V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

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The burning of coal beds in the ground is not a new event, many fires having been started by nature, not by humans, over the last few million years. In situ coal seams cannot burn for the lack of oxygen, but combustion can arise as they become exposed to the aeration zone as a result of tectonic uplift and erosion. Coal-bed fires are ignited by lightning, wildfires, steppe fires or spontaneous combustion. In some places, the coal is degassed and oxidized by the time it is exposed to the air and therefore it does not combust.

In the Kuznetsk basin the coaliferous sediments have experienced combustion metamorphism (CM), which left signature in abundant CM rocks. The most extensive coal burning has been on the north (Kemerovo), west (Salair) and south-west (Kondoma) deformed basin periphery and in the basin center (Erunakovo). The tectonic framework of the Kuznetsk basin has been controlled by growth of the flanking mountains in Early Triassic, Late Jurassic-Early Cretaceous, and Neogene-Quaternary time. The deformation acted mostly upon the basin periphery rather than in the center and was the most intense in the west. In Kuzbass coal fires arose in Pleistocene time and were unavoidable event of Late Cenozoic geological history of the basin [1,2,3].

Fig. Correlation of kuznetsk coal fires episodes and glaciations and interglaciations of Pleistocene

* Regional stratigraphic scheme of Pleistocene deposits in West Siberia after [8];

On the x-axis is distance from Tyrgan thrust dividing coal-bearing sediments of the Kuznetsk basin from Devonian coal-free rocks of the Salair

In the center of the Kuznetsk basin ancient fires arose in gently dipping (<20°) seams (1.5-15 m) of crushed medium-grade coal (from flame to fat, R = 0.7-1.0%, Q_i^r = 21.7-27.0 MJ/kg) prone to spontaneous ignition [4]. CM rocks occur most often in valley sides and their depths are limited by the level of first (upper) river terraces. Hence in the basin centre dispersed fire foci arose with erosional propagation of present drainage network (ravines and river valleys). According to Yavorski V.I. its inception occurred during Kazantsevian interglaciation (0.13-0.10 Ma) [5]. The CM complexes are weakly eroded and covered with 1.5-3 m thick weathering residue lying under gray or brown loam and loess (1-7 m). The loess and loam sequences have columnar jointing and contain only layer of paleosoil and thus appear to have been deposited since Late Neopleistocene (≤ 23 ths) [6]. It makes us possible to conclude that the age of coal fires in Kuzbass center refer to the time interval 0.13-0.02 Ma. This inference is consistent with the ⁴⁰Ar/³⁹Ar-ages of paralavas (0.06±0.1 Ma). Most likely coal burning appeared in warm dry periods of the Late Neopleistocene: Kazantsevian interglaciation and/or interglaciation in Ermakovo time (fig.). Ignition may have been either triggered by external agents (most probably steppe fires) or spontaneous.

On the west deformed periphery of Kuznetsk basin (the Salair zone) main fields of ancient coal fires are associated with coal seams of Ishanovskaya Suite (P₁ is): Burnt, Prokopievski and Thick. Combustion involved 3.5-24 m thick steeply dipping seams of crushed high-grade (forge and non-baking, R=0.8-1.8 %, Q_i^r 27.6-31.1 MJ/kg) coal. The greatest number of large natural combustion events occurred in sheared vitrain-clarain coal, which has highest oxygen and methane adsorption capacity. In the Salair zone CM rocks are commonly found in weakly deformed hinges or arches or less often in limbs of anticlines [7].

On the west periphery of the basin coal-bearing sediments were exposed by neotectonic events (movement of the Salair block), which produced three piedmont steps intermediate in height between the Salair mountains and the Kuznetsk basin plainland [3]. Block uplift of the coaliferous sediments intensified erosion processes those were the main factor, provided air to the coal. Most of CM fields are encountered in the area of intermediate (second) piedmont step within a narrow belt between the Taiba reverse fault in the west and the Afonino-Kiselevsk reverse fault in the east. In the area of low (third) piedmont step there are two known large CM fields.

Coal of the Salair zone is classified as resistant to oxidation [4]. Coal does not ignite spontaneously in bared coal seams, storage places and demonstrates high heating resistance in mines at high fire hazard. Many natural coal fires remained restricted to one seam and failed to propagate to the neighbor coal beds. This fact, which would appear strange given the close spacing and steep dips of coal seams, is evidence of a low natural fire hazard. The natural fires in the Salair zone always propagated from the roof to the base of coal seams. Therefore, following Butov P.I. and Yavorsky V.i. [1] and Usov M.A. [2], we suggest that natural fires in the area have had external triggers rather than being a result of spontaneous ignition. Unusually large depth of the fires (70-200 m), greatly exceeding present ground water level, indicative of a climate control of combustion events. The burn zones always remain above the aquifers, which prompts an origin of ancient coal fires during the warm and dry periods of post-Pliocene time [2]. In view of the above, steppe fires occurring in dry and warm climate appear to be the most probable causes of coal ignition. In Kuznetsk coal basin in the Quaternary the warmest and the driest climatic conditions are reconstructed in Late Eopleistocene and Late Neopleistocene ( Kazantsevian time ) [6].

In the Salair zone within the area of intermediate (second) piedmont step the low- and medium-grade CM rocks that originally topped the section were fully eroded, and the weathering products were redeposited in ravines and buried under brown or gray loam, loess, and soils. The ⁴⁰Ar/³⁹Ar-ages of the melted CM rocks (clinker and paralava) of this area vary from 1.0±0.1 to 1.2±0.1 Ma. The ⁴⁰Ar/³⁹Ar-ages and geological data indicate that on the west periphery of the basin the first large-scale combustion events have occurred exactly in the Eopleistocene. The yang ⁴⁰Ar/³⁹Ar-ages (0.2±0.1 Ma) of clinkers from this area record cutting-in of Tugai river during Kazantsevian time (0.13-0.10 Ma) accompanied by burning of successively lower beds of coal as they became exposed (Fig.).

The ⁴⁰Ar/³⁹Ar-age of paralava (0.3±0.3 Ma) from CM-complex localized in the area of the low (third) piedmont step allows us to suppose that there combustion events have occurred no earlier than 0.6 Ma ago. This inference is consistent with geological data. In CM complexes the rocks of medium degree of alteration are preserved. In a specified time interval equiprobable climatic prerequisites were in Tiltim and Kazantsevian interglaciation and in interglaciation in Tazian and Ermakovo time [6] (Fig.).

References:

1. Butov P.I., Yavorsky V.I. (1922) Materials for the geology of Kuznetsk coal basin (south-west district of the basin). Petersburg, Publ. H. of Ivan Fedorov, 58 pp.

2. Usov M.A. (1935) Tectonics of the Kuznetsk coal basin. Problemy Sovetskoi Geologii, v. 5, no. 2, p. 113-134.

3. Novikov I.S., Sokol E.V., Travin A.V., Novikova S.A. (2008) Signature of Cenozoic orogenic movements in combustion metamorphic rocks: mineralogy and geochronology (example of the Salair-Kuznetsk Basin transition). Russian Geology and Geophysics, 49(6), p.378-396.

4. Manukyan P.A. (1947) Underground fires in coal mines. Moscow, Ugletekhizdat, 111 pp.

5. Yavorsky V.I. (ed.) (1967) Geology of the USSR. Book XIV. West Siberia (Altai region, Kemerovo, Novosibirsk, Omsk, Tomsk regions). Part I. Geological description. Moscow-Leningrad, Publ. H. Gos. Izd. Geol. Lit, 708 pp.

6. Fronova I.V. (1998) Quaternary small mammals of southern West Siberia (Kuznetsk basin): climate implications, in: Problems of Holocene and Pleistocene Climate Reconstructions for Siberia. Institute of Archaeology and Ethnography, p. 290-299.

7. Sokol E.V., Kudinov E.V., Kiriltseva N.A., Korzhova S.A. (2010) Geological prerequisites of late Cenozoic coal fires in the Kuznetsk coal basin, West Siberia, Russia. Proceedings of "ICCFR 2 Second International conference on coal fire research", dbb forum Berlin, Germany, p. 86-92.

8. Alexeev M.N., Devyatkin E.V., Arkhipov S.A., Laukhin S.A., Grinenko O.V., Kamaletdinov V.A. (1984) Quaternary geological problems of Siberia. Proceedings of the 27th International Geological Congress, v.3, p. 1-21.

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