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1.
通过对红花生物礁露头的精细解剖和微相分析,研究了礁的内部构成和成礁模式。红花生物礁发育3期礁体旋回：礁A、礁B和礁C。礁A由生屑泥晶灰岩和骨架岩构成;礁B由生屑泥晶灰岩、粘结岩、骨架岩和生屑灰岩构成;礁C由粘结岩、骨架岩和生屑灰岩构成。红花生物礁造礁生物有钙质海绵、钙藻类、苔藓虫和水螅类,附礁生物为有孔虫、腕足类、双壳类、腹足类和棘皮动物等。单个礁体内,由下往上的生物演化为：腕足类+双壳类+有孔虫组合→钙藻类→钙质海绵+水螅类+钙藻类+苔藓虫组合→生物碎屑;岩性演化为：生屑泥晶灰岩→粘结岩→骨架岩→生屑灰岩。礁B的生屑滩内生屑间为泥晶充填,生屑分选、磨圆较好,是由相邻的高能生屑滩侵蚀搬运到礁B侧翼低能区沉积形成。3期礁都发育在碎屑滩上,礁A为低能环境下形成的礁,礁B和礁C在礁A形成的高地上成礁,为高能环境礁;单个礁体的完整成礁模式为：在浅滩之上,钙藻类大量生长、粘结吸附颗粒固结基底,钙质海绵和钙藻类在硬质基底上繁茂生长,形成具有抗浪格架的生物礁,礁体暴露水面死亡后遭波浪、水流改造形成生屑滩。 相似文献
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在三叠纪-白垩纪，特提斯海域内发育有大量的生物礁岩，羌塘盆地内亦广泛发育。生物礁沉积序列可划分为4类：(1)浅滩相生屑灰岩、核形石灰岩→生物礁岩→开阔台地相泥晶灰岩→斜坡相角砾岩；(2)开阔台地相灰岩→生物礁岩→局限台地相白云质灰岩；(3)浅滩相生屑灰岩→滨岸相碎屑岩；(4)潮坪相粉砂岩、泥灰岩→潮坪砂坝生屑灰岩→生物礁岩→潮坪砂坝生屑灰岩→潮坪相泥灰岩。生物礁岩可分为骨架岩、障积岩与粘结岩3种成因类型。成岩作用有重结晶、胶结、压实、压溶与破裂、溶蚀作用等。主要孔隙类型包括原生孔隙、裂隙与溶蚀孔、缝三类。晚三叠世造礁生物主要为珊瑚和藻类；中晚侏罗世以珊瑚礁与海绵礁为主。生物礁岩以海进体系为主，高位体系域也有发育，低位体系域中少见。可分为珊瑚礁、海绵礁、藻礁和层孔虫礁。南羌塘坳陷西部以珊瑚礁为主，东部以海绵礁为主；北羌塘坳陷西部为藻类珊瑚礁、层孔虫礁，台地内部有珊瑚礁、海绵礁。按古地理位置可分为台地边缘礁和台地内部礁，前者分布规模大，在地震剖面上也有明显的反映，有可能成为重要的油气储层。生物礁的发育状况全球海平面升降所控制。 相似文献
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江苏句容地区的上石炭统船山组碳酸盐沉积发育泥晶灰岩、细晶灰岩、生物碎屑泥晶灰岩、泥晶生物碎屑灰岩、含生物碎屑泥晶灰岩和含生物屑核形石灰岩6种岩性,可区分出潮坪、开阔台地、台盆、浅滩和生物礁5种相类型。这套碳酸盐沉积序列由S1、S2和S3三个层序构成,其中S1为Ⅰ型层序,S2和S3为Ⅱ型层序。依据岩性、沉积相、古生物与元素地球化学特征编制出的相对海平面变化曲线,反映船山组经历了3次海平面相对上升和下降。相对海平面升降控制了层序的发育,层序S1由滨海砾岩-潮坪结晶灰岩(低水位体系域)、台盆相含生物碎屑泥晶灰岩-泥晶灰岩(海侵体系域)和浅滩相含生物屑核形石灰岩(高水位体系域)构成;层序S2由生物礁相泥晶生物碎屑灰岩与开阔台地相生物碎屑泥晶灰岩-含生物碎屑泥晶灰岩-泥晶灰岩(海侵体系域)和开阔台地相含生物碎屑泥晶灰岩(高水位体系域)构成;层序S3由开阔台地相泥晶灰岩(海侵体系域)和浅滩相含生物屑核形石灰岩-潮坪相结晶灰岩-块状灰岩(高水位体系域)构成。下扬子区船山组是在构造相对稳定背景下发育的一套以开阔台地为主局部有滩有盆出现的碳酸盐沉积。开阔台地和台盆环境中沉积的灰岩是良好的生油层位,礁滩环境中的灰岩具备良好的储集性能。 相似文献
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黔南惠水雅水剖面下石炭统上司组底部含有厚度约7m的生物沉积和建造灰岩,为上下生屑滩夹含5m厚的珊瑚格架礁,微相类型丰富,可划分出4种类型:1泥粒状灰岩;2粒泥状灰岩;3珊瑚障积岩;4层孔虫盖覆岩。在生屑滩沉积生物碎屑泥粒状灰岩中,珊瑚、腕足类、钙藻、介形类、海百合茎等破碎程度高,分选性低,为较强水动力条件远距离或频繁搬运后快速沉积的产物,此时不适宜大型珊瑚原地生长保存,仅见少量而单调的单体四射珊瑚和复体四射珊瑚。珊瑚格架礁以生物碎屑粒泥状灰岩、珊瑚障积岩为主,发育少量层孔虫盖覆岩和藻类粘结作用。腕足类、钙藻、有孔虫、海百合茎、介形类、腹足类较为丰富,苔藓虫以及层孔虫的丰度未达到支撑程度,昭示珊瑚格架礁生长期的水动力条件减弱,为单体四射珊瑚、笛管珊瑚(Syringopora)和丛管珊瑚(Siphonodendron)为主体的珊瑚群落形成原地礁灰岩创造了条件。 相似文献
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四川宣汉盘龙洞上二叠统长兴组沉积相与岩石地层精细划分 总被引：5，自引：0，他引：5
