Carbon isotopes of organic matter from the upper Jurassic oil shales from the Volgian-Pechora shale province and its accumulation mechanisms

The isotopic composition of carbon from the organic matter of late Jurassic oil shales from the Volgian-Pechora shale province is studied. The existence of a dependence between C_org content in the rock and the isotopic composition of kerogen carbon is ascertained. The content of the heavy carbon isotope increases with increasing C_org. This dependence is accounted for by the progressive accumulation of isotopically heavy hydrocarbons of the initial organic matter due to sulfurization. The data on the isotopic composition of individual n-alkanes of bitumen in the rocks and the data on the absence of isotopic fractionation between thermobitumen and the residual kerogen from oil shales from the Volgian-Pechora shale province obtained by treating shale in an autoclave in the presence of water are presented first in this paper.     

Black shales and other sediments with high organic matter contents in Phanerozoic climatic cycles: Communication 2. Black shales during the Pangea existence

According to recent concepts, the Earth surface was permanently transformed during its geological history. Some stages of its evolution were marked by the convergence of separate continental blocks to result in the formation of supercontinents, which resisted successfully centrifugal processes. Other stages were characterized by the opposite tendency: after their long existence, the supercontinents became disintegrated into several large and small blacks, the motion of which was accompanied by opening of new sea basins and closure of former basins with the oceanic crust. The second half of the Paleozoic was marked by amalgamation of large continental blocks. In the Devonian, collision between Laurentia and Baltica culminated in the formation of the Euroamerica continent. After the closure of the Ural paleocean in the terminal Carboniferous–initial Permian, it was united with the Siberian and Kazakhstan continental blocks. These events provided the prerequisites for the formation of a new supercontinent (Pangea), which acquired its final configuration at the end of the Permian. One of its segments located mainly south of the equator included Gondwana. Another segment located northward included Euroamerica, Kazakhstan, Siberian, and two China continental blocks. During its geological history, Pangea suffered many dramatic events including several extinctions of organisms. The most significant event took place in the terminal Permian–initial Triassic and at the transition between the Triassic and Jurassic periods.     

Pyrite in ammonite-bearing shales from the Jurassic of England and Germany

Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected-light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro-environments, while the sediment generally was not wholly anoxic. Most pyrite pre-dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ³⁴S values ranging from ?55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas-filled. In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro-nodules. Larger nodules show a variety of forms, some of which post-date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ³⁴S values in shales are grouped about a mode near ?20. Pyrite formed over a long time-span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void-fillings and concretions, and which help define the diagenetic history of a shale.     

Stable carbon and oxygen isotopes distribution in carbonates from the Central Barents Sea shelf sediments

Carbonate macrofaunal remains and diagenetic tubes collected from a number of structures in the Central Barents Sea area during the 18th TTR (Training Through Research) cruise were subdivided into three groups according to the results of stable carbon and oxygen isotopes analysis. The first group includes carbonates that were formed from bicarbonate only from surrounding sea waters. The carbonates of the second group were formed during diagenesis with use of mixed sources of bicarbonate. The fourth group includes methane-derived carbonates that were formed as a result of anaerobic oxidation of methane (AOM).     

K-feldspar rich shales from Jurassic bedded cherts in southeastern Slovenia

The study area in southeastern Slovenia is part of the transitional zone between the internal and the external Dinarides. Within Jurassic bedded cherts there are up to 2 cm thick shale intercalations, consisting of laminated, soft, fine-grained, green to brown material whose origin has been in question. In the majority of Tethyan cherts, the interbedded material is reported to be volcanogenic and/or terrigenous, although a detailed mineralogical analysis of the material is lacking. An XRD analysis confirmed the presence of quartz, illite, chlorite and K-feldspar, which is the prevailing component in some samples. Major and trace element data exclude both a volcanogenic and an hydrothermal origin. Several discrimination diagrams indicate the upper crustal terrigenous nature of shales and a biogenic silica source. The source material was probably from a Variscan crust, which at the time of deposition had already been weathered to kaolinite, and some sporadic muscovite. The MnO/Al₂O₃ ratio suggests a slow sedimentation rate of cherts and a faster one for shales, which probably settled from distal turbidity currents. The negative Ce anomaly indicates prolonged contact with ocean water. Sediments were deposited on a Tethyan passive margin, originally as silica-rich carbonate beds intercalated with mud. During late diagenesis, the mixing of marine and meteoric waters caused the further silicification of limestone and simultaneous potassium enrichment of shale which led to their alteration into illite or chlorite and, in sediments already rich in K-minerals, into K-feldspar.     

