multistage damming of the rhine river by tephra fallout during the 12 900 bp plinian laacher see eruption germany syn eruptive rhine damming i abstract the rhine the largest river in western europe was dammed during the plinian laacher see eruption lse 12 900 bp damming during the climactic plinian episode of lse occurred both upstream and downstream of the broad tectonic lower neuwied basin lnb that interrupts the narrow rhine canyon we here document details of the upstream damming at the bottleneck entrance to the lnb near the present city of koblenz our reconstruction is based on a high resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked laacher see tephra in the lnb tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4 major flooding events even at the base of side channels that had been active prior to the lse and that had been flooded by a preceding flooding event this demonstrates that flooding occurred generally during breaks and not during fallout events the repeated formation and breach of a dam at the upstream entrance of the lnb koblenz dam consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the lnb followed by large magnitude flooding in rapid succession the strongly pulsating nature of the lse reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics the rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries the temporary dam collapsed during eruptive breaks this is the first recorded example of tephra fallout damming a major watercourse the extremely low gradient of the rhine river allowed the repeated accumulation of large volumes of water in a long multi phase dammed up lake lake koblenz that extended along the upstream course of the river for up to c 30 km despite the low height damming of a major river while a large plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation this is especially pertinent for an area close to and downwind from the vent and therefore simultaneously affected by massive plinian fall such as in the lnb

comparison of granite related uranium deposits in the beaverlodge district canada and south china a common control of mineralization by coupled shallow and deep seated geologic processes in an extensional setting abstract many uranium deposits are related to granitic rocks but the mineralization ages are much younger thus excluding a direct magmatic hydrothermal link between the mineralization and the granites two such examples are the proterozoic vein type uranium deposits in the beaverlodge district in canada and the mesozoic granite related uranium deposits in south china both areas have been extensively studied but the critical factors that control the mineralization remain unclear the uranium mineralization in the beaverlodge district occurs in quartz carbonate albite veins and breccias developed within and near major deformation zones and are mainly hosted by ca 2 33 1 90 ga granitic rocks and ca 2 33 ga murmac bay group amphibolite these rocks are unconformably overlain by the martin lake basin which was formed during a period of regional extension in the later stage of the trans hudson orogeny and is filled with red beds a ca 1820 ma mafic magmatic event is manifested as volcanic rocks occurring within the martin lake basin and as dikes crosscutting the basement rocks and lower martin group strata uraninite u pb and pb pb ages range from ca 2290 ma to the granite related uranium mineralization in south china occurs mainly as quartz carbonate fluorite veins and as disseminations in the host rocks adjacent to fracture zones the deposits are hosted by or occur adjacent to granitic rocks and are spatially close to cretaceous tertiary red bed basins that were formed in a basin and range like tectonic setting related to roll back of the pacific plate these red bed basins contain intervals of bimodal volcanic rocks and are crosscut by coeval mafic dikes uraninite u pb ages dominantly range from 100 to 50 ma which is much younger than the host granites 145 ma and broadly contemporaneous with development of the red bed basins and mafic magmatism comparison of the two study areas reveals striking similarities in geologic attributes related to uranium mineralization specifically the development of large volumes of granitic rocks that are relatively enriched in uranium and the development of red bed basins with accompanying coeval mafic magmatism in extensional tectonic settings it is proposed that oxidizing basinal fluids from the red bed basins circulated into the underlying fertile granitic rocks along high permeability structural zones acquired uranium in the path through fluid rock interaction and precipitated the uranium where they encountered reducing agents elevated geothermal gradients associated with the mantle derived magmatism greatly enhanced the mineralization by facilitating the uranium extraction and transport processes thus it is the coupling of shallow red bed basin and oxidizing basinal fluid development and deep seated mantle derived magmatism and related thermal activity processes together with the pre enrichment of uranium in basement rocks particularly granitic rocks and pre existing deformation zones that controlled the formation of the granite related uranium deposits in both beaverlodge and south china and perhaps elsewhere in the world with similar geologic setting

analysing the relation between rainfall characteristics and lahar activity at mount pinatubo philippines the eruption of mount pinatubo in june 1991 altered the conditions of the surrounding river catchments pyroclastic flows and tephra fall were deposited over extensive areas stripping off the forest cover and burying drainage divides these recent deposits are very loosely consolidated and generally consist of sand sized particles which commonly mobilize into lahars in response to rainfall of a certain magnitude several devastating lahar occurrences have buried settlements covering tens to several hundred square kilometres in a single event correlation of storm rainfall intensities and durations with lahar activity as recorded by acoustic flow monitors is used to investigate trends in the initiation conditions for lahar activity this research confirms that the relationships of rainfall intensity and duration with lahar initiation threshold values are not linear but rather approximate a power relation different relations were found for lahar initiation in different years from 1991 to 1997 as a result of the dynamic changes in hydrologic and geomorphic conditions of the affected catchments data from acoustic flow monitors are used to distinguish debris flow and hyperconcentrated flow activity from that of muddy water copyright 2005 john wiley sons ltd

spatial modelling and uncertainty assessment of pyroclastic cover thickness in the sorrento peninsula assessing spatial variability of soil thickness is a critical issue for understanding and predicting slope processes the present work was aimed at estimating the spatial scales at which the variation of pyroclastic cover thickness occurs in a sample area in the sorrento peninsula italy stochastic simulation was used to understand the spatial variability of pyroclastic cover thickness on mount pendolo and to assess its spatial uncertainty in the study area covering about 0 7 km2 thickness measurements were collected using electrical resistivity tomography profiles continuous core drillings and steel rod penetrometric tests variographic analysis revealed the occurrence of an anisotropic behaviour along the n50 and n140 directions in the latter anisotropic direction a nested variogram was fitted including 1 a long range component which could be related to large scale factors like the curvature of the slope and contributing area and 2 a shorter scale variation which is probably associated with the occurrence of denudation processes or to the articulate cover bedrock interface to assess the spatial variability and uncertainty of pyroclastic cover thickness a stochastic simulation algorithm was used and 500 equally probable images of cover thickness were yielded the results showed that a better thickness distribution map can be drawn by simulating the data collected on the slope and at the footslope separately the approach also allowed delineating the areas characterized by greater uncertainty suggesting supplementary measurements to further improve the cover thickness distribution model thus reducing the uncertainty

erosion of tephra from the 1980 eruption of mount st helens erosion of tephra on hillslopes north of mount st helens was measured at intervals of several months between 1980 and 1983 using large arrays of stakes rill erosion was initially greater than sheetwash but its importance decreased comparatively quickly both rill erosion and sheetwash decreased by one to two orders of magnitude over the three year period because a stable rill network developed and more permeable and less erodible substrates were created or exposed the decrease occurred prior to recovery of plants the causes for the rapid decline in erosion at mount st helens explain patterns of erosion following several other recent tephra eruptions basin wide annual rates of erosion were computed by mapping the surface cover the hillslope gradient and the texture and thickness of tephra and by using statistical relations developed between them and erosion rates annual erosion decreased from 26 mm between may 1980 and may 1981 to 1 8 mm between may 1982 and may 1983 projecting this rate of decline into the future indicated that only about one sixth of the tephra on hillslopes will be removed by water erosion before soil creep and other forms of mass wasting again dominate hillslope evolution

afromontane foragers of the late pleistocene site formation chronology and occupational pulsing at melikane rockshelter lesotho this paper provides a preliminary chronostratigraphic and palaeoenvironmental framework for the late pleistocene archaeological sequence at melikane rockshelter in mountainous eastern lesotho renewed excavations at melikane form part of a larger project investigating marginal landscape use by late pleistocene foragers in southern africa geoarchaeological work undertaken at the site supports in eld observations that melikane experienced regular often intensive input of groundwater via ssures in the shelter s rear wall this strong hydrogeological connection resulted in episodic disturbances of the sedimentary sequence exacerbated by other processes such as bioturbation despite this taphonomic complexity a robust chronology for melikane has been developed based on tightly cross correlated accelerator mass spectrometry ams 14 c with acid base wet oxidation stepped combustion abox sc pretreatment and single grain optically stimulated luminescence osl dating the results show that human occupation of melikane was strongly pulsed with episodes of late pleistocene occupation at w80 w60 w50 w46e38 and w24 ka at least three additional occupational pulses occurred in the holocene at w9 ka w3 ka and in the second millennium ad but these are dealt with only brie y in this paper implications of the late pleistocene pulsing for the colonisation of high elevations by early modern humans in africa ahead of dispersals into challenging landscapes beyond the continent are discussed

effective discharge in rocky mountain headwater streams summary whereas effective discharge q eff in mountain streams is commonly associated with a moderate flow such as bankfull discharge q bf this study found that the maximum discharge q max and not bankfull discharge is the channel forming or effective flow for gravel transport in plane bed streams where partial bed mobility causes steep gravel transport rating curves q eff may approach bankfull flow in some step pool channels where gravel moves over a static cobble boulder bed our conclusions are based on magnitude frequency analyses conducted at 41 gauged rocky mountain headwater streams because these gauged streams lacked gravel transport data as is typical comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites the measured flow frequency distributions were characterized by two fitted power functions the steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude frequency analyses to compute q eff

rate of sediment yield following small scale volcanic eruptions a quantitative assessment at the merapi and semeru stratovolcanoes java indonesia sediment yields were calculated on the anks of merapi and semeru volcanoes in java indonesia using two different methods during the rst year following the 22 november 1994 eruption of merapi a sediment yield in excess of 1 5 105 m3 km 2 yr 1 was calculated in the boyong river drainage basin based on the volumes of sediment that were trapped by ve check dams at semeru sediment discharges were assessed in the curah lengkong river from direct measurements on the lahars in motion and on the most signi cant stream ows the calculated rate of sediment yield during one year of data in 2000 was 2 7 105 m3 km 2 yr 1 sediment yields are dominated by rain triggered lahars which occur every rainy season in several drainage basins of merapi and semeru volcanoes mostly during the rainy season extending from october to april the return period of lahars carrying sediment in excess of 5 105 m3 is about one year in the curah lengkong river at semeru at merapi the volume of sediments transported by a lahar did not exceed 2 8 105 m3 in the boyong river during the rainy season 1994 95 on both volcanoes the sediments are derived from similar sources pyroclastic ow surges deposits rockfalls from the lava domes and old material from the riverbed and banks however daily explosions of vulcanian type at semeru provide a more continuous sediment supply than at merapi therefore sediment yields are larger at semeru copyright 2004 john wiley sons ltd

influence of sand on hydraulics and gravel transport in a braided gravel bed river measurements in a braided reach of the proglacial white river washington are used to compare the bed load capacity and competence of flow over the predominant gravel bed and over a narrow ribbon of mobile sand which extended alongside the talweg analysis of 144 velocity profiles and 222 bed load samples shows that shear stress and roughness height were significantly lower over sand than gravel but maximum bed load diameter and total gravel transport rate significantly higher as were transport rates of given size fractions at similar shear stresses these differences in competence and capacity are explained by relative size effects on the threshold shear stress for entrainment the influence on gravel transport of local variation in bed material size may explain the self sorting of bed load sheets described by other workers however in channels like that described here the continuous longitudinal ribbon of sand allows the flow to develop a lower shear stress over the smoother bed which partly offsets the relative size effects on competence and capacity

