geochemical and textural evidence for early shallow diagenetic growth of stratigraphically confined carbonate concretions upper devonian rhinestreet black shale western new york laterally persistent carbonate concretionary horizons are conspicuous to the upper devonian frasnian rhinestreet black shale of the western new york state appalachian plateau field observations including randomly tilted concretions and differential compaction of host sediment laminae around concretions are consistent with early diagenetic growth in unconsolidated sediment further estimates of pre cementation host sediment porosity based on the volume percentage of caco3 cement 74 93 and perhaps most importantly the preservation of a cardhouse clay fabric in mudstones collected from concretion pressure shadow regions suggest that concretionary growth occurred rapidly within perhaps a meter of the seafloor petrographic examination sem of matrix carbonate samples collected from six concretions reveals a relatively porous 4 7 framework of calcite micrite and microsparite euhedral pyrite most abundant near concretion rims disseminated pyrite framboids and randomly oriented detrital clay platelets and domains c isotopic compositions of rhinestreet concretion matrix carbonate range from 13 9 to 1 7 pdb suggesting derivation of c from a mixed source that included depleted and isotopically heavy c slightly to moderately depleted o isotopic compositions 3 8 to 6 8 pdb do not corroborate the shallow depth of concretion growth suggested by field and textural observations the vertically confined nature of most rhinestreet concretions their large size oblate ellipsoidal shape variable c isotope compositions and absence of any evidence that concretions were sited at local concentrations of organic matter suggest an origin by anaerobic ch4 oxidation early pre concretion sulfate reduction and associated pyrite framboid formation that occurred throughout the host sediment was renewed in thin shallow zones of uncompacted sediment the concretionary horizons by ch4 and isotopically heavy dissolved carbonate diffusing upward from the zone of biogenic methanogenesis to the base of the sulfate reduction zone probably little more than a meter below the seabed local variations in the mix of isotopically light ch4 and heavier dissolved carbonate may have been responsible for the wide range of 13c isotopic compositions of rhinestreet concretions crucial to this mechanism is a pause in deposition that would have held the zone of carbonate precipitation at a fixed distance below the ocean floor long enough for the large concretions to grow evidence that sedimentation slowed or even stopped during concretion growth includes 1 laminae that can be traced through concretions with no observed systematic changes in thickness and 2 minimal concretion center to edge variations in estimated pre cementation porosity a resumption of sedimentation and related burial brought concretion growth in each horizon to an end the moderately depleted 18o values of rhinestreet concretions probably reflect the resetting of o isotopic compositions by warm fluids migrating through the permeable concretions during burial

stable carbon and oxygen isotope evidence of cretaceous sea level fluctuations recorded in septarian concretions from pueblo colorado u s a abstract limestone concretions in the blue hill member of the carlile formation middle middle turonian exposed in the lake pueblo state park colorado u s a range in size from 20 cm diameter approximately spherical forms to flattened spheroids 2 m in width and 1 m in height the earliest matrix and septarian calcite cement with 13c and 18o values ranging mostly from 18o oo to 8o oo pdb and 3o oo to 8o oo pdb respectively precipitated from diluted seawater during shallow burial where sulfate reduction largely controlled pore water bicarbonate composition in contrast the latest septarian cements exhibit very depleted 18o values 16o oo with 13c values approaching values of 1o oo to 3o oo reflecting a strong meteoric water component to pore fluids overall 13c and 18o values are inversely covariant concretion matrix calcite and septarian cements precipitated from mixtures of modified marine and meteoric fluids pore water reversals resulted from sea level fluctuations leading up to and associated with the formation of a sequence boundary or possibly the development of parasequences near the top of the blue hill member relative sea level falls caused pore water freshening as the coastal mixing zone migrated seaward similarly modified marine derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding carbonate cements precipitated during this active hydrologic history recorded the isotopic characteristics of their parent fluids negative 18o values of the meteoric end member calcite compositions suggest derivation from high altitude precipitation in the sevier highlands to the west septarian cements in concretions despite their enclosure in relatively impermeable host sediment can act as sensitive indicators of relative sea level fluctuations

phyllosilicate orientation demonstrates early timing of compactional stabilization in calcite cemented concretions in the barnett shale late mississippian fort worth basin texas u s a abstract calcite cemented zones in the prolific gas producing barnett shale ft worth basin texas preserve very early compactional fabrics that contrast markedly with fabrics in the enclosing host rocks the compactional state of these zones based upon alignments of phyllosilicates as determined by high resolution x ray goniometry hrxtg complemented by x ray powder diffraction xrpd back scattered electron imaging and elemental x ray mapping points to a prelithification near syndepositional timing of initial authigenic calcite emplacement thus these carbonate cemented zones are of particular interest for the information they provide on the character of grain assemblages and sediment types at the time of deposition and also on the early post depositional environmental chemistry of the barnett shale strata

origin of siderite concretions in microenvironments of methanogenesis developed in a sulfate reduction zone an exception or a rule abstract the mississippian shales from wardie scotland were deposited in a large normally brackish water basin they host siderite concretions with 13c values ranging from 3 5 to 12 1 which show that sulfate reduction and methanogenesis contributed to the production of bicarbonate fossils are found within each of the concretions suggesting that the decay of these organic remains provided a catalyst for concretionary growth the pore water was freshened which is indicated by the very negative 18o values from 12 8 to 4 7 many features indicate that the concretions are very early diagenetic precompactional precipitates the concretions are mostly homogeneous only a few exhibit complex concentric zoning the microstructure of the homogeneous concretions indicates a pervasive growth however their isotopic composition reveals a clear trend towards lighter 13c and 18o values from the center to the edge it is proposed that the concretions grew due to methanogenesis in microenvironments around large organic remains which acted as nuclei we suggest that this process may be one of the reasons for the very common occurrence of cements as mineral concretions not as disseminated precipitates along beds these microenvironments developed entirely within the sulfate reduction zone which resulted in the establishment of isotopic gradients in bicarbonate dissolved in the pore water across these microenvironments these gradients were caused by fast consumption of sulfate by the intensive oxidation of organic remains this resulted in the isotopic trends preserved in the homogeneous concretions the formation of the zoned concretions proceeded generally concentrically but detailed observations suggest that pervasive growth also contributed to their formation afterwards all the concretions were deeply buried probably within the oil window where late diagenetic phases crystallized mainly kaolinite quartz and sulfides

constraining pathways of microbial mediation for carbonate concretions of the miocene monterey formation using carbonate associated sulfate abstract carbonate concretions can form as a result of organic matter degradation within sediments however the ability to determine specific processes and timing relationships to particular concretions has remained elusive previously employed proxies e g carbon and oxygen isotopes cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen nitrate metal oxides and sulfate as electron acceptors in addition to degradation by thermal decarboxylation here we employ concentrations of carbonate associated sulfate cas and 34scas along with more traditional approaches to determine the specific nature of concretion authigenesis within the miocene monterey formation integrated geochemical analyses reveal that at least three specific organo diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample site specific one calcitic concretion from the phosphatic shale member at naples beach yields 34scas values near miocene seawater sulfate 22 vcdt abundant cas ca 1000 ppm depleted 13ccarb 11 vpdb and very low concentrations of fe ca 700 ppm and mn ca 15 ppm characteristics most consistent with shallow formation in association with organic matter degradation by nitrate iron oxides and or minor sulfate reduction cemented concretionary layers of the phosphatic shale member at shell beach display elevated 34scas up to 37 cas concentrations of 600 ppm mildly depleted 13ccarb 6 moderate amounts of mn ca 250 ppm and relatively low fe ca 1700 ppm indicative of formation in sediments dominated by sulfate reduction finally concretions within a siliceous host at montana de oro and naples beach show minimal cas concentrations positive 13c values and the highest concentrations of fe ca 11 300 ppm and mn ca 440 ppm consistent with formation in sediments experiencing methanogenesis in a highly reducing environment this study highlights the promise in combining cas analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity

petrology chemistry and diagenesis of calcite concretions in silurian shales from central sweden calcite concretions in the lower silurian rastrites shale from central sweden were formed within 1 m depth of the seafloor micrite 1 4 m was formed initially but recrystallized into microspar 4 50 m and subsequently transformed into pseudospar 50 150 m skeletal calcite in the concretions and in the host shale has been replaced by microspar which is chemically similar to that occurring in the concretion body the 13cpdb values mostly between 2 and 2 suggest that carbonates were mainly derived from seawater some of the carbon within calcite was derived from the oxidation of organic matter recrystallization has resulted in increasingly negative 18 opdb values 7 to 4 3 whereas the 13c was not changed significantly

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

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

influence of host rock texture on the morphology of carbonate concretions in a meteoric diagenetic environment abstract the integrated stratigraphic petrographic geochemical study of quaternary marine terraces that have undergone well constrained sea level and water table oscillation s over geologically short time periods can provide important clues on diagenetic processes at the marine continental interface the punta di i cani marine terrace quaternary saint florent corsica unconformably overlies the well cemented low permeability arenites of the miocene monte s angelo formation and is erosively overlain by unlithified high permeability holocene continental sediments the terrace is composed of a retrogradational fining upward succession from fluvial pebbly sandstone to beach and shoreface deposits the entire thickness of the terrace hosts concretions made of low mg calcite concretion geometry is linked to the sedimentological facies of the host sediment i massive fluvial pebbly sandstone at the base of the terrace contains knobby subhorizontal concretions somewhat resembling turbidite flute casts ii very low angle cross bedded well sorted beach deposits contain pipy subhorizontal concretions iii trough crossbedded upper shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae locally obliterated by a patchy isotropic cementation overall the degree of cementation decreases from the bottom to the top of the terrace cement chemistry and morphology as well as stable carbon and oxygen isotopic signatures indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast elongate concretions are invariably parallel to the topographic gradient consistent with the notion that the growth of concretions was controlled by advection smectitic vadose cement is locally present its crosscutting relationships with calcite cement point to periodic oscillations of the water table the results of this study i show that elongate concretions can be used as interstitial paleoflow indicators and ii suggest that concretion geometry in an advection dominated diagenetic environment can depend on the small scale permeability distribution as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies

carbon oxygen and sulphur isotope variations in concretions from the upper lias of n e england carbon oxygen and sulphur isotope data for transects across two pyrite bearmg carbonate concretions and their host sediments from the upper lias of n e england show symmetrical zonation 13cpdb values of the calcite cement 12 9 to 15 4 indicate that most of it originated from organic matter by bacterial reduction of sulphate augmented with marine and to a lesser extent fermentation derived carbonate organic carbon 13cpdb 26 1 to 37 0 reflects the admixture of allochtho nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation two phases of pyrite are present in each concretion the earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has 34scd values of 22 to 26 indicating formation by open system sulphate reduction the later euhedral phase is more abundant and reaches values of 2 5 to 5 5 at concretion margins this phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system the 13c and 34s data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface the 18o values of the calcite in one concretion 18opdb 2 3 to 4 8 indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater the other concretion is isotopically much lighter 18opdb 8 9 to 9 9 and large 18o differences between concretions in closely spaced horizons imply that local factors control the isotopic composition of pore waters

permian endemic bivalves of the irati anoxic event parana basin brazil taphonomical paleogeographical and evolutionary implications abstract anoxic to dysoxic conditions must have existed in the parana basin during the deposition of the permian oil bearing shales of the irati formation reflecting the marine isolation of the basin from the panthalassic ocean in this environment benthic invertebrates are extremely rare in contrast to the overlying serra alta and teresina formations which were deposited in dysoxic oxic waters respectively hence the abundant shallow burrowing endemic bivalves recorded in the shales of the basal portion of the irati formation are one of the most distinctive features of this stratigraphic interval their preservation in offshore deposits is however a product of storm flows in shallow waters that swept the shells to distal settings subsequently these were sorted by long lasting shelf currents that produced dense 2 5 shells 20 cm2 pavements in which shells in a hydrodynamically stable posture convex up predominate forming thin complex shell concentrations despite their simple internal stratigraphy the new data presented here have important paleoecologic paleogeographic and evolutionary implications and indicate that a during the deposition of the oil rich shales of the irati formation numerous benthic bivalves thrived in the contemporary shallow water bottoms of the parana basin b the mono to paucispecific nature of the studied shell rich pavements is in part due to hydrologic shell transport and c restricted connections to open ocean waters panthalassa existed during irati times therefore the origin of the endemic passa dois molluscan fauna occurred somewhere between the interval represented by the underlying palermo formation and the basal portion of the irati formation finally these endemic bivalves appeared at least 10 million years earlier than previously thought

carbon and oxygen isotope analysis of hydrate associated oligocene authigenic carbonates using nanosims and irms abstract petrography and stable c and o isotope compositions of methane derived carbonates from the polish outer carpathians were analyzed two methods were employed in isotopic determinations conventional irms measurements on powdered bulk samples were performed with high precision errors 0 06 for 13c and 0 08 for 18o 1 whilst in situ measurements were made using nanosims with a high spatial resolution 5 m but with low precision errors 3 0 for 13c and 2 3 for 18o 2 the nanosims study proved very useful in identifying the sources of the fluids precipitating the carbonates they were formed predominantly by the anaerobic oxidation of biogenic methane aom the very fine spatial resolution of the analyses revealed highly variable 13c values even in individual crystals from extremely negative related to aom to positive related to methanogenesis me other methods including laser ablation and standard sims would fail to detect this variability and would not indicate the true sources of the fluids because they would show average values therefore any straightforward interpretation of the sources of authigenic carbonates based on these methods may be attempted only if the 13c values are extreme i e resulting either from aom or me unusual clast like druses from the carbonates were analyzed in detail with nanosims they are filled with strongly 13c depleted down to 54 and 18o enriched up to 6 fringe calcite which precipitated on the walls of the druses from heavy oxygen containing water released from the dissociation of gas hydrates the rocks can thus be termed clathrites the bulk rock isotope analyses of powdered samples of these druses show no evidence of hydrates and alone would not point to the true origin of these rocks precipitation of the carbonate cements started when hydrates were still present so they are a type of melt and seal fabric the recognition of such features may be crucial evidence of the former occurrence of hydrates in fossil methane derived carbonates the isotopic results have demonstrated that nanosims can be very useful in reconstructing the diagenetic evolution of pore water by analyzing the stable isotope compositions of even microcrystalline cements this provides a basis for the application of nanosims to tracing diagenetic pathways from various even complexly formed authigenic precipitates

time capsule concretions unlocking burial diagenetic processes in the mancos shale using carbonate clumped isotopes abstract septarian carbonate concretions contain carbonate precipitated during progressive growth of the concretion and subsequent fracture filling as such they have been used to track variations in 13c and 18o of pore waters during diagenesis and to define diagenetic zones in clastic rocks however the 18o value of the carbonate is dependent on precipitation temperature and the 18o value of the pore fluid from which it precipitated interpretations must assume one of these parameters both of which are highly variable through time in diagenetic settings carbonate clumped isotopes of the cement can provide independent estimates of temperature of precipitation allowing the pore water 18o to be back calculated here we use this technique on carbonate concretions and fracture fills of the upper cretaceous prairie canyon member mancos shale colorado we sampled concretions from two permeable horizons separated by a 5 m shale layer with one permeable horizon containing concretions with septarian fractures we show cores precipitated at cooler temperatures 31 c 660 m burial depth than the rims 68 c 1980 m burial depth and relate that to the 13ccarbonate values to suggest the concretion core precipitated in the methanogenic zone with increasing input from thermogenically produced co2 the two concretion bearing horizons have different back calculated 18oporewater values mean 2 65 and 1 13 vsmow for cements formed at the same temperature and similar 13c values suggesting the shale layer present between the two horizons acted as a barrier to fluid mixing additionally the 18ocarbonate of the septarian fractures 13 8 vpbd are due to precipitation at high temperatures 102 to 115 c from a fluid with a mean 18oporewater of 0 32 vsmow therefore we can tie in the cementation history of the formation to temporal and spatial variations in 18oporewater

the role of diagenetic carbonate concretions in the preservation of the original sedimentary record the upper cretaceous organic rich limestones and marls of the tarfaya basin of southwest morocco contain numerous calcite concretions which formed during early diagenesis relative textural similarities are observed both in the concretions and in the host sediments however the biological content of the concretions is considerably higher than in the host marls and limestones evidence for fossil dissolution in the host marls and the absence of concretions in some fossil rich zones suggest that the difference in fossil abundance between the concretions and host rock is a function of dissolution rather than preferential precipitation in fossil rich areas consequently the carbonate concretions appear to represent the memory of the sediment and allow quantification of the original biological components and are potential tools for estimating the original biological material deposited in the soft sediments

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

siderite concretions indicators of early diagenesis in the gammon shale cretaceous the upper cretaceous gammon shale in the northern great plains contains abundant siderite concretions that formed during the early compactional history of the fine grained marine sediments the relative depth of siderite precipitation can be estimated on the basis of textural mineralogical and isotopic evidence siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate 75 85 preserve uncompacted sediment textures and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater delta 18 o nearly equal e 0 to 2 per thousand pdb in contrast lower volume percent carbonate values 55 75 and relative decreases in 18 o 16 o values characterize concretions that formed longer after deposition and or at greater depths in the accumulating sediments both volume percent carbonate and delta 18 o values are strongly correlated with independently determined sediment accumulation rates concretions in rapidly deposited sediments formed at shallow depths 200 m later in the burial history the gammon served as both source rock and reservoir for accumulations of biogenic methane and like the siderite concretions the methane was generated during early diagenesis both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen because of chemical requirements siderite did not precipitate until after the onset of methane generation therefore if the depth of siderite formation is known the minimum depth for the onset of methane accumulation can be inferred the presence of abundant pyrite in slowly accumulated gammon sediments suggests by analogy to data from recent sediments that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate in slowly accumulating sediments most organic matter was probably consumed by aerobic organisms and sulfate reducing anaerobic bacteria whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation

isotopic variations in dolomite concretions from the monterey formation california abstract isotopic analysis of dolomite concretions from the miocene monterey formation of california yields a covariance of 18o and 18c within every concretion analyzed the total range for all concretions in 18o smow is 27 3 to 33 6 the total range in 13c pdb is 10 2 to 10 8 variations of up to 3 6 in oxygen and 12 2 in carbon are encountered within a single concretion decreasing 18o is interpreted in terms of increasing temperatures of formation as the concretion grows during burial the claypool and kaplan 1974 model can be successfully used to interpret carbon isotope variations as the result of progressive concretion growth in the zones of microbial sulfate reduction microbial methane production and thermocatalytic decarboxylation monterey dolomite concretions are too rich in carbonate to have originated by simple cementation of initially porous sediment much of the originally diatomaceous sediment appears to have been replaced by dolomite isotopic zonation within one concretion is found to be complex suggesting nucleation at several sites which then grew together complex growth patterns may result from local variations in lithology and porosity during precipitation multiple sampling of several concretions at a single outcrop yields a more complete early diagenetic history than by sampling only one concretion isotopic diagenetic profiles inferred from the dolomite concretions are similar to those seen in deep sea drilling project cores where deep sea dolomitization is occurring today single samples from a concretion cannot reveal the range or the trend of the diagenetic reactions that have occurred multiple sampling of diagenetic carbonate concretions is clearly essential in determining the early diagenetic history from isotopic measurements

widespread bedding parallel veins of fibrous calcite beef in a mature source rock vaca muerta fm neuquen basin argentina evidence for overpressure and horizontal compression in the neuquen basin of argentina beef bedding parallel veins of fibrous calcite is widespread within late jurassic black mudstones a typical vein consists of two grey inner zones and two white outer zones the inner zones contain inclusions of wall rock and hydrocarbons calcite fibres are perpendicular to the margins in the outer zones the angle between fibre and margin varies from about 45 at the vein tips to 90 in the centre imprints of fossils are offset proving that the fibres have grown antitaxially we infer that the veins opened in two phases during phase 1 the opening was vertical against gravity during phase 2 the veins resisted tectonic shortening so that shear stresses acted at the margins the senses of shear account for the fibre angles at outcrop igneous intrusive rocks have cut and metamorphosed the veins from burial curves maturity calculations growth strata and ages of igneous intrusions we estimate that the inner zones of the beef formed in the oil window during the aptian to albian and that the outer zones formed in the gas window during the cenomanian to campanian we infer that the beef is evidence for fluid overpressure during hydrocarbon generation

