eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

climate volcanism feedback and the toba eruption of 74 000 years ago abstract a general feedback between volcanism and climate at times of transition in the quaternary climate record is suggested exemplified by events accompanying the toba eruption 74 000 yr ago the largest known late quaternary explosive volcanic eruption the toba paroxysm occurred during the 18o stage 5a 4 transition a period of rapid ice growth and falling global sea level which may have been a factor in creating stresses that triggered the volcanic event toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co ignimbrite ash clouds to heights of at least 32 5 km which may have led to dense stratospheric dust and sulfuric acid aerosol clouds these conditions could have created a brief dramatic cooling or volcanic winter followed by estimated annual northern hemisphere surface temperature decreases of 3 to 5 c caused by the longer lived aerosols summer temperature decreases of 10 c at high northern latitudes adjacent to regions already covered by snow and ice might have increased snow cover and sea ice extent accelerating the global cooling already in progress evidence for such climate volcanic feedback following milankovitch periodicities is found at several climatic transitions

lithological discrimination based on statistical analysis of multi sensor drill core logging data in the matagami vms district quebec canada in mineral exploration traditional core logging is largely based on the visual inspection of drilled cores a process which is often subjective and non reproducible however a number of physical chemical and mineralogical properties of rocks can now be measured at high spatial resolution on drill cores the resulting large multiparameter databases can help geologists to quantitatively discriminate between lithologies study hydrothermal alteration and potentially vector towards mineralization multivariate statistical methods are important tools to assist geologists in interpreting such large databases we present an application of model based cluster analysis in the matagami base metal mining district more specifically to improve lithological discrimination at the zinc rich bracemac and mcleod volcanogenic massive sulfide vms deposits the model based cluster analysis method is able to efficiently discriminate different geological units encountered even in cases where two units are visually similar or in the presence of strong hydrothermal alteration

possible giant metamorphic core complex at the center of artemis corona venus hundreds of circular features on venus known as coronae are characterized by annular fractures and commonly associated radial fractures and lava flows coronae are thought to have been produced by buoyant mantle diapirs that flatten and spread at the base of the lithosphere and cause fracturing uplift and magmatism the interior of artemis corona by far the largest corona at 2100 km diameter is divided in half by a northeast trending deformation belt that contains numerous rounded ridges resembling antiforms the largest of these ridges located at the center of artemis corona is 5 km high on its steep northwest flank where it is adjacent to a flat bottomed 10 km wide trough interpreted as a rift valley the 280 km long antiformal ridge is marked by perpendicular grooves that cross the 50 km wide ridge and extend southeastward as far as 120 km across adjacent plains the grooves abruptly terminate northwestward at the rift trough the large antiformal ridge terminates southwestward at a transform shear zone that parallels the grooves these features rift valley antiformal uplift grooves and transform shear zone are morphologically and geometrically similar to grooved elevated submarine metamorphic core complexes on the inside corners of ridge transform intersections of slow spreading ridges on earth as with submarine core complexes the grooved surface on venus is interpreted as the footwall of a large displacement normal fault and the grooves are inferred to be the product of plastic molding of the footwall to irregularities on the underside of the hanging wall followed by tectonic exhumation of the molded grooves and conveyer belt like transport up and over the large antiform and across the southeastern plains according to this interpretation the trend of the grooves records the direction of extension which is perpendicular to the thrusts at the leading edge of the annular thrust belt 1000 km to the southeast both may have formed at the same time as a result of uniform southeastward displacement of the southeastern half of the interior of artemis corona the location of this grooved core complex at the center of artemis corona may reflect genesis above the buoyant ascending tail of the corona producing mantle diapir

multidomain petrophysically constrained inversion and geology differentiation using guided fuzzy c means clustering abstractgeophysical inversion methods are used as part of an interpretation process that seeks to differentiate geologic units to improve the reliability of geologic differentiation based on recovered images from geophysical inversions we have developed a multidomain clustering inversion algorithm that can incorporate statistical petrophysical data into a deterministic geophysical inversion framework through the use of the fuzzy c means clustering technique petrophysical data are treated in the parameter domain in the same way that geophysical data are treated in the spatial domain and these two different types of data are simultaneously inverted in their respective domains through the minimization of a single common objective function the resulting physical property model honors the geophysical and petrophysical data and therefore can represent the earth better than geophysical inversion models that solely honor the geophysical data geophysical inversion and geology differentiation are generally tr

rheological differences between archaean and younger crust can determine rates of quaternary vertical motions revealed by fluvial geomorphology surface uplift is widely observed in subaerial regions of continental crust that are distant from plate boundaries often revealed by river terrace staircases with altitude ranges of several hundred metres which typically span the quaternary this characteristic geomorphology has been explained by forcing of flow in the weak lower crustal layer by surface processes non steady state erosion and cyclical loading by ice sheets and sea level fluctuations this study tests this interpretation by showing that analogous river terrace staircases are absent in archaean crust which lacks the weak lower crustal layer found elsewhere

volcanic ash layers in the upper cretaceous of the central apennines and a numerical age for the early campanian at montagna della maiella and at gola del furlo central apennines two discrete layers of bentonic clay are intercalated within the pelagic furlo and turbiditic pelagic limestones maiella of the upper cretaceous basinal succession of the umbrian basin scaglia facies the bentonite layers are dated by planktonic foraminifera to the globotruncanita elevata zone early campanian and by calcareous nannofossils to the aspidolithus parcus zone cc 18 they fall into the reversed interval of chron 33 detailed correlation shows the layers to be of exactly the same age the upper layer is dated by u pb on magmatic zircons to 81 67 0 21 ma an age compatible with the cretaceous time scale of obradovich the mineralogy of the bentonitic clays is almost pure montmorillonite and contrasts sharply with the clay mineral assemblage of the enclosing pelagic and turbiditic limestones which is dominated by soil derived smectite and illite in different proportions the bentonite seams are interpreted as the submarine alteration products of wind borne volcanic ashes they can be followed with only minor changes in thickness over 200 km and must be derived from distant volcanic sources and related to extreme volcanic events a possible source area is present in the dinarides where upper cretaceous subduction related magmatic rocks are widespread

