Syndepositional cements associated with nannofossils in the Marmolada Massif: Evidences of microbially mediated primary marine cements? (Middle Triassic, Dolomites, Italy)
Résumé
The Marmolada platform is characterized by striking globose masses (“evinosponges”), arranged in concentric bands, of fibrous calcite cements ranging in size from centimeters to several decimeters. The lithogenetic importance of these peculiar cements has been recognized in many Middle Triassic buildups of the Western Tethys. EDS microanalyses revealed that these fibrous cements contain 1–3 mol% of Mg and detectable amounts of Sr, over 1000 ppm, sometimes exceeding 10,000 ppm. The strontium geochemical signature in neomorphic calcite could reflect the replacement of an aragonitic carbonate precursor. The boundaries between the fibrous calcite bands are marked by alignments of microcrystalline aggregates of fluorapatite and/or dolomite. The presence of fluorapatite may indicate a depositional microenvironment eutrophic or rich in microbial communities. Epifluorescence analyses showed bright bands alternated to dark ones, confirming that organic matter remains occur within the studied cements and could have played a significant role in supporting the widespread syndepositional cementation. High magnification SEM observations on bright epifluorescent bands showed the presence of widespread, more or less spherical bodies ranging in size 100–300 nm. These bodies could represent the relicts of nannobacterial cells.
Domaines
Géochimie
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