Paleoceanographic Indicators for Early Cambrian Black Shales from the Yangtze Platform, South China: Evidence from Biomarkers and Carbon Isotopes

The lower Cambrian Niutitang Formation, a widespread black shale deposition, is of geological interest because of its polymetallic beds, Cambrian explosion, depositional ages, dramatic environmental changes and so on. Previous study focused mainly on inorganic geochemistry and few studies have investigated the organic fractions of upper Neoproterozoic-lower Cambrian strata in South China. Here we report a study of biomarkers plus organic carbon isotopes for black shales from Ganziping, Hunan Province （China）. All the saturated hydrocarbon fractions have a unimodal distribution of n-alkanes, a high content of short-chain alkanes and maximize at C 19 or C 20 （C 23 for sample Gzh00-1）. The C 27 /C 29 sterane ratio ranges from 0.77 to 1.20 and 4-methylsteranes are in low abundance. These parameters indicate that algae and bacteria are the important primary producers. Furthermore, biomarker maturity proxies show the samples to be higher maturity. The low Pr/Ph values （0.7） suggest that the samples were deposited under anoxic conditions and, likely, under stratified water columns. In addition, 25-norhopanes and gammacerane are present as diagnostic indicators of normal marine salinity and dysoxic to anoxic conditions. During the Early Tommotian, known to coincide with a transgression event, small shelly fossils increased in abundance and diversity. Moreover, positive δ 13 C org excursions close to 1.4‰ occur at the base of the Tommotian stage. In summary, the Early Cambrian black shales were deposited under dramatic paleoenvironmental changes, including oceanic anoxia, higher primary productivity and sea-level rise.

The Trace Element Distribution Patterns of Ediacaran-Early Cambrian Black Shales and the Origin of Selenium in the Guangning Area, Western Guangdong Province, South China

The Ediacaran and early Cambrian black shales are widespread across the South China Craton （Yangtze and Cathaysia blocks）. However, the trace element distribution patterns of the Ediacaran and early Cambrian black shales in the Cathaysia Block are still unclear. In this study, thirty- four black shale samples in the Lechangxia Group （Ediacaran） and thirteen black shale samples in the lower Bacun Group （early Cambrian） from Guangning area, western Guangdong Province, South China, were analyzed for major and trace elements concentrations. Compared to the upper continental crust, the Ediacaran black shales exhibit strongly enriched Se, Ga, and As with enrichment factor values （EF） higher than 10, significantly enrichedBi and Rb （10〉EF〉5）, weakly enriched Mo, Ba, Cs, V, In, Be, TI, and Th （5〉EF〉2）, normal U, Cr, Cd, Sc, Pb, Cu, and Li （2〉EF〉0.5）, and depleted Ni, Zn, Sr, and Co. Early Cambrian black shales display strongly enriched Se, Ga, and As, significantly enriched Ba, Bi, and Rb, weakly enriched Mo, Cs, Cd, V, U, Be, In, and TI, normal Sc, Th, Cr, Li, Cu, Ni, and Pb and depleted Co, Zn, and Sr. Moreover, Se is the most enriched trace element in the Ediacaran and early Cambrian black shales： concentrations vary from 0.25 to 30.09 ppm and 0.54 to 5.01 ppm, and averaging 4.84 and 1.72 ppm, with average EF values of 96.87 and 34.32, for the Ediacaran and early Cambrian shales respectively. The average concentration of Se in the Ediacaran black shales is 2.8 times higher than that of early Cambrian black shales. Se contents in the Ediacaran and early Cambrian black shales exhibit significant variation （P = 0.03）. Provenance analysis showed that Se contents of both the Ediacaran and early Cambrian black shales were without detrital provenance and volcanoclastic sources, hut of hydrothermal origin. The deep sources of Se and the presence of pyrite may explain the higher Se contents in the Ediacaran black shales. Similar with the Se-rich characteristics of the contemporaneous black shales in the south Qingling Mountain and Yangtze block, the Ediacaran and early Cambrian black shales in Guangning area, Cathaysia, are also enriched in Se, which may provide a clue for looking for the selenium-rich resources in western Guangdong Province.

