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doi:10.2204/iodp.proc.340.103.2013

Lithostratigraphy

Unit A

• Depth: Hole U1393A = 0–4.24 mbsf

Only one lithostratigraphic unit, consisting of volcaniclastic deposits, was identified at the top of Hole U1393A. Unit A extends to 4.24 mbsf and likely represents volcanic products from the recent (1995–present) dome-forming eruption of the Soufrière Hills Volcano on Montserrat. Because of limited core recovery in Hole U1393A, the base of the unit coincides with the bottom of the APC core barrel. As a result, the lower stratigraphic boundary was not identified.

The uppermost 10 cm of the unit (0–0.10 mbsf) contains mud clasts in a sandy matrix and is moderately disturbed. This disruption is mostly likely a result of coring. A second flat-lying mud clast was identified at 20 cm (0.20 mbsf). Below this clast, the unit consists of dark brownish gray-black volcaniclastic sand containing medium to very coarse sand-sized grains with occasional granules (≤4 mm). The grains themselves are subangular to angular and are predominantly andesitic lava with rare carbonate material. Occasional larger clasts (≤3 cm) of andesitic lava are present. Generally, the unit is moderately well to well sorted, massive, and normally graded. The only structures present are isolated patches (5–10 cm long) of coarse sand surrounded by finer medium sand. The origin of this particular structure is unclear. It is possible that it may have resulted from coring, as the patches of coarse sand were separated from the edge of the core by thin (<5 mm) bands of medium sand. Alternatively, the isolated patches could represent a soft-sediment deformation process of mixing between two fluidized regions. The lower 32 cm of the unit (3.92–4.24 mbsf) consists of andesitic lava clasts as large as 3 cm, set in a coarse sand matrix. Some clasts are slightly hydrothermally altered, and most are subangular in shape.

Overall, the single unit in Hole U1393A is similar in composition to volcaniclastic material previously found in the same area below seafloor (Trofimovs et al., 2008). The angular nature of the grains and the slight hydrothermal alteration present on some clasts suggest the unit originated as a subaerial pyroclastic density current from the collapse of a lava dome, with some rounding in a shallow subaqueous environment as the flow transitioned to a turbidity current on entering the sea (Trofimovs et al., 2006).

Below 4.24 mbsf, no continuous cores were recovered. Material recovered from the core catcher at 4.72, 12.96, 18.89, and 28.63 mbsf consists of a variety of andesitic lava clasts with occasional carbonate material. Most of the clasts are gray and angular, some of them are hydrothermally altered, and many were reddish brown in color, suggesting oxidation in a subaerial environment. Because of the depth from which they were recovered, these clasts likely originate from older volcanic activity on Montserrat.

Igneous and alteration petrology

Observations from thin sections showed that the larger clasts within Unit A contain an amphibole-rich andesite, typical of the lava erupted from the Soufriere Hills Volcano, Montserrat.

Alteration is rare and is restricted to occasional pebble-sized clasts present in material recovered from the core catcher. The yellow to white color of the alteration is indicative of fumarolic or shallow hydrothermal activity.

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