Petrography and biosedimentology of the Rosso Ammonitico Veronese (middle-upper Jurassic, north-eastern Italy)

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 µm-sized micrite. Controversial smaller nanograins (0.1-0.5 µm) 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 µm 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.A. Préat ( ) · B. Mamet

Iron bacteria in Devonian carbonates (Tafilalt, Anti-Atlas, Morocco)

“…Early Devonian in the Czech Republic (Mamet et al 1997), Late Devonian in Belgium and France (Préat et al 1999;Boulvain et al 2001;Mamet and Préat 2005), Carboniferous of Spain (Della Porta et al 2006), Jurassic in Italy Préat 2003, 2005;Préat et al 2006) and Spain (Mamet and Préat 2007) have all conWrmed these observations. By no means is this to deny that abiotic detrital iron can also form red limestones as we have observed in the French Jurassic Prétry Limestone near Tournus, or Sampans Limestone near Dole (Chauve 1975).…”

“…Oxygen and iron concentrations are relatively low; for instance, the red Italian Ammonitico Rosso present small iron contents (generally <350 ppm of Fe in the carbonates, Préat et al 2006) and since free O 2 is linked to Fe at the time of sedimentation, it is also limited. It is the remarkably small size of the hematite crystals, mostly inferior to the original carbonate micrite crystals, that Wnally explains the distribution and spread of the red pigmentation .…”

Microbially-induced Fe and Mn oxides in condensed pelagic sediments (Middle-Upper Jurassic, Western Sicily)

“…We have previously studied the role of iron microfossils in the red pigmentation of the original Rosso Ammonitico from the Trento area, in the classical area of Altopiano di Asiago (Rosso Ammonitico Veronese, Mamet and Préat, 2003;Préat et al, 2006). However the Rosso Ammonitico term seems to have been used to describe different facies types and a controversy (Elmi, 1981) arose in the interpretation of the bathymetry (deep water bathyal versus cratonic shallow water).…”

“…In this context iron and manganese precipitated as oxides in the marginal areas where the contact with oxygenated water masses provided the necessary redox conditions. We have reported that dysoxic conditions allowing the precipitation of Fe oxy-hydroxides led to the red pigmentation of the micritic matrix in the RAV (Préat et al, 2006) and in numerous other red Phanerozoic limestones (Mamet and Préat, 2006a). Due to the particular sedimentary dynamics along an escarpment a major goal of this paper is to assess potential mechanisms that resulted in the iron and manganese formations.…”

“…The deposition and early diagenesis have been affected by bottom currents which were episodically prolonged enough to erode the substrate or prolonged enough to conduct to sediment bypassing (Martire, 1996). Préat et al (2006) have shown that the different coloured facies in the RAV are linked to the variations of oxygen gradients, the grey facies consisting of peloidal packstones indicating higher hydrodynamic energy levels and the red facies quiet environments with low sedimentation rate and dysoxic conditions which favoured the spread of iron bacterial biofilms at the water-sediment interface. Typical RAV features are the presence of hardgrounds, highlighted by Fe encrustations recording breaks in sedimentation (Clari et al, 1984).…”