the orange tuff a late pleistocene tephra fall deposit emplaced by a vei 5 silicic plinian eruption in west java indonesia a vei 5 dacite eruption emplaced the orange tuff about between 34 3 cal kbp and 17 2 cal kbp gunung salak is the unit s source and the orange tuff represents the most recent such eruption from any of the volcanoes southwest of bogor indonesia the orange tuff is the region s first such documented tephra fall deposit whose characteristics and phenocryst geochemistry make it readily identifiable over at least 1250 km2 magnetite compositions and temperature and fo2 estimates inferred from fe ti oxide compositions are particularly useful for identifying the unit deposit characteristics suggest that the eruption lasted 1 11 h with mass eruption rates of 1 0 8 3 108 kg s and a column height of 31 40 km the eruption s column height and the deposit s 2 5 11 km3 volume suggest that the unit was dispersed over a much wider area than mapped the unit is a marker bed throughout its mapped distribution and has potential to be applied over a much broader area as a regional marker bed the large population and infrastructure proximal to salak suggest that the unit should be considered in hazards assessments despite its age and the lack of subsequent similar eruptions

direct geologic evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n comment and reply comment macleod et al 2002 1 made a strong case that subhorizontal corrugated surfaces adjacent to the slowly spreading mid atlantic ridge near 15 45 n are exhumed fault surfaces that have been uncovered by normal faulting associated with oceanic lithosphere divergence see also cann et al 1997

local and regional scale impacts of the 74 ka toba supervolcanic eruption on hominin populations and habitats in india abstract the toba supervolcanic eruption of 74 000 years ago is argued to have devastated homo sapiens populations causing a human population bottleneck through a combination of rapid global climatic deterioration and ecological disruption following the widespread ash fall this dramatic eruption the largest in at least the last 2 million years is hypothesized to have shaped the genetic structure and diversity of human populations today past assessments of toba s human impacts however have been predominantly theoretical using archaeological and geological evidence from sites in india this study addresses toba s local scale impacts on hominin technology behavior demography and habitats in two valleys in india the jurreru and middle son valleys using paleoclimatological and volcanological data regional scale impacts throughout india are inferred a refugia hypothesis is presented that argues for variation in the scale of toba s impact throughout india driven by regional differences in monsoonal dynamics geography and topography areas of north west india the indo gangetic plain and parts of the deccan to the east of the western ghats are argued to have been the most affected whereas areas of southern and eastern india may have preserved the largest refugia for hominin populations

p wave velocity tomography of the venezuelan region from local arrival times arrival times from the local seismological network of venezuela were used to estimate a three dimensional p wave velocity model for the region between longitude 60 74 w and latitude 6 14 n to a depth of more than 80 km the inversion was carried out by damped least squares describing the media by homogeneous velocity blocks the resolved lateral velocity variations in the first layer 0 30 km depth showed a correlation with the main stratigraphic features of the area while second layer 30 50 km depth showed the influence of moho depth variations through the region generating a pattern well correlated to the bouguer anomaly map lithospheric seismic velocities below the moho appear to be influenced by the major crustal fault systems an important low velocity zone is present below the triple junction of the fault systems of oca bocono and moron in northwestern venezuela farther south a similar low velocity zone is present below the junction of the bocono and the santa marta fault systems those are the two continental corners of the triangular maracaibo block below 80 km depth the fourth layer the low velocity zones show a connected pattern that follows or is adjacent to the crustal fault zones the presence of subducted atlantic lithosphere below the eastern venezuelan basin could explain the high velocity zone at this location a similar interpretation emerges from the tectonic wedging model previously proposed to explain the pronounced minimum of the gravity anomaly

a statistical methodology for deriving reservoir properties from seismic data the use of seismic data to better constrain the reservoir model between wells has become an important goal for seismic interpretation we propose a methodology for deriving soft geologic information from seismic data and discuss its application through a case study in offshore congo the methodology combines seismic facies analysis and statistical calibration techniques applied to seismic attributes characterizing the traces at the reservoir level we built statistical relationships between seismic attributes and reservoir properties from a calibration population consisting of wells and their adjacent traces the correlation studies are based on the canonical correlation analysis technique while the statistical model comes from a multivariate regression between the canonical seismic variables and the reservoir properties whenever they are predictable in the case study we predicted estimates and associated uncertainties on the lithofacies thicknesses cumulated over the reservoir interval from the seism

heat flow gravity and structure of the abitibi belt superior province canada implications for mantle heat flow the results of new heat flow measurements are presented for eleven sites located in the abitibi greenstone belt of the archean superior province these new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the abitibi subprovince and to determine the mantle heat flow beneath the canadian shield in order to analyze the long wavelength variations in heat flow and bouguer gravity anomaly the crust was divided into three layers with distinctive composition and physical properties average density heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data the dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density the models require mantle heat flow to range between 10 and 14 mw m 2 the westward increase in heat flow requires the tonalitic layer to thicken from the grenville front toward the kapuskasing uplift at the expense of the greenstone supracrustals long wavelength gravity data are more affected by changes in crustal thickness than by composition and the low bouguer anomaly near the eastern grenville front is caused by 5 6 km of crustal thickening in greenstone terranes the long wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data

porosity prediction from seismic data the inversion of seismic traces allows the estimation of reservoir porosity from an analysis of transit times derived from the pseudo velocity logs a four step computational procedure is illustrated consisting of i inversion of seismic traces and calculation of interval velocities ii accurate stratigraphic interpretation iii determination of the petrophysical parameters for the porosity evaluation iv analysis of the reliability of the results and final corrections both the possibilities and the limits of the method are discussed one of the causes of error is the fact that impedances and not velocities are readily obtainable from seismic data moreover the porosity due to fracturation contributes only slightly to velocity while it often contributes most importantly to the permeability results are shown for two of the most significant reservoir types i e carbonatic and clastic two cases belonging to the latter type will be examined in the first case the primary porosity is dominant the second case is very complex and both primary and secondary porosity are present

major element geochemistry of glass shards and minerals of the youngest toba tephra in the southwestern south china sea an ash layer newly discovered in core md01 2393 from the southwestern south china sea has been studied in order to characterize its major element features the layer 4 0 cm thick light grayish and silt size occurs right at the marine isotope stage 4 5 transition ca 74 kyr ago the morphology and geochemistry of glass shards combined with oxygen isotope and carbonate stratigraphy confirm the youngest toba eruption in northern sumatra as the origin of the ash layer major element data on mineral crystals i e biotite plagioclase and hornblende from the ash layer suggest that biotite is phenocrystic while hornblende and some plagioclase are xenocrystic implying that these xenocrysts were incorporated into the youngest toba magma before the eruption q 2005 elsevier ltd all rights reserved

application of seismic reflection data to discriminate subsurface lithostratigraphy a correlation between lithology and quantitative parameters abstracted from seismic reflection data is established the concept and methodology developed on synthetic data has been successfully applied to discriminate between two different kinds of lithologies a particular hydrocarbon bearing formation in a sedimentary basin in western india has been considered part of which is dominantly sandy lithological composition sand 53 percent shale 21 percent coal 26 percent and another part which is dominantly shaly sand 37 percent shale 60 percent coal 3 percent these two different lithologies are mathematically modeled using one step markov chains their seismic responses when scrutinized in time and frequency domain and subjected to statistical discriminant analysis give a fair idea about synthetic subsurface lithostratigraphy seismic reflection data from the same area were considered for a similar analysis on subjecting the data to discriminant analysis it was again possible to dis

numerical zonation of log suites and logfacies recognition by multivariate clustering zonation of well logs and the correlation of zones between wells are routine fundamental tasks in subsurface geological analysis objective numerical multivariate clustering is an effective procedure to simultaneously comprehend a suite of well logs and partition it into geologically meaningful zones and for identifying zone specific logfacies discrimination of zones and logfacies from log suites was established by a hierarchical clustering algorithm that defines clusters so that their within cluster dispersion is minimal logfacies are defined by an unconstrained analysis of unzoned data whereas the special requirements of the stratigraphic context of the zonation are observed by an adjacency constraint which prohibits merging individual depth levels or lower order clu ters if their members are not vertically contiguous two test cases one involving a succession composed of several alternating lithologies and the other entailing a uniform carbonate sequence with only subtle changes demonstrate the applicability and efficacy of the method in both cases the numerical zonation matched the manual lithostratigraphic division extremely well the geological validity of the numerically defined zones was also borne out by density neutron crossplots the unconstrained analysis successfully identified logfacies which are zone specific indicating that logfacies can serve as important leads for the recognition of reverse faults in boreholes and for interwell correlations

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

lithologic tomography an application to geophysical data from the cadomian belt of northern brittany france abstract a probabilistic description of subsurface lithologic structures can be established by inverting multidisciplinary geophysical data constrained by geological and geostatistical priors the methodology is based on the joint modelling of several media properties and on a statistical description of the relationships between them the information provided by the geophysical data and the geological and geostatistical priors is represented by probability density functions pdf that are combined into a posterior pdf composed by 1 a prior pdf in the space of the primary lithologic model parameters 2 a pdf of the secondary physical model parameters conditional to the primary model parameters and 3 a joint likelihood function that is the product of the independent likelihood functions for each observed geophysical field applying a markov chain sampling method enables a large sample of joint models to be generated from the posterior pdf the true configuration of the media is then determined from the representation of models pulled from the chain and the elaboration of statistics from the large sample of posterior joint models the method was used to invert gravity and magnetic data jointly characterising the mass density field the magnetic susceptibility field and the lithotype field along two 2 d sections of the geological units in the cadomian belt of northern brittany besides generating 10 6 joint models consistent with the observations and priors some features of the joint models and the statistical tomographic images provided additional insights to the geologic configuration of the area for example the main cadomian thrust shows an irregular geometry that could have resulted from the belt emplacement and or from variscan tectonism and the hercynian granitic intrusion shows a deep subsurface continuation the cosimulation of magnetic susceptibility and mass density inside each lithologic region was performed according to a multivariate gaussian model or a mixed multivariate gaussian functions model that was developed specially to describe multimodal distributed properties

measurement of thermal properties of select intact and weathered granulites and their relationship to rock properties abstractunderstanding the thermal properties of granulites a group of high grade metamorphic rocks representing composition of lower crust of the earth is necessary to arrive at effective crustal heat flow modeling however the thermal conductivity and diffusivity of intact and weathered granulites and parameters that influence them are poorly known the above properties for a few felsic mafic and intermediate granulites and associated soils from the southern granulite terrain india were investigated using a cost effective self fabricated setup the results generated by this technique were precise reproducible and comparable with other published procedures and values because the samples analyzed were of intact in situ specimens the thermal conductivity and diffusivity could be directly used in practical applications among several parameters it was observed that sio2 mgo fe2o3 ratio grain size and bulk density were the principal factors to influence the conductivity and diffusivity of fresh

