SOME REMARKS ON THE TECTONICS AND PALEOGEOGRAPHY OF THE IRSHA-BORODINO BROWN COAL DEPOSIT

L. A. Pukhlyakov

(Presented by Professor A. G. Sivov)

The coal-bearing sequence of the Irsha-Borodino brown coal deposit is of Jurassic age and is overlain by Quaternary deposits. In the course of stripping operations in these deposits, the lower jaw of a woolly rhinoceros, the large tibia of a bison (the identifications were made by Professor L. L. Khalfin), as well as tusks and other skeletal remains of a mammoth were found.

The sequence of Jurassic rocks exposed by the coal-mining open pit is represented in its lower part by the Borodino bed of brown coal, up to 40 m thick or more; in its middle part by an aleuritic-clayey unit about 20 m thick, containing the Rybinsk coal bed up to 3 m thick or more; and in its upper part by a substantially sandy unit up to 30 m thick or more, containing several small coal seams.

Within the exposed rocks the author observed two small disjunctive disturbances: the first in the floor of the Borodino bed (strike azimuth 33°, dip angle 88°, displacement amplitude approximately 1 m), and the second in the Rybinsk bed (strike azimuth 250°, dip angle 73°, displacement amplitude about 0.5 m). The author was unable to trace these disjunctive disturbances in the vertical and horizontal directions; however, there is every reason to suppose that the zones affected by them were very limited, since at the first observation both were confined within 3–4 m (one bench), and during a repeated visit to the open pit, when its southern wall had shifted by approximately 100–150 m, no signs of these disjunctive disturbances could be found in the corresponding places of the exposed sequences. A large number of similar disjunctive disturbances in the mines of this deposit were observed by A. V. Aksarin and V. L. Kokunov in 1955–56. The appearance of these disjunctive disturbances is apparently connected with seismic phenomena in this area during the period of formation of the coal-bearing sequence.

Along the eastern wall of the open pit, a rather complex tectonics of burning is observed. Evidently, in the past, owing to denudation, some part of the bed became exposed and ignited. As the coal burned out, the overlying sandy-clayey sequence subsided and formed the bizarre folds and ruptures that are observed here at the present time.

If the eastern wall of the coal open pit is excluded from consideration, it may be said that the Borodino coal bed and the aleuritic-clayey unit overlying it have preserved their original mode of occurrence—

All the elements available for identification lie almost horizontally. Only in the eastern and western parts of the southern margin is a slight (up to \(5^\circ\)) dip observed toward the axial part. This fact is consistent with the conclusion that the coal seams were formed under conditions of a lacustrine-bog regime. The same may be said of the aleurite-clayey member, since in many places it displays microlamination, with the thickness of the microlayers about \(0.2\) mm. Microlamination of this type is a characteristic feature of lakes of the temperate belt, with their seasonal climatic fluctuations.

The situation is somewhat different with the sandy member. It cannot be recognized as lacustrine, first of all because the interbeds distinguished within it occur now more gently, now more steeply, than the clayey interbeds underlying them. Of particular interest in this respect is the coal interbed lying near the base of the sandy member. Its footwall as a whole descends toward the axial part of the deposit, forming dips sometimes up to \(30^\circ\). At the same time its thickness, reaching \(1\) m in the lowest parts of the surface underlying it, decreases in the elevated parts to \(0.5\)—\(0.2\) m, and in places (in the most elevated parts) it pinches out completely. The absolute excess of some of its parts over others reaches \(12\) m. The bed of this interbed represents a typical channel. Evidently, the lacustrine regime here was from time to time replaced by a fluvial one.

In the eastern part of the southern margin, the coal interbed just considered forms two small folds, separated from one another by \(150\) m and extending along an azimuth of about \(80^\circ\). The traced length of these folds is about \(250\) m, and there is every reason to suppose that one of them (the southern one) continues farther as well. At least, its dimensions, when observed in different sections (Fig. 1), were approximately the same. The beds within this fold are broken by disjunctive disturbances, different in the different sections. Changes in the thickness of individual beds are also observed. The second fold (the northern one) is an undisturbed anticline up to \(3\) m high and up to \(9\) m wide (after correction for oblique section, about \(6\) m). Eastward, the width of this fold decreases to \(3\) m and its height to \(1\) m. The thickness of the dislocated sequence is about \(10\) m, of which approximately \(4\) m belong to the aleurite-clayey member and about \(6\) m to the sandy member. The underlying rock beds are completely undislocated. Under present-day conditions, structures of this type are sometimes formed in landslides and are expressed in uplifts of river beds [1, p. 248]. Therefore these folds may be regarded as landslide folds. During their formation, movement apparently took place from the north toward the channel axis and ceased within the limits of the southern fold.

For a final resolution of the question of the genesis of the sands overlying the coal interbed under consideration, five samples were taken from them for granulometric analysis. The sampling sites are shown in Fig. 1-III. The samples were treated in accordance with the method of L. B. Rukhin [2, 3] in the following order.

1. The samples were sieved through sieves with openings of the following diameters: \(2.0\) mm, \(1.0\) mm, \(0.5\) mm, \(0.3\) mm, \(0.25\) mm, \(0.15\) mm, \(0.12\) mm, \(0.1\) mm, \(0.09\) mm, and \(0.075\) mm. The fractions obtained were carefully weighed, and on the basis of the data obtained, in coordinates \(\lg d, P\), where \(d\) is the diameter of the openings of the corresponding sieves in millimeters and \(P\) is the weight of the fraction coarser than the sieve openings, as a percentage of the total weight of the sample, a cumulative curve was constructed (Fig. 2).

2. On the basis of the cumulative curve obtained, the fractional composition of the sediment was calculated, which should have been obtained during dis—

sieving through sieves with openings of the following diameters: 2.0 mm, 1.43 mm, 1.0 mm, 0.705 mm, 0.5 mm, 0.354 mm, 0.25 mm, 0.178 mm, 0.126 mm, 0.09 mm, and 0.064 mm. These diameters were selected so that the boundaries of the fractions, in logarithms, differed from one another by the constant value \(\omega = 0.150\).

Fig. 1. Southern landslide fold in various sections observed as stripping operations advanced: I—according to the observation of August 31, 1960; II—according to the observation of September 25, 1962; III—according to the observation of September 11, 1963. In the third section, Arabic numerals indicate the sampling sites for granulometric analysis.

and so that some of them coincided with the diameters of actual sieves. As a result, it turned out that the value of \(\omega\) in some cases came to differ from 0.150 (Table 1).

1. For each of the fractions obtained, the half-sums of the logarithms of the limiting diameters \(h\) were determined (Table 1).

2. The number of grains was determined for the individual fractions. For this purpose, the weight of the fraction \(P\) was multiplied by the coefficient \(a\), by which was meant the third power of the ratio of the mean diameter of the first fraction to the mean diameter of the given fraction.

Fig. 2. Cumulative curve for sample No. 1.

1. The share of the individual fractions in the total number of grains of the given sample was determined, in percent \(f\).

Table 1

Constant characteristics of the fractions of the sieved samples

1. Conditional moments were determined. For this purpose, a zero was placed opposite the fraction taken as the basis of the calculation, and opposite the remaining ones, special-

Table 2

Calculation of conditional moments for sample 1

... multiplier \(x\), whose values had a plus sign if the fraction was above the principal one, and a minus sign if it was below the principal one. Then the share of the fraction by number of grains, in percent \(f\), was multiplied by this multiplier first to the first power (when determining the first conditional moment, \(\nu_1\)), and then to the second power (when determining the second conditional moment, \(\nu_2\)). The products obtained were summed, and their sums were converted from percentage expression to natural expression (reduced 100-fold). The values obtained were regarded as the sought conditional moments: the first, \(\gamma_1\), and the second, \(\gamma_2\). As an example, the calculation of the conditional moments for sample 1 is given (Table 2).

1. The conditional sorting coefficient \(\sigma\) and the mean diameter by number of grains, \(M_a\), were determined. The calculation was made by the formulas

\[ \sigma=\omega\sqrt{\nu_2-\nu_1^2}, \]

\[ \lg M_a=h+\omega\nu_1. \]

Here, as the calculated \(h\), one-half the sum of the logarithms of the limiting diameters of the fraction that was taken as the basis of the calculation was used. In particular, for sample 1 these quantities were expressed by the relations

\[ \omega=0.150\sqrt{2.79-0.83^2}=0.150\cdot1.45=0.217. \]

\[ \lg M_a=\overline{1}.175+0.150\cdot(-0.83)=\overline{1}.175-0.124=\overline{1}.051. \]

\[ M_a=0.113. \]

The results of the calculation for the remaining samples are summarized in Table 3.

Table 3

Results of granulometric analysis for samples taken in the third section of the southern fold (Fig. 1-III)

1. The values obtained in this way were plotted on L. B. Rukhin’s genetic diagram: the conditional sorting coefficient along the horizontal axis, and the mean diameter by number of grains along the vertical axis; and, depending on the field in which a given sample fell, it was assigned to one or another facies (Fig. 3). Of the samples studied, three (samples 1, 2, and 5) fell within the field of channel deposits, i.e., are undoubtedly fluvial, and two (samples 3 and 4) in the field of uncertainty. Thus, these two samples also do not contradict the earlier assumption that the deposits of the sandy member are fluvial.

The coal seams of the Irsha-Borodino deposit are broken by several systems of joint fractures. Most clearly expressed is a system of fractures with approximately vertical dip and a submeridional strike (between 0° and 25°). This system of fractures cuts even those coal seams which, as a result of chemical weathering, have been transformed into soot that crumbles at the first touch. A somewhat less clearly expressed system consists of fractures striking at an azimuth of approximately 140° and also dipping almost vertically. In the terrigenous sediments, similar joint fractures are not observed.

Fig. 3. Reflection of the results of granulometric analysis on L. B. Rukhin’s genetic diagram. The numbers indicate sample numbers.

Visible labels in the diagram: \(M_a\); \(\sigma\); “Lake shallow-water sands (zone of strong agitation)”; “Lake deep-water sands (zone of weak agitation)”; “Aeolian sands”; “Channel deposits”; “Field of unreliability”; samples 1, 2, 3, 4, 5.

REFERENCES

1. I. V. Mushketov. Physical Geology, vol. II. Gosizdat, 1926.
2. L. B. Rukhin. The Granulometric Method of Studying Sands. Leningrad University Press, 1947.
3. L. B. Rukhin. Granulometric analysis of loose and weakly cemented sedimentary rocks. In: Methods for Studying Sedimentary Rocks, vol. 1. Gosgeoltekhizdat, 1957.