aragonite relic preservation in jurassic calcite replaced bivalves shells of the aragonite bivalve neomiodon great estuarine group jurassic scotland replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure the presence of these relics in such old altered shells as well as the high sr content of the replacement calcite indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events but rather a rapid one step process aragonite relics once encased in neomorphic spar will survive as unequivocal evidence of original aragonite mineralogy barring total remobilization of the enclosing stable calcite a generally unlikely event the retention of this residual aragonite and high sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration recrystallization invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or as in the case of our neomiodon material deeper burial environments pseudopleochroism of the replaced neomiodon shells appears to be due to organic largely graphitic relics not to the aragonite relics

δ18o values of coexisting brachiopods and fish temperature differences and estimates of paleo water depths to estimate vertical thermal gradients and paleo water depths to marine platforms we present a new method based on the difference between 18 o values of contemporaneous brachiopod carbonate and fish phosphate present day marine fauna of well known ecology from the surface to the sea floor record isotopic temperatures that agree with measured temperatures we predict distributions of isotopic data that result from sampling strategy basin morphology and fauna ecology and discuss limitations application of the method to the jurassic paris london basin gives vertical thermal variations of up to 14 c associated with depths varying from a few meters to 170 30 m the estimated depths are minimum values and are greater than those estimated from sedimentological criteria for this ancient epicontinental sea

the origin of deep buried channels of elsterian age in northwest germany buried channels cut to a depth of over 100m into the underlying substratum of north germany are almost exclusively of elsterian age evaluation of geophysical well logs in the hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles it can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence fining from proximal to distal channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet the channel infill reflects these episodic outbursts of meltwater during deglaciation the channel remnants became infilled with fine sediments the channels ceased to exist as morphologically detectable forms at the end of the holsteinian stage

geodynamic control on carbonate diagenesis petrographic and isotopic investigation of the upper jurassic formations of the paris basin france abstract the oxfordian and kimmeridgian carbonates of the eastern edge of the paris basin display poor reservoir properties stable isotope investigation of these non recrystallized cements through both microdrilling sampling and in situ sims secondary ion mass spectrometry analysis shows that they originated from slightly buffered meteoric fluids 3 smow to 8 smow flowing through the malm carbonate aquifers of the eastern edge of the paris basin after a review of the post upper jurassic history of the paris basin it appears that meteoric water supplies leading to such cementations were associated with the evolution of the basin s geodynamic setting and notably with the lcu late cimmerian unconformity major event which corresponds to the uplift of the london brabant massif denudation of the oxfordian and kimmeridgian carbonates on the southern flank of the london brabant massif to the north of the study area then lead to lateral recharging of the associated aquifers with meteoric fluids this scenario is similar to the one proposed to explain cementations of the jurassic carbonates in the wessex weald basins which emphasizes the main role played by the cretaceous unconformities over carbonate diagenesis at the scale of the northwestern tethys margin

volcanism co2 and palaeoclimate a late jurassic early cretaceous carbon and oxygen isotope record a composite tethyan late jurassic early cretaceous carbon and oxygen isotope curve is presented c isotope data provide information on the evolution and perturbation of the global carbon cycle o isotope data are used as a palaeotemperature proxy in combination with palaeontological information the resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification positive c isotope anomalies in the valanginian and aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide a major warming pulse accompanies the aptian but not the valanginian c isotope event the observed change in early aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion both c isotope anomalies are accompanied by biocalcification crises that may have been triggered by p co 2 induced changes in climate and in surface water chemistry elevated nutrient levels in river influenced coastal waters and in upwelling regions further weakened marine calcification these conditions contrast with normal trophic conditions prevailing in the latest jurassic and favouring biocalcification the c and o isotope curves record a stable mode of carbon cycling and stable temperatures we conclude that biocalcification is mostly triggered and inhibited by co 2 conditions in the atmosphere ocean system

the origin and timing of multiphase cementation in carbonates impact of regional scale geodynamic events on the middle jurassic limestones diagenesis paris basin france the middle jurassic carbonates of the eastern part of the paris basin display surprisingly low values of porosity and permeability 15 and k 0 5 md the main objective of this study is to determine the causes and timing of the cementation that altered the petrophysical properties of these carbonates thereby destroying their potential as oil reservoirs a fate that did not befall their equivalents in deeper central parts of the paris basin using petrographic and geochemical analyses stable o and c isotopes sr isotopes major elements we identify six calcitic spar stages two dolomite stages and several episodes of fracturing and stylolitization ordered in paragenetic sequence cement quantification shows the predominance of two blocky calcite cement stages 75 of total cementation o and sr isotopes from these calcite cements suggest that the parent fluids resulted either from a mixing of trapped jurassic seawater and meteoric water or from buffered meteoric waters in the geological history of the paris basin major meteoric water inputs were possible during the early cretaceous when the middle jurassic carbonates cropped out at its northern and eastern borders lateral meteoric recharge may have occurred as a result of two separate uplift events late cimmerian unconformity and late aptian unconformity and the related exposures of carbonates to the north of the study area this palaeohydrological circulation brought about a significant reduction of porosity from 40 to 10 through calcite cement precipitation the early cretaceous events are of great importance in the diagenetic evolution of the sedimentary basins bordering the london brabant massif across all of northwestern europe a subsequent dolomite and calcite cementation stage accounts for about 5 of the total cement volumes this late cementation may have been caused by hydrothermal fluids ascending along permeable fractures during the late oligocene extension episode

