Aspects of problematic acid-resistant,organic-walled microfossils (acritarchs) in the upper proterozoic of the north atlantic region

Proterozoic acritarchs are a heterogeneous group of acid-resistant, organic-walled microfossils of obscure biological affinities. The acritarch record is regarded as strongly baised by a combination of mechanisms, such as the character of the original ecosystems, depositional conditions, diagenetic history, and thermal and tectonic events.Three distinctive acritarch assemblages are recognized in Upper Proterozoic sequences in Scandinavia and Greenland. They can be compared with Upper Proterozoic (Upper Riphean and Vendian) assemblages previously reported from the Russian and Siberian platforms, China and elsewhere. Substantial taxonomic changes characterize these assemblages and are usually connected with the existence of major unconformities in the investigated rock sequences. The acritarch assemblages can be traced over vast areas and are considered as constituting a promising tool for the biostratigraphic correlation of the Upper Proterozoic and Lower Cambrian.     

内蒙古科尔沁右翼中旗地区古生界疑源类化石及其时代

内蒙古东部科尔沁右翼中旗地区出露一套古生代浅变质地层,长期以来在缺少大化石依据的前题下,仅依据岩性组合的区域对比,将其划归二叠系。近年结束的1∶5万区调,在该套地层中发现9属疑源类化石。将该9属疑源类化石与国内外已知时代地层的疑源类化石进行了对比研究,认为研究区发现的疑源类化石组合显示早寒武世的生物组合面貌,地层时代应厘定为早寒武世。研究区下寒武统的确定,为进一步研究大兴安岭中南段古生代地质构造演化提供了新的资料。     

后生动物遗迹的拓扑结构与演化

后生动物遗迹的拓扑结构可归并为３ 族、８ 种基本拓扑型、３ 种组合拓扑型和２１ 种拓扑遗迹类, 它们与习性类有较好的成因联系。中元古代以来后生动物遗迹的演化过程在质上经历了由简单到复杂, 由低级到高级的发展、演变过程。现今所看到的后生动物遗迹的拓扑类别和欧氏几何形态格局在显生宙初期即已形成; 至少从晚古生代以来, 后生动物遗迹不论在非同胚演化方面, 还是在同胚演化方面均无重大变化。后生动物遗迹的演化过程, 在量上具积累演化特征, 即拓扑类和习性类只有新生, 没有绝灭。新元古代以Ｅｄｉａｃａｒａ动物群为代表的后生动物大爆发和寒武纪生物大爆发分别奠定了后生动物遗迹的非同胚和同胚演化格局     

An Early Paleozoic Tectonic Mélange at the Western Margin of West Cathaysia: Constraints from Organic-walled Microfossils

The Jiangshan-Shaoxing-Pingxiang Fault(JSP Fault) is traditionally considered as the boundary between the Yangtze and Cathaysia blocks in South China. Whether the previously defined Shenshan and Kuli formations located along the JSP fault and near the Xinyu City, Jiangxi Province, are continuous strata or parts of a tectonic mélange is important for understanding the geological history of South China. A carbonaceous phyllite from the area, previously considered as part of the Neoproterozoic Shen...     

Carbon Isotope Features of the Sugetbrak Section in the Aksu?Wushi Area, Northwest China: Implications for the Precambrian/Cambrian Stratigraphic Correlations

