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1.
The Wilde Kirche reef complex (Early-Late Rhaetian) grew as an isolated carbonate structure within the shallow Kössen Basin. At the Triassic/Jurassic boundary a single brief (c. 10–50 ka) period of subaerial exposure occurred. The preserved karst profile (70 m thick) displays a vadose zone, enhanced dissolution at a possible palaeo-watertable (5–15 m below the exposure surface), and a freshwater phreatic zone. Karst porosity was predominantly biomouldic. Primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone; cavity networks developed preferentially in patch reef facies. Resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and Fe-Mn crust deposition on a sediment-starved passive margin. Fibrous calcite (FC). radiaxial fibrous calcite (RFC) and fascicular optic calcite (FOC) cements preserved as low Mg calcite (LMC) are abundant in primary and karst dissolution cavities. FC cement is restricted to primary porosity, particularly as a synsedimentary cement at the windward reef margin. FC, RFC and FOC contain microdolomite inclusions and show patchy non-/bright cathodoluminescence. δ18O values of non-luminescent portions (interpreted as near original) are − 1.16 to − 1.82%0 (close to the inferred δ18O of calcite precipitated from Late Triassic sea water). δ13C values are constant (+3 to + 2.2%0). These observations suggest FC, RFC and FOC were originally marine high Mg calcite (HMC) precipitates, and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression. The HMC to LMC transition may have occurred in contact with meteoric water only in the case of FC cement. The most altered (brightly luminescent) portions of RFC/FOC cements yield δ18O=−2.44 to − 5.8%0, suggesting HMC to LMC alteration at up to 34°C. in the shallow burial environment at depths of 180–250 m. Abundant equant cements with δ18O =−4·1 to −7.1%0 show crisp, uniform or zoned dull luminescence. They are interpreted as unaltered cements precipitated at 33–36°C at 200–290 m burial depth, from marine-derived fluids under a slightly enhanced geothermal gradient. Fluids carrying the equant cements may have induced the HMC to LMC transition in the fibrous cements.  相似文献   

2.
Giant calcite-cemented concretions, Dakota Formation, central Kansas, USA   总被引：1，自引：0，他引：1  
Giant spheroidal concretions (cannonball concretions; some nearly 6 m in diameter) in fluvial channel‐fill sandstones at two localities of the Dakota Sandstone formed by import of cement constituents at a burial depth of <1 km. During cannonball concretion growth a self‐organizational process restricted concretions to a relatively few but widely spaced, and locally, evenly spaced, sites. Other forms of calcite cements at these localities are cement patches in the form of intergrown grape‐size concretions (grapestone), and, locally, pervasive cement. An early episode of invasion by thermogenically generated H2S, which reacted with iron oxides on detrital grains, generated scattered pyrite crystals and decimetre‐scale spheroidal pyrite concretions. Intergranular volumes (IGV) in the concretions range from 36% to 27%. The absence of a trend in IGV and of carbon and oxygen‐isotope ratios from cannonball centres to margins indicates that these concretions did not cement progressively outwards from the centre. Rather, the modern spheres represent the spatial extent of nucleation sites that were not otherwise organized within that volume. Carbon and oxygen‐isotope values for concretion calcites plot along a swath between depleted values of δ18C of ?36‰ and δ18O of ?13‰ and enriched values of ?4‰ and ?6‰, respectively. Four groups of calcites are evident on the basis of trace‐element content and suggest that the calcite precipitated across a range of oxidation conditions that do not correlate strongly with the isotopic compositions. Although fluvial overbank sandstones have some pedogenic calcite, the channel sandstones have at most a trace of pedogenic calcite and carbonate rock fragments, so that the bulk of cement components were imported to the sandstones. Carbon and calcium sources for calcite cement include marine limestone, carbonate shells, and anhydrite in addition to HCO derived from oxidized methane, most likely derived from beds underlying or laterally in communication with Dakota sandstones. HCO in ascending formation waters, released during compaction, mixed with meteoric water whose temperature and composition varied with time, to generate the 7‰ range in δ18Ocalcite values measured.  相似文献   

3.
The Gordon Group carbonates consist of biota of the Chlorozoan assemblage, diverse non‐skeletal grains and abundant micrite and dolomite, similar to those of modern warm water carbonates. Cathodoluminescence studies indicate marine, meteoric and some burial cements. Dolomites replacing burrows, mudcracks and micrite formed during early diagenesis.

