Late Miocene to Pliocene sediment supply to the Northwest Black Sea; the importance of the axial East Carpathian foreland basin drainage system

Late Miocene to early Pliocene siliciclastic sedimentary systems deposited a ~300 km wide, ~2 km thick succession on the northwest Black Sea shelf and fed a complex of slope to basin floor turbidite fan systems farther to the southeast. Intuitively, much of this sediment input may be attributed to the continental-scale Danube River. However, multiple datasets indicate that sediment sourced by the Danube first reached the Black Sea less than ∼4 m.y. ago. Thus an alternative source for the sediment constructing the prograding late Miocene to early Pliocene clinoforms in the northwest Black Sea must be sought.

In this presentation, we propose that the axial drainage of the East Carpathian foreland basin was the major sediment supply system to the northwest Black Sea in the late Miocene to early Pliocene. This hypothesis, developed following our analysis of the sedimentary fill of the foreland in eastern Romania, Moldova, and southwest Ukraine, is based on a compilation of existing and original data, the latter being obtained during multiple field campaigns between 2011 and 2016 when nearly 600 field outcrops were studied.

Our detailed observations indicate that a major axial sediment system prograded through the East Carpathian foreland basin from the north-northwest to the south-southeast during the late Miocene. The origin of the prograding system lies in the asymmetric, diachronous evolution of the Carpathians. These formed as a result of the collision of the Tisza-Dacia and ALCAPA (Alps, Carpathians, Pannonia) mega-units with the European margin. The last foreland-propagating thrusts, which just predate continental collision, are Langhian to early Serravallian in age in the West Carpathians and are Tortonian in age in the East Carpathians.

Continental collision first triggered an increase in mountain belt topography and associated sediment flux, while cessation of thrust propagation slowed down the creation of accommodation space in the foreland. Thus by the late Miocene, the foreland of the West Carpathians had become overfilled as a result of collision, while a last phase of thrust propagation led to rapid subsidence and an eastward expansion of the East Carpathian foreland basin farther to the south-southeast. From 12 Ma, excess sediment from the overfilled western foreland was shed laterally into the East Carpathian foreland basin, initiating the axial sediment transport system. The sediment flux to this system was further enhanced by collision-related, thick-skinned deformation and out-of-sequence thrusting in the East Carpathians between 11–8 Ma.

Collision was followed by northwest-southeast–migrating slab detachment, leading to diachronous post-orogenic uplift, erosion, and a gentle southeast tilt of the basin fill. This is exemplified by the current 200–350 m altitude of late Miocene shoreface deposits, and the up to several hundred metres of fluvial incision. Progressively older strata crop out toward the northwest, suggesting a gradual increase in exhumation in this direction. The sediment flux resulting from the erosion of the foreland basin was transmitted down the axial drainage system.

The axial system of the East Carpathian foreland basin was ideally positioned to supply sediment to the adjacent Black Sea Basin. In the early Tortonian, there was still significant accommodation space in the foreland, with clinoform height suggesting a maximum water depth of ∼400 m along the Carpathians. Sediment delivery to the Black Sea was periodic and facilitated by several large-amplitude (250–600 m), short-duration (400 k.y.) base-level falls that were superimposed on the longer duration progradation of the axial system. The last significant marine transgression onto the delta top occurred in the late Messinian (6.1 Ma), indicating that since then, the foreland has been overfilled, and all axially supplied sediment was shed into the Black Sea and Focsani Depression. This explains why distinctive clinoform progradation into the northwest Black Sea is evident on seismic sections from 5 Ma.

In summary, our field research has revealed a large south-southeast–prograding axial sediment transport system in the East Carpathian foreland basin, which evolved due to the asymmetric, diachronous evolution of the Carpathians and their foreland. The system was the major sediment supplier to the northwest Black Sea in the late Miocene to early Pliocene, when sediment supplied by the palaeo–Danube River remained trapped in the upstream Pannonian Basin. Between 9 and 5.8 Ma, when accommodation space was still available in the foreland, sediment fluxes from the East Carpathian foreland basin to the Black Sea were periodically triggered by base-level falls exceeding the water depth in the foreland basin. At 5.8 Ma, the foreland basin became permanently overfilled. From then on, the axial system persistently shed ∼10×103 km3/m.y. of sediment into the Black Sea, resulting in the striking progradation of the northwest Black Sea shelf and of by-pass into deeper water. The sediment flux from the East Carpathian foreland basin eventually diminished when the effects of collision and slab detachment in the Carpathians waned. In the late Pliocene to early Pleistocene, the Danube River finally took over as the main sediment supplier.

The switch from sediment supplied by the axial system of the East Carpathian foreland basin to the Danube system, not only resulted in a switch in provenance, but also a switch in the composition of sand shed into the northwest Black Sea. This switch is predicted to result in a degradation of reservoir quality. An understanding of the regional context, composition and sediment flux of sand supply systems to the northwest Black Sea is therefore important if we are to fully evaluate the large-scale petroleum systems hosted in their sediments.