On the Question of the Age and Stratigraphy of Deposits in the Area of Lake Igerkul

V. D. Tomashpolskaya

(Presented by Professor, Doctor K. V. Radugin)

The area of Lake Igerkul is located on the northeastern margin of the Batenev Ridge and, administratively, lies within the territory of the Bograd District of the Khakass Autonomous Oblast. The nearest populated locality is the settlement of Son station on the Krasnoyarsk Railway.

The map sheet within which the described area is located has repeatedly been subjected to areal and thematic investigations; nevertheless, the questions of the stratigraphy of the formations composing it have remained insufficiently clarified, which is explained by the complexity of the geological structure and by the scarcity of paleontological data hitherto available for determining the age of the oldest deposits.

For a number of years the author of the present article has been engaged in the study of the stratigraphy of the Cambrian deposits of the northeastern margin of the Batenev Ridge. In an article recently published by him, the author’s views on the stratigraphy of the Lower and Middle Cambrian deposits for the area of the village of B. Erba (Potekhino) and the village of Karasuk were set forth, and the guiding forms of trilobites and brachiopods were given for the suites and horizons distinguished (Sivov and Tomashpolskaya, 1957).

In other parts of the Batenev Ridge, apart from the area of the village of B. Erba, finds of trilobite and brachiopod fauna had not previously been known, and only isolated finds of archaeocyaths in limestones in the area of Lake Igerkul were mentioned (Sivov and Tomashpolskaya, 1957).

In 1955, with the participation of students of group 253-II of the Geological Prospecting Faculty of the Tomsk Polytechnic Institute, we discovered a very rich trilobite fauna in a limestone sequence that we named the Igerkul Suite.

In connection with this, in order to establish the stratigraphic position of the Igerkul Suite, regional and detailed geological mapping was carried out over the area adjoining Lake Igerkul and Son station.

Among the published works on the Lake Igerkul–Son station area, the works of A. N. Churakov and Ya. S. Edelstein are known.

On the geological map of the surroundings of Son station, A. N. Churakov distinguishes Proterozoic formations represented by deposits of the Yenisei and Kutenbuluk suites, which, according to Churakov’s data, form several anticlinal and synclinal folds of northeastern strike.

Churakov describes the Yenisei Formation as a sequence of bituminous bedded limestones, in the upper horizons of which there is an interbed of black siliceous shale.

The Kuten-Buluk Formation, according to Churakov, is a sequence of graywackes, clayey and talc shales with interbeds of dark bituminous limestones, cut by dikes and small intrusions of ancient diabases.

This formation is shown by Churakov at the base of Son Mountain in the form of several bands extending northeastward to the north and south of Son Station, in the cores of synclinal folds. Both of these formations are shown as conformably bedded, and upon them, lying unconformably, are shown remnants of Cambrian limestones distinguished by their light, often white coloration, absence of bedding, and distinct massive jointing. Churakov determines the Cambrian age of such remnants on the basis of isolated finds of archaeocyathid fauna (Churakov, 1916).

In addition, on the geological map of the vicinity of Son Station, A. N. Churakov shows and describes outcrops of reddish-gray clayey sandstones with conglomerates and covers of porphyrites of Upper Silurian age.

Legend:

• Yenisei Formation. Dark-gray bedded limestones with interbeds of shales and sandstones.
• Son Formation. Variegated sandstones, conglomerates, limestones, shales.
• Middle effusives of the Son Formation.
• Igerkul Formation. Light massive limestones with archaeocyathid and trilobite fauna.
• Granodiorites. Budinsky batholith.
• Loose Quaternary deposits.
• Bedding of strata.
• Trilobite fauna.
• Archaeocyathid fauna.

Fig. 1. Schematic geological map of the area of Lake Igerkul—Son Station.

Red sandstones are indicated by him in the form of two scarps right by the railway track near the station, on the right bank of the Son River 2 km north of Son Station, and also north of Lake Igerkul and north of the Sharlyak Lakes.

Almost simultaneously with A. N. Churakov, the area of Son Station was visited by Ya. S. Edelstein, who carried out a geological survey of the entire Son plateau and other areas of the Batenev Range.

