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

Abstract

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 freshwater 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 tidal processes. The basin was filled with brackish-water, enhancing frequent hyperpycnal plumes, ichnofossils activity, enrichment in organic material and the preservation of in situ brackish-and freshwater 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 from 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. A 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.

A Late Miocene – Early Pliocene Paratethyan Type Ostracod Fauna From the Denizli Basin (Sw Anatolia) and Its Palaeogeographic Implications

“…Ilyocypris gibba (Ramdohr, (Meisch, 2000). In the fossil record I. gibba has been described from the lower Miocene and Pliocene of Central Anatolia (Tunoğlu & Celik, 1995) and in Plio-Pleistocene deposits of the Dacian Basin (Romania) (Van Bakk et al, 2015;Jorissen et al, 2018). It has also been mentioned by Agalarova et al (1961) and Mandelstam and Schneider (1963) within Pliocene and post-Pliocene deposits of Azerbaijan, Turkmenistan SSR, Kirmea and Caucasus.…”

“…in the Central Paratethys (Carbonnel and Jiříček, 1977;Jiříček, 1983). The species has been commonly observed in Khersonian -Romanian brackish water deposits of the Slănicul de Buzău section as well as in the freshwater sediments of Romanian deposits (Van Baak et al, 2015;Jorissen et al, 2018;Lazarev et al, 2020 (submitted)). In the Mediterranean the species was first recognized in middle Tortonian brackish water deposits of Crete in the Eastern part of the Mediterranean (Sissingh, 1974).…”

“…Other occurrences include Plioceneearly Pleistocene deposits from Central Italy (Valle Orsa Fm. ; Spadi et al, 2018) and deposits from the Slănicul de Buzău section (Dacian -Romanian stage) in the Dacian Basin (Van Baak et al, 2015;Jorissen et al, 2018), upper Miocene freshwater strata from Turiec Basin (Slovakia; Pipík and Bodergat, 2007) and the upper Pontian of the Dacian Basin (Ter Borgh et al, 2014). Ecology.…”

“…This species has been described in the fossil record from the Miocene and Pliocene (Kazmina 1975;Tunoğlu and Çelik 1995) and the Pleistocene to Recent (Meisch, 2000). Inside the Paratethyan domain it has been obtained from a variety of basins, e.g., the upper Maeotian (Bădislava and Toplog sections) and Dacian (Slănicul de Buzău) of the Dacian Basin (Romania) (Floroiu et al, 2016;Jorissen et al, 2018), the upper Productive Series (Surakhany Fm., Kimmerian age) and the Akchagylian-Apsheronian of the South Caspian Basin of Azerbaijan (Van Baak et al, 2013;Lazarev et al, 2019) and the middle Pannonian of the Gelibolu Peninsula (NW Turkey) (Tunoğlu and Ünal, 2001). Further is appears during the Piacenzian-Gelasian in the Tiberino Basin in the Central Apennines of Italy (Spadi et al, 2019) and the Quarternary deposits of the Gorgan plain in North Iran (Yassini, 1987).…”

A conservation palaeobiological approach to assess faunal response of threatened biota under natural and anthropogenic environmental change

“…Sedimentation rates are estimated at 0.083 cm yr −1 for the lower parts of the cores and at 0.126 cm yr −1 for the upper parts. Sedimentation rates in the RSL are high in comparison to other regional examples of Miocene-Pliocene deltaic and lacustrine environments (De Leeuw et al, 2013;Jorissen et al, 2018), which may be explained by a lack of post-depositional compaction of the sediments in these young lagoon and lacustrine sediments. We excluded admixed reworked faunas (high taphonomic scores) to reduce a possible bias in our age model.…”