ON KEY ISSUES OF CAMBRIAN STRATIGRAPHY IN THE WESTERN SAYAN

A. G. SIVOV

The effusive-sedimentary Cambrian accumulations widely distributed in the Western Sayan, cut by coeval intrusive bodies that differ in composition, are characterized by a distinctly expressed geosynclinal type of section. At one time it was subdivided, from bottom to top, into the following units:

The Akkol Formation of metamorphic schists of the lower part of the Lower Cambrian (?); break.

The Chinginsk Formation of greenstone effusives and phyllites, containing lens-shaped bodies of microquartzites and limestones with remains of algae, and, in places, archaeocyathids of the second third of the Lower Cambrian.

The West Sayan Formation of hyperbasites; break; unconformity.

The Babakov Formation of reefogenic limestones with remains of archaeocyathids, trilobites, brachiopods, and algae from the upper part of the Lower Cambrian.

The Kair Formation of diorites; break; unconformity.

The Lower Monok Formation of spilite-keratophyre rocks of the lower part of the Middle Cambrian.

The Main Formation of trondhjemites; break; unconformity.

The Upper Monok Formation of conglomerates, sandstones, and reefogenic limestones with remains of algae, brachiopods, stromatoporoids, archaeocyathids, and trilobites from the lower part of the Middle Cambrian; break; unconformity.

The Arbat Formation of sandstones and siltstones with rare lenses of limestones containing remains of trilobites and brachiopods from the lower part of the Upper Cambrian [19; 20; 21].

This scheme of Cambrian stratigraphy in the Western Sayan, reflected to a certain extent in the Resolutions of the Interdepartmental Conference on the development of unified stratigraphic schemes for Siberia (16), was later variously modified and supplemented by our researchers.

Thus, for example, A. S. Salun and A. P. Shcheglov (17; 23), having established within the Cambrian of the northern slope of the Western Sayan the Ui, Izyrlen (?) and Monok series, listed here in ascending order, combined in the first of these, as suites, the deposits of the Akkol and Chinginsk formations and, under the name of the Shushensk suite, the lower parts of the Lower Monok Formation. In the Izyrlen series the above-mentioned researchers included the lower parts of the Arbat Formation, in particular those distributed along the Tabat River near the mouth of the Chazryk River [23]. In the Monok series they combined the middle and upper members of the Lower Monok Formation, the entire Upper Monok Formation, and the upper components of the Arbat Formation.

A. S. Salun assigned the Lower Monok and Upper Monok suites (in his, of course, understanding of their extent) to the Silurian; moreover, the archeocyathid-trilobite limestones characteristic of these subdivisions, including the Sanashtykol limestones, he regarded partly as disjunctive wedges and partly as erosional projections of the ancient basement [17].

The views set forth by A. S. Salun were disputed by A. P. Shcheglov [23; 24], who demonstrated that the age of the Lower Monok and Upper Monok suites is Middle Cambrian, and that the Sanashtykol limestones characteristic of the latter are syngenetic and of the same age as its clastic accumulations.

Subsequently, G. G. Semenov [18], N. I. Kazakov [8], and A. N. Mossakovsky [9], following N. V. Pokrovskaya, insisted on a Lower Cambrian age for the Lower Monok and Upper Monok formations that I had distinguished and, calling them suites, considered them a facies analogue of the Chinginskaya formation (suite).

Finally, a particularly sharp discussion arose over the question of the stratigraphic position of the Babakov formation, the limestones of which many researchers called Babakov limestones and assigned to the lower part of the Middle Cambrian.

Indeed, A. S. Salun [17], recognizing the trilobites of the Babakov limestones as Middle Cambrian, considered them a component of the Shushenskaya suite, which he, together with the underlying rocks of the Uy series (the Chinginskaya and Akkolskaya suites), for some reason assigned to the Lena stage of the Lower Cambrian.

In turn, A. P. Shcheglov [22] considered the limestones mentioned to be characteristic formations of his Lower Monok suite, whereas A. N. Mossakovsky and N. N. Kheraskov [9] combined the same limestones and the rocks surrounding them into the Chazyryk suite and placed it stratigraphically above the Upper Monok suite or formation as I understand its extent. The number of such examples could be greatly increased, but those cited are quite sufficient, especially since I shall return to some of them later. Here I will merely emphasize that such contradictory disagreements have arisen on key questions of the Cambrian stratigraphy of the Western Sayan that it is by no means easy to overcome them, and I have not set myself such a task. The purpose of the present article is, on the basis of all the accumulated material, to clarify the extent and age of the Lower Monok, Upper Monok, and Babakov formations, which are the reference stratigraphic subdivisions of the Cambrian of the Western Sayan. In accordance with the recommendations of the Novosibirsk Conference on the doctrine of geological formations, in the further discussion they are referred to as suites.

One may agree with those researchers who show that the Babakov limestones do not belong to the Lower Monok suite, but together with the effusives that cover them constitute a special Babakov (Chazyryk) suite, located, possibly, stratigraphically above the Upper Monok suite. In turn, it should be acknowledged that the thickness of pyrogenic accumulations of a diagenetized aspect, and of the clastic rocks associated with them, distributed east of the headwaters of the Chazyryk River, the right tributary of the Tabat River, and lying there with sharp unconformity upon the intensely schistose rocks of the Chinginskaya formation (series), does not belong in its entirety to the Lower Monok suite, as a number of researchers believed, but is divided into several separate subdivisions, of which the lower one, following A. S. Salun and A. P. Shcheglov, is here called the Shushenskaya suite. In the type section at its base, exposed on the left bank of the M. Shusha River, 1 km above the village of Pervomayskoye, a basal breccia has been established, overlain by keratophyra-

which upward along the section (the right slope of the Srednyaya Shusha River below and above the village of Subbotino) are replaced by diabases and spilites having pillow texture and enclosing thin beds of lydites, sandstones, and conglomerates.

