late ediacaran magnetostratigraphy of baltica evidence for magnetic field hyperactivity abstract long term changes in characteristics of the geomagnetic field may signal evolutionary changes within the earth s interior among these long term variations the magnetic field reversal rate is probably the best known and most frequently used parameter to evaluate concomitant changes in the earth s core although the reversal rate is well known for the last 150 ma and reasonably constrained back to 300 ma knowledge of the early paleozoic and earlier times is limited hence the haunting question is whether the reversal pattern for the last 300 myr is a good representation of the field behavior over a much longer interval a paleomagnetic study of upper ediacaran sediments from the zigan fm baltica in the western south urals revealed a high temperature dual polarity component that is of primary origin levashova et al 2013 in this paper we present the results of a magnetostratigraphic study from the same sections that can be summed up as follows 1 30 magnetozones are found in the ca 110 meters thick composite section of the zigan fm 2 a similar reversal pattern was reported from upper ediacaran sediments from the white sea coast north russia and sw siberia but has never been found in clastic rocks of any other age 3 the reversal pattern in three remote ediacaran sections could not result from remagnetization or extremely slow sedimentation 4 the reversal rate in the late ediacaran exceeded 20 reversals per million years 5 the field appears to behave in a bipolar fashion during the period of hyperactivity

changes in optical properties chemistry and micropore and mesopore characteristics of bituminous coal at the contact with dikes in the illinois basin abstract changes in high volatile bituminous coal pennsylvanian near contacts with two volcanic intrusions in illinois were investigated with respect to optical properties coal chemistry and coal pore structure vitrinite reflectance ro increases from 0 62 to 5 03 within a distance of 5 5 m from the larger dike and from 0 63 to 3 71 within 3 3 m from the small dike elemental chemistry of the coal shows distinct reductions in hydrogen and nitrogen content close to the intrusions no trend was observed for total sulfur content but decreases in sulfate content towards the dikes indicate thermochemical sulfate reduction tsr contact metamorphism has a dramatic effect on coal porosity and microporosity in particular around the large dike the micropore volume after a slight initial increase progressively decreases from 0 0417 cm3 g in coal situated 4 7 m from the intrusive contact to 0 0126 cm3 g at the contact strongly decreasing mesopore and micropore volumes in the altered zone together with frequent cleat and fracture filling by calcite indicate deteriorating conditions for both coalbed gas sorption and gas transmissibility

changing tectonic controls on the long term carbon cycle from mesozoic to present tectonic drivers of degassing and weathering processes are key long term controls on atmospheric co2 however there is considerable debate over the changing relative importance of different carbon sources and sinks existing geochemical models have tended to rely on indirect methods to derive tectonic drivers such as inversion of the seawater 87sr 86sr curve to estimate uplift or continental basalt area here we use improving geologic data to update the representation of tectonic drivers in the copse biogeochemical model the resulting model distinguishes co2 sinks from terrestrial granite weathering total basalt weathering and seafloor alteration it also distinguishes co2 sources from subduction zone metamorphism and from igneous intrusions we reconstruct terrestrial basaltic area from data on the extent of large igneous provinces and use their volume to estimate their contribution to degassing we adopt a recently published reconstruction of subduction related degassing and relate seafloor weathering to ocean crust creation rate revised degassing alone tends to produce unrealistically high co2 but this is counteracted by the inclusion of seafloor alteration and global basalt weathering producing a good overall fit to mesozoic cenozoic proxy co2 estimates and a good fit to 87sr 86sr data the model predicts that seafloor alteration and terrestrial weathering made similar contributions to co2 removal through the triassic and jurassic after which terrestrial weathering increased and seafloor weathering declined we predict that basalts made a greater contribution to silicate weathering than granites through the mesozoic before the contribution of basalt weathering declined over the cenozoic due to decreasing global basaltic area

new magnetobiostratigraphic results from the ladinian of the dolomites and implications for the triassic geomagnetic polarity timescale abstract we investigated for magnetostratigraphy the rio nigra and rio frommer stratigraphic sections from alpe di siusi seiser alm dolomites northern italy in order to improve the calibration of the triassic time scale both sections are characterized by ammonoid and conodont associations typical of longobardian late ladinian middle triassic age moreover the rio nigra section is constrained by a u pb zircon date of 237 77 0 05 ma building on the recently verified newark hartford astrochronological polarity timescale for the late carnian rhaetian plus the hettangian and through magnetostratigraphic correlations of an updated inventory of tethyan marine stratigraphic sections from the literature some of which are provided with u pb zircon age constraints we propose a revised geomagnetic polarity time scale for the entire triassic

late devonian carbonate magnetostratigraphy from the oscar and horse spring ranges lennard shelf canning basin western australia a b s t r a c t the late devonian was a time of major evolutionary change encompassing the fifth largest mass extinction the frasnian famennian event in order to establish a chronological framework for global correlation before during and following the frasnian famennian mass extinction we carried out a coupled magnetostratigraphic and biostratigraphic study of two stratigraphic sections in the upper devonian carbonate reef complexes of the lennard shelf in the canning basin western australia magnetostratigraphy from these rocks provides the first high resolution definition of the late devonian magnetic polarity timescale a 581 m reference section and an 82 m overlapping section through the marginal slope facies napier formation of the oscar range as well as a 117 m section at horse spring virgin hills formation were sampled at decimeter to meter scale for magnetostratigraphy conodont biostratigraphy was used to correlate both sections and link magnetostratigraphic polarity zones to a globally established biostratigraphy as table characteristic remanent magnetization chrm with dual polarities ne shallowly upward and sw shallowly downward is recovered from 60 of all samples with magnetite inferred to be the chief magnetic carrier from thermal demagnetization characteristics these directions define a geomagnetic pole at 49 5 s 285 8 e and 95 2 4 n 501 placing the canning basin at 9 9 s during the late devonian consistent with carbonate reef development at this time ac onservative interpretation of the magnetostratigraphy shows the recovery of multiple reversals from both sections not including possible cryptochrons and short duration magnetozones field tests for primary remanence include positive reversal tests and matching magnetozones from an overlapping section in the oscar range as trong correlation was found between magnetic polarity stratigraphies of the oscar range and horse spring sections and we correlate 12 magnetostratigraphic packages the relative stratigraphic thicknesses of the isochronous sediments from these two sections indicate that carbonate accumulation was 4 5 faster in the middle slope deposits at oscar range than in the more distal lower slope horse spring deposits for the middle frasnian through famennian the magnetic field during the late devonian underwent a relatively high reversal frequency with good potential for regional and global correlation and should prove useful in deciphering a high resolution chronostratigraphy across the lennard shelf to enable higher confidence examination of reef development across a major biotic crisis

slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity subducting slabs can stagnate in the lower mantle high pressure laboratory experiments show that the viscosity of a dominant mantle phase increases dramatically at shallow lower mantle depths which could cause the slabs to halt their descent

change in the magnetic properties of bituminous coal intruded by an igneous dike dutch creek mine pitkin county colorado abstract magnetization measurements have been made on natural coke coal samples collected at various distances from a felsic porphyry dike in a coal seam in dutch creek mine colorado to help characterize the nature and distribution of the iron bearing phases the magnetization passes through a maximum at the coke to coal transition about 31 cm from the dike contact the magnetic measurements support the geochemical data indicating that magmatic fluids along with a high temperature gas pulse moved into the coal bed interaction of the magmatic fluids with the coal diminished the reducing power of the thermal gas pulse from the dike to a point about 24 cm into the coal the hot reducing gas penetrated further and produced a high temperature 400 525 c zone at about 31 cm just ahead of the magmatic fluids metallic iron found in this zone is the principal cause of the observed high magnetization beyond this zone the temperature was too low to alter the coal significantly

petrography carbonate mineralogy and geochemistry of thermally altered coal in permian coal measures hunter valley australia abstract carbonate minerals commonly occur in coals of many ages and from a utilisation viewpoint can be deleterious several studies have been undertaken of the carbonates in the permian coals of the hunter valley australia but few studies use a multi technique approach for this study a combined petrographic geochemical and mineralogical approach was used to determine the distribution and residence of carbonate minerals in coal that had been intruded by a dyke the dominant carbonate assemblages comprise primary siderite in inertinite rich microlithotypes and secondary calcite ankerite dolomite dawsonite in vitrinite rich microlithotypes the secondary carbonates were found in both the aureole of heated coal and also in an unheated mine face sample it is believed that the secondary carbonate minerals precipitated from magma derived fluids percolating through the coal following the emplacement of the intrusions the textures and distribution of the secondary carbonate minerals suggest that the temperature and pressure of the fluids may be just as important in developing fractures near dykes particularly those that have multiple phases of geometries cleat mineralogy and coal textures as direct heating from the intrusion the partitioning of primary siderite with inertinite and secondary carbonates with vitrinite indicates that it can be reasonably expected that there would be a partitioning of minerals in various density fractions derived from float sink tests and consequently a partitioning of elements with inertinite rich fractions containing elevated fe levels and vitrinite rich fractions containing elevated ca mg and al this partitioning has implications for the behaviour of the coal during washing and combustion and the composition of combustion products

