Home – Home – Jornals – Russian Geology and Geophysics 2005 number 5

The Late Precambrian Uchur-Yudoma hypostratotype includes the Uchur-Maya plate (a depression in the southeast of the Siberian Platform), the Yudoma-Maya pericratonic trough, and the Okhotsk microcontinent. The sequence of stratigraphic units ranked groups and formations has surely been established for all these structures, but the location and nature of boundaries between them as well as their age are still debatable. The information available casts doubt upon many universally accepted concepts. It is proven that the Maya Group giving rise to the Mayanian, the lower Neoproterozoic unit, overlaps the entire region under study, being in contact with diverse older units. However, its starting Kerpyl Subgroup is not coeval with the entire Ennin Formation completing the Lower Riphean in the western Uchur-Maya plate but only with a separate member of deposits erroneously considered its part. The pre-Kerpylian tectonic rearrangement contributed much to the specific paleogeography of the region, which is also commonly associated with the pre-Vendian events alone. The Kerpylian was preceded by an accretion to the Siberian craton of its surrounding microcontinents while the supercontinent Rodinia was completing its formation. Nothing of the kind occurred prior to the formation of the Lakhanda Subgroup closely connected with the Kerpyl one. The pre-Kerpylian changes in phytolith and microfossil biotas served as a base for the paleontological substantiation of the lower boundary of the Upper Riphean. Their specific character that appeared in the Kerpylian continued to develop in the Lakhanda Subgroup. Therefore, there is no need to distinguish the Lakhandinian as a separate group to oppose it to the Kerpylian. The data reported show that the lower boundary of the Neoproterozoic is not younger than 1100 Ma.
A persistent sequence of K-Ar and Rb-Sr ages corresponding to the available paleontological data shows that the Ui Group formed in the range 750-650 Ma. This does not contradict U-Pb and Sm-Nd ages (about 1000 Ma) obtained from its cutting dikes. These figures indicate the age of the magma chamber rather than the time of their intrusion. The Ui Group dated in this way and its analogs in other regions correspond only to the upper half of the Baikalian and Cryogenian of the International Scale (850-650 Ma). This conclusion is in discrepancy with the common opinion that the Ui Group is closely connected with the Lakhandinian. In the same way, a number of formations disappear from the Ui Group leaving no trace, while on the eastern slope of the Omnya rise, the Ust'-Kirbi Subgroup completing the Ui Group rests immediately upon the Lakhandinian. These relationships are due to the regressive structure of the Ui deposits, where the event nature marking the beginning of a new group is masked by the considerable subsidence of the Yudoma-Maya trough. It is related to the extension that accompanied the breakup of the supercontinent Rodinia. In other parts of the Siberian craton where this process was accompanied by intense compression, the beginning of the Baikalian has a distinct event basis and an appropriate complex of basal deposits. Because of the complex event nature of the Baikalian in Siberia some geologists refer it partly to the Middle Riphean using U-Pb ages of dikes, whereas the others date some part of the Baikalian by the Vendian. Neither thinking is true because of the Late Riphean age of the paleontological remains. The event nature of the Vendian, the upper unit of the Neoproterozoic, is due to the repeated accretion to the Siberian craton of its surrounding microcontinents, which triggered the Caledonian tectonic activity. The principal character of the events limiting the main Neoproterozoic stratons in Siberia makes them promising as units of the General Stratigraphic Scale of Precambrian.