Chert Nodules Research Articles

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.

Petrography and Geochemistry of Limestone Upper Cretaceous Kometan Formation (Azmir Anticline): A Preliminary Assessment for Cement Production Suitability in the Kurdistan Region Northeast Iraq

A succession of slightly siliceous limestone of the Kometan Formation is studied geochemically and strati graphically. The studied area is located on the SW limb of the Azmir anticline at the northeastern of Sulaimaniyah city. The studied section is situated at longitudes 45°38'310"E and latitudes 35°36'129''N inside the Folded Zone, Kurdistan Region, northeastern Iraq. The exposed section is sampled for lab studies, and inspected by eyes and hand lenses in the field. In this study, petrographic and geochemical analyses of 17 limestone samples were carried out. This study aims to investigate the major oxides of the limestone samples, using the X-Ray Fluorescence (XRF), for preliminary assessment for possible cement production. A petrographic study was also carried out, in order. Depending on the petrographic study, three major microfacies were recognized: lime mudstone microfacies, lime wackestone microfacies, and lime packstone microfacies, which subdivided into four submicrofacies are planktonic foraminifera lime mudstone submicrofacies, planktonic foraminifera lime wackestone submicrofacies, and planktonic foraminifera lime packstone submicrofacies and heterohelix lime packstone submicrofacies. The geochemical analysis shows that a range of variations in constituents: LOI (35.90% to 42,72%), SiO2 (3.77% to 14.66%), CaO (42.59% to 49.95%), Al2O3 (0.44% to 2.22%), Fe2O3 (0.02% to 0.78%) and MgO (1.50% to 2.26%); K2O and Na2O are present in traces. The high CaO (49.95%) indicates that calcite is the primary carbonate mineral. Due to mineralogical considerations, CaO with LOI shows a positive correlation, whereas CaO with SiO2 shows a negative correlation. The low MgO contents in calcitic limestone and calcitic limestone with chert nodules recommend a lack of dolomitization process. The silica modulus and lime saturation factor from geochemical data indicated that the siliceous limestone samples from the Kometan Formation and Azmir Anticline can be used in the cement-making process provided they are processed for silica removal treatment to reduce its percentage to less than 8%, as required level, where the CaO can be automatically enriched. Depending on field observation, petrography, and geochemistry analysis, we conclude that the source of the chert or silica minerals is diagenetic processes and depositional environment origin.

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.

Trace element evidence for diverse origins of superheavy pyrite in Neoproterozoic sedimentary strata

Sedimentary pyrite has long been used as an archive of marine environments in Earth history. To capture reliable paleoenvironmental signals, however, we need to first evaluate pyrite in sedimentary strata as it can be altered and masked by later diagenetic and/or hydrothermal processes. Here, we trained two supervised machine learning algorithms on a large LA-ICP-MS pyrite trace element database to distinguish pyrite of different origins. The analysis validates that two models built on the co-behavior of 12 trace elements (Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Au, Tl, and Pb) can be used to accurately predict pyrite origins. Further statistical analysis suggests four trace element clusters behaving differently among sedimentary (syngenetic and early diagenetic), synsedimentary hydrothermal (syngenetic hydrothermal), and post-sedimentary hydrothermal (epigenetic hydrothermal) pyrite, which is probably driven by chemical and physical properties of source fluids, interactions between elements, competition among coprecipitating minerals, and pyrite growth rate. Armed with this initial success and aided by new LA-ICP-MS trace element data from 9 samples, we then demonstrated the efficacy of this approach in identifying the origins of pyrite from two Neoproterozoic sedimentary successions in South China. The first set of samples contain isotopically superheavy pyrite (i.e., whose bulk-sample δ34S values greater than those of contemporaneous seawater sulfate and whose origins remain controversial) from the Cryogenian Tiesi’ao and Datangpo formations. The second set of samples contain pyritic rims (associated with fossiliferous chert nodules and thought to be critical in exceptional fossil preservation) from the Ediacaran Doushantuo Formation. For the superheavy pyrite, the models consistently show high confidence levels (mostly > 80 % probability) in identifying its genesis type, and three out of four samples were given sedimentary origins. For the pyritic nodule rims, the models suggest that early diagenetic pyrite was subsequently altered by hydrothermal fluids and therefore shows mixed signals. The study highlights the importance of pyrite trace elements in deciphering and distinguishing the origins of pyrite in sedimentary strata.