the internal structure of carbonate concretions in mudrocks a critical evaluation of the conventional concentric model of concretion growth abstract carbonate concretions have traditionally been viewed as forming concentrically through the progressive addition of carbonate to the outer edge of the growing concretion this conventional model of concretion growth is based mainly upon center to edge textural and geochemical trends that are consistent with concentric growth however a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model in some cases complexly zoned concretions contain late stage cements that precipitated throughout the concretion body none of the center to edge relationships used to support the conventional concentric model exclude more complex modes of origin as concretions that fit the conventional model may be the exception rather than the rule high resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data

septarian concretions from the oxford clay jurassic england uk involvement of original marine and multiple external pore fluids septarian concretions occur at several horizons within the oxford clay formation a marine mudstone containing pristine aragonite and immature biomarker molecules they record the passage of at least four generations of pore fluids the first of marine origin and the last still present in cavities concretion bodies formed cracked and calcite and pyrite precipitated in and around the cracks within the sulphate reduction zone as demonstrated by c o s and sr isotopic composition pore fluid 1 before major compaction sandstone dykes were intruded locally and baryte precipitated followed by coarse calcite cements with isotopically light oxygen and radiogenic strontium indicating the introduction of meteoric derived water pore fluid 2 later coarse celestine within concretions has distinct sulphur isotopic composition and requires a further geographically restricted water source pore fluid 3 celestine bearing concretions contain water in tight cavities whose isotopic composition is close to that of modern precipitation its chemistry shows that it is equilibrating with pre existing minerals implying a relatively recent origin pore fluid 4 the mineralogy of the oxford clay concretions shows that complex results can follow from a simple burial and uplift history and that multiple generations of pore fluids can pass through a low permeability clay

patterns of carbonate authigenesis at the kouilou pockmarks on the congo deep sea fan abstract different types of seep carbonates were recovered from the kouilou pockmarks on the congo deep sea fan in approximately 3100 m water depth the carbonate aggregates are represented by pyritiferous nodules crusts and slabs tubes and filled molds the latter are interpreted to represent casts of former burrows of bivalves and holothurians the nodules consisting of high mg calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic anme 1 group whereas aragonitic crusts resulted from the activity of archaea of the anme 2 cluster evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts probably reflecting higher porosities of the host sediment during carbonate formation this finding is supported by lower 18 o values of crusts agreeing with precipitation from pore waters similar in composition to seawater the aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate rich pore waters similar to seawater moreover the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that anme 1 archaea tend to occur deeper in the sediment than members of the anme 2 group methane represents the predominant carbon source of all carbonates 13 c values as low as 58 9 v pdb and the encrusted archaeal biomarkers 13 c values as low as 140 v pdb oxygen isotope values of some nodular carbonates ranging from 3 9 to 5 1 v pdb are too high for precipitation in equilibrium with seawater probably reflecting the destabilization of gas hydrates which are particularly abundant at the kouilou pockmarks

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

distinct sedimentary processes reflected in the isotopic signatures of dolomitic concretions in the miocene pohang basin southwestern east sea abstract dolomitic concretions in diatomaceous hemipelagic sediments of the miocene pohang basin in the southwestern east sea sea of japan preserve distinct signals of two independent sedimentary processes which controlled the extents of isotopic compositions variable 18o 9 1 to 0 7 and high 13c 3 1 to 17 9 values suggest that the concretions formed in the methanogenic zone with alteration of the residual mid miocene seawater by volcanogenic sediments remarkable 18o and 13c values show a strong linear relationship indicating that distinctly independent depositional processes operated during the formation of the concretions the degree of methanogenesis was enhanced during rapid hemipelagic sedimentation of organic rich particles resulting in higher 13c values and the effect of volcaniclastics was diluted maintaining the original properties of ambient mid miocene seawater in contrast lower 18o and 87sr 86sr values characterize the effect of volcaniclastic sediments that were transported by intermittent gravity flows and interacted with mid miocene seawater the input of volcaniclastic sediment probably degraded the role of methanogenesis by lowering the contents of organic matter and thereby decreased the 13c values within the concretions isotopic signals recorded within the concretions highlight understanding of the depositional environment and evolution of the pore water chemistry

origin of spheroidal chert nodules drunka formation lower eocene egypt chert nodules of the drunka formation lower eocene are mostly spherical have diameters from 40 to 120 cm are quasi uniformly spaced 2 3 m apart in the plane of bedding have concentric internal structure and except for rare small 6 cm solid chert nodules are less than 85 chertified nodules formed after moderate alteration of limestone by meteoric water 18ocalcite 4 to 8 at shallow 100 m burial depths more extensive alteration of limestone 18o 10 to 16 by meteoric water followed nodule growth chertification was by low temperature meteoric water 18oquartz 18 in margins to 22 in nuclei at shallow burial depths meteoric water may have invaded the drunka formation in association with shelf progradation during the early eocene or during the development of a middle eocene unconformity replacement of carbonate mud by microcrystalline quartz was the dominant chertification process but fossils were replaced in part by fine grained equant megaquartz quartzine and chalcedony the last of these occurs in places as beekite opal a secreting marine organisms are the inferred source of silica but none are preserved there is no compelling evidence of an opal ct precursor so quartz may have formed by direct precipitation self organization processes of enigmatic character established the spacing pattern of the nodules and also the liesegang banded internal structure of the chert nodules nodules grew chiefly by diffusive supply of silica although one locality has elongate nodules that grew when there was some porewater advection chertification patterns and 18o values of both calcite and quartz indicate that nodule growth was complex and variable some nodules probably grew from the centre outwards many nodules however initially grew simultaneously across the entire nodule but late stage growth was predominantly at the outer margins or at selective internal sites

biomarkers in a lower jurassic concretion from dorset uk textural petrographic and stable isotopic evidence suggest that a zoned concretion birchi bed lower lias west dorset uk formed under very shallow burial with carbonate cement passively filling the pore spaces the calcitic core of the concretion formed initially whilst the intermediate and outer edge cements which are dominated by a dolomite and a calcite respectively precipitated successively mixtures of calcite and dolomite occur in the intermediate zone and outer rim suggesting initial incomplete cementation and later back filling the enveloping fibrous calcite vein beef formed later by displacive crystallization the concretionary carbonates preferentially preserve labile organic compounds i e unsaturated fatty acids not found in the surrounding shales fatty acid distributions in the concretion are distinct and informative the presence of 10 methylhexadecanoic acid provides direct evidence for sulphate reducing bacteria in the calcitic core whereas the abundance of unsaturated fatty acids in the concretion as a whole is attributed to localized bacterial production and its early formation the core probably formed in the sulphate reduction zone whereas dolomite in the intermediate zone was derived largely from methanogenesis and iron reduction although direct biomarker evidence for methanogenesis was not found the outer rim and the fibrous calcite vein probably resulted from late bacterial processes probably including renewed sulphate reduction the complex biogeochemistry of the sedimentary environment is reflected by the concentrations and distributions of biomarkers and by the detailed petrography nevertheless carbonate concretions can provide a snapshot of early diagenesis in ancient mudstones

microbial processes controls on the shape and composition of carbonate concretions abstract shape size and mineralogical chemical compositions of carbonate concretions vary considerably the questions to be answered are why do they form at all what processes are involved why do they so often form as regular spheroids why do they sometimes form laterally extensive bands what controls whether the mineral composition is calcite dolomite rhodochrosite or siderite or mixtures of them the questions are addressed and the answers illustrated by selected examples carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents organic matter microbes take advantage of the energy source and are believed to be involved in most of the various processes pore waters in steadily accumulated sediments show a depth related succession of chemical and carbon stable isotope compositions this results from degradation of organic matter proceeding via an orderly succession of processes during burial each mediated by a specific microbial group aerobic oxidation using dissolved oxygen occurs first followed by mn iv reduced to mn ii as the oxidant fe iii reduction and sulphate reduction are probably more significant methanogenesis is the final deepest microbial process the common product of all these reactions is co 2 or dissolved hco 3 which can result in carbonate supersaturation the specific process controls the chemical and isotopic compositions of pore water and leads to a characteristic mineralogy of any carbonate precipitated however the processes may not proceed successively and localised micro environments occur recent work on siderite concretions in very young salt marsh sediments norfolk u k has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth concretions in ancient rocks can be described in terms of the same processes observed in modern sediments laterally extensive siderite bands in bullhouse quarry westphalian u k resulted from interaction of two sets of complementary processes shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation the model is supported by a mass and charge balance calculation similar complementary processes formed spheroidal concretions gammon shale usa but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction the spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model kimmeridge bay uk dolomite ledges are marine analogues to the bullhouse examples for the pyrite rimmed calcite jet rock concretions early jurassic yorkshire u k the rate of production of sulphide from sulphate reduction locally exceeded availability of fe ii to precipitate pyrite outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore water evolution however all represent an expression in ancient rocks of microbiological processes similar to those operating today

depositional environments of the lower permian dwyka diamictite and prince albert shale inferred from the geochemistry of early diagenetic concretions southwest karoo basin south africa abstract the upper dwyka and lower ecca groups in the karoo basin of south africa document the climatic and palaeoenvironmental changes associated with the final permo carboniferous deglaciation of the gondwana supercontinent the depositional environments of these groups have until recently been interpreted on the basis of sedimentological and palaeontological evidence here we use the geochemistry of early diagenetic concretions septarian calcite concretions from the upper dwyka group and phosphatic chert concretions and beds from the lower ecca group to infer the depositional environment of these rocks in the southwestern karoo basin 18o values 7 8 to 8 9 smow suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than permian seawater 13c values 15 to 3 pdb indicate that the carbon was derived from a mixture of craton derived calcareous material and organic matter bacterially degraded in the lower sulphate reduction to upper methanogenesis zones during early burial diagenesis the rare earth element ree patterns sr concentrations and 87sr 86sr ratios 0 716 0 737 significantly greater than permian seawater 0 708 together also support the interpretation that calcite and phosphatic concretions formed in glacial fresh water sediments

alteration styles in the orion central volcanic complex fort a la corne kimberlite field saskatchewan and their effects on primary volcaniclastic textures implications for facies mapping and diamond exploration diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes which are generally determined from volcaniclastic facies characteristics however the effects and distribution of secondary alteration styles which usually mask primary kimberlite facies have been given little attention at the fort a la corne kimberlite field saskatchewan complex overprinting alteration textures and mineral phases are recognized this study documents the characteristics overprinting relationships and spatial distribution of alteration styles within a crater filling volcaniclastic deposit body 219 of the orion central volcanic complex data were acquired from macroscopic drill core observations microscopic petrography sem imaging eds element mapping and xrd analyses olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite late stage serpentine has migrated into the interstitial medium bulk rock alteration textures include 1 fine grained crystal rich domains which are alteration pseudoclasts and or relict juvenile pyroclasts 2 a heterogeneous interstitial medium varying from early stage disseminated calcite rich serpentine to amorphous serpentine and crystalline calcite and 3 overprinting serpentine carbonate and magnetite veins and serpentine nodules alteration styles defined by unique alteration characteristics are spatially zoned through the body 219 volcaniclastic deposit and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit recognition of alteration processes their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies and by consequence the distribution of diamond grades

