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doi:10.2204/iodp.proc.344.208.2019

Site description

Sites U1378 and U1380 are located on the middle slope of the Costa Rica margin (8°35.9980′N, 84°4.4037′W) (Figure F1). The upper 560 m of the sediment section was sampled at Site U1378, and deeper penetration was achieved by drilling Site U1380. The sites lie above the unlocked portion of the plate boundary, as indicated by interplate earthquake relocation and geodetic measurements (LaFemina et al., 2009). The primary goal in choosing these sites was to investigate the nature, composition, and physical properties of the upper plate framework.

The sediments at Site U1378, dominantly composed of a monotonous sequence of silty clay to clay that alternates with widely interspersed centimeter-scale sandy layers, were divided into two main lithostratigraphic units (Figure F1C). Unit I (0–128 meters below seafloor [mbsf]) is composed of terrigenous silty clay with interbedded fining-upward sequences consisting of lithic sands. Unit II (128–514 mbsf) is predominantly well-consolidated terrigenous clayey silt that remains unlithified to 560 mbsf. This site was abandoned during Expedition 334 because the thick slope sediment package (~750 m) prevented the drill from reaching the underlying basement rock of interest, whereas the thinner slope cover of Site U1380 provided easier access.

At Site U1380, the margin consists of a ~550 m thick section of slope sediment (Unit I) overlying upper plate framework rock (Figure F1). Drilling in Hole U1380A during Expedition 334 was terminated at 482.4 mbsf; this site was revisited during Expedition 344, and penetration in Hole U1380C reached 797.4 mbsf. In Unit I, three sandstone-rich horizons were recovered with centimeter- to decimeter-thick sandy layers cemented with calcite. Unit II (553–772 mbsf) is visually defined by a relatively sharp lithologic change into greenish gray clayey siltstone with intercalated sandstone and conglomerate layers. In the upper 120 m of Unit II, occasional thin sandstone horizons are well lithified by calcite cement; however, most are poorly consolidated. Shells, shell fragments, and lithic fragments are abundant and conspicuous components in the sandstone. Below 564 mbsf, the sandstone layers are abundant and massive (as thick as 80 cm), normally graded, and become thicker and coarser with depth. The siltstone, sandstone, and conglomerate layers are moderately well lithified by calcite cement. Toward the bottom of the sequence, rip-up siltstone clasts and fragmented (as large as 8 cm) calcite-cemented breccia are more frequent. Siltstone and sandstone smear slides indicate that the dominant matrix composition is volcanogenic with sedimentary lithic fragments, along with common feldspar and glass. The tuff layers are highly altered. The biogenic components decline precipitously and are absent in the lowermost part of this unit. Unit III is a 29.44 m thick massive, well-lithified, very dark greenish gray silty claystone with intercalated sandstone layers. The silty claystone contains common calcite-cemented horizons (Figure F1D). The matrix contains mostly terrigenous material dominated by lithic fragments and feldspar but is nearly devoid of biogenic material.

A major finding of these expeditions was that the margin wedge is composed of well-indurated, cemented clastic sediment (Expedition 334 Scientists, 2012a; Harris et al., 2013a; Bangs et al., 2011). The anomalous porosity profiles and conspicuous seismic reflectors in the margin wedge have now been attributed to the lithologic changes associated with cementation, which is pervasive throughout the 800 m drilled at this site (Expedition 334 Scientists, 2012a; Harris et al., 2013a).

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