origin of late cements in oolitic reservoir facies a fluid inclusion and isotopic study mid jurassic southern england abstract during bathonian times a carbonate ramp was constructed over southern england by the progradation of oolitic skeletal and oncoidal grainstones and packstones the grainstones provide a significant hydrocarbon reservoir facies although these carbonates suffered early diagenesis under marine and more significantly meteoric conditions later diagenetic cements such as saddle dolomite and more particularly mildly ferroan calcite are volumetrically more important in causing the occlusion of porosity on a regional scale we have analysed systematically these late cements employing a number of techniques standard petrography including cl and uvl c and o isotopes sr isotopes and fluid inclusion studies integration of the results of these approaches constrains both the pore fluid composition and the temperature regime at the time of cement precipitation saddle dolomite was emplaced before the mildly ferroan calcite at temperatures between 75 and 85 c and from highly saline brines 18 19 5 wt nacl equivalents mildly ferroan calcite precipitated over a generally lower temperature range and from less saline brines 6 5 15 wt nacl equivalents five types of fluid inclusion have been recognised 1 monophase aqueous liquid 2 monophase oil liquid 3 two phase aqueous liquid vapour 4 two phase oil liquid hydrocarbon vapour 5 three phase aqueous oil liquid hydrocarbon aqueous liquid vapour during burial warm 75 c brines passed through the limestones and precipitated saddle dolomite later slightly cooler oil rich brines precipitated mildly ferroan calcite wherever hydrocarbon accumulation had not already occurred our sr isotopic studies suggest that both these brines were derived from underlying and more deeply buried lias shales and that these brines were generated from late jurassic times onwards brine movements are likely to have been largely complete by the late cretaceous

reaction textures in al rich metabasite implication for metamorphic evolution of the eastern border of the middle austroalpine basement units abstract al rich metabasite from the speik complex in the eastern alps contains coexisting kyanite staurolite garnet and calcic amphibole compositional zoning and inclusions trails in minerals suggest a prograde metamorphic history to high pressure amphibolite facies conditions the formation of kyanite and staurolite was controlled by bulk rock composition mainly by high al content in some metabasites textural relations and thermodynamic calculations indicate isothermal decompression from 1 1 1 2 gpa 580 c to 0 5 gpa 580 c that resulted in formation of ca rich plagioclase and new amphibole replacing garnet and locally corundum substituting for clinozoisite and kyanite the replacement of amphibole by biotite and chlorite and plagioclase by margarite and the formation of prehnite and pumpellyite were due to isobaric cooling to very low grade conditions

cement distribution in a carbonate reservoir recognition of a palaeo oil water contact and its relationship to reservoir quality in the humbly grove field onshore uk abstract the distribution of mineral cements total porosity microporosity and permeability have been determined for the humbly grove oolitic carbonate reservoir middle jurassic great oolite formation weald basin onshore uk using a combination of optical petrography electron microscopy fluid inclusion analysis quantitative xrd wireline data analysis and core analysis data grainstone reservoir facies have porosities ranging between 5 and 24 but are mostly between 11 and 24 permeabilities vary from

magnesian calcite cements and their diagenesis dissolution and dolomitization mururoa atoll the most ubiquitous syn sedimentary cements affecting mururoa atoll are composed of magnesian calcite three main types are distinguished fibrous bladed and sparitic on the basis of petrography morphology and mgco3 concentration of the constituting crystals while peloid infills a particular form of hmc chemical precipitation also exist petrographic evidence and isotopic signatures are compatible with marine precipitation mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of mg calcite cements these alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology the first stage of the partial dissolution of mg calcite induces the development of chalky white friable zones within the initially crystalline hard cement layers at ultrascale this is due to the creation of micro voids along the elongate cement fibres advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and or between superposed cement layers mg calcite dissolution is related to meteoric diagenesis during periods of quaternary exposure the creation of voids within mg calcite layers is due to the mechanical removal of previously altered calcium carbonate a process suggesting marine or non marine water flow probably in the vadose environment selective dolomitization of mururoa cements involves alternations of calcite and dolomite which form successive cement like rinds within primary cavities at mururoa these alternations are the result of selective dolomitization of the pre existing mg calcite cements rather than successive precipitation of calcite and dolomite selective dolomitization of mg calcite cements at mururoa indicates that a given cement succession is not necessarily a simple chronological sequence oxygen isotope values of dolomites are enriched in 186 by about 3 pdb within calcite dolomite pseudo alternations the dolomitizing fluid at mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization

metamorphic amphiboles composition and coexistence amphiboles have long been key players in metamorphic petrology the concept of facies was developed for rocks of basaltic composition eskola 1920 1939 fyfe et al 1958 and within this concept amphiboles had a leading role the first appearance of amphiboles the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene bearing assemblages mark a number of major facies boundaries the transition from greenschist to amphibolite facies is marked in part by hornblende replacing actinolite and the granulite facies is defined by the breakdown of hornblende to orthopyroxene bearing assemblages the blueschist facies is defined by the appearance of glaucophane while the transition from blueschist facies to eclogite facies conditions is indicated by the breakdown of glaucophane to omphacitic pyroxene bearing assemblages references to amphibole names actinolite glaucophane or rocks characterized by certain amphiboles amphibolite hornblende figure prominently in six facies names turner 1981 liou et al 1985 oh et al 1988 evans 1990 peacock 1993 a large part of the literature that is devoted to metamorphic petrology of amphiboles concerns reactions which produce amphiboles from more hydrated mafic igneous rocks or lead to the destruction of amphiboles as they break down to form anhydrous or nearly anhydrous mineral assemblages consequently amphiboles can be thought of as occupying the wide intermediate portion of p t xfluid space in which metamorphic rocks are found the number of citations of the published amphibole nomenclature serves as a yardstick meterstick of the interest in amphiboles and a minimum estimate of the amount of literature devoted to this mineral two versions of the nomenclature of amphiboles have appeared 1978 and 1997 leake 1978 leake et al 1997 and they have been cited a over 2300 times in the past 28 years this works out to a minimum

