ON THE STRATIGRAPHIC POSITION AND AGE OF THE ELAN AND OBRUCHEV HORIZONS

A. G. SIVOV, V. D. TOMASHPOlSKAYA

The question posed in our article has been discussed repeatedly in the literature and was raised with particular urgency at the Interdepartmental Stratigraphic Conference in Novosibirsk in 1965; however, even now it cannot be considered confidently resolved.

This question is most closely connected with the question of the lower boundary of the Middle Cambrian and therefore is of fundamental importance.

As is well known to a broad circle of geologists, the Elan horizon of the Siberian Platform and the Obruchev horizon of the Sayan-Altai region are considered to be of the same age and are identified by the presence in the beds of archaeocyathids and also erbiid and trilobites, of which the most frequently encountered forms are: Edelsteinaspis ornata Lerm., Kooteniella slatkowskii (Schm.) and other species, Erbia sibirica (Schm.), Granularia obrutchevi, and several other genera and species that are not obligatory for every occurrence with the Obruchev faunal complex.

The stratotype of the Elan horizon is considered to be the Elan suite; the stratotype of the Obruchev horizon is considered to be the limestones of Mount Dolgii Mys*).

The analysis of the stratigraphic position of the Elan and Obruchev horizons and of their paleontological characteristics, given below, shows that the correlation of these horizons should be approached more cautiously, and that geological subdivisions not corresponding to a single stratigraphic level are very often included in the concepts of the “Obruchev” and “Elan” horizons.

Before proceeding to the presentation of the material directly on the subject of the article, we consider it necessary to give a very brief characterization of the Lower and Middle Cambrian deposits of the Batenëv Range and the Azyr-Tala Range, and thereby to determine the stratigraphic position of the beds with the Obruchev faunal complex.

We assign to the Lower Cambrian the Bagrad and Chesnokov suites, a detailed characterization of which is given in an article by one of the authors (13).

The Bagrad suite, composing the limbs of the Bagrad syncline, is represented, from bottom to top, by: 1) light massive or shaly limestones with interbeds of light dolomites and dark bedded limestones, with a total thickness of 1300 meters. In the dark bedded and distinctly platy—

*) NE of the village of V. Erba, 3.5 km.

limestones at several stratigraphic levels contain gastropod remains; 2) light-gray massive and coarse-slabby limestones with interbeds of dark-gray limestones, more rarely dolomites and siliceous rocks, with a total thickness of 600 meters. In interbeds of dark limestones on the summit of the mountain with elev. 590.3, hyoliths have been found, and near the top of the Bagrad Formation, in interbeds of dark limestones, abundant trilobites of the genus Bullaiaspis and olenellids occur (13). In age we correlate these deposits with the Kolba Formation. The overlying Chesnokov Formation is widespread over most of the sections of the Batenev Ridge and the Azyr-Tal Range and has a very uniform composition; it is represented by massive light-gray and white limestones with a large and diverse assemblage of archaeocyaths and trilobites.

In the lower part of the formation, on the basis of the assemblage of archaeocyaths and trilobites, the presence of the Bazaikha and Kameshkov faunal assemblages is established (8, p. 28), and in the middle and upper parts—the Sanashtykgol and Aidash assemblages (the latter transitional from the Sanashtykgol to the Obruchev) (13, 15).

Within the higher stratigraphic subdivisions on the Batenev Ridge we distinguish: 1) deposits of an age transitional from the Lower to the Middle Cambrian, represented by the Yuliya Formation with two subformations and two biostratigraphic horizons: the Obruchev and Azyrtal; and 2) deposits of unquestionably Middle Cambrian age, represented by the Karasuk Series, consisting of three formations: the Sukhoerba, Sladkokorenev, and Eldyakh formations*). In our earlier works these deposits were subdivided somewhat differently (15).