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

the geochemistry of concretions from the kimmeridge clay formation of southern and eastern england concretions from the kimmeridge clay formation are of three types calcareous concretions septarian calcareous concretions and pyrite calcite concretions and nodules which occur within different mudstone facies isotopic and chemical analysis of the concretionary carbonates indicate growth in the fe reduction sulphate reduction and decarboxylation zones the septarian concretions show a long and complex history with early initiation of growth and development spanning several phases of burial each often resulting in the formation of septaria growth apparently ceased in the transitional zone between the sulphate reduction and the methanogenesis zones very early growth in the fe reduction zones is also seen in one sample the non septarian concretions began growth later within the sulphate reduction zone and have had a simpler burial history while the pyrite calcite concretions show carbonate cementation in the sulphate reduction methanogenesis transition zone a ferroan dolomite calcite septarian nodule with decarboxylation zone characteristics also occurs development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment the latter determining the organic matter input to the sediments calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones pyrite calcite concretions occur in organic rich mudstones deposited under higher sedimentation rates in the basins while the ferroan dolomite nodule grew under very high sedimentation rates

septarian concretions internal cracking caused by synsedimentary earthquakes septarian concretions are abundant in many phanerozoic marine and marginal marine shales and mudstones they range from a few centimetres to several metres in size and are spherical or ellipsoidal in morphology in general formation by localized calcite or siderite cementation in argillaceous sediments began under less than a few metres of burial septarian cracks vary widely in shape and configuration included are networks of wide vertically radially and sometimes concentrically oriented lenticular shrinkage cracks and narrower parallel sided straight to irregular tension cracks locally accompanied by brecciation and plumose and en echelon sigmoidal cracks indicative of shear stresses crack types are intergradational many concretions exhibit multiple cracking events enclosed macrofossils and isopachous fibrous calcite cement that lines earlier formed cracks are commonly broken and displaced in some cases cracks contain injected lime mud and silt these features taken together testify that cracking involved a spectrum of responses in concretion interiors from loss of shear strength dewatering and shrinkage to brittle failure demonstrating variations in and contrasts between the rheological properties of the matrix and enclosed objects localization to interiors and outward tapering of lenticular cracks make a case for fracture partitioning and indicate that interiors were softer than exteriors at the time of shrinkage parallel sided cracks point to greater stiffness and evolving crack shape in multiply cracked concretions shows that rigidity increased with time crack orientations indicate highly variable tensile and shear stress directions within individual concretions rupture brecciation displacement of fragments loss of shear strength liquefaction and injection of unconsolidated granular sediment suggest that deformation events were rapid if not virtually instantaneous previous explanations for the internal cracking such as gas generation spontaneous chemical dehydration or localized overpressuring due to compaction seem either untenable or fail to account for the spectrum of observed features however syndepositional earthquake induced shaking of cementing bodies of varying rigidity at shallow burial depths seems to be a plausible source for the requisite short lived variable to anisotropic high stress regime inside the concretions septarian concretions may thus preserve a signature of basin seismicity as it relates to their cementation history

spheroidal cannonballs calcite cemented concretions from the faiyum and bahariya depressions egypt evidence of differential erosion by sand storms

material flux and porosity changes during sediment diagenesis understanding the changes in porosity that occur during burial of sediments is of importance both in modelling fluid flow in sedimentary basins and for prediction of petroleum reservoir quality when deposited up to 50 of the volume of sands is intergranular pore space porosity generally decreases with depth because of compaction and precipitation of diagenetic minerals but some sandstones retain an anomalously high porosity whether this is due to dissolution and removal of material or to its local redistribution is not clear we originally intended to quantify losses of major chemical components during burial of a complete sedimentary unit we have investigated ten reservoir sandstones of permian to tertiary age from oilfields worldwide these show consistent results and here we present detailed data from four of them we found that the silica content of these sandstones actually increased by 220 to 350 kg m 3 following compaction no statistically significant changes were observed for aluminium potassium or sodium the flux of silica cannot be modelled satisfactorily using currently accepted values of permeability silica solubility and flow rates posing a challenge for existing models of fluid flow and ore emplacement

growth mechanisms and geochemistry of carbonate concretions from the cambrian wheeler formation utah usa carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments typically they form in organic rich mudstones where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments as a result carbonates that comprise concretions are usually characterized by broad ranges in 13c and include values that are significantly depleted relative to seawater this article reports results from a physical petrographic and geochemical analysis of 238 concretions from the wheeler formation cambrian series 3 utah usa which are unusual in several respects most prominently they formed in organic poor mudstones total organic carbon 0 1 to 0 5 and are characterized by a narrow range of 13c that onlaps the range of contemporaneous seawater values subtle centre to edge trends in 13c demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters the large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix which occurred via both displacive and replacive carbonate precipitation during early diagenesis stable isotope data from cogenetic pyrite 34s and silica 18o phases provide insight into the evolution of biogeochemical processes during concretion growth and suggest that concretions were formed almost entirely during sulphate reduction with only minor modification thereafter concretions of the wheeler formation appear to represent an end member system of concretion formation in which rapid growth was promoted by ions supplied from sea water as such they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments

deep burial alteration of early diagenetic carbonate concretions formed in palaeozoic deep marine greywackes and mudstones bardo unit sudetes mountains poland carbonate concretions formed in bathyal and deeper settings have been studied less frequently than those formed in shallow marine deposits similarly concretions affected by catagenetic conditions have rarely been reported calcite concretions in deep marine mudstones and greywackes of the bardo unit sudetes mountains poland formed during early diagenesis and were buried to significant depths petrographic and geochemical elemental and stable c and o isotopic analyses document their formation close to the sediment water interface prior to mechanical compaction within the sulphate reduction zone and their later burial below the oil window although the concretions were fully formed during early diagenesis the effects of increased temperature and interaction with late diagenetic interstitial fluids can be discerned during maximum burial the concretions underwent thorough recrystallization that caused alteration of fabric and elemental and o isotope composition the initial finely crystalline cement was replaced by more coarsely crystalline sheaf like poikilotopic calcite in the concretions these large calcite crystals engulf and partially replace unstable detrital constituents the extremely low 18o values down to 21 2 vienna pee dee belemnite in the concretions are the result of the increased temperature in combination with alteration of volcanic glass both causing a significant 18o depletion of bicarbonate dissolved in the interstitial fluids recrystallization led to uniform o isotope ratios in the concretions but did not affect the c isotope signature the 13c values of the late diagenetic cements precipitated in the greywacke and in cracks cutting through concretions imply crystallization in the catagenetic zone and decarboxylation as a source of the bicarbonate these late diagenetic processes took place in a supposedly overpressured setting as suggested by clastic dykes and hydrofractures that cut through both concretions and host rock all of these features show how the effects of early and late diagenesis can be distinguished in such rocks

jurassic septarian concretions from nw scotland record interdependent bacterial physical and chemical processes of marine mudrock diagenesis septarian concretions in the staffin shales formation kimmeridgian isle of skye allow controls on concretion rheology and septarian cracking to be investigated stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite intergranular volumes range from 77 to 88 and calcite 13c and 18o values in most concretion bodies range from 10 0 to 17 3 and 0 3 to 0 6 respectively consistent with rapid and pervasive cementation in marine pore fluids septarian rupture occurred during incipient cementation with a sediment volume reduction of up to 43 crack lining brown fibrous calcite records pore fluid re oxygenation during a depositional hiatus followed by increasing fe content and 13c related to bacterial methanogenesis brown colouration results from an included gel like polar organic fraction that probably represents bacterially degraded biomass a new hypothesis for concretion growth and septarian cracking argues that quasi rigid proto concretions formed via binding of flocculated clays by bacterial extracellular polysaccharide substances eps this provided rheological and chemical conditions for tensional failure subcritical crack growth volume contraction calcite nucleation and incorporation of degraded products into crack lining cements bacterial decay of eps and syneresis of host muds provided internal stresses to initiate rupture at shallow burial development of septarian shrinkage cracks in muds is envisaged to require pervasive in situ bacterial colonization and to depend on rates of carbonate precipitation versus eps degradation and syneresis subsequent modification of septarian concretions included envelopment by siderite and calcite microspar hydraulic fracturing associated with cretaceous shallow burial or palaeogene uplift and cementation by strongly ferroan yellow sparry calcite that records meteoric water invasion of the host mudrocks an abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products and 13c data suggest a predominantly methanogenic bicarbonate source however the wide 18o range for petrographically identical cement 1 3 to 15 6 is difficult to explain

