subaqueous explosive eruption and welding of pyroclastic deposits silicic tuffs infilling an ancient submarine caldera at mineral king in california show microscopic fabrics indicative of welding of glass shards and pumice at temperatures 500 degrees c the occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water which would cause chilling intracaldera progressive aggradation of pumice and ash from a thick fast moving pyroclastic flow occurred during a short lived explosive eruption of approximately 26 cubic kilometers of magma in water 150 meters deep the thickness high velocity and abundant fine material of the erupted gas solids mixture prevented substantial incorporation of ambient water into the flow stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general both above the vent and along any flow path may be the main process giving rise to buoyant convective subaqueous eruption columns and attendant fallout deposits

little ice age aridity in the north american great plains a high resolution reconstruction of salinity fluctuations from devils lake north dakota usa the little ice age was an interval between about ad 1500 and 1850 characterized by advancing glaciers in mountainous regions of europe and western north america however it is unclear whether this cool moist period was truly global in extent or how it was manifested in other regions with different climatic controls a high resolution reconstruction of salinity fluctuations in devils lake north dakota based on fossil diatoms ostracode shell geochemistry and bulk carbonate geochemistry indicates that saline conditions prevailed throughout much of the recent past these results suggest an arid climate in the northern great plains throughout the little ice age and that during this interval climatic gradients between the great plains and regions to both the east and west may have been quite steep

the late holocene history of prairie brush prairie and jack pine pinus banksiana forest on outwash plains north central minnesota usa pollen diagrams from six sites on outwash plains of north central minnesota are interpreted as records of similar extralocal vegetation changes since c 5000 bp all of the study sites have undergone a shift from prairie to an aspen oak community followed by the jack pine dominated vegetation that now occupies the sites all of the species capable of directly invading prairie aspen bur oak willow and some entomophilous shrubs were probably established on the surrounding moraines prior to their invasion of the adjacent prairie occupied outwash plains only short distances 20 km were involved in these local invasions and the chronology of their establishment does not indicate a regional westward migration of these taxa in the late holocene because climate varies little over the short distances separating the study sites the asynchroneity of the dates for the establishment of aspen brush zone 3 zone 4 boundary and the following establishment of jack pine zone 2 zone 3 boundary indicates that locally varying conditions determined where and when these vegetational shifts occurred sloughs and wet swales are apparently prerequisite for aspen to invade prairie and they probably appeared on the outwash plains at different times because locally varying hydrologic conditions determined the absolute increase in precipitation required for the water table to rise and flood depressions the rate at which afforestation proceeded culminating with the establishment of jack pine appears to be related to local variability in the distribution of wet depressions as they serve as firebreaks where peat filled channels and chains of lakes occur afforestation occurred first and was relatively rapid where lakes and peatlands are scattered afforestation occurred later and required more time where wet depressions are rare some patches of prairie have persisted since the mid holocene

the saturation behaviour of pumice and some sedimentological implications abstract caldera forming explosive eruptions from rhyolitic volcanoes typically produce huge quantities of pyroclastic material blanketing the landscape and triggering long term voluminous and widespread sedimentary responses pumice is a major component of such eruptions but its behaviour in the sedimentary environment is little understood its most notable feature is its very low density which frequently enables pumice to float on water for long periods of time before eventually waterlogging enough to sink once fully saturated pumice behaves in a broadly similar way to normal clastic material in terms of its hydrodynamic behaviour modified by its lower density however relatively little is known about its earlier behaviour in the sedimentary environment and in particular the manner and rate by which initially dry pumice absorbs water in this paper we develop a mathematical model which predicts that the time required for a pumice clast to saturate is proportional to the square of its radius tests on samples of real pumice accord with predicted trends and match lacustrine pumiceous facies observed in the field the ability to predict the time required for large pumice clasts to sink may provide a tool for estimating the timescales over which certain rapid post eruptive sedimentary processes operate

