The “ Rzehakia beds” on the northern shelf of the Pannonian Basin: biostratigraphic and palaeoenvironmental implications

The Miocene genus Rzehakia, an endemic brackish bivalve that lived in the Paratethys Sea, recorded in late Ottnangian sediments deposited during the regression of the Central Paratethys Sea in the semi-enclosed Alpine-Carpathian foreland basin, is closely related to the Rzehakia found hundreds of kilometres away in the early Badenian transgressive sequence situated in the Western Carpathians hinterland. The finding of the “ Rzehakia beds” from the Malý Krtíš site in the Novohrad-Nógrád Basin provides insight into the palaeobiogeography of the Central Paratethys. The present study documents Rzehakia -bearing deposits of an alternating wave-dominated, river-influenced environment and of a fluvial-dominated, tide-influenced, wave-affected environment, probably located on the prodelta slope of a compound deltaic clinoform. Corresponding calcareous nannofossil assemblages, marked by the absence of Sphenolithus belemnos and the co-occurrence of Helicosphaera ampliaperta with Sphenolithus heteromorphus without Helicosphaera waltrans, indicate the NN4 Zone. Furthermore, the presence of the regional benthic foraminiferal marker Uvigerina graciliformis indicates a Karpatian age and the planktic Globorotalia transsylvanica restricts the profile stratigraphic range to the upper Burdigalian-Langhian part of the NN4 Zone (i.e., upper Karpatian-lower Badenian). In accordance with updated palaeogeographic data, these results led us to the conclusion that the “ Rzehakia beds” in the Novohrad-Nógrád Basin are synchronous with or younger than those from the Alpine-Carpathian Foredeep and are possibly related with their middle Miocene (late Kotzakhurian–early Tarkhanian) occurrences in the Eastern Paratethys.