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

the moeraki boulders anatomy of some septarian concretions abstract the moeraki boulders are large to 2 m calcite concretions with septarian veins of calcite and rare late stage quartz and ferrous dolomite the concretions are enclosed by paleocene marine mudstones with vitrinite reflectance values of 0 29 r suggesting maximum burial temperatures of 25 30 c microprobe analyses of these concretions indicate a ca enrichment mg depletion trend with growth calcite in the core contains about 88 mole ca and about 93 mole ca in the rim both rim and cores contain less than 1 mole fe and mn septarian veins also show a systematic calcium enrichment with growth of the vein the initial vein fills are similar in composition to the outer rim of concretionary bodies suggesting septarian cracks form contemporaneously with attainment of final concretion size the later stage calcite vein fills have less than 1 5 mole mg and up to 11 mole fe mn these elements systematically decrease both into and outward along the vein with increasing calcium substitution the above compositional trends in veins suggest continued crystallization during propagation of the fractures the carbonate composition trends reflect interaction between the growing concretion and the enclosing mudstone pore fluid system the observed fe mn mg depletion trends probably reflect depletion of elements released by short time scale diagenetic events of finite size the growth time of the larger concretions is estimated at about 4 million years based on published diffusion growth models extrapolation of compositional trends versus growth time from these concretion bodies suggests that septarian veins form on a time scale of several million years

diagenetic origin of nodules in the sheepbed member yellowknife bay formation gale crater mars the sheepbed member of the yellowknife bay formation in gale crater contains millimeter scale nodules that represent an array of morphologies unlike those previously observed in sedimentary deposits on mars three types of nodules have been identified in the sheepbed member in order of decreasing abundance solid nodules hollow nodules and filled nodules a variant of hollow nodules whose voids have been filled with sulfate minerals this study uses mast camera mastcam and mars hand lens imager mahli images from the mars science laboratory curiosity rover to determine the size shape and spatial distribution of the sheepbed nodules the alpha particle x ray spectrometer apxs and chemcam instruments provide geochemical data to help interpret nodule origins based on their physical characteristics spatial distribution and composition the nodules are interpreted as concretions formed during early diagenesis several hypotheses are considered for hollow nodule formation including origins as primary or secondary voids the occurrence of concretions interpreted in the sheepbed mudstone and in several other sedimentary sequences on mars suggests that active groundwater systems play an important role in the diagenesis of martian sedimentary rocks when concretions are formed during early diagenetic cementation as interpreted for the sheepbed nodules they have the potential to create a taphonomic window favorable for the preservation of martian organics

isotopic constraints on growth conditions of multiphase calcite pyrite barite concretions in carboniferous mudstones carbonate concretions in the lower carboniferous caton shale formation contain diagenetic pyrite calcite and barite in the concretion matrix or in different generations of septarian fissures pyrite was formed by sulphate reduction throughout the sediment before concretionary growth then continued to form mainly in the concretion centres the septarian calcites show a continuous isotopic trend from 1 3 c 28 7 pdb and 1 8 o 1 6 pdb through to 1 3 c 6 9 pdb and 1 8 o 14 6 pdb this trend arises from 1 a carbonate source initially from sulphate reduction to which was added increasing contributions of methanogenic carbonate and 2 burial temperature effects or the addition of isotopically light oxygen from meteoric water the concretionary matrix carbonates must have at least partially predated the earliest septarian cements and thus used the same carbonate sources consequently their isotopic composition 1 3 c 12 0 to 10 1 pdb and 1 8 o 5 7 to 5 6 pdb can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and directly or indirectly also from skeletal carbonate concretionary growth was therefore pervasive with cements being added progressively throughout the concretion body during growth the concretions contain barite in the concretion matrix and in septarian fissures barite in the earlier matrix phase has an isotopic composition 1 4 s 24 8 cdt and 1 8 o 16 4 smow indicating formation from near surface sulphate depleted porewaters barites in the later septarian phase have unusual isotopic compositions 3 4 s 6 to 11 cdt and 1 8 o 8 to 11 smow which require the late addition of isotopically light sulphate to the porewaters either from anoxic sulphide oxidation using ferric iron or from sulphate dissolved in meteoric water carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments

clumped isotope constraints on cement paragenesis in septarian concretions septarian concretions exhibit multiple generations of cements that include body fringe and spar phases classic paragenetic interpretations include initial precipitation of the body followed by fringe s and then by spar in more or less discrete events traditional approaches e g carbon and oxygen isotope analyses are generally unable to distinguish paragenetic trends as they relate to specific formation environments e g precipitation during burial or with meteoric influx here we present carbonate clumped isotope 13c 13ccarb and 18o 18ocarb values for septarian concretions taken from four host units in order to assess cement paragenesis and overcome traditional shortcomings clumped isotope and 18ofluid data exhibit a wide range of values with carbonate precipitation temperatures between 20 and 50 c and 18ofluid compositions of 14 to 4 vsmow in stable isotope cross plots specific cement phases group together and confirm the paragenesis indicated by superposition in some cases samples analyzed from concretion bodies yield temperature and 18ofluid values that indicate formation at shallow depths consistent with independent data e g high minus cement porosity external laminae deflection in contrast other concretion body analyses indicate relatively high body temperatures that conflict with shallow formation indices petrographic and backscatter scanning electron microscopy sem reveal that concretion bodies partially consist of a secondary replacement phase which could explain the higher temperatures expressed in bulk body samples based on data for different phases in these septarian concretions we suggest that initial body cement precipitation occurred at relatively shallow depths from unmodified seawater followed by fringe formation at elevated temperatures that likely coincided with the emplacement of the secondary body phase when considered together late stage spar phases exhibit temperatures and 18ofluid values supportive of spar precipitation from fluids with a significant meteoric component possibly during uplift

early diagenesis of the upper devonian escuminac formation in the gaspe peninsula quebec sedimentological and geochemical evidence abstract clay mineral investigations pyrolysis of organic matter and microscopic observation of palynomorphs are used to interpret the diagenetic conditions of the upper devonian escuminac formation quebec canada pyrolysis of kerogen vitrinite reflectance and the presence of smectite show that the sediments of this formation have been subjected only to a shallow burial diagenesis hence the geochemical signatures are informative on the palaeoenvironment concretions are observed in the escuminac formation sedimentological and geochemical evidence suggest that these concretions were formed during early diagenesis from biodegradation of organic matter the well known fish fauna of the escuminac formation show good preservation due in part to extensive early diagenetic carbonate cementation the occurrence of calcite cements and fibrous calcite is attributed to the early diagenetic transformation of pre existing organic matter

diagenesis of upper jurassic concretions from the antarctic peninsula abstract early diagenetic calcite concretions are widespread in mudstone tuff sequences deposited in late jurassic oxygen deficient basins of the antarctic peninsula although obscured or destroyed in host rocks original components and sedimentary structures are well preserved in concretions in spite of significant mineralogical and geochemical change during growth early diagenesis led to a rapid and almost total variation in the mineralogy within the concretions independently of the original host lithologies siliceous particles were replaced by chlorite and zeolites in suboxic alkaline and moderately reducing conditions on the sea floor and in the first centimeters of burial these conditions also led to localized early formation of phosphate concretions pyrite developed subsequently because of high sulfide abundance in the sulfate reduction zone most calcite precipitation took place in the lower part of the sulfate reduction zone in a strongly alkaline environment where sulfide content had been lowered by pyrite precipitation in this stage the remaining siliceous elements were dissolved or replaced by calcite later diagenetic changes include enlargement of zeolites minor changes in clay minerals and the formation of calcite veins the wide range of 13c values in cement and vein calcite 1 47o oo and 20 23o oo pdb is explained by mixed carbon sources from organic matter oxidation and dissolution of calcareous shells the low 18o of calcite 2 34o oo to 19 72o oo pdb is the result of recrystallization in contact with high temperature diagenetic or hydrothermal fluids flushed through micro fractures in the concretions which also produced extensive vein formation

depositional environment of the lower cretaceous upper albian winton formation at isisford central west queensland australia inferred from sandstone concretions abstract numerous vertebrate and plant fossils have been found in ex situ sandstone concretions near isisford in central west queensland since the mid 1990s these concretions are found in the lower cretaceous portion upper albian 100 5 102 2 ma of the winton formation the lower most winton formation is thought to have formed in a fluvial channel or flood basin setting proximal to the eromanga sea but due to the scarcity of good exposures the local depositional environment at isisford has not been ascertained minimal compression of vertebrate and plant fossils a lack of grain suturing predominantly cement supported fabric and fractures running through calcite cement as well as fossil bone and framework grains indicates that concretions formed during early diagenesis pre compactional or syndepositional calcite stable isotope 18ovpdb values range from 12 25 to 4 indicating mixed marine and meteoric pore waters and 13cvpdb values range from 5 3 to 4 1 indicative of both sulfate reduction and methanogenesis of organic material including decaying vertebrate soft tissues in the burial environment the mixed marine and freshwater signature suggests a marginal marine setting possibly deltaic or estuarine connected to the regressive epicontinental eromanga seaway at around 102 100 ma this is not inconsistent with the lithology from nearby cores coupled with isisford fossil vertebrate ecology personal observation our research demonstrates the utility of investigating ex situ concretions to refine paleoenvironments at localities where little or no outcrop is available and traditional facies analysis is impractical

stable oxygen and carbon isotopes of carbonate concretions of the miocene yeonil group in the pohang basin korea types of concretions and formation condition abstract carbonate concretions in the miocene sedimentary rocks of the yeonil group in the pohang basin korea were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation carbonate concretions are widely distributed in all the sedimentary rocks in the pohang basin showing that the calcitic concretions are preserved within the mass flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks diatomites concretions can be classified into four different types on the basis of the stable isotopic signatures each of which represents its own geochemical range type i concretions are calcitic and are composed of micrite to microspar they occur in the conglomerates and sandstones which were deposited by mass flows debris flow to turbidity current it shows relatively lower 18 o 14 0 to 9 3 and 13 c 19 6 to 8 4 values these concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments type ii concretions are also calcitic composed mostly of micrite with minor microspar and found in the sandstones these concretions are characterized by relatively high 18 o 1 8 to 2 4 and variable 13 c 17 3 to 0 4 values these isotopic signatures reflect that type ii concretions formed from just beneath the sediment water interface down to the sulfate reducing zone through the early stage of methanogenesis type iii concretions are also calcitic and composed largely of micrite with a minor contribution of microspar they are observed in hemipelagic mudrocks which were deposited under the influence of mass flows they are characterized by intermediate to high 18 o 4 6 to 1 6 and high 13 c 1 3 to 8 8 values these concretions grew in a methanogenic zone by residual ambient seawater and or seawater slightly modified by reaction with volcanogenic sediments type iv concretions are dolomite with calcite inclusion and occur in hemipelagic siliceous rocks these concretions are mostly composed of micrite and characterized by variable 18 o 9 1 to 0 7 and high 13 c 3 1 to 17 9 values suggesting formation in the methanogenic zone although the residual ambient seawater is slightly modified by volcanogenic sediments the same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature this means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan delta systems regardless of their localities further the different types of concretions are also found at the different but closely spaced stratigraphic levels in the same locality displaying the distinctive diagenetic conditions for each type such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system therefore caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin

authigenic dolomites in the eocene oligocene organic carbon rich shales from the polish outer carpathians evidence of past gas production and possible gas hydrate formation in the silesian basin abstract eocene oligocene dolomite concretions and beds from the grybow and dukla units of the polish outer carpathians were studied these rocks occur in the organic carbon rich marine and fine grained deposits of hemipelagic or turbiditic origin mineralogic elemental and stable c and o isotopic composition of the dolomites was determined results indicate that the rocks were formed by precipitation of predominantly fe rich dolomite cement close to the sediment water interface prior to significant compaction the main source of bicarbonate for dolomite formation was bacterial methanogenesis as evidenced by the high 13c values up to 16 6 the main source of alkalinity was probably weathering of silicate minerals which might have also liberated ca and mg ions for the dolomites to form the distribution of these dolomites indicates that microbial methane production was widespread in the silesian basin moreover formation of some dolomites in the eastern part of the dukla unit was probably associated with gas hydrates as suggested by the elemental and oxygen isotopic composition of dolomitic matrix therefore the dolomites may serve as a proxy of areas where biogenic methane was produced where the rocks had high hydrocarbon potential and where hydrates could have existed detailed mineralogic and petrographic analyses allowed for the reconstruction of the diagenetic sequence and the evolution of pore fluids textural relationships between successive cement generations indicate that the central parts of the composite dolomite crystals experienced corrosion and that the latest ankerite cement filled the secondary intragranular cavities within those crystals this observation shows that reconstructions of pore fluid evolution based on core to rim analyses of such composite crystals may lead to wrong interpretations septarian cracks developed in the dolomites are often filled with multistage cements the earliest generations are ferroan dolomite and ankerite cements which precipitated within the cracks simultaneously to the ferroan dolomite and ankerite cements from the matrix of the dolomitic rocks which shows that septarian cracking occurred very early during the final stages of concretionary formation these cements were followed by the late diagenetic precipitates mainly quartz kaolinite and blocky calcite this calcite is commonly associated with bitumen which shows that it precipitated during or after oil migration in the decarboxylation zone

septarian carbonate concretions in the permian rio do rasto formation birth growth and implications for the early diagenetic history of southwestern gondwana succession abstract between the late carboniferous and early triassic the southwestern gondwana supercontinent was characterized by the development of a huge intracratonic basin a large confined epeiric sea and the accumulation of a transgressive regressive sequence were formed by continuous subsidence related to tectonic effects caused by the sanrafaelic orogeny and the consequent generation of accommodation space the permian rio do rasto formation documents the last progradational cycle related to the complete continentalization of this epeiric sea the basal member of the rio do rasto formation serrinha is believed to have been deposited in a shallow epicontinental water body subjected to storms and influenced by episodic deltaic incursions one of the most remarkable characteristics of the serrinha member is the presence of carbonate concretions hosted in mudstones and very fine sandstones here we combine sedimentological and petrographic descriptions coupled with geochemical and stable carbon and oxygen isotopic data to elucidate the nature of these carbonate concretions the non deformed internal structure decreasing proportion of carbonate cements relative to detrital grains toward the concretion edges core to rim isotopic variations and perhaps most importantly the preservation of a well developed cardhouse fabric support an early diagenetic origin for these structures at shallow burial depths of tens of meters stable isotope analyses of micritic calcite cements and calcites filling the septarian fractures reveal major negative excursions in both 18 o and 13 c values oxygen isotope ratios obtained for the micritic calcite cements vary between 12 1 and 2 6 the calcite filling septarian fractures also exhibit negative values of 18 o 14 2 to 13 8 with an average of 14 the 13 c values of micritic calcite cements range from 5 0 0 2 the carbon isotopic data from the calcite filling septarian fractures are also negative 4 4 to 3 3 the 18 o signatures suggest that the early diagenetic carbonate concretions precipitated in a shallow freshwater environment rather than in a marine setting the 13 c values suggest that the carbon isotopes were derived from a source with slightly depleted 13 c supporting at least a partial organogenic contribution with weak sulfate reduction rates typical of freshwater systems sedimentological analysis shows that the epicontinental water body in which the serrinha member was deposited was constantly supplied by rivers and meteoric waters which suggests that an enormous freshwater basin with restricted marine connections to the panthalassa ocean once existed

geochemical paleogradient in pore waters controlled by aom recorded in an oligocene laminated limestone from the outer carpathians abstract a 20 cm thick laminated limestone of oligocene age from the outer carpathians is composed mainly of coccoliths and authigenic calcite 13 c values unequivocally indicate that the authigenic calcite precipitated as a result of methane oxidation the 13 c and 18 o values change gradually from the base to the top of the bed from 38 0 to 17 6 and from 1 1 to 5 7 respectively this trend reflects a geochemical gradient which developed in the zone of anaerobic oxidation of methane aom due to the consumption of sulfate by methane this gradient was related to an inverse relationship between the concentrations of both compounds in the pore waters of the aom zone methane was the source of isotopically light carbon it was accompanied by heavy oxygen containing water liberated from decomposition of gas hydrates the methane bearing fluids migrated upwards to the base of the sulfate reduction sr zone where methane was oxidized ch 4 so 4 2 hco 3 hs h 2 o and the aom zone developed the bicarbonate produced in this reaction was significantly depleted in 13 c and 16 o in the sr zone organic matter was oxidized by sulfate 2ch 2 o so 4 2 h 2 s 2hco 3 the bicarbonate released in that reaction was the source of isotopically heavier c and lighter o the proportion of bicarbonate produced by oxidation of methane to bicarbonate produced by oxidation of organic matter decreased upwards the resultant isotopic composition of bicarbonate dissolved in the pore waters changed upwards towards higher 13 c and lower 18 o values and has been recorded in the limestone s isotopic composition

spatial distribution of carbon and oxygen isotopes in laterally extensive carbonate cemented layers implications for mode of growth and subsurface identification abstract carbonate concretions and laterally extensive cemented layers are common features in the prairie canyon member of the mancos shale at its type locality in western colorado stable isotope and petrographic data were obtained from these cemented zones in an attempt to determine 1 how the cemented layers form 2 whether or not they can be chemically correlated and 3 if it is possible to differentiate them from concretions in the subsurface the cement in both the layers and concretions consists of ferroan dolomite and lesser amounts of stoichiometric dolomite and ankerite samples for stable isotope analysis were drilled from 10 grids with a typical grid spacing of 10 cm10 cm gridded samples were obtained from two continuous cemented layers at several locations along the layers 2 3 m to 1 5 km apart on the same layer and from isolated and stratabound concretions taken together all the cements regardless of spatial position show a remarkably linear correlation r2 0 98 between 13c and 18o systematic center to edge decreases in both 13c and 18o occur in each of the cemented zones and are persistent over lateral intervals of at least 1 5 km the cores of the cemented zones have positive 13c values up to 10o oo pdb which result from incorporation of 13c enriched carbon during methanogenesis the edges of the cemented zones have negative 13c values down to 9o oo pdb which suggests incorporation of 13c depleted carbon from thermocatalytic decarboxylation reactions oxygen isotope paleothermometry indicates that precipitation may have started at moderate temperatures 25 c and ended at temperatures as high as 92 c the spatial distribution of the isotopic data in the cemented layers can be explained as resulting from the coalescence of stratabound concretions followed by growth above and below the coalesced concretions such growth may have occurred in a complex manner in which early cements precipitated throughout the thickness of the layer but were more abundant in the core zone given the lateral persistence of the pattern of isotopic variation in the cemented layers geochemical correlation of the layers may be possible in the subsurface in addition because laterally persistent cemented layers in the prairie canyon member exhibit patterns of isotopic variation different from concretions layers have a more asymmetrical vertical distribution it should be possible to distinguish between the two styles of cementation from subsurface core data alone

petrographic examination and re interpretation of concretionary carbonate horizons from the kimmeridge clay dorset sections through two concretionary carbonate horizons yellow ledge and maple ledge from the kimmeridge clay type locality of the dorset coast have been examined using back scattered imaging electron microscopy x ray microanalysis and x ray diffraction the yellow ledge has experienced a complex cementation history involving precipitation of several carbonate phases samples from both the centre and margins of this ledge contain juxtaposed low mg low fe calcite and ferroan dolomite this indicates that the ledge did not simply grow from the centre outwards during progressive burial by contrast the maple ledge was found to be compositionally and texturally homogeneous being cemented throughout by interlocking subhedral ferroan dolomite crystals the observations made in this study indicate that a detailed petrographic examination of concretionary layers must be undertaken if geochemical and isotopic data obtained from the analysis of bulk samples are to be interpreted correctly

determining the diagenetic conditions of concretion formation assessing temperatures and pore waters using clumped isotopes carbonate 18 o paleothermometry is used in many diagenetic studies to unravel the thermal history of basins however this approach generally requires an assumed pore water 18 o 18 o pw value a parameter that is difficult to quantify in past regimes in addition many processes can change the original isotopic composition of pore water which further complicates the assignment of an initial 18 o pw and can lead to erroneous temperature estimates here we use clumped isotope thermometry a proxy based on the 13 c 18 o bond abundance in carbonate minerals to evaluate the temperatures of concretion formation in the miocene monterey formation and the cretaceous holz shale california these temperatures are combined with established carbonate water fractionation factors to calculate the associated 18 o pw results demonstrate that diagenetic processes can modify the 18 o of ancient pore water confounding attempts to estimate diagenetic temperatures using standard approaches clumped isotope based temperature estimates for monterey formation concretions range from 17 to 35 c up to 12 c higher than traditional 18 o carbonate water paleothermometry when 18 o pw values are assumed to equal miocene seawater values calculated 18 o pw values range from 0 3 to 2 5 vsmow higher than coeval miocene seawater likely due to 18 o pw modification accompanying diagenesis of sedimentary siliceous phases clumped isotope temperatures for the holz shale concretions range from 33 to 44 c about 15 to 30 c lower than temperatures derived using the traditional method calculated 18 o pw values range from 5 0 to 2 9 and likely reflect the influx of meteoric fluids we conclude that the use of clumped isotopes both improves the accuracy of temperature reconstructions and provides insight into the evolution of 18 o pw during diagenesis addressing a longstanding conundrum in basin evolution research

