Composition,nucleation, and growth of iron oxide concretions

Iron oxide concretions are formed from post depositional, paleogroundwater chemical interaction with iron minerals in porous sedimentary rocks. The concretions record a history of iron mobilization and precipitation caused by changes in pH, oxidation conditions, and activity of bacteria. Transport limited growth rates may be used to estimate the duration of fluid flow events. The Jurassic Navajo Sandstone, an important hydrocarbon reservoir and aquifer on the Colorado Plateau, USA, is an ideal stratum to study concretions because it is widely distributed, well exposed and is the host for a variety of iron oxide concretions.Many of the concretions are nearly spherical and some consist of a rind of goethite that nearly completely fills the sandstone porosity and surrounds a central sandstone core. The interior and exterior host-rock sandstones are similar in detrital minerals, but kaolinite and interstratified illite–smectite are less abundant in the interior. Lepidocrocite is present as sand-grain rims in the exterior sandstone, but not present in the interior of the concretions.Widespread sandstone bleaching resulted from dissolution of early diagenetic hematite grain coatings by chemically reducing water that gained access to the sandstone through fault conduits. The iron was transported in solution and precipitated as iron oxide concretions by oxidation and increasing pH. Iron diffusion and advection growth time models place limits on minimum duration of the diagenetic, fluid flow events that formed the concretions. Concretion rinds 2 mm thick and 25 mm in radius would take place in 2000 years from transport by diffusion and advection and in 3600 years if transport was by diffusion only. Solid concretions 10 mm in radius would grow in 3800 years by diffusion or 2800 years with diffusion and advection.Goethite (α-FeO (OH)) and lepidocrocite (γ-FeO (OH)) nucleated on K-feldspar grains, on illite coatings on sand grains, and on pore-filling illite, but not on clean quartz grains. Model results show that regions of detrital K-feldspar in the sandstone that consume H⁺ more rapidly than diffusion to the reaction site determine concretion size, and spacing is related to diffusion and advection rates of supply of reactants Fe²⁺, O₂, and H⁺.     

Giant calcite-cemented concretions, Dakota Formation, central Kansas, USA

Giant spheroidal concretions (cannonball concretions; some nearly 6 m in diameter) in fluvial channel‐fill sandstones at two localities of the Dakota Sandstone formed by import of cement constituents at a burial depth of <1 km. During cannonball concretion growth a self‐organizational process restricted concretions to a relatively few but widely spaced, and locally, evenly spaced, sites. Other forms of calcite cements at these localities are cement patches in the form of intergrown grape‐size concretions (grapestone), and, locally, pervasive cement. An early episode of invasion by thermogenically generated H₂S, which reacted with iron oxides on detrital grains, generated scattered pyrite crystals and decimetre‐scale spheroidal pyrite concretions. Intergranular volumes (IGV) in the concretions range from 36% to 27%. The absence of a trend in IGV and of carbon and oxygen‐isotope ratios from cannonball centres to margins indicates that these concretions did not cement progressively outwards from the centre. Rather, the modern spheres represent the spatial extent of nucleation sites that were not otherwise organized within that volume. Carbon and oxygen‐isotope values for concretion calcites plot along a swath between depleted values of δ18C of ?36‰ and δ18O of ?13‰ and enriched values of ?4‰ and ?6‰, respectively. Four groups of calcites are evident on the basis of trace‐element content and suggest that the calcite precipitated across a range of oxidation conditions that do not correlate strongly with the isotopic compositions. Although fluvial overbank sandstones have some pedogenic calcite, the channel sandstones have at most a trace of pedogenic calcite and carbonate rock fragments, so that the bulk of cement components were imported to the sandstones. Carbon and calcium sources for calcite cement include marine limestone, carbonate shells, and anhydrite in addition to HCO derived from oxidized methane, most likely derived from beds underlying or laterally in communication with Dakota sandstones. HCO in ascending formation waters, released during compaction, mixed with meteoric water whose temperature and composition varied with time, to generate the 7‰ range in δ18O_calcite values measured.     

