The Black Sea basin forms part of the interior zone of the Alpine mobile belt of southeastern Europe. It is a true sea with a depth of more than 6,000 feet. Its formation started in Upper Cretaceous time, and thus is related to other depressions of the Alpine mobile belt.

The block occupied by the Black Sea depression is surrounded by Hercynian and Alpine structures of the Great Caucasus, Small Caucasus, Pontian, and Balkan tectonic units. Characteristics of the Black Sea depression are a deep central portion and very little developed shelf areas.

On the southeast, the Black Sea depression narrows, shallows, and merges into the intermontane depression that separates the structure of the Great Caucasus from that of the Small Caucasus. Beyond, to the southeast, the trough merges with the deep depression of the southern Caspian Sea.

It is apparent that in the Upper Cretaceous and Tertiary a very thick series of sediments was deposited in the Black Sea depression. These were possibly geosynclinal sediments of Mediterranean. From then to the end of the Pliocene the Black Sea was inhabited by a special so-called Pontian fauna, characteristic of brackish waters.

There were frequent changes of salinity in the Black Sea basin during Quaternary time. The first increase of salinity was in the Mindel Riss interglacial epoch. This was caused by a connection with the salty Mediterranean waters through the Dardanelles and Bosporus. At that time the Mediterranean fauna reappeared in the Black Sea basin after a long absence. The reason for these sharp changes of salinity in the Black Sea basin is not clear. They could have been created by tectonic movements of the southern (Anatolian) coast. The salinity was also affected by changes of climate in glacial and interglacial epochs, which decreased or increased the inflow of fresh water from the land.

A characteristic of the present Black Sea basin is the poisoning of the water by hydrogen sulfide to almost its full depth of 6,000 feet. The waters contain oxygen only to a depth of 450 to 550 feet. The H₂S poisoning of the Black Sea basin started in Wurm time, or a little later, in the so-called late Euxinian basin. The deposits of the marine terraces and of the deeper bottom reflect clearly changes of salinity, oxidation, rate of sedimentation, organic matter accumulation, and other environmental factors.

The recent deposits of the Black Sea are represented by different kinds of muds. Some are very rich in organic matter. Maximal absolute accumulations of the organic matter on the bottom of the Black Sea basin are in its shallow portion, where the water still contains oxygen and where fresh water from rivers flows in. The minimal absolute accumulations of the organic matter are in the zone poisoned by H₂S, which are the pelagic areas far from the shores.

Some geologist have thought that the accumulation of the organic matter in the muds of the Black Sea varies primarily with the extent of H₂S poisoning. It is now known that the effect of this poisoning on accumulation of organic matter on the bottom of the Black Sea is exaggerated. As now understood, accumulation of organic matter on the bottom of the Black Sea depends on the whole process of sedimentation--its velocity, the mechanical and chemical composition of the sediments, and biological productivity of the basin.

It is hardly possible to compare the Black Sea basin with other recent or any ancient basins, where organic matter was accumulating.

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