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1.
Calcimicrobialites across the Permian-Triassic boundary in Huayingshan region were investigated using the fluorescence microscopic measurements to understand the occurrence of organic matter. The microbialites are composed of micrite matrix and coarse spar cement. Abundant rhombic or magnetic needle-like carbonate minerals were observed adrift within the cement. The fluorescence microscopic measurement indicates the micrite matrix in microbialites shows the most abundant organic matter, with the rhombic or magnetic needle-like carbonate minerals and coarse spar cement coming to the 2nd and the 3rd, respectively. Organic matter is mainly preserved in the space between the grains of the micrite minerals but almost evenly distributed in the rhombic or magnetic needle-like carbonate minerals. As one of the common diagenesis types, dolomitization is observed to occur in the microbialites in Huayingshan. However, the carbonate cement in microbialites still has high content of element Sr as shown by the microprobe analysis, reflecting that the dolomitization might have happened in a restricted environment. Observation under the fluorescence microscope shows that dolomitization just led to the redistribution of organic matter in the grain space of dolomite minerals, inferring that the diagenesis has a slight effect on the preservation, and thus on the content of organic matter in the microbialites. 相似文献
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微生物岩的原始定义指底栖微生物主导形成的沉积体或岩石体。笔者对该概念进行了扩充,认为微生物岩除了包括叠层石、凝块石、纹层石、核形石、均一石之外,还应该包括微生物骨架岩、微生物粘结岩、非钙化浮游或漂浮微生物形成的模铸岩、矿化浮游或漂浮微生物形成的颗粒岩和泥粒岩。P-T界线地层微生物岩的特征是具有由较粗矿物晶体(主要是方解石,其次是白云石)组成的斑点状、树枝状、网状结构的灰岩。这3种结构分别称为斑点状体、树枝状体、网状体,在露头上呈暗色,在薄片中呈浅色,一般由无定形的亮晶充填体和其间的微亮晶组成。亮晶充填体是指无定形的孔洞被不同成岩期形成的矿物充填形成的结构体,因成岩作用各异,造成不同层位、不同地点的亮晶充填体内部的矿物类型和充填顺序存在差异;所有的亮晶充填体都不具有壁,故不是钙化化石。通过形态、大小和生态比较,以及形成演化分析,认为亮晶充填体的前身是漂浮蓝细菌微囊菌,胶鞘是微囊菌形成模铸化石的关键因素。亮晶充填体是表层水漂浮生活的微囊菌沉入海底后,被泥晶沉积物掩埋或者被早期海底胶结物胶结,在泥晶沉积物半固结或固结之后腐烂留下的孔洞被后期成岩作用形成的矿物充填形成的。P-T界线地层微生物岩段顶部遭受成岩作用程度高,树枝状体和网状体中的亮晶充填体的轮廓基本都被破坏,变成微亮晶和亮晶,以前被学者解释为凝块石;但斑点状体、树枝状体、网状体是成岩流体沿着亮晶充填体或其他化石丰富的地方运移形成较粗的晶体而造成的,并不符合凝块石的定义。同时,少数学者把树枝状体本身当成底栖生物,也是没有充分认识树枝状体的矿物组成而做的解释。该微生物岩段含有钙化的小球状化石和同心层状化石,但它们不是组成微生物岩的主体。 相似文献
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Diagenesis in shallow cores from the Lower Cretaceous Edwards Limestone was investigated in thin sections and with the scanning electron microscope (SEM). The SEM is a particularly useful tool in the study of diagenesis in porous fine-grained carbonate rocks because of its good resolution and depth of field.The Edwards Group was deposited in shallow-marine environments and underwent normal early diagenesis. Dolomite and evaporite minerals such as gypsum formed penecontemporaneously in some tidal-flat sediments. Slightly later, when the carbonate sediments were flushed by fresh water, carbonate mud recrystallized to micrite and aragonite allochems altered to calcite or were leached. Some cementation by calcite occurred in a fresh-water phreatic environment.The Edwards Limestone was divided into two zones by Miocene faulting along the Balcones Fault Zone. On the upthrown side of the fault a circulating fresh-water aquifer developed, whereas relatively stagnant brackish water remained present on the down-thrown side. Differences in the chemistry of the interstitial fluids in these zones resulted in different types of diagenesis. The presence of fresh water caused extensive oxidation, solution along fractures, recrystallization of micrite to coarse microspar and pseudospar, precipitation of equant sparry-calcite crystals in a variety of shapes and sizes, and extensive dedolomitization. The dedolomitization is thought to have been caused by the high Ca/Mg ratio of the circulating fresh water in a shallow subsurface environment.In the brackish-water zone, textures and fabrics related to deposition or early diagenesis, such as primary porosity, unoxidized organic material, framboidal pyrite, and evaporite minerals have been preserved. Some precipitation of authigenic dolomite, celestite, and kaolinite occurred in the brackish-water zone. In contrast to the fresh-water zone, precipitation of coarse calcite spar, dedolomitization, and recrystallization of micrite to microspar occurred only rarely in the brackish-water zone. 相似文献
4.