The crystallographic fabric and texture of siderite in concretions: implications for siderite nucleation and growth processes

The crystallographic fabric of siderite in siderite concretions has been determined for upper Carboniferous (Westphalian‐A) non‐marine concretions and lower Jurassic (Pliensbachian) marine concretions. Compositional zoning indicates that individual siderite crystals grew over a period of changing pore water chemistry, consistent with the concretions being initially a diffuse patch of cement, which grew progressively. The siderite crystallographic fabric was analysed using the anisotropy of magnetic susceptibility, which is carried by paramagnetic siderite. The siderite concretions from marine and non‐marine formations exhibit differences in fabric style, although both display increases in the degree of preferred siderite c‐axis orientation towards the concretion margins. The Westphalian non‐marine siderites show a preferred orientation of siderite c‐axes in the bedding plane, whereas the Pliensbachian marine siderites have a preferred orientation of c‐axes perpendicular to the bedding. In addition, a single marine concretion shows evidence of earlier formed, inclined girdle‐type fabrics, which are intergrown with later formed vertical c‐axis siderite fabrics. The marine and non‐marine fabrics are both apparently controlled by substrate processes at the site of nucleation, which was probably clay mineral surfaces. Siderite nucleation processes on the substrate were most probably controlled by the (bio?) chemistry of the pore waters, which altered the morphology and crystallographic orientation of the forming carbonate. The preferred crystallographic orientation of siderite results from the orientation of the nucleation substrate. Fabric changes across the concretions partially mimic the progressive compaction‐induced alignment of the clay substrates, while the concretion grew during burial.     

太湖冲击坑溅射物的发现及其意义

太湖冲击成因说由来已久,但始终未成定论。近几年在太湖及周边湖泊的淤泥层中发现了许多形态各异的奇石,经多种方法测试研究,确定为太湖冲击坑的溅射物。根据成份,溅射物分为两大类。一类富铁质,以菱铁矿及其胶结的碎屑为主,包括微小球粒、棍状及各种形态的块状和片状体 ;另一类贫铁质,以方解石及其胶结的长英质碎屑为主,碎屑为锐角状的石英晶屑及少量粘土和长英质岩屑。溅射物的大小从厘米级块体到毫米级球粒,再至微米级尘粉都有。溅射物外形多具有旋转扭曲形态及熔壳特征,显示了熔融、塑性- 半塑性特征。这些特征显示其成因经历了冲击震碎、熔融、挖掘抛射、空中飞行,最后溅落在冲击坑及其周围。溅射物的成分反映了太湖靶岩基岩岩性特征。太湖冲击坑溅射物的发现是继太湖诸岛石英砂岩中石英晶体的冲击变质微结构发现之后,又一重大突破。综合其它特征,可以确定太湖为一冲击坑。     

论“太湖冲击坑溅射物”——来自太湖现代沉积物中铁质结核的证据

针对太湖发现由菱铁矿组成的“铁质溅射物”和“贫铁溅射物”是太湖形成于陨石冲击的观点,本文对这些实为铁
质结核样品进行了详细的矿物学、地球化学和年代学测试。数据表明,这些样品是在太湖现代沉积淤泥层中形成的菱铁矿
和褐铁矿结核。结核中含有大量水生植物碎片、植物蛋白石、细菌和少量动物壳体碎片。菱铁矿高温灼烧后全部变成赤铁
矿,说明太湖地区不存在菱铁矿熔融状的高温相。菱铁矿14C测年结果表明各个样品形成于年代,不是瞬间形成的。δ13C 数
据指示为淡水无机碳来源,并非湖区碳酸盐岩直接来源。所谓的“贫铁溅射物”实际是黄土层的成岩钙结核。这些证据表明,
太湖发现的这些结核是沉积成岩期的产物,并非冲击高温溅射熔融形成的溅射物。     

Formation of siderite-Mg-calcite-iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England

Concretions cemented mainly by siderite, Mg-calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate-reducing bacteria are active. δ¹³C values ranging from ?3 to ?11·8% (mean ?5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ¹⁸O values ranged from ?6·4% to + 0·8% (mean ?1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to-sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg-calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron.     

Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise

Sedimentological, mineralogical, stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth. High δ¹³C values (6·4‰ on average) indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated. The development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp, related to a rise of the ground‐water table. Large amounts of decaying plant debris led to efficient oxygen consumption within the pore‐water in the peat. Oxygen depletion, in combination with a decrease in sedimentation rate, promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation. This shows how a change in sedimentary conditions can have a profound impact on the early‐diagenetic environment and carbonate authigenesis. The concretions contain numerous rhizoliths; they are cemented with calcium‐rhodochrosite, a feature which has not been reported before. The rhodochrosite cement has negative δ¹³C values (?16·5‰ on average) and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction. The exceptional preservation of the epidermis/exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water‐logged sediments. Rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots, while siderite cementation occurred simultaneously around them in anoxic conditions. These suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits. This model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions, which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial, can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures.     

川北地区下寒武统筇竹寺组钙质结核特征及成因机制

不同成因的钙质结核在研究沉积水体、早期成岩环境及泥页岩的压实程度方面具有重要意义。通过对川北地区旺苍县下寒武统筇竹寺组钙质结核抛光,发现结核内部具有层理、暗色圈层和张裂缝构造。结核和围岩的层理关系和具有漂浮状的微观结构特征表明,该地区结核是形成于压实作用之前的成岩早期结核。对结核中Al、Ti、Ca、K、Na元素分析表明,结核的元素分布受到结核内部裂缝的影响。结核内部富含生物碎屑和其形成深度为数十米的特征,推测结核主要是在硫酸盐还原带生长。结核具有暗色圈层之间层理不弯曲和元素移动的特征,揭示了结核为透入性生长,在整个结核生长阶段就开始大量同时结晶。Mg/Ca和Sr/Ca的比值说明,结核内部钙质成分为富镁方解石-生物成因方解石。结核中富含钙质成分的原因可能是围岩中的生物成因方解石大量向结核运移所致。裂缝的形态特征表明,裂缝为张裂缝。对结核上部地层的沉积特征研究表明,张裂缝的成因很可能与筇竹寺顶部的浊积岩和沧浪铺组底部的似瘤状灰岩快速搬运沉积有关。     

Spherical and columnar, septarian, 18O-depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids

Calcite septarian concretions from the Permian Beaufort Group in the Maniamba Graben (NW Mozambique) allow controls on the composition and nature of diagenetic fluids to be investigated. The concretions formed in lacustrine siltstones, where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm long) forms within three closely spaced, discrete beds totalling 2·5 m in thickness. Cementation began at an early stage of diagenesis and entrapped non‐compacted burrows and calcified plant roots. The cylindrical concretions overgrew calcified vertical plant roots, which experienced shrinkage cracking after entrapment. Two generations of concretionary body cement and two generations of septarian crack infill are distinguished. The early generation in both cases is a low‐Mn, Mg‐rich calcite, whereas the later generation is a low‐Mg, Mn‐rich calcite. The change in chemistry is broadly consistent with a time (burial)‐related transition from oxic to sub‐oxic/anoxic conditions close to the sediment–water interface. Geochemical features of all types of cement were controlled by the sulphate‐poor environment and by the absence of bacterial sulphate reduction. All types of cement present have δ¹³C ranging between 0‰ and −15‰(Vienna Peedee Belemnite, V‐PDB), and highly variable and highly depleted δ¹⁸O (down to 14‰ Vienna Standard Mean Ocean Water, V‐SMOW). The late generation of cement is most depleted in both ¹³C and ¹⁸O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, ¹⁸O‐depleted, reducing, ice‐meltwaters accumulated in the underlying coal‐bearing sediments during the Permian deglaciation. The invariant δ¹³C distribution across core‐to‐rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates. Syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks. After cracking, the concretions retained a small volume of porosity, allowing infiltration of anoxic, Ba‐bearing fluids, resulting in the formation of barite. The results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift‐bound, lacustrine depositional systems.     

Nature and origin of arsenic carriers in shallow aquifer sediments of Bengal Delta,India

