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The paper presents new data on the age of Triassic–Jurassic cherts replaced up section by siliceous mudstones, then by mudstones, which together with basalts form thin (up to hundreds of meters) plates and lenses that wedge out along strike at different levels of the multikilometer terrigenous section of the Ulban terrane (zone). These deposits in the Nimelen subzone (Albazino gold deposit area) formed in the Norian–Early Toarcian; host terrigenous deposits formed in the Pliensbachian–Bathonian. The age of cherts and siliceous–clayey rocks in the Mewachan subzone is Middle Triassic–Callovian, and that of host terrigenous rocks is Middle–Late Jurassic (including the Tithonian). Comparison of deposits of the Ulban terrane with well-studied deposits of the Samarka terrane similar in age and composition allows us to consider the Ulban terrane (zone) as part of the Jurassic accretionary complex of the Sikhote-Alin orogenic belt. There are also sufficient signs to consider that its structural features are consistent with the subduction model. This idea is based on the fact that Triassic–Jurassic cherts and siliceous–clayey rocks, which accumulated over about 45 Ma at a rate of about 1.8 m/Ma, are considered the sedimentary cover of the oceanic plate. At the same time, a gradual transition from cherts to mudstones is evidence of movement of the oceanic plate into the subduction zone. The occurrence of its fragments in the terrigenous section is the result of their detachment in the trench and under the overhanging slab. Tectonostratigraphic columns of the subzones of the Ulban terrane and a zonal scheme of the Jurassic accretionary wedge, including four structural-age levels, have been compiled. [ABSTRACT FROM AUTHOR]

This study characterizes the microstructure and mineralogy of 132 (ODP sample), 1000 and 1880 million‐year‐old chert samples. By using ultra‐small‐angle X‐ray scattering (USAXS), wide‐angle X‐ray scattering and other techniques, the preservation of organic matter (OM) in these samples is studied. The scarce microstructural data reported on chert contrast with many studies addressing porosity evolution in other sedimentary rocks. The aim of this work is to solve the distribution of OM and silica in chert by characterizing samples before and after combustion to pinpoint the OM distribution inside the porous silica matrix. The samples are predominantly composed of alpha quartz and show increasing crystallite sizes up to 33 ± 5 nm (1σ standard deviation or SD). In older samples, low water abundances (∼0.03%) suggest progressive dehydration. (U)SAXS data reveal a porous matrix that evolves over geological time, including, from younger to older samples, (1) a decreasing pore volume down to 1%, (2) greater pore sizes hosting OM, (3) decreasing specific surface area values from younger (9.3 ± 0.1 m2 g−1) to older samples (0.63 ± 0.07 m2 g−1, 1σ SD) and (4) a lower background intensity correlated to decreasing hydrogen abundances. The pore‐volume distributions (PVDs) show that pores ranging from 4 to 100 nm accumulate the greater volume fraction of OM. Raman data show aromatic organic clusters up to 20 nm in older samples. Raman and PVD data suggest that OM is located mostly in mesopores. Observed structural changes, silica–OM interactions and the hydrophobicity of the OM could explain the OM preservation in chert. [ABSTRACT FROM AUTHOR]

Palaeozoic (Silurian and Devonian) bedded cherts from the Balkan Terrane in western Bulgaria occur in three tectonic units: Svoge, Lyubash-Golo Bardo, and Morava. The Silurian and Devonian ages of the chert-bearing lithostratigraphic successions have been determined by conodonts and graptolite macrofauna. The silica source and depositional settings of the cherts have been interpreted based on the received mineralogical, petrographic, and geochemical (major, trace, and rare earth elements) data. The presence of radiolarian tests, the Si/Si+Al+Fe+Ca ratios, and Al–Fe–Mn diagram plotting at the non-hydrothermal field, testify that the source of silica in the studied rocks is mostly biogenic or both biogenic and terrigenous in origin. Differential thermal analysis results suggest that the Silurian and Devonian siliceous rocks were presumably formed in predominating anoxic oceanic conditions and only sporadically in oxic waters. The MnO/TiO2 ratio and received Al2O3, Fe2O3, TiO2, Lan, and Cen contents indicate that the studied cherts have been deposited in continental margin environments. Most probably, they have been formed in slope and outer-shelf settings on the passive margin of northern peri-Gondwana. [ABSTRACT FROM AUTHOR]

The Neolithic and Bronze Age communities that settled the eastern Carpathian Forelands and Carpathian Foothills used a variety of local and non-local siliceous raw materials. Raw materials identified in the archaeological record differ in quality and usefulness for making tools. Obsidian, Jurassic flint from the Cracow-Częstochowa Upland, chocolate flint, or Świeciechów and Volhynian flints represent the best quality. On the other hand, some local raw materials were also in use, most popular among them being siliceous marls and cherts. Sources of siliceous marls and cherts are known from many locations in the Dynów, Strzyżów and Przemyśl foothills. Moreover, systematic field surveys in this area have provided new information on the availability of cherts and siliceous marls at many new locations in the region. They appear in the primary autochthonous, secondary autochthonous, and more rarely in sub-autochthonous or residual, sources. Exposures on steep hill slopes and dissected river valleys provide easy access to the best quality raw materials in the primary autochthonous sources. Raw materials from secondary autochthonous sources in the riverbeds were also available, but they were of lesser quality than those from the exposures. The aim of this paper is to present natural exposures of siliceous marls and cherts and discuss them as a potential source of raw materials for the Neolithic and Bronze Age communities inhabiting loess areas of the eastern Carpathian foreland (Rzeszów Settlement Region).

