Paleoenvironmental evolution of the East Carpathian foredeep during the late Miocene-early Pliocene (Dacian Basin; Romania)

摘要

The thick and continuous Mio-Pliocene sedimentary successions of the Focsani Depression in the Dacian Basin of Romania provide an excellent opportunity to study the paleoecological changes in the Eastern Paratethys during the time when the Mediterranean and Black Sea experienced major sea level fluctuations related to the closure and re-opening of the marine connection to the Atlantic Ocean during the Messinian Salinity Crisis. These successions form the basis of high-resolution magneto-biostratigraphic studies that allow a detailed correlation to the standard Geological Time Scale. Here, we analyze the paleoenvironmental evolution of the East Carpathian foredeep by integrating micro- and macropaleontological data and sedimentological analyses. The ostracod and mollusc fossil associations from the Ramnicu Sarat river section indicate that the late Maeotian depositional environment was characterized by shallow waters and littoral to fluvio-deltaic sediments. The Maeotian-Pontian boundary (6.04 Ma) is marked by a marine ingression, comprising benthic (agglutinated and calcareous) and planktonic (Streptochilus spp.) foraminifera and nanofossils. Following this marine ingression, the Lower Pontian (Odessian; 6.04-5.8 Ma) fauna shows an increased bathymetry of the basin. The presence of ostracod species with eye tubercles indicates depositional environments within the photic zone (＜100m). The Middle Pontian (Portaferrian; 5.8-5.5 Ma) is marked by a widespread sea level lowering resulting in dominant fluvio-deltaic conditions. This ecostratigraphy demonstrates that the main Messinian sea-level draw down (at 5.6-5.5 Ma) occurred in mid-Portaferrian times. Paleoenvironmental indicators show that the water level in the Focsani Depression dropped less than 100 m during Mediterranean desiccation. The Dacian Basin remained filled with water, suggesting a positive hydrological balance for the region. This is compatible with the presence of a shallow barrier at Dobrogea (the Galati passage), separating the Dacian Basin from the Black Sea Basin during the late Miocene. The Portaferrian-Bosphorian boundary (5.5 Ma) is marked by a second transgressive event, but this time without marine foraminifera. We conclude that the Dacian Basin formed a semi-isolated entity during the Portaferrian and experienced connectivity to the Black Sea domain during the Odessian and Bosphorian.