Dating of the Black Sea Basin: new nannoplankton ages from its Geological Society, London, Special Publications

The Eocene uplift and inversion of a part of the Black Sea margin in the Central Pontides, allows us to study the stratigraphic sequence of the Western Black Sea Basin (WBS). The revision of this sequence, with 164 nannoplankton ages, indicates that subsidence and rifting started in the Upper Barremian and accelerated during the Aptian. The rifting of the western Black Sea Basin lasted about 40 Ma (from late Barremian to Coniacian). In the inner, inverted, Black Sea margin, the syn-rift sequence ends up with shallow marine sands. The uppermost Albian to Turonian was a period of erosion or non deposition. This regional mid-Cretaceous stratigraphical gap might result from rift flank uplift, as expected in the case of a thick and cold prerift lithosphere. However, coeval collision of the Kargi Block, along the North Tethyan subduction zone at the southern margin of the Pontides, might also have contributed to this uplift. A rapid thermal post-rift subsidence of the margin occurred during the Coniacian–Santonian. Collision of the Kirşehir continental block commenced in Early Eocene time (zone NP12) giving rise to compressional deformation and sedimentation in piggyback basins in the Central Pontides, whereas the eastern Black Sea was still opening. It is commonly accepted that the Black Sea Basin opened as a back-arc basin during the Mesozoic, as a consequence of the northward subduction of the Neotethys ocean (Hsü et al. 1977; Letouzey et al. 1977; Zonenshain & Le Pichon 1986). Alternatively, it may have opened under an extensional regime following the Palaeo-Tethyan collision and overthickening of the crust (Yiğitbaş et al. 1999, 2004). However, its precise timing of opening is still under debate (e.g. Nikishin et al. 2003). The eastern Black Sea Basin (EBS) (Fig. 1) is supposed to have rifted in the Upper Paleocene (Robinson et al. 1995; Robinson 1997). This Paleocene (postDanian) age of rifting is supported by the presence of an almost complete Mesozoic to Lower Paleocene series in exploration wells drilled on the Shatsky Ridge (Fig. 1) (e.g. Robinson et al. 1996). The western Black Sea Basin (WBS) (Fig. 1) is generally considered to have rifted during the middle Cretaceous (Late Barremian or Aptian–Albian– Cenomanian; e.g. Finetti et al. 1988; Görür 1988; Manetti et al. 1988; Görür et al. 1993; Robinson et al. 1996). This age is based on facies and thickness variations in the Cretaceous stratigraphic sequence of the Central Pontides (Görür 1988, 1997; Görür et al. 1993). However, pointing out that arc magmatism started in the Western Pontides only in the Turonian, Tüysüz (1999) then Sunal & Tüysüz (2002) suggested that the main opening phase had occurred during the Turonian– Maastrichtian. Moreover, based on heat-flow data, Verzhbitsky et al. (2002) obtained a 70–60 Ma age (Maastrichtian–Danian) for the lithosphere of the western and eastern basins. Surface data concerning the rifting and evolution of the Black Sea can be obtained from the thrust belt of the Pontides, which extends all along its southern margin. The Eocene compression and thrusting have uplifted sediments of the of the Black Sea margin. Therefore, the Cretaceous ‘syn-rift’ sequence can be precisely dated by onshore studies (Fig. 2). We focused our work in the Central Pontides Belt (Fig. 1) where good outcrops of the MesozoicPalaeogene sedimentary sequence are present From: Sosson, M. , Kaymakci, N. , Stephenson, R. A. , Bergerat, F. & Starostenko, V. (eds) Sedimentary Basin Tectonics from the Black Sea and Caucasus to the Arabian Platform. Geological Society, London, Special Publications, 340, 113–136. DOI: 10.1144/SP340.7 0305-8719/10/$15.00 # The Geological Society of London 2010. (Görür 1988; Tüysüz 1999), while the Eastern Pontides are mainly constituted by an Upper Cretaceous-Oligocene magmatic arc (e.g. Yılmaz et al. 1997). The Central Pontides Belt results from the inversion of part of the southern margin of the WBS. Thus it may comprise sequences related to the opening of the WBS, and therefore the oldest syn-rift deposits of the Black Sea. In order to constrain the timing of the western Black Sea opening, we have collected 164 samples from the Cretaceous to Palaeogene sedimentary sequences, in 143 localities of the Central Pontides (Table 1). The samples are dated by nannofossils, which provided precise ages for the stratigraphic sequence of the Black Sea margin. The observed main nannofossil assemblages used for each age determination are summarized in Table 2. Overview of the stratigraphic sequence