Vol 54, No 3 (2019)

The West Siberian marine basin of the Volgian–initial Berriasian ages is described. It is shown that a marginal filter (according to A.P. Lisitsyn) functioned in the basin. The main mass of terrigenous sediments was deposited within the eastern margin of the sea. The central part of the basin only received a small amount of the terrigenous material. Water area of the West Siberian Sea was 2 mln 530 thou km²; eastern marginal filter, 535 thou km²; and open epicontinental marine basin, 1 mln 994 thou km². Depth of the Volgian Sea was 500 m. Mass of sediments in the West Siberian Sea by the end of late diagenesis was 228.4 Tt (recalculated to the anhydrous material), with sediments in the eastern marginal filter accounting for 121.7 Tt. Bioproductivity of the Volgian–Berriasian West Siberian Sea was extremely high. The mass of living matter was composed of archaea, bacteria, and protozoan unicellular eucaryotes (organic-walled), as well as organisms with the siliceous (radiolarians) and carbonate skeleton (foraminifers and others). The rock mass formed from sediments of the central deep-water part of the basin at the stage of diagenesis was 106.7 Tt (recalculated to the anhydrous material), including the mass of organic matter (OM) accounting for 15.8 Tt; mineral (siliceous and carbonate) relicts of organisms, 67.8 Tt; and allothigenic components (clay minerals and iron hydroxides), 23.1 Tt. Analysis of the composition of kerogen (polymerlipids) revealed that the amount of OM transported to sediments was 15–20 times higher than the present-day amount in rocks of the Bazhenov Formation. At the stage of early diagenesis, the OM mass in sediments was as high as 235–320 Tt (recalculated to the anhydrous material). The Bazhenov Sea represented a huge natural ecosystem favorable for the generation, reworking, and accumulation of living matter relicts. At the stage of catagenesis, unique oil-and-gas resources were generated from OM masses deposited in this system.

Clay minerals (fraction <0.001 mm) of Upper Pleistocene clayey–sandy–silty sediments recovered by DSDP Holes 481 and 481A in the Northern Trough, Guaymas Basin, Gulf of California, were studied by X-ray methods based on the modeling of diffraction patterns and their comparison with experimental diffractograms. Terrigenous clay minerals are represented mainly by dioctahedral micaceous varieties (mixed-layer disordered illite–smectites, illite) with the chlorite admixture and by kaolinite in the upper section of unaltered sediments. Intrusion of hot basalt sills (total thickness of the complex is about 27 m) provoked alterations in the phase composition of clay minerals in sediments (7.5 m thick) overlying the sill complex. These sediments include newly formed trioctahedral layered silicates (mixed-layer chlorite–smectites, smectite). Sediments inside the sill complex include trioctahedral mixed-layer mica–smectite–vermiculite or trioctahedral smectite. The trioctahedral mixed-layer chlorite–smectite coexisting with smectite was found in a single sample of the same complex.

Ferromagnesian carbonate metasomatites in limestones of the Lower Riphean Suran Formation in the Avzyan ore region (Bashkir meganticlinorium) are represented by large Fe-magnesite deposits (e.g., Ismakaevo deposit) and breunnerite stocks (Bogryashka ore occurrence). Metasomatic zonation is manifested as alterations in the series: limestone → dolomite → Fe-magnesite (breunnerite). Fe-magnesite contains up to 8 mol % FeCO₃; breunnerite, from 10 to 40 mol % FeCO₃. Metasomatic fluid represented the Ca–Na–Mg chloride brine with Fe admixture and was linked with processes of the remobilization of evaporite brines buried in Lower Riphean rocks. Salinity and homogenization temperature of fluid inclusions are: 20–26 wt % equiv NaCl and 200–240°C, respectively, in magnesites; 10–15 wt % equiv NaCl and 140–190°C, respectively, in breunnerites. Interaction of the fluid with terrigenous rocks in a tectonically active zone of the Mashak riftogenic graben ensured the enrichment of brine with different ligands and the accumulation of MREE and HREE in metasomatic products. Upon the migration of fluid across the limestone sequence and its cooling, Fe-magnesite was formed in the frontal zone (Ismakaevo deposit); breunnerite, in the rear zone adjacent to the terrigenous shales (Bogryashka ore occurrence).