δ18O values (‐5 to ‐7%ō PDB) of the non‐luminescent fauna and marine cement are lighter than those of modern counterparts but are similar to those existing within low latitudes during the Ordovician because of the light δ18O values of Ordovician seawater (‐3 to ‐5%o SMOW). The δ18O difference (2%o) between marine and meteoric calcite indicates that Ordovician meteoric water was similar to that in modern subtropics. Values of δ13C relative to δ18O indicate that during the Early Ordovician there were higher atmospheric CO2 levels than at present but during the Middle and Late Ordovician they became comparable with the present because of a change from ‘Greenhouse’ to glacial conditions. δ18O values of Late Ordovician seawater were heavier than in the Middle Ordovician mainly because of glaciation.

Dolomitization took place in marine to mixed‐marine waters while the original calcium carbonate was undergoing marine to meteoric diagenesis.  相似文献   

4.
ABSTRACT Field, geochemical, and petrographic data for late Pleistocene dolomites from southeastern Barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters. The dolomite is localized in packstones and wackestones from the algalAmphistegina fore-reef calcarenite facies. Stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization. Carbon isotopes in pure dolomite phases average about -15%0 PDB. This light carbon is attributed to the influence of soil gas CO2, and precludes substantial mixing with seawater. A narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint. Dolomitization likely occurred with as little as a five per cent admixture of seawater. Strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy. The dolomite is characterized by low sodium values. Low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization. A sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix. Limpid, euhedral dolomite cements precipitated in primary intra- and interparticle porosity subsequent to replacement dolomitization. As waters became progressively less saline, dolomite cements alternated with thin bands of syntaxial calcite cement. The final diagenetic phase precipitated was a blocky calcite spar cement, representing diagenesis in a fresh-water lens. This sequence of diagenetic features arose as the result of a single fall in eustatic sea-level following deposition. A stratigraphic-eustatic-diagenetic model constrains both the timing and rate of dolomitization in southeastern Barbados. Dolomitization initiated as sea-level began to fall immediately following the oxygen isotope stage 7–3 high stand, some 216 000 yr bp . Due to the rapidity of late Pleistocene glacio-eustasy, dolomitization (locally complete) is constrained to have occurred within about 5000 yr.  相似文献   

5.
A peculiar facies of the Norian–Rhaetian Dachstein‐type platform carbonates, which contains large amounts of blackened bioclasts and dissolutional cavities filled by cements and internal sediments, occurs in the Zlatibor Mountains, Serbia. Microfacies investigations revealed that the blackened bioclasts are predominantly Solenoporaceae, with a finely crystalline, originally aragonite skeleton of fine cellular structure. Blackening of other bioclasts also occurs subordinately. Solenoporacean‐dominated reefs, developed behind the platform margin patch‐reef tract, were the main source of sand‐sized detritus. The blackened and other non‐blackened bioclasts are incorporated in automicrite cement. Radiaxial fibrous calcite cements in the dissolutional cavities are also black, dark grey or white. Reworked black pebbles were reported from many occurrences of peritidal deposits; in those cases, the blackening took place under pedogenic, meteoric diagenetic conditions. In contrast, in the inner platform deposits of the Ilid?a Limestone, the blackening of bioclasts occurred in a marine–meteoric mixing‐zone, as indicated by petrographic features and geochemical data of the skeleton‐replacing calcite crystals. Attributes of mixing‐zone pore waters were controlled by mixing corrosion, different solubility of carbonate minerals and microbial decomposition of organic matter. In the moderate‐energy inner platform environment, large amounts of microbial organic tissue were accumulated and subsequently decomposed, triggering selective blackening in the course of early, shallow burial diagenesis. The δ18O and δ13C values of the mixing‐zone precipitates and replacive calcite do not produce a linear mixing trend. Variation mainly resulted from microbial decomposition of organic matter that occurred under mixing‐zone conditions. The paragenetic sequence implies cyclic diagenetic conditions that were determined by marine, meteoric and mixing‐zone pore fluids. The diagenetic cycles were controlled by sea‐level fluctuations of moderate amplitude under a semi‐arid to semi‐humid climate.  相似文献   

