elastic wave velocity evolution of shales deformed under uppermost crustal conditions conventional triaxial tests were performed on a series of samples of tournemire shale along different orientations relative to bedding 0 90 experiments were carried out up to failure at increasing confining pressures ranging from 2 5 to 80 mpa and at strain rates ranging between 3 10 7s 1 and 3 10 5s 1 during each experiment p and s wave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress this extensive velocity measurement set up allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation the evolution of elastic anisotropy was quantified using thomsen s parameters wich were directly inverted from measurement of elastic wave velocity brittle failure was preceded by a change in p wave anisotropy due to both crack growth and mineral re orientation anisotropy variations were largest for samples deformed perpendicular to bedding at the onset of rupture anisotropy reversal was observed at the highest confining pressures for samples deformed parallel to bedding the p wave anisotropy change is weaker

behavior of transversely isotropic shale observed in triaxial tests and brazilian disc tests abstract shale rock is widely distributed in nature and must be taken into account in some rock engineering applications in this research three kinds of experiments namely the conventional triaxial compression test reducing confining pressure test and brazilian test were conducted on a type of shale rock specimen with a horizontal bedding plane and a vertical bedding plane collected from lushan city jiangxi province china to further investigate the bedding effect on the mechanical properties of shale rock an acoustic emission ae monitoring system was used to study the fracture process under the conventional triaxial compression test whereas a digital image correlation dic system was employed to monitor the deformation of disk faces during the failure process under the brazilian test another key issue addressed in this research is the failure process and mechanism of shale specimens with horizontal and vertical bedding planes under the reducing confining pressure test the conventional triaxial compression test shows that the bedding plane could easily affect the elasticity modulus and failure pattern and the failure of the bedding plane can lead to stronger energy release and a higher ae count level the reducing confining pressure test shows that the dissipated energy magnitude of unloading path ii reducing confining pressure and increasing axial stress is much larger than that of unloading path i reducing confining pressure and keeping axial strain constant and that continuous energy input and larger energy dissipation led the specimens under unloading path ii to fail faster than those under unloading path i the brazilian test further revealed that the bedding plane has a significant effect on the tensile strength and failure pattern of shale rock

middle eocene to earliest oligocene development in the eastern north sea basin biostratigraphy magnetostratigraphy and palaeoenvironment of the kysing 4 borehole denmark abstract the cored 154 5 m deep kysing 4 borehole in central jutland is unique for high northern latitudes because it comprises an almost complete middle eocene to lowermost oligocene succession mostly in a highly calcareous facies with abundant well preserved microfossils and from an upper bathyal environment it has therefore been possible to carry out biostratigraphical analyses of calcareous nannofossils benthic and planktonic foraminifera as well as dinoflagellate cysts in order to correlate the section to the geomagnetic polarity time scale a detailed palaeomagnetic analysis has been performed the magnetobiostratigraphic calibration allowed a well constrained identification of nearly all magnetochrons between chron c21n and chron c13n the eocene deposits consist mostly of calcareous ooze sovind marl formation with three new members formally defined whereas the oligocene viborg formation consists of moderately calcareous mud overall the lithology of the deposits is in agreement with previous interpretations of a british provenance for the eocene deposits and a scandinavian provenance for the oligocene deposits cyclic shifts in the clay carbonate content during the middle eocene chrons c19 c17 are attributed to orbitally controlled climate shifts obliquity seems to have been the dominating factor but intervals dominated by precession end eccentricity signals also occur a 10 m thick hitherto unknown black mud moesgaard member of scandinavian provenance and deposited very rapidly during the late eocene chron c16n 1n is interpreted as a precursor event to the oligocene mode of sedimentation both the moesgaard member and the lower oligocene viborg formation are associated with sea level falls