Ya. S. Edelstein distinguished two divisions within the deposits of the Batenev Range: a lower, carbonate division—of age \(Cm_1—Cm_2\), and an upper, essentially terrigenous division—of Cambro-Silurian age (Edelstein, 1912).

On Ya. S. Edelstein’s geological map, the environs of Son Station are colored as the lower division, while the outcrops of red sandstones are assigned to the upper division.

In later works on the area under study, questions of stratigraphy are treated in even less detail, and the variegated deposits of the Son Station and Lake Igerkul area are assigned by some authors to the Devonian, by others to the Proterozoic, or are not noted at all.

Our investigations, both in the area of Lake Igerkul and Son Station and in the adjacent area to the north and south along the valley of the Son River, have established that the following stratigraphic units take part in the structure of the sedimentary-metamorphic sequence of the described area and of the Son plateau as a whole: the Yenisei Formation, the Son Formation, and the Igerkul Formation.

Yenisei Formation

By the Yenisei Formation we describe a sequence of rocks represented by an alternation of homogeneous, bedded, bituminous gray and dark-gray limestones with members consisting chiefly of argillaceous and siliceous shales with small interbeds of limestone. In different parts of the section the thickness of individual horizons of carbonate and terrigenous rocks varies widely, but on average does not exceed 100–200 meters.

This character of the Yenisei Formation is clearly established in natural and artificial exposures along the valley of the Son River north of the settlement of Son, and in the railway cutting to the north and south of the tunnel. The same sequence is exposed in the extreme western part of the Son plateau—in the upper reaches of the Ulen River.

The clearly expressed and well-maintained northwestern dip excludes the possibility of constructing several folds here, as A. N. Churakov does.

Thus, in the concept of the “Yenisei Formation” we have to some extent combined two of Churakov’s formations: the Yenisei and Kutenbuluk formations; that is, we accept the Yenisei Formation as it was first described by Bogdanovich (who proposed this term).

In his earliest works, A. N. Churakov himself likewise defined the content of the Yenisei Formation.

Son Formation

Stratigraphically above the Yenisei Formation, and moreover with a distinct angular unconformity, lie deposits of the Son Formation, which are represented by a complex assemblage of variegated sandstones and conglomerates, interbeds of limestones and siliceous and argillaceous shales, and numerous dikes of acidic and basic rocks.

This sequence of rocks forms the base of Son Mountain in its eastern and southern parts, where it is exposed by a railway cutting that bends around Son Mountain in a semicircle; on the right bank of the valley of the Son River this same sequence is exposed in natural outcrops on the right side of the valley of the Son River.

2 km north of the settlement, and also by numerous ditches and quarries made during prospecting for phosphorites in the ridges near the very northern end of the settlement. In 1955 we established a second outcrop of the Son Formation in the area south of Lake Igerkul, 3–3.5 km away.

On comparing A. N. Churakov’s geological map with the one compiled by us, it is not difficult to see that under the Son Formation we have combined one band of A. N. Churakov’s so-called “Kutenbuluk Formation” and the outcrops of variegated deposits that he assigned to the Upper Silurian.

The grounds for distinguishing the Son Formation were as follows.

The reddish-gray clayey sandstones and conglomerates described by A. N. Churakov, lying on Yenisei limestones with a dip to the SW in the area south of Son station, in fact have a wide distribution and are lithologically very heterogeneous, as will be seen from the section of the Son Formation.

The completely analogous variegated sandstones and conglomerates discovered by us in 1955 in the area south of Lake Igerkul, where they have a northwestern dip, indicate a synclinal occurrence of the variegated sequence and posed anew the question of its stratigraphic position.

Among the dark-gray bituminous limestones and siliceous shales with diabase dikes on the southern foot of Mt. Son, 150–200 m east of the station building (here Churakov shows the central band of the Kutenbuluk Formation), there are outcrops of red sandstones syngenetic with the limestones enclosing them. Red sandstones and conglomerates with lava cement, and interbedded bodies of effusives of intermediate composition, make up the southeastern part of the foot of Mt. Son (near the depot). The same dikes occur in the limestones composing the eastern foot of Mt. Son; they also continue onto the right side of the valley of the Son River.

The independence of the Son Formation is also supported by the fact that it lies on different horizons of the Yenisei Formation and is overlain by limestones of the Igerkul Formation (see map).

Noting the presence of red sandstones near the railway track at Son station, A. N. Churakov colors this area on the geological map as the Kutenbuluk Formation.

It will not be superfluous to note that in earlier works (Churakov, 1916), for deposits of similar variegated composition with conglomerates, A. N. Churakov distinguished the so-called Khazazi Formation; he also established that it lies unconformably on the foul-smelling black limestones.

Subsequently Churakov abandoned the distinction of the Khazazi Formation, and all deposits that had previously been assigned to it he called the Kutenbuluk Formation.

It seems to us that, having abandoned the distinction of the Khazazi Formation, Churakov united under the concept of the “Kutenbuluk Formation” geological complexes that are in fact different.

Below we give a section of the Son Formation.

In good rocky exposures on the side of the valley of the Son River, a continuous section of the lower part of the Son Formation can be clearly seen for more than one hundred meters. Farther on, the section is reconstructed from fragmentary observations.

The section begins with red and greenish-red fine-grained sandstones 3–4 m thick. They are overlain by conglomerates of pebbles of strongly crushed limestone. The cement is tuffaceous; thickness 3 m. Above lie light greenish-gray shaly limestones 5 m thick.

Over the next 2 m lies a fine-pebble conglomeratic sandstone with calcareous cement.

Still higher lies a conglomerate of limestone pebbles and green effusives, 40 m thick. The limestone among the pebbles is more often dark gray and even black, analogous to the limestones of the Yenisei Formation, but light-colored limestones also occur, similar to the limestones of the Kameshkov horizon. The color of the conglomerate is variegated—red and green.

From the next conglomerate horizon, 50 m thick, described above, it is separated by a layer of greenish-yellow fine-grained sandstone 8 m thick.

The size of the pebbles in the conglomerates varies from 1 to 10 cm. The composition of the pebbles in both conglomerate horizons is almost in no way different; the rocky outcrop described is capped by thin-bedded, fissile, clayey sandstones of brownish-red color, 60 m thick.

After a short interval, highly porous tobacco-colored limestones are exposed, several tens of meters thick.

Variegated sandstones and conglomerates similar to those described make up the southern limb of the synclinal fold and are exposed in the area of Lake Igerkul at a distance of about 2500 meters from the lake along an azimuth of SSW 200°.

The conglomerates of this southern limb form very thick, cliff-like exposures on an isolated knoll rising amid a swampy and heavily forested part of the log, and are hidden on all sides by dense forest. This alone can explain why this exposure was not known; in A. N. Churakov’s work, lidites are shown almost at this place in the limestones of the Yenisei Formation. Such rocks are present, but somewhat south of the conglomerate exposures.

In the composition of the pebbles, the character of the cement, and the general appearance, as well as on the basis of comparison under the microscope, the conglomerates of the southern limb are completely identical to the rocks of the northern limb and, consequently, establish the presence of a synclinal fold in the deposits of the Son Formation.

The conglomerates of the southern limb could not be traced along strike for more than 150 meters because of poor exposure. In the southern limb as well, the deposits of the Son Formation rest on the limestones of the Yenisei Formation, but only on its lower horizons. Red-colored deposits with conglomerates also occur within the deposits of the Son Formation. These include the conglomerate exposures at the depot of Son station and the red clayey sandstones exposed along the foot of Mount Sonskaya opposite the station, by the railway track and on the southern slope of the mountain.

The central part of the deposits of the Son Formation is composed of a variegated member of rocks consisting of interbedded siliceous and clayey shales, light and dark limestones, and dike bodies of intermediate and basic composition.

This part of the Son Formation is exposed at the eastern and southern foot of Mount Sonskaya; on the right side of the valley of the Son River it is also exposed by numerous trenches of old and recent years.

Dike rocks of intermediate composition, passing through limestones and shales, usually have a concordant character, are often fissile, and along the eastern foot they are so strongly kaolinized and ocherized¹ that their magmatic character can scarcely be determined even under the microscope. Diabase dikes and small diabase intrusions more often have a crosscutting character.

Dikes and intrusions are not traced into the overlying Igerkul Formation. The limestones themselves usually have a lenticular character. One of the largest lenses from the central part of the deposits of the Son Formation is exposed in a quarry on the right bank of the Son River, just beyond the settlement. The limestones here are light-colored, massive, and in outward appearance very much resemble the limestones of the summit of Mount Sonskaya, which probably served as the basis for Churakov to distinguish here, as on Mount Sonskaya, “remnants of Cambrian limestones among the deposits of the Kuten-Buluk Formation.” The digging of ditches, which exposed a considerable area in the vicinity of the quarry, showed: 1) that the lens of light-colored limestones exposed by the quarry is indeed a constituent part of the deposits enclosing it; 2) that, in addition to this largest lens, there is a whole series of lenticular bodies and interbeds of limestone, identical in strike and differing only in color and thickness.

Siliceous and clayey-siliceous shales within the Son Formation are widely distributed and form interbeds in the limestones up to 20–25 m thick. Very often they are cut by intrusions and concordant diabase dikes. The visible thickness of the Son Formation is estimated at 350–400 m.

The effusive rocks exposed to the WSW of Son station were described by A. N. Churakov as covers, and, like the red-colored deposits, were assigned by him to the Upper Silurian. We include them in the Son Formation, but their stratigraphic position is not determined by us with sufficient certainty.

Igerkul Formation

Still higher stratigraphically, above the deposits of the Yenisei and Son formations, lies the Igerkul Formation, composed of grayish-white and pinkish-white limestones of massive texture with distinctly expressed platy jointing.

These limestones compose the entire summit and near-summit part of Mount Sonskaya; farther to the southwest they form a chain of elevations extending along both sides of the road from Son station to the Ulen mine and, gradually widening, reach Mount Vizim.

Lithologically they are rather homogeneous, but even in them an alternation of varieties is observed: now of a completely light color with crimson streaks, now noticeably darker, gray and even dark-gray, with clearly expressed parallelism in bedding.

The distinctly expressed platy jointing, coinciding with the bedding, made it possible to establish that the limestones of the Igerkul Formation form the core of a synclinal fold and occur at different horizons of the Yenisei and Son formations; the axis of the fold runs approximately along the valley of the Son River and the Ulen River.

In natural rocky exposures in these limestones, at a distance of about three kilometers to the SSW of Lake Igerkul, we encountered a rich trilobite fauna; approximately at the same geographic point, on A. N. Churakov’s map, the find of a single specimen of an archaeocyath is shown. There are no other data, either in repository or in printed literature, on finds of trilobite fauna¹).

According to identifications by Doctor of Geological-Mineralogical Sciences A. G. Sivo[v], among the collected fauna the most abundant forms are: Edelsteinaspis ornatus Lerm and other species; in smaller, but also considerable, numbers there are Kooteniella Slatkovska (Schm), Proerbia (?) duplicata

¹) From the unpublished materials of V. M. Yaroshevich (collections of KGU) it is known that, on the southwestern continuation of our Igerkul Formation, which he shows as the Bolshaya Erba Formation with age cm₁, he found archaeocyath fauna.

Sivov sp. nov., Chondragraulos minussensis Lerm., Tobatopigellina babakovensis Siv. sp. nov., rarely Polielina iregularia, and others.

The far from complete list of trilobite fauna cited here corresponds almost entirely to the fauna of the Babakovo horizon, whose age is determined within the limits of the upper Lower or lower Middle Cambrian (Sivov, 1955).

In the B. Erba—Karasuk area, in deposits with paleontologically proven horizons of the lower division of the Middle Cambrian, we did not encounter a complete faunal assemblage identical to the fauna of the Igerkul Formation. Trilobites most characteristic of the Igerkul Formation, such as the genus Edelsteinaspis, both in deposits of undoubtedly Middle Cambrian age and in deposits of the so-called Sanashtyk-Gol horizon on the southern limb of the Loshchenkovskaya anticline¹, occurred very rarely and as small specimens.

There is reason to believe that the Igerkul Formation should be assigned to lower horizons of the Middle Cambrian than the deposits of the B. Erba—Karasuk area; accordingly, on the basis of stratigraphic position, the age of the Son Formation should be determined as Lower Cambrian, and that of the Yenisei Formation as Late Precambrian.

When correlated with the section of Cambrian deposits in the area of the village of B. Erba, the deposits of the Son and Igerkul formations, in all probability, should lie above the limestones of the Kameshki horizon and beneath the Middle Cambrian deposits, i.e., between the so-called Chesnokov Formation and the limestones of G. Sladkie Koreni, where, according to A. G. Vologdin’s stratigraphic scheme, a hiatus is indicated and the possibility of the existence of intermediate links was envisaged (Vologdin, 1947).

In the area of the village of B. Erba these deposits, apparently, are overlain by covers of Devonian effusives.

Thus, the deposits of the Lake Igerkul—Son Station area fill the missing links in the general section of Cambrian deposits on the northeastern margin of the Batenyov Range.

REFERENCES

1. Vologdin A. G.—On the stratigraphy of the eastern slope of the Kuznetsk Ala-Tau. Soviet Geology, No. 27, 1947.

2. Zhuravleva I. T.—Archaeocyaths of the Cambrian of the eastern slope of the Kuznetsk Ala-Tau. Academy of Sciences of the USSR, Transactions of the Paleontological Institute, LVI, 1955.

3. Krasnopeeva P. S.—Algae and archaeocyaths of the oldest strata of the Potekhin planchet in Khakassia. Materials on the Geology of Krasnoyarsk Krai, No. 3, 1937.

4. Krasnopeeva P. S.—New data on the stratigraphy of archaeocyaths of Western Siberia. Reports of the Academy of Sciences of the USSR, No. 99, 1954.

5. Lermontova E. V.—Class Trilobita. Atlas of Index Forms of Fossil Faunas of the USSR, 1940.

6. Pokrovskaya N. V.—Stratigraphy of Cambrian deposits of the south of the Siberian Platform. Problems of the Geology of Asia, vol. I, Jubilee collection for the 90th anniversary of V. A. Obruchev, 1955.

7. Suvorova N. P.—On the Lena Stage of the Lower Cambrian of Yakutia. Problems of the Geology of Asia, vol. I, 1955. Jubilee collection for the 90th anniversary of V. A. Obruchev.

8. Sivov A. G.—The Cambrian System. Atlas of Index Forms of Fossil Fauna and Flora of Western Siberia, 1955.

9. Sivov A. G. and Tomashpolskaya V. D.—On the question of the age of archaeocyath-trilobite assemblages of the Sanashtyk-Gol horizon. Stratigraphic Collection of ZSGU, 1957.

10. Churakov A. N.—Materials for the tectonics of the Kuznetsk Ala-Tau. Transactions of the Geological Committee, new series, issue 145, 1916.

1. Churakov, A. N.—The present state of our knowledge of the stratigraphy and tectonics of ancient deposits in the southern part of Central Siberia. Izv. AN SSSR, Section of Mathematical and Natural Sciences, Nos. 1–4, 1931.

2. Edelshtein, Ya. S.—Geological investigations in the gold-bearing regions of Siberia, issue XII, 1912.

3. Edelshtein, Ya. S.—A note on the geological section in the area of the settlement of B. Erba (Potekhino) in the Khakass Okrug, Yenisei Governorate. Collection of Geographical-Economic Studies of the Institute of Leningrad State University, 1926.

4. Edelshtein, Ya. S.—The area of the village of Bolshaya Erba (Potekhino), Siberian excursion of the XVII International Geological Congress, Krasnoyarsk Krai, 1937.

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.

1. In E. V. Lermontova’s work (1940), for the form genus Edelsteinaspis ornatus, closely similar to the Igerkul form, a very inaccurate locality is given—Minusinsk Krai (Potekhin). ↩↩