silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits an example of meteoric dominated diagenesis from the triassic of central spain cauliflower shaped nodules are widespread in a single red mudstone bed in the buntsandstein triassic facies of the iberian range they consist mostly of quartz dolomite and calcite but other minerals such as barite kaolinite and iron oxyhydroxides are also present the nodules are spherical ovoid or elongate in shape and range from 1 to 8 cm across the surface of the nodules is irregular and some show a pedogenic coating of microspar the sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of ca2 and so4 2 in a vadose environment occasionally under the influence of pedogenic processes partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting as indicated by the presence of organic filaments on the quartz crystal surfaces in type a nodules the dissolution of the innermost anhydrite was complete and different types of quartz cement filled the porosity fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules in type b nodules the dissolution of the anhydrite was incomplete inhibiting quartz cementation and enabling later dolomitization of the anhydrite dolomitization appears to have been driven by sulphate reduction as indicated by the presence of bacterial bodies within the dolomite crystals dedolomitization and precipitation of barite kaolinite and calcite spar cements occurred later under the influence of meteoric solutions the nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions

impact of atmospheric co2 and galactic cosmic radiation on phanerozoic climate change and the marine δ18o record 1 a new model is developed and applied to simulate the phanerozoic evolution of seawater composition dissolved ca sr dissolved inorganic carbon alkalinity ph 18o marine carbonates sr ca 87sr 86sr 13c 18o atmospheric co2 and surface temperature the marine carbonate records sr ca 87sr 86sr 13c are used to reconstruct changes in volcanic tectonic activity and organic carbon burial over the phanerozoic seawater ph is calculated assuming saturation with respect to calcite and considering the changing concentration of dissolved ca documented by brine inclusion data the depth of calcite saturation is allowed to vary through time and the effects of changing temperature and pressure on the stability constants of the carbonate system are considered surface temperatures are calculated using the geocarb iii approach considering also the changing flux of galactic cosmic radiation gcr it is assumed that gcr cools the surface of the earth via enhanced cloud formation at low altitudes the 18o of marine carbonates is calculated considering the changing isotopic composition of seawater the prevailing surface temperatures and seawater ph repeated model runs showed that the trends observed in the marine 18o record can only be reproduced by the model if gcr is allowed to have a strong effect on surface temperature the climate evolution predicted by the model is consistent with the geological record warm periods cambrian devonian triassic cretaceous are characterized by low gcr levels cold periods during the late carboniferous to early permian and the late cenozoic are marked by high gcr fluxes and low pco2 values the major glaciations occurring during these periods are the result of carbon cycling processes causing a draw down of atmospheric co2 and a coevally prevailing dense cloud cover at low altitudes induced by strong gcr fluxes the two moderately cool periods during the ordovician silurian and jurassic early cretaceous are characterized by both high pco2 and gcr levels so that greenhouse warming compensated for the cooling effect of low altitude clouds the very high jurassic 18o values observed in the geological record are caused by low ph values in surface waters rather than cold surface conditions

the timing of ordovician magmatism in the english lake district and cross fell inliers volcanic and hypabyssal intrusive rocks of the lower palaeozoic english lake district and cross fell inliers are elements of the ordovician destructive plate margin system of microcontinental avalonia two igneous sheets within the marine sedimentary skiddaw group of these inliers previously described as lavas are reinterpreted as sills sedimentary rocks enclosing these sills are of late tremadoc early arenig c 493 ma and early llanvirn 476 ma age and breccias along the upper contacts of both were produced by steam explosivity and fluidization ahead of theadvancing tips of the intrusions previous interpretation of the breccias on the older sheet as sediment deposited on the eroded top of a lava flow implied an early ordovician onset of arc magmatism such early magmatism would have been virtually coincident with the latest tremadoc initiation of arc magmatism in wales but evidence for such a near synchronous response tothe putative onset of subduction is lacking respective onsets of magmatismwere probably separated by at least 17 m y and possibly by as much as 29 m y the apparent contemporaneity of mid and late ordovician volcanic episodes in england and wales and similarities in extensional tectonic style suggest that the two areas then were part of the same subduction system responding similarly to plate scale magma generating and tectonic processes the early ordovician situation is uncertain but the absence of arc volcanic rocks of this age in the english lake district suggests that this area and wales are not tectonically juxtaposed elements of a former simple linear arc

