a middle cambrian trilobite faunule from the meguma group of nova scotia the meguma group of southern nova scotia is a thick succession of mostly deep water siliciclastic sedimentray rocks of uncertain age previous fossil finds have been in the halifax formation of the upper part of the sequence and have provided early ordovician ages a newly discovered trilobite bearing bed from the underlying goldenville formation yields middle cambrian taxa of acado baltic affinity specimens belonging to paradoxides s l agraulos dorypyge and ellipsocephalidae indicate that the meguma group is an exotic succession with faunal similarity to the adjacent avalon terrane and to middle cambrian successions in europe the middle east and west africa

early paleozoic avalon gondwana unity an obituary response to palaeontological evidence bearing on global ordovician silurian continental reconstructions by r a fortey and l r m cocks

the south western branch of the variscan belt evidence from morocco abstract this work is based on the compilation and re evaluation of the most significant data either personal or from the literature concerning the moroccan variscides the latter constitute the only moderately disturbed or even undisturbed part of the south western branch of the variscan belt facing directly nw gondwana they include two orogenic segments namely the northern mauritanides and the meseta domain exposed in the saharan and atlas meseta regions respectively and a foreland belt cropping out essentially in the anti atlas the eastward thrust units of saharan morocco oulad dlim mostly originate from the west african craton wac border in an area of thin palaeozoic sedimentation thin skinned fold thrust foreland arcs develop progressively northward zemmour at the expense of the increasingly thick palaeozoic series whereas thick skinned deformation characterizes the inverted proximal paleomargin in the anti atlas domain as suggested by the meseta and anti atlas stratigraphic similarities the meseta domain corresponds to a collage of moderately displaced thinned crustal blocks from the distal gondwana paleomargin variscan deformation is dominated by nw verging thrusts and metamorphism developed in the thickened tectonic prism in relation with crustal anatexis at depth the meseta anti atlas boundary is a major ene trending transpressional dextral fault referred to as the south meseta fault smf discussing the correlations between the variscan segments of morocco and sw iberia allows us to suggest that a latitudinal transform zone similar to the smf separated these segments during the late palaeozoic subduction of the rheic ocean crust would have been directed se ward along both the iberian and moroccan meseta and nw ward south of the smf i e along the wac

eastward propagating intraseasonal oscillation represented by chikira sugiyama cumulus parameterization part ii understanding moisture variation under weak temperature gradient balance abstractthe eastward propagating intraseasonal oscillation represented by the chikira sugiyama cumulus scheme in a general circulation model was investigated focusing on the variation of the free tropospheric humidity the net effect of the vertical advection and cloud process amplifies the positive moisture anomaly in the mature phase supporting the moisture mode theory the horizontal advection causes the eastward propagation of the field the variation of the moisture profile is accurately understood by using environmental vertical velocity outside cumuli the velocity is regulated by a thermodynamic balance under a weak temperature gradient a nondimensional parameter plays an important role in the moisture variation which characterizes the efficiency of moistening drying induced by external heating cooling in the middle and lower troposphere the major moistening factor is the radiative warming anomaly which induces the upward environmental vertical velocity anomaly the reevaporation of th

late cambrian middle furongian shallow marine dysoxic mudstone with calcrete and brachiopod olenid lotagnostus faunas in avalonian cape breton island nova scotia the common belief that organic rich mudstones formed in quiescent distal settings is further weakened by study of an upper cambrian leptoplastus lower peltura superzones succession in the chesley drive group in avalonian cape breton island that is comparable to alum shale successions in baltica dramatic sea level likely eustatic changes are now recognized by punctuation of deposition of shallow wave influenced black mudstone with brachiopod orusia lenticularis and olenid trilobite bearing limestones by offlap and formation of a subaerially cemented calcrete clast conglomerate subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with leptoplastus superzone l ovatus zone trilobite sclerites dynamic processes are shown by wave ripples in the mudstone and limestone sorting and winnowing of fossil rudstones and pre compactional fracture of the conglomerate and rudstones orusia rudstones in the succession below the conglomerate are regarded as analogues of eoorthis and billingsella rudstones in the biomere extinction intervals of the laurentian basal sunwaptan the lowest orusia rich beds are no older than the p spinulosa zone but as elsewhere in avalonia they range into the higher leptoplastus cape breton and even the peltura britain new brunswick superzones rare agnostoid sclerites in lower peltura superzone ctenopyge tumida zone olenid rudstone resemble those traditionally assigned to lotagnostus trisectus in avalonian britain and sweden and are distinct from laurentian l americanus an l americanus zone cannot be identified in avalonia or baltica and the first appearance datum fad of purported l americanus is not suitable as a standard for the base of the highest cambrian stage

