First record of stable isotopes (δ13C, δ18O) and element ratios (Mg/Ca, Sr/Ca) of Middle to Late Jurassic belemnites from the Indian Himalayas and their potential for palaeoenvironmental reconstructions

Middle to Late Jurassic belemnites from the Spiti and Zanskar valleys in the Indian Himalayas were used for stable isotope (δ13C, δ18O) and element (Mg/Ca, Sr/Ca) analyses. Although the Himalayan orogeny deformed and altered a large portion of the collected fossils, cathodoluminescence and scanning electron microscopy in combination with analyses of iron and manganese contents allowed the identification of belemnites believed to still retain their original chemical composition. Results indicate a long-term temperature decrease from the Middle Callovian–Oxfordian to the Tithonian, which is proposed to have been caused by a concomitant drift of eastern Gondwana into higher palaeolatitudes. Reconstructed absolute temperatures depend on the used equation and assumed δ18O value of seawater, but most likely varied between 17.6 °C to 27.6 °C in the Kimmeridgian and Tithonian with average values between 22 °C to 24 °C. This way, temperatures were similar to slightly warmer than today at comparable latitudes. The reconstruction of absolute temperatures for the Middle Callovian–Oxfordian was hindered by a larger number of poorly preserved belemnites representing this time interval.

Petrographic and geochemical characterization of the Middle‒Upper Jurassic Fe–Mn crusts and mineralizations from Monte Inici (north-western Sicily): genetic implications

Fe–Mn concretions and mineralizations, associated with condensed horizons and hardground, are significant archives in ancient carbonate rocks. Their petro-chemical study allows an assessment of the palaeoenvironmental context in which they were formed also connected to their biotic or abiotic origin. At the western side of the Monte Inici (Fornazzo section, north-western Sicily) a well exposed outcrop of condensed pelagic limestones (Rosso Ammonitico facies: Middle‒Upper Jurassic) is well-known and thoroughly studied. In this section, the base of the Rosso Ammonitico facies consists of a very condensed level rich in fossils with a variable thickness deposited from the early Bathonian to the early/middle Callovian. It is characterized, at the top, by the noticeable presence of Fe–Mn concretions, typical of the Tethyan Jurassic and related to very low sedimentation rates. For this study, Fe–Mn crusts and mineralizations from the Fornazzo section were investigated using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, ICP and stable-isotope mass spectrometry. The collected samples, deposited in submarine conditions (as testified by stable oxygen and carbon isotopes), have been subdivided in two typologies with different macroscopic and mineralogical features. The Fe–Mn crusts consist of hematite, todorokite and birnessite and are characterized by a relatively low Mn/Fe ratio. Their content in trace elements, rare earths and yttrium (REY) is compatible with a hydrogenetic origin involving the oxy-hydroxides colloids precipitation directly from seawater. Microbially mediated processes are here testified by the recognition of filamentous and coccoid-shaped microstructures referable to coexistence of chemosynthetic fungi and photosynthetic cyanobacteria and accounting for a deposition in the deep euphotic zone. An average growth rate of ~ 8.5 mm/Myr for the Fe–Mn crusts, estimated by cobalt concentrations, suggests a time elapsed for deposition of ~ 3.5 ± 1 Myr. This value is compatible with the stratigraphic gap embracing the time span from the early/middle Callovian to the middle Oxfordian. In the neighbouring pelagic limestones, Fe–Mn deposits are present in the form of micro-dendrites mainly consisting of pyrolusite, sometimes associated with carbonato-fluorapatite. The geochemical composition gives evidence of a prevalent early diagenetic origin with precipitation, at the sediment/water interface or in the first centimeters of sediments, of metals diffused from the crusts as consequence of fluctuating redox conditions. Although the well-preserved Frutexites texture is commonly related to a microbial activity, other bacterial microstructures have not been recognized, having probably been obliterated during the growth of the dendrites. Nevertheless, it is possible to suppose a deepening in the bathymetry consistent with the involvement of chemosynthetic microorganisms in the formation of Frutexites structures.

Discovery of the Middle Callovian ammonite Erymnoceras in the Blue Nile Basin (Ethiopia)

The discovery of the ammonite Erymnoceras cf. coronatum (Bruguière), typical of the Middle Jurassic Coronatum Zone, from the Dejen area of the Blue Nile Basin (central western Ethiopia) records the presence of marine Middle Callovian rocks in Ethiopia for the first time. Additionally, a brief note on the stratigraphy of the Blue Nile Basin is also provided.

Palaeoenvironments of northwestern Bulgaria and northeastern Serbia during the Jurassic

Jurassic rocks in the Danube Region of northwestern Bulgaria and northeastern Serbia have been the subject of numerous earlier studies that have shown notable similarities between their sedimentary facies and depositional environments. In terms of regional palaeotectonic zonation, this area represents the westernmost parts of the Vidin Early–Middle Jurassic Complex Horst and the Mihaylovgrad Early–Middle Jurassic Graben in NW Bulgaria, as well as the easternmost part of the Lower Danubicum in NE Serbia, which collectively take part of the Jurassic Moesian Platform. For compiling an overall conception of the palaeoenvironments that existed during the Jurassic, nine palaeoenvironmental sketch maps, from the Aalenian to the late Tithonian, have been composed in this study, based on reinterpretation of the data borrowed from previous literature. It became evident that the Middle Jurassic sedimentary successions of the Danube Region record an evolution from an initially isolated lacustrine-palustrine depositional setting (Aalenian) to rapidly expanding shallow to moderately deeper-marine sandy-calcareous setting (late Bajocian–early Callovian). From the middle Callovian and onwards, during the Late Jurassic, the region became an area of laterally extensive pelagic and platform carbonate deposition. This interpretation is consistent with the available earlier data, but it links the facies and their respective settings from NW Bulgaria to NE Serbia, which has not been made to date and will be of benefit for future regional correlations.

Detailed biostratigraphy of the Middle Callovian – lowest Oxfordian in the Mikhaylov reference section (Ryazan region, European part of Russia) by ammonites

Study of the sections near the Mikhaylov (Ryazan region) has made it possible to improve the infrazonal subdivision of the Middle Callovian – Lower Oxfordian of the European Russia by ammonites. The Athleta Zone is the most complete in the studied sections. A sequence of kosmoceratid biohorizons (phaeinum, proniae, rowlstonense and kuklikum) is established here, as well as infrazonal units based on a phylogenetic sequence of species of the genus Funiferites. A study of the paleobiodiversity and frequency of occurrence of ammonites at different intervals of the section showed that the ammonite assemblages of the Athleta Zone are Sub-Boreal or Sub-Tethyan, while in the Lamberti Zone they are Boreal, and in the Lower Oxfordian they are Arctic. It is also shown that the change in ammonite composition in the sections coincides with the cycles of transgressions and regressions of Boreal basins. New species and subspecies of the ammonites Funiferites allae compressum, Cadoceras (Eichwaldiceras) intermedium, Brightia (B.) lominadzei, B. (B.) progzhellensis, B. (B.) eccentrtica, B. (Glyptia) canaliculata stankevitchae, Zieteniceras rarecostatum are described.

Brittle stars from the British Oxford Clay: unexpected ophiuroid diversity on Jurassic sublittoral mud bottoms

Three new ophiuran species, Enakomusium whymanae n. sp., Aspidophiura? seren n. sp., and Ophiotitanos smithi n. sp., and an unnamed specimen assignable to the genus Dermocoma are described from the Callovian to Oxfordian Oxford Clay Formation of Great Britain. These determinations are based on new finds and a critical reassessment of historic specimens. The Oxford Clay ophiuroids represent two loose assemblages, one from the middle Callovian Peterborough Member and the other from the lower Oxfordian Weymouth Member. Both assemblages accord well with coeval midshelf mud bottom ophiuroid communities in terms of taxonomic composition and relative abundance of taxa. The British Oxford Clay ophiuroids are particularly significant as they are one of the rare instances where multiple species are represented, almost exclusively, by exceptionally preserved articulated skeletons. This provides an important window into the understanding of mid-Upper Jurassic ophiuroid paleobiology.

Calcareous nannofossil zonation and sequence stratigraphy of the Jurassic System, onshore Kuwait

ABSTRACT This paper presents the calcareous nannofossil zonation of the Middle and Upper Jurassic of onshore Kuwait and formalizes current stratigraphic nomenclature. It also interprets the positions of the Jurassic Arabian Plate maximum flooding surfaces (MFS J10 to J110 of Sharland et al., 2001) and sequence boundaries in Kuwait, and correlates them to those in central Saudi Arabia outcrops. This study integrates data from about 400 core samples from 11 wells representing a nearly complete Middle to Upper Jurassic stratigraphic succession. Forty-two nannofossil species were identified using optical microscope techniques. The assemblage contains Tethyan nannofossil markers, which allow application of the Jurassic Tethyan nannofossil biozones. Six zones and five subzones, ranging in age from Middle Aalenian to Kimmeridgian, are established using first and last occurrence events of diagnostic calcareous nannofossil species. A chronostratigraphy of the studied formations is presented, using the revised formal stratigraphic nomenclature. The Marrat Formation is barren of nannofossils. Based on previous studies it is dated as Late Sinemurian–Early Aalenian and contains Middle Toarcian MFS J10. The overlying Dhruma Formation is Middle or Late Aalenian (Zone NJT 8c) or older, to Late Bajocian (Subzone NJT 10a), and contains Lower Bajocian MFS J20. The overlying Sargelu Formation consists of the Late Bajocian (Subzone NJT 10b) Sargelu-Dhruma Transition, and mostly barren Sargelu Limestone in which we place Lower Bathonian MFS J30 near its base. The lower part of the overlying Najmah Formation consists of the Najmah Shale, which is subdivided into three subunits: (1) barren Najmah-Sargelu Transition, (2) Late Bathonian to Middle Callovian (lower Zone NJT 12) Lower Najmah Shale, and (3) Middle Callovian to Middle Oxfordian (upper Zone NJT 12 to NJT 13b) Upper Najmah Shale. Middle Callovian MFS J40 and Middle Oxfordian MFS J50 are positioned near the base and top of the Upper Najmah Shale. The upper part of the Najmah Formation is represented by the Late Oxfordian (Subzone NJT 13b) Najmah Limestone, and is overlain by the Kimmeridgian (Zone NJT 14) Jubaila Formation. Early Kimmeridgian MFS J60 and Late Kimmeridgian MFS J70 are positioned near the base and top of the Jubaila Formation. The positions of Late Jurassic MFS J80, J90 and J100 are not constrained by our biostratigraphic data and are positioned in the Gotnia Formation. The Upper Tithonian MFS J110 and the Jurassic/Cretaceous boundary are positioned in the Makhul Formation.