Nummulites assemblages, biofabrics and sedimentary structures: The anatomy and depositional model of an extended Eocene (Bartonian) nummulitic accumulation from the Transylvanian Basin (NW Romania)

Extensive high-resolution palaeontological, stratigraphic and sedimentological data offer a new and comprehensive view of the genesis and evolution of the most extended Eocene nummulitic accumulation in the northern Neotethyan realm. The preserved Bartonian (SBZ 17) Nummulites assemblages consist of large, granulate Nummulites perforatus and/or small, radiate Nummulites beaumonti. The assemblages differ in taxonomic content and/ or in the relative abundance of test type (A-versus B-forms): Assemblage 1 is dominated by N. beaumonti A-forms, N. perforatus A-forms are sporadic, while B-forms are rare or missing; Assemblage 2 consists of N. perforatus A and B-forms; and Assemblage 3 consists essentially of N. perforatus A-and B-forms with only rare N. beaumonti A and B-forms. These assemblages are interpreted as autochthonous and autochthonous to para-autochthonous, deposited on a low-angle inner shelf. Their distribution in space and time reflects the interplay between the palaeoenvironment and the ability of N. perforatus and N. beaumonti to colonize new habitats. Thus the genesis and development of the studied nummulitic accumulations mirrors the ecological preferences of the two Nummulites species, the relative sea-level, the sediment supply and possibly the climate history. The shelf around the storm wave base was first colonized by N. beaumonti and rare N. perforatus, then the mass nummulitic accumulation was formed by the accumulation of N. perforatus tests between the storm wave base and the fair weather wave base. The accumulation of abundant N. perforatus and rare N. beaumonti around the fair weather wave base occurred in the last phase.