TOWARD THE IDENTIFICATION OF UPPER TRIASSIC DEPOSITS IN THE NORTHEAST OF THE WEST SIBERIAN LOWLAND

S. N. GULYAEV

(Presented by Professor A. V. Aksarin)

Triassic deposits in the northeast of the West Siberian Lowland have long been known in the area of the Malo-Khetsky swell. However, previously only Lower Triassic formations had been established here [2, 3], which were assigned to the Turin Series. The most complete section of the Turin Series has been studied in the Malo-Khetsky area, where its exposed thickness in borehole 14-R reaches 972 m. On lithological grounds the Turin Series is divided into two units. The lower, variegated effusive–tuff–argillite unit, more than 420 m thick, is represented by interbedding of reddish-brown, grayish-green, dark-gray, more rarely bluish-green argillites, siltstones, fine-grained sandstones, tuffs, and tuffites. In the lower part of the unit there are thin sheets of diabase porphyrites, and in the upper part—sills of diabases. This unit extends beneath the Jurassic from the Malo-Khetsky area in southwesterly and northeasterly directions; moreover, on the crest of the Tochino uplift (borehole 11-R) there remains of it a 25-meter effusive–tuff bed directly overlying Silurian carbonates.

The upper effusive–tuffite–argillite unit, up to 400 m thick, is composed of dark-gray and black argillites and siltstones with interbeds of dark-gray and greenish-gray tuffites, fine-grained sandstones, and sheet-like bodies of diabases and diabase porphyrites. Numerous phyllopods characteristic of the Lower Triassic have been found here in the argillites. This unit is traced only to the southwest of the Malo-Khetsky uplift.

In 1962, in the Dolgan area, situated on the distant plunge of the southeastern limb of the Malo-Khetsky swell, borehole 2-R, beneath Lower Jurassic sediments in the depth interval 2619–3009 m, penetrated a predominantly clayey unit, somewhat different in lithological composition from the Lower Triassic formations discovered in the crest part of the Malo-Khetsky swell. Limestones occur in this unit and volcanogenic formations are absent. These lithological features allowed A. S. Zaporozhtseva to assign it to the Permian, and M. Kh. Sapir [4] even to the Devonian. However, the spore–pollen assemblage found by A. N. Sheiko in the argillites of the 3002–3009 m interval (whose age assignment the author left open—?) rules out these assumptions. In the spore–pollen assemblage here, pollen of gymnosperms predominates (79.2%). Among the pollen the dominant forms are Conferae (40.8%), Protopicea sp. (14.4%), Araucariaceae (6%), Bennettites (4%), Gingo sp. (3.6%), Gingo

Fig. 1. Section of Triassic deposits in well 2-R, Dolgan area (compiled by S. N. Gulyaev): 1 — argillites, 2 — siltstones, 3 — limestones, 4 — clayey limestones, 5 — silty limestones.

parva (Naum.) Bloch. (3.6%), Cycadsceae (2.4%), Pinus sp. (2.0%), Picea sp. (1.2%), Podocarpus sp. (1.2%). Spores of Coniopteris sp. (18.8%), Tripartina varicebilis Mal. (1.2%), Leiotriletes Naum. (0.4%), Osmunda sp. (0.4%) are recorded. The cited assemblage is close to the typical Rhaetian assemblage studied by I. Z. Fadeeva in the rocks of the northern part of the Tanala and Orsk depressions [1], which makes it possible to assign the enclosing sediments to the Upper Triassic. Deposits of the upper division of the Triassic system are identified in the northeast of the West Siberian Lowland for the first time and are represented by the Chirkovo Formation.

Chirkovo Formation. The name of the formation is derived from the Chirkova River, which flows into the Yenisei east of the Dolgan uplift. The stratotype of the formation is penetrated by well 2-R-D on the Dolgan area beneath Plinsbachian–Toarcian sediments of the Dzhangoda Formation, in the depth interval 2619–3009 m. The uniform distribution of core material through the sections and the clear differentiation of the rocks composing it on the electric-log diagram, where limestones correspond to high (800–2500 ohm·m) and argillites to low or comparatively small (30–400 ohm·m) resistivities, make it possible to characterize the formation fairly completely (Fig. 1). In the composition of the formation, the principal role belongs to dark-gray arg-

argillites, among which interbeds of siltstones are sometimes encountered; and in the middle part of the section (interval 2710–2838 m) beds of dark-gray, more often silty and clayey limestones appear, with thicknesses from 6 to 62 m. The exposed thickness of the formation is 390 m. The discovery of Upper Triassic deposits on the Dolgan area, the presence of Lower Triassic sediments in the arch part of the Malokhetsky swell, and geophysical data make it possible to suppose a broad development of Triassic formations in the western part of the Yenisei-Khatanga trough, where, under favorable conditions, they may be of interest with respect to oil-and-gas potential.

REFERENCES

1. E. M. Andreeva, N. V. Kruchinina, A. A. Lyuber et al. Paleopalynology, vol. 2. Moscow, “Nedra,” 1966.

2. A. S. Zaporozhtseva. The tuffaceous-argillite sequence of the Ust-Yenisei region and its origin. Transactions of NIIGA, vol. 67, 1958.

3. V. N. Saks, I. S. Egorova. Geological structure of the western part of the North Siberian Lowland. Transactions of NIIGA, vol. 81, 1957.

4. M. Kh. Sapir. Jurassic deposits of the western part of the Yenisei-Khatanga trough. Scientific Notes of NIIGA, “Regional Geology,” issue 14, 1969.