ON THE TRANSITIONAL UPPER JURASSIC–LOWER CRETACEOUS SEQUENCE OF THE ZEYA–BUREYA DEPRESSION

A. A. TIMOFEEV

(Presented by Prof. I. V. Lebedev)

The Zeya–Bureya Depression, geomorphologically, is a lowland plain surrounded on all sides by mountain structures. The most ancient sedimentary rocks exposed at the surface within its limits are the deposits of the Tsagayan Formation of Upper Cretaceous age. These deposits, together with loose Cenozoic sediments, cover the territory of the plain with a thick mantle; therefore, before geophysical and drilling work was undertaken, its deep structure remained unstudied. For a long time it was believed that the Tsagayan deposits rest directly on the complexly dislocated and metamorphosed rocks of the basement.

Only V. G. Vasil'ev, V. T. Veber, and M. M. Mandel'baum [2], as a result of interpreting the geophysical materials obtained, suggested the presence in the geological section of the depression of sedimentary rocks analogous to the Lower Cretaceous deposits of the Mongolian People’s Republic. This supposition was later confirmed by drilling. In the Belogorsk reference borehole and in a number of structural-parametric and hydrogeological boreholes, Lower Cretaceous deposits and the sedimentary-effusive formations lying beneath them were established in the section of the sedimentary cover; some authors assigned the latter to the Jurassic [4], others to the Lower Cretaceous [1], and even to the Paleozoic [5, 6].

Recently, two deep boreholes—2-SPB and 1-SPI—have been drilled in the Zeya–Bureya Depression; they penetrated the complete section of sedimentary deposits and the upper part of the basement. The first of these boreholes is located in the southwestern part of the Ekaterinoslavka–Romanovka trough and characterizes the section of the central part of the trough, while the second is on the Itikut structure of the Belogorsk trough.

The basement rocks have a rather varied petrographic composition. They include granites, granodiorites, crystalline schists, gneisses, and granite-gneisses.

The most ancient basement formations are apparently crystalline schists and paragneisses, representing a metamorphosed sedimentary substratum into which large intrusive bodies of the batholith type were emplaced. Intrusions of granite bodies of this type into metamorphosed sequences are observed in an outcrop near the city of Blagoveshchensk and in the neighboring Bureya massif.

It is not yet possible to judge reliably the age of the basement rocks penetrated by boreholes beneath the cover of Mesozoic–Cenozoic sediments.

is considered possible because their absolute age has not been determined.

The similarity in composition, structural features, and the nature of the relationships among the basement rock types of the Zeya–Bureya depression and its surrounding uplifts makes it possible to regard them as elements of a single structural edifice that arose as a result of the final phases of Paleozoic folding.

On the eroded surface of the basement rocks there lies a sequence of normally sedimentary rocks characterized by an Upper Jurassic–Lower Cretaceous spore-pollen assemblage. We have combined these deposits into the Ekaterinoslavka Formation.

The deposits of the Ekaterinoslavka Formation fill the most subsided parts of the basement and usually wedge out toward the marginal parts of the troughs. They are represented mainly by coarse-clastic rocks. Among them are distinguished conglomerates, gravelites, and sandstones. Siltstones and, more rarely, argillites, which make up no more than 30% of the section, are of subordinate importance. Beds of ash tuffs occur.

In the section of borehole I-SPI, on the granodiorites of the basement, there lies a horizon of loose rocks (interval 2632–2652 m), consisting of angular fragments of quartz, feldspars, and granitic rocks immersed in a clayey matrix.

Above lie conglomerates (interval 2568–2632 m), consisting of unrounded and unsorted large boulders of granodiorites, granites, and pebbles of the same composition. Upsection, the conglomerates pass into a sequence represented by an alternation of gravelly-sandy rocks and siltstones. The gravel material, consisting of fragments of granitoid, more rarely effusive rocks and quartz, is present in the rocks both as inclusions and as independent interbeds 4–5 m thick. Its amount in the section decreases from bottom to top.

The main part of the section (54%) consists of light-gray massive sandstones, among which interbeds of gravelites, siltstones, and more rarely argillites occur. Carbonized plant stems are frequently encountered in the sandstones; sometimes large-scale cross-bedding is observed, due to interbeds of rocks of differing grain-size composition.

Siltstones and argillites, which make up about 30% of the section, are dark gray and form interbeds 2–8 meters thick; their amount and thickness increase from bottom to top. In the siltstones, parallel bedding is often observed, caused by accumulations along the bedding of finely comminuted carbonized plant material and micaceous matter. The angle of dip of the laminae is 15–20°.

The clastic portion of the rocks of the Ekaterinoslavka Formation is represented by quartz (5–44%), feldspars (10–50%), and rock fragments, which are constantly present in the coarse-clastic varieties (conglomerates, gravelites, sandstones) and make up 5–88%.

The feldspars are represented by plagioclases and K-feldspars. Plagioclases usually predominate; they are sericitized, corroded, and often replaced by carbonates and zeolites.

The rock fragments consist of granite and granodiorite and are usually corroded. The terrigenous material shows no rounding.

The composition of the minerals of the heavy fraction is rather diverse. Among them, sphene, hornblende, epidote, magnetite, zircon, and leucoxene predominate, accounting for 95% of the fraction. Pyroxenes, muscovite, and titanium-bearing minerals are constantly present, although in small amounts. Limonite and corundum are frequently encountered. Garnet, rutile, tourmaline, anatase, brookite, and apatite occur as single grains.

The clastic material is cemented by clayey, clayey-micaceous, zeolitic, and more rarely carbonate cement. Sometimes tuffogenic material is present in the cement. The amount of cement ranges from 30 to 20%, and sometimes reaches 40%.

The upper boundary of the Yekaterinoslavka Formation in the section of borehole 1-SPI is fairly clearly distinguished macroscopically by the appearance of effusive rocks. The total thickness of the formation is 629 m.

In the section of borehole 2-SPE the lower boundary of the Yekaterinoslavka Formation was not reached; however, the main part of it is evidently present in the section. Here the Yekaterinoslavka Formation is represented by a monotonous flysch-like sequence of frequent alternation of sandstones, siltstones, argillites, and ash tuffs. The transition between the rock varieties is gradual.

In contrast to the section of borehole 1-SPI, the sandstones here are represented by finer varieties; conglomerates are absent, and gravel material is encountered rarely. Sandstones make up 45% of the section. The clastic part of the sandstones does not differ from the sandstones penetrated by borehole 1-SPI; however, the role of fragments of igneous rocks decreases sharply. Among them appear fragments of siliceous and effusive rocks. The cement of the sandstones is clayey, siliceous-micaceous, carbonate, and fluorapatite, and constitutes 5–30% in the rocks, sometimes reaching 50%.

Oblique bedding is often observed in the sandstones, caused by changes in the grain-size composition of individual interbeds.

Siltstones and argillites, which make up 40%, form interbeds 2–10 m thick, are considerably enriched in organic material, which gives them a dark-gray and black color. Parallel bedding is observed, with dip angles of 15–20°.

The tuffs are yellowish-gray and form interbeds 2–4 m thick; packets up to 90 m thick are sometimes encountered.

The penetrated thickness of the formation is 730 m.

The mineralogical composition of the heavy fraction is the same as in borehole 1-SPI, but the quantitative content of individual minerals changes sharply. Here, in the heavy fraction, zircon, leucoxene, limonite, titanium-bearing minerals, and magnetite predominate. A characteristically significant presence of pyrite (10.6%) and biotite (26.2%) is noted.

The listed minerals constitute 95% of the fraction. Garnet and muscovite are constantly present. Epidote, rutile, tourmaline, and sphene are frequently encountered.

In individual samples hornblende, chlorite, pyroxenes, andalusite, topaz, apatite, spinel, and corundum are present.

The upper boundary of the Yekaterinoslavka Formation in the section of borehole 2-SPE is fairly clearly fixed by the change in the mineralogical composition of the heavy fraction. The penetrated thickness of the formation is 730 meters.

The age of the deposits of the Yekaterinoslavka Formation is determined from spore-pollen analysis data. According to the conclusion of N. A. Zamoshnikova, the spore-pollen assemblage in the interval 2490.3–2222.6 m (borehole 1-SPI) is characterized by an abundance of spores of Coniopteris sp.—35%, by species diversity and a considerable quantity of spores of the genera Osmunda and Osmundites—6.5%. Among them characteristic are Osmundites plicatus (K.—M.) Bolch., Osmunda papillata Bolch., Osmunda jurassica K.—M., Osmunda sp. Cyathea sp.—3.5%, Adiantum sp.—3.5% are present in noticeable quantities; Alsophila chetaensis (K.—M) Bolch. occurs singly. Pteridium solidum Bolch.—1%, Dennstaedtia tubeensis Bolch.—2%.

Especially important is the presence of the species Cheiropleuria congregata Bolch., Cheiropleuria compacta Bolch.—2%, which are the most typical representatives of the Jurassic of the Far East. Also characteristic is the presence of the moss Sphagnum glabrescens (Mal) Bolch.—2%, the lycopsids Licopodium subrotundum K.—M.—2.5% and Licopodium perlicatum Bolch.—0.5%. The listed assemblage, with a considerable quantity of spores of Coniopteris sp. and the presence of forms of Cheiropleuria, sufficiently characterizes the vegetation of Late Jurassic time.

An analogous spore-pollen assemblage, according to M. A. Sedova, is contained in deposits of the Talyndzhan Formation of the Bureya coal basin, which overlie Upper Jurassic marine deposits. The age of the Talyndzhan Formation, on the basis of the flora, is regarded as Late Jurassic [3].

The spore-pollen assemblage from samples of the interval 2078.9–2022.4 differs from that described above by the appearance of representatives of the family Schizaeaceae: Aneimia tricostata Bolch.—0.4%, Aneimia exilioides (Mal) Bolch.—1–2%, Aneimia macrorhysa (Mal) Bolch.—2%, Mohria striata (Naum) Bolch.—0–1%. Here there are also spores of Gleichenia laeta Bolch., Gleichenia umbonata Bolch.; characteristic is the presence of the index Lower Cretaceous form Onychiopsis elongata (Geyl) Jok—2.5%.

Among forms passing over from more ancient assemblages there are spores of Coniopteris sp., but already in significantly smaller quantity—3.5–4%, Salvinia perpulchra Bolch.—1.4%, Trichomanes crassis Bolch.—up to 2%, which give the assemblage a tinge of antiquity. This is also clearly traceable in the pollen of conifers, among which pollen of Picea sp.—21.5% and ancient conifers of Jurassic aspect are abundant: Protoconiferus grandis Bolch., Protoconiferus funarius (Naum) Bolch., Pseudopicea variabiliformis Bolch., and Pseudopicea monstruosa Bolch., reaching a total of 33%.

The indicated spore-pollen assemblage makes it possible to assign the rocks containing it to the Lower Cretaceous (presumably Valanginian).

The material presented permits the following conclusions.

1. The oldest Mesozoic sediments of the Zeya-Bureya depression are deposits of transitional Upper Jurassic–Lower Cretaceous age, lying on crystalline basement rocks. The supposition of the presence of an intermediate sequence of Paleozoic parageosynclinal formations is not confirmed.

2. The effusive-sedimentary deposits previously assigned to the Upper Jurassic occupy a higher stratigraphic position (Lower Cretaceous).

REFERENCES

1. A. V. Abramov, V. B. Olenin, E. I. Taranenko, A. A. Trofimuk. Zeya-Bureya possible oil-and-gas-bearing basin. Novosti neftyanoy i gazovoy tekhniki, No. 7, 1962.

2. V. G. Vasil’ev, V. T. Veber, M. M. Mandel’baum. New data on the geological structure and prospects of oil-and-gas occurrence in the Zeya-Bureya interfluve. Geologiya nefti i gaza, No. 7, 1957.

3. V. A. Vakhrameev, M. P. Doludenko. The Upper Jurassic and Lower Cretaceous flora of the Bureya basin and its significance for stratigraphy. 1961.

4. V. S. Volkhonin, E. N. Lishnevskii, A. P. Tarkov, S. P. Sudakov. Lower Cretaceous deposits of the south of the Zeya-Bureya depression in connection with their possible oil-and-gas potential. Geologiya i geofizika, No. 5, 1961.

5. I. N. Sulimov. New data on the structure of the Zeya-Bureya depression. Novosti neftyanoy i gazovoy tekhniki. Geological series, No. 12, 1961.

6. A. P. Tarkov. Deep structure of the Zeya-Bureya plain according to geophysical investigations. Sovetskaya geologiya, No. 7, 1963.