Toarcian Sections Research Articles

The Toarcian Posidonia Shale at Salem (North Alpine Foreland Basin; South Germany): hydrocarbon potential and paleogeography

AbstractThe Posidonia Shale in the basement of the North Alpine Foreland Basin of southwestern Germany represents an important archive for environmental changes during the Toarcian oceanic anoxic event and the associated carbon isotope excursion (T-CIE). It is also an important hydrocarbon source rock. In the Salem borehole, the Posidonia Shale is ~ 10 m thick. The lower 7.5 m (1763.5–1756.0 m) of the Posidonian Shale and the uppermost part of the underlying Amaltheenton Formation were cored and studied using a total of 62 samples. Rock–Eval, palynological, maceral, biomarker and carbon isotope data were collected to assess variations in environmental conditions and to quantify the source rock potential. In contrast to most other Toarcian sections in southwest Germany, TOC contents are high in sediments deposited during the T-CIE, but reach a peak in post-CIE sediments. Biomarker ratios suggest that this reflects strong oxygen-depletion during the T-CIE (elegantulum to lower elegans subzones), but also during the falciferum Subzone, which is also reflected by a prolonged dinoflagellate cyst blackout. While sediments of the tenuicostatum Zone to the elegans Subzone are thinner than in neighbouring sections (e.g., Dotternhausen), sediments of the falciferum Subzone are unusually thick, suggesting that increased subsidence might have contributed to anoxia. The T-CIE interval is very thin (0.75 m). δ13C values of n-alkanes show that the maximum negative isotope shift predates the strongest basin restriction during the T-CIE and that the carbon isotope shift is recorded earlier for aquatic than for terrigenous organisms. In Salem, the Posidonia Shale is thermally mature and highly oil-prone. The residual source petroleum potential is about 0.8 tHC/m2. Graphical Abstract

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n-alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ13C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).

Sedimentary organic matter and early Toarcian environmental changes in the Lusitanian Basin (Portugal)

The objective of this study is to investigate the relationship between early Toarcian climatic events and the composition of kerogen assemblages in the Lusitanian Basin (Portugal). In particular, we aim to understand how the Pliensbachian–Toarcian Event (Pl–Toa Event) and Toarcian Oceanic Anoxic Event (TOAE) affected the continental areas of the western Iberian margin and how possible variations in continental sources of organic matter (OM) were expressed in marginal-marine and hemipelagic depositional environments during the early Toarcian.We present here a characterisation [total organic carbon (TOC), total sulphur (TS), δ13C in kerogen concentrate (δ13CKerogen), and palynofacies] of kerogen assemblages from several uppermost Pliensbachian (emaciatum ammonite Zone)–lower Toarcian sections in the Lusitanian Basin (western Iberian margin), including the Peniche section, which contains the Toarcian GSSP. In all sections, TOC concentration is low (average 0.4 wt.%), with the highest values reaching up to 2.1 wt.% in a discrete level located approximately 10.5 m above the base of the levisoni Zone at Peniche. The TOAE negative carbon isotope excursion is observed in kerogen concentrate at the base of the levisoni Zone throughout the basin. Palynofacies analysis demonstrates that the kerogen assemblages are mostly of terrestrial affinity, with the dominance of the Phytoclast Group and terrestrial palynomorphs, and with punctual increases in amorphous organic matter, freshwater (e.g. Botryococcus sp.) and marine microplankton (dinoflagellate cysts, acritarchs, and prasinophyte algae) in specific stratigraphic intervals. A change in palynofacies assemblages associated with the TOAE is observed around the base of levisoni Zone. Although with slight differences between sections, the TOAE interval records an increase in non-opaque, translucent, phytoclasts (NOP) and cuticle fragments, and is also associated with an increased contribution of terrestrial palynomorphs (increase in sporomorphs and Classopollis in tetrads and agglomerates) and decrease in marine palynomorphs. The increases in NOP and terrestrial palynomorphs support the postulated enhancement of the hydrological cycle and increased export of terrestrial OM into marine environments during the early Toarcian, especially during the TOAE, in the western Iberian margin. Understanding the impact of the early Toarcian climatic events on land-based ecosystems may provide important insights into current climate change.

Synchronization of the astronomical time scales in the Early Toarcian: A link between anoxia, carbon-cycle perturbation, mass extinction and volcanism

The Late Pliensbachian–Early Toarcian is a pivotal time in the Mesozoic era, marked by pronounced carbon-isotope excursions, biotic crises and major climatic and oceanographic changes. Here we present new high-resolution carbon-isotope and magnetic-susceptibility measurements from an expanded hemipelagic Late Pliensbachian–Early Toarcian section from the Middle Atlas Basin (Morocco). Our new astronomical calibration allows the construction of an orbital time scale based on the 100-kyr eccentricity cycle. The Early Toarcian Polymorphum Zone contains 10 to 10.5 repetitions of the 100-kyr eccentricity both in the carbon-isotope and the magnetic-susceptibility data, leading to an average duration of 1.00±0.08 myr. We also show that the Late Pliensbachian–Early Toarcian global carbon-cycle perturbation has an average duration of 0.24±0.02 myr. These durations are comparable to previous astrochronological time scales provided for this time interval in the most complete sections of the Tethyan area, and longer than what has been provided in condensed sections. Anchoring this framework on published radiometric ages and astrochronological time scales, we estimate that the carbon-cycle perturbation of the Late Pliensbachian–Early Toarcian corresponds with the early phase of the Karoo and Chonke Aike large igneous provinces. Likewise, our new age constraints confirm that the Toarcian oceanic anoxic event is synchronous to the main phase of the Ferrar volcanic activity. Thus, these successive and short phases of the volcanic activity may have been at the origin of the successive phases of the mass extinctions observed in marine biotas in the Pliensbachian and Toarcian times.

Sedimentary evidence for enhanced hydrological cycling in response to rapid carbon release during the early Toarcian oceanic anoxic event

A pronounced excursion in the carbon-isotope composition of biospheric carbon and coeval seawater warming during the early Toarcian (∼183 Ma) has been linked to the large-scale transfer of 12C-enriched carbon to the oceans and atmosphere. A European bias in the distribution of available data means that the precise pattern, tempo and global expression of this carbon cycle perturbation, and the associated environmental responses, remain uncertain. Here, we present a new cm-scale terrestrial-dominated carbon-isotope record through an expanded lower Toarcian section from Japan that displays a negative excursion pattern similar to marine and terrestrial carbon-isotope records documented from Europe. These new data suggest that 12C-enriched carbon was added to the biosphere in at least one rapid, millennial-scale pulse. Sedimentological analysis indicates a close association between the carbon-isotope excursion and high-energy sediment transport and enhanced fluvial discharge. Together, these data support the hypothesis that a sudden strengthening of the global hydrological cycle occurred in direct and immediate response to rapid carbon release and atmospheric warming.

New multiproxy record of the Jenkyns Event (also known as the Toarcian Oceanic Anoxic Event) from the Mecsek Mountains (Hungary): Differences, duration and drivers

AbstractThe oceanic anoxic event in the Early Toarcian, often referred to as the ‘Torcian Oceanic Anoxic Event’, led to widespread deposition of organic‐rich black shales and geochemical anomalies in elemental distribution and multiple isotope systems in the Early Jurassic ocean. Best characterized by its hallmark carbon isotope anomaly, the event is widely regarded as a prime example of rapid greenhouse warming‐related changes in the Mesozoic Earth system. However, despite numerous studies, details of its forcing mechanisms, exact duration and the role of regional effects remain debated. This article presents new data (high‐resolution organic carbon isotope, calcareous nannofossil and elemental geochemical analyses) from the black shale‐bearing Lower Toarcian section in the Réka Valley, Hungary, with the aim of assessing any regional differences in the sedimentary and geochemical record and their bearing on the underlying oceanographic and climatic processes. Following a short segment with a positive trend at the base of the section, values of the carbon isotope data here are turning to a negative trend with a steep, stepwise drop in two negative shifts, reaching their minimum before a positive trend with oscillations characterizing the top part of the section examined. The shape of the curve and nannoplankton biostratigraphy (recognition of zones NJ5b, NJ6 and NJ7) allow reliable correlation of these data with the global carbon isotope perturbation recorded elsewhere in the Early Toarcian. It is proposed here that it would be fitting to rename the Torcian Oceanic Anoxic Event as the ‘Jenkyns Event’, to honour the seminal contributions of Hugh Jenkyns. The cyclostratigraphic analysis in this study suggests that the duration of the negative isotope excursion at Réka Valley is 200 kyr, 350 kyr or 1 Myr, depending on which astronomical forcing parameter controls the most prominent cyclicity. Spectroscopic analyses suggest that the source of the organic matter, marine algae according to previous studies, did not change considerably during the main negative carbon isotope excursion. The variability observed in major element concentrations and enrichments relative to the average shale in the Réka Valley black shales can be regarded as mixtures of terrigenous aluminosilicates and calcium carbonate as two endmembers. Consequently, the terrigenous compositional endmember of the studied black shales consists of a mixture of an illitic/smectitic and a kaolinitic clay, which supports previous suggestions of increased weathering under extremely humid climate in the hinterland during the Jenkyns Event.