Within the breccias and conglomerates of the Shushenskaya Formation, which reaches a thickness of 1.5 km, there are fragments and pebbles of bedded marbles, quartzites, greenstone effusives, and sandstones, i.e., rocks characteristic of the Chinginskaya Series. In the middle course of the Baraksan River (a left tributary of the Migna River), the Shushenskaya spilites are overlain by a unit of siliceous-argillaceous rocks interbedded with lydites and enclosing beds of spongolites, sandstones, and conglomerates. This unit, about 600 m thick, has been named by me the Baraksanskaya Formation. Along the same Baraksan River, the Baraksanskaya Formation is overlain by rocks of the albitophyre-keratophyre series, which on the right bank of the Bolshaya Shusha River (southeast of the settlement of Kaskanok) lie directly on the Shushenskaya Formation and there constitute the lower parts of the Nizhnemonokskaya Formation. Its middle and upper members, expressed predominantly by tuffs of albitophyres and keratophyres with a noticeable participation of spilites, diabases, and, locally, augite and pyroxene porphyries, form an almost continuous belt extending from the headwaters of the Chazryk River in the northeast to the southern spurs of the Abakan Range in the southwest.

Throughout the entire area of distribution of the Nizhnemonokskaya Formation, among its pyrogenic rocks there are occasionally observed thin beds of lydites, sandstones, and lens-shaped bodies of reefogenic limestones situated at various stratigraphic levels. An example of this may be the section of the Gerasimov Spring, where, 1.2 km above its mouth (Fig. 1), the upper parts of the Nizhnemonokskaya Formation are represented by tuff-conglomerates of albitophyres, which enclose more than a dozen rounded bodies of limestones ranging in diameter from several centimeters to two tens of centimeters and which are overlain by a lens-shaped body of varicolored limestones up to 70 m thick. They contain remains of algae, archaeocyaths, and, very rarely, trilobites, among which, according to determinations by a number of paleontologists, the following forms are most characteristic: algae—Epiphyton fasciculatum Chapm., Renalcis granosus Vologd.; archaeocyaths—Ajacicyathus ijizkii (Toll), A. immanis (Vologd.), A. proscurjakovi (Toll), A. uricus (Vologd.), A. dwigthi (Walc.), A. arteintervallum (Vologd.), Archaeolynthus crassimurus (Vologd.), Bicyathus ertaschkensis Vologd., Carinocyathus bagenovi Vologd., Clathricoscinus infirmus (Vologd.), Coscinocyathus dianthus Born., C. dianthus Born. var. stabilis Krasn., C. calathus Born., C. convexus Vologd., C. torgaschinensis Vologd., Ethmophyllum ratum Vologd., E. gracilis Vologd., E. ulmensis Vologd., Leptosocyathus sp., Loculicyathus membranivestites Vologd., Nochoroiocyathus mirabilis Zhur., Protopharetra lata Born., Szecyathus sp., Syringocnema eleganta Vologd., Thalamocyathus tersiensis (Vologd.), T. kuznetzovi (Vologd.), T. gerassimovensis Krasn., Tumulocyathus admirabilis Vologd., Uralocyathus sp., Uralocyathus regularis Vologd. Among the trilobites, a representative of the genus Poliellina has so far been established.

The limestones of the Nizhnemonokskaya Formation, established in the lower course of the Rudnaya River, which flows into the Oya River on the left, in the middle course of the Bolshoy Monok River, along the Abakan River below the Kirginsk Springs, and in the upper course of the Kamzas River, a left tributary of the Abakan, also contain remains of archaeocyaths and algae, but the state of their study leaves much to be desired. On both sides of the axial part of the Abakan Range, effusives of the Nizhnemonokskaya Formation adjoin the crystalline rocks of the Trubenskaya Formation of the Early Precambrian, while on the northern slope of this

ridge, in particular, on the right bank of the Mrassa River between its tributaries, the Kamzas and Sykzas Rivers, under the name of the Sykzas effusives, overlie the carbonate-terrigenous rocks of the Adiak (Mrassa) suite (subsuite). It rests, with an erosional break, on the sedimentary-effusive rocks that I have distinguished as the P’yankovskaya suite, which, in turn,

Fig. 1. Schematic geological structure of the basin of the upper reaches of the Monok River (northern slope of the Western Sayan).
1 — Devonian deposits; 2 — Arbat suite of the late Middle Cambrian; 3 — Upper Monok suite of the early Middle (?) Cambrian; 4 — Maina plagiogranite intrusion of the late Early Cambrian; upper (5) and lower (6) parts of the Lower Monok suite of the late Early Cambrian; a, b, v — occurrences of archaeocyathid-algal limestones of the Lower Monok suite; g, d, e, zh — occurrences of archaeocyathid-algal limestones of the Upper Monok suite.

rests discordantly on the carbonate rocks of the West Siberian suite (formation) of the Late Precambrian (Fig. 2).

Along the left slope of P’yankovsky Creek, north of the settlement of Semyachkina, the aleurolites of the Adiak suite are overlain by massive Sakharov limestones, at least 500 m thick. On the left bank of the Kamzas River, above the settlement of Srednii Kamzas, dark bedded limestones of the same Adiak suite are already overlain by massive porphyrites of a diagenetically altered appearance, containing a bed of conglomerates composed of pebbles of effusives and dark limestones of the Adiak suite. Beneath the porphyrites, which have a thickness of up to 300 m, lies a bed of bedded quartzites¹), up to 10 m thick, which upward in the succession are replaced by Sakharov limestones more than 60 m thick; the latter are overlain by diabases with a visible thickness of more than 250 m.

