Sedimentary architecture and depositional controls of a Pliocene river-dominated delta in the semi-isolated Dacian Basin

Sedimentological facies models for (semi-)isolated basins are less well developed than those for marine environments, but are critical for our understanding of both present-day and ancient deltaic sediment records in restricted environments. Restricted basins form unusual depositional settings, where depositional forcing mechanisms often differ from the open ocean. Outcrops of deltaic deposits that accumulated in (semi-)isolated basins are relatively rare, but can provide wider spatial and temporal insights into drivers of sedimentary facies and fauna distributions.

This study considers an 835m-thick sedimentary succession of mid-Pliocene age (Dacian regional stage), which accumulated in the Dacian Basin, a former embayment of the Black Sea. Sediments were deposited along the northern margin of the basin, in the SE Carpathians foredeep. Detailed sedimentological and palaeontological analyses reveal a regression from distal prodelta deposits with brackish water faunas to delta-top deposits with freshwater faunas. Deltaic progradation created thin, sharply-based sand bodies formed by multiple terminal distributary channels, covering a wide depositional area. The system experienced frequent delta-lobe switching, resulting in numerous thin parasequences. Parasequences are overlain by erosive oxidized shell beds, formed under sediment starvation on top of abandoned delta lobes after a flooding event. A robust magnetostratigraphic time frame allows for comparison between the observed sedimentary cyclicity and the amplitude and frequency of astronomical forcing cycles. Our results indicate that parasequence frequencies are significantly higher than the number of time equivalent astronomical cycles. This suggests that delta-lobe switching was due to autogenic processes.

We consider the observed facies architecture typical for a delta prograding on a low-gradient slope into a shallow, brackish, protected, semi isolated basin. Furthermore, in the absence of significant wave and tidal influence, sediment progradation in such a protected depositional setting shaped a strongly river-dominated delta. Facies models developed in this paper may form good analogues for more poorly exposed or subsurface deltaic successions in (semi-)isolated basins.