Late Oxfordian Research Articles

Uplift, deflation and marine onlap of a Jurassic rift dome, illustrated by a backstepping Middle–Upper Jurassic shelf‐to‐slope succession, Geographical Society Ø, East Greenland

Abstract“Doming–rifting–spreading” constitute the main phases of classical plate tectonics. The Middle–Upper Jurassic succession of East Greenland is interpreted as reflecting formation, deflation and onlap of a rift dome, similar to contemporaneous rift domes in the North Sea, Denmark and southern Sweden. The succession forms a northwards backstepping and onlapping early rift succession in the N–S oriented basin. Deposition ranged from coastal over shallow marine and slope sands, through offshore siltstones to deep basinal mudstones. An outlier on the island Geographical Society Ø fills an important gap in the documentation of the doming, deflation and progressive onlap. Bajocian deposition took place in a shallow marine shelf. This was followed in the late Oxfordian by slope and base‐of‐slope deposition. The backstepping succession onlaps progressively older rocks towards the north, illustrating the gradual deflation, erosion and onlap of the dome, constituting the early part of the doming–rifting–spreading phases of classical plate tectonics.

Paleoenvironmental variations in a sedimentary Jurassic sequence from Lusitanian Basin (Portugal)

Gaiteiros-1 borehole, located in the central Lusitanian Basin (Portugal), drilled a continuous Jurassic (Hettangian to Kimmeridgian) sedimentary sequence. Forty-one samples were selected for this study to characterize the dispersed organic matter and its vertical variation along Gaiteiros-1 borehole and to evaluate the environmental changes based on palynofacies and bulk geochemistry analyses.Gaiteiros-1 borehole lithological sequence is characterized by a low organic carbon content, which varies between 0.10 wt% and 0.83 wt%, in Candeeiros carbonates and Lower Jurassic Coimbra Formation, respectively. The insoluble residue content indicate a carbonated character in almost the entire sequence, except for Kimmeridgian samples. The palynofacies analysis evidences the presence of the main groups of kerogen (Phytoclats, Amorphous and Palynomorphs) with a dominance of the amorphous group in the carbonated samples and phytoclasts in Kimmeridgian detritic samples. Different types of amorphous particles were identified, suggesting periods of reworking by bacterial activity and primary bioproductivity in depleted oxygen environments. Despite the dominance of amorphous particles, the kerogen groups vertical variation provides information that reveals changes in the depositional environment. The kerogen assemblage of the Candeeiros facies suggests a stratified water column, with well-oxygenated layers, responsible for high primary productivity periods, which allowed for the development of zooplankton, preserved in the form of zooclasts fragments and foraminiferal test-linings. The contribution of the Palynomorph group is scarce during the Lower-Middle Jurassic but increases in the Upper Jurassic. This modification in the kerogen assemblage reflects changes in the sea level, oxygenation conditions, climate and proximity to the terrestrial source area. The presence of Coleochaete-like freshwater algae was detected for the first time in the Lusitanian Basin, suggesting more restrictive conditions during the Late Oxfordian. During the Kimmeridgian, the amorphous material disappears and phytoclasts, mainly opaque phytoclasts, come to dominate the kerogen assemblage, as a result of terrigenous sediments input, due to the great proximity to the terrestrial source area.

Oxfordian brachiopods from the ammonitico rosso-type Fony\xe1sz\xf3 Limestone formation at Zeng\u0151v\xe1rkony, Mecsek Mountains, Hungary and their palaeoecological, palaeobiogeographical and palaeopathological significance

Recent discovery of a previously unknown outcrop in the vicinity of the Zengővárkony lime-kilns (Mecsek Mountains, Hungary) provided a few identifiable upper Oxfordian brachiopods that exhibit a truly Mediterranean (Tethyan) character. Dating of the outcrop is based on a rich ammonite fauna: Benetticeras benettii; Trimarginites ex gr. trimarginatus; Orthosphinctes (Orthosphinctes) ex gr. tiziani clearly indicate the Late Oxfordian. The brachiopod fauna indicates a deep-water marine environment and well-oxygenated sea floor. Nucleata bouei and Pygope catulloi are recorded for the first time from the Mecsek Mountains. A pathologic specimen of Pygope catulloi is also recognized. Its ventral valve was injured in an early developmental stage that caused deformation of the left side, which was overgrown by the healthy right side and created an asymmetric adult shell shape. Cause of the injury is unclear but it provides further evidence for subsequent healing of brachiopods after being injured. This is the first description and illustration of Oxfordian brachiopods from the Mecsek Mountains, Hungary. The occurrence of Tethyan originating pygopid brachiopods in the Oxfordian strengthens earlier observations that from the Bathonian/Callovian Tethyan influence became overwhelming in the Mecsek Mountains fauna. Pygope catulloi strengthens records from Algeria that pygopid brachiopods may have occurred very early on the periphery of the Western Tethys.

