Tologoi key section: A unique archive for pliocene-pleistocene paleoenvironment dynamics of Transbaikalia, Bikal rift zone
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.
Section snippets
Materials and methods
The Tologoi Key section was studied with multidisciplinary approaches, e.g. the geological structure was analyzed using lithological, granulometric and mineralogical methods; and the geochemical method was first tried in this research. When describing the color of sediments, the properties of the Munsell Soil Color Chart are used (describing color by hue, value, and chroma). Paleontological data are based on newly acquired results and detailed revision of previously gained materials. Fossil
Paleoenvironment background
In Early Pliocene, the tectonic uplift of the mountain, surrounding Lake Baikal, produced the major orographic barrier separating the Transbaikal region from the influence of the western humid Atlantic cyclones (Prokopenko et al., 2001; Kuzmin, Yarmoluyk, 2006). This uplift seems to be the main reason for generating aridization conditions in the Transbaikal region. The Late Pliocene warm and semiarid climate changed to cooler and arid climate. The savanna-like forests and steppes gradually
Geological setting
Exposed in the Tologoi locality, the Late Pliocene-Holocene sediments are widely distributed throughout the Transbaikal region. All the researchers involved in the study of Tologoi section agree in distinguishing three sedimentary units in the sequence: Tologoi 1, Tologoi 2 and Tologoi 3 (Fig. 2 A). The successive fossil soils are present in three units (Ravsky et al., 1964; Bazarov, 1968; Vangengeim et al., 1966; Erbajeva, 1970; Alexeeva, 2005).
The lower UNIT deposits (Tologoi 1 section)
Paleontology and biostratigraphy
Fossils of the earliest stage are derived from the beds below the calcareous concretion horizon in the lower unit (Tologoi 1 section). They belong to Hypolagus sp.; Ochotonoides complicidens Boule et Teilhard de Chardin, 1928; Ochotona sp.; Cricetinus cf. varians Zdansky, 1928; Promimomys cf. stehlini Kormos, 1931; Cseria gracilis Kretzoi, 1959; Mimomys cf. minor Feifar, 1961; Villanyia cf. eleonorae Erbajeva, 1975; Mesosiphneus praetingi Teilhard de Chardin, 1942 (Alexeeva, 2005). These
Grain size analyses
Fig. 3 presents variation of all granulometric parameters with depth.
The study of granulometric data allowed to refine the thicknesses and to indicate specific characteristics of some layers. The formal parameter for recognition is dynamic factor F, which is the ratio of physical sand (the sum of sand and coarse silt fractions) content to physical clay (the sum of fine silt and clay fractions) content. Its values < 1 (or closer to 1) point to the predominance of secondary post-sedimentation
Lithology
Acquired data pinpoint that sediments of the section primarily consist of sands, sandy loam and loam alternating cyclically, as shown in Fig. 3. The sedimentation cyclicity is mostly vivid on the plots of sand fractions (see Fig. 4). Cyclic recurrence results in the reduction of coarse sand supply, in some periods to zero, and increase of amount of this sand fraction to 20–30% (at depth 400–500 cm – to 70%) in the other periods. Four plotted cycles (Tologoi 2 and Tologoi 3 units) terminate with
Conclusions
The detail multidisciplinary study of the Tologoi section has revealed some sedimentation features and allow to recognize specified location and thickness of fossil soil horizons in the section:
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Four cycles of sedimentation were recognized within Tologoi 2 and Tologoi 3 units, each cycle terminated with formation of soil horizons. The soil horizon in the lower part of the section (Tologoi 1 unit) (2900–3000 cm) also reflects the end of sedimentation cycle.
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During the sedimentation time, the
Acknowledgments
The research is supported by the Russian Foundation for Basic Research (projects no. 15-05-01858, 16-05-00586, 18-05-00215)), the Russian Science Foundation (project no. 16-17-10079 (chemical analysis, paleontology)) and Integration Project No. 0341-2016-001. The study was conducted using the research facilities of the Center for Geodynamics and Geochronology at the Institute of the Earth's Crust SB RAS, Irkutsk, Russia. M. A. Erbajeva thanks Prof. W. Zech for the professional assistance.
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