The lower subformation of the Yuliya Formation throughout the area studied has a predominantly uniform composition and is represented by massive and coarse-slabby limestones of light-gray and white color, sometimes with dark coffee-colored beds. To this subdivision we assign: the limestones of Mt. Dolgii Mys (near the village of V. Erby); limestones exposed along the right and left sides of the Sukhaya Erba River in the area between the Yuliya mine and the Karasuk mine; and, on the Azyr-Tal Range, the Solontsov limestones exposed in the very near-summit part of the mountain with elev. 803.5 (17, p. 147) also belong to the same level. The faunal assemblage in the lower subformation of the Yuliya Formation is defined as Obruchev. Before proceeding to characterize this assemblage, it should be emphasized that in all sections, except the area of Sukhie Solontsy, the base of the Yuliya Formation is not characterized faunistically. In the area of Sukhie Solontsy, deposits of the lower subformation of the Yuliya Formation in the eastern part**) constitute a sequence of uniform limestones that form, lithologically, a single whole with the underlying Chesnokov Formation, characterized by Sanashtykgol and lower faunas (2, 8).

Analysis of the fauna of this horizon reveals its very uniform composition at all localities. The most complete Obruchev assemblage has been established in the limestones of the mountain 803.5, from which, according to our collections and determinations, faunal lists have been given in published works (12, 17). Less complete but very similar assemblages have also been established by us at other localities of the subdivision described. Careful analysis of all the lists made it possible to establish certain general regularities in the composition of this assemblage and its differences from the higher Azyrtal assemblage. In our earlier works, as well as in the works

*) This formation was first distinguished by R. T. Bognibova (1).

**) In the western part of the Sukhie Solontsy locality, the lower beds of the Yuliya Formation have a terrigenous-carbonate composition.

those who relied on these works arrived at erroneous conclusions, which are explained by the mixing of two assemblages. Within the Obruchev assemblage proper, representatives of the genera Kooteniella and Kootenia are widely developed, and moreover predominantly granulated forms: Kooteniella acuta N. Tchern., K. cellulifera Lerm., Kootenia sibirica Lerm., K. vologdini Lerm., and, comparatively rarely, Kooteniella slatkowskii (Schm.). In almost all localities in this assemblage there occur such characteristic forms as Batenoides lermontovae Rep., Bonnia globosa Tomaschp., Kocorynexochus convexus Tomaschp., Granularia sp., Edelsteinaspis ornata Lerm., Erbia sibirica (Schm.), Erbia aff. inflata Roman.; Chondragraulos ujaricus Rep. occurs rarely, and Ch. minussensis Lerm. very rarely. Among other forms characteristic of the Obruchev assemblage, in the same localities there are almost always forms characteristic of the Ketemen horizon of the Siberian Platform and not extending beyond the limits of the Obruchev horizon: Eospencia,—Samanioia, as well as forms called by L. N. Repina index forms for the Solontsy horizon, allegedly situated between the Sanashtykol and Obruchev horizons (8). In the paper by a group of authors (18) it is shown that the forms of the so-called Solontsy horizon: Laminurus, Solontzella, Onchocephalina, occur in the same beds as the Obruchev forms, and in the vicinity of the Podtemnogo log I. I. Koptev (18) established that beds with forms of the Solontsy horizon overlie beds with the Obruchev assemblage. We have also identified, within the Obruchev assemblage, representatives of several new genera: Prolaminurus, Namanoiella, and Olenophalus *) (12). To all that has been said above about the Obruchev assemblage it should be added that the archaeocyath assemblage encountered in the same beds differs noticeably from the Elan assemblage by the presence of a large number of genera and species known from demonstrably lower stratigraphic subdivisions (15).

To the upper sub-suite of the Yuliya Formation we assign: 1) the strata of Mt. Ostroy **) (14), 2) the Azyrtal Formation in the Sukhie Solontsy locality (12), and 3) the carbonate sequence exposed over a large area of the Batenev Ridge in the area of the Yuliya mine—Karasuk mine—village of B. Erba, where on the geological map of R. S. Krasnopeeva the Policatov horizon is shown (5). The boundary with the underlying lower sub-suite can be drawn only very conditionally, and only on the basis of fauna.