lithology discrimination from physical rock properties the estimation of lithology from multiple geophysical survey methods needs to be addressed to develop advanced tomographic methods an initial requirement for lithology discrimination is that lithology should be discriminable from the media properties physically related to the geophysical observations to test this condition for different combinations of the most common crustal rocks we performed several lithology discrimination exercises on rock samples under laboratory conditions the physical properties included mass density compressional velocity shear velocity electric conductivity thermal conductivity and magnetic susceptibility a categorical description of the sample lithology was followed hence the inference consisted of predicting the sample rock category lithotype membership the joint information provided by the physical properties of the rocks allowed us to discriminate the sample lithotype correctly with an overall success rate of 100 in the most favorable situation and over 85 in the least favorable situation we obtained successful classification results for a variety of common lithotypes granite gabbro limestone tuff marble basalt and gneiss using three common classification methods clustering analysis gaussian classification and discriminant analysis although discrimination was positive with each of these multivariate classification techniques discriminant analysis showed some advantages for the classification and graphic analysis of the data these results support our postulate that lithology can be estimated reliably if multiple geophysical observations are considered jointly

trace based and geostatistical inversion of 3 d seismic data for thin sand delineation an application in san jorge basin argentina argentina s san jorge basin which straddles the southern provinces of chubut and santa cruz in the heart of patagonia has been producing hydrocarbons since 1907 the region currently accounts for 32 of the country s production the existence and early geologic evolution of the basin is due to the same rift process responsible for the opening of the atlantic ocean in early jurassic times extensive direct faulting and local erosion accompanied the rift evolution thereby facilitating the accumulation of chiefly nonmarine terrigenous sediments well into the early cretaceous at this point andean tectonism became a major source of pyroclastic concentration in the sedimentary column it was also responsible for pervasive batholitic intrusions clastic deposition in the hydrocarbon producing zone is characterized by thick shale laminations of lacustrine and floodplain origin interspersed with much thinner and laterally sparse sand bodies the hydrocarbon reservoirs

river response to quaternary climatic fluctuations evidence from the son and belan valleys north central india abstract the last glacial period was cold and dry in peninsular india in north central india the interval from 39 9 to 16 3 ka was associated with widespread and prolonged aggradation in the son and belan valleys the aggradation ended with sustained vertical incision after 16 3 ka and reflects a return to warmer wetter conditions in this region it would appear that terminal pleistocene to holocene river incision was broadly synchronous with a strong summer monsoon regime and higher levels of river discharge and the preceding river aggradation with lower discharge and a weaker or more variable summer monsoon regime two older phases of prolonged aggradation followed by vertical incision are evident in the son and belan valleys before 39 ka one of these phases is centred towards 73 4 ka when ash from the toba mega eruption in sumatra was deposited across peninsular india the following phase of aggradation has yielded infrared stimulated luminescence ages of 58 6 and 45 12 ka the youngest phase of aggradation began towards 5 5 ka and seems to mark a return to a weaker summer monsoon regime

new heat flow density and radiogenic heat production data in the canadian shield and the quebec appalachians new heat flow density hfd measurements were performed at 10 sites in quebec for five of the sites located in the superior province the heat flow density varies between 24 and 35 mw m2 26 and 37 mw m2 after adjustment for pleistocene climatic variations in the grenville province the values obtained range between 25 and 28 mw m2 29 and 31 mw m2 after adjustment for two nearby sites in the gaspe region appalachians the heat flow density is 47 mw m2 48 mw m2 after adjustment radiogenic heat production was also measured at the sites located in the meta volcano sedimentary sequences of the superior province the heat production is low 0 1 0 6 w m3 and it does not always correlate with the surface heat flow in the grenville province the hfd is close to slightly higher than the reduced heat flow of the superior the higher hfd in the appalachians is partly explained by the higher crustal heat production and partly by higher deep heat flow

a three dimensional gravity inversion applied to sao miguel island azores gravimetric studies are becoming more and more widely acknowledged as a useful tool for studying and modeling the distributions of subsurface masses that are associated with volcanic activity in this paper new gravimetric data for the volcanic island of sao miguel azores were analyzed and interpreted by a stabilized linear inversion methodology an inversion model of higher resolution was calculated for the caldera of furnas which has a larger density of data in order to filter out the noncorrelatable anomalies least squares prediction was used resulting in a correlated gravimetric signal model with an accuracy of the order of 0 9 mgal the gravimetric inversion technique is based on the adjustment of a three dimensional 3 d model of cubes of unknown density that represents the island s subsurface the problem of non uniqueness is solved by minimization with appropriate covariance matrices of the data resulting from the least squares prediction and of the unknowns we also propose a criterion for choosing a balance between the data fit which in this case corresponds to residues with rms of the order of 0 6 mgal and the smoothness of the solution the global model of the island includes a low density zone in a wnw ese direction and a depth of the order of 20 km associated with the terceira rift spreading center the minimums located at a depth of 4 km may be associated with shallow magmatic chambers beneath the main volcanoes of the island the main high density area is related to the nordeste basaltic shield with regard to the caldera furnas in addition to the minimum that can be associated with a magmatic chamber there are other shallow minimums that correspond to eruptive processes

submarine lineated sheet flows a unique lava morphology formed on subsiding lava ponds lineated sheet flows are flat lying glassy lava flows characterized by a regular surface pattern of parallel grooves or furrows aligned with the flow direction they are unique to the submarine environment we propose that the lineations are developed within the collapsed interiors of partially ponded lobate sheet flows that initially inflate and then drain out during emplacement during lava drainout the original lobate crust founders and a new crust begins to grow on the subsiding lava surface lineated flow texture is created where molten lava emerges laterally from beneath a growing crust the lineations are formed by raking of the emerging lava surface by irregularities on the bottom edge of the crust and are preserved owing to rapid chilling by seawater therefore lineated sheet flows are the product of a specific sequence of events over a short period of time during the course of a deep submarine eruption

new geochemical evidence for the youngest toba tuff in india abstract new geochemical data on tephra samples from a layer present at several archeological sites in india support correlation of this layer to the youngest toba tuff erupted from northern sumatra about 74 000 yr ago the data show that the indian tephra layer is not a correlative of older tephra erupted from toba as has been suggested on the basis of artifact assemblages previously published geochemical data on the indian tephra beds was based on bulk ash samples containing mineral and clay contaminants and the resulting variability in analyses did not allow identification or discrimination of individual eruptive events our new data were collected on individual glass shards and small purified glass separates which have greater resolving power in fingerprinting acheulian and paleolithic artifacts found at some of the indian tephra sites do not reflect the antiquity of the tephra bed as they occur in fluvial sediments and may be reworked

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

new occurrence of youngest toba tuff in abyssal sediments of the central indian basin abstract volcanic glass and pumice found in siliceous abyssal sediments of the central indian basin south of the equator have previously been assigned various origins including intra basin volcanism indonesian arc and krakatau rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the youngest toba tuff erupted at 74 ka from northern sumatra the correlation extends the distribution of coarse 63 m glass shards some 1500 km south of the previously known fallout zone and into the southern hemisphere reaching latitudes of ca 14 s this provides evidence for bi hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption such dispersal could have facilitated the global impact of the eruption

submersible study of an oceanic megamullion in the central north atlantic recently discovered megamullions on the seafloor have been interpreted to be the exhumed footwalls of long lived detachment faults operating near the ends of spreading segments in slow spreading crust we conducted five submersible dives on one of these features just east of the rift valley in the mid atlantic ridge at 26 35 n and obtained visual rock sample gravity and heat flow data along a transect from the breakaway zone where the fault is interpreted to have first nucleated in 2 0 2 2 ma crust westward to near the termination 0 7 ma our observations are consistent with the detachment fault hypothesis and show the following features in the breakaway zone faulted and steeply backtilted basaltic blocks suggest rotation above a deeper shear zone the youngest normal faults in this sequence are interpreted to have evolved into the long lived detachment fault in younger crust the interpreted detachment surface rises as monotonously flat seafloor in a pair of broad gently sloping domes that formed simultaneously along isochrons and are now thinly covered by sediment the detachment surface is locally littered with basaltic debris that may have been clipped from the hanging wall the domes coincide with a gravity high that continues along isochrons within the spreading segment modeling of on bottom gravity measurements and recovery of serpentinites imply that mantle rises steeply and is exposed within 7 km west of the breakaway but that rocks with intermediate densities prevail farther west within 5 km of the termination small volcanic cones appear on the detachment surface indicating melt input into the footwall we interpret the megamullion to have developed during a phase of limited magmatism in the spreading segment with mantle being exhumed by the detachment fault 0 5 m y after its initiation increasing magmatism may eventually have weakened the lithosphere and facilitated propagation of a rift that terminated slip on the detachment fault progressively between 1 3 m y and 0 7 m y identifiable but low amplitude magnetic anomalies over the megamullion indicate that it incorporates a magmatic component we infer that much of the footwall is composed of variably serpentinized peridotite intruded by plutons and dikes

an integrated approach to lithologic inversion part i theory for the complex problem of lithologic inversion a new formalism is proposed using a spatial distribution of seismic velocities cp cs and densities as input at each subsurface grid point the inversion result consists of the most likely lithotype gas sand shale etc together with its related litho parameters frame strength porosity etc the inversion method is based on the bayesian theory of parameter estimation allowing specification of inaccuracies in the input and the incorporation of geological knowledge an important advantage of the proposed inversion method is that for each practical situation the necessary accuracy of cp cs and can be predetermined to distinguish between specific lithotypes or to estimate a specific litho parameter with a prespecified accuracy for instance inversion results show that water saturation can only be estimated for an unrealistic accurate input on the other hand for the estimation of porosities it is generally sufficient to have compressional ve

multivariate analysis of cross hole georadar velocity and attenuation tomograms for aquifer zonation 1 we have investigated the potential of combining cross hole georadar velocity and attenuation tomography as a method for characterizing heterogeneous alluvial aquifers a multivariate statistical technique known as k means cluster analysis is used to correlate and integrate information contained in velocity and attenuation tomograms cluster analysis allows us to identify objectively the major common trends in the tomographic data and thus to reduce the information to a limited number of characteristic parameter combinations the application of this procedure to two synthetic data sets indicates that it is a powerful tool for converting the complex relationships between the tomographically derived velocity and attenuation structures into a lithologically and hydrologically meaningful zonation of the probed region in addition these synthetic examples allow us to evaluate the reliability of further petrophysical parameter estimates we find that although absolute values of the tomographically inferred petrophysical parameters often differ significantly from the actual parameters the clustering approach enables us to reliably identify the major trends in the petrophysical properties finally we have applied the approach to a cross hole georadar data set collected in a well studied alluvial aquifer a comparison of the clustered tomographic section with well log data demonstrates that our approach delineates the hydrostratigraphic zonation

geochemical fingerprinting of the widespread toba tephra using biotite compositions toba caldera sumatra is one of the largest and most explosive volcanoes on earth erupting some of the most voluminous volcanic deposits chronologically these eruptions are 790 ka older toba tuff 500 ka middle toba tuff and 74 ka younger toba tuff ash from these eruptions is dispersed over the entire region from india through malaysia to indonesia where it forms isochronous markers that have been documented in a variety of sedimentary sequences including palaeoclimate archives and archaeological sites the chemistry of the volcanic glass shards usually enables distal volcanic ash units to be correlated to a known eruption however the magmas involved in the three largest toba eruptions are compositionally similar and consequently the glass chemistry that is used to differentiate eruption units cannot be used to identify distal deposits here we show that the composition of biotite crystals which occur with glass in the distal deposits can be used to fingerprint the deposits of the younger toba tuff biotite in younger toba tuff has a lower feo mgo 2 1 2 6 than in the products of older eruptions 2 8 3 7 correlations using these distinct biotite compositions indicate that the ash found in malaysia and in archaeological sites in india was from the 74 ka younger toba tuff eruption

