贵州五河南沱组形成的氧化还原环境:地球化学与矿物学证据

新元古代海洋氧化还原条件是影响这一时期生物演化的重要原因,海相沉积物记录的各种地球化学信息是研究古海洋氧化还原条件的主要方式.南沱冰期由于缺乏含铁建造等化学沉积岩及黑色页岩,极大地制约了对这一时期氧化还原环境的研究.本文通过对贵州剑河五河剖面南沱组红色富铁杂砾岩开展沉积学、矿物学和地球化学研究,认为杂砾岩中Si、Al、Fe含量与大陆上地壳组成类似,但Ca、Na、Mg显示显著的亏损特征,反映冰期大陆的氧化风化作用.杂砾岩中的氧化还原敏感元素Mo和U反映出不同程度的富集,其协变趋势指示了铁、锰氧化物在氧化还原分层的海水中的穿梭作用.杂砾岩的红色色调是其中赤铁矿的富集所致,红色富铁杂砾岩在南沱组中的不规则分布表明赤铁矿是同冰期海水中亚铁离子氧化后,经后期脱水形成.该剖面南沱组中流水波痕构造、氧化还原敏感元素Mo和U的富集特征和红色杂砾岩中赤铁矿的形成,反映华南地区南沱冰期极端缺氧的还原环境.     

峡东南华系南沱组底部冰川底碛沉积特征及其意义

峡东地区是我国原震旦系标准剖面所在地,该地区南沱组直接覆盖莲沱组之上,中间缺失下冰期和间冰期地层,究其原因尚不清楚.通过聚焦南沱组与莲沱组接触层位,利用现代冰川沉积物研究方法开展系统沉积学研究,结果表明:九龙湾周缘南沱组和莲沱组之间存在一层紫红色混合杂砾岩层,其内部砾石定向性、形态、磨圆、岩性以及显微构造与之上南沱组典型灰绿色杂砾岩区别明显,青林口剖面南沱组底部发育微观尺度上的变形沉积构造.表明南沱组底部与莲沱组接触层位为冰川底碛成因,而南沱组主体为冰海沉积成因.证明峡东地区南沱组与莲沱组之间地层缺失是由盛冰期的冰川剥蚀所致,南沱组可能仅代表了盛冰期之后冰消期的沉积记录.      

大庸市四都坪震旦系南沱组岩石特征及成因探讨

文中提出以砾、砂、泥作三端员成分进行陆源沉积岩结构分类的三角图解方案,并用于四都坪南沱组岩石的划分,将其划分为杂屑岩、角砾岩、杂砂岩、粉砂质泥岩等。杂屑岩在南沱组剖面中占82％,其中的砾石成分复杂,有砂岩、板岩、大理岩、花岗岩等,大砾石表面有冰川擦痕和磨光面。角砾岩中角砾成分单一,无大漂砾,具颗粒支撑,并见夹有塑性变形的含锰灰岩。杂砾岩中发育砂纹层理及冲刷构造。因此,认为南沱组在成因上与冰川活动有关,在冰浅海环境中堆积,同沉积断裂的存在提供斜坡条件,冰川重力及埋藏冰消融等引发重力流,使冰川沉积不时受颗粒流、碎屑流干扰,而在冰期末又遭冰水及海水的改造。     

鄂西地区南沱组地层沉积特征及成因分析

以沉积学，流体力学的基本原理为基础，运用沉积对比，粒度统计等方面对鄂西地区南沱组杂砾岩的成因机理进行了探讨，通过岩石类型，剖面层序，岩石的砾石表面特征、碎屑颗粒分布规律及沉积物构造研究后认为，南沱组杂砾岩并非冰碛砾岩，而属海缘盆地下水泥石流和浅海相沉积。     

滇东南沱冰期质疑

本文较详细地研究了滇东南沱组紫红色杂砾岩的特征,认为它不是冰碛岩,而是泥石流堆积物。其形成环境以小江断裂为界,可大致划分为泥石流沟和泥石流扇两个亚相。古气候的分析结果表明.南沱冰期很可能不存在。     

扬子克拉通北缘神农架-崆岭地区中-新元古代地层厘定——兼论“神农架群底界”

综合近几年来扬子克拉通北缘中新元古代最新研究成果,厘定扬子克拉通北缘神农架-崆岭地区中新元古代地层序列,自下而上依次为：吴家台组、神农架群、马槽园组、南沱组、陡山沱组和灯影组。吴家台组仅出露于距神农架不到20 km的北崆岭西汊河地区,为一套浅变质砾岩-含砾砂岩-细砂岩-白云岩组合,与下伏古元古界黄凉河岩组、上覆南华系南沱组均呈角度不整合接触,详细的年代学工作限定其形成时代为~1800-1500 Ma,代表扬子克拉通核部中元古代最早期沉积记录,可视作与神农架群底部-下部相当的层位（吴家台组与神农架群未见直接的接触关系露头）。神农架群主要岩性为藻礁白云岩、泥粉晶白云岩、粉砂岩等,夹砾岩、细砂岩、泥岩、火山岩层,与上覆马槽园组角度不整合接触,不见底,形成于~1.4-1.0 Ga。马槽园组为一套白云质或砂质胶结的杂砾岩、白云质砾岩,角度不整合于神农架群和南沱组之间。南沱组为杂砾岩-含砾粉砂质泥岩-碳质泥岩组合,形成时代为~657-635 Ma,属 Marinoan冰期产物。陡山沱组、灯影组为碳酸盐岩建造,是扬子克拉通基底之上第一套稳定盖层,沉积时限为~635-541 Ma,也是早期生命演化的关键时期。     

扬子北部三峡地区南沱组冰碛岩的物源特征:锆石年龄和地球化学证据

扬子陆块新元古代地层发育完整并广泛分布,三峡地区南沱组冰碛岩是扬子北部南华纪的冰成杂砾岩,不整合地覆盖在新元古代黄陵花岗岩之上.南沱组冰碛岩成分复杂,分选性差,缺乏可对比的标志,其沉积时代和物源组成研究较少且存在争议.对南沱组冰碛岩开展锆石U-Pb定年工作,得到最年轻的锆石年龄为706±7 Ma,印证了南沱组沉积时代应不早于700 Ma的说法,并可与Marinoan冰期对应.所获得锆石U-Pb年龄谱峰值主要分为~3.0 Ga,2.5~2.3 Ga,2.05~1.95 Ga和900~700 Ma,反映了扬子陆块4次地壳再造事件,其中古元古代和太古代年龄与崆岭杂岩相似,900~800 Ma和800~700 Ma的锆石年龄与附近黄陵花岗岩和扬子北缘岩体一致.冰碛砂岩的微量元素和同位素组成分异明显,可能是扬子北缘岩体、黄陵花岗岩和崆岭杂岩不同程度混合的结果.南沱组冰碛岩中包裹的花岗质砾石表现出不同的微量元素特征,具有偏低的稀土含量和亏损的重稀土组成,结合其锆石U-Pb年龄和Sr-Nd同位素特征,这些花岗质砾石主体可能来自于附近黄陵花岗岩和崆岭岩基.南沱组冰碛岩的物质组成特征反映了新元古时期,扬子地区强烈的裂谷岩浆作用所导致的北高南低的地势环境.      

贵州东部及邻区震旦纪铁丝坳期和南沱期沉积相与环境演化及构造属性探讨

贵州东部及其毗邻的汀西,川南东南区,下震旦统铁丝坳组和南沱组的沉积相类型可概括为冰附石相,陆源碎悄滑塌－重力流相及碳酸盐岩滑塌－重力流相,扇砾岩相。     

皖西凤台组重力流及滑塌沉积

分布于皖西淮南、霍丘一带的震旦系上统凤台组,是一套以杂砾岩为主的碎屑岩系,其中富含碳酸盐岩成份。在区域上,它与豫西罗圈组层位大致相当。对凤台组杂砾岩的成因,长期存在着几种不同认识,但大多数研究者认为是冰川形成的底碛和冰湖沉积。根据作者对该组杂砾岩的沉积构造、岩石学及地球化学特征的分析研究,认为它的成因颇为复杂,不仅具有冰碛岩的某些特征,而且还有重力流及滑塌沉积的一些特点。因此,凤台组可能是在冰前海湾(冰海)环境中的沉积。总之,这套杂砾岩显示出许多再沉积的特征。     

峡东陡山沱组丝状蓝藻化石的新资料

三峡东部震旦系广泛分布于黄陵背斜两翼,发育良好,是研究我国震旦系的典型地区。本区震旦系自下而上分为下统莲沱组和南沱组,上统陡山沱组和灯影组。     

Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway

ABSTRACT There is much debate regarding the intensity and geographic extent of glaciation during the Neoproterozoic, particularly in response to recent geochemical work suggesting that the Neoproterozoic earth was at times ice covered from equator to poles (the ‘Snowball Earth’ hypothesis). A detailed sedimentological analysis of the Neoproterozoic Smalfjord Formation of northern Norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation. In the Tarmfjorden area, the Smalfjord Formation consists of a stacked succession of diamictites interbedded with fine‐grained laminated mudstones containing rare outsized clasts. Diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin. In the Varangerfjorden area, chaotically interbedded diamictites, conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds; onediamictite unit at Bigganjargga overlies a striated pavement. The Varangerfjorden outcrops appear to record deposition on a subaqueous debris apron. Although diamictites contain rare striated and faceted clasts, suggesting a glacial sediment source, their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution. Rare lonestones may be associated with floating ice in the basin, which may be of glacial or seasonal origin. Glacial ice may have contributed poorly sorted glacial debris to the basin margin, either directly or through fluvioglacial systems, but there is no evidence of direct deposition by ice at Varangerfjorden or Tarmfjorden. The overall fining‐upward trend identified in the Smalfjord Formation and overlying Nyborg Formation is consistent with depositional models of rift basin settings. This fining‐upward trend, the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this Neoproterozoic succession. The Smalfjord Formation at Tarmfjorden and Varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball Earth hypothesis.     

Characteristics of the sinian glaciogenic rocks of the Shennongjia region,Hubei Province,China

Sinian glaciogenic rocks are widespread in Shennongjia, Hubei. They are best preserved in western Shennongjia where the glacial Gucheng and Nantuo Formations are separated by the non-glacial Datanpo Formation, which is chiefly composed of carbonaceous silty shale and siltstone. In eastern Shennongjia most of the Lower Sinian succession is, however, absent and the Nantuo Formation rests with angular unconformity on the pre-Sinian basement.The glacial sedimentary facies mainly comprise three types; namely the lodgement facies, meltwater facies and proglacial subaqueous facies. The lodgement facies is predominant in the eastern area but outwash deposits are more abundant in the western area. Major glacial transport during the Nantuo Ice Age is inferred to have been from the ESE, based on pre-glacial palaeogeography, provenance of clasts and regional variations in glacial sedimentary facies.During the Nantuo Ice Age, the Jingshan Mountain, which is situated to the east of Shennongjia, was a glacial source area. Jianghan glacial erosion and deposition was in the form of an incomplete ring-shape; the Mufu Mountain area, which is situated at the juncture between southeastern Hubei and northwestern Jiangxi provinces, and was far removed from the source areas, received outwash deposits.     

皖南新元古代冰期地层再认识

前人认为皖南地区新元古代冰期地层存在两套冰碛岩。通过对皖南蓝田新元古界剖面和皖南地区的另外3条新元古界剖面的重新观察,发现该剖面只存在一套冰碛层和一套盖帽白云岩,均未发现间冰期地层。植被覆盖和地层错断是造成前人认为本地区新元古代冰期地层存在两套冰碛层和两套盖帽白云岩的主要原因。这一套冰碛层与中国南方扬子区南华系南沱组相当。     

中国北方埃迪卡拉纪冰碛岩及冰川作用

中国北方从东到西绵延3000 km范围上的华北克拉通、塔里木克拉通及其中间微地块上都保存有埃迪卡拉纪冰川沉积记录,但有关它的年代学、冰川规模、古地理重建和大地构造背景等存有争论。本文基于中国北方埃迪卡拉纪冰碛岩空间分布、地层与沉积层序,沉积环境与沉积相等,结合以往国内外文献,系统分析了埃迪卡拉纪冰川上述有关问题。研究揭示,埃迪卡拉纪时期,国内外应存在年轻于(Gaskiers)580 Ma的冰期;中国北方埃迪卡拉纪冰川时限约562.5～551 Ma,堆积了冰下、冰缘和冰前沉积相(物),构成垂向上(由下至上)从冰下至冰前与海相冰碛物沉积层序,符合大陆冰川(盖)沉积响应样式;此外,国内外埃迪卡拉纪冰碛岩及冰川剥蚀地貌均十分发育;部分冰碛岩之上还可见盖帽白云岩,并呈现与成冰纪盖帽白云岩类似的沉积构造,但彼此碳同位素剖面却不尽相同;本研究推测,埃迪卡拉纪时期,原特提斯洋及周缘大陆(群)可能普遍存在至少是洲际性大陆冰盖,甚至是全球性的冰期。研究认为,埃迪卡拉纪时期的亚洲陆块群应与冈瓦纳大陆缺乏构造亲缘性的若干重要证据。本文研究结果在埃迪卡拉纪大陆及其古地理重建和大地构造背景恢复方面具有重要科学意义...     

Basinal setting and origin of thick (1·8 km) mass‐flow dominated Grand Conglomérat diamictites,Kamoa, Democratic Republic of Congo: Resolving climate and tectonic controls during Neoproterozoic glaciations

The Kamoa sub‐basin, in the south‐eastern part of the Democratic Republic of Congo, is a rift basin that hosts a world‐class stratiform copper deposit at the base of a very thick (1·8 km) succession of matrix‐supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet‐wide glaciation. Newly available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault‐related mass flow conglomerates and breccias (the ‘Poudingue’) with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over‐steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity, and was accompanied by syn‐depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or indirect, glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically‐driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic timescales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.     

