• The acritarchs derive from siliciclastic rocks of the Khamaka Formation, the lowermost part of the Vendian to Cambrian sedimentary succession in the central part of the Siberian Platform. Assemblages from the lowermost part of the sedimentary succession in the Nepa-Botuoba region. The lower part of the succession in the study area consists of siliciclastic rocks of variable thickness succeeded by thick carbonates, which are, in turn, overlain by evaporites. The siliciclastic deposits consist of sandstones, mudstones and shales ranging in thickness from approximately 30 to 300m. Occasionally, conglomerates occur at the base of the sequence (Borehole V-Ch 96; Rudavskaya and Vasileva 1989). Sandstones are predominant, forming thick units with numerous discontinuity surfaces and unconformities.

•Mudstones occur as thin intercalations in the sandstones or as discrete beds reaching 20 to 40 m in thickness. Carbonates overlying the siliciclastic portion of the sequence range in thickness from 170 to 400 m. They consist

•largely of dolostones or more rarely limestones, and clayey dolostones(Grausman and Zhernovskjj 1989; Rudavskaya and Vasileva 1989). The carbonate succession is overlain by thick (250 m)evaporites, comprising mainly of halite, with lesser amounts of anhydrite.