holocene climatic change in the northern midwest pollen derived estimates mapping of holocene pollen data in the midwestern united states has revealed several broadscale vegetational changes that can be interpreted in climatic terms these changes include 1 the early holocene northward movement of the spruce dominated forest and its later southward movement after 3000 yr b p and 2 the eastward movement of the prairie forest border into southwestern wisconsin by 8000 yr b p and its subsequent westward retreat after 6000 yr b p when certain basic assumptions are met multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation to maximize the reliability of the regression equations we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis reconstructions of precipitation during the holocene indicated that from 9000 to 6000 yr b p precipitation decreased by 10 to 25 over much of the midwest while mean july temperature increased by 0 5 to 2 0 c at 6000 yr b p precipitation was less than 80 of its modern values over parts of wisconsin and minnesota after 6000 yr b p precipitation generally increased while mean july temperature decreased in the north and increased in the south the time of the maximum temperature varies within the midwest and is earlier in the north and later in the south

high precision pb isotope measurements reveal magma recharge as a mechanism for ore deposit formation examples from lihir island and conical seamount papua new guinea abstract lihir island part of the tabar lihir tanga feni island chain in the sw pacific hosts the giant ladolam gold deposit polymetallic gold mineralization also was discovered offshore at conical seamount in 1998 during rv sonne cruise so 133 the seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites although some monzonites are found on lihir sr isotopic analyses of mineralized samples from the island and the seamount suggest that most of the sr is derived from the local alkaline lavas elevated 87sr 86sr ratios in some of the samples suggest that during the waning stages of the hydrothermal system part of the sr was contributed either from seawater or the thick sequence of marine sediments underlying the island high precision pb isotopic analyses conducted with mc icp ms show that the ores and volcanic rocks share similar pb isotopic compositions suggesting that the pb in the mineralized zones was ultimately derived from local magmatic sources the pb isotopic data however reveal small but significant differences between the mineralized zones and the associated host lavas mineralized samples from lihir have slightly less radiogenic lead isotopic ratios than their host lavas these lead isotopic compositions are similar however to some of the fresh lavas recovered from conical seamount and to a monzonite intrusion underlying the ladolam deposit lead isotopic ratios in mineralized samples from conical seamount are slightly more radiogenic than their host lavas and similar to those of fresh lavas recovered from nearby tubaf and edison seamounts petrographic observations and microprobe elemental analyses reveal a complex magmatic history for the magma chamber inferred beneath conical seamount based on zoning patterns in the conical seamount clinopyroxenes it appears that a sub seamount magma chamber was recharged with a mafic magma similar to the most primitive volatile rich and xenolith bearing lavas recovered from tubaf and edison seamounts rapid cooling of this mafic magma accompanied by exsolution of metal bearing fluids in the relatively shallow magma chamber is proposed as the mechanism that ultimately led to the formation of the gold mineralization similarity of the lead isotopic ratios between the ores and the mafic magmas in the area indicate that the ore metals were primarily derived from the recharging magma not by hydrothermal leaching of the host lavas suggesting that ore deposit genesis can be related to a single volatile rich magmatic pulse within otherwise barren polyphased volcano magmatic complex

u pb dating of zircon in subsurface hydrothermally altered pyroclastic deposits and implications for subsidence in a magmatically active rift taupo volcanic zone new zealand abstract recognising and correlating hydrothermally altered rock units within buried volcanic sequences in the taupo volcanic zone tvz in new zealand is difficult this is because of broad similarities in the lithologies of many major ignimbrite units and the destruction by hydrothermal alteration of distinctive chemical and mineralogical characteristics however magmatic zircons are commonly present are highly resistant to hydrothermal alteration and yield crystallisation ages in intensely altered rocks crystallisation age spectra have been obtained by sims techniques shrimp rg on zircons extracted from cores from altered ignimbrites penetrated by drillholes at the waiotapu te kopia and orakei korako geothermal fields in the central tvz at waiotapu the thick up to 350 m densely welded waiotapu ignimbrite returned a zircon age spectrum with a probability density function pdf peak of 0 79 ma consistent with an eruption age from 40ar 39ar techniques of 0 71 0 06 1 s d ma three older ignimbrite sheets yielded age spectra that were consistent stratigraphically the shallowest of the three yielded sparse zircons that gave a pdf peak of 1 24 ma and it may correlate with the 1 18 0 02 ma ahuroa ignimbrite the middle sheet although 220 m thick yielded an age spectrum identical to that obtained from pumice in the widespread 1 21 0 04 ma ongatiti ignimbrite extending earlier estimates of the likely volume of this large deposit the deepest sheet has a spectrum consistent with an eruption age of 1 45 0 05 ma it has no surficial correlative but its likely coeruptive ash forms part of a concentrated group of primary or secondary tephra in sediments on the ocean floor east of new zealand and in sedimentary basins across the north island these three ignimbrites were previously correlated with either major ignimbrites exposed on the paeroa fault scarp 10 km to the west or the akatarewa ignimbrite that occurs in drillholes at te kopia and orakei korako but these correlations are disproved from our age data drillholes at te kopia tk and orakei korako ok reach but do not penetrate the base of one or more deposits collectively termed the akatarewa ignimbrite samples from 62 tk and 165 m ok below the top contact of this unit yielded closely similar age spectra and pdf maxima of 1 00 tk and 1 02 ma ok implying that the same deposit was penetrated at both fields this ignimbrite is not correlated with a specific surface equivalent several candidate deposits exist but all are areally restricted the akatarewa ignimbrite at tk and ok is however not the same deposit as those earlier correlated with it at waiotapu the age data are consistent with there being two zones of subsidence on both sides of a horst aligned with the three geothermal fields sampled the eastern zone is the reporoa basin which has high heat flow but little surface faulting and the western one is the axis of the taupo rift currently with low heat flow but abundant surface faulting overall subsidence rates at the three geothermal fields studied vary between 2 and

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

subaqueous rhyolitic dome top tuff cones a model based on the devonian bunga beds southeastern australia and a modern analogue the relics of two late devonian subaqueous rhyolitic dome top tuff and pumice cone successions are preserved in the bunga beds outlier of the boyd volcanic complex southeastern australia these cone successions and other rhyolitic volcanics of the bunga beds are associated with turbidite and other deep water massflow sedimentary rocks the two cone successions have a generally similar stratigraphy at the base flow banded variably autobrecciated and quench fragmented rhyolite representing an intrusive to extrusive dome is overlain by rhyolitesediment breccia representing extrusion of the dome through the deep water sediment pile and resedimentation down its flanks in the northern cone succession an overlying succession of bedded pumiceous crystal rich to crystal poor tuffs represents the onset of pyroclastic activity and growth of a tuff cone an overyling debris flow deposit represents degradation of part of the cone the topmost unit a stratified pumice succession is thought to represent another cone building eruptive phase and is separated from the underlying strata by a major slide surface the southern cone succession contains less tuff and abundant pumice and is also terminated by a debris flow deposit indicating cone degradation a modern analogue for the inferred eruptive style and sequence is the 1953 1957 rhyolite eruption that formed the tuluman island lava tuff cone complex in the bismarck sea the eruptions were often cyclical consisting of an initial inferred submarine lava forming stage passing into a pumicecone forming stage in some cases a subaeriallava forming stage and a final stage following the cessation of volcanism during which the cones collapsed gravitationally or were destroyed by wave erosion using observations from both the tuluman island eruptions and the preserved stratigraphies of the devonian tuff cones a dynamic model is proposed for the formation of subaqueous rhyolitic dome top tuff and pumice cones

evolution of the taupo hikurangi subduction system abstract the present day taupo hikurangi subduction system is a southward extension of the tonga kermadec arc system into a sediment rich continental margin environment it consists of a shallow structural trench the hikurangi trough a 150 km wide imbricate thrust controlled accretionary borderland the continental slope shelf and coastal hills of eastern north island a frontal ridge the main greywacke ranges of north island and a volcanic arc and marginal basin the taupo volcanic zone structural elements become progressively more elevated and subduction more oblique towards the south the whole nne trending system is truncated at a largely strike slip transform boundary that extends along the southwestern part of the hikurangi trough and the hope fault of south island to the main alpine fault the volcanic arc is 200 270 km from the structural trench and comprises a nne trending chain of andesite dacite volcanoes extending along the eastern side of the taupo volcanic zone most of the andesites are olivine bearing and have been erupted within the last 50 000 years it is suggested the taupo hikurangi margin has evolved by rotation of accretionary elements from an original nw trending subduction system north of new zealand the older elements of the prism were associated with subduction of a re entrant of the pacific plate and perhaps the south fiji basin in mid tertiary times they subsequently became separated from their nw trending volcanic arc by dextral strike slip movement along curved faults east of the main greywacke ranges during the plio pleistocene oblique subduction and accretion intensified as the taupo hikurangi margin rotated into line with the nne trending kermadec system and a marginal basin was developed along a similar trend to form the taupo volcanic zone within the last 50 000 years olivine bearing andesite volcanism has commenced along the eastern side of the taupo volcanic zone

reconciling pyroclastic flow and surge the multiphase physics of pyroclastic density currents abstract two end member types of pyroclastic density current are commonly recognized pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows we provide scaling relations that unify these end members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the stokes number s t the stability factor t and the dense dilute condition d d we recognize five types of particle behaviors within a fluid eddy as a function of s t and t 1 particles sediment from the eddy 2 particles are preferentially settled out during the downward motion of the eddy but can be carried during its upward motion 3 particles concentrate on the periphery of the eddy 4 particles settling can be delayed or fast tracked as a function of the eddy spatial distribution and 5 particles remain homogeneously distributed within the eddy we extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current this stratification may favor particle interactions in the basal part of the flow and d d determines whether the flow is dense or dilute using only intrinsic characteristics of the current our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents

mihi breccia a stack of lacustrine sediments and subaqueous pyroclastic flows within the taupo volcanic zone new zealand abstract the taupo volcanic zone tvz new zealand encompasses a wide variety of arc related strata although most of its small volume non caldera forming eruptions are poorly exposed and extensively hydrothermally altered the mihi breccia is a stratigraphic sequence consisting of interbedded rhyolitic pyroclastic flows and lacustrine sediments with eruption ages of 281 18 to at least 239 6 ka uncertainties at 2 in contrast to other small volume rhyolitic eruptions within the tvz mihi breccia is relatively well exposed within the paeroa fault block and contains minimal hydrothermal alteration pyroclastic flow characteristics and textures include 1 prismatically jointed juvenile clasts 2 lack of welding 3 abundant ash rich matrix 4 lack of fiamme and eutaxitic textures 5 lack of thermal oxidation colors 6 lack of cooling joints 7 exclusive lacustrine sediment lithic clasts and 8 interbedding with lacustrine sediments all indicating that mihi breccia strata originated in a paleo lake system this ephemeral paleo lake system is inferred to have lasted for 50 kyr based on mihi breccia age constraints and referred to as huka lake mihi breccia pyroclastic flow juvenile clast geochemistry and petrography correspond with similar aged 264 8 263 10 and 247 4 ka intra caldera rhyolite domes filling the reporoa caldera source of the 281 ka kaingaroa formation ignimbrite these exposed intra caldera rhyolite domes as well as geophysically inferred subsurface domes are proposed to be source vents for the mihi breccia pyroclastic flows soft sediment deformation associated with mihi breccia strata indicates either seismic shock rapid sediment loading during pyroclastic flow emplacement or both thus the mihi breccia reflects a prolonged series of subaqueous rhyolite dome building and associated pyroclastic flows accompanied by seismic activity emplaced into a large paleo lake system within the tvz

late holocene history of waldsea lake saskatchewan canada abstract the post hypsithermal history of waldsea lake a saline meromictic lake located in south central saskatchewan has been deduced from a study of the changes in physical mineralogical and paleobiological parameters in sediment cores from the basin six lithostratigraphic units and three palynological zones are identified in the most recent sediment these units and zones indicate that a shallow hypersaline lake with extensive mudflats existed about 4000 yr b p in response to the subsequent trend toward a cooler and wetter climate deeper water conditions ensued and by about 3000 yr ago a relatively deep stratified lake occupied the waldsea basin a short climatic reversal about 2500 yr b p again caused low water and mudflat conditions but by 2000 yr ago the lake had regained its higher levels the past 2000 yr of waldsea s history have been relatively uneventful except for a minor lowering of the lake about 700 yr b p

the use of total lake surface area as an indicator of climatic change examples from the lahontan basin abstract variation in the size of lakes in the lahontan basin is topographically constrained river diversion also has played a major role in regulating lake size in lahontan subbasins the proper gage of lake response to change in the hydrologic balance is neither lake depth level nor lake volume but instead lake surface area normalization of surface area is necessary when comparing surface areas of lakes in basins having different topographies to a first approximation normalization can be accomplished by dividing the paleosurface area of a lake by its mean historical reconstructed surface area

explosive destruction of a pliocene hot lava dome underwater dogashima japan transition from effusive to explosive volcanism is common during subaerial eruptions and here we demonstrate that this behavior is also possible underwater the pyroclastic facies produced underwater are distinctive and can be used to distinguish subaqueous from subaerial eruptions and depositional settings the pliocene dogashima formation izu peninsula japan is a pumice rich succession originally deposited in an open marine below wave base setting jutzeler et al 2014a a thick clast supported gray andesite breccia composed of very coarse dense andesite clasts with quenched margins was sourced from disintegration of an active lava dome overall the gray andesite breccia is gradationally to sharply overlain by thick graded clast supported white pumice breccia chiefly composed of angular pumice clasts and free broken crystals regional setting and distinctive facies show that this succession was produced by a fully underwater magmatic volatile driven pumice forming explosive eruption the gradational contact between the two breccias compositional similarities rare mingled clasts and fluidal textures in the gray andesite clasts suggest that the explosive eruption destroyed a hot lava dome and generated an eruption fed high concentration density current in most places the coarsest hot lava dome fragments were deposited first followed by the lower density white pumice clasts the low amount of fine 2 mm components well developed hydraulically controlled grading and sorting clast angularity and very coarse dome derived clasts some including well defined quenched margins and common fluidal textures distinguish the products of subaqueous effusive to explosive eruptions from their subaerial counterparts

fallout of pyroclastic debris from submarine volcanic eruptions volcanic fallout deposits on land being widespread and accessible for study have received much attention and have revealed a great deal about subaerial eruption mechanisms in contrast virtually nothing is known about equivalent deposits produced by submarine volcanoes despite the probable abundance of such material in today s oceans and in accreted volcanic arc terrains many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns experiments on water saturated pumice and pieces of rock lithics show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters pieces of pumice may be five to ten times as large as codeposited lithic fragments similar material erupted into the air and deposited on land displays less well developed bimodality pumice diameters are generally two to three times as large as associated lithics submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain

the behaviour of subaerially produced pyroclastic flows in a subaqueous environment evidence from the roseau eruption dominica west indies abstract the roseau tephra 30 000 yrs b p with its source on the island of dominica represents the largest eruption in the caribbean in the last 200 000 years pyroclastic flows generated by the eruption flowed offshore into the grenada basin giving rise to a widespread layer of sediment gravity flow deposits on land the flow deposits are valley ponded and their granulometric characteristics allow them to be classified as weakly fines depleted ignimbrites offshore the characteristics of the sediment gravity flow deposits are consistent with deposition by numerous small scale events rather than a single large scale event a comparison of the on land and offshore deposits reveals an increase in the crystal lithic ratio in the latter this change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water mixing and fragmentation occurred not at the air water interface but as flows descended the steep submarine slopes of the island of dominica

characteristics of submarine pumice rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline field and experimental assessment this study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the znp marine tephra in japan and flume experiments where hot tephra sample interacted with water the znp is a very thick pumice rich density current deposit that was sourced from subaerial pyroclastic flows entering the japan sea in the pliocene notable characteristics are well developed grain size and density grading lithic rich base pumice rich middle and ash rich top preponderance of sedimentary lithic clasts picked up from the seafloor during transport fine ash depletion in coarse facies and presence of curviplanar pumice clasts flume experiments provide a framework for interpreting the origin and proximity to source of the znp tephra on contact of hot tephra sample with water steam explosions produced a gas supported pyroclastic density current that advanced over the water while a water supported density current was produced on the tank floor from the base of a turbulent mixing zone experimental deposits comprise proximal lithic breccia medial pumice breccia and distal fine ash experiments undertaken with cold water saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia results suggest the characteristics and variations in znp facies were strongly controlled by turbulent mixing and quenching proximity to the shoreline and depositional setting within the basin presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching subsequent transport in water supported pumiceous density currents preserves the fragmental textures careful study is needed to distinguish the products of subaerial versus subaqueous eruptions

century scale paleoclimatic reconstruction from moon lake a closed basin lake in the northern great plains estimates of past lake water salinity from fossil diatom assemblages were used to infer past climatic conditions at moon lake a climatically sensitive site in the northern great plains a good correspondence between diatom inferred salinity and historical records of mean annual precipitation minus evapotranspiration p et strongly suggests that the sedimentary record from moon lake can be used to reconstruct past climatic conditions century scale analysis of the holocene diatom record indicates four major hydrological periods an early holocene transition from an open freshwater system to a closed saline system by 7300 b p which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate a mid holocene period of high salinity from 7300 to 4700 b p indicating low effective moisture p et a transitional period of high salinity from 4700 to 2200 b p characterized by poor diatom preservation and a late holocene period of variable lower salinity during the past 2 200 yr indicating fluctuations in effective moisture

seismites an attempt at critical analysis and classification abstract the main types of seismites are reviewed following the proposed classification and are illustrated by our own case studies the large variety of seismites depends on the sedimentologic hydrodynamic and diagenetic characters of the deposits subject to seismic shocks on a given site generally different types of seismites cane be related to the same seismic event these peculiar but widespread sedimentary structures are not always univocal and may often be related to different causes therefore the structures discussed must necessarily be placed in a detailed geological context

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

marine emplacement of welded ignimbrite the ordovician pitts head tuff north wales the pitts head welded ignimbrite records a subaerial large volume eruption and associated pyroclastic current that flowed into the sea the current 15 km from source was steadily sustained with high mass flux and high particle concentration towards its base and it entered the sea without substantial mixing with water and thus without large scale hydroexplosivity and without general cooling it continued to aggrade ignimbrite at 580 c for at least 3 4 km from the original shoreline in water initially 50 m deep entirely subaqueous hot state progressive aggradation and welding of the ignimbrite occurred where the water could not be wholly displaced by the current although eventually the deposit displaced the shore 4 km seawards wet sea floor sediments buried by the ignimbrite were heated and locally fluidized by steam and several square kilometres of hot ignimbrite with variable thicknesses of sedimentary substrate detached and slid downslope directions of sliding and the order of piling up of slide sheets are shown by hot state rheomorphic deformation fabrics and the geometric relations of detachment surfaces extensional disruption of the ignimbrite is marked by breaks in the sheet via which fluidized sediments were mobilized locally to form rootless vents both the initial incursion of the pyroclastic current into the sea and the subsequent submarine sliding of the ignimbrite are likely to have caused tsunamis similar occurrences at modern coastlines presently susceptible to incursion of high mass flux pyroclastic currents e g taupo volcanic zone new zealand neapolitan region italy vicinity of manila taal philippines would according to tsunami propagation behaviour cause significant near and far field coastal hazard

age and eruptive center of the paeroa subgroup ignimbrites whakamaru group within the taupo volcanic zone of new zealand we here explore the temporal and spatial relationships between the contrasting sources for two eruptive episodes that collectively represent the whakamaru group the largest ignimbrite forming sequence in the 2 m y history of the taupo volcanic zone in new zealand at 349 4 ka weighted mean at 2 the 1500 km 3 widespread whakamaru group ignimbrites and 700 km 3 rangitawa tephra fallout were erupted in association with collapse of the 40 km long by 25 km wide rectilinear whakamaru caldera new 40 ar 39 ar age data presented here show that the co magmatic 110 km 3 paeroa subgroup ignimbrites previously included as part of the whakamaru group are slightly younger and were erupted at 339 5 ka weighted mean at 2 new field evidence also presented here demonstrates that the paeroa subgroup ignimbrites came from a source geographically separated from vents for the widespread whakamaru group ignimbrites the presence of co ignimbrite lag breccias sizes of vent derived lithic clasts thicknesses of exposed and subsurface deposits and morphologies of deposits imply that eruptions of the paeroa subgroup occurred from a linear source the paeroa linear vent zone coinciding with the present day northeast striking paeroa fault and outside northeast of the earlier whakamaru caldera collapse area no separate caldera has been recognized although three nearby areas may have undergone eruption related subsidence residual magma from the whakamaru or adjacent kapenga caldera areas may have migrated toward the paeroa linear vent zone during eruptive episodes resulting in subsidence in either or both of these areas shallow plutons are also inferred to lie beneath near source fault blocks paeroa and te weta on each side of the fault and eruption related subsidence may have been expressed as movement across the paeroa fault and localized subsidence in the southern paeroa fault block subsequent secular rift related displacement along the paeroa fault has obscured the paeroa linear vent zone

late quaternary history of lake manitoba canada abstract the postglacial history of lake manitoba has been deduced from a study of the changes in physical mineralogical and chemical variables in sediment cores collected from the lake six lithostratigraphic units are recognized in the south basin of the lake weakly developed pedogenic zones reflecting dry or extremely low water conditions in the basin separate five of these six units the initial phase of lacustrine sedimentation in the lake manitoba basin began shortly after 12 000 yr b p as water was impounded in front of the receding glacier to form lake agassiz by 11 000 yr ago continued retreat of the ice sheet opened lower outlets to the east and much of lake agassiz drained including the lake manitoba basin water levels again rose at 9900 yr b p but by about 9200 yr b p the south basin was again dry for the next 4700 yr there was an alternation of wet and dry conditions in the basin in response to the interaction of a warmer and drier climate and differential crustal rebound of the basin about 4500 yr ago a new phase of lake manitoba sedimentation was initiated when the assiniboine river began to discharge into the south basin the assiniboine river was diverted out of the lake manitoba watershed about 2200 yr ago erosion and redistribution of the sandy deltaic sediments deposited by the assiniboine river has created the barrier beach that now separates the extensive marsh to the south of the lake from the main lake

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

reconstruction of holocene precipitation patterns in europe using pollen and lake level data abstract lake level data can be used to refine palaeoclimate reconstructions based on pollen data this approach is illustrated for the european holocene estimates of p pet precipitation minus potential evapotranspiration were first inferred from modern pollen analogues the pollen based estimates were then compared with the status of lakes within a 5 radius analogues with p pet anomalies inconsistent with the lake level changes were rejected the constrained sets of analogues were used to estimate continental scale patterns of annual mean temperature and annual precipitation at 3000 yr intervals estimated temperature anomalies differed only slightly from the unconstrained reconstructions estimated precipitation anomalies however showed improved spatial coherence and increased regional contrast and were occasionally reversed in sign the effect of the constraint was to impose a rational selection among almost equally similar modern pollen analogues with similar temperatures but widely varying moisture regimes the resulting maps showed clear spatially coherent patterns of change in precipitation as well as temperature suitable for comparison with climate model results further improvement of these maps will become possible as a more extensive coverage of lake level data is obtained

depositional controls and characteristics of subaqueous bedded volcaniclastics of the lower devonian snowy river volcanics abstract the mt johnson member represents a relatively deep quiet marine enclave within the largely subaerial lower devonian snowy river volcanics it is intersected by seven diamond drill holes allowing a detailed analysis of the constituent sediment sequences normal sedimentation is represented by a mudstone facies which consists of interbedded black mudstones and turbiditic siltstones however this fine grained deposition was periodically interrupted by the emplacement of thick amalgamated volcaniclastic mass flow sequences six mass flow sequences are defined and two types can be recognized according to their overall bedding thickness and grainsize profile upward thinning and fining sequences are characterized by thickly bedded basal deposits of juvenile magmatically fragmented pyroclastic debris emplaced as cold water supported mass flows these are interpreted to be essentially syn eruptive arising either from flow transformation as pyroclastic flows ingested water upon entering the sea or from immediately post eruptive slumping of unreworked pyroclastic debris ponded at the shoreline these juvenile deposits are overlain by intervals of thinner bedded mass flows which are of a similar provenance but are considerably depleted in fines the second type of mass flow sequence the complex bedding thickness sequences has a basal zone of bedded fines depleted mass flow deposits with juvenile deposits in the mid to upper portions these mass flow sequences are interpreted to have arisen either from volcanic eruptive events which did not initially deposit primary pyroclastic debris subaqueously or from epiclastic events which eroded previously erupted unconsolidated volcaniclastic debris the depletion of fines in much of the volcaniclastic debris within the mt johnson member indicates that it was subjected to a degree of epiclastic reworking since this cannot have occurred in the deep quiet marine host environment it indicates an external source for the parental magmatic eruptions the eruptions probably occurred in an adjacent subaerial hinterland drained by an alluvial fluvial deltaic or fan deltaic system this system would have been a natural site for ponding and storage of freshly erupted debris and would have allowed the complex interaction between eruptive and epiclastic events which appear to have been responsible for the mass flow sequences

paleohydrological indicators in playas and salt lakes with examples from canada australia and africa abstract the sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region these records are comprised of siliciclastic sediments and chemically precipitated minerals that typically vary temporally and spatially in some cases the interpretation is straightforward with mineralogical sequences related to changing brine concentrations and hydrology as well as to climate in other cases it is difficult to identify the controlling factors which include climate groundwater geological setting and basin morphology in addition diagenesis commonly affects the sedimentary record models of lake level fluctuations are given for basins in the canadian prairies and southeastern australia the lake manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper water sediment during low water stages smaller canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators such as specific carbonate and salt mineralogy nature of bedding grain size and secondary alterations decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates to salts and then back to clastics as groundwater levels fall below the lake floor similar parameters are used in australian lakes the hydrological model for the lake tyrrell playa in victoria whose present ionic composition is similar to seawater shows clastics being deposited when average lake levels are high followed by deposition of primary carbonates gypsum and halite once seasonal drying on the lake floor begins secondary gypsum grows within already deposited sediment and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes continued water level decline leads to pedogenesis subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle and a complete cycle in the sedimentary record appears to be uncommon

late holocene climate changes in eastern north america estimated from pollen data well dated pollen profiles from six sites from maine to minnesota record vegetation changes indicative of summer temperature and annual precipitation variations over the past 2000 yr laminations in the sediment provide accurate time control multiple regression techniques were used to calculate calibration functions from a spatial network of modern pollen and climate data when applied to the six pollen diagrams these calibration functions yielded estimates that show a long term trend toward lower summer temperature superimposed on this long term trend are short term fluctuations that are frequently in phase at the sites departures from the long term cooling trend are positive around 1500 yr ago indicating relative warmth and negative between 200 and 500 yr ago indicating relative cold annual precipitation showed a slight increase at several sites during the past 1000 yr relative to the previous 1000 yr

stratigraphy and structure of the ngatamariki geothermal system from new zircon u pb geochronology implications for taupo volcanic zone evolution abstract recent drilling at the ngatamariki geothermal field taupo volcanic zone new zealand has provided new constraints on the stratigraphy and volcanic evolution of the region over 2800 m thickness of volcanic products are present at ngatamariki mainly comprised of rhyolitic ignimbrites linked to large caldera forming events at sources outside the field area but locally sourced andesite and rhyolite lavas and domes are also encountered most of the rocks are allocated to the pre 0 35 ma tahorakuri formation crystallisation age spectra and consequent best estimates of eruption age have been obtained by u pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by secondary ion mass spectrometry techniques using a shrimp rg instrument the oldest rock dated is an ignimbrite which yields an eruption age estimate of 1 85 0 06 ma this ignimbrite plus comparable aged units dated at the adjacent rotokawa and ohaaki geothermal fields are interpreted to represent the oldest silicic volcanic deposits in the area and onlap the basal andesite lava pile that is best developed at rotokawa to the south other pyroclastic units and associated volcaniclastic sediments with another intercalated andesite lava unit return age estimates between 1 85 0 06 and 0 701 0 039 ma between 0 7 and 0 35 ma contemporaneous surface lithologies in the ngatamariki area are dominated by sediments with subordinate lava domes between 0 716 0 017 and 0 655 0 016 ma at least three shallow to 5 km 3 the intrusions generated a large alteration halo 25 km 3 minimum and intense silicification of the wall rocks at 0 35 and 0 34 ma the area was buried by two ignimbrite packages of the whakamaru group erupted from sources just west and well north respectively of the field ignimbrites of the waiora formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0 282 0 011 and 0 257 0 011 ma coeval with more extensive volcanic activity in the maroa dome complex west of the field sediments of the huka falls formation and deposits of the 25 4 0 2 ka oruanui eruption then cap and seal the system the new u pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the ngatamariki geothermal field the field despite 2 8 km of subsidence does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of quaternary age two chemically and temporally distinct hydrothermal events are located at the ngatamariki field with no evidence of continuity between the two

the dali ash island of rhodes greece a problem in interpreting submarine volcanigenic sediments in the southern part of rhodes greece rhyolitic subaqueous pyroclastic deposits are interbedded with tertiary deep water marine sediments the lowermost and best exposed of these deposits the dali ash is described here the deposit has been previously described as a deep water welded subaqueous ignimbrite this paper shows that there is no evidence of welding and texture previously reported were misidentified the dali ash consists of a lower massive unit 5 m thick overlain by a sequence of ash turbidites 2 5 m thick the lower unit was deposited by a high concentration turbidity current and the ash turbidites by dilute turbidity currents foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water particulate systems a palaeomagnetic study also suggests they were deposited cold