simplified quantitative risk assessment of rainfall induced landslides modelled by infinite slopes abstract rainfall induced landslides vary in depth and the deeper the landslide the greater the damage it causes this paper investigates quantitatively the risk of rainfall induced landslides by assessing the consequence of each failure the influence of the spatial variability of the saturated hydraulic conductivity and the nature of triggering mechanisms on the risk of rainfall induced landslides for an infinite slope are studied it is shown that a critical spatial correlation length exists at which the risk is a maximum and the risk is higher when the failure occurs due to a generation of positive pore water pressure

seepage beneath water retaining structures founded on spatially random soil the effect of stochastic soil permeability on confined seepage beneath water retaining structures has been studied random field concepts for the generation of soil permeability properties with a fixed mean standard deviation and spatial correlation structure have been combined with finite element methods to perform monte carlo simulations of the seepage problem analyses have been performed for a dam with two cut off walls the results of parametric studies to gauge the effect of the standard deviation and correlation structure of the permeability on the output statistics relating to seepage quantities exit gradients and uplift pressures are presented in all cases comparison is made with results that would be achieved on a deterministic basis flow rates and other quantities are shown to be affected significantly by the standard deviation and the correlation structure of soil permeability l article etudie l influence de la permeabilite d un sol stochastique sur l infiltration confinee existant sous

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

plio pleistocene environments of bed i and lower bed ii olduvai gorge tanzania abstract the plio pleistocene sediments of olduvai gorge tanzania are well known for their records of both hominid evolution and the paleoenvironments where these early hominids lived this paper investigates the paleohabitat reconstructions of bed i and lower bed ii of olduvai and provides an analysis using bovid tribes as habitat indicators the habitat inferences based upon the bovids are compared with paleoenvironmental reconstructions from studies of the micromammals pollen profiles isotope geochemistry and geology these various habitat indicators are shown to corroborate each other closely with regard to both the kind of habitat present and the timing of inferred habitat change the general agreement on habitat types appears to present especially strong support for the paleoecological reconstructions of bed i and lower bed ii paleohabitats ranged from closed woodland or forest to a mixture of open woodland and grassland to open grassland the pattern of paleohabitats through time supports the idea of a climatic trend towards greater aridity but with intermittent shifts to more humid conditions the plio pleistocene climatic history of the olduvai basin may prove useful in understanding the evolution of east africa climates

revised magnetostratigraphy of the plio pleistocene sedimentary sequence of the olduvai formation tanzania abstract the magnetic stratigraphy of the plio pleistocene sedimentary formation of the olduvai gorge tanzania which contains a well known series of prehistoric pebble cultures and hominid remains has been reinvestigated sixty one sites were sampled along 90 metres of stratigraphic section extending from the base of bed i into the masek beds palaeo and rock magnetic analyses reveal that magnetizations are stable carried by low coercivity magnetic minerals titano magnetites and are likely to be of depositional origin a revised magnetostratigraphic scale is proposed the base of the major normal magnetozone n1 interpreted as the olduvai sub chron lies ca 6 m below tuff ia i e significantly below the previously defined position this revision associated to 40ar 39ar ages recently determined on the tuffs of bed i supports the validity of the astronomical calibration of the geomagnetic polarity time scale for the olduvai sub chron the correlation of the normal magnetozone n2 with jaramillo or jaramillo cobb mountain is suggested by the discovery of a reverse magnetozone r3 in the uppermost part of the pleistocene sequence masek beds and implies that the fossiliferous sequence of the olduvai formation was entirely deposited before the brunhes matuyama boundary

volcaniclastic debris avalanche on motomachi area of izu oshima japan triggered by severe storm phenomenon and mechanisms abstract heavy rainfall and strong wind associated with typhoon wipha hit izu oshima island japan and triggered catastrophic landslides on the western slope of oshima volcano on 16 october 2013 these landslides were shallow but large in scale and the displaced landslide materials were characterized by rapid and long runout movement a massive landslide resulted in 36 people dead and 3 missing with 46 buildings being completely destroyed in the downstream area of motomachi district to understand the initiation and movement of these shallow landslides we surveyed the landslide area and found that in addition to heavy rainfall strong wind may also have played a key role in increasing the instability of the slopes we took samples from the source areas and examined their static and dynamic shear behaviors under naturally drained or undrained conditions test results showed that high pore water pressure can be built up and maintained within the displaced landslide material and increase landslide mobility even though the thickness of displaced landslide materials is 0 7 1 2 m this may then give a rational explanation on the fluidization process of the displaced landslide materials we also examined the windthrow features of trees subjected to different wind speed through monitoring the ground motion by seismometer and portable anemometer and then performed cyclic shear tests on the materials laying above the sliding surface the test results suggest that the windthrow may have promoted the buildup of excess pore water pressure within the saturated soil layer and thus favoured the occurrence of slope failure therefore it is suggested that in the prediction of shallow landslides on forested slopes subjected to heavy storm both the effects of rainfall and strong wind should be taken into account

zeolitic diagenesis of late quaternary fluviolacustrine sediments and associated calcrete formation in the lake bogoria basin kenya rift valley late quaternary fluviolacustrine siltstones mudstones and claystones loboi silts on the northern margins of the saline alkaline lake bogoria in the kenya rift valley contain up to c 40 authigenic analcime and minor natrolite the zeolitic sediments are reddish brown and up to 1 m thick the amount of analcime increases upward in the profile but decreases with distance from the lake the altered sediments show many pedogenic features including zeolitic root mats rootmarks concretions and carbonate rhizoliths residual patches of calcrete locally cap the zeolitic rocks the profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake the analcime occurs as submicroscopic 0 5 2 5 m subhedral and euhedral crystals which have an average si a 1 ratio of 2 33 as determined by x ray microanalysis or 2 18 d value of 639 analcime peak the analcime formed in lake marginal sediments soils by reaction of silicate detritus with na2co3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration poorly ordered clay minerals were probably the main reactants authigenic illite may have been a by product of the reactions chemical analyses suggest that pore waters supplied some of na and possibly k and sio2 the associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration the ca2 may have originated from infiltrating dilute runoff and groundwater authigenic smectite was precipitated in open porosity following analcime formation the zeolitic alteration at lake bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline alkaline lake sediments

chemical sedimentology and paleoenvironmental history of lake olduvai a pliocene lake in northern tanzania lake olduvai was a saline highly alkaline closed basin lake in which lacustrine sediments of bed i and lower bed ii of olduvai gorge were deposited between ca 1 92 and 1 70 ma the lake consisted of a lacustrine area the central basin cb surrounded by lake margin terrain that was alternately flooded and dried the lacustrine deposits consist largely of authigenic clay and carbonate minerals zeolites k feldspar chert and pyrite claystones of the cb and most of the eastern lake margin elm terrain consist chiefly of mg rich authigenic illite smectite and interlayered illite smectite i s these clays were formed penecontemporaneously with detrital clay deposition by interaction of the detrital clays with the saline highly alkaline water the lake olduvai clays average 65 70 mg 2 sio 2 and alkali ions from lake water and the remaining 30 35 is detrital clay lacustrine clays of the elm indicate that this area was flooded by saline water for substantial periods of time hominids occupied the elm at times of lower lake levels when the clays were dried calcite is the most abundant carbonate mineral and occurs chiefly as sand size euhedral crystals dispersed in claystone and less commonly as calcite crystal limestones and nodules the sand size calcite crystals were formed diagenetically at shallow depths possibly from a gaylussite and or micritic calcium carbonate precursor dolomite occurs chiefly in the form of dolostones some of which are probably of replacement origin the oxygen isotope composition of lake water and pore fluid of the cb varied over 11 as determined from calculated delta 18 o h 2 o values of authigenic minerals clay minerals were formed chiefly in the more saline waters sand size calcite crystals were generated mostly from less saline pore fluids and limestone nodules were formed chiefly from dilute pore fluid dolostones were formed from fluids of varying salinity hydrogeology of the central basin was complex and postdepositional fluid movements are documented by differing delta 18 o h 2 o values for different authigenic minerals of the same samples pore fluids moved within the lake deposits for a considerable period of time after deposition as shown by 18 o values of quartz and k feldspar formed at different times downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits lake olduvai became overall shallower during the deposition of bed i and the subaerial exposure increased leading to occasional playa conditions these changes very likely resulted from increasing aridity and tectonic activity the lake that deposited lower bed ii was larger than that responsible for upper bed i and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the bed i lake

