an evaluation of the single grain zircon evaporation method in highly discordant samples comparison of results from the isotope dilution and the single grain zircon evaporation methods on highly discordant samples shows that the evaporation method can yield accurate crystallization ages on some zircon populations that display complex discordant patterns three granite gneiss samples from the western gneiss region norway which crystallized at approximately 1660 ma and experienced profound lead loss in zircon during caledonian metamorphism at 400 ma were analyzed by both the isotope dilution tucker et al 1990 and evaporation methods 207ph 206pb evaporation ages on one sample agree with the isotope dilution crystallization age two other samples produced a range of anomalously young 207pb 206pb evaporation ages analyses at the highest evaporation temperatures from one of these two samples were consistent with crystallization ages determined by the isotope dilution method but it would be difficult to interpret them correctly without independent age information the anomalously young results are generally easy to recognize by the pattern of increasing 207ph 206ph ages with increasing evaporation temperature a more serious problem however is that some grains produced 207ph 206pb age plateaus over a range of increasing evaporation temperatures that cannot be correlated with any documented geologic event in the region the evaporation analyses yielding the youngest 207pb 206pb ages also display the most common lead suggesting that common lead may diffuse into the zircon grains during loss of radiogenic lead a pegmatite from massachusetts usa contains zircons with visible xenocrystic cores the 207ph 206pb evaporation ages at lower temperatures agree well with the 299 ma crystallization age determined by the isotope dilution method on the zircon rims r d tucker unpubl data at higher evaporation temperatures a range of older 207pb 206pb ages are produced by mixing pb evaporated from both the zircon rims and xenocrystic cores large changes in the 206pb 208pb ratio accompany the increase in 207pb 206pb ages and indicate mixing of two distinct zircon domains thus in addition to providing useful 207pb 206pb age information the evaporation method may help distinguish between the discordance produced by diffusional lead loss and the mixing of two domains of differing age

coeval sedimentation magmatism and fold thrust development in the trans hudson orogen propagation of deformation into an active continental arc setting wekusko lake area manitoba early structures in the southeastern part of the trans hudson orogen tho wekusko lake area manitoba comprise northwest verging folds and a set of steep southeast dipping faults that occur at a low angle to bedding kinematic indicators are rare however the vergence of the associated folds supports reverse movement on the faults these folds and faults formed prior to the peak of regional metamorphism and therefore must correlate with the widely recognised deformational d2 event which developed pre to syn metamorphism and involved development of a regional south to southwest directed fold thrust belt the d2 folds and reverse faults in the study area are interpreted to have originally formed as gently dipping southwest verging structures i e as inclined folds and thrust faults which were then rotated into a steep northwest verging orientation during later deformation d3 in the southeastern tho regional d2 fold and thrust structures are cut by ca 1839 1830 ma plutons indicating that

sedimentary provenance and palaeoenvironment of the baixo araguaia supergroup constraints on the palaeogeographical evolution of the araguaia belt and assembly of west gondwana

microstratigraphy trilobite biostratinomy and depositional environment of the lower cambrian ruin wash lagerstatte pioche formation nevada abstract the uppermost 43 cm of dyeran strata at the ruin wash lagerstatte chief range lincoln county nevada contain nonmineralized invertebrates and exceptionally preserved articulated olenelloid trilobites however the environmental factors responsible for the preservation of olenelloids in this unusual state at ruin wash have received little study and are therefore poorly understood microstratigraphic analysis of this interval reveals that the strata comprised almost exclusively of clay sized particles and rather monotonous and featureless in outcrop were deposited as a series of thin event beds interpreted as tempestites the energy of deposition progressively waned then waxed through the interval and benthic redox conditions shifted concurrently biostratinomic data for more than 500 trilobite specimens encountered in the measured section reveal that the trilobite fauna was autochthonous although winnowed surfaces rich in bioclasts occurring at the base and top of the section indicate intervals of prolonged pre burial exposure time and or in situ reworking trilobite biostratinomy was strongly influenced by subtle environmental shifts the degree of disarticulation and sclerite fracturing correlate positively with event bed energy and inferred bottom water oxygen content these results demonstrate that 1 preservation of articulated trilobites is favoured near the distal limit of scouring associated with depositional events where rapid sedimentation occurs in the absence of substrate reworking and 2 differential taphonomic overprint on trilobite preservation can occur within a monofacial succession driven by subtle environmental change insufficient to leave a distinct lithological signature this highlights the need for careful microstratigraphic sedimentological and ichnological analysis prior to comparison of palaeobiological or palaeoecological parameters inferred from fossil assemblages even when the assemblages occur within the same lithofacies

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

exceptional fossil preservation during co2 greenhouse crises abstract exceptional fossil preservation may require not only exceptional places but exceptional times as demonstrated here by two distinct types of analysis first irregular stratigraphic spacing of horizons yielding articulated triassic fishes and cambrian trilobites is highly correlated in sequences in different parts of the world as if there were short temporal intervals of exceptional preservation globally second compilations of ages of well dated fossil localities show spikes of abundance which coincide with stage boundaries mass extinctions oceanic anoxic events carbon isotope anomalies spikes of high atmospheric carbon dioxide and transient warm wet paleoclimates exceptional fossil preservation may have been promoted during unusual times comparable with the present co 2 greenhouse crises of expanding marine dead zones oceanic acidification coral bleaching wetland eutrophication sea level rise ice cap melting and biotic invasions

cerium anomaly and th u fractionation in the 1 85 ga flin flon paleosol clues from ree and u rich accessory minerals and implications for paleoatmospheric reconstruction the 1 85 ga paleosol at flin flon manitoba canada was one of the first paleoweathering profiles taken as evidence for a dramatic rise in the oxygen content of the paleoproterozoic atmosphere diagenesis and greenschist facies metamorphism have modified the abundances of some major elements e g k and fe in the flin flon paleosol inductively coupled plasma mass spectrometry icp ms analyses reveal a positive ce anomaly on both chondrite and amisk basalt normalized ree patterns and marked th u fractionation in the uppermost maroon paleosol the ce anomaly is confirmed by the occurrence of cerianite and by the compositions of other ree and u rich accessory minerals i e secondary monazite and uraninite in the uppermost maroon paleosol th u fractionation is supported by the presence of uraninite in both the paleosol and overlying missi sedimentary rocks chemical ages of uraninite 1 85 to 1 0 ga suggest that this mineral might have formed during weathering or diagenesis but was susceptible to disturbance although igneous monazite and zircon are well preserved in even the uppermost maroon paleosol primary fluorapatite exhibits variable degrees of weathering in the basal green paleosol and has been obliterated completely in the upper maroon paleosol published paleomagnetic data suggest a tropical paleolatitude for the flin flon region at the time of formation of the paleosol the positive ce anomaly in the flin flon paleosol confirms previous studies for an oxic atmosphere p o 2 10 1 5 to 10 2 pal at 1 85 ga however extreme caution must be exercised in the interpretation of th u fractionation in paleosols moreover this study demonstrates the advantage of a detailed characterization of ree and u rich accessory minerals in the application of trace element geochemistry of paleosols for paleoatmospheric reconstruction eh ph diagrams for the stabilities of cerianite and uraninite in their respective systems ce p c h o and up c h o at 25 c and 1 bar reveal the importance of phosphate complexes in the formation of ce anomalies and th u fractionation in both modern and ancient weathering environments

plutonism deformation and metamorphism in the proterozoic flin flon greenstone belt canada limits on timing provided by the single zircon pb evaporation technique the authors have used the single zircon pb evaporation technique to determine the age of granitoids in the western flin flon domain of the trans hudson orogen in canada and to constrain the timing of molasse sedimentation deformation metamorphism and mesothermal gold mineralization the plutons were intruded between 1 860 and 1 834 ma about 30 m y after volcanism and provide further evidence of a major period of intrusive activity throughout the trans hudson orogen at that time the older plutons 1 860 to 1 848 ma are synchronous with p2 deformation and the early stages of peak metamorphism whereas the younger plutons are synchronous with or postdate the p3 deformation event molasse sedimentary units are deformed and metamorphosed thus deposition must have begun at about 1 860 ma mesothermal gold mineralization postdates all plutons and may be related to high grade peak metamorphism in adjacent terranes at about 1 815 ma this study also provides further support for the use of the pb evaporation technique as a suitable method for dating single zircons and indicates that the sup 208 pb sup 206 pb ratio of zircons may be a useful tool in detecting the presence and source of xenocrystic zircons

constraints on the timing of crustal imbrication in the central trans hudson orogen from single zircon 207pb 206pb ages of granitoid rocks from the pelican thrust zone saskatchewan the pelican thrust is a major ductile high strain zone in the reindeer zone trans hudson orogen northern saskatchewan it is interpreted as the main sole thrust separating stacked juvenile paleoproterozoic allochthons and underlying archean microcontinental crust in this central part of the orogen exposed nonmylonitic rocks in the footwall of the thrust consist of the sahli monzocharnockite and the smaller more highly retrograded macmillan point granite protomylonitic to ultramylonitic gneisses in the thrust zone derive from a variety of prethrust protoliths a footwall internal suite mainly comprises quartzofeldspathic orthogneisses q gneisses and high grade migmatitic paragneisses hanging wall external suite mylonitic gneisses include feldspar porphyroclastic hornblendic grey gneisses probably derived from arc plutons and laminated amphibolites derived from volcanic rocks the overlying allochthon mainly comprises protoliths equivalent to those of the porphyroclastic orthogneisses and lam

a proposed nomenclature for stages and series for the cambrian of laurentia an internally consistent succession of six named regional stages from youngest to oldest sunwaptan steptoean marjuman delamaran dyeran montezuman with their boundaries defined by regional extinction events within the trilobite record is proposed for the trilobite bearing rocks of laurentia sub trilobite stage s are recommended but unnamed the three youngest stages have been named previously but the lower boundary of the marjuman is shifted downward the three older stages within the trilobite record are new if deemed necessary four regional series are also suggested from youngest to oldest millardan lincolnian waucoban and begadean each of the three youngest series accommodates two stages

age and origin of the jan lake complex a glimpse at the buried archean craton of the trans hudson orogen the largely buried sask craton is a continental fragment or microcontinent that collided with and underthrust the paleoproterozoic flin flon glennie protocontinent along the pelican decollement zone at 1840 1830 ma rocks of the sask craton are exposed in three tectonic windows those of the pelican window have been named the jan lake complex and comprise 2960 ma arc derived leucocratic orthogneisses migmatitic paragneisses and a tholeiitic within plate igneous suite comprising 2488 ma dioritic to gabbroic rocks and 2450 ma enderbitic and charnockitic rocks including the sahli granite ages of ca 2450 ma are also common from the other two tectonic windows to the sask craton suggesting that emplacement of the igneous suite was widespread and perhaps part of the coeval matachewan igneous event the absence of rocks in the 2960 and ca 2450 ma age ranges on both the adjacent superior and rae hearne cratonic margins makes it improbable that the sask craton was derived by simple fragmentation without large scale tectonic transport the overthrust paleoproterozoic rocks represent a high grade northwestern extension of the flin flon volcanic belt and include 1856 ma leucotonalite and 1843 ma quartz monzodiorite an 1830 ma suite of homogeneous calc alkaline enderbitic rocks which intrude burntwood semipelitic migmatites throughout the kisseynew domain has also been emplaced in mylonites of the pelican decollement zone zircon and monazite ages in the 1812 1803 ma range record a high grade metamorphic event that resulted from collision between the sask craton and flin flon glennie complex

hydrothermal zircon and baddeleyite in val d or archean mesothermal gold deposits characteristics compositions and fluid inclusion properties with implications fortiming of primary gold mineralization zircon and baddeleyite occur within quartz tourmaline veins at four gold deposits in the val d or district of the archean abitibi southern volcanic zone host rocks have experienced intense metasomatic enrichment of zr hf y and rare earth elements the zircons contain primary inclusions of quartz tourmaline pyrite albite k mica scheelite and gold and gold occurs in primary fluid inclusions in zircons magmatic zircons in host rocks do not have this suite of inclusions consequently a wall rock inheritance model for the vein zircons is implausible compositionally the zircons feature pronounced interzone and intergrain variations of hf y yb th and u and sporadic anomalous ce contents of 1100 ppm distinct from magmatic counterparts two principal types of primary fluid inclusion occur in the vein zircons type 1 h2o co2 inclusions have low salinities variable quantities of co2 and homogenization temperatures of 260 380 c and type 2 co2 rich inclusions contain minor h2o and ch4 the vei

substrate control of benthos in a middle cambrian near shore epeiric palaeoenvironmental setting late middle cambrian mail change limestone a thin b30 m lime mudstone unit of the epeiric middle and late cambrian georgina basin australia is unusually rich in epifaunal linguliform brachiopods especially acrotretids it is the only unit of this abundantly fossiliferous cambrian basinal succession where the representation of this group is more prolific than that of trilobites the diminutive size of acrotretids generally less than 3 mm in shell diameter in the case of common taxa from the mail change limestone raises questions of how an epizoic lifestyle was achieved on muddy substrates facies analysis indicates that lime mud substrates associated with deposition of the mail change limestone included microbial firm ground which developed in upper shoreface settings and ephemeral evaporate horizons of supratidal or high intertidal association during the shallow water phases of repeating parasequence cycles lime muds of shoreface association are brachiopod rich suggesting that firm ground substrates were favoured attachment sites for populations of anchoring epizoic acrotretids the conical shell form typical of this group evolved in part to minimize current drag reducing stress on pedicle attachment and in part to induce turbulence at the shell gape to facilitate feeding the high incidence of undamaged acrotretid valves representing a spectrum of growth stages obtained from the mail change limestone suggests that failure of pedicle attachment was a major contributor to mortality in this group firm ground was also characterised by ubiquitous fine scale planolites like open burrows which housed substantial populations of wormlike suspension feeders polychaetes which also took advantage of firm ground substrates near shore substrates were also host to suspension feeding lingulella and protospongia and a deposit feeder responsible for thalassinoides ichnofossils substrates associated with deposition of the mail change limestone were unfavorable to colonization by vagile seafloor feeding ptychopariid trilobites which are poorly represented in the unit in contrast agnostid trilobites of nektonic habit were less limited by trophic opportunities and are more commonly represented diverse agnostids ranged into very shallow marine shoreface environments represented by the mail change limestone the association of abundant epifaunal linguliforms and infaunal suspension feeders with firm ground suggests that substrate stability strongly influenced the nature and biomass of middle cambrian sessile benthos evolutionary strategies among sessile benthos to take full advantage of shallow marine level bottom environments were not realised until the ordovician biodiversity event

growth mechanisms and geochemistry of carbonate concretions from the cambrian wheeler formation utah usa carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments typically they form in organic rich mudstones where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments as a result carbonates that comprise concretions are usually characterized by broad ranges in 13c and include values that are significantly depleted relative to seawater this article reports results from a physical petrographic and geochemical analysis of 238 concretions from the wheeler formation cambrian series 3 utah usa which are unusual in several respects most prominently they formed in organic poor mudstones total organic carbon 0 1 to 0 5 and are characterized by a narrow range of 13c that onlaps the range of contemporaneous seawater values subtle centre to edge trends in 13c demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters the large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix which occurred via both displacive and replacive carbonate precipitation during early diagenesis stable isotope data from cogenetic pyrite 34s and silica 18o phases provide insight into the evolution of biogeochemical processes during concretion growth and suggest that concretions were formed almost entirely during sulphate reduction with only minor modification thereafter concretions of the wheeler formation appear to represent an end member system of concretion formation in which rapid growth was promoted by ions supplied from sea water as such they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments

geochemistry and origin of cordierite orthoamphibole gneiss and associated rocks at an archaean volcanogenic massive sulphide camp manitouwadge ontario canada abstract cordierite orthoamphibole gneiss amphibolites quartz biotite schist and calc silicate rocks occur in spatial association with archaean metamorphosed volcanogenic massive sulphide vms deposits at manitouwadge ontario trace element characteristics particularly those of the high field strength elements hfse support field relationships that most of the cordierite orthoamphibole gneisses represent metamorphosed equivalents of chlorite rich rocks that were derived hydrothermally from tholeiitic basalts as an integral part of the syngenetic ore forming processes on the archaean seafloor however a garnet rich cordierite orthoamphibole dike that is characterized by low tio2 content and high concentrations of zr y nb and ree and has single zircon pb evaporation 207 pb 206 pb ages of 2715 2743 ma may have been derived from a synvolcanic felsic dike evidently the seafloor hydrothermal alteration at manitouwadge resulted in similar alteration products from contrasting protoliths and therefore was a fluid dominant system the amphibolite lenses within the cordierite orthoamphibole gneiss also have been affected by the seafloor hydrothermal alteration and some of them most likely represent metamorphosed equivalents of calcsilicate alteration products formed on the seafloor mass balance calculations indicate that hfse and ree were mobilized during the seafloor hydrothermal alteration the mobility of hfse and ree was controlled by both their host minerals and the hydrothermal fluids in particular a significant portion of igneous zircon in the garnet rich dike has survived the seafloor hydrothermal alteration and the subsequent high grade regional metamorphism the regional metamorphism did not change the bulk rock ree characteristics but was important in redefining the mineralogy of ree trace element geochemistry of mafic metavolcanic rocks at manitouwadge are similar to those of the adjacent schreiber hemlo greenstone belt and are consistent with a volcanic island arc setting however further work is required to determine whether the manitouwadge and adjacent greenstone belts of the wawa subprovince were originally a larger continuous terrane

prefrontal wind shift lines in the plains of the united states abstract in the central united states cold fronts are sometimes preceded by a surface wind shift by several hours these wind shifts are examined to determine their origins and characteristics a climatology and a case study of an autumn prefrontal wind shift are presented data collected by the recently implemented oklahoma mesonet and a network of wind profilers permit finescale analyses of the autumn prefrontal wind shift it is found that prefrontal wind shifts are associated with both lee troughs and drylines the climatology reveals that in the central united states fronts are most frequently up to 60 of them preceded by wind shifts just east of the rockies where lee troughs and drylines originate this paper focuses on a case study of a prefrontal wind shift associated with a lee trough that forms when a front approaches the lee trough the prefrontal wind shift associated with the lee trough forms in the lee of the mountains but propagates away from the mountains when downslope flow decreas

burgess shale type biotas were not entirely burrowed away burgess shale type biotas occur globally in the cambrian record and offer unparalleled insight into the cambrian explosion the initial phanerozoic radiation of the metazoa deposits bearing except

geodynamic evolution and thermal history of the central flin flon domain trans hudson orogen constraints from structural development 40ar 39ar and stable isotope geothermometry shear zones in the flin flon area record five episodes of development these have been studied using structural analysis and 40ar 39ar thermochronology deformation occurred as a result of collision between the archean rae hearne and archean superior plates during the trans hudson orogeny 1900 to 1700 ma the geodynamic evolution encompasses a transition from island arc supracrustal development 1900 ma to tectonic mountain building under brittleductile conditions 1860 ma two episodes of ductile lineation schistosity l s shear zones on the prograde metamorphic path 1860 1810 ma and two episodes of retrograde displacement between 1790 and 1690 ma during final suturing of the archean cratons with paleoproterozoic assemblages metamorphism evolved in three stages contact aureoles to granitoid bodies formed at 1860 1840 ma hornblende homfels facies 40ar 39ar ages indicate cooling through 500 c at 1840 ma this was followed by regional heating burial which produced regional peak thermal recrystallization at 1820 1790 ma under lower greenschist to lower amphibolite conditions lastly retrograde metamorphism took place during unroofing between 1790 and 1690 ma with a regional cooling rate of 2 c m y

thermochronology of the needle falls shear zone a post collisional high strain zone of the trans hudson orogen the needle falls shear zone is a major paleoproterozoic discontinuity separating an archean craton from accreted paleoproterozoic terranes in the western trans hudson orogen it is a 1 km wide subvertical mylonite zone forming the southwestern sector of a northeast trending shear system extending for more than 400 km kinematic data indicate oblique east side up dextral strike slip displacement throughout its evolution from ductile through late brittle deformation for most of its exposed length it marks the northwestern margin of the wathaman batholith the oldest pb evaporation ages of zircons from mylonite average 1857 ma consistent with published zircon ages of the wathaman batholith confirming the batholith is protolith for much of the shear zone the synkinematic metamorphic assemblage of amphibole biotite quartz feldspar signifies deformation under amphibolite facies conditions 500 630 c oxygen isotope fractionations between quartz hornblende and quartz biotite pairs in textural equilibrium

elongated thoracic spines as potential predatory deterrents in olenelline trilobites from the lower cambrian of nevada abstract repaired injuries offer the opportunity to study predation in the fossil record and test hypotheses concerning predation intensity anti predatory characters and predator selectivity many studies have used the late palaeozoic and mesozoic records but quantitative explorations of early palaeozoic predation especially during the cambrian are lacking trilobites a group of biomineralised arthropods with a high early palaeozoic disparity and diversity are an ideal study system for exploring repaired injury frequencies the ruin wash lagerstatte nevada usa stage 4 cambrian series 2 provides a bulk sample of closely related but morphologically distinct taxa the five most abundant taxa olenellus chiefensis o fowleri o gilberti o terminatus and nephrolenellus geniculatus differ in the spinosity of the cephalon axial lobe of the thorax and thoracic pleural lobes a total of 26 repaired injuries on 330 articulated trilobite cephalothoraces reveal that o terminatus has the highest incidence of repaired injuries repair frequency 0 18 and n geniculatus has the lowest repair frequency 0 043 the role of enlarged spines on the third thoracic segment which are most elongate in n geniculatus are suggested as predation deterrents predator selection for prey of a larger size may have played a minor role though we did not find statistical support for this idea three cephala with repaired injuries were noted in a sample of 685 specimens a much lower repair frequency compared to thoraces this indicates that cephalic attacks were more often fatal when compared to thoracic attacks our study provides the first quantitative support for species specific predation on cambrian trilobites

three dimensional collisional structure of the trans hudson orogen canada abstract the three dimensional structure of the eastern trans hudson orogen tho part of a paleoproterozoic continent continent collision zone in central north america is revealed through a network of lithoprobe seismic reflection profiles the seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust e w profiles show strong e dipping reflections extending throughout the crust while n s profiles record events outlining antiformal and synformal structures this allows the identification of decollements that may have localized along pre existing structures e g possible basin bounding extensional faults and at major rheological boundaries e g basement cover contact upper middle crust transition the present topographic surface displays oblique crustal sections with 10 15 km of structural relief generated during post collisional intracontinental transpression of tho as a result of crustal scale folding and faulting

age and source of detrital zircons from the missi formation a proterozoic molasse deposit trans hudson orogen canada the missi formation in the flin flon basin forms part of a discontinuous series of molasse type sediments found throughout the early proterozoic trans hudson orogen in northern saskatchewan and manitoba the flin flon basin contains a sequence of proximal fan to braided stream fluvial conglomerates and sandstones which unconformably overlie subaerially weathered amisk group volcanic rocks stratigraphic way up indicators have been preserved even though these rocks have undergone greenschist facies metamorphism and polyphase deformation the sedimentary rocks are crosscut by intrusive rocks which provide a minimum age of sedimentation of 1840 7 ma detrital zircons from each of the six stratigraphic subdivisions of the flin flon basin were analyzed using the single zircon pb evaporation technique euhedral to slightly rounded zircons dominate each sample and these zircons give ages of between about 1854 and 1950 ma the missi sediments were thus deposited between 1840 and 1854 ma possible sources for

structure and evolution of winter cyclones in the central united states and their effects on the distribution of precipitation part ii arctic fronts abstract the structure and evolution of a shallow but intense cold front commonly referred to as an arctic front and its associated precipitation features that passed through the central united states from 0000 utc 9 march to 0000 utc 10 march 1992 are studied with the aid of observations and outputs from a numerical simulation using the pennsylvania state university national center for atmospheric research mesoscale model mm4 located above the arctic front was a region of midtropospheric frontogenetical confluence that was attended by a thermally direct vertical circulation a large banded precipitation feature for the most part located behind the arctic front was produced by ice crystals from upper level clouds formed by the frontogenetical confluence falling into low level stratocumulus associated with the arctic front the arctic front at the surface separated a region where the precipitation reaching the ground was solid from an adjacent region where the precipitation was liquid a westward m

upb geochronology and tectonic development of the southern flank of the kisseynew domain trans hudson orogen canada abstract the southern flank of the kisseynew domain northern manitoba underwent polyphase deformation and metamorphism during the palaeoproterozoic trans hudson orogen and represents a lithological metamorphic and structural transition from greenschist grade volcano plutonic rocks of the flin flon domain to amphibolite grade paragneisses of the central kisseynew domain the timing of geological events has previously been constrained using widely dispersed sample sites but this study has integrated u pb geochronology with detailed lithological and structural mapping in the cleunion lake area pink gneisses interpreted as missi supracrustal rocks were deposited after 1855 ma but prior to 1835 ma and thus are similar in age to fluvial sedimentary rocks in the lower grade flin flon domain all the zircons in the pink gneisses appear to be inherited or resorbed during later amphibolite grade metamorphism and a late archaean component is also indicated corundum bearing gneisses in the missi group and leucosomes in burntwood river group rocks are deformed by f1 and are indicative of an early high t low p metamorphic event m1 that may have been synchronous with the intrusion of 1835 to 1827 ma intrusions and related to regional crustal extension the above rocks are folded during a continuous compressional deformation event d2 related to terminal collision between the palaeoproterozoic rocks of the trans hudson orogen and the superior province three main fold generations f1 f2 f3 have been identified which occurred during d2 a 1818 ma granitic vein is approximately synchronous with f2 and peak metamorphic m2 conditions in the cleunion lake area cooling uplift and intrusion of pegmatites during f3 at 1800 ma provides a time limit on the waning stages of compressional deformation the main phase of compressional deformation d2 in the southern flank of the kisseynew domain thus lasted for about 30 million years

