CORRELATION OF FRASNIAN-FAMENNIAN BOUNDARY DEPOSITS IN WESTERN REGIONS OF THE MAGNITOGORSK MEGAZONE (EASTERN FLANK OF SOUTHERN URALS)

Abstract

Upper Devonian clastic complexes (the Zilair Formation flysch, Biyagoda mixtite, Koltuban limestone conglomerate, Bugodak volcanogenic-sedimentary sequence) are considered. In the eastern flank of the Southern Urals, the Zilair Formation is shown to be represented by two sequences similar in appearance but different in composition, consisting of clastic material derived from different sources. Accordingly, it is reasonable to rise the subdivision rank up to a group (by the analogy with that in the western slope of the Urals) combining two flysch formations (which are referred to as "Prisakmara" and "Bol'shoi Kizil") and underlying mixtites and conglomerates. The Karantau flysch underlying the Biyagoda mixtite represents a part of the Sakmara Formation that formed due to erosion of the Uraltau cordillera. Based on conodont and brachiopod faunas, lumpy limestone conglomerates (the Koltuban sequence) are proved to constitute only one level of rudaceous rocks. Complicated interrelations between calcirudites and cherts of the Mukasovo Formation are accounted for the slumping origin of conglomerates. It is shown that limestone fragments in these conglomerates range in age from the middle Frasnian to earliest Famennian. The conglomerates formed during the period corresponding to the second half of the conodont triangularis Zone and initial crepida Zone, when the global sea-level drop caused destruction of shelf limestones in an island-arc inactive area. The formation of the Biyagoda mixtite was related to the same event, though under conditions of active volcanism in the island arc. The Biyagoda mixtite and silicate components of the Koltuban conglomerates are fully analogous in composition to the Bol'shoi Kizil Formation overlying them. These subdivisions are genetically interrelated, and the unnamed rudaceous sequences also belong therefore to the Zilair Group. The lower boundary of the group corresponds to the top of the Mukasovo Formation (the conodont rhenana Zone). Accumulation of the Sakmara Formation commenced prior to deposition of the Bol'shoi Kizil Formation in the middle of the triangularis hemera, i.e., at the incipient moment of the linguiformis time.