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doi:10.2204/iodp.proc.340.111.2013 LithostratigraphySediment cored at Site U1401 consists of three lithostratigraphic units (A–C) (Fig. F2). Different lithologies are observed from top to base. The cores are dominated by a combination of hemipelagic mud with interbedded tephra layers and/or volcaniclastic turbidites. Each lithology is described in detail in “Lithostratigraphy” in the “Site U1394” chapter (Expedition 340 Scientists, 2013). Unit A
Unit A extends from 0 to 8.4 mbsf in Hole U1401A, from 0 to 7.6 mbsf in Hole U1401B, from 0 to 5.7 mbsf in Hole U1401C, and from 0 to 3.70 mbsf in Hole U1401D. Unit A is divided in two subunits (A-1 and A-2). Subunit A-1Subunit A-1 extends from 0 to 0.5 mbsf in Hole U1401A, from 0 to 1 mbsf in Hole U1401B, from 0 to 0.57 mbsf in Hole U1401C, and from 0 to 0.12 mbsf in Hole U1401D. Immediately below the seafloor, there is a thick volcanic sand layer (from 12 cm in Hole U1401D to 100 cm in Hole U1401B) that may have been emplaced during recent volcanic eruptions of Montagne Pelée, such as those in 1902 and 1929. If this is the case, this layer is located ~35 km from its source. The uppermost layer is generally massive, ungraded, or very weakly graded and locally contains some light-colored lava/pumice granules. The base of the layer (from 2 cm in Hole U1401D to 20 cm in Hole U1401B) includes sediment in which a zone of hemipelagic mud occurs as a clast within volcanic sand. The base of the layer contains a mixture of grains: pumice (~50%), individual crystals (~42%), scoria (~1%), massive lava (5%), and only a small fraction (~2%) of carbonate. Subunit A-2Subunit A-2 extends from 0.5 to 8.4 mbsf in Hole U1401A, from 1 to 7.6 mbsf in Hole U1401B, from 0.57 to 5.7 mbsf in Hole U1401C, and from 0.12 to 3.7 mbsf in Hole U1401D. Subunit A-2 is composed of hemipelagic mud with abundant interbedded volcanic sand layers (at least 44 tephra layers in Hole U1401A). Most of these layers have relatively sharp bases and tops, poor sorting, and, in some cases, spaced planar lamination. The layers are dark brown-black and generally 2–15 cm thick, although some of them are thicker: 25 cm or even 87 cm. In a few cases, well-sorted volcanic layers are recognized, but they are rare. Most of these layers can be interpreted as tephra layers. The thicker layers have spaced planar lamination, which together with their poor sorting features suggests their emplacement by density flows rather than by fallout from an eruption plume. They have few (2%) or no carbonate grains and variable amounts of pumice, scoriae, massive lava, vesicular lava, and crystals. Most of these tephra layers can be correlated with pumiceous eruptions well identified on land on the western flank of the Montage Pelée Volcano. In a few places, the sandy tephra layers are separated by fine brown mud, which is most likely volcanic in origin. Unit B
Unit B extends from 8.4 to 14.4 mbsf in Hole U1401A, from 7.6 to 12.9 in Hole U1401B, from 5.7 to 10.3 mbsf in Hole U1401C, and from 3.7 to 9.25 mbsf in Hole U1401D. Unit B is composed of a series of coarse to very coarse volcaniclastic turbidites. These turbidites are thick (≤2 m) and generally normally graded with a basal part rich in gravels and a top part composed of fine to medium sand. At least three or four distinct turbidites are observed. These turbidites contain pumice and scoria in variable proportions, as well as vesiculated, massive, and oxidized lava clasts and crystals. The basal part of the turbidites is rich in granules and clasts of gray to reddish gray dense lava; some of them reach 7 cm in size. The clasts comprise mainly two-pyroxene andesites or dacites. Pyrite veins are present in some dense lava clasts, indicating hydrothermal fluid circulation in situ on the volcano. This juvenile material was likely transported from the volcano into the sea by pyroclastic density currents. Carbonate encrusted lava clasts occur, and carbonate clasts are present but very rare (<2%). The normal grading may be generated during initial deposition or possibly during core recovery (i.e., a coring artifact). No hemipelagic mud is present between the turbidites in Holes U1401A and U1401C. In Hole U1401B, 50 cm of hemipelagic mud is interbedded between two of the turbidites (10.5–11 mbsf). This mud contains two tephra layers and a zone of mixed ash and mud. In Hole U1401D, this sequence of hemipelagic mud is observed at the base of the hole (8.3–9.25 mbsf). It contains a higher proportion of volcanic sand layers (10), most of which are interpreted as tephra layers. The presence of scoria in the majority of the turbidites and tephra layers can be related to well-known subaerial deposits from Montagne Pelée. The absence of hemipelagic mud between several turbidite deposits indicates a short time between their deposition and the erosion of interlayered sediments during their emplacement. Unit C
Unit C extends from 14.4 to 81.5 mbsf in Hole U1401A. Unit C is characterized by abundant mafic andesite clasts. This unit is poorly recovered in core catcher samples, which are not in situ. It comprises loose pebbles, as long as 7 cm, made of vesicular mafic andesite, as well as subsidiary amounts of clasts of two-pyroxene andesite. |
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