6.
Meteoric sphaerosiderite lines (MSLs), defined by invariant δ18O and variable δ13C values, are obtained from ancient wetland palaeosol sphaerosiderites (millimetre‐scale FeCO3 nodules), and are a stable isotope proxy record of terrestrial meteoric isotopic compositions. The palaeoclimatic utility of sphaerosiderite has been well tested; however, diagenetically altered horizons that do not yield simple MSLs have been encountered. Well‐preserved sphaerosiderites typically exhibit smooth exteriors, spherulitic crystalline microstructures and relatively pure (> 95 mol% FeCO3) compositions. Diagenetically altered sphaerosiderites typically exhibit corroded margins, replacement textures and increased crystal lattice substitution of Ca2+, Mg2+ and Mn2+ for Fe2+. Examples of diagenetically altered Cretaceous sphaerosiderite‐bearing palaeosols from the Dakota Formation (Kansas), the Swan River Formation (Saskatchewan) and the Success S2 Formation (Saskatchewan) were examined in this study to determine the extent to which original, early diagenetic δ18O and δ13C values are preserved. All three units contain poikilotopic calcite cements with significantly different δ18O and δ13C values from the co‐occurring sphaerosiderites. The complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses. The Dakota and Swan River samples ultimately yield distinct MSLs for the sphaerosiderites, and MCLs (meteoric calcite lines) for the calcite cements. The Success S2 sample yields a covariant δ18O vs. δ13C trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters. The calcite cements in the Success S2 Formation yield meteoric δ18O and δ13C values. A stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end‐member compositions. Modelled hyperbolic fluid mixing curves for the Success S2 Formation suggest precipitation from fluids that were < 25% sea water.  相似文献   

7.
The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ13C and δ18O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO2 at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ13C and δ18O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ18O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ18O and highly variable δ13C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.  相似文献   

8.
通过对塔中地区上奥陶统数百块碳酸盐岩薄片和抛光面中各种方解石胶结物特征和成因的观察,并结合微量元素和碳、氧同位素分析,共识别出11种胶结物类型。详细描述了各胶结物的特征,并确定了胶结物的分布和形成次序。在此基础上对该区碳酸盐岩方解石胶结物形成环境进行了分析,共划分出海底成岩环境、大气水成岩环境和埋藏成岩环境。从而为该区孔隙演化史的建立和有利勘探目标预测提供了有力的依据。  相似文献   

9.
This paper addresses the diagenesis of carbonate conglomerates in that it assesses the potential of conglomerates in refining the paragenetic history in complex structural areas, such as the Albanian foreland fold‐and‐thrust belt. Of major interest are stylolites (burial and tectonic) which are restricted to conglomerate fragments or which crosscut the conglomerate matrix. Based on the inferred age of stylolite development in relation to burial, uplift and tectonic history, and the Lower to Middle Miocene age of the conglomerates, the succession of diagenetic events was subdivided into several stages. The Poçem polymict transgressive carbonate conglomerate (Kremenara anticline, central Albania) was deposited in a shallow marine environment. These conglomerates are covered by intertidal rhodolithic packstones–grainstones. The stable‐isotope signature of these packstones–grainstones (δ18OV‐PDB = −1·0 to +0·7‰; δ13C = +1·0 to +1·4‰) plots is within the range of marine Early and Middle Miocene values. Shortly after deposition of the conglomerates, micritization, geopetal infill and acicular calcite cementation took place. A first calcite vein generation is interpreted as having formed from a Messinian brine during shallow burial. Burial stylolites developed during further burial in the Pliocene. These stylolites serve as an important diagenetic time marker. The post‐burial stylolite meteoric calcite vein cement probably precipitated during the following telogenetic stage. Karstification and calcite concretion precipitiation pre‐date overturning of the western limb of the anticline. Reopening of subvertical fractures and tectonic stylolites in the western limb of the Kremenara anticline, followed by oil migration, represents one of the latest diagenetic events. These fractures and stylolites provide major pathways for hydrocarbon production.  相似文献   

