Laminated Eocene maar-lake sediments from Eckfeld (Eifel region, Germany) and their short-term periodicities

The Eocene age Eckfeld/Eifel maar lake is interesting both because of the excellently preserved fossil remains and also for the laminated sediment sequence deposited under subtropical conditions. The latitude, however, has a higher seasonal insolation difference than comparable recent environments due to the northward shift of climatic zones during the Eocene. The overall aim of this investigation was to compare sediment types and lake development of a Tertiary (subtropical) maar lake with recent maar lakes, to discuss the origin of laminations, and search for their possible periodicities. The main sediment types, which were exposed during recent paleontological excavations, are laminated oilshales, partly laminated bituminous siltstones, and graded beds (mostly turbidites), which sometimes include laminated clay tops. Some deeper parts of the profile drilled in 1980, are dominated by diatomaceous laminites. The amount of organic carbon (around 8.5%) is comparable with recent lakes with low sedimentation rates. The high C/N ratios (25–30) of the Eckfeld sediments, as well as the fact that the highest C_org values are found in the graded beds, shows the importance of terrestrial plant debris for these lake sediments. Lamination of the oil shales was caused by the complex interrelationship of clastic supply (mineral and plant detritus), autochthonous organic deposition (mainly green algae), and early diagenetic siderite formation. The darker sublaminae represent the coarser clastic (organic and inorganic) input together with the autochthonous supply from the lake (green algae), whereas the lighter sublaminae are composed of settled fine-grained, mostly siliciclastic detritus derived from the probable yearly turnover of the density stratification of the lake. The thickness of the fine siliciclastic and organo-clastic layers, the morphology and palaeogeographical situation of the maar lake, and the occurrence of algal layers, strongly suggest that the laminated sediments are varves (although there is no definitive proof of this). Thickness variations from ca. 700 oil shale laminae were measured and computed. Time series analysis yielded at least one certain period of 5.5 yr, accepting that the laminae are indeed varves. The duration of the laminated lake sequence from Eckfeld maar, as deduced from sedimentation rates of the different sediment types is ca. 82,000 yr and, thus, within the typical life time of other small maar lakes.     

The late‐stage “ferruginization” of the Ediacara Member (Rawnsley Quartzite,South Australia): Insights from uranium isotopes

The paleoenvironmental setting in which the Ediacara Biota lived, died, and was preserved in the eponymous Ediacara Member of the Rawnsley Quartzite of South Australia is an issue of long‐standing interest and recent debate. Over the past few decades, interpretations have ranged from deep marine to shallow marine to terrestrial. One of the key features invoked by adherents of the terrestrial paleoenvironment hypothesis is the presence of iron oxide coatings, inferred to represent the upper horizons of paleosols, along fossiliferous sandstone beds of the Ediacara Member. We find that these surficial oxides are characterized by (²³⁴U/²³⁸U) values which are not in secular equilibrium, indicating extensive fluid‐rich alteration of these surfaces within the past approximately 2 million years. Specifically, the oxide coatings are characterized by (²³⁴U/²³⁸U) values >1, indicating interaction with high‐(²³⁴U/²³⁸U) fluids derived from alpha‐recoil discharge. These oxides are also characterized by light “stable” δ^238/235U values, consistent with a groundwater U source. These U isotope data thus corroborate sedimentological observations that ferric oxides along fossiliferous surfaces of the Ediacara Member consist of surficial, non‐bedform‐parallel staining, and sharply irregular patches, strongly reflecting post‐depositional, late‐stage processes. Therefore, both sedimentological and geochemical evidence indicate that Ediacara iron oxides do not reflect synsedimentary ferruginization and that the presence of iron oxides cannot be used to either invoke a terrestrial paleoenvironmental setting for or reconstruct the taphonomic pathways responsible for preservation of the Ediacara Biota. These findings demonstrate that careful assessment of paleoenvironmental parameters is essential to the reconstruction of the habitat of the Ediacara Biota and the factors that led to the fossilization of these early complex ecosystems.     

Facies distribution patterns of some marine benthic faunas in early paleozoic platform environments

Worldwide Late Cambrian—Silurian lithofacies patterns indicate that the platforms of that time were sites of accumulation of two essentially different rocks suites: the platform carbonate rocks and the platform terrigenous rocks. Most of the platform rocks accumulated as sediments in shallow marine environments similar to those of the present but far more widely spread.Present-day marine benthic faunas are distributed in depth zones which are primarily controlled by temperature. Faunas tend to occur in substrate-related discrete clusters (communities) within each life zone; similar substrates in different depth zones commonly have different faunal associations. Individual phyletic stocks may encounter environmental optimum or near-optimum conditions in certain areas, that commonly are revealed by an abundance of species and individuals within species in each stock. Environmental optimum conditions depend upon availability of food that may be utilized, modes of feeding of the animals present, water motion, and substrate, among other factors. Organisms in past seas were distributed in patterns similar to those of the present.Carbonate platforms were particularly widespread during the latest Cambrian—Early Ordovician. Intertidal environments spread widely across those platforms during that time and characteristic faunal associations developed in them. Saukiid and related tribolites dominated latest Cambrian carbonate platform intertidal faunas. The Early Ordovician carbonate platform intertidal was dominated by archeogastropod-nautiloid cephalopod faunas. These animals were joined by tabulate corals and certain brachiopods during the latter part of the Ordovician and Silurian as prominent faunal elements in the carbonate platform intertidal—shallow subtidal. Cruziana and related trace fossils, bivalves, and certain tribolites (notably homalonotids and dalmanitids) dominated most terrigenous platform intertidal—shallow subtidal faunas of the Ordovician and Silurian.Articulate brachiopods (primarily orthoids, strophomenoids, and rhynchonelloids) appear to have been relatively prominent during the Early Ordovician in shallow subtidal environments on both carbonate and terrigenous platforms and to have spread down the bathymetric gradient into increasingly deeper subtidal areas of both platforms during the latter part of the Ordovician. Tribolites dominated faunas in relatively moderate to deep subtidal environments on both platforms during the early part of the Ordovician. They were gradually replaced by brachiopods in first the shallower, and later the deeper subtidal as dominant members of the faunas. Brachiopods (primarily pentameroids and spiriferoids) dominated nearly all Silurian warm-water subtidal environments from the shallow subtidal to the edges of the platforms.Platform uplifts in the Middle Ordovician and glacio-eustatic sea-level fluctuations in the Late Ordovician caused environmental changes across the platforms that were accompanied by marked replacements among marine benthic faunas in all environments. The distribution of Ordovician carbonate platforms and glacial deposits suggests that an Ordovician polar region may have been close to present-day equatorial Africa and that Ordovician warm temperate-tropical regions lay close to the present-day North Pole.     

A taphonomic approach to the genetic interpretation of clypeasteroid accumulations from the Miocene of Tarragona,NE Spain

Belaústegui, Z., Nebelsick, J.H., Gibert, J.M. de, Domènech, R. & Martinell, J. 2012: A taphonomic approach to the genetic interpretation of clypeasteroid accumulations from the Miocene of Tarragona, NE Spain. Lethaia, Vol. 45, pp. 548–565. Clypeasteroid accumulations are common in Cenozoic shallow marine sediments, particularly in the Neogene, as they are also in analogue modern environments. In this article, four clypeasteroid accumulations from Miocene (Serravallian) shallow marine carbonates of the El Camp de Tarragona Basin (NE Spain) are studied. Two of them are dominated by flat‐shaped Parascutella, whereas the other two consist exclusively of bell‐shaped Clypeaster. The combination of the taphonomic analysis of the tests, the palaeoecological and taphonomic information provided by the associated fossils, and the stratigraphical and sedimentological context of each one of these beds allows interpreting them as autochthonous/parautochthonous accumulations resulting from in situ reworking in moderate energy settings, except for one that constitutes a true sand dollar coquina deposited by a storm event. This contribution explores the potential of thorough taphonomic analysis of marine invertebrate skeletal concentrations and substantiates the importance of clypeasteroid echinoids as producers of shell beds in the Neogene. □ Clypeasteroids, Echinoids, Miocene, Taphonomy, Tarragona.     

Late Silurian plant microfossil assemblage from Guangyuan,Sichuan, China

A dispersed plant microfossil assemblage is described from Late Silurian deposits from Guangyuan, Sichuan, China. These strata are interpreted as nearshore, shallow marine deposits, and brachiopods suggest a late Ludlow–early Pridoli age. The palynomorph assemblage is dominated by terrestrial forms, including cryptospores and trilete spores, tubular structures and cuticle-like sheets, although rare marine acritarchs are also present. This microfossil assemblage is comparable to coeval assemblages from around the world (South and Southwest Wales; Libya; Canada; Southeastern Turkey; Northwest Spain; and Jiangsu, China). The sporomorphs from this assemblage indicate the existence of early land plants during the late Ludlow–early Pridoli in Guangyuan, Sichuan, China; and suggests that floras of this age were cosmopolitan and exhibited little palaeogeographical differentiation.     

广西、贵州和川东二叠纪生物礁的钙藻化石及其古生态环境

广西,贵州和川东二叠纪礁相岩石和礁后相岩层内获得了红藻Solenoporella,Gymnocodium,Permocalcu-lus;绿藻Anthracoporella,Mizzia以及显微疑难藻类化石Pseudovermiporella,Tubiphytes等,除了广西隆林有中二叠世茅口期藻类化石外,其它均属于晚二叠世乐平世的分子,这些藻类植物一般生活于热带和亚热带,正常盐度的浅海水内,其水深不大于30m,川东,鄂西晚二叠世礁属于浅水海绵礁,而非深水礁。     

Trilobites from the Silurian “Lower Red Beds” of Wuhan,South China: stratigraphic and paleogeographic implications

The age of the Silurian “Lower Red Beds” (LRBs), which occur widely in South China, has long been controversial because of the scarcity of index fossils. Here, we report a fauna with abundant trilobites from the LRBs in Wuhan, Middle Yangtze region, consisting of Aulacopleura (Paraaulacopleura) pijiazhaiensis, ‘Encrinuroides’ yichangensis, Gaotania hubeiensis, Hyrokybe gaotanensis, Japonoscutellum yichangense, Latiproetus latilimbatus, and Ptilillaenus lojopingensis. These trilobites are common in the Leijiatun, upper Xiangshuyuan, Shihniulan, and Lojoping formations in the Upper Yangtze region, and belong to the middle to upper Aeronian Ptilillaenus or ‘Encrinuroides’–Ptilillaenus–Latiproetus assemblages. The discovery of this trilobite assemblage suggests that the LRBs are probably partly of middle–late Aeronian age in the Middle Yangtze region. Thus, the lower boundary of the LRBs is likely diachronous in the Yangtze region, ranging from the middle–late Aeronian to the early Telychian. The discovery of the Ptilillaenus assemblage in the Wuhan LRBs expands the paleogeographic distribution of this assemblage and provides evidence of deeper depositional environment of the LRBs in the study area versus shallow waters in other areas of the Yangtze region.     

从塔北隆起奥陶纪钙藻化石探讨奥陶纪的古环境

塔里木盆地塔北隆起轮南46井、英买1、2井奥陶纪石灰岩内含有大量的钙藻化石、蓝细菌以及疑难微体化石。这些钻井岩芯均在深达5000-6200m的地下深部取得。钙藻主要为绿藻类的Dasyporella,Ver‘miporella,Moniliporella以及?Plexa;红藻类的管孔藻类Solenoporaceans;钙化蓝细菌则有Girvanella,Botomaella,?Subtifloria等;疑难微体化石有Bevocastria,Nuia,Rothpletzella。这些钙藻生活于热带或亚热带正常盐度的浅海水内,其水深不到20m。世界各地的奥陶纪Vermiporella均位于古赤道的两侧,这表明它们是在气候炎热、温暖海水中生活的一类海洋藻类。Girvanella以藻灰结核和内碎屑最为常见,某些球粒可能代表Girvanella破碎后形成的单管或小棒。塔里木盆地钙藻植物群相似于哈萨克斯坦、波罗的海周围地区以及北美同时代植物群,这表明这些钙藻和蓝细菌化石具有遍布于全球的性质。塔北隆起早、中奥陶世沉积属于典型的碳酸盐岩台地相沉积。到晚奥陶世时,碳酸盐岩沉积被浅水陆棚沉积所取代,以陆源碎屑岩为主,夹少量的碳酸盐岩。     

Climatic control on vegetation and sedimentary dynamics during the Miocene (Burdigalian to Langhian) in northeastern Tunisia

Palynological and sedimentological analyses were performed on Miocene sediments of North-East Tunisia in order to detect the changes in depositional environments, including those linked to eustasy, along with changes in vegetation and climate. The specific integration of palynological (pollen and dinocysts) and sedimentological (including facies analysis) data indicate that shallow marine settings persisted until the early Burdigalian–Langhian, and that open marine environments developed progressively in the late Langhian. Since the early Serravallian, deltaic environments developed under a fluctuating, but predominantly warm climate. The palynological data support a subtropical climate during the Burdigalian, with tropical conditions prevailing at the Langhian–Serravallian transition. The observed high frequency values of megathermic and mega-mesothermic pollen taxa represent the vegetation response to the Miocene climatic optimum (MCO).     

Oldest known Dicoelosia and Epitomyonia,deep water brachiopods from the Beiguoshan Formation (Middle Katian,Upper Ordovician), Shaanxi,north China

Abstract: A diverse brachiopod fauna from a relatively deep water carbonate facies of the Upper Ordovician Beiguoshan Formation (uppermost Caradoc – lower Ashgill, middle Katian) is characterized by small shells and contains the oldest known Dicoelosia and Epitomyonia, two diagnostic taxa of deep water brachiopod palaeocommunities during the Late Ordovician and Silurian. Three new species are recognized: Dicoelosia cordiformis sp. nov., Dicoelosia perbrevis sp. nov. and Epitomyonia fui sp. nov. These pioneer forms of the family Dicoelosiidae show a relatively high degree of morphological plasticity. The shells of Dicoelosia from the Beiguoshan Formation range from the typical slender‐lobed form with a concavoconvex profile to the strongly equibiconvex, fat‐lobed morphotype that was not known previously until the late Silurian. The Beiguoshan dicoelosiids point to an important attribute of the deep water brachiopods: small generalists with high morphological plasticity, which make them ideal candidates as progenitors for the evolution of shallow water brachiopod faunas in shelf and platform depositional environments.     

The sedimentology and context of Late Ordovician glacial marine sediments from Sierra Leone,West Africa

Glacial marine deposits (the Waterfall Formation of the Saionia Scarp Group) from northern Sierra Leone, West Africa, correlate with sequences in Guinea and Senegal and indicate a Late Ordovician age. The sediments (150 m in thickness) are laminated mudstones (rhythmites) of alternating fine laminae and lenticular, graded, coarse laminae a few millimetres thick. Dispersed clasts of various rock types occur within the rhythmites, which also locally contain slump structures. Interbedded with the rhythmites and dropstones are turbidites, with sole structures, amalgamated and graded beds. Transport directions are from the north.Deposition from low-density melt-water suspension currents derived from a shelf glacier is suggested for the rhythmites, with the clasts emplaced by iceberg rafting.Underlying the glacial sediments are subarkoses with heavy-mineral laminae and cross-bedded, super-mature quartz arenites attributed to a shallow-water tidal origin.The occurrence of glacial marine sediments in Sierra Leone provides a southwestern boundary to the ice-sheet which is well-documented as covering much of the Sahara and Northwest Africa during Late Ordovician time, and further emphasizes the great area affected by this glaciation.     

Darriwilian–Sandbian (Middle–Late Ordovician) shallow warm-water deposits in the Upper Yangtze region,South China: A case study from the Qiaojia area,northeastern Yunnan Province

The Middle–Late Ordovician Nuoduo and Huadan formations are well exposed at Songliang of northeastern Yunnan Province, representing rare neritic carbonate sediments deposited in the Yangtze region, South China during that time. Detailed sedimentological analyses support the recognition of ten microfacies from the succession, including shale (MF 1), argillaceous mudstone/dolostone (MF 2), mollusk-ostracod wacke-packstone (MF 3), peloidal wacke- to pack-grainstone/dolostone (MF 4), ooid-cortoid-peloid pack-grainstone/dolostone (MF 5), peloidal bindstone (MF 6), bioclastic wacke-packstone (MF 7), green algae mud-wackestone to floatstone (MF 8), bioclast-lithoclast float-rudstone (MF 9) and green algae bindstone (MF 10). Types A, B and C of shallowing-upward sequences are constructed according to vertical associations of MFs. The Nuoduo Formation mainly comprises Type A and Type B, both being characterized by abundant peloids, ooids, cortoids and aggregate grains. MF 5 and MF 6 cap the Type A and Type B sequences, respectively, suggesting an overall restricted marine environment shifting cyclically from lagoon to shoal, or lagoon to tidal flat, respectively. Type C sequences, occurring in the topmost Nuoduo to Huadan formations, contain various green algae indicative of open-sea, euphotic subtidal zones. The transition from A and B to C sequences is marked by ferruginous ooidal intervals, possibly correlated to relative sea-level rise and/or tectonic activities. A continuous neritic, warm-water environment is proposed for this area from late Middle to Late Ordovician, with the Nuoduo Formation filling the absence of warm-water deposits in the tropical or subtropical Yangtze region during Middle Ordovician. The increasing diversity and abundance of green algae in the Huadan Formation might represent a global radiation event of marine flora during the studied interval. Unique fossils and environments distinct from those of the interior Yangtze region are documented here, leading to a better understanding of the Ordovician System regionally and globally.     

Quantifying CaCO3 Microprecipitates Within Developing Surface Mats of Marine Stromatolites Using GIS and Digital Image Analysis

The unique geochemical coupling of organic molecules and mineral CaCO₃ provides a fluorescence signature detectable using conventional confocal scanning laser microscopy (CSLM). The surface microbial mats of open-water marine stromatolites (Bahamas) exist in a continuum of states ranging from a Type 1 (i.e., nonlithifying) to Type 2 (i.e., lithified micritic laminae present) to Type 3 (i.e., fused grain layer). An approach was developed here, that utilizes geographical information systems (GIS) and digital image analysis, coupled with CSLM to estimate concentrations of calcium carbonate precipitates in developing marine stromatolites. We propose that the area occupied by particles within each image can be used to estimate concentrations of precipitates. Fluorescent polymeric microbeads and bacteria were used to calibrate the approach. We used this approach to demonstrate that CaCO₃ precipitates in lithifying layers were quantifiable and significantly different (p < 0.0001) from those in nonlithifying layers. The approach provided a useful tool for the unambiguous assessment of relative changes in microbial precipitates occurring over small ( μ m to mm) spatial scales, and that characterize the formation of lithified layers (micritic laminae) in open-water marine stromatolites.     

甘肃敦煌西湖荒漠-湿地生态系统土壤水分空间异质性及其影响因子研究

土壤水分是内陆荒漠区湿地生态系统中重要的限制因子,为了揭示该区域土壤水分空间分布特征,采用传统统计学和地统计学相结合的方法,对甘肃敦煌西湖国家级自然保护区0~200cm内各层土壤水分的空间变异性及海拔、土壤质地和植被对其的影响进行了研究,旨在为极干旱区湿地生态系统植被修复和保育提供科学依据。结果表明:(1)本研究所得各变量的变异系数、块金方差、基台值、变程和结构比分别为36.51%~88.65%、0.007~0.098、0.112~0.549、116~453和76.6%~97.6%,各变量均为中等变异,存在高度异质性,具有较强空间自相关。(2)深层(60~200cm)土壤水分含量较浅层(0~60cm)变异大,且不同层次土壤水分含量的空间异质性差别也较大,空间变异主要发生在较小尺度上(分维数D在1.902~1.989之间)。(3)海拔是影响该区域深层土壤水分空间变异的主导因子。(4)土壤质地与浅层(0~60cm)土壤水分含量的相关性大于与深层(60~200cm)土壤含水量的相关性,它们与海拔相关性表现相反;草本植被盖度与浅层土壤水分含量呈较高的正关联关系,灌木根量与深层土壤水分含量呈较高的负关联关系。     

Euryhalinity of Palaeozoic articulate brachiopods

OMonotypic and very low diversity virgianid shell beds from the Upper Ordovician to Lower Silurian dolomites of North Greenland were formed in marginal marine quiet-water hypersaline environments. In the light of this evidence the salinity tolerances of other Palaeozoic articulate brachiopods is evaluated. There are only a small number of species apparently invading hypersaline or brackish environments, but it is significant considering that previously all articulate brachiopods were thought to be fully marine. Two types of occurrence are noted, those species specifically related to marginally marine environments, disappearing with the introduction of fully marine faunas, and the majority of species which extend their normal marine range into marginal conditions. No brachiopod species appears to have invaded very hypersaline or truly brackish conditions. No single group of articulate brachiopods specifically specialised in colonising marginal marine environments, apart from possibly the virgianid pentamerids. Palaeozoic, Upper Ordovician, Lower Silurian, Brachiopoda. Pentamerida, Virgianidae, Greenland, palaeoecology. hypersaline environments, brackish environments .     

泥质对志留系礁滩生长的抑制作用:黔北桐梓韩家店组的例证

黔北桐梓的戴家沟剖面和狮溪剖面志留系兰多维列统特列奇阶下部的韩家店组出露完好,该组泥岩、粉砂岩中夹有厚度1—3m、直径4—7m的小型点礁。礁核相多具典型的障积格架岩特征,但生长时限短暂,群落分异度低,仅见床板珊瑚、单体四射珊瑚、苔藓虫和海百合茎,礁间为珊瑚、苔藓虫和海百合茎碎片堆积的滩相,伴生丰富的遗迹化石。在陆源碎屑快速沉积的背景下,浑浊海水频繁的富营养化过程限制了礁体纵横向生长和朝高分异度群落发展的可能性。     

Paleosol caliche in the New Haven Arkose,Newark Group,Connecticut

The New Haven Arkose is a 1500–2500 m sequence of fluvial red beds of Late Triassic and probably Early Jurassic age that accumulated in a tropical rift valley. Streams flowing from highlands along the valley deposited braided channel sand interbedded with overbank mud. At most outcrops in Connecticut, some of the beds of red mudstone and sandstone contain paleosol caliche with densely packed root casts and rhizoconcretions. These calcified mudstones and sandstones commonly show the initial stages of calcification where subspherical to vertically elongate calcite nodules with crystic plasmic (microspar) fabric replace up to 50% of the host rock. In more mature paleosols, coalescing calcite nodules replace 50–95% of the rock, accompanied by paleosol mosaics of pedotubules, peds, and laminated crystallaria. The best developed caliche profiles are capped by thin (0–10 cm) layers of nearly pure limestone with complex fabrics of nodules, microspar, veins, caliche clasts, laminae, and calcite cement. The caliche profiles accumulated when rates of sedimentation were low. Much of the time the tropical paleoclimate was semiarid, perhaps with 100–500 mm of seasonal rain and a long dry season.     

Pliocene facies and fossil contents of Qaret El-Muluk formation at Wadi El-Natrun depression, Western Desert, Egypt

Summary A sequence of late Pliocene sediments (Qaret El-Muluk Formation) covers large areas within Wadi El-Natrun depression; at the western periphery of the Nile Delta. The exposed sequence is composed of friable sands. mudstones, shales and minor limestones with combined total thickness of approximately 50 m. The fossil content consists of fresh and brackish water elements; charophytes, ostracods, gastropods, oysters and benthonic foraminifera. Planktonic forams which suggest marine influence have been also recorded. Terrestrial and aquatic continental vertebrate fauna includes mammals, reptiles, fish and aves. Several subtypes of ichthyoliths have been discovered in this study. Both lithological and paleontological data testify to a depositional environment which belongs to a marginal region where fluvial and marine deposition meet in shallow brackish lagoons. The sediments display changes from channelled fluvial sands to deltaic interbedded sands and mudstones to lagoonal shales and limestones to lacustrine chara limestones. Vertical stacking of facies is indicative of a series of transgressive-regressive events. The transgression brought shallow lagoonal conditions to the area. The transgressions were incomplete due to high rate of fluvial input. The lagoon dried up intermittently and calcretes and red brown soils were formed. Each regression is indicated by river generated clastic influx. During periods of active fluvial input, fluvial sediments prograded lagoonward. The clastic material was probably a mixture of reworked detritus from older sediments. An admixture of first cycle material from the basement rocks of the Red Sea hills cannot be ruled out. The diversity of provenance argues for several channels of a large river which existed during the late Pliocene.     

Stromatolite-dominated microbialites at the Permian–Triassic boundary of the Xikou section on South Qinling Block,China

Permian–Triassic boundary microbialites (PTBMs) are organosedimentary carbonates formed immediately after the end-Permian mass extinction. All those reported PTBMs constrained by convincing conodont biozones are present stratigraphycally not higher than the Hindeodus parvus zone and most of them are dominated by thrombolites. This paper provides the first record of a brief, but spectacular development of stromatolite-dominated PTBMs within the basal Isarcicella isarcica conodont zone of the earliest Triassic from the Xikou section of South Qinling Block that was at the margin of the North China Block during the Permian–Triassic transition and was geographically separated from the major occurrence of post-extinction microbialites in the South China Block. This stromatolite cap overlies a 3.7-m-thick oolitic limestone and is composed of a lower 0.2-m-thick bed and an upper 0.5-m-thick bed, separated by a 0.2-m-thick greyish green siliciclastic mudstone. These two stromatolite beds mainly consist of columnar stromatolites with subordinate domal stromatolites. The intercolumn and interstitial spaces within the stromatolites are filled with oolitic grainstones. At the microscopic scale, laminoid structures in stromatolites comprise wavy, millimetric-domical and tangled laminae. The increased grain and fossil contents and/or bioturbation in the domical and tangled laminae indicate that the formation of these laminae is likely related to an increase in the populations and the disruptions by benthic metazoans, as well as an influx of sediment grains. The δ¹³C_carb values fluctuate between 2‰ and 3‰ in the uppermost Permian strata; a distinct negative shift of 1.9‰ occurs at the topmost oolitic grainstone, just below the lower stromatolite bed, and the lowest value of −0.1‰ is located at the base of the upper stromatolite bed. The stratigraphic succession from stromatolites to thrombolites of the PTBMs may represent a transgressive succession and/or a transient ecosystem recovery immediately after the end-Permian mass extinction. The thrombolites-dominated PTBMs mainly developed in near-equator shallow marine geographic locations, and stromatolite-dominated PTBMs mainly developed at higher latitude settings, which probably indicates that a relatively lower diversity and abundance of marine benthic metazoans existed at higher latitudes after the end-Permian mass extinction.     

Cyclocrinitids from the Lower Palaeozoic Tethyan sequence of Spiti,India

Well preserved cyclocrinitids (calcareous green algae) are reported from early Palaeozoic deposits of Spiti, Himachal Pradesh, India. Casts of the algal fossils are preserved in calcareous grey siltstone unit of the Takche Formation (Late Ordovician–Early Silurian) which appears rusty/earthy brown due to weathering. The assemblage includes Cyclocrinites favus, Cyclocrinites pyriformis, Cyclocrinites cf. welleri, Cyclocrinites sp. and Cyclocrinites globosus. Both Cyclocrinites cf. welleri and Cyclocrinites globosus are reported for the first time from the entire Tethyan Himalaya, India. The described algal flora basically denotes relatively shallow marine depositional setting and low to moderate hydrodynamic conditions.