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Two meter-scale pyroclastic levels are interbedded within the early Miocene succession of the Estancia 25 de Mayo (Patagoniense transgression) and Santa Cruz formations in the foreland Austral (or Magallanes) Basin, Argentina. The Lower Pyroclastic Level (LPL) is a tabular body interbedded within offshore marine deposits, laterally continuous for 30 km and varying in thickness from few centimeters to around 4 m. Grain-size grades from coarse to extremely fine ash with upward-fining along with a northeastern-fining trends. Structureless fine to very fine tuffs dominate and rare parallel laminations are the only tractive sedimentary structures. The Upper Pyroclastic Level (UPL) lies within low energy fluvial deposits and is laterally discontinuous, and it is composed by lenticular bodies reaching a maximum of 15 m thick and 100 m wide, with a concave-up base and a plane top. Grain-size range is similar to the LPL but it coarsens upward. The lower portion of the UPL shows parallel lamination, current ripple lamination and mud drapes with large pumice lapilli and plant debris, whereas the upper portion shows parallel lamination and trough cross-stratification. Both pyroclastic levels are composed mainly of pumice grains and glass shards with minor proportions of quartz and plagioclase crystals and lithic fragments. The LPL shows no mixing with epiclastic material whereas the UPL shows an upward increase in epiclastic material, and an upward increment in the scale of cross-bedding. The large thickness in relation to the possible emission center and the content of plant debris of the LPL does not suggest a direct, submarine, ash-fallout origin. The LPL is interpreted as a deposit of hyperpycnal-flows generated at the coastal zone when tephra-laden rivers plunged into the ocean. Large amounts of well preserved plant debris support the hypothesis of a terrestrial source of the sediments. The UPL is entirely composed of tractive deposits, so an ash fallout origin is disregarded. This, together with the lenticular shape and the alluvial plain origin of the encasing sediments, suggests accumulation within fluvial channels. Cycles of upper-flow-regime parallel lamination, current-ripple lamination and mud drapes at the lower portion, suggest short-lived turbulent flows that initially filled semi-abandoned channels. They were followed by sheet floods and channel reactivation, expressed by large-scale cross-bedding. The low degree of particle mixing observed in both levels is explained by the inability of streams to erode the substrate as they are suddenly over-saturated with pyroclastic sediments during and after the eruption. The grain-size distribution of the LPL and geochemical data indicate a contemporaneous volcanic source located to the west/southwest in the Andean ranges, where the South Patagonian Batholith is presently located. Fil: Cuitiño, José Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Scasso, Roberto Adrian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; Argentina

article i nfo Article history: Sedimentation and welding processes of the high temperature dilute pyroclastic densitycurrents and fallout erupted at 7.3 ka from the Kikai caldera are discussed based on the stratigraphy, texture, lithofacies characteristics, and components of the resulting deposits. The welded eruptive deposits, Unit B, were produced during the column collapse phase, following a large plinian eruption and preceding an ignimbrite eruption, and can be divided into two subunits, Units Bl and Bu. Unit Bl is primarily deposited in topographic depressions on proximal islands, and consists of multiple thin (b 1m )flow units with stratified and cross- stratified facies with various degrees of welding. Each thin unit appears as a single aggradational unit, composed of a lower lithic-rich layer or pod and an upper welded pumice-rich layer. Lithic-rich parts are fines-depleted and are composed of altered country rock, fresh andesite lava, obsidian clasts with chilled margins, and boulders. The overlying Unit Bu shows densely welded stratified facies, composed of alter- nating lithic-rich and pumice-rich layers. The layers mantle lower units and are sometimes viscously deformed by ballistics. The sedimentarycharacteristics of Unit Bl such as welded stratified orcross-stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma-water interactions. It is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot, lighter pumice-rich particles, and that this process repeated many times. It is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption, through surge-generating ex- plosions(UnitBl),intoanagglutinate-dominatedfallouteruption(UnitBu).Onthebasisof fielddata,welded