characterization and origin of permeability porosity heterogeneity in shallow marine carbonates from core scale to 3d reservoir dimension middle jurassic paris basin france abstract nuclear magnetic resonance nmr stable isotope geochemistry of micro sampled cores nmr well logs and 3d modeling are used to investigate the carbonate permeability porosity heterogeneity along 230 m thick limestones of the paris basin despite the global low porosity and permeability of the limestones two aquifers units with porosity greater than 15 were identified these two aquifers are very different in terms of pore through radii and nmr signal the first one a1 aquifer 1 is a 7 m thick mudstone unit dominated by extended microporosity with pore throat radii of 0 25 m to 0 3 m the second one a2 aquifer 2 is a 15 m thick oolitic grainstone units showing macropores reaching 100 m and pore throat radii of 32 m from core descriptions and wireline logs on 26 wells a 3d static geological model is build the fine tuning of permeability calculations from nmr logs realized along 12 of the wells allows porosity and permeability heterogeneity to be distributed within a 3d model at the reservoir scale area of about 2000 km2 which match the flow behavior illustrated by well tests associated with early meteoric calcite cements and poorly developed burial blocky calcite cements the porous and permeable intervals may be predicted in two stratigraphic and diagenetic considerations firstly the syn sedimentary meteoric dissolution or neomorphism of the initial high magnesium calcite and aragonite particles or clasts into low magnesium calcite particles or cements prevented most mechanical and chemical compaction during the first steps of burial secondly the regional stratigraphic architecture reveals the presence of local permeability barriers which prevented early cretaceous lateral meteoric water circulation and the associated burial calcite cementation

impact of basin burial and exhumation on jurassic carbonates diagenesis on both sides of a thick clay barrier paris basin ne france several diagenetic models have been proposed for middle and upper jurassic carbonates of the eastern paris basin the paragenetic sequences are compared in both aquifers to propose a diagenetic model for the middle and late jurassic deposits as a whole petrographic optical and cathodoluminescence microscopy structural fracture orientations and geochemical 18o 13c ree studies were conducted to characterize diagenetic cements with a focus on blocky calcite cements and their connection with fracturation events four generations of blocky calcite cal1 cal4 are identified cal1 and cal2 are widespread in the dominantly grain supported facies of the middle jurassic limestones about 90 of the cementation whereas they are limited in the oxfordian because grain supported facies are restricted to certain stratigraphic levels cal1 and cal2 blocky spars precipitated during burial in a reducing environment from mixed marine meteoric waters and or buffered meteoric waters the meteoric waters probably entered aquifers during the late cimmerian jurassic cretaceous boundary and late aptian early cretaceous unconformities the amount of cal2 cement is thought to be linked to the intensity of burial pressure dissolution which in turn was partly controlled by the clay content of the host rocks cal3 and cal4 are associated with telogenetic fracturing phases the succession of cal3 and cal4 calcite relates to the transition towards oxidizing conditions during an opening of the system to meteoric waters at higher water rock ratios these meteoric fluids circulated along pyrenean oligocene and alpine fractures and generated both dissolution and subsequent cementation in oxfordian vugs in mud supported facies and in poorly stylolitized grainstones however these cements filled only the residual porosity in middle jurassic limestones in addition to fluorine inputs fracturation also permitted inputs of sulphur possibly due to weathering of triassic or purbeckian evaporites or h2s input during paleogene times

chronology of fracture sealing under a meteoric fluid environment microtectonic and isotopic evidence of major cainozoic events in the eastern paris basin france abstract a detailed reappraisal of the sequence of tectonic stages that has affected the eastern part of the paris basin from the late jurassic to the end of cainozoic is presented in order to improve knowledge of the relationships between tectonics and fluid mass palaeo transfers in that basin this goal was performed in two steps i establishing a relative tectonic chronology on the basis of numerous observations and detailed analysis of geometrical relationships between striated planes vertical and bedding stylolites and tension gashes and ii discussing the geochemical signatures 18 o and 13 c of calcite from fracture infillings well positioned within the relative tectonic chronology the resulting tectonic sequence agrees with the main events described by previous papers but the present work details the pyrenean s l eocene oligocene and alpine miocene pliocene convergence stages and explains the mechanisms of the oligocene miocene transition between the two previous events the different tectonic stages are as follows i late jurassic to early cretaceous extension starting with a wnw ese direction but changing to e w ii the pyrenean convergence eocene changing from nne ssw to ne sw and finally ene wsw compression iii oligocene transition from pyrenean to alpine convergence marked by a drastic evolution of main stress which swaps from a horizontal position to a vertical one and radial extension marked by a strong reactivation of bedding stylolitisation iv the alpine convergence late miocene to present day changing from wnw ese to nnw sse direction of shortening 18 o and 13 c values of calcite from tension gashes associated to each tectonic stage spread from 18 to 27 smow and from 0 8 to 3 pdb respectively according to the maximum thermal history of this part of the basin t max 40 c or 60 c according to the available published estimates the calculated 18 o values of fluids 7 to 4 involved in mass transfer processes are interpreted as meteoric the lowest 18 o values 18 20 are located around the main regional faults and associated with 13 c values that are rather different from those of host rocks in contrast calcites from non faulted area present a higher 18 o 20 and 13 c values close to those of host rocks these data are explainable in terms of differences in the fluid rock ratios prevailing during carbonate redistribution the highest ratios being associated to discontinuities faults sometimes with hydraulic breccias that were active during the tertiary tectonic events outside the fault zones the isotopic compositions of calcite from tension gashes are similar at the regional scale to those of calcite from geodes or cements analysed in previous studies on localized areas as a result we emphasize the fact that a significant part of the carbonate redistribution in the middle to upper jurassic limestones from the eastern paris basin occurred during the tertiary s l enlightening the influence of distant orogenic systems extensive or compressive in the matter redistribution within sedimentary basins at variance of models inferring a porosity cementation predominantly during burial

