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1.
Ins Eymard Andrs Bilmes María del Pilar Alvarez Rodrigo Feo Gabriel Hunger Crisogono Vasconcelos Daniel Arizteguí 《The Depositional Record》2019,5(3):498-514
In a seminal paper regarding the mechanisms of carbonate stromatolite formation, Ginsburg (1991, Controversies in Modern Geology, pp. 25–36) emphasized the need to question the relative role of microbes versus environment in their formation. The Maquinchao Basin is a continental lacustrine system in southern Argentina. It provides an ideal site to study carbonate buildups, the role of microbes and environmental stressors in their development and their implications in palaeoenvironmental reconstructions. Presently, the basin encompasses two lakes (Carri Laufquen Grande and Carri Laufquen Chica) joined by the ephemeral Maquinchao River. Fossil microbialites are found south and southwest of the largest lake. Preferential areas of development for fossil microbialites have been mapped using a high‐resolution differential Global Positioning System. Outcrops are located between 820 and 830 m elevation, higher than actual lake levels and the Maquinchao River where living microbialites have been observed. Field data along with microscopical observations and X‐ray diffraction analyses have revealed a heterogeneity in both distribution and macro‐morphotypes since carbonate buildups display different morphologies such as crust, columns, open flower‐like, rounded and ellipsoids. Conversely, on the meso and micro‐scale they show more homogeneous morphologies including laminations and shrubs. These microbial buildups are associated with basaltic substrates of variable size from pebbles to boulder. The homogeneity in meso and micro‐structures argue in favour of stable intrinsic parameters (i.e. microbial communities) whereas the variable macro‐morphotypes indicate changing extrinsic constraints such as steepness, energy and turbidity. The occurrence of distinctive morphotypes in buildups separated by outcrop and topography suggest that the Maquinchao microbialites are indicative of a former larger lake. Thus, the Maquinchao microbial buildups are a valuable proxy for water‐level evolution and therefore palaeoenvironmental reconstructions. They can be further used to interpret the apparently random distribution of morphological types and extension of microbialites in the geological past. 相似文献
2.
CHRISTINA GLUNK CHRISTOPHE DUPRAZ OLIVIER BRAISSANT KIMBERLEY L. GALLAGHER ERIC P. VERRECCHIA PIETER T. VISSCHER 《Sedimentology》2011,58(3):720-736
Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments. 相似文献
3.
Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ13C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ18O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ18O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ18O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ18O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ29Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages. 相似文献
4.
利用大量地质及地球化学资料,开展了柴达木盆地西部地区渐新世下干柴沟组上段盐湖沉积特征研究,重点识别岩石类型,恢复古沉积环境,建立沉积模式,探讨控制因素。研究认为：盐湖环境主要发育两大类、五小类沉积相组合,盐湖边缘沉积相包括滨岸斜坡带相组合、缓坡带相组合、陡坡带相组合,盆内沉积相包括水下隆起带相组合和盐湖深水区相组合;盐湖可划分早、中、晚3个沉积演化阶段,分别对应半咸水湖泊、咸水湖泊和盐湖;形成碳酸盐岩与钙质砂岩、富含石膏的碳酸盐岩与泥岩、厚层石盐与薄层碳酸盐岩等主要岩石组合;盐湖沉积主控因素为气候、古地貌与构造运动。 相似文献
5.
