Место издания: Florence , Год издания: 2004

Manganese carbonates of the West Siberian marine basin have been found in the Upper Jurassic Georgiev Formation, which age is determined from the upper part of the Upper Oxfordian to the lower part of the Lower Volgian (Tithonian) ages. The Georgiev Formation is the condensed one. Being spreads on the area more than 1 million kilometre square, the Formation is no more than 10 m thick. Manganese carbonates are mainly situated within the western margin of the Georgiev sea. Another rocks associated with manganese carbonates are glauconitites, argillites, phosphorite concretions. From the morphological point of view the massive, stromatholitic, oncolithic structures of carbonate rocks can be distinguished macroscopically. Microscopically in the thin sections the microstromatoliths and microonkoliths in the massive carbonates are often found. Ultramicroscopically (in SEM) the cocco-like and tubular biomorphic forms are revealed. Mineralogically the manganese carbonate rocks composed of rhodochrosite and manganocalcite. The main rock-forming components of their composition are: MnO - 15-34%, CaO - 5-22%, SiO2 - 5-33%, Al2O3 -1-7%, P - 0,05-1,7%, CO₂- 15-34%. The distinguishing feature of the studied manganese carbonate rocks is their high siliceous nature. The ratio SiO2/Al2O3 sometimes is as high as 12-15. Silica is finely disseminated in the rocks or can be seen in thin sections as the cracks filling. All morphological types of manganese carbonate are similar in their mineralogical and chemical composition. Others important rocks of the Georgiev Formation are glauconitites and argillites. The first of them are related to the lower and upper parts of the Formation and the thickest bed of glauconitite is found in its basement. Argillite beds occur in the different parts of the Formation section interbedding with carbonate ones. It should be noted that argillites and glauconities are not silicified. The ratio. SiO2 /Al2O3 for the rocks ranges from 2.8 to 4.1. We relate the glauconitite bed in the basement of the section to the maximum of the transgression and stromatolith-bearing bed - to the maximum of the regression. The facial series from the coastal area to the more deep parts of the sea can be presented as the transition from the stromatolithic carbonates to the massive and oncolithic ones and then to the glauconitites. We relate the Georgiev Formation supply with manganese to the weathering crust of the nearby Ural area.