did a katian large igneous province trigger the late ordovician glaciation a hypothesis tested with a carbon cycle model abstract the ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period leading to the hirnantian glaciation and the first of the big five mass extinctions of marine life the cause of this cooling event remains unclear several authors correlate it with a drop in atmospheric p co 2 to a threshold permitting the ice accumulation on the gondwana supercontinent however what are the reasons of the atmospheric p co 2 decrease here we follow the hypothesis initiated by barnes 2004 that an ordovician superplume event occurred during the middle to late ordovician such an event would not only have a large impact on the ordovician biodiversification barnes 2004 but it would also be responsible for the climatic upheaval during the late ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric p co 2 there is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model including carbon alkalinity and phosphorus cycles coupled with a 1d climate model energy balance model ebm francois and walker 1992 that such a scenario allows to explain both the mid ashgill katian global warming event known as the boda event fortey and cocks 2005 and the subsequent late ordovician hirnantian glaciation because silicate weathering is enhanced upon warm and wet climate we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric p co 2 to the threshold proposed by herrmann et al 2004 a b to initiate a glaciation on gondwana

arenigian early ordovician sea level history and the response of conodont communities western newfoundland four cluster analyses were performed which recognized 17 conodont communities in the arenigian lower ordovician of western newfoundland the analyses include 69 598 identifiable conodont specimens recovered from 153 conodont bearing samples from four stratigraphical sections representing the environmental settings of the platform upper proximal slope lower proximal slope and distal slope the distribution of conodont communities along the platform to slope environmental gradient shows that sea level changes simultaneously affected the development and replacement of the conodont communities in the different facies the pattern of change in conodont communities allows an interpretation of sea level change that is correlated precisely into the detailed graptolite biozonation a gradual transgression lasted most of tetragraptus approximatus zone time which was followed by a brief regression a transgression regression cycle occurred in the t akzharensis zone time a major transgression caused a highst

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

long term cycles of triassic climate change a new δ18o record from conodont apatite jat and isw gratefully acknowledge financial support from the australian research council dp1096252 mr is grateful for financial support from progetto d ateneo 2009 cpda090175 09 and 2012 cpda 121100 and prin 2008 2008bef5z7 005

orbital scale climate change and glacioeustasy during the early late ordovician pre hirnantian determined from δ18o values in marine apatite this study focuses on the 10 m y before the latest ordovician hirnantian glaciation we test whether orbital scale climatic fluctuations controlled the growth and melting of continental glaciers resulting in glacioeustatic sea level changes and the development of widespread marine sedimentary cycles 18o values of conodont apatite from 14 late ordovician katian cycles range from 17 to 21 isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow water facies supporting the hypothesis that glacioeustasy was the dominant control on water depth changes measured intracycle 18o changes of 0 7 2 5 were controlled by changes in ice volume 60 m sea level changes sea surface temperatures 5 c and potentially local increases in seawater evaporation during drier and or windier glacial stages these interpreted orbital scale climate changes and resultant large glacial ice volume changes support recent interpretations of a dynamic and prolonged ordovician greenhouse to icehouse transition

evolution of the early paleozoic cordilleran margin of laurentia tectonic and eustatic events interpreted from sequence stratigraphy and conodont community patterns several field seasons in the canadian cordillera have allowed the measurement description and sampling of over 20 000 m of lower paleozoic strata from 26 stratigraphic sections across four platform to basin transects with the recovery of over 100 000 conodonts from more than 1200 4 5 kg samples this work was part of the lithoprobe slave northern cordillera lithospheric evolution snorcle project but is also being extended through a pan lithoprobe project to understand the tectonic and eustatic response of much of the laurentian plate through the early paleozoic based on the abundant field data the complex stratigraphic framework is interpreted in terms of sequence stratigraphy and a derived relative sea level curve using detailed conodont taxonomic and biostratigraphic results cluster analysis of conodont distributional data identified an evolving series of conodont communities through space and time these communities were partitioned across the platform to basin gradient and provide an additional sensitive indicator of relative sea level change these two independent approaches generated comparable eustatic curves for this cordilleran laurentian margin during much of the early paleozoic and identified some global eustatic events noted by earlier workers this part of laurentia was not a simple passive margin during the early paleozoic but rather was affected by four main tectonic events complicated by six principal eustatic changes some success was achieved in filtering the global and regional tectonic eustatic effects and in proposing causes for some of the events

ordovician back arc foreland and ocloyic thrust belt development on the western gondwana margin as a response to precordillera terrane accretion 1 the famatina belt southern central andes records a circa 470 m y shortening episode ocloyic orogeny affecting the peri gondwanan back arc basin in response to the accretion of the precordillera terrane collision created distinct features across and along the margin some of which persisted into the present andean structure from east to west the ocloyic deformation is recorded across the telescoped region of famatina represented by 1 an east vergent fold thrust belt evftb involving a major angular unconformity between early and middle ordovician rocks 2 a highly deformed volcano sedimentary belt and 3 an east vergent ductile shear zone affecting famatinian granites axis attitudes of ordovician refolded anticlines in the evftb show west east tectonic transport in agreement with kinematics of ocloyic high strain belts indicating shortening orthogonal to the present pacific margin eastward displacement of the ordovician volcanic arc at 27 s latitude together with a documented sinistral slip zone across the proto andean margin suggests tectonic indentation

