Overturned Eocene – Lower Pliocene alluvial stratum on the southern coast of Lake Baikal and its neotectonic significance

Abstract

The study is focused on a section of sediments exposed on the right bank of Mishikha River, Russia. These sediments have a wide range of ages, from the Eocene to the Lower Pliocene. The stratigraphic subdivision of the section is based on the lithogeochemical data and X-ray phase analysis of the mineral compositions. The particle-size analysis shows the alluvial origin of the deposits. Their ages are constrained by spore-pollen spectra in three palynozones: I – Eocene – Oligocene, II – Early – Middle Miocene (subzone a – Tsuga, Picea in the lower part, and Quercus, Taxodiaceae, Momipites, Carya in the upper part; subzone b – Fagus, Quercus, Tsuga), and III – the Late Miocene – beginning of the Pliocene (subzone ν – Ulmus, Juglans, Carya; subzone g – Carya, Alnus). The section shows a combination of normal and overturned sedimentary layers. The tectonic displacement of the block with its flip was accompanied by the entry into contact of the unlithified Pliocene sediments with a rigid bed and the development of a landslide. The lower age limit of deformations is constrained from the youngest (beginning of the Pliocene) spore and pollen spectrum extracted from deformed layers. It is suggested that the overturned layers result from strike-slip deformations of the sediments at the beginning of the late orogenic stage of the Baikal rift development. The regional correlations of the sedimentary strata give grounds to conclude that the Mishikha section is characteristic of alluvial sedimentation that dominated at the eastern end of the Tankhoi tectonic step (Mishikha-Klyuevka paleovalley), in contrast to the Tankhoi block in the central part of the step, wherein a thick Lower Miocene stratum of swampy-oxbow sediments accumulated. The stratons of the Mishikha section correlate with sedimentary units detected by drilling in the Selenga delta at the central part of the South Baikal basin.