Authigenic Silica Research Articles

Mineral diagenesis in a carbonate-rich mudstone: the Lower Carboniferous Hodder Mudstone, UK

Abstract Mineral authigenesis in mudstones responds differently across the compositional spectrum of fine-grained rocks, resulting in a complex array of possible rock fabrics that affect the mechanical capacity of mudstones. Within carbonate-rich mudstones, geochemical controls on silicate and carbonate mineral reactions are variable and poorly understood. Here we analyse the diagenetic minerals of the carbonate-rich Hodder Mudstone from the Carboniferous Bowland Basin, UK, using SEM petrography combined with X-ray-based inorganic geochemical data. Our findings show that up to 90% of quartz cements in the samples are diagenetic, originating mainly from biogenic silica dissolution and clay mineral reactions. It is also evident that due to the varying diagenetic silica-yielding reactions, authigenic silica overgrowths and isolated authigenic quartz crystals are localized in argillaceous samples, while calcareous samples are characterized by silica replacement textures and quartz/calcite intergrowths. Moreover, euhedral dolomite crystals are concentrated within argillaceous units, whereas calcareous units are characterized by anhedral dolomite precipitation and replacement textures. These finding presents a case for facies-selective cementation, as both early and burial diagenetic alterations were observed to be controlled by primary depositional components of biogenic debris and extrabasinal silicate detritus that resulted in complex and variable precipitation of authigenic minerals.

Гетерогенність літогенезу відкладів силуру Волино-Поділля

The paper examines the features of the lithological-lithmological structure and the regime of catagenesis of the Silurian sediments of the Lishchynska and Rava-Ruska sections of Volyno-Podillya. It is shown that in the direction from the southwest to the northeast, the role of carbonate rocks in the composition of the stratum increases, which is connected with the established facies zonation. However, the structure of the section in these areas is different: in Rava-Ruska, it is more thinly layered. The sediments are also characterized by the spatial and age heterogeneity of the distribution of carbonate lithmites: in the first case, they tend to the boundary of the Upper and Lower and the middle of the Upper Silurian, and in the second case, they are developed in the tops of the Lower, in the lower, middle, and upper parts of the Upper Silurian. Attention is drawn to the significant role of clay and the absence of marl formations in the deposits of the Rava-Ruska-1 well, which testifies to the heterogeneity of sedimentation conditions in the mesopelagial of the Silurian basin. The cyclic nature of Silurian sedimentation is established. At the same time, four regressive episodes are recorded in the Lishchynska area, and five in Rava-Ruska, which may indicate a certain specificity of sedimentation conditions in different parts of the basin. The latter directly affects the peculiarity of the spatial-age distribution of reservoir rocks and aquifers. It is shown that the post-sedimentation transformations are mainly related to the development of authigenic silica and calcite, which is found in both clayey and carbonate rocks. A significant difference in the history of the formation of the oil and gas systems of the Lishchynska and Rava-Ruska areas has been established, which allows us to assess their prospects differently. Thus, in the first case, the generation potential of organic matter of Silurian sediments was largely exhausted by the end of the Mesozoic. In the second, large-scale processes of generation and migration of hydrocarbon fluids began only in Paleogene-Neogene time.

Authigenic silica in continental lacustrine shale and its hydrocarbon significance

Taking lake basin shales of the Triassic Yanchang Formation in the Ordos Basin, NW China and the Cretaceous Qingshankou Formation in the Songliao Basin, NE China as research objects, the characteristics and origins of different types of silica in the shales have been studied by means of core observation, thin section identification, cathodoluminescence, X-ray diffraction analysis, scanning electron microscope (SEM), electron probe and rock pyrolysis. The results shows that the origins of silica include felsic mineral dissolution, tuffite devitrification, clay mineral transformation and siliceous mineral metasomatism. The silica formed by feldspar dissolution commonly appears as spots and veins, with low degree of crystallization, and is largely aqueous opal mineral, with an average SiO2 content of 67.2%. Silica formed by devitrification of tuffite mainly occurs in two forms, amorphous silica and authigenic quartz with better crystal shape. The authigenic silica formed during the transformation of clay minerals is embedded in the clay minerals in the form of micron-scale plates and small flakes, or mixed with clay minerals in a dispersed state. The authigenic quartz formed by siliceous mineral metasomatism is in better angular crystal shape, and has an average SiO2 content of 87%. The authigenic siliceous mineral content is positively correlated with the content of terrigenous felsic minerals. The pressure solution of felsic minerals is the main source of authigenic siliceous minerals, followed by the transformation of clay minerals, and the organic matter has some boost on the formation of authigenic silica. The authigenic siliceous materials of different origins have different geological characteristics and occurrence states from terrigenous quartz, which would affect the storage performance, seepage capacity and fracturing effect of continental shale. Although the organic-rich shale has high silica content, different from terrigenous quartz, authigenic silica in this kind of shale mostly floats and disperse in clay minerals, which would have negative effect on the formation of complex fractures in fracturing, fracture support ability after fracturing, and formation of effective seepage channels. Calculating the brittleness index of shale intervals only based on the composition of brittle minerals cannot accurately characterize mechanical characteristics of continental shale oil reservoirs, and would affect comprehensive evaluation and selection of continental shale oil “sweet spots”.

