The fissility of some Carboniferous shales

Laminations control the fissility of a sequence of shales and mudstones from the Upper Carboniferous Coal Measures in Britain. This was established by measuring the thickness of pairs of laminations in unweathered borehole material and comparing with the thickness of shale fragments at the outcrop of the same sequence. The parallel orientation of the clay minerals is not responsible for the fissility and it could be that its importance has been overestimated. The processes responsible for the formation and preservation of the laminations will also produce a well orientated clay fabric and hence fissility and clay orientation may be indirectly related.     

Fluvial sedimentation in the lower carboniferous of Clew Bay,County Mayo,Ireland

A succession of about 300 m of fluvial sediments from the Lower Carboniferous of northwest Ireland is described and interpreted. A lower, mainly red, formation contains fluvial channel deposits dominated by flat laminated sandstone. These are separated by interbedded sandstones and mudrocks with local caliche horizons and abundant mudcracks interpreted as levee and flood basin deposits. An upper, mainly non-red, formation contains fluvial channel deposits with common trough cross-stratification and epsilon cross-stratification also separated by interbedded sandstones and mudrocks. Evidence of desiccation is less common in the uppermost beds which pass transitionally upwards into marine sediments.The change in fluvial channel style is interpreted as due to increasing sinuosity and permanence of flow which may have been partly temporally and partly spatially controlled. The predominance of coarse sediments is thought to be largely controlled by limited subsidence. The Lower Carboniferous transgression was the major overall control of alluviation.     

Nanoscale Pore Characteristics of the Upper Permian Mudrocks from a Transitional Environment in and around Eastern Sichuan Basin,China

Nanoscale pore characteristics of the Upper Permian Longtan transitional mudrocks and their equivalent strata Wujiaping Formation marine mudrocks in and around the eastern Sichuan Basin was investigated using field emission scanning electron microscopy (FE-SEM) and low-pressure N2 adsorption experiments. The results indicate that the Upper Permian mudrock is at a mature stage with total organic carbon (TOC) values ranging between 0.47% and 12.3%. The Longtan mudrocks mainly contain vitrinite, and their mineral composition is primarily clay. In contrast, the Wujiaping mudrocks are dominated by sapropelinite and solid bitumen, and their mineral compositions are mainly quartz and a notably high amount of pyrite. The FE-SEM reveals that clay mineral pores and microcracks are the common pore types in the Longtan mudrocks. The specific surface area and pore volume depend on the clay content but are negatively correlated with the TOC. The generation of nanometer pores in the Longtan mudrocks is caused by high clay mineral contents. Meanwhile, the Wujiaping mudrock mainly contains OM pores, and the pore parameters are positively correlated with the TOC. The OM pore development exhibits remarkable differences in the Longtan and Wujiaping mudrocks, which might be related to their sedimentary facies and maceral fractions. Vitrinite and inertinite appear as discrete particles in these mudrocks and cannot generate pores during thermal maturation. Sapropelinite often contains many secondary pores, and solid bitumen with large particles, usually with several pores, is not the major contributor to the pore system of the investigated mudrock.     

Clay orientation in some Upper Carboniferous mudrocks, its relationship to quartz content and some inferences about fissility, porosity and compactional history

The degree of preferred orientation of clay minerals in a series of fifteen fine-grained sediments has been precisely measured by transmitted X-ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid-point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine-scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present-day muddy sediments. On this basis it is concluded that preferred orientation in clay-rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity.     

Petrological and geochemical interpretation of Triassic–Jurassic boundary sections from northern Iraq

Triassic–Jurassic sedimentary successions (Baluti and Sarki formations) in northern Iraq record a variety of environmental changes that may be related to global Triassic–Jurassic (Tr/J) boundary events. The diversity of some benthic fauna decreases through the transitional boundary beds. The coastal marine environment of the lower part of the Baluti Formation is followed by shallower tidal flat and supratidal marginal marine environments at the transitional boundary with the Jurassic‐age Sarki Formation. The alternating calcareous mudrocks and dolomitic limestones of the transitional succession are overlain by a succession of calcareous mudrocks and dolomicrites that form a dolocrete bed in the latest Triassic. The early Jurassic carbonates (lower part of Sarki Formation) were deposited in a shallow‐marine to lagoonal environment. Geochemical evidence supports this interpretation. TOC% increases towards the Tr/J boundary and the lower part of the Sarki Formation. This increase can be interpreted as resulting from the primary precipitation of dolocrete as palaeosol horizons. The variations in the oxygen isotope ratios mainly reflect the facies and diagenetic effects. Th/K ratio is generally constant and shows an increase in the calcareous mudrock beds of the upper part of the Baluti Formation, possibly related to the degradation of K‐bearing clay minerals. Low Th/U ratios are due to the depletion in thorium, typical of many marine carbonates rather than to an increase in authigenic uranium. This explanation is also corroborated by the presence of abundant fossils in some of the studied carbonates. Copyright © 2013 John Wiley & Sons, Ltd.     

