hydrocode simulation of the chicxulub impact event and the production of climatically active gases we constructed a numerical model of the chicxulub impact event using the chart d squared csq code coupled with the analytic equation of state aneos package in the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact related extinction hypothesis carbonates calcite and evaporites anhydrite simulations explored the effects of different projectile sizes 10 to 30 km in diameter and porosity 0 to 50 the effect of impact speed is addressed by doing simulations of asteroid impacts vi 20 km s and comet impacts vi 50 km s the masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event ranging from 350 to 3500 gt for co2 from 40 to 560 gt for s and from 200 to 1400 gt for water vapor while our results are in good agreement with those of ivanov et al 1996 our estimated co2 production is 1 to 2 orders of magnitude lower than the results of takata and ahrens 1994 indicating that the impact event enhanced the end cretaceous atmospheric co2 inventory by at most 40 consequently sulfur may have been the most important climatically active gas injected into the stratosphere the amount of s released by the impact is several orders of magnitude higher than any known volcanic eruption and with h2o is high enough to produce a sudden and significant perturbation of earth s climate

metal contents of phytoplankton and labile particulate material in the north atlantic ocean abstract phytoplankton contribute significantly to global c cycling and serve as the base of ocean food webs phytoplankton require trace metals for growth and also mediate the vertical distributions of many metals in the ocean we collected bulk particulate material and individual phytoplankton cells from the upper water column

vitric compositions in the onaping formation and their relationship to the sudbury igneous complex sudbury structure the 1 85 ga sudbury structure is the largest well preserved exposed example of a terrestrial impact crater it contains world class ni cu platinum group element pge ores associated with the igneous component a new understanding of the geology and geochemistry of the vitric crater fill sequence the onaping formation provides constraints on the evolution of 1 85 ga igneous rocks in the sudbury structure the onaping formation has a well defined mappable stratigraphy composed of three members seven stratigraphic units and two types of intraformational dikes that are overprinted by an impact generated hydrothermal system stratigraphic and alteration mapping provided a basis for well constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit including vitric bombs and blocks and intraformational vitric dikes significantly the identification of a least altered composition of the vitric onaping formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater fill sequence and the sudbury igneous complex and offset dikes the least altered vitric onaping formation represented by the massive xenolith poor cores of vitric bombs blocks and intraformational dikes is andesitic in composition consistent with a crustally derived melt the melt composition 61 6 wt sio 2 4 28 mgo 60 ppm ni is remarkably uniform throughout the onaping formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative nb and ti anomalies incompatible trace element ratios and bulk concentrations of the onaping formation coincide with the main mass norite and offset quartz diorite compositions of the underlying sudbury igneous complex but differ distinctly from the granophyre and quartz gabbro root mean squared differences in composition of the vitric onaping formation with the units of the sudbury igneous complex and quartz diorite offset dikes show that the vitric onaping formation does not directly match any of these igneous units but more closely resembles the composition of the ministic and manchester offset dikes the quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the sudbury igneous complex in detail there are small compositional differences between north and south range offset dikes and between these offset dikes and the concentric manchester offset dike however such differences are lacking in the onaping formation we propose that the least altered vitric composition of the onaping formation represents the best estimate of the bulk composition of an initial quenched melt or shock melt formed early in the evolution of the sudbury structure and is a logical starting liquid to model the evolution of the sudbury main mass and offset dikes the offset dike compositions can be derived from onaping formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater this mafic contaminated melt or impact melt developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the sudbury complex fractionation of the mafic contaminated melt by plagioclase and pyroxene accumulation produced the north range felsic and mafic norite and the south range norites the chalcophile metal content of the onaping formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores the vitric onaping formation and unmineralized offset dikes have low liquidus temperatures 1 115 1 125 c and if these units represent the source of metals in the sulfide deposits unusually high mass distribution coefficients for cu and ni and high r factors are necessary to account for the metal endowment

