ON THE STRATIGRAPHY OF TRIASSIC DEPOSITS IN TOMSK OBLAST

A. A. KURBATOVA

(Presented by the Interdepartmental Seminar of the GRF)

Triassic deposits on the West Siberian Plain were established from spore-pollen complexes only in its southwestern part. V. S. Malyavkina (1964) indicated their presence in the Zavodoukovskaya borehole (depth 1374—1830 m); later L. V. Rovnina (1968) found them in the boreholes of Rakitinskaya (depth 1800—1804.6 m), Pokrovskaya No. 8 (depth 1852.95—1840 m), and Tyumenskaya reference.

The petroleum-exploration work that unfolded in Tomsk Oblast also made it possible to discover Lower Triassic deposits in two boreholes: in borehole 14, Strezhevaya area (vicinity of the town of Strezhevoy, at a depth of 2827.8—2912.8 m), and borehole 136 of the Ketskaya area, located 350 km south of borehole 14 (depth 2846—2856).

Spore-pollen complexes were obtained from 4 samples of borehole 14 and 2 samples of borehole 136.

The spore-pollen complex from borehole 14 is characterized by a predominance of spores over pollen. The spore portion of the complex is distinguished by a predominance of smooth trilete spores from the group of articulates; second place is occupied by fern spores with spinose and tuberculate exine sculpture; the finely tuberculate exine sculpture of fern spores from the family Marattiaceae is indicative of this complex. Spores of representatives of the families Schizaeaceae, Osmundaceae, Dipteridaceae, and other ferns are invariably present in the complex. Lycopod spores are few, but noticeable: these are Selaginellaceae and Lycopodiaceae.

Pollen of gymnosperms is scarce, but among it are not rare cordaitalean, poorly preserved conifers of the Alisporites and Psophosphaera types, more rarely bisaccate conifers; pollen grains of ginkgoaleans, Ginkgocycadophytus. Monosulcate forms with ribbed exine sculpture of the Gnetaceaepollenites type and bisaccate Striatoconifera occur.

Permian relicts in the form of cordaitalean grains of the Lebachitites and Cordaitina types, striate bisaccates of the Striatoconifera type, ancient conifers of the Psophosphaera type, as well as dark smooth spores, sometimes with a trilete laesura, are an invariable supplement to these spore-pollen complexes.

All the same features are characteristic of the complex from borehole 136 (depth 2846—2856): predominance of spores over pollen; among the spores, broad development of smooth trilete spores from the group of articulate plants (up to 35% in the complex), fern spores from the families Marattiaceae, Osmundaceae, Schizaeaceae, Dipteridaceae, etc. Lycopod spores and gymnosperm pollen are few.

Similar spore-pollen assemblages were described by us earlier from the Lower Triassic deposits of the Kuznetsk Basin (the Babii Kamen’ area on the Tom’ River) and of the Siberian Platform (outcrop 71 on the Taimura River, left bank, 5 km above the mouth of the Maitunnakan River) (Kurbatova, 1962, 1971).

The assemblages obtained contain common forms of spores and pollen characteristic of the Lower Triassic deposits of the Kuznetsk and Tunguska basins and of the West Siberian Plain: Lebaehites lebachitiformis Mal., Oblatinella vulgaris Mal., Psophosphaera sp., Striatoconifera Sed., Cordaitina rotata Sam., Ginkgocycadophytus Sam., Cycloequisetites typiciformis Mal., Leiotriletes Naum., L. microdiscus K.-M., Coniopteris tajmyrensis K.-M., Cyathidites sp., Osmundopsis spinosa K.-M., Angiopteris (?), Trachytriletes cf. sibiricus Medv., Matonia cf. triassica K.-M., Lycopodium sp., Lophotriletes with finely tuberculate exine sculpture and L. with coarsely tuberculate exine sculpture, Densosporites, Gnetaceae-pollenites, Dictyophyllidites. Dark, smooth Aletes and dark, smooth Leiotriletes.

In comparison with the Lower Triassic spore-pollen assemblages described from the southwestern margins of the West Siberian Plain by L. V. Rovnina (1968), our assemblages show similarity with the assemblage from Pokrovskaya borehole No. 8 (depth 1852.95–1840 m), which L. V. Rovnina considers Lower Triassic.

The above is characteristic of the Lower Triassic spore-pollen assemblages of Western Siberia and distinguishes them from contemporaneous assemblages of the European part of the Union.

REFERENCES

1. E. N. Kara-Murza. Palynological substantiation of the stratigraphic subdivision of the Mesozoic deposits of the Khatanga Depression. Transactions of NIIGA, vol. 109, 1960.

2. A. A. Kurbatova. The spore-pollen assemblage of the Triassic of the Kuznetsk Depression. Transactions of SNIIGGiMS, issue 22, 1962.

3. A. A. Kurbatova. On the palynological characteristics of the Triassic of the Kuznetsk and Tunguska basins. Palynology of Siberia. For the Second International Palynological Conference (Utrecht, Netherlands, 1966), “Nauka,” 1966.

4. V. S. Malyavkina. Spores and pollen from the Triassic deposits of the West Siberian Lowland. “Nedra,” 1964.

5. L. V. Rovnina. On the question of the age of the rocks of the Turinsk and Chelyabinsk series within the western part of Tyumen Oblast. Transactions of ZapSibNIGNI, issue 7, 1968.

6. A. A. Kurbatova. Palynological characteristics of the Early Triassic of Siberia. Materials on the Palynology of Western Siberia. (For the 3rd International Palynological Congress). Transactions of SNIIGGiMS, issue 117, 1971.