Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of ²⁴Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of ²⁶Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ²⁶Mg_sw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ²⁶Mg_sw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ²⁶Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in ²⁴Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ²⁶Mg_sw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ²⁶Mg_sw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ²⁶Mg of the Huangjin limestone makes it difficult to reconstruct δ²⁶Mg_sw using bulk carbonate/calcareous sediments. Finally, δ²⁶Mg_sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.     

Fluvial channel and overbank deposits from the Westphalian of the Durham coalfield, NE England

The Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England were deposited in lower and upper delta plain environments. Distributary channels crossed the plain and were separated by shallow, interdistributary lakes and bays. Detailed observation of three-dimensional (3-D) opencast (surface) mine exposures, in collaboration with subsurface borehole analysis, has revealed the existence of five varieties of channel deposits and two associated overbank facies within the Durham Coal Measures. Major distributary channels were the major avenues of sediment transport across the Coal Measures plain, were variably sinuous, mostly 1–2 km wide and deposited elongate belts of sand mostly up to 5 km wide. Proximal, major crevasse splay channels formed by the breaching of major channel banks during flood events, were straight, sand-filled and up to 400 m wide. Minor distributary channels formed by the sustained operation of such crevasses, varied from straight to highly sinuous, and deposited ‘shoestring’ sand/mud belts up to a few hundred metres wide. Minor crevasse channels, generally straight and up to 50 m wide, were formed through bank breaching of minor, and in a few cases major, distributaries. Distal feeder channels formed down-palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems, were generally straight and up to 200 m wide. Of the two types of channel overbank (levee) deposits recognized, one, comprising thinly interbedded fine-grained sandstone and siltstone/claystone, is mostly, though not exclusively, associated with major distributary channels. The other, consisting of ‘massive’ siltstones with regularly spaced, thin claystone bands, is uniquely developed at the margins of minor distributary channels. The lower part of the Westphalian A succession in the northern Pennines records a change in the depositional environment upwards from a lower to upper delta plain. Through this transition, major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology. Channel sedimentation was profoundly influenced by regular, possibly seasonal, variations in flow stage and sediment load.     

Fluvial deposits within the Pleistocene Crag at Thorpe St Andrew,Norwich, England

This paper records the findings at a temporary exposure at Thorpe St Andrew near Norwich, Norfolk, UK in Early and early Middle Pleistocene Crag deposits. The British Geological Survey (BGS) describes the particular formation exposed as Norwich Crag consisting of Early Pleistocene shallow marine sediments. The section shows a succession of sorted sands and gravels overlain by a sandy diamicton. Based on field evidence and clast analysis, the sands and gravels are interpreted as the product of point bar and overbank sedimentation and represent the product of a river cutting into and aggrading within the more widespread shallow marine deposits. Composition of the sediments indicates derivation, primarily from Wroxham Crag Formation, with a contribution from Norwich Crag. The sandy diamicton is interpreted as late Middle Pleistocene Corton Till that is recorded in the area. A distinct pattern of colour changes at the top of the sands and gravels is interpreted as a soil that developed on the fluvial sediments before being overridden by the glacier that deposited the Corton Till. The existence of the fluvial sediments within the regional shallow marine deposits suggests that a fall of sea-level, possibly due to climate cooling, while the elevation of the sediments and the adjacent Crag implies that the site has been uplifted since sedimentation. This is the first observation of terrestrial sediments within the shallow marine Crag. The paper also makes a contribution to understanding the diagenetic processes that give deposits within this region some distinctive colour and sediment patterns.     

内蒙古布日敦湖湖泊沉积物粒度的风沙活动指示意义

本文通过研究位于东亚季风过渡带的布日敦湖湖泊沉积物粒度组成和碳、氧同位素特征,结合²¹⁰Pb/¹³⁷Cs年代数据,建立布日敦湖近50年的粒度变化序列。通过与中国北方100个气象站数据的年平均沙尘暴日数进行对比,建立研究区可靠的风沙活动指标。研究发现,布日敦湖湖泊沉积物粒度特征对研究区风沙活动具有指示意义。其中,细颗粒(4~16μm)含量变化与翁牛特旗沙尘暴日数变化趋势较吻合,可以指示区域风沙活动;而粗颗粒组分(>64μm)含量变化趋势与沙尘暴日数变化差异较大,推测可能受到大规模人类活动导致的湖区附近土地沙化影响,所以不能指示风沙活动。研究区生态环境脆弱,大规模人类活动会导致生态环境恶化和土地沙化,为现代北方沙尘活动提供沙源,并为未来的沙尘活动恶化埋下隐患,因而研究区不适宜进行大规模人类活动。本文的研究成果对保护研究区生态环境和区域防风固沙具有重要意义。     

