upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

isotropy or weak vertical transverse isotropy in d beneath the atlantic ocean 1 shear velocity properties of d beneath the central atlantic ocean are explored using predominantly european seismic recordings of intermediate and deep focus 100 km south american earthquakes broadband data are analyzed and when possible corrected for upper mantle models of receiver side anisotropic structure regional shear velocity heterogeneity in d is mapped by analysis of 306 s sks differential times that have been corrected for three dimensional seismic velocity structure above d using a whole mantle tomographic model this correction yields modest less than 2 estimates of seismic velocity heterogeneity in d with a transition from high to low seismic velocities traversing from west to east beneath the central atlantic in agreement with global tomographic models additionally shear wave splitting of s and sdiff for the same recordings was analyzed to assess seismic anisotropy in d the highest quality data provide 105 splitting times between sh and sv onsets that are mostly within the 1 s uncertainty level the few larger values generally exhibit sv delayed relative to sh assuming an anisotropy geometry involving vertical transverse isotropy vti as preferred in most regions of d that have been studied to date 0 5 anisotropy strength within a 100 km thick layer or 0 25 anisotropy within a 300 km thick layer are compatible with the data these values are low in comparison to those found in high velocity regions beneath the circum pacific ocean or in the low velocity region beneath the central pacific and many observations are in fact consistent with isotropic structure the lack of strong vti relative to other regions may be due to 1 the absence of stress from overlying midmantle downwelling 2 relatively weaker shear flow in the d boundary layer and or 3 lack of chemical heterogeneity that could develop either lattice preferred orientation or shape preferred orientation the azimuthal sampling of this region of d is quite limited thus the precise geometry and mechanism of any anisotropy are difficult to constrain it remains possible that this region may contain subtle azimuthal anisotropy that could couple the sv and sh signals however amplitude observations suggest that any such coupling is minor

d shear velocity heterogeneity anisotropy and discontinuity structure beneath the caribbean and central america the d region in the lowermost mantle beneath the caribbean and central america is investigated using shear waves from south american earthquakes recorded by seismic stations in north america we present a large scale composite study of volumetric shear velocity heterogeneity anisotropy and the possible presence of a d discontinuity in the region our data set includes 328 s sdiff sks differential travel times 300 scs s differential travel times 125 s sdiff and 120 scs shear wave splitting measurements and 297 seismograms inspected for scd the seismic phase refracted from a high velocity d layer broadband digital data are augmented by high quality digitized analog wwssn data providing extensive path coverage in our study area in all data from 61 events are utilized in some cases a given seismogram can be used for velocity heterogeneity anisotropy and discontinuity analyses significant mid mantle structure possibly associated with the ancient subducted farallon slab affects shear wave travel times and must be corrected for to prevent erroneous mapping of

ordovician alluvial fan to marine shelf transition in sw sardinia western mediterranean sea tectonically sardic phase influenced clastic sedimentation abstract a localized lower ordovician early caledonian structural event sardic phase is recorded in sw sardinia by an angular and erosional unconformity faulting and mild folding this event was accompanied by sedimentation of a transgressive clastic unit puddinga beds which is particularly well developed along a major fault system gonnesa fault zone there the puddinga beds range from basal interfingering fans some formed by coarse carbonate and schist clasts others by schist clasts only to fluvio lacustrine mud and gravelly sandstone to coastal muddy and sandy deposits and to fossiliferous restricted marine shelf sediments the existence of the slightly compressive sardic phase in sw sardinia and quasi coeval calc alkaline volcanism and an extensional structural phase sarrabese phase in central east sardinia suggest transcurrent faulting the calc alkaline acid volcanism is also interpreted by some authors to indicate that the rifted basins developed in backarc position the non volcanic sequence of sw sardinia analyzed in this paper was probably formed in basins developed on the northern margin of the gondwanaland craton this sardinian system may be the eastward extension of transcurrent rifts which have also been recently reported from nw spain structural activity has been the predominant control on sedimentation however worldwide recurring low sea level stands may have influenced the sedimentation of the lower part of the puddinga beds and the deglaciation of the saharan ice sheet has contributed to the uppermost ordovician early silurian deepening of the euxinic shelf

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

possibility of a low p wave velocity layer in the outermost core from global smks waveforms abstract a global data set consisting of 1211 smks m 2 waveforms has been analyzed to investigate the radial seismic velocity structure around the core mantle boundary cmb band passed f 0 02 0 1 hz and stacked waveforms coincide with reflectivity synthetic ones for prem very well whereas those for other global models iasp91 ak135 and sp6 yield disagreements a preliminary waveform modeling for the outermost core with prem as initial structure results in a core surface layer with a p wave velocity of 7 95 km s at the core top and thickness of 90 km based on a 3 d global s wave velocity model smks phases collected in this study mainly pierce slightly high velocity regions at the base of the mantle and the predicted differential travel time residuals for s3ks skks are expected to be negative approximately 0 5 s for the averages however positive s3ks skks residuals of which average is 0 3 s are predominant in the stacked waveforms moreover a waveform modeling for the d structure can result in a 30 km thick layer with a 10 s wave velocity reduction at the mantle bottom ulvz in which waveform fitness for the part of mainly s3ks is improved on the other hand the waveform data is not well explained by the lowermost mantle structures with thickness of several hundred kilometers these suggest that the smks data is still affected by the structure of the lowermost several tens kilometers in the mantle that is not sufficiently modeled by a global tomography although the thin low s wave velocity model is not conclusive the possibility of a low p wave velocity layer in the outermost core is remained because the waveform fitness for the part of s4ks is improved by the combination of the ulvz and a 140 km thick layer with a 0 8 p wave velocity reduction at the core top