reconstructing the avalonia palaeocontinent in the cambrian a 519 ma caliche in south wales and transcontinental middle terreneuvian sandstones an early cambrian caliche on the st non9s formation emended is the base of the caerfai bay formation unit term changed at caerfai bay south wales subaerial exposure and the caliche mean the two formations were not genetically related units the st non9s is an older sand sheet likely tidalitic not delta related referred to avalonian depositional sequence ads 2 and the caerfai bay is a shallow mud basin unit refered to ads 4a the similar random formation upper ads 2 in north american avalonia has a basal age of c 528 ma and is unconformably overlain by red mudstones or sandstones in fault bounded basins on the avalonian inner platform coeval british sandstones lower hartshill wrekin st non9s brand hills are unconformably overlain by latest terreneuvian ads 3 or epoch 2 ads 4a units dates of 519 ma on caerfai bay ashes give an upper bracket on the late appearance of avalonian trilobites and suggest an ads 2 4a hiatus of several million years post st non9s and post random basin reorganization led to abundant caerfai bay formation volcanic ashes and sparse brigus formation ashes in newfoundland the broad extent of erosional sequence boundaries that bracket lithologically similar to identical units emphasize that east and west avalonia formed one palaeocontinent the inner platform in southern britain was larger than the midlands craton a tectonically defined later palaeozoic area unrelated to terminal ediacaran early palaeozoic depositional belts the cool water successions of early palaeozoic avalonia were distant from coeval west gondwanan carbonate platforms

mass mortalities on an irish silurian seafloor abstract recurrent deep water benthic communities dominated by diverse tabulate corals colonized soft substrates during the late llandovery transgression in western ireland a number of autochthonous shelly assemblages dominated by brachiopods corals and crinoids occur in the upper parts of the kilbride formation the highest and most distal levels are dominated by in situ tabulate corals that are mantled by a thin layer of volcanic ash most of the coral colonies could cope with limited sedimentation growth ceased after the volcaniclastic surge this catastrophic mode of preservation permits an accurate analysis of an early stage of community development in deep water benthos on a soft llandovery seabed periodically swept by volcaniclastic debris the kilbride faunas have implications for the palaeoenvironments and taphonomy of deep water marine faunas associated with areas of active volcanism

quantitative risk assessment of landslides triggered by earthquakes and rainfall based on direct costs of urban buildings abstract this paper outlines a framework for risk assessment of landslides triggered by earthquakes and rainfall in urban buildings in the city of medellin colombia applying a model that uses a geographic information system gis we applied a computer model that includes topographic geological geotechnical and hydrological features of the study area to assess landslide hazards using the newmark s pseudo static method together with a probabilistic approach based on the first order and second moment method fosm the physical vulnerability assessment of buildings was conducted using structural fragility indexes as well as the definition of damage level of buildings via decision trees and using medellin s cadastral inventory data the probability of occurrence of a landslide was calculated assuming that an earthquake produces horizontal ground acceleration a h and considering the uncertainty of the geotechnical parameters and the soil saturation conditions of the ground the probability of occurrence was multiplied by the structural fragility index values and by the replacement value of structures the model implemented aims to quantify the risk caused by this kind of disaster in an area of the city of medellin based on different values of a h and an analysis of the damage costs of this disaster to buildings under different scenarios and structural conditions currently 62 of valle de aburra where the study area is located is under very low condition of landslide hazard and 38 is under low condition if all buildings in the study area fulfilled the requirements of the colombian building code the costs of a landslide would be reduced 63 compared with the current condition an earthquake with a return period of 475 years was used in this analysis according to the seismic microzonation study in 2002

a physically based multi hazard risk assessment platform for regional rainfall induced slope failures and debris flows abstract rainfall induced slope failures and debris flows are two major hazards in mountainous areas a physically based multi hazard risk assessment platform for regional rainfall induced slope failures and debris flows has been developed in this study the platform enables prompt assessment of risks posed by regional rainfall induced slope failures and debris flows across multiple catchments which is required in landslide risk management in a large area it considers the contribution of slope failures to debris flows and the scenario of a location impacted by multiple slope failures or debris flows or both the contribution of slope failures to debris flows is considered by adding the increased amount of channel deposit from slope failures to the source material of debris flows the platform is applied to a highway near the epicentre of the 2008 wenchuan earthquake the platform predicts the impact areas and runout distances of regional debris flows reasonably well the risk assessment results indicate that both slope failures and debris flows pose a great danger to travellers along the road shortly after the earthquake due to the presence of a large amount of loose landslide deposits on steep terrains the materials from the slope failures triggered during a storm substantially increase the channel deposit volume leading to significantly increased debris flow volume and risk a multi hazard risk assessment approach is necessary to consider the scenario of a location impacted by multiple slope failures or debris flows or both since assessing risks of slope failures and debris flows separately may underestimate the risk

subaqueous eruption and emplacement of ot2 in the middle miocene iizuka formation noto peninsula japan abstract ot2 okada tuff 2 is one of six rhyolitic tuff beds that occur in a middle miocene marine diatomite lower iizuka formation and are known as the okada tuff ot2 is well sorted and thins and thickens infilling channels 1 3 m across and 0 30 cm deep with a thickness up to 30 cm small channels 10 30 cm across and 1 10 cm deep are commonly superimposed within wider channels a thin diatomite layer is ripped from the substrate up to a level immediately above the small channels the basal part beneath the ripped up diatomite layer is dominated by ash components with vague wavy stratifications above the ripped up diatomite layer diatomite clasts are dispersed and pumice lapilli increase upward in amount and size with local interruption at a middle lower level by a poorly defined pumice lapilli poor thin layer many glass and pumice shards are blocky with curviplanar surfaces and jigsaw cracks presumably produced by phreatomagmatic explosions the poorly defined two layers of ot2 are interpreted to be ash turbidites individually produced by discrete phreatomagmatic explosions lenticular bedforms and channels indicate that the currents were fully turbulent and unsteady fed by the submarine eruptions upward coarsening of pumice probably represents settling lag of hot pumice ot2 perhaps provides potential diagnostic features to identify eruption fed turbidites

facies model of a semiarid freshwater wetland olduvai gorge tanzania the sedimentology and stratigraphy of a freshwater wet land in early pleistocene 1 75 ma volcaniclastic deposits olduvai gorge tanzania are characterized using texture mineralogy micro fossils and stratigraphic description to create a depositional facies model the freshwater wetland was located on the margin of a semi arid closed basin containing a playa lake the lake margin deposits are dominated by two distinct lithologies waxy claystones and earthy sediments waxy sediments are olive green dense claystones that con tain authigenic minerals such as trona and represent a fluctuating saline alkaline lake hay 1976 earthy sediments are beige friable siltstones that contain siliceous microfossils bone fragments and minor pebbles they represent the freshwater wetland microfossils from the earthy sediments and diatomites indicate freshwater marsh biota with some rare salt tolerant species associated lithofacies are diatomite carbonate and sandstone interpretation of vertical and lateral lithofacies variations in the 2 km 2 wetland based on 50 excavations identified seven distinct sub environments spring deposits perennial marsh pool ephemeral wet land fringing wetland wetland channel and ephemeral stream the stratigraphic sequence estimated to represent an 40 50 kyr long interval indicates that persistent 10 3 yr wet periods prevailed dur ing the deposition of thick waxy clays when the lake expanded and flooded the wetlands and equally long intervening dry periods caused the lake to contract allowing the flourishing of wetlands and accretion of earthy sediment the lithofacies associations in the olduvai paleo wetland provides an important first step in the development of a de positional environmental model for freshwater wetlands that can be tested elsewhere particularly in arid and semiarid settings

two examples of subaqueously welded ash flow tuffs the visean of southern vosges france and the upper cretaceous of northern anatolia turkey abstract pyroclastic deposits interpreted as subaqueous ash flow tuff have been recognized within archean to recent marine and lacustrine sequences several authors proposed a high temperature emplacement for some of these tuffs however the subaqueous welding of pyroclastic deposits remains controversial the visean marine volcaniclastic formations of southern vosges france contain several layers of rhyolitic and rhyodacitic ash flow tuff these deposits include from proximal to distal settings breccia lapilli and fine ash tuff the breccia and lapilli tuff are partly welded as indicated by the presence of fiamme fluidal and axiolitic structures the lapilli tuff form idealized sections with a lower coarse and welded unit and an upper bedded and unwelded fine ash tuff sedimentary structures suggest that the fine ash tuff units were deposited by turbidity currents welded breccias interbedded in a thick submarine volcanic complex indicate the close proximity of the volcanic source the lapilli and fine ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm wave base in northern anatolia thick massive sequences of rhyodacitic crystal tuff are interbedded with the upper cretaceous marine turbidites of the mudurnu basin some of these tuffs are welded as in southern vosges partial welding is attested by the presence of fiamme and fluidal structures the latter are frequent in the fresh vitric matrix these tuff units contain a high proportion of vitroclasis and were emplaced by ash flows welded tuff units are associated with non welded crystal tuff and contain abundant bioclasts which indicate mixing with water during flowage at the base basaltic breccia beds are associated with micritic beds containing a marine fauna the welded and non welded tuff sequences are interbedded in an alternation of limestones and marls these limestones are rich in pelagic microfossils the evidence above strongly suggest that in both examples tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment the sources of the pyroclastic material are unknown in both cases we propose that the ash flows were produced during submarine fissure eruptions such eruptions could produce non turbulent flows which were insulated by a steam carapace before deposition and welding the welded ash flow tuff deposits of southern vosges and northern anatolia give strong evidence for existence of subaqueous welding