geochemistry and u pb protolith ages of eclogitic rocks of the asis lithodeme piaxtla suite acatlan complex southern mexico tectonothermal activity along the southern margin of the rheic ocean recent data indicating that the piaxtla suite acatlan complex southern mexico underwent eclogite facies metamorphism and exhumation during the devono carboniferous suggest an origin within the rheic ocean rather than the iapetus ocean the asis lithodeme piaxtla suite consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks u pb zircon data indicate that the metasediments were deposited after c 700 ma and before intrusion of c 470 420 ma quartz augen granite the metasedimentary rocks contain abundant mesoproterozoic detrital zircons c 1050 1250 ma and a few zircons in the range of c 900 992 and c 1330 1662 ma their geochemical and sm nd isotopic signature is typical of rift related passive margin sediments derived from an ancient cratonic source which is interpreted to be the adjacent mesoproterozoic oaxacan complex megacrystic granites were derived by partial melting of a c 1 ga crustal source similar to the oaxacan complex amphibolitic layers exhibit a continental tholeiitic geochemistry with a c 0 8 1 1 ga source t dm age and are inferred to have originated in a rift related environment by melting of lithospheric mantle in the ordovician this rifting may be related to the early ordovician drift of peri gondwanan terranes e g avalonia from gondwana and the origin of the rheic ocean

tectonic significance of late ordovician silicic magmatism avalon terrane northern antigonish highlands nova scotia12 1 this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2 contribution to international geological correlation programme igcp project 497 avalonia was a microcontinent during most of the ordovician separating the iapetus ocean to the north from the rheic ocean to the south in the northern antigonish highlands nova scotia volcanic rocks dunn point and mcgillivray brook formations underlie early silurian early devonian strata arisaig group and were thought to represent extension that heralded the development of the basin into which arisaig group strata were deposited however recent u pb zircon thermal ionization mass spectrometry data from rhyolite in the dunn point formation dpf yielded an age of 460 0 3 4 ma and here we report a concordant age of 454 5 0 7 ma for an ignimbrite in the overlying mcgillivray brook formation mbf these data confirm a ca 10 million year gap between volcanism and onset of arisaig group deposition which occurred after accretion of avalonia to baltica the dpf and mbf both resemble a type sio2 rich magmas the mbf has very high concentrations of zr 745 1965 ppm y 65 213 ppm nb 57

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

cambrian ordovician boundary age and duration of the lowest ordovician tremadoc series based on u pb zircon dates from avalonian wales two thin volcaniclastic sandstone beds in the bryn llin fawr road section in north wales overlie an apparent sequence boundary within the uppermost cambrian acerocare zone and are overlain by lowest ordovician lower tremadoc rhabdinopora faunas u pb geochronology of zircons from these sandstones yields a maximum cambrian ordovician boundary age of 489 0 6 ma this age indicates both that the tremadoc series lowest ordovician may be shorter in duration than was previously thought and that the duration of the middle and late cambrian c 22 ma was much less than that of the early cambrian c 33 ma cambrian trilobite zones locally had an average duration as short as 1 ma

terminal cambrian and lowest ordovician succession of mexican west gondwana biotas and sequence stratigraphy of the tinu formation the tinu formation of oaxaca state is the only fossiliferous lower palaeozoic unit between the laurentian platform in northwest mexico and gondwanan successions in andean south america the tinu traditionally has been referred to the lower ordovician tremadoc and regarded as having a provincially mixed fauna with laurentian avalonian and gondwanan elements bio and lithostratigraphic re evaluation demonstrates that the tinu is a gondwanan passive margin succession it includes a lower thin to 16 m condensed uppermost cambrian yudachica member new the yudachica nonconformably overlies middle proterozoic basement as a result of very high late late cambrian eustatic levels the yudachica changes from storm dominated but slightly dysoxic shelf facies fossil hash limestone and shale in the south to an upper slope facies with debris flows 50 km to the north three biostratigraphically distinct depositional sequences comprise the yudachica the yudachica has gondwanan aspect trilobites with low diversity conodonts characteristic of unrestricted marine temperate facies the upper tinu or rio salinas member new is a lower ordovician tremadoc depositional sequence that records strong early but not earliest tremadoc eustatic rise marked by graptolite and olenid bearing dysoxic mudstones higher strata shoal upward into shell hash limestones and proximal tempestite sandstones with upper lower tremadocian unrestricted marine temperate conodonts new taxa include orminskia rexroadae landing gen et sp nov from the cordylodus andresi zone this euconodont is related to hyaline coniform genera best known from ordovician tropical platform successions cornuodus clarkei landing sp nov resembles the coeval upper lower tremadoc tropical species scalpellodus longipinnatus ji barnes

first middle ordovician biota from southern new brunswick stratigraphic and tectonic implications for the evolution of the avalon continent a limestone boulder in the triassic lepreau formation near saint john new brunswick has yielded the first diverse marine fauna from the sub caradoc ordovician of the western avalon continent this fauna includes the first arenig conodonts recovered from avalon and represents an unexposed interval in southern new brunswick association of the conodonts drepanoistodus and baltoniodus and the trilobites neseuretus nileus and stapeleyella emphasizes the faunal dissimilarity of avalon and laurentia through the late middle ordovician extension of the ranges of neseuretus cf neseuretus parvifrons and stapeleyella from britain into new brunswick further emphasizes that eastern and western avalon were confluent parts of a unified insular avalon continent that originated in the latest precambrian this fauna correlates with the lower amorphognathus lenodus variabilis zone kundan stage of baltica and the terminal arenig upper middle ordovician lower darriwilian stage of avalonian britain available evidence suggests that an arenig cover sequence with local shallow water hematitic iron ore quartz arenite and rare limestone extended across the avalonian marginal and inner platforms from eastern newfoundland to the boston massachusetts region this western avalonian arenig shows the greatest similarity with the arenig of the welsh borderlands phosphatic fossils from the boulder have a thermal alteration index much lower than that of nearby lower paleozoic outcrops and suggest derivation of the boulder from a weakly heated avalonian succession brought into the bay of fundy region by post ordovician transcurrent faulting

reconstructing the avalonia palaeocontinent in the cambrian a 519 ma caliche in south wales and transcontinental middle terreneuvian sandstones an early cambrian caliche on the st non9s formation emended is the base of the caerfai bay formation unit term changed at caerfai bay south wales subaerial exposure and the caliche mean the two formations were not genetically related units the st non9s is an older sand sheet likely tidalitic not delta related referred to avalonian depositional sequence ads 2 and the caerfai bay is a shallow mud basin unit refered to ads 4a the similar random formation upper ads 2 in north american avalonia has a basal age of c 528 ma and is unconformably overlain by red mudstones or sandstones in fault bounded basins on the avalonian inner platform coeval british sandstones lower hartshill wrekin st non9s brand hills are unconformably overlain by latest terreneuvian ads 3 or epoch 2 ads 4a units dates of 519 ma on caerfai bay ashes give an upper bracket on the late appearance of avalonian trilobites and suggest an ads 2 4a hiatus of several million years post st non9s and post random basin reorganization led to abundant caerfai bay formation volcanic ashes and sparse brigus formation ashes in newfoundland the broad extent of erosional sequence boundaries that bracket lithologically similar to identical units emphasize that east and west avalonia formed one palaeocontinent the inner platform in southern britain was larger than the midlands craton a tectonically defined later palaeozoic area unrelated to terminal ediacaran early palaeozoic depositional belts the cool water successions of early palaeozoic avalonia were distant from coeval west gondwanan carbonate platforms

the early palaeozoic of the north atlantic region as a test case for the use of fossils in continental reconstruction abstract fossils have an important contribution to make in the reconstruction of early palaeozoic palaeogeography the north atlantic regions include well known early palaeozoic fossil faunas which should relate to the dispositions of continental plates during the period however different versions of ordovician palaeogeography based on fossils are described in the literature and in this paper we evaluate the influences which produce these discrepancies the most reliable results are obtained by combining analysis of faunal similarity especially shelf faunas with mapping the distribution of those biofacies which relate to former deep water sites planktic organisms may not be reliable palaeocontinent discriminants cladistic analyses of different groups of organisms living at the same early ordovician time point to the usefulness of trilobite data for palaeogeographic reconstruction such data indicate a three fold division of the north atlantic region into baltica laurentia and gondwana from the cambrian to at least the mid ordovician paris and robardet 1990 denied the existence of a tornquist s sea separating baltica from gondwana in the early ordovician it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos on the other hand there is no evidence of deeper water biofacies along the supposed line of rheic ocean separation of avalonia from gondwana until the late ordovician however there is some evidence of deeper biofacies along the nantes sardic line new term extending from southern armorica to sardinia

upper lower cambrian depositional sequence in avalonian new brunswick the hanford brook formation emended is a thin up to 42 m upper lower cambrian depositional sequence that is unconformably bounded by the lower lower cambrian random formation and the middle middle cambrian fossil brook member of the chamberlain s brook formation these stratigraphic relationships of the trilobite bearing hanford brook formation indicate deposition on the avalonian marginal platform in the saint john new brunswick region and provide more evidence for a uniform latest precambrian cambrian epeirogenic history and cover sequence in avalon the hanford brook formation is a deepening shoaling sequence with i lower transgressive sandstone deposited in episodically high energy environments st martins member new ii highstand regressive dysaerobic mudstone fine grained sandstone with volcanic ashes somerset street member new and iii upper regressive planar and hummocky cross stratified sandstone long island member new trilobites are common in the distal somerse

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

large scale carbon isotope fractionation in evaporites and the generation of extremely 13c enriched methane petrographic fluid inclusion geochemical and gas stable isotope data are reported here for a permian zechstein evaporite sequence this deposit is a geochemically unaltered sequence bromine concentrations show a continuous evaporation profile with little postdepositional alteration in halite chemistry bacterial fermentation gases identified in primary inclusions change from an n 2 h 2 s composition in the lower middle halite series to a ch 4 h 2 composition in the upper halite and potash series carbon isotope results for ch 4 show a 1 3 c enrichment up sequence from typical biogenic values of 45 to 50 to extremely unusual 1 3 c enriched values as high as 21 the d values for these 1 3 c enriched ch 4 gases range from 240 to 377 a model is proposed for the formation of the ch 4 gases whereby the dominant isotopic fractionation process controlling the system was evaporation of the brines this generated a progressive 1 3 c enrichment in the carbon in the residual brines due to preferential loss of 1 2 co 2 to the atmosphere the resulting ch 4 generated in the sediments as evaporation and precipitation advanced recorded this 1 3 c enrichment in the carbon reservoir therefore the isotopic profile observed in this sequence today represents a primary feature with little evidence for postdepositional migration

late neoproterozoic to carboniferous genesis of a type magmas in avalonia of northern nova scotia repeated partial melting of anhydrous lower crust in contrasting tectonic environments avalonian rocks in northern mainland nova scotia are characterized by voluminous 640 600 ma calc alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc however after the cessation of arc activity repeated episodes of felsic magmatism between ca 580 ma and 350 ma are dominated by a type geochemical characteristics sm nd isotopic data combined with zircon saturation temperature estimates indicate that these magmas were formed by high temperature 800 1050 c melting of the same anhydrous crustal source regional tectonic considerations indicate that a type felsic magmatism was produced 1 at 580 ma in a san andreas type strike slip setting 2 at 495 ma as avalonia rifted off gondwana 3 at 465 and 455 in an ensialic island arc environment and 4 at 360 350 ma during post collisional intra continental strike slip activity as avalonia was translated dextrally along the laurentian margin these results attest to the importance of crustal source rather than tectonic setting in the generation of these a type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane they also suggest that the shallow crustal rocks in northern mainland nova scotia were not significantly detached from their lower crustal source between ca 620 ma and 350 ma a time interval that includes the separation of avalonia from gondwana its drift and accretion to laurentia as well as post accretionary strike slip displacement