Sinemurian-Pliensbachian Boundary Research Articles

Late Sinemurian and Early Pliensbachian Radiolaria from Mount Rettenstein (Northern Calcareous Alps, Austria). Part 2. Entactinaria and Spumellaria

On Mount Rettenstein (Northern Calcareous Alps, Austria) a 95-m-thick Lower Jurassic succession of grey siliceous limestone and marl allowed us to study well-preserved and diverse radiolarian assemblages. The succession crossing the Sinemurian–Pliensbachian boundary yielded seven productive radiolarian samples, five of which were assigned to the Upper Sinemurian and two to the Lower Pliensbachian. In this article, we present 24 species and nine genera belonging to the order Entactinaria, and 66 species and 27 genera to the order Spumellaria. One genus (Tetractoma Cifer nov. gen.) and six species are described as new: Charlottalum austriacum Cifer nov. sp., Crucella optima Cifer nov. sp., Hexapyramis? sphaericus Cifer nov. sp., Paronaella triangularis Cifer nov. sp., Pseudoheliodiscus rotaformis Cifer nov. sp., and Tetractoma tollmanni Cifer nov. gen. nov. sp. In the Sinemurian, by far the most abundant genus amongst either Spumellaria and Entactinaria is Gorgansium. The Pliensbachian samples are, on the other hand, dominated by the genera Archaeocenosphaera and Praeconocaryomma. Compiled stratigraphic and geographic ranges as well as synonymies, descriptions and remarks are provided.

Shallow-water carbonates of the Coimbra Formation, Lusitanian Basin (Portugal): contributions to the integrated stratigraphic analysis of the Sinemurian sedimentary successions in the western Iberian Margin

An integrated stratigraphic analysis of the Coimbra Formation was performed in the S. Pedro de Moel outcrops of the Lusitanian Basin (Portugal). This unit is dated from the lower–upper (Oxynotum Chronozone) Sinemurian and is subdivided into eight informal subunits. Except for its base, consisting of dolostones and microbialites, much of the succession consists of bioclastic and bioturbated micritic centimetric–decimetric limestones (sometimes rich in benthic macrofauna) alternating with millimetric–centimetric marly layers, all deposited in shallow-water carbonate ramp environments. Organic-rich sediments occur throughout, with total organic carbon reaching up to 12 wt%. At a broader scale, the Coimbra Formation is transgressive and part of a long-lasting 2nd-order transgressive–regressive facies cycle ending around the Sinemurian–Pliensbachian boundary. The vertical variation of δ 13 C determined in bulk carbonate is characterized by relatively normal marine values (0–2.5‰); however, several negative shifts are associated with the organic-rich sediments, with a maximum amplitude of ∼8‰ in the Obtusum Chronozone. These shifts in bulk carbonate δ 13 C are interpreted to be of diagenetic origin and, therefore, of local significance. However, it cannot be discarded that a regional/global signal is imprinted on the observed trends.

Integrated stratigraphy (radiolarians, calcareous nannofossils, carbon and strontium isotopes) of the Sinemurian–Pliensbachian transition at Mt. Rettenstein, Northern Calcareous Alps, Austria

