Fluidal Peperite Research Articles

Fluidal Peperites Recorded in the Cretaceous Lacustrine Sediments in the Southern Korean Peninsula: Syn-Magmatic Sediment Fluidization and Its Influence on the Peperite Formation

Fluidal Peperites Recorded in the Cretaceous Lacustrine Sediments in the Southern Korean Peninsula: Syn-Magmatic Sediment Fluidization and Its Influence on the Peperite Formation

Interactions of pāhoehoe and ‘a‘ā lavas and fluvial sediments on an alluvial plain (the Cretaceous Gyeongsang Basin, Republic of Korea)

Two lava units of basaltic composition named the Cheongryongsa and Hakbong basalts are intercalated in the Cretaceous nonmarine backarc basinfill of Korea. The Cheongryongsa Basalt is 5 to 6 m thick and laterally extends for about 12 km. It is underlain by massive sandy mudstone and overlain erosionally by a basaltic conglomerate, suggesting lava emplacement on a floodplain, followed by fluvial erosion and deposition. The basalt comprises flow-base breccia throughout the 12 km-long outcrop extent and is overlain by flow-top breccia at one locality. These breccias are composed mainly of irregular/fluidally shaped clasts in a massive mudstone matrix, and are interpreted as fluidal peperite produced by molten lava and wet sediment mingling. The Hakbong Basalt comprises three lava units, a few meters to several tens of meters thick, and intervening stratified sandstones. Three breccia types occur beneath or above the basalt: 1) irregular-clast breccia, 2) angular-clast breccia, and 3) fluidal-clast breccia. They are interpreted to be clinker, blocky peperite, and fluidal peperite, respectively, associated with ‘a‘ā lavas. The clinker could be distinguished from the peperite based on undeformed sedimentary structures in the interstitial sediment and the contrast in matrix grain size with the substrate sediment. Peperite formation was possible when the lava was emplaced upon very fine sandy to muddy sediment, which was probably under-consilidated and susceptible to liquefaction. The liquefied sediment needed to penetrate the carapace of clinker to come into contact with the coherent lava and generate peperite. Blocky peperite was produced by the interaction of semi-solidified lava with very fine sand, whereas fluidal peperite was produced by the mingling of molten lava and mud. These basalts show that various breccia types can be produced along subaerial lavas, involving autobrecciation, quench fragmentation, and peperitic fragmentation of lava. Many factors are interpreted to have been involved in forming these breccias, including the sediment type and consolidation state, the viscosity and emplacement setting of lava, and even a coincidental depositional event in the basin.

Basaltic phreatomagmatic fissure at 71 Gulch Part 1: sediment magma mingling and eruptive behavior

71 Gulch Volcano, located in the western Snake River Plain, southwestern Idaho (USA), was formed by a basaltic fissure eruption into Pliocene Lake Idaho. Deposits at and below the eruptive surface record the nature of explosive and non-explosive sediment-magma and water-magma interactions. The paleoenvironment and the volcanic plumbing system of 71 Gulch eruption were reconstructed from a detailed study of the now-exposed eruptive and subsurface deposits. Estimated water levels during the eruption range from 1 to 53 m across the entire volcano based on the distribution of pillow lavas. Billowed dikes and fluidal peperites are two types of intrusive features found ≤ 12 m below the syn-eruptive surface that have similar continuous margins representing ductile behavior and emplacement into soft, deformable sediment. In comparison, tabular dikes and blocky peperites occur throughout 41 m thick of exposed deposits below the syn-eruptive surface. A conical, diatreme-like vent structure dominates the subsurface exposure, indicating excavation by phreatomagmatic explosions. Localized spatter within the main subsurface vent suggests temporary exclusion of water and thus a period of brief vent growth above the lake level. Evidence of sediment-magma mingling through blocky and fluidal peperites shows how differing subsurface depths impact the resulting mingling textures.

