Bathonian Research Articles

METHOD FOR ASSESSING THE IMPACT OF GEOLOGICAL HETEROGENEITY ON OIL SATURATION DISTRIBUTION

This article examines a method for assessing the impact of reservoir heterogeneity on the distribution of oil saturation in pay zones using the example of the Bajossian and Bathonian stages of the Jurassic system of the Kalamkas field. Within this field, the Jurassic oil and gas stratum under consideration lies on the eroded surface of various horizons of the Middle and Lower Triassic and is represented by uneven alternation of sand-siltstone and clay rocks, as a result of which these deposits have a complex geological structure, due in the lithological plan to uneven alternation of sand-siltstone and clay rocks. As a result, a rather complex distribution of oil (water) saturation may occur in the formation, which differs from the standard ideas about the geological structure of the object under study, i.e. the presence of such «unconventional» reservoir boundaries (oil-bearing contour) as capillary barriers of the first kind is possible. As a result of this approach, the following is obtained. For the conditions of occurrence of productive formations of the Bajossian stage of the field in question, in order to ensure that the dimensions of the capillary barrier in the reservoir do not exceed the height of 1 m at an average value of the porosity coefficient Kп = 27,6 % it is necessary to fulfill the inequality of Kпр ≥ 6,1∙10−3 μm2. For the conditions of occurrence of productive formations of the Bathonian stage, in order for the dimensions of the capillary barrier in the reservoir not to exceed the height of 1 m with an average value of the porosity coefficient Kп = 28,0 % it is necessary to Kпр ≥ 7,5∙10−3 μm2. Also, on the basis of this method, various ratios of Kпр permeability coefficients and porosity were obtained Kп at which the dimensions of the capillary barrier in the deposit will exceed the heights of 1.0, 1.5 and 2.0 m above the level of the accepted oil-water contact (OWC) for the conditions of occurrence of the Kalamkas field in question. It was noted that this approach makes it possible to assess uncertainties in the position of fluid contacts when justifying the boundaries of the oil potential contour and the distribution of oil reserves in formations with a wide range of reservoir properties.

The Ammonite Catacadoceras barnstoni Beds and the Problem of the Separation of the Middle and Upper Substages of the Bathonian Stage in Northern Siberia

The Ammonite Catacadoceras barnstoni Beds and the Problem of the Separation of the Middle and Upper Substages of the Bathonian Stage in Northern Siberia

The Ammonite Catacadoceras Barnstoni Beds and the Problem of Separation of the Middle and Upper Substages of the Bathonian Stage in Northern Siberia

The paper presents the results of studying the taxonomic composition of ammonites of the family Cardioceratidae in the Bathonian of northern Siberia in the range of distribution of the last representatives of the genus Arcticoceras and the first representatives of the genus Catacadoceras to solve the problem of distinguishing between the Middle and Upper Bathonian. The incorrectness of all previous identifications of the Middle Bathonian species Arcticoceras cranocephaloide Callomon et Birkelund, 1985 based on the northern Siberian material has been established. These determinations were mainly based on Late Bathonian ammonites belonging to the species Catacadoceras barnstoni (Meek, 1859) or the form characterized by a narrow umbilicus and described herein as Cat. aff. barnstoni. An updated zonal scale of the Middle–Upper Bathonian of Siberia based on ammonites is proposed, which is recommended for use both in the regional stratigraphic schemes of the Jurassic deposits of Eastern (Middle) Siberia and in the Boreal (Siberian) zonal standard of the Bathonian Stage. The extreme importance of preserving an auxiliary biostratigraphic subdivision in this scale, the Cat. barnstoni Beds, is noted, which is due to the wide distribution and abundance of the index species in Siberian sections, as well as its significance for the correlation of the lower part of the Upper Bathonian of northern Canada, northern Siberia, Franz Josef Land and East Greenland. In addition, two intervals, namely interval with Cat. aff. barnstoni and interval with Cat. perrarum, are fixed as benchmarks, which are useful for the rapid age estimation and correlation of the Upper Bathonian deposits in Arctic sections within the upper part of the Cat. barnstoni Beds.

