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Emanuela Mattioli, Stéphane Reboulet, Bernard Pittet, Olivier Proux, François Baudin:
Calcareous nannoplankton and ammonoid abundance in Valanginian limestone-marl couplets: a dominance of carbonate dilution or productivity?
(Poster)

Limestone-marl alternations, containing thin, organic-rich levels, were analysed from the Early Valanginian (Campylotoxus Biozone: Figure 1) of the Vocontian Basin (Vergol section, SE France). In order to elucidate the mechanisms driving this rhythmic sedimentary pattern, calcareous nannoplankton, benthic and nektonic faunas were studied, and geochemical analyses applied. Different fine-fraction elements were quantified in the nannofacies, namely micarb, pyrite, clays, etc. In the present work, a quantitative approach allows us to precisely evaluate the amount of calcareous nannofossils and ammonoids per volume of rock, through a regular and continuous sampling of the section (4.5m thick).

Ammonoids represent almost the totality of the nektonic macrofauna. Statistical analyses show significant variations of the absolute abundance of the six ammonite families which dominate the assemblage with respect to the carbonate content.

Calcareous nannofossils are more abundant in calcareous marls, marls and shales than in limestones all along the section. Quantification of calcareous nannofossils in marls and organic-rich shales has evidenced a net dominance in the assemblage of coccoliths over nannoconids. The coccoliths are mainly represented by different species of the genus Watznaueria that, in different samples, constitute approximately 80-90% of the coccolith assemblage. In the calcareous marls, nannoconids are slightly more abundant, even though they do not represent more than 25% of the nannofossil assemblage. Large nannoconids (larger than 16µm) have been recorded in calcareous marls. In limestones, both coccoliths and nannoconids become very rare.

Such a record is thought to be independent of preservational effects, since: 1) no substantial differences of coccolith preservation have been observed in different lithotypes; and 2) Watznaueria and Nannoconus, which constitute the majority of the nannofossil assemblage, are among the most-resistant specimens to diagenesis.

Decrease in abundance of both phytoplankton and macrofauna with increasing carbonate content suggests dilution cycles by imported carbonate mud (probably of platform origin). Neither nannoconids nor coccoliths seem to have produced the bulk of carbonate mud, even if their contribution to the overall carbonate content is not negligible in discrete layers. The last organic carbon-rich layer (2.115m, 3.47% TOC, 46.5% CaCO₃) is of particular interest because it displays the greatest abundance of benthic macrofauna (gastropods, bivalves) and of the Family Bochianitids, likely nektobenthic ammonoids. Nannoplankton are much more abundant at 2.115m than in the other organic carbon-rich layers. Coccoliths, and mainly the genus Watznaueria, are here largely dominant over the other elements forming the nannofacies (i.e. micarb). Such data may support the hypothesis of a high pelagic carbonate productivity (from coccolithophores more than from nannoconids) and sea-bottom eutrophication during deposition of the organic carbon-rich layer at 2.115m. Changes in the biotic composition (nannoconids vs. coccoliths, nektobenthic vs. nektonic ammonoids) may indicate that primary productivity, trophic level and food-chain structure have been an important control on the formation of marl-limestone alternations, and on organic-rich shale deposition.

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

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