The paleoenvironmental distribution of Phanerozoic wrinkle structures

Wrinkle structures are microbially induced sedimentary structures that are known to have formed in ancient marine siliciclastic environments. This study reviews the sedimentary context and paleoenvironmental distribution of these sedimentary structures throughout three distinct intervals of the Phanerozoic: the Cambrian, the post-Cambrian Paleozoic, and the Lower Triassic. During these times, wrinkle structures are found primarily within two sedimentary environments, storm-dominated subtidal environments and the intertidal zone. Subtidal occurrences of wrinkle structures during the post-Ordovician Phanerozoic only appear to occur during intervals of environmental stress, while earlier examples from the Cambrian and lowermost Ordovician likely formed prior to significant increases in extent and depth of bioturbation that hallmark most of the Phanerozoic. Intertidal examples occur regardless of severe environmental stress, and may have formed under low levels of bioturbation due to inherent vagaries of a marginal marine setting. These environmental preferences appear to be conservative throughout the Phanerozoic, and may very well extend throughout Earth's history.     

Ichnofabric characterization of a deep‐marine clastic system: a subsurface study of the Middle Eocene Ainsa System,Spanish Pyrenees

This paper documents a subsurface trace fossil and ichnofabric study of the proximal parts of a structurally confined and channelized sand‐rich, lower slope and proximal basin‐floor deep‐marine system in the Middle Eocene Ainsa basin, Spanish Pyrenees. Five depositional environments are recognized based on sedimentary facies associations, depositional architecture and stratigraphic context (channel axis, channel off‐axis, channel margin, leveé‐overbank and interfan), as well as a channel abandonment phase. Each environment is characterized by distinct and recurring ichnofabrics. Ichnological measurements and observations were recorded from six cores recovered from six wells drilled at a spacing of between 400 m and 500 m at outcrop, and totalling 1213 m in length. From channel axis to levée‐overbank environments, there is a trend of increasing bioturbation intensity and ichnodiversity. Ichnofabrics in channel axis and channel off‐axis environments are characterized by low bioturbation intensity and low ichnodiversity. Thalassinoides‐dominated firmground ichnofabrics associated with erosive sediment gravity flows are common in these environments. In contrast, channel margin and levée‐overbank environments are characterized by ichnofabrics associated with high bioturbation intensity and ichnodiversity. Sediments of the interfan are characterized by the highest bioturbation intensity, associated with burrow mottling and an absence of primary sedimentary structures. This paper demonstrates that in core‐based studies, ichnofabric analysis is an important and valuable tool in discriminating between different environments in channelized deep‐marine siliciclastic systems. The results of this study should find wide applicability in reservoir characterization studies in the petroleum industry, in field‐based analogue ichnofabric studies and other core‐based studies in deep‐water siliciclastic systems worldwide such as the Integrated Ocean Drilling Program.     

Feldspar albitization in Cretaceous non-marine mudrocks, Gyeongsang Basin, Korea

Plagioclases and K-feldspars in the sandstones and mudrocks of the Cretaceous non-marine Gyeongsang Basin, Korea, were partially to completely albitized. The preservation of fresh plagioclase grains in early micrite-cemented sandstones suggests the diagenetic origin of albite. Albitization textures in mudrocks were examined using backscattered electron images. In contrast to completely albitized plagioclase grains in sandstones, those in mudrocks are mostly partially albitized. It suggests that mudrocks can be more useful for a provenance study than sandstones by preserving detrital minerals better. K-feldspar is unaltered to partially albitized in both sandstones and mudrocks. In mudrocks albitization starts preferentially along microfractures, cleavages and grain margins. Albitization along grain margins seems to be a characteristic feature in mudrocks where development of microfractures in silt-sized feldspar grains by physical compaction is limited by clayey matrix as well as by overpressure. The extent of albitization in mudrocks is mainly controlled by composition of the detrital plagioclase. Mudrocks containing calcic plagioclase grains tend to be more extensively albitized than those containing sodic varieties.     

