fossil charcoal in cretaceous tertiary boundary strata evidence for catastrophic firestorm and megawave abstract organic matter separated from calcareous sandstone from the upper portion of a deep water tsunami deposit at arroyo el mimbral taumalipas mexico which marks the biostratigraphically defined cretaceous tertiary boundary consists primarily of fossil charcoal including semifusinite and pyrofusinite analytical pyrolysis gas chromatography mass spectrometry revealed the highly aromatic and polyaromatic character of the organic matter assemblage typical of the products of partial combustion the organic matter probably originated as terrestrial vegetation that was caught in a firestorm and subsequently transported far offshore in the backwash of a megawave these data are consistent with the hypothesis of combustion of large masses of vegetation triggered by a giant extraterrestrial impact in the gulf caribbean region probably forming the chicxulub crater in yucatan at the very end of the cretaceous period

the pre quaternary history of fire abstract although evidence for land vegetation comes from the silurian and maybe even earlier the first record of fossil charcoal fusain is from the late devonian for this period there are only one or two isolated records not until the early carboniferous is there a record of extensive charcoal deposits mainly preserved in near shore clastic sediments which provide evidence of significant and widespread wildfires by the late carboniferous charcoal was common or abundant in a wide range of facies including tropical wetland peats wildfire played an important role in shaping the environment at this time the latest palaeozoic and early mesozoic records of charcoal are fewer whereas important deposits of late mesozoic age are found worldwide the occurrence of charcoal at the cretaceous tertiary boundary has been highlighted as evidence for a global fire following a meteorite impact but this interpretation is questionable charcoal has been widely reported from tertiary sediments and its appearance in the quaternary and recent is not solely as a result of human impact through the past 400 million years there have been major changes in atmospheric oxygen levels that affected fire intensity and frequency fire systems thus have a long history and their impact on shaping the environment is assessed

thermal modeling of convergent and extensional tectonic settings for the development of low grade metamorphism in the welsh basin the lower paleozoic welsh basin of the u k caledonides shows a metamorphic transition from zeolite to low greenschist facies for which there are two radically opposing models of development the traditional model is of a syntectonic metamorphism associated with crustal thickening during basin inversion and cleavage development the alternative model links the metamorphism not to traditional ideas of crustal thickening but to thermal anomalies associated with the extensional development of a back arc basin these two models are tested by applying quantitative thermal modeling to crustal thickening and crustal extension tectonic settings specific to the welsh basin this basin has a unique and very well constrained record of its stratigraphic sedimentological and chronologic histories that is used here to provide the framework from which the dynamics of crustal evolution can be modeled crustal thickening models suggest that a higher pressure facies series with a pumpellyite actinolite to greenschist facies transition should be dominant in contrast extensional modeling suggests that a low pressure fades with a transition from prehnite pumpellyite to greenschist facies should be dominant the thermal evolution of the extensional setting is more compatible with the lower pressure metamorphic series that is recorded in metabasites as well as pelitic rocks of the region the metamorphic evolution is regarded as developing initially as a burial style which is then over printed by fabric development while at or near peak p t conditions thus giving an apparent syntectonic style of metamorphism

syntectonic monazite in low grade mudrocks a potential geochronometer for cleavage formation monazite paragenesis during low grade metamorphism is poorly understood the first unequivocal instance of the mineral having crystallized synchronously with compressional cleavage formation is reported compositionally these monazites resemble diagenetic forms but differ texturally by displaying syntectonic microfabrics their predominant association with organic walled fossils implies that organic material may play an important role in controlling the behaviour of light rare earth elements lree during low grade metamorphism probably by releasing diagenetically sequestered lree and organically bound phosphorus during thermal maturation devolatilization and or by promoting a favourable environment for monazite precipitation potentially such monazites could provide a powerful geochronometer for cleavage formation

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

short paper a note on rb sr whole rock ages from cleaved mudrocks in the welsh basin two cleaved epizone grade palaeozoic mudrock suites from snowdonia yield a mean rb sr whole rock age of 409 11 ma this is consistent with resetting of the rb sr whole rock isotope systems of the mudrocks during cleavage development in end silurian early devonian acadian deformation in contrast cleaved palaeozoic anchizone mudrocks from north and central wales give a silurian mean age of 430 9 ma this dates either a pre tectonic isotopic event preserved through acadian anchizone metamorphism or mid silurian deformation and metamorphism at anchizone grade unaffected by the acadian event the precise timing of the main caledonian deformation in wales has never been established firmly powell phillips 1985 suggest that it occurred in north and south wales in latest silurian time whereas ashgill and end llandovery tectonic events were dominant in central wales see also george 1963 however woodcock 1984 1987 and soper et al 1987 have argued in favour of a single tectonic event of early to middle devonian age this chronological study was undertaken to address this controversy the only previous attempt to define precisely the timing of cleavage development in wales was the k ar dating of cleavage parallel chlorite from a volcanic greywacke from north wales fitch et al 1969 when recalculated using modern decay constants these data give an age of 399 10 ma bath 1974 demonstrated the feasibility of obtaining rb sr ages from cleaved mudrocks of the welsh caledonides and this work produced caledonian ages of 409 20ma and 393 22ma from north wales and late

