Reference Igetei Section (Middle Neopleistocene‒Holocene, Cis-Baikal Region): Reconstruction of Conditions and Assessment of Sedimentation Environments Based on the Geochemical Data

Abstract

The paper presents the results of a geochemical study of terrigenous rocks in the reference Igetei section (Middle Neopleistocene–Holocene, Cis-Baikal region), where 17 layers have been identified. Based on the grain size composition, the section is divided clearly into two parts: upper silty–sandy and lower silty–clayey, with the boundary between them passing above the top of the upper soil of the Igetei pedocomplex. Based on a significant content of clays, fine-grained silts, and organic matter in layers 1, 3, 7, 9, 10, 12, and 16, these sediments can be interpreted as fossil soils. The clay component in the lower part of the section and paleosols is dominated by a chlorite–illite mixture, whereas illite prevails in the upper part. Analysis of the lithogeochemical characteristics of sediments reveals their significant similarity to Early Jurassic rocks of the Irkutsk Amphitheater. Hence, the primary source of clastic material during Late Quaternary sedimentogenesis was represented by sedimentary rocks (Dabat and Cheremkhovo formations) and igneous rocks (Prisayan Formation). The contribution of mafic rocks was most prominent during the accumulation of the lower part of the Igetei section (layers 9 and 14). In the upper part (layers 5, 6, and 8), weathering products of felsic rocks dominate, as evidenced by low Ni/Co and V/Ni ratios and weathering trends on the CIA–ICV diagram. The ⁸⁷Sr/⁸⁶Sr ratio in gastropods in the Igetei section varies from 0.70933 (layers 6, 9, and 11) to 0.70996 (layers 7–8). Maximum values recorded in gastropods from layer 8 are consistent with the lithogeochemical data showing that the erosion area was dominated by arkoses and felsic rocks. Paleosol horizons affected by cryogenesis (layers 1, 7, 8, and 10–12) are characterized by reduced La_N/Sm_N values and the fractionation of MREE and HREE due to the formation of diagenetic Fe–Mn hydroxides and carbonate cements, reflecting a high REE mobility under increased alkalinity of pore solutions. Prior to the accumulation of paleosols of the Igetei pedocomplex (layers 9 and 10), sediments of the lower (Middle Neopleistocene) part of the section were accumulated under conditions of low paleoenvironmental humidity and reduced chemical weathering, i.e., in a relatively arid setting. Sediments of the upper part of the section (layers from 8 to 2) were accumulated primarily under colder and humid conditions.