Lower Callovian Research Articles

First record of the heteromorph ammonite genus Parapatoceras Spath, 1924 from the lower Callovian (Middle Jurassic) of Algeria

First record of the heteromorph ammonite genus Parapatoceras Spath, 1924 from the lower Callovian (Middle Jurassic) of Algeria

Microbial mats and their palaeoenvironmental analysis in offshore - shelf facies of the Los Molles Formation (Toarcian - Lower Callovian) in the Chacay Melehue area, Neuquén Basin, Argentina.

This contribution presents the first study focused on the analysis of microbial mats in the Los Molles Formation (Toarcian - Early Callovian), Neuquén Basin, Argentina. This unit mainly represents offshore-to-shelf environments affected by storms and density currents. The Los Molles Formation is one of the oldest source rocks in the Neuquén Basin and constitutes an unconventional shale gas reservoir of great economic importance. The aim of this work was to identify the microbial activity from the description and interpretation of microbially induced sedimentary structures (MISS), to determine the paleoenvironmental and paleoecological conditions under which they formed, and to establish a possible relationship between these structures and the trace fossil Trichichnus. Samples from the levels with MISS were analyzed and described from macroscopic and binocular observations, petrographic microscope thin sections, and SEM samples with EDS analyses. The results showed several levels of microbial mats presenting diverse MISS, including biolaminations and Kinneyia-like wrinkles structures that were described at the macroscopic level. In thin sections, biolaminations, filament-like microstructures with different degrees of development, oriented grains and pyrite were observed. SEM images and EDS analyses showed different types of filaments, coccoids and EPS with high concentrations of carbon. These results revealed that the studied levels fulfill the established biogenicity criteria, guaranteeing that they have a bacterial origin. The abundance of the trace fossil Trichichnus sp. throughout the section and the proximity to some Kinneyia-like wrinkle structures levels suggests that the same organisms may have generated them. Furthermore, they revealed that the Los Molles Formation, at the time of its deposition, experienced paleoecological and paleoenvironmental conditions appropriate for the establishment and development of microbial mats. The extensive levels of microbial mats in the study area suggest that they may have been a source of organic matter for the generation of hydrocarbons from the Los Molles Formation.

Tracing ecological fluctuations during the Middle to Late Jurassic of Jhura, Kutch, India, using foraminifera

The aim of this work is to examine ecological patterns by analyzing the distribution of foraminifera in the Jurassic sediments of the Jhura area, Kutch, India. 87 foraminiferal species were recovered, which were analyzed for paleoecological interpretations. Three techniques were employed: 1) Diversity indices, in which four episodes are identified based on different indices values; 2) Clustering and nMDS ordination approaches identify four assemblages of benthic foraminifera, which differ in biotic and abiotic characteristics; 3) Morphogroup analysis, which divides the studied sequence into four paleoecological units. Four depositional episodes are recognized within the studied interval: Episode I (Bajocian to Bathonian), Spirillina polygyrata assemblage shows the calm environment with a high diversity of foraminiferal species and enough nutrients in the outer shelf zone having normal salinity and high level of dissolved oxygen. Episode II (Lower Callovian), Miliammina cf. M. valdensis assemblage occurred in a stressed condition with low diversity of species in a shallower portion of mid-shelf in a hypo-saline environment and reduced oxygen condition having low to average food influx. Episode III (Middle to Upper Callovian), Lenticulina subalata assemblage indicates that this portion deposited in stress-free environmental conditions having normal salinity and well‑oxygenated waters in the deepest section of the outer shelf region with high nutrient supply. Episode IV (Oxfordian), Lenticulina bulla assemblage thrived in hyposaline conditions with low oxygen levels and enough nutrients in the deeper portion of the mid-shelf region. An unstable oceanic shelf is interpreted during the deposition of the examined section because of the fluctuations in all inferred parameters.

Middle and Late Jurassic record of sea-level, sequence development, and carbon-isotope fluctuations, Tethyan Adriatic Carbonate Platform, Croatia

The Adriatic Platform, Croatia is a large isolated carbonate platform that developed under a warm greenhouse climate interspersed with cooler episodes in western Tethys. However, the δ13C record and high-resolution sequence stratigraphy of the Middle and Upper Jurassic portion generally is poorly known. The ~1900 m thick (30 Myr duration) mainly Middle to Upper Jurassic succession of the platform was studied bed-by-bed to track the fluctuations of δ13C, facies stacking into bundles, superbundles, and sequences and potential sea-level changes within ongoing studies of evidence of astronomical forcing of the units. The study interval consists of cyclic subtidal intervals (upper Toarcian-lower Aalenian, lower Callovian, Oxfordian, lower Kimmeridgian, and lower Tithonian) interspersed with highly cyclic peritidal units (upper Bajocian-lower Bathonian, upper Kimmeridgian, and upper Tithonian-basal Berriasian). Ages were constrained by biostratigraphy and δ13C chemostratigraphy. The absolute δ13C values obtained from the shallow-marine bulk carbonate matrix are similar to those from coeval pelagic carbonates and exhibit similar trends, are characterized by overall little data spread (<2‰), and generally show little resetting associated with disconformities and/or burial. This rock-buffered record likely preserved at least relative marine δ13C values with several major excursions, including the BjBaE (Bajocian-Bathonian), CaOxBE (Callovian-Oxfordian boundary), EOxE, and MOxE (Early and Middle Oxfordian). The studied interval provides a detailed record of Milankovitch induced platform-flooding and exposure at scales from ~1Myr to ~20 kyr. The study suggests that detailed research of similar thick Jurassic isolated platform successions could provide valuable records of δ13C, climate, and sea-level changes over relatively long geological intervals.

Discovery of chitinoidellids and calpionellids from the Blue Nile Basin and the Jurassic-Cretaceous boundary in Ethiopia

The deposition of Mesozoic carbonates in the Horn of Africa is pervasive and forms an important marker for inter- and intra-basinal correlation. In central Ethiopia, this time interval is represented by transgressive deposits of the Antalo Limestone Formation in the Blue Nile and the Mekelle basins, and the Hamanlei Limestone Formation in the Ogaden Basin. These carbonate formations have traditionally been assigned to range from ?Lower to Upper Jurassic. Recently, based on calcareous nannofossils from the Blue Nile Basin, the Antalo Limestone Formation was constrained to between the lower Callovian and upper Tithonian with a suggestion of the presence of Cretaceous strata in the Blue Nile Basin, based on the record of Uniplanarius sissinghii in the Mugher section. Here, based on the discovery of chitinoidellids and calpionellids from two sections of the Blue Nile Basin, we confirm the presence of Cretaceous deposits and conclusively extend the age of the Antalo Limestone Formation to late Berriasian. Three calpionellid zones are recorded from the Blue Nile Basin: Bonetinae and Chitinoidellids/Primitive Calpionellids subzones, both assigned to the upper Tithonian Microcanthum ammonite Zone and the Calpionellopsis Zone (?Simplex‒Oblonga/Oblonga-Praecalpionellites Subzone) corresponding to the upper Berriasian ammonites Boissieri Zone. Two stratigraphic hiatuses: Kimmeridgian‒lower Tithonian and uppermost Tithonian‒lowermost upper Berriasian are noted in both sections. This discovery of calpionellids has a profound effect not only on the biostratigraphy of Ethiopia but also has allowed the integration of the study region of eastern Africa, for the first time, within the wide calpionellid domain of the South Tethysian margin.

Lower Callovian of Kanev Dislocations, Cherkasy Oblast, Ukraine: Ammonites and Stratigraphy

Lower Callovian of Kanev Dislocations, Cherkasy Oblast, Ukraine: Ammonites and Stratigraphy

Middle Jurassic (Upper Bathonian and Lower Callovian) jaws of Kosmoceratid ammonites of Central Russia

Middle Jurassic (Upper Bathonian and Lower Callovian) jaws of Kosmoceratid ammonites of Central Russia

SOURCE ROCK POTENTIAL AND DEPOSITIONAL ENVIRONMENT OF MIDDLE – UPPER JURASSIC SEDIMENTARY ROCKS, BLUE NILE BASIN, ETHIOPIA

This study investigates the hydrocarbon generation potential, kerogen quality, thermal maturity and depositional environment of Middle – Upper Jurassic sedimentary rocks in the Blue Nile Basin, Ethiopia, using organic petrography, Rock‐Eval pyrolysis and molecular organic geochemistry. Thirty‐seven outcrop samples were analysed for their total organic carbon (TOC) and inorganic carbon (TIC) contents. The samples came from a Toarcian – Bathonian transitional glauconitic shale‐mudstone unit, the overlying Upper Bathonian Gohatsion Formation, and the Lower Callovian – Upper Tithonian Antalo Limestone Formation. Thirteen samples with sufficient TOC contents for further analysis of the organic matter, eight from the Antalo Limestone Formation and five from the glauconitic shale‐mudstone unit, were selected and analysed using Rock‐Eval pyrolysis. Vitrinite reflectance (VRr) was measured on random particles, and qualitative maceral analysis was performed under normal incident and UV light. Nine samples were selected for molecular organic‐geochemical analyses. All the samples originating from the Gohatsion Formation showed TOC values which were too low for further analyses of the organic matter.The TOC contents of shales and limestones from the Antalo Limestone Formation and and of shales from the glauconitic shale‐mudstone unit were 3.43‐6.43% (average 4.85%) and 0.76‐3.15% (average 1.72%), respectively, and two coaly shale samples from the latter unit have average TOC values of 18.48%. HI values are very high for shales in the Antalo Limestone Formation (average 575 mg HC/g TOC) but lower for the shales in the glauconitic shale‐mudstone unit. The vitrinite reflectance of shales from the Antalo Limestone Formation ranged between 0.21% and 0.47%; coaly shales from the glauconitic shale‐mudstone unit have VRr% of between 0.29% and 0.35%. Pr/Ph ratios for samples of the Antalo Limestone Formation shales ranged from 0.8 to 1.1, indicating anoxic to suboxic depositional conditions; while shales in the glauconitic shale‐mudstone unit show higher values of up to 4.9.In terms of organic petrography, the Antalo Limestone Formation samples are dominated by finely dispersed liptinite particles and alginite; the organic material in the glauconitic shale‐mudstone unit is of higher land plant origin, with abundant vitrinite and inertinite. Sterane and hopane biomarker ratios suggest an anoxic/suboxic depositional environment for the Antalo Limestone Formation shales and limestones. These values together with Rock‐Eval Tmax (average 414 °C), the high ratio of pristane and phytane over the n‐alkanes C17 and C18, and hopane biomarker ratios indicate that the Middle – Upper Jurassic succession is of low thermal maturity in the central parts of the Blue Nile Basin. The Antalo Limestone Formation shales have a high petroleum generation potential, making them a viable target for future exploration activities.

Middle Jurassic crustacean microcoprolites and their association with terebellid polychaetes in prodelta deposits from the Neuquén Basin, Argentina

We present the oldest record of crustacean microcoprolites in the Neuquén Basin, and the first record of these coprolites for the Bajocian–Callovian of South America. They correspond to three crustacean microcoprolite ichnospecies in prodelta and turbiditic fan facies from Los Molles Formation: 1) Octotriangulella bonenburgensis Blau, Lukas and Stein from the upper Bathonian - lower Callovian (Rotundum and Vergarensis Standard Zones), 2) Palaxius salataensis Brönnimann, Cros, and Zaninetti, from the upper Bathonian (Steinmanni Standard Zone), and 3) Parafavreina ziczac Rindsberg and Kopaska-Merkel from the upper Bathonian (Steinmanni Standard Zone). Octotriangulella bonenburgensis also occurs in association with terebellid agglutinated polychaetes that make use of these coprolites to build the wall of their tubes. Sedimentological evidence indicates that microcoprolites were transported from shallower waters, whereas the presence of terebellids indicates dysoxic, low-energy microenvironments within prodelta facies with high sedimentation rates.

Oscillating redox conditions in the Middle–Late Jurassic Alpine Tethys: Insights from selected geochemical indices and 57Fe Mössbauer spectroscopy

Abstract: The Middle–Late Jurassic greenhouse conditions included CO2 levels that have been estimated to be four to five times higher than they are presently. By analysing the pattern of selected trace elements as redox proxies (V, U, and Mo) and the variations in the chemical state of iron using 57Fe Mossbauer spectroscopy, we demonstrate the occurrence of major changes in sediment redox conditions from the Late Bajocian through the Late Oxfordian in the pelagic environment of the Alpine Tethys in its Carpathian domain. Our interpretation is based on analysis of siliceous limestones rich in radiolarians intercalated with red nodular limestones of Ammonitico Rosso-type facies from the Križna Nappe in the eastern part of the Tatra Mountains, Poland. The presented redox potential (Eh values) shows its changeable pattern along the section (from −0.74 V to 0.16 V). The low values of Eh (from −0.74 V to −0.4 V) create the long-term (ca. 7 Myr) trend of the medium-scale fluctuations comprising the Upper Bajocian through the Lower Oxfordian interval of predominately grey and green radiolarian siliceous limestones. This interval comprises the three short-term (less than 0.5 Myr) periods of slightly increasing Eh (from −0.35 V to −0.17 V) observed in the uppermost Bajocian, Lower Bathonian, and Lower Callovian, as well as the significant changes in sediment redox conditions from oxic to dysoxic in the Upper Bajocian. The overlying succession corresponding to the Middle–Upper Oxfordian (ca. 4 Myr) is characterized by red radiolarian siliceous limestones with positive Eh values that indicate deposition under more oxic conditions than those during any other period in the study section. The negative values of Eh, attributed to the upwelling periods, coincide with the decreasing sea surface water temperature (SST) and increasing organic matter flux. The largest negative shift is noted in the Upper Callovian, which coincided with the cool climatic phase. The positive Eh values indicate oxic conditions due to the lowering of organic matter input, coinciding with an increased SST caused by surface water stratification. Based on the observed long-term variation in the redox, we postulate thermally driven east–west atmospheric circulation along the equatorial Tethys Ocean, caused by the sharp contrast in sea surface temperature, similar to the Walker Circulation operating across the modern Pacific. This phenomenon would have been the dominant periodical mode over the tropics and subtropics regions of the NW Tethys during the Middle–Late Jurassic. Global warming reduces the long-term strength of the mean tropical atmospheric circulation, leading to sea water stratification near the surface and consequently lowering the organic matter flux. However, additional seaways of heat transport could also exist, as they are associated with the opening/closing of the Hispanic Corridor.

Middle Jurassic terebratulide brachiopods from the Jordan Valley (northwestern Jordan)

ABSTRACT The Middle Jurassic terebratulide brachiopods from the Jordan Valley, northwestern Jordan, are herein revised and systematically described and new taxa are added to the faunal list. A new genus and species Jordanithyris ardainensis gen. et sp. nov., is introduced from the upper Bathonian (Hamam Formation) and lower Callovian (Mughanniyya Formation) of the Arda section. In the upper Bathonian Hamam Formation (consisting of limestone, dolomite, alternating sandstone, and marlstone beds), Ptyctothyris quillyensis occurs. The assemblage Arabatia bihinensis, Ectyphoria sinaiensis, Ptyctothyris quillyensis, Dorsoplicathyris dorsoplicata, and Tubithyris jurayfahiensis is recorded in the lower Callovian limestone beds with marlstone interbeds of the Mughanniyya Formation. Brachiopods of the Jordanian Hamam and Mughanniyya formations can be correlated with the fauna of the Aroussiah Formation in Sinai and the Zohar and Matmor formations in southern Israel. The sedimentary environment is interpreted as the protected shallow inner shelf, and nutrient rich habitat resulting in high faunal diversity. The high degree of endemism of this fauna is yet another confirmation of the representative pronounced Middle Jurassic endemism along the southern Tethyan margin of the Ethiopian Province.

The evolution of Oppel's ‘Macrocephalusbett’ (Callovian, Middle Jurassic)

In his basic work of 1856–1858, Albert Oppel subdivided his ‘Juraformation’ into eight stages and 32 zones. There is a long history of confusion as to the meaning of Oppel's zones. Using his ‘Macrocephalusbett’ and his ‘Zone des Amm. macrocephalus’, it is explained how his concept has been developed until today. Both and also Oppel's ‘Macrocephalus Schichten’ are chronostratigraphical units and correspond to today's Lower Callovian. The Lower Callovian was first subdivided by Buckman and Spath into three zones: Herveyi, Koenigi and Calloviense. These units hold good for the Subboreal Province in Central Europe and England and became the primary standard. In addition, there are four more secondary standard zonations in other faunal provinces. In terms of the ammonites, the amount and quality of material available improved and also the methods used to assess morphological similarities. That makes a finer resolution possible, and today, the Lower Callovian of England and Central Europe consists of eight subzones with about 20 biohorizons.

