ON THE CAMBRIAN STRATIGRAPHY OF THE MOUNT KOSHKULAK AREA

(Right Bank of the Bely Iyus River, Kuznetsk Alatau)

V. E. NOMOKONOV

(Presented by Prof. Dr. A. G. Sivov)

The area we investigated is situated on the eastern slope of the Kuznetsk Alatau Range, south of the boundary of the latter with the Chebakovo depression of the Minusinsk intermontane basin, on the right bank of the Bely Iyus River, in the basins of the Tyurim, Kolodzhul, Aspad, and Kulbyurstug streams.

Despite its comparatively easy accessibility and good exposure, in our opinion the study of the geological structure of this part of the Kuznetsk Alatau has undeservedly not received proper attention up to the present. Yet it is precisely here that the most complete section of the ancient strata of this region has been preserved, and their stratigraphic relationships are well established.

The first relatively complete account of the geological structure of the right bank of the Bely Iyus River was given by Ya. S. Edelshtein as a result of work carried out here in 1907–1912. In a later period the area was studied and visited by many geologists: A. V. Rudneva—1943–1945, M. P. Kortusov—1947–1950, A. L. Dodin—1953, E. D. Sulidi-Kondratyev—1954–1955, V. V. Khomentovsky—1956, V. M. Yaroshevich—1953–1955, D. I. Musatov—1958, and others.

However, the investigators just named have no single opinion either on the geological structure as a whole or on the stratigraphy of the ancient strata of the area under study. Thus A. L. Dodin and D. I. Musatov assign the carbonate sequence exposed in the headwaters of the Tyurim and Tarzhul streams to the Usinsk Formation. At the same time, D. I. Musatov considers it older than the carbonate–effusive–terrigenous sequence that is exposed in the middle course of the Kulbyurstug River (the Syiskaya Formation of D. I. Musatov). A. L. Dodin correlates the same carbonate–effusive–terrigenous sequence with the Belsin Formation and, consequently, recognizes its higher stratigraphic position in comparison with the Usinsk. Following A. L. Dodin, E. D. Sulidi-Kondratyev also distinguishes the carbonate sequence exposed along the Tarzhul stream under the name of the Usinsk Formation. E. D. Sulidi-Kondratyev correlates the carbonate–effusive–terrigenous sequence with A. L. Dodin’s Kondom Formation, while limiting the area of distribution of the latter to the valley of the Kulbyurstug stream. In addition, E. D. Sulidi-Kondratyev distinguishes the Kanym Formation of the Middle Cambrian, into which, according to our data, he combines an entire complex of formations.

The factual material collected by us in 1960–1961 agrees with none of the existing stratigraphic schemes, either in scope or in content.

The geological structure of the area, according to our materials, may be presented as follows. The southern part of the area is composed of granitoid rocks, combined by M. P. Kortusov into the Belyo-Iyus intrusive complex. The northern boundary of their distribution extends in a sublatitudinal direction and passes near the watershed line of the Aspada and Tyurim streams. Immediately north of this line, carbonate, carbonate-terrigenous, and volcanogenic sequences are exposed, among which we distinguish (from bottom to top): the Tarzhul, Syya, Kolodzhul, Malokoshkulak, and Koshkulak suites. Beyond the limits of the area under consideration, in addition to those indicated, a number of other suites of the Cambrian system are also developed.

The oldest formations of the district are probably the deposits of the Tarzhul suite, exposed along the Tarzhul River and the Kulbyurstog and Tyurim streams in their upper courses. Characteristic of the sequence is the predominance in its composition of light-colored bedded dolomites, the presence of stromatolitic algae, and a distinctive, rather poor, complex of accessory elements. The thickness of the suite is more than 5 km. On the basis of the study of algae, the age of the deposits of the suite is determined as Late Precambrian. We provisionally correlate the Tarzhul suite with the known Yenisei suite, despite the absence of a complete analogy in lithological composition.

In the middle courses of the Tyurim and Kulbyurstog streams there is exposed a sequence represented by dark anthraconite marbles with interbeds of lydite, diabase porphyrites, tuffaceous sandstones, tuff conglomerates, graywackes, sandstones, and shales. The rocks of the sequence have undergone deep metamorphism, are strongly schistose, are crumpled into tight folds of NE trend, and are cut by linear bodies of titaniferous gabbro. Along strike the sequence can be traced in a SE direction to the settlement of Malaya Syya, where its section was first described in detail by V. M. Yaroshevich (1954), who included this sequence in the Syya series. Later D. I. Musatov described these same deposits under the name of the Syya suite.

