Structure and Depositional Environment of the Upper Cenozoic Ulan-Zhalga Reference Section, Western Transbaikalia

Abstract

The results of a comprehensive study of the Upper Cenozoic Ulan-Zhalga reference section in western Transbaikalia are presented. The paleontological, paleomagnetic and petromagnetic, lithological-mineralogical, and geochemical data obtained allowed us to identify and characterize the Lower, Middle, and Upper Pleistocene and Holocene deposits in the section and to reveal the features and conditions of sedimentation. Five members recognized in the section are combined into two sequences: the lower (Beds 1–27, units 1–3) and the upper (Beds 28–35, units 4–5). The boundary between Matuyama and Brunhes chrons (0.773 Ma) is determined at a depth of 15 m and the upper boundary of the Jaramillo subchron (0.990 Ma) is determined at a depth of 23 m. The Matuyama/Brunhes boundary coincides with the boundary of Lower and Middle Pleistocene faunal assemblages. The formation of the section corresponds to two major stages of sedimentation with the boundary between the units 3 and 4 (depth of 11.8 m). Changes in rock magnetic and grain size parameters through the section has a climatic nature and reflects environmental changes. The formation of the lower part of the section (unit 1), which was accompanied by active soil formation, is characterized by the most heat- and moisture-enriched conditions. The upper sequence of the section accumulated under colder and drier conditions with increased dynamics of aeolian processes.