Evolution of organic matter degradation in Cretaceous black shales inferred from authigenic barite: A reaction-transport model

A reaction-transport model was used to infer the long-term evolution of anaerobic organic matter degradation in Cretaceous black shales from the distribution of authigenic barite in sediments drilled at Demerara Rise (ODP Leg 207, Site 1258). In these sediments, sulfate-reduction and methanogenesis are the major pathways of organic matter decomposition and the depth-distribution of authigenic barite serves as an indicator for the temporal evolution of the sulfate-methane transition zone (SMTZ), the strength of the biogenic methane flux and, ultimately, the organic matter reactivity in the black shales over geological timescales. Organic matter degradation is described according to the reactive continuum model approach and parameters values are determined by inverse modeling, based on present-day porewater and authigenic barite profiles. Fully transient simulations were performed over a period of 100 Myrs and indicate that important features of the biogeochemical dynamics are associated to changes in the boundary forcing. Hiatuses in sediment accumulation rate result in quasi-steady-state conditions and lead to distinct accumulations of authigenic barites in the SMTZ. The inversely determined parameters reveal that the reactivity of the organic matter was already low (apparent first order rate constant ) at the time of its deposition in the Cretaceous. The geochemical characteristics of sediments drilled at Demerara Rise, as well as the presence of specific biomarkers, suggest that this low reactivity is most likely due to the euxinic palaeo-conditions which favored the sulfurization of the organic matter. Simulation results predict average initial organic carbon contents between 8.1 and 9.5 wt%, implying a high preservation efficiency of the organic matter (between 79% and 89%). Calculated mass accumulation rates (between 0.43 and 0.5 ) compare well with estimations for the western basin of the Cretaceous southern North Atlantic. Simulation results thus indicate that the enhanced preservation of organic matter under euxinic conditions may have been the main cause for the formation of organic-rich Cretaceous black shales at Demerara Rise.     

湘西震旦—寒武纪交替时期古海洋环境的恢复

前寒武纪至寒武纪的交替时期是地质史和生命史的重要转折。中国南方地区广泛发育了上震旦统、下寒武统的黑色岩系。文中运用沉积学和沉积地球化学的理论和方法对湘西黑色岩系的岩石类型、矿物组成、元素地球化学特征、干酪根的结构以及碳同位素的组成等方面进行了研究 ,讨论了黑色岩系的形成条件 ,恢复了震旦纪 /寒武纪地史转折期的古环境。研究认为 ,湘西地区在晚震旦世和早寒武世地史转折期的生物爆发和高有机质的产率是形成缺氧环境的重要因素之一。     

湘西黑色岩系地球化学特征和成因意义

本文采用ＧＣ－ＭＳ,ＩＣＰ－ＭＳ,形态硫分析,硫同位素,物相分析等方法综合研究了湘西晚震旦世－早寒武世黑色岩系的地球化学特征和形成条件,湘西黑色岩系以富有机物质为特征,其来源为藻,菌类生物,形成于缺氧环境。缺氧标志包括有机碳,黄铁矿含量,硫同位素,稀土元素的分布,δＵ,ＤＯＰ和生物标志物等参数,在黑色页岩中富集Ｖ,Ｎｉ,Ｍｏ等微量元素,并在局部地区形成具有工业价值的矿床,研究认为,生物,有机质对一     