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

middle ordovician late dapingian darriwilian conodonts from the cow head group and lower head formation western newfoundland canada1this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology middle ordovician late dapingian darriwilian conodonts from the shallow bay and green point formations cow head group and the lower head formation are recorded from three sections in gros morne national park the collection was investigated to clarify local age relationships between the uppermost part of the cow head group and the interbedded to overlying sediments of the lower head formation conodonts from st pauls inlet north section indicate a middle dapingian age for the upper lower bed 13 latest dapingian to early darriwilian age for the upper bed 13 an early darriwilian dw 1 age for the top beds or bed 15 of the shallow bay formation at lower head and the lower head formation is referred to the darriwilian the uppermost part of the lower bed 13 contains periodon hankensis n sp gothodus sp a and diaphorodus delicatus followed by periodon macrodentatus ansella longicuspica erraticodon n sp a and spinodus wardi n sp in the lowermost part of upper bed 13 the fauna with p macrod

a major conodont extinction event during the early ordovician within the midcontinent realm ji z and barnes c r 1993 a major conodont extinction event during the early ordovician within the midcontinent realm palaeogeogr palaeoclimatol pataeoecol 104 37 47 the major change in the lower ordovician conodont faunas of the midcontinent realm was recognized by earlier workers our recent intense study of midcontinent conodonts from the st george group port au port peninsula western newfoundland has shown that seven major conodont lineages involving some 33 multielement species completely terminate in the late tremadoc series within an interval of a few metres at a level between the lower ordovician conodont fauna c and fauna d the extinction event occurs during the peak of a regressive phase within the deposition of the st george group and at a time of apparent global eustatic fall at or slightly below the tremadoc arenig series boundary this important extinction bioevent has been recognized throughout the midcontinent realm in north america

carbonate platform dominated by peloidal sands lower ordovician la silla formation of the eastern precordillera san juan argentina the lower ordovician la silla formation of the precordillera of west central argentina is part of the west facing early palaeozoic tropical carbonate platform succession that comprises the core of the cuyania terrane up to 360 m thick it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of about 35 km la silla formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert it accumulated on a more or less uniformly subsiding passive margin copious precipitation of microcrystalline calcite probably influenced by microbial activity to varying degrees led to the generation of peloids ooids and aggregates of these grains as well as small amounts of lime mud intraclasts stromatolites and thrombolites rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites bioturbation is present locally the array of carbonate rock types is grouped into eight recurring lithofacies in order of decreasing abundance i peloidal grainstone ii laminated dolostone iii intraclastic rudstone iv microbial laminite v peloidal packstone vi ooidal grainstone vii thrombolite boundstone and viii mudstone these facies represent sediments that formed solely in a shallow subtidal marine environment with no evidence of restricted conditions hypersalinity or subaerial exposure no events of eustatic sea level change are recorded by far the dominant facies is grainstone composed of well sorted fine sand sized peloids and peloidal aggregates in homogeneous tabular to gently undulating medium to thick beds cross lamination is scarce clusters of sub metre sized microbial patch reefs developed sporadically the shallow platform is envisaged to have been covered by extensive peloidal sand flats and low relief banks and little lime mud was generated the setting was probably microtidal and may not have been affected by strong trade winds it was washed by frequent relatively gentle wave action but without experiencing powerful storms in the middle member anomalous lenses of intraclastic rudstone and laminated dolostone occur as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift faulting seaward of the platform margin the remarkably uniform peloidal grainstone composition over a broad area shows that given the appropriate combination of climate environmental and ecological factors large portions of some early palaeozoic platforms were dominated by grainy sediment and remained under well agitated conditions within fair weather wave base without distinct lateral facies differentiation or tidal flat aggradation

the impact of paleogeography pco2 poleward ocean heat transport and sea level change on global cooling during the late ordovician abstract we performed sensitivity experiments with the global climate model genesis on two stages of the upper ordovician caradocian 454 ma ashgillian 446 ma under a range of atmospheric p co 2 values 8 18 pal pre industrial atmospheric level high and low sea level and two values of poleward ocean heat transport in order to determine the importance of these variables on global cooling we then coupled a three dimensional ice sheet model to the global climate model in order to investigate the necessary boundary conditions for ice sheet formation all simulations with a high sea level and normal heat transport remain free of ice sheets even with p co 2 levels as low as 8 pal in the caradocian simulations ice sheets form in three scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport 2 with p co 2 of 8 pal and a high sea level and reduced 50 of normal poleward ocean heat transport and 3 with p co 2 of 15 pal and a low sea level and reduced poleward ocean heat transport in the ashgillian simulations ice sheets form in only two scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport or 2 with p co 2 of 8 pal and a high sea level and reduced poleward ocean heat transport the ice sheets in the ashgillian experiments are larger and thicker than the ice sheets in the caradocian simulations because the southward movement of gondwana increased land area in the higher southern latitudes where ice sheets could grow the threshold for glaciation under ashgillian paleogeography is 8 pal and either a low sea level exposed shelves or a reduced poleward ocean heat transport while the paleogeographic evolution and a drop in p co 2 during the late ordovician cooled the global climate changes in additional factors were required to initiate ice sheet formation such as a drop in sea level a reduction in poleward ocean heat transport or a combination of both