Sedimentary Environment and Mineralization of the Black Shale Polymetallic Layer in the Early Cambrian,SW China:Constraints from in situ LA-ICP-MS Analysis of Pyrite

A polymetallic layer is usually developed at the bottom of the early Cambrian black shale in Guizhou Province.The mineral that makes up the polymetallic layer is related to the sedimentary facies.To analyze the differentiation mechanism between polymetallic deposits(Ni-Mo and V),the Zhijin Gezhongwu profile located in the outer shelf and the Sansui Haishan V deposit located in the lower slope are selected to study the in situ sulfur isotopes and trace elements of pyrite.The results show that δ^(34)S values of pyrite vary widely from−7.8‰to 28‰in the Gezhongwu profile,while the δ^(34)S values are relatively uniform(from 27.8‰to 38.4‰)in the Haishan profile.The isotopic S composition is consistent with the transition that occurs in the sedimentary phase from the shelf to the deep sea on the transgressive Yangtze platform;this indicates that the δ^(34)SO_(4)^(2−)values in seawater must be differently distributed in depositional environments.The sulfur in the Ni-Mo layer is produced after the mixing of seawater and hydrothermal fluid,while the V layer mainly originates from seawater.Overall,the Ni-Mo and V deposits have been differentiated primarily on the basis of the combined effect of continental weathering and hydrothermal fluid.

Carbon and sulfur isotopic compositions of Early Cambrian black shales, NW Hunan, China: Implications for the Paleoceanographic sedimentary environment

In order to better understand the paleoceanographic sedimentary environment of the Lower Cambrian black shales extensively distributed in South China, outcropped along the present southern margin of the Yangtze Platform with a width of ca. 200-400 km and a length of more than 1500 km, we present new paired δ13C data on carbonates (δ13Ccarb) and associated organic carbon (δ13Corg) and δ34Spy data on sedimentary pyrite in black shales from three sections (Ganziping, Shancha and Xiaohekou) located in NW Hunan, China. In these sections, a total of 82 Lower Cambrian black shale samples have δ13Ccarb values ranging from -4.0‰ to 1.7‰ with an average value of -2.1‰, and δ13Corg values between -34.9‰ and -28.8‰, averaging -31.9‰. The ?34Spy values of 16 separated sedi-mentary pyrite samples from the black shales vary between +10.2‰ and +28.7‰ with an average value of +19.5‰, presenting a small isotope fractionation between seawater sulfate and sedimentary sulfide. The model calculation based on credible data from the paired analyses for δ13Ccarb and δ13Corg of 11 black shale samples shows a high CO2 concentration in the Early Cambrian atmosphere, about 20 times higher than pre-industrial revolution values, consis-tent with previous global predictions. The small sulfur isotope fractionation between seawater sulfate and sedimen-tary sulfide in black shales, only 15.5‰ on average, implies a low sulfate level in the Early Cambrian seawater around 1 mmol. In combination with a high degree of pyritization (DOP) in the black shales, it is suggested that sul-fidic deep-ocean water could have lingered up to the earliest Cambrian in this area. The black shale deposition is envisaged in a stratified marine basin, with a surface euphotic and oxygenated water layer and sulfidic deeper water, controlled by a continental margin rift.

Nitrogen isotope stratigraphy of the Early Cambrian successions in the Tarim Basin:Spatial variability of nitrogen cycling and its implication for paleo-oceanic redox conditions

The Early Cambrian represents a critical time period characterized by extraordinary biological innovations and dynamic redox conditions in seawaters.Nitrogen isotopic signatures of ancient sediments have the potential to elucidate the evolutionary path of marine redox states and the biogeochemical nitrogen cycle within the water column of the Early Cambrian ocean.While existing research on this topic has predominantly focused on South China,the exploration of other continental margins has been limited,leaving contradictory hypotheses untested.In this study,pairedδ^(15)N andδ^(13)C org analyses were performed on the Lower Cambrian successions from the Shiairike section(inner ramp)and Well Tadong 2(deep shelf/basin)in the northwestern and eastern Tarim Basin,respectively.Our data from the Shiairike section reveal a discernible shift in the operation of different nitrogen cycles for the black chert-shale unit,also referred to as the black rock series in Chinese literature,of the Yurtus Formation(Fortunian stage to lower Stage 3).Oscillatingδ^(15)N values for its lower part are suggestive of alternating anaerobic assimilation of NH 4+and denitrification/anammox.This is likely attributed to a shallow,unstable chemocline consistent with the upwelling and incursion of deep,anoxic waters during a major transgression.In contrast,aerobic nitrogen cycling,indicated by positiveδ^(15)N values of>2‰,dominated the upper part alongside a reduction in upwelling intensity.On the other hand,theδ^(15)N signatures of Xishanbulake and Xidashan Formations of Well Tadong 2,which encompass a time interval from the Cambrian Fortunian Age to Age 4,are indicative of N_(2)fixation by diazotrophs as the major nitrogen source.The two studied intervals,although not time-equivalent,exhibit separated states of nitrogen cycling at least during the deposition of the Yurtus black rock series.The spatially different nitrogen cycling of the studied sections is compatible with a redox-stratified ocean during the deposition of the Yurtus black rock series.The build-up of a NO_(3)−reservoir and aerobic nitrogen cycling in seawater was largely restricted to near-shore settings whereas anaerobic nitrogen cycling dominated by N_(2)fixation served as the main nitrogen uptake pathway in off-shore settings.

