Interplay of global climatic and regional tectonic controls in marginal basins, with an example of the Cergowa Beds deposition (Outer Carpathians) during the Oligocene icehouse

Abstract

<p>The culmination of climate cooling at the Eocene-Oligocene boundary, known as the Terminal Eocene Event (TEE), forced the eustatic sea level fall crucial for isolation of the Paratethys from the Tethys Ocean. The isolated, northern marginal region contained starved Carpathian Flysch basins characterised by reduced circulation, dysaerobic bottom conditions and increased influx of riverine freshwater. The Cergowa Beds (Lower Oligocene) serve as an example of the icehouse period-related deposition of sandstones and subordinate sandstones-mudstones emplaced by sediment gravity flows in a predominantly anoxic depository dominated by dark shales and associated fine-grained facies of the Menilite Beds. Autochthonous calcareous nannoplankton species indicative of brackish water conditions reflect the Cergowa basin isolation and strong influence by freshwater influx during the zone NP23. This stage was dominated by high volume, high-density sediment gravity flows, occasionally triggered by hyperpycnal effluents. Coalified terrestrial organic matter, especially abundant in the proximal sector and including tree trunk fragments up to 2 m in length, suggests direct connection existed between the fluvial supply and redeposition by sediment gravity flows, probably via a shelf-edge delta supplying the Cergowa basin. The marginal character of the Cergowa basin enables to detect even subtle episodes as: (i) the CCD fluctuations, reflecting coccolithophorid-rich productivity, which is recorded as the laminated pelagic Tylawa Limestones, or (ii) local slope disequilibria reflected by hybrid flows<em> </em>interpreted as resulting from synsedimentary tectonic deformations of the basin floor. Generally, the Alpine orogenic movements enhanced the basin isolation and shoaling by tectonic uplift of the source area. However, the late stage of the Cergowa basin development, dated by the nannoplankton zone NP24, represents an open sea realm supplied by turbidity currents of decreasing density, with time suppressed and finally replaced by the anoxic sediments of the Menilite Beds type. Therefore, the deepening of the marine environment progressed against the prevailing global cooling and continuing eustatic sea-level fall. This apparent discrepancy emphasises the importance of the regional tectonic deformations of the basin that superseded the global climatic influence.</p>