ON THE BOUNDARY BETWEEN SEDIMENTATION CYCLES IN THE MESOZOIC COMPLEX OF THE WEST SIBERIAN PLATE

S. N. GULYAEV

(Presented by Professor A. V. Aksarin)

In the Mesozoic–Cenozoic platform cover of the West Siberian Plate, two regional sedimentation cycles are distinguished, but the boundary between them is placed by different researchers at different stratigraphic levels. V. D. Nalivkin and other geologists [6] place it between the Barremian and the Aptian, where a hiatus is recorded in some areas. However, the absence of late-regressive continental and coastal-continental facies in the Neocomian of the Mansy syneclise, where the Barremian and partly Lower Aptian marine clays form part of the Frolov Formation, uniform in its composition, does not allow this boundary to be accepted [5].

Yu. N. Karagodin [5] places the boundary between the sedimentation cycles between the Turonian and the Cenomanian, where, in his opinion, a hiatus is observed on some local uplifts, and the Upper Albian–Cenomanian sediments are represented by more continental formations than the underlying ones. However, there is no regional erosion between the Turonian and the Cenomanian [4], and the ratio of continental and marine facies in space and time is not a rigid criterion for drawing the boundary between sedimentation cycles. Moreover, at the early transgressive stage of formation of the first sedimentation cycle in Early–Middle Jurassic time, continental sediments accumulated over the greater part of the plate [2], which brings them closer to the Upper Albian–Cenomanian deposits. This is explained by the fact that, at the first stage of transgressions, over a considerable area of sedimentary basins, subsidence is more than compensated by intensive sediment accumulation as a result of the sharp uplift of the bordering structures—the sources of clastic material. Such conditions of formation of the lower part of transgressive complexes led to the predominance of sandy-silty rocks in it and, as a consequence, to the massive type of reservoirs of many of the largest and unique oil (Burgan in Kuwait, Sarir in Libya) and gas (Slochteren in the Netherlands, Urengoy in the USSR) fields.

A sharp uplift of the heterogeneous bordering structures and adjacent parts of the plate, which led to significant erosion of Mesozoic deposits in its outer part, occurred in pre–Upper Albian time. Thus, in the lower course of the Turukhan River (wells 8-T, 6-T, 36-T, and 37-T) and its tributary the Nizhnyaya Baikha (wells 19-B, 5-B, 6-B, 7-B, and 9-B), the Upper Albian–Cenomanian Makov Formation transgressively overlies, with erosion, different ...

Fig. 1. Geological map of the Turukhan–Yenisei interfluve without the cover of Upper Albian and younger sediments. Compiled by S. N. Gulyaev: 1 — Aptian stage–middle Albian substage (Yakovlevskaya Formation); 2 — Aptian stage; 3 — Hauterivian–Barremian stages; 4 — Berriasian–Valanginian stages; 5 — Upper Jurassic; 6 — Middle Jurassic; 7 — eastern boundary of the distribution of the transgressive Makovskaya Formation (Upper Albian substage–Cenomanian stage); 8 — boundary of the West Siberian Plate; 9 — core-drilling wells; 10 — deep exploratory wells.

horizons of the Lower Cretaceous and Upper Jurassic (Fig. 1). The stratigraphic unconformity is most sharply expressed in the southern part of the Farkovo swell (wells 19-B, 5-B, and others), where the Makovskaya Formation, composed predominantly of sandy rocks with interbeds of conglomerates, gravelites, and bauxites with rare pollen of angiosperm plants, overlaps faunistically characterized sandy-clayey deposits of the Upper Jurassic. The transgressive occurrence of the Makovskaya Formation along the eastern margin of the West Siberian Plate can be traced for hundreds of kilometers and is recorded 165 km farther south in the section of core well 1-B, located on the left bank of the Yenisei opposite the settlement of Nizhne-Imbatskoye [3]. At the extreme southeastern margin of the West Siberian Plate, in the Chulym region, the Cenomanian Kiyskaya deposits rest with a break upon the Neocomian–Lower Aptian sandy-clayey formations of the Ilek Formation.

suite, composed in its lower part of coarse-grained whitish sands with interbeds of conglomerates, and in its upper part of variegated clays [1].

Also noteworthy is the sharply expressed erosion of Albian deposits on the Sherkalin local uplift, which is already located in the northwestern part of the plate.

Thus, the factual material considered shows that a sharply expressed pre-Upper Albian break, traceable for hundreds of kilometers, separates two regional sedimentary cycles: the Lower Jurassic–Middle Albian and the Upper Albian–Pliocene. The lower parts of the transgressive complexes of these cycles are associated with numerous large and very large gas fields (Urengoy, Zapolyarnoye, Medvezhye, Yamburg, Luginetskoye, and others). This is a regular relationship, caused by the combination of the Kimmeridgian–Berriasian and Turonian–Senonian regional clay seals, corresponding to the maxima of transgressions, and the underlying sandy-siltstone regionally persistent reservoirs, formed during the first stage of the transgressions.

REFERENCES

1. A. A. Bulynnikova, A. N. Rezanov, V. V. Puchkina, N. N. Storozhenko. Geological structure and prospects for oil-and-gas potential of the Yenisei part of the West Siberian Lowland. Tr. SNIIGGiMS, issue 41, 1968.

2. A. V. Golbert, L. G. Markova, N. D. Polyakova, V. N. Saks, Yu. V. Teslenko. Paleolandscapes of Western Siberia in the Jurassic, Cretaceous, and Paleogene. “Nauka,” 1968.

3. S. N. Gulyayev. On the geological structure of the northeast of the West Siberian Lowland (Turukhansk district). Izv. TPI, vol. 166, 1969.

4. F. G. Gurari, K. I. Mikulenko, L. Ya. Grushkova. On regional erosional breaks in Mesozoic deposits in Siberia. Tr. SNIIGGiMS, issue 89, 1969.

5. Yu. N. Karagodin. Classification, structure, and nomenclature of rhythms and of the lithostratigraphic, hydrogeological, and oil-and-gas-bearing subdivisions subordinate to them. In: Problems of the Oil Potential of Siberia. “Nauka,” 1971.

6. V. D. Nalivkin et al. Principal stages in the history of the geological development of the West Siberian Plate. Tr. VNIGRI, issue 272, 1969.