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doi:10.2204/iodp.proc.344.207.2017

Drill sites

Two of the sites that were drilled on the overriding plate sampled the upper and middle slope regions, and at Site U1412 the prism toe was drilled; the other two sites were drilled on the incoming Cocos plate (Fig. F1). (For complete descriptions of the drilling results see the Expedition 344 summary chapter [Harris et al., 2013a]).

Overriding plate sites

Site U1380 is a complementary site to the adjacent Site U1378, drilled during Expedition 334 to ~480 meters below seafloor (mbsf) and abandoned because of hole instability. The hole was washed to ~450 mbsf and drilled to ~800 mbsf; however, pore fluids were only recovered between ~450 and ~600 mbsf. At greater depths the sediments were cemented and no pore-fluids were recovered. Site U1380 was drilled to investigate the deeper portions of the upper slope sequence and underlying wedge sediments. The sedimentary succession recovered from the wedge consists of alternating terrestrial turbiditic upper slope to shelf sequences, with some deltaic-derived sediment.

Site U1413 was also drilled in the middle slope region along a 3-D seismic line (Bangs et al., 2013). Three lithostratigraphic units were distinguished in the sediment section of Site U1413 (see the Upper slope Site U1413 chapter [Harris et al., 2013e]).

The frontal sedimentary prism at the base of the slope was drilled at Site U1412. The primary goal at this site was to penetrate the décollement and to investigate the fluid flow regime within the underthrust sediments as well as within the oceanic crust. Three units were distinguished in the sedimentary section; unfortunately, due to hole instability, the décollement, underthrust sediment, and basement were not sampled at this site.

Incoming plate sites

Two sites were drilled on the subducting aseismic Cocos Ridge in order to understand seismogenic processes in subduction zones. The incoming sediment thickness at Site U1381 is just <100 m, and the site is located higher on the Cocos Ridge. The upper 50 m at this site consists of predominantly monotonous sequences of silty clay to clay underlain by a more pelagic sequence with abundant biogenic sediment. The contact between the basement and the overlying sediment was recovered at ~95 mbsf in Hole U1381B and at ~104 mbsf in Hole U1381C.

The sediment thickness at Site U1414 is ~380 m; the sediment section consists of pelagic/hemipelagic silty clay to clay in the upper section with increasing nannofossil-rich calcareous ooze at greater depth plus interspersed tephra layers, mostly missing at Site U1381. Below ~345 mbsf the sediments were too indurated for pore fluid extraction (see the Input Site U1414 chapter [Harris et al., 2013b]).

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