Core Description and a Preliminarily Sedimentology Study of Site 1202D, Leg 195, in the Southern Okinawa Trough

Abstract

ODP Site 1202 of Leg 195 was designed primarily for a high-resolution study of the paleoceanography of the Kuroshio Current in the southern Okinawa Trough off NE Taiwan. Four holes were drilled in which Hole 1202D is described in detail in this study for an assessment of core quality for paleoceanography study and understanding of sedimentological features, especially turbidite sedimentation and the sediment provenances during the Late Quaternary in the southern Okinawa Trough. Pelagic mud with insignificant silt or sand layers is observed from the core top down to 133 m (mbsf; Marine Isotope Stages 1-3), but the silt-sand layer ratio (SLR: total thickness of silt and sand layers/1.5 m of core) increases gradually from a value of ＜10% between 133 and 167 m to values ＞50% between 223 and 279 m, followed by decreases to values ＜10% between 310 and 337 m and to ＜3% between 337 and 407 m (Stage 4). These silt-sand layers were most likely derived by fine-grained turbidite gravity flows, which were very active during Stages 3 and 4. Slate fragments, quartz grains, mica flakes and volcanic detritus are the major components in the coarse fraction of wash residues (＞250μm). Slate fragments are commonly found in fine-grained turbidite dominant intervals (160-280 m), while mica flakes can be observed in the muds throughout the core. The major detrital components were derived primarily from the Miocene slate belt of the pre-collision accretionary prism of the Central Range in northern Taiwan. The occurrence of volcanics could represent submarine volcanic activity in the active-opening Okinawa Trough back-arc basin off NE Taiwan. Shallow-marine fossils including benthic foraminifers, echinoids, bryozoans and mollusks are also found in the fine-grained turbidite dominant intervals. These fossil assemblages could have been deposited in the shallow shelf and then transported to the depositional site along with voluminous terrigenous materials derived from Taiwan, via submarine channels or by slope failures due to frequent earthquakes induced by plate convergence/collision and extension in the southwestern Okinawa Trough off NE Taiwan. It is concluded that the top 133 m of the core is better suited for paleoceanographic reconstruction.