On Issues of the Deep Geology of the Depression Between the Kansk Horst and the South Yenisei Ridge

K. V. Radugin

1. The South Yenisei Ridge at the latitude of the city of Kansk is a broad projection (more than 100 km) of Precambrian crystalline rocks. Being elongated from northwest to southeast, this ridge, somewhat north of the Trans-Asiatic railway, suddenly breaks off along its strike and is replaced by a broad field of younger formations of the Jurassic of the Rybinsk Basin and the Devonian. Here lies the Kansk tectonic depression, bordered by the South Yenisei Ridge on the north, the Kansk Horst on the south, the Krasnoyarsk Ridge on the west, and the Eastern Sayan on the east. In the northeast it passes into the main field of the Central Siberian Platform.

2. The geological structure of the Kansk tectonic depression has not yet been studied systematically by means of areal geological surveying. However, a whole series of investigations already carried out (by K. I. Bogdanovich, I. A. Molchanov, M. K. Korovin, K. V. Radugin, A. V. Aksarin, A. S. Khomentovsky, and others) has already, and still does in many respects, clarified the composition and structure of this interesting geological region, which may be briefly presented as follows.

Until recently, the oldest rocks of the Kansk tectonic depression were considered to be Devonian diagenetized effusives, very similar to the effusives of the Telbes Formation of Western Siberia [3, 14—20]. Stratigraphically higher lie thick deposits of the Lower (?), Middle, and Upper Devonian of a rather complex composition. These deposits, very closely resembling the sediments of the Minusinsk Basin, are subdivided into a whole series of suites and even form more than one formation. In the Kansk region Upper Paleozoic sediments are known.

After the deposition, dislocations, and denudation of the Devonian and Upper Paleozoic sediments, the next stage of Jurassic coal-bearing deposits of small thickness was formed, nevertheless reaching, according to drilling exploration work, 485 m [1]. The Jurassic deposits, according to the observations of student A. F. Belousov in 1947, were pierced on the Kan River above Kansk by trap magma, which produced there caustic metamorphism of the Jurassic deposits with Phoenicopsis angustifolia Heer.¹)

According to observations in the northwest of the depression [3, 30], there are Lower Paleogene or Cretaceous deposits. The Tertiary–Mesozoic weathering crust near Balai Station was characterized by I. A. Molchanov. Quaternary deposits are distributed almost everywhere. Jurassic deposits were discovered by us in 1947 at one more locality: near the village of Kiyai, on the road from the village of Shalinsky to the settlement of Narva.

The origin of the Kansk tectonic depression, situated along the strike of the projection of crystalline rocks (of the South Yenisei Ridge),

¹) As determined by Associate Professor of the Tomsk Polytechnic Institute named after S. M. Kirov, I. V. Lebedev.

seems at first glance incomprehensible, but a careful look at the geological map makes this origin quite regular and natural. In fact, to the west of the Krasnoyarsk Range there was a large Chulym–Yenisei tectonic depression, including the Balakhta field of Jurassic deposits. This depression is filled with Devonian effusives and red-colored deposits. Along its entire northeastern extent it passes precisely into the area of the Kansk tectonic depression that interests us.

The subsidence of the latter probably occurred before the outpouring of the Devonian lavas established by the author in 1936 near Zykovo and Sorokino stations in the northwest of the depression. As the reconstruction of paleogeographic conditions shows, there was a volcanic plateau there, later cut by a deep (more than 1 km) canyon, and individual volcanoes (Obruchev volcano, etc.). Soon afterward, abundant clastic material was carried into this same part of the Kansk depression from the side of the Krasnoyarsk Range, forming typical fanglomerates of mountain foothills. Then the Kansk depression, lying next to a desert mountain country, was filled with Devonian red-colored deposits.

For this reason, it would seem impossible to judge the composition of the basement of these deposits. However, our position in this respect is not so hopeless. What, then, are the facts in favor of such an opinion?

3. First of all, let us note an exposure of the most ancient rocks within the Kansk tectonic depression. This exposure was discovered by us in 1936 during a route to the settlement of Narva on the Mana River. Here, by the road, metamorphic conglomerates with pebbles of quartz, metamorphic sandstones and shales were found, overlain by a thick red-colored sequence. The latter is exposed along the right side of the valley of the Mana River and was assigned to the Devonian. It is interesting that this outcrop of ancient basement rocks is situated along the strike of the axial zone of the Chulym–Yenisei tectonic depression. Nevertheless, Precambrian, not Cambrian, rocks are exposed here. This is a rather serious argument in favor of the fact that Cambrian deposits, if they existed, were subsequently washed away to a significant extent from the area of the Kansk tectonic depression.

A second fact apparently confirms the participation of Cambrian deposits in its structure. It was observed near the village of Zhernovka on the Berezovka River, which comes out to the Siberian railway near the Maganskoye siding. Opposite this village, at the foot of the right slope of the valley, there are pits—traces of burning—and individual blocks of white limestone, of a form typical for Cambrian limestones. Higher up, the slopes are composed of Devonian deposits, in particular cross-bedded millstone sandstones, from which the village derived its name. This village is situated not far from the northeastern edge of the Krasnoyarsk Range. On the way to it, the Cambrian is hidden beneath the Devonian.

There are also still unverified assumptions about the presence of Cambrian along the Kan River in the section between the Kansk horst and the mouth of the Agul.

Thus, at least near the village of Zhernovka, the Cambrian participates in the structure of the basement of the Kansk tectonic depression. Possibly it forms a belt extending toward the Kan near the mouth of the Agul.

The third fact, which it is appropriate to recall here, was established east of Krasnoyarsk, between the Yenisei and the lower course of the Esaulovka. In this area it proved possible [3, 15] to establish the direct superposition of Devonian effusives upon the gneisses of the South Yenisei Range.

Let us also recall the observations of I. A. Molchanov [2, 4—6] near Balai station, where it turned out that the crystalline basement of the Kansk tectonic depression is covered by the Jurassic and by a Tertiary–Mesozoic weathering crust. It is composed of

by gneisses and granites. The cover of Devonian formations is often absent there, but farther south it acquires regional development.

Finally, let us note one more interesting fact. In the summer of 1947 we established a broad development, apparently, of Devonian effusives along the right bank of the Mana River above Narva (above the Tyulyup River). Earlier, epicontinental Devonian deposits had been shown here on maps. Among the effusives, porphyrites, felsites, and others were noted. Thus, in one dike in the xenoliths, fragments of metamorphic rocks with relict metamorphism were found. These fragments were evidently captured by magma and brought up from depth. These are true “messengers from the depths,” indicating that here too the basement of the Devonian is most likely crystalline schists of the Precambrian.

1. The observations presented relate to the northern and southern margins, as well as to the axial zone of the Kansk tectonic depression (in its western part). Especially important are the latter observations along the extent of the Chulym–Yenisei tectonic depression. They indicate the occurrence beneath the Devonian of crystalline Precambrian, linking into a whole, different in age but unified in composition and facies, the exposures of the same Precambrian now observed at the surface in the Kansk horst and in the South Yenisei horst. In addition, as noted above, Cambrian also occurs locally beneath the Devonian.

CITED LITERATURE

1. A. V. Aksarin—Recent data on the stratigraphy of the Rybinsk and Sayano-Partizansky districts of the Kansk Basin, 1947.

2. I. A. Molchanov—Deposits of kaolin and other minor mineral resources of the Balai district, Izv. SOGK, VI, 1927.

3. K. V. Radugin—Geology of the southwestern margin of the Yenisei Ridge, Materials on the Geology of Krasnoyarsk Krai, No. 2, 1937.