Evolution of the Riphean-Vendian biolithogenesis settings in the Baikal mountainous area

Three major stages of the evolution of sedimentation settings have been ref ined by lithofacies, geochemical, and microphytological studies. They corre spond to the sequential changes of paleobasins: taphrogenic passive margin of the continent, back-are basin, and foreland. Peculiarities of the paleog eographic zonation of each paleobasin have been determined according to fac ies analysis of transverse sections. The volume of stagnant conditions tend s to increase up the section, reaching its maximum in the pre-Vendian time. On the basis of the reconstructions and actuopaleontological comparison, a n ecobiological interpretation of some morphotypes of organic-walled microf ossils (OM) has been proposed. Green and brown algae and bacterial associat ions characteristic of paleobasin zones of various depths have been identif ied. It has been proven that the appearance of new traits in some OM occurr ed at the Riphean-Vendian boundary, against the background of a change in t he sedimentation pattern, The newly appearing species belong mainly to OM c onsidered to be ancestors of green algae. It is suggested that the formatio n of the carbon component of the Precambrian aleuropelites was considerably contributed by benthic bacterial associations.