Depositional processes in a kimberlite crater: the Upper Cretaceous Orapa South Pipe (Botswana)

The Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross‐cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain‐size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the ～70 m thick stratiform crater‐fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater‐fill are interpreted as rock‐fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub‐unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub‐unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small‐scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra‐basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub‐aerial settings.     

Volcaniclastic resedimentation in distal fluvial basins induced by large-volume explosive volcanism: the Ebisutoge–Fukuda tephra, Plio-Pleistocene boundary, central Japan

The Ebisutoge–Fukuda tephra (Plio‐Pleistocene boundary, central Japan) has a well‐recorded eruptive style, history, magnitude and resedimentation styles, despite the absence of a correlative volcanic edifice. This tephra was ejected by an extremely large‐magnitude and complex volcanic eruption producing more than 400 km³ total volume of volcanic materials (volcanic explosivity index=7), which extended more than 300 km away from the probable eruption centre. Remobilization of these ejecta occurred progressively after the completion of a series of eruptions, resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins, more than 100 km from the source. Facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins. The distal tephra (≈100–300 km from the source) comprises two different lithofacies, primary pyroclastic‐fall deposits and reworked volcaniclastic deposits. The resedimented volcaniclastic succession shows five distinct sedimentary facies, interpreted as debris‐flow deposits (facies A), hyperconcentrated flow deposits (facies B), channel‐fill deposits (facies C), floodplain deposits with abundant flood‐flow deposits (facies D) and floodplain deposits with rare flood deposits (facies E). Resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic, large ignimbrite‐forming eruption. Factors controlling inter‐ and intrabasinal facies changes are (1) temporal change of introduced volcaniclastic materials into the basin; (2) proximal–distal relationship; and (3) distribution pattern of pyroclastic‐flow deposits relative to drainage basins. Thus, studies of the Ebisutoge–Fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large‐magnitude eruptions, an aspect of volcaniclastic deposits that has often been ignored or poorly understood.     

Formation of Subaqueous Felsic Domes and Accompanying Pyroclastic Deposits on the Foça Peninsula (Izmir,Turkey)

In western Anatolia, a thick volcanic succession of andesitic to rhyolitic lavas and volcaniclastic rocks crops out extensively. On Foça Peninsula, the westernmost part of the region, a dominantly rhyolitic sequence is exposed where massive rhyolites occur as dome or domelike stubby lava flows. These rhyolite domes vertically and laterally pass into blanketing volcaniclastic sequences. The gradational boundary relations and the facies characteristics of the surrounding volcaniclastic sequences indicate that the silicic domes directly intruded a subaqueous environment and were shattered upon sudden contact with water to form hyaloclastic blankets.

In and around these rhyolite domes, we have defined six different volcanic and volcaniclastic facies, consisting of: (1) massive rhyolite; (2) massive perlite; (3) hyaloclastic breccias; (4) rhyolite pumice and lithic fragment-bearing volcaniclastic rocks; (5) subaqueous welded ignimbrites; and (6) brecciated perlite. The massive rhyolite facies have distinct structures from the centers to the peripheries of the domes and stubby lava flows. Massive lava facies gradually pass into hyaloclastic breccias and massive perlite facies, indicating water-magma interaction during the emplacement. Phreatomagmatic explosive activity and doming caused the subaqueous pyroclastic flows on the flanks of the volcanic center. Welding in the upper parts of these pyroclastic flow deposits indicates the high-temperature emplacement of the pyroclastic material and relatively slow cooling caused by the cushioning effect of the gas-vapor mixture and rapid deposition of younger pyroclastic units.     

Impacts of explosive volcanism on distal alluvial sedimentation: Examples from the Pliocene–Holocene volcaniclastic successions of Japan

Volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation (lahar) processes after the cessation of eruptive activity. Although there are many studies dealing with the assessment and mitigation of volcanic hazards, these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes. However, the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence, and can ultimately result in severe hazards in downstream areas.Examination of the volcaniclastic successions of non-marine Pliocene–Holocene sedimentary basins in Japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano. Impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material. Typical volcaniclastic beds comprise centimetre- to decimetre-thick primary pyroclastic fall deposits overlain by metre- to 10s of metres-thick resedimented volcaniclastic deposits, intercalated in sedimentary successions of non-volcanic provenance. The relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that: 1) potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone; and 2) the majority of resedimented material was likely derived from erosion of non-welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes.The nature, distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of: 1) proximity to ignimbrite, but not directly with the distance to the eruptive centre; 2) ignimbrite nature (non-welded or welded) and volume; 3) temporal changes in sediment flux from the source area; 4) the physiography and drainage patterns of the source area and the receiving basin, and any intervening areas; and 5) the formation of ephemeral dam-lakes and intra-caldera lakes whose potential catastrophic failure can impact distal areas. Models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation.     

