Transitional marine environments from the Lower Pleistocene Rio Basin (Western Greece) Tsoni M.1, Valavani D.1,2, Papageorgiou G.1, Iliopoulos G.1 1.Laboratory of Palaeontology and Stratigraphy, Department of Geology, University of Patras, Rio, Greece 2. Dept. of Βiology, University of Patras, Rio, Greece Introduction The Corinth-Patras Rift system is one of the most seismically active zones in Europe, attracting thus the interest of geoscientists. The Rift system has been divided into three domains (Doutsos et al. 1988): (1) The Corinth graben to the east; (2) the Rio graben to the middle, and (3) the Patras graben to the west. Although several publications related to the Rift system have been published, some parts of the rift such as the Rio basin have not been studied yet in detail, despite the fact that back in 1943 Psarianos was one of the first researchers investigating the sediments of this area. In this study the syn-rift evolution of the western part of the southern coast of the Corinth-Patras Rift system is revisited providing new palaeontological data. a1,a2 Fig1:Location map of the study area h Regional setting The study area is located between the villages Sichena and Velvitsi on the west side of Charadros River (Fig.1). Rio basin is an asymmetric graben with a NW-SE trending, located between the mainland of Greece and NW Peloponnesus (Kontopoulos & Zelilidis 1997). This basin contains lower Pleistocene silty-muddy, lagoon, lacustrine and marine deposits that are overlain by red terrestrial conglomerates. The vertical sequence of the lithofacies varies among the different sections and thus not all lithofacies are represented everywhere. f Methods & Results 300 sediment samples were collected from the sections Vigla 1 (a1 a2, h, f) and Vigla 2 (abc, st, θ, k, imnx) and analyzed stereoscopically. Herein we report our first data on the study of microfossils (foraminifera and ostracoda). Multivariate analyses of samples and species of these organisms have been performed to provide an accurate description of these assemblages. Census data were obtained for different species in each sample and the statistically significant fractional abundances values (≥10%) were analyzed using Q-mode cluster analysis and correspondence analysis. The sedimentary sequence is mainly composed of siltstones. According to the microfaunal analysis the section is composed of 116 different species of benthic foraminifera and ostracoda. The most abundant species in the samples from the studied sequences are the brackish taxa Ammonia tepida, Haynesina depressula, Cyprideis torosa f. torosa and Cyprideis torosa f. littoralis. The most abundant and widespread ostracoda and foraminifera species that have been determined in the marine sequences are: Ammonia beccarii, Cassidulina carinata, Bullimina spp., Elpidium spp., Costa edwardsii, Cytheridea neapolitana. The assemblages in the greater part of the section contained abundant microfaunal elements, whereas these were scarce or completely absent from the respective assemblages of the lower 18 meters (k, imnx). abc Discussion The sequences of Vigla 1 and Vigla 2 correspond to a transitional marine environment, which due to continuous eustatic changes, as well as to tectonism that occurred during the early Pleistocene caused gradual but continuous adjustments to the local palaeoenvironments. The environmental distribution differs from the lower to the upper part of the sequence, being a typical example for the evolution of the basin during the Quaternary ( Fig2). Several authors have proposed different ages for the sediments of NW- Peloponnesus. Frydas (1989) in his nannoplankton study of West Peloponnesus indicated that marine deposits were dating back to the Early Pleistocene. On the other hand Palyvos et al. (2007) stated that the marine deposits of the footwall block of the Rion-Patras fault zone date from the Late Pleistocene. Three years later, Palyvos et al. (2010) reported deposits from the western Corinth basin of Middle to Late Pleistocene age. According to our study the deposits around Charadros River have Early to Middle Pleistocene age being in agreement with Flotte (2005) and Frydas(1989). st References Doutsos T., Kontopoulos N., Poulimenos G., 1988. The Corinth-Patras rift as the initial stage of continental fragmentation behind an active island arc (Greece), Department of Geology, University of Patras, Greece. Frydas, D., 1989. Biostratigraphische Untersuchungen aus dem Neogen der NW- und W- Peloponnes, Griechenland. N.Jb.Geol.Palaont.Mh. Η.6, 321-344, Stuttgart. Kontopoulos N., Zelilidis A., 1997. Depositional environments of the coarse- grained lower Pleistocene deposits in the Rio-Antirio basin, Greece. Palyvos, N., Pantosti, D., Stamatopoulos, L., De Martini, P. M., 2007. Geomorphological reconnaissance of the Psathopyrgos and Rion-Patras fault zones (Achaia,NW Peloponnesus). Bulletin of the Geological Society of Greece , Proceedings of the 11th International Congress, Athens, May. Palyvos, N.,Mancini, M., Sorel, D., Lemeille, F., Pantosti, D., Julia. R.,Triantaphyllou, M., De Martini, P.M. 2010. Geomorphological, stratigraphic and geochronological evidence of fast Pleistocene coastal uplift in the westernmost part of the Corinth Rift (Greece). Geological Journal, Geol.J. 45:78-104. θ Acknowledgements The present work is being carried out within the framework of project E038 K. Karatheodori basic programme 2013 Fig2: Stratigraphic column with palaeoenvironmental interpretation