10.
Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca2+ cations to negatively charged EPS, together with CO2-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca2+ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.  相似文献   

11.
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   

12.
通过岩心和薄片观察,利用荧光显微镜、阴极发光显微镜、同位素质谱仪、冷热台等设备,对鄂尔多斯盆地长7油层组强钙质胶结砂岩及其附近含油砂岩开展研究。结果表明：钙质胶结是致密砂岩储层含油非均质性的主要因素,胶结期次主要为一期;簇同位素揭示该期钙质胶结物的形成温度为18~42℃,对应地质时代为中晚三叠世—中侏罗世,为早成岩期产物,推测与盆地早期小规模构造运动相关;相邻的含油砂岩中油气包裹体伴生的同期盐水包裹体的均一化温度为90~120℃,结合盆地模拟揭示油气主要为一期充注,充注期为100~130 Ma,处于早白垩世;长7油层组不含油致密砂岩内钙质胶结物形成时间早于含油砂岩内石油的充注时间。  相似文献   

13.
Secular variations in stable carbon‐isotope values of marine carbonates are used widely to correlate successions that lack high‐resolution index fossils. Various environmental processes, however, commonly may affect and alter the primary marine carbon‐isotope signal in shallow epicratonic basins. This study focuses on the marine carbon‐isotope record from the carbonate–evaporite succession of the upper Katian (Upper Ordovician) Red River Formation of the shallow epicratonic Williston Basin, USA. It documents the carbon‐isotope signal between the two major Ordovician positive shifts in δ13C, the early Katian Guttenberg and the Hirnantian excursions. Eight δ13C stages are identified based on positive excursions, shifts from positive to negative values and relatively uniform δ13Ccarb values. A correlation between carbon‐isotope trends and the relative sea‐level changes based on gross facies stacking patterns shows no clear relation. Based on the available biostratigraphy and δ13C trends, the studied Williston Basin curves are tied to the isotope curves from the North American Midcontinent, Québec (Anticosti Island) and Estonia, which confirm the Late Katian age (Aphelognathus divergens Conodont Zone) of the upper Red River Formation. The differences in the δ13C overall trend and absolute values, coupled with the petrographic and cathodoluminescence evidence, suggest that the carbon‐isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated Williston Basin, and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids.  相似文献   

14.
鄂尔多斯奥陶系碳酸盐岩碳氧同位素特征及其意义*   总被引：1，自引：1，他引：0       下载免费PDF全文
鄂尔多斯古生代海相地层沉积厚度巨大。鄂尔多斯奥陶系碳酸盐岩的碳氧同位素组成受后期成岩作用影响较小,基本保留了原始海洋的同位素组成： δ13C值分布于-7.30‰～2.26‰之间,均值-0.30‰;δ18O值分布于-13.14‰～-1.94‰之间,均值-6.38‰,碳氧同位素组成与全球基本一致。区域上,鄂尔多斯西缘具有相对较高的δ13C值,南缘次之,而东缘最低。纵向上,碳同位素组成逐渐增重,并在中晚奥陶世发生明显的正向偏移,δ13C均值由马家沟组的-0.36‰增加到平凉组的0.15‰,至背锅山组增加至0.68‰。碳同位素的区域分布差异表明鄂尔多斯西缘水体相对较深,南缘次之,东缘相对较浅,由早奥陶世至晚奥陶世水体逐渐加深,碳同位素组成反映的海平面变化趋势与沉积相演化一致。鄂尔多斯西南缘中晚奥陶世碳同位素组成的正向偏移,标志着较高的生产力和有机碳埋藏率,具有重要的石油地质学意义,西南缘的平凉组/乌拉力克组和背锅山组是下古生界最重要和有效的烃源岩层。  相似文献   