Late Middle Jurassic aridification event in the Qaidam Basin, Northwest China: palynological and geochemical evidences

Jurassic is characterized by a highly fluctuating climate. During the late Middle to Late Jurassic, Northwest China, including the Qaidam Basin, underwent a major climate transition from humid to arid conditions. In this study, we focused on the climate transitional interval (Bathonian Stage, Jurassic) and conducted high-resolution palynological and geochemical analysis to constrain more accurately this aridification event. We found that at the uppermost part of the Middle Jurassic Shimengou Formation, the relative abundance of Classopollis pollen increases along with a dramatic drop in the chemical index of alteration (CIA) and C-value, indicating a significant climate transition from humid to arid conditions during late Bathonian. Other geochemical proxy indicators, such as salinity and bulk carbonate carbon and oxygen isotopes, suggest the lacustrine environment of a closed saline lake. These significant changes indicate an environment of flourished xerophytic vegetation, weakened chemical weathering and enhanced lake salinization in the Qaidam Basin during late Bathonian, a critical turning point of the late Middle Jurassic climate from humid to arid conditions in the area, instead of Callovian (the red bed) as previously suggested.

Source of Roman stone for Aquae Sulis (Bath, England): field evidence, facies, pXRF chem-data and a cautionary tale of contamination

The Roman town of Bath (Aquae Sulis), renowned for its Temple to Minerva and thermal baths complex, is estimated here to have required around 500,000 m3 of stone for its construction. This huge amount of stone was likely to have been supplied from quarries within 5 km of the town, located towards the tops of the hills around Bath. Observations at the many old quarries show few features indicating Roman exploitation except for one Lewis bolt-hole and reports of chisel marks. The features of the majority of the stone in the Roman Baths-Temple Complex all suggest that the stone was sourced largely from the Combe Down Oolite Member (CDO; Jurassic, Bathonian stage), rather than the Bath Oolite Member (BO), of the Great Oolite Group. A portable X-Ray Fluorescence (pXRF) instrument, used to determine the trace element geochemistry of Bath Stone for comparison with the Roman stone, shows that the CDO and BO are very similar, except for different contents of Si, Al, Fe and Mn. These likely reflect variations in clay and organic matter content. However, with regard to the Roman stone and sculptures in the Baths-Temple Complex, all analyses of surfaces show enrichment in virtually all elements, but especially in P, Si, K, Al, S, Cl, Fe, Pb, Zn, Nb, and As. This contamination is largely attributed to the buried nature of the site (5�8 metres) from the 5th century AD until the end of the 19th century, during which time the stone would have been affected by groundwater, mostly derived from the hot-springs, with its high content of many elements. Analyses of cores cut into blocks of Roman stone show that the contamination is absent after 1�2 cm. This study demonstrates that care must be exercised in using geochemical analyses of ancient building materials for provenance studies, and that fresh surfaces of the material may well be required.

Sauropod teeth from the Middle Jurassic of Madagascar, and the oldest record of Titanosauriformes

AbstractHere we describe 31 fossil teeth, deposited in the palaeontological collections of the Museo di Storia Naturale di Milano (MSNM), that come from the inland portion of the Mahajanga Basin, NW Madagascar, namely from the Sakahara Formation (classically known as Isalo IIIb subunit), which is dated to the Bathonian stage of the Middle Jurassic. Based on detailed morphological characters, the eight morphotypes recognized herein are tentatively referred to four sauropod taxa: Archaeodontosaurus descouensi, ‘Bothriospondylus madagascariensis’, Lapparentosaurus madagascariensis and an indeterminate specialized eusauropod, which may represent a new species and provides the first evidence of a Bathonian diplodocoid in Madagascar. The identification of the teeth is corroborated by comparative examination of morphometric data. We provide evidence that Titanosauriformes were present in the Bathonian, on the basis of seven specimens referable to this clade. We also discuss in detail some dental characters that support the existence of a clear niche partitioning between the abovementioned taxa that co‐existed in the Malagasy Middle Jurassic terrestrial ecosystem. We hypothesize, for the first time, a direct correlation between the pattern drawn on the tooth crown by the enamel wrinkles and the feeding ecology of sauropod dinosaurs. The enamel wrinkles probably played a structural function: coarse wrinkles were associated with a diet composed mainly of hard foodstuff, whereas fainter wrinkles, which appeared in more derived morphologies, were associated with a diet composed of softer foodstuff.

