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1.
The microfossils studied are discovered for the first time in the Riphean-Lower Vendian deposits, which have been recovered in 2002 by the Kel’tminskaya-1 deep parametric borehole in the Vychegda depression, the northeastern margin of the East European platform. The sampled interval of core section (4825–2347 m) consists of three units: the lower (depth range 4825–3995 m, 5 samples) and middle (depth range 3687–2961 m, 17 samples) carbonate successions overlain by sandstone-siltstone beds (depth range 2907–2347 m, 58 samples). Based on lithological criteria and/or composition of stromatolites, the carbonate successions are correlative with the Yshkemes and Vapol formations of the Upper Riphean of the Timan ridge succession, while the overlying, mostly siltstone succession was correlated with the Vychegda Formation of the southern Timan according to similarity in lithology and mineral composition. Microfossils found in 56 samples occur at 20 microphytological levels and represent different microbiotas. The Yshkemes and Vapol microbiotas of low diversity characterize six lower levels and represent one assemblage, while the diverse and abundant Vychegda microbiota typical of fourteen upper levels is divisible into three successive assemblages. The Vapol stromatolites Inzeria djejimii and Poludia polymorpha along with giant Chuaria and Navifusa present in the Yshkemes-Vapol assemblage suggest that their host deposits correspond to the upper Upper Riphean. The Vychegda assemblages, each of peculiar biostratigraphic specifics and unique in composition, consist of different morphotypes, primarily of large acanthomorphic acritarchs Cavaspina, Polyhedrosphaeridium, Cymatiosphaeridium, Asterocapsoides, and Tanarium, which are known in Scandinavia, Siberia, China, Australia, and India only in the Lower Vendian microbiotas of the Perthatataka type. The comprehensive microphytological characterization of the Lower Vendian in the Vychegda depression and earlier data on the Middle-Upper Riphean microbiotas from the adjacent Mezen syneclise enable a high-resolution biostratigraphic subdivision of the Riphean and Vendian successions in the vast region under consideration.  相似文献   

2.
《Earth》2008,88(3-4):61-93
Sedimentological observations and palaeomagnetic data for Cryogenian glacial deposits present the climatic paradox of grounded glaciers and in situ cold climate near sea-level, glaciomarine deposition, and accompanying large (up to 40 °C) seasonal changes of temperature, all in low to near-equatorial (< 10°) palaeolatitudes (equated with geographic latitudes). Neither the “snowball Earth” nor the “slushball Earth” hypothesis can account for such strong seasonality near the palaeoequator, which together with findings from sedimentology, chemostratigraphy, biogeochemistry, micropalaeontology, geochronology and climate modelling argue against those scenarios. An alternative explanation of glaciation and strong seasonality in low palaeolatitudes is offered by a high (> 54°) obliquity of the ecliptic, which would render the equator cooler than the poles, on average, and amplify global seasonality. A high obliquity per se would not have been a primary trigger for glaciation, but would have strongly influenced the latitudinal distribution of glaciers. The principle of low-latitude glaciation on a terrestrial planet with high obliquity is validated by theoretical studies and observations of Mars. A high obliquity for the early Earth is a likely outcome of a single giant impact at 4.5 Ga, the widely favoured mechanism for lunar origin. This implies that a high obliquity could have prevailed during most of the Precambrian, controlling the low palaeolatitude of glaciations in the early and late Palaeoproterozoic and Cryogenian. It is postulated that the obliquity changed to < 54° between the termination of the last Cryogenian low-palaeolatitude glaciation at ≤ 635 Ma and the initiation of Late Ordovician–Early Silurian circum-polar glaciation at 445 Ma.The High Obliquity, Low-latitude Ice, STrong seasonality (HOLIST) hypothesis for pre-Ediacaran glaciation emerges favourably from numerous glacial and non-glacial tests. The hypothesis is in accord with such established or implied features of Cryogenian glaciogenic successions as extensive and long-lived open seas, an active hydrological cycle, aridity and palaeowesterly (reversed zonal) winds in low palaeolatitudes, and the apparent diachronism or non-correlation of some low-palaeolatitude glaciations. A pre-Ediacaran high obliquity also offers a viable solution of the faint young Sun paradox of a warm Archaean Earth. Furthermore, reduction of obliquity during the Ediacaran–early Palaeozoic would have yielded a more habitable globe with much reduced seasonal stresses and may have been an important factor influencing the unique evolutionary events of the Ediacaran and Cambrian. The palaeolatitudinal distribution of evaporites cannot discriminate unambiguously between high- and low-obliquity states for the pre-Ediacaran Earth. Intervals of true polar wander such as postulated by others for the Ediacaran and Early Cambrian imply major mass-redistributions within the Earth at those times, which may provide a potential mechanism for reducing the obliquity during the Ediacaran–early Palaeozoic.  相似文献   

3.
尹崇玉 《地层学杂志》2001,25(4):253-258
首次报道贵州瓮安震旦系陡山沱组磷块岩中发现大型具刺疑源类 Papillomembrana compta。对震旦纪陡山沱期已发现的特色硅化或磷酸盐矿化大型具刺疑源类组合及国内外地层对比进行综合评述 ,结合同位素测年、化学地层和其他有关资料综合分析 ,对陡山沱组的年代问题进行探讨。现有研究资料表明 ,陡山沱组含大型具刺疑源类组合的层位应低于俄罗斯北部、澳大利亚、乌克兰、西伯利亚北部及加拿大纽芬兰等地发现多种典型伊迪卡拉期生物群的层位 ,年代应早于以伊迪卡拉期生物群爆发为标志的早期后生动物多样化的时段。  相似文献   

4.
A highly diversified, advanced and exceptionally well preserved microfossil assemblage, dominated by a planktic community, has been recorded from petrographic thin sections of chert belonging to the Sirbu Shale Formation, Bhander group, upper Vindhyans, Rajasthan. Recently, it was noticed that the assemblage also contains well preserved, large-sized acanthomorphic acritarchs, Trachyhystrichosphaera considered to be an age-marker microfossil of the Cryogenian (850-630 Ma). It is reported for the first time from any Indian microfossil assemblage of Proterozoic succession. The other microfossils of the Sirbu Shale Formation are: well preserved simple, small and large-sized sphaeromorphs; complex acanthomorphs, cyanobacterial community; especially a very small-sized but exceptionally well preserved Obruchevella, a form resembling Volvox colonies; cf. vase-shaped microfossils and morphologies, possibly inclining towards fungal affinity, or lichenlike symbiotic associations of algae and fungi. Till date, Trachyhystrichosphaera has so far not been reported from successions older than the Tonian (1000850 Ma). It is believed that acanthomorphs attained maximum size in Ediacaran (630-542 Ma), and further decreased in size in the Cambrian. The global paleontological literature indicates that Trachyhystrichosphaera ranges in age from Tonian-Ediacaran (1000-542 Ma). The present record of Trachyhystrichosphaera as well as the earlier studies of micro and megascopic life of the Bhander Group in general and the Sirbu Shale in particular (aided by the absence of any Cambrian fossil record) indicate that in all possibilities, age of the Sirbu Shale should lie near Cryogenian (850-630 Ma) and the uppermost Bhander group, may incline towards the Ediacaran (630-542 Ma).  相似文献   

5.
成冰纪全球冰期是地球历史上最极端的冰室气候事件,冰川作用波及赤道区域,全球可能都遭受了冰封,海洋广泛缺氧,生物演化进程迟滞。然而,冰期结束之后,大气氧浓度迅速升高,海洋发生逐步氧化,大型带刺疑源类和真核多细胞藻类在埃迪卡拉纪开始繁盛,出现最早的动物,地表生物圈发生了翻天覆地的变化。显然,成冰纪全球冰期事件是地球系统演化的重要转折。认识冰期的环境效应是认识埃迪卡拉纪生物演化的关键,也是打开地表宜居环境演化的钥匙。本文总结了近年来成冰纪全球冰期的气候假说、冰期沉积特征、海洋氧化还原条件及冰期后的大气与海洋环境剧变等方面的研究进展,简要分析了全球冰期研究中存在的问题,并对该领域未来研究提出了展望与建议。  相似文献   