三水盆地古近系莘庄组顶部至土布心组红岗段的全岩碳酸盐稳定同位素分析结果表明其形成期间经历了多次环境变迁。根据碳酸盐氧、碳同位素比值及其相互关系的变化,可识别三次海水入侵期。其时δ18O值大幅度向正值漂移。而由于受有机质降解的影响,相应时期的δ13 C均表现为低值。在不直接受海洋影响的湖相沉积阶段,δ18O与δ13 C的相关程度虽然未达到典型的封闭型湖泊水平,但仍呈现一定的正相关变化 (r =0.6 5 ),表明其湖水滞留时间较长。而频繁出现的石膏薄层沉积也指示湖盆的封闭性较好。这些均表明这一时期的三水盆地可能是一周期性封闭型湖泊。其稳定同位素组成主要受制于蒸发量/降雨量平衡的变化。而δ13 C比值往往还受有机质活动的控制,更多的是反映有机质生产力、埋藏与降解率。 相似文献
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The composite δ13C record of Neoproterozoic carbonates is characterized by large magnitude (<18‰ Vienna Pee Dee Belemnite) swings that have been interpreted to record (a) changes in dissolved inorganic carbon in sea water driven by a carbon cycle with different boundary conditions, (b) pervasive diagenetic alteration or (c) local controls similar to those active in Cenozoic platformal successions. A sedimentological study of the Trezona Formation in the Flinders Ranges of South Australia was conducted to determine the palaeoenvironmental context of the Trezona anomaly, one of the most widely recognized examples of a Neoproterozoic mega‐excursion that occurs beneath glaciogenic sediments of the Marinoan ice age (ca 635 Ma). Stratigraphic mapping identified an unconformity at its base separating it from the shelf deposits of the underlying Enorama Shale. The lower Trezona Formation is dominantly calcareous mudstone with mudcracks, channels, and mud chip and algal flake breccias interpreted to record exposure and desiccation on mudflats, shallow pools and lagoons. A marked increase in microbial carbonate and oolites in the upper Trezona Formation records a relative decline of mud input that terminates with karst. The Trezona Formation is thus interpreted to have been deposited in shallow, localized, salt withdrawal basins on the exposed shelf creating a mosaic of poorly connected lagoons and (alkaline) lakes that were intermittently restricted or isolated from the sea. Variations in δ13C and δ18O correspond with facies changes and reflect locally evolving depositional conditions influenced by sills and channels that shifted the sea water to freshwater balance. This restricted setting and range of δ13C values is more similar to Cenozoic platformal deposits (ca 12‰) than the muted (ca 3‰) global sea water variation recorded in pelagic sediments. The anomalous δ13C values in the Trezona Formation may instead provide constraints on the changing terrestrial biosphere directly preceding the evolution and rapid expansion of metazoan life. 相似文献
7.
辽西地区下白垩统义县组是热河生物群产出的主要地层,其中产出大量精美的生物化石,但是由于受到后期构造破坏和风化作用影响,难以开展野外工作,热河生物群产出的古地理环境及盆地沉积环境的研究程度仍然较低。该组地层中湖相碳酸盐岩比较发育,本次工作主要对辽西义县组老公沟—金刚山剖面4个沉积层中产出的多层湖相碳酸盐岩沉积层进行了深入研究。通过野外工作结合室内镜下鉴定,分析了义县组湖相碳酸盐岩的产出状态和岩石类型,划分出5个岩相类型和两个相组合。结合野外实测剖面中碳酸盐岩与其它沉积岩层的组合关系对义县组湖相碳酸盐沉积环境进行了探讨,发现义县组湖相碳酸盐岩主要为洪水—漫湖沉积体系中较浅的储水洼地和滨浅湖半咸水沉积环境,并建立了义县组湖相碳酸盐岩沉积相模式。 相似文献
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湖相碳酸盐氧碳同位素的相关性与生油古湖泊类型 总被引：15，自引：1，他引：15
对山东东营盆地和南海珠江口盆地早第三纪生油层中的湖相碳酸盐氧碳同位素进行了分析。结果显示,前者δ^18O多为负值,δ^13C基本为正值,两者之间相关性强,相关系数大于0．8,而后者δ^18O和δ^13C均为负值,两者不相关。通过与现代湖粕碳酸盐氧碳同位素特征的比较,得出东营盆地早第三纪生油湖泊为封闭型咸水或半咸水湖,珠江口盆地早第三纪生油湖泊为开放型淡水湖,这一结论与两盆地生油层的沉积学和古生物学证据相一致,说明利用湖相碳酸盐氧,碳同位素的特征及其相关性可以判断生油古湖泊的类型。δ 相似文献
9.
