Trace Fossils and Sulfur Isotopes in Mudstones around the Kuroko Deposits in the Hokuroku Basin, Northeast Japan: An Attempt to Delineate the Depositional Environment

Abstract. A comprehensive investigation was carried out on the distribution of both trace fossils and sulfur isotopes in mud-stones in the Hokuroku district, northeast Japan, in the hope of delineating the depositional environment of the mudstones in which the Kuroko deposits are embedded. The mudstones are generally massive in structure and usually contain large trace fossils, being indicative of an aerobic biofacies. On the other hand, some mudstones in and above the Kuroko ore horizon are partly laminated and usually contain smaller trace fossils, being assignable to an anaerobic or dysaerobic biofacies. The δ³⁴S values of sulfides in the mudstones above and below the ore horizon range from -40 to -12 %o, indicating mostly oxic depositional conditions in equilibrium with the inferred aerobic biofacies. In the mudstones in the ore horizon, the δ³⁴S values exhibit regionally discriminated variations: -44 to -12 %o in areas far (>1 km) from the known Kuroko deposits and -24 to +6 %o in areas closer to them. The latter high δ³⁴S group implies the temporal occurrence of local anoxic basins in the vicinity of the known Kuroko deposits. At the time of late Nishikurosawa Stage (i.e. the currently assumed Kuroko metallogenic epoch), an intense oceanic stagnation is suggested to have taken place to form the local anoxic basins responsible for the formation and preservation of Kuroko deposits. This oceanic environmental event is considered to be most likely due to increasing biological productivity primarily triggered and enhanced by upwelling of NADW in the paleo-Sea of Japan at that time.     

滇东黔西上二叠统含煤岩系泥岩粘土矿物组成及环境意义*

滇东黔西上二叠统含煤岩系形成于陆相到海陆过渡相环境中,其泥岩的粘土矿物组成可以反映这种环境背景。该区泥岩粘土矿物类型以高岭石和伊利石/蒙皂石间层矿物为主,伊利石次之,局部层段见较少绿泥石及蒙皂石。垂向上,高岭石在富源后所矿区、盘县盘关矿区剖面中下部（即宣威组中、下段和龙潭组）含量偏高;伊利石在织金矿区剖面上部（长兴组）和下部（龙潭组下段）含量明显偏高,而在剖面中部（龙潭组上段）偏低;伊利石/蒙皂石间层矿物在后所矿区、盘关矿区、纳雍矿区占绝对优势,而在织金矿区仅在剖面中部含量较高,并与伊利石呈明显的负相关关系;蒙皂石含量很少,仅出现于盘关矿区和织金矿区剖面的上部和下部。粘土矿物的上述分布特征可大致反映出龙潭早期为海侵期,龙潭晚期为海退期,长兴期为新的更大的海侵期。平面上,从陆到海,即从滇东的冲积平原河流体系,向东经盘关向斜的上三角洲平原、纳雍的过渡带三角洲平原,到织金的下三角洲平原及潮汐平原,高岭石含量急剧减少,伊利石含量逐渐增加。最为明显的是从滇东河流体系到黔西三角洲—潮坪体系之间高岭石含量骤然减少。     

内蒙古西部阿拉善右旗芒罕超克地区上石炭统—下二叠统阿木山组烃源岩的特征

对芒罕超克地区阿木山组（C2P1a）岩性组合与沉积相纵向发育特征研究,结合区域沉积相展布分析,认为阿木山组沉积期为巴丹吉林海相裂谷盆地发育的鼎盛时期,沉积了一套厚度巨大的浅海陆棚相泥页岩,是区内烃源岩发育的主要层段。以TOC≥0.3%为下限,折算烃源岩厚度121.39m。其中,TOC含量中等—较高的烃源岩厚度达89.11m,占烃源岩分布层段厚度的58.9%;干酪根C同位素、甾烷C27与C29相对含量、饱和烃特征等地球化学指标研究表明,干酪根类型以Ⅱ型为主;姥鲛烷（Pr）/植烷（Ph）所反映的原始沉积环境表明,有机质沉积环境主要为还原环境,对有机质保存有利;镜质组反射率、最大热解温度和伊利石结晶度分析表明,烃源岩演化进入过成熟阶段,主要受构造动力作用和华力西晚期、印支期岩浆热接触作用的影响,不存在区域变质,不影响区域上阿木山组作为重要的烃源岩层段。     

Experimental study of slaking properties of red-bed mudstones from the Three Gorges Reservoir area

Abstract

The slake durability; grain quantity, size, roundness, and slake degree of red-bed mudstones are experimentally investigated using the slake durability test and the cyclic wetting and drying test. Results indicate that the complete slaking process exhibits three distinct stages. During the process, the slake durability index decreases linearly with increasing cycle number. Slight variation in grain quantity is observed. Smaller grain size fractions tend to have higher slaking degree indexes in earlier cycles. Grain roundness fluctuates. Two parameters, namely, fractal dimension and surface energy, are revealed to have the ability to represent the degree of slaking of red-bed mudstones.     

