Tologoi key section: A unique archive for pliocene-pleistocene paleoenvironment dynamics of Transbaikalia, Bikal rift zone

Abstract

The multi-layered key section of Tologoi is situated on the Selenga River left bank, 15 km southwest of Ulan-Ude city (51º45′ N, 107º29’ E), in the Western Transbaikal region. It holds a long record of Quaternary sedimentation since early Late Pliocene through Holocene, with the only minor gap in the beginning of Early Pleistocene. Comprehensive studies have been carried out on the lithology, geochemistry, fauna succession and paleoenvironment reconstructions. This paper summarizes the data acquired on the geology and stratigraphy of the Tologoi section. The authors investigated the composition, structure and formation conditions of the section, as well as the intensity of weathering, rock geochemistry, grain size and biostratigraphy.

Introduction

The multi-layered key section Tologoi is situated on the Selenga River left bank, 15 km southwest of Ulan-Ude city (51° 45′ N, 107° 29’ E) in the western Transbaikal region (Fig. 1). Stretching at 51º-56° N and 104º-118° E, the study area is located in the Baikal rift zone enclosed within Central Asia. It is noteworthy that this region experienced complicated geological evolution through the Pliocene and Pleistocene time. This region is known for diversity of landscapes: the territory covers deep intermountain depressions alternated by mountain ranges, their altitudes varying from 1000 m in the south to over 2600 m in the north. The North-Asian Arctic boreal areas are covered with mountainous taiga and meadows, deep forests and intermountain meadow-steppes being common for the semiarid landscapes. The Central-Asian arid landscapes with mountainous and dry steppes are available as well.

The extremely continental climate is typified by considerable amplitudes of both daily and annual temperature fluctuations. The mean January temperatures commonly vary from −20° to −30 °C, absolute minimum reaches −45°. In summer time, the mean July temperature is about 20 °C, absolute maxima range from 35° to 38°. As to the precipitation rates, they are typically low both in winter and in summer. Remarkably that in the Baikal region the weather is predominantly sunny and bright during winter time. In general, the climate is characterized by a negative mean annual air temperature (−1.25 °C). The annual precipitation is 250–400 mm (Sochava, 1967).

The Tologoi Key Section, discovered by Okladnikov A.P. in 1951, presents a sequence of continental deposits which had accumulated in the Ivolga depression since Late Pliocene through the Holocene, with the only minor gap in the beginning of Early Pleistocene. Primary information on this study area and its fauna was given by Bibikova et al. (1953).

Although the intensive investigations on geology of the Tologoi locality and its fauna are still in progress, acquired results seem fairly significant for paleoenvironment reconstructions and biostratigraphy not only for the Transbaikal, but for the entire Baikal region.

The Tologoi section contains the stratigraphic types of the Tologoi fauna complex and the Ivolga fauna, as well as the stratigraphic type of Tologoi (= Chikoy) Formation (Logatchev et al., 1964; Ravsky et al., 1964; Vangengeim et al., 1966; Bazarov, 1968; Erbajeva, 1970; Erbajeva, Alexeeva, 2000; Alexeeva, 2005; Erbajeva et al., 2005).

In the section, the magnetostratigraphy has been studied for the first time, and fossil remains have been recovered from both under and above the Matuyama/Brunhes reversal (Gnibidenko et al., 1976; Alexeeva, 2005). The earliest permafrost evidence in the Western Transbaikal region (dated as Early Pleistocene) was also found in this section (Vogt et al., 1995; Alexeeva, Erbajeva, 2000). However, Bazarov (1968) and Ravsky (1972) previously assumed that permafrost appeared in the Transbaikal region in the Riss time.

Cooperation with Prof. W. Zech (Germany, Bayreuth) provided new information on the Holocene and Late Pleistocene fossil soil structure and data on fossil formation. The sediments and fossil soil were dated (Zech et al., 2017). Moreover, the lithology, geochemistry, fauna succession and paleoenvironment reconstructions have recently been surveyed in detail. This work is mainly targeted at reporting acquired results.