the upper ordovician black shales of southern quebec canada and their significance for naturally occurring hydrocarbons in shallow groundwater abstract shale gas exploration in the st lawrence platform of southern quebec eastern canada focussed on the upper ordovician utica shale from 2006 to 2010 during which 28 wells were drilled 18 of which were fracked the st lawrence platform is thus considered as a pristine geological domain where potential environmental effects of fracking can be evaluated relative to the natural baseline conditions of the shallow aquifers in the saint edouard area southwest of quebec city it has been shown that groundwater carries variable and locally high levels of naturally occurring dissolved hydrocarbons in which thermogenic ethane and propane can be found fifteen shallow 30 147 m wells were drilled into bedrock and sampled cores and cuttings with the purpose of characterizing the shallow bedrock in a shale gas pre development context the shallow bedrock geology is made of three upper ordovician clastic formations the lotbiniere and les fonds formations are time and facies correlative with the utica shale present at a depth of 1 5 to 2 km in this area they are dominated by calcareous black shales with minor siltstone and micrite beds the nicolet formation is the youngest unit of the area and consists of gray to dark gray shales with locally abundant thick siltstone and fine grained sandstone beds the organic matter in the lotbiniere and les fonds formations is represented by solid bitumen with subordinate liptinite algae graptolites and chitinozoans representing normal marine type ii kerogen both formations are at the post peak hydrocarbon generation as indicated by the equivalent random vitrinite reflectance of 0 94 to 1 04 rock eval data support the type ii nature of the kerogen and the late oil window maturation level hydrocarbon extracts from the three formations have yielded high to low concentrations of c1 to c6 for all units an upward decrease in total volatiles c1 c2 c3 together with an increase in the gas dryness ratio c1 c2 c3 is recorded the transitions occurring at depths shallower than 50 m where the shales are more fractured the upward increase in the gas dryness ratio results from the more significant reduction of ethane and propane concentrations compared to that of methane consistent with the dryness ratio trend the 13 c vpdb values of methane change from thermogenic values 50 for deeper samples to more biogenic negative values 2 h vsmow trend of more negative values at shallower depths is noted the 13 c vpdb and 2 h vsmow values of rock hosted methane indicate that samples at shallow depth recorded a microbial influence it is proposed that diffusion and some microbial degradation of hydrocarbons are responsible for the decrease of rock volatiles and the in situ generation of biogenic methane in the shales at shallow depths to mix with the in situ thermogenic methane the utica shale is a very good source rock with high generation potential however thermogenic volatiles can also originate from shallower units with much shorter migration pathways the mixed thermogenic and biogenic methane in the groundwater results from fracture enhanced diffusion and biodegradation of volatiles at shallow depths

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

distal orogenic effects on peripheral bulge sedimentation middle and upper ordovician of the nashville dome abstract a major switch in depositional style in the ordovician carbonates of the nashville dome corresponds closely with the onset of the late middle ordovician taconic orogeny this time marks a shift from tropical type to temperate type carbonates the initiation of widespread major phosphate deposition the introduction of large amounts of terrigenous silt and clay the occurrence of widespread seismically induced soft sediment deformation and a change from a low energy flat topped carbonate shelf to a high energy doubly dipping carbonate ramp soft sediment deformation and the introduction of siliciclastics are direct effects of the taconic orogeny the switch from tropical type to temperate type carbonates the initiation of phosphate deposition and the switch in carbonate ramp are lar ely oceanographic effects triggered by the orogeny in particular phosphate deposition and the switch to temperate type limestones appears to have been driven by upwelling along the eastern side of the nashville dome within the newly deepened taconic foreland basin a fourfold decrease in the rate of relative sea level rise occurred on the nashville dome nearly 3 m y following the onset of thrusting and foreland basin initiation subsidence rates were constant before and after this decrease and no evidence of a change in subsidence rates is seen to coincide with the onset of thrusting the slowing of subsidence may reflect viscoelastic uplift of the nashville dome but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models

a mid ordovician temperate carbonate shelf the black river and trenton limestone groups of southern ontario canada abstract mid ordovician limestones of southern ontario are usually considered to be tropical shelf limestones and interpreted using facies models derived from recent tropical carbonate environments nevertheless depositional rates grain types faunas erosion surfaces mineralogy and geochemistry are more compatible with a temperate or even cold shelf environment in view of the low latitudes 20 s indicated by paleomagnetism this suggests deposition in relatively cold seas deposition below a tropical thermocline or that the paleomagnetic data are wrong similar features in other caradocian limestones of eastern north america show that these inferred cooler conditions were widespread if deposition occurred below a tropical thermocline most shallow water caradocian limestones must be re interpreted as deeper water deposits more likely in view of the evidence for temperate conditions in shallow water is that the whole eastern seaboard of north america was affected by cold currents arising from southern polar ice caps and thus was a temperate shelf environment

the utica shale and gas play in southern quebec geological and hydrogeological syntheses and methodological approaches to groundwater risk evaluation abstract the risk of groundwater contamination from shale gas exploration and development is a major societal concern especially in populated areas where groundwater is an essential source of drinking water and for agricultural or industrial use since groundwater decontamination is difficult or nearly impossible it is essential to evaluate exploration and production conditions that would prevent or at least minimize risks of groundwater contamination the current consensus in recent literature is that these risks are primarily related to engineering issues including casing integrity and surface activities such as truck traffic equipment and fluid haulage waste management mainly drill cuttings and water storage and treatment when hydraulic fracturing is utilized concerns have also been raised with respect to groundwater contamination that could result from potential fracture or fault interconnections between the shale unit and surficial aquifers which would allow fracturing fluids and methane to reach the surface away from the wellbore despite the fact that groundwater resources are relatively well characterized in some regions there is currently no recognized method to evaluate the vulnerability or risks to aquifers resulting from hydrocarbon industry operations carried out at great depths this paper focuses on the utica shale of the st lawrence platform quebec where an environmental study aiming to evaluate potential risks for aquifers related to shale gas development has been initiated to provide the context of these research efforts this paper describes the regional tectono stratigraphic evolution and current stress regime of the cambrian ordovician st lawrence platform as well as the utica shale internal stratigraphy mineralogy and thermal maturation then the hydrogeological context of the st lawrence platform is discussed finally the methodology for this environmental study based on geological geophysical geomechanical hydrogeological and geochemical data is presented

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

diachronous tectonic collapse of the ordovician continental margin eastern canada comparison between the quebec reentrant and st lawrence promontory the upper ordovician trenton group of southern quebec represents the last taconian foreland basin carbonate unit in the quebec reentrant prior to final collapse of laurentia s continental margin and its burial under synorogenic flysch the trenton group either conformably or unconformably overlies the black river group and is in turn conformably overlain by the utica shales the tripartite trenton carbonate unit records progressive deepening 1 very shallow to shallow subtidal 2 shallow to deep carbonate ramp and 3 shallow to deep outer shelf regional facies distribution lithotectonic elements and thickness variations indicate that the trenton shelf was dissected by extensional faults delineating blocks subsiding at various rates this scenario compares favourably with taconian foreland basin development in the middle ordovician table head group at the st lawrence promontory newfoundland a similar stratigraphic succession and tectono sedimentary history occurring 10 15 ma earlier at the s

sediments and history of the rottnest shelf southwest australia a swell dominated non tropical carbonate margin abstract the rottnest shelf is a narrow wave dominated open shelf on the passive continental margin of southwest australia adjacent to a hinterland of low relief and sluggish drainage high physical energy low nutrients in cool subtropical waters and rapid postglacial transgression have limited carbonate productivity restricted grain types and reworked the transgressed surface to form only a thin given sufficient time the rottnest shelf could recover from drowning and form blanket like skeletal carbonates thin

the late ordovician dawson point formation timiskaming outlier ontario key to a new regional synthesis of richmondian hirnantian carbonate and siliciclastic magnafacies across the central canadian craton current paleogeographic reconstructions extend late ordovician taconic derived siliciclastics across the cen tral canadian craton prior to the terminal ordovician glacioeustatic lowstand revision of the late ordovician dawson point formation of the timiskaming outlier greatly reduces the distribution of these siliciclastics and documents a greater spread of shallow water carbonate of richmondian age as revised the dawson point formation contains two informal members a deep water graptolitic shale that grades upward into shallow water siliciclastic redbeds and an up per member of shallow water muddy crinoidal limestone with interbedded shale likely representing low energy shoals on a muddy shelf deep water shale accumulation began in the upper manitoulinensis graptolite zone following foun dering of the regional foreland carbonate platform basin development documents a northward younging 1 million years from southern ontario foreland basins in keeping with regional tectonic driven transgression along eastern north america the shale to carbonate succession of the dawson point formation correlates with the georgian bay forma tion on manitoulin island wherein the upper carbonate dominated divisions of both formations are equivalent to the siliciclastic queenston formation of southern ontario in absence of additional biostratigraphic information the upper member of the dawson point formation is likely richmondian or late ashgillian in age the revised late ordovician history of the timiskaming outlier may identify a once significant volume of shallow water carbonate across the central canadian craton with related sequestration of carbon dioxide possibly aiding global cooling erosion of the carbonate driven by developing glacioeustatic lowstand conditions was likely contemporaneous with early hirnantian peritidal de position of the uppermost queenston formation in southern ontario

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

platforms of the nicaraguan rise examples of the sensitivity of carbonate sedimentation to excess trophic resources the nicaraguan rise is an active tectonic structure in the western caribbean carbonate accumulation on its platforms has not kept pace with relative holocene sea level rise despite a tropical location remote from terrigenous sedimentation trophic resources apparently exceed levels favoring coral reef development because sponge algal communities dominate the drowning western platforms in contrast to mixed coral algal benthos on pedro bank and well developed coral reefs along the north coast of jamaica concentrations of biotic pigments in sea surface waters show a corresponding west east gradient oceanic waters flowing over the western banks carry nearly twice as much biotic pigment as oceanic waters north of jamaica sources enriching the western caribbean are terrestrial runoff upwelling off northern south america and topographic upwelling over the nicaraguan rise that relatively modest levels of trophic resources can suppress coral reef development holds important implications for understanding carbonate platform drownings in the geologic record

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

maturation thermique des roches meres dans les bassins des basses terres du saint laurent et dans quelques buttes temoins au sud est du bouclier canadien caradocian source rocks belonging to the utica group occur around and overlay the southeastern part of the canadian shield from the ottawa embayment in the southwestern part of st lawrence lowlands to the anticosti island basin the kerogen of the source rocks is mainly made up of amorphous organic matter transmitted light resembling a solid bitumen framework in reflected light and contains a small portion of zooclasts graptolites and chitinozoans the kerogen is often too mature to have preserved the structure of the original constituents except those of graptolites and chitinozoans otherwise uv fluorescent colonial alga and tasmanites are observed in the eastern half of the st lawrence lowlands stratigraphic and tectonic burial is the main control of source rock thermal maturation from an area located 50 km west of quebec city the estimated vitrinite reflectance ro std of the source rock increases in both northeastern montmorency falls and southwestern st maurice river directions from ro std of 1 2 to 1 5 and 1 7 respectively the source rocks of the ottawa embayment and montreal area western half of the basin are overmature due to hydrothermal and igneous activities 2 3 ro std 2 8 in chicoutimi and lac saint jean outliers the source rocks show reflectances between 0 9 and 1 2 slightly less than reflectance values in the quebec city area ro std between 1 2 and 1 5 on anticosti island the thermal maturation is related to burial only and increases downdip of the inclined platform from the northeast toward the southwest ro std between 0 9 and 2 1 these observations suggest that the st lawrence platform and the southeastern canadian shield were buried by at least 3 5 km of paleozoic strata prior to erosion to grenville basement this implies that most potential hydrocarbon fields are presumably also lost by erosion

do all fractions of organic matter contribute equally in shale porosity a case study from upper ordovician utica shale southern quebec canada abstract in many unconventional hydrocarbon systems organic matter om substantially contributes to total porosity of the rock that led to a positive correlation between total organic carbon toc and porosity while organic porosity is an important reservoir property of unconventional hydrocarbon systems toc of these rocks consists of different macerals or petrographic fractions these toc fractions can be geochemically divided and quantified by the revised extended slow heating esh rock eval analysis this study discusses the contribution of different fractions of om on porosity variation in the mature to overmature utica shale in southern quebec using esh rock eval analysis helium pycnometery scanning electron microscopy sem and x ray diffraction xrd mineralogy selected samples from three wells at depths of 400 700 and 2000 m covering a range of thermal maturity from late peak oil window vroeqv 1 to dry gas zone vroeqv 2 1 were analyzed in this study the predominant om constituents are solid bitumen chitinozoans and minor graptolites with mean toc of 1 1 0 4 wt the mean porosity for oil and gas window samples decreases with depth from 3 9 0 9 to 2 6 0 5 respectively the sem images reveal that organic porosity is limited to pores developed in solid bitumen while regardless of thermal maturity chitinozoans have a non porous structure throughout the oil to dry gas window this observation suggests that the impact of thermal maturity on porosity is different for different om fractions i e macerals leading to different contribution of om fractions to total porosity the latter conclusion is further supported by the improved correlation observed between the solid bitumen fraction and porosity compared to toc porosity correlation for oil window samples our entire data set suggests that om i e bitumen fraction especially in the dry gas window is not the major controlling factor on the total porosity in the utica shale whereas intergranular porosity has the dominant control on total porosity this observation suggests that the hydrocarbon storage potential may largely reside in inorganic pores rather than organic pores in the analyzed intervals of the utica shale the decrease in total porosity observed from shallower samples i e oil window to deeper samples i e dry gas window also suggests that intergranular porosity is the dominant porosity component which decreases with burial

permian limestone in the southeastern bowen basin queensland an example of temperate carbonate deposition abstract during the permian the bowen basin a foreland basin in eastern australia was influenced by cold to cool temperate climatic conditions at a paleolatitude of 60 s limestones are rare in the sequence except in the southeastern bowin basin area where two limestone bearing sequences are present the limestones are mainly skeletal grainstones and rarer packstones skeletal grains include crinoids bryozoans brachiopods molluscs ahermatypic corals foraminifera and sponge spicules crinoid remains are dominant but brachiopod rich and coral rich limestones are present locally non skeletal carbonate grains are absent from the limestones terrigenous components range from negligible to dominant comparison of the limestones with others in the permian sequences in eastern australia reveals a consistency in sedimentary style the only variations being in the relative proportions of the skeletal fragments the permian limestones share similar characteristics with temperate cenozoic limestones of new zealand suggesting that differences in carbonate sedimentation between tropical and non tropical regions have been consistent through time and reflect real sedimentological differences

blackriveran carbonates from the subsurface of the lake simcoe area southern ontario stratigraphy and sedimentology of a low energy carbonate ramp the lowest upper ordovician blackriveran strata in the lake simcoe area represent the base of a major transgressive sequence of peritidal carbonates and siliciclastics from detailed petrographic

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

evolution of a lower paleozoic continental margin carbonate platform northern canadian appalachians the western margin of the northern appalachian orogene and adjacent craton is composed of a lower paleozoic low latitude carbonate platform which originally lay along the northern margin of iapetus ocean parts of the platform interior are now exposed in quebec much lies beneath the gulf of st lawrence and the outer shelf and deep water deposits crop out in western newfoundland the shelf break and upper slope are nowhere exposed but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes four separate phases recording platform growth and demise can be differentiated reflecting the interplay between tectonics eustasy and the evolving lower paleozoic biota phase 1 preplatform shelf reflects initial siliciclastic deposition on rifted crystalline basement followed by a short phase of carbonate sedimentation dominated by archeocyathan buildups and ooid shoals that was terminated by offlap of thick quartzarenites phase 2 narrow high energy platform is characterized by mixed siliciclastic carbonate peritidal sedimentation throughout manifest as three grand cycles contemporaneous deep water sediments comprise basal welded conglomerates overlain by quartzose carbonate turbidites phase 3 wide low energy platform is an onlap package of muddy fossiliferous subtidal and peritidal carbonates arranged inmore the form of two unconformity bounded megacycles the adjacent deep water slope was a narrow belt of debris flows and wide carbonate shale apron deposited in the lower part of an oxygen minimum zone phase 4 foundered platform documents the initial uplift faulting subsidence and fragmentation of the platform in a sequence of peritidal to subtidal to deep water carbonates reflecting the initial stages of the taconic orogeny less

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

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

seismic tomographic inversion of russian pne data along profile kraton tomographic inversion of russian peaceful nuclear explosion pne data collected along the 3500 km east west striking profile kraton has provided new information about the two dimensional p wave velocity structure of the upper mantle beneath the siberian platform the 8 discontinuity is clearly identified as the top of a zone in the 100 200 km depth range this zone appears as a low velocity zone in large parts of the profile and has zero velocity gradient along the profile the 400 km discontinuity is imaged as an increase in velocity of at least 0 4 km s at 380 420 km depth

a pre hirnantian late ordovician interval of global cooling the boda event re assessed facies analysis suggests that the early mid ashgill late ordovician boda event of fortey and cocks fortey r a cocks l r m 2005 late ordovician global warming the boda event geology 33 5 405 408 was an episode of global cooling not of warming as they proposed limestones are widely developed at this level from mid latitude to high latitude commonly interrupting thick ordovician successions of fine clastic sediments bryozoan carbonate mud mounds in a laterally extensive cool water carbonate belt along the high palaeolatitude margin of w gondwana are associated with glacial diamictites and grew during glacial lowstand with sea ice prior to the hirnantian glaciation from high to mid palaeolatitudes contemporaneous cool water carbonates with mud mounds including stromatactis both in gondwana and in baltica developed in mid outer ramp settings during regression in the dominantly clastic welsh basin of avalonia phosphatic facies on an iapetus facing carbonate ramp indicate upwelling cold waters down ramp yet up ramp coralliferous algal limestones suggest warm surface waters we propose that the late ordovician cool water carbonates developed in response to global cooling and expansion of gondwanan terrestrial and sea ice that lowered eustatic sea level and on the w gondwanan shelf reduced or shut down fluvial clastic input previously poorly circulated salinity stratified oceans that had restricted benthic dispersal in mid low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas an earlier caradoc carbonate interval in low mid palaeolatitudes that shares many facies characteristics with the ashgill cool water limestones and correlates with a widely recognised isotope excursion is interpreted as indicating an earlier global cooling episode

stratigraphy of the ordovician of the lac saint jean and chicoutimi outliers quebec four previously defined formations within the chicoutimi outlier are extended to the lac saint jean outlier these formations are gradational into one another the lowest of these formations the t

modern carbonate and terrigenous clastic sediments on a cool water high energy mid latitude shelf lacepede southern australia the wide lacepede shelf and narrow bonney shelf are contiguous parts of the south eastern passive continental margin of australia the shelves are open generally deeper than 40 m covered by waters cooler than 18 c and swept by oceanic swells that move sediments to depths of 140 m the lacepede shelf is proximal to the delta of the river murray and the coorong lagoon shelf and upper slope sediments are a variable mixture of holocene and late pleistocene quartzose terrigenous clastic and bryozoa dominated carbonate particles bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m they can be grouped into four depth related assemblages coralline algae the only calcareous phototrophs are important sediment producers to depths of 70 m active benthic carbonate sediment production occurs to depths of 350 m but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions the shelf is separated into five zones the strandline is typified by accretionary sequences of steep shoreface beach and dune carbonate siliciclastic sediments similar shoreline facies of relict bivalve limestone cobble ridges are stranded on the open shelf the shallow shelf c 40 70 m deep is a wide extremely flat plain with only subtle local relief it is a mosaic of grainy quartzose palimpsest facies which reflect the complex interaction of modern bioclastic sediment production dominated by bryozoa and molluscs numerous highstands of sea level over the last 80 000 years modern mixing of sediments from relatively recent highstands and local introduction of quartz rich sediments during lowstands the middle shelf c 70 140 m deep is a gentle incline with subtle relief where holocene carbonates veneer seaward dipping bedrock clinoforms and local lowstand beach complexes carbonates are mostly modern uniform clean coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes the deep shelf edge c 140 250 m deep is a site of diverse and active bryozoa growth resulting accumulations are characteristically muddy and distinguished by large numbers of delicate branching bryozoa the upper slope between 250 and 350 m depth contains the deepest platform related sediments which are very muddy and contain a low diversity suite of delicate branching cyclostome bryozoa this study provides fundamental environmental information critical for the interpretation of cenozoic cool water carbonates and the region is a good model for older mixed carbonate terrigenous clastic successions which were deposited on unrimmed shelves

the middle caradoc facies and faunal turnover in the late ordovician baltoscandian palaeobasin abstract a late ordovician episode of remarkable biotic climatic sea level and facies changes named here as the middle caradoc facies and faunal turnover took place in the baltoscandian area this paper presents an integrated overview of these changes in the critical middle caradoc interval data are given on carbonate rock composition distribution and grain size composition of the siliciclastic material and the carbon isotopic composition of whole rock carbonates in cores of estonia and sweden the middle caradoc facies and faunal turnover can be described as a succession of related environmental changes the turnover began with a positive excursion in carbonate 13c and continued with sea level changes that led to a sedimentary hiatus on the shelf and a change from carbonate dominated to siliciclastic dominated sedimentation in the basin the turnover ended with an extinction event and associated microfaunal crisis the middle caradoc turnover in baltoscandia is comparable to a similar succession of changes in north america the turnover affected two palaeocontinents and reflects a widespread possibly global environmental change onset of glaciation on gondwana and or increased orogenic activity might have initiated the changes in ocean circulation and led to the initial carbon isotope excursion the following sea level rise and faunal changes affected several different continents