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

chicxulub impact ejecta from albion island belize impact ejecta from the albion formation are exposed in northern belize the ejecta come from the outer portion of the continuous ejecta blanket of the chicxulub crater which is located 360 km to the northwest the basal unit of the albion formation is a 1 m thick clay and dolomite spheroid bed composed of up to four discrete flows the clay spheroids are altered impact glass and the dolomite spheroids are accretionary lapilli the upper unit is a 15 m thick coarse diamictite bed containing altered glass large accretionary blocks striated polished and impacted cobbles and rare shocked quartz the abundance of accretionary clasts evidence for atmospheric drag sorting and the presence of multiple flows in the albion formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters

structural characteristics of the sudbury impact structure canada impact induced versus orogenic deformation a review orogenic deformation both preceding and following the impact event at sudbury strongly hinders a straightforward assessment of impact induced geological processes that generated the sudbury impact structure central to understanding these processes is the state of strain of the sudbury igneous complex the solidified impact melt sheet its underlying target rocks overlying impact breccias and post impact sedimentary rocks this review addresses 1 major structural metamorphic and magmatic characteristics of the impact melt sheet and associated dikes 2 attempts that have been made to constrain the primary geometry of the igneous complex 3 modes of impact induced deformation as well as 4 mechanisms of pre and post impact orogenic deformation the latter have important consequences for estimating parameters such as magnitude of structural uplift tilting of pre impact huronian strata and displacement on major discontinuities which collectively have not yet been considered in impact models in this regard a mechanism for the emplacement of offset dikes is suggested that accounts for the geometry of the dikes and magmatic characteristics as well as the occurrence of sulfides in the dikes moreover re interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact uncertainties still exist as to whether the sudbury impact structure was a peak ring or a multi ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification

rapid honeycomb weathering tafoni formation in greywacke s e australia the development of honeycomb weathering on seawalls of non marine lower cretaceous greywacke built in 1943 and 1949 shows that under certain conditions this type of tafoni is a significant factor in the retrogradation of rocky coasts

the sudbury structure ontario canada a tectonically deformed multi ring impact basin the occurrence of shock metamorphic features substantiates an impact origin for the 1 85 ga old sudbury structure but this has not been universally accepted recent improvements in knowledge of large scale impact processes combined with new petrographic geochemical geophysical lithoprobe and structural data allow the sudbury structure to be interpreted as a multi ring impact structure the structure consists of the following lithologies sudbury breccia dike breccias occurring up to 80 km from the sudbury igneous complex sic footwall rocks and footwall breccia brecciated shocked crater floor materials in part thermally metamorphosed by the overlying sic sublayer and offset dikes main mass of the sic and basal member of the onaping formation of geochemically heterogeneous coherent impact melt complex ranging from inclusion rich basal unit through a dominantly inclusion free to a capping inclusion rich impact melt rock grey member of of melt rich impact breccia suevite green member of of thin layer of fall back ejecta black member of of reworked and redeposited breccia material onwatin and chelmsford formations post impact sediments observational and analytical data support an integrated step by step impact model for the genesis of these units analysis of the present spatial distribution of various impact related lithologies and shock metamorphic effects result in an estimated original rim to rim diameter of the final crater of 200 or even 280 km for the sudbury structure prior to tectonic thrusting and deformation during the penokean orogeny

geochemical and nd pb isotopic evidence for the provenance of the early proterozoic virginia formation minnesota implications for the tectonic setting of the animikie basin the early proterozoic animikie basin is located at a major crustal boundary between archean crust of the superior craton and penecontemporaneous early proterozoic crust of the wisconsin magmatic terranes the provenance of sedimentary rocks within the animikie basin constrains the tectonic evolution of the early proterozoic margin of the superior craton published geological petrographic and nd pb isotopic data indicate that the pokegama quartzite lower passive margin unit of the animikie group has a late archean provenance in contrast the virginia formation upper deep water turbidite shale unit of the animikie group was derived from dominantly early proterozoic sources with isotopic compositions similar to igneous rocks of the wisconsin magmatic terranes shales of the virginia formation have nd depleted mantle model ages 2 14 to 2 35 ga and mu 1 of 7 81 to 8 09 ashes within the virginia are more evolved geochemically than shales and have younger model ages of 1 86 and 1 99 ga and simila

the penokean orogeny in the lake superior region abstract the penokean orogeny began at about 1880 ma when an oceanic arc now the pembine wausau terrane collided with the southern margin of the archean superior craton marking the end of a period of south directed subduction the docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back arc extension both in the initial arc and adjacent craton margin to the north a belt of volcanogenic massive sulfide deposits formed in the extending back arc rift within the arc synchronous extension and subsidence of the superior craton resulted in a broad shallow sea characterized by volcanic grabens menominee group in northern michigan the classic lake superior banded iron formations including those in the marquette gogebic mesabi and gunflint iron ranges formed in that sea the newly established subduction zone caused continued arc volcanism until about 1850 ma when a fragment of archean crust now the basement of the marshfield terrane arrived at the subduction zone the convergence of archean blocks of the superior and marshfield cratons resulted in the major contractional phase of the penokean orogeny rocks of the pembine wausau arc were thrust northward onto the superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 ma in the south baraga group rocks and 1835 ma in the north rove and virginia formations a thick succession of arc derived turbidites constitutes most of the foreland basin fill along with lesser volcanic rocks in the southern fold and thrust belt tectonic thickening resulted in high grade metamorphism of the sediments by 1830 ma at this same time a suite of post tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the penokean orogeny the penokean orogen was strongly overprinted by younger tectonic and thermal events some of which were previously ascribed to the penokean principal among these was a period of vertical faulting in the archean basement and overlying paleoproterozoic strata this deformation is now known to have post dated the terminal penokean plutons by at least several tens of millions of years evidence of the penokean orogen is now largely confined to the lake superior region comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive penokean orogen have been removed or overprinted by younger tectonic events

the sudbury impact layer in the paleoproterozoic iron ranges of northern michigan usa a layer of breccia that contains fragments of impact ejecta has been found at 10 sites in the paleoproterozoic iron ranges of northern michigan in the lake superior region of the united states radiometric age constraints from events predating and postdating depo si tion of the breccia are ca 1875 ma and 1830 ma the major bolide impact that occurred at 1850 ma at sudbury ontario 500 700 km east of these sites is the likely causative event the michigan sites described here along with previously described sites in minnesota and ontario defi ne an extensive ejecta bearing deposit throughout the paleoproterozoic iron ranges of the lake superior region that we refer to as the sudbury impact layer the layer at the sites in michigan exhibits a range of thicknesses lithologic characters and sedimentary settings the diversity of rock types and internal stratigraphic details of the layer imply that several different processes of transport and deposition are represented but the detailed investigations needed to document them are incomplete many of the sites had been described and interpreted previously as products of common terrestrial processes but the presence of relict shock induced planar deformation features in quartz indicates that the breccia layer is in fact the product of an extra terrestrial impact at most localities this layer also contains relict fragments of altered devitrifi ed glass and or accretionary lapilli one immediate use of the impact layer is as an ultraprecise time line that ties together the well known stratigraphic sequences of the various geographically separated iron ranges the correlation of which has remained controversial for many decades the sudbury impact layer most commonly lies at a horizon that records a signifi cant change in the character of sediments across the region the impact layer marks the end of a major period of banded iron formation deposition that was succeeded by deposition of ficlastic rocks commonly black shales the impact may have produced regional if not global changes in the environment that resulted in this widespread synchronous change in sedimentation style

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

the tectonostatigraphic evolution of the huronian basement and the subsequent basin fill geological constraints on impact models of the sudbury event abstract whereas both meteorite impact and volcanic mechanisms have been proposed to explain the rocks and associated ore deposits within the sudbury structure 1 85 ga few models take into consideration the paleogeographic constraints implied by the history of the region before and following the sudbury event huronian depositional style south of the sudbury structure appears to have been strongly influenced by the early development of transtensional extensional basins these basins were initiated by sinistral strike slip movement along part of a paleoproterozoic continental margin which later evolved into a passive margin transition from a rifted strike slip margin to a passive margin was accompanied by major slope failure and soft sediment deformation south of the murray fault diabase dikes were intruded into unconsolidated sediments at considerable depth producing round stone breccias which are texturally similar to sudbury breccia minor compressional folding occurred prior to emplacement of the nipissing diabase suite at around 2 2 ga the abundance of diabase intrusions near the sudbury structure suggests that the area may have been a focal point for magma emplacement the post event fill of the paleoproterozoic sudbury basin reflects initial deposition in a deep water anoxic setting there is no evidence to suggest that sedimentation was influenced by the steep walls of an impact crater carbonate in the basal vermilion member is of sedimentary exhalative origin and was not derived from a shallow marine shelf turbidites in the chelmsford formation show no evidence of centripetal fill as might be expected from a restricted circular basin they appear to have been emplaced along the depositional axis of an elongate foreland basin developed in front of the rising penokean mountain chain penokean deformation continued after the sudbury event and influenced strata well into the archean craton deformation of huronian strata in paleovalleys north and west of the sudbury structure does not appear to be genetically related to the sudbury event if superfaults were generated around the sudbury structure in response to compression and crustal rebound following a meteorite impact these do not appear to have caused offset of pre existing steeply dipping contacts between the huronian and underlying archean basement a new model consistent with these observations needs to be developed

