Structure of the upper part of subaeral cover of Biya-Chumysh plateau

Vygranenko T.M.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics; Novosibirsk State University, Novosibirsk, Russia

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The Biya-Chumysh plateau is composed by Pleocene-Quaternary deposits of lake and subaerial genesis. The upper part of Late Quaternary cover is represented by pale-gray and brown subaerial loess-like loam, sandy loam and sand 20-35 m in total thickness. Majority of researchers describe these deposits as a uniform not subdivided strata thickness. The purpose of this study is the detailed subdivision of Late Quaternary deposits in order to reveal changes in sedimentary environment on the basis of field observations, grain-size analysis and rock-magnetics.

The field observations consisted of detailed level-by-level geological description of the section, including color determination with the Menzel table, character of layer borders, structure, porosity, fracturing and secondary transformations of the deposits. The grain size analysis includes the study of 60 fractions (from 704 to 0.026 µ) in volume percent for each sample, obtained with the help of Microtrac Х100 grain size analyzer (IPGG SB RAS). Magnetic measurements are fulfilled with dual-frequency susceptibility meter Bartington MS2, coercivity spectrometer designed by Jasonov (Kazan State University, Curie balance designed by Vinogradov (IEP RAS).

We studied the deposits of two sections of Biya-Chymysh plateau up to the thickness of 6.5 m. Maksarovo section (53,4846°N; 85,1028°E) is located in eastern part Ob-Chumysh watershed and Polkovnikovo section (53,225°N; 84,6517°) - in its western part. According to Raukas’s classification the grain size fractions have been united in four classes of size: sand, coarse silt, fine silt and clay. Loess-paleosol sequence of Polkovnikovo section consists of clay silt or sandy-clay silt. Sandy streaks in the middle part of the section are represented by silty sand while the underlying sand horizon consists of medium-fine grained sand (fig.). Paleosols are clearly identified in the section by a relatively high (to 23 %) content of clay fraction. The same-age deposits in Maksarovo section are characterized by higher clay content and basically are represented by silty clay. Clay content in paleosols horizons reaches 35-40 %, and “physical clay” (sum of clay and fine silt fractions) reaches 85-90 %. The underlying horizon is composed by clayey silty-sand.

Fig. Grain size and rock magnetic parameters of the deposits from Polkovnikovo section.

Rock magnetic properties of the deposits also distinctly indicate a separate horizons, especially of different genesis. Sandy streaks are characterized by the raised values of magnetic susceptibility, saturation magnetization and the increased size of magnetic grain in comparison with loess and paleosols of the section. Paleosols differ by the high contribution of the paramagnetic minerals and the raised coercivity. Loess has intermediate values of magnetic characteristics. The content of magnetic material in loess is coming nearer to sand while paramagnetic contents and the contribution of high coercivity magnetic minerals are close to paleosols. Rock magnetic and grain size parameters are well correlated all over the section with relatively high correlation coefficients (till 0.75). Figure testifies the greatest concentration of ferromagnetic minerals in coarse-grained fractions while paramagnetic minerals and high coercivity minerals define magnetic properties of the clay fraction. Ration between terrigenous and authigenic minerals (grain size above 50 and below 10 µ, respectively) is well fixed by the ratio of ferromagnetic and paramagnetic components (Jfer/Jpar). Similar ratios are found for the deposits of Maksarovo section.

The both sections demonstrate a similar structure, but the thickness of individual horizons, grains sizes and values of magnetic characteristics are different. The distinctions can be connected with a spatial position of the sections cuts and distinctions in sources of aeolian material.

This work was supported by RFBR grant №10-05-01029.

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