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doi:10.2204/iodp.proc.340.110.2013 LithostratigraphyThree holes were drilled at Site U1400. Correlations can be established between Holes U1400A and U1400B in the uppermost tens of meters and between Holes U1400B and U1400C between 15 and 213.4 mbsf (213.4 mbsf is the base of Hole U1400B). Figure F2 summarizes the correlations between units in all three holes. Site U1400 sediment was divided into seven lithostratigraphic units (A–G). Overall, the hole is dominated by a combination of hemipelagic mud with interbedded tephra layers, volcaniclastic turbidites, and deformed sediment. Units A–G consist of varying proportions of these lithologies. Each lithology is described in detail in “Lithostratigraphy” in the “Site U1394” chapter (Expedition 340 Scientists, 2013b), and the deformed sediment is described in “Lithostratigraphy” in the “Site U1398” chapter (Expedition 340 Scientists, 2013c). Unit A
Unit A extends from 0 to 27 mbsf in Hole U1400A and from 0 to 2.94 mbsf in Hole U1400B. In Hole U1400C, core recovery began at 15 mbsf, so Unit A is not observed. Unit A can be divided into three subunits (A-1 to A-3). Subunit A-1Subunit A-1 is thin, measuring 16 cm in Hole U1400A and 5 cm in Hole U1400B. It consists of dark gray massive volcanic sand. It can probably be associated with the last historic 1902–1905 and 1929–1932 dome-forming eruptions of Montagne Pelée, as they are the upper deposits in the core. These deposits contain pumice that is not found in dome-forming eruptions. As such, they cannot be interpreted as primary deposits but are more likely reworked material that includes older pumiceous deposits. Subunit A-2Subunit A-2 measures 14 cm in Hole U1400A and 23 cm in Hole U1400B; it consists of oxidized brown hemipelagic mud with high water content. This subunit is generally found at the top of the core in the other holes, as no historic deposits occur above it. Subunit A-3Subunit A-3 extends from 0.3 to 27 mbsf in Hole U1400A and from 0.3 to 2.94 mbsf in Hole U1400B. This subunit is composed of alternating layers of hemipelagic mud, tephra, and turbidites. Pumice is present in all the deposits, ranging from <5% to >60%. Scoria was also identified in some of the tephra layers. This subunit is considerably thinner in Hole U1400B than it is in Hole U1400A. Unit B
Unit B extends from 27 to below 51 mbsf in Hole U1400A, from 2.94 to 25.5 mbsf in Hole U1400B, and from 15 to 22 mbsf in Hole U1400C. Unit B is composed of thick volcaniclastic sand with no hemipelagic sediment or tephra layers interbedded. Both pumice and scoria were found as clasts in this unit. The base of this unit is not observed in Hole U1400A because the hole was terminated at 51 mbsf. Unit C
Unit C extends from 25.5 to 35 mbsf in Hole U1400B and 22 to 24.8 mbsf in Hole U1400C. Unit C is dominantly composed of mottled gray-green hemipelagic mud with several tephra layers and two thin turbidites. Pumice is abundant in both the tephra layers and the turbidites; some of the layers contain as much as 90% pumice. Hemipelagic mud accounts for >90% of this unit. Green laminae containing glauconite frequently occur in the hemipelagic mud interval. Unit D
Unit D extends from 35 to 58 mbsf in Hole U1400B and from 24.8 to 59 mbsf in Hole U1400C. From the top of Unit D to the base of Unit F (212.5 mbsf in Hole U1400B; 390 mbsf in Hole U1400C), the core material consists of more or less deformed sediment with interbedded tephra layers and, in a few places, turbidites. Deformation is principally identified by inclined tephra layers, inclined contacts between turbidites and hemipelagic mud, and inclined glauconite-rich green laminae in the hemipelagic mud. Inclination can be shallow (a few degrees) to high (≤80°). No debrites are observed in this thick sequence of sediment in Hole U1400B, and only a few debrite intervals are observed in Hole U1400C. This paucity of debrites contrasts with the more frequent debrite intervals seen in cores from Site U1399. Unit D is composed of gray mud and, in some places, highly mottled gray-green-brown mud with interbedded tephra layers (and a few turbidites in Hole U1400C). With the exception of the uppermost 3 mbsf in Hole U1400B, where the inclination of layers is high (60°–70°), most of the sediment is weakly deformed (inclined at 20°) or contorted. Despite a relatively shallow present-day burial depth, the sediment is occasionally highly indurated (see “Physical properties”). Green laminae are abundant and help show the deformation of the sediment. Most tephra layers contain pumice in various proportions. The few turbidites that are observed in Hole U1400C contain as much as 60% pumice. Unit E
Unit E extends from 58 to 212.5 mbsf in Hole U1400B and from 59 to 190 mbsf in Hole U1400C. Unit E is divided into three subunits (E-1 to E-3) based on the proportion of volcanic layers interbedded with the hemipelagic mud and by the intensity of the deformation. Subunit E-1In Subunit E-1 (58–67 mbsf in Hole U1400B; 59–76 mbsf in Hole U1400C), deformation is more intense than it is in Unit D, with beds inclined at an average of 40°. This subunit contains a high proportion of tephra layers (for example, 33 layers between 64.9 and 66.6 mbsf in Hole U1400B). Most of the tephra layers contain pumice, with proportions varying between 5% and 60%. Subunit E-2Subunit E-2 (67–111.6 mbsf in Hole U1400B; 76–101 mbsf in Hole U1400C) contains primarily hemipelagic mud with many interbedded tephra layers and numerous turbidites that range in thickness from a few tens of centimeters to several meters. As with previous units, turbidites contain a significant amount of pumice. In general, the sediment is weakly deformed, with most of the tephra layers featuring subhorizontal contacts. A 2 m thick, pink-colored volcanic ash rich in pumice and glass shards (75%) is observed in Hole U1400C at 82.4–84.5 mbsf. The ash forms a good reference layer that is also observed in Hole U1400B at 84 mbsf, where it appears to be thinner. Subunit E-3Subunit E-3 (111.6–212.5 mbsf in Hole U1400B; 101–190 mbsf in Hole U1400C) is generally composed of highly deformed hemipelagic sediment with steeply inclined (≤70°) contacts. Only a few zones of weakly deformed hemipelagic mud are included. As with Subunit E-2, there are abundant tephra layers and many of them contain pumice. Only a few volcaniclastic turbidites are present in Hole U1400C, whereas a 2.3 m thick mixed (bioclastic-volcaniclastic) turbidite is observed in Hole U1400B that contains a large proportion of mud clasts. Units F and GUnits F and G are only observed in Hole C. They are composed of highly indurated sediment that becomes progressively more lithified with depth, eventually occurring as mudstone. Unit F
Unit F extends from 190 to 390 mbsf in Hole U1400C. This unit is composed of a succession of deformed (and lithified) hemipelagic sediment. The inclination of bedding and contacts is ≤80°. The unit locally contains many tephra layers. In some sections, hemipelagic mud is mixed with volcanic sand, attesting to the important deformation and mixing of the sediment and volcanic layers. Unit F represents probably the most highly deformed part of the hole. A few muddy sand debrite intervals, frequently rich in pumice clasts, are observed at different depths within the unit. Unit G
Unit G extends from 390 to 436 mbsf in Hole U1400C. The top of Unit G marks the lower boundary of the deformed sediment, below which only flat-lying hemipelagic muds and partly lithified mudstones are found. The sediment lacks inclined contacts or green laminae. Two small pumice-rich debrite layers are observed in the upper part of Unit G. |
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