relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

diagenesis of siliciclastic and volcaniclastic sediments in the cretaceous and miocene sequences of the nw african margin dsdp leg 47a site 397 abstract the lower cretaceous and miocene sequences of the nw african passive continental margin consist of siliciclastic volcaniclastic and hybrid sediments these sediments contain a variety of diagenetic carbonates associated with zeolites smectite clays and pyrite reflecting the detrital mineralogical composition and conditions which prevailed during opening of the north atlantic in the lower cretaceous siliciclastic sediments siderite 6 to 0 7 18 o pdb 19 6 to 0 6 13 c pdb was precipitated as thin layers and nodules from modified marine porewaters with input of dissolved carbon from the alteration of organic matter microcrystalline dolomite layers lenses nodules and disseminated crystals 3 0 to 2 5 18 o pdb 7 2 to 4 9 13 c pdb predominate in slump and debris flow deposits within the lower miocene sequence during the opening of the atlantic volcanic activity in the canary islands area resulted in input of volcaniclastic sediments to the middle and upper miocene sequences calcite is the dominant diagenetic carbonate in the siliciclastic bioclastic volcaniclastic hybrid and in the volcaniclastic sediments which commonly contain pore rimming smectite diagenetic calcite 22 to 1 6 18 o pdb 35 7 to 0 8 13 cpdb was precipitated due to the interaction of volcaniclastic and bioclastic grains with marine porewaters phillipsite is confined to the alteration of volcaniclastic sediments whereas clinoptilolite is widely disseminated occurring essentially within foraminiferal chambers and formed due to the dissolution of biogenic silica

diagenesis and formation water chemistry of triassic reservoir sandstones from southern tunisia the fluvial triassic reservoir subarkoses and arkoses 2409 5 2519 45 m of the el borma oilfield southern tunisia were subjected to cementation by haematite anatase infiltrated clays kaolinite and k feldspar at shallow burial depths from meteoric waters subsequently basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths magnesian siderite minor ferroan magnesite and anhydrite the enrichment of siderite in 12c isotope 13cpdb 14 5 to 9 results from derivation of carbon from the thermal decarboxylation of organic matter during further burial the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred followed by the formation of microcrystalline k feldspar and quartz chlorite and illite prior to the emplacement of oil present day formation waters are na ca cl brines evolved by the evaporation of seawater and water mineral interaction and are in equilibrium with the deep burial 3 1 km minerals these waters are suggested to be derived from the underlying silurian and devonian dolomitic mudstones

mineralogical and isotopic record of biotic and abiotic diagenesis of the callovian oxfordian clayey formation of bure france abstract the callovian oxfordian cox clayey unit is being studied in the eastern part of the paris basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal the present study combines new mineralogical and isotopic data to describe the sedimentary history of the cox unit petrologic study provided evidence of the following diagenetic mineral sequence 1 framboidal pyrite and micritic calcite 2 iron rich euhedral carbonates ankerite sideroplesite and glauconite 3 limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks 4 chalcedony 5 quartz calcite pyrite in bioturbations shows a wide range of 34s 38 to 34 5 providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions the most negative values 38 to 22 measured in the lower part of the cox unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply the most positive pyrite 34s values 14 up to 34 5 in the upper part of the cox unit indicate pyrite precipitation in a closed system celestite 34s values reflect the last evolutionary stage of the system when bacterial activity ended however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution the 87sr 86sr ratio of celestite 0 706872 0 707040 is consistent with deposition from jurassic marine derived waters carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates siderite only present in the maximum clay zone has chemical composition and 18o consistent with a marine environment its 13c is however lower than those of marine carbonates suggesting a contribution of 13c depleted carbon from degradation of organic matter 18o values of diagenetic chalcedony range between 27 and 31 suggesting precipitation from marine derived pore waters late calcite crosscutting a vein filled with chalcedony and celestite and late euhedral quartz in a limestone from the top of the formation have lower 18o values 19 suggesting that they precipitated from meteoric fluids isotopically close to present day pore waters of the formation finally the study illustrates the transition from very active biotic diagenesis to abiotic diagenesis this transition appears to be driven by compaction of the sediment which inhibited movement of bacterial cells by reduction of porosity and pore sizes rather than a lack of inorganic carbon or sulphates

root related rhodochrosite and concretionary siderite formation in oxygen deficient conditions induced by a ground water table rise sedimentological mineralogical stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth high 13c values 6 4 on average indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated the development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp related to a rise of the ground water table large amounts of decaying plant debris led to efficient oxygen consumption within the pore water in the peat oxygen depletion in combination with a decrease in sedimentation rate promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation this shows how a change in sedimentary conditions can have a profound impact on the early diagenetic environment and carbonate authigenesis the concretions contain numerous rhizoliths they are cemented with calcium rhodochrosite a feature which has not been reported before the rhodochrosite cement has negative 13c values 16 5 on average and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction the exceptional preservation of the epidermis exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water logged sediments rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots while siderite cementation occurred simultaneously around them in anoxic conditions these suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits this model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures

constraints on palaeoenvironments in the lower cretaceous wealden of southern england from the geochemistry of sphaerosiderites abstract sphaerosiderites are millimetre scale concretions of feco 3 that form predominantly in waterlogged environments and are of use in palaeoenvironmental and palaeohydrological studies we present petrographic elemental and stable isotopic 13 c 18 o data from sphaerosiderites collected from the lower cretaceous wealden sediments of southern england the sphaerosiderites are composed of very pure feco 3 with only small amounts of ca mg and mn present suggesting that they are well preserved and were precipitated from fresh groundwater 13 c values display a relatively large range from about 5 to 30 vpdb in contrast 18 o values are relatively invariant 1 0 83 with an average of 3 01 vpdb the relationship between carbon and oxygen suggests that the sphaerosiderites faithfully record palaeo groundwater 18 o likely 18 o values for groundwater were calculated using a published experimentally derived siderite water fractionation equation and plausible assumed values for palaeotemperature the average estimate was 4 9 smow the wealden sphaerosiderite oxygen isotope values are comparable with other early cretaceous data from similar palaeolatitudes but lower than estimated holocene values for equivalent latitudes this observation may indicate greater rainfall at 30 n during the cretaceous relative to the holocene and also suggests relative stability of palaeoclimatic conditions during the early cretaceous at this palaeolatitude

stratigraphy and age estimate of neogene shallow marine fossiliferous deposits in brunei darussalam ambug hill tutong district abstract the ambug hill in brunei is an exceptional geological site where a series of siliciclastic rocks crops out with some layers extremely rich in marine fossils such fossiliferous outcrops are extremely rare in the northern part of borneo and their description is of primary importance as their fossil content can be used to correlate the regional depositional sequences with global biostratigraphic zonations in this work we present for the first time a detailed sedimentary profile completed with sr isotopes and biostratigraphic dating the succession is divided into four sedimentary units the first unit comprises bioturbated sandstone followed by a second unit of clay silt rich levels whose first 9 5 m contain rich marine fossil assemblages calcareous nannoplankton data indicate a late tortonian early messinian nn11 age which is confirmed by sr isotope dating derived from bivalves giving a numerical age range from 8 3 to 6 2 my after a major emersion surface the third sedimentary unit of sand and siltstone lacks suitable fossils for bio and isotope stratigraphy the age of emersion and the related sedimentary gap can be either correlated with the me1 7 25 my or the me2 5 73 my sequence boundary a fourth thin sedimentary unit is recognized on the top of the profile with silt and claystone beds without age diagnostic remains and calcareous fossils

megaflora and palaeoclimate of a late oligocene tropical delta makum coalfield assam evidence for the early development of the south asia monsoon abstract the fossil megaflora from the late oligocene deltaic sediments exposed in the tirap coal mine assam india 27 17 20 n 95 46 15 e laid down at a palaeolatitude of 10 15 n records the composition and climate of tropical vegetation during the last episode of pronounced global warmth before the progressive overall cooling to present day conditions using a new calibration of the climate leaf analysis multivariate program clamp that includes 18 sites from modern vegetation in india the analysis of 80 fossil leaf morphotypes from the tirap mine indicates a mean annual temperature mat of 26 1 2 7 c a warm month mean temperature wmmt of 27 9 3 3 c and a cmmt of 20 1 4 3 c with adjustments for evapotranspirational cooling and underestimation of the wmmt at high temperatures these can be revised upwards to a minimum mat of 28 3 3 7 c a cmmt of 23 5 5 c and a wmmt of 33 6 5 2 c maximum estimates based on evaporative cooling seen in a modern analogue of the tirap environment yield an mat of 32 9 3 8 c a cmmt of 26 8 6 c and a wmmt of 39 7 1 c all uncertainties are 2 sigma precipitation estimates reveal a marked annual variation in rainfall showing a wet season with 20 times the rainfall of the dry season this is a similar regime to that seen in the sunderbans in the modern ganges brahmaputra meghna delta we interpret this to suggest that the south asian monsoon was already established by late oligocene times at an intensity similar to that of today

palaeoenvironment of the upper cretaceous coniacian concretion bearing lagerstatten from poland abstract abundantly fossiliferous upper cretaceous sediments including decapod remains in the region of stary waliszow southern poland have been known for many years to date however no study has dealt specifically with detailed palaeoenvironmental and palaeoecological reconstruction the origin of these sediments is still uncertain and has been the source of considerable confusion and controversy mainly due to the strong bioturbation and decalcification of these sediments by using the fossil record comparative taphonomy and geochemical analyses of concretions we provide new insights into the paleoenvironment and the paleoecology of this unique coniacian ecosystem on the basis of ichno and body fossil assemblages composed almost exclusively of benthic organisms such as decapods molluscs and echinoderms which are commonly disarticulated but rarely abraded we suggest that the fossiliferous sediments in stary waliszow were formed in shallow normal saline and well oxygenated waters of the upper sublittoral zone with moderate sedimentation rate and low turbulence of waters the presence of angular terrigenous clasts well preserved delicate land flora and the near absence of planktonic and nektonic fossils strongly imply proximity to the land recorded taxa inhabited a soft bottom environment some organisms were fossilized inside siderite concretions these concretions were formed within the sediment below the zone of active bioturbation during the early stage of diagenesis remarkable preservation of decapod remains is due to phosphatization which replaced the cuticle