The porphyrites and diabases described, by their position in the section and their petrographic composition, represent varieties of the Sykzak or Lower Monok effusives. The Sakharov limestones should evidently also be regarded as belonging to the Lower Monok suite. They contain remains of archaeocyaths and trilobites; among the latter I have identified Bidjinella, Protypus, and Poliellina. In the composition of the

Fig. 2. Schematic geological structure of the southwestern part of the Abakan Range in the Western Sayan.
1 — red-colored rocks of the Synda suite, Lower Devonian. Middle Cambrian: 2 — clastic rocks of the Arbat (a) and Uytak (b) suites; 3 — Kurlan (Tarlan) effusives of basic composition; 4 — carbonate-clastic rocks of the Upper Monok suite; 5 — Upper Mazassian (Sakharov) limestones. Upper Lower Cambrian; 5 — Lower Mazassian (Sakharov) limestones; 6 — pyroclastic rocks of the Lower Monok suite; 7 — clastic-carbonate rocks of the Adyak (Mrass) suite; 8 — carbonate-clastic-pyroclastic rocks of the Pyzhakov suite. Upper Precambrian; 9 — metamorphosed effusive-sedimentary rocks of the Ust-Kamzas (Trubinsk) suite; 10 — dolomites of the West Siberian suite; 11 — disjunctive fault.

archaeocyaths, recently published by A. G. Pospelov and A. P. Shcheglov [4], the following forms predominate: Ajacicyathus cf. patulus (Born.), Aptocyathus sp., Carinocyathus bagenovi Vologd., Clathricoscinus aff. infirmus (Vologd.), Coscinocyathus aff. torgaschinensis Vologd., Dictyocyathus javorskii Vologd., Leptosocyathus sp., Loculicyathus tuberculatus Vologd., Tersia longa Vologd., Thalamocyathus gerassimovensis Krasn., Uralocyathus sp.

Within the area of distribution of the Lower Monok suite there occur chiefly small intrusive plutons of the type of the Maina plagiogranite intrusion. They are known near the village of Novopokrovka, in the middle course of the M. Monok River, in the Kirsa and Khansyn ranges, and also along the northern slope of the Abakan Range.

All investigators of the Western Sayan acknowledge that the predominantly pyroclastic accumulations of the Lower Monok suite, widespread on its northern slope and with a thickness estimated at 1.7 km, are overlain by clastic-carbonate rocks of the Upper Monok suite, which take part in the structure of a belt extending from the headwaters of the Sos—

¹ They may possibly belong to the West Siberian suite of the Precambrian.

in the northeast to the upper reaches of the Abakan River in the southwest. Packs of the Upper Monok Formation of insignificant thickness are exposed on the right bank of the Bolshaya Shusha River, where they overlie albitophyres of the lower parts of the Lower Monok Formation. In the type section along Gerasimov Creek, the Upper Monok Formation, directly overlying the limestones of the upper parts of the Lower Monok Formation, is subdivided into the following units, from bottom to top (Fig. 1).

1. A pack of coarse-grained gravelites consisting of fragments of feldspar, quartz, and, more rarely, limestone; thickness about 10 m.

2. A pack of bouldery conglomerates, gradually replaced upward by fine-clastic conglomerates; thickness up to 150 m.

3. A pack of medium-grained sandstones containing lens-shaped bodies of reefogenic limestones up to 3 m thick, with remains of archaeocyaths, among which P. S. Krasnopeeva identified from my collections: Ajacicyathus ijizkii (Toll.), Ethmophyllum ratum Vologd., Tercyathus altaicus Vologd., Tegerocyathus diploperforatum (Vologd.), and others. The total thickness of the pack is about 75 m.

4. A pack of dolomitized limestones with abundant remains of archaeocyaths and algae; thickness up to 25 m. In the archaeocyath assemblage from repeated collections by many researchers, a number of paleontologists identified a large number of forms, of which the following are most commonly found: Ajacicyathus salebrosus (Vologd.), A. ijizkii (Toll.), A. immanis (Vologd.), A. khemtschikensis (Vologd.), Archaeofungia dissepimentalis Vologd., Aptocyathus gordoni Vologd., Carinocyathus loculatus Vologd., Coscinocyathus dianthus Born., Coscinocyathellus parvus Vologd., Claruscyathus solidus Vologd., Ethmophyllum ratum Vologd., E. ulmensis Vologd., E. grasilis Vologd., Loculicyathus membranivestites Vologd., Syringocnema eleganta Vologd., Tegerocyathus pothechini Krasn., Tercyathus altaicus Vologd.

5. A pack of conglomeratic sandstones, cut by a dike of post-Arbat diabases; thickness about 150 m.

6. A pack of stylolitic limestones; thickness up to 130 m.

7. A pack of medium-grained sandstones; thickness up to 45 m.

8. A pack of stylolitic limestones with rare remains of algae and poorly preserved archaeocyaths; thickness 130 m.

9. A pack of conglomeratic sandstones with a visible thickness of about 150 m. Then, for more than 200 m, outcrops are absent, and only in tree throws are fragments of stylolitic limestones and sandstones found here and there, apparently representing the very uppermost parts of the Upper Monok Formation. In any case, on the watershed between Gerasimov, Ippolitov, and Terbishev creeks there are rocky exposures of distinctive gravelites, which I have taken to be the base of the Arbat Formation.

The total thickness of the Upper Monok Formation in its stratotypic section is unlikely to be less than 1.5 km, if one takes into account that the rocks composing it, over a distance of more than 1.7 km, dip uniformly to the southeast at an angle of 65°. In other areas of distribution of the Upper Monok Formation, its vertical profile changes appreciably and is often expressed chiefly by clastic rocks (the section on the right slope of the Abakan River above the mouth of Maskalak Creek), and sometimes predominantly by limestones (sections on the right slope of the middle and lower reaches of the Kyzas River, as well as Sanashtygol Creek).

Among the clastic rocks of the Upper Monok Formation, conglomerates clearly predominate; they are distinguished by varying sizes of fragments, belonging mainly to the rocks of the Lower Monok Formation, more rarely

boulders and pebbles of the Main plagiogranite intrusion, fragments of greenstone rocks of the Chingis Series and of the Kair microdiorite intrusion, as well as fragments of crystalline schists of the Dzhebash Precambrian Series.

However, the most characteristic rocks of the Upper Monok Formation are limestones. Forming, among its clastic rocks, bedded bodies of varying extent and thickness, they were mistakenly regarded by A. S. Salun as disjunctive wedges, protrusions of the ancient basement; the details of this were discussed in detail in the article by A. P. Shcheglov [24], and there is no need to dwell on them here.

I shall note only that the thickest and most extensive bodies of reefogenic limestones are grouped in the northeastern (Monok) and southwestern (Kyzass–Sanashtygol) areas of distribution of the Upper Monok Formation. In the Monok area, in its type section along the Gerasimov Spring, three reefogenic limestone bodies have been established, clearly interbedded with clastic rocks and attaining a total thickness of up to 125 m, with the length of the thickest body about 50 m. In the Kyzass–Sanashtygol area only a single body of reefogenic limestone has been found, but it extends for several kilometers and has a thickness of about 400–500 m. In other areas of distribution of the Upper Monok Formation, the number of reefogenic limestone bodies often increases appreciably, but at the same time their thickness sometimes decreases to several centimeters. Such thin limestone bodies, arranged en echelon along the strike of its clastic rocks, are most often accompanied by products of the near-reef facies, in which limestone fragments sometimes reach large sizes (Ippolitov Spring, Terbishchev Spring).

In many bodies of reefogenic limestones of the Upper Monok Formation and in their fragments of the near-reef facies there are remains of algae, archaeocyaths, accompanied locally by remains of trilobites, stromatoporoids, brachiopods, pelecypods, sponges, and others. Especially diverse are the archaeocyaths, whose composition, numbering about one hundred species [1], shows a known variability in the vertical section. Thus, according to Ippolitov Spring, at the very base of the Upper Monok Formation there lies a conglomerate composed of boulders, fragments, pebbles, effusive rocks, and large blocks of limestone containing the following archaeocyath remains: Ajacicyathus aricus (Vologd.), A. patulus (Born.), A. proscurjakovi (Toll), A. arteintervallum (Vologd.), Archaeolynthus cf. polaris (Vologd.), Aptocyathus gordoni Vologd., Bronchocyathus sp., Coscinocyathus cornucopiae Born., Coscinocyathellus parvus Vologd., Clathricyathus robustus Vologd., Thalamocyathus nikitini (Vologd.), Tercyathus altaicus Vologd. The composition of archaeocyaths characteristic of the higher parts of the Upper Monok Formation is given in the description of its section along Gerasimov Spring (p. 50). The archaeocyath assemblage in its very upper members is clearly expressed in limestones distributed on the watershed of the Sos River—Mount Monok. From these limestones S. V. Chernysheva, from my collections, identified: Archaeocyathus erbiensis Zhur., A. kurmini (Vologd.), Clathricyathus altaricus Vologd., Coscinocyathellus parvus Vologd., Coscinocyathus elegans Vologd., Ethmophyllum ratum Vologd., E. disparvallum Vologd., Sajanocyathus sp., Tegerocyathus abakanensis (Vologd.), Tercyathus altaicus Vologd., Thumulocyathus sp.; and I. T. Zhuravleva, from the collections of A. S. Salun, established [17]: Ajacicyathus khemtschikensis (Vologd.), A. salebrosus (Vologd.), Archaeocyathus densus (Vologd.), Archaeofungia dissepimentalis Vologd., Ethmophyllum spinosum Vologd., Ethmophyllum poletaevae Vologd., Loculicyathus membranivestites Vologd., Tegerocyathus edelsteini (Vologd.). The forms just listed

archaeocyaths in association with Ajacicyathus aff. acutus (Born.), A. laevus (Vologd.), Annulocyathus pulcher Vologd., Aptocyathus gordoni (Vologd.), Archaeocyathus condensus (Vologd.), Archaeofungia neodissepimentalis (Vologd.), Clathricoscinus infirmus (Vologd.), C. vassiljevi (Vologd.), Coscinocyathus dianthus Born., Densocyathus sanaschtykolensis Vologd., Dictyocyathus yavorkii Vologd., Erbocyathus sp., Formosocyathus bullinnikovi Vologd., Piamaecyathus fossaengulatus (Vologd.), Syringocnema eleganta Vologd., Syringocyathus minuta Vologd., Tercyathus batovi Vologd., Tumulocyathus admirabilis Vologd., T. pustulatus Vologd., and others have long been established in the limestones of the Sanashtyk-gol key, which flows into the Abakan on the left, almost opposite the mouth of the Bolshoy Kyzas River. In the limestones of the Sanashtyk-gol key, which apparently make up the uppermost parts of the Verkhnemonok Formation, trilobites are also present; these were first described monographically by O. K. Poletaeva [11]. Later they were studied by me [2], N. V. Pokrovskaya, and A. N. Repina. The trilobite determinations by O. K. Poletaeva were revised by E. V. Lermontova [1], T. Kobayashi [25], N. V. Pokrovskaya [5], by me, and by O. K. Poletaeva herself [13].

As a result of all these studies, the following forms have been identified among the trilobites from the limestones of the Sanashtyk-gol key: Dolichometopus sp., Olenoides sp., Chakasskia minussensis Polet., Chondragraulina (Sanaschtykgolia) semisphaerica Polet., Chondragraulos anomalis Sivov, Erbia sp., Erbiopsis grandis Lerm., Edelsteinaspis sp., Kooteniella sp., Kootenia abakanica (Polet.), Lermontoviella tuberosa Pokr., Poliellina lermontovae Polet., Poliellaspis sajanicus (Polet.), Laticephalus trapecoidalis Pokr., Inouyina quadratica Polet., Shivelicus parvus Pokr., Paleofossus sp., Lermontoviella sp., Kootenia sp. From the same Sanashtyk-gol limestones, I have recently also identified the following trilobites: Abakanaspis primaeva Sivov, Abakanaspis sanaschtykgolensis Sivov, Sajanocephalus abakanicus Sivov.

In addition to the Sanashtyk-gol key, trilobites together with archaeocyaths are known from the limestones of the Verkhnemonok Formation in the Ippolitov key, on the watershed of the Sos—M. Monok rivers, and in the Rudnaya Keni, Chakhshi-Agol, Bolshoy Kyzas, M. and B. Karakol, and Ada rivers. It is appropriate here to note that, in the limestones of the Verkhnemonok Formation widespread in the lower reaches of the Bolshoy Kyzas and M. Karkol rivers, which undoubtedly lie below its Sanashtyk-gol limestones, the trilobite assemblage includes Erbiopsidella sp., Poliellina lermontovae Polet., Erbiopsis aff. grandis Lerm., Shivelicus sp., Laticeptalus trapecoidalis Pokr., and others (6).

Throughout the area of distribution of the Verkhnemonok Formation, rocks of magmatic origin are not encountered very often within it; moreover, being expressed as dikes and sills, they differ in composition and age. Some of them, represented by diorite-diabases (Gerasimov key, Terbishchev key) and augite porphyrites (lower reaches of the Bolshoy Kyzas River, environs of the Abakan iron-ore deposit), cut through and metamorphose not only the deposits of the Verkhnemonok Formation but also those of the Arbat Formation. Others, represented by quartz porphyrites, are apparently pre-Arbat formations.

In the Cambrian section of the Western Sayan, as noted above, the Babakov (Chazryk) Formation is clearly distinguished; it is typically expressed in the upper reaches of the Chazryk River, which flows into the Tabat River on the right. There the Babakov Formation lies directly upon the spilite-keratophyre rocks of the Shushen Formation, being separated from the latter in places by a bed of conglomeratic sandstones. Stratigraphically above them, at the mouth of the Izvestkovy spring, lies a thick body of reef limestone known as the Babakov limestone. On the right bank of the spring

The Babak limestones, through a thin breccia of sedimentary origin, are overlain by a bed of lydite, above which occur spilitized diabases and spilites interbedded with augite porphyries. In different parts of the limestones of the Babak Formation, which reaches a thickness of up to 1 km, remains of algae, trilobites, more rarely brachiopods, and especially rarely archaeocyaths have long been found; of these, the second group of fossils has been studied in relatively great detail. Among the trilobites I have established the following forms: Chondragraulos minussensis Lermont., Ch. babakovensis Sivov, Chondranomocare sp., Erbia sibirica (Schmidt), E. granulosa Lermont., Granularia obrutchevi Polet., G. babakovia Sivov, Kooteniella slatkowskii (Schmidt), K. immanis Sivov, Tabatopygellina babakovensis Sivov, Olenoides sajanica Sivov, Proerbia (?) bidzhenia Sivov, Proasaphiscus sp., Amphaton sibirica Sivov. To this list should be added also the following forms: Olenoides convexus Lerm., Solenopleura sp., Glabrella babacovica Repina, and Agnostidae, identified by L. N. Repina from her own collections. Remains of archaeocyaths were first discovered in the Babak limestone along the Babak Brook by I. K. Bazhenov in 1934 [3]. In 1946 he also established them on the continuation of the Babak limestone (without an exact indication of the locality), and the archaeocyath assemblage from this collection, studied by P. S. Krasnopeeva, was repeatedly cited in the geological literature [17; 19]. In 1951, students of Tomsk State University and the Tomsk Polytechnic Institute, who were undergoing practical field training in a prospecting and exploration party of the Krasnoyarsk Geological Administration headed by O. I. Makarov, discovered archaeocyaths in limestones distributed on the right slope of the Chazryk River (north of Maralnik) and undoubtedly belonging to the Babak Formation. Unfortunately, the archaeocyath collection gathered from the locality mentioned was subsequently lost. In the same year, 1951, together with O. I. Makarov, I collected a fossil assemblage from limestones cropping out on the left side of the Babak Brook, 0.5 km above its mouth; part of this assemblage was sent for study to the Geological Institute of the Academy of Sciences of the USSR. In this collection, in separate specimens (Nos. 26 and 47), together with the trilobites Granularia grandis Lerm., Kooteniella slatkowskii (Schm.), Kooteniella sp. (identified by N. V. Pokrovskaya), archaeocyaths were discovered, including, according to I. T. Zhuravleva’s determinations: Ethmophyllum? sp., Ajacicyathus sp., Claruscyathus sp., Labyrinthomorpha lata Vologd., Leptosocyathus uricus Vologd. Later, a group of geologists and paleontologists, having failed to find remains of archaeocyaths in the Babak limestone, denied their presence in this limestone and, incidentally, unjustly discredited the fact of their finds made by previous investigators [6; 9]. On the left bank of the Chazryk River, between its left tributaries, the Babak Brook and the Chistogudezh, the Babak limestones are overlain by conglomeratic sandstones, in whose pebble beds fragments of the same Babak limestones are found. Toward the southwest, in the direction of Mount Turuntash, the sandstone described as conglomeratic rests upon basic effusives, probably of the Babak Formation, and is overlain by sandy-shaly-aleurolitic deposits composing the lower parts of the well-known Arbaty Formation. In my opinion, the deposits distributed in the lower course of the Chazryk River and assigned by S. A. Satun and A. I. Shcheglov to the Izyrlen Formation [23; 17] also represent the lower parts of the Arbaty Formation. Indeed, O. K. Poletaeva and I. E. Shatrov [12] showed that somewhat above the confluence of the Chazryk River with the Arbaty Brook there can be traced a conglomeratic sandstone, which contains boulders of the Maina intrusion, keratophyres, and rare porphyrites of effusive or, in modern terminology, sho—

of the Shushenskaya Formation. This conglomeratic sandstone is overlain by polymictic sandstones interbedded with strongly schistose and noticeably chloritized shales, mistakenly assigned by its authors to the Izyrlen Formation. It is worth emphasizing that in the conglomeratic interbeds of the chloritized shales mentioned above, occurring at the mouth of the Chazryk River (Izyrlen Formation), I found pebbles of keratophyres and spilites of the Shushenskaya Formation. To this I shall add that one cannot agree with I. N. Kazakov, who included the Babakov limestones in the Arbat Formation [8], if one takes into account that in the Tabat River basin it lies clearly transgressively upon the underlying rocks, covering now the pyrogenic accumulations of the Babakov, now those of the Shushenskaya formations. At the same time, in the basin of the Tabat River and the adjoining areas the Verkhnemonok Formation, contrary to the statements of A. A. Mossakovsky and N. N. Kheraskov [9; 10], is absent; consequently the Shushenskaya Formation widespread there, cut through by the Mainskaya intrusion (right slope of the Chazryk River above the Izvestkovyi settlement), is directly overlain by the Babakov Formation. In the watershed area of the Karamkol—Babak—Chistogrudezh settlements it is cut by several diorite plutons which, apparently, are not analogues of the Kair intrusion and differentiates of the Mainskaya intrusion, as I once recognized [19], but belong to an independent Karamkol intrusion. Its upper age boundary cannot yet be strictly established.

In the present state of study it does not appear possible to determine the areal distribution of the Babakov Formation, but it may be stated with confidence that its limestones were erroneously correlated with the limestones of the Nizhnemonok Formation exposed in the section of the lower course of the Gerasimov settlement and on the right slope of the Monok River, 150 m upstream from the Terbishchev settlement [19].

Thus, on the northern slope of the Western Sayan, the Cambrian section, in ascending order, comprises the following subdivisions (excluding intrusive formations): the Shushenskaya, Baraksan, Nizhnemonok, Verkhnemonok, Babakov, and Arbat formations, of which the last four formations are characterized by remains chiefly of algae, archaeocyaths, more rarely trilobites and other fossils.

A. S. Salun’s ideas [17] on the Silurian age of the Nizhnemonok, Verkhnemonok, upper Babakov, and Arbat formations are in sharp contradiction with the facts set out above and have found no support among geologists of the Western Sayan. The Chingin Series, previously referred to the Lower Cambrian and subdivided into the Maloshushenskaya and Kyzasskaya formations (formations), in reality probably corresponds to the Late Precambrian. Suffice it to say that the Chingin Formation at one time mistakenly included deposits containing remains of archaeocyaths—for example, the limestones of the Rudnaya River, a tributary of the Oya River; the limestones of the Gryaznyi settlement, a tributary of the M. Arbat River; those in the vicinity of the former Petropavlovsky mine, located on the left side of the transverse Kyzas; and also limestones exposed at the junction between the transverse Kyzas and the Bolshoi Kyzas [19]. The deposits of the Chingin Series, pierced by plutons of hyperbasites and Kair microdiorites, were folded, schistosed, and metamorphosed long before the accumulation of rocks of the Shushenskaya Formation. In this connection the statements of G. G. Semenov and I. N. Kazakov [18; 8] that the Chingin Formation allegedly represents a stratigraphic analogue of the Nizhnemonok, Verkhnemonok, and partly Arbat formations are an accidental misunderstanding on which there is no need to dwell. At the same time, the Lower Cambrian age

the Shushenskaya and Baraksan formations is here determined conditionally, for the spicules of sponges present in their deposits remain as yet unstudied. There are only grounds to suppose that these formations may prove to be stratigraphic analogues, respectively, of the P’yankovskaya and Adyakskaya formations of the northern slope of the Abakan Range, the Lower Cambrian age of which is beyond doubt.

It is sufficient to say that archaeocyaths have recently been found in limestone lenses of the P’yankovskaya Formation, and trilobites of late Early Cambrian age have been described from the limestones of the Adyakskaya Formation [22].

In considering the refinement of the age of the unquestionably Cambrian subdivisions of the northern slope of the Western Sayan, it must be acknowledged that the Middle Cambrian age of the Babakovskaya Formation can no longer be in doubt. It is quite possible that the Arbatskaya Formation as well will, in terms of age, correspond to the Middle Cambrian, if one takes into account its position in the section and the presence in its deposits, widespread in the lower reaches of the Rudnaya Keya River, of remains of archaeocyaths [23], trilobites (Abakanopleura kenensis Sivov), and brachiopods.

As for the age of the Nizhnemonokskaya and Verkhnemonokskaya formations, this question requires somewhat more detailed treatment. As noted earlier, in the lower course of the Gerasimova River, the above-described gravels at the base of the Verkhnemonokskaya Formation rest directly upon the eroded surface of the limestones of the Nizhnemonokskaya Formation, containing remains of archaeocyaths and, very rarely, trilobites. Further, in the lower course of the Kamzas River, limestones with archaeocyaths and rare trilobites occur within the pyroclastic rocks of the Nizhnemonokskaya Formation, which overlies the deposits of the Adyakskaya (Mrasskaya) Formation, containing remains of trilobites (Hebediscus, Calodiscus, Erbia, Bergeroniaspis, Proerbia) and archaeocyaths (Ajacicyathus, Archaeolynthus, Szecyathus) and other fossils of the late Early Cambrian [14].