6.
The Adelaide Rift Complex is arguably one of the most complete and best studied Neoproterozoic to early Cambrian successions worldwide, preserving evidence of the breakup of Rodinia, two Cryogenian glaciations and the interglacial phase, and one of the best documented Ediacaran to early Cambrian biotic transitions. However, the complex and protracted tectono-sedimentary history of this 0.8–0.5 Ga province is still being debated. We present new and published UPb ages and Hf and O isotope data for detrital zircons from the Adelaide Rift Complex, representing the most complete assembly of such data for this succession. Deposition during initial mid-Tonia extension was largely sourced locally from rift shoulders. As the basin evolved from rift- to sag-phase following continental breakup in the Cryogenian the provenance regions extended to more distal late Mesoproterozoic terranes to west and northwest. New data from Sturtian Glacial Epoch deposits are consistent with termination of this event at 0.66 Ga, with most deposition during deglaciation. Uplift of the Musgrave region during the Ediacaran to early Cambrian Petermann Orogeny led to dominant sediment supply from that terrane at that time in the north. In the south, earliest Cambrian deposition followed local tectonism, initially revitalising local proximal basement sources. An abrupt change in provenance occurred at the base of the Cambrian Kanmantoo Group, the youngest sediment package in the south. Paleocurrent data indicates transports from the south, probably from formerly contiguous Antarctica, possibly reflecting the onset of convergent tectonics and deposition in a foreland basin, consistent with the near depositional age of the dominant detrital zircon population. Whilst several episodes of significant crustal reworking are identified in the Hf and O isotope data, many of the zircon TDM ages lie within 0.5 Ga of the UPb ages indicating that new additions from the mantle were common in the provenance regions.  相似文献   

7.
Postulated extreme sea-level rise of up to 1-1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious,as is the great rapidity(<104 yr)of the sea-level rise.Such extreme glacioeustatic events,if real,would have been global and affected all continents.In South Australia,up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin.The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise.In the central Adelaide Rift Complex,diamictite with faceted and striated clasts occurs at the top of the Elatina Formation<1-2 m beneath the early Ediacaran Nuccaleena Formation’cap carbonate’.One hundred kilometres to the south,~30 m of siltstone and sandstone followed by^6 m of clast-poor diamictite with clasts 10+cm long occur between tidal rhythmites and the cap carbonate.Three hundred kilo metres further south,~70 m of siltsto ne,dolo mitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata.Hence the rhythmites were deposited during a high stand(interstadial or interglacial),not during post-glacial sea-level rise.Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition,and water depth and other palaeoenvironmental factors are uncertain,casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition.The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations.A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate,and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 10^5-10^6 yr.This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California,and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates.It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.  相似文献   

8.
The Borborema Province is a complex neoproterozoic orogen in northeastern Brazil, made of a mosaic of fault-bounded terrains and several metassedimentary sequences. In the present work, new zircon U–Pb provenance data for metasedimentary rocks in the Western Pernambuco-Alagoas Domain, southern part of the Province, are reported. Detrital zircon ages range from Archean to Neoproterozoic. Three samples of the Cabrobó Complex were investigated: (i) sillimanite-kyanite-garnet-biotite schist, which presented mostly Ediacaran and Cryogenian detrital zircon ages (youngest zircon at ca. 554 Ma) indicating erosion of neoproterozoic sources, (ii) garnet-biotite schist, which has a dominant Tonian/Stenian population, a less abundant Cryogenian (youngest zircon age at ca. 643 Ma) as well as Paleoproterozoic and Archean zircon grains, and (iii) tourmaline-muscovite quartzite, which contains detrital zircon varying in age between ca. 2.08 Ga and 1.57 Ga, and an abundant population close to the Meso/Paleoproterozoic boundary, possibly associated with the erosion of rocks formed during the Statherian taphrogenesis, known in the central part of the São Francisco Craton as well as in other areas of the Borborema Province. Two samples of the Riacho Seco Metasedimentary Complex were also investigated: (i) a biotite schist with a dominant population presenting ages mostly between 2.3 and 2.7 Ga (youngest zircon age at ca. 2023 Ma) and (ii) a magnetite-biotite-muscovite quartzite, having detrital zircon grains with ages ranging between ca. 1.9 and 2.7 Ga. The sedimentary rocks of the Riacho Seco Complex may have their origin related to the erosion of sources within the São Francisco Craton. The data for the Riacho Seco metasedimentary rocks, however, are not conclusive with respect to the depositional age of the original sedimentary rocks. The sequence might represent exposure of an old (Paleoproterozoic) sedimentary pile or, alternatively, it comprises a neoproterozoic passive margin sequence, with the original sediments derived from the erosion of the cratonic areas to the south.  相似文献   

9.
The Avalon biota (Ediacaran Period, 570–559 Ma) marks the first appearance of macroscopic and complex benthic communities in the fossil record. This assemblage is known from a few localities worldwide, mainly in Canada and England. Here, we report for the first time the presence of Ediacaran macrofossils in deposits of similar age from Gondwana (Itajaí Basin, southern Brazil). Our new radiometric date (~563 Ma) indicates that the Itajaí Basin can be chronocorrelated with the classic Avalonian deposits and thus represents one of the oldest records of the Ediacaran biota in Gondwana. We describe the presence of the Ediacaran genus Palaeopascichnus, as well as discs (Aspidella and Nimbia), and other problematic forms. Contrary to the deep-marine macroorganisms of the Avalon Assemblage, the Itajaí fossils are associated with abundant and exceptionally preserved three-dimensional microbial mats and microbially induced sedimentary structures (MISS) in relatively shallow settings (upper slope and distal delta front deposits). In this sense, the Itajaí biota could represent early adaptations of benthic macrobiota to the shallower and more photic environments that characterize the later White Sea Assemblage.  相似文献   

10.
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.  相似文献   

11.
The Cryogenian to Uppermost Ediacaran successions of the Tandilia System (Sierras Bayas Group and Cerro Negro Formation) in central-eastern Argentina is exceptional because of its unmetamorphosed and nearly undeformed character, its sediment provenance trend and the absence of any identified glacial deposit and the deposition of warm water carbonates. We decipher a dramatic change in the basin evolution from small-scale depositional areas during the Neoproterozoic to a larger basin related to an active continental margin throughout the Uppermost Ediacaran. The base of the succession is represented by immature detritus of alkaline composition (Villa Mónica Formation), but towards the top of this formation, the material is sorted and reworked, nonetheless still reflecting in its detritus the local rocks. The clastic deposition is interrupted by diagenetic overprinted dolomites. The unconformable overlying quartz-arenitic Ediacaran Cerro Largo Formation reworked the Cryogenian Villa Mónica Formation and contains mainly felsic granitic and metamorphic basement material of slightly wider variety, while the dominant alkaline geochemical signature in rocks of the Villa Mónica Formation disappears. Based on diagenetic, petrographic and sedimentological data, we can interpret the unconformity representing a longer time of erosion. The Cerro Largo Formation shows a transition to mudstones and the heterolithic facies of the Olavarría Formation. The top of the Sierras Bayas Group is represented by limestones (Loma Negra Formation), which are discordantly overlain by the Uppermost Ediacaran Cerro Negro Formation. The latter displays detrital material derived from a continental arc, mafic and felsic sources. Several arc-related geochemical proxies (Th/Sc?<?0.8; Zr/Sc?<?10; La/Sc?<?2; Ti/Zr?>?20) are recorded in the sediment detritus, as are syn-depositional pyroclastites. The absence of volcanic material in the underlying rocks allows proposing that the Cerro Negro Formation is related to an active continental margin fringing Gondwana (??Terra Australis Orogen??) as a retro-arc or retro-arc foreland basin.  相似文献   

12.
《Gondwana Research》2016,29(4):1449-1465
We report here in-situ U–Pb and Hf isotopic results of detrital zircons from sixteen Cambrian–Silurian siliciclastic samples across the Nanhua foreland basin, South China. Together with published data from Ediacaran–Silurian sandstones in the region, we establish the temporal and spatial provenance evolution across the basin. Except for samples from northeast Yangtze, all other Ediacaran–Silurian samples exhibit a prominent population of 1100–900 Ma, moderate populations of 850–700 Ma and 650–490 Ma, and minor populations of 2500 Ma and 2000–1300 Ma, grossly matching that of crystalline and sedimentary rocks in northern India. Zircon Hf isotopes further reveal four episodes of juvenile crustal growth at 2.5 Ga, 1.8 Ga, 1.4 Ga and 1.0 Ga in the source regions. Utilizing the basin history and late Neoproterozoic to early Paleozoic paleogeography of South China, we conclude that the Ediacaran–Cambrian sediments in the Nanhua foreland basin were mainly sourced from northern India and adjacent orogens, and the Ordovician–Silurian sediments were derived from both locally recycled Ediacaran–Cambrian rocks and eroded Cathaysian basement. The Wuyi–Yunkai late-orogenic magmatic rocks also contributed to the Silurian sediments in the basin. The upper-Ordovician to Silurian samples in northeast Yangtze received higher proportions of local Cryogenian (850–700 Ma) magmatic rocks which were uplifted during late-Ordovician to Silurian time. We speculate that there was an Ediacaran–Cambrian collisional orogen between South China and northern India, shedding sediments to the early Nanhua foreland basin. Far-field stress during the late stage of this collisional orogeny triggered the Ordovician–Silurian intraplate Wuyi–Yunkai orogeny in South China, and erosion of the local Wuyi–Yunkai orogen further provided detritus to the late Nanhua foreland basin.  相似文献   

13.
The second half of the Ediacaran period began with a large impact e the Acraman impact in South Australia, which was accompanied by a negative d13Ccarb anomaly and an extinction-radiation event involvi...  相似文献   

14.
The Carrancas Formation outcrops in east-central Brazil on the southern margin of the São Francisco craton where it comprises the base of the late Neoproterozoic Bambuí Group. It is overlain by the basal Ediacaran cap carbonate Sete Lagoas Formation and was for a long time considered to be glacially influenced and correlative with the glaciogenic Jequitaí Formation. New stratigraphic, isotopic and geochronologic data imply that the Carrancas Formation was instead formed by the shedding of debris from basement highs uplifted during an episode of minor continental rifting. Reddish dolostones in the upper Carrancas Formation have δ13C values ranging from +7.1 to +9.6‰, which is a unique C isotopic composition for the lowermost Bambuí Group but similar to values found in the Tijucuçu sequence, a pre-glacial unit in the Araçuaí fold belt on the eastern margin of the São Francisco craton. The stratigraphic position below basal Ediacaran cap carbonates and the highly positive δ13C values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high δ13C values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative δ13C excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitaí Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of São Francicso craton. Documentation of Cryogenian interglacial strata on the São Francisco craton reinforces recent revisions to the age of Bambuí Group strata and has implications for the development of the Bambuí basin.  相似文献   

15.
曹芳  刘新秒 《地质论评》2003,49(1):53-58
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。  相似文献   

16.
Large acanthomorphic acritarchs have been found in chert of the Ediacaran Doushantuo Formation at several localities in South China, including the East Yangtze Gorges of Hubei Province, Weng’an area of Guizhou Province, and elsewhere. However, their potential for biostratigraphic subdivision and correlation of Ediacaran successions is limited by facies control, taphonomic biases, and taxonomic problems. In the Yangtze Gorges, the Doushantuo Formation is generally subdivided into four lithologic members. However, in the Weng’an area, the Doushantuo Formation comprises just a lower and upper part separated by a mid-Doushantuo erosional surface. In the Yangtze Gorges at the Zhangcunping section, the Doushantuo succession is similar to that at Weng’an. So far, the correlation between the Yangtze Gorges and Weng’an area, and elsewhere has been an issue of debate. To resolve the debate, we selected eight sections in the Yangtze Gorges area and systemically sampled chert nodules of the Doushantuo Formation, focusing in particular on the upper Doushantuo Formation. Our data confirm two different assemblages appearing separately in the second and third members, which are separated by a negative δ13C excursion (EN2). The lower assemblage is characterized by Tianzhushania and a diverse suite of large acanthomorphic acritarchs. The upper assemblage is distinguished from the lower assemblage, by (1) absence of Tianzhushania; (2) occurrence of abundant, 100–150 μm, smooth-walled spherical microfossils; (3) occurrence of highly diverse acanthomorphic acritarchs including species extending from the second member and new forms in this member; (4) occurrence of unnamed new forms of protist; and (5) occurrence of the tubular microfossil Sinocyclocylicus guizhouensis. Since the Tianzhushania-dominated assemblage is not present in Australia, it seems that only the upper acanthomorph assemblage is present and thus the lower Doushantuo acanthomorph assemblage is missing in Australia.  相似文献   

17.
《Comptes Rendus Geoscience》2007,339(3-4):186-199
The Neoproterozoic-age Mali Group of the southwestern Taoudéni Basin, NW Africa, represents, in the Walidiala Valley, a glaciogenic and post-glacial succession that brackets the Cryogenian–Ediacaran period boundary. At its base, debris flows and turbidite-like, sandy units of the Pelel Member pass upward into siltstone and shale of the Diagoma Member. These two units represent the progressive evolution from some portion of a fan delta fed by a nearby ice shelf to a more distal environment disturbed only by the occasional fallout from passing icebergs. The appearance of coarse-grained, cross-bedded sandstone beds and gravels of the overlying Tanagué Member heralds a return to a shallower, fluvially influenced environment before abrupt transgression caps the glaciogenic succession. The transgressive unit consists of a regionally extensive, 2–7 m-thick, silty dolostone, the Bowal Member, which is isotopically and petrographically indistinguishable from ca. 635-Ma cap dolostone units elsewhere in NW Africa and worldwide. The Bowal Member comprises microcrystalline dolomite in turbidite-like depositional sheets disrupted by internal brecciation, fracturing and cementation by first chert and then dolomite. The stratigraphic succession in the Walidiala Valley closely resembles facies models relating to glacial retreat in a proximal glaciomarine environment affected by glacioeustasy. A large volcaniclastic debris flow has caused slumping and soft-sediment deformation within the cap dolostone of the Bowal Member. The widespread association of pyroclastic deposits with cap dolostone throughout the Taoudéni Basin implies that volcanism and deglaciation were roughly contemporaneous across a huge area. We consider that the volcaniclastic debris flow and soft-sediment deformation within the underlying Tanagué Member were possibly triggered by seismic activity during deglaciation, caused by isostatic relaxation of the lithosphere. However, fitted brecciation of cap dolostone beds here and elsewhere in the world is more consistent with pervasive dolomite cementation.  相似文献   

18.
先伊迪卡拉期的淮南生物群   总被引：17，自引：1，他引：17       下载免费PDF全文
在廿世纪中叶以前,人们对于先寒武纪的生物化石发现得很少,除藻类遗迹化石及一些可疑化石外,几乎没有什么可靠的生物化石。与此相反,在古生代地层中,特别是在早寒武世的地层中,却发现了极其丰富的发育较为高级的多门类的生物化石。当时,人们为了解释这种所谓突变现象,就提出了多种假说。但是,在廿世纪中叶以后,随着生产的发展和科学研究的深入,陆续地在晚先寒武纪地层中,发现了许多较大的生物化石、微古植物与微体化石等。由于晚先寒武纪生物化石资料的不断丰富,使得“灾变说”等不攻自破了。  相似文献   

