NEW DATA ON THE STRATIGRAPHY OF THE RIGHT BANK OF THE TUBA RIVER (EASTERN SAYAN)

G. V. POLYAKOV

(Presented by Professor, Doctor Yu. A. Kuznetsov)

Cambrian deposits are widely distributed over the extensive territory of the Eastern Sayan. For the most part, they are in active relationships with later intrusive formations and often contain deposits of iron and other useful minerals. At the same time, questions of the geology and stratigraphy of these deposits are resolved with great difficulty and very slowly. This is due to two circumstances. First, the Cambrian deposits are considerably metamorphosed, as a result of which the paleontological remains contained in them are characterized by extreme scarcity and poor preservation. Second, the results of geological investigations on the stratigraphy of the Cambrian of the Eastern Sayan are very poorly represented in print. All this is especially characteristic of the Cambrian of the southwestern margin of the Eastern Sayan, which includes the right bank of the Tuba River.

Systematic and summarizing works on the study of the Cambrian deposits of the above-mentioned region end with the works of A. G. Vologdin, the results of which were regularly published up to 1937. During the subsequent 20 years, prospecting and surveying work was carried out in the territory of the region of interest to us by the forces of production organizations. However, no new information of any kind was obtained on the stratigraphy of the Cambrian of the Tuba region.

This is explained above all by the fact that, over the extensive territory of the right bank of the Tuba River, from the time of the work of A. G. Vologdin to the present, only two reference points of faunistically characterized Cambrian have been known: the Kameshki section (village of Kameshki) and the Malo-Irba limestone quarry (village of Malaya Irba). Thus, the stratigraphy of the Cambrian of this part was compiled by the method of correlating various strata according to their material composition and structural features, without a sufficient faunistic basis.

As a result of the summer field work of 1955, carried out by the Irba thematic detachment of the Tomsk Polytechnic Institute, consisting of graduate student G. V. Polyakov and fifth-year students Yu. S. Nadler and F. M. Chernov, interesting paleontological remains were collected, additionally characterizing two upper divisions of the Cambrian deposits of the Tuba region.

In the area of the village of Malaya Irba, additional trilobite fauna was collected which, together with that previously collected by S. A. Salun and op-

identified by N. K. Pokrovskaya, makes it possible to regard the ancient deposits of the Irba area as the lower beds of the Middle Cambrian \((Cm_2^1)\).

x—places where fauna was found.

Fig. 1

Interesting finds were made in the area of the village of Moiseevka (Moiseevskii Saibar). Here, for the first time, fauna was found and collected in the limestones of the Shilovka River. This fauna is represented by forms of the family Protoleniidae, rare in the territory of the south of Krasnoyarsk Krai, confirming the Lower Cambrian age of the limestones. Here also, in the “Cambro-Silurian,” according to A. G. Vologdin (1932), sequence of gray-green sandstones and siltstones, a fauna of trilobites and brachiopods was encountered, characteristic of the lower beds of the Middle Cambrian \((Cm_2^1)\). Such an age determination for the sandstones and siltstones of this area makes one doubt the correctness of distinguishing “Cambro-Silurian” deposits in neighboring areas, where they are assigned considerable extent.

Below is a description of stratigraphic sections in whose deposits new organic remains were discovered.

Area of the village of Malaya Irba

The village of Malaya Irba is located in the Kuragino District of Krasnoyarsk Krai, 30 km NE of the district village of Kuragino. The geological setting may briefly be characterized as follows.

The oldest formations accessible to direct observation are the rocks of the Cambrian effusive-tuffaceous sequence, known as the Knyshinsko-Irbinskaya Formation (S. A. Salun, 1950). The rocks of the formation are considerably metamorphosed and are gathered into small folds, with steep (60–80°) dips of the limbs, of meridional strike.

The Knyshinsko-Irbinskaya Formation is overlain with a clear angular unconformity by an effusive sequence, the age of which is determined by all investigators as Lower Devonian.

In the lower parts of the effusive sequence, more basic varieties of effusives predominate—diabase and andesite porphyrites and their tuffs. The andesite porphyrites, for the most part, have been transformed into albitophyres. The upper parts are composed of effusives of subalkaline composition—orthophyres, in places altered into keratophyres.

The Cambrian and Lower Devonian rocks are intruded by an intrusion of quartz alkaline syenites of hypabyssal type, represented by the Irba pluton. The marginal parts of the pluton, especially in contact with the basic Cambrian rocks, are composed of more basic varieties: gabbro-diorites, diorites, and monzonites. Intrusive and effusive ro-

rocks are cut by a series of dike bodies of basic (diabases and diabase porphyrites) and alkaline (alkaline granite-porphyries and quartz albitites) composition. The latter are undoubtedly connected with the intrusion of quartz syenites. The Cambrian and Devonian rocks enclosing the intrusion underwent intense contact alteration, accompanied by the appearance of hornfelses and skarns, often with magnetite mineralization.

The deposits of the Knyshinsko-Irbinskaya Formation, containing organic remains, have a very limited distribution in the area of the village of Malaya Irba and are represented incompletely. The rocks composing the formation are exposed in the section of the B. Irba River over a small area \((1.5 \times 3\ \text{km})\), and structurally they form a small synclinal fold of northern strike. Acid varieties of effusives—strongly altered banded felsites and siliceous shales—are of subordinate importance.

The principal component of the formation consists of diabases, diabase porphyrites, and their tuffs. In essence, such a name reflects not the present, but the primary composition of these rocks. At present the diabases and diabase porphyrites are dark-green, slightly dirty-looking rocks with a fine-grained groundmass and often a porphyritic structure, which have undergone profound greenschist alteration. Under the microscope the rocks possess a prismatic-granular structure and are substantially feldspathic in composition. That feldspar had a basic composition can only be inferred, since it has been entirely replaced by albite. The dark-colored minerals are decomposed and replaced by chlorite. Secondary minerals become widely developed: epidote, calcite, sericite. Thus, in their present manifestation the described effusives are, in composition and structure, closer to rocks of the spilite type. The very characteristic pillow jointing of this group of effusives indicates the submarine conditions of their formation.

Macroscopically, the tuffs are distinguished only with great difficulty from the effusives. By the character of their fragments they belong to lithocrystalloclastic varieties. The fragments are dominated by effusives, plagioclases, and, rarely, carbonates. As a rare variety of the effusives of the formation, mention should be made of gray hornblende porphyrites encountered 1 km upstream along the M. Irba River from its mouth.

Among the effusives and their tuffs described above, carbonate rocks of organogenic origin occur in the form of a small lens. Their most typical representative is a grayish-white, sometimes yellowish, dense marbled limestone, exposed in the quarry on the northern outskirts of the village of M. Irba. It contains abundant, well-preserved trilobite fauna and sparse, poorly preserved remains of brachiopods. Other, essentially structural, varieties of this group are milky-white saccharoidal and spotted breccia-like crystalline limestones, exposed at the base of the left side of the B. Irba River near the southern outskirts of the village of B. Irba. These varieties occur in immediate proximity to the intrusive body of quartz syenites and are, in all probability, the result of recrystallization of limestones of the preceding type.

Of still more subordinate importance in this sequence are the above-mentioned gray-green banded and black-gray siliceous shales.

The fauna from the limestones of the quarry of the village of M. Irba has long been known and has repeatedly been studied by various investigators; however, its description has not been published anywhere.

A. G. Vologdin and E. V. Lermontova identified the following forms in the Irba limestones: Dorypyge Slatkowski Toll, Liostracus Ang.,

Kutorgina cingulata. Later, organic remains were collected from this same locality by S. A. Salun (1950), among which N. V. Pokrovskaya identified new forms: Erbia sp., Chandrogr ualos sp., Kooteniella sp.

Among the trilobite remains collected by us, A. G. Sivoв identified the following forms: Kooteniellina tubaenia Siv., Liostracus aff. aculeatus Ang., Chondragraulos irbiensis sp. nov., Kooteniella slatkovskii (Schm.), Chondragraulos (?) anomalus Siv.

Area of the villages of Moiseevka and Shilovka

The villages of Moiseevka and Shilovka (Troitskoe), situated adjacent to one another (3 km apart), are located in the Krasnoturansk District of Krasnoyarsk Krai, 30 km west of the Sorokino landing stage (Yenisei River).

The oldest rocks of the area are limestones of the mouth part of the Shilovka River, which could be grouped into the Moiseevka Formation. They are represented by milky-white, sometimes yellowish, marbled and locally dolomitized varieties, developed at the confluence of the Shilovka and Moiseevka rivers (Belyi Saibar). Upstream along the Shilovka River, the white marbled limestones of Belyi Saibar are replaced by dark-gray, dense, platy limestones. Thin lenses of siliceous-limestone shales occur within the limestone sequence. In our opinion, all the listed varieties lie conformably in one limb of a large fold, since they all have approximately the same latitudinal strike and a relatively gentle (30–50°) dip to the north. Until very recently no organic remains had been found in this sequence, and its age was determined on the basis of analogy with similar limestones of the B. Telekskii Baitak area (village of B. Telek, Idrinskii District), which in turn were compared with the Torgashino limestones in the environs of Krasnoyarsk.

In 1955, in the Shilovka dark-gray platy limestones, we for the first time discovered and collected trilobite remains, in particular, as identified by A. G. Sivov, Anabaraspis irbiensis sp. nov., Binodaspis sp., and representatives of the family Protolenidae. This fauna clearly indicates a Lower Cambrian age for the limestones of Belyi Saibar.

If one follows the Shilovka River farther upstream, then, after a rather long break in exposures, at the southeastern edge of the village of Shilovka one can encounter bedrock outcrops of a younger sandy-shale sequence, which we unite under the name of the Troitskoe Formation. This sequence is composed in its lower part of calcareous banded shales with thin interbeds of sandy material and dense siliceous shales. Small lenses of marbled limestones occur in the shales. The upper part consists of gray-green fine- and medium-grained sandstones and siltstones, containing remains of trilobite and brachiopod fauna (Cm²). The sandstones and siltstones lie hypsometrically above the rocks of the described carbonate sequence, occupying smoothed watershed landforms (the watershed of the Shilovka River and the Teplyi Klyuch River). Here the sequence is represented by gray-green dense platy siltstones, which are gradually replaced up and down the section, over short intervals (1.5–2 m), by fine- to coarse-grained gray-green sandstones with similar parting.

Under the microscope the siltstones have a clearly clastic character, possessing an evenly grained siltstone structure. The ratio of cement to fragments is 1 : 1. Among the fragments are quartz (30%), calcite (40%), feldspars (15%), ore and micaceous minerals (10%). All of them

by size, well sorted (mean \(d = 0.04\) mm) and rounded. Quartz grains have rounded forms. Some of them are corroded. Calcite grains have uneven, sometimes bizarre, dentate outlines. Elongated tabular and scaly forms are characteristic of fragments of feldspars and micas. The feldspars, represented by plagioclase of intermediate basicity, have undergone strong alteration with the formation of complete pseudomorphs of sericite. The micaceous mineral is identified as muscovite. The ore mineral is represented by limonite, which, in all probability, has replaced pyrite. The cement is basal, clayey-carbonate, with signs of blastesis. At present, owing to partial recrystallization, the cement is represented by a fine-grained aggregate of calcite, quartz, sericite, and chlorite.

The siltstones and sandstones of the northern slopes of the watershed facing the village of Nikolaevka bear, visible in the field, traces of recrystallization and metasomatic transformations, in individual cases turning into typical quartz-epidote hornfelses.

The sandstones adjacent to the siltstones have a composition analogous to them. Somewhat different are the sandstones exposed on the left side of the Shilovka River near the southeastern outskirts of the village of Troitskaya. More correctly, they should be called tuffaceous sandstones. Macroscopically these are gray-green, clearly metamorphosed fine-grained clastic rocks, identified in the field as sandstones. Under the microscope they possess a blasto-psammitic structure with a microlepidoblastic structure of the groundmass. It is difficult to establish the quantitative ratio of cement and fragments, since both have undergone strong recrystallization. Among the fragments, uneven, jagged grains of carbonate clearly predominate (40%); 40% is accounted for by feldspars and quartz, preserved in the rock in the form of relics. The feldspar is represented by laths of plagioclase, completely replaced by sericite. Quartz occurs in the form of rare, strongly corroded residual grains, often enclosed in a calcite shell. No less than 20% of the clastic fraction is represented by scaly sericite and small disseminated grains of ore mineral (limonite), clinozoisite, and epidote. The latter, moreover, occurs in the rock in the form of small veinlets. The cement, completely recrystallized and replaced, is represented by an aggregate of scaly chlorite, sericite, small grains of carbonate, and minerals of the epidote group. Chlorite is distributed unevenly and usually occurs in the rock in the form of separate grains, giving the structure a nodular appearance. Here, near the contact with the granodiorites of the Saibar pluton, completely recrystallized gray-green rocks with relict bedding were encountered. Under the microscope they are identified as typical quartz-amphibole-epidote hornfelses, formed at the expense of the sandstones described above.

The sandy-shale sequence, exposed at the southeastern outskirts of the village of Troitskaya and assigned by us to the Troitskaya suite, is cut by dikes of syenite-porphyries and microgranites having a direct connection with the Saibar pluton, as well as by dikes of diabases and diabase porphyrites, which are the roots of the effusives \((S_2—D_1)\) that overlie the Troitskaya suite.

The ancient Cambrian deposits and the younger effusives \((S_2—D_1)\) are distinctly cut by the intrusion of Mt. B. Saibar, composed of quartz, alkaline, and nepheline syenites.

The Troitskaya suite has a considerable development along a stream parallel to the Shilovka River but situated somewhat to the west of it (on A. G. Vologdin’s map it is incorrectly called the Moiseevka River). In this

in places the suite is represented by green, fissured argillaceous shales, gray fine-grained limestones, and siliceous-bedded limestones. According to the views of A. G. Vologdin (1932) and our own, the sandy-shaly sequence (the Troitskaya Suite) lies conformably upon the underlying limestones of Bely Saibar with a fauna of Lower Cambrian trilobites. This impression is formed when studying the section along the Shilovka River. Here one may even note a certain kinship in the composition of these sequences: in the shaly sequence of the Troitskaya Suite there occur lenses of limestone similar to the underlying ones, while in the limestone sequence of Bely Saibar there occur lenses of shales in many respects analogous to the shales of the Troitskaya Suite. Nevertheless, A. G. Vologdin separated them considerably in time, regarding the limestones as Lower Cambrian deposits, and the shales and sandstones (the Troitskaya Suite) as formations lying at the boundary of the Upper Cambrian and Lower Silurian (\(\mathrm{Cm}_3—\mathrm{S}_1\)). On A. G. Vologdin’s map these deposits are assigned a Silurian age (S). This age was adopted on the basis of finds in the rocks of this sequence of very doubtful tubular remains, identified by A. G. Vologdin as remains of Algae of the Rhabdoporella type.

In the summer of 1955, workers of the Irbinskii party collected a trilobite fauna in the sandstones and siltstones of the Troitskaya Suite on the watershed of the Shilovka River and the Teplyi Klyuch River. Among the trilobites, O. K. Poletaeva identified a new species of the genus Schistocephalus and a number of other new species and genera. Taken together, the trilobites indicate an age of early Middle Cambrian.

Thus, from what has been set forth above it follows that, on the southwestern margin of the Eastern Sayan, in addition to the long-known reference Cambrian sections (the village of Kameshki and the village of M. Irba), new Cambrian sections have been identified, the deposits of which contain relatively abundant organic remains not previously known. On the basis of these organic remains, it appears possible to determine the age of the enclosing deposits within relatively narrow limits of the Cambrian period. Thus, the limestones of Bely Saibar cannot be older than the Lower Cambrian, and the sandy-shaly deposits of the Troitskaya Suite cannot be older than the lower part of the Middle Cambrian.

At the same time, grounds have appeared for doubting the correctness of earlier ideas concerning the age of part of the Lower Paleozoic deposits of the southwestern margin of the Eastern Sayan.

In the Tuba region and beyond its limits, a considerable development is observed of ancient metamorphic rocks, united by A. G. Vologdin into a single complex under the name of Cambro-Silurian deposits. The section along the Shilovka River, described by us above as part of the Troitskaya Suite, was cited by A. G. Vologdin as the most characteristic for the Cambro-Silurian (1932). The age of the Troitskaya Suite is reliably determined by us as the lower part of the Middle Cambrian (\(\mathrm{Cm}^{1}_{2}\)), which gives grounds for doubting the correctness of the age of the “Cambro-Silurian” deposits at other points of the region as well.

More thorough searches for fauna in the corresponding deposits of other localities may yield new finds that will make it possible to resolve the question of the “Cambro-Silurian” unambiguously for the entire region.

LITERATURE

Vologdin A. G.—The Tuba-Sisim Region. Report on geological investigations, 1924–1928. Transactions of the All-Union Geological Prospecting Association of the NKTP, USSR, 1932.

Corrections and Noted Misprints

In S. A. Stroitelev’s article “Study of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.