It is true that among the archaeocyaths of the limestones of the lower course of the Kamzas River and the lower course of the Gerasimova Creek, not so very many forms in common are established, namely: Carinocyathus bagenovi Vologd., Clathricoscinus infirmus (Vologd.), Coscinocyathus torgaschinensis Vologd., Leptosocyathus sp., Thalamocyathus gerassimovensis Krasn.; but the assignment of the enclosing rocks to the Nizhnemonokskaya Formation is established on proper grounds. This means that the Nizhnemonokskaya Formation is paleontologically characterized and is distinguished on the basis of its own assemblage of archaeocyaths, associated in places with poorly studied trilobites, brachiopods, and algae. Being younger than the assemblage of archaeocyaths and trilobites of the Adyakskaya Formation of the late Early Cambrian and older than the assemblage of archaeocyaths and trilobites of the Verkhnemonokskaya Formation, it is transitional from the Adyakskaya assemblage to the Sanashtykgol’skaya one. The position of the Nizhnemonokskaya Formation in the section, and the younger age of its archaeocyath–trilobite assemblage in comparison with the Adyakskaya assemblage, may serve as an important argument for determining its age within the upper half of the Early Cambrian.

This means that the Verkhnemonokskaya Formation, which unconformably overlies the Nizhnemonokskaya Formation, will probably, in terms of age, constitute already the base of the Middle Cambrian; this does not contradict the composition of the archaeocyaths and trilobites of the latter, which differs in certain features. Indeed, among its archaeocyaths there are: Ajacicyathus laevus (Vologd.), Annulocyathus pulcher Vologd., Clathricyathus robustus Vologd., Archaeocyathus latus (Vologd.), A. densus (Vologd.), Ethmophyllum grandiperforatum (Vologd.), Tegerocyathus abakanensis (Vologd.), species of the genera Piamaecyathus, Densocyathus, Ladae-

ocyathus, and the trilobites Chakasskia, Paleofossus, Shivelicus, Laticephalus, Erbiopsis, Dolichometopus, and others, whose appearance and flourishing are limited only to Upper Monok time. Associated with the forms listed are also such forms which, having appeared in Upper Monok time itself, disappear much later. Such forms include species of the genera: Sajanocyathus, Erbocyathus, Claruscyathus, Tegerocyathus edelsteini (Vologd.), T. erbiensis Krasn., Archaeocyathus altaicus Krasn., A. kuzmini (Vologd.), as well as the genera Olenoides, Chondragraulos, Edelsteinaspis, Granularia, Kooteniella. Finally, among the archaeocyaths and trilobites of the Upper Monok Formation a noticeable role is played by “transitional” forms that appeared as early as Lower Monok time and even earlier and that disappear, in overwhelming majority, at the end of Upper Monok time. Without dwelling on the enumeration of forms of this kind, I shall merely indicate that many of them have been recognized by a number of paleontologists and geologists [4; 6; 7] as “index” forms for the Upper Monok Formation and its stratigraphic analogues. Such “index” forms, in particular, were the genera Aptocyathus, Annulocyathus, Tercyathus, Carinocyathus, Ethmophyllum ratum Vologd., E. grasilis Vologd., E. uimеnsis Vologd., Clathricoscinus infirmus (Vologd.), Syringocnema eleganta Vologd., the genera Poliellina, Poliellaspis, Inouyina, Chondrograulina (Sanaschtikgolia), Lermontoviella, Sajanella (Botomella).

These features of the composition of the archaeocyath–trilobite fauna of the Upper Monok Formation were not always taken into account in its analysis, and consequently in determining its age. Suffice it to say that I. T. Zhuravleva, L. N. Repina, and others [7; 4; 6], in characterizing the paleontological composition of the Sanashtykgol horizon, for some reason fail to mention that in its deposits of the Western Sayan there are species of the genera Olenoides, Kooteniella, Chondragraulos, Erbia.

As was noted earlier, stratigraphic relationships between the Babak and Upper Monok formations cannot be established, and the first of them is recognized as younger than the second only on the basis of paleontological documents. It is now generally accepted that the Babak Formation in the Western Sayan constitutes the lower parts of the Amga stage, which to a certain extent indicates a pre-Amga age for the Upper Monok Formation. But it is, of course, by no means a stratigraphic analogue of the Olekma horizon of the Lena stage of the Lower Cambrian of the Siberian Platform, as I. T. Zhuravleva, L. N. Repina, and others [7; 4; 6] believed, but occupies in the Cambrian section of Siberia possibly a higher position than even the Elanka horizon of the same Lena stage; the details were considered in one of my articles.

However the question of the age of the Upper Monok Formation may be resolved, it is quite obvious that it is characterized by a distinctive composition of archaeocyaths and trilobites, on the basis of which it was adopted as the stratotype for distinguishing the Upper Monok stage [2] or substage. In the stratotype itself it permits subdivision into two horizons, of which the upper should continue to be called the Sanashtykgol horizon, while the lower is proposed to be called the Ippolitov horizon. This is not the place to dwell on the paleontological characterization of these horizons, but I shall point out that in the deposits of the Sanashtykgol horizon, not only in a single thin reef body but even sometimes in a single large specimen, forms are found of the Sanashtykgol (Aptocyathus, Tercyathus), Solontsy (Claruscyathus solidus Vologd., Archaeocyathus radiathus Vologd.), Obruchev (Erbocyathus, Edelsteinaspis, Kooteniella), and even Irba (Olenoides) “horizons” in the understanding of their scope by I. T. Zhuravleva, L. N. Repina, and other researchers [4; 6; 7]. Refinement of the Cambrian stratigraphy of the Western Sayan will continue further, but,

in solving the problem posed, it would also be useful to take into account the facts that are set forth in the present article.

REFERENCES

1. Atlas of guide forms of the fossil faunas of the USSR. Cambrian I. Gosgeolizdat, 1940.

2. Atlas of guide forms of the fossil fauna and flora of Western Siberia. Gosgeoltekhizdat, 1955.

3. Bazenov I. K. Western Sayan. Essays on the Geology of Siberia. Academy of Sciences of the USSR, 1934.

4. Biostratigraphy of the Paleozoic of the Sayan-Altai Mountain Region, vol. I. Proceedings of SNIIGGiMS, issue 19, 1960.

5. Zaitsev N. S. and Pokrovskaya N. V. On the structure of the adjacent parts of the Western Sayan and Tuva. Academy of Sciences of the USSR, geological series, No. 6, 1950.

6. Zhuravleva I. T., Repina L. N., and Khomentovsky V. V. Biostratigraphy of the Lower Cambrian of the folded frame of the Minusinsk Basin. BMONT, geological department, No. 2, 1959.

7. Zhuravleva I. T., Repina L. N., and Khomentovsky V. V. Scheme of subdivision of the Lower Cambrian deposits of the Sayan-Altai folded region. Geology and Geophysics, Siberian Branch, Academy of Sciences of the USSR, 1962.

8. Kazakov I. N. An outline of the geological structure of the Western Sayan. Materials on the regional geology of the Altai-Sayan folded region. VSEGEI, vol. 58, 1961.

9. Kats Ya. G., Krasilnikov B. N., Mossakovsky A. A., Sukhinin-Kondratyev E. D., Kheraskov N. N. Stratigraphy of the Paleozoic deposits of the Minusinsk depression and its mountain frame. Materials on the regional geology of VAGT, Gosgeolizdat, 1958.

10. Mossakovsky A. A. Tectonic development of the Minusinsk basins and their mountain frame in the Precambrian and Paleozoic. Author’s abstract of the Candidate dissertation, Moscow, 1962.

11. Poletaeva O. K. The fauna of Cambrian trilobites of the Sanashtyk-Gol limestones of the Western Sayan. Materials on Geology of the West Siberian Territory, issue 35, 1936.

12. Poletaeva O. K. and Shatrov I. E. Geological structure of the northern slope of the Western Sayan in the area of the Tabat, B. and M. Arbat rivers. Materials on Geology of the West Siberian Territory, issue 33, 1936.

13. Poletaeva O. K. New genera and species of Cambrian trilobites of Western Siberia. Materials on Paleontology and Stratigraphy of West Siberia (SNIIGGiMS). Gostoptekhizdat, 1960.

14. Pospelov A. G., Aksarina N. A., Boyarinov A. S., Nadler Yu. S., Fedyanina E. S. On the stratigraphy of the Cambrian of Mountain Shoria. Materials on Geology and Mineral Resources of Krasnoyarsk Territory. Krasnoyarsk Book Publishing House, 1961.

15. Radugin K. V. Elements of the stratigraphy and tectonics of Mountain Shoria. Materials on Geology of the West Siberian Geological Trust, issue 37, 1936.

16. Decisions of the Interdepartmental Conference on the development of unified stratigraphic schemes for Siberia. Gosgeoltekhizdat, 1959.

17. Salun A. S. On the age and stratigraphic affiliation of the Sanashtyk-Gol limestones of the Western Sayan. Bulletin of the Moscow Society of Naturalists, Geological Department, issue 3, 1957.

18. Semenov G. G. New data on the stratigraphy of the western part of the northern facies of the Western Sayan. Materials on Geology and Mineral Resources of Krasnoyarsk Territory. Krasnoyarsk Book Publishing House, 1961.

19. Sivov A. G. The Lower Cambrian of the Western Sayan. Proceedings of Tomsk Polytechnic Institute named after S. M. Kirov, vol. 74, issue 2, 1953.

20. Sivov A. G. The Cambrian system. Atlas of guide faunas and floras of Western Siberia. Gosgeolizdat, 1955.

21. Sivov A. G. The Cambrian of the Western Sayan and adjacent regions. Author’s abstract of the dissertation for the degree of Doctor of Geological-Mineralogical Sciences. Tomsk, 1954.

22. Fedyanina E. S. Trilobites of the Mrassian suite from the Pyankovsky spring (Mountain Shoria). Materials on the Geology of Western Siberia. Publishing House of TSU, Tomsk, 1962.

23. Shcheglov A. P. Cambrian of the northern slope of the Western Sayan. Materials on Paleontology and Stratigraphy of Western Siberia. Proceedings of SNIIGGiMS, Gostoptekhizdat, 1960.

24. Shcheglov A. P. Conditions of occurrence of limestones with the Sanashtyk-Gol fauna of archaeocyaths in the Cambrian of the Western Sayan. Materials on Paleontology and Stratigraphy of Western Siberia. Proceedings of SNIIGGiMS, Gostoptekhizdat, 1960.

25. Teichi Kobayashi. Cambria Faunas fo Siberia. Journ. of the Faculty of Sciens, Imperial University of Toki. Sec. II, Vol. VI, yt. 12, 1943.

Table 1

Fig. 1. Conocardium ornatissimum (Barrande) subsp. altaica. General view of the left valve, ×7, holotype.
Fig. 2. The same, view from the upper margin, ×3.
Fig. 3. Actinopteria (?) scala (Barrande) subsp. altaica. Right valve, ×8, holotype.
Fig. 4. The same, ×2.5.
Fig. 5. Actinopteria (?) scala (Barrande) subsp. altaica. Right valve, ×8.5, paratype.
Fig. 6. The same, ×4.5.
Fig. 7. Prantliella palliata (Barrande) subsp. altaica. Character of the sculpture, ×3, holotype.