The age of the deposits of the Syya suite was dated by E. D. Sulidi-Kondrat'ev and D. I. Musatov as Lower Cambrian on the basis of trilobite fauna, first collected by V. V. Khomentovskii (1956), and then by a group of geologists headed by D. I. Musatov (1958), at one and the same place on the right bank of the Bely Iyus River. According to our data, the platy limestones in which the fauna was collected by the above-mentioned investigators do not belong to the sequence being described; consequently, its age was determined incorrectly. Nevertheless, we consider it advisable to retain for it the name Syya suite, taking into account that the stratotype is located in the area of the settlement of Malaya Syya, where it was first described.

In 1961, on the left side of the valley of the Tyurim River, in gray fine-grained marbles, we found formations resembling archaeocyaths, which A. G. Pospelov identified as archaeocyaths of the Lower Cambrian. If A. G. Pospelov’s determinations are correct, the Syya suite should occupy a higher stratigraphic position in comparison with the Tarzhul suite and may be provisionally correlated with the known Kondoma suite, distinguished by a number of investigators of the Kuznetsk Alatau. However, we note that there are no direct mutual relations between the Tarzhul and Syya suites in the area we studied, while the more intense tectonic structure of the rocks of the Syya suite, the presence of concordant bodies of gabbro, which are not encountered in other sequences, and the deeper metamorphism of the rocks make one question the determination of the age of the sequence from the fossils,

made by A. G. Pospelov. It is possible that the Syiskaya Formation will prove to be more ancient in comparison with the Tarzhulskaya*.

The Kolodzhulskaya Formation was first distinguished by us in 1961. The rocks of the formation are exposed in the upper reaches of the Tyurim and Kolodzhul rivers on the western and southwestern slopes of Mt. Koshkulak. The formation lies on the eroded surface of the dolomites of the Tarzhulskaya Formation and is represented (from bottom to top) by brown and reddish-brown effusive rocks of basic composition with all the features characteristic of subaerially erupted rocks, gray and dark-gray interbedded limestones, calcareous sandstones, and sandy limestones. Characteristic of the carbonate-sandstone Kolodzhulskaya Formation is the presence of a large number of accessory elements, in contrast to the dolomites of the Tarzhulskaya Formation. The thickness of the formation is 500–600 m. In the carbonate members of the formation, throughout its entire thickness, we found trilobites: Bulaiaspis cf. limbata Repina, Bulaiaspis cf. sajanica Repina, Kolbinella cf. prima Repina, Binodaspis sp., Inouyina sp., Bigotina sp. nov., Tungusella cf. manica Repina, Lenaspis sp., Rondocephalus sp., Kolbinella sp. nov., forms close to Solenopleurella sp., Kadyella? sp., and others, oncolites, academnolites, and brachiopods (determinations by I. I. Koptev).

In terms of the fossil assemblage contained in the Kolodzhulskaya Formation, it is well correlated with the Kolbinskaya Series of the Eastern Sayan and with formations of the Tolbachan–Olyokma time in sections of the Stanovaya part of the Siberian Platform.

The Malokoshkulakskaya Formation was first distinguished by us in 1960. The rocks of the formation are exposed on the western and southwestern slopes of Mt. Koshkulak and rest unconformably directly on the Kolodzhulskaya Formation. At its base are basal conglomerates containing pebbles of effusive rocks, sandstones, and limestones with trilobites of the Kolodzhulskaya Formation. Above them occur dark bituminous limestones, sandy shales, gravelites and fine-pebble conglomerates, graywackes, and platy black limestones. The section is crowned by white, light-gray massive cavernous limestones. The above-mentioned conglomerates and gravelites divide the formation into two parts, for which sharply distinct faunal assemblages are characteristic. This gives us grounds to divide the formation at least into two subformations, or even independent formations: a lower one (up to the gravelites) and an upper one. In the lower subformation, whose visible thickness is 110–130 m, we found trilobites: Hebediscus cf. lermontovae Rep., H. cf. tumidus Koptev (MS), Neocobboldia sp., Boiomella (Sajanella) polet (Lerm.), Chondragraulos minussensis Lerm., Kooteniella acuta N. Tchern., Proerbia cf. torgaschinica Rep., Pagetia sp., Chondranomocare planum Roman. and several new genera. On this basis we consider it possible to assign the deposits of the lower subformation to the upper part of the Lena stage. In the carbonate rocks of the upper subformation (formation), whose visible thickness is 250–300 m, we collected a fossil assemblage; as a result of its study, I. I. Koptev determined here the trilobites: Kooteniella slatkovskii (Schm.), Chondranomocare bidjensis Poletaeva, Chondragraulos minussensis Lerm., Erbia sibirica (Schm.), Erbia granulosa Lerm., Glabrella cf. babakoviensis Repina, Ehmaniella sp., Gaphuraspis kalievi Ivsch., Granularia obrutchevi Polet., Anabaraspis sp., Pagetia schoriensis Poletaeva, Kootenia sp., Chondragraulos sp., and others; brachiopods: Nisusia sp., Kutorgina sp.