内蒙古西部额济纳旗及邻区二叠纪暗色泥岩微量元素和稀土元素地球化学特征

对泥岩微量元素系统分析表明,额济纳旗及邻区二叠纪整体为半咸水环境;纵向上,二叠纪水体盐度逐渐降低,海水逐渐淡化的过程与裂谷盆地演化消亡的过程同步;横向上,东部（雅干以东）古盐度明显高于西部（北山地区）,这与区内水体东深西浅的沉积古地理格局有关,与古生物分布特征反映的古盐度一致。综合泥岩V/V+Ni、Ceanom分析,认为区内二叠纪整体为缺氧、还原环境,该环境有利于有机质保存;区内水体的还原性西强东弱,北强南弱,其特征控制着该区有机质的分布,这对进一步寻找潜在有利烃源岩具有一定的指导意义。综合泥岩稀土元素配分模式及野外地质调查资料分析表明,区内泥岩的原始物质应来自上地壳,其母岩可能为花岗岩和粘土岩的混合成因。     

Dryland continental mudstones: Deciphering environmental changes in problematic mudstones from the Upper Triassic (Carnian to Norian) Mercia Mudstone Group,south‐west Britain

The sedimentology, petrography and mineralogy of the seemingly monotonous Late Triassic Mercia Mudstone Group across the Severn Basin and Bristol Channel region reveals a bipartite division of mudrock facies above and below the Arden Sandstone Formation. Frequently cryptic sedimentary and pedogenic features reveal diverse alluvial, aeolian, playa‐lacustrine and pedogenic processes operating at different times, locations and scales probably in response to climate change. The current study found no evidence of significant marine influence in either of the two mudrock facies associations that are described here. Blocky claystones dominate the lower Mercia Mudstone Group (Sidmouth Mudstone Formation), with pedogenic features such as slickensides, mottling, tubules and carbonate/sulphate nodules common and widespread. The claystones are of alluvial/lacustrine origin with subordinate sheet sandstones, themselves overprinted by pedogenic features, reflecting occasional high intensity rainfall events. These facies reflect a seasonal (wet/dry) semi‐arid climate favouring development of transformed/neoformed (smectite‐rich) clay minerals of intrabasinal origin. Massive to weakly stratified silty mudstones dominate the upper Mercia Mudstone Group (Branscombe Mudstone Formation). Commonly conchoidally weathered, locally gypsiferous, but with distinct sedimentary structures scarce, they alternate with subordinate blue‐grey laminated silty mudstones. Together they reflect fluctuating hydrological conditions within extensive saline mudflats and ephemeral playa lakes, with laminated facies deposited under subaqueous conditions during more humid phases whereas massive mudstones reflect modification through interstitial growth/dissolution of sulphates and deflation of surface sediments during drier episodes. These facies reflect increased aridity during deposition of the upper Mercia Mudstone Group compared with the lower Mercia Mudstone Group, favouring development of detrital/transformed (illite–chlorite) clay minerals of extrabasinal origin. The described facies associations and the sedimentary fabrics and structures that characterize them, occur widely in the Mercia Mudstone Group across the United Kingdom and comparable facies associations may be anticipated in other fine‐grained red bed successions. Recognition of these facies may aid palaeoenvironmental interpretation of such sequences on Earth and, potentially, on Mars also.     

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Septarian concretions are abundant in many Phanerozoic marine and marginal‐marine shales and mudstones. They range from a few centimetres to several metres in size and are spherical or ellipsoidal in morphology. In general, formation by localized calcite or siderite cementation in argillaceous sediments began under less than a few metres of burial. Septarian cracks vary widely in shape and configuration: included are networks of wide, vertically, radially and sometimes concentrically oriented, lenticular shrinkage cracks; and narrower, parallel‐sided, straight to irregular tension cracks locally accompanied by brecciation, and plumose and en échelon sigmoidal cracks indicative of shear stresses. Crack types are intergradational; many concretions exhibit multiple cracking events. Enclosed macrofossils and isopachous fibrous calcite cement that lines earlier formed cracks are commonly broken and displaced. In some cases, cracks contain injected lime mud and silt. These features, taken together, testify that cracking involved a spectrum of responses in concretion interiors, from loss of shear strength, dewatering and shrinkage to brittle failure, demonstrating variations in, and contrasts between, the rheological properties of the matrix and enclosed objects. Localization to interiors and outward tapering of lenticular cracks make a case for fracture partitioning and indicate that interiors were softer than exteriors at the time of shrinkage. Parallel‐sided cracks point to greater stiffness, and evolving crack shape in multiply cracked concretions shows that rigidity increased with time. Crack orientations indicate highly variable tensile and shear stress directions within individual concretions. Rupture, brecciation, displacement of fragments, loss of shear strength, liquefaction and injection of unconsolidated granular sediment suggest that deformation events were rapid, if not virtually instantaneous. Previous explanations for the internal cracking, such as gas generation, spontaneous chemical dehydration or localized overpressuring due to compaction, seem either untenable or fail to account for the spectrum of observed features. However, syndepositional earthquake‐induced shaking of cementing bodies of varying rigidity at shallow burial depths seems to be a plausible source for the requisite short‐lived, variable to anisotropic, high‐stress regime inside the concretions. Septarian concretions may thus preserve a signature of basin seismicity as it relates to their cementation history.     