recurrent neomorphic and cement microtextures from different diagenetic environments quaternary to late neogene carbonates great bahama bank abstract the cores clino and unda taken near the western margin of great bahama bank contain a diverse suite of quaternary to late neogene carbonates that have a variety of neomorphic and cement microtextures the microtextures of each of the different neomorphosed aragonitic grain types such as corals mollusks and plates of the calcareous green algae halimeda are very similar wherever the neomorphosed grains are present in the cores limestone samples that contain neomorphosed aragonitic fossils are cemented predominantly with blocky calcite despite the similar neomorphic and cement microtextures light stable isotope data reveal that these calcites precipitated in different diagenetic environments in different parts of the cores the neomorphic calcites and associated calcite cements in the upper parts of clino and unda precipitated in meteoric influenced pore waters as indicated by 18o values ranging from 1 9 to 3 7 whereas in the lower part of clino the calcites precipitated in marine pore waters as indicated by 18o values ranging from 0 1 to 2 4 essentially identical diagenetic microtextures were thus produced in different diagenetic environments revealing a significant limitation in the use of microtextural data to determine the diagenetic histories of carbonate rocks

geochemical trends and palaeohydrological significance of shallow burial calcite and ankerite cements in middle jurassic strata on the east midlands shelf onshore uk this paper discusses isotopic and geochemical trends in shallow burial 1500 m carbonate cements in limestones from a middle jurassic shale dominated marine to paralic succession situated on the east midlands shelf the results are used to investigate carbonate shale interactions between the near surface realm of microbially mediated diagenesis and the deep thermobaric regime of clay mineral transformations and hydrocarbon generation they are also combined with published data from cements in stratigraphically and geographically adjacent strata to better understand the late mesozoic cenozoic palaeohydrology of central eastern england east midlands shelf midland platform shallow burial cements sampled from the blisworth limestone formation consist of ferroan calcite and calcic ankerite which are associated with ferroan neomorphic calcite and beef cone in cone calcite veins calcite cements are petrographically complex and variable yet record consistent compositional trends carbon isotope values 13c 1 2 to 1 7 pdb show that organic matter breakdown did not contribute significant carbon to the shallow burial pore fluids oxygen isotope data 18o 4 5 to 11 2 pdb are consistent with precipitation from modified marine pore fluids that displaced the original brackish depositional fluids from the limestone during palaeocene burial the same fluids accompanied cementation in underlying bajocian limestones ferroan calcite cements neomorphic spar and beef record a surprising correlation in fe2 and mg2 concentration that suggests a common source for the ions in the adjacent shales this trend is not present in the bajocian limestones and indicates that ionic mass transfer was on a smaller scale than the regional plumbing system the results suggest that middle jurassic limestone palaeoaquifers on the east midlands shelf hosted a westerly directed up dip flow from compacting upper and lower jurassic marine mudrocks in the sole pit basin during the cretaceous and early tertiary however early cretaceous meteoric groundwater palaeoaquifers recorded from cements in laterally equivalent middle jurassic limestones on the midlands platform and in the weald basin appear not to have extended northward into the east midlands shelf

diagenetic evolution of the eocene ramp carbonates a paradigm from the nile valley egypt petrographical and geochemical attributes abstract carbonate rocks of east nile valley in egypt have been investigated as a paradigm for revealing the diagenetic evolution of the eocene ramp carbonates the integration of petrographical elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes i e eogenesis mesogenesis and telogenesis and are related to four diagenetic environments i e marine phreatic meteoric phreatic burial and uplifting environments the paragenetic sequence of the studied carbonates commences with the eodiagenetic episode which includes the marine and meteoric phreatic diagenetic environments the marine phreatic diagenetic environment comprises micritization glauconitization pyritization marine cementation fibrous calcite and syntaxial overgrowth rim cements early dolomitization formation of fine crystalline matrix replacive dolomites and early stages of mechanical compaction before significant burial the meteoric phreatic diagenetic environment includes the neomorphism dissolution meteoric cementation earlier phases of sparry calcite cements and precipitation of megaquartz in the void spaces of the peritidal carbonates during the mesogenesis episode the studied carbonates were exposed to burial diagenesis the shallow burial diagenesis resulted in the creation of point tangenetial contacts between the allochemical constituents and the precipitation of a medium coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores the deep burial diagenetic processes embody the chemical compaction condensed fitted interpeneteration and pressure dissolution fabrics fracturing and burial cementation the telodiagenesis represents the last episode in the diagenetic history of the studied carbonates it witnessed the fracturing meteoric vadose cementation and hematitization processes

