On the Palynology of the Early Triassic of Siberia

A. A. Kurbatova

(Presented by the scientific seminar of the departments of historical geology, general geology, and combustible minerals)

Triassic deposits in Siberia have a limited distribution: according to palynological data, they have been established along the southwestern margin of the West Siberian Lowland (L. V. Rovnina, 1968), in the Kuznetsk Basin (A. A. Kurbatova, 1962), and on the Siberian Platform in the Tunguska syneclise (A. A. Kurbatova, 1966).

Recently, in connection with expanded geological exploration work in the territory of the West Siberian Plain in Tomsk Oblast, Triassic deposits were penetrated in the vicinity of the town of Strezhevoy, well 14, at a depth of 2827.8–2912.8 m. In four samples from the above-mentioned well, represented by sandy-clayey formations, we obtained a spore-pollen assemblage characterized by a fairly large presence of spores of the group Leiotriletes Naum., single grains of cordaites—relicts of the Permian flora—a low content of bisaccate striate pollen grains of ancient conifers with air sacs indistinctly delimited from the body, and a few pollen grains of the Gnetaceaepollenites type (Kurbatova, 1971). This spore-pollen assemblage is very reminiscent of Early Triassic assemblages from the lower horizons of the Korvunchan Formation of the Tunguska syneclise at outcrop 71 (Taimyr River, left bank, 5 km above the Maigunnakan River) and of the Lower Maltsevo Formation of the Kuznetsk Basin (the “Babii Kamen” outcrop on the Tom River).

Here as well, in Tomsk Oblast, 350 km south of well 14, in the Kenga well 136 at a depth of 2846–2856 m, an assemblage analogous to the above-described spore-pollen assemblage was also obtained from three samples of sandy-clayey deposits. In the latter, in addition to spores and pollen of satisfactory preservation, there is a large amount of transparent plant organic matter: small pieces of fragments of leaf cuticle and woody tissues.

The assemblages described contain common forms of spores and pollen characteristic both of Early Triassic deposits of the Tunguska syneclise and of the West Siberian Lowland: Lebachites lebachiiformis Mal., Oblatinella vulgaris Mal., Protopicea oblonga K-M., Psophosphaera sp., Striatoconifera Sed., Cordaitina rotata Sam., Gingocyfadephytus Sam., Cycloequisetites typiciformis Mal., numerous Leiotriletes Naum., among which forms of the type L. microdiscus K. M., of the type Coniopteris tajmyrensis K-M., Cyathidites sp., Osmundopsis spinosa K. M., Selaginella obtusosetosa (Lub). var. triassica K-M., Angiopteris (?) sp., Trachytriletes cf., sibiricus Medv., Matonia cf. triassica K-M., Ly-

Cycadopites sp., Lophotriletes with finely tuberculate exine sculpture and Lophotriletes with coarsely tuberculate exine sculpture, dark smooth Aletes and dark smooth Leiotriletes, Stenozonotriletes. In the Early Triassic assemblages obtained from borehole No. 14 and borehole 136, attention is drawn to the fairly large presence of Leiotriletes, among which there is a noticeable number of forms closely resembling spores of Coniopteris, fine-meshed Lycopodium of poor preservation, a small amount of sack-like pollen of the Ginkgocycadophytus type, and even fewer conifers.

The pollen grains of the latter are characterized by poor preservation, small forms with indistinct reticulation and weakly differentiated air sacs; the obligatory presence of pollen of the Gnetaceaepollenites type (1–3%), pollen grains of the Striatoconifera type (not more than 3%), cordaite pollen (1–2%), and the presence of dark smooth spores.

All the above is characteristic of the Early Triassic spore-pollen assemblages of Siberia, the floristic composition of which is sufficiently varied and at the same time distinctive.

In comparison with the Early–Middle Triassic assemblages described by L. B. Rovnina (1968) from the southeastern margin of the West Siberian Lowland, the greatest similarity of our assemblages is observed with the Early Triassic assemblage from the Pokrovskaya borehole (depth 1852–95–1840 m).

The described spore-pollen assemblages are undoubtedly Early Triassic and are correlated with those from the Korvunchan Formation of the Siberian Platform and the Nizhnemaltsevskaya Formation of the Kuznetsk Basin.

Thus, the new materials obtained establish the presence of Early Triassic deposits in the central part of the West Siberian Plain, where they had not previously been recorded by anyone. The Early Triassic deposits in the north of Tomsk Oblast directly underlie the commercially oil-bearing strata of the Middle Jurassic.

LITERATURE

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

2. A. A. Kurbatova. Spore-pollen assemblage of the Triassic of the Kuznetsk Basin. Tr. SNIIGGiMS, issue 22, L., 1962.

3. A. A. Kurbatova. On the palynological characterization of the Triassic of the Kuznetsk and Tunguska basins. Palynology of Siberia. Toward the II International Palynological Congress (Utrecht, Netherlands, 1966). “Nauka,” M., 1966.

4. A. A. Kurbatova. Palynological characterization of the Early Triassic of Siberia. Tr. SNIIGGiMS, issue 117, 1971.

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

6. L. V. Rovnina. On the question of the age of the rocks of the Turin and Chelyabinsk series within the western part of Tyumen Oblast. Tr. ZapSibNIGNI, issue 7, Tyumen, 1968.