Petrographical features of upper frasnian volcaniclastic graywackes in Jalangaskul region (West-Magnitogorsk zone of South Ural)

The Frasnian deposits of the West Magnitogorsk Zone of the Southern Urals have been studied for several decades. Despite this, the authors discovered sequence of high-silica volcaniclastic graywackes and chert in Yalangaskul Lake region. Our study revealed previously unknown features of the geological development of the territory at the end of the Frasnian. The detected sequence was traced along by route observations and airborn photographs. As a result, it was found that the sequence is a lens with length of 30-35 km and a maximum thickness of 350 meters. The petrographic study of sandstones allowed determining them as greywacke and feldspar greywacke. Clasts of high-silica vulcanites, quartz and feldspars are dominated, fragments of medium and basic volcanites, cherts and quartzites are found in smaller amounts. Four genetic groups of fragments were identified: pyroclastic, volkanoterrigenous, intraclastic and quartzites with unclear genesis. Mortar texture in sandstones was described. The presence of interbedding, cyclicality and association sandstones with cherts points out the deep-sea environment and supply sediments by means of turbidity current. The source area was region of volcanic activity, where explosion material, products of erosion of lithified volcanic rocks and soft siliceous fine-clastic sediments were involved in sedimentary process. On the base of high-silica volcaniclastic greywackes position they can be attributed presumably to the bottom of the Biyagodinskaya suite, which corresponds to the upper horizons of the Frasnian.