new age determinations for the eocene oligocene boundary sediments in the fayum depression northern egypt the fayum depression of northern egypt contains the richest collection of fossil vertebrates known from the afro arabian paleogene this fauna has been collected for over a century and yet the sediments containing the fossil localities have remained undated paleomagnetic sampling was carried out through the fossil bearing jebel qatrani formation and the underlying dir abu lifa member of the qasr el sagha foundation to determine a correlation between the paleomagnetic reversal stratigraphy and the geomagnetic reversal time scale grts a new date of 23 64 pm 0 035 ma was determined for the overlying widan el faras basalt by the laser incremental heating 40 ar 39 ar dating method the results of this study indicate that the majority of the vertebrate fossil localities occur in chrons c13 c15r late eocene to early oligocene in age these age determinations provide a minimum date for the origin of higher primates

investigation of the damage induced by desiccation and heating of tournemire argillite using digital image correlation abstract a set of samples taken from the tournemire site was submitted to a desaturation process in an oven at different temperature levels of 40 c 50 c 80 c 105 c 150 c and 200 c the resulting hydro mechanical behavior was studied by measuring the strain fields obtained from a non invasive method called digital image correlation dic after a few dozens of minutes from the beginning of the thermal loading the induced strains concentrated into narrow zones or bands indicating strain localization these narrow zones were precursors of desiccation cracks which were also detected from the first temperature level at 40 c or 50 c our results confirmed the fundamental role of the textural and sedimentological heterogeneities in the cracking process desiccation cracks appeared at the interface between clay matrix and macroscopic non clayey heterogeneity i e calcite concretion and were predominantly parallel to the bedding planes at a temperature level of 105 c the desiccation cracks were no longer observable and seemed to close this healing process was interpreted as a thermally driven process when the shrinkage limit was reached a thermal dilation of the water clay minerals system occurred leading to a gradual closure of desiccation cracks

correlation of eocene oligocene marine and continental records orbital cyclicity magnetostratigraphy and sequence stratigraphy of the solent group isle of wight uk the magnetostratigraphy clay mineralogy cyclostratigraphy and sequence stratigraphy of the estuarine and continental solent group isle of wight hampshire basin uk which is of late eocene early oligocene age were investigated a new magnetostratigraphy for the solent group is correlated to the chronostratigraphic standard using limited biostratigraphical data and it is concluded that the base of the oligocene falls close to the base of the bembridge limestone formation a long time series of clay mineral xrd data was generated which shows striking variation in illitic clay abundance illite is interpreted to have formed in gley palaeosols through repeated wetting and drying in response to high seasonality high illitic clay values are tuned to c 400 ka eccentricity maxima to develop an age model in addition to a very strong c 400 ka signal in the data spectral analysis of the clay data also confirms the influence of short eccentricity c 100 ka and obliquity c 40 ka cycles the succession displays seven conspicuous 10 20 m thick sequences which represent transitions from transgressive estuarine environments through highstand floodplains to freshwater lakes the sequences correspond exactly to the long eccentricity c 400 ka cycles a sea level curve is derived using the amount of incision as a minimum measure of eustatic fall but there is no evidence of a major eustatic drop of 30 90 m corresponding to the early oligocene glaciation of antarctica it is likely that incision was suppressed by rapid rates of subsidence

geology biostratigraphy and carbon isotope chemostratigraphy of the palaeogene fossil bearing dakhla sections southwestern moroccan sahara new palaeogene vertebrate localities were recently reported in the southern dakhla area southwestern morocco the eocene sediment strata crops out on cliffs along the atlantic ocean coast vertebrate remains come from five conglomeratic sandstone beds and are principally represented by isolated teeth belonging to micromammals selachians and bony fishes a proboscidean assigned to numidotherium sp and many remains of archaeocete whales basilosauridae from fieldwork five lithostratigraphic sections were described essentially based on the lithological characteristic of sediments despite the lateral variations of facies correlations between these five sections were possible on the basis of fossil bearing beds a1 b1 b2 c1 and c2 and five lithological units were identified the lower part of the section consists of rhythmically bedded chert rich marine siltstones and marls with thin black phosphorite with organic matter at the base the overlying units include coarse grained to microconglomeratic sandstones interbedded with silts indicating deposition in a shallow marine environment with fluvial influence the natural remanence magnetization of a total of 50 samples was measured the intensity of most of the samples is too weak however before or after the first step of demagnetization the palaeomagnetic data from the samples are very unstable except for eight from three similar sandstone levels which show a normal polarity matched with biostratigraphic data on rodents primates the selachian sirenian and cetacean faunas the new carbon isotope chemostratigraphy on organics 1 refines the age of the uppermost c2 fossil bearing bed to earliest oligocene time and 2 confirms the priabonian age of the b1 to c1 levels

