On the Characterization of the Triassic Deposits of the Uropskoye Deposit in the Kuzbass

A. A. Kurbatova, N. A. Dorofeeva

(Presented at the interdepartmental scientific seminar)

Sedimentary Triassic deposits form a narrow belt fringing the Karakan Mountains, located in the Erunakovo district of the Kuzbass. These deposits have been penetrated by a number of boreholes along the Inskaya and II exploration lines, situated within two new large coal-bearing areas, named the Uropskoye and Karakanskoye deposits.

In these deposits the Upper Permian coal-bearing sequence is gradually replaced upward in the section by the Triassic. Outwardly, the deposits are indistinguishable: they are light greenish-gray sandy-clayey rocks. The same pattern is observed in a number of other sections of the Kuzbass: at “Babii Kamen” on the Tom River, and on the Bolsherechensky profile—and it is not unexpected. Usually, in such sections the boundary between the Permian and the Triassic is taken to be the replacement of coal-bearing rocks by “tuffaceous” ones.

In the practice of geological exploration, the question of drawing the boundary between the Paleozoic and the Mesozoic is of considerable importance, since the Upper Permian deposits in the above-mentioned fields contain a large number of workable seams of high-grade coals suitable for open-pit mining, whereas the Triassic deposits are coal-barren. Refinement of this boundary became possible with the aid of palynological analysis (Kurbatova, 1962), which made it possible to draw this boundary in the “Babii Kamen” section 15 m stratigraphically above the last coal interbed in the section, and 6 m stratigraphically above the coal seam in the Bolsherechensky profile.

There are also other types of sections in which bleached rocks of limestone–montmorillonite composition appear at the base of the green-colored Triassic in the Kuzbass. A section of this type was noted by O. A. Betekhtina and Yu. P. Kazansky (1959) in boreholes 687 and 682 of the North-Talda exploration line, located in the western part of the Erunakovo district.

In the present article, materials of lithological and pa—

paleontological study of the Triassic deposits from borehole 1074, located on the Inskaya exploration line of the Uropskoye deposit. Borehole 1074 is situated approximately 40 km northwest of the Severo-Taldinsky section.

The section of Triassic deposits in borehole 1074 was penetrated to a thickness of 120 m. The predominant rocks in the section are dark greenish-gray polymictic-pyroclastic siltstones, among which interbeds of lighter, but likewise greenish-gray pyroclastic graywacke sandstones occur. At the base of the section there is a bed of clay-like limestone-montmorillonite rocks, resembling those previously described from the Severo-Taldinsky section by Betekhtina and Kazansky (1959).

The material composition of the Triassic formations in borehole 1074 is as follows: 1. Sandy rocks, in which up to 20% of redeposited volcanogenic material is noted, strongly decomposed, including well-rounded fragments of quartz, feldspar, effusive rocks, and fragments of groundmass with plagioclase microlites.

1. Under the microscope, polymictic siltstones consist of angular, subrounded quartz grains ranging in size from 0.1 to 0.01 mm; feldspar grains; flakes of hydromica and chlorite. In addition, authigenic minerals are present: rounded grains of lepto-chlorites from brownish-green to brown tones, zeolite, and pyrite, but strongly oxidized. Sorting of the clastic material is poor; along with sandy fragments, clay minerals are present (kaolinite and hydromica, which could have formed through the destruction of tuffogenic fragments). Roundness is moderate. The cement is semibasal, clayey-calcareous-micaceous. Among accessory minerals, garnet, zircon, sphene, and apatite were found. Clayey impurities range within 15–20%. The total percentage composition of the pyroclastic part of the siltstones is as follows: quartz, 17–20%; potassium feldspars, 1–3%; fragments of effusive rocks, 25–30%; heavy fraction, within 0.2–0.1%. Sporadically, in some samples confined to the stratigraphically lower horizons of the section (depth 62.5 m), an increased content of zeolites and leptochlorites is observed in the cementing mass; there is also an increased content of clay components (kaolinite and hydromica), and the content of carbonates and iron oxides increases. At a depth of 72 m and below, the appearance of clay minerals of the beidellite and montmorillonite type is noted; the presence of these clay minerals was confirmed by electron microscopy and staining (Fig. 1).

The palynological study carried out on the section of this borehole made it possible to identify, at a depth of 110 m, a typical Lower Maltsevo spore-pollen assemblage, close to that previously obtained (Kurbatova, 1962) on the “Babiy Kamen.” Both spores (59.2%) and pollen (40.8%) were encountered in it. The spores are represented by selaginellaceous, marattiaceous (10%), and horsetail (15%) forms; in smaller quantities, cyatheaceous, gymnophyllous, gleicheniaceous, and schizaeaceous forms were found. Spores of uncertain systematic affiliation of the types Limbella Mal., Leiotriletes Naum., and others are obligatory. Dark spores are obligatory. Representatives of the marattiaceous type Danaeopsites Mal. are distinguished by a finely tuberculate exine. The pollen of gymnosperms consists of grains of ginkgoalean forms (9.7%), ancient Pinaceae (12%), Protopinus Bolch., Paleoconiferus Bolch., Oblatinella vulgaris Mal., and others. Striatoconifera Sed. and Stachycarpites Mal. are solitary.

Pollen of Upper Paleozoic gymnosperms is obligatory: both cordaitalean forms (up to 3%) and Lebachites Mal. (1.5%); araucariaceous, taxodiaceous, and podozamitaceous forms were found as single grains. Caytonialeans make up a fairly large percentage in the assemblage (7%). The presented

Fig. 1. No. 4. Aleurolite. Hydromica isometrically platy, with an admixture of needles of magnesian silicates, grains of kaolinite, and a rare admixture of montmorillonite. Borehole 1074, sample 4, depth 62.5 m.

No. 6. Kaolinite with a poorly expressed pseudohexagonal particle shape, with isometrically platy hydromica, with an admixture of iron oxides and needles of magnesian silicates. Borehole 1074, sample 6, depth 67.0 m.

No. 9. Aleurolite. Hydromica—platy, elongated grains with indistinct contours of kaolinite. Some grains show a transition of hydromica into montmorillonite. Borehole 1074, sample 9, depth 72 m.

[[unclear: beginning of line]] data make it possible confidently to correlate the obtained assemblage with the Nizhnemal’tsevo assemblage from the “Babii Kamen” section, noting similarity both quantitatively and qualitatively, and to assign the deposits containing it to the Nizhnemal’tsevo Formation.

Conclusion

1. A detailed lithological-palynological study of the Triassic deposits makes it possible to refine the boundary between the Paleozoic and the Mesozoic in the Kuzbass, which is a matter of considerable importance in the practice of geological exploration work.

2. At the base of the Triassic deposits in borehole 1074, bleached rocks of limestone–montmorillonite composition were noted; this apparently is characteristic of the lower horizons of the Triassic in the Erunakovo district.

3. The spore-pollen assemblage in the deposits studied at a depth of 110 m proved similar to that from the lower horizons of the Triassic of the “Babii Kamen” section and permits the age of the deposits containing it to be regarded as Lower Triassic.

References

1. O. A. Betekhtina and Yu. P. Kazansky. On the boundary between the Permian and Triassic in the western part of the Erunakovo district of the Kuzbass. Problems of the Geology of the Kuzbass, vol. 2, 1959.

2. A. A. Kurbatova. The Lower Triassic spore-pollen complex of the Kuznetsk Basin. Proceedings of SNIIGGiMS, issue 23, 1962.

3. A. A. Kurbatova. On the question of the position of the Permian–Triassic boundary in the Kuznetsk Basin according to data from spore-pollen analysis. Reports of the Paleobotanical Conference, TSU, 1962.

4. I. V. Lebedev. The Mesozoic of the Kuznetsk Depression. Problems of the Geology of the Kuzbass, vol. 1, 1956.