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Study sites

The main aim of Expedition 334 was to characterize the sediments in the upper (Site U1379) and middle (Site U1378) slope of the Caribbean plate and the incoming sediment and igneous crust of the Cocos plate (Site U1381). Complete descriptions of the drilling results are in each site chapter of this volume.

Site U1378

Site U1378 is located above the unlocked portion of the plate boundary, as indicated by interplate earthquake relocation and geodetic measurements (LaFemina et al., 2009). The margin here consists of an upper plate framework wedge underlying about 564 m of slope sediments. Drilling at this site penetrated a landward dipping reflector interpreted as a normal fault cutting through the whole upper plate, which may be directly linked with the plate boundary fluid system. Site U1378 is dominantly composed of a monotonous sequence of silty clay to clay that alternates with widely interspersed centimeter-scale sandy layers, which was divided into two main lithostratigraphic units (Fig. F2). Unit I (~128 m thick) is composed of mainly soft, dark greenish gray terrigenous silty clay. Unit II (~386 m thick) consists predominantly of massive, well consolidated, olive-green terrigenous clayey silt(stone) and silty clay(stone). Shipboard sedimentologists identified 82 tephra layers, which seem to be quite heterogeneous in composition. The lowermost core recovered from Hole U1378B (Core 334-U1378B-63X) contains a 0.97 m thick sequence of extensively fractured silty claystones.

Site U1379

The shallowest site along the CRISP transect, Site U1379 is thought to overly the locked portion of the subduction zone seismogenic zone in an area where the plate boundary is 4.5 km below seafloor. The sediments are predominantly a monotonous sequence of silty clay to clay that alternates with widely interspersed decimeter-scale sandy layers. Sediments from Site U1379 are divided into five lithostratigraphic units. Unit I, the relatively thin top unit, consists of medium- to coarse-grained sand with abundant shell fragments. Unit II (~650 m thick) is composed of mainly olive-green clayey silt(stone) and silty clay(stone) with minor layers of tephra. The sediments in this unit are massive and well consolidated; the tephra layers are unlithified. Hardened concretions of carbonate mud are present in the interval between Cores 334-U1379C-16H and 40X (~79–305 meters below seafloor [mbsf]). Unit III (~229 m thick) consists of fining- and coarsening-upward sequences (decimeter thick) of olive-green silty sands and sandstone. Tephra layers are sparse in this unit, accumulating mainly in one sequence within the upper part of the unit. Unit IV (~2 m thick) consists of carbonate-cemented medium- to coarse-grained sand with well-rounded, lithic pebble-sized clasts and thick-walled shell shards. Unit V (~67 m thick) is composed of matrix-supported breccia with clasts of limestone, basalt, and mudstone in a fine sandy matrix intercalated. Pore waters were sampled only from sediments recovered from lithostratigraphic Units I to III (Fig. F3). Overall, 53 tephra layers (2–45 cm thick) were identified intercalated in the background sedimentation of the different units, the majority of them below 324 mbsf.

Site U1381

Site U1381 targeted the sediment and basement sequences in the incoming Cocos plate. The upper 50 mbsf of sediment at this site is composed of a predominantly monotonous sequence of silty clay to clay (Unit I); which is underlayed by a more pelagic sequence (Unit II), characterized by abundant biogenic components (Fig. F4). The contact between the basement and the overlying sediment was recovered at ~95 mbsf.