undrained failure mechanisms of slopes in random soil abstract since the charts of taylor 1937 it has been well known in engineering geological investigations that the location of the critical failure mechanism in a homogeneous undrained clay slope goes either deep tangent to a firm base or shallow through the toe depending on whether the slope angle is less than or greater than about 53 in reality natural soils always exhibit spatial variability and the above expectations no longer hold true the objective of this note is to investigate the failure mechanisms and probability of failure of slopes in random undrained soil over a wide range of slope angles an elastic perfectly plastic finite element method in combination with random field generation well known as the random finite element method rfem is employed rfem represents a powerful tool for slope stability analyses that allow mechanisms to develop naturally within soil masses it is found that for certain combinations of random field properties relatively flat slopes may display a significant number of shallow mechanisms and steeper slopes may display a significant number of deep mechanisms for a steep slope the more variable the undrained shear strength the less likely the slope is to display a toe mechanism understanding the uncertainty of failure mechanisms is important because the consequences may be more serious in a deep failure as it involves a greater volume of soil

hydrological responses and stability analysis of shallow slopes with cohesionless soil subjected to continuous rainfall understanding the hydrological and physical responses of shallow slopes subject to rainfall events is vital for the efficiency of a warning system setup in this research a series of experiments were undertaken to evaluate the hydrological responses of shallow slopes of varying steepness and when subjected to varying intensities periods and inter storm periods of rainfall an analysis of infinite slopes was also undertaken to develop a fundamental understanding of rainfall induced shallow slope failure characteristics the hydrological and physical responses were characterized in the infiltration and saturation phases during the infiltration phase the maximum magnitude of water content was found behind the wetting front termed as the water content behind the wetting front wb for a certain soil type the magnitude of wb was found to be dependent on the magnitude of rainfall intensity regardless of the slope gradient and initial water content based on the relative depth of the failure plane th

core stratigraphy constrains bed iv archaeological record at heb site olduvai gorge tanzania abstract the olduvai gorge deposits contain a rich archaeological record documenting the evolution of hominin behavior over the last 2 million years while archaeological assemblages in the lower sedimentary layers beds i ii are well preserved in relatively secure chronostratigraphic contexts the age of overlying beds is poorly constrained due to discontinuous exposures and lack of marker horizons abundant erosional contacts and lack of unweathered dateable material as a result beds iii iv and the masek beds have yet received little attention compared to the underlying beds recently the olduvai gorge coring project ogcp acquired high resolution stratigraphically continuous sedimentary cores of palaeolake olduvai through scientific drilling in order to improve the geological and palaeoenvironmental contexts of the hominin record while other studies reported in this volume have used palaeoenvironmental datasets from the cores to refine the beds i ii records this study combines outcrop and core stratigraphic methods to improve the chronology of bed iv archaeological sites our recent excavation at heb site demonstrates the importance of this approach to improve the stratigraphic positioning and relative age of bed iv archaeological materials our results suggest that the heb site is younger than previously thought because it was formed after depositional fill of an incised valley which substantially removed much of lowermost bed iv in the heb area the lithofacies at this site indicate lake expansion and development of ngorongoro highland sourced braided streams near the lakeshore at this time which coincided with rich assemblages of artifacts fossil fauna and bones bearing butchery marks this setting likely afforded a variety of resources to hominins including potable water raw materials refuge trees and animal food making heb an important site for inferring the cultural and feeding behaviors of homo erectus

rainfall infiltration in a landslide soil deposit importance of inverse particle segregation abstract coarse particles move towards the front and surface of the deposit while fine particles stay in the rear and bottom accordingly the coefficient of permeability of the soil statistically tends to decrease from the surface to the bottom and from the toe to the apex of the deposit this feature can be described by trend functions that describe the changes in particle size and permeability inside the soil deposit in this paper attempts are made to include the physical background in the characterization of geotechnical uncertainties the objectives of this paper are to simulate non stationary fields of particle size and coefficient of permeability in a landslide soil deposit and to analyse rainfall infiltration in the non stationary random fields the soil deposit is assumed to consist of inter layers parallel to the slope surface the soil deposit is also assumed to be axisymmetric therefore any vertical section of the deposit can be selected to generate two dimensional non stationary random fields each of the non stationary random fields is formulated by adding a trend surface to a residual stationary random field with orthogonal major and minor scales of fluctuation the produced random fields follow an exponential correlation structure subsequently steady state rainfall infiltration in the anisotropic non stationary heterogeneous random fields is analysed the effects of variations of saturated permeability and scale of fluctuation on the pore water pressure regime are also investigated

effects of degree of saturation on shallow landslides triggered by rainfall the empirical rainfall threshold concept and the physical based model are two commonly used approaches for the assessment of shallow landslides triggered by rainfall to investigate in detail the rainfall triggered shallow landslides many physical based models coupling the infinite slope stability analysis with the rainfall infiltration modeling in variably saturated soil were developed however in those physical based shallow landslide models the unit weight and the unsaturated shear strength were assumed constant rather than depending on the degree of saturation in this study the effects of the unit weight and the unsaturated shear strength as function of degree of saturation on rainfall triggered shallow landslides are examined several designed scenarios and a real case scenario are used to conduct the examinations the results show that not only the occurrence of shallow landslides but also the failure depth and the time to failure could be misassessed if the influences of degree of saturation on the unit weight and the unsaturated shear strength are neglected

an ash flow tuff emplaced in shallow water early miocene koura formation southwest japan abstract the t 4 tuff in non marine shallow water deposits of the early miocene koura formation southwest japan consists mainly of poorly sorted ash pumice and lithic lapilli and is about 20 m thick the tuff grades upward into a parallel to cross stratified surge like layer and further to a fine tuff layer and locally possesses gas escape pipes extending vertically from the base to the top the basal part of the tuff includes ripped up clasts and load marks are common on the sole armored lapilli commonly occur in the upper part especially in the stratified layer the features of the t 4 tuff are mostly those of subaerial ash flow tuff but load marks on the sole and ripped up clasts in the basal part are typical of subaqueous ash flow tuff armored lapilli imply that the ash flow was wet the ash flow from which the t 4 tuff was deposited is interpreted to have advanced across a shallow water body while pushing water aside and becoming wet the t 4 tuff may be a typical facies of ash flows emplaced in quite shallow water bodies

origin of mudstone clasts in turbidites of the miocene ushikiri formation shimane peninsula southwest japan abstract some turbidites of the miocene ushikiri formation in the shimane peninsula southwest japan include many mudstone clasts within their normally graded and or massive lower parts the mudstone clasts are at most 1 2 m long and 10 20 cm thick they are undulated folded or pulled apart and are often concentrated at certain levels within the lower parts the turbidites with mudstone clasts usually have sharp planar soles with no distinct channels but occasionally they penetrate and tear up the underlying mudstone layers in addition most if not all mudstone clasts lithologically resemble the underlying mudstone these observations on outcrops suggest that the most of mudstone clasts were torn off by turbidity currents from underlying hydroplastic muds that possess a breaking strength and are entrained into the currents and deformed into various attitudes during transport the force which played such an important role in the process probably came from large shear stress developed within the turbidity currents

diagenetic siderite and other ferroan carbonates in a modern subsiding marsh sequence abstract mineralogical and carbon and oxygen stable isotope compositions in siderite rich concretions from a marsh within the mississippi river deltaic plain are highly variable the concretions have sideritic interiors and become more calcitic and dolomitic ankeritic near their margins 13c values of these concretions range from 10 0 to 7 4 pdb and 1 o values range from 2 4 to 8 9 pdb the stable isotope compositions exhibit two general trends one is a growth trend that shows 13c an 18o depletion from the centers to the margins of the nodules and bands the other is a progressive burial trend that shows 13c enrichments and 18o depletions at the centers of the concretions with depth a zonal model for carbonate precipitation with emphasis on an early fresh water non sulfidic methanic phase followed by a sulfidic stage associated with marine inundation of the marsh explains much of the observed carbon isotopic variations mixing of fluids plus precipitation in a partially closed chemical microsystem accounts for most of the oxygen isotope fractionations documented between growth layers and casts doubt on the utility of change in 18o shifts as an indicator of depth or relative temperature of concretion formation for concretions that form in coastal environments