With Impressions Chosen from Another Time: Core Technologies and Debitage Production at the Lower Palaeolithic Site of Notarchirico (670–695 ka; layers F to I2)

The earliest evidence of bifaces in western Europe is dated to the initial phase of the Middle Pleistocene (la Noira, Notarchirico, Moulin Quignon, 700–670 ka), with the findings of Barranc de la Boella (1.0–0.9 Ma) considered to be an earlier local evolution. No transition assemblages are recorded during this time frame, and the “abrupt” appearance of bifaces during this time frame is associated with significant cognitive shifts in human technological behaviours (Acheulean techno-complex). The new investigations conducted at the site of Notarchirico unearthed 30 ka of repeated human occupation (695–670 ka, layers F-I2) during MIS 17, with evidence of bifacial tools in layer G (680 ka) and F along with other heavy-duty implements (LCTs, pebble tools, etc.). Massive production of debitage products realised on local raw materials collected in situ through simple and efficient core technologies characterises a large part of the lithic assemblage with a high ratio of diversified light-duty tools, including modified chert nodules. Despite core and flake assemblages being a recurrent trait of Lower Pleistocene contexts, the increase in retouched implements recorded at the onset of the Middle Pleistocene has been considered a significant technological shift. The technological analysis of the debitage products presented in this work highlights recurrent and systematic technological behaviours of the hominins of Notarchirico—who proved to efficiently overcome the raw materials dimensional constraints—even in the layers without bifaces. This may shed light on the meaning of cultural and behavioural innovation that the Acheulean techno-complex is thought to bring over Europe. It is plausible that given the substantial homogeneity of the lithic strategies within the sequence of Notarchirico, which only the “introduction” of the bifaces in the upper layers seems to interrupt, a supposed behavioural or cultural change in the site might have already occurred in the lowermost portion of the sequence. In this work, we evaluate the degree of change—if any—from a technological perspective by analysing the debitage reduction sequences.

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.

Hydrothermal silicification and hypogene dissolution of an exhumed Neoproterozoic carbonate sequence in Brazil: Insights from fluid inclusion microthermometry and silicon‐oxygen isotopes

AbstractHypogene dissolution‐precipitation processes strongly affect the petrophysical properties of carbonate rocks and fluid migration pathways in sedimentary basins. In many deep carbonate reservoirs, hypogene cavernous voids are often associated with silicified horizons. The diagenesis of silica in carbonate sequences is still a poorly‐investigated research topic. Studies exploring the complexity of silica dissolution‐precipitation patterns in hypogene cave analogues are therefore fundamental to unravel the diagenetic and speleogenetic processes that may affect this kind of reservoir. In this work, we investigated an exhumed and silicified Neoproterozoic carbonate sequence in Brazil hosting a 1.4 km‐long cave. Quartz mineralization and silicified textures were analyzed with a multidisciplinary approach combining petrography, fluid inclusion microthermometry, silicon‐oxygen stable isotope analyses and U‐Th‐Pb dating of monazite crystals. We found that an early silicification event caused the replacement of the dolostone layers with micro‐crystalline quartz forming chert nodules. This event was likely associated with mixing fluids (ancient Neoproterozoic seawater and hydrothermal solutions sourced from the underlying Mesoproterozoic basement) at relatively low temperatures (ca. 50–100°C) and shallow depth. After the tectonic deformation produced by the Brasiliano orogeny, silica dissolution was promoted by high temperature and alkaline hydrothermal solutions rising from the quartzite basement along deep‐rooted structures. Hypogene hydrothermal alteration promoted the dissolution of the cherty layers and the precipitation of chalcedony and megaquartz. Homogenization temperatures from primary fluid inclusions in megaquartz cement indicate minimum formation temperatures of 165–210°C. Similar temperature estimates (110–200°C) were obtained from the δ30Si and δ18O isotope systematics of quartz precipitated from hydrothermal solutions. The dissolved salts in the fluid inclusions were evaluated as NaCl + CaCl2 from microthermometric data combined with cryogenic Raman spectroscopy, corresponding to salinity ranging between 17 and 25 wt.%. No reliable age constraints for hydrothermal silica dissolution‐precipitation phases were obtained from monazite U‐Th‐Pb dating. However, our results, interpreted in the regional context of the São Francisco Craton, suggest that the Cambrian tectono‐thermal events could have been amongst the possible drivers for this hypogene process in the basin.