three dimensional massively parallel electromagnetic inversion ii analysis of a crosswell electromagnetic experiment summary a 3 d electromagnetic inversion scheme has been successfully applied to crosswell electromagnetic data collected at the richmond field station near berkeley california by comparing images of data collected before and after the injection of 50000 gallons of salt water a 3 d image of the plume has been developed which shows the location of zones of maximum permeability surrounding the injection well through which the salt water has migrated a resolution analysis has determined that the location of the plume is fairly accurate however the image of the geology will be distorted due to incomplete data coverage this latter problem is further complicated by the fact that data residuals indicate that the wells deviate from the vertical these deviations have been shown to cause artefacts within the images and thus further reduce the accuracy of the images with respect to the geology

ultramafic and gabbroic exposures at the mid atlantic ridge geological mapping in the 15 n region abstract the outcrops of mantle derived ultramafic rocks in the 15 n region are the most extensive yet reported for the mid atlantic ridge north of the fifteen twenty fracture zone these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall ultramafic rocks also crop out extensively south of the fifteen twenty fracture zone based on dredging and on a morphological analysis of the bathymetric map we propose that ultramafic outcrops may be common in the crust formed between 14 30 n and 15 50 n during at least the past 2 4 m y moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops diving observations also show that the ultramafic rocks are capped in stratigraphic contact by a thin layer of basalt this suggests that these rocks were tectonically emplaced at the axial seafloor or very close to it then uplifted in the footwall of the faults that bound the axial valley the occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region instead of one master shear zone there would be a complex array of cross cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds

the toba volcanic event and interstadial stadial climates at the marine isotopic stage 5 to 4 transition in the northern indian ocean abstract the toba volcanic event one of the largest eruptions during the quaternary is documented in marine sediment cores from the northeastern arabian sea on the crest of the murray ridge and along the western indian continental margin we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5 4 boundary with the chemical composition of the youngest toba tuff time series of the uk 37 alkenone index planktic foraminiferal species magnetic susceptibility and sediment accumulation rates from this interval show that the toba event occurred between two warm periods lasting a few millennia using toba as an instantaneous stratigraphic marker for correlation between the marine and ice core chronostratigraphies these two arabian sea climatic events correspond to greenland interstadials 20 and 19 respectively our data sets thus depict substantial interstadial stadial fluctuations in sea surface temperature and surface water productivity we show that variable terrigenous eolian sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record within the provided stratigraphic resolution of several decades to centennials none of these proxies shows a particular impact of the toba eruption however our results are additional support that toba despite its exceptional magnitude had only a minor impact on the evolution of low latitude monsoonal climate on centennial to millennial time scales

tropical rainforest vegetation climate and sea level during the pleistocene in kerala india abstract the southwestern ghats region of the indian peninsula is unique for its extant endemic rainforest flora supported by high rainfall throughout the year the record of tropical rainforest corresponding to the dynamic series of pleistocene interglacial glacial cycles is poorly known from peninsular india this communication discusses the palynological study of organic matter om deposits 40 ka bp in two well sections chaganachery kerala from the indian peninsula west coast a rich archive of tropical rainforest pollen spores and marine dinoflagellate cysts indicates anoxic fluvio marine estuarine depositional environments during warmer climates with an intensified asian monsoon the geochemical fingerprinting of glass shards indicates the presence of youngest toba ash of 74 ka from northern sumatra and therefore establishes a time controlled stratigraphy thus the depositional time period of the om is related to the sea level highstand of marine isotopic stage 5 1 80 ka which was the host to the ytt shards the late quaternary pollen spores diversity suggests that the modern climatic conditions in the southwestern ghats have facilitated the conservation of moist evergreen rainforest and dry moist deciduous forest the pollen grains show its lineage with the extant flora and some of the fossil pollen recorded during the mesic tertiary period from the indian peninsula thus it appears that the tropical rainforest survived here as plant refugia in xeric glacial quaternary periods perhaps as riparian vegetation and was rejuvenated during the holocene as modern extant flora

laboratory measurements of ultrasonic wave velocities in rocks from the campi flegrei volcanic system and their relation to other field data the campi flegrei caldera located near the city of naples italy has shown signs of unrest characterized by large ground deformation seismic activity and changes in the hydrothermal system since 1970 any attempt to model the processes giving rise to these phenomena requires some knowledge of the subsurface structure and of the physical properties of the rocks at depth in this study we report the results of laboratory measurements of ultrasonic compressional and shear wave velocities in rock samples cored in 4 different geothermal wells reaching a maximun depth of 3 km the wells are located in the san vito sv and mofete mf areas at about 3 km north and 4 km west respectively from the center of the caldera we selected 10 core samples 7 from the sv wells and 3 from the mf wells the attention being focused on the most important rock formations underneath campi flegrei and on the lower sections of the wells where data had been lacking the measurements involved the transmission of ultrasonic waves along three orthogonal directions through dry and water saturated rocks at room conditions the samples are only moderately anisotropic 13 for one sv sample and 8 for all the other samples wave velocities generally increase with the depth of the sample the range of velocities is 3 5 4 km s 1 for p waves and 1 5 2 9 km s 1 for s waves for the sv samples water saturation generally implies slightly larger 10 vp values and smaller 20 vs values as compared with dry velocities for the mf samples water saturation implies both higher vp and vs velocities up to 35 and 18 respectively these results may be explained by the different type of porosity of the sv samples vesicular and the mf samples fissurai the ultrasonic measurements on water saturated samples are slightly higher than the available in situ vp sonic measurements 25 for one sv sample and 15 for all the other samples these differences can be explained by velocity dispersion between ultrasonic and sonic frequencies and by the different pressure temperature conditions existing at depth

multivariate statistical analyses applied to seismic facies recognition one of the most important goals of seismic stratigraphy is to recognize and analyze seismic facies with regard to the geologic environment the first problem is to determine which seismic parameters are discriminant for characterizing the facies then to take into account all those parameters simultaneously the second problem is to be sure that there is a link between the seismic parameters and the geologic facies we are investigating this paper presents a methodology for automatic facies recognition based upon two steps the first or learning step begins with the definition of learning seismic traces for each facies we wish to recognize the choice of learning traces is based upon either well data or a seismic stratigraphic interpretation a large number of seismic parameters are then computed from the learning traces multidimensional analyses are carried out in order to validate the choice of learning traces and to select among all the available parameters those that discriminate best at this st

direct geological evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n from a detailed survey and sampling study of corrugated massifs north of the fifteen twenty fracture zone on the mid atlantic ridge we demonstrate that their surfaces are low angle detachment fault planes as proposed but not previously verified spreading direction parallel striations on the massifs occur at wavelengths from kilometers to centimeters oriented drill core samples from the striated surfaces are dominated by fault rocks with low angle shear planes and highly deformed greenschist facies assemblages that include talc chlorite tremolite and serpentine deformation was very localized and occurred in the brittle regime no evidence is seen for ductile deformation of the footwall synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low angle fault surface at very shallow levels directly beneath the ridge axis strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient

discriminant function analysis of glass chemistry of new zealand and north american tephra deposits major trace and rare earth element analyses of volcanic glass are used separately or in combination for correlating quaternary tephras often by graphical or simple comparative methods we have taken a statistical approach using discriminant function analysis dfa to assess the relative discriminating power of the different elements in volcanic glasses from several tectonovolcanic provinces we found that major oxides are powerful discriminating variables for widespread tephras from the taupo volcanic zone in new zealand and here they can be more discriminating than trace elements a wide selection of tephras from the western united states can also be distinguished on major oxides alone particularly those from cascade range volcanoes for tephras from large intracontinental calderas such as long valley or yellowstone ree and trace elements are more effective at discriminating than major oxides however tephras erupted from the long valley area can be distinguished on major oxide composition by dfa despite their similar chemistry the selection and relative significance of different elements for discriminating tephras depends on the total data set being compared as well as the source volcano and the individual eruptive events caution must be exercised in the nonstatistical selection of compositional data for characterizing tephras dfa is a more powerful and objective tool for the comparison of tephra chemistry

did the 73 ka toba super eruption have an enduring effect insights from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models abstract the potential impact of the 73 ka toba super eruption upon global and regional climate terrestrial ecosystems and prehistoric human populations remains unclear evidence from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models has provided some useful working hypotheses further progress requires a substantial improvement in the accuracy precision and resolution of the chronologies of each of the marine and terrestrial proxy records used to reconstruct the environmental impact of this extreme event

the great arc of dispersal of modern humans africa to australia abstract during the late pleistocene anatomically modern humans amh dispersed out of africa across the continents their routes obeyed the limitations placed on any large terrestrial mammal dependent on daily drinking water following certain climate permissive corridors amh first spread north with game across the sahara to the levant during the eemian interglacial c 125 ka but failed to continue to europe then occupied by neanderthals the savannah ecosystem in north africa and the middle east then dried up and amh vanished from the levantine fossil record being replaced there by neanderthals later amh successfully left africa as a single group by the southern route to india the added ability to make short but deliberate open water crossings allowed them first to cross the mouth of the red sea from eritrea and subsequently wallace s line to reach the isolated sahul continent at least by 48 000 years ago and possibly by 60 50 000 years ago they only finally arrived in europe from south asia around 45 50 000 years ago probably linked to climatic amelioration during ois 3

geophysical inversions applied to 3d geology characterization of an iron oxide copper gold deposit in brazil abstract mineral exploration dynamics often requires an efficient and objective means of evaluating a prospect in early exploration stages when few holes have been drilled in the case of deep prospects or prospects under cover this evaluation will mostly be based on geophysical data to develop an objective interpretation method capable of combining all the information available we have developed an integrated interpretation scheme of geophysical models and sparse geologic data our method is based on the relationship between recovered physical properties obtained from 2d and 3d inversions aiming to find patterns associated with geologic units such as iron formation copper ore and host rock the interpretation is guided by theoretical relations of the minerals of interest chalcopyrite and magnetite and the sparse geologic information available it is suitable for prospects in the initial stages of exploration when only limited mineralogical information is available from say one drillhole we h

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

geologic continuous casting below continental and deep sea detachment faults and at the striated extrusion of sacsayhuaman peru in one common type of industrial continuous casting partially molten metal is extruded from a vessel through a shaped orifice called a mold in which the metal assumes the cross sectional form of the mold as it cools and solidifies continuous casting can be sustained as long as molten metal is supplied and thermal conditions are maintained i propose that a similar process produced parallel sets of grooves in three geologic settings as follows 1 corrugated metamorphic core complexes where mylonitized mid crustal rocks were exhumed by movement along low angle normal faults known as detachment faults 2 corrugated submarine surfaces where ultramafic and mafic rocks were exhumed by normal faulting within oceanic spreading centers and 3 striated magma extrusions exemplified by the famous grooved outcrops at the inca fortress of sacsayhuaman in peru in each case rocks inferred to have overlain the corrugated surface during corrugation genesis molded and shaped a plastic to partially molten rock mass as it was extruded from a moderate to high temperature reservoir

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

integrated data analysis for mineral exploration a case study of clustering satellite imagery airborne gamma ray and regional geochemical data suites abstractpartitioning cluster algorithms have proven to be powerful tools for data driven integration of large geoscientific databases we used fuzzy gustafson kessel cluster analysis to integrate landsat imagery airborne radiometric and regional geochemical data to aid in the interpretation of a multimethod database the survey area extends over 3700 km2 and is located in the northern cape province south africa we carefully selected five variables for cluster analysis to avoid the clustering results being dominated by spatially high correlated data sets that were present in our database unlike other more popular cluster algorithms such as k means or fuzzy c means the gustafson kessel algorithm requires no preclustering data processing such as scaling or adjustment of histographic data distributions the outcome of cluster analysis was a classified map that delineates prominent near to surface structures to add value to the classified map we compared the detected structures to mapped geology an

depositional processes of reworked tephra from the late pleistocene youngest toba tuff deposits in the lenggong valley malaysia two fundamental issues for tephrostratigraphic work are the differentiation of primary from reworked tephra and the characterization of reworking mechanisms we study the depositional processes of four deposits of youngest toba tuff in the lenggong valley malaysia we focus on site stratigraphy particle size distributions magnetic susceptibility and mineralogical associations reworked tephra display variable sedimentological characteristics including polymodal and unimodal very fine to coarse grained distributions and variable concentrations of ash particle size distributions from this study are similar to published analyses for primary deposits demonstrating that particle size alone cannot distinguish primary from secondary tephra the tephra sequences are associated with fluvial and colluvial deposition three facies are identified flood flow mudflow and slumping the ash accumulated rapidly over a period of a few days to months in this valley the ideal site for paleoenvironmental reconstructions is kampung luat 3 where ash accumulated at least in two distinct phases despite the rapid accumulation the lenggong sites are not well suited for paleoenvironmental studies of the ytt impact the time lag between the primary deposition and the floods is unknown and the records could have been modified by site specific characteristics such variables should be considered when proposing paleo environmental reconstructions based on reworked tephra

porosity from seismic data a geostatistical approach using a geostatistical technique called cokriging the areal distribution of porosity is estimated first in a numerically simulated reservoir model then in an oil bearing channel sand of alberta canada the cokriging method consistently integrates 3 d reflection seismic data with well measurements of the porosity and provides error qualified linear mean square estimates of this parameter in contrast to traditional seismically assisted porosity mapping techniques that treat the data as spatially independent observations the geostatistical approach uses spatial autocorrelation and crosscorrelation functions to model the lateral variations of the reservoir properties in the simulated model the experimental root mean square porosity error with cokriging is 50 percent smaller than the error in predictions relying on a least squares regression of porosity on seismically derived transit time in the reservoir interval in the alberta reservoir a cross validation study at the wells demonstrates that the co

toba ash on the indian subcontinent and its implications for correlation of late pleistocene alluvium the toba ash occurs extensively in the indian subcontinent and marks a ca 74 000 yr old event in the bay of bengal and indian ocean it is about 10 cm thick whereas in several alluvial basins it is usually 1 3 m thick the latter occurs in a partly reworked state but as nearly chemically pure first cycle sediments the ash has a broad northwesterly dispersal pattern samples of ash from the indian subcontinent compare closely with the youngest 74 000 yr b p toba tuff and the deep sea toba ash in bulk chemical composition ree signature and bubble wall shard morphology however a more proximally located and thicker 2 5 m ash bed from the alluvial basins in the gneissic area and close to east coast has a lower magnitude negative eu anomaly possibly because of minor contamination by feldspathic silt quaternary sediments in the central narmada and middle son basins contain rich late and middle pleistocene mammalian and cultural records based on the presence of the ash layer marker and stratigraphic relations late pleistocene sediments within the subcontinent can be correlated with those from central india and the deep sea

the 73 ka toba super eruption and its impact history of a debate abstract toba volcano in northern sumatra is located at the intersection of two major tectonic lineaments in one of the most seismically active regions in the world and its caldera is considered the largest quaternary caldera on earth the most recent toba explosive eruption at 73 ka was an order of magnitude larger than that of tambora in 1815 and has a volcanic explosivity index of 8 the exceptional magnitude of this super eruption and the widespread distribution of the younger toba tuff ytt in marine cores in the indian ocean the arabian sea and the south china sea have prompted sustained debate as to its global and regional impact on climate ecosystems and prehistoric human populations however despite climate modelling and appeals to a variety of proxy climatic data including geochemistry pollen analysis archaeology and stable isotopes the extent of the impact remains unclear in order to achieve better progress in the future greater care is needed to obtain higher resolution and more precise chronologies from the marine and terrestrial proxy records used to test whether or not the 73 ka toba super eruption had a significant environmental impact records from speleothems and ice cores have much to offer in this regard

individual glass shard trace element analyses confirm that all known toba tephra reported from india is from the c 75 ka youngest toba eruption uncertainty over the identity and age of toba tephras across peninsular india persists with radiometric age dates contradicting earlier compositional data which have been used to identify this important stratigraphic marker as the youngest toba tuff ytt to address this issue new single glass shard analyses have been performed for samples from morgaon and bori north western india which have recently been dated at c 800 ka these and indeed all toba tephra samples thus far analysed from india show the presence of four populations of glass shards defined by their ba y ratio which uniquely identifies them as products of the c 75 ka youngest toba eruption confirmation that the ytt fingerprint is characteristic comes from new analyses of oldest toba tuff ott glass shards from five sites in the indian ocean these are compositionally identical to layer d from the odp site 758 sediment core c 800 ka and belong to a single low ba population clearly different from ytt these analyses show that there is essentially no reworked ott material in the ytt eruption and indicate unequivocally that all known toba tephra occurrences in india belong to the c 75 ka youngest toba eruption