Calcareous nannofossil biostratigraphy and magnetostratigraphy of the Upper Miocene and Lower Pliocene of the Northern Aegean (Orphanic Gulf–Strimon Basin areas), Greece

Abstract

The transitional position of the Northern Aegean makes it an important area for studying the relation between the Mediterranean and the Eastern Paratethys. A chronostratigraphic framework has been obtained for the Late Neogene deposits along the Orphanic Gulf and in the Strimon Basin of Northern Greece by analysing calcareous nannofossil data and paleomagnetic results from eight sections. The local formations and major lithological units have been correlated with the Upper Miocene and Lower Pliocene stages. Furthermore, the obtained framework has facilitated a more detailed reconstruction of the effects of the Messinian salinity crisis on the sedimentary record. Magnetostratigraphic data from the Strimon Basin result in an age estimate of 6.3 Ma for the Dafni–Choumnikon Formation transition, between normal polarity subchrons C3An.2n and C3An.1n. Since this Late Miocene age estimate corresponds closely to estimates of the Maeotian–Pontian boundary in the Eastern Paratethys (Dacic Basin), we believe this transition from marine to brackish conditions corresponds to this latter boundary. The gypsiferous unit in the basal part of the Choumnikon-equivalent beds along the Orphanic Gulf can then be correlated with the evaporites in the Northern Aegean and the Lower Evaporites of the Mediterranean Basin. Consequently, we assume that these Northern Aegean evaporites have been deposited between 5.96 and 5.59 Ma. Freshwater deposits above the evaporites are easily traceable across the Orphanic Gulf area, as a gravel or sandstone unit topped by a travertine marker bed, and have a wide lateral continuation. Since their position is intermediate between the shallow marine gypsum/anhydrite and the more open marine clastics of the Pliocene, we argue that these beds correlate with the desiccation phase of the Mediterranean and Black Sea basins. This intra-Messinian event probably occurred between 5.59 and 5.50 Ma, thus predating the Upper Evaporites or Lago Mare of the Mediterranean. A clear recognition of the Miocene–Pliocene boundary remains problematic in the Orphanic Gulf area. Nannofossil assemblages in the sections suggest a higher position of the local base of the Pliocene than indicated by the level with the first appearance of planktonic and benthic foraminifera. Discontinuous occurrences of levels containing nannofossil assemblages of Zone MNN11b–c are found between the travertine unit and the lowermost Pliocene beds, representing brief marine ingressions. The latter show that in the latest Messinian, (presumably) temporary connections of Atlantic/Mediterranean waters with the Northern Aegean Basin existed. These marine ingressions likely extended further into the Eastern Paratethys, as demonstrated by the presence of nannofossil assemblages in the Middle and Upper Pontian of the Dacic and Euxinic Basins. The events around the Miocene–Pliocene transition resulted in a fairly abrupt change from brackish to open marine conditions in a relative deep basin, which was subject to increased rates of subsidence from the onset of evaporite deposition in the Late Messinian onward.