A review of microbial-environmental interactions recorded in Proterozoic carbonate-hosted chert.

The record of life during the Proterozoic is preserved by several different lithologies, but two in particular are linked both spatially and temporally: chert and carbonate. These lithologies capture a snapshot of dominantly peritidal environments during the Proterozoic. Early diagenetic chert preserves some of the most exceptional Proterozoic biosignatures in the form of microbial body fossils and mat textures. This fossiliferous and kerogenous chert formed in shallow marine environments, where chert nodules, layers, and lenses are often surrounded by and encased within carbonate deposits that themselves often contain kerogen and evidence of former microbial mats. Here, we review the record of biosignatures preserved in peritidal Proterozoic chert and chert-hosting carbonate and discuss this record in the context of experimental and environmental studies that have begun to shed light on the roles that microbes and organic compounds may have played in the formation of these deposits. Insights gained from these studies suggest temporal trends in microbial-environmental interactions and place new constraints on past environmental conditions, such as the concentration of silica in Proterozoic seawater, interactions among organic compounds and cations in seawater, and the influence of microbial physiology and biochemistry on selective preservation by silicification.

The Depositional Mechanism of Hydrothermal Chert Nodules in a Lacustrine Environment: A Case Study in the Middle Permian Lucaogou Formation, Junggar Basin, Northwest China

Although chert deposits are limited in geological distribution, their geological and geochemical characteristics can provide important information to reconstruct paleoenvironmental and diagenetic processes. For the Permian period, cherts are utilized to trace global silicon cycles and hydrothermal activities in relation to the Permian Chert Event. In Northwest China, Permian chert nodules have recently been discovered in both the southeastern and northwestern margins of the Junggar Basin. We conducted an analysis of the mineralogy, petrology and geochemistry of chert nodules of the Lucaogou Formation in the southeastern margin of the Junggar Basin to identify silicon sources and determine the precipitation mechanism of chert nodules. As evidenced by petrology, the chert nodules were mainly composed of crypto-microcrystalline silica (94.33% on average), with development of a soft-sediment deformation structure, indicating the synsedimentary deposition of silicon. Proven by trace elements, high Eu/Eu* ratios (average 2.14), low total rare earth element content (average 6.03 ppm), low LaN/YbN ratios (average 0.17) and low Y/Ho ratios (average 25.25) in chert nodules supports the hydrothermal source of silicon. The wide distribution of authigenic metal-bearing minerals and the significant positive Eu anomalies observed suggest that the chert depositions in the Lucaogou Formation intermittently received high-temperature (&gt;250 °C) hydrothermal fluids, likely associated with the initiation of the Bogda Rift in the middle Permian. Following rapid cooling down and differential compaction, siliceous sediments dehydrated and deformed, finally forming chert nodules.

