A. B. TRAVIN

ON THE CONDITIONS OF ACCUMULATION AND FACIES VARIABILITY OF THE COALS OF ONE OF THE SEAMS OF THE PROKOPYEVSK DISTRICT OF THE KUZBASS

During 1946–47 the author of the present article carried out a petrographic study of one of the seams of the Prokopyevsk district of the Kuzbass, with the aim of clarifying the causes of the qualitative variability of the coals observed in the seam within the district. In all, 17 seam samples were studied, taken from 17 points located along the strike and across the strike of the seam at varying distances from one another. The maximum distances between the outermost samples reach 5.5 km across the strike and up to 6.0 km along the strike of the seam. ^1)

The seam described occurs in the upper part of the Balakhon suite developed in the district. The thickness of the seam in the sampled area varies within rather broad limits, reaching in its extreme values a ratio of 1:2. These fluctuations are explained, on the one hand, by changes in the conditions of accumulation of the organic mass over the area of the original peat bog and, on the other hand, by intra-seam displacements occurring in the coal seam during folding and accompanied by squeezing out of the coal mass, with losses in seam thickness. Reduction of the latter as a result of intra-seam movements can usually be readily established by the presence in the seam of interbeds of tectonically soft and crushed coal, which in most cases occur either at the contact with the roof and floor of the seam, or at the contact of two coal layers differing in mechanical strength—dull and bright; moreover, the latter, as a rule, proves to be crumpled. In individual cases the reverse picture is observed, i.e., at the boundary of two different coal layers the stronger of them—the more dull coal—is subjected to crumpling. In the seam there are occasionally small interbeds of coaly argillite (1–3), with a thickness from 0.01 to 0.16 m, which are mainly confined to the lower part of the seam. Their number and total thickness generally increase westward and northwestward.

According to data from other investigators, in the floor of the seam in the northwest of the district there is here and there an interbed of coal up to 0.25–0.30 m thick, separated from the seam by clayey argillite 0.30–0.40 m thick. Still farther to the northwest it already occurs in the form of a double seam.

The floor and roof of the seam are gray siltstone and argillite. Only in samples No. 32 and 3, located in the eastern part of the area, is sandstone observed in the floor of the first sample and in the roof of the seam at the second sample. Study of 17 sections of the seam showed that the seam is composed of the same petro-genetic varieties of coal. Being present in different quantitative proportions with one another, they sometimes create considerable diversity

^1) If the folding is taken into account and the seam is unfolded into a horizontal position, then the distances between the outermost eastern and western samples, located across the strike of the seam, increase to 14–18 km.

in the petrographic composition of individual samples of the seam. However, this does not prevent one from observing, quite distinctly, two petrographic bands traceable along the extent of the seam, constituting its lower and upper parts. The lower part, or band, of the seam is composed mainly of bright and semibright types of coal with subordinate interlayers of semidull coal. Here there are also thin interbeds of barren rock and mineralized coal, often represented by spherosiderite formations cemented by a structureless organic mass. The thickness of the band increases westward, where in individual samples it amounts to as much as 60% of the total thickness of the seam, as against 25–35% in the eastern part of the deposit.

The upper part of the seam has a less stable petrographic composition and is represented mainly by varieties transitional between semidull and semibright coals, with predominance of the former. Here there is an irregular interbedding of coal varieties, with frequent wedging-out and replacement of some by others along the strike. This indicates a lesser constancy of accumulation conditions during the period of formation of the upper band, while the more dull composition of the coals composing it indicates a predominance of aerobic conditions during decomposition of the plant matter. The existence of the latter is also evidenced by fairly thick interlayers of fusainized mineralized coal observed in the upper part of the seam. At the same time, within the upper band two horizons are clearly outlined, corresponding to periods of considerable flooding of the peat bog. They are represented chiefly by semibright varieties of coal and are distinguished by an uneven thickness, increasing toward the southwest and south.

A general increase in the content of bright and semibright coals is observed not only westward, but it is manifested especially clearly in the southerly direction, more sharply in the west and more weakly at the extreme east of the deposit. The latter is clearly illustrated by stacked diagrams compiled along the strike of the seam along lines I—I, II—II, III—III, and IV—IV (Fig. 1). To construct such diagrams, in each sample all interlayers of like-named petrographic constituent parts of coal and interlayers of barren rock were summed. After this, the combined thicknesses of the various components were plotted at the same scale for all samples. In such diagrams, proposed by I. I. Ammosov, both the ratio of the petrographic constituent parts of the coal and the variability of the seam composition along its strike and dip are reflected.