SPECTRAL ANALYSIS OF TOARCIAN SEDIMENTS FROM THE VALDORBIA SECTION (UMBRIA-MARCHE APENNINES): THE ASTRONOMICAL INPUT IN THE FORAMINIFERAL RECORD

Toarcian sections studied mainly in Europe have revealed the incidence of Milankovitch forcing with a well-developed, highly stable, 405 ky component of eccentricity, a short-term eccentricity of ~100 kyr, the cycle of obliquity ~36 kyr, and the precession signal at ~21 kyr. Cyclostratigraphic analysis of the Toarcian succession at the Valdorbia section (Umbria-Marche Apennines) was conducted based on time-series of foraminiferal assemblages. Well-developed cyclic patterns were obtained, with several significant cycles corresponding to thicknesses of 3.8-4.1 m / 5.8-6.3 m / 8.2 m / 10.4 m. Comparison with previous studies at the Valdorbia section led us to interpret the cycle of ~4 m as directly related with the short-term eccentricity (95-105 kyr). The rest of the cycles could be assigned to a periodicity of ~140-160 kyr, ~200 kyr and ~250 kyr, and interpreted as indirect signals of the long-term eccentricity, obliquity and precession, whose record would be impeded by the incompleteness of the studied succession and the sampling interval. Studied components in the foraminiferal assemblage show variable cyclostratigraphic patterns, allowing for a differentiation of groups based on similar registered cycles. These groups reveal different responses by the foraminiferal assemblage, associated with particular requirements, to the palaeoenvironmental changes of Milankovitch origin.

A high-resolution shallow marine record of the Toarcian (Early Jurassic) Oceanic Anoxic Event from the East Midlands Shelf, UK

The global deposition of organic rich-mudrocks, a mass extinction, and marked geochemical changes in the sedimentary rocks and fossils deposited during the early Toarcian indicates a period of extreme environmental change and an oceanic anoxic event. This study investigates at a high-resolution the environmental and biotic changes that occurred during the event in a shallow-marine area using rocks deposited on the East Midlands Shelf, UK. We present a new graphic log, geochemical data (δ13Corg, total organic carbon, CaCO3, total sulphur and total nitrogen), and benthic macroinvertebrate ranges from North Quarry, Holwell, Leicestershire, UK.Comparison of the ammonite ranges between the Cleveland Basin (Yorkshire) and East Midland Shelf successions in the UK shows that there is a hiatus on the shelf across the Dactylioceras semicelatum–Cleviceras exaratum Subzone boundary. The new high-resolution geochemical data show that an ~−5‰ δ13Corg excursion occurs similar to other lower Toarcian sections in the world, δ13Corg shifts ‘A’ to ‘C’ are missing in the hiatus, but that δ13Corg shift ‘D’ is present.Similar to the Cleveland Basin, the C. exaratum Subzone of the East Midlands Shelf succession is dominated by three epifaunal bivalve species, but their ranges differ significantly between the successions. An increase in the faunal diversity occurs within the upper C. exaratum Subzone at Holwell and other UK sections. The biotic data indicate that conditions on the East Midlands Shelf were more hospitable than in the Cleveland Basin. The bivalve populations on the shelf may have provided a source of new recruits for the basins.

First record of the leptonectid ichthyosaur Eurhinosaurus longirostris from the Early Jurassic of Switzerland and its stratigraphic framework

An incomplete skull of the leptonectid ichthyosaur Eurhinosaurus longirostris found in the Rietheim Member (previously “Posidonienschiefer”; Toarcian, Early Jurassic) of Staffelegg, Canton Aargau, is the first record from Switzerland of this taxon and supports the status of Eurhinosaurus longirostris as a palaeobiogeographic very widespread ichthyosaur species in the Early Toarcian of Western Europe. Being from either the Bifrons or Variabilis zone, it is one of the youngest records of Eurhinosaurus and one of the few diagnostic ichthyosaur finds from this time interval. The partial skull is well articulated and preserved three-dimensionally in a carbonate concretion. Both the mode of preservation of the ichthyosaur and an associated ammonoid (Catacoeloceras raquinianum) provided the age of the concretion, which had been collected from scree. Taphocoenosis and taphonomy show the C. raquinianum to be one of few non re-worked fossils recorded from the Early to Late Toarcian boundary (Bifrons/Variabilis zone) of northern Switzerland in general and of this ammonite species in particular. The Toarcian section at Staffelegg differs from other localities where strata of the same age are exposed with respect to facies variations of the Rietheim Member (previously “Posidonienschiefer”, Early Toarcian) and the extraordinarily high thickness of the Gross Wolf Member (previously “Jurensis-Mergel”, Late Toarcian).

Expression of the Early Toarcian negative carbon-isotope excursion in separated carbonate microfractions (Jurassic, Paris Basin)

The causes of the pronounced negative excursion in carbon-isotope values that was recorded during the Early Toarcian Oceanic Anoxic Event (T-OAE) are still under debate, particularly with regard to the local versus global pattern of the excursion, and the extent to which recorded signals are under a diagenetic control. In this study we employ a novel microseparation technique in order to investigate the isotopic and mineralogical characteristics of different size fractions of the carbonate content from a Toarcian section recovered from the Sancerre–Couy borehole, southern Paris Basin. Beyond the recognition of a − 6‰ δ 13C excursion in the bulk carbonate content, our data also demonstrate that biogenic particles (such as coccoliths) and inorganic grains precipitated as early diagenetic phases (including dolomite) both record the excursion with the same magnitude. Although several black shales occur through the Paris Basin Toarcian section, it is only that associated with the onset of the OAE that coincides with a large negative carbon-isotope excursion. Taken together these observations indicate that during this event, the entire water column was characterized by homogeneous carbon-isotope values; such a pattern is incompatible with the idea that the negative excursion was generated simply through the upwelling of bottom waters enriched in re-mineralized organic carbon (cf. “the Küspert model”), since this would have required a strong vertical gradient in the water column. Additionally, the Paris Basin data show that the decrease in carbonate δ 13C values during the OAE occurred in several discrete steps (each of some − 2‰), as has previously been found for organic carbon substrates in other European sections. The stepped nature of the isotopic profile, which is part of a stratigraphic signature previously ascribed to Milankovitch forcing, is compatible with regular pulsed input of light carbon into the whole atmosphere–ocean system from a climatically sensitive source such as gas hydrate, or from thermal methanogenesis of organic-rich sediments in the Karoo–Ferrar large igneous province. Contrasts in the amplitude of the negative carbon-isotope excursion on a regional scale remain an important unexplained aspect of the Toarcian record.

