Submarine deposits from pumiceous pyroclastic density currents traveling over water: An outstanding example from offshore Montserrat (IODP 340)

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 Soufrière 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 pyro clastic 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.

Counterfactual Analysis of Runaway Volcanic Explosions

“…While these characterizations of the volcano's magma storage and supply came much later, in autumn 1997 concern was expressed by some volcanologists (but not all) at the Montserrat Volcano Observatory (MVO) that the Soufrière Hills Volcano might be capable of producing a major sub-Plinian or even Plinian eruption, given the way activity was escalating 1 . At the time, this was regarded by some as, at best, a contentious if not implausible scenario because there was no indisputable evidence for Plinian deposits from any previous Soufrière Hills eruption (this was to change substantively, later: Smith et al, 2007;Le Friant et al, 2008Jutzeler et al, 2017). One of us (WA) was MVO Chief Scientist in June 1997, when the first Vulcanians occurred following the major, and fatal dome collapse of June 25, 1997.…”

The use and beauty of ultra-high-resolution seismic reflection imaging in Late Quaternary marine volcaniclastic settings, Napoli Bay, Italy

“…Seismic interpretation suggests that the HSF may be interpreted as the morphological expression of the submarine counterpart of pyroclastic flow and surge deposits of the Vesuvian Eruption of 79 CE (unit 4) that entered the seawater after the destroying of Herculaneum (MILIA et al 2008, POPE et al 2018. Particularly, the massive, homogeneous texture of the coarse grained pumice, lapilli and ash of unit 4 may result from the underwater modification of primary pyroclastic currents into turbulent flows (FREUNDT 2003, LE FRIANT et al 2009, TROFIMOVS et al 2012, JUTZELER et al 2017, POPE et al 2018, SLOOTMAN et al 2019 and references therein).…”

“…Explosive volcanism produced large volumes of volcani clastic sediments over the Campania coastal region, including pyroclastic air-fall deposits and flow/surge deposits that have been rapidly delivered to sites of deposition, also causing remarkable lateral facies changes as a result of seaward transformation of pyroclastic flows entering the seawater (e.g. LE FRIANT et al 2009, TROFIMOVS et al 2012, JUTZELER et al 2017.…”

Pumice clasts in cross stratified basalt-dominated sandstones and conglomerates. Characteristics and depositional significance: Huarenchenque Fm (Neuquén, Argentina)

“…However, small pumice fragments that are waterlogged (Manville et al, 2002;Dufek et al, 2007;Vella and Huppert, 2007;Allen et al, 2008;Patel et al, 2013) have a much higher density. Consequently, the pumice sinks resulting in its addition to sedimentation (Houghton and Wilson, 1989;Cashman and Fiske, 1991;Manville et al, 1998;Riggs et al, 2001;Allen and Freundt, 2006;Fauria et al, 2017;Jutzeler et al, 2017). This accumulation is mainly produced by highdensity currents (Sohn et al, 1999;Fülöp, 2001;Kataoka and Nakajo, 2002) or by hydraulic currents (White et al, 2001;Manville et al, 2002;Fülöp, 2004;Kataoka, 2005).…”

“…The Huarenchenque Fm that fills former fluvial paleovalley varies considerably in thickness, attaining 80 -100 m towards Loncopué, whereas to the SE of the Neuquén province the values attain 30 -40 m. In conglomerates, a mixture of basalt and pumice clasts is uncommon given their differences in density. Despite the occurrence of pumice clasts in non-marine deposits in the fossil record, there are many data on their sinking process in marine environments (Manville et al, 2002;Jutzeler et al, 2017). Several volcanic rock fragments are incorporated as clasts into different fluvial contexts (Cole, 1991;Paredes et al, 2015;D' Elia et al, 2016) as a result of reworking processes produced by hydraulic flows on previously accumulated 3 volcanic materials (Smith, 1987;Cole and Ridgway, 1993;Paredes et al, 2007;Sohn et al, 2013).…”