PHOSPHORITES OF THE RUSSIAN CRATON

Abstract

Phosphorites of various types occur in the Phanerozoic sedimentary cover of the Russian Craton. They were deposited in six phosphorite basins, all of which contain economic deposits. These basins are grouped into peri- and epicontinental categories. Examples of the former are: (1) Podolian Basin, where Vendian spherical radially-built concretionary phosphorites occur embedded in black shales; (2) Baltic Basin, where phosphorites composed of phosphatic shells of inarticulate brachiopods were deposited at the Cambrian/Ordovician transition; (3) West Ural Basin with laminated phosphorites occurring in a Permian carbonate succession; (4) Volyn Basin with Upper Cretaceous (Cenomanian) granular phosphorites deposited on the northern Tethyan shelf. Epicontinental phosphorites were more widespread on the Russian Craton than anywhere else. They accumulated in the Late Jurassic–Early Cretaceous Volga–Ural and Late Cretaceous Trans-European Seaways. In both cases, the phosphorites are nodular. Epicontinental phosphorites differ in composition and geochemistry (particularly rare-earth element distributions) to those of the pericontinental basins. The latter are believed to have formed in association with upwelling systems with the World ocean providing a phosphorus source, whereas the former had a local supply with phosphorus being introduced largely by river input.