is there evidence for cretaceous tertiary boundary age deep water deposits in the caribbean and gulf of mexico over most of the gulf of mexico and caribbean a hiatus is present between the lower upper maastrichtian and lowermost tertiary deposits sedimentation resumed 200 ka upper zone pla after the k t boundary current bedded volcaniclastic sedimentary rocks at deep sea drilling project dsdp sites 536 and 540 which were previously interpreted as impact generated megawave deposits of k t boundary age are biostratigraphically of pre k t boundary age and probably represent turbidite or gravity how deposits the top 10 to 20 cm of this deposit at site 536 contains very rare micula prinsii the uppermost maastrichtian index taxon as well as low values of ir 0 6 pbb and rare ni rich spinels these indicate possible reworking of sediments of k t boundary age at the hiatus absence of continuous sediment accumulation across the k t boundary in the 16 gulf of mexico and caribbean sections examined prevents their providing evidence of impact generated megawave deposits in this region our study indicates that the most complete trans k t stratigraphic records may be found in onshore marine sections of mexico cuba and haiti the stratigraphic records of these areas should be investigated further for evidence of impact deposits

formation and modification of metastable intermediate sodium potassium mica paragonite and muscovite in hydrothermally altered metabasites from northern wales

experimental study of marine pipelines on unstable and liquefied seabed experimental investigation was carried out in a wave flume to study the stability of marine pipelines on mobile and liquefied seabed a wide range of different pipeline specific gravities and wave condition were examined the results showed that for the given soil the seabed response is governed mainly by liquefaction and the pipeline behaviour on unstable seabed is strongly dependent on its specific gravity

low grade and very low grade regional metabasic ordovician rocks of llŷn and snowdonia gwynedd north wales metabasic assemblages belong to the prehnite pumpellyite and greenschist facies the isograds pumpellyite in pumpellyite out clinozoisite in and biotite in have been mapped and divide the area into four metamorphic zones field and petrographic evidence indicate that the metamorphic crystallization was largely syn and immediately post tectonic viz end silurian to devonian the use of published schreinemakers bundles indicates that most of the prehnite pumpellyite bearing assemblages formed at high grades within the facies and that the greenschist facies rocks crystallized in the higher pressure field as the amount of calcite increases so diagnostic ca al silicates are reduced and eventually eliminated from assemblages contrary to recent claims the evidence indicates that the gwna group and the gwyddel felsitic beds of ll n were metamorphosed prior to the arenig

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

40ar39ar whole rock dating and the age of cleavage in the finnmark autochthon northernmost scandinavian caledonides abstract four variably cleaved samples of red shale slate were collected from the nyborg formation vestertana group exposed in the finnmark autochthon northernmost scandinavian caledonides quartz normalized illite crystallinity values suggest metamorphism varied between upper diagenesis lower anchizone east and lower anchizone middle anchizone west the samples display similarly discordant 40 ar 39 ar whole rock age and k ca spectra with total gas ages ranging between 606 and 661 ma spectra complexity is a result of evolution of gas from relatively non retentive chlorite white mica and relatively refractory detrital plagioclase apparent ages defined by intermediate temperature increments are relatively constant within each sample but show marked intersample variations 550 575 to 635 650 ma three size fractions 1 2 2 4 and 4 8 m were separated from the sample containing the least chlorite the two coarser fractions contain numerous rock fragments and display 40 ar 39 ar age and k ca spectra similar to the whole rock the 1 2 m fraction is largely comprised of individual mineral grains gas increments evolved between 375 and 495 c record similar k ca ratios but systematically increasing apparent ages between 637 and 783 ma these results suggest that the 1 2 m fraction is composed of detrital white mica grains which were affected by a diagenetically related geologic disturbance at 635 ma although the affects of a late cambrian early ordovician orogenic event finnmarkian orogeny have been locally described from portions of the overlying caledonian allochthons no clear record of correlative tectonothermal activity has been documented within samples from the finnmark autochthon

dating incipient metamorphism using 40 ar 39 ar geochronology and xrd modeling a case study from the swiss alps six samples of a single carbonate rich unit of the swiss prealpes progressively metamorphosed from diagenesis to deep anchizone yield 40ar 39ar spectra with variably developed staircase patterns consistent with mixtures of detrital mica and neocrystallized mixed layer illite smectite the lowest temperature heating steps for different size fractions 2 6 m and 6 20 m converge to 40 ma providing an imprecise maximum age of regional metamorphism a method is described for distinguishing and quantifying the amount of pre existing detrital mica versus neoformed illite layer in the illite smectite formed during tertiary alpine metamorphism by comparison of x ray diffraction patterns with newmod simulations in the least metamorphosed samples the illite smectite contains 65 neoformed illite and this illite accounts for approximately 17 of all dioctahedral phyllosilicate minerals in the rock e g detrital mica and illite smectite in contrast the illite smectite from the more strongly metamorphosed samples contains 97 neoformed illite which accounts for 70 to 90 of all dioctahedral phyllosilicate minerals phyllosilicate morphologies viewed by scanning electron microscopy are consistent with these estimates a process of dissolution reprecipitation is inferred as a mechanism for the growth of the neoformed phyllosilicates a plot of neoformed illite content versus 40ar 39ar total fusion age yields a near linear curve with an extrapolated age of 27 ma for 100 neoformed dioctahedral phyllosilicates this age is interpreted as the time of incipient metamorphism and is consistent with independent biostratigraphic constraints model 40ar 39ar age spectra constructed with the xrd simulation results correspond well to the experimental data and illustrate the changes in degassing properties of progressively metamorphosed mixtures of detrital mica and neoformed illite

dating cleavage formation in slates and phyllites with the 40ar 39ar laser microprobe an example from the western new england appalachians usa determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts however when applied to cleavages in slates and phyllites conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations in situ high spatial resolution 40ar 39ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains three regionally developed cleavages s2 s3 and s4 are present in the northern taconic allochthon of vermont and new york representative samples were studied from a variety of localities where these cleavages which are defined by white micas are well developed in the suite of samples only s3 and s4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40ar 39ar geochronology mean 40ar 39ar laser microprobe ages for these domains are 370 7 1 0 myr for s3 and 345 5 1 7 myr for s4 because estimates of the ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining s3 and s4 these values are interpreted as periods since cleavage formation this interpretation is consistent with independent geochronological constraints on the age of the acadian orogeny in the region

precise dating of low temperature deformation strain fringe analysis by 40ar 39ar laser microprobe pyritized graptolites from the welsh basin united kingdom slate belt acted as rigid bodies during cleavage formation and epizonal white micas formed within the resulting strain shadows orthogonal to the principal stress orientation although the quantities of mica are small they are a pure synkinematic mineral and have been dated by ar 40 ar 39 infrared laser microprobe as a means to dating cleavage four samples of strain fringe mica from different hand samples yielded ages ranging from 394 4 3 1 to 397 8 1 8 ma 2sigma with a mean age of 396 1 1 4 ma 2sigma by focusing on minerals that are unequivocally synkinematic this technique provides a novel solution to the problems of isotopically dating slaty cleavage previous studies have predominantly relied on dating whole rock slate samples or separated illite grains by ar 40 ar 39 techniques problems encountered included 1 separating the effects of isotopic contamination by detrital phases 2 ar 39 loss during the irradiation of illite mineral separates and 3 thermally induced ar 40 loss in nature from fine grained minerals by circumventing these problems this new method provides the first unequivocal and high precision age data for acadian deformation in the well characterized welsh basin slate belt with such precision the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent continent collision

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

dating the transition of smectite to illite in palaeozoic mudrocks using the rb sr whole rock technique the interpretation of mudrock rb sr whole rock regression ages using stratigraphically well constrained samples from the ordovician and silurian of central wales is re examined two new rb sr whole rock regressions are presented along with k ar and rb sr data from the

investigations of fusain transition fossils from the lower carboniferous comparisons with modern partially charred wood plant fossils showing vitrinite semifusinite fusinite transitions and permineralised semifusinite transitions were collected from lower carboniferous locations within the british isles several were obtained from east kirkton quarry bathgate scotland and one fusinite semifusinite vitrinite semifusinite fusinite transition fossil was collected at muckros head county donegal ireland the muckros head fossil was found in a limestone sea cliff face where it had been weathered proud of the rock matrix thus enabling a detailed examination and interpretation of the macroscopic appearance of the specimen the distinctive sandwich like cross section of the fossil is interpreted as being caused by the burial and coalification of a partially charred plant stem with recent exposure and weathering resulting in the loss of its shattered edges under s e m and reflected light microscopy non permineralised transition fossils are seen to grade from structureless vitrinite into compressed and bogen structured semifusinite and occasionally continue through to structured uncompressed fusinite with corresponding increases in reflectance these gradations parallel the reflectances seen in modern partially charred stems stable carbon isotopic analysis of the transition fossils c13 17 19 0 to 19 26 0 22 46 0 to 24 54 0 24 47 0 24 47 0 to 26 20 0 show a significant difference between the vitrinite and semifusinite fusinite with the semifusinite fusinite isotopically heavier the various techniques employed to investigate these transition fossils all indicate that they represent coalified partially charred stems correlation of the properties and characteristics of the semifusinite fusinite in transition fossils with the properties and characteristics of isolated semifusinite and fusinite fragments collected from rocks of the lower carboniferous through to the tertiary supports the view that most of these isolated fragments also represent fossil charred wood and charcoal