Lithofacies types, reservoir characteristics and silica origin of marine shales: A case study of the Wufeng formation–Longmaxi Formation in the Luzhou area, southern Sichuan Basin

Shale, which is a fine-grained sedimentary rock, differs in reservoir quality (RQ) and completion quality (CQ) as the result of variations in its mineral and organic-matter compositions. The RQ and CQ directly affect shale gas exploitation. Therefore, classification of the lithofacies types of shale, analysis of the reservoir differences of different lithofacies, and identification of the superior shale lithofacies with the best RQ and CQ are important for decisions on well placement, targeting the “sweet spot” zone and optimizing the expected ultimate recovery of a shale gas well. In this study, several methods including microscopic observation of thin sections, X-ray diffraction analysis, core analysis and elemental geochemistry were used to investigate the lithofacies types, reservoir characteristics and silica origin of the marine shales of the Wufeng Formation–Longmaxi Formation (WF Fm.–LMX Fm.) in the Luzhou area, southern Sichuan Basin, China. The lithofacies characteristics exhibit marked differences through the WF–LMX shale succession. Specifically, the lower gas-bearing zone (LMX-1) is mainly composed of siliceous shales and siliceous–argillaceous mixed shales, whereas the upper gas-bearing zone (LMX-4) mostly contains argillaceous shales and argillaceous–siliceous mixed shales. The lower shale gas reservoirs have a higher silica content and lower clay and carbonate contents than the upper zones, and thus superior petrophysical qualities. The upper part of the WF Fm. (WF-2) and the lower part of the LMX Fm (LMX-1, LMX-2) are rich in bio-chemical authigenic silica, whereas the silica minerals of other intervals are mainly derived from allochthonous terrigenous material. The siliceous shales and siliceous–argillaceous mixed shales with a biogenic quartz content greater than 40% have the best RQ and CQ and are the most promising lithofacies for shale gas in the Luzhou area. Although the upper gas-bearing zones represented by LMX-4 are also good shale gas reservoirs, the RQ, CQ and gas content are generally worse than those of the lower gas-bearing zones because of the silica origin and lithofacies types; thus, extracting shale gas from the upper zones would be a greater challenge.

Evidence for Biogenic Silica Occurrence in the Lower Silurian Longmaxi Shale in Southeastern Chongqing, China

The gas shale in the Lower Silurian Longmaxi Formation contains a considerable amount of biogenic silica. Various originated silicas in shale, derived from different depositional environment, are commonly associated with different degrees of organic matter enrichment, resulting in different mechanical and physical properties of shale reservoirs. Thin section identification, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, and X-ray fluorescence (XRF) spectroscopy were used to investigate the Lower Silurian Longmaxi shale from Well Yuye 1 in southeastern Chongqing, China to obtain a better understanding of the origin of silica in the Longmaxi Shale. The results show ubiquitous cryptocrystalline silicas with poorly crystalline morphology, which differs from that of the detrital silica, authigenic silica, and hydrothermal silica, proving that the cryptocrystalline silicas may have a biogenic origin. Major element and mineral composition analysis indicate no correlations between K2O/Al2O3 and SiO2/Al2O3 and between illite and SiO2, and negative correlations between TiO2 and SiO2/Al2O3, between illite and quartz and excess Si, and between Al2O3 and excess Si, and all samples being located in the area of non-hydrothermal origin in the Al-Fe-Mn diagram, excluding silicas of terrigenous detrital origin, clay mineral transformed origin, and hydrothermal origin. Moreover, the fact that almost all samples plot above the illite Si/Al line in the cross-plot of Si versus Al and the mean values of Al/(Al + Fe + Mn) and Si/(Si + Al + Fe + Ca) are close to the values of biogenic silica prove that the silicas are primarily of biogenic origin. Positive correlations between TOC and quartz and excess Si and numerous siliceous organisms are observed, indicating that the silicas are associated with siliceous organisms. The postmortem siliceous organisms underwent silica diagenesis via a dissolution-precipitation mechanism following the sequence of opal-A → opal-CT → cryptocrystalline biogenic silica as the burial depth and temperature increased.