The Cretaceous Kusandong Tuff, Korea, is a thin (1–5 m thick) but laterally extensive (~ 200 km) silicic ignimbrite emplaced in a fluviolacustrine basin adjacent to a continental volcanic arc. The tuff has been used as an excellent key bed because of its great lateral continuity and unique lithology, characterized by the virtual absence of juvenile clasts and an abundance of quartz and feldspar crystals (up to 55–73 vol.%). The tuff is mostly massive and ungraded and locally shows crude internal layering, basal inverse grading and near-top normal grading of crystals, either erosional or non-erosional lower surfaces, and flat-lying to imbricated grain fabrics. Fragile intraformational clasts of mudstone and tuff are also included. These features provide only ambiguous information on the properties of the responsible pyroclastic density currents: i.e. whether they were dense and laminar or dilute and turbulent. The overall lateral continuity and sheet-like geometry of the tuff suggests, however, that the transport system of the currents was highly expanded, dilute, and turbulent. A plug-flow or slab-flow model cannot explain the origin of crude internal layering, imbricated grain fabrics, and the high crystal content, which is most likely the result of vigorous sorting processes within a dilute and turbulent current. Features indicative of deposition from a dense and laminar transporting medium are locally present, suggesting that a dense and laminar depositional system could develop locally at the base of the dilute and turbulent transport system. The virtual absence of juvenile clasts in the tuff is interpreted to be due to rapid ascent, sudden decompression, and full fragmentation of silicic magma into fine glass shards and crystals. Scarcity of basement-derived accidental components together with the absence of pumiceous fallout deposits beneath the tuff is interpreted to be due to shallow-level fragmentation of magma followed by immediate generation of pyroclastic density currents from shallow-level blasts at the onset of eruption. The eruption occurred through multiple vent sites in a short period of time, producing a seemingly single but actually composite ignimbrite unit. Such an eruption was probably possible because of a regional tectonic event within the basin or in its vicinity. It is proposed that a composite ignimbrite with the characteristics of the Kusandong Tuff can be an exemplary product of syntectonic volcanism that can provide an insight into the interpretation of structural and stratigraphic evolution of a sedimentary basin.

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.

Large-scale ignimbrite eruptions from rhyolitic caldera volcanoes can trigger geologically instantaneous changes in sedimentary systems over huge areas by either burying existing environments or overloading them with vast quantities of unconsolidated particulate material. The post-eruption readjustment of the landscape to such perturbations is one of the most dramatic processes in physical sedimentology, exemplified here by the 1.8 ka Taupo eruption in the central North Island of New Zealand. This eruption generated voluminous fall deposits, then climaxed with emplacement of a c. 30 km 3 non-welded ignimbrite over a near-circular area of c. 20 000 km 2 . Approximately 90% of the area, but The headwaters of many of the North Island's largest rivers were impacted by both the primary pyroclastic fall and flow material. Large-scale post-eruption remobilisation of this material, coupled with the re-establishment of fluvial systems, occurred in a distinct sequence as recorded by the evolution of sedimentary facies in different sub-environments. Following an initial period dominated by mass flows, re-establishment of fluvial systems began with the headward erosion of box canyons through the ponded ignimbrite deposits, a process often associated with the break-out of temporary lakes. Aggradational streams developed in these channels rapidly evolved from shallow, ephemeral, sediment-laden outbursts associated with flash flood events to deeper, permanent braided rivers, before declining sediment yields led to retrenchment of single thread rivers and a return to pre-eruption gradients and bedloads years to decades later. Typically the modern profile of many streams and rivers follow closely their pre-eruption profiles, and incision and erosion is overwhelmingly confined to the deposits of the eruption itself. Although the general remobilisation pattern is similar for all impacted river systems, detailed studies of the Waikato, Rangitaiki, Mohaka, Ngaruroro and Whanganui catchments show that the relative timing and scale of each eruption response phase differs between each catchment. These reflect differences in catchment physiography and hydrology, and the volume and type of pyroclastic material deposited in each. Ultimately, the landscape response reflects the relative spatial distributions of, and the volumetric ratios between, the volumes of pyroclastic debris, water, and accommodation space in the basin (cf. Kataoka and Manville, this volume).

The Miocene Namurungule Formation crops out on the eastern flank of the Kenya Rift, representing basin fill that developed in association with Miocene rifting. The formation is characterized by large volumes of volcaniclastics supplied passively by pyroclastic fall. Facies analysis reveals that this formation consists mostly of lacustrine delta deposits with minor alluvial fan deposits at its base. The conspicuous occurrence of a flood plain facies with infrequent channel fill deposits in the lower part of this formation suggest that the drainage area was limited in the early stage of deposition. Pyroclastic fall would therefore have been an important source of sediment during the early stages of rift development. The delta deposits are divided into two distinct successions based on lithological characteristics, separated by a thick pyroclastic layer in the middle part of the formation. The stacking pattern of the lower succession is retrogradational, whereas the upper succession is characterized by a pile of prograding bodies. Both delta successions are interpreted to have accumulated in an underfilled basin. The change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake-level rise. Assuming constant sediment supply, the apparent difference in flooding scale between the lower and upper successions is attributed to the topographical widening of the basin flat. Similar successions also occur in another basin in the East African Rift (Ngorora Formation, central Kenya), in which pyroclastic sediments are dominant and upward decrease in sedimentation rate is recognized. The differing in the stacking patterns between the Namurungule Formation and the Ngorora formation is probably induced by basin width. The narrow Namurungule basin seems to have been sensitive to increased sediment supply owing to the expansion of the catchment area.

A multidisciplinary study, including stratigraphic, sedimentological, biostratigraphic, petrographic, magnetic fabric and SEM analyses, has been performed on six volcaniclastic layers (VLs) interbedded in the Burdigalian shelf succession of the Monferrato (NW Italy). The aim was to distinguish between the volcanic and sedimentary processes that produced these deposits, to suggest a depositional model for volcaniclastic sedimentation in a shelf environment and to discuss the use of VLs for stratigraphic correlations. Two kinds of VLs have been distinguished: single volcaniclastic layers (SVLs) and multiple volcaniclastic layers (MVLs). SVLs are single beds of well sorted vitric siltites, mainly consisting of volcanic components and minor terrigenous and intrabasinal grains; the vitric fraction mainly consists of blocky fragments. They show a very low magnetic anisotropy degree and a prominent magnetic lineation. These VLs are interbedded in outer shelf marls and are interpreted as primary pyroclastic fall deposits. MVLs, which can be up to 10 m thick, show limited lateral continuity and are made up of several-decimetre-thick graded beds, separated by erosional surfaces and consisting of vitric arenites and siltites with about 15% non-volcanic components. Two kinds of MVLs have been distinguished: (1) Type 1 MVLs, interbedded in storm-dominated glaucony-rich calcarenites and showing rough, low-angle cross-stratification (hummocky cross stratification), water escape and load structures. These deposits are characterized by a slightly foliated magnetic fabric and are interpreted as storm layers, deposited between fairweather and storm wave base. (2) Type 2 MVLs are interbedded in outer shelf marls, and are characterized by parallel lamination and by a well developed magnetic foliation. They are interpreted as storm-induced, distal shelf turbidites, triggered by storm activity acting in the more internal part of the shelf. The Monferrato VLs resulted from explosive eruptions of volcanic edifices, located outside of the basin, that produced an extensive tuff blanket that was uniformly distributed on a carbonate-dominated shelf. Above storm wave base the VLs were immediately reworked by storm activity, and the resulting deposits are type 1 MVLs. Below storm wave base, primary pyroclastic fall deposits were preserved, corresponding to SVLs. Storm-induced turbidity currents gave rise to type 2 MVLs, that were deposited below storm wave base. Preservation of VLs in a shelf environment is hampered by the high-energy conditions of the shelf. Consequently, these deposits are characterized by a restricted lateral continuity and their use as correlation tools may be misleading.

The Mid-Miocene to Early Pliocene (14-3 Ma) Miura Group exposed on the Miura and Boso peninsulas around Tokyo Bay in southern Honshu comprises at least a 2 km thick succession of volcaniclastic sediments. These accumulated in a forearc basin or basins on the eastern side of a proto Izu-Bonin arc prior to and during accretion of the Miura block onto the Honshu arc and its subsequent dextral transcurrent eduction. The basin fill shows a complex admixture of facies that result from multiple processes, which operated on a relatively shallow-slope basin. The main facies groups include dark-coloured, mostly coarse-grained scoriaceous beds; pale-coloured, muddy-silty pumiceous bioturbated sediment; yellowish and whitish-coloured tuffaceous horizons; and chaotic slump, debris flow and injection units. Many of the facies and beds in the older Misaki Formation can be readily interpreted as the result of: (a) direct pyroclastic fall through the air and water; (b) downslope resedimentation via turbidity currents and related processes, in some cases derived from subaqueous base-surge pyroclastic flow or vertical density currents; (c) hemipelagic settling, commonly under the influence of thermohaline bottom currents. In other cases, composite beds are believed to result from the interaction of these processes in more complex events. The younger Hasse Formation is dominated by more shallow-water, marine-currents and storm events and in parts, by fluvial or proximal fan-delta processes. The distribution of these different facies illustrates the evolution and shallowing of the Miura-Boso forearc basins during accretion and emplacement.