siliceous islands in a carbonate sea modern and pleistocene spring fed wetlands in ngorongoro crater and oldupai gorge tanzania abstract a biogenic mechanism is proposed that produces a diagnostic geochemical signature of perennial spring fed wetlands in arid basins especially the saline and alkaline volcanic basins of east africa respiration by vegetation and decay of organic matter within marshes in ngorongoro crater tanzania locally increases aqueous equation the resulting lowered ph inhibits the precipitation of calcite and decreases the rate of silica dissolution steep geochemical gradients are produced these are represented mineralogically by siliceous biomineralization within the perennial marsh and carbonate precipitation in surrounding ephemeral wetlands mudflats and lacustrine environments as a test of this model of geochemical sedimentation the mineralogy of the early pleistocene siliceous earthy claystone of oldupai gorge is consistent with its interpretation as a paleo spring wetland deposit because such deposits are expressions of the paleo water table this approach has potential applications in many studies of basin paleohydrology paleoclimatology paleoecology and archaeology

influence of spatial variability of permeability property on steady state seepage flow and slope stability analysis in recent years spatial variability modeling of soil parameters using random field theory has gained distinct importance in geotechnical analysis in the present study commercially available finite difference numerical code flac 5 0 is used for modeling the permeability parameter as spatially correlated log normally distributed random variable and its influence on the steady state seepage flow and on the slope stability analysis are studied considering the case of a 5 0 m high cohesive frictional soil slope of 30 degrees a range of coefficients of variation cov from 60 to 90 in the permeability values and taking different values of correlation distance in the range of 0 5 15 m parametric studies using monte carlo simulations are performed to study the following three aspects i e i effect ostochastic soil permeability on the statistics of seepage flow in comparison to the analytic dupuit s solution available for the uniformly constant permeability property ii strain and deformation pattern and iii stability of the given slope assessed in terms of factor of safety fs the results obtained in this study are useful to understand the role of permeability variations in slope stability analysis under different slope conditions and material properties c 2009 elsevier b v all rights reserved

chronology and paleohydrology of late quaternary high lake levels in the manyara basin tanzania from isotopic data 18o 13c 14c on fossil stromatolites abstract superimposed phases of stromatolite buildup are observed on the eastern margin of lake manyara and depict a paleoshoreline at about 20 m above modern lake level radiocarbon and th u measurements permitted the dating of the last two phases of stromatolite formation at ca 90 000 yr and between 27 000 and 23 000 yr b p respectively the th u chronology is based on the decay of a strong 230th excess over 234u inherited with the detrital particles cemented into the stromatolites the various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels this indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation a comparison with similar stromatolitic units from the nearby lake natron lake magadi basin shows that such conditions occurred during only a few of the late quaternary humid episodes known in eastern africa and that they are different in each basin stromatolites do not necessarily represent all high lake levels that lake manyara experienced during the late pleistocene and holocene

orbital rhythms monsoons and playa lake response olduvai basin equatorial east africa ca 1 85 1 74 ma the lithostratigraphy at the pliocene pleistocene boundary exposed in olduvai gorge east africa was used to determine the duration and cause of climate variations wet dry periods the area has rich paleontological and archaeological records and the time interval is a critical period in human evolution five episodes of lake expansion and contraction can be interpreted from a stratigraphic interval defined by tuffs between ca 1 85 and 1 74 ma suggesting precession controlled cyclicity the geochronometry of the sequence was determined using the paleomagnetic record top of subchron cn2 1 785 ma the ages of two tuffs tuff ib and tuff if and sedimentation rates insolation w m 2 variations calculated as a function of eccentricity and precession predict five cycles of varying insolation 10 between 2 0 and 1 5 ma rainfall would have increased by one third between dry and wet portions of each 21 000 k y cycle this study represents the first dated terrestrial record at the pliocene pleistocene boundary with sufficient resolution to link lake cycles a climate proxy to astronomic forcing the wet dry climate pendulum may have been a factor in the natural selection processes of hominin evolution and the first wave of hominin migrations out of africa 1 8 ma

hydrogeochemistry of lake turkana kenya mass balance and mineral reactions in an alkaline lake abstract lake turkana in northwestern kenya is a closed basin alkaline ph 9 2 lake of moderate salinity tds 2500 ppm principal ions are na hco 3 and ci the lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters the omo river is the principal influent providing some 80 90 of water input to the lake chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water the bottom sediments are predominantly detrital and fine grained yet mineral water reactions are very important for the geochcmical budget ca 2 is precipitated as calcite na is removed as an exchangeable cation on smectite mg 2 is probably incorporated into a mg silicate phase most likely poorlycrystalline smectite as it enters the lake water k may be used in illite regradation cation exchange is a very important process in the mass balance of this lake over 40 of incoming na is removed as an exchangeable cation after cation exchange and interstitial water burial na has a response time of 2650 years which compares favorably with that of chloride these processes seem to occur rapidly within the water mass of the lake other reactions may be important in regulating interstitial water compositions several changes occur in the upper 3m of sediment interstitial water ph drops to 8 3 and alkalinity increases slightly with depth so 2 4 decreases slightly and amorphous silica saturation is approached these changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water high rates of sedimentation up to 1 cm per year may minimize the effects of diffusion between the interstitial waters and the lake water although burial of interstitial water assumes considerable importance

anthropocene and early human behavior when is the baseline for anthropocene the first indications of human impact on earth closely relate to early human behavior and use of resources first agricultural societies modified the landscap

model for the prediction of shear strength with respect to soil suction experimental studies on unsaturated soils are generally costly time consuming and difficult to conduct shear strength data from the research literature suggests that there is a nonlinear increas

biogeochemical evidence from ogcp core 2a sediments for environmental changes preceding deposition of tuff ib and climatic transitions in upper bed i of the olduvai basin abstract sediment cores recovered by the olduvai gorge coring project ogcp in 2014 afford the opportunity to examine the coupling of biogeochemical evidence for climatic and environmental change within the context of hominin evolution at this renowned east african locality investigations of elemental total organic carbon toc or corg wt carbon to nitrogen ratio c n molecular source specific biomarkers and isotopic 13corg 2hnc31 compositions of organic matter provide evidence for temporal changes in sedimentary materials derived from terrestrial plants c3 c4 and aquatic producers algae sponges cyanobacteria and in precipitation the 13 kyr record of upper bed i immediately preceding tuff ib extends high resolution stratigraphic profiles of precession scale alternations of wetter and drier conditions to encompass the entire interval from the bed i basalt to tuff ib the second wetter interval designated w2 records local influences on climate and water supply consistent with evidence for discrete sediment sources based its physical properties e g gamma radiation magnetic susceptibility compared with the overall sequence the 13ctoc values for wetter interval w2 reveal two millennial scale 2 5 kyr drier episodes followed by a shift in the dominance of c3 over c4 plants accompanying the transition to a drier climate d3 moreover biogeochemical data for upper bed i show that changes from drier to wetter conditions occur more rapidly 900 yr than wetter to drier transitions 2 6 kyr based on interpolated ages in addition biomarker profiles indicate that aquatic plants primarily algae and macrophytes may have been subject to more profound and faster fluctuations than variations in terrestrial vegetation expressed in terms of the relative proportions of woodland and grassland settings thus environmental and climatic changes not only influenced the availability of resources of food and shelter for hominins within the olduvai region but also led to their variation on centennial to millennial to precessional timescales

probabilistic analysis of seepage that considers the spatial variability of permeability for an embankment on soil foundation abstract in this study probabilistic analysis of seepage through an embankment on soil foundation was performed the traditional seepage analysis method was extended to develop a probabilistic approach that accounts for the uncertainties and spatial variation of the hydraulic conductivity in a layered soil profile the hydraulic conductivity of soil shows significant spatial variations in different layers because of stratification further it varies on a smaller scale within each individual layer it was assumed that the statistics of the hydraulic conductivity is different for different layers and that the hydraulic conductivity in a layer is uncorrelated with that in the other layers two dimensional random fields were generated on the basis of the karhunen loeve expansion in a manner consistent with a specified marginal distribution function and an autocorrelation function a series of seepage analyses of embankment foundation systems were performed using the generated random fields to study the effects of uncertainty due to the spatial heterogeneity of the hydraulic conductivity on the seepage flow the results showed that the probabilistic framework can be used to efficiently consider the various flow patterns caused by the spatial variability of the hydraulic conductivity in seepage assessment for an embankment on soil foundation

influences of soil water characteristic curve on rainfall induced shallow landslides the analysis of slope instability induced by rainfall was usually performed using the main drying curve as the measurement of the main wetting curve is a more time consuming and costly task in this study the influences of the main drying and wetting curves on rainfall induced shallow landslides are examined three designed scenarios and a real case scenario are used to conduct this examination the prediction of shallow landslide occurrence is related to the main drying and wetting curves due to the strong relation between groundwater pressure head and hysteresis effect the main wetting curve may have a less minimum landslide triggering rainfall amount and a less rainfall duration threshold for landslide occurrence than the drying wetting curve for safety s sake an underestimation of shallow landslide occurrence may be produced by the commonly used main drying curve in addition besides the shallow landslide occurrence the failure depth and the time to failure are also influenced by the main drying and wetting curves the hysteresis effect should be taken into account for assessing rainfall induced shallow landslides