facies related diagenesis and multiphase siderite cementation and dissolution in the reservoir sandstones of the khatatba formation egypt s western desert in the fluvio deltaic quartz arenitic sandstones of the jurassic khatatba formation in the salam field egypt9s western desert diagenesis and sedimentary facies control reservoir quality fluvial channel sandstones have the highest porosities 10 15 and permeabilities 100 600 md in part because of siderite cementation which 1 inhibited compaction and quartz cement and 2 was later dissolved creating intergranular secondary porosity 1 4 of total porosity fluvial crevasse splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive shallow burial fe dolomite cement respectively siderite precipitation was multiphase and separated by distinct dissolution events the earliest siderite precipitated near surface within suboxic tropical coastal swamps containing predominantly meteoric waters some influence of marine waters is indicated by local enrichments in mg and ca the next major siderite generation shows a trend to decreasing mn and ca contents and is of shallow burial origin the last major siderite phase is mg rich and interpreted as deeper burial in origin some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities the most important dissolution however occurred during deep burial resulting in 1 a major corrosion surface predating the last mg rich zone 2 selective dissolution of some earlier zones and 3 secondary porosity this burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults other diagenetic phases observed include early diagenetic pyrite kaolin quartz bitumen and late diagenetic barite and illite kaolinite precipitated at shallow depths 130 c during and or after rapid late cretaceous burial quartz developed in two phases separated by oil migration this paper illustrates that in fluvio deltaic quartz arenites deposited under the influence of humid tropical climate reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies this paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages including early shallow burial and deep burial diagenesis the present study also shows that in multiphase siderite cements the earlier growth zones can be selectively dissolved and replaced by later siderite zones i e recrystallized during burial diagenesis these findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier formed siderites are essentially stable during diagenesis

polygenetic history of paleosols in middle upper pennsylvanian cyclothems of the illinois basin u s a part ii integrating geomorphology climate and glacioeustasy abstract distinct lateral and stratigraphic trends in paleosol morphology and clay mineralogy in conjunction with the stable isotope composition of sub millimeter scale spherulitic siderite sphaerosiderite and flint clay kaolinite in middle upper pennsylvanian cyclic coal bearing strata of the illinois basin ib presented herein provide proxy records of middle late pennsylvanian equatorial terrestrial environments collectively these data provide a better understanding of the polygenetic history of ancient soils preserved in cyclic strata and indicate that low latitude pennsylvanian ib soil profiles were influenced by a combination of autogenic and allogenic controls lateral variations in paleosol morphology from the interior of the basin to the northern margin indicate that soil development was dominantly influenced by autogenic controls such as differences in local paleohydrology i e groundwater fluctuations and subsidence rates across the basin conversely allogenic controls i e glacioeustasy and climate are interpreted to be principally responsible for the preservation of features in gleyed protosols gleyed vertisols and gleyed calcic vertisols that occur in the interior of the ib these paleosols are characterized by a polygenetic development history that includes 1 an initial period of soil formation in well drained environments with seasonal wetting and drying of the profile followed by 2 a subsequent period of waterlogging and development of reducing conditions paleosols that lack evidence for extensive periods of waterlogging are restricted with the exception of two examples to the northern margin of the basin and they become abundant in the stratigraphic record near the middle upper pennsylvanian boundary uppermost desmoinesian the stratigraphic distribution of these paleosols in conjunction with a decrease in the relative abundance of kaolinite in the 2 m size fraction of ib paleosols near the desmoinesian missourian d m boundary is interpreted to reflect a shift in the overall low latitude regional climate to drier conditions the stable isotope compositions of sphaerosiderite and flint clay kaolinite in middle upper pennsylvanian strata of the ib provide an additional proxy record of past equatorial terrestrial environments and are used to constrain the magnitude of paleoclimate change across the d m moscovian kasimovian boundary sphaerosiderite 18ov pdb and 13cv pdb values range from 3 6 to 0 5 and 12 8 to 3 2 respectively in particular sphaerosiderite 18o values from the b horizons of paleosols display an approximate 1 5 shift across the d m boundary this stratigraphically short isotopic shift occurs across the d m boundary on the order of one cyclothem 400 kyr and is followed by a subsequent shift on the order of one cyclothem back to more positive siderite 18o values the average 18ov smow and dv smow values of kaolinite isolated from the 0 2 m size fraction of a latest desmoinesian flint clay breccia are 21 0 and 46 5 these values correspond to a latest desmoinesian crystallization temperature of 27 2 c and a meteoric water 18ov smow value of 3 1 combination of sphaerosiderite 18o values and co occurring flint clay kaolinite d and 18o values suggest a possible warming of up to 6 c across the d m boundary corresponding to an approximate temperature change of siderite crystallization from 27 to 33 c this temperature increase in conjunction with distinct stratigraphic trends in the clay mineralogy and morphology of ib paleosols is consistent with previous paleoclimate interpretations and it suggests that low latitude pennsylvanian paleoclimate became drier and warmer across the d m boundary

the origin of coexisting carbonates in banded iron formations a micro mineralogical study of the 2 4 ga itabira group brazil this is the first petrographic and chemical study on the origin of co existing calcite dolomite ankerite and sideroplesite in the 2 4 ga itabira group brazil the itabira group hosts silicic and dolomitic bifs known as quartz itabirites and carbonate itabirites which are composed of micritic quartz and dolomite with inclusions of nano and micro hematite nanometric si bearing ferrihydrite clusters are observed in the micritic dolomite mineralogical and textural features indicate a primary oxidizing depositional environment for the itabira group at 2 4 ga limestones of the itabira group include early diagenetic hematite nodules intergrown with dolomite silica in the ferrihydrite structure increases its stability up to 500 c and the nano and micro hematite included in dolomite and quartz were partially protected from later transformation by being enclosed within the dolomite and quartz crystals positive eu and y anomalies and heavy rare earth element enrichments of all lithofacies along with their low terrestrial element abundance excluding the shales indicate their primary precipitation from a mixed hydrothermal marine environment in the case of the carbonate itabirites fluids were co2 rich dolomite precipitated abiotically due to higher temperatures 100 c and high mg ca ratios the facies changes from carbonate rich to quartz rich are attributed to transgression and regression episodes and or a change in the upwelling hydrothermal fluid composition post depositional reductive dissolution affected the quartz itabirites facilitating the precipitation of ankerite sideroplesite and magnetite sulphides from a reducing diagenetic solution rich in mn mg and fe either as lenses or disseminated in the quartz itabirites and shales the sideroplesite richest samples have the most negative 13c values quartz itabirite 9 49 shale 5 76 indicating that at least part of the c is of organic origin either through the oxidation of organic matter or introduced via the diagenetic fluids specularite is replacive and likely related to a post depositional hydrothermal event this study shows that careful mineralogical investigations combined with ree y and trace element geochemistry is necessary in order to decipher the depositional environments which are oxidizing mixed marine hydrothermal post depositional reductive diagenesis masks in part these primary conditions

in situ stable isotopic evidence for protracted and complex carbonate cementation in a petroleum reservoir north coles levee san joaquin basin california u s a abstract knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation the stevens sands is a sequence of marine shales and deep sea fan sands that was deposited within the miocene monterey formation in the south central part of the san joaquin basin california during the upper miocene 10 6 ma rapid high precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi collection mode in conjunction with electron microprobe analyses indicate that carbonate cement zones within the stevens sands at north coles levee ncl have had a complex and protracted fluid history three main generations of carbonate cement were identified the relative timing of carbonate cement precipitation within the stevens sands at ncl was estimated using the thermal and burial history of the san joaquin basin in situ oxygen isotope data and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite water and dolomite water precipitation of these cement zones began soon after sediment deposition 7 ma and is ongoing early dolomite was precipitated at a temperature of 10 c near the sediment water interface and soon after sediment deposition calcite cements which are the most abundant variety precipitated semicontinuously between 4 ma and 5 ma at temperatures between 50 c and 65 c and depths of 800 m to 1300 m fe dolomite which is paragenetically late appears to have precipitated at temperatures near 100 c in response to pore pressure reduction which accompanied exploitation of the gas cap within the last 35 years carbon in these cements was likely derived from several sources including marine maturing hydrocarbons and a zone of methanogenesis

sideritic ironstones as indicators of depositional environments in the weald basin early cretaceous se england the lower cretaceous wealden sideritic ironstones have a wide occurrence and great potential to aid the reconstruction of the depositional environments of the weald basin in se england however mineralogical and geochemical datasets on the ironstones are scarce in the literature geochemical and mineralogical data on the sideritic ironstones are presented from the wadhurst clay formation within the weald basin the mineralogy of the ironstones was examined using a panalytical x pert pro x ray diffractometer and panalytical s highscore plus software elemental composition of the ironstones was measured using a panalytical minipal2 ed xrf benchtop x ray spectrometer the examination of the mineralogy of the wealden ironstones confirms the presence of early diagenetic siderites the trace element assemblage shows that the sideritic ironstones are chemically pure pointing to a freshwater origin the sideritic ironstones reveal anoxic conditions and palaeo salinity in the basin more generally it is suggested that the composition of the host rocks has significant controls on the composition of sideritic ironstones in sedimentary basins this work reinforces the importance of the composition of sideritic ironstones as useful non traditional data for understanding the depositional settings of sedimentary basins especially when traditional datasets are not readily available or insufficient

distinguishing base level change and climate signals in a cretaceous alluvial sequence we present the results of oxygen isotope and electron microprobe analyses of sphaerosiderites obtained from cretaceous paleosols in iowa the sphaerosiderite d 18 o values record creta ceous meteoric groundwater chemistry and an overall waning of brackish groundwater inundation during alluvial plain aggrada tion and soil genesis we focus on horizons that precipitated from freshwater in which d 18 o values ranging from 23 3 to 26 8 relative to the peedee belemnite standard are interpreted to record variations in the cretaceous atmospheric hydrologic cycle during relative sea level highstands moisture was derived from the cre taceous western interior seaway whereas during lowstands when the seaway narrowed and occasionally withdrew from the midcon tinent the dominance of hemispheric scale atmospheric moisture transport initiated in the tropical tethys ocean led to decreased precipitation rates these processes did not operate like a switch but rather as a continuum of competing moisture sources and mechanisms of transport between the nearby epicontinental sea and the distant tropics the sphaerosiderite data demonstrate 1 temporal variation in the intensity of hemispheric scale atmospher ic moisture transport and 2 long term amplification of the global hydrologic cycle marked by extreme 18 o depletion at the albian cenomanian boundary

diagenetic overprinting of the sphaerosiderite palaeoclimate proxy are records of pedogenic groundwater delta18o values preserved meteoric sphaerosiderite lines msls defined by invariant d 18 o and variable d 13 c values are obtained from ancient wetland palaeosol sphaerosiderites millimetre scale feco3 nodules and are a stable isotope proxy record of terrestrial meteoric isotopic compositions the palaeoclimatic utility of sphaerosiderite has been well tested however diagenetically altered horizons that do not yield simple msls have been encountered well preserved sphaerosiderites typically exhibit smooth exteriors spherulitic crystalline microstructures and relatively pure 95 mol feco3 compositions diagenetically altered sphaerosiderites typically exhibit corroded margins replacement textures and increased crystal lattice substitution of ca 2 m g 2 and mn 2 for fe 2 examples of diagenetically altered cretaceous sphaerosideritebearing palaeosols from the dakota formation kansas the swan river formation saskatchewan and the success s2 formation saskatchewan were examined in this study to determine the extent to which original early diagenetic d 18 o and d 13 c values are preserved all three units contain poikilotopic calcite cements with significantly different d 18 o and d 13 c values from the co occurring sphaerosiderites the complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses the dakota and swan river samples ultimately yield distinct msls for the sphaerosiderites and mcls meteoric calcite lines for the calcite cements the success s2 sample yields a covariant d 18 o vs d 13 c trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters the calcite cements in the success s2 formation yield meteoric d 18 o and d 13 c values a stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end member compositions modelled hyperbolic fluid mixing curves for the success s2 formation suggest precipitation from fluids that were 25 sea water

the crystallographic fabric and texture of siderite in concretions implications for siderite nucleation and growth processes the crystallographic fabric of siderite in siderite concretions has been determined for upper carboniferous westphalian a non marine concretions and lower jurassic pliensbachian marine concretions compositional zoning indicates that individual siderite crystals grew over a period of changing pore water chemistry consistent with the concretions being initially a diffuse patch of cement which grew progressively the siderite crystallographic fabric was analysed using the anisotropy of magnetic susceptibility which is carried by paramagnetic siderite the siderite concretions from marine and non marine formations exhibit differences in fabric style although both display increases in the degree of preferred siderite c axis orientation towards the concretion margins the westphalian non marine siderites show a preferred orientation of siderite c axes in the bedding plane whereas the pliensbachian marine siderites have a preferred orientation of c axes perpendicular to the bedding in addition a single marine concretion shows evidence of earlier formed inclined girdle type fabrics which are intergrown with later formed vertical c axis siderite fabrics the marine and non marine fabrics are both apparently controlled by substrate processes at the site of nucleation which was probably clay mineral surfaces siderite nucleation processes on the substrate were most probably controlled by the bio chemistry of the pore waters which altered the morphology and crystallographic orientation of the forming carbonate the preferred crystallographic orientation of siderite results from the orientation of the nucleation substrate fabric changes across the concretions partially mimic the progressive compaction induced alignment of the clay substrates while the concretion grew during burial

long term observation of permeability in sedimentary rocks under high temperature and stress conditions and its interpretation mediated by microstructural investigations in this study a series of long term intermittent permeability experiments utilizing berea sandstone and horonobe mudstone samples with and without a single artificial fracture is conducted for more than 1000 days to examine the evolution of rock permeability under relatively high temperature and confining pressure conditions effluent element concentrations are also measured throughout the experiments before and after flow through experiments rock samples are prepared for x ray diffraction x ray fluorescence and scanning electron microscopy coupled with energy dispersive x ray spectroscopy to examine the mineralogical changes between pre and postexperimental samples and also for microfocus x ray ct to evaluate the alteration of the microstructure although there are exceptions the observed qualitative evolution of permeability is found to be generally consistent in both the intact and the fractured rock samples the permeability in the intact rock samples increases with time after experiencing no significant changes in permeability for the first several hundred days while that in the fractured rock samples decreases with time an evaluation of the damkohler number and of the net dissolution using the measured element concentrations reveals that the increase in permeability can most likely be attributed to the relative dominance of the mineral dissolution in the pore spaces while the decrease can most likely be attributed to the mineral dissolution crushing at the propping asperities within the fracture taking supplemental observations by microfocus x ray ct and using the intact sandstone samples a slight increase in relatively large pore spaces is seen this supports the increase in permeability observed in the flow through experiments

xas evidence for ni sequestration by siderite in a lateritic ni deposit from new caledonia mineralogical and spectroscopic analyses were conducted on a lateritic ni deposit from southern new caledonia results show that ni is incorporated in siderite feco3 found between 37 and 40 m depth in the laterite and saprolite units of the regolith sem edxs analyses of siderite rich samples indicate that a significant amount of nickel can be hosted by this crystalline phase 0 8 wt nio ni and fe k edge extended x ray absorption fine structure exafs spectroscopic analyses of the siderite rich samples from the regolith as well as comparison with synthetic ni bearing and ni free siderites demonstrate isomorphous substitution of ni2 for fe2 in the siderite structure linear combination fitting lcf of the ni k edge exafs data reveals that this ni bearing siderite species accounts for more than 90 of the total ni pool 1 wt nio in the siderite rich horizons of the regolith in addition lcf analysis of the exafs spectra indicates that goethite and serpentine are the major ni hosts in the upper horizons laterite and lower horizons saprolite of the regolith respectively formation of siderite an uncommon mineral species in such oxidized environments is attributed to the development of swampy conditions in organic rich lateritic materials that accumulated at the bottom of dolines these results thus show the importance of siderite as a host for nickel in lateritic ni deposits that have been affected by late hydromorphic and reducing conditions

high latitude meteoric δ18o compositions paleosol siderite in the middle cretaceous nanushuk formation north slope alaska siderite bearing pedogenic horizons of the nanushuk formation of the north slope alaska provide a critical high paleolatitude oxygen isotopic proxy record of paleoprecipitation supplying important empirical data needed for paleoclimatic reconstructions and models of greenhouse world precipitation rates siderite 18 o values were determined from four paleosol horizons in the national petroleum reserve alaska npr a grandstand 1 core and the values range between 17 6 and 14 3 peedee belemnite pdb with standard deviations generally less than 0 6 within individual horizons the 13 c values are much more variable ranging from 4 6 to 10 8 pdb a covariant 18 o versus 13 c trend in one horizon probably resulted from mixing between modified marine and meteoric phreatic fluids during siderite precipitation groundwater values calculated from siderite oxygen isotopic values and paleobotanical temperature estimates range from 23 0 to 19 5 standard mean ocean water smow minor element analyses show that the siderites are impure having enrichments in ca mg mn and sr minor element substitutions and mg fe and mg ca mg ratios also suggest the influence of marine fluids upon siderite precipitation the pedogenic horizons are characterized by gleyed colors rare root traces abundant siderite abundant organic matter rare clay and silty clay coatings and infillings some preservation of primary sedimentary stratification and a lack of ferruginous oxides and mottles the pedogenic features suggest that these were poorly drained reducing hydromorphic soils that developed in coal bearing delta plain facies and are similar to modern inceptisols model derived estimates of precipitation rates for the late albian of the north slope alaska 485 626 mm yr are consistent with precipitation rates necessary to maintain modern peat forming environments this information reinforces the mutual consistency between empirical paleotemperature estimates and isotope mass balance models of the hydrologic cycle and can be used in future global circulation modeling gcm experiments of greenhouse world climates to constrain high latitude precipitation rates in simulations of ancient worlds with decreased equator to pole temperature gradients

origin and timing of siderite cementation in upper ordovician glaciogenic sandstones from the murzuq basin sw libya the origin and timing of siderite cementation have been constrained in relation to depositional facies and sequence stratigraphy of upper ordovician glaciogenic sandstones from the murzuq basin sw libya optical microscope backscattered electron imagery and carbon and oxygen stable isotope analysis have revealed that siderite is of eo and mesogenetic origin eogenetic siderite is mg poor with a mean composition of fe91 7mg1 5ca0 3mn6 5 co3 and occurs in paraglacial tide dominated deltaic highstand systems tract hst sandstones in paraglacial foreshore to shoreface hst sandstones and in postglacial gilbert type deltaic lowstand systems tract lst sandstones this siderite is typically of meteoric water origin that influxed into the lst and hst sandstones during relative sea level fall and basinward shift of the strandline mesogenetic siderite which engulfs and thus postdates quartz overgrowths and illite is mg rich with a mean composition of fe72 2mg21 7ca0 8mn5 3 co3 and occurs in the paraglacial tide dominated deltaic hst sandstones in paraglacial foreshore to shoreface hst sandstones in glacial tide dominated estuarine transgressive systems tract tst sandstones in postglacial gilbert type deltaic lst sandstones and in postglacial shoreface tst sandstones 18ov pdb values of this siderite which range between 22 6 and 13 8 suggest that precipitation has occurred from evolved formation waters with 18o values between 14 0 and 1 0 and was either meteoric mixed marine meteoric and or marine in origin by assuming postdating quartz overgrowths and illite temperature between 80 and 130 c

meteoric water incursion into marine turbditic sandstones evidence from the andrew formation paleocene uk central graben north sea abstract petrographic and stable carbon and oxygen isotope study revealed extensive meteoric water incursion during diagenesis of deep water marine paleocene turbidite sandstones in the north sea andrew formation uk central graben meteoric water diagenesis resulted in the dissolution and kaolinitization of framework silicates and cementation by calcite and methanogenic siderite and fe dolomite ankerite incursion is envisaged to have occurred over large distances along well connected permeable shelf slope and turbidite sand bodies during major fall in sea level which resulted in exposure of the continental shelf this work urges for consideration of the impact of undersaturated meteoric waters on reservoir quality modifications during hydrocarbon exploration in marine turbidite sandstones

morphologic clues to the origins of iron oxide cemented spheroids boxworks and pipelike concretions navajo sandstone of south central utah u s a abstractconcretions cemented by iron oxide are abundant and diverse in the jurassic navajo sandstone of southern utah some of these structures are considered terrestrial analogs for concretions imaged on mars the navajo concretions can be spheroidal pipelike or tabular with multicompartmented boxworks these iron oxide concretions typically display a rinded structure dense sandstone rinds cemented by iron oxide surround pale iron poor sandstone cores within a single structure the iron rich rinds may be single or multiple pseudomorphs of siderite are present in local residual iron rich cores of boxworks workers in the late nineteenth through mid twentieth centuries many of whom found evidence for siderite precusors concluded that many spherical rinded iron oxide cemented concretions were formed by centripetal precipitation of iron oxide inward from the perimeter of the concretion in contrast the walls of pipelike concretions of iron oxide grew centrifugally outward we interpret the navajo

petrology and palaeoenvironmental significance of authigenic iron rich clays carbonates and apatite in the claiborne group middle eocene ne texas the claiborne group eocene in north east texas consists of clayey sandstones and mudrocks both with variable proportions of dark green to brown clay pellets deposited in a marginal to open marine setting on the gulf coast margin the composition of concretionary carbonate and clay pellets from 5 localities has been investigated using a wide variety of analytical techniques our palaeontological sedimentological c s and stable isotope data support an interpretation of marginal marine facies with limited freshwater input for the claiborne group dark green mature pellets are predominantly associated with transgressive bioturbated marine sediments from the weches formation through to the youngest crockett formation there is a trend from pellets composed mostly of dark green serpentine rich mixed layer clay to those of mostly brown immature nontronite rich mixed layer clay the serpentine is intermediate between odinite and berthierine and may have been originally odinite that has undergone partial transformation to berthierine pellet maturity apparently corresponds with the degree of winnowing and sedimentation rate it does not appear to correspond with global temperature fluctuations though the serpentine rich rather than glauconite rich composition may do siderite cement mostly in the form of concretions post dates the iron rich clay pellets and pre dates calcite cement carbonate and apatite concretions show a wide range of mineral cement textures reflecting variation in sedimentary environment and early diagenetic processes based on water temperatures derived from verneriacardia bivalves a precipitation temperature of 20 c is assumed for the concretionary siderite using this temperature we surmise that the siderite precipitated from marine pore waters with intermittent meteoric water mixing

