Biostratigraphic and isotopic record of the Cenomanian–Turonian deposits in the Ohaba-Ponor section (SW Hat xeg, Romania) Mihaela Carmen Melinte-Dobrinescu a, * , Ana-Voica Bojar b a National Institute of Marine Geology and Geo-ecology, Dimitrie Onciul Street, No.23-25, RO- 024053 Bucharest, Romania b Karl-Fra ¨nzens Universita ¨t, Institut fu ¨r Erdwissenscaften, Heinrichstrasse 26, A-8010 Graz, Austria article info Article history: Received 8 February 2006 Accepted in revised form 4 May 2008 Available online 17 June 2008 Keywords: Middle Cenomanian–early Turonian Nannofloras Stable isotopes Southern Carpathians abstract The middle Cenomanian–lower Turonian deposits of Ohaba-Ponor section (Southern Carpathians) were studied from biostratigraphic and isotopic points of view. Both the qualitative and semiquantitative nannofloral analyses, as well as the stable isotope (d 13 C and d 18 O) data support significant palaeoenvironmental changes in the investigated interval. Two d 13 C positive excursions were recognized: (1) an excursion up to 1.8& (PDB) within the middle/late Cenomanian boundary; (2) an excursion up to 2.2& (PDB) in the Cenomanian/Turonian boundary interval. The oldest d 13 C positive excursion recorded (placed within the Acanthoceras jukes-brownei/Eucalycoceras pentagonum Ammonite Zone boundary interval, and in the NC11 Calcareous Nannofossil Zone respectively) could be assigned to the middle Cenomanian Event II (MCEII). During the above-mentioned event, significant increase in abundance of Watznaueria barnesae, followed by successive blooms of Biscutum constans and Eprolithus floralis, were observed. The youngest d 13 C positive excursion was identified in the Cenomanian/Turonian boundary interval (in the NC12 and lower part of the NC13 Calcareous Nannofossil Zones). Even the amplitude of this d 13 C positive excursion is lower in the Ohaba-Ponor section, as generally reported, this may represent the regional record of the OAE2. The successive peaks of the nannofossils Biscutum constans, Zeugrhabdotus erectus and Eprolithus floralis indicate episodes of cooler surface water and high fertility, which preceded and lasted the Cenomanian/Turonian boundary event. Additionally, fluctuations of d 18 O values between 2 and 6& suggest also cooler conditions within the Cenomanian/Turonian boundary interval. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The investigation of the Upper Cretaceous deposits of the Hat xeg area (SW of the Southern Carpathians) started already in the early 1900’s, when in this region the first dinosaur fossils from the Carpathian belt were discovered (von Nopcsa, 1902, 1914; Laufer, 1925). Since the last two decades, the study of the continental Upper Cretaceous deposits of this ‘‘forgotten land’’ was revitalized by the works of Grigorescu (1983, 1990), Grigorescu et al. (1985, 1994, 1999), Csiki and Grigorescu (1998) and Bojar et al. (1998, 2005). Although the Hat xeg region is generally famous for its rich continental macrofaunas, and particularly for its dinosaur remains, widespread marine Cretaceous deposits also occur in the NW and SE of this area. So far, the published works were focused on the lithology and palaeontology of the marine deposits of Hat xeg (Mamulea,1953; Pop et al., 1973, 1990; Szasz, 1976; Ianoliu et al., 1980; Antonescu et al., 1983; Stilla, 1985; Grigorescu and Melinte, 2001 , among others). These studies revealed various lithofacial features in both NW and SE of the Hat xeg region, indicating different palaeoenvironmental settings, especially during the Late Cretaceous. The main aim of this paper is to identify changes in lithofacies, fossil assemblages, lithology, and geochemistry in Cenomanian - Turonian successions which crop out in the SE of Hat xeg (in the Ohaba-Ponor area). Another goal of this work is to evaluate the use of the identified stratigraphical markers for regional and global correlation. 2. Geological Setting The Upper Cretaceous deposits of the Hat xeg area represent the sedimentary cover of the Median Dacides (sensu S andulescu, 1984), which overlies the Precambrian crystalline basement of the Getic Nappe of the Southern Carpathians. The present structure of the Southern Carpathians is the results of several middle and late Cretaceous collision phases. The study region, as the whole Southern Carpathian belt, was involved in Cretaceous movements, * Corresponding author. E-mail address: melinte@geoecomar.ro (M.C. Melinte-Dobrinescu). Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes 0195-6671/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2008.05.018 Cretaceous Research 29 (2008) 1024–1034