gauging the impact of glacioeustasy on a mid latitude early silurian basin margin mid wales uk the early silurian llandovery gondwanan south polar ice sheet experienced episodes of ice retreat and re advance marine base level curves constructed for the interval are widely assumed to provide a record of the associated glacioeustasy in revealing a series of progradational sequences progrades bounded by flooding surfaces recent work on the type llandovery succession in mid wales uk has provided an opportunity to test this hypothesis the grouping of these progrades into three composite sequences underpins the construction of both low order small amplitude high frequency and high order large amplitude low frequency base level movement curves revised biostratigraphical datasets for the type succession permit the accurate dating of base level events the composite sequences record progradational acmes in the acinaces lower convolutus and upper sedgwickii halli graptolite biozones a series of transgressions that postdate the hirnantian glacial maximum culminated in an upper persculptus biozone high stand maximum flooding events also occurred during the revolutus and lower sedgwickii biozones and the base of the early telychian guerichi biozone also marked the onset of a pronounced deepening a review of 62 published datasets including global and other regional base level curves records of glacial activity isotope data patterns of facies and faunal flux and putative climate models permits an evaluation of the origins of these local base level events the concept of a eustasy index is introduced and shows that the impacts of global sea level movements can only be demonstrated within narrow eustatic windows coincident with times of ice sheet collapse at other times the geometry of llandovery area progrades reflects their accumulation across a faulted basin margin where during periods of slow ice sheet advance epeirogenic processes outstripped sea level movements as the dominant forcing factors increased levels of telychian subsidence at first enhanced and then overwhelmed the influence of glacioeustasy as part of the region s response to the north european scandian deformation

high resolution surface and subsurface sequence stratigraphy of late middle to late ordovician late mohawkian cincinnatian foreland basin rocks kentucky and virginia the late middle to late ordovician late mohawkian to cincinnatian supersequence in kentucky and virginia was deposited in a tectonically active foreland basin during a transition from the ice free early ordovician to the glacial late ordovician this sequence was deposited in less than 12 m y is 250 500 m thick and is composed of four large third order sequences each 40 150 m thick that are regionally correlative in outcrops cores and gamma ray logs smaller scale third order parasequence sets up to 20 m thick and component parasequences 1 8 m thick make up the larger sequences and are only locally correlative subtidal dominated parasequences comprise the basal part of each sequence whereas shallower subtidal or peritidal dominated parasequences compose upper parts of sequences each large third order sequence is asymmetric and marked by lowstand transgressive and highstand systems tracts with unique lithologic and gamma ray log response characteristics ramp margin wedges rmw are poorly developed and consist of marine siltstone or grainstone packstone sheets extending into deep ramp settings these sheets have high gamma ray values in their base and lower blockier gamma ray responses in their tops transgressive systems tracts tst are thin composed of high energy locally phosphatic grainstone copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 7 1996 revised manuscript received may 27 1997 final acceptance june 16 1997 2department of geology virginia tech blacksburg virginia 24061 0420 present address department of earth atmospheric and planetary sciences massachusetts institute of technology cambridge massachusetts 02139 3department of geology virginia tech blacksburg virginia 24061 0420 this study was supported by nsf grant ear 9316057 to j f read a grant from mobil oil company and grants in aid of research from the geological society of america and appalachian basin industrial associates to mike pope we thank john haynes brian keith and rick major for thorough and insightful reviews of an earlier version of this manuscript

stochastic finite fault ground motion simulation in a wave field diffusive regime case study of the mt vesuvius volcanic area abstract the main aim of the present work consists in the validation of stochastic method for simulating weak ground motion in a diffusive regime due to low to moderate magnitude earthquakes and in particular in its application to a volcanic area we simulated the peak ground acceleration and the response acceleration spectra caused by two earthquakes scenarios m d 4 3 and m d 5 4 at mt vesuvius volcanic area by using the stochastic finite fault simulation method we validated the stochastic methodology by combining source path and site parameters of the investigated area considering the time duration parameter t rms calculated on the study seismograms the values of time durations are confirmed by calculating the same parameter t rms on the seismogram energy envelope described by multiple scattering models in terms of scattering and the intrinsic dissipation coefficient initially the simulations were evaluated for 10 local earthquakes 1 7 m d 3 6 that occurred at mt vesuvius in 1999 and are then compared with the observed data the comparison between simulated and observed seismograms has been used to calibrate the stochastic procedure and has been considered as the starting point for simulating ground motion for the scenario earthquake m d 3 6 that could occur in the study area the scenario earthquake and the relative fault features were chosen on the base of statistical tectonic structural and historical studies of the study area we simulated ground motions for a maximum magnitude value m max of 4 3 determined from examination of the gutenberg richter law for the study area and also for an m max 5 4 a magnitude that is associated with the earthquakes that struck the ancient town of pompei 17 yr before the eruption of mt vesuvius that occurred in a d 79 the largest values of a max for the m d 4 3 seismic event are in the range of 0 140 g 0 029 g in the case of m d 5 4 we obtain peak ground acceleration values in the range between 0 17 g and 0 55 g

the east siberian basin in the silurian evidence for no large scale sea level changes according to conventional ideas numerous eustatic sea level changes occurred during the phanerozoic with amplitudes 20 100 m and periods t 1 3 myr third order cycles at the same time it is widely recognized that tectonic factors have an important role in such water depth changes rapid large scale sea level changes have been suggested for the silurian for which detailed and continuous stratigraphic successions are available in the east siberian sedimentary basin v2u10 6 km 2 in size these show slow deposition at depths 95 10 m proceeded for 10 20 myr in many regions and enable the mathematical analysis of maximum eustatic sea level changes with periods of t 1 3 myr third order that could have occurred during the silurian the amplitudes of such events could not exceed v20 30 m and for some epochs could not be larger than 8 10 m at the same time rapid water depth changes of up to v100 m occurred in some other cratonic areas as the silurian had a relatively stable sea level these must be attributed to rapid crustal uplift and subsidence the absence of third order eustatic fluctuations with amplitudes v10 20 m has also been demonstrated for the cambrian and earliest ordovician it is probable that no significant sea level changes also occurred at many other epochs while rapid changes in water depth resulted from large scale vertical crustal movements fl 2001 published by elsevier science b v

reconciling late ordovician 440 ma glaciation with very high 14x co2 levels geochemical data and models suggest a positive correlation between carbon dioxide changes and climate during the last 540 m y the most dramatic exception to this correlation involves the late ordovician 440 ma glaciation which occurred at a time when co2 levels may have been much greater than present 14 16x since decreased solar luminosity at that time only partially offset increased radiative forcing from co2 some other factor needs to be considered to explain the glaciation prior work with energy balance models ebms suggested that the unique geographic configuration of gondwanaland at that time may have resulted in a small area of parameter space permitting permanent snow cover and higher co2 levels however the crude snow and sea ice parameterizations in the ebm left these conclusions open to further scrutiny herein we present results from four experiments with the genesis general circulation model with co2 levels 14x greater than present solar luminosity reduced 4 5 and an orbital configuration set for minimum summer insolation receipt we examined the effects of different combinations of ocean heat transport and topography on high latitude snow cover on gondwanaland for the no elevation simulations we failed to simulate permanent summer snow cover however for the slightly elevated topography cases 300 500 m permanent summer snow cover occurs where geological data indicate the ordovician ice sheet was present these results support the hypothesis based on ebm studies further results indicate that although average runoff per grid point increases substantially for the ordovician runs the decreased land area results in global runoff 10 30 less than present with largest runoff reductions for flat topography this response has implications for co2 runoff weathering parameterizations in geochemical models finally simulated tropical sea surface temperatures ssts are the same or only marginally warmer than present this result is consistent with evidence from other warm time intervals indicating small changes in tropical ssts during time of high co2