eustatic sea level changes interpreted from seismic stratigraphy a critique of the methodology with particular reference to the north sea jurassic record seismic stratigraphic methods for interpreting eustatic sea level change are based on generalized stratigraphic models and chronostratigraphic interpretations that have not been substantiated by publication of the necessary basic supportive data chronostratigraphic correlations of sea level change curves between basins are based partly on circular reasoning onlap and offlap seismic models may not indicate sea level change but flexural subsidence facies shifts or changes in sea level in the opposite direction to that proposed resolution of some of these problems may be achieved by publication of basic data now in confidential industry files the widely acclaimed vail curves should not be used as templates for basin correlation until such data have been provided and such use should be supported by appropriate conventional lithostratigraphic and biostratigraphic documentation from each new basin where such data have been made available as with the north sea jurassic serious questions of interpretation are raised

constraining outermost core velocity with smks waves smks waves m 2 3 4 seismic waves that travel as s waves in the mantle p waves in the core and reflect m 1 times on the underside of the core mantle boundary are well suited for constraining outermost core vp structure this is due to closeness of the mantle paths and also the shallow depth range these waves travel in the outermost core high quality digitized wwssn and canadian network recordings from a deep focus java sea event which sample the outer core beneath the northern pacific the artic and northwestern north america roughly th of the globe are utilized as an example to show the strength of smks waves to resolve outermost core structure s3ks s2ks and s4ks s3ks differential travel times were measured and compared to those from reflectivity synthetics created from core models of past studies constraints and uncertainties in outer core structure using smks are discussed for the event studied the prem core model with possibly a small vp decrease in the outermost 50 km of the core provides a good fit to the data

sedimentary patterns across the lower middle cambrian transition in the esla nappe cantabrian mountains northern spain in the carbonate platforms of the western gondwana margin the extinction recorded at the lower middle cambrian boundary is accompanied by a profound change in the style of carbonate deposition the lancara formation of the esla nappe cantabrian mountains northern spain contains a distinct sedimentary turnover due to a combination of tectonism eustatic fluctuations and immigration and colonization of new benthic communities such as the youngest archaeocyathan assemblage of the entire iberian peninsula during latest early cambrian times a regressive trend is recorded in the lancara formation this regression was recorded on a peritidal dominant homoclinal ramp that is topped by a tectonically induced discontinuity d1 the latter surface marks the beginning of a last prograding regressive tendency recorded on an intra shelf ramp with ooidal bioclastic shoals protecting archaeocyathan microbial patch reefs the overlying discontinuity d2 corresponds to a major erosive unconformity which coincides with the lower middle cambrian boundary in the cantabrian mountains the subsequent long term earliest middle cambrian rise in relative sea level allowed deposition of low relief bioclastic shoals bearing a diverse and cosmopolitan assemblage of benthic fauna finally the previous evolution is bounded by a third discontinuity d3 which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognised throughout southwestern europe two associations of calcimicrobes occur in the latest early cambrian regressive trend of the lancara formation i proaulopora and subtiflora are identified in peritidal high energy settings lacking self supported structures whereas ii intergrowths of epiphyton renalcis and girvanella encrusted branching colonies and solitary archaeocyaths in protected back shoal patch reefs the latest early cambrian regression is correlated in southwestern europe in both siliciclastic iberian chains and ossa morena and carbonate dominant platforms cantabrian mountains montagne noire and sardinia its tops are recognised as diachronous unconformities ranging in age from early bilbilian to the bilbilian leonian or lower middle cambrian boundary

the palaeozoic variscan oceans revisited abstract geological evidence supported by biogeographical data and in accord with palaeomagnetic constraints indicates that one ocean models for the variscides should be discarded and confirms instead the existence of three gondwana derived microcontinents which were involved in the variscan collision avalonia north armorica franconia and thuringia subdivided by a failed vesser rift and south armorica central iberia armorica bohemia all divided by small oceans in addition parts of south eastern europe including adria and apulia are combined here under the new name of palaeo adria which was also peri gondwanan in the early palaeozoic oceanic separations were formed by the break up of the northern gondwana margin from the late cambrian onwards most of the oceans or seaways remained narrow but much like the alpine cenozoic oceans gave birth to orogenic belts with hp uhp metamorphism and extensive allochthons the saxo thuringian ocean between north and south armorica and the galicia moldanubian ocean between south armorica and palaeo adria only the rheic ocean between avalonia and peri gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism and its north western arm closed before or during the emsian in europe ridge subduction under the northernmost part of armorica in the emsian created the narrow and short lived rheno hercynian ocean it is that ocean and not the rheic whose opening and closure controlled the evolution of the rheno hercynian foldbelt in south west iberia south west england germany and moravia czech republic devonian magmatism and sedimentation set within belts of early variscan deformation and metamorphism are probably strike slip related the first arrival of flysch on the forelands and or the age of deformation of foreland sequences constrains the sequential closure of the variscan seaways galicia moldanubian in the givetian saxo thuringian in the early famennian rheno hercynian in the tournaisian additional mid to late devonian and partly early carboniferous magmatism and extension in the rheno hercynian saxo thuringian and galicia moldanubian basins overlapped with variscan geodynamics as strictly defined the early carboniferous episode was the start of episodic anorogenic heating which lasted until the permian and probably relates to tethys rifting