15.
The diagenesis of carbonate platform sediments is controlled by the original facies and mineralogy, climate, sea-level changes and burial history; these controls are clearly seen in the diagenesis of the Urgonian platform carbonates of SE France. Early diagenesis in the Urgonian platform included the precipitation of marine cements, dissolution of rudist shells and minor karstification. Diagenetic features produced during this phase were controlled by several falls in relative sea-level during the Barremian to mid-Aptian punctuating platform sedimentation, the original mineralogy of the sediment and the prevailing semi-arid/arid climate in the region at this time. Following a relative sea-level rise and further sedimentation, progressive burial of the platform led to minor compaction, followed by precipitation of coarse, equant, zoned to non-luminescent, calcite cement. This cement was cut by later stylolites, suggesting a relatively shallow-burial origin. Stable isotope (mean values - 7.94%δ18O and 0.36%δ13C) and trace element (mean values of Fe 334 ppm, Mn 92 ppm and Sr 213 ppm) data suggest that these cements precipitated from meteoric fluids at temperatures slightly elevated relative to depositional temperatures. A variable thickness of replacive dolomite which occurs preferentially within the shelf-margin facies of the lower part of the Urgonian post-dates mechanical fracturing and chemical compaction, but pre-dates the main phase of stylolitization. It is probable that the dolomitizing fluid was sourced by the early compaction-driven release of connate fluids held within the underlying muddy units. The burial history of these rocks suggests that calcite cementation and dolomitization took place at relatively shallow burial depths (1–1.5 km). The overall diagenetic history of the Urgonian Limestone Formation is a reflection of the pre-conditioning of the platform limestones by climate, sea level, tectonics and the shallow burial depths experienced by the platform during the later Mesozoic.  相似文献   

16.
基于西沙永兴岛上最新钻孔（SSZK1）取得的55.92 m岩芯的 U?Th定年、矿物、薄片、主微量元素及碳氧稳定同位素等资料,开展了西沙群岛晚第四纪碳酸盐岩沉积相和淡水成岩作用的研究。根据不同的矿物组成特征,可将SSZK1钻孔岩芯分为上、中、下三段: 下 段（33.89~55.92 m,主要为低镁方解石）、中 段（18.39~33.89 m,主要为文石和低镁方解石）、上段（0~18.39 m,主要为文石、高镁方解石和低镁方解石）。由于下段碳酸盐岩几乎全为稳定的低镁方解石组成,碳氧同位素值的严重负偏和小幅度变化,可推断其经历了程度较大的淡水成岩作用。中段和上段还存在不稳定的文石和高镁方解石,碳氧同位素值相对下段正偏和高幅高频变化,推测其淡水成岩作用的程度比下段要小。中段碳氧同位素值高幅高频变化同时也说明该段的矿物纵向变化较复杂。这种矿物组成的复杂变化可能是由于晚第四纪海平面频繁变化,该段被大气水渗流带和潜流带交替占据引起的。主微量元素的变化同时受到淡水成岩作用和沉积环境的影响。在中段、下段中可识别出sq1、sq2、sq3、sq4四个完整的相旋回。Na2O,S,Sr 和碳氧同位素受到的淡水成岩作用而被消耗和负偏,且由于老一期的旋回经历了更长时间的淡水成岩作用,新、老旋回间的 Na2O,S,Sr含量值和碳氧同位素值有明显差异。利用新、老时期形成的旋回间淡水成岩作用剩余Na2O,S,Sr含量和碳氧同位素值的差别可以将新、老两个旋回区分开来。  相似文献   

17.
The precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform‐building process during deposition of the 2560–2520 Ma Campbellrand‐Malmani carbonate platform, South Africa. Aragonite fans and fibrous coatings are common in unrestricted, shallow subtidal to intertidal facies. They are also present in restricted facies, but are absent from deep subtidal facies. Decimetre‐thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies. The proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0% to > 65% depending on the facies. Subtidal facies commonly contain 20–35%in situ precipitated carbonate, demonstrating that Neoarchaean sea water was supersaturated with respect to aragonite, carbonate crystal growth rates were rapid compared with sediment influx rates, and the dynamics of carbonate precipitation were different from those in younger carbonate platforms. The abundance of aragonite pseudomorphs suggests that sea‐water pH was neutral to alkaline, whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans.  相似文献   

18.
扬子区下、中奥陶统大湾组及其同期地层   总被引：7，自引：2，他引：5  
华南扬子区不同地点奥陶纪的岩相、生物相变化显著。下、中奥陶统大湾组及其同期地层的总体走向为南西—北东向,横向上呈带状分布,从近岸到远岸呈渐变过渡。对四川长宁大官山组、贵州桐梓和沿河湄潭组以及湖北宜昌大湾组的比较研究发现,扬子区下、中奥陶统阿仑尼格期(Arenig)的地层按照其离岸的远近,在地层厚度、沉积相以及腕足动物分异度上呈现出规律性的变化。华南早、中奥陶世腕足动物的辐射首先发生在处于正常浅海底域环境的桐梓和沿河一带。  相似文献   

19.
The Albian-Danian limestones of Cauvery Basin show a wide range of d13C and d18O values (–13.2 to +1.1% and –9.0 to –2.5%, respectively). The cement samples show negative carbon and oxygen isotope values (–18.9 to –3.9% and –9.0 to –4.3%, respectively). The petrographic study reveals the presence of algae, molluscs, bryozoans, foraminifers and ostracods as major framework constituents. The limestones have microspar and equant sparry calcite cements. The pore spaces and vugs are filled with sparry calcite cement. The bivariate plot of d13C and d18O suggests that most of the samples fall in the freshwater limestone and meteoric field, while few samples fall in the marine limestone and soil calcite fields. The presence of sparry calcite cement, together with negative carbon and oxygen isotope values, indicates that these limestones have undergone meteoric diagenesis.  相似文献   

20.
Molar tooth (MT) structures are enigmatic, contorted millimetre‐ to decimetre‐long veins and spheroids of microcrystalline calcite that formed during very early diagenesis in Precambrian sediments. MT structures in the ca 2·6 Ga Monteville Formation are 600–800 Myr older than previously reported occurrences and establish that conditions necessary for MT genesis were met locally throughout much of the Precambrian. In the Monteville Formation, MT structures were formed shallow subtidally, extending to depths near storm wave base, in shale host sediments intercalated with storm‐generated carbonate sand lenses. They are filled with microcrystalline calcite and rare pyrite. Microcrystalline calcite identical to that in MT structures fills other pore space, including porosity between grains in carbonate sand lenses, moldic porosity in sand grains, sheet cracks in columnar stromatolites, and shallow cracks on sandy bedding planes. Relationships in the Monteville Formation demonstrate that microcrystalline CaCO3 precipitated in fluid‐filled cracks and pores; microcrystalline calcite characteristics, as well as the paucity of carbonate mud in host rocks, are inconsistent with injection of lime mud as the origin of MT structures. Locally, MT cracks were filled by detrital sediment before or during precipitation. Precipitation occurred in stages, and MT CaCO3 evolved from granular cores to a rigid mass of cores with overgrowths – allowing both plastic and brittle deformation of MT structures, as well as reworking of eroded MT structures as rigid clasts and lime mud. Crystal size distributions and morphology suggest that cores precipitated through nucleation, Ostwald ripening and size‐dependent crystal growth, whereas overgrowths formed during size‐independent crystal growth.  相似文献   

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