A change from primary to reworked volcaniclastic deposits in the Lower Pliocene Ohta tephra bed has been identified, using facies analysis. The Ohta tephra bed is a widespread marker-tephra, distributed over a distance of more than 130 km. The source volcano of this tephra is inferred to lie 50 to 100 km north of the most proximal Ohta tephra bed. The Ohta tephra bed shows a change in volcaniclastic facies in distal areas. Eight sedimentary facies are recognized. These are interpreted as pyroclastic fall, pyroclastic flow, and reworked (mudflow, flood flow, channel fill, swamp or backswamp, lacustrine and ephemeral pond) deposits. In underlying and overlying non-volcaniclastic beds, four sedimentary facies are recognized, indicating a fluvio (-lacustrine) depositional system on an alluvial plain. From the successions of volcaniclastic sedimentary facies, deposition of the Ohta tephra bed can be divided into stages I to VI. Each depositional stage is intimately related to volcanic activity. Stage I is the first eruption, during which only the northern part of the study area was covered with fallout ash. The second eruption stage of stage II involved fallout ash distributed over all the study area. These two fallout ash beds are derived from precursor eruptions. Stage III reflects a period of volcanic quiescence and development of widespread lacustrine conditions. The fallout ash may have destroyed vegetation, and prompted greater discharge to form the large lacustrine deposits of stage III. A pyroclastic flow stage in stage IV is associated with an explosive eruption, and resulted in pyroclastic flow deposits reaching the northern part of the study area. Stage V is characterized by mudflow deposits. Widespread mudflow deposits also originated from high water and clastics discharge on the unvegetated surface. Mudflows extend more than 180 km from the source volcano. Abundant supply of volcaniclastic material and high water and clastic discharge enhanced a rapid progradation of alluvial plains. Final reworking occurred during stage VI, when volcaniclastic sediments were deposited from a fluvio-lacustrine system. The reworking system comprises initial mudflows, followed by a normal fluvial system (normal stream flow). The mudflow deposits of stage V are traceable with roughly constant thickness, but the volcaniclastic content of the deposits decreases in distal areas. The fluvial system deposits of stage VI are thicker and have less abundant volcaniclastics in distal areas. Fluvial systems were braided streams promoted by volcanic eruption, while they were meandering streams during the pre-eruption period. Some eruption-induced braided streams remained for a while following the cessation of volcanic activity.

Fine- to very fine-grained, well sorted, distal rhyolitic tephras of Quaternary age, are widely preserved in the Auckland region, New Zealand. Deposits are thick, up to 15 m, with respect to grain size and distance, approximately 200 km, from the probable source areas in the Taupo Volcanic Zone. Constituent grains are dominated by vesicular pumice, bubble wall shards and equant to blocky glass grains, reflecting the earlier fractionation of more dense lithic and crystal grains during transportation. Glass geochemistry and grain morphology indicate that at least two, and possibly more eruptive types are represented in the deposits. There are five possible mechanisms for introduction of these materials to the Auckland area: (1) direct plinian fall forming a mantling blanket layer of ash over a large part of the Auckland area; (2) direct distal pyroclastic flows forming valley filling deposits which may not retain the characteristics of pyroclastic flows observed in more proximal settings; (3) co-ignimbrite ash associated with distal pyroclastic flows forming localised, extremely fine-grained mantling tephra layers, which may be displaced from primary flow deposits; (4) a buoyant ash cloud at the distal margins of a pyroclastic flow forming a mantling ash layer; (5) co-ignimbrite ash clouds generated by the entry of distal pyroclastic flows into Auckland's harbours, forming relatively localised mantling ash layers. Reworking of the deposits inhibits correlation with proximal tephras and obscures evidence for evaluation of the modes of transportation and deposition.