358 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



The deposition of productive sediments 



Thicks layers of productive formation covered with deposits lie in Azerbaijan 

 on the Pontic layers ; these layers are found on other parts of the Caspian 

 shores, being made up of sand, clayey sand and clay. Their fauna is very poor, 

 consisting of either purely fresh-water or land molluscs. During the period 

 of the accumulation of the productive deposits the body of water was con- 

 siderably reduced in size and, in the opinion of V. Baturin (1931), it was 

 limited to a southern basin with the waters of Volga, Samur, Kura, Uzboi 



Fig. 175. Basins of Cimmerian era (Archangelsky, 1927), 

 and of the productive zone era (Baturin, 1931). 



(from the east) and other rivers flowing into it, and its water then became 

 almost fresh (Fig. 175). The productive formation, apparently connected by 

 its deposits with the river deltas, contains in its layers huge accumulations 

 of petroleum, the origin of which may be due to vigorous delta growths, 

 whereas the North Caucasian petroleum beds, as has been pointed out by A. 

 Archangelsky (1927), belong to the deep-water environment of the Middle- 

 Maeotic basin in the depths of the Chokraksky and Karagatsky Seas con- 

 taminated with hydrogen sulphide. 



Akchagyl basin 



The next deposits of Precaspian marine sediments (after the Pontic ones) 

 were those of the Akchagyl basin, when the waters of the Caspian basin 

 moved north on a wide front and, to a lesser degree, spread east and west 

 (Fig. 176) as the result of the submersion of the Precaspian region. 



The fauna of the Akchagyl basin, characterized by its considerable salinity, 

 differs fairly sharply from that of the Pontic basin. It includes numerous 

 species of Mactra, Cardium, calcareous sea-weed, Avicularia and other 

 marine forms, which suggests that the salinity of the Akchagyl basin was 

 quite high. No explanation has yet been put forward for the high salinity of 

 the Akchagyl waters and the marine aspect of its fauna, in spite of the fact 



