Mayanian (1100-850 Ma); Prebaikalian upper Riphean of Siberia

The Mayanian (1100-850 Ma) is an all-Siberian stratigraphic subdivision of the Neoproterozoic distinguished on the basis of generalized geological and paleontological data for the Maya Group in the southeast of the Siberian craton and for its counterparts in other structure-facies areas (SFA). The Mayanian deposits in all principally different structures of every SFA, such as intracratonic depressions, pericratonic troughs, and marginal parts of blocks with oceanic crust, were incorporated into the craton in the Vendian. It is separated from the surrounding stratigraphic divisions, Baikalian and Aimchanian, by distinct tectonic boundaries coinciding in time with the formation and starting breakup of the supercontinent Rodinia, i.e., with global events. In the adjacent parts of the platform they were reflected in erosion and considerable gaps in sedimentation. These events are well-expressed where they occurred under intense contraction or, on the contrary, are veiled where extension was predominant. The boundary between the Kerpylian and Lakhandinian, subdivisions of the Mayanian, is not bearing on important events and is hardly traceable throughout the region. Therefore, the base of the Kerpylian is preferable to be taken as the lower boundary of the Upper Riphean. There are attempts to establish this boundary, as it is made for the Phanerozoic, from paleontological data in compositionally similar deposits. These attempts, however, fail in matching this boundary with the base of the Lakhandinian. Distinctive transitions in communities of microfossils and stromatolites appear to be linked to the Mayanian-Baikalian and, undoubtedly, Middle Riphean-Aimchanian boundaries. In the Lakhandinian, on the contrary, the changes that appeared in the Kerpylian only tend to continue. Some taxa typical of the Lakhandinian appear as early as the Kerpylian. In the Lakhandinian their amount gradually increases upsection, until the top of this division. Thus, it is reasonable to start the Upper Riphean with the lower boundary of the Mayanian. All the main subdivisions of the Siberian Neoproterozoic - Mayanian, Baikalian, and Vendian - are linked to the most important stages of development of this vast and structurally heterogeneous territory. Each of the above-mentioned stratons can be divided, according to historico-geological and paleontological data, into two or three independent interregional subdivisions. Thus, the stratigraphic scheme of the Neoproterozoic of Siberia seems to be promising for the general scale of this period of the Late Precambrian.