THE UPPER PART OF THE YERUNAKOVSKAYA FORMATION AND ITS BOUNDARY WITH THE TRIASSIC ALONG THE NORTH TALDINSKAYA EXPLORATION LINE IN THE YERUNAKOVSKY DISTRICT OF KUZBASS

N. A. Lobova and N. I. Shcherbakov

For a number of years, the Uskatskaya and then the Prokopyevskaya geological exploration parties of the trust “Kuzbassuglegeologiya” carried out coal prospecting and study of the geological section in the western part of the Yerunakovsky district of Kuzbass. In 1956, drilling was completed on the so-called North Taldinskaya prospective line (Fig. 1). Along this line it proved possible to obtain a continuous section of the upper horizons of the Yerunakovskaya Formation, to establish the contact of the latter with the Mal’tsevskaya Series, and to study in part the lithological composition of the Triassic deposits overlying the Paleozoic.

Up to the present time only one such section has been known, described by G. P. Radchenko from the Tailuganskoye exposure on the left bank of the Tom’ River. This description cannot be regarded as entirely convincing, owing to certain debatable assumptions in correlating individual parts of the section mentioned. From this standpoint, the section obtained along the North Taldinskaya line is more reliable. The correlation of the factual material obtained here is presented in Fig. 2. At the same time, this section makes it possible, by comparison, to evaluate the Tailugansky section and, on the whole, to approach the solution of the question of the full thickness of the Yerunakovskaya Formation in the Yerunakovsky district. The foregoing determines the theoretical interest and practical importance of these new materials, to the presentation of which this article is devoted.

In the western part of the Yerunakovsky district, deposits of Triassic and Permian age were encountered in 1939 by G. P. Radchenko in the area of the village of Kyrgai. In connection with the very modest scope of the work carried out here, only a small section was obtained, and the composition of the deposits of the Yerunakovskaya Formation below its contact with the Triassic remained undetermined. On the basis of considerably more complete materials obtained in 1954–1956 in the North Taldinskaya area as a result of prospecting work by the Prokopyevskaya party, the section of the deposits developed here appears as follows.

Yerunakovskaya Formation \((P^{er}_{2})\), immediately downward from the contact with the Triassic, was penetrated in a continuous section by boreholes along the North Taldinskaya exploration line, with a total thickness of 1860 m.

Lithologically, the deposits of the formation are represented by thick beds of sandstones, siltstones, and, less thickly, argillites, coaly argillites, and coals (Fig. 3).

The lower part of the section of the formation up to seam 31 differs from the overlying section by the predominance of a clayey complex of rocks and by the presence in the section of a large number of coal seams of medium thickness.

(mainly up to 4 m). The overlying part of the formation section between seams 31 and 22 is characterized by a predominance of sandstones, the presence of thick coal seams (up to 14 m), evenly distributed through this part of the section, and the complete absence of thin coal seams and interbeds. In the deposits of the very upper horizons of the formation, clayey rocks again predominate. Here a gradual upward decrease in the thickness of the coal seams and an increase in their number are observed. On the whole, clayey rocks occupy a dominant position in the section (Table 1).

Table 1

Figure 3 shows a schematic subdivision of the Yerunakovskaya Formation section in accordance with the unified scheme adopted in Leningrad in 1956. The Lenin and Gramoteino subformations have been studied in the Yerunakovsky district over a whole series of deposits.

According to the subdivision carried out for the section obtained along the North Taldinskaya line, the thickness below seam 39 belongs to the Lenin subformation ($P_2^{ln}$). The exposed thickness of the subformation is 500 m; it contains 14 workable coal seams with a total thickness of 26 m. The workable coal content is 5.2%.

The Gramoteino subformation ($P^{gr}$) is distinguished within the boundaries between the floor of seam 39 and the roof of seam 26. The total thickness of the subformation is 425 m. The subformation contains 12 workable coal seams with a total thickness of 54 m. From coeval deposits exposed at other fields of the district, the Gramoteino subformation in the Taldinskaya area differs by a higher workable coal content, reaching 12.7%, instead of 9–10% at the Yerunakovskoye deposit.

The Tailugan subformation ($P_2^{?}$) comprises all the remaining upper section of the Yerunakovskaya Formation. Its thickness is 935 m. The lower part of the subformation section is characterized by a coarser granu-

Fig. 1. Overview map of the Yerunakovsky district.
1 — basalts; 2 — formation boundaries; 3 — coal seams; 4 — disjunctives; 5 — boundary of the Yerunakovsky district.

Fig. 2. Geological cross-section along the North Taldinskaya exploration line. Compiled by N. I. Shcherbakov and N. A. Lobova.

1 — gravelite; 2 — sandstone; 3 — siltstone (a — normal, b — tuffaceous); 4 — argillite; 5 — interbedded packages of sandstones and siltstones (a — normal, b — tuffaceous); 6 — carbonaceous argillite and siltstone; 7 — coal; 8 — sites where samples were taken for chemical-petrographic analysis; 9 — burned rocks; 10 — faults; 11 — boundaries of suites and subsuites.

mineralogical composition of the rocks composing it, as well as by the presence of thick coal seams (12–13 m). Toward the upper part of the section the coal seams decrease in thickness and are distributed in groups of 2–4 seams.

In all, 24 workable seams have been established in the deposits of the Tailugan subformation, with a total thickness of the coal benches of 114 m. The workable coal content of the subformation is 12.2%.

To characterize the deposits composing the formation, Table 1 gives the quantitative and percentage content of rocks by subformations.

Among the known sections of the Yerunakovskaya Formation in Kuzbass, on the northeastern limb of the Demyanovskaya anticline, along the North Taldinskaya line, a section with the maximum coal content has been obtained.

The prevailing strike of the limb within the Taldinsko-Northern area is NW 325–330°. The angles of dip on the limb vary from 60–70° in the SE part of the line to 40–45° toward the NE, gradually flattening as the flexure-like fold that complicates the limb is approached.

The coal seams in the middle part of the Yerunakovskaya Formation reach thicknesses of 12–14 m. Most of them have a complex structure. In all, within the formation, in a continuous section, 66 coal seams have been uncovered, of which 50 seams have workable thickness (more than 0.70 m). The total thickness of all workable coal seams in the section of the formation is 194 m, and the workable coal content is 10.4%.

The qualitative characteristics of the coals of the Yerunakovskaya Formation in the Taldinsko-Northern area were studied chiefly from core samples from columnar boreholes. In all, 46 workable coal seams were sampled.

The group of upper seams up to and including seam 24, according to GOST 8162—56, belongs to the long-flame grade D and has the following average parameters: ash content ($A^c$)—9–25%, volatile matter—($V^r$)—38–42%. The plastic layer is absent or only faintly indicated.

The remaining seams of the exposed part of the formation belong to grades G and GZh. The ash content of the lower group of seams most often ranges from 4 to 14%, the yield of volatile matter from 34 to 38%, and the thickness of the plastic layer from 6 to 12 mm, increasing toward the lower part of the section.

The section of the upper part of the Yerunakovskaya Formation along the North Taldinskaya line is of exceptional interest: it is the second section, after Tailugan, that completely encompasses the Gramoteinskaya and Tailugan subformations.

In Fig. 3 a provisional correlation of these two sections is given. The correlation is made mainly according to lithological features and the coal content of the lower part of the section.

The cited section of the Yerunakovskaya Formation along the bank of the Tom River is borrowed from the work of P. V. Neutrievskaya on the calculation of coal reserves in the eastern part of the Yerunakovsky district. This section consists of two parts: the Yerunakovskaya—the lower—and the Tailugan—the upper—compiled in different places and not having a reliable tie between them. The correlation of the sections given by us in Fig. 3 should be regarded as quite tentative, mainly pursuing the aim of showing the striking difference between the section of the Tailugan subformation in the Taldinsko-Northern area and the section at Tailugan.

D. G. Samylkin and G. P. Radchenko [2] indicate that the strata exposed between the mouth of the B. Kuksha River and the settlement of Tailugan, “in its external appearance, lithological composition, the presence of thin interbeds of hard coal and carbonaceous shale, and the abundance of finds here of the Pelecypoda fauna, is very similar to the lower parts of the Yerunakovskaya subformation.” Therefore, we have some doubt as to the correctness of the compara-

Table 2

Table of results of chemical-petrographic analysis of samples from boreholes Nos. 682 and 687

(Continuation of Table 2)

(Continuation of Table 2)

Fig. 3. Correlation scheme of the section of the Yerunakovskaya Formation in the North Taldinskaya area (I) with the section along the bank of the Tom River near the settlement of Taytugan (II).

Annotations in the figure:

• I — North Taldinskaya area.
• Upper seam.
• Taldinskaya Subformation, \(P_2^{t}\).
• Yerunakovskaya Formation, \((P_2^{er})\).
• Gramoteinskaya Subformation, \(P_2^{gr}\).
• Leninskaya Subformation, \(P_2^{len}\).
• Ilyinskaya Formation.

• Uskatskaya Subformation, \(P_2^{usk}\).

• II — Taytugan section.

• Taytugan section (after G. R. Radchenko).
• Northern limb of the Narykskaya anticline.
• Southeastern limb of the Taldinskaya syncline.
• Section between the Barzas and Kuksha rivers.
• Yerunakovskii section (after G. P. Radchenko and V. I. Yavorskii).

Legend:

1 — sandstone;
2 — clayey sandstone;
3 — siltstone;
4 — tuffaceous siltstone;
5 — argillite;
6 — interbedding of siltstones and sandstones;
7 — interbedding of tuffaceous siltstones and sandstones;
8 — carbonaceous argillite;
9 — coal.

... of the low-coal-bearing Tailugan section with the section along the Taldinskaya line, which has high coal-bearing capacity¹.

With such a parallelization of these sections it is necessary to assume a very sharp decrease in coal-bearing capacity eastward: over a distance of 26 km, the coal-bearing capacity of the upper part of the formation (the Tailugan subformation) changed from 12.2% in the Taldinsky area to 2.4% in the Tailugan section.

In accordance with the scheme for correlating the sections adopted by us (Fig. 3), and also taking into account the sections of the lower part of the Yerunakovskaya and the upper part of the Ilyinskaya formations in deposits located south of the Taldinsky area, the total thickness of the Yerunakovskaya Formation is estimated at 2000–2100 m, and the thickness of the continuous productive section of the Kolchugino Series, including the tops of the Ilyinskaya Formation, for the western part of the Yerunakovsky district is 2650–2800 m. In the eastern part of the district the thickness of the productive section will be smaller because of the loss of workable thickness by the group of seams of the Uskat subformation.

The Nizhnemaltsevskaya Formation ($T_1^m$) in its lower part (153 m thick) was penetrated by two boreholes (682 and 687) in the eastern part of the North Taldinskaya line. It includes thick beds of dirty greenish-gray and greenish-black siltstones and units of interbedded sandstones and siltstones.

In these rocks, which have a clearly tuffogenic character, shell-like parting is distinctly expressed. All the rocks are carbonate to varying degrees. In the tuffogenic rocks there are numerous inclusions of pink zeolite and calcite in the form of elongated crystals. In the clastic material the principal component is quartz. Coaly particles are present in considerable quantity. Fragments of fine-grained siliceous and calcareous rocks occur, as do fragments of siltstones and argillites, probably derived from the underlying rocks. The results of chemical-petrographic study of rock samples from the Nizhnemaltsevskaya and Yerunakovskaya formations are given in Table 2.

Triassic rocks in the section of the North Taldinskaya exploration line rest on the underlying rocks of the Yerunakovskaya Formation with a break, but without visible angular unconformity. The boundary between the Triassic and the Paleozoic in the line section is established distinctly by the change in lithological features; it is drawn along a bed of bleached, loose, and weakly cemented rocks, resembling in outward appearance a weathering crust. This bed is noted in the extreme eastern boreholes (682 and 687), three meters stratigraphically above the Verkhny seam, and is a marker horizon.

The weathering crust noted in the core from the indicated boreholes testifies to the presence of a break in sediment accumulation between the Paleozoic and the Triassic. Judging from the small thickness of the bleached bed, it may be assumed that the break was rather short-lived; this, however, requires further clarification.

The correctness of drawing the boundary at the indicated place is confirmed by paleontological data: 13 m above the Verkhnii seam, in a bed of interbedded rocks, a fern flora was encountered that differs sharply from the Paleozoic flora of the Kolchugino Series. According to O. A. Betekhtina, this flora has a typically Mesozoic character. And seven meters below the Verkhnii seam, in a bed of siltstone, cordaites, ferns of the type Pecopteris anthriscifolia, and other Kolchugino flora were encountered.

Thus, according to the paleontological data, the boundary between the Permian and Triassic lies within the narrow limits of a stratigraphic interval 20 m thick. The marker horizon of bleached weathering products has been accepted by us as this boundary.

CITED LITERATURE

1. Questions of the Geology of Kuzbass. Materials of the Second Conference on the Stratigraphy of Coal-Bearing Deposits. Ugletekhizdat, Moscow, 1956.
2. Samylkin D. G. and Pachinko G. N. Yerunakovskii District. Geology of the USSR, vol. XVI (Kuznetsk Basin), 1940.
3. Fomichev V. D. Essays on the Geology of Siberia, issue II, Kuznetsk Coal Basin. 1940.

Trust “Kuzbassuglegeologiya.”

1. The pelecypod fauna from the Tailugan section, studied by L. L. Khalfina, and from the upper part of the North Taldinskaya section, studied by O. A. Betekhtina, possesses one important feature distinguishing it from the fauna of the lower and middle parts of the Yerunakovskaya Formation, namely: the diminution of representatives of the most important group of the Kolchugino fauna—the genus Anthraconauta Pruvost.

   This can apparently be explained by the fact that in the final stages of its existence, before complete extinction, this Permian fauna was in a state of decline. The correlation of the Tailugan section with the upper part of the North Taldinskaya section (from the boundary with the Triassic to seam 20) is substantiated paleontologically (see the article by O. A. Betekhtina and Yu. P. Kazansky published in the present volume). (Editor’s note.) ↩