holocene sediment dynamics on a cool water carbonate shelf otway southeastern australia the otway shelf is covered by cool ocean waters and veneered by bryozoan dominated carbonate sediments radiocarbon dating and stratigraphy of shelf vibracores and slope gravity cores document late pleistocene holocene deposition shelf sediments of the late pleistocene highstand 60 26 ky are rare either never having been deposited or having been removed during the following sea level fall 26 17 ky during the subsequent lowstand the shelf was exposed facies shifted basinward and beach dune complexes were constructed near the shelf edge the deep shelf was characterized by nondeposition and hardground formation and the shelf margin became locally erosional upper slope bryozoan sponge assemblages continued to grow actively and lower slope foraminifera and nannofossil ooze was increasingly enriched in hemipelagic terrigenous mud swept off the wide shelf coarse shelf debris and lowstand dune sands were erosively reworked and transported onto the upper slope and redistributed to deep slope aprons during early transgression 17 10 ky this event locally terminated shelf edge bryozoan and sponge growth continued erosive shoreface retreat formed a bored and cemented ravinement surface mantled by bivalves and lithoclasts across the open shelf base level rise reduced the amount of terrigenous material reaching the continental slope slower transgressive flooding from 10 to 6 5 ky allowed infilling of shallow embayments on the shelf and outer shelf sediment accumulation began by 7 ky sea level stabilized near its present level by 6 5 ky and the area is now partitioned into three zones by an energetic wave and swell regimen 1 inboard protected embayments and or beach dune complexes with high sediment accumulation rates about 100 cm ky 2 an open shelf where although sediment production is active accumulation is patchy 3 5 cm ky to nonexistent within the zone of wave abrasion and minimal about 23 cm ky between maximum abrasion depth and swell wave base and 3 a deep shelf edge and upper slope where sediment is accumulating at rates of 2 50 cm ky interpreted deep water 100 m bryozoan sponge mounds are growing at rates of 105 cm ky accumulation decreases progressively down slope with deep slope pelagic carbonate sedimentation at rates of 5 cm ky the late quaternary shelf record resembles that of flat topped warm water platforms with holocene sediment overlying pleistocene tertiary limestone but for different reasons much of this high energy shelf is still within lag depth more than 10 ky after flooding the steady state deep water shelf margin bryozoan factory has been largely unaffected by 50 100 m eustatic changes in sea level and has contributed to slow but continuous platform progradation the slow growth potential uniform profile of sediment production and distribution and inability of constituent organisms to construct rigid frameworks favor maintenance of a shallow ramp profile and makes the cool water carbonate system an excellent modern analog for interpretation of many ancient ramp successions

uppermost mantle velocities and anisotropy beneath europe pn data collected within southern europe and the mediterranean are used to tomographically image variations in both seismic velocity and seismic anisotropy seismic anisotropy is an essential part of the inversion and without it several low velocity features within the uppermost mantle could not be properly imaged the technically active mantle of southern europe has much lower seismic velocities 7 6 8 1 km s than the more stable mantle of the sub african plate of the adriatic sea 8 3 km s however the most dramatic features within europe s uppermost mantle relate to the apennine dinaride and hellenide arcs these arcs all have extremely low 5 amounts of arc parallel anisotropy the tyrrhenian and aegean back arc regions also show low velocities 7 7 7 9 km s but less anisotropy the same may be true for the pannonian basin but the tomography has poorer resolution there a model explaining these observations focuses on subducted water metasomatizing the mantle wedge the addition of water causes melting creates arc volcanism lowers the seismic velocity and enhances the formation of anisotropy due to preferential olivine orientation for collisional arcs of the northern mediterranean arc parallel anisotropy has formed in response to compression across them and extension along them within back arc regions water is no longer a major factor instead convection associated with subduction and back arc extension controls the anisotropy

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

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

ordovician intracratonic sediments from the lac st jean and chicoutimi areas quebec eastern canada the transgressive sequence of the ordovician intracratonic basin of the present day lac st jean and chicoutimi areas is characterized by a variety of sediments that were deposited in a number of en

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

uppermost mantle velocities beneath china and surrounding regions 1 seismic p wave velocity variations in the uppermost mantle beneath china are tomographically imaged using pn wave travel time data the average velocity of the mantle lid beneath china is 8 05 km s with a range from 0 30 km s to 0 35 km s eastern china has slower mantle velocities and a thinner crust than western china different inversions performed for shorter ray path and longer ray path data sets separated the shallow and deep structure within the mantle lid velocities in the lower lid are no more than 0 2 km s faster than at the top the fastest velocities are beneath cratons which remained relatively undeformed despite orogenic events around their margins the pn velocity beneath southern tibet is faster than that beneath northern tibet suggesting that this contrast delineates the northern edge of the indian lithosphere on the southeastern margin of the tibetan plateau the panxi rift slow pn velocity exists in the lithosphere due to the transextensional tectonic environment in eastern china extension of the north china block produced a series of nne ssw trending rifts that cause slow mantle lid velocities these slow velocities extend to the base of the mantle lid suggesting that late cenozoic pacific subduction drove the rifting both high temperature and hydration may contribute to these low uppermost mantle velocities the shanxi rift along the east margin of the ordos platform is characterized by a slow mantle velocity in contrast the yinchuan rift on the west margin does not show any uppermost mantle velocity anomaly

paleoclimate of some permo triassic carbonates of malaysia abstract four hundred thin sections of permian chuping and summalayang limestones and triassic kodiang limestone carbonates of malaysia were studied mainly to interpret probable climatic zones during their formation chuping limestone is rich in skeletal grains and intraclasts formed in shallow high energy environments original mineralogy of skeletal grains and early diagenetic cements were chiefly calcites with some aragonite the faunal assemblage is similar to that of subpolar carbonates the brachiopods are characterized by heavy 13 c and light 18 o and these values lie in the same field as an early permian fauna from eastern australia the 18 o values of fauna show unrealistic temperatures because the fauna has equilibrated with melt waters however calculated original 18 o values of the fauna from 13 c indicate temperatures ranging from 2 to 13 c with w of 1 2 these features reflect cool temperate to subpolar conditions summalayang limestone is rich in fusulinids and was deposited in bar to shelf environments fusulinids are often filled with fine equigranular sparry calcite early diagenetic origin of these cements is indicated by erosion of cements during transportation and also by the cross cutting relationship of veins with cements these cements were originally mg calcites the foramol faunal assemblage and predominantly mg calcite mineralogy of both fauna and cements reflect temperate climate kodiang limestone was deposited in peritidal environments this limestone formation has all the characteristic features of modern warm tropical carbonates such as chlorozoan assemblage diverse non skeletal grains abundant early diagenetic dolomites and predominance of aragonitic fauna non skeletal grains and cements the formation of temperate carbonates during the permian suggests that malaysia was probably part of gondwanaland

ostracods from upper ordovician katian carbonate lithofacies in southwest scotland the ordovician craighead limestone formation of southwest scotland was formed on a carbonate platform on the eastern tropical margin of the laurentia palaeocontinent during the early katian c 456 ma it yields the most diverse and well preserved ostracod fauna yet recovered from the scottish ordovician succession with some 25 species divisible into two distinct marine biotopes comprising shallow lagoonal and deeper platform margin settings respectively the ostracods show strong biogeographic links at species level with sandbian and early katian faunas of north america including krausella arcuata steusloffina cuneata monoceratella teres and species of levisulculus and platybolbina however many of the ostracod genera that characterize the craighead limestone formation have earlier origins in the baltica palaeocontinent suggesting enhanced migration of species from mid baltica about 30 s to low laurentia latitudes in the late sandbian and early katian interval such baltica origin genera include distobolbina kiesowia and platybolbina notable is the wide biogeographic occurrence of steusloffina cuneata extending from warm tropical laurentia to cooler high latitude gondwana an enormous latitudinal range for a shelf dwelling marine species the possible tvaerenellidae taxon duoarcus levigatus gen et sp nov is described

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

a skeletal assemblage classification system for non tropical carbonate deposits based on new zealand cenozoic limestones abstract cenozoic limestones in new zealand are mainly skeletal grainstones and packstones formed under non tropical climatic conditions in open marine shelf or ramp environments following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by lees and buller 1972 for temperate region carbonate deposits the seven assemblages are given contracted names as follows a barnamol barnacle bivalve dominated b bimol bivalve dominated c bryomol bryozoan bivalve dominated d echinofor echinoderm benthic foraminiferal dominated e nannofor nannofossil planktonic foraminiferal dominated f rhodalgal calcareous red algal dominated and g rhodechfor calcareous red algal echinoderm benthic foraminiferal dominated a composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components the diagram successfully characterises more than 85 of the new zealand cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non tropical carbonate deposits limitations relate mainly to locally common skeletal types e g serpulids brachiopods that are presently not incorporated into the new zealand based scheme the general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats consequently as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers the scheme can assist with the paleoenvironmental interpretation of non tropical skeletal carbonate facies

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

a late ordovician high energy temperate water carbonate ramp southern quebec canada implications for late ordovician oceanography the trenton group late ordovician the youngest carbonate unit in the taconic foreland basin of southern quebec is a tripartite unit with a distinctive coarse grained middle part the deschambault formation lithofacies of the deschambault formation are dominated by coarse grained bioclastic intraclastic limestones finer grained lithofacies are ubiquitous but subordinate the complete spectrum of lithofacies indicates sedimentation ranging from above fairweather to below storm wave base skeletal components are indicative of the modern temperate water bryomol association non skeletal elements are represented by peloids and intraclasts accretion rates from areas of continuous sedimentation were low 14 cm 103 years from sedimentological and faunal evidence it is proposed that the late ordovician deschambault ramp was bathed by temperate waters the model compares favourably with modern cool water shelves rimming the southern edge of the australian continent palaeomagnetic data locate southern quebec in a low latitudinal setting during the late ordovician upper ordovician facies distribution in eastern canada and progressive disappearance of some faunal provinces through late ordovician time are used to conclude that the initiation of the late ordovician glaciation that covered most of gondwana was instrumental in easing northward movement of cold oceanic currents this resulted in the rapid contraction of the southern hemisphere warm water tropical belt from a 30 latitudinal wide zone in the early caradoc to a 15 zone in the late caradoc

cool water carbonates in a paleoequatorial shallow water environment the paradox of the auernig cyclic sediments upper pennsylvanian carnic alps austria italy and its implications distinct parts of limestones within the upper paleozoic auernig group of the carnic alps austria and italy are characteristic of cool water carbonates the carnic alps were between 5 n and 10 s paleolatitude during the late carboniferous a position confirmed by dasyclad algae and fossil plants the floral association occurrence of coal seams and absence of evaporites indicate a humid tropical environment the entire section lacks abiotic components of typical warm water limestones no ooids and no aggregates occur within the auernig group parts of the limestones show surprisingly a cool water association of high diversity bryozoans brachiopods crinoids red algae sponge spicules and entomozoan ostracodes the genesis of these limestones atypical for a paleoequatorial setting cannot be explained by changes in salinity bathymetry or terrigenous input the water temperature possibly linked with upwelling nutrient supply and paleoceanographic currents is the most convincing cause of this unusual association paleoceanographic changes are interpreted as linked to contemporaneous glaciation deglaciation cycles in gondwana this paper shows that cool water carbonates in shallow water environments are not necessarily nontropical as generally interpreted future studies should consider water temperature oceanic circulation and availability of nutrients supplementary to bathymetry salinity and latitude related climate in interpretation of carbonate components

cool water carbonates in an eocene palaeoestuary norseman formation western australia abstract numerous palaeovalleys formed extensive drowned estuaries during eocene transgressions along the southwestern part of the southern margin of australia the tertiary sediments of the cowan palaeovalley have been extensively drilled revealing deposition of the norseman formation during the middle eocene tortachilla transgression initial deposition occurred during transgression of the valley to form a drowned estuary sediments consisted of coarse grained muddy lithic iron and glauconite rich sands and gravels of mixed carbonate and quartz pure carbonates accumulated during the highstand produced by a typical shallow temperate water assemblage of bryozoans coralline algae echinoids and molluscs and were swept into shoals by strong tidal currents minor tropical components in the form of large benthic foraminifers and dasycladacean algae are present coarse bryozoan and trough cross bedded carbonate sands accumulated in the margins of the estuary and fine bryozoan sands in the deeper parts rhodoliths accumulated to form shoals in sheltered localities the spencer gulf and gulf st vincent of south australia provide close modern analogues to the cowan palaeovalley and the norseman formation modern carbonate sediments off esperance on the south coast of western australia contain tropical faunal elements within an otherwise temperate skeletal assemblage and also provide a modern analogue the norseman formation thus provides an excellent example of cool water carbonate deposition in near shore tide dominated environments this study complements and contrasts existing cool water shelf facies models based on tertiary carbonates deposited on deep shelves elsewhere in southern australia

an atmospheric pco2 threshold for glaciation in the late ordovician the late ordovician glaciation differs from other major phanerozoic glaciations because of its short duration 1 m y a satisfactory explanation for this glaciation must be able to account for its sudden initiation short duration and rapid termination mechanisms operating on tectonic time scales appear to be precluded given recent studies of a major perturbation in the carbon cycle during the glaciation a climate model investigation of the sensitivity of the late ordovician climate to changes in atmospheric p co2 was undertaken under the condition of a 4 5 reduction in solar luminosity permanent snow cover taken as a key indicator of potential for glaciation is dramatically different between five experiments the range of 18x present atmospheric level co2 ice free to 8x runaway icehouse lies within the uncertainty of previous geochemical estimates of late ordovician atmospheric p co2 the strong sensitivity to the modest direct forcing from p co2 changes is due to the ice albedo feedback a plausible increase in organic carbon burial could have drawn down enough atmospheric co2 to have briefly lowered the climate system below a critical glacial inception threshold at the end of the ordovician conversely a high p co2 is required for the rest of the early paleozoic which was essentially ice free

sequence stratigraphy in the middle ordovician shale successions mid east korea stratigraphic variations and preservation potential of organic matter within a sequence stratigraphic framework abstract the jigunsan formation is the middle ordovician shale dominated transgressive succession in the taebaeksan basin located in the eastern margin of the north china platform the total organic carbon toc content and some geochemical properties of the succession exhibit a stratigraphically distinct distribution pattern the pattern was closely associated with the redox conditions related to decomposition bulk sedimentation rate dilution and productivity to explain the distinct distribution pattern this study attempted to construct a high resolution sequence stratigraphic framework for the jigunsan formation the shale dominated jigunsan formation comprises a lower layer of dark gray shale deposited during transgression and an upper layer of greenish gray siltstone deposited during highstand and falling stage systems tracts the concept of a back stepped carbonate platform is adopted to distinguish early and late transgressive systems tracts early and late tst in this study whereas the highstand systems tracts and falling stage systems tracts can be divided by changes in stacking patterns from aggradation to progradation the late tst would be initiated on a rapidly back stepping surface of sediments and just above the surface exhibits a high peak in toc content followed by a gradually upward decrease this trend of toc distribution in the late tst continues to the maximum flooding surface mfs the perplexing toc distribution pattern within the late tst most likely resulted from both a gradual reduction in productivity during the late tst and a gradual increase in dilution effect near the mfs interval the reduced production of organic matter primarily incurred decreasing toc content toward the mfs when the productivity was mainly governed by benthic biota because planktonic organisms were not widespread in the ordovician results of this study will help improve the understanding of the source rock distribution in mixed carbonate siliciclastic successions within a stratigraphic framework particularly for unconventional shale reservoirs

sedimentary model and high frequency cyclicity in a mediterranean shallow shelf temperate carbonate environment uppermost miocene agua amarga basin southern spain uppermost tortonian to lower messinian temperate carbonates crop out in the agua amarga basin se spain they consist of four units the lower three units can be tentatively assigned to the lowstand systems tract of a fourth order sequence constituting in turn the lowstand megatrough unit transgressive breccia unit and highstand bedded unit stages of a higher order cycle all these materials were deposited in a small pull apart basin related to the sinistral carboneras strike slip fault system the best represented is the bedded unit up to 25 m thick which consists of bioclastic bryozoan bivalve dominated calcarenites calcirudites with abundant fragments of echinoids barnacles benthic foraminifers coralline algae brachiopods and solitary corals facies trends within this unit are roughly arranged in an e w direction with the coastline to the north of the basin the depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts seaward of the shoals was the factory area where most organisms lived and maximum carbonate production took place from this area some of the skeletons were washed landwards by waves and or currents during storms and incorporated into the shoals and beaches and others moved downslope along the ramp as mass flows accumulating to form the fan bedded zone the factory area and fan bedded sediments intercalate five well defined thick beds of calcarenites fine grained calcirudites they show bar morphologies single or amalgamated or make up sand waves with very consistent tabular cross bedding pointing landwards these beds formed in a very shallow wave current influenced coastal environment the bars and sand waves in the fan bedded zone developed during lowstands while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages prograding beaches shoals factory facies and fan bedded layers developed during the highstands net skeletal production occurred mainly during the highstands sediment accretion values of these sediments are similar to those of present and ancient shallow marine temperate carbonates considering that the whole bedded unit was deposited in a 100 000 year interval equivalent to the short eccentricity cycle the five cycles inside the bedded unit would therefore correspond to the c 20 000 year precession cycles of the milankovitch band

biotic response to a middle ordovician paleoceanographic event in eastern north america global paleontological events such as mass extinctions have been the focus of attention in recent years yet regional biotic events account for a greater proportion of extinction in the fossil record and should receive greater attention if we are to fully understand extinction processes over the phanerozoic we use a sequence stratigraphic framework to document a major extinction and migration episode that occurred in the middle and late ordovician of north america this event is characterized in eastern north america by coordinated changes in faunal assemblages carbonate lithologies siliciclastic influx and phosphatization we ascribe these changes to a regional paleoceanographic event brought on by thrust driven flexural subsidence and sea level fluctuations that lowered water temperature and increased turbidity and nutrient input

low latitude ordovician to triassic brachiopod habitat temperatures bhts determined from δ18o brachiopod calcite a cold hard look at ice house tropical oceans abstract some researchers have suggested that phanerozoic land based ice sheets extended occasionally into the tropical realm if true the tropical ocean at those times must have been distinctly colder than at the last glacial maximum lgm when northern hemisphere ice sheets did not extend below 38 latitude low latitude ocean temperatures derived from oxygen isotopes of brachiopod shells test this hypothesis by comparing the temperature regime for paleozoic and early mesozoic low latitude settings to the tropical temperature regime of the modern interstadial ocean and to mean temperatures estimated for the tropical ocean at lgm a running mean of ph adjusted brachiopod habitat temperatures bhts shows that paleozoic low latitude oceans were on average cool to cold relative to the modern interstadial tropical ocean at times during pennsylvanian serpukhovian tournaisian and ordovician silurian glaciations these tropical seas were indeed significantly colder on average than at the lgm ice sheets within tropical latitudes can be reasonably predicted at those times abundant and diverse paleozoic brachiopod communities reflect these cool tropical oceans consistent with modern brachiopod ecological preference for colder waters amplified paleozoic temperature oscillations suggest recurring global warming events which episodically drove these cold tropical oceans to temperatures significantly higher than the warmest modern tropical ocean

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

forced regressive deposits of a deglaciation sequence example from the late quaternary succession in the lake saint jean basin quebec canada differentiating between forced regressive deposits from deglacial periods in high latitude domains and forced regressive deposits from the onset of glacial periods in low latitude domains is fundamental for the accurate interpretation of glacial cycles within the geological record and then for the reconstruction of palaeogeography and palaeo climate a forced regressive deglacial sequence is documented from the lake saint jean basin quebec canada in this area the late pleistocene to holocene sediments have recorded the laurentide ice sheet retreat accompanied by the invasion of marine waters laflamme gulf from ca 129 cal kyr bp subsequently fluviodeltaic and coastal prograding wedges were deposited they followed the base level fall due to glacio isostatic rebound this succession representing a transition from glacial to post glacial periods within a previously glaciated area was investigated through recent mapping preserved landforms facies analysis and new optical stimulated luminescence and radiocarbon dates three basin scale geological sections share a common lower part made of isolated ice contact fan deposits overlying bedrock throughout the entire basin ice contact fans are capped by glacimarine muds above fluvial and coastal prograding systems were deposited and evolved through four steps i deltaic systems progressively increased in width ii coastal influence on sedimentation increased iii hydrographic drainage systems became more organised and iv deltas graded from steep gilbert delta to low angle foresets mouth bar delta deposited during the base level fall from glacioisostatic rebound the complete succession has been designated as a single falling stage system tract referred to as a deglacial falling stage system tract it is representative of a deglaciation sequence in areas previously covered by ice during glacial periods i e medium to high latitude domains diagnostic criteria are provided to identify such a deglacial falling stage system tract in the geological record which may aid identification of previously unknown glacial cycles

response of late ordovician paleoceanography to changes in sea level continental drift and atmospheric pco2 potential causes for long term cooling and glaciation abstract we performed sensitivity experiments using an ocean general circulation model at two stages of the late ordovician caradoc 454 ma ashgill 446 ma under a range of atmospheric pco2 values 8 18 pal pre industrial atmospheric level at high and low sea level the model results indicate that the long term cooling trend during the late ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric pco2 sea level change and paleogeographic change these results also explain the occurrence of low latitude cool water carbonates in north america in all simulations a drop in sea level led to a reduction in poleward ocean heat transport this indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the late ordovician alterations in poleward ocean heat transport linked to changes of atmospheric pco2 also indicate that there is a threshold of 10 pal above which ocean current change cannot be responsible for glaciation in the late ordovician continental drift could explain the observed global cooling trend in the late ordovician through a combined poleward ocean heat transport feedback and increased ice albedo effect if atmospheric pco2 was low during the entire late ordovician the model results further indicate that the response of meridional overturning to changes in paleogeography atmospheric pco2 and sea level is stronger than the response of surface circulation to these perturbations because the overturning circulation is so strong meridional overturning was the dominant mechanism for described changes in heat transport in the late ordovician

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana