Chert Formation Research Articles

Geochemistry of cherts from the northern Jiangxi region, South China: Implication for paleoenvironment

The extensive bedded cherts deposited during the Ediacaran–Cambrian (E–C) transition period play a crucial role in understanding the geological evolution of this period, yet the origin of these cherts remains disputed. Here, we present new geochemical data for cherts of the Piyuancun (PYC) Formation deposited during the Late Ediacaran and the Hetang (HT) Formation deposited during the Early Cambrian in northern Jiangxi region, Lower Yangtze region, South China. The PYC cherts contain a small amount of monaxons sponge spicules and radiolarian fragments, while the HT cherts lack siliceous organism evidence. Major and trace element analysis, coupled with discriminant diagrams, indicate a possible shift in redox conditions of seawater during the E–C transition in the northern Jiangxi region. The shift suggests a change from weakly–moderately restricted euxinic conditions to strongly restricted euxinic conditions. Furthermore, the location of both cherts are distant from the source area of siliceous organisms. Fossil evidence, as well as the values of Fe/Ti and Fe/(Mn+Ti), Eu anomalies, Post-Archean Australian Shale (PAAS) normalized REE+Y patterns, and various discriminant diagrams, support the conclusion that the PYC and HT cherts originated primarily from direct seawater precipitation, with the PYC cherts exhibiting weak hydrothermal evidence. Upwelling contributes to the formation of HT cherts and organic matter (OM) accumulation. Ocean acidification, triggered by OM degradation and biodegradation processes during the E–C transition period, leads to the extensive silica precipitation and preservation. These results enhance our understanding of the geological processes during the E–C transition.

Nanoscale characterization of Ediacaran microfossils from lower Doushantuo formation chert nodules

Microfossils identification, especially in very old rocks, is extremely challenging because morphological and chemical signatures of microfossils are subtle and inevitably altered with aging. Chert nodules from the Doushantuo Formation in the Yangtze Gorges area of South China have captured significant interest due to their remarkable preservation of Ediacaran microfossils. To understand the taphonomic processes leading to an exceptional preservation of microbes in these rocks, we studied the morphological and chemical features of microfossils within the lower Doushantuo Formation chert nodules down to atomic scales via advanced microscopy techniques. Our results align with previous studies, confirming that most microfossils there are preserved by silicification. Further analysis of a representative filamentous microfossil, Polytrichoides lineatus, reveals that both the cell wall (or extracellular sheath) and cytoplasm are preserved by loose aggregates of subrounded or irregular quartz grains, along with patchy organic residuals, which are distinctive from the closely packed and angular-shaped quartz grains in the non-fossil matrix of the chert nodule. The cell wall or extracellular sheath likely provided narrower spaces and more nucleation sites for silicification, resulting in smaller quartz grains (i.e., 115 ± 42 nm) compared to those formed in the cytoplasm (i.e., 1150 ± 258 nm). The permeation and precipitation of quartz grains within the cell wall or extracellular sheath could contribute to an exceptional preservation of subcellular structures. This study offers valuable insights into the preservation of microbes in the Doushantuo Formation chert nodules and even older siliceous sedimentary rocks.

Opaline silica precipitations in the Permian Fengcheng Formation indicate hot spring environments in north Mahu Sag, NW China

The Permian Fengcheng Formation represents a series of deposits formed in an alkaline saline lake, characterized by high proportions of cherts, located in the Northwestern Junggar Basin in China. While discussions on silica-rich hot springs as sources of silica for brine and subsequent chert deposition have been extensively conducted for Quaternary and Mesozoic-Cenozoic alkaline saline lakes worldwide, investigations into Paleozoic alkaline saline lakes are scarce due to challenges associated with identifying evidences of hot spring environments. In this study, we present detailed SEM-EDS analyses and examine silica isotope compositions as well as oxygen isotope compositions to discuss hydrothermal-associated silica precipitation processes. Our findings suggest that migrating silica-rich hot springs along deep-rooted faults released silica into the brine, leading to chert deposition during cooling periods and induction of thermophilic microorganism degradation. Microorganism-associated siliceous precipitates primarily consist of opal-A spheres adhered by extracellular polymeric substances (EPS), while numerous microorganism spheres have undergone silicification. Opal-A spheres within these structures were partially transformed into well-crystallized quartz through multiple stages of dissolution-precipitation processes during prolonged burial history. Furthermore, our results indicate that deep-rooted fault-related hot springs can occur both in marginal and subaqueous areas within alkaline saline lakes, with chert formations observed in shallow-marginal saline lake settings as well as deeper regions. The findings of this study provide valuable insights into the paleoenvironmental conditions, which can be further explored in investigating the accumulation of organic matter in chert-rich successions formed within an alkaline saline lake.

Formation conditions of Jixian System cherts in the Qishan area, Ordos Basin: Implications for marine redox conditions and paleoecology

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 Siex 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 SiO2 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.

Primary microfossiliferous chert in the Aptian Barra Velha Formation

ABSTRACTThe Barra Velha Formation and other Aptian pre‐salt deposits record the history of the proto‐Atlantic basin and the rifting of Gondwana. Studies have sought to characterize the depositional environment of the basin with a focus on carbonate fabrics and magnesium silicate clays. However, the water chemistry and fluid sources in the basin, the silica cycle, and how the basin evolved over time are not fully constrained. Additionally, current understanding of the microbiota that inhabited this basin is incomplete because microfossils have rarely been identified in pre‐salt deposits, especially on the Brazilian margin. This study describes authigenic chert in the Barra Velha Formation that preserves distinct, organic‐rich structures and textures. The petrographic relationships between the chert and carbonate suggest that both formed as authigenic phases, but their formation was temporally decoupled. These relationships and δ30Si and δ18O data suggest that chert post‐dates the formation and subsequent dissolution of the carbonates, and may have formed from a different fluid. By characterizing the chert–carbonate paragenesis and mechanism of chert formation, this study provides new insights into the fluid sources and complexity of the basin. Together, the results of this research suggest that the chert precipitated as primary, authigenic phases after karstification of the carbonate from a newly introduced, low temperature, freshwater fluid that was chemically distinct from the lake water. The chert preserves organic matter that is compositionally and texturally distinct from the void‐filling bitumen associated with the classically studied carbonate facies. Based on the composition and morphologies of organic structures, this is likely primary organic matter and a morphologically diverse microfossil assemblage preserved in place at the time of chert formation. Thus, this early chert provides new insights into the water chemistry, fluid sources and silica cycle in the basin, and represents a unique taphonomic window that helps us characterize the pre‐salt basin microbiota.

Formation of Ediacaran stromatolitic cherts through a combination of biogenic and abiotic processes: New insights from the Bohemian Massif

Stromatolites are remarkable organosedimentary laminated structures that grow gradually through time due to the activity of microorganisms, mostly algae and cyanobacteria. The appearance of stromatolites in formations as old as ∼ 3.7 Ga represents the earliest macroscopic traces of life on Earth. Yet, the biogenic origin of some presumed stromatolites has been questioned as abiotic processes (e.g., fluid precipitation, diagenesis and metamorphism) can also be responsible for the formation of stromatolite-like textures. This is especially true in case of pervasively silicified stromatolites (stromatolitic cherts) that have rarely been reported from Neoproterozoic successions. Here, we present a detailed study of the Ediacaran stromatolitic cherts from the Blovice accretionary complex (Bohemian Massif) that combines petrography, trace element and Si–O isotopic compositions and Raman spectroscopy of organic matter to decipher the complex processes leading to their formation and evaluate the biogenicity of their precursor. Textural evidence (i.e. the presence of pseudomorphs after carbonate/evaporate and micro-ooids, variable thickness and spacing of organic-rich laminae) as well as trace element/isotopic compositions argue for a multi-stage formation of stromatolitic cherts. The primary carbonaceous stromatolitic precursors first formed in shallow-water lagoons with variable salinity at seamount slopes, were then fragmented by surface processes (erosion, mass wasting), and finally silicified by low-temperature hydrothermal fluids. The primary biogenic character of stromatolitic textures is evidenced by the non-equidistant alternation of dark (organic matter-rich) and light laminae with markedly different oxygen isotopic compositions reflecting probably seasonal cycles during stromatolite deposition, enrichment of bio-essential elements (e.g., Cu, Zn, Co and Mn) in dark laminae, and Raman spectra of organic matter indicative for variably matured kerogens. The presence of late-stage diagenetic modifications might be related to subsequent burial and low-grade metamorphism of stromatolitic cherts within the accretionary wedge.