effects of relative humidity and mineral compositions on creep deformation and failure of a claystone under compression abstract during the last decades clayey rocks have been widely investigated in many countries as a potential geological barrier for underground disposal of radioactive waste clayey rocks are heterogeneous materials at a micrometer scale mechanical behaviors of clayey rocks are generally influenced by mineralogical compositions and or water content a large number of studies have been so far performed on short term behaviors of clayey rocks however very limited data are available on time dependent deformation and failure process this paper is devoted to investigating the effects of relative humidity and mineralogical compositions on creep deformation of a hard claystone and the possibility of creep failure is studied influences of confining pressure and structural anisotropy are also investigated for this purpose four groups of creep tests are carried out under triaxial compression conditions on claystone samples drilled from the underground research laboratory at bure in france in the first group creep tests are performed on samples with three different values of relative humidity rh 59 85 and 98 under a confining stress of 6 mpa in the second group claystone samples with different mineralogical compositions drilled from different geological unities are investigated in the third group for a selected value of relative humidity rh 59 triaxial creep tests are performed under different values of confining stress 2 6 and 12 mpa finally effects of structural anisotropy of claystone at hr 59 are investigated by considering samples respectively drilled in parallel and perpendicular directions to the bedding planes a series of new experimental data are obtained and analyzed it is found that at the tested level of relative humidity the confining stress seems to increase creep strain and rate of the claystone the clay minerals especially the smectite can play an essential role in time dependent deformation and increase creep strain of the claystone at rh 59 further creep strains are more important under higher values of relative humidity than lower ones in very few cases the failure due to acceleration creep was observed but for high deviatoric stress level the experimental data obtained in this work can be used as a background for numerical modeling of time dependent behavior of re desaturated clayey rocks in the context of geological disposal of radioactive waste

integrated biomagnetostratigraphy of the alano section ne italy a proposal for defining the middle late eocene boundary the alano section has been presented at the international subcommission on paleogene stratigraphy isps as a potential candidate for defi ning the global boundary stratotype section and point gssp of the late eocene priabonian stage the section is located in the venetian southern alps of the veneto region ne italy which is the type area of the priabonian being exposed along the banks of the calcino torrent near the village of alano di piave it consists of 120 130 m of bathyal gray marls interrupted in the lower part by an 8 m thick package of laminated dark to black marlstones intercalated in the section there are eight prominent marker beds six of which are crystal tuff layers whereas the other two are bioclastic rudites these distinctive layers are useful for regional correlation and for an easy recognition of the various intervals of the section the section is easily accessible crops out continuously is unaffected by any structural deformation is rich in calcareous plankton and contains an expanded record of the critical interval for defi ning the gssp of the priabonian in order to further check the stratigraphic completeness of the section and constrain in time the critical interval for defi ning the priabonian stage we performed a high resolution study of integrated calcareous plankton biostratigraphy and a detailed magnetostratigraphic analysis here we present the results of these studies to open a discussion on the criteria for driving the golden spike that should define the middle eocene late eocene boundary

in situ x ray micro ct for investigation of damage evolution in black shale under uniaxial compression crack damage evolution of shale is crucial to the hydraulic fracturing treatment and engineering stability although many effects have been done on the macroscopic characteristics of shale yet the microscopic failure mechanism is not well understood a uniaxial compressive test on black shale was conducted under topographic monitoring using in situ x ray micro tomography ct a series of high resolution reconstruction images were obtained by carrying out ct scans at six key points throughout the test to obtain the internal structure of shale sample in addition the ct values for the purpose of crack damage evolution in shale were identified clear 2d 3d ct images ct value analysis and image segmentation analysis reveal that the sample experiences compression damage cracking crack propagation and collapse stages crack geometry and distribution in the shale sample is visualized by rendered ct images and a combined tension and shear failure mode is observed from the fracture rose diagram this work suggests that formation and propagation of fractures are influenced by the stratified structure and weak cementation medium between layers

