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Integrated stratigraphy of the Oxfordian global stratotype section and point (GSSP) candidate in the Subalpine Basin (SE France)

Pellenard P., Fortwengler D., Marchand D., Thierry J., Bartolini A., Boulila S., Collin P., Enay R., Galbrun B., Gardin S., Huault V., Huret E., Martinez M., Smith C.

Volumina Jurassica

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2014

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Vol. 12, no. 1

1--44

EN

An integrated biostratigraphic approach, based on ammonites, calcareous nannofossils, dinoflagellates, combined with sedimentology, carbon-isotope and physical stratigraphy, is proposed for the Subalpine Basin (Thuoux and Saint-Pierre d’Argençon sections). Within the expanded marl deposits of the Terres Noires Fm., the Callovian-Oxfordian boundary is particularly well defined by ammonite taxa from different families (i.e. Cardioceratidae, Oppeliidae, Aspidoceratidae and Perisphinctidae), calcareous nannoplankton (first occurrence of large-sized Stephanolithion bigotii) and dinoflagellate cysts (first occurrence of Wanaea fimbriata). This precise biostratigraphy and diversity of ammonites permit long-range correlations with Boreal and Pacific domains. The exceptional outcrop conditions and continuous sedimentation allow high-resolution chemostratigraphy and cyclostratigraphy to be used. Variations of the δ13C record (1.5‰ increase during the Lamberti Zone), consistent with data from other sections of the Subalpine Basin and with the Paris Basin, provide additional markers to characterise the Callovian-Oxfordian boundary. Promising results from cyclostratigraphy should serve to constrain the duration of biostratigraphic units and thus improve the Late Jurassic Geologic Time Scale. Comparisons are made with the two other candidates for an Oxfordian GSSP, Redcliff Point (UK) and Dubki (Russian Platform). Finally, the advantages of the Subalpine Basin sections support the proposal of Thuoux as a suitable GSSP candidate for the base of the Oxfordian Stage

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Proposal of the Muller Canyon section (New York Canyon area, Nevada, USA) as stratotype for the Triassic/Jurassic boundary

Guex J., Taylor D., Rakús M., Bartolini A., Atudorei V., Carter E., Lucas S.

Volumina Jurassica

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2006

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Vol. 4, no. 1

283-285

EN

Completeness of the sedimentary and biostratigraphic record is the most important characteristic of a global stratotype intended to record the boundary between two geological stages. Such a reference section must also contain fossils with proven chronological value. In the Jurassic, ammonites are usually considered the ideal chronological marker and all high-resolution zonations concerning that period are based on this fossil group. Figure 1 demonstrates that the Muller Canyon section (New York Canyon area, Nevada, USA) provides by far the most complete biochronological ammonite sequence known in the world for this interval of time. The oldest Jurassic ammonites found in the area are the smooth Psiloceras tilmanni and P. spelae, which are characterized by nodes in their earliest stage of development (Knötchenstadium). Odoghertyceras, a genus found also in Peru and Canada, occurs just above that fauna. Other smooth ammonites found between the first occurrence of P. spelae and the last occurrence of Choristoceras crickmayi belong to a group that we call "intermediate phylloceratids". Such forms lack the nodose juvenile stage and are geometrically intermediate between rhacophyllitids and psiloceratids. In our region, C. crickmayi (younger than C. marshi) co-occurs with the last Arcestes, Placites and abundant Rhacophyllites. In our sections we have found the first Jurassic-type bivalves below the first occurrence of Psiloceras, in beds that we assign to the topmost Rhaetian. In our view, the occurrence of bivalves is strongly controlled by the nature of the sediments and by local ecological conditions and this group does not seem suitable for use as index for the base of the Jurassic. The absence of Triassic ammonoids in the classical NW European sections precludes the possibility of defining the stratotype for the Triassic/Jurassic boundary (TJB) in that region and the ammonite faunas are not well diversified in the classical Tethyan Alpine sections. At present, there are three potentially good GSSP candidates: Chilingote (Peru), Kunga Island (BC, Canada) and Muller Canyon (Nevada). As far as ammonite record is concerned, Nevada ranks foremost. However, an interesting solution would be to propose the Kunga Island section (British Columbia) as parastratotype because it records a complete sequence of radiolarians across this interval, and this group is not preserved around the TJB in the New York Canyon area.

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Micropalaeontologic, geochemical and cyclostratigraphic approach for the timing of the Early Toarcian Oceanic Anoxic Event in the Paris Basin (GPF-Sancerre borehole)

Clemence M., Huret E., Bartolini A., Galbrun B., Gardin S., Hinnov L., Beaumont V.

Volumina Jurassica

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2006

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Vol. 4, no. 1

154-155

EN

The marine sedimentary record of the Toarcian exhibits evidence for a perturbation of the global carbon cycle associated with high burial of organic matter, known as the Early Toarcian Oceanic Anoxic Event (OAE). It is accompanied by climate warming, elevated rates of marine faunal extinction and short-lived, strong negative isotope excursion in both oceanic and terrestrial reservoirs. The timing and the pattern of the negative shift in 13C are critical for understanding the possible mechanism of this isotopic event, and the nature and the origin of the Early Toarcian OAE. To improve our understanding of the palaeoenvironmental background of the Early Toarcian OAE, we have investigated the sedimentary record of the GPF-Sancerre borehole from the southern Paris Basin (Cher, France) by integrating geochemical analyses (13Corg, TOC and CaCO3) with synecological analyses of benthic foraminifers and calcareous nannofossils; and assessing the duration of the 13C excursion by cyclostratigraphic analysis using magnetic susceptibility (MS) and CaCO3 data. Our results indicate progressive environmental deterioration from Domerian/Toarcian to a paroxysm coincident with the Early Toarcian OAE (highest TOC values and negative excursion of 13Corg). This deterioration is marked by a high fertility period that precedes anoxic conditions. The OAE coincides with a major crisis in the benthos and with a decrease of calcareous nannofossils. Following this major dysoxic episode, the water column is characterized by a succession of alternating suboxic and stagnation phases that correlates well with positive values of 13Corg. These results attest of a highly perturbed environment, characterized by the presence of opportunist species both in the benthos and nannoplankton communities. To quantify the timing of these events, high resolution cyclostratigraphic analysis is applied to MS and CaCO3 data (sampling interval – 2 cm). Cycles of ca. 0.5, 1 and 2.5 m are observed. The calculation of cycle frequency ratios matches that of the Milankovitch orbital cycles. The duration of the negative isotope excursion can be estimated by counting cycles to 120,000š40,000 yrs. The diminution of cycle thicknesses at the Domerian/Toarcian boundary (353-358 m interval) reflects a decrease of sedimentation rate. We interpreted this result as an evidence for a potentially condensed level. This multi proxies approach is innovative and promising to better understand the water column dynamic during the OAE (nannofossil and foraminifer association), shows that the OAE is a multiphase event (beginnings, acme and recovery) and estimates brief events (<200,000 yrs), follows the sedimentation rate evolution and highlights potential hiatuses (condensed zones).

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Aalenian carbon-isotope stratigraphy: a tool for the ammonite radiolarian and nannofossil zonation calibrations in the Western Tethys

Sandoval J., Aguado R., Bartolini A., Bill M., Bruchez S., O'Dogherty L., Perilli N.

Volumina Jurassica

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2006

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Vol. 4, no. 1

212-213

EN

Here we report a detailed carbon-isotope stratigraphy for the Aalenian (Middle Jurassic) pelagic carbonates in the Betic Cordillera (southern Spain), which represents an ideal region to directly tie the stable carbon-isotope curves to ammonite zones and subzones, and thereby for the first time achieve an accurate chronostratigraphic calibration. For this purpose we have selected two sections (Agua Larga in the province of Granada and Cerro de Mahoma in the province of Murcia) which represent basinal deposits of the southern Iberian palaeomargin. In these sections ammonite are common and relatively wellpreserved. Biostratigraphic analyses enable the recognition of the uppermost Toarcian (Aalensis Zone), the complete Aalenian (Opalinum, Murchisonae, Bradfordensis and Concavum zones) and the lowermost Bajocian (Discites Zone). Calcareous nannofossils and radiolarians (at discrete beds of the Upper Aalenian - Lower Bajocian) are also common in these Median Subbetic hemipelagic sections. The Subbetic Aalenian is characterized by a monotonous and rhythmical alternance of marlstones and marls in continuous sedimentation throughout the analysed interval. We present a ?13C curve very detailed (bed by bed) for the uppermost Toarcian - lowermost Bajocian interval. The curve shows a relative minimum (around 1‰) in the Upper Toarcian, a weak positive shift (around 2‰) in Lower Aalenian (Comptum Subzone), a decreasing values (newly around 1‰) in the Middle Aalenian (Bradfordensis Zone), a positive peak of 2.7‰ in the Upper Aalenian (Concavum Zone and Subzone) and a new relative minimum (1.5-1.7‰) at Aalenian/Bajocian boundary. A good correlation among the isotope curve and the different biostratigraphic zonations is accurately presented. We explore the powerfulness of this well-calibrated ?13C curve as a tool for stratigraphic correlation. In this sense, the biochronostratigraphic position of the radiolarian UAzones 1-2 (mainly based on Italian sections with scarce record of ammonites, Baumgartner et al. 1995) are here redefined by means of the isotope reference curve of the Subbetic. We present the results of a biostratigraphic study of nannofossils assemblages throughout the studied interval. In addition, on the base of a semiquantitative study, we have selected the most abundant taxa (>10%) from the whole assemblage to perform a multivariate analysis (principal components): Biscutum dubium, B. intermedium, Crepidolithus crassus, Carinolithus superbus, Discorhabdus striatus, Schizosphaerella spp., Watznaueria contracta and the genus Lotharingius. This multivariate analysis reveals a strong correlation between the abundances of oligotrophic (C. crassus and Schizosphaerella spp.) and eutrophic (B. dubium and B. intermedium) taxa and the fluctuation of the C degree curve.