precambrian accretionary tectonics in the bou azzer el graara region anti atlas morocco ophiolites of the anti atlas of morocco occur as highly dismembered slivers of mafic and ultramafic rocks sandwiched between a precambrian craton and a magmatic arc tectonic slices of accretionary melange ophiolites and forearc basins have been juxtaposed by oblique subduction and transpression in the forearc region of a late proterozoic subduction zone synkinematic to postkinematic calc alkalic magmatism in these terranes indicates migration of the magmatic front over the deformed forearc terranes

marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

δ18o values of coexisting brachiopods and fish temperature differences and estimates of paleo water depths to estimate vertical thermal gradients and paleo water depths to marine platforms we present a new method based on the difference between 18 o values of contemporaneous brachiopod carbonate and fish phosphate present day marine fauna of well known ecology from the surface to the sea floor record isotopic temperatures that agree with measured temperatures we predict distributions of isotopic data that result from sampling strategy basin morphology and fauna ecology and discuss limitations application of the method to the jurassic paris london basin gives vertical thermal variations of up to 14 c associated with depths varying from a few meters to 170 30 m the estimated depths are minimum values and are greater than those estimated from sedimentological criteria for this ancient epicontinental sea

high resolution sequence stratigraphy of lower paleozoic sheet sandstones in central north america the role of special conditions of cratonic interiors in development of stratal architecture well known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental scale tectonism oceanographic dynamics of epeiric seas and eustasy we used a multi disciplinary approach to construct a high resolution stratigraphic framework for lower paleozoic strata in the cratonic interior of north america within this framework these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins settings markedly different from the cratonic interior parasequences parasequence stacking patterns systems tracts maximum flooding intervals and sequence bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification the similarity of cratonic interior and foreland basin successions in size geometry constituent facies and local stacking patterns of nearshore parasequences is especially striking this similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same despite marked differences in tectonism physiography and bathymetry between the two settings our results do not support the widespread perception that paleozoic cratonic interior successions are so anomalous in stratal geometries and constitute such a poor record of time that they are poorly suited for modern sequence stratigraphic analyses the particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another thus paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography special conditions of exceptionally slow subsidence rate shallow bathymetry and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings these results suggest that if other cratonic interior successions are as anomalous as reported a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture the results also lead us to suggest that a nonvegetated lower paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones particularly their widespread yet thin geometry and a scarcity of shale and siltstone

carbon isotope evidence for deep water invasion at the marjumiid pterocephaliid biomere boundary black hills usa a common origin for biotic crises on late cambrian shelves an integrated carbon isotope sedimentologic and biostratigraphic study across a late cambrian mass extinction boundary marjumiid pterocephaliid biomere boundary in the deadwood formation black hills south dakota indicates invasion of the carbonate shelf by deep cold marine waters carbonate samples stratigraphically below the extinction boundary show an apparent trend of upwardly increasing 13 c values from 0 4 to 0 1 and relate well to secular 13 c trends of late cambrian limestones coincident with the extinction horizon is a negative shift in 13 c values of 0 4 to 0 7 this shift can be attributed to introduction of cold deep 12 c enriched seawater accompanying invasion of opportunistic cold water trilobites onto depopulated shallow water continental shelves a similar abrupt negative shift in 13 c values has been observed at the suprajacent pterocephaliid ptychaspid biomere boundary others have found that decreases in 87 sr 86 sr values synchronous with increases in 13 c values accompany renewed sea level rise in this part of the late cambrian abrupt negative shifts in 13 c values at biomere boundaries require a companion process responsible for the extinction events this process may be impingement of a shallow thermocline on the shelf or overturning of a stratified cambrian ocean coupling of sea level rise with location of the thermocline and shelf geometry may create conditions responsible for recurrent extinctions of shallow shelf communities throughout the late cambrian

equilibrium versus disequilibrium of barchan dunes abstract barchans are crescentic dunes which occur in mainly mono directional winds shape aspect ratios and velocities of these dunes have been studied as if they were in equilibrium however following a study of the shape and migration of 11 barchans of different sizes for 18 months in the field on moroccan atlantic sahara we show that they only appear to be in a stationary state if studied over a long timeframe at the scale of the year or several years but are never in equilibrium at the scale of weeks or months rather they are always trying to reach a possible equilibrium state but never have enough time to accomplish this this may be the main reason for the large variation observed in previous measurements and justifies some caution in what can be deduced from them

the oldest uhp eclogites of the world age of uhp metamorphism nature of protoliths and tectonic implications coesite bearing eclogitic rocks from the pan african belt in northern mali were dated by multichronological methods rb sr sm nd and ar ar rb sr and sm nd isotope analyses on whole rock and mineral separates of an omphacite kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 ma this is interpreted as the time of the eclogitisation which represents the oldest ultrahigh pressure uhp metamorphic event so far recorded in the continental crust the nearly identical rb sr and sm nd ages suggest very rapid exhumation as observed in many phanerozoic uhp rocks 40ar 39ar dating of phengites from two uhp micaschists gave much older ages of 1045 and 760 ma phengite from a quartzite collected from the same site yielded a plateau date of 623 3 ma which is in agreement with the rb sr and sm nd dates the two 40ar 39ar ages older than 625 ma testify once more the recurrent problem of excess ar in ultrahigh pressure phengites the occurrence of the mali uhp eclogites indicates that by the late precambrian the earth has cooled enough to sustain the formation and preservation of deeply subducted uhp metamorphic rocks the new ages of ca 620 ma also constrain the amalgamation in nw gondwana to have occurred in the latest proterozoic but not in the middle cambrian this is supported by several recent chronological data for the southern segment of the major suture zone in the dahomeyide orogenic belt

history of the west african neoproterozoic ocean key to the geotectonic history of circum atlantic peri gondwana adrar souttouf massif moroccan sahara abstract bordered by the archaean basement of the reguibat shield to the e and s the adrar souttouf massif is located in the southern regions of the moroccan sahara and represents the northern part of the mauritanide belt the central areas of this massif comprise the dayet lawda and sebkha matallah units that are mainly composed of mafic rocks ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect to their morphology u th pb and lu hf isotope composition additionally 155 apatite grains from six samples were also dated by the u th pb method mostly well rounded zircon grains commonly showed bright cathodoluminescent cl overgrowth domains and leaching zones indicating metamorphic overprint around unaffected oscillatory zoned core areas all samples but one yielded two significant zircon age populations 605 ma and 634 ma with some inherited grains in the range of 650 740 ma and very scarcely up to 1190 ma for six samples the hf isotope composition suggests a major contribution of neoproterozoic juvenile magmas from depleted mantle source the zircon hf data for the remaining samples points to a predominant recycling of older archaean to palaeoproterozoic crust based on the geochemical composition of the rocks an island arc setting is assumed at the periphery of the west african craton close to the cryogenian ediacaran boundary subsequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605 ma a minor variscan overprint could be demonstrated for only one sample this case study exemplifies the great potential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of the west african craton for palaeogeographic and geodynamic reconstructions including the peri gondwanan terranes during the late neoproterozoic

p and s wave velocity models of shallow dry sand formations from surface wave multimodal inversion abstractsurficial formations composed of loose dry granular materials constitute a challenging target for seismic characterization they exhibit a peculiar seismic behavior characterized by a nonlinear seismic velocity gradient with depth that follows a power law relationship which is a function of the effective stress the p and s wave velocity profiles are then characterized by a power law trend and they can be defined by two power law exponents p s and two power law coefficients p s in case of depth independent poisson s ratio the p wave velocity profile can be defined using the vs power law parameters and poisson s ratio because body wave investigation techniques e g p wave tomography may perform ineffectively on such materials because of high attenuation we addressed the potential of surface wave method for a reliable seismic characterization of shallow formations of dry uncompacted granular materials we took into account the dependence of seismic wave velocity on effective pressure

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

updated geochronology and isotope geochemistry of the vila de cruces ophiolite a case study of a peri gondwanan back arc ophiolite

u pb zircon ages from volcanic and sedimentary rocks of the ediacaran bas draâ inlier anti atlas morocco chronostratigraphic and provenance implications abstract u pb la icp ms analyses on zircon extracted from felsic ignimbrites and sandstones of the bas dra succession bds constrain the post pan african evolution in the anti atlas aa at the northern margin of gondwana the 2000 m thick ediacaran bds consists of four volcanic units including i rhyolitic ignimbrite sheets ao i ii high silica andesitic and rhyolitic lavas ao ii iii silica poor lava and pyroclastics ao iii and iv rhyolitic lava dome complexes ao iv ao iii is dominated by basaltic and andesitic lava and it contains at least three sandstone lenses the bds is capped by an angular unconformity which is defined by sedimentary rocks of the adoudou formation of the taroudant group the u pb data set 99 zircon grains with 90 110 concordance from the ignimbrite yields an age of 565 6 ma 564 8 5 6 ma interpreted as the crystallization age which represents the early volcanic activity in the bds for the upper bds maximum ages of ca 552 and ca 541 ma are indicated by u pb ages of detrital zircons extracted from sandstone these age constraints confine the bds to the upper ouarzazate supergroup most detrital zircon ages three sandstone samples with 367 zircon grains analyzed fall in the range of ca 606 541 ma this main ediacaran peak is interpreted to have been derived from the post pan african magmatic complexes surrounding the west african craton wac secondary age peaks at ca 652 ma and ca 655 ma are inferred to originate from early to late pan african magmatism of the anti atlas region due to the complete absence of pre neoproterozoic zircon ages our data set is interpreted to indicate a large voluminous volcanic episode that covered the palaeoproterozoic and cryogenian basement or the river system was confined to the bds basin facies and provenance analyses of bds sandstones indicate that the detrital input was mostly locally derived with a minor amount from an external source in addition large peaks of older zircon ages are recognized only on the lower sandstone lens bd 86 aoiii thus the drainage system was large within the lower part and became more localized at the end of bds evolution the maximum depositional age for the zircons separated from the uppermost ao iii sandstone lens 541 ma shifts the precambrian cambrian boundary down from the lower adoudou formation as proposed by previous studies to the uppermost ouarzazate supergroup or to the hiatus between both units the volcanic activity within the bds is linked to a transtensional regime that operated along the gondwana margin during the ediacaran the bds volcanic activity is contemporaneous with the widespread volcanic volcanoclastic and siliciclastic rocks of upper ouarzazate supergroup

the case for dynamic subsidence of the u s east coast since the eocene the dynamic subsidence of the united states east coast is addressed using the discrepancy between regional and global estimates of sea level elevation of paleoshorelines and adjoint models of mantle convection that assimilate plate motions and seismic tomography the positions of eocene and miocene paleoshorelines are lower than predicted by global sea levels suggesting at least 50 m and possibly as much as 200 m of subsidence since the end of the eocene dynamic models predict subsidence of the east coast since the end of eocene although the exact magnitude is uncertain this subsidence has been occurring during an overall global sea level fall with the eustatic change being larger than the dynamic subsidence this results in a regional sea level fall in the absence of land subsidence dynamic subsidence is consistent with the difference between eustasy and regional sea level at the new jersey coastal plain

evidence of a paleoproterozoic basement in the moroccan variscan belt rehamna massif western meseta abstract for the first time an eburnian magmatic event has been identified in the rehamna massif moroccan variscan belt western meseta located north of the south meseta fault the best estimate of the crystallization age of rhyolitic porphyry is given by a weighted mean 207pb 206pb age of 2050 6 3 ma rhyacian orosirian the new u pb age obtained for rhyolitic porphyry from the rehamna massif is of great relevance for improving geological knowledge about the boundaries of the wac because i it overlaps the older eburnian magmatic event described in the anti atlas belt and the icartian magmatic event of the european variscan belt ii this suggests that exists paleoproterozoic basement in the western meseta a hundred kilometers further to the north of the south meseta fault as old continental crust slivers preserved in the cadomian and variscan belts and iii this means that the cambrian transgression in the western meseta probably took place based on a more complex structural architecture affecting the precambrian basement composed not only of ediacaran rocks as has been suggested in the literature but also with paleoproterozoic rocks ca 2 05 ga as discovered in this study

comparing oxygen isotope records of silurian calcite and phosphate δ18o compositions of brachiopods and conodonts oxygen isotope compositions of silurian calcitic brachiopod shells from gotland are compared with those of phosphatic inarticulate brachiopod shells and conodonts from identical stratigraphic horizons the 18ophosphate values of inarticulate brachiopod shells vary widely 13 0 to 17 5 v smow and are significantly lower than those of coeval conodont samples 17 5 to 19 5 v smow conodont samples from individual beds have homogenous 18ophosphate values with variations close to the analytical precision 0 2 together with a significant correlation of conodont 18ophosphate and articulate brachiopod shell 18ocarbonate values r 0 85 this observation suggests that primary marine oxygen isotope compositions are better preserved in conodont apatite than in inarticulate brachiopod shell apatite however with decreasing 18ocarbonate values the 18ophosphate values of conodonts are higher than expected for theoretical isotopic equilibrium with brachiopod calcite different habitats of conodonts and brachiopods cannot account for the observed offset in 18o because both organisms were thriving on a broad shallow marine carbonate platform whereas 18ophosphate values of silurian conodonts are close to 18ophosphate values of modern and mesozoic tropical biogenic apatites most silurian calcitic brachiopod shells from gotland are depleted in 18o when compared to modern or mesozoic brachiopods we therefore suggest that silurian conodonts from gotland record paleotemperature and 18o of silurian sea water more faithfully than coeval calcitic brachiopod shells

comparative analysis of methods used to define eustatic variations in outcrop late cambrian interbasinal sequence development interbasinal correlation of late cambrian cyclic carbonates from the appalachian and cordilleran passive margins the texas craton and the southern oklahoma aulacogen defines six major third order depositional sequences graphic correlation of biostratigraphically constrained strata was used to establish equivalency of stratigraphic sequences between the individual sections relatively isochronous biomere boundaries were used as time datums for lithostratigraphic correlation although the individual sections are composed of different types of meter scale cycles and component lithofacies that reflect the various environmental settings of the localities the overall upward shallowing character of individual sequences is evident the sequences are late cedaria mid crepicephalus late crepicephalus aphelaspis to earliest elvinia elvinia to early saukia and saukia to the cambrian ordovician boundary interbasinal correlation of stratigraphic sequences permits an evaluation of quantitative techniques for determining accommodation history correlation of fischer plots of cyclic successions from separate basins supports a eustatic control of late cambrian sequence development r2 r3 curves derived from subsidence analysis of the late cambrian sections provide good resolution of the second and third order scales of accommodation change and interbasinal correlations of r2 r3 curves also support eustatic control on sequence development comparing the accomodation curves and subsidence analysis with paleobathymetric trends of late cambrian cyclic strata suggests that themore curves may approximate the form of the eustatic sealevel signal a composite eustatic sealevel curve for late cambrian time in north america was created by qualitatively combining the accommodation curves defined by the different techniques for each of the four localities 129 refs 16 figs 3 tabs less

the middle late cambrian reef transition and related geological events a review and new view abstract the reefs of the middle late cambrian cambrian epoch 3 furongian have long been simply considered as microbialites that flourished in the aftermath of the archaeocyath extinction thorough review of the reefs in these periods shows however that the cambrian epoch 3 and the furongian actually yield different types of reefs the cambrian epoch 3 reefs are dominated by thrombolites and dendrolites largely constructed by the calcified microbes epiphyton and renalcis on the other hand the furongian reefs consist mainly of maze like maceriate reefs and columnar stromatolites the maceriate reefs most likely formed by siliceous sponges and calcified microbes including girvanella and tarthinia whereas the columnar stromatolites were mainly constructed by girvanella tarthinia and minor siliceous sponges other microbial reefs e g non columnar stromatolites persisted during the cambrian epoch 3 and the furongian mainly as small patch reefs or reefal crusts lithistid sponge microbial reefs initially formed in the cambrian epoch 3 and occurred throughout the furongian but occupied only a minor portion during these periods several geological events occurred across the boundary between the cambrian epoch 3 and the furongian including positive carbon and sulfur isotope excursions a eustatic sea level drop major faunal turnover of trilobites and diversification of new organisms the coincidence of these events and the transition in reefal fabrics suggest that reef ecosystems were significantly influenced by at least some of these events among the events it is likely that sea level change would have affected reefs although further studies are required in order to test whether and how the other events influenced the reef transition epiphyton and renalcis and thrombolites and dendrolites formed by them declined greatly most likely due to sea level fall and thus decreased in shallow marine habitat at the end of cambrian epoch 3 the cambrian epoch 3 thrombolites and dendrolites were gradually replaced by a sponge microbial association of the maze like maceriate reefs and the columnar stromatolites in the furongian coincident with a sea level rise

geophone ground coupling for frequencies much lower than the coupling resonant frequency the geophone accurately follows the ground motion but for higher frequencies the coupling can alter both the amplitude and phase of the seismic signal the normal planting of vertical geophones in the field results in coupling adequate for conventional recording that uses frequencies less than 100 hz however for very loose soils or for high frequency seismic recording i recommend that the geophones be buried to place the geophones in firmer soil the coupling resonant frequency for vertical geophones is determined by the firmness of the soil and i have measured resonant frequency ranging from 100 to 500 hz at different locations because the firmness of the soil increases with depth the coupling resonant frequency can be increased by burial of the geophones or by the use of longer spikes the rocking of horizontal geophones causes a low frequency coupling resonance it is crucial that horizontal geophones be planted with their bases firmly contacting the soil geophones so planted have a resonance around 130 hz whereas those 1 cm off the ground can have a resonance of 30 hz or lower soil conditions have little effect on the resonant frequency horizontal geophones with 1 inch spikes are as well coupled as those with longer spikes but the best coupling is achieved by burial of the geophones modified journal abstract

oxygen isotope variations in phosphate of biogenic apatites ii phosphorite rocks abstract phosphorites from sedimentary sequences ranging in age from archaean to recent were analysed for 18 o in both the po 4 18 o p and co 3 18 o c in the apatite lattice the oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts the use of the longinelli and nuti 8 temperature equation yields temperatures for recent phosphorites that are in good agreement with those measured in the field the 18 o p values of ancient phosphorites decrease with increasing age these changes with time are not likely to be due to post depositional exchange changes in 18 o values of seawater and variations of temperature with time can account for the 18 o p time trend but the latter explanation is preferred in ancient phosphorites 18 o c in structurally bound carbonate in apatite is not a reliable geochemical indicator

a newly discovered orogenic event in morocco neoproterozic ages for supposed eburnean basement of the bou azzer inlier anti atlas mountains abstract the bou azzer inlier within the anti atlas orogenic belt of morocco exposes a dismembered ophiolite long considered to mark a late neoproterozoic suture between the west african craton wac in the south and neoproterozoic arcs to the north southern parts of the bou azzer inlier include meta igneous units paragneiss schist and muscovite bearing granite that have until now been regarded as the c 2 ga eburnean wac basement onto which the ophiolite was obducted an example of supposed paleoproterozoic basement occurs at tazigzaout the main units within this complex are metagabbro augen granite gneiss finely banded granite gneiss and variably sheared syn tectonic leucogranite all of which are intercalated within a complex sub vertical upper greenschist facies ductile dextral shear zone the meta igneous rocks are crosscut by a sheeted muscovite biotite granite pluton three concordant u pb analyses of zircon from an augen granite gneiss provide a date of 753 1 2 ma zircon from a nearby metagabbro provide an indistinguishable date of 752 2 2 4 ma both dates are considered best estimates of the crystallization ages of their igneous protoliths analyses of zircon from two crosscutting leucogranite bodies provide younger dates of 705 2 3 ma and 701 2 1 ma the initial nd values of all lithological units in the complex range from 4 9 to 6 0 with depleted mantle model tdm ages of 890 790 ma the high positive nd values for all of these igneous units is indicative of an isotopically juvenile source such as depleted mantle derived material and precludes substantial interaction with old evolved crustal sources these data demonstrate that the basement rocks of the tazigzaout complex are neoproterozoic intrusions not c 2 ga eburnean units as found in more southerly anti atlas inliers the structural relationships and u pb ages are interpreted as recording dextrally transpressive deformation of c 755 ma juvenile crust after c 750 and before 700 ma this neoproterozoic deformation occurred as much as 100 ma prior to the top to the north thrusting of the meta igneous units over units of the ophiolite and arc and provides strong evidence for two discrete pan african events in morocco

seismic ground roll absorption and reemission by sand dunes this work was performed at schlumberger cambridge research where m i arran was a case student intern and e muyzert is a senior research scientist

proterozoic collisional basins in a pan african suture zone anti atlas mountains morocco abstract the anti atlas mountains constitute a late proterozoic suture zone produced by northward subduction of oceanic lithosphere culminating in the pan african orogeny southward migration of thrust slices associated with the destruction of the forearc terrane resulted in the uplift and erosion of previously deposited basin sediments these sediments were subsequently reincorporated into collisional basin deposits of the tiddiline formation the tiddiline formation consists of coarsening upwards sequences of maroon siltstones sandstones and intraformational conglomerates these rocks unconformably overlie metamorphosed volcaniclastic rocks of the relict forearc basin and accretionary terrane syn and post depostional deformation has resulted in folding about gently plunging fold axes folds were subsequently cut by strike slip faults that strike at a high angle to the basin axis deformation of the tiddiline formation is attributed to transpressional suturing of the relict forearc terrane to the west african craton to the south collisional basins of the anti atlas mountains serve as ancient analogs for the destruction of forearc basins in an obliqueconvergent margin setting such as those of the western pacific region

evaluating the relationship between the carbon and sulfur cycles in the later cambrian ocean an example from the port au port group western newfoundland canada abstract we present a high resolution 34 s sulfate and pyrite and 13 c carbonate record from the middle upper cambrian port au port group a mixed carbonate siliciclastic succession exposed in western newfoundland canada the results illustrate systematic 34 s sulfate shifts of 15 over relatively short stratigraphic intervals 10 m likely 34 s sulfate pyrite ca 23 and a generally positive coupling between changes in 13 c carbonate and 34 s sulfate together these results indicate that middle to late cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to change than it was in either terminal neoproterozoic or cenozoic oceans however a simple carbon c and sulfur s isotope box model of the late cambrian ocean illustrates that low sulfate concentrations alone fail to account for the 15 34 s sulfate shifts recognized in port au port strata such large shifts can be generated only if fluctuating oceanic redox is invoked marine anoxia forces reduced c s burial and elevated 34 s driving larger 34 s changes per mole of organic carbon buried the conclusion that later cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late early cambrian extinction

δ18o composition of conodont apatite indicates climatic cooling during the middle pridoli phosphatic microfossils such as conodonts and fish microremains dermal scales from the upper silurian pridoli of lithuania have been studied for their oxygen isotope composition the conodont c

the last stages of the avalonian cadomian arc in nw iberian massif isotopic and igneous record for a long lived peri gondwanan magmatic arc abstract the upper allochthonous units of nw iberian massif contain an extensive cambrian magmatism c 500 ma covering felsic to mafic compositions the magmatic activity generated large massifs of granitoids and gabbros with calc alkaline and tholeiitic compositions respectively petrological and geochemical features of these massifs are characteristic of volcanic arc the plutons intruded siliciclastic sedimentary series deposited in the periphery of the west africa craton u pb hf isotopic compositions of detrital zircon in the siliciclastic host series indicate continental arc activity between c 750 ma and c 500 ma it was characterized by a large variety of isotopic sources including from very old continental input even archean to the addition of a significant amount of juvenile mafic material these isotopic sources experienced an extensive mixing that explains the composition and isotopic features ehft from 50 until 15 of the represented cambrian plutons the cambrian igneous rocks of the upper units of nw iberia are related to the latest activity of the avalonian cadomian arc from the middle cambrian arc activity in the periphery of gondwana was replaced by pronounced extension associated with the development of continental rifting which finally led to separation of the microcontinent avalonia subsequent drifting of avalonia to the north caused progressive opening one of the main paleozoic ocean the rheic ocean

stratigraphic and oxygen isotope evidence for my scale glaciation driving eustasy in the early middle devonian greenhouse world article i nfo million year scale 3rd order depositional sequences few 10s to 100 m thick are common in the devonian marine record and are correlated between sedimentary basins and across widely separated continents implying a eustatic origin tectonically driven changes in mid ocean ridge spreading rates lengths are too slow to account for the calculated 3rd order rise fall rates and typical orbital frequencies 20 400 ky are too fast we analyzed oxygen isotopes from conodont apatite to evaluate if early middle devonian 3rd order sea level changes were controlled by paleoclimatically driven ice volume glacio eustasy and seawater temperature change thermo eustasy two successive depositional sequences in the western u s nevada and czech republic prague basin were sampled the 18 o values range from 17 8

structural control magmatic hydrothermal evolution and formation of hornfels hosted intrusion related gold deposits insight from the thaghassa deposit in eastern anti atlas morocco in the moroccan eastern anti atlas the thaghassa intrusion related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the ikniwn granodiorite fields studies reveal three tectono magmatic stages controlling the formation of the deposit i the first stage refers to the top to the south asymmetry and the syn kinematic ikniwn pluton emplacement controlled by a compressional or transpressional strain regime ii the second stage is characterized from older to younger and further away from the intrusion by metatexite with leucocratic stromatic bands aplo pegmatite sills intermediate veinlets composed of quartz k feldspar and muscovite and then gold bearing striped foliation veins all these features are assumed to have been emplaced during a large scale ene wsw dextral shearing process that results from an ese wnw shortening direction during transtensive tectonics we suggest that the progressive and continuous shearing was initiated since the aplo pegmatite stage and achieved during the hydrothermal phase the existence of intermediate veins characterized by quartz rich core and apatite muscovite feldspar rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character at least until the onset of the hydrothermal process iii the late stage developed large volcanic dyke swarm and brittle faulting zircon u pb la icp ms dating yields a concordia age of 563 5 6 3 ma for the ikniwn granodiorite intrusion the fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted i a hot aquo carbonic h2o nacl co2 fluid with n2 and ch4 evolving from vapour rich n2 and ch4 inclusions for the magmatic stage 550 c to ch4 co2 biphased inclusions for the ongoing hydrothermal stage 450 300 c and ii an always secondary low t 200 c saline aqueous type probably nacl free from volatiles with very variable salinity eventually we highlight that in the hornfels hosted irg deposits fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks

structural and geochronological constraints on the evolution of the bou azzer neoproterozoic ophiolite anti atlas morocco abstract the bou azzer ophiolite in the moroccan anti atlas represents the suture of the neoproterozoic pan african orogen the bou azzer inlier includes from ne to sw three main geological units a volcanic arc an ophiolite and a continental platform several subduction related plutons intrude the ophiolite and the volcanic arc and syn to late orogenic deposits unconformably cover the three terranes the eburnean 2 ga continental basement observed in other inliers of the anti atlas appears not to crop out at bou azzer where rocks traditionally attributed to that old basement are in fact neoproterozoic rocks either metagranitoids of the continental margin or ophiolitic metagabbros the age of a metagabbro of the ophiolite is reported here for the first time as 697 8 ma shrimp u pb on zircons u pb geochronological data are also presented for two syn kinematic subduction related plutons which together with previous data establish a time span of ca 655 635 ma for these arc type rocks moreover the oumlill granite previously attributed to the eburnean basement has been dated as 741 9 ma the earliest tectonometamorphic event affecting the ophiolite is recorded as a relic foliation and a garnet rutile metamorphic assemblage which might attest to an early peak pressure exceeding the 5 6 kbar medium pressure reported so far thus we suggest a first stage of ophiolite underthrusting below a volcanic arc followed by ophiolite exhumation and the concomitant development of the main low grade planar linear tectonic fabric the ages of the calc alkaline syn kinematic subduction related plutons 655 635 ma might be taken as the end of subduction and the beginning of the ophiolite exhumation obduction the author s work in the bou azzer region supports the ophiolite continental platform interpretation of leblanc 1975 and leblanc and moussine pouchkine 1994 rather than the tectonic melange interpretation proposed by saquaque et al 1989b and others a recumbent anticlinal and the basal thrust of the ophiolitic terrane are the main macrostructures formed during the obduction stage characterized by top to the w sw kinematics this deformational event is also recorded in the rocks of the underlying continental platform a syn orogenic unit namely the tiddiline formation unconformably overlies the arc volcanics the ophiolite and the continental platform further collisional deformation affected the tiddiline sediments giving way to ese wsw striking upright folds and oblique reverse faults the latest structures in this neoproterozoic suture were left lateral strike slip faults pointing to an increase in the left lateral component of the continental collision

small scale physical modeling of seismic wave propagation using unconsolidated granular media laboratory physical modeling and laser based experiments are frequently proposed to tackle theoretical and methodological issues related to seismic prospecting e g when experimental validations of processing or inversion techniques are required lasers are mainly used to simulate typical field acquisition setups on homogeneous and consolidated materials assembled into laboratory scale physical models pms of various earth structures we suggested the use of granular materials to study seismic wave propagation in unconsolidated and porous media and target near surface exploration and hydrogeologic applications we designed and tested the reproducibility of an experimental procedure to build and probe pms consisting of micrometric glass beads gbs a mechanical source and a laser doppler vibrometer were used to record small scale seismic lines at the surface of three gbs models when guided surface acoustic mode theory should prevail in such unconsolidated granular packed structure under gravity we only considered elastic wave propagation in stratified media to interpret recorded data thanks to basic seismic processing and inversion methods first arrivals and dispersion analyses we were able to correctly retrieve the gradients of pressure and shear wave velocities in our models a 3d elastic finite difference simulation of the experiment offered despite significant differences in terms of amplitudes a supplementary validation of our approximation as far as elastic properties of the medium were concerned

sedimentary structure of large sand dunes examples from dumont and eureka dunes california the current research presents ground penetrating radar images up to 30 m in depth of two large desert dunes in california usa a barchanoid ridge in the dumont field and a linear dune in the eureka expanse the radar images show a complicated structure of internal layering with ascending cross strata cross bedding and bounding surfaces cutting through layers additional research using seismic refraction surveys and sand sampling refine the image of the subsurface 5 m structure the sedimentary structure of the dune shows a strong internal layering with a cemented structure that may immobilize and influence migration of dune expanses the subsurface features of the sand dune fields in the mojave desert provide evidence of dune building wind regime and precipitation history

design and application of a towed land streamer system for cost effective 2 d and pseudo 3 d shallow seismic data acquisition to reduce the field effort required for 2 d and 3 d shallow seismic surveying we have developed a towed land streamer system it was constructed with self orienting gimbal mounted geophones housed in heavy 1 kg cylindrical casings sturdy seismic cables with reinforced kevlar sheathing robust waterproof connectors and a reinforced rubber sheet that helped prevent cable snagging maintained geophone alignment and provided additional hold down weight for the geophones each cable had takeouts for 12 geophones at 1 m or 2 m intervals by eliminating the need for manual geophone planting and cable laying acquisition of 2 d profiles with this system proved to be 50 100 faster with 30 40 fewer personnel than conventional procedures costs of the land streamer system and total weight to be pulled could be minimized by employing nonuniform receiver configurations short receiver intervals e g 1 m at near offsets were necessary for identifying and mapping shallow 50 m reflections whereas larger re

geochemistry of precambrian ophiolites from bou azzer morocco the upper proterozoic ophiolite complex of bou azzer morocco includes ultramafic rocks cumulate gabbros sheeted dykes pillow lavas and diorite quartz diorite intrusions and an overlying volcano sedimentary sequence the gabbroic cumulates basaltic flows and dykes have compositions similar to recent ocean floor rocks n and or t type among other features they have comparable light ree depleted patterns and relations of ti zr and la nb although fractional crystallization played an important role in the evolution of these rocks the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and or by a dynamic partial melting process diorites quartz diorites and the volcanic rocks of the overlying sequence are calc alkaline genetically unrelated to the tholeiitic suite and indicative of an island arc setting a possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc

elucidating the relationship between the later cambrian end marjuman extinctions and spice event abstract the late cambrian early ordovician transition contains several discrete marine extinction events the first of these extinctions the end marjuman occurs in two phases and is thought to coincide with the beginning of the steptoean positive carbon isotope excursion or spice a large excursion in the marine carbon isotope record that represents a large perturbation to the carbon cycle during this time additionally the carbon isotope record from the deadwood formation in the black hills of south dakota usa displays a small negative 13c excursion near the end marjuman extinctions here we examine the carbon isotope stratigraphy of the upper cambrian portion of the conasauga group of the southern appalachians usa to determine the relative timing between the extinction events and changes in the carbon cycle represented by excursions within the carbon isotope record previous high resolution biostratigraphic studies have identified a thick stratigraphic record of end marjuman extinctions within the conasauga group making it an excellent target for a high resolution chemostratigraphic study in the conasauga group there is no change in carbon isotope stratigraphy across the first phase of the end marjuman extinctions suggesting no major change occurred in the carbon cycle during this time further a negative 13c excursion is absent in the conasauga group across the interval that contains the end marjuman extinctions this suggests that the excursion in the deadwood formation is either a local oceanographic signal or a diagenetic feature finally the onset of the spice occurs at the same stratigraphic point as the second phase of the end marjuman extinctions and at the appearance of a low diversity potentially low oxygen tolerant trilobite fauna the stratigraphic positions of these biological and geochemical events suggest a role for marine anoxia in the second phase of the end marjuman extinctions

u pb zircon geochronology of the ediacaran volcano sedimentary succession of the ne saghro inlier anti atlas morocco chronostratigraphic correlation on the northwestern margin of gondwana abstract we report a new regional correlation for the ediacaran succession in the anti atlas belt on the northwestern margin of gondwana based on u pb la icp ms zircon geochronology of volcanic rocks in the ne edge of the saghro inlier the thick volcano sedimentary succession comprises a diverse suite of rhyolitic ignimbrite basaltic to andesitic lava fields rhyolitic lava mafic hydroclastic complex fallout and surge deposits pyroclastic dyke interbedded clastic sediment and subvolcanic bodies ten volcanic rocks yield crystallization ages ranging from 573 to 547 ma consistent with a lower and upper ouarzazate supergroup affinity respectively inherited zircon ages range from 623 ma to 600 ma analogous to zircon peaks in the older volcano sedimentary rocks of the bou salda and saghro groups in the anti atlas suggesting the continuity of the saghro group beneath the ouarzazate supergroup at the ne edge of the saghro inlier rocks with a lower ouarzazate supergroup affinity include lithic poor ignimbrites which yield ages of 573 6 1 9 ma 571 8 4 2 ma 571 3 2 6 ma and 567 4 2 9 ma two fallout deposits which yield ages of 563 5 2 1 ma and 569 2 1 9 ma a surge deposit dated at 571 6 2 8 ma and a rhyolite lava dated at 562 5 3 1 ma two lithic poor ignimbrites from the upper ouarzazate supergroup are dated at 557 3 2 6 ma and 547 9 3 1 ma volcanic activity at the ne edge of the saghro inlier is related to west african cadomian orogenic wacadomian activity between 620 and 560 ma during this period the saghro and bou salda groups were deposited followed by the lower ouarzazate supergroup later extension along the gondwanan margin took place close to ediacaran cambrian boundary contemporaneous with upper ouarzazate supergroup deposition

100 myr cycles of oceanic lithosphere generation in peri gondwana neoproterozoic devonian ophiolites from the nw african iberian margin of gondwana and the variscan orogen

paleomagnetism of a well dated marine succession in south china a possible late cambrian true polar wander tpw abstract the cambrian true polar wander tpw hypothesis remains controversial largely because of the uncertainties in the quality and or fidelity of the paleomagnetic data as well as their chronological control testing the tpw hypothesis requires high quality paleomagnetic data of sufficient spatial and temporal resolutions here we present paleomagnetic results of a continuous cambrian shallow marine succession from south china where available detailed biostratigraphy provides exceptional chronological constraints forty three sites of paleomagnetic samples were collected from this limestone dominated succession stepwise thermal demagnetization generally reveals three component magnetizations low and intermediate temperature components can be cleaned by 330 c and the high temperature component htc was isolated typically from 350 to 450 c a positive fold test and the presence of reversed polarity in the strata together with rock magnetic data as well as the scanning electron microscopic sem and transmission electron microscopic tem results collectively suggest that the htcs are likely primary a directional shift of the htcs occurs between the lower middle cambrian and the upper cambrian strata in the succession and is tentatively interpreted to indicate a 57 polar wander from 500 5 to 494 ma because the rate of polar wander is too fast to be a tectonic origin this polar wander is interpreted to represent a late cambrian tpw this tpw appears coeval with the steptoean positive carbon isotope excursion spice and the major trilobite mass extinctions suggesting a potential link between the tpw and the late cambrian biotic and climatic changes because the proposed tpw event is exceptionally well dated it should be testable through examination of other worldwide sections

episodic magmatism during the growth of a neoproterozoic oceanic arc anti atlas morocco abstract we present an integrated study combining detailed field geochronological and geochemical data of a neoproterozoic intra oceanic arc systems exposed in the pan african belt of the moroccan anti atlas the arc rock units exposed in bou azzer and sirwa inliers consist of a tectonic patchwork of back arc ophiolitic sequences to the north thrusted onto accreted arc complexes to the south arc complexes are composed of amphibolite granodioritic and granitic gneisses intruded by various undeformed hydrous ultramafic hornblendite mafic hornblende gabbro diorite and felsic granodiorite tonalite granite arc lithologies we show that these complexes are remnants of a long lived 120 myr neoproterozoic oceanic arc punctuated by three successive magmatic episodes 760 730 ma 710 690 ma 660 640 ma respectively interspersed with periods of magmatic quiescence the typical geochemical arc signature and positive ndt values for the igneous rocks emplaced during each magmatic episode medians at 7 1 5 4 and 5 7 from older to younger attest that their parental magmas derived from a depleted mantle source without substantial assimilation by the wac older crustal basement trace element geochemistry i e sr y la yb of intermediate to felsic arc rocks produced during each magmatic pulse suggests that the arc crust was thickened 30 35 km over a short time period between the first and second magmatic episodes 730 710 ma which coincides with an important regional shortening event soft docking of the oceanic arc on a buoyant transitional margin is invoked to explain tectonic inversion in overriding plate leading to shortening and related thickening of the arc crust concomitant magmatic shutdown resulting from a reorganization of subduction dynamics i e change in slab geometry flip in subduction polarity a non tectonic critical thickening of the arc crust is invoked to explain the second magmatic shutdown 680 660 ma by freezing the subarc mantle influx this lull period is followed by a third magmatic episode which is likely triggered by delamination of the dense lower crust and reactivation of subarc mantle flow this is supported by the bimodal chemical signature of evolved magmatic products suggesting two distinct sources partial melts from the foundered lower crust and new magmatic products which differentiated from a post delamination thinned crust

oxygen isotope variations in phosphate of biogenic apatites i fish bone apatite rechecking the rules of the game abstract the major advantage of the oxygen in phosphate isotope paleothermometry is that it is a system which records temperatures with great sensitivity while bone and teeth building organisms are alive and the record is nearly perfectly preserved after death fish from seven water bodies of different temperatures 3 23 c and different 18 o values 16 to 3 of the water were analysed the 18 o values of the analysed po 4 vary from 6 to 25 the system passed the following tests a the temperatures deduced from isotopic analyses of the sequence of fish from lake baikal are in good agreement with the temperatures measured in the thermally stratified lake b the isotopic composition of fish bone phosphate is not influenced by the isotopic composition of the phosphate which is fed to the fish but only by temperature and water composition isotopic analysis of fossil fish in combination with analysis of mammal bones should be a useful tool in deciphering continental paleoclimates

geophone ground coupling with flat bases seismic acquisition can be costly and inefficientwhen using spiked geophones in most cases such as the desert the most practical solution is the use of flat bases where geophone ground coupling is based on an optimal choice of the mass and area of contact between the receiver and the ground this optimization is necessary since areas covered by sand are loose sediments and poor coupling occurs other cases include ground coupling in stiff pavements for instance urban areas and ocean bottom nodes we consider three different approaches to analyse coupling and model the geophone with a flat base plate resting on an elastic half space two existing models based on the full wave theory which we refer to as the wolf and hoover o brien models predict a different behaviour with respect to the novel method introduced in this work this method is based on the transmission coefficient of upgoing waves impinging in the geophone ground contact where the ground is described as an anelastic halfspace the boundary conditions at the contact have already been used to model fractures and are shown here to provide the equation of the damped oscillator this fracture contact model depends on the stiffness characteristic of the contact between the geophone base plate and the ground the transmission coefficient from the ground to the plate increases for increasing weight and decreasing base plate area the new model predicts that the resonant frequency is independent of the geophone weight and plate radius while the recorded energy increases with increasing weight and decreasing base plate area as shown from our own experiments and measurements by krohn which is contrary to the theories developed bywolf and hoover o brien the transient response is obtained by an inverse fourier transform optimal geophoneground coupling and energy transmission are required the first concept meaning that the geophone is following the motion of the ground and the second one that the signal is detectable as a final example we simulate seismic acquisition based on the novel theory showing the differences between optimal and poor ground to geophone energy transmission

singing sand avalanches without dunes 1 singing sand dunes have attracted curiosity for centuries and are now the subject of controversy we address here two aspects of this controversy first the possible link between the frequency heard and the shear rate for a gravity avalanche on a dune slip face scaling as 0 4g d with d the mean grain diameter and second the assumed necessity of a layered dune structure under the avalanche that acts as a resonator field recordings of singing dunes over the world reveal that they can present very different spectral characteristics a dune with polydisperse grains produces a very broad and noisy spectrum while a dune with sorted grains produces a well defined frequency performing laboratory avalanches on a hard plate with singing dune sand shows that there is no need for a dune below the sand avalanche to produce the singing sound anda fortiorineither for the dune s layered structure nor for its particular sound transmission by sieving the polydisperse grains the same well defined frequency is obtained to that of the dune with sorted grains with the same diameter frequency relation the various frequencies heard in the field avalanches match the shear rates not calculated from the average size but from the various peaks of the grain size distributions

booming sand dunes booming sand dunes have a remarkable capacity to produce sounds that are comparable with those from a stringed instrument this phenomenon in which sound is generated after an avalanching of sand along the slip face of a dune has been known for centuries and occurs in at least 40 sites around the world a spectral analysis of the sound shows a dominant frequency between 70 and 110 hz as well as higher harmonics depending on the location and time of year the sound may continue for several minutes even after the avalanching of sand has ceased this review presents historical observations and explanations of the sound many of which contain accurate and insightful descriptions of the phenomenon in addition the review describes recent work that provides a scientific explanation for this natural mystery which is caused by sound resonating in a surface layer of the dune

proterozoic blueschist bearing melange in the anti atlas mountains morocco abstract blueschists from the bou azzer inlier provide compelling evidence for late proterozoic subduction in the anti atlas mountains of morocco high pressure low temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbar together with regional relationships the geologic setting of the blueschists constrains the polarity of pan african subduction in this region which occurred from 750 to 600 ma blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix the melange belt containing the blueschist facies rocks is intruded by a number of diorite plutons one of which has yielded a u pb radiometric age of 650 ma together with transaharan belt to the southeast the anti atlas suture zone exposed within the bou azzer inlier contains among the oldest known blueschist bearing ophiolitic melanges in the world

carbon isotope stratigraphy of upper cambrian steptoean stage sequences of the eastern great basin record of a global oceanographic event a large global positive excursion in 13 c from 0 5 to 4 5 during the late cambrian pterocephaliid biomere steptoean stage aphelaspis elvinia zones is documented at high stratigraphic resolution in three sections in the eastern great basin the excursion which we refer to as the steptoean positive carbon isotope excursion or spice excursion began coincident with a world wide extinction event the 13 c data from the great basin reflect global seawater values in a wide range of lithologies including oolitic grainstones wackestones thrombolitic boundstones and flat pebble conglomerates we use a section at shingle pass in the southern egan range to divide the pterocephaliid biomere into eight isotope steps that represent equal divisions of the 13 c rise and fall 4 this provides a basis for recognition of a revised chronostratigraphic framework for the pterocephaliid biomere steptoean stage strata deposited during the beginning of the spice excursion record a major change in the pattern of sedimentation in the eastern great basin this is reflected in a siliciclastic carbonate transition at shingle pass nevada and a carbonate siliciclastic transition at the house range and lawson cove sections in utah a regional siliciclastic influx recognized throughout the great basin occurs near the peak of the spice excursion carbon isotope analyses from cratonal sections in wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the spice excursion the correlated changes in 13 c relative sea level and the marine biota during the spice excursion provide remarkably detailed records of a major paleoceanographic event we speculate that changes in sea level climate or tectonics may have triggered the spice excursion and coeval extinction event subsequent burial of organic carbon caused the increase in 13 c and may have led to an interval of global cooling the results of this study lend confidence to carbon isotopic studies of pre mesozoic rocks

a late cambrian positive carbon isotope excursion in the southern appalachians relation to biostratigraphy sequence stratigraphy environments of deposition and diagenesis abstract a positive carbon isotope excursion is recorded within the upper cambrian sedimentary succession in the southern appalachians that consists of the nolichucky shale the maynardville formation and the copper ridge dolomite the lower part of the succession contains aphelaspis zone fauna early steptoean the extensively dolomitized and poorly fossiliferous nature of the upper part of the succession precludes any detailed biostratigraphic determinations correlation with similar positive carbon isotope excursions in coeval successions elsewhere suggests that this excursion represents a perturbation in the global cycling of carbon comparison of excursions at different localities in north america provides a means for the application of carbon isotope stratigraphy in the southern appalachians the excursion started during deposition of the upper nolichucky shale maximum 13c values 4 to 5o oo pdb are associated with the conformable interval at the maynardville copper ridge dolomite transition which has been interpreted as a correlative conformity in sequence stratigraphic terms the excursion ended during deposition of the lower copper ridge dolomite in western north america the excursion started at the base of the pterocephaliid biomere near the base of the aphelaspis zone this well documented excursion ended prior to the end of the pterocephaliid biomere with the maximum excursion at the sauk ii sauk iii unconformity this supports the correlation between late steptoean dresbachian franconian sea level fall and the sequence boundary at the end of cambrian grand cycle deposition in the southern appalachians the cause of this carbon isotope excursion remains unclear the excursion most likely reflects the enhanced burial of organic carbon promoted by ocean stratification a warm nonglacial climate and a sea level maximum during the early late cambrian the onset of regression may have contributed to the maximum carbon isotope excursion by enhancing sedimentation rates and by increasing organic productivity because of increased nutrient availability the removal of carbon from the ocean surface may have caused a decrease in pco2 of the atmosphere the resulting cooling episode could have triggered an oceanic overturn bringing 12c enriched bottom waters to the surface which in conjunction with oxidation of organic matter during the sea level fall ended the carbon isotope excursion comparison of 13c and 18o values of matrix samples to the associated cement phases provides insights into the relationship between isotope variations and depositional and diagenetic environments 13c values of peritidal dolomicrite define a rather smooth stratigraphic variation curve whereas the values for subtidal micrite have significant scatter resulting from involvement of organic matter in diagenesis fibrous to bladed calcite cement from the subtidal deposits has 13c and 18o values comparable to the associated micrite suggesting precipitation from marine water and similar diagenetic modifications meteoric diagenesis may be responsible for the depletion of 13c and 18o in equant calcite cement relative to the micrite for saddle dolomite cement the depletion of 18o and 13c values similar to those for the peritidal dolomicrite are consistent with formation during burial at elevated temperatures in a rock dominated system this study demonstrates the potential of applying carbon isotope stratigraphy developed in highly fossiliferous successions to stratigraphic intervals with poorly constrained biostratigraphy such studies require evaluation of the effects of depositional environments and diagenesis upon the preservation of marine isotope signatures

the late cambrian spice δ13c event and the sauk ii sauk iii regression new evidence from laurentian basins in utah iowa and newfoundland carbon isotope data from upper cambrian sections in three laurentian basins in northern utah central iowa and western newfoundland record a large positive d 13 c excursion spice event of up to 1 5 peak d 13 c ratios are well dated by trilobite collections to the middle of the steptoean stage dunderbergia zone and occur during maximum regression associated with formation of the sauk ii sauk iii subsequence boundary on the north american craton max imum regression was marked by an influx of quartz sand into carbon ate platform settings in all three widely separated basins in northern utah this quartz sand formed a thick sequence known as the worm creek quartzite which marks a conspicuous interruption of carbonate deposition during the middle to late cambrian in the region in west ern newfoundland the thickness of the quartz sand unit is much re duced but still marks a brief shutdown of the carbonate factory that is unique to the cambrian shelf succession of the area in the central iowa area of the cratonic interior an upward shallowing carbonate succession culminates in cross stratified trilobite grainstones at the peak of the spice in dunderbergia zone time and the lowest point on the relative sea level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface although it is difficult to determine precisely the departure from baseline d 13 c that marks the beginning of the spice excursion in the stratigraphic successions analyzed our results are consistent with a rise and subsequent fall in d 13 c tracking a major regressive transgressive event recorded across northern laurentia the correlation of a major d 13 c excursion with regression is similar to that described for the late ordovician for which the pattern has been attributed to either in creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic matter containing silicates at high latitudes or high productivity and organic carbon burial driven by oceanic overturn the lack of known steptoean age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic carbon burial was the likely cause of the spice event we suggest that increased weathering and erosion rates during relative sea level fall sauk ii iii increased the burial fraction of organic carbon in an expanded region of fine grained siliciclastic deposits in shelf and upper slope environments during the steptoean

oxygen isotope trends and seawater temperature changes across the late cambrian steptoean positive carbon isotope excursion spice event the globally recognized late cambrian steptoean positive c isotope excursion spice is characterized by a 3 5 positive 13 c shift spanning org and pyrite we analyzed 18 o values of apatitic inarticulate brachiopods from three upper cambrian successions across laurentia to evaluate paleotemperatures during the spice 18 o values range from 12 5 to 16 5 estimated seawater temperatures associated with the spice are unreasonably warm suggesting that the brachiopod 18 o values were altered during early diagenesis despite this all three localities show similar trends with respect to the spice 13 c curve suggesting that the brachiopod apatite preserves a record of relative 18 o and temperature changes the trends include relatively high 18 o values at the onset of the spice decreasing and lowest values during the main event and an increase in values at the end of the event the higher 18 o values during the global extinction at the onset of the spice suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters onto the shallow shelf decreasing and low 18 o values coincident with the rising limb of the spice support the hypothesis that seawater warming and associated reduced thermohaline circulation rates contributed to decreased dissolved o 2 concentrations which enhanced the preservation burial of c org causing the positive 13 c shift

elasticity profile of an unconsolidated granular medium inferred from guided waves toward acoustic monitoring of analogue models abstract the monitoring of materials elastic parameters during geological analogue experiments would highly improve the interpretation of involved processes it would more particularly help the implementation of joint analytical or numerical analyses we propose laser doppler vibrometry as a probing tool for the systematic characterisation of analogue models we therefore develop an experimental set up to record small scale seismic lines at the surface of an unconsolidated granular medium in laboratory conditions pressure wave first arrival times and surface wave dispersion are then inverted for a one dimensional pressure and shear wave velocity structure inferred profiles appear to match previously thoroughly estimated properties of the probed medium thus validating our approach

geophones on blocks a prototype towable geophone system for shallow land seismic investigations in recent years some authors have given a certain amount of attention to towed seismic reflection acquisition systems based on some of these works we sought to design and test a system making use of currently available geophones instead of specifically designed sensors as employed in some previous works thus bearing in mind that the geophone s characteristics are achieved in the conditions that they are originally designed to be employed in i e coupled with a spike driven into the ground we devoted our attention to some of the variables involved in the geophone s performance namely the total weight the effect of a spikeless geophone and the surfaces on which the geophone is placed previously we had experimentally verified some variations in the signal response due to coupling geophones in different surface materials such as hard soil asphalt and concrete pavement and we noticed that these surface materials were in fact an important factor in the overall response hence these materials or as we also called them coupling agents could be employed as a base material in the construction of a mobile seismic acquisition device composed of blocks of a certain size on which the geophone would be then inserted and thus making it into a spikeless surface towable system therefore various materials were tested in order to select one that could maintain a similar fidelity to that of the spike coupled geophone and thus contribute towards building a more time efficient and towable geophone and block system pinging tests revealed variations in the coupling frequency and damping characteristics of each coupling agent and from all of these tested materials one was selected for field comparative tests with the normally planted geophones with spike coupling finally a seismic reflection profile was acquired simultaneously with both systems i e spike coupling versus cement block coupled geophones this field test showed similar results in terms of signal levels and frequency content and therefore it was possible to observe the presence of the same reflectors and other seismic events in either of the time sections with this experiment we propose not only a system that allows a time efficient seismic field operation but we also aim to encourage more research into the response dependency of the coupling agent of which the towable base block is made of

unusual massive magnetite veins and highly altered cr spinels as relics of a cl rich acidic hydrothermal event in neoproterozoic serpentinites bou azzer ophiolite anti atlas morocco abstract if magnetite is a common serpentinization product centimetric massive and pure magnetite veins are rarely observed in serpentinites unique example in the ait ahmane ultramafic unit bou azzer neoproterozoic ophiolite anti atlas morocco allows to assess the hydrothermal processes that prevailed at the ending precambrian in this study rock magnetism petrography mineral and bulk chemistry are combined to assess iron behavior in these meta ultramafics in order to constrain the serpentinites alteration and magnetite veins formation processes very high cr low mg high fe 3 and low ti content of cr spinels cores reflect a supra subduction zone origin for the ait ahmane serpentinites precursor typical lizardite chrysotile pseudomorphic texture in fresh serpentinites reveals an initial oceanic like serpentinization involving temperature 200 c magnetic measurements reveal a lower magnetite content in hydrothermalized serpentinites hosting the magnetite veins with lowest values down to 0 58 wt for bleached serpentinites constituting the wall rock of the veins these magnetic data are consistent with bulk rock chemistry showing a lower total iron content in hydrothermalized serpentinites hysteresis parameters and thermomagnetic measurements denote a magnetic grains size that increases with the alteration associated with the emergence of a new magnetic phase cr magnetite produced by cr spinels alteration a new proxy based on thermomagnetic measurements the crm m ratio provides a quantification of its contribution to the magnetic susceptibility mineral chemistry allowed to identify the cr spinels alteration sequence and reveals an important chlorine enrichment in serpentine phases from hydrothermalized serpentinites these results suggest that a cl rich acidic hydrothermal event involving temperatures below 350 c produced an intense magnetite leaching in the host serpentinite and an advanced cr spinels alteration to ferritchromite and cr magnetite iron provided by this leaching have led to the formation of unique magnetite veins in the ait ahmane ultramafic unit two different settings are proposed for the hydrothermal event 1 a continental hydrothermal system as advanced for the co ni as ores in the bou azzer inlier or 2 an oceanic black smoker type hydrothermal vent field on the neoproterozoic seafloor

δ18o and ree contents of phosphatic brachiopods a comparison between modern and lower paleozoic populations 18o values of both carbonate 18 7 25 1 and phosphate 15 1 16 7 components of the francolite from the fossil brachiopod shells are interpreted as the result of a partial resetting of pristine isotopic compositions the similarity of hat shape ree patterns of some lower ordovician linguloids with those of extant littoral specimens suggests the preservation of a 500 myr old record of ree marine chemistry in agreement with the known paleogeography the phosphatic brachiopods have been deposited in a coastal environment under shallow and oxygenated marine waters however the contents and distribution of ree in some strongly altered brachiopod shells reveal an alteration of the original negative ce anomalies and both nd and sm enrichments that transformed the initial hat shape patterns of linguloids into strongly convex or bell shape patterns

reply to comment by b andreotti et al on solving the mystery of booming sand dunes this reply addresses three main issues raised in the comment of andreotti et al 2008 first the turning of ray paths in a granular material does not preclude the propagation of body waves and the resonance condition described by vriend et al 2007 the waveguide model still holds in the dune for the observed velocities even with a velocity increase with depth as implied by andreotti et al 2008 secondly the method of initiation of spontaneous avalanching does not influence the booming frequency the frequency is independent of the source once sustained booming starts it depends on the subsurface structure of the dune thirdly if all data points from vriend et al 2007 are included in the analysis and not an average or selection no correlation is observed between the sustained booming frequency and average particle diameter

revised phosphate water fractionation equation reassessing paleotemperatures derived from biogenic apatite oxygen isotopes of biogenic apatite have been widely used to reassess anomalous temperatures inferred from oxygen isotope ratios of ancient biogenic calcite more prone to diagenetic alteration however recent studies have highlighted that oxygen isotope ratios of biogenic apatite differ dependent on used analytical techniques this questions the applicability of the phosphate water fractionation equations established over 25 years ago using earlier analytical techniques to more recently acquired data in this work we present a new phosphate water oxygen isotope fractionation equation based on oxygen isotopes determined on fish raised in aquariums at controlled temperature and with monitored water oxygen isotope composition the new equation reveals a similar slope but an offset of about 2 to the earlier published equations this work has major implications for paleoclimatic reconstructions using oxygen isotopes of biogenic apatite since calculated temperatures have been underestimated by about 4 to 8 c depending on applied techniques and standardization of the analyses

the implication of the oxygen isotope records in coexisting cherts and phosphates 18 o values have been determined for 53 coexisting chert phosphate pairs from the phanerozoic and precambrian in addition 18 o values were also determined for 18 biogenic phosphates from the phanerozoic the oxygen isotope data for the chert phosphate pairs permit the calculation of the temperatures of formation of the chert phosphate pairs as well as the 18 o of the waters in which they equilibrated in turn the calculated 18 o of the waters permit the calculation of temperatures from the isotopic data of the biogenic phosphates the temperatures for the tertiary and mesozoic samples are compatible with the literature values determined from oxygen isotope data for well preserved calcareous marine skeletal remains the temperatures calculated from the chert phosphate pairs of the paleozoic are usually higher and vary in cycles of about a 150 million year period the early precambrian temperatures are as high as 80 c and decrease with time the use of chert phosphate pairs for temperature determinations are sufficiently promising to warrant further investigations of this type on well preserved chert phosphate or carbonate phosphate pairs

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

solving the mystery of booming sand dunes desert booming can be heard after a natural slumping event or during a sand avalanche generated by humans sliding down the slip face of a large dune the sound is remarkable because it is composed of one dominant audible frequency 70 to 105 hz plus several higher harmonics this study challenges earlier reports that the dunes frequency is a function of average grain size by demonstrating through extensive field measurements that the booming frequency results from a natural waveguide associated with the dune the booming frequency is fixed by the depth of the surficial layer of dry loose sand that is sandwiched between two regions of higher compressional body wave velocity this letter presents measurements of the booming frequencies compressional wave velocities depth of surficial layer along with an analytical prediction of the frequency based on constructive interference of propagating waves generated by avalanching along the dune surface

geochemistry and significance of metavolcanic rocks from the bou azzer el graara ophiolite morocco abstract the bou azzer el graara ophiolite has been interpreted as oceanic or backarc crust overlain by a calc alkalic arc sequence or as a composite forearc and accretionary melange seventy nine new major and trace element whole rock analyses of the metavolcanic rocks from throughout the complex support the latter interpretation the results of this study indicate that the bou azzer metavolcanics comprise three petrographically and geochemically distinct groups 1 a a siliceous metavolcanic series ranging from basaltic andesites to rhyolites with calc alkalic characteristics is the principal component of the northern section of the complex it includes distinct quartz bearing rhyolite flows isolated massive pyroclastic layers are typically associated with this series these rocks are interpreted as forearc basin volcanics and pyroclastics produced during late proterozoic north dipping subduction 2 b low ti metabasalts which display island arc tholeiitic affinities occur as isolated flows or blocks in most of the ophiolite these rocks may represent fragments of the forearc basement below the siliceous series 3 c high ti metabasalts which range from enriched tholeiitic zr y 4 compositions are the principal volcanic block in the southern melange like terrane of the complex these have high ba and sr contents 100 200 ppm average ba 260 ppm average sr they are identical to basaltic compositions from tachdamt and bleida on the southern margins of the ophiolite which have previously been interpreted as a rifted continental margin sequence these rocks could represent seamount volcanic products which have been thrust into an accretionary prism along the north dipping subduction zone or rift margin basalts which have been assembled in a tectonic melange as a consequence of a forearc continent collision both the low ti and high ti metabasalt groups include samples previously identified as ocean ridge or back arc volcanics however there is no compelling evidence for the occurrence of these lava compositions in this region the distribution of rock types within the ophiolite is most consistent with a previously proposed model which views the complex as three allochthonous terranes which were juxtaposed during collision to form the bou azzer el graara complex

cobalt nickel copper arsenide sulfarsenide and sulfide mineralization in the bou azzer window anti atlas morocco one century of multi disciplinary and geological investigations mineral exploration and mining

marine anoxia and sedimentary mercury enrichments during the late cambrian spice event in northern scotland elevated mercury concentrations in ancient sedimentary rocks are used as a fingerprint for large igneous province lip volcanism because there is a tight association between known lips and coeval sedimentary hg anomalies while nonvolcanic processes of sedimentary hg enrichment including redox variations have been demonstrated in modern settings interpretations of ancient sedimentary hg records have focused on lip volcanism here we document a link between sedimentary hg enrichment and marine redox changes during the late cambrian steptoean positive isotopic carbon excursion spice event a time with no known lip we report a new occurrence of the spice event from the eilean dubh formation of northern scotland which preserves a series of coeval hg enrichments abundant glauconite a redox sensitive iron bearing mineral co occurs stratigraphically with the spice and hg enrichments but is rare to absent from the rest of the section and bioturbation is low in strata spanning the spice we suggest that local hg enrichments were driven by changing marine redox conditions during the spice event rather than emplacement of a lip redox oscillations should be considered as an additional control on hg enrichments in the geologic record

geochronological constraints on late precambrian intrusion metamorphism and tectonism in the anti atlas mountains the bou azzer inlier anti atlas morocco is a critical element for understanding gondwanan geology because it exposes rocks formed during the paleosuturing of the gondwanan margin with peri gondwanan arc terranes numerous intrusions within the inlier allow specific tectonic events associated with the tectonic evolution of the gondwanan margin to be bracketed detailed examination of plutons within the inlier reveal down temperature magmatic to solid state fabrics and contact relationships indicative of emplacement during oblique collision between the margin and one or more arc terranes u pb geochronological data for the ao t abdulla diorite ma and 653 8 1 6 bou offroh granodiorite ma provide a limit to the onset of collision and regional metamorphism a 653 0 1 3 precise age of ma for the ousdrat quartz diorite provides an indication of the lower limit of the collision 640 8 1 4 in the region tectonothermal activity in the anti atlas at ca 650 ma correlates with similar periods of tectonism in the peri gondwanan terranes of avalonia and cadomia and points to the emergence of continent wide tectonism in western gondwana as early as 650 ma

oxygen isotope variations in phosphate of biogenic apatites iii conodonts abstract the method for measurement of the isotopic composition of oxygen in phosphates has been improved and adapted for analysis of small quantities of apatite down to 10 mg this extension enables one to analyze hand picked conodont samples with an analytical reproducibility better than 0 5 1 46 samples of conodonts ranging from the ordovician to the pennsylvanian of north america were analysed some insoluble phosphatic residues ichthyoliths and inarticulate brachiopods of the same time range were also measured the range of the 18o values of the analysed conodonts is between 15 and 19 it shows a general trend of decreasing d18o with increasing age from an average value of about 19 in the pennsylvanian to 17 in the ordovician this trend parallels that previously detected in marine phosphorites for the time range between the devonian and pennsylvanian our data are in agreement with independent paleoclimatic information specifically we detect maximum18o enrichment at the end of the pennsylvanian and minimum enrichment at the end of the devonian the difference between these two extremes is equivalent to about 10 c from about 40 to 30 c assuming constant isotopic composition of ocean water the success of oxygen isotopic analysis of conodonts raises the possibility of their use in paleozoic paleo oceanography in a similar way to foraminifera in the mesozoic cenozoic

u pb baddeleyite ages and geochemistry of dolerite dykes in the bas draa inlier of the anti atlas of morocco newly identified 1380 ma event in the west african craton in the bas draa inlier anti atlas morocco the paleoproterozoic basement which is cut by the ediacaran taourgha granite is also crosscut by numerous dykes of a variety of trends mostly of uncertain age two doleritic dykes are dated by the id tims u pb method on baddeleyite and yield emplacement ages of 1381 8 ma mswd 0 84 and 1384 6 ma mswd 1 4 determined for a n135 degrees e and a n40 degrees e trending dyke respectively these dates represent the first geochronological evidence of a mesoproterozoic magmatic event in the anti atlas this magmatic event falls in the previously considered ca 1 7 1 0 ga mesoproterozoic gap in geological activity in the anti atlas the poorly dated taghdout and taarotihate sequences could represent remnants of the ca 1380 ma magmatism and rift related sedimentation the mesoproterozoic sedimentary succession of the atar group in the taoudeni basin mauritania could also represent a good candidate for rift related sedimentation but it postdates the 1380 ma magmatic event by 270 ma the dated 1380 ma dykes are transitional to mildly alkaline basalts not unlike some hawaiian lavas however these dykes have a distinct negative nb anomaly a common features in many large igneous provinces lips and this requires interaction with the lithosphere this interaction may have occurred at the level of the lithospheric mantle or the crust these newly dated 1380 ma dykes may converge to the north speculatively suggesting a magmatic center associated with a 1380 ma mantle plume along the northern margin and possibly linked to rifting and possible breakup on that margin and also to a regional uplift that largely removed the evidence of a 1380 ma cover sequence contemporaneous 1380 1390 ma magmatism is reported elsewhere on other crustal blocks and that in northeastern laurentia northern greenland northern siberia anabar shield and baltica southern urals can be reconstructed with that of the bas draa inlier anti atlas region of the west african craton wac into a single large igneous province lip extending over an area of 1 million km 2 and associated with the final fragmentation of the columbia nuna supercontinent c 2012 elsevier b v all rights reserved

azimuthal seismic anisotropy constrains net rotation of the lithosphere 1 net rotation nr of the lithosphere is found in hot spot reference frames important for tectonics and plume models but difficult to constrain using mantle flow and crystallographic texture modeling i show that nrs lead to mantle shearing which is recorded in azimuthal seismic anisotropy the nr amplitude in some hot spot models is so large that it degrades the model fit to anisotropy significantly smaller nrs such as predicted by flow models with stiff continental keels are consistent with seismology however

temporal persistence and stability of cambrian biofacies sunwaptan upper cambrian trilobite faunas of north america abstract during the upper cambrian sunwaptan stage ptychaspid biomere trilobite faunas diversified following a mass extinction at the base of the unit during and immediately after the extinction interval a small number of low diversity trilobite biofacies were spread widely over the shelf turnover at all taxonomic levels was brisk and in all facies there was a sequence of rapid biofacies replacements by the end of the sunwaptan a well developed spectrum of environmentally circumscribed trilobite biofacies was developed in carbonate depositional systems and tracking of lithofacies by biofacies can be demonstrated however rapid turnover still took place at the species level so persistence and stability of biofacies was expressed at the clade level a preliminary assessment of other sunwaptan faunal elements including articulate and inarticulate brachiopods suggests that they were characterized by more persistent stable assemblages even at the species level the pattern of stability of cambrian trilobite biofacies stands in marked contrast to the species level coordinated stasis documented for the articulate brachiopod rich biofacies of the devonian of new york state the difference in behavior of these biofacies may simply be an expression of differences in the extinction and speciation rates of component taxa biofacies assembled from volatile taxa such as cambrian trilobites will inevitably display rapid turnover whereas coordinated stasis is likely to be characteristic of less volatile groups such as articulate brachiopods or bivalves the extent of biofacies stability will therefore be controlled ultimately by factors influencing evolutionary rates such as behavioral complexity niche breadth population size and dispersal ability

post variscan evolution of the anti atlas belt of morocco constrained from low temperature geochronology the anti atlas belt of morocco extends ene wsw over more than 600 km from the atlantic margin in the west to the interior of the african plate in the east it exhibits precambrian rocks outcropping as basement inliers and surrounded by marine ediacaran cambrian sequences around the axis of the mountain range the belt which has for a long time been interpreted as of variscan age is now revealed to have experienced major vertical movements through mesozoic and cenozoic times thereby the anti atlas domain appears to be affected by two episodes of exhumation separated by an episode of subsidence the initial episode occurred in the late triassic and led to the exhumation of 7 5 10 5 km of crustal rocks by the end of the middle jurassic ca 160 150 ma the following phase resulted in 1 3 km of basement subsidence and occurred during the late jurassic and most of the early cretaceous the basement rocks were then slowly brought to the surface after experiencing 2 3 5 km of exhumation throughout the late cretaceous and the cenozoic the timing of these episodes of exhumation and subsidence coincides with major tectonic and thermal events in relation with the evolution of the atlantic and tethys oceans indicating that the effects of their rifting and drifting extended beyond their presumed margins

post variscan exhumation of the central anti atlas morocco constrained by zircon and apatite fission track thermochronology the origin of the anti atlas relief is one of the currently debated issues of moroccan geology to constrain the post variscan evolution of the central anti atlas we collected nine samples from the precambrian basement of the bou azzer el graara inlier for zircon and apatite fission track thermochronology zircon ages cluster between 340 20 and 306 20 ma whereas apatite ages range from 171 7 ma to 133 5 ma zircon ages reflect the thermal effect of the variscan orogeny tectonic thickening of the ca 7 km thick paleozoic series likely enhanced by fluid advection apatite ages record a complex mesozoic cenozoic exhumation history track length modelling yields evidence that i the precambrian basement was still buried at ca 5 km depth by permian times ii the central anti atlas was subjected to erosional exhumation during the triassic early cretaceous then buried beneath ca 1 5 km thick cretaceous paleogene deposits iii final exhumation took place during the neogene contemporaneously with that of the high atlas

sedimentary and volcanic evolution of a neoproterozoic continental margin bleida anti atlas morocco abstract a tentative geodynamic evolution of the neoproterozoic continental margin of the northern border of the west african craton is reconstructed using both sedimentological observations and geochemical arguments from volcanic rocks the rifting stage was marked first by intermittent subsidence of a very shallow shelf environment leading to deeper water sedimentation conditions with local surges of deltaic channels this rifting stage was also characterized by the eruption of a 500 m thick pile of submarine basic lava flows with geochemical characteristics similar to those generally observed in basaltic lavas erupted during continental rifting this first rifting stage 790 ma clauer 1976 was succeeded by a drastic change both in sedimentation and volcanism the younger sediments are deep water distal turbidites with associated pyritic black shales they include a few highly differentiated tholeiitic flows and sedimentary exhalative copper deposits linked to felsic volcanism these changes are attributed to the development of deep water oceanic basins along the northern border of the west african craton the closure of this pan african oceanic domain occurred by transpression tectonics along this major lithospheric transform zone which is roughly e w trending

the moroccan anti atlas the west african craton passive margin with limited pan african activity implications for the northern limit of the craton abstract the moroccan anti atlas region located south of the south atlas fault has been viewed traditionally as containing two segments separated by the anti atlas major fault these two segments are said to consist of a 600 700 ma pan african segment located in the northeast and b 2 ga eburnian segment situated to the southwest on the basis of observations in the zenaga and saghro inliers and of a recent literature review we suggest that this subdivision is inappropriate in that eburnian and pan african materials occur throughout the anti atlas region the entire anti atlas is underlain by eburnian crust unconformably overlain by a lower neoproterozoic passive margin allochthonous pan african ocean crustal slices were thrust onto the west african craton wac passive margin sequence 685 ma ago as a result of pan african accretion tectonics high level high k calc alkaline and alkaline granitoids locally intruded the anti atlas sequence as a whole at the end of the pan african orogeny at 585 560 ma the intervening 100 m y interval was marked by quiescence this succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the pan african events that occurred to the east in the tuareg shield and to the north in avalonian terranes this model implies that the actual northern limit of the wac is located at the south atlas fault saf and not at the anti atlas major fault aamf we propose that the aamf corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the wac during early neoproterozoic times this is consistent with the development of the gourma aulacogen on the eastern side of the wac this model further suggests that the northeastern boundary of the wac occurs north and not south of ougarta algeria support for this model is provided by geological and geophysical evidence

shrimp u pb geochronological constraints on the timing of the intra alcudian cadomian angular unconformity in the central iberian zone iberian massif spain new shrimp u pb ages of detrital zircons from neoproterozoic low grade metasandstones of the schist graywacke complex central iberian zone iberian massif sampled just below and above the intra alcudian unconformity at two selected locations contribute to reconstruct the geodynamic evolution of iberia during the cadomian orogeny in the north gondwana margin the youngest zircons i e maximum depositional age in the lower alcudian are c 580 576 ma while those in the upper alcudian are c 555 552 ma the obtained remarkable time gap of about 21 ma supports the existence of a tectonic event in between this event resulted in moderate folding without related foliation metamorphism that verticalized the lower alcudian previous to the deposition of the upper alcudian additional evidence of late cadomian tectonothermal events elsewhere in iberia also fit in the interval c 560 550 ma combined with other geological data the most probable maximum depositional ages are c 580 560 ma for the lower alcudian previous to the late cadomian folding event and c 550 540 ma for the upper alcudian previous to the deposition of the overlying pusian group and lower cambrian sandstones and limestones a comparison of the new zircon age spectra with possible source areas verifies recent studies that point to the cadomian foreland in the north gondwana continent affected by the pan african orogeny the west african craton and or the saharan metacraton furthermore ongoing cadomian arc related magmatism in iberia c 605 545 ma could have contributed as a local zircon source the end of the cadomian activity is marked by a transient stage ephemeral lower cambrian platform which preceded widespread cambro ordovician rifting of north gondwana

terrestrial marine carbon cycle coupling in 500 m y old phosphatic brachiopods carbon isotope compositions 13c of inarticulate brachiopod shells from upper cambrian sandstone in the cratonic interior of laurentia record a 5 positive excursion that correlates biostratigraphically with the global steptoean positive isotopic carbon excursion a consistent 6 negative displacement in brachiopod 13c relative to carbonate values is interpreted to represent an onshore offshore gradient in the isotopic composition of dissolved inorganic carbon in cambrian seawater thus these 500 m y old chitinophosphatic brachiopod shells preserve evidence for carbon cycle coupling between the ancient atmospheric oceanic and terrestrial reservoirs in the time before embryophytic land plants

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

a global carbon isotope excursion spice during the late cambrian relation to trilobite extinctions organic matter burial and sea level abstract the steptoean positive carbon isotope excursion spice marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue water trilobites the spice excursion is documented from sections in laurentia kazakhstan china and australia where it begins with the mass extinction at the base of the pterocephaliid biomere steptoean stage in laurentia and at coeval extinction horizons in gondwana and perigondwana terranes the peak of spice 5 coincided with a time of maximum regression in laurentia spice is similar in this regard to excursions that coincide with glacio eustatic falls such as in the late ordovician a plausible scenario involves the transformation of ocean circulation between two states which led to enhanced coastal upwelling and benthic extinctions the lack of evidence for glaciation indicates that the coeval sea level fall sauk ii sauk iii event resulted from tectonic or hydrologic changes that remain poorly understood at this time

a fundamental problem with the kinematic interpretation of geological structures abstract non steady flows are ubiquitous in nature not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non steady flow histories in modern kinematic analysis flow apophyses the instantaneous stretching axes and the vorticity vector collectively referred to as the eigen directions of deformation are used in place of bruno sander s kinematic axes for deformation with a steady flow history this practice is well justified and has led to great advances in structural geology but for non steady flow histories the geometrical relationships among eigen directions flow pattern vary with time this makes it inappropriate to correlate an association of structures and fabrics with certain time invariant flow patterns and hence raises the question how should we interpret geological structures and fabrics kinematically without the assumption of homogeneous and steady deformation we suggest that the answer lies primarily in forward modeling of deformation based on a knowledge of rock properties

comment on solving the mystery of booming sand dunes by nathalie m vriend et al 1 we show here that the standard physical model used by vriend et al 2007 to analyse seismograph data namely a non dispersive bulk propagation does not apply to the surface layer of sand dunes according to several experimental theoretical and field results the only possible propagation of sound waves in a dry sand bed under gravity is through an infinite yet discrete number of dispersive surface modes besides we present a series of evidences most of which have already been published in the literature that the frequency of booming avalanches is not controlled by any resonance as argued in this article in particular plotting the data provided by vriend et al 2007 as a table it turns out that they do not present any correlation between the booming frequency and their estimate of the resonant frequency