The Ordovician Mukteian Horizon of the Siberian Platform: paleontological substantiation, distribution, and correlation of sections

The new-generation regional stratigraphic chart of the Siberian Platform was accepted by the Interdepartmental Stratigraphic Committee of Russia in April 2013. Two main supplements to the chart of 1979 were made: (1) the new scale of stages, accepted by the International Commission on Stratigraphy in 2008, was presented for the Ordovician System along with the previous British standard and (2) the nomenclature of the regional stratigraphic units was supplemented by the Mukteian Horizon, which occupies an intermediate position between the Vikhorevian and Volginian Horizons. The first substantiation of the necessity of viewing this stratigraphic interval as an independent horizon was made in 1980, after it had become possible to use well cores for a more precise definition of the paleontologic characteristics and boundaries of the Vikhorevian Horizon, originally detected from several isolated sections in the Irkut amphitheater with an indistinct boundary with the overlying Volginian Horizon. Recent data, particularly those from drilling, permit a more complete paleontological substantiation of the Mukteian Horizon, consideration of its distribution in different structure-facies zones, and correlation of type sections.