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1.
Evaporitic sediments of Early Archaean age from the Warrawoona Group, North Pole, Western Australia 总被引：2，自引：0，他引：2
Chemical sediments are common and diverse in the c. 3500 Myr old North Pole chert-barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi-permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea-shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect. 相似文献
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Datangpo-type sedimentary manganese deposits, which are located in northeastern Guizhou province and its adjacent areas, are Mn carbonate-type deposits hosted in black carbonaceous shale that represent a series of medium to large deposits containing a huge tonnage of reserves. PAAS-normalized rare earth element distribution patterns of manganese ores record “hat-shaped” REY (REE + Y) plots characterized by pronounced middle rare earth element enrichment, evident positive Ce anomalies, weak to strong positive Eu anomalies and negligible negative Y anomalies. These REY geochemical characteristics are different from those of country rocks and record the processes and features of sedimentation and diagenesis. Manganese was precipitated as Mn-oxyhydroxide particles in oxidized water columns with the sorption of a certain amount of rare earth elements, subsequently transforming from Mn-oxyhydroxides to rhodochrosite and redistributing REY in reducing alkaline pore-water during early diagenesis. A number of similarities can be observed through a comparison of Datangpo-type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on their rare earth elements. The precipitation of Datangpo-type manganese ores is similar to that of hydrogenetic crusts and nodules based on their positive Ce anomalies and relatively higher total REY concentrations. However, several differences also exist. Compared to hydrogenetic crusts and nodules, Datangpo-type manganese ores record smaller positive Ce anomalies, lower total REY concentrations, unobvious fractionation between Y and Ho, and weak to strong positive Eu anomalies. These were caused by quicker sedimentary rates in the oxic water columns of the shallower basin, after which pore water became strongly reducing and alkaline due to the degradation of organic matter in the early diagenetic stage. In addition, compared to typical deposits in the world, Datangpo-type manganese ores are similar to hydrogenetic deposits and different than hydrothermal deposits. All of these characteristics of manganese ores indicate that Datangpo-type manganese ores, the principal metallogenic factors of which include oxidation conditions during deposition and reducing conditions during early diagenetic stages, represent hydrogenetic deposits. 相似文献
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Late Cretaceous epicontinental phosphorites, porcelanites/cherts, dark-coloured shales, glauconitic sandstones and bioclastic and fine-grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon-rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea-level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution-collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron-oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters. This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters. 相似文献
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山东临朐山旺地区中新世山旺组硅藻土极为发育。通过对该地区硅藻土纹层的研究,确定其分类和成因,为进一步确立年纹层的存在、利用其进行高分辨率测年、探讨高频古气候旋回奠定基础。山旺组硅藻土的单层纹层厚度一般为100μm左右。依据纹层的物质成分,单层纹层可分为硅藻纹层、有机纹层和粘土纹层三种类型。各类单层纹层组合成为明暗相间的层偶,层偶厚度一般为100～400μm,常常表现为浅色的硅藻纹层逐渐向深色的粘土纹层或有机质纹层渐变过渡。纹层层偶可分为:薄层硅藻纹层-有机质层偶,厚层藻纹层-有机质层偶,薄层粘土纹层-有机质层偶,厚层粘土纹层-有机质层偶和硅藻纹层-粘土纹层层偶。山旺组硅藻土中不同类型的单层纹层以及层偶的形成主要受控于当时湖盆内粘土物质的沉积速率、硅藻的繁盛程度和有机物质的沉积和保存条件。 相似文献
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Liyao Li Jiuchuan Wei Huiyong Yin Jun Xie Yusheng Ding Shouqiao Shi 《Arabian Journal of Geosciences》2018,11(15):432
The Paleogene clastic sediments of the Funing Formation, which is characterized by slump-fan, delta-front, and beach-bar deposits, are important targets for oil and gas exploration in the Wanglongzhuang Oilfield, Subei Basin, eastern China. Three categories of reservoir quality are distinguished on the basis of petrographic and petrophysical experiments such as nuclear magnetic resonance (NMR) and mercury injection (MI). A paragenetic sequence for diagenetic evolution of each sedimentary facies has been reconstructed to determine the mechanisms for the variations in reservoir quality of the formation and the influence of the sedimentary facies on the distribution of diagenetic features in each of its