BIOSTRATIGRAPHIC SCHEME FOR THE SUBDIVISION OF THE LOWER MIDDLE CAMBRIAN OF THE BATENEVSKY RIDGE

A. K. SEMASHKO

(Presented by the scientific seminar of the departments of general geology, historical geology, and combustible minerals)

Over a number of years the author carried out biostratigraphic studies in the northern part of the Batenevsky Ridge. Geological sections of the lower Middle Cambrian, situated in the area of the Yuliya mine—Katyushkino village, were studied. The results of these investigations have been published in several of our works.

The most complete section of the lower Middle Cambrian is located on Mount Balykhtag [8], where the deposits of the Efremkino Formation, according to lithological features and successively replacing one another faunal assemblages, are subdivided into three subformations (members).

At the base of the section lie light-colored massive limestones of the lower subformation of the Efremkino Formation (member “A”), forming the core of an anticlinal fold (the base of the sequence is not exposed), with numerous trilobites of the genera Erbia, Proerbia, Edelsteinaspis, Kootenia, Kooteniella, Bonnia, Eocorynexochus, Paleoorictocephalus, Chondranomocare, Chondragraulos, Pseudanomocarina (?), Taniaspidella, Bunerus, Elrathiella (?) and others, as well as archaeocyaths of the genera Ajacicyathus, Ethmophyllum, Szecyathus, Tegerocyathus, and others.

Above, with a gradual transition, occur black bedded limestones of the middle subformation of the Efremkino Formation (member “B”) with trilobites of the genera Erbia, Proerdia, Kooteniella, Bonnia, Olenoides (one specimen), Amgaspis (?), Ogygopsis, Chondragraulos, Ch. (Antagmopleura), Gaphuraspis, Menneraspis, and archaeocyaths Tegerocyathus sp., Claruscyathus sp., Flindersicyathus sp.

The dark limestones of the middle subformation are replaced, with a sharp transition, by yellowish massive limestones of the upper subformation of the Efremkino Formation (member “V”), in whose deposits there occur trilobites of the genera Amgaspis (two species), Kooteniella, Kootenia, Olenoides (two species), Chondragraulos, Pseudanomocarina (?), Chondranomocare, Erbia, Granularia; archaeocyaths are absent.

The lower part of the section of the Efremkino Formation on Mount Balykhtag is well correlated with the sections at the mouth of Podtemny Log [5] and at the Kureney locality. Thus, the light-colored massive limestones of the section at the mouth of Podtemny Log are correlated with member “A” of the Balykhtag section on the basis of the common occurrence of such trilobites as Chondragraulos ujaricus Rep., Edelsteinaspis ornata Lerm., Proerbia torgaschinica Rep., Kootenia sp., Kooteniella sp., Bonnia sp., and others, and also by the appearance of such typically Middle Cambrian forms as Chondranomocare sp., Ch. (Antagmopleura) sp. Some archaeocyaths are also common. The deposits of this interval

are absent in the section of the Kurenei locality; they correspond to a hiatus established between the deposits of the Kurenei Formation of the Lower Cambrian and the middle subformation of the Efremkino Formation.

Stratigraphic analogues of the limestones of member “A” of the Balykhtag section occur in the area of the Bogradskii Most locality [6] and in the region of the “Khakasskii” fur farm [9].

The light massive limestones in the section at the mouth of Podtemnyi ravine are overlain by dark-gray limestones, which are characterized by the following trilobites: Onchocephalina pertenuis Rep., O. arguta Rep., Laminurus aff. insuetus Rep., Proerbia aff. anomalica Rep., Bonnia sp., Jangudina quadrata Kopt., Prohedinella sp.

We correlate these limestones with the limestones of member “B” of the section of Mount Balykhtag on the basis of their stratigraphic position (occurrence upon coeval light massive limestones) and the lithological similarity of the rocks.

The stratigraphic analogue of member “B” of the Balykhtag section in the area of the Kurenei locality is the gray massive limestone with trilobites Olenoides (?) sp., Kooteniella sp., Ptarmigania sp., Ogygopsis aff. exacutus (E. Rom.), Gaphuraspis sp., Bonnia sp., Erbia sp., Prohedinella sp., Onchocephalina sp., Laminurus sp., Proerbia sp., and archaeocyaths.

The deposits of the Efremkino Formation in different sections are overlain by carbonate-terrigenous and carbonate rocks of the unnamed or Karasuk formations, with trilobites of the genera Olenoides, Oryctocephalops, Oryctocephalina, Pachyaspis, Schistocephalus, Peronopsis, Juliaspis, Binella.

As a result of the study of the stratigraphic succession, the correlation of sections of the lower part of the Middle Cambrian, and analysis of the trilobite fauna in them, it appears possible to distinguish at least three successive assemblages corresponding to three biostratigraphic levels—the Khakass, Balykhtag, and Mundybash horizons. The first two horizons are being distinguished for the first time.

The stratotype of the Khakass horizon is taken to be the light massive limestones of the lower subformation of the Efremkino Formation in the section of Mount Balykhtag (member “A”). The stratotype of the Balykhtag horizon is also located on Mount Balykhtag and is represented by the dark-gray limestones of the middle subformation of the Efremkino Formation (member “B”). The boundaries, general paleontological characteristics, and geographic distribution of the horizons distinguished will be discussed below.

The paleontological characterization of the Khakass horizon, compiled on the basis of materials from the area studied, is as follows: trilobites—Edelsteinaspis gracilis Lerm., E. ornata Lerm., Kootenia anomalica Rep., Kooteniella acuta N. Tchern., K. slatkowskii (Schm.), K. edelsteini Lerm., K. cellulifera Lerm., Bonnia globosa Tom., Eocorynexochus sp., Erbia sibirica (Schm.), E. granulosa Lerm., Proerbia torgaschinica Rep., Paleooryctocephalus angulatus Rep., Chondragraulos minussensis Lerm., Ch. ujaricus Rep., Ch. (Antagmopleura) sp., Chondranomocare cf. planum Rom., Pseudanomocarina (?) sp., Batenoides sp., Koptura sp., Solontzella enorma Sem., Onchocephalina sp., Laminurus sp., Granularia obrutchevi Polet., Taniaspidella plana Sem., Taniaspidella eleganta Sem., Taniaspidella asinosa Sem., Bunerus mobilis Sem. (SP), and others. Numerous archaeocyaths, similar to archaeocyaths of the Obruchev assemblage, also occur here. Unfortunately, the lower boundary of the horizon is not known in the stratotype; however, in sections located in the immediate vicinity of Mount Balykhtag, deposits of the Efrem—

...of the [[unclear: -kin?]] Formation overlie, with a major hiatus, the Precambrian dolomites of the Martukhin Formation. On this basis it may be assumed that, in the stratotype of the Khakassian Horizon as well, the underlying deposits for the beds of the Khakassian Horizon are most likely the Martukhin dolomites.

The upper boundary of the Khakassian Horizon in the continuous section of the stratotype is the base of the Balykhtakh Horizon.

As is evident from the foregoing, a particularly important point is the establishment of the lower boundary of the Khakassian Horizon in a continuous section. For this purpose the most promising, in our opinion, is the Dolgii Mys—Ostraya Mountains area, whose geological structure has been studied by us personally and from published data. The oldest deposits of the area are the light-colored massive limestones of Dolgii Mys Mountain, with the fauna of the stratotype of the Obruchev Horizon. On the southwestern slope of Ostraya Mountain, in erosional windows amid volcanogenic-sedimentary rocks, higher horizons of light-colored massive limestones, making up Dolgii Mys Mountain, are exposed. That is, we may suppose that during denudation the deposits corresponding to the Khakassian Horizon and to the overlying effusive-sedimentary sequence on Dolgii Mys Mountain were eroded, as a result of which an older level, corresponding to the Obruchev Horizon, was uncovered. However, the possibility is not excluded that over the large area occupied by the limestones of Dolgii Mys Mountain, younger deposits have also been preserved in places. The trilobite fauna encountered in one of the erosional windows near the summit of Ostraya Mountain corresponds almost completely to the assemblage of the stratotype of the Khakassian Horizon. According to R. T. Bognibova, I. I. Koptev, A. G. Sivov, and others [2], the following trilobites were found in this locality: Kooteniella slatkowskii (Schm.), K. acuta N. Tchern., Edelsteinaspis ornata Lerm., Erbia sibirica (Schm.), E. granulosa Lerm., Laminurus sp., Pagetia ferox Lerm., Chondranomocare bidjensis Polet., Kootenia minima Ivsh., Chondragraulos ujaricus Rep., Binodaspis sp. Archaeocyaths of the genera Erbocyathus, Tegerocyathus, Ethmophyllum, and others were also found here. In one of the erosional windows, in light-colored massive limestones, we found Paleoryctocephalus angulatus Rep. and Taniaspidella plana Sem.

Thus, in the continuous section of this stratigraphic interval, the lower boundary of the Khakassian Horizon is the Obruchev Horizon.

The second biostratigraphic level distinguished by us corresponds to the middle subformation of the Efremkino Formation. It is characterized by the following faunal assemblage: trilobites—Ogygopsis sp., Ogygopsis aff. exacutus (E. Rom.), Olenoides sp., Kooteniella slatkowskii (Schm.), K. acuta N. Tchern., K. insolita Sem. (MS), Kootenia hirsuta Suv., Bonnia sp., Ptarmigania sp., Amgaspis (?) sp., Gaphuraspis aff. gaphuri Ivsh., Gaphuraspis sp., Erbia sibirica (Schm.), Proerbia anomalica Rep., Menneraspis delicata Jegor., Jangudina sp., Chondragraulos minussensis Lerm., Ch. (Antagmopleura) sp., Chondranomocare sp., Prohedinella sp., Onchocephalina sp., Laminurus sp.; archaeocyaths—Tegerocyathus edelsteini (Vol.), Flindersicyathus sp. and some new forms.

When this faunal assemblage is compared with the assemblage of the Khakassian Horizon, we see that, along with a certain continuity of the fauna, there is a considerable difference in its composition toward a predominance of unquestionably Middle Cambrian forms. We propose to give this level the name Balykhtakh Horizon, after the place where its stratotype was established on Mount Balykhtakh. The base of the Balykhtakh Horizon in the stratotype is the Khakassian Horizon; its top is the Mundybash Horizon.

For the deposits of the described level, different names were proposed at different times by various investigators: the Suyaryk Horizon (M. K. Vinkman, A. B. Ginzinger, M. F. Romanenko, et al. [3], E. V. Romanenko, M. F. Romanenko [7], R. T. Bornibova [1]) and the Azyrtal Horizon (A. B. Ginzinger, M. K. Vinkman, et al. [4]). The name “Suyaryk Horizon” cannot be accepted, since, in the opinion of most geologists, the fauna in its stratotype is redeposited. The term “Azyrtal Horizon” was first proposed by V. D. Tomashpolskaya. In extent this horizon corresponds to the Khakassian and Balykhtak horizons taken together. The Azyrtal Horizon of A. B. Ginzinger, M. K. Vinkman, et al. [4], correlated with the Suyaryk Horizon of Gorny Altai, in its stratotype (Sukhie Solontsy level—the Azyrtal Formation) has very few forms in common with the latter. This applies not only to trilobites, but also to archaeocyaths. If, however, we compare the Azyrtal assemblage with the Khakassian, we see their almost complete identity, except for the absence in the Azyrtal assemblage of Edelsteinaspis sp. The latter is evidently explained by the facies confinement of representatives of this genus to pure limestones, whereas the limestones of the Azyrtal Formation contain much clayey material. Of definite interest is the presence in the Azyrtal assemblage of numerous representatives of the genus Solontzella and individual forms of the genus Batenoides, the latter representatives of which in the Batenevsky Ridge do not range above the Khakassian Horizon. By contrast, such typical representatives of the Balykhtak (Suyaryk) assemblage as Ogygopsis sp., Ptarmigania sp., Olenoides sp., and others are absent from the stratotype of the Azyrtal Horizon.

Thus, we cannot accept the term “Azyrtal Horizon” for two reasons: 1) one and the same name is used by different authors for deposits of different extent; 2) the extent of the Azyrtal Horizon does not correspond, in our opinion, to the concept invested in it by A. B. Ginzinger, M. K. Vinkman, et al.

Deposits lying stratigraphically above the unnamed and Karasuk formations belong to the Mundybash Horizon. The difference in the qualitative composition of the trilobites of the two formations is explained, in our view, not so much by the difference in age of these deposits (although this factor, too, undoubtedly plays a certain role), as by their difference in facies.

Biostratigraphic detailing of the indicated deposits was not the purpose of our investigations.

Summing up the above and taking as the lower boundary of the Middle Cambrian the base of the Obruchev Horizon, we propose the following biostratigraphic scheme for the lower part of the Middle Cambrian of the Sayan-Altai folded region:

References

1. R. T. Bognibova. The Middle Cambrian of the Batenevsky Ridge and adjacent areas of the south of Krasnoyarsk Krai. Abstract of a Candidate’s dissertation. Tomsk, 1966.

2. R. T. Bognibova, A. P. Sheglov, A. K. Semashko et al. The Amgan Stage of the Altai–Sayan region. Transactions of SNIIGGiMS. Issue III, 1971.

3. M. K. Vinkman, A. B. Gintsinger, A. G. Pospelov et al. A stratigraphic scheme of the Lower Cambrian and the lower part of the Middle Cambrian deposits of the Altai–Sayan folded region. Materials on the Regional Geology of Siberia. Transactions of SNIIGGiMS. Issue 24, 1962.

4. A. B. Gintsinger, M. K. Vinkman et al. Sections of the Precambrian and Lower Paleozoic of the western part of the Altai–Sayan folded region. Krasnoyarsk: Krasnoyarsk Book Publishing House, 1969.

5. I. I. Koptev, A. K. Semashko, V. A. Shipitsin. On the question of the age of the sequence of carbonate-siliceous limestones in the area of the Katyushkino village (Batenevsky Ridge). Proceedings of TPI, vol. 166. Tomsk: TSU Publishing House, 1969.

6. I. I. Koptev, V. E. Nomokonov, A. K. Semashko, V. A. Shipitsin. On the stratigraphy of the ancient sequences in the area of the upper reaches of the Sukhaya Erba River. Proceedings of TPI, vol. 177. Tomsk: TSU Publishing House, 1971.

7. E. V. Romanenko, M. F. Romanenko. On the biostratigraphic subdivision of Middle Cambrian deposits of Gorny Altai. Transactions of the Interdepartmental Conference on the Development of Unified Schemes of Siberia. Krasnoyarsk Book Publishing House, 1967.

8. A. K. Semashko, V. A. Shipitsin. On the biostratigraphy of Cambrian deposits in the area of Mount Balykhtag (Batenevsky Ridge). Proceedings of TPI, vol. 166. Tomsk: TSU Publishing House, 1969.

9. A. K. Semashko, T. V. Yankauskas. On the stratigraphy of Middle Cambrian deposits in the area of the settlement of Oktyabrsky (Batenevsky Ridge). Proceedings of TPI, vol. 177. Tomsk: TSU Publishing House, 1971.