glacial rebound and plate spreading results from the first countrywide gps observations in iceland iceland is one of the few places on earth where a divergent plate boundary can be observed on land direct observations of crustal deformation for the whole country are available for the first t

segmented lateral dyke growth in a rifting event at barðarbunga volcanic system iceland crust at many divergent plate boundaries forms primarily by the injection of vertical sheet like dykes some tens of kilometres long1 previous models of rifting events indicate either lateral dyke growth away from a feeding source with propagation rates decreasing as the dyke lengthens2 3 4 or magma flowing vertically into dykes from an underlying source5 6 with the role of topography on the evolution of lateral dykes not clear here we show how a recent segmented dyke intrusion in the bar arbunga volcanic system grew laterally for more than 45 kilometres at a variable rate with topography influencing the direction of propagation barriers at the ends of each segment were overcome by the build up of pressure in the dyke end then a new segment formed and dyke lengthening temporarily peaked the dyke evolution which occurred primarily over 14 days was revealed by propagating seismicity ground deformation mapped by global positioning system gps interferometric analysis of satellite radar images insar and graben formation the strike of the dyke segments varies from an initially radial direction away from the bar arbunga caldera towards alignment with that expected from regional stress at the distal end a model minimizing the combined strain and gravitational potential energy explains the propagation path dyke opening and seismicity focused at the most distal segment at any given time and were simultaneous with magma source deflation and slow collapse at the bar arbunga caldera accompanied by a series of magnitude m 5 earthquakes dyke growth was slowed down by an effusive fissure eruption near the end of the dyke lateral dyke growth with segment barrier breaking by pressure build up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries

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

sea level driven changes in ocean chemistry at an upper cambrian extinction horizon a combined stratigraphic and isotopic study across a late cambrian mass extinction horizon pterocephaliid ptychaspid biomere boundary in cratonal and miogeoclinal sections of the western united states reveals evidence for a pattern of sea level rise decreasing 87 sr 86 sr ratios and increasing 13 c the positive shift in 13 c reaching maximum values a few metres above the extinction interval is interpreted to reflect elevated primary productivity and an increase in the rate of burial of organic matter during expansion of the oxygen minimum zone the 87 sr 86 sr decrease is interpreted to reflect some combination of increased sea floor spreading rates and decreased continental weathering rates the 13 c and 87 sr 86 sr trends independently support the field based hypothesis that a sea level rise led to catastrophic oceanic overturn possibly culminating in anoxic conditions that ultimately proved lethal to a large fraction of the existing shelf fauna

seismicity of the reykjanes peninsula 1971 1976 abstract the reykjanes peninsula oblique rift a segment of the mid atlantic plate boundary connects the reykjanes ridge to the rift zones of iceland the axis of the deformation zone of the plate boundary can be traced as a rather narrow epicentral zone of earthquakes that is highly oblique to the spreading direction of the north america plate with respect to the eurasia plate the seismicity of the area is episodic active periods with a duration of about a decade are separated by more quiet periods of one to two decades our observation period spans a good part of an episode that began in 1967 and ended in 1975 nine earthquake swarms illuminate a 50 km long segment of the plate boundary the epicentral zone cuts obliquely across the fissure swarms of the volcanic systems of the peninsula geothermal areas are preferentially located at the intersection of the fissure swarms with the seismic zone outside the intersections with the seismic zone the fissure swarms show very low seismicity the tectonic characteristics gradually change along the plate boundary segment from being similar to those of a spreading segment in the western part to being more like a transform zone in the eastern part this is seen in the type of earthquake sequences maximum magnitude and type of faulting

controlling factors on earthquake swarms associated with magmatic intrusions constraints from iceland abstract the currently two most important surveying tools in volcano monitoring are observations of seismicity and surface deformation magma migration within the earth s crust is frequently associated with seismic activity often occurring as distinct earthquake swarms understanding the exact nature and significance of these swarms is important when evaluating volcanic crisis situations there seems however to be no general correlation between the amount of seismic energy release and the rate and volume of magma on the move which may complicate an immediate risk evaluation it has previously been shown that stressing rate appears as the controlling factor on the occurrence of seismicity during intrusion however in this paper we emphasize that this is only true under specific circumstances other factors influence the evolution and resulting seismic energy release and each intrusion scenario needs to be evaluated separately three icelandic cases where seismic swarms have been confirmed through deformation measurements to be related to magmatic movements were selected we show how the relationship between volume change and resulting seismicity varies greatly between cases and suggest that in a general approach the most important factor governing the level of seismic energy release is the background stress state tectonic setting regional stresses and tectonic history are therefore all of great importance when evaluating a magmatically induced seismic crisis the stressing rate only plays the dominant role when either a stress in the intruded area is close to the yield strength before intrusion or possibly b when the intrusion is extraordinarily voluminous and rapid

coseismic stress changes and crustal deformation on the reykjanes peninsula due to triggered earthquakes on 17 june 2000 1 a large mw 6 5 earthquake struck the south iceland seismic zone sisz on 17 june 2000 the 17 june main shock triggered increased seismicity over a large area and significant slip on at least three distinct faults on the reykjanes peninsula up to 87 km to the west of the event a second large mw 6 4 earthquake in the sisz occurred on 21 june 2000 about 17 km west of the 17 june main shock this event does not appear to have triggered as much activity on the reykjanes peninsula as the 17 june main shock although the epicenter was closer crustal deformation signals due to the june 2000 earthquakes on the reykjanes peninsula were observed with campaign and continuous gps and synthetic aperture radar interferometry with the largest coseismic deformation signal near lake kleifarvatn we model the faults using three uniform slip rectangular dislocations in an elastic half space best fit uniform slip models consistent with seismic and geodetic data indicate that all three faults trend n s and the motion on them was primarily right lateral strike slip our study suggests that the event near kleifarvatn had a significantly larger moment than seismic estimates indicating a component of aseismic slip on the fault lasting no more than several hours static coulomb failure stress change calculations indicate that the event at kleifarvatn increased the coulomb stress at the hypocenter of the nupshlidarhals event by 0 1 0 2 mpa as well as loading the hvalhnukur fault

the structure of seismogenic strike slip faults in the eastern part of the reykjanes peninsula oblique rift sw iceland abstract a considerable part of the seismic risk to the capital area of iceland is provided by a series of strike slip faults at the mid atlantic plate boundary within a distance range of 15 35 km about two thirds of the icelandic population lives within this distance range from the source areas of m 6 6 5 earthquakes the structure of the plate boundaries in sw iceland is relatively complex three branches meet in the hengill triple junction the oblique rift of the reykjanes peninsula the south iceland seismic zone a transform zone and the western volcanic zone a rift zone the reykjanes peninsula oblique rift has an over all trend of 70 highly oblique with respect to the spreading direction 101 in this region it contains both volcanic systems and seismogenic strike slip faults the fissure swarms of individual volcanic systems contain normal faults and fissures with a ne trend also quite oblique to the plate boundary the fissure swarms fade out towards the ne and sw as they extend into the plates on either side overprinting this pattern of volcano tectonic structures are sets of parallel northerly striking transcurrent faults that generate the largest earthquakes in this zone their surface expressions are en echelon fracture arrays and push up structures the sense of displacement is right lateral in the 15 km long section of the rift studied here we find evidence for at least six faults of this type the length of individual faults may exceed 12 km the distance between them varies from 1 to 5 km and together they form a bookshelf type fault system taking up the left lateral component of plate movements across the oblique rift zone