A 95 m thick succession of grey siliceous limestone and marl on Mount Rettenstein in the Northern Calcareous Alps allowed us to study well-preserved and diverse radiolarian assemblages across the Sinemurian–Pliensbachian boundary. The distribution of 31 most characteristic radiolarian species is presented. Ammonites of the Jamesoni Subzone, the topmost subzone of the Jamesoni Zone, have been previously found in the upper part of the section. The radiolarian data are complemented with calcareous nannofossil biostratigraphy, and carbon and strontium isotope analyses of bulk carbonate samples. A negative stable carbon isotope excursion (CIE) occurs in the middle part of the section and is correlated to the supraregional Sinemurian – Pliensbachian Boundary Event. Radiolarian assemblages below the negative CIE belong to the C anutus rockfishensis – Wrangellium thurstonense and Jacus ? sandspitensis radiolarian zones. Katroma clara and several other species belonging to the subsequent Canutus tipperi – Katroma clara Radiolarian Zone first occur at the beginning of the negative CIE. The first occurrence of the nannofossil Similiscutum cruciulus , which defines the base of the NJ4 Calcareous Nannofossil Zone, was recognized near the top of the negative CIE, 10 m above the inferred stage boundary. The strontium isotope ratio is due to diagenetic alteration of bulk micrite systematically shifted to higher values, but clearly decreases across the stage boundary. This decrease is compatible with the trend of the standard 87 Sr/ 86 Sr curve established in skeletal calcite. This is the first time radiolarian zones across the Sinemurian–Pliensbachian boundary in Europe can be tied to calcareous nannofossil zones and chemostratigraphy and also calibrated to chronostratigraphy. These results contribute to the stability of Lower Jurassic radiolarian zones and will allow the recognition of the Sinemurian–Pliensbachian boundary in deep-marine successions where ammonites are absent. • Dating Sinemurian–Pliensbachian boundary in deep-marine sediments with radiolarians. • Lower Jurassic radiolarian zones calibrated to chemostratigraphy. • Lower Jurassic radiolarian zones calibrated to calcarenous nannofossils. • Sinemurian–Pliensbachian negative CIE recognized in the Northern Calcareous Alps.

High-resolution record of multiple organic carbon-isotope excursions in lacustrine deposits of Upper Sinemurian through Pliensbachian (Early Jurassic) from the Sichuan Basin, China

Abstract The Sinemurian-Pliensbachian boundary event (ca. 193 Ma) is recorded as a global perturbation of the carbon cycle, as evidenced by a large negative carbon-isotope excursion recorded in many marine sedimentary successions. Whereas multiple lines of evidence testify that the Sinemurian-Pliensbachian boundary event was associated with environmental and climatic changes, sea-level oscillations, and biotic turnovers in marine settings, the record and effects of the event on continents are poorly known. In this paper, we report a high-resolution δ13Corg record and palynological data from the Lower Jurassic lacustrine succession of the Sichuan Basin that allow a prominent 8‰ negative carbon-isotope excursion to be identified at the Sinemurian-Pliensbachian transition. We therefore interpret this perturbation as the expression of the Sinemurian-Pliensbachian boundary event in the Sichuan Basin, and we propose a correlation with the marine realm. Facies evolution illustrates that the Sinemurian-Pliensbachian boundary event coincided with a phase of expansion of the lacustrine systems. Palynological analyses indicate a sharp shift from arid to humid climate conditions coincident with the carbon-isotope perturbation that supports a scenario of lake expansion driven by increased rainfall. In contrast to observations in the Sichuan Basin, where deep lake conditions persisted across the Sinemurian-Pliensbachian boundary event, a global drop in the sea level is documented at the onset of the isotope perturbation. This suggests that eustatic oscillations due to increased continental water storage in lakes and aquifers in the context of a wetter climate phase may have been associated with the early stages of the Sinemurian-Pliensbachian boundary event.

Early Jurassic long-term oceanic sulfur-cycle perturbations in the Tibetan Himalaya

The Early Jurassic is an important interval characterized by several global carbon-isotope (δ13C) perturbations. Although the δ13C records are becoming better documented during this time interval, we have a relatively poor understanding of the associated long-term environmental and climatic changes. In order to decipher these events, we here present new stable sulfur-isotope data of carbonate-associated sulfate (δ34SCAS) for the Sinemurian–Pliensbachian interval from the Wölong section in the Tibetan Himalaya that was located palaeogeographically in the southern hemisphere. An overall positive shift in δ34SCAS coincides with the negative δ13C excursion around the Sinemurian–Pliensbachian boundary, suggesting an increased 34S-depleted pyrite burial rate. The ensuing overarching negative δ34SCAS shift coincides with the upper Pliensbachian positive δ13C excursion. The initial falling limb of the δ34SCAS shift suggests a transient δ34S-depleted sulfate input, but this trend was soon reversed to become positive, likely caused by a persistently enhanced 32S-rich pyrite burial flux in the latest Pliensbachian.Modeling results show that maximum oceanic sulfate concentration likely decreased during the Sinemurian–Toarcian interval, probably due to large-scale evaporite deposition in the western Tethys and proto-Atlantic and enhanced pyrite burial in a number of marine settings. The concentration of seawater sulfate could have been high enough to maintain a homogeneous sulfur-isotope ocean in the late Sinemurian, but its persistent decrease may have initiated a spatially heterogeneous ocean after the Pliensbachian: an oceanic geochemical state that was amplified during the Toarcian Oceanic Anoxic Event.

An Early Jurassic (Sinemurian–Toarcian) stratigraphic framework for the occurrence of Organic Matter Preservation Intervals (OMPIs)

Lower Jurassic sedimentary successions in the Atlantic margin basins include several organic-rich intervals, some with source rock potential; time-equivalent units are also identified in on- and offshore areas worldwide. Despite decades of research, it is still unclear which mechanisms lead to the deposition of organic-rich sediments during the Early Jurassic. The objectives of this study are to construct a detailed temporal and geographical framework of Sinemurian–Toarcian organic matter preservation intervals (OMPIs; subdivided into local, regional, and superregional) and roughly constrain the relationship of OMPIs with the Lower Jurassic δ 13 C record. This survey combines an in-depth analysis of literature on the distribution of organic-rich facies in the Sinemurian–Toarcian with new geochemical studies [total organic carbon (TOC) and organic matter pyrolysis] from selected outcrop sections from Portugal, Spain, and Morocco. Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ 13 C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressive in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ 13 C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ 13 C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ 13 C (recovery) trend following the δ 13 C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of organic matter (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval. The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian (not taking into account the T-OAE related OMPIs) regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).

Million-year-scale alternation of warm–humid and semi-arid periods as a mid-latitude climate mode in the Early Jurassic (late Sinemurian, Laurasian Seaway)

Abstract. Clay mineral and stable isotope (C, O) data are reported from the upper Sinemurian (Lower Jurassic) of the Cardigan Bay Basin (Llanbedr–Mochras Farm borehole, northwestern Wales) and the Paris Basin (Montcornet borehole, northern France) to highlight the prevailing environmental and climatic conditions. In both basins, located at similar palaeolatitudes of 30–35∘ N, the clay mineral assemblages comprise chlorite, illite, illite–smectite mixed layers (R1 I-S), smectite, and kaolinite in various proportions. Because the influence of burial diagenesis and authigenesis is negligible in both boreholes, the clay minerals are interpreted to be derived from the erosion of the Caledonian and Variscan massifs, including their basement and pedogenic cover. In the Cardigan Bay Basin, the variations in the proportions of smectite and kaolinite are inversely related to each other through the entire upper Sinemurian. As in the succeeding Pliensbachian, the upper Sinemurian stratigraphic distribution reveals an alternation of kaolinite-rich intervals reflecting strong hydrolysing conditions and smectite-rich intervals indicating a semi-arid climate. Kaolinite is particularly abundant in the upper part of the obtusum zone and in the oxynotum zone, suggesting more intense hydrolysing conditions likely coeval with warm conditions responsible for an acceleration of the hydrological cycle. In the north of the Paris Basin, the succession is less continuous compared to the Cardigan Bay Basin site, as the oxynotum zone and the upper raricostatum zone are either absent or highly condensed. The clay assemblages are dominantly composed of illite and kaolinite without significant stratigraphic trends, but a smectite-rich interval identified in the obtusum zone is interpreted as a consequence of the emersion of the London–Brabant Massif following a lowering of sea level. Following a slight negative carbon isotope excursion at the obtusum–oxynotum zone transition, a long-term decrease in δ13Corg from the late oxynotum–early raricostatum zones is recorded in the two sites and may precede or partly include the negative carbon isotope excursion of the Sinemurian–Pliensbachian Boundary Event, which is recognised in most basins worldwide and interpreted to signify a late pulse of the Central Atlantic Magmatic Province volcanism.

High-precision Re[sbnd]Os dating of Lower Jurassic shale packages from the Western Canadian Sedimentary Basin

The Lower Jurassic rock record contains abundant organic-rich mudrocks suitable for refining Early Jurassic time using ReOs organic-rich shale (ORS) geochronometry, such as those associated with the Sinemurian–Pliensbachian boundary event or the Toarcian Ocean Anoxic Event (T-OAE). Despite this, most studies to date have failed to obtain high precision, geologically meaningful, ages for such Lower Jurassic rocks. The cause of imprecision in ReOs ORS age dates, at least for the Toarcian, has been linked to subtle chemostratigraphic variations in 187Os/188Osi values. Here, we report the first high-precision ReOs ages of organic-rich shale packages of the Gordondale and basal Poker Chip Members of the Lower Jurassic Fernie Formation of Northwestern Alberta, Canada from the Western Canadian Sedimentary Basin (WCSB). Our ReOs age results bracket sediment deposition between the late Sinemurian-early Pliensbachian (192.11 ± 0.77 Ma [2σ] and 193.83 ± 1.34 Ma [2σ]) and late Toarcian (180.10 ± 2.68 Ma [2σ]). When coupled with chemo- (87Sr/86Sr and 187Os/188Os isotopes) and bio-stratigraphy (ammonite and coccolith), our ReOs ages correlate with regional and global stratigraphic tie points corresponding to pivotal points of Early Jurassic time, such as the Sinemurian–Pliensbachian boundary event, T-OAE, and late Toarcian, that are otherwise weakly constrained by isotopic methods. This integrative approach helps to place these events in a more robust temporal framework. Based on seawater Os isotope data, we also speculate that a previously unidentified positive Os isotope excursion exists at the Sinemurian–Pliensbachian boundary - perhaps triggered by late-stage Central Atlantic Magmatic Province (CAMP) volcanism.

Climate and environmental response to the break-up of Pangea during the Early Jurassic (Hettangian-Pliensbachian); the Dorset coast (UK) revisited

Abstract Between the end-Triassic mass extinction and the Toarcian Oceanic Anoxic Event, the Early Jurassic witnessed important changes in carbon burial, palaeogeography and paleoceanography, which were linked to the initial breakup of Pangaea. In order to better understand the climate and environmental impact of this phase of major tectonic rearrangement we revisited a key section along the Dorset coast (UK) and used a multi-proxy geochemical approach for its analysis. Carbon isotopes (δ13C) measured on organic matter were corrected for the influence of changes in the type of organic matter. The thus obtained δ13C-HI index and an additional δ13Ccarb record measured on bulk carbonate indicate a suite of carbon-isotope excursions (CIEs) including the negative main CIE (Triassic-Jurassic boundary to early Sinemurian) the negative Sinemurian-Pliensbachian boundary CIE and the positive margaritatus Zone (late Pliensbachian) CIE. Based on elevated hydrogen index values and enrichments in Mo, U, V, total organic carbon (TOC) and pyrite, the Hettangian-Sinemurian interval was interpreted to have been characterised by high productivity conditions and strong oxygen depletion of the basin. The chemical alteration index (CIA), clay mineralogy, and δ18O data provide independent evidence for increased continental runoff triggered by humid climate conditions on the adjacent continent during this time period. The strongest oxygen-depleted conditions were inferred for the lowermost Hettangian interval, coincident with the onset of the main negative CIE. A further less strongly oxygen-depleted interval was recorded through the negative CIE near the Sinemurian-Pliensbachian boundary (jamesoni and ibex Zones). This interval was characterised by drier conditions and possibly cooler water temperatures (e.g. CIA, δ18Ocarb and kaolinite/illite ratio). Based on low CIA values and kaolinite/illite ratios, the Pliensbachian climate is considered to have been generally drier, superimposed by more humid episodes restricted to the davoei and margaritatus Zones. The long-term trend to drier conditions is explained by: (1) diminished atmospheric CO2 contents due to a reduction in emissions of volcanogenic greenhouse gases, and/or; (2) changes in the palaeogeography and current patterns related to the break-up of Pangaea and more specifically to the opening of the Hispanic corridor.

Chemostratigraphy of Late Sinemurian – Early Pliensbachian shallow-to deep-water deposits of the Central High Atlas Basin: Paleoenvironmental implications

Shallow water carbonate rocks are especially prone to diagenetic alteration. As such they are sometimes problematic archives of past carbon cycle perturbations, casting doubt on the reliability of shallow-water carbonates carbon isotopes analyses for chemostratigraphic purposes. In this paper, bulk organic carbon isotopes (δ13Corg) is used as a robust replacement of the more sensitive carbonate carbon isotope systems for the establishment of a refined chronostatigraphic framework for the evolution of Sinemurian-Pliensbachian neritic carbonate systems in the Central High Atlas Basin of Morocco. These data show very similar patterns and characteristics in the organic carbon isotope curves of basinal and platform sections and thus illustrate the reliability of these archives. Simultaneously, we explore the expression of the global carbon isotope excursion at the Sinemurian-Pliensbachian boundary (SP event) in the study area in Morocco. This event has been previously described as a negative carbon isotope excursion of ∼2‰ associated with a large transgression in numerous basins. In Morocco and other neritic carbonate records, the SP event is better described as a return to previous values after a positive carbon isotope excursion coinciding with a major latest Sinemurian regression recognised on a global scale. Similar patterns are also found in other sections, although sometimes poorly expressed. However, it shows, that the SP event has ambiguous characteristics and questions the conventional hypothesis of it being linked to the massive injection of 13C-depleted carbon into the atmosphere-ocean system.

A new TOC, Rock-Eval, and carbon isotope record of Lower Jurassic source rocks from the Slyne Basin, offshore Ireland

This paper presents an integrated assessment of total organic carbon, Rock-Eval pyrolysis, carbonate δ13C and δ18O, and total organic carbon δ13C record of Upper Sinemurian–Aalenian age from the Slyne Basin (well 18/25-1), offshore Ireland. Total organic carbon (up to 5.2 wt%) and Rock-Eval pyrolysis data highlights the Portree Shale Formation Equivalent (Lower–?Middle Toarcian) as a mature (Tmax ∼440 °C) source rock dominated by type II kerogens. This interval is more than 90 m thick.In a previously unexplored paleogeographical domain regarding stable carbon isotopes, the δ13C record shows positive values during the Upper Sinemurian, a negative trend followed by a minor positive trend during the uppermost Sinemurian–Lower Pliensbachian, a negative trend during the lowermost Lower Toarcian, and then the return to more positive δ13C values. Despite the biostratigraphic uncertainty, the coarse resolution of the dataset, and the complex assessment of diagenetic effects, we suggest that the δ13C record of the studied well records the Raricostatum Zone positive CIE, the Sinemurian-Pliensbachian Boundary Event, and the negative CIE associated with the T-OAE. The T-OAE positive CIE is not clearly defined in the obtained dataset. The slight post T-OAE negative CIE recovery to more positive values and the continuation of a relatively high organic content is coeval with a regional organic matter preservation interval observed in several Northern-European locations.These new datasets and their interpretation add to the understanding of the paleoenvironmental dynamics during the Early–?Middle Jurassic in the Northern European domain and opens new possibilities for hydrocarbon exploration in Ireland's offshore.

Climatic and palaeoceanographic changes during the Pliensbachian (Early Jurassic) inferred from clay mineralogy and stable isotope (C-O) geochemistry (NW Europe)

The Early Jurassic was broadly a greenhouse climate period that was punctuated by short warm and cold climatic events, positive and negative excursions of carbon isotopes, and episodes of enhanced organic matter burial. Clay minerals from Pliensbachian sediments recovered from two boreholes in the Paris Basin, are used here as proxies of detrital supplies, runoff conditions, and palaeoceanographic changes. The combined use of these minerals with stable isotope data (C-O) from bulk carbonates and organic matter allows palaeoclimatic reconstructions to be refined for the Pliensbachian. Kaolinite/illite ratio is discussed as a reliable proxy of the hydrological cycle and runoff from landmasses. Three periods of enhanced runoff are recognised within the Pliensbachian. The first one at the Sinemurian-Pliensbachian transition shows a significant increase of kaolinite concomitant with the negative carbon isotope excursion at the so-called Sinemurian Pliensbachian Boundary Event (SPBE). The Early/Late Pliensbachian transition was also characterised by more humid conditions. This warm interval is associated with a major change in oceanic circulation during the Davoei Zone, likely triggered by sea-level rise; the newly created palaeogeography, notably the flooding of the London-Brabant Massif, allowed boreal detrital supplies, including kaolinite and chlorite, to be exported to the Paris Basin. The last event of enhanced runoff occurred during the late Pliensbachian (Subnodosus Subzone of the Margaritatus Zone), which occurred also during a warm period, favouring organic matter production and preservation. Our study highlights the major role of the London Brabant Massif in influencing oceanic circulation of the NW European area, as a topographic barrier (emerged lands) during periods of lowstand sea-level and its flooding during period of high sea-level. This massif was the unique source of smectite in the Paris Basin. Two episodes of smectite-rich sedimentation (‘smectite events’), coincide with regressive intervals, indicating emersion of the London Brabant Massif and thus suggesting that an amplitude of sea-level change high enough to be linked to glacio-eustasy. This mechanism is consistent with sedimentological and geochemical evidences of continental ice growth notably during the Latest Pliensbachian (Spinatum Zone), and possibly during the Early Pliensbachian (late Jamesoni/early Ibex Zones).

Sinemurian–Pliensbachian calcareous nannofossil biostratigraphy and organic carbon isotope stratigraphy in the Paris Basin: Calibration to the ammonite biozonation of NW Europe

The biostratigraphy of Sinemurian to lower Toarcian calcareous nannofossils has been investigated in the Sancerre-Couy core (Paris Basin), which contains a mixed assemblage of species with affinities to the northern and southern areas of the peri-tethyan realm, thus allowing for the use and calibration of the Mediterranean Province (Italy/S France) and NW Europe (UK) biozonation schemes. This study is based on semi-quantitative analyses of the calcareous nannofossil assemblage performed on 145 samples and the recorded bioevents are calibrated to the NW European Ammonite Zonation and to a new organic carbon isotope curve based on 385 data points. The main bioevents, i.e. the first occurrences of Parhabdolithus liasicus, Crepidolithus pliensbachensis, Crepidolithus crassus, Mitrolithus lenticularis, Similiscutum cruciulus sensu lato, Lotharingius hauffii, Crepidolithus cavus and Lotharingius sigillatus as well as the last occurrence of Parhabdolithus robustus, have been identified. However, we show that a large number of standard biostratigraphic markers show inconsistent occurrences at the base and top of their range, possibly accounting for some of the significant discrepancies observed between the different domains. In addition to the nine main bioevents used for the biozonation of the core, we document an additional 50 distinct bioevents, evaluate their reliability and discuss their potential significance by comparison to previous studies. A total of five significant negative organic carbon isotope excursions are identified and defined in the Paris Basin including the well-documented Sinemurian–Pliensbachian boundary event. One positive excursion is further defined in the Pliensbachian interval. Our calibration of high-resolution calcareous nannofossil biostratigraphy to ammonite biostratigraphy and organic carbon isotopes represents a new stratigraphic reference for the Lower Jurassic series.

An extensional syn-sedimentary structure in the Early Jurassic Trento Platform (Southern Alps, Italy) as analogue of potential hydrocarbon reservoirs developing in rifting-affected carbonate platforms

This work focuses on the 3D modeling and structural analysis of the Monte Testo syn-sedimentary structure, developed in the Early Jurassic Calcari Grigi Group of the Trento carbonate platform (Southern Alps, Italy). Significant changes in the facies architecture of the platform sedimentary units, occurred across a global perturbation of the Carbon cycle at the Sinemurian-Pliensbachian boundary, are associated with evidences of syn-sedimentary tectonics. In particular, an early cemented oolitic sedimentary body with a high initial porosity (Loppio Oolitic Limestone) was broken-up and tilted by a pulse of rifting and overlain by tight marls and marly limestones (lower Rotzo Formation) that display sharp changes in thickness across the syn-sedimentary faults. This complex setting creates conditions potentially favorable to hydrocarbon accumulation. In this work, the Monte Testo structure is presented as a conceptual analogue of a hydrocarbon reservoir that may develop thanks to the overlap of the effects of extensional tectonics and climate change-induced modifications in the carbonate platform facies. A 3D geo-model was realized to obtain information about the genesis and tectonic evolution of the structure. Hence, a potential porosity distribution in the 3D model was evaluated showing that such extensional structure, which has a vertical extent of 500 m and covers an area of 15 km2, could have been associated to a total pore volume of 2.24 × 107 m3 at the time of its formation. Results suggest that in rifting contexts the combined effect of syn-sedimentary faulting and facies variations related to perturbations in the global carbon cycle could generate potential reservoirs in carbonate platforms.

Astronomical constraints on the duration of the Early Jurassic Pliensbachian Stage and global climatic fluctuations

The Early Jurassic was marked by multiple periods of major global climatic and palaeoceanographic change, biotic turnover and perturbed global geochemical cycles, commonly linked to large igneous province volcanism. This epoch was also characterised by the initial break-up of the super-continent Pangaea and the opening and formation of shallow-marine basins and ocean gateways, the timing of which are poorly constrained. Here, we show that the Pliensbachian Stage and the Sinemurian–Pliensbachian global carbon-cycle perturbation (marked by a negative shift in δ13C of 2–4‰), have respective durations of ∼8.7 and ∼2 Myr. We astronomically tune the floating Pliensbachian time scale to the 405 Kyr eccentricity solution (La2010d), and propose a revised Early Jurassic time scale with a significantly shortened Sinemurian Stage duration of 6.9±0.4 Myr. When calibrated against the new time scale, the existing Pliensbachian seawater 87Sr/86Sr record shows relatively stable values during the first ∼2 Myr of the Pliensbachian, superimposed on the long-term Early Jurassic decline in 87Sr/86Sr. This plateau in 87Sr/86Sr values coincides with the Sinemurian–Pliensbachian boundary carbon-cycle perturbation. It is possibly linked to a late phase of Central Atlantic Magmatic Province (CAMP) volcanism that induced enhanced global weathering of continental crustal materials, leading to an elevated radiogenic strontium flux to the global ocean.

Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin

Carbon isotope stratigraphy and ammonite biochronostratigraphy across the Sinemurian-Pliensbachian boundary in the western Iberian margin

Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys

High-resolution carbonate carbon-isotope stratigraphy of Lower Jurassic marine shallow-water limestones are compared with the distribution of the Lithiotis Fauna on the Trento Platform (Southern Alps, Italy). The Lithiotis Fauna is the first example of globally distributed mound-building bivalves in the geological record and experienced global diffusion in the Early Jurassic. A set of carbonate carbon-isotope excursions of 2–3‰, illustrating three distinct negative shifts followed by positive rebounds, are recorded in the isotope stratigraphy and can be correlated with the global negative δ13C shift of the Sinemurian–Pliensbachian boundary Event (S–P Event) and to the subsequent phase of C-isotope perturbation that characterized the lower Pliensbachian. In the studied stratigraphic sections, the S–P Event likely triggered eutrophic conditions illustrated by the presence of organic-rich facies and by fossil associations characteristic of poorly oxygenated waters. After the eutrophic phase, the amelioration of environmental conditions was marked by a positive ~3‰ rebound of the δ13Ccarb values, and by the occurrence of marine stenotypic faunas. On the Trento Platform, the stabilization of the δ13Ccarb values coincided with the appearance of the Lithiotis Fauna that subsequently became widely distributed in the entire range of platform environments and thrived in the late Pliensbachian when metric-scale bivalve mounds were generated. During the same time, the maximum proliferation of the Lithiotis Fauna is recorded both in the Tethyan and Panthalassa regions. Hence, the reported relationships between the δ13Ccarb data and the distribution and ecological characteristics of the genera contained in the Lithiotis Fauna suggest that the S–P Event and its aftermath, possibly coupled with the undergoing coeval continent reorganization that led to the opening of the Hispanic Corridor, could have set the stage for the rapid diffusion of these unusual bivalves across many parts of the globe.

Tectonostratigraphic evolution of the Jurassic extensional basins of the eastern southern Alps and Adriatic foreland based on an integrated study of surface and subsurface data

Successions that characterize the eastern southern Alps have been compared with coeval units drilled in the Alpine foreland (Po and Veneto plains, northern Adriatic Sea). The eastern southern Alps are composed of a carbonate platform-plateau, drowned in the Early Jurassic (Trento platform and plateau); a basin formed in the Early Jurassic (Belluno Basin) and a carbonate platform that lasted from the Jurassic to the Cretaceous (Friuli platform). Integration of stratigraphic and geophysical data illustrates the extensional architecture of the Alpine foreland subsurface. At the beginning of the Jurassic, peritidal successions were widespread everywhere except for the Belluno Basin. A reorganization of the Early Jurassic paleogeography affected the southern Alps around the Sinemurian–Pliensbachian boundary: the Pliensbachian successions were deposited in the central-western areas of the Trento platform whereas, elsewhere in the same platform and in the northern Friuli platform, Pliensbachian units are missing, replaced by an unconformity surface covered by crinoidal sands that have also been found in the subsurface. The Belluno Basin is recognizable in seismic profiles under the Veneto Plain. Southward (northern Adriatic Sea and Po plain), the basin between the Trento and the Friuli platforms, here called the “northern Adriatic Basin,” possesses a stratigraphy different from that of the Belluno Basin. The northern Adriatic Basin drowned later, and seismic profiles indicate that it was wider and bounded by groups of small synsedimentary faults instead of the major faults displacing the Belluno Basin. The northern Adriatic Basin can be interpreted as the northeastern extension of the Umbro-Marchean Basin of central Italy.

Shallow marine carbon and oxygen isotope and elemental records indicate icehouse‐greenhouse cycles during the Early Jurassic

For much of the Mesozoic record there has been an inconclusive debate on the possible global significance of isotopic proxies for environmental change and of sequence stratigraphic depositional sequences. We present a carbon and oxygen isotope and elemental record for part of the Early Jurassic based on marine benthic and nektobenthic molluscs and brachiopods from the shallow marine succession of the Cleveland Basin, UK. The invertebrate isotope record is supplemented with carbon isotope data from fossil wood, which samples atmospheric carbon. New data elucidate two major global carbon isotope events, a negative excursion of ∼2‰ at the Sinemurian–Pliensbachian boundary, and a positive excursion of ∼2‰ in the Late Pliensbachian. The Sinemurian–Pliensbachian boundary event is similar to the slightly younger Toarcian Oceanic Anoxic Event and is characterized by deposition of relatively deepwater organic‐rich shale. The Late Pliensbachian strata by contrast are characterized by shallow marine deposition. Oxygen isotope data imply cooling locally for both events. However, because deeper water conditions characterize the Sinemurian–Pliensbachian boundary in the Cleveland Basin the temperature drop is likely of local significance; in contrast a cool Late Pliensbachian shallow seafloor agrees with previous inference of partial icehouse conditions. Both the large‐scale, long‐term and small‐scale, short‐duration isotopic cycles occurred in concert with relative sea level changes documented previously from sequence stratigraphy. Isotope events and the sea level cycles are concluded to reflect processes of global significance, supporting the idea of an Early Jurassic in which cyclic swings from icehouse to greenhouse and super greenhouse conditions occurred at timescales from 1 to 10 Ma.

The Global Boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic), Wine Haven, Yorkshire, UK

Following votes by the Pliensbachian Working Group, the Jurassic Subcommission and the International Commission on Stratigraphy, IUGS rati.ed the proposed Global Boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage (Lower Jurassic) at the base of bed 73b in the Wine Haven section, Robin Hood's Bay, Yorkshire Coast, UK. This level contains the characteristic ammonite association Bifericeras donovani Dommergues and Meister and Apoderoceras sp. Complementary data include: a) Strontium-isotope stratigraphy, based on analysis of belemnites which yield a calcite 87Sr/86Sr ratio for the boundary level of 0.707425 and data supporting interpretation of continuous sedimentation; b) Belemnite oxygen-isotope data indicate a signi.cant temperature drop (~5 oC) across the boundary at this locality; c) A Transgressive Systems Tract (TST) initiated in the Aplanatum Subzone (uppermost Sinemurian) continues into the Lowermost Pliensbachian (Taylori Subzone); it forms part of a transgressive facies cycle sensu Graciansky et al. (1998); d) The Upper Sinemurian to lowermost Pliensbachian at Wine Haven section has a predominantly normal magnetic polarity, but two discrete reversed polarity magnetozones are present. The .rst spans much of the latest Sinemurian Aplanatum Subzone. It terminates <0.5m below the Sinemurian-Pliensbachian boundary and may prove a valuable chronostratigraphic marker. The second extends from the latest Oxynotum Subzone probably through to the lower part of the Raricostatum Subzone.