evaluating the mechanical compaction of quartzarenites the importance of sorting llanos foreland basin colombia abstract mechanical compaction is the main porosity reducing process in sandstones including high reservoir quality rigid grain sandstones for such sandstones the extrapolation of theoretical or experimental compaction algorithms needs calibration with rocks having well constrained burial histories evaluating the compaction of these rocks is achieved by comparing current intergranular volume igv with depositional igv which is strongly dependent on sorting however because sandstone sorting is difficult to measure accurately its impact on depositional porosity and compaction state is largely underestimated we use the quartzarenites of the oligocene carbonera formation in the subsurface of the hydrostatically pressured llanos basin to illustrate the importance of sorting when evaluating the compaction of rigid grain sandstones igv and sorting were measured in core samples using a combination of transmitted light and cathodoluminescence images resulting in improved accuracy over standard procedures the compaction state of clean quartzarenites at given depths is best described using igv versus sorting plots which are used to derive compaction curves for specified sorting values the igv versus sorting trends are displaced to lower igv values with increasing burial depth the differences in igv caused by differences in sorting exceed the differences in igv resulting from 1000 m of burial illustrating the high impact of sorting when evaluating compaction contrasting with published experimental results the compaction of the llanos basin ductile grain poor quartzarenites is independent of grain size and grain rearrangement is the main compaction mechanism during the first 1 6 km of burial based on the llanos data we have generated igv versus depth curves for clean pure quartzarenites of specific sorting which can be used to predict their maximum primary porosity up to moderate burial depths differences with other published burial curves are probably related to unaccounted variations in sorting ductile grain content and framework strengthening cements however the llanos basin quartzarenites contain virtually no cements explaining their high degree of compaction relative to other rigid grain sandstones and making them ideal to isolate the effects of compaction on the igv of quartzarenites the llanos basin data suggest that below 2 5 km of depth clean well to moderately well sorted quartzarenites continue reducing their igv by mechanical compaction below the 26 limit which should apply only to extremely well sorted rigid grain uncemented sandstones

variation of magnetic properties in sediments from lake towuti indonesia and its paleoclimatic significance abstract we investigated the mineral magnetic behavior of sediments from lake towuti located in the eastern sulawesi ophiolite belt indonesia rock magnetic analysis supplemented by x ray diffraction and scanning electron microscopy analysis were performed on sediment core tow10 9b from the north basin of lake towuti to give insights on the environmental and sedimentary processes controlling the magnetic properties of the sediment and its paleoclimatic significance the results show that the core has three distinct zones of varying magnetic properties careful examination demonstrates that these zones correspond to varying levels of iron oxide dissolution and magnetite precipitation that are climatically and environmentally dependent the magnetically strongest zone is characterized by weak iron oxide dissolution and intense magnetite precipitation likely driven by changes in the stratification and or water level of the lake during dry conditions in marine isotope stage 2 mis2 period whereas the two magnetically weaker zones are characterized by signs of dissolution and correspond to relatively wet conditions respectively during marine isotope stage 3 mis3 and the holocene although our data show that major changes in concentration dependent parameters such as magnetic susceptibility and saturation isothermal remanent magnetization sirm in lake towuti sediment correlate with changes in regional rainfall many of the concentration changes are more strongly affected by in situ chemical processes than by changes in erosion and terrestrial sediment supply these findings urge caution in the interpretation of magnetic mineral concentration profiles as indicators of clastic sediment inputs

thin section grain size analysis revisited existing proposals for converting thin section data to their sieve equivalents are all flawed in various ways while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using units and of non spherical ellipsoidal grains using both millimetres and units have not been satisfactorily resolved in the literature it can be shown mathematically that the mean thin section diameters of spherical grains or axis lengths of a series of parallel sections through ellipsoidal grains will underestimate the dimensions of the corresponding central section i e one passing through the grain centre by 0 2023 when measured on a volume frequency basis in order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains the dimensions of a series of sections cut in 49 unique directions symmetrically arranged and evenly spread with respect to the ellipsoidal axes were calculated this calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations the data indicated that the mean true nominal diameters d of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters d and major axes of the central sections derived from the observed values by multiplying the millimetre means by 1 1318 and subtracting 0 2023 from the means using the following equations a rough estimate to within c 5 of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millimetres or units

cross correlation weighted migration towards high resolution mapping of mantle heterogeneities summary we present a new type of born migration scheme to obtain a high resolution image of smallscale scatterers in the mantle seismic migration is a technique that projects scattered energy back to its origin by shifting and stacking seismograms using theoretical traveltimes migration methods in global seismology because of the limited distribution of sources and receivers tend to suffer from considerable smearing owing to isochronal artefacts this type of artefacts is caused by migration grids that have the set of theoretical traveltimes from a given source to all receivers similar to that for a true scattering point previously the information of slowness and backazimuth was explicitly explored to reduce such migration artefacts here we propose a simpler and more computationally efficient method but with similar effects in which the coherency of shifted seismograms measured by cross correlation is exploited to suppress isochronal artefacts the new migration scheme makes it possible to compute a large number of migrated images efficiently and thus evaluate the robustness of observed scattering objects by statistical approaches such as bootstrap resampling

transient d discontinuity revealed by seismic migration 1 the d region beneath north central asia is investigated using a weighted migration based on the generalised radon transform we image complex structure including a d discontinuity with variable topography and velocity contrast we also present observations of an area where the d reflector stops using synthetic data sets generated for distributions of point scatterers we show that this is not due to deficiencies in the data coverage but requires that the d discontinuity is not a continuous surface the termination of the discontinuity appears to restrict it to a region high seismic velocity this could be interpreted as confirmatory evidence for the existence and behaviour of the post perovskite phase transformation however compositional variations may also play a role