Felsic magma-water interaction in shallow intrusive environments: Timing between fluidal peperites and intrusive hyaloclastites in a jurassic cryptodome from the eastern Deseado Massif (patagonia, Argentina)

This work describes the facies architecture, morphology and textures of a Jurassic rhyodacitic cryptodome outcropping in the Don Nicolás Mine, an important Au–Ag producing mine located in the Deseado Massif, Argentinean Patagonia. The access to 70 m deep surface mining operation and to a large number of diamond drill cores offer a unique chance to explore the volcanic rock assemblage of this body, allowing to study intrusion and growth mechanisms of a shallow subvolcanic body, and also providing insights into the timing between different brecciation processes occurring during magma intrusion into a poorly consolidated and water saturated pyroclastic sequence. Facies analysis allowed to infer this body as the result of a rhyodacitic magma intruding in a single and steady endogenous inflation pulse, resulting in the development of a flow banded core and a fluidal peperite envelope due to the combination of relatively low magma viscosities (for a rhyodacitic melt), the development of stable vapor films insulating the flowing magma and also allowing the ductile deformation of the rhyodacite clasts, and the fluidization of the unconsolidated and water saturated host rocks. After the main inflation stage stopped, the collapse of the vapor films resulted in quench fragmentation modifying the originally fluidal shape of many clast at the peperites, specially at the outer portion of the breccia envelope where fluidal clast shapes were almost completely erased due to this process. At the same time water invaded the inner portions of the intrusion, causing widespread quenching and the development of intrusive hyaloclastites at the edges of the coherent facies.

Characteristics of lava-sediments interactions during emplacement of mid-Tertiary volcanism, Northeastern Desert, Egypt: field geology and geochemistry approach

Mid-Tertiary volcanism in the Northeastern Desert of Egypt was associated with the African-Arabian plate rifting and opening of the Red Sea, a period of 30 Ma (Late Oligocene). The resulted lava flows, predominately basaltic sheets, have mingled with the underlying sediments, forming peperite along the contact between them. Field geology, petrography, and geochemistry methods have been used to identify the behavior of the lava-sediment interaction which produced peperite. Two types of peperite have been recognized, fluidal at Naqb Ghul and El-Yahmum areas, and blocky at El-Qattamiya area. Sometimes, the two types are represented together at the El-Qattamiya area. Fluidal peperite is characterized by globular and equant juvenile clasts and with irregular or ragged margins suggesting low viscosity, ductile fragmentation, and sediment fluidization. Blocky peperites are characterized by angular, polyhedral, platy juvenile clasts and a jigsaw-crack texture, resulted from quenching of magma in a brittle regime. The coexistence of fluidal and blocky clasts together reflects progressive disintegration and suggests decreasing temperature and increasing viscosity during fragmentation. The presence of sediments in vesicles and in fractures in the peperite zone indicates a non-explosive phase of interaction. Geochemical data suggests that different composition across the magma/sediment boundary is a result of silicification and sediment fluidization.

Petrologic and chronological characteristics and formation mechanism of peperite in the Sanjiang Orogenic Belt in western Yunnan, China

Jamda-Weixi continent marginal arc belt is an important component to understand the evolution of Paleotethyan. For the first time, the occurrence of peperite is being reported at the bottom of Pantiange Formation in western Yunnan, China. Two types of peperites have been identified, namely fluidal peperite and blocky peperite. LA-ICP-MS U–Pb dating results (253.6±0.7Ma) of dacitic lava showed that the peperite was formed during the late Permian. The peperite was formed during the alternation of active period and quiet period of the volcano in an apparent shallow ocean margin.

Emplacement of lobate sheet flows with hyaloclastites onto soft sediment: The Erquy Neoproterozoic lava pile, Armorican Massif (France)

The Neoproterozoic volcano-sedimentary succession of the Heussaye Headland (Erquy) belongs to the Cadomian block located in the north-east of the Armorican Massif (France). The volcanism, dated at approximately 608 ± 7 Ma (zircon U/Pb data), occurred in a back-arc basin. This site exhibits a noteworthy verticalized pile of moderately metamorphosed (greenschist facies) basaltic andesite flows of arc/back-arc tholeiite affinity, alternating with metasedimentary layers (sandstones and siltstones). A large range of volcanic facies, characteristic of an underwater soft-substrate environment, can be continuously studied for over than 200 m parallel to the flow edges. They include pillow lavas, lava lobes, fluidal peperites and sediment-matrix hyaloclastites. The presence of parallel lava lobes in a sector where one lava flow is capped by a variably thick hyaloclastite carapace, together with the occurrence of asymmetric turbiditic folds in interbedded metasedimentary layers, suggests the existence of an easterly-facing gentle paleoslope. The sudden thinning of this hyaloclastite carapace in two spots located approximately 60 m from each other, is attributed to the synvolcanic activity of two conjugate normal paleofaults. This hypothesis is supported by the observation of a hyaloclastite accumulation in one of the two spots close to a strike-slip fault, which may have been extensional at the time of the volcanism. Water entered the lava flow along the faults, resulting in an interstitial network of hyaloclastites by quench fragmentation in the upper part of the hanging wall block. All these observations are consistent with a model of lava emplacement along a subaqueous slope resulting from synmagmatic extensional faulting in a marginal basin context.