盘龙洞剖面长兴组与普光气田同属于川东北的碳酸盐台地边缘相礁滩相层序,其岩石地层和沉积相研究结果对于认识普光气田、龙岗气田的储层发育和分布规律具有很好的参考作用。根据对薄片的仔细观察分析以及对相关地层的认识,对剖面长兴组的结晶白云岩地层进行了原岩恢复。根据古生物面貌及岩石组构特征,将盘龙洞剖面长兴组自下而上划分四段:下生屑岩段、礁灰岩段、上生屑岩段、微生物岩段。下生屑岩段包括了下部含小型有孔虫的深水陆棚相泥晶灰岩、以及上部含腕足类和棘皮类的浅水陆棚相生屑泥粒岩。礁灰岩段厚达108m,由海绵骨架岩和障积岩组成。上生屑岩段厚约45.3m,已经白云石化为细晶白云岩,导致前人往往误划分为蒸发台地相。仔细观察后仍然可以进行原岩恢复,主要由生屑泥粒岩、粒泥岩组成,含棘皮类、有孔虫、粗枝藻、等组成的群落。微生物岩段厚约8.7m,不仅地层以不规则薄层状为特征,而且含有特征的微生物群落。与普光6井、湖北利川见天坝长兴组剖面对比后发现,它们有相似的沉积层序:下部的深水相灰岩、中部的生物礁岩、上部的开阔台地相或相似环境的生屑灰岩。这应代表台缘礁滩相带的典型层序,可作为认识台缘礁滩相储层的基础。前人研究未能识别长兴组顶部的微生物岩段。这直接导致了二叠系—三叠系界线的无法正确确定。本文提出可以把微生物岩段作为一个岩石地层单位对待,可以作为确定二叠系—三叠系界线的标志层,把界线放在微生物岩段的中部。 相似文献
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通过野外剖面测制、室内岩石薄片观察、古生物鉴定,运用沉积学、矿物岩石学、层序地层学、构造地质学等的方法原理,对黔桂之交天生桥镇-安然地区晚二叠世乐平统吴家坪-长兴期碳酸盐岩微相特征进行分析鉴定,试图阐述沉积演化及古地理意义。分析表明,研究区未见长兴组及大隆组,合山组与吴家坪组均为孤立碳酸盐岩台地,且属同时异相关系。合山组发育局限台地、开阔台地、受簸选台地边缘砂、生物建造等4种沉积亚相,发育MF1-生屑粉微晶灰岩、生屑泥微晶含泥质灰岩,MF2-含泥质砂岩夹含粉砂质泥岩或煤层,MF3-泥晶介壳屑蜓屑灰岩或泥晶生屑灰岩,MF4-泥、微晶或亮晶生屑藻砂屑灰岩,MF5-生屑藻粘结灰岩等5种沉积微相;吴家坪组发育受簸选台地边缘砂、生物建造及前缘斜坡等3种沉积亚相,发育MFⅠ-泥微晶藻砂屑生屑灰岩、海绵礁生物灰岩,MFⅡ-亮晶藻砾屑生屑灰岩,MFⅢ-复成分灰岩角砾岩或礁前角砾岩等3种沉积微相。二叠世乐平统吴家坪-长兴期经历了4期相对连续完整的次级海进-海退旋回,总体为海退进积层序的古地理特征,并形成相应的沉积矿产资源。 相似文献
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浙江长兴长兴组火山岩和放射虫的发现 总被引：5，自引：2，他引：5
<正> 我国南部长兴阶的层型剖面在浙江长兴煤山葆青采石场。层型剖面经我所二迭—三迭纪地层队研究(1978),厚约40米,岩性大致可分上下二段:上段以灰—浅灰色中层状生物屑灰岩为主,次为含有孔虫微晶灰岩和海绵骨针微晶灰岩,普遍发育水平层理。含胶磷矿、沥青,产古(虫筳)、有孔虫、假蠕孔藻、棘皮类、腕足类、三叶虫、苔藓虫和海绵骨针。厚约 相似文献
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柴达木盆地西部地区古近系和新近系湖相碳酸盐岩主要分布于下干柴沟组上段到油砂山组,其中,下干柴沟组上段和上干柴沟组的碳酸盐岩更发育。碳酸盐岩主要岩石类型有泥晶灰岩、藻灰岩和颗粒灰岩等三大类,此外,还普遍发育由石灰质、白云质和陆源碎屑等3种组分构成的混积岩。碳酸盐岩沉积相可划分为滨湖灰泥坪、滨湖藻坪、浅湖颗粒滩、浅湖藻丘以及半深湖泥灰岩相。滨湖灰泥坪的主要岩石类型有泥晶灰岩、含陆屑泥晶灰岩、陆屑泥晶灰岩以及陆屑泥灰岩等;滨湖藻坪为藻泥晶灰岩、藻纹层灰岩、含陆屑藻泥晶灰岩;浅湖颗粒滩有亮晶或泥微晶的鲕粒灰岩、生屑灰岩和内碎屑灰岩,其次为含陆屑颗粒灰岩;浅湖藻丘为藻叠层灰岩、藻团块灰岩、藻泥晶灰岩和含陆屑藻泥晶灰岩;而半深湖泥灰岩相的主要岩石类型为泥晶灰岩、泥灰岩以及含少量陆屑泥和粉砂的泥晶灰岩或泥灰岩。碳酸盐岩沉积相表现出很强的由西南向东北的迁移性。 相似文献
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2012年底在云南省泸西县东山镇地区中三叠统关岭组中发现大规模典型台地边缘鲕粒浅滩和生物礁,同时在向阳乡方摆村附近发现典型台地前缘斜坡相泥晶灰岩,滑塌构造发育.台缘生物礁垂向3分结构明显,礁基为浅滩相鲕粒灰岩、豆粒灰岩;礁核为粘结海绵-水螅骨架岩,蓝绿藻粘结结构和亮晶栉壳状胶结结构发育;礁盖为台缘斜坡相深灰色薄层泥晶灰岩和泥质灰岩.生物礁横向展布也具有3个明显特征:礁后为碳酸盐台地相灰岩,礁核为粘结海绵-水螅骨架岩,礁前为台缘斜坡相薄层灰岩和泥岩,局部发育滑塌构造和滑塌角砾岩.该发现可为滇东南地区继续寻找油气资源开辟新的勘探方向. 相似文献
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一种特殊类型硅质岩的特征与成因研究 总被引：5，自引：0，他引：5