an enhanced understanding of the basinal bowland shale in lancashire uk through microtextural and mineralogical observations abstract variability in the lower bowland shale microstructure is investigated here for the first time from the centimetre to the micrometre scale using optical and scanning electron microscopy om sem x ray diffraction xrd and total organic carbon content toc measurements a significant range of microtextures organic matter particles and fracture styles was observed in rocks of the lower bowland shale together with the underlying pendleside limestone and worston shale formations encountered the preese hall 1 borehole lancashire uk four micro texture types were identified unlaminated quartz rich mudstone interlaminated quartz and pyrite rich mudstone laminated quartz and pyrite rich mudstone and weakly interlaminated calcite rich mudstone organic matter particles are classified into four types depending on their size shape and location multi micrometre particles with and without macropores micrometre size particles in cement and between clay minerals multi micrometre layers and organic matter in large pores fractures are categorized into carbonate sealed fractures bitumen bearing fractures resin filled fractures and empty fractures we propose that during thermal maturation horizontal bitumen fractures were formed by overpressuring stress relaxation compaction and erosional offloading whereas vertical bitumen bearing resin filled and empty fractures may have been influenced by weak vertical joints generated during the previous period of veining for the majority of samples the high toc 2 wt low clay content 50 wt and the presence of a multi scale fracture network support the increasing interest in the bowland shale as a potentially exploitable oil and gas source the microtextural observations made in this study highlight preliminary evidence of fluid passage or circulation in the bowland shale sequence during burial

large scale glaciation and deglaciation of antarctica during the late eocene approximately 34 m y ago earth s climate transitioned from a relatively warm ice free world to one characterized by cooler climates and a large permanent antarctic ice sheet understanding this major climate transition is important but determining its causes has been complicated by uncertainties in the basic patterns of global temperature and ice volume change here we use an unusually well exposed coastal incised river valley complex in the western desert of egypt to show that eustatic sea level fell and then rose by 40 m 2 m y prior to establishment of a permanent antarctic ice sheet this fall in sea level is associated with a positive oxygen isotope excursion that has been interpreted to reflect global cooling but instead records buildup of an antarctic ice sheet with a volume 70 of the present day east antarctic ice sheet both the sea level fall and subsequent rise were coincident with a transient oscillation in atmospheric co2 concentration down to 750 ppm which climate models indicate may be a threshold for southern hemisphere glaciation because many of the carbon emission scenarios for the coming century predict that atmospheric co2 will rise above this same 750 ppm threshold our results suggest that global climate could transition to a state like the late eocene when a large permanent antarctic ice sheet was not sustainable

fission track evidence on the initial rifting of the red sea two pulses no propagation fission track analyses indicate that the red sea initially opened simultaneously along its entire length two distinct pulses of uplift and erosion characterized the early stages of rifting in the red sea throughout egypt and in southwestern saudi arabia the first pulse began at 34 million years ago ma the second pulse began in the early miocene 21 to 25 ma and marked the start of the main phase of extension these data support a rigid plate model for continental extension these results also indicate that the initiation of rift flank uplift and therefore rifting and volcanism occurred nearly simultaneously this conflicts with classical models of active and passive extension that predict sequential development of these features