Stratigraphic styles of the Upper Paleocene-Lower Eocene rocks along the escarpment of Jabal Waddan, NW Libya

Abstract&#x0D; Upper Paleocene-Lower Eocene carbonate rocks along the escarpment of Jabal Waddan of NW Libya are displaying two major sedimentary cycles based on stratigraphic and foraminiferal attributes. The first one is representing by a regressive-transgressive sequence and is attributed to the Shurfah Formation. It indicates that during the Thanetian (Upper Paleocene) the deposition of the lower part (the Bú Rá's Member) was in shallow water depths of the subtidal environment under low energy conditions and is represented primarily by Dasycladacean and Codiacean green algae. Their association up-levels with small shallow benthic foraminifera such as miliolids and nonionids, however, may indicate a broad nearshore environment of deposition. The decline of the green algae and the prevalence of small-sized (&lt;1 cm) and thick operculinids witnesses in the middle and upper parts (i.e. the Qaltah and Ammúr members) are characteristic of an open-shelf setting down to a water depth of a few dozen meters and typify warmwater habitats. Evidence of further deepening is indicated up-section by the existence of common shallow shark-teeth in a water depth limited to the photic zone ((&lt;120 m)&#x0D; The second sedimentary cycle is showing a transgressive-regressive sequence and is ascribed to the Bishimah Formation. It shows that during the Ypresian (Lower Eocene) the deposition of the lower, middle and upper parts (i.e. Khayir, Wadi Zakim and Rawaghah members) of the Bishimah Formation was in the shallow marine neritic environment. The association of alveolinids with common textulariids in the lower and middle members points to a shallow inner shelf with water depth up to 60 m whereas their association with miliolids, nonionids and elphidiids seen at the upper levels indicate innermost shelf or restricted platform conditions. Evidence of further shallowing, however, is indicated up-section (the Rawaghah Member) by the presence of cross-bedded dolomitic limestone and chert nodules which delineate an advanced episode of the second sedimentary cycle. The closing part for the second cycle, however, is attributed to the overlying Al Jir Formation, elsewhere from the study area, in which a thick sequence of evaporates and chalky dolomites is reported to include a rich Spirolina-miliolid assemblage with globular alveolinids similar to those recovered from the studied deposits. Consequently, we recommend considering the Al Jir Formation as the upper member of the Bishimah Formation as originally described in the local literature. The boundary between the Ammúr Member of Shurfah Formation and the overlying khayir Member of the Bishimah Formation, however, is erosional everywhere in the study area and sets the contact between the first and second sedimentary cycles. It has been considered here to represent the Upper Paleocene/Lower Eocene boundary. Based on planktonic foraminiferal evidence, described from the subsurface of the Sirt Basin, the lower part of the khayir Member has been considered in the current study to represent a Ypresian (Lower Eocene) rather than Thanetian (Upper Paleocene) in age as previously thought

Description and characteristics of chert nodules in Appollonia Formation, Al Hmmeda road-cut, Al Jabal Al Akhdar, NE Libya

Physical, petrographic and bulk elements analysis of Chert Nodules in Appollonia Formation, Al Hmmeda road-cut were conducted and showed that the origin of these chert by replacement. The suspected siliceous organic matters are of low crystallized silica and were dissolved under the influence of increasing temperature and pH. Most of the chert was localized in the limestone and dolomitic limestone of Appollonia Formation (nodular zone) by percolation of silica-rich solution along microfractures and porous/permeability zones. This local porosity and permeability variations, mostly produced mixing of the diffused digenetic fluids. This process produces the nodular form of the cherts in the Appollonia mudstone facies in the nodular zone, while less marked permeability variations could have produced more regular tabular bodies of chert, which were not observed at least in the studied section. The absence of chert nodules in some adjacent rocks and even at higher elevation in the Appollonia stratigraphic section (nonnodular zone), might reflect effective exhaustion of the silica supply, fewer microfractures condition and perhaps dilution by the marine water of highly reduced silica-rich solution.

