MIDDLE RIPHEAN SEDIMENTARY BASIN IN THE JUNCTION AREA BETWEEN THE RUSSIAN PLATFORM AND SOUTH URALS: LITHOLOGY, FACIES, PALEOGEOGRAPHY, AND EVOLUTIONARY TREND

Abstract

The paper considers the structure, lithology, and facies characteristics of the Yurmatin and Serafimovka groups of the Bashkir meganticlinorium and Volga–Urals region. The analyzed distribution schemes of facies associations and complexes are plotted, based on these data, for the accumulation periods of the Zigal’ga, Tukaevo, Zigazino–Komarovo formations, Akberdino Horizon, and Avzyan and Ol’khovka formations. Accumulation of thick volcanogenic and siliciclastic, mainly peritidal deposits in the easternmost areas was characteristic of the initial stages of the basin’s evolution (approximately 1.34–1.35 Ma ago). The westerly areas represented at that time an elevated and differentiated land, whose eastern boundaries are marked by large lenses and beds of pebble–boulder conglomerates. In the mid-Middle Riphean time, sediments accumulated mainly in shallow-water, often well-aerated settings. In the paleogeographic aspect, the basin of this period probably represented a sea with flat bottom surrounded by islands. By the end of the Middle Riphean, the expanding transgression resulted in accumulation of several carbonate successions, mostly consisting of subtidal and moderately deep-water sediments in the eastern areas, which graded westward into siliciclastic associations of shallow and very shallow settings.