thermal history of low metamorphic grade paleoproterozoic sedimentary rocks of the penokean orogen lake superior region evidence for a widespread 1786 ma overprint based on xenotime geochronology abstract paleoproterozoic strata in northern michigan wisconsin and minnesota were deposited between 2 3 and 1 75 ga within the rifted margin and subsequent foreland basin of the penokean orogen these strata show evidence for multiple regional metamorphic events previously attributed entirely to the penokean orogeny 1875 1835 ma metasandstones from the marquette range supergroup and the animikie mille lacs and north range groups were sampled at multiple localities across minnesota wisconsin and michigan for metamorphic xenotime suitable for in situ shrimp u pb geochronology all samples are from the northern penokean foreland basin where the metamorphic grade is greenschist to sub greenschist and the strata are virtually undeformed xenotime u pb ages in these samples have a bimodal population with means of 1786 4 ma n 32 and 1861 10 ma n 9 xenotime of both ages are contained in metasandstones from the basal chocolay group in michigan and wisconsin and the mille lacs group and north range groups in minnesota the older age records a regional low temperature thermal event that is slightly older than the overlying menominee group in michigan and the animikie group in minnesota and ontario this 1861 ma event coincides with regional uplift that led to the formation of the unconformity between the menominee group and the overlying baraga group in michigan hence xenotime growth must have occurred at shallow burial depths younger units from the menominee and baraga groups in michigan and the animikie group in minnesota record only the 1786 ma event a dominant 1800 1790 ma metamorphic monazite population that overprints penokean interval monazite has been documented within amphibolite to granulite facies rocks immediately north of the niagara fault zone within the vicinity of gneiss domes and granitic plutons in contrast the 1786 ma xenotime ages are from low grade virtually undeformed rocks 50 150 km from the high grade zones and thus do not appear to reflect a local thermal imprint rather the geographic extent of the 1786 ma xenotime growth event suggests that it reflects a basin wide subtle thermal pulse it is proposed that the xenotime ages record widespread subtle heating triggered by renewed subduction along the orogen due to yavapai interval convergence the 1800 1700 ma yavapai terrane forms an accretionary belt throughout the central and southwestern u s and truncates the southern part of the penokean orogen in central wisconsin and southeastern minnesota about 200 km south of the sample sites alternatively an 1800 1765 ma interval of gravitational collapse of overthickened crust of the penokean orogen immediately north of the niagara fault zone may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the penokean foreland and resulted in xenotime growth

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

sub million year age resolution of precambrian igneous events by thermal extraction thermal ionization mass spectrometer pb dating of zircon application to crystallization of the sudbury impact melt sheet a new approach to zircon dating is described that potentially offers a considerable improvement in the accuracy of 207 pb 206 pb ages on a wider variety of samples and at less cost than that generally available from conventional techniques zircon is first preheated in a vacuum to evaporate pb from altered domains leaving predominantly pb with isotopes that preserve the primary age of the sample refractory pb is removed from the zircon by thermal extraction at higher temperature directly into a silica melt ionization activator within a thermal ionization mass spectrometer this produces strong ion beams that allow 207 pb 206 pb isotope ratios to be measured to high precision 10 4 with almost negligible contamination from common pb isotope fractionation appears to be more reproducible than with conventional solution analysis allowing routine age determinations with precision of 0 2 m y for the precambrian as a test application zircon from a noritic boundary phase of the sudbury impact melt gives 1849 53 0 21 ma while a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849 11 0 19 ma 95 confidence errors the enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating

terrestrial chondrules glass spherules and accretionary lapilli from the suevite ries crater germany abstract until recently no terrestrial analogues of meteoritic and lunar chondrules were known only rare glass spherules from the lonar crater india and black magnetic spherules from various localities have been recorded the impact breccia suevite of the no rdlinger ries crater germany contains both chondrules and glass spherules and in addition accretionary lapilli all of which are found imbedded within the fine grained matrix of the suevite the chondrules display many of the textural features characteristic of meteoritic and lunar chondrules lithic chondrules and fluid drop chondrules are present the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition on the whole fluid drop chondrules lithic chondrules and glass spherules are rare in the suevite with fluid drop chondrules prevailing detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules accretionary lapilli also represent material formed as a result of impact

age of volcanic rocks and syndepositional iron formations marquette range supergroup implications for the tectonic setting of paleoproterozoic iron formations of the lake superior region a rhyolite in the hemlock formation a mostly bimodal submarine volcanic deposit that is laterally correlative with the negaunee iron formation yields a sensitive high resolution ion microprobe shrimp u pb zircon age of 1874 9 ma but also contains inherited archean zircons as old as 3 8 ga this precise age determination for the classic paleoproterozoic stratigraphic sequence of northern michigan the marquette range supergroup mrs necessitates modification of previous depositional and tectonic models our new data indicate that the menominee group previously ascribed to continental rifting during early pre collision phases of the penokean orogenic cycle is coeval with arc related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit we interpret these to be second order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin along with a recently reported age for the gunflint formation in ontario of 1878 2 ma our data suggest that an extensive foredeep in the western lake superior region was the locus of iron formation deposition during arc accretion from the south further we interpret the lower mrs chocolay group a glaciogenic and shallow marine succession that lies atop archean basement to be equivalent to the upper part of the huronian supergroup of ontario and to represent the original continental rifting and passive margin phase of the penokean cycle the upper mrs baraga group represents deeper marine basins dominated by turbidites and lesser volcanic rocks resulting from increased subsidence and continued collision a stitching pluton which cuts correlatives of the hemlock formation in a thrust sheet yielded a u pb zircon age of 1833 6 ma consistent with other post tectonic plutons in wisconsin and northern michigan indicating that penokean convergence lasted no longer than 40 million years

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

physical and chemical evidence of the 1850 ma sudbury impact event in the baraga group michigan an ejecta layer produced by the sudbury impact event ca 1850 ma occurs within the baraga group of northern michigan and provides an excellent record of impact related depositional processes this newly discovered 2 4 m thick horizon accumulated in a peritidal environment during a minor sea level lowstand that punctuated a period of marine transgression common ejecta clasts include shock metamorphosed quartz grains splash form melt spherules and tektites accretionary lapilli and glassy shards suggesting sedimentation near the terminus of the continuous ejecta blanket sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact generated tsunami wave train observed platinum group element anomalies ir rh and ru within the sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite documenting and interpreting the detailed characteristics of the sudbury ejecta horizon in michigan have yielded a fingerprint to identify this chronostratigraphic marker in other paleoproterozoic basins for the first time a foundation exists to assess the consequences of the sudbury impact on precambrian ocean chemistry and early life

a precambrian proximal ejecta blanket from scotland ejecta blankets around impact craters are rarely preserved on earth although impact craters are ubiquitous on solid bodies throughout the solar system on earth they are rapidly effaced and few records exist of the processes that occur during emplacement of ejecta the stac fada member of the precambrian stoer group in scotland has previously been described as volcanic in origin however shocked quartz and biotite provide evidence for high pressure shock metamorphism while chromium isotope values and elevated abundances of platinum group metals and siderophile elements indicate addition of meteoritic material thus the unit is reinterpreted here as having an impact origin the ejecta blanket reaches 20 m in thickness and contains abundant dark green vesicular devitrified glass fragments field observations suggest that the deposit was emplaced as a single fluidized flow that formed as a result of an impact into water saturated sedimentary strata the continental geological setting and presence of groundwater make this deposit an analogue for martian fluidized ejecta blankets

discovery of distal ejecta from the 1850 ma sudbury impact event a 25 70 cm thick laterally correlative layer near the contact between the paleoproterozoic sedimentary gunflint iron formation and overlying rove formation and between the biwabik iron formation and overlying virginia formation western lake superior region contains shocked quartz and feldspar grains found within accretionary lapilli accreted grain clusters and spherule masses demonstrating that the layer contains hypervelocity impact ejecta zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca 1878 ma and 1836 ma the sudbury impact event which occurred 650 875 km to the east at 1850 1 ma is therefore the likely ejecta source making these the oldest ejecta linked to a specific impact shock features particularly planar deformation features are remarkably well preserved in localized zones within the ejecta whereas in other zones mineral replacement primarily carbonate has significantly altered or destroyed ejecta features

formation mechanisms of planar deformation features in naturally shocked quartz abstract shock waves induce peculiar defects in quartz shock mosaics high pressure polymorphs coesite and stishovite diaplectic glass and planar deformation features pdfs together these features are the indices of shock metamorphism pdfs appear under the optical microscope as straight and narrow defects parallel to the 10 1n rhombohedral planes with n 3 and 2 as most frequent values although n 1 4 and i e basal plane are also found we report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of pdfs in shocked quartz grains from a variety of sites slate islands la malbaie and manson in north america the vredefort complex in south africa the ries crater in germany and the cretaceous tertiary boundary layer at raton basin colorado we distinguish four pdf fine structures 1 bands of dislocations 2 lamellae of mixtures in various proportions of amorphous silica and small crystallites 3 thin brazil twin lamellae 4 short parallel lamellae forming serrated ladder structures the possible formation mechanisms of these pdfs are discussed in the second part of the paper it is suggested that the dislocation bands are probably not original pdf structures but stem from a later recrystallization stage the brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave the other pdf structures which consist of partial amorphization in the 10 1 n planes must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 gpa the growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave

the dimensions of the chicxulub impact crater and impact melt sheet the chicxulub impact crater which is the principal source of impact debris in cretaceous tertiary boundary sediments is currently buried by a 1 km thick sequence of carbonate sediments because the crater is not exposed for direct scrutiny its size has remained uncertain and in particular estimates ranging from 100 to 300 km have been made on the basis of gravity and magnetic field data in this study the physical properties of the k t boundary impact ejecta and chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater s size on the basis of the measured thickness of impact ejecta in haiti and north america the calculated volume of the impact melt sheet and the chemical and isotopic composition of the melt sheet the chicxulub impact crater is inferred to be 180 km in diameter and to contain a 3 to 7 km thick melt sheet and breccia sequence within a centrally located 100 km diameter region that corresponds to the limits of the transient cavity

shocked rocks and impact glasses from the el gygytgyn impact structure russia the elgygytgyn impact structure is about 18 km in diameter and is located in the central part of chukotka arctic russia the crater was formed in volcanic rock strata of cretaceous age which include lava and tuffs of rhyolites dacites and andesites a mid pliocene age of the crater was previously determined by fission track 3 45 0 15 ma and 40ar 39ar dating 3 58 0 04 ma the ejecta layer around the crater is completely eroded shock metamorphosed volcanic rocks impact melt rocks and bomb shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event clasts of volcanic rocks which range in composition from rhyolite to dacite represent all stages of shock metamorphism including selective melting and formation of homogeneous impact melt four stages of shocked volcanic rocks were identified stage i 35 gpa lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant pfs and pdfs coesite and stishovite occur as well stage ii 35 45 gpa quartz and feldspar are converted to diaplectic glass coesite but no stishovite stage iii 45 55 gpa partly melted volcanic rocks common diaplectic quartz glass feldspar is melted and stage iv 55 gpa melt rocks and glasses two main types of impact melt rocks occur in the crater 1 impact melt rocks and impact melt breccias containing abundant fragments of shocked volcanic rocks that were probably derived from now eroded impact melt flows on the crater walls and 2 aerodynamically shaped impact melt glass bombs composed of homogeneous glass the composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target cobalt ni and ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks only the cr abundances show a distinct enrichment which points toward an achondritic projectile however the present data do not allow one to unambiguously identify a meteoritic component in the elgygytgyn impact melt rocks

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated