The limestones and dolomites of the Sukhaya Tunguska Formation, which are referred to the Middle-Upper Riphean boundary horizons (the isochron Pb-Pb age 1017 +/- 91 Ma), are subdivided into five units composing two subformations. The silicified microfossils known predominantly from the upper subformation are mainly represented by the following taxa: (1) morphologically simple filamentous and coccoidal microfossils of genera Siphonophycus, Gloeodiniopsis, Eosynechococcus, Eoentophysalis, and others, which are the remnants of cyanobacteria of a wide stratigraphic range, and (2) remnants of a specific stalked cyanobacterium Polybessurus bipartitus unknown in the deposits below the upper part of the Middle Riphean. In addition, single remnants of supposed acanthomorphic acritarchs cf. Trachyhystrichosphaera were found in one site in the lower subformation. The facies reconstructions for each unit of the formation show that the Sukhaya Tunguska deposits form a continuous shallowing-up succession deposited in the poorly differentiated carbonate platform dipping gently northwestward. There were two stages in the platform development corresponding approximately to two Sukhaya Tunguska subformations. The early stage was during the high sea level stand and resulted in predominantly open-marine environments varying laterally. The late stage corresponded to the low sea level stand and uniform landscape in the upper subtidal zone and, at the end of the stage, in the intertidal zone. Most of the early diagenetic fossiliferous cherts are confined to the shallow-water and supershallow-water sediments of the late stage. The relatively deepwater deposits of the early stage reveal selective silicification rarely conserving microfossil remnants. The paleofacies reconstructions for the fossiliferous strata based on sedimentological records are concordant with results of the actualistic interpretation of paleoecological settings of certain microfossil morphotypes.