塔里木盆地西北缘下寒武统黑色岩系差异性成岩演化及其影响因素

尽管泥页岩在埋藏过程中经历的复杂成岩改造会显著影响页岩油气储层发育潜力,但对其成岩作用,特别是泥页岩中成岩矿物的类型、成岩演化及其影响因素、成岩矿物元素在低孔低渗孔隙系统中的迁移等问题的认识仍很薄弱,因而制约了对泥页岩储层的深入认识。为此,本文以塔里木盆地西北缘下寒武统玉尔吐斯组发育的两套黑色岩系为例,在沉积环境认识的基础上,通过矿物组分、黏土矿物组合、有机碳含量及有机质成熟度等测定分析,着重研究了黑色岩系自生矿物的分布及在成岩演化阶段的变化特征,阐述了成岩变化的时空分布及其影响因素。研究结果表明：（1）在浅水缓坡台地环境下,玉尔吐斯组受两个较为完整的海进-海退沉积旋回控制而形成的两套黑色岩系主要经历了早成岩阶段和中成岩阶段;（2）受沉积环境和热液作用的影响,这两套黑色岩系在石英、重晶石、碳酸盐胶结物和黏土矿物等自生矿物的组合、分布及形成顺序上存在显著的差异,呈现出差异性成岩演化的特点;（3）伊利石、伊蒙混层、绿泥石、白云石、石英胶结等中成岩阶段成岩产物的广泛分布,反映出在成岩过程中特别是中成岩阶段低孔低渗的泥页岩仍存在一定程度的内部和外部来源的矿物元素迁移。本次研究对深入认识塔里木西北缘下寒武统黑色岩系成岩机制,以及分析页岩储层发育特征提供了重要的依据。     

湘西黑色岩系沉积演化与含矿序列

研究了湘西晚震旦世-早寒武世形成黑色页岩的沉积环境和含矿性.黑色岩系形成于沉积盆地斜坡或以下地带,与海平面上升有关.晚震旦世到早寒武世早中期的沉积层序可划分为三个II级层序.海平面上升的结果使下层海水成为还原环境,有机质得以保存,沉积了大面积的黑色岩系.湘西上震旦统、下寒武统分别构成了黑色页岩 磷块岩 硅质岩组合和磷结核 重晶石 石煤 多金属富集层两个成矿系列,含矿层段集中发育于海平面上升期和凝缩层沉积中,在受海底火山、喷泉活动提供丰富物源的背景条件下,有机质参与元素的迁移、富集作用,经成岩期的转化富集成黑色岩系型矿床.     

Multiple Sources of Metals of Mineralization in Lower Cambrian Black Shales of South China: Evidence from Geochemical and Petrographic Study

Black shales of the Lower Cambrian Niutitang Formation in southern China (Huangjiawan mine, Zunyi region, northern part of the Guizhou Province) host regionally distributed stratiform polymetallic Ni‐Mo‐platinum group elements (PGE)‐Au phosphate‐ and sulfide‐rich ores. These are confined to a ≥0.2‐m thick ore horizon composed of mineralized bodies of algal onkolites, phosphate nodules, and sulfide and shale clasts in a mineralized phosphate‐ and organic matter‐rich matrix. Compared to footwall and hanging wall shales, the ore bed is strongly enriched in Ni (up to 100‐fold), As (up to 97‐fold), Mo (up to 95‐fold), Sb (up to 67‐fold), Rh (up to 49‐fold), Cu (up to 37‐fold), Pd (up to 33‐fold), Ru (up to 24‐fold), Zn (up to 23‐fold), Pt (up to 21‐fold), Ir (up to 15‐fold), Co (up to 14‐fold), and Pb (up to 13‐fold). Even footwall and hanging wall black shales are significantly enriched by Mo (21‐fold) and Ni (12‐fold) but depleted in Cr in comparison to average Cambrian black shale. Organic matter is represented by separate accumulations dispersed in the rock matrix or as biotic bitumen droplets and veinlets in ore clasts. Similar organic carbon (C_org) values in an ore bed and enclosing footwall and hanging wall shales of little mineralization indicate that metal accumulation was not controlled only by biogenic productivity and organic matter accumulation rate. Evaporitic conditions during sedimentation of the basal part of the Niutitang Formation were documented by an occurrence of preserved Ni‐, V‐, Cr‐, and Cu‐enriched phosphate‐rich hardground with halite and anhydrite pseudomorphs on the paleosurface of the underlying Neoproterozoic carbonates. Neoproterozoic black shales of the Doushantuo Formation are characterized by increased metal concentrations. Comparison of metal abundances in both hardground and Doushantuo black shales indicate that black shales could have become a source of metal‐rich hardground during weathering. The polymetallic Ni‐Mo‐PGE sulfide‐rich ore bed is interpreted to represent a remnant of shallow‐water hardground horizon rich in metals, which originated in a sediment‐starved, semi‐restricted, seawater environment. During the Early Cambrian transgression an influx of fresh seawater and intensive evaporation, together with the hydrothermal enrichment of seawater in a semi‐restricted basin, resulted in the formation of dense metalliferous brines; co‐precipitation of metals together with phosphates and sulfides occurred at or above the oxic–anoxic sediment interface. Metal‐enriched hardground was disintegrated by the action of waves or bottom currents and deposited in a deeper part of the anoxic basin. Contemporaneously with the formation of a polymetallic Ni‐Mo‐PGE‐Au sulfide ore bed, economic sedimentary exhalative (SEDEX)‐type barite deposits were forming in a stratigraphically and geotectonically similar setting. The results of geochemical study at the Shang Gongtang SEDEX‐type Ba deposit indicate that concentrations of Ag, As, Cr, Cu, Fe, Mn, Ni, Pb, Sb, V, Zn and other metals decrease from top of the barite body toward the hanging wall black shale. Lower Cambrian black shales of the Niutitang Formation above the barite body also display similar element abundances as Neoproterozoic black shales of the Doushantuo Formation, developed in the footwall of the barite body. But the geochemical composition of the sulfide layer is different from the Ni‐Mo ore bed, showing only elevated Pb, Cu, Ni and Mo values. It is suggested that hydrothermal brines at Shang Gongtang might have leached metals from footwall Neoproterozoic sequences and became, after mixing with normal seawater, an additional source of Ag, Cr, Cu, Pb, Sb, Zn, Ni, PGE, V and other metals.     

塔里木盆地北部下寒武统底部黑色页岩形成的次氧化条件

富有机质黑色页岩的形成长期以来被认为是缺氧沉积环境的证据,并被作为全球缺氧事件的标志。塔里木盆地下寒武统底部黑色页岩的岩石学特征说明其沉积在受陆源有一定影响的陆棚环境中。矿物学研究表明,硫均以硫酸盐状态存在,没有见到黄铁矿,说明其形成在硫酸盐还原带以上。黑色页岩中结核状氟磷灰石的存在以及剖面底部结核状磷块岩的出现,表明了一种与现代热带太平洋东海岸和阿拉伯海相类似的广泛发育上升洋流的大陆边缘沉积环境。黑色页岩的地球化学特征中,Re和Mo的高度富集、高的Re/Mo比以及含有一定量的Mn,Cd和U的相对富集以及低的Cd/U比等,这些特点均支持了黑色页岩沉积于次氧化条件的认识。     

Orogenesis vs. diagenesis: Can we use organic-rich shales to interpret the tectonic evolution of a depositional basin?

Black shales from the southern Appalachian Basin and the southwest Welsh Basin have anomalous U–Pb and Nd model ages suggesting syn- and post-depositional resetting of the Sm–Nd and U–Pb isotopic systems. This alteration to the primary detrital signature of these two shale sequences is indicative of black shale diagenetic/depositional processes that obscure paleo-environmental and provenance information recorded prior to and during deposition. The trace element and isotopic signatures of these two shale sequences reveal a syn-/post-depositional history that is de-coupled from the coeval orogenic history of the region making it difficult to reconstruct the tectonic and oceanographic conditions present at the time of deposition.Both the Ordovivian Welsh Basin and the Devonian Appalachian Basin sequences host REE- and U-bearing diagenetic phosphate minerals that play a critical role in the whole rock REE and U budgets. In the Welsh Basin shales, early diagenetic apatite and a later monazite phase dominate the REE budget and cause the redistribution of REE early in the basin's history (ca. 460 Ma). This redistribution is recorded by the Sm–Nd system (450 ± 90 Ma) and the Nd model ages that are anomalously old by as much as 20% (T_DM > 2.0 Ga). This early history is complicated by a Permo-Triassic fluid event affecting the whole rock U-budget and resetting the U–Pb isotopic system at 193 ± 45 Ma. The Appalachian Basin sequence appears to have a much less complicated history yet still records a significant disturbance in both the Sm–Nd isotopic system (392 ± 76 Ma) and the Pb isotopic system (340 ± 50 Ma) at about the time of deposition (ca. 365 Ma).These two sequences suggest a pattern of diagenetic disturbance common to black shales. These processes are unique to black shales and must be considered when interpreting provenance and paleo-environmental information from the black shale sequences. Although these rocks are susceptible to alteration, the alteration may provide extensive information on the post-depositional history of the basin while still retaining some primary depositional information. If black shale processes are considered during the interpretation of isotopic and trace element signatures from organic-rich shales, it may be possible to recover an extensive basin history.     

Uraniferous Black Shale and Related Uranium Mineralization Features in South China

: Black shales are marine sediments with argillaceous, silty and siliceous compositions and high contents of organic materials, disseminated pyrite and uranium. Uraniferous black shale has uranium content of more than 20 ppm.Black shales are widely distributed in 17 provinces or autonomous regions in northwestern and southern-central China. Their sedimentary ages are from the Sinian to the Tertiary and uraniferous black shales are mainly exposed in Yunnan, Guizhou, Sichuan, Hunan, Hubei, Jiangxi, and Zhejiang provinces and Guangxi Zhuang Autonomous Region and the economically significant uranium deposits associated with black shale occur in Hunan and Jiangxi provinces and Guangxi Zhuang Autonomous Region.Uranium mineralization associated with black shale has the following main features: (1) forming stratabound deposits; (2) controlled by structures such as interlayer and intersected faults and fractures; (3) associated with different ore-forming processes such as leaching and hydrothermal reworking; (4)     

Evidence of Bio-Organic Mineralization

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Geology of the Devonian black shales of the Appalachian Basin

Black shales of Devonian age in the Appalachian Basin are a unique rock sequence. The high content of organic matter, which imparts the characteristic lithology, has for years attracted considerable interest in the shales as a possible source of energy. The recent energy shortage prompted the U.S. Department of Energy through the Eastern Gas Shales Project of the Morgantown Energy Technology Center to underwrite a research program to determine the geologic, geochemical, and structural characteristics of the Devonian black shales in order to enhance the recovery of gas from the shales. Geologic studies by Federal and State agencies and academic institutions produced a regional stratigraphic network that correlates the 15 ft black shale sequence in Tennessee with 3000 ft of interbedded black and gray shales in central New York. These studies correlate the classic Devonian black shale sequence in New York with the Ohio Shale of Ohio and Kentucky and the Chattanooga Shale of Tennessee and southwestern Virginia. Biostratigraphic and lithostratigraphic markers in conjunction with gamma-ray logs facilitated long-range correlations within the Appalachian Basin. Basinwide correlations, including the subsurface rocks, provided a basis for determining the areal distribution and thickness of the important black shale units. The organic carbon content of the dark shales generally increases from east to west across the basin and is sufficient to qualify as a hydrocarbon source rock. Significant structural features that involve the black shale and their hydrocarbon potential are the Rome trough, Kentucky River and Irvine-Paint Creek fault zone, and regional decollements and ramp zones.     

沉积过程对自生黄铁矿硫同位素的约束

自生黄铁矿是海洋沉积物中还原态硫的主要赋存形式,其形成过程与有机质矿化相关,影响全球的C-S-Fe生物地球化学循环。自生黄铁矿硫同位素分馏主要受微生物硫酸盐还原的控制,但近期的研究成果表明局部沉积环境的改变也可以影响黄铁矿硫同位素的组成,特别是在浅海环境。在浅海非稳态沉积环境内,物理再改造和生物扰动作用,导致硫酸盐还原带内生成的硫化物被再氧化,进而影响黄铁矿的硫同位素值。浅海沉积过程容易受到古气候和海平面变化的影响,引起沉积速率的剧烈波动,导致有机质和活性铁输入的不稳定,进而影响成岩系统的开放性和硫酸盐还原速率,最终影响黄铁矿的硫同位素值。另外,沉积速率的改变还影响硫酸盐—甲烷转换带的迁移,造成有机质和甲烷厌氧氧化硫酸盐还原的相互转化,产生不同的硫同位素信号。东海内陆架泥质区为研究沉积过程对自生黄铁矿的形成及其硫同位素组成的约束机制提供了很好的研究材料。该区域有很好的沉积学研究基础,自生黄铁矿丰富、并且个别层位有生物气（甲烷为主）存在,是研究边缘海C-S-Fe循环的理想场所。     

Source/process apportionment of major and trace elements in sinking particles in the Sargasso sea

Elemental composition of the particle flux at the Oceanic Flux Program (OFP) time-series site off Bermuda was measured from January 2002 to March 2005. Eighteen elements (Mg, Al, Si, P, Ca, Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Sr, Cd, Ba and Pb) in sediment trap material from 500, 1500 and 3200 m depths were quantified using fusion-HR-ICPMS. Positive Matrix Factorization (PMF) was used to elucidate sources, elemental associations and processes that affect geochemical behavior in the water column.Results provide evidence for intense elemental cycling between the sinking flux material and the dissolved and suspended pools within mesopelagic and bathypelagic waters. Biological processing and remineralization rapidly deplete the sinking flux material in organic matter and associated elements (N, P, Cd, Zn) between 500 and 1500 m depth. Suspended particle aggregation, authigenic mineral precipitation, and chemical scavenging enriches the flux material in lithogenic minerals, barite and redox sensitive elements (Mn, Co, V, Fe). A large increase in the flux of lithogenic elements is observed with depth and confirms that the northeast Sargasso is a significant sink for advected continental materials, likely supplied via Gulf Stream circulation.PMF resolved major sources that contribute to sinking flux at all depths (carbonate, high-Mg carbonate, opal, organic matter, lithogenic material, and barite) as well as additional depth-specific elemental associations that contribute about half of the compositional variability in the flux. PMF solutions indicate close geochemical associations of barite-opal, Cd-P, Zn-Co, Zn-Pb and redox sensitive elements in the sinking flux material at 500 m depth. Major reorganizations of element associations occur as labile carrier phases break down and elements redistribute among new carrier phases deeper in the water column.Factor scores show strong covariation and similar temporal phasing among the three trap depths and indicate a tight coupling in particle flux compositional variability throughout the water column. Seasonality in flux composition is primarily driven by dilution of the lithogenic component with freshly-produced biogenic material during the late winter primary production maximum. Temporal trends in scores reveal subtle non-seasonal changes in flux composition occurring on month long timescales. This non-seasonal variability may be driven by changes in the biogeochemical properties of intermediate water masses that pass through the region and which affect rates of chemical scavenging and/or aggregation within the water column.     

四川盆地五峰—龙马溪组黑色页岩有机岩石学特征研究

目前五峰—龙马溪组黑色页岩中显微组分缺乏统一的分类方案,命名也较为混乱,给页岩气勘探与评价造成了困难。为此,采用全岩光片和有机地球化学(TOC、δ¹³C_org)分析等方法,对四川盆地五峰—龙马溪组黑色页岩中显微组分进行有效识别与特征总结,探讨其可能成因,并判断其有机质类型。研究结果表明:五峰—龙马溪组页岩中显微组分主要由海相镜质组、腐泥组、动物有机碎屑组和次生组组成。其中,海相镜质组由无结构镜质体组成,呈浑圆状或长条状,具有强的光反射能力,但分布并不广泛;腐泥组主要由无结构腐泥体组成,为藻类遭受热降解过程而形成的无结构且无固定形态的显微组分,在富有机质页岩中广泛分布;动物有机碎屑组主要包括笔石表皮体、几丁虫和放射虫有机碎屑体;次生组由次生沥青体组成,广泛分布在页岩基质孔隙中,呈无固定形状。五峰—龙马溪组页岩中主要发育腐泥组和次生组,其次为动物有机碎屑组和海相镜质组,其有机质类型以Ⅰ-Ⅱ₁型干酪根为主,且腐泥组和次生组含量越高,有机质类型越好,其生烃潜力越大。     

