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 rock magnetic, 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 (layers 1–27, units 1–3) and the upper (layers 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) at a depth of 23 m. The Matuyama/Brunhes boundary coincides with the boundary of Lower and Middle Pleistocene faunal complexes. The formation of the section corresponds to two major stages of sedimentation, which boundary is between the units 3 and 4 (depth 11.8 m). Changes in rock magnetic and granulometric parameters over the section has a climatic nature and reflect environmental changes. The formation of the lower part of the section (unit 1), which was accompanied by active pedogenesis, is characterized by the most heat- and moisture-enriched conditions. The upper strata of the section accumulated in colder and drier conditions with increased dynamics of aeolian processes.