The geological subdivision described is characterized by a large and diverse assemblage of archaeocyath and trilobite fauna, in which, alongside forms characteristic of the Obruchev horizon, isolated Middle Cambrian genera and species appear, their number gradually increasing upward. This subdivision is most fully characterized paleontologically on the northern limb of the Sukhoerbinsk anticline ), where the rejuvenation of the fauna is clearly established in a continuous carbonate section upward through the section, and the gradual change of the Obruchev assemblage in such a direction that forms such as Kooteniella slatkowskii (Schm.) and Chondragraulos minussensis Lerm., Erbia sibirica (Schm.), Erbia granulosa Lerm. acquire predominant development; single specimens of Middle Cambrian genera appear for the first time: Chondranomocare, Amgaspis, Koptura, Proasaphiscus, Kootenia—(characteristic of the Agyrek horizon), and in individual cases even Olenoides* and others. Alongside such advanced forms, in this assemblage, which we call the Azyrtal,

) A similar form was later named by L. N. Repina as Paleoryctocephalus (8).
) The area of the village of V. Erba, east of Mt. Dolgiy Mys, 3–4 km.
**) On the left bank of the Sukhaya Erba River between the concentrating mill and the Karasuk mine.

occur, but very rarely, genera characteristic of the Obruchev complex: Onchocephalina, Eosopencia Laminurus, Solontzella, as well as Edelsteinaspis and species of the genus Kooteniella, widely developed in the Obruchev complex. All the forms listed from the Azyrtal complex are recorded in almost all localities of the subdivision being described, and predominantly in carbonate sequences. In the sequence of Mt. Ostraya, where beds of interbedded terrigenous rocks, acidic effusives, and dark-coffee-colored platy limestones with thin lenticular interbeds of light limestones are predominantly developed, the paleontological complex is very distinctive and differs from everything that has been said in characterizing the Azyrtal Horizon. Namely, here, in the lower part of the sequence, where it has a carbonate composition and forms a single geological whole with the limestones of the underlying lower subsuite of the Yulinskaya Formation (limestones of Mt. Dolgii Mys), there is also established a rejuvenated Obruchev complex very close to the Azyrtal one in the presence of the genera Ptoasaphiscus, Chondranomocare. In the middle and upper parts of the sequence of Mt. Ostraya, representatives of the family Oryctocephalidae occur in interbeds of thinly platy dark-coffee-colored limestones, along with the new genera Paraoryctocephalops and Oryctocarella (14).

In the same sequence there are numerous small bodies of light massive limestones, in which a large and diverse complex of archaeocyaths and trilobites, completely identical in composition to the forms of the Obruchev complex, is readily collected; this is apparently determined by the more ancient age of these limestones and by their not being syngenetic with the sequence in which they occur. Apparently, small bodies of limestones occurring in the Azyrtal Formation at Sukhie Solontsy are in the same stratigraphic position, which is confirmed by the corresponding composition in them of the fauna of the Obruchev, and not the Azyrtal, complex. Some geologists, on the basis of the presence in the sequence of Mt. Ostraya of representatives of the family Oryctocephalidae, correlate it in age with the Sladkokorenevskaya Formation. We strongly object to such an identification, not only because the generic composition of the oryctocephalids on Mt. Ostraya is distinguished by a more primitive structure, but also on geological grounds: 1) the sequence of Mt. Ostraya, in stratigraphic position and lithology, is very close to the Azyrtal Formation, for which occurrence beneath beds with an undoubtedly Middle Cambrian fauna is clearly established; and 2) in the near-summit part of the mountain, at elevation 667.5 (NNW of Mt. Sladkie Korenya by 1.5–2.0 km), on the archaeocyath–trilobite limestones of the Obruchev complex lies a thin bed of platy dark-coffee-colored limestones with oryctocephalids of the Mt. Ostraya type, and above lie light-coffee-colored limestones of the Sukhoerbinskaya Formation with a very rich Middle Cambrian complex of trilobites.