influence of the state of stress on the brittle ductile transition in granitic rock evidence from fault steps in the sierra nevada california left lateral strike slip faults in the lake edison granodiorite central sierra nevada california are composed of en echelon segments relative displacements across the faults apparently are transferred between segments by ductile shearing at right steps and by extensional fracturing at left steps the granodiorite within right steps displays mylonitic foliation and thin sections show textures in quartz associated with dislocation glide recovery processes and dynamic recrystallization whereas textures in feldspar are related to fracturing only centimetres outside the right steps the rock fabric is approximately isotropic and deformation is accommodated by mineralized opening mode fractures the stress field calculated for the right step geometry when a boundary element model is used shows an increase in mean compressive stress of up to 25 mpa within the step relative to that outside this difference in stress apparently produced the contrasting behaviors of the granitic rock experimentally derived power law flow laws do not predict these behaviors

sulfate reduction related to the early diagenetic degradation of organic matter and black spot formation in tidal sandflats of the german wadden sea southern north sea stable isotope 13c 34s 18o and other geochemical results abstract a stable isotope geochemical investigation 34 s 32 s 13 c 12 c 18 o 16 o has been carried out to study the development of anoxic surface sediments black spots in the backbarrier tidal area of spiekeroog island southern north sea microbial sulfate reduction related to the degradation of mainly marine organic matter under anaerobic conditions led to the accumulation of hydrogen bisulfide up to 8 mm in the interstitial waters of the sandy sediments while the carbon isotopic composition of organic matter reflected the downcore preferential degradation of labile marine organic matter sulfur and oxygen isotope variations in the dissolved sulfur species provided information about the microbiologically mediated complex interactions between the sulfur carbon and oxygen cycles

low grade and very low grade regional metabasic ordovician rocks of llŷn and snowdonia gwynedd north wales metabasic assemblages belong to the prehnite pumpellyite and greenschist facies the isograds pumpellyite in pumpellyite out clinozoisite in and biotite in have been mapped and divide the area into four metamorphic zones field and petrographic evidence indicate that the metamorphic crystallization was largely syn and immediately post tectonic viz end silurian to devonian the use of published schreinemakers bundles indicates that most of the prehnite pumpellyite bearing assemblages formed at high grades within the facies and that the greenschist facies rocks crystallized in the higher pressure field as the amount of calcite increases so diagnostic ca al silicates are reduced and eventually eliminated from assemblages contrary to recent claims the evidence indicates that the gwna group and the gwyddel felsitic beds of ll n were metamorphosed prior to the arenig

modeling of fate and transport of coinjection of h2s with co2 in deep saline formations 1 the geological storage of co2 in deep saline formations is increasingly seen as a viable strategy to reduce the release of greenhouse gases into the atmosphere however costs of capture and compression of co2 from industrial waste streams containing small quantities of sulfur and nitrogen compounds such as so2 h2s and n2 are very expensive therefore studies on the coinjection of co2 containing other acid gases from industrial emissions are very important in this paper numerical simulations were performed to study the coinjection of h2s with co2 in sandstone and carbonate formations results indicate that the preferential dissolution of h2s gas compared with co2 gas into formation water results in the delayed breakthrough of h2s gas coinjection of h2s results in the precipitation of pyrite through interactions between the dissolved h2s and fe2 from the dissolution of fe bearing minerals additional injection of h2s reduces the capabilities for solubility and mineral trappings of co2 compared to the co2 only case in comparison to the sandstone siliciclastic formation the carbonate formation is less favorable to the mineral sequestration of co2 in sandstone and carbonate formations the presence of fe bearing siliciclastic and or carbonate is more favorable to the h2s mineral trapping

tectonic controls of ordovician arc and marginal basin volcanism in wales ensialic destructive plate margin volcanism persisted in wales from late tremadoc to caradoc times late tremadoc uplift and erosion in nw wales is attributable to onset of subduction and development of a frontal ridge system fore arc south east of this in the vicinity of the harlech dome and in sw wales more localized uplift was associated with the development of arc volcanoes at a volcanic front in north wales the arc volcanism occurred in a regime of e w crustal extension and was centred on a deep crustal discontinuity during arenig times the arc terrane was transgressed subsequently marine conditions with local and temporary emergence persisted through arenig to caradoc episodes of marginal basin volcanism while there was widespread and persistent subsidence in wales during development of the marginal basin the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence the grabens mark the sites of deep and steep crustal discontinuities fractures one of which controlled the earlier arc volcanism changing sites of volcanism in wales can now be seen as due to transfer of active extension from one fracture to another there is evidence that the fractures were present by tremadoc times and they almost certainly formed before development of the cambrian marine rift basin they are most likely to have been strike slip faults or shear zones and their pattern in wales indicates that they may represent part of a complicated deep crustal strike slip duplex system the volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults flower structure which channelled magmas through the cover during extension across the fractures the fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during late caledonian compression and transpression in north wales the volcanically active grabens trended broadly n s oblique to the exposed ne sw basement tectonic grain and dominantly e w extension seems to have persisted throughout the ordovician volcanism while there is considerable uncertainty concerning ordovician plate tectonic configurations and displacements the extension is best explained by invoking a n s convergence vector between the oceanic and over riding microcontinental plates with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin the model highlights the suspect nature of the relationship between the welsh ordovician terrane and others to the north

rare earth element systematics in hydrothermal fluids abstract rare earth element concentrations have been measured in hydrothermal solutions from geothermal fields in italy dominica valles caldera salton sea and the mid atlantic ridge the measured abundances show that hydrothermal activity is not expected to affect the ree balance of either continental or oceanic rocks the ree enrichment of the solutions increases when the ph decreases high temperature solutions 230 c percolating through different rock types may show similar ree patterns

ordovician magnetostratigraphy a correlation of global data palaeomagnetic studies where primary magnetizations are established from well dated rocks have been compiled to construct a magnetostratigraphic time scale for the ordovician period an excellent correlation of magnetozones is observed between independent studies of the baltic and south siberian platform sequences supplemental datasets from other continents concur with the baltic and siberian polarity data early ordovician times were mainly characterized by a reverse field rapid reversals then occurred during llanvirn and llandeilo times late ordovician times were dominated by a normal polarity field some gaps remain in the polarity record notably for late caradoc early ashgill and mid tremadoc times

late diagenetic calcitization of anhydrite from the mississippian of saskatchewan western canada mississippian nodular anhydrites beneath an unconformity in the subsurface of southern saskatchewan are locally replaced by calcite pyrite and celestite triassic clastics above the unconformity are green rather than red and a usually developed subunconformity alteration zone where carbonates are dolomitized and porosity is filled with anhydrite is absent the unconformity lacks karstic features unlike in the usa and probably formed in a hyperarid climate mississippian anhydrites near the unconformity are not preferentially dissolved nor were they extensively hydrated anhydrite calcitization occurred only after the unconformity was shallowly buried by redbeds and it probably involved sulphate reducing bacteria hydrogen sulphide generated by bacteria reduced redbed pigments the replacement calcite contains pseudomorphs and relicts of anhydrite and pseudomorphs of secondary gypsum these indicate calcitization occurred only after original mississippian gypsum was altered to anhydrite and this in turn was partially converted back to secondary gypsum beneath the unconformity replacement occurred concurrently with the formation elsewhere of the dolomitized zone beneath the unconformity sulphur isotopic ratios of replacement pyrite are depleted relative to mississippian sulphate values consistent with the activities of sulphate reducing bacteria carbon isotopic ratios of replacive calcites however do not support this interpretation and are identical to those of mississippian limestones simple replacement of sulphate by pore water bicarbonate in equilibrium with host limestones is unlikely because protons generated during the reaction should have created acidic conditions in which calcite would have dissolved a full explanation of the calcitization remains elusive but may involve replacement occurring in an active groundwater system and or bacterial sulphate reduction occurring upstream of the site of calcitization

rare earth element and yttrium rey geochemistry in carbonate reservoirs during deep burial diagenesis implications for rey mobility during thermochemical sulfate reduction abstract the impact of burial diagenesis especially deep burial related processes such as thermochemical sulfate reduction tsr on the distribution of rare earth elements and yttrium rey in diagenetic minerals in carbonate reservoir has gained little attention even though they may be unique indicators of the diagenetic system being closed or open to external influx of material trace element and rey concentrations 87 sr 86 sr 18 o and 13 c have been determined for limestone host dolomite pore filling calcite calcite 2 and late stage fracture filling calcite calcite 3 barite barite 2 and anhydrite anhydrite 3 from lower triassic feixianguan formation reservoirs in northeast sichuan basin china calcite 2 calcite 3 barite 2 and anhydrite 3 precipitated during deep burial from 110 c to 220 c demonstrated by petrology and fluid inclusion thermometry sr isotope analysis revealed that diagenetic carbonate minerals and anhydrite have largely identical 87 sr 86 sr ratios to triassic seawater indicating negligible input of sr from terrigenous sources and probably a relatively closed diagenetic environment for precipitation of these minerals carbon isotope analysis showed that calcite 2 has relatively low 13 c values down to 18 9 v pdb suggesting that they are tsr calcites with the carbonate derived from oxidized isotopically light hydrocarbons unlike the seawater like rey patterns of the limestone and dolomites calcite 3 and anhydrite 3 are enriched in rare earth elements ree and show light rare earth element enrichment and heavy rare earth element depletion exhibiting a chevron like pattern of shale normalized rey trends in contrast tsr calcite calcite 2 has similar ree to the host dolomite but a relatively flat rey sn pattern suggesting strong variations in elemental and ree compositions of the burial fluids significantly tsr calcite shows a prominent positive eu anomaly and an unusually high chondritic y ho ratio both yttrium versus holmium fractionation and eu 2 oxidation to eu 3 must have occurred during thermochemical sulfate reduction hence a positive eu anomaly and an elevated y ho ratio may be used as effective proxies to differentiate calcite resulting from tsr from ordinary calcite cement this is especially useful when carbon isotope analysis cannot be used to give an unambiguous interpretation of the origin of the calcite

constraining the fluid history of a co2 h2s reservoir insights from stable isotopes ree and fluid inclusion microthermometry reservoirs that host co2 h2s bearing gases provide a key insight into crustal redox reactions such as thermochemical sulphate reduction tsr despite this there remains a poor understanding of the extent duration and the factors limiting this process on a reservoir scale here we show how a combination of petrography fluid inclusion rare earth element ree and carbon 13c oxygen 18o and sulphur 34s stable isotope data can disentangle the fluid history of the world s largest co2 accumulation the labarge field in wyoming usa the carbonate hosted labarge field was charged with oil around 80 ma ago which together with nodular anhydrite represent the reactants for tsr the nodules exhibit two distinct trends of evolution in 13c with both 34s and 18o that may be coupled to two different processes the first trend was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulphur and the oxidation of organic compounds and associated precipitation of calcite during tsr in contrast the second trend was interpreted to be the result of the hydrothermal co2 influx after the cessation of tsr in addition mass balance calculations were performed to estimate an approximate tsr reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the tsr process such an approach provides a tool for the prediction of tsr occurrence elsewhere and advancing our understanding of crustal fluid interactions

new uppermost cambrian u pb date from avalonian wales and age of the cambrian ordovician boundary a crystal rich volcaniclastic sandstone in the lower peltura scarabaeoideszone at ogof ddu near criccieth north wales yields a u pb zircon age of 491 1 ma this late late cambrian date indicates a remarkably young age for the cambrian ordovician boundary whose age must be less than 491 ma hence the revised duration of the post placentian trilobite bearing cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by ordovician graptolites and mesozoic ammonites