members. The maturity of the organic material, textural relationships, and the distribution pattern of diagenetic minerals suggest that the study area underwent mainly mesodiagenesis. The mesodiagenesis A and mesodiagenesis B stages induced different diagenetic features in the target reservoir; the beach-bar sand of the E1f2 unit and the delta-front sand of the E1f4 unit underwent mostly only cementation; the delta-front deposits of the E1f3 unit, however, suffered more from mechanical and chemical compaction than from cementation. The loss of porosity in the fluxoturbidites of E1f4 due to compaction was just as large as the loss by cementation. The gravity-flow channel, sand-bar, and distributary-channel deposits have a higher porosity and permeability than the sand-beach and estuarine-bar deposits. 相似文献
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The lithology and geochemistry of upper Oxfordian sedimentary rocks enriched in marine organic matter (OM) have been studied. These rocks occur as a persistent unit of Upper Jurassic rocks exposed on the right bank of the Unzha River in the Kostroma district. The OM was investigated in detail in both the carbon-rich rocks and their hosts. It was established that the OMs from the Oxfordian rocks are characterized by a low degree of thermal (catagenetic) maturity and their geochemical signature reflects specific features of synsedimentary and early diagenetic processes. Kerogen in the carbon-rich sedimentary rocks is markedly enriched in Sorg, and its formation was related to the early diagenetic sulfate reduction (sulfurization of the lipid fraction of the initial OM). The composition of kerogen from the host clay is sharply distinct in many parameters. No derivatives of isorenieratene were revealed in the aromatic fraction of bitumen in the carbon-rich rocks. The Oxfordian carbonaceous rocks are distinguished by slightly enriched in S, Mo, V, and Ni. Anoxic conditions were unstable in the water column during the deposition of carbon-rich sediments (such conditions were probably episodic). The Corg-rich unit formed due to a short-term abrupt increase in the productivity of phytoplankton related to eutrophication of water, probably, as a result of the recycling and redistribution of biophile elements. 相似文献
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Manganese-rich brown layers in Arctic Ocean sediments: Composition, formation mechanisms, and diagenetic overprint 总被引：4，自引：0，他引：4
C. März A. Stratmann A.-K. Meinhardt B. Schnetger R. Stein 《Geochimica et cosmochimica acta》2011,75(23):7668-7687
We present inorganic geochemical analyses of pore waters and sediments of two Late Quaternary sediment cores from the western Arctic Ocean (southern Mendeleev Ridge, RV Polarstern Expedition ARK-XXIII/3), focussing on the composition and origin of distinct, brown-colored, Mn-rich sediment layers. Carbonate enrichments occur in association with these layers as peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg, suggesting enhanced input of both ice-rafted and biogenic carbonate. For the first time, we show that the Mn-rich layers layers are also consistently enriched in the scavenged trace metals Co, Cu, Mo and Ni. Distinct bioturbation patterns, specifically well-defined brown burrows into the underlying sediments, suggest these metal enrichments formed close to the sediment-water interface. The geochemical signature of these metal- and carbonate-rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover. Both rivers and sea ice delivered trace metals to the Arctic Ocean, while enhanced seasonal productivity exported reactive organic matter to the sea floor. The coeval deposition of organic matter, Mn (oxyhydr)oxides and trace metals triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, and the degradation of labile organic matter. With the onset of cooler conditions, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, resulting in deposition of grayish-yellowish, metal-poor sediments. Oxygen depletion of Arctic bottom waters under these cooler conditions is not supported by our data, and did not cause the sedimentary Mn distribution. While the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes, pore water data show that diagenetic Mn redistribution is still affecting the organic-poor deeper sediments. Given persistent steady state conditions, purely authigenic Mn-rich brown layers may form, while others may be partly or completely dissolved. The degree of diagenetic Mn redistribution largely depends on the depositional environment, the Mn and organic matter availability, and apparently affected the Co/Mo ratios of Mn-rich layers. Thus, brown Arctic layers are not necessarily synchronous features, and should not be correlated across the Arctic Ocean without additional age control. 相似文献
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Hydrothermal manganese and ferromanganese deposits associated with Neyriz ophiolite colored mélange occurred as small ore deposits in the Abadeh-Tashk area, SE of Fars Province, SW Iran. The deposits are found in three types: a) banded syngenetic ores, b) massive boudin and lens shaped diagenetic ores and c) vein and veinlet epigenetic ores. Microtextural, geochemical and mineralogical data associated with petrographic Raman, FTIR and SEM studies indicated that the primary Fe compounds formed series of microbially mediated biomats and Mn compounds were precipitated as an amorphous oxide on an active oxide surface accompanying silica gels. Field relationships between ore and host rock, high Mn/Fe ratio (17.43 to 40.79), ΣLREE, positive Eu and negative Ce anomalies in syngenetic ore types reveal that the ores were formed by hydrothermal fluid in an oceanic floor environment. Manganese was fractionated from iron due to physicochemical changes as well as microbial activities in the sedimentary environment. Microbial remains as filamentous beads with regular circular shapes, vermiform structures, series of Fe-rich biomats, traces of embedded organic material besides trace metals and REE concentrations in Mn ores emphasize the role of microorganisms in Fe and Mn precipitation. Syngenetic mineralization took place under suboxic neutrophilic conditions, while diagenetic processes resulted in variably reduced Fe- and Mn-oxides via organic matter decomposition, forming rhodochrosite as the end product. Braunite formation occurred most probably as a biogeochemically mediated early diagenetic product. Diagenetic and epigenetic Mn ores were formed when primary Mn deposits underwent subsequent diagenetic and remobilization–redeposition events respectively. 相似文献
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Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area. 相似文献
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The Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault-bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north-western Red Sea has cored, below a massive salt unit, an anhydrite-bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca-sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine-grained laminated sediments to coarse-grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken-wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom-rich oozes formed in marine, shallow-water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca-sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope-foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine-grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows. 相似文献
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重晶石沉积类型丰富,具有多种成因过程。通常,沉积型重晶石可分为生物、热液、成岩和冷泉重晶石四种类型。富钡与富硫酸盐的流体（海水、早成岩孔隙水或热液流体）及其相互作用过程（水柱、热液系统、沉积柱、沉积物-水界面附近）决定了重晶石的沉积环境、宏微观产出方式、同位素组成及相应的地质意义。另外,根据扬子地区下寒武统富重晶石沉积的地质特征,简述了其各种富集机制的适用性及争论。据此建议,结合埃迪卡拉纪-寒武纪转折时期的古海洋背景,对其进行详细的沉积学及地球化学分析,有助于深化成因认识,弥合分歧。 相似文献
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沉积成岩作用研究的若干问题 总被引：7，自引：2，他引：5
20世纪60年代由于物理化学动力学的引入和低温低压下矿物相平衡的研究,沉积成岩作用的研究有了突破性的进展,有机地球化学大大促进了沉积成岩作用的研究。现在沉积成岩作用的研究已经成为包括大地构造、盆地分析、沉积学、物理化学、有机地球化学等学科、范围包括大、中、小尺度的各种作用的综合研究。对油气和固体矿产资源的需求促进了沉积成岩作用的深入研究,石油天然气的形成和聚集、储层性质的变化涉及沉积成岩的全过程;一些低温低压的层控固体矿产的富集涉及盆地沉积充填、盆山转化的岩相及流体的动力和物理化学行为,自上世纪70年代以来,研究工作都有重要进展。近年来有关海洋沉积的研究、硅、锰、碳酸盐等沉积物形成、大地构造背景、同位素、有机物形成演化等的成岩过程的研究都有重要进展,也成为研究的热点问题。 相似文献
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沉积盆地动力学与盆地流体成矿 总被引：13，自引：1，他引：12
刘建明 《矿物岩石地球化学通报》2000,19(2):76-84
沉积盆地实际上是地球动力学系统中一个巨大的动态化学反应器,其所处的地球动力学背景制约着它的边界条件、外部环境及其与外部环境的物质和能量交换。盆地内则是一个由固体无机,有机沉积物颗粒和盆地流体组成的多相反应体系,包括各种流-岩-烃及流-流反应、与有机质成熟演化有关的各种有机反应、在浅部还有细菌参与的生物化学反应。体系的上部边界不断有新沉积物颗粒和流体的加入,下部则有来自基底的流体或来自盆地边缘大气降水的加入循环,同时还有盆地的构造沉降和基底热能的交换,以及可能岩浆物质,能量的加入。沉积物的固结成岩和后生变化,以及其中储存的油气和金属矿床都是这个巨型化学反应器的一部分(并非全部)产物。因此,只有站在盆地动力学的高度,才可能从整体上去认识这些共存的复杂因素和作用过程的耦合关系,才可能从本质上理解有关矿床的形成机理和产出规律。 相似文献