Late Pliensbachian–Early Toarcian (Early Jurassic) environmental changes in an epicontinental basin of NW Europe (Causses area, central France): A micropaleontological and geochemical approach

We present an integrated work based on calcareous nannofossil and benthic foraminiferal assemblages, and geochemical analyses of two Upper Pliensbachian–Lower Toarcian sections located in the central-South France. The studied sections, Tournadous and Saint-Paul-des-Fonts, represent the proximal and the distal part, respectively, of the Jurassic Causses Basin, one of the small, partly enclosed basins belonging to the epicontinental shelf of the NW Tethys. At the transition from Late Pliensbachian to Early Toarcian, the Causses Basin recorded an emersion in response to the global sea-level fall. Our data indicate severe environmental conditions of marine waters, including salinity decrease and anoxia development, occurring in the Early Toarcian. The acme of this deterioration coincides with the Early Toarcian Anoxic Event (T-OAE) but, due to the restricted nature of the basin, anoxia persisted until the end of the Early Toarcian, mainly in the deeper parts of the basin. The micronutrients and organic organic-matter fluxes were probably high during the entire studied time interval, as shown by nannofossil and foraminiferal assemblages. However, nannoplankton production drastically decreased during the T-OAE, as demonstrated by very low nannofossil fluxes, and only taxa tolerant to low-saline surface waters could thrive. At the same time, benthic foraminifers temporarily disappeared in response to sea-bottom anoxia. Our study demonstrates that environmental changes related to the T-OAE are well-recorded even in small, partly enclosed basins of NW Europe, like the Causses Basin. Within this area, the effects of global changes, like sea sea-level and temperature fluctuations, are modulated by local conditions mainly controlled by the morphology of the basin.

Foraminifera and ostracodes across the Pliensbachian-Toarcian boundary in the Arctice Realm (stratigraphy, palaeobiogeography and biofacies)

Abstract The analysis of samples from numerous Pliensbachian and Toarcian sections from the northern regions of Russia and northern Alaska, as well as published data, allow us to estimate the correlative significance of foraminifera and ostracodes, and to develop an Arctic zonal standard based on these microfauna. During the Late Pliensbachian-Early Toarcian depositional period in the Arctic Basin and northwestern seas of Western Europe, a succession of almost simultaneous biotic and abiotic events occurred: widespread development of black bituminous shale at the beginning of the Early Toarcian, and a microbiota crisis (mass extinction event) in the Northern Hemisphere. The Early Toarcian microbiotic crisis was very sharp in both Arctic and Western European palaeobasins. In the Arctic seas, the generic and family composition of ostracode communities was completely replaced by new taxa. The species composition of foraminiferal assemblages changed considerably, while the generic composition of the foraminifera is only characterized by partial changes. In the Western European seas, the Early Toarcian microbenthos crisis caused taxonomic changes generally at the species level among foraminifera and at the generic level among ostracodes. The comprehensive analysis of the biogeographical distribution of Late Pliensbachian and Early Toarcian foraminifera and ostracodes, as well as Jaccard cluster analysis, allow us to define several biogeographical units within the Arctic and Boreal-Atlantic realms. A pattern of ecological distribution of microbenthos in bathymetric zones in Siberian palaeobasins has been developed, providing the basis for palaeoenvironmental reconstructions.

Paleoceanographic changes of the Late Pliensbachian–Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event

Secular records of the elemental and isotopic composition of belemnite calcite were studied in Pliensbachian and Toarcian sections from the Yorkshire coast, UK, and Southern Germany, to investigate oceanographic change during an interval prior to and including the Toarcian Oceanic Anoxic Event (OAE). Records from Southern Germany are correlated to the UK stratigraphy using strontium isotope stratigraphy. The geochemical trends measured from belemnite calcite are consistent between the two sections, and are interpreted in terms of temperature and salinity of the northwest European epi-continental sea. The data suggest that a dramatic environmental change coincided with the Toarcian OAE. Belemnite Mg/Ca, Sr/Ca, and Na/Ca ratios increase by a factor of between 1.7 and 2 coincident with a 3‰ negative shift in δ 18O from the mid- tenuicostatum zone until the lower falciferum zone of the UK ammonite biostratigraphy (a period of ∼0.6–0.7 Myr). Taken at face value, the Mg/Ca and δ 18O data argue for an abrupt warming of 6–7°C and substantial freshening during this interval. Global warming accompanied by an accelerated hydrological cycle and increased runoff is proposed to explain these changes. Prior to these events, data from lower in the Yorkshire section suggest a possible cooling accompanied by a shift to more saline waters during the period from the upper Pliensbachian margaritatus zone to the Toarcian lower tenuicostatum zone. This earlier event may also have been important in causing density stratification in the northwest European epi-continental sea.

Petroleum geochemistry of the Lower and Middle Jurassic in Atlantic margin basins of Ireland and the UK

Abstract Potential hydrocarbon source rocks of Lower and Middle Jurassic age have been reported from outcrop, shallow boreholes and exploration wells in Atlantic margin basins of the UK (Hebrides, West of Shetlands and flanking the NE Rockall Trough) and, recently, in the continuation of this trend offshore Ireland (Slyne, Erris and Porcupine basins). Previously these organic-rich mudrocks were considered to be of little economic importance, due largely to their perceived limited areal distribution and low maturity. However, recent geochemical studies of oils and shales from exploration drilling of these basins shows the Lower and Middle Jurassic to have considerable potential as effective hydrocarbon source rocks, supplanting the Late Jurassic-Early Cretaceous Kimmeridge Clay Formation equivalents as the only viable oil source rock in the region. Flanking the Atlantic margin in the Irish and UK sectors, rich oil source potential occurs in two transgressive mudrock cycles of Lower Jurassic age. These are the Sinemurian-Pliensbachian interval and the overlying Toarcian section, present in basins such as the Solan, Minch, Hebrides, Slyne, North Celtic Sea, St George’s Channel and Central English Channel. The Middle Jurassic source rocks have a more limited areal distribution and occur in the Faroe-Shetland, Solan, West Lewis, West Flannan, Hebrides, Slyne and North Porcupine basins with oil source potential in regressive marginal marine to lacustrine facies mudrocks. Geochemical studies were undertaken on mudrocks from the Lower and Middle Jurassic sections in Atlantic margin basins (outcrop, shallow borehole core and exploration well cores and cuttings samples) and on oils from drill stem test and shows (core and cuttings extracts). Detailed analyses using GC, GC-MS and carbon isotopes allowed both characterization of the source rocks and oil-to-source correlation. Biomarker and carbon isotope studies of oils from the Faroe-Shetland Basin (Foinaven and Schiehallion fields), the Porcupine Basin (Connemara accumulation), the Wessex Basin (Wytch Farm and Kimmeridge oil fields) and wells in the Slyne Basin show strong correlations to the various source rock developments in the Lower and Middle Jurassic. The mixed biodegraded Foinaven and Schiehallion oils have a major waxy component and correlate with lacustrine Middle Jurassic source rocks in the Solan and West Lewis/West Flannan basins. Middle Jurassic sourcing of the Connemara oils is also suggested, while oils in the Slyne Basin appear to have been largely sourced by the Lower Jurassic Pabba Shale Formation. Oils in the Wessex Basin (Wytch Farm and Kimmeridge) appear to have been sourced by Hettangian-Sinemurian mudrocks and those in the North Celtic Sea Basin by Toarcian source rocks. The results from this study, in combination with previously published data, show that rich, effective oil-prone source rocks occur in both the Lower and Middle Jurassic of the Atlantic margin basins offshore Ireland and the UK. These source rocks can be correlated with indigenous oils, indicating the existence of a previously under-evaluated petroleum system.

Calibration of early–middle Toarcian nannofossil events based on high-resolution ammonite biostratigraphy in two expanded sections from the Iberian Range (East Spain)

This work deals with refined nannofossil biostratigraphy in two well-documented lower–middle Toarcian sections, in the Iberian Range. The sampled shallow carbonate ramp deposits belong to the Turmiel Fm. and consist of an alternation of marls and limestones. Both lithologies yield common and well-preserved ammonite assemblages, which provide a refined ammonite biostratigraphic framework. A significant calcareous nannofossil composition change has been recognized within the Serpentinus Zone, and a useful succession of nannofossil events has been calibrated to the ammonite biostratigraphy. This succession includes: FO of Lotharingius sigillatus (Mirabile Subzone); LO of Mitrolithus jansae (Semicelatum Subzone); FO of Carinolithus cantaluppii (Strangewaysi Subzone); LOs of Biscutum grande and Biscutum finchii (lower part of the Serpentinus Zone); FO of Carinolithus superbus (Strangewaysi–Falciferum boundary); FO of Lotharingius velatus (Falciferum Subzone); FOs of Discorhabdus striatus and Discorhabdus ignotus (Serpentinus–Bifrons boundary) and “acme end” of Lotharingius hauffii (Variabilis–Illustris boundary). In the studied sections, the composition changes, the biohorizons recognized and their succession are closely comparable with other early–middle Toarcian Boreal sections.

Lower Jurassic epicontinental carbonates and mudstones from England and Wales: chemostratigraphic signals and the early Toarcian anoxic event

ABSTRACTSections through Lower Jurassic epicontinental carbonates from Southern Britain (Junction Bed and equivalent) show a positive carbon‐isotope excursion (δ13Ccarbonate), detectable in bulk rock, in the falciferum Zone of the lower Toarcian. Isotopic data from organic matter in more clay‐rich sections from Wales and north‐east England, together with determinations on belemnite calcite, indicate that highest δ13C values are localized in the upper exaratum Subzone of the falciferum Zone. Levels of particular enrichment in organic carbon were developed in the early to mid‐exaratum Subzone and hence pre‐date this δ13C maximum. These phenomena reflect the impact of the early Toarcian oceanic anoxic event in the British area. Similar isotopic trends have been recorded in other Toarcian sections from Tethyan Europe and are interpreted as reflecting the chemistry of sea water. On the assumption of isotopic correlation between the English and Tethyan sections, the δ13C maximum would be everywhere dated as latest exaratum Subzone in terms of the north European ammonite scheme.Absolute oxygen‐isotope values in carbonates probably reflect both early diagenetic cementation and later temperature‐related burial diagenesis, although a palaeotemperature maximum is tentatively identified as characterizing the early falciferum Zone. Subsequent climatic deterioration may have been triggered by drawdown of CO2, related to regional excess carbon burial during the oceanic anoxic event. Using the positive δ13C excursion as a correlative level in sections from different faunal provinces (Britain, Italy and Spain) implies that lower Toarcian zonal stratigraphy is diachronous between northern and southern Europe. There is evidence for partitioning of water masses between the north European shelf and the Tethyan continental margin during the Early Jurassic.

Geochemical effects of petroleum migration and expulsion from Toarcian source rocks in the Hils syncline area, NW-Germany

For mature, oil-prone source rocks of Toarcian-age in the Hils syncline area, NW-Germany, the geochemical effects of primary migration of petroleum are shown. Expulsion has occurred at high efficiencies primarily via a network of macro- and micro-fractures. The effects of this fracture-controlled expulsion with respect to gross and molecular composition of C 15+-soluble organic matter are documented. Samples from fractured intervals have expelled total C 15+-soluble organic matter more efficiently than adjacent unfractured intervals. A systematic comparison of samples taken at cm-intervals away from macro-fractures reveals compositional gradients which are interpreted to reflect preferential depletion of saturated and aromatic hydrocarbons from the source rock shale into the fractures. Extracts of shale samples directly at fracture walls are enriched in NSO-compounds and highly polar asphaltene-like fractions. The composition of residual petroleum fluids extracted from the fracture-fill materials is variable. In some cases there is a close match with that of the shale at some distance away from the fractures. In other cases there is geochemical evidence to suggest that petroleum fluids were chemically altered during lateral flow along the fractures. Based on available geological and geochemical evidence a conceptual model is proposed to explain fracture development and fluid flow processes. Rapid heating by a nearby intrusive body at shallow depths when the Toarcian sediments still contained about 15–20% pore water has played an important role: overpressuring of aqueous pore fluids with abundant dissolved carbonate caused intense fracturing of the rich intervals in the mature Toarcian sections. Finally, the closely interdependent relationships between petroleum generation and petroleum migration processes are documented. Especially, the evolution of these relationships with maturity progress from immature (0.48% R m) to mature (0.68 and 0.88% R m) and overmature (1.45% R m) stages is shown. For example, in the maturity interval from 0.68 to 0.88% R m most of the measured shale compaction and porosity decrease is achieved by expulsion of petroleum. For some samples there is an inverse relationship between kerogen quality and extractable hydrocarbon yields suggesting that expulsion efficiencies were higher from the better-quality intervals because the petroleum achieved higher saturations and hence higher relative permeabilities.