Late Jurassic temperatures for the southern Tethyan margin based on belemnites δ18O from the Ouarsenis Mountains, northwestern Algeria

This study presents the first record of stable carbon and oxygen isotopes of well-preserved belemnites and bulk sediment from the Upper Jurassic pelagic carbonates of the Ouarsenis Mountains (northwestern Algeria), which were deposited at the southern margin of the Tethys ocean. Cathodoluminescence and scanning electron microscopy was used to eliminate potentially diagenetically altered specimens (brachiopods, aptychi, and some belemnites). Ultimately, 55 belemnite specimens were used to reconstruct water temperatures for the Middle Oxfordian to the Early Tithonian. Their δ18Ocarb values ranged from −0.35‰ to −3.88‰, suggesting warm and mostly stable temperature conditions during the Late Jurassic. Using a δ18Osea value of 0‰ for the tropical palaeolatitude of 15–20°N, the δ18Ocarb values of the well-preserved belemnite rostra translate into average temperatures of 27.8 °C in the Middle Oxfordian (Gregoryceras transversarium Zone), 27.0 to 27.5 °C in the Late Oxfordian (Epipeltoceras bimmamatum and Idoceras planula zones, respectively), 26.2 °C in the Early Kimmeridgian, and 27.3 °C in the Early Tithonian (Hybonoticeras hybonotum Zone).

Latest Oxfordian – earliest Kimmeridgian ammonite dominated fauna and microfacies from the ammonitico rosso-type Fonyászó Limestone Formation at Zengővárkony (Mecsek Mountains, Hungary)

Field work has resulted in the recognition of a previously unknown outcrop in the vicinity of the Zengővárkony lime-kilns (Mecsek Mountains, South Hungary) which has provided a remarkably rich, but poorly preserved, uppermost Oxfordian – earliest Kimmeridgian fauna dominated by ammonites probably representing the Hypselum–Bimammatum zones. This is the first description and illustration of Oxfordian–Kimmeridgian ammonites from the Mecsek Mountains. The cephalopod fauna comprises Phylloceras div. sp., Holcophylloceras div. sp., Sowerbyceras sp., Lytoceras sp., Lissoceratoides erato, Trimarginites cf. trimarginatus, Glochiceras (Coryceras) cf. microdomum, Subdiscosphinctes sp., ?Wegelea sp., Passendorferiinae gen. et sp. ind., ?Orthosphinctes sp., Euaspidoceras cf. radisense, Aspidoceras sp., A. cf. binodum, Physodoceras sp. ex gr. altenense-wolfi, Lamellaptychus sp., Laevaptychus sp. ex gr. hoplisus-obliquus. The fauna has a Mediterranean character (55% of Phylloceratidae and Lytoceratidae) and is similar to the Tethyan assemblages of the Venetian Alps (Italy), and Palma de Mallorca. The spirochaete tube worm Spiraserpula spirolinites is also the first record from the Mecsek Mountains. The Saccocoma wackestone-packstone microfacies is recorded with benthic foraminifera (Lenticulina sp. and Spirillina sp.) indicating well oxygenated and nutrient-rich bottom conditions.

СТРАТИГРАФИЯ И УСЛОВИЯ ФОРМИРОВАНИЯ КЕЛЛОВЕЙСКИХ И ВЕРХНЕЮРСКИХ ОТЛОЖЕНИЙ ЦЕНТРАЛЬНОЙ ЧАСТИ КАЗЫМ-КОНДИНСКОГО РАЙОНА (ЗАПАДНАЯ СИБИРЬ)

The results of stratigraphic and facies studies of the Callovian and Upper Jurassic of the Kazym-Konda facies region (West Siberia) are presented. The sequence of subdivisions of the boreal zonal standard is established. The section is composed of rocks with predominantly shallow marine genesis; their composition depended on sedimentation cyclicity and sea level fluctuations. The highest incoming rates of sedimentary material were associated with transgressive episodes in the Callovian, Middle Oxfordian and Middle Volgian. Stratigraphic breaks and periods of deficient sedimentation in the Late Oxfordian, Kimmeridgian, Early Volgian and Late Volgian correspond to regressive episodes in the section.

Benthic Foraminifera from the Saïda clay formation (Djebel Brame, northwestern Algeria) biostratigraphy and paleoecology

The analysis of benthic foraminiferal assemblages was performed in the Oxfordian deposits of the “Argile de Saida” in northwestern Algeria. This formation consists mainly of clay and sandstone alternations of the Upper Jurassic. It is located in the western part of the Tlemcenian domain. The first results of the micropaleontological study of Djebel Brame indicated that the microfauna of Foraminifera was quite original and widely abundant during that period. In addition, their biostratigraphic distribution exhibited three microfaunal associations, largely dominated by Nodosariidae. These results also revealed the renewal of the assemblage composition and allowed identifying several species of biostratigraphic divisions which were found consistent with those established for the ammonites. In addition, the palaeoenvironmental interpretations were based on the spatio-temporal distribution of benthic foraminiferal associations with hyaline and agglutinated tests. Besides, the geochemical analyses (calcimetry) reflected the basin infill during the Late Oxfordian.

The semimammatum and semiarmatum ammonite biohorizons (Late Oxfordian, Hypselum Zone) in the Upper Jurassic of Southwestern Germany

The Late Oxfordian ammonite faunas of the semimammatum and semiarmatum biohorizons are described, mostly based on material from the Impressamergel Formation of the Plettenberg Quarry near Dotternhausen (western Swabian Alb). 21 ammonite are recorded, including some previously undescribed from the Upper Jurassic of SW Germany. Previously, the semimammatum Biohorizon has been studied in Franconia, Western France and central Poland and the semiarmatum Biohorizon only in Western France, respectively. Compared to the semimammatum Biohorizon, the semiarmatum Biohorizon is characterized by a significant increase of Submediterranean and Tethyan taxa and a total decline of Subboreal ones. This phenomenon is interpreted as the result of a seawater warming trend towards the above following bimammatum Biohorizon, which correlates with the recently suggested Oxfordian/Kimmeridgian stage boundary. This article summarizes the current state of knowledge of the Hypselum Zone in SW Germany. A lectotype is designated for Richeiceras lochense (Oppel, 1863).

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.

Provenance, tectonic setting, and source area weathering of Middle Jurassic siliciclastic rocks of Chari Formation, Jumara Dome, Kachchh Basin, Western India: Sedimentological, mineralogical, and geochemical constraints

Kachchh Basin is well known for its Jurassic sediments and hydrocarbon prospects, but geochemically, these sediments remained less explored. Here, we present petrography, mineralogy, and geochemistry of the Middle Jurassic sandstone and shale of the Chari Formation exposed at Jumara Dome, Kachchh Basin, to understand their provenance, tectonic setting, source area weathering, and paleoclimate. Petrographic analysis shows that the sandstone consists of abundant quartz, feldspar (K‐feldspar and plagioclase), mica (muscovite and biotite), and lithic fragments. The X‐ray diffraction analysis of both sandstone and shale reveals the presence of abundant quartz, and lesser amount of kaolinite, illite, and chlorite. The presence of these clay minerals was also attested by scanning electron microscope analysis. Based on framework composition and geochemistry, the sandstone is classified as arkose to subarkose and shale as graywackes. Detrital modes and geochemical data suggest the derivation of these clastic rocks from a felsic source, partially influenced by sediment recycling and deposited in a passive margin setting in a rifted basin. Values of weathering indices including chemical index of alteration, plagioclase index of alteration, chemical index of weathering, and index of compositional variability indicate that the rocks of Jumara Dome are mature and the source area has undergone a moderate to high degree of chemical weathering under warm and humid climatic conditions. The petrography, bulk mineralogy, scanning electron microscope analysis, and geochemical composition in conjunction with paleocurrent data affirm the supply of detritus for Jumara Dome from the Aravalli Craton situated to the east and northeast and Nagar Parkar Massif situated to the north and northwest of the basin. This study endorses the formation of Transgondwanan seaway route during the Late Oxfordian, which caused a marine transgression onto the land.

New marine reptile fossils from the Oxfordian (Late Jurassic) of Greenland

AbstractKnowledge about marine reptile diversity and disparity during the Late Jurassic is increasing. This contribution describes marine reptile skeletal elements (ichthyosaur and plesiosaur) from Kingofjeld mountain in NE Greenland. The assemblage is early Late Oxfordian (Late Jurassic) in age, and consists ofc. 100 disarticulated skeletal elements. The location is of biogeographic importance as it was at the time situated between the Boreal realm and the Tethys Sea and is promising in terms of future prospecting.

СТРОЕНИЕ КЕЛЛОВЕЯ И ВЕРХНЕЙ ЮРЫ ЕМ-ЕГОВСКОГО МЕСТОРОЖДЕНИЯ (ЗАПАДНАЯ СИБИРЬ)

The results of litho-bio-stratigraphic studies of Callovian and Upper Jurassic of the Em-Ega oil field (Western Siberia) are presented. The formation of sediments took place in the sea basin and had a pulsating character. Episodes of sediment accumulation were replaced by periods of a sharp shortage of terrigenous material. The main stages of the arrival of terrigenous material were Callovian, Middle and Late Oxfordian, Early Kimmeridgian and Middle and Late Volgian. In Early Oxfordian, Late Kimmeridgian, Early Volgian deposits did not accumulate.

Stratigraphy and ecostratigraphic distribution of foraminiferal morphogroups from the Upper Jurassic of the Makar’yev section (Unzha River, Volga River basin)

Investigation of the Upper Callovian to Lower Kimmeridgian microfossils from the Makar’yev reference section (Unzha River, East European Platform) has been carried out. The section is characterized by ammonite debris and abundant associations of benthic and planktic foraminifers. It is a perfect object for stratigraphic and paleoecological researches. The biostratigraphic distribution of foraminifers from the Makar’yev section allows one to identify standard foraminifera zones of the East European Platform, as well as to upgrade some of them. The analysis of vertical and lateral ammonites and foraminiferal distribution, completed with litho stratigraphy, has precised the stratigraphic volume and position of boundaries of several lithological units.An improved stratigraphic scheme for the Kostroma area of the Moscow Depression is proposed. Analysis of the composition, structure, and dynamic changes of the foraminiferal assemblages has been performed. The morphofunctional analysis of foraminiferal genera has for the first time identified how foraminiferal morphogroups differing in their life style and feeding strategy varied with short-term paleoenvironmental changes. These morphogroup changes allow establishing four ecostratigraphic levels. These paleoecological data have been calibrated along with geochemical factors. They have shown a crisis of foraminiferal association during the Late Oxfordian and Early Kimmeridgian. A similar crisis has also been discovered in the north of Siberia, which may be an argument for its global distribution. The analysis of the taxonomic composition and the density of foraminiferal associations, in parallel with the structure of the association, has revealed a succession of transgressive and regressive events during the Late Callovian-Early Kimmeridgian. It allows the typification of each assemblage in relation with each event and underlines the occurrence of second-order sea-level fluctuations (middle part of the Middle Oxfordian and the earliest Kimmeridgian).

Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea

Understanding rift topography is essential for determining source areas, sediment pathways, and the type of sediment delivered to a rift basin; factors essential for interpreting petroleum systems in ancient rifts. Here we investigate Upper Jurassic sediments from the Josephine Ridge region of the Central Graben, North Sea, by integrating geophysical, palynological, petrophysical and sedimentological datasets to analyse the palaeoenvironments of the Jade and Judy horsts, the tops of which are not preserved. Interpretation of geophysical and petrophysical data together with core descriptions shows study wells to step progressively away from the Josephine Ridge into adjacent grabens. One hundred and five palynological samples from six wells range from the Oxfordian to the Lower Tithonian, spanning the syn-rift period of the Central Graben. Samples from the adjacent grabens and the Jade Horst are rich in dinoflagellate cysts and possess <20% terrestrial palynomorphs. Samples from the Judy Horst contain a wide range of terrestrial palynomorphs, dominated by lycopsid, fern and moss spores, representing c.50% of the recovered palynomorphs. Correspondence analysis of the assemblages implies that Jade did not possess a terrestrial ecosystem; Judy samples define seven groupings of related miospores, all interpreted to represent very low lying, relatively early successional type environments. This implies subaerial exposure of the Judy Horst during the Late Jurassic, which is interpreted to have formed an isolated, low relief, footwall crest island. This study provides a new methodology for investigating rift topography, particularly in cases where the tops of horsts were subsequently removed by erosion. The Judy Island would have separated the Central Graben into its eastern and western arms earlier than previously predicted, in the Late Oxfordian, with consequences for distribution of shallow and deep-marine reservoir quality sediments.

The evolution of Late Callovian to Early Kimmeridgian foraminiferal associations from the central part of the Russian Sea (Makar'yev section, Volga River Basin, Russia)

Foraminiferal associations from the Upper Callovian to Lower Kimmeridgian beds of the Makar'yev reference section (Volga Basin, East European Platform) provide insights into contemporaneous palaeoecological, palaeoenvironmental and palaeoceanographic conditions. Foraminiferal associations were studied using semi-quantitative methods, morphogroup analyses and various biodiversity indices, and were compared with different abiotic factors, including transgressive and regressive events, variations in temperature, and the content of total organic carbon (TOC). Analyses of the resulting data show that the Makar'yev section was strongly affected by variations in sea level during Oxfordian time and suggest that the environment was mainly associated with the outer part of the middle sublittoral zone on the outer platform.A regressive event strongly affected the foraminiferal communities at the Early/Middle Oxfordian boundary. This event was followed by a transgressive phase and an increase in water temperature that allowed the opening of a marine pathway and migration of planktonic foraminifera from tropical and sub-boreal latitudes to boreal and sub-arctic latitudes. This phenomenon may explain the increase in foraminiferal biodiversity during the Middle Oxfordian. However, a crisis in microbenthic communities greatly affected foraminifera during the Late Oxfordian. This crisis was probably the result of a combination of two events rather than a single large event: the strong input of organic matter during maximum flooding, and a subsequent second-order regressive event at the transition between the Middle and the Late Oxfordian. This massive injection of organic matter would have increased the quantity of organic carbon as well as decrease the amount of oxygen in bottom water. It ultimately resulted in the development of shallow-water and potentially deep-water infaunal foraminifera, as well as the disappearance of non-adapted taxa from the Late Oxfordian and, particularly, the Kimmeridgian.

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

ABSTRACTThis 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.

Seawater temperatures and carbon isotope variations in central European basins at the Middle–Late Jurassic transition (Late Callovian–Early Kimmeridgian)

Oxygen and carbon isotope values and elemental ratios of well-preserved belemnite rostra, brachiopod shells and bulk-carbonates from the Upper Callovian–Lower Kimmeridgian of the Polish Jura Chain, Kujawy (Poland) and Swabian Alb (Germany) are investigated to reconstruct environmental conditions and perturbations in the marine carbon cycle. Belemnite δ18O values show relatively constant temperatures (ca. 12°C) of bottom waters in the Polish Jura Chain basin during a major part of the Late Callovian–Middle Oxfordian, except for a short-term cooling (to ca. 9°C) at the Callovian–Oxfordian transition. Belemnite and brachiopod δ18O values show a gradual increase in temperature during the Submediterranean Late Oxfordian; the highest temperatures (ca. 16°C) are calculated for the Submediterranean Oxfordian–Kimmeridgian transition. Belemnite and brachiopod Mg/Ca and Sr/Ca ratios are disregarded as a paleotemperature proxy because of their weak correlation with δ18O values.The belemnite and brachiopod isotope data confirm that the carbon isotope composition of belemnite rostra is affected by a metabolic effect, which results in a depletion of belemnite calcite in the 13C isotope. Belemnite rostra are considered, nevertheless, as a valuable tracer of temporal variations in the carbon isotope composition of marine carbonates. Belemnite δ13C data show the presence of two positive excursions (in the Upper Callovian and the Middle Oxfordian) in the carbon isotope record of peri-Tethyan carbonates. The excursions are divided by a Lower Oxfordian interval characterized by decreased δ13C values. This is most likely a regional feature caused by upwelling. Lowest belemnite and brachiopod δ13C values are observed in the lower part of the Submediterranean Upper Oxfordian and are linked to a well-mixed state of the seawater in the basins studied. The carbon isotope record of bulk carbonates differs from those of belemnites and brachiopods probably because of strong variations in carbonate production in the Polish Jura Chain basin.

Palaeogeography of a shallow carbonate platform: The case of the Middle to Late Oxfordian in the Swiss Jura Mountains

The Oxfordian (Late Jurassic) carbonate-dominated platform outcropping in the Swiss Jura Mountains offers a good biostratigraphic, sequence-stratigraphic, and cyclostratigraphic framework to reconstruct changes in facies distribution at a time-resolution of 100 ka. It thus allows interpreting the dynamic evolution of this platform in much more detail than conventional palaeogeographic maps permit. As an example, a Middle to Late Oxfordian time slice is presented, spanning an interval of about 1.6 Ma. The study is based on 12 sections logged at cm-scale. The interpreted depositional environments include marginal-marine emerged lands, fresh-water lakes, tidal flats, shallow lagoons, ooid shoals, and coral reefs. Although limestones dominate, marly intervals and dolomites occur sporadically.Major facies shifts are related to m-scale sea-level changes linked to the orbital short eccentricity cycle (100 ka). The 20-ka precession cycle caused minor facies changes but cannot always be resolved. Synsedimentary tectonics induced additional accommodation changes by creating shallow basins where clays accumulated or highs on which shoals or islands formed. Autocyclic processes such as lateral migration of ooid and bioclastic shoals added to the sedimentary record. Climate changes intervened to control terrestrial run-off and, consequently, siliciclastic and nutrient input. Coral reefs reacted to such input by becoming dominated by microbialites and eventually by being smothered. Concomitant occurrence of siliciclastics and dolomite in certain intervals further suggests that, at times, it was relatively arid in the study area but there was rainfall in more northern latitudes, eroding the Hercynian substrate.These examples from the Swiss Jura demonstrate the highly dynamic and (geologically speaking) rapid evolution of sedimentary systems, in which tectonically controlled basin morphology, orbitally induced climate and sea-level changes, currents, and the ecology of the carbonate-producing organisms interacted to form the observed stratigraphic record. However, the interpretations have to be treated with caution because the km-wide spacing between the studied sections is too large to monitor the small-scale facies mosaics as they can be observed on modern platforms and as they certainly also occurred in the past.

The systematics and paleobiogeographic significance of Sub-Boreal and Boreal ammonites (Aulacostephanidae and Cardioceratidae) from the Upper Jurassic of the Bohemian Massif

Abstract Upper Jurassic marine deposits are either rarely preserved due to erosion or buried under younger sediments in the Bohemian Massif. However, fossil assemblages from a few successions exposed in northern Bohemia and Saxony and preserved in museum collections document the regional composition of macro-invertebrate assemblages and thus provide unique insights into broad-scale distribution and migration pathways of ammonites during the Late Jurassic. In this paper, we focus on the systematic revision of ammonites from the Upper Oxfordian and Lower Kimmeridgian deposits of northern Bohemia and Saxony. The ammonites belong to two families (Aulacostephanidae and Cardioceratidae) of high paleobiogeographic and stratigraphic significance. Six genera belong to the family Aulacostephanidae (Prorasenia, Rasenia, Eurasenia, Rasenioides, Aulacostephanus, Aulacostephanoides) and one genus belongs to the family Cardioceratidae (Amoeboceras). They show that the Upper Jurassic deposits of the northern Bohemian Massif belong to the Upper Oxfordian and Lower Kimmeridgian and paleobiogeographically correspond to the German-Polish ammonite branch with the geographical extent from the Polish Jura Chain to the Swabian and Franconian Alb. Therefore, the occurrences of ammonites described here imply that migration pathway connecting the Polish Jura Chain with habitats in southern Germany was located during the Late Oxfordian and Early Kimmeridgian in the Bohemian Massif.

Zonal stratigraphy and biogeography of the West Siberian Oxfordian based on ammonites

The Oxfordian Stage of West Siberia contains Boreal ammonites Cardioceratidae. The authors’ bank of paleontological data includes ~500 definitions of Cardioceratinae, permitting a considerable refinement of the official Oxfordian regional zonal scale. The lower substage is divided into the Cardioceras (Scarburgiceras) obliteratum, C. (S.) scarburgense, and C. (S.) gloriosum Zones instead of beds with C. (S.) spp., whereas the C. (Cardioceras) percaelatum and C. (C.) cordatum Zones are recognized instead of beds with C. (C.) spp. We have found new ammonites typical of the Middle Oxfordian C. (Subvertebriceras) densiplicatum and C. (Miticardioceras) tenuiserratum Zones. The first of these zones is divided into two subzones. The Upper Oxfordian includes the Amoeboceras glosense and A. serratum Zones instead of beds with A. spp., and the A. regulare Zone and beds with A. rosenkrantzi are recognized instead of the A. ex gr. regulare Zone. The genus Ringsteadia (Aulacostephanidae) is observed only in the northwestern part of the region, along the eastern slope of the North Urals; therefore, two upper units of the biostratigraphic scale correspond to beds with Ringsteadia marstonensis.In the Oxfordian, West Siberia and northern Siberia belonged to the North Siberian province of the Arctic realm. Only in the latest Oxfordian did the northwestern West Siberian basin become part of the Boreal-Atlantic realm, as evidenced by the distribution of Ringsteadia on the eastern slope of the Cis-Polar Urals.