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

geotherms evolution of the lithosphere and plate tectonics abstract for the purpose of calculating precambrian geotherms modern average geotherms need to be defined curves calculated from estimated plate sections are close to those of clark and ringwood 1964 thicknesses of lithospheric plates at any time in the past can be estimated from fossil geotherms because subduction of oceanic plates depends upon their density average densities of precambrian plates should place important constraints on the beginning of subduction this method however only indicates that in all probability subduction did not begin before 1000 ma ago geological data suggest a date of about 800 or 900 ma ago formation of archean continents would have proceeded from partial melting of basaltic crust and the end of that phase of earth s evolution would be a thermally controlled feedback mechanism lowering geothermal gradients in sub continental mantle the beginning of subduction was also thermally controlled and probably accompanied the increasing stability of eclogite in the upper mantle

strength anisotropy of shales deformed under uppermost crustal conditions conventional triaxial tests were performed on three sets of samples of tournemire shale along different orientations relative to bedding 0 45 90 experiments were carried out up to failure at increasing confining pressures ranging from 2 5 to 160 mpa at strain rates ranging between 3 10 7s 1 and 3 10 5s 1 this allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure results show that the orientation of principal stress relative to bedding plays an important role on the brittle strength with 45 orientation being the weakest we fit our results with a wing crack micromechanical model ashby and sammis 1990 and an anisotropic fracture toughness we found low values of internal friction coefficient and apparent friction coefficient in agreement with friction coefficient of clay minerals between 0 2 and 0 3 and values of kic comparable to that already published in the litterature we also showed that strain rate has a strong impact on peak stress and that dilatancy appears right before failure and hence highlighting the importance of plasticity mechanisms although brittle failure was systematically observed stress drops and associated slips were slow and deformation always remained aseismic no acoustic emission were detected this confirms that shales are good lithological candidates for shallow crust aseismic creep and slow slip events

geology geochemistry and isotopic characteristics of the archaean kaap valley pluton barberton mountain land south africa abstract formed approximately 3500 ma ago the kaap valley tonalite pluton is one of the oldest and the largest of a suite of tonalite trondhjemite gneiss plutons in the immediate environs of the barberton greenstone belt although similar in many respects to the other members of this suite the kaap valley pluton is distinctively more mafic in composition containing hornblende instead of biotite as its dominant mafic mineral and is enriched in rare earth elements these characteristics are not compatible with an origin simply involving a greater degree of partial melting than that required to form more felsic trondhjemites from the same precursor the parental magma to the kaap valley tonalite is considered rather to originally have been a trondhjemitic magma enriched in minor phases such as zircon and apatite and possibly also dissolved co2 vapour which has been contaminated by a mafic component the form and composition of hornblendes in the tonalite are inconsistent with a xenocrystic origin by incomplete digestion of surrounding mafic metavolcanic country rocks and consequently the contamination is inferred to have taken place prior to magma emplacement possible mechanisms include magma mixing and complete assimilation at depth of mafic rocks a subdued form of crystal fractionation occurred during solidification of this magma resulting in the segregation of discrete hornblende and hornblende biotite tonalite phases the tonalite was subsequently diapirically emplaced into its present position perhaps about 3200 ma ago a process which resulted in deformation of the surrounding greenstone belt successions diapirism also perturbed the original probably sub horizontal compositional zoning the present geometry of which can best be explained in terms of an orthogonal pattern of intersecting structural lineaments

paleogene initiation of the western branch of the east african rift the uplift history of the rwenzori mountains western uganda abstract the two branches of the east african rift system ears are believed to have initiated diachronously however a growing body of work continues to suggest the onset of rifting in the western branch occurred in the paleogene coeval to the eastern branch due to a lack of pre miocene stratigraphy attempts to resolve the geological history of the western branch must study the uplift and erosional histories of the modern rift topography in this study the rock uplift history of the rwenzori mountains western uganda is resolved to better our understanding of the tectonic history of the western branch of the ear through the application of low temperature thermochronology mapping and the modelling of river profiles we show that rock uplift of the rwenzori dates back to the oligocene with thermal history models suggesting uplift induced exhumation may date back as far as the eocene this provides tangible evidence that extension began in the region in the paleogene coeval with the eastern branch and not the late neogene these results have broad implications for the tectonic evolution of the entire east african rift system and suggest our current understanding of the region s rift history remains incomplete

upb zircon ages bearing on the nature of early archaean greenstone belt evolution barberton mountainland southern africa abstract u pb zircon systematics for granitic and gneissic clasts from the moodies conglomerate of the barberton greenstone belt south africa exhibit discordancy patterns that suggest variable and multi episodic lead loss and indicate crystallisation ages for the parent granitoids between 3 3 and 3 47 ga metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2 8 2 9 ga ago that is ascribed to a regional thermal event as also shown by published rb sr data for a nearby granitoid pluton since the moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early archaean granite greenstone terrains the maximum age of 3 3 ga for the moodies group also shows that greenstone belt evolution in the barberton region took place over a period of at least 150 ma from about 3450 to 3300 ma ago the kaap valley and stentor plutons intrude the lower part of the greenstone succession and their diapiric emplacement caused large scale deformation of earlier structures in the entire greenstone belt sequence at about 3 23 3 25 ga ago detrital zircons in the sediments of the fig tree group date the mean age of their source terrain at c 3520 ma which is also the maximum age of deposition for these strata this age is indistinguishable from the oldest age reported from the barberton greenstones and suggests the possibility that pre greenstone granitoid crust may exist in the eastern kaapvaal craton

sm nd age and isotopic systematics of the bimodal suite ancient gneiss complex swaziland studies of the development and stabilization of the archaean crust often focus on the relative temporal relationships between the metamorphosed basaltic to ultramafic volcanic units greenstone belts and the sialic gneiss terrains that make up the oldest sections of the terrestrial crust at the heart of this interest are the questions of the processes responsible for crust formation in the archaean and whether or not the various units of an archaean crustal section represent new additions to the crust from the mantle or are products of the reprocessing of even older crustal materials one area where this controversy has been particularly pronounced is the archaean crustal section of south west africa1 6 the oldest rocks in the kaapvaal craton consist of the onverwacht group of mafic to ultramafic metavolcanics of the barberton greenstone belt and a grey gneiss complex termed the ancient gneiss complex agc of swaziland we report here the results of a whole rock sm nd isotopic study of the agc and the implications these data may have for crustal evolution in the kaapvaal craton

large scale glaciation and deglaciation of antarctica during the late eocene comment peters et al 2010 1 present a hypothesis for a late eocene glaciation by interpreting an incision surface at wadi al hitan egypt as indicating a 40 m fall in eustatic sea levels we argue that there is no evidence for a major fall in sea level and that their calibration of the event is

experimental investigation of the effect of temperature on the mechanical behavior of tournemire shale this paper is devoted to the experimental investigation of the effect of temperature on the mechanical behavior of a typical anisotropic clayey rock the tournemire shale hydrostatic and conventional triaxial compression tests are first performed under room temperature for two principal loading orientations respectively parallel and perpendicular to the bedding planes the obtained results confirm an anisotropic mechanical behavior observed by the previous works further hydrostatic and triaxial compression tests with different confining pressures are carried out under different temperatures up to 250 c with the increase of temperature there is a significant decrease of young s modulus and the compression failure strength but an increase of the overall deformability of material the temperature change also affects the anisotropic response related to the deformation of bedding planes

palaeobiogeographic relationships and palaeoenvironmental implications of an earliest oligocene tethyan ichthyofauna from egypt the fayum depression of egypt has produced a great diversity of fossil material including marine and freshwater fishes in contrast to the eocene formations of the fayum the oligocene jbel qatrani formation has been more or less consistently considered to be deposited in a freshwater environment however the ichthyofauna indicates a more complex picture cenozoic fishes have been convincingly used to interpret the palaeoenvironment in which sediments were deposited based on the elasmobranch and osteichthyan faunas of the jbel qatrani formation we interpret that this formation was not deposited entirely in fresh waters but had some marine influence particularly in the lower part of the formation the mixture of freshwater elements such as polypterids and alestids with brackish and marine elements including myliobatid stingrays in the quarry e site suggests a local palaeoenvironment that was very close to the shoreline in a less protected area or under more seasonal influence than the rest of t

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

stratigraphic and sedimentological evidence bearing on structural repetition in early archean rocks of the barberton greenstone belt south africa abstract the 3 5 3 3 ga old swaziland supergroup in the barberton greenstone belt south africa includes a lower predominantly volcanic unit the onverwacht group and an upper largely sedimentary section the fig tree and moodies groups although viljoen and viljoen concluded that the onverwacht group exceeds 15 km in stratigraphic thicknesses more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting our mapping indicates that the upper half of the group includes at least 8 km of unrepeated stratigraphic section inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the onverwacht group on the west limb of the onverwacht anticline shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform highly complex zone of deformation up to 2 km wide the displacement across the zone is unknown but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the group has not been significantly altered structures previously mapped as major fold hinges within the upper part of the onverwacht group can be shown to have formed by local deformation and reflect neither large scale folding nor structural repetition of the rocks several kilometers higher in the section rocks of the uppermost onverwacht and fig tree groups are stacked and repeated in a series of thrust nappes individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks they were apparently emplaced by faulting or gravity sliding at or near the earth s surface their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying much weaker serpentinized ultramafic komatiites locally the sheets were deformed to form fold nappes the lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact the thrust faults are the loci of small hypabyssal igneous bodies that appear to be co magmatic with a suite of upper fig tree volcanic rocks this style of faulting has not affected rocks of the overlying moodies group although estimates of the timing of deformation are as yet imprecise these features suggest that it occurred during a late fig tree phase of tectonism and volcanism

diagenetic control on mineralogical suites in sand silt and mud cenozoic nile delta implications for provenance reconstructions abstract this nile delta case study provides quantitative information on a process that we must understand and consider in full before attempting provenance interpretation of ancient clastic wedges petrographic and heavy mineral data on partly lithified sand silt and mud samples cored from the up to 8 5 km thick post eocene succession of the offshore nile delta document systematic unidirectional trends with increasing age and burial depth quartz increases at the expense of feldspars and especially of mafic volcanic rock fragments heavy mineral concentration decreases drastically transparent heavy minerals represent progressively lower percentages of the heavy fraction and zircon tourmaline rutile apatite monazite and cr spinel relatively increase at the expense mainly of amphibole in pliocene sediments and of epidote in miocene sediments recent studies have shown that the entire succession of the nile delta was deposited by a long drainage system connected with the ethiopian volcanic highlands similar to the modern nile since the lower oligocene the original mineralogy should thus have resembled that of modern delta sand much more closely than the present quartzose residue containing only chemically durable heavy minerals stratigraphic compositional trends although controlled by a complex interplay of different factors document a selective exponential decay of non durable species through the cored succession that explains up to 95 of the observed mineralogical variability our calculations suggest that heavy minerals may not represent 20 of the original assemblage in sediments buried less than 1 5 km 5 in sediments buried between 1 5 and 2 5 km and 1 for sediments buried 4 5 km no remarkable difference is detected in the intensity of mineral dissolution in mud silt and sand samples which argues against the widely held idea that unstable minerals are prone to be preserved better in finer grained and therefore presumably less permeable layers intrastratal dissolution acting through long periods of time at the progressively higher temperatures reached during burial can modify very drastically the relative abundance of detrital components in sedimentary rocks failure to recognize such a fundamental diagenetic bias leads to grossly mistaken paleogeographic reconstructions as documented paradigmatically by previous provenance studies of ancient nile sediments

eocene initiation of nile drainage due to east african uplift abstract the late eocene and early oligocene sedimentary succession in the fayum egypt records the progressive development of northerly flowing nile type african drainage new biostratigraphic dating of these units allows the calibration of the paleomagnetic record the combination of dating methods enabling a detailed chronology of events to be studied between about 38 and 35 ma there was a dramatic change in sedimentary regime and vast quantities of clastic material were transported into the area smothering the underlying carbonate platform and initiating a stepwise progradation of clastic units the sudden change in sediment availability coincides with the beginning of uplift and volcanic activity in the turkana region of east africa cutting off preexisting easterly drainage from the middle of the continent the fayum succession therefore records the initiation of northerly drainage of central and eastern africa and the origins of the modern nile watershed the development of the current route of the nile with the incision of the current nile valley was slightly later and related to mid oligocene uplift of the red sea margins and messinian base level fall

thermomechanical properties of deep argillaceous formations abstract a difficulty in the interpretation of mechanical and thermo mechanical tests on specimens drawn from large argillaceous formations is the strong inhomogeneity of void ratio clay minerals and carbonates content in this paper a relationship is developed to link strength and the maximum preconsolidation stress to the initial void ratio and carbonate content compressibility is also correlated to carbonates thermal strains in drained and undrained conditions for a spanish a belgian and an italian natural clay are compared in the elastic state strains are comparable while in the plastic range thermal strains are highest for the belgian clay lower in the spanish cemented clay and lowest in the italian clay very stiff and cemented

trans african drainage system of the sahara was it the nile an exciting result of the first spaceborne radar scans of the eastern sahara has been the recognition of drainage channels buried at depths of several metres below the dry desert sand we relate these observations to the megageomorphological evolution of africa and conclude that the radar river valleys may have been parts of an old nile river system rather than courses cut by a postulated westward flowing trans african drainage system tads extending from the red sea hills to the niger delta

weathering before the advent of land plants evidence from unaltered detrital k feldspars in cambrian ordovician arenites potassium chelation by land plants and the consequent depletion of k ion activity in soil waters make k feldspars unstable in present day soils other factors remaining constant the absence of land plants in pre silurian time would make k feldspars stable dissolution of plagioclase under the action of soil water would be the same in both pre and post silurian time the presence of abundant fresh k feldspars and the absence of plagioclase in the cambrian ordovician arenites of ohio wisconsin minnesota missouri texas wyoming and montana are probably due to the absence of potassium chelating agents in the soils of the time

the initiation and evolution of the river nile abstract the nile is generally regarded as the longest river in the world knowledge of the timing of the nile s initiation as a major river is important to a number of research questions for example the timing of the river s establishment as a catchment of continental proportions can be used to document surface uplift of its ethiopian upland drainage with implications for constraining rift tectonics furthermore the time of major freshwater input to the mediterranean is considered to be an important factor in the development of sapropels yet the river s initiation as a major drainage is currently constrained no more precisely than eocene to pleistocene within the modern nile catchment voluminous cenozoic continental flood basalts cfbs are unique to the ethiopian highlands thus first detection of their presence in the nile delta record indicates establishment of the river s drainage at continental proportions at that time we present the first detailed multiproxy provenance study of oligocene recent nile delta cone sediments we demonstrate the presence of ethiopian cfb detritus in the nile delta from the start of our studied record c 31 ma by 1 documenting the presence of zircons with u pb ages unique within the nile catchment to the ethiopian cfbs and 2 using sr nd data to construct a mixing model which indicates a contribution from the cfbs we thereby show that the nile river was established as a river of continental proportions by oligocene times we use petrography and heavy mineral data to show that previous petrographic provenance studies which proposed a pleistocene age for first arrival of ethiopian cfbs in the nile delta did not take into account the strong diagenetic influence on the samples we use a range of techniques to show that sediments were derived from phanerozoic sedimentary rocks that blanket north africa arabian nubian shield basement terranes and ethiopian cfb s we see no significant input from archaean cratons supplied directly via the white nile in any of our samples whilst there are subtle differences between our nile delta samples from the oligocene and pliocene compared to those from the miocene and pleistocene the overall stability of our signal throughout the delta record and its similarity to the modern nile signature indicates no major change in the nile s drainage from oligocene to present day

role of a thin komatiite rich oceanic crust in the archean plate tectonic process in the archean tectonic regime the time available for oceanic lithosphere to cool on its journey from ridge to convergent plate boundary was far less than at present under these conditions only an inherently dense lithosphere could have acquired the negative buoyancy necessary for it to subduct at this time plate tectonics could have operated only if 1 the oceanic crust was rich in komatiite lava and 2 the thickness of this crust was not much greater than the thickness of modern oceanic crust