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

biostratigraphy and palaeoecology of middle late ordovician conodont and graptolite faunas of the las chacritas river section precordillera of san juan argentina a conodont graptolite biostratigraphic study was carried out on the top strata of the san juan las chacritas and las aguaditas formations in the la trampa range precordillera of san juan in western argentina significant conodont records in the san juan and las chacritas formations allow for the recognition of the yangtzeplacognathus crassus eoplacognathus pseudoplanus microzarkodina hagetiana and m ozarkodella subzones and eoplacognathus suecicus zones of darriwilian age index species and co occurrences of graptolites and conodonts were recorded in the las aguaditas formation allowing the identification of the nemagraptus gracilis and the pygodus anserinus zones which represent the sandbian stage these data indicate a hiatus between the las chacritas and the las aguaditas formations corresponding to the pygodus serra zone and the pterograptus elegans and hustedograptus teretiusculus zones upper darriwilian a total of 7287 identifiable conodont elements were recorded from the study section the species frequency registered for each zone shows that periodon and paroistodus are the most abundant taxa which are indicative of open marine environments the records of particular conodont taxa such as histiodella periodon microzarkodina eoplacognathus and baltoniodus allow a precise global correlation with other regions such as south central china baltoscandia north america great britain southern australia and new zealand the graptolite fauna identified here are recognized worldwide in equivalent strata in the baltic region great britain north america china southern australia and new zealand the presence of graptolites in the ribbon limestones of the las chacritas formation is documented for the first time

the early paleozoic evolution of the argentine precordillera as a laurentian rifted drifted and collided terrane a geodynamic model paleontologic and stratigraphic evidence points to the early paleozoic precordilleran terrane of western argentina as being the conjugate rift pair of the appalachians stratigraphic similarities of the cambrian and early arenig carbonate series and very strong affinities among trilobite conodont and brachiopod faunas show their close relationship the most probable provenance areas are the appalachian ouachita rifted margin and the newfoundland appalachians although the former fits better with geometric and drifting paths suggested by faunal affinities increasing celtic and baltic brachiopod genera and divergent stratigraphy since the arenig indicate the drifting of the precordilleran terrane collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed the collision of the precordillera with western gondwana occurred during the mid llanvirn to llandeilo a magmatic arc related to eastward subduction present coordinates was active in the famatina range east of the precordillera this region of celtic affinity shows faunal exchange with the precordilleran terrane since the late arenig and may represent accreted intra iapetus volcanic island arc complexes the rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the precordilleran terrane from periequatorial to peripolar latitudes between the cambrian and latest ordovician the deep ocean to the west of the precordillera started to close by the late ordovician with the eastward drift of the chilenian terrane absence of volcanic or pyroclastic arc derived rocks in the precordillera indicate west dipping subduction as chilenia approached the continental margin a new forebulge was established on the former collided precordilleran terrane developing an erosional unconformity in central precordillera talacasto tambolar arch a gondwanic signature was fully developed by the middle silurian when the malvinokaffric clarkeia fauna flourished before then the late ordovician glacial record and associated hirnantia fauna were the first clear tie to gondwana during the silurian the marginal basin behaved as a foreland with lithosphere rheology and eustasy governing the sequence stratigraphy wrench faulting along its eastern boundary displaced the precordillera toward the south continued shortening during closure with the chilenian terrane in the mid devonian produced thrust loading of the basin and generated a thick graywacke succession final accretion of chilenia late devonian generated a regional angular unconformity between the lower and upper paleozoic new eastward subduction was initiated west of the accreted chilenian terrane during the late permian triassic as indicated by the choiyoi volcanic complex which presently outcrops in the frontal cordillera

