The eocene/oligocene boundary in Hungary in the light of the study of larger foraminifera

Abstract

The most important larger foraminifera in the study of the Eocene/Oligocene boundary are the nummulites, especially Nummulites intermedius. However, the revision of the nummulite faunas of the Hungarian transitional Eocene-Oligocene formations ( Nummulites-Discocyclina-Lithothamnium Limestone, Bryozoan Limestone, Buda Marl, Hárshegy Sandstone) revealed that N. intermedius is absent in these faunas, which emphasizes the importance of other nummulites and larger foraminiferal species. The studies on the transitional sequences and their larger foraminiferal faunas from the Buda Mountains, Bakony Mountains, the Ságvár-Balatonbozsok-Tabajd-Csákvár Belt, the Nagyegyháza and Dorog Basins, Kósd and the Mátra and the Bükk Mountains are discussed. The most complete transitional sequences appear in the Buda Mountains. The studies on the larger foraminiferal fauna of these sections resulted in the discovery of a Nummulites species, which gives important data concerning the Eocene/Oligocene boundary. This can now be drawn between the Hárshegy Sandstone of Várerdöhegy at Solymár on the basis of N. vascus, a characteristic Lower and Middle Oligocene species, and the underlying Nummulites-Discocylina-Lithothamnium Limestone, on the basis of N. fabianii, N. chavannesi, N. pulchellus and N. incrassatus, belonging to the Upper Eocene. In addition to the stratigraphically most important nummulitids and lepidocyclinids, the spatial distribution of other larger foraminifera from these transitional formations is discussed. Finally, a comparison is made between the most important boundary faunas in Hungary and elsewhere. The Hungarian larger foraminiferal faunas show a conspicuous agreement with the Mediterranean faunas. From the latter, those of the Priabonian stratotype and the sections near Cluj-Napoca, Romania, are reviewed in detail.