ON THE LOWER BOUNDARY OF THE MIDDLE CAMBRIAN IN THE SAYAN–ALTAI REGION

A. G. SIVOV

The stratigraphic scheme of the Cambrian of the Sayan–Altai folded region, developed in 1956 at the Interdepartmental Conference on the preparation of unified stratigraphic schemes for Siberia, is of great scientific and practical significance, although in many of its links it remains debatable and does not reflect reality.

Among such unresolved questions is the question of the lower boundary of the Middle Cambrian in the region mentioned; the presentation of its details, only weakly reflected in the published geological literature, is the purpose of the present article.

In addressing this task, one cannot fail to dwell on the following exceptionally important circumstances, which follow from the results of many years of research by a large group of stratigraphers, paleontologists, and geologists of local and central industrial, scientific-research, and higher-educational collectives.

At present many geologists, including I. T. Zhuravleva, L. N. Repina, and V. V. Khomentovskii, recognize the long-established fact that the Kameshkovskii horizon represents a subdivision of the late Lower Cambrian, and not the early Lower Cambrian, as I. T. Zhuravleva once supposed [9]. Included in this horizon, in addition to the Kameshkovskaya Formation of the southwestern margin of the Eastern Sayan, are now confidently placed the upper parts of the Chesnokov limestones of the Batenev Ridge, the Adnakskaya (Mrasskaya) Formation of the upper reaches of the Mrassa River in Mountain Shoria, where trilobites were discovered in the deposits of the latter subdivision by L. I. Egorova, and in those of the former by V. D. Tomashpolskaya jointly with archaeocyaths, among them representatives of the genus Bulaiaspis were identified.

Apparently the Saryasakskaya Formation [3], widespread along the Biya and Isha rivers of Mountain Altai and containing remains of archaeocyaths and trilobites—including, among the latter, according to the determination of O. K. Poletaeva [11], Sajanaspis and Paleolenus—may be referred to the Kameshkovskii horizon, as may also the Kolbinskaya Formation of the Eastern Sayan, in which representatives of the genus Bulaiaspis, according to I. I. Koptev, are found in almost all its members, including the Novovasil’evskaya Subformation.

At the same time it has been rigorously demonstrated that the Mondybashskaya, Kaimskaya, and Karasukskaya formations of Mountain Shoria, Mountain Altai, and the Batenev Ridge of the Kuznetsk Alatau, respectively, are isochronous and constitute a single biostratigraphic subdivision, which should be called the Bol’sheerbinskii substage, but not the Irbinskii horizon—

[7]. This substage is clearly distinguished on the basis of the characteristic composition of the trilobites, and locally also of the archaeocyaths. Among the trilobites, representatives of the genera Chondragraulos, Erbia, Kooteniella, Kootenia, Granularia, Antagmopleura, Schistocephalus, Chondranomocare, and Olenoides are widespread in deposits of the substage under consideration, occurring together with Pachyaspis, Ptarmigania, Gaphuraspis, Oryctocephalus, Edelsteinaspis, and archaeocyaths in the Kaim and Karasuk formations. In addition, in the Karasuk Formation the trilobites include Peronopsis, Amgaspis, Proasaphiscus, Bathynotus, Prohediniella, and Oryctocephalops, while in the Kaim Formation they include Kochiella, Taxiora, Paradoxides, and Dinesus, the last two forms also having been established in the Mondybash Formation.

The archaeocyaths found in the Karasuk Formation are confined to its middle members, exposed north of the village of B. Erba on the Gladkaia Mys and Sladkie Korenya heights; being represented by Erbocyathus heterovallum (Vologd.), Tegerocyathus edelsteini (Vologd.), Ethmophyllum ratum Vologd., Archaeocyathus erbiensis Zhur., Archaeocyathus altaicus Krasn., and A. inyrgensis Krasn., they constitute, together with the trilobites associated with them, the Obruchevsky (more properly, Karasuk) assemblage.

In the Kaim Formation, widespread near the settlement of Ulus Cherga, M. K. Vinkman [5] cites, among the archaeocyaths, Ajacicyathus sp., Coscinocyathus sp., and Ethmophyllum; and at the mouth of the Suyaryk Brook—Tegerocyathus erbiensis Krasn. The composition of the archaeocyaths and trilobites from the latter locality was combined by M. F. Romanenko [13] into an independent Suyaryk assemblage.

On the basis of the list of trilobites given above, the age of the lower Middle Cambrian beds of the Bolshe-Erbinsky substage cannot be in doubt, although, in our opinion, it does not constitute the base of the Middle Cambrian in the territory of the Sayan-Altai region.

Leaving aside consideration of the details of the stratigraphic relations of the Bolshe-Erbinsky substage with the underlying rocks, I shall merely point out that the Kaim Formation in the Prikatun region of the Altai Mountains lies with a break upon the carbonate-terrigenous Kayancha Formation, while in Mountain Shoria, in the basin of the Amzas (Emzas) River, the Mondybash Formation sharply unconformably overlies pyroclastic accumulations that I have assigned to the Tarlash Formation (the Bachat Formation of M. A. Usov). As is known, the Kayancha Formation was at one time adopted by me as the stratotype for the designation of a stage of the same name, younger [2; 14] than the Kameshki Horizon.

It has now been established that the Kayancha Stage consists of carbonate rocks only in certain parts of its distribution, while for the most part it is represented by pyroclastic-sedimentary accumulations of the Tarlash Formation type, enclosing bodies of reefogenic limestones of small thickness and extent. It has also become clear that the archaeocyath–trilobite assemblages of the stage that interests us display certain peculiarities in composition, which is of great importance in solving problems of both long-distance and local correlation, and consequently also in determining age.

I shall give examples. First example. All paleontologists who have studied the remains of archaeocyaths and trilobites of the Kayancha Formation, on the basis of the presence among the former of representatives of the genera Tegerocyathus and Erbocyathus, and among the latter of Erbia, Chondragraulos, Kooteniella, Edelsteinaspis, and Gaphuraspis, have correlated them with the Obruchevsky assemblage, although forms more characteristic of the Kameshki Horizon are associated with them;

for example, Ajacicyathus, Tersia cf. nodosa Vologd., Coscinocyathus and especially Weymouthia, Bergeroniellus, Namania, Parapoliella.

Second example. In the basin of the upper course of the Mrassu River in Gornaya Shoriya, K. V. Radugin distinguished the Mazas carbonate suite, which there overlies the Dokameshkov deposits and which the author united into the Kayancha Stage. In its upper parts L. I. Egorova recently found remains of trilobites and identified among them Erbia, Kooteniella, Chondragraulos, Edelsteinaspis, Gaphuraspis, Taxioura, which, in her opinion, belong to the Obruchev complex.

However, in the same upper parts of the Mazas limestones, remains of archaeocyaths were long ago established, including representatives of the “Bazaikha” (I. T. Zhuravleva) and “Sanashtyk-Gol” (A. G. Pospelov) complexes, completely unknown in the composition of the Obruchev (Karasuk) complex.

The Mazas limestones at the mouth of the Aizaz River apparently are unconformably overlain by terrigenous accumulations constituting the Aizaz suite. No organic remains have yet been found in its deposits, and it is provisionally correlated with the aforementioned Tarlash suite, in whose reef limestones the following archaeocyaths were identified: Erbocyathus heterovallum (Vologd.), Ethmophyllum ratum Vologd., Retecyathus sp. (the “Obruchev” complex, according to I. T. Zhuravleva and P. S. Krasnopeeva), as well as Nochorojcyathus sp., Szecyathus sp., Ajacicyathus clarus (Vologd.), A. amplus (Vologd.), and others, which are more characteristic of the Mazas complex and have not been established in the Obruchev (Karasuk) complex.

Third example. On the northwestern margin of the Eastern Sayan, one of the units of the Kayancha Stage is the Torgashin suite, representing the stratotype of the Torgashino Substage (Kayancha), established by A. G. Vologdin [6].

In its vertical section, I. T. Zhuravleva, L. N. Repina, and V. V. Khomentovsky [7] distinguished six successive and heterochronous archaeocyath–trilobite complexes, which was clearly an exaggeration, for there are no more than two of them. They have now been studied in relatively great detail by G. G. Ilyina, L. N. Kashina, and I. I. Koptev, who showed that, in the composition of the lower (Bazaikha) complex, stratigraphically confined to the lower parts of the Torgashin suite (the Bazaikha member), exposed on the right slope of the lower course of the Bazaikha River, together with the “Bazaikha” complex of archaeocyaths, trilobites are associated, including Hebediscus, Poliellina, Pagetiellus, Fordaspis.

In the composition of the upper (Torgashin) complex, stratigraphically connected with the upper parts of the Torgashin suite, occupying space also on the northern slope of the Torgashin Range, together with many forms characteristic of the preceding complex, there are present: Aptocyathus, Archaeocyathus, Loculocyathus membranivestites Vologd., Ethmophyllum poletaevae Vologd., Tumulocyathus admirabilis Vologd., Proasaphiscus, Erbiopsis, Granularia, Namania, Parapoliella, Poliellina, Chondragraulos, Edelsteinaspis, Elrathia, Elrathiella, Olenoides, Redlicina, Shivelicus.

The Torgashin complex displays a certain similarity with the Kayancha complex (Cheposh) and a closer similarity with the Aidachikha complex, identified in the suite of the same name, distributed in the Batenyov Range of the Kuznetsk Alatau and lying there, undoubtedly, stratigraphically above the Chesnokov limestones of the Kameshki horizon and below the Karasuk suite of the Bolshe-Erba Substage. Among the archaeocyaths of the Aidachikha complex there are present: Annulocyathus, Archaeofungia, Tegerocyathus, Archaeolynthus, and among the trilobites—Chondragraulos, Edelste-

inaspis, Erbia, Erbiopsis, Kooteniella, Shivelicus, Olenoides, Paleofossus, Milaspis, of which only the last two forms have not been found in the composition of the Torgashino assemblage. However, all the forms just listed, in association with species of the genera Erbocyathus, Sajanocyathus, Densocyathus, Sanaschtykgolia, and others, have been established in the composition of the Sanashtykgol (Upper Monok) assemblage of the Upper Monok clastic-carbonate formation of the Western Sayan.

In the section of the Batenevsky Ridge, analogues of the Bazaikha assemblage of the Torgashino Formation have not yet been discovered, but they have been identified in the lower parts of the Maza Formation, the Lower Monok Formation of the Western Sayan, and their stratigraphic analogues.

Thus, the Kayanchin Stage is characterized by many archaeocyathid-trilobite assemblages that are close in composition but not identical. At its different stratigraphic levels there are quite a few ancient forms passing upward from the Kameshki Horizon, both among the archaeocyathids—Ajacicyathus khemtschikensis (Vologd.), A. ijizkii (Toll.), Clathricoscinus infirmus (Vologd.), Coscinocyathus dianthus Born., Dictyocyathus javorskiji Vologd., Tersia nodosa Vologd., Thalamocyathus howelli (Vologd.), Archaeolynthus, and others—and among the trilobites (Inouina, Bergeroniellus, Binodaspis, Bonnia, Redlichina, Poliellina, Poliellaspis, Parapoliella, Pseudoeteraspis, Bergeroniaspis, Namanoia). It is appropriate to note here that the last four forms on the Siberian Platform are known only in the Doelan parts of the Lena Stage of the Lower Cambrian. In turn, almost at the very base of the Kayanchin Stage there appears a large number of forms of abrupt renewal; some of them, such as, for example, Annulocyathus, Aptocyathus, Syringocyathus, Sajanocyathus, Tercyathus, Clathricyathus, Archaeocyathus densus (Vologd.), A. latus Vologd.), Ethmophyllum poletaevae Vologd., E. katunicum Krasn., E. kourense Krasn., E. gracilis Vologd., E. grandiperforatum Vologd. (archaeocyathids), Paleofossus, Shivellicus, Sanaschtykgolia, Erbiopsis, do not cross the upper boundary of the stage.

Others, however—for example, Archaeocyathus erbiensis Zhur., A. altaicus Krasn., Ethmophyllum ratum Vologd., Erbocyathus, Claruscya­thus, Retecyathus kuzmini Vologd., Gaphuraspis, Kooteniella, Erbia, Chondragraulos, Edelsteinaspis, Granularia, Amgaspis, Amecephalina, Kooteniellina, Olenoides, Proasaphiscus, Taxioura, Elrathiella, Elrathia—crossing the upper boundary of the Kayanchin Stage, enjoy a notable distribution also in the overlying Bol’shaya Erba Substage of the lower Middle Cambrian; moreover, the last four forms are excellent indicators of a fauna of the same age, characteristic of the Pacific province.

The Kayanchin Stage almost throughout the entire Sayan-Altai region lies transgressively on the underlying rocks; moreover, its Torgashino Formation, as I. I. Koptev has shown, overlaps the Kolbinskaya Formation, the upper parts of which are correlated with the Ketemen (Pre-Delamaran) Horizon of the Lena Stage of the Siberian Platform. In connection with this, and especially with the paleontological data, it seems possible to regard the Kayanchin Stage and the Elan Horizon as a whole as formations of the same age.

Recently I. T. Zhuravleva [9] noted that on the territory of the Siberian Platform in Elan time, among the archaeocyathids the greatest flourishing was enjoyed by Archaeocyathus densus (Vologd.), Erbocyathus heterovaeilum (Vologd.), Tegerocyathus edelsteini, and N. P. Suvorova [15] showed that in the same interval of time, in the same territory, representatives of the genera Erbia, Chondragraulos, Kooteniella first appeared among the trilobites.

and typically Middle Cambrian forms, in particular Gaphuraspis, Proasaphiscus.

Obviously, with the presence of Middle Cambrian forms as a whole, the paleontological associations—and consequently also the deposits containing them—of the lower parts of the Kayanchin Stage and of the Elan horizon will also correspond to the Middle Cambrian, but, quite understandably, only to its most ancient links, the Doamtin and Dobolshcherbin.

In light of the circumstances set forth, it becomes evident that the lower boundary of the Middle Cambrian, not only in the Sayan-Altai region but also beyond it, should be established not at the base of the Oryctocephalos—Schistocephalus zone of the Amga Stage, as is reflected in the decisions of the Cambrian, Ordovician, and Silurian Stratigraphy Section of the Interdepartmental Conference of 1956 [12], but much lower: at the base of the Kayanchin Stage and its isochronous analogues, for example the Elan horizon of the Siberian Platform.

It is appropriate to note here that the possibility is not excluded that the lower parts of the Bol’shcherbin Substage of the Sayan-Altai region are of a more ancient age than the Oryctocephalops—Schistocephalus zone of the Amga Stage.

It is enough to recall that the fauna of the latter includes such archaic representatives as, for example, archaeocyaths, and, among trilobites, Bathynotus, Edelsteinaspis, which are completely unknown in the Amga Stage of the Siberian Platform.

This means that throughout the entire time of accumulation of the sediments of the lower parts of the Bol’shcherbin Substage, and possibly also the upper parts of the Kayanchin Stage, in the Sayan-Altai geosynclinal region there prevailed on the territory of the Siberian Platform a break in sedimentation, described by a number of researchers and, in particular, by N. V. Pokrovskaya [10].

The Kayanchin Stage and the Bol’shcherbin Substage overlying it permit subdivision into more detailed biostratigraphic units (horizons), the details of which I am not able to dwell on here. I shall indicate only that within the Kayanchin Stage two parts are clearly distinguished, with its upper half characterized by the greatest flourishing of the archaeocyath fauna and trilobites of the so-called “Sanashytykgol” complex.

The proposal advanced here to place the lower boundary of the Middle Cambrian in the Sayan-Altai region at the base of the Kayanchin Stage, which contains many forms of abrupt renewal both among the archaeocyaths (Erbocyathus, Sajanocyathus, Aptocyathus) and among the trilobites (Kooteniella, Chondragraulos, Olenoides, Shivelicus), is not something new, but represents a development of views on the question under consideration that were expressed earlier by E. V. Lermontova and especially by A. G. Vologdin [1].

This question will probably continue to be a subject of discussion in the future, but in discussing it one can no longer insist on the erroneous concept that archaeocyaths on a planetary scale supposedly do not cross the upper boundary of the Lower Cambrian.

LITERATURE

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

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

3. Belousov A. F., Sennikov V. M. Cambrian of Northeastern Altai. In: Materials on regional geology, Gosgeolizdat, 1960.

4. Biostratigraphy of the Paleozoic of the Sayan-Altai Mountain Region. Transactions of SNIIGGiMS, vol. 1, issue 19, 1960.

1. Vinkman M. K. Stratigraphy of the Proterozoic, Sinian, and Cambrian deposits of the Gorny Altai. Materials on Regional Geology, Gosgeolizdat, 1959.

2. Vologdin A. G. The stratigraphic significance of archaeocyathids. Reports of the Academy of Sciences of the USSR, vol. III, no. 1, 1956.

3. Zhuravleva I. T., Repina L. N., Khomentovskii V. Z. Biostratigraphy of the Lower Cambrian of the folded margin of the Minusinsk Basin. Bulletin of the Moscow Society of Naturalists, Geological Section, no. 2, 1959.

4. Zhuravleva I. T., Repina L. N., Khomentovskii V. V. Lower Cambrian horizons of Gornaya Shoria. Reports of the Academy of Sciences of the USSR, vol. 128, no. 5, 1959.

5. Zhuravleva I. T. Archaeocyathids of Siberia and their significance for the stratigraphy of the Lower Cambrian. Stratigraphy of the Late Precambrian and Cambrian. International Geological Congress, XXI Session, Reports of Soviet Geologists, 1960.

6. Pokrovskaya N. V. Stratigraphy of the Cambrian deposits of the south of the Siberian Platform. Problems of the Geology of Asia, Academy of Sciences of the USSR, 1954.

7. Poletaeva O. K. New genera and species of Cambrian trilobites of Western Siberia. Materials on Paleontology and Stratigraphy of Western Siberia, Gosgeolizdat, 1960.

8. Resolution of the Interdepartmental Meeting on the Development of Unified Stratigraphic Schemes for Siberia. Gosgeolizdat, 1959.

9. Romanenko M. F. On the biostratigraphy of the ancient strata of Gorny Altai. Bulletin of the West Siberian and Novosibirsk Geological Administrations, no. 2, 1960.

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

11. Suvorova N. P. Cambrian trilobites of the east of the Siberian Platform. Olenellids—Granulariids. Transactions of the Paleontological Institute of the Academy of Sciences of the USSR, issue 2, 1960.