the structure and evolution of a continental winter cyclone part i frontal structure and the occlusion process the frontal structure and occlusion process in a cyclone of moderate intensity that affected the central united states in january 1995 is examined the deep warm frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that although characterized by cold air advection near the surface had many of the characteristics of a warm front aloft in fact this feature had a structure similar to the so called bent back fronts previously documented only in association with explosively deepening maritime cyclones the development of a warm occluded structure was investigated with the aid of a numerical simulation of the event performed using the university of wisconsin nonhydrostatic modeling system the development of the warm occluded structure was asynchronous in the vertical occurring first at midtropospheric levels and later near the surface in contrast to the classical occlusion process near the surface the warm occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical coldfrontal zone at midtropospheric levels the warm occluded structure formed as a result of the cold frontal zone approaching and subsequently ascending the warm frontal zone in accord with a component of the classical occlusion mechanism the observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper level potential vorticity pv anomaly that controls the cyclogenesis it is suggested that the occlusion process begins aloft where the associated vortex strength is greatest and gradually penetrates downward toward the surface during the cyclone life cycle additionally a characteristic treble clef shape to the upper level pv anomaly is shown to be a sufficient condition for asserting the presence of a warm occluded structure in the underlying troposphere

kinetics of pb release during the zircon evaporation technique abstract the release of silica and pb from zircon by the thermal evaporation technique is shown to relate to the conversion of zircon to a porous baddeleyite structure the zircon baddeleyite reaction occurs at a sharp reaction front which progressively migrates towards the centre from the grain margin significant reaction in non metamict zircon commences in the mass spectrometer at temperatures of the order of 1500 c the progressive migration of zircon baddeleyite transformation towards grain interiors explains why the evaporation technique with its inherent advantage of not requiring chemical separation of pb and u progressively samples pb in zones towards the centre of the grain and gives 207 pb 206 pb ages as accurate as conventional u pb dating methods the rates of pb and silica loss are primarily determined by the rate of advance of this de silicification reaction front towards the interior of the grain most estimates of pb diffusion coefficients in zircon imply that diffusion distances in the unreacted zircon are negligible 10 2 m at the temperatures and duration of the pb evaporation experiments isothermal experiments demonstrate the rate of zircon baddeleyite transformation with time and temperature and provide an estimate of 130 10 1 kcal mol 1 for the activation energy of the process significantly higher than estimates of pb diffusion activation energy

mineral composition of host sediments influences the fossilization of soft tissues cambrian lagerstatten host rocks are frequently composed of kaolinite and chlorite in varying amounts accordingly our goal was to study the preservation potential of crustaceans in these two clays we conducted long term experiments 12 18 months the longest duration of actualistic taphonomy experiments from published literature on the decay of artemia salina in these clay sediments the degree of preservation transformed mineralogical composition of the sediments and the elemental composition of the nauplial remains were examined we demonstrate that the kaolinite and chlorite sediment enhanced the preservation in the kaolinite the effect was considerably higher than in the chlorite compared with the sediment free control ph inside the sediments dropped to 6 5 7 1 and was even lower 4 around the buried carcasses facilitating the dissolution of clays this phenomenon was confirmed by mineralogical analyses of the experimental sediments which showed mineralogical signatures of such dissolutio

chronology of crustal growth and recycling in the paleoproterozoic amisk collage flin flon belt trans hudson orogen canada u pb zircon ages have been compiled for magmatic and sedimentary rocks from the low metamorphic grade portion of the flin flon greenstone belt now recognized as a paleoproterozoic tectonic collage

taphonomy and palaeoecology of the emuellid trilobite balcoracania dailyi early cambrian south australia monospecific assemblages of the trilobite balcoracania dailyi occur in lower cambrian strata within the adelaide geosyncline in south australia biostratinomic data from single bedding plane assemblages within the warragee and coads hill members of the billy creek formation and white point conglomerate reveal a range of taphonomic signatures from census to within habitat timeaveraged assemblages these assemblages are interpreted as having inhabited protected shallow marginal marine environments size frequency distributions coupled with taphonomic data show that the warragee member census assemblage represents a living population in a physically stressful environment within a tidally influenced lagoon while the original population structure of the coads hill member and white point conglomerate assemblages has been lost due to varying degrees of taphonomic overprinting integration of taphonomic stratigraphic and sedimentological data supports the interpretation of b dailyi as representing an opportunistic species a preserved body cluster from the warragee member assemblage is considered to characterise a congregation formed for the purpose of synchronous reproduction and ecdysis representing one of the oldest examples of gregarious behaviour in the arthropod fossil record furthermore by analogy with modern horseshoe crabs the high number of juveniles i e protaspides and early meraspides within the same assemblage are believed to be constituents of a nursery within the intertidal zone with adults migrating into the shallows to copulate and spawn preserved moult ensembles from the coads hill member and white point conglomerate have enabled the description of exuviation techniques for b dailyi 2007 elsevier b v all rights reserved

taphofacies models for epeiric sea environments middle paleozoic examples abstract skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and therefore are useful in discriminating taphonomic facies for purposes of taphonomic comparisons skeletonized marine taxa are reduced to five generally defined types massive arborescent univalved bivalved and multielement different skeletal types display different attributes which reflect specific responses to physical chemical and or biological processes the taphonomic properties of disarticulation reorientation and sorting fragmentation corrosion and abrasion skeletal dissolution and early diagenesis display patterns which are predictable with respect to a variety of inter related physico chemical parameters these can be quantified through indices which evaluate relative frequencies of occurrence sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event related properties thereby generating composite taphofacies such events produce taphonomic signature which in themselves correspond to specific environmental conditions i e water depth and event intensity indeed background and event related processes interact to generate complex but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments the comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties deductively derived taphofacies models corroborated by empirical data from paleozoic strata illustrate the distribution of taphonomic properties with respect to environmental energy turbulence background sedimentation rate and sediment oxygenation these parameters also indirectly influence other less obvious factors including bioturbation and sediment chemistry trends in the distribution of taphonomic properties taphonomic gradients permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore offshore transect within a generalized epeiric sea setting taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena moreover taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies

testing patterns of association between brachiopod bioclast deposits and stratigraphic discontinuities a case study in middle upper devonian carbonate dominated settings of north america abstractnondepositional hiatuses tend to favor the dense accumulation of skeletal material because they are associated with minimal sedimentary dilution of bioclast input however bioclast durability or production must be sufficient to counterbalance the rigors of delayed burial and especially the repeated events of seafloor exposure and reworking that bioclasts can experience during sedimentary bypass or starvation associated with the formation of hiatuses detailed field investigations of skeletal concentrations in middle upper devonian brachiopod rich carbonate stratigraphic records from the continental margin of nevada and the cratonic interior of iowa reveal a surprising lack of abundant skeletal material mantling major stratigraphic discontinuities rather the distribution of densely packed brachiopod deposits tends to be determined by physical processes that operate within the timescale of deposition of a single lithofacies producing skeletal concentrations that are mostly associated with beddin

single zircon age constraints on the tectonic juxtaposition of the archean abitibi greenstone belt and pontiac subprovince quebec canada abstract zircons from metasediments and granitoids in the high grade lacorne block within the low grade archean abitibi greenstone belt have been dated by the single zircon pb evaporation technique yielding 207pb 206pb minimum ages detrital zircons in the mature clastic metasediments of the lacorne block display a range of ages from 2691 8 ma to 3042 6 ma the younger zircon ages 2691 8 2695 4 2715 16 and 2719 4 ma thus impose an upper limit for deposition and indicate that the high grade lacorne block is not basement to the abitibi supracrustal sequence 2747 2680 ma existence of abundant 69 older detrital zircons 2750 ma suggests in turn that the abitibi supracrustal rocks are not the source of the lacorne sediments two generations of granitoids occur in the lacorne block an early monzodiorite monzonite granodiorite syenite series mmgs 2671 4 to 2675 24 ma and a younger s type garnet muscovite granite series gmg 2630 20 to 2655 8 ma this contrasts with granitoid magmatism in the abitibi greenstone belt which ended at 2675 ma the pontiac subprovince to the south of the abitibi greenstone belt shares all of the above features of the lacorne block including detrital zircon ages as well as the composition and timing of granitoid magmatism this is interpreted to indicate that the lacorne block was originally part of the same tectonic terrane as the pontiac subprovince after development of the mmgs magmatism 21670 2680 ma the pontiac subprovince locally underthrust the abitibi greenstone belt and crustal thickening promoted partial melting of underthrust pontiac metasediments to form the 2644 13 ma s type granites later differential uplift exposed the lacorne block as a high grade tectonic window in the abitibi greenstone belt

th u zonation in zircon derived from evaporation analysis a model and its implications abstract zircon evaporation analyses often exhibit the existence of spatial extraneous variations in 208 pb 206 pb compared to 207 pb 206 pb as revealed by increasing evaporation temperature and duration of analysis as 208 pb solely stems from the decay of 232 th variations are interpreted to directly reflect changing th u ratios within different zircon domains which are probed at different evaporation temperatures and evaporation duration such strongly varying th u is interpreted as reflecting the magmatic growth zonation in the zircon crystal constant th u over several evaporation steps is interpreted as being characteristic for homogeneous zircon domains which probably originate from zircon recrystallisation or growth during a metamorphic event stepwise evaporation of zircon crystals and the monitoring of th u ratios thus provide some means to decipher the internal structure of the analysed zircon and therefore allows some conclusions about magmatic vs recrystallised or newly grown metamorphic origin to be drawn

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

a new burgess shale type assemblage from the thin stephen formation of the southern canadian rockies a new burgess shale type assemblage from the stephen formation of the southern canadian rocky mountains is described herein it occurs near stanley glacier in kootenay national park 40 km southeast of the type area near field british columbia while at least a dozen burgess shale localities are known from the thick stephen formation the stanley glacier locality represents the first discovery of burgess shale type fossils from the thin stephen formation the cathedral escarpment an important regional paleotopographic feature has been considered important to the paleoecologic set ting and the preservation of the burgess shale biota however the stanley glacier assemblage was preserved in a distal ramp setting in a region where no evidence of an escarpment is present the low diversity assemblage contains eight new soft bodied taxa including the anomalocaridid stanleycaris hirpex n gen n sp new genus new species pelagic or nektobenthic predators represent the most diverse group whereas in relative abundance the assemblage is dominated by typical cambrian shelly benthic taxa the low diversity of both the benthic taxa and the ichnofauna which includes diminutive trace fossils associated with carapaces of soft bodied arthropods suggests a paleoenvironment with restrictive conditions the stanley glacier assemblage expands the temporal and geographic range of the burgess shale biota in the southern canadian rockies and suggests that burgess shale type assemblages may be common in the thin ste phen formation which is regionally widespread