In addition, to some extent they express the total components of vertical oscillatory movements during the period of formation of bright and dull coals, as well as interlayers of barren rock, which differ from the former in the rate and scale of accumulation. For example, during the formation of interlayers of barren rock, the rate of subsidence of the basin floor was considerably higher than during the accumulation of bright and especially dull coals. Therefore, on the basis of such stacks, compiled in different sectors with different total contents of the above-mentioned components, one may approximately judge the degree of mobility of these sectors.

Among the varieties of coal composing the seam described, the greatest volume is occupied by semibright and semidull banded coals, which on average constitute respectively 30 and 24% of the total thickness of the seam. In individual samples their content varies greatly, changing from 12 to 44% in the case of semibright coal and from 5 to 53% in the case of semidull coal. The next place in occupied volume belongs to transitional varieties from semibright coal to semidull coal, which make up on average 12% of the total thickness of the seam. Next come bright banded coal—6%, semidull striated coal—6%, and semibright striated coal—1.3%. Bright indistinctly banded and dull mineralized coals, as well as interlayers of barren rock, each separately constitute from 2 to 3%

Bar diagrams,

characterizing the change in petrographic composition by the principal varieties of coal

across the strike of the seam

along the strike of the seam

Fig. 1. 1—bright coal, 2—semibright coal, 3—semidull coal, 4—mineralized coal, 5—argillite.

of the total thickness of the seam. Semi-dull massive coal takes a negligibly small part (fractions of a percent) and is observed only in two samples, Nos. 26 and 23.

Thus, within the seam, the greatest distribution is enjoyed by the semi-bright, semi-dull, and intermediate varieties of banded coal, of which the first, by their petrographic indices, belong to coking coals. Coals of the blast-furnace type (semi-dull streaky varieties) play a subordinate role in the composition of the seam and make up on average about 6% of its thickness. The total percentage content of bright and semi-bright coals is approximately 52%. Semi-dull banded and massive coals, together with varieties transitional to semi-bright coals, amount to 37%; dull mineralized coal—about 3%, and partings of barren rocks—2%. Such a percentage ratio of the principal petrographic component parts in the seam is favorable for coking purposes, representing a natural coking charge for the given initial coal substance and degree of its coalification. The good coking properties of this mixture are confirmed both by the data of technical analysis and by plastometric indices of average seam coal samples.

Blast-furnace coals, making up a small thickness in the seam and occurring as small interbeds, have no independent practical significance.

Microscopic study of the coals showed that their bright varieties are characterized by: 1) an increased content of structureless vitrified mass (from 80–90% in bright coals to 60–70% in semi-bright coals); 2) a low content of fusainized elements; and 3) a considerable content of sphaerosiderite, especially in bright coals. Dull and semi-dull varieties, in contrast to bright ones, are distinguished by: 1) a low content of structureless substance (not more than 50–60%); 2) an increased content of fusainized components; and 3) the complete absence of sphaerosiderite.

By the predominant composition of the original plant material, the coals of the seam described belong to the type of woody humolites, and by the conditions of accumulation—to autochthonous formations. Allochthonous varieties of coal occupy a small volume in the seam. These include: dull mineralized coals containing a sandy-clayey admixture, and semi-dull lenticular-streaky and massive attrital coals. All other varieties, bright and dull, belong to autochthonous coals. The autochthonous nature of the overwhelming mass of the coals explains the comparatively consistent average petrographic composition of the seam along its strike and dip. True, over large distances and in the direction from the floor to the roof of the seam this composition, as we saw above, changes noticeably, but over small distances it remains fairly well maintained.

The microscopic composition of the coals of the seam described indicates that the bright varieties, rich in vitrified mass and containing sphaerosiderite, were formed under conditions of more or less considerable waterlogging. Conversely, the dull varieties, rich in fusainized components and containing no sphaerosiderite,\[^1\] formed under conditions of a more “dry” peat bog.

The above-mentioned percentage ratio of bright and dull varieties of coal in the seam (approximately 1:1) indicates that the seam as a whole formed under conditions of a significantly less waterlogged peat bog than, for example, in the case of coals of the Kolchugino type.

\[^1\]: Siderite, as is known, can form only in a reducing environment at a comparatively high position of the reduction–oxidation boundary, i.e., in a setting opposite to that in which fusain forms.

Bright coals usually occur as interbeds from 0.08 to 0.15 m thick in the lower and partly in the upper member. They are found in all the sampled benches except two samples—Nos. 17 and 32. Semi-bright banded coal is observed in all samples in the form of interbeds from 0.07 to 0.93 m, predominantly in the lower part of the seam. Semi-dull banded coal and varieties transitional to semi-bright coal constitute chiefly the upper member of the seam, occurring in all samples as interbeds from 0.6 to 1.76 m thick. Semi-dull striated varieties are also observed, mainly in the upper part of the seam, as interbeds from 0.04 to 0.38 m. They have limited development and in many samples are entirely absent. Dull mineralized coal occurs as small interbeds of 0.02–0.05 m in the lower and, more rarely, in the upper part of the seam in the eastern and southeastern samples. Sometimes interbeds of mineralized coal reach 0.18–0.22 m.

In most cases there is frequent alternation and mutual pinching-out of some coal varieties in the seam by others, especially in its upper half. Only in the area where samples such as 16, 15, and 17 were taken is a more homogeneous petrographic composition of the seam observed. There, according to the samples taken, the seam is composed of thick, up to 1.25–1.75 m, layers of bright, semi-bright, and semi-dull coal, successively replacing one another in order of decreasing luster from the floor to the roof. The seam in this place was evidently accumulated under calmer conditions, in an environment of slow subsidence of the basin floor and gradual “drying” of the peat bog, without frequent and abrupt replacement of anaerobic conditions of decomposition of organic matter by aerobic ones, and vice versa. This, apparently, was some more or less remote part of the peat bog, away from the shore zone.

In general, as noted above, substantial changes in the petrographic composition of the seam along its extent are observed only over large distances. Over short intervals (100–300 m) these changes are small, and the average petrographic composition of the seam in this case usually remains more or less constant.

Among the principal primary factors that caused differences in the petrographic composition of the seam (its facies variability), one must include epeirogenic movements and, as a consequence of them, changes in the microclimate and microrelief that took place continuously and intermittently over the long history of formation of the thick ancient peat bog. These changes in microclimate and microrelief caused fluctuations in the degree of waterlogging of the peat deposit, as well as differing duration and intensity of aerobic and anaerobic processes of transformation of plant tissues, which in turn led to changes in the petrographic composition of the seam in vertical and horizontal directions.

The change in petrographic composition observed in the horizontal direction and expressed in an increase in the content of dull coals in the seam toward the east of the deposit should apparently be attributed chiefly to changes in microrelief. The changes observed in the vertical direction and expressed in the replacement of the bright coals of the lower member of the seam by duller coals in its upper half may also be explained by phenomena of microclimate. The latter, over the long history of formation of the thick peat bog, could have changed substantially. Undoubtedly, the chemical composition of the groundwaters that fed the peat bog also played a certain role here. Their composition, changing over time, could either increase or decrease the acidity of the environment (even to alkaline), thereby creating different conditions for the biochemical decomposition of plant remains. However, in this case as well, the leading importance remains with epeirogenic movements, which, by redistributing the sources of supply in time and space, led to changes in the chemistry of the groundwaters.

Along with oscillatory movements, one of the reasons for the variability of the petrographic composition of the bed and of its thickness in the horizontal direction is the primary relief of the bogged area of the lithosphere. Causes of a more local type include, for example, such phenomena as local rupture of the aquifer and discharge of groundwater, accompanied by partial or complete drainage of the peat bog; seasonal fluctuations of the groundwater level; migration of water flows over the area of the peat bog; suffosion; landslide and other phenomena of an atectonic order.

However, among the principal factors determining the general course of development of a thick peat bog over broad areas of its distribution, tectonic movements in the form of epeirogenic oscillations should nevertheless be included.

Analyzing the contacts between the coal bed and its floor, one may note that in the western and central samples the bright and semibright coals rest directly on clastic rocks of the substrate, i.e., here there is observed an abrupt replacement of barren rocks by organic sediments, indicating, on the one hand, a sharp interruption in the deposition of sandy-clayey material and, on the other, the onset of a period of extremely slow subsidence of this part of the lithosphere, under which conditions the possibility was created for the development of vegetation and peat accumulation.

As one moves eastward, interlayers of mineralized coal with inclusions of spherosiderite appear at the base of the bed. By means of these interlayers, the bright and semibright coals of the lower bench of the bed are connected by gradual transitions with the rocks of the floor. This latter circumstance indicates a more prolonged struggle that took place here between peat accumulation and the sedimentation of clastic mineral deposits, in comparison with that which occurred in the western part of the area.

The eastern part of the deposit at this time probably represented a coastal zone, characterized by the frequent encroachment of surface waters from the shore and the introduction of a considerable quantity of sandy-clayey material, which, accumulating here, retarded the growth and dispersal of vegetation. After this, in connection with the apparently begun slow subsidence and bogging of the surface of the lithosphere in the east of the deposit, favorable conditions were created for the development of vegetation and for its migration in an easterly direction over the territory of the area under consideration. Such dispersal of the flora was accompanied by the accumulation of organic mass and the formation of interlayers of bright and dull coals, alternating many times with one another vertically. The frequent interbedding of bright and dull coals, among which a significant role belongs to dull mineralized and semidull streaky attrital varieties of allochthonous origin, is a highly characteristic feature of this part of the bed and testifies to the unstable regime that existed during the formation of this part of the peat bog. The presence of traces of local erosion and redeposition of plant mass, expressed in the bed in the form of interlayers of semidull streaky coal, indicates that the portion of the peat bog under consideration periodically experienced weak uplifts, during which partial erosion and redeposition of the organic mass took place in places. An indication that this area was formed under more “dry,” and consequently more elevated, conditions than the western one is also provided by the increased content of dull coals in the composition of the bed. Here, evidently, the rate of subsidence of the peat bog was so slow that the reduction–oxidation boundary did not often rise above its daytime surface.

It should be noted that, according to the data of I. I. Ammosov, northward (toward Kiselevka) this bed, along with an increase in thickness, becomes more

matte in petrographic composition partly owing to dull attrital coals¹.

The western part of the seam is distinguished by less facies variability and, as a consequence, by a more uniform petrographic composition. The coals composing it are predominantly bright, and dull varieties predominate only in the upper part of the seam. In contrast to the eastern part, there is no frequent alternation here of interbeds of bright and dull coals, which indicates a more stable regime of peat accumulation under conditions of a slowly and for a long time subsiding basin floor. At times the rate of subsidence of the area increased, and a brief partial flooding of the peat bog occurred, with accumulation of thin interbeds of barren rocks among the bright coals of the lower member. During the formation of the upper member of the seam, the rate of subsidence of the peat deposit everywhere slowed down (both in the west and in the east of the area), and so markedly that the rate of accumulation of organic matter in many cases began to exceed it. This period corresponded to the time of formation of dull coals in the upper part of the seam, which were then rather sharply replaced by sandy-clayey roof rocks as a result of the subsequent rapid subsidence of the basin floor and burial of the peat bog. The cessation of peat accumulation occurred non-simultaneously and not everywhere. Flooding and subsidence of the peat bog proceeded in a direction from west to east. This was reflected: 1) in the enrichment of the seam in the west of the deposit with bright coals and interlayers of barren rocks, testifying to a more considerable subsidence and watering of this part of the peat bog, and 2) in the reduction of the seam thickness westward, indicating earlier burial of the western part of the original peat bog in comparison with its northern and eastern parts. The same pattern of earlier covering of the peat bog by the roof is observed partly in the south of the Prokop’evsk district, where the seam is somewhat reduced in thickness and acquires a brighter composition.

Thus, within the Prokop’evsk deposit proper, the migration of the original peat bog apparently developed in the general direction from southwest to northeast, with earlier cessation of peat accumulation in the southwest and continuing sedimentation of plant matter in the north and northeast of the deposit. In connection with this, in the same direction an increase in seam thickness is observed; it rises sharply toward the north, reaching in the Kiselevsk district a value twice as great as in the south. Along with the general increase in seam thickness from south to north, an increase in its thickness is observed eastward in the south of the deposit and, conversely, westward in the north of the area. All this, taken together, testifies to the considerable mobility of this part of the lithosphere, which existed at the moment of formation of the peat bog and was due to its marginal position in the system of the Kuznetsk basin.

The area in the west underwent a more energetic downwarping, which is confirmed not only by our observations but also by data from other investigators of the Kuzbass (I. I. Ammosov and others), who point to an increase in the role of mineral clastic formations in the composition of the seam in the western part of the deposit.

The massive influx of clastic mineral material, which led to the cessation of peat accumulation, came from the west—from the side, probably, of the rising Salair.

The migration of the original peat bog during its life did not take place strictly in one direction, but in a spiral, in the form of a zigzag curve shown in Fig. 2, which schematically reflects the epeirogenic periodic-rhythmic movements of the described part of the lithosphere, gradually

Block diagram,

characterizing the facies variability of one of the seams of the Prokopyevsk deposit of the Kuzbass

Compiled by A. B. Travin, 1949

NE

Across the strike of the seam along line I–I′

14–18 km

2.5 km along the strike of the seam (along IV–IV)

Schematic curve of peat-bog development

Fig. 2. 1—bright and semibright coals; 2—dull and semidull coals; 3—dull attrital coals; 4—mineralized coal; 5—argillite; 6—siltstone; 7—sandstone; 8—isolines showing the upper boundaries of the distribution of the peat bog; 9—petrographic column samples; 10—development of the peat bog in the western direction; 11—development of the peat bog in the eastern direction.

varying both in their amplitude and in direction (upward or downward).

In the very earliest moments of formation of the peat deposit, the most favorable conditions for peat accumulation were created in the west of the area, where the bright coals of the lower bench of the seam accumulated. This stage on our curve corresponds to the segment a′—a. Then in the west there occurred a short interruption in the deposition of organic matter and an accumulation of interbeds of barren rocks; by contrast, in the east of the area at that time the most favorable setting for coal sedimentation was created (accumulation of bright coals). This stage corresponds to the segment a—a′ on the curve. After this, purer bright coals again began to accumulate in the west. The latter was reflected on our curve in the form of the segment a′—B.

Higher in the section, the bright coals were replaced by an interbed of dull, partly redeposited coal of the durain type. The appearance of such an interbed meant the onset here of less favorable conditions for peat accumulation. On the diagram this was expressed as a shift of the curve in an easterly direction (segment B—B′). Then a favorable setting for the formation of bright coals was again created in the west of the area. Accordingly, the curve shifted in the same westerly direction (segment B′—B). The period of formation of the upper bench of the seam, containing essentially dull coals, corresponds to segments on the curve distinguished by a small amplitude of vertical swing (B—B′, etc.). They reflect a more retarded rate of subsidence of the peat bog. Finally, the displacement of the entire curve to the east schematically shows the general direction of migration of the peat bog along the horizontal. By means of such a curve we attempted, in simplified form, to represent graphically the general character of the development of the peat bog in time and space. This is shown in greater detail and at the appropriate scale in the block diagram (Fig. 2).

In constructing the block diagram, we selected the central field of the seam in the territory of the Prokopyevsk district, bounded by the lines of sections I—I, IV—IV′, II—II.¹) As control points in constructing longitudinal sections along the indicated lines, petrographic samples—columns Nos. 26, 20, 16, 15, 6, 3, 33, PK—23, 24, and 23—were used, and the upper boundary of the lower bright bench of the seam was taken as the datum line, as a line approximately corresponding to the horizontal position of the water in the peat bog. From this line, the thicknesses of the coal and mineral interbeds composing the seam were then plotted downward and upward at the appropriate scale. In this way it proved possible, with rough approximation, to reconstruct the position of the floor and roof of the seam that existed during the period of its formation. At the same time, a clear sagging of the bottom of the peat bog in the southwest of the area and an earlier covering of the seam in this place by roof rocks became apparent.

Lithologically identical portions of the roof in different samples were connected by isolines of the same composition. The first such isoline was drawn through samples Nos. 16 and 26, the second through samples Nos. 15, 17, and 19, in whose roof lithologically similar rocks were found (siltstone in the first case and argillite in the second, Fig. 2). The subsequent isolines were drawn parallel to the first. All of them were drawn along the upper boundaries of like-named interbeds of barren rocks. The northern ends of the isolines were turned somewhat westward in connection with the fact that the seam in the north of the deposit (in the Kiselevsk district) has a greater thickness than in the southeast and, consequently, remained unburied here for a longer time. At the moment when in the south of the area the seam had already been covered by clastic sediments, in the north the process of coal sedimentation and increase in seam thickness was still continuing.

¹) The sections were compiled after the seam, gathered into folds, had first been unfolded into a horizontal position.

From the arrangement of the isolines on the block diagram it is clearly evident how the peat bog migrated in a northeastern direction. The isolines thus show the displacement of the shoreline of the peat bog, or the upper limit of its spread.

Reconstruction, with the aid of a block diagram, of the initial position of the seam in space and of its subsequent development in the course of formation is the first attempt to depict graphically such development of a seam in time and space within the area of the Prokopyevsk deposit.

The construction of such block diagrams is not only of methodological interest, but also has a definite practical significance, since, as we have seen above, it makes it possible to draw a number of practical conclusions concerning changes in the petrographic composition of the seam in horizontal and vertical directions, the facies conditions of its accumulation, the duration of peat accumulation in various parts of the deposit, and so on.

All this, taken together, gives the author grounds to recommend the construction of such block diagrams as a graphical method in the petrographic study of coal seams in general, all the more so because the compilation of separate, isolated seam bench sections—as has been practiced up to now—does not provide a sufficiently complete picture of the conditions of accumulation of a coal seam as a whole, as a single natural-historical body.

1. In our understanding, streaky semi-dull coals. ↩