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Marie-pierre Dabard - One of the best experts on this subject based on the ideXlab platform.
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environmental control on Concretion forming processes examples from paleozoic terrigenous sediments of the north gondwana margin armorican massif middle ordovician and middle devonian and sw sardinia late ordovician
Sedimentary Geology, 2012Co-Authors: Marie-pierre DabardAbstract:Abstract Concretions of various compositions are common in the Paleozoic terrigenous successions of the north Gondwana margin. This study focuses on phosphatic (P) and siliceous (Si) Concretions present in some successions of the Armorican Massif (NW France) and SW Sardinia (W Italy). It shows that they consist of mudstones, fine- to very fine-grained sandstones or shellbeds with a more or less abundant P-cement and form a continuum between a phosphatic end-member and a siliceous biogenic end-member. The P 2 O 5 contents are ranging from 0.26% to 21.5% and are related to apatite. The SiO 2 contents vary from 25% to 82% and are linked both to a terrigenous phase and to a biogenic silica phase. Concretions showing the lower P-contents (P 2 O 5 2 /Al 2 O 3 > 5). Comparison with the surrounding sediments shows that all the Concretions are enriched in chlorite and in Middle Rare Earth Elements (La s /Gd s : 0.12–0.72) and some of them in Y (up to 974 ppm), Rare Earth Elements (more than 300 ppm) and Sr (260–880 ppm). The Concretions with highest biogenic silica concentrations are contained in the outer shelf sediments whereas the other Concretions are present from the proximal part of the inner shelf to the outer shelf. A genetic model in two stages is proposed. During early diagenesis, the dissolution of shells and degradation of organic matter progressively enrich the pore water in dissolved Si, Ca and P. When the suboxic zone is reached, P-precipitation begins, leading to the formation of protoConcretions. In shallow environments, the relative permeability of sediments and the winnowing or reworking of the upper few centimetres by bottom currents allow for suboxic conditions to be maintained, leading to P-rich Concretion formation. In deeper environments, the anoxic zone is reached more rapidly, thereby preventing extensive phosphogenesis. Nevertheless in the protoConcretions the early P-cement preserves pore spaces from compaction. In the presence of biogenic siliceous particles, the fluids are enriched in dissolved silica and diffuse towards the protoConcretions. Silica precipitation can thus occur later in the intergranular spaces.
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Environmental control on Concretion-forming processes: Examples from Paleozoic terrigenous sediments of the North Gondwana margin, Armorican Massif (Middle Ordovician and Middle Devonian) and SW Sardinia (Late Ordovician)
Sedimentary Geology, 2012Co-Authors: Marie-pierre Dabard, Alfredo LoiAbstract:Concretions of various compositions are common in the Paleozoic terrigenous successions of the north Gondwana margin. This study focuses on phosphatic (P) and siliceous (Si) Concretions present in some successions of the Armorican Massif (NW France) and SW Sardinia (W Italy). It shows that they consist of mudstones, fine- to very fine-grained sandstones or shellbeds with a more or less abundant P-cement and form a continuum between a phosphatic end-member and a siliceous biogenic end-member. The P2O5 contents are ranging from 0.26% to 21.5% and are related to apatite. The SiO2 contents vary from 25% to 82% and are linked both to a terrigenous phase and to a biogenic silica phase. Concretions showing the lower P-contents (P2O5b1.5%) are often enriched in biogenic silica (SiO2/Al2O3>5). Comparison with the surrounding sediments shows that all the Concretions are enriched in chlorite and in Middle Rare Earth Elements (Las/Gds: 0.12-0.72) and some of them in Y (up to 974 ppm), Rare Earth Elements (more than 300 ppm) and Sr (260-880 ppm). The Concretions with highest biogenic silica concentrations are contained in the outer shelf sediments whereas the other Concretions are present fromthe proximal part of the inner shelf to the outer shelf. A geneticmodel in two stages is proposed. During early diagenesis, the dissolution of shells and degradation of organicmatter progressively enrich the pore water in dissolved Si, Ca and P.When the suboxic zone is reached, P-precipitation begins, leading to the formation of protoConcretions. In shallow environments, the relative permeability of sediments and the winnowing or reworking of the upper few centimetres by bottom currents allow for suboxic conditions to be maintained, leading to P-rich Concretion formation. In deeper environments, the anoxic zone is reached more rapidly, thereby preventing extensive phosphogenesis. Nevertheless in the protoConcretions the early P-cement preserves pore spaces from compaction. In the presence of biogenic siliceous particles, the fluids are enriched in dissolved silica and diffuse towards the protoConcretions. Silica precipitation can thus occur later in the intergranular spaces.
