四川盆地东部中二叠统茅口组顶部钙结壳的发现及其发育模式

对四川盆地东部涪陵地区角帮沟剖面中二叠统茅口组顶部钙结壳进行的岩石学和矿物学系统分析发现,该剖面中钙结壳的矿物组成和含量呈规律变化,可分为基岩带、过渡带及钙结带。岩石学分析表明,基岩带为灰白色中厚层泥晶生物灰岩,其内发育碳酸盐岩砂和岩溶角砾充填的岩溶系统;过渡带为黄白色、黄褐色、褐红色含燧石条带或团块状泥晶生物灰岩,其内发育钙质胶结的原地角砾充填的侵蚀漏斗;钙结带为局部钙结岩化的灰色亮晶生屑灰岩叠覆土黄色多孔块状钙结岩,其内发育多期裂缝、根模、肺泡结构和次生碳酸钙胶结物等典型钙结壳结构。矿物学分析表明,自过渡带至钙结带CaCO₃含量呈逐渐上升的趋势,推测内部次生碳酸钙逐步胶结富集是钙结岩形成的主要原因。结合四川盆地茅口组顶部以发育铝土质泥岩和铝土岩的岩石组合为特征,且其与中二叠世上扬子地区位于赤道附近的古地理背景相吻合,表明四川盆地东部地区茅口组顶部钙结壳形成于湿热气候条件下;并通过与经典钙结壳特征的比较,总结出湿热气候条件下的钙结壳发育模式。该研究成果不仅是对四川盆地中二叠统茅口组表生成岩作用产物的补充,而且还可为国内外同类型钙结壳研究提供详实的材料。     

Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania

Pedogenic calcretes are closely associated with Pliocene to Holocene wind-worked deposits of volcanic ash in the Olduvai and Ndolanya Beds of northern Tanzania. The typical profile with calcrete consists of an unconsolidated sediment layer, an underlying laminar calcrete, and a lowermost massive calcrete. The laminar calcrete is a relatively pure limestone, whereas massive calcrete is aeolian tuff cemented and replaced by calcite. An Olduvai calcrete profile can develop to a mature stage in only a few thousand years. Carbonatite ash was the dominant source for most of the calcite in the calcretes. Replacement was a major process in formation of the massive calcretes, and oolitic textures have resulted from micrite replacing pelletoid clay coatings around sand grains. Phillipsite and possible other zeolites were extensively replaced in the massive calcretes. Replacement of clay by micrite in the Olduvai calcretes is accompanied by dissolution or leaching of phengitic illite and the formation of clay approaching the composition of halloysite or kaolinite. In the upper calcrete of the Ndolanya Beds, montmorillonite was altered to a kaolinite-type mineral and to dioctahedral chlorite. Authigenic dolomite, zeolite, and dawsonite in the Olduvai calcretes probably received at least some of their components from replaced materials.     

Paleoclimatic implications (Late Cretaceous–Paleogene) from micromorphology of calcretes,palustrine limestones and silcretes,southern Paraná Basin,Uruguay