stratiform siderites from the mesoproterozoic xiamaling formation in north china genesis and environmental implications abstract stratiform or layered siderites are much less common than siderite concretions in the stratigraphic record in the middle part of the mesoproterozoic xiamaling formation ca 1 40 1 35 ga layered siderites are widespread across the north china platform facies analysis indicates that the xiamaling siderites were formed in shallow water environments above the storm wave base high fe al and v al ratios and the lack of pyrites in siderite bearing strata indicate ferruginous bottom water conditions during the formation of siderites the low thermal maturity of organic matter and clay mineral structures suggest that the layered siderites were formed at burial temperatures 90 c the negative 13c values 24 to 12 of siderites and the presence of m scale hematite inclusions in siderite grains suggest that dissimilatory iron reduction dir during early diagenesis has played critical roles in siderite formation the source of fe oxyhydroxides required for dir could be from fe ii oxidation in shallow seawater above the redoxcline in response to enhanced hydrothermal activities or alternatively directly from continental fe oxide input the positive cerium ce and europium eu anomalies of clay minerals in siderite bearing strata and their positive correlation with total fe contents tfe2o3 seem to favor a hydrothermal origin of iron syndepositional hyalophane concretions intermittent tuff layers and remarkable enrichment of cu and zn in some of the siderite hosting rocks provide further support to hydrothermally sourced iron the enhanced hydrothermal activity for the development of stratiform siderites in the xiamaling formation probably related to the final breakup of the north china craton from the columbia supercontinent during which active hydrothermal systems provided sufficient fe ii for fe redox recycling and formation of layered siderites

early diagenesis and its relationship to depositional environment and relative sea level fluctuations upper cretaceous marshybank formation alberta and british columbia early diagenesis of the upper cretaceous late coniacian to early santonian marshybank formation was controlled by depositional environment composition of depositional water fe and organic content of the sediment sedimentation rate proximity to the shoreline and influx of meteoric water related to relative sea level fall five depositional environments each characterized by a distinct early diagenetic mineral assemblage have been recognized offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions composed of early calcite and siderite intense sulphate reduction promoted by the dysaerobic depositional water was the primary influence on early diagenesis offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions composed of two generations of siderite s1 and s2 in this environment methanogenesis rather than sulphate reduction was more important early diagenesis of the inner shelf sands was generally limited however in sands deposited proximal to the shoreline mixing of marine and meteoric waters promoted crystallization of fe rich chlorite and siderite the shoreface was characterized by dissolution of detrital minerals in the upper portion and precipitation of kaolinite or illite smectite in the lower portion in the coastal plain environment brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite ankerite rather than siderite is the typical early diagenetic carbonate the 18o values of the earliest cements i e early calcite siderite s1 inner shelf siderite indicate crystallization from a low 18o marine derived porewater assuming crystallization at 25 c a 18o value of about 7 smow can be estimated for the seaway during marshybank formation time similar calculations for the overlying dowling member puskwaskau formation suggest that the 18o value of the seaway increased to about 4 smow consistent with its transgressive nature very low 18o values are exhibited by siderite s2 these results indicate crystallization during intermediate diagenesis 60 c from meteoric water 15 smow that entered the marshybank formation during sea level lowstand

early diagenetic siderite as an indicator of depositional environment in the triassic rewan group southern bowen basin eastern australia early concretionary and non concretionary siderites are common in subsurface triassic sandstones and mudrocks of the rewan group southern bowen basin a detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks the siderites are extremely pure containing 85 97 mol feco3 and are commonly enriched in manganese 13c pdb values are highly variable ranging from 18 4 to 2 9 whereas 18o pdb values are very consistent ranging from 14 0 to 10 2 mean 11 9 1 0 the elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation implying that host rocks accumulated in totally non marine environments the carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate carbon dioxide generated by methane oxidation and methanogenesis very low 13c values demonstrate that contrary to current views highly 13c depleted siderite can be produced at shallow burial depths in anoxic non marine sediments

dolomitisation of the ardross limestones dinantian east fife scotland abstract the ardross limestones are two thin marine units within the otherwise dominantly delta top sediments of the upper calciferous sandstone measures of east fife they are entirely dolomitised bioclastic wackestones which are cut by complex dolomite ankerite veins the matrix of these limestones is composed of brown microcrystalline dolomite together with a significant amount of framboidal pyrite and small amounts of siderite the matrix dolomite usually has compositions of between 50 and 51 mol caco3 8 15 mol feco3 and up to 1 5 mol mnco3 matrix 18o 8 31 to 1 58 the veins comprise mn enriched 18o depleted inclusion rich dolomite succeeded by 18o enriched ankerite dolomitisation appears to have been a two stage process in which an early fine grained protodolomite phase recrystallised in hydrothermal fluids associated with namurian volcanic emplacement the protodolomite was precipitated during earliest burial from marine dominated fluids largely within the zone of sulphate reduction the later recrystallisation of this material was accompanied by precipitation of the earliest vein dolomites later ankerite veins were precipitated from deep diagenetic brines released during some phase of post carboniferous regional extension geochemical and petrographic differences between dolomite in the ardross limestones and that in similar crinoidal wackstones of the overlying lower limestone group in east fife can be attributed to primary sedimentary differences and contrasting early burial histories

late permian continental sediments in the se iberian ranges eastern spain petrological and mineralogical characteristics and palaeoenvironmental significance abstract a detailed mineralogical and petrological study and the analysis of paleosol profiles in continental alluvial sediments of the late permian in the se iberian ranges spain allow us to infer the significant environmental changes that occurred during this time period three parts have been distinguished in the late permian sediments alcotas formation the lower part includes abundant and well preserved carbonate paleosol profiles and fine grained sediments made up by quartz feldspar hematite and illite with scarce kaolinite the preservation of dolomicrite in some paleosols suggests that they originally developed as dolocretes in an arid to semi arid climate with marked seasonality a change towards more humid and acid conditions can be deduced from the presence of siderite and goethite in paleosols in the middle part of the alcotas formation moreover the presence of plant remains coal beds and or carbonaceous shales at the top of the middle part and the lack of carbonate paleosols in the upper part of the formation would indicate a further step towards acid conditions these conditions would increase until the early triassic as indicated by the lack of carbonates and the presence of sr rich aluminium phosphate sulphates aps minerals at the base of the triassic canizar formation which clearly indicates extreme acid conditions during the permian triassic transition of the study area

sedimentology palynostratigraphy and palynofacies of the late oligocene makum coalfield assam india a window on lowland tropical vegetation during the most recent episode of significant global warmth abstract the sedimentology palynofacies and pollen and spore assemblages together with observations on the preserved megaflora from the tirap coal mine assam india 27 17 20 n 95 46 15 e provide important insights into the composition and distribution of plant communities in a late oligocene tropical delta palaeolatitude approximately 18 n these insights take on additional significance because the late oligocene included the last significantly warm interval in earth s history before progressive neogene cooling to the present day icehouse the lower two thirds of a 188 m studied section represents lower delta plain environments with only a small degree of brackish water marine influence raised mires autochthonous swamps and allochthonous organic matter accumulations comprise the vitrinite rich high volatile bituminous b coals the upper third of the section represents upper delta plain environments with high sediment pass through here standing fossil forests attest to frequent inundation and recovery of the arborescent interfluve vegetation throughout the period of deposition of the studied section several 100 kyr sediment supply exceeded increases in accommodation space afforded by basin subsidence relative sea level rise a dominance of spores attributed to cyatheaceous and other fern taxa indicates that tree ferns and other pteridophytes were a major component of the vegetation throughout the environments represented by the section despite the absence of ferns in the megafossil record nypa and mangrove megaremains together with mangrove pollen and acrostichum spores attest to the presence of a mangrove community similar in composition to those of modern southeast asia palms and megathermal tree taxa such as those belonging to the malvaceae meliaceae alangiaceae anacardiaceae and pellicieraceae inhabited the interfluve and swamp forests perennial herbs of the family gunneraceae occur throughout the section palynofacies analysis shows a dominance of anoxic dysoxic environments with a small part of the lower part of the section exhibiting more oxic conditions associated with circumstantial evidence for mild marine possibly tidal influence most palynodebris fluoresce only weakly consistent with the relatively high rank for coals of this age a small number of reworked gondwanic permian grains suggest sediment erosion from the highlands to the north the late oligocene tropical delta vegetation has much in common with modern lowland wet megathermal forests from southeast asia however tree ferns appear more prevalent in the late oligocene

climatic depositional and burial controls on diagenesis of appalachian carboniferous sandstones qualitative and quantitative methods abstract carboniferous quartz lithic and feldspar rich sandstones from different stratigraphic intervals within the central appalachian basin contain evidence for eogenetic and mesogenetic diagenesis important eogenetic products include fe oxide oxyhydroxide siderite kaolinite chlorite and calcite whereas quartz albite illite sericite fe carbonate and secondary porosity are the most important mesogenetic components correspondence and detrended correspondence analysis of the framework grain and diagenetic mineral data set has guided diagenetic interpretations and leads to two major conclusions first minerals that formed early appear to have been influenced by stratigraphic position the distribution of siderite and iron oxide oxyhydroxide is considered to reflect a second order paleoclimatic signature recognized throughout the carboniferous where siderite formed during everwet periods and iron oxide oxyhydroxide during semi arid conditions reflecting differences in redox second framework grain composition controlled the distribution of diagenetic products in the burial environment lithic arenites are deficient in authigenic quartz yet have undergone various degrees of illitization quartz deficiency is attributed to compaction related loss of primary porosity relatively early which inhibited sufficient fluid flow to account for much higher concentrations of quartz cement in the more competent quartz rich sandstones

the habit and origin of siderite spherules in the eocene coal bearing prestfjall formation faroe islands abstract siderite spherules are described and chemically analysed from two localities holi i helli section and rokhagi coal mine within the coal bearing prestfjall formation the formation marks a significant hiatus in the eruption of lava flows of the paleogene faroe islands basalt group and shortly follows the paleocene eocene thermal maximum the spherules are chemically pure although feco 3 80 6 99 2 mol shows a marked substitution for mnco 3 0 1 13 9 mol the low concentrations of mgco 3 0 0 0 4 mol and the lack of associated sulphide minerals e g pyrite confirm the freshwater lake environment affinity of the formation the siderite spherules from the holi i helli section contain significantly higher concentrations of mnco 3 3 may also account for the abundance of siderite spherules formed by the split crystal process in the holi i helli section whereas prefect spherules consisting of individual crystals growing concentrically around a cryptocrystalline centre are dominant from the rokhagi coal mine

