V. A. KUZNETSOV

STRATIGRAPHIC POSITION OF THE HYPERBASITES OF TUVA AND THE WESTERN SAYAN

Clarifying the question of the age of the ultrabasic intrusions of Tuva and the Western Sayan, which are also widely developed in Gorny Altai, Salair, and the Kuznetsk Altai, is of not only great theoretical interest, but also major practical importance for refining geological criteria in organizing searches for mineral resources connected with these intrusions.

The question of the stratigraphic position of the Tuvan and Western Sayan hyperbasites is not new and already has a fairly long history.

At the first stages of investigation of both Tuva and the Western Sayan, the age of the ultrabasic intrusions was considered Caledonian, but relatively young. Researchers of the Western Sayan believed that these intrusions were emplaced during the epoch of powerful Caledonian folding that occurred at the boundary between the Upper Silurian and the Lower Devonian, i.e., that they were associated with the Erian phase of Caledonian tectogenesis. The first investigators of the geology of Tuva, in particular Z. A. Lebedeva, arrived at a similar conclusion.

Later, as a result of the 1932 investigations in Western Tuva, in which I participated, it was established that the hyperbasites of this region of Tuva are clearly older than the Chergak Formation of the lower Upper Silurian, in whose basal conglomerates abundant pebbles of already serpentinized hyperbasites were found. Consequently, the Erian age of the latter, at least for Western Tuva and the adjoining southern slope of the Western Sayan, fell away, and the Taconian epoch of tectogenesis had to be regarded as the upper age limit. At the same time, the confinement of hyperbasites exclusively to the so-called Aktovrak Formation of Cambrian age was noted, as was their complete absence among all the other strata of the region. This regular association of the hyperbasites with a greenstone effusive-sedimentary Cambrian sequence, highly characteristic in its composition, was established from the literature data for broader territories of Tuva, the Western Sayan, and the adjacent regions of northwestern Mongolia, Gorny Altai, and the Eastern Sayan. From this there followed the idea of a close connection of the hyperbasites with the Cambrian strata not only spatially but also in age, i.e., of a Cambrian or Salairian age of this intrusion, and then of the existence in southern Siberia of a powerful Altai–Sayan serpentinite belt of Cambrian age, corresponding to an ancient zone of deep faults [1, 63; 7].

In M. A. Usov’s well-known work “The Hyperbasite Formation of Western Siberia,” written in 1937 [9, 59—77], all the material that had accumulated by that time was summarized, not only for the Western Sayan but also for the entire southern mountainous part of Western Siberia. In this work the regional distribution of hyperbasites in almost all mountain regions of the territory was emphasized, and it was established that almost all the hyperbasites of the territory belong to a single intrusive formation of Middle Cambrian age. It should be noted, incidentally, that M. A. Usov regarded the hyperbasite intrusive formation

the least studied formation of the region, requiring special monographic study [9, 70].

In subsequent years the question of the age of the Tuvan and Western Sayan hyperbasites was reconsidered anew. In her summary work on the geology of Tuva, Z. A. Lebedeva indicated that, alongside ancient serpentinites, in some areas of Tuva there are serpentinites that allegedly cut paleontologically characterized Upper Silurian deposits. As an example of such an area, she cited the area of the Otuk-Tash hills, on the left bank of the Ulu-Khem River, opposite the mouth of the Baingol River [5, 228]. Consequently, the view was expressed that in Tuva there are hyperbasite intrusions of different ages, including relatively young ones.

In the Western Sayan, on the contrary, a tendency has emerged to lower the age of this intrusion to the Lower Cambrian. A proponent of this is A. G. Sivov, who recently came to the conclusion that the West Sayan formation enclosing the hyperbasites [8], i.e., the Uzu-Naryg (A. G. Sivov, 1939) or Chinginskaya (A. G. Sivov, 1946) formation, is the oldest of the Cambrian formations of the Western Sayan and, apparently, belongs not to the Middle, as was previously thought, but to the lower division of the Cambrian system. This conclusion, not yet confirmed by indisputable paleontological material, is based chiefly on stratigraphic-tectonic analysis. Together with this formation, A. G. Sivov lowers to the Lower Cambrian the age of the Western Sayan hyperbasites as well.

Naturally, the question arises whether in the Western Sayan and in Tuva there are hyperbasite intrusions of different ages, and, consequently, the former ideas on this question were erroneous, or whether, on the contrary, only the newly stated views are erroneous. Naturally, in connection with this, during our recent investigations in Tuva and on the southern slopes of the Western Sayan I carefully collected material for one or another solution of this question. An unambiguous solution of this question has always seemed to me very important, in particular for solving certain problems of applied geology, and therefore already in a 1934 work I emphatically drew the attention of researchers to this question [1, 63]. At present this question is becoming especially acute, in view of the heightened interest being attracted by the hyperbasites of our region on the part of a number of geological organizations and a large number of investigators.

Our three-year investigations, and, taking into account my work in 1932, four-year investigations in Tuva and the southern ranges of the Western Sayan adjoining the Tuvan depression¹, have shown the following.

Massifs of hyperbasites are widely developed and are concentrated chiefly along the northwestern margin of the Tuvan depression, i.e., along the junction of the latter with the Western Sayan. All hyperbasite massifs are situated exclusively within Cambrian strata and in no case occur among younger formations. This applies, in particular, to the Otuk-Tash area as well, where, according to Z. A. Lebedeva, an intrusion of post–Upper Silurian serpentinites allegedly occurred. In fact, in this sector the serpentinites, together with their accompanying metadiorites and metasomatic silicilites, cut the Cambrian, so-called Baingol suite of graywacke sandstones and are completely absent in the nearby strata of the Upper Silurian. This area is described in greater detail in my previously published work [2].

Neither in Tuva nor in the Western Sayan is there yet known a single case of occurrence of hyperbasites in strata younger than the Middle Cambrian.

¹ During the last two years, G. V. Pinus took the most active part in them.

In Tuva we know of, and have already studied, more than two dozen hyperbasite massifs of various, chiefly small, dimensions. Almost all of them are situated exclusively within the so-called Aktovrak Formation, first distinguished by me in Western Tuva as early as 1932 [1; 7], and subsequently studied in greater detail and over broader areas in 1946–1947 [3].

In the composition of this formation, basic and intermediate porphyrites predominate in the lower parts, accompanied by tuffs, various agglomerates, and also sandstones and clay shales, with subordinate, usually small, lenticular bodies of limestone. In the upper parts of the formation, developed, in particular, on the Utuk-kaya hill, immediately east of the Aktovrak deposit area, there occurs a bedded sequence of sandstones and limestones, often marly and rich in an archeocyathid and trilobite fauna. As has already been said, the formation is intruded by a large number of small and large hyperbasite massifs and by diorites accompanying the latter, and contains numerous lenticular and irregular bodies of metasomatic quartzites.

In composition the hyperbasites are rather uniform and are represented mainly by serpentinized peridotites of the harzburgite type, as well as by pyroxenites and still more widely developed serpentinites. It has already been noted that in many cases metamorphosed, saussuritized gabbros and diorites are closely associated with the hyperbasites. Deposits of asbestos, as well as occurrences of chromite mineralization, are connected with the hyperbasites.

The hyperbasites form clearly concordant massifs of lenticular shape, subordinate to the structure of the enclosing Aktovrak Formation. At the same time, they are arranged in a regular manner as a continuous garland of bodies over a great distance, forming, in the aggregate, a clearly expressed belt. This hyperbasite belt, which we distinguish as the Tuvan hyperbasite belt and which is a large branch of the powerful Altai–Sayan belt, has already been traced by us for about 200 km. It is traced from the Khopsek locality and the area of the Aktovrak deposit in the west to the Utuk-kaya hill in the east, and then to the area of the upper course of the Chakol River and to the Karatag and Argalykty hills, on the southern slopes of which both the Aktovrak Formation and a series of small serpentinite massifs are developed. Farther on, this belt is thrown over to the right bank of the Ulu-khem River, where, between the Ilikhem and Temirsuk rivers, and also in the valley of the Idzhim River, a number of serpentinite massifs is observed. Branches of this same belt occur, on the one hand, to the south in the Otuk-tash hill, and, on the other hand, to the north in the Bura Range, where serpentinites are exposed in the middle courses of the Ilikhem and Temirsuk rivers. As a whole, the belt is arranged en échelon with respect to a similar, but still larger and still more distinctly expressed, South Sayan hyperbasite belt, traced approximately along the watershed of the Kurtyushibin Range from the valley of the Yenisei River in the west to the headwaters of the Amyl River in the northeast.

Throughout the entire extent of the Tuvan belt, the sequence enclosing the hyperbasites is, as has already been said, in almost all cases the Cambrian Aktovrak Formation. Only in the eastern part of the belt do we know hyperbasites in the so-called Baingol Formation, but the latter, in its stratigraphic position, is very close to the Aktovrak Formation, being an analogue of the upper horizons of the latter. However, it would be wrong to think that every outcrop of the Aktovrak Formation, or every sequence of similar stratigraphic position, necessarily includes hyperbasite massifs. On the contrary, for example, in the Eastern Tannu-Ola Range there is very widely developed a sequence very similar to the Aktovrak Formation, likewise consisting predominantly of effusives with subordinate sedimentary members. Judging from the fauna of archeocyathids and trilobites, it is a stratigraphic analogue of the Akto-

of the Aktovrak Formation. However, hyperbasites are practically almost absent in Eastern Tannu-Ola. This, evidently, indicates that, for the localization of the hyperbasite massifs and the formation of the Tuvan hyperbasite belt mentioned above, in addition to the presence of the Aktovrak Formation, special tectonic conditions must have existed.

It may be pointed out that, in my view, the Tuvan and South Sayan hyperbasite belts correspond to zones of ancient mobile sutures at the boundary between tectonically heterogeneous structures: the West Sayan folded zone of Caledonian age and the Tuvan median massif in the Caledonian geosynclinal region of southern Central Siberia. This question, however, is a special one that lies beyond the scope of the present topic, and is treated by me in a special work on the stratigraphy and tectonics of Central and Western Tuva.

At the same time, irrespective of questions concerning the conditions of localization of the hyperbasites, etc., it is necessary to emphasize the fact of the very close spatial association of the hyperbasites in Tuva and on the southern slopes of the Western Sayan precisely with Cambrian strata of a definite composition and stratigraphic position—an association so close that the thought involuntarily arises of the proximity in age of the hyperbasite intrusions and of these enclosing strata.

In this connection, determination of the age of these strata, and first of all of the Aktovrak Formation, is of extreme importance. In this respect our latest data, in particular the discovery of a trilobite fauna, apparently are of special interest.

The fact is that the age of the Aktovrak Formation has hitherto been determined within rather broad limits. At first, according to the works of 1932, when in the Aktovrak Formation we had discovered only calcareous algae of the type Osagia or Anomas, as they were then identified, the Aktovrak Formation was assigned to the Upper Cambrian, in accordance with the then prevailing conceptions of the exclusively carbonate composition of the Middle and Lower Cambrian deposits of Siberia [7].

Later, after the publication of the already mentioned summary work by Z. A. Lebedeva [5], data appeared in favor of the Aktovrak Formation being, apparently, older than the Upper Cambrian. The archaeocyathid fauna from the hills of Utuk-kaya, along the continuation of the Aktovrak horizons proper, was determined as Middle Cambrian. Certain other limestone outcrops of the same formation situated farther east were also referred to the Middle Cambrian. In other cases, however, from certain localities situated on the direct continuation of the same Aktovrak Formation, the archaeocyathid fauna was determined as Lower Cambrian (the Karatag and Khairkhan hills, etc.).

The view arose that here there are strata of different ages, of Lower and Middle Cambrian. The question became still more complicated and confused because doubt was expressed as to the very existence of the Aktovrak Formation of complex effusive-sedimentary composition. Z. A. Lebedeva, for example, asserted that the Cambrian in Tuva is supposedly represented only by carbonate facies; that all cases of occurrence of archaeocyathid limestones in Tuva among clastic or effusive strata are tectonic injections of a diapiric type; and that the Aktovrak Formation, described by me together with P. M. Tatarinov, is also supposedly a tectonic complex of strata of different ages and different character [5].

It is understandable that, since distrust had been expressed toward my previous observations and conclusions, in the course of the field work of the last three years I devoted special attention to the study of the Cambrian strata of Tuva, and in particular of the Aktovrak Formation. The abundant material collected in the process confirmed the indisputable existence of the Aktovrak Formation of the composition briefly described above. It should be noted that the correctness of my observations

and conclusions on the question of the composition of the Cambrian deposits in Tuva and, conversely, the erroneousness of Z. A. Lebedeva’s views were later confirmed by other investigators of Tuva, in particular by V. P. Maslov [6]. However, the question of whether, in the regions of Northwestern Tuva, there are Lower- and Middle-Cambrian strata, or whether, if the matter really concerns one and the same stratum, as it seemed to me, what its age is—whether it belongs to the middle or to the lower division of the Cambrian—still remained unclear. The question was complicated by the fact that until now we had finds only of archaeocyathid fauna, which in many cases gives insufficiently precise data for determining age and, in the opinion of many geologists, is not in general a reliable document in this respect. It seemed urgently necessary to find trilobite fauna in the Aktovrak Formation. This task appeared realistically possible after my finds, in 1945, of trilobite fauna along the Shivelik-khem River in the Eastern Tannu-Ola Range [2]. And indeed, in 1946 I found as yet indeterminable remains of trilobites in the marly limestones of Mount Utuk-kaya, and in 1947 it proved possible to find trilobite fauna in the argillites of the same Aktovrak Formation in the Shagonar district of the Tuva Basin, near the southern slope of the Western Sayan Range.

The trilobite fauna was found by me on the right bank of the Ulu-khem River, in a group of small hills situated between the valleys of the Ulu-khem and Ilikhem rivers.

In this area, in small hills dissected by ravines, the following incomplete section of part of the Aktovrak Formation was observed (from north to south): 1) a stratum of gray, brown, and lilac plagioclase porphyries. Among them are lenses of serpentinites with the usual associates of the latter in the form of small lenses of diorites and quartzites; 2) lilac plagioclase porphyries; 3) intraformational fine-pebble conglomerate, with pebbles of porphyries, more rarely of other rocks, up to 30 m thick; 4) light-gray platy limestone without fauna, occurring as a bed about 6 m thick; 5) gray argillite, replaced along the strike by fine-pebble conglomerate 0.5 m thick; 6) light olive-gray, thin-bedded argillite, with trilobite fauna, up to 10 m thick.

The entire stratum is dislocated in an ENE direction, with a steep dip to the NNW. Farther to the south, basal loose conglomerates of the Upper Silurian Chergak Formation lie sharply unconformably upon the heads of the argillites.

The trilobite fauna from this section, very interesting according to the specialists’ conclusion, has unfortunately not been studied in sufficient detail. According to preliminary determinations, representatives of the genus Protolenus sp. have been established in the fauna; this genus, as is known, is indicative of the lower division of the Cambrian system and does not occur in the Middle Cambrian. At the same time, this fauna is comparable with the trilobite fauna found by us along the Shivelik-khem River in the Eastern Tannu-Ola Range, although the latter was at the time, for a number of considerations—in particular on the basis of comparison with the known faunistic complex from the Sanashtyk-gol limestones of the Western Sayan—assigned to the lower parts of the Middle Cambrian [2]. Obviously, it now becomes necessary to reconsider the question of the stratigraphic position of the Shivelik-khem Cambrian section. It should be taken into account here that, in the composition of the archaeocyathids from this section, according to P. S. Krasnopeeva, there are forms very characteristic both of the Sanashtyk-gol limestones and, to an equal degree, of the Kameshkov horizon of the Lower Cambrian of the Eastern Sayan. Apparently, it will be more correct to assign both the Cambrian stratum from the Shivelik-khem section and the Aktovrak Formation, analogous to it in age, to the lower division of the Cambrian, and most likely to the upper parts of the lower division.^1)

^1) After the preparation of the present article, which is the content of a report delivered in the spring of 1948, information appeared that more detailed investigations by N. V. Pokrovskaya

Thus the lower age boundary of the hyperbasite intrusion in Tuva and the southern chains of the Western Sayan is also determined. Taking into account that the intrusion was emplaced into an already dislocated sequence, the most substantiated view is that the intrusion took place in the initial stages of the Salair tectogenic cycle, apparently at the boundary between the Lower and Middle Cambrian.

It should be noted that the upper age boundary of this intrusion has also now been lowered. As is known, the former upper age boundary was the Upper Silurian, the Takonian phase of tectogenesis, since pebbles of serpentinites were known from the basal conglomerates of the lower suite of the Upper Silurian of Tuva. We have now found conglomerates with pebbles of all the rocks of the Aktovrak formation, including derivatives of the intrusion, within a sequence of metamorphosed argillaceous shales and sandstones that belongs to the lower part of the Lower Silurian. Such finds were made by G. V. Pinus in the area of the settlement of Kholsek in Western Tuva in 1947. Consequently, the age limits of the hyperbasites are narrowed to the Middle–Upper Cambrian.

Thus, new factual material makes it possible to refine the views of earlier investigators, and in particular those of M. A. Usov, on the question of the age and stratigraphic position of the hyperbasites in Tuva and the Western Sayan.

As for views concerning the presence in the areas under consideration of hyperbasites of another, younger age, these ideas have as yet been confirmed by nothing. I have already noted that up to now not a single case is known of hyperbasites being found in sequences younger than Cambrian. On the contrary, all the known material agrees well with the idea that, ordinarily, all hyperbasite massifs within a single belt are of the same age [10].

It remains to touch upon the question of whether the hyperbasites that form part of the hyperbasite belts of the northern slopes of the Western Sayan are of a different age. It seems to me that this is unlikely. Analysis of the geological map of the Western Sayan, on which the hyperbasite belts stand out quite distinctly, indicates that all these belts are structurally closely connected with one another, being branches of the powerful, single Altai-Sayan belt. Regardless of how the question of the age of the Chinginskaya formation is resolved, in light of the foregoing there are no serious grounds for distinguishing a special, more ancient hyperbasite intrusion in the north of the Western Sayan.

In conclusion I note that, in the petrography laboratory of the Mining-Geological Institute of the West Siberian Branch of the Academy of Sciences of the USSR, a comprehensive study of the hyperbasites of Western Siberia has been begun and is developing, and this work is already yielding interesting results. In particular, it is becoming clear that the hyperbasite massifs not only in Tuva and the Western Sayans, but also in the Gorny Altai, in the Salair, and in the Kuznetsk Alatau, are distributed in a strictly regular manner, within definite, geologically distinct linear zones or hyperbasite belts. These belts are already quite clearly outlined on geological maps. The overwhelming majority of the hyperbasite belts of the Altai-Sayan mountain system, and the numerous hyperbasite massifs participating in their structure, are Cambrian. Besides them, in Western Siberia there apparently occur hyperbasites of another age and type, but they have neither the wide distribution nor the great practical significance that the Cambrian hyperbasites have and are acquiring, especially in recent times [4].

proved the affiliation to the Lower Cambrian both of trilobites from the described sequences of Tuva and of the fauna complex from the Sanashtykgol limestones in the Western Sayan.

References

1. Kuznetsov V. A. New data on the geology of the Western Sayan (serpentine belt of the Sayan). Bulletin of ZSGGT, 2, 1934.

2. Kuznetsov V. A. New data on the geological structure of Tuva. Izv. Academy of Sciences of the USSR, geological series, 5, 1946.

3. Kuznetsov V. A. Tectonics of Western Tuva at the junction with the Gorny Altai. Izv. Academy of Sciences of the USSR, geological series, 1, 1948.

4. Kuznetsov V. A. Hyperbasite belts of the Sayano-Altai mountain system. Doklady of the Academy of Sciences of the USSR, vol. LX, 2, 1948.

5. Lebedeva Z. A. Principal features of the geology of Tuva. Transactions of the Mongolian Commission of the Academy of Sciences of the USSR, 26, 1938.

6. Maslov V. P. On the composition of the Cambrian deposits of Central Tuva. Izv. Academy of Sciences of the USSR, geological series, 3, 1947.

7. Tatarinov P. M., Kuznetsov V. A., et al. Geological investigations in the area of the Aktovrak asbestos deposit. Transactions of TsNIGRI, issue 13, 1934.

8. Usov M. A. Phases and cycles of tectogenesis. Western Siberian Territory, 1936.

9. Usov M. A. Hyperbasite formation of Western Siberia. Collection dedicated to Academician V. A. Obruchev, publ. Academy of Sciences of the USSR, vol. II, 1939.

10. Hess G. G. Island arcs, gravity anomalies, and serpentinite intrusions (on the problem of ophiolites). Transactions of the XVII Session of the International Geological Congress, vol. II, 1939.