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 sediment records in restricted depositional environments. Our proposed facies model considers an 835m-thick sedimentary succession, accumulated in a semi-isolated brackish embayment of the mid-Pliocene Black Sea. We investigated the sedimentary processes and depositional controls responsible for the sedimentary architecture of a delta flowing into this semi-isolated basin. The deltaic progradation caused a regressing from distal shelf deposits with brackish-water faunas to proximal fluvial deposits with fresh-water faunas. The observed facies architecture is typical for a river-dominated delta. The deltaic progradation into a restricted depositional environment probably resulted in the river domination and in a near-absence of sediment redistribution by wave- or tide-dominated processes. The protected brackish-water basin enhanced frequent hyperpycnal plumes, ichnofossils activity, enrichment in organic material and the preservation of in situ faunas. The delta prograded into a shallow basin on a low-gradient slope, creating thin sharp based sand bodies in numerous thin parasequences, due to a multiplication of the terminal distributary channels, covering a wide depositional area. The parasequences are bounded by reddish oxidized shell-rich indurated flooding surfaces, formed by sediment starvation on the top of the abandoned delta lobes, due to frequent delta-lobe switching. As a result, a succession of 64 parasequences occurs, with a thinning from 15 to 95m towards the top of the section. These high-frequency parasequences combine into nine low-order regressive sequences of around 83m and into three high-order regressive sequences of around 300m. The robust magnetostratigraphic time frame permitted to compare the observed sedimentary cyclicity with the amplitude and the frequency of various climatic cycles including astronomical forcing. Our results show that the frequencies of the parasequences and sequences are not in line with any Milankovitch climatic cycle. This suggests that astronomical climate forcing didn’t influence autogenic delta-lobe switching.