Sedimentologic and petrographic analyses of outcroping and subsurface calcretes, palustrine carbonates, and silcretes were carried out in the southern Paraná Basin (Uruguay). The aim of this work is to describe the microfabric and interpret the genesis of these rocks through detailed analyses, since they contain significant paleoenvironmental and paleoclimatic evolution information.The main calcrete and silcrete host rock (Mercedes Formation) is represented by a fluvial thinning upward succession of conglomerate and sandstone deposits, with isolated pelitic intervals and paleosoils. Most of the studied calcretes are macroscopically massive with micromorphological features of alpha fabric, originated by displacive growth of calcite in the host clastic material due to evaporation, evapotranspiration and degassing. Micromorphologically, calcretes indicate an origin in the vadose and phreatic diagenetic environments. Micrite is the principal component, and speaks of rapid precipitation in the vadose zone from supersaturated solutions. The abundance of microsparite and secondary sparite is regarded as the result of dissolution and reprecipitation processes.Although present, brecciated calcretes are less common. They are frequent in vadose diagenetic environments, where the alternation between cementation and non-tectonic fracturing conditions take place. These processes generated episodes of fragmentation, brecciation and cementation. Fissures are filled with clear primary sparitic calcite, formed by precipitation of extremely supersaturated solutions in a phreatic diagenetic environment. The micromorphological characteristics indicate that calcretes resulted from carbonate precipitation in the upper part of the groundwater table and the vadose zone, continuously nourished by lateral migration of groundwater.The scarcity of biogenic structures suggests that they were either formed in zones of little biological activity or that the overimposed processes related to water table fluctuations produced intense recrystallization completely obliterating the biogenic fabric.Limestone beds containing terrestrial gastropods are geographically restricted. Situated at the top of the calcrete successions, they exhibit brecciated and peloidal-intraclastic textures but lack lamination, edaphic structures, aggregates and vertical rhizoliths. This indicates they correspond to low-energy palustrine deposits, generated in shallow, local and ephemeral ponds developed in topographic depressions. When water table levels dropped, the palustrine deposits were exposed. This favours the presence of terrestrial gastropods, seeds and insect nests. The combination of calcretes and palustrine carbonates indicates periods and areas with a reduced clastic input and a predominantly semiarid climate, with well-defined humid and dry seasons.Characteristics of the later developed massive and nodular horizons of silcretes, such as, preservation of the internal structure of the host rock, the small areal extent, the formation of massive lenses, the complex pore infillings and the lack of a columnar upper section, indicate that they were generated from groundwaters. Every silcretized horizon shows different positions of the groundwater table and relates to the dissection of landscape.The age of calcretization and silcretization is bracketed between the Late Cretaceous (Campanian–Maastrichtian) and the Early Eocene. Paleoclimate indicates changing conditions from warm and humid at the end of the Cretaceous (Mercedes Formation) to semiarid and seasonal during Paleocene (groundwater calcretes and palustrine deposits) and subtropical and seasonal in the early Eocene (Asencio Formation).     

Early diagenetic spherulitic siderites from Pennsylvanian palaeosols in the Boss Point Formation, Maritime Canada

The Boss Point Formation of Maritime Canada comprises alternating successions of braidplain sandstone, lacustrine mudstone, and lacustrine fluvio-deltaic sandstone and mudstone. These rocks were deposited within an active strike-slip basin during the early Pennsylvanian (Westphalian A), at a palaeolatitude of 8°S. Palaesols comprise a small but distinct proportion of the formation, and typically occur at the tops of fining upward sandstone-siltstone cycles. Spherulitic siderites from the Boss Point Formation occur within sandy and silty palaeosols. They occur as ‘large’ (>200 μm) and ‘small’ (<200 μm) diameter spherulitic nodules within calcrete, and show either a radially arranged ferroan calcite and siderite spar morphology, or contain a core of ferroan calcite, surrounded by a later formed sheath of siderite. Analytical data indicate that with respect to Ca-Mg-Mn, wide compositional variation occurs within the siderites. The Boss Point Formation siderites are similar to, but contain less Ca for a given Mg/Mn ratio, than previously published siderite compositional data. Published data, together with the results from the Boss Point Formation, indicate that a continuum exists in freshwater siderites, between the relatively Mn-depleted and the Mg-enriched analytical fields that were previously reported. The relatively low Ca values determined in the Boss Point Formation samples reflect the general lack of calcium in the sedimentary basin during sedimentation. Both chlorite and smectite clays may have been important sources of iron. It is concluded that the spherulitic siderites are eogenetic, and formed within small anoxic ponds rich in organic matter, under reducing and low dissolved sulphate conditions. With time these pools dried up, ferroan calcite precipitating as calcrete nodules around the earlier formed siderite spherulites.     

Pedogenic-phreatic carbonates on a Middle Devonian (Givetian) terrigenous alluvial-deltaic plain, Gilwood Member (Watt Mountain Formation), northcentral Alberta, Canada

In the Muskeg Trough of northcentral Alberta the Gilwood Member contains widespread carbonate deposits that formed within terrigenous mudstone and sandstone hosts. Stratigraphic, depositional and petrographic relationships indicate that these carbonates represent calcretes and dolocretes. Calcretes, observed best with cathodoluminescence, display microcrystalline alpha fabrics, circumgranular cracks, root networks, displacive growth fabrics, elongate channel voids and rare coloform growths with flower spar. Similarly, dolocretes have microcrystalline alpha fabrics, brecciation, gradational contacts with host mudstones, extensive layered nodular horizons and are associated with anhydrite and pyrite. δ¹³C values range between ?7‰ to +1‰ and –6‰ to +3‰ for calcretes and dolocretes, respectively. Oxygen isotopes are more variable and differ with host lithologies. δ¹⁸O of calcretes ranges between ?11‰ to ?8‰ for sandstones and ?8‰ to ?3‰ for mudstones, whereas δ¹⁸O of dolocretes ranges between ?3‰ to 1‰ for marine mudstones and ?6‰ to ?2‰ for pedogenic mudstones. Regional mapping indicates that calcretes thicken towards the deepest parts of the Muskeg Trough. Widespread dolocretes extend beyond the eastern and western limits of Muskeg Trough and are useful marker intervals for regional correlations. Dolocretes of restricted lateral extent are found within gleyed palaeosol mudstones next to calcretized channel sandstones. Calcrete isotopic values are interpreted as indicative of carbonate precipitation from waters with meteoric water input. However, the higher δ¹⁸O values in dolocretes are indicative of a contribution from an isotopically heavier source such as seawater. Stratigraphically, calcretes are most common along the western and northern edges of Muskeg Trough; thus, calcrete accumulation was further controlled by meteoric water in-flow from the highland to the west and sluggish groundwater flow in Muskeg Trough. In contrast, regionally widespread dolocrete horizons appear to have formed from mixing of fresh waters derived from the highland to the west and seawaters introduced from the east. Regionally restricted dolocretes which are found next to channel sandstones formed from groundwater out-flow from the permeable channel sandstones which resulted in calcretization in channel proximal mudstones and dolomitization in channel distal mudstones.     

Quantitative analysis of the clay minerals in the Shurijeh Reservoir Formation using combined X-ray analytical techniques

The Shurijeh Reservoir Formation of Neocomian age is represented by a sandstone sequence, occasionally interbedded with shale, in the Gonbadli gas field, Kopet-Dagh Basin, Northeastern Iran. In this study X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques were used together to characterize the Shuirjeh clay minerals in 76 core samples collected from two deep Gonbadli wells. The results of XRF analysis showed high percentages of silicon and moderate to low percentages of aluminum, sulfur, calcium, potassium, sodium, magnesium, and iron in both wells. The XRD analysis indicated that the above elements were concentrated in the form of quartz, anhydrite, dolomite, calcite, plagioclase, K-feldspar, hematite, and clay minerals. Further XRD examination of the clay fraction revealed that illite, chlorite, and kaolinite were the major types of clay minerals. Unlike, glauconite, smectite, and a mixed layer clays of both the illite–smectite and chlorite–smectite types were observed only in very few samples. The percentages of individual clay minerals were determined using external standard calibration curves and successfully validated by a system of simultaneous linear equations acquired from detailed elemental information based on the XRF analysis. The error reached ± 5% for the main mineral constituent and ± 15% for minor minerals. A local regression relationship was also derived, based on the XRF elemental information, which can be used to estimate the clay contents of other Shurijeh drilled wells with data of pulsed-neutron spectroscopy tools. According to the proposed quantitative approach, the amount of illite varied considerably, reaching 18.3%. In contrast, the amounts of kaolinite and chlorite were generally small, i.e., less than 8.4%. The amount of total clay minerals changed greatly from a minimum of 5% to a maximum of 32.5%. An increase in illite with increasing burial depth and temperature was an obvious indication of deep burial diagenesis in this formation.     

Lithofacies associations and stable isotopes of palustrine and calcrete carbonates: examples from an Indian Maastrichtian regolith

Detailed information on semi‐arid, palustrine carbonate–calcrete lithofacies associations in a sheetwash‐dominated regolith setting is sparse. This is addressed by studying the Lower Limestone of the Lameta Beds, a well‐exposed Maastrichtian regolith in central India. The general vertical lithofacies assemblage for this unit comprises: (a) basal calcareous siltstones and marls with charophytes, ostracods and gastropods; (b) buff micritic limestones associated in their upper parts with calcretized fissure‐fill sandstones; (c) sheetwash as fissure‐fill diamictites and thin pebbly sheets, locally developed over a few metres; and (d) sandy, nodular, brecciated and pisolitic calcretes at the top. The sequence is ‘regressive’, with upsection filling of topographic lows by increased sheetwash. Lateral lithofacies change is marked, but there are no permanent open‐water lake deposits. In topographic lows close to the water table, marshy palustrine or groundwater calcretes formed, whereas on better drained highs, brecciation and calcretization occurred. Prolonged exposure is implied, suggesting that shrinkage was the main cause of brecciation. Evidence for rhizobrecciation and other biological calcrete fabrics is sparse, contrasting with the emphasis on root‐related brecciation in many studies of palustrine lithofacies. Stable isotope (δ¹⁸O and δ¹³C) values are consistent with the palustrine limestones being fed from meteoric‐derived groundwater with a strong input of soil‐zone carbon. There is overlap of both δ¹⁸O and δ¹³C values from the various palustrine and calcrete fabrics co‐occurring at outcrop. This suggests that, in groundwater‐supported wetlands, conversion from palustrine carbonate to calcrete need not show isotopic expression, as the groundwater source and input of soil‐zone carbon are essentially unchanged. Cretaceous–Tertiary δ¹⁸O and δ¹³C values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands. Hydrologically closed lakes and marine‐influenced water bodies tend to result in low negative palustrine δ¹⁸O and δ¹³C values. During brecciation and calcretization, the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland. Marked biological activity (e.g. rhizobrecciation or root mat development) during calcretization may lower δ¹³C values where C₃ plants are abundant but, in shrinkage‐dominated systems, δ¹³C values will be largely inherited from the palustrine limestones.     