Geochemical and textural investigations of the Eoarchean Ukaliq supracrustals, Northern Québec (Canada)

Structural, geochronological and geochemical studies of pre-3.75 Ga rocks of volcano-sedimentary protoliths in the Inukjuak domain in the Superior Province in Québec have been mostly focused on the Nuvvuagittuq Supracrustal Belt (NSB). The Porpoise Cove outcrops – at the southwestern limit of the NSB – are the de-facto “type locality” for the supracrustals of the Inukjuak Complex. Yet, it remains unclear whether the NSB rocks are geochemically distinct from, or are in fact common to, a host of other supracrustal enclaves locked in the dominantly gneissic Inukjuak domain. Here, we report detailed textural and geochemical studies for a suite of rocks from the Ukaliq Supracrustal Belt (USB), located approximately 3 km northeast of the NSB. We find that the USB and NSB have a similar protracted metamorphic history; both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets, quartz-magnetite rocks (banded iron-formation s.l.) and quartz-biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions. If the various Inukjuak supracrustal belts were once a part of a larger coherent (now dismembered) terrane, they should show similar emplacement ages and source chemistry. New zircon UPb geochronology from five gneissic units and two quartz-biotite (metasedimentary) schists reveal the oldest emplacement ages across all units of each individual rock type to be 3.68 ± 0.07 Ga (granitoid gneisses) and 3.65 ± 0.06 Ga (quartz-biotite schists). These new ages are similar to those documented as likely minimum emplacement ages of the NSB determined by UPb geochronology. Zircons from the quartz-biotite schist were also analyzed by ion microprobe for their UPb geochronology and were found to yield statistically identical, albeit more precise, ages than those obtained by LA-ICP-MS. Possible detrital zircons from the USB quartz-biotite schists were analyzed by ion microprobe for their coupled δ30SiNBS28 and δ18OVSMOW values with respective values between −0.75 and − 0.07‰ (δ30SiNBS28), and + 5.61 and + 6.59‰ (δ18OVSMOW). The δ18OVSMOW values, which are on average above mantle-derived zircon, indicate contribution of altered, non-mantle, derived material into the source of the rocks that weathered to form the quartz-biotite schists. Zircon mineral inclusions (quartz, feldspar, apatite, biotite, muscovite and other unrecognized Fe/Al/Si rich phases), along with the major- and trace element contents for the rocks were analyzed to substantiate this interpretation. Together with δ30SiNBS28, δ18OVSMOW, our results suggest that lithologies like authigenic silica and serpentinized rocks re-melted to form the parent melts that gave rise to zircons found in the USB quartz-biotite schists. Additional LuHf studies reported here on the same zircons also show similarities with NSB zircons. The εHf values showed a positive correlation with the measured UPb age from −22.7 ± 0.8 to +1.9 ± 1.1. The LuHf system also reveals that the USB, extracted at ca. 3.8 Ga, carries isotopic signatures of an older Hadean reservoir, having been formed from an Eoarchean mafic melt that incorporated them. Taken together, this supports a co-genetic origin for the NSB and the USB.

A record of dust deposition in northern, mid-latitude Pangaea during peak icehouse conditions of the late Paleozoic ice age

ABSTRACT The Wordiekammen Formation, a carbonate ramp on Spitsbergen developed on the Northern Pangaean margin in Moscovian (Carboniferous) through Sakmarian (Permian) time at a paleolatitude of 30–35° N. The study site on the Nordfjorden High was isolated from any source of fluvio-deltaic input, such that detrital material that occurs in this system experienced eolian transport, thus forming a proxy for atmospheric dust loading. We analyzed two intervals, of Moscovian (10 m) and Asselian (27 m) age, at 20 cm resolution, and identified five mid-ramp subtidal facies organized in upwardly shallowing, high-frequency sequences 3–5 m thick. High-frequency sequence boundaries commonly exhibit signs of subaerial exposure (e.g., Microcodium) developed atop subtidal facies, recording glacioeustatic falls (glacial phases), although the Moscovian section has a severe karst overprint attributable to prolonged exposure on a paleohigh. Samples were processed to isolate the silicate-mineral fraction (SMF), which includes both detrital silicate material and authigenic silica mostly in the form of (fine-sand-size) doubly terminated quartz crystals. Detrital cores in these crystals, together with other evidence, indicate recrystallization from fine-grained (silt- and clay-size) dust. Analysis of the dust record demonstrates that the Asselian (peak icehouse) had a significantly higher atmospheric dust load than the Moscovian (moderate icehouse). In the Asselian interval, dust input varies commensurate with glacial–interglacial cyclicity. Highest dust contents correspond to transgressive facies immediately above sequence boundaries, indicating peak atmospheric dust loading at lowstand to incipient interglacial times. Provenance data from detrital-zircon and whole-rock geochemistry indicate two distinct source regions for the dust. Dust from the Moscovian and lower Asselian intervals reflects a continental island-arc signature consistent with sourcing from the basement of northeast Greenland. Dust from the upper Asselian interval is more consistent with recycling from Devonian and Carboniferous strata of the east Greenland Caledonides, likely deflated from fluvial systems draining this orogenic system, indicating an expansion of regions of eolian deflation.

Origin of chert within the Turonian carbonates of Abu Roash Formation, Abu Roash area, Egypt: Field, petrographic, and geochemical perspectives

This paper tries to elucidate the origin of the siliceous rocks (cherts) within the Turonian flint‐bearing chalky limestone member of Abu Roash Formation, Abu Roash area, north Western Desert, Egypt. Understanding the formation mechanism of the studied chert was gained through field and petrographic investigations, supplemented by geochemical analyses of both cherts and host Turonian carbonates. The field observations reveal that the chert is found in two forms: banded and nodular. The microscopic investigation determined that the cherts exhibit two types of silica: replacive silica (cryptocrystalline and microcrystalline quartz) and silica cement (length‐fast chalcedony and megaquartz). The chert texture corroborates that the examined authigenic silica was formed diagenetically by the replacement and void‐filling of the host carbonate rocks. The silica was most likely derived from the biogenic opal‐A of the siliceous radiolarian tests. The silica‐phase transformation started with the dissolution of the biogenic opal‐A and precipitation of opal‐CT. Subsequently, opal‐CT was recrystallized into crypto‐ and microquartz at low pH and high temperature. The length‐fast chalcedony and megaquartz occur later as infill to the cavities and pore spaces. The formation of chert is interpreted to be a result of mixing marine–meteoric waters. The effect of the freshwater diagenesis on carbonates is manifested by the dissolution of the allochems, precipitation of granular, drusy, and blocky calcite (meteoric cements), aggrading neomorphism of matrix, and calcitization of the skeletal particles. Geochemical analyses of the host carbonates indicate nearly flat REE patterns and absence of a negative Ce anomaly. This confirms the deviation from the typical seawater and supports the effect of phreatic meteoric water in the chert formation. The paragenetic relationships reveal that the chertification process was performed after dolomitization and before the carbonate meteoric cementation.

Grain assemblages and diagenesis in the tarl-dominated Lower Silurian mudrock succession of the western margin of the east European craton in Poland and Lithuania

Detrital grain assemblages in mudrocks establish a key starting condition for the diagenetic pathways that drive the evolution of rock properties in unconventional petroleum systems. This case study is based on petrographic examination (light microscopy, elemental mapping, and cathodoluminescence imaging) of 41 subsurface samples of Silurian mudrocks from the western margin of the East European Craton (EEC) in Poland. About 2/3 of the samples have grain assemblages composed by >90% of grains of quartz, feldspar, mica, and clay of extrabasinal derivation. Mechanical compaction is the principal cause of porosity decline in these terrigenous-argillaceous mudrocks (tarls). Where thermal maturation has reached a level sufficient to induce smectite illitization, minute amounts of authigenic silica (≪1 vol%) are observed in the small (~100 nm) primary intergranular pores that survived compaction. Organic-rich mudrocks in this succession also display prominent compaction and contain little organic-matter-hosted porosity. Although these organic-rich tarls may have served as effective source rocks as unimpeded compaction drove expulsion of generated hydrocarbons, they do not in general have sufficient porosity and stiffness to serve as effective reservoirs.Around 27% of the sample set has grain assemblages that include biogenic grains in amounts over 10%. Most of these are calcareous tarls (calcareous-terrigenous argillaceous mudrocks), but do not display discernible carbonate cements. Only three of the studied samples contain grain assemblages that originally included a significant component of biogenic silica. These three samples contain microquartz cement (up to 30% of rock volume) and migrated pore-filling organic matter that has developed secondary porosity. Where thermally mature and sufficiently thick, these local intervals of silica-cemented mudrocks may offer interesting targets for development of unconventional gas.

A geochemical view into continental palaeotemperatures of the end-Permian using oxygen and hydrogen isotope composition of secondary silica in chert rubble breccia: Kaibab Formation, Grand Canyon (USA)

The upper carbonate member of the Kaibab Formation in northern Arizona (USA) was subaerially exposed during the end Permian and contains fractured and zoned chert rubble lag deposits typical of karst topography. The karst chert rubble has secondary (authigenic) silica precipitates suitable for estimating continental weathering temperatures during the end Permian karst event. New oxygen and hydrogen isotope ratios of secondary silica precipitates in the residual rubble breccia: (1) yield continental palaeotemperature estimates between 17 and 22 °C; and, (2) indicate that meteoric water played a role in the crystallization history of the secondary silica. The continental palaeotemperatures presented herein are broadly consistent with a global mean temperature estimate of 18.2 °C for the latest Permian derived from published climate system models. Few data sets are presently available that allow even approximate quantitative estimates of regional continental palaeotemperatures. These data provide a basis for better understanding the end Permian palaeoclimate at a seasonally-tropical latitude along the western shoreline of Pangaea.

Mixed biogenic and hydrothermal quartz in Permian lacustrine shale of Santanghu Basin, NW China: implications for penecontemporaneous transformation of silica minerals

The cycling of various isomorphs of authigenic silica minerals is a complex and long-term process. A special type of composite quartz (Qc) grains in tuffaceous shale of Permian Lucaogou Formation in the sediment-starved volcanically and hydrothermally active intracontinental lacustrine Santanghu rift basin (NW China) is studied in detail to demonstrate such processes. Samples from one well in the central basin were subject to petrographic, elemental chemical, and fluid inclusion analyses. About 200 Qc-bearing laminae are 0.1–2 mm and mainly 1 mm thick and intercalated within tuffaceous shale laminae. The Qc grains occur as framework grains and are dispersed in igneous feldspar-dominated matrix, suggesting episodic accumulation. The Qc grains are bedding-parallel, uniform in size (100 s µm), elongate, and radial in crystal pattern, suggesting a biogenic origin. Qc grains are composed of a core of anhedral microcrystalline quartz and an outer part of subhedral mega-quartz grains, whose edges are composed of small euhedral quartz crystals, indicating multiple episodic processes of recrystallization and overgrowth. Abundance of Al and Ti in quartz crystals and estimated temperature from fluid inclusions in Qc grains indicate that processes are related to hydrothermal fluids. Finally, the Qc grains are interpreted as original silica precipitation in microorganism (algae?) cysts, which were reworked by bottom currents and altered by hydrothermal fluids to recrystalize and overgrow during penecontemporaneous shallow burial. It is postulated that episodic volcanic and hydrothermal activities had changed lake water chemistry, temperature, and nutrient supply, resulting in variations in microorganic productivities and silica cycling. The transformation of authigenic silica from amorphous to well crystallized had occurred in a short time span during shallow burial.

Pore characterization and shale facies analysis of the Ordovician-Silurian transition of northern Guizhou, South China: The controls of shale facies on pore distribution

The Ordovician-Silurian Wufeng-Longmaxi Formations in the Upper Yangtze Platform have been considered as the most promising shale play in China. However, few studies of detailed shale facies and their influence on pore distribution have been conducted. Therefore, it is important to describe the heterogeneity of shale facies in detail and elucidate their relationship to pore networks and development. Seven distinct facies were identified on the basis of their characteristic physical, biological, and chemical attributes. Ion-milled samples from all facies were examined with SEM for identification of pore types. Point-counting was used to quantify porosity. Four major pore types appear dominant: framework pores, phyllosilicate framework (PF) pores, intrapores within inorganic grains, and organic matter (OM) pores. Due to variability in composition and depositional setting of the different facies, pore networks are not uniform. Framework pores between detrital grains and PF pores are widespread in bioturbated claystones, Pores in black siliceous shales are dominated by framework pores supported by authigenic silica. Most of these are filled with kerogen/bitumen that is host to “foam” and “bubble” type OM pores. Intrapores within calcite or dolomite grains are common in fossiliferous mudstones and black dolomitic siltstones, but are isolated and do not contribute to productive porosity. In muddy siltstones, PF pores are well developed within silty bands. Initial framework pores within silty beds or laminae, however, were subsequently cemented by calcite or destroyed by compaction. As most of primary PF pores and primary frameworks pores are lost during compaction, porosity is highly dependent on the population of framework pores between authigenic quartz grains. Secondary cracking of amorphous organic matter added porosity after initial infilling of porosity by oil and bitumen migration. Therefore, porosity is mainly controlled by abundance of authigenic silica and TOC, in spite of differences of burial diagenesis among various facies. Understanding these main controls on porosity should be helpful to predict and find and more porous facies and have a significant impact on future gas exploration in the Wufeng-Longmaxi Formations.

Insights Into Marls From Optical and Back-Scattered Electron Petrography: An Example From the Outer Carpathians (Poland)

Abstract: This paper is a study of grain provenance, depositional processes, and the relationship of those processes to composition and diagenesis of fine-grained rocks, based on high-resolution petrography. The marly successions deposited in siliciclastic flysch and related to the paleotectonics of the northern domain of the Tethys Ocean (Outer Carpathian Basin) were examined. The following marls were studied: Upper Jurassic marls from the Vendryně Formation (Goleszow Marls); Upper Cretaceous Siliceous-Fucoid, Wegierka, Weglowka, Frydek, Jasienica, and Żegocina Marls; Eocene Łącko, Budzow, Zembrzyce, and Leluchow Marls; and Oligocene Sub-Cergowa and Grybow Marls. These marls are products of the redeposition of shelf sediments into deeper parts of the basin. Knowledge of their diagenetic history can contribute to the evaluation of the petroleum potential of the Polish Outer Carpathians. Thin-section analysis by optical and electron back-scattered microscopy (FESEM-BSE) highlights the similarity of all these marls with respect to composition, grain size, sedimentary features, and diagenetic changes. They differ only in the quantitative proportion of constituents, the relative extent of key microfacies, and diagenetic effects. The diagenetic modifications recognized in the rocks studied are characteristic of the processes operating in the sedimentary environment and during burial diagenesis. Thin-section petrography of the marls reveals fine, mostly silt-size siliciclastic material and bioclasts supported by matrix. Siliciclastic particles comprise mostly quartz, micas, feldspars, glauconitic nodules, and lithoclasts. Calcareous and siliceous bioclasts, mostly microfossils (foram and sponge spicules, and a few radiolarians and diatoms) and coccospheres, are variably preserved; they may occur as calcite, silica, and clay pseudomorphs. Additional components include peloids and stringers of brown organic matter. Millimeter-scale fabric analysis provides detailed insight into the processes of transport and deposition. Five microfacies are identified: parallel-laminated, wavy and lenticular-laminated, homogeneous exhibiting preferred orientation of elongated components, bioturbated, and bioclast-bearing. The range of microfacies suggests that the marl sediment was deposited while currents were active. The depositional settings identified range from traction, reworking by bottom currents, supply by nepheloid flow (indistinctly bounded laminae), possible eolian input (laminae a few grains thick), and biogenic reworking of sediments. Back-scattered electron images reveal that the matrix consists of coccolithophorid remains with varying abundances of clay, silica, and carbonate cements. Five varieties of matrix are distinguished: 1) carbonate–clay–silica bearing, 2) carbonate–clay supported, 3) carbonate–silica supported, 4) clay–silica supported, and 5) clay dominated. Clay appears as rosette and lamellar aggregates, silica as nanoquartz (microspheres, clusters, coatings), quartz overgrowths, and occasionally opal CT as lepispheres and aggregates moss-like in appearance, and calcite as overgrown coccoliths, pseudomicrite, micrite, and microspar. Pyrite framboids are present in some samples. Intrabiotic and intergranular cements are developed. Also present are partially dissolved coccolith placoliths, silica- and clay-replaced micarb, and grains similar in appearance to volcanic glass shards that now consist of calcite and quartz. Bioclasts may have been a source for calcite and silica cements. Volcanic glass may have been a source for authigenic silica and clay minerals; this is suggested by the close spatial juxtaposition of silica microspheres and authigenic clay (cauliflower-like aggregates). The authigenic origin of the clay in these marls is also suggested by the presence of overgrown coccoliths because recrystallization of shields is reduced when clay is present, so that the overgrowths must have been formed in an environment where clay minerals were scarce initially. Despite diagenetic changes, the presence of pyroclasts is still recognizable. There are quartz crystal chips, shard-shaped grains, volcanic rock fragments, and minerals (biotite, apatite, zircon). The marls studied could be described as impure fine-grained detrital limestones chalky in type, with evidence of redeposition and possible volcanic-ash input. The marl depositional system within the Outer Carpathian Basin was associated with tectonic movements which initiated chalk-type facies, and this was particularly widespread in the Upper Cretaceous and Paleocene.

Transformation of silicon in a sandy beach ecosystem: Insights from stable silicon isotopes from fresh and saline groundwaters

Dissolved silicon isotope compositions (δ30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736μM, but homogeneous δ30Si of +1.0±0.15‰. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3μM, and consequently characterised by high δ30Si of +3.0‰. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23–192μM), and concentrations increase with depth at all sampling sites. The corresponding δ30Si values are highly variable (+0.3‰ to +2.2‰) and decrease with depth at each site. All groundwater δ30Si values are lower than seawater and most values are lower than dissolved δ30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5μmolSig−1). Silica discharge from the beach to the coastal ocean is estimated as approximately 210molSiyr−1 per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input.

Palaeoredox geochemistry and bioturbation levels of the exceptionally preserved early Cambrian Indian Springs biota, Nevada, USA

The Sirius Passet Lagerstatte (SP), Peary Land, North Greenland, occurs in black slates deposited at or just below storm wave base. It represents the earliest Cambrian microbial mat community with exceptional preservation, predating the Burgess Shale by 10 million years. Trilobites from the SP are preserved as complete, three-dimensional, concave hyporelief external moulds and convex epirelief casts. External moulds are shown to consist of a thin veneer of authigenic silica. The casts are formed from silicified cyanobacterial mat material. Silicification in both cases occurred shortly after death within benthic cyanobacterial mats. Pore waters were alkali, silica-saturated, high in ferric iron but low in oxygen and sulphate. Excess silica was likely derived from remobilized biogenic silica. The remarkable siliceous death mask preservation opens a new window on the environment and location of the Cambrian Explosion. This window closed with the appearance of abundant mat grazers later as the Cambrian Explosion intensified.

Quartz types, authigenic and detrital, in the Upper Cretaceous Eagle Ford Formation, South Texas, USA

Lithologic heterogeneity of the Eagle Ford Formation in South Texas arises from mixing of extrabasinal grains of siliciclastic composition with intrabasinal grain assemblages composed dominantly of marine carbonate with a lesser component of biosiliceous debris. Detrital quartz in particular is derived from both extrabasinal and intrabasinal sources, posing a challenge for the use of bulk compositional data for mudrock classification. Extrabasinal detrital quartz supplied along a major axis of siliciclastic influx, the Woodbine depositional system of East Texas, is reduced to a minor part of the grain assemblage in South Texas. Petrographic evidence and point-count results indicate that around 85 percent of total quartz in these rocks, equal to about 12.6 volume percent, is authigenic. Thus, significant quantities of authigenic silica are not restricted to siliceous mudrocks, but can be found in carbonate-rich mudrocks as well. Formerly opaline skeletons of radiolaria, the dominant source of silica for authigenic quartz precipitation, are only poorly preserved by replacements including calcite, dolomite, pyrite, and quartz. Dissolved silica released by dissolution of radiolarians, and perhaps also by volcanic glass dissolution is re-precipitated in a variety of forms, including matrix-dispersed microquartz cement, fillings within primary intragranular pores, and grain replacement of both calcareous and siliceous allochems. The mass balance of dissolved silica mobilized from radiolarians and other reactive silicates and the precipitation of authigenic quartz is uncertain because the initial volumes of now-dissolved detrital material versus the final volume of authigenic material (quartz and other authigenic silicates) cannot be determined with accuracy.

Provenance of sandstone on the western flank of Anambra Basin, Southwestern Nigeria

Petrographic and heavy mineral studies were carried out on clastic deposits that crop out in Ikpeshi, Auchi and Fugar localities in order to determine the provenance of the sediments and the tectonic setting of the source area. The localities are in the western flank of the Anambra basin, southwestern Nigeria. Petrographic study shows that the sandstone deposits are composed of variable amounts of quartz, feldspars and lithic fragments with minor occurrence of authigenic silica and chlorite cements. Quartz is the predominant detrital mineral in all the samples accounting for over 87 % of the total framework composition. The representative samples from all the localities contain 88-94 % quartz, 1-8 % lithic fragment, 1-4 % cement and 0-4 % matrix. Feldspar is present in small amount only in samples from Fugar locality. The sandstones each classify as quartz arenites and sublitharenites. Both opaque and transparent heavy minerals occur in the sandstones. The opaques are magnetite, ilmenite and leucoxene. Together, they account for 35 – 55 % of the total heavy mineral abundance. The transparent heavy minerals are zircon, tourmaline, rutile, staurolite, and apatite listed in order of decreasing abundance. The heavy mineral assemblages in the localities are similar except for the absence of rutile from the sediments of Ikpeshi locality. The heavy mineral grains are subangular to rounded. The zircon-tourmaline-rutile (ZTR) index for the sediments from the three localities are generally high and range from 90.5 to 100% indicating high degree of mineralogical maturity of the sandstones. The heavy mineral suites and the petrographic signatures of the sandstones suggest derivation mainly from acid igneous rocks, gneisses and older sandstones in both stable tectonic and recycled orogenic settings. However, the absence of rutile in sandstone from Ikpeshi may suggest derivation from lithologically more restricted provenance.

A Kungurian oceanic upwelling on Yangtze platform: Evidenced by δ13Corg and authigenic silica in the lower Chihsia Formation of Enshi Section in South China

The Late Paleozoic Ice Age across Carboniferous and Permian had a significant impact on the Kungurian (Upper Cisuralian series of Permian) Chihsia Formation in South China. This resulted in a unique interval with features such as the lack of reef in Chihsian limestone, widespread stinkstone and nodular/bedded chert. The Chihsia limestone (Kungurian stage) deposited during a time of cooling was resulted from oceanic upwelling. Here we present evidence for this upwelling using several geochemical analyses: bulk organic carbon isotope, biomarker molecular geochemical data, and authigenic silica of the stinkstone member in the lower Chihsia Formation of the Kuangurian stage from the Enshi Section in western Hubei Province, South China. The lower part of the stinkstone member shows a rapid organic carbon isotope excursion with a −3‰ shift triggered by the upwelling of 13C-depleted bottom water. The concurrent rapid increasing of authigenic silica content resulted from the enhanced supply of dissolved silica in the upwelling water mass. This upwelling at the Enshi Section also led to relative high TOC content, accounting for the widespread stinkstone in the lower Chihsia Formation during the Kungurian stage in Permian.

Constraining silica diagenesis in methane-seep deposits

Silicified fossils and authigenic silica are common in ancient seep limestones. Silicification of calcareous fossils facilitates the preservation of even fine details and is therefore of great interest to paleontologists, permitting a reliable taxonomic identification of the chemosynthesis-based taxa that lived at hydrocarbon seeps. Four methane-seep limestones of Paleozoic, Mesozoic, and Cenozoic age with abundant silica phases are compared in this study; one, an Eocene seep deposit on the north shore of the Columbia River at Knappton, western Washington State, USA, is described for the first time. Its lithology and fabrics, negative δ13Ccarbonate values as low as −27.6‰, and 13C-depleted biomarkers of archaea involved in the anaerobic oxidation of methane (AOM) reveal that the carbonate rock formed at a methane seep. The background sediments of the studied Phanerozoic seep limestones contain abundant siliceous microfossils, radiolarian tests in case of the Carboniferous Dwyka Group deposits from Namibia and the Triassic Graylock Butte deposits from Oregon (USA), as well as diatom frustules in case of the Eocene Knappton limestone and an Oligocene seep deposit from the Lincoln Creek Formation (Washington State, USA). These microfossils are regarded as the source of dissolved silica, causing silicification and silica precipitation. Silica cements formed after AOM-derived cements ceased to precipitate but before equant calcite formed. Numerical experiments using the computer code PHREEQC confirmed that (1) AOM increases the pH of pore waters and that (2) this pH increase subsequently mobilizes biogenic silica, (3) followed by the re-precipitation of the dissolved silica in the periphery of the AOM hotspot. The experiments revealed that degassing of carbon dioxide has the potential to significantly increase the local pH of pore waters, exerting an even stronger control on the local pH and silica dissolution than the rate of AOM alone.

Bentonite evolution at elevated pressures and temperatures: An experimental study for generic nuclear repository designs

Geologic disposal of spent nuclear fuel in high-capacity metal canisters may reduce the repository footprint, but it may yield high-thermal loads (up to 300 °C). The focus of this experimental work is to expand our understanding of the hydrothermal stability of bentonite clay barriers interacting with metallic phases under different geochemical, mineralogical, and engineering conditions. The hydrothermal experiments were performed using flexible Au/Ti Dickson reaction cells mounted in an externally heated pressure vessel at 150-160 bars and temperatures up to 300 °C for five to six weeks. Unprocessed Wyoming bentonite, containing primarily montmorillonite with minor amount of clinoptilolite, was saturated with a K-Ca-Na-Cl-bearing water (~1900 mg/L total dissolved solids) at a 9:1 water:rock mass ratio. The bentonite and solution combination contained either steel plates or Cu-foils and were buffered to low Eh using magnetite and metallic iron. During reactions, pH, K+, and Ca2+ concentrations decreased, whereas SiO2(aq), Na+, and SO42- concentrations increased throughout the experiments. Pyrite decomposition was first observed at ~210 °C, generating H2S(aq,g) that interacted with metal plates or evolves as a gas. The aqueous concentrations of alkali and alkaline earth cations appear to be buffered via montmorillonite and clinoptilolite exchange reactions. Illite or illite/smectite mixed-layer formation was significantly retarded in the closed system due to a limited K+ supply along with high Na+ and SiO2(aq) concentrations. Precursor clinoptilolite underwent extensive recrystallization during the six weeks, 300 °C experiments producing a Si-rich analcime in addition to authigenic silica phases (i.e., opal, cristobalite). Analcime and feldspar formation partially sequester aqueous Al3+, thereby potentially inhibiting illitization. Associated with the zeolite alteration is a ~17% volume decrease (quartz formation) that translates into ~2% volume loss in the bulk bentonite. These results provide chemical information that can be utilized in extending the bentonite barriers’ lifetime and thermal stability. Zeolite alteration mineralogy and illitization retardation under these experimental conditions is important for the evaluation of clay barrier long-term stability in a spent nuclear fuel repository.