Physical properties and AMS characteristics of ODP Site 185-1149 sediments

During Ocean Drilling Program Leg 185, we studied progressive changes of microfabrics of unconsolidated pelagic and hemipelagic sediments in Holes 1149A and 1149B in the northwest Pacific at 5818 m water depth. We paid particular attention to the early consolidation and diagenetic processes without tectonic deformation before the Pacific plate subduction at the Izu-Bonin Trench. Shape, size, and arrangement of pores were analyzed by scanning electron microscope (SEM) and were compared to anisotropy of magnetic susceptibility (AMS) data. The microfabric in Unit I is nondirectional fabric and is characterized by large peds of ~10-100 µm diameter, which are made up of clay platelets (mainly illite) and siliceous biogenic fragments. They are ovoid in shape and are mechanically packed by benthic animals. Porosity decreases from 0 to 60 meters below seafloor (mbsf) in Unit I (from 60% to 50%) in association with macropore size decreases. The microfabric of coarser grain particles other than clay in Unit II is characterized by horizontal preferred orientation because of depositional processes in Subunit IIA and burial compaction in Subunit IIB. On the other hand, small peds, which are probably made of fragments of fecal pellets and are composed of smectite and illite (3-30 µm diameter), are characterized by random orientation of clay platelets. The clay platelets in the small peds in Subunit IIA are in low-angle edge-to-face (EF) or face-to-face (FF) contact. These peds are electrostatically connected by long-chained clay platelets, which are interconnected by high-angle EF contact. Breaking of these long chains by overburden pressure diminishes the macropores, and the clay platelets in the peds become FF in contact, resulting in decreases in the volume of the micropores between clay platelets. Thus, porosity in Subunits IIA and IIB decreases remarkably downward. The AMS indicates random fabric and horizontal preferred orientation fabric in Units I and II, respectively. This result corresponds to that of SEM microfabric observations.In Subunit IIB, pressure solutions around radiolarian tests and clinoptilolite veins with normal displacement sense are seen distinctively below ~170 mbsf, probably in correspondence to the transition zone from opal-A to opal-CT.

Chemistry of earliest Cretaceous bentonites

Bentonites (i.e., smectite-dominated, altered volcanic ash layers) were discovered in Berriasian to Valanginian hemipelagic (shelfal) to eupelagic (deep-sea) sediments of the Wombat Plateau (Site 761), Argo Abyssal Plain (Sites 261, 765), southern Exmouth Plateau (Site 763), and Gascoyne Abyssal Plain (Site 766). A volcaniclastic origin with trachyandesitic to rhyolitic ash as parent material is proved by the abundance of well-ordered montmorillonite, fresh to altered silicic glass shards, volcanogenic minerals (euhedral sanidine, apatite, slender zircon), and rock fragments, and by a vitroclastic ultra-fabric (smectitized glass shards). For the Argo Abyssal Plain, we can distinguish four types of bentonitic claystones of characteristic waxy appearance: (1) pure smectite bentonites, white to light gray, sharp basal contacts, and a homogeneous cryptocrystalline smectite matrix, (2) thin, greenish-gray bentonitic claystones having sharp upper and lower contacts, (3) gray-green bentonitic claystones mottled with background sedimentation and a distinct amount of terrigenous and pelagic detrital material, and (4) brick-red smectitic claystones having diffuse sedimentary contacts and a doubtful volcanic origin. For the other drill sites, we can distinguish between (1) pure bentonitic claystones similar in appearance and chemical composition to Type 1 of the Argo Abyssal Plain (except for gradual basal contacts) and (2) impure bentonitic claystones containing textures of volcanogenic smectite and pyroclastic grains with terrigenous and pelagic components resulting from resedimentation or bioturbation. The ash layers were progressively altered (smectitized) during diagenesis. Silicic glass was first hydrated, then slightly altered (etched with incipient smectite authigenesis), then moderately smectitized (with shard shape still intact), and finally, completely homogenized to a pure smectite matrix without obvious relict structures. Volcanic activity was associated with continental breakup and rapid subsidence during the "juvenile ocean phase." Potential source areas for a Neocomian post-breakup volcanism include Wombat Plateau, Joey and Roo rises, Scott Plateau, and Wallaby Plateau/Cape Range Fracture Zone. Westward-directed trade winds transported silicic ash from these volcanic source areas to the Exmouth Plateau and, via turbidity currents, into the adjacent abyssal plains. The Wombat and Argo abyssal plain bentonites are interpreted, at least in parts, as proximal or distal ash turbidites, respectively.