comparative lower middle ordovician conodont oxygen isotope palaeothermometry of the argentine precordillera and laurentian margins abstract stratigraphic palaeontologic and palaeomagnetic data support a hypothesis that argues for the argentine precordillera rifting from the southwestern margin of laurentia in low latitudes during the cambrian migrating across the iapetus ocean colliding with the gondwanan margin in the late middle ordovician and receiving glaciogenic sediments in the late ordovician an alternative model proposes that the precordillera originated as a low latitude segment of gondwana migrated southward through major transform faulting toward high latitudes in the late middle ordovician to reach its present position in the devonian new conodont oxygen isotope compositions 18ophos have been determined by ion microprobe shrimp ii using samples from both the precordillera and laurentia marathon area of texas wilcox pass in alberta and western newfoundland significantly the 18o values of conodonts from all four widely separated areas show a consistent pattern of a cyclic but overall increasing trend in 18o ca 16 to 18 hence ocean cooling through the early and middle ordovician an apparent change occurs at the basal late ordovician where 18o values obtained from conodonts in the uppermost sample from the precordillera are significantly higher 1 5 than those from laurentia albeit from a single sample this higher value implies significantly cooler conditions as would be anticipated with a southerly poleward migration of the precordillera irrespective of either hypothesis the virtual absence of conodont bearing carbonates in most of the precordilleran upper ordovician precluded analysis of younger samples when combined with existing macrofaunal and palaeomagnetic data the oxygen isotope data would tend to favour the model of a drift of the precordillera from tropical to higher latitudes during the ordovician however further studies are needed to determine unequivocally whether the precordillera originated from southern laurentia ouachita embayment these new oxygen isotope values provide the best and regionally most consistent data through the early middle ordovician

high dependence of ordovician ocean surface circulation on atmospheric co2 levels abstract life on earth radiated dramatically during the ordovician period 485 444 ma at all taxonomic levels during an evolutionary episode known as the great ordovician biodiversification event abundant scientific literature has documented the pattern of biodiversification for numerous organism groups however fully understanding this event also requires detailed knowledge about the spatial distribution of the organisms the full potential of the palaeobiodiversity patterns remains under explored due to the lack of knowledge about past ocean circulation surface circulation significantly conditions the dispersal of organisms in creating preferential ocean migration pathways or in contrast by isolating specific regions and thus establishing major physical barriers these uncertainties persist because neither direct observational data nor proxy data independent from the fossil record are available to reconstruct the ocean surface circulation here we present new maps of ordovician ocean surface circulation based on simulations using the general circulation model foam this is a coupled ocean atmosphere general circulation model and represents a methodological improvement over previously published work in addition we use the most up to date palaeogeographical reconstructions to increase compatibility with recent palaeontological databases in providing clear synthetic maps of ocean surface circulation in addition to raw model outputs we aim to facilitate data model comparison to assist in interpretation of palaeontological datasets and to promote renewed discussion about ordovician biogeography finally we investigate the sensitivity of the circulation pattern to the atmospheric co2 content which remains poorly constrained in the ordovician potentially ranging from more than 15 times the preindustrial atmospheric co2 level pal to 5 pal maps are systematically constructed for a high 16 pal medium 8 pal and low 4 pal co2 value we show that the circulation pattern is much more sensitive to co2 than previously suggested

implications of fault scarp degradation for brent group prospectivity ninian field northern north sea although the majority of the oil reserves contained in the ninian field are being produced from tilted middle jurassic sandstones of the brent group significant volumes of oil have also been discovered within a poorly documented subtle structural trap that lies in the immediate footwall to the field s controlling eastern boundary fault integration of well data and the interpretation of seismic surveys allow the definition of an area characterized by major footwall degradation the data demonstrate that the 1 25 mi 2 km wide area affected lies to the east of the zone of brent group truncation along the whole 15 mi 25 km length of the field a combination of sedimentary and stratigraphic information shows that the degradation and products obtain a thickness that locally exceeds 450 ft 126 m the degradation process appears to have been triggered by accelerated extension during the callovian early oxfordian deposition of the heather formation the development of pronounced fault terraces due to footwall collapse copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 26 1996 revised manuscript received august 12 1996 final acceptance december 24 1996 2department of geology and geophysics the university of edinburgh kings buildings west mains road edinburgh eh9 3jw scotland e mail jru glg ed ac uk 3chevron uk ltd ninian house crawpeel road aberdeen ab12 3lg scotland 4department of earth sciences university of manchester oxford road manchester m13 9pl england we would like to thank the ninian partners chevron uk ltd murphy petroleum ltd oryx uk energy company sands petroleum formerly neste north sea ltd sun oil britain ltd and ranger oil for permission to publish this article however the views expressed here are ours and are not necessarily those of chevron or the ninian partnership help was received from rick davis robert heming bryan man bill higgs andrew harding and john connor of chevron gerry white is thanked for drafting the diagrams interpretations carried out using geoquest ies and iesx software mounted on edinburgh s seismic workstation facility were supported by the petroleum science and technology institute psti norsk hydro shell expro and esso exploration and production uk ltd the study forms part of a wider research program studying the occurrence and effects of fault scarp degradation and subtle stratigraphic trap formation in the brent province currently being supported at the university of edinburgh by a natural environmental research council nerc realising our potential award ropa grant gr3 r9521 finally we would like to acknowledge aapg reviewers alan roberts and j brint for helpful and constructive comments