牛蹄塘组页岩矿物的成因类型及其地质指示意义

以场发射扫描电镜为主要研究手段,在微米级-纳米级尺度上观测南方下古生界牛蹄塘组（包括相当层位）页岩的主要组成矿物。通过揭示矿物质与有机质的生长发育形貌特征,从而获得判断矿物成因的直观依据。研究结果表明,牛蹄塘组页岩矿物的成因类型有陆源碎屑成因、自生成因（包括蚀变成因、生物成因等）、后生成因、热液成因、表生成因等,其中以陆源碎屑和自生成因为主。不同成因的矿物具有不同的地质指示意义,对页岩储层物性也有不同的影响。生物成因石英（包括硅质）与有机质的交互共生关系指示页岩气勘探的甜点层位。      

台湾国姓地区中新世海相菱铁矿的成因

菱铁矿很好地记录了过去地质流体的信息,能够用于示踪生物地球化学反应相关的成岩作用带。台湾国姓地区中新世海相泥页岩中发育自生的菱铁矿结核,其成因尚未厘清。野外观察发现菱铁矿以不连续透镜体平行散布于泥页岩中,主要由自生碳酸盐菱铁矿（78.63%）等矿物组成。菱铁矿的稀土元素配分模式为轻稀土亏损、中稀土富集,无Ce异常,指示菱铁矿形成于弱氧化的沉积环境,弱氧化的环境促进了菱铁矿在次氧化带的沉淀。菱铁矿的δ13C_VPDB和δ18O_VPDB值分别为-3.69‰~+0.08‰和-1.09‰~+0.25‰,指示菱铁矿形成于次氧化带,碳源很可能是海水和有机质降解混合产生。研究表明自生菱铁矿能够被用于识别沉积物中的生物地球化学过程和指示成岩作用带。     

Origins and accumulation of organic matter in expanded Albian to Santonian black shale sequences on the Demerara Rise, South American margin

Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian–Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ¹³C_org values lie mostly between −30‰ and −27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C₃ land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ¹³C_org values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian–Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.     

黔北下寒武统黑色岩系的沉积环境与地球化学响应

下寒武统黑色岩系在我国华北、华南及塔里木盆地等广泛发育,但富有机质泥页岩的分布特征及其控制因素未有定论.本文对黔北地区下寒武统牛蹄塘组(及同时期地层)不同沉积相带典型剖面中的富有机质泥页岩及其上下层位进行了地球化学分析,识别出寒武纪早期不同沉积相带氧化还原环境的明显差异.台内凹陷相主要岩性为黑色页岩,底部为不等厚的薄层...     

Influences of the Basin Brines on Hydrocarbons of the Anthracosia Shales from Southwestern Poland

Previous studies have shown that the oxidizing brines from the Early Permian Rotliegende sequence have influences on the organic matter of Kupferschiefer. However, inside the Rotliegende sequence there are two other black shales: the Lower and Upper Antracosia shales, which have not been studied as much in detail as in Kupferschiefer. In the present study 12 samples from the Lower and Upper Antracosia shales were analyzed by organic geochemical methods in order to clarify the influences of the oxidizing brines on organic matter. The results indicate that the organic matter of the samples from the Upper Antracosia shale and the bottom of the Lower Antracosia shale was oxidized under the influences of the oxidizing brines. The oxidation resulted in a depletion of saturated hydrocarbons and the alky Is of the aromatic compounds.