groundwater table fluctuations recorded in zonation of microbial siderites from end triassic strata in a terrestrial triassic jurassic boundary succession of southern sweden perfectly zoned sphaerosiderites are restricted to a specific sandy interval deposited during the end triassic event underlying and overlying this sand interval there are several other types of siderite micromorphologies i e poorly zoned sphaerosiderite spheroidal ellipsoid siderite spherical siderite and rhombohedral siderite siderite overgrowths occur mainly as rhombohedral crystals on perfectly zoned sphaerosiderite and as radiating fibrous crystals on spheroidal siderite concretionary sparry microspar and or micritic siderite cement postdate all of these micromorphologies the carbon isotope composition of the siderite measured by conventional mass spectrometry shows the characteristic broad span of data probably as a result of multiple stages of microbial activity sims secondary ion mass spectrometry revealed generally higher 13c values for the concretionary cement than the perfectly zoned sphaerosiderite spheroidal siderite and their overgrowths which marks a change in the carbon source during burial all the various siderite morphologies have almost identical oxygen isotope values reflecting the palaeo groundwater composition a pedogenic freshwater origin is supported by the trace element compositions of varying fe mn ratios and low mg contents fluctuating groundwater is the most likely explanation for uniform repeated siderite zones of varying fe mn ratios reflecting alternating physiochemical conditions and hostility to microbial life activity bacterially mediated siderite precipitation likely incorporated mn and other metal ions during conditions that are not favourable for the bacteria and continued with fe rich siderite precipitation as the physico chemical conditions changed into optimal conditions again reflecting the response to groundwater fluctuations

changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half graben an example from the triassic ischigualasto villa union basin argentina well exposed triassic rift strata from the ischigualasto villa union basin nw argentina include a 80 to ca 515 m thick lacustrine dominated package that can be correlated across a half graben using key stratigraphic surfaces sequence boundaries lacustrine flooding surfaces and forced regressive surfaces the characteristics of the synrift lacustrine fill in different parts of the half graben have been examined and the mechanisms controlling sedimentation inferred a variety of sedimentary environments are recognized including volcaniclastic floodplain mildly saline lake and playa lake offshore lacustrine delta front to fluvial dominated and wave dominated deltas distributary and fluvial channel and interdistributary bay the succession can be divided into four stratigraphic sequences ss1 to ss4 the oldest of which ss1 contains volcaniclastic fluvial and saline lake deposits it is thickest close to the western border fault zone reflecting more rapid subsidence here accommodation exceeded sediment and water input during ss1 the second and third sequences ss2 and ss3 mark the onset of widespread lacustrine sedimentation reflecting a balance between accommodation creation and water and sediment fluxes sequences ss2 and ss3 are represented by offshore meromictic lacustrine and deltaic deposits the latter mostly sourced from the flexural and southern axial margins of the half graben the presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone the youngest sequence ss4 is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies palaeocurrents and sandstone composition suggesting a switch in sediment supply to the footwall margin to the nw the change in the sediment source is related to reduced footwall uplift the possible presence of a relay ramp and or supply from a captured antecedent drainage network during ss4 the rate of creation of accommodation was exceeded by the sediment and water discharge the stratigraphic evolution of lacustrine strata in the half graben was mainly controlled by tectonic processes including subsidence rate and the growth and evolution of the border fault zone but changing climate inducing changes in water balance and lake level and autocyclic processes delta lobe switching were also important

high latitude meteoric δ 18 o compositions from the cenomanian bastion ridge formation axel heiberg island canadian arctic archipelago a palaeoclimate proxy from the sverdrup basin

sedimentology and oceanography of early ordovician ironstone bell island newfoundland ferruginous seawater and upwelling in the rheic ocean abstract the bell island and wabana groups together comprise a ca 150 m thick succession of interbedded clastic and chemical sedimentary rocks composed of eight distinct lithofacies that accumulated along the northern margin of the rheic ocean lithofacies stacking patterns indicate that deposition occurred during a marine transgression with superimposed small scale sea level fluctuations producing at least six parasequences parasequences containing ironstone are 10 to 20 m thick and composed of hummocky cross stratified sandstone interbedded with organic rich mudstone and phosphatic fe silicate bearing siltstone which is overlain by hematitic granular ironstone capped by an erosive flooding surface this lithofacies association is interpreted to record the deposition of upwelling related ironstone on a storm dominated shelf the close relationship between fe silicates and phosphorite typical of upwelling systems suggests that fe was delivered from deep anoxic nutrient rich seawater that also stimulated high surface productivities the result was the precipitation of authigenic sedimentary apatite in anoxic organic rich sediments that accumulated near the upwelling front the gradual advection of fe rich waters away from the upwelling front initiated precipitation of fe silicate coated grains and cements in suboxic pore waters iron pumped into shallower environments through advection and fe redox cycling is interpreted to have precipitated fe oxyhydr oxide grains in sediment of the oxygenated middle shelf these coated grains were subsequently concentrated by fairweather and storm currents on the shoreface to create granular economic fe deposits we challenge conventional models of paleozoic ironstone deposition that rely on a continental source of fe by proposing a hydrothermal source that supplied fe2 to the shelf through upwelling it also highlights the potential connection between the delivery of anoxic ferruginous seawater to the margins of the rheic ocean and the early ordovician extinctions that punctuated the beginning of the great ordovician biodiversification event

optimized sequential extraction for carbonates quantification and δ13c analysis of calcite dolomite and siderite siderite is present in diverse types of rocks and sediments but its quantification is cumbersome when present in relatively low contents a new analytical method for the sequential separation of different carbonate phases is presented the separation quantification and characterization of the carbonates is based on different kinetic reactions during acid digestion in combination with monitoring the solution composition and the concomitant release of co2 trapping the released gasses permits the isotopic characterization 13c of the different phases based on published rate laws an idealized model was built to identify ph and temperature conditions for optimal separation of the various carbonates by taking the size distribution of carbonate particles into consideration results from the theoretical survey were implemented in an extraction scheme which was optimized and evaluated by tests on pure mineral phases and their mixtures further tests on a diverse array of sediment and rock samples validated the specificity and reproducibility of the method

widespread nearshore and shallow marine deposition within the lower jurassic precipice sandstone and evergreen formation in the surat basin australia abstract in the surat basin of eastern australia the lower jurassic precipice sandstone and evergreen formation are a highly prospective reservoir seal pair for notional future carbon capture and storage however the succession remains poorly constrained from a paleodepositional standpoint and this has impacted the capacity to construct predictive reservoir models here we integrate sedimentological ichnological and palynological data from ten cores located across a large region of the northern and central basin to produce conceptual depositional models for the interval our analysis shows that the lower jurassic series consists of fifteen recurring sedimentary facies that are arranged into six facies associations braidplain lower delta plain subaqueous delta delta influenced shoreface tidally influenced shoreline and restricted marine shoals the facies associations occur in the context of large scale fluvio deltaic systems that occupied the basin these results are supported by ichnological indications of marine and brackish water a coastal suite of palynomorphs and marine palynomorphs including rare dinocysts acritarchs and copepod fragments the very low abundance of marine palynomorphs are confined to the upper portion of the evergreen formation and in combination with the sedimentology and ichnology suggest that marine influence increased through time as the basin was filled the elucidation of marine influenced deposition contravenes all but the most recent facies interpretations of the precipice sandstone and evergreen formation and suggests that the paleogeography of the mesozoic of eastern australia needs to be reconsidered importantly the nearshore and shallow marine depositional affinity has important implications for the size orientation and distribution of geobodies when building geologically realistic static reservoir models for dynamic flow simulation

anoxic to suboxic mesoproterozoic ocean evidence from iron isotope and geochemistry of siderite in the banded iron formations from north qilian nw china abstract banded iron formations bifs provide important constraints on geochemical cycling of fe and ancient marine environment globally the formation of bifs peaked during late archean and early paleoproterozoic with limited reappearance during neoproterozoic here we investigate a rare example of mesoproterozoic ca 1 3 ga bifs from jingtieshan in the north qilian region of nw china we present rare earth element and yttrium ree y compositions together with iron isotope features of siderite from the bifs and integrate these with our published data on carbon isotope compositions from the siderite rich bifs including carbonate facies bifs and mixed carbonate oxide facies bifs siderite from the bifs shows ree y characteristics that are consistent with submarine hydrothermal fluids mixed with seawater including positive eu eu anomalies and high y ho ratios the siderite also displays homogeneous iron isotope compositions 56fe range from 0 71 to 0 41 one sample down to 1 62 and moderate negative carbon isotope compositions 13c values 8 40 to 3 0 these findings argue against the traditional view that siderite associated with bifs was formed through microbial dissimilatory iron reduction dir instead siderite from jingtieshan bifs is in equilibrium with submarine hydrothermal fluids and seawater suggesting an inorganic origin and was directly precipitated from the co2 and fe oversaturated water column with only insignificant role for dir these mesoproterozoic bifs are also characterized by ce anomalies and the siderite falls within the pe log s t and po2 pco2 stability fields suggesting that the bifs were deposited in an anoxic to suboxic ocean the results from our study suggest the presence of iron rich mesoproterozoic deep ocean in the north qilian area

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

resolving original signatures from a sea of overprint the geochemistry of the gungri shale upper permian spiti valley india abstract the geochemistry of black shales can faithfully record paleoenvironmental conditions however the original geochemical signatures are obscured by post depositional events such as diagenesis and weathering resolving the relative influence of these processes on the geochemistry of shales requires a combination of geochemical and chemometric chemical statistics approaches to data analysis that identify the sources of variance in major and trace element geochemistry the permian gungri shale kuling group at attargoo spiti valley himachal pradesh india is a gray to black shale capped unconformably by an iron rich pebble sand layer ferruginous layer marking ostensibly the permian triassic boundary high resolution analysis of the gungri at attargoo 3 cm intervals reveals a geochemical record complicated by low grade metamorphism diagenesis and modern weathering using multivariate chemometric techniques e g principal component analysis we identified variance in the geochemical record associated with the original detrital and depositional conditions we also identified 4 beds within the gungri that mark depositional events which may correlate to those identified in other pt sections e g guryul ravine kashmir identification of sources of variance allowed us to identify geochemical signatures that can be used for paleooceanographic reconstruction the attargoo section records a sequence of discrete events including transient iron enrichments attributed to euxinic depositional conditions and late diagenetic formation of siderite and regression condensation surfaces associated with pulsed transgression regression towards the uppermost permian our results show that chemometric analysis of geochemical data enables the resolution of paleoenvironmental conditions in shales impacted by post depositional alteration

diagenetic alterations related to marine transgression and regression in fluvial and shallow marine sandstones of the triassic buntsandstein and keuper sequence the paris basin france abstract the distribution of diagenetic alterations in triassic fluvio deltaic quartzarenitic to sublitharenitic lowstand systems tract lst sandstones of the gres a voltzia formation anastomosing fluvial quartzarenitic transgressive systems tract tst sandstones of the gres a roseaux formation and shallow marine quartzarenitic to sublitharenitic tst sandstones of the gres coquiller formation the paris basin france can be linked to transgression and regression events and thus to the sequence stratigraphic context near surface eogenetic alterations which display a fairly systematic link to the depositional facies and sequence stratigraphic framework include i cementation by meteoric water calcite 18 o 8 9 and 13 c 9 1 in the fluvio deltaic lst sandstones ii cementation by mixed marine meteoric calcite 18 o 5 3 to 2 6 and 13 c 3 9 to 1 3 and dolomite 18 o 4 6 to 2 6 and 13 c 2 9 to 2 3 in the foreshore tst sandstones and below parasequence boundaries pb and transgressive surface ts and in the shoreface tst sandstones below maximum flooding surfaces mfs being facilitated by the presence of carbonate bioclasts iii dissolution of detrital silicates and precipitation of k feldspar overgrowths and kaolinite particularly in the fluvio deltaic lst sandstones owing to effective meteoric water circulation and vi formation of autochthonous glauconite which is increases in abundance towards the top of the fluvio deltaic lst sandstones and along ts and in the shoreface tst sandstones by alteration of micas owing to the flux of seawaters into the sandstones during transgression whereas parautochthonous glauconite is restricted to the ts sandstones owing to marine reworking mesogenetic alterations which include cementation by quartz overgrowths and illite display fairly systematic link to fluvio deltaic lst sandstones this study has revealed that linking of diagenesis to transgression and regression events enables a better understanding of the parameters that control the spatial and temporal distribution of diagenetic alterations in sandstones and of their impact on reservoir quality evolution