Fluidization of host sediments and its impacts on peperites-forming processes, the Cretaceous Buan Volcanics, Korea

In the Cretaceous Buan Volcanics (SW Korea), blocky and fluidal peperites are developed in a bed of poorly sorted, massive pumiceous lapilli tuff (hot sediments) as a result of the vertical to subvertical intrusion of the trachyandesitic dikes into the bed. Blocky peperites are composed of polyhedral or platy juvenile clasts with a jigsaw-crack texture. Fluidal peperites are characterized by fluidal or globular juvenile clasts with irregular or ragged margins. The blocky peperites are ubiquitous in the host sediments, whereas the fluidal peperites only occur in fine-grained zone (well sorted fine to very fine ash) that are aligned parallel to the dike margin. The development of the fine-grained zone within the poorly sorted host sediments is interpreted to form by grain size segregation caused by upward moving pore water (fluidization) that has resulted from heat transfer from intruding magma toward the waterlogged host sediments during intrusion. With the release of pore water and the selective entrainment of fine-grained ash, the fine-grained zone formed within the host sediments. Subsequent interactions between the fine-grained zone and the intruding magma resulted in ductile deformation of the magma, which generated fluidal peperites. Outside the fine-grained zone, because of the relative deficiency of both pore water and fine-grained ash, intruding magma fragmented in a brittle manner, resulting in the formation of blocky peperites. The results of this study suggest that redistribution of constituent particles (ash) and interstitial fluids during fluidization resulted in heterogeneous physical conditions of the host sediments, which influenced peperite-forming processes.

The Ophiolitic Mélanges in Strike‐slip Fault Zones in West Junggar, Xinjiang, NW China

The Ophiolitic Mélanges in Strike‐slip Fault Zones in West Junggar, Xinjiang, NW China

Relations between basalts and adakitic–felsic intrusive bodies in a soft-substrate environment: the South Ouessant Visean basin in the Variscan belt, Armorican Massif, France

The metasedimentary and magmatic terranes in the southern part of the Ouessant Island (Western Brittany, France) are the offshore prolongation of the Léon Variscan metamorphic domain. They mainly consist of micaschists and subordinate amphibolitic lenses (meta-pillow lavas and volcaniclastic successions) cut by a swarm of trondhjemite sills, together with a large porphyritic monzogranite body, newly dated at 336 Ma, and later syeno-leucogranitic intrusions. A large spectrum of fluidal peperites, including spectacular “fiamme”-bearing breccias, is observable at the contact between metasediments and most of the intrusives. The coexistence of amphibolitized basalts, adakitic trondhjemites, and peraluminous granites in the inferred South Ouessant basin is assigned to a variety of deep subcontemporaneous processes, including asthenospheric partial melting, high-pressure fractionation in lithospheric reservoirs (or partial remelting of deep crystallized mafic intrusions), and continental crust melting. Implications of these new results are discussed in the Visean basinal framework of the Armorican Massif, formed at an early stage of the Variscan orogeny.

Volcanological aspects of the northwest region of Paraná continental flood basalts (Brazil)

Abstract. There has been little research on volcanological aspects of Paraná continental flood basalts (PCFBs), and all investigations have mainly been concentrated on the internal portions of the lava flows. Thus, this study describes for the first time morphological aspects of lava flows and structural characteristics caused by lava–sediment interaction in the northwestern PCFB province (NW-PCFB). Early Cretaceous (134 to 132 Ma) tholeiitic rocks of the PCFB were emplaced on a large intracratonic Paleozoic sedimentary basin (Paraná Basin), mainly covering dry eolian sandstones (Botucatu Formation). As this sedimentary unit is overlain by the basic lava flows of the PCFB, the interaction of lavas and unconsolidated sediments resulted in the generation of fluidal peperites. This aspect is significant because it shows that restricted wet environments should have existed in the Botucatu desert. The peperite zones of the NW-PCFB are associated with compound pahoehoe-type (P-type) flows and are always related to the first volcanic pulses. These flows have dispersed vesicles and sand-filled cracks in their base and top borders, as well as the presence of interlayered sandstones with irregular contacts and varied thicknesses. It is remarkable that, to the best of current knowledge, only in this area of the whole PCFB did the volcanic activity start with low-Ti basalt flows of Ribeira type (TiO2 < 2.3 wt%), which are scarce in the province.