The upper Qigeblaq Formation （Fm） dolostones and the Yurtus Fm phosphatic cherts, black shales, limestones, and dolostones are widely distributed in the Precambrian/Cambrian transitional succession of the Aksu-Wushi area. Negative δ13C excursion above the Yurtus Fm/ Qigeblaq Fm boundary was determined in this study. The pronounced negative carbon isotope excursion occurs in the phosphatic chert layers at the bottom of the Cambrian Yurtus Fm, below which the first appearance of the Asteridium- Heh＇osphaeridium-Comasphaeridium （AHC） acritarch assemblage zone. The δ13C curve of the lower part of the Yurtus Fm in the Aksu-Wushi area was found to be correlated with the early Cambrian δ13C curves of the Zhujiaqing Fm （Daibu Member）, the lower part of the Yanjiahe Fm on the Yangtze Platform in China, the lower Tal Fm in India, the Sukharikha Fm in Siberia, and the upper part of the Tsagaan Oloom Fm in Mongolia through biostigraphy. The lower part of the Yurtus Fm in the Tarim Basin is at the Nemakit-Daldynian stage, and the Precambrian/Cambrian boundary of the Aksu-Wushi area may be located in the phosphatic chert unit which just below the first appearance AHC acritarch assemblage zone. The negative δ13C excursion （N1） across the Precambrian/Cambrian boundary in the studied section may have resulted from oceanic overturning and sea level rise.     

鄂尔多斯地区西部和南部奥陶纪海槽边缘沉积特征与天然气成藏潜力分析

鄂尔多斯地区自中—晚元古代至早古生代继承性地发育区域性海相沉积,大体经历了中—晚元古代大陆裂谷及拗拉槽、寒武纪陆表海台地、奥陶纪局限海台地及构造转换等三个主要演化阶段。分析认为,奥陶纪在盆地西部和南部存在对天然气成藏较为有利的海槽/台地边缘沉积相带。中—上奥陶统平凉组—背锅山组是盆地西部和南部下古生界天然气成藏的主要烃源层;奥陶系礁滩体岩性圈闭是最主要的圈闭类型。盆地西部的天环北段和南部的麟游北为两个近期天然气勘探的有利目标。     

Submarine hydrothermal contribution for the extreme element accumulation during the early Cambrian,South China

Throughout the geological history of the Earth, submarine hydrothermal activity has played an important role in seawater chemistry, biological evolution and enrichment of metals in the Earth crust. However, the prospect of hydrothermal activity for extreme element accumulation during the early Cambrian, a key geological period, in South China has not been well-constrained. This study reports geochemical (e.g. REE and Sr isotope) investigations of a coarse-grained limestone layer and associated calcite veins in Zunyi and Nayong areas, Guizhou Province, to constrain the hydrothermal activity and evaluate the significance of hydrothermal contribution to extreme element accumulation during the early Cambrian, South China. Our results reveal positive Eu anomalies and higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7083–0.7150) for carbonate samples than those of early Cambrian seawater, indicating the presence of hydrothermal processes. Combined with constraints from the spatial relationships and coincidence with adjacent mineralization, these hydrothermal processes provide the most probable contribution for polymetallic Ni–Mo–PGE mineralization. Furthermore, there are abundant hydrothermal dolomite and barite-calcite veins in the dolostone of the Dengying Formation, indicating the occurrence of a variety of hydrothermal fluids. Overall, multi-stage hydrothermal pulses with different fluid compositions spanned the Ediacaran–Cambrian transition in South China. In particular, these hydrothermal fluids with positive Eu anomalies and enriched radiogenic Sr, originating from Proterozoic mafic/ultramafic rocks, may have flowed through the underlying Precambrian silicate clastic rocks (e.g., Xiajiang, Banxi and Lengjiaxi Groups) and may have been crucial for the marine environment, biological diversity and extreme element accumulation during the early Cambrian, South China.     

Sea-level changes,facies changes and the late Precambrian—Early cambrian evolutionary explosion

An explosive increase in the fossil record coincided with a major rise in sea level during latest Precambrian—early Cambrian times. The artefactual and real effects of an onshore—offshore environmental gradient upon taxonomic diversity are considered, with particular reference to 14 fossiliferous sections from around the world. Within-taxon and within-habitat evolution through the early Cambrian are also discussed. The evidence suggests that evolution was a diversity-dependent (i.e. kinetic) growth process, made possible by the colonisation and gradual filling of habitats that were growing both in variety and area. Archaeocyatha diversified rapidly at first but may have suffered later decline and extinction because facies changes in late early Cambrian times reduced their area of habitat.     

