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1.
In order to constrain spatial variability in watermass conditions within the European Epicontinental Seaway prior to, during and after the Toarcian Oceanic Anoxic Event, carbon (δ13Cbel, δ13Ccarb) and oxygen (δ18Obel, δ18Ocarb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a north–south transect across the European Epicontinental Seaway. As the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes, an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite. Intra‐rostral carbon and oxygen‐isotope data from six belemnite specimens belonging to the genus Passaloteuthis were collected. Intra‐rostral carbon‐isotopes are influenced by vital effects, whereas oxygen‐isotopes reflect relative changes in temperature and salinity. Palaeotemperatures calculated from δ18Obel‐isotope records from the Grands Causses Basin confirm relatively low temperatures throughout the Late Pliensbachian. Similar cool water conditions have previously been shown in Germany, England, Spain and Portugal. A temperature increase of up to 6 °C is observed across the Pliensbachian–Toarcian boundary. A pronounced negative shift of at least ?3‰ (Vienna‐Pee Dee Belemnite) is recorded in bulk carbonate carbon during the lower Harpoceras serpentinum zone, typical of the Toarcian Oceanic Anoxic Event. Before and after the Toarcian Oceanic Anoxic Event, a good correlation between δ13Ccarb and δ13Cbel exists, indicating well‐ventilated bottom‐waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong north–south gradient in salinity stratification and surface‐water productivity for the Western Tethyan realm. This study thus lends further support to a pronounced regional overprint on carbon and oxygen‐isotope records in epicontinental seaways.  相似文献   

2.
This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark-colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearasfalciferrum Zone. Therefore,the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ^13C values of the kerogen (δ^13Ckerogen) fluctuating from -26.22 to -23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ^13Ckerogen values. The biological assemblage,characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon-isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths.  相似文献   

3.
Severe global climate change led to the deterioration of environmental conditions in the oceans during the Toarcian Stage of the Jurassic. Carbonate platforms of the Western Tethys Ocean exposed in Alpine Tethyan mountain ranges today offer insight into this period of environmental upheaval. In addition to informing understanding of climate change in deep time, the effect of ancient carbon cycle perturbations on carbonate platforms has important implications for anthropogenic climate change; the patterns of early Toarcian environmental deterioration are similar to those occurring in modern oceans. This study focuses on the record of the early Toarcian Oceanic Anoxic Event (ca 183.1 Ma) in outcrops of the north‐west Adriatic Carbonate Platform in Slovenia. Amidst environmental deterioration, the north‐west Adriatic Platform abruptly transitioned from a healthy, shallow‐water environment with diverse metazoan ecosystems to a partially drowned setting with low diversity biota and diminished sedimentation. An organic carbon‐isotope excursion of ?2.2‰ reflects the massive injection of CO2 into the ocean‐atmosphere system and marks the stratigraphic position of the Toarcian Oceanic Anoxic Event. A prominent dissolution horizon and suppressed carbonate deposition on the platform are interpreted to reflect transient shoaling of the carbonate compensation depth to unprecedentedly shallow levels as the dramatic influx of CO2 overwhelmed the ocean’s buffering capacity, causing ocean acidification. Trace metal geochemistry and palaeoecology highlight water column deoxygenation, including the development of photic‐zone anoxia, preceding and during the Toarcian Oceanic Anoxic Event. Ocean acidification and reduced oxygen levels likely had a profoundly negative effect on carbonate‐producing biota and growth of the Adriatic Platform. These effects are consistent with the approximate doubling of the concentration of CO2 in the ocean‐atmosphere system from pre‐event levels, which has previously been linked to a volcanic triggering mechanism. Mercury enrichments discovered in this study support a temporal and genetic link between volcanism, the Toarcian Oceanic Anoxic Event and the carbonate crisis.  相似文献   

4.
The sedimentary record of the Arabian Shelf offers a unique opportunity to study the Cretaceous (Albian–Turonian) greenhouse climate from a palaeoequatorial perspective. In particular, hemipelagic to pelagic carbonate successions from the extensive Shilaif intra‐shelf basin have the potential to produce an excellent record of carbon cycle perturbations during this interval. This study presents a 269 m thick chemostratigraphic (carbonate δ13C and δ18O) record from the Middle Albian to Early Turonian of central Abu Dhabi (United Arab Emirates), representing over 14 Myr of uninterrupted carbonate sedimentation. The Mauddud to Shilaif formations represent outer ramp to basinal intra‐shelf carbonates with variations from laminated organic‐rich to clean bioturbated intervals. Isotopic evidence of the latest Albian Anoxic Event (Oceanic Anoxic Event 1d), Middle Cenomanian Event I and the Cenomanian–Turonian Anoxic Event (Oceanic Anoxic Event 2) are confirmed and biostratigraphically calibrated by means of calcareous nannofossils. The carbon isotope record allows correlation with other regional records and well‐calibrated records across the Tethyan Ocean and represents a significant improvement of the chronostratigraphic framework of the United Arab Emirates (Shilaif) and Oman (Natih) intra‐shelf basins. The study further confirms that low carbon isotope values corresponding to the two source rock intervals in the Shilaif Formation clearly precede the isotopic expressions of Oceanic Anoxic Event 1d and Oceanic Anoxic Event 2.  相似文献   

5.
Pressure–Temperature–time (P–T–t) estimates of the syn‐kinematic strain at the peak‐pressure conditions reached during shallow underthrusting of the Briançonnais Zone in the Alpine subduction zone was made by thermodynamic modelling and 40Ar/39Ar dating in the Plan‐de‐Phasy unit (SE of the Pelvoux Massif, Western Alps). The dated phengite minerals crystallized syn‐kinematically in a shear zone indicating top‐to‐the‐N motion. By combining X‐ray mapping with multi‐equilibrium calculations, we estimate the phengite crystallization conditions at 270 ± 50 °C and 8.1 ± 2 kbar at an age of 45.9 ± 1.1 Ma. Combining this P–T–t estimate with data from the literature allows us to constrain the timing and geometry of Alpine continental subduction. We propose that the Briançonnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision.  相似文献   

6.
Briançonnais units are squeezed between two Mesozoic eclogitic belts (Piemont-Ligurian ocean and Valaisan ocean) along the ECORS-CROP seismic line in the Italian-French Western Alps (France, Italy). The metamorphic evolution of this area plays a key role for understanding the evolution of the Western Alps and is discussed on the basis of detailed petrographic investigations carried out on weathered sediments issued from the erosion of the Hercynian belt, especially on lower Permian to Mesozoic sediments. In the Zone Houillère, as well in the Permo-Triassic cover of the Briançonnais basement, the index metamorphic mineral assemblage is mainly composed of white micas with varying chemical composition, chloritoid and garnet. This same assemblage occurs within different lithologies (metaarkose, metapelite, metasandstone). Consequently, equilibrium phase diagrams were computed for different whole rock compositions using DOMINO software. The results of the P-T investigations clearly show that each unit underwent a different sequence of metamorphic reactions. An increase in metamorphic grade from greenschist facies conditions in the Northwest (Zone Houillère) to the transition between blueschist and eclogite facies conditions in the Southeast (Internal Briançonnais) is observed. A major discontinuity in metamorphic grade is located at the contact between Zone Houillère and Ruitor unit, as documented by a pressure gap of ~ 7 kbar. In general, the observed metamorphic field gradient is inverted and is interpreted to represent different depths of burial during subduction, which correlates with the paleogeographic position of the different units.  相似文献   

7.
Abstract

The tectonic significance of the Eocene unconformity in the Briançonnais domain, classically regarded as recording a compressional event, is re-evaluated, based on field studies in the Marguareis massif, Maritime-Ligurian Alps. In this external, weakly metamorphic Briançonnais unit, we describe N-trending, folded paleo-normal faults. These paleofaults operated during the Late Cretaceous-Late Eocene, and control both the thickness of the Senonian-Paleocene calcschists and the distribution of the disconformable Middle Eocene-Early Priabonian formations, i.e. the channelised, resedimented Nummulitic limestones, associated with sandy turbidites, and the sandy- calcareous Lower Flysch Noir. The chaotic Upper Flysch Noir (Priabonian), which includes olistoliths from the Helminthoid Flysch nappes, disconformably overlies the Late Cretaceous-Middle/Late Eocene levels. At the scale of the whole Briançonnais domain of the French-Italian Alps, the superimposed Senonian-Eocene disconformities would indicate extensional faulting and block tilting, associated with a regional uplift which caused emersion of part of the domain (most internal Briançonnais, Corsica). Extension and coeval uplift would record the crossing of the frontal flexure (external bulge) of the European/Briançonnais lower plate situated west of the Alpine subduction zone between 80–70 Ma and ～40 Ma, i.e. before the subduction of the Briançonnais plateau around 38–35 Ma. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.  相似文献   

8.
Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian-Toarcian boundary with resolution to the zonal level. The oxygen-isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ13C value of 4·52%PDB is attained. These intervals are known to be favoured periods of organic-rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian-Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’. Some Italian sections spanning this Lower Jurassic interval contain organic-rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δPDB and, in one section examined in detail, correlate with the calcium-carbonate content of the shales; they may reflect a partial change to a non-calcified planktonic biota during deposition of this lime-poor interval, possibly responding to upwelling and increased fertility of near-surface waters. The onset of upwelling may have been as early as spinatum-tenuicostatum Zone time, that is, at the Pliensbachian-Toarcian boundary.  相似文献   

9.
The Toarcian Oceanic Anoxic Event (T-OAE, ～183 Ma) was characterized by enhanced carbon burial, a prominent negative carbon-isotope excursion (CIE) in marine carbonate and organic matter, and numerous geochemical anomalies. A precursor excursion has also been documented at the Pliensbachian/Toarcian boundary, but its possible causes are less constrained. The T-OAE is intensively studied in the Cleveland Basin, Yorkshire, UK, whose sedimentary deposits have been litho-, bio- and chemostratigraphically characterised. Here, we present new elemental data produced by hand-held X-ray fluorescence analysis to test the expression of redox-sensitive trace metals and detrital elements across the upper Pliensbachian to mid-Toarcian of the Cleveland Basin. Detrital elemental concentrations (Al, Si, Ti, Zr) are used as proxies for siliciclastic grain content and thus, sea-level change, which match previous sequence stratigraphic interpretations from the Cleveland Basin. The timescale of the event is debated, though our new elemental proxies of relative sea level change show evidence for a cyclicity of 350 cm that may be indicative of ～405 kyr eccentricity cycles in Yorkshire. Trends in total organic carbon and redox-sensitive elements (S, Fe, Mo, As) confirm scenarios of widespread ocean deoxygenation across the T-OAE. The correlation of comparable trends in Mo across the T-OAE in Yorkshire and the Paris Basin suggests a similar oceanic drawdown of this element accompanying widespread anoxia in the two basins. Data from Yorkshire point to a transgressive trend at the time of the Mo drawdown, which contradicts the “basin restriction” model for the euxinic conditions that characterise the CIE interval.  相似文献   

10.
Extreme environmental change during the Toarcian Oceanic Anoxic Event had widespread impacts on marine biota. This study provides new evidence, from the Yorkshire coast sections, UK, that the event was associated with periods of elevated fish and ammonite mortality. Using a synthesis of pelagic macrofaunal changes, benthic macrofaunal data and geochemical proxies we show that there are stratigraphical correlations between: (1) pelagic macrofaunal ranges and abundance, (2) benthic macrofaunal abundance, and (3) geochemical proxies that indicate deoxygenation. We identify eight stratigraphical intervals of differing character. Results suggest two major phases of relatively persistent deoxygenation with photic zone euxinia. The cyclostratigraphic timescale indicates that each phase lasted at least tens of thousands of years. Belemnite migration during the event probably resulted from increased seawater temperatures and low food supply similar to that observed for many marine taxa, including squid, within the present-day oceans.  相似文献   

11.
In the internal zone of the European Alps, late Carboniferous to Permian sediments have been detached from their basement (e.g. the Zone Houillère in the Briançonnais Zone). The Pinerolo Unit (Dora‐Maira Massif) is the deepest unit exposed in the stack of the Western Alps and is considered to be Carboniferous in age based on lithological considerations. Detrital zircon grains from the Pinerolo Unit and the Zone Houillère display similar age patterns, with the youngest and largest population being Carboniferous (340–330 Ma). The distribution of Carboniferous magmatism in the Alps and surrounding areas suggests that the detritus was transported from Maures‐Corsica and possibly from the Helvetic Zone into the Zone Houillère and the Pinerolo basin. Our results highlight the potential of detrital zircon geochronology for deciphering the sources of detrital material in meta‐sediments, even if they have been affected by metamorphic overprints.  相似文献   

12.
Chemostratigraphic analyses (87Sr/86Sr, δ13Ccarb) of limestones from two Jurassic platform‐carbonate sequences in Italy (Trento and Campania–Lucania Platforms) illustrate previously established trends found in pelagic sediments and skeletal carbonates from biostratigraphically well‐calibrated sections elsewhere in Europe. Chemostratigraphic correlations between the platform‐carbonate successions and appropriate intervals from well‐dated reference sections allow the application of high‐resolution stratigraphy to these shallow‐water peritidal carbonates and, furthermore, elucidate the facies response to the Early Toarcian Oceanic Anoxic Event (OAE). Lower Jurassic (Toarcian) levels of the western Trento Platform (Southern Alps, Northern Italy) contain spiculitic cherts that appear where rising carbon‐isotope values characterize the onset of the OAE: a palaeoceanographic phenomenon interpreted as driven by increased nutrient levels in near‐surface waters. There is a facies change to more clay‐rich facies at the level of the abrupt negative carbon‐isotope excursion, also characteristic of the OAE, higher in the section. The Campania–Lucania Platform (Southern Apennines, Southern Italy) records a change to more clay‐rich facies where carbon‐isotope values begin to rise at the beginning of the OAE but the negative excursion, higher in the section, occurs within oolitic facies. Although, in both examples, the Early Toarcian OAE can be recognized by a change to more clay‐rich lithologies, this facies development is diachronous and in neither case did the platform drown. Although the Trento Platform, in the south‐west sector studied here, was adversely affected by the OAE, it did not drown definitively until Late Aalenian time; the Campania–Lucania Platform persisted throughout the Jurassic and Cretaceous. Differential subsidence rates, which can be calculated using comparative chemostratigraphy, are identified as a crucial factor in the divergent behaviour of these two carbonate platforms: relatively fast in the case of the Trento Platform; relatively slow in the case of the Campania–Lucania Platform. It is proposed that where water depths remained as shallow as a few metres during the OAE (Campania–Lucania Platform), dissolved oxygen levels remained high, nutrient levels relatively low and conditions for carbonate secretion and precipitation remained relatively favourable, whereas more poorly ventilated and/or more nutrient‐rich waters (Trento Platform) adversely influenced platform growth where depths were in the tens of metres range. The stage was thus set for drowning on the more rapidly subsiding western margin of the Trento Plateau and a pulse of oolite deposition post‐dating the OAE was insufficient to revitalize the carbonate factory.  相似文献   

13.
Sediments of Early Aptian age in Bulgaria can be assigned to four different facies: platform carbonates (Urgonian complex), shallow-water siliciclastics, hemipelagic and flyschoid siliciclastics. The taxonomic analysis of the ammonite faunas of 18 sections from these four different facies resulted in a revision of the existing ammonite zonation scheme so far applied in Bulgaria and adjoining areas. A new biostratigraphic scheme, which bridges the western and eastern Tethys, is thereby proposed for the Lower Aptian of Bulgaria.The Upper Barremian Martelites sarasini Zone is characterized in its upper part by the Pseudocrioceras waagenoides Subzone in the shallow-water sections and by a horizon with Turkmeniceras turkmenicum in the deep-water settings. The Upper Barremian/Lower Aptian boundary is fixed by the first appearance of Paradeshayesites oglanlensis. For the Lower Aptian the following ammonite zones were established (from bottom to top): The Paradeshayesites oglanlensis Zone, the Deshayesites forbesi Zone (= formerly Paradeshayesites weissi Zone) including the Roloboceras hambrovi Subzone in the upper part, the Deshayesites deshayesi Zone including the Paradeshayesites grandis Subzone in the upper part and the Dufrenoyia furcata Zone. The Lower–Middle Aptian boundary has been defined by the appearance of species belonging to the genera Epicheloniceras and Colombiceras.The Lower Aptian ammonite faunas of Bulgaria, allow an interregional correlation with other areas of the Tethyan Realm. The presence of Turkmeniceras in the Upper Barremian enables a correlation with the Transcaspian region, whereas Roloboceras, Koeneniceras and Volgoceratoides found in the middle part of the Lower Aptian are more typical representatives of the ammonite faunas in northern Europe (England, Germany, Volga region).The analysis of the ammonite successions in combination with sedimentological observations enable us to conclude that the marls and marly limestones of the Lower Aptian studied here also cover the interval of the Oceanic Anoxic Event 1a. An interval of thin-laminated clays, rich in organic matter, was identified in the upper part of the D. forbesi Zone (Roloboceras hambrovi Subzone). This interval is characterized by a total lack of benthic faunas.  相似文献   

14.
《Geodinamica Acta》2013,26(3):117-137
In the Ligurian Alps (South-Western Italian Alps), Zn-Pb deposits occur within late Palaeozoic meta-sedimentary units belonging to the Briançonnais Zone near Casario (Tanaro valley). Different types of sulphide-rich, lens-shaped mineralizations are recognized: sphalerite-galena massive sulphide bodies, pyrite-rich lenses and sulphide-rich quartz–carbonate-chloritoid granofels. Sulphide lenses and host rocks are affected by at least three ductile deformation phases and by a polyphase alpine metamorphism, whose climax conditions are estimated, based on P-T pseudosection calculations, at T = 300-325 °C and P = 0.55-0.60 GPa. In all the mineralized lenses the ore minerals are represented, in variable amount, by Fe-poor sphalerite, galena, pyrite and arsenopyrite (± tetrahedrite, chalcopyrite and pyrrhotite); the gangue consists of quartz, carbonate (sideritemagnesite ± rhodochrosite s.s.), Fe-chloritoid, muscovite-phengite and chlorite. The mineralizations are associated with chloritoid – carbonate micaschists displaying a finely bedded texture, with sharp between-bed compositional contrast, which suggests their exhalative origin.

In spite of the tectono-metamorphic overprint, some pre-metamorphic features of the hydrothermal system are still recognized, like relics of the hydrothermal feeding system, primary growth textures and sulphide-rich microbreccias. These massive sulphide lenses, which share many characters with the SEDEX deposits, testify to the occurrence of an exhalative event of Upper Carboniferous age previously unrecognized in the Ligurian Briançonnais Unit.  相似文献   

15.
The Gargano Promontory of southern Italy, located on the eastern margin of the Apulia Platform, represents a peculiar Tethyan area where the transition between carbonate platform and adjacent basins is exposed on land. The Aptian stratigraphic record, represented in shallow-water, slope and deep-water deposits, provides a good opportunity to investigate the regional response to the worldwide documented climatic, biotic and palaeoceanographic changes related to Oceanic Anoxic Event 1a (OAE1a). A synthesis of data previously published (Ischitella and Coppitella sections), together with original data (Val Carbonara and Coppa della Guardia sections), from four stratigraphic sections from different depositional settings (proximal to distal) is provided, using an integrated, high-resolution micropalaeontological (planktonic foraminifera and calcareous nannofossils) and, for one section, geochemical (stable carbon and oxygen isotopes) approach. Organic matter preservation is confined to the more distal areas and consists of two thin intervals of black shales in the Aptian portion of the Marne a Fucoidi Formation. Biostratigraphic data assign the older black shale (5 cm thick) to the Selli Level equivalent (OAE1a, Lower Aptian); this carbon-rich interval is immediately followed by another black shale (7–10 cm thick) of early Late Aptian age.OAE1a is generally interpreted as a high-productivity event during a warming interval, followed by a cooling trend. In the Gargano Promontory, although the oxygen isotope curve indicates the above-mentioned climatic evolution, the micropalaeontological data do not support high fertility of the surface water, whereas micropalaeontological and geochemical data for the younger black shale do record high productivity (radiolarian increase in overall abundance, low nannofossil and foraminiferal species richness, increase in abudance of nannofossil fertility indices) associated with a cooling trend. The carbon and oxygen isotope record is in line with evidence from the curves documented elsewhere, whereas, among the biotic events, only the “nannoconid crisis” preceding OAE1a is revealed to be globally correlable.Environmental models for the two episodes of organic matter preservation are proposed, taking into account both global and local controlling factors.  相似文献   

16.
Sections through Lower Jurassic epicontinental carbonates from Southern Britain (Junction Bed and equivalent) show a positive carbon-isotope excursion (δ13Ccarbonate), detectable in bulk rock, in the falciferum Zone of the lower Toarcian. Isotopic data from organic matter in more clay-rich sections from Wales and north-east England, together with determinations on belemnite calcite, indicate that highest δ13C values are localized in the upper exaratum Subzone of the falciferum Zone. Levels of particular enrichment in organic carbon were developed in the early to mid-exaratum Subzone and hence pre-date this δ13C maximum. These phenomena reflect the impact of the early Toarcian oceanic anoxic event in the British area. Similar isotopic trends have been recorded in other Toarcian sections from Tethyan Europe and are interpreted as reflecting the chemistry of sea water. On the assumption of isotopic correlation between the English and Tethyan sections, the δ13C maximum would be everywhere dated as latest exaratum Subzone in terms of the north European ammonite scheme. Absolute oxygen-isotope values in carbonates probably reflect both early diagenetic cementation and later temperature-related burial diagenesis, although a palaeotemperature maximum is tentatively identified as characterizing the early falciferum Zone. Subsequent climatic deterioration may have been triggered by drawdown of CO2, related to regional excess carbon burial during the oceanic anoxic event. Using the positive δ13C excursion as a correlative level in sections from different faunal provinces (Britain, Italy and Spain) implies that lower Toarcian zonal stratigraphy is diachronous between northern and southern Europe. There is evidence for partitioning of water masses between the north European shelf and the Tethyan continental margin during the Early Jurassic.  相似文献   

17.
High-resolution clay-mineral analyses were performed on upper Hauterivian to lower Aptian sediments along a platform-to-basin transect through the northern Tethyan margin from the Neuchâtel area (Switzerland), to the Vocontian Trough (France) in order to investigate links between climate change, carbonate platform evolution, and fractionation patterns in clay minerals during their transport.During the Hauterivian, the northern Tethyan carbonate platform developed in a heterozoan mode, and the associated ramp-like topography facilitated the export of detrital material into the adjacent basin, where clay-mineral assemblages are dominated by smectite and kaolinite is almost absent, thereby suggesting dry-seasonal conditions. During the Late Hauterivian Balearites balearis ammonite zone, a change to a more humid climate is documented by the appearance of kaolinite, which reaches up to 30% of the clay fraction in sediments in the Vocontian Trough. This prominent change just preceded the Faraoni Oceanic Anoxic Event and the onset of the demise of the Helvetic Carbonate Platform, which lasted to the late early Barremian.From the Late Barremian onwards, the renewed growth of the northern Tethyan carbonate platform in a photozoan mode and the associated development of a marginally confined platform topography fractionated the clay-mineral assemblages exported into hemipelagic settings: kaolinite particles were preferentially retained in proximal, platform settings, due to their size and their relatively high specific weight. In the inner platform environment preserved in the Swiss Jura, an average of 32% of kaolinite in the clay fraction is observed during the latest Barremian–earliest Aptian, whereas clay-mineral assemblages of coeval sediments from deeper depositional settings are dominated by smectite and show only minor amounts of kaolinite.This signifies that besides palaeoclimate conditions, the morphology and ecology of the carbonate platform had a significant effect on the distribution and composition of clay assemblages during the Late Hauterivian–Early Aptian along the northern Tethyan margin.  相似文献   

18.
Carbon and oxygen isotope data from Cenomanian–Turonian sediments from the southwest of the Crimea are presented. The sediments consist of limestones, marls and organic-rich claystones, the latter with total organic carbon values up to 2.6 wt. %, representing Oceanic Anoxic Event 2. A shift to more negative δ18O values through the uppermost Cenomanian into the lowermost Turonian may be the result of warming; however, petrographic analysis shows that the samples have undergone a degree of diagenetic alteration. The carbon isotope data reveal a positive excursion from 2.7‰ to a peak of 4.3‰ at the Cenomanian/Turonian boundary; values then decrease in the early Turonian. This excursion is comparable to those of other Cenomanian–Turonian sections, such as those seen in the Anglo-Paris Basin, and is thought to be due to global changes in the oceanic carbon reservoir. On this curve are a number of negative δ13C excursions, just below the Cenomanian/Turonian boundary. It is suggested that these negative excursions are associated with the uptake of light carbon derived from the oxidation and deterioration of organic material during localised exposure of the sediments to oxic or meteoric diagenetic conditions, possibly during sea-level fluctuations.  相似文献   

19.
The late Turonian to early Campanian calcareous nannofossil biostratigraphy of the Austrian Gosau Group is correlated with ammonite and planktonic foraminiferal zones. The standard Tethyan zonations for nannofossils and planktonic foraminifers are applied with only minor modifications. The basal marine sediments of the Gosau Group, bearing late Turonian-early Coniacian macrofossils, belong to the Marthasterites furcatus nannofossil Zone (CC13). The Micula decussata Zone (middle Coniacian to early Santonian) is combined with the Reinhardtites anthophorus Zone because of the rare occurrence of Renhardtites cf. R. anthophorus already in the Coniacian and taxonomic problems concerning the correct identification of this species. The Santonian-Campanian boundary lies within the Calculites obscures Zone (CCl7).  相似文献   

20.
The belemnite species Praeactinocamax primus (Arkhangelsky, 1912) and Belemnocamax boweri Crick, 1910 are described from the Cenomanian of the abandoned limestone quarry section of Hoppenstedt (Sachsen-Anhalt, northern Germany). They co-occur in the upper part of a prominent tripartite bioclastic limestone bed associated with the ammonite Acanthoceras rhotomagense, indicating the primus Event of the lower middle Cenomanian A. rhotomagense ammonite Zone. An integrated stratigraphical calibration including carbon stable isotope correlation to southern England suggests that the belemnite event horizon at Hoppenstedt occupies exactly the same chronostratigraphical position as elsewhere, highlighting the strictly isochronous character of the primus Event across northwestern Europe. Furthermore, stratigraphical gaps in the Hoppenstedt succession are evaluated.  相似文献   

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