Abstract
The lowermost part of the Eocene section in the district of Pleven, represented by strongly glauconitic sand marls and clayey - calcareous sandstones, belongs to the Beloslav Formation, while the clayey marls of its middle and upper parts belong to the Avren Formation. The sandstones, outcropping near the village of Opanec, are differentiated as a sandy formation, on account of their unclear distribution. On the basis of a recent study on the nummulites fauna, a Late Ilerdian Age is assigned to the Beloslav Formation, and to the sandy formation Early Lutetian. There are no nummulites found in the sediments of the Avren Formation. Their chronostratigraphie position has been determined on the basis of data on microforaminifers reported in Bulgarian literature, where their stratigraphic distribution is interpreted from a modern point of view. The age of the Avren Formation in Pleven district is determined within the interval of Early-Middle Eocene (Cuisian, Lutetian, Bartonian). The following Late Ilerdian species are described: Nummulites aff. gammardensis, N. spileccensis, N. aff. soerenbergensis, N. cf. involutus, Assilina leymeriei (Beloslav Formation), as well as the following Early Lutetian species: Nummulites perplexus, N. obesus, N. messinea, which fill up the sandy formation.
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