Towards understanding the early Gondwanan margin in southeastern Australia

This review synthesizes the Proterozoic and early Paleozoic geology of Tasmania, Bass Strait and western and central Victoria. We examine the many different conflicting hypotheses that have been proposed to solve the paradoxical relationships between Tasmanian geology and that of mainland Australia, most notably the prevalence of Proterozoic basement of western and central Tasmania, while immediately across Bass Strait evidence of Proterozoic rocks is much more cryptic. We conclude that the Selwyn block model is the most satisfactory hypothesis to date, since it fits best with the obvious patterns in the magnetic and gravity data. This model proposes that the central Victorian Melbourne Zone is underlain by the northern extension of thin Tasmanian Proterozoic and Cambrian crust under Bass Strait, and that the Silurian to Middle Devonian Melbourne Zone was shortened along a décollement during the Tabberabberan Orogeny. The Ordovician rocks of eastern Tasmania correlate more closely with the Tabberabbera Zone than the Melbourne Zone in Victoria; however the Silurian and Devonian correlations are less certain. Major unresolved issues are the origins of the Proterozoic and Early Cambrian lithostratigraphic packages, tectonic models for their assembly during the Tyennan Orogeny, and how these models fit with those for mainland Australia.     

雪峰山隆起北缘热历史及其对牛蹄塘组生烃作用的影响

对于复杂构造带的古老- 深层页岩，烃类的生成过程可能是多阶段的。准确恢复页岩的成熟演化过程是研究页岩气富集机理的先决条件。本文依托雪峰山隆起北缘新钻探的地质调查井，恢复雪峰山北缘古生代以来的热历史，从热演化的角度讨论牛蹄塘组页岩的生烃潜力。元素分析显示，热液活动对牛蹄塘组黑色页岩有机质的富集产生了积极影响，特别是下部页岩热液指示指标异常高、显著的Ce负异常、Eu正异常和Y正异常，都证明了牛蹄塘组早期受热液作用的影响。古温标联合反演结果显示，雪峰山北缘自古生代以来先后经历了三次升温—降温过程。三次热演化高峰依次出现在晚奥陶世末期、中三叠世末期和早白垩世末期，所达到的最高温度依次降低。三次升温过程分别受到早古生代拉张作用和岩浆活动、晚古生代—早中生代快速沉降作用和早白垩世岩浆活动的控制。受沉积埋藏作用和早期热事件的影响，牛蹄塘组页岩在寒武纪—早志留世快速经历了生油高峰、原油裂解高峰等生烃关键时期，在晚奥陶世达到过成熟阶段。随后第一次的抬升剥蚀作用，破坏了页岩及上覆盖层的封闭性，形成有利于气体扩散的裂缝或断层通道，使得早期形成的烃类散失。     

鲁西地区早前寒武纪地质研究新进展

近年来在鲁西地区早前寒武纪结晶基底进行锆石SHRIMP测年结果显示,泰山岩群形成年龄大约在2770～2750Ma。新太古代发生3期岩浆侵入活动,每期岩浆侵入活动开始都有地幔岩浆侵入。早期构造岩浆活动主要形成2740～2700Ma的英云闪长质片麻岩、条带状英云闪长质片麻岩;中期构造岩浆活动主要形成2650～2600Ma的TTG质花岗岩;晚期构造岩浆活动最强烈,从地幔岩浆侵入到地壳深熔形成大规模的钾质花岗岩,导致大规模陆壳的形成。古元古代早期和中元古代,有少量岩浆沿太古代刚性陆壳裂解形成的张性裂隙侵入。对鲁西地区前寒武纪侵入岩按岩性＋时代进行了重新划分,变质深成侵入体（片麻岩）按时代＋gn,依据其不同成分和不同结构构造的第一个英文大写字母进行表示,探讨了鲁西地区早前寒武纪陆壳演化历史。     

Differentiation and correlation of the Riphean rocks of Eastern Siberia

The Riphean (upper) portion of the Proterozoic interval is treated as an era divided into four systems on the basis of stromatolith morphology. The evolution of the stromatoliths is traced into the Lower Cambrian in order to demonstrate that divisions of system rank can be identified by the stages of stromatolith morphogenesis, from columnar through tabular and, in the Lower Cambrian, "nodular" and laminar. The Riphean stromatoliths, in the main, represent Baicalia and Conophyton, even through other major taxons are present. The authors, at the present stage in the the development of Riphean stratigraphy, seem to assign the rank of genera to Baicalia and Conophyton. --Mark E. Burgunker.     

A New Progress of the Proterozoic Chronostratigraphical Division

The Precambrian, an informal chronostratigraphical unit, represents the period of Earth history from the start of the Cambrian at ca. 541 Ma back to the formation of the planet at 4567 Ma. It was originally conceptualized as a "Cryptozoic Eon" that was contrasted with the Phanerozoic Eon from the Cambrian to the Quaternary, which is now known as the Precambrian and can be subdivided into three eons, i.e., the Hadean, the Archean and the Proterozoic. The Precambrian is currently divided chronometrically into convenient boundaries, including for the establishment of the Proterozoic periods that were chosen to reflect large-scale tectonic or sedimentary features(except for the Ediacaran Period). This chronometric arrangement might represent the second progress on the study of chronostratigraphy of the Precambrian after its separation from the Phanerozoic. Upon further study of the evolutionary history of the Precambrian Earth, applying new geodynamic and geobiological knowledge and information, a revised division of Precambrian time has led to the third conceptual progress on the study of Precambrian chronostratigraphy. In the current scheme, the Proterozoic Eon began at 2500 Ma, which is the approximate time by which most granite-greenstone crust had formed, and can be subdivided into ten periods of typically 200 Ma duration grouped into three eras(except for the Ediacaran Period). Within this current scheme, the Ediacaran Period was ratified in 2004, the first period-level addition to the geologic time scale in more than a century, an important advancement in stratigraphy. There are two main problems in the current scheme of Proterozoic chronostratigraphical division:(1) the definition of the Archean–Proterozoic boundary at 2500 Ma, which does not reflect a unique time of synchronous global change in tectonic style and does not correspond with a major change in lithology;(2) the round number subdivision of the Proterozoic into several periods based on broad orogenic characteristics, which has not met with requests on the concept of modern stratigraphy, except for the Ediacaran Period. In the revised chronostratigraphic scheme for the Proterozoic, the Archean–Proterozoic boundary is placed at the major change from a reducing early Earth to a cooler, more modern Earth characterized by the supercontinent cycle, a major change that occurred at ca. 2420 Ma. Thus, a revised Proterozoic Eon(2420–542 Ma) is envisaged to extend from the Archean–Proterozoic boundary at ca. 2420 Ma to the end of the Ediacaran Period, i.e., a period marked by the progressive rise in atmospheric oxygen, supercontinent cyclicity, and the evolution of more complex(eukaryotic) life. As with the current Proterozoic Eon, a revised Proterozoic Eon based on chronostratigraphy is envisaged to consist of three eras(Paleoproterozoic, Mesoproterozoic, and Neoproterozoic), but the boundary ages for these divisions differ from their current ages and their subdivisions into periods would also differ from current practice. A scheme is proposed for the chronostratigraphic division of the Proterozoic, based principally on geodynamic and geobiological events and their expressions in the stratigraphic record. Importantly, this revision of the Proterozoic time scale will be of significant benefit to the community as a whole and will help to drive new research that will unveil new information about the history of our planet, since the Proterozoic is a significant connecting link between the preceding Precambrian and the following Phanerozoic.     

The southern border of the Guaporé Shield in western Brazil and Bolivia: An interpretation of its geologic evolution

The geology of the southeastern and southwestern margins of the Guaporé Shield is described. The Shield rocks comprise high-grade granulite and low-grade greenschist sequences. These rocks are poorly known. Oldest recorded ages include Transamazonian datings (± 2 Ma). The margins of the shield show the development of a mobile belt formed by rocks of Upper Proterozoic to Cambrian age. Within this mobile belt, the Cuiabá Group of uncertain (but possibly Upper Proterozoic) age outcrops to the east and shows tectonic contacts with younger rocks that span the Vendian (Upper Proterozoic) to Cambrian age. The rocks of Vendian to Cambrian age form three isolated elongated basins which show evidence of fault control. The axes of these basins are equally separated by angles of ～ 120° and meet at a triple junction near Corumbá. The stratigraphy of the Vendian to Cambrian basins is correlated from Bolivia to Brazil, based on tectonic continuity, sedimentological similarity and fossil content. The central location at Corumbá shows instability since the Middle Proterozoic when important igneous activity is recorded at Rincon del Tigre (Bolivia) and in Brazil. This igneous activity is interpreted as being related to hot-spot activity in an intercratonic setting from which the younger Upper Proterozoic basins originated.     

Tectonic significance of deformation patterns in granitoid rocks of the Menderes nappes, Anatolide belt, southwest Turkey

Deformation fabrics in Proterozoic/Cambrian granitic rocks of the Çine nappe, and mid-Triassic granites of the Bozdag nappe constrain aspects of the tectonometamorphic evolution of the Menderes nappes of southwest Turkey. Based on intrusive contacts and structural criteria, the Proterozoic/Cambrian granitic rocks of the Çine nappe are subdivided into older orthogneisses and younger metagranites. The deformation history of the granitic rocks documents two major deformation events. An early, pre-Alpine deformation event (D_PA) during amphibolite-facies metamorphism affected only the orthogneisses and produced predominantly top-to-NE shear-sense indicators associated with a NE-trending stretching lineation. The younger metagranites are deformed both by isolated shear zones, and by a major shear zone along the southern boundary of the Çine submassif. We refer to this Alpine deformation event as D_A3. D_A3 shear zones are associated with a N-trending stretching lineation, which formed during greenschist-facies metamorphism. Kinematic indicators associated with this stretching lineation reveal a top-to-south sense of shear. The greenschist-facies shear zones cut the amphibolite-facies structures in the orthogneisses. ²⁰⁷Pb/²⁰⁶Pb dating of magmatic zircons from a metagranite, which crosscuts orthogneiss containing amphibolite-facies top-to-NE shear-sense indicators, shows that D_PA occurred before 547.2ǃ.0 Ma. Such an age is corroborated by the observation that mid-Triassic granites of the Çine and Bozdag nappes lack D_PA structures. The younger, top-to-south fabrics formed most likely as a result of top-to-south Alpine nappe stacking during the collision of the Sakarya continent with Anatolia in the Eocene.     

Growing Gondwana and Rethinking Rodinia: A Paleomagnetic Perspective

The formation of Gondwana during the late Neoproterozoic to early Cambrian times (550-530 Ma) was traditionally viewed as the welding of two, more or less contiguous, Proterozoic continental masses called East and West Gondwana. The notion of a united West Gondwana is no longer tenable as a wealth of geochronologic and structural data indicate major orogenesis amongst its constituent cratons during the final stages of greater Gondwana assembly. The idea that East Gondwana may also have formed through the amalgamation of a collage of cratonic nuclei during the Cambrian is controversial. Recent paleomagnetic, geochronologic and structural data from elements of East Gondwana indicate that its formation may have extended well into Cambrian time. Thus, the terms ‘East’ and ‘West’ Gondwana may be relegated to convenient geographical terms rather than any connotation of tectonic coherence during the Proterozoic. In addition, the paleomagnetic data also challenge the conventional views of the Neoproterozoic supercontinent Rodinia and the SWEAT fit. Alternative variants including Protopangea and AUSWUS are not supported by paleomagnetic data during the interval 800–700 Ma.     