Sepiolite and Other Authigenic Mg-Clay Minerals Formation in Different Palustrine Environments (Madrid Basin, Spain)

Lithofacies belonging to mud-flat and palustrine deposits (lake margin) in the Miocene of the Madrid Basin (Spain) have been studied. Four lithofacies corresponding to mud flat (1 and 3) and palustrine (2 and 4) deposits have been differentiated. Units 1 and 3 consist mainly of mudstones and carbonates (calcretes and diolocretes). The clay fraction is dominated by trioctahedral smectite (up to 79%) with illite and kaolinite as minor components. The d(060) spacing value shows reflections at 1.52 and 1.50 Å indicating also the presence of dioctahedral phyllosilicates. Unit 2 consists predominantly of lutites (claystones), locally with carbonate and chert nodules. The clay fraction is dominated by sepiolite (up to 96%) with variable contents of smectite and subordinate illite. The d(060) spacing value shows reflections at 1.51 and 1.52 Å indicating trioctahedral clay minerals. Unit 4 consists mostly of carbonates (limestones) with mudstone and lutite inserts. The clay fraction shows different contents of sepiolite, palygorskite and dioctahedral smectite. The analysis of a selection of trace elements (Cr, Co, Th, La, Sc) has allowed us to determine the characteristics of the source area as dioritic, somewhat different from those of the nearby materials from the Batallones sector. Sepiolite shows FWHM values ranging between 0.68 and 1.10 (2θ), indicating “low crystallinity sepiolite”. Differences in the conditions of formation of magnesian smectite and palygorskite have been observed in the mud-flat and palustrine deposits. The formation of sepiolite mainly by neoformation in palustrine deposits with different hydrochemistry is remarkable, leading to differences in fibre size and crystallinity of the fibrous clay mineral. Authigenic transformation processes from previous Al-rich phases would be responsible for the formation of saponite and palygorskite in mud flat and palustrine environments, with different pH conditions.

Can Eu Anomaly Indicate a Hydrothermal Fluid Si Source? A Case Study of Chert Nodules From the Permian Maokou and Wujiaping Formations, South China

The Middle–Late Permian witnessed an unusual chert accumulation event along the margin of the Pangea and Paleo-Tethys realms, known as the “Permian Chert Event (PCE).” The PCE is well recognized in the Permian limestone from South China, in the forms of nodular and bedded cherts. Previous studies suggested that PCE was caused by hydrothermal fluids related to the Emeishan large igneous province (ELIP). Meanwhile, another hypothesis supported the biogenic origin of PCE, i.e., the Permian chert derived from biosilicification of abundant sponges and radiolarian. Thus, sources of silica from the Permian chert remain uncertain. To understand linkages among PCE, biosilicification mechanism, and the ELIP event, this study focused on chert nodules collected from the Permian Maokou and Wujiaping formations in the Lianziya and Maoertang sections, South China. We measured germanium/silicon ratios (Ge/Si) and rare earth element (REE) compositions of chert nodules on the basis of petrographic analysis. Ge/Si ratios range from 0.14 to 0.63 μmol/mol with an average of 0.33 μmol/mol (n=18) in the Lianziya section and from 0.02 to 0.75 μmol/mol with an average of 0.18 μmol/mol (n=45) in the Maoertang section, both of which are close to the seawater value. The REE pattern is characterized by LREE depleted with a positive Eu anomaly ranging from 0.66 to 2.16 in the Lianziya section and from 1.05 to 9.57 in the Maoertang section. Our results indicate that the silica of the Permian chert predominantly originated from seawater with limited contributions from hydrothermal fluids. To further quantify the contributions of hydrothermal fluids, we applied a binary (seawater and hydrothermal fluid) mixing model based on two geochemical proxies, i.e., the Ge/Si ratio and Eu anomaly. The modeling results suggest a mixing of 0.5 vol% to 1 vol% hydrothermal fluids with contemporaneous seawater, verifying the dominant seawater source of silica in the PCE. Although it has been widely accepted that positive Eu anomaly points to the hydrothermal fluid origin of silica, our study demonstrates that positive Eu anomaly could also be present in cherts that was predominantly derived from normal seawater. Therefore, the analysis of the Ge/Si ratio or REE compositions is highly recommended when determining the Si source of cherts.

