STRATIGRAPHY OF LOWER HORIZONS IN THE SIBERIAN HYPOSTRATOTYPE OF THE RIPHEAN (THE SOUTHEASTERN SIBERIAN PLATFORM)

Abstract

A multidisciplinary approach was used to study the Lower Riphean deposits in the Siberian hypostratotype of the Riphean in the Uchur region of the southeastern Siberian platform and to clarify their relationships with the Lower Proterozoic (Upper Karelian) rocks. As is shown, the regional section of the Lower Riphean begins with the Uyan Group divisible into the Birindya, Konkula, and Adargai formations, the latter distinguished for the first time. An important unconformity is established below basal beds of the Uyan Group overlying the deeply eroded surface of weathered acid volcanics of the Elgete Formation of the Upper Karelian Ulkan Group, which are intruded by granites having the U-Pb isotopic ages of 1676-1721 Ma. The Uchur Group including the Gonam, Omakhta, and Ennin fort-nations terminates the Lower Riphean in the Uchur-Maya region and overlies deposits of the Uyan Group with a stratigraphic hiatus and azimuthal discordance. The basal conglomerate of the Gonam Formation includes pebbles of stromatolitic dolomites derived from the underlying Adargai Formation. This geological boundary separates the Ulkan and Uyan rock complexes of contrast lithological composition. The boundary is geologically distinct and close in isotopic age to the lower boundary of the Riphean that is accepted to be 1650 50 Ma old in the Precambrian stratigraphic scale of Russia.