Early Diagenetic Silicification Research Articles

Greensand formation, siliceous earth deposition and coastal metal drawdown in the Danubian Cretaceous Basin (Bavaria, Germany)

The Late Cretaceous epoch was characterized by extreme greenhouse climates and widespread glaucony formation in shallow marine settings. However, in the Danubian Cretaceous Basin (DCB, Bavaria, SE-Germany), contemporaneous shallow marine deposition of glauconitic sandstones of the Regensburg Formation in the eastern and the glaucony-free Neuburg Siliceous Earth deposits of the Wellheim Formation in the western parts of the basin during Cenomanian–early Turonian times is puzzling. An integrated approach of sedimentology, stratigraphy, mineralogy and geochemistry reveals that the striking lithological and mineralogical differences can be attributed to the geological structure of the hinterland and the nature of element input: in the eastern DCB, deeply chemically weathered granites and gneisses of the Bohemian Massif were leached due to the warm climate and high precipitation rates as indicated by high values of the modified Chemical Index of Alteration (CIXrev). Elements crucial for glaucony formation (K, Fe, Si, Al) were amply supplied by rivers, fueling a shallow marine glaucony factory. Slightly reducing conditions and a temporally increased primary productivity further promoted favorable conditions for geologically fast shallow water glaucony authigenesis. In the western DCB, in contrast, a hinterland consisting of karstified Upper Jurassic carbonates devoid of essential elements for glaucony formation and a lack of significant fluvial input inhibited shallow marine glaucony formation. Furthermore, we suggest that the depositional environment of the Neuburg Siliceous Earth was affected by submarine discharge of silica-rich groundwater resulting in intense early diagenetic silicification. Additionally, our geochemical data provide the first evidence of a trace metal drawdown during Oceanic Anoxic Event 2 (approx. 94 Ma) in shallow water/coastal settings in Germany, as shown in very low V/Al, Ni/Al, Cu/Al and As/Al ratios, much lower than the average shale and upper continental crust.

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Permian mixed carbonate–siliciclastic lagoon coastal system in West-Central Gondwana

Although in recent years mixed carbonate–siliciclastic platforms have been increasingly recognized in the Permian of Western Gondwana, there is no detailed description of these mixed systems in West-Central Gondwana. This study focuses on the Permian of the Sergipe–Alagoas Basin (NE Brazil), with the aim of filling this knowledge gap. Based on sedimentological data and microfacies analysis, six facies associations (FA) were identified: FA1 is an eolian–fluvial system, composed of stratified sandstones, FA2 and FA3 represent a mixed tidal flat–lagoon system composed of microbialites, pelitic/micritic sediments and intraclastic packstones, FA4 represents shoreface wave deposits composed of laminated sandstones and silicified grainstones, and FA5 and FA6 are interpreted as a barrier–tidal inlet system, composed of oolitic/intraclastic grainstones, subtidal microbialites, and mixed rudstones. Facies associations reflect the paleoenvironmental conditions of a coastal environment, with dunes and small rivers adjacent to a mixed carbonate–siliciclastic ramp. This is defined as a lagoon system associated with tidal flats protected from the high-energy region by carbonate shoals and microbial constructions. These deposits represent the eastern portion of a shallow epicontinental sea that covered lowlands in West-Central Gondwana within the arid–semiarid belt, whose climate characteristics are reflected in eolian deposits, desiccation structures, mixed carbonate–siliciclastic facies, and fossil records. The main diagenetic feature observed in petrographic analyses was an early diagenetic silicification. Although the absence of spiculites and shelf configuration did not allow a correlation with the Permian glass ramp model, similarities were observed with Eocene glass ramps of Australia, and a relationship with the Permian Chert Event is suggested.

Anatomical preservation of silicified Corylites J.S.Gardner leaves from the Paleocene maar lake of Menat (Puy-de-Dôme, France)

Uhl D., Jasper A. & Wuttke M. 2020. — Anatomical preservation of silicified corylites J.S.Gardner leaves from the Paleocene maar lake of Menat (Puy-de-Dôme, France). geodiversitas 42 (26): 485-492. http://geodiversitas.com/42/26The late Paleocene fossil lagerstätte Menat in France is well known for its wealth of excellently preserved fossil plants and insects. Although the flora from this locality is regarded as characteristic for the late Thanetian in Western and Central Europe, there is a noteworthy lack of modern paleobotanical studies on this locality. The few existing studies on plant megafossils utilized only the gross morphology of leaves and carpological material for taxonomic questions, whereas anatomical data (i.e. from cuticles and permineralizations) have been completely neglected. The present study provides the first data on anatomically preserved internal tissues of leaves assigned to Corylites macquarrii (Forbes) Heer from this locality. Cell walls are preserved as silicates, whereas cell lumina are mostly empty. On occasion, cell lumina are filled with foam-like, porous silica. Anatomical preservation of these tissues is probably related to early diagenetic silicification of plant cell walls. Although at the moment nothing can be said about the source of the SiO2 it is likely that it is related to the volcanic origin of the Menat maar and/or volcanic activities in the vicinity of the lake during deposition of the sediments.

Entrapment and transformation of post-bloom radiolarians in cyanobacterial mats as a factor enhancing the formation of black cherts in the Early Silurian sea

ABSTRACTBlack radiolarian cherts are widespread in the lithological records of the Silurian; however, the processes governing their formation remain unclear. Analyses of Early Silurian cherts of Poland have revealed that they are composed of degraded remains of cyanobacterial mats enclosing variable numbers of dissolved radiolarian tests. In optical and scanning electron microscope (SEM) images, this cyanobacterial necromass, containing entrapped radiolarians and forming massive or laminated cherty deposits, exhibits a complex taphonomic and diagenetic history. These cyanobacterial mats underwent syndepositional to early diagenetic silicification in a peculiar environment which was highly enriched with dissolved silica, due mainly to a massive deposition of opaline tests of post-bloom radiolarians, which settled on mats covering large areas of the sea bottom. It is assumed that precipitation of the chert precursor silica occurred in the highly porous mats and, to a great extent, was governed by the activity of sulfate-reducing bacteria (SRB), which resulted in changes in pH in the mat profile. This process was a key factor governing the dissolution of the radiolarian tests and subsequent reprecipitation of dissolved radiolarian silica in the mats' microenvironment. Photosynthesis carried out by cyanobacteria and the decompositional activity of sulfate-reducing bacteria resulted in: i) a shift of the oxygen gradient below the mat, and ii) strong pH fluctuation within the mat profile. During the first stage of bacterial degradation of the cyanobacterial necromass (SRB I stage), an increase in pH caused dissolution of the radiolarian tests. During the second stage of bacterial degradation (SRB II stage), as the decomposition of organic matter proceeded, a significant amount of organic acid was produced, causing a reduction in pH in the mat interior. Presumably this triggered rapid silica precipitation from the pore waters, which were highly saturated with silica. The availability of radiolarian tests and the rate of their dissolution was therefore a key factor governing the rate of silicification in Early Silurian cyanobacterial mats.

The Taphonomy of Proterozoic Microbial Mats and Implications for Early Diagenetic Silicification

The complex nature of growth and decomposition in microbial mats results in a broad range of microbial preservation. Such taphonomic variability complicates both the description of microbial elements preserved within geologic materials and the potential interpretation of microbial biomarkers. This study uses a taphonomic assessment to explore the preservation of different microbial components within silicified microbial mats of the late Mesoproterozoic (~1.0 Ga) Angmaat Formation, Bylot Supergroup, Baffin Island. The Angmaat Formation consists of unmetamorphosed and essentially undeformed strata that represent intertidal to supratidal deposition within an evaporative microbial flat. Early diagenetic silicification preserved microbial communities across a range of environments, from those episodically exposed to persistently submerged. Here, we present the development of a new methodology involving the use of high-resolution image mosaics to investigate the taphonomy of microfossils preserved in these mats. A taphonomic grade is assigned using a modified classification that accounts for both the taphonomic preservation state (good, fair, poor) of individual microfossils, as well as the degree of compaction of the overall mat. We show that although various taphonomic states occur within each of the silicified mats, the overall taphonomic assessment differentiates between well-preserved mats that are interpreted to have been silicified during active growth, to highly degraded and compacted mats that are interpreted to represent preservation during later stages of biological decomposition. These data indicate that even small changes in the timing of silicification may have substantial implications on our identification of microbial biomarkers and, therefore, our interpretation of early Earth ecosystems.

Sunspot cycles recorded in siliciclastic biolaminites at the dawn of the Neoproterozoic Sturtian glaciation in South China

Well-preserved siliciclastic biolaminites from the Neoproterozoic Wuqiangxi Formation (ca. 810–715 Ma) in South China show submillimeter-scaled couplets that consist of alternating dark and light laminae. The laminar couplets have been interpreted of annual origin, and the dark and light laminae mainly record warm spring to summer and cold autumn to winter deposition, respectively. Power spectrum and evolutionary fast Fourier transform (FFT) analyses of the laminar couplet thickness variations and geochemical series (Al, Th, Ba/Ti, Cu/Ti, Zn/Ti, V/Ti and U/Ti) revealed prominent periodicity of 10.0–11.4 yr and 18.9–25.6 yr. These two coherent periodic modes match well with the 11-yr Schwabe sunspot cycle and 22-yr solar Hale cycle, respectively. The observed layering pattern is thus interpreted as recording solar induced climate changes that may have modulated sedimentation, microbial growth and biomass production in shallow marine environments between fair-weather and storm-wave bases. Preservation of the sunspot cycles may have been facilitated by quiet depositional environment, microbial mat protection, and early diagenetic silicification. The documented Neoproterozoic cyclicity, in combination with previously reported examples, implies the stability of sunspot periodicity since the Paleoproterozoic. The presence of stable sunspot cycles in the upper Wuqiangxi Formation also suggests that the solar activity did not change significantly at the dawn of the Sturtian glaciation.

Redox architecture of an Ediacaran ocean margin: Integrated chemostratigraphic (δ13C–δ34S–87Sr/86Sr–Ce/Ce*) correlation of the Doushantuo Formation, South China

Early diagenetic silicification and phosphatization of the Ediacaran Doushantuo Formation (ca. 635 to 551Ma) in South China offer extraordinary taphonomic windows into the early evolution of multicellular eukaryotes, including various algal groups and potentially animals. In order to understand how the ecological and taphonomic distribution of these Ediacaran eukaryotes was controlled by oceanic redox conditions, it is critical to reconstruct the redox architecture of the sedimentary basin. Recently two alternative redox models have been proposed to account for the geochemical and sedimentary features of the Doushantuo Formation. One argues that the unit was deposited on a continental margin where a metastable sulfidic wedge was dynamically maintained by a sulfate concentration gradient between shelf and basinal environments. The other contends that the sulfidic water mass was largely restricted to the intra-shelf basin behind a rimmed margin. These two models make different predictions about the stratigraphic completeness and correlation of the Doushantuo Formation. To test these predictions, we generated high-resolution time-series trends of multiple isotopic and elemental tracers, including δ34S, 87Sr/86Sr and Ce/Ce*, to facilitate an integrated chemostratigraphic correlation between inner shelf (Xiaofenghe), intra shelf (Jiulongwan), and outer shelf (Yangjiaping and Zhongling) sections. Our correlations suggest that both the inner and outer shelf sections are stratigraphically incomplete relative to the intra shelf section. The euxinic wedge model should be reconsidered insofar as it is based on a miscorrelation between sections. Viewed from our revised chemostratigraphic framework, euxinic conditions on the platform appear to have been largely restricted to the intra shelf basin. Carbonates in the upper Doushantuo Formation at Jiulongwan and their stratigraphic equivalents are characterized by a profound negative carbon isotope anomaly (i.e., the Shuram Excursion) coincident with a drop in pyrite sulfur isotope values and a significant rise in 87Sr/86Sr from 0.7080 to 0.7090. The integrated stratigraphic data from South China suggest that the onset of the Shuram Excursion is associated with enhanced oxidative continental weathering that delivered radiogenic strontium, as well as sulfate, to the Ediacaran basin.

Germanium/silicon of the Ediacaran-Cambrian Laobao cherts: Implications for the bedded chert formation and paleoenvironment interpretations

Sedimentary strata of the terminal Ediacaran (635–542 Ma) to early Cambrian (542–488 Ma) Laobao-Liuchapo bedded cherts in the South China Block include the Ediacaran Oxidation Event and the Cambrian explosion. Understanding the origin and depositional environment of the bedded cherts may provide insight into how the Earth's surface environment changed between the Proterozoic and Phanerozoic. We measured major and trace element compositions and Ge/Si ratios of the Laobao cherts from northern Guangxi province. The Laobao cherts were deposited in the deep basinal environment of the South China Block. We show that the composition of the Laobao cherts is determined by a mixture of four components: quartz, clay, carbonate, and pyrite/iron-oxide. The quartz component is the dominant component of the Laobao cherts. The maximum estimated Ge/Si of the quartz component is between 0.4 and 0.5 μmol/mol, which is close to the Ge/Si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids. These Ge/Si systematics suggest that normal seawater rather than mid-ocean ridge hydrothermal fluids is the primary Si source for the Laobao cherts. The Ge/Si of the clay component varies between 1 and 10 μmol/mol, which is comparable to the Ge/Si of typical marine clays, but 10–100 times lower than that of chert nodules from early Ediacaran beds (the Doushantuo Formation) predating the terminal Ediacaran Laobao cherts studied here. Our observations indicate that the clay component Ge/Si ratio decreased from the early Ediacaran to the late Ediacaran. We speculate that high Ge/Si ratios in clays reflect the preferential chelation of Ge by dissolved organic compounds adsorbed onto clays. If so, this suggests that the decrease in Ge/Si ratio of the clay component in the Ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the Ediacaran-Cambrian transition, consistent with oxidation of the oceans during the late Ediacaran. Finally, the seawater origin of the Laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation. Instead, direct precipitation from seawater or early diagenetic silicification of calcareous sediments, perhaps due to the emergence of Si-accumulation bacteria, may have been responsible for the bedded Laobao-Liuchapo chert formation in South China Block.

Sedimentary geology of the Palaeoarchaean Buck Ridge (South Africa) and Kittys Gap (Western Australia) volcano-sedimentary complexes

The two Palaeoarchaean volcano-sedimentary complexes of the Buck Ridge (Barberton Greenstone Belt, South Africa) and Kittys Gap (Coppin Gap Greenstone Belt, East Pilbara, Australia) have a similar geological setting and age (∼3.45 Ga). The predominantly volcaniclastic sediments are concentrated at the top of these complexes, and experienced thorough, (very) early diagenetic silicification. In many places the silicification process has led to excellent preservation of the primary sedimentary structures. Elsewhere it has resulted in their obliteration or replacement by diagenetic structures. The Buck Ridge chert forms a regressive-transgressive succession, deposited around base level, with lacustrine and littoral marine facies. Deposition of the Kittys Gap Chert was also close to base level, almost exclusively subaqueous, with tidal influence and a regressive sequential trend. In both volcano-sedimentary complexes, these low-energy sediments are juxtaposed with high-energy breccia pods and layers, with often a high Fe-oxide content. The breccias are interpreted as being the result of explosive hydrothermal activity. Sedimentation was strongly controlled by normal faulting.

Diagenetic Fe-carbonates in Paleoarchean felsic sedimentary rocks (Hooggenoeg Formation, Barberton greenstone belt, South Africa): Implications for CO2 sequestration and the chemical budget of seawater

In order to evaluate the potential of felsic sediments as a CO2-sink in the Archean, we studied felsic volcaniclastic/epiclastic sedimentary rocks of the 3.45Ga Hooggenoeg Formation, Barberton greenstone belt, which were affected by metasomatic processes during seafloor alteration and diagenesis. Water–rock interactions leading to K-, Si- and CO2-metasomatism were quantified. The precursor rock, a K-metasomatized dacite, was leached of Ca, Mg, Fe, Na, Sr, and Ba and enriched in K, Rb and Si prior to erosion and deposition. The formation of K-mica and quartz (and minor K-feldspar) in the dacites suggests low pH metasomatic conditions, likely produced by hydrothermally induced circulation of hot, acidic and reduced Archean seawater in equilibrium with a high PCO2 atmosphere. Erosion and transport of K-metasomatized dacitic detritus away from the felsic volcanic centers resulted in the deposition of conglomerate, sandstone and shale by mass flow processes. Early diagenetic silicification affected mainly the fine-grained sediments with higher silica sorption capacity, forming impermeable layers, while sand-rich sediments were partly silicified and remained permeable. Trapped fluids precipitated two generations of Fe-rich dolomites and finally calcite. Up to 30vol.% of siliciclastic coarse-grained sediment was replaced by carbonates in a shallow-burial, high heat-flow diagenetic regime (depth: ∼750m, temperature: 80–160°C), and likely throughout deposition of overlying volcano–sedimentary units. The carbon isotopic composition of Fe-rich dolomites (δ13CPDB=+1.9 to +2.4‰) and the strong Fe–Ca–Mg leaching of the Paleoarchean volcanic formations support the influence of seawater-derived fluids throughout CO2-metasomatism. For each gram of eroded dacite, the overall chemical exchange involved by K–Si–CO2-metasomatism was characterized by a mass transfer of Fe (1.2mmol/g), Na (2.1mmol/g) and O2− (2.0mmol/g) to seawater. In contrast, seawater was depleted in Si (10mmol/g), Ca (0.51mmol/g), Mg (0.43mmol/g), K (1.5mmol/g) and H (0.93mmol/g) during incorporation of these elements in the volcanic and sedimentary rocks. The average CO2 uptake by the sedimentary rocks studied here is estimated to be 1.8mmol/g, in the same order of magnitude as previous estimates for the Paleoarchean basaltic crust. Although mafic-ultramafic rocks are the most abundant rocks in Paleoarchean greenstones belts, and represent the most important atmospheric CO2-sink upon seafloor alteration in the Paleoarchean, coarse felsic sedimentary rocks provide a non-negligible contribution to the build-up of the continental CO2 reservoir.

