ON THE IDENTIFICATION OF SUITES IN THE PRECAMBRIAN BASEMENT OF THE NORTHEASTERN PART OF THE WEST SIBERIAN PLATE

S. N. GULYAEV

(Presented by Professor A. V. Aksarin)

Precambrian formations in the northeastern part of the West Siberian Plate are exposed by a number of core and deep boreholes in the valley of the Yenisei River, and a brief characterization of some of them is given in the work of V. D. Nakaryakov, devoted to the tectonics of this region [5]. However, no detailed study of the Precambrian formations has been carried out here. In the present essay, on the basis of a study of geological materials obtained as a result of drilling, an attempt is made to determine the stratigraphic position of the sections penetrated by boreholes at the latitude of the Arctic Circle in the area of the settlement of Ermakovo, in relation to the Precambrian sections of the northwestern margin of the Siberian Platform.

In the Ermakovo area, located 10–15 km west of the Yenisei River, the Precambrian is exposed by boreholes 1-RE and 3-RE directly beneath Jurassic deposits, respectively in the intervals 1420–2000 m, 1437–1666 m, and 1565–1705 m (Fig. 2). In the Shchuchya area, situated 5 km farther west, basement rocks are penetrated by borehole 1-RSh in the interval 1668–1893 m. The most complete core material characterizes the Precambrian section in borehole 1-RE, and to a lesser extent in the Shchuchya area. However, the clear differentiation of the rocks composing it on electrical logging diagrams—where carbonates correspond to high resistivities (500–7000 ohm·m), and shales to low or comparatively small resistivities (10–300 ohm·m)—makes it possible to characterize the exposed formations in sufficient detail for all boreholes (Fig. 2). In terms of the composition of the constituent rocks, the Precambrian section is clearly subdivided into two sequences: an upper, shale sequence, and a lower, shale-carbonate sequence, which allowed N. F. Ivlev and M. Kh. Sapir, in manuscript, to name the first of them the Kedrovka Suite and the second the Ermakovo Suite. At the same time, the authors did not provide descriptions of stratotype sections and did not adhere to the accepted stratigraphic nomenclature [6]. The detailed study of the Precambrian formations in

Fig. 1. Overview sketch map of the work area: 1 — deep exploratory boreholes.

in the area of the settlement of Ermakovo, and an analysis of the available materials on the Precambrian of the northwestern margin of the Siberian Platform [1, 2, 3, 4], indicate the necessity of distinguishing here two new suites, for one of which the name proposed by M. Kh. Sapir and N. F. Ivlev may be retained.

Poloi Suite. The name of the suite is derived from the settlement of Poloi, located on the bank of the Yenisei River north of the Arctic Circle. The deposits of the suite have been traced on the Ermakovo and Shchuch’ya areas (Fig. 2), but its stratotype section is exposed by borehole 1-RE, located 13 km northwest of the settlement of Ermakovo. In the composition

Fig. 2. Correlation scheme of Precambrian deposits in the area of the settlement of Ermakovo: 1 — marmarized limestones, 2 — schists.

of the suite the principal role belongs to dark-gray, black, and occasionally gray marmarized limestones. Among the carbonate rocks throughout the section there occur beds, 2 to 25 m thick, of dark-gray and black graphitic schists with mineral associations: quartz–biotite–chlorite–calcite, quartz–muscovite–biotite–chlorite. In the upper part of the suite quartz–sericite schists are sometimes encountered, and lower in the section, tremolite calcite schists. The exposed thickness of the suite is 514 m. Carbonates similar in composition, with interbeds and individual beds of schists, are exposed by borehole 3-RE, located 1.2 km northwest of the first borehole, and on the Shchuch’ya area, where their exposed thicknesses are, respectively, 55 and 51 m.

In degree of metamorphism, the deposits assigned to the Poloi Suite differ substantially from the Late Precambrian of the Turukhansk, Igarka, and Norilsk regions of the western margin of the Siberian Platform [1, 2, 3, 4]. Moreover, marmarized limestones have not been encountered

in any one of the listed areas. This makes it possible to consider that, in the area of the settlement of Ermakovo, a more ancient sequence of the Upper Proterozoic has been penetrated, which is not exposed and is not encountered by boreholes within the northwestern margin. The rocks underlying the Poloi Formation have not been penetrated, and it is overlain by shales of the Kedrovo Formation.

Kedrovo Formation. The name of the formation is derived from Lake Kedrovoe, located in the area of the Ermakovo site. The stratotype of the formation has been penetrated by a number of deep boreholes in the Ermakovo and Shchuchya areas (Fig. 2), but the most complete section is represented in borehole 2-PE in the area of Lake Kedrovoe. Here, directly beneath Jurassic deposits, at an interval of 1437–1455 m, greenish-gray, green sericite-chlorite, quartz-chlorite, and quartz-sericite, locally brecciated shales were penetrated. Lower in the section, in the interval 1455–1663 m, dark-gray and black graphitic shales were encountered, in places with numerous veinlets of sulfides along the schistosity: sericite-biotite, chlorite-biotite, quartz-sericite, quartz-sericite-biotite, and, more rarely, muscovite shales, among which there are interbeds of shales with mineral associations: quartz–muscovite–chlorite–calcite and quartz–muscovite–biotite–calcite. The penetrated thickness of the formation in this borehole is 226 m.

Shales similar in composition, significantly affected in the upper parts of the sections by weathering processes, have been encountered in boreholes 1-PE, 3-PE, and 1-RSh, where their penetrated thicknesses are, respectively, 66 m, 85 m, and 170 m. The shales of this formation are underlain by marbled limestones of the Poloi Formation, and in the upper part they are truncated by erosion. In degree of metamorphism and in their composition, the shales assigned to the Kedrovo Formation are to some extent close to the shales of the Proterozoic of the Igarka district of the western margin of the Siberian Platform [2]. However, the absence in its section of quartzites and basic effusive rocks, which occur in the Proterozoic sequence of the Igarka district [2], and its position in the section above the rocks of the Poloi Formation, make it possible to consider that older formations have been penetrated here.

REFERENCES

1. O. A. Votakh, N. F. Ivlev, S. M. Mikutskii. Precambrian of the Igarka district. Doklady AN SSSR, vol. 154, 1964.

2. V. A. Dashenko. Stratigraphy of the Precambrian and Cambrian deposits of the northwest of the Siberian Platform. Tr. NIIGA, vol. 133, 1963.

3. V. M. Krymskii. Stratigraphic correlation of the Igarka Formation with the Strel’nogorsk and Nizhnesukharikha effusives. Doklady AN SSSR, vol. 164, No. 5, 1965.

4. S. P. Mikutskii et al. Geological structure and prospects for oil-and-gas potential of the northwest of the Siberian Platform. Tr. SNIIGGiMS, issue 28, 1963.

5. V. D. Nakaryakov. Structure of the Pre-Yenisei zone of the Domesozoic basement of the West Siberian Platform. Geologiya i geofizika, No. 3, 1961.

6. Stratigraphic classification, terminology, and nomenclature. Moscow, “Nedra,” 1965.