Re-evaluation of quartz vein history in the Dolgellau Gold-Belt,North Wales,United Kingdom

Conspicuous, gently inclined, lenticular quartz–carbonate veins with fibre-textured infills dissect the steep, gold-bearing quartz veins in the Dolgellau Gold-Belt. Similar quartz veins fill the necks of boudin-like structures affecting the main gold-bearing veins at Clogau mine. They also dissect the post-mineralization ENE–WSW ‘Clogau Stone’ dykes. The uniformly oriented regional cleavage shows marked deflection around the gently inclined quartz veins. These late quartz veins were emplaced whilst the mudstones of the Clogau Formation, which host the steep gold-bearing quartz veins, were undergoing ductile deformation. The stretching involved in the late vein and boudin development requires these structures to be contemporaneous with some stage in the development of the regional sub-vertical cleavage and upright fold system. The earlier gold-bearing veins show evidence of brittle fracture and still retain some drusy space-filling textures. Cleavage is deflected adjacent to these early veins and in breccia clasts within the veins. The rare minor folds seen in the gold-bearing quartz veins have axial planar cleavages. It is concluded that the gold-bearing veins pre-date cleavage development. Thus they were forming when large amounts of water were still available in the sedimentary sequence, removing the severe source and transport problems that are implicit in the previous view that the veins were post-tectonic. Copyright © 1999 John Wiley & Sons, Ltd.     

Mineral and chemical composition of Karak mudstone, Kohat Plateau, Pakistan: implications for smectite-illitization and provenance

The Karak mudstone interbedded in an Eocene evaporite sequence, is dominated by R-1 ordered illite-smectite with a 20 to 30% expandable component. Minor phases include kaolinite, chlorite, illite/muscovite, plagioclase, potash feldspar, quartz, dolomite and pyrite. The present illite-smectite was probably originally smectite or highly expandable illite-smectite which underwent conversion to illite-smectite with a low expandable component in a comparatively low-temperature (ca. 100°C) closed-system sedimentary basinal diagenetic environment at a depth of ca. 5 km. Al³⁺ and K⁺ necessary for the conversion reaction were provided through the breakdown of potash feldspar. Burial under a 5 km thick pile of sediments produced some of the observed structures. Whole-rock chemistry presented here suggests that the mudstone formed by severe weathering of acidic source rocks. The influx of freshwater probably flushed out Ba, Rb, Ca and Mn from the restricted basin.     

Organic material accumulation of Carnian mudstones in the North Qiangtang Depression,eastern Tethys: Controlled by the paleoclimate,paleoenvironment, and provenance

The Late Triassic mudstones are considered to be the most significant hydrocarbon source rocks (TOC: 0.54%–3.29%) in the North Qiangtang Depression, eastern Tethys. Here, we present geochemical data from the Woruo Mountain Carnian mudstones, in order to investigate their paleoclimate, paleoenvironment, and provenance and to analyze the mechanism of organic material accumulation. The paleoclimate condition was warm and humid during the Carnian mudstones deposition, as indicated by moderate chemical index of alteration (CIA; 73–76), which may be connected with the Late Triassic Carnian stage global climate event in the Tethys. The low U/Th (0.17–0.25) and C_org/P_tot (7–33) ratio values and moderate manganese contents, reflect the oxidizing conditions during the Carnian mudstones deposition. The relatively high primary productivity in this study is supported by the relatively high P concentrations. The Al₂O₃–(CaO* + Na₂O)–K₂O ternary plot and Th/Sc–Zr/Sc crossplot reflect that the source areas have undergone a medium chemical weathering with weak sedimentary recycling. The TiO₂–Zr, Co/Th–La/Sc, La/Th–Hf, and La/Yb–∑REE bivariate diagrams indicate that the provenance of Carnian mudstones was primarily from felsic igneous rocks. The collision setting has been identified based on the multi-major elements discriminate plots in the present study. The Riwanchaka and Mayigangri masses to the southwest and south of the study area consisting mainly of Middle-Triassic granodiorite and Late-Triassic granite are likely responsible for supplying provenance to the Woruo Mountain Carnian mudstones, which have similar REE patterns. The relatively high TOC contents of Carnian mudstones are related to high paleoproductivity and fast sedimentation rates, which will lead to preservation of some organic matter even when bottom waters are completely oxidizing. The detrital input during the Carnian mudstones deposition would result in dilution of organic matter.