layer induced seismic anisotropy from full wave sonic logs theory application and validation thin isotropic elastic layering in the earth is one cause of seismic vti anisotropy along with intrinsic anisotropy and fractures an important issue related to the routine use of vti seismic data processing is the estimation of the necessary parameters the full set of layer induced vti anisotropy parameters can be computed from full wave sonic and density log data using backus averaging intrinsic anisotropy can be incorporated if it is known from laboratory analysis the isotropic layering method is applied to six wells in eastern saudi arabia and the estimated anisotropy parameters are persistent across distances of many kilometers this leads to the possibility of parameter estimation at sparse well locations for use in seismic data processing validation is demonstrated by direct numerical simulation of elastic wavefields in original and backus averaged earth models with various window lengths we observe precise equivalence of the full wavefield when the averaging length is less than or equal to one third of the minimum dominant wavelength first arrival information used in depth migration is preserved with much longer averaging windows up to twice the minimum dominant wavelength

processing induced anisotropy processing of seismic data is often performed under the assumption that the velocity distribution in the subsurface can be approximated by a macromodel composed of isotropic homogeneous layers or blocks despite being physically unrealistic such models are believed to be sufficient for describing the kinematics of reflection arrivals in this paper we examine the distortions in normal moveout nmo velocities caused by the intralayer vertical heterogeneity unaccounted for in velocity analysis to match p wave moveout measurements from a horizontal or a dipping reflector overlaid by a vertically heterogeneous isotropic medium the effective homogeneous overburden has to be anisotropic this apparent anisotropy is caused not only by velocity monotonically increasing with depth but also by random velocity variations similar to those routinely observed in well logs assuming that the effective homogeneous medium is transversely isotropic with a vertical symmetry axis vti we express the vti parameters thro

carnian norian biomagnetostratigraphy at silicka brezova slovakia correlation to other tethyan sections and to the newark basin correlations of upper triassic magnetic stratigraphies from tethyan sections have been hampered by difficulties with conodont biostratigraphy and taxonomy and discontinuous sedimentation particularly in the hallstatt limestones of turkey and austria the magnetic stratigraphy and conodont biostratigraphy from the upper carnian to upper norian limestones exposed at silickabrezova slovakia can be correlated to other tethyan sections and to the continental succession in the newark basin the resulting correlations help to resolve some of the apparent discrepancies in existing conodont zonations and result in a revised correlation to north american terrestrial vertebrate and palynological zones the correlations imply that the norian rhaetian boundary lies within newark polarity zone e17r at v207 ma the carnian norian boundary lies close to the base of newark polarity zone e7r at v226 ma this implies durations for the norian and rhaetian stages of 19 myr and 7 myr respectively 2002 elsevier science b v all rights reserved

pore scale modeling of elastic wave propagation in carbonate rocks abstractthe relationship between p wave velocity and porosity in carbonate rocks shows a high degree of variability due to the complexity of the pore structure this variability introduces high uncertainties to seismic inversion amplitude variation with offset analysis porosity estimation and pore pressure prediction based on velocity data elastic wave propagation in porous media is numerically modeled on the pore scale to investigate the effects of pore structure on p wave velocities in carbonate rocks we built 2d models of porous media using pore structure information and the similarity principle then we simulated normal incidence wave propagation using finite element analysis finally the velocity was determined from received modeled signals by means of crosscorrelation the repeatability and accuracy of this modeling process was verified carefully based on the modeling results a simple formulation of sun s frame flexibility factor k aspect ratio ar the ratio of the major axis to the mi

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

low latitude and southern hemisphere origin of anisian triassic bedded chert in the inuyama area mino terrane central japan paleomagnetic and paleontologic studies of middle triassic to earliest jurassic radiolarian bedded chert in the inuyama area southern mino terrane of central japan have shown that the chert was deposited in an equatorial region with a paleolatitude between 10 n and 10 s and in particular the lower to middle anisian chert depositional basin was situated in the southern hemisphere the chert has four characteristic components of remanent magnetization the highest blocking temperature component carried by hematite shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition the other three components are interpreted as a present field overprint and two postfolding remagnetizations the absolute tilt corrected inclinations of the primary component are less than 20 for all the samples of middle triassic to earliest jurassic age with the mean inclination of the lower to middle anisian chert being 11 0 4 3 paleomagnetic polarity reversals observed for the lower to middle anisian chert are well correlated with those of the european triassic sections under paleontologic age constraints using radiolarians and conodonts provided that the anisian chert in inuyama was deposited in the southern hemisphere this conclusion illuminates the paleogeographic evolution of the chert bearing accretionary complexes not only in japan but along the eastern margin of asia as well

azimuthally dependent anisotropic velocity model update abstractwe consider a case where a 3d depth migration has been performed in the local angle domain lad using rich azimuth seismic data e g conventional land surveys the subsurface geologic model is characterized by considerable azimuthally anisotropic velocity variations the background velocity field used for the migration can consist of azimuthally independent e g vertical transverse isotropy and or azimuthally dependent e g orthorhombic velocity layers the resulting 3d full azimuth reflection angle gathers generated by the lad migration represent in situ high resolution amplitude preserved reflectivities associated with opening angles between incident and reflected slowness vectors in the specular directions residual moveouts rmos automatically picked on these 3d image gathers along major horizons can indicate considerable residual periodic azimuthal variations this situation is typical in depth imaging applied to unconventional shale plays where the background velocity model does

comparison of the ages of large body impacts flood basalt eruptions ocean anoxic events and extinctions over the last 260 million years a statistical study many studies have linked mass extinction events with the catastrophic effects of large body impacts and flood basalt eruptions sometimes as competing explanations we find that the ages of at least 10 out of a total of 11 documented extinction events over the last 260 myr 12 out of 13 if we include two lesser extinction events coincide within errors with the best known ages of either a large impact crater 70 km diameter or a continental flood basalt eruption the null hypothesis that this could occur by chance can be rejected with very high confidence 99 999 the ages of large impact craters correlate with recognized extinction events at 36 two impacts 66 145 and 215 myr ago and possibly an event at 168 myr ago and the ages of continental flood basalts correlate with extinctions at 66 94 116 183 201 252 and 259 myr ago and possibly at 133 myr ago furthermore at least 7 periods of widespread anoxia in the oceans of the last 260 myr coincide with the ages of flood basalt eruptions with 99 999 confidence and are coeval with extinctions suggesting causal connections these statistical relationships argue that most mass extinction events are related to climatic catastrophes produced by the largest impacts and large volume continental flood basalt eruptions

