Turbidite sedimentology and history at DSDP Site 89-585

At Site 585 in the East Mariana Basin, a 900-m section of Aptian-Albian to Recent sediments was recovered. The upper 590 m are pelagic components (carbonate, siliceous, and clay); small-scale graded sequences and laminations are common. The underlying sediments are volcaniclastic sandstones with a large proportion of shallow-water carbonate debris; sedimentary structures including complete Bouma sequences, cross-laminae, and scouring are common. These structures indicate that the entire section was deposited by turbidity currents. The change in lithology upward in the section reflects the evolution of the surrounding seamounts, from their growth stages during the middle of the Cretaceous to the later subsidence phases. Several black layers containing pyritized organic debris and associated turbidite structures were cored near the Cenomanian/Turonian boundary; this material has been transported from the flanks of the seamounts where it was deposited within a shallow anoxic zone. Seismic data extends the stratigraphy across the entire Basin, showing the reflectors onlapping the seamounts, and indicating at least 1200 m of sediment at Site 585. The crust is placed at 6900 m after correcting for sediment loading, and the subsidence curve indicates that the Basin has been deeper than 5500 m since before the Aptian.

Rb-Sr isotope systematics and Sr/Ca-Ba/Ca ratios at DSDP Hole 89-462A

Four samples of Nauru Basin basalts (Cores 94 to 109 of Hole 462A, sub-bottom depth 1077-1209 m) have 87Sr/86Sr ratios in the range 0.7037 to 0.7038, which is distinctly higher than the ratios of N-type MORB. The Rb contents of the samples are depleted in comparison with those of MORB and ocean-island basalts. These chemical and isotopic characteristics are identical to those of the basalts previously drilled during Leg 61 (Cores 75 to 90 of Hole 462A), and are explained in terms of inhomogeneity of the source region in the mantle or later alteration effects. Sr/Ca-Ba/Ca systematics of 15 samples from Cores 462A-94 to 462A-109 and 14 samples from Cores 462A-75 to 462A-90 suggest that the Nauru Basin basalts are derived from a mantle peridotite by 20 to 30% partial melting with subsequent Plagioclase crystallization.

Mineralogy and diagenesis of slope sediments at DSDP Leg 84 holes

The distribution and composition of minerals in the silt and clay fraction of the fine-grained slope sediments were examined. Special interest was focused on diagenesis. The results are listed as follows. (1) Smectite, andesitic Plagioclase, quartz, and low-Mg calcite are the main mineral components of the sediment. Authigenic dolomite was observed in the weathering zones of serpentinites, together with aragonite, as well as in clayey silt. (2) The mineralogy and geochemistry of the sediments is analogous to that of the andesitic rocks of Costa Rica and Guatemala. (3) Unstable components like volcanic glass, amphiboles, and pyroxenes show increasing etching with depth. (4) The diagenetic alteration of opal-A skeletons from etching pits and replacement by opal-CT to replacement by chalcedony as a final stage corresponds to the typical opal diagenesis. (5) Clinoptilolite is the stable zeolite mineral according to mineral stability fields; its neoformation is well documented. (6) The early diagenesis of smectites is shown by an increase of crystallinity with depth. Only the smectites in the oldest sediments (Oligocene and early Eocene) contain nonexpanding illite layers.