Vol 51, No 1 (2016)

Buried ferromanganese nodules and their stratiform accumulations (nodule horizons), which are widespread in sedimentary sequences of the Magellan Seamounts, have been studied in core obtained by shallow drilling on its guyots in the course of expeditions of the R/V Gelendzhik in 2001–2010. The study included description of morphological features, structure, and composition of buried nodules, the assessment of their geological age, and the analysis of host geological environments. It was revealed that nodule horizons are developed in areas with unstable hydroand lithodynamic conditions. The frequent changes of these conditions were responsible for the formation of such nodule horizons and overlying ferromanganese crusts. It is shown that the crusts and nodules developed at the surface of bottom sediments and nodules buried within them are characterized by the similar structure and composition, which indicates their belonging to the same ore formation, i.e., ferromanganese ores of seamounts. The age of buried nodule horizons is correlated with that of layers in ferromanganese crusts, on the one hand, and with the age of host sedimentary complexes defined in the Magellan Seamounts area. The buried nodules include both Cenozoic and Late Cretaceous varieties. The finds of Cenomanian nodule horizons provide grounds for the assumption that the formation of ferromanganese ores on gyuots of the Magellan Seamounts could start prior to the Senonian.

According to recent concepts, the Earth surface was permanently transformed during its geological history. Some stages of its evolution were marked by the convergence of separate continental blocks to result in the formation of supercontinents, which resisted successfully centrifugal processes. Other stages were characterized by the opposite tendency: after their long existence, the supercontinents became disintegrated into several large and small blacks, the motion of which was accompanied by opening of new sea basins and closure of former basins with the oceanic crust. The second half of the Paleozoic was marked by amalgamation of large continental blocks. In the Devonian, collision between Laurentia and Baltica culminated in the formation of the Euroamerica continent. After the closure of the Ural paleocean in the terminal Carboniferous–initial Permian, it was united with the Siberian and Kazakhstan continental blocks. These events provided the prerequisites for the formation of a new supercontinent (Pangea), which acquired its final configuration at the end of the Permian. One of its segments located mainly south of the equator included Gondwana. Another segment located northward included Euroamerica, Kazakhstan, Siberian, and two China continental blocks. During its geological history, Pangea suffered many dramatic events including several extinctions of organisms. The most significant event took place in the terminal Permian–initial Triassic and at the transition between the Triassic and Jurassic periods.

Contents and distribution of U, Th, and K in rocks of the so-called normal and anomalous sections of the Upper Jurassic–Lower Cretaceous Bazhenov Formation in the West Siberian marine basin are examined. In normal sections of this unit, the most widespread clayey–siliceous rocks deposited in the course of slow “background” sedimentation, are characterized by high (relative to black shales) contents of U (66.5 ppm) but moderate contents of Th (5.07 ppm) and K (0.81%). In mudstones identified as turbidite sediments with high sedimentation rate, contents of these elements are 11.68 ppm, 11.00 ppm, and 2.70%, respectively. In rare clayey–silty sediments of anomalous sections marked by maximal deposition rates, contents of these elements are 2.15 ppm, 3.78 ppm, and 2.15%, respectively. Uranium is associated mainly with organic matter and partly with apatite (phosphate clasts of fish skeletons) and calcite; thorium, with clay minerals; potassium, with clay minerals and K-feldspar. The U content is 345 ppm at high apatite concentration (7%).