THE LOWER-MIDDLE RIPHEAN BOUNDARY IN THE SOUTHERN URALS: NEW ISOTOPIC U-PB (SHRIMP II) CONSTRAINTS

Abstract

In the general Precambrian stratigraphic scale of the former USSR approved at the 2nd All-Russia meeting on general problems of Precambrian stratigraphy (Ufa, 1990), the Riphean representing the largest Upper Proterozoic subdivision comprises three chronostratigraphic units—the Lower Riphean (Burzyanian), Middle Riphean (Yurmatinian), and Upper Riphean (Karatavian)—with the lower isotopic boundaries at 1650 ± 50, 1350 ± 20, and 1000 ± 50 Ma, respectively. The Riphean–Vendian boundary is placed at 650 ± 20 Ma. Since that time, the stratigraphic position of some of these boundaries has changed. For example, based on Pb–Pb dating of limestones from the Neruen Formation (Lakhanda Group, Yudoma–Maya region) and carbonate rocks from the Sukhaya Tunguska Formation (Turukhansk area), the Yurmatinian–Karatavian boundary is now drawn at 1030 ± 30 Ma. The Riphean–Vendian boundary is now substantially younger (600 Ma). Only the Burzyanian–Yurmatinian boundary is still accepted in its former position at 1350 Ma.