Alfredo Loi - One of the best experts on this subject based on the ideXlab platform.
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Environmental control on Concretion-forming processes: Examples from Paleozoic terrigenous sediments of the North Gondwana margin, Armorican Massif (Middle Ordovician and Middle Devonian) and SW Sardinia (Late Ordovician)
Sedimentary Geology, 2012Co-Authors: Marie-pierre Dabard, Alfredo LoiAbstract:Concretions of various compositions are common in the Paleozoic terrigenous successions of the north Gondwana margin. This study focuses on phosphatic (P) and siliceous (Si) Concretions present in some successions of the Armorican Massif (NW France) and SW Sardinia (W Italy). It shows that they consist of mudstones, fine- to very fine-grained sandstones or shellbeds with a more or less abundant P-cement and form a continuum between a phosphatic end-member and a siliceous biogenic end-member. The P2O5 contents are ranging from 0.26% to 21.5% and are related to apatite. The SiO2 contents vary from 25% to 82% and are linked both to a terrigenous phase and to a biogenic silica phase. Concretions showing the lower P-contents (P2O5b1.5%) are often enriched in biogenic silica (SiO2/Al2O3>5). Comparison with the surrounding sediments shows that all the Concretions are enriched in chlorite and in Middle Rare Earth Elements (Las/Gds: 0.12-0.72) and some of them in Y (up to 974 ppm), Rare Earth Elements (more than 300 ppm) and Sr (260-880 ppm). The Concretions with highest biogenic silica concentrations are contained in the outer shelf sediments whereas the other Concretions are present fromthe proximal part of the inner shelf to the outer shelf. A geneticmodel in two stages is proposed. During early diagenesis, the dissolution of shells and degradation of organicmatter progressively enrich the pore water in dissolved Si, Ca and P.When the suboxic zone is reached, P-precipitation begins, leading to the formation of protoConcretions. In shallow environments, the relative permeability of sediments and the winnowing or reworking of the upper few centimetres by bottom currents allow for suboxic conditions to be maintained, leading to P-rich Concretion formation. In deeper environments, the anoxic zone is reached more rapidly, thereby preventing extensive phosphogenesis. Nevertheless in the protoConcretions the early P-cement preserves pore spaces from compaction. In the presence of biogenic siliceous particles, the fluids are enriched in dissolved silica and diffuse towards the protoConcretions. Silica precipitation can thus occur later in the intergranular spaces.
Lorenz Schwark - One of the best experts on this subject based on the ideXlab platform.
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aromatic hydrocarbons provide new insight into carbonate Concretion formation and the impact of eogenesis on organic matter
Organic Geochemistry, 2020Co-Authors: Kliti Grice, Chloe Plet, Alan G Scarlett, Wolfgang Ruebsam, Alex I Holman, Lorenz SchwarkAbstract:Abstract Investigations of aromatic biomarkers extracted from carbonate Concretions can contribute to characterization of the enhanced microbial activity that mediates carbonate Concretion formation. This microbial footprint can be further inferred from the stable isotopic values of carbonate (δ13C) and pyrite (δ34S). Here, we used a combination of GC–MS and GC × GC-ToF-MS to compare the aromatic fractions of two Toarcian carbonate Concretions from the H. falciferum ammonite zone of the Posidonia Shale (SW-Germany) and their host sediment. The results revealed that n-alkylated and phytanyl arenes were enhanced in the Concretions, relative to the host sediment. These findings support a very early diagenetic (eogenetic) microbial source for alkylated and phytanyl arenes derived from the microbial ecosystem mediating Concretion formation. In contrast, aromatic compounds formed by thermal maturation (e.g. polycyclic aromatic hydrocarbons, aromatic steroids, organic sulphur compounds) remained invariant in host rock and Concretion samples. When combined with bulk sediment and Concretion properties, the distribution of aromatic compounds indicates that eogenetic microbial activity upon Concretion growth does not diminish organic matter quality.
