Peperite Formation 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

The Tennis Ball Marker in the south-eastern Bushveld Complex: comparison with the Boulder Bed of the Western Bushveld Complex and proposed genesis by disaggregation of intrusive sills in processes akin to those involved in the formation of peperites and mafic enclave swarms

Abstract The Tennis Ball Marker (TBM) is a distinctive lithology that is particularly well developed near the base of the Rustenburg Layered Suite (RLS) on the farm Middelkraal 221 JS, approximately 20 km south of the town of Roossenekal in the Eastern Limb of the Bushveld Complex. The name refers to a texture in which approximately tennis ball-sized spheroidal aggregates of feldspathic orthopyroxenite or melanorite occur within a lighter-coloured gabbronorite host rock. We have identified two well-defined layers in which pyroxenitic spheroids are densely packed, with spheroids more sparsely distributed elsewhere in the host gabbronorite. The TBM at Middelkraal has previously been described as a contact phenomenon where the Main Zone (MZ) has been contaminated by footwall lithologies that include basaltic lavas of the Dullstroom Formation. Our geochemical data, in tandem with new geological mapping, suggest that the TBM is in fact hosted by the Marginal Sill Phase (MSP), and not by the MZ. The MSP is a regional feature that separates the overlying units of the RLS from its floor rocks throughout this region. Sills that make up the MSP were injected on a regional plane of weakness in the primary stratigraphy prior to the intrusion of the remainder of the RLS. The MSP does not represent the chilled carapace of a magma chamber, as implied in previous studies, and there is no evidence of a genetic relationship with the MZ. Our geochemical data indicate, furthermore, that the gabbronorite constituting the dominant lithology of the MSP that hosts the TBM at Middelkraal formed from a sequence of several magma influxes. The TBM is ascribed to one or more subsequent intrusions of pyroxenitic magma into the still hot gabbronoritic host sequence. There are no comparable lithologies reported from other layered intrusions, and the closest analogy to the TBM is the Boulder Bed in the Upper Critical Zone (UCZ) of the Western Bushveld Complex, which has received considerably more attention in the literature than has the TBM. The Boulder Bed has been ascribed variously to in-situ agglomeration of clusters of orthopyroxene resulting from liquid immiscibility, in situ breakup of a pre-existing pyroxenite layer, possibly due to seismic events, or the disaggregation of a late-stage pyroxenitic sill intruding into the magma chamber. We subscribe to the latter mechanism for both the TBM and the Boulder Bed, drawing on similarities with mafic magmatic enclaves, where mafic sills have been shown to have disaggregated on intrusion into earlier bodies of felsic to intermediate igneous rock, or peperites, where magma intruded fluidised sediments.

Skarn and peperite formation within the frame of rifting dynamics, sedimentation, and magmatic activities, Hammam Faraun, Gulf of Suez, Egypt

The Gulf of Suez Rift is a classic example of a rift zone worldwide for studying sedimentation, tectonics, hydrocarbon potentiality, and magmatism. Detailed field studies of the Gabal Hammam Faraun sub-block on the eastern flank of the Gulf of Suez revealed the interaction of sedimentation and rift-related magmatism. These interactive processes develop skarns along the basaltic sill-pre rift Late Cretaceous chalk and peperites along the Early Miocene carbonate-sill contact. generations of both rocks influence hydrocarbon potentiality by modifying reservoir rock porosity and permeability. Skarn rocks were recorded along the contacts of the basaltic sill and the pre-rift Late Cretaceous chalks. Peperites were recorded along the Early Miocene carbonates-sill contacts. The formational settings and associated processes responsible for developing skarn rocks and peperites during the early rifting stage have been postulated in an explanatory model based on mineralogical and textural criteria. The skarn rocks under study underwent four distinct evolutionary stages: an isochemical metamorphic, a prograde anhydrous, a retrograde metasomatic, and a meteoric event. The intermingling of syn-sedimentary basaltic sill emplaced under subaqueous conditions with the Early Miocene unconsolidated calcareous sediments of various rheologies produced fluidal and blocky peperites. The age of the rift initiation issue could be settled by discovering peperites in the Early syn-rift sediments in the study area and adjacent parts of the Gulf of Suez. The existence of magmatic bodies and the distribution of skarn and peperites across carbonate reservoirs are two key features that must be considered when developing a reservoir architectural model.

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.

Volcanoes in the estuaries: Insights into Earth’s oldest (3.22 Ga) terrestrial microbial habitats, Moodies Group, Barberton Greenstone Belt

The Moodies Group of the Paleoarchean Barberton Greenstone Belt (BGB; ca. 3.22 Ga) provides an exceptional window into Archaean sedimentary and magmatic processes interacting with microbial ecology. Its magmatic component, the Moodies Igneous Complex (MIC), has to date been largely overlooked. The MIC consists of several mafic-to-intermediate sills, peperitic dike stockworks, extensive basaltic lava flows, and various volcaniclastic deposits and tuffs, all of Moodies depositional age. Excellent outcrop exposures illustrate diverse and dynamic interactions between volcanic activities and shallow-water sedimentary processes; they constrain the conditions in which microbial communities thrived in estuarine and tidal environments: (1) When the Moodies Basaltic Lava flooded the basin, some lava fragments and pillows were reworked as boulder-sized conglomerate clasts and in gravelly sandstones while still plastic and presumably hot. (2) Aerodynamically shaped ejecta of mafic lava and common tephra shards in cross-bedded sandstones demonstrate explosive lava-water interactions. (3) Dacitic (lapilli) tuffs up to several m thick were eroded by estuarine channels and filled back-beach lagoons, modifying and smothering concurrent aqueous transport of epiclastic sediment. (4) The paleo-groundwater table and the MIC interacted vigorously by in situ fluidization of poorly to unconsolidated sediment. This led to entrainment of grains and sandstone xenoliths in mafic (sub-)volcanics, widespread peperite formation, and formation of lava-filled fractures and abundant quartz veins in surficially consolidated sandstones.The proximity of numerous shallow intrusives, lavas, and volcanic tephra to the tidal-zone habitat of abundant benthic microbial mats provided microorganisms with readily accessible thermal and chemical energy. Heavy mineral lags in the estuaries, combined with potentially aggressive weathering of felsic to ultramafic volcanic rock, released ample nutrients as colloidally-bound transition metals and phosphorus. Drying-and-wetting cycles on tidal flats and coastal plains, the high porosity and permeability of the well-sorted quartzose sand in shallow water, the variably energetic current regime, and the anoxic, reducing-to-oxidizing chemistry of these habitats also benefitted microbial communities greatly.

Contemporaneously emplaced submarine volcaniclastic deposits and pillow lavas from multiple sources in the island arc Brook Street Terrane, Southland, New Zealand

ABSTRACT Volcaniclastic and coherent volcanic rocks of the Permian island arc Brook Street Terrane crop out along the Southland coast, New Zealand, near Riverton. Extensive along-strike exposures reveal lateral variations in these submarine deposits and provide a rare opportunity to examine deposits of an arc environment within a thin slice of time. We describe pillow lavas, dikes, hydroclastic breccias, tuffs, lapilli tuffs and argillites, all hydrothermally altered and metamorphosed to prehnite-pumpellyite facies. Trace element geochemical data indicate multiple primitive island arc to MORB-like magma sources. Two separate volcanic centres apparently tapped different mantle domains in a source region transitional between the arc and back-arc. The centres’ proximity and inferred small sizes suggest they were satellite vents of a larger arc volcano. Tuff and lapilli tuff were formed as hyaloclastite during eruption of pillow lava, and redeposited by subaqueous mass flows. Peperitic contacts indicate that the lavas intruded unconsolidated volcaniclastic deposits. We infer that hydroclastic fragmentation associated with pillow emplacement produced debris that was then intruded by later pillows, with local peperite formation, as eruption persisted. The hydroclastic debris from these satellite vents was resedimented in turbidite fans with channels, and periodic turbidity currents delivered fine-grained material from the volcano’s main vent.

The Ayer Chawan Facies, Jurong Formation, Singapore: Age and observation of syndepositional pyroclastic sedimentation process with possible peperite formation

The Ayer Chawan Facies, Jurong Formation, Singapore: Age and observation of syndepositional pyroclastic sedimentation process with possible peperite formation

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

In the Northern Owyhee Mountains (SW Idaho), a >200-m-thick flow of the Miocene Jump Creek Rhyolite was erupted on to a sequence of tuffs, lapilli tuffs, breccias and lacustrine siltstones of the Sucker Creek Formation. The rhyolite lava flowed over steep palaeotopography, resulting in the forceful emplacement of lava into poorly consolidated sediments. The lava invaded this sequence, liquefying and mobilising the sediment, propagating sediment subvertically in large metre-scale fluidal diapirs and sediment injectites. The heat and the overlying pressure of the thick Jump Creek Rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the Sucker Creek Formation units, and the formation of metre-scale loading structures (simple and pendulous load casts, detached pseudonodules). Density contrasts between the semi-molten rhyolite and liquefied sediment produced highly fluidal Rayleigh-Taylor structures. Local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment. The result is a 30–40-m zone beneath the rhyolite lava of extremely deformed stratigraphy. Brittle failure and folding is recorded in more consolidated sediments, indicating a differential response to loading due to the consolidation state of the sediments. The lava-sediment interaction is interpreted as being a function of (1) the poorly consolidated nature of the sediments, (2) the thickness and heat retention of the rhyolite lava, (3) the density contrast between the lava and the sediment and (4) the forceful emplacement of the lava. This study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture.

Emplacement of the Cabezo María lamproite volcano (Miocene, SE Spain)

Lamproite volcanoes are uncommon in the geological record but are exceptionally well preserved in the Betic Cordilleras of SE Spain, where they erupted during the Late Miocene (Tortonian to Messinian stages). The parent melts are thought to have been channelled through major lithospheric faults to erupt at or near the faulted margins of Neogene sedimentary basins. Lamproite magmas are thought to be relatively CO2-poor (<1 wt %) and are typically characterised by an effusive eruption style and the development of lava lakes and scoria cones. Cabezo Maria is a relatively small (∼550 m diameter) lamproite volcano that was emplaced within the shallow-water marine-influenced Vera Basin. The lamproites are compositionally similar to those of the Roman Province and generally less potassic (K2O<5 wt%) than other (ultra-) potassic rocks in SE Spain (e.g. Cancarix, Fortuna). The initial eruption stages were dominated by explosive magma-water interactions and the formation of peperites. These are characterised by angular fragments of glassy lamproite lavas (and isolated lobes) incorporated in sediments, locally showing the effects of thermal metamorphism. Further, elutriation pipes and ‘jigsaw-fit’ textures are observed in the peperites. The lavas and peperites are overlain by outward-dipping well-stratified scoria deposits defining part of a cinder cone, which is inferred to have emerged above sea level. Steep internal contacts with inward-dipping, structureless breccias likely represent the inner wall deposits of a central conduit. The deposits are cross-cut by late-stage dykes, which supplied fissure eruptions of geochemically similar lavas capping the scoria cone. The transition from explosive to effusive behaviour may reflect the decreased availability of water, possibly due to downward migration of the feeder conduit below the level of water-saturated sediments.

청송 주왕산 페퍼라이트의 산출상태 및 형태학적 특징

청송 주왕산 화산암체의 최하부층인 대전사 현무암에서 산출되는 페퍼라이트는 화성활동과 퇴적작용의 동시성을 보여준다. 본 연구에서는 야외 조사와 이미지 분석, 편광현미경 분석, XRD 같은 실내 조사를 통해 주왕산 페퍼라이트의 형태를 분류하고 형성과정을 유추함으로써 반응 메커니즘과 고환경을 연구하였다. 야외 조사 결과, 하부에서는 구형, 중부에서는 아각형, 상부에서는 불규칙형 페퍼라이트가 대표적으로 나타나며, 노두 규모에서 페퍼라이트 영역은 판상형, 유동형 그리고 공급통로형의 특징적인 구조로 산출된다는 것을 알 수 있다. 페퍼라이트의 형태는 마그마 또는 용암과 퇴적물의 종류에 의해 좌우된다고 연구되어 왔지만, 동일한 공급물에도 불구하고 다양한 형태로 페퍼라이트가 산출되는 것은 형성과정이 단일 메커니즘으로 이루어진 것이 아니라 여러 메커니즘의 복합적인 결과임을 나타낸다. 상하부에 따른 변화 중 가장 뚜렷한 변화를 보이는 퇴적물비로부터 생성 당시 모퇴적물의 공급량이 페퍼라이트의 형성과정에 영향을 미쳤을 것으로 추정한다. Peperite exposed along the Dajeonsa basalt in Mt.Juwang, Cheongsong emerged as a consequence of the simultaneity of volcano-sedimentary sequences. This study aims to classify peperites as morphological characteristics and infer the formation process by the field investigation, image analysis, X-Ray Diffractometer and polarization microscope for the magma-sediment interaction mechanism and paleoenvironment. As a result of the field investigation globular(fluidal) peperite is the representative feature at the bottom of Dajeonsa basalt, sub-angular peperite at the middle and irregular peperite at the top. Peperite domains range from sheet and pod or feeder conduit. Although the study that the morphologies of peperite are controled by the sorts of sediment and lava or magma has tended to center around the peperite, the study addresses not single mechanism but the multi-stage mechanism because Mt.juwang peperite, under the same conditions, varies in shapes and sizes and also the ratio of sediments that show the most change of the peperite affects to the formation of peperite.

Sill genesis in the Paleoproterozoic tectonic evolution of the Onega Trough, Baltic shield

This study considers the role of sill genesis in the tectonic evolution of the Onega Trough during the Middle to Late Paleoproterozoic (Jatulian-Vepsian). The evolution of the Onega Trough is divided into three stages: pre-sill, or preparatory, subsynchronous, and post-sill. Sill magmatism manifested itself most completely at the subsynchronous stage of the evolution of the Onega Trough within the initial, principal, and final phases of sill genesis. Sill formation followed the stage of regional downwarping of the area reaching its maximum during the Early Ludicovian. Paragenesis of sills and high carbon shungite rocks was accompanied by the formation of peperites, while sills influenced the structure of the host rocks. A model reflecting the regular patterns of manifestations of sill genesis identified in the Onega Trough has been proposed.

Intra-vent peperites related to the phreatomagmatic 71 Gulch Volcano, western Snake River Plain volcanic field, Idaho (USA)

The western Snake River Plain volcanic field in SW Idaho contains up to 400 basaltic vents and centers that produced lava shields, pahoehoe lava fields, scoria cones, and a great variety of phreatomagmatic volcanoes between late Miocene and middle Pleistocene time. Tephra deposits produced by phreatomagmatic eruptions are particularly well exposed in the walls of the Snake River canyon, where thick accumulations of pyroclastic rocks indicate widespread phreatomagmatic eruptive events throughout most of the volcanic history of the region. Previously, many of the phreatomagmatic deposits were considered to be the products of subaqueous eruptions that took place on the floor of one or more large freshwater intra-continental lakes. Recent field based observations confirm the presence of widespread phreatomagmatic pyroclastic rocks; however, some that had been interpreted as being subaqueous exhibit textural features that are more consistent with subaerial depositional environments. Intrusive and extrusive magmatic bodies with features associated with peperite formation have also been identified. Most of these peperites can be attributed to magma–sediment mixing in intra-crater/conduit or vent settings, and therefore they can only be used as widespread paleoenvironmental indicators with limitations to demonstrate magma and surface water (e.g. lake) non-explosive interaction. One of the studied sites (“71 Gulch Volcano”) was previously used to indicate the presence of a shallow lake. At this site there is clear field evidence that peperitic feeder dykes contacted muddy, sandy siliciclastic sediments forming globular peperite. The peperitic feeder dykes transition to pillowed, ponded lava up section. The ponded lavas are partially surrounded by a ~ 5-m-thick unit composed of gently dipping, dune bedded, volcanic glass shard-rich, unsorted, tuff and lapilli tuff containing abundant impact sags caused by volcanic lithics. We suggest that the 3D architecture of the erosional remnant of “71 Gulch Volcano” does not require the presence of a lake at the time of its formation; it is equally possible that that it represents a subaerial phreatomagmatic upper conduit — crater filling succession. This interpretation opens up many questions about the Mio/Pliocene evolution of SW Idaho, the timing of the volcanism, and its association with the evolution of the lacustrine systems in the region. In addition, re-evaluations of the volcanic features in SW Idaho have some general implications for the usage of phreatomagmatic pyroclastic rocks for paleoenvironmental reconstruction.

백악기 계화리층 내 현무암질 안산암-실트암 페퍼라이트

전북 부안군 백악기 계화리층 하부에 나타나는 현무암질 안산암-실트암 페퍼라이트의 산출양상과 특성을 보고한다. 페퍼라이트는 판상의 현무암질 안산암에 수반되어 산출되며, 안산암은 적색의 범람원 기원 실트암과 실트질 사암교호층 내에 조화적으로 협재되어 있다. 페퍼라이트가 안산암 상부 경계부에서 산출되는 점과 경계부에서는 분산형 페퍼라이트가 우세하나 안산암 내부쪽에서는 밀집형 페퍼라이트가 우세해지는 점은 안산암이 관입기원(?관입암상)임을 지시한다. 마그마 관입과 페퍼라이트 형성은 계화분지 형성 초기부터 퇴적동시성 화산활동이 활발하였음을 시사한다. 안산암의 K-Ar 전암 절대연령은 백악기 후기(Santonian)로 측정되었다. This paper presents the occurrence and characteristics of the basaltic andesite-siltstone peperite in the lower part of the Gyehwari Formation (Cretaceous), Buan-gun, Jeonbuk province, SW Korea. The peperite is associated with tabular basaltic andesite body, concordantly intercalated with red siltstone and silty sandstone interbeds of floodplain facies. Development of the peperite along the upper margin of the andesite and its textural transition from a dispersed blocky type inward into a closely packed type collectively indicate an intrusive origin (?sill) of the andesite. Magma intrusion and subsequent peperite formation suggest an active syndepositional volcanism since the early stage of evolution of the Gyehwa Basin. The andesite is dated at Late Cretaceous (Santonian) by K-Ar whole-rock radiometric method.

How do you grade peperites?

A spectacular, graded peperite of volcanic origin from the Palaeogene Mull Lava Field of NW Scotland, the Carraig Mhór Bed (CMB), consists of a basal facies of pale, sub-horizontally-aligned, fluidal-shaped clasts of hydrothermally-altered basalt set in a brown, complexly-laminated, silt- to clay-grade, host siliciclastic sediment. The basal facies of the CMB grades upwards over an interval of ∼ 10 m into an upper facies dominated by smaller and more angular clasts of identical composition, set in a subordinate proportion of the same sediment matrix. The distinctive textural characteristics of the CMB, together with its remarkably uniform grading motif, may be explained in terms of the mingling of basaltic magma and fluidised siliciclastic sediment, resulting in the ductile fragmentation of magma, and production of the fluidal clasts which dominate the basal facies. Cooling of the system led to limited brittle fragmentation, yielding the finer-grained, sediment-poor upper facies of the CMB. During cooling, the larger fluidal-shaped clasts settled to the base of the unit, through the saturated sediment, producing the vertical (stratigraphic) grading now preserved. Grading occurred, essentially, in situ during peperite formation and cannot be attributed to remobilisation, mass flow or pyroclastic phenomena.

Carbonate melting and peperite formation at the intrusive contact between large mafic dykes and clastic sediments of the upper Palaeozoic Saint‐Jules Formation, New‐Carlisle, Quebec

AbstractThe base of an upper Palaeozoic graben‐fill in eastern Canada was affected by mafic dyke intrusions shortly after deposition, resulting in the formation of peperite. Complex magma–sediment interactions occurred as the melts mingled with the wet and poorly consolidated clastic material of this sedimentary basin, which is separated from underlying rocks by the Acadian unconformity (Middle Devonian). As a result of these interactions, the mafic rocks are strongly oxidized, albitized and autobrecciated near and above the unconformity, where blocky juvenile clasts of mafic glass and porphyritic basalt have mingled with molten or fluidized sediments of the upper Palaeozoic Saint‐Jules Formation, forming a peperite zone several metres thick. In contrast to most peperite occurrences, the New‐Carlisle peperites are associated with the tip of dykes rather than with the sides of sills or dykes. We argue that more heat can be concentrated above a dyke than above a sill, as the former provides a more efficient and focused pathway for heated waters to invade the poorly consolidated host sediments. Superheated groundwaters that issued from the sides of the dykes appear to have promoted melting of carbonate components in calcareous sedimentary rock clasts of the Saint‐Jules Formation, locally generating carbonate melts that contributed to the mingling of juvenile and sedimentary clasts in the peperite. Copyright © 2005 John Wiley &amp; Sons, Ltd.

O vulcanismo Serra Geral em Torres, Rio Grande do Sul, Brasil: empilhamento estratigráfico local e feições de interação vulcano-sedimentar

At the mounts found in the city of Torres, southern Brazil, three facies associations were identified (i) basalts, (ii) sandstones and (iii) features of volcanic-sedimentary interaction. From field and petrographic descriptions it is possible to suggest the genetic mechanisms of the volcanicsedimentary interactions. The striations on top of sandstones are produced by the flow of lava over unconsolidated sand. The clastic dikes are produced by the infilling of aeolian sand in the disjunctions of the basalt, or the injection of sand towards the lava while it is still flowing. This mechanism of injection, when dealing with plastic lava, leads to the formation of peperite, which is also found as a result of traction and fragmentation of the front and base of the lava flow. It is possible to identify, at Torres, three individual lava flows, each one of them defining a distinct stratigraphic level. These levels are composed by a cycle of events, initiated by the lava flow, which is responsible for the formation of peperite, and this succession is recovered by aeolian deposition of sand. With the definition of these levels it becomes clear that the sandstone outcropping at Morro das Cabras (Guarita Park) is intertrapic sandstone from the Serra Geral Formation, and does not belong to the underlying Botucatu Formation. Key words: Serra Geral volcanism, stratigraphy, volcanic-sedimentary interactions, peperite.

Peperitic lava lake-fed sills at Ság-hegy, western Hungary: A complex interaction of a wet tephra ring and lava

Abstract Ság-hegy is the remnant of a complex volcano consisting of several phreatomagmatic pyroclastic sequences preserved in immediate contact with a thick ( c. 50 m) coherent lava body. Due to the intensive quarrying, the inner part of the lava has been removed, leaving behind a castle-like architecture of pyroclastic rocks. The outcrop walls thus demonstrate the irregular morphology of the lava, which was emplaced in a NW-SE-trending ellipsoidal vent zone in a phreatomagmatic volcano. Pyroclastic beds in the quarry wall are cross-cut by dykes and sills, inferred to have been fed from a central magma zone. Thin (&lt;10 cm) strongly chilled, black, angularly jointed aphanitic basaltic lava mantles the pyroclastic sequence, and has a corrugated margin as a consequence of sudden chilling against the cold and wet phreatomagmatic tephra in the inner wall of the tuff ring crater. These corrugated zones are inferred to be a characteristic textural feature, indicating extensive mixing of lava and host tephra which led to peperite formation along the outer rim lava lake. A spectrum of peperite formed along the lake margin, and fluid oscillation, due to fluidization of the wet tephra, disrupted a steam envelope formed around the lava, causing basaltic magma to invade and mix with the phreatomagmatic tephra. The presence of unconformities in the tephra ring facilitated the formation of sills fed from the central lava body.

Water/magma interaction: some theory and experiments on peperite formation

Experiments, using molten thermite as a magma analog, produce peperite when the melt interacts with wet sand. These experiments also show explosive behavior, developing Strombolian- and Surtseyan-like bursts. The results demonstrate that the application of fuel–coolant interaction (FCI) theory is appropriate for interpretation of peperites. The theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma ( R s), which determine thermal equilibrium temperature limits and contact interface dynamics. The dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation. A vapor film layer develops at the interface and acts both as an insulating barrier, promoting passive quenching, as well as a potential energy reservoir that can cause magma fragmentation, mingling of the magma with wet sediments, and explosive quenching when the vapor film becomes unstable. An important parameter in determining the behavior of the vapor film is the value of R s, which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma. If R s>1 for fully saturated sediments, there is enough water in the sediments to make convective heat flow effective in quenching the magma, but below that value, there is the potential that the vapor film will be unstable, producing highly dynamic phenomena, including explosive fragmentation. At values of R s<0.1 there is insufficient water to allow the escalation of explosive fragmentation.

Peperite as a record of early-stage phreatomagmatic fragmentation processes: an example from the Hopi Buttes volcanic field, Navajo Nation, Arizona, USA

Peperite is the product of near-surface magma–wet sediment mingling. Some peperites are inferred to record ‘frozen’ coarse mixing-stage fuel–coolant interactions (FCIs), and hence can be used in attempts to determine specific parameters related to phreatomagmatic fragmentation, such as water/magma mass ratios, confining pressures, and vent-region hydrology. Two lamprophyric dikes in the Miocene–Pliocene Hopi Buttes volcanic field, Arizona, contain ellipsoidal regions of dilation (buds) associated with peperite. These peperite-rimmed zones occur within identifiable stratigraphic horizons, and at known depths beneath the elevation of the local pre-eruptive surface. The presence of both fluidal and blocky juvenile clasts, combined with physical aspects of the sediment–lamprophyre contacts, suggest that coarse mixing-stage FCI was a potential fragmentation mechanism within these systems. Confining pressures calculated for sites of single buds range from 0.6 to 1.0 MPa. Average inferred water–magma ratios within peperite bodies at the time they formed vary from 0.09 to 0.16, consistent with experimental values for water–melt mixing ratios necessary for explosive heat transfer. A number of factors may have inhibited further explosive activity in these peperite-forming mixing events, including anisotropic saturation of the host sediment, low confining pressures and water/melt ratios, and damped explosive interactions resulting from increased density, viscosity, and surface tension of sediment–water coolants. Nevertheless, these deposits record localized thermohydraulic interactions between magma and wet sediment, and provide valuable information about the geometry and mixing ratios present during peperite formation.

Dynamic mingling of magma and liquefied sediments

Hydrodynamic mingling of magma and liquefied sediments is generally accepted to represent the key process in the formation of some peperites. Experimental studies on simulant liquids and calculations based on recent empirical findings in the field of polymer research were undertaken to investigate the effectiveness of this process. These studies show that for a wide range of shear rates a laminar flow behaviour of the system magma–liquefied sediment can be expected, i.e. turbulent mingling is not a realistic scenario. Formation of peperitic fabrics with grain sizes on a cm scale can hydrodynamically be explained under realistic intrusion velocities. In addition to the hydrodynamics, cooling processes must also be considered during peperite formation. Calculations of the cooling history of the juvenile magmatic component under realistic cooling conditions demonstrate a significant limitation of the hydrodynamic mingling time. The consequence for peperite formation under normal intrusion conditions is that domains of magmatic melt and domains of liquefied sediment smaller than 10 cm are not probable. However, as smaller domain sizes (peperitic grain sizes) exist in nature, we conclude that either those peperites represent the products of explosive events or that hydrodynamic mingling was accompanied by additional fragmentation processes.