Turbidite Sedimentology and History of the East Mariana Basin

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. SITE 585 SEDIMENTS Lithologies Drilled at Site 585Two holes were drilled at Site 585.The first hole (585) was washed to 256 m after one core was collected at the sediment/water interface and then cored continuously until drilling was impeded at 764 m.The second hole (585A) was spot-cored in a few intervals of interest (Cretaceous/Tertiary boundary, the Cenomanian/Turonian boundary), then cored continuously below 772 m until bit failure at 893 m.The sediments recovered at this site are divided into six units on the basis of composition and degree of diagenesis and lithification (Table 1).The upper five units (I-V) are composed primarily of pelagic components with varying proportions of carbonate, siliceous debris, and clay.The sediments are generally fine clays and silty clays, with radiolarian-rich sandy layers occurring primarily in Unit V. Unit VI is composed of volcaniclastic material with grain sizes ranging from clay to cobble-sized clasts.Evidence for sediment reworking exists throughout the section.Diagnostic features include the occurrence of shallow-water carbonate material, the discordant ages of microfossils, internal sedimentary structures, and high sedimentation rates.Shallow-water carbonate material is common in the lower third of the section (590-893 m); this debris includes benthic foraminifers, echinoid fragments and spines, rare algal fragments, sponge spicules, bryozoans, mdist fragments, and ooids.Fauna typically associated with reefal environments, such as mdist fragments and abundant algal debris, are rare.Sedimentary structures indicative of resedimentation include partial or complete Bouma sequences, basal scour, 175 C