Organic geochemistry and petrography of Lower Cretaceous Wealden black shales of the Lower Saxony Basin: The transition from lacustrine oil shales to gas shales

Ninety-seven Wealden black shale samples from three wells in the Lower Saxony Basin have been studied by organic geochemical and organic petrographical methods to determine their maturity, organic facies and depositional environment. The maturities of the three wells range from early mature (Ex-A), late to postmature (Ex-C) to overmature (Ex-B) as determined by vitrinite reflectance measurements, diamondoid ratios and other geochemical maturity parameters. Ex-C and Ex-B show distinct petrographic features related to oil generation and migration. In particular, the occurrence of dispersed solid bitumen replacing initial type I kerogen suggests a formerly active petroleum system. Structural and textural differences between early mature alginites and solid bitumen in postmature to overmature samples show an alteration of the pore system with increasing maturity. A freshwater depositional environment is indicated by widespread occurrence of botryococcus algae and other small alginite particles predominating in the immature well. These alginites are absent in the more mature gas shales of wells Ex-C and Ex-B. Geochemical evidence of algae and phytoplankton in general is provided by numerous biomarker parameters, while the occurrence of β-carotane in some samples indicates events of increased salinity, although no hypersaline conditions are inferred due to very low gammacerane indices. Increased amounts of vitrinite and inertinite in samples of Ex-B suggest locally significant terrigenous input of organic matter for some periods during Wealden Shale deposition. High sulfur/organic carbon ratios provide evidence for sulfate rich waters and (partly) anoxic bottom water conditions. While the lower mature lacustrine source rocks generate paraffinic/waxy oils, gas and condensates are produced at post-mature stages. Furthermore, maturity distribution maps from 3D numerical petroleum systems modeling reveal substantial differences in respect to petroleum generation.     

Facies and depositional processes in an Upper Jurassic to Lower Cretaceous pelagic sedimentary sequence, Antarctica

The Nordenskjöld Formation (?Oxfordian-Berriasian age) is exposed on the east coast of the Antarctic Peninsula, where it consists of interbedded ash layers and biosiliceous mudstones which accumulated under anaerobic to dysaerobic bottom waters. The mudstones were deposited by pelagic settling and the ash layers by pelagic settling from suspension or as fallout from subaerial eruption columns. The lower part of the succession accumulated in a basinal setting under anaerobic bottom waters and is characterized by parallel bedding. Mudstones deposited in this setting preserve abundant zooplankton faecel pellets. Compaction of these pellets has given rise to a bedding parallel fissility. The upper part of the succession accumulated under dysaerobic bottom waters in a slope setting. The sequence is wavy bedded and contains abundant evidence of post-depositional sediment instability and resedimentation, much of which was caused by tectonic activity. Discrete slide masses are absent from the slope sequence and it appears that slope processes were dominated by creep. Examination of the mudstones shows that as levels of dissolved oxygen in bottom waters increase, pelleted mudstones give way to structureless mudstones before visible bioturbation is noted.     

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ¹³C values are found in the Tarim and Qaidam basins (δ¹³C₁: − 19.0 to − 29.9‰; δ¹³C₂: − 18.8 to − 27.1‰), and those with lowest δ¹³C values occur in the Turpan-Harmi and Junggar basins (δ¹³C₁: − 40.1 to − 44.0‰; δ¹³C₂: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C_5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ¹³C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.     

Biofacies of Upper Jurassic and Lower Cretaceous sediments of central West Siberia

Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater.     

Carbon and oxygen isotopes: a tool for Jurassic and early Cretaceous pelagic correlation (southern Spain)

The isotopic evolution of δ¹³C and δ¹⁸O is reported for the Jurassic and early Cretaceous in two pelagic sections of the External Zones in the Betic Cordilleras (SE Spain). Stable isotope curves from pelagic trough and swell sections display similar patterns. Variations in δ¹⁸O and δ¹³C values from strata at equivalent age probably reflect both early diagenetic cementation and later temperature‐related burial diagenesis. Comparison of global isotope curves with those presented in this work allows the differentiation of global from local events. Thus, the anoxic event during the early Toarcian (falciferum Zone) is characterized by elevated δ¹³C and depressed δ¹⁸O values. The isotopic record also allows the detection of the middle Oxfordian transgression. There are other peaks for the late Toarcian, early Bajocian, Callovian and early Berriasian that can also be used as tools for correlation purposes. Copyright © 2002 John Wiley & Sons, Ltd.     

页岩孔隙结构研究进展及下扬子古生界页岩孔隙特征

天然气可以吸附或以游离态赋存在富有机质泥页岩及其夹层中,使页岩成为一种重要的非常规储集体。页岩的微米—纳米级孔隙是页岩气富集的重要场所,常规孔隙研究手段难以适用,目前,国际上尚没有关于页岩孔隙类型的统一分类方案。因此,页岩的孔隙结构特征成为页岩气勘探开发中的关键问题。总结近年来页岩孔隙结构的主要研究手段和分类,并初步分析了浙西—皖南下扬子地区古生界页岩的孔隙特征。     

Speculations on the genesis of crinoidal limestones in the Tethyan Jurassic

Crinoidal limestones in the Tethyan Jurassic often occur capping reef or carbonate-platform successions, and these bioclastic deposits commonly exhibit a lensoid form. They may also occur in tectonic fissures or as turbidites in basinal sequences. Their age is commonly Liassic, though they may be younger.The crinoidal lenses are here interpreted as sand-waves formed on current-swept pelagic seamounts; and a parallel may be drawn with the Blake Plateau where partly lithified carbonate sand bodies, albeit composed of globigerines and pteropods, have recently been described. The Liassic was a time when many seamounts were formed by block-faulting of reefs and carbonate platforms, and this explains the widespread distribution of crinoidal deposits of this age. Autochthonous crinoidal calcarenites may characterise an early shallow-water phase of seamount evolution.The presence of kaolinite in many of the west Sicilian examples of this rock type is taken as suggesting the former presence of exposed and lateritised areas, oceanic islands, which may also have furnished a food supply for the crinoid gardens.The formation of nodular crinoidal limestones, which occur in some Tethyan localities, is attributed to diagenetic segregation of calcium carbonate.

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Zusammenfassung Crinoiden-Kalke treten im Jura der Tethys oft im unmittelbar Hangenden von Riff-Komplexen oder Karbonat-Plattformen auf, wobei es sich meist um linsenförmige Gesteinskörper handelt. Außerdem treten diese bioklastischen Sedimente in tektonischen Spalten oder als Turbidite mit Becken-Sedimenten assoziiert auf. In den meisten Fällen sind die Crinoiden-Kalke liassischen Alters, zuweilen finden sie sich aber auch in jüngeren Schichtreihen.Wir möchten die hier beschriebenen Crinoiden-Kalklinsen als sand-waves interpretieren, welche von Strömungen auf seamounts in einem pelagischen Ablagerungsraum gebildet wurden. Sie lassen sich mit Ablagerung des Blake Plateau vergleichen, von wo teilweise lithifizierte Karbonatsande, welche allerdings von Globigerinen und Pteropoden aufgebaut werden, beschrieben wurden. Während des Lias entstanden viele seamounts durch differentielle Absenkung der durch liassische Brüche zerstückelten Riff-Komplexe und Karbonat-Plattformen, und dies mag das gehäufte Auftreten von Crinoiden-Kalken im Lias erklären. Autochthone Crinoiden-Kalkarenite scheinen deshalb für erste, relativ untiefe Absenkungsphasen von seamounts charakteristisch zu sein.Kaolinit tritt in vielen liassischen Crinoiden-Kalken West-Siziliens auf, was für gleichzeitige Emersion und Lateritisierung begrenzter Gebiete, wahrscheinlich in der Form von ozeanischen Inseln sprechen würde. Diese Gebiete hätten gleichzeitig organisches Material zur Ernährung der Crinoiden-Rasen geliefert.Knollige Crinoiden-Kalke, welche ebenfalls im Jura der Tethys vorkommen, werden durch frühdiagenetische Umlagerung von Calciumkarbonat erklärt.

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Résumé Parmi les dépÔts jurassiques de la Tethys, des calcaires à CrinoÏdes reposent souvent directement sur des récifs ou sur des plateformes carbonatées. Dans la plupart des cas, il s'agit de dépots à extension lenticulaire. En outre, les sédiments bioclastiques sont déposés dans des fissures tectoniques ou ils sont intercalés comme turbidites dans les sequences sédimentaires des bassins. Le plus souvent, leur âge est liasique, plus rarement on les trouve dans des niveaux plus élevés.Les dépÔts lenticulaires à CrÏnoides sont interprétés ici comme des dunes sous-marines («sand-waves») qui se forment sur des plateaux plus ou moins profonds («seamounts») dans le domaine pélagique de la mer. Ils peuvent Être comparés avec des dépÔts actuels sur le Blake Plateau. On y a décrit récemment des sables carbonatés, en partie consolidés, formés par des coquilles de Globigérines et de Ptéropodes. Dans la région méditerranéenne, de nombreux plateaux sous-marins se sont formés pendant les temps liasiques où des récifs et des plateformes carbonatées ont été souvent morcelés par des failles. Ceci peut expliquer la fréquence des calcaires à CrinoÏdes pendant l'époque liasique. Des calcarenites à CrinoÏdes déposés sur place pourraient caractériser une première phase d'approfondissement modéré dans l'évolution des plateaux sous-marins.Dans de nombreux calcaires à CrinoÏdes liasiques de la Sicile occidentale, l'on trouve de la caolinite. On pourrait en déduire que des Îlots océaniques à sol latéritique émergaient dans le voisinage. Ils auraient pu fournir en mÊme temps le matériel organique nécessaire pour alimenter les prairies de CrinoÏdes.La génèse des nodules observés dans les calcaires à CrinoÏdes provenant de plusiers localités dans la Tethys pourrait Être expliquée par un déplacement du calcaire pendant les premières phases de la diagénèse.

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, , , , . - , . — , sand waves, seamounts . Blake Plateau, , . Seamounts , . , seamounts. — - , , . , , .

     

Uppermost Triassic to Lower Jurassic sediments of the island of Ireland and its surrounding basins

The uppermost Triassic to Lower Jurassic interval has not been extensively studied across the island of Ireland. This paper seeks to redress that situation and presents a synthesis of records of the uppermost Triassic and Lower Jurassic from both onshore and offshore basins as well as describing the sedimentological characteristics of the main lithostratigraphical units encountered. Existing data have been supplemented with a re-examination and logging of some outcrops and the integration of data from recent hydrocarbon exploration wells and boreholes. The Late Triassic Penarth Group and Early Jurassic Lias Group can be recognised across the Republic of Ireland and Northern Ireland. In some onshore basins, almost 600 m of strata are recorded, however in offshore basins thicknesses in excess of two kilometres for the Lower Jurassic have now been recognised, although little detailed information is currently available. The transition from the Triassic to the Jurassic was a period of marked global sea-level rise and climatic change (warming) and this is reflected in the lithostratigraphical record of these sediments in the basins of Northern Ireland and offshore basins of the Republic of Ireland. In general, the sediments of this interval are thicker than those in Great Britain and have potential for detailed study of climatic and sea-level fluctuation.     