graptolite conodont biostratigraphy and biofacies of the middle ordovician cerro viejo succession san juan precordillera argentina abstract high resolution graptolite conodont biostratigraphy and biofacies analysis have been carried out for the lower member of the los azules formation exposed in the cerro viejo area san juan precordillera western argentina the rock succession is composed of alternating rich fossiliferous argillites and virtually barren volcanic ash beds k bentonites of darriwilian da2 age graptolite and conodont collections from the los azules and amarilla creeks sections are studied graptolites are the most common fossils in the lower member of the los azules formation and represent the undulograptus dentatus and holmograptus lentus zones a diverse conodont fauna recovered from the uppermost limestones of the underlying san juan formation and conodont elements preserved on bedding plane surfaces in the lower los azules formation indicate the lenodus variabilis and eoplacognathus suecicus zones micaceous siltstone with bergstroemograptus crawfordi harris and abundant biserial graptolites close to the top of the lower member is notable this bed yielded the first specimens of polonodus magnum albanesi this species precedes the entrance of pygodus anitae bergstrom which is typical from the upper part of the e suecicus zone characteristic graptolites of the diplograptus decoratus zone da3 or equivalent faunas were not recorded in the studied sections suggesting the presence of a hiatus between the lower and middle members which may indicate a critical event in the history of the basin a rich conodont assemblage dominated by planktic pelagic forms periodon paroistodus protopanderodus drepanoistodus an abundant shelly fauna and scarce graptolites are recorded in the upper san juan limestone after drowning of carbonate platform the conodont diversity dramatically dropped becoming graptolites the dominant elements of the faunas in the lower member of the los azules formation the periodon pygodus biofacies represents a low diversity deep water conodont community present both in the upper part of the lower member and the middle member graptolite faunas represent the isograptid biofacies including pandemic and pacific forms of the epipelagic and deep water biotopes they are related to a transgressive event that diachronically flooded the precordilleran carbonate platform during the early middle ordovician

chronology of motion in a complete thrust belt the precordillera 30 31 s andes mountains in the precordillera thrust belt of west central argentina faulted rotated and onlapping relations of piggyback basin units and foreland basin units yield an accurate but imprecise chronology of thrust motion addition of data from provenance history strata accumulation rates and presence of thick conglomerate units in the bermejo foreland basin results in a thrust chronology that is more precise but not entirely independent of assumptions about foreland basin dynamics seven new radiometric dates allow substantial revision of the interpreted age of first deformation in the precordillera at about 30 south units previously assigned to the paleozoic and mesozoic are of oligocene and miocene ages a redbed unit that began before 21 6 pm 0 8 ma appears to have pre dated thrusting deposits in the early bermejo foreland basin were dominated by eolian sandstones unlike the younger fluvial and alluvial units an andesitic volcanic center within the miocene foreland basin had long term effects on sub

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

did intense volcanism trigger the first late ordovician icehouse oxygen isotopes measured on late ordovician conodonts from minnesota and kentucky united states were studied to reconstruct the paleotemperature history during late sandbian to katian mohawkian cincinnatian time this time interval was characterized by intense volcanism as shown by the prominent deicke millbrig and other k bentonite beds a prominent carbon isotope excursion guttenberg 13 c excursion gice postdates the millbrig volcanic eruptions and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling the oxygen isotope record in conodont apatite contradicts this earlier interpretation an increase in 18 o of 1 5 vienna standard mean ocean water just above the deicke k bentonite suggests an abrupt and short lived cooling that possibly initiated a first short term glacial episode well before the major hirnantian glaciation the decrease in 18 o immediately after the mega eruptions indicates warming before the gice and no cooling is shown in the gice interval the coincidence of the deicke mega eruption with a cooling event suggests that this major volcanic event had a profound effect on late ordovician late mohawkian climate

oxygen isotopic evidence for late triassic monsoonal upwelling in the northwestern tethys the oxygen isotopic composition of conodonts from the pizzo mondello section in the sicani basin sicily italy on the northwestern tethys margin has been used to reconstruct regional paleoseawater conditions the 18 o phos values 21 21 9 measured by sensitive high resolution ion microprobe shrimp ii are consistent with previously published 18 o phos data from the contemporaneous marginal hallstatt meliata basin 20 3 22 5 located farther north but are distinctly higher than those previously reported from the open ocean setting of the nearby lagonegro basin 18 7 20 0 this 2 offset represents a temperature differential of 8 c suggesting that sea surface temperatures along the northwestern margin sicani and hallstatt meliata basins of the tethys ocean were lower than in the open ocean lagonegro basin during the late carnian tuvalian 2 to early norian lacian we interpret the cooler waters of the marginal basins to reflect coastal upwelling driven by strong monsoonal systems as predicted by existing paleoclimate models for this region

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

persistent oceanic anoxia and elevated extinction rates separate the cambrian and ordovician radiations recurrent mass extinction events at biomere a biostratigraphic unit boundaries characterize the middle cambrian to early ordovician tremadocian time interval that is between the major cambrian and ordovician radiations of animal life a role for anoxia in maintaining elevated extinction rates in the late cambrian has been proposed based on coincidence of an extinction with positive excursions in 13ccarb and 34scas cas carbonate associated sulfate here we examine an early ordovician extinction event at the base of the north american stairsian stage upper tremadocian and demonstrate concurrent onset of positive excursions in 13c and 34s inferred to reflect enhanced organic matter burial under anoxic waters sea level rise may have brought anoxic waters onto the shelf to initiate extinctions the evidence for 13c excursions and elevated extinction rates appears to wane in the tremadocian consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the great ordovician biodiversification event

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

conodont apatite δ18o values from a platform margin setting oklahoma usa implications for initiation of late ordovician icehouse conditions abstract the uppermost sandbian lower katian stratigraphic record documents a critical time period in earth history lithological evidence and geochemical proxy records suggest that the uppermost sandbian lower katian interval could represent a transitional phase from a long cambrian ordovician greenhouse world to the late ordovician icehouse world to examine this interval of earth s history oxygen isotopes of conodont apatite from the bromide and viola springs formations in southern oklahoma were analyzed in order to provide sea surface temperature estimates at a location that had little influence by terrigenous sedimentation our results are inconsistent with 1 previous suggestions that global cooling occurred across a prominent sequence boundary close to the sandbian katian boundary across north america and 2 the hypothesis that there is a direct connection between volcanic mega eruption s and a significant long term impact on climate during the sandbian katian of laurentia furthermore 18 o values spanning the guttenberg carbon isotope excursion gice an interval that has been interpreted as evidence for an early onset of the late ordovician glaciation are variable a 1 5 18 o shift is observed in the lower gice that is followed by a return to 18 o values that fluctuate around a mean of 19 0 in the upper gice together these observations suggest a short lived cooling event within the lower gice this cooling event may represent enhanced carbon burial and drawdown of atmospheric p co 2 followed by a return to more stable low latitude temperatures in the upper gice

did the taconic appalachians continue into southern south america the appalachian mountains now terminating abruptly at the gulf of mexico coastal plain may have formerly continued into southern south america rocks forming the basement of the argentine andes can be interpreted as remnants of an early paleozoic orogen the famatinian belt not unlike the taconic appalachians both orogens are bordered to the west present coordinates by lower paleozoic carbonate platforms bearing the olenellid trilobite fauna that is characteristic of laurentia paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain possibly extending into antarctica during ordovician closure of an ocean basin southern iapetus between laurentia and gondwana the taconic and famatinian segments of the chain may have been truncated during late ordovician separation of laurentia and gondwana along the preexisting late neoproterozoic to cambrian rift system that initiated formation of the ouachita embayment and the southern margin of north america

the rio sassito sedimentary succession ordovician a pinpoint in the geodynamic evolution of the argentine precordillera in the precordillera of western argentina an isolated outcrop of llandeilian siliciclastics and caradocian limestones rio sassito succession reveals a complex interplay between the tectonic and the sedimentary history of the precordillera during middle and late ordovician times the succession is composed of a lower siliciclastic interval and an upper carbonate interval and is bounded below and above by erosional unconformities dating of these unconformities which in many places merged to form one single surface demonstrates that the most important erosional event took place prior to the deposition of the rio sassito succession this erosional event is correlated to extensional tectonics during continental breakup and the separation of the precordillera from laurentia block faulting with the formation of horst and graben structures provided the topography for the establishment of a pelagic carbonate platform during the caradoc in our view there are no indications that these phenomena are related to the accretion of the precordillera to gondwana or to the formation of an ordovician supercontinent the carbonate sediments are typical of temperate water settings characterized by the absence of ooids oncoids and algae and by the presence of abundant abraded bioclasts intraclasts and peloids the inference of a temperate water environment does not as previously supposed indicate the accretion of the precordillera to gondwana but is more likely related to global cooling prior to the ashgillian glaciation

the proposed cambrian ordovician global boundary stratotype and point gssp in western newfoundland canada sections exposing the cambrian ordovician boundary interval at broom point in western newfoundland have been proposed earlier for a global systemic boundary stratotype these lie within the cow head group a late middle cambrian to early middle ordovician allochthonous unit of limestone shale and conglomerate deposited at the toe of the ancient continental slope and on the adjacent continental rise several recent studies have further investigated the stratigraphy sedimentology and palaeontology of the cow head group and others are under way on magnetostratigraphy and chemostratigraphy these aspects are reviewed for six key boundary sections representing proximal to distal facies cow head ledge broom point south broom point north st pauls inlet quarry martin point and green point in particular new data are presented from 260 conodont samples that yielded 15500 conodonts this intense sampling has allowed the discrimination of minor hiatuses in the proximal to intermediate facies where conglomerates have eroded and cannibalized underlying strata new conodont data from broom point north have lowered the base of the c lindstromi zone into unit 74 conglomerates thereby making this section unsuitable as a boundary stratotype new collections from green point have yielded abundant conodonts and over 9400 conodonts have been recovered from 77 samples the conodont graptolite and trilobite biostratigraphy through the boundary interval is documented allowing accurate correlation between sections and more precisely revealing small hiatuses in the proximal and intermediate facies the sequence of conodont zones is eoconodontus notchpeakensis cordylodus proavus c caboti c intermedius c lindstromi and c angulatus these can be correlated with trilobite zones established from both in situ and clast faunas from the proximal to intermediate facies and with graptolite assemblages of cooper 1979 especially in the intermediate to distal facies three new species of cordylodus are described c andresi c hastatus and c tortus and the full apparatus of iapetognathus preaengensis is illustrated the criteria for selecting a global boundary stratotype and point gssp are reviewed in terms of the cow head sections the green point section is shown to meet and largely surpass the prerequisites required of a stratotype the green point section is proposed to be the global boundary stratotype with the base of the ordovician system defined at the base of unit 23 which is the base of the broom point member green point formation at a level coincident with the base of the cordylodus lindstromi zone in addition to an abundant and superbly preserved conodont fauna this section preserves the best sequence of earliest planktic graptolites through a 40 m interval the first nematophorous graptolites of assemblage 1 occur in unit 25 6 9 m above the base of the c lindstromi zone this level can be readily correlated into the proximal facies where both deep and shallow water trilobites in situ and in clasts respectively show the base of the c lindstromi zone to lie within the symphysurina brevispicata trilobite subzone