Pencil structure and strain in weakly deformed mudstone and siltstone

Weakly deformed mudstone and siltstone (Middle Ordovician Knobs Formation) of the Appalachian Valley and Ridge Province, south-western Virginia, U.S.A. show strain-dependent transitions between bedding fissility, pencil structure and cleavage. Pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain (Z) ranges between 9 and 26% shortening (assuming a plane strain and constant volume). Where strains are less, bedding fissility dominates. Pencil fragments are defined by intersecting fracture sets subparallel to the pre-existing bedding fissility and cleavage. Their long axes are both parallel to the bedding-cleavage intersection and to the inferred Y axis of the tectonic strain ellipsoid. Pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post-tectonic stress relaxation. Pencils show variations in size and shape depending on lithology (grain size and composition), degree of initial clay preferred orientation, degree of cleavage development, type of cleavage, total bulk strain and degree of strain homogeneity. Their shape (l/w) however is a direct measure of total Y/Z strain since strain determinations from chlorite pressure-fringes on framboidal pyrite within the pencil fragments give

$\text{(}\text{Y}\text{Z}\text{) = 0.913 + 0.019 (l/w)}$

. Pencil structure is therefore a potential strain marker in weakly deformed rocks.     

海底生物扰动作用及其对沉积过程和记录的影响

海底生物扰动作用主要由底栖生物的摄食、掘穴和栖所建造等活动造成,它可以充分改变沉积物的物理、化学特性,扭曲或模糊地层学信号,影响早期成岩过程和沉积记录以及沉积物的稳定性。海底生物扰动作用的研究,对于全面、精确地掌握海洋中各种物质的生物地球化学循环过程,正确回溯古气候、古海洋和古生态记录,合理调节和控制海底生态系统的多样性具有十分重要的意义。简要回顾了海洋生物扰动作用的研究进展,并着重对海洋沉积物中生物扰动作用的观测和定量研究方法、主要影响因素及其对沉积过程和记录的影响进行了评述。     

Facies and depositional processes in an Upper Jurassic to Lower Cretaceous pelagic sedimentary sequence, Antarctica

The Nordenskjöld Formation (?Oxfordian-Berriasian age) is exposed on the east coast of the Antarctic Peninsula, where it consists of interbedded ash layers and biosiliceous mudstones which accumulated under anaerobic to dysaerobic bottom waters. The mudstones were deposited by pelagic settling and the ash layers by pelagic settling from suspension or as fallout from subaerial eruption columns. The lower part of the succession accumulated in a basinal setting under anaerobic bottom waters and is characterized by parallel bedding. Mudstones deposited in this setting preserve abundant zooplankton faecel pellets. Compaction of these pellets has given rise to a bedding parallel fissility. The upper part of the succession accumulated under dysaerobic bottom waters in a slope setting. The sequence is wavy bedded and contains abundant evidence of post-depositional sediment instability and resedimentation, much of which was caused by tectonic activity. Discrete slide masses are absent from the slope sequence and it appears that slope processes were dominated by creep. Examination of the mudstones shows that as levels of dissolved oxygen in bottom waters increase, pelleted mudstones give way to structureless mudstones before visible bioturbation is noted.     

Magnetic fabric characteristics of bioturbated wave-produced grain orientation in the Bridport-Yeovil Sands (Lower Jurassic) of Southern England

There is little visible primary hydrodynamic lamination preserved in the Bridport-Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave-produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non-bioturbated fine-grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long-axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non-bioturbated sandstone.     

Clay fabrics in relation to the burial history of shales

Quantitative appraisal of compaction strain is essential for the study of the burial history of shales in sedimentary basins. The results of a preliminary fabric analysis of Westphalian and Zechstein shales in the Campine Basin (Belgium) show that clay fabric analysis, using an X-ray pole figure goniometer, is suitable for this purpose. Clay fabrics, in the range studied, are independent of depth and therefore cannot be used as depth indicators. This suggests that in the early stages of the burial history a stable clay fabric has to develop, which will basically remain unchanged during the subsequent burial history. The degree of clay particle preferred orientation not only reflects the compaction strain, but is also determined by mineralogical parameters: the presence of non-platy particles and the relative concentrations of the different clay minerals. This degree of preferred orientation furthermore determines the degree of fissility of the shales. These mineralogical factors limit the use of clay fabrics as truly quantitative strain markers. Their use as semi-quantitative strain markers remains advantageous, mainly because of the common occurrence of clay fabrics in the geological record. Moreover, the relative ease of measurement and the possibility of distinguishing compaction from tectonic strains favour the use of clay fabrics in the quantitative strain analysis of argillaceous rocks.     

藏南白垩系泥、页岩微量、稀土元素特征及氧化-还原环境分析

为了解藏南定日-岗巴盆地和江孜盆地白垩纪沉积时底层水体的氧化一还原环境,采用ICP-MS方法进行微量元素及稀土元素测试分析.结果表明:研究区微量元素在时空上均发生了变化,这与各元素对沉积水体的氧化一还原敏感程度有关.元素Al标准化后变化趋势也不尽相同,其垂向上的变化反映了沉积物底层水体氧化-还原状况、生物产率等沉积环境,与海平面升降、陆源碎屑输入、成岩作用再活化及沉积后氧化作用等多因素有关.V/(V+Ni)、U/Th、V/Cr、Ni/Co、Re/Mo、Uau及δU比值表明早白垩世及晚白垩世中、晚期藏南地区盆地内底层水体环境总体上为中等分层的含氧环境,但定日一岗巴盆地内晚白垩世早期Cenomanian-Turonian界线附近微量元素发生突变,显示为强烈的还原环境.稀土元素球粒陨石标准化后均表现为轻稀土富集的右倾型,LREE/HREE介于7.42～14.80之间,LaN/YbN变化范围为7.93～18.54,δEu负异常,δCe异常不明显,表明为含氧环境.因此微量、稀土元素微观分析与野外宏观露头观察相结合,不但可以作为地层划分对比的直接依据,且能更准确、有效地判断地层沉积时底层水体的环境状况.     

Aphaenogaster ants as bioturbators: Impacts on soil and slope processes

Australian ants belonging to the genus Aphaenogaster excavate dense and frequently relocated nest systems in topsoil and deposit ephemeral, highly erodible (type-I) mounds at their funnel-shaped nest entrances. Rates of mounding are generally higher for this genus than for other Australian ant species, and are amongst the highest rates recorded for ant mounding anywhere in the world. Furthermore, tentative analysis of subsurface mixing suggests that overall rates of Aphaenogaster bioturbation are higher than indicated by mounding alone. This bioturbation has pronounced implications for soil and landscape processes, particularly in modifying soil fabric and texture and in impacting on soil hydrology and erosion. Aphaenogaster bioturbation may also be viewed as a form of ecosystem engineering, and affects the distribution of soil nutrients and the dispersal of seeds. This can lead to localized increases in soil fertility, although Aphaenogaster are notorious as a nuisance in agricultural landscapes.     

X射线照像术在泥质岩岩石学研究中的应用

前言X射线照像术是采用X射线这一种穿透性放射线来做照片的技术。自1895年以来,这种技术已广泛地应用于医学、工业和各种自然科学领域。1962年,美国学者W·K·Hamblin首次将X射线照像术应用于沉积岩石学研究。自那以后,岩石薄板经X射线照像制成的照片,为研究沉积物(岩)的内部结构提供了一种有效、快速和廉价的方法,并得到越来越多的沉积学家的重视。在国外,X射线照像术已应用于现代沉积物、砂岩、碳酸岩、泥质岩、煤等领域的研究。     

海南岛全新世海滩岩的胶结作用与成岩环境的关系

我国海南岛及南海诸岛沿岸,广泛发育海滩岩。1980年我队赴海南岛考察现代沉积时,对海滩岩的分布、岩性特征及其与周围环境的关系进行了观察和采样。样品采自崖县鹿回头三亚湾水尾岭海蚀崖、西洲岛、小东海、东瑁岛、西瑁岛、天涯海角,乐东县莺歌海,文昌县渔业等地(图1)。有关的地质、地貌及岩性特征等,已有许多描述,对海滩岩的岩石学及成岩作用也有许多研究。本文侧重探讨海滩岩的胶结作用及与成岩环境的关系。     

Remote Sensing of Biologically Reworked Sediments: A Laboratory Experiment

The present study aims to test the application of remote sensing to address the impact of bioturbation on physical sediment properties. Therefore, a laboratory experiment was developed, using microcosms mimicking a marine intertidal water–sediment interface to test the influence of Corophium volutator densities on sediment properties. Three main variables (water content, clay content, and mean grain size) were measured in three treatments (no Corophium, 5,000 Corophium per square meter, and 20,000 Corophium per square meter) after 16 days of bioturbation. Results obtained with conventional—destructive—techniques showed a significant increase of water content and a significant, but small decrease of clay content in the presence of Corophium. The remote sensing technique detected the impact of Corophium on water content as an increase in absorption at 1,450 nm, but was not able to detect the animal impact on clay content. This study demonstrates that remote sensing data could be significantly modified by bioturbation activities and that remote sensing can be applied in the laboratory to address the impact of bioturbation on sediment properties. This possibly opens new perspectives for long-term experiments concerning the role of bioturbation on sedimentary processes.     

Using ichnological relationships to interpret heterolithic fabrics in fluvio-tidal settings

In fluvio-tidal settings, the sediment is dominantly derived from the river systems. However, the importance of landward tidal transport of sediment in tidally influenced sedimentary environments is difficult to assess, particularly in the rock record. This problem is addressed using two intervals within the Lower Cretaceous McMurray Formation, each representing a distinct inclined heterolithic stratification motif. The ichnological variation between the heterolithic intervals is analyzed to determine which lithosomes are associated with brackish-water (tidally influenced) colonization windows. From this, the relative fluvial influence responsible for the deposition of the fine and coarse members can be determined. Both of the inclined heterolithic stratification fabrics studied record the deposition of fluvio-tidal point bars wherein the heterolithic bedding represents variations in river discharge. The first fabric comprises inclined heterolithic stratification in which bioturbation only occurs in mudstone beds. This fabric indicates that deposition occurred in more proximal positions within a fluvio-tidal system (i.e. the outermost inner to middle estuary or distributary channels). In this example sand deposition is interpreted to represent high-energy, freshwater dune migration within a fluvial-dominated setting, whereas mud beds reflect brackish-water suspension deposition during times of low river discharge. The second fabric, which is interpreted to have developed in more distal depositional positions (i.e. the middle estuary or seaward of the turbidity maximum in deltas), consists of inclined heterolithic stratification with laminated mudstone and bioturbated sandstone. In these inclined heterolithic stratification successions the mudstone beds were deposited under the influence of freshwater and heightened sedimentation rates, whereas bioturbated sandstone was colonized under brackish-water conditions and in the presence of tidally facilitated sediment transport. In both examples, the bioturbated lithosomes are related to colonization windows that indicate the predominance at that time of marine or tidally influenced processes over fluvial processes.     

The applicability of modern tidal analogues to pre‐vegetation paralic depositional models

In modern siliciclastic environments terrestrial and aquatic vegetation binds substrate, controls weathering and erosion rates, influences run‐off, sediment supply and subsequent depositional architecture. This study assesses the applicability of modern depositional models that are impacted by vascular vegetation, as analogues for ancient pre‐land plant systems. A review of pre‐Devonian published literature demonstrates a paucity of described tidal successions; this is possibly due to the application of modern analogues for interpreting the record when there is a lack of tidal indicators. This paucity suggests a need for revised models of tidal deposition that consider the different environmental conditions prior to land plant evolution. This study examines the Ordovician–Silurian Tumblagooda Sandstone, which is exposed in the gorge of the Murchison River and coastal cliffs near Kalbarri, Western Australia. The Tumblagooda Sandstone comprises stacked sand‐rich facies, with well‐preserved bedforms and trace fossils. Previous interpretations of the depositional setting have proposed from a mixed sheet‐braided fluvial and intertidal flats; to a continental setting dominated by fluvial and aeolian processes. An enigmatic element is the rarity of mud‐rich facies preserved in the succession. Outcrop logging, facies and petrographic analysis record dominantly shallow water conditions with episodes of emergence. Abundant ichnotaxa indicate that marine conditions and bi‐directional flow structures are evidence for an intertidal and subtidal depositional environment. A macrotidal estuary setting is proposed, with evidence for tidal channels and repeated fluvial incursions. Physical and biogenic sedimentary structures are indicative of tidal conditions. The lack of clay and silt resulted in the absence of flaser or lenticular‐bedding. Instead cyclic deposition of thin beds and foreset bioturbation replaced mud drape deposits. Higher energy conditions prevailed in the absence of the binding activity of plants in the terrestrial and marine realm. This is suggestive of different weathering processes and a reduction in the preservation of some sedimentary features.