SOME RESULTS OF PALEOMAGNETIC STUDIES IN THE BALEY GRABEN (EASTERN TRANSBAIKALIA)

V. M. PEREDERIN, Z. A. MYSHKO

(Presented by Prof. D. S. Mikov)

Until now, the physical properties of the continental Mesozoic of the Undino-Dainskaya depression have remained almost unstudied; that is, additional important sources of information have not been used for geological-geophysical mapping and for resolving questions of the stratigraphy of sedimentary deposits. Therefore, in 1964–65 we carried out reconnaissance study of the magnetic properties of rocks of the Baley graben as a characteristic and geologically best-studied structural unit of the Undino-Dainskaya depression. At the same time, the question of the suitability of the rocks of the area for paleomagnetic study was addressed.

According to geological investigations [3], the following subdivision has been established through the section of the volcanogenic-sedimentary deposits of the Baley graben. On the eroded surface of the Unda granites lies the effusive-sedimentary Talangui Formation \((I_3)\); above it are situated the sedimentary formations of the Tergen \((I_3—Cr_1ter)\) and Baley \((I_3—Cr_1bal)\) formations. The latter is subdivided, according to lithological features, into four horizons. On the Baley Formation lie the Kamenka conglomerates, the age of which still remains disputed. The framing of the graben is composed mainly of granitoids of the Borshchovochny and Unda complexes.

Five hundred specimens were studied. Measurements were carried out on an astatic magnetometer with a sensitivity of 0.022 γ/div. The results of the investigations were presented in the form of stereographic projections of vectors and histograms of the distribution of scalar characteristics of magnetic properties. On the basis of the histograms, using the template of N. K. Razumovsky [4], rectified cumulative frequency curves were constructed, from which the most probable value of the given quantity and the limits of its variation were determined. As a result, the following picture was obtained for the distribution of the most probable values of the magnetic parameters through the section.

The sandstones of the Kamenka conglomerates possess a magnetic susceptibility \(\chi\) equal to \(15 \times 10^{-6}\) CGS, a remanent magnetization

\[ I_n = 4 \times 10^{-6}\ \text{CGS} \]

and a factor \(Q = 0.61\). No regularity was established in the distribution of the azimuths of the vectors \(I_n\), although the inclination angles are positive in most cases and lie near \(50^\circ\).

By scalar characteristics, the Baley Formation can be divided into two members. The first member has \(\chi = 60—80 \times 10^{-6}\) CGS, \(I_n = 6 \times 10^{-6}\) CGS, and factor \(Q = 0.17\). For the second member, \(\chi\) is \(15—20 \times 10^{-6}\) CGS, \(I_n = 3 \cdot 10^{-6}\) CGS, \(Q = 0.3—0.4\).

The deposits of the underlying Tergen Formation are clearly distinguished from the lower parts of the Baley Formation by elevated values of \(\chi = 56 \cdot 10^{-6}\) CGS, \(I_n = 5 \cdot 10^{-6}\) CGS, and by reduced values of \(Q = 0.2\).

The Talangui Formation, composed chiefly of effusives, has the highest values: \(\chi = 190 \cdot 10^{-6}\) CGS, \(I_n = 26 \cdot 10^{-6}\) CGS, although \(Q = 0.45\).

The distribution of the directions of the vectors \(I_n\) for the lower part of the Balei Formation proved distinctive: four groups of directions are distinguished, differing considerably from the direction of the present magnetic field. The first group has a general northern direction of the vectors with \(I_n\) close to \(50^\circ\); the second group is southern with \(I_n = 45^\circ\); the third group is predominantly western, with a scatter of \(I_n\) from 30 to \(80^\circ\); the fourth group has large inclination angles, equal to \(70^\circ\), and scatter in azimuth. The inclination angles of the vectors are mostly positive.

The section of the graben ends with the Unda granites, for which: \(\chi = 17—60 \cdot 10^{-6}\) CGS, \(I_n = 2—4 \cdot 10^{-6}\) CGS, \(Q = 0.13—0.14\). The directions are eastern; positive inclination angles of about \(50^\circ\) predominate.

A stock of microdiorites in the Unda granites (southern periphery of the graben) has magnetic parameters close to those of the granites, although the directions of the remanent-magnetization vectors are predominantly western.

Lamprophyre dikes have different values of the scalar parameters and directions of the magnetization vectors; one of the three dikes studied is negatively magnetized.

The Borshchovochny granites (northern periphery of the graben) are characterized by the following values of the magnetic parameters: \(\chi = 15 \cdot 10^{-6}\) CGS, \(I_n = 5 \cdot 10^{-6}\) CGS, \(Q = 1.0\). The remanent-magnetization vectors have northwestern and southeastern directions and negative magnetization angles. Effusives of the Talangui Formation on the western periphery of the graben have \(\chi = 16 \cdot 10^{-6}\) CGS, \(I_n = 3 \cdot 10^{-6}\) CGS, \(Q = 0.69\). A southwestern direction of the remanent-magnetization vectors is distinguished.

As a result of the investigations carried out, the following conclusions may be drawn:

1. The rocks composing the Balei graben possess low values of magnetic properties, of approximately the same order for sedimentary and igneous rocks. Most of the rocks are suitable for paleomagnetic study.

2. A regular change in the magnetic properties of the rocks along the section is indicated, i.e., definite values of the scalar magnetic parameters correspond to a definite stratigraphic subdivision.

3. A change in the directions of \(I_n\) is indicated within the lower part of the Balei Formation.

4. The granitoids of the Unda and Borshchovochny complexes are magnetized with opposite signs and differ in \(Q\), which is 0.4 for the former and 1.0 for the latter.

5. The dikes of the southern periphery of the graben have different magnitudes and directions of remanent magnetization; moreover, two closely situated dikes proved not only to differ strongly in the values of magnetization, but also to be magnetized oppositely.

REFERENCES

1. A. G. Vetrov, V. N. Dobryakov. On the use of the cumulative frequency curve in processing observations. Problems of Exploration Geophysics, issue 1, 1962.

2. Magnetism of Rocks and Paleomagnetism. Materials of the Fifth All-Union Conference on Paleomagnetism. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, Krasnoyarsk, 1963.

3. N. V. Petrovskaya, P. S. Bernshtein, S. G. Mirchink, M. G. Andreeva. Geological structure, mineralogy, and features of the genesis of the gold-ore deposit of the Balei ore field (Eastern Transbaikalia). Transactions of TsNIGRI, Moscow, 1961.

4. N. K. Razumovsky. The logarithmically normal law of distribution of matter and its properties. Proceedings of the Leningrad Mining Institute, vol. 20, 1948.