THE VENDIAN OF THE BAIKAL-PATOM UPLAND, SIBERIA

Abstract

A large gap is documented in the Patom Upland, at the base of the Zherba Formation. The closely connected Tirbes Formation is now considered as its completing member. The Tinnaya Formation is separated from the Zherba and the overlying Nokhtuisk Formations by gaps. Small shelly fossils found there for the first time gave grounds for substantiation the lower boundary of the Cambrian at the bottom of the Nokhtuisk Formation and for referring the Tinnaya Formation to the Nemakit-Daldyn Stage of the Vendian. The more precise isotope chemostratigraphy of the Vendian-Cambrian deposits has shown that here, as in the other sections of the Siberian Platform, a negative marking excursion of δ 13 C matches the paleontologically substantiated lower boundary of the Cambrian. The adjacent peaks helps to draw the boundaries of the regularis and lenaicus Zones and permit two conclusions. First, fossils of the trisulcatus Zone in the Siberian sections occur below the base of the Manykai Formation, whereas some representatives of the Ediacaran fauna penetrate there from below. Therefore, the Manykai Stage recognized on basis of this formation has no biostratigraphic sense and its lower boundary cannot be used for supporting the Vendian-Cambrian border. Second, the Zherba Formation corresponds to the Ediacaran Stage of the Khorbusuonka section of the Olenek uplift. The Lower Vendian deposits in the region were preserved only in paleodepressions. Their most complete section is known in the Bodaibo-Baikal pericratonic trough. The Lower Vendian is linked there, through gradual transitions, to the host rocks. Glacial formations and cap-dolomites are absent from it and, therefore, cannot be used as the bottom of the terminal Neoproterozoic of the International Scale. Quite promising for this purpose are pre-Vendian and intra-Vendian tectonic rearrangements whose ages reliably suggest that the Lower Vendian of the region formed in the interval from 650 to 612–620 Ma. Thus, the section of the Upper Vendian of the Baikal-Patom Upland is well substantiated by direct data at the Neopro-terozoic III of the International Scale.