Description of sandstones in the Ulaanbaatar area, Mongolia Description of sandstones in the Ulaanbaatar area, Mongolia
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The texture and the grain composition of sandstone provide information about the tectonic setting of basins of deposition and associated provenances. The geological history of the Khangay-Khentey belt composed of pre-Carboniferous accretionary complex and Carboniferous shallow-marine strata was linked with the evolution of the Mongol-Okhotsk Ocean. This paper describes the sandstones of the Gorkhi, Altan-Ovoo & Orgioch- Uul Formations and Carboniferous shallow marine system, that occurs around Ulaanbaatar city The sandstones in the Gorkhi and Altan-Ovoo & Orgioch-Uul Formations are classified as feldspathic arenite. They are clast supported and poorly-sorted, with dominantly angular to very angular grains mainly composed of quartz, potassium feldspar, plagioclase and rock fragments of rhyolite and tuff. The modal framework grain analysis shows that sandy detritus was derived from "continental crust" where erosion has cut deep into volcanic cover. The sandstone in the Carboniferous shallow marine system classified as feldspathic wacke is rich in muddy matrix and poorly-sorted, dominantly includes rounded to little angular grains of quartz, potassium feldspar, plagioclase and rock fragments of rhyolite, felsic tuff, andesite and basalt. The Qm-F-Lt diagram suggests that the sandstone is originated from the "transitional arc". The sandstones from the Gorkhi and Altan-Ovoo & Orgioch-Uul Formations are significantly similar and they can not be distinguished to each other in their textures, grain compositions and poor in rock fragments, and they were originated from the same kinds of source, i.e. continental crust where erosion has cut deep into volcanic cover which related to the subduction of oceanic plate. However, the lithostratigraphies of these formations are quite different each other and they were likely to have been formed at different places, or formed in different ages each other. The volcanism evidenced by the felsic tuff intercalations in the Altan-Ovoo & Orgioch-Uul Formation at the provenance area was likely enough immature which could not provide much volcanic rock fragments to the basin. The sandstone of the "Carboniferous formation," rich in volcanic rock fragments, strongly indicates that the volcanic activity had developed to supply volcanic rock fragments into the sedimentary basin of this formation.
名古屋大学博物館報告 (28), 27-38, 2012