一种事件沉积的标志——陆相流水波痕

本文从现代洪水事件作用可以形成大片不对称流水波痕人手,"将今论古",发现古代地层剖面中也有大量水下至水上的沉积旋回序列,每个旋回单元与下伏岩层为突变接触。水下沉积环境富含不对称流水波痕,水上则为富含泥裂,它们系为洪水事件沉积产物。这些洪水事件具有不同的规模和级别,在地学上具有重要意义。     

与细菌活动有关的成矿作用——两个欧洲铅锌矿床实例

法国布·玛和西班牙拉·弗矿床分别产于富含干酪根的白云岩和沥青的铁质白云岩中。正构烷、烷基环已烷、藿烷和甾烷等生物标志化合物研究揭示了成矿时发生过细菌活动。细菌在沉积－成岩早期发育于还原水体中，或在成岩期－成岩期后由大气降水自地表带来，其赖以生存的有机质既可以是生物聚合体（布·玛矿床），亦可以是地质聚合体（拉·弗矿床）。这些微生物在１００℃以下的温度条件下生存繁殖，使硫酸盐还原生成硫化氢，导致铅锌硫化物沉淀     

河流相古土壤及其在河流沉积地层对比中的应用

本文以济阳坳陷孤东油田馆上段河流相古土壤为例,对古土壤这一新的领域进行探索,主要论述河流相古土壤的识别及成因,并以古土壤为对比标志进行河流沉积地层的对比,为解决河流相储层对比这一人所公认的技术难题提出了一个新的方法。     

Trend-surface analysis as an aid in exploration for Mississippi valley-type ore deposits

An examination of the characteristics of Mississippi Valley-type ore deposits shows that these deposits have many similarities which appear to be structurally controlled. To define the relationship between the ore deposits and these structural conditions, trend-surface analysis using the Precambrian surface as the datum plane was performed within the general Missouri area. Examination of the residual maps prepared from this analysis shows that all the ore deposits have the same spatial relationship to areas of strong residual highs. A more detailed study was conducted in the area of the Viburnum Trend using a datum plane just below the ore horizon. The spatial relationship shown on the Precambrian residual map was even better defined in this study. From this study, it appears that the optimum structural conditions necessary for the deposition of Mississippi Valley-type ore deposits exist in regions located next to and slightly downgradient from areas which exceed one positive standard deviation from the trend surface. The methods described in this paper could comprise an extremely useful tool in exploration programs as they can define high probability locations in which to concentrate further exploration efforts.This paper was presented at Symposium 116.3, Quantitative Strategy for Exploration, held as part of the 25th International Geological Congress, Sydney, Australia, August 1976.     

Geological mapping of rapid mass movement deposits as an aid to land-use planning

Rapid mass movements are common features of hilly or mountainous terrains, and depending on the type of movements or processes involved these give rise to characteristic deposits. Identification of these deposits in mapping programmes gives a clue as to their mode of origin (rockslide/rockfall, snow avalanche, debris flow, fluidal sediment flow) and this can be used to predict rapid mass movements in the future.

A case story from western Norway is described and the value of mapping such deposits for land-use planning and hazard estimation stressed.     

Using the geologic setting of talc deposits as an indicator of amphibole asbestos content

This study examined commercial talc deposits in the U.S. and their amphibole-asbestos content. The study found that the talc-forming environment directly influenced the amphibole and amphibole-asbestos content of the talc deposit. Large talc districts in the U.S. have mined hydrothermal talcs that replaced dolostone. Hydrothermal talcs, created by siliceous fluids heated by magmas at depth, consistently lack amphiboles as accessory minerals. In contrast, mineable talc deposits that formed by contact or regional metamorphism consistently contain amphiboles, locally as asbestiform varieties. Examples of contact metamorphic deposits occur in Death Valley, California; these talc-tremolite deposits contain accessory amphibole-asbestos. Talc bodies formed by regional metamorphism always contain amphiboles, which display a variety of compositions and habits, including asbestiform. Some industrial mineral deposits are under scrutiny as potential sources of accessory asbestos minerals. Recognizing consistent relations between the talc-forming environment and amphibole-asbestos content may be used in prioritizing remediation or monitoring of abandoned and active talc mines.     

The comparison of strata-bound massive sulfide deposits using the fuzzy-linguistic diagnosis of the Zlaté Hory deposits,Czechoslovakia, as an example

The Zlaté Hory ore deposits — as an example of volcanogenic, strata-bound, massive, base-metal sulfides occurring in the Devonian formations of the Jeseníky Mts. — are compared with distinctive Phanerozoic types representing this type of ore deposit, i.e., with Kuroko-type, Rosebery-type, Besshi-type and Cyprus-type deposits. The results of comparison performed with fuzzy-linguistic diagnosis indicate close resemblance of the Zlaté Hory deposits to Rosebery-type massive sulfides with regard to primary features; however, individual features point to certain original similarity with Kuroko-type deposits, too. The metamorphic history of the ore deposits studied was similar to that of the Rosebery and Besshi types.     

Numerical Analysis of Fluvial Landscapes

The Smith and Bretherton model for fluvial landsurfaces consists of a pair of partial differential equations: one governing water flow and one governing sediment flow. Numerical solutions of these equations have been shown to provide realistic models of the evolution of fluvial landscapes. Further analysis of these equations shows that they possess scaling laws (Hack’s Law) that are known to exist in nature. The preservation of these scaling laws in simulations is highly dependent on the numerical method used. Two numerical methods, both optimized for overland flow, have been used to simulate these surfaces. The implicit method exhibits the correct scaling laws, but the explicit method fails to do so. These equations, and the resulting models, help to bridge the gap between the deterministic and the stochastic theories of landscape evolution. Slight modifications have been made to this model to make the resulting surfaces more realistic. The most successful of these was the addition of an abrasion term to assist in the channelization of rivers.     

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D_1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D₁, D₃, D₄, and D₅, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D₁ and syn- to post-D₃. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D₁ veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D₁) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D₃ and D₄ phases. Little is known about the nature of the D₁ ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D₁ indicates a high temperature for the fluid. The D₃ hydrothermal phase coincided with peak metamorphism. D₄ fluids were hypersaline, high temperature, CO₂-poor, and H₂S-poor. Editorial handling: L. Meinert     

人类活动强度空间化方法综述与展望

地球系统受到气候变化和人类活动的双重影响,相比于气候变化研究相关的空间数据在数据采集、制备方法、制备速度、时空分辨率和数据质量等各方面的快速发展和提高,人类活动强度空间数据还处于探索发展阶段。人类活动强度空间数据有助于更好的理解人类活动影响的强度和范围,对于探究人类活动对地球系统各圈层及其变化的影响,以及促进全球和区域可持续发展具有重要意义。基于此,通过对人类活动强度空间数据相关研究进行梳理和分析,首先提出人类活动强度空间化的概念,然后归纳总结六类主要的空间化方法,并以青藏高原地区为例重点展示和分析四类应用较为广泛的方法,包括净初级生产力人类占用法、土地类型变化法、全球干扰指数法和人类足迹指数法等。最后讨论目前人类活动强度空间化研究中存在的问题,并对未来的发展提出建议。     

河流相溢岸沉积特征及其层序地层意义——以大庆长垣扶余油层为例

相对于河道内沉积,作为河流相沉积主要部分的溢岸沉积占河流相地层比例可达70%。基准面旋回控制下的溢岸频率和沉积速率等因素决定了溢岸沉积的结构、厚度和横向展布等特征。分析不同基准面旋回位置内溢岸沉积特征,可以建立基准面旋回内溢岸沉积的演化模式;反之,通过溢岸沉积结构特征和地层叠加样式可以判断和识别基准面信息,指导划分层序界面和建立层序地层格架。以大庆长垣扶余油层为例,通过岩心观察识别溢岸沉积不同岩相类型及其岩相组合和序列特征,统计研究区120井决口河道、决口扇、决口扇复合体的垂向厚度分布,并分析在扶余油层整体基准面上升过程中,河流相溢岸沉积发育特征。研究表明,河流相地层内溢岸沉积和河道沉积存在此消彼长的相对关系:随着基准面的上升,河道溢岸沉积逐渐发育,河道溢岸沉积速率和溢岸频率逐渐增加,河道逐渐呈分散状态,地层内溢岸沉积厚度、延展范围和比例逐渐增大;基准面下降期,其特征相反。区域地层发育较薄、溢岸沉积不发育或较少发育位置,可作为区域基准面下降到上升的转换面;而地层发育较厚、溢岸沉积发育程度较大位置,可作为基准面上升到下降的转换面。     

基准面旋回在砂岩型铀矿勘查中的应用--以东胜铀矿床为例

近年来以基准面旋回为参照面的高分辨率层序地层学迅速发展,其最大的优势在于能够适用于受多重因素控制的陆相沉积盆地,并有效提高层序地层分析的准确度和分辨率。基准面旋回原理也同样适用于砂岩型铀矿的勘探开发研究和实践,它控制了砂岩、泥岩的空间分布和岩石孔渗、封闭性能,决定了地层结构的叠加样式。文章以东胜铀矿床为例,分析了基准面旋回在砂岩型铀矿勘探开发中的应用。认为:基准面升降旋回中形成的砂、泥岩地层结构配置良好;长期基准面上升早期形成的辫状河河道砂岩具连通性好、粒度粗、碎屑含量高、杂基含量低及孔渗条件好等特点,是重要的含矿层。     

矿浆型铁矿的氧同位素判别标志:以宁芜玢岩铁矿为例

自然界是否存在矿浆型铁矿以及如何判别矿浆型铁矿是地质学家争论探索了几十年的问题。大量地质现象和实验研究证实,中酸性岩浆在高氧逸度、富磷等挥发分和助熔剂的条件下,硅酸盐熔体与铁氧化物熔体之间可以发生液态不混熔,熔离出富铁氧化物熔体或富铁岩浆。但有些学者认为实验无法直接一次性熔离出高品位铁矿浆,因此不存在矿浆型铁矿。实际上,高品位富铁矿浆可能是经过多次熔离富集形成的,而非一次简单熔离完成;磁铁矿中钛含量偏低,可能与矿浆型铁矿遭受后期热液改造、钛大量丢失有关,最典型的例子莫过于智利拉科铁矿。长江中下游玢岩铁矿是我国重要铁矿资源类型,其中是否发育矿浆型铁矿也一直存在激烈争论。为了避免不必要的争议,本文将铁矿浆限定为由岩浆熔离形成的铁氧化物浓度30%的富铁熔体,由铁矿浆演化形成的铁矿床称为矿浆型铁矿。根据宁芜成矿岩体中锆石的钛温度计确定了岩浆的温度,根据锆石的氧同位素组成及磁铁矿与锆石之间的氧同位素分馏方程,计算出岩浆温度下直接从熔体中熔离出来的磁铁矿的δ~(18)O_(Mt)值为4.2‰。据此建立了宁芜玢岩铁矿中矿浆型铁矿的氧同位素判别标志,如果矿体中磁铁矿的δ~(18)O_(Mt)≥4.0‰,则为矿浆型铁矿,否则为热液型或浆-液过渡型铁矿。判别结果表明,钟姑山矿田矿浆型和热液型矿体同时存在,梅山铁矿介于矿浆型-热液型之间,而凹山矿田铁矿则为热液型,与野外观测及前人研究结果一致。     

Floodplain lakes as an archive for the metal pollution in the River Elbe (Germany) during the 20th century

The German Elbe River floodplains rank under the most polluted areas in Europe. A sudden concentration increase of potentially harmful elements (PHEs) is documented in sediment profiles of Elbe bayous. The increase is dated to the mid of the 20th century (¹³⁷Cs) and indicates industrialization of the former German Democratic Republic (GDR) as the source of intense pollution. The collapse of the GDR and the industries in the 1990s is traced by a concentration decrease in young sediments. The contamination is restricted to an increase of unstable binding forms; the hydroxide binding form is predominant. The geogenic concentration portions are of normal level and remain stable throughout the profiles (0–2 m). The equilibrium of contradictory binding forms in the sediments makes it mandatory not to interfere with the thermodynamic conditions and to keep the Elbe floodplain as an undisturbed ecological system.     

Oceanic circulation as an element in palaeogeographical reconstructions: the Arenig (early Ordovician) as an example

A new conceptual palaeo-oceanographical model is outlined in this paper. The model differs from previous models by using shifts in Hadley circulation caused by orbital variations, and the rotation rate of the Earth, to locate the position of the planetary oceanic low-and high-pressure systems, around which the planetary ocean surface currents flow. Adapting the model to the Arenig (early Ordovician) the temperate low pressure zones were found to be located at 50° latitude and the subtropical high pressure zones at 25° latitude.
Traditionally, most Palaeozoic palaeogeographical recon-structions are reconstructed using palaeomagnetic data supplemented with data from climate-sensitive lithofacies and palaeo-biogeographical distributions. However, as a new approach in palaeogeographical reconstructions, the con-ceptual palaeo-oceanographical model is combined with palaeobiogeographical data for the Arenig series, comple-menting the palaeomagnetic data, and resulting in a new, refined palaeogeographical reconstruction.     

风成砂沉积和古气候研究

作为一类常见的陆相沉积岩类型,风成砂岩产出于太古代到新生代的岩石记录中。风成砂沉积的形成与古气候、古环境密切相关,因而地质历史中的风成砂沉积是研究古气候和古地理环境的重要窗口。本文回顾了国内外风成砂沉积的研究进展,着重讨论了气候因素控制下风成砂沉积的成因及其形成过程。当前,国际研究注重风成沙丘形成过程的推理和模拟以及风成砂的沉积成岩过程,在风成沙丘形成过程、计算模拟、沉积保存的四维时空模型、风成相等方面取得了许多新认识。鉴于国际研究动态,国内需要在地层记录中鉴别和剖析风成砂的宏观和微观形态特征基础上,加强风成砂沉积动力学过程研究,增强对风成砂沉积(微)环境的理解和认识。