conodont biostratigraphy of the latest cambrian early ordovician upper mckay group southeastern british columbia the age of the upper mckay group based on conodont biostratigraphy is latest cambrian cordylodus proavus zone to late early ordovician middle floian oepikodus communis zone a collection of 12 940 conodont elements was recovered from 306 samples of upper mckay group strata exposed in the western main ranges of the southern canadian rocky mountains southeastern british columbia the conodont fauna is assigned to 53 species representing 30 genera twelve zones are recognized two of which are cosmopolitan cordylodus proavus zone and iapetognathus zone seven midcontinent realm zones in ascending order include polycostatus falsioneotensis rossodus tenuis rossodus manitouensis low diversity interval scolopodus subrex and acodus kechikaensis zones and tropodus sweeti subzone of the oepikodus communis zone three atlantic realm zones in ascending order include cordylodus angulatus acodus deltatus and paroistodus proteus zones the zonation for the upper mckay group establishes correlation w

cambrian ordovician conodonts from slump deposits of the argentine precordillera new insights into its passive margin development fil voldman gustavo gabriel consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet cordoba centro de investigaciones en ciencias de la tierra universidad nacional de cordoba facultad de ciencias exactas fisicas y naturales centro de investigaciones en ciencias de la tierra argentina

darriwilian middle ordovician delta c 13 carb chemostratigraphy in the precordillera of argentina documentation of the middle darriwilian isotope carbon excursion mdice and its use for intercontinental correlation although documented from estonia latvia sweden eastern north america and china the middle ordovician darriwilian positive delta c 13 excursion known as the mdice has previously not been recognized with certainty in south america the most promising region in south america for detailed middle ordovician carbon isotope research is the precordillera of western argentina where there are many excellent exposures of biostratigraphically well dated carbonate successions spaning the tremadocian through sandbian stratigraphic interval for this project we collected numerous isotope and conodont samples from the middle darriwilian las chacritas and aguaditas formations at their type localities which yielded important biostratigraphic data as well as informative delta c 13 carb values in the e pseudoplanus zone in the upper half of the las chacritas formation there is a relatively modest but distinct delta c 13 carb excursion because its stratigraphic position and magnitude closely agree with the mdice in other parts of the world we recognize it as the first firm record of this excursion in south america the fact that the delta c 13 carb curve from the las aguaditas formation shows no such excursion is due to the existence of a stratigraphic gap between the lower and middle members of this formation that cuts out the excursion interval the precordilleran mdice is used for detailed long range correlations with successions in baltoscandia newfoundland and china illustrating the usefulness of also this delta c 13 carb excursion as a global stratigraphic tool a recent proposal of a greatly extended chronostratigraphic range of the table head group on newfoundland is rejected based on well established biostratigraphic evidence c 2013 elsevier b v all rights reserved less

modeling the early paleozoic long term climatic trend the early paleozoic climate has been described as warm and equable however recent data based on conodont oxygen isotopic composition reveal a large long cooling trend through the ordovician followed by an abrupt cooling during the late ordovician glaciation this long term climate change is associated with a major radiation in the earth life history nonetheless the driving mechanisms for this cooling trend remain unknown carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change however the impact of the plate motion context has not been explored yet although it might have a major impact on atmospheric co 2 levels simulations with a climate model coupled to a biogeochemical model geoclim show that the atmospheric co 2 decreased from more than 20 pal 5600 ppmv in the furongian down to approximately 10 pal 2800 ppmv in the llandovery before rising again in the early devonian we suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large co 2 drawdown that led to the onset of cooler to glacial conditions from the middle ordovician to the llandovery the weathering of fresh volcanic rocks is itself responsible for 33 of the late ordovician atmospheric co 2 decrease the rest being related to the continent motion through the intertropical convergence zone itcz mean annual continental temperature falls by 3 c in the early ordovician reaching 13 5 c during the glacial interval and rises to 16 c in the early devonian

the allochthony of the argentine precordillera ten years later 1993 2003 a new paleobiogeographic test of the microcontinental model abstract recent and new faunal data from the cambrian to silurian rocks of the precordillera famatina and northwest argentina basins are used to discriminate between different paleogeographic models and especially to establish to what extent they are compatible with a previous conclusion that the precordillera is a laurentian derived microcontinent there is no paleontological evidence to support a para autochthonous gondwanan origin of the precordillera the strong differences in the cambrian trilobite faunas and lithologic successions preclude a common origin of the precordillera terrane eastern antarctica and south africa recent discoveries of brachiopods and organisms of the phylum agmata strengthened laurentian affinities during the cambrian the latest cambrian early ordovician faunas that inhabited the autochthonous northwest argentina basin including the western puna volcaniclastic successions are mostly peri gondwanan the early ordovician brachiopods ostracods and trilobites display mixed laurentian baltic and avalonian biogeographical links supporting a drifting of the precordillera across the iapetus ocean increasing gondwanan elements during the llanvirn along with varied geological evidence indicate that the first stages of collision may have begun at that time involving a major change in the plate kinematics the distribution of facies and faunas basin development and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the precordillera terrane with the gondwana margin the high level of endemism of caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike slip movement of the colliding precordillera plate with respect to the gondwana margin suggested relationships between facies distribution geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late cambrian to silurian

lower paleozoic stratigraphic and biostratigraphic correlations in the canadian cordillera implications for the tectonic evolution of the laurentian margin the ancient laurentian margin rifted in the latest neoproterozoic to early cambrian but appears not to have developed as a simple passive margin through a long post rift drift phase stratigraphic and conodont biostratigraphic information from four platform to basin transects across the margin has advanced our knowledge of the early paleozoic evolution of the margin in northeastern british columbia two northern transects span the macdonald platform to kechika trough and ospika embayment and a third transect spans the parautochthonous cassiar terrane in the southern rocky mountains new conodont biostratigraphic data for the ordovician succession of the bow platform is correlated to coeval basinal facies of the white river trough in total from 26 stratigraphic sections over 25 km of strata were measured and 1200 conodont samples were collected that yielded over 100 000 conodont elements key zonal species were used for regional correlation of uppermost cambrian to middle devonian strata along the cordillera the biostratigraphy temporally constrains at least two periods of renewed extension along the margin in the latest cambrian and late early ordovician alkalic volcanics associated with abrupt facies changes across the ancient shelf break intervals of slope debris breccia deposits and distal turbidite flows suggest the margin was characterized by intervals of volcanism basin foundering and platform flooding siliciclastics in the succession were sourced by a reactivation of tectonic highs such as the peace river arch prominent hiatuses punctuate the succession including unconformities of early late ordovician sub llandovery possibly early to middle silurian and early devonian ages

paleomagnetic confirmation of the laurentian origin of the argentine precordillera abstract several recent tectonic models have portrayed the argentine precordillera has been recently portrayed as an early paleozoic laurentian derived exotic terrane now found in southwestern south america these models have primarily been based on strong biogeographic and stratigraphic evidence however no paleomagnetic data have previously been available to independently test them a paleomagnetic study was therefore carried out on the early cambrian cerro totora formation exposed in the northern reaches of the argentine precordillera after stepwise thermal demagnetization a pre folding remanence was identified in ten sites of this formation yielding a paleomagnetic pole ct at 37 0 n 314 1 e a95 5 8 this pole is not consistent with the latest proterozoic early paleozoic apparent polar wander path for gondwana but it agrees with the early cambrian section of the laurentian path if the argentine precordillera is positioned as the conjugate margin of the ouachita embayment in southeast laurentia this result confirms that the precordillera is an allochthonous terrane derived from laurentia in cambrian times that was later accreted to gondwana probably in middle ordovician times

ordovician metamorphic event in the carbonate platform of the argentine precordillera implications for the geotectonic evolution of the proto andean margin of gondwana an ordovician metamorphic event is recorded by means of conodont paleothermometry in the slope sedimentary sequences of the western precordillera allochthonous conodonts from reworked deposits of the eastern carbonate platform with cai 4 5 5 and autochthonous conodonts from the olistostrome matrix with cai 3 allow for constraining the metamorphic age within the paltodus deltifer and lenodus variabilis zones i e ca 480 465 ma the conodont data support an ordovician instead of an alternative devonian collision for the cuyania terrane with the proto andean margin of gondwana which is consistent with the earlier proposed but still contentious microcontinent model in the latter scenario the collision of the cuyania terrane caused unroofing and cannibalism of the leading edge of the carbonate succession and basement rocks the erosion of these rocks supplied detritus and metamorphosed conodonts to the deeper basinal settings of the western precordillera as the deformation front migrated into the peripheral foreland of the eastern and central precordillera nonmetamorphosed conodonts were eventually incorporated into the west prograding synorogenic clastic wedge the occurrence of allochthonous and autochthonous conodonts in the slope facies of the western precordillera provides a time constraint for the early accretion of the cuyania terrane and its transition to an evolving foreland mountain system in the ordovician period