Peperites in the Permian Tarim large igneous province in Northwest China and their constraints on the local eruption environments

Peperites are special kinds of volcaniclastic materials generated by mingling of magma and unconsolidated sediments. They directly demonstrate the contemporaneity of volcanism and sedimentation, and hence they can be used to constrain the local paleoenvironments during volcanic eruptions. We identified peperites in the lower sequence of the northwest outcrops (Inggan-Kalpin area) of Permian Tarim large igneous province (TLIP), Northwest China. In Inggan, blocky peperites were observed at the base of lava flows generated in the second eruption phase. This kind of peperites is generated by quenching of magma in a brittle fragmentation mechanism. While in Kalpin, both the second and the fourth eruption phases preserved peperites in the base of lava flows. Not only blocky but also fluidal peperites can be observed in Kalpin. The fluidal peperites were generated in vapor films, which insulated the magmas from cold sediments and avoided direct thermal shock, and therefore kept the fluidal forms of magma. All of these peperites are hosted by submarine carbonates. In lava sequences generated in the same eruption phases but located in Kaipaizileike, ∼15 km east to Inggan, terrestrial flood basalts developed while peperites are absent, implying a paleoenvironmental transition between Kaipaizileike and Inggan-Kalpin area. Gathering information from observed peperites, TLIP lava flows, and the Lower Permian sedimentary strata, we precisely constrained the spatial distribution and temporal evolution of sedimentary facies of the early stage of TLIP. As a result, two marine transgressions were identified. The first transgression occurred contemporaneous with the second eruption phase. The transition from submarine to subaerial is located between Kaipaizileike and Inggan. The second transgression occurred contemporaneous with the forth eruption phase, and the transition from submarine to subaerial occurred between Inggan and Kalpin.

The facies architecture of a paleoproterozoic basaltic andesite synvolcanic intrusion, Flin Flon, Manitoba, Canada

The Paleoproterozic 1920 unit occurs at the base of the Hidden formation, overlying greater than 90 million tons of volcanogenic massive sulfide deposits at Flin Flon, Manitoba, Canada. The 1920 unit is a laterally discontinuous, Fe-Ti-P-rich basaltic andesite (Fe2O3 12 wt.%, TiO2 1.1 wt.% and P2O5 0.30 wt.%), consisting of massive, pillowed and in situ brecciated basaltic andesite. The 1920 unit is characterised by facies, textures and structures similar to those associated with subaqueous basaltic lavas. However, the presence of blocky and fluidal peperite along the upper contact, and lens-shaped inclusions of tuff within and along its basal contact, as well as the discordancy of the upper contact suggest that the predominantly massive, basaltic andesite was emplaced as a high-level, synvolcanic sill into wet, unconsolidated volcaniclastic deposits. The presence of a breccia containing clasts of the 1920 unit within the package of volcaniclastic rocks overlying the 1920 unit, and a pillowed facies within the 1920 unit, suggest that, locally, the 1920 unit breached the seafloor. An abundance of synvolcanic dikes within the footwall to the 1920 unit, including the presence of a basaltic andesite dike that merges into the 1920 unit, indicates the possible location of the feeder for the 1920 unit in the northwestern portion of the area.

Vitriclastic lithofacies from Macquarie Island (Southern Ocean): compositional influence on abyssal eruption explosivity in a dying Miocene spreading ridge

Macquarie Island is composed of a complete section of oceanic crust that formed in a slow-spreading mid-ocean ridge 2.0 to 3.5 km below sea level. Vitriclastic facies preserved on the island have both pyroclastic and hyaloclastic characteristics. Monomict hyaloclastic breccia facies are widespread across the island and are predominantly composed of near-primitive (~7.9 wt% MgO) subalkaline/transitional (~0.7 wt% K2O) sideromelane shards and crystalline basalt clasts with low vesicularity (LV, < 15% vesicles). Breccias are thick bedded and structureless with matrix-supported angular pillow fragments, bomb-sized fluidal mini-pillows, and globular glass lapilli. Clasts are lithologically similar to interbedded pillow basalts and laterally grade into fine-grained sandstone facies. These sandstones are normal-graded, well-laminated, thin bedded, and interstratified with red pelagic mudstone. Lithofacies associations indicate that the hyaloclastic breccias were formed proximal to a source vent via quench-fragmentation, and subsequently reworked by ocean-bottom currents into distal epiclastic sandstone facies. During eruption, co-genetic pillow lava and hypabyssal intrusions mingled with the breccia, forming fluidal peperite. Rare polymict pyroclastic facies only occur in the highest stratigraphic levels and are mostly composed of highly vesicular (HV, 15–50% vesicles) sideromelane shards and crystalline basalt clasts with alkaline (~1.0 wt% K2O) fractionated (~6.8% MgO) compositions. Minor lithic grains are composed of subalkaline (~0.7 wt% K2O) to very highly alkaline (~1.7 wt% K2O) LV sideromelane shards, and amphibole-bearing diabase. The pyroclastic facies contains medium to thick beds of lapilli-tuff that exhibit both reverse and normal grading, diffuse lamination, and planar-grain fabric. These beds are locally overlain by thin fine-grained tuff beds entirely composed of cuspate to very thin elongate bubble-wall shards. These characteristics indicate that explosive deep-marine eruptions produced high-density coarse-grained gravity flows that were covered by slower suspension settle-out of delicate bubble-wall shards. Stratigraphic relationships suggest that explosive eruptions started during the waning stages of more alkaline volcanism along the proto-Macquarie spreading center.

Peperite formed by lava flows over sediments: An example from the central Paraná Continental Flood Basalts, Brazil

Pahoehoe flows interbedded with sediments have been identified in the superior portion of Paraná Continental Flood Basalts (PCFB), west portion of Paraná State, southern Brazil. In the study area peperites are generated by the interaction between lava flows and wet lacustrine sediments (silt and clay). Evidence that the sediments were unconsolidated or poorly consolidated and wet when the lava flowed over them includes vesiculated sediment, sediment in vesicles and fractures in lava flow and in juvenile clasts in the peperite and soft sediment deformation. Hydrodynamic mingling of lava and wet sediments (coarse mingling) is predominant and volcanic rocks and textures related to explosive phase of Molten Fuel Coolant Interaction (MFCI) are not observed in study area. Locally centimeter-sized areas display direct contact between ash-sized juvenile clasts and sediments formed by the collapse of a vapor film. The textures of fluidal peperites in the central PCFB indicate that the relevant factors that led to a coarse mingling between lava/sediment are (1) lava properties (low viscosity); (2) fine grained, unconsolidated or poorly consolidated wet sediment; and (3) a single episode of interaction between lava flows and sediment.

Blocky versus fluidal peperite textures developed in volcanic conduits, vents and crater lakes of phreatomagmatic volcanoes in Mio/Pliocene volcanic fields of Western Hungary

Volcanic fields in the Pannonian Basin, Western Hungary, comprise several Mio/Pliocene volcaniclastic successions that are penetrated by numerous mafic intrusions. Peperite formed where intrusive and extrusive basaltic magma mingled with tuff, lapilli-tuff, and non-volcanic siliciclastic sediments within vent zones. Peperite is more common in the Pannonian Basin than generally realised and may be also important in other settings where sediment sequences accumulate during active volcanism. Hajagos-hegy, an erosional remnant of a maar volcano, was subsequently occupied by a lava lake that interacted with unconsolidated sediments in the maar basin and formed both blocky and globular peperite. Similar peperite developed in Kissomlyó, a small tuff ring remnant, where dykes invaded lake sediments that formed within a tuff ring. Lava foot peperite from both Hajagos-hegy and Kissomlyó were formed when small lava flows travelled over wet sediments in craters of phreatomagmatic volcanoes. At Ság-hegy, a large phreatomagmatic volcanic complex, peperite formed along the margin of a coherent intrusion. All peperite in this study could be described as globular or blocky peperite. Globular and blocky types in the studied fields occur together regardless of the host sediment.

Magma composition and viscosity as controls on peperite texture: an example from Passamaquoddy Bay, southeastern Canada

Silurian mafic lava flows and rhyolite sills in the Passamaquoddy Bay area, southeastern Canada, interacted with wet sediment to produce peperitic breccia. The pahoehoe-type mafic flows flowed over and bulldozed into wet, unconsolidated silt and sand to produce both blocky and fluidal peperite at their lower contacts. Soft-sediment deformation structures in the siltstone adjacent to the flows, irregularly shaped vesicles in the siltstone, and siltstone-filled vesicles in the mafic lava all indicate the sediment was wet at the time of flow emplacement. Sills of rhyolite emplaced into the same sedimentary sequence also interacted with the wet sediment and produced peperite at both the upper and lower contacts. Evidence for the rhyolitic magma interacting with wet sediment includes sediment-filled vesicles in rhyolite, quartz-filled vesicles in siltstone and angular, non-vesicular shards of rhyolite in clastic dykes. Peperite formed by this process is exclusively blocky. The rhyolitic magma in the Passamaquoddy Bay sequence and other examples of high-viscosity magma producing only blocky peperite suggest that in contrast to low-viscosity magmas, which produce a combination of blocky and fluidal peperite, only blocky peperite will form associated with highly viscous rhyolite magma. The sediment that interacted with both the mafic and rhyolitic lavas is of similar texture indicating that, in this case, sediment texture did not control peperite type. The change from fluidal to blocky peperite texture may also be controlled in part by the magma temperature and/or confining pressure, and the generation and maintenance of a steam film at the magma/wet sediment interface.

Characteristics and origin of peperite involving coarse-grained host sediment

Peperite involving basalt and polymictic volcanic conglomerate occurs in the Pliocene Ba Volcanic Group at Yaqara in northern Viti Levu, Fiji. Because the host sediment is coarse-grained and dominated by basalt clasts, the peperite could be easily overlooked and mistaken for another coarse volcaniclastic facies. However, the presence of groups of basalt clasts that show jigsaw-fit texture, fluidally shaped basalt clasts with complete glassy margins, gradational contacts with adjacent sedimentary facies and the absence of stratification indicate that molten basalt mingled with unconsolidated gravel. Using these criteria, we show that other superficially similar, coarse, polymictic facies with fluidal basalt clasts are not peperite. Both blocky and fluidal basalt clasts occur together in the peperite. The amoeboid basalt clasts in the fluidal peperite result from dismembering of ductile, low-viscosity, relatively hot magma. At this stage, propagating magma lobes were probably insulated from direct contact with the wet sediment by a vapour film. The angular, polyhedral basalt clasts in the blocky peperite indicate brittle disintegration of somewhat cooler, higher-viscosity magma. The presence of jigsaw-fit texture and polyhedral clasts with glassy margins suggest that quench fragmentation of the basalt was important in the formation of the blocky peperite. Although there is no positive evidence for steam explosivity, the presence of steam could be recorded by small quartz-filled cavities that occur within the host sediment. The co-existence of fluidal and blocky basalt clasts is interpreted to reflect successive ductile then brittle fragmentation of intruding magma. The change from fluidal to blocky peperite might have resulted from progressive cooling of the magma during intrusion, and also from the breakdown of fluidisation when the limited supply of fine sediment in the host gravel was exhausted.

Peperite: a review of magma–sediment mingling

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.

Morphology of a crater-filling lava lake margin, The Peninsula tuff cone, Tule Lake National Wildlife Refuge, California: implications for formation of peperite textures

A crater-filling lava lake basalt at The Peninsula tuff cone in northeastern California has features on its base formed by interaction of the basalt with wet hydrovolcanic tuff. A spectrum of features, including elongated, streamlined, flute-shaped lobes, and irregular corrugations, are exposed on the base of the lava lake. Flute-shaped lobes range in amplitude from several centimeters to 0.5 m, plunge downdip with respect to the subjacent layered tuff toward the center of the crater, and are better defined at depths more than 3 m below the lava lake surface. These may have been formed by growth of Rayleigh–Taylor instability waves caused by the density contrast between the wet tuff and basalt, combined with directed stress from flow of the lava lake radially outward from the crater. Formation of an insulating vapor film at the wet tuff–basalt interface and downward pressure exerted by the lava lake allowed the edge of the lava lake to quench gradually, and prevented formation of peperite at greater depths in the lake. Corrugations exposed on the upper 1.5 m of the lava lake margin increase in size from several millimeters to 10 cm in amplitude toward the top of the lava lake and approximately parallel the surface. Corrugations may be the remnant of fluidal peperite preserved on the edge of the lava lake, where the peperite was eroded, or may be deformation features formed within an insulating vapor film due to oscillation of the vapor film. These were formed higher in the lava lake, where the lava lake basalt was thinner, wet tuff deposits confining the lava lake were thinner, and temperature differences between the wet tuff and lava were greater. If corrugations represent deformation of the lava edge within the vapor film, rather than peperite formation, they preserve an intermediate process that is involved in the formation of peperite. This study offers a unique opportunity to evaluate some of the initial mechanisms involved in the formation of peperite based on features preserved in the lava where the host rock, in this case hydrovolcanic tuff, has been eroded but the properties of the host rock are known from adjacent exposures.