On the Stratigraphy of the Jurassic Deposits of the North Sudzha Region

Yu. P. Kazanskii

(Presented by Professor Doctor Yu. A. Kuznetsov)

Jurassic coal-bearing deposits north of the city of Anzhero-Sudzhensk were first discovered by members of the Kuzbassuglegeologiya trust. This area was named the Katat brown-coal deposit (Elisafenko, 1956).

The Jurassic deposits of the North Sudzha region, according to lithological features, are divided into three suites: lower, middle, and upper. The lower suite lies on Devonian deposits. The composition of the suite is variable and changes from southwest to northeast. The greatest thickness is observed in the southeastern part of the region (275 m). In boreholes in the northeastern part of the region only the upper half of the section of the suite has been penetrated. The lower suite is composed of pebble beds, gravelites, and, to a lesser extent, sandstones and siltstones. The pebbles are often kaolinized. The amount of coarse clastic material decreases from south to north. In the upper part of the suite, interbeds and seams of coal appear. The heavy fractions of the lower suite are characterized by an abundance of ore minerals and zircon. In the deposits of this suite, I. G. Kovalevskaya, a member of ZSTU, discovered a spore-pollen flora of Lower Jurassic—Upper Triassic age.

The middle suite, composed of pebble beds, sandstones, and siltstones, lies on the Lower Jurassic deposits. The thickness of the suite varies from 15 m in the southwest to 150 m in the northeast. In the same direction there is an increase in the thickness and number of interbeds of fine-grained rocks. In the heavy fractions of the middle suite an abundance of epidote is noted. The deposits of this suite are confidently correlated with the lower non-coal-bearing horizons of the Middle Jurassic deposits of the Chulym-Yenisei basin.

The upper suite crowns the Jurassic section of the North Sudzha region. It lies on rocks of the middle suite and has a thickness of 80 to 170 m, with thicknesses increasing from southwest to northeast. In the same direction the grain size in the sediments decreases. The rocks of the upper suite are represented by pebble beds, sandstones, siltstones, carbonaceous rocks, and coals. As established, in almost all the sections studied the upper suite contains a stratum of light-colored rocks distinguished by its lithological composition. The upper suite contains an epidote-zircon association of heavy minerals. Epidote is absent in the stratum of light-colored sandstones and siltstones. The upper suite has been correlated with the coal-bearing horizons of the Middle Jurassic of the Itat region.

Thus, in the Jurassic section of the North Sudzha region, two rhythmic members are distinguished—the Lower Jurassic and the Middle Jurassic. Each of them is enriched at the base with coarse-clastic material, and upward contains seams of brown coal.

In the light of these data it is of some interest to consider the stratigraphic scheme of the Jurassic of this region proposed by I. I. Elisafenko (1956). Without citing any new materials, she places the boundary between the Lower Jurassic (Makarovka) and Middle Jurassic (Itatka) strata at the base of the IV (lower) coal seam, i.e., within the Middle Jurassic sequence, approximately 100 m above the naturally indicated stratigraphic boundary. Apparently, the author of the article did not know the precise stratigraphic position of the samples containing Lower Jurassic flora, which were collected by K. V. Ivanov from sections of the lower suite.

To substantiate the position of the upper stratigraphic boundary of the Jurassic sequence, I. I. Elisafenko (1956, p. 238) uses members of bleached rocks occurring within the Katatka and Itatka deposits. In the sections of the Katatka deposit, interbeds of bleached rocks usually occur beneath the first (from the top) coal seam. In the Itatka region, a member of bleached sands and sandstones up to 90 m thick is known in the southwestern part of the deposit, where it underlies the thick Itatka seam. If one uses the scheme of stratigraphic correlation given by Elisafenko (1956), then in the Katatka section the bleached rocks are confined to the upper part of the Itatka strata, whereas in the Itatka region they are situated at the base of these strata and also include the upper part of the Makarovka strata. Thus, these materials show that the sequence of bleached rocks occupies an inconsistent stratigraphic position, and it is inadvisable to use it for substantiating age in the present case. The formation of members of bleached rocks, as we see it, is connected with epochs of redeposition of the products of the pre-Jurassic and Jurassic weathering crusts. In order to prove the contemporaneity of these periods of erosion in different regions, a sufficient amount of paleontological material is required, and this is absent from the article by I. I. Elisafenko.

The examples cited show that the stratigraphic scheme proposed by I. I. Elisafenko (1956) lacks sufficient paleontological and lithological substantiation and, in the form in which it is presented by the author, cannot be used.

LITERATURE

1. Elisafenko I. I.—The Mesozoic of the northern continuation of the Kuzbass. Collection Problems of the Geology of the Kuzbass, vol. I, pp. 235–247, 1956.