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The Kathmandu and Banepa Basins, Central Nepal, are located in a large syncline of the Lesser Himalayas. The Older Kathmandu Lake evolved during the Pliocene and early Pleistocene; the Younger Kathmandu Lake, which is the focus of this study, is infilled with late Quaternary sediments. Three formations, arranged in stratigraphical order, the Kalimati, Gokarna and Thoka Formations formed during the infilling stage of this lacustrine basin. Structural and textural sedimentological analyses, a chemical survey across the basin and mineralogical investigations of fine‐grained sediments form the basis of this palaeogeographical study. The basin under investigation was covered by a perennial freshwater lake before 30 000 yr BP. The lake was infilled with alluvial and fluvial sediments delivered mainly from the mountains north of the basin. A fairly low gradient was favourable for the formation of diatomaceous earths, carbonaceous mudstones and siltstones, which were laid down in the centre of the lake and in small ponds. Towards the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. Crevasse splays and anastomosing rivers mainly delivered suspended load for the widespread siltstones and mudstones. The proximal parts of the alluvial–fluvial sedimentary wedge contain debris flows that interfinger with fine‐grained floodplain deposits. Three highstands of the water‐level (>30 000 yr BP, 28 000–19 000 yr BP, 11 000–4000 yr BP (?)) have been recognised in the sedimentary record of the younger Kathmandu Lake in the Late Quaternary. Second‐order water‐level fluctuations are assumed to be triggered by local processes (damming by tectonically induced landslides). First‐order water‐level fluctuations are the result of climatic changes. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
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The behavior of phosphorus is considered at major stages of the sedimentary cycle beginning with supply sources for its delivery into the ocean to precipitation and its sedimentation, localization and subsequent diagenetic redistribution in bottom sediments. River runoff represents the main phosphorus source in the ocean. It delivers annually about 1.5 Mt of dissolved phosphorus and more than 20 Mt of suspended phosphorus into the ocean. Up to 80% of the suspended phosphorus incorporated in the lithogenic material precipitates within submarine continental margins. Phosphorus dissolved in seawater repeatedly participates in biogeochemical processes owing to its assimilation by phytoplankton that annually consumes from 1.5 to 2.5 Gt of phosphorus. Dissolved phosphorus is incorporated in organic remains and precipitates from seawater by a biogenic mechanism, too. Only a part of phosphorus settled onto the bottom is buried in sediments. Due to reducing diagenetic processes, up to 30–40% of the primarily precipitated phosphorus diffuses from the upper layer of sediments into bottom water. Diffusion flux into the ocean significantly exceeds the supply of dissolved phosphorus from river runoff. The absolute mass phosphorus dispersed in sediments is several orders of magnitude greater than the mass concentrated in phosphorite deposits. However, the majority of phosphorite formation epochs coincide with the intensification of total phosphorus accumulation in marine sediments in conditions of humid climate, intense chemical weathering of rocks on continents, and considerable expansion of the oceanic shelf area. 相似文献
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Kais Jacob Mohamed Daniel Rey Belen Rubio Federico Vilas Thomas Frederichs 《Quaternary Research》2010,73(3):507-520
Integrated analyses of magnetic, geochemical and textural data on six cores from the northwestern Iberian continental shelf allowed the reconstruction of the paleoenvironmental evolution of this area since the last glacial maximum (LGM). Four sedimentary units were identified, representing a succession from fluvial and subaerial settings to high and finally low-energy marine deposits subsequent to the post-LGM sea-level rise. The uppermost unit was deposited during the Holocene and its magnetic properties were controlled by the interplay between detrital input and early diagenetic reductive dissolution of magnetic minerals. Identification of a primary steady-state early diagenetic signal allowed the recognition of periods of increased detrital input, bounded by intervals of lower detrital input and intensified reductive diagenesis related to intensified upwelling in the area. These paleoenvironmental alternations are consistent with the climatic evolution of the late Holocene. During the Roman Warm Period and Medieval Warm Period, the combined effect of greater humidity and intense agricultural and mining activities led to a greater erosion and transport of detrital sediments to the shelf. In contrast, enhanced diagenetic reduction intervals, caused by upwelling intensification, were roughly coincident with the colder Dark Ages and the Little Ice Age. 相似文献