 100 m above the uppermost hatteras fan and has buried the transition from the mouth of hatteras transverse canyon to uppermost hatteras fan 4 a zone of cyclic steps on upper hatteras fan that suggests supercritical turbidity currents performed a series of hydraulic jumps and formed large upstream migrating bedforms 5 several knickpoints in the channel thalwegs of both hatteras transverse canyon and hatteras canyon one 40 m high that suggest both canyon channels are out of equilibrium and are in the process of readjusting either to the channel blockage by the extensive landslide deposits or by a readjustments to increased sedimentation during the last eustatic lowstand 6 a large area of outcrop on the lower margin between pamlico and hatteras canyons that previously was interpreted as an area of slumps blocky slide debris and mud waves 7 headward erosion in the head region of hatteras transverse canyon where it has intercepted the lowest reaches of albemarle canyon channel as well as headward erosion in a small side channel that has eroded into hatteras outer ridge and 8 sections of bedforms on hatteras outer ridge that are partially buried by sediment from washington norfolk canyon channel as well as by sediment transported from hatteras abyssal plain the newly discovered features add a new level of detail to understand the recent processes that have profoundly affected hatteras transverse canyon hatteras canyon and to a lesser degree hatteras outer ridge

micro and meso scale deformation structures in experimental fault zones abstract kinematic structures of brittle brittle ductile and ductile shear zones were developed in experimental simple shear deformation transparent films of paraffin wax with different initial fabrics were progressively deformed in a manually driven stage mounted shear apparatus producing heterogeneous simple shear 0 36 2 64 at variable strain rates of from 3 3 10 5 to 4 08 10 5 s 1 en echelon fracture arrays compressive and tensile bridges spaced cleavage stylolite like cleavage and sigmoidal cleavage trails were developed in brittle and brittle ductile deformation structures produced in ductile experiments were c s and c s fabrics and type asymmetric tails shear lenses and active folds of pre existing and induced anisotropy planes the experiments offer insight into the microstructural processes involved and demonstrate development of a progression of shear structures in a single non steady deformation

the july 1996 flood deposit in the saguenay fjord quebec canada implications for sources of spatial and temporal backscatter variations in july 1996 a major rainstorm and flood took place in the saguenay fjord backscatter strength measurements made with a simrad em1000 multibeam echosounder in 1993 1997 and 1999 have shown spatial and temporal variations which are interpreted in relation to the occurrence of the flood after empirical calibration of the different maps the data show an overall diminution of 5 db in backscatter strength in 1997 1 yr after the flood took place while the data sets obtained in 1993 and 1999 show comparable levels of backscatter strength the different data sets show a similar pattern of low and high backscatter patches which represent variations of the backscatter strength of a few decibels several grain size and water content measurements were also carried out on sediment box core and grab samples from the fjord bottom in 1997 and 1999 these have shown that the areas with higher acoustic backscatter correspond to the finer sediments while the low backscatter patches correspond to the coarser material the data show little relation between water content and backscatter strength thus indicating a poor dependence between the impedance terms bulk density and sound speed and the backscatter having taken this into account the major contribution to backscatter strength is assumed to result from differences in surface and volume roughness of the sediment since finer grain sizes offer higher backscatter the grain size of the sediment is not considered as being a major contributor to roughness and hence backscatter strength the major factor that seems to control roughness generation in the saguenay fjord is considered to be bioturbation the areas where geological processes most physically disturb the bottom which coincide with the areas where also most accumulation takes place and the coarser grain sizes are deposited are more sparsely colonized by organisms this results in a lower degree of bioturbation lower roughness and thus backscatter strength this hypothesis helps explain the variation in backscatter strength observed between the different measurement years the several million tons of sediment deposited in the fjord bottom after the 1996 flood buried the benthic fauna which was still recovering 1 yr later the less bioturbated sediment at this time would present lower roughness with respect to the 1993 and 1999 data resulting in a lower backscatter strength

the andoya slide and the andoya canyon north eastern norwegian greenland sea based on gloria side scan sonar imagery echo sounder records 3 5 khz profiles multichannel seismics and gravity cores the andoya slide and andoya canyon north eastern norwegian greenland sea were mapped and interpreted the andoya slide covers an area of about 9700 km2 of which the slide scar area comprise ca 3600 km2 the slide has a total run out distance of about 190 km slope failure is inferred to have occurred during the holocene because the slide scar has prominent relief on the present sea floor the area of sediment removal is characterised by an irregular relief were relatively consolidated sediments are exposed at the sea floor little or no unconsolidated sediments overlies the slide deposits earthquake activity is inferred to have triggered the slide a holocene age of the andoya slide implies that three giant slides the storegga traenadjupet and andoya slides have occurred along the continental slope of norway during the last 10 000 years a large canyon the andoya canyon is located immediately south of the andoya slide on the upper slope the canyon has been incised about 1000 m in the bedrock and the maximum width at the bottom and between the canyon shoulders is 2 and 12 km respectively the andoya canyon represents the upper part of the lofoten basin channel based on analogy with other deep sea canyon channel systems the andoya canyon lofoten basin channel is possibly of pre quaternary age holocene sediments recovered from within the canyon and draping the flanking channel deposits indicate that the andoya canyon is not presently active and has probably not been active during the holocene during the holocene the canyon acted as a trap for sediments settling from the winnowing norwegian current

variation in fold geometry in the yuso basin northern spain implications for the deformation regime first generation structures in greywackes of the yuso group from the cantabrian mountains of northern spain show a distinct variation in geometry with depth in a regional synclinal structure curavacas and lechada synclines they are easily distinguished from other deformation events in the structurally uppermost level we find flap folds flap folds are recumbent structures with the inverted limb preserved below this level cascade folds are found these structures have a vergence opposite to that of parasitic folds the nomenclature adopted is from harrison and falcon characteristically these structures have shallowly dipping axial surfaces in agreement with the shallow dip of the axial plane regional cleavage in the lowermost structural level upright parasitic folds with a steep cleavage are present the variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth in the absence of overprinting relationships the f1 fold geometries are included in a single deformation event the steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub vertical in the upper part of the syncline to sub horizontal in the lower part with increasing depth the deformation regime during f changed from bending to buckling the deformation regime on the regional scale however is associated with basement subsidence and passive formation of the regional synclinal structure furthermore the absence of a distinct microfabric for the different f1 folds indicates that on a small scale a similar deformation regime was present we conclude therefore that the scale at which we study a structure only reflects the deformation regime at that particular scale consequently the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone

strain probe best fitting a homogeneous strain to the motions of scattered points abstract in laboratory experiments or e g in studies of glacier motion one sometime wants to estimate a finite strain from images or surveys of scattered marker points taken at successive times similarly crustal geodetic strain can be estimated by measuring velocities of geodetic stations the strain probe presented here is a simple formula that calculates a best fit strain from the motions of n marker points distributed over an area or a volume and monitored in a cartesian reference frame specifically it is the product of two 2 2 or 3 3 in 3 d symmetric matrices whose components are populated from the known positions and the measured displacements of the markers two examples implemented in excel workbooks illustrate how to combine the strain probe with the processing of the data and of its results speck motions in a means urai see through experiment are an example of a finite strain calculation published geodetic site velocities 120 stations in central anatolia are an example of infinitesimal strain

the formation of sedimentary strata in an allochthonous shelf environment the washington continental shelf nittrouer c a and sternberg r w 1981 the formation of sedimentary strata in an allochthonous shelf environment the washington continental shelf mar geol 42 201 232 models of fine scale 120 m shelf regions the inner shelf sand and mid shelf sandy silt are the predominant accretionary deposits on the shelf important sedimento logical observations obtained by boxcoring the upper 1 2 m of these deposits are a progressive decrease in grain size with distance from the columbia b downward coarsening within the seabed c loss of distinct sedimentary structure homogenization of sediment with distance from the columbia non dimensional parameters based on sediment transport and benthic biological studies can be used to relate rates of sediment mixing re working to rates of accumulation these parameters have the potential for quantitative prediction of strata formation from measurements of active processes physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the washington shelf and accumulation rate is known from pb 210 geochronology to decrease away from the columbia together these factors tend to increase the ratio of mixing to accumulation with distance from the columbia and predict the sedimentological observations listed above

the ephemeral development of c shear bands a numerical modelling approach abstract c shear bands are ubiquitous structures in shear zones but their development is poorly understood previous research has determined they mostly occur in rocks with a high mechanical strength contrast using numerical models of viscoplastic deformation we studied the effect of the proportion of weak phase and the phase strength contrast on c shear band development during simple shearing to a finite strain of 18 we found that c shear bands form in models with 5 weak phase when there is a moderate or high phase strength contrast and they occur in all models with weak phase proportions 15 contrary to previous research we find that c shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards this occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate c shear bands form on planes of low strain rate and stress and not in sites of maximum strain rate as has previously been suggested c shear bands are ephemeral and they either rotate backwards to the c plane once they are inactive or rotate into the field of shortening and thicken to form x and triangle shaped structures

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

shear bands crenulations and differentiated layering in ice mica models abstract thin sheets of composite ice mica have been deformed in order to simulate the development of cleavages in quartz mica rocks a strong initial mica preferred orientation was variably oriented to the shortening direction deformation parallel to the foliation results in a crenulation type cleavage developing from shear bands initiated after a component of pure shear deformation oblique to the foliation produces a differentiated cleavage and involves a large component of shear strain subparallel to the original anisotropy the strain is accommodated by intra and intercrystalline processes that produce extensive grain elongation and rearrangement of the ductile matrix thereby forming ice vs mica rich regions on the other hand there is no drastic morphological change when a sample is shortened perpendicular to an original foliation that is where the micas lie in the plane of no shear strain instead the mica fabric is strengthened and the grains in the ductile matrix are flattened two models are presented for the initiation propagation and evolution of the observed crenulation versus differentiated cleavage types these depend on mica stacking and orientation relative to the transverse properties of the sample and also on the direction of anisotropy to the xy plane of the bulk strain ellipsoid the models invoke shear on planes of high shear strain and rotation of the shear bands and rigid mica grains into a direction approximately parallel to the bulk extension direction

mineral fish their morphological classification usefulness as shear sense indicators and genesis mineral fish are sheared and commonly asymmetric mineral grains or clusters of grains this work reports 11 sub types of mineral fish showing a top to se sense of ductile shearing in the karakoram metamorphic complex kmc the mineral fish are of three broad geometries sigmoid lenticular and parallelogram reliable senses of shear are indicated by the overall asymmetry and inclination of mineral fish on the other hand the true shear sense is not always indicated by either the orientations of their cleavage planes or those of the individual grains in composite mineral fish the ranges of local orientations of single sigmoid mineral fish that include the lower values 23 in the kmc indicate their extensive ductile shearing the studied mineral fish were products of a range of deformation mechanisms including homogeneous deformation simple shear intra granular slip crystal plastic deformation fracturing and synthetic shearing additionally some examples might have undergone duplex slips and a few nucleated and grew either prior to or during the top to se shearing

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

the interaction of deformation and metamorphism in slates abstract the distribution microchemistry and internal structure of phases present in a slate are reported and used as a basis for a discussion of the interaction between deformation and metamorphism in slates the study reveals details of the interactions between mechanical and chemical processes involved in slaty cleavage evolution evidence for the involvement of mechanical rotation solution and crystallisation recrystallisation and metamorphic reactions is presented and amalgamated into a model of cleavage development the model involves a synchronous variation in the type location and rate of cleavage evolution processes and a change in the importance of particular processes during the total history the early stages of cleavage development are dominated by mechanical rotation associated with crenulation and possibly accompanied by grain boundary sliding and solution processes later stages of cleavage evolution are more heterogeneous and occur by a complex interaction of deformation and metamorphic growth processes which are concentrated at the interfaces between domains generated earlier in the strain history the variations in grain chemistry and orientation together with changes in the strain rate or stress level are suggested to control the different deformation behaviour of different grains along these strain accommodation zones and to dictate whether grains bend kink dissolve recrystallise or react and undergo growth evidence for the involvement of reactions ranging from ionic exchange to crystallisation from solution and solid state reordering is presented and the influence of deformation on the location and rates of these reactions discussed approximately ten potential reactions are involved in the cleavage evolution of the slate studied the model presented incorporates these reactions into the deformation processes and accounts for the concentration of phengite and iron poor chlorites in to one microstructural domain and paragonite quartz and fe rich chlorites into the other domain two transformations appear to be fundamental in the slate studied 1 1 the transformation of illite to phengite 2 2 the transformation of fe rich chlorite to fe poor chlorites and both can be related to particular responses of grains to the reorientation processes at domain interfaces

sandy submarine braid plains potential deep water reservoirs sandy submarine braid plains like their fluvial counterparts on land are sand rich depositional environments that may display excellent reservoir characteristics in terms of sediment volume porosity and permeability the submarine examples may be laterally associated with potential source rocks such as the fine grained levee deposits of deep sea channels a side scan sonar study of the central labrador sea revealed the existence of a more than 700 km long and up to 120 km wide submarine sand and gravel plain that has been supplied with sediment by high density turbidity currents possibly resulting from subglacial lake outburst flooding in the hudson strait the side scan imagery of parts of the plain displays a conspicuous streaky pattern of alternating high and low backscatter intensity high resolution 3 5 khz seismic profiles and 12 khz bathymetric profiles show that the pattern represents a furrow and ridge erosional or channel and bar depositional topography similar to a braided alluvial plain the furrows or channels have low acoustic backscatter are less than 10 m deep and are separated by ridges or bars having high backscatter some channels terminate in depositional lobes individual channels and bars or furrows and ridges are less than 100 m wide and can be followed up to 40 km downcurrent on sleeve gun seismic profiles the total sand thickness appears to be between 200 m proximal and 100 m distal piston cores from the plain recovered massive sand layers up to 4 m thick buried under 1 m of holocene hemipelagic ooze texturally the sands and gravelly sands display a trend of improving sorting with increasing mean grain size some very coarse grained samples are moderately well sorted and almost matrix free the flooding events that deposited the sands might be the submarine counterpart of heinrich events but need not be restricted to such events radiocarbon ages of about 10 k y from the base of the ooze overlying the youngest sand gave a minimum age for the sand that is similar to the age of heinrich event 0 estimates for the discharge volume of individual events are poorly constrained and range from 103 to 105 km3 braided channel patterns in deep water begin page 1500 sandy depositional environments are not restricted to high latitudes but also have been identified in various submarine fan settings in the lower latitudes for example the orinoco var and monterey deep sea fans and in the santa monica basin the largest examples however are known from high latitudes suggesting that melt water discharge from continental ice sheets may favor the formation of this habitat of giant sands in the deep sea the occurrence of sandy braided deep water environments having favorable reservoir characteristics in a variety of tectonic settings makes this type of environment a potentially interesting deep water target

the port mouton shear zone intersection of a regional fault with a crystallizing granitoid pluton abstract the peraluminous tonalite monzogranite port mouton pluton is a petrological geochemical structural and geochronological anomaly among the many late devonian granitoid intrusions of the meguma lithotectonic zone of southern nova scotia the most remarkable structural feature of this pluton is a 4 km wide zone of strongly foliated 040 subvertical monzogranites culminating in a narrow 10 30 m straight zone of compositionally banded rocks that extends for at least 3 km along strike the banded monzogranites consist of alternating melanocratic and leucocratic compositions that are complementary to the overall composition of that part of the pluton suggesting an origin by mineral melt and mineral mineral sorting biotite and feldspar are strongly foliated in the plane of the compositional bands these compositional variations and foliations originated by a process of segregation flow during shearing of the main magma with a crystallinity of 55 75 subsequent minor brittle fracturing of feldspars twinning of microcline development of blocky sub grains in quartz and kinking of micas demonstrate overprinting by a high temperature deformation straddling the monzogranite solidus small folds and late sigmoidal dykes indicate dextral movement on the shear zone this port mouton shear zone pmsz is approximately co linear with the only outcrops of late devonian mafic intrusions in the area two of which are syn plutonic with well developed mingling textures in the marginal tonalite of the port mouton pluton also closely co linear with the mafic intrusions are a granitoid dyke that extends well beyond the outer contact of the port mouton pluton a swarm of large aligned angular xenolithic slabs a zone of thin wispy schlieren banding a large be bearing pegmatite and a breccia pipe with abundant garnetiferous metapelitic xenoliths in various ways the shear zone may control all of these features the port mouton shear zone is parallel to many other ne trending faults and shear zones in the northern appalachians probably related to the docking of the meguma zone along the cobequid chedabucto fault system

sea bed mixing and particle residence times in biologically and physically dominated estuarine systems a comparison of lower chesapeake bay and the york river subestuary biologically dominated lower chesapeake bay and the physically dominated york river subestuary are contrasted in terms of the dynamics of sediment mixing strata formation and sea bed particle residence times two lower bay sites were examined both are located within the bay stem plains and are characterized by muddy sand and an abundance of large deep dwelling organisms x radiographs indicate extensive biological reworking of sediments with no long term preservation of physical stratification 210pb profiles reveal low sediment accumulation rates at both lower bay sites 80vs6 30 cm2year 1 in contrast the york river site located within a partially filled palaeochannel is predominantly mud with a depauperate benthic community dominated by small short lived shallow dwelling organisms although210pb accumulation rates at the york river site 0 2 cm year 1 are similar to those measured in the lower bay there is little bioturbation in addition transient bed forms at the york river site form laterally persistent linear ridges and furrows sub parallel to the channel spaced 10 20 m apart these observations coupled with evidence of episodic erosion and deposition from radioisotope and porosity profiles and x radiographs suggest that the upper 60 120 cm of the sea bed are dominated by physical mixing deep mixing and low accumulation rates result in long residence times of particles in the mixed upper portion of the sea bed 102year at both locations despite different mixing controls i e biological diffusive vsphysical advective

fluid expulsion sites on the cascadia accretionary prism mapping diagenetic deposits with processed gloria imagery point discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near surface 10 m below seafloor mbsf hemipelagic sediment the contrasting clastic and diagenetic lithologies should be apparent in side scan images however sonar also responds to variations in bottom slope so unprocessed images mix topographic and lithologic information we have processed gloria imagery from the oregon continental margin to remove topographic effects a synthetic side scan image was created initially from sea beam bathymetric data and then was subtracted iteratively from the original gloria data until topographic features disappeared the residual image contains high amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge based on submersible mapping ocean drilling program drilling and collected samples diagenetic deposits are concentrated 1 near an out of sequence thrust fault on the second ridge landward of the base of the continental slope 2 along zones characterized by deep seated strikeslip faults that cut transversely across the margin and 3 in undeformed cascadia basin deposits which overlie incipient thrust faults seaward of the toe of the prism there is no evidence of diagenetic deposition associated with the frontal thrust that rises from the decollement if the decollement is an important aquifer apparently the fluids are passed either to the strike slip faults which intersect the decollement or to the incipient faults in cascadia basin for expulsion diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates

spatial and temporal patterns in erosion and deposition in the york river chesapeake bay va abstract seasonal patterns of seabed erosion and deposition in a dynamic estuarine environment the york river usa are examined through acoustic studies and seabed sampling to identify the dominant environmental controls on sediment mobility areas near clay bank in the york river were surveyed monthly using a dual frequency sonar for a 12 month period in addition six stations within these areas were sampled during the surveys strong spatial and temporal variations of a soft mud layer were identified providing new insight to recurring seasonal and spatial patterns in deposition and erosion in the middle reaches of the york river estuary sub bottom data indicated maximum thickness 5 22 cm 4 cm of the soft mud layer during spring and winter from april 2008 through march 2009 minimum thicknesses 1 12 cm 4 cm of the soft mud layer were observed during summer and fall maximum thickness coincided with the occurrence of the secondary turbidity maximum near clay bank in the york river spatial variations in the soft mud layer thickness were also identified between and within the york river secondary channel the inactive oyster reef area the south west flank of the main channel and the main channel the upriver secondary channel the south west channel flank and the main channel areas were physically dominated the downriver secondary channel area was biologically dominated and the inactive oyster reef area reflected both physical and biological processes water content trends showed no correlation with seabed stability in this area observed changes in soft mud thicknesses are interpreted to result from both biological and physical factors which vary markedly between winter spring and summer fall seasons and among the different environments of the estuary s channel

the northern mid atlantic and reykjanes ridges spreading center morphology between 55 50 n and 63 00 n seamarc ii and hydrosweep acoustic imagery and bathymetry reveal that the northern mid atlantic ridge mar and reykjanes ridge between 55 50 n and 63 00 n are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from iceland the northern 100 km of the mar 55 50 n 56 47 n is oriented 005 perpendicular to the spreading azimuth and contains four third order spreading segments separated along strike by right and left stepping axial discontinuities the boundary between mar and reykjanes ridge occurs at 56 47 n where the bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092 this is the only transform fault between 55 50 n and 63 00 n and is inferred to have generated the bight fracture zone the reykjanes ridge lies between the bight transform and iceland s reykjanes peninsula and is oriented 30 oblique to the spreading orthogonal direction axial faults and volcanic systems on the reykjanes ridge are arranged in a right stepping en echelon pattern and display strikes intermediate between the spreading orthogonal direction and the strike of the ridge which is compatible with experimental models of oblique extension spreading segments on the northern mar and reykjanes ridge are defined structurally and bathymetrically at the smallest scale the volcanic axis is segmented into individual volcanic systems 4 45 km long which are separated along strike by intervolcano gaps or offsets these features are analogous to fourth order spreading segments documented elsewhere on the mar axial volcanic systems are superimposed on intermediate wavelength 13 65 km axial topographic highs that constitute third or second order spreading segments segment boundaries occur at axial depth maxima which correspond to offsets in the volcanic axis four primary characteristics of the spreading axis axial valley depth axial boundary fault throw relief along the neovolcanic axis and degree of inter segment structural discontinuity decrease toward the north this variation is consistent with higher crustal temperatures near iceland due to either thicker crust or elevated mantle temperatures

shoreline erosion and decadal sediment accumulation in the tar pamlico estuary north carolina usa a source to sink analysis abstract estuaries contain vital habitats and it is important to understand how these areas respond to human activities and natural processes such as sea level rise and wave attack as estuarine shorelines erode or become modified with hard structures there is potential for significantly altering the availability of sediment and the filling of coastal systems this study used a source to sink approach and quantified rates of shoreline erosion in the tar pamlico sub estuary a tributary of the larger albemarle pamlico estuarine system apes the average shoreline change rate scr determined using an end point method was 0 5 0 9 m yr 1 for the tar pamlico incorporating bulk density estimates this contributes 0 6 105 tons of fine sediment to the system annually or after accounting for fluvial input about 40 of the total sediment supply to the sub estuary the role of the tar pamlico as a sink for these sediments was addressed using the radionuclide tracers 210pb and 137cs radionuclide activities and sediment accumulation rates identified several depositional regions in particular in the middle of the estuary linear sediment accumulation rates ranged from 0 10 0 02 to 0 38 0 02 g cm 2 yr 1 and total storage of fine sediment in the system was 1 6 105 t yr 1 it was not possible to confidently discern a change in the rate of shoreline erosion or seabed accumulation a preliminary budget for fine sediments grain size

sonar acoustic facies and sediment distribution on an area of the deep ocean floor abstract long range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor seven acoustic facies that arise from differing sediment or rock types have been mapped over the low relief saharan continental rise and madeira abyssal plain these have been calibrated with sampling profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data the mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise abysaal plain transition over 1000 km from the nearest continental slope

comparison of flat topped stellate seamounts on earth s seafloor with stellate domes on venus using side scan sonar and magellan synthetic aperture radar abstract the diameters of many domes on venus imaged by the magellan synthetic aperture radar sar are an order of magnitude larger than terrestrial subaerial domes however surveys of the seafloor using sonar imaging systems such as gloria geological long range inclined asdic and seabeam have revealed flat topped volcanic features with similar diameters volumes and slope angles suggested analogies between domes on venus and seamounts on the northern east pacific rise and off hawaii have been tested by d k smith j volcanol geotherm res 73 1996 47 64 using comparisons of size and shape and significant differences identified gloria images of flat topped seamounts near hawaii show them to have stellate planforms over half of the total population of 375 domes on venus have been modified and many have stellate planforms we have focused on the similarities in stellate planforms between flat topped seamounts and modified domes on venus we have used gloria sonar and magellan radar data to constrain the formation and evolution of both flat topped seamounts on the ocean floor and volcanic domes on venus to see if the processes that formed them are similar we propose that flat topped seamounts on the hawaiian ridge are built on sediments overlying the oceanic crust and that available evidence supports three processes that can form stellate margins 1 radial dike intrusions related to flank rift zones 2 dike intrusions which cause slope failures and 3 gravitational slope failures unrelated to intrusions for venus where low erosion rates and the absence of a sediment layer results in lavas being emplaced directly onto the crust the available evidence supports stellate margins originating by gravitational slope failures unrelated to diking though intrusions cannot be ruled out with existing data

geometry classification and kinematics of s c and s c fabrics in the mushandike area zimbabwe planar fabrics in small dextral shear zones in the mushandike area of the zimbabwe archaean craton consist of a sigmoidal primary foliation s displaced by discrete shears c or c these planar fabrics can be divided into porphyroclastic megacrystic and banded types depending mainly on the values of the angles between s and c or c surfaces the structures that define s surfaces and the spacing of c or c surfaces the values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones the values of the angles depend on microstructures and strain microstructural evidence shows that slip did not occur on s surfaces but only on c or c surfaces a model for the development of the fabrics proposes that s surfaces lie parallel to the xy plane of the local finite strain ellipsoid in domains of simple shear between c or c surfaces c surfaces may form in the orientation of a coulomb failure surface the field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between c or c surfaces as the s surfaces rotate with the local finite strain ellipsoid

dynamic extraction of sediment attenuation from subbottom acoustic data reliable and accurate measurements of acoustic attenuation enable correlations between the acoustic responses and properties of sediments to be attempted this paper proposes a dynamic modeling technique which allows for a direct assessment of attenuation from changes in the signal spectral shape caused by absorption losses in the sediment using the frequencies corresponding to the signal spectral maximum and a point 3 db below the maximum attenuation data can be dynamically extracted laboratory experiments on three types of sand were conducted to validate the proposed method although the concept can be applied using any broadband sound source a parametric array source was used in the study because of its broad bandwidth and definable wave shape a comprehensive discussion on the different sets of data collected is given with an emphasis on the attenuation coefficient k and the exponent n of frequency the results of the study demonstrate close agreement when compared with the accepted literature values

the effects of fine scale surface roughness and grain size on 300 khz multibeam backscatter intensity in sandy marine sedimentary environments abstract high frequency acoustic backscatter intensity that is returned from sediment is affected by a number of seabed characteristics including sediment grain size distribution the nature and magnitude of seabed surface roughness and volume scattering by subsurface scatterers in order to develop quantitative remote sediment classification techniques it is important to determine the relative and combined effects of the dominant variables to which the acoustic signal is sensitive data used in this study were collected at two sandy sites off the south shore of long island new york and include 300 khz multibeam simrad em 3000 backscatter intensity data as well as grab samples and stereo bottom photographs multivariate statistical analyses were used to investigate the relationships between grain size properties seabed roughness and backscatter intensity at a number of sample locations at each study site encompassing a moderate range of backscatter intensities from 32 5 to 24 db the results of this study demonstrate that the relative importance of variables that affect backscatter intensity varies at different sites suggesting that backscatter intensity alone cannot be used to quantitatively predict seabed characteristics however our study indicates that in sandy sediments the median grain size the standardized magnitude of seabed roughness and the sorting of sediment can together be used to predict 300 khz backscatter intensity with high significance and that the relative importance of these variables varies from site to site we present an empirical model derived from a combined dataset that includes two sites of similar sedimentary and hydrodynamic conditions to predict the magnitude of seabed roughness based on backscatter intensity and sediment grain size characteristics the spatially coherent patterns in predicted seabed surface roughness revealed through the use of this kind of model greatly improve our understanding of variations in backscatter intensity in sandy marine sedimentary environments as a result the multivariate statistical approach that we employ improves our overall ability to remotely assess the details of the sedimentary environment that are significant with respect to benthic habitats and physical processes and are applicable to other geophysical datasets including sidescan sonar data

the influence of the san gregorio fault on the morphology of monterey canyon abstract a side scan sonar survey was conducted of monterey canyon and the san gregorio fault zone off shore of monterey bay the acoustic character and morphology of the sonar images enhanced by seabeam bathymetry show the path of the san gregorio fault zone across the shelf upper slope and monterey canyon high backscatter linear features a few kilometers long and 100 to 200 m wide delineate the sea floor expression of the fault zone on the shelf previous studies have shown that brachiopod pavements and carbonate crusts are the source of the lineations backscatter in monterey canyon the fault zone occurs where the path of the canyon makes a sharp bend from wnw to ssw 1800 m here the fault is marked by nw se trending high reflectivity lineations that cross the canyon floor between 1850 m and 1900 m the lineations can be traced to ridges on the northwestern canyon wall where they have 15 m of relief above the low relief ridges bowl shaped features have been excavated on the canyon wall contributing to the widening of the canyon we suggest that shear along the san gregorio fault has led to the formation of the low relief ridges near the canyon wall and that carbonate crusts as along the shelf may be the source of the high backscatter features on the canyon floor the path of the fault zone across the upper slope is marked by elongated tributary canyons with high backscatter floors and u shaped cross sectional profiles linear features and stepped scarps suggestive of recent crustal movement and mass wasting occur on the walls and floors of these canyons three magnitude 4 earthquakes have occurred within the last 30 years in the vicinity of the canyons that may have contributed to the observed features as shown by others motion along the fault zone has juxtaposed diverse lithologies that outcrop on the canyon walls gully morphology and the canyon s drainage patterns have been influenced by the substrate into which the gullies have formed

modeling reactive transport in sediments subject to bioturbation and compaction current bioturbation models are marked by confusion in their treatment of porosity different equations appear to be needed for different biodiffusion mechanisms i e interphase mixing where biological activity causes bulk mixing of sediment affecting both tracer and porosity profiles versus intraphase mixing where the solid components are intermixed but the porosity is left unchanged another issue is whether the model depends upon the particle type with which tracers are associated e g 137 cs on small clay particles versus 210 pb on larger grains this uncertainty has lead to conflicting conservation equations for radiotracers and in particular to the question whether the porosity should be placed inside or outside of the differential term that governs the biodiffusive flux we have reexamined this situation in the context of multiphase multicomponent continuum theory most importantly we prove that under the assumption of steady state porosity there exists only one correct form of the steady state conservation equation for a radiotracer regardless of biodiffusion mechanism and particle type i e

incremental deformation and fabric development in a kcl mica mixture abstract the history of deformation and of mica reorientation is described in a kcl mica mixture in the middle of a kink like experimental shear zone the history was reconstructed by deforming several specimens to successively greater total strains and assuming the more deformed specimens passed through the states of strain and fabric exhibited by the less deformed specimens a pseudo differentiated layering is described which arises from the original network distribution of the mica grains without benefit of any differentiation process a comparison is made between the observed mica fabrics at various strains and those predicted by the march model taking the measured initial preferred orientation of the micas into account the actual reorientation processes in the material are found to be more efficient than the march transformation at shortening strains less than about 70 but less efficient at higher strains these departures from marchian behavior are explaned qualitatively comparison is also made between observed and predicted intensities of mica preferred orientation assuming a hypothetical random orientation of micas in the starting material for the march calculation

multiple closely spaced transform faults in fast slipping fracture zones the 400 km long offset of the east pacific rise spreading axis between the quebrada fracture zone lat 3 5 s and the gofar fracture zone lat 4 6 s has been surveyed using the long range sidescan sonar gloria the offset is accomplished in three distinct fracture zones each of which contains up to four actively slipping transform faults separated by short spreading centers transform spacings range from 5 to 16 km each transform is associated with its own transverse valley but at quebrada fracture zone and to some extent at the other two these valleys merge into one large slightly oblique fracture zone valley the morphological trends of fracture zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement multitransform fracture zones appear to be much commoner on the fast spreading east pacific rise than on slower spreading ridges the tectonic spreading fabric which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them is modified to a sigmoidal pattern between the closely spaced transforms even midway between transforms these faults remain oblique to the spreading direction the degree of obliquity increases as the transform separation decreases showing progressive rotation of the stress ellipse as transforms are approached

polycyclic aromatic hydrocarbon distribution within lower hudson river estuarine sediments physical mixing vs sediment geochemistry abstract sediments and porewaters from two cores one from the east river new york and one from newark bay new jersey were sampled for polycyclic aromatic hydrocarbons pahs and other geochemical markers including 210 pb specific activities despite what appears to be similar sources of pahs to sediments within each core as inferred by ratios of alkylated to non alkylated pahs and pah isomer ratios sediments and associated pahs have undergone distinctly different depositional processes at each site sediments from the core in the east river appear to have been subject to intense physically driven resuspension resulting in selective depletion of low molecular weight pahs within the core presumably due to processes such as enhanced desorption and scouring of particle surfaces although sediment pah concentrations were higher in the east river core compared to the newark bay core porewaters were depleted of the majority of pahs analysed in the east river core relative to the newark bay core precluding quantifying sediment porewater pah distribution coefficients in this core sedimentary variables such as c n ratio particle surface area and sedimentary soot carbon were not related to organic carbon normalized pah distribution coefficients k oc obs rather in the core from the newark bay site k oc obs were influenced by sediment mixing and porewater dissolved organic carbon doc results indicate that in urban coastal areas the physical factors influencing time scales of sediment resuspension and deposition may be of primary importance over particle geochemistry in determining whether pahs will likely equilibrate between sediments and pore waters

gloria survey of the east pacific rise near 3 5 s tectonic and volcanic characteristics of a fast spreading mid ocean rise abstract a long range sidescan sonar gloria survey of an approximately 100 km square area of the east pacific rise crest between 3 s and 4 s extends results obtained in the same area by lonsdale 1977 using deep tow the axial linear volcano was found to be continuous over a distance of 75 km the presence of major inward and outward facing fault scarps was confirmed but the gloria data show several distinct differences between the two fault sets the inward dipping faults are more numerous more closely spaced and longer than the outward dipping ones and their dip slopes backscatter sound more extensively than those of the outward dipping faults moreover most of them appear to be formed within 2 km of the axis whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis these differences suggest that the two sets of faults have different origins the horizontal pattern of inward dipping faults is similar to those observed on other mid ocean rises at all spreading rates though the lengths and separations of the scarps are slightly and their throws considerably less than on slower spreading rises this common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate horizontal tension is probably the dominant factor but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2 8 km from the axis the similarity of major fault spacings on the east pacific rise to those on slower spreading rises suggests that faulting is invariant in space rather than time and that the lithosphere where these faults are formed about 2 km from the spreading axis has a similar small thickness for all spreading rates this is attributed to the regulating effect of hydrothermal circulation and plate cooling

toward a unified conceptual framework for shear sense indicators abstract the growing lexicon for shear sense indicators has created a need for a unified conceptual system of classification many common shear structures can be described by linear combinations of two kinematic variables which quantify the relative importance of objects vs foliations in controlling the shear zone velocity field

latest holocene evolution and human disturbance of a channel segment in the hudson river estuary abstract the latest holocene sedimentary record of a cohesive channel and subtidal shoal in the lower hudson river estuary was examined to elucidate natural sea level rise sediment transport and anthropogenic bulkheading dredging influences on the recent morphodynamic evolution of the system to characterize the seafloor and shallow subbottom 100 km of high resolution seismic reflection profiles chirp were collected within a 20 km reach of the estuary and correlated with sediment lithologies provided by eight vibracores recovered along seismic lines sediment geochronology with 137cs and 14c was used to estimate intermediate and long term sedimentation rates respectively and historical bathymetric data were analyzed to identify regional patterns of accretion and erosion and to quantify changes in channel geometry and sediment volume the shoal lithosome originated around 4 ka presumably with decelerating eustatic sea level rise during the latest holocene long term sedimentation rates on the shoal 2 3 2 6 mm yr are higher than in the channel 2 mm yr owing to hydrodynamic conditions that preferentially sequester suspended sediment on the western side of the estuary as a result the shoal accretes oblique to the principal axis of tidal transport and more rapidly than the channel to produce an asymmetric cross section shoal deposits consist of tidally bedded muds and are stratified by minor erosion surfaces that seismic profiles reveal to extend for 10s of meters to kilometers the frequency and continuity of these surfaces suggest that the surficial shoal is catastrophically stripped on decadal centennial time scales by elevated tidal flows tidal erosion maintains the shoal at a uniform depth below sea level and prevents it from transitioning to an intertidal environment consequently the long term sedimentation rate approximates the rate of sea level rise in the lower estuary 1 3 mm yr after the mid 1800s the natural geometry of the lower hudson river estuary changed rapidly in response to engineering works that forced the channel to self deepen analysis of historical bathymetric data indicates that the channel lost an estimated 3 106 tons of sediment between ca 1939 and 2002 50 000 tons yr average by subaqueous erosion increasing in depth by as much as 4 m in places erosion appears to have been concurrent with systematic bulkheading of the shoreline after ca 1865 which decreased the estuary surface area by 19 overall evidently self deepening of the channel is a morphodynamic adjustment to reestablish equilibrium cross sectional area yet the state of this change locally and elsewhere in the estuary is unknown subaqueous erosion documented in this study is a significant source of sediment with implications to the sediment budget and environmental quality of the hudson river estuary

structural analysis of mylonitic rocks in the cougar creek complex oregon idaho using the porphyroclast hyperbolic distribution method and potential use of sc type extensional shear bands as quantitative vorticity indicators abstract mylonitic rocks of the cougar creek complex of northeastern oregon and west central idaho provide an opportunity to document the deformational structures produced during general non coaxial shear within quartz feldspar mylonites and to explore the potential role of sc type extensional shear bands in vorticity analysis well developed feldspar porphyroclasts within six mylonite zones were utilized to estimate bulk kinematic vorticity w k using the porphyroclast hyperbolic distribution phd method w k values for the cougar creek mylonites range from w k 0 26 to w k 0 37 synthetic and antithetic shear band inclinations were measured relative to observed shear zone boundaries within five mylonite zones with estimated w k values and compared to the non coaxial flow field geometries and eigenvector orientations in each mylonite zone synthetic sc type shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the acute bisector ab of the eigenvectors similarly antithetic shear band populations show a range of inclination near the obtuse bisector ob of the eigenvectors we infer that sc type extensional shear bands form initially parallel to ab and ob and rotate towards the flow plane with progressive deformation decreasing their inclination relative to the shear zone boundary ab and ob have significance in the strain field in that they represent orientations of maximum angular strain rate thus planes perpendicular to ab and ob are mechanically favorable for small zones of localized simple shear shear bands within the heterogeneous bulk strain of the mylonite orientation analysis of populations of sc type shear bands may provide a direct quantitative means of estimating w k

the effect of sediment mixing on pb 210 accumulation rates for the washington continental shelf abstract nine cores from the washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from pb 210 profiles th 234 profiles indicate mixing coefficients of 140 cm 2 yr 1 for the seabed offshore from the columbia river and 47 cm 2 yr 1 for the seabed of the mid shelf silt deposit 75 km north of the columbia river these large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer 10 cm within one year after emplacement at the seabed surface observed depths of cs 137 penetration within the seabed are compared with depths predicted from the surface mixed layer thickness and the pb 210 accumulation rate the observed and predicted depths agree well at all but one station at this station a combination of th 234 co 60 pb 210 and cs 137 profiles suggests that active mixing occurs below the intensely mixed surface layer the agreement of pb 210 and cs 137 data at the other stations however indicates a general absence of deep mixing on the washington shelf thus the apparent accumulation rates calculated from pb 210 profiles below the intensely mixed surface layer on the washington shelf generally reflect the true rates of sediment accumulation

rheology microstructure and crystallographic preferred orientation of matrix containing a dispersed second phase insight from experimentally deformed ice abstract we utilize in situ neutron diffraction to continuously track the average grain size and crystal preferred orientation cpo development in ice during uniaxial compression of two phase and pure ice samples two phase samples are composed of ice matrix and 20 vol of second phases of two types 1 rheologically soft platy graphite and 2 rigid rhomb shaped calcite the samples were tested at 10 c below the ice melting point ambient pressures and two strain rates 1 10 5 and 2 5 10 6 s 1 to 10 and 20 strain the final cpo in the ice matrix where second phases are present is significantly weaker and ice grain size is smaller than in an ice only sample the microstructural and rheological data point to dislocation creep as the dominant deformation regime the evolution and final strength of the cpo in ice depend on the efficiency of the recrystallization processes namely grain boundary migration and nucleation these processes are markedly influenced by the strength shape and grain size of the second phase in addition cpo development in ice is further accentuated by strain partitioning into the soft second phase and the transfer of stress onto the rigid second phase

correlation of sidescan backscatter with grain size distribution of surficial seabed sediments the dependence of acoustic backscatter on sediment grain size distribution is examined using dual frequency 100 and 410 khz sidescan sonar and 22 sediment grab samples from the loch linnhe artificial reef site on the west coast of scotland the sidescan data were processed to remove an empirically estimated average grazing angle dependence on backscatter the processed data were analysed by forming histograms of pixels extracted from a 20 m 2 box around each ground truth site a positive correlation r 0 73 between mean backscatter intensity and mean grain size was obtained i e the coarsest samples had the brightest backscatter a positive correlation r 0 59 was also found between the standard deviations of the backscatter and grain size distributions i e poorly sorted sediments gave the most variable backscatter the performance of the sidescan data was compared to results from a co incident single beam echo sounder roxann survey the roxann roughness index e1 compared well with the sidescan whilst the roxann hardness index e2 did not this may be due to a physical link between the acoustic measures the comparison showed the sidescan to have delivered a significantly higher resolution image of the seabed for a similar amount of ship time imaging of the artificial reef modules themselves was found to be frequency dependent d 2004 elsevier b v all rights reserved

morphology and processes in lake tahoe california nevada lake tahoe was surveyed using a state of the art high resolution multibeam mapping system to provide an accurate base map for the myriad of ongoing environmental studies in and around the lake the newly defined basin morphology shows steep basin margins on the northern eastern and western sides and a gentle margin on the southern side two large flat plateaus several kilometers wide extend from the shore to about 40 m water depth in the northern and northwestern sections of the basin a series of ridges in the west and north are presumed traces of faults some of which border the lake basin and some of which traverse across the northern section of the lake and converge in mckinney bay mckinney bay is a large reentrant in the western margin that was created by a failure of the western margin that occurred about 300 ka the failure generated a major debris avalanche that carried large blocks some more than 1000 m long and 80 m high across the basin apparently the debris avalanche was deflected by the eastern margin of the basin and flowed to the north and south small debris flows and slides have continued to occur in this area small debris aprons occur along the northern western and eastern margins some apparently the remnants of collapsed terminal moraines formed in the basin from the 160 ka tahoe glaciation which reached the edge of the basin eroded plateaus and ridges occur on a glacial outwash plain that covers the gentle southern margin the plateaus and ridges are inferred to be remnants of another large terminal moraine of the tahoe glaciation

a long range side scan sonar gloria survey of the romanche active transform in the equatorial atlantic abstract a long range side scan sonar survey along the westernmost 720 km of the active section of the romanche fracture zone shows a narrow lineament which can be traced eastwards almost continuously along the north wall of the romanche trough until it cuts across the trough at its deepest point vema depth this lineament is inferred to be the presently active trace of the romanche transform fault the almost continuous ponded floor along the deep axis of the romanche trough was also mapped in detail as were tectonic trends to the north and south of the trough estimated poles of rotation based on both the active fault trace and the sediment pond show considerable differences between the poles describing the motions between north america and africa and between africa and south america the transform motions near the equator therefore do not appear to be simply coupled to the movement of the bordering continental blocks the location of the active transform fault along the younger northern flank of the romanche trough surveyed here is consistent with a gravimetric model that indicates that the plate boundary should be located under the younger flank of the fracture zone trough alternatively should the active fault be shown in future surveys not to cross over to the southern younger flank of the trough east of vema depth then some shift in the transform motions is indicated

response of sediment dynamics in the york river estuary usa to tropical cyclone isabel of 2003 tropical cyclone isabel of 2003 generated large storm surge strong waves and subsequent river flooding in the york river estuary usa during its passage across the chesapeake bay region a 3d model was used to investigate the changes of sediment concentration sediment flux and the recovery time of the york river estuary to its naturally evolved condition without the storm the results showed that two sediment concentration peaks appeared during the storm event the first one was induced by the large upstream flow and waves during the storm surge rising period and the later one was caused by the strong downstream flow during the descent of the storm surge the advection which was induced by the barotropic gradient dominated the sediment flux during the storm event the sediment fluxes increased by a factor of 100 during the rise and descent of the storm surge a large amount of sediment that was transported into the estuary and eroded from the seabed during the rising of the storm surge was quickly transported out of the estuary during the descent of the storm surge waves played a key role in stirring the seabed and increasing the sediment concentration during the storm subsequent high freshwater inflow changed the sediment loading and hydrodynamics in the estuary and thus influenced the estuarine turbidity maximum etm dynamics profoundly the etm moved downstream with the river flooding initially and returned upstream with the waning of river flooding and the re establishment of gravitational circulation the effect of river flooding on sediment concentration varied spatially and depended on the changes of etm locations and vertical mixing the model results suggest that a large amount of sediment was transported out of the estuary during the storm event and the subsequent river flooding had a larger impact on recovery time of the estuary

seafloor characterization and backscatter variability of the almeria margin alboran sea sw mediterranean based on high resolution acoustic data abstract an acoustic study of the almeria margin eastern alboran sea sw mediterranean was carried out by means of an integrated dataset comprising deep towed tobi sidescan sonar swath bathymetry topas high resolution seismics and sediment gravity cores the almeria margin is a complex system dominated by volcanic and tectonic features shaped by recent sedimentary processes in which a regional hemipelagic sedimentation is intercalated by sporadic gravitative events the aim of this work is twofold the first objective is to recognize the sedimentary and tectonic features that shape the almeria margin the main morpho sedimentary and morpho structural elements observed are the almeria canyon channel turbidite system the dalias tributary valley system landslides active faults and folds and volcanic banks the second aim is to explore and quantify the surface and subsurface seafloor parameters that characterize the backscatter of the study area sedimentary geomorphological and biological elements that play a role in the acoustic signature of the almeria margin were detected nevertheless it should be noted that seafloor acoustic backscatter is also affected by geometrical and instrumental factors despite the wide variability of the environments studied a low average acoustic backscatter reflects a diffused draping of hemipelagic sedimentation in the area higher backscatter values were observed at the top of some volcanic banks along the almeria canyon and in the shallower sector of the dalias tributary valley system the influence of subseafloor properties in the acoustic signature of the area was revealed by backscatter grain size correlations which were carried out for different depth intervals in sediment cores collected in the almeria turbidite system a poor relationship was found between backscatter and superficial silty sediments of the area whereas a higher correlation resulted in the upper 50 cm the presence of subsurface turbidites in the cores associated with the higher backscatter values suggests volume interface scattering of these sandy layers as a controlling factor of the acoustic signature obtained in the almeria margin on the other hand in rough settings such as the ones observed in the dalias tributary valley system topographic relief of up to 1 1 5 m strongly interacted with the tobi acoustic pulse this suggests that large scale roughness is an additional parameter that characterizes the acoustic strength of the area extremely high reflective patches distributed along some of the volcanic banks chella and pollux banks for depths ranging from 230 to 470 m coincide with areas where carbonate cold water coral mounds develop in the tobi images coral facies reach the maximum value of acoustic backscatter which is probably related to the rough morphology of coral ecosystems the position of coral mounds in the banks suggests that the occurrence of strong bottom currents and reduced sedimentary inputs are environmental factors that favour their settling and development in the almeria margin

the active part of charlie gibbs fracture zone a study using sonar and other geophysical techniques a short survey with gloria side scan sonar and other geophysical instruments has revealed new information about charlie gibbs fracture zone between 29 and 36 w the traces of two transform faults have been clearly delineated they fit small circles about the pole of rotation with an rms error of only about 1 km but they do not always follow the deepest parts of the transform valleys the transforms are joined by a short spreading center at 31 45 w the median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable vine matthews magnetic anomalies a transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature considerable thicknesses of sediment appear to have been deposited in the northern transform valley from norwegian sea overflow water passing through the fracture zone but transverse ridges have prevented the sediment reaching the southern valley

crystal growth during a single stage opening event and its implications for syntectonic veins abstract the crystallization of a fluid filled vein or cavity and interpretation of the microstructure in a deformed rock is an important problem for geologists dynamic crystallization experiments using ice and an aqueous solution have been used as an analogue system for single stage opening events and have shown that the presence or absence of a deviatoric stress is an important factor in the development of texture as a result of crystallization from a fluid the textures can vary from 1 coarse crystal aggregates involving static growth where there is no significant macroscopic deviatoric stress at the time of crystallization the free face growth model to 2 polycrystalline aggregates involving static growth which develop from a mixture of skeletal crystals and fluid in a stressed environment in the latter situation the crystals and fluid coexist until the macroscopic stress can be transmitted between impinging crystals and at this stage fast dynamic recrystallization and grain boundary migration occurs this recrystallization has been termed the contact growth model and the resulting metamorphic texture is compared with examples in natural quartz veins during a single stage vein opening uncomplicated by intragranular deformation either free face and or contact growth textures can develop from the fluid filled void depending upon the kinematic environment