a study of the outermost liquid core using differential travel times of the sks skks and s3ks phases abstract differential travel times of sks skks and s3ks seismic phases are analysed to specify the structure of the outermost liquid core which is poorly sampled by the pkp waves currently used for core tomography the data are mainly broadband records at epicentral distances of 83 140 they have been selected to obtain a good global earth coverage skks sks and s3ks skks travel time residuals have been computed with respect to the mean earth model preliminary reference earth model prem and corrected for ellipticity the skks sks residuals plotted as a function of epicentral distance exhibit a general positive slope for distances greater than 105 with mean positive values of approximately 2 s at 120 130 c these features may be accounted for by slight perturbations in prem involving a 2 5 s velocity increase in the d layer at the base of the mantle and a small decrease in the p velocity in the upper 800 km of the liquid core the structure in the uppermost 100 km of the core is mainly constrained by the s3ks data a high velocity gradient followed by a low gradient is obtained immediately beneath the core mantle boundary this may suggest the existence of a thin low velocity layer at the top of the liquid core as a result of a possible accumulation of light elements beneath the core mantle boundary the skks sks residuals exhibit strong coherent regional variations whereas the s3ks skks residual variations remain small this indicates that the lateral heterogeneitics at the core side of the core mantle boundary if any are very small the simultaneous analysis of s3ks sks and skks sks residuals suggests a prevailing contribution of long wavelength heterogeneities in d

analysis of seismic sv waves in the core s penumbra vertically polarized s waves sv attenuate rapidly as they diffract around the earth s core whereas horizontally polarized s waves sh propagate to large distances in the core shadow zone the amplitude decay of diffracted sv signals is so strong that few studies have been made of these phases despite their acute sensitivity to velocity structure just above the core mantle boundary we analyze sv signals in the penumbra of the core s shadow finding systematic waveform complexities indicative of local stratification and apparent anisotropy at the base of the mantle with implications for the dynamic processes in this internal boundary layer

rise and fall of an isolated unstable carbonate platform the cambrian of southwestern sardinia strong effects of tensional tectonics dominate the lower and middle cambrian sequence of southwestern sardinia a thinning of the continental crust and the possible development of a passive margin in the cambrian is indicated by different stages of evolution of a mainly carbonatic platform a clastic carbonatic homoclinal ramp with algal archaeocyathan mounds epiphyton renalcis in the west clastic shallow marine to tidal sequences in the east b carbonatic clastic ramp or rimmed shelf with an ooid shoal complex prograding towards the west the back shoal area contains peloidal mudstones algal archaeocyathan biostromes girvanella and increasingly tidal deposits clastic and carbonatic towards the east c isolated platform aggraded to sea level rimmed by slope deposits with slumps and breccia beds in the southeast and northwest d isolated flooded platform barriers towards the open sea partly broke down e isolated platform with raised rims and deep interior often with thick breccia beds in uppermost parts f break down of the platform late early cambrian marked by nodular limestones and limestoneshale intercalations g terrigenous clastics cover the former platform

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

the stably stratified outermost core revisited the p wave velocity structure of the outermost 200 km of the outer core the e region is analyzed using a suite of differential travel times for sks skks and s waves bottoming below within and above this depth range core penetrating phases with paths beneath alaska exhibit similar sks skks differential travel time patterns to phases sampling the core beneath the central pacific but very different sks s differential travel times these patterns can be explained by a core velocity model in which the outermost 50 to 100 km has 1 2 slower p wave velocities than standard earth models perhaps due to a stably stratified chemical boundary layer combined with 3 lateral variations in s wave velocity in the lowermost mantle

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

tomographic inversion of s sks times for shear velocity heterogeneity in d degree 12 and hybrid models differential travel times between s or diffracted s and sks were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle the d region commencing with 3000 s sks times with variable qualities we minimize uneven path coverage by thinning redundantly sampled regions e g the fiji tonga to north america corridor and collect additional data for sparsely sampled areas especially in the southern hemisphere about 1500 paths were retained distributed to reconcile both high density and homogeneous sampling we compare 1 spherical harmonic and 2 equal area block parameterizations of d shear velocity heterogeneity for identical minimum resolving lengths and mean model errors we show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4 5 4 5 and maximum spherical harmonic degree of 40 we demonstrate synthetically that a high degree l inversion followed by a lower degree ls spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects tomographic inversion of this data set yields a global distribution of d heterogeneity robust up to degree 12 in our preferred model l 40 ls 12 rms 1 surface hotspots and estimated lower mantle plume roots are located in or at edges of low velocity regions vs 2 for both the atlantic indian low velocity corridor as well as the low velocities beneath the central pacific although the above parameterizations entail no intrinsic difference in resolvability higher resolution is regionally achieved by removing the derived degree 12 model from the data then inverting the corrected residuals using 4 5 4 5 blocks this hybrid method prevents power loss at intermediate to long wavelengths from otherwise severe damping and recovers smaller scale structure where constraints are better than the global average such as beneath the pacific and eurasia a synthetic recovery test yields resolvability maps that reflect both the path geometry and the quality of data which help to identify robust features in both the degree 12 and the hybrid d models

palaeogeographical and palaeoecological aspects of the cambro ordovician radiation of echinoderms in gondwanan africa and peri gondwanan europe abstract ecology and tempo of the lower palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in cambro ordovician faunas from laurentia and the northern gondwana margin the cambrian agronomic revolution triggered a global radiation of echinoderms with the progressive disappearance of biomat related lifestyles and the colonisation of new environments both in laurentia and on the northern gondwana margin soft substrate echinoderm assemblages related to cold and or deep environments were dominated by blastozoans and stylophorans these assemblages show a pattern of continuous diversification from the middle cambrian to the middle ordovician the major radiation of crinoids in the lower to middle ordovician of laurentia is only a local diversification correlated with the development of hardgrounds in warm shallow environments no comparable major ordovician radiation event occurred in the cooler environments of the northern gondwana margin

relations between submarine fissures internal breccias and mass flows during triassic and earlier rifting periods deep fissures in shallow water limestones filled with deep water sediments are described from many places especially in the triassic and jurassic of the tethys and in the middle to upper devonian of the variscian mountains mass flows occur in the same formations and areas on the island of hydra greece it was possible to show that both phenomena are linked by internal breccias of well fitted limestone fragments they are indicative of slope or near slope positions and are very probably caused by flexure like deformations of shallow burial limestones seismic shocks producing overpressure may be instrumental in the disintegration of the semi lithified limestones thus providing the material for mass flows these events are associated with a tectonical subsidence as shown by deep water matrix in the brecciated shallow water limestones they occur in areas and periods of submarine rifting especially in late devonian and triassic time at least four triassic brecciation events can be traced from greece to the alps and correspond presumably to major transgressions in the cratonic basin of middle europe