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

life span and fate of basins the life span of each main class of sedimentary basin is estimated from published data life spans vary over at least three orders of magnitude from 100 m y for passive margin and intracratonic basins the life span estimates are used to calibrate a chart of basin groups that focuses on the likely basin fates i e depositional deformational or thermal consequent fates predetermined by the tectonic setting of a particular basin class are distinguished from contingent fates which are independent of basin type accretion of trench basin fill is for example a consequent fate whereas inversion far field shortening of a rift basin is a contingent fate life span data are also used to calibrate the wilson cycle and indicate that it has an average duration of 260 m y this is certainly an underestimate because basin life spans are an imperfect proxy for the duration of subduction and collision zones

fire across the k t boundary initial results from the sugarite coal new mexico usa abstract it has been hypothesized that major global fires occurred following a bolide impact at the cretaceous tertiary boundary evidence for this has been mainly from soot or black carbon at a number of marine boundary sites as well as the occurrence of inertinites fusinite in coals above the boundary in addition the occurrence of fossil charcoal inertinites in potential tsunami deposits has been used to strengthen this idea however fire is known to have been widespread throughout the cretaceous based on the distribution of fossil charcoal and it is necessary to evaluate the claims for a global fire at the k t boundary within the context of this more extensive record of ancient fires the occurrence of the k t boundary within the sugarite coal sequence in the raton basin new mexico offers the opportunity to assess the incidence of fire in local peat forming vegetation in the latest cretaceous across the cretaceous tertiary boundary interval and in the earliest tertiary the distribution of fire products i e fossil charcoal is assessed using a combination of inertinite group macerals in polished blocks of coal and plant particles released by chemical maceration of coal inertinite group macerals fusinite semifusinite inertodetrinite which represent fossil charcoal and particles of charcoal in maceration residues are abundant throughout the sugarite coal sequence both before during and after the k t boundary event samples from below the boundary yield inertinites that constitute more than 20 of the coal three horizons are inertinite rich 50 fire was obviously an important element of the terrestrial environment during the latest cretaceous high inertinite values and abundant charcoal particles characterize the latest cretaceous part of the coal the carbonaceous shale just below the boundary the boundary interval itself and tertiary coals and carbonaceous shales above the boundary charcoal in mineral rich units of the latest cretaceous immediately below the boundary in the boundary interval and in the tertiary is dominated by small inertodetrinite particles these might have been wind blown from regional fires or they may reflect reworking during erosion which led to sediment input into the mire in either case the charcoal signature in these units is the same irrespective of their position in the sequence our data combined with that from two other sites in terrestrial sequences in north america show that fires were an integral part of mire ecosystems through the latest cretaceous and into the early tertiary therefore evidence for distinctive global wildfires at the cretaceous tertiary boundary would have to be sought from careful consideration of other aspects of charcoal deposition including flux rates for charcoal and soot

the sequence of sediment behaviour during wave induced liquefaction this paper presents the results of an experimental investigation of the complete sequence of sediment behaviour beneath progressive waves the sediment was silty with d50 0 060 mm two kinds of measurements were carried out pore water pressure measurements across the sediment depth and water surface elevation measurements the process of liquefaction compaction was videotaped from the side simultaneously with the pressure and water surface elevation measurements the video records were then analysed to measure i the time development of the mudline ii the time development of liquefaction and compaction fronts in the sediment and iii the characteristics of the orbital motion of the liquefied sediment including the motion of the interface between the water body and the sediment the ranges of the various quantities in the tests were wave height h 9 17 cm wave period t 1 6 sec water depth 42 cm and the shields parameter 0 34 0 59 the experiments reveal that with the introduction of waves excess pore pressure builds up which is followed by liquefaction during which internal waves are experienced at the interface of the water body and the liquefied sediment the sequence of processes known from a previous investigation this sequence of processes is followed by dissipation of the accumulated excess pore pressure and compaction of the sediment which is followed by the formation of bed ripples the present results regarding the dissipation and compaction appear to be in agreement with recent centrifuge wave tank experiments as for the final stage of the sequence of processes formation of ripples the ripple steepness normalized with the angle of repose for sediment with liquefaction history is found to be the same as that in sediment with no liquefaction history

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

studies of the influence of non linear stress strain characteristics in soil structure interaction recent field and laboratory studies have shown that even at very small strains many soils exhibit non linear stress strain behaviour nevertheless because of its convenience linear elasticity will continue to play an important role in the analysis of such problems as settlement deformation and soil structure interaction in this paper the measured non liqear stress strain properties of a low plasticity clay are used in the finite element analysis of footings piles excavations and pressuremeter tests to assess the influence of small strain non linearity in comparison with linear elastic behaviour in all cases non linear behaviour results in the concentration of strain and deformation towards the loading boundaries this is shown to have important consequences for soil structure interaction problems such as settlement profiles pile group interaction and contact stress distributions small strain non linearity also has a significant influence on the interpretation in terms of equivalent elastic modu

wave induced dynamics of marine pipelines in liquefiable seabed abstract this paper presents a simple but workable modeling method to simulate the wave induced liquefaction scenarios around a marine pipeline within the framework of the biot s theory incorporating the main features such as relation for the consolidation describing the pore volume reduction hysteretic stress strain behavior of soil skeleton and soil pipe contact effect in this context special attention is paid to the implementation of a well calibrated cyclic soil model for hysteretic and nonlinear stress strain behavior i e strain softening and cyclic degradation associated with a semi empirical shear volume coupling equation for capturing the accumulative volumetric change which links the increment of volumetric strain per cycle of wave with the shear strain occurring during that particular cycle the proposed modeling framework is then incorporated into an explicit time matching finite difference analysis procedure allowing a full non linear dynamic analysis of the intensive interactions between the pipeline and the seabed undergoing buildup of pore pressure and residual liquefaction retrospective simulation of the wave flume test performed by sumer et al 2006c using the proposed model shows good agreement calibrating the reliability of the modeling method for the prediction of wave induced liquefaction of sandy seabed and failure process of the buried pipelines finally the liquefaction mechanism around a buried pipeline under a nonlinear wave loading is investigated by numerical examples the obtained results interpret the cause of liquefaction and the resulting consequence for pipeline stability in wave environment

40 ar 39 ar dating of cleavage formation in tuffs during anchizonal metamorphism 40ar 39ar incremental release analyses were carried out on whole rock and constituent white mica illite rich size fractions 0 63 1 to 6 3 20 m within two very low grade penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade northeastern rheinisches schiefergebirge federal republic of germany one sample from the upper anchizone displays internally concordant 40ar 39ar spectra with plateau ages ranging between ca 316 and 325 ma these are similar to conventional k ar ages determined for the whole rock and size fractions together the isotopic results suggest that cleavage formed at ca 320 ma during a concomitant very low grade metamorphism this is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the westphalian

laser 40ar39ar dating of microgram size illite samples and implications for thin section dating laser 40ar 39ar ages were obtained from a series of replicate illite samples ranging in mass from 14 to 77 pg using vacuum encapsulated irradiation and the results were compared with earlier analyses of the same material doug et al 1995 this test confirmed that measured argon ages in particular argon retention ages are independent of sample size since the argon retention age does not require the 39ar recoil gas fraction it is clear that retention ages can be measured on illite within thin sections laser spot ages were obtained from thin sections of the same welsh basin shale analyzed as illite separates early results on untreated thin sections yielded a wide range of ages some much younger than those measured previously however apparent age was negatively correlated with measured clk ratios suggesting the presence of a young water soluble component which had been removed during mineral separation prolonged water rinsing of the thin sections reduced or virtually eliminated the young phase yielding age results consistent with earlier work on illite separates this indicates that dating of clays in thin section has the potential of being a powerful tool in unraveling complex histories of diagenesis and metamorphism

compositional controls on coexisting prehnite actinolite and prehnite pumpellyite facies assemblages in the tal y fan metabasite intrusion north wales implications for caledonian metamorphic field gradients the tal y fan intrusion is a 110 m thick sub concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of ordovician age in north wales despite low grade metamorphism primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization this has resulted in a vertical sequence of secondary mineral assemblages through the intrusion during early hydrothermal alteration k feldspar replaced plagioclase micropheno crysts in the marginal and contact zones and olivine in the central zone was replaced by saponite subsequent regional metamorphism resulted in the development of metastable prehnite pumpellyite epidote assemblages in two sub zones characterized by high fe2o3 elsewhere the assemblage prehnite actinolite epidote developed except in the contact and marginal zones where activity of co2 suppressed both prehnite and pumpellyite both assemblages contain excess albite quartz and chlorite and on the basis of uniform mineral compositions over the area of an individual thin section are considered to represent buffered equilibrium assemblages indicative of prehnite pumpellyite and prehnite actinolite facies conditions a metamorphic temperature of 310 c at 1 85 kbar is obtained using the p t x grid of liou maruyama cho 1985 which implies a field gradient of 44 c km 1 assuming that metamorphism relates to burial an overburden thickness of 7 km is indicated total maximum thicknesses however of ordovician silurian and lower devonian strata in the area do not exceed 6 km indicating a field gradient of 52 c km 1 these relatively high gradients may possibly be related to concealed late caledonian intrusions or alternatively may result from high heat flow as a consequence of crustal thinning rapid sedimentation and intense magmatic activity in a marginal basin setting

isotopic dating of basin inversion the palaeozoic cobar basin lachlan orogen australia abstract the age of inversion of the early devonian cobar basin is poorly defined by field geological data which have to be extrapolated from the coeval shelf west of the basin in order to better constrain the age of deformation a whole rock isotopic dating program was undertaken and yields the following results rb sr isotopic dates predate the depositional age giving a regional isochron of 442 4 ma three k ar dates from zone 1 are mutually indistinguishable 402 8 402 8 405 8 ma suggesting a mid to late early devonian age of cleavage formation four whole rock 40ar 39ar spectra from the high strain zone 1 give intermediate temperature plateaux ranging from ca 389 ma to 404 ma in the lower strain zone 2 total gas ages from three samples range from ca 401 ma to ca 423 ma an approximate intermediate temperature plateau for one sample indicates an age ca 405 ma the other sample yields a good plateau of ca 437 ma these data are interpreted as reflecting a low grade cleavage forming inversion event which took place at ca 395 400 ma isotopic data from zone 1 suggest near complete rejuvenation of detrital phases during the 395 400 ma event in contrast the older zone 2 rb sr ages and some of the 40ar 39ar ages reflect at least in part basement provenance ages preserved within detrital micas and feldspars which did not undergo complete rejuvenation because of lower deformational strain and or temperature of metamorphism the intense 395 400 ma late early devonian inversion event in zones 1 and 2 of the former cobar basin largely bypassed the southwestern part of the basin and the adjacent shelf to the west these only underwent minor block faulting and uplift at this time but were more strongly deformed along with the overlying fluviatile mulga downs group in the carboniferous

the influence of strain lithology and stratigraphical depth on white mica illite crystallinity in mudrocks from the vicinity of the corris slate belt wales implications for the timing of metamorphism in the welsh basin low and very low grade metamorphism in a mudrock dominated succession ranging from upper cambrian to wenlock is characterized by the white mica illite crystallinity technique kubler indices indicate that grade ranges from late diagenetic to low epizonal the prograde reaction most easily observed by xrd study is the progressive ordering of k na micas diagnostic 2m 1 polytypic reflections become recognizable at 0 4 2 and strengthen as grade advances grade generally correlates with thickness of overburden in the 9 km thick succession however strain exercises additionally a profound influence on grade and the distribution of strain was related to decollement resulting from inhomogeneities and ductility contrasts within the succession strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates the distribution of metamorphic grade implies that the lower palaeozoic succession was augmented by several kilometres of lower devonian old red sandstone strata at the metamorphic culmination the culmination was syntectonic and represents an acadian rather than an early diastathermal event subsequent dextral movement on the bala fault displaced the regional isocryst pattern by c 4 km retrogressive smectite appears to be localized to the vicinity of the bala fault and is probably of variscan age

high spatial resolution 40ar 39ar dating of pseudotachylites geochronological evidence for multiple phases of faulting within basement gneisses of the outer hebrides uk abstract pseudotachylites within the neoarchaean to palaeoproterozoic basement of the lewisian gneiss complex have been analysed using the high spatial resolution 40 ar 39 ar uv laserprobe technique it has been possible to measure 40 ar 39 ar age and apparent9 age variations that would be beyond resolution by lower spatial resolution or bulk 40 ar 39 ar dating methods we report a range of complexities including significantly younger narrow 40 ar 39 ar ages for pseudotachylite forming events in lewis the c 1900 ma event may relate to early stages of laxfordian reworking in barra and south uist a 1300 1200 ma event is recorded whereas in barra south and north uist a second event is recorded at c 700 ma which is likely to be associated with nw and west directed thrusting and could represent far field effects of tectonic events recorded at this time east of the moine thrust in nw scotland supplementary material 40 ar 39 ar data are available at http www geolsoc org uk sup18372

excess argon 40are uptake during slate formation a 40ar 39ar uv laserprobe study of muscovite strain fringes from the palaeozoic welsh basin uk syn kinematic pure muscovite strain fringes grew around pyritized graptolites in response to the early devonian acadian closure and uplift of the peri gondwanan welsh basin they record ar isotope information on the timing of low temperature deformation and the mudrock to slate transition here we have applied three different 40ar 39ar laserprobe dating methods to strain fringes from the central welsh basin to test for variations in age and to explore the technical limitations of the method infrared laserprobe step heating analyses yielded only occasional plateau ages varying between ca 403 ma and 422 ma and strong evidence for discordance in the ar system likewise ages from infrared laserprobe total fusion analysis are scattered and range from ca 390 ma to 651 ma by contrast ultraviolet laserprobe intra fringe age profiles revealed a complex pattern of ages across individual strain fringes this is the highest spatial resolution technique available and the resulting age profiles consistently showed an oscillatory pattern comprising troughs or baseline ages that are all within error of ca 397 ma and peaks or apparent ages ranging up to ca 550 ma these apparent ages we interpret as the results of repeated periodic assimilation of excess 40ar 40are during deformation and the mudrock to slate transition here basinal fluids carrying 40are are driven through bulk mudrock during contractional phases of the acadian orogen and 40are is taken up by muscovite strain fringes the concentration of 40are varies with metamorphic grade and is inversely proportional to the temperature of metamorphism and we suggest is controlled by the solubility of ar in water which has potentially quite severe implications for 40ar 39ar dating of k bearing minerals in any mudrock these results provide a first record of ar isotope information in strain fringes and a first record of orogenically driven fluid flow through bulk deforming mudrock they also demonstrate how the high spatial resolution ultraviolet laserprobe 40ar 39ar method can access ar isotope information that is obscured by lower spatial resolution laser analysis or bulk sampling methods

dating diagenetic monazite in mudrocks constraining the oil window authigenic monazite nodules increasingly recognized in pre mesozoic mudrock successions provide a means for dating diagenesis by exploiting trace uranium within the monazite structure but dating is difficult in practice because of zonation and abundant host rock inclusions we report here a refined technique that has yielded a date of 419 14 ma 2 mswd 1 2 for a 17 point pb pb regression age from a telychian silurian late llandovery mudrock from central wales shrimp analyses of monazite nodules from the same horizon are consistent with this and additionally demonstrate concordance of the u pb and th pb systems these results open the door to providing robust dates for a phase of burial related diagenesis which in wales seems approximately coincident with and perhaps was related to hydrocarbon expulsion

analysis of wave induced liquefaction of sand beds this paper describes finite element analyses of wave induced liquefaction of sand beds the authors were motivated by previous experimental results in which a series of centrifuge wave tank tests were performed on loosely packed fresh deposits of sand with viscous scaling introduced the principal finding was that the resistance to liquefaction under progressive waves was considerably smaller than the resistance exhibited under standing waves an important factor for this marked difference was suggested to be the rotation of the principal stress axes that occurs under progressive wave loading the present study extends an existing cyclic plasticity constitutive model to account for the effect of stress axis rotation of sands this model was incorporated into a finite element analysis procedure which was applied to soil responses to progressive wave and standing wave loading the predicted results compare well with observed soil behaviour the predicted effective stress paths and associated stress strain

direct dating of folding events by 40ar 39ar analysis of synkinematic muscovite from flexural slip planes abstract timing of folding is usually dated indirectly with limited isotopic dating studies reported in the literature the present study investigated the timing of intracontinental multi stage folding in upper proterozoic sandstone limestone and marble near beijing north china and adjacent regions detailed field investigations with microstructural backscattered electron bse images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations striations or along thin flexural slip surfaces both developed during the complex deformation history of the study area involving repeated compressional extensional and strike slip episodes muscovite sericite separates from interlayer slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite 40 ar 39 ar plateau ages of 158 159 ma whereas those from folded marble and limestone samples yield ages of 156 1 ma muscovite from thin flexural slip planes on fold limbs and hinges yields ages within analytical error of 155 165 ma further muscovite samples collected from extensionally folded limestone and strike slip drag folds yield younger ages of 128 125 ma with well defined plateaus to assess the potential influence of the detrital mica component of the host rock on the age data two additional muscovite samples were investigated one from a folded upper proterozoic cambrian sandstone outside the western hills of beijing and one from a folded sandstone sampled 20 cm from folding related slip planes muscovite separates from these samples yield significantly older ages of 575 2 ma and 587 2 ma suggesting that the timing of folding can be directly determined using the 40 ar 39 ar method this approach enables the identification and dating of distinct deformation events that occur during multi stage regional folding 40 ar 39 ar dating can be used to constrain the timing of muscovite and sericite growth at moderate to low temperatures

synchronous very low grade metamorphism contraction and inversion in the central part of the welsh lower palaeozoic basin a metamorphic map white mica crystallinity based on 663 data points has been generated for the recently published british geological survey 1 50 000 geological sheets 178 llanilar and 179 rhayader epizone upper anchizone lower anchizone and sub anchizone are delineated metamorphic cross sections are drawn and superimposed on structural cross sections assumptions made in order to draw metamorphic cross sections are 1 the anchizone spans 100 c 2 the metamorphic field gradient was 36 ckm 1 3 metamorphic surfaces were initially generated parallel to the contemporary ground surface restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults development of back thrusts footwall short cuts and limited growth of precursor folds had taken place metamorphism continued locally during later stages of the contractional phase as a strain related phenomenon finally tensional faulting followed the cessation of folding and metamorphism

wave induced liquefaction of beds of sand in a centrifuge the behaviour of beds of fine grained sand under fluid wave trains was investigated using centrifuge modelling three sets of centrifuge wave tank tests with viscous scaling were performed such that time scaling laws for wave propagation as well as consolidation were matched the test programme consisted of either progressive or standing wave tests on loosely packed fresh deposits of soil as well as repeated travelling wave tests with intervening periods of consolidation the results from the progressive and standing wave tests indicate that for each of the wave loading regimes there exists a critical cyclic stress ratio below which liquefaction does not occur the critical value for the progressive wave loading series was found to be considerably smaller than that for the standing wave loading series moreover the wave induced liquefaction of the sand beds was of a progressive nature in fact under the action of severe travelling waves liquefaction was first induced in the uppermost layer of the

the early caledonian finnmarkian event reassessed in finnmark 40ar 39ar cleavage age data from nw varangerhalvoya n norway abstract the relative significance of early finnmarkian and late scandian caledonian deformation in n norway is uncertain early studies suggested pervasive finnmarkian deformation whilst later results indicated a restricted finnmarkian domain the present work suggests it was more widespread than accepted and that inter finnmarkian scandian deformation occurred 40 ar 39 ar dating of 2 6 and 6 11 m pelitic fractions from the lower to mid greenschist facies tanahorn nappe five samples base middle allochthon and the epizone lokvikfjellet and barents sea groups three samples north varanger region in the north scandinavian caledonides show slightly discordant spectra most spectra from the tanahorn nappe preserve possible evidence of an early caledonian event in the high temperature steps with recoil excess ar effects in the low temperature steps no pre caledonian relict component has been recorded the results indicate finnmarkian deformation continued to 460 ma with scandian reactivation at 425 415 ma from the north varanger region a strongly crenulated sample yielded plateau ages 444 442 ma means of combined young steps from weakly to uncrenulated samples gave 470 450 ma suggesting penetrative strike slip deformation occurred in the late finnmarkian to inter finnmarkian scandian period no scandian ages were recorded in the north varanger region reassessment of published data from the laksefjord nappe and gaissa thrust belt suggests they were affected by finnmarkian deformation

a transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence north wales uk high resolution transmission electron microscopy hrtem measurements of the thickness of white mica crystallites were made on three pelite samples that represented a prograde transition from diagenetic mudstone though anchizonal slate to epizonal slate crystallite thickness measured normal to 001 increases as grade increases whereas the xrd measured 10 a peak profile the kubler index decreases the mode of the tem measured size population can be correlated with the effective crystallite size n 001 determined by xrd the results indicate that the kubler index of white mica crystallinity measures changes in the crystallite size population that result from prograde increases in the size of coherent x ray scattering domains these changes conform to the scherrer relationship between xrd peak broadening and small crystallite size lattice strain broadening is relatively unimportant and is confined to white mica populations in the diagenetic mudstone rapid increases in crystallite size occur in the anchizone coincident with cleavage development changes in the distribution of crystallite thickness with advancing grade and cleavage development are characteristic of grain growth by ostwald ripening the kubler index rapidly loses sensitivity as an indicator of metapelitic grade within the epizone

isotopic dating of strain fringe increments duration and rates of deformation in shear zones the time scales over which deformation in the earth9s crust remains localized in shear zones are poorly known as are the associated strain rates we have determined the longevity and rates of deformation using rubidium strontium rb sr microsampling dating of increments of fibrous strain fringes from a pyrenean shear zone the fibers grew quasi continuously through a protracted deformation history between 87 and 50 million years ago over a period comparable to that of an orogeny during a short interval between 66 and 62 million years a rise in strain rate from 1 1 10 15 to 7 7 10 15 seconds 1 occurred this acceleration correlates with an abrupt change in fiber growth direction and a stress field inversion from gravitational collapse to renewed horizontal crustal shortening

upb pbpb and smnd dating of authigenic monazite implications for the diagenetic evolution of the welsh basin abstract authigenic monazite nodules from silurian turbiditic rocks in the central welsh basin are zoned with respect to nd isotope composition the cores are relatively radiogenic and the rim to core trend projects toward the composition of associated very early authigenic apatite concretions these authigenic phases therefore equilibrated with changing fluid composition as they formed the apatites and the monazite nodule cores may have included significant nd from organic matter or seawater whereas the rees in the outer parts of the monazite nodules were derived from clastic detritus the apparent sm nd age obtained from these phosphates is a mixing line which gives an ordovician age for these silurian rocks the u pb isotope system does not show similar trends and shows promise as an alternative method for dating such phosphates a date of 417 11 ma 2 mswd 107 agrees well with recent estimates of the time of the diagenetic smectite illite transition in the rocks of this study a pb pb isochron gave an age of 422 24 ma 2 mswd 0 9 this shows that the pb is in isotopic equilibrium and is undisturbed and that the elevated mswd of the u pb regression is caused by slight subsequent mobility in the u pb system the event dated by the monazite is probably compactional dewatering during accelerated mid silurian sediment loading in the central welsh basin

liquefaction and flow failure during earthquakes two aspects of seismically induced liquefaction are discussed which are of vital engineering significance the triggering condition and the consequences of liquefaction the triggering condition is examined with respect to liquefaction analysis note being taken of the onset condition which is governed by cyclic strength consequences of liquefaction are discussed with respect to post seismic stability analysis in which the residual strength plays a major role procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced and the applicability of this method for estimating associated ground settlements is discussed the evaluation of residual strength requires a better understanding of undrained sand behaviour results of extensive laboratory rests on japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better the results of laboratory tests on silty sands are examined in the same way ah the results are

incipient metamorphism in the lower palaeozoic marginal basin of wales a diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the lower palaeozoic marginal basin of wales by examination of variations in phyllo silicate mineralogy illite crystallinity and bo parameter of white micas this transition represents a temperature range from 150 c to 400 c and the metamorphism is of a low pressure facies series type with a geothermal gradient of 40 ckm 1 variations in grade can be correlated largely with the original basin and shelf form suggesting a depth related metamorphism however in areas closer to the site of caledonian plate collision an increasingly syn tectonic metamorphic event is apparent correlation of pelite data with metabasite assemblages is variable the most consistent relationship being between epizone crystallinity values andepidote actinolite greenschist facies assemblages diagenetic clay mineral assemblages are found associated with prehnite pumpellyite assemblages in metabasites and it is suggested that the latter represent non buffered and therefore non diagnostic assemblages