Acritarchs and microfossils from the Mesoproterozoic Bangemall Group, northwestern Australia.

Three microfossil assemblages occur in the Mesoproterozoic Bangemall Group (1625-1000 Ma) of northwestern Australia, each occupying a different environmental and taphonomic setting. In peritidal environments, benthic prokaryotic filaments and spheroids of matting habit and small size were permineralized by early diagenetic silicification of stromatolitic carbonates. In shallow subtidal environments, benthic filaments of large size and nonmatting habit and planktonic sphaeromorph acritarchs with thin walls and moderate dimensions were compressed in mildly kerogenous shale. In deeper subtidal environments, planktonic megasphaeromorph acritarchs with thick walls were initially entombed in concretionary nodules in highly kerogenous shale and then permineralized by silica during later diagenesis. Taxonomic diversity and numerical abundance evidently decrease offshore. The three assemblages have typical Mesoproterozoic aspects: peritidal benthic habitats were dominated by Siphonophycus-Sphaerophycus-Eosynechococcus-Myxococcoides-Palaeopleurocapsa, shallow subtidal settings were occupied by Siphonophycus-Leiosphaeridia-Pterosphermopsimorpha-Satka, and offshore plankton consisted solely of very large chuarid acritarchs. Because of its taphonomic restriction to mid-intertidal stromatolites, the peritidal assemblage can be equated in microenvironment with a similar assemblage in the Neoproterozoic Draken Conglomerate, suggesting that ecological stasis at the community level can last for intervals up to 900 million years. In the deeper subtidal assemblage, the common chuarid has an unusual mode of preservation, in three dimensions in early diagenetic concretions, revealing that it possesses a thick multilamellate wall. Because of this distinctive ultrastructure, the new genus Crassicorium is erected for these fossils, which are among the oldest indubitable eukaryotes. Very large (34-55 micrometers in diameter) filaments from shallow subtidal habitats are assigned to the emended species Siphonophycus punctatum.

Proterozoic ministromatolites with radial‐fibrous fabric

ABSTRACTSmall digitate stromatolites with diameters in the range of 0·2–20 mm (ministromatolites) are common in Early and Middle Proterozoic carbonate sequences, and extend stratigraphically from the Archaean to the Holocene. An occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial‐fibrous fabric and microscopically crinkled (microcrenate) lamination is described from the ˜ 1·9 Gyr old Belcher Supergroup (McLeary Formation) in southeastern Hudson Bay, Canada. The structures, which can be considered to be a variety of tufa, are unusually well preserved because of early diagenetic silicification. Columnar types are referable to Pseudogymnosolen (Asperia), and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern Canada, where the fabric is normally preserved by a secondary mosaic of dolomite. The textural evidence of angulate cross‐sections and rectilinear divergent patterns indicates that the radial‐fibrous fabric represents primary or very early diagenetic precipitation, and that pseudogymnosolenids with mosaic dolomite originally also had radial‐fibrous structure. The precipitation may have been within, or on, microbial mats.

Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)

ABSTRACTIn Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro‐ and cryptocrystalline quartz, with local chalcedonic quartz (both length‐fast and length‐slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro‐ and cryptocrystalline quartz, length‐slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg‐rich environment.Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica‐rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.