trilobites in early cambrian tidal flats and the landward expansion of the cambrian explosion reply the timing of the early invasion of the continents the routes to the land and the environmental breadth of the cambrian explosion are important topics because they are at the core of our understanding of early evolutionary breakthroughs illuminating some aspects of these problems are trilobite trace fossils in tidal flat deposits from the lower cambrian rome formation in the southern appalachian mountains of tennessee usa morphologic details and size range of the trace fossils suggest production by olenellid trilobites which occur as body fossils in the same unit the occurrence of this ichnofauna together with physical structures indicative of periodic subaerial exposure desiccation cracks and deposition within the intertidal zone flat topped ripples shows that trilobites forayed into the upper intertidal zone during the cambrian our finding supports the migration of subtidal organisms into marginal marine intertidal settings at the dawn of the phanerozoic suggesting that trilobites contributed to the establishment of the intertidal ecosystem during the cambrian the sequence of events involved in the colonization of early paleozoic tidal flats is consistent with the idea that most terrestrial taxa originated from marine rather than freshwater ancestors and that direct routes to the land from marginal marine ecosystems were involved in the colonization of continental environments early in the phanerozoic

herpetogaster from the early cambrian of nevada series 2 stage 4 and its implications for the evolution of deuterostomes the pioche formation of se nevada preserves a diverse soft bodied fauna from the early and middle cambrian series 2 3 stage 4 5 while the fauna is dominated by arthropods animals belonging to other taxa can be found here we document the first occurrence of herpetogaster collinsi outside the burgess shale further the specimens are from the nephrolenellus multinodus biozone and thus represent the oldest occurrence of the species as well as possibly the earliest soft bodied deuterostomes in laurentia

thermotectonic studies in the paleoproterozoic glennie domain trans hudson orogen canada abstract the glennie domain of the trans hudson orogen tho is characterized by paleoproterozoic island arc volcanogenic assemblages and granitoid rocks thrust as sheets over the archean saskatchewan craton prior to terminal collision between the archean rae hearne and superior cratons four discrete deformation events have been recorded in the domain the d1 and d2 events are associated with the southwestward directed imbrication of paleoproterozoic allochthons over the saskatchewan craton while the d3 and d4 events are associated with terminal collision and transpression respectively discernable metamorphic episodes are associated with the d2 and d3 events with peak thermal metamorphism during d3 using inveq thermobarometry metamorphic conditions during d3 in the santoy lake area were ca 655 705 c and 6 1 7 3 kbar and ca 530 595 c and 3 2 4 8 kbar for the brownell lake area 40ar 39ar hornblende and biotite ages suggest the santoy lake area remained above ca 490 c until ca 1720 ma when it underwent rapid cooling through the biotite ar closure temperature of ca 330 c 40ar 39ar biotite ages from the brownell lake area suggest moderate cooling from peak thermal metamorphism at ca 1810 1755 ma when it passed through the ca 310 c biotite ar closure temperature differences between cooling rates for the two areas are interpreted as differential exhumation of smaller lithotectonic blocks bound by domain wide deep crustal faults and shear zones zircon pb evaporation ages of variably deformed granitoid rocks from the santoy lake area show that the d2 event occurred after ca 1857 ma the oldest granitoid in the santoy lake area provides a minimum age of mafic volcanism in the area of ca 1886 ma as well as a significant hiatus ca 50 ma between deposition of the two supracrustal assemblages in the domain

geochemistry and zircon geochronology of the i type high k calc alkaline and s type granitoid rocks from southeastern roraima brazil orosirian collisional magmatism evidence 1 97 1 96 ga in central portion of guyana shield abstract the understanding of the geological evolution of the uatuma anaua domain in southeastern roraima central region of guyana shield is of major significance in the study of the amazonian craton this region lies between some major paleoproterozoic geological geochronological provinces tapajos parima or ventuari tapajos dominant maroni itacaiunas or transamazon northwest and central amazonian or central amazon southeast and east geological mapping of the northern area of uatuma anaua domain integrated with whole rock geochemistry data and previous and new pb evaporation and u pb zircon geochronology point out for a plutonic collisional magmatic event 1975 1968 ma represented by i type high k calc alkaline martins pereira and s type serra dourada granitoid rocks this magmatism was probably generated from crustal sources by partial melting during amalgamation of the ttg like anaua magmatic arc 2028 ma with transamazonian 2 2 2 0 ga and central amazonian older than 2 3 ga terranes local cumulatic leucogranites fills planar structures of the martins pereira granites these leucogranites show younger ages 1909 ma and several inherited zircons 2354 2134 1997 and 1959 ma suggesting origin from crustal sources

conventional u pb dating versus single grain pb evaporation dating of complex zircons from a pegmatite in the high grade gneisses of southwestern sweden u pb multi grain dating of zircons from a pegmatite dyke in southwestern sweden yield analyses which define a discordia with upper and lower intercept ages of 1510 32 ma and 631 64 ma respectively dating by the single zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 20 ma in addition four oval zircon grains yield a 207pb206pb age of 969 27 ma the older single zircon evaporation age is interpreted as representative of pegmatite emplacement whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late sveconorwegian metamorphic event to explain the false intercept ages in the u pb concordia diagram this study suggests a three stage model including 1 crystallization of zircons during pegmatite intrusion 2 episodic lead loss and new zircon growth during high grade metamorphism and 3 a recent lead loss event as the dykes are strongly deformed the protolith age of the pegmatite provides an upper age limit for the gneiss forming event the single zircon age of 969 27 ma is interpreted to be associated with this event which took place during high grade metamorphic conditions