Petrography, chemistry and origin of early diagenetic concretions in the Lower Carboniferous of the Isle of Man

Three silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz. The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore-waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater. The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods. A four-fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions. Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth.     

Geochemistry of the Hepworth Carboniferous sediment sequence and origin of the diagenetic iron minerals and concretions

The major element geochemistry of a mudstone sequence of Westphalian age has been studied in detail and mineralogical distributions calculated. Quartz and zircon trends indicate the energy of the depositional environment and this is closely reflected in the clay mineralogical composition. Of the detrital clays kaolinite is concentrated in the finer-grained sediments relative to illite. Chlorite and interstratified illite-smectite probably underwent diagenetic modification and small amounts of authigenic clays may have formed.Siderite, which occurs in both concretionary and disseminated form, and pyrite developed during diagenesis under reducing conditions and account for most of the iron present. Pyrite grew first deriving its iron partly from detrital clays. Carbon isotopic evidence suggests that early siderite precipitated in pore space as nodules and later in dispersed form. A second concretionary phase, pistomesite, grew during late dewatering. Siderite iron was derived mainly from Mn-rich oxidate phases.Titanium occurs as anatase uniformly distributed through the sequence. Phosphorus is mainly in diagenetic apatite and the $\text{sodium}\text{potassium}$ ratio is controlled by the micas. Adsorbed water is closely related to grain size and clay content.     

Lower and Middle Carboniferous oil-bearing limestones of southeastern Tatarstan and reconstruction of the paleoenvironment of their formation

The study of Lower and Middle Carboniferous limestones within the eastern flank of the Melekess Depression and the South Tatar Arch allowed us to reconstruct their formation environment. Bioclastic phyto-and zoomorphic limestones occur as wide zones parallel to the shoreline. Therefore, lithogenetic typification of limestones was used for the stratification of Tournaisian and Visean deposits. Serpukhovian and Bashkirian zoogenic, clastic, and pelitomorphic limestones formed away from the shoreline. Such deposits are represented by biogenic buildups (possibly, biostromes), fan limestones, synsedimentary crushing (clastic) limestones, and interbiostrome deposits (pelitomorphic limestones). Therefore, their horizon-scale stratification is impossible.Commercial oil pools in the carbonate part of the section are confined to bioclastic zoomorphic limestones subjected to secondary leaching. The confinement of leaching to the lithogenetic limestone types mentioned above is related to their composition (shell remains and calcite cement). Shell remains making up the rock skeleton are protected against leaching by the organic material that prevents rocks from compaction.     

The mineralogy and origin of interbedded clay wayboards in the Lower Carboniferous of the Derbyshire Dome

Interbedded clays, or clay wayboards, from the Lower Carboniferous Limestones of Derbyshire and Staffordshire, are discussed in terms of their geological relationships, textural features and mineralogy, and comparison is made between these clays and other interbedded argillaceous materials from the area. It is concluded that the clay wayboards are the argillation products of contemporaneous volcanic ashfalls, and lie within the definition of K-bentonites. Stratigraphical and palaeogeographical aspects of clay wayboard distribution are briefly discussed, and it is shown that despite the probable widespread extent of individual clay horizons they are only of limited value for the purposes of correlation.     

云南保山地区的下石炭统

<正> 云南保山地区下石炭统及其生物群曾引起许多学者的兴趣(Reed l927;王鸿桢,1945;段丽兰,1973,1985;宋学良,1982;金玉玕、方润森,1983;杨宗仁,1983;陈重泰,1984;陈根保,1984;金苏华,1992),其主要原因在于丰富而特殊的生物类群,悬而未决的地层问题以及独特的生物地理位置。本文以4条剖面(图1)为基础,依据多门类的生物化石对诸如本区杜内/维宪阶界线、Siphonophyllia-Keyserlingophyllum(Humboldtia)动物群的时代、冷水(凉水)珊瑚动物群、岩石地层单位等问题进行讨论。     

Occurrence of diagenetic pseudobreccias within the paleokarst zone of the upper Dammam Formation in Kuwait, Arabian Gulf

