CLIMATE VS TECTONICS: THE CERGOWA BEDS (OUTER CARPATIANS) AS THE CASE STUDY OF A MARGINAL BASIN SEDIMENTARY RECORD

The Eocene-Oligocene climate cooling brought about the eustatic sea level fall crucial for the Paratethys isolation from the Tethys Ocean. Such climatic change influenced the Carpathian basins, marginal to the oceanic realm, resulting in reduced circulation and anoxic conditions. This is recorded in the Outer Carpathians by the Menilite Formation (Oligocene) typified by dark shales. The Menilite Fm. also contains a lenticular lithosome of the Cergowa Beds composed of sandstones and subordinate sandstone-mudstone associations dated by two nannoplankton zones NP 23-24. Deposition of the Cergowa Beds during the NP23 zone took place from mainly dense, sandy sediment gravity flows, including high-density turbidity currents. Brackish water conditions, indicated by nannoplankton species, reflect significant freshwater influx into a confined basin isolated from the Tethys Ocean. The abundance of coalified plant matter suggests a direct connection between the basin and the terrestrial source via a fluvial system and probably a shelf-edge delta. Basin shallowing, and the resulting isolation, was caused by a combined effect of global cooling-related eustatic sea-level fall and tectonic uplift during the Alpine orogenesis. Deposits and nannoplankton of the succeeding zone NP24 indicate an open sea realm with deposition by low-density turbidity currents declining with time, and eventually returning to anoxic conditions typical for the Menilite Fm. Therefore, the sea deepening progressed against the continuing global cooling and eustatic sea-level fall. This apparent discrepancy shows that the local subsidence rate in the Cergowa Basin must have exceeded the efficiency of supply in detrital sediment from the source. The marginal character of the Cergowa Basin relative to the Tethys Ocean resulted in the stratigraphic record of relatively subtle episodes like: (i) the CCD fluctuations, reflecting coccolithophorid-rich productivity recorded as the laminated pelagic Tylawa Limestones; (ii) local slope instability reflected by slumps and hybrid flows interpreted as a record of synsedimentary tectonic deformations of the sea bottom; (iii) consequences of salinity changes between open marine and restricted brackish basin; (iv) interplay between variations in subsidence and sediment supply rates.