Copyright of Advances in Earth Science (1001-8166) is the property of Advances in Earth Science Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

New perspectives in the archaeometric investigation of the chert used in lithic tool assemblages allow us to learn more about the socio−economic behavior of prehistoric human groups. The case study presented here is from the Artofago Cave in South Tuscany, Italy. Among other findings, an Upper Paleolithic lithic complex was excavated from a large fireplace near the entrance. The purpose of this study is to gather data aimed at determining the raw material used in the production of these lithic tools, thereby enhancing research into its geographic source area. Here we highlight the significance of the geological age of the rock as one of key factors enabling us to formulate hypotheses about its geological provenance. It showcases a successful application of radiolarian biostratigraphy in dating cherts from the archaeological record. Specifically, 67 chert samples from the Upper Paleolithic lithic complex of the Artofago cave were treated with HF to isolate the radiolarian assemblages. The samples had been carefully chosen in advance between broken chert pieces (chert debris) in the waste material given that the analysis is partially destructive. Chert samples can be assigned to a Middle − Late Jurassic age, more specifically middle Callovian−early Oxfordian to late Kimmeridgian−early Tithonian (UAZ 8-11). These ages are comparable with those of the Diaspri di Monte Alpe Formation of the Ligurian Vara Unit, the uppermost tectonic unit of the Northern Apennines. [ABSTRACT FROM AUTHOR]

Paleozoic (Silurian and Devonian) cherts from Western Bulgaria occur in three tectonic units: Svoge, Lyubash-Golo Bardo and Morava. The SiO2 content in these rocks varies from 76.06% to 96.02%. The silica source and depositional settings of the cherts have been interpreted based on the received geochemical major element data. The calculated Si/(Si+Fe+Al+Ca) ratio and Al-Fe-Mn diagram plotting at non-hydrothermal field suggest that most of silica in the cherts is biogenic or both biogenic and terrigenous in origin. On the other hand, all studied siliceous samples plot into the field of the continental margin sediment environments on the Fe2O3/TiO2 vs. Al2O3/(Al2O3+Fe2O3) diagram. The calculated MnO/TiO2 ratio also indicates deposition in continental shelf or slope environments. [ABSTRACT FROM AUTHOR]

Abundant chert bands and nodules are discovered throughout the Mesoproterozoic Jixian System in the Ordos Basin. These cherts faithfully record the Jixian period oceanic conditions and paleoecology. However, the diagenetic mechanism of the cherts remains unclear and controversial. To understand the origin of these cherts, we performed a multitracer study by combining field reconnaissance, petrological analyses, Si isotope analysis, and major and trace element analysis of chert samples from the Qishan section. The results show that Jixian cherts had a high Al/(Al + Fe + Mn) value (~0.48), showed a flat distribution of rare earth elements in seawater, and most of the Fe/Ti values are <20. A weak negative Ce anomaly (~0.97) indicates the contribution of REY to weakly oxygenated seawater. The high isotopic composition of silicon (δ30Si = 0.74 ‰-1.35 ‰) and the average Si ex value of 37.52 indicate that there is a source of biological silica, and the relationship between Eu/Eu* and Y/Ho and δ30Si shows that the hydrothermal and volcanic influences were less and correlated with seawater. The chert bands were closely related to biological activities. The chert nodules were influenced by hydrothermal activity. Secondary cherts were related to diagenesis, but the nature of siliceous fluids remains unchanged, still characterized by biogenic activity origins. The sea oxygen concentration in the Jixian period had increased, with periodic variations in the intensity of biological activities affecting the pH of the water body, or biological photosynthesis binding SiO 2 colloid in water, affecting chert sediments, leading to frequent interlayering between dolomite and chert bands. Episodic hydrothermal fluids and late-stage diagenetic processes jointly influenced the formation of cherts. This study bears significant significance in enhancing our understanding of the sedimentary environment and the origin of cherts during the Jixian period in the Ordos Basin. • Cherts of the Jixian System formed in a low-oxygen water environment. • Cherts were of biogenic and chemical origins, and some of them had a replacement origin. • Silica sourced from dissolved silicon in seawater, organisms, and occasional hydrothermal fluids. • Biological activities affected chert deposition. [ABSTRACT FROM AUTHOR]

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Neolithic and Bronze Age communities that settled the eastern Carpathian Forelands and Carpathian Foothills used a variety of local and non-local siliceous raw materials. Raw materials identified in the archaeological record differ in quality and usefulness for making tools. Obsidian, Jurassic flint from the Cracow-Częstochowa Upland, chocolate flint, or Świeciechów and Volhynian flints represent the best quality. On the other hand, some local raw materials were also in use, most popular among them being siliceous marls and cherts. Sources of siliceous marls and cherts are known from many locations in the Dynów, Strzyżów and Przemyśl foothills. Moreover, systematic field surveys in this area have provided new information on the availability of cherts and siliceous marls at many new locations in the region. They appear in the primary autochthonous, secondary autochthonous, and more rarely in sub-autochthonous or residual, sources. Exposures on steep hill slopes and dissected river valleys provide easy access to the best quality raw materials in the primary autochthonous sources. Raw materials from secondary autochthonous sources in the riverbeds were also available, but they were of lesser quality than those from the exposures. The aim of this paper is to present natural exposures of siliceous marls and cherts and discuss them as a potential source of raw materials for the Neolithic and Bronze Age communities inhabiting loess areas of the eastern Carpathian foreland (Rzeszów Settlement Region). [ABSTRACT FROM AUTHOR]

Solid-state nuclear magnetic resonance, which enables the characterization of structures of a variety of materials whatever their crystallinity/amorphous state, is used in the present work to determine the provenance of two raw materials, namely marbles and cherts. Regarding marbles, the 13C NMR signal of the carbonate function contains information about both the Fe content in its area and the presence of calcium substitutions in the calcite crystal in its linewidth. Regarding cherts, discriminant information is provided by both 29Si and 27Al NMR: the 29Si area signal depends on the paramagnetic ion content of the material, and the 27Al spectra give information both on the aluminosilicate content and its distribution in tecto- and layer-lattice aluminosilicates. As an application, we use the differences observed from one source to another to determine the provenance of archaeological finds.

Research subject. The West Magnitogorsk zone of the Southern Urals in the vicinity of the Ishkildino village features a subaerially exposed basaltic sequence superposed by cherts and siliceous-clay shales. The basalts and the overlying shales are assumed to have formed during the Ordovician and Silurian (?)–Early Devonian (up to the conodont zone excavates inclusive) periods, respectively. The aim of this research was to reconstruct, using geochemical data, the conditions under which the rocks present in this geological location were formed.Materials and methods. Five samples of the basalts (XRD and ICP-MS methods), 27 samples of the siliceous-clay shales and 10 samples of the cherts (XRD and ICP-AES methods) were analyzed.Results. According to the ratio of SiO2, Na2O and K2O, the volcanic rocks from the lower part of the section are represented by basalts and trachybasalts. Their geochemical composition corresponds to the N-MORB and is established to be similar to that of the basalts in the Polyakovskaya formation (the Middle–Upper Ordovician). In terms of main elements, the shales under study consist of quartz and illite with a slight admixture of organic matter, goethite, quartzfeldspar fragments, etc. The degree of the sedimentary material weathering according to the CIA, CIW and ICV index values is shown to be moderate. The values of Strakhov’s and Boström’s moduli correspond to sediments without the admixture of underwater hydrothermal vent products. The values of Cr/Al, V/Al and Zr/Al correspond to those characteristic of deposits in deep-water zones remote from the coasts of passive and active continental margins, basalt islands and areas adjacent to mid-ocean ridges. For most samples, the values of Ni/Co, V/Cr, Mo/Mn are typical of deposits formed under oxidative conditions. However, several samples from the upper part of the section, which is comparable to the kitabicus and excavatus conodont zones, demonstrate the Ni/Co, V/Cr, and Mo/Mn values corresponding to deposits formed under reducing atmospheres. An assumption is made that the existence of these deposits can be associated with the Bazal Zlichov event.Conclusion. The investigated pre-Emsian shales have shown no signs of volcanic activity in the adjacent areas. The studied deposits are established to correspond to the central part of the Ural Paleoocean.

Upper Triassic (upper carnian - rhaetian) grey cherty limestone are known in the internal Dinarides under the name of “Grivska Formation”. Sediments of the Grivska Formation are characterized by microfossils only - conodonts and radiolarians, and did not yield any macrofossils. Micropa - laeontological research of Upper Triassic siliceous rocks was performed at the locality Lim river, in the vicinity of Bistrica Village in SW Serbia. Radiolarian assemblages are characterized by such species as Capnodoce anapetes De Wever, C. sarisa De Wever, Sarla hadrecaena (De Wever), Praehexasaturnalis tenuispinosus (Donofrio & Mostler), Xiphothecaellla longa (Kozur & Mock). According to the radiolarian data, the investigated cherts are of latest carnian to early norian age. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. 176015]

The base of the Guadalupe Group, in the Tunja area of Colombia, contains cherts, porcellanites, mudstones, and siltstones with subordinate quartz arenites. The lithostratigraphic description of two stratigraphic sections showed that the dominant facies have fine granular textures and siliceous compositions, which considerably differ from those of the prevailing sandy terrigenous facies described in the type locality in the Eastern Hills of Bogotá, in the Arenisca Dura Formation, the basal unit of the Guadalupe Group in this sector. The units that form the Guadalupe Group (base of the Guadalupe Group, Plaeners, and Arenisca Tierna) markedly differ from each other morphologically, which facilitates their mapping because the base and top units generate a steep morphology, and the intermediate units form surface depressions or valleys, similar to the morphology of the Guadalupe Group in its type locality in the Eastern Hills of Bogotá. The base of the Guadalupe Group consists of cherts and porcellanites toward the NW of the study area (Alto del Gavilán section), with mudstones, siltstones, quartz arenites, and to a lesser extent porcellanites and cherts prevailing toward the SE (Vereda Salitre section). Geochemical analysis of total rock samples by XRD and XRF confirmed the primarily siliceous nature of the base of the Guadalupe Group, with SiO2 ranging from 62 to 98%, CaO less than 3.0%, and P2O5 peaking at 15.0%. Etayo-Serna (2015) conducted paleontological determinations of ammonites found in the stratigraphic section of Alto del Gavilán and assigned the base of the Guadalupe Group mainly to the Lower Campanian.

Solid-state nuclear magnetic resonance, which enables the characterization of structures of a variety of materials whatever their crystallinity/amorphous state, is used in the present work to determine the provenance of two raw materials, namely marbles and cherts. Regarding marbles, the 13C NMR signal of the carbonate function contains information about both the Fe content in its area and the presence of calcium substitutions in the calcite crystal in its linewidth. Regarding cherts, discriminant information is provided by both 29Si and 27Al NMR: the 29Si area signal depends on the paramagnetic ion content of the material, and the 27Al spectra give information both on the aluminosilicate content and its distribution in tecto- and layer-lattice aluminosilicates. As an application, we use the differences observed from one source to another to determine the provenance of archaeological finds. [ABSTRACT FROM AUTHOR]

In this study, siliceous nodules from the world-famous Myrtos beach, as well as from Avithos beach, in the western flanks of Kefalonia Island in Greece are examined by means of petrographical, mineralogical, geochemical and micropaleontological methods. The objectives of this study are to characterize the textural and compositional features of the nodules, with the aim to provide an initial interpretation of their origin and their diagenetic evolution. The studied siliceous nodules are hosted within Lower Cretaceous thin-bedded limestones at Myrtos and Upper Eocene limestones at Avithos. Nodules from both areas display a characteristic concentric texture at a macroscopic and microscopic scale. They both have a dense fine-grained siliceous sedimentary fabric, composed mainly of microcrystalline or cryptocrystalline quartz and moganite with common residual calcite in the case of Avithos. These results, and in particular the shape of the nodules, along the textural and compositional characteristics, indicate different conditions of formation in the two localities, both during the early epigenetic stages, as well as later during the diagenetic processes. Myrtos nodules originated from Si-precursors deposited in a pelagic environment, going through intense Si-replacement. Avithos nodules were deposited in a more proximal environment, being influenced by a less intense silicification. Nevertheless, the higher degree of recrystallization of Avithos samples indicates a syn- or post-diagenetic tectonic activity that resulted in the circulation of geothermal fluids. The conclusions drawn from this work demonstrate the usefulness of thorough studies of siliceous nodules in order to get a more comprehensive understanding of the initial depositional conditions, as well as diagenetic pathways and processes.

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Solid-state nuclear magnetic resonance, which enables the characterization of structures of a variety of materials whatever their crystallinity/amorphous state, is used in the present work to determine the provenance of two raw materials, namely marbles and cherts. Regarding marbles, the 13C NMR signal of the carbonate function contains information about both the Fe content in its area and the presence of calcium substitutions in the calcite crystal in its linewidth. Regarding cherts, discriminant information is provided by both 29Si and 27Al NMR: the 29Si area signal depends on the paramagnetic ion content of the material, and the 27Al spectra give information both on the aluminosilicate content and its distribution in tecto- and layer-lattice aluminosilicates. As an application, we use the differences observed from one source to another to determine the provenance of archaeological finds. [ABSTRACT FROM AUTHOR]

Marine siliceous deposits of cherts are known to have formed since the Archean and reflect the biogeochemical Si cycle, comprising the burial of Si in sediments during diagenesis. The actualistic assumption of biological control over the Si cycle was outlined in the seminal models of Si evolution in Earth history sketched by Maliva et al. (1989) , Maliva and Siever (1989a, 1989b) and Siever (1991, 1992). Their assumptions were treated as the dominant paradigm in subsequent geological studies and outlined the approach to be taken towards the relationship between the productivity of silicifiers and the formation of siliceous rocks. In light of new paleontological studies of evolution of silicifers, as well as chert facies distribution analysis in combination with geochemical (REE + Y, Y/Ho, Eu/Eu⁎, Al-Fe-Mn, Hg) and isotopic (δ30Si) studies of siliceous rocks, the biological control of siliceous sponges and radiolarians over the Palaeozoic Si cycle is a phenomenon which is difficult to understand. This review provides a critical discussion of the seminal model of Si evolution presented over a quarter of a century ago and, based on new studies, considers the endogenous source of dSi (volcanic and hydrothermal activity, weathering of freshly erupted basalts) as an overlooked controlling factor of Si cycling in the Archean-Palaeozoic. • The assumption of biological control over siliceous deposition is of limited significance. • The significant amount of Palaeozoic chert were deposited on abiotic way. • The overlooked source of Si for cherts were endogenous (volcanic and hydrothermal activity). • The updated volcanic press-pulse model explains the DSi source for chert gensis. [ABSTRACT FROM AUTHOR]

Cherts have been thought to originate from biosilicification, terrestrial inputs and hydrothermal activity. The study of cherts is helpful in understanding the paleo-ocean environment and tectonic–sedimentary processes. Large amounts of cherts occur widely in the Maokou Formation in the Sichuan Basin, which may be largely connected to the Permian Chert Event (PCE). However, the source of silica and the formation process of cherts remain debated. Here, we analyze the petrographic and geochemical features of the cherts from the Guadalupian Maokou Formation (~268–259 Ma) in six sections in the Sichuan Basin. Two main types of cherts, nodular and bedded, are recognized in the Maokou Formation. The formation of nodular cherts was mainly affected by hydrothermal fluids, whereas the bedded cherts are mainly of biogenetic origin. The Emeishan large igneous province (ELIP) caused the activation of deep faults, accompanied by intense hydrothermal activities. Correspondingly, the cherts of significant hydrothermal origin developed near the active deep faults. The intensified hydrothermal activities may provide extra silica supplies and flourish the silica-secreting organisms by the associated volcanogenic upwellings that facilitated the enrichment of cherts. The study of Maokou cherts can help to record the volcanic- and silicon-related biological activities in the eastern Paleo-Tethys Ocean and can provide significant implications for chert enrichment in analogous settings.

Graphical abstract Highlights • Paleomagnetic results of Cretaceous cherts in Zhongba, southern Tibet are reported. • The results yield a reliable, ∼120–93.9 Ma paleopole for Tethyan Himalaya (TH). • Apparent polar wander paths of TH and cratonic India exhibit similar patterns with no separations. • The new paleomagnetic results do not support the Greater India Basin hypothesis. • The new evidence argues against the proposed dual collisions between India and Asia. Abstract An oceanic Greater Indian Basin was hypothesized to have formed between ∼120 Ma and ∼65 Ma by regional extension that separated the cratonic India from its northern margin of Tethyan Himalaya (TH). However, the lack of paleomagnetic data from this critical time interval in the TH marks an important gap in our knowledge about the tectonic evolution of the Greater India. We have conducted a paleomagnetic study of a Cretaceous marine succession in Zhongba, southern Tibet that consists of predominantly purple reddish cherts and siliceous limestone. Paleomagnetic samples from 12 sites were subjected to stepwise thermal demagnetization that revealed either two- or three-component magnetizations with the high-temperature component (HTC) typically unblocked at ∼650 °C. The HTCs exhibit dual polarities that yield a positive reversed test at 95% confidence level, suggesting a primary origin of the remanence. With chronologic constraints from the combined analysis of the magnetostratigraphic and radiolarian data, the HTCs yield a mean of D s = 336.9°, I s = −50.7°, k s = 18.0, α 95 = 5.3° for the ∼120.8–93.9 Ma interval of the TH, which corresponds to a paleopole at 25.0°N, 285.7°E (dp/dm = 4.8/7.1°). The apparent polar wander path (APWP) of the TH overall resembles that of cratonic India, suggesting no polarward separation that is sufficiently large to merit an oceanic basin between the TH and cratonic India. Therefore, our new reliable paleomagnetic data from the critical time interval do not support the Greater Indian Basin hypothesis and its implied dual collisional processes between India and Asia. On the other hand, the APWPs exhibit relative shifts for the post-120 Ma segment and the pre-130 Ma segment, indicating relative motions between the TH and cratonic India that most likely resulted from shearing along boundary-parallel faults during the India-Asia collision. [ABSTRACT FROM AUTHOR]

Cherts are quite frequently occurring rocks in the Internal Dinarides, an extremely complex area composed of several tectonostratigraphic units in which oceanic sediments, ophiolites and partly metamorphosed parts of the distal continental margin of Adria are preserved. Therefore, these cherts differ in age and the original depositional environment in which they were formed. Results of investigations carried out in the chert blocks found in the melange in the vicinity of Jasenovo village on SE slopes of Zlatibor Mt. are presented here. Radiolarian cherts from the studied localities represent blocks in melange of the East-Bosnian-Durmitor Unit, exposed in a large tectonic window below the Triassic carbonates of Drina-Ivanjica Unit. Biostratigraphic data revealed Callovian-early Kimmeridgian ages of the studied chert blocks, thus implying a Kimmeridgian or younger age of obduction of the West Vardar ophiolites. [Projekat Ministarstva nauke Republike Srbije, br. 176015]

The oxygen isotope composition of the Earth's oceans is buffered by high- and low-T exchange with the lithosphere. We present a triple oxygen isotope mass balance model for the Earth's oceans. The model is based on triple oxygen isotope measurements of rocks from various reservoirs including high- and low-T alteration products. The modern ocean water composition can be well-matched if the ratio between continental weathering and high-T seafloor alteration is ~25% higher than previously assumed. The mass balance suggests that putative Precambrian low-δ 18 O ocean water would fall on a trend with slope λ = 0.51 passing through “modern” ice-free-world seawater. Exemplified application to a published Phanerozoic and Archean chert data suggest precipitation in cool oceans with modern-like δ 18 O followed by diagenetic alteration with involvement of meteoric water. [ABSTRACT FROM AUTHOR]

This paper considers lithic raw material procurement of Epigravettian hunter-gatherers from Kopačina Cave on the island of Brač (Dalmatia, Croatia). The most significant group among the determined petrographic categories are different cherts, and a significantly smaller group of radiolarites. Cherts are both locally and regionally available, while radiolarites originate from more distant areas. Use of raw material that could have been procured within 20 km radius (local) and in the range of 20–50 km (regional) is predominant and very similar in all phases. Raw material that could have been procured from distances ranging from 20 to 50 km shows a gradual trend of increase in frequency from the earliest to the latest phase, while the raw material procured from distances greater than 50 km (extra-regional) has an obvious drop in frequency from the eldest to the youngest phase. Temporal trends in lithic raw material use suggest certain continuity during the whole Epigravettian sequence as well as change which shows itself in larger exploitation areas or more intensive long-distance contacts in earlier Epigravettian phases in Kopačina than in the later ones, and possibly a higher degree of hunter-gatherers’ mobility. Presence of radiolarites in Kopačina's Late Upper Paleolithic layers, as well as their potential outcrops suggest movements of hunter-gatherers deeper in east Adriatic continental hinterland where for now only a few traces of human settlements from Late Glacial have been found. Raw material of several found artifacts indicates possible contacts with the west Adriatic coast. [ABSTRACT FROM AUTHOR]

The elemental analysis of cherts is very important for understanding their geological history, formation and origin. In this work, chert samples collected from a mining complex located in the Gargano promontory (Apulia, Italy) have been investigated by laser-induced breakdown spectroscopy (LIBS) using a commercial handheld (h) LIBS instrument. In order to discriminate chert samples according to their mining site, the broadband LIBS spectra acquired on the specimens were subjected to multivariate data analysis. In particular, machine learning (ML) algorithms, including discriminant analysis (DA), support vector machine (SVM), and k-nearest neighbors (KNN), were applied to the data set from the averaged LIBS spectra of each sample dimensionally reduced by principal components analysis (PCA). Among the classifiers tested, the highest accuracy (72.34%) in identifying the chemical features of five different chert suites and discriminating their sources was achieved by Leave-One-Out Cross-Validation (LOO-CV) using the cosine KNN ML algorithm. The exclusion of PC1, which appeared to be mainly influenced by elements featuring large intra-class variation and engraved the clusterization, allowed to improve the accuracy of the classification reaching the value of 80.85% using the linear DA algorithm. The classification and discrimination between 2 classes (1 × 1) reached a high rate of correct predictions, which implied that this discrimination could be used as a subsequent mean to confirm if the class predicted in the previously performed multiclass classification was correct. In conclusion, the results of this study demonstrated that even chemically similar geological materials could be discriminated using LIBS analysis combined with appropriate multivariate chemometric data processing, in particular with the aim of recognizing and differentiating specific geological chert sources and identifying the origin of chert archaeological artifacts. [Display omitted] • Handheld LIBS analysis was used for the geochemical classification and provenance identification of cherts. • PCA of the whole experimental hLIBS spectra was used to find the class clusterization in the scores plot. • Three types of supervised machine learning (ML) algorithms were tested to classify the samples. [ABSTRACT FROM AUTHOR]

The Pangaea assemblage resulted in significant tectonic and eustatic changes, but also in a well-known desertification triggered by the increasing continentalization. While the northern hemisphere basins have been thoroughly studied, several recent studies on southern hemisphere basins bring new insight on how these events affected the Gondwana hinterlands. In this study, sedimentological and geochemical analyses of chert deposits from the Pennsylvanian Piauí Formation of the Parnaíba Basin are examined as new evidence for the onset of continentalization and aridification in the northwestern Gondwana mid-latitudes. The chert deposits correspond to three facies (stromatolites, ooidal arenites and breccias) that together suggest deposition in a shallow water environment and biomarkers analysis of stromatolites indicated predominance of organic matter produced by cyanobacteria in an environment characterized by high salinity and elevated pH. Such evidence suggests that the Piauí Formation cherts deposited in a shallow, saline-alkaline lacustrine environment during a period marked by increasing arid climate conditions. • Contribution to the discussion on Pangaea aridification based on southern hemisphere data. • The onset of aridification process can be reliably positioned in Pennsylvanian period. • First ever evidence of saline-alkaline lake in Gondwana mid-latitudes during Pennsylvanian age. • Panthalassa Ocean transgression and withdrawal related to tectonic event and arid conditions. [ABSTRACT FROM AUTHOR]

We present a new model to explain the origin, emplacement and stratigraphy of the Nicoya Complex in the NW part of the Nicoya Peninsula (Costa Rica) based on twenty-five years of field work, accompanied with the evolution of geochemical, vulcanological, petrological, sedimentological and paleontological paradigms. The igneous-sedimentary relation, together with radiolarian biochronology of the NW-Nicoya Peninsula is re-examined. We interpret the Nicoya Complex as a cross-section of a fragment of the Late Cretaceous Caribbean Plateau, in which the deepest levels are exposed in the NW-Nicoya Peninsula. Over 50% of the igneous rocks are intrusive (gabbros and in less proportion plagiogranites) which have a single mantle source; the remainder are basalts with a similar geochemical signature. Ar39/Ar40 radioisotopic whole rock and plagioclase ages range throughout the area from 84 to 83 Ma (Santonian) for the intrusives, and from 139 to 88 Ma (Berriasian-Turonian) for the basalts. In contrast, Mn-radiolarites that crop out in the area are older in age, Bajocian (Middle Jurassic) to Albian (middle Cretaceous). These Mn-radiolaritic blocks are set in a “matrix” of multiple gabbros and diabases intrusions. Chilled margins of magmatites, and hydrothermal baking and leaching of the radiolarites confirm the Ar39/Ar40 dating of igneous rocks being consistently younger than most of the radiolarian cherts. No Jurassic magmatic basement has been identified on the Nicoya Peninsula. We interpret the Jurassic-Cretaceous chert sediment pile to have been disrupted and detached from its original basement by multiple magmatic events that occurred during the formation of the Caribbean Plateau. Coniacian-Santonian (Late Cretaceous), Fe-rich radiolarites are largely synchronous and associated with late phases of the Plateau.

Blocks of cherty rocks and Aptychus Limestone embedded into ophiolite mélange south of Avala Gora (Serbia) contain radiolarians of different ages. We distinguished here Late Jurassic (middle Oxfordianearly Tithonian), Middle-Late Jurassic (Bathonian-early Tithonian), and Middle Triassic (early Ladinian) radiolarian assemblages. The respective stratigraphic data suggest that the ophiolite mélange was formed after the early Tithonian. [ABSTRACT FROM AUTHOR]

Abstract: Sedimentary structures interpreted as early diagenetically silicified post-bloom macroaggregates are described from early Silurian black radiolarian cherts of Poland. They occur as whitish lenses and/or sheet-like layers of various thickness and lateral extension distributed in certain horizons of the studied radiolarian chert sections. The macroaggregates originated from decomposing plankton bloom biomass comprised mainly of large acanthomorph acritarchs with an admixture of radiolarians and graptolites, detrital quartz, and amorphous organic matter. After accumulation on the sea floor the macroaggregates were overgrown by mats of benthic cyanobacteria and became a source of phosphate and barium diffusing to the surrounding sediment, causing phosphatization of the large bloom-forming acanthomorph acritarchs buried in their proximity. Modern macroaggregates show extremely low fossilization potential and the finding of comparable early Silurian material represents a unique preservation explainable by their rapid burial and very early silicification. The presence of fossil macroaggregates in black radiolarian cherts is testimony of periodic high productivity within early Silurian epicontinental seas. [Copyright &y& Elsevier]

Abstract: Devonian radiolarian ribbon cherts are found as olistoliths and as a thin (<100m) tectonic slice in pervasively deformed sandstone and shale in a Triassic subduction-accretion complex (Upper Karakaya Complex) in Northwest Turkey. The subduction-accretion complex also comprises exotic blocks of Lower Carboniferous and Upper Permian limestone. It lies tectonically over a thick metabasite series and is unconformably overlain by little deformed continental to shallow marine sedimentary rocks of Jurassic age. The Devonian radiolarian cherts, along with the earlier descriptions of Carboniferous and Permian radiolarian cherts from the Karakaya Complex suggest the subduction of a Late Paleozoic ocean, the Paleo-Tethys, along the southern margin of the Pontides. [Copyright &y& Elsevier]

Cherts in the Laobao Formation in the Silikou section, South China were deposited in terminal Ediacaran deep-water. The speciations of iron in cherts, such as pyrite Fe (FeP), HCl-extractable Fe (FeH), and total Fe (FeT), and their interrelationships, are proxies for distinguishing oxic, anoxic but ferruginous, and sulfidic conditions, which is an effective method for studying the terminal Ediacaran ocean environment. For chert samples from the Laobao Formation, pyrite concentrations are generally low (<0.56%), degree of pyritization (DOP) ranges from <0.01 to 0.78 and most are <0.45; (FeP+FeH)/FeT ratios are between 0.27 and 0.84 and most exceed 0.38, and most of FeT/Al ratios range from 0.55 to 5.87 (averaging 1.1). Relatively high content of highly reactive Fe and low content of pyrite indicate a scarcity of H2S and Fe-enriched deep-water. These features suggested that the deposition setting of Laobao cherts was anoxic and ferruginous. Terminal Ediacaran deep-water in the South China Basin was most likely anoxic and ferruginous, which played a key role in the emergence and evolution of early multicellular life. [ABSTRACT FROM AUTHOR]

In this study, siliceous nodules from the world-famous Myrtos beach, as well as from Avithos beach, in the western flanks of Kefalonia Island in Greece are examined by means of petrographical, mineralogical, geochemical and micropaleontological methods. The objectives of this study are to characterize the textural and compositional features of the nodules, with the aim to provide an initial interpretation of their origin and their diagenetic evolution. The studied siliceous nodules are hosted within Lower Cretaceous thin-bedded limestones at Myrtos and Upper Eocene limestones at Avithos. Nodules from both areas display a characteristic concentric texture at a macroscopic and microscopic scale. They both have a dense fine-grained siliceous sedimentary fabric, composed mainly of microcrystalline or cryptocrystalline quartz and moganite with common residual calcite in the case of Avithos. These results, and in particular the shape of the nodules, along the textural and compositional characteristics, indicate different conditions of formation in the two localities, both during the early epigenetic stages, as well as later during the diagenetic processes. Myrtos nodules originated from Si-precursors deposited in a pelagic environment, going through intense Si-replacement. Avithos nodules were deposited in a more proximal environment, being influenced by a less intense silicification. Nevertheless, the higher degree of recrystallization of Avithos samples indicates a syn- or post-diagenetic tectonic activity that resulted in the circulation of geothermal fluids. The conclusions drawn from this work demonstrate the usefulness of thorough studies of siliceous nodules in order to get a more comprehensive understanding of the initial depositional conditions, as well as diagenetic pathways and processes. [ABSTRACT FROM AUTHOR]

Abstract: A survey of the anatomically preserved flora of the Grand-Croix cherts (Late Pennsylvanian, Saint-Etienne basin, France) is presented following a study of collections preserved in the Swedish Natural History Museum, Stockholm in complement to previous investigations on the original material kept in France. An exhaustive list of the flora is provided, confirming the particular importance of this permineralized plant assemblage that is one of the most diverse for the Late Pennsylvanian. The observed occurrence of taxa and organs confirms a previous quantitative study based on material in the French collections: cordaiteans are both the most common taxa and the most important contributors to peat volume, whilst leaves are the most common and abundant organs. However, the seed ferns are the most diverse group of plants before cordaiteans and ferns; the exceptional diversity of medullosan ovules is of particular interest. The Grand-Croix flora is compared with that of the nearby Early Permian basin of Autun. The two assemblages have 21 genera, but only 6 species, in common, and this may be explained by differences in age and taphonomy. The Grand-Croix flora is also similar to that of American coal balls of the same age and, despite taxonomic differences, it is significant that several species of ferns and seed ferns are common to the vegetation of the French limnic and American paralic basins. [Copyright &y& Elsevier]

The reported data present the stratigraphy of several sections across a Middle-Late Jurassic Radiolaritic Unit, well exposed in different thrust sheets pertaining to the Maghrebian chain of Southwestern Sicily. The aim was to define the chronostratigraphical distribution of the Jurassic biosiliceous sedimentation in the Sicanian palaeogeographical zone, a deep water basin belonging to the Southern Tethys continental margin. The radiolarian biostratigraphy indicates that the switching from carbonate to siliceous sedimentation in the Sicanian Basin is referable to the Bajocian, as shown by the section of Campofiorito, near Corleone. The biostratigraphical dataset allows the correlation between the onset of biosiliceous sedimentation and the fall of biodiversity in the Sicanian basin with the carbonate productivity crisis, indicated by the highest eutrophication that affected Western Tethys during Middle Jurassic times. Sono state studiate alcune sezioni dell’Unitá radiolaritica di etá Giurassico Medio-Superiore del Bacino Sicano, affioranti nel settore esterno della catena Appenninico-Maghrebide in Sicilia sud-occidentale. Lo scopo di questo lavoro è di definire la distribuzione cronostratigrafica dell’intervallo siliceo in questo settore paleogeografico della Tetide. In queste successioni la distribuzione di ben preservate associazioni a radiolari, consente di riferire la base delle radiolariti al Bajociano (sezione di Campofiorito). Questo dato rafforza l’ipotesi di una possibile crisi di produttività carbonatica, dovuta a eutrofizzazione, di estensione regionale nella Tetide occidentale durante il Giurassico Medio. [ABSTRACT FROM AUTHOR]

Abstract: The recognition of five standard conodont biozones and five radiolarian zones in the same set of samples from cherts within the Frasnian–Famennian (F–F) boundary interval at Bancheng, southeastern Guangxi, China allowed precise placement of the F–F boundary, and demonstrated the multi-episodic pattern of the F–F mass extinction. Frasnian pelagic radiolarian and conodont faunas had an acme in the lower Palmatolepis linguiformis Zone. In the upper Pa. linguiformis Zone, until the level of the upper Kellwasser extinction (Kw.), members of the radiolarian family Ceratoikiscidae and of the conodont Frasnian populations disappeared. Above the Kw, in the lower Palmatolepis triangularis Zone, new populations of the conodont Pa. triangularis gradually appeared and the survivor radiolarian genus Trilonche gradually re-appeared, both blooming in the lower part of the middle Pa. triangularis Zone. The Nehden extinction (Nd) in the uppermost middle Pa. triangularis Zone interrupted the recovery of the conodont and radiolarian faunas, but above this level the conodont species became more numerous, and the post-extinction radiolarian faunas (including Entactiniidae) recovered gradually. [Copyright &y& Elsevier]

Abstract: New data gathered during mapping of c. 3490–3240 Ma rocks of the Pilbara Supergroup in the Pilbara Craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism. For most cherts, fossil hydrothermal fluid pathways are preserved as silica±barite±Fe-bearing veins that cut through the footwall and up to the level of individual bedded chert units, but not above, indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events. Silica±barite±Fe-bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units, and occurred under alternating high-sulphidation and low-sulphidation conditions. These veins provided pathways to the surface for elements leached from the footwall (e.g., Si, Ba, Fe) and volcanogenic emissions from underlying felsic magma chambers (e.g., CO2, H2S/HS−, SO2). Stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of Warrawoona Group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and/or degassing of these magmas. Heat from these intrusions drove episodes of hydrothermal circulation. Listric normal faulting during caldera collapse produced basins with restricted circulation of seawater. Eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components, thereby providing habitats suitable for early life forms. Fossil stromatolites from two distinct stratigraphic units in the North Pole Dome grew in shallow water conditions, but in two very different geological settings with different morphologies. Stratiform and domical stromatolites in the stratigraphically lower, c. 3490 Ma, Dresser Formation of the Warrawoona Group are intimately associated with barite and chert precipitates from hydrothermal vents, suggesting that component microbes may have been chemoautotrophic hyperthermophiles. Evidence of shallow water to periodically exposed conditions, active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water, tectonically active caldera and concentrated therein to produce an extreme habitat for early life. Widespread conical and pseudocolumnar stromatolites in the c. 3400 Ma, Strelley Pool Chert at the base of the unconformably overlying Kelly Group occur in shallow marine platform carbonates. Silicification was the result of later hydrothermal circulation driven by heat from the overlying, newly erupted Euro Basalt. The markedly different morphology and geological setting of these only slightly younger stromatolites, compared with the Dresser Formation, suggests a diversity of microbial life on early Earth. The biogenicity of putative microfossils from this and younger hydrothermal silica veins in the Warrawoona Group remains controversial and requires further detailed study. [Copyright &y& Elsevier]

Stephen Jay Guld, the prokaryotes and their evolution in a geological context. Stephen Jay Gould (Full House, Harmony Books, New York, 1996) emphasised the importance of the bacterial (prokaryote) world right from the beginnings of the history of life, up to the present day and, even into the future. Moreover, he suggested that the various forms of life on the planet today represent a diversification, or increase in complexity, from these simple organisms, rather than a directed evolution towards complexity. On the scale of evolution, the oldest fossils, dating back to almost 3.5 billion years ago, comprise simple unicellular organisms, i.e. prokaryotes that occur adjacent to the left wall of complexity. As far as can be ascertained from the fossil record, this early life had already exploited all the possible ramifications of diversification within an anaerobic environment and within the habitats available on the early Earth. Further evolution from this stage involved the development of organisms undertaking oxygenic photosynthesis that opportunistically invaded the newly formed, sunlight-bathed, shallow water continental platforms. To cite this article: F. Westall, C. R. Palevol 2 (2003). [Copyright &y& Elsevier]

Repetitive sequences of carbonate deposits, occurrence include in the lower part of the Paleogene Carbonate Unit (northeast border of the Madrid Basin), have been studied, defining regressive lacustrine sequences and early diagenetic processes. Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray observations, and X-ray diffraction analyses joint to isotopic studies (δ18O and δ13C) have been used to characterise the facies and environments.The sequences consist of a lower uncemented carbonate mud unit, calcitic or dolomitic in composition, and an upper carbonate unit (arenites, bioclastic limestones, and microbial laminated limestones with cherts). Visual features (vertical prismatic structures, fissures, massive nodulization, rhizoliths, brecciation) and microscopic features (micrite micronodules, vug porosity, circumgranular cementation, gypsum lenticular crystals) outline pedogenic processes. These features, found locally in bioclastic and microbial laminated limestones or in dolomite uncemented muds, define, respectively, palustrine limestones or dolocretes. Facies analysis allows us to define several lacustrine sub-environments (basinal, littoral, eulittoral and supralittoral) and characterises different types of shallowing upward lacustrine sequences, either with or without subaerial exposure. The isotopic values of the bioclastic and microbial laminated limestone (from −5.77‰ to −6.78‰ for δ13C, and from −5.25‰ to −5.53‰ for δ18O) and those of uncemented calcitic muds (from −5.80‰ to −7.01‰ for δ13C, and from −4.98‰ to −5.58‰ for δ18O) establish that both types of carbonates precipitated in equilibrium with meteoric waters. In the palustrine carbonate deposits, the δ13C values suggest a strong organic contribution.The dolomitization that only affected the uncemented carbonate muds is early interpreted because of the structural and compositional characteristics of the dolomite (micro-rhombic dolomicrite, nearly stoichiometric and poorly ordered), and because the calculated average Δ18O dol–cal for calcitic and dolomitic uncemented muds is about 6‰. The δ18O isotopic values indicate that the dolomite precipitated from water that was slightly more enriched in heavy isotopes than the calcite, because of an increase in evaporation rates.Nodules and nodular levels of cherts occur in the upper units of littoral and eulittoral sequences, probably as a consequence of the existence of microbial mats that could include the silica source. According to their mineralogy (Opal CT and quartz/moganite) and structures (double nodules, lamination and bioturbation), three types of chert are described (TB, MB and WO). These types define three general stages of silicification during the early diagenesis, from the recently buried to the postcompactation of carbonate deposits. The δ18O values of quartz show that the silicification and ageing of opaline phases occurred from meteoric waters, which were lighter than the calcite or dolomite precipitating waters. The cherts included in palustrine limestones show δ18O values of quartz that record more evaporated waters than those of the general stages of silicification. [Copyright &y& Elsevier]

Archean carbonaceous meta-cherts (upper-greenschist to amphibolite facies) from the Nickol Well Unit, the Cleaverville Group (∼3.2Ga) in the Roebourne district in West Pilbara are characterized by variable Al2O3/TiO2, Zr/TiO2 and Cr/Th values and by high concentrations of carbon and some trace metals. Distribution patterns of the cherts on a newly proposed diagram (Al2O3/TiO2–element/Al2O3) indicate that concentrations of Cr, V and Ni can be explained by contributions of clastic components, whereas concentrations of Cu, Zn and Pb are much higher than those expected solely from clastic contributions. Detrital material in the cherts was derived from a wide variety of volcanic sources (mafic–ultramafic–felsic sources) consistent with the inferred geological setting of the Cleaverville Group (immature island arc setting). Excess Cu, Zn and Pb are probably from hydrothermal solutions, as suggested by positive Eu anomalies and enrichment in heavy rare-earth elements (REE) in chondrite-normalized patterns. High concentrations and light isotopic values of C (−17 to −24 per mil) in the cherts indicate that the precursory sediments were enriched in organic materials. Although it is unclear whether the organic material was derived from organic particles that settled from the ocean surface or benthic microbes, in either case, it is possible that this material played an important role in the accumulation of heavy metals (Cu, Zn and Pb) in the sediments. [Copyright &y& Elsevier]

Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO2 (78.40%-89.28%) and high Al2O3 (3.42%-11.02%). Low Si/Al ratios (6.3-23) and tight negative correlation between Si/Al and Al2O3 of the samples indicate that they are muddy cherts containing high and variable contents of pelitic detritus. Geochemically, they are characterized by Al2O3/(Al2O3+Fe2O3) = 0.51-0.90, shale-normalized Lan/Cen = 0.76-1.11, Ce/Ce* = 0.91-1.22, V<20μg/g, V/Y<2.6 and Ti/V>40, resembling those of cherts formed in the continental margin regimes. It is therefore concluded that these late Paleozoic radiolarian muddy cherts were most likely formed in a continental margin regime, and not genetically related to the ophiolite suite in NE Jiangxi. It is also unlikely that an oceanic basin existed between the Yangtze and Cathaysia blocks during the late Paleozoic. [ABSTRACT FROM AUTHOR]

Graphitic cherts are interbedded within terrigenous sediments in the Cadomian orogenic belt of end-Proterozoic age. In the Armorican Massif (NW France), the graphitic cherts are of two types: massive cherts essentially composed of quartz (SiO2 > 96%) and with rare sedimentary structures; laminated cherts containing up to 3·4% Al2O3 and 92–98% SiO2. Sedimentary structures observed in the laminated cherts are indicative of a restricted hypersaline tidal or supratidal environment. The origins of both types of chert are to be found in the diagenetic processes of silification of terrigenous and mixed terrigenous-evaporitic facies. These processes, which could be mediated by the presence of organic matter, were controlled by the migration of the freshwater/saltwater mixing zone during periods of relative sea-level change. The proposed diagenetic origin for the cherts places a number of constraints on their use in the establishment of stratigraphic correlations. [ABSTRACT FROM AUTHOR]