paleopedology and paleohydrology of a volcaniclastic paleosol interval implications for early pleistocene stratigraphy and paleoclimate record olduvai gorge tanzania abstract a cumulative red paleosol interval developed on volcaniclastic parent material under semiarid conditions in olduvai gorge tanzania it contains a complex history of pedogenesis that was affected by 1 episodic pyroclastic and debris fan processes and 2 episodic expansions and contractions of an adjacent alkaline lake and the associated fluctuations in the water table the paleosol interval is 130 320 cm thick represents 25 ka of the 50 kyr duration of lowermost bed ii and is defined by early pleistocene 1 75 ma tuffs if and iia the paleosol interval records a paleocatena related to both landscape and drainage the slope position on a pyroclastic fan relative to an alkaline lake the proximity to freshwater wetlands and the position of water table biogenic paleosol structures include grass and sedge root traces zeolite rhizocretions and soil fauna termite and ant traces abundant pedogenic features sensitive to soil moisture conditions including redoximorphic mottles in the paleosol matrix fe oxide glaebules grain and pore coatings illuviated clay grain and pore coatings and vadose siliciclastic and zeolite crystal silt record episodic water table fluctuations the geochemistry of whole rock samples distinguishes two parent materials early low ti zr weathered volcaniclastic sediment and late high ti zr tuffaceous sediment which represent two distinct pedogenic phases the lower paleosol developed at both sites whereas the upper paleosol developed only at the upslope site mass balance calculations indicate greater weathering higher eh and ph and greater zeolite precipitation at the upslope site than at the downslope site these relationships are compatible with the upslope site having had a lower overall water table and better drained conditions than the downslope site which had a higher water table and poorly drained conditions the lower paleosol provides evidence of a fluctuating water table consistent with a wetter climate followed by a prolonged arid period the upper paleosol began to form after a return to wetter conditions and ended under arid conditions the position of the olduvai subchron c2n 1 942 1 785 ma in bed i directly beneath the paleosol interval is used to make a tentative correlation at 1 75 ma with global climate dust records wet dry cycles

stability analyses of rainfall induced landslides the slope stability issues concerning rainfall induced slope failures are investigated and presented specifically the effect of both negative and positive pore water pressures on the stability of initially unsaturated slopes are carefully explained and coupled with infinite slope analysis methods in order to present a predictive formulation of slope failures that occur as a result of rainfall events the formulation serves as a baseline analysis method for evaluating potentially unstable soil slopes that are subject to surface infiltration and explains the various triggering mechanisms that may occur based on individual combinations of the slope geometry soil strength and infiltration parameters a procedural method is outlined for utilizing the analytical formulation to predict the change in the factor of safety for a slope subject to infiltration and a detailed analysis of a case study is presented to verify the method quantitative statements are made concerning the time and depth of failure in relationship to the soil slope and rainfall parameters

random finite element method for spudcan foundations in spatially variable soils abstract spudcans are large diameter 20 m conical foundations holding up mobile jack up platforms for offshore oil and gas developments in shallow to moderate water depths during self installation the spudcan often penetrates several tens of meters into seabed soils where large variations often occur in the shear strength of the soils although the failure mechanisms and bearing capacity of spudcan foundations have been studied in homogeneous soil and layered uniform soil the effect of the spatial variability in soil strength on the spudcan response is still unclear this study presents a random finite element method rfem for assessing the failure mechanisms and the bearing capacity of a spudcan foundation embedded in spatially varied seabed soils a comprehensive review of seabed soils discusses typical ranges of the mean the coefficient of variation and the scale of fluctuation of the undrained shear strength random fields are generated and mapped into a non linear finite element analysis to reveal the failure mechanisms of the spudcan in spatially varied soils the influence of the scale of fluctuation in different directions is investigated the results indicate that ignoring the spatial variability of soil strength leads to an overestimation of the bearing capacity it is unconservative for the foundation design to assume an isotropic random field model in the rfem analysis

an isotopic study of paleosol carbonates from olduvai gorge the isotopic composition of paleosol carbonates from olduvai gorge can be used to give estimates of the paleoclimatic history of east africa during the plio pleistocene oxygen isotopes are indicative of major changes in the isotopic composition of meteoric water while carbon isotopes are indicative of major changes in the fraction of biomass using the hatch slack c4 photosynthetic pathway three major periods of climatic change in this terrestrial setting occurred at about 1 67 1 3 and 0 6 myr these involved major changes in flora as well as major changes in the isotopic composition of meteoric water overall the climate has become warmer and more arid over the past 2 2 myr

geochemical fingerprints for olduvai gorge bed ii tuffs and implications for the oldowan acheulean transition bed ii is a critical part of early pleistocene olduvai gorge tanzania its deposits include transitions from humid to more arid conditions with associated faunal changes from homo habilis to erectus and from oldowan to acheulean technology bed ii 1 8 1 2 ma is stratigraphically and environmentally complex with facies changes faulting and unconformities making site to site correlation over the 20 km of exposure difficult bed ii tuffs are thinner less evenly preserved and more reworked than those of bed i five marker tuffs tuffs iia iid bird print tuff bpt plus local tephra were collected from multiple sites and characterized using stratigraphic position mineral assemblage and electron probe microanalysis of phenocryst feldspar hornblende augite titanomagnetite and glass where available composition lowermost bed ii tuffs are dominantly nephelinitic middle bed ii tuffs bpt tuff iic have basaltic components and upper bed ii tuff iid is trachytic the bpt and tuff iid are identified widely using phenocryst compositions high ca plagioclase and high ti hornblende respectively though iid was originally hay 1976 misidentified as tuff iic at loc 91 shk annexe in the side gorge this work helps establish a high resolution basin wide paleolandscape context for the oldowan acheulean transition and helps link hominin faunal and archaeological records

palaeovegetation changes recorded in palaeolake olduvai ogcp core 2a 2 09 2 12 ma naibor soit formation olduvai gorge tanzania abstract for five decades olduvai gorge has been a key site to reconstruct and understand the relationship between environmental and landscape conditions and use of affordances by early african hominin populations following the first olduvai gorge coring project ogcp during 2014 a multiproxy microbiological analysis which includes phytoliths pollen diatoms sponge spicules and chrysophyte cysts was undertaken on samples collected from various borehole cores the aim of the study is to better understand palaeoenvironmental and palaeovegetation conditions and changes through time and their relationship to hominin presence and evolution this study details the first palaeobotanical and palaeoenvironmental study of borehole 2a at olduvai gorge it represents the as yet oldest known sedimentary sequence in the olduvai basin for a portion of the pre bed i naibor soit formation this is a unit that is not accessible in any natural exposures in olduvai gorge and has only recently been encountered by drilling here we present the results from a particularly phytolith prone portion between 2 09 ma and 2 12 ma phytolith results indicate a savannah environment dominated by grasses where poaceae were a key component and where the c3 pooideae grasses were mostly dominant alternating with c4 grasses oscillations between grass subfamilies c3 pooideae c4 chloridoideae and c4 panicoideae to lesser degrees indicate five substantial climatic shifts varying between more humid and arid conditions associated with phytoliths freshwater indicators such as diatoms sponge spicules and chrysophyte cysts were also identified suggesting the presence of wetlands in the lake catchment area pollen is extremely rare in the sediments but when present comprises fungal spores and poaceae pollen thus supporting the wetland and grassland reconstructions respectively these results offer for the first time a whole picture of the palaeovegetation and associated palaeoenvironments for this pre bed i period together with previous results from other areas and chronological periods they improve our understanding of the evolution and adaptation of early hominins and their close relationship to the surrounding landscape

tuff fingerprinting and correlations between ogcp cores and outcrops for pre bed i and beds i ii at olduvai gorge tanzania abstract sediment cores retrieved from the pleistocene olduvai basin by the olduvai gorge coring project ogcp provide a high resolution record of tuffs and other volcaniclastic deposits together with a lacustrine sedimentary record full of paleoenvironmental indicators correlating tuffs between the cores and outcrops at olduvai where these tuffs are identified at paleoanthropologically important sites is critical for applying the new paleoenvironmental data to the conditions under which hominins lived tuffs and other volcaniclastic deposits from three cores were analyzed for mineral assemblages and glass and mineral major element compositions feldspar augite hornblende titanomagnetite and glass where possible to compare to published geochemical fingerprint data based on marker tuffs from outcrop equivalents at olduvai gorge in combination with stratigraphic position these mineralogical and geochemical data were used to correlate between the cores and outcrops providing direct temporal tie lines between the cores and sites of paleoanthropological interest direct correlations are most certain for olduvai bed i where all major tuff markers from outcrop are identified for one or more of the three core sites and for the upper part of the underlying ngorongoro formation which includes the coarse feldspar crystal tuff cfct and naabi ignimbrites exposed in the oldest pleistocene exposures of the western gorge also characterized were the mineral and glass compositions of tuffs and ignimbrites pre dating the oldest exposed outcrop units extending our record of explosive events from the ngorongoro volcano while no specific correlations can be confirmed between individual bed ii tuffs in the cores and in outcrops correlations are possible between the cores themselves using newly identified tuff compositions and some potential correlations non unique based on individual mineral phases between core and outcrop can be used in conjunction with other stratigraphic tools to help constrain the intervals in question

