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doi:10.2204/iodp.proc.314315316.217.2011

Site locations and lithologies

Site C0002

Site C0002, drilled during Expedition 315, is located at the southern margin of the Kumano forearc basin (Figs. F1A, F1B) (see the “Expedition 315 Site C0002” chapter [Expedition 315 Scientists, 2009]). The lithology of this site is divided into four units: upper forearc basin (Unit I), lower forearc basin (Unit II), basal (starved) basin (Unit III), and upper accretionary prism (Unit IV) (Fig. F1D).

Units I and II are composed mainly of dark gray hemipelagic silty clay with minor fine sand, silt turbidites, and volcanic ash. The dominant lithologies of Unit III are greenish-gray, gray, and gray-brown silty claystones. Calcareous nannofossils are abundant in Unit III, and a suite of Pliocene biostratigraphic events was recognized in the short depth interval within this unit (Fig. F1D).

The boundary between Units III and IV is an unconformity. The dominant lithology of Unit IV is gray to greenish-gray silty claystone. The silty claystones here are highly fractured. Unit IV is interpreted to be of late Miocene age on the basis of biostratigraphic analysis. This unit is interpreted as accreted sediments.

One sample from Unit II (forearc basin sediments) and two samples from Unit IV (accreted sediments) are examined in this study.

Site C0004

Site C0004, drilled during Expedition 316, is located at the edge of the megasplay fault seaward of Site C0001 (Figs. F1A, F1B) (see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009]). This site is divided into four units on the basis of lithology and age (Fig. F1C).

Unit I is composed mainly of greenish-gray silty clay containing nannofossils (up to ~25%). The boundary between Units I and II is an unconformity.

Unit II is divided into two subunits on the basis of lithology (Fig. F1C). Subunit IIA comprises greenish-gray synsedimentary breccia with rounded to subangular clasts of pebble size. Both the matrix and the clasts are composed of dark greenish-gray silty clay. This unit is interpreted as slump or mass wasting deposits (see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009]). Subunit IIB also consists of mostly dark greenish-gray silty clay, but the breccia is only rarely observed.

Unit III includes relatively abundant ash layers within hemipelagic silty clays. The boundaries between Units II and III and between Units III and IV might be faults because biostratigraphic age reversals have been identified around the boundaries.

The dominant lithology of Unit IV is dark olive-gray silty clay with a moderate amount of calcareous nannofossils. This unit is considered to have formed in a lower trench-slope environment and underthrust the footwall of the megasplay fault.

In this study, one sample from Subunit IIA (slope apron sediments) and two samples from Unit IV (underthrust sediments) were examined. The samples from Subunit IIA were selected from a homogenized part within the synsedimentary breccia.