BIOSTRATIGRAPHY OF THE VENDIAN-CAMBRIAN SUKHARIKHA RIVER SECTION, NORTHWESTERN SIBERIAN PLATFORM

The Sukharikha River section contains more than 800 m of fossiliferous Vendian and Lower Cambrian carbonate rock deposited in ramp, shelf, a nd slope environments. A diverse fauna of small shelly fossils, calcib ionts, brachiopods, trilobites, and archaeocyaths has allowed us to de velop a multi-taxa biostratigraphic framework for this section. A dear th of distinctive fossils low in the Sukharikha Formation prevents us from determining the position of the Vendian-Cambrian boundary. Abunda nt small shelly fossils and archaeocyaths in the uppermost Sukharikha Formation and low in the Krasnoporog Formation provide ample biostrati graphic control near the base of the Tommotian Stage, but the Nemakit- Daldynian - Tommotian boundary, as defined at Ulakhan-Sulugur on the A ldan River, is temporally ambiguous. For this reason there is no preci se definition of this boundary. In the Sukharikha River section we hav e provisionally placed the base of the Tommotian Stage at the first oc currence of Nochoroicyathus sunnaginicus Zone archaeocyaths, about 1.5 m below the top of the Sukharikha Formation. However, we suppose that this horizon actually predates the deposition of nominally basal Tomm otian taxa in the Aldan region. A new global stratotype section for th e Nemakit-Daldynian Tommotian boundary should be selected, and this se ction may turn out to be a good candidate. The paleontological richnes s (especially small shelly fossils and archaeocyaths), the apparent ab sence of long depositional hiatuses, and the presence of well-preserve d limestones suggest that the Sukharikha River section contains the co mbination of paleontological, sedimentological, and isotopic data to r esolve some fundamental problems in Early Cambrian stratigraphy.