towards absolute plate motions constrained by lower mantle slab remnants the movement of tectonic plates before the cretaceous period is poorly understood a global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18 compared with tectonic reconstructions for the same period

ediacaran palaeomagnetism and apparent polar wander path for australia no large true polar wander summary we report new palaeomagnetic data for red beds from the ediacaran brachina and wonoka formations in the adelaide geosyncline south australia and discuss their place with previously determined poles in the ediacaran apparent polar wander path for australia both formations behave similarly on thermal demagnetization displaying high temperature components that decay to the origin at 680 c consistent with haematite being the only magnetic mineral present restoring the strata to the palaeohorizontal yielded positive fold tests for both units at 99 per cent confidence indicating that acquisition of magnetization occurred before the early palaeozoic delamerian orogeny for the brachina formation n 91 specimens the mean direction after unfolding is declination d 178 2 inclination i 22 6 95 4 4 indicating a palaeolatitude 11 8 2 5 and a pole position at latitude p 46 0 s longitude p 315 4 e with confidence semi axes dp 2 4 and dm 4 6 the mean direction for the wonoka formation after unfolding n 70 is d 255 9 i 23 7 95 6 4 indicating 12 3 3 8 3 4 and a pole position at latitude p 5 2 s longitude p 30 5 e dp 3 6 and dm 6 8 the mean directions for these units and other ediacaran units in the adelaide geosyncline are significantly different from each other which excludes blanket remagnetization of the units before delamerian folding and therefore gives strong preference to their magnetization dating from close to the time of deposition the late cryogenian ediacaran cambrian apparent polar wander path for south australia spans 150 myr from 635 to 490 ma and places australia in low palaeolatitudes throughout the interval studied the poles differ significantly from each other suggesting australia underwent continual drift during that time whereas the directional difference between the late cryogenian elatina formation and early ediacaran nuccaleena formation is mainly in inclination for most other contiguous stratigraphic units the differences are mainly in declination with minor inclination differences indicating australia was rotating about a nearby euler pole in low palaeolatitudes the large and perhaps rapid polar shifts at 615 590 and 575 565 ma in the laurentian apparent polar wander path are not evident in the ediacaran apparent polar wander path for australia because true polar wander should be recorded globally in the absence of evidence for any major stratigraphic break in the south australian succession we conclude that large true polar wander did not occur during the ediacaran

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

the polarity of the silurian magnetic field indications from a global data compilation silurian palaeomagnetic data from all continents are compiled in an attempt to define the polarity history of the earth s magnetic field during the period data sets from llandovery ludlow and pridoli strata often display evidence of field reversals i e the presence of both normal and reverse polarity magnetization directions despite the fact that the palaeomagnetic sample collections may cover only a limited stratigraphic range in each case palaeomagnetic data from rocks of wenlock age are exclusively of normal magnetic polarity a comparison of the available silurian data with published ordovician palaeomagnetic results suggests that the frequency of magnetic field reversals increased from ordovician to silurian times palaeomagnetic data sets are not yet available from a continuous silurian stratigraphic sequence of any great duration so the silurian magnetostratigraphy is still at a much more preliminary stage than that for the ordovician apart from the wenlock all silurian series have some intervals with frequent

convergence of tectonic reconstructions and mantle convection models for significant fluctuations in seafloor spreading for 50 years of data collection and kinematic reconstruction efforts plate models have provided alternative scenarios for plate motions and seafloor spreading for the past 200 my however these efforts are naturally limited by the incomplete preservation of very old seafloor and therefore the time dependence of the production of new seafloor is controversial there is no consensus on how much it has varied in the past 200 my and how it could have fluctuated over longer timescales we explore how seafloor spreading and continental drift evolve over long geological periods using independently derived models a recently developed geodynamic modelling approach and state of the art plate reconstructions both kinematic reconstructions and geodynamic models converge on variations by a factor of 2 in the rate of production of new seafloor over a wilson cycle with concomitant changes of the shape of the area age distribution of the seafloor between end members of rectangular triangular and skewed distributions convection models show that significant fluctuations over longer periods 1 gy should exist involving changes in ridge length and global tectonic reorganisations although independent both convection models and kinematic reconstructions suggest that changes in ridge length are at least as significant as spreading rate fluctuations in driving changes in the seafloor area age distribution through time

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

magnetic reversal frequency and apparent polar wander of the siberian platform in the earliest palaeozoic inferred from the khorbusuonka river section northeastern siberia summary marly limestones that crop out along the khorbusuonka river northeastern siberian platform define a well dated lower and middle cambrian sequence which was sampled for magnetostratigraphy in the upper part of the section from the uppermost lower cambrian toyonian stage to the middle cambrian amgan and mayan stages thermal demagnetization isolates two magnetic components carried either by magnetite haematite or both the first is parallel to the recent geomagnetic field at the site and is removed in the low to middle temperature range the second component is isolated at higher temperatures and has dual magnetic polarities this component yields a positive but partial since we consider only two sites fold test with other middle cambrian data previously obtained from the distant kulumbe river section and was probably acquired during sedimentation or at an early stage of diagenesis it records a very high magnetic reversal frequency of at least six to eight reversals per myr during the entire middle cambrian assuming a 10 myr duration for this epoch a much more complex palaeomagnetic behaviour is observed within the upper tommotian lower atdabanian lower cambrian part of the khorbusuonka section demagnetization diagrams show large overlaps between magnetic components which prevents the isolation of a clear magnetic component at high temperatures analysis of these overlaps is however consistent with the existence of an ill determined primary magnetic direction for the lower cambrian which is very different from that isolated in the middle cambrian although still in need of further confirmation the possibility that a large drift in siberian apparent polar wander occurred over a rather short time during the lower cambrian appears to be supported by our data

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

empirical prediction of porosity and permeability in sandstones current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process oriented models empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models of these parameters the most important are detrital composition grain size sorting and temperature history or pressure history or both despite its limitations the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within 2 porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest the effectiveness of the empirical predictive approach is illustrated by case studies from the taranaki basin new zealand and yacheng field people s republic of china these studies indicate that empirical predictions are basin specific or even play specific and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target process oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability until adequate quantification of the sandstone diagenesis processes is achieved empirical models have a distinct advantage over process oriented models in providing reliable predictions of reservoir quality in many sandstone intervals

thermal alteration and rank variation of coals in the matanuska field south central alaska abstract the matanuska valley of south central alaska is a geologically unique region that shows a variety of contact metamorphic phenomena resulting from the thermal metamorphism of coals by igneous intrusives the coals of the paleocene chickaloon formation vary widely in rank from subbituminous to meta anthracite coals of the lower or western matanuska valley are predominantly high volatile bituminous coals of the central matanuska valley are predominantly medium to low volatile bituminous and coals of the upper or eastern matanuska valley are predominantly anthracite the matanuska valley contains relatively small approximately 135 million metric tons but important resources of bituminous coal and minor quantities of anthracite structural deformation in the matanuska valley increases from west to east as does the size and frequency of igneous intrusive masses the intrusives represent one general period of pliocene volcanic activity that antedates the tectonic disturbance that caused the maximum deformation of the coal bearing rocks the intrusive rocks vary in composition from felsic to mafic varieties and in size from stocks to small dikes and sills although locally the intrusives have caused beneficial improvement in rank they have generally introduced factors that have made an undetermined percentage of the coal areas of the central and eastern matanuska valley of doubtful commercial value the thermal effects of contact metamorphism have produced a large amount of natural coke along with some anthracite and graphite petrologic compositions of matanuska valley coals show that vitrinite contents increase with rank whereas liptinite and inertinite contents generally decrease with rank the sample analyses show an overall range in vitrinite reflectance from 0 47 to 5 34 moist mineral matter free heating contents range from 12 322 to 14 890 btu lb 1 chemical compositions of coal ashes show that fe 2 o 3 cao p 2 mgo so 3 bao sro and mr 3 o 4 contents increase with rank whereas k 2 o and sio 2 contents decrease with rank

subduction history reveals cretaceous slab superflux as a possible cause for the mid cretaceous plume pulse and superswell events abstract subduction is a fundamental mechanism of material exchange between the planetary interior and the surface despite its significance our current understanding of fluctuating subducting plate area and slab volume flux has been limited to a range of proxy estimates here we present a new detailed quantification of subduction zone parameters from the late triassic to present day 230 0 ma we use a community plate motion model with evolving plate topologies to extract trench normal convergence rates through time to compute subducting plate areas and we use seafloor paleo age grids to estimate the thickness of subducting lithosphere to derive the slab flux through time our results imply that slab flux doubled to values greater than 500 km3 yr from 180 ma in the jurassic to 130 ma in the mid cretaceous subsequently halving again towards the cretaceous paleogene boundary largely driven by subduction zones rimming the pacific ocean basin the 130 ma spike can be attributed to a two fold increase in mid ocean ridge lengths following the break up of pangea and a coincident increase in convergence rates with average speeds exceeding 10 cm yr with one third of the total 230 0 ma subducted volume entering the mantle during this short 50 myr period we suggest this slab superflux drove a surge in slab penetration into the lower mantle and an associated increase in the vigour of mantle return flow this mid cretaceous event may have triggered or at least contributed to the formation of the darwin rise mantle superswell dynamic uplift of the south african plateau and the plume pulse that produced the ontong java hikurangi manihiki and kerguelen plateaus among others the models presented here contribute to an improved understanding of the time evolving flux of material consumed by subduction and suggest that slab superflux may be a general feature of continental dispersal following supercontinent breakup these insights may be useful for better understanding how supercontinent cycles are related to transient episodes of large igneous province and superswell formation and the associated deep cycling of minerals and volatiles as well as leading to a better understanding of tectonic drivers of long term climate and icehouse to greenhouse transitions

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

detrital zircon as a proxy for tracking the magmatic arc system the california arc example coupled age and trace element geochemical analyses of detrital zircons provide criteria that may be applied to describe average melt compositions through virtually the entire history of a long lived magmatic arc detrital zircon data suggest that the california western united states cordilleran arc was characterized by five mean magmatic states three geochemically distinct pulses were characterized by high th u and progressively heavy rare earth element hree depleted melts the first and last pulses were also characterized by higher than average u yb suggesting that progressive crustal thickening coupled with variable fluid inputs from the subducting slab modulated pulse volumes lulls between pulses were states generally characterized by low magmatic volumes and low th u and u yb suggesting fluid poor conditions with minimized crustal involvement in magmatism

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

magnetic polarity stratigraphy and palynostratigraphy of the mississippian pennsylvanian boundary interval in eastern north america and the age of the beginning of the kiaman magnetic polarity data from the maritimes basin in eastern canada reveal patterns of magnetic reversal through 17 m y of late mississippian and early pennsylvanian time we collected samples from three sections on cape breton island a fourth that extends a section studied previously and a fifth from a deep cored well on prince edward island we compared the polarity data from these sections to previous results from lower parts of the joggins section of eastern canada all sections are constrained by new palynological data and collectively span the late mississippian brigantian pendleian and much of the arnsbergian substages a major unconformity at the pennsylvanian mississippian boundary makes correlation difficult the late mississippian arnsbergian substage is dominated by normal polarity whereas the pendleian and brigantian are characterized by normal and reversed polarity intervals in about equal proportions in the united states the mississippian pennsylvanian boundary is a major hiatus that juxtaposes strata of early pennsylvanian age on mississippian strata ranging in age from brigantian to mid to late arnsbergian data for the mauch chunk formation in the united states provide polarity reversal patterns for correlative sequences in the central appalachian basin field mapping and magnetic polarity data show closely comparable mississippian pennsylvanian relationships in the maritimes basin more than 1500 km northeast from the u s sections the eustatic event that coincides with the mississippian pennsylvanian boundary unconformity is late arnsbergian to yeadonian in both the central appalachians and maritime basins basal pennsylvanian strata of yeadonian to langsettian age on cape breton island reveal normal paleomagnetic polarity through several hundred meters of section these strata are overlain by a thick section showing only reversed polarity we believe that this polarity boundary is the base of the kiaman an early langsettian position ca 318 ma for the base kiaman superchron is indicated by these new data in eastern nova scotia and new brunswick the sedimentary rocks just above the unconformity are of reversed polarity it is clear that the sedimentation in the two areas is affected by the unconformity and no magnetic or palynological correlation is possible above the unconformity only saetosa zone sediments are present

review and update of the applications of organic petrology part 1 geological applications abstract organic petrology developed as coal petrology at the beginning of the 20th century dedicated mainly to the study of coals because of their utilization in industry coal petrology was then considered a branch of coal science later with the development of specialized nomenclature classification of coal components and the standardization and improvement of analytical microscopical methods this discipline expanded in interests and name becoming organic petrology organic petrology carries a broader context being as well a tool applied in the study of dispersed organic matter in sedimentary rocks due to its importance in exploration for fossil fuel resources at present organic petrology is a discipline widely recognized for its role in fundamental and applied research with respect to both coal utilization and in geosciences throughout the 20th century several important monographs have been published on the discipline of organic petrology including stach s textbook of coal petrology 1st edition 1935 2nd 1975 3rd 1982 updated as the more general organic petrology by taylor et al 1998 more recently the text applied coal petrology the role of petrology in coal utilization was published by suarez ruiz and crelling 2008 this review is the first in a two part review series that describes and updates the role of organic petrology in geosciences a second part complementing this one and focused on the applications of organic petrology to other scientific fields will follow

porosity enhancement from chert dissolution beneath neocomian unconformity ivishak formation north slope alaska discussion 1 shanmugam and higgins 1988 concluded that chert dissolution was the dominant control of porosity and permeability trends in the fluvial facies of the ivishak formation in the prudhoe bay field chert dissolution in turn was interpreted to be a function of proximity to the lower cretaceous unconformity in their opinion the data presented by shanmugam and higgins are not sufficient to justify their conclusions the authors apparently neglected to consider the effects of fundamental geological parameters grain size and sorting and processes burial history on porosity and permeability in this discussion the authors specifically address the effects of grain size sorting and burial history on porosity and permeability to provide an alternative explanation of reservoir quality trends in the ivishak reservoir 3 figs

asymmetry in the reversal rate before and after the cretaceous normal polarity superchron abstract the geomagnetic reversal chronology shows a decreasing reversal rate from about 160 ma to about 118 ma a cessation in the reversal process the cretaceous normal polarity superchron until about 83 ma and then an increasing until recent times visually the rate of decrease in leading into the superchron appears larger than the rate of increase in following the superchron statistical tests developed here confirm the visual perception of an asymmetry in the rate of change in this asymmetry could be caused by a relatively rapid breakdown of d with thermal energy being advected away followed by a slower thickening of the layer through thermal conduction however in the absence of good thermal models as to how different core mantle boundary conditions affect the geodynamo there are probably many apparently viable speculations as to the origin of the observed rate asymmetry

testing the limits of chronostratigraphic resolution in the appalachian basin late devonian middle frasnian eastern north america new u pb zircon dates for the belpre tephra suite the belpre tephra suite from the chattanooga shale in eastern tennessee and the lower rhinestreet shale in western new york has yielded high precision chemical abrasion thermal ionization mass spectrometry ca tims u pb zircon dates of 375 55 0 10 ma from tephra 01 and 375 25 0 13 ma from tephra 06 at little war gap hancock county tennessee and 375 14 0 12 ma from tephra 7 67 at eighteenmile creek erie county new york while the latter two ages provide an apparently isochronous marker horizon for stratigraphic correlation the conodont zonation for the two localities is disjunct the tephra beds from the chattanooga shale at little war gap are at least in part frasnian zonation fz 8 based on the co occurrence of ancyrognathus barba and palmatolepis housei while the tephra bearing interval from the rhinestreet shale in erie county new york is apparently older in fz 7 based on the occurrence of ancyrognathus sp l of klapper and the goniatite naplesites inyx we discuss various hypotheses to explain this chronostratigraphic conflict including the proposition that we have reached the current limits of resolution of both radioisotopic and biostratigraphic methodologies in this epoch

quantifying the timing and rate of crustal evolution global compilation of radiometrically dated detrital zircon grains abstractmultiple models steady state episodic and early growth followed by crustal reworking have been postulated to explain the evolution of earth s continental crust an independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using u pb and hf isotopes a compilation of 200 000 published zircon dates reveals a global age distribution u pb with six prominent statistically significant peaks 3 2 3 0 2 7 2 5 2 0 1 7 1 2 1 0 0 7 0 5 and 0 3 0 1 ga in most cases these peaks are recorded on all seven continents most of the peaks are detectable also in modern sediments a comparison of grains with both u pb ages and hf isotope model ages available for 5100 detrital zircons suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth the zircon age distributions and associated data provide consistent ev

geodynamo reversal frequency and heterogeneous core mantle boundary heat flow abstract we analyze more than two hundred and forty magnetic polarity reversals and excursions over forty million years of time from numerical dynamos with heterogeneous boundary heat flux comparisons between a dynamo with uniform boundary heat flux dynamos with boundary heat flux patterns consisting of a single spherical harmonic and dynamos with tomographic heat flux patterns proportional to the large scale lateral variation of seismic shear wave velocity in the lower mantle reveal that boundary heat flux heterogeneity tends to increase the average frequency of polarity reversals relative to uniform boundary conditions particularly if the heterogeneity increases the average equatorial heat flux non axisymmetric spherical harmonic degree l 1 and 2 boundary heterogeneity and the seismically derived tomographic heterogeneity produce comparable reversal and excursion frequencies in our models suggesting that geomagnetic polarity reversal rates may be relatively insensitive to the non axisymmetric planform of core mantle boundary heat flux in contrast the average polarity reversal frequency in our models is quite sensitive to the total boundary heat flow and to the total heat flow at the equator with non reversing behavior at low heat flow and frequent reversals at high heat flow conditions reversal frequency also increases with the amplitude of the boundary heterogeneity although it is less sensitive to this parameter our results suggest that geomagnetic superchrons may correspond to times with reduced core mantle boundary heat flow either globally or equatorially and conversely periods with high paleomagnetic reversal frequency may correspond to times with increased heat flow at the core mantle boundary

dike induced thermal alteration of the springfield coal member pennsylvanian and adjacent clastic rocks illinois basin usa abstract a basaltic dike intrusion caused a short term and short distance contact aureole in the vitrinite rich springfield coal member of the carbondale formation pennsylvanian and in the underlying paleosol and overlying black shale in the illinois basin parallel series of samples from all three horizons were analyzed for vitrinite reflectance r o and other parameters to evaluate the lateral extension of the thermal aureole r o values in the porous and fluid permeable coal increase from a background value of 0 66 at 4 0 m distance from the dike to more than 5 0 at the intrusive contact r o values in the underlying paleosol increase from a background of 0 65 at 3 m distance to 1 74 at the dike contact in contrast roof rock shale close to the contact had a higher r o value than the paleosol r o 2 56 these differences cannot be explained by thermal conductivity of heat alone or the coal having lower thermal conductivity would show lower vitrinite reflectance values apparently the coal having much higher r o values experienced enhanced lateral convective heat transfer in comparison to the denser and less permeable but more conductive rocks in the paleosol and overlying black shale the advection of relatively cold fluid in the lower part of the coal seam streaming towards the dike may have provided cooling for floor rock close to the coal near the contact in contrast roof rock close to the contact had a higher r o value than the paleosol r o 2 56 in the absence of cooling

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

insights into the effects of matrix retention and inert carbon on the petroleum generation potential of indian gondwana shales abstract the rock eval pyrolysis and total organic carbon toc analysis technique is widely used for organic geochemistry screening of source rocks and potential unconventional petroleum reservoirs the rock eval derived parameters the hydrogen index hi which is the ratio between hydrocarbons released under the s2 curve hydrocarbon formed by thermal pyrolysis s2 is an indicator of petroleum generation potential and total organic carbon toc can be used to infer the type of organic matter present in a rock however hi is often under estimated due to retention of some hydrocarbons included under the s2 curve by the rock matrix and the presence of inert carbon within the rock here we describe and correct the matrix retention and inert carbon effects on hydrocarbon generation from a suite of shale samples from indian gondwana shale reservoirs removal of the petrographically identified inert carbon component from the samples tested leads to less scatter in the toc s2 relationship obtained the ratio of volume percentage of organic matter identified through optical microscopy to toc is calculated and that ratio was least in the one heat affected sample but higher in low toc shales 12 5 with decreasing toc content in the sample set analyzed the corrected hi values calculated using the s2 toc intercept increase significantly this correction can therefore lead to false indications about the type of organic matter present a key novel finding of this work is the need while correcting hi for matrix retention effects to consider samples with a specific range of toc contents and to match them to the kerogen types present e g types iii and iv in the samples analyzed filters should also be applied to adjust for the degree of thermal maturity and organic facies

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

a paleomagnetic power spectrum we construct a power spectrum of geomagnetic dipole moment variations or their proxies that spans the period range from some tens of million down to about 100 years empirical estimates of the spectrum are derived from the magnetostratigraphic time scale from marine sediment relative paleointensity records and from a time varying paleomagnetic field model for the past 7 kyr the spectrum has the most power at long periods reflecting the influence of geomagnetic reversals and in general decreases with increasing frequency decreasing period the empirical spectrum is compared with predictions from simple models discrepancies between the observed and predicted spectra are discussed in the context of i changes in reversal rate ii overall average reversal rate iii cryptochrons iv the time taken for a reversal to occur and v long term paleosecular variations and average estimates of the field strength and variance from other sources 2005 elsevier b v all rights reserved

linking lowermost mantle structure core mantle boundary heat flux and mantle plume formation abstract the dynamics of earth s lowermost mantle exert significant control on the formation of mantle plumes and the core mantle boundary cmb heat flux however it is not clear if and how the variation of cmb heat flux and mantle plume activity are related here we perform geodynamic model experiments that show how temporal variations in cmb heat flux and pulses of mantle plumes are related to morphologic changes of the thermochemical piles of large scale compositional heterogeneities in earth s lowermost mantle represented by the large low shear velocity provinces llsvps we find good correlation between the morphologic changes of the thermochemical piles and the time variation of cmb heat flux the morphology of the thermochemical piles is significantly altered during the initiation and ascent of strong mantle plumes and the changes in pile morphology cause variations in the local and the total cmb heat flux our modeling results indicate that plume induced episodic variations of cmb heat flux link geomagnetic superchrons to pulses of surface volcanism although the relative timing of these two phenomena remains problematic we also find that the density distribution in thermochemical piles is heterogeneous and that the piles are denser on average than the surrounding mantle when both thermal and chemical effects are included

extreme geomagnetic reversal frequency during the middle cambrian as revealed by the magnetostratigraphy of the khorbusuonka section northeastern siberia abstract we present new magnetostratigraphic results obtained for the drumian stage 504 5 500 5 ma epoch 3 middle cambrian from the khorbusuonka sedimentary section in northeastern siberia they complement previous data that did not allow the determination of a reliable estimate of the geomagnetic reversal frequency during this time magnetization of the samples is carried by a mixture of magnetite and hematite in various proportions thermal demagnetization makes it possible to distinguish two magnetization components the low unblocking temperature 20 reversals per myr during the drumian to 1 5 reversals per myr during the furongian upper cambrian such a sharp decrease is consistent with a transition at a 1 myr timescale probably caused by threshold effects in core processes between two geodynamo modes one characterized by reversals occurring at frequencies ranging from 1 to 5 reversals per myr and the other marked by hyperactivity of the reversing process with reversal rates 15 reversals per myr

lower mantle superplume growth excites geomagnetic reversals abstract seismic images of the lower mantle reveal two large scale low shear wave velocity provinces beneath africa and the pacific that are variously interpreted as superplumes plume clusters or piles of dense mantle material associated with the d layer here we show that time variations in the height of these structures produce variations in heat flux across the core mantle boundary that can control the rate at which geomagnetic polarity reversals occur superplume growth increases the mean core mantle boundary heat flux and its lateral heterogeneity thereby stimulating polarity reversals whereas superplume collapse decreases the mean core mantle boundary heat flux and its lateral heterogeneity inhibiting polarity reversals our results suggest that the long stable polarity geomagnetic superchrons such as occurred in the cretaceous permian and earlier in the geologic record were initiated and terminated by the collapse and growth of lower mantle superplumes respectively

modelling the palaeo evolution of the geodynamo although it is known that the geodynamo has been operating for at least 3 2 ga it remains difficult to infer the intensity dipolarity and stability occurrence of reversals of the precam brian magnetic field of the earth in order to assist the interpretation of palaeomagnetic data we produce models for the long term evolution of the geodynamo by combining core ther modynamics with a systematic scaling analysis of numerical dynamo simulations we update earlier dynamo scaling results by exploring a parameter space which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to earth in the precambrian our analysis highlights the central role of the convective power which is an output of core thermodynamics and the main input of our updated scalings as the thermal evolution of the earth s core is not well known two end member models of heat flow evolution at the core mantle boundary cmb are used respectively terminating at present heat flows of 11 tw high power scenario and 3 tw low power scenario the resulting models predict that until the appearance of the inner core a thermal dynamo driven only by secular cooling and without any need for radioactive heating can produce a dipole moment of strength comparable to that of the present field thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence the observed lack of strong long term trends in palaeointensity data throughout the earth s history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter specifically the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation but the dynamo becomes deeper seated in the core thus largely cancelling the increase at the core and earth surface and diminishing the prospect of observing this event in palaeointensity data our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history in the precambrian we predict a dynamo with similar dipolarity and less frequent reversals than at present times due to conditions of generally lower convective forcing quantifying the typical cmb heat flow variation needed for the geodynamo to cross the transition from a reversing to a non reversing state we find that it is unlikely that such a variation may have caused superchrons in the last 0 5 ga without shutting down dynamo action altogether

changes in geochemistry and mineralogy of thermally altered coal upper hunter valley australia abstract igneous intrusions thermally and geochemically alter coal commonly causing economic and safety problems for many coal mines the effects of two dykes on the inorganic content of the late permian bituminous upper wynn seam were determined from analyses of 44 samples that were collected along transects approaching the intrusions petrographic and xrd data were used to determine sample mineralogy and inaa and xrf spectrometry were utilised to determine the contents of 57 elements the mineralogy of the unaltered coal altered coal and dyke is dominated by carbonates particularly dawsonite which formed by epigenetic precipitation at a late stage after thermal alteration ankerite and siderite are the products of thermal alteration and are restricted to the altered coal and dykes principal component analysis correlations and compositional trends approaching the intrusions were used to subdivide the elements into groups and to identify the mineralogical affinity of each group geochemical data are more sensitive than mineralogy for defining the size of the alteration halo and three zones ranging from unaltered coal through altered coal to highly coked coal are recognised as each intrusion is approached the alteration halos differ in extent 9 5 and 56 m wide but their size is not a multiple of the size of the causative intrusion at the contact between coal and intrusion concentrations of elements with affinities to some aluminosilicates oxides carbonates sulphides and organic components are enriched while other aluminosilicate related elements are depleted in the altered coal towards the edge of the alteration halo some aluminosilicate elements are enriched and oxide elements are depleted

controls on geomagnetic reversals and core evolution by mantle convection in the phanerozoic abstract numerical dynamos driven by non uniform heat flux at the core mantle boundary are used to investigate the connections between geomagnetic field structure geomagnetic reversal frequency core evolution and mantle convection through phanerozoic time polarity reversal sequences and time average magnetic field structures are calculated using dynamos driven by two representations of lower mantle history a reconstruction of mantle convection with plate motions by zhang and zhong 2011 that produces time variable core mantle boundary cmb heat flux and an irregular evolution of the core and a second model based on hotspot locations with a time independent pattern of cmb heat flux derived from the present day seismic shear wave heterogeneity of the lower mantle that produces a monotonic evolution of the core for both mantle histories present day values of the dynamo control parameters are tuned to match geomagnetic polarity time scale reversal statistics for 0 5 ma and the time dependences of the dynamo control parameters are determined from the thermal evolution of the core including time variability of cmb heat flow inner core size inner core chemical buoyancy flux and rotation rate the dynamo with time independent cmb heat flux shows minor fluctuations in reversal frequency with age whereas the dynamo with time variable cmb heat flux shows reversal rate fluctuations including stable polarity at 275 and 475 ma and frequent reversals at other times this dynamo also produces departures from geocentric axial dipole symmetry during the time of supercontinent pangaea and a heterogeneous growth history of the inner core

nature of the jurassic magnetic quiet zone the nature of the jurassic quiet zone jqz a region of low amplitude oceanic magnetic anomalies has been a long standing debate with implications for the history and behavior of the earth s geomagnetic field and plate tectonics to understand the origin of the jqz we studied high resolution sea surface magnetic anomalies from the hawaiian magnetic lineations and correlated them with the japanese magnetic lineations the comparison shows the following i excellent correlation of anomaly shapes from m29 to m42 ii remarkable similarity of anomaly amplitude envelope which decreases back in time from m19 to m38 with a minimum at m41 then increases back in time from m42 and iii refined locations of pre m25 lineations in the hawaiian lineation set based on these correlations our study presents evidence of regionally and possibly globally coherent pre m29 magnetic anomalies in the jqz and a robust extension of hawaiian isochrons back to m42 in the pacific crust

subduction flux modulates the geomagnetic polarity reversal rate abstract ascertaining the cause of variations in the frequency of geomagnetic polarity reversals through the phanerozoic has remained a primary research question straddling paleomagnetism and geodynamics for decades numerical models suggest the primary control on geomagnetic reversal rates on 10 to 100 ma timescales is the changing heat flux across the core mantle boundary and that this is itself expected to be strongly influenced by variations in the flux of lithosphere subducted into the mantle a positive relationship between the time dependent global subduction flux and magnetic reversal rate is expected with a time delay to transmit the thermal imprint into the lowermost mantle we perform the first test of this hypothesis using subduction flux estimates and geomagnetic reversal rate data back to the early paleozoic subduction area flux estimates are derived from global full plate tectonic models and are evaluated against independent subduction flux proxies based on the global age distribution of detrital zircons and strontium isotopes a continuous phanerozoic reversal rate model is built from pre existing compilations back to 320 ma plus a new reversal rate model in the data sparse mid to early paleozoic cross correlation of the time dependent subduction flux and geomagnetic reversal rate series reveals a significant correlation with a time delay of 120 ma with reversals trailing the subduction flux this time delay represents a value intermediate between the seismologically constrained time expected for a subducted slab to transit from the surface to the core mantle boundary 150 300 ma and the much shorter lag time predicted by some numerical models of mantle flow 30 60 ma while the reason for this large discrepancy remains unclear it is encouraging that our novel estimate of lag time represents a compromise between them although important uncertainties in our proposed relationship remain these results cast new light on the dynamic connections between the surface and deep earth and will help to constrain new models linking mantle convection the thermal evolution of the lowermost mantle and the geodynamo

dynamic topography and long term sea level variations there is no such thing as a stable continental platform abstract globally uniform i e eustatic sea level trends with time scales approaching 100 myr have been inferred from both seismic and backstripping stratigraphic analysis at a small set of geographic sites that are presumed to lie on stable continental platforms and passive continental margins characterized by simple thermal subsidence histories e g the new jersey margin the western african margin we demonstrate using mantle flow simulations based on high resolution seismic tomography that both the new jersey margin and the conjugate western african margin have been subject to orders of 100 m of dynamic i e flow induced topography change over the last 30 myr we also show that the changing pattern of downwelling mantle flow associated with plate subduction is a significant contributor to the background eustatic sea level trend which is also of order 100 m during the 30 myr time window therefore late cenozoic variations of dynamic topography on these passive margin sites are comparable to the eustatic sea level changes and can partially mask the latter furthermore even if the observed trend could be accurately corrected for local dynamic topography variation the residual eustatic signal does not merely reflect changes in mean spreading rates at mid ocean ridges we conclude that the observed long term sea level variations at so called stable sites cannot be interpreted as eustatic moreover previous analyses that have used long term sea level trends as a proxy for spreading rates and geochemical fluxes must be revisited

astronomical tuning of the end permian extinction and the early triassic epoch of south china and germany abstract the timing of the end permian mass extinction and subsequent prolonged recovery during the early triassic epoch can be established from astronomically controlled climate cycles recorded in continuous marine sedimentary sections astronomical cycle tuning of spectral gamma ray logs from biostratigraphically constrained cyclic stratigraphy through marine sections at meishan chaohu daxiakou and guandao in south china yields an integrated time scale for the early triassic which is consistent with scaling of magnetostratigraphy from climatic cycles in continental deposits of the germanic basin the main marine mass extinction interval at meishan is constrained to less than 40 of a 100 kyr kilo year cycle i e c 13 is estimated to have lasted c 13 from 2 to 4 across the smithian spathian boundary at chaohu was completed in 50 kyr the earliest marine reptiles in the mesozoic at chaohu that are considered to represent a significant recovery of marine ecosystems did not appear until 4 7 myr million years after the end permian extinction the durations of the griesbachian dienerian smithian and spathian substages including the uncertainty in placement of widely used conodont biostratigraphic datums for their boundaries are 1 4 0 1 0 6 0 1 1 7 0 1 and 1 4 0 1 myr implying a total span for the early triassic of 5 1 0 1 myr therefore relative to an assigned 251 902 0 024 ma for the permian triassic boundary from the meishan gssp the ages for these substage boundaries are 250 5 0 1 ma for base dienerian 249 9 0 1 ma for base smithian base of olenekian stage 248 2 0 1 ma for base spathian and 246 8 0 1 ma for the base of the anisian stage this astronomical calibrated timescale provides rates for the recurrent carbon isotope excursions and for trends in sedimentation accumulation through the early triassic of studied sections in south china

