Material Composition, Petrogenetic Types, and Structure of Thick Coal Seams of the Tataurovo Brown-Coal Deposit, Chita Oblast

O. A. Dulzon, V. L. Kukulov

(Presented by Professor A. V. Aksarin)

The Tataurovo brown-coal deposit is located 65 kilometers southwest of the city of Chita. The deposit is confined to the central part of the Chita–Ingoda intermontane depression and constitutes a closed trough elongated in a northeastern direction.

The area of the Tataurovo deposit is composed of continental coal-bearing deposits which, according to the regional stratigraphic scheme for subdividing the continental Mesozoic formations of Transbaikalia proposed by G. G. Martinson, are divided into three formations: the Bukachacha—$I_2$, the Ulanganga—$I_3—Cr_1$, and the Turginovitimo—$Cr_1$ [2].

The Upper Jurassic age of the Ulanganga Formation is substantiated by a complex of plant remains identified for the Chernovskoye deposit by V. A. Khakhlov [4], A. N. Krishtofovich [1], V. D. Prina and N. N. Srebrodolskaya [3], and for the Tataurovo deposit by A. V. Aksarin.

In the present work, the results are presented of macro- and microscopic study of the material of the Tataurovo coals (Table 1), their petrogenetic types, and the structure of the thick coal seams. In all, 360 coal thin sections were studied; the samples for their preparation were taken from 16 boreholes: 353, 406, 402, 367, 405, 403, 366, 362, 404, 400, 240, 363, 401, 173, 232, 220. The boreholes studied are mainly extended along the front of the future Tataurovo open-pit coal mine; one profile of boreholes was studied along the dip of the seams.

The principal seams of the Tataurovo deposit are composed only of representatives of the humite class. They are subdivided into a number of petrogenetic types of coals, which are representatives of the corresponding facies of coal accumulation. The Tataurovo coals include the following petrogenetic types:

1. Clarain—as a representative of a stagnant, boggy forest swamp characterized by a low groundwater level. Clarain coals constitute from 33 to 65.4% of the coal seams, with the maximum of clarain coals observed in seam III, and the minimum in seam II.

2. Dureno-clarain—as a representative of the facies of forest peat bogs with weak water flow and likewise characterized by a low standing level of groundwater. This type of coal composes from 23 to 56.7% of ...

Table 1

Microcomponent composition of the main coal seams
of the Tatayurovo deposit

coal seams, with their maximum observed in seam II and their minimum in seam III.

1. The clarene-durene type—as a representative of the facies of forest peat bogs with a very low groundwater level. This type of coal constitutes, in seams I and III, from 7,7 to 9,1% of the coal seams.

2. The durene type of coal is distinguished as representative mainly of the facies of swampy forests, and to a lesser degree of flowing forest bogs. This type of coal is also observed only in seams I and III and is of limited occurrence, constituting from 3,9 to 9,0% of the coal mass of the seams.

The coals of the Tatayurovo deposit may be divided by luster into dull, semidull, and semibright. Dull coals account for about 52% of the total coal mass; semidull and semibright coals make up 48%, with seam III being the richest in semibright varieties, seam II to a lesser degree, and seam I to an even lesser degree.

The structure of the Tatayurovo coals is governed by the alternation of bands and lenses of fusinite and vitrinite, with a sharp predominance of banded structure in seams I and II. Seam III has mainly a banded and banded-striped structure.

Before proceeding to the characterization of individual microcomponents of the coals, it should be noted that the brown coals of the working seams of the Tatayurovo deposit are composed mainly of remains of higher plants; lower plants are encountered in them only extremely rarely.

During the geological history of the formation of the deposit, plant remains were subjected to processes of gelification and fusenization; as a result, in the Tatayurovo coals there are observed mainly lignin-cellulose elements in the form of gelified and fusenized components, and only in insignificant quantities are resinite elements encountered.

The coal seams studied have both simple and complex structures over the area of the deposit, but their principal property is the fact that at the southwestern periclinal termination of the Tataurovo trough all the seams split, forming a “horse-tail” structure. Hence the following conclusion may be drawn: in the northeastern part of the Tataurovo trough one should expect genetic wedging-out of seams I, II, and III.

It should be noted that the most persistent in thickness in that part of the deposit area where the seams do not yet split is seam III, which here has a simple structure, i.e., in most cases it contains no rock partings. At the floor of this seam, as a rule, there lies a bed of siltstones from 0.5 to 1.5 meters thick, which near the contact with the coal matter of seam III grades into argillites.

The roof of seam III, judging from the described borehole sections, is as a rule composed of argillites.

Seam II has a much less persistent thickness and, judging from the geological map of the Tataurovo deposit, often wedges out, being replaced by carbonaceous rocks and even siltstones. Correspondingly, the rocks of the floor and roof of this seam also vary lithologically from argillites to sandstones. Like seam III, seam II has a simple structure, i.e., it contains no rock partings.

In contrast to the seams described above, seam I most often has a complex structure, since it contains from three to one rock partings, composed either of sandstones or siltstones, less often of argillites. In the floor of seam I, siltstones most often occur, grading in the contact zone with the coal matter of the seam into argillites. In the roof of the seam sandstones most often occur.

The petrographic structure of the coal seams of the Tataurovo deposit is presented stratigraphically from bottom to top, and within each individual seam the distinguished coal varieties are likewise described from bottom to top.

Seam III has a simple structure and the greatest thickness recorded during coal-petrographic studies—16.6 meters (the description of seam III is given for borehole No. 173). The following coal types take part in the structure of seam III: clarain, dureno-clarain, clareno-durain, and durain. Coals of the clarain type make up 65.4% of the total volume of seam III; coals of the dureno-clarain type make up 23% of the volume of seam III; and the remaining types account for only 11.6% of the volume of this seam.

According to the alternation of petrographic types of coals composing seam III, it may be divided into 4 benches, each of which is apparently a peculiar rhythm of coal accumulation, consisting in its lower half of coals of the clarain type, and in its upper half of coals of either the durain, clareno-durain, or dureno-clarain types. The petrographic structure of seam III, stratigraphically from bottom to top, may be represented as follows.

The first bench is four meters thick and is composed, in the lower half of the bench, of coals of the clarain type; from bottom to top these are xylovitreno-vitrain and vitrino-cutinito clarain, i.e., mainly stem clarains containing an insignificant admixture of fusinite. The upper half of the first bench is composed of coals of the clareno-durain and durain types, with the thickness of each of the layers approximately the same.

The second bench is 4.5 meters thick and for three quarters is represented by coals of the clarain type, but, as in the first bench,

ends with a layer of fusinite-semifusinite durain-clarain. The clarain layers of this member begin with a layer of sporinite clarain about one meter thick; then comes a meter-thick layer of fusinite-semifusinite clarain and a layer of the same thickness of xylovitrinite-vitrinite clarain.

The third member has a more complex structure and its thickness reaches 5.4 meters. This member begins with a layer of vitrinite-cutinite clarain 1 meter thick; then follows a layer of fusinite-semifusinite clarain 1.2 meters thick; and, finally, there are two layers of fusinite-semifusinite durain-clarain, separated by an interbed of vitrinite-cutinite clarain 0.70 meter thick. The total thickness of the durain-clarain coals of this member is 2.5 meters.

The fourth member has a thickness of 3.2 meters. It, like the preceding members, is composed of coals of the clarain type and ends with an interbed of coals of the durain-clarain type.

The clarain coals begin at the bottom with a layer of sporinite clarain 1.4 meters thick. Then comes a layer of fusinite-semifusinite clarain 1.3 meters thick. The fourth member is crowned by a layer of fusinite-semifusinite durain-clarain 0.5 meter thick.

Such, in general, is the petrographic structure of seam III. This seam has considerable thickness and contains no rock interbeds. An example of the structure of seam III in the present case, when its thickness decreases, may be the section of seam III in borehole No. 220. In this borehole its total thickness is only 7 meters, and here it also has a simple structure.

Seam III in this borehole begins with a layer of vitrinite-cutinite clarain 0.4 meter thick. Above lies a layer of fusinite-semifusinite durain-clarain 1.6 meters thick, on which rests a thick member of clarain coals, beginning with a layer of fusinite-semifusinite clarain 2.5 meters thick and crowned by a layer of cutinite clarain also 2.5 meters thick. Thus, in the case of a decrease in the thickness of the seam, the content of coals of the clarain type increases in it.

Finally, it is necessary to note that the amount of clarain coals in seam III increases in the direction from the axial part of the trough toward the place where the seam passes beneath the alluvium.

Seam II. A distinctive feature of the petrographic structure of this seam is the complete absence of regularity in the alternation of petrographic layers, which testifies to the extreme variability of the facies environment during the accumulation of this seam. Thus, in borehole No. 220 seam II has a thickness of 6 meters and consists of two layers. The lower layer, 2 meters thick, is represented by fusinite-semifusinite durain-clarain, and the upper layer, 4 meters thick, is composed of fusinite-semifusinite clarain. In borehole No. 173 seam II has a thickness of 4.5 meters and is subdivided into three layers, composed of more gelified varieties of coals than in borehole No. 220.

The first layer from below of seam II in borehole No. 173 is composed of xylovitrinite-vitrinite durain-clarain 2 meters thick; then follows a meter-thick layer of xylovitrinite-vitrinite clarain, and seam II ends with a layer of xylovitrinite-vitrinite durain-clarain 1.5 meters thick.

Seam I. This seam, in contrast to those described above, has a complex structure, since it contains rock interbeds. At the same time it also has a complex petrographic structure and strong variability of petrographic composition over the area of its distribution.

Let us indicate the general regularity in the distribution of the petrographic types of coals in this bed, which consists in the fact that each bench of this complex bed begins with fusinite-semifusinite varieties of coals of the durain and clarain-durain types and ends with coals formed in a more flooded setting, i.e., coals containing non-fusainized but gelified plant fragments—coals of xylovitrinite-vitrinite character. In boreholes No. 401 and No. 173, bed I has the greatest total thickness.

REFERENCES

1. L. N. Krishtofovich. Plant remains from lacustrine Jurassic deposits of Transbaikalia. Proceedings of the Mineralogical Society, issue 1, 1916.

2. G. G. Martinson. Mesozoic and Cenozoic mollusks of continental deposits of the Siberian Platform, Transbaikalia, and Mongolia. Proceedings of the Baikal Limnological Station, issue 19, 1961.

3. V. D. Prinada. The Mesozoic flora of Transbaikalia and its stratigraphic distribution. Materials on the Geology and Mineral Resources of Eastern Siberia, issue 22, 1950.

4. V. A. Khakhlov. Plant remains of the Chernovskie mines of Transbaikalia. Proceedings of the Siberian Technological Institute, vol. 47 (1), issue 3, Tomsk, 1927.