extraction of high molecular weight hydrocarbons from source rocks an example from the green river formation uinta basin utah abstract results of extraction experiments performed on immature green river formation source rock samples from the black shale facies uinta basin utah demonstrate the potential of extracting high molecular weight hydrocarbons hmwhc with sequential dichloromethane methanol soxhlet and p xylene sonication techniques the first soxhlet extraction step with dcm meoh removes lower molecular weight alkanes from the source rock while the hmwhc remain in the rock residue sonication of the rock residue with p xylene recovers these hmwhc showing distinct hydrocarbon distributions and preferences in high temperature gas chromatograms extracts obtained by using p xylene soxhlet with original source rocks and sequential soxhlet extraction instead of sonication have relatively lower concentrations of hmwhc sequential extraction of two other source rock types mahogany shale and black shale facies coal did not result in detection of similar hmwhc in the extract this indicates that the presence of these compounds in the black shale facies of the green river formation is a primary source signal and a specific characteristic of this source rock

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

paleomagnetism of triassic bedded chert from japan for determining the age of an impact ejecta layer deposited on peri equatorial latitudes of the paleo pacific ocean a preliminary analysis abstract bedded chert samples from the norian upper triassic sakahogi section of the mino terrane in the inuyama area southwest japan have been collected for paleomagnetic study in order to determine the age of an impact ejecta layer interbedded within it and the paleolatitude of its deposition thermal demagnetization of the bedded chert samples revealed four distinct remanent magnetization components the last demagnetized component with both polarities is interpreted to be the primary magnetization which produced the first magnetostratigraphic data of the middle to upper norian from panthalassa consistent with tethyan magnetostratigraphic sections tie points were derived from paleomagnetic and paleontological data from which the stratigraphic position of the ejecta layer was compared with the astronomically tuned geomagnetic polarity time scale apts the age of ejecta layer is estimated to be about 212 ma this magnetostratigraphic age is consistent with the radiometric age of the manicouagan impact that formed the second largest known crater in the phanerozoic at quebec canada the results of this study as well as previous studies suggest that the chert section which includes the ejecta layer was deposited within a paleolatitude range of 0 3 14 4 n this indicates that the material ejected by the manicouagan impact event reached near the equatorial region of the paleo pacific ocean

high resolution u pb ages from the upper triassic chinle formation new mexico usa support a diachronous rise of dinosaurs abstract though the late triassic preserves major paleoenvironmental fluctuations and is key for understanding the evolution of mesozoic and modern terrestrial ecosystems comparisons of late triassic non marine sedimentary and fossil records are difficult because global correlations lack precise radioisotopic ages and have instead been based upon unconstrained biostratigraphic ranges of palynomorph and vertebrate fossils the chinle formation in southwestern north america preserves a major late triassic record of paleoenvironmental and biotic change including significant early dinosaur fossils previous high resolution radioisotopic age constraints for the formation are limited to a single u pb zircon age from the upper third of the formation the extraction of a geologically meaningful age is challenging from these redeposited units and preference is given to considering the youngest age of a deposit as a maximum age and closest approximation of the depositional age because calculating a weighted mean age or median age from a group of ages from such deposits is often not adequate the precision of our two new ca tims single crystal zircon u pb ages from the chinle formation of new mexico is limited to ca 0 3 or 0 7 ma of the youngest crystal age our 206 pb 238 u age of 218 ma from the blue mesa member in six mile canyon western new mexico demonstrates that strata palynomorphs and vertebrate fossils previously considered to be late carnian in age are actually middle norian in age our new age of 212 ma from the hayden quarry within the petrified forest member at ghost ranch northern new mexico provides the first maximum age for important vertebrate assemblages from this area that record the rise of dinosaurs and demonstrates that basal dinosauromorphs dinosaur precursors co existed with dinosaurs for at least 18 ma these new radioisotopic data allow a new correlation of the chinle formation to the late triassic timescale suggesting that most if not all of the lower chinle is norian in age this new correlation has global implications as it allows us to make more precise comparisons with early dinosaur assemblages from the ischigualasto formation of argentina indicating that chinle dinosaur assemblages are significantly younger than those from south america the revised age of the chinle formation also demonstrates that dinosaurs were much rarer in north america at a time when they were abundant in south america supporting hypotheses of paleolatitudinal variation during the rise of dinosaurs

ubiquitous occurrence of high molecular weight hydrocarbons in crude oils abstract for many years the presence of waxes in crude oils has been associated with organic matter derived from terrigenous sources high temperature gas chromatography htgc has been used to establish the ubiquitous presence of high molecular weight hmw hydrocarbons extending as high as c 120 in crude oils hmw hydrocarbons c 40 have been observed in crude oils derived from terrigenous lacustrine and marine source materials modified extraction techniques have also led to the detection of hmw hydrocarbons in source rock extracts the fractions of hmw hydrocarbons are complex mixtures of long chain aliphatic and alkyl aromatic hydrocarbons and can represent up to 8 of a whole oil the carbon preference index cpi of hmw alkylcyclopentanes in the c 42 c 46 range provides useful information on the characteristics of the original depositional environment age maturity and degree of biodegradation do not appear to affect the cpi of alkylcyclopentanes used for evaluating depositional environments differences in the distributions of the hmw hydrocarbons between the source rock extracts and the crude oils in the reservoirs may reflect changes occurring due to migration

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

assessing the record and causes of late triassic extinctions accelerated biotic turnover during the late triassic has led to the perception of an end triassic mass extinction event now regarded as one of the big five extinctions close examination of the fossil record reveals that many groups thought to be affected severely by this event such as ammonoids bivalves and conodonts instead were in decline throughout the late triassic and that other groups were relatively unaffected or subject to only regional effects explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms regression during the rhaetian with consequent habitat loss is compatible with the disappearance of some marine faunal groups but may be regional not global in scale and cannot explain apparent synchronous decline in the terrestrial realm gradual widespread aridification of the pangaean supercontinent could explain a decline in terrestrial diversity during the late triassic although evidence for an impact precisely at the boundary is lacking the presence of impact structures with late triassic ages suggests the possibility of bolide impact induced environmental degradation prior to the end triassic widespread eruptions of flood basalts of the central atlantic magmatic province camp were synchronous with or slightly postdate the system boundary emissions of co2 and so2 during these eruptions were substantial but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved a substantial excursion in the marine carbon isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion although the triggering mechanism and climatic effects of such a release remain uncertain d 2003 elsevier b v all rights reserved