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
10.
《Geological Journal》2017,52(5):808-827
With the recent discovery of new oil fields around the Dongying Depression, Bohai Bay Basin, a better understanding of the origin of its high‐quality lacustrine carbonate reservoirs has become important. The lacustrine carbonates of the upper fourth member of the Eocene Shahejie Formation (ES4s) have been examined using a variety of methods, including core and thin section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The δ13C and δ18O values of the Dongying carbonates differ significantly from Eocene marine values, and the 87Sr/86Sr ratios of carbonate minerals are higher than Eocene marine water and the same as the surrounding Palaeozoic carbonate bedrock. Additionally, the distribution of the carbonates is controlled by the surrounding Palaeozoic carbonate bedrock, all of which indicate their lacustrine origin. Six major carbonate lithofacies are distinguished in the southern gentle slope zone. Bioclastic, oolitic and intraclastic grainstones dominate the littoral to sublittoral zones, while mudstones are mainly distributed in the profundal zone. The ES4s interval is composed of a third‐order depositional sequence that contains several small‐scale, higher frequency depositional cycles. Overall, stacking patterns of high‐frequency depositional cycles are consistent with the general facies variation from basin to shore. Shallow‐water grainstones contain both primary intergranular and secondary dissolution pores, whereas the dolomitized mudstones in the profundal zone contain intercrystalline pores and fractures. Diagenetic processes that improve reservoir quality include dissolution and dolomitization. Reservoir quality was diminished by burial calcite cementation and compaction. High‐quality reservoirs are mostly distributed in dolomitized grainstones and wackestones, which are deposited in the shoal facies of shallow lakes. Variations in reservoir quality in the Dongying Depression are due to a combination of lithofacies type, dissolution intensity, early dolomitization and tectonic‐related deformation. Due to the influx of meteoric waters and shallow burial depth, the grainstones in the littoral zone have the best reservoir quality. Integrated studies have helped to understand the reservoir heterogeneity and hydrocarbon potential of the Dongying lacustrine carbonates. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Recent work has established that marine teleost (bony) fish represent a prolific source of mud grade, mainly high‐Mg calcite, carbonate sediment by means of primary precipitation within the intestine. Previously documented crystalline products display a diverse array of morphologies, many unique in shallow tropical marine settings, and have a wide range of magnesium contents (from 18 to 39 mol% MgCO3). This study utilizes scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray diffraction and liquid ion chromatography to provide a more extensive and expansive morphological, mineralogical and chemical characterization of the crystalline forms produced by a wider range of piscine functional groups (covering 21 different fish species common in The Bahamas). Several crystal morphologies not previously described in fish‐derived carbonates are documented, and chemical composition is found to be more variable than previously reported: in addition to high‐Mg calcites with >18 mol% MgCO3, high‐Mg calcites with lower MgCO3 contents and low‐Mg calcites are identified. From the expanded species range, MgCO3 content in fish‐derived carbonates ranges from ca 0˙5 to > 40 mol%, and particle length is in the range 0˙1 to >50 μm (typically <2 μm for individual crystals). Amorphous Mg‐carbonates (with detectable CaCO3 of <2 mol%) are also found to varying extent in the precipitates of many species. Dominant mineralogy and MgCO3 content varies with producing species and crystal morphology (itself a species‐dependent variable). Given the very small grain size and often high MgCO3 contents of these carbonates, interesting questions arise about their preservation potential. Thus, the extent to which carbonates produced by different species may follow different post‐excretion preservation pathways is considered. 相似文献
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Paul L. Broughton; 《The Depositional Record》2024,10(5):637-669
A tufa mound developed at La Saline Lake, an oxbow of the Athabasca River in the Athabasca Oil Sands deposit of north-east Alberta, is characterised by an unusual emplacement of a gypsum caprock. This two-tiered architecture resulted from the bicarbonate-saturated groundwater flow along Upper Devonian limestone being redirected deeper and encountering the halite–anhydrite dissolution trend within the underlying Middle Devonian Prairie Evaporite Formation, 175 to 200 m below. Migrations up-section of sulphate-saturated brine resulted in the gypsum caprock on the tufa mound and discharge of a sulphate-saturated brine spring with total dissolved solids of ca 80,000 mg/L. The brine spring is bifurcated with flows to the south-west and north-west. Calcite–gypsum thrombolytic bottom sediments along the south-western branch were covered by a halite deposit and subsequently a gypsum crust with a microbial community dominated by the cyanobacteria Coleofasciculus chthonoplastes. The thrombolite contact zone with the halite–gypsum encrustation has a more diversified community with the cyanobacteria Dactylococcopsis. Cyanobacterial mats that wrap around the bulbous gypsum crust protuberances distributed along the brine pool bottom surfaces have significant eukaryotic diversity, represented by the heterotrophic Ochrophyte Paraphysomonas. In contrast, sediments accumulated along the adjacent spring branch flow to the north-west were thicker and clogged by abundant decomposed and fermented floral debris, unlike the deposit accumulated along the south-western branch. This resulted in an oxygen-depleted anaerobic environment dominated by sulphate-reducing bacteria resulting in a calcite-rich and sulphate-starved anaerobic sediment with ca 20% elemental sulphur and emanation of H2S gas. 相似文献
14.
Mar Simonet Roda;Daeyeong Kim;Alexander T. Brasier;Erika Griesshaber;Jeong-Hyun Lee; 《Sedimentology》2024,71(7):2448-2469
Lacustrine stromatolites serve as important archives for recording environmental changes, and the detailed examination of their microfabrics is essential for understanding their formative processes and the environmental changes embedded within them. This study explored the application of Electron Backscatter Diffraction combined with Energy-Dispersive X-ray Spectroscopy to investigate a well-preserved middle Cretaceous lacustrine stromatolite from south-eastern Korea, unveiling ultra-high-resolution sedimentary processes that are often challenging to observe using conventional methods. Two types of microsparitic layers and one type of crystalline layer are distinguished based on their texture, crystal morphology and elemental composition. Both microsparitic layers are micrometre-thick and are characterized by poorly co-oriented calcite grains, but differ in their composition. Type 1 is depleted in magnesium but enriched in detrital elements such as silicon and aluminium, likely originating from the trapping and binding of detrital sediments on