late ordovician turinian chatfieldian carbon isotope excursions and their stratigraphic and paleoceanographic significance five positive carbon isotope excursions are reported from platteville decorah strata in the upper mississippi valley all occur in subtidal carbonate strata and are recognized in the mifflin grand detour quimbys mill spechts ferry and guttenberg intervals the positive carbon isotope excursions are developed in a platteville decorah succession in which background 13c values increase upward from about 2 at the base to about 0 vienna pee dee belemnite vpdb at the top a regional north south 13c gradient with lighter values to the north and heavier values to the south is also noted peak excursion 13c values of up to 2 75 are reported from the quimbys mill excursion and up to 2 6 from the guttenberg excursion although there are considerable local changes in the magnitudes of these events the quimbys mill spechts ferry and guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities and completely starve out in deeper more offshore areas closely spaced chemostratigraphic profiles of these sculpted pyrite impregnated hardground surfaces show that they are associated with very abrupt centimeter scale negative 13c shifts of up to several per mil possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events

widespread prolonged late middle to late ordovician upwelling in north america a proxy record of glaciation late middle to late ordovician subtidal ramp carbonates of new mexico texas and oklahoma contain abundant spiculitic chert to 70 chert by volume and phosphate 1 5 wt indicating that these rocks formed in an extensive upwelling zone upwelling began in the late middle ordovician ca 454 ma and persisted until the end of the ordovician late ordovician cherty carbonates also occur along the u s cordilleran margin in board of organic rich graptolitic shale and chert the widespread occurrence of late ordovician cherty and phosphatic carbonates on southern and western laurentia in addition to phosphate rich cool water carbonates over much of the north american midcontinent suggests vigorous thermohaline circulation related to prolonged 10 14 ma gondwana glaciation

an alternative model for positive shifts in shallow marine carbonate 13c and 18o positive shifts in global seawater d 13 cdic are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in d 18 o here an alternative model is proposed for the formation of local positive isotope shifts in shallow marine settings the model involves geochemically altered platform top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition both mechanisms are active on modern sub tropical carbonate platforms and result in low carbonate d 13 ca ndd 18 o relative to typical oceanic values during high amplitude transgressive events the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced and 13 c depleted platform top water mixes with open oceanic water masses having higher isotope values both factors are recorded as a transient increase in carbonate 13 c and 18 o relative to low background values these processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas

reconstruction of reflecting structures from vertical seismic profiles with a moving source when a vertical seismic profile vsp is recorded the illuminated part of a reflector depends upon the shape and position of the reflector itself as well as on the seismic velocities and the positions of sources and receivers a preferable arrangement for the investigation of structures of reflectors is to fix the receiver s at constant depth s in the well and move the source horizontally along a line at the earth s surface usually called a multioffset vsp msp or walkaway vsp as a test of the resolution power of this survey geometry synthetic records were generated from a subsurface model by inverse kirchhoff migration three different methods were applied for the reconstruction wavefront construction leads to the correct shape of the reflectors thus assuring the validity of the modeling method applied reflection point mapping delivered a near similarity to the model but without focusing fault edges kirchhoff migration resulted in a detailed image of the reflectors with fault edges focuse

differential deformation of gravity driven deep water fold and thrust belts along the passive continental margin of east africa and their impact on petroleum migration and accumulation abstract passive margin deep water fold and thrus belts dwftbs have been the focus of a number of studies in recent decades because many oil and gas fields have been discovered in traps associated with them nevertheless the impact of dwftbs on petroleum plays remains unclear in this study we developed models for petroleum migration and accumulation in the offshore rovuma basin and lamu basin dwftbs located along the passive continental margin of east africa based on an integrated analysis of seismic geochemical and geological data using available high quality seismic data in the depth domain we were able to quantify the evolution of thrust anticline growth by applying the area depth strain ads method our results indicate that the offshore lamu basin dwftb was active from the late cretaceous period to the early miocene period while the offshore rovuma basin dwftb has been active from the oligocene period geochemical parameters and basin modeling show that widespread lower jurassic mudstone is the dominant source of petroleum resources with peaks of oil and gas generation in the cretaceous and cenozoic periods respectively based on this we suggest that the traps within the offshore lamu basin dwftb contain oil generated from lower jurassic sources that migrated along faults and accumulated during the late cretaceous period and gas generated from the same sources during the cenozoic period conversely there is an insufficient amount of oil in the offshore rovuma basin dwftb owing to the absence of oil migration pathways or traps as a result of the dwftb s more recent formation from the oligocene period

depth imaging of basalt flows in the faeroe shetland basin summary we use a grid of long offset seismic profiles to map the thickness of subsurface basalt flows that extend southeastward across the faeroe shelf offsets of up to 38 000 m were achieved using two seismic acquisition vessels towing multichannel streamers and shooting alternately wide angle reflections and refractions from the basalt and underlying strata allow us to build a velocity model down to the basement lewisian crust the velocity models give a first order picture of the geology of low velocity sediments of presumed early palaeocene and mesozoic age beneath the basalt flows they also provide a control for pre stack depth migration of the entire seismic data set thus allowing us to produce good depth images of the structure pre stack depth migration of selected wide angle arrivals such as those from the base of the basalt and the underlying basement produce strong reflection images that allow these interfaces to be identified unambiguously on the higher resolution images produced from the entire data set in particular we can distinguish between the primary and multiple reflections we thus use the very long offset arrivals to tag the velocities of the geological interfaces from which they are returned and hence to improve both the imaging and the interpretation of the basalt flows and the underlying structure

correlation of the ordovician deicke and millbrig k bentonites between the mississippi valley and the southern appalachians 1 two widespread rocklandian age middle ordovician k bentonite beds the deicke and millbrig can be correlated from southern minnesota and northwestern iowa through eastern missouri across kentucky and central tennessee to the valley and ridge of the southern appalachian mountains they are equivalent to beds previously called t 3 or pencil cave and t 4 or mud cave metabentonites and thus constitute ordovician chronostratigraphic marker horizons throughout much of the eastern mid continent previous correlations of these beds in the mississippi valley by chemical fingerprinting is extended toward the southern appalachian basin by tracing in surface exposures and on wireline logs both beds thicken toward the southeast indicating a volcanic source area that was probably ast of south carolina the interval between the k bentonite beds also thickens toward the southeast owing to increased rates of carbonate deposition between the two volcanic episodes the areal extent of known correlatives of the deicke and millbrig is minimally estimated to be 600 000 sq km and at least 1122 cu km of pre compaction deicke k bentonite accumulated as ash in eastern north america the volume of both beds suggests they rank among the largest air fall ash deposits documented in the stratigraphic record

vsp normal moveout stretch the vertical seismic profiling common depth point vspcdp transformation is a technique used to image the reflected waves in an offset vsp this coordinate transformation procedure maps reflection amplitudes from receiver depth and reflection times to lateral offset and reflector depth wyatt and wyatt 1981 the process involves the application of moveout correction followed by stacking this moveout correction results in a distortion of the original waveform and a smear of the reflection event that consequently degrades the lateral resolution of the transformation owusu 1991 therefore to determine the optimum processing parameters for the transformation we need to understand the effects of the vsp moveout stretch

theory and seismic applications of the eigenimage discrete wavelet transform discrete wavelet transforms are useful in a number of signal processing applications to improve the scale resolution a joint function of time scale and eigenvalue that describes the energy density or intensity of a signal simultaneously in the wavelet and eigenimage domains is constructed a hybrid method which decomposes eigenimages in the wavelet domain is developed and tested on field data with a variety of noise types several illustrative examples examine the ability of wavelet transforms to resolve features at several scales successful applications to time lapse seismic reservoir monitoring are presented in reservoir monitoring the scale dependent properties of the eigenstructure of the 4d data covariance matrix enable us to extract the low frequency time lapse signal that is the result of internal diffusive losses caused by fluid flow

complex polarization analysis of particle motion knowledge of particle motion polarization aids in identifying phases on three component seismograms the scheme of montalbetti and kanasewich 1970 is extended to analytic three component seismograms where the imaginary part of the signal is the hilbert transform of the real part this scheme has only one free parameter the length of the time window over which the polarization parameters are estimated so it can be applied in a routine way to three component data the azimuth and dip of the direction of maximum polarization and the degree of elliptical polarization as a function of time for the seismograms are obtained polarization analysis of strong motion data from the 1971 san fernando earthquake aids in the discrimination between wave types which is important for the understanding of the complicated earthquake induced shaking observed in basins most arrivals are incident on the receivers in the direction of the back azimuth to the epicenter which suggests that despite the complicated motions two dimensional finite difference methods are sufficient to understand the effect on seismic waves of the los angeles and san fernando basins vidale and helmberger 1986b

late middle ordovician environmental change and extinction harbinger of the late ordovician or continuation of cambrian patterns positive excursions in carbon isotope compositions of carbonate 3 and organic carbon 4 6 from the late middle ordovician middle caradocian of the midcontinent and the eastern united states indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric p co 2 precipitating global cooling although not necessarily ice sheet formation these climatic changes were associated with regional orogenic uplift a relative rise in sea level changes in epeiric sea circulation patterns and carbonate platform destruction that led to regional extinction of marine benthos the combination of sea level rise changing ocean circulation and extinction in the middle caradocian is similar to the suite of environmental changes described for cambrian biomere boundaries suggesting shared causes for these events in contrast middle caradocian environmental changes are markedly different from the environmental patterns associated with the late ordovician mass extinction despite the evidence for long term cooling from the middle to the late ordovician

an isotopic study of biogeochemical relationships between carbonates and organic carbon in the greenhorn formation carbon isotopic compositions of total carbonate inoceramid carbonate micritic carbonate secondary cements total organic carbon and geoporphyrins have been measured in 76 different beds within a 17 m interval of a core through the greenhorn formation an interbedded limestone and calcareous shale unit of cretaceous age cenomanian turonian from the western interior seaway of north america results are considered in terms of variations in the processes of primary production which led to the biosynthesis of the molecular precursors of the geoporphyrins and in secondary processes those mediating the transformation of primary organic material into sedimentary total organic carbon it is shown that the porphyrin isotopic record reflects primary isotopic variations more closely than the toc isotopic record and that in these sediments toc is enriched in 13c relative to its primary precursor by 0 6 to 2 8 this enrichment is attributed to isotope effects within the consumer foodweb and is associated with respiratory heterotrophy variations in this secondary enrichment are correlated with variations in the isotopic composition of marine carbonate this correlation is attributed to effects of environmental changes on the marine foodweb these may have included increased atmospheric oxygen associated with the cenomanian turonian oceanic anoxic event the isotopic fractionation associated with fixation of carbon by primary producers is observed to have varied by 1 5 during the interval of deposition it is suggested that this change is due to a variation in the makeup of the community of primary producers and or to a decrease in the atmospheric abundance of co2 during the oceanic anoxic event

reverse time migration of offset vertical seismic profiling data using the excitation time imaging condition to apply reverse time migration to prestack finite offset data from variable velocity media the standard time zero imaging condition must be generalized because each point in the image space has a different image time or times this generalization is the excitation time imaging condition in which each point is imaged at the one way traveltime from the source to that point reverse time migration with the excitation time imaging condition consists of three elements 1 computation of the imaging condition 2 extrapolation of the recorder wave field and 3 application of the imaging condition computation of the imaging condition for each point in the image is done by ray tracing from the source point this is equivalent to extrapolation of the source wave field through the medium extrapolation of the recorded wave field is done by an acoustic finite difference algorithm imaging is performed at each step of the finite difference extrapolation by extracting from the propagating wave field the

characterization of volcanic units using detailed velocity analysis in the atlantic margin west of shetlands united kingdom the offshore faroe shetlands basin northwest of scotland and south of the faroe islands figure 1 has seen renewed exploration interest following development of schiehallion foinaven and loyal fields and opening of the first licensing round of the faroe islands figure 1 significant basalt wells rock property studies and key velocity inversion profiles in area of study interpretation of volcanic data in the area is very challenging and many local oil and gas prospects involve clastic objectives above or below volcanic zones high uncertainty about the thickness and morphology of these zones results in high uncertainties in structural geometry reservoir definition and hydrocarbon migration and charge risk only a few wells have penetrated these volcanic zones to date sonic logs from these wells and regional rock property trends for tertiary aged clastic sediments suggest that significant rms root mean square velocity changes occur at or near these volcanic boundaries boundaries not easily identified with conventional 2 d or 3 d data and interpretation techniques this case study discusses how interpretation was improved via a focus on velocity coherence analysis data were reprocessed using various demultiple and filtering techniques specifically designed to improve velocity coherence analysis emphasis was placed on identification of bona fide velocity coherences and associated inflection points within stacking velocities that might indicate bulk changes to true interval velocities the geology of the late cretaceous and early tertiary in the vicinity of the faroe islands is dominated by incipient continental break up and associated rifting between greenland and northwest europe this break up and its association with a major mantle plume has created one of the world s major basalt provinces the faroe islands are predominantly made of these basaltic lavas intercalated with tuff and relatively minor sedimentary rocks faroese lavas are broadly equivalent in age to those encountered in the british isles greenland and offshore norway and have

sea level forcing of carbon isotope excursions in epeiric seas implications for chemostratigraphy five new 13c excursions are reported from the upper ordovician galena group of iowa the 13ccarb and 13corg trends are compared to sea level trends based on conventional sedimentological and novel chemostratigraphic end methods it is found that rising 13c values occur during rising sea level peak magnitude 13c values are coincident with inferred peak flooding and declining 13c values occur with declining sea levels the relationship to sea level is explained by a simple model of episodic flooding of the galena platform by cool oxygen depleted nutrient rich bottom waters originating from the sebree trough which was itself connected to the nearby iapetus ocean flooding of the inner galena shelf during sea level transgressions caused local changes in the carbon isotope balance of inner shelf waters due to 1 locally increased primary productivity and organic carbon burial 2 locally increased exchange of 13c enriched dissolved inorganic carbon dic with surrounding epeiric sea water masse

paired δ13ccarb and δ13corg records of upper ordovician sandbian katian carbonates in north america and china implications for paleoceanographic change late sandbian early katian marine carbonates from two sections in north america and one in south china were analyzed for paired 13ccarb and 13corg and revealed similar 13ccarb but varying trends in 13corg stratigraphy while all sections recorded a well known positive 13ccarb shift these 13corg records are the first through the guttenberg 13ccarb excursion gice in china oklahoma and west virginia in south china a positive excursion in 13corg is associated with the latter part of the positive 13ccarb excursion and closely resembles trends from iowa and pennsylvania that have been interpreted to reflect a lowering of atmospheric pco2 levels the 13corg trends from oklahoma and west virginia differ significantly from what is observed in china this likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the dominant control on 12c fractionation in oklahoma and west virginia masking a possible global signal of lowered atmospheric pco2 levels observed in china

migration of mixed mode vsp wavefields1 two dimensional vsp surveys are often conducted to provide structural illumination of the subsurface away from the borehole the illumination is achieved through offsetting the source with respect to the downhole geophone this inevitably gives rise to mode conversions in both downgoing and upgoing wavefields migration of mixed mode wavefields is complex because the velocity profile used for wavefield extrapolation is valid only for a particular propagation mode the other mode always propagates at a different velocity it is therefore advisable to separate the wave types p wave and sv wave prior to migration this may be achieved through wavemode filtering a multichannel process which exploits the relation between propagation velocity slowness of events at the recording array and particle motion the necessary information about particle motion is available only if the vsp data are acquired with a three component downhole geophone assembly the wavemode filter partitions wave types at the recording array it provides no information about the various changes of propagation mode experienced by the energy as it travels from source to geophone for the purpose of migration the intermediate modes of propagation must be deduced much of the energy arriving at the receivers is p wave which has followed the p wave velocity profile from the source it can therefore be imaged by conventional kirchhoff migration as an example of sv wave imaging a common mode code is p wave from source to reflector and sv wave from reflector to geophone migration of such data calls for back propagation of the geophone array wavefield at sv wave velocity to the point in the subsurface where it is time coincident with the forward propagated downwave at p wave velocity

techniques for assessing spatial heterogeneity of carbonate δ13c values implications for craton wide isotope gradients the sedimentary record of carbonate carbon isotopes 13ccarb provides one of the best methods for correlating marine strata and understanding the long term evolution of the global carbon cycle this work focuses on the late ordovician guttenberg isotopic carbon excursion a ca 2 5 positive 13ccarb excursion that is found in strata globally substantial variability in the apparent magnitude and stratigraphic morphology of the guttenberg excursion at different localities has hampered high resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle this work investigates the magnitude spatial scale and sources of isotopic variability of the guttenberg excursion in two sections from missouri usa centimetre scale isotope transects revealed variations in 13ccarb and 18ocarb greater than 2 across individual beds linear 13ccarb to 18ocarb mixing lines together with petrographic and elemental abundance data demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components these patterns facilitated objective sample screening to determine the least altered data a 18ocarb filter based on empirical 18ocarb values of well preserved carbonate mudstones allowed further sample discrimination the resulting least altered 13ccarb profile improves the understanding of regional as well as continental scale stratigraphic relations in this interval correlations with other laurentian sections strongly suggest that i small scale variability in guttenberg excursion 13ccarb values may result in part from local diagenetic overprinting ii peak guttenberg excursion 13ccarb values of the midcontinent are not distinct from their taconic equivalents and iii no primary continental scale spatial gradient in 13ccarb for example arising from chemically distinct aquafacies is required during guttenberg excursion time this study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for 13ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry

seismic reflection amplitudes seismic amplitude variations with offset contain information about the elastic parameters prestack amplitude analysis seeks to extract this information by using the variations of the reflection coefficients as functions of angle of incidence normally an approximate formula is used for the reflection coefficients and variations with offset of the geometrical spreading and the anelastic attenuation are often ignored using angle of incidence as the dependent variable is also computationally inefficient since the data are recorded as a function of offset improved approximations have been derived for the elastic reflection and transmission coefficients the geometrical spreading and the complex travel time including anelastic attenuation for a 1 d medium these approximations are combined to produce seismic reflection amplitudes p wave s wave or converted wave as a taylor series in the offset coordinate the coefficients of the taylor series are computed directly from the parameters of the medium without using the ray parameter for primary reflected p waves dynamic ray tracing has been used to compute the offset variations of the transmission coefficients the reflection coefficient the geometrical spreading and the anelastic attenuation the offset variation of the transmission factor is small while the variations in the geometrical spreading absorption and reflection coefficient are all significant the new approximations have been used for seismic modelling without ray tracing the amplitude was approximated by a fourth order polynomial in offset the traveltime by the normal square root approximation and the absorption factor by a similar expression this approximate modelling was compared to dynamic ray tracing and the results are the same for zero offset and very close for offsets less than the reflector depth

decomposition of multicomponent seismic data into primary p and s wave responses1 inversion of multicomponent seismic data can be subdivided in three main processes 1 surface related preprocessing decomposition of the multicomponent data into primary p and s wave responses 2 prestack migration of the primary p and s wave responses yielding the angle dependent p p p s s p and s s reflectivity of the subsurface 3 target related post processing transformation of the reflectivity into the rock and pore parameters in the target this paper deals with the theoretical aspects of surface related preprocessing which consists of two steps 1 decomposition of the multicomponent data pseudo p and s wave responses into true p and s wave responses 2 elimination of the surface related multiple reflections and conversions

vertical seismic profile migration using the kirchhoff integral wave equation migration can form an accurate image of the subsurface from suitable vsp data the image s extent and resolution are determined by the receiver array dimensions and the source location s experiments with synthetic and real data show that the region of reliable image extent is defined by the specular zone of illumination migration is achieved through wave field extrapolation subject to an imaging procedure wave field extrapolation is based upon the scalar wave equation and for vsp data is conveniently handled by the kirchhoff integral the migration of vsp data calls for imaging very close to the borehole as well as imaging in the far field this dual requirement is met by retaining the near field term of the integral the complete integral solution is readily controlled by various weighting devices and processing strategies whose worth is demonstrated on real and synthetic data

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

anisotropic migration of coincident vsp and cross hole seismic reflection surveys seismic depth migration may result in false reflector positioning and destructive interference when an incorrect velocity field is used to convert from time to depth the assumption of isotropy to describe anisotropic rocks is one major source of error in the velocity model although individual survey images may not be impaired by such an approximation when different survey types such as vsp and cross hole reflection seismics have coincident illumination of the subsurface it is important not only to produce consistent images upon depth migration but also to determine a consistent velocity model using real data sets as examples both objectives are successfully achieved when anisotropy is incorporated into the velocity model

conodont apatite δ18o values from a platform margin setting oklahoma usa implications for initiation of late ordovician icehouse conditions abstract the uppermost sandbian lower katian stratigraphic record documents a critical time period in earth history lithological evidence and geochemical proxy records suggest that the uppermost sandbian lower katian interval could represent a transitional phase from a long cambrian ordovician greenhouse world to the late ordovician icehouse world to examine this interval of earth s history oxygen isotopes of conodont apatite from the bromide and viola springs formations in southern oklahoma were analyzed in order to provide sea surface temperature estimates at a location that had little influence by terrigenous sedimentation our results are inconsistent with 1 previous suggestions that global cooling occurred across a prominent sequence boundary close to the sandbian katian boundary across north america and 2 the hypothesis that there is a direct connection between volcanic mega eruption s and a significant long term impact on climate during the sandbian katian of laurentia furthermore 18 o values spanning the guttenberg carbon isotope excursion gice an interval that has been interpreted as evidence for an early onset of the late ordovician glaciation are variable a 1 5 18 o shift is observed in the lower gice that is followed by a return to 18 o values that fluctuate around a mean of 19 0 in the upper gice together these observations suggest a short lived cooling event within the lower gice this cooling event may represent enhanced carbon burial and drawdown of atmospheric p co 2 followed by a return to more stable low latitude temperatures in the upper gice

kirchhoff elastic wave migration for the case of noncoincident source and receiver in taking into account both compressional p and shear s waves more geologic information can likely be extracted from the seismic data the presence of shear and converted shear waves in both land and marine seismic data recordings calls for the development of elastic wave migration methods the migration method presently developed consists of simultaneous migration of p and s waves for offset seismic data based on the kirchhoff helmholtz type integrals for elastic waves a new principle of simultaneously migrating both p and s waves is introduced the present method named the kirchhoff elastic wave migration has been tested using the 2 d synthetic surface data calculated from several elastic models of a dipping layer including a horizontal layer a composite dipping and horizontal layer and two layers over a half space the results of these tests not only assure the feasibility of this migration scheme but also demonstrate that enhanced images in the migrated sections are well formed moreove

nature and regional significance of unconformities associated with the middle ordovician hagan k bentonite complex in the north american midcontinent stratal patterns of the middle ordovician hagan k bentonite complex and associated rocks show that the black river trenton unconformity in the north american midcontinent formed through the complex interplay of eustasy sediment accumulation rates siliciclastic influx bathymetry seawater chemistry and perhaps local tectonic uplift the unconformity is diachronous and is an amalgamated surface that resulted from local late turinian lowstand exposure followed by regional early chatfieldian transgressive drowning and sediment starvation the duration of the unconformity is greatest in southern wisconsin northern illinois and northern indiana where the deicke and millbrig k bentonite beds converge at the unconformity on the basis of published isotopic ages for the deicke and millbrig beds it is possible that in these regions erosion and nondeposition spanned a period of as much as 3 2 m y two broad coeval depositional settings are recognized within the north american midcontinent during early chatfieldian time 1 an inner shelf subtidal facies of fossiliferous shale spechts ferry shale member and ion shale member of the decorah formation and argillaceous lime mudstone and skeletal wackestone guttenberg and kings lake limestone members extended from the canadian shield and transcontinental arch southeastward through minnesota wisconsin iowa and missouri 2 a seaward relatively deep subtidal sediment starved middle shelf extended eastward from the mississippi valley region to the taconian foreland basins in the central and southern appalachians and southward through the pericratonic arkoma and black warrior basins in the inner shelf region the black river trenton unconformity is a composite of at least two prominent hardground omission surfaces one at the top of the castlewood and carimona limestone members and the other at the top of the guttenberg and kings lake limestone members both merging to a single surface in the middle shelf region the inner and middle shelves redeveloped later in approximately the same regions during devonian and mississippian time

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

sub basalt decoupled walkaway vsp imaging walkaway vertical seismic profile vsp data acquired in basalt covered areas can be used to improve knowledge of the sub basalt structure a synthetic example and a case study from the north atlantic uk show that elastic two way downward continuation migration combined with the stationary phase principle is well suited to the processing of vsp data vector data are processed using decoupled elastic migration algorithms in both isotropic and anisotropic media to illustrate the value of decoupled imaging equations conventional pp imaging is carried out on the enhanced vsp data and compared with the decoupled scheme decoupled vector migration operates directly on the displacement vector and uses various wave modes downgoing waves are migrated to image basalt lava flows and measure their anisotropy upgoing waves are used for high resolution sub basalt imaging

coal seismic surveying over near surface basalts experience from central queensland australia abstractseismic reflection surveying in basalt covered areas often fails to image underlying reflectors to gain insights into the nature of the problem and obtain potential solutions we have conducted experimental 2d seismic reflection and offset vsp surveys at two coal mines in the bowen basin of australia at the first mine the basalt is relatively deep 114 m and relatively thin 20 m conventional seismic acquisition and processing of a 2d seismic line provide poor results however upgoing reflections from layers below the basalt are clearly evident in the vsp survey and prestack depth migration is able to improve the continuity of the reflectors beneath the basalt at the second mine the 360 m wide basalt is at a depth of 40 m and has a thickness of about 40 m it is fresh and unweathered and consists of multiple flows which are interlayered with unconsolidated sediments long offset data acquisition combined with prestack depth migration was expected to produce satisfactory results but this i

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

carbon cycling across the southern margin of laurentia during the late ordovician abstract the guttenberg isotope carbon excursion gice a positive carbon isotope excursion that occurs near the base of the katian stage is thought to be a global event possibly related to late ordovician cooling documenting how much regional and global variability exists in carbon isotopic trends prior to and during the gice is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle paleoclimate and chemostratigraphic correlation during the ordovician to investigate carbon isotopic trends along the southern margin of the laurentian carbonate platform during the late ordovician we measured bulk carbonate 13 c carb and 18 o carb as well as organic carbon 13 c org values from four locations in alabama these sections are excellent study sites because they are well exposed contain the regionally well correlated deicke and millbrig k bentonites and are in a region where 13 c trends have not been studied carbonate 13 c carb results from one section i e fort payne in northeastern alabama record a 1 5 positive excursion above the millbrig k bentonite we interpret the 1 5 positive excursion as a primary feature and identify it as the gice the characteristic gice excursion is absent in the other study sections and we attribute this absence to the presence of unconformities either non depositional or erosional in these sections we find neither evidence for sea level control on 13 c trends nor conclusive evidence for 13 c dic gradients along the southeast margin of laurentia s epeiric sea combined with the proximity to the iapetus ocean these results suggest that carbon isotopic trends in alabama are a reliable proxy for open ocean conditions therefore despite the absence of gice excursion in all but one of our study sections increasing 13 c values in alabama prior to the gice provides evidence for a steady 13 c enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric co 2 during the early late ordovician