The volcanoes of an oceanic arc from origin to destruction: A case from the northern Luzon Arc

Volcanoes were created, grew, uplifted, became dormant or extinct, and were accreted as part of continents during continuous arc–continent collision. Volcanic rocks in Eastern Taiwan’s Coastal Range (CR) are part of the northern Luzon Arc, an oceanic island arc produced by the subduction of the South China Sea Plate beneath the Philippine Sea Plate. Igneous rocks are characterized by intrusive bodies, lava and pyroclastic flows, and volcaniclastic rocks with minor tephra deposits. Based on volcanic facies associations, Sr–Nd isotopic geochemistry, and the geography of the region, four volcanoes were identified in the CR: Yuemei, Chimei, Chengkuangao, and Tuluanshan. Near-vent facies associations show different degrees of erosion in the volcanic edifices for Chimei, Chengkuangao, and Tuluanshan. Yuemei lacks near-vent rocks, implying that Yuemei’s main volcanic body may have been subducted at the Ryukyu Trench with the northward motion of the Philippine Sea Plate. These data suggest a hypothesis for the evolution of volcanism and geomorphology during arc growth and ensuing arc–continent collision in the northern Luzon Arc, which suggests that these volcanoes were formed from the seafloor, emerging as islands during arc volcanism. They then became dormant or extinct during collision, and finally, were uplifted and accreted by additional collision. The oldest volcano, Yuemei, may have already been subducted into the Ryukyu Trench.     

Stratigraphy and volcanology of the Türkbükü volcanics: products of a stratovolcano in the Bodrum Peninsula,SW Anatolia

The Middle‐Upper Miocene Bodrum magmatic complex of the Aegean region, southwestern Turkey, is mainly represented by intermediate stocks, lavas, pyroclastic and volcaniclastic deposits. Monzonitic stocks and connected porphyry intrusions and extrusions are the first products of the magmatism. These are followed by a volcanic succession consisting of andesitic‐latitic lavas, autobrecciated lavas, pyroclastic and volcaniclastic deposits. The final stage is represented by basaltic and basaltic andesitic flows and dykes intruded into previous units. The volcanic succession crops out in the northern part of the Bodrum peninsula. In the lower part of this succession are widespread pyroclastic deposits, composed of pyroclastic fall and flow units, alternating with epiclastic deposits. Grain size, volume and thickness of the pyroclastic deposits were mainly controlled by the type, magnitude and intensity of the eruption. Further up the section, there are two horizons of debris avalanche deposits forming the coarsest and thickest deposits of the volcaniclastic succession. The debris avalanche deposits indicate at least two different flank collapses coeval with the volcanism. The stratigraphy and map pattern of these volcanic units imply that the northern part of the Bodrum peninsula was the north‐facing flank of a stratovolcano during the mid‐Late Miocene. Copyright © 2006 John Wiley & Sons, Ltd.     

Syndepositional volcanism in the Rietgat and arenaceous Bothaville Formations,Ventersdorp Supergroup (late Archaean—early Proterozoic), in South Africa

The volcanic-sedimentary succession of the Ventersdorp Supergroup which is virtually undisturbed tectonically and of low-grade (greenschist facies) metamorphism, affords a unique opportunity for studying the interplay between volcanic and sedimentary processes. The transitional sequence between the Rietgat and Bothaville Formations consists of a number of lithofacies. These are a basal breccia representing pyroclastic and laharic deposits, an overlying breccia—arenite—conglomerate (BAC) which formed by debris flow and fluvial processes, an arenite deposited offshore during a transgression, and an upper conglomerate laid down on a beach. In the volcaniclastic BAC and arenite lithofacies the presence of thin tuff beds, deformed acid lava fragments (bombs?) and glass shards in the arenaceous matrix suggest syndepositional volcanism.Sedimentation took place along the flanks of an asymmetrical, actively volcanic, domal structure which consisted partly of unstable pyroclastic deposits in the east. Resedimentation of the pyroclastic debris by subaerial debris flows and braided streams built a volcaniclastic fan lobe at the foot of the domal structure. As volcanic activity subsided, sands derived from a granitic terrain, mixed with minor air-fall debris to subsequently cover the fan lobe during a regional transgression.     

The fluvial and pyroclastic deposits of the Cagayan Basin, Northern Luzon, Philippines—an example of non-marine volcaniclastic sedimentation in an interarc basin

ABSTRACT The Cagayan basin of Northern Luzon, an interarc basin 250 km long and 80 km wide, contains a 900 m thick sequence of Plio-Pleistocene fluvial and pyroclastic deposits. These deposits are divided into two formations, the Ilagan and Awidon Mesa, and three lithofacies associations. The facies, which are interpreted as meandering stream, braided stream, lahar, and pyroclastic flow and fall deposits, occur in a coarsening upward sequence. Meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits; lahars and ignimbrites. The coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent Cordillera Central volcanic arc. Uplift of the arc resulted in the progradation of coarser clastics further into the basin, the development of an alluvial fan, and migration of the basin depocentre away from the arc. The coarsening of the pyroclastic deposits reflects the development of a more proximal calc-alkaline volcanic belt in the maturing volcanic arc. The Cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins. This is because the tectonic and volcanic processes which controlled sedimentation in the Cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins.     

The lower Devonian Kowmung volcaniclastics: A deep‐water succession of mass‐flow origin,northeastern Lachlan Fold Belt,N.S.W.

The Lower to ?Middle Devonian Kowmung Volcaniclastics form the upper part of a succession of Upper Siluran to mid‐Devonian flyschoid rocks in the Yerranderie area of N.S.W., and contain two major facies associations. (1) A mudstone facies association represents the ambient, background sedimentation, comprising predominantly buff mudstone that is host to an assemblage of coarser‐grained sediments, including graded‐bedded to massive siltstone, sandstone, conglomerate, allodapic limestone, and large allochthonous limestone blocks and associated limestone breccia. Bouma sequences are common, sole structures occur and maximum bed thickness is about 3 m. (2) A volcaniclastic facies association intrudes and interrupts the accumulation of the ambient mudstone facies association, and contains massive to partly graded, quartzofeldspathic siltstone, sandstone, breccia and conglomerate. Sedimentation units in the volcaniclastic facies association are up to 120 m thick. The two facies associations interfinger. Stratigraphically, the base of the Kowmung Volcaniclastics is taken as the first sedimentation unit of the volcaniclastic facies association. The mudstone facies association below this level is part of the Siluro‐Devonian Taralga Group.

Both facies associations were deposited in relatively deep‐water. The dominant transport process in both associations was mass‐flow, involving granular mass‐flows (turbidity currents, grain flows), debris flows and avalanches. Massive mudstone is hemipelagic in origin. The volcaniclastic facies association probably represents a submarine volcanic apron around the emergent, volcanic Bindook Complex. Grossly, the succession coarsens upwards, and there is evidence of several sources of sediment, rather than a single point at the head of a submarine fan.

Provenance is diverse. In the mudstone facies association, framework grains in sandstone are microlitic volcanic‐rock fragments with a mafic to intermediate volcanic source. Clasts in conglomerate and breccia are consistent with derivation from the regionally extensive, quartzose Ordovician flyschoid successions. Clasts of ?penecontemporaneous limestone also occur. The volcaniclastic facies association was probably derived largely from the nearby, coeval Bindook Complex, which consists of silicic ash‐flow and ash‐fall tuff, lava, associated sediment and granitoids. Detritus was either derived directly from volcanic eruptions or was worked in fringing littoral and fluvial environments prior to redeposition by mass‐flow. Quartzite boulders mixed with volcanic clasts in the conglomerate suggest that Ordovician quartzarenite was also exposed around the volcanic complex. Tentative provenance correlations have been made between the different rock units in the Kowmung Volcaniclastics and their possible sources in the northern part of the Bindook Complex.     

Sedimentary processes caused by felsic caldera-forming volcanism in the Late Miocene to Early Pliocene intra-arc Aizu basin, NE Japan arc

Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma.     

Modern analogues for Miocene to Pleistocene alkali basaltic phreatomagmatic fields in the Pannonian Basin: “soft-substrate” to “combined” aquifer controlled phreatomagmatism in intraplate volcanic fields Research Article

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km². These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.     

An interplay of syn- and intereruption depositional processes: the lower part of the Jangki Group (Miocene), SE Korea

The lower part of the Jangki Group (Miocene), SE Korea consists of pyroclastic mass-flow-dominated facies and epiclastic stream-flow-dominated facies which reflect sedimentation during syn- and intereruption periods, respectively. On the basis of pyroclastic composition, sedimentary structures and bed geometry, they are organized into two facies associations: (1) dacitic and basaltic debris-flow and hyperconcentrated-flood-flow deposits of eruption periods, and (2) epiclastic stream-flow and interchannel deposits of intereruption periods. The lateral relationship between the syn- and intereruption deposits varies significantly over short distances (2 km). In the western part of the study area, syneruption deposits are predominant, and fluvial deposits occur as small-scale channel-fill gravelstone bodies encased within dacitic debris flow deposits. In the eastern part, however, intereruption deposits are dominated with thick sequences of interbedded channel and interchannel deposits. The abrupt lateral change indicates alternation of epiclastic axial fluvial system with pyroclastic-rich volcaniclastic aprons. The syneruption deposits are enriched in vitric ash but lack contemporary volcanic rock fragments (dacitic or basaltic). They are sharply differentiated from intereruption deposits that mostly consist of epiclasts and are deficient in vitric ash. The vertical transition suggests that streams drained a hinterland of igneous basement rocks during intereruption periods and became bulked with pyroclasts during syneruption periods.     

Intra- and extra-caldera volcaniclastic facies and geomorphic characteristics of a frequently active mafic island–arc volcano, Ambrym Island, Vanuatu

Ambrym is one of the most voluminous active volcanoes in the Melanesian arc. It consists of a 35 by 50 km island elongated east–west, parallel with an active fissure zone. The central part of Ambrym, about 800 m above sea level, contains a 12 kilometre-wide caldera, with two active intra-caldera cone-complexes, Marum and Benbow. These frequently erupting complexes provide large volumes of tephra (lapilli and ash) to fill the surrounding caldera and create an exceptionally large devegetated plateau “ash plain”, as well as sediment-choked fluvial systems leading outward from the summit caldera. Deposits from fall, subordinate base surge and small-volume pyroclastic (scoria) flows dominate the volcaniclastic sequences in near vent regions. Frequent and high-intensity rainfall results in rapid erosion of freshly deposited tephra, forming small-scale debris flow- and modified grain flow-dominated deposits. Box-shaped channel systems are initially deep and narrow on the upper flanks of the composite cones and are filled bank-to-bank with lapilli-dominated debris flow deposits. These units spill out into larger channel systems forming debris aprons of thousands of overlapping and anastomosing long, narrow lobes of poorly sorted lapilli-dominated deposits. These deposits are typically remobilised by hyperconcentrated flows, debris-rich stream flows and rare debris flows that pass down increasingly shallower and broader box-shaped valleys. Lenses and lags of fines and primary fall deposits occur interbedded between the dominantly tabular hyperconcentrated flow deposits of these reaches. Aeolian sedimentation forms elongated sand dunes flanking the western rim of the ash-plain. Outside the caldera, initially steep-sided immature box-canyons are formed again, conveying dominantly hyperconcentrated flow deposits. These gradually pass into broad channels on lesser gradients in coastal areas and terminate at the coast in the form of prograding fans of ash-dominated deposits. The extra-caldera deposits are typically better sorted and contain other bedding features characteristic of more dilute fluvial flows and transitional hyperconcentrated flows. These outer flank volcaniclastics fill valleys to modify restricted portions of the dominantly constructional landscape (lava flows, and satellite cones) of Ambrym. Apparent maturity of the volcanic system has resulted in the subsidence of the present summit caldera at a similar rate to its infill by volcaniclastic deposits.     

辽河东部凹陷火山岩相测井响应特征及储集意义

在郯庐断裂多期构造活动作用下,辽河坳陷东部凹陷发生了多期火山活动,致使中基性火山岩广泛发育,以玄武岩和粗面岩为主,可分为5相14亚相。以常规测井曲线特征为基础,结合电成像的分析,识别出了爆发相（火山碎屑流和热基浪亚相）、溢流相（玻质碎屑岩、板状熔岩流和复合熔岩流亚相）、侵出相（内带、中带和外带亚相）和火山沉积相（含外碎屑和再搬运火山碎屑沉积亚相）10种岩相/亚相：火山碎屑流亚相具"焊接"特征,热基浪亚相具层理特征;玻质碎屑岩亚相具高CNL的特征,板状熔岩流亚相DEN、CNL和AC测井曲线显示微齿平滑的特征,而复合熔岩流亚相测井曲线呈指状叠加的特征;侵出相内带→中带→外带电阻率逐渐减小;含外碎屑和再搬运火山碎屑沉积亚相GR值范围不同,并且再搬运火山碎屑沉积亚相具水平层理特征。火山岩相控制原生储集空间的类型,并作用于后期的次生改造,从而影响储层的物性、储集性和有效性。复合熔岩流亚相储层孔隙发育,物性好,但由于内部纵向上结构不一致,非均质性强,因而储层含油性差。火山碎屑流亚相内部在纵向上岩性及结构相对一致,因而储层物性相对均一、分布集中,为有利的火山岩储层,可以作为东部凹陷进一步勘探开发的有利相带。     

The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings.     

Volcaniclastic resedimentation on the northern slope of Vesuvius as a direct response to eruptive activity

A new archaeological excavation on the northern slope of Vesuvius has provided invaluable information on the eruptive activity and post-eruptive resedimentation events between the late Roman Empire and 1631. A huge Roman villa, thought to belong to the Emperor Augustus, survived the effects of the 79 a.d. Plinian eruption, but was mainly engulfed in volcaniclastic materials eroded and redeposited immediately after a subsequent eruption or during repose periods. Primary pyroclastic deposits of the 472 a.d. eruption are only few centimeters thick but are overlain by reworked volcaniclastic deposits up to 5 m thick. The resedimented volcaniclastic succession shows distinct sedimentary facies that are interpreted as debris flow deposits, hyperconcentrated flow deposits, and channel-fill deposits. This paper has determined that the aggradation above the roman level is about 9 m in 1,200 years, leading an impressive average rate of 0.75 cm/year.     

The Ilchulbong tuff cone, Cheju Island, South Korea

The Ilchulbong mount of Cheju Island, South Korea, is an emergent tuff cone of middle Pleistocene age formed by eruption of a vesiculating basaltic magma into shallow seawater. A sedimentological study reveals that the cone sequence can be represented by nine sedimentary facies that are grouped into four facies associations. Facies association I represents steep strata near the crater rim composed mostly of crudely and evenly bedded lapilli tuff and minor inversely graded lapilli tuff. These facies suggest fall-out from tephra finger jets and occasional grain flows, respectively. Facies association II represents flank or base-of-slope deposits composed of lenticular and hummocky beds of massive or backset-stacked deposits intercalated between crudely to thinly stratified lapilli tuffs. They suggest occasional resedimentation of tephra by debris flows and slides during the eruption. Facies association III comprises thin, gently dipping marginal strata, composed of thinly stratified lapilli tuff and tuff. This association results from pyroclastic surges and cosurge falls associated with occasional large-scale jets. Facies association IV comprises a reworked sequence of massive, inversely graded and cross-bedded (gravelly) sandstones. These facies represent post-eruptive reworking of tephra by debris and stream flows. The facies associations suggest that the Ilchulbong tuff cone grew by an alternation of vertical and lateral accumulation. The vertical buildup was accomplished by plastering of wet tephra finger jets. This resulted in oversteepening and periodic failure of the deposits, in which resedimentation contributed to the lateral growth. After the eruption ceased, the cone underwent subaerial erosion and faulting of intracrater deposits. A volcaniclastic apron accumulated with erosion of the original tuff cone; the faulting was caused by subsidence of the subvolcanic basement within the crater.     

松辽盆地改造残留的古火山机构与现代火山机构的类比分析

现代火山机构形态有盾状、锥状和穹状,可按喷发样式进一步划分为7种类型。据此分类,在松辽盆地周缘剖面及其北部徐家围子断陷区可识别出4类火山机构：盾状火山机构,由喷溢相熔岩组成,可夹有薄层爆发相火山碎屑岩;层火山机构,由互层的熔岩与火山碎屑岩组成,喷溢相与爆发相交替的序列明显;火山碎屑锥,几乎全部由火山碎屑（熔）岩组成,爆发相为主;熔岩穹丘由高粘度的流纹质、英安质熔岩堵塞火山口后缓慢挤出形成,喷溢相和侵出相发育,兼有火山通道相。盆地内埋藏火山机构最小坡度为3°,最大坡度为25°,底部直径为2～14 km,分布面积为4～50 km²,火山岩厚度为100～600 m;总体上呈现出数目多、个体规模小、受区域大断裂控制、具裂隙式-多中心喷发、彼此相互叠置的特征。火山岩岩性和岩相是控制松辽盆地古火山机构类型及形态的主要因素。