Occurrence of calcrete in the Phu Kradung Formation of the Mesozoic Khorat Group, NE Thailand

The Phu Kradung Formation of the Mesozoic Khorat Group is deposited by meandering river system. Floodplain deposits in the Nong Bua Lamphu section, northeastern Thailand contain paleosols with abundant calcretes. Calcretes occur within about 60 horizons in the studied section. Occurrences of calcretes are related with traces of life, such as roots and burrows. Microstructures of calcretes are mixture of biogenic and non-biogenic origin. It is suggested that the calcrete formation in the Phu Kradung Formation was affected by abundant biological activity.     

东昆仑造山带东段下中三叠统研究进展

东昆仑造山带东段下中三叠统包括下三叠统洪水川群和中三叠统闹仓坚沟组.根据实体化石和遗迹化石的研究, 结合区域上的岩性对比, 将洪水川群划分为3个岩性单元, 即底部砂砾岩段、中部灰岩夹砂岩段和上部砂板岩段, 属于下三叠统印度阶和奥伦尼克阶.闹仓坚沟组包括闹一段砂砾岩和闹二段砂板岩, 从中三叠统早安尼锡期延续到中安尼锡期.砂岩的常量元素分析结果表明, 大部分砂岩为杂砂岩, 属于复理石盆地的浊积岩范畴; 砂岩稀土元素的特征值接近于活动大陆边缘和大陆岛弧元素的参数值.      

Calcretes in the Thar desert: Genesis,chronology and palaeoenvironment

The calcretes in the Thar desert occur in a variety of settings, including the piedmonts, sheetwash aggraded plains; and this study adds calcretes in regolith and colluvio-alluvial plains to the group of settings in which calcretes occur in the region. Field logs, morphological details and analytical data such as petrographic, cathodoluminescence and geochemical characteristics are described along with a discussion on their implications. Sand dunes and sandy plains dating to < 20 ka have weakly developed calcretes. The better-developed calcrete horizons occur in piedmonts, interdunes or in areas that have sufficient groundwater. Deep sections in the region show phases of calcrete development in aeolian sand aggradation at ∼ 150, ∼ 100, ∼ 60 and 27–14 ka. The extensive sheetwash plains have mature calcretes and date to mid-Pleistocene. Our studies indicate that these calcretes represent a hybrid process, where carbonate enrichment of the originally calcareous host occurred due to periodically raised groundwaters, and its differentiation into nodules occurred under subaerial environment i.e., after recession of groundwater. Deep sections also show a stack of discrete calcretes that developed in individual aggradation episodes with hiatuses as indicated by ESR dating results. Nodules display a multiplicity of carbonate precipi tation events and internal reorganization of calcitic groundmass. The process is accompanied by degradation and transformation of unstable minerals, particularly clays and with a neosynthesis of palygorskite. The ancient calcretes are dated from the beginning of the Quaternary to ∼ 600 ka and show more evolved morphologies marked by brecciation, dissolution, laminar growth on brecciated surfaces, pisolites and several generations of re-cementation. Mica/chlorite schists and such other rocks are particularly vulnerable to replacement by carbonate. In an extreme case, replacement of quartzose sandstone was observed also. The presence of stretches of alluvio-colluvial plains in an area presently devoid of drainage bespeaks of occasional high-energy fluvial regime, under a semi-arid climate. The mid-Pleistocene period saw a shift towards more arid climate and this facilitated sheetwash aggradation. Finally, during the late Pleistocene, aggradation of aeolian sands indicated a progressively drier climate. However, this does not find its reflection in stable isotope data. The amount of carbonate in the form of calcretes is substantial. The present studies indicate that aeolian dust or rainwater are minor contributors to the carbonate budget. A more important source was provided by the pre-existing calcretes in the sheetwash aggraded plains and detrital carbonate in the aeolian sediments. The original source of carbonate in the region, however, remains unresolved and will need further investigations. Electron spin resonance protocols for the dating of calcretes were developed as a part of this study and the results accorded well with geological reasoning     

