Paleogene quartz sandstones of the Sosnovka Formation of the Uljanovsk-Syzran Volga region (Eastern Russian Platform): Mineral composition and origin

Abstract

The Paleogene quartz sandstones of the Sosnovka formation are widely occurred in the Uljanovsk-Syzran Volga region and are used as valuable building materials. The articlecomprises an improvement for thecontinental origin of the Sosnovkasandstones based on the results of a comprehensive study of the features of the mineral composition and microstructure of quartz sandstones of the Sosnovka formation obtained by using petrographic, XRD and electron-microscopic analyses. Therelevance of thestudy is based on the debatability of the origin ofsandstones, the depletion ofthemineralresource base ofthistype ofmineralrawmaterials andthe needto develop new approachesto its prediction. Accordingto theresults ofthestudy, itwasfoundthat opokas and diatomites ofthe Lower Syzransubformation, brought uptothe denudationsurface,could be the source of free silica for the cementation of sand grains of the Sosnovka sandstones. The deficit of siliceous cement and its extremely irregular distribution among the clastic grains indicate the local and restrict occurance of the source of solutions saturated with free silica, which, most likely, was controlled by paleorelief. Electron microscopic images and microprobe analysis of individual sandstone microcomponents indicate strong bacterial activity and frequent change of the land paleoenvironments to extensive marshy ones. The formation of pyrite framboids in the cementitious mass is a sign of oxygen deficit in the continental basin and the activity of magnetotactic bacteria. Whereas the decay of framboids is associated with dissolution of organic matter inside the framboids due to oxidation of the paleobasin. The results of the study leave no doubt about the non-marine origin of the sandstones studied, hence, we should not assume the presence of extended bodies of sustained thickness when solving prognostic-mineralogical problems. The Sosnovka sandstones are of extremely irregular distribution in the sandy sequence and formation in extremely continental pedological conditions will allow us to solve the important task of improving approaches to the prediction of new objects of this valuable mineral resource in the Ulyanovsk-Syzran Volga region.