Sediments from shallow aquifers in Bengal Delta, India have been found to contain arsenic. Rivers of Ganga-Brahmaputra system, responsible for depositing these sediments in the delta, have created a store of arsenic. Geomorphological domains with different depositional styles regulate the pattern of distribution of zones with widely different content of groundwater arsenic. The high arsenic zones occur as narrow sinuous strips confined to channel deposits. A few iron-bearing clastic minerals and two post-depositional secondary products are arsenic carriers. Secondary siderite concretions have grown on the surface of the clastic carriers in variable intensity. The quantity of arsenic in all clastic carriers is in excess of what is generally expected. Excess arsenic is contributed by the element adsorbed on the concretion grown on the surface of the carriers, which adds up to the arsenic in the structure of the minerals. Variable abundance of concretions is responsible for the variable quantity of arsenic in the carriers and the sediment samples. Fe²⁺ for the growth of siderite concretions is obtained from the iron-bearing clastic carriers. The reaction involves reduction of trivalent iron to bivalent and the required electron is obtained by transformation of As³⁺ to As⁵⁺. It is suggested that oxidation of As³⁺ to As⁵⁺ is microbially mediated. In the Safe zone arsenic is retained in the carriers and groundwater arsenic is maintained below 0.05 mg/l. In the Unsafe zone sorbed arsenic is released from the carriers in the water through desorption and dissolution of concretion, thereby elevating the groundwater arsenic level to above 0.05 mg/l.     

Significance of the chemistry and morphology of diagenetic siderite in clastic rocks of the Mesozoic Scotian Basin

Siderite (FeCO₃) is a widespread minor diagenetic mineral in clastic sedimentary basins. Although eodiagenetic authigenesis of siderite is well-known, siderite formed during burial diagenesis shows habits and chemical compositions that are poorly understood. This study tests the hypothesis that diagenetic siderite cements in sandstones in the Scotian Basin, offshore eastern Canada, show systematic variability in chemistry and habit that is a response to recrystallization and changing composition of basinal fluids. Mineral textures were determined from backscattered electron images, and chemistry mostly from electron microprobe analyses. Five chemical types of siderite are identified using k-means cluster analysis, based on the amount of substitution of Ca, Mg and Mn for Fe. Eodiagenetic microcrystalline coated grains, concretions and intraclasts in sandstones are principally Fe-rich siderite and locally have recrystallised to blocky equant crystals. Mesodiagenetic Mg-rich siderite partly replaced these equant crystals and also framework mica and K-feldspar grains, showing textural evidence for coupled dissolution–reprecipitation. Slender Mg-rich siderite rhombs (lozenges, bladed or wheat-seed siderite) have precipitated before and after the formation of quartz overgrowths in geochemical microenvironments. Magnesium substitution reflects Mg-rich formation waters resulting from smectite to illite conversion. Equivalent Ca-rich siderite occurs where sandstones overlie a Jurassic carbonate bank. Late Mn-rich siderite has complex textures resembling those of Mississippi-Valley type ores, with spheroidal rims, a honeycomb-like mesh and concentric infill around secondary pores. It also occurs in veins or replacing intraclasts, post-dating late ferroan-calcite cements in sandstones that show strong dissolution by hot basinal brines. The Ca, Mg and Mn content of diagenetic siderite, coupled with textural evidence for recrystallization, can thus be used to track changes in ambient formation fluids. Siderite habits and chemistry described from the Scotian Basin are found in many clastic basins, suggesting that the observed recrystallization textures and variation in chemical type are of broad application.     

Origin and diagenetic evolution of Ca–Mg–Fe carbonates in some coalfields of Japan

Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca-Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish-marine sediments whose pore waters abound in Ca²⁺ and Mg²⁺ originating in seawater and calcareous shells. Carbonates formed during Stage II (500–2000 m) include high Ca-Mg siderite, ankerite, Fe dolomite and Fe–Mg calcite in freshwater sediments. The assemblage of Stage II carbonates in brackish-marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage III (> 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.     

Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah,USA)

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ¹³C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ¹³C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ¹³C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ³⁴S) and silica (δ¹⁸O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.     

Genesis and diagenesis of hiatus- and breccia-concretions from the mid-Cretaceous of Texas and northern Mexico

A study of septarian concretions from late Cretaceous shale sequences of Texas and northern Mexico reveals complex burial-exhumation histories. First generation concretions and pre-fossilized moulds formed in silty clays before significant compaction occurred. Syneresis cracks developed and were filled by length slow fibrous calcite with a decrease in iron content toward the centre of each vein; in some cases this was succeeded by growth of equant or bladed calcite. Subsequent erosion of enclosing sediment caused collapse and break up of concretions, leaving fragments and some unbroken concretions exposed to encrustation and boring on the sea floor. These were subsequently buried and enclosed in a second generation of concretions, which also developed syneresis cracks. The calcite that filled these latter cracks was generally bladed (sometimes followed by equant calcite) and contains more iron toward the centre of the vein. Differences between the calcite filling first and second generation septarian veins indicates changing diagenetic regimes during burial in the marine environment, without a major shift in diagenetic conditions, such as the introduction of meteoric waters. These conclusions have significant implications to cementation in ancient limestones, which show textural and chemical sequences similar to those present in the septarian concretions described.     

