REVISITING THE QUESTION OF THE AGE OF THE BAGRAD FORMATION

V. D. Tomashpolskaya

(Presented by the scientific seminar of the Department of General Geology)

The Bagrad Formation was first distinguished by us jointly with R. B. and N. I. Karpinsky in 1958 (1963) in the area of the village of V. Erba-Bagrad, where it occurs on the limbs of the Bagrad syncline upon dolomites of the Martaikha Formation and is overlain by light-colored massive limestones of the Usi Formation, which make up the closure of the fold.

At that time, within the Bagrad Formation we distinguished two subformations. The lower subformation is represented mainly by massive or schistose limestones with thin interbeds of light-colored dolomites and interbeds of gravellites. In the middle and upper parts of the subformation there are interbeds of distinctly thin-platy gray-black limestones, in which we first established remains of the gastropods Scenella and Helcionella (determinations by N. A. Aksarina).

The upper subformation is composed of light-gray massive and coarse-platy limestones with interbeds of dark-gray limestones, rarely dolomites and siliceous rocks. In interbeds of dark limestones in the lower part of the subformation, hyoliths were discovered, and near the top, jointly with R. B. and N. I. Karpinsky, we found for the first time on the Batenev Ridge trilobites of the genus Bulaiaspis taseevica Repina et al. The first determinations were made by us*, and later, from the same beds, L. P. Repina identified olenellids (Repina, 1964, p. 28). On the SW limb of the same syncline, the deposits assigned by us to the Bagrad Formation differ sharply in lithological composition and, as shown below, are very similar to the rocks of the Kutebuluk Formation in its stratotype. Organic remains on the SW limb were not established by us at that time.

Later, this sequence was divided by M. K. Vinkman et al. (1969), taking into account the works of L. N. Repina et al. (1964) and A. Yu. Rozanov et al. (1966) (in which this sequence was paleontologically characterized), into two formations: the lower—the Brodovskaya—and the upper—the Bagrad; consequently, the Bagrad Formation in the scope distinguished by us should be transferred to the rank of a series. Below is given a description of the Bagrad Series**.

The lower part of the Bagrad Formation Series is stratigraphically simple in terms of its content of gastropod remains: Scenella and Helcionella. These beds are exposed by bedrock outcrops along the Bagrad—V. Erba road

* D. M. Yaroshevich (1962) reports this fact in a highly distorted form.
** With the use of the works of L. N. Repina et al. (1964), A. Yu. Rozanov et al. (1966), M. K. Vinkman et al. (1969).

in the last third of the way, where gastropods were first collected by us together with R. B. and N. I. Karpinsky in the summer of 1958. Such organic remains are known to be characteristic of the Ust-Kundat horizon of the Lower Cambrian in the Kuznetsk Alatau.

The Brodovskaya Formation is correspondingly assigned a unit represented in both limbs of the syncline by massive light-gray, saccharoidal limestones with interbeds and packets of dolomitic limestones. In this unit, in the SE limb, the work of A. Yu. Rozanov et al. (1966) established the presence of cataraphytes of the Vendian complex, while in the NW limb of the same syncline, in similar deposits, M. K. Vinkman et al. (1969) showed the presence of oncolites, spicules, sponges, and Lower Cambrian algae of the genus Epiphyton, on the basis of which those same geologists determine the age of the Brodovskaya Formation as the very base of the Lower Cambrian. The section of the Brodovskaya Formation established in the area of V. Erba–Bagrad is almost the only one for the entire Batenyov Range, as well as for other districts of the Sayano-Altai region, as is emphasized in the work of M. K. Vinkman et al. (1969). We have distinguished deposits of the Brodovskaya Formation only along the Poperechny Log in the area of the Bagrad bridge, where they rest on deposits of the Litvinskaya Formation and, moreover, unconformably and even with a conglomerate at the base (according to the data of V. E. Nomokonov) and are composed of limestones with packets of dolomites, in which cataraphytes of the Vendian complex and Lower Cambrian algae of the genus Epiphyton*** are found.

The Bagrad Formation in the same section between the villages of V. Erba and Bagrad, in the NW and SE limbs of the same syncline, has dissimilar lithological and paleontological characteristics. Thus, in the NW limb of the Bagrad syncline, above beds with gastropods and hyoliths that form the base of the formation, there lie massive light-gray and pinkish limestones with thin interbeds of dolomites, in which there are small packets, in terms of thickness, of dark-gray platy limestones. Trilobites of the genus Bulaiaspis taseevica and olenellids of the genus Follotaspidella were identified, as indicated above, in similar limestones in the top of the formation; and according to the data of B. G. Kraevsky (M. K. Vinkman et al., 1969), the same packets of bedded black limestones, 10–40 m thick, with trilobites of the genera Bulaiaspis and Bagradia, were established there at several levels in light-colored limestones.

In the SE limb of the same syncline, the Bagrad Formation is represented by an alternation of thin- and thick-platy limestones and siliceous shales. In this unit, in its lower part, according to the work of A. Yu. Rozanov et al. (1966), algae of the genera Epiphyton and Rasumovskia, hyolithellids, and isolated archaeocyaths were found, known in the Ust-Kundat Formation of the Kiysky section in the Kuznetsk Alatau. In the beds situated near the top of this unit and beneath the massive light-colored limestones of the Usinskaya Formation, L. N. Repina distinguished two packets: the lower, represented by dark-gray bedded limestones 400 m thick with trilobites of the Kiysky complex, or beds with Resimopsis, and the upper—100 m thick, of brownish-gray platy limestones with trilobites of the Kameshkovsky complex, or beds with Sayanaspis. The archaeocyath complex in both packets, according to the data of Yu. A. Rozanov (1966), is almost identical and as a whole is characterized as Subsanash-Tykgol. By decision of the Interdepartmental Conference (1965), it is also called the Erbinsky. In the same section, M. K. Vinkman et al. (1966) distinguish the beds with Sayanaspis at the base of the massive limestones of the Usinskaya Formation, and accordingly they are the top of the Bagrad Formation

* Oral communications.
** Finds by S. S. Gudymovich.

are placed along the top of the beds with Resimopsis, and the beds with Bulaiaspis and olenellids of the NW limb of the Bagrad syncline are assigned to the same stratigraphic level as the beds with Resimopsis of the SE limb. L. N. Repina et al. (1964) assign these same beds to the level of the beds with Sayanaspis, which seems to us more reliable, since the beds with Sayanaspis contain trilobites of the genus Bulaiaspis, and also because in both limbs, below, there occur homogeneous massive grayish-white limestones in which, from the very base, there are numerous archaeocyaths containing certain forms that are undoubtedly Sanashtyk-Gol forms. The beds with Sanashtyk-Gol trilobites make up the closing part of the Bagrad syncline. To the level of the Bagrad Formation, in our opinion, should also be assigned the terrigenous-carbonate sequence—the Kutenbuluk Formation, exposed along the Kutenbuluk depression and composing the central part of the Kutenbuluk syncline, where it was first distinguished by A. N. Churakov in the upper reaches of the Yenisei Formation with a Cambrian age. It should be emphasized that, in lithological composition, the Kutenbuluk Formation in the stratotype is very similar to the deposits of the Bagrad Formation in the SE limb of the Bagrad syncline; their paleontological characteristics are also very similar.

The first finds of algae of the genus Epiphyton in this formation were made by A. A. Predtechensky and A. P. Shcheglov (1960), and thereby its Cambrian age was determined. Later G. M. Ekhanin found archaeocyaths of the genera Coscinocyathus and Archaeolynthus, while M. K. Vinkman et al. also indicate trilobite carapaces, brachiopods—Acrothelle—and gastropod shells; on this basis they determine the age of the formation as no older than the Bagrad horizon (1969, p. 86). It seems to us that there is insufficient evidence for such a conclusion, and that it is more correct to place this formation at the level of the Ust-Kundat horizon.

M. K. Vinkman et al. (1969) assign to the Bagrad Formation a siliceous-terrigenous-marly-limestone sequence first distinguished by I. I. Koptev under the name of the Kurenin Formation. This sequence is exposed on the eastern limb of the Katyushkin syncline in the area of the Podtemnyi ravine. As is known from the works of I. I. Koptev (1964) and the later works of A. N. Repina (1967), M. K. Vinkman et al. (1969), and E. V. Shirokova (1967), this sequence contains in its upper part the Sanashtyk-Gol faunal assemblage, while at the base the presence of beds with the Ust-Kundat assemblage is not excluded. Thus, as a whole, this sequence, in our opinion, should be placed at the level of the entire Usin Formation, and only its lowest parts may be comparable with the level of the Bagrad Formation proper. Consequently, as it seems to us, the Kurenin Formation as a whole cannot be called the Bagrad Formation in the sense that it has in its stratotype, i.e., within the scope of the Ust-Kundat horizon and the beds with Sayanaspi and Resimopsis.

REFERENCES

1. B. G. Kraevsky. On the stratigraphy of the Lower and Middle Cambrian of the basin of the Belyi Iyus River. Tr. SNIMGGIMS, No. 24, 1962.

2. V. A. Borisov. Age and biostratigraphic subdivision of the Yenisei series of the Batenëv Ridge and the Azyrtal Ridge. Krasnoyarsk, 1963.

3. M. K. Vinkman et al. Sections of the Upper Precambrian and Lower Paleozoic of the western part of the Altai-Sayan folded region. Krasnoyarsk, 1969.

4. I. I. Koptev, A. K. Semashko. On the stratigraphy of ancient sequences of the upper reaches of the Sukhoy Erbin River (Batenëv Ridge). Izv. TPI, vol. 218, 1970.

5. N. V. Pokrovskaya, N. S. Zaitsev. On the structure of the Kuznetsk Alatau. “Sovetskaya geologiya,” 1958, No. 6.

6. L. N. Repina, V. T. Khomentovsky et al. Biostratigraphy of the Lower Paleozoic of the Sayan-Altai folded region. “Nauka,” 1964.

7. L. N. Repina, I. T. Zhuravleva, V. V. Khomentovsky. Scheme of biostrati-

stratigraphic subdivision of the Lower Cambrian of the Sayan–Altai folded region. Siberian Branch of the Academy of Sciences of the USSR, Institute of Geology and Geophysics, Krasnoyarsk, 1967.

1. A. Yu. Rozanov, V. V. Missarzhevsky. Biostratigraphy and fauna of the lower horizons of the Cambrian. “Nauka,” 1966.

2. A. Yu. Rozanov and V. V. Missarzhevsky. On the question of the lower boundary of the Cambrian. “Geology and Geophysics,” 1962, No. 3.

3. V. D. Tomashpolskaya, R. B. Karpinsky. On the discovery of trilobites of the genus Bulaiaspis in Lower Cambrian deposits of the Batenev Ridge. Proceedings of TPI, vol. 121, 1965.

4. V. D. Tomashpolskaya. Stratigraphy and paleontology of the Cambrian of the eastern part of the Batenev Ridge. Abstract of Candidate’s dissertation, Tomsk, 1964.

5. V. D. Tomashpolskaya. On the question of the correlation of Precambrian deposits of the Batenev Ridge and the Azyrtal Range. Geology. Proceedings of the conference dedicated to the 75th anniversary of the Institute; TPI Publishing House, 1973.

6. E. V. Shirokova, D. M. Bondareva. The Son-Katyushkinsky reference section of the Precambrian and Cambrian in the Kuznetsk Alatau. Stratigraphy of the Precambrian and Cambrian of Central Siberia, Siberian Branch of the Academy of Sciences of the USSR, Krasnoyarsk, 1967.