the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

diagenesis of the lower eocene thebes formation gebel rewagen area eastern desert egypt abstract the diagenesis of lower eocene shallow water carbonates with flint was studied in the gebel rewagen area eastern desert egypt the carbonates are mainly wackestones to packstones with benthic bioclasts embedded in a dark red luminescent micrite matrix the studied succession displays a complex diagenetic history that involves syngenetic and late diagenetic processes silica which exists either as persistent bands nodules and or silicified benthic bioclasts shows a distinctive pattern regarding its distribution source depositional environments and timing three lines of evidence support a syngenetic origin of the chert bands 1 they alternate in a cyclic manner within the host carbonates and 2 they exhibit noticeable lateral persistence throughout the investigated area following the strata boundaries and 3 there is a lack of any carbonate dissolution in limestone adjacent to chert bands the deposition of silica bands in association with shallow water carbonates is possibly related to eustatic sea level changes which were accompanied by episodic variations in silica and carbonate productivities with a relative sea level fall and the establishment of a lowstand period at the end of the early eocene a basinward shift of the groundwater zones is expected within the carbonate platform during this period some late diagenetic processes took place which involve 1 the formation of siliceous and carbonate concretionary growths 2 partial silicification of bioclasts 3 neomorphic stabilization of the caco3 bioclasts and 4 the formation of equant calcite cement siliceous and carbonate concretions are believed to have taken place within microenvironments created and controlled by sulphate reducing bacteria and physico chemical and kinetic factors near a marine meteoric water mixing zone this is inferred from the distribution of iron sulphides the non ferroan nature of all concretions and the depleted 13c 5 4 to 6 0 pdb and 18o 5 8 to 6 8 pdb values of the carbonate concretions the silica fabrics of all the partly silicified benthic bioclasts with abundant pyrite cubes argue for a delayed silicification process within the sulphate reduction zone the close and relatively depleted 18o values of the low mg calcite micrite matrix 3 0 to 4 4 pdb and benthic bioclasts 3 4 to 4 8 pdb indicate that both matrix and bioclasts have probably suffered neomorphic stabilization from freshwater dominated solutions the initial marine 13c values of both low mg calcite matrix 0 2 to 2 2 pdb and altered benthic bioclasts 0 2 to 0 9 pdb are retained and not affected by diagenesis the luminescence character and the extremely depleted oxygen values 8 3 to 10 1 pdb of the equant void filling calcite mosaics are consistent with their formation within a meteoric phreatic realm

quartz cementation in late cretaceous mudstones northern north sea changes in rock properties due to dissolution of smectite and precipitation of micro quartz crystals abstract late cretaceous mudstones from two wells located in the northern north sea and the norwegian sea have been examined with respect to quartz cement two different types of quartz cement type 1 and type 2 have been identified using sem eds cl analysis of drill bit cuttings at depths 2370 2670 m 80 85 c type 1 appears as relatively large aggregates 30 100 m of depth temperature related crypto or microcrystalline to macrocrystalline irregular quartz cement formed by local re crystallization of biogenic silica the cl responses of type 1 quartz cement give a clear indication of an authigenic origin type 2 quartz cement represents relatively high amounts of extremely fine grained micro sized 1 3 m crystals embedded as discrete short chains or small clusters nests within the illitized clay matrix the cl responses of micro quartz crystals indicate an authigenic origin the micro quartz is most probably sourced from silica released during the smectite to illite dissolution precipitation reaction the petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones the silica released produce a subtle inter connected micro quartz network interlocked with aggregates of micro quartz and authigenic clay crystals this micro quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime this is indicated by an abrupt increase in well log velocity vp and change in seismic facies close to 2500 m 80 85 c

morphological development of basin scale silica diagenetic fronts revealed with 2d seismic reflection data offshore sakhalin russian far east two dimensional 2d seismic reflection data reveal two silica diagenetic boundaries located in the north sakhalin basin offshore sakhalin island these boundaries form distinctive high amplitude seismic reflections that predominantly cross cut miocene stratigraphy the boundaries represent the opal a to opal ct cristobalite and tridymite transition and the subsequent deeper opal ct to quartz transition and cover an area of c 107 000 km 2 the reflections represent diagenetic reaction fronts that advance through silica rich host sediments during burial the oldest strata within which the diagenetic fronts have been identified are of miocene age and the fronts commonly show a parallel relationship to a late miocene unconformity these diagenetic fronts develop a variety of morphological forms in the north sakhalin basin these front geometries are probably the result of the interference of the diagenetic fronts with inclined and folded stratigraphy together with lateral changes in the composition and therefore depth of transformation of the sediment we define criteria for the recognition of these 2d cross sectional morphological elements and by so doing provide the basis for the objective description and categorization of diagenetic front geometry

monterey formation porcelanite reservoirs of the elk hills field kern county california oil and gas production from monterey formation porcelanite at the elk hills field in california s san joaquin basin occurs from intervals that have quartz phase mineralogy however characteristics differ from chert and porcelanite reservoirs of the coastal california monterey formation in that matrix porosity is more typical of the opal ct phase petroleum storage is mostly in the matrix and natural fracture patterns are dominantly small scale several elk hills reservoirs located on two large anticlines produce from porcelanite the 29r ab and 31s d are the most productive porcelanite reservoirs each having cumulative oil production of about 40 million bbl although interbedded with siliceous shale sandstone and dolomite most of the porous reservoir rock is laminated porcelanite porosity averages between 20 and 25 and is evenly distributed throughout the porcelanite as extremely small pores ranging in size from 1 to 10 m matrix permeability averages 0 8 md but flow of oil and gas is enhanced through fractures parallel with and perpendicular to bedding higher than anticipated porosity may be in part due to migration of hydrocarbons into the porcelanite reservoirs while still in opal ct phase mineralogy the dissolution of opal ct and precipitation of quartz occurs in place and the resulting quartz phase mineral structure mimicks the porous opal ct framework

accelerated transformation of organic matter below the silica transition zone in immature sediments from the japan sea abstract notable compositional changes of organic matter are observed below the silica transition zone in thermally immature sediments the increase of bitumen ratio and hopane and sterane isomerization parameters indicate an acceleration of the kinetics of the chemical reactions which transform the organic matter this phenomenon is probably due to the numerous mineral and textural changes induced by the transformation of amorphous biogenic silica into crystalline authigenic silica

depositional and diagenetic signatures of late eocene oligocene sediments south carolina abstract surficial and near surface soils of the south carolina coastal plain reflect a variety of lithologies and depositional environments that are difficult to differentiate because of intense leaching and abrupt or laterally inconsistent facies changes binocular microscopic examination scanning electron microscopic energy dispersive x ray sem edx observations and x ray diffraction xrd analyses indicate that onshore late eocene to late oligocene barnwell group sediments are transitional facies ranging from high energy fluvial deposits to offshore siliciclastic shelf sands interfingering of the units results in alternation of mineralogic signatures within a low gradient fluvial transitional marine depositional system late eocene and early oligocene offshore sediments were deposited in a mixed carbonate siliciclastic middle to outer shelf environment that was subjected to periods of erosion or non deposition during transgressive events detrital and diagenetic characteristics of the onshore kaolinite enriched late oligocene upland unit sediments reflect deposition in a high to low energy fluvial system differentiation between these uppermost sediments and the underlying low energy fluvial deposits of the late eocene tobacco road sand is based on distinctive hydroxy interlayered vermiculite hiv signatures intervals of hiv enrichment are coincident with accumulations of carbonaceous material and identified as paleosols these soils are used to infer offshore transgressive periods onshore sediments of the late eocene dry branch formation contain high concentrations of smectite and flocculated relatively poorly crystallized kaolinite flakes reflective of marine depositional conditions at the base of this unit authigenic ca minerals ca zeolites and calcite and quartz lepispheres opal ct form coatings on and between sand grains late eocene siliceous microfossils that contribute to opal ct formation are identified in southwestern north atlantic continental margin shelf slope and rise deposits and are a valuable correlation tool for onshore sediments

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

a numerical study of the genesis of extratropical convective mesovortices part i evolution and dynamics abstract the purpose of this study is to understand the genesis of extratropical convective mesovortices and the large scale environmental features that influence the vortex formation a hypothesis is proposed that mesovortices form in the stratiform rain regions of mesoscale convective systems mcss due to the reduction of static stability that reduces the effective local rossby radius in such regions a conceptual model of the mesoscale convective cyclogenesis is introduced which describes the three stages of the mesovortex formation a modified version of the pennsylvania state university national center for atmospheric research three dimensional hydrostatic mesoscale model is used to simulate mesovortex genesis in analytically generated pre mcs large scale environments the model simultaneously incorporates parameterized convection and a grid resolvable convective scheme containing the effects of hydrostatic water loading condensation evaporation freezing melting and sublimation a control si

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

fossilisation of a silica diagenesis reaction front on the mid norwegian margin the diagenetic transformation of biogenic silica from opal a to opal ct was recognised on seismic reflection data over an area of 78 103 km2 on the mid norwegian margin the opal a ct diagenetic boundary appears as a positive high amplitude reflection that generally cross cuts the hosting stratigraphy we demonstrate that it is not a sea bottom simulating reflection bsr and also that is not in thermal equilibrium with the present day isotherms we present arguments that three styles of deformation associated with the opal a ct reflection polygonal faulting regional anticlines and synclines and differential compaction folding indicate that the silica diagenesis reaction front is fossilised at a regional scale isochore maps demonstrate the degree of conformity between the opal a ct reflection and three seismic horizons of late miocene to early pliocene age that potentially represent the paleo seabed when fossilisation of the reaction front took place the seismic interpretational criteria for recognition of a fossilised diagenetic front are evaluated and the results of our study are integrated with previous studies from other basins of the ne atlantic in order to determine if the arrest of silica diagenesis was diachronous along this continental margin

formation of analcime and moganite at lake lewis central australia significance of groundwater evolution in diagenesis abstract lake lewis basin in the northern territory is a hydrologically closed intermontane basin in which clay rich palaeolacustrine sediments are infused with highly evolved brines acquisition of dissolved silica and sodium by groundwaters from the proterozoic arunta granite gneiss terrain and from arkosic alluvium and weathering products sourced from these bedrocks is important to diagenesis at the salt lake supersaturation of groundwaters with respect to sio 2 and the availability of catalytic ions coupled with high rates of evaporation favour neoformation of opal and chalcedonic silica from solution near the groundwater discharge zone moganite a distinctive silica polymorph that is commonly associated with evaporites has been identified amongst precipitated silica species down gradient from the zone of silica precipitation beneath the playa brines are sio 2 deficient long periods of weathering and diagenesis of lacustrine clays immersed in semi stagnant brines has transformed detrital minerals into substantial proportions of amorphous or gelatinous aluminosilicate material aluminosilicate phases have become metastable in the presence of evolved sio 2 deficient interstitial brine analcime a sodium bearing zeolite group mineral is stable in the presence of the na rich brine and is crystallizing authigenically below the watertable from the amorphous aluminosilicate material in the lacustrine sediment no volcanic rocks or pyroclastic sediments are involved in this diagenetic occurrence of analcime the appearance of analcime in the evolution of the salt lake system probably commenced within the last 100 000 years and is occurring in the present day hydrologic and climatic regime of this arid zone area

porosity reduction during diagenesis of diatomaceous rocks with lithification and burial diatomaceous rock transforms from opal a to opal ct and then quartz this study documents two different patterns of porosity reduction and their associated mineralogical and density changes using samples from the monterey formation california samples that have one set of properties are called group 1 and those showing other trends are assigned to group 2

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

springs and streams of the taro ceno valleys northern apennine italy reaction path modeling of waters interacting with serpentinized ultramafic rocks article i nfo in the area of the taro ceno valleys northern apennine emilia romagna region italy waters of meteoric origin interact with ophiolite rocks of the external ligurides fresh water springs issuing from basalts have a ca hco3 composition whereas freshwater springs from ultramafites vary in composition from ca hco3 or mg hco3 to na oh or na so4 types and in ph values from 7 3 8 8 up to ph 11 respectively in addition the boron content of the alkaline waters is up to 13 mg l which is unusually high for freshwaters in general and ultramafites that have undergone oceanic serpentinisation in particular and gives a boric alkalinity to the waters the springs waters show evidence of recent low temperature continental serpentinisation and the process is modeled by reaction paths using an updated geochemical thermodynamic database consistent with the local primary and secondary serpentinite paragenesis for the model bicarbonate waters evolve to alkaline waters supersaturated in ca mg carbonate based on the assumption that the dissolution of serpentinite results in supersaturation with respect to kaolinite ferrihydrite vermiculite fe 2 mg 2 saponite and poorly crystalline serpentine the alkaline composition and the chloride content of the waters suggest a prolonged interaction with the rocks at depth that led to dissolution of albite and leaching of olivine hosted fluid inclusions a similar evolution is also proposed for the more developed springs issuing from the ultramafic rocks of the voltri group liguria region where solutions are supersaturated in brucite and are in equilibrium with enstatite and or chlorite

effect of sedimentary environment on the formation of organic rich marine shale insights from major trace elements and shale composition abstract sedimentary environment and redox conditions play a significant role in the formation of organic rich shale the vertical variations of major trace elements toc total organic carbon and mineral compositions were investigated for the lower silurian longmaxi typical marine shale in the upper yangtze platform south china to decipher sedimentary environment redox conditions detrital input sedimentation rate sr paleoproductivity and watermass restriction this gives insight to the effects of variations in sedimentary environment on the formation of organic rich shale the results show that the lower longmaxi formation is composed of three systems tracts tst transgressive systems tract ehst early highstand systems tract and lhst late highstand systems tract from tst to lhst the contents of toc quartz and pyrite decline steadily whereas clay minerals and carbonate show an increasing trend thus organic rich siliceous orss organic moderate mixed omms and organic lean argillaceous olas shales are dominant for tst ehst and lhst respectively due to a continues fall of the relative sea level a deep water shelf environment with anoxic condition high paleoproductivity and low detrital input in tst gradually evolves into a semi deep water environment with dysoxic oxic conditions moderate paleoproductivity and moderate detrital influx in ehst further to an environment of oxic condition low paleoproductivity and high detrital flux in lhst as evidenced by mo toc and mo u patterns orss was deposited in a moderately restricted watermass weaker than black sea which progressively evolved into an enhanced restriction degree during the deposition of omms and olas due to a fall of relative sea level major trace elements and excess silica ex sio2 concentrations illustrate that origins of quartz in the lower longmaxi shale are primarily terrigenous biogenic and authigenic smectite illitization the average proportions of ex sio2 biogenic and authigenic quartz are 52 34 24 86 and 6 57 for orss omms and olas respectively biogenic quartz is the dominant contributor to ex sio2 while the contribution of authigenic quartz is limited and the formation of the former is earlier than the latter according to the diagenetic transformation temperature as both biogenic quartz and organic matter are firmly related to the abundance and preservation of paleoorganisms anoxic condition high paleoproductivity low detrital flux and moderate restriction collectively control the high enrichment and preservation of organic matter and biogenic quartz while oxic condition low paleoproductivity high detrital flux and stronger watermass restriction are unfavorable for the enrichment of biogenic quartz and organic matter

on the relationship between silica and carbonate diagenesis in deep sea sediments silica diagenesis and carbonate diagenesis are interrelated this is confirmed by observations of dsdp sites 462 463 465 466 and 577 carbonate sediments containing chert 1 tend to be more indurated and display more advanced diagenetic alterations regardless of sub bottom depth and 2 microfossil components are more strongly affected overgrown and or dissolved while the amount of micritic particles and larger euhedral calcite crystals is greater in addition mass physical properties porosity in particular vary more widely in sediment sections containing chert furthermore in the studied similarly composed sediments recrystallization of biogenic opal is indicated by a significant reduction of the specific surface area reaching a minimum value when quartz is formed

different formation mechanism of quartz in siliceous and argillaceous shales a case study of longmaxi formation in south china abstract brittle mineral especially quartz exert significant effect on the development of shale gas however the formation mechanism of quartz in shale is still unclear here we present x ray diffraction data scanning electron microscopy sem determination cathodoluminescence and geochemical data to investigate longmaxi formation which is a key target for shale gas exploration and development in china recently siliceous and argillaceous mudstones were recognized from 5 outcrops of longmaxi formation which were examined with respect to the formation mechamism of quartz three different types of quartz silt size detrital grain with bright luminescence micro sized grains and quartz overgrowths with bright luminescence were identified by sem eds cl both in the argillaceous and siliceous shale siliceous shale deposited by suspension sediment settling with high sea level because of anoxia condition indicated by lower th u and higher v cr ratios the quartz in siliceous shale display a positive covariation with babio as well as a good correlation between toc and quartz suggesting biogenic quartz is an important constituents argillaceous shale was likely dominated by suspension alternate with gravity current in consideration of the mixing of clay and silt laminae there is no correlation between quartz and toc and positive correlation between excess silica and zr indicating terrestrial quartz in argillaceous is more important relative to siliceous shale additionally reorientation of phyllosilicate particles and positive correlation between ki and quartz contents r2 0 57 suggest the indispensable diagenetic quartz illitization of smectite near or far from the clay minerals both in siliceous and argillaceous shales this process also result in high quartz saturation in silt rich laminae thus that quartz overgrowths encased in dolomite cements is common in argillaceous shale based on the geochemical analysis the argillaceous shale contains 86 detrital silica 14 digenetic silica however the siliceous shale approximately contains 33 detrital silica 11 digenetic silica and 56 biogenic silica occurrence and origin of quartz are of great significance for physical properties of shale and hydraulic fracturing during the development of shale gas

fabric development in proterozoic bedded chert penganga group adilabad india sedimentologic implications laterally persistent horizons of interstratified chert limestone manganese ores within a siliceous micritic limestone are interpreted to have been deposited on a distally steepened deep carbonate ramp in the proterozoic penganga group of the pranhita godavari valley south india the association of intraformational limeclast conglomerate turbidites and pelagic micritic limestone indicates deposition of the chert bearing horizons at the toe of a distally steepened ramp slope environment the chert is characterized by a wide variety of fabrics including cryptocrystalline and microcrystalline quartz equant megaquartz and chalcedony cryptocrystalline and microcrystalline quartz are most common they occur mainly as lenses of mosaic quartz and define laminae megaquartz in contrast occurs as irregular patches as fenestroids within cryptocrystalline or microcrystalline fabrics or in complex aggregates and disrupted mosaics the origin of the cryptocrystalline and microcrystalline quartz by maturation parallels the morphological evolution of many deep sea cherts that form by maturation of biogenic opal a to quartz chert through an opal ct stage within the quartz stage the microcrystalline fabric formed by pervasive space centered grain growth of cryptocrystalline crystals and porphyroid growth resulted in porphyrotopic fabric the megaquartz formed directly from pore water in continuity with maturation of opal ct the disrupted and curdled mosaics attest to high fluid activity within semilithified sediments the variation in fabric types at the microscale suggests fabric evolution at low temperature the presence of chert clasts in debris flow conglomerates and development of a plethora of water escape structures suggest maturation of silica at shallow burial depth maturation at shallow burial was favored by very low detrital content of these cherts and a mg rich pelagic carbonate depositional geochemical milieu estimated sedimentation rates from 2 to 10 mm ky super 1 closely match the average sedimentation rate from modern pelagic siliceous deposits

geographic changes in the aegean sea since the last glacial maximum postulating biogeographic effects of sea level rise on islands abstract in order to assess how the last sea level rise affected the aegean archipelago we quantified the magnitude and rate of geographic change for the aegean islands during the last sea level rise episode 21 kyr bp present with a spatially explicit geophysical model an island specific area distance change adc typology was constructed with higher adc values representing a higher degree of change the highest fragmentation rates of the aegean archipelago occurred in tandem with the largest rates of sea level rise occurring between 17 kyr and 7 kyr ago sea level rise resulted in an area loss for the aegean archipelago of approximately 70 spatiotemporal differences in sea level changes across the aegean sea and irregular bathymetry produced a variety of island surface area loss responses with area losses ranging from 20 to 90 per island in addition sea level rise led to increased island isolation increasing distances of islands to continents to 200 for some islands we discuss how rates of area contractions and distance increases may have affected biotas their evolutionary history and genetics five testable hypotheses are proposed to guide future research we hypothesize that islands with higher adc values will exhibit higher degrees of community hyper saturation more local extinctions larger genetic bottlenecks higher genetic diversity within species pools more endemics and shared species on continental fragments and higher z values of the power law species area relationship the developed typology and the quantified geographic response to sea level rise of continental islands as in the aegean sea present an ideal research framework to test biogeographic and evolutionary hypotheses assessing the role of rates of area and distance change affecting biota

identification and tectonic implications of nano particle quartz 50 nm by synchrotron x ray diffraction in the chelungpu fault gouge taiwan abstract to determine the ultrafine nano scale grains in a fault gouge the chelungpu fault gouge was sampled and analyzed it was separated into different particle size ranges and was analyzed by synchrotron x ray diffraction and transmission electron microscopy the minerals of gouge are predominantly composed of quartz plagioclase smectite illite chlorite and kaolinite the mineral association of

sediment with porous grains rock physics model and application to marine carbonate and opal we offer an effective medium model for estimating the elastic properties of high porosity marine calcareous sediment and diatomite this model treats sediment as a pack of porous elastic grains the effective elastic moduli of the porous grains are calculated using the differential effective medium dem model whereby the intragranular ellipsoidal inclusions have a fixed aspect ratio and are filled with seawater then the elastic moduli of a pack of these spherical grains are calculated using a modified scaled to the critical porosity upper hashin shtrikman bound above the critical porosity and modified lower carbonates and upper opal hashin shtrikman bounds below the critical porosity the best match between the model predicted compressional and shear wave velocities and ocean drilling program odp data from three wells is achieved when the aspect ratio of intragranular pores is 0 5 this model assigns finite nonzero values to the shear modulus of high porosity marine sediment unlike the suspe

the hot spring and geyser sinters of el tatio northern chile abstract the siliceous sinter deposits of el tatio geothermal field in northern chile have been examined petrographically and mineralogically these sinters consist of amorphous silica opal a deposited around hot springs and geysers from nearly neutral silica saturated sodium chloride waters water cooling and evaporation to dryness are the main processes that control the opal a deposition in both subaqueous and subaerial settings in close spatial relation to microbial communities all fingerprints of organisms observed in the studied sinter samples represent microbes and suggest that the microbial community is moderately diverse cyanobacteria green bacteria and diatoms the most important ecological parameter is the temperature gradient which is closely related to the observed depositional settings 1 geyser setting water temperature 70 86 c boiling point at el tatio 4200 m a s l coarse laminated sinter macrostructure with rapid local variations biota comprises non photosynthetic hyperthermophilic bacteria 2 splash areas around geysers water temperature 60 75 c laminated spicule and column macrostructure locally forming cupolas

textural preservation in siliceous hot spring deposits during early diagenesis examples from yellowstone national park and nevada u s a abstract a petrographic mineralogical and geochemical investigation reveals evidence for selective preservation of microfacies in siliceous sinters plio pleistocene sinters from steamboat springs nevada contain identical but fewer petrographic textures than pleistocene sinters from artist point yellowstone national park wyoming the latter are preserved in altered sediments of unknown origin a sequence of pore filling and mineralogical changes explains the excellent preservation of nine microfacies and two petrographic textures plio pleistocene steamboat springs sinters are preserved in successions of andesitic basalt flows and were likely subjected to a more extreme thermal and a different hydrological environment than those from yellowstone national park most microfacies were obliterated during postdepositional heating of steamboat spring sinters differences in diagenetic histories related to depositional environment water chemistry and or subsequent burial must account for the loss of textural evidence between the diagenetic stage represented by artist point sinters and that of steamboat springs sinters hence early postdepositional history affects both the likelihood and quality of microfossil preservation

diagenesis of pleistocene reef associated sediments from the red sea coastal plain egypt abstract pleistocene reef associated sediments from the red sea coastal plain of egypt consist of carbonate to mixed carbonate siliciclastic sediments the recognized facies are calcareous sandstone skeletal rudstone grainstone wackestone and micritic crusts evidence from cement fabrics aragonite high mg calcite and gypsum suggests that cementation mainly occurred in the intertidal zone under marine vadose conditions however the interaction of these sediments with meteoric waters resulted in the modification and stabilization of the metastable carbonates the replacement of detrital feldspars by carbonates and the silicification of carbonates

primary relationships of basaltic intrusive rocks and siliceous sedimentary rocks in the northern chichibu belt shikoku island southwest japan abstract understanding the primary relationships between basaltic rocks and fossiliferous sedimentary rocks provides direct information on the timing and geologic setting of past oceanic igneous activity though the basalts associate with cherts in the northern chichibu belt have been petrographically identified as oia oit or morb such primary relationships of them are not well understood this paper provides the primary intrusive relation of basaltic rocks contact with chert and calcareous siliceous shale of sugyo area in the northern chichibu belt on shikoku island the sediments are included within the basalt as xenoliths and in the contact zone with the basalt they have been recrystallized and contain metamorphic minerals possibly produced by contact metamorphism due to the basalt intrusion these indicate that the basalts are sills or dykes that have intruded the sediments the cherts are radiolarian chert which alternate with the calcareous siliceous shale and include minerals of oceanic volcanic origins asselian radiolarian fossils are yielded from a subangular chert xenolith that was intruded by the basalt possibly before the chert had undergone complete diagenesis the petrography of the sills indicates they are alkali basalts x ray fluorescence analyses indicate that the basaltic rocks include subalkaline basalts and oceanic island tholeiite or oceanic island alkaline basalts we have concluded that the basaltic dykes and sills were products of asselian or younger intra plate volcanism at an oceanic area covered by a pile of pelagic siliceous sediments these asselian cherts are earlier than the sakmarian wordian and capitanian limestones that overlie intra plate basalts of ohnogahara part of the northern chichibu belt the basalt sills may therefore represent the early stages of intra plate igneous activity in the paleo pacific ocean

sequences of silica phase transitions effects of na mg k al and fe ions abstract hydrothermal experiments on silica phase transitions in artificial solutions tested the effects of dissolved aluminum and iron on the evolution of solution composition and the mineral compositions of solid phases dissolved aluminum and iron come from detrital aluminosilicate phases including their coatings aluminosilicate phases retard formation of opal ct a metastable silica phase but they may promote transformation of opal ct to quartz the stable phase the four starting solutions all had alkalinity greater than seawater either 90 or 8 mm the four solutions were dominated by different cations one solution contained magnesium ion one magnesium and sodium ions one potassium ion and one sodium ion compositions of the solutions changed within 4 weeks quartz formed under a higher initial alkalinity of 90 mm with or without magnesium minor phases including opal ct clay minerals and melanophlogite were also observed in some experiments opal ct formed in the presence of magnesium and an initial alkalinity above 8 mm dissolved aluminum or iron reduced concentrations of dissolved silica causing precipitation of an initially more ordered opal ct in the presence of magnesium and quartz in its absence these effects influence the rates and sequences of silica diagenesis

development of early diagenetic silica and quartz morphologies examples from the siri canyon danish north sea abstract the siri canyon has proved to be a perfect area for investigating various morphologies of diagenetic silica in sandstones the development in silica morphologies can be observed from very shallow 1700 m to increased burial depth 3000 m and increased proximity to the central graben distance from 0 to 65 km hydrocarbons and pore fluids now found in the siri fairway have at least partly originated from the central graben the siri canyon is a submarine canyon system eroded into the uppermost chalk deposits and filled with palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstone units deposited from sandy mass flows and sandy turbidites which originated on the stavanger platform several hydrocarbon exploration and production wells have been drilled in the siri canyon seven of which are included in this study nini 3 nini 1 na 2p sofie 1 siri 4 celilie 1a and augusta 1 the reservoir sandstones in these wells all contain authigenic silica of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy the silica morphologies in some places are classic and well documented in the literature whereas others at least to our knowledge have never previously been described some of the silica morphologies presented here show gradual transition from one to another and others are stand alone forms without clear relationships to other forms the silica morphologies can be expressed in the following way 1 opal rims characteristic of the initial phase of the silica diagenesis in most sandstone units in the siri canyon thick opal rims characterise the sandstone parts adjacent to the mudstone units in the stine segment of the siri field 2 microquartz quartz crystals with a size of 1 5 m seen as coatings on the opal rims both ordered and random 3 cavity overgrowth found as quartz outgrowths in circular and angular cavities formed by dissolution of early authigenic phases angular cavities in the microquartz coatings origin from dissolution of clinoptilolite possibly with a source in volcanic ash whereas dissolution of lepispheres probably resulted in circular cavities 4 quartz ridges syntaxial quartz overgrowths a few micro millimetre in width in parallel lines defined in this paper 5 quartz mountains syntaxial irregular 5 10 m large quartz crystals defined in this paper developed in the water filled sandstone intervals where fluids from the central graben were introduced possibly together with the hydrocarbon and where the continued growth was not retarded by the presence of hydrocarbons in the pore fluids 6 macroquartz syntaxial quartz overgrowths 20 m seen as continued growths from quartz mountains this takes place in the deeper reservoir sandstones promoted by fluids originating from the central graben and increased burial depth 7 quartz with sutured grain contacts occurring only in the deepest well 3000 m

thermogenetic degradation of early zeolite cement an important process for generating anomalously high porosity and permeability in deeply buried sandstone reservoirs abstract diagenetic explanations to anomalously high porosities and permeabilities in deeply buried sandstones have been presented for many reservoirs for example grain coatings of microquartz or chlorite can preserve porosity by preventing succeeding quartz overgrowths and dissolution of grains or cement can generate secondary porosity anomalously high porosity and permeability up to 25 and 500 md respectively characterize the deeply buried 5 km upper jurassic shoreface sandstones of the gert member heno formation in the hejre field in the danish central graben optical cathodoluminescence and scanning electron microscopy were used to characterize the petrography and diagenetic changes that resulted in this high pressure and high temperature hpht reservoir this was supplemented by chemical analyses icp oes and icp ms mineralogical composition xrd and oxygen isotope composition of quartz and calcite cement sims the petrographic investigation shows that none of the known explanations for anomalously high porosity and permeability can be applied hence a new hypothesis for deeply buried sandstones in contact with lavas and volcaniclastic lithologies is proposed the new hypothesis includes reducing mechanical compaction due to early patchy zeolite cement which was sourced from adjacent lavas and volcaniclastic rocks locally stylolites developed where zeolite cement was less effective hence the present overpressure formed at a later stage increased temperatures during burial resulted in thermal instability of the zeolite cement which was dissolved and authigenic k feldspar and quartz precipitated instead this resulted in a reduction of the cement volume of up to 70 if the zeolite was k heulandite and a corresponding relative porosity increase thus a reduction in cement volume was generated by replacing zeolite of an open crystal structure with mineral phases k feldspar and quartz of denser crystal structures the thermogenetic breakdown of zeolite was forced by increased temperatures during burial this process may explain previously enigmatic highly porous and permeable deeply buried sandstone reservoirs

reservoir quality in the jurassic sandstone reservoirs located in the central graben north sea abstract this study investigates the diagenesis and reservoir quality of upper jurassic sandstones of ula formation from the central graben petrophysical and petrographical studies have been done on cored interval from well 2 1 6 of ula field precipitation of quartz cement is the main porosity destroying process in deeply buried quartz rich sandstone reservoirs of the north sea quartz cement precipitates in the form of syntaxial overgrowth over detrital grain of quartz grain coatings like micro quartz and illite are the main reasons for preservation of porosity in sandstones as they cover the grain and inhibit the quartz overgrowth petrographical data in this study clearly indicates that grain coatings are present in the studied samples micro quartz grain coating is the most common grain coat in the upper jurassic sandstones of ula which is generated from the transformation of siliceous sponge spicules known as rhaxella perforata clay grain coats like illite and chlorite are also present relation between intergranular volume igv versus matrix and quartz cementation versus porosity have also been studied igv is strongly affected by mechanical compaction grain size grain shape quartz and carbonate cements sandstones with high amount of matrix and fine grains have high igv as compared to coarse grains because coarse grains are compacted more when they are subjected to mechanical compaction grain shape also has a pronounced effect on porosity angular grains tend to lose porosity easier as they are subjected to stress

tracking crystallinity in siliceous hot spring deposits siliceous hot spring deposits sinters entrap paleoenvironmentally significant components and are used as extreme environment analogs in the search for early earth and extraterrestrial life however sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals for ancient hydrothermal settings including those close to the dawn of life these transformations have long since occurred so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change three young sinters preserve the entire diagenetic sequence of silica phases from opal a to quartz the 6000 to 11 500 years bp 70 years sinter at steamboat springs nevada the 1600 1900 160 years bp opal mound sinter at roosevelt hot springs utah and the 456 35 years bp deposit at sinter island taupo volcanic zone new zealand provide an opportunity to track crystallographic mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches worldwide sinter forms from cooling alkali chloride waters as noncrystalline opal a transforming first into noncrystalline opal a ct then paracrystalline opal ct moganite paracrystalline opal c and eventually to microcrystalline quartz in this study these changes were identified by the novel and combined application of electron backscatter diffraction x ray powder diffraction and scanning electron and optical microscopy techniques we show that mineralogical changes precede morphological and accompanied crystallographic transformations during this modification silica particles grow and shrink several times from the micron to nano meter scales via dissolution reprecipitation and recrystallization and diagenesis follows the ostwald step rule all deposits followed nearly identical diagenetic pathways with time as the only variable in the march toward physicochemically stable quartz crystals diagenesis alters original environmental signatures trapped within sinters after five silica phase changes filamentous microfossils are modified but still remain recognizable within sinter from the opal mound and steamboat springs deposits and during the opal a to opal ct silica phase transformations at sinter island therefore delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot spring deposits

red palaeosols sequence in a semiarid mediterranean environment region in this work we study one of the most palaeopedological sequence formed in central spain which is located on the pliocene pleistocene erosional surface in the madrid basin we also analyse its relationship to erosive and sedimentary pleistocene events in order to obtain new data for a correct interpretation of the origin and evolution of forms at the top of tabular lands in this site the geomorphic features and the properties of a sequence of very red palaeosols that developed on this old surface can help us in the understanding of the palaeoclimatic evolution of central spain in a mediterranean climate they were examined to identify pedologic and climatic changes during the quaternary the soil sequence comprises intercalated palaeoargillic and palaeopetrocalcic horizons the clay minerals are mainly illite kaolinite smectite and sepiolite the alternation of argillic and calcic horizons limestone debris cryoclastic colluvions and aeolian sands suggests succeeding periods of phytostability and phytoinstability biostasis rhexistasis argillation rubification and calcium carbonate accumulation were repeated throughout the pleistocene and it is hypothesised that climatic conditions during numerous stages of this period were not very different from the present conditions

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

the discovery of the barmer basin rajasthan india and its petroleum geology the discovery of oil and gas in the barmer basin in northwest india was one of the more significant global discoveries in the decade 2001 2010 the basin s presence was suspected from gravity and magnetic data in the late 1980s but not confirmed until 1999 from seismic and drilling the basin is a lacustrine failed rift biostratigraphic data however indicate it was intermittently connected to marine waters via either the cambay basin the kutch basin or across the devikot high temporarily forming a large shallow estuary at least six major tectono stratigraphic events have caused relative lake level falls and translation of clastic reservoirs basinward upward of 6 km 20 000 ft of cenozoic and mesozoic sedimentary rocks have been preserved prolific source rocks occur from the mesozoic through eocene strata tectonically the basin is divided into a northern and a southern province the north province continues to undergo inversion and erosion and has not been buried as deeply as the south kinetics of the major source facies in the north are substantially different from those in the south as well as the present day and paleo heat flow these differences have made the northern part of the basin predominantly an oil province and the southern part a mixed oil and gas province the prolific paleocene fatehgarh formation contains the bulk of the 7 3 billion barrels of stock tank oil in place stoip identified to date but other reservoirs from the mesozoic to the late cenozoic are common and may yield significant future resource additions

pleistocene cold climate groundwater silicification jbel ghassoul region missour basin morocco surficial silicifications have been long considered to be indicative of warm and dry climates here we describe various forms of supergene silicification in a miocene lacustrine sequence in the missour basin near jbel ghassoul morocco in a landscape with accentuated relief the silicification is almost exclusively limited to a 10 40 m wide zone from the edges of scarp and mesa landforms this distribution is interpreted to record the locations where groundwaters which produced the silicification discharged from a higher level palaeolandscape the main component of the silica was imported late and significantly post dates the deposition of the sediments this implies that significant volumes of silica bearing solutions flowed through these formations in response to a hydraulic gradient generated by relief silicification thus occurred only after uplift and incision of the sedimentary fill of the missour basin the zones of silicification of the jbel ghassoul sequence can be linked geomorphologically to remnants of high level pediments that have been dated in the literature to the early to middle pleistocene and interpreted to have been formed during cold climates low temperatures in outcrops near the discharge zones during cold periods are considered to be a key factor in silica precipitation from groundwaters

the chemistry of eolian quartz dust and the origin of chert abstract among the numerous models that have been suggested for the primary and predominant source of silica for chert we suggest that eolian dust is worthy of further considerations such considerations are supported by the common association of phanerozoic chert with evaporites limestones dolomites or other strata that were deposited within or near arid paleoclimates this association suggests a direct or indirect causal link between aridity and chert formation in addition eolian processes export large quantities of quartz rich dust from arid climate ergs or loess lands the chemistry of abraded quartz particles derived therefrom is consistent with chert formation abrasion of quartz particles produces an amorphous surface layer and underlying lattice disorder an inverse relation between particle size distribution 64 m and the enthalpy of solution of abraded quartz is indicative of the degree of lattice disorder as particle size decreases the enthalpy of solution increases the amorphous surface layer and underlying lattice disorder enhance both the rate and amount of dissolution of quartz dust particles the solubilized silica can then be reprecipitated and diagenetically altered to the various silica polymorphs that occur in chert an eolian supply and the chemistry of eolian abraded quartz particles may account for chert formation in disparate depositional environments that encompass deep sea chert chert in epicontinental seas including shallow shelves and chert that formed in continental eolinites lakes and soils arid climate silcretes

impact of silica diagenesis on the porosity of fine grained strata an analysis of cenozoic mudstones from the north sea silica diagenesis has the potential to drastically change the physical and fluid flow properties of its host strata and therefore plays a key role in the development of sedimentary basins the specific processes involved in silica diagenesis are however still poorly explained by existing models this knowledge gap is addressed by investigating the effect of silica diagenesis on the porosity of cenozoic mudstones of the north viking graben northern north sea through a multiple linear regression analysis first we identify and quantify the mineralogy of these rocks by scanning electron microscopy and x ray diffraction respectively mineral contents and host rock porosity data inferred from wireline data of two exploration wells are then analyzed by multiple linear regressions this robust statistical analysis reveals that biogenic opal a is a significant control and authigenic opal ct is a minor influence on the porosity of these rocks these results suggest that the initial porosity of siliceous mudstones increases with biogenic opal a production during deposition and that the porosity reduction during opal a ct transformation results from opal a dissolution these findings advance our understanding of compaction dewatering and lithification of siliceous sediments and rocks moreover this study provides a recipe for the derivation of the key controls e g composition on a rock property e g porosity that can be applied to a variety of problems in rock physics

a proposed mechanism for the growth of chalcedony the structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica although the paragenesis of chalcedony has provoked marked disagreement among researchers a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures 100 c these conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short chain linear polymers via bridging silica monomers this assembly occurs through a spiral growth mechanism activated by a screw dislocation withb n 2 110 wheren is an integer the proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale such as 1 the direction of fiber elongation along 110 rather than 001 2 the periodic twisting of chalcedony fibers about 110 3 the high density of brazil twin composition planes 4 the common intergrowth of moganite within chalcedony

integrated biostratigraphy and carbon isotope stratigraphy for the upper cretaceous kanguk formation of the high arctic sverdrup basin canada