ON THE AMYL, SYUTKHOL, AND ISHKIN FORMATIONS OF WESTERN TUVA

(Anyyak-Orug River Basin)

V. P. Korobeinikov

(Presented by Professor A. G. Sivov)

The region of the upper course of the Anyyak-Orug River in Western Tuva is perhaps not the only place where normal stratigraphic relationships are clearly observed between the Lower Cambrian volcanogenic formations and the barren sequences of metamorphic and metamorphosed rocks on the left bank of the Khemchik River. These sequences are widely known under the name of the Ust-Ushkin series of the Upper Cambrian [4], which unites the Syutkhol and Ishkin formations, distinguished by G. M. Vladimirsky [3], and the analogues of the Alasut formation of V. A. Blagoravov [1]. A brief description of the section and structure of this area is given by L. P. Zonenshain [5]. However, L. P. Zonenshain uses the section of the upper course of the Anyyak-Orug River to substantiate the stratigraphic position of the Amyl formation, which, according to the views of the geologists of the West Tuva Geological-Prospecting Trust, occupies a position in the section between the volcanogenic formations of the Lower Cambrian and the deposits of the Ust-Ushkin series of the Upper Cambrian. Thus, in the opinion of its authors, the Amyl formation has a provisionally Middle Cambrian age and plays the role of a connecting link in a continuous section. In 1960, however, detailed surveying work by Yu. M. Kolletanov, G. N. Spiridonov [8], and other researchers showed that in the area of the stratotype of the Amyl formation on Mount Pobeda in the upper reaches of the Amyl River there are developed metamorphic schists of the Dzhebash formation of the Upper Proterozoic. There are no objective data for distinguishing these formations as a new Middle Cambrian Amyl formation.

In connection with this, the region of the upper course of the Anyyak-Orug River is all the more interesting, since, according to L. P. Zonenshain, “under favorable structural conditions (on a gentle periclinal closure), it is possible to trace a continuous section from the lower volcanogenic-schist sequence to the Alasut formation of the Upper Cambrian” [5, p. 21].

According to the author’s observations, the geological structure of the area is a tectonic wedge in which, however, the periclinal closure of an anticline has been preserved; the core of this anticline is composed of volcanogenic formations, in accordance with overlying metamorphosed terrigenous sediments. The deposits underlying the volcanogenic formations do not crop out at the surface (Fig. 1).

In the composite section of the area the following take part (from bottom to top):

1. Dark-green, dark-gray to black hornfelsed amygdaloidal basalt porphyrites with albitized plagiocla-

... in the apodiabase main mass. In some exposures a weakly expressed pillow-like jointing is observed. In the upper part of the member there are thin interbeds of green-violet bedded tuffs and dark-gray to black microquartzites. The visible thickness is more than 500 m.

1. Lilac-gray porous or amygdaloidal, silicified and hornfelsed plagioclase porphyrites with secondary albite in the form of isolated phenocrysts. Thin interbeds of tuffaceous rocks are observed. Thickness 300 m.

Fig. 1. Geological structure of the upper course of the Anyyak-Orug River. 1 — volcanogenic unit \(Cm_1\) (members 1–4); 2–3 — Syutkhol (?) and Ishkin formations, undivided \((Cm_1)\): 2 — metamorphosed siltstones and sandstones with interbeds of shales (member 5), 3 — rhythmically bedded metamorphosed sandstones and siltstones (member 6); 4 — Alasug Formation \((Cm_{2-3})\); 5 — a) normal boundaries, b) disjunctives; 6 — bedding elements.

1. Green mottled plagioclase porphyrites and their tuffs. Thickness 300 m.

2. Dark-violet amygdaloidal effusives with amygdules filled with chlorite; dark-green hornfelsed diabases and diabase porphyrites with abundant pyrite disseminations, tuffs of diabase porphyrites, thin-bedded tuffaceous rocks, thin interbeds of siliceous shales. Thickness 200 m.

3. Grayish-violet and dirty-green metamorphosed platy siltstones, silty sandstones, and fine-grained sandstones with thin interbeds of siliceous-hematite and quartz-chlorite-carbonate shales. Thickness 400 m.

4. Green rhythmically bedded metamorphosed epidote-feldspar-carbonate, quartz-chlorite-epidote-carbonate sandstones and siltstones with wavy-curved bedding surfaces. Thickness 400–500 m.

5. Dirty-green, gray-green, and lettuce-green bedded and thin-bedded weakly metamorphosed siltstones, fine- and medium-grained polymictic sandstones. Thickness 300 m.

6. Bluish-green massive fine-grained quartz-feldspar sandstones, grayish-violet siltstones, violet and green phyllites. Thickness 500 m.

1. Green and lilac rhythmically alternating sandstones and siltstones. Thickness 200 m.

2. Green and lilac rhythmically alternating conglomerates, sandstones, and siltstones. The conglomerates are medium- to coarse-pebble; the cement is sandy; the pebbles consist of sandstones, quartzites, red jaspers, quartz, marbleized limestones, tuffs, and granitoids. Thickness 800 m.

The effusives and tuffs of units 1–4 are assigned by the author to the volcanogenic sequence of the Lower Cambrian. A consequence of the active influence on these formations of the Shapshal pluton of Upper Cambrian granitoid intrusion is a very intensely manifested contact metamorphism. The contact-metamorphic alteration of the rocks of the volcanogenic sequence is expressed in their silicification and hornfelsing. In part they have lost their primary composition and structure. Nevertheless, as is evident from the brief description of the section, the volcanogenic sequence, in its composition, is very close to the spilite-diabase formations of the Kurtushibin Range, which are stratigraphic analogues of the Lower Monok Formation of the northern slope of the Western Sayan. On the other hand, as L. P. Zonenshain [5] rightly notes, the volcanogenic sequence of the Anyyak-Orug differs substantially from the Lower Cambrian deposits of the ophiolite belts that make up the Chingin Formation. These differences are expressed in the character of folding, in the absence, within the volcanogenic sequence of the Anyyak-Orug, of thick horizons of quartzites and siliceous shales, and in the absence of hyperbasites in the area of development of the rocks of this sequence. Thus, the volcanogenic sequence of the lower part of the Anyyak-Orug section is correlated with the Lower Monok Formation of the Western Sayan and is provisionally placed at the level of the Bagrad horizon according to the unified scheme of 1965.

The deposits of units 5–6 overlie the volcanogenic sequence in apparent conformity and are subdivided, according to L. P. Zonenshain, into analogues of the Amyl Formation of the Lower–Middle Cambrian and the undifferentiated deposits of the Syutkhol and Ishkin formations of the Upper Cambrian. We shall return to the substantiation of the stratigraphic position of these deposits somewhat later, after considering the overlying beds.

The latter comprise units 7–10 of the section presented and lie, in complete conformity, with gradual transitions, on the underlying formations, differing from them by a lesser degree of metamorphism and by the clearly expressed rhythmic structure of their upper part (units 9–10). Drawing the boundary between this complex and the underlying rocks appears somewhat difficult. In particular, this concerns the siltstones and sandstones of unit 7, since it is obvious that, with such relationships among monotonous rocks that are inconspicuous in appearance and composition, any drawing of boundaries is to a considerable extent arbitrary.

On the basis of the results of mapping work by T. S. Gosteva and of personal observations, L. P. Zonenshain assigns the deposits corresponding to units 7–10 of the section presented by the author to the Alasyg Formation of the Upper Cambrian. Since this name, as applied to the upper part of the complex of flyschoid sediments on the left bank of the Khemchik River, has already become established in the geological literature and in map legends, one may agree with L. P. Zonenshain and apply this name to the deposits of the upper part of the Anyyak-Orug section. At the same time it must be borne in mind that the Alasyg Formation in Western Tuva and in the Western Sayan is distinguished only on the basis of the lithological similarity of the corresponding deposits to the sedimentary complex of the Alasyg Formation proper of V. A. Blagonravov [1], which he distinguished in Eastern Tuva from the com-

...of the Khemterek Formation of A. G. Sivov [10]. A number of circumstances, which we set out earlier [9], as well as certain considerations of a structural-tectonic nature, compel us to assign the lower part of the Alasu Formation to the Middle Cambrian, and to regard the entire formation as Middle–Upper Cambrian and to correlate it, following V. A. Blagonravov [1], with the Arbat Formation of the northern slope of the Western Sayan. Thus, considering, on the basis of lithological similarity, the siltstones, sandstones, and conglomerates of units 7–10 of the Anyyak-Orug section to be analogues of the Alasu Formation, we provisionally accept a Middle Cambrian age for these deposits.

Returning to the underlying formations of units 5–6, it should be emphasized that, first, they occupy a position between the volcanogenic sequence of the lower Lower Cambrian and the lower beds of the Alasu Formation of Western Tuva, of provisionally Middle–Upper Cambrian age; secondly, they have a thickness of no more than 1000 m¹ and constitute, in essence, a homogeneous sequence that can only with great difficulty be provisionally subdivided by lithological features into two parts: a lower one—metamorphosed platy sandstones and siltstones with interbeds of green metamorphic schists, and an upper one—somewhat more weakly metamorphosed rhythmically bedded sandstones and siltstones. According to the views of N. N. Kheraskov and L. P. Zonenshain [5], the Amyl Formation “has a very distinctive composition,” which is defined by quartz-sericite, quartz-chlorite-carbonate, quartz-chlorite-albite, chlorite-epidote-actinolite metamorphic schists, siliceous-chlorite schists, porphyroids, and microquartzites, usually with a thickness of not less than 3000 m. From this brief characterization alone it is quite obvious that formations of similar composition and thickness have no place in the Anyyak-Orug section. If, moreover, one takes into account that, according to the ideas of the geologists of VAGT, between the Amyl Formation of the Middle Cambrian and the Alasu Formation of the Upper Cambrian, in the continuous section of the Lower Paleozoic of the Western Sayan, there occur deposits of the Syutkhol and Ishkin formations, each of which is characterized by very considerable thicknesses, then the impossibility of distinguishing all these three subdivisions in the continuous section of the Anyyak-Orug River basin will raise no doubt.

From the author’s point of view, in this section, which may be regarded as one of the reference sections for the stratification of the unfossiliferous flyschoid sequences of Western Tuva, above the volcanogenic sequence and, correspondingly, below the analogues of the Alasu Formation, a sequence of metamorphosed sandstones and siltstones should be distinguished which, paying tribute to tradition, may be regarded as the undivided Syutkhol and Ishkin formations. The age of these deposits will probably be determined within the limits of the Sanashtygol and Oruchey horizons of the Lower Cambrian, since they lie conformably upon the most probable analogues of the Nizhnemonok Formation and are conformably overlain by the presumed analogues of the Arbat Formation of the northern part of the Western Sayan. Thus, in terms of age, these deposits most closely correspond to the Verkhnemonok Formation of the North Sayan zone.

The very low degree of metamorphism of the deposits of the lower part of this sequence and its insignificant thickness as a whole, in comparison with the sections of the Ustyushkin Series in the basins of the Aldy-Ishkin—Usty-Ishkin rivers, compel one to raise the question of the validity of distinguishing the Syutkhol

formation as part of the Cambrian system, below the deposits of the Ishkin Formation, which, according to G. M. Vladimirsky [3], conformably overlies the Syutkhol Formation. Researchers’ attention has repeatedly been drawn to the absolute-age figure of 1115 million years, obtained by the argon method for schists of the Syutkhol Formation in the area of its typical development [6]. Relying chiefly on this determination, T. N. Ivanova [7], V. V. Velinsky and A. P. Shcheglov [2], and M. A. Chernomorsky²) are inclined to regard the deposits of the Syutkhol Formation as Precambrian.

From our point of view, within the deposits currently distinguished as the Syutkhol Formation, deposits of different ages are artificially combined: some of them (for example, in the Anyyak-Orug River basin) are of Lower Cambrian age, while another part consists of analogues of the Dzhebash Formation of the Upper Proterozoic. It should be stated at once that everything said concerning the stratigraphic position of the Ishkin and Syutkhol formations applies only to the deposits of the left bank of the Khemchik River. As for the metamorphic schists developed along the northern part of the Kurtushibin Range and distinguished by geologists of the WAGT as the Syutkhol and Ishkin formations, we believe that they are Upper Proterozoic and form the smaller southern branch of the Dzhebash uplift [9].

Thus, it may be supposed that in the southwestern part of the Western Sayan, identified by us as the Aksug trough [9], there existed a zone (the Anyyak-Orug River basin) in which sedimentation proceeded continuously throughout the entire Cambrian period. This zone may be regarded as an area of stable compensated downwarping, with manifestations of submarine volcanism at the beginning of the Lower Cambrian and terrigenous flyschoid sedimentation that took place from the middle of the Lower Cambrian to the end of the Upper Cambrian, and possibly also the beginning of the Ordovician. In the eastern part of the Aksug trough, Lower Cambrian volcanogenic formations are absent, which is apparently due to the predominance of upward movements in this area in the Early Cambrian. However, from the second half of the Lower Cambrian, downwarping also occurred here, and a single marine basin existed on the territory of the Aksug trough; its final closure took place in post-Alasug time.

REFERENCES

1. V. A. Blagonravov. On the Alasug and Sistigkhem formations of the basins of the Sistig-Khem, Uzy, Chavash, and Ala-Suga rivers. Transactions of WAGT, issue 5, 1959.

2. V. V. Velinsky, A. P. Shcheglov. Volcanogenic-sedimentary formations of the Western Sayan. Geology and Geophysics, No. 9, 1965.

3. G. M. Vladimirsky. New data on the stratigraphy of the Paleozoic of the western part of the southern slope of the Western Sayan. Information Collection of VSEGEI, No. 21, 1959.

4. G. M. Vladimirsky, M. A. Chernomorsky. Folded structures of the Upper Cambrian (?) deposits of the left bank of the Khemchik River in Western Tuva. Transactions of VSEGEI, new series, vol. 58, 1961.

5. L. P. Zonenshain. Tectonics of the Western Sayan. Gosgeoltekhizdat, 1963.

6. T. N. Ivanova, N. I. Polevaya, G. M. Vladimirsky, E. F. Dorofeeva, D. M. Orlov, E. K. Stankevich, V. A. Unksov. Absolute age of some magmatic and metamorphic rocks of the central part of the Altai-Sayan region. Transactions of VSEGEI, new series, vol. 58, 1961.

7. T. N. Ivanova. Structural-facies zoning of Tuva for the epoch of the Early and beginning of the Middle Cambrian. Transactions of VSEGEI, new series, vol. 58, 1961.

8. Yu. M. Kolleganov, G. N. Spiridonov. New data on magmatism, stratigraphy, and tectonics of the northeastern part of the Western Sayan. Materials of the NTO conference, city of Minusinsk, 1964.

9. V. P. Korobeinikov, V. M. Isakov. On the tectonics and stratigraphy of the metamorphic complexes of the Precambrian and Cambrian of the Western Sayan. Transactions of SNIIGGiMS, issue 34, 1965.

10. A. G. Sivov. On the “Lower Silurian” of the Western Sayan. Proceedings of the Tomsk Industrial Institute, vol. 62, issue 1, 1944.

²) Personal communication.

1. In the southeastern part of the area (Fig. 1) the visible thickness of the corresponding deposits reaches a value on the order of 1500–1800 m. However, the structure of these beds is not entirely clear, and their true thickness may prove to be considerably less (author’s note). ↩