Iodine diagenesis in pelagic deep-sea sediments

Nineteen sediment cores from the Madeira, Seine, Tagus and Nares Abyssal Plains and the Alboran Sea have been used to evaluate the speciation, fluxes and diagenesis of iodine in the deep sea. The sediments have surficial molar I/C ratios of 10–30 × 10⁻⁴ in excess of previous reported values for planktonic material (~1 × 10⁻⁴). Solid phase I contents decrease exponentially with depth corresponding to decomposition rate constants of 5–260 × 10⁻⁶ yr⁻¹ which vary with the carbon accumulation rate.Iodine species in the pore waters follow a vertical sequence of four zones: 1. a zone of I⁻ production where total dissolved iodine (∑I) concentrations initially increase at the seawater-sediment interface; 2. a zone of I⁻ oxidation where interconversion of I⁻ to IO⁻₃ occurs; 3. a zone of IO⁻₃ reduction where interconversion of IO⁻₃ back to I⁻ occurs which corresponds to the suboxic part of the sediment column; and 4. a further zone of I⁻ production which is confined to the lower anoxic part of the sediment column. Benthic ∑I fluxes in the Madeira Abyssal Plain measured from shipboard incubation experiments and calculated from porewater gradients are similar, averaging 0.55 and 0.36 × 10⁻⁸ μmol cm⁻² sec⁻, respectively.In the surface sediment the observed I enrichment results from a quasi-closed cycle for iodine initially involving release of I⁻ from decomposing marine organic matter followed by rapid removal onto organic matter at the sediment-seawater interface where I/C regeneration ratios of up to 200 × 10⁻⁴ are found, lodate reduction occurs during suboxic diagenesis, after denitrification and before MnO₂ reduction, consistent with the sequence of reactions predicted from the free energy yields for organic matter oxidation. There is some further I⁻ production in the anoxic section of sediments but at much smaller rates than occur during the interfacial diagenetic cycling.     

Carbon and sulfur isotopes in Precambrian sediments from the Canadian Shield

Carbon and sulfur isotope ratios have been determined for more than 200 samples of Precambrian graphitic and sulfidic metasediments from the Superior and Churchill Provinces of Canada. The sediments were deposited in small sedimentary basins related to various Canadian greenstone belts. The age of the Archean samples is approximately 2.7 Ga., the Proterozoic samples about 1.8 Ga.,

The Archean organic material shows C isotope values between −47 and −15%. vs. PDB. The Proterozoic metasediments show a smaller range, with δ¹³C between −30 and −17%.,

A few carbonate samples yielded C isotopic compositions between −5.9 and −4.7%.,

Associated iron sulfides have δ³⁴S-values ranging from −6 to +8%., which are consistent with isotopic fractionation effects either by inorganic or bacterial reactions in the sedimentary sulfur cycle.

The great variability of organic C isotopic composition, especially in the late Archean, although affected by extensive postdepositional alteration, might indicate differences in the environmental conditions and/or the organic content of these sedimentary basins. Some extremely ¹³C-depleted organic carbon values support a global occurrence of environments likely dominated by methanotrophic organisms at the end of the Archean.     

Geochemistry of barium in pelagic sediments

High concentrations and accumulation rates of Ba occur on active (spreading) ridges, and in some areas of high biological productivity. These patterns show that Ba is deposited on the deep sea floor by both volcanic and biological processes.     

Aerobic respiration in pelagic marine sediments

Analyses for dissolved oxygen, nitrate and total CO₂ in the interstitial water have been combined with solid phase sediment analyses of carbon and nitrogen to calculate the rates of reaction and stoichiometry of decomposing organic matter in central Equatorial Pacific pelagic sediments. The diagenesis is dominated by aerobic respiration and nitrification.Organic carbon and total nitrogen decrease exponentially with depth in both red clay and carbonate ooze sediments. In addition, there is a correlation between surface organic carbon and total nitrogen with distance from the equator. Fixed NH₄ is relatively constant with depth and constitutes 12 to 64% of the total nitrogen. The remainder is considered to be organic nitrogen.The $\text{C}\text{N}$ ratio of the decomposing organic matter was obtained using three approaches. Using the correlations of organic carbon with total nitrogen or organic nitrogen the molar ratios varied from 3.4 to 18.1. The average of all stations was 12.6 using total nitrogen and 13.7 using organic nitrogen. The Redfield ratio is 6.6. Approaches using interstitial water chemistry gave lower ratios. The average value using correlations between dissolved oxygen and nitrate was 8.1. The same approach using total CO₂ and nitrate gave an average of 9.1. Due to difficulties in unambiguously interpreting the solid phase data we favor the ratios obtained from the pore water analyses.The rate of organic matter decomposition can be obtained from model calculations using the dissolved oxygen and solid organic carbon data. Most gradients occur in the upper 10 to 20 cm of the sediments. Assuming that bioturbation is more important than sedimentation we have calculated first order rate constants. The average values using organic carbon and dissolved oxygen was 3.9 kyr^? and 4.2 kyr^? respectively using a biological mixing coefficient of 100 cm² kyr^?1. These rate constants decrease in direct proportions to the mixing coefficient.     

Remobilization of transition metals in surficial pelagic sediments from the eastern Pacific

The top thirty centimeters of sediment at two sites in the eastern equatorial Pacific contain evidence of post-depositional remobilization of Mn, Fe, Co, Ni, Cu, and Zn. Remobilization takes place as Mn and Fe oxyhydroxides are released to the pore water during the microbially-mediated decomposition of organic matter. Precipitation of the dissolved metals in near-surface more oxic strata controls the solid-phase distribution of Mn, Fe, and Zn. The solid-phase redistribution of Co and Ni requires only suitable material for readsorption. Comparison of pore water fluxes with solid-phase metal distributions in the solid sediment indicates no loss of dissolved metal to the overlying water column at the present time. Loss of Mn during the Quaternary is indicated by the composition of the sediments, however. Leaching experiments suggest that portions of the mobile Mn, Fe, Co, Ni, and Cu are fixed by incorporation in authigenic smectite in the surficial sediments.     

Geology and geochemistry of Jurassic pelagic sediments, Scisti silicei Formation, southern Apennines, Italy

A better understanding of genesis and palaeoenvironmental setting of the Scisti silicei Formation (Lagonegro units, southern Italy) was achieved by means of geochemical analysis integrated with new stratigraphic information. Data show that major and trace element geochemistry of ancient clay-rich beds and banded cherts add new insights into the Mesozoic evolution of the Lagonegro basin. Sedimentary contributions to Jurassic shales sampled during this study were mainly derived from two major sources: (i) a dominant terrigenous fine-grained component, having affinity with average upper continental crust that had not undergone intense weathering and (ii) biogenic siliceous material. The latter component occurs in clay-rich layers from the “basal member” of the Scisti silicei Formation.

Composition varies up section and accounts for changes in the detrital supply due to bathymetric oscillations. The compositional variations from the basal to the overlying member are consistent with a distal source passing in time to a more “proximal” source, as indicated by sharp changes in the concentrations of detrital elements (Ti, Zr and Nb). It is likely that increased detrital input occurred through turbidity current deposition. Finally, the chemical features of the clay-rich layers from the upper cherty portion of the studied succession imply a progressive deepening of the basin.

The lack of any mafic and hydrothermal contributions in the Jurassic shales as well as the continental nature of detrital input suggests that the Lagonegro basin was located between two carbonate platforms, in accordance with the classical restoration of the African–Apulian palaeomargin. Thus, the basin acted as a preferential sink connected to the African cratonic areas through a southern entry point.     

Relative sea-level change and carbon isotopes: data from the Upper Jurassic (Oxfordian) of central and Southern Europe

Studies on Cretaceous pelagic limestones have shown a close correspondence between putative sea-level curves and the carbon-isotope profile, suggesting a possible relationship between the two phenomena. Such a relationship could relate to shelf-sea area governing the global burial rate of organic carbon which, in turn, controls the ¹³C/¹²C ratio of dissolved inorganic carbon in the oceanic reservoir. In order to see whether or not this relationship might be of more general applicability, the major mid-Oxfordian (Jurassic) transgression is documented to zonal level and appropriate carbon-isotope data are presented. Although the applicability of ammonite zonation to the mid Oxfordian is problematic on a regional scale, a clear δ¹³C excursion is present in the trunsversariurn Zone, which is coincident with regional overstep and/or evidence of shoreline retreat and/or evidence of bathymetric deepening in Europe and elsewhere. The use of the carbon-isotope curve as a proxy for shelf-sea area or relative sea level is worthy of further exploration.