Accumulation Conditions of Organic Matter during the Deposition of Black Shales of the Buxin Formation (Early Paleogene) in the Sanshui Basin, South China

Analyses of organic carbon, nitrogen, sulfur and iron have been performed in order to understand sources and preservation of organic matter in black shale of the Buxin Formation (Early Paleogene) from the Sanshui Basin. The C/N ratios show that the organic matter is characterized by a mixture of terrestrial and phytoplanktonic contributions. The relative importance of different sources depend on climate conditions and most of organic matter is of terrestrial origin. The relationships between C, S and Fe indicate that the brackish environment with alternation of anoxia and low-O2 developed in the bottom waters during the deposition of these organic-rich sediments as a result of a mixed setting of thermal stratification and salinity stratification, the latter being the consequence of intermittent sea water incursion. Bacterial sulfate reduction is the most effective early diagenesis affecting the preservation of organic matter. The intensity of sulfate reduction is related to the relative proportion of metabolizable organic matter supplied to sediments.

Organic carbon isotopes of the Sinian and Early Cambrian black shales on Yangtze Platform,China

Organic matter of the Sinian and early Cambrian black shales on the Yangtze Platform belongs to the light carhon group of isotopes with the δ13C values from - 27% to - 35 %, which are lower than those of the contempomneously deposited carbonates and phosphorites. A carbon isotope-stratified paleooceanographic model caused by upwelling is proposed, which can be used not only to interpret the characteristies of organic carbon isotopic compositions of the black shales, but also to interpret the paleogeographic difference in the organic carbon isotope compositions of various types of sedimentary rocks.

Contribution of Oxygenic Photosynthesis to Palaeo-Oceanic Organic Carbon Sink Fluxes in Early Cambrian Upper Yangtze Shallow Sea: Evidence from Black Shale Record

The extensive transgression that occurred on the Yangtze Plate in Early Cambrian led to a massive organic carbon pool in the Niutitang Formation. A black shale core section from 3 251.08 to 3 436.08 m beneath the Earth＇s surface was studied to estimate the contribution of oxygenic photosynthesis to organic carbon sink fluxes in Early Cambrian Upper Yangtze shallow sea. Results indicate that the oxygenic photosynthesis played the most important role in carbon fixation in Early Cambrian. Organic carbon sink was mainly contributed by photosynthetic microorganisms, e.g., cyanobacteria, algae and archaea. The Niutitang Formation was formed in a deep anoxic marine shelf sedimentary environment at a sedimentation rate of ~0.09±0.03 mm/yr. The initial TOC abundance in Niutitang shale ranged from 0.18% to 7.09%, with an average of 2.15%. In accordance with the sedimentation rate and initial TOC abundance, organic carbon sink fluxes were calculated and found to range from 0.21 to 8.10×10~3 kg/km^2·yr^（-1）, especially the organic carbon sink fluxes in depth between 3 385 and 3 470 m range from 3.80 to 8.10×10~3 kg/km^2·yr^（-1）, with an average of ~6.03×10~3 kg/km^2·yr^（-1）, which is much higher than that of contemporary marine sediments. The organic carbon sink fluxes of Niutitang shale are equal to 0.56 to 21.61×10~3 kg/km^2·yr^（-1） net oxygen emitted into the Early Cambrian ocean and atmosphere, this emitted oxygen may have significantly promoted the oxygen level of the Earth＇s surface and diversification of metazoans.

Characteristics of sulfur species and their implications in Lower Cambrian black shales from southern margin of Yangtze Platform

There are different sulfur forms in the black shales from theEarly Cambrian of the Yangtze platform. With its emphasis on pyrite and organosulfur, this paper discusses their distribution and formation. The research shows that sulfur phases take regular variations laterally as well as vertically in the research areas. In western researched profile with high terrigenous supply at the time it formed, there exists a larger amount of pyrite and less organosulfur, and pyrite amount declines while organosulfur content increases upwards along the profile. This black shale profile is characterized by relatively light sulfur isotope composition with evolution trend of becoming heavier both for pyrite and organosulfur from bottom to top along the profile. Opposite situation occurs in eastern profiles which were located farther away from terrigenous land. Here pyrite amount obviously decreases and organic matter has combined more sulfur, although these two kinds of sulfur species take similar trend in content variation along profiles to that for western profile. At the same time more 34S is accumulated in sulfur species of black shale samples from eastern profile, and sulfur isotope composition gradually turns lighter from bottom to top. In combination with other information of iron, organic carbon contents and petrographic features, it can be established that sea-level change, supply of terrigenous matters, tectonic background and natures of paleoceanic chemistry have exerted great influence on the distribution of sulfur species in these black shales.

Controls on the organic carbon content of the lower Cambrian black shale in the southeastern margin of Upper Yangtze

Control of various factors, including mineral components, primary productivity and redox level, on the total organic carbon(TOC) in the lower Cambrian black shale from southeastern margin of Upper Yangtze(Taozichong, Longbizui and Yanbei areas) is discussed in detail in this article. Mineral components in the study strata are dominated by quartz and clay minerals. Quartz in the Niutitang Formation is mainly of biogenic origin, and the content is in positive correlation with TOC, while the content of clay minerals is negatively correlated with TOC. Primary productivity, represented by the content of Mobio(biogenic molybdenum), Babio(biogenic barium) and phosphorus, is positively correlated with TOC. The main alkanes in studied samples are nCC, and odd–even priority values are closed to 1(0.73–1.13), which suggest the organic matter source was marine plankton. Element content ratios of U/Th and Ni/Co and compound ratio Pr/Ph indicate dysoxic–anoxic bottom water, with weak positive relative with TOC. In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation:(1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environment was favorable for living organisms;(2) high productivity provided the material foundation for organic generation;(3) the redox conditions impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.

Study of Noble Metal Elements in Lower Cambrian Black Rock Series of Guizhou-Hunan Provinces, China

Systematic analyses of noble metal elements in the Lower Cambrian black rock series of South China are reported. Correlations of w (Os)/ w (Ir), w (Au)/ w (Ir), w (Ag)/ w (Au), w (Pt+Pd)/ w (Os+Ru+Rh+Ir), relations of noble metal and platinum group element (PGE) distribution patterns reveal that the noble metals are not directly from extraterrestrial materials. Studying the data of 9 aspects, the authors conclude that the noble metals were mainly from ultramafic mafic igneous rocks and their enrichment in black rocks is mainly controlled by hydrothermal fluid.

Simulating experiment on the enrichment of precious metals in Lower Cambrian black shale series of Hunan and Guizhou provinces

Adsorption experiments were made at room temperature and neutral pH value on different types of min-erals associated with the Lower Cambrian black shale series polymetallic layers in Hunan and Guizhou provinces on nanometer-sized Pt colloids and PtCl42--bearing ionic solutions with an attempt to constrain the relationship between the different types of minerals in the polymetallic layers and the enrichment of platinum group elements (PGEs). Experimental results showed that the different types of minerals show strong selectivity to the adsorption of nano-meter-sized Pt colloids and PtCl42--bearing ionic solutions. Metallic sulfides, organic matter and clay minerals are the strong adsorbents of PGEs, while quartz, albite, muscovite and other silicate minerals show a week adsorbility to both of them. This phenomenon is well consistent with the geological fact that metallic sulfides, organic matter and clay minerals in the polymetallic layers of the black shale series are the major carrier minerals of PGEs, giving a thorough explanation to the mechanism of enrichment of previous metal elements. Adsorption may be a principal mechanism of enrichment of precious metal elements under lower temperature conditions. The presence of the aforementioned strong adsorbents is the good geochemical barriers for the enrichment of PGEs.