The association of gold with calcrete

The discovery that Au accumulates in calcrete (pedogenic carbonate or caliche) was made in 1987 by CSIRO. Calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi-arid terrains around the world. The principal constituent of calcrete is calcite while Au is a noble metal. Calcrete has been a significant tool in a number of Au deposit discoveries, so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery. Numerous laboratory experiments, case histories and exploration models have been published (most from Australia) yet we do not fully understand the mechanisms involved. It is timely, therefore, twenty-five years on since the first publication of this phenomenon, to review this highly unusual but economically important association.Critical to any review on Au in calcrete is to first consider calcretes themselves. The nature of a particular calcrete, where it has formed and mode of formation is relevant to how, where and why Au accumulates within it. This review commences with a background, nomenclature, history, classification and some examples of calcrete types found near Au deposits. How calcretes form, their origins and the role of biota is considered. Their locations in the regolith and landscape, as well as exploration models for Au in calcrete are discussed. A section on the chemistry of Au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us. Following on is a summary of practical aspects of identifying, collecting and analysing samples for exploration purposes. Selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings. Concluding sections include a summary and implications of this accumulated knowledge to discovering Au deposits.     

THE TERTIARY STRATIGRAPHIC SEQUENCE OF YANYUAN BASIN IN THE SOUTHEASTERN MARGIN OF THE QINGHAI—TIBET PLATEAU

THE TERTIARY STRATIGRAPHIC SEQUENCE OF YANYUAN BASIN IN THE SOUTHEASTERN MARGIN OF THE QINGHAI—TIBET PLATEAUYoungGeologistofGMR(Qn979816 )andNSFCGeneralProjects(4980 2 0 13)     

An unsuccessful attempt at U/Th dating of soil calcretes from the Doukkâli area (western Morocco) and environmental implications

A large number of calcrete samples from topsoils of the Doukkâli area, western Morocco, were studied by U-disequilibrium series methods. The ²³⁰Th/²³⁴U ages are rather uniformly distributed between>350 and 2 ka BP. Homologous samples for occurrence and structure display quite different ages, the ²³⁰Th/²³⁴U ages are considered apparent. This is explained by repeated deposition of secondary calcite in the calcrete pores, which caused lowering of the original ages. However, the apparent ages may be considered minimum ages of the calcrete formation, indicating that calcium carbonate mobilization and deposition has been taking place repeatedly since >350 ka.

The age range obtained is quite comparable with that of calcretes from southern Spain, suggesting similar conditions for the calcrete formation in the two areas.     

'Pseudobreccias' revealed as calcrete mottling and bioturbation in the Late Dinantian of the southern Lake District, UK

Two types of ‘pseudobreccia’, one with grey and the other with brown mottle fabrics, occur in shoaling‐upward cycles of the Urswick Limestone Formation of Asbian (Late Dinantian, Carboniferous) age in the southern Lake District, UK. The grey mottle pseudobreccia occurs in cycle‐base packstones and developed after backfilling and abandonment of Thalassinoides burrow systems. Burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence. Mottling formed when an early diagenetic ‘aerobic decay clock’ operating on buried organic material was stopped, and sediment entered the sulphate reduction zone. This probably occurred during progradation of grainstone shoal facies, after which there was initial exposure to meteoric water. Microspar calcites then formed rapidly as a result of aragonite stabilization. The precipitation of the main meteoric cements and aragonite bioclast dissolution post‐date this stabilisation event. The brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete, which developed beneath palaeokarstic surfaces capping cycle‐top grainstones and post‐date all depositional fabrics, although they may also follow primary depositional heterogeneities such as burrows. They consist of coarse, inclusion‐rich, microspar calcites that are always very dull to non‐luminescent under cathodoluminescence, sometimes with some thin bright zones. These are interpreted as capillary rise and pedogenic calcrete precipitates. The δ¹⁸O values (?5‰ to ?8‰, PDB) and the δ¹³C values (+2‰ to ?3‰, PDB) of the ‘pseudobreccias’ are lower than the estimated δ¹⁸O values (?3‰ to ?1‰ PDB) and δ¹³C values of (+2‰ to +4‰ PDB) of normal marine calcite precipitated from Late Dinantian sea water, reflecting the influence of meteoric waters and the input of organic carbon.     