19.
Neoproterozoic successions in western Shandong of North China Craton contain fine-grained siliciclastic rocks which offer a unique opportunity to explore the Neoproterozoic biosphere. In this study, we have revealed a well-preserved and diversified microfossil assemblage from the Tongjiazhuang Formation, Tumen Group in western Shandong, North China Craton, which is characterized by abundant smooth-walled sphaeromorphic acritarchs and cyanobacterium-like filamentous forms and relatively low abundance of more complex acritarchs. A total of 40 microfossil taxa belonging to 20 morphological genera have been identified, most of which are described for the first time from the Tongjiazhuang Formation of the Tumen Group in western Shandong, including Simia sp., Trachyhystrichosphaera aimika, T. botula, Germinosphaera bispinosa, Unnamed sp. A, Jacutianema solubila, and Pololeptus rugosus. The Tongjiazhuang assemblage revealed in this study is much more diverse than previously reported and well comparable to the Tonian Liulaobei and Gouhou assemblages in the Xuhuai region. Particularly, the occurrence of T. aimika, T. botula, G. bispinosa, J. solubila, and P. rugosus indicates a late Mesoproterozoic to Tonian age for the Tongjiazhuang Formation. In addition, the occurrences of macroscopic carbonaceous compressions, including Chuaria-Tawuia assemblage from the Tongjiazhuang Formation and Tawuia and Sinosabellidites from the Shiwangzhuang Formation, are consistent with geochronological data, suggesting a Tonian age for the Tongjiazhuang, Fulaishan, and Shiwangzhuang formations of the middle and upper Tumen Group, while the Heishanguan and Erqingshan formations of the lower Tumen Group are likely late Mesoproterozoic to Tonian in age. Thus, available biostratigraphic data, lithostratigraphic correlation, and geochronological data suggest that the whole Tumen Group is late Mesoproterozoic to Tonian in age, rather than Cryogenian or Ediacaran. This study significantly improves our knowledge about the diversity of the Tonian biosphere and implies a ∼200–300 million years unconformity (the ‘Great Unconformity’) between the late Mesoprotoerozoic to Tonian Tumen Group and the early Cambrian Liguan Formation in western Shandong.  相似文献   

20.
Doushantuo acanthomorphic acritarchs are large morphologically complex organic-walled microfossils broadly constrained between the 635 Ma Nantuo glaciation and the 551 Ma Miaohe Biota. They are potential biostratigraphic tools for subdivision and correlation of the Ediacaran System in South China. However, major variations in sedimentary facies and stratigraphic thickness present challenges in understanding the spatial and temporal distribution of these acritarchs. Further, the distribution of acritarchs in the Doushantuo Formation is associated with the presence of early diagenetic chert and phosphate, implying a certain degree of preservational bias and/or environmental control. The purpose of this paper is to document the stratigraphic distribution of Doushantuo acritarchs and to quantitatively evaluate their biostratigraphic significance and possible taphonomic–environmental biases, based on high-resolution paleontological data from six sections over 100 km in the Yangtze Gorges area, South China.A total of 1082 acritarch fossils were recorded from 84 chert horizons in the six sections of the study area. These chert horizons are not uniformly distributed throughout the Doushantuo Formation, thus the presence/absence of early diagenetic chert does play a role in controlling the distribution of acritarchs. The sampled chert horizons can be grouped into two stratigraphic intervals in the lower and upper Doushantuo Formation, respectively, based on regional stratigraphic correlation. Quantitative analysis shows that the two intervals are distinct taxonomically and largely independent of taphonomic or facies controls. Thus, the two intervals can be regarded as assemblage biozones. The lower biozone (biozone 1) is numerically dominated by Tianzhushania spinosa (n = 587; 68.3%) and Meghystrichosphaeridium magnificum (n = 74; 8.6%), whereas the upper biozone (biozone 2) is dominated by Ericiasphaera rigida (n = 104; 47.1%) and Tianzhushania spinosa (n = 34; 14.1%). The three most common genera, Meghystrichosphaeridium, Tianzhushania, and Ericiasphaera, have been identified in all sections. Correspondence analysis (CA), detrended correspondence analysis (DCA), discriminant analysis (DA), and pairwise comparisons of samples (Spearman rank coefficient and Jaccard–Chao index), all consistently support biostratigraphic zonations. Thus, the distribution of the Doushantuo acanthomorphs is primarily controlled by biostratigraphic position of samples, with facies and taphonomic differences playing a secondary role. Our case study suggests that acanthomorphic acritarchs can offer a viable tool for regional correlation of the Ediacaran System.  相似文献   

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