the development of a single cleavage in an area of repeated folding abstract structural investigations in the variscan fold interference pattern of sw sardinia italy document an intriguing example of the relationship between cleavage folding and finite strain although palaeozoic sediments were affected by two major folding events both under identical low grade metamorphic conditions only a single penetrative cleavage formed which is not unambiguously correlated with either of these events cleavage orientation changes smoothly along sigmoidally curving trajectories throughout the interference pattern thus the same cleavage can be axial planar to either the first or the second phase folds or transect both when curving from one set into the other the singularity of the cleavage is supported by microscopic studies which show no evidence of a pre existing or overprinting tectonic cleavage finite strain determinations and numerous field observations demonstrate that cleavage always parallels the principal plane of finite strain even though local strain varies according to the relative position in the fold interference pattern similar strain patterns were achieved by forward modelling of superposed buckle folds based on our findings we propose i that the finite strain pattern is due to the superposition of the heterogeneous strain fields associated with the individual folding events and ii that cleavage reflects the cumulative deformations that effected the rock this has consequences for the regional interpretation of cleavage in our study area and elsewhere and for the distinction between deformation phases in general

a reassessment of the archean mesoproterozoic tectonic development of the southeastern chhattisgarh basin central india through detailed aeromagnetic analysis abstract we constrained the geological framework over polydeformed paleoproterozoic sonakhan greenstone belt and addressed the tectonic evolution of singhora basin in the fringes of bastar craton central india by utilizing aeromagnetic data interpretation 2 5d forward modelling and 3d magnetic susceptibility inversions the sonakhan greenstone belt exposes volcano sedimentary sequences of the sonakhan group within nnw sse to nw se trending linear belts surrounded by granite gneisses which are unconformably overlain by sedimentary rocks of chhattisgarh basin the orientations of aeromagnetic anomalies are coincident with geological trends and appear to correlate with lithology and geologic structure regional magnetic anomalies and lineaments reveal both nnw sse and ne sw trends prominent e w trending linear high amplitude magnetic anomalies are interpreted as the trans chhattisgarh aeromagnetic lineament tcal nw se trending aeromagnetic signatures related to sonakhan greenstone belt extends below the singhora sedimentary rocks and forms the basement in the west the analysis suggests that tcal is a block fault with northern block down thrown and affected the basement rocks comprising the sonakhan greenstone belt and samblapur granitoids the episode of faulting represented by the tcal is pre singhora sedimentation and played a vital role in basin evolution the basement configuration image generated by estimates of depth to magnetic basement suggests a complex pattern of nne ssw to ne sw trending depressions separated by a linear n s trending basement ridge it is inferred from the 3d magnetic susceptibility inversion that the thickness of sediments is more towards the eastern basin margin and the n s ridge is a manifestation of post sedimentary faulting results of 2 5d modelling of a wnw ese profile across the singhora basin combined with results from 3d inversion suggest suggests the basin subsidence was controlled by ne sw trending regional faults in an active system the basin geometry evolved by e w block faulting overprinted by ne sw trending pre to syn depositional normal faults generating ne sw depression which are affected by n s trend post sedimentary faulting though the present work relates the basin evolution with the initiation of rift basin it warrants further work to establish the deformation within the basin pertaining to the proximal thrust and uplift along the craton fringe

the early palaeozoic of the north atlantic region as a test case for the use of fossils in continental reconstruction abstract fossils have an important contribution to make in the reconstruction of early palaeozoic palaeogeography the north atlantic regions include well known early palaeozoic fossil faunas which should relate to the dispositions of continental plates during the period however different versions of ordovician palaeogeography based on fossils are described in the literature and in this paper we evaluate the influences which produce these discrepancies the most reliable results are obtained by combining analysis of faunal similarity especially shelf faunas with mapping the distribution of those biofacies which relate to former deep water sites planktic organisms may not be reliable palaeocontinent discriminants cladistic analyses of different groups of organisms living at the same early ordovician time point to the usefulness of trilobite data for palaeogeographic reconstruction such data indicate a three fold division of the north atlantic region into baltica laurentia and gondwana from the cambrian to at least the mid ordovician paris and robardet 1990 denied the existence of a tornquist s sea separating baltica from gondwana in the early ordovician it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos on the other hand there is no evidence of deeper water biofacies along the supposed line of rheic ocean separation of avalonia from gondwana until the late ordovician however there is some evidence of deeper biofacies along the nantes sardic line new term extending from southern armorica to sardinia

origin of peritidal carbonate cycles early cambrian sardinia abstract cyclic shallow water carbonates of early cambrian age were investigated in sardinia they consist of muddy lagoonal deposits at the base followed by a thin bed of intertidal sediments they in turn are overlain by spillover oolites cycles are locally restricted and bordered laterally by non cyclic sequences these coeval sediments consist of platform margin oolites on one side and mostly terrigenous clastics of an intertidal regime on the other side sedimentary conditions within these environments were probably stable during deposition no proof of periods of emergence is given therefore sea level changes seem less likely as the driving mechanism of cycle formation instead cyclicity is supposed to be intrinsically controled the model developed is based on a changing sedimentation rate high accumulation and building up to sea level intertidal stage and drop of sedimentation rate lag time increasing water depth after that sedimentation recovered and a new cycle started

stratabound ore deposits related to synsedimentary tectonics south west sardinia italy and sierra de aracena spain a comparison deposition of stratabound zn pb ba ores in the lower cambrian carbonates in south west sardinia show a distinct relation with the prominent tensional tectonics in the area a part of the mineral deposits are syngenetic sedimentary exhalative other massive sulfides and or barite are early diagenetic other occurrences mvt are from early to late diagenetic they are mostly confined to matrix and cementbreccias which might be dependent on the cambrian tectonics