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isotope and elemental geochemistry of black shale hosted fossiliferous Concretions from the cretaceous santana formation fossil lagerstatte brazil
Sedimentology, 2017Co-Authors: Ulrich Heimhofer, Patrick Meister, Stefano M Bernasconi, Daniel Ariztegui, David M Martill, Aristoteles De Moraes Riosnetto, Lorenz SchwarkAbstract:Carbonate Concretions hosted within organic carbon-rich shale sequences represent unique archives of often exceptionally preserved fossil biota. Besides providing high-fidelity preservation, their geochemical signatures can provide insight into the physical and chemical processes during early and later-stage Concretion growth. Here, two fossiliferous carbonate Concretions of the late Early Cretaceous Santana Formation (Araripe Basin, north-east Brazil) are analysed with an integrative geochemical approach including μ-XRF scanning, δ13C, δ18O, 87Sr/86Sr and Δ47 (clumped-isotope thermometry). Individual Concretions show a concentric internal zonation with the outermost layer being composed of millimetre-thick cone-in-cone calcite. A strong covariance of δ13C and δ18O values of the fine-crystalline Concretion body indicates mixing of two different carbonate phases and supports a scenario of temporally separated pervasive growth stages. Microbially-mediated formation of an early porous calcite framework was controlled by the combined processes of fermentation and methanogenesis around the decaying carcass, forming localized environments within a zone of sulphate reduction. Microbial sulphate reduction is indicated by the concentric enrichment of pyrite in the outer part of the Concretion body and by high pyrite abundance in the surrounding shale. Information on the later-stage diagenetic processes affecting the Santana Concretions can be derived from the outermost fringing cone-in-cone calcite. The carbonate precipitating fluid was characterized by a more or less marine δ18O composition (calculated δ18Oporewater = -1.0 to -1.8‰) and by radiogenic Sr-isotope ratios (up to 0.713331 ± 7.0*10-6), the latter probably reflecting modification due to interaction with the surrounding shale or, alternatively, with underlying evaporitic sulphate deposits influenced by strong continental inflow or with crystalline basement rocks. The Δ47-derived temperature estimates range between 37 to 42°C ± 5, indicating precipitation of the cone-in-cone calcite at a depth of 650 to 850 m, which is only half as much as the maximum burial depth derived from existing fission-track data. Overall, the study of fossiliferous carbonate Concretions in organic carbon-rich sedimentary sequences can reveal a complex growth history spanning incipient microbially-influenced precipitates as well as later-stage burial diagenetic phases. This article is protected by copyright. All rights reserved.
Kliti Grice - One of the best experts on this subject based on the ideXlab platform.
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aromatic hydrocarbons provide new insight into carbonate Concretion formation and the impact of eogenesis on organic matter
Organic Geochemistry, 2020Co-Authors: Kliti Grice, Chloe Plet, Alan G Scarlett, Wolfgang Ruebsam, Alex I Holman, Lorenz SchwarkAbstract:Abstract Investigations of aromatic biomarkers extracted from carbonate Concretions can contribute to characterization of the enhanced microbial activity that mediates carbonate Concretion formation. This microbial footprint can be further inferred from the stable isotopic values of carbonate (δ13C) and pyrite (δ34S). Here, we used a combination of GC–MS and GC × GC-ToF-MS to compare the aromatic fractions of two Toarcian carbonate Concretions from the H. falciferum ammonite zone of the Posidonia Shale (SW-Germany) and their host sediment. The results revealed that n-alkylated and phytanyl arenes were enhanced in the Concretions, relative to the host sediment. These findings support a very early diagenetic (eogenetic) microbial source for alkylated and phytanyl arenes derived from the microbial ecosystem mediating Concretion formation. In contrast, aromatic compounds formed by thermal maturation (e.g. polycyclic aromatic hydrocarbons, aromatic steroids, organic sulphur compounds) remained invariant in host rock and Concretion samples. When combined with bulk sediment and Concretion properties, the distribution of aromatic compounds indicates that eogenetic microbial activity upon Concretion growth does not diminish organic matter quality.
