THE UPPER RIPHEAN TANGAUR SUBGROUP OF THE ALATAU ANTICLINORIUM IN THE SOUTH URALS

Abstract

Within the Alatau anticlinorium of the South Urals (the Zilim River middle course), structure and sedimentation settings of terrigenous and carbonate deposits of the Tangaur Subgroup of the Upper Riphean Karatau Group are investigated. Based on the analysis of 15 bed-by-bed described sections, siliciclastic and carbonate beds are correlated, and their lateral relations along and across the regional paleoslope are considered. It is established that the Conophyton-bearing strata at the base of the Min'yar Formation and overlying dolomite beds ranging up to the level with abundant Minjaria forms are most stable constituents of the Tangaur Subgroup. The Podinzer limestone beds are also easily traceable throughout the study region, but their facies affinity is laterally variable. A member of phytogenic limestones crowning the beds occurs only locally. Small erosion marks and desiccation cracks abundant in the beds imply a partial draining of the basin and existence of a major event boundary at that level. The intricate Inzer Formation that includes two carbonate and two terrigenous sequences is traceable only along the western periphery of the Bashkir meganticlinorium. East of the Zilim River middle courses, it is replaced by a siliciclastic succession. Main facies systems are reconstructed, and the basin evolution is characterized in terms of the event stratigraphy. The appearance level of cherts in the Min'yar Formation is shown to be diachronic. In general, the accumulation time of Tangaur Formation along the western periphery of the Bashkir meganticlinorium seems to be corresponding to the existence period of a large epicontinental sea basin that had a complex evolution.