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

algen als faziesindikatoren bioherme und biostrome im unter kambrium von sw sardinien archaeocythan limestones of the lower cambrian nebida fm sw sardinia form bioherms lower nebida fm matoppa mbr as well as biostromes upper nebida fm punta manna mbr in both cases algae are the main rock forming organisms they act as sediment binding and sediment baffling organisms and thus are responsible for the growth of these carbonates

shear wave splitting and waveform complexity for lowermost mantle structures with low velocity lamellae and transverse isotropy 1 shear waves that traverse the lowermost mantle exhibit polarization anomalies and waveform complexities that indicate the presence of complex velocity structure above the core mantle boundary synthetic seismograms for horizontally and vertically polarized shear waves sh and sv respectively are computed using the reflectivity method for structures with low velocity sheets lamellae and for comb like models approximating long wavelength vertical transverse isotropy vti motivated by evidence for partial melt in the deep mantle lamella parameter ranges include 1 vp from 5 to 10 vs 3 vp 2 100 to 300 km thickness of vertical stacks of lamella 3 lamella spacing and thickness varying from 0 5 to 20 km and 4 lamellae concentrated near the top bottom or throughout the d region at the base of the mantle such lamellae represent in effect horizontally emplaced dikes within d excessively complex waveforms are produced when more than 20 of d volume is comprised of low velocity lamellae many lamellae models can match observed sdiff splitting 1 10 s delays of svdiff but typically underpredict scs splitting 1 4 s delays of scsv vti model parameters are selected to address d observations and include 1 0 5 to 3 anisotropy 2 discontinuous d shear velocity increases up to 3 3 d thicknesses from 100 to 300 km and 4 vti concentrated at the top bottom or throughout d vti models readily match observed splits of scs and sdiff we discuss lamellae and vti model attributes in relationship to waveform complexities splitting magnitude triplications from a high velocity d discontinuity and apparently reversed polarity svdiff onsets the possible presence of melt filled lamellae indicates that local chemical or thermal perturbations can produce regions that exceed the solidus within d such melt could occur in the bulk of d because the melt is either close to neutral buoyancy advective velocities exceed percolative velocities or both

early rifting deposition examples from carbonate sequences of sardinia cambrian and tuscany triassic jurassic italy an analogous tectono sedimentary and climatic context lower cambrian ceroide limestone sardinia and lower jurassic massiccio limestone tuscany belong to sequences deposited in analogous tectono sedimentary context the former linked to the caledonian sardic phase the latter to the alpine orogeny both units consist of massive pure limestone characterized by marginal and lagoonal sequences repeatedly interfingering in the same geological structure this distribution indicates a morphology of the platforms composed of banks marginal facies and shallow basins lagoonal facies comparable with a bahamian complex dolomitization affects patchily the massive limestone bodies and karstic features breccias and sedimentary dikes occur at their upper boundary both units overlie early dolomite and evaporites sabkha facies containing siliciclastic intercalations in their lower and or upper part and are unconformably covered by open shelf red hematitic nodular limestone ammonitico rosso facies the sedimentary evolution of the two sequences appears to have been controlled by synsedimentary tectonics whose major effects are the end of the terrigenous input the bank and basin morphology of the platform the irregular distribution of the dolomitization and the nodular fabric of the overlying facies the end of the bahamian type system is marked by the karstification of the emerged blocks and is followed by their differential sinking and burial under red nodular facies from amore geodynamic viewpoint sequences composed of bahamian like platform carbonates followed by ammonitico rosso facies imply deposition along continental margins subjected to block faulting during an extensional regime connected with the beginning of continental rifting moreover the variation from sabkha to bahamian conditions suggests the drifting of the continent from arid to humid tropical areas less

relationship between eustatic sea level changes and basement subsidence geohistory reconstructions based on well data provide a direct and quantitative method of estimating basement subsidence geohistories in addition to responding to rates of sediment accumulation and to local and regional subsidence mechanisms also reflect eustatic sea level changes we have determined quantitatively that component of subsidence that is common to a worldwide set of wells after compensating for the response of the crust to sedimentation we find some correlations between the global rate of subsidence and low frequency changes in the height of eustatic sea level as determined from the stratigraphic interpretation of seismic data these results suggest that basement subsidence data which are independent of the techniques used previously may be used to re ognize variations in eustatic sea level however we are unable to determine the size of the eustatic sea level excursions because of difficulties in determining and separating variations in lithospheric rigidity thermal subsidence and sediment supply independent of tectonic subsidence and sea level variation we compare subsidence curves in an empirical fashion with published eustatic sea level curves thus avoiding the application of geologic models involving crustal properties we cannot quantify geohistory analysis is also a powerful means of studying subsidence on a regional scale and so of deducing the regional tectonic history we illustrate this point by subsidence maps through time for the north sea

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

effects of d anisotropy on seismic velocity models of the outermost core we explore effects of shear wave anisotropy in d on seismic waves utilized in constructing models of the outermost core reflectivity synthetic seismograms approximating transverse isotropy ti in d yield absolute and differential time perturbations of s sks and skks of up to several seconds especially if laterally varying anisotropy is considered several studies have used anomalies of this magnitude to infer a 0 5 2 5 p velocity reduction in the outermost 50 150 km of the core suggesting a stably stratified layer just beneath the core mantle boundary our data samples the mantle and core beneath alaska and northern pacific a region with strong lateral variations in d anisotropy ti models that predict the magnitude of shear wave splitting seen in the data can account for much but not all of the anomalies that isotropic models attribute to outermost core velocity reductions future outermost core seismic studies should address such d anisotropy effects

on seismic resolution of lateral heterogeneity in the earth s outermost core issues concerning resolution of seismically determined outermost core properties are presented with an example from three earthquakes in the fiji tonga region travel time behavior of the commonly used family of smks waves which travel as s in the mantle p in the core reflecting m 1 times at the underside of the core mantle boundary cmb are analyzed over a large distance range 125 165 data having wavepaths through an area of known d heterogeneity 2 exhibit systematic anomalies in smks differential times two dimensional wave propagation experiments demonstrate how large scale lower mantle velocity perturbations can explain long wavelength behavior of such anomalous smks times though heterogeneity on smaller scales may be responsible for the observed scatter about these trends if lower mantle heterogeneity is not properly accounted for in deriving a core model misfit of the mantle model maps directly into core structure the existence of outermost core heterogeneity is difficult to resolve at present owing to uncertainties in global lower mantle structure resolving a one dimensional chemically stratified outermost core also remains difficult owing to the same uncertainties inclusion of the slowly accruing broadband data should help in this regard restricting study to higher multiples of smks m 2 3 4 can help reduce the effect of mantle heterogeneity because of the closeness of the mantle legs of the wavepaths smks waves are ideal in providing additional information on the details of lower mantle heterogeneity

genesis of stromatactis cavities between submarine crusts in palaeozoic carbonate mud buildups it is concluded that the sedimentary structure stromatactis is a calcite cement fill of a swarm of labyrinthine cavities developed in marine carbonate muds of palaeozoic bioherms commonly with geopetal sediment field and petrographic evidence is examined from the ordovician of dalarna sweden the devonian of the canning basin australia and of belgium and the carboniferous of lancashire u k in all four occurrences the host microspar encloses masses of radiaxial fibrous calcite with sub horizontal floors following other workers radiaxial fibrous calcite is regarded as an altered marine cement a conclusion supported by the presence of overlying geopetal sediments with marine fossils widespread evidence of early submarine cementation in palaeozoic mud mounds isotopic analyses and the presence of microdolomites the stromatactis masses are separated from each other by laminae of host biomicrospar which show discontinuities resembling fractures which are healed with the early marine cement or filled with marine internal sediments the fractures are therefore of submarine origin the evidence points to episodic submarine lithification of the carbonate muds yielding successive crusts between which cavities were eroded and then filled the crusts were broken and rotated during bioturbation differential compaction or shearing during burial

shelf to basin transition in the cambrian lower ordovician of sardinia italy reply in southern and central sardinia two different cambrian lower ordovician sedimentary belts occur the former is exposed in southwestern sardinia iglesiente and sulcis and is characterized by shallow marine siliciclastic and carbonate sediments settled on a continental shelf with lowangle morphology the latter exposed in the nappe zone central and southeastern sardinia consists of a thick monotonous sequence of siliciclastic deposits settled in a deeper water basin

facies analysis and palaeogeography of the rytteraker formation lower silurian oslo region norway abstract the llandovery rytteraker formation is a carbonate dominated lithologic unit which extends over the whole oslo region eight composite sections reveal five characteristic facies associations these represent different laterally coexisting depositional settings of an incipient storm dominated carbonate platform which developed out from the transition zone between an open and a restricted marine basin a mixed carbonate siliciclastic shelf lagoonal unit a bioclastic shoal barrier unit a bioherm unit a tidal delta shallow shelf association and a mud dominated deeper neritic unit are presented with their typical inventory of sedimentary structures a possible tidal inlet fill sequence is reported from the skien district where it is incised into bioclastic shoal deposits biostratigraphic correlation points to diachroneity of facies units and reveals their lateral and vertical association as being due to the migration of depositional belts general palaeogeographic implications of the observed facies pattern are discussed and an attempt is made to place them into their global palaeoclimatic and geotectonic context

on the inversion of sd particle motion for seismic anisotropy in d in order to make accurate estimates of anisotropy in d as measured from observations of sdi f f sd it is necessary to correct the observed waveforms for anisotropy outside of the deepest mantle as well as side refraction along the path and to pick arrivals of svd and shd as accurately as possible we show that corrections based on estimates of upper mantle anisotropy inferred from sks skks splitting measurements are sometimes insufficient and propose a master event method to directly obtain more accurate corrections on specific source station paths our study illustrates the complexity of structure that needs to be taken into account when addressing issues of d properties using seismic body wave methods we also describe a method to accurately measure the arrival time of the corrected sv d wave

enhanced resolution of geological structures from magnetic data an example from the abitibi greenstone belt of northern ontario structural information about that portion of the economically important destor porcupine deformation zone and its associated faults which lie buried under many metres of clay belt sediments in the region north of matheson ontario must rely principally on interpretation of geophysical data in this area the archean greenstones of the abitibi are crosscut by at least three generations of strongly magnetic diabase dykes the predominant dyke set the matachewan trend north south orthogonal to structures in the greenstones directional horizontal derivative analysis of the total magnetic field which has a much sharper angular cutoff than that provided by standard fourier based filters provides a method for separating the magnetic contributions from these two sources this analysis revealed the existence of a previously unidentified fault the winslow fault located to the north of and parallel to the economically significant pipestone fault we also define a second new fault the carr fault that is

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

les deformations ordoviciennes dans la zone des sehoul maroc septentrional une orogenese caledonienne en afrique du nord the sehoul zone of northern morocco differs from the remainder of the hercynides in having the structural signature of an earlier event of deformation folds and thrusts overturned to the south aff

cryptalgal metazoan bioherms of early ordovician age in the st george group western newfoundland bioherms are common in the st george group a sequence of shallow water carbonate rocks deposited on the western continental shelf of iapetus ocean they range from small heads and metre sized mounds to extensive banks and complexes many metres thick and hundreds of metres in lateral extent the cores of these bioherms are principally composed of thrombolites unlaminated branching columnar stromatolites structures quite distinct from laminated stromatolites which are common in intertidal beds associated with thrombolites is a diverse fauna of burrowing invertebrates trilobites nautiloids pelmatozoans brachiopods gastropods rostroconchs and archaeoscyphiid sponges on the basis of framework building components three main bioherm types are distinguished 1 thrombolite mounds 2 thrombolite lichenaria or sponge mounds and 3 thrombolite lichenaria renalcis reef complexes the framework of the last is the most complex with abundant cavities and a demonstrably uneven growth surface of thrombolites corals and free standing renalcis heads walls and roofs some cavities were active sediment conduits while others were protected their roofs draped with renalcis and their walls coated by cryptalgal laminites these bioherms possess the attributes of shallow water ecologic reefs they span a critical time gap in the development of reefs the transition period from algal dominated bioherms of the precambrian and cambrian to the metazoan dominated bioherms of the middle ordovician and remaining phanerozoic

microbial signatures in peritidal siliciclastic sediments a catalogue a catalogue of microbial structural signatures is presented based upon the coupling of fundamental biogeochemical microbial processes and local morphogenetic determinants it summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones temperate humid mellum island southern north sea subtropical arid coast of southern tunisia textural geometries reveal a high structural diversity but their determinants are primarily based upon six major parameters 1 intrinsic biofactors structural diversification of sedimentary microbial films and mats inherent in the organisms i e their construction morphology growth taxis and behaviour and local abundance of specific morphotypes most prominent are the ensheathed filamentous cyanobacteria microcoleus chthonoplastes and lyngbya aestuarii and the sheathless filamentous cyanobacterium oscillatoria limosa 2 biological response to physical disturbances sediment supply erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats 3 trapping binding effects physicobiological processes give rise to grain orientations and wavy to lenticular lamina lamina specific grain arrangements and sucrose calcium carbonate accumulations 4 secondary physical deformation of biogenic build ups mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures 5 post burial processes textural fabrics that evolve from mechanical effects of gas formation from decaying mats and features related to the formation of authigenic minerals calcium carbonates calcium sulphates pyrite 6 bioturbation and grazing post depositional structures such as skolithos type dwellings traces of burrowing insects gastropod grazing traces and faecal pellets in synopsis the catalogue firstly comprises a sound set of ubiquitous signatures this uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings the catalogue secondly documents signatures that are extremely sensitive to tidal position hydrodynamic regime and overall climatic conditions these kinds of signature indicate narrow facies zones which often coincide with the activity or dominance zones of certain organisms an overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings

jabali a zn pb ag carbonate hosted deposit associated with late jurassic rifting in yemen the jabali deposit 3 8 mt at 16 zn 2 pb and 132 g t ag is hosted by dolomitized platform carbonates of kimmeridgian age at the southwestern edge of the oil producing wadi al jawf rift basin in northern yemen paleogeographical reconstructions demonstrate that tensional synsedimentary tectonic activity from the late jurassic to early cretaceous was responsible for the thick accumulation of argillaceous and evaporitic sediments in the subsident rift basin the unstable margin of which was the site of rapid facies changes local disconformities and periods of emergence as well as of dolomitization along the wnw and nnw striking boundary fault system in the jabali area the upper part of the jurassic sequence underwent two stages of dolomitization before emergence and deep karstic erosion solution cavities and depressions in the eroded surface were filled by dolomite sand and black pyritic mudstone prior to a last marine transgression of limited extent subsequent ore deposition and associated late dolomitization sealed the network of solution cavities impregnating the dolomite sands and the host dolomites sphalerite i and wurtzite followed by silver bearing zoned sphalerite ii associated with galena crystallized from a cyclic influx of low temperature 75 100 c saline solutions lead isotope geochemistry indicates that the lead zinc and silver probably originated from an early proterozoic basement the dissolved metals were likely derived from the basal aquifer detrital material of basement origin of the evaporite bearing sequence filling the wadi al jawf trough migrating metalliferous brines from the basin to the uplifted jabali area where ore deposition was favoured by a reducing environment were probably channelled by the boundary fault system during the last stages of synsedimentary tectonic activity

the absence of echinoderms from the lower cambrian chengjiang fauna of china palaeoecological and palaeogeographical implications abstract although deuterostome phylogeny has now achieved a good degree of consensus echinoderm emergence and early evolution remain highly enigmatic different phylogenetic scenarios have emerged based partially on early cambrian enigmatic animals of the chengjiang biota yuanshan formation cambrian series 2 yunnan china interpreted as primitive echinoderms they include cambrofengia yunnanensis hou et al 1999 cotyledion tylodes luo and hu 1999 and two vetulocystids dianchicystis jianshanensis and vetulocystis catenata shu et al 2004 their interpretation as echinoderms is here critically revised and c tylodes based on newly sampled specimens is tentatively re interpreted as a cnidarian polyp as a result it appears that the chengjiang biota one of the most diverse and earliest recorded communities is most probably devoid of echinoderm representatives this is not interpreted as a taphonomic artefact but as a palaeoecological signal of the radiation of echinoderm during the early cambrian on carbonate substrates up to ten exceptional biotas are described from the yangtze platform south china from cambrian stage 2 to early stage 5 in siliciclastic environments varying from shallow platform to basin slope the absence of echinoderms in those burgess shale type deposits from cambrian stages 2 and 3 along with their report from laterally coeval carbonate sediments argue for a proximal radiation of echinoderms during early cambrian times in carbonate environments they would have later colonized siliciclastic proximal and distal environments the same colonization sequence is generally observed at a global scale with the first appearance of echinoderms varying in time according to local palaeogeographical controls

cambroclaves from the cambrian of sardinia italy and germany constraints for the architecture of western gondwana and the palaeogeographical and palaeoecological potential of cambroclaves cambroclaves represent a group of problematic microfossils previously known from strata close to the early middle cambrian boundary of only a few palaeogeographic regions kazakhstan china australia because of their frequent occurrence as disarticulated remains they have hitherto been assumed to be provincially restricted microfossils of unclear palaeobiological affinity discoveries of cambroclaves from the early to early middle cambrian of southern sardinia and central germany europe point to a much wider distribution during their short stratigraphic range and imply closer palaeogeographic relations between the european shelf of western gondwana and the areas from which cambroclaves were previously known these relations are also supported by the common occurrence of other small shelly fossils these facts support the existence of a widespread uniform facies belt shelf around parts of gondwana during the early middle cambrian time interval and contradict the interpretation of the european depositional areas as isolated basins or as distinctly separate cambrian terranes the western gondwana cambroclaves occur in carbonate successions indicative of special palaeoecological conditions the specimens are limited to distinct layers formed during transgressive phases that opened inner and partly restricted platform areas to open marine and more distal deeper subtidal environments possibly accompanied by a transition from a rather arid to more humid climatic conditions because of the short stratigraphic window of occurrence and of distinctive facies characteristics cambroclaves are palaeoecologically and palaeobiogeographically useful and consequently contribute important evidence for both the reconstruction of the perigondwana realm and the relations to other palaeocontinents in the cambrian 7 2003 elsevier science b v all rights reserved

an unusual occurrence of an exploitable k feldspar deposit hosted in the ordovician porphyroids southern sardinia geology mineralogy geochemical features and economic potential abstract an atypical potassium feldspar deposit of real economic potential has been found at arcu de su bentu southern sardinia italy the massive mineralization occurs in the middle ordovician porphyroids metavolcanic complex which consists of dominant metarhyolite and minor metarhyodacite lavas and or ignimbritic flows erupted in a sub aerial environment during variscan orogenesis the mineralized area was affected by low grade anchizone to greenschist facies metamorphism the deposit generally shows uniform macroscopic textural features over the entire outcrop the significant presence of k feldspar sanidine and quartz megacrystals the latter being generally smaller than the former mafic minerals do not occur in significant proportions although minor amounts of biotite and muscovite occur locally besides the uncommon lithological type the typical feature of this deposit is the unusual coarseness 10 to 20 cm in diameter of the potassium feldspar crystals the favourable geomorphological conditions of the area high k 2 o contents 6 to 8 wt high proportion of k feldspar in the mineralized rock 35 to 51 vol and large reserves estimated at 4 368 000 t in just one restricted part of the deposit are all favourable conditions for future economic exploitation in spite of this mineralogical and chemical impurities may adversely affect the possible use of k feldspar from arcu de su bentu in some conventional applications e g manufacture of high quality products such as ceramics to assess the real future economic development of the deposit market specific beneficiation is required although it is important to note that the bulk raw materials even if they have a lower market value could be still be targeted to specific industrial sectors e g red gres

northeast trending early proterozoic dykes of southern superior province multiple episodes of emplacement recognized from integrated paleomagnetism and u pb geochronology integrated paleomagnetic and u pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of early proterozoic tholeiitic dykes of northeast trend in the southern superior province previously referred to collectively as preissac dykes it is demonstrated that they are readily separated on the basis of paleomagnetism into subsets referred to as the biscotasing and senneterre swarms in addition a pair of unnamed dykes may be associated with the north and northwest trending matachewan swarm farther west biscotasing dykes have a down west magnetization of single polarity with a corresponding paleopole at 27 8 n 136 7 w dm 12 3 and dp 9 4 senneterre dykes carry an up north or occasionally down south direction with corresponding paleopole at 15 3 s 75 7 w dm 7 0 dp 4 4 the senneterre direction is indistinguishable from the primary n1 remanence direction that dominates the magnetization of nipissing sills of the southern province paleomagnetic fiel

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

models for generation of carbonate cycles computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences to model carbonate cycles the authors can use water depth dependent sedimentation rate for each facies an initial lag time linear subsidence tidal range and period and amplitude of sea level oscillation about a horizontal datum tidal flat capped cycles up to a few meters thick result from low amplitude sea level oscillation of a few meters and short lag times nonerosive caps reflect sea level lowering being balanced by subsidence and basinward migration of the shoreline not exceeding tidal flat progradation rate when higher amplitude sea level oscillations occur the tidal flats are abandoned on the inner shelf during sea level fall because seaward movement of the strandline outpaces progradation rate of flats increased amplitude also results in sea level falling faster than flats can subside so that disconformities with thick vadose profiles develop high amplitude 100 m or more oscillations result in incipient drowning of platforms and juxtaposition of deep water facies against shallow water facies within cycles sea level falls before the platform can build to the sea level highstand and the shoreline migrates much more rapidly than tidal flats can prograde thus cycles are disconformity bounded and lack tidal flat caps 10 references

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

evidence for anisotropy in the deep mantle beneath alaska we consider the possibility that the velocity structure of d is anisotropic the data we examined consist of seismograms from 9 deep japanese earthquakes recorded at wwssn receiver stations in north america the source receiver combinations span distances of 70 106 with associated s waves passing through d beneath alaska differential travel times of the s scd scs and sks phases are used to constrain the velocity structure in d shear waves refracted by d are observed beyond 72 2 and provide a sensitive measurement of the velocity structure in d beyond 93 the vertically polarized sv and horizontally polarized sh shear waves often appear distinctly split although at distances less than 89 the components are more nearly synchronous near 94 sh occurs as a double arrival sv in this range however remains a single arrival roughly synchronous with the second sh arrival we have been unable to reproduce these effects in isotropic model synthetics synthetics for transversely isotropic models have been computed that do match these waveforms the anisotropy was constrained to be only within d with a vertical symmetry axis we conclude that these observations may be explained by an anisotropic d layer the d discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above the core mantle boundary

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios