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

regional scale pressure shadow controlled mineralization in the principe orogenic gold deposit central brazil abstract the principe gold deposit situated in the natividade gold district resulted from establishment of the regional transbrasiliano strike slip system during the late neoproterozoic brasiliano event the host rocks however were formed in the paleoproterozoic with sm nd model ages from 2250 to 2500 ma therefore the principe gold deposit is epigenetic the local lithological association is a tonalitic granodioritic suite formed in an island arc environment that later experienced continental collision tectonics leading to the intrusion of the peraluminous principe granite with an age of 2169 42 ma by zircon u pb during the neoproterozoic these units underwent deformational greenschist facies metamorphism and hydrothermal alteration forming metamorphic tectonites inside the principe granite and hydrothermally altered rocks at the northern termination of the granite where the principe gold deposit is located a strong structural control is identified in the deposit with n10 w n20 e trending brittle ductile shear zones that are visible at all scales from thin sections to regional satellite images these shear zones played an important role in the formation of the deposit acting as pathways that promoted a channelized fluid flow to metal precipitation sites hydrothermal alteration processes of different grades occur along these structures mainly characterized by saussuritization biotitization chloritization sericitization carbonatization silicification and sulfidation reactions producing a neoformed mineral assemblage the principe granite acted as a regional scale porphyroclast for migration of the hydrothermal solutions as well as deformation forming two pressure shadows that acted as structural traps for the ore as stress release zones these structures concentrated hydrothermal solutions during seismic fault movement events under a transpressive regime they also changed the physicochemical conditions in the fluid by abruptly reducing the pressure and increasing the fluid rock interaction rate producing a structural framework characterized by a pervasively interconnected system of discontinuous and anastomosed quartz carbonate sulfide gold veins fieldwork petrographic analysis and mineral chemistry characterization suggest that the fluid had an aqueous carbonic nature and low salinity values thermometric studies suggest that gold mineralization occurred between 250 c and 400 c stable isotopes of s c and o indicate a metamorphic source for the fluid whole rock geochemistry and microprobe studies highlight hydrothermal alteration processes involving fe ca mg k na au and ag metasomatism the principe gold deposit belongs to the orogenic class but it is distinguished from other orogenic type occurrences in the natividade gold district and from most orogenic deposits of the world for not being located along a shear zone instead it is situated within a regional scale auriferous pressure shadow that was brought about by a short lived pressure drop during the brasiliano cycle

ordovician k bentonites issues in interpreting and correlating ancient tephras abstract prominent upper ordovician k bentonites include the deicke and millbrig beds in the late ordovician of eastern north america and the kinnekulle k bentonite in the uppermost sandbian of baltoscandia these beds are thought to represent some of the largest ash fall deposits known in the phanerozoic record this report presents a comprehensive study of biotite compositions from the three beds covering a far more extensive geographic range than has previously been described these data show that the kinnekulle and millbrig are multiple event ash beds some parts of which are indistinguishable from one another tectonomagmatic discrimination diagrams combined with mg number data indicate that the deicke millbrig kinnekulle sequence represents the progressive transformation from calc alkaline to peraluminous magmatic sources consistent with a model of progressively evolving magmatism during the closure of the iapetus ocean it is concluded that the millbrig and kinnekulle beds are coeval and represent primarily simultaneous episodes of explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event

apatite phenocryst compositions demonstrate a miscorrelation between the millbrig and kinnekulle k bentonites of north america and scandinavia the ordovician millbrig k bentonite of north america and the kinnekulle big bed of scandinavia have been considered correlative thus representing one of the largest volcanic eruptions of the phanerozoic whether these two k bentonites correlate has implications for the paleogeographic reconstructions of laurentia and baltica transoceanic biostratigraphic correlations of graptolites and conodonts and the possibly global occurrence of the guttenberg carbon isotope excursion trace element analyses of apatite phenocrysts have proven to be good discriminators for other ordovician k bentonites and it is shown here on the basis of significantly different mg cl mn fe ce and y concentrations in apatite phenocrysts from the kinnekulle and millbrig k bentonites that the beds must be viewed as representing two or more different eruptions vertical subsampling of several layers from each bed suggests that both beds are likely composed of multiple components but none of the components coincide with one another these results indicate that the millbrig and kinnekulle beds cannot both be derived from a single ultra plinian eruption as previously proposed

revised sequence stratigraphy of the upper katian stage cincinnatian strata in the cincinnati arch reference area geological and paleontological implications abstract the upper ordovician cincinnatian series is a critical upper katian reference succession previously six 3rd order depositional sequences c1 to c6 have been recognized and frequently used as regional time slices recent research however indicates a need for revision and additions including the recognition of additional 3rd and 4th order sequences we propose amendments based upon high resolution correlations using a combination of regional disconformities cycle stacking patterns bioevents marker beds and chemostratigraphy several disconformities including a large regionally angular mid richmondian unconformity are newly recognized or reinterpreted in addition distinctive depositional patterns aid in the recognition of systems tracts with a thick proximal laminated peritidal facies representing early transgressive systems tracts tsts or lowstands in part lsts b shell rich limestone packages representing later tsts and overlain by c more shale prone highstand systems tracts hsts themselves overlain locally by d siliciclastic dominated falling stage systems tract fsst deposits that are commonly removed beneath substantial unconformities sequence boundaries at present we recognize eight 3rd order cincinnatian sequences designated c1 through c8 and 16 4th order subdivisions and their component systems tracts a younger hirnantian sequence may be present at some localities as the enigmatic centerville formation and or basal belfast member of the brassfield formation the implications of these new correlations and sequence interpretations include a new standard for relative sea level changes and insights into basin dynamics this framework also permits a more nuanced history of biotic changes that include geologically brief pulses of immigration associated with the transgressions of the c1 lower kope formation c5c clarksville member of waynesville formation and c7a basal whitewater formation

mass mortalities on an irish silurian seafloor abstract recurrent deep water benthic communities dominated by diverse tabulate corals colonized soft substrates during the late llandovery transgression in western ireland a number of autochthonous shelly assemblages dominated by brachiopods corals and crinoids occur in the upper parts of the kilbride formation the highest and most distal levels are dominated by in situ tabulate corals that are mantled by a thin layer of volcanic ash most of the coral colonies could cope with limited sedimentation growth ceased after the volcaniclastic surge this catastrophic mode of preservation permits an accurate analysis of an early stage of community development in deep water benthos on a soft llandovery seabed periodically swept by volcaniclastic debris the kilbride faunas have implications for the palaeoenvironments and taphonomy of deep water marine faunas associated with areas of active volcanism

nature and origin of tuff beds in jurassic strata of the surat basin australia implications on the evolution of the eastern margin of gondwana during the mesozoic abstract volcanogenic rocks in the great australian superbasin provide one of the principal records of contemporaneous volcanism in eastern australia during the mesozoic however the paucity of primary jurassic to early cretaceous age extrusive or intrusive igneous bodies on the australian continent makes it particularly challenging to deduce their source and character this in turn makes it difficult to ascertain how the eastern margin of gondwana evolved during this timeframe despite some studies of this enigmatic volcanism there have been little or no detailed analyses of these jurassic to early cretaceous aged sediments based on the multidisciplinary analyses of age constrained air fall tuffs 168 to 148 ma from the jurassic walloon coal measures of the surat basin we suggest from zircon grains of a similar age that the tuffs were erupted from volcanoes fed by intermediate to felsic magmas supported by their quartz and feldspars rich composition and from zircon grains with low to moderate nb values 0 5 to 100 ppm and high u and th values 30 to 1000 ppm the mapping of tuff isopachs and a mean zircon crystal size of 170 m supports the source being from volcanoes approximately 280 to 1000 km from the palaeoeast southeast with a volcanic explosivity index vei of 8 our results indicate the tuffs originated from a continental arc setting associated with the whitsunday igneous association and the long lived late palaeozoic to cretaceous westward subduction of the palaeo pacific oceanic crust beneath eastern australia such a tectono magmatic environment would help constrain the timing of the transition of eastern gondwana from a convergent to a divergent margin

gigantic ordovician volcanic ash fall in north america and europe biological tectonomagmatic and event stratigraphic significance biostratigraphical geochemical isotopic and paleogeographic data suggest that the millbrig k bentonite one of the thickest and most widespread ordovician volcanic ash beds in eastern north america is the same as the so called big bentonite in baltoscandia this is the first time that the same k bentonite has been identified in both north america and europe and it serves as a unique event stratigraphic marker over a large portion of the northern hemisphere this eruption produced at least 340 km sup 3 of dense rock equivalent ash that was deposited in a layer up to 1 2 m thick over several million square kilometers as much as 800 km sup 3 of additional ash may have fallen into the iapetus ocean for a total of 1 140 km sup 3 trace element geochemistry shows that the ash was derived from a felsic calc alkalic magmatic source characteristic of volcanism in a continental crust based destructive plate margin setting this is one of the largest if not the largest ash falls recorded in earth s phanerozoic stratigraphic record but its recognizable effect on faunas and floras was minimal and it did not result in a global extinction event the millbrig big bentonite bed provides accurate time control for sedimentologic paleoecologic and paleogeographic reconstructions across plates positionedmore in tropical laurentia and temperate baltica latitudes during middle ordovician time less

upper ordovician mohawkian carbon isotope δ13c stratigraphy in eastern and central north america regional expression of a perturbation of the global carbon cycle abstract the guttenberg carbon isotope excursion gice documented from eastern north america demonstrates the effects that regional geochemically distinct water masses upwelling and ocean circulation have on the carbon isotope record from carbonate platforms late turinian chatfieldian carbonates from oklahoma kentucky virginia and west virginia record a positive carbon isotope excursion 3 0 the gice excursion the gice excursion has relations to established biostratigraphy beginning in the north american midcontinent phragmodus undatus conodont zone and continuing through the plectodina tenuis zone sequence and event stratigraphy previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the gice excursion where geochemically distinct water masses are defined for the upper ordovician a major eustatic sea level rise before the gice promoted a greater exchange of open ocean waters onto the carbonate platform of laurentia however restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent iapetus ocean were still apparent local variations documented in the gice excursion are directly related to upwelling of nutrient rich isotopically light waters increased primary productivity and the subsequent organic carbon production and burial

biogenic chert and the ordovician silica cycle abstract analysis of 161 nodular and bedded cherts shows that shallow water biosiliceous facies which were common in the lower ordovician had mostly retreated from peritidal lagoonal facies to deeper settings by the middle ordovician for decades this retreat was thought to have spanned the cambrian and ordovician periods our data suggest that the basinward shift may have taken approximately 2 14 m y in the late lower to early middle ordovician a likely cause for the departure of these often siliceous sponge cherts from shallow waters was depletion of shallow water dissolved silica dsi increased use of dsi by radiolarians may have commenced in the middle ordovician when radiolarians increased just as a pulse of radiolarite deposition began siliceous sponges may have been forced to relocate to deeper waters because radiolarians depleted shallow water dsi apparent low relative sea level in the middle ordovician may have hampered the formation and or preservation of shallow water cherts by reducing available shelf area to host such deposits however the resurgence of shelf chert in the upper ordovician while peritidal cherts remained virtually absent suggests that biology was critical to driving the basinward shift of cherty facies formation and preservation of abundant latest darriwilian through sandbian cherts may have been favored in deep water settings during a possible warming interval marked by sea level rise and extensive anoxia and euxinia that coincided with an ongoing decline in 87sr 86sr ratios and increased sea floor volcanic activity that probably enriched the dsi content of deep ocean waters ocean circulation associated with the onset of cooling in the katian may have optimized chert accumulation by stimulating surface water biosiliceous productivity before dsi rich chert preserving anoxic basins were ventilated as late ordovician glaciation intensified deep ocean ventilation by thermohaline sinking of oxygen rich waters probably decreased preservation potential of biosiliceous deposits while increasing oceanic recycling of silica

triggers of permo triassic boundary mass extinction in south china the siberian traps or paleo tethys ignimbrite flare up abstract assessment of the synchroneity between the siberian traps and the permo triassic boundary ptb mass extinction has led to the proposition that the siberian flood volcanism was responsible for the severest biotic crisis in the phanerozoic however recent studies suggest that the siberian traps may have postdated the main extinction horizon in this paper we demonstrate using stratigraphy a time and intensity coincidence between ptb volcanic ash and the main extinction horizon geochemistry of the ptb volcanic ashes in five sections in south china indicates that they were derived from continental magmatic arc zircons extracted from the ptb volcanic ashes have negative e hf t 12 9 to 2 0 and 18 o 6 8 to 10 9 consistent with an acidic volcanism and a crustal derived origin and therefore exclude a genetic link between the ptb mass extinction and the siberian traps on the basis of spatial variation in the number of the ptb volcanic ash layers and the thickness of the ash layers in south china we propose that the ptb volcanic ash may be related to paleo tethys continental arc magmatism in the kunlun area ignimbrite flare up related to rapid plate subduction during the final assemblage of the pangea super continent may have generated a volcanic winter which eventually triggered the collapse of ecosystem and ultimately mass extinction at the end of the permian the siberian traps may have been responsible for a greenhouse effect and so have been responsible for both a second pulse of the extinction event and early triassic ecological evolution

upper ordovician facies in the lac saint jean outlier quebec eastern canada palaeoenvironmental significance for late ordovician oceanography upper ordovician caradocian carbonates of eastern north america were deposited along the iapetus continental margin and record a transition from warm to cool water settings despite this margin having been within the southern hemisphere tropical belt this event has been documented from virginia usa to southern quebec canada although not previously from areas close to the palaeoequator field petrographic and major element geochemistry data have been gathered from the poorly known upper ordovician carbonate succession outcropping in the lac saint jean outlier in central quebec the succession consists of a lower siliciclastic formation tremblay overlain by three limestone formations simard shipshaw and galets and capped by shales pointe bleue shale from macro and microfaunal evidence carbonate sedimentation occurred during the late caradoc and is younger than the early to mid caradoc carbonate succession present farther south relative sea level fluctuations recorded in the sediments suggest an overall sea level rise briefly halted by a minor end caradocian sea level fall the lower limestone formation simard consists of muddy sediments with algal coral stromatoporoid boundstones green algae are abundant this unit reflects low energy sedimentation on a shallow warm water carbonate ramp colonized by a diverse chlorozoan fauna the upper limestone formation galets is typified by coarse grained bioclastic sediments punctuated by numerous phosphate rich hardgrounds with evidence for high energy shallow marine conditions faunas were dominated by crinoids and bryozoans this unit represents high energy sedimentation on a cool shallow water carbonate ramp colonized by a brynoderm faunal association between both units a deeper marine outer shelf limestone formation shipshaw was developed in the lac saint jean area a transition from warm to cool water carbonate ramps occurred in latest caradoc times and is litho and biofacies wise similar to what is documented for lower caradocian limestones present farther south upwelling of nutrient rich cool bottom oceanic waters was a probable cause for this transition

nd isotope composition and rare earth element distribution in early paleozoic biogenic apatite from baltoscandia a signature of iapetus ocean water analyses of the nd isotopic composition and ree distribution in biogenic apatite organophosphatic brachiopods and conodont elements from the cambrian and ordovician sequences of the baltic plate give new insights into the development of the southeastern segment of the continental margin bounding the iapetus ocean the nd isotope analyses show nd t of 8 0 for the cambrian indicating that the main source of the sedimentary deposition came from weathered sedimentary rocks of vendian and cambrian age the increase of nd t to 5 0 for the early ordovician indicates the appearance of a new source of radiogenic nd in the surrounding area most likely a volcanic arc along the western borderland of baltic plate from arenigian time samples of cambrian biogenic apatite show significantly lower total amounts of ree than do the ordovician samples and this is probably due to a shorter exposure to seawater before burial during ordovician sedimentary accumulation in baltoscandia these preliminary results suggest that biogenic apatite from the baltoscandian basin preserves geochemical signatures of the water masses that will be important for understanding the evolution of the iapetus ocean during the early paleozoic

early cambrian palaeogeography and the probable iberia siberia connection abstract the end of the proterozoic beginning of the cambrian is marked by some of the most dramatic events in the history of earth the fall of the ediacaran biota followed by the cambrian explosion of skeletonised bilaterians a pronounced shift in oceanic and atmospheric chemistry and rapid climatic change from snowball earth to greenhouse conditions all happened within a rather geologically short period of time these events took place against a background of the rearrangement of the prevailing supercontinent some authors view this as a sequence of individual supercontinents such as mesoproterozoic midgardia neoproterozoic rodinia and early cambrian pannotia assembled in the mesoproterozoic this supercontinent appears to have existed through the neoproterozoic into the early cambrian with periodic changes in configuration the final rearrangement took place during the precambrian cambrian transition with the cadomian and related phases of the pan african orogeny the distribution of early cambrian molluscs and other small shelly fossils ssf across all continents indicates a close geographic proximity of all major cratonic basins that is consistent with the continued existence of the supercontinent at that time subsequently rodinia experienced breakup that led to the amalgamation of gondwana separation of laurentia baltica siberia and some small terranes and the emergence of oceanic basins between them spreading oceanic basins caused a gradual geographic isolation of the faunal assemblages that were united during the vendian early cambrian

40ar 39ar and nd sr isotopic characteristics of mid ordovician north american k bentonites a test of early paleozoic laurentia gondwana interactions highly altered tephra layers k bentonites of mid ordovician age are widespread throughout the midcontinent region of the united states and the appalachian taconic foreland basin the numerous ash layers their wide areal extent and their large volume provide evidence that extensive volcanic activity accompanied taconic orogenesis the mineralogical and geochemical characteristics of these widespread ordovician tephra layers can be used to constrain the nature of their source region and thereby help test recent hypotheses that gondwana collided with laurentia in mid ordovician time initial 143nd 144nd and 87sr 86sr ratios of two k bentonites that occur traced throughout the midcontinent are 0 51183 0 51187 and 0 7101 0 7120 respectively isotopic ratios for k bentonites within the taconic basin are 0 51178 0 51201 and 0 70564 0 70905 the isotopic composition of the tephra layers and the ubiquitous presence of proterozoic zircon xenocrysts within the k bentonites is consistent with an origin by crustal anatexis the cooling history of the source is constrained by 842 996 ma 40ar 39ar apparent ages of hornblende xenocrysts contained within two taconic basin k bentonites a possible south american source region of the k bentonites is the arequipa massif of peru this region contains early paleozoic arc related rocks many of which contain mid proterozoic zircon xenocrysts and early proterozoic gneisses which were affected by grenville age granulite grade metamorphism gneisses within the adirondack highlands of the grenville province have similar end 450 ma values and hornblende cooling ages to the k bentonites consistent with this region as a source caradocian k bentonites in sweden have sr isotopic ratios similar to the north american beds if the swedish and north american k bentonites were produced from the same source then baltica and laurentia were proximal in ordovician time precluding a gondwana laurentia link if caradocian k bentonites with north american characteristics are found in western south america support for a laurentia gondwana link would be established if such k bentonites cannot be found considerable doubt would be cast on this model

late triassic tuff intervals in the ordos basin central china their depositional petrographic geochemical characteristics and regional implications abstract tuff intervals of upper triassic yanchang formation are laterally widespread in the ordos basin central china this paper focuses on magmatic origins and potential source regions of these tuff intervals through detail depositional petrographic and geochemical analyses most of the tuff intervals are well documented at the bottom of the chang7 oil reservoir unit and can be correlated laterally and certain tuff beds are reworked by turbidity current or seismic activity petrographic studies of the chang7 tuffs indicate that they are composed of crystal shards lithic shards and altered glass shards and the crystal shards include plagioclase quartz and biotite alteration of the chang7 tuffs is ubiquitous thus most of these tuffs transformed into illite smectite i s mixed layers which are identified by scanning electron microscopy sem and x ray diffraction xrd less common minerals are also detected in the chang7 tuffs such as zircon hematite siderite anatase major elements are determined by the x ray fluorescence xrf analysis the results indicate that the chang7 tuffs are enriched in k2o average 4 21 the ratio of sio2 al2o3 ranges from 1 73 to 2 85 average 2 17 and the ratio of tio2 al2o3 varies between 0 006 and 0 032 average 0 017 which imply that the chang7 tuffs originated from a felsic parental magma trace elements are determined by inductively coupled plasma mass spectrometry icp ms indicating the total rare earth element ree concentrations are variable and range from 117 46 to 466 83 ppm average 251 88 ppm ree distribution pattern of the chang7 tuffs presents a lree rightward incline with flat hree curve the value of eu ranges from 0 151 to 0 837 average 0 492 suggesting a strong to weak negative eu anomaly the chang7 tuffs show positive anomalies in rb th and u and negative anomalies in nb sr and eu on a primitive mantle normalized spidergram a preliminary analysis of the geochemical composition of the chang7 tuffs suggests a parental magma origin of rhyodacite dacite which came from volcanic arc related setting along an active continental margin combined with the chronology and geochemical studies of the synchronous tianshui rhyolite in the west qinling mountains we propose that the west qinling mountains is one of the potential source regions of these tuffs and the middle late triassic terminal closure of eastern tethys provided the arc related magma

high resolution stratigraphy of the deep water lower cloridorme formation ordovician gaspe peninsula based on k bentonite and megaturbidite correlations the ordovician cloridorme formation is a thick foreland basin turbidite succession that for several decades has served as a natural laboratory for the long distance physical tracing and study of single turbidites a robust temporal correlation of these deposits is a prerequisite for the evaluation of sedimentary processes published stratigraphic subdivisions of the lower part of the cloridorme formation have impeded rather than facilitated such studies because of errors in the physical correlation of contemporaneous deposits tracing of the deposits of exceptionally large gravity flows known as megaturbidites was previously recognized as a useful stratigraphic tool here we strengthen and revise the stratigraphic framework based on megaturbidites by considering nine widely traceable and geochemically fingerprinted tuffs k bentonites two discriminant functions incorporating the abundances of six trace elements v sm nd th er zr permit the unambiguous distinction of the lower four tuffs the

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

far field tilting of laurentia during the ordovician and constraints on the evolution of a slab under an ancient continent during a brief period of 10 to 15 million years in the middle ordovician the michigan basin departed from its bull s eye subsidence pattern and tilted toward the east opening to the appalachian basin this tilting is observed in maps of tectonic subsidence estimated for the black river and trenton formations and extends over 300 km across the michigan basin and into eastern wisconsin contours of constant tectonic subsidence rate are approximately parallel to the inferred position of the laurentian iapetus convergent margin the distance between the inferred position of the subduction zone to the limit of tilting is approximately 1000 km three alternative models for the tilting are tested two relying on the rigidity of the continental lithosphere and a third on the viscous flow generated by a subducted slab in the first elastic model we assume the edge of the elastic plate is simply loaded from above by a fold and thrust sheet volcanic pile etc this model however cannot simultaneously satisfy the space of tectonic subsidence and subsidence rate even for lithospheres which have a rigidity of 10 28 nm in the second elastic model the laurentian continental margin descends into a trench of an eastward dipping slab that is a slab descending under the iapetus ocean this process cannot generate any significant far field displacements even for extremely rigid plates and must be rejected for the third model we use finite element solutions of a negatively buoyant slab in a viscous medium with a faulted lithosphere such slabs can easily generate not only realistic trenches on the under thrusting plate but also significantly tilt the lithosphere as much as 1000 km from the plate margin the magnitude and distribution of far field displacements depend on the age length and dip angle of the slab in contrast to the elastic models penetration of a west dipping slab beneath the continent can reproduce both the extent magnitude and rate of tectonic subsidence observed in the trenton and black river formations the observed data are best fit by an old slab 140 ma which initially descended at a moderate dip 20 30 for 10 15 m y which then steepened as the slab penetrated deeper into the mantle at the position of the previous ordovician plate margin there are narrow block faulted basins which underwent rapid subsidence 10 m y before the michigan basin tilted toward the east we propose that these earlier subsidence events were caused by the initial descent of the slab under the preexisting cambro ordovician passive margin the time lag of 10 m y may be due to the time it takes the slab to penetrate the upper mantle this result is important for understanding the time evolution of mantle convection and mechanisms for the initiation of subduction

origin and palaeogeographical distribution of the viruan iron and phosphate ooids in estonia evidence from mineralogical and chemical compositions abstract the occurrence of iron ooid beds in the ordovician sequence of baltoscandia has been known for a long time in estonia the best developed ooid beds are known to occur at two levels in the lower oelandian to middle ordovician viruan boundary interval being restricted to the kunda and aseri lasnamagi stages respectively according to outcrop and subsurface data these ooid beds can be traced from hiiumaa island estonia to the st petersburg area russia in the east the geographic and stratigraphic distribution of the viruan oolite body indicates that the source material was derived from a land area located northwest to north northwest of tallinn the presence of well developed hardgrounds at the base of viruan oolite suggests that ooid formation took place during a deepening event two mineralogically different ooid groups have been identified in this study brown ooids composed of iron oxy hydroxides and apatite and light grey ooids composed primarily of carbonate fluorapatite the latter probably formed through early diagenetic alteration of the brown ooids brown ooids contain over 60 of iron oxy hydroxide and remnants of an unidentified silicate mineral the concentric lamination is composed of alternating laminae of phosphate francolite and iron oxy hydroxide the trace element abundances in the ooids are strongly influenced by the phosphate content as is the observed ree distribution pattern the chondrite normalized ree patterns are characterized by enrichment of the lighter ree and have a typical negative eu anomaly of about 0 7 shale normalized ree patterns have a hat shape similar to ree patterns found in conodonts and other biogenic apatite thin volcanic ash beds have been recorded at this level in sweden in estonia however there are no visible indications of volcanic material

gas reservoir potential of the lower ordovician beekmantown group quebec lowlands canada the beekmantown group in the quebec lowlands was deposited as part of an extensive early ordovician coastal and shallow marine complex on the eastern margin of the north american craton the beekmantown is stratigraphically equivalent to the beekmantown knox arbuckle and ellenburger rocks of the united states and is subdivided into two formations the sandstone rich theresa formation and the overlying dolomite rich beauharnois dolomites of the beekmantown provide an important exploration target in both the autochthon and the overlying thrust sheets of the canadian and u s appalachians the reservoir potential of the autochthonous beekmantown group in the quebec lowlands can be determined from seismic data well logs cuttings and petrographic analyses of depositional and diagenetic textures deposition of the beekmantown occurred along the western passive margin of the iapetus ocean by the late ordovician the passive margin had been transformed into a foreland basin faulting locally positioned upper ordovician utica source rocks against the beekmantown and contributed to forming hydrocarbon reservoirs the largest beekmantown reservoir found to date is the st flavien field with 7 75 bcf of original gas methane in place in fractured and possibly karst influenced allochthonous dolomites within a thrust fault anticline the beekmantown below the thrust sheets forms a northward thinning wedge of peritidal and subtidal deposits seven major depositional units can be distinguished in cuttings and correlated with wireline logs most of these units form northward thinning sediment wedges and were deposited on a gently dipping ramp quartz sandstones dominate updip whereas shallow subtidal pelletal to skeletal limestones dominate downdip a widespread blanket of shaly dolomite is the uppermost unit of the beekmantown but is of poor reservoir quality dolomites in the beekmantown contain vuggy moldic intercrystalline and fracture porosity early porosity formed at the top of the major depositional units in peritidal dolomites however much of this porosity was later filled by late stage calcite cement after hydrocarbon migration thus a key to finding gas reservoirs in the autochthonous beekmantown is to define ordovician paleostructures in which early and continuous entrapment of hydrocarbons prevented later cementation

bentonite geochronology marine geochemistry and the great ordovician biodiversification event gobe abstract attribution of ordovician climate forcing to explosive volcanism and the potential global importance of volcanism in ordovician biodiversification suggest the necessity of evaluating the relationships between k bentonite deposition and increasingly high resolution records of marine biogeochemical change globally ordovician strata preserve an extensive record of explosive volcanism including the widely recognized lower to middle ordovician famatina k bentonite suite in argentina and the upper ordovician millbrig deicke kinnekulle suite of north america and europe here we present high resolution id tims u pb zircon ages of k bentonites from measured sections of the san juan formation talacasto and cerro la chilca section of the argentine precordillera k bentonites from the argentine precordillera provide stratigraphically consistent i e younging upward ages that range from 473 45 0 70 ma to 469 53 0 62 ma and constrain the age of a low magnitude 2 globally recorded negative carbon isotope excursion evaluation of the timing of k bentonite deposition in the argentina precordillera relative to marine biostratigraphic and biogeochemical records provides insight into relationships between explosive volcanism and regional to global environmental change from a regional standpoint these ages provide critical direct evidence for a dapingian to earliest darriwilian age of the upper san juan formation at sampled localities these ages are consistent with carbon isotope data suggesting that the san juan formation in the region of its type section is coeval with only the base of the often correlated table head group of western newfoundland this data thus highlights the difficulties in using regional biostratigraphic data particularly within erosionally truncated or otherwise diachronous units to define the timeframe of carbon isotope chemostratigraphy new geochronological data also indicate that a discrete negative carbon isotope excursion within the san juan and table head formations is correlative to a globally recognized pre mdice negative excursion and indicates that this aspect of the marine carbon isotope record can be used as a discrete chronologic marker san juan formation bentonites however cannot be discretely correlated with observed environmentally significant changes in the middle ordovician marine geochemical records of carbon sulfur strontium or sea surface temperature these results suggest that the extent of volcanism represented by the famatina bentonite suite was insufficient to affect global surface environments and that the relationship between explosive volcanism and environmental change may not be straightforward as previously suggested

a new age model for the ordovician sandbian k bentonites in oslo norway abstract during the late ordovician large explosive volcanic eruptions deposited worldwide k bentonites including the millbrig and deicke k bentonites in north america and the kinnekulle k bentonite in scandinavia we have studied a classical locality in oslo containing one of the most complete sections of k bentonites in europe in a 53 m section of sandbian age we discovered 33 individual k bentonite beds the most notable beds being the kinnekulle and the upper grimstorp k bentonite magnetic susceptibility ms measurements on two intervals show significant periodicity peaks interpreted as milankovitch cycles and thus astronomically forced changes in sediment supply and composition these cycles fit remarkably well with both the expected milankovitch periodicities for the ordovician as well as the radiometric ages presented in this study and may represent one of the most convincing demonstrations of milankovitch cycles from the lower paleozoic so far five of the k bentonites have been dated by high precision chemical abrasion thermal ionization mass spectrometry ca tims u pb zircon geochronology where the kinnekulle k bentonite gives an age of 454 06 0 43 ma we have integrated the new data and calculated an age model showing the sedimentation rates through the section and thereby the ages of each of the 33 k bentonites using the age model we further present a new age for the sandbian katian stage boundary the section in oslo provides the highest resolution window into the upper ordovician k bentonite succession so far and helps shed more light on the chronology of one of the most intense volcanic periods of the paleozoic and the relationship with the global carbon cycle changes that followed

a revision of ordovician series and stages from the historical type area this paper is a revision of the standard chronostratigraphy of the ordovician of the historical type area in england and wales the revision is a response to the need for more precise definitions of series and stages especially for practical international correlation in some cases this has entailed moving horizons for series bases away from those in the classical sections the scheme will be used as the standard for a forthcoming revision of the correlation chart of the british ordovician system the ordovician is divided into five series tremadoc arenig llanvirn caradoc and ashgill the llanvirn is extended to include part of the classical llandeilo series which is included within it as a stage where stage subdivisions of standard series have not yet been defined as in the tremadoc they are proposed in this paper however bancroft s fine stadial divisions of the caradoc which have been criticized for their brevity and local utility are combined into four new stages which will have wider application the former caradoc subdivisions are retained as substages for the purposes of regional correlation

k bentonites a review pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the proterozoic to the tertiary this altered tephra variously called bentonite or k bentonite or tonstein depending on the degree of alteration and chemical composition is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past k bentonite and bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high mg content and that leach alkali elements smectite composition is partly controlled by parent rock chemistry studies have shown that k bentonites often display variations in layer charge and mixed layer clay ratios and that these correlate with physical properties and diagenetic history the following is a review of known k bentonite and related occurrences of altered tephra throughout the timescale from precambrian to cenozoic

origin and correlation of tuffs in the permian newcastle and wollombi coal measures nsw australia using chemical fingerprinting abstract felsic tuffs are common throughout the late permian newcastle and wollombi coal measures ncm wcm of the northern sydney basin petrographic studies reveal that they are composed of fragmented crystals of plagioclase quartz and less common degraded biotite and k feldspar together with lithic fragments and relict altered glass shards alteration during burial at t 62 78 kaolinite siderite ankerite and in extreme circumstances tonsteins dominated by i s the latter is formed by the alteration of vitric tuffs zr tio 2 and nb y ratios and rock primordial and chondrite normalised ree patterns la yb 4 05 to 11 37 indicate that the tuffs have been derived from rhyodacitic to dacitic continental arc calc alkaline magmas the accuracy of a recent lithostratigraphic correlations between the ncm and the wcm has been tested by determining the chemical composition of four stratigraphically well defined tuffs in the ncm and comparing them with those obtained from tuffs of the wcm which are thought to be stratigraphically equivalent the comparison was carried out using multivariate statistical analysis the analysis revealed that two of the tuffs awaba nobbys in the ncm could be distinguished the others mt hutton warners bay showed considerable scatter this contrasted with the tonsteins formed from the tuffs which apart from the mt hutton tuff were able to be separated in the wcm three of the tuffs nalleen unnamed tuff equivalent to the mt hutton tuff monkey place creek could be separated however the fourth unnamed tuff equivalent to the warners bay tuff exhibited some scatter treating tuffs in the wcm separately as unknowns in the discriminant model to determine possible correlatives in the ncm revealed that the nalleen tuff is equivalent to the awaba tuff in the ncm confirming the lithostratigraphic correlation the remainder of the tuffs in the wcm however show moderate to poor correlation with the proposed stratigraphically equivalents in the ncm thus the analysis shows that correlation based on lithostratigraphic grounds may be incorrect

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

phanerozoic large igneous provinces lips heatt haline euxinic acidic thermal transgression episodes and mass extinctions abstract the phanerozoic consists of three mutually exclusive climate states icehouse greenhouse and hothouse the greenhouse is the default state representing 70 of the phanerozoic it is characterized by a weak thermal mode pole driven deep water formation with little or no continental sea ice and weakly oxic cool water sinking near the poles nitrogen limitation characterizes greenhouse oceans in which ammonium may be the dominant bioavailable form of nitrogen the steady state greenhouse default must be forced to produce either of the other two states continent continent collision produces the intense chemical weathering that draws down atmospheric co2 and results in the icehouse state that comprises 20 25 of the phanerozoic icehouses have ice caps in contact with the ocean that produce icebergs and foster sea ice formation sea ice formation fills the ocean with highly oxic phosphorus limited icewater except for a thin layer of warm surface water in the tropics of the 23 most conspicuous mass extinctions of the phanerozoic including the late ediacaran 4 are associated with ice volume related regressions and 3 are problematic we suggest that the other 16 mass extinctions occurred when greenhouse climate was forced to hothouse climate large igneous provinces lips can trigger the heatt haline euxinic acidic thermal transgression mechanism which can force greenhouse climate to a hothouse climate state the rapid voluminous co2 emissions of lips cause a series of events that result in very slow but pronounced haline mode circulation mid latitude driven deep water formation that produces warm saline bottom water at very dry hot subtropical and middle latitudes this water upwells at the poles causing warm and moist equable climate hothouse development promotes deep sea anoxia that is generated in concert with co2 buildup and warming the spread of anoxia favors reduced nitrogen over nitrate and limited iron dust curtails nitrogen fixation this leads to dominance of primary productivity by primitive green algae and or cyanobacteria progression to euxinia that reaches the photic zone severely curtails primary productivity via poisoning even as availability of usable nitrogen plummets the result is eukaryotic mass extinction we suggest that the 16 phanerozoic mass extinctions referred to above resulted at least in part from the heatt process evidence for this relationship at 11 of the most severe extinctions is convincing data sets for the other 5 are strongly suggestive of heatt related mass extinction but do not yet have enough documentation to reliably characterize them as heatt episodes

occurrence and significance of silurian k bentonite beds at arisaig nova scotia eastern canada the most extensive succession of k bentonite beds known in the silurian of north america occurs at arisaig on the northern coast of nova scotia at least 40 ash beds are present in the llandoverian

reefs during the multiple crises towards the ordovician silurian boundary anticosti island eastern canada and worldwide multiple latest ordovician rawtheyan hirnantian glaciations in central africa with concomitant global sea level lowstands and cooler restricted equatorial carbonate shelves and ramps interrupted by warmer interstadial highstands had a dramatic global impact on the tropical shallow water reef ecosystem and carbonate production with the ordovician silurian boundary strata on anticosti island as a global standard for a carbonate shelf ramp setting the latest ordovician and earliest silurian reveal three reef phases ended by three extinctions the first extinction towards the end of the rawtheyan affected the last richmondian type reefs vaureal formation mill bay member the second extinction was less pronounced ending with reefs at the base of the prinsta member ellis bay formation interpreted as the top of the normalograptus extraordinarius graptolite subzone the third and most severe extinction phase capped the laframboise patch reef complex ellis bay formation at the top of the no

structural inversion and origin of a late ordovician trenton carbonate buildup evidence from the tanglewood and devils hollow members lexington limestone central kentucky usa abstract throughout east central united states lexington trenton limestones are largely late ordovician chatfieldian mid caradoc argillaceous skeletal calcarenites which are transgressive upward into late ordovician late chatfieldian edenian mid late caradoc shales and interbedded micrograined limestones in central kentucky however the same sequence is partially truncated by an unconformity and passes upward into a thicker younger edenian late caradoc regressive shoal complex of coarse calcarenites and calcirudites that is not generally typical of the lexington trenton sequence elsewhere the shoal complex is interpreted to have been a carbonate buildup and is partially bound by extant faults with basement precursors comparison of isopachous maps for pre and postunconformity buildup units with basement structural lineaments suggests that the buildup is related to reactivation and in one prominent case inversion of the basement structures the coincidence of structural reactivation inversion and buildup development with other major regional cratonic changes may reflect a reorganization of cratonic far field forces accompanying a nearly coeval reversal in the polarity of taconic subduction

