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1.
Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones 总被引：1，自引：0，他引：1
The Coinstone is a well known hiatus-concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay-hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea-floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack-lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV-fluorescent and inclusion-rich, and is similar to the first calcite generations seen in many other clay-hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post-exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today. The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized. 相似文献
2.
贵州紫云县猴场镇扁平村的上石炭统叶状藻礁及其周边灰岩中广泛发育大量的各类胶结物。通过对胶结物的形态、结构和阴极发光特征以及胶结物间的接触关系的研究,可以确定成岩作用的先后并识别成岩环境。浅海海底同生成岩阶段大的孔隙中形成等厚环边针状胶结物、葡萄状胶结物,小的孔隙里形成微晶胶结物。早成岩阶段形成微亮晶和斑块状亮晶方解石胶结物和放射纤维扇状胶结物,表生成岩阶段的溶蚀作用和胶结作用强烈,胶结物类型有斑块状或等粒的方解石胶结物和等厚环壁柱状胶结物,等厚环壁柱状胶结物在所有胶结物中体积是较大的。早期胶结作用使叶状藻礁灰岩孔隙度大为降低。中、晚成岩阶段,孔隙被等厚环壁刃状胶结物和晶簇或斑块状亮晶方解石所充填,有些先成的胶结物被热液改造。后生作用阶段发生的主要是构造破裂作用,其中少数裂隙被红褐色含Fe2O3微晶层和晶体粉砂及渗流豆粒充填。叶状藻礁灰岩的孔隙在晚成岩阶段前或中被胶结而之后没有创造出大且连通的孔隙,是它没能成为油气储集层的原因之一。 相似文献
3.
ABSTRACT
Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite-rich zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased. Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite microspar crystals were enlarged.
The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about 550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200 and 500 m during the second burial.
Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of which is marked by the pyrite-enriched zone. Dissolving shells, and possibly minor methanogenic carbonate, slowly diluted sulphate reduction-zone carbonate during deeper burial. During early concretion growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg decreased as calcite precipitated, implying relatively closed systems for these elements.
Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge Clay became overpressured during the later stages of both burials. 相似文献
Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite-rich zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased. Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite microspar crystals were enlarged.
The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about 550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200 and 500 m during the second burial.
Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of which is marked by the pyrite-enriched zone. Dissolving shells, and possibly minor methanogenic carbonate, slowly diluted sulphate reduction-zone carbonate during deeper burial. During early concretion growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg decreased as calcite precipitated, implying relatively closed systems for these elements.
Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge Clay became overpressured during the later stages of both burials. 相似文献
4.
Thomas Mann Andr Wizemann Marleen Stuhr Yannis Kappelmann Alexander Janßen Jamaluddin Jompa Hildegard Westphal 《Sedimentology》2022,69(1):282-300
Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively, this study shows that early-diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories. 相似文献
5.
VICTOR A. MELEZHIK ANTHONY E. FALLICK† RICHARD A. SMITH‡ DANTA M. ROSSE§ 《Sedimentology》2007,54(6):1389-1416
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 formed in lacustrine 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 2·5 m in 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 δ13C ranging between 0‰ and −15‰(Vienna Peedee Belemnite, V‐PDB), and highly variable and highly depleted δ18O (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, 18O‐depleted, reducing, ice‐meltwaters accumulated in the underlying coal‐bearing sediments during the Permian deglaciation. The invariant δ13C 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. 相似文献
6.
The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England 总被引：1，自引：0，他引：1
I. C. SCOTCHMAN 《Sedimentology》1991,38(1):79-106
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. 相似文献
7.
