a sulfate control on marine mid depth euxinia on the early cambrian ca 529 521 ma yangtze platform south china abstract given the high oxygen demands of complex metazoans a sound understanding of marine redox conditions is essential to reconstruct the factors that lead to the dramatic increases in ecosystem complexity and biological diversity in the early cambrian in this study we report new fe mo c s geochemical data from black shale rich strata of the early cambrian continental margin yangjiaping section in the yangtze platform south china integrated fe speciation and mo abundance data from the yangjiaping section in combination with previously reported data from shatan inner shelf songtao slope and longbizui basinal sections suggest mid depth euxinic waters coexisting with ferruginous deep waters and oxic surface waters this redox structure is similar to conditions previously suggested for the ediacaran yangtze platform and other proterozoic marine basins indicating that proterozoic layered ocean redox structure in some basins persisted into the early cambrian furthermore the fe mo data demonstrate that the mid depth euxinia was widespread during a marine transgression but shrank gradually along with widespread shelf oxygenation when the basin evolved into subsequent regression excess highly reactive iron fe hr in deep water with a hydrothermal origin may have had a control on the mid depth euxinia given geochemical and geological evidence for basinal hydrothermal activities on the other hand a sulfate limitation on bacterial sulfate reduction bsr and sulfide generation was suggested by a weak correlation r 2 0 22 between pyrite iron versus fe hr and total organic carbon toc form anoxic deep water sections and high toc averagely 6 in the deepest ferruginous water longbizui section furthermore a spatial gradient was observed in sulfur isotopic composition of pyrite averagely from 0 2 to 21 0 consistent with a lateral gradient of sulfate in concentration or and in isotope from shore to distal waters on the transgressive yangtze platform we accordingly proposed that two distinct sources riverine input and ocean reservoir provided sulfate for bsr dominantly in near shore and distal areas respectively and the euxinic retreat observed for the regression can be explained as a response to the progressive sulfate consumption of the ocean reservoir by intensive bsr and pyrite formation during preceding transgression therefore our results support a sulfate control on the spatiotemporal distribution of mid depth marine euxinia on the yangtze platform in early cambrian when complex metazoans rapidly evolved

temporal geomagnetic and related attributes of kimberlite magmatism at ekati northwest territories canada abstract this paper outlines the development of a multi disciplinary strategy to focus exploration for economic kimberlites on the ekati property high resolution aeromagnetic data provide an over arching spatial and magnetostratigraphic framework for exploration and kimberlite discovery at ekati and hence also for this investigation the temporal geomagnetic spatial and related attributes of kimberlites with variable diamond content have been constrained by judiciously augmenting the information gathered during routine exploration with detailed laboratory based or field based investigations the natural remanent magnetisation of 36 ekati kimberlites has been correlated with their age as determined by isotopic dating techniques and placed in the context of a well constrained geomagnetic polarity timescale kimberlite magmatism occurred over the period 75 to 45 ma in at least five temporally discrete intrusive episodes based on current evidence the older kimberlites 75 to 59 ma have low diamond contents and are distributed throughout the property younger kimberlites 56 to 45 ma have moderate to high diamond contents and occur in three distinct intrusive corridors with nne to ne orientations economic kimberlite pipes erupted at 55 4 0 4 ma along the a154 lynx intrusive corridor which is 7 km wide and oriented at 015 and at 53 2 0 3 ma along the panda intrusive corridor which is 1 km wide and oriented at 038 the intrusion ages straddle a paleopole reversal at chron c24n consistent with the observation that the older economic kimberlites present as aeromagnetic low anomalies while the younger economic pipes are characterised as aeromagnetic highs the aeromagnetic responses for these kimberlites are generally muted because they contain volcaniclastic rock types with low magnetic susceptibility kimberlites throughout the ekati property carry a primary natural magnetic remanence nrm vector in ti bearing groundmass magnetite and it dominates over vectors related to induced magnetisation magnetostratigraphic correlation of ekati kimberlites may therefore present a powerful adjunct to existing exploration techniques mainly because the diamond content of ekati kimberlites apparently is related more to the age of eruption than to any other parameter investigated in this work

criteria for interpreting kimberlite as coherent insights from the muskox and jericho kimberlites nunavut canada the jurassic muskox and jericho kimberlites northern slave province nunavut canada contain a variety of facies exhibiting different geometries contact relationships internal organisation country rock abundance and olivine shapes although many have similar matrix groundmass mineralogies and textures five facies are examined that either have characteristics consistent with coherent rocks in general i e intrusive and extrusive non fragmental rocks or are mineralogically and texturally similar to kimberlite described as coherent or apparent coherent three facies are interpreted as coherent on the basis of 1 geological setting 2 apparent porphyritic texture 3 sharp contacts with fragmental kimberlite 4 relative abundance of elongate and unbroken olivine crystals and 5 paucity of country rock xenoliths while the remaining two facies are interpreted as fragmental on the basis of 1 the gradational contacts with demonstrably fragmental kimberlite 2 relative abundance and range of sizes of country rock lithic clasts and 3 numerous broken olivine crystals comparisons are made with coherent and apparent coherent kimberlite from the literature our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock ckd in terms of internal organisation low abundance of country rock xenoliths and apparent porphyritic texture conversely our two fragmental facies share attributes with previously described pipe filling coherent and apparent coherent kimberlite ckpf in terms of geometry internal organisation and abundance of country rock xenoliths we conclude that ckd and most ckpf although similar in matrix groundmass mineralogy and texture can be distinguished on the basis of internal organisation country rock lithic clast abundance texture e g apparent porphyritic texture and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported

endolithic biodegradation of cool water skeletal carbonates on scott shelf northwestern vancouver island canada abstract biodegradation of shell material is widespread in the cool water skeletal carbonate deposits on scott shelf northwestern vancouver island and is especially evident in the large aragonitic bivalves glycymeris and humilaria major primary sediment contributors ten types of endolithic microborings have been identified in the shells including representatives of green algae e g ostreobium quekettii blue green algae e g plectonema terebrans scytonema sp fungi bacteria and clionid sponges as well as macroborings of phoronids polychaetes and naticid gatropods microcrystalline carbonate is not precipitated in vacated bores boring physically weakens the shells rendering them more prone to mechanical abrasion during sediment transport and bioturbation and to biological abrasion by grazing benthos tumbling experiments demonstrate that the rate of carbonate mud production is much greater for bored as compared to fresh bivalve shells and that mud production rates decrease with tumbling time because most endolithic microborings are confined to the periphery of grains boring also increases significantly the porosity and surface area of skeletal grains and destroys their organic matrix making them susceptible to maceration and dissolution on cool water shelves fostered by the generally low rates of carbonate production and accumulation many aragonitic bivalve shells on scott shelf have become thoroughly degraded through a combination of endolithic microboring maceration and dissolution within about 1000 years in ambient sea water in geologic terms such selective taphonomic loss of skeletal material may be considerable in ancient temperate shelf limestones and should be evaluated when interpreting their paleoecology and paleoenvironments

petrogenesis of the late cretaceous northern alberta kimberlite province abstract at present 48 late cretaceous ca 70 88 ma kimberlitic pipes have been discovered in three separate areas of the northern alberta the mountain lake cluster the buffalo head hills field and the birch mountains field the regions can be distinguished from one another by their non archetypal kimberlite signature mountain lake or in the case of kimberlite fields primitive buffalo head hills to evolved birch mountains magmatic signatures the dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine which acts as the main criteria to distinguish between primitive and evolved group i type kimberlite fields in the northern alberta this is important from the viewpoint of diamond exploration because the majority about 80 of the more primitive buffalo head hills kimberlites are diamondiferous whereas the more evolved birch mountains pipes are barren of diamonds for the most part petrographically the buffalo head hills samples are distinct from the birch mountains samples in that they contain less carbonate have a smaller modal abundance of late stage minerals such as phlogopite and ilmenite and have a higher amount of fresh coarse macrocrystal 0 5 mm olivine consequently samples from the buffalo head hills have the highest values of mgo cr and ni and have chemistries similar to those of primitive hypabyssal kimberlite in the northwest territories based on whole rock isotopic data the buffalo head hills k6 kimberlite has 87sr 86sr and end values similar to those of south african group i kimberlites whereas the birch mountains legend and phoenix kimberlites have similar end values between 0 and 1 9 but distinctly higher 87sr 86sr values 0 7051 0 7063 the lack of whole rock geochemical overlap between kimberlite and the freshest least contaminated mountain lake south pipe rocks reflects significant mineralogical differences and mountain lake is similar geochemically to olivine alkali basalt and or basanite intra field geochemical variations are also evident the k4 pipe buffalo head hills and xena and kendu pipes birch mountains are characterized by anomalous concentrations of incompatible elements relative to other northern alberta kimberlite pipes including chondrite normalized rare earth element distribution patterns that are less fractionated than the other kimberlite samples from the buffalo head hills and birch mountains the xena pipe has similar major element chemical signatures and high al clinopyroxene similar to or trending towards the mountain lake pipes in addition k4 and kendu have higher 87sr 86sr and lower end than bulk earth and plot in the bottom right quadrant of the nd sr diagram we suggest therefore that the k4 and kendu pipes contain a contribution from old lree enriched low sm nd lithosphere that is absent from the other kimberlites are affected by crustal contamination or both based on xenocryst populations the northern alberta kimberlite province mantle is dominated by carbonate saturated lherzolitic mantle higher levels of melt depletion characterize the buffalo head hills mantle sample despite high diamondiferous to barren pipe ratios in the buffalo head hills pipes mineral indicators of high diamond potential such as g10 garnet diamond inclusion composition chrome spinels and high sodium eclogitic garnet are rare

reconstruction of a kimberlite eruption using an integrated volcanological geochemical and numerical approach a case study of the fox kimberlite nwt canada abstract an integrated approach involving volcanology geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the fox kimberlite pipe the observed deposits within the vent include a basal massive poorly sorted matrix supported lithic fragment rich eruption column collapse lapilli tuff extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments dense juvenile pyroclasts and olivine crystals triggering column collapse 40 dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature where enrichment in al2o3 and na2o compared to average values for coherent kimberlite is seen the wide deep open vent provided a trap for a significant proportion of the collapsing column material preventing large scale run away in the form of pyroclastic flow onto the ground surface although minor flows probably also occurred a massive to diffusely bedded poorly sorted matrix supported accretionary lapilli bearing lithic fragment rich lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage caused by the explosive interaction of external water with residual magma correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble boulder breccia was deposited ash and accretionary lapilli rich tephra deposited on the crater rim during the late phreatomagmatic stage was subsequently resedimented into the vent incompatible elements such as nb are used as indicators of the proportion of the melt fraction or kimberlite ash retained or removed by eruptive processes when compared to average coherent kimberlite the ash rich deposits exhibit 30 loss of fines whereas the column collapse deposit exhibits 50 loss this shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption indicating the existence of a high sustained eruption column the deposits within fox record a complex eruption sequence showing a transition from a probable violent sub plinian style eruption driven by instantaneous exsolution of magmatic volatiles to a late phreatomagmatic eruption phase mass eruption rate and duration of the sub plinian phase of the eruption have been determined based on the dimensions of milled country rock boulders found within the intra vent deposits calculations show a short lived eruption of one to eleven days for the sub plinian magmatic phase which is similar in duration to small volume basaltic eruptions this is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters such as predictions of magma ascent rates in kimberlite dykes

reconstruction of a multi vent kimberlite eruption from deposit and host rock characteristics jericho kimberlite nunavut canada abstract the jericho kimberlite 173 1 1 3 ma is a small 130 70 m multi vent system that preserves products from deep 1 km portions of kimberlite vents pit mapping drill core examination petrographic study image analysis of olivine crystals grain size distributions and shape studies and compositional and mineralogical studies are used to reconstruct processes from near surface magma ascent to kimberlite emplacement and alteration the jericho kimberlite formed by multiple eruptions through an archean granodiorite batholith that was overlain by mid devonian limestones 1 km in thickness kimberlite magma ascended through granodiorite basement by dyke propagation but ascended through limestone at least in part by locally brecciating the host rocks after the first explosive breakthrough to surface vent deepening and widening occurred by the erosive forces of the waxing phase of the eruption by gravitationally induced failures as portions of the vent margins slid into the vent and in the deeper portions of the vent 1 km by scaling as thin slabs burst from the walls into the vent at currently exposed levels coherent kimberlite ck dykes

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

rb sr and u pb geochronology and setting of the buffalo head hills kimberlite field northern albertathis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers new rb sr phlogopite and u pb perovskite age determinations for 12 of 38 known ultramafic bodies in the buffalo head hills area of north central alberta show that kimberlitic magmatism occurred in at least two separate episodes during late cretaceous and paleocene time nine kimberlites yield coniacian to campanian ages of between 88 5 ma u pb perovskite k5a and 81 2 2 3 ma rb sr phlogopite k252 a danian u pb perovskite isochron age of 63 5 0 7 ma was obtained for the bm2 kimberlite and selandian rb sr ages of 59 6 2 8 and 60 3 0 8 ma were determined for the k1a and k19 bodies respectively these specific periods of magmatism correspond to characteristic intra field features such as kimberlite spatial distribution diamond content rock classification and mechanisms of emplacement the 88 81 ma group generally occurs in the northwestern part of the field and defines the diamond window for the buffalo head hills kimberlite field this volcanism is contemporaneous with sedimentary host

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

an estuarine facies model for the middle jurassic sleipner formation beryl embayment north sea the middle jurassic sleipner formation is sub divided into three units in the beryl embayment area of the north sea the lowermost two units the 9interbedded unit9 and 9lower coal marker9 overlie lower jurassic marine silts and were probably deposited in progressively shallowing coastal environments during a eustatic sea level fall an estuarine deposit overlies the 9lower coal marker9 and was deposited following sea level rise or tectonic subsidence of the area the 9estuarine succession9 forms the bulk of the formation and is composed of seven facies grouped into three facies associations representing sub tidal inter tidal and salt marsh sub environments the sub tidal facies association is dominated by sandstones deposited in tidal channels and sand flats the inter tidal association is characterized by alternations of sandstone and siltstone on a scale of millimetres to several metres and consists of tidal channel sandstones lenticular bedded siltstones and other tidally influenced facies the salt marsh association is dominated by coals and rooted siltstones with minor amounts of creek fill sandstones the estuary mouth probably lay towards the northern end of the embayment where a connection existed through the viking graben to the boreal ocean

a geochemical investigation of fragmentation and physical fractionation in pyroclastic flows from the soufriere hills volcano montserrat abstract geochemical analysis is used to investigate fragmentation and physical fractionation in pyroclastic flows bulk analyses of the matrices

pyroclastic kimberlite deposits from the victor northwest pipe ontario canada the transition from phreatomagmatic to magmatic explosivity magmas of all compositions including kimberlites may undergo both magmatic and phreatomagmatic fragmentation during emplacement in this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the victor northwest kimberlite northern ontario canada detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles each consisting of three repeated eruptive phases pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine grained poorly sorted deposits containing broken olivine crystals angular country rock fragments accretionary lapilli and variably vesicular irregular shaped juvenile pyroclasts all observations indicate these deposits formed by phreatomagmatism subsequent clastogenic coherent kimberlite deposits were formed as a result of hawaiian fire fountaining during the second phase of both cycles these phases were followed by mass wastin

late wisconsin ice flow history in the buffalo head hills kimberlite field north central alberta ice flow of the last glaciation in the buffalo head hills kimberlite field of northern alberta is reconstructed from landform interpretations and clast orientations for the purpose of aiding kimberlite exploration in the region the paucity of bedrock outcrop and the absence of preserved striae and other erosional ice flow indicators on the soft cretaceous marine sediments inhibit detailed interpretations on glacial flow chronology poorly developed bedrock drumlins on the buffalo head hills and erosional ice flow indicators preserved on the kimberlite outcrops indicate southwestward ice flow during the maximum extent of ice during the last glaciation during the deglaciation of northern alberta later phases of ice flow were controlled by lobes of surging ice which surged into proglacial lakes west of the buffalo head hills the maximum phase of southwest flow was followed by southeastward ice movement of the peace river ice lobe similarly east of the buffalo head hills the maximum phase of ice flow was superceded by a south southwest ice advance of the wasbasca ice lobe

a new approach to understanding diamond surface features based on a review of experimental and natural diamond studies abstract diamonds originate deep in the earth s mantle since billions of years ago through their long history diamonds accumulate information about the earth s evolution and preserve it owing to their extreme chemical and mechanical stability the surface of natural diamonds shows a variety of growth and dissolution features which reflect the diversity of conditions in the mantle and in kimberlite magma providing an important clue for understanding the deep regions of subcratonic mantle however such studies are hampered by an absence of a systematic approach for studying diamond surface features and morphology this review integrates studies of natural diamonds with the results of diamond dissolution experiments to explore the origin of the most typical resorption features of diamonds and the information they provide it uses detailed studies of over 3500 diamonds from eight kimberlite bodies in the northwest territories in canada and orapa kimberlite cluster in botswana and the data from diamond dissolution experiments covering a pressure range of between 0 1 mpa 7 5 gpa temperature range of between 900 c 1750 c and over 12 log units of oxygen fugacity values examining the effects of these parameters on diamond resorption morphology shows that the shape and size of the etch pits depends on the temperature and h2o co2 ratio in the fluid whereas pressure affects the efficiency of diamond crystal shape transformation from octahedral into rounded resorbed forms the effect of pressure on the physical properties of the reacting fluid melt controls the character of diamond etching a comparison between the experimentally induced and naturally occurring diamond resorption demonstrates a clear difference between the features developed in kimberlite magma and features inherited from the mantle source kimberlite induced resorption on diamonds shows a strong correlation with the geology and emplacement mode of the hosting kimberlite unit low relief surfaces develop on diamonds from pyroclastic kimberlites in all kimberlite classes whereas surface features on diamonds from coherent kimberlites differ between kimberlite localities and often show corrosive character diamond resorption morphology can offer a robust method to better understand emplacement processes in different kimberlite localities which are a matter of significant debate the proposed here classification scheme for diamond resorption features is based on the features observable under a stereomicroscope it helps differentiating resorption produced in the mantle source from that in the kimberlite magma and assigning diamond resorption to a particular mode of kimberlite emplacement or a mantle metasomatic event

global warming and south indian monsoon rainfall lessons from the mid miocene precipitation over india is driven by the indian monsoon although changes in this atmospheric circulation are caused by the differential seasonal diabatic heating of asia and the indo pacific ocean it is so far unknown how global warming influences the monsoon rainfalls regionally herein we present a miocene pollen flora as the first direct proxy for monsoon over southern india during the middle miocene climate optimum to identify climatic key parameters such as mean annual temperature warmest month temperature coldest month temperature mean annual precipitation mean precipitation during the driest month mean precipitation during the wettest month and mean precipitation during the warmest month the coexistence approach is applied irrespective of a 3 4 c higher global temperature during the middle miocene climate optimum the results indicate a modern like monsoonal precipitation pattern contrasting marine proxies which point to a strong decline of indian monsoon in the himalaya at this time therefore the strength of monsoon rainfall in tropical india appears neither to be related to global warming nor to be linked with the atmospheric conditions over the tibetan plateau for the future it implies that increased global warming does not necessarily entail changes in the south indian monsoon rainfall

ultrafresh salty kimberlite of the udachnaya east pipe yakutia russia a petrological oddity or fortuitous discovery an ultrabasic ultramafic composition of kimberlite magmas is difficult to reconcile with existing models of the kimberlite mantle source and melting conditions inferred magma temperatures and rheological properties and the style of magma ascent and emplacement the inconsistencies in current thinking indicate serious flaws in understanding kimberlite magma compositions much of the uncertainty over true kimberlite compositions may stem from almost ubiquitous hydration and leaching of kimberlite rocks this study presents petrographic and geochemical data for kimberlite samples largely unaffected by postmagmatic modification from the devonian udachnaya east pipe in siberia these samples are unusually enriched in chlorine and sodium yet they are essentially anhydrous these features are consistent with the phase composition of the groundmass which is dominated by minerals such as na ca carbonates na k chlorides and sulphates which appear to be in our samples co magmatic with common silicates and oxides but are unknown in other kimberlites or rarely found within magmatic assemblages we suggest that a kimberlite parent melt of essentially non silicate composition with high concentrations of alkalis co2 and cl may be a viable alternative to the currently favoured water rich high mg model primary melt entrainment of mantle silicates into such a melt en route to the surface followed by gravitational accumulation of mantle olivine and liquidus oxides perovskite cr spinel at the bottom of vertically extensive magma bodies after emplacement would explain the observed properties of kimberlite magma rock notably enrichment in olivine and trace elements in the hypabyssal kimberlite facies a carbonate melt composition would retain attributes of the standard model such as trace element enrichment via low degrees of partial melting it would explain low temperatures of crystallisation and the exceptional rheological properties that enable kimberlite primary melts to segregate from the lithospheric source and buoyantly ascend at high speed while mixing and reacting with country rocks

molybdenum isotope fractionation in pelagic euxinia evidence from the modern black and baltic seas here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the black sea and two deeps of the baltic sea dissolved molybdenum in all water samples is depleted in salinity normalized concentration and enriched in the heavy isotope 98mo values up to 2 9 compared to previously published isotope data of sedimentary molybdenum from the same range of water depths furthermore 98mo values of all water samples from the black sea and anoxic deeps of the baltic sea are heavier than open ocean water the observed isotope fractionation between sediments and the anoxic water column of the black sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions an extrapolation to a theoretical pure mos42 solution indicates a fractionation constant between mos42 and authigenic solid mo of 0 5 0 3 measured waters with all thiomolybdates coexisting in various proportions show larger but non linear fractionation the best explanation for our field observations is mo scavenging by the thiomolybdates dominantly but not exclusively present in the form of mos42 the mo isotopic compositions of samples from the sediments and anoxic water column of the baltic sea are in overall agreement with those of the black sea at intermediate depth and corresponding sulphide concentrations the more dynamic changes of redox conditions in the baltic deeps complicate the black sea derived relationship between thiomolybdates and mo isotopic composition in particular the occasional flushing mixing of the deep waters affects the corresponding water column and sedimentary data 98mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations the results can be explained by non conservative behaviour of mo under suboxic to anoxic conditions in the shallow bottom parts of the basin most pronounced on the nw shelf of the black sea

canadian kimberlites geological characteristics relevant to emplacement in canada more than 770 kimberlites have erupted in diverse tectonic settings over a period of 1000 ma and over an area at least 5000 km across the kimberlites represent at least 30 comagmatic fields one pipe type dominates each of the different kimberlite fields even with the discovery of a significant number of new kimberlite fields this review of recent data substantiates the previous proposal that there are at least three distinct classes of kimberlite pipes which show a correlation with the nature of the country rocks into which they were emplaced new data show that a variation on the prairies pipe type occurs when kimberlites were emplaced into competent paleozoic sediments resulting in steeper sided pipes infilled with similar pyroclastic kimberlite new data for each pipe type are summarised and the contrasting characteristics of the three pipe types are considered to reflect fundamentally different styles of eruption and deposition apparently uniform near surface olivine and volatile rich kimberlite magmas were modified differently during the contrasting emplacement processes resulting in distinct textural rock types pyroclastic kimberlite pk tuffisitic kimberlite tk and associated hypabyssal kimberlite hk or resedimented kimberlite rvk dominating each type of pipe the substantiated correlation of pipe type with country rock geology could indicate variable constraints on volatile exsolution which affected the nature of the magmatic eruptions in different settings

testing symbiotic morphology in trilobites under dysoxic and oxic conditions from cambrian to early ordovician lagerstatten dysoxic conditions were prevalent in cambrian and early ordovician oceans adaptations of trilobites to these conditions yield insight into the selective forces driving their evolution and the potential for modern arthropods to adapt to anthropogenically driven dysoxia in the early paleozoic olenid trilobites may have adapted to dysoxia through olenimorphy special morphologic adaptations facilitating symbiosis with sulfur oxidizing bacteria olenimorphy s link to symbiosis has never been tested and might instead be related to phylogeny here we tested whether olenimorphy occurs predominately in dysoxic environments as expected or whether it occurs within certain trilobite groups regardless of environment trilobites were selected from nine fossil lagerstatten representing the early to middle cambrian prior to olenid evolution and the middle cambrian early ordovician alum shale with olenids putative exemplars of symbiosis trilobite morphology was scored as ranked characters and reduced via non metric multidimensional scaling nmds to a single metric describing suitability for symbiosis results indicated that olenimorphy was significantly related to phylogeny and not environment in the early to middle cambrian dysoxic corynexochids and oxic redlichiids were equally unsuitable for symbiosis while ptychopariids from both oxic and dysoxic conditions were equally well suited for symbiosis in the alum shale asaphids and ptychopariids from both oxic and dysoxic conditions were equally well suited for symbiosis and redlichiids from oxic conditions were poorly suited olenids were not the best suited trilobites within their data set nor even within their order secular trends in thoracic segmentation did not bias our results trilobite hard parts do not appear to provide evidence for symbiosis which is consistent with modern arthropods that have few morphological adaptations for dysoxic conditions in fact arthropods are rare in persistently dysoxic conditions raising concerns about the spread of anthropogenically triggered benthic dysoxia in the future particularly given the extinction of trilobites under similar conditions at the end of the permian

the rate of taphonomic loss in modern benthic habitats how much of the potentially preservable community is preserved abstract the post mortem fate of shells added by natural mortality to modern death assemblages was studied to determine the importance of taphonomic loss during the initial stages of the formation of a fossil assemblage individuals are added to the living community in pulses usually by larval settlement these input pulses usually are followed by discrete input pulses into the death assemblage the decay of these discrete input pulses in the death assemblage can be used to measure the rate of taphonomic loss as expressed by the half life of a pulse for instance if a species has a half life of 125 days and the input pulse begins with 500 individuals then the number of shells remaining after one half life 125 days is 250 shells at two sites on the texas coast the longest half lives did not exceed one year for any of the 13 species which produced input pulses during the study period little net addition to the death assemblage could be detected in spite of the transitory addition of hundreds of shells as input pulses these rapid decay rates show that death assemblages do not accumulate at the rate at which organisms die some species such as the infaunal bivalve tagelus plebeius did not decay measurably we predict that decay rates drop substantially with depth in the sediment so that individuals which die at depth or are rapidly reworked below the sediment surface have a greater chance of preservation the final composition of death assemblages is probably little related to the actual numerical input from the potentially preservable component of the living community

storm deposits from the quaternary outardes delta quebec canada abstract sand laminae interbedded with thicker pelitic sediments in the uplifted post glacial succession near outardes falls quebec are interpreted as storm deposits the sands were put into suspension by strong wave action in the coastal zone and transported offshore shallow water benthos were also carried offshore and deposited with the sand their presence indicates the coast was ice free at this time in places the pelitic units with sand laminae are confined to bathymetric lows suggesting that they may have been transported by low density turbidity currents the transition to this type of flow may have been aided by the presence of high subaqueous slopes and high concentrations of suspended fine sediments

the dorothy bentonite an extraordinary case of secondary thickening in a late campanian volcanic ash fall in central alberta the late campanian 73 ma dorothy bentonite outcrops in the marine bearpaw shale for 20 km along the red deer river valley east of drumheller alberta and is up to 13 5 m thick an isopach map based upon 230 sub surface and surface thickness measurements illustrates an elongated southwest northeast lobe with maximum dimensions of about 300 km by 50 km the volume is 57 km3 distributed within an area of 11 000 km2 this bentonite is the altered product of what is believed to be a short lived plincan type eruption from part of the howell creek instrusives in southeastern british columbia fortuitous preservation of the original ash in an up to 2 5 m thick calcite cemented tuffaceous zone near the middle of the bentonite shows the original ash to have been 99 glass shards and pumice the remainder of the ash is a crystal component consisting mostly of plagioclase and biotite the silica content of the isotropic glass shards of about 77 and a refractive index of 1 503 0 001 suggest a magma of rhyo

are diamond bearing cretaceous kimberlites related to low angle subduction beneath western north america abstract diamond bearing cretaceous kimberlites of western north america were emplaced 1000 1500 km inboard of the farallon plate subduction margin and overlap with the development of the western interior seaway shut down of the sierra nevada arc and the laramide orogeny these events are consistent with a decrease in subduction angle along much of the margin which placed the subducted farallon plate in close proximity to the continental interior at the time of kimberlite magmatism our numerical models demonstrate that low angle subduction can result from high plate convergence velocities and enhanced westward motion of north america further rapid subduction allows hydrous minerals to remain stable within the cool interior of the subducting plate to more than 1200 km from the trench destabilization of these minerals provides a fluid source that can infiltrate the overlying material potentially triggering partial melting and kimberlite lamproite magmatism

systematic variations in xenocryst mineral composition at the province scale buffalo hills kimberlites alberta canada abstract the buffalo hills kimberlites define a province of kimberlite magmatism occurring within and adjacent to proterozoic crystalline basement termed the buffalo head terrane in north central alberta canada the kimberlites are distinguished by a diverse xenocryst suite and most contain some quantity of diamond the xenocryst assemblage in the province is atypical for diamondiferous kimberlite including an overall paucity of mantle indicator minerals and the near absence of compositionally subcalcic peridotitic garnet g10 the most diamond rich bodies are distinguished by the presence of slightly subcalcic chromium rich garnet and the general absence of picroilmenite with the majority forming a small cluster in the northwestern part of the province barren and near barren pipes tend to occur to the south with increasing proximity to the basement structure known as the peace river arch niobian picroilmenite compositionally restricted low to moderate cr peridotitic garnet and megacrystal titanian pyrope occur in kimberlites closest to the arch major element data for clinopyroxene and trace element data for garnet from diamond rich and diamond poor kimberlites suggests that metasomatism of lithospheric peridotite within the diamond stability field may have caused destruction of diamond and diamond source rocks proximal to the arch were the most affected

macrocrystal phlogopite rb sr dates for the ekati property kimberlites slave province canada evidence for multiple intrusive episodes in the paleocene and eocene abstract new rb sr age determinations using macrocrystal phlogopite are presented for 27 kimberlites from the ekati property of the lac de gras region slave province canada these new data show that kimberlite magmatism at ekati ranges in age from at least late paleocene 61 ma to middle eocene time 45 ma older perovskite bearing kimberlites from ekati extend this age range to late cretaceous time 74 ma within this age range emplacement episodes at 48 51 53 55 56 and 59 61 ma can be recognized middle eocene kimberlite magmatism of the previously dated mark kimberlite 47 5 ma is shown to include four other pipes from the east central ekati property a single kimberlite aaron may be younger than the 47 5 ma mark kimberlite the economically important panda kimberlite is precisely dated in this study to be 53 3 0 6 ma using the phlogopite isochron method and up to six additional kimberlites from the central ekati property have early eocene ages indistinguishable from that of panda including the koala and koala north occurrences late paleocene 55 56 ma kimberlite magmatism represented by the diavik kimberlite pipes adjacent to the southeastern ekati property is shown to extend onto the southeastern ekati property and includes three and possibly four kimberlites a precise eight point phlogopite isochron for the cobra south kimberlite yields an emplacement age of 59 7 0 4 ma eight other kimberlites from across the ekati property have similar late paleocene rb sr model ages the addition of 27 new emplacement ages for kimberlites from the ekati property confirms that kimberlite magmatism from the central slave province is geologically young despite ages ranging back to cambrian time from elsewhere in the slave province with the available geochronologic database lac de gras kimberlites with the highest diamond potential are currently restricted to the 51 53 and 55 56 ma periods of kimberlite magmatism

kimberlite sourced bentonite its paleoenvironment and implications for the late cretaceous k14 kimberlite cluster northern albertathis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers kimberlite sourced bentonite layers have been discovered in late cretaceous sedimentary drill cores located within the buffalo head hills kimberlite field north central alberta two bentonites are unambiguously differentiated from common intermediate to felsic volcanic derived alberta bentonite by having similar whole rock geochemical composition to ultramafic rocks from the buffalo head hills kimberlite field and worldwide crater facies kimberlite the results demonstrate that the geochemical analysis of bentonite can provide a quick cost effective means of testing for low volume kimberlite volcanism the kimberlite sourced bentonite is associated with a cluster of buffalo head hills kimberlites known as the k14 complex the k14a kimberlite occurrence has a previously reported 206pb 238u perovskite emplacement age of 86 8 2 1 ma this age is compatible with early or mid cenomanian 99 96 ma and mid coniacian to santonian 88 84 ma palynological results for mudstone underlying and bracketing the

diamond resorption features as a new method for examining conditions of kimberlite emplacement the study develops a new approach utilizing parameters of trigonal etch pits on diamond crystals to infer the conditions of diamond residence in kimberlite magma diamond crystals from dissolution experiments conducted at 1 gpa and 1150 1350 c in the presence of h2o rich or co2 rich fluid were studied with atomic force microscopy afm the afm data of resorbed diamond surfaces show that much deeper surface relief was produced in co2 fluid it also clearly distinguishes the profiles of the trigonal etch pits forming regular flat bottomed trigons in h2o fluid and round or pointed bottomed trigons in co2 fluid the relationship between the diameter and the depth of the trigonal pits is found to be another important indicator of the fluid composition dissolution in h2o fluid develops trigons with constant diameter and variable depth where the diameter increases with temperature trigons developed in co2 fluid have a large range of diameters showing a strong positive correlation with the depth the developed criteria applied to the natural diamond crystals from three ekati mine kimberlites indicate significant variation in co2 h2o ratio and temperature of their magmatic fluid this conclusion based on diamond resorption agrees with the mineralogy of microphenocrysts and groundmass of the studied kimberlites offering new method to study crystallization conditions of kimberlite magma

shallow marine dysoxia across the permian triassic boundary evidence from pyrite framboids in the microbialite in south china abstract the end permian mass extinction resulted in the collapse of reef ecosystems and the subsequent formation of microbialites in shallow carbonate platforms in south china large quantities of pyrite framboids are preserved in the microbialites but absent in the underlying skeletal limestone indicating an abrupt shift in water column redox conditions in association with the shallow marine faunal mass extinction detailed study of size distributions of 1374 pyrite framboids found in 9 horizons in laolongdong section in chongqing municipality south china were performed to evaluate this shift in paleo redox condition of the shallow marine environment pyrite framboids in the microbialite average 8 41 m in size comparable with those from modern sediments underlying dysoxic water columns this infers that dysoxic waters in the latest permian might have expanded to reach the shallow marine environments in south china mean diameters of pyrite framboids from mudstones overlying the microbialite are 7 63 m 7 68 m likely indicating continuing intensification of oxygen deficiency in the bottom waters microgastropods ostracods and some foraminifers which survived and flourished in the microbialite almost disappeared during the mudstone deposition possibly reflecting a consequence of oxygen depletion

fragmentation in kimberlite products and intensity of explosive eruption the explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes shapes and textures however all pyroclastic deposits of kimberlite comprise two main types of pyroclasts 1 pyroclasts of kimberlite with or without enclosed olivine crystals and 2 olivine crystals which lack coatings of kimberlite here we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption 1 olivine crystals break during kimberlite eruption and 2 kimberlite melt can be efficiently separated from crystals during eruption these ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at diavik nt olivines are expected to break because of rapid pressure changes during the explosive eruption disruption of kimberlite magma and pyroclast production is driven by ductile deformation processes rather than by brittle fragmentation the extent to which melt separates from olivine crystals to produce kimberlite free crystals is a direct consequence of the relative proportions of gas melt and crystals lastly the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions a fragmentation index is proposed for kimberlite eruption based on a crystal size distributions of olivine and on b ratios of selvage free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

on the welding of pyroclasts from very low viscosity magmas examples from kimberlite volcanoes abstract lithofacies in kimberlite pipes in southern africa exhibit features consistent with welded pyroclastic rocks these include flared conduit filling geometries abundant lithic clasts lithic clast layers subhorizontal clast fabrics gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash and lapilli and melt coated particles the welding dynamics of kimberlite pyroclasts differ from those of glassy vesiculated pyroclasts in silicic volcanic systems low melt viscosity 0 1 pa s results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets the glassy state is difficult to form in silica deficient magmas because the crystallization kinetics are fast in low viscosity melts three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition 1 densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure mel

paleomagnetic dating of the northern alberta kimberlites due to the vast amount of economic interest diamonds have created in northern alberta there is a need to pro duce an accurate geological model for the northern alberta kimberlite province to aid in the development of such a model the emplacement ages of two kimberlite occurrences k5 and k6 from the buffalo head hills region of north central alberta and the ultramafic mountain lake body from northwestern alberta were estimated using paleomagnetic methods paleomagnetic poles obtained in our study do not differ statistically from the reference poles for late mesozoic cenozoic for north america besse and courtillot 2002 with the aid of polarity determinations palynology and radio genic dating the paleomagnetic results allow for new constraints on the emplacement age of the selected ultramafic occurrences the paleomagnetic emplacement ages established for the k5 k6 and mountain lake bodies are 90 83 ma 83 0 79 1 ma and not older than 79 1 ma respectively resume en raison du grand intereteconomique que les diamants ont creedans le nord de l alberta il est necessaire de developper un modele geologique precis de la province kimberlitique du nord de l alberta afin d aider au developpement d un tel modele les ages de la mise en place de deux occurrences de kimberlite k5 et k6 de la region des collines buffalo head du centre nord de l alberta et de l amas ultrabasique mountain head du nord ouest de l alberta ont eteestimes par une methode paleomagnetique les poles paleomagnetiques obtenus dans notre etude ne different statistiquement pas des poles de reference pour l amerique du nord au mesozoo que tardif cenozoo que besse and courtillot 2002 grace aux determinations de polarite ala palynologie et ala datation radiogenique les resultats paleomagnetiques limitent l age de la mise en place des occurrences ultramafiques selectionnees les ages paleomagnetiques de la mise en place etablis pour les amas k5 k6 et mountain lake sont respectivement de 90 83 ma 83 0 79 1 ma et pas plus vieux que 79 1 ma traduit par la redaction

the geology of kimberlite pipes of the ekati property northwest territories canada abstract this paper reviews key characteristics of kimberlites on the ekati property nwt canada to date 150 kimberlites have been discovered on the property five of which are mined for diamonds the kimberlites intrude archean basement of the central slave craton numerous proterozoic diabase dykes intrude the area the precambrian rocks are overlain by quaternary glacial sediments no phanerozoic rocks are present however mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late cretaceous and tertiary cover likely

geochemical trends in kimberlites of the ekati property northwest territories canada insights on volcanic and resedimentation processes abstract the ekati property northwest territories canada hosts in excess of 150 volcanic kimberlite pipes occupied by a wide variety of rock types including coherent magmatic material and a range of fragmental volcaniclastic deposits of both pyroclastic and resedimented origin geochemical analysis of a suite of samples from several of these bodies provides valuable insight into the nature of their components and the processes by which they form observed variations in the bulk composition of coherent kimberlites correlate with their mode of emplacement and petrographic characteristics high volume bodies of coherent kimberlite occurring within and in several cases dominating volcanic pipes pipe fill ck are depleted in co2 and key incompatible elements e g ti and nb but enriched in ni and sio2 relative to samples from narrow kimberlite dyke intrusions the composition and certain textural features of pipe fill ck can be interpreted to reflect formation by pyroclastic processes that involved a loss of volatiles and fines and a concentration of olivine crystals relative to intrusive magmatic kimberlite if this is the case then the apparent coherent character of these rocks suggests that they represent coalescence of hot fluidal pyroclasts i e spatter generated by fire fountain style eruptions samples of massive pyroclastic kimberlite pk are geochemically similar to pipe fill ck but generally show a greater degree of depletion of incompatible elements suggestive of more explosive eruption processes and a greater degree of physical fractionation the geochemical data support petrographic observations that ekati pk contains very little xenolithic material resedimented volcaniclastic kimberlite rvk shows variable degrees of enrichment in al2o3 relative to pk the extent of al enrichment correlates with the proportion of dark fine grained clastic matrix material in the rvk samples and their composition can be explained as a mixture of a very olivine rich fines depleted juvenile component with varying amounts of argillaceous sediment likely derived from surface deposits present at the time of kimberlite eruption however the increased al2o3 content in rvk is also positively correlated with nb concentration indicating that the matrix consistently includes a small amount of kimberlitic ash geochemical trends in massive volcaniclastic kimberlite mvk and fine grained volcaniclastic kimberlite vk from the fox pipe are very different to those of other vk samples from ekati and appear to reflect mixing of juvenile kimberlitic material with granodiorite the data suggest very little if any incorporation of exotic fine grained sediments into these rocks geochemical modelling suggests that the eruption s forming fox mvk involved a significant loss of fines similar to that interpreted for pk in other ekati kimberlites however the large quantity of fragmented granodiorite 30 to 45 within fox mvk indicates that in contrast to other localities the fox eruption was unable to completely clear its vent and disperse the excavated granodiorite prior to infilling the pipe the composition of fox vk provides no evidence for a loss of fines and suggests that these were either retained during eruption or reintroduced by mixing with ash deposits from previous eruptions

the paleoredox setting of burgess shale type deposits abstract burgess shale type bst biotas occur globally in lower and middle cambrian shales and were preserved under specific and exceptional circumstances among the circumstances surrounding fossilization benthic redox conditions have been the subject of particular disagreement while most authors have assumed that the sites of preservation were anoxic and therefore inhospitable to metazoans several have recently argued that bst assemblages were preserved in situ under habitable benthic conditions here we use field and laboratory intensive fine scale methods to investigate the paleoredox settings of fossil assemblages in two burgess shale type deposits the wheeler and marjum formations of western utah cambrian series 3 and review recently published data for other principal burgess shale type deposits in the wheeler and marjum formations fossil assemblages and ichnofabrics were evaluated at the millimeter scale an ichnological model was then applied to reconstruct relative paleo oxygen content of bottom waters at a bed to bed scale through multiple intervals sampled continuously this approach permits fine scale evaluation of the relationships of discrete fossil assemblages to paleoredox conditions over 7800 individual beds were logged in this study these data demonstrate that the wheeler and marjum formations were deposited under dynamic benthic redox conditions in both deposits beds representing deposition under dysoxic conditions may be interbedded at the millimeter centimeter scale with those representing deposition under anoxic conditions although the beds deposited under both types of redox conditions may be richly fossiliferous the distribution of fossil assemblages within the wheeler and marjum formations was regulated by dissolved oxygen content of bottom waters as a first order control bst preservation occurs within a microfacies inferred to represent deposition under anoxic conditions and contains transported fossils along a recognizable proximal distal gradient under dysoxic benthic conditions assemblages of skeletonized body fossils dominated by trilobites occur in both formations in the wheeler formation the junction of these two benthic redox environments is characterized by dense monospecific associations of the opportunistic trilobite elrathia kingii but a comparable exaerobic taxon does not appear to be present in the marjum formation these results demonstrate the control of near bottom anoxia over preservation of bst assemblages in both formations and are consistent with the findings from the burgess shale and the chengjiang

the sedimentology stratigraphy and preservation of intertidal deposits in the minas basin system bay of fundy abstract intertidal sediments of the minas basin a macrotidal estuary at the head of the bay of fundy can be subdivided into six major facies tidal marsh beach mudflat sandflat channel lag and sandbar each is characterized by its grain size distributions sedimentary structures and biological communities these sedimentary facies occur in three types of high to low tide zonation depending on the local sediment source and wave and current energy the holocene stratigraphy presently preserved in the upper intertidal zone of minas basin consists of a relatively thin on the average less than 1 2 m wedge of sediment overlying bedrock only in a few sheltered locations where deposits accumulate to several meters is the marine transgressive stratigraphy well represented the hypothetical progradational or marine regressive stratigraphy of upper intertidal sediments would probably resemble from top to bottom the high to low tide sequence of facies occurring today on sheltered tidal flats lebensspuren would provide the most reliable representation of the ecology of intertidal sediments in minas basin they are useful in delineating communities as well as being sensitive indicators of sediment deposition microfauna are too easily transported or reworked field data from relict intertidal deposits dated 2300 3400 yr b p indicate calcareous organism hard parts would probably provide a biased fossilized record indirect evidence is discussed which suggest in situ dissolution of shells in certain intertidal sediments

tidal bar cyclicity record in the lower permian the rio bonito formation parana basin southern brazil abstract tidal bars register gravitational interactions between the earth moon and sun and allow inferences about astronomical positioning through the cyclic variation of sandstone layer thickness throughout depositional events measurements of sandstone layers thickness in a tidal bar of the rio bonito formation parana basin southern brazil and further spectral analysis allowed the definition of the tide patterns imprinted on rock layers the study was based on a remarkable tidal bar outcrop located in the candiota paleovalley in the southern basin candiota town brazil with sedimentary characteristics that suggest to a tidally dominated estuary depositional environment the tidal bar shows a medium to large scale cross stratification reactivation surfaces bidirectional structures and tidal bundles the spectral analysis resulted in a significant frequency peak 0 0329 which corresponds to a period of 30 45 layers per cycle six of these cycles are recognizable in the tidal bar by its cyclic thickening thinning of layers that range from 1 5 to 11 cm this cyclic pattern is interpreted as being produced by the neap spring tide cycle our study also allows us to infer that the south parana basin permian tide had a semi diurnal pattern with a neap spring tide period of 14 9 days comparison with previous studies points to small changes in the duration of the neap spring tide cycle in the last 900 ma 15 2 days in the cambrian 14 9 days in the early permian and 14 7 days today explained by the distance and acceleration of the movement of the moon s revolution and decrease in the speed of rotation of the earth

pyrite framboid study of marine permian triassic boundary sections a complex anoxic event and its relationship to contemporaneous mass extinction size analysis of pyrite framboids has been undertaken on epicontinental permian triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia mid paleolatitude sections from the margins of the boreal spitsbergen greenland and neotethyan oceans western australia reveal intense anoxia throughout the permian triassic boundary interval with euxinic conditions frequently developing and dysoxia encountered even in relatively shallow water settings above storm wave base at equatorial paleolatitudes weakly oxygenated dysoxic conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition although euxinia was rare western and eastern tethyan locations re veal a complex and unstable redox history anoxia in the hindeodus praeparvus zone was replaced by oxygenated facies in the permian triassic boundary interval h changxingensis to h parvus zones oxygen poor deposition returned during the succeeding isarcicella isarcica zone the more persistent and intense development of oxygen restriction in cooler water mid paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of permian triassic bound ary anoxia in higher paleolatitudes the ben thic invertebrate extinctions occurred during a prolonged phase of oxygen poor deposition while in equatorial tethyan locations benthic losses occurred at the end of the fi rst anoxic phase in the late h praeparvus zone

sequence stratigraphy cyclic facies and lagerstatten in the middle cambrian wheeler and marjum formations great basin utah abstract recurrent taphofacies including conservation lagerstatten are identified within a spectrum of facies in fourth order sequences in the middle cambrian wheeler and marjum formations of the drum mountains and house range west central utah these sequences are 3 20 m thick and commence with sharply based compact oncolitic oolitic or pelletal pack and grainstones with sharply defined corroded and mineralized upper contacts that record drowning discontinuities and early transgressive systems tracts tsts overlying intervals of calcareous shale and thin bedded wacke to packstones with abundant disarticulated polymerid and agnostoid trilobites represent late tsts these are commonly followed by lavender gray mudstones rich in sponge spicules and comminuted fossil debris that reflect condensed maximum flooding zones the overlying early highstand hst intervals of black fissile shales are typically barren except for indistinct circular carbonized algae but in rare instances include soft bodied animal remains a combination of lower dysoxic anoxic conditions with a fluctuating oxycline and relatively rapid episodic influx of fine grained detrital sediment favored repeated burial and preservation of abundant organic detritus and rarely soft bodied animals interbedded dark gray shales include abundant articulated agnostoid trilobites and diminutive polymerids e g jenkinsonia overlying platy calcareous bedding planes covered with articulated bodies and molts of the polymerid elrathia indicate rapid blanketing of undisturbed seafloors by calcareous mud layers these beds grade upward successively into interbedded sparsely fossiliferous platy to flaggy shale and thin pale gray weathering calcisiltites and burrow mottled to nodular limestones recording late hst to falling stage fsst carbonate shedding thin calcisiltites include fossil debris and articulated larger polymerid trilobites and the eocrinoid gogia preserved by obrutionary deposits the repeated recurrence of these patterns provides the rudiments of a predictive model that not only explains the differing modes of preservation but may also aid in prospecting for new lagerstatten

geology of northeast british columbia and northwest alberta diamonds shallow gas gravel and glaciersthis article introduces a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers this special issue reports on some of the results of a multi disciplinary research program conducted in the boreal plains of northwest alberta and northeast british columbia several innovative approaches to conducting geoscientific research in this remote drift covered region are presented including geochemical analysis of bentonites for evaluating kimberlite potential the use of electromagnetic surveys for mapping buried aggregate deposits and paleo topographic mapping techniques to define buried channels results of the program include the discovery of several large aggregate deposits the first kimberlite indicator minerals in northeast british columbia a significant sphalerite dispersal train in northwest alberta the first documented report of kimberlite sourced bentonites and numerous previously unknown interglacial sites together these papers provide a greatly enhanced understanding of the glacial history quaternary stratigraphy and kimberlite geology of northwest alberta and northeast bri

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

a sedimentological evaluation of the devonian carboniferous boundary stratotype in southern ireland abstract the stratotype section for the base of the british dinantian devonian carboniferous boundary is currently located at the interface of two marine formations at the old head of kinsale this corresponds with the boundary of the ln and vi miospore subzone floral assemblages the succession immediately below the interface records an upward change from inner shelf into transgressive backbarrier sediments due to the initiation and progressive influence of a storm dominated migrating microtidal barrier complex the formation junction is considered to reflect a non sequence arising from subsequent erosive shoreface retreat and destruction of the barrier superstructure which was followed by apparently sudden deepening and the deposition of pro delta facies comparisons with other sections at this stratigraphical level in southern ireland indicates that barrier construction with landward migration into backbarrier bays occurred during a destructive phase of deltaic development in latest devonian times a rapid transgression associated with a cessation of sand supply then took place resulting in variable reworking of the underlying sequences and discontinuous shoreline retreat this was followed by the deposition of storm influenced calcareous shelf and inshore bay sediments over laterally diverse delta platform successions with continued deepening a pro delta environment was established over the area in the earliest carboniferous vi miospore subzone which thus records the relatively unusual preservation of barrier and backbarrier sequences in a regionally transgressive situation

kimberlite eruptions as triggers for early cenozoic hyperthermals 1 the early cenozoic experienced at least three short but major hyperthermals associated with disruptions of the global carbon cycle the largest among those the paleocene eocene thermal maximum was associated with a negative carbon isotopic excursion of 2 5 that appears to be best explained by the thermal dissociation of methane hydrates due to an initial period of warming the cause of the initial warming has been attributed to a massive injection of carbon co2 and or ch4 into the atmosphere however the source of the carbon is as yet unknown the emplacement of a large cluster of kimberlite pipes at 56 ma in the lac de gras region of northern canada may have provided the carbon that triggered early warming in the form of exsolved magmatic co2 our calculations indicate that the estimated 900 1100 pg of carbon required for the initial 3 c of ocean water warming associated with the paleocene eocene thermal maximum could have been released during the emplacement of a large kimberlite cluster the coeval ages of two other kimberlite clusters in the lac de gras field and two other early cenozoic hyperthermals indicate that co2 degassing during kimberlite emplacement is a plausible source of the co2 responsible for these sudden global warming events

distal sedimentation in a peripheral foreland basin ordovician black shales and associated flysch of the western taconic foreland new york state and ontario black shale is characteristic of early siliciclastic fill along the distal western flank of the middle to late ordovician taconic peripheral foreland basin this facies referred to as the utica black shale magnafacies includes at least five intervals bounded by unconformities and or condensed beds each unit records a pulse of subsidence and subsequent siliciclastic sedimentation in the foreland basin progressive episodic shifts from carbonate to organic rich mud deposition may be due to steepening and subsidence of the western carbonate ramp followed by onlap of axially dispersed sediment in general silt poor organic rich mud was deposited on a subsided carbonate ramp that sloped eastward toward the nearest siliciclastic sediment source this mud may have accumulated from the pelitic tails of axial fan turbidites derived from source areas to the east and possibly the south the resultant black shale does not necessarily represent the deepest part of the basin the deepest part of the basin the basin axis contained coarser grained axial submarine fan deposits throughout the later part of the middle ordovician and the first half of the late ordovician the basin axis migrated westward over 100 km yet the orientation of the axis and the basin margins were persistent this basin orientation generally parallel to the orogenic belt along with the large scale basin shape resulted from a combination of deformational loading of the continental margin and progressive flexure in the foreland smaller scale structural elements normal fault bounded basement blocks were superposed on large scale taconic foreland basin geometry abrupt shifts from carbonate ramp sedimentation to deeper water organic rich mud deposition may reflect movement along these faults during the orogeny

in situ u pb age determination and nd isotopic analysis of perovskites from kimberlites in southern africa and somerset island canada abstract determination of the emplacement ages and initial isotopic composition of kimberlite by conventional isotopic methods using bulk rock samples is unreliable as these rocks usually contain diverse clasts of crustal and mantle derived materials and can be subject to post intrusion sub aerial alteration in this study 8 samples from 5 kimberlites in southern africa and twelve samples from 7 kimberlites from somerset island canada have been selected for in situ perovskite u pb isotopic age determination and nd isotopic analysis by laser ablation using thin sections and mineral separates these fresh perovskites occur as primary groundmass minerals with grain sizes of 10 100 m they were formed during the early stage of magmatic crystallization and record data for the least contaminated or contamination free kimberlitic magma u pb isotopic data indicate that the majority of the southern africa kimberlites investigated were emplaced during the cretaceous with ages of 88 3 to 97 6 ma although one sample yielded an early paleozoic age of 515 6 ma twelve samples from somerset island yielded ages ranging from 93 4 ma to 108 5 ma and are contemporaneous with other cretaceous kimberlite magmatism in central canada 103 94 ma although whole rock compositions of the kimberlites from southern africa have a large range of end t values 0 5 to 5 1 the analysed perovskites show a more limited range of 1 2 to 3 1 perovskites from somerset island have end t values of 0 2 to 1 4 these values are lower than that of depleted asthenospheric mantle suggesting that kimberlites might be derived from the lower mantle this study shows that in situ u pb and nd isotopic analysis of perovskite by laser ablation is both rapid and economic and serves as a powerful tool for the determination of the emplacement age and potential source of kimberlite magmas

in vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement an example from the fox kimberlite ekati diamond mine nwt canada abstract the origins of massive poorly sorted fragmental kimberlite and kimberlite breccias in the diatremes of kimberlite volcanic pipes is currently poorly understood studies of the textural features of the major infill of the fox kimberlite ekati diamond mine nwt canada show that it may have formed as a result of the collapse of the explosive eruption column above the vent into the deep open pipe during the climactic stage of the eruption as the eruption intensity increased the column became critically overloaded with dense particles and rapidly collapsed onto itself unlike deposits typically preserved from column collapse events such as ignimbrites some portion of the collapsing mass was trapped in the deep pipe forming a distinct type of pyroclastic deposit in vent vent fill column collapse lapilli tuff this deposit has previously been tentatively identified as tuffisitic kimberlite tk or massive volcaniclastic kimberlite mvk based on broadly similar textural characteristics with these lithologies though every individual deposit will have formed under slightly different circumstances mass emplacement from column collapse is proposed as a viable model for massive volcaniclastic kimberlite emplacement based on extrapolation of modern volcanological processes

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the