四川盆地城口地区下寒武统水井沱组有机质富集控制因素

【目的】四川盆地下寒武统水井沱组黑色页岩是四川盆地重要的烃源岩和页岩气勘探层位,但目前对该黑色页岩有机质富集机理还存在不同的看法。【方法】通过对川东北城口地区下寒武统水井沱组下部黑色页岩的沉积相与沉积微相分析、环境成因草莓状黄铁矿形态与粒径变化规律研究,结合黑色页岩总有机碳含量、主量元素、微量元素等地球化学数据分析,重建了四川盆地东北部水井沱组沉积期古地理、古环境演化模式,探讨了有机质富集主控因素与形成机制。【结果】水井沱组黑色页岩可以划分出与深水陆棚—斜坡环境有关的五个沉积微相。生产力指标Ba_(XS)、P/Ti及陆源碎屑输入指标Ti、Al表明,水井沱组黑色页岩沉积环境处于较低的古生产力水平,陆源碎屑输入较稳定且与TOC没有明显相关性。因此,古海洋初级生产力及陆源碎屑输入不是水井沱组黑色页岩有机质富集的主控因素。氧化还原敏感元素指标U/Th、U_(EF)、Mo_(EF)及草莓状黄铁矿研究表明,有机质含量与古水体氧化还原指标呈明显正相关,指示下寒武统水井坨组有机质富集主要受到水体氧化还原条件变化的控制。【结论】基于底水氧化还原条件变化规律、沉积相及沉积充填演化序列等认识,认为寒武纪第二阶—第三阶(529~514 Ma)时期,水井沱组黑色页岩有机质富集受大规模海侵事件导致的底水氧化还原条件波动控制,并提出了有机质富集模式。

The Origin of the Black Shale Series and Bentonites from the Wufeng Formation in the Southwestern Upper Yangtze:Implications for the Convergence of the Yangtze and Cathaysia Blocks in the Late Ordovician

Early Paleozoic black organic sediments and bentonites occur widely in the craton basin within the Yangtze block and are generally believed to be genetically related to a specific tectonic setting on the cratonic boundary.However,the intimate relationship between their origins and the dynamic mechanisms are unclear,as exemplified by the genesis of the black shale series and bentonites from the Wufeng Formation during the Ordovician–Silurian transition(OST).In order to reveal the relationship between the Wufeng Formation and the convergence of the Yangtze and Cathaysia blocks(i.e.,the intracontinental Kwangsian Orogeny),two stratigraphic sections respectively in Zhaotong area(Northeast Yunnan)and Puyi area(Northwestern Guizhou)that were located in the semi-restricted inner Yangtze Sea during the OST were systematically studied,on the basis of whole-rock geochemical composition,pyriteδ^(34)S(δ^(34)Spy),total organic carbon(TOC),stable Sr isotope,pyrite framboid size distribution and zircon U-Pb age,trace elements.The evidence shows that the paleo-oceanic environment changed significantly at the turn of the early–late Katian and formed the black shale series in the Wufeng Formation.These acritarch assemblages were formed in the transition process of the Upper Yangtze Basin from passive continental margin basin to foreland basin during this interval.Based on previous research on the genetic relationship between black shale series and plate tectonic movement,a basin-mountain evolution model suitable for South China in the Late Ordovician is presented.The two bentonites in the Wufeng Formation with U-Pb ages of 445.5±0.8 Ma and 441.9±2.4 Ma primarily originated from the intermediate–acid volcanic eruption during the collision and convergence between the Yangtze and Cathaysia blocks in the Late Ordovician,the provenance region probably being located in the Jiangnan orogenic belt.Thus,we believe that the appearance of the black shale series and bentonite in the Wufeng Formation at the turn of the early–late Katian may represent the initiation of basin-mountain transformation and the Kwangsian Orogeny in South China,which provides important evidence for the collision and convergence of the Yangtze and Cathaysia blocks in the Late Ordovician.

下寒武统黑色页岩金属富集机理与小行星撞击

扬子地台寒武系下统存在富含V、Ni、Mo和铂族元素(platinum group element, PGE)等的黑色页岩,局部地区有U或者Ba、Hg、As等的富集,研究这些元素富集机理有利于寻找相关矿产或者探究地质事件。6 500万年前的小行星撞击地球产生了高PGE含量的地层或者化石,依据这一事实和陨石的特征及其他地质地球化学证据,推断下寒武统黑色页岩中Ni-Mo-PGE富集是陨石撞击结果,PGE中的Ir、Os富集最明显是因为二者最抗淋滤。U在康滇地轴东侧黑色页岩中最高可达480×10^(-6),因康滇地轴本身就存在混合型铀矿,推测黑色页岩中U来自它的风化。据V和Ba-Hg-As等的地化特性,认为V富集是受生物活动影响,但因V易于在热泉水中富集,故寒武纪早期泛非运动的构造-热事件可导致V富集成矿,Ba、Hg、As也因该事件产生构造裂隙后,它们优先随热液沿裂隙上升富集,并在海底局部成矿。此外,5.65亿年前的埃迪卡拉动物群发生地基本上就是5.8亿年前小行星撞击地点,提出此次行星撞击可能与埃迪卡拉生物群出现存在因果关系。由于在中国存在寒武纪澄江生物群大爆发事件,推测该次生物大爆发之前存在的下寒武统黑色页岩也存在陨石撞击痕迹。寒武系下统的含岩屑细砂岩夹黑色页岩甚至砾岩,反映了黑色页岩沉积时存在的突变沉积环境,加上其他地球化学特征也支持小行星撞击说。

筇竹寺组高角度裂缝储层取心复杂处理技术

川南地区页岩气井下寒武统筇竹寺组是长宁—威远国家级页岩气示范区勘探开发与产能建设重要的接替地层。Z201井在筇竹寺地层开展取心作业,面临高角度楔形裂缝发育、分布点多、井段长的地质复杂情况,取心起钻完采用常规敲击方式时出现了地面出心困难的技术难题,严重影响取心作业的质量与效率。针对上述问题,围绕敲击效果提升与敲击力控制方法,自主设计研发了快速出心工具,并形成了配套处理技术:①改变敲击位置,缩短敲击点与堵点岩心距离,增加震动效果;②优化工具结构,扩大敲击接触面,减少震动应力,控制对取心筒的伤害;③优化作业工艺,精准控制敲击力,强化使用后的检验、维护、保养,减少对取心筒的损伤。现场应用中,Z201井筇竹寺地层完成14趟取心作业,取心进尺171.50 m,心长170.50 m,取心收获率高达99.42%,岩心完整率100%,取心筒完好率100%。

湘鄂西下寒武统牛蹄塘组黑色页岩发育特征及沉积环境

湘鄂西地区下寒武统牛蹄塘组是我国南方页岩气勘探的重要目标。通过鄂西古城村和湘西岩屋潭2个剖面的下寒武统牛蹄塘组黑色页岩层系的岩石学特征与地球化学分析,开展了黑色页岩沉积的物源和构造背景研究,并探讨了其沉积环境。研究结果表明:(1)鄂西古城村和湘西岩屋潭2个剖面的牛蹄塘组黑色页岩沉积差异明显,古城村剖面牛蹄塘组页岩主要为泥质页岩,厚度小,底部发育少量硅质页岩;岩屋潭剖面牛蹄塘组页岩则以硅质页岩为主,厚度大,底部富集硅磷质结核及镍、钴、钒等多种金属。岩屋潭剖面的石英含量远高于古城村剖面,并且黄铁矿也更富集。(2)研究区2个剖面的黑色岩系沉积环境、物源背景与构造背景均存在明显差异。古城村剖面物源以长英质火成岩和石英质沉积岩为主,沉积于被动大陆边缘大陆架上的弱还原-弱氧化环境;岩屋潭剖面物源以长英质火成岩、石英质沉积岩和大陆拉斑玄武岩为主,并伴随热液物源的输入,沉积于被动大陆边缘大陆坡上的还原环境。(3)早寒武世,鄂西鹤峰古城村的沉积以黑色炭质页岩和灰岩为主,且有机质与硅质含量均较低,水体相对较浅,受海平面波动影响明显,为陆棚环境;湘西的沉积厚度大,有机质含量高,且由于靠近深大断裂,热液硅来源丰富,为大陆坡环境。

贵州遵义松林寒武系底部热液喷口及与喷口相关生物群特征

贵州遵义寒武系底部黑色页岩中含Mo、Ni、U等重金属的富集层,它们一直被认为与热水沉积作用有关。本文通过对贵州遵义松林小竹一带的寒武系底部地层的系统的岩石、矿物、地球化学研究,认为遵义松林小竹一带寒武系底部黑色页岩中富含Mo、Ni、U等重金属层属于热液(水)喷流沉积,主要依据有:①含大量的黄铁矿、针镍矿、硫钼矿及较多的锑硫镍矿、黄铜矿、闪锌矿、重晶石、石英、石膏等矿物;②具有大量的同生砾屑,砾屑成分为黑色碳质体和碎屑状黄铁矿、针镍矿、硫钼矿等矿石,它们堆积成层状结构,碎屑呈被撕裂状、棱角状、熔蚀港湾状等结构,球状、脸盆状、枕头状等构造,它们类似海底喷流沉积;③硅同位素、锶同位素、Re/Os值、Th/So值、Th/U值都表明有深部物质加入;④黑色岩系中夹碳酸盐岩透镜体,碳同位素具有强烈负异常特征,表明有热液作用;⑤超强富集元素Mo、As、Se、Re和Tl都属于典型的气相迁移元素,而钼是典型的高温气相迁移元素,暗示了可能存在火山射气作用;而强富集的Ni、U、Au、Ag、PGE元素组合是超基性岩浆活动的产物,表明有热液活动。研究发现在热液(水)喷流沉积形成的富集Mo、Ni、U等重金属硫化物层之上,产出丰度大、分异度低、含大型海绵类、双壳类、菌藻类和少量大型蠕虫化石的生物群,它的特征与现代太平洋海底热喷口附近的生物群类似,因此认为遵义小竹一带寒武系底部生物群属于与海底热喷口相关生物群,这一新发现对研究寒武纪生态和生物爆发具有重要意义。

贵州早寒武世早期黑色页岩中生物化石保存及生态学研究

早寒武世早期黑色页岩中以前只发现少量的海绵骨针,但随着研究的深入,各类动物化石被发现。最近,贵州早寒武世早期黑色页岩中发现碳质和薄膜型化石,它们包括大量的简单线状遗迹、碳质膜状高肌虫、完整海绵动物、管虫、薄壳双壳动物、碳质膜状软舌螺以及一些圆形膜状化石等。通过对黑色页岩的粘土矿物定量和定性分析,结合地球化学特征,认为早寒武世早期小壳动物繁盛期之上有一个大型动物繁盛期,随后由于地壳拉张作用,大规模的海底热液和火山喷发作用的加强,海洋环境变成酸性环境,不利于生物生存,发生生物绝灭作用,随后海洋环境逐渐趋于碱性,大量的生物又开始繁盛。

湘中下寒武统黑色页岩土壤的地球化学特征

以湘中发育于下寒武统黑色页岩之上的土壤为研究对象,选择安化东坪、烟溪,桃江,宁乡等地的典型土壤及相应成土母岩,利用等离子质谱(ICP-MS)、X射线荧光光谱(XRF)等分析技术,对土壤、成土母岩(黑色页岩)的主量元素和微量元素(包括重金属元素、稀土元素等)进行了较系统的分析测定。结果表明,湘中下寒武统黑色页岩土壤风化作用强烈,风化指数CIA均在73以上。强烈的风化使得土壤具有明显贫CaO、Na2O,而富Al2O3、Fe2O3的化学组成特征。土壤因继承成土母岩(黑色页岩)的特征而富集Mo、Cd、Sn、Sb、U、V、Cr、Co、Ni、Cu、Zn、Tl、Pb、Th等多种重金属元素,其综合富集指数(EI值)平均在3以上,最高达17。地质累计指数(Igeo)评价结果显示,土壤重金属的富集已达到污染程度,土壤存在Cd、Mo、Sb、U、Sn、V、Cu、Tl、Ba等重金属的污染,并以Cd、Mo、Sb等重金属污染最强,达中度至极强污染程度。重金属与主量元素的相关性分析显示,土壤中的重金属主要赋存于黏土矿物和铁氧化物(针铁矿)等矿物相中,其中Ba、Sn、Th、Cu、Sc等主要赋存黏土矿物中;Zn、Ni、Mn、Co、Cd、Tl、Pb等则主要赋存于铁氧化物矿物(针铁矿)中;而Cr、V、Mo、Sb、U等则不受黏土矿物和铁氧化物矿物的控制。此外,不同地区土壤的Zr/Hf、Ta/Nb、Nd/Sm等元素比值相对稳定,依次为36.20、0.085、5.30(n=73),并与相应的成土母岩(黑色页岩)相应值基本一致。土壤与成土母岩具有相同的稀土配分型式,且成土过程中稀土元素不发生明显的分异。微量元素比值和稀土元素特征指示土壤中的重金属来自成土母岩(黑色页岩)本身,为自然污染源。