calcite cemented concretions in cretaceous sandstone wyoming and utah u s a abstract spheroidal cannon ball calcite cemented concretions some of gigantic size were studied from three cretaceous shelf sandstone units ferron frontier and second frontier the concretions have diameters between 2 cm and 6 m those larger than 40 cm in diameter in the ferron and frontier have septarian structure rare concretions in the second frontier with cone in cone also have septaria the pattern of carbon and oxygen isotopic data and intergranular volume igv data across concretions indicate that concretion growth was complex not all concretions grew progressively from their centers toward their edges in many concretions displacive fascicular optic high mg calcite foc precipitated simultaneously throughout the concretions pervasive growth pattern although the earliest cementation was concentrated in and near their centers remaining pores were subsequently filled by poikilotopic calcite of lower mg content displacive foc cement in the bodies of concretions and in cone in cone structure and the large igv values indicate that these carbonate phases formed at burial depths no greater than tens of meters igv values indicate that spar cement continued to depths of at least 50 m and possibly deeper the association of organic matter and microbial structures or microbial byproducts with displacive foc and cone in cone calcite raises the possibility that biogenic processes were responsible for the apparently rapid growth of the displacive calcite the oxygen isotope composition of pore waters was fairly uniform in the range of 18o of 4 to 6 smow during 80 of concretion growth but became even more depleted in 18o during the last stage of growth and during septarian fracture filling ranging into values as depleted as 14 smow 13c in calcite is negative 4 to 18 v pdb but variable reflecting a range of microbial processes under both oxidizing and reducing conditions earlier calcites fall mostly in the range of 10 to 16 v pdb relatively later calcite in veins and on concretion margins is more enriched in 13c fe and mn concentrations in calcites have a rough positive correlation with higher 13c values the concentration of mg derived from seawater and altered volcanic rock fragments covaries positively with the degree of meteoric influence lower mg contents correlate positively with relatively depleted 18o values 87sr 86sr values are typical of sr derived from coeval seawater or marine skeletal debris growth of concretions 4 to 6 m in diameter apparently in 5 my requires some spatial localization of growth sites by a self organization process and probably a large supply of carbonate shells in the original sediment elongation of larger concretions in the plane of bedding suggests that advective rather than diffusive supply of cement components dominated late in concretion growth history concretions formed close to the tops of parasequence boundaries

spherical and columnar septarian 18o depleted calcite concretions from middle upper permian lacustrine siltstones in northern mozambique evidence for very early diagenesis and multiple fluids calcite septarian concretions from the permian beaufort group in the maniamba graben nw mozambique allow controls on the composition and nature of diagenetic fluids to be investigated the concretions formedinlacustrine siltstones where they occur in spherical 1 to 70 cm in diameter and columnar up to 50 cm long forms within three closely spaced discrete beds totalling 2ae5 min thickness cementation began at an early stage of diagenesis and entrapped non compacted burrows and calcified plant roots the cylindrical concretions overgrew calcified vertical plant roots which experienced shrinkage cracking after entrapment two generations of concretionary body cement and two generations of septarian crack infill are distinguished the early generation in both cases is a low mn mg rich calcite whereas the later generation is a low mg mn rich calcite the change in chemistry is broadly consistent with a time burial related transition from oxic to sub oxic anoxic conditions close to the sediment water interface geochemical features of all types of cement were controlled by the sulphate poor environment and by the absence of bacterial sulphate reduction all types of cement present have d13c ranging between 0 and 15 vienna peedee belemnite v pdb and highly variable and highly depleted d18o down to 14 vienna standard mean ocean water v smow the late generation of cement is most depleted in both 13c and 18o the geochemical and isotopic patterns are best explained by interaction between surface oxic waters pore waters and underground 18odepleted reducing ice meltwaters accumulated in the underlying coal bearing sediments during the permian deglaciation the invariant d13c distribution across core to rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks after cracking the concretions retained a small volume of porosity allowing infiltration of anoxic ba bearing fluids resulting in the formation of barite the results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift bound lacustrine depositional systems

origin of giant calcite cemented concretions temple member qasr el sagha formation eocene faiyum depression egypt abstract the temple member of the qasr el sagha formation contains spheroidal calcite cemented concretions in sandstone ranging from 70 cm to 9 m in diameter although most are 1 to 4 m in diameter most concretions are solitary oblate spheroids but some are prolate coalesced joined laterally or are compound enclose smaller concretions half the concretions have domains with a fairly uniform center to center spacing of 4 6 m but half have domains of random spacing both clusters and segregated concretions or occur in parallel rows of coalesced concretions miocene marine carbonate shells now preserved only inside concretions were the source of calcite cement as further supported by carbon and strontium isotopic signatures of cement 13c 2 to 1o oo pdb 87sr 86sr 0 70765 to 0 70770 concretions formed at shallow burial depths in meteoric pore water most 18o values of calcite are from 4o oo to 2o oo pdb at temperatures probably between 10 and 20 c spheroidal uniformly spaced concretions formed where carbonate shells were uniformly distributed and where nucleation sites were influenced by low carbonate saturation levels and widely spaced sites or at sites established by some self organization process concretions in orthogonal rows in the plane of bedding probably formed where shells were concentrated during deposition randomly spaced concretions formed where shells were unevenly spaced or where nucleation of concretions was rapid some temple concretions reached exceptional size in less than 20 my owing to a combination of conditions large initial aragonite and mg calcite shell concentration widely spaced concretion growth sites locally presence of a stable hydrologic regime over a long period of time and possibly bacterially mediated precipitation of calcite

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

fluid sources and hydrothermal architecture of the sudbury structure constraints from femtosecond la mc icp ms sr isotopic analysis of hydrothermal epidote and calcite abstract this study presents new sr isotopic data on hydrothermal mineral assemblages from different stratigraphic levels within the sudbury impact structure determined using femtosecond la mc icp ms these data provide new insights into the origin of the hydrothermal fluids that have circulated through and around the sudbury igneous complex sic and further define the overall hydrothermal architecture of the structure early blocky miarolitic epidote from the sic has a narrow range of initial 87 sr 86 sr 0 70710 to 0 70742 similar to the sic 0 70648 to 0 70736 that is consistent with the precipitation from orthomagmatic fluids derived from the sic the 87 sr 86 sr values of later replacement miarolitic and vein epidote from different stratigraphic levels of the sic and of replacement and amygdule epidote from the overlying onaping formation are distinctly higher up to 0 71753 than those of the blocky miarolitic epidote these data suggest that the fluids responsible for this late epidote were not derived from the sic and more likely represent basement derived fluids that passed through the sic and onaping formation the sr isotopic composition of syngenetic laminated calcite in the post impact vermillion formation suggests precipitation from proterozoic seawater 87 sr 86 sr 0 70295 whereas some vermilion formation related syngenetic laminated calcite and epigenetic discordant and replacement mineralized calcite with more radiogenic values suggest precipitation from seawater modified through interaction with the vitric and lithic fragments present in the onaping formation the formation of some replacement and amygdule carbonate in the onaping formation also involved mixing between seawater and basement derived fluids of similar chemical and isotopic composition to those responsible for the replacement epidote hosted by the sic and onaping formation the sr isotopic composition of hydrothermal epidote from throughout the sudbury structure indicates a fluid connection between the footwall and hanging wall of the sudbury igneous complex

contrasting diagenetic histories of concretions and host rock lion mountain sandstone cambrian texas white elliptical calcite cemented concretion nuclei up to 2 m long contrast markedly in color composition and diagenetic history from more glauconite rich concretion rinds and from dark green glaucarenite host rocks concretion nuclei are placers of loosely packed trilobite carapaces and minor quartz and glauconite tightly cemented with an average of 58 calcite cement the nuclei are shell lag deposits that were cemented by calcite at the sea floor or after burial of a few meters concretion rinds composed of sub equal amounts of quartz and compactionally deformed glauconite have an average of only 32 cement which consists of minor quartz overgrowths and later pore occluding calcite cement the rinds underwent burial for several million years to tens of million years to depths of several hundred meters before they were cemented the host rock is predominantly glauconite with very minor quartz and calcite cement 87 sr 86 sr ratios of host rock calcite cement are variable 0 7084 to 0 7093 but the lowest value suggests precipitation during the middle ordovician in the absence of significant amounts of carbonate cement the host rock underwent complete dissolution of trilobite carapaces and maximum compaction with total loss of porosity through squashing of glauconite grains maximum burial during this stage was completed by the end of ordovician time carbon and oxygen isotopic values of calcite from 16 samples show no differences between concretion nuclei rinds and host rocks 13 c ranges from 2 2 to 2 9 pdb and probably reflects derivation of carbon from trilobite carapaces and other skeletal grains 18 o ranges from 4 6 to 6 2 pdb and reflects either the original composition of the cement and trilobites or the modification of original grains by meteoric dominated ground water

paleogeographic significance of oriented calcareous concretions in the triassic katberg formation south africa abstract elongated calcareous concretions in sandstones of the fluvial katberg formation beaufort group south africa display a distinct preferred orientation that coincides with the regional paleoslope as deduced from cross stratification and primary current lineations these concretions formed after deposition of the sandstone but before the strata were affected by tectonic movement presumably by groundwater flowing down the regional paleoslope similar sandstone hosted concretions in other non marine successions could potentially yield valuable information on paleo groundwater flow and by inference depositional paleoslopes

physical and chemical growth conditions of ordovician organogenic deep water dolomite concretions implications for the δ18o of early palaeozoic sea water the estimated depth of formation of authigenic dolomite concretions in the middle ordovician cloridorme formation quebec ranges from 350 mbsf 25 17 porosity in the lower ordovician levis formation outward decreasing 13cvpdb values 10 2 0 8 suggest precipitation in the methane generation zone with an increasing contribution of light carbonate derived by advection from thermocatalytic reactions at depth anomalously low 18ovpdb values centre to margin variations of 0 4 to 7 5 give reasonable temperatures for the concretion centres only if the 18o of ordovician sea water was negative 6 and the bottom water was warm 15 c the 3 5 lower values for the concretion margins compared with the centres can be explained if in addition volcanic ash alteration organic matter decomposition and or advection of 18o depleted water lowered the 18o of the pore water further by 2 0 4 0 during the first 25 200 m of burial reasonable growth temperatures for the margins of 17 20 c are compatible with a lowering of the isotopic ratios by 1 to 1 3 as a temperature effect the systematic concentric isotope zonation of the concretions suggests that the well ordered near stoichiometric dolomite is a primary feature and not the result of recrystallization diagenetic dolomite beds of the cloridorme formation appear to have formed by coalescence of concretions as shown by randomly sampled traverses that indicate formation at different subsurface depths growth of the cloridorme dolomites was probably limited by calcium availability at least 50 of which was derived from connate water and the remainder by diffusion from sea water dolomite precipitation was favoured over calcite by very high sedimentation rates the abundance of marine organic matter in the host sediment and a correspondingly thin sulphate reduction zone deep seated concretion growth in the levis formation required either internal sources for the participating ions carbonate dissolution event or porewater advection along faults

on cooling of the earth s core abstract we have constructed a self consistent model for cooling of the earth s core in which the thermal history of the core is computed as a function of the time evolution of the heat flux delivered to the mantle across the core mantle boundary the temperature profile in the convecting core is first assumed to be adiabatic and its evolution in time is calculated with the only constraint that energy be globally conserved when the temperature at the centre drops below the freezing point of the core alloy the inner core starts growing and cools by conduction it is found that it cannot have reached its present size in more than 1 7 billion years if the heat flux delivered to the mantle becomes less than that conducted down the adiabat the temperature profile becomes subadiabatic in a shell at the top of the core through which heat is evacuated by conduction although it is stable against thermal convection this shell is not necessarily stagnant and may be the seat of motions owing to compositional convection

degradative calcification of a modern siliceous sponge from the great bahama bank the bahamas a guide for interpretation of ancient sponge bearing limestones abstract organic colloids are involved in the early calcification of the modern siliceous sponge spheciospongia vesparium lamarck 1815 from the great bahama bank electron microscopy and in situ fluorescence microspectometry studies indicate that colloids attached within or onto a collagen network promote the precipitation of aragonite crystals in these sponges calcification occurs within those portions of the sponge that are buried in the sediment preferentially in regions of agglutinated sediment particles with sponge connective tissue being subjected to necrosis and significant degradation of the extracellular collagen matrix the dismantling of collagen bundles leads to collagen scaffolds which act as a sorbent and have significant adhesive effects for ions and or organic colloids bacteria and other microorganisms support tissue degradation but neither act as a substrate for aragonite precipitation or are present in significant numbers at calcification sites this process of early calcification may explain the origin of fossil calcified siliceous sponges sponge mummies and tuberoids as well as the occurrence of patchy calcified sponge materials thereby calling into question the commonly accepted idea that pelletoidal texture associated with these fossil sponges indicates that bacteria are directly responsible for the calcification

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

eogenetic siderite as an indicator for fluctuations in sedimentation rate in the oligocene boom clay formation belgium abstract horizons with septarian concretions are a salient feature of the marine boom clay formation at most horizons the concretions consist of ferroan calcite with variable amounts of pyrite but at stratigraphic level s60 they also contain siderite s60 is situated at the centre of an intensely bioturbated zone that is underlain by a pyrite rich layer furthermore the enclosing clay is strongly enriched in iron manganese and phosphorous the sedimentological and chemical zoning is indicative for low sedimentation rates which allowed the concentration of iron in the aerobic zone of the sediment concentration of iron was the prerequisite for the formation of the siderite containing concretions the co precipitation with pyrite is an argument for a formation in the sulphate reduction zone and is indicative for a high rate of iron reduction the latter was due to the rapid burial of the iron enriched layer below the redox boundary the abrupt fluctuations in sedimentation rate were a response to the maximum flooding event of the second rupelian third order relative sea level cycle which caused a brief pushback of the detrital sediment wedge to its source areas as this response is logically explained by the general sequence stratigraphic model spec publ soc econ paleontol mineral 42 1988 109 early diagenetic siderite may be widespread at maximum flooding surfaces in rapidly prograding marine mudstones

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

importance of volcanic glass alteration to sediment stabilization offshore japan a minor amount ca 1 wt of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the nankai trough subduction zone throughout the shikoku basin the distribution and nature of the cement were examined via secondary and backscattered electron imaging the amorphous silica occurs as altered material in contact with volcanic glass coating grains including grain contacts and filling pores based on chemical and petrographic evidence the cement is probably sourced from volcanic glass this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay dominated sediment throughout the shikoku basin measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the shikoku basin at sites separated by 500 km the cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration i e central america cascadia and the gulf of alaska

bedding parallel lenticular soft sediment deformation structures a type of seismite in extensional settings abstract soft sediment deformation structures ssdss with bedding parallel lenticular morphology have been occasionally reported and called different names such as loop bedding or pillow bed however there is no understanding of the formative conditions that give rise to them versus other ssdss similar bedding parallel lenticular dolomite bodies tens of centimeters thick with quite variable lengths 30 cm 30 m occur across a large area 3000 km2 exclusively in the st louis member late vasean mississippian of the slade formation in east central kentucky usa field investigations indicate that the lenticular structures along with other associated ssdss represent a persistent deformed horizon that is correlative with and occurs symmetrically around the more intensely deformed big sinking bed which was previously interpreted as earthquake induced ssdss based on multiple lines of evidence the lenticular structures are interpreted to represent distal expressions of the same seismic event that generated the proximal big sinking bed the lenticular structures were subsequently dolomitized and their selective dolomitization is attributable to their brittle deformation during the earthquake which resulted in preferentially developed fracture porosity that allowed more efficient circulation of mg2 fluids throughout the lenticular structures petrographic and stable isotopic evidence also suggests that the selective dolomitization postdates the deformation probably related to a brine reflux system developed during the first marine transgression following subaerial exposure of the st louis carbonates based on comparative analysis with other global cases the bedding parallel lenticular structures are interpreted to represent a distinct type of ssds comparable to boudinage in tectonic deformation their formation reflects a situation in which seismically elevated pore fluid pressure is lower than vertical confining stress but higher than horizontal confining stress a prominent occurrence of lenticular type ssds is a potential indicator of syndepositional extensional stress regime which is testified by tectonic settings of all known cases including the kentucky dolomite bodies

isotope and elemental geochemistry of black shale hosted fossiliferous concretions from the cretaceous santana formation fossil lagerstatte brazil carbonate concretions hosted within organic carbon rich shale sequences represent unique archives of often exceptionally preserved fossil biota besides providing high fidelity preservation their geochemical signatures can provide insight into the physical and chemical processes during early and later stage concretion growth here two fossiliferous carbonate concretions of the late early cretaceous santana formation araripe basin north east brazil are analysed with an integrative geochemical approach including xrf scanning 13c 18o 87sr 86sr and 47 clumped isotope thermometry individual concretions show a concentric internal zonation with the outermost layer being composed of millimetre thick cone in cone calcite a strong covariance of 13c and 18o values of the fine crystalline concretion body indicates mixing of two different carbonate phases and supports a scenario of temporally separated pervasive growth stages microbially mediated formation of an early porous calcite framework was controlled by the combined processes of fermentation and methanogenesis around the decaying carcass forming localized environments within a zone of sulphate reduction microbial sulphate reduction is indicated by the concentric enrichment of pyrite in the outer part of the concretion body and by high pyrite abundance in the surrounding shale information on the later stage diagenetic processes affecting the santana concretions can be derived from the outermost fringing cone in cone calcite the carbonate precipitating fluid was characterized by a more or less marine 18o composition calculated 18oporewater 1 0 to 1 8 and by radiogenic sr isotope ratios up to 0 713331 7 0 10 6 the latter probably reflecting modification due to interaction with the surrounding shale or alternatively with underlying evaporitic sulphate deposits influenced by strong continental inflow or with crystalline basement rocks the 47 derived temperature estimates range between 37 to 42 c 5 indicating precipitation of the cone in cone calcite at a depth of 650 to 850 m which is only half as much as the maximum burial depth derived from existing fission track data overall the study of fossiliferous carbonate concretions in organic carbon rich sedimentary sequences can reveal a complex growth history spanning incipient microbially influenced precipitates as well as later stage burial diagenetic phases this article is protected by copyright all rights reserved

convection in the earth s core driven by lateral variations in the core mantle boundary heat flux moving core fluid maintains an isothermal core mantle boundary cmb so lateral variations in the cmb heat flow result from mantle convection such variations will drive thermal winds even if the top of the core is stably stratified these flows may contribute to the magnetic secular variation and are investigated here using a simple non magnetic numerical model of the core the results depend on the equatorial symmetry of the boundary heat flux variation large scale equatorially symmetric es heat flux variations at the outer surface of a rapidly rotating spherical shell drive deeply penetrating flows that are strongly suppressed in stratified fluid smaller scale es heat flux variations drive flows less dominated by rotation and so less inhibited by stratification equatorially anti symmetric flux variations drive flows an order of magnitude less energetic than those driven by es patterns but due to the nature of the coriolis force are less suppressed by stratification the response of the rotating core fluid to a general cmb heat flow pattern will then depend strongly on the subadiabatic temperature profile imposing a lateral heat flux variation linearly related to a model of seismic tomography in the lowermost mantle drives flow in a density stratified fluid that reproduces some features found in flows inverted from geomagnetic data

significance of stable carbon isotope trends in carbonate concretions formed in association with anaerobic oxidation of methane aom middle and upper devonian shale succession western new york state usa abstract carbonate concretions are concentrated along discrete stratigraphic horizons within middle and upper devonian carbonaceous black and organic deficient gray shale of western new york state textural characteristics preserved throughout studied concretions including a clay grain microfabric typical of flocculated clay and spherical algal cysts are consistent with a model entailing the formation of low density compaction resistant calcium carbonate masses at shallow burial depth within the zone of bacterial sulfate reduction burial of nascent concretions to the sulfate methane transition zone smtz was accompanied by the nearly complete infilling of porosity the diagenetic consequence of the anaerobic oxidation of methane aom each concretionary horizon then defines the position of the smtz stabilized during an episode of reduced sedimentation rate most analyzed concretions 75 of the middle devonian marcellus formation into the upper devonian gowanda formation display center to edge profiles of increasing bulk 13c far fewer concretions are characterized by radial profiles of decreasing 13c the common radial profiles of increasing 13c can be attributed to aom related kinetic carbon isotope fractionation under diagenetic conditions of limited methane which favors the production of dissolved inorganic carbon of increasing 13c less common concretions displaying center to edge profiles of diminishing 13c may record the anaerobic oxidation of more strongly 13c depleted methane produced by carbon dioxide reduction methanogenesis related to the degradation of organic matter within and near the smtz or aom back flux an enzyme mediated equilibrium isotope effect these processes are favored by sulfate limited diagenetic conditions more typical of the lower smtz and immediately underlying upper methanogenic zone mez contradictory isotope profiles of texturally similar concretions can be accounted for by diagenesis associated with an extended smtz esmtz comprising the methane limited smtz and the underlying sulfate deficient upper mez vertical shifts of the diagenetic horizons of the esmtz induced by variable methane flux would have juxtaposed concretions displaying seemingly incompatible isotopic histories