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1.
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 δ13C=?28·7‰ PDB and δ18O=?1·6‰ PDB through to δ13C=?6·9‰ PDB and δ18O=?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 (δ13C=?12·0 to ?10·1‰ PDB and δ18O=?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 (δ34S=+24·8‰ CDT and δ18O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ34S=+6 to +11‰ CDT and δ18O=+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.  相似文献   

2.
Porewaters from a variety of Recent, Pleistocene, and Eocene lithified marine carbonate frameworks displayed similar chemical characteristics: highly depleted concentrations of dissolved oxygen (>20 μM), elevated levels of dissolved methane (25-5000 nM), and near-seawater sulphate levels. These porewaters also had low pH values (7·5-7·9), and contained elevated concentrations of sulphide (4–10 μM), dissolved inorganic carbon (2·05–2·46 mM), and inorganic nutrients. Hydrocarbon composition data indicate that the methane is biogenic, whereas the methane δ13C values (–47·4 ± 2·7%0) suggest that it has been subject to oxidation. The porewater dissolved inorganic carbon δ13C values varied from –0·6 to –39%0, suggesting input of carbon dioxide from organic matter oxidation. We conclude that anaerobic diagenesis involving bacterial degradation of organic matter is a common process in lithified marine carbonates and hypothesize that it may be an important factor controlling their carbonate geochemistry.  相似文献   

3.
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 δ13C 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 δ13C 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.  相似文献   

4.
Early diagenesis of the Upper Cretaceous (late Coniacian to early Santonian) Marshybank Formation was controlled by depositional environment (composition of depositional water, Fe and organic content of the sediment, sedimentation rate, proximity to the shoreline) and influx of meteoric water related to relative sea-level fall. Five depositional environments, each characterized by a distinct early diagenetic mineral assemblage, have been recognized. Offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions, composed of ‘early’ calcite and siderite. Intense sulphate reduction, promoted by the dysaerobic depositional water, was the primary influence on early diagenesis. Offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions, composed of two generations of siderite (S1 and S2). In this environment, methanogenesis, rather than sulphate reduction, was more important. Early diagenesis of the inner shelf sands was generally limited. However, in sands deposited proximal to the shoreline, mixing of marine and meteoric waters promoted crystallization of Fe-rich chlorite and siderite. The shoreface was characterized by dissolution of detrital minerals in the upper portion, and precipitation of kaolinite or illite/smectite in the lower portion. In the coastal plain environment, brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite. Ankerite, rather than siderite, is the typical early diagenetic carbonate. The δ18O values of the earliest cements (i.e. ‘early’ calcite, siderite S1, inner shelf siderite) indicate crystallization from a low-18O, marine-derived porewater. Assuming crystallization at 25°C, a δ18O value of about ?7‰ (SMOW) can be estimated for the seaway during Marshybank Formation time. Similar calculations for the overlying Dowling Member (Puskwaskau Formation) suggest that the δ18O value of the seaway increased to about ?4% (SMOW), consistent with its transgressive nature. Very low δ18O values are exhibited by siderite S2. These results indicate crystallization during intermediate diagenesis (≥60°C) from meteoric water (≥? 15‰ SMOW) that entered the Marshybank Formation during sea-level lowstand.  相似文献   

5.
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 δ13C 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 δ13C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ13C 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 (δ34S) and silica (δ18O) 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.  相似文献   

6.
Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km2 in size. The δ18OPDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ13CPDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The 87Sr/86Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca2+) ions with relatively minor amounts of Ca2+ derived from the chemical weathering of bedrock.  相似文献   

7.
The estimated depth of formation of authigenic dolomite concretions in the Middle Ordovician Cloridorme Formation, Quebec, ranges from < 1 m to 150–200 m below sea floor (mbsf) (mostly between < 1 and 25 mbsf), based on centre‐to‐margin variations in minus‐cement porosity (80–90% to 45–75%). Formation depths are > 350 mbsf (25–17% porosity) in the Lower Ordovician Levis Formation. Outward‐decreasing δ13CVPDB 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 δ18OVPDB values (centre‐to‐margin variations of ?0·4 to ?7·5‰) give reasonable temperatures for the concretion centres only if the δ18O 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 δ18O 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.  相似文献   

8.
Early concretionary and non-concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85–97 mol% FeCO3, and are commonly enriched in manganese. δ13C (PDB) values are highly variable, ranging from - 18·4 to +2·9‰, whereas δ18O (PDB) values are very consistent, ranging from - 14·0 to - 10·2‰ (mean= - 11·9 ± 1·0‰). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non-marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low δ13C values demonstrate that, contrary to current views, highly 13C-depleted siderite can be produced at shallow burial depths in anoxic non-marine sediments.  相似文献   

9.
The surface sediments of two mud mounds (“Mound 11” and “Mound 12”) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δ18Ocarbonate values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δ13Ccarbonate values from ?52.2 to ?14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The δ18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (δ18Oporefluid = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δ18Oporefluid ≈5 ‰) in Mound 11. A positive correlation between δ13Ccarbonate and δ18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δ13Cporefluid ≈0 ‰) and (2) bicarbonate which formed during the AOM (δ13Cporefluid ≈?70 ‰). Furthermore, the δ18Oporefluid composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δ13CCH4 = ?38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).  相似文献   