Early diagenesis of the lower Vectis Formation,Wealden Group Lower Cretaceous,Barremian), Sandown,Isle of Wight

The Vectis Formation was deposited in a restricted lagoonal environment that periodically dried up, as indicated by the presence of desiccation surfaces. The fauna indicates that salinities fluctuated significantly during deposition, from fresh to brackish-marine. Pre-burial berthierine-rich clay replaced faecal pellets in the sediment, infilling desiccation cracks, during re-flooding of the lagoon. Concretions formed by early pyrite and apatite cementation during initial burial in lake sediment, with organic debris of fish and wood, acting as nuclei and a trigger for cementation. With subsequent partial or complete exhumation oxidation of the pyrite occurred, prior to cementation by Mn siderite. All further concretion cementation occurred within the oxic to sub-oxic diagenetic zones. Textural relationships indicate that commonly occurring baryte formedafter pyrite oxidation and represents the last diagenetic mineral phase.     

Fluvial architecture and diagenesis of the Mt Eclipse Sandstone,northern Ngalia Basin,Australia

The Upper Devonian to Carboniferous Mt Eclipse Sandstone is a basin-wide host to uranium mineralisation in the Ngalia Basin, NT. The fluvial depositional architecture and diagenesis of the Mt Eclipse Sandstone at the Bigrlyi uranium deposit on the northern margin of the basin are deduced from hyperspectral mineral results captured from 200 drill holes, combined with core and outcrop observations across a ~10 km strike length. The succession hosting the uranium mineralisation is interpreted to be deposited in the lower parts of a mature alluvial fan system with low slope angle and dominated by immature, kaolinised, medium-grained subarkosic sandstones and patchy calcite cement. This study reveals the fluvial channel sequence is estimated to be 3 km wide, 100–200 m thick and sourced from the north. This multidisciplinary study also uncovers the complex interaction and codependencies between fluvial/groundwater activity, evaporation, oxidation, fluctuating pH and detrital mineralogy controlling early diagenetic processes in the alluvial fan sediments. Carbon isotope data identify calcite cements as groundwater calcrete, while strontium isotope data suggest limited prolonged water–rock interaction prior and during calcite cementation. The petrographic data reveal the importance of early calcite cement occluding all pore space and preserving detrital minerals from later diagenetic processes. The hyperspectral results highlight the intermittent distribution of the calcite cement and the commonly repetitive mineralogical zonation throughout the 200 drill holes, including the inverse spatial correlation between kaolinite ± goethite ± gypsum vs white mica ± hematite dominated zones. X-ray diffraction and the hyperspectral data reveal the scarcity of early diagenetic clay minerals such as montmorillonite. This paper is the first to report on a systematic mineralogical and sedimentological study for the Mt Eclipse Sandstone. By focusing on the diagenesis and fluvial architecture of this stratigraphic unit, a framework to support exploration for sediment-hosted uranium deposits is established.     

Fluvial to tidal transition in proximal,mixed tide‐influenced and wave‐influenced deltaic deposits: Cretaceous lower Sego Sandstone,Utah, USA

Facies models for regressive, tide‐influenced deltaic systems are under‐represented in the literature compared with their fluvial‐dominated and wave‐dominated counterparts. Here, a facies model is presented of the mixed, tide‐influenced and wave‐influenced deltaic strata of the Sego Sandstone, which was deposited in the Western Interior Seaway of North America during the Late Cretaceous. Previous work on the Sego Sandstone has focused on the medial to distal parts of the outcrop belt where tides and waves interact. This study focuses on the proximal outcrop belt, in which fluvial and tidal processes interact. Five facies associations are recognized. Bioturbated mudstones (Facies Association 1) were deposited in an offshore environment and are gradationally overlain by hummocky cross‐stratified sandstones (Facies Association 2) deposited in a wave‐dominated lower shoreface environment. These facies associations are erosionally overlain by tide‐dominated cross‐bedded sandstones (Facies Association 4) interbedded with ripple cross‐laminated heterolithic sandstones (Facies Association 3) and channelized mudstones (Facies Association 5). Palaeocurrent directions derived from cross‐bedding indicate bidirectional currents which are flood‐dominated in the lower part of the studied interval and become increasingly ebb‐directed/fluvial‐directed upward. At the top of the succession, ebb‐dominated/fluvial‐dominated, high relief, narrow channel forms are present, which are interpreted as distributary channels. When distributary channels are abandoned they effectively become estuaries with landward sediment transport and fining trends. These estuaries have sandstones of Facies Association 4 at their mouth and fine landward through heterolithic sandstones of Facies Association 3 to channelized mudstones of Facies Association 5. Therefore, the complex distribution of relatively mud‐rich and sand‐rich deposits in the tide‐dominated part of the lower Sego Sandstone is attributed to the avulsion history of active fluvial distributaries, in response to a subtly expressed allogenic change in sediment supply and relative sea‐level controls and autocyclic delta lobe abandonment.     

Carbonate petrography, kerogen distribution, and carbon and oxygen isotope variations in an early Proterozoic transition from limestone to iron-formation deposition, Transvaal Supergroup, South Africa

The transition zone comprises Campbellrand microbialaminated (replacing "cryptalgalaminate") limestone and shale, with minor dolomite, conformably overlain by the Kuruman Iron Formation of which the basal part is characterized by siderite-rich microbanded iron-formation with minor magnetite and some hematite-containing units. The iron-formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones. The sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees C and pressures of 2 kbars. Carbonate minerals in the different rock types are represented by primary micritic precipitates (now recrystallized to microsparite), early precompactional sparry cements and concretions, deep burial limpid euhedral sparites, and spar cements precipitated from metamorphic fluids in close contact with diabase sills. Paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them. Kerogen occurs as pigmentation in carbonate crystals, as reworked organic detritus in clastic-textured carbonate units, and as segregations of kerogen pigment around late diagenetic carbonate crystals. Locally kerogen may also be replaced by carbonate spar. Carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies. Integration of sedimentary, petrographic, geochemical, and isotopic results makes it possible to distinguish between depositional, early diagenetic, deep burial, and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence. Major conclusions are that deep burial thermal decarboxylation led to 13C depletion in euhedral ferroan sparites and 13C enrichment in kerogen (organic carbon). Metamorphic sparites are most depleted in 13C. Carbonates in oxide-rich iron-formations are more depleted in 13C than those in siderite-rich iron-formation whereas the kerogens in oxide banded iron-formations (BIF) are more enriched. This implies that the siderite-rich iron-formations were not derived from oxide-rich iron-formation through reduction of ferric iron by organic matter. Organic matter oxidation by ferric iron did, however, decrease the abundance of kerogen in oxide-rich iron-formation and led to the formation of isotopically very light sparry carbonates. Siderite and calcmicrosparite both represent recrystallized primary micritic precipitates but differ in their 13C composition, with the siderites depleted in 13C by 4.6 per mil on average relative to calcmicrosparite. This means that the siderites were precipitated from water with dissolved inorganic carbon depleted in 13C by about 9 per mil relative to that from which the limestones precipitated. This implies an ocean system stratified with regard to total carbonate, with the deeper water, from which siderite-rich iron-formation formed, depleted in 13C. Iron-formations were deposited in areas of very low organic matter supply. Depletion of 13C may, therefore, derive not from degradation of organic matter but from hydrothermal activity, a conclusion which is supported by 18O composition of the carbonate minerals and trace element and rare earth element (REE) compositions of the iron-formations.     

Inhibition of dissolution within shallow water carbonate sediments: impacts of terrigenous sediment input on syn-depositional carbonate diagenesis

The early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron‐rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore‐water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe‐rich bauxite sediment contamination. At non‐impacted sites sediments are dominated by CaCO₃ (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO₄/Cl^? ratios) and minor CaCO₃ dissolution (increased Ca²⁺/Cl^? ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite‐impacted sites, the sediment is composed of up to 15% non‐carbonate and contains up to 6000 μg g^?1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre‐date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous‐impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.