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mudstones and embedded Concretions show differences in lithology related but not source related biomarker distributions
Organic Geochemistry, 2017Co-Authors: Sabine K Lengger, Ines Melendez, Roger E Summons, Kliti GriceAbstract:Abstract The mudstones of the Western Australian Gogo Formation harbour numerous carbonate Concretions which often contain preserved fossils of Lagerstatte-like quality. These are especially notable in places where the mudstone has eroded, giving way to nodule fields, which afford valuable paleobiological records. It is, however, a challenge to determine their paleoenvironmental context. Here, we analysed Concretions from a core drilled in the Canning Basin Gogo Formation. At two different depths, Concretions were compared to the surrounding mudstone found at the same depth. Electron microscopy and X-ray spectroscopy showed that the Concretions were carbonate-rich and contained detrital fragments. Biomarker data showed that mudstone and Concretions had very similar distributions and presented marine biosignatures including indicators of anoxic depositional conditions, a stratified water column, and photic zone euxinia. The Concretions contained higher amounts of C 27 steranes, indicating that more labile organic matter such as animal remains could have triggered Concretion formation. Statistical analyses of the results showed that Concretions and shales differed ( p p 31 homohopanes, and hopane/sterane ratios ( p
David M Martill - One of the best experts on this subject based on the ideXlab platform.
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isotope and elemental geochemistry of black shale hosted fossiliferous Concretions from the cretaceous santana formation fossil lagerstatte brazil
Sedimentology, 2017Co-Authors: Ulrich Heimhofer, Patrick Meister, Stefano M Bernasconi, Daniel Ariztegui, David M Martill, Aristoteles De Moraes Riosnetto, Lorenz SchwarkAbstract:Carbonate Concretions hosted within organic carbon-rich shale sequences represent unique archives of often exceptionally preserved fossil biota. Besides providing high-fidelity preservation, their geochemical signatures can provide insight into the physical and chemical processes during early and later-stage Concretion growth. Here, two fossiliferous carbonate Concretions of the late Early Cretaceous Santana Formation (Araripe Basin, north-east Brazil) are analysed with an integrative geochemical approach including μ-XRF scanning, δ13C, δ18O, 87Sr/86Sr and Δ47 (clumped-isotope thermometry). Individual Concretions show a concentric internal zonation with the outermost layer being composed of millimetre-thick cone-in-cone calcite. A strong covariance of δ13C and δ18O values of the fine-crystalline Concretion body indicates mixing of two different carbonate phases and supports a scenario of temporally separated pervasive growth stages. Microbially-mediated formation of an early porous calcite framework was controlled by the combined processes of fermentation and methanogenesis around the decaying carcass, forming localized environments within a zone of sulphate reduction. Microbial sulphate reduction is indicated by the concentric enrichment of pyrite in the outer part of the Concretion body and by high pyrite abundance in the surrounding shale. Information on the later-stage diagenetic processes affecting the Santana Concretions can be derived from the outermost fringing cone-in-cone calcite. The carbonate precipitating fluid was characterized by a more or less marine δ18O composition (calculated δ18Oporewater = -1.0 to -1.8‰) and by radiogenic Sr-isotope ratios (up to 0.713331 ± 7.0*10-6), the latter probably reflecting modification due to interaction with the surrounding shale or, alternatively, with underlying evaporitic sulphate deposits influenced by strong continental inflow or with crystalline basement rocks. The Δ47-derived temperature estimates range between 37 to 42°C ± 5, indicating precipitation of the cone-in-cone calcite at a depth of 650 to 850 m, which is only half as much as the maximum burial depth derived from existing fission-track data. Overall, the study of fossiliferous carbonate Concretions in organic carbon-rich sedimentary sequences can reveal a complex growth history spanning incipient microbially-influenced precipitates as well as later-stage burial diagenetic phases. This article is protected by copyright. All rights reserved.
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the age of the cretaceous santana formation fossil konservat lagerstatte of north east brazil a historical review and an appraisal of the biochronostratigraphic utility of its palaeobiota
Cretaceous Research, 2007Co-Authors: David M MartillAbstract:This paper is concerned with the famous fossil-bearing carbonate Concretions of the Romualdo Member of the Santana Formation Konservat Lagerstatten of north-east Brazil. This palaeontologically important horizon was first dated as Cretaceous by the French palaeoichthyologist Louis Agassiz on the basis of fish fossils obtained by Bavarian explorers Spix and Martius between 1817 and 1820 and Scottish botanist and explorer George Gardner between 1836 and 1841. Gardner equated the Concretion level with the English Albian ‘Upper Greensands’ on the basis of an imagined similarity of stratigraphic sequence with that of the Isle of Wight, southern England. Since then high precision dating of this remarkable deposit has proved elusive and the Concretion-bearing part of the Santana Formation has been variously dated as early Late Cretaceous or late Early Cretaceous. Attempts at greater precision over the last 30 years have cited its age variously as Aptian, Albian or possibly Cenomanian, but few reliable data have been presented to support these dates.