四川盆地东北部地区长兴组碳酸盐岩成岩作用及储层孔隙演化 总被引：4，自引：0，他引：4
以四川盆地东北部长兴组碳酸盐岩为研究对象,应用薄片、铸体、阴极发光和X射线衍射等室内分析技术,对其成岩作用、成岩演化序列、储层孔隙演化及其特征进行了详细研究。研究区常见的成岩作用有：压实压溶作用、胶结作用、充填作用、交代作用、白云化作用、重结晶作用、溶蚀作用、破裂作用等成岩作用。其中对储层孔隙发育影响较大的主要是胶结作... 相似文献
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微生物岩储层是当前的研究热点之一.通过对塔中北坡微生物岩样品的岩芯以及铸体薄片的观察,发现研究区域的微生物岩储层以裂缝-孔隙型储层为主.对其进行全直径孔渗性检测发现,微生物岩储层具有较好的天然气储集物性.通过对微生物岩的形成机制及特点的分析,结合应力敏感实验和成像测井资料,总结出微生物岩储集空间发育的优势因素.原生白云石的生成可以促进方解石的溶解,而塔中北坡微生物岩发生早成岩岩溶作用,部分地区叠加晚期深成热液改造,提高了储集物性;有机质含量较丰富不仅可以在埋藏成岩阶段生成有机酸造成溶蚀,还能够降低微生物岩的极限强度从而容易在构造作用下发生断裂形成裂缝,因此塔中北坡微生物岩储集空间主要以次生作用为主.通过岩石压缩系数的测定,得出微生物岩的抗压实能力一般,但藻屑骨架对藻孔的保护有利,抗压实能力最强的是受到热液作用而硅化的微生物岩,其次是白云石化微生物岩. 相似文献
6.
Origin of subaerial Holocene calcareous crusts: role of algae, fungi and sparmicritisation 总被引：3，自引：0，他引：3
CHARLES F. KAHLE 《Sedimentology》1977,24(3):413-435
The Pleistocene Miami Limestone that crops out on the lower Florida Keys is overlain by thin (16 cm or less), discontinuous, Holocene calcareous crusts (caliche) that are usually laminated, composed dominantly of calcite micrite and may or may not incorporate part of the underlying limestone. Both allochems and sparry calcite cement in the former unit contain endolithic algae and fungi, borings and unicellular algae. Biogenic structures identical to those in the Miami Limestone also occur in the calcareous crusts but are somewhat less abundant in the latter unit versus the former unit. The calcareous crusts were formed in the vadose diagenetic environment. Some of the CaCO3 necessary for the micrite that comprises the bulk of the crusts was probably derived from solution of carbonate from a soil cover and some from wind blown salt spray. Most of the micrite, however, was formed by replacement of the uppermost portions of the Miami Limestone. Replacement involved micritisation of allochems and a previously unreported process, sparmicritisation, the degrading recrystallization of sparry calcite to micrite. Minor sparmicritisation was caused by micrite calcification of endolithic fungi or algae within sparry calcite cement or by micrite precipitation in empty borings within such cement. Most sparmicritisation took place by dissolution of sparry calcite and concomitant precipitation of micrite in the space occupied previously by the dissolved spar. Such sparmicritisation is interpreted to be caused by chemical reactions involving the crystals, pore water which is moving slowly but steadily and organic compounds released during bacterial decomposition of fungi, algae or both. It is recognized that sparmicritisation occurs in the marine diagenetic environment and is not, therefore, necessarily indicative of vadose diagenesis. Incomplete sparmicritisation is responsible for some of the clotted textures typically found within calcareous crusts and may explain such textures in many other carbonate rock types. A combination of sparmicritisation and micritisation has probably greatly influenced the porosity of many reefs and, in some cases, led to the formation of ‘micritic reefs’. 相似文献
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8.
