Filling of sedimentary basins and the birth of large rivers: The lower Danube network in the Dacian Basin, Romania

Abstract

Large rivers typically form in the final phase of evolution of sedimentary basins. By that time the sediment supply balances out the accommodation, local rivers coalesce into larger rivers and seek pathways to flow out of the basin into areas where accommodation is available. Very often, this is controlled by tectonics controlling sediment supply and accommodation or facilitating opening inter-basinal gateways. Using subsurface data (seismic and well logs), this paper discusses the controlling factors that lead to the birth of large, continental-scale rivers using the Danube River in the Carpathians foreland basin as a study example. The Proto-Danube originated in the North Alpine Foreland basin around 19–18 Ma and prograded southwards across the Vienna basin (14–13 Ma) and Pannonian basin (10–5 Ma) before reaching the Dacian basin. The Dacian basin took shape during the mid-Miocene and initially consisted of a number of individual and loosely connected depocenters, which were subsequently filled by local sediment sources. These sources typically reflect the uplift and erosion history of the closest orogenic segment. As the accommodation filled, the fluviatile networks coalesced and merged with the Paleo-Danube around 4 Ma, which flowed closer to the Carpathians, about 100 km north of the present path. Continued subsidence in the eastern part of the foreland basin controlled the further evolution of the Paleo-Danube that formed a large delta system, a precursor of the present-day Danube Delta. Eventually, the delta depocenter advanced into the Black Sea in the Romanian/Pleistocene. We interpret this to be the result of increased sediment supply due to renewed uplift of the Carpathians. Comparing the evolution of the Danube River drainage with other continental scale rivers around the world, we can conclude that the Danube represents an uncommon river system characterized by transiting multi-basin drainages. Initially, multiple small rivers filled sub-basins from different directions and these coalesced at later stages. Most large modern rivers, similar in size, formed within a simpler tectonic frame with sediments commonly supplied along a well-defined tectonic trend.