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doi:10.2204/iodp.proc.340.205.2016

Introduction

Integrated Ocean Drilling Program Expedition 340 investigated volcaniclastic sediments and landslide deposits in the Lesser Antilles arc with the primary objective of furthering understanding of the evolution and eruptive history of the arc, as well as gaining insight into the emplacement of large landslides and debris avalanches in the submarine environment along the Lesser Antilles arc.

During Expedition 340, four sites adjacent to the island of Montserrat, in the northern part of the Lesser Antilles arc (Fig. F1), were drilled. One of these sites, U1396, was drilled in order to recover cores for investigation of the long-term volcanic history of Montserrat. Site U1396 is located 55 km southwest of Montserrat on a topographic high. A detailed overview of Site U1396 geography, objectives, and preliminary results is provided by Le Friant et al. (2015) and is in the “Expedition 340 summary” chapter (Expedition 340 Scientists, 2013). Drilling at Site U1396 recovered cores from three holes. Holes U1396A and U1396C are continuous to ~138 meters below seafloor (mbsf), whereas core recovered from Hole U1396B was used to supplement a section of core in Hole U1396A that was disturbed during recovery. All samples analyzed in this data report are from cores recovered from Holes U1396A and U1396B.

Over the last 20 y, Montserrat has been the focus of considerable work on the processes, deposits, and effects of volcanism in both subaerial and submarine environments (e.g., Aspinall et al., 1998; Young et al., 1998; Druit and Kokelaar, 2002; Wadge et al., 2014). However, the majority of the research has been on the Soufriere Hills Volcano, which has been erupting since July 1995. Consequently, only limited data on the history of the older volcanic centers on Montserrat exists. At present, studies of Montserrat have identified three main centers of volcanism; starting with the oldest they are Silver Hills (~2600 to ~1200 ka), Centre Hills (950–550 ka), and the South Soufriere Hills-Soufriere Hills (~282 ka to present) (Roobol and Smith, 1998; Harford et al., 2002; Smith et al., 2007; Le Friant et al., 2008).

During onboard and postcruise core description and logging, 187 visible tephra layers were identified in Hole U1396A (including 9 in Core 340-U1396A-2H). A further 11 visible tephra layers were identified in core recovered from Hole U1396B, giving a total of 190 visible tephra layers in the combined Holes U1396A and U1396B stratigraphy. According to preliminary paleomagnetic ages determined on board the R/V JOIDES Resolution during Expedition 340, the oldest tephra dates to 4.45 Ma (the early Pliocene), ~2 m.y. older than the oldest known age for subaerial volcanism on Montserrat. For the purposes of this data report, the focus has been on analysis of samples from visible tephra layers within the uppermost six cores recovered (i.e., the uppermost 53 mbsf). The oldest tephra has a paleomagnetic age of ~2.5 Ma and therefore corresponds to the oldest known age for subaerial volcanism on Montserrat.

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