GEOLOGICAL STRUCTURE OF THE TATAUROVO BROWN-COAL DEPOSIT IN CHITA OBLAST

A. V. Aksarin, V. L. Kokunov

The Tataurovo brown-coal deposit is located 65 km southwest of the city of Chita and 0.5 km from the railway branch of the Lesnaya–Provyannaya station of the Trans-Baikal Railway, and administratively lies within the territory of Uletovsky District, Chita Oblast. The deposit is confined to the central part of the Chita–Ingoda intermontane depression and is an enclosed syncline of the same name, elongated in a northeastern direction, i.e., in accordance with the elongation of the Chita–Ingoda depression. The width of the syncline in the central part of the deposit, within the contour of the lowermost seam of workable thickness, is 5.2 km, and its length is 10.3 km. Its area within the same contour is 38.6 sq. km. The dip angles of the limbs of the syncline range from 4 to 7°. Toward the center of the syncline the dip of the beds gradually flattens to 1–2°.

No major disjunctive faults cutting the coal-bearing sequence were found during exploration work at the deposit; however, the possibility cannot be excluded that small faults with amplitudes of several meters, analogous to the faults in the sequence of the neighboring Chernovskoe deposit, may be present, since with an exploration grid of 250–330 m they could not have been detected.

The area of the Tataurovo deposit is composed of continental coal-bearing deposits which, according to the regional stratigraphic scheme for the subdivision of continental Mesozoic formations (Transbaikalia) proposed by G. G. Martinson (1961), are divided into three suites: the Bukachacha — \(I_2\), the Ulanganga — \(I_3—Cr_1\), and the Turgino–Vitim — \(Cr_1\). However, according to the investigations of E. V. Barabeshev and I. N. Fomin, carried out in 1956 and perhaps somewhat earlier, in the Chita–Ingoda depression the Middle Jurassic sediments were assigned the name Kharyulatin suite, after one of the small rivers where these deposits were studied. Consequently, the section of continental deposits filling the Chita–Ingoda depression should have a threefold division and consist of the following suites (from bottom to top): Kharyulatin — \(I_2\), Ulanganga — \(I_3—Cr_1\), and Turgino–Vitim \(Cr_1\). According to the local stratigraphic subdivision, the Ulanganga suite is subdivided by L. I. Ferdman (1964, 1965) into the Byrshin and Doronin suites on the basis of lithologic composition, while the Turgino–Vitim suite is called the Titnin suite. At the same time, according to data from V. F. Korolev (1965), the Byrshin suite of Late Jurassic age is represented by andesite-basalts, andesitic porphyries, quartz and quartz-bearing-

porphyries in the form of isolated spots, which occur, properly speaking, already in the foothills of the Chersky Range and have not been recorded by boreholes within the territory of the Tataurovo deposit. The Dzhartalantui suite of effusive formations of Middle–Upper Jurassic age, distinguished by local geologists, is not distributed in the section of the Tataurovo deposit.

Let us now turn to a comparison of the units of the local stratigraphic scale with the subdivisions of the regional stratigraphic scale of the Mesozoic deposits of Transbaikalia.

At the base of the section of continental deposits of the Tataurovo deposit, resting with angular unconformity on the erupted rocks of the basement, lies the Kharoylatin suite (Table 1), the age of which is substantiated by finds of Middle Jurassic flora, identified by Prof.

Table 1

A. V. Aksarin and B. M. Rempel. This suite of the regional stratigraphic scale corresponds to the suite of basal conglomerates of V. P. Plotnikov and V. A. Kolicheva (1954) of the Chernovka deposit and to the conglomerate unit of V. F. Korolev (1965), but with a reservation concerning the persistence of the rocks composing its section.

During exploration of the deposit it was not penetrated by boreholes, since it lies deeper.

The Upper Jurassic Ulanganginskaya suite rests conformably on the deposits of the Kharoylatin suite; in volume it corresponds to the suite of coal-bearing formations of V. P. Plotnikov and V. A. Kolicheva of the Chernovka deposit and to the coal-free and coal-bearing units of V. F. Korolev for the Tataurovo deposit. The Upper Jurassic age of the suite is substantiated by an assemblage of plant remains identified for the Chernovka deposit by V. A. Khakhlov, A. N. Krishtofovich, V. D. Prinada, and N. N. Srebrodolskaya, and for the Tataurovo deposit by A. V. Aksarin. Deposits of this suite were penetrated by boreholes during exploration of the Tataurovo deposit through almost their entire thickness, with the exception of the very lowest parts of the suite. However, the possibility is not excluded that borehole No. 347 penetrated the basal member of sandstones of the Ulanganginskaya suite, resting on a thin interbed of effusives of the Byrchinskaya suite, and then it may be considered that borehole No. 375 already penetrated the top of the Kharoylatin suite, represented by siltstones.

