Академическое издательство «Гео»

Keywords: Cenozoic, Baikal, sedimentary deposits, stratigraphy, evolution of topography, pyrometamorphism.


Pyroclastics as the indicator of the uplift of the Ikatskii Ridge relative to the Barguzinskaya depression of the Baikal Rift Zone

S.V. Rasskazov*, **, I.S. Chuvashova*, **, T.A. Yasnygina*, M.V. Usol’tseva***,
N.A. Rudneva*, **, D.Yu. Mit’kin**, and A.Yu. Fedin****

*Institute of the Earth’s Crust SB RAS, 664033, Irkutsk, Lermontova str., 128, Russia,
rassk@crust.irk.ru, chuvashova@crust.irk.ru, ty@crust.irk.ru, rudneva@crust.irk.ru
**Irkutsk State University, 664003, Irkutsk, Karla Marxa str., 1, Russia,
rassk@crust.irk.ru, chuvashova@crust.irk.ru, rudneva@crust.irk.ru, meat_dan@mail.ru
***Limnological Institute SB RAS, 664033, Irkutsk, Ulan-Batorskaya str., 3, Russia, usmarina@inbox.ru
****Sosnovgeologiaya, 664039, Irkutsk, Gogolya str., 53, Russia, aleksandrfedin24@rambler.ru

Pyroclastic material was discovered on the pediment of the Barguzinskaya depression adjacent to the Ikatskii Ridge. It is found that the contents of SiO2 in it vary from 56.35 to 62.67 wt%, Na2O + K2O from 4.59 to 5.9, and Al2O3 from 20.4 to 26.5 wt%. According to K-Ar dating of a sample of aleurite containing glass, the lower limit of the baking and fusion of sedimentary deposits is estimated at 8.5 million years. For identifying the source of pyroclastics, a borehole in the area of the discovery exposed a sequence of sedimentary deposits of the Miocene–Lower Pliocene coal-bearing Tankhoi and Pliocene/Eo-Pleistocene ocherous Anosov formations. The profile of the Tankhoi formation revealed six layers with the transition from bog-oxbow deposits of layers 1–3 to lake deposits of layers 4–6. We identified considerable weathering of material in unite 3 characterized by an accumulation of brown coal layers as thick as 10 m. A comparative analysis of major oxides and trace elements of the sedimentation sequence of pyroclastics showed that the source of baking were represented by lacustrine aleurites of layer 4 overlaying directly the coal seam from layer 3. In most samples of pyroclastics, the interval Fe2O3/FeO — 0.06–0.15 is correlated with Fe2O3/FeO in the brown coal unite, which might point to fusion of sedimentary material under the action of the flow of reduced gases liberated from the coal intercalation. It is suggested that the coal seam self-ignited when exposed to the air during the rise of the Ikatskii Ridge at the time of restructuring of the northeastern Baikal Rift Zone 9–5 Ma.

Keywords: Cenozoic, Baikal, sedimentary deposits, stratigraphy, evolution of topography, pyrometamorphism.

DOI: 10.21782/GiPR0206-1619-2016-5(117-127)