Thus, a significantly lower position of the sequence of Mt. Ostraya than that of the Sladkokorenevskaya Formation is established with certainty. Without dwelling on the paleontological characteristics of the Karasuk series as a whole, since its Middle Cambrian age raises no doubts in anyone, and the faunal complexes of the deposits of Mt. Sladkie Korenya*) have repeatedly been described in published works (1, 15), we consider it necessary to focus on the characterization of the Sukhoerbinskaya Formation, which is necessary for solving the problem we have posed. In our earlier works, deposits with the Sukhoerbinskaya complex were also, apparently erroneously, included in the composition of the Yulinskaya Formation.

The Sukhoerbinskaya Formation differs noticeably from the deposits underlying it—

*) The terrigenous-carbonate sequence assigned, on the basis of the faunal complex, to the Sladkokorenevskaya Formation was called the Sonskaya Formation in our earlier works.

…both by the light coffee color of the limestones and by a sharply different faunal assemblage and the absence of archaeocyaths. Its geological position in relation to the underlying Yulian Formation can by no means always be established.

The Sukhoerba Formation is exposed most distinctly in the limbs, and especially well in the southern limb of the Sukhoerba anticline, where these deposits rest now on beds with the Obruchev, now on beds with the Azyrtal faunal assemblage and, consequently, unconformably.

Beds with an entirely similar faunal assemblage in generic and species composition are exposed on the western slope of Mt. Arkheotsiata, on hill 667.5, and in many other localities, where their boundary with the underlying deposits of the Yulian Formation is well determined by the faint coffee color of the limestones and the absence of archaeocyaths. They differ from the overlying deposits of the Sladkokornev Formation by a lighter coffee color and a lower stratigraphic position. The thickness of the deposits of the Sukhoerba Formation is usually very small, and therefore in places the Sladkokornev Formation lies directly on beds with the Azyrtal, and sometimes even on beds with the Obruchev faunal assemblage.

The composition of the trilobites of the Sukhoerba Formation differs noticeably from the composition of the trilobites of the Amga Stage of the Siberian Platform by the absence of oryctocephalids and paradoxidids, which occur in the Sladkie Koreni Formation, and is characterized by numerous species of such genera as Chondranomocare, Pachyaspis, Olenoides, Erbia, Chondragraulos, Prohedinella, Kootenia, Solenopleura. The genus Kooteniella is represented both by undoubtedly Middle Cambrian species: Kooteniella mutabilis N. Tchern., and by species passing from the Obruchev Horizon: K. edelsteini Lerm., K. cellulifera Lerm., etc.

Thus, the Sukhoerba assemblage is characterized by a marked predominance of Middle Cambrian genera and species and is readily recognized. In many cases, however, gradual transitions may be observed between the Azyrtal and Sukhoerba faunal assemblages.

Turning to the Siberian Platform, it seems to us that the correlation of the subdivisions distinguished on the Batenev Ridge at the level of the Obruchev Horizon—Middle Cambrian—may be made as follows.

The Elan Horizon in the section of the upper part of the Khomustakh Formation on the Amga River, characterized by N. E. Chernysheva (16, 19), contains:
a) in the lower part (thickness 30 m), typically Obruchev forms: Kooteniella acuta N. Tchern., Edelsteinaspis ornata K. Lerm., Erbia sibirica (Schm.), Namanoia incerta N. Tchern., Eospencia amgensis N. Tchern., Chondragraulos minussensis f. infida N. Tchern., Bathyuriscellus quadratus N. Tchern., Kooteniella slatkowskii (Schm.), Jakutus exiguus and archaeocyaths.

In the upper part, Jakutus exiguus N. Tchern. and Eospencia amgensis N. Tchern. continue and here end their existence. The species Chondragraulos minussensis Lerm. and Erbia sibirica (Schm.) pass still higher; in this same part, single specimens of Middle Cambrian genera appeared: Amgaspis and Koptura (16, 29). Higher in the described section lie deposits with the fauna of the Amga Stage. It seems to us that, by analogy with the section of similar deposits of the Batenev Ridge, the upper part of the Khomustakh Formation should be assigned to the Azyrtal Horizon (Table 1), and at the base of the Amga Stage one should assume the presence of a significant hiatus, into which a considerable part of the Azyrtal Horizon and the entire Sukhoerba Formation with the Sukhoerba faunal assemblage will also enter.

For a characterization of the Elan Horizon in the stratotype on the Lena River (Yakutia, the area of the village of Elanka), we use the materials of N. P. Suvorova), from which it is evident that the lower parts of the Elan Formation (15 meters thick) contain forms characteristic chiefly of the Azyrtal) Horizon: Kooteniella slatkowskii (Schm.), Chondragraulos minussensis Lerm., Erbia sibirica (Schm.), as well as genera Paramicmacca Lermontova; and in the upper parts—(25 m thick)—Kooteniella slatkowskii (Schm.), Erbia sibirica (Schm.), E. granulosa subsp. Suv., Granularia obrutchevi Polet., Edelsteinaspis ornata Lerm., Chondranomocare sp., Ragetia cf. ferox Lerm., Anabaraspis sp., as well as Olenoides sp. and Schistocephalis sp., Ampioton sp., Amgaspis sp., Ogygopsis (Taxioura), several species of the genus Chilometopus, and also forms of a low stratigraphic level: Bonnia, Onchocephalina. Archaeocyaths are present. The complex fauna cited from the upper parts of the Elan Formation can only very provisionally be correlated with the Azyrtal and transitional-to-Sukhoerba complexes (Table 1). In our earlier works we considered it possible to draw the lower boundary of the Middle Cambrian at the base of the beds with the Obruchev fauna complex and even lower (15)**). In the present article we argue that this boundary should be drawn at the base of the Azyrtal Horizon, without ruling out that the Azyrtal complex ought to be distinguished as an independent biostratigraphic subdivision with an age transitional from the Lower to the Middle Cambrian. In our opinion, the Sukhoerba complex should also be assigned to an independent subdivision, since the composition of its fauna differs noticeably from the fauna complex of the Amga Formation and is linked by transitions with the Azyrtal complex.

We propose that these two subdivisions be united into the Sukhoerba Stage, with an age transitional from the Lower to the Middle Cambrian and the lower parts of the Middle Cambrian.

With the Amga Stage, as it seems to us, only the Sladkokornev and Eldyak formations should be correlated (Table No. 1).

In conclusion, we consider it necessary to emphasize that the deposits with the Obruchev fauna complex, studied in the stratotype of the Obruchev Horizon on Mount Dolgii Mys and in the parastratotype at Sukhie Solontsy, where, alongside the so-called guide Obruchev forms, there are numerous forms of the Ketemen Horizon, and where in the immediately underlying beds rich Sanashtykgol and Aydashev trilobite complexes are collected (12, 15), should more correctly be assigned to the Lena Stage, with an age of the upper part of the Lower Cambrian. The criterion for drawing the boundary between the Obruchev and Azyrtal horizons should be considered the appearance of certain Middle Cambrian genera or species while such forms as Edelsteinaspis ornata, Kooteniella slatkowskii, etc., continue to exist; these naturally should be regarded as having lost their guiding significance as forms of the Obruchev Horizon.

As can be seen from a recently published work by a group of authors (20), fauna complexes are also distinguished in the deposits of the Gorny Altai: for deposits above the Obruchev beds, with the Ust-Cherga complex, which, according to the characterization given to it, can be correlated with the Azyrtal; and still higher—the Suyaryk complex, which apparently should be correlated with the Sukhoerba. Similar deposits, apparently, can also be identified in other districts of the Sayan-Altai region.

) With her kind permission.
) In our interpretation.
**) A. G. Sivo remains of the former point of view.

Table 1

Scheme of the stratigraphic subdivision of the Lower and Middle Cambrian deposits of the Batenev Ridge and the Azyr-tash Range
Compiled by V. D. Tomashpolskaya

References

1. R. T. Bognibova. Stratigraphic position and age of the unit of archaeocyathid-trilobite limestones in the area of Mount Arkheotsiatovaya and Sladkie Korniya (Batenyov Ridge). Materials on the Geology, Geophysics, and Mineral Resources of Siberia, Tr. SNIIGGiMS, issue 34, 1965.

2. R. T. Bognibova. New data on the stratigraphy of Cambrian deposits in the Sukhie Solontsy area (Azyr-tal Ridge). Materials on the Geology, Geophysics, and Mineral Resources of Siberia. Tr. SNIIGGiMS, issue 34, 1965.

3. M. G. Vinkman, A. B. Gnilitsner. On the correlation of Cambrian deposits of the western part of the Sayano-Altai region. Tr. SNIIGGiMS, issue 24, 1962.

4. A. G. Vologdin. On the stratigraphy of the eastern slope of the Kuznetsk Alatau. Soviet Geology, No. 27, 1947.

5. P. S. Krasnopeeva. Algae and archaeocyathids of the oldest strata of the Potekhin district of Khakassia. Materials on the Geology of Krasnoyarsk Krai, issue III, publ. of the West Siberian Geological Trust, 1937.

6. E. V. Lermontova. Section “Trilobites.” In: Atlas of Guide Fossils of the USSR, vol. 1. Gosgeolizdat, 1940.

7. O. K. Poletaeva. Fauna of Cambrian trilobites of the Sanashtykol limestone of Western Sayan. Materials on the Geology of the West Siberian Krai, No. 35, 1936.

8. L. N. Repina, V. V. Khomentovskii, I. T. Zhuravleva, A. Yu. Rozanov. Biostratigraphy of the Lower Cambrian of the Sayano-Altai Region. Moscow: Nauka, 1964.

9. Decisions of the Interdepartmental Conference on the Development of Unified Stratigraphic Schemes for Siberia. 1959. Gosgeoltekhizdat.

10. A. G. Sivov. On the lower boundary of the Middle Cambrian in the Sayano-Altai region. Proceedings of the Tomsk Polytechnic Institute, vol. 121, 1963.

11. N. P. Suvorova. Trilobites of the order Corynexochida and their historical development. Transactions of the Paleontological Institute, Academy of Sciences of the USSR, Nauka Publishing House, vol. CIII, 1964.

12. V. D. Tomashpolskaya. On the stratigraphy of the Batenyov Ridge. Materials on the Geology of Western Siberia, Izv. Tomsk., 1962.

13. V. D. Tomashpolskaya, R. B. Karpinskii. On finds of trilobites of the genus Bulaiaspis in Lower Cambrian deposits of the Batenyov Ridge.

14. V. D. Tomashpolskaya, R. B. Karpinskii. On the find of Middle Cambrian trilobites in the area of the village of Sukhaya Erba (Batenyov Ridge). Proceedings of the Tomsk Polytechnic Institute, vol. 120, 1961, Tomsk.

15. V. D. Tomashpolskaya. Stratigraphy and paleontology of the Cambrian of the Batenyov Ridge. Author’s abstract of dissertation. 1964.

16. N. E. Chernysheva. Stratigraphy of the Cambrian of the Aldan syneclise and paleontological substantiation for distinguishing the Amga Stage. 1961.

17. A. P. Shcheglov, V. D. Tomashpolskaya, N. P. Suvorova, I. I. Koptev et al. On the lower boundary of the Middle Cambrian of Western Siberia. Stratigraphy of the Precambrian and Cambrian of Central Siberia, Krasnoyarsk, 1967.

18. A. P. Shcheglov, V. D. Tomashpolskaya, I. I. Koptev, R. T. Bognibova. On the Solontsy and Ketemen horizons in the Sayano-Altai region. Krasnoyarsk, 1967. Ibid.

19. N. E. Chernysheva. Principal questions of the stratigraphy of Middle Cambrian deposits of Central Siberia. Ibid.

20. E. V. Shirokova, M. F. Romanenko, N. A. Aksarina. On the early Middle Cambrian of the Sayano-Altai region. Ibid.