the use of heavy mineral correlation for determining the source of impact ejecta a manicouagan distal ejecta case study due to the nature of distal impact ejecta layers on earth preserved deposits are rare and are commonly altered by diagenetic weathering processes to establish the source of ejecta trace element signatures of impact glasses have typically been used to establish the source crater of a deposit however in many cases impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition this is the case for the late triassic ejecta deposit of sw britain where impact melt spherules have been completely altered to clay this late triassic ejecta deposit was originally believed to be derived from two possible sources the rochechouart or manicouagan impact structures to accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion ar ar dating of shocked biotite u pb dating of zircon was also used but proved inconclusive radiogenic dating of shocked biotites observed exclusively in the ejecta deposit yielded ages consistent with the grenvillian target rocks at manicouagan and excluded rochechouart as a potential source garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the manicouagan target rocks and impactite rocks a strong garnet composition signature correlation between the samples from manicouagan and the ejecta deposit provides convincing evidence that manicouagan is the source of the sw britain ejecta deposit furthermore we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta

backus averaging scattering and drift differences between traveltimes from sonic to seismic frequencies commonly known as drift can be attributed to a combination of multiple scattering and absorption the portion due to scattering can be estimated directly by calculating synthetic seismograms from sonic logs a simple alternative approach is suggested by the long wave equivalent averaging formulae for the effective elastic properties of a stack of thin layers which gives the same traveltime delays as the low frequency limit of the scattering dispersion we consider the application of these averaging formulae over a frequency dependent window with the hope of extending their use to frequencies higher than those allowed by the original validity conditions however comparison of the time delay due to window averaging with the scattering dispersion predicted by the o doherty anstey formula reveals that it is not possible to specify a form of window that will fit the dispersion across the spectrum for arbitrary log statistics a window with a width proportional to the wavelength squared matches the behaviour at the low frequency end of the dispersive range for most logs and allows an almost exact match of the drift across the entire spectrum for exponential correlation functions we examine a real log taken from a hole in nearly plane layered geology which displays strong quasi cyclical variations on one scale as well as more random smaller scale fluctuations the details of its drift behaviour are studied using simple models of the gross features the form of window which gave a good theoretical fit to the dispersion for an exponential log correlation function can only fit the computed drift at high or low frequencies confirming that there are at least two significant scale lengths of fluctuation a better overall fit is obtained for a window whose width is proportional to the wavelength the calculated scattering drift is significantly less than that observed from a vertical seismic profile but the difference cannot be wholly ascribed to absorption this is because the source frequency of the sonic tool is not appropriate for its resolution receiver spacing so that the scattering drift from sonic to seismic frequencies cannot be fully estimated from the layer model derived from the log

estimation of effective anisotropy simultaneously with locations of microseismic events passive seismic tomography in which the event locations and the velocity model are inferred simultaneously is seldom used to process microseismic surveys acquired in the oil and gas industry we discuss advantages of applying tomographic ideas to typical microseismic data recorded in a single nearly vertical well to monitor hydraulic stimulation of a shale gas reservoir microseismic events are conventionally located in the energy industry applications using a velocity model derived from sonic logs and perforation shots instead of fixing the model as is normally done we alter it while locating the events this added flexibility not only makes it possible to accurately predict traveltimes of the recorded p and s waves but also provides a convincing evidence for anisotropy of the examined shale formation while we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing the obtained models are suggestive of possible time laps

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

sedimentary pge signatures in the late triassic ejecta deposits from japan implications for the identification of impactor abstract positive platinum group element pge concentration and negative os isotope anomalies reported from a claystone layer in the upper triassic bedded chert succession of the sakahogi section mino belt central japan are thought to have been derived from an impact event stratigraphic variations and concentrations of pge were examined in the sakahogi section to determine the type of the impactor upper triassic claystone layers where pge anomalies have been newly discovered in bedded chert successions in southwest japan were also examined these include i the unuma section in the inuyama area mino belt ii the hisuikyo section in the kamiaso area mino belt and iii the enoura section in the tsukumi area chichibu belt radiolarian and conodont biostratigraphic data indicate that these claystone layers are of upper middle norian age reconstruction of bedded chert in these sections suggests that they originate from open ocean pelagic deep sea sediments deposited in the panthalassa ocean the relatively flat ci chondrite normalized patterns of the least mobile pges ir ru and rh and the ru ir ratio determined by linear regression analysis suggest that a chondritic impactor is the source of the pge anomalies preserved in claystone samples from the study sections although ru ir ratios cannot conclusively distinguish chondrites from iron meteorites the cr ir ratios of the claystone layers range from 10 4 to 10 5 clearly indicating contribution from chondritic materials the chondritic impactor of the suggested size 3 3 7 8 km in diameter implies that a large amount of debris and or climatically active gasses e g sulfur oxides would have been released from the impactor which would have had a marked effect on the environment

severe selenium depletion in the phanerozoic oceans as a factor in three global mass extinction events abstract selenium se is one of the key trace elements required by all animal and most plant life and se deficiencies in the food chain cause pathologies or death here we show from new geochemical analyses of trace elements in phanerozoic marine pyrite that sustained periods of severe se depletion in the past oceans correlate closely with three major mass extinction events at the end of the ordovician devonian and triassic periods these represent periods of se depletion 1 5 2 orders of magnitude lower than current ocean abundances being within the range to cause severe pathological damage in extant se reliant organisms se depletion may have been one of several factors in these complex extinction scenarios recovery from the depletion extinction events is likely part of a natural marine cycle although rapid rises in global oxygen from sudden major increases in marine productivity and plant biomass after each extinction event may also have played a crucial role

seismic anisotropy in sedimentary rocks part 1 a single plug laboratory method a single plug method for measuring seismic velocities and transverse isotropy in rocks has been rigorously validated and laboratory tested the method requires only one sample to measure the velocities needed to derive the five independent elastic constants for transversely isotropic materials in this method piezoelectric transducers are fitted to the top bottom and sides of the cylindrical sample laboratory velocity and anisotropy can be measured as functions of pressure temperature fluid saturation and fluid displacement because this method uses a horizontal core plug that has much higher permeability than a vertical core plug it is especially suitable for low permeability shale measurements it reduces the sample preparation and velocity measurement time by more than two thirds

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

a comparison of laboratory and field measurements of p wave anisotropy1 p wave velocity anisotropy determined from a cross hole survey at the imperial college borehole test site compares favorably with that measured in the laboratory on core from the holes the holes penetrate a layered sequence of sandstones shales and carbonates of the namurian upper limestone group the laboratory measurements of the vertical and horizontal velocities of core samples indicate that the shales exhibit p wave anisotropies of over 20 but that the sandstones and limestones are only slightly anisotropic these discrete measurements have been used in combination with wireline data to produce a log of p wave anisotropy including the anisotropic information vastly improves the match between observed and synthetic traveltimes from the cross hole data set this implies that there is little frequency dependence of intrinsic p wave anisotropy inversion of the cross hole traveltimes highlights the need for good angular coverage in order to resolve the anisotropy parameters the observed p wave anisotropy of the field data is due to the combined effect of sedimentary layering and the intrinsic anisotropy of the rocks the intrinsic anisotropy is found to be the dominant factor

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

carbon and nitrogen isotope disturbances and an end norian late triassic extinction event major perturbations of organic carbon and nitrogen isotope ratios from a norian rhaetian late triassic boundary section in british columbia coincide with an extinction of the dominant deep water invertebrate fauna of the late triassic monotids and most ammonoids the carbon isotope excursion is attributed to the development of widespread oceanic stagnation that favored organic rich shale deposition the coincident nitrogen isotope excursion suggests that progressively more nitrate limited productivity forced a change to nitrogen fixing cyanobacteria populations as ocean stagnation created nutrient starved conditions the biotic crisis and geochemical events of the norian rhaetian boundary predate the latest rhaetian end triassic mass extinction thus the late triassic interval was marked by more than one extinction event

late triassic compositional changes of aeolian dusts in the pelagic panthalassa response to the continental climatic change abstract the carnian pluvial event cpe is considered as the drastic environmental change that occurred in the western tethys during the carnian late triassic although some studies have suggested that the cpe was a global event no studies have referred to pelagic environment related to the cpe to reveal the pelagic environment during the carnian x ray absorption fine structure xafs analysis was used to determine the iron fe species in chert samples collected from the ladinian carnian strata in the tamba mino ashio belt japan the x ray absorption near edge structure xanes analysis shows that all the red cherts contain hematite with a combination of clay minerals such as illite chlorite and smectite the existence of these minerals is identified using extended xafs exafs the xanes analysis of the green and the white cherts does not show any hematite signatures while the exafs analysis indicated increasing amounts of chlorite the stratigraphic change in the fe bearing compositions observed in this study can be divided into three stages i a stage where a relatively stable mineral composition chlorite illite hematite is observed in cherts deposited during the early carnian julian ii the abrupt change in mineral composition lack of chlorite and appearance of smectite observed in the middle area in the studied section and corresponding to the middle part of carnian late julian early tuvalian and iii the recovery of the previous stable mineral composition during the late carnian tuvalian these variations in the mineralogical composition of pelagic cherts can be interpreted to mean that the continental climate conditions changed during the carnian and different clay fractions were delivered to the pelagic region these data combined with the results of previous studies performed around the continental realm can be interpreted as follows i there was a relatively stable dry climate before the cpe ii there were increasing amounts of precipitation owing to the cpe and iii the climate returned to dry conditions similar to those prior to the cpe

where have all the craters gone earth s bombardment history and the expected terrestrial cratering record here we make the first quantitative estimates of the maximum total number of large terrestrial impact craters that can survive until the current era on earth our estimates of crater survival are made using the age of earth s crust and the expected flux of large extraterrestrial bodies striking earth this analysis neglects effects such as weathering and burial which tend to preferentially erase smaller craters thus our results represent the maximum number of craters we expect to exist on earth today although the constant recycling of crustal material creates a strong bias toward younger ages our model suggests that the terrestrial cratering record can be used to determine the size frequency distribution of the impactors that bombarded earth over the past 3 5 g y however our model suggests that the cratering record cannot be used to distinguish between scenarios with a constant flux of impactors over time and scenarios where the bombardment rate was considerably higher in earth s ancient history luckily layers of distal impact ejecta may act as records of impacts even when the source craters have been tectonically recycled we suggest that searches for these distal impact ejecta layers will be more fruitful for constraining the bombardment history of earth than searches for large craters

more than one event in the late triassic mass extinction the recent hypothesis that mass extinctions are discrete phenomena that have occurred with great regularity during the history of life1 2 is testable in several ways two essential elements of the hypothesis are 1 that each extinction event represents a significant departure from normal or background rates of extinction and 2 that such mass extinction events are spaced equally in time the analyses of the cyclicity of mass extinctions so far have concentrated on the past 250 myr with the first event occuring at the permian triassic boundary 245 myr ago1 5 the second event followed 26 33 myr later1 6 in the late triassic here i present a detailed analysis of the fossil record of marine and non marine life during the late triassic which suggests that there were at least two phases of mass extinction during that time separated by 12 17 myr

magnetostratigraphic dating of the proposed rhaetian gssp at steinbergkogel upper triassic austria implications for the late triassic time scale the global stratotype section and point gssp for the rhaetian stage has recently been proposed at steinbergkogel in austria we re sampled the steinbergkogel sections stk a and stk b c in high resolution to establish a robust magnetostratigraphy that allows global correlation the palaeomagnetic signal at steinbergkogel is composed of three components which can be separated by thermal demagnetization the highest temperature component revealed between 280 300 and maximum 600 c is of dual polarity and is interpreted as primary rock magnetic experiments showed that the signal is carried by magnetite our results allow correlation between the two individual steinbergkogel outcrops subsequently we correlate the two key biostratigraphic horizons for the base of the rhaetian the fo of m hernsteini and the fad of m post hernsteini to other sections of the tethys domain the correlation to the astronomically dated continental successions of the newark basin indicates that these positions for the base of the rhaetian are most likely determined in chrons e16n and e16r respectively this correlation is confirmed by cyclostratigraphic control on the marine pizzo mondello italy section where a combination of long period milankovitch cycles 175 myr and short eccentricity cycles 100 kyr provide additional correlation constraints respectively supporting a long duration of the rhaetian our study implies that the norian and rhaetian stages have durations of 17 and 9 myr

