diapiric transfer of melt in kilauea iki lava lake hawaii a quick efficient process of igneous differentiation kilauea iki lava lake formed in 1959 is a large pond of picritic basalt average mgo content 15 34 by weight which has cooled and crystallized as a small self roofed magma chamber repeated drilling of the upper crust of the lake down to its molten core and more recent 1981 drilling through the thermal maximum and part way through the lower crust have made it possible to monitor the differentiation processes in the lake in detail differentiation processes recognized as active in the lake include rather inefficient settling of the larger 2 10 mm olivine phenocrysts formation of segregation veins and formation of diapir like vertical olivine rich bodies all processes which occur in one or more of the other kilauean lava lakes as well in addition most of the central part of kilauea iki has been affected by diapiric melt transfer in this process relatively low density liquid present at 1145 1160 c rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt passing through the hotter but denser melt in the core of the lake without mixing the source volume from which the low density liquid was extracted is depleted in tio 2 and other incompatible elements and enriched in feo and cao the upper part of the lake shows the opposite effects the crystalline assemblage present was olivine augite minor plagioclase the crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m the process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others calculations suggest that between 21 and 42 wt liquid has been extracted from the depleted zone at 56 78 m in the center of the lake making this a very efficient process of chemical differentiation

macaronichnus and contourite depositional settings bottom currents and nutrients as coupling factors abstract the trace fossil macaronichnus segregatis degiberti 7 15 mm in diameter is recorded for the first time in deep marine contourites which were deposited in a late miocene outer shelf to upper slope of the rifian corridor morocco macaronichnus is typical of shallow marine mainly foreshore environments it is produced by deposit feeding opheliid polychaetes in high to intermediate latitudes this record supports the significance of bottom current processes on environmental conditions as a major limiting factor affecting the macrobenthic trace maker community mediterranean overflow water during the late miocene enabled a high nutrient supply to the seafloor favouring colonization by macaronichnus producers thus m s degiberti should be used as an indicator for nutrient availability instead of mainly a palaeodepth indicator this finding provides significant information for oceanographic and environmental reconstructions related to contouritic processes as well as to contourite characterization

ichnofacies distribution in the eocene early miocene petra tou romiou outcrop cyprus sea level dynamics and palaeoenvironmental implications in a contourite environment ichnological analysis of the carbonate contourite drift at the petra tou romiou outcrop southern cyprus reveals a relationship between sea level dynamics shoreline position sedimentation rate flow hydrodynamics and trace fossil assemblages the base of the outcrop is composed of chalky deposits showing the zoophycos ichnofacies attributes the transition to the first contouritic interval beds is marked by the gradual disappearance of zoophycos and dominance of horizontal traces this change is caused by a gradual sea level fall determining an increase in proximity and energy conditions in the environment that favoured the development of distal cruziana ichnofacies upper contouritic interval beds contain abundant burrow systems with vertical components increased ichnodiversity particularly among vertical morphologies coincides with the appearance of teichichnus gyrolithes and rare ophiomorpha revealing a transition to the proximal cruziana ichnofacies related with the maintenance of the previous trend toward more proximal and higher energy settings the differentiated ichnofacies in the petra tou roumiou carbonate drifts are controlled by complex interactions between variable environmental factors with sea level dynamics playing a major role

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

large burrow systems in marine miocene deposits of the betic cordillera southeast spain abstract large burrow systems occur in the miocene marine deposits of the almeria nijar basin betic cordillera southeast spain they consist of galleries of dense burrow segments locally forming irregular networks cylindrical branched y shaped structures with diameters ranging from 8 to 22 cm and minimum lengths of 30 60 cm are abundant in addition galleries longer than 70 80 cm are common and burrow segments larger than 200 cm are locally present burrows have smooth walls without pellet reinforcements but locally possible scratch marks are present burrow fill is structureless and has a similar composition to the host rock these structures are assigned to the ichnogenus thalassinoides ehrenberg displaying similarities with t suevicus identification of the possible tracemaker is not straightforward but decapod crustaceans probably shrimp or shrimp like organisms are the most likely candidates

grain boundary contact effects during faulting of quartzite an sem ebsd analysis abstract during low temperature faulting of cambrian quartzite assynt nw scotland stress concentrations develop at grain contacts either at the onset of deformation prior to the establishment of a through going fault plane or within the damage zone remote from the main displacement segment such concentrations contribute to the development of intragranular microfractures cataclastic microstructures and fault rocks this contribution considers the progressive deformation sequence that precedes microfracturing and cataclasis the complexity of this deformation is revealed by scanning electron microscope sem electron backscattered diffraction ebsd dauphine twinning is a widespread feature associated with grain contact stress concentration and forms distinctive ebsd microstructures automatic sem ebsd analysis reveals that whilst initial indentation causes dauphine twinning of many grains continued indentation results in the formation of an arcuate array of subgrains via low temperature plasticity and or microcracking which overprint the dauphine twins these observations are consistent with transmission electron microscopic analysis of quartz crystals used for microhardness indentation tests which reveal that indentation causes an intensely deformed region to develop comprising a high density of microfractures and a submicron scale blocky microstructure that accommodates any plastic deformation deformation mechanisms and associated microstructures develop sequentially with progressive indentation and may provide sites of microfracture nucleation via low temperature ductile fracture the new microstructures assist diffusive mass transfer dmt processes by the formation of a cellular or subgrain array that represents a reduction of several orders of magnitude in apparent grain size and hence in diffusion path length concomitantly associated microfracturing perturbs local thermodynamic equilibrium leading to enhanced dmt crack healing and cementation overgrowths together these processes form the aseismic creep and sealing components of fault zone development

water films at grain grain contacts debye huckel osmotic model of stress salinity and mineralogy dependence abstract water film diffusion is one of the mechanisms proposed to explain the deformation of rocks by pressure solution during geological processes in the upper crust this mechanism assumes that matter is dissolved inside the contact between two grains the resulting solutes are transported in the pore fluid through diffusion in an adsorbed water film the main problem of this theory is that it requires the presence of a water film that is believed to be stable under large deviatoric stresses inside the contact between two grains in this paper we show that the electrically charged surface of a mineral can attract counter ions from the pore and by the related change of osmotic pressure keep water within the contact this is due to the counter ions in the water film that increase the salinity in the film relative to that in the pore this lowers the free energy of water in the contact zone to a degree that balances the increase in free energy of water due to the elevated pressure in the film these notions are made more precise by combining the theory of the debye huckle double layer with equations of osmotic pressure the resulting d h o theory predicts the dependence of the water film thickness on stress across the contact composition of the pore fluid and the identity of the minerals involved

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

sea level dynamics and palaeoecological factors affecting trace fossil distribution in eocene turbiditic deposits gorrondatxe section n spain abstract ichnological analysis of the upper ypresian lower lutetian interval at the gorrondatxe section w pyrenees n spain reveals a relationship between sea level dynamics and the eco sedimentary factors influencing trace fossil assemblages the 600 m thick section of deep sea turbiditic deposits contains 41 ichnospecies belonging to 28 ichnogenera which are typical of the nereites ichnofacies and mostly of the paleodictyon ichnosubfacies suggesting deposition in a basin plain to fan fringe setting the trace fossil diversity and abundance fluctuate irrespective of turbidite frequency these ichnological features are strongly affected by trophic level changes related partly to sea level dynamics according to the sequence stratigraphic interpretations for the studied section temperature oxygenation and substrate changes are also considered as relevant factors increased ichnodiversity particularly among graphoglyptids coincides with moderate oligotrophy and stable ecological conditions eutrophisation lowered oxygenation and drop of temperature typical of low sea level can reduce ichnodiversity

quantification of brittle deformation in burial compaction frio and mount simon formation sandstones abstract brittle deformation together with grain rearrangement and ductile grain deformation is a key mechanism of compaction in sandstones quartz cementation by virtue of its impact on sandstone mechanical properties is expected to affect the compaction progress by these various mechanisms sandstone samples of different ages and compositions taken from two basins with contrasting burial histories are used to quantify the relationship between brittle deformation and quartz cementation in the context of burial compaction exponential trends of increasing deformation by microfracturing are observed in both the lithic rich frio formation from the gulf of mexico basin r2 0 81 and in the quartz rich mount simon formation from the illinois basin r2 0 78 the two formations contrast in terms of the observed rate of grain fracture increase with depth a larger number of quartz grains in the mount simon formation undergo fracturing at shallow burial 3 km a larger number of quartz grains are fractured in the frio sandstones cathodoluminescence cl imaging conducted on an sem shows that grain fracturing and subsequent cementation of fractures by quartz can occur before during and after normal quartz overgrowth precipitation although fracturing is generally concurrent with cementation the intensity of brittle deformation for a single fractured grain is independent of depth as illustrated by plotting the ratio of the number of particles in fractured grains versus depth r2 0 21 for frio and 0 03 for mount simon suggesting that once fractured grains do not tend to become the locus of subsequent fracturing apparent fracture apertures in the frio grains are slightly wider average 5 m than in mount simon grains average measurable aperture width 4 m quartz grains in the frio have a variety of fracture morphologies including wedge shaped apertures intense comminution at grain contacts and grains with exploded fabrics apparent fracture apertures in mount simon grains are thinner and transect grains as straight uniform traces differences in fracture intensity and pattern are attributed to the fact that cementation in the frio began at greater depth than cementation in the mount simon combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures here referred to as cf or volume of intragranular cement allows calculation of the amount that brittle deformation influences compaction in the frio formation 0 12 to 8 37 of porosity loss due to compaction can be attributed to brittle deformation whereas the values for the mount simon formation lie between 0 25 to 2 16 the larger values within both formations are affiliated with deeper samples in which a majority of grains manifest fracturing

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

crystal mush compaction in the cohassett flood basalt flow hanford washington abstract the 40 m thick central part of the cohassett flow which was probably emplaced by inflation of an earlier more ti rich flow exhibits textural evidence of crystal mush compaction during solidification although most of the crystallization started in the roof zone as water circulated through fractures in the crust plumes of dense crystal mush must have dripped from the roof to the floor of the sheet thus displacing upward the colonnade entablature boundary the mush initially consisted of loosely stacked plagioclase crystals and liquid surrounding rapidly grown pyroxene oikocrysts containing radial chadacrysts of plagioclase upon compaction in the floor zone the liquid between the loosely stacked plagioclase crystals was expelled as the crystals packed tightly together to form sub parallel grains wrapped around the pyroxene oikocrysts which recrystallized into granular aggregates of augite and pigeonite stacks of sinking plagioclase crystals and rising liquid accumulated on the upper and lower sides respectively of horizontal plagioclase phenocrysts this asymmetric texture clearly indicates the relative motion of solids and liquid the degree of compaction as determined from variations in the amount of interstitial liquid and quantitative measures of textural anisotropy accounts for the chemical variation through the central part of the flow with compatible elements being concentrated in the lower part of the flow and incompatible elements concentrated in the upper part modeling of the compaction process using the iridium melt infiltration and reaction program of boudreau a 2003 iridium a program to model reaction of silicate liquid infiltrating a porous solid assemblage computers and geosciences 29 423 429 shows that the observed chemical and compaction dilation profiles are to be expected in a 40 m thick sheet of basaltic magma solidifying on the earth s surface compaction of crystal mush should be even more likely in slowly cooled intrusive bodies

cooling history and differentiation of a thick north mountain basalt flow nova scotia canada a thick 175 m north mountain basalt flow at mckay head nova scotia canada shows 25 cm thick differentiated layers separated by 130 cm of basalt in its upper 34m upper layers 5 m below the lava top are highly vesicular whereas lower ones are pegmatitic and contain a thin 2 cm rhyolite band the layering of the flow closely resemble that of some hawaiian lava lakes the eesicular basalts and mafic pegmatites are inferred to be liquid rich segregations which drained into horizontal cracks that formed within a crystalline mush the cracks resulted from a thermal contraction associated with cooling and shrinkage of the mush rhyolites were formed by in situ differentiation gas overpressures fractured the pegmatites and gas effervescence filter pressing forced silicarich residual liquid from pegmatite interstices into the fractures creating bands chemical differences between the pegmatitic layers and early formed highly differentiated upper vesicular layers may reflect a role for volatiles in the differentiation process along with crystal fractionation

potassium leached zones at the contacts of the garn formation mid norwegian continental shelf evidence for the role of shale in sandstone diagenesis abstract

the formation of plagioclase chains during convective transfer in basaltic magma the basaltic rock in the lower part of the thick holyoke lava flow in connecticut and massachusetts has been shown to have a remarkable texture with crystals of feldspar linked together in a continuous three dimensional network of chains1 heating experiments have revealed that this network persists to temperatures where the rock is 75 liquid2 and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow3 4 despite the texture s importance to our understanding of how such basalt flows form the origin of the texture has remained uncertain1 2 3 here we show that although the network is present in the lower third of the flow it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush convection of this type has been postulated for intrusive magma chambers but corroborative field evidence has been equivocal especially in lava lakes and flows preservation of the roof generated texture in the lower part of a thick flood basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet

toarcian oceanic anoxic event induced unusual behaviour and palaeobiological changes in thalassinoides tracemakers abstract the early toarcian early jurassic global marine mass extinction is associated with one of the most important environmental perturbations of the mesozoic the toarcian oceanic anoxic event t oae usually interpreted as a global oceanic anoxic event in the lusitanian basin lb one of the reference areas for the study of the t oae despite an elevated extinction rate among marine invertebrates the anoxia hypothesis has been discarded based on geochemical and isotopic data in the fonte coberta section the t oae is characterised by the extinction of the previously well represented brachiopod taxa a decreased abundance of ammonoids and the establishment of a well developed endobenthic community mainly composed by thalassinoides tracemaker which indicates favourable environmental parameters at least for this community thus the palaeoenvironmental changes associated with the t oae event bear a minor incidence on the macrobenthic tracemaker community at the level of extinctions and changes in diversity or abundance yet they exert a major effect on the behaviour and palaeobiology of tracemakers the thalassinoides specimens show an exclusively horizontal development and unusual architectural features during the extinction interval tied to behavioural adaptations to nutrient availability and or substrate firmness smaller burrow diameters are registered at the base of the t oae and then an increase is observed during the t oae this could be associated with a palaeobiological response lilliput effect to palaeoecological changes at the beginning of the t oae followed by a progressive change to pre event environmental parameters this study therefore sheds light on how environmental alterations can determine specific behavioural and palaeobiology changes of the producer as reflected by variations in significant features of thalassinoides architecture

hawaiian basalt and icelandic rhyolite indicators of differentiation and partial melting in spite of the voluminous basaltic volcanism on the island of hawaii rhyolite is not produced iceland on the other hand exhibits common rhyolitic volcanism amounting to some 10 12 of its surface rocks this contrast is investigated using the fundamental igneous processes exhibited by sheet like hawaiian lava lakes and shonkin sag laccolith in montana highly differentiated residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes only when a large initial phenocryst population is present from which a thick basal cumulate can rapidly form is it possible to supply highly differentiated melt into the active i e eruptable portion of the magma chamber although there is protracted control of differentiation at hawaii by settling of olivine further differentiation occurs within the solidification fronts only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front 7 mgo and 51 5 sio2 crystal size distributions csds for hawaii and shonkin sag are used to demonstrate the inferred physical and chemical processes of solidification including the kinetics of crystallization

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

contourite facies model improving contourite characterization based on the ichnological analysis abstract the calcareous contourites from the late oligocene and early miocene in the petra tou romiou type section southern cyprus have been studied in detail being regarded as outstanding examples of fossil bioclastic contourites and representative of the standard facies model of contourite bigradational sequence these bigradational sequences of sand dominated contourites consists of whitish calcarenite beds with wavy layering in the middle portion of the sequence gradual alternations of whitish and greenish calcilutites appear above and below these calcarenites the internal wavy layering is composed by compacted interlayers and non compacted interstratified layers in the present study an in depth ichnological analysis of the bigradational sequence has been conducted focusing on ichnological features such as length shape diameter and orientation of individual burrow segments the configuration of burrow systems as well as external features fill material and finally taphonomy trace fossil assemblages differ containing planolites isp chondrites isp and thalassinoides isp at the compacted interlayers and non compacted layers bearing only planolites structures the compacted interlayer structures are flattened and elliptical containing similar fill material as the calcilutite host sediment though lighter in color in the non compacted layers planolites are near un deformed being cylindrical and tubular with circular to sub circular cross sections these are located mainly at the base or occupy the entire layer locally showing longitudinal striae planolites reinecki ksi ziewicz and filled with calcarenite material the different trace fossil assemblages in the compacted layers and non compacted interlayers varying in fill material evidence an original primary differentiation presumably indicative of fluctuating paleoenvironmental conditions in consecutive non compacted layers the distribution of planolites is separated by compacted interlayers this indicates that the middle portion of the contourite type facies with internal wavy layering underwent a multiphase deposition in the non compacted layers the almost undeformed planolites marked by striae is consistent with a higher substrate consistency which is interpreted as being associated with sedimentary condensation or omission immediately before the next compacted interlayers were deposited this evidence clearly supports the contention that sedimentary processes governing contourite deposition were intermittent rather than continuous as traditionally proposed these findings raise basic questions regarding the bottom current processes and consequent bedform development in these deep marine environments

deformation enhanced metamorphic reactions and the rheology of high pressure shear zones western gneiss region norway microstructural and petrological analysis of samples with increasing strain in high pressure hp shear zones from the haram garnet corona gabbro give insights into the deformation mechanisms of minerals rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions scanning electron microscopy with electron backscattering diffraction sem ebsd compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization plagioclase with a shape preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the 0 0 1 1 0 0 slip system in omphacite crystallographic preferred orientations indicate slip on 1 0 0 0 0 1 and 1 1 0 0 0 1 during deformation fine grained garnet deformed by diffusion creep and grain boundary sliding ilmenite deformed by dislocation glide on the basal and at higher strains prism planes in the a direction relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500 650 c and 0 5 1 4 gpa and continued during prograde metamorphism to ultra high pressure uhp conditions both products and reactants show evidence of syn and post kinematic growth indicating that prograde reactions continued after strain was partitioned away the restriction of post kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away demonstrating that deformation played an important role in enhancing metamorphic reactions two important consequences of deformation observed in these shear zones are i the homogenization of chemical composition gradients occurred by mixing and grain boundary migration and ii composition changes in zoned metamorphic garnet by lengthening diffusion distances the application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100 150 mpa and lower limits for strain rates of 10 12 to 10 13 s 1 as deformation conditions for the shear zones in the haram gabbro that were produced during subduction of the baltica craton and resulted in the production of hp and uhp metamorphic rocks

microcodium an extensive review and a proposed non rhizogenic biologically induced origin for its formation abstract microcodium has been previously described as a mainly cenozoic calcification pattern ascribed to various organisms a review of the available literature and our data reveal two peaks in microcodium abundance the moscovian early permian and the latest cretaceous paleogene a detailed analysis of late paleozoic and cenozoic examples leads to the following new conclusions typical microcodium forming unilayered corn cob aggregates of elongated grains and thick multilayered palisade replacing structures cannot be linked to smaller grained intracellular root calcifications as became widely accepted after the work of klappa klappa c f 1979 calcified filaments in quaternary calcretes organo mineral interactions in the subaerial vadose environment j sediment petrol 49 955 968 typical microcodium is recognized from the early carboniferous with doubtful devonian reports to quaternary as a biologically induced mineralization formed via dissolution precipitation processes in various aerobic ca rich soil and subsoil terrestrial environments morphology and 13c signatures of microcodium suggest that neither plants algae or roots and root associated mycorrhiza regulate the formation of these fossil structures non recrystallized microcodium grains basically consist of slender 1 5 4 m curved radiating monocrystalline prisms with occasionally preserved hyphae like morphology thin 0 5 3 m hypha like canals can also be observed these supposed hyphae may belong to actinobacteria however thin fungal mycelia cannot be excluded we propose a model of microcodium formation involving a mycelial saprotrophic organism responsible for substrate corrosion and associated bacteria capable of consuming acidic metabolites and caco3 reprecipitation into the microcodium structures

mechanical compaction of deeply buried sandstones of the north sea abstract sandstones experience mechanical compaction when the overburden load exceeds the compressive strength petrographic evidence is rarely sufficient to determine the timing of mechanical compaction it is often assumed from indirect evidence such as regional porosity depth trends that mechanical compaction is a process that occurs exclusively during shallow or intermediate burial mechanical compaction without grain fracturing may occur at depth following pervasive framework grain dissolution and or if anomalously high porosity has been preserved due to the presence of small amounts of cement we describe examples from the fulmar sandstone formation of the central graben north sea that experienced late stage mechanical compaction following sponge spicule dissolution and microcrystalline quartz cementation deep burial mechanical compaction involving grain crushing may occur if the rate of grain contact quartz dissolution and or quartz overgrowth development cannot compete with the rate of stress increase at grain contacts some rotliegendes sandstones of the southern north sea that have been buried to 4 5 km offer a good example where the suppression of chemical compaction due to the presence of grain coating clays resulted in pervasive grain fracturing mineral veins are frequently associated with sandstones that have experienced pervasive mechanical compaction during deep burial these may reflect the sudden development of overpressure resulting from the transfer of load to the fluid during collapse of the sandstone framework

cretaceous depositional systems in the norwegian sea heavy mineral constraints deep water cretaceous sandstones in the norwegian sea were deposited by a number of distinct sediment transport systems tapping different sediment source terrains three distinct sandstone types k1 k2 and k3 have been identified distinguished and mapped on the basis of a combination of heavy mineral parameters sandstone type k1 occurs on the trondelag platform halten terrace and nordland ridge but does not appear to be present in the deeper water voring basin sandstone type k1 was ultimately derived from the scandinavian landmass with detritus sourced from metasediments of the caledonian fold belt intrusives of the trans scandinavian igneous belt and to a smaller extent svecofennian basement k1 sandstones were deposited on an unstable slope by debris flows and slumps with minor reworking by bottom currents sandstone type k3 was derived from the western gneiss region farther south on the scandinavian landmass sandstone type k2 occurs in more basinal locations in the norwegian sea and was not supplied by the systems operating along the scandinavian margin because its mineralogy contrasts with that of k1 and k3 k2 mineralogy is not consistent with a source in the lofoten area or east greenland therefore k2 is believed to represent the deposits of a separate axial transport system fed by a source that lay in northeast greenland k2 zircon ages indicate involvement of early proterozoic approximately 2000 ma and archean basement together with metasediments of the caledonian fold belt previous sedimentological models for the cretaceous of the norwegian sea suggest that sand deposition occurred as slumps and debris flows along the continental slope resulting in discontinuous and unpredictable sandstone units unless they become amalgamated into thick reservoir sequences the mineralogical evidence indicates that this model can be applied only to sandstone types k1 and k3 by contrast sandstone type k2 represents sediment introduced from the conjugate margin of the basin and occurs up to 200 km from its detrital source region therefore k2 is likely to occur as more predictable sheetlike bodies on the basin floor

palaeohydrogeological control of palaeokarst macro porosity genesis during a major sea level lowstand danian of the urbasa andia plateau navarra north spain abstract an extensive palaeokarst porosity system developed during a pronounced mid paleocene third order lowstand of sea level is hosted in danian limestones of the urbasa andia plateau in north spain these limestones were deposited on a 40 50 km wide rimmed shelf with a margin characterised by coralgal buildups and coarse grained bioclastic accumulations the sea level fall that caused karstification was of approximately 80 90 m magnitude and 2 5 ma in duration during the exposure a 450 m wide belt of sub vertical margin parallel fractures developed a few hundred metres inboard of the shelf edge most fractures are 90 100 m deep average 1 m in width and are associated with large erosional features created by collapse of the reefal margin inland from the fracture belt three superimposed laterally extensive cave systems were formed over a distance of 3 5 km perpendicular to shelf edge at depths ranging from 8 31 m below the exposure surface the palaeocaves range from 0 3 to 2 m in height average 1 5 m high they show no evidence of meteoric processes and are filled with thanetian grainstones rich in reworked microcodium a lithology that also occurs infilling the fractures the caves are interpreted as due to active corrosion at the saline water fresh water mixing zone caves are missing from the shelf edge zone probably because the fractures beheaded the meteoroic lens preventing mixing zone cave development beyond the fracture zone towards the platform interior each cave system passes into a prominent horizon averaging 1 m in thickness of spongy porosity with crystal silt infills and red fe oxide coatings the spongy horizons can be traced for 5 5 km inboard from the cave zone and occur at 10 5 m 25 m and 32 m below the exposure surface in the inland zone two additional horizons with the same spongy dissolution have been recognised at depths of 50 m and 95 m all are analogous to swiss cheese mixing zone corrosion in modern carbonate aquifers and probably owe their origins to microbially mediated dissolution effects associated with a zone of reduced circulation in marine phreatic water in the most landward sections a number of collapse breccia zones are identified but their origin is unclear the palaeokarst system as a whole formed during the pulsed rise that followed the initial sea level drop with the three main cave spongy zones representing three successive sea level stillstands recorded by stacked parasequences infilling large erosional scallops along the shelf margin the geometry of the palaeo mixing zones indicates a low discharge system and together with the lack of meteoric karstic features favours a semi arid to arid climatic regime which is further supported by extensive calcrete bearing palaeosols occurring in coeval continental deposits

from morphology to behaviour quantitative morphological study of the trace fossil helminthorhaphe abstract the ichnogenus helminthorhaphe is a meandering trace fossil valuable to the study of movement behaviours of the trace making marine invertebrates in this work we studied systematically the morphology of selected examples of helminthorhaphe from the upper cretaceous to miocene by means of primary morphological constructs morphometrics and theoretical morphological models eleven morphological constructs encompassing meander arrangement meander tip and meander course properties are established to illustrate the essential morphological features of meanders which cover key aspects of the motion and navigation capacities of helminthorhaphe makers tunnel diameter meander width and meander length are extracted to explore the size range of helminthorhaphe from which an exceptionally large end member is recognised h magna ichnosp nov meander width versus tunnel diameter ratio and the fractal dimension of helminthorhaphe are two prominent morphometric parameters that characterise the average closeness and space usage of the meanders respectively which aid differentiation between ichnospecies helminthorhaphe miocenica h magna h flexuosa and h japonica generally conform to a series of increasing closeness and space usage of the meanders theoretical morphogenetic analysis of the meander generating process of helminthorhaphe based on the bilateral phobo thigmotactic robot car model reveals that the tempo and hierarchy of bilateral movement differentiation contribute to the formation of primary terminal u turn at the meander tip as well as lower level meander course undulations the meander tip property proves to be an important morphological feature with biomechanical and neurobiological implications on the body flexibility and lateralisation in the nervous system of helminthorhaphe producers larger scale meander arrangement styles suggest that helminthorhaphe makers potentially possessed certain radial or concentric spatial cognition of the environment with an emphasis on theoretical morphogenetic analysis this study provides a preliminary attempt in linking ichnology with behavioural neurobiological and intelligence sciences

the effect of stylolite spacing on quartz cementation in the lower jurassic sto formation southern barents sea abstract the shallow marine lower jurassic quartz arenites of the sto formation in the southern barents sea comprise 1 intervals where dispersed detrital clay is absent and where the spacing between clay rich laminae that evolved into stylolites upon burial is exceptionally large up to several meters and 2 intervals where minor detrital clay matrix occurs clay laminae are very common and stylolite spacing is typically less than a centimeter point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large 4 11 versus 10 20 outside these intervals there is also a correlation between distance to nearest stylolite and volume of quartz cement samples located a centimeter or less from a stylolite contain 10 20 quartz overgrowths for distances of 3 20 cm quartz cement content is 4 10 and only 3 8 when the closest stylolite is more than 20 cm distant modeling of quartz cementation with the exemplartm diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size degree of grain coating or content of quartz grains i e the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites on the contrary the modeling suggests that the samples situated more than 20 cm from stylolites contain 5 8 less quartz cement than what would have been the case given a more normal stylolite abundance this study indicates that sandstones with exceptionally few clay rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories however such sandstones seem to be highly unusual on the norwegian continental shelf this suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation

dauphine twinning as evidence for an impact origin of preferred orientation in quartzite an example from vredefort south africa large meteorite impacts cause great changes to geologic features on earth ranging from cratering and ejecta blankets to phase transformations in minerals in this report we describe the effect of a shock wave on the crystallographic orientation of quartz crystals in quartzite from the vredefort impact site in south africa preferred orientation of bulk quartzite samples was measured by time of flight neutron diffraction with a random distribution of c axes a weak but distinct difference between the orientation distribution of positive and negative rhombs was observed as illustrated with pole figures and inverse pole figures results for the natural vredefort sample are compared with deformation

detrital quartz and quartz cement in upper triassic reservoir sandstones of the sichuan basin characteristics and mechanisms of formation based on cathodoluminescence and electron backscatter diffraction analysis abstract the relationship between detrital quartz grains and silica cement is significant because cement is often a major cause of porosity reduction in oil and gas reservoirs thus degrading hydrocarbon reservoir quality on the other hand the attributes of grain surfaces may inhibit cementation thus preserving porosity in the reservoirs in this study we focus on the microstructures in quartz grains and secondary overgrowths quartz cement in the heavily cemented sandstones from uplifted upper triassic siliciclastic rocks in the southern sichuan basin china electron backscatter diffraction ebsd optical and cathodoluminescence cl imaging methods have been used to address the microstructural and crystallographic attributes of the detrital grains and their overgrowths this study confirms that 1 the overgrowth zones of quartz are clearly visible in optical and cl images 2 all silica overgrowths are perfectly crystalline quartz 3 quartz cements have a syntaxial relationship with the host detrital grain indicating crystallographic continuity between the detrital quartz and its overgrowth and 4 crystallographic maps confirm that dauphine twins commonly occur both in the detrital grains and in the quartz cement most dauphine twins are developed at grain grain contacts including the margins of detrital grains where they are in contact with quartz cement some twins are limited to a single grain whereas others cross grain cement boundaries based on these twin distributions we can distinguish inherited twins from twins that formed during after burial compaction this study documents the grain deformation structures induced by sedimentary compaction and these structures can be accurately illustrated by ebsd data

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

petrologic evolution of a nipigon diabase sill ontario canada insights from compositional and textural profiles petrographic and geochemical profiles collected through a 250 m thick nipigon diabase sill near black sturgeon lake ontario constrain the complex emplacement and differentiation processes that controlled the magmatic evolution of these rocks several nested reinjections of phenocryst bearing magma compactiondriven redistribution of mildly evolved interstitial liquids in the central portion of the sill and segregation of highly evolved interstitial liquids into micropegmatitic veins near the upper contact these processes impact not only the silicate mineralogy of the rocks but also distribution of sulfide phases both petrographic and geochemical data demonstrate that the sill was gradually inflated via several discrete injections of olivine bearing magma plagioclase crystal size trends exhibit several reversals that are correlated with increases in olivine abundance and bulk rock mgo content these textural and compositional reversals are attributed to reinjections of olivine bearing magma into partially solidified magma from earlier injections mixing between evolved resident magma and reinjections of less evolved magma is commonly proposed as a mechanism for triggering sulfide mineralization in this sill which contains no significant mineralization localized minor enrichments and depletions in copper content are associated with the margins of a well constrained reinjection horizon chemical differentiation of the magma was controlled by three distinct processes flow differentiation crystal mush compaction and solidification front instability although the mineralogy of the sill is dominated by plagioclase and clinopyroxene major oxide compositional variations are largely controlled by the mechanical concentration of olivine via flow differentiation or phenocryst redistribution within the individual reinjection pulses this redistribution of phenocryst phases is the primary factor controlling the major element composition of the rocks in most of the sill in the central portion of the sill the abundances of incompatible trace elements are influenced by the compaction of partially crystalline mush and the expulsion of incompatible rich interstitial liquid this is recognized by a negative correlation between the alignment of plagioclase crystals and concentrations of incompatible trace elements such as zr finally there is a 2 5 m thick silicic segregation approximately 32 m below the upper contact whose composition is consistent with residual liquid after 65 to 85 fractional crystallization of magma with a composition equivalent to that of the marginal rocks similar veins and patches are common throughout the upper 50 m of the sill indicating that this segregation process was common and widespread during the cooling and crystallization of the sill the complexity of intrusion and differentiation in this sill despite its generally unremarkable texture and composition suggests that many diabase sills may have experienced a range of processes that have previously been recognized to impact the petrologic evolution of larger igneous systems and that these processes can be identified in minor intrusions such as sills and dikes using simple and robust analytical techniques

habitat of hydrocarbons on the norwegian continental shelf eds a m spenceret al norwegian petroleum society graham and trotman 1986 isbn 0 86010 833 3 price 65 00

ichnological analysis of contourites past present and future abstract contourite deposits are significant facies in deep water sedimentary environments because they record critical paleoclimatological and paleoceanographic data contourite research also informs slope stability geological hazard assessment and petroleum exploration scientific consensus has not converged on several key aspects of contourite interpretation however some researchers interpret bioturbation as a diagnostic feature that differentiates contourites from associated facies e g turbidites while other researchers consider bioturbation to be of second order importance this work reviews existing literature and relevant field data to better define the role of ichnological analysis in contourite research review of significant contributions by earlier studies including formalization of terminology bioturbation burrow etc can help constrain wider usage of ichnological concepts e g ichnofacies and ichnotaxonomic analysis this paper also revisits ichnological data in paleoenvironmental interpretations reservoir characterization and the rare example of ichnofacies and ichnofabric analyses designation of diagnostic criteria can promote the use of ichnological information in contourite research regardless of consensus concerning bioturbation as a diagnostic criterion in contourite characterization ichnological data can still inform interpretations and integrate with other techniques to advance understanding of contourite sedimentary facies for a broad range of applications

zoophycos in deep sea sediments indicates high and seasonal primary productivity ichnology as a proxy in palaeoceanography during glacial interglacial variations trace fossils provide valuable palaeoenvironmental information in hemipelagic settings this is particularly true in the case of zoophycos an easily recognizable trace fossil in core at iodp site u1385 zoophycos was found throughout an interval representing 1 5 ma covering 45 glacial interglacial cycles mediated by obliquity 41 ka and short term eccentricity 100 ka zoophycos is most common in sediments deposited during glacial times and when the sedimentation rate was intermediate and primary production was high and seasonal occurrences of zoophycos elsewhere support a similar relationship with seasonal organic matter deposition this is particularly significant considering that seasonality of organic matter deposition is difficult to decipher from the sediment record zoophycos appears to represent a useful proxy to characterize high and seasonal organic matter deposition and primary productivity in neogene hemipelagic deposits

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

diorite segregations in gabbro geochemical characteristics and conditions for origin assessed at diorite gabbro contacts abstract silicic segregation veins in the basement sill dry valleys antarctica represent fracture filling liquids differentiated from mid jurassic ferrar tholeiitic basalt magmas geochemical and mineralogical characterizations for several of these veins and for their host gabbros within centimeters of sharp contacts with the veins provide information about silicic liquid produced from basalt in closed systems the basement sill silicic veins are coarse to pegmatite textured diorites 60 wt sio2 1 6 2 6 mgo composed of fe rich clinopyroxene cpx fs20 60 and orthopyroxene and pigeonite an50 60 plagioclase and 20 30 vol mesostases of micrographic quartz alkali feldspar or80 90 the host gabbros 52 54 wt sio2 5 5 9 2 mgo within 2 cm of veins contain pyroxene and feldspar with compositions that range from overlapping those in the diorite veins to those closer to characteristic of gabbro e g cpx fs20 an60 80 but unlike the more primitive mineral compositions representing t

petroleum migration faults and overpressure part ii case history the haltenbanken petroleum province offshore norway abstract petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration overpressure development and palaeo leakage of oil from currently dry structures in the haltenbanken vest area the existence of fluorescent oil type inclusions in quartz in the smorbukk asgard 2 field suggest that oil migrated into this structure 70 50 million years before present ma bp this is also the case for the dry structures 6506 12 4 6506 11 3 and 6506 11 1 west of the main smorbukk fault zone black oil inclusions with medium gas oil ratio gor occur in these fields together with condensate type petroleum inclusions this suggests that the dry structures transformed from containing oil to condensate before leakage petroleum extracted from inclusions in these structures and in nearby fields have identical marine type ii kerogen signatures source rocks at the spekk formation level in the current drainage area of smorbukk and these dry structures were immature 70 50 ma bp and the smorbukk sor asgard 3 field did not fill at this early time thus oil must initially have entered into smorbukk from areas to the w sw through the currently pressure sealing smorbukk fault zone which today marks the westward limit of the smorbukk field diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506 12 4 6506 11 3 and 6506 11 1 structures as overpressure built up regionally petroleum loss from these structures with their often thick seals must have occurred via self propagating open fracture induced mechanisms lack of petroleum in the cretaceous strata above these structures suggests that leakage occurred to even shallower strata this could imply that the cretaceous strata in halten vest were overpressured at the time of leakage in contrast the oil in the cretaceous lysing and lange formation above the jurassic reservoirs in smorbukk and smorbukk sor most likely originated based on geochemistry and gors from the jurassic reservoirs below and not from cretaceous strata this migration event would have been facilitated if it occurred before these sands became overpressured as they are today modelling suggests that the spekk formation became mature in the smorbukk sor region

permeability and melt flow in simulated and natural partially molten basaltic magmas abstract a lattice boltzmann method is used to calculate grain scale low reynolds number single phase interstitial porous melt flow in two independent igneous microstructures 1 a microstructure simulated using a stochastic algorithm for progressive crystallization and 2 a natural microstructure obtained through x ray computed tomography ct of a partially melted basalt in both cases the error in the calculated flow field due to finite discretization and domain size has been estimated or demonstrated to be insignificant visually the porous melt flow tends to localize into high flux channels especially with increasing crystallinity and this impression is quantitatively confirmed by increasing skewness of the velocity distribution in the direction parallel to the imposed pressure gradient driving the flow a change from uniform to localized melt flow in naturally occurring situations may have a profound effect on the distribution of trace elements and if the melt is reactive the chemical and structural evolution of the igneous microstructure through localized phase change permeabilities of both microstructures are determined from the calculated steady flow field the permeabilities are then fitted to two different correlation models which are based on the rumpf gupte and carman kozeny relations in these models we express permeability as functions of melt fraction and either the mean crystal length or specific surface area extrapolated permeability to higher melt fractions using both correlation models for the partially melted basalt is shown to be within a factor of four of experimentally determined permeability of a similar sample we determine permeability estimates in the relatively unexplored melt fraction range of 30 80 and find consistency with previous work for permeability estimates in the melt fraction range of 20 30

microcodium revisited root calcification products of terrestrial plants on carbonate rich substrates abstract microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the cretaceous and tertiary continental and marine successions of the peri tethyan realm the main controversy about the microcodium structures is their origin and possible relation with calcified plant roots microcodium and rhizogenic root formed microfabrics were studied in calcrete profiles within the paleocene shallow marine carbonate succession in southwestern slovenia the prominent laminar calcrete horizons contain abundant calcite aggregates 150 m to 1 cm in size with perfectly preserved structural details of plant root tissues morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells the morphologies intermediate between the typical microcodium aggregates composed of a single layer of individual elongate pyramidal or prismatic crystals of calcite measuring 100 500 m in length and 20 70 m in width and calcified roots with multilayer arrangement of isodiametric cells were observed and this supports previous rhizogenic interpretations of microcodium structures intermediate forms show that the typical microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells which distorted the cell shape calcification of roots and the creation of microcodium structures can be explained as an effective nutrient acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient poor calcareous substrates the widespread occurrence of microcodium in almost unaltered shallow marine limestones indicates that its formation took place during early stages of paleosol development probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure

dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic carbonate sediments northwestern qaidam basin northwest china abstract petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata with reservoir quality being a key the formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated however the mechanism of mixed siliciclastic carbonate reservoirs has not been well studied in this paper we address this issue using a case study in the northwestern qaidam basin northwest china n1 e1 2 age 2500 4500 m dissolution pores and fractures dominate the reservoirs however only the fractures were the focus of previous studies thus here we investigate dissolution and its impacts on reservoir formation providing a complementary understanding of the reservoir formation in the basin using core observations and examining thin section it was discovered that dissolution was both random and followed bedding the dissolved components primarily included calcareous and gypsum minerals with fingerprints that are characteristic of burial dissolution further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation the fluids passed through faults and fractures dissolution pores were an important component of the reservoir providing approximately 60 of the porosity in addition segments of high porosity generally above 5 are associated with dissolution based on these observations a schematic model was established to explain the impacts of dissolution on reservoir formation specifically organic acidic formation fluids enter tectonic fractures resulting in dissolution as they pass through them this dissolution enhances the size of the pore space and the reservoir properties of the rocks eventually developing a fracture dissolution reservoir

mechanisms of compaction of quartz sand at diagenetic conditions the relative contribution of cracking grain rearrangement and pressure solution during experimental compaction of quartz sand at diagenetic conditions was determined through electron and optical microscopy and image analysis aggregates of st peter sand 255 60 m diameter grain size and porosity of approximately 34 were subjected to creep compaction at effective pressures of 15 34 5 70 and 105 mpa temperatures of 22 and 150 c nominally dry or water saturated pore fluid pressure of 12 5 mpa conditions and for times up to one year all aggregates displayed transient decelerating creep and volume strain rates as low as 2 10 10 s 1 were achieved the intensity of fracturing and degree of fragmentation increase with volume strain and have the same dependence on volume strain at all conditions tested indicating that impingement fracturing and grain rearrangement were the main mechanisms of compaction throughout the creep phase the increase in fracture density and decrease in acoustic emission rate at long times under wet conditions reflect an increase in the contribution of subcritical cracking no quantitative evidence of significant pressure solution was found even for long term creep at 150 c and water saturated conditions comparison of our findings to previous work suggests that pressure solution could become significant at temperatures or times somewhat greater than investigated here

evaluating quartz crystallographic preferred orientations and the role of deformation partitioning using ebsd and fabric analyser techniques quartz crystallographic preferred orientations cpo from three distinct orthogneisses using both the electron back scatter diffraction ebsd and fabric analyser fa techniques reveal a clear trend from basal and rhomb slip for high p t conditions 670 20 degrees c 9 kbar rhomb and basal slip for medium p t 590 15 degrees c 6 kbar and a dominance of prism slip for lower p t conditions 570 degrees c 4 5 kbar the textural variations are interpreted in terms of a temperature field gradient and microscale strain partitioning controlled by a weak feldspar matrix that can locally invert the expected slip system sequences locally quartz cpus are different within one thin section and in comparison to bulk orientation measurements both ebsd and the fabric analyser techniques are ideal to determine such textural heterogeneities while the ebsd is a powerful technique to determine the full cpo measurements from similar locations inside several quartz grains from the orthogneisses and an annealed and undeformed quartzite show that the fa is an accurate tool to determine cpus of c axis orientations in uniaxial materials in a cpo focussed study the fa is a cheap alternative to ebsd as the analysis of whole thin section can be accomplished in a very short time with minimal sample preparation with the fa it is possible to evaluate the cpus of several samples quickly with an accuracy that allows identification of the main slip systems and their homogeneity

stylolite interfaces and surrounding matrix material nature and role of heterogeneities in roughness and microstructural development rough pressure solution interfaces like stylolites are one of the most evident features of localized slow deformation in rocks of the upper crust there is a general consensus that the development of these rough structures is a result of localized stress enhanced dissolution of material along a fluid filled interface but little is known on the initiation of this roughness the aim of this article is to reveal the role of heterogeneities initially present in the host rock on roughness initiation this should give insights on whether stylolite roughness is generated by a stress induced instability or by the presence of disorder in the material i e quenched noise we use a microstructural approach based on sem ebsd analysis combined with orientation contrast oc image analysis of stylolites in limestones we found that the stylolite roughness is induced by heterogeneities in the host rock clay particles and detrital quartz grains in our case in addition close to mature stylolite interfaces matrix modifications occur which can be attributed to the compaction along the stylolite the grain size decreases by 15 25 and a pre existing shape and lattice preferred orientation spo lpo are significantly modified in the vicinity of the stylolite the results presented here imply that localized pressure solution along stylolites is not necessarily restricted to the actual interface but influences the adjacent matrix the heterogeneity data might serve as a quantitative basis for elaborate numerical models of localized compaction

the effect of dauphine twinning on plastic strain in quartz we present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies t 300 400 c granitoid protomylonite from the arolla unit of the nw alps mechanical dauphine twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane z almost orthogonal to the compression direction z twin orientation twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb r nearly parallel to the compression direction r twin orientation in contrast we report the lack of twinning in two porphyroclasts already oriented with one of the r planes orthogonal to the compression direction one twinned porphyroclast has been investigated with more detail it shows the localization of much of the plastic strain into discrete r twins as a consequence of the higher amount of elastic strain energy stored by r twins in comparison to z twins the presence of dauphine twins induced a switch in the dominant active slip systems during plastic deformation from basal regions without twinning to and pervasively twinned regions dynamic recrystallization is localized along an r twin and occurred dominantly by progressive subgrain rotation with a local component of bulging recrystallization part of the recrystallized grains underwent rigid body rotation approximately about the bulk vorticity axis which accounts for the development of large misorientation angles the recrystallized grain size piezometer for quartz yields differential stress of 100 mpa the comparison of this palaeostress estimate with literature data suggests that mechanical dauphine twinning could have a potential use as palaeopiezometer in quartz bearing rocks

calciclastic submarine fans an integrated overview abstract calciclastic submarine fans are rare in the stratigraphic record and no bona fide present day analogue has been described to date possibly because of that and although calciclastic submarine fans have long intrigued deep water carbonate sedimentologists they have largely been overlooked by the academic and industrial communities to fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans thus offering an updated view of this type of carbonate slope sedimentary system calciclastic submarine fans range in length from just a few to more than 100 km three different types can be distinguished 1 coarse grained small sized most calciclastic submarine fans were formed on low angle slopes and were sourced from distally steepened carbonate ramps subjected to high energy currents under these conditions shallow water loose grainy sediments were transferred to the ramp slope and eventually funnelled into the submarine fan by sediment gravity flows these conditions seem to have been more easily met on leeward margins in which the formation of reefs was hampered by cool waters nutrient enrichment or oligophoty another circumstance that contributes to the transfer of shallow water sediments to the distal ramp slope is a low sea level forcing the carbonate factory closer to the slope break and destabilizing sediments by increased pore water pressure however the most important factor controlling the development of calciclastic submarine fans was the existence of an efficient funnelling mechanism forcing sediment gravity flows to merge downslope and build up a point sourced sedimentary accumulation in most cases this occurred through a major slope depression associated with tectonic structures an inherited topography or large scale mass failures

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

origin of pegmatitic segregation veins within flood basalts subhorizontal veins and lenses of coarse grained rock composed of plagioclase augite and fe ti oxides in a glassy and vesicular mesostasis occur within several thick subaerial basalt flows in the columbia river basalt province of washington and in the eastern north american mesozoic basalt province the veins and lenses referred to as pegmatitic segregation veins are typically 1 10 cm thick and are enriched in fe ti k p cu zr ba and ree by a factor of about 1 1 to 3 over the host basalt al ca sr mg ni and cr are lower and si and na are typically unchanged we interpret the pegmatitic segregation veins as the product of residual melt carried in vapor bubbles from the lower crystallization front of partially crystallized flows to the crystal mush within the upper crystallization front movement through the elevated temperatures of flow interiors fused any crystal nuclei in the segregation melt diminished nucleation density during crystallization may partially account for coarse grain sizes in addition diffusion of ions to large rapidly growing skeletal crystals was probably enhanced by the viscosity reduction effect of water enrichment the water content of accumulated segregation melt may have increased whenever the escape of volatiles from partiallymore crystallized flows was temporarily sealed off under a rigid barrier of solidified basalt rupture of the barrier by downward propagation of columnar joints would allow the resumption of effervescence and any remaining partially crystallized segregation melt phase would be degassed and quenched into a glassy and vesicular mesostasis 59 refs 8 figs 2 tabs less

ichnology of prodeltaic hyperpycnite turbidite channel complexes and lobes from the upper cretaceous prairie canyon member of the mancos shale book cliffs utah usa

slump and debris flow dominated upper slope facies in the cretaceous of the norwegian and northern north seas 61 67 n implications for sand distribution a regional sedimentological study of cretaceous sequences in the mid norway region norwegian sea and in the agat region agat field area northern north sea reveals that these sequences were predominantly deposited in an upper continental slope environment by slumps and debris flows examination of nearly 500 m of core from 14 wells shows eight distinct lithofacies facies 1 contorted conglomerate and pebbly sandstone represents deposits of sandy slumps and debris flows possibly in a channel setting facies 2 contorted sandstone is the most widespread and is the product of sandy slumps and debris flows facies 3 contorted mudstone indicates deposition from muddy slumps and debris flows facies 4 rippled sandstone suggests bottom current reworking facies 5 gr ded sandstone represents turbidity current deposits and is very rare facies 6 laminated mudstone is a product of pelagic or hemipelagic deposition facies 7 cross bedded sandstone is indicative of tidal processes and facies 8 laminated sandstone represents delta front and shelf deposits these facies and their association suggest a shelf edge delta to upper slope environment of deposition existing core data document deltaic facies only in the mid norway region the proposed shelf edge delta and upper slope model has important implications for sand distribution 1 this model provides an alternative to the conventional submarine fan model previously applied to these sequences 2 although slump and debris flow emplaced sands are usually discontinuous and unpredictable highly amalgamated slump and debris flow sands may develop thick reservoirs 3 by using the eocene frigg formation as an analog it is predicted that externally mounded seismic facies in the study area may be composed of sandy slumps and debris flows

danian lower selandian microcodium rich calcarenites of the subbetic zone se spain record of nereites ichnofacies in a deep sea base of slope system abstract the olivares formation a lower paleogene unit of the subbetic zone in southern spain is mostly composed of calcarenites comprising 60 90 of resedimented microcodium remains which are intercalated in hemipelagic marls rich in planktonic foraminifera microcodium is formed on the roots of terrestrial plants adapted to live on carbonate rich substrates its occurrence in the olivares formation entailing a massive resedimentation from a subaerially exposed carbonate source area new field observations demonstrate that the unit was deposited at the base of a faulted slope within a graben flanked by massifs of mostly jurassic carbonates while an ichnological analysis shows an abundant and moderately diverse trace fossil assemblage consisting of 30 ichnospecies belonging to 21 ichnogenera the nongraphoglyptid assemblage 24 ichnospecies 16 ichnogenera is dominated by chondrites ophiomorpha planolites and scolicia zoophycos isp is common while cladichnus gordia helminthopsis multina nereites parahaentzscheliana polykampton spirophycus strobilorhaphe and thalassinoides isp are scarce or rare in the graphoglyptid assemblage 6 ichnospecies 5 ichnogenera helminthorhaphe is common scarce and belorhaphe desmograpton paleodictyon and urohelmintoida are scarce rare this trace fossil assemblage with abundant scolicia and nereites and frequent graphoglyptids can be assigned to the deep sea nereites ichnofacies within the nereites ichnofacies the ophiomorpha rudis and paleodictyon subichnofacies are readily differentiated and the nereites subichnofacies probably also occurs pointing to vertical changes in the turbiditic system most likely related to a coeval sea level rise that eventually resulted in the flooding of the emergent microcodium producing carbonate massifs

compaction microstructures in quartz grains and quartz cement in deeply buried reservoir sandstones using combined petrography and ebsd analysis abstract sandstone diagenesis involves porosity loss by mechanical and chemical compaction in this study we have examined relations of diagenesis and microstructures in quartz grains and quartz cement in sandstones from relatively deep burial depths offshore mid norway electron backscatter diffraction analysis ebsd has been combined with optical and cathodoluminescence petrography study of samples with different degrees of compaction and quartz cementation quartz cement is shown to be syntaxial to the nearest host domain grain or subgrain both for monogranular polygranular undeformed and deformed quartz the quartz cement growth may have been initiated at different subgrain surfaces and preserving pre depositional deformation structures ebsd inverse pole figure imaging shows dauphine twins to be common in all samples both in quartz grains and quartz cement the dauphine twins appear in grain grain contacts and in cement crystal boundaries and commonly crossing grain cement boundaries on basis of twin distributions we suggest that both inherited twins from the source area and twins formed by compaction induced grain boundary deformation are present a general problem in sedimentary rocks is to distinguish inherited from sedimentary compaction induced intra grain deformation structures this study shows that the role of ductile deformation mechanisms e g twinning in sediment compaction can be clarified by more comprehensive following up studies using ebsd analysis in combination with other techniques

testing ethological hypotheses of the trace fossil zoophycos based on quaternary material from the greenland and norwegian seas abstract despite the fact that the trace fossil zoophycos has been found in rocks from most of the phanerozoic little consensus has been reached on how and why this intricate burrow was constructed the cenozoic morphotypes of zoophycos typically include a helically coiled spreite arranged around a vertical shaft connected to the sediment surface morphological details and environmental preferences of the zoophycos producer were studied using 156 cores from the norwegian and greenland seas in order to test the different ethological hypothesis proposed for this trace fossil the spreiten were constructed during interglacial or interstadial intervals and consist of a repeated alternation of lamellae consisting of fine grained pelleted material and coarse grained unpelleted material respectively spreiten were encountered in vigorously bioturbated sediment in turbidite layers and in layers dominated by coarse ice rafted debris this indifference to the composition of the substrate effectively rules out ethological models based on different forms of deposit feeding and the large size and wide spacing of the whorls of the spreiten also make the cesspit model unlikely rather the observed features best agree with a cache behaviour where the main purpose of the deep penetration was to store food and to prevent access by other burrowers likely combined with some gardening of microbes however no indubitable evidence of a reworking of the cached material could be found and probably the answer to how the cache was accessed by the producer is to be found in the marginal tube the difference in diameter between marginal tube and spreiten lamellae together with the presence of both open and filled marginal tubes indicate that the marginal tube is the result of a far more complex behaviour than simply a lateral shift through the sediment

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself