湖北宜昌樟村坪埃迪卡拉系陡山沱组硅磷质结核中的微体化石

本文报道的微体化石产于湖北宜昌樟村坪万家沟剖面埃迪卡拉纪陡山沱组第10层的硅磷质结核中,化石组合包括疑源类Appendisphaera grandis、Ericiasphaera spjeldnaesii、Knollisphaeridium maxi mum、Leiosphaeridia tenuissi ma、Meghystrichosphaeridium perfectum、Tianzhushania polysiphonia、T.spinosa、T.ornata;丝状蓝藻Oscillatoriopsis obtusa、Polytrichoides induviatus、P.lineatus、Salome hubeiensis、Siphonophycustypicum;多细胞藻类Sarcinophycus palilloformis、Wengania minuta。该微体化石组合面貌与黄陵背斜东、南翼陡山沱组二段硅质结核中以大型具刺疑源类Tianzhushania为特征的组合相同,与贵州瓮安地区陡山沱组上磷块岩下部保存的微体化石组合面貌基本一致。     

The Baykonurian glaciohorizon of the Late Vendian

Glacial deposits occurring close to the Vendian-Cambrian boundary were discovered long ago in several regions of Asia and West Africa. New data of recent years confirm earlier observation and specify age constraints of these deposits. In certain sections, glacial sediments rest on beds with the Upper Vendian fossils (Vendotaenia sp., Sabellidites remains, and microfossils). In the Hoggar Highland, they discordantly overlie magmatic rocks ca. 560 Ma old. In the East Sayan Mts., the Upper Vendian Cloudina sp. have been found in the diamictite matrix. In Central Asia and Northwest China, cap dolomites conformably overlying glacial deposits grade upward into the chert-phosphatic sediments containing fossils (Tarim region), which are characteristic of the Anabarites trisulcatus Zone in the Nemakit-Daldynian Stage. Fossils of this zone have been found directly above glacial deposits in the East Sayan. According to all these data, the respective Baykonurian glaciation, as it is termed in this work, either antedated the Nemakit-Daldynian Age or was concurrent to its commencement. The glaciation is correlative with a significant negative δ¹³C excursion of ?5 to ?10‰. Judging from recent paleotectonic reconstructions, the Baykonurian glaciation was not regional in rank and left records in both hemispheres of the Earth. This glaciation resumes the well-known succession of the great glaciation event in the terminal Late Precambrian, i.e., the Sturtian, Marinoan, and Gaskiers glaciations. Hence, one cannot ignore the Baykonurian glaciation when analyzing the geological, biospheric, and biotic history of the Late Precambrian.     

Microfossils in cherts from the Mesoproterozoic (Middle Riphean) Debengda Formation, the Olenek Uplift, northeastern Siberia.

Silicified shallow-water marine carbonate deposits of the Proterozoic Debengda Formation (the Olenek Uplift, northeastern Siberia) contain well preserved microfossils. One or two distinct assemblages consists only of filamentous Siphonophycus microfossils, which are presumably the extracellular sheaths of hormogonium cyanobacteria. The other is dominated by coccoidal microfossils, first by the entophysalidacean cyanobacterium Eoentophysalis. The coccoidal assemblage was recognized in the layered carbonate precipitate structures of a superficially stromatolite appearance. Despite its simple composition, the microfossil assemblage supports the generally accepted Mesoproterozoic (middle Riphean) age of the Debengda Formation. This conclusion corresponds to the available data on isotopic geochronology, and to the composition of columnar stromatolites from the Dehengda Formation. Both the structural features and carbon isotopic composition of its rocks are comparable to those of rocks of known Mesoproterozoic age, but differ from the characteristics of definitely Neoproterozoic deposits.     

The room for the Vendian in the International Chronostratigraphic Chart

The Vendian was proposed by B.S. Sokolov as a stratigraphic subdivision comprising the last of the Proterozoic glacial periods (the Laplandian Glaciation) and the overlying strata delineated by a full stratigraphic range of fossil soft-bodied organisms. For over three decades the Vendian had been an informal part of the Standard Global Chronostratigraphic Chart, until 2004, when it gave place to the new Ediacaran System. Further research has shown that the Ediacaran System significantly exceeds the stratigraphic range of the Vendian in Sokolov’s definition and includes stratigraphic analogs of the Laplandian Glaciation in sections across North America, Australia, Newfoundland, Scotland, Ireland, Chinese Tien Shan Range, and Tasmanian microcontinent. Carbon isotope variations in carbonates provide criteria for subdivision of the Ediacaran into two series. If a relationship between the Laplandian Glacial Period (600–580 Ma) and the negative excursions EN2 and EN3 on the 5¹³C curve for the Doushantuo Formation of China is established, the Vendian might take its place in the Standard Global Chronostratigraphic Chart as a formal upper series of the Ediacaran System. The Vendian Series, in turn, might be further subdivided into the Laplandian, Redkinian, Belomorian, and Kotlinian stages typified by regional stages of the Vendian of the East European Platform.     

华南埃迪卡拉纪陡山沱海洋中无机碳同位素组成变化

报道了贵州江口桃映深水相剖面陡山沱组碳酸盐岩δ13C值的变化特征,结合其他已报道的数据,分析了华南扬子地区埃迪卡拉纪陡山沱海洋不同沉积环境,包括盆地相、斜坡相、台地相、台地边缘相碳酸盐岩中δ13C变化趋势及绝对值的异同,发现浅水区剖面记录的δ13C漂移次数多于深水区剖面,且不同相区δ13C值也有差异。δ13C值的差异与埃迪卡拉纪陡山沱海洋的阶段性演化密切相关。基于不同相区的δ13C值变化,埃迪卡拉纪陡山沱海洋的演化历史可分为3个时期:1)陡山沱组1段盖帽白云岩沉积期华南扬子地区很可能为一个开阔台地,白云岩中δ13C值可能继承于幔源CO2的碳同位素特征,深水区和浅水区碳酸盐中δ13C值无显著差异;2)陡山沱组2段和陡山沱组3段下部沉积时期盆地深水区中δ13C值显著低于浅水区,且深水区δ13C值与陡山沱组1段时期无显著差异,浅水区的δ13C值则显著升高;3)陡山沱组3段上部和陡山沱组4段沉积时期陡山沱盆地中δ13C值均显著下降,且不同沉积环境中的δ13C值差异度降低。盖帽后埃迪卡拉纪陡山沱海洋阶段性演化主要与不同时期深水区DOC库的逐步氧化有关。     

Late Neoproterozoic (Ediacaran)-Early Paleozoic (Cambrian) acritarchs from the Marwar Supergroup,Bikaner-Nagaur Basin,Rajasthan

Characteristic latest Neoproterozoic and Early Paleozoic acritarchs and associated organic-walled microfossils are recorded from the sediments of Marwar Supergroup encountered in BGW-A well (Bikaner-Nagaur Basin) from 1123–481 m depth. Six distinct acritarch assemblages, broadly comparable with globally known Ediacaran (Vendian) and Cambrian assemblages are recognised. The recovered microfossils provide precise age for different units of the Marwar Supergroup whose ages, till now, were poorly understood due to absence or paucity of invertebrate and other mega and microfossils.     

Association of macro- and microfossils in the Vendian (Ediacaran) postglacial successions in Western Mongolia

The Vendian (Ediacaran) beds of the Zavkhan Basin, in the upper part of the Tsagaanolom Formation (<632 ± 14 Ma), yielded a new “Zavkhan” association of algae, microfossils, and problematic organisms, which is established in the series of alternating chert-carbonate shale with phosphorite interbeds. This association is distinct in the predomination of large (250 μm and over) sphaeromorphic microfossils of the genera Tasmanites, Archaeooides, and Leiosphaeridia, whereas acanthomorph acritarchs are represented by rarely found Cavaspina sp. and Tanarium sp. Multicellular algae included fragments of encrusting or foliate thalli with pseudoparenchymatous structure of polygonal cells characteristic of Rhodophyta algae (Thallophycoides sp.), and cordlike thalli of Vendotaenid algae Tyrasotaenia podolica. These layers of siltstone contain imprints of the problematic Vendian macrofossil Beltanelliformis brunsae. In their stratigraphic position, chemostratigraphic data, and fossil assemblage, the “Zavkhan” association can be assigned to the Upper Vendian.     

关于峡东陡山沱组具刺疑源类化石研究的补充

在峡东陡山沱组黑色燧石中前人报道了多种多样的具刺疑源类化石，笔者最近重新研究了陡山沱组岩石切片中的具刺疑源类，发现微化石三维地保存了几乎没蚀变的条件下，这个生物群落包括Filis phaeridi-um,Baltisphaeridium,Comasphaeridium和Tianzhushania等分子，它们中的Tianzhushania在外表及形态上可与产自挪威斯瓦尔巴德群岛晚里菲岩层中的Trachyhstrichosphaera aimika比较，Trachyhystrichosphaera aimika是晚前寒武纪最特殊和分布最广的一个分子，目前已知至少产自世界上15个地区的晚里菲岩层中，对于新古元代前文德地层似乎是一个出色的指示化石，产自陡山沱组的Tianzhushania似乎支持Butterfield的观点，即：陡山沱组硅化碳酸盐沉积的年代早于伊迪卡拉后生物多样化的时期，但是，由于化石只发展于少数岩石薄片中，因此，这些化石或许只代表峡东陡山沱组局部的生态环境。文中还讨论了陡山沱组的沉积环境及化石的保存。     

Biogenic Microstructures in Stromatolites of the Baikal–Patom Highland: Results of Complex Study

Precambrian stromatolites were studied with a complex approach including two complementary methods. The biogenic origin of ultramicrostructures examined with SEM was supported by the traditional optical microscopy. The paper addresses columnar-stratiform stromatolites of the Vendian Chencha Formation in the Ura Uplift (Baikal–Patom Highland, Central Siberia) and fossilized remains of coccoidal and filamentous microorganisms therein: cyanobacteria Eoentophysalis, hormogonian cyanobacteria Siphonophycus and, probably, Eomicrocoleus. The unraveled community of stromatolite builders includes the major organisms commonly observed in the Precambrian microbiota. Stromatolites of the Chencha Formation contained not only cyanobacteria, but also remains of eukaryotic microorganisms, including likely testate amoebae and acanthomorphic acritarchs. It is shown that the complex approach rules out incorrect determination of biota and significantly enhances concepts of the origin and formation of stromatolites, as well as the participation of microorganisms in their formation.     

The Yudomian of Siberia,Vendian and Ediacaran systems of the International stratigraphic scale

In Russia, the terminal Neoproterozoic formally includes the Vendian of western part of the East European platform and the concurrent Yudoma Group of Siberia. As is shown in this work, the designated subdivisions correspond in the stratotypes only to the upper, Yudomian Series of the Vendian. In the Siberian platform, the Ust-Yudoma and Aim horizons of the Yudomian are tightly interrelated. The lower of them, bearing remains of Ediacaran Fauna, represents the Ediacarian Stage, whereas the upper one containing small-shelled fossils (SSF) corresponds to the Nemakit-Daldynian Stage divided into the trisulcatus and antiqua superregional zones. In more complete sections of the platform periphery, sediments of these subdivisions conformably rest on siliciclastic succession that should be ranked as basal subdivision of the Yudomian. The succession is concurrent to the Laplandian Stage of the East European platform. According to geochronological dates obtained recently, the Yudomian Series spans interval of 600–540 Ma. In the East European platform, the Upper Vendian (Yudomian) begins with the Laplandian basal tillites of synonymous stage. In the west of the platform, tillites are dated at 600 Ma like the Upper Vendian base in Siberia. The next Ediacarian Stage of the East European platform is stratigraphic equivalent of the Redkino Horizon, while summary range of the Kotlin and Rovno horizons is concurrent to that of the Nemakit-Daldynian Stage. The Vendian of Russia is conformably overlain by the Tommotian Stage of the Lower Cambrian. Intense pre-Vendian events constrained distribution areas of the Lower Vendian sediments in Russia. The Lower Vendian deposits of the East European platform are most representative and well studied in the central Urals, where they are attributed to the Serebryanka Group. In Siberia, separate subdivisions representing the Lower Vendian are the Maastakh Formation of the Olenek Uplift, two lower members of the Ushakovka Formation in the Baikal region, and the Taseeva Group of the Yenisei Range. Chronological interval of the Lower Vendian corresponds to 650–600 Ma. The Marinoan Glaciation dated in Australia at 650–635 Ma is concurrent to basal part of the pre-Yudomian interval of the Vendian in Russia, whereas the Laplandian Tillite and Gaskiers Glaciation (600–580 Ma) correspond to onset of the Yudomian Epoch. The new Ediacaran System (Knoll et al., 2004) legalized in the International Neoproterozoic scale is close in range to the entire Vendian (635–544 Ma), although without basal beds (Marinoan Tillite) it deprives the terminal Neoproterozoic of its original sense. Inferiority of the system consists also in its indivisibility into stages. Hence, it is clear that the Vendian System subdivided in detail in Russia should be retained in the rank of terminal system of the Precambrian, one of the basic in general scale of the Neoproterozoic.     

Radiometric and stratigraphic constraints on terminal Ediacaran (post-Gaskiers) glaciation and metazoan evolution

Radiometric constraints on mid-Ediacaran Period glaciation (Gaskiers) in Newfoundland narrowed the known temporal gap between widespread ice ages and the evolution of complex metazoans to several million years. To further evaluate this claim we studied an Ediacaran glacial diamictite at the base of the Fauquier Formation of northern Virginia, and discovered a conformable relationship between the post-glacial cap carbonate and overlying volcanic rocks of the Catoctin Formation. U/Pb zircon age constraints for the rift-related volcanic flows suggest initial emplacement around 571 million years ago. Application of the Catoctin age to the Fauquier succession indicates the occurrence of an ice age about 10 million years younger than the 582 Ma Gaskiers event, supporting the view of multiple Ediacaran Period glaciations. Furthermore, the age constraint from eastern Laurentia falls within radiometric uncertainty of fossiliferous strata in Avalonia, indicating that the Fauquier glaciation was coincident with early metazoan evolution.     

上扬子区震旦系层序地层格架及其形成的古地理背景

上扬子区的震旦系包括陡山沱组和灯影组,前者包含较多的黑色页岩,而后者则以发育大套白云岩为特征。从岩相序列到米级旋回,从沉积相序列到三级层序,可将浅水台地背景的震旦系划分为5个三级层序。这些三级层序可以进一步归为一个二级层序。以各个主要剖面的层序划分为基础,依据地层记录中的两种相变面和两种穿时性,可以建立研究区震旦系的层序地层格架。其中包含较多的深水黑色页岩系的陡山沱组,反映了大冰期之后的海侵作用的特点;与寒武系的变化特征不同,自北西而南东沉积环境由浅变深,该沉积背景中沉积了原生沉积组构保存较为完好的灯影组白云岩,这种变化可能反映了前寒武纪末期特殊的白云石化作用特征,同时也代表了许多有待于进一步研究的沉积学问题。从陡山沱组黑色页岩系到灯影组白云岩,不但构成了一个更大级别的总体向上变浅的沉积相序列,还反映了一个与三级和二级海平面变化过程相响应的缓坡型碳酸盐台地的生长发育过程,该过程较为清楚地显示在系列层序地层格架栅状图和岩相古地理图之中。     

The Baikalian and Vendian sequences in the Lower Angara area (southwestern Siberian Platform)

In the section of Agaleevskaya BH-4 drilled in the lower reaches of the Angara River, Vendian and Baikalian sediment sequences have been recognized within Neoproterozoic strata. The Vendian sequence is formed by terrigenous-carbonate deposits of the Tetere, Soba, and Katanga Formations of the Danilovo Horizon, referred to as the Upper Vendian Nemakit-Daldyn Stage, as well as by terrigenous deposits of the Taseeva Group. The Baikalian Horizon is composed of the Brus Formation, earlier recognized only on the Chadobets uplift, and is separated from the Vendian deposits by a stratigraphic gap. In the Brus Formation, a microfossil complex similar to earlier described biotas of the Siberian Baikalian strata was found. The underlying deposits of the Terina Formation contain microfossils lacking below the basement of the Lakhanda Horizon (Neoproterozoic).     

U–Pb zircon age of the base of the Ediacaran System at the southern margin of the Qinling Orogen

Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive δ¹³C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deep-water basinal cap carbonate section with strong negative δ¹³C values in the southern margin of the Qinling Orogen, Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U–Pb age of 634.1 ± 1.9 Ma (1σ, MSWD_CE = 0.31, Probability_CE = 1.000, n = 20). The age is in good agreement with previously reported TIMS U–Pb ages for the termination of Marinoan glaciation and provides a geochronological constraint for the Ediacaran successions in the Qinling Orogen.     

