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doi:10.2204/iodp.proc.340.108.2013

Lithostratigraphy

Two holes were drilled at Site U1398: Hole U1398A was drilled to 260.5 mbsf, and Hole U1398B was drilled to 255 mbsf. A few preliminary correlations can be made between the holes, but there is considerable variability in the stratigraphy even though the holes are ~20 m apart. Correlation in the lower parts of the two holes is problematic because of the low recovery in Hole U1398A.

The lithostratigraphy of Site U1398 comprises seven units (A–G). The upper part of the core is dominated by volcaniclastic turbidites, whereas the lower portion is composed of various combinations of hemipelagic muds, volcaniclastic turbidites, and tephra layers. Each lithology is described in detail in “Lithostratigraphy” in the “Site U1394” chapter (Expedition 340 Scientists, 2013b).

Unit A

  • Depths: Hole U1398A = 0–40 mbsf, Hole U1398B = 0–41 mbsf

Unit A extends from 0 to 40 mbsf in Hole U1398A and from 0 to 41 mbsf in Hole U1398B. The top of Unit A comprises 70 cm of hemipelagic mud, with the uppermost 30 cm composed of oxidized brown hemipelagic mud with high water content. Below 70 cm, the main part of Unit A is composed of a series of thick (mainly massive) volcaniclastic turbidites. This series of turbidites is continuous in Hole U1398B and represents >95% of the unit, with hemipelagic mud representing <5% of this interval. Most of the sand layers are massive (i.e., not laminated), and some are normally graded. These layers generally contain pumice (≤20%, with the exception of one turbidite that contains 40%). The upper turbidite is thick and massive and contains abundant pumice clasts (≤5 cm). The dominant component is represented by mineral crystals, which vary in proportion from 50% to 70%. The other components are massive lava clasts (<20%) and sparse bioclastic or detritic carbonates (10%). In Hole U1398A, pumiceous turbidites are observed in the uppermost 10 mbsf; lack of recovery between 10 and 32 mbsf prevents correlation with Hole U1398B. In general, turbidites in Hole U1398A are fewer in number and thinner than those in Hole U1398B. The upper few tens of centimeters of each core consistently contain an abnormal concentration of large pumice and lava clasts. This coarse deposit results from the collapse during coring of pumice and lava clasts from above and is a coring artifact.

Unit B

  • Depths: Hole U1398A: 40–57 mbsf, Hole U1398B = 41–58 mbsf

Unit B extends from 40 to 57 mbsf in Hole U1398A and from 41 to 58 mbsf in Hole U1398B. The upper part of Unit B consists of hemipelagic sediment interbedded with thin tephra layers; this finer grained part of Unit B is several meters thick in Holes U1398A and U1398B. The lower part of Unit B comprises a succession of massive, normally graded turbidite units, which can be as thick as 6 m. Turbidites occasionally display compositional laminations, grain-size layering, and parallel stratification in the upper part of each layer.

Unit C

  • Depths: Hole U1398A = 57–80 mbsf, Hole U1398B = 58–79 mbsf

Unit C extends from 57 to 80 mbsf in Hole U1398A and from 58 to 79 mbsf in Hole U1398B. Hemipelagic mud is interbedded with multiple thin tephra layers and thin turbidites in Unit C. Hemipelagic sediment makes up most of this unit. Debrites and a relatively thick turbidite (>2 m thick) also occur at 60 and ~70 mbsf, respectively, in Holes U1398A and U1398B. Turbidites generally contain pumice clasts, which are normally graded, and hemipelagic clay.

Unit D

  • Depths: Hole U1398A = 80–104 mbsf, Hole U1398B = 79–99 mbsf

Unit D extends from 80 to 104 mbsf in Hole U1398A and from 79 to 99 mbsf in Hole U1398B. This unit is composed of a succession of massive turbidite units that are a few meters thick. The turbidites, many of which display normal grading, are separated by hemipelagic sediment interbedded with thin tephra layers. The lower boundary of Unit D is the upper of two distinctive pink-colored ash layers.

Unit E

  • Depths: Hole U1398A = 104–115 mbsf, Hole U1398B = 99–115 mbsf

Unit E extends from 104 to 115 mbsf in Hole U1398A and from 99 to 115 mbsf in Hole U1398B. This unit is characterized by multiple tephra layers. It also contains a few turbidites that are generally <1 m thick and interbedded with hemipelagic sediment. In Hole U1398A, only the top 8 m of Unit E was recovered. Two pinkish, 1 cm thick, glassy ash layers are present in the upper half of Unit E. These ash layers are key markers for this site, which allow preliminary correlations between the two cores.

Unit F

  • Depths: Hole U1398A = 115–171 mbsf, Hole U1398B = 115–171 mbsf

Unit F extends from 115 to 171 mbsf. This unit is very well recovered in Hole U1398B but poorly recovered in Hole U1398A. The top of this unit is composed of a 3 m thick debrite followed by a succession of thick massive turbidites. Intercalation of multiple thin ash fall and small-scale turbidites occur below the massive turbidites. Deformed volcaniclastic sand layers are present at 127 and 133 mbsf in Hole U1398B, and they represent unusually strong deformation during coring.

Unit G

  • Depths: Hole U1398A = 171–260.5 mbsf (bottom of hole), Hole U1398B = 171–255 mbsf (bottom of hole)

Unit G extends from 171 to 260.5 mbsf in Hole U1398A and from 171 to 255 mbsf in Hole U1398B. In this unit, core recovery was poor in both Holes U1398A and U1398B. Unit G mostly comprises well-lithified, heavily bioturbated hemipelagic mudstones. This unit occasionally contains interbedded volcaniclastic sand layers that are potential tephra layers. A whitish to pinkish colored, 1 cm thick, glassy ash layer occurs at 232 mbsf in Hole U1398A and at 246 mbsf in Hole U1398B. This layer is probably a correlatable tephra for the two holes at this site.