Abstract:Investigations of the Lower Miocene of the Transylvanian Basin reveal particularly high abundances ( > 90 % of total foraminifera) of small sized biserial foraminifera (Bolivina/Streptochilus). This biotic event has not been noticed in the Transylvanian Basin so far probably owing to the facies misinterpretation and the small size of the specimens. SEM investigations allow more precise identification of biserial planktonic taxa and more accurate taxonomic interpretations. The high abundance of Bolivina/Streptochilus assemblages provide evidence for paleogeographic connections to the IndoPacific area and support new paleoenvironmental interpretations at the transition from the Early to Middle Miocene in relation to the paleoceanographic events. Both planktonic foraminifera and calcareous nannoplankton suggest a late Burdigalian age. A new Bolivina/Streptochilus Abundance Biozone is proposed just below the Early/Middle Miocene boundary.

Abstract. The Early -Middle Miocene transition can be studied in several sections from the north-western Transylvanian Basin, across the boundary between the Hida and Dej Formations. The Pâglişa and Râpa Dracului (Dej locally outcrop) sections offer a very good record of the changes in the sedimentary facies and micropaleontological content across the boundary. The lower parts of the sections display the upper Hida Formation with prodelta fine-grained sediments covered by coarser grained sediments related to a delta front progradation. Four sedimentary facies associations have been separated based on grain size and associated sedimentary structures. Micropaleontological assemblages are moderately to poorly preserved, with a low abundance and diversity. Planktonic Streptochilus pristinum is the main taxon associated with rare (< 10%) calcareous benthic foraminifera (Bulimina, Bolivina, Cibibicidoides). The Dej Formation consists of stacked channel lag deposits associated with lateral accretion bars and crevasse splays followed by fall-out tuffs and redeposited low density tuffites interbedded with low density fine siliciclastics. The deposits of Dej Formation from the investigated areas were separated in four facies associations based on grain size/petrography and primary sedimentary structures. Micropaleontological assemblages contain typical early Badenian planktonic foraminifera (species of Orbulina, Praeorbulina, Globigerinoides, and Globorotalia) associated to the main phase of the marine transgresion from the beginning of the Middle Miocene.

. Palaeoenvironmental changes in the Transylvanian Ba sin dur ing the Early Mio cene re vealed by the foraminifera as sem blages. Geo log i cal Quar terly, 60 (1): 167-180, doi: 10.7306/gq.1245The evo lu tion of the Transylvanian Ba sin dur ing the Early Mio cene has been re con structed from the suc ces sion of palaeoenvironments in ferred from sedimentological trends and the suc ces sion of spe cific foraminiferal as sem blages from Lower Mio cene Tihãu sec tion in the north west ern Transylvanian Ba sin. Plank tonic foraminifera sug gest a Burdigalian age and re cord sea level changes, cli ma tic and pro duc tiv ity events. Ben thic foraminifera of fer data on the palaeoenvironmental evo lu tion, with a large-scale progradational (coars en ing up) sed i men tary suc ces sion in flu enced by re gional tec ton ics. The suc ces sion of depositional events in cludes: (1) transgressive coarse grained de pos its with typ i cal Med i ter ra nean as semblages of bi valves in beach en vi ron ments; (2) a glauconitic fa cies which can be as so ci ated with the max i mum flood ing surface of the trans gres sion; (3) sed i men ta tion con tin ued on a nar row shelf in flu enced by del tas dur ing the highstand; (4) the in flu ence of re gional tec ton ics and sub se quent fill ing with turbidites as so ci ated with fan del tas.