the evidence and implications of polar ice during the mesozoic abstract the mesozoic perhaps representing the longest period of warmth during phanerozoic earth history contains in general sparse and frequently equivocal evidence for polar ice although this time is undoubtedly punctuated by oscillations in climate whether sufficient to lead to polar cooling and the formation of polar ice an inviting mechanism to account for faunal and floral distribution patterns and large scale sea level change has been widely debated mesozoic evidence for glacial conditions includes abraded rock surfaces generally unsorted stone rich beds and the presence of dropstones rafted by ice within a finer grained host sediment faunal and floral evidence has also been utilised to determine the presence or absence of cold or sub freezing polar conditions together with more indirect evidence for glacial conditions derived from general circulation model gcm simulations of climate the analysis of clay mineral distributions glendonite abundance and palaeontological and sedimentological evidence for globally synchronous sea level change the extent of possible glacial environments during the mesozoic has been established by plotting published reports of glacial sediments on palaeogeographic reconstructions the general clustering of evidence at high palaeolatitudes suggests that the extent of polar ice during the mesozoic is likely to have been approximately one third the size of the present day based on such evidence a number of episodes of cold or sub freezing polar climates during the bajocian bathonian tithonian volgian valanginian and aptian are recognised evidence exists possibly also for a cold episode during the early jurassic pliensbachian although poorly constrained owing to limited biostratigraphical control the longevity of these events may be represented by a cold snap within a stage although undoubtedly the possible smearing of ages may have had the effect of lengthening and hence promoting the importance of these proposed events the climate regime of the earth during these times may be hypothesised to be characterised by a relatively steep pole to equator temperature gradient where low latitude regions are as warm or warmer than today the evidence to support such reasoning includes sharp increases in the areal distribution of both glendonites and deposits with affinities to glacial tillites and dropstones for these times coincidental falls of sea level arid events and an increased bipolarity of faunas at these times imparts further confidence that these events were of a magnitude to effect the earth as a whole

oxygen isotope salinity relationship in the northern indian ocean we analyze the surface 18o salinity relationships of the bay of bengal and the arabian sea in the northern indian ocean known for their contrasting hydrological conditions new measurements of these tracers show a very low 18o salinity slope associated with the strong dilution in the bay of bengal but a slope more typical of this latitude in the arabian sea although this region is marked by a complex monsoonal regime numerical modeling using a box model and a general circulation model is able to capture the 18o salinity slope and its geographical variation both models clearly show that the low 18o salinity slope is due to the evaporation minus precipitation balance with an important contribution of the continental runoff in the bay of bengal although the low value of these slopes 0 25 makes past salinity reconstructions uncertain insight into the last glacial maximum conditions shows a probable stability of these slopes and limited error on paleosalinity

influence of original fabric on subsequent porosity evolution an example from the corallian upper jurassic reefal limestones the weald basin southern england abstract the corallian reefs of the weald basin southern england developed over a prograded oolitic sandbody during a marine transgression the diagenesis and porosity evolution of these reefs is directly related to their original fabric a reflection of original environment of formation a variety of facies related diagenetic processes and products has been recognized including early marine cementation skeletal diagenesis and burial cementation the early marine peloidal cements are abundant in the corallian subsurface reefs and have played a key role in the overall reservoir quality these cements by virtue of occurring in stromatolitic crusts and having abnormally light stable isotopic values are interpreted as microbial products the skeletal diagenesis is mainly controlled by skeletal and matrix permeability with dissolution predominantly occurring in the upper relatively high energy part of the reefs and neomorphism commonly developing in the lower part of the reefs which grew in a low energy setting burial cementation has involved both calcite and dolomite precipitation over a wide range of burial depths various forms of calcite and dolomite cements have been recognized including relatively early ferroan drusy poikilotopic calcite and zoned dolomite and relatively late saddle dolomite and non ferroan coarsely crystalline calcite these carbonate cements by virtue of post compactional distribution and having depleted 18o values 18 o pdb 6 to 9 are interpreted as having formed under burial conditions perhaps in the range of 600 to 2000 m the porosity evolution is essentially controlled by the combined effects of these diagenetic processes with the best reservoir potential developing in the upper part of the reefs keep up highstand systems tract which underwent extensive skeletal dissolution porosity was poorly developed in the middle part of the reefs catch up highstand systems tract due to the extensive early marine cementation the lower part of the reefs transgressive systems tract virtually has no reservoir potential due to the relatively fine grained sediment texture and intensive compaction

vertical distribution of helium and 40ar 36ar in porewaters of the eastern paris basin bure haute marne constraints on transport processes through the sedimentary sequence

a u pb and 40ar 39ar time scale for the jurassic published time scales provide discrepant age estimates for jurassic stage boundaries and carry large uncer tainties the u pb or 40 ar 39 ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration a radiometric age database consisting of fifty u pb and 40 ar 39 ar ages was com piled to construct a revised jurassic time scale accepted ages have a precision of 5 ma 2 or better and are con fined to no more than two adjacent stages the majority of these calibration points result from integrated bio and geochronologic dating in the western north american cordillera and have not been previously used in time scales di rect dates are available only for the triassic jurassic boundary and the initial boundary of the crassicosta chron and the callovian stage the chronogram method was used to estimate all early and early middle jurassic zone boundaries attempted here for the first time late middle jurassic substage boundaries and late jurassic stage boundaries signif icant improvement is achieved for the pliensbachian and toarcian where six consecutive zone boundaries are deter mined the derived zonal durations are disparate varying between 0 4 and 1 6 ma the latest jurassic isotopic database remains too sparse therefore chronogram estimates are improved using interpolation based on magnetochronology the initial boundaries of jurassic stages are proposed as follows berriasian jurassic cretaceous 1418 18

rapidity of freshwater calcite cementation implications for carbonate diagenesis and sequence stratigraphy abstract in less than 10 years subaerially exposed holocene oolitic sand on eleuthera island bahamas was completely case hardened by freshwater vadose calcite cementation this phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains in part why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed it further proves contrary to what the exxon conceptual sequence stratigraphic models imply that carbonate deposits do in fact respond differently to subaerial exposure than siliciclastic deposits case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early near surface freshwater diagenesis helps explain why shallow water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands during lowstands rapid subaerial lithification of carbonate sediments retards their reworking by wind or water once lithified these deposits are further stabilized by vegetation and karstification inhibiting their transport as unconsolidated sediment into adjacent deeper water basins observations from eleuthera island also document the reciprocal processes associated with freshwater carbonate diagenesis calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore filling low magnesian calcite cement this reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10 20 cm of the subaerially exposed body of oolitic sand below this case hardened surface oolitic sands remained unconsolidated only very minor amounts of calcite cement were required for effective casae hardening

