Yu. B. Bogdanov, N. G. Berezhnaya, S. A. Anisimova, A. S. Nikonova, N. V. Rodionov

Stratigraphy problems of the Gimoly Superhorizon formations (Upper Lopian) in the Karelian region

Abstract. The data cover a complex study of published and original geological and geochronological materials on the stratigraphy of the Gimoly (Upper Lopian) and Sumian (Lower Proterozoic) superhorizons in the Karelian region. The base of the Sumian Ozhijarvi Formation should locate the Archean–Proterozoic boundary. The Gimoly Superhorizon is divided into the Kostomuksha (lower) and Tunguda (upper) horizons with the timeframe of about 2,760 Ma. Mainly sedimentary rocks form the Kostomuksha Horizon, whereas felsic and basic volcanites construct the Tunguda one. Sumian basic volcanites underlying the Ozhijarvi formation should belong to the Tunguda Horizon. The completeness and age distinguish three types of superhorizon sections — West Karelian, Central Karelian, and North Karelian, which form independent structural and formational zones. The rocks of the Tunguda and Kostomuksha horizons represent the West Karelian and North Karelian zones, only the Tunguda Horizon describes the Central Karelian one. The geological and geochronological analysis reveals a number of unresolved issues. The major ones include the location and age of the Tunguda Horizon lower boundary, the age and structure of the Moroznoozerskaya strata and Pebozero series sections in the stratotypic section of the north-east part of the Lekhta structure.

Keywords: stratigraphy; Upper Lopian; Sumi; regional stratigraphic chart; Karelia; West Karelian, Central Karelian, and East Karelian section types; Kostomuksha and Tunguda horizons