Creating frames of reference for chert exploitation during the Late Pleistocene in Southwesternmost Iberia.

Southwestern Iberia has played a key role in characterizing Late Pleistocene human ecodynamics. Among other aspects of human behavior, chert procurement and management studies in this region have received increasing attention in the past two decades, especially focusing on the sites showing repeated human occupation, such as the case of Vale Boi (Southern Portugal). However, these studies have been very limited in their geographical scope, and mostly focused on brief macroscopic descriptions of the raw materials. To further our knowledge of the relationship between regional availability of raw materials and its impact on human adaptations and mobility, a more detailed approach to characterizing geological sources is needed. This paper characterizes chert raw materials location, diversity, and availability in a geologically well-defined region of southern Portugal - the Algarve. Through macroscopic and petrographic approaches, we provide a detailed characterization of geological chert sources to build a frame of reference for chert exploitation in the region. Our results show that there are four main chert formations in Algarve, and that despite the within-source variability, sufficient differences at macroscopic and petrographic levels are present to allow clear source attribution. These results provide a baseline for raw material studies in archaeological assemblages across southwestern Iberia, that will be essential to further characterize the dynamics of human behavior in some of the most important eco-cultural niches.

Influence of the parent rock nature on the mineralogical and geochemical composition of ferralsols used for sedentary agriculture in the Paleoproterozoic Franceville sub-basin (Gabon)

The aim of this study was to assess the influence of parent rock nature on the mineralogical and geochemical properties of ferralsols used by sedentary subsistence farmers in the Proterozoic Franceville subbasin of Gabon. Thus, soils developed from cherts, black shales and sandstones were investigated for their mineralogical composition and their heavy metal contamination of topsoil using the X-ray diffraction (XRD), and Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). Results show that the dominant mineralogical assemblage is made of quartz, illite, kaolinite and smectite, and this is reflected in the major-element chemistry which include essentially SiO2 (46.4 - 89.2%), Al2O3 (4.3 - 19.8%) followed by Fe2O3 (0.7 - 15.3%), with highest amounts of SiO2 and lowest amounts of Al2O3 and Fe2O3 found in soils developed from sandstone. The geochemical data revealed some doleritic intrusions through the chert formations with highest values of Fe2O3 and TiO2 in the overlying soils. Results indicated serious health concern associated with a geogenic source of As, Ba, Cd, Cu, La, Pb, Rb, Th and U in soils developed from cherts and black shales. Consequently, only the uncontaminated soils developed from sandstone could be appropriate for smallholder farming communities.

Seawater silica cycling and chert formation at the Neoproterozoic–Cambrian transition: Insights from δ30Si and Ge/Si systematics of hydrothermal cherts from the Bohemian Massif

The genesis of cherts, rocks with >90 wt% SiO2, is suggested to be linked with dissolved seawater silica and its elevated precipitation in the Phanerozoic due to massive biological activity in the Earth's oceans compared to the Precambrian where hydrothermal fluids played a more dominant role. Because cherts may record a history of global seawater Si cycle, their Si isotope systematics may serve as a proxy for their origin and depositional environment. In this study, Si isotope compositions, combined with Ge/Si ratios, were determined for a suite of shallow-water, deep-water and stromatolitic cherts from the Blovice accretionary complex (Bohemian Massif, Czech Republic), representative of deep marine and subduction settings, to establish the nature of cherts and processes of their formation, identify the sources of Si and Ge, and discuss the possible effect of the Neoproterozoic–Cambrian transition. The predominantly high δ30Si values > +0.23‰, associated with low Ge/Si, in deep-water cherts suggest their precipitation from seawater-derived low-temperature fluids at the seawater–sediment interface with subordinate admixture of terrigenous material. Moreover, the tight relationship between δ30Si and organic matter contents in these deep-water cherts is indicative of either equilibrium precipitation of isotopically lighter Si as the organic content of the medium increases or global decrease of δ30Si values of seawater during the Meso–Neoproterozoic related to coupled Si and C cycles. On the other hand, a δ30Si variation of ∼2.5‰ (−0.94‰ to +1.60‰) in shallow-water cherts argues for precipitation from seawater interacting with hydrothermal fluids of variable temperature. Organic matter, Fe-(oxy)-hydroxides and clays represent important sinks for Ge (and Si in case of organic matter) and thus play a determining role in global marine Si−Ge cycling. This, together with the presence of abundant cherts at the Neoproterozoic–Cambrian transition, appears to be associated with redox-controlled Fe cycle which itself is related to microbial reduction and the increasing presence of organic matter towards the Phanerozoic.

Discriminating chert origins using machine‐learning approaches

Automatic techniques have not been applied in the geochemical discrimination of marine, terrestrial and hydrothermal cherts, which is critical to retrieving the paleoenvironmental information of ancient sedimentary basins. Here, we applied three supervised machine‐learning methods to analyse the labelled geochemical compositional data (32 major and trace elements) of cherts, including the support vector machine (SVM), sparse multinomial regression and linear discrimination analysis. Our results show that these machine‐learning methods can successfully discriminate different chert types, as the lowest classification score is 79%. Among the methods, the SVM is the most powerful approach (classification score 88%, on average), even though it is hard to be geochemically interpreted. Sparse multinomial regression and linear discrimination analysis are also efficient approaches (mean classification score 84%), which can be used to extract the important elements and the most representative sample for each chert type. Especially, the linear discrimination analysis can construct binary discrimination diagram superposed with biplot rays and posterior probability‐based decision boundaries, which could be quickly applied and explained by other researchers. Furthermore, the successful discrimination of different chert types indicates that cherts derived from different silica sources have different geochemical signatures, albeit mixing between different sources of silica during chert formation is common. Last, the general geochemical features of each chert type were summarised based on geochemical information extracted by machine‐learning methods and previous geochemical studies.

Triple silicon isotope insights into the formation of Precambrian cherts

Cherts are pervasive in the Precambrian and their O and Si isotopic compositions have been used extensively for reconstructing seawater paleo-temperatures. However, these reconstructions are obscured by the fact that cherts exhibit, at a given age, a large isotopic (O and Si) heterogeneity at bulk and micrometer scales. This is understood as reflecting the fact that cherts are an assemblage of silica precursors having different origins, and that diagenesis and metamorphism have modified the isotopic compositions acquired during the last equilibration with seawater. To test the contributions of the different processes that control the Si isotopic composition of cherts, we developed high-precision triple Si isotope measurements by Multicollector-Inductively Coupled-Plasma Mass-Spectrometer (MC-ICP-MS). This approach allows to identify deviations from the equilibrium Si isotopic fractionation at the level of a few ppm. Triple Si isotope data combined with trace elements and O isotope data in Precambrian cherts demonstrate that three different sources of silica were involved in the formation of cherts, and that both equilibrium and kinetic isotopic fractionation took place for Si. Archean seawater is predicted to have a triple Si isotopic composition kinetically fractionated relative to bulk silicate Earth. This requires a large kinetic isotopic fractionation during hydrothermalism of the oceanic crust and appears consistent with warm oceans in the Precambrian. However more work is required on the triple isotope systematics of Si inputs and output to seawater by rivers and authigenic clays formation. Triple Si isotope variations can thus be used as a new tool to unravel the origin of cherts and of their O and Si isotopic compositions. The present evidence of kinetic isotopic fractionations for Si isotopes also implies that Si isotopes should be associated to triple O isotope for paleotemperature reconstructions.

Conditions for the formation of siliceous rocks of the Lower-Middle Cambrian in the southeast of the Siberian Platform

Based on the study of microstructural and geochemical features of bedded cherts of the Inikan Formation (Є 1–2 in ) from coastal outcrops of the river Yudoma found that the key role in their formation apparently belongs to siliceous organisms — radiolarians and sponges. The Ge/Si ratio indicates that there is no direct contribution of fluids to the formation of cherts in this part of the paleobasin. The studied chalcedony-quartz rocks were formed from primary biogenic silica subjected to diagenetic redistribution and post-diagenetic processes.

EOCHEMICAL, MINERALOGICAL AND DIAGENETIC CHARACTERISTICS OF MARINE CHERT IN THE HATAY REGION, S-TURKEY: ITS ORIGIN AND DEPOSITIONAL ENVIRONMENT

Trace elements and rare earth elements (REEs) were studied to determine the formation of chert and siliceous limestones, which are predominantly observed in the Okçular formation. Cherts are divided into four types according to their varying forms depending on the deposition and diagenesis stages: (Type-I) layered cherts formed by direct precipitation from sea water, (Type-II) nodular cherts with rim formed by replacement, (Type-III) homogeneous chert nodules without rim, (Type-IV) chert micro nodules with high porosity. The porosity of stylolite-fracture networks and the chemical composition of seawater played a role in the formation of different types and sizes in cherts. The rim of Type II nodular cherts has significant color changes and porosity up to 20%. Anomalies of major and trace elements, especially La, Y, Sc, Ce, Th, U and Gd values, indicate direct deposition and replacement of early diagenetic pelagic deposits of cherts in the oxidized marine environment. Total (REE)+Y values, high LREE/HREE ratio and low Ce values indicate that dissolved seawater of chemical or biogenetic origin has a role in silicification. According to the similarity of the hierarchical cluster analysis of the chemical contents of the Okçular formation and the ophiolites, the silicon in the chert formation originates from the ophiolites.

Multiple sulphur isotope record of Paleoarchean sedimentary rocks across the Onverwacht Group, Barberton Greenstone Belt, South Africa

This study presents multiple sulphur isotope (32 S, 33 S, 34 S, 36 S) data on pyrites from silicified volcano-sedimentary rocks of the Paleoarchean Onverwacht Group of the Barberton greenstone belt, South Africa. These rocks include seafloor cherts and felsic conglomerates that were deposited in shallow marine environments preserving a record of atmospheric and biogeochemical conditions on the early Earth. A strong variation in mass independent sulphur isotope fractionation (MIF-S) anomalies is found in the cherts, with Δ33 S ranging between -0.26‰ and 3.42‰. We explore possible depositional and preservational factors that could explain some of this variation seen in MIF-S. Evidence for microbial activity is recorded by the c. 3.45 Ga Hooggenoeg Formation Chert (HC4) preserving a contribution of microbial sulphate reduction (-Δ33 S and -δ34 S), and a c. 3.33 Ga Kromberg Formation Chert (KC5) recording a possible contribution of microbial elemental sulphur disproportionation (+Δ33 S and -δ34 S). Pyrites from a rhyo-dacitic conglomerate of the Noisy Formation do not plot along a previously proposed global Felsic Volcanic Array, and this excludes short-lived pulses of intense felsic volcanic gas emissions as the dominant control on Archean MIF-S. Rather, we suggest that the MIF-S signals measured reflect dilution during marine deposition, early diagenetic modification, and mixing with volcanic/hydrothermal S sources. Given the expanded stratigraphic interval (3.47-3.22 Ga) now sampled from across the Barberton Supergroup, we conclude that large MIF-S exceeding >4‰ is atypical of Paleoarchean near-surface environments on the Kaapvaal Craton.

A Pennsylvanian saline-alkaline lake in Gondwana mid-latitude: Evidence from the Piauí Formation chert deposits, Parnaíba Basin, Brazil

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.

Geochemical evidences on the origin and paleo‐oceanic depositional setting of the Naga Hills Ophiolite cherts, North East India

The Jurassic‐Cretaceous chert of the Naga Hills Ophiolite (NHO) is tectonically juxtaposed with the mafics (mostly basalts) and ultramafics along the Indo‐Myanmar Ranges (IMR), which constitute the southeastern extension of the Indus‐Yarlung‐Tsangpo Suture. Petrographic and geochemical studies were taken up to determine their origin and paleo‐oceanic depositional environments. Cryptocrystalline quartz is intimately mixed with iron oxides and clay minerals containing poorly preserved radiolarians. Geochemical signatures indicate a predominantly biogenic origin for the NHO cherts, which were derived from radiolarians and other siliceous microfossils. Rare earth elements and trace element abundances indicate predominant contribution by scavenging from seawater with the minor influence of low‐temperature hydrothermal activity. Most of the NHO cherts were deposited in an open ocean basin where oxic conditions prevailed. Geochemical signatures endorsed by field relations of chert, pillow basalts, and limestone within the oceanic plate indicate that chert formation continued in the vicinity of oceanic islands. Organic matter played a pivotal role in regulating oxic‐anoxic conditions during deposition, as evidenced by redox‐sensitive trace elements. These cherts, envisaged to have been originally part of the Tethys, were tectonically emplaced during the Middle Eocene. They now constitute part of the NHO along the IMR.

Protracted formation of nodular cherts in marine platform: new insights from the Middle Permian Chihsian carbonate successions, South China

Protracted formation of nodular cherts in marine platform: new insights from the Middle Permian Chihsian carbonate successions, South China

Origin of the Multiple-Sourced Cherts in Maokou Carbonates in Sichuan Basin, South China

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.

Quartz Crystallinity Index of Arabian Sands and Sandstones

AbstractAlthough quartz crystallinity index (QCI) was developed more than four decades ago, it remains a relatively new tool used in provenance studies. This study examines QCI values of Arabian sands (Quaternary sands from Nafud, Dahna, and AlRub'AlKhali deserts), and Arabian sandstones (Paleozoic and Mesozoic sandstones from northern and central Arabia). QCI values of sandstones range from 8.2 to 9.8. For Quaternary occurrences, QCI values of Nafud sands range from 7.4 to 9.5, Dahna sands range from 7.9 to 9.4, and AlRub'AlKhali sands range from 8.8 to 9.4. The data confirm that Arabian sands are derived mainly from early Paleozoic sandstones, with subordinate contributions from late Paleozoic and Mesozoic sandstones. They also highlight the role that sediment homogenization plays in narrowing QCI values of AlRub'AlKhali sands. Highest and lowest QCI values raise questions about the role of chert in controlling of QCI values of Arabian sands and sandstones. Arabian sandstones of Late Ordovician, Late Devonian, Late Permian, Late Triassic, and Cretaceous age possess the highest QCI values (&gt;9.5). These time intervals coincide with lower abundances of siliceous sponges, and therefore fewer potential sources of chertification/silicification. Cambrian sandstones and Quaternary sands possess the lowest QCI values (&lt;8.5). This coincides with the fact that both are of eolian origin, environments, which usually contain large quantities of quartz‐rich dust that are documented to be a primary source of silica for chert formation. Therefore, we propose that the presence of chert is responsible for the extreme values of QCI in Arabian sands and sandstones.

Geochemical characterization of the El Cortijo Formation metacherts: New evidence of an oceanic rock sequence in the Palaeoproterozoic basement of the Tandilia Belt, Argentina, Río de la Plata Craton

Archaean-Palaeoproterozoic cherts are rare and contain valuable information to understand the chemical evolution of oceans in the past, given that during most of this period an abiological deposition process has been interpreted for their formation, in which silica precipitated directly from seawater. In the Palaeoproterozoic basement of the Tandilia Belt (Argentina), Río de la Plata Craton (RPC), cherts have been mentioned as part of the El Cortijo Formation, which includes low-grade metamorphic rocks, such as metacherts, metawackes and metabasites. In order to provide conclusive evidence about the origin of this unit, detailed geochemical analysis and petrographic and field studies of the metacherts were performed. They allowed two groups of metacherts to be identified: gray-whitish (GWM) and green-bluish metacherts (GBM). The GWM present nodular and lens morphologies and are mainly composed of micro- and mesoquartz crystals with evidence of recrystallization processes. The GBM show a layered structure that consists of micro- and mesoquartz crystals recrystallized in fine bands, interspersed with bands composed of epidote, zoisite, chlorite and sericite. The multielement diagram (normalized to post-Archaean Australian shale) shows positive anomalies of La, Eu and Y and a slight enrichment of the heavy rare earth elements (HREE) with respect to the light rare earth elements (LREE), similar to seawater signatures, for the GWM. Eu anomalies between 1.2 and 1.4, a flat distribution of LREE and depleted contents of HREE are shown for the GBM. These characteristics indicate that the GWM derived from C-cherts (chemical cherts) sensu-stricto, while the GBM protolith is compatible with silicification of volcanic rocks, or S-cherts (silicified cherts), also considering the high lithophile element contents. Furthermore, the Ge/Si atomic ratios, from 1.48 x 10−6 to 4.79 x 10 −6, and Y/Ho ratios lower than 43.36 (for both chert types) indicate an influence of hydrothermal silica source. The study of the El Cortijo Formation cherts has revealed conclusive evidence about the origin of these marine rocks allowing a reinterpretation of the whole formation as an Oceanic Plate Stratigraphy complex in the Tandilia Belt basement, southern margin of the RPC.