综合利用铸体薄片、X射线衍射、阴极发光、扫描电镜、物性分析及镜质体反射率和包裹体温度等资料,结合岩心观察法,对Bongor盆地下白垩统碎屑岩储层基本特征及成岩作用进行了研究。依据胶结物的体积分数及形态、黏土矿物组分的分布、岩石结构镜下特征及孔隙演化、有机质热演化等证据划分了岩石成岩阶段,并分析了成岩作用对物性的影响。结果显示：该区储层岩石类型以长石砂岩为主,胶结物以碳酸盐和黏土矿物为主;溶孔-粒间孔组合型储集性最好,粒间孔-溶孔组合型次之;成岩作用是控制储层物性的主要因素,成岩作用主要包括压实作用、胶结作用、交代作用和溶蚀作用;压实作用和胶结作用是破坏性成岩作用,二者分别使原始孔隙度损失54.93%和31.30%,岩石颗粒中长石、碳酸盐胶结物等易溶物质发育,溶蚀作用是主要建设性成岩作用;目的层成岩阶段以中成岩A期为主,成岩作用弱-中等。 相似文献
9.
Physico-chemical environment of pedogenic carbonate formation in Devonian vertic palaeosols, central Appalachians, USA 总被引：1，自引：0，他引：1
The morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red-brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non-luminescent to dull luminescent, whereas later spar cements exhibit bright yellow-orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow-orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn2+ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe2+ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 13C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO2. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO2; the use of nodule compositions may result in overestimation of PCO2 by as much as 30%. 相似文献
10.
Microbialites (benthic microbial carbonate deposits) were discovered in a hypersaline alkaline lake on Eleuthera Island (Bahamas). From the edge towards the centre of the lake, four main zones of precipitation could be distinguished: (1) millimetre‐sized clumps of Mg‐calcite on a thin microbial mat; (2) thicker and continuous carbonate crusts with columnar morphologies; (3) isolated patches of carbonate crust separated by a dark non‐calcified gelatinous mat; and (4) a dark microbial mat without precipitation. In thin section, the precipitate displayed a micropeloidal structure characterized by micritic micropeloids (strong autofluorescence) surrounded by microspar and spar cement (no fluorescence). Observations using scanning electron microscopy (SEM) equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities. These extracellular polymeric substances (EPS) are progressively replaced with high‐Mg calcite. Discontinuous EPS calcification generates a micropeloidal structure of the micrite, possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria. At high magnification, the microstructure of the initial precipitate consists of 200–500 nm spheres. No precipitation is observed in or on the sheaths of cyanobacteria, and only a negligible amount of precipitation is directly associated with the well‐organized and active filamentous cyanobacteria (in deeper layers of the mat), indicating that carbonate precipitation is not associated with CO2 uptake during photosynthesis. Instead, the precipitation occurs at the uppermost layer of the mat, which is composed of EPS, empty filamentous bacteria and coccoids (Gloeocapsa spp.). Two‐dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat. In combination, these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for CaCO3 precipitation and dissolution. Theoretically, partial degradation of EPS by aerobic heterotrophs or UV fuels sulphate‐reducing activity, which increases alkalinity in microdomains, inducing CaCO3 precipitation. This degradation could also be responsible for EPS decarboxylation, which eliminates Ca2+‐binding capacity of the EPS and releases Ca2+ ions that were originally bound by carboxyl groups. At the end of these processes, the EPS biofilm is calcified and exhibits a micritic micropeloidal structure. The EPS‐free precipitate subsequently serves as a substrate for physico‐chemical precipitation of spar cement from the alkaline water of the lake. The micropeloidal structure has an intimate mixture of micrite and microspar comparable to microstructures of some fossil microbialites. 相似文献
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山东东营牛庄洼陷沙三中亚段浊积砂体储层特征及影响因素 总被引：15，自引：3，他引：15
山东东营牛庄洼陷沙三中亚段浊积砂体储层以长石细砂岩、岩屑细砂岩及粉砂岩为主 ,结构成熟度及成分成熟度均较低。储层微观特征研究表明 ,砂体中发育了剩余原生粒间孔、粘土矿物杂基内的微孔隙、粒间溶孔、粒内溶孔及微裂缝等储集空间类型 ,孔隙结构以低渗细喉型为主 ,为物性较差的中低孔、低渗型储层。通过对储层物性影响因素的研究得出 ,岩石组构、沉积微相、成岩作用和异常超压的形成与分布是影响本区浊积砂体储层物性的主要因素。其中 ,碳酸盐胶结物、硅质胶结物及自生粘土矿物是致使储层物性变差的主要原因 ,沉积物中有机质的大量生烃以及粘土矿物在转化过程中所释放出的酸性水的溶解作用是产生次生孔隙的重要因素 ,而异常超压的存在、高的地温场及烃类的早期进入是部分原生孔隙和次生孔隙得以保存的重要条件。 相似文献
13.
JOHN D. HUMPHREY 《Sedimentology》1988,35(2):327-348
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
14.
黄骅坳陷三马地区中-深部储层成岩作用及主控因素分析 总被引：12，自引：2，他引：12
黄骅坳陷三马地区储集岩样品的显微特征及成岩矿物的组合关系分析结果表明，三马地区下第三系中深层储层主要处于晚成岩A期，成岩现象包括石英、长石的次生加大、碳酸盐矿物胶结作用、矿物交代作用、溶解作用和粘土矿物转化作用等。其中石英次生加大于2700m左右开始出现，随埋深增大，石英增生加强；长石次生加大现象不十分普遍；碳酸盐胶结作用包括早期自生泥晶方解石胶结作用、连晶方解石和晚期白云石(或铁方解石)的胶结作用。在扫描电镜下可见到方解石与含铁方解石集合体充填在粒间孔隙中或与石英、钠长石及粘土矿物共生，白云岩多呈菱形粒状分布于粒间；常见的交代现象主要有方解石交代长石、石英及粘土矿物，粘土矿物交代长石、石英等；溶解作用主要有长石的溶解、碳酸盐颗粒的溶解和方解石胶结物的溶解等，粘土矿物的转化主要为蒙脱石、高岭石向伊利石(或绿泥石)转化。还讨论了储层岩相、岩性特征、孔隙水中有机酸含量、早期方解石的充填、烃类早期注入等因素对储层成岩演化的控制作用。 相似文献
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Microbial primary dolomite from a Norian carbonate platform: northern Calabria, southern Italy 总被引：1，自引：1，他引：1
ADELAIDE MASTANDREA EDOARDO PERRI FRANCO RUSSO ALESSANDRA SPADAFORA MAURICE TUCKER† 《Sedimentology》2006,53(3):465-480
The origin of fine‐grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High‐relief thrombolites and stromatolites characterize sub‐tidal facies, and low‐relief and planar stromatolites, with local oncoids, typify the inter‐supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub‐tidal facies. Some 75% of the succession consists of fabric‐preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end‐members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5–60 μm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter‐coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic‐rich micritic laminae and clots of the inter‐supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron‐sized coccoid bacteria and spheroidal–ovoidal nanometre‐scale dwarf‐type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive δ18O values. The occurrence of fossilized bacteria and organic matter in the fabric‐preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn‐sedimentary dolomite. 相似文献
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四川东北部罗家寨气田的储层为下三叠统飞仙关组孔隙性鲕粒白云岩。其沉积相属碳酸盐台地的鲕粒滩微相，为台地潮下较高能环境沉积，主要受潮汐作用的控制。原生岩石为亮晶鲕粒石灰岩和泥晶鲕粒石灰岩，白云石化作用使其岩石结构大多遭受破坏。对储层起建设性作用的成岩作用主要有白云石化作用、埋藏溶解作用。白云石化作用是形成优质储层的基础。埋藏溶解作用主要在粒间半充填剩余缝隙、交代白云石的晶间孔内进行，这进一步溶蚀造就了大量超大孔隙、粒间溶孔、粒内孔、铸模孔等。对储层起到破坏作用的主要是充填作用，表现为次生溶蚀孔隙、裂缝、溶洞中充填自生矿物，它们的包裹体测温数据证明充填作用发生在埋藏时期。各种自生矿物充填的不均匀性揭示了岩石与孔隙水之间相互作用的复杂性，以及岩溶水在不同时期是变化的。 相似文献
17.
以四川盆地川中高石梯-磨溪地区钻井岩芯和水样资料为主,盆地周边地面剖面露头资料为辅,采用微量元素、阴极发光、包裹体、同位素等分析手段进行岩石学和地球化学分析,在此基础上讨论了灯影组碳酸盐岩的成岩作用变化和成岩作用过程中孔隙流体的演化。研究结果表明:灯影组碳酸盐岩沉积物在沉积作用和成岩作用过程中广泛地受到孔隙流体和成岩作用变化的影响,发生过海水的胶结作用、早期大气水成岩作用（溶解作用和沉淀作用）、表生喀斯特化作用、构造控制的热液白云石化和密西西比谷型矿化作用、碳氢产生时的溶解作用、油热裂解时焦沥青的充填孔洞作用等,其中喀斯特化和热液白云石化对储层的发育起着重要的作用。然而,灯影组孔隙流体广泛地被白云石化作用和喀斯特化作用的修饰。现今地层水资料分析表明这些成岩作用明显地影响蒸发海水的地层水流体。地层水分析资料在Ca-Br及Mg-Br关系图上显示出蒸发海水的特征,Ca的浓度比蒸发海水分布曲线要高,Mg的浓度比蒸发海水曲线要低,为灯影组沉积物在成岩作用过程中发生过白云石化作用的结果。 相似文献
18.
Marine cements in mid-Tertiary cool-water shelf limestones of New Zealand and southern Australia 总被引：1，自引：0，他引：1
Large areas of southern Australia and New Zealand are covered by mid‐Tertiary limestones formed in cool‐water, shelf environments. The generally destructive character of sea‐floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates, yet these limestones include occasional units with marine cements: (1) within rare in situ biomounds; (2) within some stacked, cross‐bedded sand bodies; (3) at the top of metre‐scale, subtidal, carbonate cycles; and (4) most commonly, associated with certain unconformities. The marine cements are dominated by isopachous rinds of fibrous to bladed spar, interstitial homogeneous micrite and interstitial micropeloidal micrite, often precipitated sequentially in that order. Internal sedimentation of microbioclastic micrite may occur at any stage. The paradox of marine‐cemented limestone units in an overall destructive cool‐water diagenetic regime may be explained by the precipitation of cement as intermediate Mg‐calcite from marine waters undersaturated with respect to aragonite. In some of the marine‐cemented limestones, aragonite biomoulds may include marine cement/sediment internally, suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences. We suggest that marine cementation occurred preferentially, but not exclusively, during periods of relatively lowered sea level, probably glacio‐eustatically driven in the mid‐Tertiary. At times of reduced sea level, there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters, and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites, which now experienced increased swell wave and/or tidal energy levels, fostering sediment abrasion and reworking, reduced sedimentation rates and freer exchange of sediment pore‐waters. Energy levels were probably also enhanced by increased upwelling of cold, deep waters onto the Southern Ocean margins of the Australasian carbonate platforms, where water‐mass mixing, warming and loss of CO2 locally maintained critical levels of carbonate saturation for sea‐floor cement precipitation and promoted the phosphate‐glauconite mineralization associated with some of the marine‐cemented limestone units. 相似文献
19.
Secondary carbonate formations, such as travertine and calcareous tufa deposits, are important archives for quaternary continental climate studies and archaeology. The extremely complex growth mechanisms result in some serious problems for precise mass spectrometric uranium-series dating. Often, detrital and organic particles contaminate the carbonate and large pore volumes yield a great potential for open system behavior. We utilized microscopic, mineralogical and geochemical methods prior to sample selection to determine the abundance of primary calcite, i.e. micrite and spar. Furthermore, the state of alteration was characterized by cathodoluminescence and trace-element analysis. We conclude that travertine and calcareous tufa are appropriate for precise U-series age determination if a) micrite and/or spar are the dominant phases; b) cathodoluminescence of both phases is weak or absent; c) Fe and Al levels are low; and d) Sr concentrations are close to the average of the studied site. We mapped and sampled solely areas of major micrite/spar abundance having minor alteration for accurate U-series dating. When this new method was applied, travertines located in eastern Germany (sites Bad Langensalza, Burgtonna and Weimar-Ehringsdorf) gave single 230Th/238U-ages consistent with the lithological growth sequence and greatly improved compared to previously published chronologies. In addition, we determined 230Th/U isochron ages on bulk samples that confirm our single ages. In contrast to primary calcite, pore cements are homogeneously distributed throughout the travertine fabric and reflect early diagenetic processes and/or weathering. 相似文献
20.
本文根据岩矿特征、沉积构造、生物组合等标志将巴楚-柯坪地区上石炭统碳酸盐岩沉积环境自西往东划分为:广海盆地相、台地边缘浅滩相、开阔台地相及滨岸浅滩相。并采用阴极发光,扫描电镜、X射线衍射分析、TAS-plus图像分析仪及测定镜质体反射率等分析手段,对该地区碳酸盐岩的压实、压溶、胶结、溶解及白云化作用进行了详细研究,推断了该岩层经历的最高成岩温度和所处的成岩阶段,并对其孔隙演化特点进行了分析,指出了次生孔隙发育的有利地带。 相似文献