Sediment description, CaCO3, density and porosity at DSDP Site 89-585

Siliceous sediments and sedimentary rocks occur as chert and silicified chalk, limestone, and claystone in Site 585 lower Miocene to Campanian sediments, with one older occurrence of chert near the Cenomanian/Turonian boundary. The recovered drill breccia in the Miocene to middle Eocene interval is dominated by bright red, orange, yellow, and brown chips and fragments of chert. In early Eocene and older sediments gray silicified limestone and yellowish brown chert fragments predominate. Recovery is poor in cores with chert because chert tends to fracture into smaller pieces that escape the drill and because the hard chert fragments grind away other sediments during rotary drilling. Thin-section and hand-sample studies show complex diagenetic histories of silicification (silica pore infill) and chertification (silica replacement of host rock). Multiple events of silicification can occur in the same rocks, producing chert from silicified limestone. Despite some prior silicification, silicified limestone is porous enough to provide conduits for dissolved silica-charged pore waters. Silicification and chert are more abundant in the coarser parts of the sedimentary section. These factors reflect the importance of porosity and permeability as well as chemical and lithologic controls in the process of silica diagenesis.

Petrology at DSDP Leg 66 Holes

The sandstone petrology of Leg 66 samples provides insights into changes through time in the geology of the source regions along the Guerrero portion of the Middle America continental margin. This in turn constrains possible models of the evolution of the Middle America Trench (e.g., de Czerna, 1971; Malfait and Dinkleman, 1972; Karig, 1974). Primarily medium-grained sands and sandstones, representing the widest variety available of trench/trench slope settings and ages, were analyzed in both light and heavy mineral studies. Standard techniques were used as much as possible in order to compare results from other margins and from ancient rocks.

Magnetic properties and sand composition at DSDP Site 87-582 and Hole 87-583D

At Site 582, DSDP Leg 87, turbidites about 560 m thick were recovered from the floor of the Nankai Trough. A turbidite bed is typically composed of three subdivisions: a lower graded sand unit, an upper massive silt unit, and an uppermost Chondrites burrowed silt unit. The turbidites intercalate with bluish gray hemipelagic mud which apparently accumulated below the calcite compensation depth. In order to investigate the nature and provenance of the turbidites, we studied the grain orientation, based on magnetic fabric measurements and thin-section grain counting, and grain size, using a photo-extinction settling tube and detrital modal analysis. The following results were obtained: (1) grain orientation analysis indicates that the turbidity current transport parallels the trench axis, predominantly from the northeast; (2) Nankai Trough turbidites generally decrease in grain size to the southwest; (3) turbidite sands include skeletal remains indicative of fresh-water and shallow-marine environments; and (4) turbidites contain abundant volcanic components, and their composition is analogous to the sediments of the Fuji River-Suruga Bay area. Considering other evidence, such as physiography and geometry of trench fill, we conclude that the turbidites of Site 582 as well as Site 583 were derived predominantly from the mouth of Fuji River and were transported through the Suruga Trough to the Nankai Trough, a distance of some 700 km. This turbidite transport system has tectonic implications: (1) the filling of the Nankai Trough is the direct consequence of the Izu collision in Pliocene- Pleistocene times; (2) the accretion of trench fill at the trench inner slope observed in the Nankai Trough is controlled by collision tectonics; and (3) each event of turbidite deposition may be related to a Tokai mega-earthquake.