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

a new k ar age from uppermost tremadoc rocks of north wales new conventional k ar dating of virtually unaltered pargasite phenocrysts extracted from a metabasalt of the uppermost tremadocian rhobell volcanic group cropping out near dolgellau gwynedd north wales yields an age of 508 11 ma this result provides a valuable new time scale calibration point although incompatible with some other versions of the radiometric time scale for the lower palaeozoic which have indicated ages of less than 508 11 ma for the tremadoc arenig boundary

the fate of co2 derived from thermochemical sulfate reduction tsr and effect of tsr on carbonate porosity and permeability sichuan basin china abstract this article discusses the role of methane in thermochemical sulfate reduction tsr the fate of tsr derived co2 and the effect of tsr on reservoir porosity and permeability and the causes of the anomalously high porosity and permeability in the lower triassic soured carbonate gas reservoirs in the northeast sichuan basin southwest china the lower triassic carbonate reservoirs were buried to a depth of about 7000 m and experienced maximum temperatures up to 220 c before having been uplifted to the present day depths of 4800 to 5500 m but they still possess porosities up to 28 9 and permeabilities up to 3360 md the present day dry gas reservoirs evolved from a paleo oil accumulation and experienced varying degrees of tsr alteration as evidenced from the abundant sulfur rich solid bitumens and varying h2s and co2 concentrations tsr occurred mainly within the oil and condensate wet gas windows with liquid hydrocarbons and wet hydrocarbon gases acting as the dominant reducing agents responsible for sulfate reduction sulfur rich solid bitumen and h2s generation and calcite precipitation methane dominated tsr was a rather late event and had played a less significant role in altering the reservoirs intensive h2s and co2 generation during tsr resulted in calcite cementation rather than carbonate dissolution which implies that the amount of water generated during tsr was volumetrically insignificant 13c depleted co2 derived from hydrocarbon oxidation preferentially reacted with ca2 to form isotopically light calcite cements and the remaining co2 re equilibrated with the 13c enriched water rock systems with its 13c rapidly approaching the values for the host rocks which accounted for the observed heavy and relatively constant co2 13c values the carbonate reservoirs suffered from differential porosity loss by tsr involved solid bitumen generation and tsr induced calcite and pyrite precipitation intensive tsr significantly reduced the porosity and permeability of the intervals expected to have relatively high sulfate contents the evaporative platform dolostones and the platform margin shoal dolostones immediately underlying the evaporative facies early oil charge and limited intensity of tsr alteration together with very low phyllosilicate content and early dolomitization accounted for the preservation of anomalously high porosities in the reservoirs above the paleo oil water contact a closed system seems to have played a special role in preserving the high porosity in the gas zone reservoirs below the paleo oil water contact the closed system which is unfavorable for deep burial carbonate dissolution and secondary porosity generation was favorable for the preservation of early formed porosity in deeply buried carbonates especially sucrosic and vuggy dolostones have a high potential to preserve such porosity

the significance of a reliable ordovician age for some highland border rocks in central scotland a lower arenig fauna with north american affinities has been rediscovered at dounans aberfoyle central scotland this discovery makes part of the highland border complex younger than the adjacent dalradian rocks and suggests a provenance from the s

the significance of isotopic dates from the english lake district for the ordovician saurian time scale the lower palaeozoic time scale is discussed in the light of a new date for the threlkeld microgranite and a reassessment of other published dates from the lake district which suggest ages of c 410 ma for the base of the devonian c 421 ma for the ashgill c 429 ma for the late caradoc c 439 ma for the early llandeilo and c 468 ma for the early llanvirn