In the shadow of giants: Calcareous nannoplankton and smaller benthic foraminifera from an Eocene nummulitic accumulation (Transylvanian Basin, Romania)

Nummulitic accumulations are an important component of the Eocene sedimentary record in the Neotethyan realm. Their age, paleoenvironmental significance and depositional model are almost exclusively based on the interpretation of the larger benthic foraminiferal assemblages. We show here, however, that the high-resolution study of the other, often neglected, microfossils (calcareous nannoplankton and benthic foraminifera) is crucial to better constrain the diversity and paleoenvironmental gradient along the nummulitic accumulation(s). The studied micropaleontological assemblages, recovered from the large Eocene (Bartonian) nummulitic accumulation in the northwestern Transylvanian Basin (Romania), suggest an inner shelf depositional environment. A depth and nutrient gradient along the nummulitic accumulation from the south to the north could be recognized. In the southern part of the study area the environment was shallow and oligotrophic with Zygrablithus bijugatus, Blackites inflatus, Neococcolithes dubius, Ericsonia spp., larger sized reticulofenestrids and epiphytic foraminifera taxa (Pararotalia, Lobatula and Cibicides) on a sandy substrate. The progressive deepening toward the north led to an increase in clay content and more nutrient-rich marine waters in which calcareous nannoplankton (especially Reticulofenestra minuta), agglutinated foraminifera and calcareous infaunal foraminifera (Nonion, Protelphidium and Reusella) proliferated. The calcareous nannoplankton and smaller benthic foraminiferal assemblages confirm and further constrain the Bartonian age of the studied nummulitic accumulation, previously based on the larger benthic foraminiferal SBZ 17 zone.