probabilistic stability analysis of rainfall induced landslides considering spatial variability of permeability abstract many regions around the world are vulnerable to rainfall induced landslides a variety of methods have been proposed for revealing the mechanism of landslide initiation current analysis methods however do not consider the effects of non homogeneous soil profiles and variable hydraulic responses on rainfall induced slope failures in korea where the depth of weathering is very shallow many slope failures occur in the layer of weathered residual soil that overlays the bedrock these failures are characterized by shallow failure surfaces located near the interface between the weathered soil and the underlying bedrock in this study probabilistic stability analyses were conducted for a weathered residual soil slope with shallow impermeable bedrock to study the failure mechanism of rainfall related landslides a series of seepage and stability analyses of an infinite slope based on one dimensional random fields were performed to study the effects of uncertainty due to the spatial heterogeneity of hydraulic conductivity on the failure of unsaturated slopes due to rainfall infiltration the results showed that a probabilistic framework can be used to efficiently consider various failure patterns caused by spatial variability of hydraulic conductivity in rainfall infiltration assessment for a shallow infinite slope

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

subaqueous intermediate to silicic composition explosive volcanism a review abstract mass flow deposits of pyroclastic debris commonly consist of two parts a lower coarse grained basal section and an upper finer grained bedded section subaqueous pyroclastic flow deposits particularly those derived from a vertically directed eruption appear better sorted than their subaerial counterparts because of the efficient sorting characteristics of water compared to air subaqueous deposits frequently are depleted in bubble wall pumice that floats off and vitric ash that remains in suspension hence the deposits show a relative enrichment of crystals especially at the bases from 1 flotation and suspension of the vitric material and 2 ingestion of water at the head of the flow a subaqueous volcaniclastic flow deposit may contain evidence indicative of both turbulent and non turbulent movement the flow may have had a turbulent head and non turbulent body or flow transformations may have occurred from 1 collapse of the eruption column 2 changes in slope and in the velocity of the flow and or 3 the amount of water mixed into the flow retention of heat in the subaqueous environment may be enhanced by lateral eruptions high eruption rates large eruptive volumes and little or no mixing of pyroclastic debris with water the lack of welded subaqueous deposits in the geologic record may be a result of 1 effusive rather than explosive eruptions in a deep water environment 2 explosive phreatic disintegration of pyroclastic flows in a shallow subaqueous environment and or 3 the inability of subaerial pumiceous pyroclastic flows to penetrate the land water interface characteristics of explosive intermediate to silicic composition subaqueous volcanic eruptions also have been summarized in terms of 1 environment of the eruption 2 orientation of the eruption and 3 composition of the juvenile debris these three aspects of explosive subaqueous volcanism are ideal end members and gradations no doubt occur

shear strength characteristics of a residual soil shallow landslides in natural residual soils slopes are common all over the world the slip surfaces associated with these landslides are often situated above the groundwater table therefore it i

a gis physically based emergency methodology for predicting rainfall induced shallow landslide zonation abstract this study proposes an original approach for predicting shallow translational slide hazards which are triggered by irregular rainfall events via the method of zonation at a regional scale in emergency situations the proposed model incorporates the infinite slope method in a gis platform to assess hazard and risk associated with slope instability this application can be integrated into the emergency management software simge 1 0 sge 2 0 the objective of the work is to provide a quick tool for managing emergencies and predicting shallow rainfall induced landslides rils the originality of this approach includes the relationship between two variables m z with m being a hydrological pattern of steady state simulating the effects of rain that cause translational slides and z being the characteristic sliding depth based on the bounded lithological unit this method provides hazard maps including zonation based on a factor of safety towards shallow landslides where stable and unstable conditions depending on precipitation are determined for managing emergencies this approach was applied in the guadalmansa river basin in benahavis malaga andalusia where numerous shallow landslides occurred in 2010 caused by extreme rainfall events and a lithological unit has been isolated for this research alpujarride schist the results obtained through this method have been verified after analysing the studied area s landslide inventory

biogeochemical evidence for environmental changes of pleistocene lake olduvai during the transitional sequence of ogcp core 2a that encompasses tuff ib 1 848 ma abstract olduvai gorge is renowned for discoveries of hominin fossils and tools in a well resolved sedimentary context representing one of the foremost sites in east africa that has afforded critical evidence of hominin evolution in 2014 the olduvai gorge coring project ogcp recovered the first deep sediment cores from this location these cores provide the first opportunity to examine an extensive stratigraphic record from one location and independent of the weathering and related degradation typical of outcrop samples samples from these cores have been correlated to bed i tuffs and basalt marker beds that are well characterized from outcrops within the gorge this study focuses on the biogeochemical investigation of a 10 m segment from ogcp sediment core 2a where age constraints indicate deposition occurred over 50 000 years from 1 8055 0 003 ma to 1 8551 0 013 ma the segment includes tuff ib and if which were deposited at a time that prior research indicates represents marked changes in the paleoenvironment at olduvai with indications that paleolake olduvai may have been completely desiccated by the time tuff if was deposited this environmental change is recorded by the composition of sedimentary organic matter om in terms of bulk organic geochemical properties observed as a decrease in toc and 13corg in addition temporal variations in source specific biomarkers n alkanes phytane steradienes a norsteranes and alkenones reflect a shift from a deeper lake setting with a prevalence of aquatic sourced om combined with input from terrestrial plant waxes to a more shallow perennial lake where evidence of burrows and erosional surfaces is associated with microbially degraded om and intermittent aquatic biomarkers thus variations in biogeochemical proxies complement the sedimentological evidence confirming increased periods of prolonged desiccation of paleolake olduvai beginning around the time of tuff ib deposition

combined roles of saturated permeability and rainfall characteristics on surficial failure of homogeneous soil slope abstract both rainfall characteristics rainfall intensity and duration and saturated permeability of soil may influence the type and mechanism of surficial slope failures in general the failures can be initiated by two mechanisms i e loss of matric suction through propagation of wetting front and rise of water table up to date there are still no clear indicators to identify the dominant parameters that control the type of failure for these shallow landslides this paper investigates the hydraulic responses of soils to the variations of rainfall characteristics and soil permeability through numerical analyses the results showed that the hydraulic responses to rainfall for a homogeneous infinite slope underlain by an impermeable layer can be divided into two stages 1 the propagation of wetting front and 2 the rise of water table based on these hydraulic responses the type and mechanism of failures were deduced from the analytical analyses both the rainfall characteristics and saturated permeability were found to be predominant in controlling the hydraulic responses of soil and hence the occurrence time depth of failure plane and type of surficial slope failures

hominin use of springs and wetlands paleoclimate and archaeological records from olduvai gorge 1 79 1 74 ma abstract an interdisciplinary study of the geologic and archaeological records of a 50 ky year period at the plio pleistocene boundary in olduvai basin tanzania revealed that a small groundwater fed wetland was a consistent focus of hominin activity during periods of climate change wet dry cycles paleoenvironmental analysis of sedimentary records from 33 excavations in lowermost bed ii lmb ii olduvai gorge 3 s revealed evidence of significant climate fluctuations that produced two and one half lake cycles between 1 79 and 1 74 ma the region experienced wet dry cycles in response to combined astronomical forcing effects of eccentricity 100 ky cycle and precession of the equinox 19 and 23 ky cycles rhythmic variability of the monsoon system produced wet periods followed by a transition into dry periods followed by a transition back into wet climate a high resolution study of the stratigraphy documented a groundwater fed wetland complex 1 km2 that flourished during low lake levels dry periods and was drowned during high lake levels wet periods stratigraphic analysis temporal changes of 916 artifacts from 13 excavations selected from a landscape transect across alluvial fan spring wetland and lake margin environments showed no notable differences in artifact type frequency and raw material type when comparing wet periods as a group and dry periods as a group however a lateral shift of hominin activity on the landscape did occur between wet and dry periods the oldowan tool kit did not vary at spring sites between wet and dry periods whereas the tool kit did vary at other lake margin sites between wet and dry periods both analyses suggest that climate affected the location of an activity not type of activity these data suggest that oldowan hominins adapted their land use patterns to exploit spring wetland resources available during dry periods a switch from wet to dry conditions likely altered surface and groundwater systems affecting food and potable water availability forcing hominins to adjust their subsistence patterns