* In 1963, in deposits of the Syiskaya Formation on the right side of the valley of the Tyurim River and in the area of the “Kommunar” mine, we found algae Newlandia Walcott, indicating its more ancient age relative to the Tartulskaya Formation.

On the basis of the trilobite-fauna assemblage, the deposits of the upper subformation are correlated by us with the Middle Cambrian Systyk-Zhul’ Formation of V. M. Yaroshevich. Its age is confidently determined as the lower part of $Cm_2$.

The Koshkulak Formation lies on the eroded surface of the Malokoshkulak Formation and is exposed in the headwaters of the Tyurim, Glukhaya, and Kolodzhula streams, occupying a comparatively large area. At the base of the sequence there is a thin horizon of conglomerates with pebbles of effusives and limestones, which higher up are overlain by agglomeratic lavas and andesitic porphyrites of lilac and reddish-brown colors. More rarely, thin interbeds and lenses of red-colored sandstones and siltstones are observed within the sequence. No fauna or flora has been found. The distinctive composition of the effusive Koshkulak Formation was noted earlier by M. P. Kortusov, who gave it this name. We tentatively regard the age of the Koshkulak Formation as Middle Cambrian, following other investigators, and correlate it with the Berikul’ Formation of the Kuznetsk Alatau.

Thus, detailed study of the area has made it possible to refine the geological structure of the region and to introduce substantial corrections into the stratigraphic schemes proposed by previous investigators.

The carbonate deposits, which most investigators combined into the Usa Formation, we were able to subdivide into at least three formations: the Tarzhul’, Kolodzhul’, and Malokoshkulak formations, separated by tectono-denudational breaks. The presence of tectono-denudational breaks has been established by us on the basis of study of the contact surfaces themselves, the conditions of occurrence of the sequences in the contact zone, the composition of the conglomerates, and the sharp difference in the geochemical features of the formations and their composition.

The discovery and study of paleontological remains contained in the rocks of the sequences made it possible to refine their age and to determine their stratigraphic position more correctly.

Of interest is the fact of the co-occurrence at one locality (locality 8134, Malokoshkulak Formation, lower subformation) of trilobites considered characteristic of the lower part of $Cm_1$ (Aldan Stage or Bazaikha Horizon), on the one hand, and trilobites considered characteristic of beds transitional from $Cm_1$ to $Cm_2$, on the other hand. Thus, in one sample I. I. Koptev identified the trilobites: Hebediscus cf. lermontovae Repina, Botomella poletaevae (Lerm.) (Sajanella), Hebediscus cf. tumidus Koptev (MS), Neocobboldia? sp.—lower part of $Cm_1$; Chondragraulos minussensis Lerm., Kooteniella acuta N. Tchern., Pagetia sp., Chondranomocare sp.—upper part of $Cm_1$.

The presence of breaks in sedimentation during Cambrian time refutes the opinions of some investigators of the Kuznetsk Alatau (V. V. Khomentovskii, V. M. Yaroshevich) regarding the conformable occurrence of Cambrian formations one upon another and an uninterrupted process of sedimentation in Cambrian time in this region. Unfortunately, these ideas have been reflected in the approved unified stratigraphic scheme, which can hardly contribute to the successful execution of geological work in the territory of the Kuznetsk Alatau.

The factual material obtained by us and presented here far from fully compels us to raise the question of the need to develop new and more refined stratigraphic schemes.

REFERENCES

1. Dodin A. L. Geological structure and principal features of the metallogeny of iron of Gornaya Shoriya and the Kuznetsk Alatau. In the collection Iron-Ore Deposits of the Altai-Sayan Mountain Region, book 2, Moscow, 1959.

1. Musatov, D. I. Stratigraphy of the Cambrian deposits of the eastern slope of the Kuznetsk Alatau. Geology and Geophysics, No. 2, Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, 1961.

2. Khomentovsky, V. V. Formations of the structural-facies zones of southwestern Siberia and the mineral deposits associated with them. Regularities in the Distribution of Mineral Deposits, vol. III, Publishing House of the Academy of Sciences of the USSR, 1960.

3. Yaroshevich, V. M. On the structural-facies zones of the Cambrian of the eastern slope of the Kuznetsk Alatau. Reports of the Academy of Sciences of the USSR, vol. 121, No. 2, 1958.