paleomagnetically defined rotations from the precordillera of northern chile evidence of localized in situ fault controlled rotations a paleomagnetic study of paleocene cretaceous and jurassic units in the precordillera of northern chile 26 s to 27 s demonstrate a complex pattern of crustal rotations the region was selected to investigate the pattern of deformation associated with the sierra castillo agua amarga fault and its associated structural subdomains which form part of the domeyko fault system the fault system that controls much of the structure of the chilean precordillera paleocene lavas from the center of the study area pass a fold test indicating the primary nature of the remanence in these rocks a second group of lavas from the south has a similar formation mean inclination after tilt correction for a uniform dip which coupled with the presence of reversals suggests that these lavas also carry a primary remanence the data indicate clockwise rotations of 35 and 42 respectively for the two sampling localities three geographically separate areas of lower cretaceous red beds also reveal primary remanences based on the presence of sites with reversed polarity at two of the localities and an inclination only fold test of the three locality mean directions for this formation these sampling areas yield remanence directions that reveal 30 of counterclockwise and 31 of clockwise rotation as well as an area without statistically significant rotation these data include the first well constrained counterclockwise rotation to be recovered from rocks in the southern central andes although mapping suggests the block involved is of limited geographic extent jurassic strata failed to yield any reliable results overall the data indicate a greater variability in the rotation pattern than has previously been observed in most comparable areas south of the arica deflection the variation cannot be explained by large scale rotation mechanisms such as oroclinal bending or domino style block rotation by widely separated transandean faults instead the rotations are consistent spatially and temporally and in their sense and magnitude with the eocene transpressional deformation associated with the domeyko fault system the deformational event that generated most of the important structures in the studied area the data emphasize the significance of local tectonics in controlling rotations in this part of the south central andes and the importance of understanding the local structure of regions sampled for paleomagnetic study

forecasting seismic scenarios on etna volcano italy through probabilistic intensity attenuation models a bayesian approach abstract in this paper we apply a probabilistic procedure to model the attenuation of the macroseismic intensity in the mt etna region which allows estimating probabilistic seismic scenarios starting from the local earthquake catalogue we select a dataset of 47 events having epicentral intensity i0 from vi to ix x ems and update the model parameters previously achieved for italy according to the bayesian paradigm for each class of epicentral intensity i0 we then estimate the probability distribution of the intensity at a site conditioned on the epicentre site distance through a binomial beta model under the assumption of a point seismic source and isotropic decay circular the mode of the distribution is taken as the expected intensity is at that site since the strongest earthquakes show a preferential propagation of shaking along the fault strike and a rapid decrease in the perpendicular direction we also consider the anisotropic decay elliptical of the intensity due to a linear source finite fault we therefore transform the plane so that the ellipse has the length of the fault rupture as maximum axis and its strike as azimuth is changed into a circle with fixed diameter then we apply the probabilistic model obtained for the isotropic case to the modified data the entire calculation procedure is implemented in the software proscen which given the location and the epicentral intensity and eventually the fault parameters of the earthquake to be simulated generates the probabilistic seismic scenario according to the isotropic and anisotropic models of attenuation the results can be plotted on grid maps representing 1 the intensity that can be exceeded with a fixed probability or 2 the probability of exceeding a fixed intensity value the first representation may also find application in seismic monitoring at etna volcano in order to produce real time intensity shakemaps based on the instrumental parameters calculated by the automatic earthquake processing system

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

assessment of hirnantian synglacial eustacy and palaeogeography in a tectonically active setting the welsh basin uk the hirnantian stratigraphy of wales is critically assessed against the global evidence for two major hirnantian synglacial eustatic lowstands the evidence for two separate lowstands of relative sea level is locally suggestive in shelf sequences but generally only permissive in the basin nowhere can the possible candidate lowstands be separated biostratigraphically and thus basinwide correlation that might rule out local tectonic control is impossible to prove consistent with palaeolatitude evidence and a situation far removed from the gondwana margin there is no facies evidence e g dropstones for local glacial conditions and the diamictites are of mass flow origin sequence stratigraphic reasoning proves helpful in the correlation of the basinal succession in central wales with the shelf sequences along the tywi lineament and the variscan front and suggests that the first appearance of normalograptus persculptus in wales does not correspond with the base of the persculptus biozone extensive new palaeocurrent data and sedimentological restudy is combined with review of earlier knowledge to argue for significant revisions to previous palaeogeographic maps for generalized lowstand conditions notably along the bala lineament the variscan front and the central wales inliers

upper proterozoic glaciogenic rocks and their stratigraphic significance abstract among more than 300 precambrian tillites mentioned in the literature only 25 can be identified as glacial rocks tillites are known from all continents with the exception of antarctica the uneven stratigraphic distribution of tillites cannot be explained as being due to differences in knowledge roughly synchronous glacial formations and groups form wide regional glaciohorizons they are concentrated in two stratigraphic intervals named glaciocomplexes one such the canadian glaciocomplex consisting of three or four glaciohorizons occurs in the lowermost lower proterozoic in north america and south africa the second glaciocomplex the african containing four or more glaciohorizons is known in the uppermost part of the precambrian in europe asia north and south america africa australia the glaciohorizons and glaciocomplexes reflect oscillation dynamics of glaciations and show a hierarchical structure of glacial events the glaciohorizons and glaciocomplexes were deposited during glacial periods and glacial eras on and marginal to the continents in high and middle paleolatitudes of both hemispheres by comparison with late cenozoic glacial events the duration of the ancient glacioperiods can be roughly estimated at between some millions and a few tens of millions of years within these limits each glaciohorizon can be asynchronous the glaciohorizons have great importance for regional correlations of the precambrian interregional and intercontinental glacial correlations however require radiometric and biostratigraphic controls glacial units can be used in construction of a global stratigrphic scale for the precambrian

the pangean ice age studies with a coupled climate ice sheet model application of an ice sheet model developed for the pleistocene to the extensive carboniferous glaciation on gondwana yields an ice sheet which has several features consistent with observations while complete deglaciation is not achieved without co2 changes the milankovich induced fluctuations in ice sheet volume are comparable to pleistocene glacial interglacial signals this result is shown to hold for a large fraction of physically reasonable parameter space the model also exhibits multiple equilibria and sharp bifurcations as infinitesimal changes in the solar constant or precipitation can lead to a qualitatively different climate the success of the model in predicting ice location in an environment quite different from the pleistocene provides additional support for the robustness of the basic model physics and suggests that the model can be applied with some confidence to other pre pleistocene glaciations

calibrating the late ordovician glaciation and mass extinction by the eccentricity cycles of earth s orbit a process based sedimentological analysis of upper ordovician glacial marine rocks in africa suggests that full glaciation of the continental shelf started in the late extraordinarius zone of the hirnantian two cycles of ice sheet growth are represented during full glaciation initial terrestrial ice sheet growth in the early extraordinarius zone influenced the first event of the late ordovician mass extinction retreat of the ice sheet from the shelf ended by the persculptus zone when the second event of the late ordovician mass extinction began eccentricity controlled ice sheet growth is assumed periodicity 0 1 m y hence two cycles of full glaciation lasted 0 2 m y the duration of the extraordinarius zone is estimated as 0 5 m y therefore the minimum duration of the first extinction event was 0 3 m y

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

late ordovician sand wave complexes on anticosti island quebec a marine tidal embayment laterally discontinuous mixed carbonate siliciclastic sandstones in the upper vaureal and lower ellis bay formations of anticosti island were deposited on an equatorial carbonate ramp with a slope of less than 1 the 10 18 m thick sandstones are interpreted as subaqueous sand wave complexes analogous to detached parts of modern shoreface connected sand ridges these record storm enhanced tidal modification of a northerly derived shoal retreat massif that may have formed in response to recovery from global sea level lowstands in the late ordovician ashgill late rawtheyan hirnantian the sand wave complexes formed within a tidal embayment that was confined by the precambrian shield to the north and northwest by rising tectonic highlands of the humber zone in newfoundland to the east and by active tectonic highlands in the quebec appalachians gaspesie to the south paleocurrent distributions parallel to the western margins of the strait of belle isle suggest that the north end of the embayment w

late ordovician glaciation under high atmospheric co2 a coupled model analysis the analysis of the geologic record has revealed a question concerning how the late ordovician glaciation could have occurred simultaneously with high co2 levels 10 18x sensitivity studies using a coupled atmosphere ocean sea ice model show that it is possible to maintain a permanent snow cover which corresponds to 60 of all the glacial deposits found on gondwana under 10x co2 levels warm fall cool spring orbital parameters a 4 5 reduction in solar luminosity a length of day of 21 5 hours and an enhanced snow sea ice albedo of 0 3 a cold summer orbit experiment with 10x co2 and a reduced snow sea ice albedo of 0 1 also sustains a permanent albeit less extensive snow cover the geographic configuration of the late ordovician results in an up to 42 increase in the global ocean poleward heat transport in the southern hemisphere relative to present day and a significant asymmetry relative to the equator

oxygen isotopes from conodont apatite of the midcontinent us implications for late ordovician climate evolution abstract the major glaciation at the end of the ordovician is associated with the 2nd largest mass extinction event of the phanerozoic growth of late ordovician ice sheets requires a dramatic cooling from the greenhouse conditions that prevailed for most of the ordovician but when and how fast this cooling occurred are controversial issues the controversy is due in large part to a lack of good geochemical constraints on the temperature history of the katian 453 445 2 ma to address this uncertainty we measured phosphate 18o values from 3 conodont species collected from sections in the midcontinent region of the united states that span an 5 7 m yr long interval covering most of the katian results reveal a statistically significant offset in 18o values between some taxa and show up to 2 differences among samples however there are no apparent long term trends within or between sections rather values fluctuate around a 18o mean of 19 vsmow our study provides the longest relatively high resolution species specific conodont record generated for this interval and we found no evidence supporting progressive cooling during the katian

spatial variation in late ordovician glacioeustatic sea level change abstract mass extinction of late ordovician marine fauna closely coincided with southern hemisphere glaciation the sequence stratigraphic architecture of shallow marine deposits informs estimates of glacioeustatic sea level change at sites both proximal and distal to the reconstructed ordovician ice sheet s and contemporaneous changes in ice volume a recent correlation framework for the stratigraphic architectures of one near and one far field late ordovician margin concluded that the late ordovician glaciation encompassed multiple long term cycles of ice volume growth and retreat with superimposed higher frequency cycles here we posit that similar to cenozoic glacial cycles glacial isostatic adjustment can preclude synchronous and similar magnitude or directional changes in late ordovician sea level between ice proximal and ice distal locations and hence distort a globally correlative sequence stratigraphy we explored whether long duration i e million year late ordovician glacial cycles should produce a globally coherent eustatic record of sea level change between ice proximal and ice distal margins using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model we adopted a late ordovician paleogeography and a synthetic continental ice sheet distribution and volume informed by the areal extent of glaciogenic deposits and geochemical records respectively we demonstrate that modeled million year late ordovician glacial cycles produce sea level histories on near and far field margins that differ from eustasy and from one another due primarily to elastic flexure and associated gravitational effects while predicted far field sea level histories faithfully preserve the temporal structure of modeled glacioeustasy their amplitude may differ from eustasy by as much as 30 40 the impact of glacial isostatic adjustment is largest at the margins of glaciated continents and these effects can be of the same order of magnitude as the eustatic and even induce a local sea level rise during an episode of ice growth and eustatic sea level fall and vice versa in this regard stratal surfaces of maximum regression and flooding expressed at near field margins need not reflect global eustatic trends in ice sheet growth and decay respectively and thus may not provide chronostratigraphic horizons for correlation with far field sequence stratigraphic architectures

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

silurian tectonic sedimentary setting and basin evolution in the sichuan area southwest china implications for palaeogeographic reconstructions abstract silurian black shales are among the most significant hydrocarbon source rocks globally and are widespread across the southern and eastern sichuan basin and adjacent areas their tremendous production potential has been demonstrated by the discovery of self sourced shale gas reservoirs in the jiaoshiba field in the fuling area of the sichuan basin in this study we reconstruct the tectonic and palaeogeographic framework of the silurian sichuan basin analyze the distribution of sedimentary facies and subdivide the sedimentary succession into a number of stages understanding the formation and evolution of the silurian basin is necessary to predict its potential for future hydrocarbon exploration and development and may also help to better understand global palaeoceanographic conditions during this interval the sichuan basin in the silurian period was located along the northwestern margin of the yangtze plate which was itself located on the northern margin of gondwana at a low near equatorial latitude the basin was initially composed of an intracratonic depression surrounded by three structural uplifts forming a semi enclosed shallow sea however the basin experienced intraplate tectonic deformation at an early stage followed by continued flexural subsidence and intracratonic deformation preserved silurian strata comprising the longmaxi shiniulan and hanjiadian formations reflect several depositional stages including 1 slow deposition of the basal black shale of the longmaxi formation in a deep water environment due to a transgressive lag following glacial melting in gondwana the resulting sea level rise and intraplate tectonic activity 2 rapid sedimentation during the later stage of longmaxi deposition 3 deposition of the shiniulan formation on a neritic carbonate platform in the southern of the basin 4 deposition of the siliciclastic hanjiadian formation in a deltaic environment

sedimentary corg p ratios paleocean ventilation and phanerozoic atmospheric po2 the corg p ratios of organic rich facies tocn1 exhibit considerable variation through the phanerozoic from b10 1 in the permian phosphoria formation to n1000 1 in some devonian black shales relative to the composition of phytoplankton c p 106 1 which provide the bulk of organic c and p to organic rich marine sediments the range of phanerozoic sedimentary corg p ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of c and p remobilization enhanced preservationoforganiccatlowerredoxpotentials combinedwithenhancedsedimentaryretentionofremineralizedorganicpathigher redox potentials results in a strong relationship between benthic redox conditions and sedimentary corg p ratios the strong secular coherence of the phanerozoic corg p record i e limited variation within narrow time slices suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale such as atmospheric po2 inversion and scaling of the phanerozoic corg p record yields a new atmospheric po2 model that has strong similarities to existing models based on elemental and isotopic mass balances yet differs in some potentially significant respects in particular the new model suggests that atmospheric po2 wasmarkedlylowerduringtheearlytomiddlepaleozoicthanpreviouslyinferred duringmostofthedevonian atmospherico2levels mayhavebeensufficientlylow b13 astopreventthesustainedcombustionofplantmaterial thisfeatureofthemodelmayaccount for ananomalous gapinthefossilcharcoalrecordduringthedevonian theredox dependentburialofpintheoceanresults inpositive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales temporal variations in the burial of p relative to c suggest that the strength of these feedback mechanisms varied over the phanerozoic 2007 elsevier b v all rights reserved

toward reconciliation of late ordovician 440 ma glaciation with very high co2 levels although phanerozoic glaciations usually coincided with times of estimated low atmospheric co2 the late ordovician 440 ma glaciation is a significant exception co2 levels during that time may have been as much as 10 times greater than present in an earlier paper we suggested that the unique geographic configuration of gondwanaland may explain such a response as the edge of the supercontinent was essentially tangent to the south pole and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass thereby allowing glacial inception one limitation to the earlier study was that it used a linear energy balance model ebm in this paper we further test the above hypothesis in a suite of experiments with a nonlinear ebm that allows for snow albedo feedback we also utilize updated estimates for co2 levels decreases in solar luminosity and variations in orbital forcing baseline experiments with no changes in luminosity or co2 resulted in an ice covered area of 6 3 106 km2 53 of the estimated area covered by the ordovician ice sheet additional experiments for different combinations of orbital forcing 7x 13x co2 and 3 5 5 0 luminosity yielded 0 35 of the estimated ice area in the late ordovician a crude estimate of possible topographic influences increased these numbers to7 47 of total estimated ice area additional factors related to ice sheet growth should increase these values somewhat these results provide additional support for the high co2 glaciation explanation with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range the estimated duration of ordovician glaciation is also consistent with the migration of gondwanaland across the south pole with a centrally located pole yielding ice free conditions in the summer thus identical levels of external forcing yield either glaciated or ice free conditions with the solution dependent on location of the landmass although more work is required on this topic our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox the results lend further support to the proposition that paleogeography significantly modifies the role of co2 in the long term evolution of climate

paleogene clockwise tectonic rotations in the forearc of central andes antofagasta region northern chile 1 for the central valley of northern chile antofagasta region a paleomagnetic analysis of data from 108 sites mainly in mesozoic and paleogene volcanic rocks has yielded stable remanent magnetization directions for 86 sites from these data we infer clockwise tectonic rotations of up to 65 within the forearc domain of the central andes the apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the incaic orogenic event of eocene early oligocene age a few paleomagnetic results obtained in neogene rocks do not show evidence of clockwise rotations hence the development of the bolivian orocline during late neogene time cannot be explained by simple bending of the whole margin these results demonstrate that tectonic rotations within the forearc and pre cordillera are key elements of early andean deformation which should be taken into account by kinematic models of mountain building in the central andes

