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Messinian-Style Drawdown in the Black Sea at the End Eocene

Abstract

A prominent erosional unconformity between the Eocene and Oligocene strata has been described from various parts of the Black Sea. Based on well and outcrop data, the typical facies of the Eocene strata underlying the unconformity indicates deposition in shallow water (e.g. neritic limestones) whereas the Oligocene overlying the unconformity with shales and marls deposited in a deeper water environment, i.e. in a deep shelf or slope setting. Based on seismic reflection data the end Eocene unconformity can be followed from the shelfal regions of the Black Sea to the deepwater parts of the basin. Particularly pronounced is the erosion associated with this unconformity in the Histria Trough, offshore Romania, and in the Kaliakra Canyon, offshore Bulgaria. In these canyons the pronounced truncation of the strata underlying the unconformity can be mapped beyond the paleo-shelf break into the basin. The geometry of the unconformity suggests erosional character in a paleo-deepwater depth more than 1000 m. Recently acquired long-offset (10+ km) seismic reflection data revealed a very distinct seismic package in the Bulgarian part of the deepwater basin. The small-scale (40–80 m) clinoforms, downlapping onto the top Eocene unconformity, could not be seen on earlier seismic data sets due to their very deep position, close to the basin center. Due to their characteristic seismic pattern, we interpret these clinoforms as part of a shallow-water delta during the early Oligocene. The delta sequence is correlated with a similar seismic sequence located on the conjugate Turkish margin. However, it is overlain there by a much larger shelf margin prograding sequence with clinoforms indicating paleo-water depth of about 200–400 m during the Early Oligocene. This shows the rapid deepening of the basin during the deposition of the earliest Maykop sediments. The above observations are interpreted as the signature of a huge base-level drop (2000+ m!) in the Black Sea Basin at the end of the Eocene. The geometry of the pronounced unconformity separating the Eocene (and older) and Oligocene strata, seen in every segment of the basin, not only on the shelf but also in the deepwater domains, suggests a major drawdown in the Black Sea. This “Messinian-style” dessication of the basin might have been triggered by the docking of the Balkans folded belt onto the Moesian Platform closing the marine connection to the West.