Below we present a brief lithological and facies cha-

characterization of the studied section of deposits filling the Tataurovo mulda of the Chita–Ingoda depression.

Kharulgatinskaya Formation—\(I_2\). The deposits of this formation were not studied by us, and therefore their brief characterization is given according to the data of V. F. Korolev (1965), who described these deposits as the “conglomerate sequence.”

Lithologically, the deposits of the Kharulgatinskaya Formation are composed of boulder–pebble formations extending along the northwestern margin of the Chita–Ingoda depression. The pebbles and boulders consist mainly of local rocks from the mountain structures surrounding the depression: gneisses, crystalline schists, leucocratic granites, diorites, acid and intermediate effusives, and, more rarely, granosyenites and syenite-porphyries. The cement of the conglomerates is arkosic-sandy, sandy-clayey, and sometimes ferruginous. The formations of this suite are typical proluvial deposits at the foot of steep mountain slopes and therefore are lithologically extremely unstable in plan: conglomerates pass into sandstones, sandstones into aleurolites, etc. On the whole, conglomerates predominate in the section of the formation, but in the central parts of the depression the amount of conglomerates in the section of the formation should be considerably smaller.

The thickness of the formation varies in different parts of the Chita–Ingoda depression. Approximately, judging from geophysical data on the depth of the basement, the thickness of the formation reaches 500–600 m.

Ulanganginskaya Formation—\(I_3\). The deposits of the upper part of the Ulanganginskaya Formation were studied in considerable detail in the sections of boreholes Nos. 401, 173, 240, 220, 232, and a number of others. The lower part of the formation, however, was studied in less detail from the sections of boreholes Nos. 341, 347, and 375.

The lower boundary of the Ulanganginskaya Formation is drawn by us, with a considerable degree of convention, on the basis of the appearance in the sedimentary section of effusive rocks encountered by borehole No. 375. The upper boundary of the formation in the section of the Tataurovo deposit is not fixed, and its relation to the overlying Turgino–Vitim (Titninskaya) Formation should also, conditionally, be regarded as conformable.

The deposits of the formation, by all investigators who have studied it, were divided at the Chernovskoe and Tataurovo deposits into three parts according to the principle of coal saturation: the lower—coal-barren, the middle—coal-bearing shale or a unit of frequent interbedding of rocks, and the upper—coal-bearing, as is commonly accepted in the practice of coal geologists at the first stages of deposit study.

Considering that at the present time a sufficient amount of material has accumulated, we propose that the subdivisions of the Ulanganginskaya Formation be regarded as subformations and that they be named according to a geographic principle, on the basis of finds of fossil flora in them. Thus, the upper part of the Ulanganginskaya Formation, containing coal seams of workable thickness at the Chernovskoe and Tataurovo deposits, owing to the discovery of the first finds of flora in coal section Torm-2 in the roof of seam III, in our opinion would best be called the Torm subformation, characteristic of both deposits. The middle part of the Ulanganginskaya Formation, corresponding in volume to the coal-bearing shale horizon of the Chernovskoe deposit, owing to the discovery in its deposits of fossil flora at the Tataurovo deposit, is proposed to be called the Tataurovo subformation, as the second floriferous horizon of the Ulanganginskaya Formation.

For the lower part of the Ulanganginskaya Formation, formerly called the coal-barren sequence, the name “lower subformation” may be retained.

Lithologically, the lower subformation is represented by sandstones, aleurolites, and argillites. The sandstones usually have a light-gray ...

in color and, in grain size, are coarse- or coarse-grained; in sorting they are unsorted. They compose the lower part of the subformation and in borehole No. 347 have a thickness of up to 40 m.

Stratigraphically higher lies a thick unit of dark-gray, greenish-tinged siltstones, interbedded with thin layers of dense, hard argillites of the same color. The thickness of this unit reaches 70 m. Thus, the thickness of the lower subformation is 110 m, but it may probably reach an even greater value. The subformation is characterized by extreme inconstancy of the rocks both across the section and along the strike of the Tataurovo deposit.

The Tataurovo subformation rests conformably on the lower one and is characterized by frequent alternation of the lithologic rock types composing it, namely gray varigrained sandstones, gray and dark-gray siltstones, dark-gray argillites, and thin interbeds of brown coals and coaly argillites. The thin coal interbeds are characterized by extreme lack of persistence and a lenticular structure. Despite their number (there are about 10 in the section of the subformation), they are intersected by no more than two or three boreholes and are of no industrial value.

The thicknesses of sandstone beds range from 2 to 10 m; the thicknesses of siltstone beds vary from 4 to 10 m. The argillite beds are usually thinner. The thicknesses of the coal interbeds range from 0.1 to 0.9 m.

The siltstones are carriers of elements of lamination, caused by, and expressed in, the presence of plant detritus on the bedding planes.

The thickness of the Tataurovo subformation within the limits of the deposit is 110 m, but in the Chernovskaya mulde its thickness increases to 320 m. Deposits of the Tormovskaya subformation crown the section of the Ulanganginskaya Formation at the Tataurovo deposit.

Lithologically, the Tormovskaya subformation is composed of an alternation of sandstones, siltstones, argillites, coaly argillites, and coal seams. Sandy rocks account for 48.5% of it, siltstones for 21%, and argillites for 10%. Thus, the coal-bearing coefficient of the Tormovskaya subformation is calculated to be 21.5%, which indicates a very high coal saturation of this subformation. The composition and conditions of formation of the deposits of this subformation have been studied by us in the greatest detail. The deposits of the Tormovskaya subformation rest conformably on the underlying formations.

The sandstones of the Tormovskaya subformation are usually represented by medium- and fine-grained varieties, but with the obligatory presence of coarse-grained and aleuritic material, and on granulometric-composition graphs they are expressed by very gentle curves, i.e., for the most part they belong to varigrained sandstones. In places these sandstones show indistinct oblique bedding, emphasized by changes in grain size. The clastic material of the sandstones is usually represented, in the lower parts of the subformation, by fragments of quartz and quartz–feldspar rocks (granites) at 13–30%, feldspars at 59–65%, including potassium feldspars at 1.5–9%. The remainder consists of fragments of pegmatites and crystalline schists, sulfide minerals, and micas.

The cement of the sandstones is weak, clayey; in mineralogical composition, as studied under an electron microscope, it is kaolinite–hydromica, with hydromicas significantly predominating over kaolinite. Interbeds of hard sandstones on carbonate cement occur rarely. The cement type is mostly basal.

The siltstones composing the lower part of the Tormovskaya subformation usually have a gray and dark-gray color and form a complex, floodplain-

a layer of the channel type, expressed by a change in grain size and emphasized by plant detritus. In the siltstones, remains of leaf flora of poor preservation and root remains of plants are abundant.

The mineralogical composition of the components of the terrigenous part of the siltstones is similar to that of the sandy rocks, but with an almost complete disappearance of potassium feldspars and a noticeable increase in the quartz content (up to 60%). The cement of the basalt-type siltstones is clayey in composition. The mineral composition of the cement is kaolinite–hydromicaceous or hydromicaceous.

The argillites in this part of the subformation have a dark-gray, brownish-gray color and parallel bedding, emphasized by layer-by-layer accumulations of leaf flora. Thin (up to 2–5 cm) interbeds of argillites often contain root remains of plants, indicating that the buried flora also grew here.

Among the siltstones and argillites of this part of the Torma subformation, two thin (1–1.5 m) seams of brown coal have been recorded.

Stratigraphically above lies a thick seam of brown coal (seam III), which is one of the principal objects of exploration.

The interseam interval between coal seams II and III is composed mainly of sandstones and gravelites with thin interbeds of siltstones and argillites. The sandstones are usually light gray, for the most part with an admixture of gravelly material. Their bedding is coarse, oblique, expressed by plant detritus or by thin interlayers of fine-grained material. According to the data of granulometric analysis, processed by the method of L. B. Rukhin, these are mainly channel and floodplain formations.

The mineralogical composition of the coarse-grained fraction of these sandstones (diameter 1–2 mm) consists of feldspars—35.8%, including up to 4% potassium feldspars; rock fragments—58.6%, represented by quartz-feldspar rocks—25%, crystalline schists—1%, and effusives. In addition, chlorite, biotite, and quartz are observed in the composition of the detrital material of the sandstones.

The cement of the sandstones is clayey, weak. Under the electron microscope, and also by the staining method, it was determined that the amount of kaolinite in the mineral composition of the cement gradually increases as one approaches the floor of seam II, and the mineral composition of the cement becomes hydromicaceous–kaolinitic, i.e., kaolinite begins to predominate in the composition of the sandstone cement.

In the interseam interval between coal seams III and II, up to 6 thin (0.4–3.7 m), rapidly wedging-out coal seams of no industrial significance are recorded. In the floor of seam II, impressions of leaf flora have been encountered. The interseam interval between coal seams II and I consists of two members. The lower member, forming the roof of seam II, is composed of sandstones; the upper, of siltstones, rarely argillites. The sandstones are gray, usually fine-grained, aleuritic to a greater or lesser degree, and with fine bedding of the floodplain type.

The siltstones are usually dark gray, sometimes dark brown, with parallel and wavy bedding of the floodplain type. Their bedding is emphasized by fine coaly detritus on the bedding planes, sometimes by an alternation of lighter and darker layers about 1–2 mm thick.

The sandstones and siltstones of the interseam interval are extremely inconsistent along strike and in thickness. In individual boreholes, the interseam interval between seams I and II is composed of a fine alternation of sandy, silty, and argillitic interlayers. The mineral composition of the terrigenous part of the sandy-silty layers is represented by quartz and feldspars.

whereas the amount of quartz begins to predominate over feldspars, which indicates a change in the nature of the processes of transformation of the source material in the source area and along the transport routes.

The mineral composition of the cementing material of the sandy–silty rocks is represented by kaolinite and hydromica with an admixture of halloysite, palygorskite, and montmorillonite. At the same time, stratigraphically from bottom to top the composition of the cement changes markedly, from kaolinite–hydromicaceous to hydromicaceous–kaolinitic and purely kaolinitic.

In the siltstones near the footwall and in the rock interbed within seam I, we collected and identified an assemblage of fossil leaf flora. The species composition of the flora, as determined by Prof. A. V. Aksarin, is as follows: Ginkgo digitata Brongh., Ginkgo flabellata Hr., Phoenicopsis speciosa Hr., Phoenicopsis stobieckii Racib., Phoenicopsis angustifolia Hr., Phoenicopsis simus Aks., Sphenobaiera magnifolia Aks., Pseudotorellia (Feildenia) ensiformis (Hr) Dolud., Pseudotorellia pulchella (Hr) Vasil., Scleropteris daurica Pryn., Coniopteris angarensis Pryn., Coniopteris burejensis (Zal) Sew., Podozamites lanceolatus Eichw., Pityophyllum nordenskioldii (Hr) Nath., Pityophyllum sp., Angariella angustufolia (Hr)., Radicites sp., CARPOLITES sp.

According to A. V. Aksarin’s conclusion, the species composition of the flora indicates an Upper Jurassic age for the enclosing sediments.

In the roof of seam I there occurs a bed of sandstones up to 30 m thick. At this section, the coal-bearing deposits in the Tataurovo trough come to an end.

The coal-bearing deposits are overlain by Quaternary formations, the thickness of which ranges from 5.0 to 15.7 m, rarely reaching 23.0 m.

The Quaternary formations are represented by gravel–pebble alluvial material in the lower part and by a layer of sand with crushed stone, 0.3–0.5 m thick, in the upper part.

LITERATURE

1. G. G. Martinson et al. Jurassic and Cretaceous Continental Deposits. In: Geology of the USSR, vol. XXXVI, part 1, Gosgeoltekhizdat, 1961.

2. V. P. Plotnikov, V. A. Kolichev. Summary geological report on the recalculation of reserves of the Chernovsk brown-coal deposit, taking into account the results of exploration. Irkutsk, 1954.

3. L. I. Ferdman. Prospects for petroleum and gas potential of the Chitino-Ingodinskaya and Onon Upper Mesozoic basins of Central Transbaikalia. Collection of geophysical investigations in solving geological problems in Eastern Siberia. Issue 3. “Nedra,” Moscow, 1964.

4. L. I. Ferdman, B. V. Kornev. Geological structure and prospects for petroleum and gas potential of the Chitino-Ingodinskaya basin of Central Transbaikalia. Geology of Oil and Gas, No. 7, Moscow, 1965.