摩洛哥前阿特拉斯山脉前寒武系-寒武系界线的地层概述

作者首先评述了前阿特拉斯山脉（摩洛哥）古生代山系前寒武纪盖层的三个部分:火山质和磨拉石质前寒武纪Ⅲ岩系、白云质阿土图期（Adoudounian）岩系和塔利文期（Taliwinnian）岩系（“酒红色（lie de vin）”岩系）,塔利文期岩系主要是陆相的,但西部则发育成海相含叠层石地层,据此有可能确定由塔利文阶到下寒武统的过渡。随后的几个章节介绍了下寒武统的发现和研究简史,以及对前阿特拉斯山脉西部下寒武统含三叶虫“阶”的概述。文章还附有若干幅阿土图期、塔列文期和早寒武世的古地理图,以及一些地层和构造剖面图。     

Glaciation and ~ 770 Ma Ediacara (?) Fossils from the Lesser Karatau Microcontinent,Kazakhstan

The Cambrian explosion, c. 530–515 Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635–542 Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~ 750–635 Ma Neoproterozoic glacial episodes by 20–30 million years. In this paper, we present evidence for the oldest discovery of the ‘Ediacara’ discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. “Marinoan”) glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata.     

Acritarchs from the Lower Palaeozoic succession on the south County Wexford coast,Ireland: new age constraints for the Cullenstown Formation and the Cahore and Ribband Groups

Lower Palaeozoic sediments crop out on the southern coast of County Wexford, Ireland, comprising three distinctive lithostratigraphical units: from west to east the Cahore Group, the Blackhall Formation of the Ribband Group and the Cullenstown Formation. The three units are largely devoid of macrofossils and thus their ages have to date been uncertain and, in the case of the Cullenstown Formation, speculative. In the Cahore Group, a diverse assemblage of acritarchs composed of seventeen species has been recorded indicating a middle Early Cambrian age. This is similar to the age of the lithologically identical Bray Group to the north, in County Wicklow. In the Ribband Group, two very distinct assemblages have been noted. Both are poorly preserved, but diagnostic species have been determined giving a biostratigrapical range of early Mid‐Cambrian to Llanvirn. Palynomorphs have been recorded for the first time from the Cullenstown Formation allowing comparison with eastern Newfoundland where a similar, less diverse assemblage has been recorded. The age indicated is latest Mid‐Cambrian to early Late Cambrian. Overall, despite generally poor preservation of the organic matter, some 45 acritarch species have been distinguished, among which one new combination is proposed: Retisphaeridium postae instead of Cymatiosphaera postae. Copyright © 2004 John Wiley & Sons, Ltd.     

Origin and variability of a terminal Proterozoic primary silica precipitate,Athel Silicilyte,South Oman Salt Basin,Sultanate of Oman

The Precambrian–Cambrian Athel Silicilyte is a 400 m thick, salt‐encased siliceous succession in the South Oman Salt Basin. It is a self‐sourcing hydrocarbon reservoir and comprises up to 95% microcrystalline quartz and exhibits wavy discontinuous lamination, comprising thin, alternating organic‐rich and silica‐rich layers. Textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water, within the water column at the sulphidic/oxic interface. The unique occurrence of the Athel Silicilyte in the terminal Proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly. Basin anoxia, coupled with the growth of microbial mats, low pH and high silica pore water saturations, created optimal chemical conditions for silica precipitation. Volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the Athel Basin. At the basin margins, and within the late highstand systems tract, volumes of microcrystalline quartz decreased as the volume of detrital sediment increased. Mass‐balance calculations indicate that silica‐enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column. In conclusion, precipitation of the Athel Silicilyte was driven by the coincidence of basin restriction, limited clastic input, availability of organic matter and water column anoxia. The observation that there are few documented examples of chert deposits younger than ca 700 Ma, prior to the Cambrian explosion, suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica‐secreting organisms, direct precipitation from sea water was restricted. This is tentatively related to the gradual increase in alkalinity of sea water through the Palaeo‐Proterozoic and Meso‐Proterozoic, such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input.