浙江江山晚石炭世藕塘底组上段发现一种较为特殊的硅质岩。该类岩石呈白色、黄白色,夹于浅海相黄白色长石石英砂岩、粉砂岩和砾岩中。其显著特征是,岩石具残余生物碎屑结构,见大量腕足类、海百合茎、珊瑚等钙质生物假象,主要为mm级完整个体,密集堆积。生物已完全硅化,但内部组构清晰可辨。常见由多个微晶石英交代的自形细晶白云石假象以及碳酸盐交代残余。岩石质地较轻,具大量圆形、菱形和不规则状孔隙,系由海百合茎等生物碎屑、白云石(或钙质成分)风化淋滤流失造成空洞所致。岩石属于交代成因,原岩可能为亮晶(或微晶)生物碎屑灰岩,为潮下浅滩沉积产物。岩石硅化前曾发生过白云化作用。地球化学特征表明,硅质来源于正常地下水,无热水或火山硅质来源的迹象。笔者将这类硅质岩取名为“钙骨假象燧石岩”。 相似文献
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Recent caliche, including nodules, pisolites, crusts, internal sediment, speleothem deposits, and spherulites, has formed within the dolomitic Cretaceous Edwards Formation of central Texas. As weathering altered the host strata, rhombic crystals of calcite were precipitated concomitantly with dissolution of the dolomite, thereby forming nodules. The highly altered dolomite (i.e. pulverulite) was then removed and spar, internal sediment, and travertine accumulated in the internodular voids. Nodular masses of calcite and dolomite are the most prominent constituent of the caliche. Some of the nodules have a well developed concentric structure as well as other characteristics similar to hypersaline pisolites. Features which appear to be useful in distinguishing caliche from hypersaline pisolites are: regional geological setting, association with other caliche and palaeosoil deposits, types of fossils present, and the presence of rhombic calcite and/or bladed sparry calcite with triangular shaped cross-section. A brick-like calcite texture and relict aragonite rays characterize hypersaline pisolites. Incipient neomorphism of the nodules and pisolites has resulted in the development of a radial pattern of spar within these structures. Geopetal deposits of internal sediment, including terra rossa soil, inhibited spar growth in the upward direction; consequently, spar is much better developed on the undersides of pisolites. Crusts and travertine flowstone (speleothem) deposits are intimately associated with the nodular masses and internal sediment. The brecciated thin crusts and travertine flowstone are end products of the same processes. The crusts formed during times of periodic desiccation of the growing surface while the flowstone formed when water was relatively abundant. Spherulitic bodies within the caliche, commonly 1–2 mm in diameter, display a radial texture and yet are composed of single crystals of calcite. The structures are the product of neomorphosed Microcodium or Microcodium-like globular bodies. 相似文献
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A. M. Alonso-Zarza Y. Sánchez-Moya† M. A. Bustillo‡ A. Sopeña§ & A. Delgado¶ 《Sedimentology》2002,49(2):303-317
Cauliflower-shaped nodules are widespread in a single red mudstone bed in the Buntsandstein (Triassic) facies of the Iberian Range. They consist mostly of quartz, dolomite and calcite, but other minerals, such as barite, kaolinite and iron oxyhydroxides, are also present. The nodules are spherical, ovoid or elongate in shape and range from 1 to 8 cm across. The surface of the nodules is irregular, and some show a pedogenic coating of microspar. The sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of Ca2+ and SO4 2– in a vadose environment, occasionally under the influence of pedogenic processes. Partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting, as indicated by the presence of organic filaments on the quartz crystal surfaces. In type A nodules, the dissolution of the innermost anhydrite was complete, and different types of quartz cement filled the porosity. Fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules. In type B nodules, the dissolution of the anhydrite was incomplete, inhibiting quartz cementation and enabling later dolomitization of the anhydrite. Dolomitization appears to have been driven by sulphate reduction, as indicated by the presence of bacterial bodies within the dolomite crystals. Dedolomitization and precipitation of barite, kaolinite and calcite spar cements occurred later under the influence of meteoric solutions. The nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin. Their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions. 相似文献
14.
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
15.
320矿床微晶石英硅质岩具独特的岩石学特征和特定的微量元素组合;产于栖霞组灰岩与当冲组硅质页岩界面的层间破碎带的上盘。按硅化强弱可分为石英质岩、微晶石英岩和硅化硅质岩3类,与铀矿化关系最密切的是黑色石英质岩和黑色微晶石英岩。根据P·尼格里的岩石化学计算法类比,笔者认为该岩石的原岩为粘土-石灰岩混合物类型。微晶石英硅质岩的形成是受区域构造应力影响的,是在一定温度、压力和裂隙溶液的共同作用下溶液中SiO_2围绕石英、方解石和生物碎屑颗粒重新结晶沉淀形成的。岩石重结晶时没有外界成分的参与。 相似文献
16.
Yongjie Hu Chunfang Cai Dawei Liu Chelsea L. Pederson Lei Jiang Anjiang Shen Adrian Immenhauser 《Sedimentology》2020,67(2):1161-1187
Neoproterozoic marine dolomite cements represent reliable, albeit complex, archives of their palaeoenvironment. Petrological and high-resolution geochemical data from well-preserved fibrous dolomite and pyrite in the upper Ediacaran (ca 551·1 to 548·0 Ma) Dengying Formation in south-west China are presented and discussed here. The aim of this research is to reconstruct the redox state of late Ediacaran shallow seawater and porewater in the Sichuan Basin using early marine diagenetic fabrics. Based on crystalline texture and axis, four basic types of fibrous dolomite cements formed penecontemporaneously in a microbialite reef setting at the platform margin: (i) bladed dolomites (replacement from a high-Mg calcite precursor); (ii) fascicular fast dolomites (replacement from an aragonitic precursor); (iii) fascicular slow dolomites; and (iv) radial slow dolomites. The latter two fabrics are considered direct marine porewater precipitates due to their length-slow character, cathodoluminescent zonation, and enriched copper and cobalt concentrations. Marine cements yield rare earth element and yttrium patterns comparable to modern seawater and represent a refined set of archive data relative to previously published bulk dolostones. Redox-sensitive elements and cathodoluminescence indicate that the fascicular fast dolomites formed in suboxic seawater, while fascicular slow and radial slow dolomites formed in euxinic marine porewaters. Microbial sulphate reduction during the formation of fascicular slow and radial slow dolomites is recognized by nanometre-scale spheroidal ankerite and sulphur-containing dolomite, and intergrown pyrite grains with U-shaped δ34S transects. Data shown here suggest predominantly suboxic shallow late Ediacaran seawater and euxinic marine porewaters, with microbial activity promoting the direct precipitation of dolomite. 相似文献
17.
18.
The Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound-shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs. The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near-normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non-reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under-represented in ancient reefs. 相似文献
19.
The Coniacian Arnager Limestone Formation is exposed on the Danish island of Bornholm in the Baltic Sea. It is composed of mound-bedded siliceous chalk, and X-ray diffraction and scanning electron microscopy indicate a content of 30–70% insoluble minerals, including authigenic opal-CT, quartz, clinoptilolite, feldspars, calcite, dolomite, and barite. Opal-CT and clinoptilolite are the most common and constitute 16–53% and 2–9%, respectively. The content of insoluble minerals varies laterally both within the mounds and in planar beds, and the opal-CT content varies by up to 10% vertically. The mounds consist of two microfacies, spiculitic wackestone and bioturbated spiculitic wackestone, containing 10–22% and 7–12% moulds after spicules, respectively.Subsequent to deposition and shallow burial, dissolution of siliceous sponge spicules increased the silica activity of the pore water and initiated precipitation of opal-CT. The opal-CT formed at temperatures around 17 °C, the precipitation lowered the silica activity and the Si/Al ratio of the pore water, resulting in precipitation of clinoptilolite, feldspar and smectite. Calcite formed synchronously with the latest clinoptilolite. Minor amounts of quartz precipitated in pore water with low silica activity during maximum burial, probably to depths of 200–250 m. The dissolution of sponge spicules and decomposition of the sponge tissue also resulted in the release of Ba2+, Sr2+, Mg2+, Ca2+ and CO32?, facilitating precipitation of barite and dolomite. Precipitation of especially opal-CT reduced the porosity to an average of 40% and cemented the limestone. The study highlights the diagenetic pathways of bio-siliceous chalk and the effects on preservation of porosity and permeability. 相似文献
20.
Subías I. Yuste A. Fanlo I. Fernández-Nieto C. González López J.M. 《Geologie en Mijnbouw》1999,78(1):87-102
Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light 18O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more 18O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in 13C of micrites represent primary secular trends, according to published 13C variations. The 13C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones. 相似文献