Provenance and timing of Late Miocene to Recent sediment delivery to the Western Black Sea through integrated outcrop-to-well studies

Heavy mineral sediment provenance studies may provide key insights to improve the predictive quality of basin models, particularly when integrated with high-resolution stratigraphic research. The Western Black Sea Basin is surrounded by sediment sources providing sand-grade material with a wide range of compositions from source areas ranging from the Alps, via the Balkans, Carpathians and Rhodope mountain ranges to Anatolia and the Pontides. To the north of the basin, the East European Craton may supply sands with excellent reservoir potential. Being able to identify in detail the pathways along which sands with good reservoir potential were transported into, and deposited in, the basin and the timing of switches in sediment provenance has proven to be a crucial prerequisite in the Black Sea for successful exploration and appraisal drilling of late Miocene and Pliocene targets.

Here we show the application of this integrated approach using reference data from the CASP sample and analytical database of present-day rivers and geological outcrops surrounding the Western Black Sea Basin to constrain the sediment provenance evolution in the late Miocene to Recent southwestern part of the Western Black Sea Basin. The basinal perspective we study from the only publicly accessible wells in the Western Black Sea Basin that penetrated the late Miocene to Pliocene stratigraphic interval; the Deep Sea Drilling Project (DSDP) cores from Site 380/380A (Leg 42 B) drilled in the late 1970s in the southwest Black Sea. We identify two changes in siltstone provenance-sensitive heavy mineral abundances at DSDP site 380/380A in the southwest Black Sea. Comparison with modern river sediment compositions indicates that siltstones above 571.5 mbsf (metres below sea floor) were supplied by the Danube, while sediments below 651.0 m were sourced by other, probably southern sediment supply systems.

In addition, our study allows us to provide detailed temporal constraints for the observed trends in sediment provenance. Establishing a high-resolution stratigraphic model for the basinal Black Sea has proven difficult in the past. This is primarily because the Black Sea since the Miocene has experienced regular large paleoenvironmental changes, switching between (anoxic) marine conditions when connected to the Mediterranean Sea and (oxic) freshwater conditions at times of basin isolation. While these regular switches of basin conditions potentially allow for a very high-resolution stratigraphic framework, independent absolute age constraints are required to provide a proper temporal context.

In the past decade, there have been significant advances in our understanding of the stratigraphy of the Black Sea through high-resolution stratigraphic studies of outcrop records on the margins of the Black Sea. These studies have focused primarily on the northern and northwestern margins of the basin at the Taman Peninsula in Russia and in the Dacian Basin in Romania. Using stratigraphic constraints from these outcrop localities, we can now improve the stratigraphic control on the DSDP wells. Unfortunately, the stratigraphic model for DSDP 380/380A is compromised by the presence of multiple 100+ m thick mass transport deposits.

Similar integrated high-resolution heavy mineral provenance and stratigraphic study as we now present for DSDP 380/380A at other basinal locations, particularly in undisturbed settings without mass transport deposits, would allow us to further develop the basin-to-margin stratigraphic correlation model. The enhanced insight created this way would provide a chance to reduce risk during further exploration and appraisal work beyond the currently known fields in the Western Black Sea Basin.