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

large scale slumping deduced from structural and sedimentary features in the lower palaeozoic anglo brabant fold belt belgium distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult especially when the scale of the slump folds exceeds outcrop scale in the southeastern part of the single phase deformed lower palaeozoic anglo brabant fold belt a comparison of cleavage fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle caradoc fine grained turbidites in the core of a large synform was overturned prior to tectonic deformation this overturning is attributed to large scale slumping which was most likely a result of middle caradoc seismic activity the exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias making up less than 5 of the exposed thickness if the beds were not overturned large scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall stable sedimentary pile this may explain why so few ancient large scale slides and slumps have been reported the small amount of internal deformation makes them very difficult to recognize especially when dealing with poorly exposed areas as such large ancient slides and slumps may be more common than suggested by the geological literature

the silurian glaciation in the eastern parnaiba basin brazil paleoenvironment sequence stratigraphy and insights for the evolution and paleogeography of west gondwana abstract glacial strata are excellent stratigraphic markers in paleozoic basins worldwide the eastern parnaiba basin in northern brazil is a highly favorable setting for studying episodes related to the ordovician silurian glaciation previous studies in this region led to the recognition of unequivocally glaciogenic sediments associated with pre and postglacial deposits favoring the understanding of lower paleozoic events that occurred in west gondwana outcrop based facies and stratigraphic analyses of the 170 m thick siliciclastic succession of the serra grande group indicate fluvial glacial and coastal depositional systems seventeen sedimentary facies are grouped into seven facies associations fas fa1 interpreted as intermediary sheet braided plain deposits that consist of tabular cross bedded coarse grained sandstones and conglomerates fa2 fa4 which correspond to subaqueous glacial deposits comprising massive to stratified diamictites cross laminated sandstones diamictites and organic matter rich black shales with dumpstone and dropstone structures fa5 fa6 constituting organic matter rich black shales fine to medium grained sandstones with sigmoidal cross bedding and occurrences of the ichnotaxon arthrophycus interpreted as delta front deposits and fa7 composed of medium to coarse grained pebbly sandstones interpreted as channelized braided plain deposits this interpretation partially confirms the previous paleoenvironmental interpretation discounting the presence of subaerial alluvial fan and outwash plain deposits for this unit the sequence stratigraphy of the serra grande group is refined mainly using the coherent interpretation of the key surfaces and comparing the systems tracts with the global sea level curve providing a more robust third order sequential evolutionary model that includes three depositional sequences the middle ordovician sheet braided deposits are the lowstand sediments of sequence 1 truncated by an extensive unconformity that possibly removed the transgressive and highstand strata this unconformity generated by glacial dynamics represents 23 myr during the late ordovician to early silurian the second sequence started with the advance of ice sheets in marginal areas of the basin causing the deposition of ablation diamicton during the lowstand the postglacial transgression is marked by ice rafted debris from icebergs deposited onto organic matter rich mud in the shoreface offshore zone the maximum flooding during the middle to late silurian was succeeded by the highstand phase marked by the progradation of deltaic deposits biological activity is marked by the presence of polychaetes that reworked the marine seafloor the early devonian featured extensive sea level fall and renewed continental drainage with the formation of a braided plain marking a lowstand phase of sequence 3 these new interpretations allow the global correlation of the serra grande group and provide an improved understanding of the role of ice sheets and postglacial transgressions that affected west gondwana during the ordovician silurian periods

the paradox of the global standard late ordovician early silurian sea level curve evidence from conodont community analysis from both canadian arctic and appalachian margins abstract relative sea level fluctuations in the late ordovician early silurian were largely driven by gondwana deglaciation three possible short lived ice readvances and the closure of the iapetus ocean this study establishes and compares sea level events on the canadian arctic and appalachian margins of laurentia based on the pattern of conodont communities and demonstrates that sea level events were not synchronous on the two margins conodont data were compared for a the arctic margin from the cape phillips formation richmondian upper ordovician through lower sheinwoodian lower silurian cornwallis island canadian arctic islands and b the appalachian margin from the upper ellis bay becscie merrimack gun river jupiter and chicotte formations richmondian upper ordovician rhuddanian aeronian and most of telychian lower silurian anticosti island quebec the pattern of late ordovician early silurian conodont communities is established based on three cluster analyses on conodonts from cornwallis island 4967 specimens representing 54 species from 77 samples cape phillips formation and anticosti island 1980 specimens representing 21 species from 25 samples upper ellis bay formation 24 839 specimens representing 42 species from 123 samples becscie merrimack gun river jupiter and chicotte formations overall the sea level curves inferred from the distribution of the conodont communities from cornwallis and anticosti exhibit gradual and rapid oscillating patterns respectively specifically during the time interval of the latest ordovician and the early silurian rhuddanian the sea level behaved differently in these two regions 1 during the latest ordovician conodont community changes show the different patterns in the two regions which reflects a transgression on cornwallis but a regression on anticosti 2 during the earliest rhuddanian similar conodont communities arose in both regions but they indicate a regression on cornwallis and a transgression on anticosti 3 in the early rhuddanian these communities were replaced by a deeper water community on cornwallis and a shallower water community on anticosti which suggest a transgression on cornwallis and a regression on anticosti 4 a highstand drove out almost all conodonts from anticosti by the end of rhuddanian which is not seen on cornwallis 5 a deep water conodont community remained almost unchanged on cornwallis from the late rhuddanian to the early aeronian whereas a shallow water conodont community returned to anticosti in the early aeronian however the sea level dropped on both cornwallis and anticosti during the late telychian and the sea level curves show a similar pattern the difference in the sea level pattern during the late richmondian rhuddanian and aeronian on both the northern and southern margins of laurentia reflected by conodont communities is supported by the studies on quaternary glaciation deglaciation events where the melting of an ice sheet was accompanied by sea level change but with variable regional isostatic effects that do not generate a uniform global eustatic signal

controls on reservoir quality in exhumed basins an example from the ordovician sandstone illizi basin algeria abstract petroleum exploration in many north african intracratonic basins targets early paleozoic sandstones as the primary reservoir objective these sandstones are often characterized by highly variable reservoir quality 0 0001 1000 md and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential the objective of this study is to characterize the primary controls on reservoir quality in an ordovician field in the illizi basin of algeria through detailed core and petrographic analysis and establish if variations in thermal history across the field have a material impact on reservoir quality the best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1 fibrous illite these lithologies are most commonly found within the high energy tidally reworked post glacial facies sandstones of the uppermost ordovician succession observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial this interpretation is supported by field wide numerical modelling of sandstone diagenesis this study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability thermal history therefore is an important consideration in reservoir quality studies in exhumed basins where variations in present day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play

the late ordovician early silurian glacial period abstract late ordovician to early silurian tillites and evidence of extensive glacial erosion were first discovered in the western sahara in the late 1950 s although the concept of a glacial period at this time had been around since the late 19th century further evidence of glaciation is now available from west africa south africa the arabian peninsula eastern north america west and central southe america and europe several glacial episodes can be distinguished the earliest evidence local mountain glaciation appears to be of caradoc age the major episode may have begun in caradoc time and lasted much of ashgill time when a major ice sheet covered at least north central and west africa which was then centered near the south pole however independent palaeontological and sea level evidence from areas far removed from the centre of glaciation suggests a time span for this episode of less than 2 ma this conflict has yet to be resolved the latest evidence of glaciation is wenlock in age but is confined to south america which by then was replacing africa over the south polar region however this latest episode may simply have been a rather intensive mountain glaciation rather than being of ice sheet dimensions the lack of land in the north polar region probably explains the absence of tillites from the then northern hemisphere land masses the total area affected by glaciation in early palaeozoic time in north africa alone is at least 6 106 km2 combined with other glaciated areas glaciation may have been on the scale of that of present day antarctica

gcm greenhouse and high obliquity solutions for early proterozoic glaciation and middle proterozoic warmth 1 the first definitive evidence of low latitude glaciation occurs in the early proterozoic approximately 2 3 billion years ago this period of glaciation occurs near the time for a significant rise in oxygen levels a reduction in the methane producing population through the oxidation of earth s atmosphere has been proposed as the cause for generating cold climatic conditions another alternative is that cold climatic conditions could have been generated by the migration of a landmass into low latitudes if earth s obliquity was considerably higher than present these two hypotheses are tested with global climate model simulations using present day and high obliquity 70 boundary conditions further the solar constant is reduced by 17 and 10 co2 is used an idealized super continent is used and methane mixing ratios vary from 0 1 ppmv to 25 ppmv in the simulations for the present day obliquity low latitude glaciation occurs when ch4 concentrations are reduced from 10 to 0 1 ppmv however the same result is produced when the oceanic heat transport is turned off in high obliquity simulations low latitude glaciation occurs when the idealized super continent is moved from middle into low latitudes finally high obliquity may explain the warm middle proterozoic as warm simulated climatic conditions occur even for a low latitude super continent using a solar constant representative of the middle proterozoic and reduced atmospheric methane mixing ratios

palaeomagnetism and age of mid palaeozoic ring complexes in niger west africa and tectonic implications summary detailed af and thermal demagnetization has been performed on 185 oriented samples from 36 sites in a series of basic to acid subvolcanic ring complexes in central and southern niger published rb sr whole rock and mineral isochrons from acid members of the suite range north to south from 480 ma to 320 ma two distinct vectors have been isolated by linear regression of demagnetization data 1 a due north and negative inclination rarely reversed component is present in basic rocks from 12 sites in four distinct northern complexes published rb sr ages date from 480 ma to 420 ma this component is carried by single domain magnetite occasionally also by titanhaematite and is considered to be primary acquired during the terminal stages of this igneous episode 40ar 39ar dating of biotite from three samples used in the palaeomagnetic study yield 435 myr ages 2 a due north and positive inclination rarely reversed direction is present as a second component in some of the basic suite sites and is the predominant component in most sites in syenitic to granodioritic rocks primary fe ti oxides are rare in these acid igneous rocks and the feldspars within them are usually pervasively altered this component carried primarily by haematite is believed to be either a modern day vrm or a secondary tcrm acquired during hydro thermal activity associated with widespread quaternary volcanism in the southern air the primary component gives a mean south pole at 43 4s 08 6e n 12 k 50 95 6 2 located southwest of cape town this position requires a rapid southward excursion of the gondwana a p w path in early silurian time but it is generally consistent with most of the other silurian devonian palaeomagnetic data from gondwana these however are from the lachlan fold belt in southeast australia whose relation to the australian craton in early palaeozoic time is not clear integrating all stratigraphic palaeoclimatic tectonic and other palaeomagnetic data for early and middle palaeozoic time suggests that the proto atlantic closed during the silurian period if a late devonian early carboniferous gondwana pole in central africa is valid the ocean may have opened again before the final closure and reassembly of gondwana in the late carboniferous

the origins of glacially related soft sediment deformation structures in upper ordovician glaciogenic rocks implication for ice sheet dynamics in the upper ordovician glaciogenic rocks of west gondwana many soft sediment deformation structures are attributed to glaciotectonic processes however little is known about the subglacial and ice marginal processes associated with this ice sheet a systematic description and interpretation of these soft sediment deformation structures is supplemented by a comparison with late quaternary and modern glacial settings to provide qualitative constraints on the character of ancient subglacial thermal regimes soft sediment striations were formed by discontinuous shear induced deformation beneath an ice sheet that oscillated between warm and cold based thermal regimes beneath warm based ice coupling across the ice bed interface was poor with sliding and loading occurring at this surface loading of soft waterlogged sediments allowed the development of fluted surfaces which are also associated with the formation of truncated folded zones during times of cold thermal regimes coupling across the ice bed interface was enhanced by freezing which caused changes in the position of the freezing front these discontinuous changes coupled with sustained subglacial shearing resulted in the development of stacked soft sediment striated surfaces in areas where the subglacial substrate had an irregular topography complex shear zones formed that comprised stacked duplex systems interlayered with highly strained units preserving pervasive folded lineations at the ice front large glaciotectonic deformation structures comparable with push moraines developed during sustained periods of re advance that created composite thrust and fold systems deformation was transmitted for up to 1 km from the ice front and reduced in intensity with distance other deformation structures include dome like folds that consist of central anticlines 30 to 70 m in width with tight rim synclines that formed by the load induced diapiric intrusion of muds an ice sheet surging mechanism is inferred for the development of composite thrust and fold systems and the coeval sediment diapirs these surges potentially caused the widespread reorganisation of a glacial load resulting in the pro to subglacial extrusion of soft sediments

peripheral bulge of a foreland basin in the oslo region during the early silurian abstract on the basis of detailed bathymetric analyses the lower and middle llandovery succession of the oslo region shows signs of a transitory uplift which advanced from the present north northwest across a gently east sloping epicontinental shelf in late llandovery time a graptolitic shale basin developed in the north northwestern part of the region where earlier land marginal conditions had existed this basin was previously interpreted as a trough in front of advancing caledonian nappes the two concepts are here linked together and a model featuring a low peripheral bulge followed by a foreland basin is outlined some information on the shape height and speed of the peripheral bulge is deduced the present interpretation provides important clues for dating movements of the caledonian thrust front in its later phases

attenuation and source parameters of shallow microearthquakes at mt etna volcano italy we estimated the attenuation laws of high frequency seismic waves in the shallow crust depth 5 km and earthquake source parameters by using a se lected data set of 320 shallow events 2 6 md 4 2 recorded at mt etna volcano during the last two flank eruptions occurring in 2001 and 2002 2003 the quality factor q was estimated from spectra of p and s waves for 24 stations of the local permanent network by applying a spectral ratio technique the results show varia tions in both qp and qs as a function of frequency according to the power law q q0f n with n ranging between 0 3 and 1 3 for p waves and between 0 2 and 0 9 for s waves as typical of volcanic environments strong azimuthal variations of qp were also found suggesting the presence of local strong lateral heterogeneities and or of fluid filled cracked volumes after correction for attenuation we estimated the source parameters seismic moment source radius and stress drop of a subset of 66 shallow events under the assumption of a circular dislocation the estimated seismic moments m0 range from 10 13 to 10 15 n m the source radii r are confined between 100 and 1000 m and stress drop dr ranges between 0 2 mpa and about 4 mpa combining the source parameters obtained in this study with those calculated by pataneet al 1997 for an old data set of smaller microearthquakes 10 9 m0 10 14 n m recorded in the same area we re evaluated the scaling relationship between seismic moment m 0 and corner frequency f c for the earthquakes with m 0 ranging between 10 13 and 10 15 n m we confirm that microearthquakes at mt etna seem not to obey a

elevated marine productivity triggered nitrogen limitation on the yangtze platform south china during the ordovician silurian transition abstract the ordovician silurian transition ost coincided with global environmental and biological changes including perturbations of the carbon and sulfur cycles oceanic redox changes and a mass extinction event but their effects on the contemporaneous marine nitrogen cycle remain poorly known in this study we measured bulk nitrogen isotopes in the datianba and shuanghe ost sections of south china which accumulated in the semi restricted inner yangtze sea the uniformly low 15nbulk mostly 1 to 1 of wufeng and longmaxi formation black shales representing the pre and post hirnantian intervals imply large losses of bioavailable nitrogen i e nitrate and ammonium via denitrification anammox as well as strong nitrogen fixation by diazotrophs persistence of low 15nbulk values within the guanyinqiao bed representing the hirnantian glacial interval indicates that denitrification anammox remained strong even during this major glacio eustatic fall and deep ocean ventilation event we hypothesize that elevated primary productivity caused by upwelling was a key factor producing a shallow redoxcline in the inner yangtze sea by comparing our nitrogen data with records from other cratons we suggest that oceanic anoxia was prevalent globally during the late katian to early rhuddanian and that ocean redox conditions were spatially heterogeneous during the hirnantian glaciation these considerations imply that bioavailable nitrogen was the limiting factor on marine primary productivity during the ost and that oceanic anoxia may have contributed to the late ordovician mass extinction especially on the yangtze platform

