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doi:10.2204/iodp.proc.340.103.2013 Site U13931Expedition 340 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1393 (proposed Site CARI-02C; 16°43.13′N, 62°5.06′W; 914 meters below sea level [mbsl]) is located close to the Soufrière Hills Volcano on Montserrat (7.4 nmi [13.7 km] from Point Shoe Rock, at the southeast tip of Montserrat) (Fig. F1). The ongoing eruption of the Soufrière Hills Volcano on Montserrat started in 1995. Activity has included lava dome growth, pyroclastic flows from dome collapse, explosive activity with tephra fall and pumice flows, flank collapse with debris avalanches, and volcanic blasts. More than 70% of erupted material from the ongoing eruption has been transported to the sea (Le Friant et al., 2009, 2010; Trofimovs et al., 2006). The rapid deposition of volcanic material into the sea has caused small tsunamis (Herd et al., 2005). Site survey data indicated that distal parts of the pyroclastic flows and some underlying, older chaotic deposits interpreted as debris avalanches have been deposited at Site U1393. The English’s Crater event, which occurred ~2000 y ago, produced Deposit 1 (Boudon et al., 2007). The deeper deposit (Deposit 2) probably resulted from a combined submarine and subaerial mass-wasting process of the eastern flank of the volcano, which included failure and deformation of submarine sediment (Le Friant, 2004; Lebas et al., 2011; Watt et al., 2012). Seismic data indicated that drilling at Site U1393 could penetrate through the erupted material from the ongoing eruption and into the underlying Deposits 1 and 2. The objective for Site U1393 was to characterize the processes occurring during debris avalanche emplacement, associated erosional processes, and tephra diagenesis. Analysis of 5 m piston cores taken in this area shows that pyroclastic material from the 2003 Soufrière Hills Volcano lava dome collapse mixed with seawater and immediately deposited the coarse components out of suspension (Trofimovs et al., 2006). Study of the coarse debris avalanche deposit will enhance our understanding of emplacement processes. Comparing the geochemical signatures (pore water and sediment) of cored material with surface sediment (from the 2007 Natural Environment Research Council [NERC] cruise) will allow us to characterize the alteration rates of volcanic material in seawater. In addition, we will examine the dependency of alteration rate and style on grain size, layer thickness, and admixture of sediment. Cores from Deposit 1 (smaller volume) and Deposit 2 (larger volume) will allow us to compare the emplacement processes of mass-transport deposits of different magnitudes. We plan to undertake a detailed lithologic, sedimentologic, and textural fabric analysis of the retrieved material at macro- and microscopic scales to investigate transport and deposition processes, the nature and magnitude of erosional processes, and interaction with the substratum (e.g., bulking) (Komorowski et al., 1991; Glicken, 1991, 1996). These data will provide valuable insights into chronology (one or several pulses) and debris avalanche mobility, which have implications for tsunami genesis. |
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