Groundwater calcrete deposits in Australia some observations from Western Australia

Groundwater calcretes are a non‐pedogenic form of calcrete occurring in broad fossil drainage systems. In Australia, they only occur north of about latitude 30°S and their formation requires arid conditions with very high potential evaporation as well as the periodic recharge of groundwater systems. They are linear, tabular limestone bodies, averaging about 10 m thick, occurring at or close to the surface and forming gentle mounds. The most extensive are located in the centre of the zone of distribution, where the climatic conditions are optimal, and are of the order of 100 km by 10 km. Some groundwater calcretes are fossil and strongly dissected, others are modern and still in process of formation.

A model of carbonate precepitation below the water table, i.e. in the phreatic zone, resulting in continuous calcrete growth, accounts for their morphology, their textures and the apparently inverted stratigraphy suggested by carbon‐14 dating. In areas of maximum growth, small anticlines develop which have structures engendered by both shear and concentric folding due to upward growth pressure.     

Soil-zone microfabrics in calcrete and in desiccation cracks from the upper Jurassic purbeck formation of dorset

Soil-zone microfabrics, alveolar-septal structure, needle-fibre calcite, and calcans are described from horizontal calcrete layers, stringers, and infillings in vertical desiccation cracks from an Upper Jurassic limestone in the Lower Purbeck Formation of Dorset. These calcrete palaeosols occur in an oolitic limestone (the Hard Cap) which represents former evaporitic lagoonal to carbonate mudflat environments. The calcretes occur 6-10 cm below the Great Dirt Bed, a former rendzina soil with rooted tree remains. Desiccation cracks and vugs formed in the oolitic sediment before Great Dirt Bed times. After formation of the Great Dirt Bed, soil-water rich in dissolved CaCO₃ preferentially flowed through natural conduits in the underlying sediment, namely the desiccation cracks and vugs. Calcrete precipitated in these cracks and vugs around decaying plant roots, and probably, during more arid (evaporative) climatic periods. These palaeosol microfabrics are among the first to be described from the British Jurassic and were probably preserved due to the semiarid Lower Purbeck climate where rapid oxidation of organic matter limited the amount and strength of carbonic acid generation, thereby limiting extensive dissolution of early formed soil-zone carbonate. Early diagenetic cementation of the sediment also aided microfabric preservation by sealing off soil-zone structures from subsequent diagenetic fluids.     

The role of fungal biomineralization in the formation of Early Carboniferous soil fabrics

Paleosols in the Lower Carboniferous limestones of South Wales commonly contain needle-fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils. The needle-fibre calcite occurs in two associations in the paleosols: as coatings on sediment grains and as rhizocretions. The former can be compared with the microbial grain coatings of Quaternary calcretes. The latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae, a symbiotic fungal sheath-root association. These findings suggest that biomineralization was important in the formation of soil fabrics during the Carboniferous as it is in present day soils.     

西湖凹陷渐新统花港组大型沿岸砂坝沉积特征及主控因素*

西湖凹陷位于东海陆架盆地东部坳陷带,是该盆地最大的富生烃凹陷,其主力储集层渐新统花港组的沉积环境与沉积体系类型一直存在较大争议。以岩心为基础,结合测井响应与地震反射特征,识别出西湖凹陷中东部花港组厚层砂岩主要有块状含砾中砂岩、冲洗层理中细砂岩、沙纹层理粉细砂岩共3种典型岩石类型,并通过统计分析沉积构造参数,明确花港组厚层砂岩主要发育在岸线附近的冲洗带,水动力强,波浪作用明显;该厚层砂岩在伽马测井曲线上整体表现为下部低幅锯齿箱状、顶部逐渐过渡为高幅锯齿状特征,地震上主要为中等振幅与连续性的、底平顶凸的丘状反射,为大型沿岸砂坝沉积。最后结合测井曲线样式、地震属性与含砂率拟合,刻画了沿岸砂坝的叠置样式与空间展布,确定西湖凹陷花港组大型沿岸砂坝主要分布在研究区东部三角洲前缘外侧,是由于基准面下降、水体变浅,早期形成的辫状河三角洲前缘受波浪作用改造而成。