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doi:10.2204/iodp.proc.340.204.2016

Past offshore work

Offshore seismic surveys have identified more than seven debris avalanche deposits with volumes varying between 0.3 and 20 km³ (Deplus et al., 2001; Lebas et al., 2011; Watt et al., 2012a, 2012b; Crutchley et al., 2013; Karstens et al., 2013; Boudon et al., 2007) (Fig. F1). The most recent large landslide occurred at 12–14 ka. Efforts at Site U1393 were unsuccessful to sample Deposit 1 (1.8 km³), which was emplaced toward the southeast of the island. A smaller landslide (Deposit 5; 0.3 km³) originated from the shelf edge southwest of the island at a similar or slightly later time (Cassidy et al., 2012, 2013; Wall-Palmer et al., 2014). A much larger landslide (Deposit 2; ~20 km³) occurred at ~130 ka and overlies an even larger landslide, Deposit 8, as shown in seismic data sets (Lebas et al., 2011; Watt et al., 2012a, 2012b; Le Friant et al., 2015). Both Deposits 2 and 8 contain areas of well-bedded reflectors, as well as large blocks (Watt et al., 2012a, 2012b). It appears that emplacement of material from the volcano onto the surrounding seafloor resulted in even more widespread failure of seafloor sediment (Watt et al., 2012a, 2012b; Le Friant et al., 2015).

A comprehensive set of >70 shallow (up to 6 m long) vibrocores and gravity cores has been collected from around the island (Fig. F1). These cores have been used to correlate and date a series of turbidites that were emplaced in the last 130 ky (Trofimovs et al., 2006, 2008, 2010, 2012, 2013; Cassidy et al., 2013, 2014a, 2014b) (Figs. F1, F2). An unusually large number of accelerator mass spectrometry (AMS) radiocarbon dates have precisely constrained the ages of events in the last ~40 ky (Fig. F2). These events include an erosive stack of turbidites at 11.5–14 ka that may be linked to emplacement of Deposit 1. Two smaller volume turbidites were emplaced at 2 and 6 ka, probably linked to later failures of English’s Crater (Boudon et al., 2007; Trofimovs et al., 2013) (Fig. F2). An oxygen isotope stratigraphy from these vibrocores helps establish the ages of older events, including basaltic-rich turbidites at 110 ka. A detailed analysis of the CAR-MON2 core collected southwest of Montserrat provides information on events during the last 250 ky (Le Friant et al., 2008). Cassidy et al. (2013, 2014a, 2014b) provide information on landslide and turbidite events during the last 110 ky on the southeast side of the island. More recently, Wall-Palmer et al. (2014) discuss events recorded at Site U1396 during the last ~250 ky. In general, this extensive set of shallower cores is very useful for understanding the stratigraphy and age of layers in the upper parts of the IODP sites described here.

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