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André Bornemann, Jens O. Herrle, Jens Lehmann, Jörg Mutterlose:
Calcareous nannofossils and stable isotopes of Late Albian black shales (SE-France)
(Poster)

For the Aptian and Albian period, numerous black shale events have been observed on a global scale. Black shales of Late Albian age (OAE 1d) are known from, e.g., Tunisia, the Western Interior Basin of N America, the Blake Nose Plateau and the Vocontian Trough (SE France). In this study, a total of 56 samples have been investigated from the Col de Palluel section (SE France). A 7m-thick interval has been studied with respect to calcareous nannofossils and stable isotopes in order to decipher the palaeoenvironmental and palaeoceanographical conditions of this interval.

The nannofossil assemblages allow an age assignment to Nannofossil Subzone CC9b (Eiffellithus turriseiffelii Zone), which partly corresponds to the Stoliczkaia dispar Ammonite Zone. Lithologically, the sediments are made up of carbonate-rich, greyish to black, laminated marls and clays. Slight differences in colour reflect variations of the carbonate content, indicating pale-dark bedding rhythms. The studied part comprises five black shale horizons with a different degree of lamination. Each of these black shales has a thickness from 20 to 80cm. A distinctive lamination and relatively high TOC values (max. 2.5%) suggest deposition under dysoxic to anoxic conditions.

The nannofossil assemblages are characterised by high diversity and high abundances of cosmopolitan taxa (Biscutum constans, Rhagodiscus spp., Zeugrhabdotus spp., especially Z. erectus, Watznaueria barnesiae, Discorhabdus rotatorius, in order of abundance). The preservation of the nannofossil assemblages is moderate to good throughout the interval examined. Only a few taxa show correlation with the black shales; in particular, nannoconids have significantly higher values throughout the examined section, where they contribute upto 6% of the nannofossil assemblage. The taxa, B. constans and Zeugrhabdotus spp. (both make up 40-50% of the assemblage), are considered to be indicators of fertility and upwelling, while W. barnesiae is interpreted as an eurytrophic species. Rhagodiscus spp. is used as a proxy for warm surface-water temperatures. The B. constans-Rhagodiscus spp.-Zeugrhabdotus spp. assemblage suggests that the black shales were deposited under eutrophic and warm surface-water conditions.

Absolute abundance of calcareous nannofossils, determined by using the random settling technique, show minor fluctuations between 1.5-3 billion nannofossils per gram of sediment. These values depend strongly on surface-water productivity, sedimentation rate and dissolution.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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