Microfacies Analysis And Depositional Environment Of Pila Spi Formation (Middle - Late Eocene) In The Nerwa Section, Berat Anticline, High Folded Zone, Kurdistan Region, Northern Iraq

Microfacies analysis and depositional environment of the Pila Spi Formation (Middle-Late Eocene) were studied in the Nerwa outcrop, Berat anticline, High Folded Zone in Iraqi Kurdistan Region. The total thickness of the formation is 13.5 m composed of medium to thick bedded yellow to white limestone and thick bedded grey to white dolomitic limestone. It is interbedded with thin beds of yellow marl in the lower part and have chert nodules in the upper part. The petrographic study of 17 thin sections of the Pila Spi carbonate showed that the majority of the carbonates are micritic mixed with some microspar. The skeletal grains involve benthonic foraminifera, ostracods, mollusca, dasycladacean green algae, pelecypods, gastropods, rare planktonic foraminifera and bioclasts. Non skeletal grain included peloids, intraclasts and extraclasts which is mainly monocrystalline quartz grain. Based on field observations and petrographic analysis, three different lithological units were distinguished in the Pila Spi Formation in the studied section. They are in ascending order: medium to thick bedded limestone interbedded with marl unit, thick bedded dolomitic limestone unit and, cherty limestone unit. Depending on detailed microfacies analysis of carbonate rocks, three main microfacies and twelve submicrofacies were recognized in the studied section of the Pila Spi Formation. From the sum of all petrographic, facies and textural analyses, it is here concluded that Pila Spi Formation in the Nerwa section was deposited at subtidal semi-restricted lagoon, in lower part gradually changed to open lagoon environment toward the upper part.

Systematic paleontology, acritarch biostratigraphy, and δ13C chemostratigraphy of the early Ediacaran Krol A Formation, Lesser Himalaya, northern India

AbstractAcritarch biostratigraphic and δ13C chemostratigraphic data from the Krol A Formation in the Solan area (Lesser Himalaya, northern India) are integrated to aid inter-basinal correlation of early–middle Ediacaran strata. We identified a prominent negative δ13C excursion (likely equivalent to EN2 in the lower Doushantuo Formation in the Yangtze Gorges area of South China), over a dozen species of acanthomorphs (including two new species—Cavaspina tiwariaeXiao n. sp.,Dictyotidium grazhdankiniiXiao n. sp.), and numerous other microfossils from an interval in the Krol A Formation. Most microfossil taxa from the Krol A and the underlying Infra-Krol formations are also present in the Doushantuo Formation. Infra-Krol acanthomorphs support a correlation with the earliest Doushantuo biozone: theAppendisphaera grandis-Weissiella grandistella-Tianzhushania spinosaAssemblage Zone. Krol A microfossils indicate a correlation with the second or (more likely, when δ13C data are considered) the third biozone in the lower Doushantuo Formation (i.e., theTanarium tuberosum-Schizofusa zangwenlongiiorTanarium conoideum-Cavaspina basiconicaAssemblage Zone). The association of acanthomorphs with EN2 in the Krol Formation fills a critical gap in South China where chert nodules, and thus acanthomorphs, are rare in the EN2 interval. Like many other Ediacaran acanthomorphs assemblages, Krol A and Doushantuo acanthomorphs are distributed in low paleolatitudes, and they may represent a distinct paleobiogeographic province in east Gondwana. The Indian data affirm the stratigraphic significance of acanthomorphs and δ13C, clarify key issues of lower Ediacaran bio- and chemostratigraphic correlation, and strengthen the basis for the study of Ediacaran eukaryote evolution and paleobiogeography.UUID:http://zoobank.org/5289fdb2-0e49-4b3b-880f-f5b21acab371.

Tracing the evolution of dissolved organic carbon (DOC) pool in the Ediacaran ocean by Germanium/silica (Ge/Si) ratios of diagenetic chert nodules from the Doushantuo Formation, South China

The Ediacaran Period (ca. 635–538 Ma) was a critical interval in Earth’s history, bridging the Proterozoic and the Phanerozoic Earth systems. It witnessed the diversification of eukaryotic phytoplanktons in the early stage, the evolution of macroscopic multicellular lives in the late stage, and frequent oscillations of carbonate carbon isotopes of large magnitudes, presumably reflecting large perturbations of the global carbon cycle. It has been proposed that a large dissolved organic carbon (DOC) pool, hundreds to thousands times larger than the present one, was developed in the Ediacaran deep ocean. The wax and wane of the marine DOC pool might be responsible for the secular variation of carbon isotopes of marine dissolved inorganic carbon (DIC) pool, which was recorded in the carbonate carbon isotope chemostratigraphy. However, neither the extent nor the evolutionary history of marine DOC pool in the Ediacaran ocean has been well constrained. In this study, we use Germanium/silica (Ge/Si) ratios of chert nodules from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, to trace the evolution of marine DOC pool. These chert nodules yield abundant microfossils, i.e. acanthomorphic acritarchs, and were precipitated in the early diagenetic replacement of calcareous silty/muddy sediments. Most chert nodules show positive correlations between Al contents and Ge/Si ratios, which can be resolved by a binary mixing between a silica (quartz) endmember and an Al-rich silicate (clay) endmember. On the one hand, the Ge/Si ratios of silica component ((Ge/Si)SiO2) are comparable to the modern seawater composition, suggesting the normal seawater rather than hydrothermal fluids was the major source of Si. On the other, the Ge/Si ratios of the clay component ((Ge/Si)clay) are one to two orders of magnitudes higher than that of normal marine clays. The Ge-enrichment of the clay component could be attributed to the extensive chelation of Ge by organic ligands and massive formation of organic-Ge-Clay complex at high DOC concentration. This observation supports the presence of a large DOC pool in the Ediacaran ocean. In addition, there is an increase of (Ge/Si)clay from the Member II to Member III of the Doushantuo Formation, suggesting an increase of DOC concentration. The expansion of marine DOC pool is coincident with the reduction of the sizes of the upper acanthomorphic acritarch assemblage in the Member III. We suggest that the increase of marine DOC pool might be attributed to the reduction of sizes of particulate organic matter, which favored DOC generation and accumulation in the Ediacaran seawater.

Spatial distribution of organic functional groups in Ediacaran acritarchs from the Doushantuo Formation in South China as revealed by micro-FTIR spectroscopy

Raman and Fourier-transform infrared (FTIR) microspectroscopic analyses were performed to examine the chemical signatures and their spatial distribution of organic-walled microfossils together with organic matter (OM) in a black chert nodule from the Ediacaran (635–551 Ma) Doushantuo Formation in China. Raman spectral parameters (I-1350/1600 values) reveal that the acritarchs, bundled filaments, and OM have different degrees of structural order, and that there is no significant difference in I-1350/1600 values among the different portions of individual acritarchs (i.e., inner structure, inner membrane, and outer wall). The IR mapping reveals portion-specific chemical signatures within the individual acritarch specimens. The inner membrane of Tianzhushania contains aliphatic C-H bonds. The acritarchs, including Tianzhushania, show chemically four types of inner structures: (1) aromatic C-H-rich type, (2) aliphatic C-H-rich type, (3) aromatic/aliphatic C-H-poor type, or (4) heterogeneous. All of the examined acritarchs have aromatic C-H rich outer wall, irrespective of the presence/absence of processes. These features support a eukaryotic cyst origin, although it is difficult to determine whether the acritarchs correspond to the cysts of Protista or Metazoa. The bundled filaments also consist of an aromatic C-H bond, but its origin is uncertain. The OM shows a high degree of variation in Raman spectral I-1350/1600 values, as does the IR spectral intensity ratio of asymmetric aliphatic CH3/CH2, alongside a heterogeneous spatial distribution of aromatic and aliphatic C-H bands. This suggests the contribution of various precursors derived from dead microbial cell debris and extracellular organic compounds. Overall, these results confirm the presence of diverse microorganisms in the Doushantuo microbiota. Furthermore, the spatial distribution of organic functional groups in individual microfossils could provide clues about the taxonomy of microfossils of unknown origin.

Diverse multicellular algae from the early Ediacaran Doushantuo chert nodules and their palaeoecological implications

Morphologically diverse multicellular algae preserved with anatomical details are recovered from the chert nodules in the member II of the early Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China. They are taxonomically identified as Archaeophycus yunnanensis, Sarcinophycus papilloformis, Thallophyca ramosa, Thallophyca sp., Thallophycoides phloeatus, Wengania exquisita, W. globosa, W. minuta and two unnamed species. An individual growth of spheroidal thallus is evidenced by a series of specimens of Wengania minuta. Archaeophycus yunnanensis and Sarcinophycus papilloformis are possibly green algae, while the others are interpreted as red algae. The radiation of multicellular algae likely enhanced the efficiency of the biological pump, influencing the ocean ecology and environment during the early Ediacaran.

Facies Analysis and Depositional Environments of the Ubaid Formation, Western Iraq

The Early Jurassic (Liassic) sequence crops out in numerous anticlines of the high folded zone of north and north-east Iraq and in the Rutba subzone (including Ubaid Formation) in west Iraq. The present study deals with siliciclastic / carbonate rocks of the 58 m-thick Ubaid Formation at Zor Hauran valley in south western Iraq. The formation consists of two parts; the lower part is composed of pebbly coarse sandstone and greenish to yellowish soft marl alternated with marly dolostone, while the upper part is characterized by light brown, well bedded dolostone, with stromatolite structure in some locations. Oval, light to dark brown nodules of chert are also present.&#x0D; A detailed field lithological description and facies analysis of the Ubaid Formation were performed for thin sections. It is composed of five main microfacies; dolomitized mudstone, dolomitized bioclastic wackstone, dolomitized pelloidal packstone, dolomitized oolitic grainstone, and bindstone, in addition to two lithofacies: marl and pebbly sandstone. These facies reflect the deposition from the environment of the lower supratidal to intertidal zone and the encompassing shallow scaffold secured shoal - marine environment with cautious water circulation.

Evolution of Sedimentary Basins as Recorded in Silica Concretions: An Example from the Ionian Zone, Western Greece

Chert concretions in thick limestone successions preserve a more complete paragenetic sequence of diagenetic minerals than their host limestone and interbedded shale. The goal of this study was to test the possible presence of a high-temperature mineralising system in the Ionian basin of western Greece. Upper Cretaceous chert nodules were sampled at Araxos, where rocks are highly faulted and uplifted by salt diapirism, and on Kastos Island, on the flanks of a regional anticline. Chert concretions have microporosity produced by recrystallisation of opal to quartz and fractures produced in the brittle chert during basin inversion. Diagenetic mineral textures were interpreted from backscattered electron images, and minerals were identified from their chemistry. Diagenetic minerals in pores and veins include sedimentary apatite (francolite), dolomite, Fe-chlorite, Fe oxide-hydroxide mixtures, sphalerite, barite and calcite. Sphalerite is restricted to Araxos, suggesting that inferred basinal fluids were hotter and more saline than at Kastos. At Araxos, the Fe oxide-hydroxide also includes minor Cu, Zn, and Ni. Whether the transported metals were derived from sub-salt clastic rocks and basement, or from enriched Mesozoic black shales, is unclear. The effectiveness of this novel approach to understanding fluid flow history in thick limestone successions is validated.