Sparry calcite fracture fills and concretion body cements in concretions from the Flodigarry Shale Member of the Staffin Shale Formation, Isle of Skye, Scotland, entrap and preserve mineral and organic materials of sedimentary and diagenetic origin. Fatty acids are a major component of the lipids recovered by decarbonation and comprise mainly n-alkanoic and α-ω dicarboxylic acids. Two generations of fracture-fill calcite (early brown and later yellow) and the concretion body microspar yield significantly different fatty acid profiles. Early brown calcites yield mainly medium-chain n-alkanoic acids with strong even predominance; later yellow calcites are dominated by α-ω dicarboxylic acids with no even predominance. Both fracture fills lack the long-chain n-alkanoic and α-ω dicarboxylic acids additionally recovered from the concretion bodies. The absence of longer chain acids in the calcite spar fracture fills is inferred to result from the transport of fatty acids by septarian mineralising fluids whereby low-aqueous solubility of longer chain acids or their salts accounts for their relative immobility.Comparative experiments have been carried out using conventional solvent extraction on the concretion body and associated shales, both decarbonated and untreated. Extracted lipid yields are higher, but the fatty acids probably derive from mixed locations in the rock including both kerogen- and carbonate-associated lipid pools. Only experiments involving decarbonation yielded α-ω dicarboxylic acids in molecular distributions probably controlled mainly by fluid transport. Alkane biomarker ratios indicate very low thermal maturity has been experienced by the concretions and their host sediments. Septarian cracks lined by brown calcite formed during early burial. Microbial CO2 from sulphate-reducing bacteria was probably the main source of mineralising carbonate. Emplacement of the later septarian fills probably involved at least one episode of fluid invasion. 相似文献
8.
9.
不同成因的钙质结核在研究沉积水体、早期成岩环境及泥页岩的压实程度方面具有重要意义。通过对川北地区旺苍县下寒武统筇竹寺组钙质结核抛光,发现结核内部具有层理、暗色圈层和张裂缝构造。结核和围岩的层理关系和具有漂浮状的微观结构特征表明,该地区结核是形成于压实作用之前的成岩早期结核。对结核中Al、Ti、Ca、K、Na元素分析表明,结核的元素分布受到结核内部裂缝的影响。结核内部富含生物碎屑和其形成深度为数十米的特征,推测结核主要是在硫酸盐还原带生长。结核具有暗色圈层之间层理不弯曲和元素移动的特征,揭示了结核为透入性生长,在整个结核生长阶段就开始大量同时结晶。Mg/Ca和Sr/Ca的比值说明,结核内部钙质成分为富镁方解石-生物成因方解石。结核中富含钙质成分的原因可能是围岩中的生物成因方解石大量向结核运移所致。裂缝的形态特征表明,裂缝为张裂缝。对结核上部地层的沉积特征研究表明,张裂缝的成因很可能与筇竹寺顶部的浊积岩和沧浪铺组底部的似瘤状灰岩快速搬运沉积有关。 相似文献
10.
Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis 总被引：3，自引：0，他引：3
JAMES P. HENDRY MICHAEL J. PEARSON† NIGEL H. TREWIN‡ ANTHONY E. FALLICK§ 《Sedimentology》2006,53(3):537-565
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
11.
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
12.
Injection clastic dykes in the Lower Oxford Clay (Jurassic) of central England: relationship to compaction and concretion formation 总被引：1，自引：0，他引：1
An injection dyke of fine-grained sandstone derived from the Kellaways Sand Formation intrudes overlying organic-rich shales and shell beds of the Lower Oxford Clay. The dyke shows cross-cutting relationships with early carbonate concretions, and fills uncompacted kosmoceratid ammonite shells both within the concretions and surrounding shales. Internally the dyke displays flow-like features, and the walls show lobate flow structures. Clasts of uncompacted Lower Oxford Clay and fragments of pyrite-rich concretions occur within the sandstone intrusions. The sandstone of the dyke was cemented by calcite identical to that precipitated in septarian cracks in the concretions. This cementation took place prior to final compaction of the Oxford Clay. The dyke has a sub-parallel relationship to the nearby Tinwell-Marholm fault suggesting that the dyke may be related to local tectonic events during the Middle Jurassic. 相似文献
13.
岩心观察鄂西黄陵背斜南翼下寒武统牛蹄塘组二段黑色页岩中发育构造挤压成因的高角度裂缝和顺层裂缝.选取不同产状的典型裂缝脉体样品,薄片镜下观察发现高角度裂缝中充填方解石脉,顺层裂缝中充填方解石-白云石复合脉.通过阴极发光、流体包裹体分析以及微区原位元素测定,开展了裂缝脉体成岩环境演化研究,分析了不同产状裂缝对页岩气层局部封闭性的影响.阴极发光结果表明,高角度方解石脉和顺层方解石-白云石复合脉均为2期流体活动形成.根据流体包裹体分析,高角度方解石脉早期成脉流体主要为变质较深的地层卤水,晚期成脉流体混入了低矿化度流体,而顺层方解石-白云石复合脉2期成脉流体均未受低矿化度流体的改造.利用不同阴极发光的裂缝脉体碳酸盐矿物的微区原位微量元素测定结果,对铁锰含量、稀土元素（REE）和氧化还原判别参数分析显示,高角度方解石脉成岩环境经历了由还原环境向氧化环境转化,而顺层方解石-白云石复合脉的成岩环境长期保持了较为还原的环境.因此认为,高角度裂缝对页岩气层的局部封闭条件可能产生破坏作用,从而造成页岩气一定程度的散失,而顺层裂缝对局部封闭条件的破坏作用相对有限,裂缝发育特征及其经历的成岩演化过程研究可为页岩气层自封闭和页岩气微观保存条件的评价提供依据. 相似文献
14.
Concretions, isotopes, and the diagenetic history of the Oxford Clay (Jurassic) of central England 总被引：1，自引：0，他引：1
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
15.
Petrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO3, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low- to intermediate-Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO3 content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light-brown turbid fibrous calcite has low MgCO3 (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ13C and δ18O values of all fibrous calcite form a tight field (δ13C=1·7 to 3·1‰ PDB, δ18O= ? 2·6 to ? 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ18O values are larger than adjacent meteoric or burial cements, which have δ18O δ ? 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First-generation equant and bladed calcite cements, which pre-date fibrous calcite, are interpreted as unaltered, low-Mg calcite marine cements based on δ13C and δ18O data (δ13C = 2·3 to 2·7‰ PDB, δ18O= ? 2·8 to ? 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ18O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ18O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed. 相似文献
16.
R. Raiswell S. H. Bottrell S. P. Dean J. D. Marshall† A. Carr‡ & D. Hatfield 《Sedimentology》2002,49(2):237-254
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
17.
MARK W. HOUNSLOW 《Sedimentology》1997,44(6):1133-1147
The burial-stress and hydrologic conditions existing during concretion formation in mudrocks are evaluated and integrated into a model for the genesis of septarian cracks. Initial concretion cement formation will lower concretion permeability through the filling of pre-existing pore space. During progressive burial, this may lead to increased excess pore pressure, localized within the concretion body causing a reduction of the effective stress. Analysis of the stress conditions and crack morphology suggests that cracks in septarian concretions result from tensional failure (sub-critical crack growth), as a consequence of this localized excess pore pressure. Conditions suitable for crack formation will depend upon the magnitude of the excess pore pressure and the stress corrosion limit of the concretion body. A review of the likely strength of such concretions indicates that cracking could be initiated at depths less than 10 m. A variety of observed crack morphologies can be explained with this model, depending upon the spatial distribution of strength and effective stress in the concretion. Crack orientations mostly reflect stress anisotropy, but are also influenced by directional anisotropy in the crack growth rates. Locally increased pore pressure also likely occurs in non-septarian concretions, but is not sufficient to cause cracking. This enhanced local pressure may assist the crystal surface growth reactions of the carbonate cement. Through this enhancement process, the shape of concretions may be a response to the local anisotropic pore-pressure contours, which reflect the permeability anisotropy of the concretion and surrounding mudrock. 相似文献
18.
Siderite (FeCO3) is a widespread minor diagenetic mineral in clastic sedimentary basins. Although eodiagenetic authigenesis of siderite is well-known, siderite formed during burial diagenesis shows habits and chemical compositions that are poorly understood. This study tests the hypothesis that diagenetic siderite cements in sandstones in the Scotian Basin, offshore eastern Canada, show systematic variability in chemistry and habit that is a response to recrystallization and changing composition of basinal fluids. Mineral textures were determined from backscattered electron images, and chemistry mostly from electron microprobe analyses. Five chemical types of siderite are identified using k-means cluster analysis, based on the amount of substitution of Ca, Mg and Mn for Fe. Eodiagenetic microcrystalline coated grains, concretions and intraclasts in sandstones are principally Fe-rich siderite and locally have recrystallised to blocky equant crystals. Mesodiagenetic Mg-rich siderite partly replaced these equant crystals and also framework mica and K-feldspar grains, showing textural evidence for coupled dissolution–reprecipitation. Slender Mg-rich siderite rhombs (lozenges, bladed or wheat-seed siderite) have precipitated before and after the formation of quartz overgrowths in geochemical microenvironments. Magnesium substitution reflects Mg-rich formation waters resulting from smectite to illite conversion. Equivalent Ca-rich siderite occurs where sandstones overlie a Jurassic carbonate bank. Late Mn-rich siderite has complex textures resembling those of Mississippi-Valley type ores, with spheroidal rims, a honeycomb-like mesh and concentric infill around secondary pores. It also occurs in veins or replacing intraclasts, post-dating late ferroan-calcite cements in sandstones that show strong dissolution by hot basinal brines. The Ca, Mg and Mn content of diagenetic siderite, coupled with textural evidence for recrystallization, can thus be used to track changes in ambient formation fluids. Siderite habits and chemistry described from the Scotian Basin are found in many clastic basins, suggesting that the observed recrystallization textures and variation in chemical type are of broad application. 相似文献
19.
The septarian carbonate concretions from the Boom Clay (Belgium) consist mainly of authigenic minerals such as micrite ( 70% bulk volume) and pyrite framboids ( 3%). These mineral phases occur between detrital grains and fossils. The septarian cracks are lined with calcite, which is sometimes covered with pyrite. The preservation of delicate sedimentological features in the concretion matrix (hardly compacted faecal pellets, burrows and uncrushed shells) points to an early origin of the concretions. Systematic geochemical variations from concretion centre to edge suggest that growth continued during shallow burial. The13C values (–17.5 to –20.5) of the concretionary carbonate show that bacterial sulphate-reduction processes were dominant. Sulphate-reduction-derived HCO3- was diluted by marine-related HCO3-, derived from dissolved bioclasts. A slight enrichment in 13C during growth is caused by the decreasing influence of sulphate reduction because of the progressive closure of the diagenetic system due to shallow-burial compaction. The 18O values (–0.5 to +1.0) of the concretionary carbonate point to a marine origin. The slightly 18O-depleted signature with respect to time-equivalent marine-derived carbonate relates to the incorporation of an 18O-depleted component, originating from sulphate and organic matter. The slight decrease in 18O during growth relates to an increasing influence of this component and to a decreasing influence of seawater-derived oxygen during early diagenesis. 相似文献
20.
塔河油田中下奥陶统碳酸盐岩储层在构造活动的作用下发育多期方解石脉体,为研究储层流体演化和油气成藏过程提供了重要素材.通过岩心观察、薄片鉴定、阴极发光和微区元素分析在塔河油田中下奥陶统碳酸盐岩储层中识别出3期裂缝方解石脉体,3期脉体在阴极发光颜色、稀土元素、碳氧锶同位素以及形成时间方面都存在差异.采用激光原位方解石U-Pb定年技术确定3期方解石脉形成的绝对年龄分别为466±11 Ma~458.6±8.8 Ma、329.7±1.6 Ma~308.1±4.1 Ma和249.3±2.6 Ma~220.5±7.3 Ma.第2期和第3期方解石成脉流体主要来源于深部地层成岩流体,并伴有成烃流体的影响.第3期方解石脉的成脉流体可能还有壳源锶的输入,导致其87Sr/86Sr比值高于第2期方解石脉.方解石脉体的绝对年龄可以用于约束多旋回叠合盆地断裂活动和油气成藏时间.在塔河油田中下奥陶统碳酸盐岩储层中识别的3期方解石脉体形成时间分别对应于加里东中期Ⅰ幕、海西中期Ⅰ~Ⅱ幕和海西晚—印支期构造运动造成的断裂活动时间.方解石脉体与烃类流体包裹体的共生关系指示中下奥陶统储层在海西中期Ⅱ幕发生过一期原油充注,在印支期之后又存在两期原油充注. 相似文献