volcanogenic mass flow deposits and seafloor diapirism following the largest insular quaternary eruption of the eastern mediterranean at nisyros island aegean volcanic arc abstract nisyros volcanic island at the eastern end of the aegean volcanic arc was mainly constructed after the ultra plinian kos plateau tuff kpt eruption at 161 ka the surrounding submarine area consists mostly of marginal mass transport deposits that commonly characterize insular stratovolcanoes this study aims at providing insights into the deposition and volume of such deposits by elucidating the products of a mega eruption and subsequent paroxysmal volcanicity typical of subduction zone island arc systems seismic reflection and multibeam bathymetry around nisyros identify pyroclastic and debris flow deposits covering areas several orders of magnitude greater than the island the entire seabed between the basins of east kos and tilos is underlain by a 7 km3 volcaniclastic unit that defines a ubiquitous basal unconformity at an average depth of 30 m below the seafloor southwest of nisyros two groups of avalanche deposits from the same unit are bathymetrically identified on the slopes of the northernmost karpathos basin apron seismic stratigraphic markers that trace the unit on the shelf of tilos suggest it is concomitant with the kpt eruption this formerly unknown submarine expansion of the kpt deposits into three neighboring basins when added to the volume of this massive unit to the rest of the nisyros surroundings gives it an explosivity index vei 6 as such it represents the largest quaternary subaqueous volcanogenic mass flow deposit of the eastern mediterranean from this dataset it can be inferred that the submarine advance of the erupted material occurred mainly through erosive gravity flows envisioned as the main dispersion mechanism of large scale subaqueous pyroclastic deposits of plinian eruptions east of nisyros unique small scale diapiric domes and seafloor mounds are attributed to loading by the kpt in combination with regional faulting subsequent nisyros effusive activity produced pyroclastic units around the northeast as well as debris avalanche and lava flows south of the island these have been correlated to known onshore formations the present study documents the intensity and impacts of a volcanic island mega eruption and paroxysmal epilogue over a cycle of 100 kyr

palaeosalinity and palaeoclimatic geochemical proxies elements ti mg al vary with milankovitch cyclicity 1 3 to 2 0 ma ogcp cores palaeolake olduvai tanzania abstract the analysis of geochemical palaeoclimate and palaeosalinity proxy elements ti mg and al derived from x ray fluorescence xrf scans of olduvai beds i and ii from olduvai gorge coring project ogcp borehole cores 2a and 3a provides a record of cyclic variation between 1 3 ma and 2 0 ma the boreholes were drilled into the depocentre of the olduvai basin between fifth and flk faults where palaeolake olduvai was most persistent and deepest during most of bed i the lake was particularly deep and probably meromictic preserving high toc contents and commonly preserving fine lamination due to lack of bioturbation accretion rates were also high during bed i when rates of basinal subsidence were maximal due to crustal stretching associated with basaltic volcanism towards the end of bimodal ngorongoro volcanism basaltic magma effusive activity is manifested as tuffs scoriaceous layers and the bed i basalt complex lava flows a magnesium anomaly is recorded in the claystone geochemistry at this time and deposition of dolomite and limestone beds are restricted to this syn volcanic phase of basin history during bed i deposition accretion rates 0 23 mm yr were high enough to permit recognition of cycles with an average periodicity of 22 3 kyr corresponding to the earth s precession only the high values during the mg anomaly are adequate for the application of the palaeosalinity proxy element ratio mg al but ti counts provide a cyclic record in both bed i and bed ii during bed ii deposition the accretion rate was much slower 0 058 mm yr and cyclicity averaged 40 4 kyr corresponding to earth s orbital obliquity the bed ii interval corresponds to mis stages 40 to 64 coincidence of precessional and obliquity minima at 1 8 ma explains the superdrought that affected the basin at the time of emplacement of tuff if when the lake was dried out the aridity of the sequence containing tuff ia is also associated with a precessional minimum the cyclic record suggests that three bed i basalt flows were extruded at 1 94 ma during a time span lasting between 6 kyr and 15 kyr

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

chronostratigraphy and age modeling of pleistocene drill cores from the olduvai basin tanzania olduvai gorge coring project abstract the olduvai gorge coring project drilled a total of 611 72 m of core 575 48 m recovered of mostly fluvio lacustrine and fan delta volcaniclastic pleistocene strata at three sites in the olduvai basin tanzania in 2014 we have developed a chronostratigraphic framework for three of the cores based on 40ar 39ar dating of core and outcrop volcanic and volcaniclastic units core paleomagnetic stratigraphy and tephrochemical correlation between cores and from core to outcrop this framework is then used to constrain bayesian stratigraphic age models which permit age estimates for desired core levels with realistic confidence intervals the age models reveal that the deepest core level reached at 245 mbs is 2 24 ma 210 kyr older than the oldest strata exposed at olduvai gorge strata net accretion rates in this early phase of basin history were relatively rapid 57 69 cm kyr but decreased within 250 kyr to 15 cm kyr in lower bed i rates rebounded partially in upper bed i but subsequently declined to

deformation structures in shale bed indicate flow direction of overlying miocene subaqueous pyroclastic flow thrusts pinch and swell structures and undulations are present within a 2 m thick layered shale bed of miocene age that is overlain by a rhyolite subaqueous pyroclastic flow deposit 2 3 m thick the deformation structures were caused by loading and lateral compression by the subaqueous pyroclastic flow probably analogous to those observed in layered muds deformed by a sand mass advancing across them prominent thrusts strike east west and dip south and the crests of undulations strike east west indicating that the subaqeuous pyroclastic flow moved northward

volcaniclastic sedimentation in a shallow water marginal basin the early miocene koura formation sw japan abstract the early miocene koura formation sw japan is composed mainly of reworked volcaniclastics and andesite to rhyolite pyroclastics and constitutes part of the volcanic zone of sw japan of that time reworked volcaniclastic sequences are thick and mostly coarse grained and based on conventional facies concepts are interpreted as shoreface to beach deposits turbidites and debris flow deposits thick acid tuffs intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash flow emplacement or in some cases both andesite pyroclastic facies include surge and fall deposits facies relationships together with the fossil record indicate deposition in a shallow lacustrine environment the deposition rate of the koura formation was probably high enough to rapidly fill the shallow lake but sedimentation was balanced by rapid subsidence of the lake similar to lakes present within the modern volcanic zones of taupo new zealand and hohi japan the koura formation grades upward into marine deposits and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back arc or intra arc rifting

fiamme formed by diagenesis and burial compaction in soils and subaqeuous sediments pumice clasts can be diagenetically altered at low temperature and flattened by compaction to form fiamme examples are described from trachytic pumice fall layers alternating with palaeosoils on sao miguel in the azores and from ordovician lacustrine volcaniclastic sedimentary rocks from the english lake district

paleoenvironmental interpretation of lake margin deposits using δ13c and δ18o results from early pleistocene carbonate rhizoliths olduvai gorge tanzania isotope analyses of lake margin rhizoliths from paleo lake olduvai ca 1 75 ma form the basis of a new model proposed here for interpreting stable isotope values of phreatic rhizolith carbonates average 18o and 13c values from rhizoliths formed in transgressive lacustrine waxy claystones are relatively low low 18o averages 5 5 reflect meteoric water values consistent with increased precipitation during wet periods that would increase the outflow of fresh water from subsurface seeps and shift a brackish groundwater zone lakeward low 13c averages 4 1 could indicate little atmospheric exchange high plant decay and or increased groundwater low 13c flow higher averages in 18o 3 6 and 13c 2 0 occur during dry periods and lake regressions earthy claystones when hydraulic head is reduced the lake recedes and water within the wetlands is subjected to intense evaporation and gas exchange with the atmosphere the isotope ratios of the rhizoliths from lowermost bed ii change in response to groundwater hydrology on milankovitch time scales but the isotopes also provide evidence of shorter term decadal to centennial scale climate fluctuations the orbitally driven climate changes are recorded faithfully by lithologic variations and stable isotope patterns

whole rock geochemistry tracks precessional control of pleistocene lake salinity at olduvai gorge tanzania a record of authigenic clays the paleoclimatic framework of human evolution is central to the study of human origins olduvai gorge tanzania has a rich paleontological record including four hominin species we analyzed 177 mudstone samples from a lacustrine section supported by a new high resolution geochronology to track changes in paleolake water chemistry six peaks in al 2 o 3 mgo indicate freshening events occurring over a generally saline and alkaline environmental background five of the six events occurred at peak climatic precession correlating with december insolation at 20 s the southern monsoon therefore may have been more effective at delivering moisture to east africa than today peak amplitude of geochemical variation preceded the ca 1 84 ma eccentricity maximum by 20 k y suggesting that eccentricity modulation was unexpectedly weak in this interval or that other factors affected the water balance or geochemical record this study demonstrates that authigenic clay geochemistry determined through whole rock analyses can preserve a record of astronomically forced environmental change and documents paleohydrologic changes in tropical africa at a time when zonal walker circulation increasingly affected global climate new stone technologies emerged and the genus homo spread beyond africa

paleogeographic and paleotectonic evolution of a basement controlled archean supracrustal sequence chibougamau caopatina quebec the 7 14 km thick archean volcano sedimentary sequence in the chibougamau caopatina region ccr composed of two mafic to felsic cycles and two thick essentially epiclastic successions is divided into a southern caopatina segment scs and northern chibougamau segment ncs the sedimentary units which evolved after cycle 1 volcanic rocks in the scs and after the combined cycles 1 and 2 in the ncs document a diachronous sedimentary basin evolution this criterion has been employed to distinguish an early evolutionary stage in the south and a late evolutionary stage in the north in each segment the basin episode represents a unique extensional event volcanism preceded the major sedimentary rifting event in both segments composition of sediments and sedimentary facies correlated with the interpreted basin margin faults support the interpretation of two phases of crustal extension volcanic construction and sill intrusion were the prevalent factors governing emergence of volcanic islands in the cc

late cambrian middle furongian shallow marine dysoxic mudstone with calcrete and brachiopod olenid lotagnostus faunas in avalonian cape breton island nova scotia the common belief that organic rich mudstones formed in quiescent distal settings is further weakened by study of an upper cambrian leptoplastus lower peltura superzones succession in the chesley drive group in avalonian cape breton island that is comparable to alum shale successions in baltica dramatic sea level likely eustatic changes are now recognized by punctuation of deposition of shallow wave influenced black mudstone with brachiopod orusia lenticularis and olenid trilobite bearing limestones by offlap and formation of a subaerially cemented calcrete clast conglomerate subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with leptoplastus superzone l ovatus zone trilobite sclerites dynamic processes are shown by wave ripples in the mudstone and limestone sorting and winnowing of fossil rudstones and pre compactional fracture of the conglomerate and rudstones orusia rudstones in the succession below the conglomerate are regarded as analogues of eoorthis and billingsella rudstones in the biomere extinction intervals of the laurentian basal sunwaptan the lowest orusia rich beds are no older than the p spinulosa zone but as elsewhere in avalonia they range into the higher leptoplastus cape breton and even the peltura britain new brunswick superzones rare agnostoid sclerites in lower peltura superzone ctenopyge tumida zone olenid rudstone resemble those traditionally assigned to lotagnostus trisectus in avalonian britain and sweden and are distinct from laurentian l americanus an l americanus zone cannot be identified in avalonia or baltica and the first appearance datum fad of purported l americanus is not suitable as a standard for the base of the highest cambrian stage

changing depocentre environments of palaeolake olduvai and carbonates as marker horizons for hiatuses and lake level extremes abstract primary carbonate and marl layers and limestone nodular horizons were intersected in ogcp boreholes 1a 2a 3a 3b drilled into the depocentre of palaeolake olduvai the various carbonate types were analysed employing petrographic including cathodo luminescence stable isotope and sequence stratigraphic techniques and recorded important information concerning lake evolution primary carbonate and marl layers are preserved at the top of lake deepening cycles lake parasequences marking maximum flooding followed by lake withdrawal and then fluvial erosion leading to the next depositional cycle presence of pelagites and a meromict lake favouring anoxic conditions show it to be not a playa lake but a deeper fault bound lake akin to present day lake eyasi carbonate and marl layers were exclusively deposited when claystone facies show a geochemical magnesium anomaly associated with a phase of basaltic volcanism affecting the basin marked by mafic tuffs and basaltic lava flows calcium was partitioned into evaporites such as gypsum anhydrite together with the nodular horizons and carbonate soil profiles in the sequence the resulting high mg2 ca2 ratio in the lake waters promoted dolomite deposition and replacement and the formation of aragonite the nodular horizons yield rainfall isotope values and mark times when the lake was empty and the lake bed fell under meteoric conditions precipitating limestone nodules just below the sediment surface under pedogenic vadose groundwater interface and groundwater phreatic conditions the nodular limestone horizons sit below erosional hiatal surfaces and show vadose micritic and fibrous types to phreatic sparry and other accretionary type textures enterolithic to chickenwire textured nodules indicate the pseudomorphing of anhydrite nodules that form beneath salt marshes thus the two carbonate types primary layers and nodular horizons provided depth gauges respectively for the extremes of lake expansion and emptying drying out in the case of nodular horizons the maturity of the carbonate profile gives an indication of the magnitude of the hiatal time gap represented 1 kyr to 2 kyr for a single horizon to 3 kyr to 9 kyr for more mature soil profiles emptying drying out episodes of palaeolake olduvai were not uncommon and often short lived a spectrum of hiatal disconformities in the olduvai basin vary from multimillennial hiatuses to mega disconformities of 40 kyr and 75 kyr

use of single grain geochemistry of cryptic tuffs and volcaniclastic sandstones improves the tephrostratigraphic framework of olduvai gorge tanzania single grain geochemical composition of volcaniclastic sandstones can be a potential tool to improve correlations of mixed pyroclastic epiclastic deposits to test this trachytic tuffs of the paleoanthropologically important flk flk n and flk nn sites of pleistocene olduvai gorge bed i tanzania are used as an established tephrostratigraphic framework against which to test volcaniclastic sandstone correlations fluvio lacustrine sandstones and tuff samples were collected from eight archeological trenches between tuffs ib and id across a 500 m transect including leakey s famous zinjanthropus flk and oh 7 oh 8 flk nn sites a previously unknown thin fine mineralogically unique black trachyandesitic fallout ash was discovered below tuff ic compositions of individual augite feldspar and titanomagnetite grains from sandstones between tuffs ib and ic reveal some ib equivalent material and a new compositional assemblage distinct from the sandwiching marker tuffs mineral compositions of the tripartite volcaniclastic sandstone between tuffs ic and id are similar to id volcaniclastic sandstone grain fingerprints further refine correlations between fluvio lacustrine sections within the area providing support for proposed high resolution stratigraphic reconstruction of the zinjanthropus and oh 7 oh 8 land surfaces this method might be applied to other sections where pyroclastic particles are admixed but distinct tuffs are not preserved

volcaniclastic lithostratigraphy and paleoenvironment of the lower eocene mcabee fossil beds kamloops group british columbia canada the lower eocene mcabee fossil beds 53 ma in south central british columbia canada represent a lacustrine sequence deposited during a time of pervasive regional volcanism previous studies on

an aem tem study of weathering and diagenesis abert lake oregon ii diagenetic modification of the sedimentary assemblage abstract this paper compares the mineralogy and chemistry of clay minerals in sediments from various depths and positions in abert lake and surrounding playa with those of the weathered materials entering the lake in order to reveal the nature and extent of post depositional mineralogical modification analytical electron microscope aem data from individual clay particles reveal that each sample is comprised of a highly inhomogeneous smectite assemblage the thin clay flakes commonly less than 10 nm wide display a complete range in octahedral sheet compositions from nearly dioctahedral to nearly trioctahedral the very abundant mg rich lake smectites with an estimated composition k 0 29 al 0 23 mg 2 16 fe 0 30 si 3 80 al 0 20 o 10 oh 2 are not formed by weathering this confirms the importance of diagenetic mg uptake lattice fringe imaging failed to reveal distinct brucite like or vermiculite like layers suggesting that interstratifications of this type are rare or absent siliceous coatings on clay particles identified by silica excess in smectite analyses seem to favor topotactic overgrowth of stevensite rather than addition of brucite like layers to the dioctahedral nuclei the growth of k stevensite dilutes the al content of the crystal and thus the increasing diagenetic modification reduces rather than supplements its illite component smectite compositions within individual samples were highly variable yet source related characteristics such as the abundance of fe rich smectite were apparent little evidence for systematic k or mg enrichment with depth was identified in samples from depths of down to 16 feet below the sediment water interface the most magnesian assemblages are associated both with weathering sources of mg rich smectite and playa environments subjected to repeated wetting and drying cycles thus the observations suggest that clay compositions primarily reflect changes in lake levels brine composition and source characteristics rather than time and depth compaction effects other diagenetic reactions in the sediment include recrystallization of na rich silica gel and diatom fragments abundant submicron sized untwinned euhedral crystals of k feldspar are interpreted to be authigenic in origin

the dali ash island of rhodes greece a problem in interpreting submarine volcanigenic sediments in the southern part of rhodes greece rhyolitic subaqueous pyroclastic deposits are interbedded with tertiary deep water marine sediments the lowermost and best exposed of these deposits the dali ash is described here the deposit has been previously described as a deep water welded subaqueous ignimbrite this paper shows that there is no evidence of welding and texture previously reported were misidentified the dali ash consists of a lower massive unit 5 m thick overlain by a sequence of ash turbidites 2 5 m thick the lower unit was deposited by a high concentration turbidity current and the ash turbidites by dilute turbidity currents foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water particulate systems a palaeomagnetic study also suggests they were deposited cold