Identifying Upper Miocene – Lower Pliocene Lacustrine Sediments in Dry Tunka Basin of the Baikal Rift Zone

Abstract

We present results of lithogeochemical, diatomic and palynological studies of sediments from the Tunka-13 well that was drilled in the southeastern part of dry Tunka basin in the Baikal rift zone. At the base of the section, there is an eroded basaltic flow of 16-15 Ma. From lithogeochemical signatures, we identify nine sedimentary units. The seven lower ones (interval 7.2-86.5 m) belong to the Tankhoi formation, the eighth (interval 2.7-6.6 m) to the Anosov formation, the ninth (interval <2.4 m) to the sandy stratum. We determined local sources of clastic material of basaltic and silicic compositions for units 1 and 2, respectively, and remote sources of silicic compositions for the overlaying units. The section shows a change from alluvial facies (units 1-3) through avandelta (unit 4) and lacustrine ones (units 5-7), again to alluvial facies (unit 8) and then to lacustrine-eolian ones (unit 9). Spore and pollen spectra from sediments of units 1-7 are divided into three palynozones (PZ), reflecting the vegetation change in the Late Miocene -Early Pliocene: PZ-1 -coniferous and deciduous forests with a small participation of thermophilic broadleaved species in moderately warm, humid climatic conditions; PZ-2 -enhancing the role of hemlock and more diverse thermophilic deciduous rocks in more humid and warm conditions; PZ-3 -a gradual reduction in the number of hemlock and other dark coniferous species, removing broadleaved species by birch and alder, growing grassy communities in wetlands due to climate cooling. We identified layers of lacustrine facies by occurrence of fossil diatoms that are absent in the layers of the alluvial and avandelta facies. In lacustrine sediments, we distinguish four diatom zones: DZ-1 denotes a relatively deep Late Miocene paleolake, marked by planktonic species, DZ-2, DZ-3 and DZ-4 -a shallow Early Pliocene lake with a developed littoral zone and short transgression. The Late-Miocene paleolake transgression, indicated by changing facies, is associated with structural reorganization, accompanied by volcanic extinction in the Tunka valley about 9-8 Ma, and the Early Pliocene short transgression with a new reorganization, reflected in volcanic rejuvenation about 4.0 Ma.

Diversity and Ecology of Diatoms in Pliocene Deposits of the Tunka Valley (Baikal Rift Zone)

“…The Tunka-13 hole was drilled in the southeastern part of the Tunka Basin near Nikolskoye village (GPS coordinates: 51 • 43 45 N; 102 • 34 35 E). At the base of the section, an eroded lava flow is present that corresponds in composition to the horizon of basaltic lavas with K-Ar ages of 16-15 Ma [11]. The overlying sediments reach an overall thickness of 86.5 m.…”

“…In the section (Figure 3), nine units were identified from lithogeochemical data of sediments [11]. These units are attributable to the Tankhoi Formation (units 1-7) in the interval of 86.5-7.2 m, to the Anosovka Formation (unit 8) in the interval of 7.2-2.5 m, and to the lacustrine-aeolian sandy unit (unit 9) in the interval of the uppermost 2.5 m. Between units 2 and 3 (in the depth interval of 66.1-71.0 m), a strongly watered loose sandy layer was found, which has not been studied.…”

“…The seven lower sedimentary members of the Tankhoi Formation show that the change of alluvial facies (units 1-3) through the avandelta diatom-free facies of a palaeolake (unit 4) to lacustrine facies is presented by grey and light grey aleurolite enriched in fossil diatoms (units 5-7). The eighth unit (interval 2.7-6.6 m) designates the alluvial facies of the Anosovka Formation, the ninth one (interval <2.4 m), the accumulation of the final lacustrine-aeolian sandy sediments [11].…”

“…These sediments have accumulated since the Miocene-Pliocene boundary on the western slope of the uplifted Elovsky interbasin spur. The accumulated Middle-Upper Miocene lava sequence was eroded almost to the bottom, and the erosion slot was filled with alluvial deposits generated due to erosion of basaltic material at the end of the Late Miocene [11].…”

“…Earlier, in order to obtain new data on the sedimentation of diatoms in the palaeoreservoir of the Tunka Valley, we drilled the Tunka-13 well and studied the composition of planktonic diatoms [11]. Benthic diatoms have not been studied by SEM.…”