Facies and Architecture of a Carboniferous Grounding-line System From the Guandacol Formation, Paganzo Basin, Northwestern Argentina

New outcrops of Middle Carboniferous glacigenic deposits found in the Guandacol Formation (western Paganzo Basin) are described in this paper. The study locality of Los Pozuelos Creek (northwestern Argentina) includes coarse-grained diamictites, rhythmites, laminated pebbly mudstones and shales that represent an expanded column of the Gondwanic glaciation in this region. Thirteen lithofacies recorded at the measured section have been grouped into three facies associations. Facies Association I is composed of coarse-grained massive and stratified diamictites (lithofacies Dmm, Dms, Dmg, Dcs), laminated siltstones with dropstones (Fld) and interstratified sandstones and mudstones (Fl, Sr). These rocks represent both tillites and resedimented diamictites closely associated to small water bodies where laminated siltstones with dropstones and stratified sandstones and mudstones were deposited. Facies Association II comprises couplets of matrix-supported thinly bedded diamictites (Dmld) and laminated mudstones with dropstones (Fld). This facies association results from the combination of three different processes, subaqueous cohesionless debris flows, coeval rainout of ice-rafted debris and settling of fine-grained particles from supension. Finally, Facies Association III is made up of laminated mudstones without dropstones, thin marl levels and scarce fine- to very fine-grained sandstones. This assemblage clearly suggests sedimentation in a deep marine environment below the wave base.The architecture of the glacigenic deposits has been investigated using photomosaic panels. The geometry of the depositional bodies and facies suggest that Los Pozuelos Creek outcrops exhibit a well preserved three-dimensional example of a grounding-line system. In particular, three different subenvironments of a morainal bank were interpreted: a bank-front, a bank-core and a bank-back. The bank-front assemblage is characterized by coarse-grained, mainly resedimented, diamictites grading laterally to prograding clinoforms composed of interbedded matrix-supported thinly bedded diamictite and mudstones. The bank-core assemblage is formed by a stacking of coarse-grained diamictites where at least five major erosional surfaces, bounding four multistory diamictite bodies, can be recognized. Finally, the bank-back assemblage corresponds to discontinuous intervals of striated lodgement till, and coarse-grained resedimented diamictites showing important post-depositional deformation. The retrogradational stacking of the morainal banks indicate an overall glacial retreat and a glacioeustatic sea-level rise. Erosional surfaces at the base of each morainal bank suggest intervening short term episodes of ice advance.The new data presented here confirm the existence of "true" tillites in western Paganzo Basin and suggest several (at least four) pulses of glacial advance and retreat during the Namurian glaciation in the region and permit a more refined interpretation of the glacial deposits in the Huaco area.     

Bolla Bollana boulder beds: A Neoproterozoic trough mouth fan in South Australia?

The Bolla Bollana Formation is an exceptionally thick (ca 1500 m), rift‐related sedimentary succession cropping out in the northern Flinders Ranges, South Australia, which was deposited during the Sturtian (mid Cryogenian) glaciation. Lithofacies analysis reveals three distinct facies associations which chart changing depositional styles on an ice‐sourced subaqueous fan system. The diamictite facies association is dominant, and comprises both massive and stratified varieties with a range of clast compositions and textures, arranged into thick beds (1 to 20 m), representing stacked, ice‐proximal glaciogenic debris‐flow deposits. A channel belt facies association, most commonly consisting of normally graded conglomerates and sandstones, displays scour and fill structure of ca 10 m width and 1 to 3 m depth: these strata are interpreted as channelized turbidites. Rare mud‐filled channels in this facies association bear glacially striated lonestones. Finally, a sheet heterolithics facies association contains a range of conglomerates through sandstones to silty shales arranged into clear, normally graded cycles from the lamina to bed scale. These record a variety of non‐channelized turbidites, probably occupying distal and/or inter‐channel locations on the subaqueous fan. Coarsening and thickening‐up cycles, capped by dolomicrites or mudstones, are indicative of lobe build out and abandonment, potentially as a result of ice lobe advance and stagnation. Dropstones, recognized by downwarped and punctured laminae beneath pebbles to boulders in shale, or in delicate climbing ripple cross‐laminated siltstones, are clearly indicative of ice rafting. The co‐occurrence of ice‐rafted debris and striated lonestones strongly supports a glaciogenic sediment source for the diamictites. Comparison to Pleistocene analogues enables an interpretation as a trough mouth fan, most probably deposited leeward of a palaeo‐ice stream. Beyond emphasizing the highly dynamic nature of Sturtian ice sheets, these interpretations testify to the oldest trough mouth fan recorded to date.     

碧口群内冰川沉积及冰期的划分与对比

碧口群内前人曾怀疑为冰碛岩的一套岩石,已取得了大量冰川证据,肯定了冰川作用的存在。其时代属早震旦世。据沉积特征、层位及上下接触关系等,将其划为长安、南沱两个冰期,南沱冰期又进一步划分为两个亚冰期,并分别与国内外有关地区作了对比。