distribution of diagenetic alterations in glaciogenic sandstones within a depositional facies and sequence stratigraphic framework evidence from the upper ordovician of the murzuq basin sw libya abstract the spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the upper ordovician glaciogenic quartzarenite sandstones in the murzuq basin sw libya which were deposited during the haritanian glaciation when the basin was laying along the continental margin of gondwana eogenetic alterations encountered include i replacement of detrital silicates mud matrix and pseudomatrix by kaolinite in paraglacial tide dominated deltaic in foreshore to shoreface highstand systems tract hst and in post glacial gilbert type deltaic lowstand systems tract lst sandstones particularly below the sequence boundaries sb kaolinite formation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline ii cementation by calcite 18 o vpdb 3 1 to 1 1 and 13 c vpdb 1 7 to 3 5 and mg rich siderite in the paraglacial tide dominated deltaic and foreshore to shoreface hst sandstones in the glacial tide dominated estuarine transgressive systems tract tst sandstones and in the post glacial shoreface tst sandstones is interpreted to have occurred from marine pore waters iii cementation by mg poor siderite which occurs in the post glacial gilbert type deltaic lst sandstones and in the paraglacial tide dominated deltaic and foreshore to shoreface hst sandstones is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline iv pervasive cementation by iron oxides has occurred in the glacial shoreface offshore tst sandstones and post glacial shoreface tst sandstones immediately below the maximum flooding surfaces mfs which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor v formation of grain coating infiltrated clays mainly in the glacial fluvial incised valley lst sandstones and in the post glacial gilbert type deltaic lst sandstones as well as less commonly in the paraglacial foreshore to shoreface hst sandstones and in the tide dominated deltaic hst sandstones below the sbs mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial fluvial incised valley lst sandstones post glacial gilbert type deltaic lst sandstones and glacial shoreface tst sandstones in which early carbonate cements are lacking illite chlorite and albitized feldspars which occur in small amounts are most common in the glacial tide dominated estuarine tst sandstones and paraglacial shoreface hst sandstones this study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir quality evolution in glacial paraglacial and post glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework

stratigraphic implications of meteoric sphaerosiderite 18o values in paleosols of the cretaceous albian boulder creek formation ne british columbia foothills canada abstract the md 80 08 coal exploration core from the albian boulder creek formation of northeastern british columbia contains 90 m of alluvial strata that underwent extensive pedogenic modification gleyed colors pedogenic slickensides clay coatings mottling sphaerosiderite and sedimentary pyrite characterize the paleosols the isotopic compositions of 30 sphaerosiderite bearing horizons commonly show trends of invariant 18o and variable 13c values equal area plots of 18o vs 13c were used to construct meteoric sphaerosiderite lines msls the msls indicate that meteoric groundwater compositions ranged from approximately 11o oo to 16o oo smow the sphaerosiderite 18o values indicate formation in shallow groundwaters predominantly recharged by local precipitation the sphaerosiderite isotopic compositions are a proxy record of mid cretaceous precipitation and are useful for quantifying changes in the hydrologic cycle during humid greenhouse periods micromorphology of a 1 15 meter interval 69 75 68 6 m with a 1 31o oo increase in the average 18o values records a polygenetic history of paleosol development the pedocomplex has a four stage developmental history characterized by 1 an initial well drained phase with wet dry cycles and relatively low baselevel 2 erosion baselevel rise and coarser grained sedimentation 3 saturated reducing soil conditions high baselevel with some marine influence on pedogenesis and 4 meteoric water dominated hydromorphic soil conditions high baselevel and sphaerosiderite precipitation the 18o excursion recorded through this interval reflects minor mixing of marine and meteoric groundwaters the modal abundance of pyrite and increased mg ca mg and mg fe cation ratios in the sphaerosiderites further substantiate marine influence on pedogenesis the alluvial section of the boulder creek formation is aggradational and several changes in baselevel recorded in the paleosols are interpreted to have been driven by minor parasequence scale changes in relative sea level

iron and phosphorus biochemical systems and the cryogenian ediacaran transition jacadigo basin brazil implications for the neoproterozoic oxygenation event abstract termination of the marinoan snowball earth ice age 635 ma marked the transition to a greenhouse world this climate change forever modified the biogeochemical cycling of iron and phosphorus ended large scale iron formation deposition began accumulation of phosphorus on marine shelves and led to the ediacaran radiation of eukaryotes the jacadigo basin brazil contains a nearly complete record of this critical transition glaciomarine diamictites and iron formation accumulated during two marinoan ice advances with hydrothermal input of iron delivered via ice margin upwelling biochemically precipitated rhythmites of siderite sedimentary apatite and hematite represent microbially mediated sub sea ice precipitation siderite laminae preserve microbial textures and have a mean 13c 8 80 pdb 0 86 reflecting degradation of organic matter at the seafloor these millimeter scale rhythmites are a sensitive record of sub ice dynamics because they formed in response to short term fluctuations of o2 due to seasonal sub ice photosynthesis they demonstrate the connection between ice cover o2 and cycling of iron and nutrient elements such as phosphorus these biochemical rhythmites suggest that cryogenian sea ice limited oxygen production prior to the onset of the neoproterozoic oxygenation event o2 increased enough to concentrate bioavailable phosphorus at the seafloor which was essential for later diversification of metazoans in the ediacaran such sub ice cryogenian biochemical systems may provide earth based analogs for life on ice covered worlds such as europa and enceladus

origin of low δ18o pre compactional ferroan carbonates in the marine sto formation middle jurassic offshore nw norway siderite delta 18 o pdb 9 3 to 5 7 parts per thousand and fe dolomite ankerite delta 18 o pdb 13 7 to 7 7 parts per thousand occur as texturally early diagenetic pre compactional ce

geochemistry of carbonate cements in the sag river and shublik formations triassic jurassic north slope alaska implications for the geochemical evolution of formation waters carbonate cements calcite siderite dolomite and ankerite formed throughout the diagenetic history of the sag river and shublik formations the trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation earliest calcites formed prior to significant compaction of the sediment are relatively enriched in mg up to 4 4 mol and have 87sr 86sr values mean 0 707898 compatible with the original marine pore waters later calcites are relatively fe rich up to 5 0 mol and are characterized by increasing 87sr 86sr values up to 0 712823 and sr content with decreasing age the fe content of zoned siderite and dolomite ankerite rhombs increases towards the outside of the rhombs i e increasing fe content with decreasing age these geochemical variations appear principally to result from changes in pore water chemistry during diagenesis the increase in 87sr 86 sr and sr content of the cements is most likely due to interaction between pore waters and 87 sr rich clay and possibly feldspar in ellesmerian mudrocks whole rock 87sr 86 sr signatures for the mudrocks are 0 716 pore water fe2 concentration was probably controlled by diagenetic alterations involving fe bearing minerals e g pyrite precipitation a reconnaissance examination of carbonate cements in the overlying kingak shale indicates that similar alterations occurred in the kingak the low 18 o value of some calcite cements 11 96 pdb suggests that an influx of meteoric water may have occurred in the mid neocomian though the low value could also result from an abnormally high geothermal gradient associated with mid neocomian rifting

evidence for episodic oxygenation in a weakly redox buffered deep mid proterozoic ocean abstract over the last two decades popular opinion about prevailing conditions in the mid proterozoic deep ocean has evolved from fully oxygenated to globally euxinic sulfidic to a more heterogeneous stratified water column with localized pockets of euxinia existing in predominantly iron rich ferruginous deep waters the animikie basin in the lake superior region has been essential in shaping our view of marine redox evolution over this time period in this study we present a multi proxy paleoredox investigation of previously unanalyzed strata of the late paleoproterozoic animikie basin using drill cores through the 1 85 ga stambaugh formation paint river group in the iron river crystal falls district of the upper peninsula of michigan usa based on previous tectonic reconstructions and analysis of sedimentary regimes the iron river crystal falls section captures strata from among the deepest water facies of the animikie basin in contrast to previous work on sedimentary rocks in this basin we find evidence from iron speciation trace metal and mo isotope data for episodes of at least local deep water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions while trace metal enrichments and iron speciation data suggest predominantly anoxic conditions the occurrence of mn rich intervals up to 12 3 wt mno containing abundant mn fe carbonate and a wide range of mo isotope data with extremely negative values 98 95mo 1 0 to 1 1 record the shuttling of mn oxides from surface waters through oxic or suboxic waters to the sediment water interface we propose that such conditions are analogous to those of locally restricted modern and holocene basins in the baltic sea which receive episodic inflow of oxygenated water producing similar geochemical signatures to those observed for the animikie basin we argue that the mid proterozoic was characterized by a lack of a strong redox buffer low sulfide ferrous iron and oxygen contents and thus was vulnerable to dramatic and at least local redox shifts including briefly oxygenated bottom waters a refined view of the mid proterozoic ocean is emerging one that was still predominantly anoxic but marked by regional heterogeneities and short term redox variability that may in part reflect a transitional state between prevailingly anoxic archean and predominantly oxic phanerozoic oceans

carbon and oxygen stable isotopic signatures of methanogenesis temperature and water column stratification in holocene siderite varves abstract manganoan siderite fe mn co 3 occurs in abundance of up to 19 dry weight as the sole endogenic carbonate within a succession of holocene organic rich varved sediments from freshwater otter lake ol michigan radiocarbon dating and varve counts from a 7 m piston core constrain periods of major siderite accumulation to sediments older than 1200 cal yr bp sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet bulk sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial scale cycles of siderite accumulation siderite 13 c and 18 o are enriched when the mineral is abundant and depleted when it is least abundant samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water but are 13 c enriched relative to modern waters samples from intervals of low siderite abundance are 13 c and 18 o depleted relative to equilibrium with modern lake water these data suggest that abundant siderite precipitation occurred when the ol water column was ferruginous iron meromictic allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon dic enriched in 13 c below the chemocline where methanogenesis in waters and sediments influenced dic composition seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution manganese substitution in siderite lowered kinetic barriers to low temperature mineral precipitation climate changes and basin filling influenced lake stratification and the rate of siderite precipitation the siderite occurrence reported here displays remarkable similarity to its occurrence in paleogene maar lake deposits suggesting that further studies of holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments

early diagenetic spherulitic siderites from pennsylvanian palaeosols in the boss point formation maritime canada the boss point formation of maritime canada comprises alternating successions of braidplain sandstone lacustrine mudstone and lacustrine fluvio deltaic sandstone and mudstone these rocks were deposited within an active strike slip basin during the early pennsylvanian westphalian a at a palaeolatitude of 8 s palaesols comprise a small but distinct proportion of the formation and typically occur at the tops of fining upward sandstone siltstone cycles spherulitic siderites from the boss point formation occur within sandy and silty palaeosols they occur as large 200 m and small 200 m diameter spherulitic nodules within calcrete and show either a radially arranged ferroan calcite and siderite spar morphology or contain a core of ferroan calcite surrounded by a later formed sheath of siderite analytical data indicate that with respect to ca mg mn wide compositional variation occurs within the siderites the boss point formation siderites are similar to but contain less ca for a given mg mn ratio than previously published siderite compositional data published data together with the results from the boss point formation indicate that a continuum exists in freshwater siderites between the relatively mn depleted and the mg enriched analytical fields that were previously reported the relatively low ca values determined in the boss point formation samples reflect the general lack of calcium in the sedimentary basin during sedimentation both chlorite and smectite clays may have been important sources of iron it is concluded that the spherulitic siderites are eogenetic and formed within small anoxic ponds rich in organic matter under reducing and low dissolved sulphate conditions with time these pools dried up ferroan calcite precipitating as calcrete nodules around the earlier formed siderite spherulites

spherulitic siderites in the holocene coastal deposits of korea eastern yellow sea elemental and isotopic composition and depositional environment abstract siderite concretions are recovered from both freshwater unit ii and tidal unit i deposits in the holocene macrotidal flat of kyunggi bay west coast of korea siderites from both units show a spherulitic and well rounded texture and well developed equant rhombs on the external surfaces the size of the concretions in unit ii 50 150 m in diameter is normally larger than that in unit i 10 20 m the siderites in unit ii are chemically pure and highly enriched in fe whereas those of unit i are relatively impure and distinctly zoned demarcating high mn up to 69 1 mol in the core and high fe up to 75 9 mol in the margin within siderites of unit i substitution for fe by ca and mg reaches up to 20 3 mol mean 15 1 mol and 16 1 mol mean 12 6 mol respectively bulk 18 o values 0 7 to 1 3 peedee belemnite pdb for siderites in unit i are relatively higher than those 7 1 to 0 5 pdb in unit ii regardless of units siderite 13 c values 21 3 to 11 2 pdb are persistently low indicating microbially mediated precipitation at shallow burial depth extensive mn ca and mg substitution coupled with enrichment of 18 o in the siderites of unit i seems to reflect marine influence during the holocene transgression the occurrence of early diagenetic siderites provides supporting evidence on the depositional environment marine vs non marine of the holocene muddy coastal deposits of korea eastern yellow sea

gold deposition by sulfidation of ferrous fe in the lacustrine sediments of the pueblo viejo district dominican republic the effect of fe c s diagenesis on later hydrothermal mineralization in a maar diatreme complex abstract the pueblo viejo district located in the cordillera central of the dominican republic contains large au ag deposits associated with acid sulfate alteration within spilites conglomerates and carbonaceous sedimentary rocks that were deposited in a maar diatreme complex much of the au mineralization occurs in pyritic carbonaceous siltstones of the pueblo viejo maar diatreme member of the cretaceous los ranchos formation pyrite is the only fe bearing phase in mineralized rock whereas siderite is the dominant fe bearing phase in siltstones distal to mineralization disseminated pyrite occurs as framboids cubes pyritohedra concretions and cement early framboids occur throughout the district au occurs as inclusions in later non framboid disseminated pyrite nfdp an occurrence that is interpreted to be indicative of contemporaneous deposition pyrite framboids exhibit a wide range of 34s cdt values 17 5 to 4 8 and are interpreted to have formed during biogenic reduction of pore water sulfate the nfdp yield restricted 34s cdt values x 5 2 s 2 4 n 43 similar to those obtained from later vein pyrite x 6 4 s 1 5 n 12 alunite and barite have 34s values ranging from 18 8 to 21 6 the interpretation that the nfdp vein pyrite alunite and barite and possibly even the framboidal pyrite share a common source of igneous sulfur is supported by the 34s data siderite occurs as concretions and cement contains abundant mg fe0 75mg0 19mn0 03ca0 02co3 and has 13c pdb and 18o smow values ranging from 2 5 to 1 1 and 14 6 to 19 5 respectively these data are consistent with the interpretation that the siderite formed in lacustrine sediments and that the carbonate in the siderite is probably methanogenic although contributions from oxidation of organic matter during biogenic sulfate reduction thermal decarboxylation of organic matter or magmatic vapor cannot be ruled out disseminated au mineralization in the sedimentary rocks formed when a hydrothermal fluid encountered reactive fe2 in diagenetic siderite the ensuing pyrite deposition consumed h2s and destabilized the au hs 2 complex leading to precipitation of au the capacity of the sedimentary rocks to consume h2s and precipitate au was controlled by the amount of non pyrite fe present as siderite the abundance of siderite was controlled by the extent of pyrite formation during diagenesis

facies variations in response to holocene sea level and climate change on bora bora french polynesia unravelling the role of synsedimentary siderite in a tropical marine mixed carbonate siliciclastic lagoon abstract five mixed carbonate siliciclastic sedimentary facies were identified in the barrier reef lagoon of bora bora using microfacies and statistical analyses of 70 sediment samples taken at high resolution from two vibrocores facies and facies successions were interpreted with respect to holocene sea level and climate changes the windward lagoon core is characterized by sideritic marly wackestones and foraminifera sideritic wackestones deposited around 7700 years bp years before present during the early mid holocene transgression at that time extensive weathering and erosion of iron bearing minerals from the volcanic island due to a wetter climate were expressed in the formation of synsedimentary siderite in lagoonal sediments the enrichment in 18o 0 32 to 0 54 in the siderite grains indicates marine to mixed marine meteoric conditions during precipitation siderite formation resulted from microbial degradation of organic material indicated by depleted 13c values 13 61 to 14 48 that led to reducing conditions in lagoonal sediments and resulted in iron reduction in the presence of dissolved bicarbonate the chemical compositions of the siderites changes upcore from relatively high fe 91 95 mol and low mn 5 6 mol at the core base to relatively low fe 83 88 mol and high mn 11 16 mol at the core top the substitutions of fe by mn ca and mg at grain margins illustrate changes in pore water chemistry towards more oxygenated conditions and reflect sea level rise and elevated rainfall during the early mid holocene a drier climate during the mid late holocene was accompanied by reduced iron input and the proportion of siderite decreased approaching zero in the upper section of the core in the leeward lagoon core siderite is again common in the lower section and decreases in abundance upcore mollusc foraminifera marly packstones and mollusc wackestones accumulated 5400 3500 years bp during the mid holocene in the windward core early in this period rotalid and miliolid foraminifera dominated these are tolerant of environmental stress such as changes in water quality nutrients or salinity from the mid holocene to the present textularid foraminifera are common in both cores and indicate normal marine lagoonal conditions since the mid late holocene sea level highstand and fall to modern level mudstones have dominated in both cores during the last 1000 years coral fragments have increased in abundance in the windward lagoon presumably as a result of lagoonward progradation of fringing reefs in the mid late holocene since the late holocene motus on the windward side of bora bora have hampered sediment transport and lagoonward progradation of sand aprons increasing numbers of peloids largely hardened faecal pellets in the windward core in the last 1000 years may reflect early submarine lithification within the lagoon our study shows that during the holocene sea level and climate change have influenced sediment import composition and distribution in the bora bora lagoon the sensitive response of the environment to external changes demonstrates the potential of tropical reef lagoons as archives of climate and sea level changes

anatomically preserved plants in siderite concretions in the shale split of the foord seam mineralogy geochemistry genesis upper carboniferous canada abstract a shale split in the well studied foord seam upper carboniferous stellarton basin nova scotia provides a rare opportunity to study little known siderite concretions with anatomically preserved plants from a limnic basin siderite concretions occur throughout the split commonly as sheet like inter connected lepidodendrid logs or less frequently as spherical masses they are typically vuggy a sample of 14 stratigraphically oriented concretions in the split shows that primary siderite varies from 45 to 80 wt due to selective replacement by 5 to 40 ferroan dolomite or ankerite in plant tissues these two minerals show large mg fe and mg ca variability and are fezoned clay content ranges from trace to 20 and quartz from 5 to 10 partial cell wall destruction by erosion and changeable oxic anoxic conditions prior to siderite permineralization resulted in preservation mainly of the more resistant plant tissues on average carbon from siderite is isotopically heavier 4 8 1 9 0 00 than that from dolomite 7 2 1 1 0 00 the distribution of and the positive correlation between 13 c and 18 o probably reflects the two fractionation processes associated with methanogenesis and dolomitization respectively in the genetic history of the siderite concretions in the split siderite concretions formed diagenetically early from bicarbonate under reducing conditions in a slightly alkaline freshwater environment that precluded calcite deposition

geology and stratigraphy of fluvio deltaic deposits in the ivishak formation applications for development of prudhoe bay field alaska significant remaining reserves in prudhoe bay field are confined within deltaic rocks at the base of the triassic ivishak sandstone the initial stratigraphic characterization of the prudhoe bay reservoir was lithostratigraphically based and it depicted this basal reservoir interval as tabular zones between marine shale and overlying coarse grained fluvial sandstones a reassessment of this interval based on cores and genetic stratigraphic correlations depicts en echelon offlapping fluvially dominated deltaic wedges reservoir quality rocks occur in distributary mouth bar distributary channel and fluvial facies associations a paleogeographic reconstruction of one delta lobe includes an alluvial plain crossed by channels of possibly braided or low sinuosity rivers this alluvial plain graded into a delta plain cut by distributary channels that fed distributary mouth bars on a broad delta front river dominance is inferred from the abundance of unidirectional current structures normally graded beds soft sediment deformation and general absence of wave formed tidal and biogenic structures slumping and growth faulting locally replaced coarsening upward deltaic successions with sharp based overthickened mouth bar and distributary channel deposits mudstones deposited following delta lobe abandonment form laterally extensive flow barriers between lobes compartmentalization is most pronounced distally where deltaic sandstones are overlain by and pass laterally into marine shale proximally fluvial and deltaic sandstones are juxtaposed across erosional contacts improving reservoir continuity this stratigraphic interpretation is corroborated by production and surveillance data plus an interference test locally stratigraphy and poor waterflood performance reflect completions in diachronous sandstones that originated in separate deltaic lobes previously poor well performances were attributed to sandstone pinch outs in some cases production can be enhanced with recompletions rather than infill drilling nonconventional wells planned and completed with the benefit of detailed facies association correlations currently are recovering millions of barrels of previously bypassed oil

interactions of a paleocene river a rising fold and early diagenetic concretions abstract the relative rates of sediment accumulation erosion and structural uplift determine whether a growing fold develops positive topographic relief is beveled by antecedent streams or is buried under thick growth strata when folds rise in subsiding basins upward convergent flow of groundwater through the permeable growth strata that underlie antecedent streams enhances the flux of ions required for concretion growth early diagenetic concretions that grow in such alluvial strata may constitute the only clasts larger than sand size available for transport when antecedent streams become erosive the first reworked concretions deposited by these streams should accurately mark the transition from aggradation to erosion as folds rise into the paths of streams in this situation the ability to differentiate reworked from in situ concretions is crucial the west vergent simpson ridge anticline a n s trending thick skinned laramide structure in east central wyoming separates the larger hanna basin from the carbon basin near the north nose of this anticline in situ iron oxide rich concretions are abundant in folded paleocene strata ferris formation and just to the east large reworked iron rich concretions are abundant in younger more gently dipping conglomerates in the basal hanna formation of the backlimb smaller reworked concretions are also present near the base of the hanna formation at least 7 km south of the anticlinal nose and just east of the fold9s axis at the anticlinal nose in situ non reworked concretions up to 3 m 1 m 1 m are abundant at the top of an 7 km thick sequence of sandstones and siltstones that constitute the late cretaceous early paleocene ferris formation reworked concretions are absent in the strata hosting these in situ concretions but reworked concretionary clasts up to 2 m in diameter are present in exposures of conglomerates in the lowermost hanna formation that lie just above the in situ ferris concretions and southeast of the anticlinal nose these early diagenetic concretions were originally cemented by siderite feco 3 oxidation of some small rinded siderite cemented clasts took place after their fluvial transport into the hanna formation but abundant angular un rinded iron oxide cemented clasts indicate that many large in situ siderite concretions had resided in the vadose zone before they were entrained the distribution of reworked concretions and the orientations of crossbeds show that antecedent hanna streams eroded a swath at least 5 km wide across the rising structure these streams transported ferris formation concretions southeastward into the carbon basin and deposited them in a conglomeratic sandstone body in the hanna formation large calcite cemented concretions many with a pipe like morphology then grew within hanna crossbeds in many cases these in situ concretions enclose transported iron rich concretions but there is no evidence any calcite cemented concretions were reworked the nw se alignment of the pipes record southeastward flow of groundwater and thus also provide evidence together with the orientation of the crossbeds of the original continuity of the hanna formation across the anticline reworked concretions reveal the interplay of deposition diagenesis and erosion due to convergent groundwater flow over growing anticlines early diagenetic concretions both in situ and as reworked clasts are especially likely to be found in growth strata

distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework evidence from the petrohan terrigenous group and the svidol formation lower triassic nw bulgaria abstract elucidation of diagenetic alterations in the petrohan terrigenous group fluvial highstand systems tract hst sandstones and svidol formation tide dominated deltaic and tidal flat transgressive systems tract tst and highstand systems tract hst respectively sandstones and calcarenite lower triassic nw bulgaria was constrained within a sequence stratigraphic framework eogenetic alterations in the fluvial hst sandstones include i formation of grain coating infiltrated clays as a result of percolation of mud rich surface waters into underlying coarse grained and permeable channel fills and crevasse splay sandstones ii formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse grained channel sandstones during their lateral avulsion and iii cementation by calcite 18 o vpdb 6 5 to 3 13 c vpdb 5 1 to 0 6 and dolomite 18 o vpdb 6 1 to 0 3 13 c vpdb 7 2 to 5 8 in the crevasse splay and floodplain sediments mesogenetic alterations that are encountered in the fluvial hst sandstones include i illitization of grain coating clays mud intraclasts and mica possibly because of simultaneous albitization of feldspars ii cementation by calcite 18 o vpdb 14 5 to 8 4 13 c vpdb 7 7 to 0 6 and dolomite 18 o vpdb 15 8 to 5 13 c vpdb 7 9 to 1 5 and iii limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain coating clays conversely the tide dominated deltaic tst sandstones and the tidal flat hst calcarenite were pervasively cemented by calcite 18 o vpdb 6 6 to 3 1 13 c vpdb 5 1 to 0 6 and siderite 18 o vpdb 7 2 to 5 7 13 c vpdb 0 3 to 0 9 particularly below marine and maximum flooding surfaces due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces the remaining open pores were cemented during mesodiagenesis by calcite 18 o vpdb 6 6 to 3 1 and 13 c vpdb 5 1 to 0 6 and dolomite 18 o vpdb 6 6 to 3 1 and 13 c vpdb 5 1 to 0 6 this study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy the model shows that tide dominated deltaic tst sandstones and tidal flat hst calcarenite were pervasively cemented by carbonates during near surface eodiagenesis owing to the presence of abundant bioclasts conversely fluvial lst sandstones remained poorly cemented during near surface eodiagenesis due to the lack of bioclasts but were cemented by mesogenetic calcite dolomite and quartz overgrowths instead

crustal structure of the southern korean peninsula from seismic waves generated by large explosions in 2002 and 2004 in order to investigate the velocity structure of the southern part of the korean peninsula seismic refraction profiles were obtained along a 294 km wnw ese line and a 335 km nnw sse line in 2002 and 2004 respectively seismic waves were generated by detonating 500 1000 kg explosives in drill holes at depths of 80 150 m the seismic signals were recorded by portable seismometers at nominal intervals of 1 5 1 7 km separate velocity tomograms were derived from first arrival times using a series expansion method of travel time inversion the raypaths indicate several mid crust interfaces including those at approximate depths of 2 3 15 17 and 22 km the moho discontinuity with refraction velocity of 7 8 to 8 4 km s has a maximum depth of 37 39 km under the southern central portion of the peninsula the moho becomes shallower as the yellow sea and the east sea are approached on the west and east coasts of the peninsula respectively the depth of the 7 6 km s velocity contour varies from 29 4 km to 36 5 km the discrepancy in depth between the seismological moho and the interpreted critically refracting interface may result from the presence of a gradual transition between the crust and mantle the velocity tomograms show particular crustal structures including 1 the existence of an over 70 km wide low velocity zone centered at 6 7 km depth under the okchon fold belt and ryeongnam massif 2 existence of high velocity materials under the gyeongsang basin and 3 the downward extension of the yeongdong fault to depths greater than 10 km

the sedimentological evolution and petroleum potential of a very thick upper cretaceous marine mudstone succession from the southern high latitudes a case study from the bight basin australia abstract during iodp expedition 369 a 690 m thick succession of silty claystone spanning the early turonian to the late santonian was encountered at site u1512 in the bight basin offshore southern australia stacking patterns sedimentary facies and palynological assemblages reveal that the succession was rapidly deposited with hyperpycnal and hypopycnal flows in a marine prodelta setting which was subject to basin restriction the dominance of clay rich facies and phytoclasts in the succession was likely the result of a major river system delivering a high sediment load into the bight basin when a warm wet climate prevailed a combination of high sedimentation rates 19 272 m myr and accelerated subsidence prevented the delta from rapidly prograding into more distal regions of the basin the complete turonian to santonian mudstone succession yields low total organic carbon 1 wt and type iv kerogens however palynofacies assemblages become progressively marine in character and total organic carbon values vary between 1 and 1 5 wt with depth this may indicate that the base of the hole at site u1512 was close to potential organic rich black shales associated with ocean anoxic event 2 low amplitude and irregular reflections on seismic data and disparities between biostratigraphic zonations suggest the upper 350 m of the turonian to santonian succession may represent a mass movement that happened during the pleistocene this study reveals that site u1512 material likely represents a near stratigraphically complete marine mudstone succession from high paleolatitudes as well as the only depositional record that was fully cored from the bight basin

bulk and stable isotopic compositions of carbonate minerals in martian meteorite allan hills 84001 no proof of high formation temperature understanding the origin of carbonate minerals in the martian meteorite allan hills alh 84001 is crucial to evaluating the hypothesis that they contain traces of ancient martian life using arguments based on chemical equilibria among carbonates and fluids an origin at greater than 650 c inimical to life has been proposed however the bulk and stable isotopic compositions of the carbonate minerals are open to multiple interpretations and so lend no particular support to a high temperature origin other methods possibly less direct will have to be used to determine the formation temperature of the carbonates in alh 84001

climate variability in the stephanian b based on environmental record of the msec lake deposits kladno rakovnik basin czech republic abstract in the stephanian b in eastern equatorial pangaea a large freshwater lake was formed in the continental basins of the west central and northeast bohemia czech republic with a total area estimated at 5000 10 000 km 2 and a lifetime of 50 to 500 ky its size exceeded that of any permian carboniferous lake in the european basins of that period so far described continual sedimentation of organic rich seasonally laminated clayey silty lake deposits produced a unique high resolution record of part of the stephanian b climate in the studied region an extraordinarily large trophic web of fish assemblages also proves the temporal stability of this meromictic oligotrophic lake the lake hydrology was inferred from the chemical and isotopic analyses of the autochthonous carbonates particularly siderite and analysis of organic matter the environmental and weathering conditions in the lake watershed were inferred from analysis of palynospectra and detrital clay minerals the lake passed through several stages of development including an initial highstand with peak productivity a stepwise increase in mineralization accompanying gradual lake regression a lowstand in the middle part of its lifetime a partial lake recovery and then a final transition to a shallow lacustrine deltaic system lake regression was triggered by a long term decrease in the p e ratio coupled with increasing seasonality and accompanied by a decrease in intensity of chemical weathering in the drainage basin considerable vegetation response to fluctuating lake levels suggests complex environmental changes during the initial lake highstand abundant and diversified assemblages of xerophilous elements prevailed spores of ferns prevailed during the regressive episode and spores of lycopsids abruptly increased at the lowstand and remained dominant in the palynospectra of the second lake highstand final transition to a lacustrine deltaic system was associated with pronounced increase in sphenopsids and ferns these changes can be attributed to milankovitch like climate change from warm wet to warm seasonal dry and may be inferred to have been responsible for stephanian continental cyclothems the entire stephanian b period was more humid than the preceding barruelian and subsequent stephanian c in the czech basins thus revealing a climate cycle at a timescale of several million years which is discussed in relation to southern gondwana ice sheet stability

geochemistry of early siderite cements from the eocene succession of whitecliff bay hampshire basin u k abstract siderite cements in the eocene of whitecliff bay hampshire basin u k occur in a range of depositional facies and morphological associations most siderites are associated with in situ glaucony or occur beneath conspicuous lithological breaks these associations illustrate the importance of slow deposition for early diagenetic carbonate precipitation siderite in the eocene sediments of whitecliff bay is impure and in many cases comprises zoned crystals with an overall decrease in substitution of ca mg and mn for fe from crystal core to rim trends in ca mg and mn from early to later siderite vary between samples with no evident environmental control siderite concretions without calcite cement occur only beneath lithological breaks where meteoric water may have been introduced precipitation temperatures have been calculated from siderite 18o data and are based on the assumption that precipitation occurred from seawater these temperatures are reasonable for typical microbial siderite precipitated in marine sediment but are slightly low for marine siderite where there may have been meteoric overprinting if the precipitation temperature for siderite cements is assumed from 18o data for time equivalent biogenic carbonate rather than the composition of the precipitating fluid the calculated isotopic composition of the precipitating fluid is generally compatible with the inferred marginal marine depositional environments