cyclostratigraphic calibration of the frasnian late devonian time scale western alberta canada until now the duration of the frasnian stage has remained very poorly constrained hampering a detailed understanding of sedimentation processes and environmental and evolutionary change in this study timeseries analyses of high resolution 10 20 k y magnetic susceptibility data identify sixteen 405 k y eccentricity cycles in the magnetic susceptibility stratigraphy of the frasnian late devonian derived from carbonate platform and surrounding slope and basin deposits in western alberta canada previous studies demonstrated the generally consistent pattern of magnetic susceptibility change across the alberta basin and thus demonstrated the utility of magnetic susceptibility stratigraphy as a refi ned regional correlation tool compared to biostratigraphy in the present study we show that the magnetic susceptibility stratigraphy of the frasnian interval in western alberta has been signifi cantly infl uenced by astro nomical forcing using the sixteen 405 k y eccentricity cycles as a geochronometer we constructed a frasnian astronomical time scale this time scale indicates a duration of 6 5 0 4 m y for the frasnian calibrating this duration to the best available devonian chronology the absolute age of the givetian frasnian boundary is recalculated to 383 6 3 0 ma and the age of the frasnian famennian boundary is recalculated to 376 7 3 0 ma these new absolute ages take into account the astronomically derived duration of the frasnian but they also yield a narrowing of the error margins of the absolute ages by several hundreds of thousands of years

terminal ordovician extinction geochemical analysis of the ordovician silurian boundary anticosti island quebec elemental abundances including ir carbon and oxygen ratios in carbonates mineral content and thin sections have been measured in samples collected across the conodont defined ordovician silurian o s boundary exposed on anticosti island quebec the ir concentrations ranged from 5 to a maximum at the boundary of 58 parts per trillion ppt however there is no evidence on the basis of these ir results for the association of a large body earth impact with the o s extinction because the ir concentrations like those of most other trace elements are simply proportional to the clay al content in the carbonate sequence the sup 13 c sup 12 c and sup 18 o sup 16 o ratios decrease abruptly at the boundary then just as abruptly increase to a long period of higher than preboundary ratios these patterns are probably related to the salinity in the seaway which was shallowing up to boundary time and then deepened and developed patch reefs fresh water input from rivers would have been most influential during the shallow water conditions 32 references 4 figures

a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

improvements to regional explosion identification using attenuation models of the lithosphere 1 regional p s amplitudes have been recognized as an effective discriminant between earthquakes and explosions while closely spaced events generally discriminate easily the application of this technique to broad regions has been hampered by large variations in the amplitude of phases due to the attenuation of the crust and upper mantle making use of a recent p wave and s wave attenuation model of the lithosphere we have found that correcting the events using our amplitude methodology significantly reduces the scattering in the earthquake population we demonstrate an application of this technique to station nil using broad area earthquakes and the 1998 indian nuclear explosion using the pn lg discriminant in the 1 2 hz passband we find that the explosion which is lost in the scatter of the earthquakes in the uncorrected discriminant clearly separates by correcting for attenuation structure we see a similar reduction in scatter and separation for the pn sn and pg lg discriminants

orbital scale climate change and glacioeustasy during the early late ordovician pre hirnantian determined from δ18o values in marine apatite this study focuses on the 10 m y before the latest ordovician hirnantian glaciation we test whether orbital scale climatic fluctuations controlled the growth and melting of continental glaciers resulting in glacioeustatic sea level changes and the development of widespread marine sedimentary cycles 18o values of conodont apatite from 14 late ordovician katian cycles range from 17 to 21 isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow water facies supporting the hypothesis that glacioeustasy was the dominant control on water depth changes measured intracycle 18o changes of 0 7 2 5 were controlled by changes in ice volume 60 m sea level changes sea surface temperatures 5 c and potentially local increases in seawater evaporation during drier and or windier glacial stages these interpreted orbital scale climate changes and resultant large glacial ice volume changes support recent interpretations of a dynamic and prolonged ordovician greenhouse to icehouse transition

pan african granitoid emplacement in the adrar des iforas mobile belt mali a rb sr isotope study abstract time and space relationships between pan african intrusives in the iforas belt northeast mali are discussed on the basis of structures lithological composition and plutonism three domains have been defined these are from west to east 1 the tilemsi accretion domain 2 the central domain characterized by an early pan african tectonism with large scale basement nappes and 3 the eastern domain a later pan african trough deposited on continental basement domains 1 and 2 are separated by a huge complex batholith geochronological results confirm the structural distinctions and the existence of two major events as shown by rb sr isochron ages domains 2 and 3 were stabilized 645 ma ago while the multistage evolution of the batholith can be bracketed between 614 and 570 ma rb sr biotite whole rock pairs give very similar ages which indicates a short time lapse between emplacement and cooling the initial ratios in the range 0 705 0 708 are as in many other batholiths slightly higher than the mantle growth curve indicating a complex origin where crust and mantle are involved as granitoids occupy some 60 of the surface area in the iforas belt the pan african epoch corresponds to an important crustal addition in the iforas segment

high resolution carbon isotope stratigraphy of the basal silurian stratotype dob s linn scotland and its global correlation since its designation as the global stratotype section and point gssp for the base of the silurian system the choice of dob9s linn southern scotland has received criticism due to the difficulties of relating its well constrained graptolite biostratigraphy to shallow water sequences elsewhere kerogen samples from across the ordovician silurian boundary interval at dob9s linn have yielded carbon stable isotope signatures consistent with those recorded elsewhere in particular showing a clear positive 13 c excursion in the terminal ordovician the architecture of the 13 c curve from dob9s linn enables very high resolution stratigraphic subdivision and direct correlation between the deep water dob9s linn section and time equivalent carbonate shelf deposits an integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites conodonts and shelly faunas has been constructed this direct correlation shows that the shallow water successions including the former stratotype candidate at anticosti island are generally incomplete with hiatuses related to the rapid sea level changes during the hirnantian stage this confirms and greatly increases the global utility of dob9s linn as a boundary stratotype

sense of shear and displacement estimates in the abeibara rarhous late pan african shear zone adrar des iforas mali the late pan african abeibara rarhous shear zone in the adrar des iforas mali is described and studied with the aim of defining the direction sense of movement and amount of displacement along the zone it is a strike slip shear zone the dextral sense of which is demonstrated at the scale of the map by the rotation of the related mylonitic foliation and at the scale of the thin section with characteristic microstructures preferred orientation of quartz c axes is tentatively used three quartz rich samples of 35 or more quartz indicate dextral strike slip movement but other samples do not show preferred orientation of quartz c axes strain measurements have been performed on one half of the shear zone using established techniques and a new technique using the thickness of mylonitic layering the results vary along the length of the shear zone when using the same method and for the same cross section when using the three methods together a mean value of 4 km is obtained for total displacement which is low when considering the apparent width of the shear zone this result is discussed in view of the assumptions involved in the strain estimation the tectonic history of the abeibara rarhous shear zone and its significance in the trans saharan pan african collisional belt are discussed

caracterisation du magmatisme proterozoique superieur en afrique de l ouest et implications geodynamiques des rifts intracratoniques au panafricain the main petrological mineralogical and geochemical features of the metavolcanites belonging to the late proterozoic tectono sedimentary units of southern mauritanides termesse and niokolo koba koulountou and western hoggar algeria timesselarsine permit a reconstruction of the geodynamic environment that controlled the evolution of the pan african ranges of western africa flows and pyroclastic beds metamorphosed under greenschist facies conditions are interbedded within detritic and carbonate platform sediments suggesting an unstable environment in the vicinity of a continental margin the association of oceanic tholeiites slightly enriched in lree and basalts more enriched in tio2 zr nb and lree displaying alkalic affinities suggests intracontinental rifting succeeded by expansion the lack of ultrabasic basic complexes oceanic paleosutures in southern mauritanides and their occurrence in the hoggar suggest that the opening was bigger in the north than in the south the later calc alkali

simulation of topographic effects on seismic waves from shallow explosions near the north korean nuclear test site with emphasis on shear wave generation 1 we performed high resolution 8 hz three dimensional simulations of ground motions from shallow explosions in the presence of rough surface topography near the north korean nuclear test site to study elastic propagation effects with emphasis on theoretical aspects of shear wave generation interaction with rough topography causes significant p to rg scattering along the surface with amplification of high frequency 2 8 hz shear waves relative to the flat earth case shear waves of different polarizations are coupled by topographic scattering rg precursors composed of p to rg conversions traveling as surface waves have the spectral amplitudes comparable to the p wave while the rg phase has the low frequency 0 5 3 hz spectral shape of the rg from the flat case plus the high frequency 3 8 hz p wave spectra motions at near vertical takeoff angles corresponding to teleseismic propagation are increased or decreased indicating that waves are focused or defocused by topographic features above the source topographic roughness has a dramatic effect as short wavelength features 2 5 km are included higher frequencies are amplified by topography including frequencies corresponding to wavelengths shorter than the shortest topographic scale length overall topography enhances energy propagating along the surface near the source amplifies surface waves and tends to balance sv and sh polarized motions all of which impact shear wave observations used for nuclear explosion monitoring further simulation studies could elucidate how the wavefield emerging from a topographically rough area ultimately propagates to regional and or teleseismic distances

seismic decoupling with chemical and nuclear explosions in salt an extensive series of simulations was performed of underground explosions in salt using both chemical and nuclear explosives in both cases the radius of the initial emplacement cavity was varied from the fully tamped configuration to as large as 80 m kt sup 1 3 when not fully tamped the cavity was assumed initially to contain air at ambient temperature and pressure in the nuclear source case the simulations are shown to be in good agreement with the salmon sterling events conducted by the united states and with recently released russian data on a similar pair of explosions in an azgir salt dome simulation of the us cowboy series of chemical explosions in a louisiana salt mine are also shown to be in very good agreement with the experimental data however the constitutive model for the salt that best explains these data is different from that derived for salmon both salt models are amply supported by laboratory and field data the main result of these simulations is that cavity decoupling with chemical explosives is much less efficient than with nuclear explosives although maximum decoupling factors f sub max near 200 may be attainable with either of the two sources the cavity size required to achievemore this value appears to be 40 m kt sup 1 3 for cavity radii half as large f sub max is roughly 4 times lower with nuclear explosives and lower by another factor of 4 with chemical sources moreover if the initial cavity radius is a more modest 10 m kt sup 1 3 f sub max 3 even with a nuclear source 24 refs 8 figs less

effects of rock damage on seismic waves generated by explosions in studying the physical processes involved in the generation of seismic waves by explosions it is important to understand what happens in the region of high stresses immediately surrounding the explosion this paper examines one of the processes that takes place in this region the growth of pre existing cracks which is described quantitatively as an increase in rock damage an equivalent elastic method is used to approximate the stress field surrounding the explosion and a micro mechanical model of damage is used to calculate the increase in damage simulations for a 1 kt explosion reveal that the region of increased damage can be quite large up to ten times the cavity radius the damage is initiated on a damage front that propagates outward behind the explosive stress wave with a velocity intermediate between that of p and s waves calculations suggest that the amount of increased damage is controlled primarily by the initial damage and the extent of the region of increased damage is controlled primarily by the initial crack radius the motions that occur on individual cracks when damage increases are converted to seismic moment tensors which are then used to calculate secondary elastic waves which radiate into the far field it is found that while the contribution from an individual crack is small the combined effect of many cracks in a large region of increased damage can generate secondary waves that are comparable in amplitude to the primary waves generated by the explosion provided that there is asymmetry in the damage pattern this process is quite effective in generating s waves thus providing a quantitative explanation of how s waves can be generated by an explosion two types of asymmetry are investigated a shear pre stress and a preferred orientation of cracks and it is found that both produce similar effects

a high resolution continuous δ13c record spanning the ordovician silurian boundary on anticosti island eastern canada1 one of the best exposed and most complete stratigraphic records from paleotropical areas spanning the ordovician silurian o s boundary is located on anticosti island eastern canada our study is

first record of the hirnantian upper ordovician δ13c excursion in the north american midcontinent and its regional implications the most prominent of the two major global delta c 13 excursions in the ordovician the hirnantian delta c 13 excursion hice which is previously recorded from the uppermost ordovician in a few sections in nevada quebec arctic canada baltoscandia scotland and china is documented for the first time from the north american midcontinent samples through the girardeau limestone and leemon formation in missouri and illinois show elevated delta c 13 values of 4 parts per thousand to 5 parts per thousand although not determined precisely the beginning of the hice is likely to be in the upper part of the orchard creek shale and it ends in the upper leemon formation being extraordinarily useful chronostratigraphically the presence of the hice makes it possible to provide a firm dating of the study interval whose age has long been controversial comparison between the study sections and coeval hice sequences in north america and europe show striking similarities especially in sea level history indicating that major local lowstands reflect eustatic sea level changes a comparison with hirnantian diamictite successions in north and south africa and argentina suggests that these lowstands correspond to two major gondwanan glacial episodes less

sea level curve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america at least 55 cycles of marine inundation and withdrawal are recognized in the mid desmoinesian to mid virgilian midcontinent outcrop sequence in north america they range from widespread major cycles classic cyclothems with deep water facies extending across the northern shelf through intermediate cycles persisting as marine horizons across the shelf to minor cycles developed on the lower shelf or as parts of major cycles biostratigraphic differentiation of the cycles should establish interbasinal correlation on a scale fine enough to allow evaluation of differential tectonics and sedimentation sequential groupings of cycles are more irregular than proposed megacyclothems or mesothems but they may be obscured by the distinctness of the major cyclothems estimates of cycle periods range from about 40 to 120 x 10 sup 3 yr for the minor cycles up to about 235 to 400 x 10 sup 3 yr for the major cyclothems the range for all cycles corresponds well to the range of periods of earth s orbital parameters that constitute the milankovitch insolation theory for the pleistocene ice ages and it further supports gondwanan glacial control for the pennsylvanian cycles even though the dominant period of the major pennsylvanian cyclothems is up to four times longer than the dominant 100 000 yr period inmore the pleistocene the shapes of both curves display rapid marine transgression rapid melting of ice caps and slow interrupted regression slow buildup of ice caps which suggest similar linkages between the climatic effects of the orbital parameters and ice cap formation and melting at the two different scales widely separated in time 28 references 3 figures less

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

geochemistry and geotectonic interpretation of granitoids from central iforas mali w africa abstract late proterozoic granitoids from central iforas mali w africa show a large range of chemical variations e g sio 2 52 8 75 6 and include diorites monzotonalites tonalites granodiorites and granites the major and trace element composition of these rocks is comparable to modern calc alkaline rocks of continental margins which were emplaced far from the trench in an area with thick continental crust the chemical variations were produced by crustal contamination and by fractional crystallization dominated by hornblende and plagioclase the granitoids are also similar in composition to pan african orogenic volcanic rocks of the hoggar geodynamic interpretations of the adrar des iforas belt are discussed in the light of these data

episodic sedimentation on an early silurian storm dominated carbonate ramp becscie and merrimack formations anticosti island canada the 150 160 m thick lowermost silurian rhuddanian becscie and merrimack formations of anticosti island canada represent continuous deposition on a shallow open marine carbonate ramp several rock types are identified a laminated and homogenous mudstone b laminated and homogenous packstone c argillaceous mudstone and packstone d calcareous shale e laminated calcisiltite f medium to fine grained grainstone and g bio intraclastic rudstone these rock types are arranged into five distinct lithofacies lf1 calcareous mudstone shale lf2 laminated homogenous mudstone lf3 calcareous grainstone shale lf4 laminated mudstone grainstone and lf5 laminated calcisiltite grainstone the sequence reflects deposition on a low energy muddy carbonate to argillaceous ramp subject to short lived episodic high energy storms these events produced fining upwards storm units 5 80 cm thick or tempestites consisting of a sharp scoured base overlain by intra bioclastic rudstone grading upwards into medium grained grainstone finely laminated calcisiltite and mudstone or shale these are interbedded with low energy fairweather mudstones and calcareous shales deposition progressed from a carbonate mud dominated ramp in the becscie formation to an argillaceous mud dominated ramp in the merrimack formation lateral tempestite proximality trends and lithofacies distribution indicate that the anticosti basin deepened to the south east into the iapetus ocean and shallowed towards a sw ne orientated shoreline to the north west vertical tempestite proximality trends and lithofacies changes identify third order eustatic sea level changes after an initial deepening at the base of the formation a shallowing deepening event dominated the sequence several higher order fluctuations defined by lithofacies and tempestite proximality trends are superimposed on these changes the fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods

structure and u pb geochronology of central hoggar algeria a reappraisal of its pan african evolution discovery of large scale deep seated thrusts in central hoggar with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation has led us to reconsider the pan african tectonic and metamorphic history in that region two areas are described and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust the age and kinematics of this structural reworking have been approached using u pb zircon dating in the tamanrasset region despite the difficulty of estimating the age of the initiation of the assumed intracontinental a type subduction the results provide a time span of 30 40 m y between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes some key ages were determined 1 2075 30 ma is the age given by granulite facies remnants which escaped from the refoliation the corresponding lower intercept at 530 70 ma confirms the pan african imprint 2 615 5 ma reflecting the age of syntectonic to late tectonic granitoids emplaced in reworked gneisses and in preserved granulites 3 580 ma the concordant age of sphenes and monazites from the same granitoids which is interpreted as corresponding to the end of medium grade conditions no evidence has been found of a 1000 ma age a kibaran event does not appear to exist in central hoggar the age similarity between the observed deep intracontinental evolution of central hoggar and the collision related tectonics of western hoggar and iforas suggests a common origin for both phenomena

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

stratigraphy and structure of the laurentian rifted margin in the northern appalachians a low angle detachment rift system the neoproterozoic early cambrian rifted margin of eastern laurentia is framed by promontories and embayments defined by northeast trending rifts offset by northwest trending transforms a regional first order synthesis of the available stratigraphic data in the northern appalachians reveals significant along strike variations in the thickness composition age and facies of synrift and early postrift stratigraphy these variations are consistent with models for low angle detachment rift systems and allow for resolution of regional structures specific to low angle detachments including upper plate margins lower plate margins and transform faults that bound zones of oppositely dipping low angle detachments

high precision u pb zircon age calibration of the global carboniferous time scale and milankovitch band cyclicity in the donets basin eastern ukraine 1 high precision id tims u pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic cyclostratigraphic and biostratigraphic framework of the carboniferous donets basin of eastern europe chemical abrasion of zircons use of the internationally calibrated earthtime mixed u pb isotope dilution tracer and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of 0 05 or 100 ka for these carboniferous volcanics this precision allows the resolution of time in the milankovitch band and confirms the long standing hypothesis that individual high frequency pennsylvanian cyclothems and bundles of cyclothems into fourth order sequences are the eustatic response to orbital eccentricity 100 and 400 ka forcing tuning of the fourth order sequences in the donets basin to the long period eccentricity cycle results in a continuous age model for the middle to late pennsylvanian moscovian kasimovian ghzelian strata of the basin and their record of biological and climatic changes through the latter portion of the late paleozoic ice age detailed fusulinid and conodont zonations allow the export of this age model to sections throughout euramerica additional ages for mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations

paleogeography of the matapedia basin in the gaspe appalachians initiation of the gaspe belt successor basin the matapedia basin consists of the uppermost ordovician lowermost silurian deep water fine grained carbonate siliciclastic rocks of the honorat garin formation and matapedia groups pabos and white head formations the lower rock assemblage of the gaspe belt in the gaspe appalachians paleogeographic maps of eight time slices from the caradocian to the llandoverian are presented to better understand the tectonosedimentary evolution of the matapedia basin deposition evolved from siliciclastic garin fm to argillaceous limestones pabos fm to limestones white head fm the overall change from terrigenous garin fm to limestone facies white head fm reflects a change in the source area paleocurrent directions and composition of sandstones indicate an orogenic source area to the south for the garin formation which is believed to be the inliers of the humber and dunnage zones in the southern gaspe and new brunswick appalachians lime muds deposited by turbidity currents coming from the nor

astronomically forced cyclicity in the upper ordovician and u pb ages of interlayered tephra oslo region norway abstract the late ordovician world experienced a series of huge volcanic eruptions recorded as the big deicke millbrig and kinnekulle bentonites together with numerous thinner beds the kinnekulle event can be traced across northwestern europe u pb zircon ages are here presented of both the kinnekulle k bentonite and the uppermost recorded tephra layer in the upper ordovician of the oslo region norway the tephras are located in the upper part of the arnestad formation sandbian south of oslo and gave ages of 454 52 0 50 ma the kinnekulle k bentonite and 453 91 0 37 ma the upper grimstorp k bentonite the dated tephras are separated by a 7 m thick shale succession with subordinate nodular limestone beds although the two u pb ages overlap within errors statistical considerations indicate a most likely time interval of about 600 kyr between the tephras high resolution magnetic susceptibility logging in the same section shows cycles that likely represent changes in sediment supply in response to astronomical forcing spectral analysis shows the presence of long 400 kyr and short 100 kyr eccentricity bands and obliquity components in the 30 kyr band precessional cycles are not detected based on this method it is possible to estimate a time interval of 766 kyr between the two tephra events and the radiometric dating therefore does not exclude a milankovitch interpretation of the cycles this opens new possibilities for understanding the evolution of one of the world s best preserved ordovician marine systems

cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences over the past 25 yr the science of stratigraphy has evolved to include time correlative data from vastly disparate components of the earth system not least of these is the global signal afforded by cyclostratigraphy which has recorded the evolution of earth s astronomical milankovitch forcing of insolation and the paleoclimate system fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies geochemistry mineralogy rock magnetism color etc in step with the documentation of astronomically forced paleoclimate from ever older older geologic times innovations in computational science have provided ever longer high accuracy astronomical model targets that can be used for time scale calibration the earth s orbit is affected by motions of other planets notably the orbital perihelia of venus and jupiter which impose a dominant 405 k y eccentricity cycle on earth s orbital evolution the large mass of jupiter stabilizes this cycle over hundreds of millions of years the cyclostratigraphic record of 405 k y cycles is therefore often used to correct chronologies affected by variable sedimentation earth s shape and rotation rate are influenced by tidal dissipation and climate friction these effects affect earth s precession rate through time thus a record of earth moon evolution is also embedded in cyclostratigraphy the geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high precision radioisotope dating which today has the potential to define the ages of stratigraphic horizons with 2 uncertainties at the scale of a precession cycle precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca 50 ma limit of accurate astronomical solutions and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time finally space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of mars with patterns suggestive of astronomical forcing analogous to earth

oxygen and carbon isotopes and event stratigraphy near the ordovician silurian boundary anticosti island quebec the growth and destruction of continental ice sheets in north africa and adjacent parts of gondwana during the hirnantian stage had profound influence on the sedimentary style of mixed carbonate siliciclastic sequences on anticosti island quebec canada these strata deposited in a tropical mid shelf setting show direct evidence of local sea level changes of up to 40 m after correction for tectonic and load induced subsidence this indicates maximum global glacio eustatic changes of 32 to 36 m this implies maximum ice volume of about 12 to 14 106 km3 and related changes in the oxygen isotopic composition of global sea water amounting to as little as 0 5 to 0 6 given that the oxygen isotope ratios in syn glacial samples from the anticosti sequence range from 8 9 to 2 8 18o it is not possible to clearly distinguish the effects of ice cap development from temperature and diagenetic controlled variation the strong negative shift in 13c values at the top of the ellis bay formation may indicate a pronounced increase in atmospheric co2 coincident with destruction of the late ordovician ice caps

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

tempestite frequency curves a key to late ordovician and early silurian subsidence sea level change and orbital forcing in the anticosti foreland basin quebec canada following partial closure of the northern iapetus ocean along the newfoundland segment of the st lawrence promontory subsidence along the anticosti platform was influenced by residual thermal subsidence renewed tectonic loading by thrust sheets to the south and sediment loading basement subsidence calculated by removing the effects of sediment loading was between 2 and 5 cm ka in the caradocian increasing to 8 6 cm ka in the pusgillian and cautleyan and reaching a maximum of 17 7 cm ka in the rawtheyan during deposition of the princeton lake and vaureal formations a marked decline in subsidence beginning in the hirnantian and continuing into the early silurian may reflect decoupling of thrust loads to the south although a further stage of thrust loading may have occurred in the aeronian during deposition of the lower part of the jupiter formation storm frequency curves produced using tempestite abundance thickness and grain size through more than a kilometre of carbonate strata on antico

a paleotropical carbonate dominated archive of carboniferous icehouse dynamics bird spring fm southern great basin usa abstract much of our far field knowledge of glaciation and climate during the late paleozoic ice age is built on decades of study of mixed carbonate siliciclastic cyclothemic successions from paleotropical euramerica far fewer carbonate dominated successions have been studied despite their high sensitivity to changes in accommodation space and environmental conditions here we present a sequence stratigraphic framework for the carbonate dominated bird spring formation southern great basin u s a and infer from it a relative sea level history for the latest mississippian through latest pennsylvanian correlation of four successions across the paleo platform documents changes in the lithofacies composition of meter scale cycles their cycle stacking patterns and cycle bounding surfaces that record substantial variation in relative sea level at several temporal scales during the pennsylvanian ten genetic sequences bounded by intervals of restricted peritidal lithofacies karst terra rossa rooted caliche and or protosols are recognized within sequences 126 parasequences stack into parasequence sets that in turn define lowstand transgressive and highstand systems tracts the stratigraphic cycle stacking patterns and across platform architecture of systems tracts suggest a mid carboniferous lowstand in relative sea level followed by a middle morrowan middle bashkirian highstand as previously suggested bishop et al 2010 subsequent progradation of the platform occurred during a gradual long term fall through the late morrowan and atokan late bashkirian early moscovian upon which repeated higher frequency events were superimposed culminating in widespread exposure across the platform proximal to the atokan desmoinesian boundary long term sea level increased into the mid desmoinesian end moscovian after which progradation late desmoinesian and aggradation missourian virgilian of shallow water and peritidal carbonates occurred across the bird spring platform the progradational to aggradational phase involved an abrupt shift from ramp to rimmed shelf geometry in the missourian kasimovian likely in response to progressively increasing sea level dampened high frequency sea level fluctuations and or to regional faulting along the bird spring platform margin inferred magnitudes of short term 105 yr fluctuations superimposed on the longer term rises and falls range from 20 to 70 m during the early to mid pennsylvanian and are substantially reduced 20 m during the late pennsylvanian the early to middle pennsylvanian sea level history inferred from the carbonate dominated bird spring platform corresponds well with a recently published pennsylvanian onlap offlap history defined from the cyclothemic succession of the donets basin and with independent near field reconstructions of glaciation during the pennsylvanian

the timedjealen alkaline ring complex and related n 1bs dyke swarms adrar des iforas mali a pb 1bsr 1bo isotopic study abstract the iforas alkaline province is composed of spectacular n s trending acid dyke swarms plateaux of ignimbritic and rhyolitic lavas and silica oversaturated ring complexes this province was emplaced just after the pan african collision which occurred between the tuareg shield and the west african craton 600 ma ago it is devoid of sn mineralization in comparison with similar paleozoic mesozoic complexes in niger and nigeria the pb 1bsr 1bo data on the timedjelalen ring complex and on contemporaneous n s dykes demonstrate the cogenetic mantle source for their parental magma with a contaimination by the lower continental crust from 10 to 50 on the basis of pb isotopes there is no evidence for incorporation of old upper continental crust these alkaline rocks show little alteration except for hydrothermal activity at ring or dyke country rock contacts as shown by lower 18o values the hydrothermal water involved appears isotopically heavy in the range 0 to 3 and may have originated in a residual sea or lake lack of mineralization can be explained by the fact that the primary source is economically barren mantle and lower crust and that the interaction with country rocks during the intrusion process was weak and mainly with barren mantle derived pan african rocks

inverting ground motion from a seismometer array to obtain the vertical component of rotation a test using data from explosions to study factors that affect reliability of the derived vertical rotational ground motions about vertical axis from translational array waveforms we have conducted a field test using explosion the dataset was collected on march 4 2008 from an active source experiment of the taiwan geodynamics research project taiger 2008 in which 750 kg of explosives were set off about 500 m away from a specially designed seismic array in free field consisting of 8 tri axial rotational sensors 13 tri axial accelerometers and 12 six channel 24 bit dataloggers with gps receivers our results show that the waveform fits were greatly improved when the explosion induced crustal strains were less than 2 10 5 for our station configuration as a result our modeling for the large amplitude part of the mainshock was not entirely successful however we have successfully recovered the first order features of the later part of the vertical rotational ground motions using translational velocity waveforms in the 0 5 20 hz bandwidth using the spudich and fletcher s procedure 2009 moreover we used a modified jaeger s 1969 formula and derived similar rotational synthetics we found that the best rotational rate waveform fits were derived using a station configuration of a dense small aperture translational array combined with a few largeoffset stations a small aperture array is needed to fulfill the uniform rotation assumption however inverting data from an array of small spatial dimension requires waveforms of high signal to noise ratio and high sampling rates surprisingly large offset translation stations provided additional constraints in time and space helping to fit the low frequency waveforms using the inverted result we forward predicted the vertical translational waveforms not used in the inversion and got excellent fits this suggests that the inverted results are of good quality for predicting the complete translational ground motions even though only horizontal components were used in the inversion array derived or directly measured rotational ground motions can be used to predict translational ground motions in the near by region lessons learned from this study could be helpful for future studies using translational ground motions to derive dynamic ground strains tilts and torsions

phytoplankton dynamics across the ordovician silurian boundary at low palaeolatitudes correlations with carbon isotopic and glacial events the late ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the phanerozoic during this interval anticosti island quebec eastern canada was located at low to intermediate palaeolatitudes 15 30 s on the eastern margin of laurentia it displays a relatively complete section across the ordovician silurian o s boundary upper ordovician to lower silurian strata of anticosti studied here comprise the vaureal katian ellis bay hirnantian and becscie latest hirnantian rhuddanian formations phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies 13c with recent palynological data from baltica and also with phytoplankton dynamics as described from near polar high latitude localities of the gondwana region two positive excursions of the carbon isotope record have been identified in the lower and upper ellis bay formation overall phytoplankton trends are as follows 1 as with chitinozoans conodonts brachiopods and corals acritarchs from anticosti display a major turn over during the early hirnantian uppermost vaureal formation lower ellis bay formation with the appearance of taxa that exhibit silurian affinities e g tylotopalla sp ammonidium sp oppilatala sp evittia sp dilatisphaera spp high polymorphism for some common taxa is observed in the interval corresponding to the first 13c positive excursion using sequence stratigraphy this period may be related to a first glacially driven sea level drawdown corresponding to terrestrial ice sheet growth on gondwana such as recorded in morocco 2 subsequently during the late hirnantian of anticosti uppermost ellis bay formation phytoplanktic communities suffered a crisis exemplified by the disappearance of several typical large late ordovician taxa i e orthosphaeridium spp peteinosphaeridium spp large baltisphaeridum spp sacculidium sp the lower becscie strata corresponding to the time of deglaciation at the o s boundary interval only contain a low diversity assemblage dominated by large acritarchs of the genus hoegklintia and other morphologically related forms and a few tolerant and long ranging taxa e g evittia remota micrhystridium spp leiospheres despite a possible taphonomic bias in the uppermost ellis bay formation the major crisis in the late hirnantian coincides with the second 13c excursion and a second stronger glacially driven sea level fall corresponding to the time of the maximum extend of the ice sheets on gondwana 3 such phytoplankton events have also been detected in gondwanan ice center area but with typical ordovician taxa crises occurring apparently later when plotted against positive carbon isotope excursions as also observed in baltica this time lag may be linked to a basin configuration of the studied areas where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the uppermost ordovician strata of anticosti in addition to their biostratigraphic and palaeobiogeographic utility acritarchs turn out to be more specific in highlighting palaeoenvironmental conditions

chemical stabilization of low mg calcite an example of brachiopods abstract trace element study of brachiopod shells from the upper ordovician ellis bay formation anticosti island quebec canada shows that they underwent only a slight 20 diagenetic equilibration with meteoric waters despite 450 million years of postdepositional history this in contrast to large trace element repartitioning observed in their enclosing rocks attests to the relative stability of low mg calcite even within the realm of meteoric waters paleozoic brachiopods apparently exerted a biological control on the distribution of na and mg in their low mg calcitic shells preferentially incorporating the former and discriminating against the latter trace constituent

time relationships between magmatism tectonics and metamorphism in three plutonic belts in southern tibet new k ar data from north to south the three plutonic belts in southern tibet are the transhimalaya or kangdese the north himalaya or lhagoi kangri and the high himalaya 25 new mineral and whole rock k ar dates added to previous data enable a precise analysis to be made of the time relationships between different magmatic tectonic and metamorphic events that occurred on both sides of the indus tsangpo suture zone these include 1 the emplacement of a large part of the eastern transhimalaya magmatism occurred during eocene time suggesting a close relationship between this emplacement and the late stages of conver gence between india and eurasia 2 south of the suture the ages obtained fall mainly between 11 and 15 ma the greatest age allows us to fix a lower limit on the timing of metamorphic recrystallization of the paleozoic north himalaya granites an intermediate age may correspond to a minimum age of emplacement of the high himalaya and part of the north himalaya cenozoic granitoids finally the youngest age may characterize the northernmost cenozoic granitoids of the north himalaya belt the consistency of these tenuous age variations with field and structural data suggests that they may be significant 3 our determinations give ages often significantly younger than those usually proposed for the same events as suggested by some authors such a discrepancy might reflect relatively late india eurasia collision in this eastern part of the suture

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse our new data address the paradox of late ordovician glaciation under supposedly high pco2 8 to 22 pal preindustrial atmospheric level the paleobiogeographical distribution of chitinozoan mixed layer marine zooplankton biotopes for the hirnantian glacial maximum 440 ma are reconstructed and compared to those from the sandbian 460 ma they demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the polar front through time from 55 70 s to 40 s these changes are comparable to those during pleistocene interglacial glacial cycles in comparison with the pleistocene we hypothesize a significant decline in mean global temperature from the sandbian to hirnantian proportional with a fall in pco2 from a modeled sandbian level of 8 pal to 5 pal during the hirnantian our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end ordovician mass extinction

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

oxygen and carbon isotopic composition of silurian brachiopods implications for coeval seawater and glaciations we collected 236 calcitic brachiopod shells covering the entire silurian period 30 m y at high temporal resolution from stratotype sections from anticosti island canada wales united kingdom the oslo region norway gotland sweden and podolia ukraine estonia latvia and lithuania data from petrography scanning electron microscopy cathodoluminescence isotopes and trace elements all confirmed that there was excellent preservation in most shells thus arguing for retention of primary isotope signals exceptions were samples from the oslo region the d18o and d13c values for the well preserved samples range from 2 to 6 5 and from 1 to 7 5 peedee belemnite respectively in terms of temporal trends oxygen and carbon isotopes vary in parallel with a slight decrease with declining age of 1 through silurian time with superimposed short term oscillations that are negatively correlated with sea level changes three successive positive d18o shifts in early aeronian latest aeronian and early wenlock time correlate with sea level lowstands and with glacial diamictite deposits in the amazon basin and in africa the high d18o and d13c values are attributed to cold episodes with low sea levels and low values to warm episodes with high sea levels during warm silurian episodes the d18o value of seawater is suggested to have been about 3 5 standard mean ocean water smow and the global tropical temperatures of about 20 30 c similar to the present day values in summer during glacial episodes seawater is proposed to have had a d18o value of 2 5 smow and temperature of about 14 23 c comparable to tropical temperatures proposed for the last glacial episode 14 to 28 c three positive d13c peaks in early and late wenlock and late ludlow time likely of regional to global significance appear to coincide with sea level lowstands but we are as yet unable to propose a convincing causative geologic scenario that would explain their origin

terminal ordovician carbon isotope stratigraphy and glacioeustatic sea level change across anticosti island quebec canada globally documented carbon isotope excursions provide time varying signals that can be used for high resolution stratigraphic correlation we report detailed inorganic and organic carbon isotope curves from carbonate rocks of the ellis bay and becscie formations spanning the ordovician silurian boundary on anticosti island quebec canada strata of the anticosti basin record the development of a storm dominated tropical carbonate ramp these strata host the well known hirnantian positive carbon isotope excursion which attains maximum values of 4 5 in carbonate carbon of the laframboise member or the fox point member of the becscie formation the excursion also occurs in organic carbon and 13 c carb and 13 c org values covary such that no reproducible 13 c 13 c carb 13 c org excursion is observed the most complete stratigraphic section at laframboise point in the west shows the characteristic shape of the hirnantian stage excursion at the global stratotype section and point gssp for the hirnantian stage in china and the silurian system in scotland we therefore suggest that the entire hirnantian stage on anticosti island is confined to the laframboise and lower fox point members by documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin we are able to reconstruct glacioeustatic sea level fluctuations corresponding to maximum glacial conditions associated with the end ordovician ice age the combined litho and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and becscie limestones the former transgresses from west to east and the latter progrades from east to west the sea level curve consistent with our sequence stratigraphic model indicates that glacioeustatic sea level changes and the positive carbon isotope excursion were not perfectly coupled although the start of the isotope excursion and the initial sea level drawdown were coincident the peak of the isotope excursion did not occur until after sea level had begun to rise carbon isotope values did not return to baseline until well after the anticosti ramp was reflooded the sea level 13ccarb relationship proposed here is consistent with the weathering hypothesis for the origin of the hirnantian 13 c carb excursion

refining 2 km of ordovician chronostratigraphy beneath anticosti island utilizing integrated chemostratigraphy new high resolution chemostratigraphy in combination with updated biostratigraphy refines the chronostratigraphic resolution of the nearly 2 km thick ordovician section below anticosti island a total of 1414 horizons spaced at 1 0 1 5 m intervals were sampled from the new associated consolidated paper nacp drill core and analyzed for major and trace elemental composition by portable x ray fluorescence analyzer pxrf select micrite and calcareous shale powders were then analyzed for 13ccarb number of samples n 364 and 87sr 86sr n 25 our results indicate a floian to early darriwilian f3 dw1 age for the romaine formation a middle darriwilian to sandbian dw2 sa1 age for the mingan formation a lower to mid katian ka1 ka2 age for the macasty formation an upper katian ka3 age for the lower vaureal formation an uppermost katian ka4 age for the upper vaureal and a hirnantian h1 age for the ellis bay formation this integrated chemostratigraphic and biostratigraphic synthesis es

les phases tardives de l orogenese pan africaine dans l adrar des iforas oriental mali lithostratigraphie des formations molassiques et geochronologie u pb sur zircon de deux massifs intrusifs two main tectono metamorphic domains are distinguished on both sides of the adrar fault adrar des iforas mali the western domain is mainly made up of retrogressed eburnean granulites and various granitoids the ibedouyen quartz diorite pluton which postdates the major tectono metamorphic event responsible for the mylonitisation of granulites gives a u pb age on zircon of 600 2ma a figure which is interpreted as its magmatic crystallisation the molassic series of the adrar fault unconformably rest on this unit the eastern domain includes 1 a highly deformed molassic series recrystallised under greenschist facies which we correlate with the unmetamorphic molassic series of the western domain 2 a pelitic unit affected by a high temperature low pressure type metamorphism and cut by several granites the tamassahart massif which has intruded this pelitic unit before the metamorphism gives a u pb zircon age of 581 67 ma this figure is also interpreted as that of the magmatic crystallisation these two new geochronologic data are thus in the range of the previously obtained data in other parts of the pan african belt from west africa they confirm that the eastern domain is a slightly younger pan african province

seismic source theory in stratified anisotropic media we extend seismic source theory to general anisotropic media for the numerical evaluation of spectral amplitudes of point sources in multilayered anisotropic crustal models first we obtain an explicit representation of the spectral elastodynamic green s tensor in general homogeneous anisotropic media as a sum of three integrals over the corresponding three slowness surfaces the multidimensional stationary phase principle is then applied to derive an asymptotic approximation at the far field in the case of azimuthally isotropic media we offer an alternative representation of the green s tensor and its ensuing displacements fields in the form of an exact hankel transform over the horizontal wave number variable the total field is specified here in terms of two potentials an sh potential and a mixed quasi transverse quasi longitudinal potential both of which assume the role of two scalar green s functions the availability of the green s tensor in analytical form enables one to obtain readily numerical solutions for a wide selection of media and sources it is shown that the radiation field of an explosion has the following new features 1 quasi transverse waves are created with four and eight lobe patterns 2 quasi longitudinal waves are generated for the collatitudinal displacement with four lobe patterns 3 the energy ratio sv p may reach the value of 20 for more than 50 of the azimuths in crustal structures such as tuff and shales and 4 radiation patterns for vertical shear waves are created which are indistinguishable from corresponding waves produced by earthquake faults our formalism allows us to establish a haskell type matrix algorithm for a multilayered azimuthally isotropic half space which enables us to calculate body waves and surface waves in real earth crustal models

long lived glaciation in the late ordovician isotopic and sequence stratigraphic evidence from western laurentia the timing and causes of the transition to an icehouse climate in the late ordovician are controversial results of an integrated 13c and sequence stratigraphic analysis in nevada show that in the late ordovician chatfieldian stage mid caradoc a positive 13c excursion in the upper part of the copenhagen formation was closely followed by a regressive event evidenced within the prominent eureka quartzite the chatfieldian 13c excursion is known globally and interpreted to record enhanced organic carbon burial which lowered atmospheric p co2 to levels near the threshold for ice buildup in the ordovician greenhouse climate the subsequent regressive event in central nevada previously interpreted as part of a regional tectonic adjustment is here attributed in part to sea level drawdown from the initiation of continental glaciation on gondwana this drop in sea level which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p co2 by suppressing shelf carbonate production signals the transition to a late ordovician icehouse climate 10 m y before the widespread hirnantian glacial maximum at the end of the ordovician

late pan african tectonics marking the transition from subduction related calc alkaline magmatism to within plate alkaline granitoids adrar des iforas mali boullier a m liegeois j p black r fabre j sauvage m and bertrand j m 1986 late pan african tectonics marking the transition from subduction related talc alkaline magmatism to within plate alkaline granitoids adrar des iforas mali in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 233 246 the pan african trans saharan belt resulting from oceanic closure and oblique collision between the west african craton and the tuareg nigerian shields around 600 ma ago displays in the iforas a rapid switch from subduction and collision related talc alkaline magmatism to typical a type granitoids 560 540 ma the transition is clearly seen in associated spectacular acid dyke swarms early talc alkaline e w trending sets being cut by alkaline n s trending arrays isotopic evidence shows that the change in chemistry involves a different more primitive mantle source structural observations indicate that the transition from talc alkaline to alkaline magmatism is related to reversals in the stress field during and after collision in the model proposed the first major reversal from d with ei ca 13s0n lloon to ds with ui ca 50 n 60 n is thought to have been responsible for the slicing of the subducted plate thereby allowing the rise to shallow depth beneath the continental lithosphere of asthenospheric mantle believed to be the source of the alkaline magmatism the tapping of this new source occurred between ds and d4 events contemporaneous with a rotation of the maximum compressive stress back from 60 n to 105 n

chitinozoan biostratigraphy of a new upper ordovician stratigraphic framework for anticosti island canada thickness and facies variations in the upper ordovician succession of anticosti island have made correlations between the western and eastern ends of the island difficult this chitinozoan study reveals that most strata of the ellis bay formation at the eastern end of the island correlate with the vaureal formation at the western end in the eastern end chitinozoans typical of the ellis bay formation occur only in the upper part of the lousy cove member whereas chitinozoans diagnostic of the mid upper vaureal occur in the rest of the formation moreover the eastern schmitt creek and mill bay members of the vaureal formation are older than the uppermost vaureal eponym members in the western end new east west correlations supported by geochemical sedimentological and sequence stratigraphic data are proposed comparison with coeval upper ordovician successions suggests a hirnantian age for the gamachiana zone that occurs in the lower members of the ellis bay formation it also indicates that the hirnantian isotopic carbon excursion hice on anticosti begins in the d anceps zone just below the base of the hirnantian stage reaches a first peak in the lower n extraordinarius zone a second modest peak high in the n extraordinarius zone and a third more important one in the lower part of the n persculptus zone these results are in agreement with the biostratigraphic and chemostratigraphic data at the global boundary stratotype section and points gssps for the base of the hirnantian and silurian in south central china and at dob9s linn in scotland

propagation velocity of shockwaves and pressure waves in soil and rock a variable pressure wave propagation velocity is developed and analyzed in ground materials which may be characterized by an exponential density pressure relationship the peak overpressure dependence is minor in the low pressure range and the propagation velocity of the pressure wave is essentially a constant in fact it is the seismic p wave propagation velocity pressure dependence becomes increasingly more significant at peak overpressures exceeding 1 kbar 108 pa at very high pressure levels encountered only in shockwaves at and above 10 kbar 109 pa the initial mechanical characteristics of the ground materials become less significant and the propagation velocity depends primarily upon the peak overpressure

two dimensional array measurements of near source ground accelerations abstract two dimensional array measurements of near source accelerations have been conducted 6 km from the 5 6 m l explosion colwick at pahute mesa nevada test site the three component nine station array measured 400 m across with interstation spacing of 100 m wavenumber spectra and beam forming stacking were used to measure apparent velocities and directions of propagation of acceleration waveforms the robustness of individual station accelerograms was measured with frequency domain coherency estimates and cross correlation functions the dependence of interstation coherency upon station spacing and frequency band was also examined wave propagation in pahute mesa can be characterized by p sv and sh wave motion in a horizontally homogeneous medium for frequencies below 5 hz strong incoherent scattered signals are present above 5 hz on all components the fraction of scattered energy can be estimated and treated as signal generated noise a strong sh pulse was observed to propagate across the array measured phase velocity arrival time and direction of propagation require cogeneration of sh with p sv energy by the explosion colwick weaker and more diffuse transverse acceleration signals arrive coincident with the p wave and during the time prior to s these arrivals are interpreted as forward scattered energy from the p wave the small array permitted a refinement of the horizontally averaged s wave velocity model for pahute mesa the mesa is characterized by a low velocity layer 600 m thick above the water table with a steep velocity gradient immediately below the coherency measurements indicate the upper 1 km or more of pahute mesa is heterogeneous at scale lengths less than 1 km

end ordovician extinctions a coincidence of causes the end ordovician hirnantian extinction was the first of the five big phanerozoic extinction events and the first that involved metazoan based communities it comprised two discrete pulses both linked in different ways to an intense but short lived glaciation at the south pole the first occurring at or just below the normalograptus extraordinarius graptolite biozone mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second within the n persculptus graptolite biozone was less focused eradicating faunas across a range of water depths in all about 85 of marine species were removed proposed kill mechanisms for the first phase have included glacially induced cooling falling sea level and chemical recycling in the oceans but a general consensus is lacking the second phase is more clearly linked to near global anoxia associated with a marked transgression during the late hirnantian most recently however new drivers for the extinctions have been proposed including widespread euxinia together with habitat destruction caused by plate tectonic movements suggesting that the end ordovician mass extinctions were a product of the coincidence of a number of contributing factors moreover when the deteriorating climate intensified causing widespread glaciation a tipping point was reached resulting in catastrophe

the stratigraphic and geochemical imprints of late ordovician glaciation on far field neritic carbonates anticosti island eastern canada abstract the storm dominated open marine neritic carbonate facies continuously exposed in coastal cliffs and wide rocky tidal flat exposures at the western end of anticosti island form a 315 m thick expanded section spanning the ordovician silurian boundary these carbonate facies display deepening shallowing trends of different orders resulting in a hierarchical stacking of sedimentary cycles the carbonate facies within these multi order cycles represent sediments that were deposited from shoreline to deeper outer carbonate ramp 80 m in water depth and exhibit sea level changes of several tens of metres in magnitude field petrographic and geochemical evidence support a well preserved primary 18o signal in close equilibrium with the original paleoenvironmental conditions the 18o intracycle variations up to 1 0 in magnitude reveal repeated decreasing and increasing values during the transgressive and regressive portions of decametre tr cycles respectively although temperature salinity and ice volume influenced the recorded primary 18o signal orbital scale glacioeustasy ultimately controlled facies and cycle development and 18o signal during the katian immediately before the latest ordovician hirnantian glaciation the 3 5 myr long 18o signal recorded by our entire succession supports a protracted period of cooling leading into the main phase of latest ordovician hirnantian glaciation spectral analysis of the 18o record also reveals decametre variations which are associated with decametre scale transgressive regressive tr cycles possibly reflecting eccentricity orbital signals our study shows the importance of a thick expanded stratigraphic succession sampled at the highest resolution possible to decipher potential linkages between facies changes multi order stratigraphic cycles 18o signal and the earth s orbital parameters