intercontinental correlation by sea level events in the early silurian of north america and china yangtze platform potential for eustatic sea level changes on continental platforms during early silurian llandovery time was highly propitious due to the double effects of lingering glaciation in gondwana and the rapid rate of sea floor spreading which assisted the general dispersion of many era tonic realms through middle paleozoic oceans following the initial mass melting of the late ordovician early silurian glaciers in north africa and arabia four smaller cycles of sea level fluctuation were recorded by recurrent shelly communities in a predominantly carbonate setting on the north american platform peaks in sea level rise occurred in late rhuddanian or early idwian a 4 b 1 positions early fronian c 1 position early telychian c 4 position and late telychian c 6 position times although absolute time control is poor the periodicity of these events may have been every 2 5 m y in the more submerged regions of the north american platform such as iowa and anticosti island diverse faunal associations distinguished by the presence of stricklandiid brachiopods were especially productive at peak times of transgression co occurrence of the calcareous green alga cyclocrinites with certain of these faunas indicates that maximum water depth was still within the photic zone at the other extreme low stands in sea level are identified by diminished faunal diversity as well as a combination of typical sedimentary structures and evaporite indicators located on a well defined precambrian craton the yangtze platform is now a large region in central and southwestern china during successive stages of the early silurian a thick sequence of graptolitic shales shelly carbonates and red beds were deposited on the platform of the four sea level peaks defined in north america three are recognized in the yangtze region there is very good evidence for a latest llandovery or earliest wenlock transgression based on the extensive distribution of the salopinella coronocephalus sichuanoceras fauna another clear and well dated sea level peak involves the occurrence of faunas with characteristic stricklandiid and pentamerid brachiopods at a level equivalent to the lower telychian c 4 position the initial silurian transgression appears to have reached its peak on the yangtze platform primarily during early idwian time b 1 position but this is based less surely on changing patterns in graptolite diversity at present there is no compelling evidence for a widespread early fronian c 1 position transgression in china successful matching of the other sea level events however substantially refines intercontinental correlation for lower silurian strata in the yangtze region of china and the central regions of north america

thrust fault growth and segment linkage in the active ostler fault zone new zealand abstract thrust faulting is a fundamental mode of crustal deformation yet many of the key geometrical attributes of thrust faults and the controls on fault rupture growth and linkage remain poorly resolved numerous surface rupturing thrust faults cut through upper quaternary glacial outwash terraces within the ostler fault zone an active thrust system in the southern alps new zealand we use these deformed marker surfaces to define the three dimensional deformation field associated with their surface expression and to map displacement and length on 40 fault segments displacement transfer across two fault segment arrays occurs in distinctly different styles in one displacement is transferred between en echelon fault segments to produce a smooth linear displacement gradient in the other large scale folding and a population of small faults transfer displacement between two non overlapping fault segments with a residual displacement minimum within the transfer zone size distribution of fault segment length and maximum displacement follow a power law scaling relationship maximum displacement d max scales linearly with and represents 1 of segment trace length l d max l data from the segments of the ostler fault zone display similar scaling to a global fault dataset this similarity is surprising given that the observed displacements along the ostler fault represent only 20 ky of slip on fault segments that are likely to be many times older

earthquakes in northern europe in 1375 1989 abstract an updated version of the catalogue of earthquakes in northern europe is presented it comprises events inside a region window of about 55 80 n and 10 w 45 e covering a 615 year period from 1375 to 1989 the data have been compiled using all the available historical publications catalogues studies and reports of the region and for events from 1984 to 1989 a regional seismic parameter database maintained by the institute of seismology of the university of helsinki the number of events exceeds 5200 of which more than 40 have been registered since 1980 the huge increase of event reports is the result of the installation of new high frequency networks for microearthquake studies and the upgrade of the seismic instrumentation because of the different degree of accuracy and homogeneity of the parameter data the events were divided into historical 1375 1964 and instrumental 1965 1989 datasets the source parameters are mainly macroseismic for historical events and obtained by iterative location procedure for instrumental data by assuming internal homogeneity between the different regional magnitude scales we estimated the catalogue to be complete for events with magnitude 4 5 since the 1880 s magnitude 4 0 4 4 since the 1940 s and magnitude 3 5 3 9 since the 1970 s although the distribution of seismicity exhibits similar patterns in both datasets the clustering of epicenters and offshore seismicity is more distinct in the instrumental period the occurrence of the greatest events can be seen more clearly in the historical data the earthquake epicenters are concentrated in the geologically younger tectonic provinces in the western part of the region and follow with some exceptions the tectonic fracture and fault zones in the fennoscandian area 80 of the earthquakes occurred in the upper 20 km of the earth s crust the deepest epicenters are in the southwestern corner of the region where the crust is thinnest

hydrogeochemical characterization of oilfield waters from southeast maracaibo basin venezuela diagenetic effects on chemical and isotopic composition free water and water in emulsion with oil from hydrocarbon wells onshore offshore lake maracaibo were extracted and analyzed the use of classical and new chemical diagrams evidenced that na hco3 and na cl are the main chemical facies whereas na so4 facies was rare and revealed only in faulted areas in which shallow groundwater was involved in deep circulation cretaceous ca cl waters were not revealed but their imprint on some samples was traced and evidenced by langelier ludwig brine differentiation and caexcess vs nadeficit diagrams these square plots allowing a discrimination between the water contributions from the different source rocks should be more exploited as hydrogeochemical tools in hydrocarbon exploration original data on monocarboxylic anions showed acetate concentration higher than ten times that of formate at sampling conditions but speciation under deep conditions indicated a reversed activity dominance calculation of saturation indexes and kinetic modeling showed an active precipitation of chalcedony kaolinite and clays especially where the temperature was lower t 100 c formation waters and lake water showed quite similar chloride concentration and hydrogen isotope composition however thermodynamic calculations at the oilfield p t conditions revealed chlorite and illite produced during previous diagenetic stages of misoa sandstones recrystallization to smectite and vermiculite in present day stage the diagenetic water produced in these reactions could have enriched the 18o isotopic composition of approximately 2 this isotope diagenetic effect was more evident in na hco3 waters which also showed a decrease in chloride at constant boron concentration as a consequence of clay dewatering in this regard a new boron geothermometer useful for formation waters is also proposed t c 5 1364 ln 1 b mg 1 5 728 0 180 0 164 0 012

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

a lancefieldian graptolite from the lake district large dictyonema species including d pulchellum t s hall have been collected from beds in the uldale fells northern lake district below strata yielding didymograptus protobalticus monsen they suggest the presence there of graptolitic beds transitional from lancefieldian to bendigonian in age approximately tremadoc arenig and suggest that the area may yet provide an internationally correlatable base to the arenig series in britain

focal mechanisms of three earthquakes in finland and their relation to surface faults abstract focal mechanisms for three recent earthquakes in finland are determined using p wave polarities together with sv p and sh p phase amplitude ratios the events occurred on may 11 2000 in toivakka central finland ml 2 4 on september 15 2000 in kuusamo northeastern finland ml 3 5 and on may 2 2001 in kolari western finnish lapland ml 2 9 in order to obtain reliable estimates of the source parameters one dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep seismic sounding results the starting models are modified by one dimensional ray tracing using the earthquake observations the events are relocated by employing p and s phase arrival times from the nearest seismic stations and the final velocity models synthetic waveforms calculated with the reflectivity method are used to further constrain and verify the source and structural parameters the toivakka earthquake indicates thrust or reverse faulting mechanism at a depth of 5 km after comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane 358 42 was the fault plane the best fitting fault plane solution of the kolari earthquake suggests pure thrust faulting at a depth of 5 km the nodal plane striking 035 30 correlates well with surface observations of the postglacial possibly listric fault systems in the source area the kuusamo earthquake focal depth 14 km has a normal faulting mechanism with the nodal planes trending 133 47 or 284 47 preference is given to the sw dipping nodal plane as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age the three earthquakes have occurred in old precambrian faults and shear zones which have been reactivated the reactivated faults are favourably oriented in the local stress field

provenance constraints from detrital zircon u pb ages in the nw iberian massif implications for palaeozoic plate configuration and variscan evolution detrital zircons from pre orogenic upper ordovician to devonian low grade metasedimentary rocks have been dated by laser ablation inductively coupled plasma mass spectrometry la icp ms to 1 constrain the variations of zircon age populations within the autochthonous sequences of nw iberia 2 compare u pb detrital zircon ages with previous data 3 test the hypothesis of the armorica microplate as a peri gondwanan terrane separated from gondwana the similarity in the ages of detrital zircons found in the palaeozoic samples studied here to those published for the upper proterozoic lower cambrian and lower ordovician sediments of nw iberia argues against the separation of armorica and points to a common source area for all of the palaeozoic detrital formations of nw iberia the west african craton and the surrounding pan african belts la icp ms u pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as early namurian this age is also a maximum age limit for thrusting and refolding in this part of the iberian massif correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of nw iberia and synorogenic sedimentation

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

lake sediment evidence of paleoseismicity timing and spatial occurrence of late and postglacial earthquakes in finland abstract sediment profiles in six lake basins located near the late and postglacial fault surface ruptures in northern finland were investigated for turbidites and soft sediment deformation structures that could reveal information on the timing and spatial occurrence of paleoearthquakes in the area we used geophysical sub bottom profiling and lithological logging of sediment cores combined with radiocarbon and paleomagnetic dating in three lakes immeljarvi aakenus and laukkujarvi sediment records provided evidence of postglacial earthquakes or landslides whereas in three others lakes pyhajarvi kallojarvi and syvajarvi no disturbance structures were observed in their sequences that cover roughly the holocene period combined with previous data on paleoseismicity in finland the results enhance the non stationary hypothesis of seismicity throughout the late weichselian and the holocene with three general maxima that cluster around the deglaciation 12 0 10 0 kyr bp the mid holocene 5 0 7 0 kyr bp and the late holocene 1 5 3 0 kyr bp a strong asymmetry of the offset profiles of the finnish postglacial fault complexes indicate that the general rupture propagation was from sw to ne the present data also indicates that the oldest and highest magnitude events mw 7 or higher generally took place in the sw sector of the finnish pgf complexes during or soon after the retreat of the continental ice sheet the ruptures then propagated towards the ne sector of pgf complexes in a cascading fashion and were generally characterized by lower magnitude events mw 6 during the holocene

major earthquake at the pleistocene holocene transition in lake vattern southern sweden lake vattern sweden is within a graben that formed through rifting along the boundary between two precambrian terrains geophysical mapping and geological coring show that substantial tectonic movements along the lake vattern graben occurred at the very onset of the holocene this is evident from deformation structures in the soft sediment accumulated on the lake floor our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a 80 km long fault system if these large displacements are from one tectonic event lake vattern must have had an earthquake with seismic moment magnitudes to 7 5 in addition our geophysical mapping shows large landslides along sections of the steep lake shores pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the younger dryas preboreal transition ca 11 5 ka we suggest that this event is mainly related to the rapid release of ice sheet load following the deglaciation this paleoseismic event in lake vattern ranks among the larger known intraplate tectonic events in scandinavia and attests to the significance of glacio isostatic unloading

distribution and morphology of landslides in northern finland an analysis of postglacial seismic activity abstract the ages and sizes of landslides occurring in seismically active areas can be used to reconstruct the seismic history of the area and estimate the maximum moment magnitudes of past earthquakes here we present a data set of 121 landslides discovered in northern finland that were analyzed for their morphometric characteristics we show that 89 debris slide type landslides in the data set are clustered close to known postglacial faults pgfs and thus provide information on the characteristics of postglacial paleoseismic events by using empirical correlations between the landslide volume area data and earthquake moment magnitude we estimate maximum moment magnitudes m w 6 9 7 7 for postglacial earthquakes in the suasselka isovaara riikonkumpu venejarvi and vaalajarvi areas where earlier estimates based on fault length and displacement have yielded magnitudes varying between m w 6 5 and 7 5 we also show that the landslides in northern finland are located within a radius of 35 km from the closest known pgf and that sizes of the landslides decrease as a function of distance from pgfs hence providing strong empirical evidence for their seismic origin as far as we are aware this is the first use of landside data in quantifying postglacial seismicity within the fennoscandian shield area