reactant product textures volume relations and implications for major element mobility during natural weathering of hornblende tallulah falls formation georgia blue ridge u s a natural weathering of hornblende from the tallulah falls formation georgia u s a takes place under thoroughly leached oxidizing conditions at the earliest stage of weathering corrosion of hornblende forms denticulated terminations with only minimal porosity apparent within the partially pseudomorphic kaolin group weathering product or between it and the reactant mineral isovolumetric replacement of hornblende by kaolin group clay or precursor products implies inheritance of parent mineral silica tetrahedra by the weathering products and largely immobile behavior of si during early weathering aluminum normally considered immobile or minimally mobile during weathering must be imported into early stage weathering products more advanced weathering of hornblende forms denticulated terminations separated from the earlier formed products by a large continuous peripheral void porosity is much more conspicuous at this stage late stage ferruginous products precipitate from fe mobilized in solution dissolved fe to form the late stage goethite can be entirely derived from the parent hornblende requiring only short transport distances for aqueous fe between the source parent mineral and the location of product precipitation textural and volumetric relationships between naturally weathered hornblende and its partially pseudomorphic weathering products suggest structurally controlled immobility of much parent mineral si and considerable mobilization of al at early stages of weathering but at more advanced stages of weathering the textural and volumetric relationships are consistent with immobility of al and fe and leaching of substantial si and all base cations

widespread cementation induced by inflow of continental water in the eastern part of the paris basin o and c isotopic study of carbonate cements abstract mesozoic limestones from the eastern part of the paris basin display mostly in strata rich in bioherms abundant macro cavities vugs and some connected micro fractures almost completely filled with euhedral calcite crystals late calcite spar the interparticle porosity of limestones is also filled with euhedral calcite spar cements similar to those observed in vugs and fractures and rarely displays the preservation of the structures typical of early marine cements these calcite spars both from vugs fractures infillings and from cemented porosity were studied in drilled cores cross cutting the kimmeridgian to bathonian series with the main objective of determining the nature and flow regime of the fluids responsible for the strong porosity reduction observed in that part of the paris basin mesozoic limestones the approach combined mineralogical and geochemical studies o and c stable isotopic compositions measured by conventional and sims methods with temperature estimates from fluid inclusion analyses the 13 c values of late calcite are close to those of the host limestones and correlate with the 13 c changes in host rocks indicating a local source of c by contrast late calcites from vugs fractures and also in the cemented interparticle porosity of limestones have 18 o values around 21 smow values which are distinct from those of bulk host limestones 25 to 27 the 18 o value of the fluid is estimated to be 6 8 to 2 5 for a crystallisation temperature between 32 and 42 c determined from fluid inclusion studies thus the parent fluid of cemented interparticle porosity and vug and fracture infillings likely contained a component of meteoric water in contrast to the previous belief that the reduction of porosity in those series was related to early marine diagenesis possible situations in which such meteoric fluid could have occurred are i a stage when the limestone aquifers were directly connected to surface water and or ii channelling of water from deeper zones in the basin through regional fault systems the o isotopic compositions of the calcite cements in the underlying 140 m thick callovo oxfordian argillites and those of the less recrystallised bathonian to kimmeridgian limestones are those expected for unmodified marine calcite 18 o ca 28 smow therefore from the isotopic point of view the callovo oxfordian argillites remained essentially unaffected by the paleo fluid circulation documented in the adjacent limestones series this indicates the argillites acted as an efficient permeability barrier in the past

u pb dating of geodic calcite new insights on western europe major tectonic events and associated diagenetic fluids this study presents the first application of the u pb dating method to highly pb depleted diagenetic geodic calcites of the jurassic formations of the paris basin that leads to a reappraisal of the palaeohydrological history of this region composite u pb ages from multiple geodes combined with 18 o analyses reveal two main phases of diagenetic fluid circulations linked with major regional tectonic events dogger formations recorded a first diagenetic fluid episode at 147 8 3 8 ma i e at the very beginning of the emersion of the basin during the tithonian period and 30 ma earlier than previously assumed 18 o results confirmed that most of the calcitic cement phases that closed the porosity of these formations precipitated at the beginning of the cretaceous period oxfordian formations recorded another major meteoric fluid circulation at 33 5 2 8 ma related to the initial stage of the european cenozoic rift system ecris consequently the porosity of the mesozoic formations of the studied area may have been closed sooner than previously thought before the main ecris rifting phase of oligocene age this study shows that u pb dating of secondary geodic calcite offers a new powerful way for reconstructing the coupled palaeohydrological and diagenetic history of sedimentary basins supplementary material pb pb diagrams 238 u and 235 u series isochron diagrams and effect of blank correction on u and pb isotopic compositions and on ages calculation are available at https doi org 10 6084 m9 figshare c 3886885

upper jurassic microsolenid biostromes of northern and central europe facies and depositional environment abstract microsolenid biostromes are a common and important reef type in many upper jurassic reefal sections comparative study of these biostromes in western europe allows the characterisation of this distinct biostrome type in terms of its taxonomic composition palaeoecology and sedimentary regime the biostromes are characterised by a coral community of relatively low generic diversity being dominated by the microsolenidae family of corals the coral skeletal biovolume is generally high and is overwhelmingly dominated by platy ecomorphotypes sedimentologically these biostromes developed in low energy marl and limestone regimes which were subject to low sedimentation rates and varying degrees of siliciclastic influx in contrast to the coral fauna the associated fauna shows a high generic diversity in particular a rich assemblage of echinoids and bivalves in addition the coral plates provide an ideal habitat for a well developed and strongly polarised encrusting and boring fauna top surfaces show limited post mortem encrustation by serpulids and cementing bivalves and are also bored by lithophagan bivalves the undersides of the plates show a highly developed and diverse cryptic fauna of serpulids thecidean brachiopods bryozoans calcareous sponges and sclerosponges it is concluded that the main control on the development of these biostromes was low light intensities though low energy level and low background sedimentation rates were also important it is suggested that relatively high nutrient levels may have also played a significant role the conditions for microsolenid biostrome development prevail in relatively deep water environments and hence these are the first coral dominated reefal structures to have developed in deeper water shallowing upward sequences the corals in these biostromes have adapted to these conditions by developing a strongly platy growth form but also reducing their growth rates substantially the microsolenids are particularly adapted to these environments in two ways 1 they secreted an extremely porous calcium carbonate skeleton which in environments where calcification rates are slow such as deep water environments is a distinct advantage 2 in response to the limited potential of phototrophy in these environments the microsolenids have developed a pennular septal structure which is suggested to be a means of improving heterotrophic feeding trophic analysis of the reef fauna reveals that the nutrient limitation model developed for present day coral reefs cannot be applied to these biostromes it is demonstrated that the microsolena and the present day agariciid leptoseris show a number of significant morphological and ecological similarities

amphibolites with staurolite and other aluminous minerals calculated mineral equilibria in ncfmash amphibolite facies mafic rocks that consist mainly of hornblende plagioclase and quartz may also contain combinations of chlorite garnet epidote and more unusually staurolite kyanite sillimanite cordierite and orthoamphiboles such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites because of the high variance of most of the assemblages the phase relationships in amphibolites depend on rock composition in addition to pressure temperature and fluid composition the mineral equilibria in the na2o cao feo mgo al2o3 sio2 h2o ncfmash model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite kyanite staurolite and cordierite except in aluminous compositions these assemblages are restricted to higher pressures the iron to magnesium ratio x fe and to a lesser extent sodium to calcium ratio have important roles in determining which if any of the aluminous minerals occur under particular pressure temperature conditions where aluminous minerals occur in amphibolites the p t x dependence of their phase relationships is remarkably similar to that in metapelitic rocks the mineral assemblages of fe rich amphibolites are typically dominated by garnet and staurolite bearing assemblages whereas their more mg rich counterparts contain chlorite and cordierite assemblages involving staurolite hornblende can occur over a wide range of pressures 4 10 kbar at temperatures of 560 650 degrees c however except in the more aluminous iron rich compositions they occupy a narrow pressure temperature window thus although their occurrence in typical amphibolites may be indicative of relatively high pressure metamorphism in more aluminous compositions their interpretation is less straightforward

basin scale thermal and fluid flow histories revealed by carbonate clumped isotopes δ47 middle jurassic carbonates of the paris basin depocentre the reconstruction of past diagenetic conditions in sedimentary basins is often under constrained this results from both the analytical challenge of performing the required analyses on the minute sample amounts available from diagenetic mineral phases and the lack of tracers for some of the diagenetic parameters the carbonate clumped isotopes thermometry 47 opens new perspectives for unravelling the temperatures of diagenetic phases together with the source of their parent fluids two parameters that are otherwise impossible to constrain in the absence of exploitable fluid inclusions here is reported the study of a large number of sedimentary and diagenetic carbonate phases from middle jurassic reservoirs of the paris basin depocentre by combining detailed petrographic observations with a large number of 47 data n 45 on a well documented paragenetic sequence including calcite and dolomite burial cements the data reveal carbonate crystallization at temperatures between 29 c and 98 c from fluids with 18owater values between 7 and 2 in response to the progressive burial and uplift of the paris basin throughout 165 myr of basin evolution coupled to the time temperature evolution previously estimated from thermal maturity modelling these temperatures allow determining the timing of four successive cementation episodes the overall dataset indicates a history of complex water mixing with a significant contribution of hypersaline waters from the triassic aquifers migrated upward along faults during the cretaceous subsidence of the basin subsequent large scale infiltrations of meteoric waters induced a dilution of these pre existing brines in response to the paris basin uplift in the tertiary overall the data presented here allow proposing an integrated approach to characterize the cementation events affecting the studied carbonate reservoir units based on temperature oxygen isotope composition and salinity of the parent fluids as well as on petrographic grounds this article is protected by copyright all rights reserved

skeletal aragonite neomorphism in plio pleistocene sandy limestones and sandstones hollywood florida usa abstract the basic mechanics of the neomorphism of aragonitic shells to calcite are generally understood but fundamental questions remain concerning the details of the process completely and partially neomorphosed aragonitic skeletal fragments recovered in cores of plio pleistocene limestones and sandstones from hollywood florida provide some insights into the process skeletal aragonite neomorphism occurred in a volume for volume manner across solution films which resulted in the preservation of ghosts of fine scale microstructures neomorphic calcite crystals that replaced skeletal aragonite in at least some instances inherited their crystallographic orientation from adjoining calcite cement crystals rather than from the host shell microstructure some neomorphic calcite crystals inherited the optical orientation of nearby echinoderm ossicles however host shell microstructure commonly controlled the direction of migration of the neomorphic replacement front anisotropies in shell solubility influenced the directional rates of growth of neomorphic calcite crystals as evidenced by the strong tendency for the boundaries between neomorphic calcite and aragonite to parallel or coinicide with shell microstructural features no examples were observed where neomorphic calcite replaced aragonite along euhedral crystal faces such as commonly occurs during dolomitization and the replacement of limestone by megaquartz

the mineral chemistry of hydrothermally altered and metamorphosed wall rocks at the stollberg fe pb zn mn ag deposit bergslagen sweden the c 1 9 ga old stollberg sulphide and mnrich skarn iron ores and sulphide ores in bergslagen south central sweden are hosted by hydrothermally altered and metamorphosed felsic volcanic and volcaniclastic rocks the ores are underlain by comformable alteration zones characterized by albite gedrite quartz and biotite muscovite plagioclase k feldspar quartz garnet assemblages the present mineralogies are interpreted as medium grade metamorphic equivalents to the original alteration mineral assemblages pt conditions during prograde regional metamorphism are semiquantatively determined to be 510 to 560 c at approximately 3 kbar with increasing modal content of gedrite and biotite in the alteration zones the mg fe ratios and xmg s in octahedral positions of these minerals also increase in the gedrite bearing strata whole rock mg fe ratios remain constant whereas in the biotite rich unit the wholerock mg fe trend is parallel to that of the biotites

are oxygen and carbon isotopes of mollusc shells reliable palaeosalinity indicators in marginal marine environments a case study from the middle jurassic of england stable isotope analyses of non luminescent texturally pristine oyster calcites praexogyra hebridica are presented from bathonian inner ramp limestones in central eastern england a relatively large range of values 18 o 2 0 to 2 3 pdb 13 c 0 9 to 3 7 pdb define a crude positive covariance which can be regionally correlated with an offshore tonearshore increase in faunal restriction this palaeoenvironmental stress gradient has previously been interpreted as a consequence of fluctuating nearshore palaeo salinities due to episodic or seasonal runoff from surrounding hinterlands covariant 18 o 13 c trends from euryhaline bivalves are commonly ascribed to regional salinity gradients the most isotopically depleted examples representing the greatest influence of fresh water however bathonian palaeogeography indictates that the most isotopically depleted oysters inhabited relatively offshore settings assuming an average growth temperature of 25 c these shells were secreted in near normal sea water 18 o 0 4 smow in contrast isotopically heavy oyster calcites were produced in highly evaporated nearshore lagoonal waters 18 o up to 4 8 smow modelling the oxygen isotope and chemical evolution of residual waters during evaporation suggests that up to 65 of the nearshore water mass was lost probably in a complex of hydrodynamically choked lagoons evaporites were not precipitated in the sediment and geochemical modelling suggests that the lagoon waters may have been brackish rather than marine in origin the regional 13 c trend is contrary to that predicted on the basis of palaeosalinity and may have resulted from variations in organic carbon production decay and fixation in early diagenesis

skeletal aragonite neomorphism quantitative modelling of a two water diagenetic system abstract the neomorphism of skeletal aragonite occurs by the simultaneous dissolution of aragonite and precipitation of calcite along thin solution films which may have an associated zone of partly dissolved aragonite chalky zone it has been proposed that pore waters at neomorphic calcite aragonite contact zones neomorphic fronts may be partly isolated from bulk pore waters and thus retain primary geochemical signatures to varying degrees iterative computer programs were developed that simulate aragonite neomorphism in two water diagenetic systems using stable isotope and trace element data neomorphism of aragonitic mollusks from the purbeck group of southern england late jurassic to early cretaceous was associated with a decrease in strontium concentration from approximately 5100 ppm to 500 ppm modelling results indicate a high bulk water rock ratio and minimal if any isolation of the neomorphic front waters for the purbeck group mollusks and also for some pleistocene corals partial isolation of neomorphic front waters from bulk pore waters is uncommon based on published data on the geochemistry of neomorphic calcites however partial isolation does occur in some thick low porosity shells such as the gastropod strombus gigas pleistocene process based modelling of fluid rock interaction provides a means for better interpreting trace element and stable isotope data in diagenetic carbonates sensitivity analysis results indicate that water rock ratios cannot be determined at much better than order of magnitude accuracies because of uncertainties over initial pore water composition fractionation factors and distribution coefficients

coupled δ13c and 87sr 86sr chemostratigraphy of kimmeridgian shoal water deposits a new composite record from the lower saxony basin germany abstract correlation of shallow marine carbonate deposits with their coeval pelagic counterparts is often hampered by the lack of open marine stratigraphic marker fossils and by the restrictions of regional biostratigraphic schemes these difficulties can partly be overcome by the use of coupled 13c and 87sr 86sr chemostratigraphy based on pristine shell calcite and bulk rock material here a new high resolution chemostratigraphic framework covering kimmeridgian strata is presented based on three stratigraphic sections located in the subboreal lower saxony basin lsb of northern germany combined with sedimentological data the bulk rock 13c signal is critically evaluated for both diagenetic alteration and local environmental effects in general the 13c signatures are considered to predominantly record the global marine signal diagenetic and facies related discrepancies are restricted to dolomitic supratidal back ramp and intertidal back ramp deposits respectively strontium isotope stratigraphic data obtained from diagenetically screened low mg calcite shells refine the existing ostracod biostratigraphic scheme in the lower saxony basin lsb and contribute to the existing global strontium isotope dataset of the kimmeridgian these new results confirm the potential of well preserved low mg calcite from shallow marine settings to preserve a global marine sr isotope signal calibrated by the integrated biostratigraphic and strontium isotope results a high resolution composite 13c record for kimmeridgian shoal water deposits is established comparison of the new composite 13c curve from the subboreal lower saxony basin lsb with existing kimmeridgian records from the peri tethyan and western tethyan realms provides new insights into the long term global carbon cycle during the kimmeridgian