temporal variation of oceanic spreading and crustal production rates during the last 180 my we present a re evaluation of seafloor spreading and generation rates mainly based on a direct measurement of the remaining surfaces of oceanic crust and isochron lengths defined in the most recent isochron maps j y royer r d muller l m gahagan l a lawyer c l mayes d nurnberg j g sclater a global isochron chart tech rep 117 austin univ of tex inst for geophys 1992 r d muller w r roest j y royer l m gahagan j g sclater digital isochrons of the world s ocean floor j geophys res 102 1997 3211 3214 our evaluation of the amount of oceanic crust per unit age da dt as a function of age which can be expressed as da dt co 1 t tm is in fairly good agreement with previous determinations j g sclater b parsons c jaupart oceans and continents similarities and differences in the mechanisms of heat loss j geophys res 86 1981 11 535 11 552 d b rowley rate of plate creation and destruction 180 ma to present geol soc amer bull 114 2002 927 933 with co 2 850 0 119 km2 year 1 and tm 180 2 9 7 ma dividing these da dt by the ridge lengths l defined as the isochron length at each epoch allowed us to compute the evolution of global half spreading rates these have been roughly constant at 25 9 3 3 mm year 1 for at least the last 150 ma we propose that the global seafloor surface generation rate is roughly constant as well with a mean half value of 1 298 0 284 km2 year 1 and varying 20 with time this study corroborates the recent conclusion of rowley d b rowley rate of plate creation and destruction 180 ma to present geol soc amer bull 114 2002 927 933 of a constant generation rate since 180 ma and completely contradicts the commonly accepted idea of high seafloor spreading and surface generation rates during a large part of the cretaceous combining the oceanic surface generation rates derived here with crustal thicknesses deduced from the chemical composition of old oceanic crusts and seismic measurements e humler c h langmuir v daux depth versus age new perspectives from the chemical compositions of ancient crust earth planet sci lett 173 1999 7 23 the magmatic flux at young 0 80 ma oceanic ridges appears to be about 18 1 3 4 km3 year 1 and was possibly 15 to 30 higher during the mesozoic we propose that mantle temperature variation provides an alternative mechanism to spreading rate for the cretaceous highstand in sea level and atmospheric co2 generation

numerical simulations of an ocean continent convergent system influence of subduction geometry and mantle wedge hydration on crustal recycling the effects of the hydration mechanism on continental crust recycling are analyzed through a 2 d finite element thermomechanical model oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method hydration is accomplished by lawsonite and serpentine breakdown topography is treated as a free surface subduction rates of 1 3 5 7 5 and 10 cm yr slab angles of 30 45 and 60 and a mantle rheology represented by dry dunite and dry olivine flow laws have been taken into account during successive numerical experiments model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material the larger the viscosity contrast between hydrated and dry mantle the larger the percentage of recycled material into the mantle wedge slab dip variation has a moderate impact on the recycling metamorphic evolution of recycled material is influenced by subduction style tpmax generally representative of eclogite facies conditions is sensitive to changes in slab dip a direct relationship between subduction rate and exhumation rate results for different slab dips this relationship does not depend on the used mantle flow law thermal regimes predicted by different numerical models are compared to pt paths followed by continental crustal slices involved in ancient and recent subduction zones making ablative subduction a suitable precollisional mechanism for burial and exhumation of continental crust

geomagnetic reversal behaviour since 100 ma abstract the new magnetic polarity time scale of cande and kent j geophys res 97 13917 13951 1992a provides an opportunity to analyse further the random character of the geomagnetic polarity sequence since 100 ma large differences in the peak amplitudes of changes in magnetic reversal frequency are observed when the previous harland et al a geological time scale cambridge university press 1990 and new cande and kent sequences are compared these differences illustrate remaining uncertainties in the significance of these changes when the successive lengths of polarity intervals are displayed as a function of their order of occurrence in the sequence a change in behaviour is observed midway through the series flat white noise like behaviour is observed for the last 100 intervals from about 25 ma to the present for older intervals a quasi periodic pattern in interval duration is observed this evolution is more apparent in the cande and kent time scale the quasi periodicity of interval duration as a function of occurrence does not translate into periodicity of reversal frequency as a function of time from 85 ma to about 25 ma our description of the sequence is reminiscent of the findings of dubois and pambrun c r acad sci 311 643 650 1990 who suggested that low dimensional deterministic chaos between 85 and 25 ma preceded a random process occurring since then the details of a long term control or memory of the geomagnetic field remain an open question addition of independent information such as palaeointensity data will probably be necessary to uncover any determinism in reversal chronology

a geomagnetic polarity timescale for the permian calibrated to stage boundaries the reverse polarity kiaman superchron has strong evidence for at least three or prob ably four normal magnetochrons during the early permian normal magnetochrons are during the early asselian base ci1r 1n at 297 94 0 33 ma late artinskian ci2n at 281 24 2 3 ma mid kungurian ci3n at 275 86 2 0 ma and roadian ci3r an at 269 54 1 6 ma the mixed polarity illawarra superchron begins in the early wordian at 266 66 0 76 ma the wordian capitanian interval is biased to normal polarity but the basal wuchiapingian begins the beginning of a significant reverse polarity magnetochron lp0r with an overlying mixed polarity interval through the later lopingian no significant magnetostratigraphic data gaps exist in the permian geomagnetic polarity record the early cisuralian magnetochrons are calibrated to a succession of fusulinid zones the later cisuralian and guadalupian to a conodont and fusulinid biostratigraphy and lopingian magnetochrons to conodont zonations age calibration of the magnetochrons is obtained through a bayesian approach using 35 radiometric dates and 95 confidence intervals on the ages and chron durations are obtained the dating control points are most numerous in the gzhelian asselian wordian and changhsingian intervals this significant advance should provide a framework for better correlation and dating of the marine and non marine permian

plate tectonics and net lithosphere rotation over the past 150 my we have developed an improved model of global digital palaeo plate boundaries and plate motion to describe the distribution and history of plates since the late jurassic from this history we computed net lithospheric rotation nr through time confirming the so called westward drift but only for the past 30 myr the nr has significantly smaller magnitudes 0 13 my past 5 my than for some other plate models it averages to 0 11 0 03 my for the past 50 my with a small but systematic increase toward the present the westward drift seen only for the past 30 my is attributed to the increased dominance of a steadily growing and accelerating pacific plate nr shows peaks with time but only an early tertiary peak of 0 33 my when the indian plate was undergoing the largest known acceleration deceleration can be interpreted with some confidence we find a linear decreasing trend in net rotation over the past 150 my but attribute this trend to increasing reconstruction uncertainties back in time as subduction consumed more than half of the oceanic crust since the jurassic after removing a linear time trend we find a nr average of about 0 12 my for the past 150 my

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

influence of lamprophyre sills on coal metamorphism coalbed gas composition and coalbed gas occurrence in the tongxin minefield datong coalfield china abstract magmatic intrusions in coal bearing strata can significantly change the physical and chemical properties of the intruded coal lamprophyre intrusions are extensive in coal seams 3 5 and 8 of the carboniferous permian coal bearing strata in the southeastern and southwestern regions of the tongxin minefield datong coalfield china they appear primarily as sills of varying magnitudes with thicknesses ranging from 0 15 to 23 51 m in this area coals can be classified as unaffected normal geothermal gradient and intrusion affected unaffected coals are of high volatile bituminous rank with values of maximum vitrinite reflectance ranging from 0 7 to 0 9 whereas intrusion affected coals have values of maximum vitrinite reflectance ranging from 0 9 to 7 2 a marked decrease in the degree of coalification can be observed with increasing distance from the lamprophyre intrusions near the intrusion coal contacts high volatile bituminous coals have been thermally altered to form natural coke the carbonate contents in intrusion affected coals 0 2 46 1 are significantly higher than those of the unaffected coal 0 2 0 the coalbed gases of coal seams 3 5 and 8 are highly variable in both their molecular compositions and overall gas contents their co2 volumes vary from 0 01 to 6 55 m3 t daf higher co2 values are found in the coals affected by intrusions and roughly decrease with increasing distance from the lamprophyre igneous intrusions toward the central region of the minefield these variations in gas compositions are mainly related to magmatic activity the coalbed gas contents in coal seams 3 5 and 8 range from 1 48 to 16 43 m3 t on a dry ash free basis the positive correlation observed between gas content and the ratio of co2 ch4 co2 occurs only in intrusion affected coals lamprophyre intrusions have negative effects on ch4 content in the study area since co2 has a higher adsorption capacity than ch4 c2h6 and n2 the injection of gaseous co2 derived from magmatic intrusions can effectively displace ch4 rich reservoir gas in addition there is a positive correlation between ash content and gas content on a dry ash free basis ash content is positively correlated with the ratio of co2 ch4 co2 occurring only in intrusion affected coals massive secondary carbonates that filled in the cleats act as small scale impermeable seals which provide a sealing effect both on gas contents and gas compositions during uplift

stationary and nonstationary behaviour within the geomagnetic polarity time scale we analyse the geomagnetic polarity time scale gpts since the upper jurassic by displaying the successive lengths of polarity intervals as a function of their order of occurrence the sequence consists of three segments between the upper jurassic and the lower cretaceous segment a comprises intervals of short duration with a mean duration of about 0 29 my and no clear long term evolution segment b begins around 130 ma displays a sudden increase of the duration of the magnetic intervals an interval of maximum duration the normal cretaceous superchron and a long and erratic sequence of intervals with decreasing average duration between 85 ma and about 25 ma from 25 ma to the present segment c consists of intervals of short duration with a mean value of 0 23 my this description suggests that the earth s magnetic field could have experienced a fairly stationary regime until slightly before the onset of the cretaceous superchron when the regime has been rapidly and strongly perturbed before progressively returning to another stationary regime about 25 ma ago a geophysical explanation for this sequence of events could be that the geodynamo has been perturbed by the arrival of some cold material at the core mantle boundary as this material would have heated up the geodynamo would have been brought back to its stationary regime

the impact of rapid heating by intrusion on the geochemistry and petrography of coals and organic rich shales in the illinois basin abstract igneous intrusion into organic rich sedimentary rocks and coals has been suggested as a factor in the large scale release of 13c depleted thermogenic ch4 which may have led to global warming and mass extinction events in the geologic past if a significant release of 13c depleted thermogenic ch4 results from the intrusion of coal or organic rich rocks then it should produce 13c enriched residual coal and dispersed organics in rocks adjacent to the intrusion due to the release of isotopically lighter ch4 gas a review of the literature suggests only minor changes in the 13corg of coals adjacent to intrusions however a few studies have shown that changes in 13corg in intruded shales may be slightly more pronounced the current study further evaluates the geochemical isotopic and petrographic changes that result from contact metamorphism and specifically compares the intrusion of coal to that of an organic rich shale collected from the same general vicinity data for two different transects of intruded pennsylvanian coal danville no 7 coal and an intruded organic rich shale in the southern part of the illinois basin are presented both transects show similar increases in mean vitrinite reflectance rr reflectance increases from background levels of 0 66 to 4 40 in the danville no 7 coal and 0 71 to 4 78 in the organic rich shale in addition both transects show the formation of isotropic coke and even development of fine circular mosaic anisotropic coke structure at and near the contact with the intrusion along with the visual loss of liptinites at higher reflectances in the danville coal transect volatile matter n h s and o decrease whereas fixed carbon c and ash increase approaching the intrusion the coal shows a marked decrease in remaining hydrocarbon potential s2 and hydrogen index hi and an increase in tmax c trends in most of the rock eval parameters for the organic rich shale are less clear due to variations in the amount of organic matter present but a significant increase in thermal maturity tmax 0c is observed no systematic changes in 13c occur in the no 7 coal transect as the intrusion is approached with 13c varying between 25 4 and 24 8 the organic rich shale transect shows a minor 1 2 enrichment in 13c from 25 2 to 24 0 within 2 m of the intrusion these isotopic shifts are not of a magnitude that would be expected if associated with a large scale release of thermogenic ch4 in addition no evidence exists in either transect for 13c depleted condensed gas or pyrolytic carbon at the intrusion contact that could have moderated the isotopic signature these data agree with those reported previously that indicate no clear isotopic evidence for large scale ch4 generation due to rapid heating by igneous intrusion into coals or sedimentary rocks

magnetostratigraphic constraints on the gondwanan origin of north china cambrian ordovician boundary results summary a significant gap in the middle palaeozoic apparent polar wander apw path precludes polarity definition of the early palaeozoic palaeopole for north china this problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals a magnetostratigraphic study was carried out on upper cambrian to lower ordovician sediments near zhaogezhuang long 118 5 e lat 39 7 n north china after stepwise thermal or thermal and alternating field demagnetizations a characteristic magnetic component with normal and antipodal reversed directions was identified these data drawn from 49 samples yield a north palaeopole at long 294 6 e lat 32 9 n dp 3 0 dm 5 3 a concordant magnetic polarity pattern around the cambrian ordovician boundary and lowest ordovician obtained from different continents favours a southern hemisphere origin 17 formation site using the cambrian ordovician apw paths between north china and gondwana we suggest that the north china block ncb was part of gondwana during the cambro lowest ordovician and started breaking away from gondwana in the lower ordovician this finding is contrary to some palaeomagnetic models where the ncb was quite separate from gondwana in the late proterozoic and was attached to the pacific side of antarctica

a geomagnetic polarity timescale for the carboniferous the geomagnetic polarity pattern for the carboniferous is incompletely known but with the best resolved parts in the serpukhovian and bashkirian hence data from both igneous and sedimentary units are also used in an additional polarity bias evaluation in the tournaisian to mid visean interval polarity is mainly derived from palaeopole type palaeomagnetic studies allowing identification of polarity bias chrons seven polarity bias chrons exist in the mississippian mi1nb to mi4nb with an additional 33 conventional magnetochrons and submagnetochrons mi4r to mi9r the moscovian and gzhelian polarity is best resolved in magnetostratigraphic studies from the donets basin and the southern urals dispute about the reliability of these data is ill founded since an assessment of supporting data from palaeopole type studies suggests that these datasets currently provide the best magnetic polarity data through the pennsylvanian polarity bias assessment indicates a normal polarity bias zone in the kasimovian in the pennsylvanian there are 27 conventional magnetochrons and submagnetochrons pe1n to ci1r and one normal polarity bias chron pe8nb the kiaman superchron begins in the mid bashkirian with clear data indicating brief normal polarity submagnetochrons within the superchron the magnetochron timescale is calibrated using 31 u pb zircon dates and a quantitative bayesian based age scaling procedure

on the increase in thermal diffusivity caused by the perovskite to post perovskite phase transition and its implications for mantle dynamics the thermal diffusivity of perovskite and post perovskite cairo3 has been measured at elevated pressure and temperatures up to 600 c using the x radiographic angstrom method at high temperatures we find that the thermal diffusivity of post perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated assuming a similar effect occurs in mgsio3 post perovskite the effect of the contrasting thermal transport properties between perovskite and post perovskite on mantle dynamics has been investigated using simple two dimensional convection models these show a reduction in extent and increase in depth of post perovskite lenses as well as increased core mantle boundary heat flux broader upwellings and more vigorous downwellings when compared to the reference constant a case

the geodynamo as a random walker a view on reversal statistics 1 the sequence of durations of the geomagnetic polarity intervals is often described in terms of a nonhomogenous poisson process with time dependent reversal rate reflecting the nonstationarity of the underlying geodynamo process this view has recently been challenged and here we show that the first passage time statistics of random walks taking place on a flat potential relief yields a much more consistent interpretation of the distribution of geomagnetic polarity intervals a possible physical explanation suggests that the stability of a polarity chron of the earth s magnetic field is controlled by a sum of statistically independent randomly behaving dynamo processes in the outer core the random walk hypothesis naturally includes the observed occurrences of very long superchrons and it provides a new considerably longer statistical estimate for the total duration of the present brunhes chron predicting that the probability for a geomagnetic field reversal within the next 30 ka is 5 and the probability that we live in a chron longer than 2 ma is about 30

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

reconstructing first order changes in sea level during the phanerozoic and neoproterozoic using strontium isotopes the eustatic sea level curves published in the seventies and eighties have supported scientific advances in the earth sciences and the emergence of sequence stratigraphy as an important hydrocarbon exploration tool however validity of reconstructions of eustatic sea level based on sequence stratigraphic correlations has remained controversial proposed sea level curves differ because of site to site changes in local tectonics depositional rates and long wavelength dynamic topography resulting from mantle convection in particular the overall amplitude of global phanerozoic long term sea level is poorly constrained and has been estimated to vary between 400 m above present day sea level to 50 m below present day sea level to improve estimates of past sea level we explore an alternative methodology to estimate global sea level change we utilise the phanerozoic neoproterozoic 87sr 86sr record which at first order represents the mix of inputs from continental weathering and from mantle input by volcanism by compensating for weathering with estimates of runoff from a 3d climate model geoclimtec a corrected 87sr 86sr record can be obtained that solely reflects the contribution of strontium from mantle sources at first order the flux of strontium from the mantle through time is due to increases and decreases in the production of oceanic crust through time therefore the changing levels of mantle derived strontium can be used as a proxy for the production of oceanic lithosphere by applying linear oceanic plate age distributions we compute sea level and continental flooded area curves we find that our curves are generally within the range of previous curves built on classical approaches a phanerozoic first order cyclicity of 250 myr is observed that may extend into the neoproterozoic the low frequency i e on the order of 10 to 100 my sea level curve that we propose while open for improvement may be used as baseline for refined sequence stratigraphic studies at a global and basin scale

geology and stratigraphy of fluvio deltaic deposits in the ivishak formation applications for development of prudhoe bay field alaska significant remaining reserves in prudhoe bay field are confined within deltaic rocks at the base of the triassic ivishak sandstone the initial stratigraphic characterization of the prudhoe bay reservoir was lithostratigraphically based and it depicted this basal reservoir interval as tabular zones between marine shale and overlying coarse grained fluvial sandstones a reassessment of this interval based on cores and genetic stratigraphic correlations depicts en echelon offlapping fluvially dominated deltaic wedges reservoir quality rocks occur in distributary mouth bar distributary channel and fluvial facies associations a paleogeographic reconstruction of one delta lobe includes an alluvial plain crossed by channels of possibly braided or low sinuosity rivers this alluvial plain graded into a delta plain cut by distributary channels that fed distributary mouth bars on a broad delta front river dominance is inferred from the abundance of unidirectional current structures normally graded beds soft sediment deformation and general absence of wave formed tidal and biogenic structures slumping and growth faulting locally replaced coarsening upward deltaic successions with sharp based overthickened mouth bar and distributary channel deposits mudstones deposited following delta lobe abandonment form laterally extensive flow barriers between lobes compartmentalization is most pronounced distally where deltaic sandstones are overlain by and pass laterally into marine shale proximally fluvial and deltaic sandstones are juxtaposed across erosional contacts improving reservoir continuity this stratigraphic interpretation is corroborated by production and surveillance data plus an interference test locally stratigraphy and poor waterflood performance reflect completions in diachronous sandstones that originated in separate deltaic lobes previously poor well performances were attributed to sandstone pinch outs in some cases production can be enhanced with recompletions rather than infill drilling nonconventional wells planned and completed with the benefit of detailed facies association correlations currently are recovering millions of barrels of previously bypassed oil

depositional facies diagenesis and reservoir quality of ivishak sandstone sadlerochit group prudhoe bay field abstract the sadlerochit group is a large fan delta system comparable in size to the modern kosi river wet alluvial fan of nepal and india braided stream processes distributed chert gravel and quartz and chert sand in radial fashion to construct the subaerial part of the fan delta fluvial energy slope of the fan surface and grain size decrease in a north to south basinward direction there is also a decrease in scale of sedimentation units from source area seaward facies of the subaerial fan delta can be broadly categorized as midfan delta alternating conglomerate and sandstone distal fan delta chiefly sandstone and abandoned channel fill overbank and pond facies mudstone siltstone fine grained sandstone seaward of the subaerial fan delta are the delta front and rodelta facies subaerial fan delta and delta front facies compose the ivishak sandstone which grades basinward into the kavik shale a prodelta facies diagenetic effects were gradually superimposed on the sediments deposited in the sadlerochit fan delta system the sedimentary facies and in particular their textural characteristics seem to control the side and degree of diagenesis including enhancement of porosity and permeability comparison of permeability trends among the facies of the ivishak sandstone with permeability patterns displayed by unconsolidated sands with analogous grain size and texture indicates that the general trends that existed in the ivishak sediments are still recognizable in spite of the diagenetic overprint end of article last page 286

do superchrons occur without any palaeomagnetic warning abstract the mainly dipolar geomagnetic field generated by the geodynamo within the earth s liquid core has reversed its polarity many times in the past this succession of intervals of alternate polarity defines the geomagnetic polarity time scale gpts usually interpreted as resulting from a gamma renewal process the rate of which would be controlled by the boundary conditions imposed by the mantle on the core in this interpretation boundary conditions would have occasionally evolved towards being unfavourable to reversals leading the reversal rate of the process to progressively decrease and reach zero at least twice causing the onset of the so called kiaman and cretaceous very long polarity intervals superchrons here we reconsider this causal link analysing the latest and best constrained gpts thanks to the continuous record provided by marine magnetic anomalies describing the past 160 myr we found that contrary to earlier claims no long term behaviour over the 40 myr preceding the cretaceous superchron can be seen in the reversal rate that could explain its onset at 120 ma more generally it turns out that hardly any special behaviour can be identified in the gpts which could have announced this superchron only the occurrence of the longest of all pre superchron intervals cm1n 3 myr before the onset of the superchron could be identified as a precursor such a behaviour could possibly be the consequence of medium term on the 10 myr time scale changes in the boundary conditions imposed by the mantle on the core but we note that the sole analysis of the gpts does not allow this to be tested in fact it appears that a single stationary process could also explain the entire pre superchron gpts with the only possible exception of cm1n this suggests that the occurrence of superchrons does not necessarily require changes in boundary conditions and could simply attest for a sudden non linear transition between a reversing and a non reversing state of the geodynamo

the cenozoic evolution of the strontium and carbon cycles relative importance of continental erosion and mantle exchanges the past variations of the seawater 87sr86sr isotopic ratio are related to changes in the relative contribution of the mantle sr input to the ocean and the sr supply from continental weathering recently it has been postulated that the cenozoic increase in the seawater 87sr86sr isotopic ratio was associated with the uplift of the himalayan and andean mountains at that time these orogenies may have changed the sr isotopic ratio of the continental rocks undergoing weathering as a result of extensive metamorphism increased the river flux of sr through enhanced weathering in these regions and possibly caused the global climatic cooling trend of the cenozoic a model of the major geochemical cycles coupled to an energy balance climate model is used to explore the possible causes of the mesozoic cenozoic fluctuations in the seawater 87sr86sr isotopic ratio the contribution of the mantle exchanges at mid ocean ridges of the recycling of seafloor carbonates through plate margin volcanism and of the alteration of seafloor basalts to the fluctuations of the seawater 87sr86sr isotopic ratio are studied finally this model tentatively describes the impact of the himalayan orogeny on the geochemical cycles of sr and c some possible effects of the extensive metamorphism associated with the india asia collision and of the himalayan uplift are modelled the model reproduces the cenozoic increase of the seawater 87sr86sr isotopic ratio however the impact of the himalayan orogeny on the c cycle appears to be limited and insufficient to generate the global climatic cooling of the cenozoic rather in the model the cenozoic cooling is mostly due to the reduction of the co2 emission from mid ocean ridge volcanism and to changes in the chemical weathering rates in the rest of the world excluding the himalayas

evolution of a late jurassic carbonate evaporite platform conecuh embayment northeastern gulf coast u s a successive time slice lithofacies maps of the smackover haynesville depositional sequence constrained by marker beds an interpreted maximum flooding surface and interpreted sequence boundaries show an onlapping and retrograding sequence of lithologies characteristic of a transgressive systems tract tst the tst is followed by several prograding sequences characteristic of a highstand systems tract hst and stacked shelf margin wedges smw comparison of stratigraphic cross sections lithofacies maps and a paleotopographic map of the sub smackover transgressive surface shows that the distribution of lithofacies within the tst was strongly controlled by two types of sub smackover paleotopographic highs 1 norphlet dune fields and 2 eroded basement rock as th tst became progressively buried during deposition distribution of lithofacies within the hst and the smws was controlled less by these early paleotopographic features and more by 1 differential compaction around shoal water facies in the tst 2 early louann salt movement 3 basement related faulting and 4 freshwater influx along the basin rim

mantle plumes link magnetic superchrons to phanerozoic mass depletion events abstract the four most recent large mass extinction events in the phanerozoic the cretaceous tertiary kt the triassic jurassic tj and the permo triassic pt and guadalupian tatarian gt doublet are associated with a major flood basalt eruption with the timing of peak volcanic activity corresponding within measurement uncertainties to the extinction event three magnetic superchrons precede the four largest phanerozoic extinctions the cretaceous long normal superchron duration 35 myr precedes the kt and the permian kiaman long reversed superchron 50 myr precedes the pt gt doublet in addition the newly recognized ordovician moyero long reversed superchron 30 myr precedes the end ordovician extinction event there is a 10 20 myr delay between the end of each superchron and the subsequent mass depletion event both of which represent distant outliers from their respective populations we propose that deep mantle plumes link these seemingly unrelated phenomena long term 200 myr variations in mantle convection possibly associated with the wilson cycle induce temporal and spatial variations in heat flow at the core mantle boundary polarity reversals are frequent when core heat flow is high and infrequent when it is low thermal instabilities in the d layer of the mantle increase core heat flow end the magnetic superchron and generate deep mantle plumes the plumes ascend through the mantle on a 20 myr time scale producing continental flood basalt trap eruptions rapid climatic change and massive faunal depletions

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

sea level and vertical motion of continents from dynamic earth models since the late cretaceous dynamic earth models are used to better understand the impact of mantle dynamics on the vertical motion of continents and regional and global sea level change since the late cretaceous a hybrid approach combines inverse and forward models of mantle convection and accounts for the principal contributors to long term sea level change the evolving distribution of ocean floor age dynamic topography in oceanic and continental regions and the geoid we infer the relative importance of dynamic versus other factors of sea level change determine time dependent patterns of dynamic subsidence and uplift of continents and derive a sea level curve we find that both dynamic factors and the evolving distribution of sea floor age are important in controlling sea level we track the movement of continents over large scale dynamic topography by consistently mapping between mantle and plate frames of reference and we find that this movement results in dynamic subsidence and uplift of continents the amplitude of dynamic topography in continental regions is larger than global sea level in several regions and periods so that it has controlled regional sea level in north and south america and australia since the late cretaceous northern africa and arabia since the late eocene and southeast asia in the oligocene miocene eastern and southern africa have experienced dynamic uplift over the last 20 to 30 m y whereas siberia and australia have experienced cenozoic tilting the dominant factor controlling global sea level is a changing oceanic lithosphere production that has resulted in a large amplitude sea level fall since the late cretaceous with dynamic topography offsetting this fall