New data on the Eocene/Oligocene boundary in the Romanian Carpathians

Abstract

In the Romanian Carpathians the boundary between Eocene and Oligocene stratigraphic sequences may be examined in three geological provinces: (1) the east Carpathians, (2) the Getic Depression (these two units containing exclusively detrital-terrigenous sediments), and (3) the northwestern sector of the Transylvanian Basin (which contains an epicontinental sequence). 1. (1) In the East Carpathians, the boundary occurs between the zones Globigerina corpulenta-gortanii + Ericsonia subdisticha (Priabonian) and Pseudohastigerina micra + extinction to Cyclococcolithus formosus (Rupelian). From a lithostratigraphic point of view, the boundary lies between the lower Fusaru sandstone (internal facies) or the Lucacesti sandstone (external facies) and the overlying mainly non-marine bituminous formations (marls with fetid opal-cherts). 2. (2) In the Getic Depression, the Priabonian is represented by the upper section of a pseuoflysch (Olanesti beds) with Globigerina corpulenta, G. officinalis (P 17) and a heterogeneous nannoplankton assemblage. The gritty-conglomerate and ingressive Lower Oligocene (Corbi or Cheia Beds) yields G. tapuriensis (P 18) and Helicopontosphaera reticulata (NP 22) assemblages. 3. (3) In the Paleogene epicontinental area of the northwestern part of the Transylvanian Basin, the formation across this boundary are represented by the Brebi marls (with bryozoans) (Upper Priabonian-Henisian). The equivalence of the (northern) Lattorfian to the upper part of the (Mesogean) Priabonian has recently become accepted. On the basis of the evolution of marine microfaunas (nannoplankton, ostracods and foraminifera), the Eocene/Oligocene boundary has been determined at a few metres below the top of the Brebit marls, at the junction of zones NP 21 and NP 22 (the extinction of Cyclococcolithus formosus is considered as a marker for the upper boundary of the Ericsonia subdisticha zone). The same boundary delimits two ostracod zones (those of the Eocene Phlyctocythere eocaenica and of the transition Eocene/Oligocene species Paijenborchella tricostata) and two zones with Mesogean planktonic foraminifera (P 17 with G. cerroazulensis and P 18 with G. ampliapertura + Pseudohastigerina micra).