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doi:10.2204/iodp.proc.340.105.2013 LithostratigraphyThe lithostratigraphy of Site U1395 consists of nine units (A–I). Each unit generally consists of various combinations of hemipelagic muds, turbidite sands and muds, and volcanic ash. Each lithology is described in detail in “Lithostratigraphy” in the “Site U1394” chapter (Expedition 340 Scientists, 2013b). As at Site U1394, boundaries for each of the lithostratigraphic units are defined by abrupt or gradational changes in the abundance of lithologies and by distinctive marker horizons such as basaltic tephra fall and unusually thick turbidites. Unit A
Unit A at Site U1395 is similar to Unit A described for Holes U1394A and U1394B and consists mostly of hemipelagic sediment interbedded with a variety of volcaniclastic and mixed bioclastic-volcaniclastic turbidites. At Site U1395, Unit A extends from the seafloor to ~12 mbsf. This unit has been divided into five subunits (A-1 to A-5). Subunit A-1Subunit A-1 extends from the seafloor to 10 cm (Hole U1395A) or 21 cm (Hole U1395B) and contains (in Hole U1395B only) a series of three turbidite layers separated by thin muddy horizons. No distinction could be made between the three turbidites. These layers are considered to be products from the recent eruptions of the Soufrière Hills Volcano, Montserrat. All three turbidites in Hole U1395B grade from very coarse to fine sand. The base of the sequence in Hole U1395B is marked by an erosive boundary. Subunit A-2Subunit A-2 contains a sequence of predominantly mixed bioclastic-volcaniclastic turbidites separated by as much as 30 cm of hemipelagic sediment. In Hole U1395A, the fourth turbidite layer is ~100 cm thick and contains more volcaniclastic material toward its base. In Hole U1395B, the equivalent layer is <10 cm thick but still contains a significant proportion of volcaniclastic material, although it appears to be concentrated more toward the top of the turbidite. Below this, another one (Hole U1395B) or three (Hole U1395A) turbidites are found. All of the intervening hemipelagic mud is slightly to moderately bioturbated. The total thickness of this unit varies from 1.2 m in Hole U1395B to 3.6 m in Hole U1395A. Subunit A-3In Hole U1395B, Subunit A-3 consists of ~1.5 m of moderately bioturbated, mottled, and banded hemipelagic mud. In Hole U1395A, this unit is only 32 cm thick. Subunit A-4Below Subunit A-3, as much as 1 m of stacked mixed bioclastic-volcaniclastic turbidites is present; these turbidites form Subunit A-4. In Hole U1395B, which contains the thickest sequence, some of the upper turbidites are laminated, whereas the lowest turbidite in the sequence measures 60 cm thick and contains a zone of dispersed felsic pumice at its base. This pumice-rich horizon is not seen in any of the turbidite packages in Hole U1395A. Subunit A-5The final sequence in Unit A, Subunit A-5, consists of a thick package of hemipelagic mud and various mixed bioclastic-volcaniclastic and volcaniclastic turbidites. Toward the base of this unit, particularly in Hole U1395A, two 2 cm thick coarse sand beds are found. These beds are normally graded and likely represent the distal extent of basaltic ash falls from the 138 ka eruption of the South Soufrière Hills Volcano on Montserrat. The basaltic ash falls were also seen at Site U1394, where they consisted of thicker and coarser scoriaceous layers. Unit B
Unit B extends from 12 to 16–18 mbsf. The top of this unit is measured from the first occurrence of a thick sequence of stacked volcaniclastic turbidites, each >1 m thick and exhibiting normal grading from very coarse to very fine sand. The matrix of these turbidites is rich in both dense and vesicular lava, as well as crystals (pyroxenes, amphiboles, and feldspars). Little to no carbonate could be identified in these turbidites. Occasionally, larger clasts (≤2 cm) of more pumiceous material were present. The sequence of stacked turbidites is much better developed in Hole U1395A, where the individual layers range in thickness from <10 cm to >1.5 m. Unit C
Unit C consists of a 65 cm thick layer of hemipelagic sediment sandwiched between the two thick turbidite packages of Units B and D. Unit D
Unit D ranges from ~17 to 20 mbsf and consists of a second thick sequence of stacked volcaniclastic turbidite beds. As with Unit B, the sequence is better developed in Hole U1395A, where several thin turbidites are found. Typically, the turbidites in Hole U1395A are similar to those seen in Unit B of Hole U1395A, containing abundant granules of andesitic lava in a crystal-rich matrix. In Hole U1395B, however, these turbidites appear different, as they are rich in pumice. Only two turbidites were identified at this level in Hole U1395B, and they were both ~1.5 m thick. These two turbidites are rich in large (1–4 cm diameter) pumice clasts, which are dispersed throughout the matrix of each turbidite. There appears to be no grading or sorting of the pumice clasts in either turbidite. The base of Unit D is marked by the beginning of a sequence of hemipelagic sediment. Unit E
Unit E extends from 20 to 63 mbsf and contains principally muddy hemipelagic sediment, within which are intercalated a small number of relatively thin sandy turbidites. The turbidite sand can be volcaniclastic, bioclastic, or mixed in character and ranges in thickness from a few centimeters to a few meters. Additionally, several volcanic fine sand layers are identified in this unit. A 70 cm thick pumice-rich, clast-supported deposit occurs at 43 mbsf in Hole U1395B. Pumice is abundant (40%–75%) in this layer. This deposit lacks reworked carbonate and might represent an unusually thick pumice-fallout deposit. Two other relatively thick (15–22 cm) well-sorted pumice-rich fallout deposits occur at 19–20 mbsf in the upper part of Unit E. Unit F
Unit F extends from 63 to 82 mbsf and starts at a thick set of stacked turbidites with little or no intervening hemipelagic mud. This unit is divided into three subunits (F-1 to F-3). Subunit F-1Subunit F-1 is composed of a dark, thick homogeneous turbidite (>6 m thick) made of mixed bioclastic-volcaniclastic material. The majority of this turbidite is ungraded (fine to medium sand), with normal grading occurring only in the uppermost part. This turbidite sequence is 4 m thick in Hole U1395A and 2 m thick in Hole U1395B. Subunit F-2Subunit F-2 is mainly composed of hemipelagic sediment with intercalated sandy bioclastic or volcaniclastic layers. These layers are massive to normally graded, changing from very fine to medium sand. Some of these layers correspond to thin bioclastic or volcaniclastic turbidites, whereas other layers are dark in color, a few centimeters thick, and composed dominantly of crystals and a few lava and scoria fragments. We interpret these thin layers as tephra fallout deposits. Subunit F-3Subunit F-3 is composed of a thick volcaniclastic turbidite deposit, 3 to 3.5 m thick, which occurs at ~70–75 mbsf in Holes U1395A and U1395B. This turbidite consists of massive medium sand and contains a concentration of poorly vesiculated white pumice at the top. The matrix is composed of crystals (70%), lava clasts (<1%), pumice (20%), and carbonates (10%). No amphibole is observed in the matrix, although it is present in the pumice. However, a few green clinopyroxenes are identified in the matrix. Unit G
Unit G extends from 82 to 113 mbsf and is divided into two subunits (G-1 and G-2). Subunit G-1Subunit G-1 is composed of hemipelagic sediment in which are intercalated numerous volcaniclastic sand layers. It is distinguished from overlying Subunit F-3 by a greater abundance of hemipelagic sediment. The majority of sand layers (which are a few centimeters thick) are massive to normally graded from medium to very fine sand and composed dominantly of crystals, lava clasts, and scoria. Plagioclase is the dominant phase. Pyroxene is also present, whereas amphibole is rare. Amphibole appears only in few layers (without clinopyroxene). These layers are likely tephra fallout deposits originating from the volcanoes in the central part of Guadeloupe. Tephra fallout deposits are particularly abundant between 90 and 100 mbsf (Cores 340-U1395A-12H and 340-U1395B-12H). Thicker sand layers correspond to turbidites. These turbidites are rare in Hole U1395A and are dominantly made of volcaniclastic material. In Hole U1395B, a succession of volcaniclastic turbidites occurs at the top of Subunit G-1. One of the turbidites (at the top of the sequence) is 5.3 m thick, massive, medium to coarse-grained sand composed of 90% volcaniclastic material (60% crystals and 30% lava) and 10% bioclastic material. A few bioclasts are observed (pteropods, otoliths, and spongy spicules) in this turbidite. Subunit G-2In Holes U1395A and U1395B, the hemipelagic sediment located at the base of Unit G is highly contorted and deformed. This deformed interval is 8.5 m thick in Hole U1395A and 7 m thick in Hole U1395B. The deformed unit comprises beds of different colors, which often exhibit strongly inclined contacts or mixed grain size. Unit H
Unit H is composed of a thick sequence of pumiceous volcaniclastic turbidites. In Hole U1395A, two thick deposits are identified between 113 and 121 mbsf. The deposits are 4 and 4.8 m thick. In Hole U1395B, two pumiceous turbidites are also present within the same depth range, and they are separated by 1.5 m of hemipelagite. All of these pumiceous deposits have the same characteristics. They are normally graded, and the matrix is dominantly composed of crystals, lava clasts, and minor pumice and carbonates. Pumice clasts measuring 1–2 cm in diameter occur at the base of the deposit and fine upward to a few millimeters in diameter at the top (Fig. F2). No amphibole is observed in these deposits, and clinopyroxene is not present in the upper part of the unit. The absence of amphibole and clinopyroxene indicates that these deposits originate from volcanoes in the central chain of Guadeloupe, as previously described for the fallout deposits in this unit, or from Montserrat. Unit I
Unit I extends from 121 mbsf to the base of Hole U1395A and from 124 mbsf to the base of Hole U1395B. This unit represents the top of a succession of indurated muddy to silty hemipelagic sediment. This sediment is composed of semiconsolidated siltstones and mudstones. Most of them are heavily bioturbated. |
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