Astrochronology of the Middle Jurassic Buqu Formation (Tibet, China) and its implications for the Bathonian time scale

The Buqu Formation in Qiangtang Basin in China was formed during the Bathonian Stage of the Middle Jurassic. The subject of this study is the formation at Well QZ11 at a depth of from 34 to 591m, located in the Southern Depression of Qiangtang Basin. The optimal accumulation rate from the result of ASM analysis was used as the “target” to tune the GR (gamma ray) and DLR (double lateral resistivity) series in QZ11. Power spectra analysis in tuned GR and DLR series demonstrate the existence of a series of prominent astronomical periods within the Buqu Formation in the Qiangtang Basin. Then ~100kyr bandpass filtered curve was used to establish a floating astronomical time scale for the Buqu Formation, and the maximum and minimum sedimentation rates without considering the factor of formation compaction were 150m/Myr and 395m/Myr, respectively. Using this scale, the lower limit for the duration of the formation is estimated to be approximately 2.09Myr. This duration can serve as a reference for temporal and chronological calibrations of the Bathonian Stage.

A new macroflora from the Middle Jurassic (Bathonian) of NW Jordan and its palaeoenvironmental significance

A new macroflora from the Middle Jurassic (Bathonian) of NW Jordan and its palaeoenvironmental significance

SYSTEM FOR EVALUATION TECHNOLOGICAL EFFICIENCY OF GEOLOGICAL AND TECHNICAL ACTIONS ON THE RESULTS OF THE INDICATOR RESEARCH

The results of indicator research of oil reservoirs, conducted in 2011 and 2013 on the same areas of injection wells oil field in Republic of Kazakhstan. Studies were conducted on layers Ю-I - Ю-X, in which productive sediments are Callovian Stage, Bathonian Stage, Bajocian Stage of Jurassic. As a result of research the values calculated the total weight of the recovered from the mouths of the producing wells of the indicator (%), calculate the volume (m3) and performance channels of LFR (m 3 / day) in the inter-well space, the actual speed of the labeled water (m / day), permeability of LFR channels (μm 2 ) and hydraulic conductivity (μm 2 *cm/cPs). The aim of the research of 2011 was study the hydrodynamic characteristics of the layers, identify of fluid flow in areas of low filtration resistance in the inter-well space. The aim of the research of 2013 were performance evaluation conducted in 2012, geological and technical measures (GTM), aimed at increasing oil recovery in the areas of indicator research 2011. The task was to create a system of evaluating the effectiveness of GTM in the field of low filtration resistance. Have been proposed evaluation criteria sweep efficiency of developed oil formations. All areas on the results of studies carried out during the period 90 - 120 days, i.e. in the areas of low filtration resistance, divided into three groups: uniform water flood, water flood ambiguous, non-uniform water flood. The article presents the parameters used for classification. Classification allows you to recommend priority areas for inclusion in the plan of GTM to increase oil production. Conducting of indicator research before and after the GTM allows to estimate the changing nature of water flood on the area and to draw conclusions on the effectiveness of these measures. Transition area injection well from the group of non-uniform displacement to uniform water flood talk about the positive result of conducted GTM.

Facies associations of the Bathonian Hamam Formation from Northwestern Jordan

Two stratigraphic sections of the Hamam Formation (Bathonian Stage, Middle Jurassic) exposed in the western part of Wadi Zarqa region, northwestern Jordan, are described and interpreted on the basis of palynoflora and facies analysis in order to reconstruct their depositional environments and sequence stratigraphic framework, which not discussed before. Five facies associations have been identified in the Hamam Formation characterized by a mixed carbonate–siliciclastic ramp setting, ranging from incised fluvial valley fill facies, beach foreshore restricted inner ramp to high-energy shoals and mid-ramp settings. The palynoflora includes well-preserved miospore assemblages which are recorded only from the incised fluvial valley fill facies for the first time and yielded 64 miospore species belonging to 40 genera. Most of these taxa are long-ranging and have been reported from Jurassic and Cretaceous rocks worldwide, except Callialasporites dampieri, Murospora florida, Granulatisporites jurassicus, Piceites expositus, Pityosporites parvisaccatus, Leptolepidites verrucatus, and Protopinus scanicus which have short ranges in the Middle Jurassic. Furthermore, these rocks are rich in shallow-marine Neo-Tethys macro-invertebrates supporting a Bathonian age. Two third-order depositional sequences bounded by three regional unconformities at the Bajocian–Bathonian and Bathonian–Callovian stage boundaries as well as within the Bathonian are defined based upon facies characteristics and stratal geometries. A regional correlation of sequence boundaries of similar age indicates that they are eustatic in origin.

