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Petrography and biosedimentology of the Rosso Ammonitico Veronese (middle-upper Jurassic, north-eastern Italy)
Preat, A.; Morano, S.; Loreau, J.P.; Durlet, C.; Mamet, B. (2006). Petrography and biosedimentology of the Rosso Ammonitico Veronese (middle-upper Jurassic, north-eastern Italy). Facies 52(2): 265-278. dx.doi.org/10.1007/s10347-005-0032-2
In: Facies. Springer: Heidelberg; Berlin. ISSN 0172-9179, more
Peer reviewed article  

Available in  Authors 
    VLIZ: Open Repository 279548 [ OMA ]

Keywords
    Callovian; Kimmeridgian; Oxfordian; Tithonian; Ammonoidea † [WoRMS]; Cyanobacteria [WoRMS]; Europe, Italy [Marine Regions]; Marine
Author keywords
    Rosso Ammonitico; bioprecipitation; pigmentation; diagenesis; micrite;Jurassic

Authors  Top 
  • Preat, A.
  • Morano, S.
  • Loreau, J.P.
  • Durlet, C.
  • Mamet, B., more

Abstract
    A petrographic and biosedimentological study of the Rosso Ammonitico Veronese from the Trento Plateau (north-eastern Italy) shows that diagenetic (neomorphism, recrystallization) and biological processes (microbial content and pigmentation) influenced the formation and alteration of the carbonate matrix. The subject of this article is the interaction of early diagenetic processes and an attempt to explain the different colors of the matrix (red, pink, grey). Nearly 200 samples derived from 14 sections (Callovian to Tithonian) located in the Verona area have been studied by means of classical, cathodoluminescence and SEM microscopy. Calcite and ferruginous microfilaments of different shapes and sizes are present and tentatively attributed to fungi and iron bacteria. These micro-organisms precipitated iron oxy-hydroxides at poorly dysoxic-anoxic sediment-water interfaces. Further liberation of submicronic hydroxides (now hematite) was responsible for the red pigmentation of the carbonate matrices, originally composed of less than 1 mu m-sized micrite. Controversial smaller nanograins (0.1-0.5 mu m) attributed to nanobacteria or planktonic picoeukaryotes have been observed in the reddish samples. Recrystallization of the micrite leads to the formation of new micritic crystals, between 2 and 4 mu m in size, then to microspar crystals. Micritic textures are linked to the different colours of the samples. The intensity of the red colour is correlated with the presence of hematite (former iron hydroxides) and the presence of planar subhedral micritic grains. In contrast, pink and greyish samples are linked to the increasingly coalescent structure of anhedral micritic and microsparitic crystals.

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