The ammonite genus Cadoceras (Cardioceratidae) in the Lower Callovian (Middle Jurassic) of the Swabian Alb and the Wutach area (Southern Germany)

The ammonite genus Cadoceras (Cardioceratidae) in the Lower Callovian (Middle Jurassic) of the Swabian Alb and the Wutach area (Southern Germany)

Ostracods of the genus Palaeocytheridea Mandelstam in the Middle and Upper Jurassic of Europe: 3. Stratigraphy and paleobiogeography

The analysis of distribution of endemic and cosmopolitan ostracods of the genus Palaeocytheridea shows that, in the Bajocian and Bathonian, the Tethyan and Boreal-Atlantic regions of Western and Eastern Europe developed in partial isolation, while beginning from the middle of the Lower Callovian, these parts of the European paleobasin were connected. For the Middle Jurassic of Western and Eastern Europe and, in particular, for Ukraine, the ostracod zones are recognized based on stratigraphic distribution of species of the genus Palaeocytheridea.

Early Callovian ingression in southwestern Gondwana. Palaeoenvironmental evolution of the carbonate ramp (Calabozo Formation) in southwestern Mendoza, Neuquen basin, Argentina

The carbonatic sequence of the Calabozo Formation (Lower Callovian) developed in southwestern Gondwana, within the northern area of the Neuquén basin, and is widespread in thin isolated outcrops in southwestern Mendoza province, Argentina. This paper describes the facies, microfacies and geochemical-isotopic analysis carried out in five studied localities, which allowed to define the paleoenvironmental conditions of a homoclinal shallow ramp model, highly influenced by sea level fluctuations, where outer, mid and inner ramp subenvironments were identified. The outer ramp subenvironment was only recognized in the south of the depocenter and is characterized by proximal outer ramp facies with shale levels and interbedded mudstone and packstone layers. The mid ramp subenvironment is formed by low energy facies (wackestone) affected by storms (packstones, grainstones and floatstones). The inner ramp subenvironment is the most predominant and is characterized by tidal flat facies (wackestones, packstones and grainstones) over which a complex of shoals (grainstones and packstones) dissected by tidal channels (packstone, grainstones and floatstones) developed. In the north area, protected environment facies were recorded (bioturbated wackestones and packstones). The vertical distribution of facies indicates that the paleoenvironmental evolution of the Calabozo Formation results from a highstand stage in the depocenter, culminating in a supratidal environment, with stromatolitic levels interbedded with anhydrite originated under restricted water circulation conditions due to a progressive isolation of the basin. δ13C and δ18O values of the carbonates of the Calabozo Formation suggest an isotopic signature influenced by local palaeoenvironmental parameters and diagenetic overprints. The δ13C and δ18O oscillations between the carbonates of the different studied sections are related with lateral facies variations within the carbonate ramp accompanied with dissimilar reactivities in relation to diagenetic fluids. The δ18O values of all sections exhibit a rather broad scatter which may be attributed to diagenesis and recrystallisation while the carbon isotopic composition has been less affected by those processes. Carbon isotope system has best retained the primary isotopic signal and δ13C values (0–3.9‰) are within the Callovian isotope range. The 87Sr/86Sr ratios of the bulk carbonates of El Plomo creek, La Vaina creek and Potimalal River sections are in agreement with the Callovian seawater Sr-isotope curve.

CALCAREOUS NANNOPLANKTON OF CALLOVIAN OF UKRAINE

Gives data of Callovian calcareous nannoplankton study of sections of the Dnieper-Donets Basin, northwestern Donbass, the Crimean Mountains. Analysis of the distribution of nanofossils in the studied sections, as well as sections of the Moscow syncline, the Saratov Volga region and Kanev dislocations can distinguish in the Callovian two nannoplankton zones: Stephanolithuin hexum (top Batonian – Lower Callovian) и Sephanolithion bigoti (middle – upper Callovian). Subspecies Sephanolithion bigoti maximum in Callovian deposits of Ukraine (and the South East European platform) is not defi ned.

Ammonites and biostratigraphy of the Cuyo Group in the Sierra de Reyes (central Neuquén Basin, Argentina) and their sequential significance

This paper deals with the stratigraphy, the systematics of the ammonite faunas and the biostratigraphy of the Cuyo Group in the Sierra de Reyes (southern Mendoza Province, Argentina). The 230 m-thick marine siliciclastic succession of the Cuyo Group is composed, from base to top, of the Bardas Blancas Formation (mudstones, sandy mudstones and fine-grained sandstones), the Los Molles Formation (black shales) and the Lajas Formation (coarse- to fine-grained sandstones). Four ammonite levels have been recognised in the Cuyo Group.&nbsp;The first two levels (I and II) are located in the lower section of the Bardas Blancas Formation. The monospecific level&nbsp;I yielded Dumortieria cf. pusilla and is regarded as upper Toarcian-lower Aalenian, whereas level II is composed of an association of Podagrosiceras cf. athleticum, Planammatoceras sp., Phylloceras sp. and Eudmetoceras? sp. which can&nbsp;be assigned to the upper Aalenian-lower Bajocian Malarguense Standard Zone. The third level appears in the middle part&nbsp;of the Bardas Blancas Formation and contains Morphoceras sp., Phylloceras sp. and Lytoceras cf. eudesianum, and is&nbsp;assigned to the lower Bathonian. The younger level (IV) was identified in the lower section of the Lajas Formation and&nbsp;yielded Iniskinites crassus, Stehnocephalites gerthi and Lilloettia cf. steinmanni. This association corresponds to the&nbsp;upper Bathonian Steinmanni Standard Zone. The ammonite record of the Cuyo Group in the Sierra de Reyes allowed&nbsp;dating the transgressive succession represented by the Bardas Blancas and Los Molles formations as late Toarcian-early&nbsp;Bathonian. The ammonite fauna identified in the lower section of the Lajas Formation (level IV) indicates that the change from transgressive to regressive conditions recorded at the upper part of the Cuyo Group occurred around early-late&nbsp;Bathonian times. The uppermost strata of the Cuyo Group (upper section of the Lajas Formation), having a well defined&nbsp;regressive signature, can be dated as lower Callovian. This relatively well constrained biostratigraphic framework of&nbsp;the Cuyo Group in the Sierra de Reyes and its comparison with neighbouring areas allowed for a better understanding&nbsp;of the sequential evolution in central Neuqu&eacute;n Basin during the late Lower to Middle Jurassic.

Organic matter from the Callovian (Middle Jurassic) deposits of Lithuania: Compositions, sources and depositional environments

Abstract This study presents the first organic geochemical and petrographical investigation of the Callovian deposits of the eastern part of the Central European Basin. It is shown that in both the terrigenous Papilė Formation (Lower Callovian) and shallow- to deeper-marine facies of the Papartinė and Skinija formations (Middle and Upper Callovian, respectively), terrestrial organic matter predominates. This is reflected by the carbon preference index values higher than 1.2 for all samples and in some cases higher than 2, as well as the occurrence of characteristic higher plant biomarkers like cadalene, dehydroabietane, simonellite and retene. Moreover, in the case of the Papilė Formation, sugiol – a natural product terpenoid produced by distinct conifer families, has been detected in clay sediments. The occurrence of such a biomolecule in the Middle Jurassic clays is reported for the first time. Its occurrence is probably connected with the presence of small wood debris in the clay sediments. In samples of the Papilė Formation, charcoal fragments co-occurring with unsubstituted polycyclic aromatic hydrocarbons were detected, indicating that wildfires took place during the Early Callovian of Lithuania and/or neighbouring areas. In the Middle and Upper marine Callovian sediments of Lithuania there is no evidence of anoxic conditions occurring in the water column. However, periodic anoxic or strongly dysoxic episodes may have occurred, most probably below the photic zone, during the deepest phase of the Late Callovian transgression, as is evidenced from pyrite framboid diameter distribution and general impoverishment of benthic fauna. Huminite reflectance ( R r ) values for the investigated area are in the range of 0.21–0.31%, suggesting the occurrence of immature organic matter. Such values indicate that these investigated deposits were close to the surface during their whole diagenetic history, and the thickness of younger cover did not exceed ca. 500 m. This is also supported by a biomarker analysis in which less thermally stable ββ -hopanes and hopenes significantly dominated.

A new decapod crustacean faunule from the Middle Jurassic of north‐west France

Abstract: Decapod crustacean material collected recently from the lower Callovian (Middle Jurassic) in Maine‐et‐Loire (north‐west France) comprises two new species of prosopid and one new species of tanidromitid crabs, of the genera Nodoprosopon and Tanidromites, respectively. Also represented in this faunule is a probable paguroid anomuran, in the form of isolated chelae here assigned to the genus Orhomalus, as well as appendicular remains of unknown affinity; some of the latter might belong to prosopid crabs. These anomurans and brachyurans co‐occur with a diverse benthic fauna in limestones with abundant iron ooids; their main interest lies in the fact that they add valuable data to the rather poor record of Middle Jurassic decapod crustaceans.

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.