PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 166 1969

POSITION OF THE BABAKOV LIMESTONE IN THE STRUCTURE AND SECTION OF THE CAMBRIAN STRATA OF THE WESTERN SAYAN

V. M. ISAKOV

(Presented by Professor A. G. Sivov)

The Babakov limestone is exposed along the northern face of the Dzhoi Range, along the valley of the Chazryk River.

In the first summary on the geology of the Western Sayan, I. K. Bazhenov (1934) assigns the Babakov limestone to the third Cambrian unit (the Chingin Formation of the Lower Cambrian, according to A. G. Sivov). In 1928 I. K. Bazhenov collected trilobites from the Babakov limestone, and in the same area he also found archaeocyaths, which are likewise associated with the Babakov limestone.

In 1934 O. K. Poletaeva, from her own collections and from the collections of I. K. Bazhenov, identified trilobites from the Babakov limestone and, on the basis of these identifications, considered its age to be Middle Cambrian (Poletaeva, Shatrov, 1936).

A. G. Sivov (1948, 1953) came to the conclusion that there exists an independent Babakov Formation composed of limestones. The trilobites of the Babakov limestone, collected and identified by A. G. Sivov, on the basis of the presence of genera and species identical to those of the trilobites of the Politsiat horizon of A. G. Vologdin, served as grounds for assigning these limestones to the upper part of the Lower Cambrian.

A. D. Shchelkovnikov and S. I. Makarov arrived at analogous conclusions regarding the stratigraphic position of the Babakov limestone. The latter, together with trilobites, collected archaeocyaths from this limestone.

In 1958 L. N. Repina repeated the collections of trilobites from the Babakov limestone. From these collections she identified, along with previously known forms, typically Middle Cambrian forms, and assigned the Babakov limestone to the Oryctocephalops—Shistocephalus zone.

Below is a list of trilobites collected from different horizons of the Babakov limestone, according to the identifications of N. V. Pokrovskaya, O. K. Poletaeva, L. N. Repina, and A. G. Sivov: Kooteniella slatkowskii (Schm.), K. edelsteini Lerm., K. immanis Siv., Chondragraulos minussensis Lerm., Erbia sibirica (Schm.), E. granulosa Lerm., Tabatopygellina sp., Granularia grandis Lerm., G. obrutchevi Polet., Proerbia (?) sp., Kootenia sp., Taiganella minuta Rep., Amphoton sp., Olenoides convexus Lerm., Chondranomocare irbinica Rep., Pseudanomocarina aojiformis N. Tchern., P. tabatica Rep., Proasaphiscus clarus N. Tchern., Solenopleura recta N. Tchern., Glabrella sp., Triplagnostus sp., Corynexochus sp.

Indeed, the last nine trilobite forms leave no doubt as to the Middle Cambrian age of the Babakov limestone; the remaining forms do not preclude such a determination of their age.

From the collections mentioned above, I. K. Bazhenov and P. S. Krasnopeev identified: Ajacicyathus otichtaschensis (Vologd.), Archaeofungia dissepimentalis (Vologd.), Bicyathus crassimurus Vologd., Archaeocyathus simplex Vologd., Tersia sp.

Detailed geological mapping of the Babakov limestone area and additional fossil collections make it possible to refine the section of the Cambrian deposits of the Western Sayan and the position of the Babakov limestone in this section (Fig. 1).

The section of Cambrian deposits in the basins of the Arbatka–Chazryk rivers begins with the Chinginskaya Formation of the lower Lower Cambrian, composed of altered diabase porphyrites, quartzites, and clayey shales; these deposits are cut by hyperbasites.

The overlying Lower Monok Formation of the Lower Cambrian is represented by effusives of the spilite–keratophyre formation. It is intruded by granites of the Maina intrusive complex. Rocks of the Upper Monok and Arbatka formations rest unconformably on the Lower Monok Formation.

Sedimentary-terrigenous rocks of the Upper Monok Formation are exposed only in the western parts of the area—in the basin of the Arbatka River and in the lower reaches of the Chazryk River. In the section of the formation, tuffaceous and sedimentary rocks occur in equal amounts. The thickness of the Upper Monok Formation on the watershed of the Arbatka–Kyndyrla rivers is 450 m.

