Pleistocene, multiple-growth, lacustrine oncoids from the Poacher's Point Formation, Etosha Pan, northern Namibia

The Poacher's Point Formation is located within the Etosha Pan, northern Namibia. It contains Pleistocene stromatolites, ooids and silica nodules. The stromatolites were constructed by cyanobacteria in very lowenergy lacustrine conditions. The internal structure of the oncoids is complex and subdivided into shells by unconformities. Each unconformity-bounded shell probably represents stromatolite growth during a distinct lacustrine event. Shallower conditions are represented by colloform growth and deeper water by smooth oncoid mat morphology. The unconformities represent hiatuses of unknown duration. At least six stromatolite growth shells are recognized. Silica concretions which may have developed during shallowing lake stages are also present. The lack of clastic material in the carbonate lithologies suggests that the lake waters may have accumulated from collecting rainwater or have been spring fed. The lowest bed of the succession is a saline clay which represents a seasonally flooded salt pan deposited in an arid to semi-arid climate similar to that of today. The overlying carbonates represent sedimentation during a series of transient lacustrine events, representing intermittent wetter periods. Finally, due to progressive aridity, the lacustrine succession was terminated by calcrete development.     

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。     

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.     

豫西渑池寒武系第三统馒头组核形石中的Girvanella

寒武纪微生物岩中的Girvanella化石在地层中广泛分布,这为微生物参与微生物岩的形成提供了更直观的证据.豫西渑池地区寒武系第三统馒头组二段碳酸盐岩中发育了数量丰富的微生物岩类型——核形石,其核心、纹层及围岩中保存的Girvanella丝状体化石具有密度大、完整度高的特点.对核形石的微观分析表明,其核心、包壳等各个部位均大量保存有Girvanella丝状体.这些丝状体为任意弯曲、不分节、不分枝的管状集合体,单管直径约为20 μm,最大延伸长度约为400 μm.核形石核心以丝状体包绕的灰泥团块或砾屑为特征,局部可见丝状体包绕生物碎屑成核;包壳内的丝状体多出现在暗色纹层中,其分布表现为缠绕叠覆、穿层、凸起和匍匐(或水平状)这4种方式.在核形石的任意部位,Girvanella丝状体均具有相似的分布形式,表明它们的生长条件是相似的.核形石中Girvanella丝状体的大量出现与其形成于低能的滨岸鲕粒滩滩间或滩后低洼地带有关,这种低能条件对Girvanella的生长及保存十分有利.对研究区核形石中的Girvanella化石在微生物岩中的分布特征、微组构等的进一步研究将为寒武纪其他微生物岩的成因及微生物与环境的相互作用提供重要实证.      

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。     

鲕粒滩中光合作用生物膜构建的高能核形石:以辽西葫芦岛三道沟剖面寒武系张夏组为例

核形石是一种具有双重属性而较为特别的微生物碳酸盐岩:①作为一种包覆颗粒类型,核形石以不平滑的圈层和较大的大小被解释为微生物成因区别于成因存在着较大争议的鲕粒;②作为一种形成在围绕着核心的序列式纹层化作用过程中的球形或椭球型的生物沉积构造,核形石还常常被解释为微生物碳酸盐岩的一种类型而被归为球状叠层石。在辽西葫芦岛三道沟剖面寒武系苗岭统张夏组顶部的块状鲕粒滩相灰岩之中,密集发育着厘米级别大小的高能泥晶核形石。另外,以下重要特征将张夏组鲕粒灰岩中的核形石特征化,包括:①厘米级别的大小;②椭圆形为主的形态学特征;③特征性的纹层状构造;④核形石皮层中普遍发育而且较高密度保存的丝状蓝细菌鞘的钙化化石等等。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将存在着巨大的挑战,也尽管形成这些核形石复杂的生物膜钙化作用机理还没有得到精确的了解,但是,辽西葫芦岛三道沟剖面张夏组中的核形石,直接的微生物化石证据,尤其是在核形石内较为丰富的丝状蓝细菌鞘化石,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;同时,也为进一步了解形成在水柱中的放射鲕粒以及形成在海底的泥晶核形石之间复杂的形成机理的差异性,提供了一个较为典型的实例。     

Carbonate tufas as archives of climate and sedimentary dynamic in volcanic settings,examples from Gran Canaria (Spain)

Three Holocene tufas from Gran Canaria volcanic island were studied with the aim of deciphering their sedimentary evolution through space and time. Las Temisas tufa (south-eastern arid part of the island) is dominantly composed of oncoids, intraclasts, phytoclasts, coated stems, minor thin stromatolites, and a high amount of siliciclastics. It was deposited in a fluvial system with variable flow velocities and palustrine conditions areas, which alternated with high energy events. Azuaje tufa (northern humid part of the island) is composed of coated stems, stromatolites, oncoids and phytoclasts, with relatively low amounts of siliciclastics, suggesting slow-flowing and palustrine conditions and a relatively low incidence of (high energy) floodings. Los Berrazales tufa (north-west of Gran Ganaria, the most humid one), is mainly composed of coated stems and crystalline crusts, formed in a laminar flow regime. Dominant clastic sedimentation in Las Temisas and high calcite growth rates in Los Berrazales led to a poor development of stromatolites in comparison with Azuaje. Las Temisas and Azuaje deposits have similar upward evolution with decreasing trend in siliciclastics and increasing trend in carbonates. However, Las Temisas has higher siliciclastic and lower phytoclastic contents suggesting a less vegetated area and more arid climate than in the other deposits. Additionally, tufas record local events common in volcanic terrains. Azuaje presents three units bounded by erosive discontinuities, which reveal significant erosion by enhanced runoff that could be caused by loss of vegetation due to wildfires related to volcanic eruptions at headwaters. Las Temisas record a possible interruption in sedimentation represented by aligned boulders due to rockfalls from the hillsides. These deposits formed from waters with similar chemistry providing to the carbonates their similar signals in δ¹³C–δ¹⁸O stable isotopes and ⁸⁷Sr/⁸⁶Sr ratios like that of the volcanic rocks. This work shows how, in volcanic areas, tufas are unique archives of the climate, vegetation and volcanic-related processes, because all imprint the sedimentary regime of tufa deposition.     

Formation of silica oncoids around geysers and hot springs at El Tatio, northern Chile

Siliceous oncoids, up to 4 cm in diameter, are common on the laterally extensive sinter aprons that surround the spectacular geysers and hot springs at El Tatio in northern Chile. Many of these complex oncoids developed close to geyser and spring vents that discharge boiling water. Internally the oncoids, which are composed of precipitated amorphous silica, are formed of complex arrays of spicules and concentric laminae as well as detrital volcanic grains. Spicular growth is dominant in most examples. The formation and growth of the spicules and concentric laminae were mediated by a microbial community which included filamentous microbes, mucus, and possibly bacteria. The microbes and mucus were silicified by replacement and encrustation. In some laminae the filamentous microbes lay parallel to the growth surface; in other laminae most filaments forming the thin mats were suberect. Amorphous silica precipitated between the filaments occluded porosity and commonly disguised the microbial fabric. The oncoids grew on the proximal sinter aprons around the geyser vents and hot spring pools. Most growth took place subaerially with the silica delivered to the precipitation sites by splashing water from the geysers and/or periodic shallow flooding of the discharge aprons. Unlike silica oncoids at other geothermal sites, vertical growth of oncoids that formed in some rimstone pools was not limited by water depth.     

Facies analysis and sequence stratigraphy of a Miocene warm-temperate carbonate ramp,Montagna della Maiella,Italy

The uppermost Oligocene/Lower Miocene to Upper Miocene ramp carbonates from Montagna della Maiella (Italy) form a supersequence bounded by deeply incised truncation surfaces. This supersequence is subdivided into four sequences. Each sequence is composed of skeletal limestones in its lower part and marly limestones in its upper part. The lower parts of the sequences are foramol limestones, which suggest deposition in the warm-temperate climate zone. Changes in climate, oceanography and relative sea level combined to control sedimentation in the four sequences. In the lower parts of the two older sequences, the skeletal sands built dunes, suggesting high-energy conditions. The dominant skeletal grains in the oldest sequence are larger foraminifers and in the next sequence they are bryozoans; this change reflects cooling around the time of the Aquitanian/Burdigalian boundary. In the lower parts of the two younger sequences, of Middle and Late Miocene age, sediment sheets with red-algal–bryozoan oncoids suggest deposition under calmer conditions. Transgressive and highstand systems tracts are recognized in all sequences; a shelf margin systems tract may be exposed in the second oldest sequence. In contrast to the situation that exists when warm-water carbonates are deposited, sedimentation of the foramol limestones on this isolated ramp was unable to balance accommodation during sea-level rise; this led to hemipelagic sedimentation during sea-level highstands. Conglomerates resulted from reworking along flooding surfaces.     

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.     

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。     

Origin of the Lower Cambrian phosphatic bed at Vassbo, Sweden

An extensive phosphatic bed characterized by a black apatite-cemented sandstone is developed in the uppermost part of the Lower Cambrian formation over the entire Vassbo area, Sweden. The thickness of the layer is generally 0.1 m, increasing in places to 0.2 m. The lower boundary of the bed is transitional to a sandstone; the upper boundary displays a contact that is both sharp and irregular with a Middle Cambrian bituminous black shale.
The phosphatic bed was diagenetically formed by inorganic in situ precipitation of apatite in a sandstone at a depth of about 0.2 m below the sediment-water interface, where reducing conditions were developed, as a result of oxygen consumption during the degradation of organic matter. Sulphur isotope data from associated pyrite suggest that organic processes, such as bacterial sulphate reduction, were also involved in the formation of the phosphatic bed. The depositional environment of the bed was a marine shelf with a zero to very low sedimentation rate. The upper part of the apatite-cemented sandstone was partly reworked by bottom currents as demonstrated by a board containing numerous pebbles. Sedimentological as well as chemical data confirm a southern hemisphere position of the Baltic Shield in the early Cambrian in which an oceanic upwelling and high organic productivity was the probable source of the phosphorus along the palaeo-west-coast.     

四川盆地东北缘灯影组核形石特征及环境意义

四川盆地紫阳麻柳地区震旦系灯影组中见有核形石发育。通过野外剖面实测、偏光显微镜和场发射扫描电镜等手段,对核形石的形态、结构、形成环境及其主控因素进行研究,结果表明:麻柳灯影组核形石主要集中在灯四段中上部,其可分为四类:Ⅰ类正常核形石,Ⅱ类正常复合核形石,Ⅲ类薄皮核形石,Ⅳ类薄皮复合核形石,且以Ⅰ类为主。核形石形态以球形为主,可见少量椭球形、棒形以及不规则形态。紫阳麻柳地区灯影组核形石的形成受灯影组充足的物质基础、适宜的中-高能水动力条件以及低陆源碎屑物质的注入等因素共同控制。通过研究该区核形石沉积环境可知,灯影组四段的沉积环境为中-高能浅缓坡沉积相带。     

Phosphate stromatolites from condensed cephalopod limestones, Upper Jurassic, Southern Spain

Upper Jurassic phosphate stromatolites of the Almola Sierra (Southern Spain) encrust macrofossils and hardgrounds, and form oncoids included within pelagic, condensed fossiliferous limestones. Their accretion was determined by bacterially mediated precipitation of phosphate, by the trapping and binding of fine siliciclastics and pelagic biomicrite and by the encrustation of benthonic foraminifera. Phosphorous, trace elements and rare-earth elements were concentrated from degraded organic matter and seawater by stromatolite-building communities, which mediated the formation of phosphate-rich and Fe-Al-Si-rich organic gels under oxic conditions, favouring the precipitation of amorphous mineral precursors (ACP and Fe-Al-Si oxyhydroxides). The observed Ce-enrichment for some stromatolites is explained by oxidative scavenging of Ce⁴⁺ from seawater by Fe—Mn oxyhydroxides. The bacterially mediated gels were able to migrate and fill the voids of the stromatolite structure, and later changed to carbonate-fluorapatite, haematite and poorly crystalline Fe-rich clays under postoxic conditions. Phosphatization of trapped carbonate particles also occurred. The phosphate stromatolites formed on a sediment-starved pelagic swell, during periods of no carbonate sedimentation and hardground development. Stromatolite lamination provides evidence for rhythmic alternation between bacterially mediated phosphogenesis, sedimentation and erosion, suggesting episodic changes in the sedimentary environment. Although some of the parameters that controlled phosphate precipitation associated with the stromatolites (local high organic productivity, sediment starvation, moderate depth of deposition and physicochemical conditions) were similar to those found in modern and ancient phosphogenic settings, the palaeogeographical framework and the intensity of sedimentary processes were different to those of the World's major phosphorite deposits.     

川西北杨坝剖面埃迪卡拉系灯影组核形石组构特征

埃迪卡拉系灯影组核形石研究对揭示该期古环境特征及演化具有重要意义。在杨坝剖面埃迪卡拉系灯影组2段核形石发育段的宏观及微观描述（564块薄片）的基础上,分析了核形石组构、类型及垂向分布。该剖面核形石发育段共分为上、下两个亚段,厚度分别为109.53 m、126.2 m,核形石累积厚度分别为22.6 m、49.5 m。核形石核心及壳层的成分、组构、形态多样;核形石类型包括不规则状核形石、椭圆状核形石、次圆状—圆状核形石、帽状核形石等4种,每种类型的粒径、形态、层位分布、形成环境不同;粒度在垂向上表现为10个反旋回。整体上核形石发育不够完善,以薄皮核形石、弥散粒为主,代表着核形石发育的最初阶段。不规则状、次圆状、帽状核形石形成于弱搅动的浅水低能环境,包括潮下低能带、潮间带等;椭球状、圆状核形石形成于连续搅动的潮下高能带。核形石粒度差异大、形态多样、垂向多旋回变化的特征反映了埃迪卡拉纪灯影期海水受限、水体相对深浅及能量频繁动荡,同时受一定物源影响的特征。     

The hot spring and geyser sinters of El Tatio, Northern Chile

The siliceous sinter deposits of El Tatio geothermal field in northern Chile have been examined petrographically and mineralogically. These sinters consist of amorphous silica (opal-A) deposited around hot springs and geysers from nearly neutral, silica-saturated, sodium chloride waters. Water cooling and evaporation to dryness are the main processes that control the opal-A deposition in both subaqueous and subaerial settings, in close spatial relation to microbial communities. All fingerprints of organisms observed in the studied sinter samples represent microbes and suggest that the microbial community is moderately diverse (cyanobacteria, green bacteria, and diatoms). The most important ecological parameter is the temperature gradient, which is closely related to the observed depositional settings: 1) Geyser setting: water temperature = 70–86 °C (boiling point at El Tatio: 4200 m a.s.l.); coarse laminated sinter macrostructure with rapid local variations; biota comprises non-photosynthetic hyperthermophilic bacteria. 2) Splash areas around geysers: water temperature = 60–75 °C; laminated spicule and column macrostructure, locally forming cupolas (< 30 cm); predominant Synechococcus-like cyanobacteria. 3) Hot spring setting: water temperature = 40–60 °C; laminated spicules and columns and subspherical oncoids characterize the sinter macrostructure; filamentous cyanobacteria Phormidium and diatoms (e.g., Synedra sp.) are the most characteristic microbes. 4) Discharge environments: water temperature = 20–40 °C; sinter composed of laminated spicules and oncoids of varied shape; cyanobacterial mats of Phormidium and Calothrix and diatoms (e.g., Synedra sp.) are abundant. El Tatio is a natural laboratory of great interest because the sedimentary macrostructures and microtextures reflect the geological and biological processes involved in the primary deposition and early diagenesis of siliceous sinters.     

The role of endolithic cyanobacteria in the formation of lithified laminae in Bahamian stromatolites

The microboring activity of endolithic cyanobacteria plays a major role in the formation of the dominant lithified laminae in modern marine stromatolites in the Exuma Cays, Bahamas. These stromatolites are composed primarily of fine-grained carbonate sand that is trapped and bound by the filamentous cyanobacteria Schizothrix sp.. Periodic introduction of coccoid endolithic cyanobacteria Solentia sp. during hiatuses in stromatolite growth associated with very low rates of sedimentation results in the formation of lithified horizons, 200–1000 μm thick. These layers consist of micritized grains that are welded together at point contacts. The micritization is caused by extensive microboring and carbonate precipitation within boreholes concurrent with endolithic activity. Grain welding occurs when boreholes cross from one grain to another at point contacts. Thus, microboring destroys original grain textures but, at the same time, plays a constructional role in stromatolite growth by forming lithified layers of welded grains. These lateral bands of fused carbonate grains help to stabilize and preserve the stromatolite deposits.     

Microbial deposits in the aftermath of the end‐Permian mass extinction: A diverging case from the Mineral Mountains (Utah,USA)

The Lower Triassic Mineral Mountains area (Utah, USA) preserves diversified Smithian and Spathian reefs and bioaccumulations that contain fenestral‐microbialites and various benthic and pelagic organisms. Ecological and environmental changes during the Early Triassic are commonly assumed to be associated with numerous perturbations (productivity changes, acidifica‐tion, redox changes, hypercapnia, eustatism and temperature changes) post‐dating the Permian–Triassic mass extinction. New data acquired in the Mineral Mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures. These data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the Smithian–early Spathian. The studied section records a large‐scale depositional sequence during the Dienerian(?)–Spathian interval. During the transgression, depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral–microbial limestones, shallow subtidal marine sponge–microbial reefs to deep subtidal mud‐dominated limestones. Storm‐induced deposits, microbialite–sponge reefs and shallow subtidal deposits indicate the regression. Three microbialite associations occur in ascending order: (i) a red beds microbialite association deposited in low‐energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure; (ii) a Smithian microbialite association formed in moderate to high‐energy, tidal conditions where microbialites include stromatolites and associated carbonate grains (oncoids, ooids and peloids); and (iii) a Spathian microbialite association developed in low‐energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes. Data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase in accommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and the traditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned.     

山东省平邑盆地古近系湖相核形石特征及环境意义

山东省平邑盆地古近系官庄组湖相核形石较富集,其特征和分布受古沉积环境的控制。在总结前人研究的基础上,运用野外现场观测、偏光显微镜和扫描电镜观察、地球化学测试等技术手段,研究了本区核形石灰岩的岩石形态、结构、古生物、构造、元素地球化学等具有环境指示意义的特征,并据此将该区核形石分成A、B两类,它们之间根本性区别在于其形成的沉积环境的不同,A类核形石形成于翻转、滚动的强水动力的,水体较浅且较混浊的滨湖岸滩-浅湖浅滩环境;而B类核形石形成于水体较深、较平静的且较清澈的浅湖湖湾环境。研究结果表明,本区的核形石发育于一个处于湖侵体系域阶段的干热湖盆的缓坡地带。     

Sedimentary signatures of cyclic growth and destruction of stratovolcanoes: A case study from Mt. Taranaki, New Zealand

The long-term behaviour of andesite stratovolcanoes is characterised by a repetition of edifice growth phases followed by collapse. This cyclic pattern represents a natural frequency at varying timescales in the growth dynamics of stratovolcanoes worldwide. Around the > 130 ka Mt. Taranaki (Egmont volcano), New Zealand, coastal–cliff successions at 20–40 km distance comprise repeating packages of lithologically and sedimentologically distinctive mass-flow deposits. Varying depositional mechanisms and source properties of these units record growth and collapse cycles of the central edifice. These are used to construct a model for cyclic volcaniclastic sedimentation in the surrounds of stratovolcanoes. During edifice-construction phases, thick packages of tabular, predominantly monolithologic, hyperconcentrated-flow and debris-flow deposits accumulate with intercalated tephra beds. The mass-flow units commonly contain large proportions of fresh pumice or juvenile-lithic andesite. Intervals of quiescence separating eruptive periods are characterised by landscape re-adjustment, accompanied by deposition of fluvial and aeolian sediments, along with steady accretion of medial ash. In contrast, brief episodes of destruction are marked by wide-spread, distinctively clay-rich, polylithologic debris-avalanche deposits and related marginal debris flow units. The growth stages can be terminated by an eruption-triggered sector collapse, or by external forces once the edifice exceeds a critical stable height or profile (dependent on eruptive style and local geo-tectonic conditions). Once the edifice becomes metastable, regional tectonic earthquakes or shallow-level intrusion events are likely triggers for collapse. Although the resulting debris avalanches represent the greatest individual hazard from such andesite stratovolcanoes, their frequency is relatively low compared with other types of mass-flows generated during edifice-growth phases. Accurate forecasts of future hazard from mass-flow events are therefore dependent on recognition of both the frequency of a stratovolcano's growth cycle and its current position in that cycle.