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doi:10.2204/iodp.proc.340.109.2013

Lithostratigraphy

Site U1399 is composed of two holes: Hole U1399A was drilled to 270 mbsf, and Hole U1399B was drilled to 182 mbsf. Good correlations between the two holes can be established to 182 mbsf.

The lithostratigraphy of Site U1399 consists of eight units (A–H). The cores are dominated by a combination of hemipelagic mud with interbedded tephra layers, volcaniclastic turbidites, and various types of deformed sedimentary intervals that occur at different depths. Each lithology is described in detail in “Lithostratigraphy” in the “Site U1394” chapter (Expedition 340 Scientists, 2013b).

Unit A

  • Depths: Hole U1399A = 0–24 mbsf, Hole U1399B = 0–12 mbsf

Unit A extends from 0 to 24 mbsf in Hole U1399A and from 0 to 12 mbsf in Hole U1399B. The top of Unit A is composed of 30 cm of oxidized brown hemipelagic mud with high water content. Below this mud, the main part of the unit is composed of hemipelagic mud with abundant interbedded tephra layers. Several thin turbidites are interbedded in the hemipelagic mud in Hole U1399A, whereas only one turbidite is identified in Hole U1399B. Most of the tephra layers and turbidites contain pumice, with proportions of pumice that vary from 5% to 65%.

Unit B

  • Depths: Hole U1399A = 24–50.5 mbsf, Hole U1399B = 12–29 mbsf

Unit B extends from 24 to 50.5 mbsf in Hole U1399A and from 12 to 29 mbsf in Hole U1399B. Unit B is composed of a thick sequence of more or less deformed hemipelagic sediment intervals with several areas of inclined and convoluted banding. The unit also contains debris flow deposits (debrites) with a muddy sand matrix. Dispersed lava clasts and deformed tephra layers are often found in the debrite intervals. In some parts of the debrites, mud clasts and pumice clasts are included in the muddy sediment. In Hole U1399A, two intervals of deformed sediment are observed: the first is a 18 m thick sequence between 24 and 41.5 mbsf, and the second is 1.50 m thick and occurs between 49 and 50.5 mbsf. In Hole U1399B there are also two sequences of deformed mud and associated debrites occurring between 13 and 15 mbsf and 19 and 29 mbsf. An interval of 4–4.5 m of hemipelagic mud separates these two deformed zones.

Unit C

  • Depths: Hole U1399A = 50.5–72 mbsf, Hole U1399B = 29–53 mbsf

Unit C extends from 50.5 to 72 mbsf in Hole U1399A and from 29 to 53 mbsf in Hole U1399B. Unit C is made of hemipelagic mud with abundant tephra layers and thin turbidites (<1 m thick). In Hole U1399A, a few turbidites contain pumice, but most of the turbidites are rich in crystals and massive lava clasts. In contrast, the proportion of pumiceous turbidites is higher in Hole U1399B, with the proportion of pumice reaching 40% in some cases. The base of Unit C is composed of a 1 m thick interval of hemipelagic mud overlain by a turbidite. This basal turbidite is 8.2 m thick in Hole U1399A but only 2.1 m thick in Hole U1399B.

Unit D

  • Depths: Hole U1399A = 72–76 mbsf, Hole U1399B = 53–68.5 mbsf

Unit D extends from 72 to 76 mbsf in Hole U1399A and from 53 to 68.5 mbsf in Hole U1399B. The upper part of Unit D is composed of deformed sediment intervals with inclined contacts between zones of different colored mud. These deformed intervals contain contorted volcanic layers and muddy sand debrites with abundant mud and pumice clasts. In Hole U1399B, 25 volcanic layers are identified in a 6 m thick sequence of deformed sediment. The lower part of the unit is composed of weakly deformed hemipelagic mud with inclined (deformed) green laminae.

Unit E

  • Depths: Hole U1399A = 76–112 mbsf, Hole U1399B = 68.5–102.7 mbsf

Unit E extends from 76 to 112 mbsf in Hole U1399A and from 68.5 to 102.7 mbsf in Hole U1399B. This unit comprises a succession of nondeformed hemipelagic mud in which there are interbedded tephra layers and thin turbidites, deformed hemipelagic mud, debrites, and thick turbidites.

The following succession is observed in Unit E in Hole U1399A from top to base:

  • 76–83.3 mbsf: hemipelagic mud that includes tephra layers and small turbidites.
  • 83.3–91.5 mbsf: deformed sediment and muddy sand debrite, rich in pumice and mud clasts.
  • 91.5–95 mbsf: pumiceous turbidites.
  • 95–96.5 mbsf: muddy sand debrites, rich in pumice and mud clasts.
  • 96.5–97.9 mbsf: hemipelagic mud with tephra.
  • 97.9–112 mbsf: the base of the unit is composed of two thick pumiceous turbidites.

In Hole U1399B the stratigraphy of Unit E is somewhat different:

  • 68.5–71.9 mbsf: a pumiceous turbidite (3.2 m thick).
  • 71.9–77 mbsf: hemipelagic mud that includes tephra layers.
  • 77–90 mbsf: a series of pumiceous turbidites and a few tephra layers within hemipelagic mud. At the base a thick (7.8 m) pumiceous turbidite is observed.
  • 90–97 mbsf: a succession of muddy sand debrites and hemipelagic mud. The hemipelagic mud intervals may represent large mud clasts in the debrites.
  • 97–102.7 mbsf: a thick pumiceous turbidite (5.3 m).

Unit F

  • Depths: Hole U1399A = 112–142.5 mbsf, Hole U1399B = 102.7–129 mbsf

Unit F extends from 114 to 142.5 mbsf in Hole U1399A and from 102.7 to 129 mbsf in Hole U1399B. This unit is composed of a thick sequence of highly deformed and contorted sediment. It also includes some turbidites and contorted tephra layers, which are observed in both holes. However, the turbidites and tephra layers are thicker in Hole U1399A than in Hole U1399B.

Unit G

  • Depths: Hole U1399A = 142.5–191.7 mbsf, Hole U1399B = 129–182.3 mbsf (bottom of hole)

Unit G extends from 142.5 to 191.7 mbsf in Hole U1399A and from 129 to 182.3 mbsf in Hole U1399B. This unit is composed of a thick sequence of pumiceous turbidites, some of which are very (>5 m) thick. It also comprises a low proportion of interbedded hemipelagic mud with a few tephra layers. Only two intervals of slightly deformed sediment are observed in Unit G: 155–160 and 170.5–178 mbsf in Hole U1399A. These deformed intervals include muddy sand debrites and contorted tephra layers.

Unit H

  • Depth: Hole U1399A = 191.7–270 mbsf

Unit H extends from 191.7 to 270 mbsf and is only observed in Hole U1399A (the depth of Hole U1399B is only 182 mbsf). Unit H is mainly composed of hemipelagic mud with interbedded tephra layers and pumiceous turbidites. The proportion of hemipelagic mud (70%) is higher than that found in preceding units, and tephra layers are frequently observed. The proportion of turbidites within the unit is low, and they are generally thinner than those in the preceding units. Pumice is always present in the turbidites, and pumice also occurs in most of the tephra layers.

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