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

evaluation of the salinic i tectonic cancaniri glacial and ireviken biotic events biochemostratigraphy of the lower siluriansuccession in the niagara gorge area canada and u s a abstract well preserved brachiopods from the niagara gorge area anticosti island britain gotland and estonia were utilised to delineate a complex isotopic evolution for llandovery wenlock seawater the sr isotope record reflects the salinic i tectophase of the late llandovery in the continuous increase in 87sr 86sr values from 0 708070 to 0 708346 the salinic ii tectophase is marked by relative constancy of sr isotope values until the late wenlock when it rises from 0 708345 to 0 708430 the second tectonic phase was therefore likely only of a regional nature the carbon isotopes during the llandovery fall within a band of about 1 to 3 a range comparable to modern low latitude brachiopods a large positive 13c excursion of about 3 identifies the ireviken event excursion characterizes the early wenlock the biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the cancaniri glaciation although a direct causative scenario is as yet unknown the oxygen isotopic trends of well preserved brachiopods clearly reflect a warm climate interval during the latest llandovery associated with the silurian sea level highstand subsequently in the early wenlock the sea level fell with the onset of the cancaniri glaciation in the southern hemisphere this is reflected in a significant positive 18o excursion particularly in brachiopods from the niagara gorge area brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative 18o values indicating a lesser cooling gradient

the attenuation of seismic intensity in italy part i theoretical and empirical backgrounds abstract we critically analyze the results on seismic intensity attenuation in italy derived by albarello and d amico 2004 and gasperini 2001 we demonstrate that due to the inadequacy of certain underlying assumptions the empirical relationships determined in those studies did not best reproduce the decay of intensity as the distance from the source increases we reconsidered some of the relevant concepts and assumptions used in these intensity attenuation studies macroseismic epicenter epicentral intensity data completeness to suggest some useful recipes for obtaining unbiased estimates in particular we suggest that 1 data for distances from the source at which an intensity below the limit of diffuse perceptibility iv is expected should be excluded from attenuation computations because such data are clearly incomplete 2 attenuation equations that include a term proportional to the epicentral intensity i 0 with a coefficient different from 1 0 must not be used because they imply a variable offset between i 0 and the intensity expected at the epicenter and 3 epicentral intensities must be recomputed consistently with the attenuation equation because those reported by the italian catalog do not generally correspond with the intensity predicted at the epicenter by the attenuation relationships so far proposed following these suggestions produces a significant reduction in the standard deviation of the model that might lead to a corresponding reduction of the estimates of seismic hazard

the iapetus model a plate tectonic concept for the variscan belt of europe abstract a plate tectonic model is presented explaining the evolution of the variscan belt of europe it regards the iapetus ocean and its branch the tornquist sea as the only ocean involved in the caledonian variscan and alleghenian mega orogen the iapetus and its marginal basins disappeared through oblique subduction and collision in the following sequence 1 1 northeastern iapetus including the tornquist sea caledonian orogeny 2 2 central iapetus acadian orogeny 3 3 adjacent marginal basins variscan orogeny and 4 4 southwestern iapetus alleghenian orogeny the area of the variscides was part of the continental margin of gondwana made up of ridges and ensialic basins no ocean lay within the variscan area during the palaeozoic although patches of oceanic crust probably existed in rift basins e g the lizard ophiolite after the silurian early devonian closure of northwestern and central iapetus the variscan area was surrounded by cratons on three sides in a manner similar to that around the modern mediterranean sea the subsequent variscan orogeny was essentially an ensialic thrusting and folding induced by a further approach of gondwana toward laurentia after the variscan collision gondwana rotated clockwise to the west by moving along a strike slip zone this rotation during the carboniferous opened the palaeo tethys in the east and caused the alleghenian collision in the west the middle ordovician to permian palaeogeography is presented on a series of maps modelling tectonic displacement and plate motions avalonia is interpreted as the ordovician volcanic island arc of gondwana separated from mainland gondwana by an oceanic gulf or a back arc basin the iberian arc may have originated by a continental escape of iberia from the center of collision into the oceanic gulf the variscan austro alpine and southern alpine units are today located near their earliest early carboniferous position

subglacial to proglacial depositional environments in an ordovician glacial tunnel valley alnif morocco abstract this paper presents the sedimentary analysis of an exceptional ordovician glacial tunnel valley in the eastern part of the anti atlas the valley infill comprises two major glacial erosion surfaces striated pavements each overlain by a fining upward glacial unit these units are composed of five distinct facies associations recording the evolution from subglacial to proglacial environments and an additional sixth facies association overtopping the tunnel valley infill and associated with post glacial environments the tunnel valley infill also records a transitional environment between the subglacial and proglacial settings which is compared with the antarctic ice sheet margin these three environments are defined by the position of the grounding line and the coupling line the new proposed depositional model also differs from usual ordovician depositional models in which the main tunnel valley infill is interpreted as essentially proglacial outwash deposits in a range of glaciomarine to glaciofluvial environments overall a substantial part of the valley infill 50 of volume was deposited in a subglacial setting the sedimentary bodies could form potentially thick and laterally extended although these were limited by the shape and extent of the subglacial accommodation space finally the sedimentary record when compared with regional analogues also provides information for the palaeogeographic reconstruction of the ordovician ice sheet in this region

detecting imminent eruptive activity at mt etna italy in 2007 2008 through pattern classification of volcanic tremor data abstract volcano monitoring aims at the recognition of changes in instrumentally observable parameters before hazardous activity in order to alert governmental authorities among these parameters seismic data in general and volcanic tremor in particular play a key role recent major explosive eruptions such as okmok aleutians and chaiten chile in 2008 and numerous smaller events at mt etna italy have shown that the period of premonitory seismic activity can be short only a few hours which entails the necessity of effective automatic data processing near on line here we present a synoptic pattern classification analysis based on self organizing maps and fuzzy cluster analysis which is applied to volcanic tremor data recorded during a series of paroxysmal eruptive episodes and a flank eruption at etna in 2007 2008 in total eight episodes were analyzed in six of these significant changes in the dynamic regime of the volcano were detected up to 9 h prior to the onset of eruptive activity and long before changes in volcanic tremor amplitude and spectral content became evident in classical analysis in two cases the state transition was

the neuquen composite section magnetostratigraphy and biostratigraphy of the marine lower jurassic from the neuquen basin argentina abstract paleomagnetic and biostratigraphic data from five marine sedimentary sections in the argentine neuquen basin have allowed to construct the lower jurassic neuquen composite section this composite section comprises 14 reversed and 11 normal polarity zones in relation to 17 andean ammonite assemblage zones spanning the hettangian toarcian early jurassic it represents the first paleomagnetic data of lower jurassic marine successions in the southern hemisphere the neuquen composite section was correlated to the international mesozoic polarity time scale which for the lower jurassic comprises 54 polarity zones and 16 standard ammonite zones the correlation between the regional and the international biomagnetostratigraphic scales supports but also refines the correlation between the andean and standard ammonite zonations correlation between the neuquen composite section and the polarity sequences recorded in each section helped to assign several unfossiliferous stratigraphical levels to the corresponding biozones

two from donegal neoproterozoic glacial episodes on the northeast margin of laurentia two glacial episodes the sturtian ca 720 ma and marinoan varangerian ca 590 ma characterize many neoproterozoic successions worldwide in contrast northeast laurentia is typically considered as containing only the latter in scotland and ireland this is represented by the port askaig tillite here we present evidence ice rafted debris for a second younger neoproterozoic glacial event in the dalradian southern highland group of donegal ireland these deposits overlie strata correlative to ca 595 ma rocks in scotland making this heretofore unrecognized glacial episode a marinoan varangerian equivalent in that the port askaig occurs 3 5 km stratigraphically below the southern highland group rocks it is most likely sturtian not marinoan varangerian in age thus the dalradian succession in the irish caledonides contains strata representing both the sturtian and marinoan varanger glaciations

reply to a comment on early jurassic palaeomagnetic study of lower jurassic marine strata from the neuquen basin argentina a new jurassic apparent polar wander path for south america fil iglesia llanos maria paula consejo nacional de investigaciones cientificas y tecnicas oficina de coordinacion administrativa ciudad universitaria instituto de geociencias basicas aplicadas y ambientales de buenos aires universidad de buenos aires facultad de ciencias exactas y naturales instituto de geociencias basicas aplicadas y ambientales de buenos aires argentina

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

sequence architecture of the bighorn dolomite wyoming usa transition to the late ordovician icehouse abstract although major late ordovician glaciation was confined to the terminal ordovician hirnantian stage lasting less than 2 million years recent isotopic evidence suggests a prolonged interval of slow growth in polar ice caps tropical carbonate platforms from passive margins and epicontinental seas offer the opportunity to monitor changes in eustatic sea level changes that accompanied this transition from a greenhouse towards an icehouse climate the late ordovician bighorn dolomite of northern wyoming and adjacent states preserves such a transition the bighorn dolomite was deposited on a northeastward facing subequatorial platform and it consists of three third order depositional sequences each is bounded by a regional erosional surface commonly with erosional truncation visible at outcrop scale and commonly mantled by a distinctive brecciated horizon the bighorn dolomite is dominated by highly bioturbated and fossiliferous open marine shallow subtidal facies but portions also contain dolomitic bioturbated restricted marine shallow subtidal and laminated peritidal facies as well as thin bedded argillaceous and fossiliferous deep subtidal limestone the three depositional sequences preserve a transition in their internal cyclicity that is consistent with computer modeling of the stratigraphic signal of increasing period and amplitude of eustatic sea level changes the lowermost sequence which spans the lander and steamboat point members consists of homogeneous acyclic shallow subtidal facies with a few meter scale peritidal parasequences near its top the middle sequence which corresponds to the leigh member contains acyclic to weakly cyclic restricted shallow subtidal and peritidal facies the uppermost sequence consisting of the horseshoe mountain member is dominated by five decameter scale parasequences composed of open marine shallow subtidal restricted shallow subtidal and peritidal facies overall the bighorn dolomite is interpreted to represent deposition during a shift from low amplitude a few meters short period 20 kyr cyclicity to moderate amplitude 10 m long period 100 kyr cyclicity indicating a shift from greenhouse conditions towards icehouse conditions a shift that agrees with recent isotopic evidence for changing late ordovician climate

chemostratigraphy of an ordovician silurian carbonate platform δ13c records below glacioeustatic exposure surfaces the use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios 13 c to be preserved in carbonate dominated strata such changes are commonly interpreted to reflect primary secular variation in ocean chemistry however negative 13 c anomalies developed in pliocene pleistocene carbonate platforms following glacioeustatic sea level fall due to remineralization of terrestrial biomass during meteoric diagenesis these anomalies are similar in structure and magnitude to some neoproterozoic 13 c records opening the possibility that the neoproterozoic 13 c anomalies have a meteoric origin derived from a large terrestrial biosphere here we test the hypothesis that a large terrestrial biosphere existed prior to the silurian devonian land plant radiation by examining 13 c records of subaerial exposure surfaces formed in a shallow water carbonate platform during the ordovician silurian icehouse the exposure surfaces include an unconformity at the ordovician silurian boundary with terra rossa and dissolution collapse breccia and a lower silurian quartz sand layer feeding a 50 m deep system of karst pipes there is no evidence for 13 c depletion beneath either exposure surface strontium concentrations in the rocks are low 10 120 ppm and covary with 18 o oxygen isotope ratios however do not positively correlate with 13 c our results suggest that there was no significant terrestrial biosphere during ordovician silurian time and by extension that neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis

comparison of albedo changes due to shelf exposure and ice during the late ordovician glacioeustatic regression in a glacioeustatic regression the amount of exposure of the continental shelves and the size of the ice cap are intimately related we use a numerical model to compare the effects on planetary albedo of the formation of ice caps and the exposure of continental shelves due to an eustatic regression such as was the known case in the late ordovician to compare the two effects in a meaningful way we link them by relating the size of the ice sheet to the change in volume of the ocean and thus to the exposure of the shelves for the conditions of the late ordovician caradocian and ashgillian the change in albedo from the exposure of continental shelves and epeiric seafloor is many times greater than that due to the formation of ice up to s 0 0233 versus s 0 0009 25 versus 1 of the global mean surface albedo p 0 0079 versus p 0 0001 2 6 versus less than 0 1 the change in planetary albedo is enough to lower the earth s effective temperature by up to three quarters of a degree kelvin we extend the hypothesis that the ashgillian ice age was intensified and possibly even triggered by the exposure of extensive areas of epeiric floor during the caradocian the effect is more pronounced for the ordovician than more recent times due to the differences in continental positions today s lack of epeiric seas differences in atmospheric composition and the absence of land plants

molybdenum contents of sulfides in ancient glacial diamictites implications for molybdenum delivery to the oceans prior to the great oxidation event abstract in order to determine whether sulfides are major reservoirs for mo in the upper continental crust ucc we determined the composition and mode of occurrence of sulfides and evaluated their contribution to the molybdenum budget in twelve glacial diamictites with ages ranging from 2900 to 300 ma the diamictites provide a snapshot of ucc mineralogy and composition at the time of their deposition and show systematic depletion in bulk rock mo concentrations after the great oxidation event goe reflecting the effects of oxidative weathering in their provenance gaschnig et al 2014 li et al 2016 sulfides are generally confined to archean and paleoproterozoic diamictites although they also have been found in one phanerozoic sample with an ancient provenance we classify the sulfides based on their compositions and morphologies detrital sulfides are generally rounded may be a single mineral or an assemblage of minerals and show a very wide range in mineralogy including a single molybdenite grain sedimentary sulfides are pyrites generally with framboidal like textures pyrites also include non framboidal textured authigenic pyrites epigenetic sulfides consist of irregular pyrrhotite aggregates sometimes pyrrhotite intergrown with chalcopyrite and cobaltite late stage euhedral pyrites and pyrite aggregates in veins high mo concentrations up to 230 ppm are found in some sedimentary framboidal like pyrites from the mesoarchean coronation and paleoproterozoic makganyene formations epigenetic pyrrhotite aggregates and chalcopyrite in the ramsay lake diamictite and in detrital sulfides in timeball hill diamictites that may have originated from hydrothermal fluids in the sedimentary basins other detrital sulfides have widely variable mo concentrations 0 5 36 ppm mass balance calculations show that sulfides can account for only

the role of long period ground motions on magnitude and damage of volcanic earthquakes on mt etna italy on october 2002 a seismic swarm occurred on mt etna one of the strongest events caused severe damage up to a european macroseismic scale inten sity of viii that contrasts with its local magnitude of 4 4 the occurrence of significant damage at such a small magnitude is repeatedly observed in the area and is tradition ally attributed to shallow source recorded strong motion accelerograms and broad band seismograms demonstrate that there is one more cause for the severe damage that is an anomalously strong low frequency 0 1 f 1 hz radiation deviating from the conventional brune 1970 spectral scaling therefore these earthquakes cause large ground displacements and long 20 sec durations of shaking the integration of digital accelerograms yields a maximum peak ground displacement as large as 1 8 cm at a distance of 18 km based on the sharp local attenuation of ground motion in the study area we infer that peak ground displacements near the epicenters did exceed 10 cm the occurrence of large displacements caused selective damage to medium rise 3 stories reinforced concrete buildings and elements like church facades the frequency cutoff below 1 25 hz in the wood anderson response attenuates the peak to peak amplitudes used to assess local magnitudes therefore ml values are not representative of the real strength of volcanic earthquakes because a prompt magnitude and damage potential assessment is crucial for civil protection actions a procedure is proposed which in near real time can be successful in identifying po tentially damaging earthquakes of mt etna through the computation of pseudove locity response spectra the procedure provides a magnitude value that is derived on a statistical basis from the housner 1952 spectral intensity computed in the low frequency band this parameter is a suitable near real time indicator of large earthquake induced building shaking and could also be applied for a preliminary es timate of the epicentral macroseismic intensity

large scale channel fill structures in late ordovician glacial deposits in mauritania western sahara abstract late ordovician siliciclastic glacial and related deposits on the north gondwana continental shelf comprise striking incised features referred to as large scale channel fill structures these are described from exposures and aerial photographs in two areas adrar and hodh in mauritania the channel fill structures are up to several kilometres in length and several hundred metres wide they are slightly sinuous narrow sandstone bodies deeply incised into the late ordovician glacial drift and the cambro ordovician bedrock palaeogeographical reconstructions indicate that these structures were located in the ice marginal zone they are preferentially oriented parallel to palaeo ice flow directions sedimentary facies analysis reveals three vertically stacked architectural units infilling a u shaped erosional basal unconformity unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris flows deltaic like progradational foresets and trough cross stratified sands in adrar architectural unit 2 shows sheet like evenly laminated fine to medium grained well sorted sandstones characteristic of a high energy marine environment in contrast in the hodh area unit 2 displays vertically stacked successions of coarse grained sandstones that indicate high discharge sediment laden flows these are interpreted as meltwater sediments in both adrar and hodh architectural unit 3 is made up of gravelly coarse grained trough cross stratified sandstone emplaced in a braided low sinuosity fluvial environment the three stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel fill structures with pleistocene tunnel valleys found in areas of low relief in cool temperate and low arctic zones such features are incised by high pressure subglacial meltwater and later infilled by proglacial to postglacial deposits based on this comparison architectural unit 1 represents subglacial or proglacial outwash high sediment discharge in unit 2 took place in a proglacial environment in the hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the adrar channels and deposits in unit 3 are fluvial postglacial sediments identification of preserved channel fill structures may be useful in reconstructing late ordovician ice sheet dynamics

a multiparameter approach to volcano monitoring based on 4d analyses of seismo volcanic and acoustic signals the 2008 mt etna eruption 1 volcanic tremor and low frequency events together with infrasound signals can represent important precursory phenomena of eruptive activity because of their strict relationship with eruptive mechanisms and with fluid flows through the volcano s feeding system important variations of these seismo volcanic and infrasound signals recorded at mt etna volcano occurred both in the medium and short term before the eruption that took place on 13 may 2008 the most significant changes were observed in the frequency content and location of lp events as well as in volcanic tremor location that allowed us to track the magma pathway feeding the 2008 eruptive activity the infrasound showed three different families of events linked to the activity of the three active vents north east crater south east crater and the eruptive fissure the seismic and infrasonic variations reported corroborated by ground deformations variations help to develop a quantitative prediction and early warning system for effusive and or explosive eruptions

body size trends in the ordovician to earliest silurian of the oslo region abstract body size is an important ecological parameter that can be understood in both evolutionary and environmental terms we present a database of changes in overall body size in brachiopods and trilobites through the ordovician and earliest silurian of the oslo region norway in contrast with global studies the limited geographical extent of our data allows correlation with environmental parameters such as climate and water depth and simplifies taxonomical standardization and stratigraphical correlation in our data set there is a clear indication of increasing size both for brachiopods and trilobites during most of the ordovician although this could result from an unbiased random walk the size increase is followed by a reduction in size during the latest ordovician trilobites show a strong increase followed by a decrease in body size during the middle ordovician while brachiopod body size peaks later in the late ordovician these changes are partly correlated with the changes in species diversity in the oslo region we discuss the results in terms of two models for change in body size cope s rule and the lilliput effect cope s rule is the theory of an increasing body size through geological time while the lilliput effect describes a decrease in body size in the aftermath of a mass extinction or a severe environmental disturbance we find no clear correlations between body size and sea level

re evaluation of the petroleum potential of the kufra basin se libya ne chad does the source rock barrier fall abstract the kufra basin is a large underexplored palaeozoic intracratonic sag basin in se libya and ne chad with extensions into nw sudan and sw egypt the basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to cretaceous geologically the basin is very similar to the murzuq basin in sw libya which recently presented libya with its largest oil discovery for over a decade most of the hydrocarbon play elements known from the murzuq basin also occur in the kufra basin thick porous cambro ordovician sandstones are present and would form good reservoirs lower silurian shales may act as effective seals and there are potential structural traps in seismically defined fault blocks however the source rock availability in the kufra basin is currently unclear one of the two main source rock candidates in the basin is a lower silurian shale unit tanezzuft formation the tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by agip between 1978 and 1981 hot shales developed at the base of this widespread silurian shale unit form important source rocks in many areas of north africa and arabia these hot shales are interpreted to have been deposited in palaeodepressions such as incised valleys of the preceding lowstand or intrashelf basins during the initial transgression after the melting of the late ordovician ice cap the areal distribution of the organic rich unit is therefore discontinuous fieldwork in the kufra basin has shown that the basal tanezzuft horizon is not exposed on the northern and eastern margins of the basin deep infracambrian rift grabens have been interpreted on seismic lines from the kufra basin and in analogy to oman and algeria could contain organic rich infracambrian deposits the infracambrian succession in the kufra basin may contain a second major potential source rock and warrants further investigation

towards an automatic monitoring system of infrasonic events at mt etna strategies for source location and modeling active volcanoes characterized by open conduit conditions generate sonic and infrasonic signals whose investigation provides useful information for both monitoring purposes and studying the dynamics of explosive processes in this work we discuss the automatic procedures implemented for a real time application to the data acquired by a permanent network of five infrasound stations running at mt etna volcano the infrasound signals at mt etna consist in amplitude transients called infrasound events the adopted procedure uses a multi algorithm approach for event detection counting characterization and location it is designed for an efficient and accurate processing of infrasound records provided by single site and array stations moreover the source mechanism of these events can be investigated off line or in near real time by using three different models 1 strombolian bubble 2 resonating conduit and 3 helmholtz resonator the infrasound waveforms allow us to choose the most suitable model to get quantitative information about the source and to follow the time evolution of the source parameters

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

probabilistic seismic hazard at mt etna italy the contribution of local fault activity in mid term assessment abstract in this work we tackle the problem of seismic hazard at etna deriving from the recurrent seismogenic activity of local faults by adopting two independent methods based on probabilistic approaches we assess the hazard in terms of macroseismic intensity and represent the occurrence probability calculated for different exposure times both on maps and at fault scale seismic hazard maps obtained by applying the site approach through the sasha code and a new probabilistic attenuation model indicate the eastern flank of the volcano as the most hazardous with expected intensity i exp in 50 years i e the standard exposure time adopted in the seismic regulations ranging from degrees ix to x ems in shorter exposure periods 20 10 5 years values of i exp up to ix are also reached in the same area but they are clearly determined by the earthquakes generated by the timpe fault system in order to quantify the contribution of local seismogenic sources to the hazard of the region we reconstruct the seismic history of each fault and calculate with sasha the probability that earthquakes of a given intensity may be generated in different exposure times results confirm the high level of hazard due to the s tecla moscarello and fiandaca faults especially for earthquakes of moderate intensity i e vi i 0 vii with probabilities respectively exceeding 50 and 20 in 10 years and 30 and 10 in 5 years occurrence probability of major events i 0 viii at the fault scale has also been investigated by statistics on intertimes under stationary assumptions we obtain a probability of 6 8 in 5 years for each structure by introducing the time dependency time elapsed since the last event occurred on each fault through a bpt model we identify the moscarello and s tecla faults as the most probable sources to be activated in the next 5 years 2013 2017 this result may represent a useful indication to establish priority criteria for actions aimed at reducing seismic risk at a local scale

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

estimates of the earth s spin geographic axis relative to gondwana from glacial sediments and paleomagnetism abstract the ice centres of the permo carboniferous and late ordovician glaciations of gondwana have been quantitatively estimated their positions relative to the inferred mean paleomagnetic poles are not significantly different from those of the quaternary ice centres relative to the present day geographic poles these data suggest it is unnecessary to postulate that the paleozoic earth had marked non dipole components in the geomagnetic field or that the average of the dipole axis was significantly different from the geographic axis or that there was a significantly different tilt to the geograpic axis the short lived late ordovician ice age is tentatively attributed to equally short episodes of faulting within gondwana these are believed to have created transient highlands which converted a previously snow free summer climate into one in which snow could accumulate and build ice caps once the faulting ceased the topography decayed and the climate reverted to its previous snow free summer slate

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

the first record of cooksonia from south america in silurian rocks of bolivia plant megafossils resembling cooksonia caledonica lang aredescribed from the kirusillas formation in southern bolivia faunal and lithological evidence together with field relation ships advocate a late silurian ludlow age palaeocontinentalreconstructions suggest that the plants grew on the western edgeof gondwana at 50 60 s and indicate that cooksonia hadspread to higher latitudes soon after its appearance in the wenlock

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time

global carbon isotopic events associated with mass extinction and glaciation in the late ordovician mass extinctions and glacioeustatic sea level changes in the lower part of the hirnantian final stage of the ashgill are accompanied by shifts in marine stable isotope compositions previously published stable isotope changes have been used to identify the onset and demise of the gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans however the existing isotopic data set had limitations because it was derived from ordovician low latitude settings and from carbonates or organic carbon in separate areas we report new data from ordovician high latitude carbonates and demonstrate parallel shifts in organic and carbonate 13c from baltica brachiopod shells from a high palaeolatitude periglacial setting in argentina have elevated 13c values similar to those described previously from low latitude sites the new data demonstrate that the positive hirnantian 13c excursion previously only recognised from low palaeolatitude areas was widespread and probably global in extent the poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material preliminary carbon isotopic data from thermally immature organic matter from estonia provide the first indication of a synchronous shift in organic and inorganic 13c in sediments from the same basin this work provides new data of critical importance for constraining models of end ordovician palaeoceanography and climate change

estimates of site seismicity rates using ill defined macroseismic data a new approach to the problem of site seismic hazard analysis is proposed based on intensity data affected by uncertainties this approach takes into account the ordinal and discrete character of intensities trying to avoid misleading results due to the assumption that intensity can be treated as a real number continuous distribution estimators attenuation relationships etc the proposed formulation is based on the use of a distribution function describing for each earthquake the probability that site seismic effects can be described by each possible intensity value in order to obtain site hazard estimates where local data are lacking the dependence of this distribution function with the distance from the macroseismic epicenter and with epicentral intensity is examined a methodology has been developed for the purpose of combining such probabilities and estimating site seismicity rates which takes into account the effect of uncertainties involved in this kind of analysis an application of this approach is described and discussed

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference