Abstract
Pyroclastic density currents have been observed to both enter the sea, and to travel over water for tens of kilometers. Here, we identified a 1.2-m-thick, stratified pumice lapilli-ash cored at Site U1396 offshore Montserrat (Integrated Ocean Drilling Program [IODP] Expedition 340) as being the first deposit to provide evidence that it was formed by submarine deposition from pumice-rich pyroclastic density currents that traveled above the water surface. The age of the submarine deposit is ca. 4 Ma, and its magma source is similar to those for much younger Soufriere Hills deposits, indicating that the island experienced large-magnitude, subaerial caldera-forming explosive eruptions much earlier than recorded in land deposits. The deposit’s combined sedimentological characteristics are incompatible with deposition from a submarine eruption, pyroclastic fall over water, or a submarine seafloor-hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline. The stratified pumice lapilli-ash unit can be subdivided into at least three depositional units, with the lowermost one being clast supported. The unit contains grains in five separate size modes and has a >12 phi range. Particles are chiefly subrounded pumice clasts, lithic clasts, crystal fragments, and glass shards. Pumice clasts are very poorly segregated from other particle types, and lithic clasts occur throughout the deposit; fine particles are weakly density graded. We interpret the unit to record multiple closely spaced (<2 d) hot pyroclastic density currents that flowed over the ocean, releasing pyroclasts onto the water surface, and settling of the various pyroclasts into the water column. Our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate. The overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column, attesting to complex sedimentation. Slow-settling particles were deposited on the seafloor together with faster-descending particles that were delivered at the water surface by subsequent pyroclastic flows. The final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies. This study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water.
Highlights
Pumice-rich volcaniclastic deposits are widely recognized in the marine record
Submarine deposits from pumiceous pyroclastic density currents we argue that this unit is the product of a twostep transport process involving multiple dilute pyroclastic density currents that traveled tens of kilometers over the sea surface, coupled with quick waterlogging and settling of pyroclasts that were delivered to the water surface and settled through the water column in multiple sedimentation pulses
Cold flotation data (20 °C) for these –1.2 to –4.5 phi particles show that 50% of pumice clasts sank within 2 h, 84% within 12 h, and 94% within 24 h (Fig. 9); a few outliers floated much longer; these occurred at all levels of the deposit
Summary
They may originate via pyroclastic density currents entering or traveling over water, pyroclastic fall onto water, submarine eruptions, or by resedimentation (e.g., Cas and Wright, 1991; McPhie et al, 1993; Carey, 2000; Kano, 2003; Manville et al, 2010) In addition to their volcanological significance and substantial effects on marine benthic life (Wiesner et al, 1995; Wetzel, 2009), they are broadly important in geological studies because they are commonly deposited as widespread blankets on the seafloor, which makes them excellent chronostratigraphic markers (Lowe, 2011; Larsen et al, 2014). Cold pumice waterlogging times range from minutes to years (Whitham and Sparks, 1986; Manville et al, 1998; White et al, 2001; Risso et al, 2002; Jutzeler et al, 2014a)
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