A paleokarstic zone capping the Middle Eocene dolomitic Dammam Formation delineates a regional disconformity with the overlying Mio-Pleistocene fluvial deposits in Kuwait. This formation outcrops in a quarry located in the southeast of Kuwait and occurs in the subsurface of Umm Ghudair water field, west Kuwait. Karstification has affected the upper member of the Dammam Formation, which is formed of extensively chertified, vuggy chalky biodolomicrite. Karstification is manifested by two phases: a dissolution phase that was responsible for the occurrence of surface and near surface karst features and a precipitation phase during which a karst carapace and dolocretic and calcitic pseudobreccias were developed. Petrographic investigations of both the biodolomicrites host rock and the karst lithotypes revealed a complex diagenetic history that encompasses the following diagenetic events, arranged in a chronological order: (a) replacive dolomitization, (b) precipitative dolomitization, (c) precipitative silica, (d) replacive silica, (e) karstification, (f) development of karst carapace [calcretization and calcitization (development of calcitic psuedobreccia)], (g) dolocretization (development of dolocretic psuedobreccia), (h) mild karstification, (i) calcite precipitation, and (j) silica precipitation. The development of the karst carapace at the unconformity surface between the Eocene Dammam Formation dolomicrites and the overlying Mio-Pleistocene Kuwait Group clastics has played a significant role in the hydrological characteristics of Kuwait usable groundwater resources. It is recommended that the occurrence of similar diagenetic processes within the Arabian carbonate shelf deposits be investigated.     

Diagenesis of early Permian high‐latitude limestones,Lower Parmeener Supergroup,Tasmania

The Darlington (Sakmarian) and Berriedale (Artinskian) Limestones are neritic deposits that accumulated in high‐latitude environments along the south‐eastern margin of Pangea in what is now Tasmania. These rocks underwent a series of diagenetic processes that began in the marine palaeoenvironment, continued during rapid burial and were profoundly modified by alteration associated with the intrusion of Mesozoic igneous rocks. Marine diagenesis was important but contradictory; although dissolution took place, there was also coeval precipitation of fibrous calcite cement, phosphate and glauconite, as well as calcitization of aragonite shells. These processes are interpreted as having been promoted by mixing of shelf and upwelling deep ocean waters and enabled by microbial degradation of organic matter. In contrast to warm‐water carbonates where meteoric diagenesis is important, the Darlington and Berriedale Limestones were largely unaffected by meteoric diagenesis. Only minor dissolution and local cementation took place in this diagenetic environment, although mechanical compaction was ubiquitous. Correlation with burial history curves indicates that chemical compaction became important as burial depths exceeded 150 m, promoting precipitation of extensive ferroan calcite. This effect resulted from burial by rapidly deposited, overlying, thick, late Permian and Triassic terrestrial sediments. This diagenetic pathway was, however, complicated by the subsequent intrusion of massive Mesozoic diabases and associated silicifying diagenetic fluids. Finally, fractures most probably connected with Cretaceous uplift were filled with late‐stage non‐ferroan calcite cement. This study suggests that both carbonate dissolution and precipitation occur in high‐latitude marine palaeoenvironments and, therefore, the cold‐water diagenetic realm is not always destructive in terms of diagenesis. Furthermore, it appears that for the early Permian of southern Pangea at least, there was no real difference in the diagenetic pathways taken by cool‐water and cold‐water carbonates.     

湖南邵东下石炭统刘家塘组的介形类

湖南邵东下石炭统刘家塘组产丰富的介形类化石，计有１８属３５种（包括６新种），它们绝大多数可与国内外下石炭统介形类对比，其地质时代为早石炭世杜内期，刘家塘组介形类明显地可分成上、下两个不同组合。本文仅描述了刘家塘组介形类６属７种（包括６新种）。     

Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ¹⁸O and δ¹³C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ¹⁸O=–2·2, δ¹³C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ¹⁸O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.     

The genus Allorisma from Lower Carboniferous of the Moscow kettlehole

The author discusses the conceptual bases of the pelecypod family Allorismidae and of the Carboniferous genera Allorisma, Allorismiella, Edmondia, Edmondiella, and Pseudedmondia. There is an extensive discussion of the hinge line structures of Allorisma based on Russian specimens, and four species of the genus from the Carboniferous of the Moscow area are described. Astaf'yeva-Urbaitis overlooked Wilson's 1959 paper in which Allorisma was shown to be a junior synonym of Edmondia; as a result Astaf'yeva-Urbaitis' nomenclature is faulty. Wilson replaced Allorisma with the new generic name Wilkingia. — J. Pojeta, Jr.     

The significance of needle-fibre calcite in a Lower Carboniferous palaeosol

Needle-fibre calcite occurs in association with root structures in a Lower Carboniferous calcrete crust. The origin of this form of calcite is discussed and the scattered literature from soil science, microbiology and geology is reviewed. It is concluded that while needle-fibre calcite is not diagnostic of any environment its origin is related to the activities of various micro-organisms, especially fungi. The use of needle-fibre calcite as a climatic indicator is reviewed and it is used, with other criteria, as evidence of fluctuating climates in the Lower Carboniferous.     

Diagenesis of the Gully Oolite (Lower Carboniferous), South Wales

Deposition of the Gully Oolite was locally interrupted by emergence and a regionally extensive palaeosol is present at the top of the unit. Early diagenetic phases include isopachous, fibrous submarine cements, nonluminescent vadose cements, and mixing zone dolomite. Subsequent nonferroan phreatic cements are non- to dully luminescent and in restricted vertical intervals predate significant compaction. More usually, however, phreatic cements postdate extensive overpacking of allochems. Ooid isotopic composition (δ¹⁸O=-7·80° to -3·10° and δ¹³C = -2·38° to +3·28°) is similar to that of associated phreatic cements and the data suggest that the bulk of ooid stabilization and cementation occurred within meteoric groundwaters. The extensive allochem overpacking appears to have occurred during the first few tens of metres of burial and intergranular macroporosity was eliminated prior to deep burial. Fracturing of the Gully Oolite during the Hercynian Orogeny and subsequent post-orogenic uplift led to localized dolomitization, several generations of calcite veins, and the restricted occurrence of ¹⁸O depleted cements in inter- and intragranular microporosity. Some of the veins clearly relate to Triassic exhumation of the Carboniferous Limestone, but others may be related to post-Mesozoic uplift and erosion of South Wales. Fracture-associated dolomitization may have occurred within a large-scale post-orogenic groundwater system, with Mg²⁺ being supplied through the release of deeply buried diagenetic brines.     

Regional studies of dolomites and their included fluids: recognizing multiple chemically distinct fluids during the complex diagenetic history of Lower Carboniferous (Mississippian) rocks of the Irish Zn-Pb ore field

This study provides a regional framework within which studies of ore-related dolomite and dolomite cements may be placed. Fluid inclusion data indicate the presence of three distinct fluids following early dolomitization: 1) a ubiquitous low-temperature, higher salinity fluid found in saddle dolomite; 2) a low-temperature, lower salinity fluid limited to sub-Waulsortian and Waulsortian carbonates; and 3) a higher temperature, lower salinity fluid found in Waulsortian and supra-Waulsortian rocks. Similar fluids have been reported in ore-associated minerals and postmineralization dolomite (Type 1) and in ore-stage carbonates and sphalerite (Types 2 and 3). The halogen geochemistry of included fluids indicates genesis from evaporated seawater. Type 1 fluids are enriched in chloride relative to bromide, reflecting a component of salinity derived via dissolution of halite or from dehydration of seawater. These data suggest that dolomitization and mineralization of Mississippian rocks in the Midlands requires both regionally extensive and localized fluid flow.     

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ¹⁸O_SMOW=+24.4 to +26.5‰δ¹³C_PDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ¹⁸O_SMOW=+18.4 to +25.8‰; δ¹³C_PDB=?2.1 to +0.2‰), and calcite (δ¹⁸O_SMOW=+21.3 to +32.6‰; δ¹³C_PDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ¹⁸O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ¹⁸O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.     

Lower Carboniferous Siderites: A Product of Bottom Seeps and Bacterial Metanogenesis (Subpolar Urals)

Complex modern micro- and spectroscopic methods for study of siderite concretions in the Lower Carboniferous terrigenous strata on the Kozhym River (Subpolar Urals) have shown that its formation was caused by destruction of clay minerals due to the activity of bacterial communities. The abundance of these bacteria was caused by gas–fluid seeps and bacterial methanogenesis processes in bottom deposits. In basins with normal marine fauna, this led to local desalination, hydrogen sulfide contamination, mass collapse of primary organisms, and the development of element-specific bacteria. The occurrence of these bacteria caused the formation of specific authigenic mineralization in the concretion of sideritic bacteriolites: the framboidal pyrite, sphalerite, galenite, barite, sulfoselenides, and tellurides.     

Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones

The Coinstone is a well known hiatus-concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay-hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea-floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack-lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV-fluorescent and inclusion-rich, and is similar to the first calcite generations seen in many other clay-hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post-exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today. The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.     

Primary versus diagenetic origin of Blue Lias rhythms (Dorset, UK): evidence from sulphur geochemistry

The question of a primary versus diagenetic origin for the limestone-shale rhythms of the Blue Lias has been addressed through a study of pyrite abundance and isotopic composition. Pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent, less calcareous, sediment. The abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation. The isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate, part of which was subsequently reduced back to pyrite before preservation by burial. Acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution, hence supersaturation of sulphides could be maintained. By contrast, in adjacent less calcareous sediments, carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron-rich pore solutions. Continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and, with burial below the zone of bioturbation, the alkalinity so generated caused cementation of the limestones. Diagenetic cementation would be enhanced during an hiatus in sedimentation, an event which might be related to a Milankovitch forcing mechanism, but which would not be recorded in bioturbated, less calcareous sediment, thus leaving an imperfect record. Only cyclicity in pre-diagenetic sedimentation patterns may be safely related to a Milankovitch forcing mechanism as proposed by Weedon.     

An Upper Carboniferous tonstein of volcanic origin

Previous work on the mineralogy of the volcanic supra-Wyrley Yard tonstein is extended to include the opaque minerals. This has led to a better understanding of the major and trace element geochemistry of the tonstein. The complete mineral assemblage is kaolinite (dominant) with minor quartz, K-mica, zircon, apatite, pyrite, sphalerite, galena, rutile, dolomite, barytes, goethite and organic matter. Information on element location in the tonstein has been obtained by using autoradiography and fission track radiography, the electron probe and by analysing separated fractions with an optical spectrograph. This has enabled the diagenetic history of individual elements to be determined, and the composition of the tonstein to be compared with igneous material. The high concentration of Th, U, Pb, Sn, Bi, Y, Be and B in the tonstein, and the low concentration of Ti, Cr, V, Ni, Cu and Co is indicative of an acidic ash. A source in the Armorican orogenic belt is postulated, based on the contrast with the basaltic Carboniferous volcanism and also the high concentrations of Sn, Pb and Zn.     

Spatial geochemistry of a Carboniferous platform-margin-to-basin transect: Balancing environmental and diagenetic factors

Conventional (one-dimensional) chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time. Whereas this approach has shown to be of value when linking isobathymetric domains, usually characterised by similar facies, more caution is needed when correlations are extended across different physiographic settings and hence different facies belts. Here, the spatial geochemical record of Pennsylvanian platform-margin-to-basin transects across a bathymetric range of about 800 m is documented and discussed in a process-oriented context. Particularly, the presence of layered palaeo-water masses and their potential control on slope facies distribution and geochemical properties requires attention. Whereas Carboniferous thermo- and/or chemo-clines most likely affected depth-related slope facies zonation, it was facies change and hence, variances in porosity–permeability properties, that controlled differential early burial diagenetic alteration. Specifically, the lower-slope related breccia facies is characterised by higher volumes of early burial carbonate cements. This implies that these sediments entered the shallow-burial domain with a considerable open pore space and gave way to an increased rock:fluid ratio. Whereas the δ¹³C record is invariant with bathymetry, the more diagenesis-sensitive δ¹⁸O proxy, records pronounced shifts observed across major facies boundaries. From this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature, it is differential rock:fluid ratios that control the first-order, spatial shifts in δ¹⁸O composition. These findings show that one-dimensional chemostratigraphy will severely underestimate the complexity of three-dimensional (bathymetric) data sets across platform margins. This is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general.