37. Geochemical History of Post-jurassic Sedimentation in the Central Northwestern Pacific, Southern Hess Rise, Deep Sea Drilling Project Site 465

Analysis of the distribution of major components and heavy metals in a section of post-Jurassic deposits of southern Hess Rise (Deep Sea Drilling Project Site 465), the results of processing analytical data by factor analysis, interpretation of information on mineralogy and lithology of sediments, and evaluation of rates of component accumulation enable distinction of the main stages in the geochemical history: (1) Late Albian-early Cenomanian (early oceanic), in which the accumulation of mostly shallow-water carbonate sediments of turbiditic nature enriched in organic matter and volcanogenic materials was proceeding; the sediments are characterized by maximally high contents of Fe, Mn, SiO2, A12O3, and associated heavy metals, present in the form of basaltic volcaniclastic material and, to a lesser extent, hydrothermal and exhalation products and materials of their post-sedimentary transformation (smectite, hydromica, Mn-Fe-Mg-carbonates); extremely high rates of accumulation of components are representative of this given stage. (2) Late Cretaceous (middle Cenomanian-late Maastrichtian), in which the accumulating sediments were considerably eroded along some hiatuses (middle Cenomanian-middle Turonian; late Coniacian; early Campanian); during the Cenomanian-Coniacian, this region is assumed to have been in the subequatorial zone of high biological productivity, within the general northward movement of the Pacific Plate (Lancelot and Larson, 1975; Lancelot, 1978; van Andel, 1974). Considering geochemical features, the sediments of this stage are similar to biogenic carbonate pelagic oozes of the open ocean. (3) Early Tertiary (early-late Paleocene), in which the beginning of the Tertiary is characterized by accumulation of carbonate nannofossil oozes admixed with siliceous remains; sedimentation was proceeding in the northern oligotrophic zone of the Pacific Ocean. (4) Tertiary-Quaternary, in which foraminifer-nannofossil pelagic oozes with geochemical features typical of these varieties were accumulated; during the Eocene-Pliocene biogenic carbonate sedimentation was repeatedly disturbed by hiatuses. Thus, the established geochemical stages of post-Jurassic sedimentation of the region reflect, as a whole, the evolution of post-Jurassic sedimentation in the central northwestern Pacific. It is noteworthy that although the timing of events of these stages (for the Cretaceous particularly) for certain regions (Mid-Pacific Mountains, Nauru Basin, etc.) are essentially different, the general tendency of geochemical evolution of the basins is common.