Some Eocene species of marine pyritized diatoms in the core of Saint-Josse (France)

Pyritized Eocene diatoms were studied in the sediments of the « Argile de Saint-Aubin » Formation in the Saint-Josse (France) borehole. A new methodology was developed in the aim to extract pyritized diatoms from any types of sediments. The methods used facilitate extraction and concentration of a good quantity and quality of diatoms from sedimentary environments previously unfeasible in diatom analysis. The degree of pyritization, which transforms siliceous frustule to internal mold of pyrite and, by epigenesis, to pyritized frustule with conservation of the ornamentations, is described. Eighteen taxa of pyritized diatoms are identified. The assemblage of pyritized diatoms of the lithostratigraphic unit studied shows that the most frequent diatom observed is Fenestrella antiqua associated with Actinoptychus senarius and Craspedodiscus moelleri. Their presences indicate that the paleoenvironment was brackish with marine influence in Lowermost Eocene.