Low-temperature emplacement of phreatomagmatic pyroclastic flow deposits at the monogenetic Mt Gambier Volcanic Complex, South Australia, and their relevance for understanding some deposits in diatremes

Abstract

The occurrence of pyroclastic flow deposits is not restricted to polygenetic and/or felsic volcanic systems, but can also occur at mafic monogenetic volcanic centres. At the c. 5 ka Mt Gambier Volcanic Complex, Australia, two small-volume pyroclastic flow deposits resulted from phreatomagmatic eruptive phases of the Blue Lake East and the Valley Lake maar craters. The facies descriptions of the two massive, poorly sorted lapilli tuff and tuff breccia deposits are given. Low-grade carbonized wood fragments in the deposits mark low emplacement temperatures (180-270°C) for these deposits with only c. 12-23% of the total thermal energy of the original magma preserved at the time of deposition. The Blue Lake East vent erupted by forming a short-lived vertical eruption column, which could not be sustained and collapsed. The Valley Lake pyroclastic flow deposit has a highly asymmetric dispersal to the west, indicating that this deposit was formed during an eruptive phase with a large westward-directed lateral component. Such massive, poorly sorted deposits outside the maar crater indicate that some similar deposits in diatremes may have originated from surface eruptions and not just from subsurface debris jets.

Evidence for the relative depths and energies of phreatomagmatic explosions recorded in tephra rings

“…This contrasts with previous work that interpreted coarse and fine beds as separate events reflecting variations in either efficiency, explosion location, or abundance of water (Wohletz and Sheridan 1983;Haller and Nemeth 2006;van Otterloo et al 2013;Chako Tchambe et al 2015). Experiments also showed that when explosions of a given energy occurred at deeper relative depths, and if a crater was already present above the explosion site, the ability of ejecta to travel ballistically out of a crater was limited and extracrater deposition occurred predominantly by dilute density currents through the expulsion of finegrained particles and gas from the explosion jet collapsing into the crater (Graettinger et al 2015a).…”

“…The presence of magmatic explosion products record additional variations during an eruption scenario. The HBVF deposits are similar to stratigraphic descriptions published on other tephra ring sequences (Fisher and Waters 1970;Sohn and Chough 1989;Aranda-Gomez et al 1992;Nemeth et al 2001;Haller and Nemeth 2006;Carrasco-Núñez et al 2007;Vazquez and Ort 2006;Geshi et al 2011;Valentine 2012;Valentine and Cortés 2013;van Otterloo et al 2013;Valentine et al 2015). The recognition of these depositional facies in published stratigraphic sections from multiple volcanic fields enables a comparison of a wide range of tephra rings.…”

“…Gambier ( Van otterloo et al 2013; Van Otterloo and Cas 2016) (Australia); Tihany Maars (Nemeth et al 2001) (Hungary); Straciacappa ) (Italy); Suona Crater (Geshi et al 2011) (Japan); Atexcac (Carrasco-Núñez et al 2007;López-Rojas and Carrasco-Núñez 2015), La Brena-El Jaguey (Aranda-Gomez et al 1992), and Tepexitl (Austin-Erickson et al 2011) (Mexico); Suwolbong tuff ring (Sohn and Chough 1989) (South Korea); and Easy Chair (Valentine and Cortés 2013), Fort Rock , Haskie Maar (Vazquez and Ort 2006), Narbona Pass ), Salt Lake Craters (Fisher and Waters 1970), Ubehebe Craters (Fisher and Waters 1970;Fierstein and Hildreth 2017), and Zuni Salt Lake Maar (Fisher and Waters 1970) (USA). Tephra rings selected had stratigraphic columns, field photos, or descriptions with sufficient detail to be compared with facies descriptions produced during field work at HBVF.…”

“…Previous models have relied on host rock lithics to determine the depth of explosions, but it is now clear that these explosions sample material that has been mixed within the vent during an eruption and reflect the cumulative explosion history rather than individual events (Lefebvre et al 2013;Graettinger et al 2015aGraettinger et al , 2016Sweeney and Valentine 2015). There is also ample evidence through stratigraphic relationships, ballistic orientations, and morphology that vent locations within a crater migrate both vertically and laterally throughout an eruption (Ort and Carrasco-Núñez 2009;van Otterloo et al 2013;Jordan et al 2013;Pedrazzi et al 2014;Agustin-Flores et al 2015;Kosik et al 2016). Individual clasts in a tephra ring therefore do not help constrain the depth of the explosion that produced the beds in which the lithics are hosted.…”

“…These deposits have a range of bed thicknesses, structures, grain size characteristics, and sorting (Sohn and Chough 1989;White 1991;Vazquez and Ort 2006;Valentine et al 2015), as well as differing proportions of juvenile versus lithic components with height and distance (Valentine 2012;Chako Tchambe et al 2015). The deposit variations, particularly coarse-grained tuff breccias or lapilli tuffs alternating with thinly stratified to cross-stratified tuffs and lapilli tuffs, have been interpreted to be a result of primary explosion processes controlled by the efficiency of phreatomagmatic fragmentation and clogging of the vent (Wohletz and Sheridan 1983;Haller and Nemeth 2006;van Otterloo et al 2013;Chako Tchambe et al 2015). The presence of deposits from Strombolian explosions, Hawaiian fallout, and lava flowsi.e., processes driven by magmatic volatiles-below, interbedded with, and capping tephra rings adds to the complexity of these eruption sequences (White 1991;Aranda-Gomez et alexplosion mechanism (Graettinger et al 2015a, b), simply due to variations in depth and energy release (parameterized together as scaled depth), and the presence or absence of a crater (Goto et al 2001;Taddeucci et al 2013;Graettinger et al 2015a, b;Sonder et al 2015).…”