On the Absence of a Thick Seam of the Nazarovo Brown Coal Deposit in the Chertovo Lake Area
Abstract Generated abstract
This paper examines the unexpected absence of the Thick coal seam of the Nazarovo brown coal deposit in the former Chertovo Lake area, where open-pit stripping and borehole data revealed a localized loss of the seam previously thought to be continuous. The authors describe geological observations from drilling, including steeply inclined beds in borehole core, and discuss two possible explanations: a diapiric fold related to vertical basement movements, or a landslide-like deformation driven by groundwater pressure and subsequent erosion. They propose additional drilling to distinguish between these hypotheses and to define the limits of the seam’s absence, emphasizing that the affected zone may be small and should not necessarily lead to rejection of the whole area as noncommercial.
Full Text
ON THE ISSUE OF THE ABSENCE OF THE THICK SEAM OF THE NAZAROVO BROWN-COAL DEPOSIT IN THE CHERTOVO LAKE AREA
V. V. Kosarev, V. N. Pulyaev, L. A. Pukhlyakov
(Presented by Professor A. G. Sinovyi)
The coal-bearing deposits of the Nazarovo brown-coal deposit are Jurassic in age and are overlain by Quaternary deposits. Among the latter, horns with part of a bison skull, as well as molar teeth, tusk fragments, and other bone remains of a mammoth were found (identified by L. Khalfin).
The coal-bearing sequence is represented by interbedding of aleurite-sandy-clayey rocks and coal seams, among which the Thick seam is of greatest interest; it occurs in the upper part of the deposit and is currently being mined by open-pit methods.
The tectonics of the deposit are generally calm. The part of the Thick seam exposed by the coal-mining open pit lies practically horizontally. As for the unexposed part, according to exploratory drilling data, the dip angles of its floor and roof do not exceed 1–2°. It was also assumed that, within the explored area, this seam has continuous distribution.
By 1968 the stripping operations had approached the area of the former (now drained and filled-in) Chertovo Lake, and here a decrease in the thickness of the Thick seam being mined by the open pit was discovered. The suggestion was made that this phenomenon was connected with the erosional activity of surface waters, and a borehole (borehole 1342) was sunk at some distance from the edge of the open pit. This borehole showed that near the middle of the former Chertovo Lake the Thick seam is completely absent, and that the cause of its absence is not merely erosion, but erosion connected with tectonics. In the core recovered from this borehole, the rock beds had an inclination of up to 70°, and this indicated that the borehole had exposed the limb of some fold. The core was unoriented, and therefore it was impossible to say anything about the azimuth of dip of the seam at the point where it was sampled. But this did not change the essence of the matter. A fold was present. A new question arose: as a result of what had this fold formed?
In the opinion of one of the authors of this article, V. V. Kosarev, the fold is diapiric and arose in connection with radial (vertical) movements of the basement. In the opinion of another author of the article, L. A. Pukhlyakov, it has approximately the same nature as the landslide folds of the Borodino brown-coal deposit, which affected a rock sequence of small thickness (up to 10 m) above the Rybinsk seam [1]; however, unlike the latter, the fold of the Nazarovo deposi-
Fig. 1. Geological section through the zone of absence of the Moshchnyi seam in the area of Chertovo Lake.
1 — drilled boreholes; 2 — planned boreholes; 3 — Quaternary deposits; 4 — brown coal; 5 — Jurassic aleuritic-sandy-clayey deposits; 6 — identified seam boundaries; 7 — inferred seam boundaries.
Visible labels on the section: SW → NE; 0–50 m; 250 m; 200 m; boreholes 1332, 1342, 1337; $J_2$; Moshchnyi seam.
genesis encompassed a somewhat greater thickness of rocks (possibly up to 50 m). Moreover, here the displacement involved strata that did not cover the coal seam, but lay below it. Apparently, in these rocks there was movement of groundwater directed toward the area of the former Chertovo Lake, where it created upward pressure. As a result of this pressure, the pores of the water-bearing bed expanded and were filled with sand grains brought here. Owing to this, the area began slowly to rise, and erosional forces came into action on its territory; as a result, the pressure on the underlying beds decreased and the process continued.
In accordance with what has been set forth, there is the fact that in the zone where the seam is absent the groundwater has a high head.
In order to test these hypotheses, and also to refine the zone of absence of the seam within the territory of the former Chertovo Lake, it is proposed to lay out several additional boreholes. If the first of the hypotheses is correct, boreholes that encounter dislocated rocks will not encounter nondislocated rocks any further. If the second is correct, beneath the dislocated rocks the borehole will encounter nondislocated rocks (Fig. 1). Refinement of the zone of absence of the seam is of still greater relevance, since it is quite probable that it is comparatively small in size, and therefore there is no need to write off the entire area as noncommercial.
REFERENCES
- L. A. Pukhlyakov. Some remarks on the tectonics and paleogeography of the Irsha-Borodino brown-coal deposit. Izv. TPI, vol. 166, Tomsk, 1969.