Peperite: a review of magma–sediment mingling

Abstract

The study of peperite is important for understanding magma^water interaction and explosive hydrovolcanic hazards. This paper reviews the processes and products of peperite genesis. Peperite is common in arc-related and other volcano-sedimentary sequences, where it can be voluminous and dispersed widely from the parent intrusions. It also occurs in phreatomagmatic vent-filling deposits and along contacts between sediment and intrusions, lavas and hot volcaniclastic deposits in many environments. Peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal, but other shapes occur and mixtures of different clast shapes are also found. Magma is dominantly fragmented by quenching, hydromagmatic explosions, magma^sediment density contrasts, and mechanical stress as a consequence of inflation or movement of magma or lava. Magma fragmentation by fluid^fluid shearing and surface tension effects is probably also important in fluidal peperite. Fluidisation of host sediment, hydromagmatic explosions, forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed. Factors which could influence fragmentation and mingling processes include magma, host sediment and peperite rheologies, magma injection velocity, volatile content of magma, total volumes of magma and sediment involved, total volume of pore-water heated, presence or absence of shock waves, confining pressure and the nature of local and regional stress fields. Sediment rheology may be affected by dewatering, compaction, cementation, vesiculation, fracturing, fragmentation, fluidisation, liquefaction, shear liquification and melting during magma intrusion and peperite formation. The presence of peperite intraclasts within peperite and single juvenile clasts with both sub-planar and fluidal margins imply that peperite formation can be a multi-stage process that varies both spatially and temporally. Mingling of juvenile clast populations, formed under different thermal and mechanical conditions, complicates the interpretation of magma fragmentation and mingling mechanisms. ß

Debris jets in continental phreatomagmatic volcanoes: A field study of their subterranean deposits in the Coombs Hills vent complex, Antarctica

“…Mafic volcaniclastic rafts -linear density (number of rafts per km in a traverse) appears quite low in field area, w/ perhaps one raft every km or less (field estimate); only 3 volcaniclastic rafts surveyed in map C compared to >20 "yellow" rafts -overall these bodies range from a few dm to several 10s of m in length; map view shapes vary from rectilinear to irregular (e.g., the largest one in map C) (Skilling et al, 2002) [In the present context, the 'sediment' (granular host) consists of existing coarse volcaniclastic material]…”

“…Mingling of juvenile clasts -mingling of existing 'sediment' (volcaniclastic debris) w/ newly-formed juvenile clasts has been attributed to various processes in the literature, including fluidisation of host (Kokelaar, 1982), forceful intrusion of magma, phreatomagmatic explosions (Busby-Spera & White, 1987;Hanson and Hargrove, 1999) & magma-host density contrasts (Skilling et al, 2002) -authors are unable to isolate a single mingling process as being the most relevant for Coombs Hills, as all appear plausible * See text for description of these zones Table 5 End-member possibilities for the behaviour of debris jets travelling through a granular host (see Fig. 11 Tables 1 and 2 3.…”

Extensive soft-sediment deformation and peperite formation at the base of a rhyolite lava: Owyhee Mountains, SW Idaho, USA

“…Deformation by liquefaction occurs homogeneously throughout the sediment unit or units. Fluidisation is defined as the transmitting of fluids through sediment causing the particles to become supported and entrained within the fluid (Allen 1982;Owen 1996;Skilling et al 2002;. The shear stress of the upward directed fluid matches the weight of the grains .…”

“…Hanson and Schweickert 1982;Kokelaar 1982;Hanson and Wilson 1993;Hanson and Hargrove 1999;Dadd and Van Wagoner 2002;Donaire et al 2002;Skilling et al 2002;Németh et al 2008). In contrast, there have been very few studies on peperite formation and sediment fluidisation at the base of a rhyolite lava flow (Henry et al 1990;Tuffen et al 2001;Branney et al 2008), where the sediment substrate is often less confined and where surface topography and environmental setting (e.g.…”

“…In contrast, there have been very few studies on peperite formation and sediment fluidisation at the base of a rhyolite lava flow (Henry et al 1990;Tuffen et al 2001;Branney et al 2008), where the sediment substrate is often less confined and where surface topography and environmental setting (e.g. water-logged sediments) are important factors (Skilling et al 2002). Determining the processes that occur at the base of rhyolite lavas is important for a number of reasons, including the interpretation of volcanic successions in difficult ancient terranes where textures can often be obscured, and understanding the flow mechanics of large volume lava flows over soft substrates.…”