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doi:10.2204/iodp.proc.340.204.2016

Stratigraphy

The following stratigraphy was seen at the three sites, as shown in “Appendix A.” Unit thicknesses are available in Table 204_ST05.CSV.

Site U1394

The upper ~6 m of both holes comprises stratigraphy seen previously in shallow cores, as described in detail by Trofimovs et al. (2013). This stratigraphy comprises the 1995–recent eruption products at the very top of the core followed by a series of bioclastic and volcaniclastic turbidites. These include 2 and 6 ka turbidites (Trofimovs et al., 2013) and a stack of turbidites previously dated to 11.5–14 ka by Trofimovs et al. (2013) (Figs. F2, F3). A prominent thick pumice-rich turbidite occurs at ~6–14 mbsf in Hole U1395B, overlain by a distinctive basaltic-rich unit inferred to be a fallout deposit. The basaltic fallout and pumicious turbidite have been the subject of detailed analysis by Cassidy et al. (2015) and are thought to be associated with landslide Deposit 2 at ~130 ka.

A series of relatively thick (3–10 m) turbidites occur between 60 and 120 mbsf (~130 and 200 ka) in Hole U1394B (“Appendix B”). These sand layers have variable composition with greater or lesser amounts of bioclastic and volcanic material. Some of these thick turbidites may have been sucked in as an artifact of piston coring (see Jutzeler et al., 2014), and such intervals are shown by shaded boxes in “Appendix A.” However, a significant number of these thick turbidite sand layers appear to be in situ.

The interval between 120 and 175 mbsf (~200 and 350 ka) in Hole U1394B comprises a greater fraction of hemipelagic muds and has thinner turbidite and fallout layers (“Appendix A”). A thick pumice-rich sand interval at 155–163 mbsf may have been flow-in during coring. Thinner bedded turbidites and fallout deposits underlie the pumice-rich sand, below which a thick sand layer forms the base of Hole U1394B.

Hole U1394B includes landslide Deposit 2, which is seen as a chaotic sequence on 2-D seismic profiles (Fig. F1) (Watt et al., 2012a, 2012b). Deposit 2 comprises two parts (Deposits 2a and 2b) in seismic data separated by an interval of continuous reflectors that extend 15 km downslope (Watt et al., 2012a, 2012b). Analysis of the seismic data suggests that Deposit 2b lies between 16 and 60 mbsf in Hole U1394B, whereas Deposit 2a is located between 60 and 95 mbsf (M. Vardy, pers. comm., 2015). Thus, only Deposit 2a is sampled in Hole U1394B, as there is very little core recovery between 16 and 60 m. Surprisingly, landslide Deposit 2a comprises a sequence of flat-lying turbidites in Hole U1394B with no evidence of deformation, despite appearing as a chaotic unit in seismic surveys.

Site U1395

This more distal site is located in the Bouillante- Montserrat Graben ~25 km from the island (Fig. F1). Advanced piston coring ensured good recovery in both holes down to 120–125 mbsf. A change to rotary drilling resulted in much poorer recovery at deeper levels in the holes (“Appendix A”).

The upper 11.5 m of both holes comprises a series of turbidites separated by hemipelagic mud, capped by a turbidite stack from the 1995–recent eruptions on Montserrat. This turbidite stratigraphy can be correlated to Site U1394 and to the shallow cores described by Trofimovs et al. (2013) (Figs. F2, F3).

A prominent thick turbidite sand layer occurs between ~11.5 and 18 mbsf at this site. This layer has been attributed to the turbidity current created by Deposit 2 emplacement (130 ka). It is most likely equivalent to the thick pumice-rich turbidite that occurs between 6 and 14 mbsf at Site U1394 (“Appendix A”).

Between 18 and 62 m (~130 and 700 ka), Holes U1395A and U1395B tend to have more hemipelagic mud and thinly bedded turbidites and tephra fallout layers. There are a small number of thicker (>1 m) turbidites. Some of these thicker turbidites may be related to flow-in during piston coring (Jutzeler et al., 2014), but others appear to be in situ. Shaded boxes in “Appendix A” show intervals that may have been sucked in. The lower part of Holes U1395A and U1395B (62–120 mbsf; 700–1000 ka) comprise numerous thin tephra layers (<10 cm) and thin volcaniclastic turbidites (<20 cm) with thin intervals (<50 cm) of hemipelagic mud.

At the base of Hole U1395B (~1 Ma) lies Deposit 8. This is the largest known debris avalanche deposits from Montserrat, with an estimated volume of 20 km³ (Boudon et al., 2007; Lebas et al., 2011) (“Appendix A,” “Appendix B”). Seismic surveys estimate that the top of Deposit 8 lies ~120 m below the seafloor (M. Vardy, pers. comm., 2015). Below 119 mbsf in Hole U1395B is a coarse, thick (5 m) volcaniclastic turbidite with some evidence of basal flow-in. This unit may be the top of Deposit 8. Unfortunately, a change from advanced piston coring to rotary drilling resulted in poor recovery within Deposit 8 below 120 mbsf.

Site U1396

Hole U1396C is generally much finer grained and more thinly bedded than Sites U1394 and U1395. Site U1396 is dominated by tephra fall deposits with additional thin (<40 cm) turbidites (“Appendix A”).

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