simulating the dispersal of tephra from the 1991 pinatubo eruption implications for the formation of widespread ash layers abstract puff and hazmap two tephra dispersal models developed for volcanic hazard mitigation are used to simulate the climatic 1991 eruption of mt pinatubo puff simulations indicate that the majority of ash was advected away from the source at the level of the tropopause 17 km several eruptive pulses injected ash and so2 gas to higher altitudes 25 km but these pulses represent only a small fraction 1 of the total erupted material released during the simulation comparison with toms images of the so2 cloud after 71 and 93 h indicate that the so2 gas originated at an altitude of 25 km near the source and descended to an altitude of 22 km as the cloud moved across the indian ocean hazmap simulations indicate that the pinatubo tephra fall deposit in the south china sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16 18 km results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height 40 km and at a level below the calculated height of neutral buoyancy 25 km simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause in contrast the movement of the so2 cloud occurred at higher levels along slightly different trajectories and may have resulted from gas particle segregations that took place during intrusion of the pinatubo umbrella cloud as it moved away from source

ordovician k bentonites in the upper plate active margin of western gondwana famatina ranges stratigraphic and palaeogeographic significance abstract numerous thin bedded tabular pale yellowish clay bands are interlayered with black shales in a biostratigraphically constrained early ordovician volcano sedimentary succession at famatina western argentina this region was part of a fairly continuous upper plate convergent volcanic chain that fringed western gondwana mineralogy on both clay and non clay fractions whole rock chemistry and field observations on these distinctive event beds indicate that they originated as relatively coarse fallout tephras altered first into bentonites and later through burial metamorphism into k bentonites metabentonites these tephras were deposited as single crystals and glassy dust or pumiceous fragments in a restricted subtidal environment the region of famatina has previously been inferred as the source of abundant distal k bentonites recorded in the adjacent lower plate allochthonous precordillera terrane however these k bentonites within the proximal arc site were unknown and rather unexpected since they are generally better preserved like distal deposits associated either with central vent plinian ultraplinian eruptions or with accompanying co ignimbrite ash clouds their chemistry and comparison with those k bentonites in the precordillera allow tracing an evolution from volcanic arcs into continental crust k bentonites described in this paper are much older than those recorded in the adjacent precordillera terrane and seem to be associated with a first eruptive period along western gondwana that has no counterpart in the argentine precordillera suggesting a significant longitudinal separation between these two regions by the early ordovician

astronomically forced cyclicity in the upper ordovician and u pb ages of interlayered tephra oslo region norway abstract the late ordovician world experienced a series of huge volcanic eruptions recorded as the big deicke millbrig and kinnekulle bentonites together with numerous thinner beds the kinnekulle event can be traced across northwestern europe u pb zircon ages are here presented of both the kinnekulle k bentonite and the uppermost recorded tephra layer in the upper ordovician of the oslo region norway the tephras are located in the upper part of the arnestad formation sandbian south of oslo and gave ages of 454 52 0 50 ma the kinnekulle k bentonite and 453 91 0 37 ma the upper grimstorp k bentonite the dated tephras are separated by a 7 m thick shale succession with subordinate nodular limestone beds although the two u pb ages overlap within errors statistical considerations indicate a most likely time interval of about 600 kyr between the tephras high resolution magnetic susceptibility logging in the same section shows cycles that likely represent changes in sediment supply in response to astronomical forcing spectral analysis shows the presence of long 400 kyr and short 100 kyr eccentricity bands and obliquity components in the 30 kyr band precessional cycles are not detected based on this method it is possible to estimate a time interval of 766 kyr between the two tephra events and the radiometric dating therefore does not exclude a milankovitch interpretation of the cycles this opens new possibilities for understanding the evolution of one of the world s best preserved ordovician marine systems

dispersal in the ordovician speciation patterns and paleobiogeographic analyses of brachiopods and trilobites abstract the middle to late ordovician was a time of profound biotic diversification paleoecological change and major climate shifts yet studies examining speciation mechanisms and drivers of dispersal are lacking in this study we use bayesian phylogenetics and maximum likelihood analyses in the r package biogeobears to reanalyze ten published data matrices of brachiopods and trilobites and produce time calibrated species level phylogenetic hypotheses with estimated biogeographic histories recovered speciation and biogeographic patterns were examined within four time slices to test for changes in speciation type across major tectonic and paleoclimatic events statistical model comparison showed that biogeographic models that incorporate long distance founder event speciation best fit the data for most clades which indicates that this speciation type along with vicariance and traditional dispersal were important for paleozoic benthic invertebrates speciation by dispersal was common throughout the study interval but notably elevated during times of climate change vicariance events occurred synchronously among brachiopod and trilobite lineages indicating that tectonic climate and ocean processes affected benthic and planktotrophic larvae similarly middle ordovician inter oceanic dispersal in trilobite lineages was influenced by surface currents along with volcanic island arcs acting as stepping stones between areas indicating most trilobite species may have had a planktic protaspid stage these factors also influenced brachiopod dispersal across oceanic basins among laurentia avalonia and baltica these results indicate that gyre spin up and intensification of surface currents were important dispersal mechanisms during this time within laurentia surface currents hurricane tracks and upwelling zones controlled dispersal among basins increased speciation during the middle ordovician provides support for climatic facilitators for diversification during the great ordovician biodiversification event similarly increased speciation in laurentian brachiopod lineages during the hirnantian indicates that some taxa experienced speciation in relation to major climate changes overall this study demonstrates the substantial power and potential for likelihood based methods for elucidating biotic patterns during the history of life

paleogeography an earth systems perspective abstract paleogeography is the subdiscipline within the geosciences that studies the physical and biological geography of the geologic past including the configuration and latitudinal distribution of continental landmasses their topographic relief climate and biota in recent years the study of paleogeography has evolved from simple continental reconstructions to more sophisticated paleocontinental maps that form the basis of paleoclimactic modeling by incorporating elements of oceanic and atmospheric circulation and their interactive relationship with landmasses a broad range of geologic elements can be reconstructed through paleogeographic studies such as mountain belts climate zones emergent land and continental configurations many of which impact directly on understanding the driving forces behind geochemical signals contained in the rock record which are used as a proxy for biogeochemical processes in earth history paleogeography is thus an essential component of an earth systems approach to interpreting the geologic past paleoconfigurations of continents and ocean basins are determined by a variety of techniques paleomagnetic studies are the most common but additional constraints are provided by faunal provincialism biogeography and abiotic continental linkages such as pre break up magmatism recent developments within paleogeography include the use of various forms of climate models to analyze the relationships between global continental reconstructions and the rock record of past climates in order to test more directly feedback between paleogeography and paleoclimate development of an extended gis database for the geological past promises new insights into quantitative analysis of earth system evolution

origin alteration and geochemical correlation of late permian airfall tuffs in coal measures sydney basin australia abstract the late permian illawarra coal measures of the southern sydney basin contain tuffs derived from altered pyroclastic airfall material these units represent ideal stratigraphic time planes because as the products of airfall volcanic ashes they are distributed over wide areas in short periods of geological time geochemical correlation of the tuffs of the illawarra coal measures with similar units in the stratigraphically equivalent newcastle coal measures offers a unique opportunity to understand the temporal and spatial relations during deposition of coal bearing units in the sydney basin thirty five core samples comprising 12 samples from each of the burragorang and farmborough claystone members and 11 samples from the huntley claystone member were analysed for major and trace elements by x ray fluorescence and instrumental neutron activation and the results were subjected to stepwise discriminant analysis the elements that served as the best discriminators between tuffs were in order of atomic number ti v sn hf and th samples for basin wide correlation were taken from thick interseam tuffs of the newcastle coal measures and treated as unknowns in the discriminant model the burragorang claystone member shows a strong geochemical correlation with the awaba tuff and the farmborough claystone member correlates with the warners bay tuff the huntley claystone member however has proved difficult to correlate with the nobby s tuff the proposed stratigraphic equivalent in the newcastle coal measures petrographic data for tuffs of the illawarra coal measures indicate that they were deposited by airfall mechanisms and consist mainly of kaolinite and mixed layer illite smectite clays quartz and plagioclase crystal fragments lithic fragments and secondary calcite and siderite zr tio2 and nb y ratios tectonic discrimination diagrams and chondrite normalised ree patterns la yb 3 3 to 11 9 are identical to those for tuffs of the newcastle coal measures and are interpreted as being derived from magma that was calc alkaline rhyodacitic to rhyolitic in composition and derived from a continental volcanic arc tectonic setting the source of the tuffs is interpreted to be an active volcanic arc to the east of the present coastline currarong orogen but a lack of evidence from below the continental shelf makes it difficult to make concrete conclusions on its likely extent and composition

late ordovician cooling event evidence from the siberian craton abstract the upper ordovician of the tungus basin on the siberian platform is represented by cool water carbonates onset of cool water conditions starts at the darriwilian and it is associated with widespread phosphatization especially in the volginian kirensko kudrinian and chertovskian regional stages mid darriwilian lower sandbian the shift from tropical type to temperate type carbonates was preceded by destruction of the warm water carbonate factory and a large input of siliciclastic material baykit sandstone during the vikhorevian and mukteian regional stages lower to mid darriwilian wide distribution of temperate type carbonates across the siberian platform located in the low latitudes in the ordovician time can be explained by upwelling of cold oceanic waters and their penetration into epicontinental seas the same situation was recorded for the upper ordovician of the north american platform newly discovered k bentonite beds in the uppermost sandbian katian of the tungus basin are also associated with cool water carbonates the k bentonite beds point to extensive volcanism on or near the western in present day orientation margin of the siberian craton in the late ordovician time timing of the volcanism is surprisingly close to the period of volcanic activity of the taconic arc near the margin of laurentia both upwelling and volcanism seem to be activated by the same plate tectonic reorganization and both these processes seem to play an important role in a global later ordovician cooling terminated by the hirnantian glaciation

lower ordovician iron ooids and associated oolitic clays in russia and estonia a clue to the origin of iron oolites abstract on the baltic platform a lower llanvirn ordovician iron oolite can be traced for a distance of 1200 km from norway to the east of lake ladoga in russia this oolite is usually thin seldom exceeding 0 5 m and is dominated by goethite limonite type ooids the easternmost part of the oolite from tallinn to ladoga is examined here the oolitic limestone is intercalated with oolitic clay beds the mineralogical chemical and isotopic composition and other indicators point to volcanic ash being the source for the clay similarities in ree distribution patterns and immobile element contents between ooids and the oolitic clay suggest that the ooids were also formed from volcanic ash

upper ordovician δ13corg chemostratigraphy k bentonite stratigraphy and biostratigraphy in southern scandinavia a reappraisal a pioneer 13corg study through the upper sandbian and katian upper ordovician succession in the rostanga 1 drill core in the classical geological outcrop area at rostanga in southernmost sweden produced a wealth of new carbon isotope data which are useful for local and regional correlations among the upper ordovician positive 13c excursions the guttenberg gice waynesville saunja whitewater moe paroveja and hirnantian hice isotopic carbon excursions are recognized but the kope rakvere 13c excursion is missing suggesting a stratigraphic gap all these isotopic excursions are tied closely to biostratigraphy especially graptolite biostratigraphy and in the case of the waynesville saunja and whitewater moe excursions for the first time anywhere in the world the rostanga gice 13corg curve from the upper sularp shale shows a striking similarity to that of the katian gssp in oklahoma suggesting the potential of trans atlantic correlation based on a projection from the katian gssp the previously poorly constrained position of the base of the katian in southern sweden appears to be in the uppermost sularp shale in strata of the upper diplograptus foliaceus zone previous interpretations of the relations between k bentonite successions in southern scandinavia are somewhat revised and the kinnekulle k bentonite is recognized for the first time in scania based on new radiometric dates this very prominent and widespread ash bed appears to be slightly older than the deicke and millbrig k bentonites in eastern north america

discovery of the ordovician millbrig k bentonite bed in the trenton group of new york state implications for regional correlation and sequence stratigraphy in eastern north america the ordovician chatfieldian millbrig k bentonite bed is a key stratigraphic marker horizon that is regionally synchronous over much of eastern and central north america this prominent marker is an independent source of correlation among the major chronostratigraphic and sequence stratigraphic units in this region the general stratigraphic position of the millbrig k bentonite has suggested to some authors that it is identical with the hounsfield k bentonite at dexter new york the traditional type area of the middle ordovician or mohawkian series in north america but previously available geochemical and biostratigraphical evidence has been insufficient to confirm this correlation analyses of apatites and melt inclusions in quartz phenocrysts from the millbrig k bentonite at eight localities in kentucky missouri virginia and pennsylvania and the hounsfield k bentonite at its type locality at dexter new york using high precision electron microprobe analysis techniques shows that the millbrig and the hounsfield have identical apatite and melt inclusion chemistry indicating a geochemical correlation this correlation is supported by conodont biostratigraphy and 13c isotope chemostratigraphy the new data demonstrate that the millbrig k bentonite and therefore the base of the chatfieldian stage of the north american mohawkian series by definition lies very close to the base of the traditional rocklandian stage of new york furthermore the millbrig k bentonite bed lies in close proximity to the base of the taconic supersequence over much of the midcontinent region and in particular lies just below the m5 sequence boundary recognized in kentucky and tennessee our results permit extension of the chatfieldian sequences into new york state and southern ontario and contributes to the resolution of the long standing uncertainty about the position of the base of the trenton group in ontario furthermore viewed in a broad context of regional stratigraphic relations we conclude that our results suggest that the persistent correlation difficulties reflect diachronous effects of widespread changes in oceanographic circulation patterns that emerged during taconic orogeny finally regional differences in the timing and character of sequence bounding surfaces and facies similarities summarized here suggest that the causes of relative sea level change during the chatfieldian may have been primarily tectonoeustatic mechanisms

trace element geochemistry of altered volcanic ash layers tonsteins in late permian coal bearing formations of eastern yunnan and western guizhou provinces china trace element compositions were determined by instrumental neutron activation analysis inaa in 30 samples of synsedimentary volcanic ash derived tonsteins and detrital claystones from coal seams within the late permian coal bearing formation of eastern yunnan and western guizhou provinces china the characteristics of trace element geochemistry in the tonsteins can be distinguished from those of detrital claystones because of the former s unique volcanic ash origin the detrital claystones are characterized by their relatively high content of v ti sc cr co and ni relatively low content of th and u th u ratio and small negative eu anomaly eu eu 0 63 0 93 overall these trace element characteristics are consistent with a mafic source similar to the composition of basalt rocks in the erosional region on the western edge of the study area in contrast the tonsteins are low in v ti sc cr co and ni contents and have a high th u ratio with a distinct negative eu anomaly eu eu normally in the range of 0 2 0 4 consistent with a silicic magmatic source within the group of tonsteins those from the lower section p2 1 of the coal bearing formation are relatively high in ti zr hf nb ta and rare earth elements ree as compared to those from the middle and upper sections p2 2 3 in trace element discrimination diagrams scatter plots of hf ta ti ta ti v hf sc lu hf and lu th tonsteins from the p2 1 horizon always fall in isolated distribution areas separate from the tonsteins of the p2 2 3 horizon these results suggest that the source materials of tonsteins from the two separate horizons were probably derived from volcanic ash falls of two distinctly different natures based on a comparison of the concentrations and assemblages of trace elements between various magmatic rocks the source materials of tonsteins from p2 1 horizon were mostly composed of calc alkalic silica poor volcanic ash similar to rhyodacitic magma whereas those from p2 2 3 were apparently more siliceous and k rich rhyolitic magma thus tonsteins from the two different horizons are characterized by unique geochemical properties which remain constant over a wide lateral extent integration of trace elemental compositions with mineralogical and textural observations makes possible the establishment of tonstein stratigraphy thus facilitating more precise and reliable coal seam correlations

gigantic volcanic eruptions and climatic change in the early eocene 23 layers of altered volcanic ash bentonites originating from the north atlantic igneous province have been recorded in early eocene deposits of the austrian alps about 1 900 km away from the source area the austrian bentonites are distal equivalents of the main ash phase in denmark and the north sea basin we have calculated the total eruption volume of this series as 21 000 km3 which occurred in 600 000 years the most powerful single eruption of this series took place 54 0 million years ago ma and ejected ca 1 200 km3 of ash material which makes it one of the largest basaltic pyroclastic eruptions in geological history the clustering of eruptions must have significantly affected the incoming solar radiation in the early eocene by the continuous production of stratospheric dust and aerosol clouds this hypothesis is corroborated by oxygen isotope values which indicate a global decrease of sea surface temperatures between 1 and 2 c during this major phase of explosive volcanism

transient landscape evolution of basement cored uplifts example of the kyrgyz range tian shan 1 the kyrgyz range located on the northern margin of the western tian shan illustrates long term 106 107 year transient landscape evolution in response to an active basement cored rock uplift eastward propagation of range uplift has progressively exposed resistant bedrock capped by a tilted formerly planar pre cenozoic unconformity we develop an approximate stream power based analytic model of transient river profile incision into progressively exposed and resistant bedrock to explore the patterns of channel development into the unconformity surface this analysis shows that the unconformity can be preserved as a geomorphic marker defined by upland headwaters and interfluvial areas though channels are not at equilibrium with rock uplift prominent knickpoints are not predicted to develop on main stem channels however knickpoints are predicted to develop on tributaries upstream of their junctions with the trunk stream because of differential erosion rates initial channel slope and transient channel form are both sensitive to the n value of the stream power model and could prove useful for calibration of n from field data accumulation of catchment area a proxy for discharge into larger catchments develops a positive feedback where larger drainages with higher stream power at a given slope undermine and capture adjacent drainage area a simple model of competitive fluvial incision illustrates the role of tributary junction position in maximizing stream power expended upon the trunk stream examples from the kyrgyz range illustrate the effects of tributary junction position on fluvial relief and we propose that adjustments to the tributary network through stream capture are ongoing within this landscape even after several kilometers of exhumation

large magnitude middle ordovician volcanic ash falls in north america and europe dimensions emplacement and post emplacement characteristics abstract middle ordovician k bentonites represent some of the largest known fallout ash deposits in the phanerozoic era they cover minimally 2 2 106 km2 in eastern north america and 6 9 105 km2 in northwestern europe and represents the coeval accumulation of plinian and co ignimbrite ash on both laurentia and baltica during the closure of the iapetus ocean the three most widespread beds are the deicke and millbrig k bentonites in north america and the kinnekulle k bentonite in northwestern europe the vents were located near the laurentian margin of iapetus on an arc or microplate undergoing collision with laurentia the volume of ash preserved in the stratigraphie record converted to dense rock equivalent dre of silicic magma is minimally estimated to be 943 km3 for the deicke 1509 km3 for the millbrig and 972 km3 for the kinnekulle the millbrig and kinnekulle beds are coeval and possibly equivalent yielding a combined dre volume of nearly 2500 km3 some unknown but probably large amount of additional ash fell into oceanic regions of the iapetus but these areas became subducted and the ash is not preserved in the geologic record the symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of ordovician paleogeography based on grain size measurements and thickness area 1 2 plots we conclude the three beds were each formed from co ignimbrite or possibly phreatoplinian eruption columns analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4 dissolved water at the time of the eruption this water provided the explosive energy during the initial gas thrust phase the implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles forming a deposit composed largely of single crystals and glassy dust conversion of the ash to k bentonite resulted in a mass loss of approximately 35 mostly in the form of si with lesser amounts of na and k

geochemical and tight reservoir characteristics of sedimentary organic matter bearing tuff from the permian tiaohu formation in the santanghu basin northwest china abstract tuff reservoirs have been found in petroliferous basins but have not been sufficiently studied the discovery of the permian tiaohu formation reservoir in the santanghu basin in northwest china has offered an excellent opportunity to further enhance our knowledge of the tuff s geochemical and reservoir characteristics the tuff reservoir has the peculiar property of being a sedimentary organic matter bearing tuff in this study an integrated analysis of the organic geochemistry elemental and mineral compositions quartz crystallinity pore types and tight reservoir characteristics was conducted based on samples from the 20 30 m thick tuffs the extracted tuff samples have total organic carbon toc values of 0 5 1 0 wt total hydrocarbon yield values of 2 6 mg g and hydrogen index values of 20 336 mg hc g toc the organic matter consists predominantly of type iii and ii 2 kerogens and the temperature of the maximum yield of pyrolysate varies from 420 to 450 c which reflects the oil generating capacity of the rock the physical properties of the tuff are characterised by high porosity varying from 10 to 25 and low permeability the air permeability mainly ranges from 0 01 to 0 50 md the devitrification of vitreous textures within the tuff primarily dictates the reservoir s characteristics i e the greater the degree of devitrification the higher the porosity of the reservoir vitric tuffs have reservoirs with higher porosity than crystal vitric tuffs and the degree of devitrification is generally controlled by the burial depth temperature and organic matter organic acid content vitric tuffs with high porosity and permeability are usually located far from the crater and within the central lower part of the reservoir this research may influence geologists to pay more attention to the exploration value of this special type of reservoir in other regions

new late middle to early late ordovician u pb zircon ages of extension related felsic volcanic rocks in the eastern pyrenees ne iberia tectonic implications pre variscan basement rocks from the pyrenees provide evidence of several magmatic episodes with complex geodynamic histories from late neoproterozoic to palaeozoic times one of the most significant episodes consisting of several granitic and granodioritic bodies and volcanic rocks mostly pyroclastic in nature dates from the late ordovician period in the eastern pyrenees this magmatism is well represented in the ribes de freser and nuria areas here the nuria orthogneiss and the ribes granophyre both dated at c 457 460 ma seem to form a calc alkaline plutonic suite emplaced at different crustal levels the presence of numerous pyroclastic deposits and lavas interbedded with upper ordovician sandbian lower katian formerly caradoc sediments intruded by the ribes granophyre suggests that this magmatic episode also generated significant volcanism moreover the area hosts an important volume of rhyolitic ignimbrites and andesitic lavas affected by alpine deformation these volcanic rocks were previously attributed to late variscan volcanism extensively represented in other areas of the pyrenees here we present the first five laser ablation u pb zircon dates for this ignimbritic succession and two new ages for the ribes granophyre the ages of the ignimbrites overlapping within error are all 460 ma suggesting a genetic relationship between the plutonic and volcanic rocks and indicating that the sandbian katian magmatism is much more voluminous than reported in previous studies and possibly includes mega eruptions linked to the formation of collapse calderas

late vendian early palaeozoic tectonic evolution of the baltic basin regional tectonic implications from subsidence analysis abstract subsidence analysis was performed on 43 boreholes penetrating the upper vendian lower palaeozoic sedimentary succession of the baltic basin the results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development tectonic subsidence patterns are consistent throughout the basin for the time period studied an extensional tectonic subsidence event possibly of two phases is indicated from the late vendian to the beginning of the middle cambrian this event is seen in the southwestern part of the baltic basin peri tornquist zone until the earliest cambrian after which it is also observed in the sw ne trending baltic depression part of the basin basin development during this time is interpreted as recording the latest stages of break up of the precambrian super continent rodinia and ultimately the formation of the tornquist sea the late middle cambrian to middle ordovician tectonic subsidence pattern of the baltic basin is characteristic of post rift thermal subsidence of the newly formed passive continental margin of baltica developed along its southwestern edge a gradual increase in subsidence rate is observed from the middle late ordovician and throughout the silurian particularly for ludlow and pridoli times creating subsidence curves with convex shapes typical of foreland basin development the rate of late silurian tectonic subsidence increases significantly towards the southwest margin of the baltic basin adjacent to the present location of the north german polish caledonides the baltic basin therefore appears to have developed primarily as a flexural foreland basin during silurian oblique collision of baltica and eastern avalonia a foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the late silurian compressional deformation structures of early devonian lochkovian age are seen in seismic sections in the central part of the baltic basin lithuania these together with a change in subsidence pattern mark the end of the caledonian stage of basin development of the baltic basin

the geochemistry of the sedimentary rocks from the huadi no 1 well in the wufeng longmaxi formations upper ordovician lower silurian south china with implications for paleoweathering provenance tectonic setting and paleoclimate abstract the late ordovician early silurian transition was a critical interval in earth s history marked by mass extinction the gondwana glaciation and volcanic events here we present mineralogical and geochemical characteristics across the ordovician silurian boundary wufeng guanyinqiao and longmaxi formations from the huadi no 1 well in the sichuan basin in order to explore the paleoweathering provenance tectonic setting and paleoclimatic conditions of the source area thin sections xrd x ray diffraction data and major element results reveal that the rocks were abundant in clay minerals quartz minor feldspar pyrite and carbonate minerals the samples have similar chondrite normalized ree patterns to those of paas which are characterized by slight lree enrichments and accompanied by flat hree trends with weakly negative eu anomalies paleoweathering indices cia and pia suggest that the sediments experienced weak to moderate chemical weathering in the source area cia values in the lower wufeng formation range from 67 11 to 72 17 indicating a warm and humid climate the cia values in the upper wufeng formation decrease from 70 35 to 66 15 and the cia values in the guanyinqiao and lower longmaxi formation are persistently low 56 64 62 87 suggesting a cold and dry climate which is consistent with the widespread presence of glaciation in the hirnantian period further upward in the upper longmaxi formation the cia returns to a high value 65 59 which may reflect that the climate started getting warm and glaciation had finished fluctuations in cia values reflect that the climate shifted from warm to cold and then to warm again during the end ordovician early silurian time period the a cn k triangular diagram and sio2 al2o3 tio2 tio2 zr and la th hf bivariate diagrams indicate that the provenance of the rocks from the three formations was primarily felsic igneous rocks discriminant function diagrams and la th sc th co zr 10 and th sc zr 10 triangular diagrams show that the clastic sediments of the wufeng longmaxi formations were derived most likely from an active continental margin finally across the late ordovician early silurian tectonic activity and the end of the ordovician glaciation dominantly controlled the deposition of black shales and siltstones in the wufeng longmaxi formations of south china

the ordovician chitinozoan biodiversification and its leading factors abstract regional and global dataset now available for almost all of the ordovician fossil groups have led to a search for the causes controlling the major biodiversification events that occurred during the ordovician period a review of the physico chemical state of the ordovician world is presented with an emphasis on the regional or global changes contemporaneous with the origination and extinction events that could have influenced the chitinozoan diversity this study focuses on the chitinozoans because these enigmatic organic walled microfossils are an important component of the ordovician palaeoplankton and provide one of the best documented dataset the intrinsic factors that initiated the ordovician biodiversification of this group are not discussed because the chitinozoans are regarded as reproduction stages of cryptic chitinozoan animals whose biological characteristics are speculative at best this study has two main goals i the evaluation of the impact of regional and global physico chemical events on chitinozoan diversity ii the comparison of the biodiversification patterns of the chitinozoans with other selected benthic and pelagic ordovician fossil groups the chitinozoan diversification was progressive and showed similar patterns in laurentia baltica and northern gondwana from the tremadocian to the late darriwilian when the group reached its acme in baltica and northern gondwana from the middle upper ordovician boundary onward the biodiversification pattern documented in laurentia diverged drastically from the two other regions in the late ordovician the contribution of the laurentian chitinozoans to the global curve was high suggesting major faunal inputs from the two other regions which significantly lowered the endemic character prevalent in the early middle ordovician in the late ordovician large anti clockwise oceanic currents developed as the result of a thermohaline circulation this marine circulation was likely to have been driven by a global cooling concomitant with a major palaeogeographic reorganisation of the southern hemisphere these oceanic climatic changes intervened in the breakdown of the existing chitinozoan endemism globally the chitinozoan biodiversity was not much higher in the mid late ordovician than in the late darriwilian diversity peak the most obvious feature is the progressive decrease in diversity during the late ordovician long before the hirnantian glaciation the influences of cosmic and volcanic parameters are excluded as they appear to have had too low impact on the late ordovician decrease of chitinozoan diversity correlation is noticed between some diversification events and sea level changes at least on a regional scale more globally however a climatic control is favoured a durable greenhouse environment gave an efficient support to the diversification conversely the onset of an icehouse environment in the early late ordovician onward culminating with the hirnantian glaciation is interpreted as a limiting factor for the chitinozoan diversification the reasons behind the onset of this icehouse episode are not fully understood but they appear to be linked to changes in the palaeogeography e g progressive closure of the iapetus northward drift of avalonia and its docking with baltica and in the carbon cycle e g high organic carbon burial with a lowering of the p co 2

organic matter accumulation of the wufeng longmaxi shales in southern sichuan basin evidence and insight from volcanism abstract the wufeng longmaxi formations wf lm fms in the sichuan basin are sets of graptolite bearing shale strata containing high frequency bentonite layers in order to investigate the influence of volcanism on the organic matter om accumulation in shales detailed observations of several typical profiles and wells were performed also analyses including total organic carbon toc contents and mineral compositions were conducted on 126 shale and 5 bentonite core samples major and trace element analyses were conducted on the selected 25 shale and 5 bentonite samples in the wf lm fms of well lu a results show that wf and lm1 4 members are the bentonites concentrated sections while lm5 6 are the bentonites relatively concentrated sections and no bentonite is found in lm7 9 members the high potassium k content of bentonites have the typical characteristics of intermediate acid calcium alkaline volcanic rocks such as andesite and dacite which probably originated from intraplate collision or volcanic arc structural backgrounds related to the collision belt of the cathaysia block to the yangtze block productivity indicators p content excess si p al cu al and ni al in the wf and lm1 3 members are all significantly higher than those in the lingxiang lx and lm4 5 members the higher biological productivity during the ordovician silurian o s transition was attributed to the massive and intensive volcanism the fertilization by volcanic ash brought abundant biologically relevant nutrient elements n p si fe and al into the ocean thus triggering the phytoplankton bloom and enhancing primary biological productivity all the redox condition indexes moef uef nief vef mo toc corg p and u th positively correlate with toc content during the o s transition the intensive and massive volcanic eruptions triggered global cooling sea level changes late ordovician mass extinction and primary productivity increases thus promoting anoxic and possibly intermittently euxinic bottom conditions in addition the deposited bentonite layers had a strong capacity for oxygen isolation volcanism should be considered when modeling the om accumulation of the wf lm shales

quartz crystal size distribution of the ordovician millbrig k bentonite abstract crystal size distribution csd analysis has been applied to quartz crystals of the ordovician millbrig k bentonite which represents one of the largest known fallout ash deposits in the phanerozoic era to establish crystal growth histories and conditions in the magma chamber prior to eruption specific csds were examined for crystal growth conditions all millbrig samples exhibit lognormal concave down shapes in agreement with previously reported csds on large silicic systems but in contrast to more mafic systems characterized by linear csds concave down csds are possibly an intrinsic property of phenocryst growth in large volume silicic magmas as documented in the literature bindeman i n 2003 crystal sizes in evolving silicic magma chambers geology 31 367 370 quartz crystals of the millbrig must have been produced by surface controlled size dependent growth with a minor ostwald ripening component which could have easily changed to supply controlled growth due to a limited supply of nutrients as the crystals grew larger csds were also used to examine the possible origin of multiple ash beds in the millbrig these ash beds appear to be a product of a series of separate eruptions that represent separate magma layers or batches each with slightly different crystal growth conditions

high resolution sequence stratigraphy of a mixed carbonate siliciclastic cratonic ramp upper ordovician kentucky ohio usa insights into the relative influence of eustasy and tectonics through analysis of facies gradients abstract detailed facies analysis and event stratigraphy of an upper ordovician rocklandian edenian cratonic ramp succession in eastern north america yields insights into eustatically driven sequence architecture and localized tectonic instability seven predominantly subtidal mixed carbonate siliciclastic depositional sequences 3rd order are identified and correlated across the length of a 275 km ramp to basin profile within the larger depositional sequences 3rd order at least two smaller orders 4th and 5th of cyclicity are recognizable three systems tracts occur within each sequence transgressive tst highstand hst regressive rst and are considered in terms of their component parasequences 5th order generally tsts are composed of skeletal grainstone rudstone facies hsts are dominated by shaly nodular wacke packstone facies and rsts are mostly calcarenite facies systems tracts sequence boundaries and their correlative conformities maximum flooding surfaces and forced regression surfaces were traced from shallow shelf to basinal settings this high resolution framework also provides insight into the timing of tectonic fluctuations on this cratonic ramp during the taconic orogeny and documents the relative influence of tectonism on lateral facies distributions and eustatically derived cyclicity

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

upper ordovician and silurian ash beds in the holy cross mountains poland preservation in mudrock facies and relation to atmospheric circulation in the southern hemisphere numerous volcanic ash beds k bentonite are recognized globally in ordovician to early devonian sedimentary successions and are important for regional stratigraphic correlations relatively little however is understood of the origin and transport of these ash deposits here we examine ash beds preserved in the upper ordovician and silurian dark shales and mudstones of the holy cross mountains poland and discuss their origin in the context of contemporaneous tectonic configurations the ordovician ash beds occur in the upper sandbian and katian mudrock succession whereas the silurian k bentonites form prominent beds in the upper wenlock and lower ludlow graptolite shales their various thickness and irregular distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors preventing physical and biological reworking and mixing of the volcanic ash the upper ordovician ash beds in the holy cross mountains as well as the baltoscandian k bentonites appear to be accumulated from pyroclastic material delivered by westerlies of the southern hemisphere from the avalonian volcanoes in the silurian the distribution of pyroclastic material was controlled by se trade winds and therefore the source area for the wenlock and ludlow k bentonites in the holy cross mountains appears to have been an active subduction zone developed close to the southeastern side of baltica however the delivery of volcanic ash to the central part of baltica might have been strongly affected by climatically driven latitudinal migration of the intertropical convergence zone

factors controlling organic matter accumulation in the upper ordovician lower silurian organic rich shale on the northeast margin of the upper yangtze platform evidence from petrographic and geochemical proxies abstract organic rich shales are widespread in the upper ordovician lower silurian wufeng longmaxi formations on the northeastern margin of the upper yangtze platform and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in china however organic matter accumulation in these shales needs further research especially in the areas which experienced an extensive upwelling and volcanism here this work integrates detailed petrographic and geochemical proxies namely redox sensitive trace elements rstes and ratios of si al ti al from two ordovician silurian sections on the northeast margin of upper yangtze platform in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the ordovician silurian transition data of si al and ti al ratios indicate that the deposition of black gray siliceous mudstones in the lower wufeng formation commenced with a transgression triggered by the boda warming event which led to a reduced detrital input based on rstes the black carbonaceous shales of the upper wufeng formation are organic rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation abundant nutrients supplied by the post glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column which lead to high concentrations of organic matter in lower longmaxi formation although the water column conditions were not conducive to the accumulation of organic matter it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle longmaxi formation the rstes especially cd mo co mn reveal that the organic matter accumulation of the wufeng longmaxi formation during ordovician silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input

was there a super eruption on the gondwanan coast 477 ma ago precise zircon and monazite id tims u pb dating of three lower ordovician altered ash fall tuff beds k bentonites in the cantabrian zone of nw iberia yielded coeval ages together with an equivalent previously studied sample 477 5 1 gutierrez alonso et al 2007 of 477 1 3 ma 477 2 1 1 ma and 477 3 1 ma with a pooled concordia age all analyses in the four samples of 477 2 0 74 ma a conservative estimation of the volume and mass of the studied k bentonite beds using exclusively the cz data yields a volume for the preserved deposits of ca 37 5 km3 volcanic explosivity index vei 6 colossal when considering other putative equivalent beds in iberia and neighboring realms i e armorica sardinia the volume of ejecta associated to this event would make it reach the supervolcanic apocalyptic status vei 8 1000 km3 at variance with most known cases of this kind of gigantic eruption events geological observations indicate that the studied magmatic event was related to continental margin extension and thinning and not to plate convergence we speculate that a geochronologically equivalent large caldera event recognized in the geological record of nw iberia could be ground zero of this super eruption

the tornquist sea and baltica avalonia docking early ordovician late arenig limestones from the sw margin of baltica scania bomholm have multicomponent magnetic signatures but high unblocking components predating folding and the corresponding palaeomagnetic pole latitude 19degreesn longitude 051degreese compares well with arenig reference poles from baltica collectively the arenig poles demonstrate a midsoutherly latitudinal position for baltica then separated from avalonia by the tomquist sea tornquist sea closure and the baltica avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for avalonia and baltica during the mid late ordovician by the caradoc avalonia had drifted to palaeolatitudes compatible with those of sw baltica and subduction beneath eastern avalonia was taking place we propose that explosive vents associated with this subduction and related to andean type magmatism in avalonia were the source for the gigantic mid caradoc c 455 ma ash fall in baltica i e the kinnekulle bentonite avalonia was located south of the subtropical high during most of the ordovician and this would have provided an optimum palaeoposition to supply baltica with large ash falls governed by westerly winds in scama we observe a persistent palaeomagnetic overprint of late ordovician ashgill age pole latitude 4degreess longitude 012degreese the remagnetisation was probably spurred by tectonic derived fluids since burial alone is inadequate to explain this remagnetisation event this is the first record of a late ordovician event in scania but it is comparable with the shelveian event in avalonia low grade metamorphism in the north sea basement of ne germany 440 450 ma and sheds new light on the baltica avalonia docking c 2002 elsevier science b v all rights reserved less

biotic response to explosive volcanism ostracod recovery after ordovician ash falls abstract the impact of two ordovician ash falls of different intensities was studied in order to determine the recovery patterns of benthic ostracods the studied sections are of sandbian age in nw estonia at poosaspea the kinnekulle k bentonite 40 cm derived from a major volcanic event and at ristna the grimstorp bentonite 6 cm in poosaspea important faunal changes are observed the three pre crisis dominant species i e tetrada memorabilis steusloffina sp nov and olbianella braderupensis were not recovered above the bentonite and at least two of them became extinct the post crisis ecosystem shows strong perturbations of diversity and abundance for a long period of time in ristna the impact of the ash fall is less dramatic all of the dominant pre crisis species survived although some changes in abundance and diversity are observed these results show that significant ash falls lead to marked rearrangement of assemblages and extinction of some taxa while less prominent volcanic episodes only result in temporary changes in the assemblage structure in both cases recovery follows several steps post event ecosystem with survival faunas very low abundance low diversity one two species opportunistic taxa e g sigmoopsis rostrata circulinella nuda dominate the assemblage early stages of recolonization with recovery faunas increasing abundance and diversity recolonization by pre event and immigrant species after a short disturbance period return to climax assemblages with specialized taxa high abundance high diversity although assemblages recovered rapidly after the crises the recovery periods still seem much longer than observed in recent foraminiferal assemblages causal links between the volcanic events and subsequent diversifications could not be demonstrated