SUBDIVISION OF THE MESOZOIC FORMATIONS OF THE BALEI GRABEN BY THE PETROPHYSICAL METHOD

V. M. PEREDERIN, Z. A. MYSHKO, G. G. NOMOKONOVA (TPI)

This article is devoted to the problem of subdividing the Mesozoic continental rocks that fill the Balei graben.

The sequence of rock succession in the section of the graben has now been established as follows (from bottom to top): on the eroded surface of the Unda granites \((I_1C_2)\) lies the undivided volcanogenic Shadaron Series \((I_3sd)\), composed mainly of tuffs, tuff conglomerates, tuff breccias, and porphyrites. Above it is the terrigenous, predominantly conglomeratic suite \((I_3—Cr_1us_1)\), also undivided. It is overlain by the Balei Suite \((I_3—Cr_1us_2)\), represented by conglomerates, breccias, sandstones, and siltstones. By lithological features it is subdivided into four members. The deposits of this suite are characterized by frequent facies transitions and replacements, especially in the marginal parts. The section is completed by the Kamenka conglomerates \((I_3—Cr_1us_3)\), whose age still remains disputed.

The Balei graben has a complex tectonic structure, since it is located at the junction of two large regional zones of dislocation. By a number of faults it is divided into three blocks displaced relative to one another vertically.

The framing of the graben is composed mainly of granitoids of the Borshchovochny and Unda complexes, as well as of volcanogenic rocks of the Shadaron Series.

Since the Balei gold-ore deposit is situated within a definite depth interval and corresponds to definite lithological members, its enriched areas are associated with individual lithological varieties of rocks of these members. Therefore, detailed subdivision and correlation of the graben deposits acquire great practical importance in prospecting, exploration, and exploitation of ore veins and zones. The tendency of the Balei–Taseevo mineralization toward the physical boundary of a division in the section of the Upper Mesozoic of Eastern Transbaikalia is noted by V. D. Lyubalin and Yu. I. Simonov (ChGU).

The regularities in the variation of the physical properties of the graben rocks were studied from the cores of 23 exploratory core-drilling boreholes,

which was taken by depth at intervals of 2–5 m. The core was oriented in the “top–bottom” position according to a set of lithological indicators. In addition, about one hundred samples were taken with “top–bottom” orientation directly from the core barrels during drilling.

The following physical parameters were measured and determined: magnetic susceptibility ($\chi$), the magnitude of remanent magnetization ($I_n$) and its inclination ($I^\circ$), the factor

$$ Q=\frac{I_n}{I_i}, $$

($I_i$ is the magnitude of induced magnetization), and bulk density ($\sigma$). According to internal and external checks, the accuracy of measurements of magnetic quantities lies within 5–10%.

The error in determining densities does not exceed 0.01 g/cm³ in absolute value.

Analysis of the regularities in the distribution of the magnetic properties of the rocks of the graben made it possible to distinguish four major physical horizons, corresponding to the principal stratigraphic subdivisions, and to subdivide the three upper horizons (formations) into subhorizons. The Shadaron Series and the Tergen Formation were subdivided by us for the first time (see Table 1).

For the first horizon of Unda granitoids, which compose the basement of the graben, high density values and very low and uniform magnetization are characteristic; these remain practically unchanged in section and in plan throughout the graben.

Anomalously high values of magnetic susceptibility have been found as zones only in four boreholes of the Eastern block. We presumably associate these zones with secondary processes that supplied ferromagnetic minerals.

The rocks of the Shadaron Series (horizon II), lying on the granites, are characterized by the highest magnetic properties of all the rocks of the graben and are divided into two well-correlated subhorizons: a lower one with reduced values of magnetic properties and an upper one with increased values. The most probable values of magnetic susceptibility and remanent magnetization of the upper subhorizon increase, respectively, by two and seven times, and the $Q$ factor by a factor of 2. In horizontal plan there is an increase in $\chi$ and $I_n$ along the southern and eastern margins of the graben. The physical boundary of the subdivision in the Shadaron Formation, first distinguished by us, proved to be very interesting. It shows that the formation of these deposits was not a simultaneous act, but reflects the evolution of the intensity of effusive processes, i.e., their intensification toward the end of Shadaron time, which was expressed geologically in the predominance of effusive formations (lava breccias, porphyrite lavas) and physically in a sharp increase in the magnetic parameters of the rocks owing to the thermoremanent character of their magnetization.

Higher up lie the conglomerates of the Tergen Formation (horizon III), clearly divided into two well-correlated subhorizons, as are the deposits of the Shadaron Series, but with the reverse alternation of magnetic characteristics. The lower subhorizon has increased magnetic values, and the upper one reduced values. The subhorizons are well recognized by a 2–3-fold difference in remanent magnetization and in the factor $Q$. A difference in magnetic susceptibility is observed, but it is not so large. These facts show that, in the process of accumulation of the thick sequence of Tergen conglomerates, their material composition did not remain constant. Namely, at first fragments of rocks with high thermoremanent magnetization predominated, then their quantity sharply decreased, which led to a decrease in $I_n$ and $Q$. Analysis of the geological materials from the boreholes confirms this, since in the lower parts of the Tergen

within the formation, pebbles of effusive rocks often predominate in the gravel fraction. Hence, the abrupt change in the values of magnetic parameters within the thickness of the Tergen conglomerates may serve as confirmation that, during the period of accumulation of this sequence, if the source area did not change, then in any case the depth of erosional truncation did change.

The distribution of the physical properties of the deposits of the Tergen Formation over the area of the graben is, on the whole, uniform; only in the upper subhorizon is there a tendency toward a decrease in density and an increase in magnetic susceptibility in the Eastern block.

The upper part of the graben section is occupied by deposits of the Balei Formation (horizon IV), which have the lowest magnetization and density values of all the sedimentary formations. Nevertheless, throughout the graben the Balei Formation is clearly subdivided into four subhorizons, corresponding to the four members distinguished from geological data. The following change in the magnetic properties of the rocks is observed upward through the section: the first member has elevated values, the second—reduced values, the third—elevated values, and the fourth again reduced values.

In addition to differences in scalar characteristics, the subhorizons also differ and are readily recognized by the character of the alternation of zones of positive and negative magnetization: within subhorizon IV₁ in the Western block four such zones are distinguished from bottom to top (+, −, +, −); the deposits of subhorizon IV₂ have predominantly positive magnetization, while in subhorizon IV₃ only two zones are clearly distinguished (positive and negative). For the upper subhorizon IV₄, such zones have not been established, since it was not possible to take oriented samples from it.

The magnetic properties of the deposits of the Balei Formation within each member are distributed over the area of the graben rather uniformly; only in subhorizon IV₃ is there a tendency toward an increase in magnetic susceptibility at the western and eastern margins. In the three lower subhorizons, a decrease in density from west to east is noticeable. Vertically, density decreases from horizon IV₁ to IV₄. The subdivision of horizons and subhorizons by magnetic parameters is generally confirmed by density, although the boundaries distinguished by it often do not coincide with the magnetic boundaries.

The distribution of physical parameters within the studied deposits shows that their variation is not random, but is governed by definite geological processes, which permits the following conclusions to be drawn:

1. The rocks composing the Balei graben, from the basement to the modern deposits, are clearly subdivided, according to the general regularities in the distribution of physical parameters, into horizons (formations) and, according to particular regularities, into subhorizons (members).

2. The mean or most probable values of the physical parameters in the distinguished formations and members within the graben vary, which is quite natural under conditions of strong facies variability; however, this influence is not so great as to obscure the regularities in the variation of the physical parameters through the section.

3. Zones of magnetization of different sign are statistically distinguished and correlate well with the other magnetic parameters; these zones make it possible to identify unambiguously coeval formations within sequences where the recovery of oriented core is possible.

4. The density properties change regularly from formation to formation, although the boundaries of horizons distinguished by density do not always coincide with the boundaries distinguished by magnetic characteristics.

1. As a result of subdividing the sedimentary cover of the graben according to physical properties, the position of a number of stratigraphic boundaries has been refined, and new, clearly defined and readily correlatable subdivision boundaries have been outlined for the first time within the Shadaron Series and the Tergen Formation.

Table 1

Note: in the numerator is the mean or most probable value of the parameter studied; in the denominator, the limits of variation of the mean value for the horizon studied within the graben.