microbial mats during rainy seasons. In contrast, type 2 is enriched in magnesium but devoid of detrital elements, interpreted to have formed by the precipitation of calcium carbonate during dry seasons. The crystalline layers comprise fan-shaped calcite crystals (ca 500 μm in length) with radiating internal structures, with their c-axes oriented perpendicular to the stromatolite layers. These structures resemble those observed in some modern freshwater stromatolites, which are interpreted as imprints of cyanobacterial fascicules. While the cyclic occurrence of types 1 and 2 microsparitic layers might imply annual seasonal cycles, the less frequent crystalline layers are interpreted as a result of environmental changes occurring every 27 to 30 years. This is the first study to apply Electron Backscatter Diffraction to stromatolites, showcasing its potential in unravelling both the formative and diagenetic processes of ancient stromatolites. 相似文献
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湖相碳酸盐岩因形成于大陆环境,通常陆源硅酸盐组分含量较高并影响元素组成特征,不应简单地套用海相碳酸盐岩的方法进行元素测试。对柴西茫东地区红山梁剖面始新统下干柴沟组—中新统下油砂山组湖相碳酸盐岩采用酸溶法、电子探针、LA等多手段进行的地球化学综合研究,结果显示,各种测试方法应用于湖相碳酸盐岩分析时都存在适用性与局限性问题。酸溶法的局限性在于所获得的K等碱金属元素含量不能用于后续分析。而对微区测试方法而言,首先LA与探针两种微区测试方法能否进行数据上的通用需要比较二者的相关性;其次由于光斑大小的限制,对泥晶组分而言,需在判断陆源的干扰程度并进行相应扣除后,才能反映沉积水体的元素组成特征。因此,在分析陆源硅酸盐组分对湖相碳酸盐岩元素含量影响程度的基础上,提出两种排除陆源硅酸盐组分对泥晶组分元素含量影响的方法,可以实现微区测试方法在湖相碳酸盐岩中的有效应用。 相似文献
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湖相碳酸盐岩在我国古代和现代各种类型的陆相盆地中非常发育,查明不同类型湖湘碳酸盐岩的沉积模式差异对指导薄层碳酸盐岩油气储层勘探开发具有重要意义,为油气储层分布规律提供依据。根据岩心、测井、地球化学、古生物等资料对歧口和泌阳凹陷湖相碳酸盐岩沉积特征差异和主要控制因素进行了对比分析,研究结果发现:歧口凹陷和泌阳凹陷湖相碳酸盐岩在岩石类型、沉积位置、沉积规模、沉积相类型和沉积旋回特征都存在明显差异,歧口凹陷湖相碳酸盐岩沉积面积大于2 000 km2,分布在凹陷西南缓坡区,具有分布面积广、厚度薄、岩石和沉积相类型多样、沉积旋回变化周期长等特点,泌阳凹陷湖相碳酸盐岩主要分布在凹陷中心部位,具有分布面积小、厚度大、沉积相类型单一、旋回变化周期短的特点。古地貌、水体开放程度和气候条件是歧口和泌阳凹陷沉积模式差异的主要控制因素。盐度、水温、生物群类型以及湖平面变化频率间接受到了古地貌、水体开放程度和气候条件的控制,也是沉积模式差异的因素。 相似文献
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王文静;刘圣乾;张震宇;高雅宣;张子微;李雷 《沉积学报》2025,43(1):108-120
东营凹陷西部沙四上亚段发育大规模湖相碳酸盐岩沉积,目前对其发育规律及成因模式的认识尚存争议。 基于岩心、薄片观察及测录井资料,对东营凹陷西部沙四上亚段碳酸盐岩的岩石类型、沉积微相及其时空展布进行了研究。 （1）研究区湖相碳酸盐岩主要发育8种岩石类型,根据岩性组合识别出礁滩、浅滩—滩间、混积—灰泥坪和风暴—重力流4种沉积微相。（2）该区湖相碳酸盐岩沉积时期宏观上为小型构造台地,沉积体系空间发育和展布主要受古地貌和古风场两个因素控制。古地貌决定了沉积微相发育的类型及分布,台地顶部向广阔湖盆方向发育礁滩及浅滩高能相带,而向陆地方向为混积—灰泥坪沉积,东部和南部的台地斜坡风暴—重力流发育。盛行东南风作用下迎风侧和背风侧的水动力差异影响了沉积体系发育,促进台地顶部礁滩复合体及大面积浅滩相带的形成,使高能沉积相带主体垂直风浪呈NE—SW向连片展布,向南和向东规模逐渐扩大。 研究区风浪—地貌联控的碳酸盐岩台地沉积模式对湖相碳酸盐岩油气勘探及有利相带预测具有一定的指导作用。 相似文献
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This work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins. Sedimentological and stable‐isotope data of microbialites and associated carbonate facies, along with regional geological information, are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings. The Vega Formation – a 150 m thick Lower Kimmeridgian siliciclastic fluvial sequence in Asturias Province (northern Spain) – constitutes a candidate for this approach. This unit includes varied facies (stromatolites; rudstones, packstones and wackestones containing oncoids, intraclasts, charophytes and shell bioclasts; marlstones and polygenic calcareous conglomerates) that formed in a low‐gradient fluvial–lacustrine system consisting of shallow, low‐sinuosity oncoid‐bearing channels and pools within marshy areas, with sporadic coarse alluvial deposition. The sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water‐discharge variations. The carbonate fluvial–lacustrine system developed near uplifted marine Jurassic rocks. The occurrence of the system was conditioned by normal faults (active during the deposition of the unit) that favoured: (i) springs of HCO3–Ca‐rich water from a Rhaetian–Sinemurian carbonate rock aquifer; and (ii) carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system. The microbialite δ13C and δ18O values support deposition in a hydrologically open system, fed by ambient‐temperature meteoric water, with riparian vegetation. Three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacterial communities. The textural pattern of lamination parallels δ13C and δ18O changes, suggesting short‐term cycles of precipitation and temperature. A moderately to strongly contrasted seasonal and/or pluriannual precipitation regime is inferred from the cyclic δ13C pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids. Thus, the isotopic and sedimentological attributes of the carbonate deposits were linked to short‐term climate changes associated with semi‐arid conditions, consistent with the studied climatic zone. 相似文献
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STEVE KERSHAW SYLVIE CRASQUIN MARIE‐BÉATRICE FOREL CARINE RANDON PIERRE‐YVES COLLIN ERDAL KOSUN SYLVAIN RICHOZ AYMON BAUD 《Sedimentology》2011,58(3):739-755
The Permian–Triassic Boundary sequence at Çürük Dag, near Antalya, Turkey, begins with a major erosion surface interpreted as being the Late Permian lowstand, on which lies ca 0·4 m of grainstone/packstone composed of ooids, peloids and bioclasts. Most ooids are superficial coats on fragments of calcite crystals presumed to be eroded from crystal fans which are no longer present. The erosion surface is smooth and shows no evidence of dissolution; the grainstone/packstone contains intraclasts of the underlying wackestone, proving erosion. Next are 15 m of microbialite comprised of interbedded stromatolites, thrombolites, plus beds of planar limestones with small‐scale erosion. The latter comprise a complex interlayering of stromatolitic, thrombolitic and peloidal fabrics and precipitated crystal fans, which form a hybrid of microbialite and inorganic carbonate, together with bioclastic debris and micrite. The Çürük Dag microbialite sequence is repetitious; the lower part is more complex, with abundant stromatolites and hybrid microbialites. Some of the stromatolites are themselves hybrids composed of peloids and crystal fans. In the upper part of the sequence stromatolites are missing and the rock is composed mostly of recrystallized thrombolites that develop upwards from tabular to domal form. The domes form directly below small breaks in microbialite growth where very thin shelly micrites and grainstones/packstones are deposited. Repetition of facies may be controlled by sea‐level change; a deepening‐up model is consistent with the evidence. Stromatolites (with abundant crystal fans) dominate in shallower water, deepening through hybrid microbialite and interlayered sediments to thrombolite, probably no more than a few tens of metres deep, followed by breaks and renewal of microbialite growth. An interpretation of open marine fully oxygenated waters for microbialite growth is consistent with ongoing parallel work that has identified Bairdioid ostracods in the microbialite, a group known to be open marine. However, other researchers have proposed low oxygen conditions for Permian–Triassic boundary facies globally, so work continues to confirm whether the Çürük Dag microbialite grew in dysoxic or normally oxygenated conditions. The principal stimulus for post‐extinction microbialites is likely to be carbonate supersaturation of the oceans. The microbialite sequence is overlain by a further 25 m of grainstone/packstone (without microbialite), followed by Early Triassic shales. Overall, microbialites form a thin aggradational sequence during an overall relative sea‐level rise, consistent with global eustatic rise following the Late Permian lowstand. 相似文献
