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doi:10.2204/iodp.proc.340.106.2013

Lithostratigraphy

Three holes were drilled at Site U1396. Holes U1396A and U1396C were drilled to 135.5 and 140.0 mbsf, respectively; Hole U1396B was only drilled to 15 mbsf. The aim of Hole U1396B was to fill out an interval of core that was poorly recovered (deformed during coring) in Hole U1396A.

This site mainly comprises a series of hemipelagic intervals and relatively thin (typically <10 cm and most often <5 cm) tephra layers in Units B and E. A distinctive very coarse grained unit rich in white felsic clasts (Unit D) provides a good marker horizon in Holes U1396A and U1396C. Massive volcaniclastic sand intervals comprise the remaining Units A and C.

Distinguishing fallout deposits from fine-grained distal turbidites

A key issue for the description of these deposits is to distinguish tephra layers deposited by fallout from eruption columns (“fallout deposits”) from distal turbidity current deposits. Both types of deposit can be normally graded and lack laminations. The criteria adopted for Expedition 340 are given below.

Fallout deposits contain very little or no reworked carbonate material. However, the amount of carbonate material is difficult to determine accurately by visual examination in fine-grained sediment. Therefore, a limited number of microscopic component analyses (as time on board permitted) were undertaken to determine composition. Importantly, fallout deposits tend to be better sorted than turbidites because of settling processes; turbidites tend to have very poor sorting. Only turbidites will display evidence of strong lateral flow, such as basal erosion and reworking as bed-load. However, basal erosion can be hard to determine unambiguously in narrow-diameter core barrels and can be overprinted by core deformation (downward bowing of basal contacts). Ripple cross-lamination will also only occur in turbidites, but it was rarely if ever seen in Site U1396 cores. Care needs to be taken to distinguish planar lamination due to tractional reworking (turbidites) from grain size and compositional zoning (tephra fallout).

Defining units

The lithostratigraphy of Site U1396, thick sequences of thin tephra layers and intervening hemipelagic units, does not lend itself to defining obvious lithostratigraphic units. Information from magnetic susceptibility curves should be considered in future studies to better constrain the stratigraphic record. For example, the cores contain hemipelagic intervals with increased sand content or variable color, which may be bioturbated tephra layers. The average accumulation rate of 3 cm/k.y. for Site U1396 means that thinner tephra layers are most likely reworked by burrowing organisms and seafloor currents.

Unit A

• Depths: Hole U1396A = 0–0.4 mbsf, Hole U1396C = 0–0.4 mbsf

Unit A extends from 0 to 0.4 mbsf and comprises 39 cm of “soupy,” high-water content, bioclast-rich fine sand, which is only seen in Holes U1396A and U1396C (Hole U1396B did not sample the interval directly below the seafloor). This sand is massive and ungraded, and it could represent a high-density turbidite.

It seems likely that this unit is related to the 1995–recent eruption on Montserrat. The unit has an oxidized brown top and is unlikely to be an artifact of coring. A similar uppermost unit was not observed previously in other piston and gravity cores in the vicinity of Site U1396 (Le Friant et al., 2008; Cassidy et al., submitted).

Unit B

• Depths: Hole U1396A = 0.4–116.1 mbsf, Hole U1396B = 5–14.5 mbsf, Hole U1396C = 0.4–116.1 mbsf

Unit B extends from 0.4 to 116.1 mbsf and comprises a thick sequence of interbedded hemipelagic mud and tephra layers, with the latter typically being <5 cm and rarely >10 cm in thickness. In general, hemipelagic mud makes up most of Unit B. Tephra layers become more common below 90–95 mbsf, as seen in the magnetic susceptibility data. Hemipelagic mud dominates the middle part of the unit.

Thin black or brown fine sand and silt layers (typically <5 cm thick) are common in Unit B and most likely represent fallout deposits. In some cases their coarse grain size (and well-sorted nature) and exclusively volcaniclastic composition is strong evidence for fallout. There may be many more cryptotephras, which are not visible upon visual examination, within the hemipelagic mud intervals.

Most cores at this site comprise hemipelagic mud and thin tephras, with the only exceptions being a few thick tephra layers that are described in more detail below.

Thick tephra layers

Tephra units thicker than 12 cm are observed in only a few instances:

• Upper 5 m: the uppermost part of Unit B (only seen in Hole U1396A) has two thick (20–30 cm) poorly sorted sand layers with a mixed bioclastic (20%–50%) and volcaniclastic (50%–80%) composition at 1.7 and 2.6 mbsf. These mixed composition units are most likely turbidites. Sandwiched within the lower unit is a well-sorted layer comprising 99% volcanic material that is a few centimeters thick and is most likely a fallout unit. Together with a 4 cm (fallout) tephra layer at 3.7 mbsf, these tephra layers occur within the uppermost 5.7 mbsf. Similarly thick tephra layers are not apparent in the 5.7 m long CAR-MON 2 core studied by Le Friant et al. (2008) that extends to 250 ka, suggesting these thick tephras have a localized distribution. The CAR-MON 2 core was located ~15 km from Site U1396, nearer to Montserrat on the same bathymetric ridge. The two thick sand layers seen in the upper 5.7 m of Hole U1396A may be correlative to the more subtle cryptotephras described by Le Friant et al. (2008).
• Two 12–15 cm mixed bioclastic-volcaniclastic poorly sorted turbidites occur in Hole U1396C at 11.2 and 10.2 mbsf.
• A ~50 cm well-sorted coarse fallout unit occurs at 34.3 mbsf in Hole U1396A and comprises at least two graded sequences.
• A 24 cm thick dark coarse tephra layer occurs at 48.7 mbsf in Hole U1396C.
• A 22 cm thick well-sorted fallout deposit occurs at 82.7 mbsf in Hole U1396A and may be equivalent to a 14 cm layer at 78.5 mbsf in Hole U1396C or a 35 cm thick layer at 84.4 mbsf in Hole U1396C.
• Three coarser units occur near the base of the unit in Hole U1396A. A distinctive 50 cm thick, dark gray, massive, poorly sorted medium sand occurs at 106.7 mbsf and is most likely a volcaniclastic turbidite.
• In Hole U1396A a 12 cm thick, coarse, well-sorted fallout unit occurs at 111.4 mbsf, and a 35 cm thick well-sorted fallout deposit occurs at 115.2 mbsf.

Isolated pebble-sized volcanic clasts

Individual outsize volcanic clasts (≤1 cm) are observed in the hemipelagic mud in a few locations. Their origin (perhaps as reworked dropstones) is poorly understood at present.

Inclined laminated sandstone

A 14 cm interval of fine to medium sand is seen in Hole U1396C at 110.7 mbsf; the interval has inclined and truncated laminations that are enigmatic.

Unit C

• Depths: Hole U1396A = 116.1–122 mbsf, Hole U1396C = 116.1–122 mbsf

Unit C extends from 116.1 to 122 mbsf in Holes U1396A and U1396C and comprises massive medium-coarse volcaniclastic sand. The sand has ~85% light-colored andesitic lava grains within a finer darker matrix made mainly of mineral crystals. The sand hue changes progressively vertically, and these changes in hue represent compositional zoning. Unit C is darker than the overlying and underlying units and has some dispersed 1 cm clasts near its base. Unit C may represent high-density turbidity current deposition.

Unit C is absent or poorly developed in Hole U1396A and was too deep for Hole U1396B. Preliminary comparison of the core logs does not indicate an obvious correlative unit in Hole U1396A at a similar depth below seafloor. If Unit C does not represent sand sucked in during APC coring (i.e., it is not an artifact of drilling), this is surprising, as Holes U1396A and U1396C are separated by just 28 m.

Unit D

• Depths: Hole U1396A = 122–123.9 mbsf, Hole U1396C = 122–123.9 mbsf

Unit D extends from 122 to 123.9 mbsf. This unit comprises a distinctive series of unusually coarse (commonly as large as centimeter-scale clasts) breccias with a pinkish color (Fig. F2). Five stacked fining-upward units make up Unit D in Holes U1396A and U1396C, with the uppermost unit being the thickest. The uppermost unit has very well sorted intervals of pebble-sized (≤2 cm) material, with little or no sand or mud-sized matrix material. These normally graded intervals are 3–62 cm thick and separated by thin (1–5 cm) layers of massive fine sand or silt. Unit D is similar in thickness and grading in Holes U1396A and U1396C. The similar stratigraphy beneath Unit D in both holes suggests it has not eroded underlying strata, which is consistent with air fall deposition.

Unit E

• Depths: Hole U1396A = 123.9–135.5 (bottom of hole), Hole U1396C = 123.9–140 mbsf (bottom of hole)

Unit E extends from 123.9 mbsf to the base of Hole U1396C at 140 mbsf. This unit comprises hemipelagic mud and relatively abundant thin (<10 cm) tephra layers. Many of these tephra layers are well-sorted and this is consistent with fallout deposition.

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