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

Highlights

• Detailed micropaleontological study of a nummulitic accumulation

• Multivariate data analyses were performed.

• Abiotic factors are the main control on the distribution of the assemblages.

• Paleoecological parameters reflect a preference for shallow water.

Abstract

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.

Introduction

The Nummulites genus of larger benthic foraminifera is a diverse group of Paleogene Rotaliida that was especially widespread and abundant in the Eocene shallow water marine realm around the Neotethyan margin (Schaub, 1981). The build-up of calcareous Nummulites tests led to the formation of nummulitic accumulations (sometimes referred to as nummulitic banks), a peculiar sedimentary record without a modern counterpart (Racey, 2001). Because these accumulations are both interesting from a paleoecological point of view and may also be important hydrocarbon reservoirs, their paleoenvironmental interpretation is an important area of scientific research (Racey, 2001; Papazzoni and Seddighi, 2018). However, published studies have often reported contradictory results because they either use a purely paleontological approach focusing on the paleoecological interpretation based on Nummulites, or a sedimentological approach in which the nummulitic tests are treated as simple grains (see Mateu-Vicens et al., 2012; Briguglio et al., 2017 and references cited therein).

To shed light on the paleoenvironmental interpretation of nummulitic accumulations, we carried out a high-resolution study of the calcareous nannoplankton and smaller benthic foraminiferal content of the well exposed and extended Eocene nummulitic accumulation in the Transylvanian Basin, Romania. This is a classic example of a nummulitic accumulation, usually interpreted as a nummulitic bank (Papazzoni and Sirotti, 1995; Kövecsi et al., 2016; Papazzoni and Seddighi, 2018) which preserves mainly in situ, or rarely para-autochthonous assemblages (see Kövecsi et al., 2016; Pleș et al., 2020). The Nummulites which form the main part of the studied nummulitic accumulation, if transported, are transported as a bed load over a short distance (decimeters, rarely meters) because they imbricate rapidly (Briguglio et al., 2017. Furthermore, the monospecific or the “odd pairs”- type (Hohenegger et al., 1999) bi-specific assemblages, and the micrit or clay filling the spaces between the Nummulites are interpreted as clues for the autochthonous origin of the studied nummulitic accumulation (Briguglio et al., 2017; Pleș et al., 2020). Even so the depositional environment and the paleoecology of its fossil groups are still not well understood. Previous studies either focused on the paleoecological interpretation of Nummulites (Papazzoni, 2008; Kövecsi et al., 2016) or sedimentological analyses (Proust and Hosu, 1996) when interpreting this sedimentary record. As a result, few studies have focused on the calcareous nannoplankton assemblages identified within and/or below the nummulitic accumulation when constraining the age of this deposit, with no reference to the paleoecology of the fossil assemblages (Bombiţă et al., 1975; Popescu et al., 1978; Gheța, 1984; Rusu et al., 2004). The smaller benthic foraminifera associated with Nummulites have been almost completely neglected, except in one study (Bombiţă et al., 1975), while those occurring just below the nummulitic accumulation were studied on a purely taxonomic basis (Popescu et al., 1978; Rusu et al., 2004) with no reference to their biostratigraphy or paleoecology. This is curious, because it is well known that marine microfossils are sensitive to external factors (biotic and abiotic) and have a high preservation potential in the sedimentary record. They can therefore be good indicators of changes in the environmental and paleoenvironmental settings (Murray, 2006). A study of the microfossil assemblages i.e., of calcareous nannoplankton and smaller benthic foraminifera that lived in the “shadow” of the larger benthic foraminiferal communities may therefore offer a better view on the paleoenvironment of nummulitic accumulations and can further constrain the age and paleoecology of these fossil communities.

Thus, we carried out biostratigraphic framing by detailed comparison with other Eocene biozonations, determined the main taxa and assemblages using statistical analysis of the micropaleontological data, and interpreted the paleoenvironmental significance of the recovered assemblages based on the ecological preferences of their main taxa.