长江三峡地区震旦系的微体植物化石

<正> 1980年,笔者同余汶、钱逸、王宗哲、侯先光、赵自强等在宜昌莲沱的王丰岗,南沱至石牌村以及秭归县庙河震旦系剖面采集了岩样(图1)。莲沱砂岩和南沱冰碛层中的化石贫乏,仅发现球形疑源类Leiominuschla sp.Protoleiosphaeridium spp.以及一些微体植物残片。陡山沱组微体植物类型多样化,数量可观。有疑源类(acritarchs)的刺球亚类(Acanthomorphitae)的分子,如微刺藻(Micrhystridium),波罗地藻(Baltishaeridium)和毛发藻(Comasphaeridium)等,其中Micrhystridium的数量多至千万计。有晚前寒武纪常见的球形亚类(sphaeromorphitae)的分子外,出现许多具有各种裂口、开口及至口盖的疑源类属种,如Leiosphaeridia?operculia sp.n.,Stictosphaeridium?apertum sp.n.,Paiaeostomc cystis angoformis,Eoaperturilites monoscissus gen. et sp. n., Eo. gonostomus gen. et sp. n., Tasmanites? sp. A。     

Problematic fossils from the Palaeo-Neoproterozoic Vindhyan Supergroup, India

Fossils of the Vindhyan Supergroup exhibit extensive diversity and variable biologic affinities represented by: bacteria, cyanobacteria, algae, fungi, acritarchs, metaphytes and metazoans (including members of the Ediacaran Fauna) and ranging from less than a micron to almost a metre in size. Besides identified fossils, a number of bizarre morphologies (due to deviation of morphology from conventional structures), present in various stratigraphic horizons, have been observed. It is very difficult to identify and decide their biologic affinities. In thin sections of Lower Vindhyan cherts, microfossils resembling lichen-like or fungal forms in which a sac encompassing a coiled filament may possibly indicate a symbiotic relationship, a Volvox colony-like structure and a vase-shaped body without an opening are unique. Among the carbonaceous fossils, very unusual and interesting fossil is a transparent disc comprising numerous appendages of an unidentified mesoscopic insect-like organism. Megascopic branching and associated Grypania-like structure is another form preserved as impression on micritic limestone. Petrographic thin sections of chert belonging to the Sirbu Shale Formation, exhibit presence of microscopic bizarre forms. The assemblage includes acritarchs and acanthomorphs of variable morphology and a dividing cell-like structure interpreted to be of rhodophycean affinity or a cleaving embryo of an animal affinity. Other peculiar morphologies among the carbonaceous fossils are: branched filaments that have attached sporangia-like vesicles, Chuaria-like body comprising cluster of very small-sized spheroids resemble scale-like structure, a chrysophycean alga or a multicellular tissue of a metaphyte. Another carbonaceous fossil represents a possible metazoan exhibiting an elongate body and a mid-gut-like structure or a Vaucheriacean alga. Although the biologic affinities of these bizarre forms can be a matter of debate, their biogenic nature is almost undoubted. The presence of such forms in the Vindhyans indicates advancement in morphology and a gradual evolution of life during the Palaeoproterozoic–Neoproterozoic period. In addition, presence of Ediacaran fossils in Bhander Group and large-sized acritarchs especially Trachyhystrichosphaera sp. in petrographic thin section of chert from the Sirbu Shale Formation, Bhander Group, Upper Vindhyans, suggests Ediacaran age as an upper age limit of the Vindhyan Supergroup.     

Paleobiology of the Mesoproterozoic-Neoproterozoic transition: the Sukhaya Tunguska Formation, Turukhansk Uplift, Siberia

Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 micrometers in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals.     

Microfossils from the Upper Sinian (Late Neoproterozoic) Doushantuo Formation in Changyang, Western Hubei, China

l.IntroductionIn1978,YinLeimingandLiZaipingreportedonProterozoicmicroplansinsouthwestChina.Alt-houghmostwerefoundinmacerationofsiliclasticsandcarbonates,thinsectionsoftheDoushantuochertfromtheTianzhushandistrictnearChangyang(samples#TianR.l9and#TianR.29,inL.YinandLi,l978)containedtwolargespinosespeciesfAs-lerocaPsoidessinensis(upto325Umindiameter)andTianzhushaniasPinosa(45opmindiameter).Subsequentst-UdiesoftheDoushantuoFormationindicatethatthemicrofossilsintheDoushantuochertandphosph…