mixed siliciclastic carbonate upward deepening cycles of the upper cambrian inner detrital belt of laurentia abstract high resolution sedimentological and biostratigraphic data recently recovered from upper cambrian strata in the northern rocky mountain and central appalachian regions reveal that meter scale cycles of very different character developed synchronously during deposition of the basal subzones of the ibexian series in both the carbonate facies belt and the distal part of the inner detrital mixed siliciclastic carbonate facies belt of the laurentian paleocontinent a typical mixed siliciclastic carbonate cycle in the snowy range formation of wyoming and montana consists of the following lithofacies in ascending order shale shale with very thin grainstone interbeds grainstone with subordinate thin shale interbeds flat pebble conglomerate and in some cycles thrombolitic bioherms internal transitions between the constituent lithofacies are mostly gradational but cycle boundaries are sharp with shale directly overlying flat pebble conglomerate and or thrombolitic boundstone we postulate that these mixed siliciclastic carbonate cycles developed in response to different depositional dynamics than meter scale cycles developed in carbonate belt facies specifically cycles in the carbonate belt were produced largely by temporal variation in accommodation space and associated filling of that space through progradation of the carbonate system and aggradation to sea level in contrast cyclic deposition within the inner detrital belt was controlled by variations in terrigenous sediment input and resulting effects on carbonate productivity in this case enhanced delivery of siliciclastic mud by rivers was triggered by regression and or a shift to more humid climatic conditions an upward increase of carbonate in the main part of the cycles reflects enhanced carbonate production in response to the reduction of terrigenous clay supply during shoreline retreat widespread cementation of fine grainstone substrates led to deposition of extensive beds of flat pebble conglomerate possibly in association with elevated sea surface temperatures and enhanced storm intensity thrombolites formed at times of maximum transgression as microbial communities thrived in the absence of turbidity colonizing the irregular surfaces of flat pebble conglomerate beds we thus interpret these wholly subtidal cycles to record full changes in paleobathymetry with the bulk of the cycles recording upward deepening rather than shoaling associated with regression comparison with age equivalent shoaling cycles of the conococheague formation of the carbonate belt of the appalachian mountains highlights the different depositional dynamics in the mixed siliciclastic carbonate systems and underscores the peril in extrapolation of depositional models derived from carbonate systems to mixed systems regardless of age or paleogeographic setting

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

structural architecture of a highly oblique divergent plate boundary segment abstract the reykjanes peninsula in southwest iceland is a highly oblique spreading segment of the mid atlantic ridge oriented about 30 from the direction of absolute plate motion we present a complete and spatially accurate map of fractures for the reykjanes peninsula with a level of detail previously unattained our map reveals a variability in the pattern of normal oblique and strike slip faults and open fractures which reflects both temporal and spatial strain partitioning within the plate boundary zone fracture density varies across the length and width of the peninsula with density maxima at the ends and at the northern margin of the zone of volcanic activity fractures with similar strike cluster into distinct structural domains which can be related to patterns of faulting predicted for oblique extension and to their spatial distribution with respect to volcanic fissure swarms additional structural complexity on the reykjanes peninsula can be reconciled with magmatic periodicity and associated temporal strain partitioning implied by gps data as well as locally perturbed stress fields individual faults show variable slip histories indicating that they may be active during both magmatic and amagmatic periods associated with different strain fields

sabkha facies and the preservation of a falling stage systems tract at the sauk ii iii supersequence boundary in the late cambrian eilean dubh formation nw scotland the durness group of northwest scotland represents a nearly one kilometer thick succession of mid cambrian to lower middle ordovician carbonates these rocks crop out in a narrow belt along the foreland of the caledonian orogen and in the moine thrust zone stretching some 170 km from loch eriboll southwestwards to the isle of skye the cambrian parts of the succession represent deposition on a paleo southeast facing low latitude passively subsiding continental margin situated at around 20 s carbonate deposition in scotland formed part of the extensive great american carbonate bank the sauk ii iii supersequence boundary occurs above an interval of thin meter scale shallowing upward peritidal cycles containing intraclast dolorudstones stromatolites tepees siliciclastic sand and surface karst evidence of former evaporites in the succession comprises pseudomorphs of nodular and bedded anhydrite crystal shaped calcite filled vugs representing growth of sulfate dissolution breccias and local halite casts these features represent sabkha facies arranged into cycles that constitute a preserved falling stage systems tract fsst the top of the fsst is marked by a pronounced karst surface of sandstone filled fissures the section preserves a rare snapshot of a seldom preserved environment on the cambrian laurentian margin its preservation during a time of falling relative sea level under slow tectonic subsidence is discussed

spatial patterns of sediment accumulation on a holocene carbonate tidal flat northwest andros island bahamas abstract to characterize spatial patterns of sedimentation and analyze the morphology of part of the modern tidal flats of northwest andros island in the bahamas this study integrated remote sensing geographic information systems gis and carbonate sedimentology the fundamental data are a landsat tm image that has been classified to create a thematic map of eight subfacies interpreted to represent a distinct tidal flat subenvironment such as adjacent marine exposed levee beach ridge pond and algal marsh spatial statistics of the thematic map characterize the patterns of sediment accumulation quantitative analysis highlights several interesting results concerning subfacies character and distribution 1 of the eight mapped subfacies low algal marsh is most widespread representing 27 5 of the total area whereas exposed levee beach ridge is the least widespread accounting for 10 of the area 2 the patches of different subfacies have different shape complexities with low algal marsh high algal marsh and mangrove ponds being the least complex and exposed levee beach ridge being the most complex 3 markov chain analysis suggests that lateral transitions between different subfacies are highly ordered and 4 frequency distribution of subfacies patch area and lacunarity gap size distribution data exhibit power law relationships over several orders of magnitude consistent with fractal characteristics the fractal nature of patch size and gaps between facies illustrate that on this tidal flat neither the size nor the spatial distribution of subfacies has a characteristic scale this statistical behavior is consistent with the presence of self organization or emergence of pattern in the absence of a template or external forcing the statistical self organization on the tidal flat is the cumulative expression of local processes but it becomes apparent only through analysis of the whole system these results are inconsistent with models suggesting that tidal flats include a migrating complex of randomly distributed randomly sized subenvironments ancient successions that include random patterns may reflect the more pronounced influence of forces external to the sedimentary system instead of an absence of those forces

plate boundary deformation and man made subsidence around geothermal fields on the reykjanes peninsula iceland abstract we present interferometric synthetic aperture radar insar data from 1992 1999 and 2003 2008 as well as gps data from 2000 2009 for the active plate boundary on the reykjanes peninsula southwest iceland the geodetic data reveal deformation mainly due to plate spreading anthropogenic subsidence caused by geothermal fluid extraction and possibly increasing pressure in a geothermal system subsidence of around 10 cm is observed during the first 2 years of production at the reykjanes geothermal power plant which started operating in may 2006 we model the surface subsidence around the new power plant using point and ellipsoidal pressure sources in an elastic halfspace short lived swarms of micro earthquakes as well as aseismic fault movement are observed near the geothermal field following the start of production possibly triggered by the stresses induced by geothermal fluid extraction

progradational holocene carbonate tidal flats of crooked island south east bahamas an alternative to the humid channelled belt model the stratigraphic record of many cratonic carbonate sequences includes thick successions of stacked peritidal deposits representing accumulation at or near sea level these deposits have provided insights into past palaeoenvironments sea level and climate change to expand understanding of carbonate peritidal systems this study describes the geomorphology sedimentology and stratigraphy of the tidal flats on the crooked acklins platform south east bahamas the crooked island tidal flats extend continuously for ca 18 km on the platformward flank of crooked island reaching up to 2 km across tidal flats include four environmental zones with specific faunal and floral associations and depositional characteristics i supratidal continuous supratidal crust and pavement ii upper intertidal with the mangrove avicennia germinans and the cyanobacteria scytonema iii lower intertidal with the mangrove rhizophora mangal and iv non vegetated heavily burrowed subtidal submarine these zones have gradational boundaries but follow shore parallel belts coring reveals that the thickness of this mud dominated sediment package generally is 2 m with depth to pleistocene bedrock gradually shallowing landward the facies succession under much of the tidal flat includes a basal compacted organic rich skeletal lithoclast lag above the bedrock contact suggesting initial flooding this unit grades upward into rhizoturbated skeletal sandy mud subtidal overlain by coarsening upward peloid foraminifera gastropod muddy sand reflecting shallowing to intertidal elevations cores from landward positions include stacked thin indurated layers with autoclastic breccia root tubules and fenestrae interpreted as supratidal conditions collectively the data reveal an offlapping pattern on this prograding low energy shoreline and these holocene tidal flats may represent an actualistic analogue for ancient humid progradational tidal flats nonetheless their vertical facies succession is akin to that present beneath channelled belt examples suggesting that facies successions alone may not provide unambiguous criteria for prediction of the palaeogeomorphology lateral facies changes and heterogeneity in stratigraphic analogues

fracture movements and graben subsidence during the 2014 barðarbunga dike intrusion in iceland abstract a dike propagated 48 km from the bar arbunga central volcano into one of its fissure swarms in august and early september 2014 the dike intrusion was accompanied by caldera subsidence in the ice covered bar arbunga central volcano as well as a propagating earthquake swarm graben subsidence and fissure eruptions in the fissure swarm most of the dike was emplaced in the glacier covered part of the fissure swarm but a 10 km long section at the distal northern end of the dike is ice free new ground fractures were observed in the ice free part during the dike propagation and prior to the outbreak of a major lava producing eruption the fractures delineate two graben structures in direct continuation of each other here we show the extent and depth of the grabens and maps of fractures and eruptive fissures that were formed or reactivated north of the vatnajokull glacier during the dike intrusion the active graben subsidence was identified 3 days after seismicity reached the ice free area then fractures delineated a 700 1000 m wide and 5 km long graben visible from the northern edge of the vatnajokull glacier farther north a narrower 250 450 m wide graben was seen a short lived minor eruption occurred between these grabens on the 29th of august 2014 2 days after the grabens were identified the northern graben was extended southwards during or before the eruption and included the eruptive fissure on the 31st of august 2014 a much larger eruption began on the same eruptive fissure the fissure quickly obtained its maximum length of 2 km this eruption lasted 6 months a minor eruption lasting 2 days or less occurred on the southern graben on the 5th of september 2014 the first two eruptions took place on the same eruptive fissure that formed the pre existing holuhraun lava presumably in the 18th century measurements in january 2015 show that the southern graben was 3 5 5 5 m deep but the northern graben had been covered by lava from the depth and the opening of the graben we estimate that the dip of the boundary faults of the graben is 75 the narrow grabens observed before the eruptions indicated that magma was at shallow depth likely much less than 300 m 2 days before it opened to the surface such observations can serve as a tool to estimate likelihood of eruptions during dike intrusions

flat pebble conglomerate its multiple origins and relationship to metre scale depositional cycles carbonate flat pebble conglomerate is an important component of precambrian to lower palaeozoic strata but its origins remain enigmatic the upper cambrian to lower ordovician strata of the snowy range formation in northern wyoming and southern montana contain abundant flat pebble conglomerate beds in shallow water cyclic and non cyclic strata several origins of flat pebble conglomerate are inferred for these strata in one case all stages of development of flat pebble conglomerate are captured within stormdominated shoreface deposits of hummocky cross stratified hcs fine carbonate grainstone a variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat pebble conglomerate these features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits failure was induced by either storm or seismic waves and many beds failed along discrete slide scar surfaces centimetre scale laminae within thick amalgamated hcs beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds in some cases buckled masses accelerated downslope until they exceeded their internal friction completely disarticulated into clasts and transformed into a mass flow of individual cm to dm scale clasts this transition was accompanied by the addition of sand sized echinoderm rich debris from local sources which slightly lowered friction by reducing clast clast interactions the resulting dominantly horizontal clast orientations suggest transport by dense viscous flow dominated by laminar shear these flows generally came to rest on the lower shoreface although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds the clasts in these beds show no evidence of extensive reworking i e not well rounded or condensation i e no rinds or coatings a second type of flat pebble conglomerate bed occurs at the top of upward coarsening metre scale cycles the flat pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or in some cases may represent transgressive lags the clasts are well rounded display borings and have iron rich coatings the matrix to these beds locally includes glauconite these beds were considerably reworked and represent condensed deposits thrombolites occur above the flat pebble beds and record microbial growth before initial transgression at the cycle boundaries a third

strain accumulation along an oblique plate boundary the reykjanes peninsula southwest iceland strain accumulation along an oblique plate boundary the reykjanes peninsula southwest iceland

earthquakes stress and strain along an obliquely divergent plate boundary reykjanes peninsula southwest iceland earthquakes stress and strain along an obliquely divergent plate boundary the reykjanes peninsula southwest iceland

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

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

carbon chemostratigraphy of the cambrian ordovician transition in a midlatitude mixed platform montagne noire france the cambrian drumian carbon isotope excursion dice and the steptoean positive carbon isotope excursion spice have been so far reported in subtropical platforms the dice event is a large negative excursion in 13ccarb record that nearly coincides with the beginning of the drumian cambrian series 3 whereas the spice event is a large positive shift in 13ccarb identified close to the base of the paibian furongian series the chemostratigraphic excursions have been recognized in carbonate dominated platforms and their application to mid and high latitude siliciclastic dominated platforms has been problematic this paper offers for the first time a high resolution stratigraphic analysis of this time span in a temperate water platform montagne noire france that recorded episodes of carbonate productivity in nearshore environments two sections were analyzed in detail a complete cambrian series 3 tremadocian succession devoid of carbonate interbeds and paleogeographically located in the distal part of the platform pardailhan nappe and a furongian tremadocian succession which has limestone interbeds and represents the proximal part of the platform minervois nappe the lower part of the la gardie formation in the pardailhan nappe displays background 13corg values of 22 punctuated by a large negative middle languedocian regional substage shift peaking at 24 5 this negative excursion is similar to the dice event reported close to the base of the drumian although the latter is exclusively based on 13ccarb values stable isotopes of 13ccarb and 13corg from both sections also indicate a common chemostratigraphic shift at the onset of the furongian in nearshore deposits of the minervois nappe the 13ccarb background values for the interbedded limestone strata of the val d homs formation gradually increase from 3 to 1 and are punctuated by a sharp increase in 13ccarb values to 3 0 by contrast in basinal shales of the la gardie formation in the pardailhan nappe the 13corg background values decrease from 22 to 27 1 this trend is directly controlled by total organic carbon toc contents and is interpreted to have been initiated by changes in the degree of biodegradation of organic matter sulfate reduction and methanogenesis before or soon after burial despite this overprinting and the very low degree of metamorphism reached by the shales the 13corg background trend is also punctuated by a sharp positive shift in 13corg values to 4 0 similar to the spice event in addition the asymmetric shape of the spice excursion in the montagne noire shows evidence for the herb event a late furongian negative carbon isotope excursion not yet accepted as a worldwide chemostratigraphic anomaly the recognition of both chemostratigraphic shifts in mixed and clayey strata opens new possibilities of chemostratigraphic correlation in mid and high latitude platforms where carbonate factories did not widely develop and strata only contain endemic fauna this implication has a major consequence because the global stratotype section and point gssp of the drumian and paibian stages occurs near the base of both excursions

sequence stratigraphy and depositional facies of lower ordovician cyclic carbonate rocks southern missouri u s a abstract lower ordovician cyclic carbonate strata of southern missouri were deposited in a warm shallow epeiric sea on a fully aggraded carbonate platform sedimentological characteristics distinguish the jefferson city and cotter dolomites from the underlying gasconade and roubidoux formations mixed carbonate siliciclastic sedimentation characterizes the roubidoux formation with sandstones accounting for up to 60 of sedimentation the gasconade jefferson city and cotter dolomites exhibit an increased occurrence of chalcedonic chert nodules in very similar shape and texture to the gypsum and anhydrite nodules common on modern sabkha supratidal flats casts of halite and ghosts of gypsum laths also exist in the jefferson city and cotter strata but are rarely found in the underlying units facies analysis from drill cores and outcrop sections provides the basis for identifying two major meter scale cycle types type i cycles consist of algal stromatolites tidal flat laminites mechanical and algal ooid grainstones wavy peloidal wackestones and quartz sandstones interpreted as peritidal facies they are the dominant components of the roubidoux formation jefferson city dolomite and cotter dolomite type ii cycles consist mostly of subtidal facies such as strongly burrowed mudstone thrombolite boundstone and stromatolites type i cycles are thinner and represent highstand systems tracts whereas the thicker type ii cycles represent transgressive systems tracts and are dominant in the gasconade dolomite the cycle stacking patterns facies changes and the intrabasinal correlatability of fischer plots made from the widely spaced sections argue for a eustatic control on sea level fluctuation on the platform interbasinal correlation with other north american basins is possible using biostratigraphic information and comparison of fischer plots five missouri sequences correlate with those described for other regions the continent wide uniformity in cycle stacking patterns indicates a primarily eustatic control on lower ordovician meter scale cycle development regional tectonic and autocyclic controls probably account for general differences in sedimentation pattern among the correlated basins

sequence stratigraphic expression of greenhouse transitional and icehouse conditions in siliciclastic successions paleozoic examples from the central appalachian basin usa abstract climates during the paleozoic varied from greenhouse to transitional to icehouse and siliciclastic successions deposited during those times have stratigraphic architectures consistent with magnitudes and frequencies of sea level fluctuations at the time of deposition three central appalachian basin paleozoic successions the early cambrian chilhowee group the late devonian foreknobs formation and the early pennsylvanian breathitt group are compared and contrasted to develop models that relate stratigraphic architectures to greenhouse transitional and icehouse conditions existing on earth within the upper portion of the early cambrian chilhowee group the hampton and erwin formations represent a succession of 2 myr 3rd order sequences consisting of stacked 40 80 kyr 5th order marine parasequences that record low magnitude high frequency sea level changes consistent with greenhouse conditions the late devonian foreknobs formation consists of a series of 0 8 1 0 myr 3rd order composite sequences comprised of stacked unconformity bounded 225 to 450 kyr 4th order sequences each containing 40 to 65 kyr 5th order marine parasequences the bases of the unconformity bounded sequences are marked by erosional surfaces regressive surfaces of marine erosion fluvial deposits or paleosols indicative of sea level drops of 10 to 45 m consistent with a transitional greenhouse to icehouse climate the early pennsylvanian breathitt group consists of stacked unconformity bounded 100 kyr 4th order sequences that lack parasequences unconformity bounded sequences are comprised of from bottom to top lowstand incised valley braided alluvial deposits transgressive tidal estuarine deposits and progradational highstand deltaic deposits depths of incised valleys and vertical transition into estuarine facies imply sea level changes of 50 m consistent with icehouse conditions this study demonstrates that siliciclastic successions preserve a record of sea level changes consistent with greenhouse transitional and icehouse conditions on the paleozoic earth

relative sea level changes in baltoscandia in the cumbrian and early ordovician the predominance of tectonic factors and the absence of large scale eustatic fluctuations abstract fluctuations in sea depth within a magnitude 20 100 m and a duration of 1 10 m y are often explained by rapid eustatic changes so called third order eustatic events considerable influence of regional tectonics on relative sea level changes has been demonstrated by many authors but because of uncertainties in the timing of short events in widely separated regions the problem of separating tectonic and eustatic factors still remains unsolved in this paper a new and simple approach is used to reveal the presence or absence of eustatic events we consider the st petersburg area and north estonia in the north eastern region of the east baltic from the late early cambrian until the middle of the tremadoc early ordovician deposition was extremely slow and the sea bed remained for a long time in a well defined peritidal zone in a water depth 10 m under such environmental conditions a sea level rise of 10 m would result in marked changes in the character of faunas and sedimentation in the time interval considered here significant sea deepening in the north eastern baltic region occurred only twice and its magnitude did not exceed 10 20 m a fall of sea level by 10 m would result in complete regression in the peritidal zone this situation also occurred in region however the preservation of a sequence of unconsolidated sands which is only a few tens of metres thick and includes all the main stratigraphic subdivisions on a regional scale indicates that the crustal surface reached a very low altitude 10 20 m above sea level these data show that in the late late cambrian to the middle of the tremadoc over a period of 40 m y long eustatic sea level changes did not exceed 10 20 m this limits the magnitude of several third order cycles eustatic events with duration of a few million years which have been proposed previously for the epoch of the transition from the cambrian to the ordovician in the late early cambrian to the late cambrian transgressions and regressions with a magnitude of 50 150 m took place in southern sweden and lithuania since these phenomena occurred when there were no comparable eustatic sea level changes they must be associated with regional tectonic movements some were rapid and could be easily misinterpreted as indications of third order eustatic changes it is probable that some of the other eustatic events that have been proposed for the phanerozoic were actually not of eustatic but of tectonic origin such rapid tectonic movements with magnitude of 50 100 m in cratonic areas can be caused by changes in the forces in the lithospheric layer with a laterally variable thickness and by phase transitions in the mafic lower crust depending on the spatial distribution of vertical crustal movements both these mechanisms could have been operating in the east baltic and southern sweden in the cambrian

seismic properties density and composition of the icelandic crust near reydarfjordur compressional and shear wave velocities densities and porosities have been measured to confining pressures of 6 kbar for 135 samples from a deep borehole in eastern iceland the measurements are from a random sampling of dikes and sills through the 2 km hole velocities within the flow units are relatively low at the tops reach maximum values within central portions and decrease near the bases this systematic variation correlates well with variations in porosity and bulk density the dike units on the other hand have relatively uniform velocities which are approximately equivalent to maximum velocities measured within flow units below 700 m the dike and flow velocities show systematic increases with depth which are related to changes in mineralogy and independent of porosity the velocity gradient dvp dz t within this region is well defined by the laboratory velocities and approximates 0 6 s 1 in good agreement with field refraction measurements high velocities near the base of the hole are related to increasing alteration with depth and in particular an increase in epidote content the abundance of epidote in the rocks recovered from the lower portion of the drillhole supports a metamorphic origin for the layer 2 layer 3 boundary in this region

sequence sequence stratigraphy sea level changes and depositional systems in the cambro ordovician of the north china carbonate platform abstract the cambro ordovician strata in north china were deposited over a very extensive craton extending some 1500 km east west and 1000 km north south the dominantly shallow water carbonate succession reaches up to 2000 m in thickness and two megasequences transgressive regressive cycles can be distinguished lower cambrian through lower ordovician strata and middle through upper ordovician strata separated by a major palaeokarst the first megasequence consists of nine sequences which are generally 50 150 m in thickness the lower cambrian sediments consist of phosphorites and phosphatic sandstones deposited during the flooding of the craton carbonates mudrocks and evaporites were deposited in the early cambrian under an arid climate laying the foundation for the subsequent long period of shallow water carbonate deposition which lasted some 70 m y the middle and lower upper cambrian sequences consist predominantly of mudrocks and storm deposits tempestites in the lower part and oolitic grainstones and tidal flat lime mudstones in the upper part these represent outer mid and mid inner ramp depositional systems respectively of the transgressive and highstand systems tracts these sequences have strong similarities with the grand cycles in the cambro ordovician successions of north america in the upper cambrian there is a distinctive unit of glauconitic purple red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence in the upper cambrian there was a phase of tilting of the north china carbonate platform to the north and storm deposits especially intraclastic conglomerates and hummocky cross stratified grainstones packstones were very common at this time also common in upper middle and lower upper cambrian strata are stromatolitic thrombolitic bioherms several metres in diameter the upper upper cambrian through lower ordovician strata are dominantly fine grained limestones and dolomites deposited in shallow subtidal and inter to supra tidal environments on a low energy epeiric sea platform this part of the succession is the regressive part of the first megasequence so that overall the platform shows an evolution from platform initiation to platform foundation to a ramp depositional system and then an extensive epeiric platform on a small scale the succession is composed of metre scale shallowing upward cycles parasequences arranged into cycle sets of 10 30 m thick the middle upper ordovician megasequence consists mostly of shallow water carbonates with several thick evaporite units minor palaeokarsts and palaeosoils separate the sequences which are composed of metre scale cycles regional uplift affected the north china platform in the ordovician and sedimentation did not resume until the carboniferous there is a good correlation of the two transgressive regressive megasequences described here with the global 2nd order relative sea level curve for the cambro ordovician

strain accumulation 1986 1992 across the reykjanes peninsula plate boundary iceland determined from gps measurements gps geodetic observations 1986 1992 spanning the oblique plate boundary on the reykjanes peninsula sw iceland show that left lateral shear strain is accumulating in the area the principal strain rates are 1 0 255 0 055 strain yr extension at n121 6 e and 2 0 190 0 053 strain yr contraction at n31 6 e the principal axis of extension is perpendicular to the strike of fractures within volcanic systems on the peninsula the direction of maximum left lateral shear strain is n76 6 e the same as the trend of the seismic zone on the peninsula tensor shear strain in this direction is 0 222 0 054 strain yr continuous aseismic slip of the north american eurasian plates below a 5 11 km locking depth on the peninsula can explain the observed depth of earthquakes the width of the deformation zone determined from recent fissuring and faulting and the gps measurements of strain

stratigraphic completeness of carbonate dominated records from continental interiors versus continental margins stratigraphic thinning occurs via condensation and omission at multiple scales abstract over phanerozoic time scales stratigraphic records from continental interiors or cratons are dramatically thinner and are generally assumed to be relatively incomplete with more numerous and longer duration hiatuses compared to records from more rapidly subsiding continental margins however this assumption need not be true for the shorter time scales i e several 106 years and shorter over which accumulation of the preserved stratigraphic record on the continental interior actually takes place this study compares carbonate dominated sedimentary records from the devonian continental interior iowa and continental margin nevada to evaluate whether the continental interior record is 1 miniaturized i e thinner but equally complete 2 comparable in thickness and quality where the sedimentary record is preserved but certain portions of the record are notably absent i e omitted or truncated or 3 so invariably preserved that equivalent stratigraphic packages cannot be recognized compared to the continental margin record this study finds that fourth to fifth order cycles in iowa are on average approximately half as thick and half as numerous compared to the continental margin of nevada moreover the stratigraphic record on the continental margin of nevada is dominated by cycles that are composed of tidal flat and shallow subtidal lithofacies capped by simple flooding surfaces and exhibit catch up or keep up depositional styles the continental interior record in iowa is dominated by cycles that are composed of subtidal lithofacies capped by a range of bounding surfaces and exhibit catch down or give up depositional styles these findings suggest that the thin record in iowa is largely a result of low sedimentation rates associated with suppressed carbonate production in this epeiric sea setting rather than minimal subsidence rates and an increased potential for subaerial exposure and associated loss of record

upper cambrian carbonate platform evolution elvinia and taenicephalus zones pterocephaliid ptychaspid biomere boundary northwestern wyoming abstract the pterocephaliid ptychaspid biomere boundary is an upper cambrian extinction horizon that can be used to correlate mixed carbonate siliciclastic strata in northwestern wyoming the biomere boundary is defined by the change between the elvinia and taenicephalus zones of the standard north american trilobite zonation it is also signaled chemostratigraphically by a positive trend in carbon isotope ratios thus both biostratigraphic horizons and carbon isotope data are used to constrain the position of the biomere boundary in northwestern wyoming study of 21 stratigraphic sections reveals the presence of distinct facies belts for the middle and upper elvinia zone and lower taenicephalus zone these are interpreted to reflect deposition in three distinct paleoenvironments 1 oolitic shoal 2 intrashelf basin and 3 inner platform margin following a major hiatus in sedimentation in wyoming during much of pterocephaliid biomere time a healthy shallow water carbonate factory was reestablished during elvinia zone time this produced abundant deposits of oolitic and skeletal packstone in the shoal environment and an areally extensive thrombolite boundstone facies in the intrashelf basin setting approximately coincident with the biomere boundary extinction event irvingella major time ooid production was terminated and the thrombolites were drowned the correlation of this major change in the pattern of sedimentation allows for the recognition of parts of two depositional sequences in the study area this drowning event sequence boundary may also be coeval with the accumulation of transported shallow water carbonate deposits in an inner platform margin environment during early taenicephalus zone time sedimentation rates were apparently reduced and the entire study area is marked by thin concentrations of trilobite and brachiopod shells the nonskeletal carbonate factory was never reestablished in the former shoal environment where thick successions of homogeneous lime mud accumulated during the rest of taenicephalus zone time flat pebble conglomerate and shale accumulated in the storm influenced intrashelf basin during this time sea level change and changes in seawater chemistry likely both played roles in the drowning event across the elvinia taenicephalus boundary a positive trend in carbon isotope values across the pterocephaliid ptychaspid biomere boundary is observed in the six sections analyzed this is consistent with previous studies that indicated that the mass extinction occurs within a regional shift towards positive values and this may reflect the development of low oxygen conditions on the shelf the global significance of these isotopic trends changes in relative sea level and the mass extinction has not been confirmed although recent study of the extinction event at the base of the pterocephaliid biomere has revealed a similar trend towards positive carbon isotope values that is worldwide

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

quantitative analysis of accommodation patterns in carbonate platforms an example from the mid cretaceous of se spain abstract for sequence stratigraphic analysis of extensive carbonate platforms hundreds of kilometres wide developed in greenhouse climates on broad passive margins less emphasis should be placed on large scale seismic geometries and more attention paid to sequence stratigraphic correlation of stratigraphic sections based usually on isolated outcrops to this end quantitative analysis of accommodation emerges as a simple useful tool that allows detailed architectural reconstructions regional chronostratigraphical correlation and systems tract interpretation in this paper a quantitative analysis of accommodation was applied to the wide platforms that developed in the southern passive continental margin of iberia during the mid cretaceous late albian to early middle cenomanian interval this analysis was based on several integrated techniques including 1 construction of total accommodation curves with the aid of backstripping techniques for calculating decompacted sedimentary accumulation through time 2 mathematical analysis of these curves and characterisation of second and third order accommodation patterns and 3 analysis of parasequence stacking patterns in peritidal cyclic successions by means of fischer plots by applying these techniques to eight individual sections logged at the decimetre scale in outcrops of the external zones of the betic orogenic belt it was possible to characterise the second and third order accommodation signal for this interval in the basin the second order curve defines a long term sigmoidal pattern of nearly six million years with low rates of accommodation generation in the first and the last part of the interval and high rates in the mid interval the third order signal defines six accommodation events of one million years average duration which controlled the development of six successive depositional sequences and their systems tracts on the basis of this new sequence stratigraphic framework a high resolution 2 d platform transect showing the spatial distribution of facies was erected and analysed the results notably complete previous qualitative sequence stratigraphic data on the platform and contribute to a better understanding of the nature of systems tracts and their boundaries in response to overlapping of second and third order accommodation patterns

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

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

surface effects of faulting and deformation resulting from magma accumulation at the hengill triple junction sw iceland 1994 1998 the hengill triple junction sw iceland is subjected to both tectonic extension and shear causing seismicity related to strike slip and normal faulting between 1994 and 1998 the area experienced episodic swarms of enhanced seismicity culminating in a ml 5 1 earthquake on june 4 1998 and a ml 5 earthquake on november 13 1998 geodetic measurements using global positioning system gps leveling and synthetic aperture radar interferometry insar detected maximum uplift of 2 cm yr and expansion between the hromundartindur and grensdalur volcanic systems a number of faults in the area generated meter scale surface breaks geographic information system gis software has been used to integrate structural field and geophysical data to determine how the crust failed and to evaluate how much of the recent activity focused on zones of pre existing weaknesses in the crust field data show that most surface effects can be attributed to the june 4 1998 earthquake and have occurred along or adjacent to old faults surface effects consist of open gashes in soil shattering of lava flows rockfall along scarps and within old fractures loosened push up structures and landslides seismicity in 1994 1998 was distributed asymmetrically about the center of uplift with larger events migrating toward the main fault of the june 4 1998 earthquake surface effects are most extensive in the area of greatest structural complexity where n and e trending structures related to the transform boundary intersect ne trending structures related to the rift zone insar gps and field observations have been used in an attempt to constrain slip along the trace of the fault that failed on june 4 1998 geophysical and field data are consistent with an interpretation of distributed slip along a segmented right lateral strike slip fault with slip decreasing southward along the fault plane we suggest a right step or right bend between fault segments to explain local deformation near the fault b 2002 elsevier science b v all rights reserved

carbonate deposition and sequence stratigraphy of the terminal cambrian grand cycle in the southern appalachians u s a abstract the cessation of shale and carbonate deposition of the conasauga group grand cycles middle to upper cambrian and the establishment of the widespread peritidal carbonate deposition of the knox group upper cambrian to lower ordovician represent a prominent change in sedimentation along the early paleozoic passive continental margin in the southern appalachians to evaluate the causes for this change this study focuses on the maynardville formation which is the uppermost carbonate unit of the conasauga group the maynardville consists of 1 a lower subtidal facies succession which is underlain by the nolichucky shale and resembles the rest of the conasauga group carbonate deposits and 2 an upper peritidal facies succession which is conformably overlain by similar peritidal deposits of the copper ridge dolomite knox group deposition of shale and subtidal carbonate took place in deep ramp nolichucky to shallow ramp subtidal shoal and lagoonal settings lower maynardville the carbonate ramp was westward sloping toward the conasauga intrashelf shale basin to the east the ramp was linked to a broad semiarid carbonate tidal flat encompassing a variety of peritidal environments upper maynardville and copper ridge the maynardville is a shallowing upward succession that formed by carbonate platform aggradation and basinward progradation the transition between the maynardville and the copper ridge dolomite is within a conformable peritidal carbonate succession that contains common siliciclastic sand size detritus this interval is interpreted as a sequence boundary correlative with the craton wide late steptoean dresbachian franconian or sauk ii sauk iii unconformity the change from a conasauga to a knox depositional style may be related to distinct stages in passive margin evolution the end of grand cycle deposition in the early late cambrian is coincident with the cessation of tectonic activity along extensional features an intracratonic graben and other fault systems and marks the transition into a mature passive margin setting the mature margin was characterized by decreased rates of thermal subsidence which coupled with the infilling of the conasauga intrashelf basin favored shallow water carbonate deposition the final stabilization of the margin is reflected in the deposition of the thick peritidal carbonate strata of the knox group

the krafla fissure swarm iceland and its formation by rifting events fissure swarms at divergent plate boundaries are activated in rifting events during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions to study the evolution of fissure swarms and the behaviour of rifting events detailed mapping was carried out on fractures and eruptive fissures within the krafla fissure swarm kfs fracture densities of dated lava flows ranging from 10 000 years bp to 30 years old were studied and the fracture pattern was compared with data on the historical m vatn rifting episode 1724 1729 and the instrumentally recorded krafla rifting episode 1975 1984 additionally the interaction of transform faults and fissure swarms was studied by analysing the influence of the husavik transform faults on the kfs during the historical rifting episodes eruptions on the fissure swarm occurred within 7 km from the krafla central volcano although faults and fractures were formed or activated at up to 60 70 km distance this is consistent with earlier rifting patterns as holocene eruptive fissures within the kfs are most common closer to the central volcano most fractures within the central krafla caldera are parallel to the overall orientation of the fissure swarm this suggests that the regional stress field is governing in the krafla central volcano while the local stress field of the volcano is generally weak a sudden widening of the graben in the northern kfs and a local maximum of fracture density at the junction of the kfs and the extrapolation of the husavik transform fault zone indicates possible buried continuation of the husavik transform fault zone which extends to the kfs eruptive fissures are found farther away from the krafla central volcano in the southern kfs than in the northern kfs this is either due to an additional magma source in the southern kfs the hei arspor ur volcanic system or caused by the husavik transform faults transferring some of the plate extension in the northern part fracture density within particular lava flow fields increases with field age indicating that repeated rifting events have occurred in the fissure swarm during the last 10 000 years bp the fracture density in the kfs is also generally higher closer to the krafla central volcano than at the ends of the fissure swarm this suggests that rifting events are more common in the parts of the fissure swarm closer to the krafla central volcano

a basin wide record of the late cambrian steptoean positive carbon isotope excursion spice in the amadeus basin australia abstract the cambrian paibian sedimentary succession of the central australian amadeus basin contains a sequence of supratidal to subtidal shallow marine siliciclastic and oolitic stromatolitic limestones and dolostones basin wide sequence stratigraphy in combination with biostratigraphy revealed the g stolidotus zone within a 3rd order transgressive systems tract tst the westward transgression caused changes from a fluvial dominated depositional environment towards a shallow marine oolitic carbonate shoal environment the eastern succession is dominated by stromatolitic oolitic carbonate rocks with 2 to 5 m 5th order shoaling upward cycles with several 4th order cycles the change from tst to hst highstand systems tract is marked by a maximum flooding surface within the goyder formation which coincides with the peak of the steptoean positive carbon isotope excursion spice the spice shows a facies independent synchronous positive 13c excursion of 5 in a 130 m interval in 8 sections across a 460 km transect the spice peak is lowest in the nearshore successions 0 4 13c and highest in the platform succession 4 9 13c and is interpreted to be related to the chemical gradient of seawater and mixing of the dic with atmospheric co2 derived i e terrestrial bicarbonate the recovery from spice is recorded by 4th order shoaling upward cycles that compose the 3rd order hst this is the first time that sequence stratigraphy and biostratigraphy has been combined with carbon isotope chemostratigraphy across the nearshore to subtidal cambrian basin succession in australia

evidence of spice related anoxia on the laurentian passive margin paired δ13c and trace element chemostratigraphy of the upper conasauga group central appalachian basin abstract this study reports data from the upper cambrian conasauga group and overlying copper ridge formation of the central appalachian region eastern u s a geochemical isotopic and petrographic analysis of core material was carried out to constrain the extent of marine anoxia and to characterize sediment fluxes on the carbonate platform and continental shelf of laurentia contemporaneous with the steptoean positive carbon isotope excursion spice a late cambrian global marine anoxic event carbonate rocks primarily dolostone record a positive 13ccarb excursion starting in the middle nolichucky formation reaching its peak 4 3 at the boundary between the maynardville and copper ridge formations strontium isotope ratios in the dolostone units are only slightly offset from the expected cambrian seawater values suggesting minimal post diagenetic disturbance of isotopic and trace element systematics selective leaching of carbonate units reveal upward decreasing trends in dissolved redox sensitive trace metals e g u ni v indicative of drawdown from regional and global marine anoxia during the peak of the late cambrian spice event

triggered fault slip on june 17 2000 on the reykjanes peninsula sw iceland captured by radar interferometry 1 dynamically triggered seismicity followed shortly after a ms 6 6 earthquake in iceland on june 17 2000 smaller earthquakes occurred on the reykjanes peninsula up to 100 km from the mainshock rupture using interferometric analysis of synthetic aperture radar images insar we measure crustal deformation associated with three triggered deformation events the largest of these occurred at lake kleifarvatn 85 km west of the mainshock epicenter modeling of the insar data reveals strikeslip on a north striking fault with a geodetic moment of 6 2 1017 nm equivalent to magnitude mw 5 8 earthquake a seismological estimate of the moment is not yet available because the seismic signature of this event is partly hidden by the mainshock waveform the paucity of aftershocks on the triggered rupture plane suggests some aseismic slip there compatible with a thin seismogenic crust high heat flow hydrothermal alteration and the presence of fluids in the area

the 1994 1995 seismicity and deformation at the hengill triple junction iceland triggering of earthquakes by minor magma injection in a zone of horizontal shear stress since july 1994 an unusually persistent swarm of earthquakes m 4 0 has been in progress at the hengill triple junction sw iceland activity is clustered around the center of the hromundartindur volcanic system geodetic measurements indicate a few centimeters uplift and expansion of the area consistent with a pressure source at 6 5 3 km depth beneath the center of the volcanic system the system is within the stress field of the south iceland transform zone and the majority of the recorded earthquakes represent strike slip faulting on subvertical planes we show that the secondary effects of a pressure source modeled as a point source in an elastic half space include horizontal shear that perturbs the regional stress near the surface shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress the recorded earthquakes show spatial correlation with areas of enhanced shear the maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 strain sufficient to trigger earthquakes if the crust in the area was previously close to failure

recognizing artifactually generated coordinated stasis implications of numerical models and strategies for field tests abstract a null model that generates the first and last occurrences of species through time in a single stratigraphic section predicts several patterns startlingly similar to those described as coordinated stasis the model is able to produce these patterns by assuming stochastically constant extinction and origination rates a gaussian form of facies control with respect to water depth and cyclically varying water depths in accord with sequence stratigraphy the model predicts that highstand and lowstand systems tracts will be characterized by faunal tracking with only minor amounts of faunal turnover sequence boundaries and flooding surfaces within the transgressive systems tract will be characterized by elevated rates of faunal turnover these pulses of faunal turnover will exhibit ecological selectivity in terms of the preferred depths facies tolerances and abundance of species where outcrop patterns of first and last occurrences match these predictions biologically driven or environmentally driven coordinated stasis may not be distinguishable from stratigraphic artifact however pulses of faunal turnover that occur within highstand and lowstand systems tracts are likely to be real turnover events multiple outcrop studies that span a significant distance along paleoslope are necessary to disprove artifactual turnover that occurs at a sequence boundary or within a transgressive systems tract

enargite luzonite hydrothermal vents in manus back arc basin submarine analogues of high sulfidation epithermal mineralization abstract active and inactive hydrothermal chimneys composed almost entirely of enargite and luzonite rare minerals in seafloor hydrothermal deposits were found at the summits of two submarine volcanoes north su and kaia natai in the manus back arc basin detailed mineralogical and geochemical studies revealed that most probably these deposits precipitated at t 200 330 c and high fs2 the negative 34s values 8 58 to 3 70 of the enargite luzonite are best explained by disproportionation reactions of magmatic so2 and suggest that the high fs2 is likely provided by direct magmatic input of so2 into the hydrothermal system fractionation of cu stable isotopes during the precipitation of enargite luzonite 65cu ranges from 0 20 to 0 35 is inferred to be associated with either rayleigh type fractionation or redox processes cu oxidation to cu2 and the mass balance of dissolved cu and cu2 species in the hydrothermal fluid the trace element composition of enargite and luzonite indicates a temporal fluctuation of the chemistry of the ore forming fluid with an increase of fe ga tl au hg pb and ag and decrease of sb sn te ge and v concentrations with time and points out that this type of deposits is the richest in au average 11 9 ppm and te average 169 ppm among all other types of seafloor metal deposits in addition to the widespread inorganic precipitation of enargite and luzonite in this setting there is evidence that this mineralization may be biogenically mediated on the external surfaces of the active vents fungi like filaments mineralized by luzonite imply that the fungi dikarya subkingdom may be implicated in a mechanism of bio sequestration of as s and cu and provide the initial substrate for luzonite precipitation the studied enargite luzonite deposits have characteristics similar to those of subaerial high sulfidation epithermal mineralization back arc basin setting acid sulfate and boiling ore forming fluids altered advanced argillic stage dacitic host rocks major enargite luzonite and minor pyrite barite and s0 34s au mineralization at depth

sea level changes and stratigraphy of the nelson limestone middle cambrian neptune range antarctica abstract the first detailed history of relative sea level changes known from lower paleozoic strata in antarctica is documented in this study of the nelson limestone basal beds of the formation constitute a nonmarine terrigenous valley fill succession thickness of these strata varies from 0 1 m to 72 m suggesting that significant topographic relief existed below the sub nelson unconformity locally base level must have risen at least 72 m because following fluvial infilling of the valley a succession of carbonates over 400 m thick was deposited as seas inundated the antarctic margin of gondwana during late middle cambrian time on a global scale the broad relative sea level rise recorded in the nelson limestone is synchronous with previously recognized eustatic events during lejopyge laevigata time in the nelson limestone smaller fluctuations of relative sea level are indicated by three sequences each about 100 m thick the lower sequence is bounded below by the sub nelson unconformity and includes the valley fill succession transgressive sandstone and highstand carbonate deposits the top of a subaerially exposed interval of carbonates in one section is interpreted as a sequence boundary with correlative conformity in the deeper water areas the subaerially exposed interval is roughly correlative with a succession of five or more slump deposits 1 2 m thick in another section subaerial exposure at nearly the same stratigraphic level as the slumps marked a fundamental shift in depositional style following marine flooding an offshore carbonate shoal with an associated restr cted lagoon was established during deposition of the middle sequence the upper sequence records marine flooding above the middle sequence and subsequent reestablishment of an offshore carbonate shoal prior to deformation and volcanism in the urea related to the ross orogeny

a review of the late cambrian furongian palaeogeography in the western mediterranean region nw gondwana abstract the cambrian ordovician transition of the western mediterranean region nw gondwana is characterized by the record of major erosive unconformities with gaps that range from a chronostratigraphic stage to a series the hiatii are diachronous and involved progressively younger strata along the gondwanan margin from sw morocco to ne montagne noire they can be related to development of a multi stage rifting further north currently connected to the opening of the rheic ocean and concomitant erosion on southern rift shoulders the platforms of this margin of gondwana occupied temperate water mid latitudes and were dominated by siliciclastic sedimentation while carbonate factories were only episodically active in the montagne noire platform the upper cambrian is devoid of significant gaps in the southern montagne noire and the iberian chains there the sedimentation took place in a transgressive dominated depositional system with common offshore deposits and clayey substrates and was bracketed by two major regressive trends the late cambrian is also associated with the record of volcanic activity e g in the cantabrian and ossa morena zones and the northern montagne noire and widespread development of a tectonic instability that led to the episodic establishment of palaeotopographies and record of slope related facies associations several immigration events are recognized throughout the latest middle cambrian late cambrian and tremadocian the trilobites show a stepwise replacement of acado baltic type families e g the conocoryphid paradoxidid solenopleurid assemblage characterized by i a late languedocian latest middle cambrian co occurrence of middle cambrian trilobite families with the first anomocarid dorypygid and proasaphiscid invaders ii a late cambrian immigration replacing previous faunas composed of trilobites aphelaspidids catillicephalids ceratopygids damesellids eulomids idahoiids linchakephalids lisariids onchonotinids and pagodiids linguliformean brachiopods acrotretids obolids scaphelasmatids siphonotretids and zhanatellids echinoderms mitrates glyptocystitid cystoids and stromatocystoids and conodonts belonging to the lower peltura zone and iii the subsequent input of new trilobites asaphids calymenids catillicephalids nileids and remopleurids which marks the base of the proteuloma geinitzi zone associated with pelmatozoan holdfasts oryctoconus and a distinct input of late tremadocian conodonts paltodus deltifer zone the biogeographic distribution of latest middle and late cambrian trilobites supports brachiopod data indicating strong affinities between the western mediterranean region east gondwana north china korea south china australia and antarctica and kazakhstania during the late languedocian which became significantly stronger during the late cambrian this major shift may suggest modification in oceanic circulation patterns throughout gondwana across the middle late cambrian transition

crustal deformation at the oblique spreading reykjanes peninsula sw iceland gps measurements from 1993 to 1998 in 1993 and 1998 a 38 point gps network was surveyed on the reykjanes peninsula sw iceland according to the nuvel 1a plate motion model the spreading rate of the north american and eurasian plates in sw iceland is 18 9 0 5 mm yr toward n102 7 1 1 e highly oblique to the plate boundary instead of oblique spreading the measurements indicate left lateral shear strain accumulation parallel to the reykjanes peninsula seismic zone n76 e at the rate of about eyx 0 2 strain yr tensor shear strain subsidence is generally observed toward the seismic zone a local maximum subsidence of 60 mm was measured in the svartsengi geothermal area subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage expansion was observed in the area of the hengill triple junction this is probably a result of magma accumulation beneath mount hromundartindur as previous seismic and geodetic measurements have indicated using a simple screw dislocation model we fit the majority of the data assuming a left lateral shear zone at depth along the seismic zone we estimate locking depth of 6 5 km and deep slip rate of 16 5 mm yr the maximum left lateral displacement predicted by the screw dislocation model 11 85 0 06 mm yr is consistent with the observed value of 11 9 0 5 mm yr if the hengill area is excluded little extension is observed across the peninsula the discrepancy between the nuvel 1a oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period

kinematic models of plate boundary deformation in southwest iceland derived from gps observations 1 we use data from gps campaign and continuous measurements from 1992 to 2004 in sw iceland to map the surface velocity field from the reykjanes peninsula to the eastern volcanic zone we divide the time series into preseismic july 1992 to june 2000 and postseismic june 2000 to may 2004 time intervals and we estimate gps station velocities for each interval as well as coseismic offsets due to the june 2000 earthquake sequence in the south iceland seismic zone sisz in addition to the plate spreading the preseismic velocity field shows the effects of inflation at hengill and hekla volcanoes whereas the postseismic velocities show deformation following the june 2000 earthquakes we consider several kinematic models to explain the preseismic velocities our preferred model includes several dislocations and point sources in an elastic half space with left lateral slip along the plate boundary on the reykjanes peninsula and below the sisz opening across the reykjanes peninsula and the western and eastern rift zones the optimal model has a locking depth of about 8 km in the central and eastern part of the reykjanes peninsula with a deep slip rate of about 17 mm yr and an opening of about 9 mm yr this locking depth is in agreement with the thickness of the seismogenic crust on the reykjanes peninsula which appears to vary between 7 and 9 km for the sisz we obtain a deep slip rate of about 19 mm yr below 16 km depth which is considerably deeper than the earthquake hypocenter depths in the area

sequence stratigraphy of the upper cambrian furongian jiangshanian and sunwaptan tunnel city group upper mississippi valley transgressing assumptions of cratonic flooding abstract new data from detailed measured sections permit comprehensive analysis of the sequence framework of the furongian upper cambrian jiangshanian and sunwaptan stages tunnel city group lone rock formation and mazomanie formation of wisconsin and minnesota the sequence stratigraphic architecture of the lower part of the sunwaptan stage at the base of the tunnel city group at the contact between the wonewoc formation and lone rock formation records the first part of complex polyphase flooding sauk iii of the laurentian craton at a scale smaller than most events recorded by global sea level curves flat pebble conglomerate and glauconite document transgressive ravinement and development of a condensed section when creation of accommodation exceeded its consumption by sedimentation thinly bedded fossiliferous sandstone represents the most distal setting during earliest highstand subsequent deposition of sandstone characterized by hummocky or trough cross stratification records progradational pulses of shallower storm and wave dominated environments across the craton before final flooding of sauk iii commenced with carbonate deposition during the middle part of the sunwaptan stage comparison of early sunwaptan flooding of the inner laurentian craton to published interpretations from other parts of north america suggests that sauk iii was not a single long term accommodation event as previously proposed

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

evolution of migrating transform faults in anisotropic oceanic crust examples from iceland major transform fault zones link extensional segments of the north american eurasian plate boundary as it transects the iceland hotspot changes in plate boundary geometry involving ridge jumps

surface effects of triggered fault slip on reykjanes peninsula sw iceland three large earthquakes mw 4 5 were triggered within 5 min 85 km west of a mw 6 5 earthquake in the south iceland seismic zone sisz we report on surface effects of these triggered earthquakes which include fresh rupture widespread rockfall disrupted rockslides and block slides field data confirm that the earthquakes occurred along n striking right lateral strike slip faults field data also support the conclusion from modeling of insar data that deformation from the second triggered event was more significant than for the other two a major hydrological effect was the draining of water through an open fissure on a lake bed lowering the lake level by greater than 4 m field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone d 2003 elsevier science b v all rights reserved