ON THE GEOLOGICAL STRUCTURE OF THE NORTHEASTERN WEST SIBERIAN LOWLAND

(Turukhansk Region)

S. N. GULYAEV

(Presented by the scientific seminar of the Department of Fossil Fuels)

In 1960–1961, along the Yenisei River, in the section from the town of Turukhansk in the north to the village of Kangatovo in the south (Fig. 1), profile core drilling was carried out. On the Vereshchagin profile, whose total length is 250 km, 7 boreholes were drilled, with depths from 78 to 500 m. These regional geological investigations, connected with an assessment of the prospects for oil-and-gas occurrence in the east of the West Siberian—

Fig. 1. Geological section along boreholes of the Vereshchagin profile. Compiled by S. N. Gulyaev. 1 — Paleozoic deposits, 2 — Lower Jurassic deposits, 3 — Middle Jurassic deposits, 4 — Upper Jurassic deposits, 5 — Neocomian deposits, 6 — Albian–Cenomanian deposits, 7 — Quaternary deposits, 8 — core boreholes, 9 — stratigraphic breaks.

...of the Siberian Lowland showed that Paleozoic deposits, which are part of the basement, and sediments of the Meso-Cenozoic cover take part in the geological structure of the region.

The stratigraphic subdivision of the Paleozoic and Mesozoic sediments penetrated by the wells has been made provisionally, since in determining the age of the former their lithological similarity to the presumably coeval formations of the right bank of the Yenisei River was taken as the basis, whereas the age of the latter was established from spore-pollen assemblages. Only in well 1-V is the identification of Jurassic and Neocomian sediments confirmed to some extent by determinations of microfaunal remains.

Paleozoic deposits in the area of the Vereshchagino profile were penetrated by wells at various depths (from 109 to 275 m), and their exposed thickness does not exceed 68 m.

The oldest formations composing the basement are light-gray and whitish-gray cavernous dolomites, encountered at the mouth of the Pakulikha River (well 2-V) and opposite the village of Tatarskoe (well 4-V), respectively in the intervals 180–241 m and 275–291 m. In the dolomites of well 4-V, dips from 20 to 25° are observed in separate intervals. On the basis of lithological similarity to the dolomites of the Kostino Formation of the western framing of the Siberian Platform, they are provisionally assigned to the Lower Cambrian.

Younger deposits are penetrated in the interval 109–177.5 m by well 5-V, situated on the left bank of the Yenisei River opposite the settlement of Sukhaya Tunguska. Here the rocks are represented by brownish-red and greenish-gray argillites. The dips of the rocks range from 20 to 35°. The upper part of the section (interval 109–125 m) bears traces of weathering and is represented by light-gray clays and light-gray, white argillites. These bleached argillites also have dips of 20–35°. In the very lowest part of the section (interval 174–177.5 m), interbeds of brownish-red clayey limestone and light-gray dolomite are observed. No faunal remains were found in the rocks, and they are provisionally assigned to the Devonian.

Formations presumably of Upper Paleozoic age [1] are penetrated by well 9-I in the interval 418–426 m, where they are represented by dark-gray, almost black argillites.

Taking into account all the foregoing and the fact that, in the area of the village of Nizhne-Imbatskoe, Devonian deposits had previously been established beneath the Mesozoic, one may suppose the presence of an exposure of Paleozoic rocks between the villages of Tatarskoe and Vereshchagino.

The pre-Jurassic basement occupies its highest hypsometric position in the area of well 5-V, where it is penetrated at a depth of 109 m. From this well, both to the north and to the south, a subsidence of the basement surface is observed; moreover, to the south the subsidence is less intense (at an angle of 6′) than to the north, where well 6-V, at a bottom-hole depth of 500 m, did not pass out of the Mesozoic sediments. But even in the northern direction the angle of inclination of the basement surface apparently does not exceed 1°.

On the eroded surface of various Paleozoic horizons, Middle Jurassic sediments occur almost everywhere with angular unconformity. An exception is the area of the mouth of the Suklemikha River, where, in well 6-V, Lower Jurassic deposits are penetrated in the interval 349–500 m. The age of these sediments is established from paleobotanical data. In the spore-pollen assemblages of these deposits, V. N. Kustova notes a sharp predominance of gymnosperm pollen over fern spores, and a high content of large-...

…large sizes, such as Pseudopinus pergrandis Bolch, as well as the presence, in considerable quantities, of ancient pollen Dipterella oblatinoides Mal. The upper boundary of the Lower Jurassic is drawn provisionally at a depth of 349 m. The Lower Jurassic deposits are represented by frequent alternation of light-gray and greenish-gray siltstones and argillite-like clays. Interbeds of gray and light-gray fine-grained sandstones play a subordinate role in the section. An increased content of siderite is noted in the rocks; in the siltstones it is sometimes the principal cementing material.

The section contains interbeds of brown coal up to 3 m thick, carbonized plant remains, and in the clays of the interval 385–391 m impressions of small pelecypods have been recorded. The exposed thickness of the Lower Jurassic deposits reaches 151 m.

In general, the Lower Jurassic of the area described is characterized by a predominance of siltstones and argillite-like clays; a comparatively light coloration of the rocks (light tones predominate), which is apparently due to a significant content of kaolinite in them; and a relatively high degree of compaction of the rocks for the near-margin part of the West Siberian Lowland, where loose or weakly compacted varieties are usually encountered.

Lower Jurassic deposits in the very near-margin part of the West Siberian Lowland have not yet been established anywhere, except in well 6-V. They are also absent in the crest part of the Farkovo uplift [2], situated 35 km west of well 6-V. These data show that the Turukhansk Mesozoic basin, located between the Farkovo uplift in the west and the Turukhansk horst of the Siberian Platform in the east, was formed as early as Lower Jurassic time, when sediment accumulation took place mainly in more westerly areas. Thus, Lower Jurassic deposits are penetrated by exploratory wells in the middle course of the Turukhan River on the Sredne-Perov uplift, 130 km northwest of well 6-V, at depths of approximately 2250–2400 m. There they rest transgressively on phyllites of presumably Proterozoic age and differ somewhat from the deposits penetrated by well 6-V. The Lower Jurassic section of the middle course of the Turukhan River is also characterized by a predominance of siltstones and argillites, rather frequent alternation of lithologic varieties, and rare faunal remains (impressions of small pelecypods); but, in contrast to the section of well 6-V, in the Lower Jurassic section of this area thicker sandstone beds appear; dark, predominantly brown tones are characteristic of the siltstones and especially of the argillites; and coal interbeds occur more rarely and are very thin (up to 3 cm). Here, at the base of the Lower Jurassic (at the boundary with the pre-Jurassic basement), there is a sandy basal horizon, the presence of which is of definite interest in the search for oil and gas. This horizon is apparently absent in the Lower Jurassic section in the area of well 6-V, since sediment accumulation here during the Lower Jurassic epoch occurred under different conditions. The change in conditions of sediment accumulation also accounts for the change in the petrographic composition of the rocks, which does not permit correlation of individual lithologic horizons of the Lower Jurassic sections of well 6-V and of the middle course of the Turukhan River, but only a comparison of the sections of these areas.

Middle Jurassic deposits on the segment of the Vereshchagino profile are penetrated by almost all wells; moreover, if in wells 5-V, 2-V, and 4-V they rest on Paleozoic formations, then in well 6-V they concordantly overlap the Lower Jurassic deposits. In the area under consideration, as everywhere in the east of the West Siberian Lowland, for the Middle…

The Jurassic is characterized by an alternation of clays, siltstones, sands, sandstones, the presence of seams of brown coals, and an almost complete absence of faunistic remains. The microfaunal remains found in the clays of the 487–499 m interval of borehole 1-V do not make it possible to determine precisely the age of the deposits enclosing them. Here V. K. Komisarenko and K. E. Tylkina identified foraminifers: Ammodiscus ex gr. incertus Orb., Ammodiscus ex gr. tenuissimus Gümb., Glomospira gordialis (Park et Jon.), Ammodiscus uglicus Ehrem. The first two ammodiscids are known in Valanginian–Volgian deposits. Glomospira gordialis (Park et Jon.) in Western Siberia is recorded in the Volgian and Oxfordian stages, while in the Nordvik–Khatanga region it is known from Bajocian deposits; moreover, here there is also a joint occurrence of shells of this species and of the ammodiscids. Ammodiscus uglicus Ehrem, separated by the author of the species into an independent zone for the lower Oxfordian, was traced by P. G. Dayn in more ancient deposits within the Bathonian–Callovian.

Taking into account all the above and the lithological features of the section of borehole 1-V, the author, like V. K. Komisarenko and K. E. Tylkina, assumes that the deposits of the 487–499 m interval are Middle Jurassic.

A somewhat anomalous section of the Middle Jurassic is observed in borehole 6-V. Here there is a high degree of compaction of the rocks, a lighter coloration, and the maximum coal saturation of these deposits for the entire northeastern part of the West Siberian Lowland. The thickness of the Middle Jurassic deposits in borehole 6-V, where they are penetrated completely, is 229 m.

Upper Jurassic deposits were encountered by boreholes 1-V, 3-V, 6-V, and 7-V, and their complete section is penetrated only by the first borehole. In the other boreholes the Upper Jurassic deposits were eroded completely (boreholes 2-V, 4-V, 5-V) or partly in pre-Quaternary, and possibly also in pre-Upper Cretaceous, time.

The Upper Jurassic section is characterized by interbedding of gray, greenish-gray, dark-gray clays and siltstones and greenish-gray and light-gray fine- and various-grained sands and sandstones. Thin (up to 0.5 m) seams of brown coals and brownish-gray siderites occur sporadically.

The rocks contain many coalified plant remains; grains and small concretions of pyrite, and pelecypod impressions occur. In beds apparently transitional from the Upper Jurassic to the Valanginian (interval 248–255 m), foraminifers were identified in borehole 1-V: Ammodiscus ex gr. incertus Orb., Glomospira ex gr. gaultina (Bert), Glomospira ex gr. gordialis (Parker et Jones).

V. K. Komisarenko and K. E. Tylkina, who carried out the determinations of the microfauna, indicate that the first of the glomospiras occurs in Valanginian deposits of the West Siberian Lowland, and in the Ust-Yenisei region it is known from the Valanginian and Volgian stages.

Comparison of the Upper Jurassic deposits of the area described with coeval deposits of more western regions (the Elogui and Turukhan reference boreholes) shows that:

1) the Upper Jurassic section of the area under consideration is characterized by more frequent interbedding of rocks of different composition;

2) individual coal seams are present in the sections of boreholes of the Vereshchagino profile and are almost entirely absent in coeval deposits of more western regions;

3) in the Upper Jurassic deposits of this area the fauna occurs rarely and is poor in composition (a monotonous assemblage of foraminifers, pelecypod impressions), whereas in the coeval deposits of the western regions the fauna

is present in greater quantity and is richer in composition (ammonites, belemnites, pelecypods, foraminifers).

The foregoing makes it possible to conclude that, in contrast to the areas of the Yelogui and Turukhansk reference wells, sediment accumulation in the given area took place not in nearshore-marine but in lagoonal conditions.

Cretaceous deposits are exposed only in wells 1-V and 3-V, where we distinguish sediments of the Neocomian and the Albian–Cenomanian. The complete or partial absence of Cretaceous deposits in the wells is explained by pre-Quaternary and, possibly, pre-Albian–Cenomanian erosion.

The deposits assigned to the Neocomian are composed of greenish-gray and gray-green clays, siltstones, and light-gray, greenish-gray fine- to medium-grained sands and sandstones. Thin (up to 5 cm) partings of brown coals occur rarely. The rocks contain many carbonized plant remains. In well 1-V, from the deposits of the interval 219–279 m, K. P. Baibarodskikh identified spore-pollen assemblages containing spores of the family Gleicheniaceae: three-lobed Gleichenia, Gleichenia delicata Bolch., Gleichenia triplex Bolch. Spores of the family Schizaeaceae are present: Lygodium sp., Mohria sp.; individual spores of Leiotriletes Naum, Coniopteris, Salvinia sp., and Cibotium were encountered. Among the pollen, Pinus sp., Picea sp., and indeterminate Coniferae predominate; Podocarpus sp., Ginkgo sp., and Psophosphaera sp. are present. These spore-pollen assemblages and the foraminifers noted in the interval 248–255 m make it possible to assign the homogeneous bed of sandy-clayey rocks in the interval 251–216 m in well 1-V to the Neocomian.

Upon the Neocomian rocks, both in well 1-V and in well 3-V, Albian–Cenomanian sediments rest with erosion. In well 1-V, above the paleontologically characterized bed (interval 251–216 m) of greenish-gray sandy-clayey Neocomian rocks, there occur gray and light-gray, predominantly sandy rocks, which likewise form a single bed in appearance and lithological features (interval 216–68 m). It is characterized by a predominance of sandy rocks in the section; the presence of variegated light-gray kaolinized sands with pebbles of siliceous rocks; and an uneven distribution in the clays and siltstones of sandy material in the form of irregular inclusions, lenses, and interbeds. Thin (up to 5 cm) interbeds of brown coal occur rarely. From the deposits of this bed of rocks in the interval 129–159 m, K. P. Baibarodskikh identified a spore-pollen assemblage of Upper Cretaceous aspect. Spores encountered here include: Polypodiaceae, Divisisporites Thomson, Sphagnum sp., Gleichenia delicata Bolch., Lygodium sp., Amentia sp.

Spores of the group Leitriletes Naum., Hausmannia alata K—M., Alsophila sp., Selaginella sp., and others are present. Pollen of gymnosperms is represented by: Pinus sp., Picea sp., Cedrus sp., Abies sp., Podozamites sp., Ginkgo sp.; pollen of the families Cupressaceae–Taxodiaceae and pollen of Angiospermae were encountered.

This spore-pollen assemblage makes it possible to assign the upper bed of Cretaceous deposits in well 1-V to the upper Albian–Cenomanian. Aptian–Lower Albian deposits are unknown here. This indicates that in well 1-V the Albian–Cenomanian deposits possibly rest with erosion on the Neocomian sediments. A similar picture is observed in well 3-V, but here a smaller part of the Neocomian section was subjected to pre-Albian–Cenomanian erosion.

Quaternary deposits, from 53 to 120 m thick, are penetrated by all boreholes. They rest with erosion on various Mesozoic horizons.

Thus, as a result of the drilling operations carried out in the Turukhan–Elogui interfluve, the following was established:

1. The shallow occurrence here of the Jurassic basement.
2. The development, on the left bank of the Yenisei River beneath the Mesozoic, of weakly dislocated Paleozoic deposits that do not differ from analogous sediments of the western framing of the Siberian Platform.
3. The presence of Lower Jurassic sediments in the Turukhansk trough (borehole 6-V) and indications of a pre-Albian–Cenomanian erosion of Mesozoic deposits in the area studied.

REFERENCES

1. V. D. Nakaryakov. Structure of the Pre-Mesozoic Basement of the Yenisei Region of the West Siberian Platform. Geologiya i geofizika, No. 3, Publishing House of the Siberian Branch of the USSR Academy of Sciences, Novosibirsk, 1961.

2. V. D. Nakaryakov. Tectonics of the Mesozoic Deposits of the Turukhan River Basin. Transactions of SNIIGGiMS, issue 14, 1961.