strong seasonality in sand loading and resulting feedbacks on sediment transport bed texture and channel planform at mount pinatubo philippines declining sand inputs to a channel with bimodal bed sediment can lead to degradation armoring and reduced bedload transport rates where sand loading is episodic channels may alternate between high sand and low sand conditions with ensuing responses in bed texture and bedload transport rates the effects of episodic sand loading are explored through flow grain size and bedload transport measurements on the pasig potrero river a sediment rich channel draining mount pinatubo philippines sand loading on the pasig potrero river is highly seasonal and channel adjustments between seasons are dramatic in the rainy season inputs from sand rich 1991 eruption deposits lead to active sand bedded braided channels in the dry season many precipitation driven sand sources are cut off leading to incision armoring and significantly lower bedload transport rates this seasonal transition offers an excellent opportunity to examine models of degradation incision and armoring as well as the effectiveness of sediment transport models that explicitly encapsulate the importance of sand on transport rates during the fall 2009 seasonal transition 7 6 km of channel incised and armored carving a 2 3 m deep channel on the upper alluvial fan bedload transport rates measured in the august 2009 rainy season were over four orders of magnitude greater than gravel bedded dry season channels surveyed in january 2010 despite having similar shear stress and unit discharge conditions within dry season incised channels bed armoring is rapid leading to an abrupt gravel sand transition bedload transport rates adjust more slowly creating a lag between armoring and commensurate reductions in transport seasonal channel incision occurred in steps aided by lateral migration into sand rich banks these lateral sand inputs may increase armor layer mobility renewing incision and forming terraces within the incised seasonal channel the seasonal incised channel is currently being reset by precipitation driven sand loading during the next rainy season and the cycle begins again copyright 2012 john wiley sons ltd

depositional processes of primary to reworked volcaniclastics on an alluvial plain an example from the lower pliocene ohta tephra bed of the tokai group central japan abstract 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

rain triggered lahar susceptibility using a shallow landslide and surface erosion model abstract lahars are mass flows containing variable concentrations of water and volcanic debris that can cause catastrophic impacts to life livelihoods and infrastructure downstream from their volcanic origin accurate and quantitative information on lahar hazards are essential for reducing the impact of these events lahar hazard assessments often focus on the use of numeric or empirical models to describe flow behaviour and inundation areas which rely on historic lahar events and expert elicitation to define model inputs this results in qualitative or semi quantitative estimates of hazard that do not account for the mechanics of lahar initiation or in the case of rain triggered lahars the dependence of rainfall intensity and duration on initiation here we develop a method for calculating rain triggered lahar susceptibility defined as the occurrence probability of a particular lahar initial volume at a specific location the model relies on terrain and deposit characteristics and a probabilistic measure of rainfall in the form of rainfall intensity frequency duration relationships results for a case study of the october 28 1995 lahar at mangatoetoenui stream ruapehu volcano new zealand indicate lahar volume is controlled by a characteristic timescale relating the deposit depth h to the hydraulic diffusivity d 0 in the ratio h 2 d 0 the timescale describes the transmission of positive pore pressures within the deposit leading to shallow failure as a consequence of this timescale rainfall duration is the most important factor determining initial lahar sediment volume rainfall intensity plays a minor role controlling the volume of water in the lahar mixture this observation is consistent with power law relationships used to determine lahar triggering rainfall thresholds the rain triggered lahar susceptibility approach developed here is anticipated to improve probabilistic lahar hazard assessments by providing quantitative reproducible estimates of initial lahar volumes

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

fluvial sediment transport and deposition following the 1991 eruption of mount pinatubo the 1991 eruption of mount pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows bedload sampling in the pasig potrero river one of the most heavily impacted rivers revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth fine grained bed dimensionless bedload transport rates in the pasig potrero river differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments the similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply in arid rivers this is attributed to a flashy hydrograph whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery this work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced d 2002 elsevier science b v all rights reserved

altered regional sediment transport regime after a large typhoon southern taiwan analyses of river suspended sediment response to record breaking regional rainfall in southern taiwan during typhoon morakot 7 9 august 2009 reveal systematic changes in the regional sediment transport regime as characterized by rating curve parameters with decreasing b typically considered to be a constant for a given gauging station and increasing intercept k values in concentration discharge c q relations c kq b these changes result in much greater sediment concentration and thus sediment transport in subsequent low flow events after the typhoon an effect that amplifies and extends the influence of such extreme events through increased low flow sediment transport our finding that the exponent b is not constant contrasts with the conventional assumption that large events influence sediment yields through increased intercept k values thereby supporting the interpretation that basin sediment delivery influences both rating parameters and increases post event low flow sediment transport surveys of landslide density and riverbed grain sizes before and after typhoon morakot support the interpretation that the observed decrease in b values and the synchronous shift from an inverse correlation between k and mean catchment slope before the typhoon to a positive correlation afterward reflect an altered sediment transport regime and a change from channel migration and bank erosion to reworking of landslide debris and enhanced bed mobility as the dominant processes supplying fluvial sediment

interpreting flash flood palaeoflow parameters from antidunes and gravel lenses an example from montserrat west indies the wavelength of stationary water surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow in sand bed flume experiments alexander et al 2001 found that lens structures were preserved during antidune growth and change and the dimension of the lenses were empirically related to antidune wavelength and thus could be used to estimate flow velocity and depth this study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting testing the alexander et al 2001 relationship at a field scale trains of stationary and upstream migrating water surface waves were prevalent during the flash flood in october 2012 in the belham valley montserrat west indies wave positions and wavelengths were assessed at 900 s intervals through the daylight hours of the event within a monitored reach the wave data indicate flow depths up to 1 3 m and velocity up to 3 6 m s 1 sedimentary structures formed by antidune growth and change were preserved in the event deposit these structures include lenses of clast supported gravel and massive sand with varying internal architecture the lenses and associated low angle strata are comparable to sand bed structures formed from stationary and upstream migrating waves in flume experiments confirming the diagnostic value of these structures using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow

late quaternary dynamics of southern africa s winter rainfall zone abstract variations in the nature and extent of southern africa s winter rainfall zone wrz have the potential to provide important information concerning the nature of long term climate change at both regional and hemispheric scales positioned at the interface between tropical and temperate systems southern africa s climate is influenced by shifts in the intertropical convergence zone the westerlies and the development and position of continental and oceanic anticyclones over the last glacial interglacial cycle substantial changes in the amount and seasonality of precipitation across the subcontinent have been linked to the relative dominance of these systems central to this discussion has been the extent to which the region s glacial climates would have been affected by expansions of antarctic sea ice equatorward migrations of the westerlies more frequent intense winter storms and an expanded wrz this paper reviews the developing body of evidence pertaining to shifts in the wrz and the evolution of ideas that have been presented to explain the patterns observed dividing the region into three separate axes along the western and southern margins of the continent and across the interior into the karoo and the kalahari a range of evidence from both terrestrial sites and marine cores is considered and potential expansions of the wrz expansions are explored despite the limitations of many of the region s proxy records a coherent pattern has begun to develop of a significantly expanded wrz during phases of the last glacial period with the best documented being between 32 17 ka while more detailed inferences will require the recovery and analysis of longer and better dated records this synthesis provides a new baseline for further research in this key region

origin and early evolution of lahar channel at mabinit mayon volcano philippines mabinit channel 5 km long 15 to 70 m wide and 2 to 22 m deep was formed by lahars on the southeastern slope of mayon volcano during its 1984 eruption in 1985 the channel was drastically modified by a lahar triggered by a typhoon sediment budget calculations from surveys conducted in 1985 and 1986 corroborate inferences from other volcanoes that lahars can grow significantly in volume by eroding their channels the 1984 eruption produced a deep summit ravine and at its base a fan of avalanche and pyroclastic flow deposits 2 km2 in area and 40 x 106 m3 in volume lahars initiated in the ravine by heavy rains from eruption updrafts carved out 5 km long mabinit channel along the west margin of the pyroclastic avalanche field a position that could not have been predicted from pre eruption topography by eroding the channel the lahars generated more than half of their own solid contents below 240 m where the volcano slope decreases from 5 to 3 the first eruption debris flows were unconfined and spread out over a 512 000 m2 area depositing a layer locally more than 4 m thick with a volume of 1 25 x 106 m3 the thin margins of these unconfined flows were non erosional however as they stopped their thick central portions continued flowing extending the new channel downslope through the depositional field subsequent eruption lahars modified the channel and overflowed at its bends a moderately intense typhoon in october 1985 triggered a single 9 hr lahar event that widened the channel by an average of 25 m and caused as much as 66 m of lateral bank erosion debris flows overtopping the channel at four sites coalesced to cover a 2 km long 200 000 m2 area with bouldery sediment in wider channel stretches debris flow margins left prominent paired levees while their central portions continued moving down channel as rigid plugs the lahar plugged the lower 0 5 km stretch of the channel and replaced it by eroding a new channel of comparable size this demonstrated how easily lahars can change the courses of channels on the upwardly convex debris apron of a stratovolcano with serious consequences for farmland and communities

extraordinary sediment delivery and rapid geomorphic response following the 2008 2009 eruption of chaiten volcano chile u s geological survey volcano science center sernageomin s programa de riesgos volcanicos conicyt fondecyt grants 1110609 1141064 11130671 conicyt fondap 15090013 vamos research centre

historical sea level changes and effects on the coasts of sorrento peninsula gulf of naples new constrains from recent geoarchaeological investigations abstract this paper describes the results of a multidisciplinary study of four geoarchaeological sites on the sorrento peninsula coast italy where the submerged ruins of roman buildings have enabled the ancient position of both the sea level and the coastline to be reconstructed the results highlight that the sea level in roman times deduced from the submersion measurement of archaeological markers did not exceed 1 2 m 0 30 m except for seiano whose submerged remains and surrounding area were heavily damaged by extreme events following the 79 ce vesuvius eruption the comparison of these results with the eustatic sea level curve in roman times together with the submersion of the tectonically stable site on the tyrrhenian coast of italy torre astura with a well preserved roman fishpond demonstrate that the sorrento peninsula has been tectonically stable over the last two millennia this behaviour started many thousands of years ago not later than the last interglacial while the nearby plain of pompeii and probably the bottom of the adjacent gulf of naples have been subsiding at rates of 1 or more mm per year in the same time span two of the investigated coastal sectors capo di sorrento and punta campanella have not suffered any significant changes because their sea cliffs are cut in hard limestone the tufa sea cliff of the sorrento plain on the other hand has been retreating by several metres on the marina di equa seiano alluvial coastal plain which suffered the effects of the 79 ce eruption the debris and hyperconcentrated flow produced a progradation of 200 m which has been totally dismantled over three centuries

sediment yield following severe volcanic disturbance a two decade perspective from mount st helens explosive volcanic eruptions perturb water and sediment fluxes in watersheds consequently posteruption sediment yields can exceed preeruption yields by several orders of magnitude annual suspended sediment yields following the catastrophic 1980 mount st helens eruption were as much as 500 times greater than typical background level and they generally declined nonlinearly for more than a decade although sediment yields responded primarily to type and degree of disturbance streamflow fluctuations significantly affected sediment yield trends consecutive years 1995 1999 of above average discharge reversed the nonlinear decline and rejuvenated yields to average values measured within a few years of the eruption after 20 yr the average annual suspended sediment yield from the 1980 debris avalanche deposit remains 100 times 10 4 mg megagrams km 2 above typical background level 10 2 mg km 2 within five years of the eruption annual yields from valleys coated by lahar deposits roughly plateaued and average yields remain about 10 times 10 3 mg km 2 above background level yield from a basin devastated solely by a blast pyroclastic current diminished to background level within five years these data demonstrate longterm instability of eruption generated detritus and show that effective mitigation measures must remain functional for decades

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

remobilized cordon caulle 2011 tephra deposits in north patagonian watersheds resedimentation at deltaic environments and its implications abstract the argentinean side of the andean north patagonia is drained by several fluvio lacustrine systems flowing eastwards from their headwaters at the andes mountains towards large glacial lakes these watersheds are recurrently affected by the deposition of tephra erupted from the many active andean volcanos located in the southern volcanic zone this research focusses on two fluvial systems pireco and totoral pt located in nahuel huapi national park which were severely impacted by the cordon caulle 2011 eruption cc2011 we investigate particularly the resedimentation of pyroclastic materials on their lacustrine deltas and its potential implications considering 1 the deltas morphology and 2 mass wasting processes developing at the delta fronts original tephra fall deposits were registered at nahuel huapi national park in 2011 based on the isopach map we performed a gis based volume estimation of the deposit for the studied watersheds delta plain progradation from 2011 to 2017 was analyzed from satellite images we studied the temporal evolution of the subaqueous morphology and mass wasting processes at the delta fronts using high resolution bathymetric grids registered in 2011 2014 and 2017 during june 2011 the studied watersheds received a tephra input of 72 x 106m3 the significant progradation experienced by both deltas especially during the first month of the eruption was favored by 1 abundant precipitation at the watersheds concomitant to heavy tephra deposition and 2 the pre existing subaqueous morphology characterized by a wide shallow terrace bordering pt delta plains about six years after the beginning of cc2011 6 of the tephra fallen over the entire basins had been resedimented at the delta environments terrace and delta front totalizing 3 x 106t evidence of pre eruption and or co eruptive mass wasting processes occurring at pt delta fronts was found on november 2011 up to 30 m thick pyroclastic deposits were accumulated at pt delta fronts since november 2011 burying mass wasting features at the upper delta fronts a suspended sediment collector installed at the prodelta in 2012 detected hyperpycnal currents transporting pyroclastic materials these currents probably explain the development of mass movements at high depths such as the one that displaced and filled up the bottom sediment trap this frontally confined landslide initiated at a depth of 60 m and involved tephra and organic materials deposited at the lower part of totoral delta front because of their recent deposition and loose consolidation we propose that fresh tephra deposits at pt delta fronts could become particularly unstable and represent a potential hazard to failure especially if a strong earthquake affects the study area in the next years a landslide induced tsunami wave generated at these deltas would impact rural settlements located along the coasts of brazo rincon and probably reach the coastal city of villa la angostura

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

alluvial and coastal hazards caused by long range effects of plinian eruptions the case of the lattari mts after the ad 79 eruption of vesuvius abstract the lattari mountains a limestone ridge about 20 km south of vesuvius received 1 2 5 m of fallout from the famous plinian eruption of ad 79 as demonstrated by many residual outcrops of thick volcanoclastic debris flow and alluvial deposits referred to here as the durece unit the pyroclastic fall was soon followed by rapid erosion and landsliding that produced 1 decametre scale aggradation of some narrow valley floors 2 reactivation of alluvial fans and 3 growth of new fan deltas extending as far as 500 m at the coast this response was primarily due to the steep topography of the area and the high erodability of the pyroclastic materials light and cohesionless pumice fragments several geo archaeological data indicate that the accelerated sedimentation had a duration of the order of decades and was followed by rapid dissection of the durece unit deposits and fast dismantling by wave action of the newly created fan deltas this case highlights the need to consider the possibly catastrophic reaction of fluvial and coastal systems to large explosive eruptions even in non volcanic terrains at some distance from the volcano

temporal variations of large wood abundance and mobility in the blanco river affected by the chaiten volcanic eruption southern chile abstract the aim of this study is to analyze the temporal variation in abundance and transport of in channel wood along a river affected by a volcanic eruption considering the mobility and retention rates in detail the 2008 eruption of chaiten volcano southern chile severely affected the blanco river basin causing changes in channel morphology and large wood lw dynamics along the fluvial corridor of the river a fourth order stream on the southern slope of the volcano temporal dynamics of lw were investigated from january 2015 until march 2016 in three 80 m long study reaches of the active channel in the lower course of the river every wood element 1 m in length and 0 1 m in diameter within the active channel of each reach was georeferenced measured tagged and characterized wood mobility rate defined as the number of tagged wood pieces repositioned or not found within each study reach to total tagged lw in the same reach and retention rate defined as the number of tagged lw pieces found within each study reach to total tagged wood pieces in the same reach were calculated considering two periods one in summer and one in autumn winter a regional flood frequency analysis estimated flood magnitude defining a recurrence interval of 1 year for summer floods and 10 25 years for autumn winter floods results highlighted how eight years after the eruption the types of lw are still influenced by the impacts of pyroclastic density currents on riparian vegetation the characteristics and abundance of monitored lw are typical of a river system with frequent wood transport the mobility rate ranges between 42 and 94 during the summer and autumn winter period respectively while the wood volume decreases during the summer and increases in autumn winter large wood mobility was also found to vary according to the predominant morphological settings with lower values in narrower channels compared to wider wetted areas in this context the greatest abundance of lw was detected after the higher floods corresponding to an input of lw up to 285 35 m3 ha 1 along the blanco river ordinary floods deliver lw mainly through fluvial transport and bank erosion during higher floods appears as an additional and important wood recruitment process the bank erosion processes with up to 5200 m2 of eroded area confirmed the widening tendency occurring in river systems following volcanic disturbances as there are dead riparian trees along the easily erodible banks it is reasonable to suppose that the blanco river will continue to widen for a long period recruiting and transporting downstream huge volumes of lw in this context these analyses appear of fundamental importance to better manage the river system also for decreasing hazards to the downstream village of chaiten

mantle derived gaseous components in ore forming fluids of the xiangshan uranium deposit jiangxi province china evidence from he ar and c isotopes abstract the xiangshan u deposit the largest hydrothermal u deposit in china is hosted in late jurassic felsic volcanic rocks although the u mineralization post dates the volcanics by at least 20 ma the mineralization coincides with intrusion of local mantle derived mafic dykes formed during cretaceous crustal extension in south china ore forming fluids are rich in co 2 and u in the fluid is thought to have been dissolved in the form of uo 2 co 3 2 2 and uo 2 co 3 3 4 complexes this paper provides he and ar isotope data of fluid inclusions in pyrites and c isotope data of calcites associated with u mineralization pitchblende in the xiangshan u deposit he isotopic compositions range between 0 1 and 2 0ra where ra is the 3 he 4 he ratio of air 1 39 10 6 and correlates with 40 ar 36 ar although there is potential for significant 3 he production via 6 li n 3 h 3 he reactions in a u deposit due to abundant neutrons nucleogenic production cannot account for either the 3 he concentration in these fluids nor the correlations between he and ar isotopic compositions it is more likely that the high 3 he 4 he ratios represent trapped mantle derived gases a mantle origin for the volatiles of xiangshan is consistent with the 13 c values of calcites which vary from 3 5 to 7 7 overlapping the range of mantle co 2 the he ar and co 2 characteristics of the ore forming fluids responsible for the deposit are consistent with mixing between 3 he and co 2 rich mantle derived fluids and co 2 poor meteoric fluids these fluids were likely produced during cretaceous extension and dyke intrusion which permitted mantle derived co 2 to migrate upward and remobilize u from the acid volcanic source rocks resulting in the formation of the u deposit subsequent decay of u within the fluid inclusions has reduced the 3 he 4 he ratio and variations in u 3 he result in the range in 3 he 4 he observed with u 3 he ratios in the range 5 17 10 3 likely corresponding to u concentrations in the fluids

late holocene changes in erosion patterns in a lacustrine environment landscape stabilization by volcanic activity versus human activity the most recent eruption of mount fuji japan the vei 5 hoei plinian eruption ce 1707 heavily impacted lake yamanaka a shallow lake located at the foot of mount fuji here we discuss the influence of the hoei eruption on the lacustrine sedimentation of lake yamanaka using high resolution geophysical and geochemical measurements on gravity cores hoei scoria fallout had two major impacts on lake yamanaka i reduction of the sedimentation rate from similar to 0 16 to similar to 0 09 cm year and ii the increase of in situ lake productivity sedimentation rates after the eruption were relatively low due to the thick scoria layer trapping underlying sediments in the catchment the lacustrine system took more than similar to 170 years to begin to recover from the hoei eruption most recently sedimentation recovery has been accelerated by changes in land use since the beginning of the twentieth century vegetated strips delimited cultivated parcels trapping sediment and minimizing anthropogenic impacts on the sedimentation rate over the last decade the decline of agriculture and the increase of other human activities has led to an increase in the sedimentation rate similar to 1 cm year this study highlights the effect of the grainsize of the volcanic ejecta on the sedimentation rate following a volcanic eruption coarse grained tephra are difficult to erode therefore their erosion and remobilization is largely limited to intense typhoons when porous scoria deposits are saturated by heavy rains moreover this study suggests that recent anthropogenic modifications of the catchment had a greater impact on the sedimentation rate than the hoei eruption

subaerial debris flow deposition in the upper paleozoic cutler formation western colorado abstract outcrops of the lower part of the cutler formation near gateway colorado contain proximal alluvial fan deposits of two principal lithofacies the diamictite lithofacies d consists of a variety of sharply bounded massive muddy beds which are interpreted as deposits of subaerial debris flows the gravelly sandstone lithofacies s consists of amalgamated beds of laminated and cross bedded conglomeratic sandstones which are interpreted as water deposited sediments clasts in s are rarely larger than 0 5 m in maximum diameter but clasts in d are commonly much larger up to 8 m both normal and inverse grading are visible in d but grading is generally not well developed in either lithofacies imbrication of clasts although more pronounced and more consistent in s is fairly commo in both lithofacies four subfacies of diamictite are distinguishable on the basis of clast packing and grading massive matrix supported dram graded matrix supported dmg inversely graded clast supported dci and massive clast supported dcm given the hydrodynamic and rheologic properties of debris flows as reported by previous workers four debris flow regimes are postulated on the basis of varying water content and clast interaction subfacies dram and drag typify deposits of flows having viscous clast interactions subfacies dci represents deposits of flows having inertial or colliding clast interactions subfacies dram comprises deposits of flows with demonstrably high yield strengths subfacies dcm represents deposits of flows in which yield strength was insufficient to support most clasts this classification specifically recognizes turbulence selective deposition and bedload transport of clasts as essential characteristics of certain dilute debris flows it also accommodates as debris flow deposits some beds which have features usually associated with aqueous deposition including clast frameworks channelization imbrication and normal grading

middle stone age archaeology at olduvai gorge tanzania abstract this paper describes the motivation procedures and results of archaeological and geological field survey of the ndutu unit olduvai gorge conducted in june and july of 2013 survey focused upon the area of olduvai gorge between the second fault and the obalbal depression although selective survey occurred in other areas in and around the gorge over 72 archaeological find spots were recorded and hundreds of middle stone age msa artifacts were recovered as was a small sample of fauna geological observations provisionally suggest that the ndutu was formed in part from a series of pyroclastic density flow and ash fallout events from neighbouring volcanoes this contrasts slightly with previous interpretations of the deposits in that at least some of the beds are thought to be in primary stratigraphic context our initial field findings are conducive to discussions of a number of issues directly relevant to the msa in east africa and overall we conclude that there is strong potential for olduvai s ndutu unit to shed light on the behavior adaptations and evolution of homo sapiens prior to during and just after its physical emergence

acute sedimentation response to rainfall following the explosive phase of the 2008 2009 eruption of chaiten volcano chile the 10 day explosive phase at the start of the 2008 2009 eruption of chaiten volcano in southern chile 42 83 s 72 65 w blanketed the steep rain forest cloaked 77 km2 chaiten river drainage basin with 3 to 100 cm of tephra predominantly fine to extremely fine rhyolitic ash fell during the latter half of the explosive phase rain falling on this ash blanket within days of cessation of major explosive activity generated a hyperconcentrated flow lahar followed closely by a complex multi day muddy flood streamflow bordering on dilute hyperconcentrated flow sediment mobilized in this lahar flood event filled the chaiten river channel with up to 7 m of sediment buried the town of chaiten 10 km downstream of the volcano in up to 3 m of sediment and caused the lower 3 km of the channel to avulse through the town although neither the nature nor rate of the sedimentation response is unprecedented they are unusual in several ways 1 nearly 70 percent of the aggradation almost 5 m in the 50 70 m wide chaiten river channel was caused by a lahar triggered by an estimated 20 mm of rainfall over a span of about 24 h an additional 2 m of aggradation occurred in the next 24 36 h 2 direct damage to the town was accomplished by the sediment laden water flood phase of the lahar flood event not the lahar phase 3 the volume of sediment eroded from hillslopes and delivered to the chaiten river channel was at least 3 8 106 m3 roughly 15 40 of the minimum tephra volume that mantled the chaiten river drainage basin 4 the acute sedimentation response to rainfall appears to have been due to the thickness and fineness of the ash blanket inhibiting infiltration of rain and the steepness of the basin s hillslopes other possible factors such as the prior formation of an ash crust development of a hydrophobic surface layer or large scale destruction of rain intercepting vegetation did not play a role

late pleistocene to holocene human palaeoecology in the tropical environments of coastal eastern africa abstract the ecological adaptations that stimulated the dispersal and technological strategies of our species during the late pleistocene remain hotly disputed with some influential theories focusing on grassland biomes or marine resources as key drivers behind the rapid expansion and material culture innovations of homo sapiens within and beyond africa here we present novel chronologically resolved zooarchaeological taxonomic and taphonomic analysis and stable isotope analysis of human and faunal tooth enamel from the site of panga ya saidi c 78 0 4 ka kenya zooarchaeological data provide rare insights into the fauna associated with and utilized by late pleistocene holocene human populations in tropical coastal eastern africa combined zooarchaeological and faunal stable isotope data provide some of the only dated on site archives of palaeoenvironments beyond the arid interior of eastern africa for this time period while stable isotope analysis of humans provides direct snapshots of the dietary reliance of foragers at the site results demonstrate that humans consistently utilized tropical forest and grassland biomes throughout the period of site occupation through a transition from middle stone age to later stone age technological industries and the arrival of agriculture in the region by contrast while coastal resources were obtained for use in symbolic material culture there is limited evidence for consumption of marine resources until the holocene we argue that the ecotonal or heterogeneous environments of coastal eastern africa may have represented an important refugium for populations during the increasing climatic variability of the late pleistocene and holocene and that tropical environments were one of a diverse series of ecosystems exploited by h sapiens in africa at the dawn of global migrations

channel bed mobility response to extreme sediment loading at mount pinatubo since the 1991 eruption of mount pinatubo the specific sediment yields from watersheds draining its slopes have been the highest ever recorded in spite of this overwhelming sediment load rivers inundated by pyroclastic flow deposits delivered almost half of the initial deposits to downslope alluvial and or debris fans by 1996 although most of this transport occurred by hyperconcentrated flows and debris flows lahars very high sediment transport rates also characterize low flow measured flow velocities and depths channel properties and the size of both mobile and stable clasts in the sacobia and pasig potrero rivers indicate median bed surface grain sizes of 2 to 8 mm grain size independent dimensionless critical shear stresses c of 0 016 to 0 041 and manning s n values of 0 017 to 0 024 values well below those previously reported for steep mountain channels the dramatic bed surface fining bed mobility and selective transport in these extremely sediment rich channels indicate that changes in grain size c values and bed roughness can increase transport capacity in response to high sediment supply our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain size dependent selective transport as end member concepts that apply to channels with low or intermittent and high or continuous sediment supply respectively

morphological evolution of the north fork toutle river following the eruption of mount st helens washington abstract the north fork toutle river nftr has undergone extensive morphological changes following the catastrophic eruption of mount st helens washington in 1980 especially the upper reaches affected by a 2 5 km3 debris avalanche deposit caused by the eruption this paper reports analysis and interpretation of vertical adjustments to the thalweg long profile at some 33 km river reaches redeveloped on the debris avalanche deposit during the 30 year period since the eruption the results confirm that adjustments in the upper part of the study reaches have generally been led by degradation while that in the lower reaches have been led by aggradation with the middle reaches acting as a hinge zone trends of change in the thalweg long profile and bedslope reveal that channel gradients have decreased nonlinearly through time and with distance downstream from the volcano values of stream power have decreased with time commensurately owing to reductions in slope and channel widening while the bed has coarsened so that rates of erosion of the debris avalanche deposit in the upper nftr have slowed to the point that the long profile now perched and slightly steeper is relaxing toward a new equilibrium or graded condition thirty year relaxation paths for thalweg elevation were simulated at seven key cross sections using newly developed comprehensive rate law models based on nonlinear decay in rates of morphological response to perturbation the results indicate that both single and multistep rate law models can simulate the observed records consequently the rate law approach provides an effective method for studying and simulating morphological response of the fluvial system to a major instantaneous disturbance such as a volcanic eruption

sedimentology and flow behavior of a rain triggered lahar mangatoetoenui stream ruapehu volcano new zealand on october 28 1995 heavy rain triggered failure of approximately 3 3 10 5 m 3 of fall deposits on the mangatoetoenui glacier and generated a lahar event that flowed east northeast of ruapehu volcano down the mangatoetoenui stream and eventually entered the tongariro river 19 km downstream the lahar left multiple stacked deposits of at least three major flow surges or pulses which traveled down slope at calculated velocities of as much as 27 m s 1 and calculated maximum peak discharges of as much as 2900 m 3 s 1 the source area of the mangatoetoenui lahar is delineated by a thin furrowed lag of unsorted ash and lapilli margined by steep headwall scarps marking the edges of undisturbed fall deposits failure of fall deposits in the source area triggered debris flows down a north and south branch of the mangatoetoenui stream these two debris flows coalesced and ponded then flowed across seasonal snowpack covering the stream in the proximal zone to 5 km downstream of the source area the deposits of the individual constituent debris flows cannot be distinguished however in the medial zone between 5 and 16 km downstream of the source area and beyond the limit of the snowpack and the junction with perennial streamflow the depositional record of this lahar shows clear evidence for downstream transformation of individual peak flows from debris flow to hyperconcentrated flow transformation began at 5 km from source due to dilution 1 by each peak flow incorporating normal streamflow in the perennial mangatoetoenui stream and tributaries and 2 through deposition of part of each peak flow9s sediment load the hyperconcentrated flow deposits frequently may be subdivided into multiple units which is evidence that the lahar flowed as a series of surges or waves we consider that these were caused by episodic but contemporaneous inputs from a divergent flow from a single source area or by ponding and episodic release of debris within the channel estimated froude numbers for the mangatoetoenui lahar are greater than 3 implying supercritical flow and the development of fluid mechanical instabilities that enhanced these flow surges

sea level in roman time in the central mediterranean and implications for recent change instrumental records indicate that ocean volumes during the 20th century have increased so as to raise eustatic sea level by f1 2 mm year and the few available records suggest that this is higher than for the previous century geological data indicate that ocean volumes have increased since the main phase of deglaciation about 7000 years ago but whether this continued into the recent past remains unclear yet this is important for establishing whether the recent rise is associated with global warming or is part of a longer duration non anthropogenic signal here we present results for sea level change in the central mediterranean basin for the roman period using new archaeological evidence these data provide a precise measure of local sea level of 1 35f0 07 m at 2000 years ago part of this change is the result of ongoing glacio hydro isostatic adjustment of the crust subsequent to the last deglaciation when corrected for this using geologically constrained model predictions the change in eustatic sea level since the roman period is 0 13f0 09 m a comparison with tide gauge records from nearby locations and with geologically constrained model predictions of the glacio isostatic contributions establishes that the onset of modern sea level rise occurred in recent time at f100f53 years before present d 2004 elsevier b v all rights reserved

continuous monitoring of bedload discharge in a small steep sandy channel this paper reports on bedload flux and texture monitored in a natural steep sandy ephemeral channel draining a small gullied sandy watershed the barranca de los pinos 1 32 ha spain bedload flux was continuously monitored with two independent reid type slot samplers bedload texture was determined from the sediment collected in the samplers channel morphology was surveyed with a high spatial resolution with a terrestrial laser scanner the monitored instantaneous bedload fluxes are among the highest measured in natural rivers characterized by high temporal and spatial variability related to the presence of bedforms shallow bars and sand sheets and to the reworking of the dry bed between and at the end of individual flow events the grain size distribution of the bedload indicates equal mobility but bedload texture fluctuates depicting the transport of coarser bar surfaces and of finer grained anabranch surfaces as well as of the overall bed subsurface

flow resistance under conditions of intense gravel transport a study of flow resistance was undertaken in a channelized reach of the north fork toutle river downstream of mount st helens washington hydraulic and sediment transport data were collected in flows with velocities up to 3 m s and shear stresses up to 7 times the critical value needed for bed load transport details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance the plane gravel bed persists up to stresses 3 times critical at which point irregular bed forms appear bed forms greatly increase flow resistance and cause velocity profiles to become distorted the latter arises as an effect of flows becoming depth limited as bed form amplitude increases at very high rates of bed load transport an upper stage plane bed appeared velocity profiles measured in these flows match the law of the wall closely with the equivalent roughness being well represented by ks 3d84 of the bed load the effects noted here will be important in very large floods or in rivers that are not free to widen such as those cut into bedrock

environmental impact of the 1 8 ka taupo eruption new zealand landscape responses to a large scale explosive rhyolite eruption abstract 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 interaction of volcanism and sedimentation in the proximal areas of a mid tertiary volcanic dome field central arizona u s a abstract exposures of an oligocene small volume dome field in central arizona usa provide an excellent example of the interaction of volcanism and sedimentation in a proximal volcanic setting volcanism in the field began with eruption of a shoshonite cinder cone and lava flows which were emplaced onto braided stream gravels stream sediments aggraded to the top of the lava flows before eruption of an amphibole trachyte dome and its derivative pyroclastic flows deposition from the pyroclastic flows was punctuated by fluvial sedimentation which took the form of both sheetflow reworking and debris flow and hyperconcentrated flow deposition the lack of pronounced channel formation may indicate that inter eruptive periods were brief two principles of volcaniclastic sedimentation are well documented in this study first active aggradation may preserve proximal deposits that are normally eroded quickly from a volcano this aggradation is seen in the interstratification of braided stream gravels derived from outside the area of volcanism with dome derived primary pyroclastic deposits the growing dome apparently did not form a large topographic feature and thus did not overwhelm the preservation caused by regional aggradation the landscape response to volcanism was highly complex and the response of the stream system to short term volcanic processes merely accentuated the effects occurring due to longer term tectonic subsidence second because sediments in the proximal areas of the dome were preserved the interaction of the regional dispersal system with the dome complex is clear pyroclastic deposits were in some cases localized in channels reworked pyroclastic deposits have structures indicative of sheetflow or mass flow but are entirely pyroclastic in composition deposits strongly influenced by the regional system are also mass flows or sheetflows but they contain appreciable concentrations of non volcanogenic material and were deposited by debris choked braided streams

the immediate environmental effects of tephra emission the earth s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere these tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents the interaction of tephra with or in receiving environments may induce an array of physical chemical and biological effects the consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent small eruptions while larger and rarer events may have acute regional scale impacts it is therefore necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems we collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post eruptive effects of tephra emission into the atmosphere onto vegetation soil or ice snow surfaces and in aquatic systems we further consider the repercussions that may be induced in the weeks to years afterwards in the atmosphere tephra can influence cloud properties and air chemistry by acting as ice nuclei in or by offering sites for heterogeneous reactions respectively tephra on vegetation causes physical damage and sustained coverage may elicit longer term physiological responses tephra deposits on soils may alter their capacity to exchange gas water and heat with the atmosphere or may have a specific chemical effect such as nutrient input or acidification on sensitive soils tephra deposition onto snow or ice may affect ablation rates rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged months or years erosion from the tephra covered catchment in the first weeks after deposition tephra leaching may affect river chemistry the abundance and speciation of phytoplankton populations in lakes may be altered by tephra induced changes in water chemistry or sediment water nutrient cycling in the oceans tephra deposition may fertilise fe limited waters with potential impacts on the global carbon cycle embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies combined with consideration of the interconnectivity of induced impacts within and between different receiving environments

preliminary results of excavations at spitzkloof rockshelter richtersveld south africa the recent excavation of spitzkloof rockshelter in namaqualand south africa is part of a larger project called amemsa adaptations to marginal environments in the middle stone age this project is aimed at answering the questions how when and under what environmental conditions were so marginal environments permanently colonized during the middle stone age with over 1500 later stone age 90 middle stone age and 50 early stone age sites in namaqualand evidence of occupation from this southern extension of the namib desert has potential to inform on how people used economic technological and social strategies to adapt to the stress of this environment in order to address these questions the shelter was recently excavated in order to establish a chronological palaeoenvironmental and archaeological record for the region following a biogeographic model to interpret the faunal remains from the bottommost layers brian and genevieve the results reflect evidence for an arid to semi arid and therefore hard environment the subsistence strategy is broad for the region and consists of arid adapted species found on the landscape today non local yellow silcrete suggests the potential for higher levels of mobility than found in later layers the presence of gypsum and trigonephrus sp land snails suggests an arid environment that may have been slightly more humid than today future research will include increasing the sample size through continued excavation while a rigorous radiometric dating program and geomorphology study will hopefully more precisely identify the time frame of this occupation

bed form dependent pulsating flow in medano creek southern colorado some sand bed streams exhibit unsteady supercritical flow in the form of periodic bores that propagate downstream the bores are formed by unstable antidunes which store water while growing in amplitude and then release the stored water when the antidunes wash out to form a plane bed a cyclic process of antidune growth oversteepening antidune breaking then bed form washout produced bores on regular periodic intervals of roughly 15 s in medano creek great sand dunes national monument colorado this periodic bore generating pulsating flow is bed form dependent and it requires sufficiently high flow velocities and sediment transport rates to form unstable breaking antidunes the froude number maximum is 1 8 below that required for slug flow pulsating flow has been observed in flumes arroyos and rivers and it may be more common than initially anticipated its high erosive capacity can influence both channel morphology and the character of associated sedimentary deposits

youngest toba tuff in the son valley india a weak and discontinuous stratigraphic marker abstract investigation of the climatic and environmental impacts of the youngest toba tuff ytt 74 ka bp eruption of toba volcano sumatra is crucial for understanding the consequences of the eruption for contemporaneous human populations the middle son valley in india was the first locality on the indian subcontinent where the ytt was reported the ash bed forms a discontinuous layer stretching for over 30 km along the river here we report on the stratigraphic contexts of ytt ash layers in alluvial deposits of the middle son valley in order to reconstruct the taphonomy of the ash deposits and the dynamic of their deposition although the distal ash has been studied since the 1980s its stratigraphic integrity and the mechanisms and pathways involved in its transport and deposition have bit previously been assessed we find that the ytt occurrences in the middle son valley may not be reliable chronostratigraphical markers for millennial scale palaeoenvironmental reconstruction

effect of oligocene volcanism on sedimentation in the trans pecos volcanic field of texas the nature and history of volcanic activity in the trans pecos volcanic field of texas controlled depositional timing environmental distribution and lithologic character of associated sediments tuffaceous and epiclastic sedimentary rocks of the tascotal and fresno formations and the perdiz conglomerate are part of the intermediate source facies of the field rhyolitic and trachytic activity in major calderas produced abundant pyroclastic debris that has been reworked into sandy alluvial fans adjacent to the caldera complexes whereas post volcanic alluvial fan sediments are tuffaceous or epiclastic conglomerates the intermediate source facies consists of four components active apron inactive or epiclastic apron eolian sand sheet and valley facies the arrangement and timing of development of these facies depend upon the eruptive history of the volcanic centers the tascotal fresno and perdiz contain five depositional units 1 the chinati apron unit recorded rhyolitic and trachytic activity in the chinati caldera during the 105 106 yr after its collapse 2 the southern apron unit formed around the san carlos sierra rica eruptive center in mexico before eruption of the san carlos ignimbrite 3 the conglomerate and limestone unit formed as epiclastic fans around the solitario uplift and as braided stream and lake deposits between aprons and the solitario 4 the eolian unit of the tascotal formed as drainage off the chinati eruptive center adjusted from carrying fine pyroclasts to coarser epiclasts 5 the perdiz conglomerate and equivalent epiclast rich conglomeratic parts of the tascotal and fresno are a bajada like accumulation of fluvial epiclastic conglomerate from non volcanic sources and the chinati eruptive center after its activity ceased framework mineral composition is a function of grain size very fine and fine tuffaceous sandstones consist mostly of reworked glass shards and rocks that have textures coarser than medium sand are mostly epiclastic vrf s active apron rocks formed by deposition from sheet floods and poorly defined channels in a bajada like accumulation that resembles sandy distal alluvial fan deposits because the volcanos provided so much sand sized debris but little clay

debris flow and hyperconcentrated flood flow deposits in an alluvial fan northwestern part of the cretaceous yongdong basin central korea abstract the alluvial fan deposits in the cretaceous yongdong basin korea consist of conglomerates with a muddy or sandy matrix and sandstones with thick or thin laminations the conglomerates and sandstones occur commonly in couplets constituting apparently single sedimentation units facies transitions in the deposits can be summed up in a tripartite facies sequence 1 a clast supported conglomerate with a muddy or sandy matrix and parallel clast alignment facies a and b 2 a matrix supported coarse tail normally graded conglomerate with random clast orientation facies c and thickly stratified pebbly sandstone facies d and 3 laminated sandstone facies e the clast supported conglomerate is interpreted as deposits of debris flows dominated by frictional grain interactions development of pervasive parallel clast alignment lacking large floating clasts and inverse grading suggests deposition via incremental aggradation rather than en masse freezing the matrix supported conglomerate an

uranium metallogenesis in south china and its relationship to crustal extension during the cretaceous to tertiary south china is rich in vein type hydrothermal uranium deposits hosted in granitic volcanic and carbonaceous and siliceous pelitic sedimentary rocks the uranium deposits are spatially associated with extensional structures and or mantle derived mafic dikes both the uranium deposits and mafic dikes are cretaceous to tertiary in age temporally coincident with the crustal extension carbon isotope analyses of calcite deposited in the main stage mineralization in the veins from 12 representative uranium deposits yield 13c values of ore forming fluids mainly from 4 to 8 per mil which are permissive of a mantle origin for the co2 in the ore forming fluids a mantle origin is consistent with the association of the deposits with mafic dikes and the 3he 4he ratios of ore forming fluids e g 0 10 2 02 ra for the volcanic hosted xiangshan uranium deposit isotopic compositions of h and o demonstrate that water in the ore forming fluids is predominantly meteoric in origin ore forming temperatures ranged approximately from 150 to 250 c uranium rich crustal rocks in south china may have been the sources for the uranium crustal extension and associated mafic magmatism are considered to have heated the rocks and allowed co2 possibly from mantle sources to migrate upward and to mix with co2 poor meteoric water the co2 rich hydrothermal fluids mobilized uranium from the source rocks and then the uranium was deposited in various host rocks to form the uranium deposits

blombos cave southern cape south africa preliminary report on the 1992 1999 excavations of the middle stone age levels the later and middle stone age levels at blombos cave bbc were excavated over four field seasons between 1992 and 1999 here we report on the results from the middle stone age msa levels the taphonomy and depositional history of the msa levels is complex due to faulting folding and spalling careful observations during excavation have assisted in understanding some of these taphonomic and site formation processes microstratigraphic analysis currently in progress will add to this information the uppermost msa level the still bay phase contains high densities of bifacial points the fossile directeur of the still bay industry placing the still bay within the msa culture sequence has been problematic in the past because still bay assemblages are rarely found in situ and previous excavations were inadequately recorded however with the regional data discussed in the text the still bay can be securely placed before the howiesons poort dated at 65 70 ka flaked stone makes up the greatest proportion of all artefacts with the highest incidence of retouch and use of fine grained non local materials found in the still bay levels the ochre assemblage is remarkable for the mass of material compared to other msa sites finds uncommon in an msa context are two pieces of ochre from the still bay phase engraved with a geometric design a fragment of deliberately engraved bone also 28 shaped and polished bone tools recovered mainly from a phase just below the still bay blombos cave is the first site where well preserved faunal remains have been recovered in association with the still bay allowing for unique insights into human subsistence behaviour and palaeoenvironmental reconstruction large fish bones marine shells seals and dolphins attest to extensive exploitation of aquatic resources and a wide range of terrestrial animals were hunted and gathered the few human teeth recovered are heavily worn and damaged thus the issue of morphological modernity cannot be addressed the bbc findings are a useful adjunct to findings from other msa coastal sites in the southern cape especially klasies river kr and die kelders cave 1 dk1 uniquely bbc provides insights into human behaviour during a phase of the msa never before studied in detail

origin of uranium deposits revealed by their rare earth element signature uranium deposits form in a wide range of geological settings including deep magmatic to surficial conditions and range in age from archaean to recent these temporal and spatial variations have given rise to an extreme diversity of ore deposits however understanding their conditions of formation has remained challenging this article reports rare earth element ree abundances measured by microbeam methods in uranium oxides for a series of worldwide uranium occurrences the ree patterns are very specific to each deposit type and directly reflect the conditions of their genesis we propose an evaluation of the first order parameters controlling the ree behaviour in each mineralised system this study demonstrates that the ree pattern is the most efficient tool for constraining the geological models of uranium deposits and for genetically discriminating new uranium discoveries this approach may form the starting point for a new procedure in the fight against nuclear trafficking

fluvial responses to volcanism resedimentation of the 1800a taupo ignimbrite eruption in the rangitaiki river catchment north island new zealand abstract the potential for the generation of dangerous and damaging lahars and floods in response to the eruption of voluminous pyroclastic debris has become increasingly appreciated in recent years the style and tempo of this response varies both between eruptions and between individual catchments impacted by a single eruption so that an understanding of the factors controlling this variation is necessary for precise hazard assessment the 1800a taupo eruption from the taupo volcanic centre in the central north island of new zealand devastated an area of 20 000 km 2 during eruption of a climactic ignimbrite impacting the headwaters of all major rivers draining radially from this region the rangitaiki river the subject of this paper differs from other catchments in that the taupo ignimbrite buried an essentially flat land surface inherited from a suite of welded ignimbrite sheets erupted between 320 340 and 230 ka the middle reaches of the catchment are characterised by narrow steep gorges alternating with low gradient basins developed in tectonic half grabens initially remobilisation of pyroclastic material in the headwaters was dominated by hyperconcentrated sheet flows resulting in shallow reworking in higher gradient areas reintegration of drainage networks was achieved by incision of deep channels and gullies assisted by breakouts from ephemeral lakelets developed in ignimbrite dammed depressions braided and later meandering streams superseded this pattern as rill and gully systems stabilised and sediment yields fell leading to a decline in drainage density gorge reaches acted as efficient conduits for remobilised material while the basins acted as local depocentres for the temporary storage of volcaniclastic sediments mediating the transfer of pyroclastic debris to the bay of plenty coast 100 km to the north reworking and resedimentation of pyroclastic debris began immediately after the eruption peaking early and then rapidly declining so that most geomorphic adjustments occurred within one or two decades of the initial disturbance

spatial and temporal patterns in fluvial recovery following volcanic eruptions channel response to basin wide sediment loading at mount pinatubo philippines the june 1991 eruption of mount pinatubo philippines was one of the largest volcanic eruptions of the twentieth century emplacing 5 6 km 3 of pyroclastic fl ow material on the fl anks of the volcano the combination of abundant relatively fi ne grained easily erodible material and intense tropical rainfall led to numerous lahars immediately following the eruption even after major lahars ended sediment yields in some basins remained orders of magnitude above pre eruption levels using data collected from 1996 through 2003 we investigated fi ve basins that experienced varying amounts of sediment loading in the 1991 eruption from 1 to 33 of the basin area covered by valley fi lling pyroclasticfl ow deposits from measurements of fl ow and bed characteristics made through time we developed a general conceptual model for channel recovery following basin wide sediment loading initially fi ner grained sediment and pumice are mobilized preferentially through selective transport once the bed is coarse enough for gravel size clasts to interact with one another clast structures develop increasing form roughness and critical shear stress and inhibiting initial clast mobility as sediment inputs continue to decline the channel incises into valley bottom sediments progressively armoring through winnowing at pinatubo incision and armoring occur fi rst as dry season phenomena due to reduced sediment inputs eventually moving to year round low fl ow bed stability observed timing of the onset of in channel biological recovery suggests that reestablishment of channel stability helps catalyze aquatic ecosystem recovery

key factors controlling volcanic related uranium mineralization in the xiangshan basin jiangxi province south china a review abstract the xiangshan basin hosts the largest volcanic related uranium deposits in china despite several decades of extensive research the key factors controlling volcanic related uranium mineralization in the xiangshan basin remain unclear in this paper we present a review of previous studies and highlight the critical features that are closely related to the uranium mineralization in the xiangshan basin with focus on the geological feathers host rocks ore controlling structures alterations mineralization fluid inclusions and geochronological and geochemical data of the uranium deposits based on these reviews a comprehensive genetic model is proposed to better explain the uranium mineralization mechanism the uranium mineralization at xiangshan is mainly hosted in felsic volcanic and sub volcanic rocks and spatially close to the cretaceous tertiary redbeds the mineralization mainly took place from 125 to 70 ma which is much younger than the ages of host volcanic rocks 135 ma and broadly contemporaneous with the development of the redbed basins and mafic magmatism the uranium mineralization occurs mainly as veins controlled by brittle structures such as fractures faults and volcanic structures the extensional reactivation of preexisting structures and development of new fractures created permeability for the circulation of the ore forming fluids and provided spaces for uranium precipitation the ore forming fluids are characterized by basinal brines with a meteoric water origin possibly mixed with mantle derived fluids the uranium was mostly contributed by the host volcanic and sub volcanic rocks the precipitation of uranium is related to redox reactions with reductants such as fe2 h2s and ch4 as well as other mechanisms related to change of physical and chemical conditions due to fluid boiling the comprehensive review of the geological characteristics related to uranium mineralization reveals that the key factors controlling the uranium mineralization in the xiangshan volcanic basin are related to the coupling of shallow development of oxidizing fluids in the redbeds and brittle structures in the volcanic rocks and deep seated thermal activities related to mafic magmatism geological processes it is proposed that meteoric water infiltrated into the high permeability redbed basins and evolved to oxidizing basinal fluids the oxidizing fluids further circulated into the volcanic basin along brittle structures and extracted uranium from the volcanic rocks the heat provided by mafic magmatic activities accompanying upward flowing deep source hot fluids promoted the uranium extraction and transportation

the geomorphological evolution of the paricutin cone and lava flows mexico 1943 1990 abstract erosion and sedimentation processes in the paricutin cone and adjacent lava fields have been evaluated 47 years after the eruption through field study of the morphological characteristics of the area previous studies indicate an initial stage of accelerated erosion decreasing quickly after the stripping of the ash layers and cessation of the volcanic activities in the cone and crater area revegetation is the main process after the first stage of stripping of fine ash the lava fields are the least affected areas by erosional processes with vegetation development in minor depressions crevices and along the edges of the lava flows the llanos or lava dammed plains are the most active physiographic units with continuous deposition of sediments they are completely covered by vegetation or under agricultural use incision of channels along the lava field edge promotes the integration of the drainage system but still most of the affected area drains into the lave field present erosion rates are higher than normal rates and a complete rehabilitation of the pyroclastic area may extend over decades filling of sediments and revegetation of the lava fields will develop in a longer period of several millennia

the nahal yatir bedload database sediment dynamics in a gravel bed ephemeral stream the nahal yatir bedload transport database arises from the northern negev river sediment monitoring programme and represents the first body of bedload information to be collected during flash floods in desert gravel bed streams bedload flux was established automatically with three slot samplers of the birkbeck type this was complemented by hydraulic measurements that allow sediment transport to be rated against channel average shear stress and specific stream power bed material and bedload grain size distributions are also provided the database is given in both printed and electronic tabular format

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

late quaternary palaeoenvironments of the southwestern cape south africa a regional synthesis the contemporary uniqueness of the southwestern cape environemnt suggests that its response to the environmental perturbations of the late quaternary may have been correspondingly distinctive despite the wide variety of palaeoenvironmental evidence which has accumulated from within the region a coherent picture of the evolution and development of its environments has remained elusive since the publication of several regional reviews over the last decade or so a considerable body of palynological and other palaeoecological evidence has emerged which prompts a fresh synthesis as to the nature of late quaternary environmental changes in the southwestern cape s fynbos biome although there are few sediments which date to the appropriate period the last glacial maximum in contrast to the situation evident in the summer rainfall region is suggested to be a period characterised by considerably cooler and wetter conditions in certain parts of the biome the first half of the holocene appears to have been somewhat drier than much of the second half and the warmer temperatures of the holocene hypsithermal are associated with reduced moisture availability intra regional contrasts are evident in the response to late quaternary environmental changes furthermore it transpires that for much of the later quaternary and for large areas of the region precipitation may have fluctuated out of phase with changes in the interior of the subcontinent a situation which has great relevance to the prediction of future environmental changes and their impacts in the southwestern cape

the influence of explosive volcanism on fluvial sedimentation the deschutes formation neogene in central oregon abstract the upper miocene to lower pliocene deschutes formation of central oregon is a sequence of interstratified volcanic and sedimentary rocks as much as 700 m thick largely derived from the adjacent cascade range the formation is correlative with the early high cascade eruptive episode that included voluminous pyroclastic eruptions and culminated in the development of an intra arc graben in the central oregon high cascades facies associations and paleocurrent data define three depositional settings 1 the eastward sloping arc adjacent alluvial plain was the depositional site by sheetfloods debris flows and hyperconcentrated flood flows their deposits from 10 to 70 m thick sequences separated by erosion surfaces with up to 60 m of relief widespread superimposed paleosols overlie these facies and constitute the upper 10 to 50 m of the section sedimentation was episodic with pulses of rapid aggradation corresponding to periods of explosive volcanism and abundant sediment supply separated by periods of nondeposition when streams incised to reestablish grade as sediment loads diminished 2 a gravel bedload river flowed northward through the basin at the foot of the alluvial plain and is represented by channel fill conglomerates and overbank sandstones and siltstones 3 a thin sequence derived from older tertiary highlands was deposited on the east side of the basin and was little affected by cascade volcanism extensive development of thick superimposed paleosols reflects the inactive nature of the eastern basin margin as a sediment source deschutes formation deposition records the response of a fluvial system to large volcanism induced sediment loads popular facies models do not adequately allow for the influence of volcanism on fluvial systems because sediment is mobilized episodically and on a scale unparalleled in nonvolcanic settings

sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt ellensburg formation late miocene central washington late miocene volcaniclastics of the ellensburg formation washington northwestern usa are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a cascade range source area two facies associations are recognized the first is composed of laterally restricted well sorted polymictic conglomerates representing a gravel bedload regime during inter eruption periods the second comprises laterally extensive sheets bounded by deeply developed paleosols composed of monomictic pebbly pumiceous dacitic sandstones with intercalated debris flow and hyperconcentrated flood flow deposits these sheets aggraded in response to eruptive events that are sometimes recorded by air fall tephras at the bases of sequences debris flow and hyperconcentrated flood flow deposits occur as far as 120 km from source but are uncommon beyond 50 km hyperconcentrated flood flow deposits initially increase in abundance away from source as debris flow deposits diminish suggesting formation of hyperconcentrated flood flows by dilution of debris flows sandy facies form broad sheets dominated by scour fill bedding in proximal and medial settings and grade to narrower trough cross bedded sheets in distal settings suggesting moderation of flood discharges with distance base level changes associated with episodic sediment influx caused incision aggradation cycles in mainstream settings and episodic impoundment of tributaries to form lakes or rapidly aggrading poorly drained floodplains although volcanism was the primary control on depositional style concurrent development of the yakima fold belt produced a structure consequent drainage pattern that determined sediment dispersal and basin subsidence permitted preservation of both syn eruption and inter eruption facies detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics but dominates deposits of small tributary streams flowing toward rather than from the cascades

lithofacies variations within sandur deposits the role of runoff regime flow dynamics and sediment supply characteristics abstract the icelandic sandur plains have for long acted as classic sites for models of braided river behaviour and of associated lithofacies assemblages however many of the icelandic sandurs are subject to repeated catastrophic flood events or jokulhlaups which are likely to have played a significant role in sandur construction established lithofacies models based on the icelandic sandurs have failed to take account of the varying runoff regimes which characterise the different sandur environments in addition sediment inputs to sandur channel systems are highly variable fluctuating in response both to varying runoff patterns and to advance and retreat of the source glacier that affect sediment availability within the proglacial zone this paper describes the main lithofacies types and vertical profiles that characterize the sandurs of south iceland and demonstrates that they can largely be attributed to extreme runoff events with few deposits typical of braided river facies occurring within the sedimentary record a preliminary qualitative model is developed that allows interpretation and prediction of vertical lithofacies profiles derived from glacial outwash in terms of i the drainage mechanism which in turn controls the dominant runoff population ii sediment availability and iii the spatial variability in flow dynamics

channel bed evolution and sediment transport under declining sand inputs 1 coupled field and laboratory investigations explore links between bed surface structure development and sediment transport as sand inputs decline on the pasig potrero river we investigated channel recovery following emplacement of sand rich pyroclastic deposits in the 1991 eruption of mount pinatubo philippines as sediment inputs declined from 1996 to 2003 surface grain size increased clast structures formed and sediment mobility and bed load transport rates declined our field results parallel those from flume experiments studying highly energetic channels with similar sand contents as the feed mixture shifted from 70 to 40 sand surface grain size and gravel interactions increased forming clusters and armor patches bed reorganization into patches initially accommodated coarsening with minimal effect on bed load transport rates roughness or friction angle within sandy transport zones as sand declined further gravel jams interrupted the connectivity of sandy transport zones lowering transport rates patch development lowered the threshold for the transition from sand dominated to gravel dominated transport focusing this transition over a narrow range of sand content

dating young geomorphic surfaces using age of colonizing douglas fir in southwestern washington and northwestern oregon usa dating of dynamic young 500 years geomorphic landforms particularly volcanofluvial features requires higher precision than is possible with radiocarbon dating minimum ages of recently created landforms have long been obtained from tree ring ages of the oldest trees growing on new surfaces but to estimate the year of landform creation requires that two time corrections be added to tree ages obtained from increment cores 1 the time interval between stabilization of the new landform surface and germination of the sampled trees germination lag time or glt and 2 the interval between seedling germination and growth to sampling height if the trees are not cored at ground level the sum of these two time intervals is the colonization time gap ctg such time corrections have been needed for more precise dating of terraces and floodplains in lowland river valleys in the cascade range where significant eruption induced lateral shifting and vertical aggradation of channels can occur over years to decades and where timing of such geomorphic changes can be critical to emergency planning earliest colonizing douglas fir pseudotsuga menziesii were sampled for tree ring dating at eight sites on lowland 750 m a s l recently formed surfaces of known age near three cascade volcanoes mount rainier mount st helens and mount hood in southwestern washington and northwestern oregon increment cores or stem sections were taken at breast height and where possible at ground level from the largest oldest looking trees at each study site at least ten trees were sampled at each site unless the total of early colonizers was less results indicate that a correction of four years should be used for glt and 10 years for ctg if the single largest and presumed oldest douglas fir growing on a surface of unknown age is sampled this approach would have a potential error of up to 20 years error can be reduced by sampling the five largest douglas fir instead of the single largest a glt correction of 5 years should be added to the mean ring count age of the five largest trees growing on the surface being dated if the trees are cored at ground level this correction would have an approximate error of 5 years if the trees are cored at about 1 4 m above the ground surface breast height a ctg correction of 11 years should be added to the mean age of the five sampled trees with an error of about 7 years published in 2006 by john wiley sons ltd

posteruption suspended sediment transport at mount st helens decadal scale relationships with landscape adjustments and river discharges received 11 december 2002 revised 30 september 2003 accepted 18 november 2003 published 17 january 2004 1 widespread landscape disturbance by the cataclysmic 1980 eruption of mount st helens abruptly increased sediment supply in surrounding watersheds the magnitude and duration of the redistribution of sediment deposited by the eruption as well as decades to centuries old sediment remobilized from storage have varied chiefly with the style of disturbance posteruption suspended sediment transport has been greater and more persistent from zones of channel disturbance than from zones of hillslope disturbance despite the severe landscape disturbances caused by the eruption relationships between discharge magnitudes and frequencies and suspended sediment transport have been remarkably consistent discharges smaller than mean annual flows generally have transported 5 but locally 15 of the annual suspended sediment loads and infrequent p 0 01 large floods have transported as much as 50 of the annual suspended sediment loads in a single day however moderate magnitude discharges those greater than mean annual flows but less than 2 year floods have transported the greatest amounts of sediment from all disturbance zones such discharges have transported on average 60 to 95 of the annual suspended sediment loads usually within cumulative periods of 1 3 weeks each year although small magnitude and largemagnitude discharges have locally and episodically transported considerable amounts of suspended sediment there has not been any notable change in the overall nature of the effective discharges moderate magnitude flows have been the predominant discharges responsible for transporting the majority of suspended sediment during 20 years of posteruption landscape adjustment index terms 1824 hydrology geomorphology 1625 1815 hydrology erosion and sedimentation 1860 hydrology runoff and streamflow keywords sediment yield suspended sediment transport mount st helens erosion volcano geomorphology

environmental response to a large explosive rhyolite eruption sedimentology of post 1 8 ka pumice rich taupo volcaniclastics in the hawke s bay region new zealand abstract the 1 8 ka eruption of the taupo ignimbrite from the taupo volcanic centre in the central north island of new zealand produced a 30 km3 ignimbrite covering an area ca 160 km in diameter centred on lake taupo remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement in the hawke s bay region in the eastern north island three large rivers rise in the ignimbrite impacted mountainous headwaters in areas that were buried by up to 40 m of loose unconsolidated pumiceous material analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment water ratios the first comprised erosion and redeposition of ignimbrite debris by lahars e g debris flows and hyperconcentrated flows the second was marked by decreasing sediment water ratios and the re establishment of braided fluvial systems in the former river valleys as recorded by such deposits as trough planar low angle and antidune cross bedded planar bedded and ripple cross laminated sands and gravels however there is no systematic chronological trend towards lower pumice contents in these fluvial facies four characteristic sets of co occurring lithofacies were produced during the sedimentary response 1 laharic remobilisation association 2 upper flow regime dominated stream flow association 3 lower flow regime dominated stream flow association and 4 lacustrine association individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice particularly its low and variable density automated settling tube analysis of water saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments but statistical sorting and mean grain size parameters do not discriminate between the deposits of tractional and overcapacity parent flows therefore particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits

lahar hazard assessment using titan2d for an alluvial fan with rapidly changing geomorphology whangaehu river mt ruapehu abstract rapid changes in small areas at the apex of alluvial fans may have devastating consequences by directing downstream flood or lahar impacts into catchments of widely varying population or infrastructure vulnerability during a series of lahars in 1995 at mt ruapehu new zealand aggradation of the whangaehu fan apex draining the eastern edifice caused the onset of avulsion of flows northward into the highly vulnerable tongariro catchment an earth training dike or bund was constructed to protect this catchment by retaining flows on the southern side and the normal lahar outlet path to the south surveys in 2001 late 2005 and following a major lahar in march 2007 now show net degradation of a channel in the whangaehu fan apex bordered by the bund this indicates a net increase in the channel capacity at this site and shows that the bund remains at its effective design capacity past hydrological modelling used for the bund design provided a large range of discharge estimates but lacked precise constraints on the size and nature of lahars from eruption and lake breakout events new modelling has been carried out using titan2d to examine the impacts of a 6 106 m3 volume granular flow down this catchment this simulates either an eruption or a lake breakout induced lahar with a historically typical volumetric bulking factor of 4 these simulations predict minimum discharges between 1800 and 2100 m3 s at the bund site by comparison the largest 1995 flow at this site was estimated at around 1200 m3 s further any single modelled flow from the normal outlet channel of crater lake could not be induced to overtop the bund because discharge appears to be limited by the narrow upper reaches of the whangaehu gorge theoretical discharge levels required to overtop the bund are estimated to be 6800 m3 s assuming no aggradation of the channel by the decelerating flow maximum potential discharge at the bund site is additionally modified by potential bifurcation of lahars above a certain size threshold at a point 2 5 km upstream of the bund a major side channel the chute effectively diverts part of the flows away to the southern side of the whangaehu fan this potential for splitting of the flow appears to have increased since 1995 from aggradation of lahar deposits and reworked sediment in the area immediately upstream of the divide despite remaining at design capacity for lahar events in the whangaehu erosion of new channels following the 2007 lahar renders the bund vulnerable to undercutting

a 300 kyr record of aridity and wind strength in southwestern africa inferences from grain size distributions of sediments on walvis ridge se atlantic abstract the terrigenous fraction of sediments recovered from walvis ridge se atlantic ocean reveals a history of southwestern african climate of the last 300 kyr end member modelling of a data set of grain size distributions n 428 results in three end members the two coarsest end members are interpreted as eolian dust the third end member as hemipelagic mud the ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the se trade winds trade winds were intensified during glacials compared to interglacials changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern african aridity late quaternary southwestern african climate was relatively arid during the interglacial stages and relatively humid during the glacial stages owing to meridional shifts in the atmospheric circulation system during glacials the polar front shifted equatorward resulting in a northward displacement of the zone of westerlies causing increased rainfall in southwestern africa the equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient leading to enhanced atmospheric circulation and increased trade wind intensity

yanshanian late mesozoic ore deposits in china an introduction to the special issue abstract the late jurassic cretaceous yanshanian orogeny or yenshan movement one of the most important tectonothermal events is first recognized in china especially eastern china this late mesozoic orogeny which was initiated most likely by a mesozoic tectonic switch strongly reworked or destructed the older continental lithospheres or cratonic keels that are manifested by alternating compressive and extensional deformation voluminous igneous rocks and a variety of characteristic magmatic hydrothermal mineral systems despite its first discovery and definition in yenshan yinshan area of north china craton the yanshanian orogeny probably is of global tectonic magmatic and metallogenic significance however there have been hot debates on the precise starting time accurate duration or time interval detailed processes and evolution linked to deep lithospheres tectonic nature and geodynamic mechanism s of the yanshanian orogeny which inevitably have hindered the understanding of the genesis mineralizing processes and geodynamic mechanism of the late mesozoic magmatic hydrothermal mineral systems this special issue captures some of the latest research results on the yanshanian ore deposits that are involved into a few main mesozoic metallogenic belts or provinces from northeast to southwest china including 1 the jiaodong peninsula metallogenic province in the north china craton 2 the middle lower yangtze river valley metallogenic belt in the central eastern china 3 the jiangnan and 4 the nanling metallogenic belts in the south china block 5 the southeastern china coast metallogenic belt and 6 the sanjiang metallogenic belt in southwest china through a multidisciplinary study this special issue re investigated and re evaluated the relationship between the late mesozoic magmatic hydrothermal mineral systems and the yanshanian tectonothermal events in the studied metallogenic belts or provinces a few important contributions to the topic in this special issue yanshanian metallogeny are summarized as followings 1 a new ore deposit type i e the intracontinental reactivation type has been suggested to interpret the genesis of those au polymetallic deposits that are hosted within older metamorphic rocks and related to the late mesozoic basin and range extensional settings 2 late mesozoic re activation of the preexisting structures by the yanshanian tectono thermal event s might be an important mechanism controlling the yanshanian large scale mineralization 3 a type granites formed by partial melting of the mesoproterozoic crust but with inputs from mantle derived melt are also favorable for sn mineralization in addition to s type and i type granites as previously recognized 4 calculated oxygen fugacities o 2 of granitic magmas based on chemical compositions of primary biotite have been confirmed to be effective proxy for distinguishing cu au mo w sn pb zn mineralized granites from barren granites 5 a significant epoch of w sn magmatic hydrothermal ore system at ca 145 135 ma has been identified in the southeastern china coast metallogenic belt and 6 in addition to traditional structural geology mineralogy petrology geochemistry and geochronology new analytical techniques e g cu isotopes and data treatment method e g bi dimensional empirical mode decomposition can be used to provide more constraints for deep exploration

a technological and morphological study of late paleolithic ostrich eggshell beads from shuidonggou north china abstract we report the results of a detailed analysis of ostrich eggshell oes beads derived mainly from cultural layer 2 cl2 of locality 2 at the shuidonggou site sdg2 in north china which is dated to ca 31 ka cal bp the eggshells belong to the extinct ostrich struthio anderssoni based on microscopic examination morphometric analysis and experimental replication we identify clear differences in morphology size technology and style results indicate that the technology of bead making is similar to that used in most middle and later stone age sites in africa and recorded ethnographically both well made and poorly crafted oes beads were produced at sdg2 drilling experiments conducted in the framework of this study show that hafted stone points were probably used to make the perforations only occasionally beads were deliberately polished on inner and outer eggshell surfaces beads morphology and technology suggest that distinct types of beads were made by different individual craftspeople this supports the hypothesis that several human groups visited the shuidonggou site and used oes beads as an information technology about 31 ka cal bp

dynamic interactions between lahars and stream flow a case study from ruapehu volcano new zealand three lahars were sampled in the whan gaehu river on the eastern flank of ruapehu volcano new zealand at 23 5 and 42 km from the source as they were flowing on september 27 and 29 and october 6 1995 the lahars were generated by water explosively ejected from the highly mineralized crater lake we used the chemical contrast between the lahars and resident stream water in their paths to describe a four phase model of a noncohesive lahar wave in a river channel 1 ambient stream water pushed ahead of the lahar in a process of miscible displacement due to hydrodynamic dispersion 2 a zone of mixing between the stream water and the lahar that increases in length with distance from source 3 a remnant of the original lahar least diluted by stream water that decreases in length and dilutes downstream 4 the tail of the lahar surge peak discharge occurs at the end of phase 1 as water is pushed ahead of the lahar peak sediment concentration occurs at the end of phase 2 where debris entrainment by the flow is at its greatest i e in front of the lahar proper deposits record only phases 2 and 3 of the lahar wave phase 1 flow left only a tide line of organic debris and phase 4 deposits were rapidly eroded by later streamflow downstream dilution by stream water eventually caused transformation of phases 2 and 3 of the lahars from debris flow to hyperconcentrated streamflow and then to normal streamflow as the flows became progressively finer grained and more turbulent

plio pleistocene pumice floods in the ancestral rio grande southern rio grande rift usa abstract at least four times during the late pliocene and early pleistocene pyroclastic eruptions in the jemez volcanic field northern rio grande rift flooded the ancestral rio grande with gravel sized pumice following as much as 400 km of fluvial transport the pumice was deposited in beds 0 2 to 2 0 m thick in the camp rice formation of the southern rio grande rift a combination of reversal magnetostratigraphy and single crystal sanidine 40 ar 39 ar dating constrains the ages of pumice clast conglomerates at 3 1 2 0 1 6 and 1 3 ma the coarsest pumice beds cobbles boulders were deposited as antidune like bedforms in a fluvial channel and as a crevasse splay sheet granule and pebble sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain the abundance of pumice clasts in the gravel fraction 60 100 suggests very rapid transport downriver probably in a few days or weeks the two older pumice clast conglomerates correlate with the puye formation in the jemez volcanic field whereas the younger two are coeval to the lower bandelier tuff and cerro toledo rhyolite

rainfall induced lahars in the belham valley montserrat west indies rain falling on loose volcanic debris over the soufriere hills volcano montserrat generates hazardous floods in the belham valley these rainfall induced lahars vary greatly in discharge and sediment concentration in space and time they differ from examples documented on other volcanoes in that 1 the eruption has been continuing since july 1995 generating repeated pulses of excess sediment 2 rainfall is the only significant trigger 3 the system is small with short distance to the sea and relatively low altitude at the catchment top repeat mapping and comparison with pre eruption data demonstrate significant geomorphological change with c 120 m shoreline progradation and c 0 4 m a 1 mean aggradation rate in the middle to lower valley the nature of the hazard and area of risk have changed as the valley has aggraded the channel widened and the runoff efficiency increased as a result of rilling and vegetation removal lahars in the belham valley correlate with days when 10 mm rain fell in 24 h with more events triggered in the late rainy season the flows are mainly newtonian but one non newtonian flow event has been demonstrated and is described in detail 20 march 2000 this flow is explained by direct volcanic ash input to the runoff

metallogeny of the zoujiashan uranium deposit in the mesozoic xiangshan volcanic intrusive complex southeast china insights from chemical compositions of hydrothermal apatite and metal elements of individual fluid inclusions abstract the zoujiashan u deposit in the mesozoic xiangshan volcanic intrusive complex is one of the largest volcanic related type u deposits in china chemical composition of hydrothermal apatite and metal contents in individual fluid inclusions of quartz and fluorite are investigated in order to understand the precipitation mechanism of u and to further constrain the origin of u and ore forming fluids of the zoujiashan deposit illitization hematitization and u mineralization successively occurred within rhyolite and porphyritic lava three types of hydrothermal apatite are identified apatite type 1 ap1 and apatite type 2 ap2 are both euhedral subhedral crystals found in hematitization zone while only ap2 shows evident irregular chemical zoning along its rims apatite type 3 ap3 occurs as anhedral aggregates closely associated with u minerals in u ore veins implying synchronous precipitation of ap3 and u ores comparable chemical compositions of ap1 and cores of ap2 including similar high cl content indicate both of them were precipitated from a cl rich fluid that induced hematitization the elevated f contents of ap3 and rims of ap2 implies that a f rich fluid was involved and altered ap2 during the main stage of u deposition ap1 and cores of ap2 have lower mn contents than ap3 and rims of ap2 indicating an oxidized character for the cl rich fluid and a reducing character for the f rich fluid metal contents of fluid inclusions in fluorite and quartz occurring in u ores suggest that two end members of fluids including a u rich and a u poor were involved in the main stage of u mineralization mixing between these two fluids led to the deposition of u and th minerals through oxidation reduction reactions and destabilization of th f complexes respectively the u rich fluid is depleted in th and sr compared with the u poor fluid based on comparison of chemical compositions of apatite and fluid inclusions we suggest that the cl rich fluid precipitating ap1 and ap2 corresponds to the u rich end member fluid while the f rich fluid precipitating ap3 represents a mixture between the two end members the high u 12 7 58 5 ppm in the u rich end member fluid is in agreement with a relatively high oxygen fugacity and a surface derived origin the u poor end member fluid has lower rb cs than the u rich end member and the unaltered host rocks indicating the u poor fluid was derived from a mantle wedge that experienced addition of cs by fluids generated from the devolatilization of the subducted paleo pacific slab

trace element and sr nd isotope geochemistry of fluorite from the xiangshan uranium deposit southeast china the trace element and sr nd isotope geochemistry of fluorite have been investigated in the xiangshan volcanic rock hosted uranium deposit in southeast china detailed sr nd isotope data of the hydrothermal fluorite closely associated with the uranium mineralization suggest that ore forming fluids were a combination of mantle derived fluid and fluids equilibrated with basement metamorphic rocks this model is consistent with previously published h o isotope data and is incompatible with meteoric water as the only ore forming fluid trace element geochemical profiles across the uranium orebody can be interpreted to reflect reaction between the host rocks and a mantle derived fluid containing high u and th with low th u ratios the mantle derived fluid is considered to have been u f and carbonate rich mixing with shallowly circulated meteoric water within the fractures of the host volcanic rocks caused dilution and cooling of the ore forming fluid and subsequent precipitation of fluorite and uranium ore