Lithological and Petrographic Characteristics of Aleuropelitic Ishimskian Deposits in the Western Part of Tobol-Ishim Interstream Area

The lithological and petrographic analysis of aleuropelitic rocks of Ishimskian suite of the Upper Miocene was conducted for outcrops “Bigila”, “Pyatkovo” and “Masali” in the south of Tyumen Region. The data obtained complete the previously performed research of material composition, physical properties and identification of preliminary age of the upper part of Ishimskian suite. The results of these studies do not record significant differences in the lithology of the studied species, presented in different exposures: rocks are identical in their mineral composition and structural and textural features, confirming the community of their formation conditions. The studied rocks are composed mainly of fine poorly rounded quartz; in small quantities contained feldspar and mica. Feldspars consist principally of plagioclase, less microcline, which is confirmed by the determination of rocks chemical composition by X-ray analysis. Micas are present in the form of thin flakes with a bright interference color. Very rarely there are small grains of round glauconite, yellow-green, the exact origin of which is not yet set. In the outcrop Masali aleuropelitic rocks are overlapped by clayey silt with a high content of dispersed organic matter and coalified plant detritus. High dispersion and predominantly quartz composition allows us to characterize the studied rocks as marshallits. In this respect, more detailed studies must be based on analytical and instrumental methods that could be applied for this type of rocks. Persistence of the thickness of deposits over a large area gives grounds to consider the formation of the Ishimskian suite along with other horizons of the Middle Cenozoic as objects for prospecting siliceous raw materials.