origin and paleoenvironmental reconstruction of phosphorus bearing sandstones of the cambrian xinji formation southwestern margin of the ordos basin china increasing exploration interest in oil and gas hosted by early cambrian strata has focused research efforts on early cambrian sandstones the origin of phosphorus and the paleoenvironment of phosph

strong seismic scatterers near the core mantle boundary west of mexico pkpphasesobservedinjapan showstrongpre cursors with clear onsets for earthquakes that occurred in south america the onset times of the pkp precursors can allbeexplainedbyexistenceofseismicscatterersinthelow ermost 100 kmof themantlewest of mexico withinan area of 200 km300 km forward modeling of pkp precursor amplitudes indicates that a p wave velocity variation of at least 6 is required to explain the large amplitudes of the observed pkp precursors suggesting that the core mantle boundary region beneath west of mexico is a highly anoma lous region characterized by strong seismic scatterers

imaging the upper mantle transition zone with a generalized radon transform of ss precursors we demonstrate the feasibility of using inverse scattering for high resolution imaging of discontinuities in the upper mantle beneath oceanic regions far from sources and receivers using broadband wavefield observations consisting of ss and its precursors the generalized radon transform grt that we developed for this purpose detects in the broadband data signals due to scattering from elasticity contrasts in earth s mantle synthetic tests with realistic source receiver distributions demonstrate that the grt is able to detect and image deep mantle interfaces even in the presence of noise depth phases ghosts phase conversions and multiples generated by reverberation within the transition zone as a proof of concept we apply the grt to 50 000 broadband seismograms to delineate interfaces in the depth range from 300 to 1000 km beneath the northwest pacific we account for smooth 3d mantle heterogeneity using first order perturbation theory and independently obtained global tomography models the preliminary results reveal laterally continuous but undulating scatter zones near 410 and 660 km depth and a weaker broader and more complex structure near 520 km depth the images also suggest the presence of multiple laterally intermittent interfaces near 350 km and between 800 and 1000 km depth that is above and below the transition zone sensu stricto filtering of the data we consider four pass bands 20 50 s 10 50 s 5 50 s and 2 50 s reveals a prominent frequency dependence of the magnitude width and complexity of the interfaces in particular of the scatter zone near 520 km depth such dependencies may put important constraints on the mineralogy and phase chemistry of the transition zone

quantitative analysis of detrital modes statistically rigorous confidence regions in ternary diagrams and their use in sedimentary petrology abstract detrital modes are the basic type of quantitative data used in sedimentary petrology they represent estimates of the petrographic framework composition of sand or sandstone obtained by point counting of thin sections at present geological inferences from analysis of these numerical data are semi quantitative only because many of the data processing methods employed in sedimentary petrology lack a firm theoretical basis a review of existing methods reveals a number of fundamental statistical problems associated with the use of univariate statistics and the construction of so called hexagonal fields of compositional variation it is shown that these problems can be overcome by using multivariate methods that honour the non negativity and unit sum constraints on compositional data and incorporate the covariance structure spread in compositional data can be modelled on two levels 1 by regarding the grain as unit of observation and the total point count as a sample from a multinomial distribution 2 by regarding the total count as unit of observation and a set of counts as a sample from an additive logistic normal distribution numerical methods are presented for constructing statistically rigorous confidence regions in ternary diagrams by inversion of significance tests for these two distributions statistical analyses of the framework composition and heavy mineral assemblages of modern beach and river sands illustrate their use the case histories are followed by a brief overview of popular multivariate methods for reconstructing sediment provenance

lower mantle seismic discontinuities and the thermal morphology of subducted slabs abstract the nature of the lowermost few 100 km of the mantle holds insights into the scale of convection of the earth s mantle here we investigate the fine structure of this region using seismic migration of shear waves reflecting from velocity discontinuities near the core mantle boundary cmb recorded by a dense network of european seismic stations two main features are found one structure exhibits large scale topography and can be explained by a sharp increase in seismic velocity 206 316 km above the cmb the second structure lies 55 85 km above the cmb and marks a sharp reduction in velocity to explain the origin of these discontinuities we appeal to numerical simulations of three dimensional 3d mantle convection incorporating rigidly moving plates the plates organize the convective flow to allow for the development of large convection cells and cool sheet like downwellings at a convergent plate boundary the results suggest that in places observed seismic complexity in d discontinuity structure may be due to the thermal gradients resulting from the accumulation of cool subducted plate material at the cmb

a two dimensional p sv hybrid method and its application to modeling localized structures near the core mantle boundary a p sv hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process the seismic motions after interacting with the heterogeneous structures are propagated back to the earth s surface analytically with the aid of the kirchhoff method anomalous long period sks spdks observations sampling a region near the core mantle boundary beneath the southwest pacific are modeled with the hybrid method localized structures just above the core mantle boundary with lateral dimensions of 250 to 400 km can explain even the most anomalous data observed to date if s velocity drops up to 30 are allowed for a p velocity drop of 10 structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core mantle boundary some important issues such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle however remain unresolved due to the nature of the data

imaging scattering structures in the lower mantle by migration of long period s waves a simplified wave field migration method is applied to long period tangential component recordings to image lower mantle heterogeneity beneath alaska observed wave field complexities are dominated by coherent arrivals between direct s or ss and the core reflected scs or sscs phases a relative waveform inversion method is used to objectively extract the time and amplitude of the three main spikes in each wavetrain removing the effects of source time function receiver and instrument responses the timing of the intermediate arrivals relative to s or scs is used to define the volumetric position of scattering ellipsoids in the lower mantle with intersections of ellipsoids illuminating possible regions of isotropic scattering while a complete waveform migration is not viable due to the very limited ray path coverage and the band limited data the kinematics and relative amplitudes of the scattered arrivals are still valuable for assessing the origin of the waveform complexity in this region which displays particularly uniform existence of an extra arrival in the data the simplest interpretation is that a scattering surface with lateral dimensions exceeding 1500 km at a depth near 2600 km in the mantle is responsible for the extra phase this is consistent with previous interpretations involving a one dimensional model with a rapid shear velocity increase or discontinuity 240 280 km above the core mantle boundary the proposed methodology can potentially image both heterogeneities or layering in the d region as well as scattering from midmantle structure although data limitations will continue to be a major obstacle

evidence for a ubiquitous seismic discontinuity at the base of the mantle a sharp discontinuity at the base of earth s mantle has been suggested from seismic waveform studies the observed travel time and amplitude variations have been interpreted as changes in the depth of a spatially intermittent discontinuity most of the observed variations in travel times and the spatial intermittance of the seismic triplication can be reproduced by a ubiquitous first order discontinuity superimposed on global seismic velocity structure derived from tomography the observations can be modeled by a solid solid phase transition that has a 200 kilometer elevation above the core mantle boundary under adiabatic temperatures and a clapeyron slope of about 6 megapascal per kelvin

the stratigraphic record and processes of turbidity current transformation across deep marine lobes sedimentary facies in the distal parts of deep marine lobes can diverge significantly from those predicted by classical turbidite models and sedimentological processes in these environments are poorly understood this gap may be bridged using outcrop studies and theoretical models in the skoorsteenberg formation south africa a downstream transition from thickly bedded turbidite sandstones to argillaceous internally layered hybrid beds is observed the hybrid beds have a characteristic stratigraphic and spatial distribution being associated with bed successions which generally coarsen and thicken upward reflecting deposition on the fringes of lobes in a dominantly progradational system using a detailed characterization of bed types including grain size grain fabric and mineralogical analyses a process model for flow evolution is developed this is explored using a numerical suspension capacity model for radially spreading and decelerating turbidity currents the new model shows how decelerating sediment suspensions can reach a critical suspension capacity threshold beyond which grains are not supported by fluid turbulence sand and silt particles settling together with flocculated clay may form low yield strength cohesive flows development of these higher concentration lower boundary layer flows inhibits transfer of turbulent kinetic energy into the upper parts of the flow ultimately resulting in catastrophic loss of turbulence and collapse of the upper part of the flow advection distances of the now transitional to laminar flow are relatively long several kilometres suggesting relatively slow dewatering several hours of the low yield strength flows the catastrophic loss of turbulence accounts for the presence of such beds in other fine grained systems without invoking external controls or large scale flow partitioning and also explains the abrupt pinch out of all divisions of these sandstones estimation of the point of flow transformation is a useful tool in the prediction of heterogeneity distribution in subsurface systems

authigenic mineralogy depositional environments and evolution of fault bounded lakes of the yunnan plateau south western china the dianchi erhai and fuxian lakes lie in faulted basins in a subtropical humid region of the yunnan plateau china three groups of authigenic minerals have been recognized in their recent sediments carbonate minerals fe bearing minerals and silica minerals the main authigenic minerals are goethite calcite aragonite siderite and quartzine goethite is chemically precipitated from a colloidal suspension calcite is a widespread chemical precipitate that is present in deep parts of the lakes and in shallow areas associated with aquatic macrophytes aragonite is mainly biochemical in origin and commonly associated with shallow benthos siderite forms in reducing environments associated with pore waters with a high pco2 that resulted from microbial degradation of organic matter it forms mainly in deep water environments quartzine which occurs mainly in delta front and prodeltaic sites forms from diatom dissolution and dissolved silica introduced by streams six authigenic mineral associations are recognized each of which can be related to depositional setting within the lake and the stage of lake development the same associations can also be recognized in a 480 m long core recovered from dianchi lake strong reducing environments and migrating pore fluids with high pco2 have led to the early diagenetic alteration of some of the initial authigenic minerals using the mineral associations from the modern lakes the pliocene to recent history of dianchi lake has been interpreted and is in general agreement with palaeoenvironmental reconstructions based upon palaeontological and other evidence

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

slowness backazimuth weighted migration a new array approach to a high resolution image summary we have developed a new array method combining conventional migration with a slownessbackazimuth deviation weighting scheme all seismic traces are shifted based on the theoretical traveltime of the scattered wave from specific gridpoints in a 3 d volume observed slowness and backazimuth are calculated for each raypath and compared with theoretical values in order to estimate slowness and backazimuth deviations subsequently stacked energy calculated by a conventional migration method is weighted by the slowness and backazimuth deviations to suppress any arrival energy whose slowness and backazimuth are inconsistent with the expected theoretical values this new method was applied to two p wave data sets which comprise 1 underside reflections at the 410 and 660 km mantle discontinuities and 2 d reflections as well as their corresponding synthetic data sets the results show that the weighting scheme dramatically increases the resolution of the migrated images and enables us to obtain wellconstrained focused images making upper mantle discontinuities and d reflections more

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

the core mantle boundary under the gulf of alaska no ulvz for shear waves the earth s core mantle boundary cmb marks the boundary between the hot molten iron core and the silicate mantle and is a thermal chemical and flow boundary previous observations of very slow compressional wavespeeds suggest that thin ultra low velocity zones ulvzs possibly composed of a mixture of molten iron and silicates exist at the base of the mantle a molten or partially molten layer would cause a large shear wavespeed decrease however this velocity drop has not been observed here we use core reflected scp phases to investigate the shear properties of ulvzs these phases reveal at least two distinct regions one region under central america which is distinctly average prem and iasp91 like and a second region under the entire gulf of alaska that produces large scp reflections with amplitudes of up to 30 times larger than calculated with average earth models the large amplitudes suggest a combination of focusing by cmb topography high shear wavespeeds at the bottom of the mantle and low attenuation high q along the scp path high shear wavespeeds and low attenuation are opposite from what would be expected from a shear wave ulvz a ulvz in compressional wavespeeds ultra low vp has previously been observed in this region however in addition to the large amplitudes the short period scp waveforms show no complexity that can be related to a ulvz thus either there is not a ulvz under the gulf of alaska or if it does exist it is restricted to compressional wavespeed changes precluding interpretation as partial melt but rather suggesting a chemical origin fl 2000 published by elsevier science b v all rights reserved

eolianite grain size distributions as a proxy for large changes in planetary atmospheric density a digitized version of the ahlbrandt 1979 global dune dataset and data for distributions in figure are available at https github com erikgoosmann aeoliandata

finite frequency effects on global s diffracted traveltimes summary many seismic observations have shown that strong heterogeneities exist in the bottom few hundreds kilometres of the mantle among different seismic phases this region that is the d layer can be most globally sampled by diffracted waves along the core mantle boundary here we assess the amplitude and distribution of s wave velocity variations in the d layer of an existing tomographic model we compare observed shdiff traveltime anomalies to synthetic ones obtained using 1 the coupled spectral element method csem which is our reference exact method 2 non linear asymptotic coupling theory nact and 3 1 d ray theory synthetic waveforms are calculated down to 0 057 hz with a corner frequency at 0 026 hz in the first part of this paper we compare the traveltime anomaly predictions from the three different methods the anomalies from csem and nact are obtained by taking cross correlations of the 3 d and 1 d synthetic waveforms both nact and standard ray theory which are used in other recent tomographic models suffer from biases in traveltime predictions for vertically varying structure near the core mantle boundary nact suffers from saturation of traveltimes due to the portion in the kernel calculation that is based on the reference 1 d model while ray theory suffers from wave front healing effects in the vertical plane exacerbated in the presence of thin low velocity layers in the second part we compare observed traveltime anomalies and predictions from csem the data consists of 506 sdiff traveltime anomalies from 15 events obtained form global seismograph network records the tomographic model does a good job at predicting traveltimes of sdiff phases especially when the path mostly samples fast s velocity regions at the base of the mantle such as beneath india china north america and northern pacific the underprediction of the positive observed traveltime anomalies seems to occur in regions where the paths sample close to the border of the large low shear velocity provinces such as beneath southern pacific south of australia philippine sea plate and northwestern pacific ocean inspection of the observed seismograms at higher frequencies indicate that 1 the gradient of structure is underestimated and 2 there are post cursors to the sdiff which bias the lower frequency traveltime measurements and which are not taken into account by the tomographic inversion because of the approximate theoretical framework

grain size distribution and cementation of the cretaceous mzamba formation of eastern cape south africa a case study of a storm influenced offshore sequence abstract the late cretaceous mzamba formation of the eastern cape south africa is a storm influenced succession deposited in an offshore marine environment the succession is composed of two fundamental lithofacies very fine sandstone and packstone shell bed lithofacies the former represents background sedimentation in a relatively deep water low energy offshore environment whereas the latter represents storm deposits occurring with variable periodicity within the quiet offshore environment the storm generated packstone lithofacies shows a bimodal grain size distribution with two saltation fractions on the cumulative frequency diagrams together with negative skewness parameters and a relatively wide kurtosis of grain size distribution these characteristics are indicative of offshore storm generated sediments initial cementation of the storm influenced sediments occurred in a marine phreatic environment when isopachous rims of phosphate isopachous fibrous rims of smectite and micrite calcite cements were formed these cementation textures also support the offshore tempestite origin of the mzamba sediments the original marine phreatic diagenetic environment of the mzamba sediments changed to a meteoric phreatic environment probably at the end of mesozoic following deposition of the overlying cainozoic sediments the mzamba formation entered a burial diagenetic environment where the sediments were affected by elevated temperature and pressure finally the sediments were entered into a meteoric vadose zone due to tectonic uplift or sea level fall new dissolution pores and pendant calcite cement were created

imaging the lowermost mantle d and the core mantle boundary with skks coda waves summary in our previous studies we developed a method for imaging heterogeneity at and near the core mantle boundary cmb with a generalized radon transform grt of transverse component broad band scs data and we developed a statistical model for producing images of the d discontinuity with variable confidence levels in these applications the background is smooth and perturbations are represented as contrasts here we extend the theory to allow known discontinuities such as the cmb in the background model the resulting imaging operator which is formally not a grt can be used either alone or along with scs for the imaging of lowermost mantle structure and in particular the d discontinuity with the scattered skks wavefield synthetic seismograms calculated with the wkbj method are used to test the performance of our approach as a proof of concept we transform 38 000 radial component skks waveforms into image gathers of a cmb patch beneath central america the skks image gathers and image traces are in good agreement with the image traces obtained from the grt transform of scs data

a statistical approach to estimate the 3d size distribution of spheres from 2d size distributions size distribution of rigidly embedded spheres in a groundmass is usually determined from measurements of the radii of the two dimensional 2d circular cross sections of the spheres in random flat planes of a sample such as in thin sections or polished slabs several methods have been devised to find a simple factor to convert the mean of such 2d size distributions to the actual 3d mean size of the spheres without a consensus we derive an entirely theoretical solution based on well established probability laws and not constrained by limitations of absolute size which indicates that the ratio of the means of measured 2d and estimated 3d grain size distribution should be 4 785 actual 2d size distribution of the radii of submicron sized pure fe 0 globules in lunar agglutinitic glass determined from backscattered electron images is tested to fit the gamma size distribution model better than the log normal model numerical analysis of 2d size distributions of fe 0 globules in 9 lunar soils shows that the average mean of 2d 3d ratio is 0 84 which is very close to the theoretical value these results converge with the ratio 0 8 that hughes 1978 determined for millimeter sized chondrules from empirical measurements we recommend that a factor of 1 273 reciprocal of 0 785 be used to convert the determined 2d mean size radius or diameter of a population of spheres to estimate their actual 3d size

intense seismic scattering near the earth s core mantle boundary beneath the comoros hotspot i report strong seismic pkp precursors recorded in the seismic stations in tanzania for earthquakes occurred in the fiji subduction zone the observed pkp precursors show clear onsets and rapid variations of amplitude for different events and stations i develop a technique to use the precursor onset times to determine the location of seismic scatterers and apply forward modeling of precursor amplitudes to place constraints on the magnitude of seismic anomalies the travel time analysis indicates that these recorded pkp precursors are caused by the seismic scattering in the lowermost 360 km of the mantle beneath the comoros hotspot the amplitudes of the observed pkp precursors suggest a rather complex seismic structure with a p velocity variation of at least 8 likely associated with partial melt

directional petrological characterisation of deep marine sandstones using grain fabric and permeability anisotropy methodologies theory application and suggestions for integration abstract different methods for quantifying preferred grain orientation or grain shape fabric in sandstones are reviewed and their value for the reconstruction of palaeocurrent direction and preferred direction of pore fluid flow is reassessed critically based on existing knowledge from literature and new grain fabric data it is shown that petrographic analysis has distinct merits because grain fabric can be determined with low equipment costs and population statistics are based on the orientation of individual grains several recommendations are given how to standardise procedures for acquisition of grain fabric through microscopy these recommendations include 1 the careful selection of cross section orientations to acquire apparent long grain axes that are representative of true long grain axes 2 the preferred measurement of the coarsest grains in cross section and 3 a new statistically founded method to select a subset of grains that is representative of the total population of grains in a sandstone sample magnetic techniques that use anisotropy of magnetic susceptibility ams as a proxy for grain orientation require specialised equipment and are more restricted in statistical evaluation but these methods benefit greatly from being able to measure a large number of grains in a three dimensional space in a short amount of time and with a lower sensitivity to user bias of the two magnetic techniques discussed in this paper the new enhanced ams method in which rock magnetic properties are enhanced by introducing magnetite into the pores and as precipitates on grain surfaces is regarded to be an improvement over the traditional natural ams method in which only the natural assemblage of magnetite particles present in the rock is analysed enhanced ams measures the preferred orientation of essentially all grains in a rock sample thus providing the shape fabric of a hydrodynamically more accurate representation of framework grains than natural ams through a small technical modification the enhanced ams method can be used to obtain a magnetic cast of the pore network of a rock sample so that the resulting ams is controlled by the shape anisotropy of the individual pores it is commonly assumed that long axes of sand grains in sedimentary deposits parallel the direction of the flows that formed the deposits this concept is challenged by a new systematic study of grain fabric in deep marine turbidites it indeed appears that grains in ripple cross laminated divisions of turbidites dominantly attain a preferred orientation parallel to the palaeoflow direction but grains in plane parallel laminated divisions have an additional flow perpendicular mode which suggests a contribution of rolling grain transport three different fabric modes occur in massive structureless sandstones and massive divisions in turbidites apart from flow aligned and flow perpendicular modes there is a statistically significant but hydrodynamically enigmatic flow oblique mode with an average azimuthal deviation of 40 this large variation of fabric types suggests that a similar variation exists in preferred paths of pore fluid flow within turbidites controlled to at least some degree by depositional process this is confirmed by independent laboratory measurements of permeability anisotropy on samples for which grain fabric data are available and thus defines grain fabric analysis as a viable alternative to traditional permeametry techniques

three dimensional passive seismic waveform imaging around the safod site california using the generalized radon transform 1 we apply a three dimensional 3d generalized radon transform grt to scattered p waves from 575 local earthquakes recorded at 68 temporary network stations for passive source imaging of near vertical structures close to the san andreas fault observatory at depth safod site the grt image profiles through or close by the safod site reveal near vertical reflectors close to the fault zone as well as in the granite to the southwest and the franciscan melange to the northeast of the main fault although slightly lower in resolution these structures are generally similar to features in 2d images produced with steep dip prestack seismic migration of data from active source seismic reflection and refraction surveys our grt images however also reveal several vertical reflectors to the northeast of the saf that do not appear in the migration images but which are consistent with local geology these results suggest that in a seismically active area inverse scattering of earthquake data for instance with the grt can be a viable and in 3d economic alternative to an active source survey

shear wave speeds at the base of the mantle we inverted 4864 scs s and 1671 s diff sks residual travel times for shear wave speed anomalies at the base of the earth s mantle we applied ellipticity corrections accounted for mantle structure outside of the basal layer using mantle tomography models and employed finite size sensitivity kernels the basal layer thickness was set to 290 km however the data allow thicknesses between 200 and 500 km the residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small euclidean norm which limited spectral leakage of higher order structures into low order wavelengths hotspots dominantly overlay slow wave speed regions nonsightings of ultralow velocity zones ulvzs most frequently appear in fast regions suggesting that slow regions at the base of the mantle are associated with ulvzs however ulvz sightings appear in both slow and fast regions recently active subduction zones do not correlate with velocity anomalies however the locations of subduction zones active prior to 90 ma correlate extremely well with fast anomalies implying that slabs descend as fast as 2 cm yr through the lower mantle the correlation continues through the historical subduction record to 180 ma suggesting that slabs remain in the deep mantle at least 90 myr fast anomalies reach 2 percent while slow anomalies extend to 5 percent if we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed thermal anomaly conversion the temperature deviations would be over 500 k cold in fastest regions and over 1000 k hot in the slowest regions which would initiate plumes much hotter than those observed at the surface author

lithospheric imaging via teleseismic scattering tomography summary the coda of the teleseismic p phase consists largely of energy scattered by small inhomogeneities in the receiver side lithosphere given large collections of teleseismic data from dense permanent networks previous workers have successfully back propagated coda energy back to scattering source points using various kinematic migration schemes as well as by inverting using an inverse scattering radon transform approach under the born approximation seismic scattering is a linear process therefore it is possible to approach coda scattering as a linear waveform inversion problem mathematically similar to transmission based tomography assuming ray theoretical propagation and rayleigh scattering we pose the inverse scattering problem in tomographic form and recover perturbations in density and p and s velocities from pp and ps scattered data the method is applied to data from the southern california seismic network scsn covering the san jacinto anza region the results show a considerable correlation between seismicity and velocity perturbation structure particularly in the region between the mission creek and banning fault branches features connecting the coyote creek and elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression

using scp precursors to search for mantle structures beneath 1800 km depth 1 if either two stratified geochemical reservoirs or a mineralogical phase change in perovskite exist in the mantle the boundary between the geochemical layers or mineralogical phases may lie at a depth between 1800 km and the core mantle boundary cmb we searched for the scp precursors s to p reflections that would be generated at the boundary by stacking hundreds of short period vertical component teleseismic waveforms from the dense pacific northwest seismic network pnsn pnsn recorded clean scp phases and saved the p to scp time window for four earthquakes we estimate our detection threshold to be impedance contrast 4 none of the data show evidence of sharp structure between 1800 km and the cmb suggesting that a sharp seismic discontinuity does not exist between 1800 km and the cmb under the gulf of alaska and mexico any discontinuity would need to be diffuse have large topography or have a small impedance change while not conclusive this observation is consistent with an earth model lacking a global boundary separating geochemical reservoirs and lacking a phase change in this depth range

high resolution imaging of lowermost mantle structure under the cocos plate 1 broadband seismic shear waves are analyzed to investigate the fine velocity structure in the lowermost mantle beneath the cocos plate a region where previous studies have indicated the presence of a shear velocity increase about 200 300 km above the core mantle boundary data from 14 south american earthquakes recorded at california broadband networks provide dense ray path sampling of the lowermost mantle in an approximately 700 km long north south corridor roughly 150 km wide application of a simplified seismic migration method that uses a homogeneous background velocity model suggests topography of the previously imaged positive impedance jump varying in depth from north to south by as much as 150 km with a weakly reflecting transition zone in between the migration approach enables examination of small scale spatial variations and out of plane scattering effects topography of the discontinuity may account for observed variations in the amplitude of reflected arrivals or there may be lateral variations in the velocity contrast across the boundary lateral variations of the shear velocity structure within the d layer may produce some apparent topography in the discontinuity image but any such volumetric structure is not yet well enough determined to incorporate in the migration a localized negative impedance contrast reflector or scatterer is imaged at depths about 100 km below the positive reflector in the northern portion of the study area several scenarios can explain these results including 1 a slab that reaches the lowermost mantle 2 the birth of an upwelling beneath a recumbent slab or 3 chemical layering in this region

a scattering region near the core mantle boundary under the north atlantic summary we analysed pcp precursors in seven events from central america recorded at the uk array of short period stations to determine the most likely source of this signal combinations of observations of slowness traveltime and backazimuth were used to investigate the properties of the phase with the result that its source is identified as p to p scattering from an anomalous region in the lower mantle within d using a joint likelihood method the best estimate of a point source for scattering is at 44 0 n 71 5 w under nova scotia and 2720 km depth analysis of the spectrum of the scattered signal indicates that the region contains small scale heterogeneities on the order of 0 6 to 0 7 km and is 700 km in size the velocity contrast between the scattering bodies and ambient mantle is as high as 0 4 to 0 7 per cent vp and 1 4 to 6 6 per cent vs on account of their small size positive velocity anomaly and association with the reconstructed position of ancient subducted material we conclude that they represent chemical heterogeneity in the lower mantle

investigating the lowermost mantle using migrations of long period s scs data summary the lowermost mantle is investigated using a data set of high quality waveforms and a phase stripping technique which removes the s and scs phases in order to isolate more subtle arrivals two migration methods were applied to locate scattering bodies in the lower mantle the first a simple backprojection is useful for identifying regions of strong scattering the second migration scheme uses weights based on the generalized radon transform we present results for a region of lower mantle beneath alaska and western canada and test the influence of experimental geometry on the migration results by migrating synthetic data sets generated for various distributions of point scatterers we show that although the exact geometry of scattering bodies is poorly constrained the majority of the features in the data can be recreated with a few simple structures the results show the presence of a d discontinuity which can be simulated using a flat scattering body 250 km above the core mantle boundary near the alaska canada border we also present evidence for positive velocity anomalies in the bottom 100 km of the mantle beneath the d reflector and near the aleutians the association of these scattering structures with a region of high velocity is consistent with a cause related to lower mantle heterogeneity introduced by a subducting slab

size shape and orientation of grains in sands and sandstones image analysis applied to rock thin sections abstract a computer based image analysis system is used to determine the grain size distribution grain shape and grain orientation from rock thin sections a monte carlo simulation allows correction of the grain size distribution for the corpuscle effect grains cut marginally in the thin section plane have smaller apparent diameters thin section derived grain size distributions of modern and ancient samples have been compared with distributions obtained from sieving of the same samples different grain shape parameters have been compared with the wadell formula the grain orientation can be easily and reliably measured in thin sections from different cuts of a sample grain orientation data can help to determine paleocurrent directions

late holocene paleoclimatic changes in western central africa inferred from mineral abundance in dated sediments from lake ossa southwest cameroon abstract minerals derived from catchment soils were determined using ftir spectroscopy in the well dated core ow4 from lake ossa a lowland rainforest area in cameroon this quantification provides a hydrologic record indicating that the magnitude of runoff events and by inference rainfall pattern has varied during the late holocene the comparison between minerogenic inputs and vegetation changes improves the understanding of the inferred climate dynamics since at least 5400 cal yr b p the paleomonsoon rainfall intensity decreased as shown by a general decrease in mineral fluxes this observation is consistent with a gradual weakening of the boreal summer insolation in tropical latitudes however the major vegetational change lags behind the onset of the decrease in mineral fluxes from 2800 to ca 1000 cal yr b p the forest receded the amount of rainforest taxa decreased and is replaced by pioneer trees and poaceae when the mineral fluxes attained their lowest values this episode of maximum dryness is attributed to an abrupt climatic event of global significance which is superimposed onto the paleomonsoon variability it is related to a cold event which in turn produced a change in the lower atmospheric circulation that was characterized by a strengthening of northern trade winds probably correlated with sea surface temperature variations in the eastern tropical atlantic area

lower mantle heterogeneity beneath eurasia imaged by parametric migration of shear waves abstract lower mantle structure beneath central eurasia is examined using a recently developed technique for migrating parametric representations of shear waves from teleseismic events the migration method of lay and young lay t young c j 1996 imaging scattering structures in the lower mantle by migration of long period shear waves j geophys res 101 20 023 20 040 assumes isotropic scattering from discrete heterogeneities to account for s wave coda arrivals and provides resolution at scale lengths of about 500 km the migration is applied to 15 s period shear wave data from 21 western pacific earthquakes recorded in europe and the middle east that exhibit extra arrivals between s and scs on the transverse components migrations are performed using prem as well as model sgle gaherty j b lay t 1992 investigation of laterally heterogeneous shear velocity structure in d beneath eurasia j geophys res 97 417 435 which has a discontinuity at 2605 km 286 km above the core mantle boundary the migration images for the most coherent coda arrival scd resemble those expected for a lower mantle discontinuity model and simulations are performed for models with discontinuities at various depths quantitative correlation of the images for the data and synthetic migrations show the optimal average depth of the discontinuity to be 2605 km migrations of subsets of the data suggest that the discontinuity depth varies from 2605 km in the northern end of the study area to 2620 km in the southern end an additional intermediate arrival scd2 is observed in a limited portion of the dataset and migrations indicate this phase is generated by localized discrete scatterers located at depths of 2750 to 2770 km under north central eurasia these results are generally consistent with previous forward modeling studies but the migration approach allows lateral variations to be modeled more systematically the heterogeneous structure near the base of the mantle appears to be a manifestation of a complex boundary layer

evidence for lake formation as a response to an inferred holocene climatic transition in brazil abstract geochemical mineralogical and palynological records from a holocene sediment core 12 7 m long from lake silvana southeastern brazil served to identify the source of detrital sediments within catchment soils the lake basin was first flooded in the early holocene 9400 14 c yr b p when a gibbsite rich b soil horizon started to accumulate four distinct hydrologic phases are consistent with vegetational changes indicated by pollen data phases of slope instability colluviation and low lake level correspond to pollen free intervals and point to a severe retreat of the vegetation cover the rapid sedimentation of slope wash sediments alternating with alluvial sediments appears to have formed a tributary fan 7 8 m thick that dammed the lake silvana basin 8500 14 c yr b p probably as a response to a drastic increase in precipitation rates

