河南登封地区寒武系第三统馒头组二段中的核形石及其意义

河南登封地区寒武系第三统馒头组二段发育有三种类型的核形石:球状、椭球状核形石与大型柱状叠层石伴生,形成于高能的潮下带;长卵形核形石与小型柱状叠层石伴生,形成于低到中等能量的潮间带;不规则状核形石与近水平状、缓波状叠层石伴生,形成于低能的潮上带和潮间带。从核形石的成因可以看出,核形石等微生物成因构造与后生动物扰动构造存在耦合关系,水动力条件是核形石形态类型变化的决定因素,泥质（陆源物质）供应是影响核形石生长及消亡的直接因素。     

广西泗顶泥盆系上统融县组中核形石的类型及其环境意义

广西泗顶泥盆系上统融县组中含有丰富的核形石,通过岩相剖面,100多块核形石手标本及50多块核形石薄片,对核形石的形态、纹层和核心等特征进行了详细的研究。核形石是由含粘液的藻类生长并捕获和粘结沉积物围绕核心而形成的颗粒。核形石的形态及纹层特征能反映它形成的条件。核形石不限定在动荡环境,如帽状、花瓣状核形石形成于浅水低能间歇性弱搅动的沉积环境。核形石的堆积环境与形成环境可以一致,也可以不一致。     

辽东半岛复州湾剖面寒武系第二统光合作用生物膜建造的核形石

作为一种经常大于鲕粒的包覆颗粒类型,核形石以其不平滑的圈层被解释为微生物成因,区别于成因存在较大争议的鲕粒,而且常与鲕粒和其他类型的碳酸盐颗粒相互共生;作为一种在围绕着生物碎屑和非生物碎屑核心的序列式纹层化作用过程中形成的球形或假球型生物沉积构造,核形石还常常单独产出和分布,所以又被解释为微生物碳酸盐岩,或者被归为球状叠层石。在辽东半岛寒武系第二统碱厂组和馒头组之中,厘米级别大小的核形石密集发育在三级层序的顶部,成为一种时间特化的相。另外,以下重要特征将辽东半岛寒武系第二统的核形石特征化,包括:(1)与凝块和微凝块共生;(2)多为球状和椭球状;(3)由不均一的非纹层状致密泥晶和微亮晶构成;(4)核形石皮层以及核形石间凝块中发育特别的蓝细菌鞘钙化化石等。尽管穿越成岩作用过滤器去解释古代核形石复杂的形成机理存在着巨大的挑战,也尽管形成这些核形石的复杂生物膜钙化作用细节需要更加深入地研究才能得到更好地了解,但是,辽东半岛第二统碱厂组和馒头组核形石中直接的微生物化石证据,尤其是核形石内较为丰富的钙化蓝细菌鞘化石,使其成为一个了解光合作用生物膜建造核形石的典型实例。     

云南施甸铺门前组灰岩的微相特征及其环境意义

<正> 保山地块位于怒江和澜沧江断裂带之间,西为西缅地块及怒江—龙陵—瑞丽缝合带,东为昌宁—双江缝合带,海进自泥盆纪开始,早石炭世发生海退,下石炭统香山组及铺门前组碳酸盐沉积仅局限分布于由保山—施甸呈袋形向南延伸的区域内(图1)。沉积间断发生于早石炭世末、晚石炭世初之间。而后,海进扩大,上石炭统丁家寨组覆于铺门前组灰岩之上。     

Decline and recovery of the Aptian carbonate factory in the southern Apennine carbonate shelves (southern Italy): Climatic/oceanographic vs. local tectonic controls

In interpreting the inception and demise of different carbonate depositional systems, climatic-oceanographic variations together with sea-level oscillations are commonly considered key elements. However, local tectonic controls cannot be ruled out. In attempts to discriminate among the main factors controlling the evolution of the southern Apennine mid-Cretaceous carbonate system, detailed facies analyses have been performed on Aptian–Albian carbonate successions in the Matese Group (southern Apennines). Since the mid Aptian, the analysed successions testify to a tectonically-driven topography, replacing the previous large tropical shallow-water domain with subdomains characterised by diversified sedimentological trends. Nevertheless, around the middle part of the Aptian, the studied successions register palaeoecological signals that cannot be linked exclusively with tectonic disturbance because of the coeval appearance of analogous signals at a global scale, including a significant change in biological assemblages and an outbreak of organisms indicative of stressful conditions in the water mass.Upper Bedoulian strata clearly record open marine settings characterised by a significant richness of the benthic communities. The biota included rudists, gastropods and echinoids plus many different benthic foraminifers and green algae. Rudists considered to be typically “tropical” forms (the caprinid Offneria nicolinae (Mainelli) and Offneria murgensis Masse, the requieniid Lovetchenia Masse and the monopleurid Agriopleura Kühn) characterise thick subtidally deposited strata in which large coral colonies and mollusc shells contributed to occasional storm-related skeletal concentrations.The analysed Gargasian strata show impoverished biota: caprinids totally disappear, both as in situ and storm-related layer components, and hermatypic corals are drastically reduced. Muddy lithofacies prevail in intertidal metric cycles in which cyanobacterial consortia, both in the form of dense laminae and coalescent oncoids, orbitolinids and small gastropods (cerithiids) suggest restricted, nutrient-rich water. Large oncoids of Bacinella irregularis/Lithocodium aggregatum and mollusc (mostly chondrodontids and gastropods) shell fragments significantly contribute to storm-related coarse skeletal intercalations, in which oligotrophic condition-adapted forms (e.g., hermatypic corals) are reduced or absent. This pattern suggests generalised conditions of stress in the water mass and in more marginal open areas.During the mid-Aptian interval, characterised by the flourishing of assemblages adapted to mesotrophic–eutrophic conditions, the southern Apennines shallow-water domain shows a progressive reduction of the previous mainly aragonite-dominated chlorozoan assemblages and an increase of calcite-dominated skeletal components, including rudists with thickened calcitic outer shell layers. Cyanobacteria and polychaetes characteristically marked the first phases of recovery in the shallow-water domains, rapidly evolving into more complex and differentiated assemblages. The outbreak of nerineid and acteonid gastropods seems to be related to a flourishing of cyanobacterial mats and related microphytae and also to the presence of polychaetes tubes (Thartarella cocumeriformis (Wahlman)). Moreover, the grazing activity of the nerineids favoured the flourishing of oyster-like bivalves. Among the rudists, the persistence and radiation of species adapted to a wide range of temperatures, such as Requieniidae and Monopleuridae as well as the first elevator Radiolitidae suggests some kind of oceanographic change (e.g., seawater chemistry and/or temperature).On the basis of the features described above, we propose the existence of a complex environmental scenario in which cooler conditions, presumably coupled with meso/eutrophic and locally oligophotic conditions, related to climatic/oceanographic global changes, cooperated in modifying the carbonate factory characterisation in a tectonically controlled setting.     

柴西南翼山地区藻灰岩层储层特征及成因分析①

对南翼山藻灰岩岩层储层特征及成因进行了系统研究。结果表明，南翼山藻灰岩沉积水体环境相对浑浊，但藻类仍能生长；该区藻灰岩岩石组成及类型复杂，主要的藻灰岩岩石类型有包心菜状叠层石、藻礁、指状叠层石、水平波状叠层石等；其主要为滨岸斜坡上低能潮上环境至高能潮下环境形成的沉积体，在重力等因素的作用下产生滑动或滑塌形成的微生物成因的滑塌混积岩；岩层物性好于其它岩层；藻灰岩层的成因决定了该地区单个沉积体规模小、分散、层薄。     

云贵地区二叠系瘤石灰岩的成因

云南东北部及贵州北部的二叠系中广泛发育了瘤石灰岩（亦称眼球石灰岩）。瘤石灰岩由颜色较浅、较纯净的石灰岩瘤和颜色较深、泥质含量较高、微缝合线发育密集的包层组成。组成瘤和包层的石灰岩类型一致,为生屑石灰岩、生屑质灰泥石灰岩、含生屑灰泥石灰岩或灰泥石灰岩。本次研究认为,瘤石灰岩是压溶成岩作用的产物,不是某种沉积环境的标志     

柴达木盆地西部地区新近系藻灰岩沉积特征及形成机制

柴达木盆地西部地区（简称“柴西地区”）新近系主要为陆源沉积岩,湖相碳酸盐岩亦广泛发育,其中藻灰岩的储集性和含油性最好。为进一步探究柴西地区新近系藻灰岩沉积特征及形成机制,主要通过详细的岩心和岩石薄片观察分析,对油泉子、黄瓜峁、南翼山、大风山等地新近系的藻灰岩进行研究。研究表明:1）柴西地区新近系藻灰岩主要有层纹石灰岩、叠层石灰岩和凝块石灰岩3种类型,以凝块石灰岩最发育。层纹石灰岩中纹层呈较连续且相互平行的水平纹层状,单个纹层最大厚度一般不超过0.5 m;叠层石灰岩主要有柱状叠层石灰岩和锥状叠层石灰岩2种类型,柱状叠层石在岩心上呈高宽比较小的灌木状,锥状叠层石则呈倾斜于地层面的指型密集簇状,叠层石最大单层厚度仅为0.25 m;凝块石灰岩在岩心上表现为表面粗糙的团块状,显微镜下藻凝块内部显微组构为不均匀的云雾状和海绵状。此外,有较多叠层石灰岩和凝块石灰岩呈不规则的砾屑状产出,且发育有明显的同生变形沉积构造。2）柴西地区新近系层纹石灰岩均为原地沉积,而叠层石灰岩和凝块石灰岩除原地沉积外,以原地垮塌沉积为主,以近原地滑塌沉积为次。3）柴西地区新近系藻灰岩同生改造有2个主要控制因素,一是藻灰岩与其基底灰质泥岩岩性硬度的差异,二是以底流为主、波浪为辅的水动力,次要因素为地震的振动、浅湖斜坡的坡度以及叠层石和藻凝块的倾斜生长等。     

Microencruster-microbial-cement framework of the Upper Jurassic reef developed on the slope of the intra-Tethyan carbonate platform (Bulgaria)

The carbonate succession in the Lyalintsi section of the western Moesian Platform (western Bulgaria) displays a shallowing-upward trend. Growth of the Tithonian–Valanginian coral biostromes and low-relief bioherms was preceded by Oxfordian–Kimmeridgian sedimentation of fine-grained peloidal-bioclastic limestones and Saccocoma-bearing limestones on the homoclinal ramp and the carbonate platform slope. In the late Kimmeridgian, boundstones with very rare corals, but with easily recognisable biohermal morphology, were developed. The main components of this reef are encrusting microorganisms, microbial crusts and synsedimentary cements. Microencrusters Labes atramentosa, Crescentiella morronensis, Perturbatacrusta leini and Radiomura cautica, as well as thin crusts of calcified sponges (sclerosponges), are the main biotic components. Corals (almost exclusively microsolenids) are sparse, whereas photophilic microencrusters (e.g., “Lithocodium–Bacinella”), are absent, although they are common in the overlying shallow-water part of the Lyalintsi sequence. Microbialites and synsedimentary cements provided additional support for the reef framework. The framework, especially the biotic components, and the reefal facies position within the sedimentary succession, implies that the high-energy upper slope of the carbonate platform was the depositional setting of the microencruster-microbial-cement reef studied. Encrusting microorganisms, except for C. morronensis and sponges, are only known from the intra-Tethyan platforms. This study supports conclusion of studies of coeval Alpine reefs that the presence of the microencruster-microbial-cement framework provides insight into the palaeobathymetry, palaeogeography and tectonic configuration of the intra-Tethyan carbonate platforms.     

平邑盆地下第三系官中段核形石成因分析

平邑盆地下第三系官中段核形石较为发育,类型多样。通过对其内部结构的研究发现核形石单元纹层有六种类型,作为核心的藻类仅在顶端发生碎屑粘结效应,而其底部则发育厚度小,规则的暗色纹层。纹层组合类型多样,主要受藻类发育程度,水动力强度和藻类种属的控制,是不同水动力环境下的产物。根据颗粒大小、纹层组合类型纹层层数、填隙物将核形石分为浅水高能成因核形石和较深水低能成因核形石。核形石成因分析和分类对沉积环境的判     

柴达木盆地西部地区古近系及新近系碳酸盐岩沉积相

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。     

Sedimentology and isotopic composition of Upper Palaeocene to Eocene non-marine stromatolites, eastern Ebro Basin, NE Spain

Stromatolites are abundant in Upper Palaeocene to Eocene fluvial deposits from the Eastern Ebro Basin, whereas they are scarce in the lacustrine facies. The fluvial stromatolites display a variety of growth forms: oncoids, domes, laminated crusts and locally irregular bioherms. These morphologies are related to the hydrological behaviour of the fluvial systems. Elongate domes, dome heads and ovoid to subspherical oncoids accreted in flowing water. Laminated crusts and small-sized cylindrical to lenticular oncoids formed during low-discharge and ponding phases. In the lacustrine environments only small-sized ovoid oncoids occur. The main characteristics of the Ebro Basin non-marine stromatolites are: (1) laminations and concentric banding exhibited by all morphologies; (2) the occurrence of well-preserved cyanobacterial filaments in several laminae, suggesting that these microbiotas are the major contributors in growth; (3) the asymmetrical growth of cortices in domes and oncoids with polar thickening on the upper side suggesting in situ growth under low energy conditions. δ¹³C values of carbonate stromatolites range from - 11.3%o (Late Palaeocene) to — 4.4%o (Middle-Late Eocene). During the Late Palaeocene, under tectonically quiescent conditions, the δ¹³C values mainly reflect the influence of soil-derived CO₂. During the Eocene, the progressive uplift of the surrounding catchment areas led to an increase of exposures of Mesozoic rocks. Consequently, the δ¹³C values of the Eocene stromatolites reflect the influence of the CO₂ derived from the dissolution of Mesozoic marine carbonates. There is a 2.5%o shift in δ¹⁸O values of stromatolites from the Late Palaeocene to Middle-Late Eocene which is consistent with the variation in δ¹⁸O of precipitation due to changes in altitude of the catchment area during this time. The isotopic values of all Middle-Upper Eocene laminated crust samples show distinct covariant trends, suggesting that these stromatolites were formed either in ponding zones of fluvial channels or in disconnected pools developed during low-discharge episodes.     

四川江油马角坝地区船山组核形石基本特征及其环境意义

四川江油马角坝地区船山组核形石十分发育,类型多样。基于对其内部显微结构的研究发现核形石核心和纹层各有5种类型,并伴随有3种圈层"构造"。基于对其核心、纹层和形态的研究将研究区核形石分为大球状核形石、小球状核形石、长棒状核形石、帽状核形石、不规则状核形石和复合核形石等6大类,并将它们划分成3种沉积环境,即台地边缘滩、开阔台地和局限台地,并探讨了碳酸盐岩地层中核形石的分布、形成环境和沉积环境的耦合关系。高含量、大颗粒、纹层厚度大于核心厚度、纹层复杂、核心组分与基质组分存在较大差异,亮晶胶结的核形石的形成环境与沉积环境不一致,即核形石形成于水动力较弱的局限台地或开阔台地环境,强水动力使核形石发生磨圆,基质生物碎屑破碎,进而沉积于水动力较大的台地边缘滩环境。中等含量、中等粒度、纹层简单、与完整生物碎屑共存且粒径类似、亮晶胶结的核形石的形成环境与沉积环境类似,即部分生物碎屑在中等水动力条件下,处于悬浮状态,捕获和粘附碳酸盐质点,当个体增大和水体环境变化时,核形石就地沉积于中等能量的开阔台地环境。含量低、粒径变化大、纹层厚度小于核心厚度、纹层简单、形状易受核心的影响,纹层外壳凹凸不平,基质生物含量低、泥晶基质含量特别高的核形石的形成环境与沉积环境一致,即形成并沉积于水动力偏弱的局限台地环境。     

Cyanobacterially generated peloids in laminated,organic-matter rich,limestones: an unobtrusive presence

The late-Kimmeridgian Southern Jura (France) is a carbonate platform where laminated limestones accumulated locally (notably at Orbagnoux). The two types of laminations encountered result either from particle settling in a very quiet environment or from bacterial mat growth. In the latter case, as revealed by acid etching, the dark-coloured laminae result from filamentous cyanobacteria developments, whereas the light-coloured laminae appear, surprisingly, to comprise packed peloids. These peloids are interpreted as originating from the in vivo or shortly post mortem calcification of sheaths of coccoid cyanobacteria in an environment loaded by temporary excess alkalinity (and possible H₂S emissions). Thus, a simple etching treatment reveals the unsuspected, albeit prominent, role of cyanobacteria in the genesis of the limestones from Orbagnoux. As such facies are widespread in Late Jurassic records, the role of cyanobacteria in their formation deserves re-evaluation.     

中国南方晚震旦世沉积相及磷块岩的时空分布

本文扼要阐述扬子地台及邻区晚震旦世地层的研究现状,沉积相带(台地相、斜坡相及盆地相)及古地理轮廓、磷块岩的时空分布(包括地理、地层及沉积相的分布)。同时还着重指出了台地边缘及斜坡相带的上部是寻找磷块岩的有利地区。     

顺托果勒奥陶系一间房组超深层灰岩储层类型及储集空间定量表征

顺托果勒地区位于塔里木盆地北部坳陷带顺托果勒低隆起之上。该区中奥陶统一间房组发育台地相碳酸盐岩沉积,地层厚度为168～220m,埋深主要在6300～7850m,超深层储层是研究的关键。经过系统的岩心、铸体薄片、电镜观察和岩心微纳米CT检测分析,发现该套储层以致密灰岩储层为主,包括砂屑灰岩、凝块灰岩、藻粘结灰岩和藻灰岩等岩石类型。储集空间包括生屑遮避孔、铸模孔、粒内孔、晶内孔、微裂缝和溶蚀孔洞,其中粒内孔是主要储集空间类型。物性分析和储集空间的三维定量表征表明,储层以台内滩相亮晶藻屑砂屑灰岩和凝块灰岩为最好,孔隙度主要为2%～4%,少量可达12.14%;其次是藻粘结灰岩和藻灰岩。构造热液交代作用形成蚀变灰岩,发育溶蚀孔洞、微孔隙和微裂缝,构成良好储集空间体系,从而形成优质储层,这对超深层天然气勘探具有重要价值。     

Mg fractionation in crustose coralline algae: Geochemical, biological, and sedimentological implications of secular variation in the Mg/Ca ratio of seawater

The Mg/Ca ratio of seawater has varied significantly throughout the Phanerozoic Eon, primarily as a function of the rate of ocean crust production. Specimens of the crustose coralline alga Neogoniolithon sp. were grown in artificial seawaters encompassing the range of Mg/Ca ratios shown to have existed throughout the Phanerozoic. Significantly, the coralline algae’s skeletal Mg/Ca ratio varied in lockstep with the Mg/Ca ratio of the artificial seawater. Specimens grown in seawater treatments formulated with identical Mg/Ca ratios but differing absolute concentrations of Mg and Ca exhibited no significant differences in skeletal Mg/Ca ratios, thereby emphasizing the importance of the ambient Mg/Ca ratio, and not the absolute concentration of Mg, in determining the Mg/Ca ratio of coralline algal calcite. Specimens grown in seawater of the lowest molar Mg/Ca ratio (mMg/Ca = 1.0) actually changed their skeletal mineralogy from high-Mg (skeletal mMg/Ca > 0.04) to low-Mg calcite (skeletal mMg/Ca < 0.04), suggesting that ancient calcitic red algae, which exhibit morphologies and modes of calcification comparable to Neogoniolithon sp., would have produced low-Mg calcite from the middle Cambrian to middle Mississippian and during the middle to Late Cretaceous, when oceanic mMg/Ca approached unity. By influencing the original Mg content of carbonate facies in which these algae have been ubiquitous, this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the Phanerozoic. The crustose coralline algae’s precipitation of high-Mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of CaCO₃. However, the algae’s nearly abiotic pattern of Mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal Mg incorporation. Therefore, the Mg/Ca ratio of well-preserved fossils of crustose coralline algae, when corrected for the effect of seawater temperature, may be an archive of oceanic Mg/Ca throughout the Phanerozoic. Magnesium fractionation algorithms that model algal skeletal Mg/Ca as a function of seawater Mg/Ca and temperature are presented herein. The results of this study support the empirical fossil evidence that secular variation of oceanic Mg/Ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time.     

Bivalves and oncoids as palaeoenvironmental indicators at Late Jurassic and Early Cretaceous dinosaur sites from Spain

Bivalves from the Late Jurassic and Early Cretaceous stratigraphic section at Las Zabacheras (Galve Sub-basin Teruel, northern Spain), are reviewed from both systematic and palaeoautoecological perspectives. For this study the Villar del Arzobispo Formation, well known for important dinosaur occurrences, was sampled from the boundary with the underlying Higueruelas Formation (Late Jurassic), to the first levels of the overlying El Castellar Formation (Early Cretaceous). The bivalve taxa have Late Jurassic affinity, pointing to a possible Jurassic-Cretaceous boundary towards the top of the Villar del Arzobispo Formation. We have sampled oncoids, whose nuclei are bivalves, through the section to study environment change in this lithostratigraphical formation. Geochemical trace elements and δ¹³C and δ¹⁸O stable isotope analysis of the oncoids enable us to determine the conditions in which the microbialites were formed and provide further palaeoenvironmental data from the deposits containing the bivalves. Bivalve taxa change from the lower part of the Villar del Arzobispo Formation, where Ceratomya excentrica and Unicardium cf. subregulare are characteristic of marine conditions, becoming more continental towards the top of the formation with the presence of Unionoidean bivalves, and in the “Wealden” facies of the El Castellar Formation, where Teruella gautieri, has been found. Bivalves and oncoids allow us to recognize continental conditions where the first dinosaur of Spain, the sauropod Aragosaurus ischiatus was found, in an open water system, where there was limited evaporation, and with enough energy to produce well oxygenated water.     

In situ accretion mechanism of concavo-convex lacustrine oncoids ('swallow nests') from the Oligocene of the Mainz Basin, Rhineland, FRG

Oligocene oncoids from the lacustrine Süβwasserschichten of the Mainz Basin exhibit a pustular surface and a characteristic cavity on their undersides, which gave rise to their designation as ‘swallow nests’. Oncoids grew in situ, under low-energy conditions during a pause in sedimentation. Three features indicate that they were never overturned: (1) oncoids can be placed in a stable position on a smooth surface; (2) laminae, which are mostly concentric around the entire oncoids, display polar thickenings on the upper side during all growth stages of oncoids; (3) the depression is always situated on the stable side of oncoids. Development of this cavity is favoured by unionid shells, in the stable position, serving as oncoid nuclei, but occurs also under flat nuclei. It is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions, on a fine-grained substrate, and is considered as diagnostic of the in situ growth of oncoids. Oncoid cortices are mainly formed by a Scytonema-like cyanobacterium, whose carbonaceous organic remnants are occasionally preserved. Growth stages of individual oncoids were image-processed and compared by means of shape parameters. Due to optimum growth conditions, cyanobacteria try to occupy the entire available space; hence, oncoids tend to achieve a spherical form even during in situ growth. Renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobacterial growth.