On the Stratigraphy of the Lower Cambrian of the Krasnoyarsk Ridge

I. I. KOPTEV

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

The area of the middle course of the Mana River is characterized by the wide distribution of paleontologically richly characterized Lower Cambrian deposits, which by the present time have been studied fairly well.

The basic principles of the stratigraphy of the Cambrian of the area were set forth by I. A. Molchanov [1] and later by V. M. Chairkin [9], who distinguished the dolomite–quartzite–sandstone, variegated Koy Formation and the conformably overlying, predominantly carbonate Kolba Formation with a trilobite fauna. Later, in 1954–1956, this scheme, with some modifications, was elaborated in detail by L. N. Repina, M. A. Semikhatov, and V. V. Khomentovskii [5], and L. N. Repina [6, 7] studied a rather diverse trilobite fauna, on the basis of analysis of which she carried out a correlation with the reference Cambrian sections of the Siberian Platform. From V. M. Chairkin’s Koy Formation they distinguished (from below) the Angul and Anastas’ino formations \(Cm_1\), and the Badzhei Formation \(Cm_3—O\). The lower carbonate part of V. M. Chairkin’s Kolba Formation and the upper part of the Koy Formation were included by them in the Zherzhul Formation, whose upper beds are correlated with the Sinsk horizon of the platform; the remaining part of V. M. Chairkin’s Kolba Formation was left under the same name and divided into three subformations (from below): the variegated Vyezhego Log subformation and the carbonate Siner and Novovasilyevka subformations, which correspond to the Tolbachan, Olekma, and Ketemen horizons of the Siberian Platform.

Beginning in 1936 the area was visited repeatedly by K. V. Radugin, who made a major contribution to the development of the stratigraphy of the ancient strata of this area. During the period from 1954 to 1959, I. I. Koptev and K. V. Radugin carried out systematic mapping of the area of distribution of the Cambrian and Late Precambrian strata of the Mana and Bazaikha river basins. As a result of the work carried out, rich paleontological material was collected and a geological map was compiled. In the sector from the settlement of Vyezhego Log to the settlement of Upper Bazaikha, within the belt of Cambrian and Late Precambrian deposits, a trilobite fauna was collected at more than 160 localities; archaeocyaths were found at 5 localities, brachiopods at 10, academiolites at 25, hyoliths at several localities, epiphytonids were encountered at no fewer than 50 localities, and stromatolitic algae were discovered at numerous localities (about 55). The material obtained

makes it possible to introduce certain changes and additions to the existing ideas on the stratigraphy of this area.

Stratigraphy and organic remains of the Late Precambrian and Lower Cambrian of the area

According to L. N. Repina, M. A. Semikhatov, and V. V. Khomentovsky [5], the Cambrian begins with the basal conglomerate of the Angul Formation, which transgressively overlies Archean and Proterozoic formations; among these they distinguished: 1) an Archean intrusive-metamorphic complex, structurally representing a horst-like uplift along major faults, as had also been noted by many previous investigators; 2) a Lower Proterozoic metamorphic complex—the Kozhelak Sequence; 3) the Derba Formation of graphitic marbles of the Middle Proterozoic; 4) the Zhaima Formation of siliceous-graphitic shales of the Middle Proterozoic; and 5) the Upper Proterozoic, represented by the Kuvai Formation of basic and intermediate greenstone effusives and their tuffs, the Pavlovsk Formation of marbles, and the Urman Formation of gray sandy shales.

In studying the base of the Koi Formation (Angul—V. V. Khomentovsky et al.), we confirmed the previously expressed opinion that part of the “Archean” rocks was formed as a result of contact-injection metamorphism of the effusives of the Kuvai Formation of the Upper Proterozoic. This fact casts doubt on the existence of Archean in this area at all, or requires, at the very least, that the “Archean” in its former understanding be divided into the Archean proper and rocks erroneously assigned to it that arose during contact-injection metamorphism in Late Proterozoic time. The metamorphism was caused by the intrusion of an acidic composition, emplaced in the form of small, numerous bodies into the sequence of older rocks, including the Kuvai Formation.

Before proceeding to characterize the Cambrian deposits proper, it is necessary to dwell somewhat on the question of the position of the lower boundary of the Lower Cambrian. The point is that, at present, this question for the given area cannot be considered finally resolved, since, first, the relationship between the Koi—in the sense of V. M. Chairkin (Angul—V. V. Khomentovsky et al.)—and the Anastasya formations remains unclear; and, second, K. V. Radugin considers the Lower Cambrian section to begin at the base of the Krol Subformation (the lower carbonate part of the Kolba Formation of V. M. Chairkin, or the upper part of the Zherzhul Formation of V. V. Khomentovsky et al.).

It was believed [5, 9] that in the southeastern part of the area, from the Mana River to the Kuvai River, there rests unconformably upon the Precambrian rocks an almost three-kilometer-thick sequence of variegated conglomerates, quartzites, and sandstones (the Koi Formation of V. M. Chairkin, or the Angul Formation of V. V. Khomentovsky et al.); and to the west—northwest of the Kuvai River, the Anastasya Formation likewise rests unconformably on the rocks of the Kuvai Formation through basal graywacke conglomerates, sandstones, and shales, with the Anastasya Formation being the spatial continuation of the Koi Formation along strike. This relative position of the formations, and the fact that the Koi Formation is underlain by quartz-rich intrusive and metamorphic rocks, whereas the Anastasya is underlain by greenstone effusives of the Kuvai Formation, gave V. M. Chairkin grounds to regard the lower graywacke part of the Anastasya Formation as a facially altered Koi Formation.

L. N. Repina, M. A. Semikhatov, and V. V. Khomentovsky, considering the Anastasya Formation younger in relation to the Angul Formation, and saying

of gradual transitions between them, do not convincingly explain the sudden disappearance northwest of the Kuvai River of a broad belt of supposedly underlying red beds and the discordant occurrence of the Anastas’ino Formation on more ancient deposits.

At present, facts have been obtained that indicate a more ancient age of the Anastas’ino Formation in comparison with the Koi Formation (the Angul Formation of V. V. Khomentovsky et al.). These facts are as follows.

1. On the watershed of the Krol—Solbiya rivers and in the upper reaches of the Kirza River (right tributary of the Kuvai River), weakly metamorphosed greenish-gray sandstones, marls, and shales occur as patches and narrow belts; gray weakly marbled limestones and white and white-pink limestones and dolomites underlie the basal conglomerate of the Koi Formation and are contained as pebbles in this overlying conglomerate. These limestone-sandstone-shale rocks in outward appearance largely resemble the rocks of the Anastas’ino Formation and the white and white-pink dolomites distributed along the Zherzhul River (the Zherzhul Formation of K. V. Radugin).

2. West of the Kuvai River the Anastas’ino Formation rests, with erosion, on the effusives of the Kuvai Formation, but red-bed rocks of the Koi Formation are not found among the pebbles of the basal conglomerate of the Anastas’ino Formation.

3. In the sections along the Mana River near the settlement of Vyezzhii Log, the Angul Formation of V. V. Khomentovsky et al. is inseparably connected with the Leiba Formation, which constitutes its upper part and rests on the former with a gradual transition. V. M. Chairkin combined these strata into a single Koi Formation and regarded the latter as lying below the Kolba Formation. This unity makes it possible to consider that the sandy dolomites, sandstones, and quartzites exposed along the Kolba River near the settlement of Cheremushki and lying undoubtedly above the Anastas’ino and Zherzhul formations are the northwestern continuation of V. M. Chairkin’s Koi Formation or of the Angul Formation of V. V. Khomentovsky et al. On the basis of these facts, it must be recognized that the Anastas’ino Formation and the white and white-pink dolomites of the Zherzhul River basin that are closely connected with it lie below V. M. Chairkin’s Koi Formation (the Angul Formation of V. V. Khomentovsky et al.) and can hardly be assigned to the Cambrian.

In addition, K. V. Radugin establishes a major break at the base of the Krol Subformation, characterized in its very upper part by fauna of the Sinsk horizon of the Lena Stage. The considerable thickness of this subformation (about 1200–1300 m) gives grounds to regard it as equivalent to the Yudoma Formation of the Aldan Stage; and if this is so, then, taking into account the break in pre-Krol time and the fact that the underlying Leiba and Koi formations are still assigned to the Cambrian, one can confidently speak of the Precambrian (Sinian?) age of the Anastas’ino and Zherzhul formations. Finds of pteropods in the Anastas’ino Formation do not preclude its Late Precambrian age.

Taking into account the above, the stratigraphic scheme of the Late Precambrian and Lower Cambrian deposits of the region is accepted in the following form.

Late Precambrian — Sinian (?) System

Included here are deposits that, in degree of metamorphism, are closely connected with the Cambrian strata, are overlain by the latter with erosion, and rest sharply discordantly on undoubtedly Upper Proterozoic formations. They also cannot be assigned to the Cambrian in view of the considerable thickness of the paleontologically uncharacterized strata included in it, lying below beds with the fauna of the Sinsk horizon.

Anastas′inskaya Formation. Its most complete and well-exposed section is the section along the Kolba River in the area of the village of Anastas′ina. In its lower part the formation is represented by basal graywacke fine-pebble conglomerates, sandstones, and shales about 100 m thick. Above, with a gradual transition, lies a member consisting of alternating dark-gray and black platy limestones, calcareous dolomites, dirty-gray marls, and graywacke calcareous-micaceous shales. The thickness of this member is about 500 m. Still higher, also with a gradual transition, lies a member of dark-gray limestones, platy in the lower part and massive in the upper part, with epiphytons. The thickness of the epiphyton limestones is about 1000 m.

Zherzhul′ Formation. This includes, in contrast to the content of the Zherzhul′ Formation of V. V. Khomentovskii et al., only white and whitish-pink dolomites and dolomitic limestones, often containing epiphytons and distributed mainly in the basin of the Zherzhul River (it was in precisely this sense that this formation was first distinguished by K. V. Radugin). On the Anastas′inskaya Formation, in the section along the Kolba River, this formation lies with gradual transitions, whereas at other points there are indications of a slight break at their boundary. The thickness of the formation, owing to the absence of indications of bedding, is not established.

Lower Cambrian

Koiskaya Formation. Its assignment to the Lower Cambrian is still provisional. We understand the formation in almost the same scope as it was first distinguished by V. M. Chairkin, except for its upper part, now separated as the independent Leibinskaya Formation, and except for the graywackes distributed west—northwest of the Kuvai River and referred to the Anastas′inskaya Formation. In its lower part the Koiskaya Formation is represented by conglomerates 100 to 500 m thick, overlain above by a 50-meter member of reddish-gray coarse-grained sandstones. Still higher lie predominantly red-colored arkosic sandstones and quartzites, 500–600 m thick, with rare interbeds of dolomitic sandstones, overlain by a member, approximately 60 m thick, of gray sandy dolomites. The section of the Koiskaya Formation ends with a member of arkosic sandstones and quartzites, 200–300 m thick, with interbeds of lilac-gray and pink dolomites and sandy dolomites. Thus, the total thickness of the formation is 900–1400 m. The described section of the formation was observed along the Krol River, a left tributary of the Mana River.

Approximately the same section and the same thickness of the formation are observed in outcrops along the Mana River near the settlement of Tyulüp. The difference between this section and the preceding one is the absence of a carbonate member and the greater coarseness of the material.

The section of the Koiskaya Formation along the Mana River near the settlement of Koi differs considerably, where the formation is represented by rather uniform brownish-greenish-gray fine-grained sandstones almost four kilometers thick. V. V. Khomentovskii et al. [5, 8] assign this sequence, together with the Narva conglomerate, to the Upper Cambrian–Ordovician, calling it the Badzheiskaya Formation. However, such facts as the presence, among the sandstones near the settlement of Koi, of interbeds of lilac and pink dolomites and sandy dolomites characteristic of the Koiskaya Formation in the section along the Krol River, and the gradual transition along strike of the sandstones occurring near the settlement of Koi into red-colored quartzites and quartzitic sandstones exposed near the settlement of Tyulüpa, and further into greenish quartzites exposed near the settlement of Kiyai (the latter rocks are, undoubtedly, more ancient in comparison with the Kolbinskaya Formation), for the time being compel us to refrain—

…to refrain from subdividing the Koi Formation into the Angul and Badzhei formations.

It should be noted that in the summer of 1959, in an exposure of conglomerates located 7 km east of the village of Oreshnaya and assigned on the geological map to the lower half of the sequence of red sandstones of the Koi Formation, we encountered, in pebbles of gray limestones, trilobites of Middle Cambrian aspect, characteristic of the limestones found in the pebbles of the Narva conglomerate; moreover, the conglomerates of the locality described are outwardly indistinguishable from the Narva conglomerates. However, until the relationship of these conglomerates to the sandstones of the Koi Formation has been clarified (Badzhei Formation according to V. V. Khomentovsky et al.), one should not speak of an Upper Cambrian age for the entire sequence of red sandstones.

Leiba Formation¹. The recognition of an independent Leiba Formation is based on the fact that the underlying Koi Formation pinches out gradually to the northwest, while the Leiba Formation already rests directly on the Zherzhul and even the Anastasino formations, with a conglomerate at its base. In the section along the Mana River near the settlement of Vyezzhii Log, the Leiba Formation lies with a gradual transition on the Koi Formation, and the boundary between them is not quite distinct here. In the area of the settlement of Murtuk and the mouth of the Leiba River, the formation rests on the Zherzhul and Kuvai formations through a thick conglomerate. The type section of the Leiba Formation is the section near the mouth of the Leiba River, where, in the lower part, there is exposed a member of gray marly platy dolomites with thin interbeds of dirty-gray micaceous shales and sandstones. The incomplete thickness of the member is 30 m. Above it, with gradual transitions, lies a 200 m member of lilac, pink, and pink-red dolomites with interbeds of cherry-colored dolomitic shales. Still higher follows a 50 m member of brown and brown-red quartzites and sandstones with subordinate interbeds of cherry-colored siltstone-shales and sandy dolomites, among which individual quartzite horizons reach 9 m. The section of the formation ends with a variegated 40 m member consisting of alternating cherry-colored and greenish micaceous thin-platy shales, gray dolomites that turn yellow on weathering, and cherry-red marly-sandy dolomites. The total thickness of the formation in this section is about 300 m.

Near the settlement of Murtuk, beneath the red-colored rocks with which the above-described section begins, there is exposed a thick sequence of graywacke sandstones and conglomerates, which are probably basal for the Leiba Formation and rest on the dolomites of the Zherzhul Formation. The pebbles of the conglomerate, in addition to all other rocks, contain effusives of the Kuvai Formation, white-pink dolomites of the Zherzhul Formation, and red quartzites, probably of the Koi Formation.

The section of the formation along the Mana River above the settlement of Vyezzhii Log likewise begins with a member of dark-gray dolomitic limestones with interbeds of dirty-gray micaceous shales, very intricately crumpled. This member is identified by V. V. Khomentovsky et al. [5, 8] as the Anastasino Formation. Farther upward, with gradual transitions, follow lilac and pink, often sandy, dolomites, in the upper part of which there appears a member of brown and brown-red quartzites. The thickness of the formation in this section is estimated at approximately 800–900 m.

The formation has exactly the same section in exposures on the right bank of the Kolba River, 1.5 km above the settlement of Cheremushki, where on white and white-pink

on the dolomites of the Zherzhul Formation lie platy dark sandy limestones and dolomites, replaced higher up by lilac and pink dolomites, and then by quartzites. The thickness of the formation in this area is 750–800 m.

It is possible that it would be more correct to assign the quartzites that complete the section of the formation to the basal beds of the overlying Kolba Formation (as K. V. Radugin does, drawing the boundary of a major break at the base of the quartzites), but the gradual transition to the quartzites from the underlying dolomites for the time being compels one to refrain from doing so.

Kolba Formation. It is the most fully paleontologically characterized and well studied. Four subformations are distinguished within it (from below): the Krol, variegated—Vyezzhii Log, Gogolev, and Ermakov.

The Krol subformation is represented predominantly by gray and dark-gray dolomites and limestones, partly by light-gray calcareous dolomites, and by rare interbeds of cherry-colored shales and yellow marls. In the uppermost beds of the subformation, in gray massive limestones, the following occur: Hyolites sp., brachiopods—Kutorgina flerovae Lerm. and Botsfordia caelata Matthew; trilobites—Judaiella inepta Repina, Krolina pressulata Rep., Manaspis oblitterus Rep., Solenaspis amara gen. et sp. nov., and two forms gen. et sp. indeterm. In the northeastern limb of the syncline (left bank of the Mana River, 1200 m below the settlement of Vyezzhii Log), stratigraphically somewhat below the beds with the fauna noted above in the Krol subformation, the trilobite Bulaiaspis taseevica Repina was found.

The middle part of the subformation is characterized by the spread of stromatolitic algae and by an abundance of siliceous inclusions and lens-shaped interbeds of chalcedony. In the very lowest part of the Kolba Formation, at the mouth of the Krol River, in the variegated member, poorly preserved brachiopods resembling Lingulella sp. were encountered. The thickness of the Krol subformation is 1200–1300 m.

V. V. Khomentovskii and others [5, 8] include the Krol subformation in the Zherzhul Formation on the grounds that light-colored dolomites, characteristic of the Zherzhul Formation proper, occur within it at Vyezzhii Log. However, many facts contradict such an artificial union.

The Krol subformation is composed predominantly of dark-gray carbonate rocks, among which rare interbeds of variegated beds occur in the section at Vyezzhii Log, and numerous ones in the section along the Solbi River. These are characteristic rocks of the Kolba Formation as a whole, not encountered in the field of development of the Zherzhul dolomites proper. This indicates the existence throughout the entire Kolba time of generally approximately similar facies conditions, with insignificant changes in time and space. Nowhere along the strike of the Krol subformation do we see a transition of dark carbonate rocks into light-colored ones (the Zherzhul dolomites proper), whereas on all maps a considerable widening of the belt of variegated beds to the NW of Vyezzhii Log is shown, occurring as a result of facies replacement of the rocks of the Krol subformation, i.e., its transformation into the variegated one.

Both in the Krol and in the other subformations of the Kolba Formation there occur Bulaiaspis taseevica Rep., Kutorgina flerovae Lerm., Botsfordia caelata Matthew, and stromatolitic algae, whereas in the Zherzhul dolomites proper we have only epiphyton algae.

In the section at Vyezzhii Log, the Krol subformation lies above the Leiba Formation, whereas in all areas the Leiba Formation lies

higher than the Zherzhul’ Subformation. All this gives grounds for including the Krol’ Subformation in the Kolba Formation.

The next subformation of the Kolba Formation is the variegated Vyezzhii Log Subformation, represented by an alternation of cherry-red, yellow, and greenish siltstones and marls, gray yellowish dolomites, dark-gray stromatolitic limestones, gray marls, and brown calcareous sandstones. This subformation is fluorite-bearing and gypsum-bearing. Desiccation cracks and traces of wave ripples are often observed on bedding surfaces. Trilobites have been found in several horizons, chiefly p. Bulaiaspis (B. sajanica Rep., B. vologdini Rep., B. taseevica Rep., B. sp. No. 1 and B. sp. No. 2—evidently new species) and one trilobite form close to the Olenellidae—Eobulaiaspis sajanicus Koptev. Stromatolitic algae are not infrequently found in dolomite interbeds. Thickness 240 m.

Still higher lies the Gogol’ Subformation of black and dark-gray limestones and calcareous dolomites (corresponding to the lower part of the Siner Subformation of V. V. Khomentovskii et al.). The thickness of the subformation in the section at Vyezzhii Log is about 300 m, but toward the northwest it apparently increases. It overlies the variegated unit with gradual transitions. The Gogol’ Subformation is everywhere saturated with diverse organic remains—trilobites, archaeocyaths, brachiopods, pteropods (hyoliths and academeolites), stromatolitic and oncolitic algae, and worm trails. Among the trilobites occurring in this subformation are: Bulaiaspis sajanica Repina, B. limbata Rep., B. taseevica Rep., Jnouyina subquadratica Rep., Tungusella manica Rep., T. obesa Rep., Bigotina coniferica Rep., Solenopleurella Kolbinica Rep., Kolbinella prima Rep., Kolbinella exacta sp. nov., Kolbinia proxima gen. et sp. nov., Termieraspis sp., Parapoliella sulcata N. Tchern., P. obrutchevi Lerm., Parapoliella commutabilia sp. nov., Jakutus planus sp. nov., Janguidaspis sp., Binodaspis spinosa Lerm., Bergeroniaspis divergens Lerm.; several new trilobite forms; and brachiopods—Kutorgina flerovae Lerm. and Botsfordia caelata Matthew.

An interesting feature of the subformation is the distribution of the trilobite fauna in the horizontal direction. In the southeastern part of the region (the settlement of Vyezzhii Log) and in the northwestern part (the settlements of Oreshnaya and Narva), chiefly species of the genera Bulaiaspis, Tungusella, Bergeroniaspis, Bigotina, and others are distributed, whereas in the central part (the settlement of Novovasil’evka) Parapoliella, Solenopleurella, and Binodaspis occur predominantly. Such a spatial separation of forms characteristic on the Siberian Platform, on the one hand, of the Tolbachan and, on the other, of the Ketemen horizons, gave L. N. Repina grounds for distinguishing an independent Novovasil’evka Subformation of the Kolba Formation and assigning it a younger age. This distinction was not confirmed during geological mapping, and now the paleontological data also contradict it. Thus, in the area of the settlement of Nizhnyaya Solbiya there are finds of Parapolilla in beds definitely belonging to the Gogol’ Subformation, underlying the light-colored limestones of the overlying Ermakovo Subformation; moreover, tubular fossils with sharply conical partitions—academeolites—characteristic of these horizons are also found here. In addition, in the central part of the area occupied by outcrops of the “Novovasil’evka” Subformation (with a predominance of Parapoliella and Solenopleurella), we found Bulaiaspis and Bigotina, again together with academeolites.

Especially indicative in this respect is the sector in the area of the settlement of Novoalekseevka. Here, proceeding along the right bank of the Bol’shoi Ungut River upstream, beginning at the mouth of the Belaya River,

first there occur brown and brownish-red quartzites (about 300–400 m), then variegated rocks (the section runs across the strike of the Kolbinskaya Formation from west to east, beginning from its base), represented by an alternation of cherry-red, red, yellow, greenish-yellow marls, shales, and calcareous sandy shales with interbeds of black limestones, and farther on—black and dark-gray limestones and dolomites. Only at the village of Novoalekseevka itself, in the central part of the syncline, do light-gray and white limestones appear. Farther east the rocks are repeated in reverse order, i.e., we have the other limb of the synclinal fold. The variegated rocks and quartzites most probably correspond to the Krolskaya Subformation and the Vyezzhii Log Subformation, the black limestones to the Gogolevskaya Subformation, and the white limestones represent small patches of the Ermakovskaya Subformation. On the geological map of V. V. Khomentovskii and others these subformations are also shown here, i.e., the Tolbachan and Olekminsk beds of the Lena Stage. However, in a bed of black limestones west of the settlement of Novoalekseevka, 300 m away, we found the trilobites Bulaiaspis sajanica Rep. and B. limbata Rep.; and on the western outskirts of the settlement, in this same bed, there were found Binodaspis spinosa Lerm., Parapoliella obrutchevi Lerm., Jakutus planus sp. nov., archaeocyathids, and Parapoliella sp. (probably a new species). Thus, in one bed of black limestones there occur trilobite forms characteristic of both the Olekminsk and the Ketemen horizons. Moreover, at a point situated on the right bank of the Bazaikha River, 1.5 km downstream from the Shakhmatov zaimka, the following occur together in the same samples: Tungusella cf. manica Rep., Bulaiaspis limbata Rep., Parapoliella sp., Binodaspis spinosa Lerm., Jangudaspis sp., Bulaiaspis sajanica Rep., Kutorgina sp., and others.

Therefore, the beds assigned by L. N. Repina to the Novovasilyevka Subformation, supposedly having a higher stratigraphic position in relation to the Gogolevskaya Subformation, should be regarded as synchronous with the latter. The reason for the difference in the compositions of the trilobite fauna, evidently, must be sought in certain differences in habitat conditions in the separate areas. Possibly the deeper-water part of the Lower Cambrian sea in the area of the settlement of Novovasilyevka (as indicated by the absence of admixture of terrigenous material in the carbonate rocks of this area) was characterized by more favorable conditions for the existence of some forms, while the shallower-water area along the margins (the wide distribution of marls and marly limestones) was favorable for others.

The archaeocyathids encountered in the Gogolevskaya Subformation also do not provide an unequivocal solution to the question of its age. At first they were identified by I. T. Zhuravleva as Kameshkovsk, later as Bazaikha, and, finally, as Sanashtyk-Gol forms.

The Gogolevskaya Subformation above is gradually replaced by the Ermakovskaya, consisting of light-gray and white limestones and dolomites. The rocks of the Ermakovskaya Subformation have mostly been eroded and are preserved as patches only in the most downwarped parts of the main syncline. The incomplete thickness of this subformation is 400 m. Organic remains are represented by indeterminate fragments of algae and burrows of boring animals; and near the settlement of Vyezzhii Log one incomplete cranidium of some very large trilobite was encountered.

The Ermakovskaya Subformation ends the section of the Lower Cambrian deposits of this area. The next complex—the Narvskaya Formation $Cm_3$, beginning with a conglomerate, in the pebbles of which occur limestones with remains of trilobites—Binodaspis spinosa Lerm., Parapoliella sulcata N. Tchern., Parapoliella obrutchevi Lerm., Solenopleurella sp., and two or three forms gen. et sp. indeterm., as well as limestones with remains of stromatolitic and oncolitic algae, worm burrows, and, possibly,

academiolites (poorly preserved fossils of an elongate-conical form). The thickness of the Narva Formation is 2.5 km.

As is evident from the above lists of fauna, the overwhelming majority of forms are new, which makes it difficult to carry out a reliable correlation of the Cambrian of this region with the reference sections of the Lower Cambrian of the Siberian Platform. In general, of course, L. N. Repina’s opinion is correct that the Kolba Formation corresponds to the Lena stage of the platform, or to some part of it, but a more detailed comparison requires a more cautious approach. To take at least the following example: the upper horizon of the Krol Subformation, on the basis of the presence of Judaiila, is correlated by L. N. Repina with the Sinsk horizon of the platform, but at the same time representatives of the genus Bulaiaspis, characteristic of the Tolbachan horizon, occur here. On the other hand, in the Variegated Subformation, correlated with the Tolbachan beds, representatives of the olenellid group occur, while in the beds of the Gogolevka Subformation, together with species of the genera Bulaiaspis, Tungusella, Inoujina, Kolbinella, Termieraspis, characteristic of the Tolbachan and Olekma horizons of the platform, there occur Parapoliella, Solenopleurella, Binodaspis, Pseudoteraspis, distributed on the platform in the beds of the Ketemen and Elan horizons; and here also occur archaeocyathids of the Bazai complex of Tolbachan time.

From this it follows that trilobite forms which characterize definite horizons in the Cambrian section of the Siberian Platform may, under geosynclinal conditions, characterize beds of a somewhat different stratigraphic position, or cover a larger stratigraphic range. An error in determining the age of beds containing a predominantly endemic fauna entails an error in establishing the stratigraphic significance of these new forms.

Thus, despite the fairly good degree of study of the Lower Cambrian deposits of the area of the middle course of the Mana River, many questions of stratigraphy still remain unresolved and require a more detailed study of the entire assemblage of organic remains, together with more detailed and thorough geological mapping. In this connection, special attention should be paid not only to changes in the composition of the fauna vertically, but also horizontally within one and the same geological-stratigraphic unit. The present state of study of the paleontological material from the Lower Cambrian of this region does not make it possible at this time to establish a reliable horizon-by-horizon linkage with the sections of the Lower Cambrian of the platform.

REFERENCES

1. Molchanov I. A. Eastern Sayan. Essays on the Geology of Siberia, issue 5, 1934.

2. Radugin K. V. The Cambrian of the Krasnoyarsk Ridge. Problems of the Geology of Asia, vol. 1, 1955.

3. Radugin K. V., Belousov A. F. On the relationship of the Torgashin limestone to the ancient strata of the Krasnoyarsk Ridge. East Siberian Branch of the Academy of Sciences of the USSR, Transactions of the Mining-Geological Institute, issue 17, 1956.

4. Radugin K. V. On the paleontological distinction of the systems of the Upper and Middle Proterozoic. Proceedings of the Interdepartmental Conference, 1957.

5. Repina L. N., Semikhatov M. A., Khomentovsky V. V. On the stratigraphy of the Cambrian deposits of the western part of the Eastern Sayan. Doklady of the Academy of Sciences of the USSR, vol. 110, no. 1, 1956.

6. Repina L. N. Paleontological substantiation of the age of Cambrian deposits of the western part of the Eastern Sayan. Doklady of the Academy of Sciences of the USSR, vol. 110, no. 2, 1956.

7. Repina L. N. Description of the genus Bulaiaspis, in the collection Materials on Paleontology, issue 12, new series, 1956.

8. Khomentovsky V. V. On the tectonics of the Eastern Sayan. Izvestiya of the Academy of Sciences of the USSR, geological series, no. 7, 1957.

9. Charkin V. M. Report on the conduct of geological-survey work. Funds of KGU, 1948.

1. The formation was first distinguished in 1954 by K. V. Radugin in the area of the settlement of Spirino under the name Spirino, but in print it first appeared under the name Leiba. ↩