10.
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 δ18O 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 δ13C 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.  相似文献   

11.
MUCHEZ  NIELSEN  SINTUBIN  & LAGROU 《Sedimentology》1998,45(5):845-854
Two calcite cements, filling karst cavities and replacing Lower Carboniferous limestones at the Variscan Front Thrust, were precipitated after mid-Jurassic Cimmerian uplift and subsequent erosion but before late Cretaceous strike-slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclusions along growth zones. The calcite lattice contains < 0·07 mol.% Fe, but Mn concentrations can be as high as 0·72 mol.% in bright yellow luminescent zones. Primary, originally one-phase, all-liquid, aqueous inclusions have a final melting temperature between ?0·2° and +0·2 °C, indicating a meteoric origin of the ambient water. The δ13C and δ18O values of the calcites are between ?7·3‰ and ?6·3‰, ?7·8‰ and ?5·5‰ on the Vienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stage B) consists of ferroan (0·13–0·84 mol.% Fe) blocky crystals with Mn concentrations between 0·34 and 0·87 mol.%. Primary, single-phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 °C . The δ13C values of stage B calcites vary between ?7·3‰ and ?2·1‰ VPDB and the δ18O values between ?7·9‰ and ?7·2‰ VPDB. A precipitation temperature below 50 °C for the stage A calcites and the presence of iron oxide/hydroxide inclusions in the crystals indicate near-surface precipitation conditions. Within this setting, the geochemistry of the nonferroan stage A calcites reflects precipitation under oxic to suboxic conditions. The ferroan stage B calcites precipitated in a reducing environment. The evolution from the stage A to stage B calcites and the associated geochemical changes are interpreted to be related to the change from semiarid to humid conditions in western Europe during late Jurassic–Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher δ13C values of the stage B compared to the stage A calcites can be explained by a smaller contribution of carbon derived from soil-zone processes than from carbonate dissolution in the groundwater under humid conditions. The small shift to lower δ18O between stage A and B calcites may be caused by a higher precipitation temperature or a decrease in the δ18O value of the meteoric water. This decrease could have been caused by a change in the source of the air masses or by an increase in the amount of rainfall during the early mid-Cretaceous. Although the latter interpretation is preferred, it cannot be proven.  相似文献   

12.
Detailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ13C, δ18O, δ34S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite-rich and dolomite-rich carbonates. C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ18O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for 18O and 1·5% for 13C in calcite. The δ34S values of pyrite are isotopically light (mean value = - 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins. The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in the Zechstein Sea.  相似文献   

13.
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. δ13CPDB 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 (δ13CPDB = ?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 δ34SCD 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 δ13C and δ34S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ18O values of the calcite in one concretion (δ18OPDB = 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 (δ18OPDB?8.9 to ?9.9%.) and large δ18O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.  相似文献   

14.
Geochemical evidence of microbial activity within ooids   总被引：1，自引：0，他引：1       下载免费PDF全文
Ooid formation remains elusive despite their importance as palaeoclimatic indicators and important contributors to global carbonate budget. Based on stable isotopes, nutrient and elemental analyses on solid components and ooidal leachates, this study supports the notion of microbial involvement in the development of ooids from Great Bahama Bank. Carbon and nitrogen isotopic analyses on organic fractions identified geochemical signatures of microbial activity. The δ13C values for organic carbon in the bulk (?11·9 to ?16·9‰); intercrystalline/intracrystalline (?11·9 to 16·7‰); and intracrystalline phases (?12·4 to ?17·7‰) were similar and, except for the more enriched values of ooids from Butterfly Beach, were within the range of photosynthesisers. The δ15N values for the bulk (+0·5 to ?0·2‰); intercrystalline/intracrystalline (?0·3‰ to ?0·7‰) and intracrystalline organic matter (?0·3 to ?1·7‰) showed a narrow range consistent with nitrogen fixation. While positive δ15N and δ18O values of the leached from the ooids provided evidence of denitrification, the carbonate associated sulphate δ34SCAS of the bulk sediments (+19·2 to +19·6‰) and δ34S of the leachates (+16·6 to +18·3‰) provided weak indication of sulphate reduction, suggesting either that high concentrations of isotopically enriched S are overriding bio‐signatures of sulphate reduction or that microbes are preferentially using as an electron acceptor. In contrast, the elevated sulphate concentrations of the leachates suggest the occurrence of microbial sulphide oxidation within ooids. The high Mg/Ca of the leachates and scanning electron microscope analyses provide putative evidence of amorphous calcium carbonate and a formative role in CaCO3 precipitation. Together, these findings indicate that a redox dependent microbial consortium may influence CaCO3 precipitation in the form of ooid accretion, cementation and micritization. It is also inferred that ooid deposits are not suitable indicators of palaeoclimate because ooids are affected throughout their life by a complex chain of abiotic and biological processes which can lead to large geochemical offsets.  相似文献   

15.
Holocene meteoric dolomitization of Pleistocene limestones, North Jamaica   总被引：3，自引：0，他引：3  
Wholesale removal of the unstable carbonate phases aragonite and Mg-calcite, and precipitation of calcite and dolomite is currently taking place where phreatic waters (the modern water table) invade 120,000-year-old Pleistocene biolithites (Falmouth Formation), North Jamaica. Pleistocene rocks presently in the vadose zone are relatively unaltered, and consist of mineralogically unstable scleractinian biolithites. At the water table, a narrow zone of solution, a ‘water table cave’ is commonly encountered. Below the water table the rocks are invariably more highly altered than those above. Mg-calcites are very rare, and considerable dissolution of aragonite has commonly occurred. Dolomite occurs as 8–25 μm, subhedral to euhedral crystals replacing micrite, or precipitated as void linings. The isotopic composition of the dolomite (δO18=-1·0 %0, δC13=-8·4 %0), and its high strontium content (3000 p.p.m.) suggest precipitation as CO2-oversaturated meteoric groundwaters invade the mineralogically unstable biolithites, dissolve Mg-calcites and Sr-rich aragonites, and de-gas. Because some dolomitized rocks are enriched in magnesium relative to unaltered biolithites, addition of magnesium to the system is necessitated, and is probably derived from sea water in the mixing zone. Phreatic meteoric diagenesis is thus demonstrated to be a rapid process, and to be capable of dolomitization.  相似文献   

16.
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 δ13C and δ18O 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 δ13C 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 δ13C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ18O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.  相似文献   

17.
Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite‐like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (δ13C) and oxygen (δ18O) in the carbonate and organic carbon (δ13Corg) were determined and compared with δ18O of water and δ13C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC0]?1) vs. (δ13C‐DIC × ([DIC]/[DIC0])) ( Sayles & Curry, 1988 ). The δ13C value of added DIC was ?13·6‰, corresponding to the dissolution of CO2 with δ13C between ?19·5 and ?23·0‰ Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing δ13C and decreasing δ18O. The lack of covariation between δ13C and δ18O indicates that precipitation of calcite was not kinetically controlled for either of the elements. δ13C and δ18O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation.  相似文献   

18.
Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ18O values from ?13·7 to ?9·7‰ VPDB, δ13C values from ?2·7 to + 1·5‰ VPDB and 87Sr/86Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (Th) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ18O values (?14·2 to ?9·3‰ VPDB), lower δ13C values (?3·0 to ?0·7‰ VPDB), higher 87Sr/86Sr ratios (≈ 0·7100) and higher Th values (171–209 °C), which correspond to trapping temperatures (Tr) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in 87Sr/86Sr and δ18O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ18O values (?18·5 to ?14·3‰ VPDB), variable δ13C values (?11·3 to ?1·2‰ VPDB) and high Th values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).  相似文献   

19.
J. D. HUDSON 《Sedimentology》1978,25(3):339-370
In interpreting the results of a petrographic and isotopic study of concretions, a range of subjects is discussed including the original texture of the Oxford Clay sediment, Jurassic palaeotemperatures, the diagenetic history of pore-waters and the palaeo-hydrology of central England. The concretions are all composed predominantly of calcite. They include precompactional, pyrite-rich concretions that later suffered an eposide of brecciation, and others that only commenced to form after compaction had crushed ammonite shells included in the bituminous clay sediment. Petrographic, chemical, and especially carbon isotope data demonstrate a dominantly organic source for the carbon in the early formed concretions. Oxygen isotopes indicate formation at the same temperatures (13-16°C) at which benthic molluscs were living. Concretion growth in pelleted, anaerobic mud proceeded concurrently with bacterial sulphate reduction and pyrite precipitation. Cracking of the concretions started at this stage: in a few concretions, the cracks were also partially filled with brown calcite. During post-compactional growth, δ13C increased and pyrite content decreased, showing waning organic influence; δ18O decreased. The brecciated concretions were intruded by clay in which baryte crystals grew; finally, most remaining voids were filled with strongly-ferroan calcite of δ18O about—7 PDB and δ13C about O PDB. This must indicate strong depletion of the pore waters in 18O. Mechanisms that might lead to this are reviewed. It is concluded that the sequence of mineralogical and chemical changes is most readily explained if originally marine porewaters, first modified by bacterial activity, were flushed from the compacting clays by water of ultimately meteoric origin. This had its source in palaeo-aquifers beneath the Oxford Clay. Speculative attempts are made to relate this history to the geology of the region.  相似文献   

20.
Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca-Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish-marine sediments whose pore waters abound in Ca2+ and Mg2+ originating in seawater and calcareous shells. Carbonates formed during Stage II (500–2000 m) include high Ca-Mg siderite, ankerite, Fe dolomite and Fe–Mg calcite in freshwater sediments. The assemblage of Stage II carbonates in brackish-marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage III (> 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.  相似文献   

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