Facies and Paleogeographic Reconstructions of Vendian Postglacial Deposits in the Southeastern Nepa–Botuoba Anteclise

Abstract

The work is based on extensive factual material, including a core collection and GIS data on 84 deep wells drilled in the southeastern Nepa–Botuoba anteclise (Siberian Platform). Lithological studies of the Vendian basal deposits formed in the postglacial epoch were carried out. These deposits characterized by significant vertical lithological heterogeneity include layers of highly radioactive coarse-grained monazite-containing rocks. Therefore, it was necessary to check the clay content and combine the chosen lithological types into paragenetic rock associations that compose relatively thick intervals of the section and can be recognized using the geophysical research methods. The results of cyclostratigraphic analysis made it possible to indentify three ranks of binomial sedimentary cyclites and a basal acyclic unit of pudding conglomerates. The population of detritus zircons from pudding conglomerates (66 grains) contains the youngest grains with an age of 600–900 Ma. Minimum age of zircons is 631 ± 14 Ma. Taking into account the presence of diamictites and pudding conglomerates in the Vendian basal deposits, the results of studying the quartz grain surface and mineralogy of the heavy fraction from the sedimentary cover base and basement rocks suggested that their formation coincided with late glacial and postglacial stages of the Gaskier glacioperiod (584–582 Ma). Structural and textural features of postglacial deposits were determined based on the hydrodynamic parameters of fluvioglacial flows, features of the postglacial relief, the distance to provenances, and the mineral composition of eroded rocks. The Vendian cyclical deposits (Nepa and Tira formations) were formed during climate changes from the glacial to arid conditions in the course of transition from the continental to coastal-marine type.