fiamme degassing structures and their implications for the post emplacement temperatures and h2o contents of high grade ignimbrites abstract a new process in post emplacement ignimbrite degassing is proposed from observations of unusual structures in a high grade ignimbrite in sardinia the structures consist of fiamme decorated above and below by vesicles termed here fiamme degassing structures fds the relative volumes of vesicles and fiamme have a positive linear correlation and the ratio between the volume of vesicles above and below fiamme asymmetry ratio is 1 wt at

non newtonian rheology of igneous melts at high stresses and strain rates experimental results for rhyolite andesite basalt and nephelinite the stress strain rate relationships of four silicate melt compositions high silica rhyolite andesite tholeiitic basalt and nephelinite have been studied using the fiber elongation method measurements were conducted in a stress range of 10 400 mpa and a strain rate range of 10 6 to 10 3 s 1 the stress strain rate relationships for all the melts exhibit newtonian behavior at low strain rates but non newtonian nonlinear stress strain rate behavior at higher strain rates with strain rate increasing faster than the applied stress the decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt the decrease in viscosity observed at the high strain rates of the present study ranges from 0 25 to 2 54 log10 pa s the shear relaxation times of these melts have been estimated from the low strain rate newtonian shear viscosity using the maxwell relationship s g non newtonian shear viscosity is observed at strain rates time 1 equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time brittle failure of the fibers occurs 2 log10 units of time above the relaxation time this study illustrates that the occurrence of non newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate high silica rhyolite melts involved in ash flow eruptions are expected to undergo a non newtonian phase of deformation immediately prior to brittle failure

pressure prediction for a central graben wildcat well uk north sea pore pressure and fracture gradient prediction formed an essential part of the planning process for a deep wildcat exploration well in the north sea central graben known vertical and lateral differences in the pressure regime even in adjacent fault blocks reduced reliance on offset wells for this reason a model was developed to predict pore pressure from first principles this model assumed that under compaction lateral transfer and hydrocarbon buoyancy were the main contributors to the significant overpressure expected the pressure regime encountered in the well was close to the prediction although it was not possible to confirm the relative contribution of the three mechanisms mentioned above

long period water surface fluctuations on a horizontal coastal shelf with a steep seaward face abstract steep slopes significantly influence the propagation of both short period waves and accompanying long period waves with the same time scale as the wave groups therefore conventional wave models are not directly appropriate for the computation of the wave field on such a slope a numerical model is proposed for the simulation of long period water surface fluctuations forced by short period wave groups this model is based on the generalized conservation equations of mass and momentum the conventional closure relationship used for radiation stresses in most previous long period wave models is upgraded for steep slopes and an explicit relationship is found for the dynamic component of the mean bottom pressure the model is verified through some wave flume data collected on a steep slope the model is then used to investigate long period water surface fluctuations on horizontal shelves with steep seaward faces it is found through numerical investigations that the long period water surface fluctuations on a steeply faced coastal shelf greatly depend on the transmission of short period waves for short period waves propagating without breaking steeply faced shelves provide large long period waves while opposite is true for breaking short period waves

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

bore like surf beat on reef coasts reef coasts because the wave height of incoming stormy high waves are extremely dampened by coral reef have been believed as calm sea areas however many coastal structures have actually been damaged by stormy waves dampened by the reef surf beat with large wave height was discovered through field observations the surf beat has been named as bore like surf beat the wave height and the wave velocity of the bore like surf beat is lager than that of individual waves numerical simulation taking into account of non linear effect of surf beat phenomenon on the reef shows good agreement with laboratory data the main reason of the disasters of coastal structures on the reef coasts is existence of the bore like surf beat resonantly excited incoming wave groups

the clay with flints deposit in northern ireland reassessment of the evidence for an early paleocene ignimbrite reassessment of key geological sections field relationships and petrographical characteristics of the northern ireland clay with flints and donald s hill ignimbrite formation show they formed dominantly by sedimentary processes the involvement of a previously postulated pyroclastic flow during early paleocene time is not recognized and as such the donald s hill ignimbrite formation stratigraphic term is discounted instead a multistage model of formation by sedimentary accumulation and remobilization is presented and the term clay with flints is retained regionally two dominant facies are recognized in most clay with flints sections facies 1 was formed by an initial accumulation of flints on a chalk landscape undergoing karstification and involved deposition of a clay matrix derived predominantly from contemporaneous erosion of subtropical soil horizons formed mainly on basalt in facies 2 evidence is observed for widespread remobilization of facies 1 deposits by high density mudflows driven by the advancement of the antrim lava group which caused the blockage of subsurface and marginalization of surface drainage a stratigraphical constraint imposed by the presence of a supposed ignimbrite in this part of the north atlantic igneous province has been problematic but this is resolved by its identification as a diachronous sedimentary deposit that formed until buried by either the lower or upper formations of the antrim lava group

deposition of rheomorphic ignimbrite d mogan formation gran canaria canary islands spain rheomorphic ignimbrite d 13 4 ma upper mogan formation on gran canaria a multiple flow single cooling unit is divided into four major structural zones that differ in fabric and finite strain of deformed pyroclasts their structural characteristics indicate contrasting deformation mechanisms during rheomorphic flow the zones are a a basal zone vitrophyre with pure uniaxial flattening perpendicular to the foliation b an overlying shear zone characterized by asymmetric fabrics and a significantly higher finite strain with an ellipsoid geometry similar to stretched oblate bodies c a central zone with a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component and d a slightly deformed to non deformed top zone where the almost random orientation of subspherical pyroclasts suggests preservation of original syn depositional clast shapes rheomorphic flow in d is the result of syn to post depositional remobilization of a hot pyroclastic flow as shown by kinematic modeling based on a the overall vertical structural zonation suggested by finite strain and fabric analysis b the relation of shear sense to topography c the interrelationship of the calculated vertical cooling progression at the base of the flow formation of vitrophyre and the related vertical changes in strain geometry d the complex lithification history and e the consequent mechanisms of deformational flow rheomorphic flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally 6 8 we suggest that every level of newly deposited pyroclastic flow material of d first passed through a welding process that was dominated by compaction pure flattening before rheomorphic deformation started

petrography and geochemistry of impactites and volcanic bedrock in the icdp drill core d1c from lake el gygytgyn ne russia the 3 6 ma old and 18 km diameter el gygytgyn impact structure in ne siberia was drilled in 2008 09 by icdp international continental scientific drilling program a 517 m long core hole d1c was drilled into the outer flank of the central uplift structure with an overall core recovery of approximately 63 thereby approximately 315 m lake sediments and approximately 202 m impactites were recovered here we present a detailed petrographic and geochemical assessment of the impact breccia and bedrock sections in this core the 97 m long lower bedrock unit 517 420 m below lake floor blf consists of an ignimbrite in the overlying upper bedrock unit 420 390 mblf the core recovered a sequence of similar ignimbrite and several decimeters of mafic rocks we interpret these units as rocks that are located close to their former preimpact position but have been somewhat rotated due to collapse of the central uplift i e it represents parautochthonous basement from about 390 to 328 mblf occurs a suevite package with an impact melt poor clast dominated matrix and lithic and mineral clasts that cover the entire range of volcanic target rocks known from the el gygytgyn region all stages of shock metamorphism unshocked to melted were observed in clasts and in microclasts of the matrix of suevite from different depths immediately below this package at the contact to the underlying bedrock occurs a 1 m wide sheared zone within vitrophyric ignimbrite which we consider the actual crater floor the uppermost approximately 12 m from 328 316 mblf depth seem to comprise reworked suevite consisting of a mixture of sediments and suevite with more and on average stronger shocked minerals than found in the main suevite unit this includes a small component of glassy spherules and impact melt fragments toward the top of this unit lake sediments progressively become the dominant material in this section we assume that this unit contains a fallback component from the ejecta plume that was mixed with the first sediments of the postimpact crater lake and possibly some rocks that slumped off the inner crater wall similar to a thin layer at the base of the sediment section of borehole lb 5a recovered in lake bosumtwi ghana

retarded compaction due to overpressure deduced from a seismic velocity depth conversion study in the danish central trough north sea as a result of overpressuring below the upper miocene seismic velocities below this level are low in the danish central trough north sea compared with areas outside the trough using the top middle miocene as a datum velocity depth functions match those functions established east of the study area and lead to improved depth conversion it is proposed that the present day velocity depth porosity depth profiles below the top middle miocene reflect the frozen pre late miocene profiles the compaction effect of late cenozoic burial was retarded with the exception of areas where the sealing capacity of the lower tertiary sediments was reduced due to permeable sandy deposits or possibly due to fracturing caused by salt diapirism in the study area the thick late cretaceous danian chalk group constitutes a significant high velocity layer depth conversion of maps covering the danish central trough down to the top middle jurassic an important deep exploration target should include at least four layers the post chalk group cenozoic excluding danian the chalk group and the cromer knoll group lower cretaceous and the upper jurassic analysis of time depth data from 76 wells shows that velocities in the study area vary as a function of depth thickness lithology presence of gas and overpressure caused by rapid late cenozoic deposition and by gas generation for wells drilled away from salt diapirs optimum depth conversion with constant velocity is obtained with velocities of 2050 4400 2950 and 2800 m s for the four layers the relative success of different depth conversion methods is evaluated by comparing the mean absolute error on the estimated thicknesses for the layers individually assuming each well in turn to be undrilled velocity anomaly depth conversion or the v0 k method is shown to be an effective way to convert travel times to depth when interval velocity increases with depth of burial in this method velocity is assumed to increase linearly with depth and laterally velocity is calibrated to wells velocity variations in the upper jurassic impose the major problem for depth conversion in the area

viscosity of a teide phonolite in the welding interval abstract the viscosity of a natural phonolitic composition with variable amounts of h2o has been experimentally determined the starting materials were crystal free phonolitic glasses from montana blanca situated within the las canadas caldera of teide dry phonolitic melt viscosities were determined using concentric cylinder viscometry in the low viscosity range the glassy quench products of these runs were then hydrated by high pressure synthesis in a piston cylinder apparatus to generate a suite of samples with water contents ranging from 0 02 to 3 75 wt samples thus hydrated were quenched rapidly and prepared cut and polished for the determination of water contents by infrared spectroscopy before and after experimental viscometry the viscosities of the melts dry and hydrated were determined at 1 bar using a micropenetration technique samples were stable under the measurement conditions up to 3 75 wt h2o homogeneity of water content was confirmed by infrared spectroscopy and total water contents were calculated using absorptivity coefficients for compositions extremely close to that investigated here the variation of viscosity as a function of water content and temperature can be described in the high viscosity interval of relevance to many welding processes by the non arrhenian expression 1 log 10 5 900 0 286 ln h 2 o 10775 4 394 8 h 2 o t 148 7 21 65 ln h 2 o whereas the high viscosity range alone is adequately described by the arrhenian expression 2 log 10 10 622 0 738 ln h 2 o 17114 3 590 4 h 2 o 1 t where is the viscosity in pa s h2o is the water content in wt and t is the temperature in k these results are particularly useful for the scaling of conditions extant during the welding of phonolitic products of montana blanca the welding of glassy phonolitic rocks is enhanced by the lower viscosity of these melts with respect to calcalkaline rhyolites the ratio of viscosities of phonolitic to calcalkaline rhyolitic melts is a complex function of temperature and water content and reaches up to 104 5 at 0 1 wt h2o and 500 c abundant evidence of welding and remobilisation of pyroclastic and spatter products of teide system volcanism are consistent with these experimental observations

overpressure retardation of organic matter maturation and petroleum generation a case study from the yinggehai and qiongdongnan basins south china sea three superimposed pressure systems developed in the yinggehai basin south china sea as indicated by seismic data well logs and direct pressure measurements the organic maturation profile is nonlinear with three nonparallel segments that correspond to the shallow normal pressured system the intermediate overpressured system and the deep strongly overpressured system respectively the intermediate and deep overpressured systems have abnormally low r0 gradients the organic maturity of these overpressured rocks is significantly lower than the maturity of normal pressured source rocks in nearby wells with similar thermal histories and does not match the thermal histories of the rocks such an organic maturity anomaly is distinctly different from those aused by variation in activation energies conductivity contrasts and hydrologic effects and is confirmed to be the result of overpressure retardation the degree to which the organic matter maturation is retarded expressed as the difference between predicted and measured vitrinite reflectance increases exponentially with increasing fluid pressure confirming that pressure increases the activation energies of organic matter maturation reactions overpressure retardation has been proven to be conditional and quite important for clearly understanding petroleum generation migration and accumulation in overpressured sedimentary basins

the transition of spatter to lava like body in lava fountain deposits features and examples from the cabezo segura volcano calatrava spain abstract the cabezo segura ii volcanic cone calatrava volcanic province iberian microplate comprises proximal wall deposits with a well defined crater wall unconformity and crater fill deposits the complex volcanic succession that shows evidence of several eruptive episodes was built by magmatic and hydrovolcanic explosions of different styles strombolian hawaiian sub plinian and phreato strombolian generated from a multiple feeder ultrabasic dyke intra crater rock units at the volcano summit include spatter deposits together with up to 10 m thick and more than 200 m long lava like bodies geological logs for the main lava like bodies define a characteristic facies model that involves a central lava like mass which grades vertically into a transition zone of apparently coherent spatter then dense spatter and finally into vuggy spatter deposits these units are inferred to have formed during pulsating lava fountain type explosive eruptions the depicted facies distribution being the result of progressive increase in welding grade and densification of the spatter in response to variations in the accumulation rate their field features may be used as a guide for the precise identification of vent sites in deposits of hawaiian eruptions also structures like those here recognised that might have survived in lava like flows could be of help to identify when lava producing eruptions represented an explosive hawaiian event lava fountains and not a purely effusive event

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

low temperature hydrothermal alteration of intra caldera tuffs miocene tejeda caldera gran canaria canary islands article i nfo the miocene tejeda caldera on gran canaria erupted 20 rhyolite trachyte ignimbrites mogan group 14 13 3 ma followed by 20 phonolitic lava flows and ignimbrites fataga group 13 8 5 ma upper mogan tuffs have been severelyaltered immediately within the caldera margin whereas extra caldera mogan ignimbrites and overlying fataga units are apparently unaltered the altered intra caldera samples contain minerals characteristic of secondary fluid rock interaction clays zeolites adularia and relics of the primary mineral assemblage identified in unaltered ignimbrites k feldspar plagioclase pyroxene amphibole and groundmass quartz major and trace element data indicate that si na k pb sr and rb were strongly mobilized during fluid rock interaction whereas ti zr and nb behaved in a more refractory manner experiencing only minor mobilization the 18 o values of the altered intra caldera tuffs are significantly higher than in unaltered extra caldera ignimbrites consistent with an overall low temperature alteration environment unaltered extra caldera ignimbrites have d values between 110 and 173 which may reflect rayleigh type magma degassing and or post depositional vapour release the d values of the altered intra caldera tuffs range from 52 to 131 with ambient meteoric water at the alteration site estimated at ca 15 interaction and equilibration of the intra caldera tuffs with ambient meteoric water at low temperature can only account for whole rock d values of around 45 given that dclay water is ca 30 at 100 c and decreases in

welding and rheomorphism of phonolitic fallout deposits from the las canadas caldera tenerife canary islands the las canadas caldera is a nested collapse caldera formed by the successive migration and collapse of shallow magmatic chambers among the pyroclastic products of this caldera are phonolitic fallout deposits that crop out in the caldera wall and on the extracaldera slopes these deposits exhibit an uninterrupted facies gradation from nonwelded to lava like and record continuous volcanic deposition densely welded and lava like facies result from the extreme attenuation and complete homogenization of juvenile clasts that destroy original clast outlines and any evidence of fallout deposition agglutination contributes significantly to the final degree of flattening observed in the welded facies after deposition rheomorphic flowage occurs emplacement temperatures for one of the welding sequences are calculated from magmatic temperatures and a model of tephra cooling during fallout results are 486 c for the nonwelded facies and 740 c for the moderately welded facies for the same welding sequence a cooling time between 25 and 54 days is estimated from published experimental and computational data as the possible duration of welding and rheomorphism following deposition and agglutination the lava like pyroclastic facies had the rheological properties of viscous lavas and flowed down the outer slopes away from the caldera some lava like masses detached from proximal areas to more distal regions during deposition the eruptive style evolved from plinian fallout to fountain fed spatter deposition this evolution was accompanied by a decrease in explosive power and a lower height of the eruptive column which produce higher emplacement temperatures and more effective heat retention of pyroclasts

emplacement and rheomorphic deformation of a large lava like rhyolitic ignimbrite grey s landing southern idaho the miocene grey9s landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the snake river plain it shows particularly intense welding and rheomorphic deformation and although parts are eutaxitic most is lava like with flow banding and no fiamme a near ubiquitous penetrative flow lamination associated with a well developed elongation lineation is folded into small intrafolial tight to isoclinal oblique and sheath folds which are refolded by larger folds in the upper parts structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot 1000 c high mass flux pyroclastic density current that flowed westward across a graben faulted landscape as hot particles were deposited they rapidly agglutinated and coalesced and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current deposit interface the shear zone is interpreted to have been less than 2 m thick it produced and deformed the rheomorphic fabric and it migrated upward with the rising current deposit interface during aggradation so that it transiently affected all levels of the resultant thick ignimbrite deformation was progressive and after the density current had dissipated viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet this folded the flow banding and f 1 intrafolial isoclines into larger sheath folds and into more upright periclines near the top of the ignimbrite we demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites

the emplacement of a large chemically zoned rheomorphic lava like ignimbrite the sgurr of eigg pitchstone nw scotland the sgurr of eigg pitchstone on the isle of eigg nw scotland is a crystal rich trachydacitic partially vitrophyric rock which has previously been interpreted both as a lava and as a sill we interpret this rock as a chemically zoned rheomorphic lava like ignimbrite that formed during a sustained pyroclastic eruption the sgurr of eigg pitchstone can be subdivided into discrete emplacement units distinguished by the following features 1 their present day weathering characteristics 2 the orientation spacing and morphology of the columnar joints 3 sharp undulating boundaries with marked topographic breaks the absence of weathered surfaces palaeosols pyroclastic fall deposits or sedimentary rocks at emplacement unit boundaries suggests deposition from a single eruption the emplacement units some of which display upper and basal vitrophyres represent distinct depositional packages that record several rapidly emplaced ignimbrites which welded cooled and devitrified as a simple essentially single cooling unit during eruption from a sustained low pyroclastic column the sgurr of eigg pitchstone displays a pervasive base parallel flow banding which is folded into intrafolial recumbent isoclinal folds the flow banding and folds indicate that rheomorphism occurred throughout deposition the sgurr of eigg pitchstone is interpreted as an erosional remnant of an extensive ignimbrite sheet the first such unit recorded within the north atlantic igneous province

numerical modeling of infragravity wave response during delilah 1 the nearshore circulation model shorecirc is applied to simulate the infragravity wave conditions at two intervals during the 1990 delilah campaign at duck n c the model has been extended to solve the energy equation for random short waves on the timescale of the wave groups which acts as a wave driver for the long wave model to simulate the wave conditions an algorithm has been derived which synthesizes a directional offshore wavefield from measured data including the bound directional low frequency components the model data comparison shows that the mean short wave transformation and mean longshore current are predicted well the structure of the infragravity spectrum as measured in the nearshore array is correctly represented albeit that the energy levels are underpredicted in most locations the 2 d frequency longshore wavenumber spectra show that the infragravity wave energy consists mostly of edge waves and that the computations qualitatively resemble the measurements it is shown that for 1 day bar trapped edge waves are present the model is also used to show that the influence of the longshore variability is small for this particular beach and that the directional spreading in the offshore wave field is critical in predicting the nearshore infragravity wave conditions correctly edge waves which propagate against the current are generated by the directionally broad short wave forcing and to a lesser extent by back scattering the nonlinear terms are relatively unimportant for the prediction of the infragravity wave climate the cross shore gradient of the onshore directed radiation stress is the most important of the forcing terms

recognition characterization and implications of high grade silicic ignimbrite facies from the paleoproterozoic bijli rhyolites dongargarh supergroup central india abstract a controversy regarding the distinction between the highly welded lava like ignimbrites sometimes showing strongly rheomorphic characters and the extensive silicic lava flow has been overwhelming in the recent literature however a rethinking after walker 1983 has brought into light the concept of grade referring to the degree and extent of welding between the pyroclasts various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites which differentiate them from lava here a comprehensive study on early proterozoic acid magmatic rocks forming lower part of the dongargarh supergroup central india has been made to suggest extensive occurrence of high grade welded rheomorphic tuffs the possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences despite having overall monolithologic look various units bear distinction on account of their nature of welding enrichment of phenocrysts and degree of stretching the presence of vitroclastic texture melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits based on these characters four facies a b c and d from bottom to the top respectively have been identified from field studies around salekasa facies a and b represent clast supported matrix supported welded pyroclastic flow deposits facies c represents extremely welded thinly laminated rheomorphic tuffs while lava like tuffs with an autobreccia carapace is represented by facies d a complete gradation of facies a b to d through c exists high to extremely high grade nature of welding in these deposits suggests a low column height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting

low temperature hydrothermal alteration of intra caldera tuffs miocene tejeda caldera gran canaria canary islands abstract the miocene tejeda caldera on gran canaria erupted 20 rhyolite trachyte ignimbrites mogan group 14 13 3 ma followed by 20 phonolitic lava flows and ignimbrites fataga group 13 8 5 ma upper mogan tuffs have been severely altered immediately within the caldera margin whereas extra caldera mogan ignimbrites and overlying fataga units are apparently unaltered the altered intra caldera samples contain minerals characteristic of secondary fluid rock interaction clays zeolites adularia and relics of the primary mineral assemblage identified in unaltered ignimbrites k feldspar plagioclase pyroxene amphibole and groundmass quartz major and trace element data indicate that si na k pb sr and rb were strongly mobilized during fluid rock interaction whereas ti zr and nb behaved in a more refractory manner experiencing only minor mobilization the 18o values of the altered intra caldera tuffs are significantly higher than in unaltered extra caldera ignimbrites consistent with an overall low temperature alteration environment unaltered extra caldera ignimbrites have d values between 110 and 173 which may reflect rayleigh type magma degassing and or post depositional vapour release the d values of the altered intra caldera tuffs range from 52 to 131 with ambient meteoric water at the alteration site estimated at ca 15 interaction and equilibration of the intra caldera tuffs with ambient meteoric water at low temperature can only account for whole rock d values of around 45 given that dclay water is ca 30 at 100 c and decreases in magnitude at higher temperatures all altered tuff samples have d values that are substantially lower than 45 indicating interaction with a meteoric water source with a d value more negative than 15 which may have been produced in low temperature steam fumaroles supported by numerical modeling our gran canaria data reflect the near surface epithermal part of a larger fault controlled hydrothermal system associated with the emplacement of the high level fataga magma chamber system in this near surface environment fluid temperatures probably did not exceed 200 250 c

a model to explain joint patterns found in ignimbrite deposits the study of fracture systems is of paramount importance for economic applications such as co2 storage in rock successions geothermal and hydrocarbon exploration and exploitation and also for a better knowledge of seismogenic fault formation understanding the origin of joints can be useful for tectonic studies and for a geotechnical characterisation of rock masses here we illustrate a joint pattern discovered in ignimbrite deposits of south america which can be confused with conjugate tectonic joint sets but which have another origin the pattern is probably common but recognisable only in plan view and before tectonic deformation obscures and overprints it key sites have been mostly studied by field surveys in bolivia and chile the pattern is represented by hundreds of meters up to kilometre long swarms of master joints which show circular to semi circular geometries and intersections that have x and y patterns inside each swarm joints are systematic rectilinear or curvilinear in plan view and as much as 900 m long in section view they are from sub vertical to vertical and do not affect the underlying deposits joints with different orientation mostly interrupt each other suggesting they have the same age this joint architecture is here interpreted as resulting from differential contraction after emplacement of the ignimbrite deposit above a complex topography the set of the joint pattern that has suitable orientation with respect to tectonic stresses may act to nucleate faults

numerical experiments on low frequency fluctuations on a submerged coastal reef abstract low frequency fluctuations on a reef coast are studied through a numerical model based on nonlinear conservation equations of mass and momentum the model is used to investigate the long wave generation and propagation phenomena under both breaking and non breaking incident short wave groups the effects of incident wave characteristics and reef topography on the long wave phenomena are discussed the results show that the long wave elevations on reef coasts are substantial which explains the large water level fluctuations observed at certain occasions

ranking welding intensity in pyroclastic deposits welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature progressive welding is recorded by changes in the petrographic e g fabric and physical e g density properties of the deposits mapping the intensity of welding can be integral to studies of pyroclastic deposits but making systematic comparisons between deposits can be problematical here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations e g oblateness of pumice lapilli and micro fabric orientation and measurements of physical properties including density porosity point load strength and uniaxial compressive strength our dataset comprises measurements on 100 samples collected from a single cooling unit of the bandelier tuff and parallel measurements on 8 samples of more densely welded deposits the proposed classification comprises six ranks of welding intensity ranging from unconsolidated rank i to obsidian like vitrophyre rank vi and should allow for reproducible mapping of subtle variations in welding intensity between different deposits the application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre welding thicknesses

rheology of welding inversion of field constraints at present the mechanisms and rheological behaviour of pyroclastic deposits during welding and compaction are poorly understood here we explore the extent to which the rheological properties of pyroclastic deposits are constrained by physical property distributions in welded ignimbrite physical properties of samples from a 20 m section of the bandelier tuff new mexico are used as proxies for strain the observed strain et is ascribed to a combination of a time dependent viscous compaction ev and a time independent mechanical compaction em described by

impact of mechanical heterogeneity on joint density in a welded ignimbrite joints are conduits for groundwater hydrocarbons and hydrothermal fluids robust fluid flow models rely on accurate characterisation of joint networks in particular joint density it is generally assumed that the predominant factor controlling joint density in layered stratigraphy is the thickness of the mechanical layer where the joints occur mechanical heterogeneity within the layer is considered a lesser influence on joint formation we analysed the frequency and distribution of joints within a single 12 m thick ignimbrite layer to identify the controls on joint geometry and distribution the observed joint distribution is not related to the thickness of the ignimbrite layer rather joint initiation propagation and termination are controlled by the shape spatial distribution and mechanical properties of fiamme which are present within the ignimbrite the observations and analysis presented here demonstrate that models of joint distribution particularly in thicker layers that do not fully account for mechanical heterogeneity are likely to underestimate joint density the spatial variability of joint distribution and the complex joint geometries that result consequently we recommend that characterisation of a layer s compositional and material properties improves predictions of subsurface joint density in rock layers that are mechanically heterogeneous

grain size distribution of volcaniclastic rocks 1 a new technique based on functional stereology the power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits grain size also forms part of lithofacies characteristics that are necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits however the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record here we show that functional stereology can be used to obtain actual 3d volume fractions of clast populations from 2d cross sectional images tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method we show that the true grain size distribution is finer grained than its representation on a random 2d section our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes we anticipate that the original grain size distribution of welded ignimbrites can also be characterized by this method our method using functional stereology can be universally applied to any type of consolidated weakly to non deformed clastic material regardless of grain size or age and therefore has a wide application in geology

thermodynamic and rheological properties of rhyolite and andesite melts the heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative enthalpy measurements made between 400 and 1800 k for the glass phases the experimental data agree with empirical models of calculation of the heat capacity for the liquid phases the agreement is less good owing to strong interactions between alkali metals and aluminum which are not currently accounted for by empirical heat capacity models the viscosity of both liquids has been measured from the glass transition to 1800 k the temperature dependence of the viscosity is quantitatively related to the configurational heat capacity determined calorimetrically through the configurational entropy theory of relaxation processes for both rhyolite and andesite melts the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

kinematics of compressional fold development in convergent wrench terranes abstract kinematic models are presented for compressional fold development in wrench and convergent wrench terranes that relate fold shortening axial rotation and axial extension fold shortening may be derived from final fold geometry existing fold geometry and axial orientation two readily measurable quantities provide the data needed to determine the relative components of shearing and convergence within the fold system analyses utilizing these kinematic models indicate that folds developed in sedimentary rocks in the wrench borderlands of both the rineonada and san andreas wrench faults in central california are the product of strongly convergent wrenching the axes of these folds have been rotated no more than a few degrees during the course of their development in contrast folds developed in the alpine schists along the alpine fault in new zealand and in pleistocene sediments along the southern limit of the san andreas fault suggest an almost pure wrench setting and large 25 axial rotations significant axial extension is inherent in wrench related compressional folds this axial extension is commonly manifest in the form of normal and strike slip faults that are internal to the folds and trend at high angles to the fold axes the relative amount of axial extension diminishes as the degree of convergence increases this axial extension and the associated extensional features can be a diagnostic indication of the influence of wrenching

wave transformation on a coral reef wave transformation of regular waves was measured in a laboratory model of a fringing reef with a steep face and an outer reef top slope gradually decreasing in the landward direction data was obtained for various wave conditions and water levels a nonlinearity parameter fco g1 25ho 0 5t2 5 hc 1 75 based upon one proposed by swart and loubser 1979 is proposed as a suitable parameter for classifying wave transformation regimes on this reef in particular when fco 150 waves plunge on the reef edge and the amount of wave energy reaching a shore or structure is small 16 when fin co 100 waves spill on the reef top but the greater part of their energy is transmitted over the reef top the maximum values of the wave height to water depth ratio on the reef top were found to be consistent with nelson s analyses for laboratory and field data which indicate that the maximum stable wave height to depth ratio h d on a horizontal bottom never exceeds 0 55 for shallow water waves fc 500 the experimental data confirms that the maximum value of h d decreases when fc decreases but that it also increases when the bottom slope increases

rheological controls on the emplacement of extremely high grade ignimbrites extremely high grade lava like welded ignimbrites are produced by many large explosive eruptions with volumes typically 101 103 km3 however understanding of the physical properties of these unusual deposits and their transport and depositional mechanisms is incomplete the lava like and rheomorphic grey s landing ignimbrite idaho western united states provides abundant field evidence supporting the upward migration of a transient 2 m thick sub horizontal ductile shear zone at the interface between the pyroclastic density current and the deposit through which all of the aggrading pyroclastic material passed here we use a combination of rheological experiments and thermo mechanical modeling to test the syndepositional shear zone model we show that syndepositional welding and ductile flow are achievable within a very restricted field of likely temperature strain rate space where rapid deformation is favored by higher emplacement temperatures 850 c the field of ductile deformation is broadened significantly by accounting for strain heating which permits a sustained temperature increase of up to 250 c within the shear zone and helps to explain the enormous extents of lava like lithofacies and the intense rheomorphism recorded in extremely high grade ignimbrites recognition of strain heating within rheomorphic ignimbrites suggests that large pyroclastic density currents may travel over a hot substrate potentially hotter than the density current itself

large scale rheomorphic shear deformation in miocene peralkaline ignimbrite e gran canaria the single ignimbrite cooling unit e average thickness 28 m volume ca 30 km3 forms the uppermost member of the miocene upper mogan formation on gran canaria it is strongly chemically zoned from basal first erupted comendite peralkaline rhyolite to late erupted trachyte and apart from an upper trachytic zone it is densely welded e was emplaced onto a surface inclined ca 2 5 from the source caldera detailed mapping of key sections up to 300 m long exposed in barranco walls ca 10 km from the caldera margin reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones the shear zones formed within the lower viscosity comenditic tuff extensional structures include mega boudinage and decapitated sequences and compression resulted in sequence repitition by overthrusting mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs compaction lateral creep folding vertical density driven diapirism cannot account for these features which reflect lateral post compactional rheomorphic movement locally in excess of 800 m we suggest the following sequence of events emplacement of the several flow units compaction with little lateral movement rheomorphic deformation during and after compaction layers of secondary porosity developed within the comenditic tuff possibly where upward escape of gas was prevented by overlying relatively impermeable layers of densely compacted ignimbrite these structurally weak layers of high porosity subsequently acted as shear zones

long waves induced by short wave groups over an uneven bottom unidirectional and periodically modulated short waves on a horizontal or very nearly horizontal bottom are known to be accompanied by long waves which propagate together with the envelope of the short waves at their group velocity however for variable depth with a horizontal lengthscale which is not too great compared with the group length long waves of another kind are further induced if the variation of depth is only one dimensional and localized in a finite region then the additional long waves can radiate away from this region in directions which differ from those of the short waves and their envelopes there are also critical depths which define caustics for these new long waves but not for the short waves thus while obliquely incident short waves can pass over a topography these second order long waves may be trapped on a ridge or away from a canyon

hydrocarbon generation as a mechanism for overpressuring in rocky mountain region overpressured reservoirs are present in the deeper parts of most basins in the rocky mountain region a variety of conditions can cause overpressuring however most high pressures in the region are caused by present day or recently active oil and gas generation or generation in the last few million years in low permeability rock successions that still contain organic matter capable of yielding thermally generated hydrocarbons rocks with above normal pressure range from devonian to tertiary and commonly are associated with low permeability tight reservoirs most of the overpressuring occurs in cretaceous and tertiary rocks overpressuring in rocks older than cretaceous is rare and where present only occurs in strata with very rich hydrocarbon source beds such as the d vonian mississippian bakken formation most organically lean pre cretaceous source beds are no longer capable of yielding enough hydrocarbons to maintain abnormal pressure nearly all overpressured reservoirs and source rocks have temperatures of approximately 200 f 93 c or higher in addition available data indicate that hydrocarbon related overpressuring does not generally occur if vitrinite reflectance ro values are less than 0 6 in oil prone strata or less than 0 7 in gas prone strata overpressured gas bearing strata generally have ro values of 0 8 or higher maximum reservoir pressures are about equal to the fracture gradient pressure in the reservoir rocks some vertical fractures in reservoir rocks are probably caused by rapid expulsion of hydrocarbons from the source beds high pressures in the rocky mountain region means that the rock succession is gas or oil saturated and discrete hydrocarbon water contacts are generally not present classical concepts of hydrodynamics do not appear to be applicable to these overpressured successions the optimum depths for exploration are generally the upper 1 000 2 000 ft 305 610 m of the overpressured succession

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

energy transfer between wind waves and low frequency oscillations on a fringing reef ipan guam field observations from a guam fringing reef are used to examine the cross reef energy exchange between high frequency sea and swell ss and low frequency infragravity ig and far infragravity fig waves energetic ss waves significant wave heights 2 4 m break at the outer reef leading to weak 1 m conditions on the shallow reef flat as ss waves shoal on the reef face before breaking ig and fig energy fluxes both increase through nonlinear energy transfer from the ss waves in contrast through the surf zone the ig energy flux decreases whereas fig flux increases the decrease in ig energy flux through the surf zone is attributed to breaking ss waves working against the incident bound ig wave energy which dominates breakpoint forced ig waves yielding a net flux decrease in contrast fig energy flux increases through the surf zone consistent with breakpoint forcing and the absence of an energetic bound fig component on the reef face ig and fig energy fluxes decay on the shallow reef flat due primarily to frictional dissipation with tidal modulations attributed to nonlinear conversion and friction forcing at fig frequencies may lead to a normal mode response on the reef with comparable incoming and outgoing fig energy fluxes at the outer reef flat depending on water level over the reef flat and the degree of frictional dissipation

mafic plinian volcanism and ignimbrite emplacement at tofua volcano tonga tofua island is the largest emergent mafic volcano within the tofua arc tonga southwest pacific the volcano is dominated by a distinctive caldera averaging 4 km in diameter containing a freshwater lake in the south and east the latest paroxysmal vei 5 6 explosive volcanism includes two phases of activity each emplacing a high grade ignimbrite the products are basaltic andesites with between 52 wt and 57 wt sio2 the first and largest eruption caused the inward collapse of a stratovolcano and produced the tofua ignimbrite and a sub circular caldera located slightly northwest of the island s centre this ignimbrite was deposited in a radial fashion over the entire island with associated plinian fall deposits up to 0 5 m thick on islands 40 km away common sub rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma water interaction the common intense welding of the coarse grained eruptive products however suggests that the majority of the erupted magma was hot water undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water we propose that the development of a water saturated dacite body at shallow 6 km depth resulted in failure of the chamber roof to cause sudden evacuation of material producing a plinian eruption column following a brief period of quiescence large scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements this similar but smaller eruption emplaced the hokula ignimbrite sheet in the northeast of the island a maximum total volume of 8 km3 of juvenile material was erupted by these events the main eruption column is estimated to have reached a height of 12 km and to have produced a major atmospheric injection of gas and tephra recorded in the widespread series of fall deposits found on coral islands 40 80 km to the east in the direction of regional upper tropospheric winds radiocarbon dating of charcoal below the tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1 000 years bp we estimate an eruption magnitude of 2 24 1013 kg sulphur release of 12 tg and tentatively assign this eruption to the ad 1030 volcanic sulphate spike recorded in antarctic ice sheet records

flow banding in obsidian a record of evolving textural heterogeneity during magma deformation we perform a quantitative textural analysis of banding in obsidian from big glass mountain bgm medicine lake volcano california and from mayor island mi new zealand the samples are compositionally homogeneous with banding defined by variable microlite content bgm or vesicularity mi the spatial distribution of alternating light and dark bands has a 1 wavenumber power spectral density as well as multifractal characteristics through the use of simple mathematical models we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma our results indicate that changes in crystallinity vesicularity were concurrent with magma deformation and are random multiples of the total amount of crystallinity vesicularity that was already present at any given time banded obsidian from bgm and mi can be found in close spatial proximity to brittle deformational textures we therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation fragmentation can result in variable rates of degassing of fragments which in turn can lead to nonuniform crystal vesicle nucleation and growth rates repeated fragmentation welding and subsequent viscous deformation can thus lead to banding formation

gas exsolution during fluid migration and its relation to overpressure and petroleum accumulation abstract petroleum systems in the tertiary sections of the gulf of mexico and niger delta among other basins are characterized by the close association between petroleum accumulation and over pressure although overpressure is generated mainly by compaction disequilibrium of the low permeability sediments exsolution of gases from water and petroleum during migration over great vertical distance may create overpressure or modify the existing overpressure in this paper the author combined relative permeability data and petroleum phase behavior modelling to explain and predict the presence of an overpressure and petroleum baffle due to gas exsolution generalized isoperm diagrams for oil gas and water phases in sandy and silty shaly sediments were created based on published data aqueous solubility of methane and carbon dioxide as a function of temperature pressure and salinity is summarized based on literature data phase diagrams of petroleum with a wide range of composition api gravity and gor were produced using phase behavior software with calibration using field data these tools were integrated to model overpressure and petroleum phase variations in two oil gas fields in the gulf of mexico the pressure gradients in these fields are likely modified by gas exsolution from the petroleum which migrated and accumulated within and below the pressure transition zone ptz petroleum trapped within ptz tends to be fractionated whereas that accumulated below ptz tends to be single phase with high api gravity these results provide a model which explains both the effect of gas exsolution on ptz and the occurrence of different types of petroleum in these fields these tools may be integrated with geological and seismic data to assess more accurately the economic value of petroleum accumulations in petroleum systems governed by overpressure and petroleum phase separation and fractionation

groundmass crystallisation and cooling rates of lava like ignimbrites the grey s landing ignimbrite southern idaho usa constraining magmatic and eruptive processes is key to understanding how volcanoes operate however reconstructing eruptive and pre eruptive processes requires the ability to see through any post eruptive modification of the deposit the well preserved grey s landing ignimbrite from the central snake river plain provides an opportunity to systematically investigate the post eruptive processes occurring through a single deposit sheet despite overall compositional homogeneity in both bulk and glass compositions the grey s landing ignimbrite does preserve differences in the abundance of li in plagioclase crystals which are strongly associated with the host lithology li abundances in plagioclase from the quickly cooled upper and basal vitrophyres are typically low average 5 ppm n 262 while plagioclase from the microcrystalline interior of the deposit has higher li contents average 33 ppm n 773 given that no other trace elemental parameter in plagioclase varies we interpret the variability in li to reflect a post depositional process groundmass crystallisation of a rhyolite like grey s landing requires 50 crystallisation of sanidine and variable amounts of a silica rich phase quartz tridymite cristobalite and plagioclase to satisfy mass balance we suggest the low affinity of li for sanidine causes migration of groundmass li into plagioclase during crystallisation even within the microcrystalline interior of the deposit the morphology of the groundmass varies the more marginal finer grained regions are dominated by cristobalite as the sio2 rich phase while tridymite and quartz are additionally found in the more slowly cooled coarser grained portions of thick sections of the ignimbrite numerical models of cooling and crystallisation tested against field observations indicate that the groundmass crystallisation occurred relatively rapidly following emplacement a maximum of a few years where the ignimbrite is thickest these numerical models also illustrate that the time at which the potential for rheomorphism ceases either the ignimbrite is a solid glass or 50 crystalline is diachronous across the deposit

comparison of long medium and short term variations of beach profiles with and without submerged geological control abstract victoria beach cadiz spain comprises a rocky flat outcrop in its northern zone and a sand rich southern zone these natural features allowed for a 5 year monitoring period and subsequent analysis of two different profiles one in each zone based on differences in bottom contours topo bathymetric data were analysed using empirical orthogonal functions eofs to determine changes over the short medium and long term several morphologic phenomena were identified generalised erosion seasonal or summer winter tilting of the profile around different hinge points berm development and its posterior destruction etc in terms of their importance in explaining the variability of the collected data for both profiles it is worth mentioning that both profiles undergo parallel regression in the medium term thus the 1st eigenfunction enabled us to identify the true regression of the beach shoreline independent of seasonal or summer winter slope changes reconstruction of profiles using eof components demonstrated that though accretion periods in the medium term were similar for both types of profiles the accretion speed was much faster in the sand rich profile than in the reef protected profile 1 01 m 3 day versus 0 33 m 3 day moreover the seasonal erosion rate and the subsequent shoreline retreat for the sand rich profile were much larger than for the reef protected profile 121 m 3 year versus 29 m 3 year analysis in the short term changes induced by a single day s storm showed an instantaneous tilting of the profile with the mobilised sand volume being much greater for the sand rich than for the reef protected profile 68 m 3 m versus 12 m 3 m in brief we can assume that the wave energy attenuation provoked by the existence of a reef flat acted as a geological control for profile evolution which is essentially different for both profiles among the most important differences special attention should be given to the larger slope smaller mobilised sand volume and slower accretion rate of the reef flat profile versus the non protected profile

the emplacement history of a remarkable heterogeneous chemically zoned rheomorphic and locally lava like ignimbrite tl on gran canaria abstract ignimbrite tl on gran canaria is a complex compositionally zoned rheomorphic tuff that locally exhibits features previously considered to be diagnostic of lavas it is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode the eastern lobe is high grade with rheomorphic zones and localised patches that are lava like the western lobe is extremely high grade more extensively lava like and welded to its top surface both parts are zoned with a basal comendite rich zone grading up through a mixed zone into an upper trachyte rich zone lithic contents and the relative proportions of comendite and trachyte pyroclasts vary with height each comendite rich zone is vitroclastic whereas each trachyte rich zone is partly lava like with local gradations into vitroclastic ignimbrite mixed zones are intermediate in character and locally show compositional banding gradational zoning in massive ignimbrite best seen in lower strain zones and welding fabrics that are pervasively lineated and oblique to bedding suggest that deposition was sustained agglutination was rapid and rheomorphic deformation began during the sustained deposition the viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize composition and temperature the hot agglutinate continued to compact and shear downslope after the density currents had dissipated causing further rheomorphic folding thrusting attenuation and autobrecciation the western lobe locally overlies the partly welded top of the eastern lobe in part because it advanced rheomorphically across it for at least 300 m hot state loading and auto intrusion occurred due to unstable density layering in the chemically zoned agglutinate deformation behaviour changed during cooling and degassing and because of heat transfer between juxtaposed agglutinates and localised retention of dissolved volatiles where there was an overlying impermeable cap

the late paleozoic felsic lava dominated large igneous province in northeast germany volcanic facies analysis based on drill cores in the deep subsurface of the northeast german basin products of extensive volcanism are present that formed during the waning phase of the variscan orogeny permian carboniferous boundary large volumes 48 000 km3 of dominantly felsic magma were emplaced in the northern foreland of the variscan mountains in a terrestrial semi arid environment most of these units were inferred by previous authors to represent the products of explosive volcanic eruptions such as ignimbrites however a detailed analysis of the volcanic texture and lithofacies association shows that most units are best interpreted as coherent lavas emplaced either as extrusive units or as shallow intrusions with important implications for the reconstruction of the paleo environment this study shows that detailed textural examination of drill cores provide important observations which can be applied as criteria for the interpretation of the mode of volcanic eruptions furthermore phenocryst logging and geochemical fingerprinting based on immobile element ratios can be employed for the classification and discrimination of individual emplacement units

welding processes in volcanology insights from field experimental and modeling studies this volume is a collection of ten papers largely derived from a special session welding processes in volcanology organized for the 2003 meeting of eug egs agu in nice france and brings together field experimental and modeling studies we briefly review advances in understanding welding as represented by this volume and point to future lines of inquiry with an emphasis on understanding the timescales of welding the metrics of strain and feedback relationships between compaction porosity permeability loss fluid transport and rheology of the deposit our analysis suggests that within the realm of field and textural studies more systematic and quantitative data are necessary to evaluate strain histories related to welding and the timing of welding relative to other processes during eruption and emplacement a new generation of experiments is needed in which load strain rate and volatile pressure can be controlled numerical models are needed to further explore the relationships and feedbacks between processes that contribute to welding d 2004 elsevier b v all rights reserved

overpressure history of fractures west maracaibo basin venezuela prediction of density of natural fractures in reservoir rocks requires evaluation of many factors including the effective stress and thus fluid pressure conditions through time in this study we use analyses of fluid inclusions in calcite filled fractures with burial and thermal history models to assess the fluid pressure history and the causes of overpressure in the west maracaibo basin we analyzed abundant oil filled and rare aqueous fluid inclusions in calcite filled fractures in the la luna formation source rock and in the underlying cogollo gp carbonate reservoir our analyses of fluid inclusion homogenization temperatures and fluorescence properties lead us to the interpretation of near lithostatic paleo fluid pressures in la luna formation and near lithostatic to hydrostatic fluid pressures in the cogollo gp fractures maturation and expulsion of oil from the la luna formation source rock is required to generate the large inferred excess pressures as compaction disequilibrium and thermal expansion of pore fluids from rapid miocene burial are insufficient to achieve near lithostatic conditions this hypothesis is supported by the observation of decreasing paleo and modern fluid pressures with depth beneath the la luna formation thus based on the wide occurrence of oil filled inclusions in calcite filled fractures and the high fluid pressures associated withmore trapped oils we infer extensive rock fracture under overpressured conditions near maximum miocene burial during inferred late source rock yield and during miocene growth of structural traps less