acritarch marine microphytoplankton diversity in an early ordovician deep water setting the skiddaw group northern england implications for the relationship between sea level change and phytoplankton diversity early ordovician acritarch microphytoplankton assemblages from an outer shelf to slope facies in northern england show changes in diversity that parallel changes in sea level with high diversity occurring during lowstands and low diversity during highstands the pattern is the reverse of that recorded in the literature in which high acritarch diversity in continental platform successions has been related to periods of transgression and highstand and low diversity to periods of regression and lowstand the apparent contradiction can be explained if high diversity assemblages move further onto continental platforms during transgression and highstand but towards basins during regression and lowstand the hypothesis that high diversity acritarch assemblages track onshore and offshore during episodes of sea level change implies that there is a direct causal link between sea level and microphytoplankton diversity at least in so far as changes in sea level influence the location of high diversity the direct physical influence of sea level change on phytoplankton diversity might have been restricted to relatively small scales however its most obvious effect being perhaps to shift the locus of maximum diversity rather than affect the overall level of diversity at larger temporal and spatial scales the direct influence of sea level change on acritarch diversity is less certain parallel large scale changes in sea level and phytoplankton diversity have been documented for the phanerozoic as a whole but possibly reflect either independent responses to other factors such as the effects of tectonic cycles and changes in palaeogeography ocean circulation and climate or else indicate an indirect influence in the sense that sea level rise created conditions under which phytoplankton diversity could increase in contrast to the results discussed in this paper other analyses of ordovician acritarch diversity at larger spatial scales than those considered here have concluded that there is no evidence for any causal link between sea level change and diversity if however the controls on diversity exerted directly by changes in sea level are scale dependent and only apparent at relatively small scales analyses at larger spatial scales that compile measures of diversity across a range of depositional environments i e that sample both onshore and offshore environments and combine the results might not evince a correlation between changes in sea level and diversity in addition to producing shifts in the location of high diversity assemblages sea level changes also appear to be accompanied by at least local turnover in species composition during the early ordovician in northern england the high diversity assemblages associated with lowstands are very different in composition elsewhere in cratonic settings maximum flooding intervals within successive early palaeozoic stratigraphic sequences are characterised by markedly different acritarch assemblages hence changes in sea level appear to be associated with local extinctions and originations as well as changes in the locus of high diversity possibly as a result of ecological stresses arising from changes in the location of acritarch assemblages

late ordovician mass extinction in the selwyn basin northwestern canada geochemical sedimentological and paleontological evidence we present a detailed study of the trace element and stable isotope geochemistry sedimentology and fossil distributions in two avalanche lake av4b av1 ordovician silurian boundary sections in the selwyn basin trilobites and conodonts indicate a profound extinction at the end of the ordovician which is constrained stratigraphically within a 3 in magnitude is recorded in the extinction interval iridium abundances 0 051 ppb in the extinction interval are low and fail to provide evidence for an impact the highest ir abundance is found to be associated with reduced sedimentation in a condensed horizon cerium anomalies indicate a s

microearthquakes illuminate the deep structure of the endglacial parvie fault northern sweden at 155 km the parvie fault is the world s longest known endglacial fault egf it is located in northernmost sweden in a region where several kilometre scale egfs have been identified based on s

relationships between syn orogenic sedimentation and nappe emplacement in the hinterland of the variscan belt in nw iberia deduced from detrital zircons flysch type syn orogenic deposits of carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the variscan belt of nw iberia new u pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry la icp ms are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting the age populations of four syn orogenic formations are compared with those of the pre orogenic sequence in the autochthon and parautochthon representing the gondwanan passive margin and in the allochthon formed by peri gondwanan and oceanic terranes in addition a new structural study has been carried out to understand the relationships between the syn orogenic deposits and the development of variscan structures the aims are to identify the sources of sediments and to establish the relationship between variscan structural evolution and syn orogenic sedimentation development of a forebulge outwards from the allochthonous front deduced from the structural study suggests the existence of depocentres that hosted the syn orogenic sediments together with the trend shown by the more recent zircons in each formation that are younger towards the external zones the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement the zircon age populations point to the allochthon as the main source of detritus for the syn orogenic basins with perhaps a limited participation of the parautochthon and autochthon in the younger formations

marine redox variability from baltica during extinction events in the latest ordovician early silurian abstract it is well documented that upper ordovician and silurian successions record multiple marine turnover events including the second largest mass extinction in the phanerozoic widespread glaciation and multiple global carbon cycle perturbations whereas causal mechanisms for the late ordovician major mass extinction event involving climate paleoceanographic variation and 13c records have been published similar records remain poorly constrained for subsequent extinction events in the early silurian here we present new organic matter carbon isotope 13corg chemostratigraphy and corresponding paleoredox proxies fe speciation mn v mo and pyrite sulfur isotopes from two organic rich drill core sections in sweden and latvia that span the upper katian through lower wenlock stages 446 431 ma pyritized fe and bulk sedimentary mn concentrations from upper ordovician strata in southern sweden suggest a local redox shift to more reducing conditions in the late hirnantian possibly including euxinic anoxic and sulfidic water column conditions that coincide with the second mass extinction pulse the new high resolution 13corg and 34spyr datasets from the late aeronian early silurian interval within both drill cores show positive excursions that are broadly coincident with the associated sandvika and sedgwickii extinction events independently fe speciation and bulk sedimentary trace metal data from this late aeronian interval record locally euxinic conditions in both the deep basinal sweden and distal shelf latvia settings before and during the late aeronian positive 13c excursion this multiproxy paleoredox dataset provides the first direct evidence for local to regional expansion of reducing marine conditions coincident with this early silurian late aeronian biotic event and positive 13c excursion additionally new 34spyr values spanning the llandovery wenlock boundary interval in the latvia core show a positive excursion coincident with fe speciation and trace metal enrichments that imply a local redox perturbation with intermittently euxinic bottom waters during the rising limb of the ireviken positive 13c excursion the combination of these geochemical data for local to regional scale more data required for global interpretations changes in marine redox conditions with paleobiological records and evidence for eustatic sea level rise indicate that environmental stresses related to an expansion of anoxic to euxinic conditions were a probable driver for several extinction events during the latest ordovician early silurian

marginal faunal belts and their structural implications with examples from the lower palaeozoic fossils provide an independent criterion for testing structural hypotheses generated from field evidence or geochemistry as well as functioning in their traditional role in the dating of rocks or determining biogeographic realms on a global scale the identification of exterior oceanic biofacies may be of use in discriminating the course of former continental margins the isograptid graptolite biofacies early to middle ordovician can be distinguished from contemporary epicratonic graptolite biofacies its worldwide occurrence corresponds with continent margins reconstructed from other criteria hence a new isograptid record from irian jaya e new guinea although based on a small sample suggests that there may have been an ordovician oceanic site there on a continental scale the tectonic juxtaposition of different biofacies or of different stratigraphical packages contributes to an understanding of tectonics and can serve to quantify displacement the accretionary prism hypothesis for the southern uplands of scotland is supported and contrasted with the rearrangement of biofacies in the cambro ordovician cow head group of western newfoundland which reveals not only the original shelf margin slope topography now vanished but also provides an estimate of the movement involved in the emplacement of one taconic allochthon on a more local scale faunal evidence is used in assessing the original position of 9suspect9 marginal areas using the simplest possible model this shows for example that anglesey was not greatly separated from gondwana in the early ordovician but strike slip movement cannot be disproved local changes in relative sea level are determinable from sediments and faunas in marginal belts these may differ from the global eustatic curve and when this happens it is a measure of local tectonic activity particular sea level curves may serve to define structurally coherent units within mobile belts new sea level curves are contrasted for a segment of n wales north of the bala fault with that for central sw wales arenig llandovery the s welsh area mostly responded passively to the eustatic curve which can adequately account for shifts in facies and faunas while n wales had a curve often at odds with eustasy and reflecting its structural history

slip accumulation and lateral propagation of active normal faults in afar we investigate fault growth in afar where normal fault systems are known to be currently growing fast and most are propagating to the northwest using digital elevation models we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0 3 to 60 km faults exhibiting the elliptical or bell shaped slip profiles predicted by simple linear elastic fracture mechanics or elastic plastic theories are rare most slip profiles are roughly linear for more than half of their length with overall slopes always 0 035 for the dominant population of nw striking faults and fault systems longer than 2 km the slip profiles are asymmetric with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero whereas slip decreases roughly linearly and tapers in the direction of overall aden rift propagation at a more detailed level most faults appear to be composed of distinct shorter subfaults or segments whose slip profiles while different from one to the next combine to produce the roughly linear overall slip decrease along the entire fault on a larger scale faults cluster into kinematically coupled systems along which the slip on any scale individual fault or fault system complements that of its neighbors so that the total slip of the whole system is roughly linearly related to its length with an average slope again 0 035 we discuss the origin of these quasilinear asymmetric profiles in terms of initiation points where slip starts and barriers where fault propagation is arrested in the absence of a barrier slip apparently extends with a roughly linear profile tapered in the direction of fault propagation

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

the ordovician silurian boundary at the eastern end of anticosti island a hirnantia fauna is recorded for the first time from anticosti island situated 6 m below the beds interpreted here as representing the maximum regression at the eastern end of the island the latter coinciding with the local top of the revised ellis bay formation the ordovician silurian boundary on the island is discussed

early devonian pre acadian magnetization directions in lower old red sandstone of south wales uk summary two components of magnetization have been resolved from the lower old red sandstone of south wales all 39 sampled sites lie north of the variscan front and all but three are largely unaffected by variscan folding thirteen sites yield a high unblocking temperature magnetization component which pre dates acadian mid devonian folding the site mean direction for this component dec 232 2 inc 31 9 95 8 5 yields a pole position lat 7 3 s long 306 7 e which can be assigned an early devonian age a lower unblocking temperature component is present at 38 of the sampled sites this component post dates both the acadian and variscan folding has reversed polarity and a mean direction dec 193 8 inc 7 2 95 2 6 which corresponds to a pole position lat 40 4 s long 338 0 e consistent with a latest carboniferous or early permian age the early devonian pole position confirms the existence of a siluro devonian inflexion in the apparent polar wander path apwp for southern britain which coincides with an inflexion in the apwp for scotland britain north of the iapetus suture indicating the closure of the iapetus ocean by siluro devonian time the early devonian palaeolatitude 17 s 5 is consistent with an early devonian configuration in which gondwana laurentia baltica and avalonia were part of a single supercontinent

postglacial rebound and fault instability in fennoscandia summary the best available rebound model is used to investigate the role that postglacial rebound plays in triggering seismicity in fennoscandia the salient features of the model include tectonic stress due to spreading at the north atlantic ridge overburden pressure gravitationally self consistent ocean loading and the realistic deglaciation history and compressible earth model which best fits the sea level and ice data in fennoscandia the model predicts the spatio temporal evolution of the state of stress the magnitude of fault instability the timing of the onset of this instability and the mode of failure of lateglacial and postglacial seismicity the consistency of the predictions with the observations suggests that postglacial rebound is probably the cause of the large postglacial thrust faults observed in fennoscandia the model also predicts a uniform stress field and instability in central fennoscandia for the present with thrust faulting as the predicted mode of failure however the lack of spatial correlation of the present seismicity with the region of uplift and the existence of strike slip and normal modes of current seismicity are inconsistent with this model further unmodelled factors such as the presence of high angle faults in the central region of uplift along the baltic coast would be required in order to explain the pattern of seismicity today in terms of postglacial rebound stress the sensitivity of the model predictions to the evects of compressibility tectonic stress viscosity and ice model is also investigated for sites outside the ice margin it is found that the mode of failure is sensitive to the presence of tectonic stress and that the onset timing is also dependent on compressibility for sites within the ice margin the evect of earth rheology is shown to be small however ice load history is shown to have larger evects on the onset time of earthquakes and the magnitude of fault instability

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

a reassessment of facies and faunas in the type llandeilo ordovician wales abstract detailed analysis of the lithological and faunal successions of the type llandeilo forms the basis of an evaluation of the sequence of depositional environments and a quantitative description of the entombed faunas comprising mainly brachiopods trilobites and ostracods recorded distribution patterns confirm the validity of the existing trilobite trinucleid biostratigraphic zones but emphasize the importance of benthic bivalved taxa which are particularly abundant in the lower llandeilo groups of assemblages comprising localized faunules or associations despite exhibiting evidence of transportation are readily identified as representative of more widespread contemporary palaeocommunities recently described from wales and the welsh borderland the relationship between these palaeocommunities and the transgressive llandeilo sequence indicates a shoreface to open shelf zonation of ecosystems controlled primarily by substrate conditions rather than depth

lingulid brachiopods and lingulichnus from a silurian shelf slope carbonate sequence delorme group mackenzie mountains northwest territories structures assigned to the ichnogenus lingulichnus commonly containing the burrow forming brachiopod were collected from an argillaceous unit in a silurian shelf dolostone sequence delorme group the sediments benthic fauna and inclined orientation of all lingulid burrows indicate the presence of a bottom current and restricted conditions on the margin of the selwyn basin high abundance low diversity faunas of lingulids and lingulichnus are indicative of limiting conditions at the substrate surface under low energy conditions they seem to be indicative of the zoophycos ichnofacies

electrical sedimentary anisotropy of landforms adjacent to postglacial faults in lapland abstract postglacial faults pgfs are common sw ne trending features in the northern part of the fennoscandian shield being attributed to former high magnitude mw 7 8 2 earthquakes arvidsson 1996 these are of interest for the long term seismic hazard of deep geological nuclear waste repositories the pgfs are still seismically active and many of the deformation features such as paleolandslides are spatially coextensive within these earthquake zones based on the 14c datings of landslide buried organic materials three postglacial seismic episodes 9 7 11 5 6 and 1 3 1 8 ky ago have been found in western finnish lapland because the model predictions by wu et al 1999 suggested that the onset of fault instability started at 15 ky and that the maximum fault instability was reached at 13 10 ky in fennoscandia we postulate that not only subaerial but also subglacial types of deformations may have been associated with the past seismic events here we present lidar dem observations and sediment electromagnetic anisotropy data on the landforms such as the rim ridged arcuate pulju moraine kujansuu 1967 roundish liquefaction bowls postdepositional liquefaction deformations on streamlined landforms as well as hummocky ridge fields of massflow sediments the sediments in the pulju moraine and massflow ridges displayed clear anisotropy indicating lateral transportation in subglacial wet based environment instead sediments in the bowl shaped morphologies next to pgfs and deformed ice stream flutings tended to be isotropic suggesting postdepositional and vertical liquefaction processes based on the lidar morphology and sedimentary anisotropy data these landforms may be linked to past earthquakes

definition of the tremadoc series and the series of the ordovician system in britain the tremadoc series and series of the ordovician system are widely used in international correlations and this review explains and summarizes how the base of each of these series is recognized in britain no type section in which the base of any one of them is displayed has ever been selected and formally designated hence the basal horizon of each series is a matter of general understanding rather than precise definition this general understanding is of particular biozones or of assemblages of fossils and their relative ages the geographical separation between type sections of series and differences in facies complicate the understanding of relative ages of biozones brachiopod and trilobite faunas of the llandeilo caradoc and ashgill series have been studied in detail in recent years and biozones proposed these biozones have proved difficult or impossible to recognize outside their type areas in britain and brachiopod trilobite faunas of the tremadoc arenig and llanvirn series are likewise highly provincial thus for correlation within and outside britain attention has been focused on the standard graptolite biozones particularly since such conodont faunas as have been found have not proved useful yet there are problems associated with these standard biozones for example the tetragraptus approximatus biozone is unknown in wales or england the relationship between the base of the glyptograptus teretiusculus biozone and the earliest llandeilo shelly faunas requires clarification and at what level the base of the caradoc series may lie in the nemagraptus gracilis biozone is unknown thus to bring precision to definitions of ordovician series in britain the identification of continuous fossiliferous sequences that will serve as stratotypes is required such a section for the basal portion of the tremadoc series has recently been proposed systematic studies related to particular biozones are needed for example early caradoc and early ashgill brachiopods and graptolites of the didymograptus bifidus and glyptograptus teretiusculus biozones the stratigraphical ranges and abundances of species in particular sections require documentation so that the type of biozone assemblage acme concurrent range etc may be recognized such work could lead towards chronostratigraphy the provision of a standard stratigraphical scale in britain for the tremadoc series and ordovician series

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

fennoscandian earthquakes whole crustal rupturing related to postglacial rebound local and regional earthquake locations provide seismic evidence that large shield earthquakes have occurred in northern fennoscandia these paleoearthquakes with fault lengths of up to 160 kilometers and average displacements of up to 15 meters were triggered by nonisostatic compressive stresses caused by the removal of the ice at the end of the last deglaciation the fennoscandian faults were probably formed by single events that ruptured through most of the crust the largest event moment magnitude mw 8 2 was larger than other known stable continent earthquakes outside failed rifts or extended crust

phylogenetic analysis of some silurian rostroconchs mollusca from northwestern canada the new genus avalanchia and the new species avalanchia pterocarina cassowarioides anisomorpha cassowarioides polgari nehedia bergeraci nehedia restricta and nehedia tricarina are proposed from exceptionally preserved silicified faunas two new but unnamed rostroconch species are also described these conocardioid rostroconchs were collected from silurian deposits late llandovery to early ludlow near avalanche lake in the mackenzie mountains northwest territories canada and when added to global rostroconch faunal lists indicate a previously unrecognized diversity of silurian conocardioid rostroconchs the morphology of conocardioid rostroconchs is discussed and a glossary of terms presented sexual dimorphism is recognized in species of cassowarioides phylogenetic analysis of hippocardiids and bransoniids supports the monophyly of several genera the hippocardiid genus bigalea pojeta and runnegar 1976 is recognized as paraphyletic and three species are assigned to a new genus redstonia p

evidence of multiple slip events on postglacial faults in northern fennoscandia abstract an analysis of high resolution lidar based digital elevation models dems trenching of quaternary deposits diamond drilling and a structural analysis of the bedrock were conducted in order to characterise the geological structure and to derive full slip profiles of the venejarvi jauhojarvi postglacial fault complex located in northern finland the venejarvi jauhojarvi fault complex consists of 17 surface ruptures striking in a sw ne direction cutting through late weichselian till and traceable for a total distance of 29 9 km based on stratigraphical and structural analysis we show that the naamivitikko fault segment which is part of the venejarvi jauhojarvi complex hosted up to three distinct slip events and it is therefore argued that the prevailing single rupture hypothesis of postglacial faults should be treated with caution based on the measured surface rupture dimensions it is conservatively estimated that the maximum moment magnitude of the paleoseismic event that the complex hosted ranged between mw 5 8 8 4 if uncertainties in the scaling laws equating surface rupture dimensions to moment magnitudes are accounted for a more realistic maximum moment magnitude range between mw 5 8 7 6 is given

la repartition geographique des mollusques bivalves du paleozoique cambrien a devonien une revue abstract paleozoic bivalves have long been neglected in faunal studies the reasons for this are considered and the conclusion reached that this neglect has no real foundation recent and growing interest in paleozoic bivalve faunas has accentuated their potential importance in paleogeographic faunal reconstructions current knowledge of bivalve faunas from the cambrian to devonian is reviewed in a framework of global plate tectonics

surficial geology indicates early holocene faulting and seismicity central sweden in sweden knowledge of the location and timing of glacially induced faulting and seismicity is critical to effective engineering of a long term nuclear disposal facility to improve understanding and modeling of the complex ice induced and tectonic stresses associated with glacially induced faulting field studies detailing the location and timing of movement of such structures are required although the fault has not been confirmed in the bedrock multi proxy surficial geologic evidence indicates that the recently discovered scarp in bollnas is such a structure machine excavated trenches across the scarp reveal landsliding down the scarp and in one location faulted and vertically offset fine grained glacial sediments the presence of water escape structures in trenches excavated on a topographic high strongly suggests a co seismic origin derived from earthquake magnitudes 5 5 numerous landslides in till exist in the region as well four slopes with landslides were examined in detail and the factors of safety for these slopes indicate stable conditions and suggest a seismic trigger basal radiocarbon dates from peat bogs located stratigraphically above the landslides provide minimum limiting ages for the co seismic landslides the oldest date indicates sliding prior to 10 180 calendar years before the present the proposed bollnas fault is 400 km south of the so called lapland fault province to date it is the southernmost confirmed glacially induced fault in sweden the results of this study are consistent with existing modeling results that indicate fault instability in this region of central sweden following deglaciation

some silicified middle silurian rostroconchs mollusca from the mackenzie mountains n w t canada a silicified rostroconch fauna is described from strata of early late llandovery to middle wenlock to early ludlow silurian age in the mackenzie mountains two new genera cassowarioides and nehedia and nine new species bigalea buskasi b tercierae bransonia foxi cassowarioides perryi c stelcki mulceodens schneideri m wilsoni nehedia bainsi and n grovesi are proposed an examination of the stratigraphic ranges of some of these taxa suggests that the rostroconchs may have some biostratigraphic utility during the silurian

comparison between hydroacoustical and terrestrial evidence of glacially induced faulting lake voxsjon central sweden the recent availability of a terrestrial high resolution digital elevation model in sweden has led to the discovery of previously unknown scarps believed to be associated with bedrock faults that ruptured to the surface during the holocene field investigations however are required to confirm these findings and determine the timing of post glacial seismicity here we present results from a unique hybrid approach where hydroacoustical data from the sediments of lake voxsjon are compared to stratigraphic and geomorphologic records from nearby terrestrial settings the hydroacoustical data are largely consistent with the terrestrial data indicating a single fault rupture shortly after deglaciation which occurred about 11 000 10 500 cal bp

ordovician to devonian sea level changes in western and northern canada lower paleozoic sediments of central cordilleran and arctic canada are dominated by two megafacies supratidal to shallow subtidal carbonates and minor evaporites of the platformal cratonic are

extinction and migration in silurian trilobites and conodonts of northwestern canada appearances and disappearances lineage turnover of trilobite and conodont evolutionary species clades in silurian shelf marginal sections of the mackenzie mountains are concentrated at particular stratigraphical levels a depth energy curve generated for this region shows three major and several minor transgressive regressive cycles in the llandovery and wenlock series a comparison of this curve with conodont and trilobite distributional data supports the thesis that there are cause effect relationships between changes in sea level and migration evolution and extinction events of these marine organisms the rate amount and direction of change in depth were important factors in the migration and extinction of clades the depth energy curve for the region fits the pattern expected for a passive downdropping shelf responding to global eustatic sea level changes emigrations and extinctions are associated with rapid transgressive episodes and concentrated at three levels lower llandovery lower wenlock and middle or lower part of the upper wenlock the early wenlock event was the most impressive a further concentration of extinctions and or migrations not considered here occurred later in the ludlow associated with a regional regression phyletic evolution is most convincingly demonstrated in fossil collections from intervals of gradual regression a model is proposed that predicts that a similar pattern of extinctions occurred around the silurian world in regions of similar palaeotectonic regime and that global extinction patterns of clades between episodes of mass extinction were not random episodes of high extinction turnover pulses were caused by perturbations of the environment foremost of which were rapid changes in the depth of the seas on the continental shelves because taxonomic loss during these extinction emigration episodes was generally at low below generic levels resolution of silurian turnover patterns requires data collected at the evolutionary species level

fault branching and rupture directivity 1 could the directivity of a complex earthquake be inferred from the ruptured fault branches it created typically branches develop in forward orientation making acute angles relative to the propagation direction direct backward branching of the same style as the main rupture e g both right lateral is disallowed by the stress field at the rupture front here we propose another mechanism of backward branching in that mechanism rupture stops along one fault strand radiates stress to a neighboring strand nucleates there and develops bilaterally generating a backward branch such makes diagnosing directivity of a past earthquake difficult without detailed knowledge of the branching process as a field example in the landers 1992 earthquake rupture stopped at the northern end of the kickapoo fault jumped onto the homestead valley fault and developed bilaterally there nnw to continue the main rupture but also sse for 4 km forming a backward branch we develop theoretical principles underlying such rupture transitions partly from elastostatic stress analysis and then simulate the landers example numerically using a two dimensional elastodynamic boundary integral equation formulation incorporating slip weakening rupture this reproduces the proposed backward branching mechanism based on realistic if simplified fault geometries prestress orientation corresponding to the region standard lab friction values for peak strength and fracture energies characteristic of the landers event we also show that the seismic s ratio controls the jumpable distance and that curving of a fault toward its compressional side like locally along the southeastern homestead valley fault induces near tip increase of compressive normal stress that slows rupture propagation

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

extent and bathymetry of north american platform seas in the early silurian the north american craton is 25 covered by lower silurian llandovery rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface this figure does not take into account extensive post silurian tectonics and erosion isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the silurian analysis of interbasin structural arches and outliers provides important clues as to the actual extent of early silurian seas information on the age of tectonic origin for 15 prominent arches is reviewed the placement and significance of 11 key outliers is also discussed evidence of this kind indicates early silurian platform seas were among the most extensive of the paleozoic probably more than 65 of the north american craton was then flooded nothing on so vast a scale serves as a recent guide to the past the pliocene holocene bahamian banks do provide however worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies the original bathymetry of early silurian environments may be determined correlated and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations the range of fully integrated facies patterns includes 1 coral stromatoporoid stromatolite paleosol cycles williston basin 2 pentamerid brachiopod coral stromatoporoid stromatolite cycles michigan basin and 3 stricklandiid brachiopod pentamerid brachiopod coral stromatoporoid cycles east iowa basin with shallow bathymetries of 0 30 m the williston class tends to represent mosaic facies while with bathymetries of 0 60 m and 10 90 m the deeper water michigan and iowa classes typically involve blanket deposits bathymetry of other basin deposits generally fits one of these three classes maximum and minimum stands in sea level during late llandovery time are represented by bathymetric maps

meteoric diagenesis and porosity enhancement in fluvio deltaic reservoir facies b 6 sands maracaibo basin venezuela the eocene northwest southeast trending fault bounded reservoirs reveal a systematic diagenetic evolution due to changing physical and chemical conditions related to an oligocene uplift separating an eocene and a subordinate miocene phase of subsidence compaction cementation and dissolution during eocene subsidence reduced original porosity to about 20 the oligocene uplift created abundant secondary porosity raising the total porosity to around 30 especially near the truncated erosion surface on the northwest here abundant lamellar porosity is characteristic intense leaching action by undersaturated meteoric co sub 2 charged waters may be responsible for the regenerated porosity the sandstones immediately below the unconformity surface show the best reservoir quality leaching of monocrystalline and polycrystalline quartz feldspar rock fragments and clays has produced a highly porous and permeable diagenetic quartzarenite porosity and permeability decrease systematically southeastward as the vertical distance from the unconformity surface increases labile grains largely persist and compaction and cementation calcite silica and clays reduce the reservoir quality a patchily welded texture observed in the southeast may be due to silica cement originated from grain dissolution near the unconformity surface on the northwest presence of highly degraded crudes lends additional support to the influence of meteoric waters on these reservoirs similar processes of silica dissolutionmore through the action of acidic meteoric water presently occur in extensive outcrops of precambrian karstic quartzarenites in venezuela these tropical analogs support the efficacy of freshwater flushing mechanism and consequent enhancement of reservoir quality in the eocene sequence less

mineralogy and geochemistry of the clay fraction of till in northern finland abstract the mineralogy and geochemistry of the clay fraction of till in different moraine types and in different bedrock areas in northern finland have been studied moraine types include ground moraine rogen moraine pulju moraine sevetti moraine and kianta moraine the bedrock is composed of granite granite gneiss and mafic volcanite x ray diffraction differential thermal and thermogravimetric analyses transmission electron microscopy x ray fluorescence and atomic absorption spectrometry were used as analytical methods quartz feldspars and amphibole are the primary rock forming minerals occurring in nearly all the samples of each study area the clay minerals proper include kaolinite vermiculite swelling lattice vermiculite chlorite illite and mixed layer minerals kaolinite occurs most abundantly only in till in the kittilaarea where a preglacial weathering crust has been the source rock poorly crystalline or amorphous fe oxides also abound in till at kittila sodium ca and si have been depleted and al mg fe k ti p and trace elements have been enriched in the clay fraction compared with the fine fraction the contents of si k and na are higher in the granitic areas and the contents of cu co ni and zn are higher in the mafic volcanite areas preglacial weathering has led to the depletion of si mg ca k and na and to the enrichment of al fe ti p and trace elements in the clay fraction of till in the kittilaarea high base metal values in the inari area suggest the possibility of sulphide mineralizations there and high as values in the kittilaarea indicate au potential there the most important factors controlling the mineralogical and geochemical composition of the clay fraction of the tills in northern finland are the composition of the bedrock and the possible occurrence of an old weathering crust the final grain size composition of the tills and consequently the quantitative proportions of the different minerals are often related to the last glacial quarrying and sorting processes therefore the mineralogical composition of the tills is to a certain extent bound also to the respective moraine types

sedimentological evidence of pleistocene earthquakes in nw poland induced by glacio isostatic rebound abstract soft sediment deformation structures are abundantly present in two levels within warthanian eemian lacustrine sediments at siekierki near the polish german border the two event horizons show intense folding collapse sag and load structures indicative of liquidization and fluidization the structures must have been caused by sudden shocks most probably resulting from earthquakes that were induced by glacio isostatic rebound probably after the warthanian deglaciation such seismites have not been previously recognized in polish quaternary sediments they provide supporting evidence for glacio isostatic movements that were interpreted up till now based on drilling lithostratigraphic geophysical and geodetic data the recognition of seismites in nw poland may help recognize other deformed event horizons in pleistocene sediments as glacio isostatic or neotectonic seismites

pre quaternary sea level changes while the paramount importance of global sea level changes has been widely accepted by quaternary geologists throughout this century it is only within the last decade that such changes in the rest of the phanerozoic have received much attention there are two principal reasons for this arousal of interest first increased exploration of the stratigraphic record across the earth both on land and under the sea and significant improvement in biostratigraphic correlation have drawn attention to what appear to have been globally synchronous events second oceanographic research has demonstrated the importance of tectonically controlled changes in the cubic capacity of the ocean basins notably variations in ridge volume which provide a ready mechanism for sea level changes even at nonglacial times the adjective eustatic was first proposed for global changes of sea level by suess 1906 in his great treatise the face of the earth the corresponding noun is eustasy not eustacy as it is commonly misspelt the unconvinced reader need only think of ecstasy eustatic sea level changes can be inferred as suess recognized by a plotting the temporal spread of marine sediments over the continents and b estimating the depositional depth variations in marine stratal sequences which can be correlated across and between continents to these methods can be added seismic stratigraphy in which biostratigraphically age determined seismic sequences on the con tinental margins are analyzed in terms of cycles of coastal onlap and offiap when the results of these methods are broadly in accord confidence in a eustatic interpretation is increased but the range of confidence can vary considerably it is widely accepted that there was an exceptionally high sea

postglacial reactivation of the suasselka pgf complex in sw finnish lapland the suasselka fault complex is recognized from high resolution digital elevation models lidar dems as a complex of 37 surface ruptures striking sw ne and cutting through weichselian glacial sediments confirming the postglacial nature of the rupturing event the analysis of lidar dems geophysical profiles and trenching of glacial sediments at two sites along the suasselka postglacial fault pgf system has provided evidence of earthquakes that post and predate the late weichselian deglaciation at the suaspalo trenching site at least two different rupture events are observed the first one predates the deposition of the lowermost till unit and potentially dates to the early or even pre weichselian the second rupture event on the other hand deforms overlying tills and is thus a post weichselian event our results indicate that multiple rupturing events can take place within postglacial fault complexes which is the second time such a characteristic has been proven for pgfs in northern fennoscandia furthermore the present results provide the first evidence of either early or even pre weichselian faulting in finland suggesting that the same fault complexes were also active after glacial phases that predated the last glacial maximum of the late weichselian these results warrant re evaluation of the long term seismic risk associated with the faults as the assumption of single rupture events is erroneous considering that the suasselka pgf complex is composed of four pgf systems and 37 isolated 150 7500 m long segments we calculated the potential moment magnitudes based on the rupture length and mean maximum vertical displacement for each individual segment and system taken that they potentially ruptured independently and finally for the entire suasselka pgf complex for the entire complex rupturing in a single event the observed displacement and length of the complex yields a moment magnitude estimate of mw 6 7 8 1 however based on geomorphological and sedimentological evidence we believe that this is a conservative estimate of the moment magnitude of the paleoearthquake if isolated pgf systems are considered to have ruptured individually the moment magnitudes estimated based on surface rupture lengths range from mw 5 5 6 7 and based on mean and maximum cumulative displacement values the estimated values range from mw 6 2 7 3 and mw 6 7 8 0 respectively as we cannot with the available data distinguish the exact number of events the age of events or which segments potentially ruptured at the same time we conclude that the events that took place in the complex ranged in moment magnitude from c mw 5 5 to 8 1 the mw 8 1 estimate representing conservative upper bound for the maximum moment magnitude

ordovician provincial signals from appalachian caledonian terranes statistical analysis of mainly mid to deep shelf ordovician brachiopod faunas from localities within and marginal to the appalachian caledonian orogen reveals biogeographical patterns not entirely consistent with those presumed for the progressive orthogonal closure of the iapetus ocean correspondence analysis of faunas of late arenig early llanvirn late llandeilo earliest caradoc and late ashgill age emphasize the importance of marginal and island faunas the role of regressions and transgressions on faunal distributions and the effects of climatic change in the definition of biogeographical trends during the ordovician

the significance of a reliable ordovician age for some highland border rocks in central scotland a lower arenig fauna with north american affinities has been rediscovered at dounans aberfoyle central scotland this discovery makes part of the highland border complex younger than the adjacent dalradian rocks and suggests a provenance from the s

silurian δ13c stratigraphy a view from north america three positive shifts in 13 c are recognized in well dated carbonate successions in north america and can be confidently correlated with shifts in the baltic region by using conodont and graptolite zonations the 13 c excursions reach distinct peaks of 3 to 4 at the ends of the amorphognathoides and siluricus conodont zones and within the ludensis graptolite zone the three events are associated with the most significant extinction horizons in the silurian and occur near clastic carbonate transitions that mark eustatic or oceanographic changes the magnitudes of the two oldest 13 c peaks compare well with the baltic region however the end siluricus excursion reaches far greater values of as much as 11 in sweden likely reflecting local modification of the global seawater signature

interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling four sets of thin section scale mode i open mode cemented microfractures are present in sandstone from the eocene misoa formation maracaibo basin venezuela the first set of microfractures is intragranular f1 formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 8c the second set of microfractures f2 is cemented by bituminite pyrite formed at temperatures between 60 and 100 8c and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks the third set of microfractures f3 is fully cemented by either quartz cement or calcite cement the former has fluid inclusion homogenization temperatures between 149 and 175 8c these temperatures are mostly higher than maximum burial temperatures 160 8c suggesting that upward flow caused by a pressure gradient transported silica vertically which crystallized into the fractures upward decompression may have also caused a pco2 drop which at constant temperature allowed simultaneous carbonate precipitation into the third microfracture set the fourth set of thin section scale microfractures f4 is open or partially cemented by siderite hematite and other iron oxides the presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the misoa formation in order to time and place constraints on the depth of formation of the fourth set of microfractures we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if in fact they could be associated with the uplift of the formation our results suggest that thermoelastic contraction caused by the formation s uplift erosion and consequent cooling is a feasible mechanism for the origin of the last fracture set hence we infer that meteoric water invasion into the fractures at the end of the uplift cause the precipitation of oxides and the transformation of siderite to hematite d 2005 elsevier b v all rights reserved

paucicanthus vanelsti gen et sp nov an early devonian lochkovian acanthodian that lacks paired fin spines the acanthodian paucicanthus vanelsti gen et sp nov is described from six body fossils from lower devonian lochkovian rocks of the southern mackenzie mountains northwest territories canada this new species is unique among acanthodians in that it lacks both pectoral and pelvic fin spines in the absence of fin spines the leading edges of the pectoral and pelvic fins are reinforced by enlarged scales the anatomy of the acanthodiform traquairichthys pygmaeus is similar to p vanelsti in that both lack pelvic fin spines although t pygmaeus also lacks pelvic fins similarly the acanthodian yealepis douglasi lacks both paired and median fin spines and its anatomy resembles that of p vanelsti based only on the loss of paired fin spines the lack of paired and or median fin spines in these three taxa contrasts with the widely held view that acanthodian fins all were preceded by spines the anatomy of p vanelsti also is similar to that of the acanthodian brochoadmones milesi in that both have a c

displacement and geometrical characteristics of earthquake surface ruptures issues and implications for seismic hazard analysis and the process of earthquake rupture there now exist about three dozen historical earthquakes for which in vestigators have constructed maps of earthquake rupture traces accompanied by de scriptions of the coseismic slip observed along the fault strike the maps and slip distributions are compiled here to place observational bounds on aspects of seismic hazard analysis and fault mechanics analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults the observations also give support to the ideas that there exists a process zone or volume of about 3 4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes to this may be added the observation that the form of earthquake surface slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike online material maps of surface ruptures digitized values and curve fits to surface slip distributions and notes and references for tables 1 and 2