The Arbatka Formation, in the basin of the Arbatka River, conformably and with gradual transitions overlies the Upper Monok Formation. The Arbatka Formation consists of graywacke and arkosic sandstones, gravelites, lettuce-green argillites, clayey shales, and limestones in the upper parts (northern slope of Mount Khudoi). The thickness of the Arbatka Formation in the basin of the Arbatka River is 1200–1500 m.

According to the views of most investigators, the section of Cambrian deposits on the northern slope of the Western Sayan ends with the Arbatka Formation. The age of the Arbatka Formation was interpreted in different ways for a long time. In 1962, Z. P. Lyubalinskaya, in the near-mouth part of the Terben-Karasuk Creek, found trilobites in limestones of the lower part of the Arbatka Formation, and L. N. Repina identified from them the following forms: Atmgaspis sp., Chondranomocare irbinica Rep., Chondranomocare sp., Antagmopleura sp., Chondragraulos minussensis Lerm., Kooteniella sp. In addition, in the limestones of the Arbatka Formation in the basin of the Arbatka River, at the mouth of Egorov Creek, Z. P. Lyubalinskaya collected trilobites: Kooteniella slatkowskii (Schm.), Erbia granulosa Lerm., Prohedinella erbiensis, Siv Chondragraulos (Antagmopleura) convexa N. Tchern. In 1964, T. V. Korobeinikova, V. P. Korobeinikov, and the author, in tobacco-green clayey shales lying stratigraphically above the limestones of Egorov Creek, found the trilobites Pthychoparia (?) sp. (identified by T. V. Korobeinikova) and brachiopods: Acrotreta sp., Lingulella sp., Acrothele (?) sp. (identified by N. A. Aksarina). On the basis of the fauna cited, the age of the Arbatka Formation is determined as the lower part of the Middle Cambrian; it is possible that its lower parts belong to the uppermost Lower Cambrian.

The Babakov limestone can be traced continuously for a distance of 12.5 km from Chistogudishche Creek as a single lens-shaped body, wedging out eastward, and has a maximum thickness of 600 m. According to measurements in the banded varieties, only southerly dips are recorded in the limestones; locally they are overturned.

Fig. 1. Schematic geological map of the area of Mount Lokhmataya.

Lower Cambrian: 1 — Chinginskaya Formation: clay shales, effusive rocks, limestones, quartzites. 2 — Nizhnemonokskaya Formation: siltstones, keratophyres. 3 — Verkhnemonokskaya Formation: conglomerates, sandstones, tuffs. Middle Cambrian: 4, 6, 7 — Arbatskaya Formation; 4 — gravelites, sandstones, aleurolites; 6 — limestones; 7 — gravelites, sandstones, aleurolites. 8 — Tigertyshskaya Formation: basic effusive rocks. 9 — granites of the Mainskii complex. 10 — gabbro-diorites of the Adal’skii complex. 11 — granites of the Adal’skii complex. 12 — Quaternary deposits. 13 — disjunctive faults. 14 — attitude elements: a) bedding, b) schistosity. 15 — fossil localities. 16 — unconformities.

The limestones are accompanied by siliceous shales and siltstones encountered in the roof and at the base of the Babakov limestone.

From the northern contact of the Babakov limestone L. N. Repina (collections of 1958) identified: Kooteniella slatkowskii (Schm.), K. edelsteini Lerm., Granularia grandis Lerm., Chondragraulos minussensis Lerm., Chondranomocare irbinica Rep., Erbia sibirica (Schm.).

From the southern contact—Pseudanomocarina aojiformis N. Tchern., P. tabatica Rep., Chondranomocare irbinica Rep., Taiganella minuta Rep., Triplagnostus sp., Kooteniella slatkowskii (Schm.), Granularia obrutchevi Polet., Amphoton sp., Olenoides convexus Lerm., Erbia sibirica (Schm.), Corynexochus sp.

In 1959 A. P. Shcheglov and the author collected from the southern contact of the Babakov limestone (the mouth of Izvestkovyi Brook), and E. S. Fedyanina identified in 1962: Olenoides optimus Laz., Pseudanomocarina cf. plana N. Tchern., Peronopsis sp., Solenopleura recta N. Tchern., Triplagnostus sp., Kooteniella sp., Corynexochus ex gr. perforatum Lerm.

The beds containing fauna from the northern contact of the Babakov limestone may be correlated in age with the limestones of Egorov’s key (see the list by Z. P. Lyubalinskaya given above) and with rocks containing trilobite fauna in the lower course of Terben-Karasuk Brook. All these beds belong to the lower part of the Amginian Stage of the Middle Cambrian. The trilobites collected at the southern contact of the Babakov limestone make it possible to correlate this assemblage with the fauna of the Pseudanomocarina plana zone of the upper Amginian Stage of the Siberian Platform. Consequently, formation of the Babakov limestone took place throughout the entire Amginian Age—from its very beginning to its very end. The uppermost beds of the Babakov limestone are situated at its southern contact.

From the north, metamorphosed sandy-shaly rocks adjoin the Babakov limestone everywhere. In them only southerly dips are recorded; rarely they stand on end. The deposits of the sandy-shaly unit have many features in common with the deposits of the Arbat Formation of the basin of the Arbat River.

For a long time the position of this unit in the section was unclear. The close composition of the trilobites of the Babakov limestone and of the limestones of the Arbat Formation of Terben-Karasuk Brook, the lithological similarity of the clastic rocks adjoining the Babakov limestone from the north to the deposits of the Arbat Formation, and also the identical orientation of the strikes of the clastic rocks and the Babakov limestone allow us to assign the limestones and these rocks to the Arbat Formation.

The general section of the Arbat Formation in the valley of the Chazyryk River may be represented as follows (from bottom to top):

1. Metamorphosed gravelites, sandstones, tuffaceous sandstones, and tuffites with epidote-carbonate cement and with fragments of basic effusives and quartzites, with a visible thickness of up to 800 m.

2. Metamorphosed carbonatized, epidotized, and chloritized arkosic sandstones, medium-grained to coarse-grained, interbedded with green and green-violet shales, about 300 m thick.

3. Babakov limestone with trilobite fauna of the Amginian Stage, up to 600 m thick.

The lower parts of the sandy-shaly unit are cut off by a disjunctive fault; therefore the true thickness of the Arbat Formation in the basin of the Chazyryk River is indeterminable. It is not excluded that the lower part of the sandy-shaly unit of the river

The Chazyryk may correspond to the upper part of the Upper Monok Formation of the Lower Cambrian.

Clarifying the relationships between the Babakov limestone and the effusives to the south of it is of great fundamental importance. The position of these effusives in the section of the Cambrian strata has been a subject of discussion. A. G. Sivov (1948, 1953) and A. P. Shcheglov (1960), for example, assigned these effusives to the Lower Monok Formation. In 1957 A. A. Mossakovsky and N. N. Kheraskov (Kats et al., 1959) distinguished the Chazyryk Formation of the Middle Cambrian and included in it the Babakov limestone together with the effusives distributed south of it.

The package of effusives of the northern slope of the Dzhoi Range has considerable thickness, but is distinguished by the relative uniformity of its petrographic composition. Complete sections of these formations are represented on the watershed of the Turuntash–Chistogudishche streams, in the valley of the Babak stream, and in the upper reaches of the Chazyryk River. Among the effusives the following varieties are distinguished: spilitized amygdaloidal basaltic aphyrites, spilitized diabase porphyrites, pyroxene-plagioclase basaltic porphyrites; andesitic plagioclase porphyrites occur more rarely. According to observations in trenches by S. I. Makarov and A. P. Shcheglov, the direct relationships of the Babakov limestone and these effusives are interpreted as the overthrusting of the effusive sequence onto the top of the Babakov limestone. The temporal proximity between the limestones and the effusives is indicated by the presence, in the effusive sequence, of small lens-like bodies of siliceous rocks similar to the siliceous formations occurring in the northern and southern contacts of the Babakov limestone. Measurements of bedding in the siliceous rocks indicate that there are no angular or azimuthal unconformities between the limestone and effusive sequences. According to the observations of A. G. Sivov on the watershed of the Babak–Maly Babak streams, the effusives in immediate proximity to the limestones contain xenoliths of limestones, which emphasizes the higher stratigraphic position of the effusives.

In the stratigraphic schemes of the Western Sayan there are no effusive sequences analogous in age to the effusives overlying the Babakov limestone. We assign this sequence of basic effusives to the Turuntash Formation. The lower boundary of the Turuntash Formation is referred to the lower part of the Maian Stage of the Middle Cambrian, on the basis of the overthrusting of the rocks of this formation onto the upper horizons of the Babakov limestone. The thickness of the formation on the watershed of the Turuntash River and Babak stream is about 950 m. The Kizir Formation of basic effusives in the Eastern Sayan and the Taidon Formation of basic effusives in the Kuznetsk Alatau are comparable in age with the Turuntash Formation.

On the northern slope of the Dzhoi Range, in the basin of the Chazyryk–Arbatskaya rivers, intrusive rocks are widely developed. They cut through the Turuntash effusives, as well as rocks of the Arbatskaya and Chinginskaya formations. In the area there is one large body of gabbro-diorites—in the upper part of the Adal stream; along the right bank of the Chazyryk River, small bodies of gabbroids and dioritoids are distributed.

The predominant rock type in the intrusive complex is diorites and gabbro-diorites, more often hornblende-bearing; in smaller amounts, pyroxene and uralite varieties occur; gabbro-norites have also been encountered; among the gabbro-diorite intrusions there are small stocks of plagiogranites. The latter are usually represented by leucocratic varieties. It can be stated with confidence that the plagiogranite intrusions are a later phase of the described complex than the gabbroids enclosing them: in the endocontact of the stocks of leucocratic plagiogranites, hybrid rocks are developed, consisting of quartz, amphiboles, basic and aci-

…of plagioclases; in addition, xenoliths of gabbro have been found in the plagiogranites (the headwaters of the Adal stream).

Since the Upper Monok conglomerates are saturated with pebbles of intrusive rocks of the Maina complex, and the Arbat conglomerates in the basin of the Bolshoi Arbat River and in the western parts of the Western Sayan also contain fragments of the Maina intrusions, the intrusive rocks of the Chazryk River basin, which cut the deposits of the Arbat and Turuntash formations, cannot be identified with the intrusions of the Maina complex. The time gap between the emplacement of the one and the other intrusions is equal to Upper Monok and Arbat time. Most likely, the effusives of the Turuntash Formation, together with the gabbro-diorite and granite intrusions that cut them, were formed as the result of a single magmatic cycle in the second half of the Middle Cambrian—early Late Cambrian. We distinguish the gabbro-diorites and granites distributed in the valley of the Adal stream and in the basin of the Chazryk River as the Adal intrusive complex, with an emplacement age of the end of the Middle Cambrian to the beginning of the Upper Cambrian. It is possible that the gabbro-diorite intrusions cutting the Arbat deposits in the basin of the Bolshoi Arbat River and farther west correspond to intrusions of the Adal complex.

Below is a stratigraphic scheme of the Cambrian deposits of the northern part of the Western Sayan and the position of the intrusive complexes in the general Cambrian section (from bottom to top):

1. Chingin Formation of the lower part of the Lower Cambrian. Hyperbasites. A break is possible.

2. Lower Monok Formation of the Lower Cambrian. Maina intrusive complex. Break.

3. Upper Monok Formation of the second half of the Lower Cambrian.

4. Arbat Formation of the upper Lower Cambrian—first half of the Middle Cambrian.

5. Turuntash Formation of the second half of the Middle Cambrian. Adal intrusive complex.

The final stage of geosynclinal accumulation in the northern part of the Western Sayan is the formation of molasse and flysch formations (the Upper Monok and Arbat formations). The vigorous volcanic and intrusive activity that occurred at the end of this stage (the Turuntash Formation and the Adal intrusive complex) may be a distinctive feature of the development of the Western Sayan and emphasizes the great complexity of the development of magmatic processes in eugeosynclines of the Western Sayan type.

REFERENCES

1. I. K. Bazhenov. Western Sayan. Publishing House of the Academy of Sciences of the USSR, 1934.

2. Ya. G. Kats, B. N. Krasil’nikov, A. A. Mossakovskii, E. D. Sulid-Kondrat’ev, N. N. Kheraskov. Stratigraphy of the Paleozoic deposits of the Minusinsk Basin and its mountainous framing. Trudy VAGT, issue 4, 1959.

3. O. K. Poletaeva, I. E. Shatrov. Geological structure of the northern slope of the Western Sayan in the area of the Tabat and Bol. and Mal. Arbat rivers. Trudy ZSGRT, 1936.

4. A. G. Sivov. The Cambrian and Precambrian of the Western Sayan. Trudy GGI ZSFAN SSSR, issue 2, 1948.

5. A. G. Sivov. The Lower Cambrian of the Western Sayan. Izv. TPI, vol. 74, issue 2, 1953.

6. A. P. Shcheglov. The Cambrian of the northern slope of the Western Sayan. Trudy SNIIGGiMS, Materialy po paleontologii i stratigrafii, issue 8, 1960.