On the Permian–Triassic Boundary in the Western Part of the Erunakovsky District of Kuzbass

O. A. Betekhtina and Yu. P. Kazansky

The uppermost horizons of the Erunakovskaya Formation of Kuzbass, as well as the boundary with the Triassic, have not yet received sufficiently complete coverage; below we present some new materials bearing on this question. In the summer of 1956, boreholes were drilled in the western part of the Erunakovsky district along the so-called North-Talda prospecting line. This line lies several kilometers north of the village of Bolshaya Talda, running from NE to SW. The easternmost boreholes of this line (Nos. 682 and 687) pass along the very southern boundary of the Maltsevskaya syncline, and in them the boundary horizons of the Permian and Triassic are exposed. Other, more western boreholes (Nos. 681, 680, 679, 678, etc.) expose the upper horizons of the Erunakovskaya Formation with a total thickness of up to 1000 m.

The sections of several boreholes along this line were described in the field by one of the authors. In addition, core material from borehole 687, as well as a paleontological collection from a number of boreholes of the North-Talda line, were made available to us by the staff of the Prokopyevsk party of the “Kuzbassuglegeologiya” trust, to whom we express our sincere gratitude. Below is given the lithological-paleontological characterization of the upper horizons of the Erunakovskaya Formation of this area, of the lower horizons of the Triassic deposits, and of the boundary between them.

Permian deposits

The uppermost horizons of the Permian deposits, which are overlain by Triassic deposits (Fig. 1), are observed in borehole 687 (at a depth below 187 m) and in borehole 682 (at a depth of 102 m). The total thickness of the studied section of the Permian deposits is approximately 1100 m. The section encompasses a series of coal-bearing deposits from seam 1, occurring near the very gra-

Fig. 1. Correlation of sections of Permian and Triassic deposits along the North-Talda line.
1) sandstones; 2) siltstones affected by Quaternary weathering; 3) tuffaceous siltstones with interbeds of argillites; 4) sandstones; 5) calcareous montmorillonite clays; 6) conglomerates; 7) siltstones and argillites; 8) coals.

…lower boundary with the Triassic, down to bed 25, which is exposed by borehole 678 at a depth of about 900 m from bed 1.

In the upper horizons of the Erunakovskaya Formation in this area, the lithological rock types usual for this formation are widespread; they were described and classified earlier [2]. Among them are distinguished deposits of the channel facies (sandstones and even conglomerates of types P₁ and P₂), floodplain facies (siltstones P₁, P₂, and P₃), and facies of overgrowing lakes and oxbow lakes (coaly siltstones, siltstones of types C₁ and C₂).

Deposits that could be assigned to the lake facies are very rare. But considerably below the upper Permian boundary, at approximately a depth of 200 m, certain rock types appear that were not included in the above classification. Such rocks include lithological varieties represented by very fine interbedding of light-colored, more sandy material and dark-colored, more clayey material. The thickness of the laminae is negligible (fractions of a mm), more rarely reaching 1–2 mm in lenticular swellings. The forms of the laminae are either horizontal and parallel, or obliquely lenticular, cutting one another. Rocks of this type, as a rule, contain no organic remains and apparently belong to the lake facies. These rocks are widely developed in the sections of boreholes 680, 679, and 678, composing intervals 10–20 m and more in thickness. In addition, rather frequently one may encounter dark-gray or gray non-bedded siltstones, usually devoid of organic remains, likewise belonging to deposits of the lake facies and composing intervals of considerable thickness. In general, at a depth of 200–250 m below the boundary with the Triassic, a noticeable change of facies is observed in the section: lake deposits begin to be widely distributed, whereas channel and floodplain sediments occupy considerably less space than in the overlying part of the section.

The petrographic characteristics of the principal varieties of Permian rocks are as follows.

The conglomerates of the upper part of the section of coal-bearing deposits are composed of rounded pebbles from 2 to 5–7 cm in diameter. The pebbles consist of fragments of quartz, quartzites, and siliceous shales. The cement of the conglomerate is sandy–calcite–siliceous. The calcite is well recrystallized. The siliceous substance is predominantly chalcedonic.

The sandstones are light gray in color, medium- and fine-grained. In composition they belong to the quartz–siliceous type. The clasts in them are represented by quartz (29–32%), more rarely chalcedony (up to 6%) and grains of siliceous and siliceous-clayey rocks (61–67%). Feldspars (up to 4%) and hydromuscovite (up to 2%) are noted in small amounts. There is comparatively little cement (20–25%). Its composition is calcite–siliceous–clayey. The calcite is crystalline, filling pores between the fragments and not infrequently corroding them. The siliceous substance (chalcedony, more rarely quartz) encrusts the quartz fragments.

The clayey substance forming films on the surface of the clastic material consists predominantly of kaolinite, which is established by staining and by measurements of the refractive index in oriented aggregates (Nm = 1.560 ± 0.001). Siltstones of various types are rich in fine plant detritus. The silt particles are represented by grains of quartz and feldspars. Up to 20% of the rock consists of small (up to 0.05 mm) siderite concretions. The clayey substance is weakly anisotropic and is represented by hydromica with Nm about 1.573. Staining with methylene blue gives blue and light-blue colors, also characteristic of hydromicas.

Minerals of the heavy fraction, 0.25–0.01 mm, separated in bromoform from sandy rocks, are represented by angular grains of ilmenite and magnetite (33–41%), leucoxene (3–4%), short, more rarely elongated…

prisms of zircon (16–19%), angular grains of colorless and pink garnet (18–27%), short prisms and rounded fragments of bluish-brown tourmaline (5–7%), short-prismatic grains of colorless and pink apatite (5–10%), and brownish-brown sphene (1%). In individual grains there are platy garnets, rutile, greenish-yellow epidote, common hornblende, andalusite, and corundum.

Paleontological remains in the Permian deposits are represented by impressions of plants, shells of pelecypods, and estheriids.

The plant remains of the upper horizons, near the boundary with the Triassic, do not differ in any essential way from the usual Kolchugino flora. Present here are ferns of the collective species Pecopteris anthriscifolia (Goepp.) Zal., cordaites, and articulates. The preservation of the flora is usually poor; mainly fragments of cordaite leaves and horsetail stems have been encountered. Undamaged impressions are rare. Of the groups mentioned above, the cordaites are the most widespread, chiefly forms with a large, broad leaf blade (usually only part of such a leaf fits within the core). Long and narrow leaves are often encountered: their width does not exceed 2 cm. Those few leaves in which the apices have been preserved may be assigned to the species: Noeggerathiopsis lanceolata Neub., Noeggerathiopsis candalepensis Zal. (large forms), Noeggerathiopsis spatulata Gor. et Radcz.

Among the articulates there are phyllothecas with a broad stem (up to 2.5 cm) and a narrow stem (less than 1 cm). There is one impression that may be assigned to the genus Koretrophyllites Radcz. Among the phyllothecas, forms with a broad stem predominate. Impressions of Lepeophyllum sp. are significantly widespread.

As is evident from the foregoing, the uppermost horizons of the Erunakovskaya Formation do not have any characteristic representatives of the flora that would indicate such a high stratigraphic position for these deposits. Rather, on the contrary, there is observed here a certain impoverishment in the composition of the flora, expressed in its uniformity and in the relatively small quantity of plant remains. But below the boundary with the Triassic, from the level of beds 6–15, the character of the flora changes; a significant number of rather diverse representatives of a flora of Mesozoic aspect appear, which in other parts of the section of the Erunakovskaya Formation are usually almost unknown. These include Yavorskiia murgatica Radcz., Iniopteris sp. (fragments), Tychopteris cuneatum (?) Zal., Chiropteris palmilobata Zal., and some others.

A horizon with such a flora is traceable in several boreholes. In contrast to the uppermost horizons of the section, here there is a general increase in the quantity of plant remains, in particular an increase in the number of coarsely ribbed articulates of the type Paracalamites sp., cordaites, etc. The character of the distribution of the flora through the section is shown in Fig. 2, where several paleontological horizons are quite clearly distinguished (in Fig. 2 only the forms with significant distribution are given).

Impressions of fauna in the section we studied are concentrated mainly in its very upper part, in the interval from bed 3 to bed 6. In its general aspect this fauna differs rather sharply from the usual Kolchugino fauna, although almost all groups and forms characteristic of that fauna are present in it: abiellas, anthraconauts, microdontellas. The peculiarity of the fauna from the upper horizons consists in the following.

1. The overwhelming majority of representatives are small in size. The only exceptions are abiellas, which attain normal and

of rather large size and with a fairly broad distribution.

1. In the fauna, the predominant group is the anthraconautids; impressions of microdontellas, so widely distributed in other sections of the Kolchugino Series, are extremely rare here, and the microdontellas are very small.

2. The character of the anthraconautids is highly peculiar. It may be said that, within this faunal horizon, almost all representatives of the Kolchugino anthraconautids are present, but in a dwarfed, suppressed form, and new peculiar forms appear, of which the most interesting are forms of a mytilid-like aspect. Among the Kolchugino anthraconautids the following species are present:

3. Anthraconauta ex gr. pseudophillipsii Fed.

4. Anthr. pseudophillipsii var. acuta Khalf.
5. Anthr. pseudophillipsii var. subaequalis Khalf.
6. Anthraconauta cuneata Khalf.
7. Anthraconauta simplex (Khalf.).
8. Anthraconauta cf. lata (Khalf.).
9. Anthraconauta obliqua (Khalf.).
10. Anthraconauta subparallela (Khalf.).
11. Anthraconauta tchernychevi (Khalf.).
12. Anthraconauta gibbosa Bet.
13. Anthraconauta khalfini Bet.
14. Anthraconauta exilis Bet.
15. Anthraconauta kostomanovi Khalf.

All these forms are of unusually small size, only a few mm. In addition to pelecypods, estherians occur in abundance.

In addition to the horizon characterized above, another faunal horizon is observed, which, according to the party’s constructions, is situated approximately 300 meters below the first, in the interval from seam 17 to seam 20. This horizon, like the first, is repeated in two boreholes (679, 683) and has a faunal composition completely identical with the first.

Below this second horizon the fauna occurs extremely rarely and has the usual Kolchugino aspect: fragments of Microdontella subovata (Jones) of various sizes, anthraconautids of various sizes.

Thus, the upper horizons of the Erunakovskaya suite, exposed in this area, contain a fauna sufficiently characteristic and distinct from the usual Kolchugino fauna. A fauna of similar aspect (small, diverse anthraconautids) was determined by L. L. Khalfin from collections by V. I. Yavorsky from the upper horizons of the Erunakovskaya suite near Babiy Kamen. A similar fauna has also been established in the upper horizons of the Erunakovskaya suite near the village of Gramoteino. A fauna very close in character was found in some boreholes (Nos. 598, 615) drilled by the left-bank party of the West Siberian Geological Administration in the Erunakovsky district. Samples with this fauna, according to the party’s data, occur at the level of seams 72–73.

On the basis of the fauna, the upper horizons of the Erunakovskaya suite, exposed along the Severo-Taldinskaya line in boreholes 680, 681, 682, 687, should be assigned to the Tailugan Subsuite of the Erunakovskaya Suite of Kuzbass, its very uppermost member [3, p. 248]. The Tailugan Subsuite thus receives a distinct faunal characterization. Its characteristic association of pelecypods differs from the fauna of the entire remaining section of the Erunakovskaya Suite partly in species composition, but chiefly in its general aspect. The lower boundary of this subsuite, if based on the constructions adopted by the Prokopyevsk party, within the Taldinsky sector passes along seam 20.

Composite section from wells 682, 681, 680

Left stratigraphic labels:

• Subformation
• Tailuganskaya
• Gramoteinskaya

Visible bed labels: bed 1, bed 2, bed 3, bed 4, bed 5, bed 6, bed 9, bed 12, bed 13, bed 14, bed 15, bed 16, bed 17, bed 18, bed 20, bed 21, bed 22, bed 23, bed 24, bed 25.

Taxa and remains shown in the distribution chart:

• Estherians
• Small anthracosids
• Astrella concina (Jones)
• Astrella sp.
• Small Microconchus [[unclear: species name]] (Jones)
• Fragments of small pelecypods
• Fragments of large pelecypods
• Prynadae sp. with a narrow stem
• Prynadae sp. with stem more than 1 cm wide
• Paracalamites sp.
• Small cordaite leaves
• Narrow and long cordaite leaves
• Large cordaite leaves
• N. cordaense dob.
• N. cordaense (narrow forms)
• N. nuzina Radcz.
• N. spartulata Nodae et Ugo ebora
• N. coascervata Neu (MS)
• N. argadensis [[unclear: abbreviated authority]]
• Annularia ensifolia Radcz.
• Forms of Mesozoic aspect
• Pecopteris anthriscifolia (Goeppert)
• Prynadae sp. with a tuberculate stem

Fig. 2. Distribution of organic remains in deposits of the Erunakovskaya Formation along the North-Talda exploration line.

Triassic Deposits

Sediments of the Maltsevo suite were penetrated in borehole 682 in the interval 12–102 m and in borehole 687 at depths of 20–187 m (Fig. 1). The boundary between the Permian and Triassic deposits is marked by a sharp change in rock composition. Deposits of Triassic age are represented mainly by pyroclastic rocks of the type of tuffaceous siltstones, in which normally sedimentary deposits occur as interbeds (sandstones, argillites).

The tuffaceous siltstones have a greenish-gray or greenish-black color. They contain a variable amount of ash material, consisting of fragments of psammitic and aleuritic size. The volcanogenic substance is represented by altered, usually rounded glass particles, angular grains of plagioclase, and chlorite replacing dark-colored minerals. Fragments of the groundmass of effusives are found more rarely. Microlites, noted by I. V. Lebedeva [4] in rounded glass particles, were not observed in our samples.

Alteration of the ash material is usually manifested in chloritization of the dark-colored minerals and replacement of glass by clay minerals of the beidellite type. Beidellite is demonstrated in the fraction smaller than 0.001 mm by staining, by measurement of refractive indices, and by the thermal method. Methylene blue stains this fraction violet-blue, which changes to green upon addition of potassium chloride; benzidine gives a blue coloration. The precipitate of the suspension is gel-like. Measurements of refractive indices in oriented particles that break up into prismatic flakes with indistinct edges give: 1) \(Ng' = 1.559\), \(Np' = 1.547\); and 2) \(Ng' = 1.561\), \(Np' = 1.550\). The thermogram of a beidellite-bearing rock is shown in Fig. 3. Endothermic reactions at 160°, 560°, 835° and the exo-effect at 870° belong to beidellite. A small negative effect at 750° apparently belongs to an admixture of montmorillonite. Terrigenous material in the tuffaceous rocks is represented by grains of quartz, rounded fragments of effusives, and, in the heavy fraction, by ilmenite and magnetite (35–68%), short-prismatic zircon (2–10%), colorless and pink garnet (about 3%), tourmaline (up to 4%), colorless and pink apatite (3–15%), and single grains of sphene, andalusite, etc. Individual fractions are enriched in epidote (up to 13–28%), common hornblende (up to 16%), and chlorite (4–13%), which may in part have formed through alteration of volcanic material (plagioclases and dark-colored minerals) in the rock, and in part may be pyroclastic material.

Fig. 3. Differential heating curves. No. 16—tuffaceous siltstone; No. 17—marl (calcareous siltstone); No. 30—clayey fraction from limestone weathering-crust clay. All samples were taken from borehole 682.

Secondary (diagenetic) formations are not uncommon in tuffaceous siltstones; among them calcite and zeolites are especially frequent. Secondary calcite, developing along cracks and replacing the original cement of the rocks, is usually coarsely crystalline. The abundance of calcite in some portions of the tuffaceous rocks makes them marl-like. The heating curve of such a marl is shown in Fig. 2. Calcite gives the usual endothermic reaction connected with dissociation at 910°. Effects with maxima at 135°, 570°, and 650° apparently belong to a clay mineral.

Among the zeolites, gray analcime with a refractive index of 1.487±0.001 and pink heulandite are distinguished. Analcime has already been described in rocks of the Maltsevo Formation [1; 4]; heulandite is recorded for the first time. It occurs in pink, worm-like crystals 1–5 mm long and up to 1 mm across, often penetrating almost the entire rock. In thin section the mineral is colorless, sometimes pink along the edges of crystals; individuals form layered aggregates intersecting at different angles; it is biaxial, positive. The refractive indices are Ng=1.501±0.001, Nm=1.496±0.001, and Np=1.495±0.001. Ng is perpendicular to the cleavage along (010); Nm makes an angle of 81° with the face (001). The mineral has noticeable dispersion.

Among the normally sedimentary rocks, sandstones predominate. They are usually medium- and fine-grained, weakly cemented. In composition they correspond to graywackes. The composition of the fragments, which constitute up to 80% in the thin sections studied, is as follows: quartz (19–27%), feldspars (6%), fragments of siliceous rocks (20–37%), fragments of clayey, tuffaceous, and effusive rocks (27–48%), and chlorite (up to 7%).

In the heavy fraction, whose yield is insignificant, the following were encountered: ilmenite and magnetite (36–55%), leucoxene (9–10%), zircon (9–17%), short-prismatic and more rarely in elongated crystals, colorless and sometimes pink garnet (11–12%), tourmaline (2–3%), epidote (1–3%), colorless and pink apatite (6–16%), sphene (1–3%), and, in isolated grains, rutile, chlorite, staurolite, andalusite, brookite, and apatite.

The cement of the sandstones is clayey-chloritic and clayey-siliceous-calcitic. The cement structure is of the contact type. The clayey substance is recrystallized, with Ng′–Np′ about 0.015. According to staining data, the predominant clayey material here is hydromica. An admixture of beidellite is possible. Chlorite, chalcedony, and calcite are later formations and fill pores between fragments.

In the lower horizons of the Triassic deposits there is a distinctive unit of light-gray, greenish, and calcareous clays, which represents an ancient weathering crust developed on tuffaceous rocks. In the section (borehole 682) the following succession of rocks is distinguished (from below):

1. Greenish-gray, slightly lightened tuffaceous siltstone with a distinct ash structure. Fragments of volcanogenic material are represented by grains of plagioclase and chloritized dark-colored rock. They are cemented by clayey substance and calcite, the clay minerals, according to staining data, being represented by montmorillonite and beidellite (?). The heavy minerals of this horizon consist of ilmenite and magnetite (29%), zircon (3%), garnet (8%), greenish-yellow epidote (28%), colorless and pink apatite (8%), light-green hornblende (16%), and chlorite (8%).

2. White calcareous clay with a relict ash structure. The clay fraction is represented by montmorillonite with Ng′=1.532 and Ng′–Np′=0.025. The heating curve (Fig. 2) gives typical endoeffects at 145°, 700°, and 835°. The endothermic reaction at 540°, apparently—

mµ, caused by a small admixture of beidellite. The sandy-silty fraction in the rock is extremely insignificant and is represented by isolated fragments of quartz, plagioclase, chlorite, ilmenite, zircon, garnet, epidote, and apatite.

1. Above follow tuffogenic rocks.

The thickness of the entire sequence of exposed rocks reaches 6 m.

Paleontological remains in the Triassic deposits are very rare. Only in two samples were fragments of flora of poor preservation established. But even these fragments indicate a sharp difference between the plant remains below and above the Permian–Triassic boundary. In the Triassic deposits it was possible preliminarily to identify: Taeniopteris sp. and Cladophlebis sp. These are typical Mesozoic forms. Further study and collecting of flora in the Triassic deposits will make it possible to clarify the question of the age of these sediments and to give them a more distinct paleontological characterization.

Conclusions

The lithological-paleontological differences between Permian and Triassic deposits are shown in Table 1. Rocks of Triassic age differ in the composition of detrital, clayey, and authigenic material. The composition of the heavy fractions of both Permian and Triassic deposits is very similar in the quantitative ratios and morphology of the minerals and, in this case, practically does not provide reliable criteria for separating these sequences.

The uppermost horizons of the Permian deposits are characterized by a distinctive complex of pelecypods, different from all the other complexes of the Kolchugino fauna. According to preliminary data, representatives resembling marine forms of pelecypods appear here. In the Triassic deposits, flora of a typical Mesozoic aspect has been encountered.

The question of the character of the boundary between the Erunakovskaya and Maltsevskaya suites deserves attention. For the Babyev Kamen area [5; 6], in the basin of the Ters River [4], a gradual transition between them is described. In the vicinity of the village of Kyrgai, normally sedimentary Permian deposits are sharply replaced by tuffogenic rocks [1]. An analogous section is exposed in boreholes of the Severo-Taldinskaya line. Here, above bed 1, sedimentary Permian rocks border on tuffogenic siltstones with an ash structure. In addition, this boundary is emphasized by the presence, in the lower beds of the Triassic deposits, of a weathering crust of montmorillonite type. Possibly, formations of a similar type were described in the lower Triassic by Adler and coauthors [1] under the name of limestone interbeds.

The formation of the weathering crust should be associated with the favorable physical-geographical setting that existed at the beginning of the Triassic period. However, the intensity of chemical weathering apparently was insufficient to substantially alter the sedimentary rocks of Permian age. The formation of eluvium became possible after a conspicuous packet of tuffogenic rocks, more subject to processes of chemical destruction, had accumulated. It should be noted that weak bleaching of the tuffogenic rocks is also observed in higher horizons of the section of the Maltsevskaya suite.

Table 1

Lithological-paleontological characteristics of the rocks of the Erunakovskaya and Maltsevskaya suites

CITED LITERATURE

1. Adler Yu. F., Karpov N. F., Neiburg M. F., Yavorsky V. I. New materials on the Triassic of the Kuznetsk Basin. Problems of Soviet Geology, No. 10. 1936.

2. Betekhtina O. A. An attempt at a lithological description of cores. Proceedings of Tomsk State University, vol. 135. 1956.

3. “Questions of the Geology of Kuzbass,” vol. 1. Ugletekhizdat, Moscow. 1956.

4. Lebedev I. V. The Mesozoic of the Kuznetsk Basin. In: Questions of the Geology of Kuzbass, 1. Ugletekhizdat, Moscow. 1956.

5. Neiburg M. F. Coal-bearing deposits of the Mesozoic of the Kuznetsk Basin. Geology of the USSR, vol. XVI, Kuznetsk Basin. 1940.

6. Radchenko G. P. Some plant remains from the area of the Ostashkino Mountains in the Kuznetsk Basin. Materials on the Geology of the West Siberian Territory, issue 35. 1936.

Tomsk Polytechnic Institute
named after S. M. Kirov