phytoplankton associations of the anisian peri tethys basin central europe evidence of basin evolution and palaeoenvironmental change abstract anisian lower muschelkalk phytoplankton associations of the nw tethys shelf and the northern peri tethys basin are characterized by acritarchs and prasinophytes stratigraphical and lateral variations in the distribution pattern of these phytoplankton groups were analyzed in key sections of the central basin germany the southern gate areas switzerland poland and the northwestern tethys shelf hungary with the aim to infer palaeoenvironmental changes related to the basin evolution stratigraphically maximum abundance of marine phytoplankton is recognized during phases of maximum flooding laterally a dominance of different phytoplankton groups within these intervals is documented in the central part of the basin prasinophytes reach 80 percent of the total plankton association sediments of the basin margin the gate areas and proximal shelf show high amounts of acritarchs ranging from 76 to 95 these signatures clearly reflect the basin evolution of the anisian lower muschelkalk peri tethys basin and the interaction between a restricted intracratonic basin and an open ocean generally the peri tethys basin is characterized by a stratified water column with oxygen depleted bottom waters and maximum abundance of prasinophytes during phases of transgression cooler waters from the shelf flood the restricted basin displayed in the acritarch dominated phytoplankton associations of the shelf and gate areas marginal parts of the basin are generally well oxygenated and dominated by acritarchs within the acritarch assemblage veryhachium spp is characteristic of the marginal and central parts of the basin whereas micrhystridium spp dominates shelf deposits and sediments of the gate areas respectively the present study highlights the significance of different phytoplankton associations in basin analysis and high resolution sequence stratigraphy

new tree ring dates for recent eruptions of mount st helens abstract distinctive patterns of growth rings in increment cores from old growth douglas fir pseudotsuga menziesii stands identify a d 1800 as a more precise date for the eruption of tephra layer t by mount st helens washington layer t was previously inferred to date to about a d 1802 growth patterns also establish a d 1480 as the date of eruption of the earlier layer wn previously estimated as dating to about a d 1500 the timing of radial tree growth places a small limitation on the seasonal resolution of these new tree ring dates

mount st helens 1980 and mount pelee 1902 flow or surge the far reaching directed blast of mount st helens and the devastating nuees ardentes of mount pelee produced deposits that in many places consist of three main layers a basal gravelly or sandy layer 1 a massive or bedded ash layer 2 and a capping ash cloud layer 3 these thin and in part landscape mantling deposits are generally ascribed to pyroclastic surges and the dune bedding seen in parts of layer 2 has reinforced this interpretation there are several reasons however for preferring a pyroclastic flow origin 1 much of layer 2 is unequivocal pyroclastic flow 2 the tripartite subdivision and landscape mantling habit is similar to that displayed by ignimbrites of the low aspect ratio type 3 the deflation of a pyroclastic surge and the subsequent deposition of particulate material will tend not to conserve the fine ash and dust that are abundant in valley ponded parts of layer 2 and cannot explain the observed tendency for layer 2 to show stronger fines depletion on ridges than in valley bottoms and 4 the coarseness and the variance of grain size found within dune bedded bed sets in mount st helens layer 2 are like those of very weak pyroclastic surges and the dune bedding most likely resulted from local minor turbulence as could have been caused by surface roughness in a mountainous terrain littered with tree stumps and fallen trees in a thin depositing pyroclastic flow the directed blast and nuees ardentes at mount st helens and mount pelee are interpreted to have been violently emplaced pyroclastic flows producing deposits of low aspect ratio type the characteristics of which stem from an exceptionally high flow velocity

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

interrelations among pyroclastic surge pyroclastic flow and lahars in smith creek valley during first minutes of 18 may 1980 eruption of mount st helens usa a devastating pyroclastic surge and resultant lahars at mount st helens on 18 may 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank the tributaries channeled the flows to smith creek valley which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank stratigraphy shows that the pyroclastic surge preceded the lahars there is no notable wet character to the surge deposits therefore the lahars must have originated as snowmelt not as ejected water saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano in stratigraphic order the smith creek valley floor materials comprise 1 a complex valley bottom facies of the pyroclastic surge and a related pyroclastic flow 2 an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris and 3 deposits of secondary pyroclastic flows these units are capped by silt containing accretionary lapilli which began falling from a rapidly expanding mushroom shaped cloud 20 minutes after the eruption s onset the smith creek valley bottom pyroclastic facies consists of a a weakly graded basal bed of fines poor granular sand the deposit of a low concentration lithic pyroclastic surge and b a bed of very poorly sorted pebble to cobble gravel inversely graded near its base the deposit of a high concentration lithic pyroclastic flow the surge apparently segregated while crossing the steep headwater tributaries of smith creek large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge the unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough brown muddy sand matrix like that of lahar deposits upvalley this unit contains irregular friable lenses and pods meters in diameter blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier the hummocky unit is the deposit of a high viscosity debris flow which formed when lahars mingled with the pyroclastic materials on smith creek valley floor overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines poor gas escape pipes and containing charred wood the deposits are thickest in topographic lows along margins of the hummocky diamict emplaced several minutes after the hot surge had passed this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides

edifice collapse and related hazards in guatemala abstract guatemalan volcanoes have at least seven debris avalanche deposits associated with cerro quemado fuego pacaya tecuamburro and an unidentified volcano the deposits range in size from less than 0 1 to in excess of 9 km 3 and from 2 5 to in excess of 300 km 2 the avalanches traveled 3 to 50 km from their sources in the guatemalan highlands three of the avalanches occurred in late pleistocene time and four in holocene time two of them within the last 2000 years the avalanches occurred at both andesitic and basaltic stratovolcanoes and at dacitic dome complexes laterally directed phreatic or magmatic pyroclastic explosions were associated with two of the debris avalanches an evaluation of factors that might lead to an edifice collapse in guatemala is based on the case studies presented in this report and a survey of the literature edifice collapses are more apt to occur if zones of weakness exist within the volcanic edifices such as unwelded pyroclastic rocks and pervasively altered rocks further the trench ward side of volcano pairs like fuego and atitlan may be more likely to fail because it may have weak zones along the contact with the older back arc volcano the direction of failure may be influenced by regional slopes which in guatemala generally trend southward toward the oceanic trench and by such structural factors as multiple vents or overly steep slopes reflecting previous activity or erosion debris avalanches are more likely to occur in drainages which have headwaters at two or more volcanoes domes are especially apt to produce small to moderate sized debris avalanches and further if the domes form a coalescing chain are most likely to fail in a direction normal to the chain these factors are used at seventeen major volcanic centers in guatemala to assess their potential for edifice collapse and most probable direction of failure

migration of widespread long lived volcanism across the galapagos volcanic province evidence for a broad hotspot melting anomaly abstract the well studied galapagos archipelago is a small part of the much larger galapagos volcanic province gvp consisting of the cocos carnegie coiba and malpelo aseismic ridges and related seamount provinces although these aseismic ridges and seamounts dominate the morphology of the region little is known about their origin due to a lack of direct age and geochemical information in order to establish how well the gvp fits with the predictions of the standard fixed hotspot and mantle plume hypotheses we conducted a first reconnaissance dredge grab sampling of submerged regions of the gvp we present here 40ar 39ar ages for many of these new basement samples and evaluate their implications for the various models put forward to explain the origin of the gvp correlating new and published sample site ages with distance from the western side of the galapagos islands show that volcanism has not progressed in narrow time progressive lines of seamounts and ridges as predicted by the conventional fixed hotspot and mantle plume hypothesis rather volcanism apparently migrated time progressively across the gvp in broad regions of long lived and possibly concurrent volcanism we propose that the most viable explanation for these observations is that the gvp is the product of cocos and nazca plate motions across a broad hotspot melt anomaly the complex spreading history of the cocos nazca spreading centre likely controlled the relative distribution of gvp volcanism between the cocos and nazca plates while creating lithosphere of variable age thickness across the region while the notion of a broad galapagos hotspot melting anomaly linked to a complex regional tectonic history requires significant testing it nevertheless highlights the need to test alternative mantle upwelling shapes and sizes compared to the widely accepted notion of a narrow continuous long lived galapagos mantle plume conduit defined by the size and location of a galapagos island

acid resistant microfossils used in the determination of palaeozoic palaeoenvironments in libya abstract a study of silurian acid resistant microfossils from the ghadames basin in tripolitania libya using palynological techniques revealed that the following may be used as palaeoenvironmental indicators spores acritarchs chitinozoa tasmanitids scolecodonts plant tissue eurypterid cuticle and graptolite siculae the palynofacies were defined by quantitative and qualitative diversification of these assemblages sedimentological features were used to confirm these proposed facies six distinct types of palynofacies were identified and related to models of distance from the shoreline it is tentatively suggested that they may relate to lagoonal intermediate and open marine environments

charcoal in the silurian as evidence for the earliest wildfire rare basal pridoli plant fossils which resemble the rhyniophytoid hollandophyton colliculum and have exceptional three dimensional cellular anatomy are preserved as charcoal as such these fossils from ludford lane in the welsh borders are evidence of the earliest recorded wildfire and are the first documented from before the devonian the fossils represent the charred remains of a low growing low diversity in situ rhyniophytoid or cryptospore bearing plant vegetation charred during a low temperature fire

fluid dynamics of the 1997 boxing day volcanic blast on montserrat west indies 1 directed volcanic blasts are powerful explosions with a significant laterally directed component which can generate devastating high energy pyroclastic density currents pdcs such blasts are an important class of eruptive phenomena but quantified understanding of their dynamics and effects is still incomplete here we use 2 d and 3 d multiparticle thermofluid dynamic flow codes to examine a powerful volcanic blast that occurred on montserrat in december 1997 on the basis of the simulations we divide the blast into three phases an initial burst phase that lasts roughly 5 s and involves rapid expansion of the gas pyroclast mixture a gravitational collapse phase that occurs when the erupted material fails to mix with sufficient air to form a buoyant column and thus collapses asymmetrically and a pdc phase that is dominated by motion parallel to the ground surface and is influenced by topography we vary key input parameters such as total gas energy and total solid mass to understand their influence on simulations and we compare the simulations with independent field observations of damage and deposits demonstrating that the models generally capture important large scale features of the natural phenomenon we also examine the 2 d and 3 d model results to estimate the flow mach number and conclude that the range of damage sustained at villages on montserrat can be reasonably explained by the spatial and temporal distribution of the dynamic pressure associated with subsonic pdcs

swift snowmelt and floods lahars caused by great pyroclastic surge at mount st helens volcano washington 18 may 1980 the initial explosions at mount st helens washington on the moring of 18 may 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano near source surge deposits on the east and west were lithic sorted lacking in accretionary lapilli and vesiculated ash not plastered against upright obstacles and hot enough to char wood all attributes of dry pyroclastic surge material deposited at the surge base on steep slopes near the volcano transformed into high concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods lahars arrived this field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars nor is there evidence that meters thick snow slab avalanches intimately mixed with the surge to form the floods the floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows as these flows removed the surface snow and pyroclasts newly uncovered snow was partly melted by the turbulent surge base this and accumulating hot surge debris in turn began flowing a self sustaining process feeding the initial flows the flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta laden sheetfloods gravity accelerated the floods to more than 100 km h as they swept down and off the volcano flanks while the snow component melted to form great debris rich floods lahars channeled into valleys

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

chitinozoan bio event in the frasnian famennian boundary beds at la serre montagne noire southern france abstract chitinozoans and other palynomorphs have been investigated in frasnian famennian boundary beds from trench c at la serre montagne noire southern france this section is located some 30 km from the coumiac quarry containing the global stratotype section and point gssp for the base of the famennian at la serre the frasnian famennian f f boundary falls within interbedded anoxic dark shales and limestones this lithology proves to be more suitable for palynological investigations than the reddish nodular limestones of the f f stratotype in the coumiac section which are virtually devoid of organic matter at la serre the uppermost meter of frasnian deposits corresponding to the upper kellwasser event level contains only a few chitinozoans but other palynomorphs especially tasmanaceans maranhites proliferate in some horizons conversely the most basal famennian bed yields an exceptional number of chitinozoans up to 19 000 specimens per gram of rock a few tracheids but virtually no tasmanaceans the absence or the extremely low numbers of miospores and acritarchs in these f f boundary beds seems related to the distal position of the deposition site we have compared the fluctuation of the chitinozoan abundance both with the abundance of the other palynomorphs and with that of the conodonts present in the same samples these data collated with sedimentological and palaeoecological information document the relationships between the anomalous abundance of the chitinozoans and the latest frasnian earliest famennian mass extinction it is concluded that the exceptional chitinozoan concentration in the most basal famennian bed at la serre is not related to a sorting process generated by turbidity currents or by a temporary increase in the hydrodynamic energy at the water sediment interface it corresponds to the conjunction of a high production of chitinozoans a very low rate of destruction of the vesicles and minimal dilution of the deposit by terrestrial mineral input the environmental factors leading to such an exceptionally high record of chitinozoans are discussed with a particular attention to consequences of the late frasnian mass extinction

palaeoenvironmental interpretation of microphytoplankton diversity trends in the cambrian ordovician of the northern sahara platform abstract analysis of vertical fluctuations in assemblage diversity and relative abundances of microphytoplankton morphological classes permits palaeoenvironmental reconstructions of the cambrian ordovician stratal sequences in the hassi r mel area north central algeria and northern rhadames basin southern tunisia abundance fluctuations of acritarch morphological classes and assemblage diversity appear to be related to changes in depositional facies the present palaeoenvironmental interpretation is primarily based on a comparison with microphytoplankton distributional trends in modern depositional environments where possible the observations are tested against palaeoenvironmental information derived independently from previous lithostratigraphical macropalaeontological and petrographical studies of the formations investigated the present data support the following palaeoenvironmental scenario during late cambrian times relatively shallow water neritic conditions prevailed throughout the study area with evidence for a more proximal environment in southern tunisia in the early tremadoc open marine offshore conditions characterized the central algerian region but a shallower water setting is indicated for southern tunisia during the late arenig llanvirn interval an open marine offshore setting is envisaged for the southern tunisian region with more proximal conditions prevailing in the algerian hassi r mel area during the entire ashgill an essentially restricted marine environment was established throughout the study area some stratigraphically important acritarch species e g acanthodiacrodium angustum appear to be facies sensitive the recognition of palaeoecological control on microphytoplankton distribution has important consequences on the biostratigraphic application of these microfossils

was the 18 may 1980 lateral blast at mt st helens the product of two explosions the 18 may 1980 lateral blast at mt st helens has been interpreted as the product of a single explosion by some stratigraphers and as two closely spaced explosions by others the stratigraphic evidence that bears on this question is inconclusive strata change dramatically over short distances and this complexity provides wide latitude for interpretation some independent non stratigraphic evidence however suggests that the blast was the product of two explosions or clusters of explosions the independent evidence comes from eyewitness accounts and photographs from satellite sensors and from seismic records this paper reviews the pertinent evidence offers a new interpretation and concludes that the blast was indeed the product of two explosions or clusters of explosions

k ar radiometric ages of lavas from cocos island eastern pacific abstract new chemical analyses microprobe investigations and the first radiometric 40 k 40 ar data carried on volcanic rocks from cocos island indicate volcanic activity with alkaline affinities between 2 4 and 1 9 m y these results are compared to the characteristics of lavas from galapagos archipelago

a model for the mechanism of the may 18 1980 mount st helens blast abstract a model is suggested for the mechanism of the blast that accompanied the may 18 1980 mount st helens eruption the model explains the large duration of the blast process and accounts for several explosive episodes recognized during the blast the mechanism is based on the idea of destruction of the near solidus porous cryptodome by a fragmentation wave termed the f wave originating from cryptodome decompression induced by a landslide the f wave propagates from a free surface to the interior of the cryptodome at velocity n the products of cryptodome fragmentation a gas pyroclast mixture are ejected into the atmosphere at velocity u calculations show that for pressure of gas in the cryptodome pores of 20 mpa and porosity 0 4 0 7 the f wave travels at 5 m s and velocity of pyroclast ejection is 100 250 m s for a cryptodome with diameter of 700 m the duration of blast fragmentation is estimated at 140 s energy released during the blast is estimated as 4 8 7 2 1015 j blast power is 3 4 5 1 1013 wt gas pyroclastic mixture mass flux is 5 0 7 5 103kg m2 s the maximum discharge is 1 9 2 9 sx 109 kg s and the weight fraction of water vapor in the cryptodome is 0 006 0 04 several explosive episodes during the blast are accounted for by fragmentation wave stoppage due to blockage of the exploding cryptodome by landslide material

alkalic marine tephra layers at odp site 1241 major explosive eruptions from an oceanic volcano in a pre shield stage highlights subplinian to plinian eruptions from cocos island tectonically controlled melt ascent ocean island evolution without passing typical growth stages abstract we report a series of fourteen marine tephra layers that are the products of large explosive eruptions of subplinian to plinian intensities and magnitudes vei 4 from cocos island costa rica cocos island is a volcanic island in the eastern central pacific ocean 500 km offshore costa rica and is situated on the northwestern flank of the aseismic cocos ridge geochemical fingerprinting of pleistocene 2 4 1 4 ma marine tephra layers from ocean drilling project odp leg 202 site 1241 using major and trace element compositions of volcanic glass shards demonstrates unequivocally their origin from cocos island rather than the galapagos archipelago or the central american volcanic arc cava cocos island and the adjacent seamounts of the cocos island province have alkalic compositions and formed on young 3 ma oceanic crust from an extinct spreading ridge bounded by a transform fault against the older and thicker crust of the aseismic cocos ridge cocos island has six times the average volume of the adjacent seamounts although all appear to have formed during the 3 1 4 ma time period cocos island lies closest to the transform fault and we explain its excessive growth by melts rising from garnet bearing mantle being deflected from the thick cocos ridge lithosphere toward the thinner lithosphere on the other side of the transform thus enlarging the melt catchment area for cocos island compared to the seamounts farther away from the transform this special setting favored growth above sea level and subaerial explosive eruptions even though the absence of appropriate compositions suggests that the entirely alkalic cocos island and seamounts never evolved through the productive tholeiitic shield stage typical of other pacific ocean islands possibly because melt production rates remained too small conditions of magma generation and ascent resembled hawaiian pre shield volcanoes but persisted for much longer 1 m y and formed evolved trachytic magmas therefore cocos island may be a unique example for a volcanic ocean island that did not pass through the typical growth stages

the spatial nearshore offshore distribution of latest permian phytoplankton from the yangtze block south china abstract in order to analyze the spatial paleoecological distribution of the organic walled microphytoplankton in the late permian five sections from the yangtze block south china displaying different sediment facies types from neritic to offshore paleoenvironments including basinal facies have been investigated palynologically based on the diversity and relative abundance of acritarch species and genera the new data from the permian provide similar patterns as those described from other geological periods 1 low diversities with 2 to 4 acritarch species occur in nearshore environments whereas the higher diversities more than ten acritarch species appear in the offshore environments 2 at the generic level the genera leiosphaeridia reduviasporonites and micrhystridium are distributed widely from nearshore facies corresponding to shallow water environments to offshore facies corresponding to deeper water settings whereas some genera such as dictyotidium and veryhachium have a narrower distribution occurring on the continental shelf and towards the basin indicating open marine environments 3 the genus schizosporis only occurs around the permian triassic boundary ptb when the sea level declined probably indicating nearshore environments with shallow water settings 4 at the specific level the species micrhystridium breve displaying short spines and leiosphaeridia minutissima are indicative of neritic facies whereas the other species of micrhystridium and veryhachium with longer spines e g micrhystridium stellatum and veryhachium hyalodermum and leiosphaeridia microgranifera indicate more open marine environments big spherical acritarch species over 80 m in diameter such as dictyotidium reticulatum indicate shallow water environments and 5 in the ptb strata the relative abundance of the enigmatic reduviasporonites interpreted by some authors as a fungal spore is not higher than 14 in every sample indicating that a spike of reduviasporonites does not occur in the yangtze area reduviasporonites chalastus 40 m in length obviously dominates in shelf environments of shallow water whereas the smaller reduviasporonites catenulatus 15 m in length are more common in deeper water

influence of pre eruptive degassing and crystallization on the juvenile products of laterally directed volcanic explosions abstract strikingly similar examples of edifice collapse and directed blast are the 18 may 1980 eruption of mount st helens msh washington usa and the 30 march 1956 eruption of bezymianny volcano bz kamchatka russia in these cases flank failures led to near instantaneous decompression and fragmentation of intra edifice cryptodome magma which produced catastrophic laterally directed blasts in both instances the blast products consisted of juvenile material with bimodal density vesicularity distributions low and high density modes at 1900 and 2400 kg m 3 for bz 1600 and 2300 kg m 3 for msh although the proportion of high density material is greater at bz blast materials also exhibit striking variety in groundmass crystallinity 90 vol despite having fairly uniform pheno crystallinities suggesting that degassing driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes new bulk rock h2o and d measurements confirm that progressive open system outgassing occurred prior to both blasts the correlations between crystallinity clast density and bulk h2o contents suggest that syn blast magma expansion was modulated both by non uniform volatile distribution within the cryptodome and rheological controls associated with non uniform crystal content spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin which probably correlates with timing of magma injection the greater proportion of high density material at bz is speculatively related to lower blast energy compared with msh

paleoenvironment reconstruction of parnaiba basin north brazil using indicator species analysis indval of devonian microphytoplankton abstract the microphytoplankton record of parnaiba basin is typically of devonian age and commonly associated with changes in marine environments the method of indicator species analysis indval was applied as a tool for reconstructing marine environment changes during the devonian pragian famennian the indval expresses which taxa are strongly associated with particular groups of samples the devonian microphytoplankton distribution reflects the paleoenvironment and the indval indices illustrated how the taxa interacted through time and reveals patterns in the changing composition of the microphytoplankton the material studied derives from three well sections arn 12 samples na 22 samples and lt 13 samples drilled through the itaim pimenteira and cabecas formations in the parnaiba basin respectively samples were prepared using the standard palynological method the indicator species coincidently are age diagnostic for the devonian succession the indval indicates fourteen taxa viz schizocystia saharica triangulina alargada bimerga paulae pragian emsian interval onondagella asymetrica tunisphaeridium tentaculiferum estiastra spp early givetian interval mediocorpore conspicuus unattributed species of unellium frasnian 1 interval petrovina connata unattributed species of acriora frasnian 2 interval winwaloeusia distracta unattributed species of stellinium frasnian 3 interval arkonites bilixus and unattributed species of duvernaysphaera frasnian famennian interval microphytoplankton typical of more nearshore settings are recorded in the basal interval pragian emsian interval accompanied by high values of phytoclasts and continental marine ratio c m a change in microphytoplankton is recorded with an increase in acritarchs and prasinophycean algae typical of open marine conditions in particular during the frasnian 3 interval the change reflects sea level rise

a global chitinozoa biozonation for the silurian a global chitinozoan biozonation for the silurian period is proposed each biozone is an interval range defined by the first occurrence of an index species selected from well defined species with a relatively short biostratigraphic range to be included in the scheme index species must have been recorded from at least two major silurian palaeocontinents where usable chitinozoa assemblages have been studied that is avalonia baltica which had already undergone collision by the silurian laurentia gondwana and southern china seventeen biozones are identified with seven in the llandovery the fragilis postrobusta electa maennili alargada dolioliformis and longicollis biozones four in the wenlock the margaritana cingulata pachycephala and lycoperdoides biozones three in the ludlow the elongata philipi and barrandei biozones and three in the p idoli the kosovensis elegans and superba biozones chronostratigraphic calibration is partly provided by reference to the range of the appropriate chitinozoa index species in the global stratotype sections and points gssp of the silurian series e g in bohemia czech republic for the p idoli and the welsh borderland in england for the ludlow and part of the wenlock when this information was not available independent biostratigraphic control was provided by calibration with graptolite biozones or in a few cases conodont or trilobite biozonal schemes the index and most characteristic species of each biozone are illustrated and their total stratigraphie range provided

the 1982 eruptions of el chichon volcano mexico 3 physical properties of pyroclastic surges two major pyroclastic surges generated during the 4 april 1982 eruption of el chichon devastated an area of 153 km2 with a quasi radial distribution around the volcano the hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation field evidence indicates voidage fraction of 0 99 in the surge cloud with extensive entrainment of air thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud the dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10 20 characterized by stoss side erosion and lee side deposition of 1 10 cm reversely graded laminae a systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals resulting in loss of at least 10 25 of the cloud mass due to winnowing out of fines during surge emplacement greatest fractionation from the 1 0 0 0 grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud extensive pumice rounding indicates abrasion during bed load transport flow of pyroclastic debris in the turbulent surge cloud was by combination of bed load and suspended load transport the surges are viewed as expanding pyroclastic gravity flows which entrain and mix with air during transport the balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence which persisted to the distal edge of the surge zone

the intensity and magnitude of holocene plinian eruptions from mount st helens volcano dispersal characteristics of the t we wn pu ps ye yn and yb plinian fall deposits of mount st helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities magma discharge based on a theoretical model of tephra dispersal new proximal thickness measurements are combined with an empirical distal extrapolation based on studies of 53 plinian deposits to calculate the magnitude erupted mass of each eruption layer yn 3510 y r b p represents the highest intensity and largest magnitude eruption at mount st helens in post glacial times modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase 26 hours several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of mount st helens peak intensity of the plinian phase was 108 kg s and the total erupted volume was 4 km3 dre of magma small plinian style eruptions are represented by layers such as ps and pu of the pine creek eruptive period 3000 2500 yr b p and have intensities of only 106 kg s when compared with plinian eruptions from other volcanoes the holocene eruptions of mount st helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate sized stratovolcanoes there is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle this relationship may be related to a time dependent process for the accumulation of differentiated and volatile rich magma within the chamber beneath mount st helens

foreland basin development and tectonics on the northwest margin of eastern avalonia the early palaeozoic convergence of avalonia and laurentia created a foreland basin at the suture zone of the former lapetus ocean sedimentological and stratigraphic evidence of shallowing and contemporaneous shortening suggests that the southern part of the basin the windermere group became detached from its basement in the late ludlow and began to invert the detachment beneath the basin rooted into a northwest dipping mid crustal thrust system contemporaneous uplift to the north of the late silurian basin involved shortening of the avalonian foreland basement by thrusting basin inversion occurred ahead of a southeastward advancing mountain front we postulate a foreland southeast prograding sequence of thrusting through the ludlow in the lake district the basin continued to migrate onto the avalonian foreland through the early devonian ahead of an advancing orogenic wedge finally coming to a stop in the emsian

the 1902 peleean deposits in the fort cemetery of st pierre martinique a model for the accumulation of turbulent nuees ardentes abstract the ancient fort cemetery located at the northwestern limit of st pierre has been left undisturbed since 1902 and therefore offers a unique opportunity to study the deposits of the eruptions that destroyed the city six eruption units were found overlying the floor of the cemetery leaving an average 1 6 m thick deposit the city was hit by four major nuees which occurred on may 8 20 june 6 and august 30 the may 20 and june 6 eruption units are each made of three beds the basal bed always lacks fines and is generally massive the middle bed is normally graded on which dune type cross bedding is superimposed in the may 20 deposit the upper bed the thinnest has the finest grain size of the eruption unit and is normally graded true parallel laminae were only observed in the upper bed the may 8 eruption unit is essentially made of fines depleted massive lapilli and tuff the evidence strongly suggests that the deposits of the middle and upper beds accumulated from turbulent suspensions the absence of structures in the basal bed is not conclusive evidence of laminar flow deposition and its characteristics are similar to deposits that have accumulated from highly concentrated turbulent suspensions the lack of fines observed only in the basal bed is attributed to gravity segregation helped by vaporization of water present in the underlying deposits and for the may 8 deposit to escaping gas produced by the destruction of the vegetation during or immediately after deposition which could explain the absence of primary structures in the basal bed the fine grained deposits of mt pelee show a primary structure sequence similar to the turbidite sequence and are therefore interpreted in terms of decreasing flow velocity and grain size with time at a fixed site

palynofacies in a late silurian regressive sequence in the welsh borderland and wales the upper whitcliffe and lower part of the downton castle sandstone formations ludlow and pridoli series have been sampled from sequences distributed over an area roughly 4800 square km in west central england and wales ludfordian age rocks have been interpreted as the deposits of shelf and basin environments and those of the lower downton group as near shore to beach sections sampled from the shelf area are in the ludlow type area and in the usk may hill and woolhope inliers basin sediments were sampled at knighton the long mountain capel horeb and kington the samples were analysed for their palynological content and all the organic walled microfossils were studied together with the fragmentary remains of larger plants and animals using a series of ratios between different groups of microfossils it is apparent that there are consistent changes through time in both the palynomorph assemblages and in the structured and unstructured organic material dominance of some microplanktic groups taxa is used to define a new microphytoplanktic phase concept and to compare the sequence of palynoevents depositional environments inferred from palynofacies interpretation of the ludlow type area are closely comparable with those proposed previously from sedimentological data palynofacies sequences differ for the shelf and basin areas but show similar trends we have also recognized a dramatic shift in palynofacies at a level within the downton castle sandstone formation and its correlatives in both shelf and basin areas this change is tentatively interpreted as a storm event

the middle rhuddanian lower silurian hot shale of north africa and arabia an atypical hydrocarbon source rock an important hydrocarbon source rock a hot shale has been identified from the middle rhuddanian llandovery lower silurian of a number of localities in north africa and arabia it is unusual in having been deposited during a eustatic fall in sea level evidence for this sea level fall in libya and jordan is manifested in a wide variety of palynological proxy data including reduced acritarch and chitinozoan total abundance reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups and from the presence of a minor positive 13corg excursion at the same stratigraphical level as the hot shale the environment of deposition was anoxic before during and after deposition of the hot shale except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three dimensional pyrite internal moulds these incursions occurred just before and during deposition of the hot shale and are interpreted as resulting from the influx of sediment laden oxygenated currents from shallower water environments enhanced organic matter preservation within the hot shale is attributed to greater productivity resulting from increased nutrient input particularly from wind blown dust more rapid descent through the water column of organic matter in post bloom marine snow macroaggregates development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the rhuddanian

impact and volcanism a momentum scaling law for erosion the controversy that raged through the 1950s 1970s over similarities and differences between meteorite impact and volcanic processes is revisited in this paper we propose that there are quantitative similarities in erosion caused by high speed ejecta produced by either impacts or volcanic processes field and petrographic data from the manicouagan impact crater canada are used to demonstrate that during the emplacement of the impact melt sheet erosion occurred at a rate of 2562 kg m 2 s 1 field data for the mount st helens lateral blast of may 18 1980 suggest an erosion rate of 2 kg m 2 s 1 and field data for a small pyroclastic flow on august 7 1980 suggest an erosion rate of 14 kg m s 1 it is proposed that these three rates were determined dominantly by the momentum of the ejecta and a quantitative formulation based on lofting theory is given a new application of the monte carlo approach to analysis provides minimum most likely and maximum estimates for both the field and the theoretical analyses the substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales

lateral blasts at mount st helens and hazard zonation lateral blasts at andesitic and dacitic volcanoes can produce a variety of direct hazards including ballistic projectiles which can be thrown to distances of at least 10 km and pyroclastic density flows which can travel at high speed to distances of more than 30 km indirect effect that may accompany such explosions include wind borne ash pyroclastic flows formed by the remobilization of rock debris thrown onto sloping ground and lahars

mapping the volcanic hazards from soufriere hills volcano montserrat west indies using an image processor we have used a digital model of the topography of montserrat a simple mathematical model of gravitational flow and some assumptions of the way in which the next eruption will develop to create a map of the volcanic hazards from soufriere hills volcano this has been done using an image processing computer to simulate the deposits produced by pyroclastic flows this technique has the advantages over more traditional cartographic methods of spatial precision rapid computation of multiple eruption models and the explicit nature of the physical model used soufriere hills volcano is a small andesitic volcano characterized by a cluster of summit domes and an apron of pyroclastic flow deposits and mudflows upon which several thousand people now live most of the flanks were covered by deposits from a series of eruptions from 24000 to 16000 a bp though there is some evidence that dome growth and small pyroclastic flows have occurred since the modelling is constrained by field evidence from the deposits of previous eruptions although the evidence is not good enough to model individual flow units the cumulative deposits can be used from the eruption deposit models we have created a new type of map specifically for emergency planning this sequential hazard zone map attempts to portray the regions that would be at hazard from pyroclastic flows during successive stages from the start of an eruption whose energy release was increasing with time

holocene geomagnetic secular variation recorded by volcanic deposits at mount st helens washington a compilation of paleomagnetic data from volcanic deposits of mount st helens is presented in this report the database is used to determine signature paleomagnetic directions of products from its holocene eruptive events to assign sampled units to their proper eruptive period and to begin the assembly of a much larger database of paleomagnetic directions from holocene volcanic rocks in western north america the paleomagnetic results from mount st helens are mostly of high quality and generally agree with the division of its volcanic deposits into eruptive episodes based on previous geologic mapping and radiocarbon dates the muddy river andesite s paleomagnetic direction however indicates that it is more likely part of the pine creek eruptive period rather than the castle creek period in addition the two fingers andesite flow is more likely part of the middle kalama eruptive period and not part of the goat rocks period the paleomagnetic data from mount st helens and mount hood document variation in the geomagnetic field s pole position over the last 2 500 years a distinct feature of the new paleosecular variation psv record similar to the fish lake record oregon indicates a sudden change from rapid clockwise movement of the pole about the earth s spin axis to relatively slow counterclockwise movement at 800 to 900 years b p

palaeophytogeographical and palaeoecological implications of a miospore assemblage of earliest devonian lochkovian age from saudi arabia abstract palynological samples from two boreholes drilled in the eastern part of saudi arabia contain a rich assemblage of cryptospores and trilete spores they are dated as most probably lochkovian in age intriguingly the cryptospores are very similar to those from a coeval continental palynomorph assemblage from the old red sandstone continent whereas the trilete spores show a strong affinity with miospore assemblages from western gondwana it is suggested that cryptospore producing plants inhabited confined damp biotopes and had a wide range of climatic tolerance on the other hand trilete spore producing plants inhabited a wider variety of biotopes but were more sensitive to climatic variations the high degree of similarity between old red sandstone and saudi arabian cryptospore assemblages favours palaeogeographic reconstructions where there is close proximity between western gondwana and euramerica and or a land connection between these palaeoplates climatic differences between the old red sandstone continent and the northern part of western gondwana can be invoked to explain the variations in the trilete spore assemblages

volcanic hazards from bezymianny and bandai type eruptions major slope failures are a significant degradational process at volcanoes slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years the historic record provides evidence for at least six of these events in the past century several historic debris avalanches exceed 1 km3 in volume holocene avalanches an order of magnitude larger have traveled 50 100 km from the source volcano and affected areas of 500 1500 km2 historic eruptions associated with major slope failures include those with a magmatic component bezymianny type and those solely phreatic bandai type the associated gravitational failures remove major segments of the volcanoes creating massive horseshoe shaped depressions commonly of caldera size the paroxysmal phase of a bezymianny type eruption may include powerful lateral explosions and pumiceous pyroclastic flows it is often followed by construction of lava dome or pyroclastic cone in the new crater bandai type eruptions begin and end with the paroxysmal phase during which slope failure removes a portion of the edifice massive volcanic landslides can also occur without related explosive eruptions as at the unzen volcano in 1792 the main potential hazards from these events derive from lateral blasts the debris avalanche itself and avalanche induced tsunamis lateral blasts produced by sudden decompression of hydrothermal and or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s 1 the ratio of area covered to distance traveled for the mount st helens and bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume the potential for large scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice as at mount st helens and bezymianny debris avalanches can move faster than 100 ms 1 and travel tens of kilometers when not confined by valley walls avalanches can affect wide areas beyond the volcano s flanks tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions the probable travel distance l of avalanches can be estimated by considering the potential vertical drop h data from a catalog of around 200 debris avalanches indicates that the h l rations for avalanches with volumes of 0 1 1 km3 average 0 13 and range 0 09 0 18 for avalanches exceeding 1 km3 h l ratios average 0 09 and range 0 5 0 13 large scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts the nature and duration of precursory activity vary widely and the timing of slope faliure greatly affects the type of associated eruption bandai type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity

devonian marine incursions into the orcadian basin scotland bp chevron well 14 6 1 drilled in the east orkney basin has penetrated devonian sedimentary rocks which can be dated palynologically as within the latest givetian to early frasnian interval this microflora contains the first substantiated record in western europe of archaeoperisaccus a spore hitherto regarded as restricted to northern laurasia three grey green intervals within the well contain marine microfossils which provide the first direct evidence for the movement of marine waters into the northern part of the old red sandstone continent onshore an eday marls section in the bay of berstane orkney contains a marine microfauna which demonstrates the presence of a marine incursion of mid late givetian age this is the first record of a devonian marine incursion in scotland combination of this palaeontological data with sedimentological studies of the eday marls and upper old red sandstone suggests that a wide flat sabkha plain extended eastwards from the onshore orcadian basin during the later stages of its infill and was subjected to episodic marine incursions these incursions are tentatively linked with marine transgressions into the argyll area of the central north sea two orders of cyclicity occur within the dated devonian interval of 14 6 1 and are interpreted as resulting from orbital cyclicity at periodicities of 39 5 and 413 9 ky this indicates that the control on the devonian sea level maxima may have been orbital cyclicity these marine incursions are speculatively correlated to high stands on the devonian sea level curve the bay of berstane marine incursion representing the taghanic onlap whilst those in 14 6 1 are the rhinestreet middlesex and genundewa transgressions reconsideration of the palaeogeography of the northern margin to the marine devonian indicates that the sea entered the orcadian basin from the east along the tornquist zone at the margin of the fenno scandian high