Assessment of the geological heritage of Cape Mondego Natural Monument (Central Portugal)

Abstract In the western coast of Portugal, Cape Mondego is a well-known set of outcrops due to its international stratigraphic relevance given by the establishment of two stratotypes: the Global Boundary Stratotype Section and Point (GSSP) for the base of the Bajocian Stage and the Auxiliary Stratotype Section and Point (ASSP) for the base of the Bathonian Stage. The remarkable geodiversity of these Jurassic outcrops justifies the implementation of strategies in order to conserve and promote the geosites, which include a rich palaeontological record of macrofossils, microfossils and dinosaur footprints. Based on the exceptional quality of the geological record, on its international importance and on its high scientific and educational values, this area was classified in 2007 as Natural Monument. However, no geosite systematic inventory was ever done. In this work, a systematic identification, characterization and assessment of geosites was done in the Natural Monument. Based on fieldwork and published data, a first set of 32 potential geosites was identified taking into account their scientific, educational and touristic values. After the application of three criteria (representativeness, singularity and proximity) this group of 32 potential geosites was reduced to 12. These 12 geosites were assessed resulting on the establishment of a medium to high ranks for both educational and geotouristic potential uses. Based on this assessment, some valuing strategies were proposed, aiming at the sustainable use and the promotion of the Natural Monument geosites, within the scope of both educational and geotouristic activities addressed to secondary school students and the general public.

RADIOLARIAN AGES FOR THE SEDIMENTARY COVER OF SEVAN OPHIOLITE (ARMENIA, LESSER CAUCASUS)

Age data of Armenian ophiolites are of great importance for understanding the palaeogeographic and geodynamic evolution of Tethys in the Caucasus area and for lateral correlations of Tethyan ophiolitic belts in the Middle East area. Moderately well-preserved radiolarian assemblages obtained during this study from the Sarinar section of eastern Armenia allow to investigate the sedimentary and volcanic history recorded in a ca. 30 m-thick radiolarite sequence associated with spilitic lavas of the Sevan ophiolitic suture zone. The latest Bajocian-early Bathonian assemblage yielded from a sample situated close to the base of the studied sequence suggests that radiolarian ooze accumulation took place on oceanic crust of late Middle Jurassic age (probably Bajocian). Several tuffite levels are intercalated within the lower to middle part of the radiolarite sequence; a late Bathonian-early Oxfordian assemblage from a sample taken just below the uppermost tuffitic bed, suggests that during this time the oceanic domain of Sevan ophiolites was close to subaerial volcanic activity.

OPHIOLITES FROM THE GRAMMOS-ARRENES AREA, NORTHERN GREECE: GEOLOGICAL, PALEONTOLOGICAL AND GEOCHEMICAL DATA

The Grammos-Arrenes ophiolites crop out between the Pindos Flysch to the East and the Meso-Hellenic basin to the West and represent the northernmost exposure of the Pindos Ophiolite belt in Greece. Field work and sampling on key outcrops of the Sub-ophiolitic Melange provided new data on the oceanization age and on the geochemistry of the effusive products. The basalts samples show a N-MOR affinity and are associated with latest Bajocian-early Bathonian radiolarian cherts. Emplacement of the ophiolites onto the continental margin is preceded by mass flow deposition of ophiolitic material in the basins facing the advancing ophiolitic nappe. Ophiolite-bearing debris flow deposits and slide blocks are recognized in the Early Cretaceous deep water sediments of the Beotian Unit and in the upper portion of the Tertiary Pindos Flysch. The intercalations of ophiolitic material in the Beotian Flysch first, and in the Pindos Flysch, later, are interpreted as the forerunners of the Ophiolitic Nappe which derived from the Vardar Ocean to the east, and was emplaced westwards onto the Adria continental margin. The collected data allow consolidating the constraints for the timing and mechanism of ophiolite emplacement in the Pindos-Grammos area. In addition, taking into consideration the geometry of the tectonic stack in the Grammos-Arrenes area and the ages of the involved sedimentary deposits it is possible to reconstruct a geodynamic history comparable to that of the other zones studied along the Dinaric-Hellenic Chain.

The Global Boundary Stratotype Section and Point (GSSP) for base of the Bathonian Stage (Middle Jurassic), Ravin du Bès Section, SE France

The Bathonian GSSP is defined at the base of limestone bed RB071 in the Ravin du Bes Section, Bas-Auran area, southern Subalpine Chains (France). This section satisfies most of the erquirements recommended by the International Commission on Stratigraphy. The exposure extends over 13 m in thickness. No vertical biofacies, ichnofacies or taphofacies changes, stratigraphic gaps or hiatuses have been recorded at the Bajocian-Bathonian transition. Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant constraints. There is a well-preserved, abundant and diverse fossil record, with key markers (ammonites and nannofossils). The base of Bathonian Stage and Zigzag Zone corresponds to the first occurrence level of Gonolkites convergens Buckman that coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils are present in all beds. The GSSP is below the Tethyan calcareous nannofossil NJT10b/11 zonal boundary (lowest Watznaueria barnesiae) and above the Bo real nannofossil NJ10/11 boundary (lowest Pseudoconus enigma). Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray data corroborate interpretation of an Early Bathonian deepening half-cycle of second order. The criteria of accessibility, conservation and protection are assured by the “Reserve Naturelle Geologique de Haute Provence”. The Cabo Mondego Section (Portugal) is the Bathonian auxiliary section and point (ASSP) .T he proposal was voted by the Bathonian Working Group in December, 2007, and by the International Subcommission on Jurassic Stratigraphy in March, 2008, approved by the ICS in June 2008, and ratified by the IUGS in Ju,ly 2008.

Formal proposal for the Bathonian GSSP (Middle Jurassic) in the Ravin du Bès Section (Bas-Auran, SE France)

The Bathonian Global Stratotype Section and Point (GSSP) is proposed at the base of limestone bed RB071 (bed 23 in Sturani 1967) in the Ravin du Bès Section (43° 57′ 38′′ N, 6° 18′ 55′′ E), Bas-Auran area, “Alpes de Haute Provence” French department. The Ravin du Bès Section, as formal candidate GSSP for the base of the Bathonian Stage, satisfies most of the requirements recommended by the International Commission on Stratigraphy: 1) The exposure extends over 13 m in thickness. At the Bajocian-Bathonian transition, no vertical (bio-, ichno- or tapho-) facies changes, condensation, stratigraphic gaps or hiatuses have been recorded. Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant constraints. 2) There is a well-preserved, abundant and diverse fossil record across the boundary interval, with key markers (ammonites and nannofossils) for worldwide correlation. The base of the Bathonian Stage and Zigzag Zone in Bas-Auran corresponds to the first occurrence level of Gonolkites convergens Buckman, which coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils, as secondary global marker, are present in all beds and allow characterizing the Bajocian-Bathonian transition. 3) Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray data corroborate an Early Bathonian deepening half-cycle of second order. 4) The criteria of accessibility, conservation and protection are assured by the “Réserve Naturelle Géologique de Haute Provence”. The Cabo Mondego Section (Portugal) is suggested as the Bathonian auxiliary section and point (ASSP) within this GSSP proposal.

GEOCHEMISTRY AND TECTONO-MAGMATIC AFFINITY OF MAFIC EXTRUSIVE AND DYKE ROCKS FROM THE OPHIOLITE MELANGE OF THE SW ZAGORJE-MID-TRANSDANUBIAN ZONE (MT. MEDVEDNICA, CROATIA)

Mafic extrusive rocks occasionally intersected by dolerite dykes are the most abundant fragments of oceanic crust in the lower Callovian to upper Valanginian ophiolite melange exposed on the Mt. Medvednica, at the SW tip of the Zagorje-Mid-Transdanubian Shear Zone. The melange includes a chaotic mixture of various extrusive rocks which may be analogues of the oceanic rocks derived from (at least) two different tectonic settings. The extrusive and dyke rocks of this melange are geochemically classified into three magmatic groups: high-Ti group of N-MORB-like with arc signatures (Ti/V = 26.3-38.5), medium- Ti group (Ti/V = 20.2-23.0) and low-Ti group showing untypical LREE depleted [(La/Sm)cn = 0.49-0.62] IAT-like signatures (Ti/V = 13.2-18.0). All three geochemical groups of magmas were successfully monitored by clinopyroxene geochemistry. The magmas have consistent initial Nd-isotopic values (eNd(T=163 Ma) +5.9 to +6.3) and initial 87Sr/86Sr ratios (0.703748-0.704286) suggesting a similar parental source related to the initial stage of intraoceanic subduction of a back-arc basin ridge. It represents the new crust formed from melts derived from LREE extremely depleted MORB-type mantle which was affected by concomitant metasomatism and partial melting in a suprasubduction forearc setting. K-Ar ages on plagioclase separates indicate uppermost Bathonian - lower Oxfordian magmatic activity between 165±3.3 Ma and 160±3.3. A geodynamic model for the Mt. Medvednica ophiolites, part of the Repno oceanic domain of the Meliata-Maliak oceanic system, is here proposed. There are many common geochemical and geological correlation features between the Mt. Medvednica and Szarvasko ophiolites, located in NE Hungary that allow to integrate them in the same oceanic domain, whereas this is not obvious in the case of the Dinaric ophiolites.

CALLOVIAN RADIOLARIANS FROM THE LOWERMOST CALCARE SELCIFERO DI FONZASO AT PONTE SERRA (TRENTO PLATEAU, SOUTHERN ALPS, ITALY)

This paper deals with radiolarian biostratigraphy of the Calcare Selcifero di Fonzaso at the type-section of Ponte Serra (Southern Alps, Italy). The Ponte Serra section is located on the easternmost part of the Trento Plateau and represents an early-drowned sector that marks the transition to the Belluno Trough.The Calcare Selcifero di Fonzaso consists of pelagic siliceous limestones of Middle to Late Jurassic age for a total thickness of about 70 m. A 24 metres thick package of resedimented limestone is interstratified. This research is focused on the lowermost part, 9 metres thick, that mainly consists of red and green siliceous micrites with interbedded clay beds and abundant chert lenses and nodules. The studied interval lies between the lower member of the Rosso Ammonitico Veronese Formation below and the resedimented limestone above. The recovered radiolarian assemblages indicate a Callovian age for the base of the Calcare Selcifero di Fonzaso. Correlation with other sections of the Trento Plateau reveals a significant diachronism in siliceous sedimentation that we relate to highly irregular sea-bottom paleotopography. The base of siliceous deposits can be even older (Bathonian) on the eastern margin of the Trento Plateau, but younger (Oxfordian) on its western margin. The Callovian succession of the Belluno Trough consists of radiolarian-bearing micrite alternating with oolitic limestone. The oolitic sands that filled in the Belluno Trough during the Middle Jurassic prograded over the eastern margin of the Trento Plateau in the early Oxfordian.

Ammonoid succession at the Bajocian-Bathonian transition in the Bas Auran area, Digne district, South-East France.

The uppermost Bajocian to lowermost Bathonian ammonoid succession has been studied in the Bas Auran area in view of choosing one of its sections as the Global Stratotype Section and Point (G.S.S.P.) of the Bathonian Stage. The Bas Auran ammonite assemblages display exceptional values of record quality. The stratigraphic distribution of 629 specimens referred to 63 species and 35 genera, collected during the last forty years from three sections (Ravin du Bes, Ravin d'Auran, Ravin des Robines), is plotted and analyzed, bed by bed. Over 85 stratigraphic levels, through 9 m in thickness, have been studied at the top of the Marno-calcaires a Cancellophycus formation, ranging from the latest Bajocian (Parkinsoni Zone, Bomfordi Subzone) to the earliest Bathonian (Zigzag Zone, Convergens Subzone) and the base of the Macrescens Subzone. Ammonoid assemblages are composed of Northwest European and Mediterranean elements, associated with Sub-Mediterranean ones, allowing chronocorrelation between the Northwest European Convergens Subzone and the Sub-Mediterranean Parvum Subzone. The basal boundary of the Zigzag Zone and of the Bathonian Stage is placed at the base of Sturani's bed 23 in the Ravin du Bes section and is identified by the first occurrence of Gonolkites convergens, and coincides with the loest occurrence of Morphoceras parvum. Features of the ammonoid succession indicate relatively homogeneous and good record quality, gradual biostratigraphic change and high degree of taxonomic similarity between the Bomfordi and Convergens subzones. Palaeontological criteria also indicate relatively high values of palaeontological and stratigraphic completeness at the base of levels RB070-RB071 (= level 23 in Sturani 1967) which corresponds to the Bajocian/Bathonian boundary. The Ravin du Bes Section, with forty-six successive ammonoid fossil-assemblages of the Convergens Subzone belonging to three biohorizons through five metres of thickness, shows maximum values of biostratigraphic and biochronostratigraphic completeness, being one of the most complete in the world. Therefore, the Ravin du Bes Section should be chosen as the Global Stratotype Section and Point for the base of the Bathonian Stage. Two biostratigraphic logs summarize the ammonite content from the uppermost Bomfordi to basal Macrescens subzones. Three plates illustrate the most characteristic ammonoid taxa at the Bajocian-Bathonian passage in the Bas Auran succession.

NEW GEOCHEMICAL AND AGE DATA ON THE OPHIOLITES FROM THE OTHRYS AREA (GREECE): IMPLICATION FOR THE TRIASSIC EVOLUTION OF THE VARDAR OCEAN

This paper presents new radiolarian biostratigraphic and volcanic geochemical data for the Agoriani Melange Unit and Fourka Unit, which represent the basal part of the Othrys ophiolitic nappe (central Greece). The Agoriani Melange Unit is the base of the Othrys ophiolitic nappe, consists of a tectono-sedimentary melange that is, in turn, tectonically overthrust by the Fourka Unit, which consists of basalt-radiolarian chert sequences. Geochemical data show that, in the study area, the Agoriani Melange includes alkaline within-plate basaltic (WPB) and normal-type mid-ocean ridge (N-MORB) sequences in the pillowfacies, while the Fourka Unit mainly consists of normal-type mid-ocean ridge basaltic (N-MORB) sequences, as well as low-K tholeiitic (L-KT) sequences, both in occurring in the pillow-facies. Alkaline WPB have a clear enriched nature, with high concentration of high field strength elements (HFSE), relative enrichment in light (L-) rare earth elements (REE) with respect to heavy (H-) REE, and high Nb/Y (> 0.7), Ce/Y (> 1.5) Ta/Hf (> 0.35), Th/Yb (> 0.7) and Ta/Yb (> 0.3) ratios. N-MORB rocks have a more depleted nature with flat N-MORB normalized HFSE patterns, flat or slightly LREE depleted chondrite normalized REE patterns, and low Nb/Y (< 0.3), Ce/Y (< 0.5) Ta/Hf (< 0.1), Th/Yb (< 0.08) and Ta/Yb (< 0.05) ratios. L-KT basalts show an evident subduction- related geochemical signature exemplified by depletion of Ta, Nb, Zr, and Ti relative to other HFSE, slight LREE/HREE enrichment, and relative enrichment of Th with respect to Ta. Biostratigraphic data for the Fourka Unit show that radiolarites associated with N-MORBs were deposited, on the whole, in Middle and Late Triassic time. In particular the late Anisian - early Ladinian age found in one stratigraphic section represents the oldest age ever reported for a MORB sequence in the Hellenides. Radiolarites deposited over the L-KT basaltic sequence are early Carnian - middle Norian/Late Norian in age. The radiolarites associated with alkaline WPB in the Agoriani Melange did not yield determinable radiolarians, while a radiolarite sequence in this melange not associated with basalts is middle Bathonian to late Bathonian/early Callovian in age. The data presented in this paper provide new constraints on the early (Middle- Late Triassic) phase of evolution of the Vardar Ocean in the Hellenides. The MORBs are interpreted to have erupted during Middle-Late Triassic time, related to the initial opening of the Vardar oceanic basin. The L-KT basalts are interpreted to have erupted during Middle-Late Triassic time (i.e., in part contemporaneously with MORBs), in the ocean-continent transition zone adjacent to the rifted continental margin. Moreover, we speculate that the alkaline WPB have formed in oceanic seamounts or in the ocean-continent transition zone adjacent to the rifted continental margin.

VARIATION IN GROWTH‐RATE AND FORM OF A BATHONIAN (MIDDLE JURASSIC) OYSTER IN ENGLAND, and ITS ENVIRONMENTAL IMPLICATIONS

Abstract: We review controls on extensional growth rate and shell thickness in the extant oyster Crassostrea. Data on these shell parameters for the ecologically similar Bathonian oyster Praeexogyra hebridica, sampled at carefully selected sites, are then used to test a hypothesis that small size in contemporaneous marine bivalves of north‐west Europe was the product of reduced salinity. The hypothesis is refuted, at least to the extent that some additional factor must be involved. The relatively low extensional growth rate and shell thickness at the highest‐salinity site, together with the elongate (‘etiolated’) shape there and the low extensional growth rate at all sites in comparison with Crassostrea, suggests that this factor is reduced primary productivity. Other faunal, sedimentological and diagenetic evidence is consistent with low productivity. We point to other possible instances of reduced productivity during the Middle Jurassic and discuss the possible role of this factor in encouraging the widespread colonization of brackish‐water environments by bivalves in the Bathonian Stage.