beds comprising debrite sandwiched within co genetic turbidite origin and widespread occurrence in distal depositional environments co genetic debrite turbidite beds occur in a variety of modern and ancient turbidite systems their basic character is distinctive an ungraded muddy sandstone interval is encased within mud poor graded sandstone siltstone and mudstone the muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite the mud poor sandstone siltstone and mudstone show features indicating progressive layer by layer deposition and are thus called a turbidite palaeocurrent indicators ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event detailed field observations are presented for co genetic debrite turbidite beds in three widespread sequences of variable age the miocene marnoso arenacea formation in the italian apennines the silurian aberystwyth grits in wales and quaternary deposits of the agadir basin offshore morocco deposition of these sequences occurred in similar unchannellized basin plain settings co genetic debrite turbidite beds were deposited from longitudinally segregated flow events comprising both debris flow and forerunning turbidity current it is most likely that the debris flow was generated by relatively shallow few tens of centimetres erosion of mud rich sea floor sediment changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition the association with distal settings results from the ubiquitous presence of muddy deposits in such locations which may be eroded and disaggregated to form a cohesive debris flow debrite intervals may be extensive 26 x 10 km in the marnoso arenacea formation and are not restricted to basin margins such long debris flow run out on low gradient sea floor 0 1 may simply be due to low yield strength 50 pa of the debris water mixture this study emphasizes that multiple flow types and transformations between flow types can occur within the distal parts of submarine flow events

sequential growth of early diagenetic freshwater siderites in the holocene coastal deposits korea spherulitic siderite grains ranging from 100 to 250 am in diameter occur abundantly in a sediment layer which is almost 8600 years old in namyang bay on the west coast of korea these large siderite grains consist predominantly of feco3 average 65 and mnco3 average 22 and have low mg ca and mg fe ratios compositional scatter plots together with the high mn and low mg contents indicate that meteoric pore water was mainly involved in the siderite precipitation examination of the siderite grains with an electron microprobe reveals a systematic micron scale center to margin compositional variation within individual grains three zones are recognized on the basis of the compositional variations zone i center in which mn decreases and fe increases zone ii intermediate in which mn increases and fe decreases and zone iii outer in which both mn and fe decrease and mg increases this zoning is interpreted as a result of depth related early diagenetic reactions with different fe mn and mg concentrations in the sediment pore waters meanwhile compositional mapping of spherulitic grains implies that a siderite grain consists of numerous tiny siderite precipitates such dust like tiny siderite precipitates appear to be adhered to the large spherulitic siderite grain in some cases large siderite grains are adjoined together and occurred as a siderite aggregate we suggest that a spherulitic siderite grain is growing sequentially by adhering of numerous tiny siderite precipitates formed in each diagenetic zone onto the siderite grain like the formation of a snowball d 2004 elsevier b v all rights reserved

compositional variations in deep sea gravity flow deposits a case study from the voirons flysch voirons wagital complex chablais prealps france abstract grain size control is frequently invoked to explain the compositional variation of detrital sediments in fluvial sediments a similar possibility is here investigated in the fossil deep sea gravity flow deposits from the voirons flysch voirons wagital complex chablais prealps about 270 samples of diverse depositional settings were analysed using the framework composition including cement and porosity mutti facies and grain size measurements six lithofacies have been identified from cluster analysis they correspond to channel l1 to l3 and lobe l4 and l5 depositional settings and to reworked turbidites l6 our results emphasize the close relationship between petrography and grain size which alters the detrital signal from extrabasinal sources the abundant polycrystalline component of proximal deposits is progressively removed while skeletal grains and additional intrabasinal grains become more abundant in distal deposits alongside porosity is reduced by the simultaneous increase of cement which constrains subsequent diagenetic modifications hence proximal depositional settings keep most of the inherited signal from extrabasinal sources whereas distal deposits will be homogenised when plotted in provenance analysis diagrams the successive depositional settings show distinct fields possibly corresponding to different tectonic settings of the dickinson model this compositional variation thus creates a bias in provenance interpretation resulting from both grain size control and flow mechanisms finally our results emphasize that petrographic data must be handled with care in provenance analyses especially when comparing various deep sea gravity flow systems with different grain size ranges

palaeo environment in an ancient low latitude arid lacustrine basin with loessite the smith bank formation early triassic in the central north sea uk continental shelf acknowledgements this work was conducted as part of the triassic mudstones joint industry project which was sponsored by bp conocophillips eon e p esso exploration and production uk limited gdf suez e p uk ltd jx nippon maersk oil shell and total the guidance of prof steve hillier of the james hutton institute in quantitative mineralogical analysis is acknowledged with gratitude dr stuart jones and an anonymous referee are thanked for their incisive and constructive reviews that significantly helped in the revision of the manuscript

anatomy of a water escape cusp in upper proterozoic torridon group sandstones scotland abstract evidence from recently published experiments is applied to the interpretation of a typical soft sediment deformation structure in coarse fluvial sandstones from the applecross formation of the upper proterozoic torridon group in northwest scotland the structure is an interfacial cusp formed at the contact between two cross bedded sandstone units in an upward coarsening sequence the characteristics of the structure suggest that deformation took place while the sediment was in a fluidized state during fluidization a water filled cavity developed near the top of the lower unit separating fluidized sediment beneath from a static couplet above the cavity became narrower and taller as fluidization continued focussing fluidization at a point primary stratification was destroyed in the fluidized zone but there is no evidence that fluidized sediment burst through the overlying zone a rupture may be out of the plane of the section or alternatively fluidization could have stopped before this stage was reached or fluid pressures could have been dissipated upwards the cavity closed leaving the lower part of the static couplet as a distinct unit estimates of the fluidization velocity and the duration of fluidization suggest the volume of water required for fluidization this could have been derived from packing changes following liquefaction of underlying sediment or from groundwater movements within the braided fluvial environment