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

environmental change during the triassic jurassic boundary interval of an equatorial carbonate platform sedimentology and chemostratigraphy of the ghalilah formation united arab emirates abstract the sedimentology and chemostratigraphy 13ccarb 13corg sr concentrations of the triassic jurassic tr j boundary interval has been investigated in detail in sections of the ghalilah formation exposed in wadi ghalilah wadi milaha and wadi naqab northeastern united arab emirates the sequence of events recorded across the tr j boundary interval in ras al khaimah reflects significant environmental change that can ultimately be related to camp volcanism uppermost triassic limestones of the sumra member rich in coral calcareous sponges and large size bivalves are interbedded with siliciclastic mudstones and deposited in an open lagoonal environment these are abruptly followed by oolites with intercalated calcareous mudstones of the sakhra member that are devoid of macrofossil thrombolitic microbialites that occur in the lowermost sakhra member are interpreted to reflect the demise of hypercalcifying marine biota during the tr j boundary interval the oolites of the sakhra member are capped by a subaerial exposure surface followed by peritidal deposits of the shuba member the available sparse biostratigraphic data of the ghalilah formation are critically discussed and support a position of the tr j boundary at the base of the sakhra member a negative excursion in 13corg occurs in the upper sumra member and possibly correlates with the late triassic spelae carbon isotope event the 13ccarb and 13corg values increase across the boundary interval and an extended positive excursion in 13ccarb occurs in the oolites of the sakhra member sr concentrations increase in the bioclastic limestones of the uppermost sumra member and peak in the oolites of the upper sakhra member high sr concentrations also occur at correlative stratigraphical levels in the lombardy basin italy and are interpreted as an early hettangian seawater signal reflecting the global scale reduction of biological aragonite precipitation and an increased continental weathering flux

geochemical characterization of free versus asphaltene sorbed hydrocarbons in crude oils implications for migration related compositional fractionations based on the existing petroleum migration models that focus on the role of organic sorption a laboratory study has been performed to simulate the migration effects on the various fractions of oil in this study one free oil fraction defined as bitumen 1 existing outside of or loosely attached to asphaltene and three to four absorbed oil fractions defined as bitumens 2 3 4 and 5 more strongly attached to asphaltene macromolecules that were obtained from three oil samples collected from the bohai bay basin east china were analyzed in detail based on the laboratory partition of oil components between the solvent phase oil and the asphaltene phase petroleum migrationrelated effect on saturate fraction was re evaluated our observation indicates that the compositional variation from bitumen 1 to bitumen 4 or 5 includes the following 1 the resin content increases while the saturate content and saturates aromatics ratio decrease 2 the relative abundance of short chain n alkanes decreases while that of the long chain n alkanes increases 3 pr nc17 ph nc18 and pr ph ratios stay virtually unchanged while the relative abundances of terpanes to n alkanes and steranes to n alkanes decrease 4 the ratios of c21 c21 c29 steranes and c23 c23 c30 terpanes in the free oil fraction are similar to or even lower than those in the whole absorbed fraction suggesting relatively stronger affinities of these smaller terpane and sterane molecules to asphaltenes in contrast to the relatively larger ones and 5 the isomerization parameters of c27 and c29 steranes are slightly higher in the free fraction than in the whole absorbed oil fraction large variations were observed in the distributions of steranes among the four bitumen fractions of the x52 oil and were considered to be related to the heterogeneity of the source rocks in addition to the partition of these compounds between the asphaltene phase and the solvent phase q 2002 elsevier science ltd all rights reserved

high precision u pb zircon geochronology of the late triassic chinle formation petrified forest national park arizona usa temporal constraints on the early evolution of dinosaurs the triassic successions of the colorado plateau preserve an important record of vertebrate evolution and climate change but correlations to a global triassic framework are hampered by a lack of geochronological control tuffaceous sandstones and siltstones were collected from the upper triassic chinle formation exposed in the petrified forest national park arizona usa within a refined stratigraphic context of 31 detailed measured sections u pb analyses by the isotope dilution thermal ionization mass spectrometry id tims method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the chinle fluvial system the base of the blue mesa member of the chinle formation is placed at ca 225 ma and the top of the petrified forest member is placed at 208 ma or younger bracketing an 280 m thick section that spans nearly the entire norian stage of the late triassic estimated sediment accumulation rates throughout the section reflect extensive hiatuses and or sediment removal by channel erosion the new geochronology for the chinle formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria a mid norian age ca 219 213 ma for the distinctive sonsela conglomeratic sandstone bed constrains the adamanian revueltian land vertebrate faunachron boundary our new data permit a significant time overlap between the lower chinle sequence and the dinosauromorph rich ischigualasto formation of northwestern argentina near contemporaneity of the trans american deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the americas

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

new early jurassic tetrapod assemblages constrain triassic jurassic tetrapod extinction event the discovery of the first definitively correlated earliest jurassic 200 million years before present tetrapod assemblage fundy basin newark supergroup nova scotia allows reevaluation of the duration of the triassic jurassic tetrapod extinction event present are tritheledont and mammal like reptiles prosauropod theropod and ornithischian dinosaurs protosuchian and sphenosuchian crocodylomorphs sphenodontids and hybodont semionotid and palaeonisciform fishes all of the families are known from late triassic and jurassic strata from elsewhere however pollen and spore radiometric and geochemical correlation indicate an early hettangian age for these assemblages because all typical triassic forms are absent from these assemblages most triassic jurassic tetrapod extinctions occurred before this time and without the introduction of new families as was previously suggested by studies of marine invertebrates this pattern is consistent with a global extinction event at the triassic jurassic boundary the manicouagan impact structure of quebec provides dates broadly compatible with the triassic jurassic boundary and following the impact theory of mass extinctions may be implicated in the cause

microcrack induced versus intrinsic elastic anisotropy in mature hc source shales recent experimental studies of ultrasonic velocity anisotropy in kerogen rich shales indicate that these rocks are characterized by a very strong anisotropic response related to a very fine bedding parallel lamination of organic matter and preferred orientation of clay particles in the rock matrix vernik and nur 1992 this intrinsic anisotropy is further enhanced in thermally mature shales by bedding parallel microcracks caused by the processes of hydrocarbon generation vernik paper in preparation however the potential of recognizing mature source rocks in situ using downhole sonic crosshole seismic or vsp data clearly depends on our ability to discriminate between these two major causes of elastic anisotropy remove the effect of the intrinsic anisotropy and estimate crack density and crack porosity from velocity measurements

linking preferred orientations to elastic anisotropy in muderong shale australia the significance of shales for unconventional hydrocarbon reservoirs nuclear waste repositories and geologic carbon storage has opened new research frontiers in geophysics among many of its unique physical properties elastic anisotropy had long been investigated by experimental and computational approaches here we calculated elastic properties of cretaceous muderong shale from australia with a self consistent averaging method based on microstructural information the volume fraction and crystallographic preferred orientation distributions of constituent minerals were based on synchrotron x ray diffraction experiments aspect ratios of minerals and pores determined from scanning electron microscopy were introduced in the self consistent averaging our analysis suggested that phyllosilicates i e illite mica illite smectite kaolinite and chlorite were dominant with 70 vol the shape of clay platelets displayed an average aspect ratio of 0 05 these platelets were aligned parallel to the bedding plane with a high degree of preferred orientation the estimated porosity at ambient pressure was 17 vol and was divided into equiaxial pores and flat pores with an average aspect ratio of 0 01 our model gave results that compared satisfactorily with values derived from ultrasonic velocity measurements confirming the validity and reliability of our approximations and averaging approach

texture and anisotropy analysis of qusaiba shales scanning and transmission electron microscopy synchrotron x ray diffraction microtomography and ultrasonic velocity measurements were used to characterize microstructures and anisotropy of three deeply buried qusaiba shales from the rub alkhali basin saudi arabia kaolinite illite smectite illite mica and chlorite show strong preferred orientation with 001 pole figure maxima perpendicular to the bedding plane ranging from 2 4 6 8 multiples of a random distribution m r d quartz feldspars and pyrite crystals have a random orientation distribution elastic properties of the polyphase aggregate are calculated by averaging the single crystal elastic properties over the orientation distribution assuming a nonporous material the average calculated bulk p wave velocities are 6 2 km s maximum and 5 5 km s minimum resulting in a p wave anisotropy of 12 the calculated velocities are compared with those determined from ultrasonic velocity measurements on a similar sample in the ultrasonic experiment which measures the effects of the shale matrix as well as the effects of porosity velocities are smaller p wave maximum 5 3 km s and minimum 4 1 km s the difference between calculated and measured velocities is attributed to the effects of anisotropic pore structure and to microfractures present in the sample which have not been taken into account in the matrix averaging

redox conditions in the end early triassic panthalassa abstract this study focuses on an upper lower triassic spathian to lowermost middle triassic anisian section representing the central panthalassic deep sea analysed organic carbon isotope ratio 13corg records from the section demonstrate that lower values in the spathian increase by up to 6 at the spathian anisian transition this trend accords with the carbonate carbon isotope 13ccarb record from shallow water carbonate sections most horizons during late spathian early anisian show features of redox conditions of not fully oxic but dysoxic conditions inferred from low mn u v mo and euhedral pyrite dominated occurrences conversely in the end spathian black coloured beds and underlying siliceous claystone beds relatively higher concentrations of redox sensitive elements such as u v mo and abundant pyrite framboids are detected as enrichment factors of redox sensitive elements are not much higher than the typical anoxic sulphidic trend and large pyrite framboids are found these trends suggest suboxic rather than strong anoxic conditions these oxygen poor conditions coincide with carbon isotope minimum values at the late spathian at the same time reducing seawater conditions have been also reported in from continental sections these coincidences imply global environmental perturbations that may have been related to the delayed recovery of life after the end permian mass extinction

the quantitation and origin of c40 n alkanes in crude oils and source rocks a gas chromatographic method using n alkane standards n c 20 n c 30 n c 40 n c 50 and n c 60 and a deuteriated alkane internal standard n c 24 d 50 is presented for the quantitative analysis of high molecular weight alkanes c 40 hmwas the samples were injected in pulsed splitless mode using this method hmwas ranging up to c 100 can be determined and a gc response factor for n c 60 of up to 83 that of n c 30 was obtained the hmwa distributions in three typical high wax oils and a source rock extract from the nanyang depression china were determined the results show that the concentrations of hmwas in the three crude oils range from 22 8 to 38 0 mg g oil accounting for about 16 of the total concentration of c 22 n alkanes the concentration of hmwas in the source rock extract from the dong 10 well is twice as large compared to the concentration in the crude oil in dong 12 well which may be associated with fractionation during migration from the source rock to the reservoir the formation of c 40 c 60 it alkanes may be associated with cutan cutin sources within kerogen org geochem 31 2000 589 and also with biochemical and geochemical processes under strongly reducing conditions the n c 21 to n c 35 n alkanes in typical high wax oils from the nanyang depression have a weak odd over even carbon number predominance whereas the n c 40 to n c 60 n alkanes show a strong odd over even carbon number predominance this phenomenon is tentatively considered to be related to the occurrence of different hydrocarbon generation and preservation characteristics of n alkanes with different carbon numbers

occurrence and distribution of high molecular weight hydrocarbons in selected non marine source rocks from the liaohe qaidam and tarim basins china abstract extracts of 18 potential source rocks from the qaidam liaohe and tarim basins china have been characterized by high temperature gas chromatography htgc and found to contain high molecular weight hydrocarbons hmwhcs above c40 these rocks were originally formed in freshwater lacustrine saline lacustrine and terrigenous bog environments with mudstone calcareous mudstone coaly shale and coal lithologies their ages range from triassic to miocene and maturities from immature to mature covering a vitrinite reflectance range ro for the liaohe and tarim samples of 0 54 0 84 the combined distributions of the n alkane and alkylcyclopentane homologues in the extracts can be used to characterize the nature of the original depositional environment in certain samples the carbon number predominance of the n alkanes changes from an odd even predominance to even odd predominance above c40 possibly suggesting different precursors for the n alkanes above and below c40 concentrations of the hmwhcs were also observed to increase with maturity of the source rocks this is the first comprehensive report on the occurrence of hmwhcs in source rocks and their discovery introduces the possibility of using these compounds for oil source rock correlation purposes in future studies along with the more conventional biomarkers

hybrid fourier finite difference 3d depth migration for anisotropic media when a subsurface is anisotropic migration based on the assumption of isotropy will not produce accurate migration images we develop a hybrid wave equation migration algorithm for vertical transversely isotropic vti media based on a one way acoustic wave equation using a combination of fourier finite difference ffd and finite difference fd approaches the hybrid method can suppress an additional solution that exists in the vti acoustic wave equation and it offers speed and other advantages over conventional ffd or fd methods alone the algorithm is tested on a synthetic model involving log data from onshore eastern saudi arabia including estimates of both intrinsic and layer induced vti parameters results indicate that vti imaging in this region offers some improvement over isotropic imaging primarily with respect to subtle structure and stratigraphy and to image continuity these benefits probably will be overshadowed by perennial land seismic data issues such as near surface distortions and multiples

petrography of late cambrian to middle ordovician radiolarian chert in kazakhstan with special reference to the emergence of benthic animals in the pelagic realm

shocked quartz at the triassic jurassic boundary in italy quartz grains that appear to have been shock metamorphosed occur within three closely spaced shale beds from the uppermost triassic rhaetian calcare a rhaetavicula in the northern apennines of italy the upper shale coincides with the abrupt termination of the distinctive uppermost triassic rhaetavicula fauna and is overlain by the hettangian lower jurassic calcare massiccio no extinctions appear to be associated with the two lower layers which occur 1 2 and 2 4 meters below the boundary shale approximately 5 to 10 of the quartz grains within these layers exhibit one or more sets of planar deformational features whose orientations cluster around the rational crystallographic planes basal and most commonly observed in shocked quartz textural and stratigraphic observations support an interpretation of at least three closely spaced impacts at the end of the triassic

tethyan magnetostratigraphy from pizzo mondello sicily and correlation to the late triassic newark astrochronological polarity time scale we present the magnetostratigraphy and stable isotope stratigraphy from an expanded 430 m thick upper triassic marine limestone section at pizzo mondello sicily and review published biostratigraphic information that can be used to defi ne the location of the conodont carnian norian and norianrhaetian boundaries in this section pizzo mondello offers good potential for magnetostratigraphic correlation of marine biostratigraphic and chemostratigraphic data with the continental newark astrochronological polarity time scale apts for development of an integrated late triassic time scale the relatively stable average values of 18 o centered on 0 are a strong indication that the cherty limestone at pizzo mondello suffered very little diagenetic overprinting the conodont carnian norian boundary is located 12 5 m above a positive shift of 13 c a statistical approach was applied to evaluate various pizzo mondello to newark magnetostratigraphic correlations two correlation options have the highest correlation coeffi cients in option 1 the base of pizzo mondello correlates with the middle part of the newark apts whereas in option 2 the base of pizzo mondello starts toward the early part of the newark apts we prefer option 2 in which the carnian norian boundary based on conodonts as well as its closely associated positive 13 c shift correspond to newark magnetozone e7 at ca 228 227 ma adopting newark astrochronology implying a long norian with a duration of 20 m y and a rhaetian of 6 m y duration these ages are in fact not inconsistent with the few high quality radiometric dates that are available for late triassic time scale calibration based on its good exposure accessibility stratigraphic thickness and continuity and multiple chronostratigraphic correlation possibilities we propose pizzo mondello as global stratigraphic section and point for the base of the norian

an intercalibrated triassic conodont succession and carbonate carbon isotope profile kamura japan abstract the kamura section located in the chichibu belt of kyushu island in southern japan contains a succession of uppermost permian to upper triassic shallow marine atoll sediments from the mid panthalassic ocean in the present study carbonate microfacies conodont biostratigraphy and carbonate carbon isotopes were analyzed making it possible to construct a detailed biochronological framework for the triassic of the central panthalassic region a total of 14 conodont zones are recognized from the uppermost changhsingian to the upper norian the lower triassic contains the hindeodus parvus isarcicella isarcica and h postparvus zones griesbachian the neospathodus dieneri zone dienerian and the novispathodus ex gr waageni parachirognathus zone smithian while no evidence of the upper lower triassic spathian substage was recovered at kamura the overlying middle triassic section consists of the chiosella ex gr timorensis cratognathus paragondolella ex gr alpina pg ex gr excelsa budurovignathus hungaricus and b mungoensis zones followed in the upper triassic by the quadralella tadpole gladigondolella malayensis q lobata q carpathica primatella cf orchardi pr permica epigondolella rigoi pg hallstattensis and e ex gr bidentata norigondolella steinbergensis zones the exact stratigraphic intervals present at kamura were refined through intercalibration of the conodont biozonation data with its 13ccarb profile which shows a characteristic series of negative and positive excursions that correspond to well established global reference profiles the permian triassic boundary is placed 0 20 m above the mitai kamura formation contact a reference datum corresponding to the end permian extinction horizon 0 m based on the first appearance of h parvus and the olenekian anisian boundary is placed 17 60 m above the reference datum based on the first appearance of ch ex gr timorensis kamura carbonates were assigned to six fabric types and 17 microfacies the microfacies succession records a deepening immediately following the end permian mass extinction a strong shallowing during the mid to late smithian development of an unconformity spanning the entire spathian and minor sea level fluctuations during the middle and late triassic the spathian unconformity is attributed to a eustatic fall related to a sharp cooling event at or close to the smithian spathian boundary that exposed the kamura atoll the subsequent resumption of shallow marine sedimentation in the earliest middle triassic was probably due to continued thermal subsidence of the oceanic crustal basement beneath the atoll the present work is the most complete analysis to date of the conodont biostratigraphic carbon isotope chemostratigraphic and eustatic records of the triassic system in the central panthalassic ocean and should serve as a regional reference standard in the future

a laboratory study of seismic velocity and attenuation anisotropy in near surface sedimentary rocks the laboratory ultrasonic pulse echo method was used to collect accurate p and s wave velocity 0 3 and attenuation 10 data at differential pressures of 5 50 mpa on water saturated core samples of sandstone limestone and siltstone that were cut parallel and perpendicular to the vertical borehole axis the results when expressed in terms of the p and s wave velocity and attenuation anisotropy parameters for weakly transversely isotropic media q q show complex variations with pressure and lithology in general attenuation anisotropy is stronger and more sensitive to pressure changes than velocity anisotropy regardless of lithology anisotropy is greatest over 20 for velocity over 70 for attenuation in rocks with visible clay organic matter laminations in hand specimens pressure sensitivities are attributed to the opening of microcracks with decreasing pressure changes in magnitude of velocity and attenuation anisotropy with effective pressure show similar trends although they can show different signs positive or negative values of q q we conclude that attenuation anisotropy in particular could prove useful to seismic monitoring of reservoir pressure changes if frequency dependent effects can be quantified and modelled

a 70 million year astronomical time scale for the deep sea bedded chert sequence inuyama japan implications for triassic jurassic geochronology abstract the astronomical time scale ats has provided high resolution geochronology however the early mesozoic ats is still under construction partly due to the lack of continuous pelagic sequences of the early mesozoic here we present ca 70 myr long ats constructed from the early mesozoic deep sea bedded chert sequence exposed in the inuyama area central japan the sedimentary rhythms of bedded chert display a full range of climatic precession related cycles 20 kyr cycle as a chert shale couplet and 100 405 2000 to 4000 and 10 000 kyr cycles as chert bed thickness variation the newly established ats inuyama ats is tuned by 405 kyr eccentricity cycle and is anchored at the end triassic radiolarian extinction level as 201 4 0 2 ma this inuyama ats gives ages consistent with the radiometric ages projected to the inuyama deep sea sequence using biostratigraphy and carbon isotope stratigraphy the inuyama ats provides the age constraints for the triassic and jurassic stage boundaries which support the long norian option of muttoni et al 2004 because the deep sea bedded chert sequence covers a long time interval before the cretaceous the ats for the bedded chert will serve as a template for the astrochronology of mesozoic and older ages

geochemical evaluation of the besikli oil field kahta adiyaman turkey abstract mesozoic source rocks and crude oils from the besikli oil field adiyaman turkey havebeen characterized by a variety of organic geochemical techniques the potential source rocksinvestigated include the karababa a b and c members and the karabogaz formations thekarababa a member and the karabogaz formation of clayey limestone and limestone areimportant source rocks in the basin the crude oils examined in this study were obtained fromreservoir formations of the same age throughout the oil field the source rock screening analysisindicates that the karababa a member and the karabogaz formation have substantialoil generation potential the karababa b and c members have no source potential because theydo not contain sufficient amount of total organic matter content 0 12 0 24 toc wt selectedsource rock and oil samples were characterized in detail and distributions and concentrations both relative and absolute concentrations of biomarkers at different oils were examined thebiomarker characteristics were combined with other geochemical and geological data to interpretthe sources depositional environments and the age of the oils oil source rock correlations weremade between source rocks and crude oils based on qualitative and quantitative biomarkerdistributions petrographic descriptions and vitrinite reflectance measurements were used toevaluate petrographic properties of source rocks and maturity levels of the formations thesource rocks for the oil from the karababa a member and karabogaz formation are not matureenough and therefore oil should have migrated from the outside of the study area

a late triassic impact ejecta layer in southwestern britain despite the 160 or so known terrestrial impact craters of phanerozoic age equivalent ejecta deposits within distal sedimentary successions are rare we report a triassic deposit in southwestern britain that contains spherules and shocked quartz characteristic of an impact ejecta layer inter and intragranular potassium feldspar from the deposit yields an argon argon age of 214 2 5 million years old this is within the age range of several known triassic impact craters the two closest of which both in age and location are manicouagan in northeastern canada and rochechouart in central france the ejecta deposit provides an important sedimentary record of an extraterrestrial impact in the mesozoic that will help to decipher the number and effect of impact events the source and dynamics of the event that left this distinctive sedimentary marker and the relation of this ejecta layer to the timing of extinctions in the fossil record

phased array compaction cell for measurement of the transversely isotropic elastic properties of compacting sediments sediments undergoing compaction typically exhibit transversely isotropic ti elastic properties we present a new experimental apparatus the phased array compaction cell for measuring the ti elastic properties of clay rich sediments during compaction this apparatus uses matched sets of p and s wave ultrasonic transducers located along the sides of the sample and an ultrasonic p wave phased array source together with a miniature p wave receiver on the top and bottom ends of the sample the phased array measurements are used to form plane p waves that provide estimates of the phase velocities over a range of angles from these measurements the five ti elastic constants can be recovered as the sediment is compacted without the need for sample unloading recoring or reorienting this paper provides descriptions of the apparatus the data processing and an application demonstrating recovery of the evolving ti properties of a compacting marine sediment sample

anisotropy estimate for the horn river basin from sonic logs in vertical and deviated wells abstract shale anisotropy must be quantified to obtain reliable information about reservoir fluids lithology and pore pressure from seismic data failure to account for anisotropy in seismic processing can lead to errors in normal moveout nmo dip moveout dmo migration time to depth conversion and amplitude variation with offset avo analysis kriged estimates of density vertical compressional wave and shear wave velocities were derived from vertical wells in an area of interest in the horn river resource play in northeastern british columbia the kriged predictions were compared with measured logs along the trajectory of a deviated well whereas the density comparison provided a blind test of kriging accuracy because density is scalar and independent of well deviation the same comparison for sonic velocities revealed that they are systematically higher than the kriged vertical velocities this difference was used to estimate anisotropy parameters at the location of the deviated well the fac

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

climatic and biotic change in the late triassic generally arid conditions that pervaded much of europe and north america during the late triassic were interrupted by a wet monsoonal climatic phase during middle and late carnian times extensive fluviatile sandstones deposited at this time throughout the region occur within a thick sequence of playa lake mudstones the sandstones occasionally contain kaolinite suggesting a humid climate an extreme 13 c depletion in a shallow marine sequence of this age in israel has been interpreted as evidence for an influx of freshwater a widespread change from carbonates to clastics in marine sequences at this time may also be climate related water course cave systems in limestone areas exposed during the late triassic indicate high levels of runoff during the middle and late carnian the marine invertebrate fauna shows a significant turnover at the end of the early carnian the terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the carnian norian boundary these periods of biotic change appear to be synchronous with the onset and cessation of a carnian humid phase the change to a monsoonal climate during this interval has been documented over more than 90 of longitude between 5 and 50 north of the triassic equator it may have been caused by rising atmospheric co 2 levels due to volcanism associated with the incipient dispersal of pangaea

deep sea record of impact apparently unrelated to mass extinction in the late triassic the 34 million year my interval of the late triassic is marked by the formation of several large impact structures on earth late triassic impact events have been considered a factor in biotic extinction events in the late triassic e g end triassic extinction event but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits here we report evidence for an impact event platinum group elements anomaly with nickel rich magnetite and microspherules from the middle norian upper triassic deep sea sediment in japan this includes anomalously high abundances of iridium up to 41 5 parts per billion ppb in the ejecta deposit which suggests that the iridium enriched ejecta layers of the late triassic may be found on a global scale the ejecta deposit is constrained by microfossils that suggest correlation with the 215 5 mya 100 km wide manicouagan impact crater in canada our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon and no contemporaneous faunal turnover is seen in other marine planktons however such an event has been reported among marine faunas and terrestrial tetrapods and floras in north america we therefore suggest that the manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm

the chicxulub impact event and its environmental consequences at the cretaceous tertiary boundary abstract an impact mass extinction hypothesis for the cretaceous tertiary k t boundary transition has been confirmed with multiple lines of evidence beginning with the discovery of impact derived ir in k t boundary sediments and culminating in the discovery of the chicxulub impact crater likewise a link between the chicxulub impact crater and k t boundary sediments has been confirmed with multiple lines of evidence including stratigraphic petrological geochemical and isotopic data the environmental effects of the chicxulub impact event were global in their extent largely because of the interaction of ejected impact debris with the atmosphere the environmental consequences of the chicxulub impact event and their association with the k t boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet