the rock elm meteorite impact structure wisconsin geology and shock metamorphic effects in quartz the rock elm structure in southwest wisconsin is an anomalous circular area of highly deformed rocks 6 5 km in diameter located in a region of virtually horizontal undeformed sedimentary rocks shock produced planar microstructures pms have been identified in quartz grains in several lithologies associated with the structure sandstones quartzite pebbles and breccia two distinct types of pms are pres ent p1 features which appear identical to planar fractures pfs or cleavage and p2 features which are interpreted as possible incipient planar deformation features pdfs the latter are uniquely produced by the shock waves associated with meteorite impact events both types of pms are oriented parallel to specific crystallographic planes in the quartz most commonly to c 0001 11 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 2 end document 2 and r z 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 1 the association of unusual structurally deformed strata with distinct shock produced microdeformation features in their quartz bearing rocks establishes rock elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact preliminary paleontological studies indicate a minimum age of middle ordovician for the rock elm structure a similar age estimate 450 400 ma is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks

sediments from lake cheko siberia a possible impact crater for the 1908 tunguska event cheko a small lake located in siberia close to the epicentre of the 1908 tunguska explosion might fill a crater left by the impact of a fragment of a cosmic body sediment cores from the lakes bottom were studied to support or reject this hypothesis a 175 cm long core collected near the center of the lake consists of an upper 1 m thick sequence of lacustrine deposits overlaying coarser chaotic material 210 pb and 137 cs indicate that the transition from lower to upper sequence occurred close to the time of the tunguska event pollen analysis reveals that remains of aquatic plants are abundant in the top post 1908 sequence but are absent in the lower pre1908 portion of the core these results including organic c n and d 13 c data suggest that lake cheko formed at the time of the tunguska event

geochemistry geology and isotopic sr s and b composition of evaporites in the lake st martin impact structure new constraints on the age of melt rock formation 1 we report new sr s and b isotopic data for evaporites gypsum anhydrite carbonates melt rocks gneisses and groundwaters recovered in and around the lake st martin lsm impact structure interlake region manitoba canada the lsm meteorite impacted devonian to ordovician carbonates and sandstones of the eastern williston basin resulting in partial melting of underlying superior province 2 5 ga gneisses of the canadian shield overlying the lsm melt rocks are red beds and evaporites anhydrite gypsum glauberite previously inferred to have been deposited during the jurassic the 87sr 86sr lowest values cluster at 0 70836 and 34scdt 23 7 0 9 of the evaporites combined with b isotope compositions of associated groundwaters 11bnbs951 25 to 28 are consistent with evaporite deposition within the impact structure near the edge of an ocean connected salina the establishment of a marine origin for the evaporites offers a method of age assignment using the secular variation of s and sr isotopes in seawater comparison of sr and s isotope results with the seawater curves precludes jurassic deposition for the evaporites or correlation with watrous and amaranth formation evaporites previously considered correlative with those at lsm the lowest sr and mean of s isotope values from the lsm evaporites are similar to seawater in the latest devonian consistent with conodonts recovered from carbonate breccia overlying melt rocks and we suggest this as an alternative age of the evaporites data presented here preclude a jurassic age for the evaporites and therefore for the impact event

the aouelloul crater mauritania on the problem of confirming the impact origin of a small crater the impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features a case in point is the 3 1 ma aouelloul crater mauritania 390 m in diameter which is exposed in an area of ordovician oujeft and zli sandstone we studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects in thin section a large fraction of the quartz grains show abundant subplanar and planar fractures many of the fractures are healed and are evident only as fluid inclusion trails a few grains showed sets of narrow and densely spaced fluid inclusions trails in one rarely two orientations per grain which could be possible remnants of planar deformation features pdfs although such an interpretation is not unambiguous in contrast an impact origin of the crater is confirmed by re os isotope studies of the target sandstone and glass found around the crater rim which show the presence of a distinct extraterrestrial component in the glass

seasonal variability in the origin of recent atmospheric mineral dust at northgrip greenland abstract isotopic sr and nd and mineralogical analyses of dust extracted from snow deposits from the last decade at the northgrip greenland ice camp 75 1 n 042 3 w confirm the eastern asian source for greenland dust and extend it to the present day the 1998 and 1999 samples which represent a 2 4 month resolution of dust deposition show that there are at least two east asian sources for the greenland dust that vary seasonally the provenance of dust during the major spring summer deposition is the takla makan desert of northwest china

boron isotope variations in nature a synthesis the large relative mass difference between the two stable isotopes of boron 10b and 11b and the high geochemical reactivity of boron lead to significant isotope fractionation by natural processes published 11b values relative to the nbs srm 951 standard span a wide range of 90 the lowest 11b values around 30 are reported for non marine evaporite minerals and certain tourmalines the most 11b enriched reservoir known to date are brines from australian salt lakes and the dead sea of israel with 11b values up to 59 dissolved boron in present day seawater has a constant world wide 11b value of 39 5 in this paper available 11b data of a variety of natural fluid and solid samples from different geological environments are compiled and some of the most relevant aspects including possible tracer applications of boron isotope geochemistry are summarized

inversion of magnetic and frequency domain electromagnetic data for investigating lithologies associated with gold mineralization in the canadian malartic area quebec canada canadian malartic is an archean low grade bulk tonnage native gold deposit the deposit is mostly located in altered clastic metasedimentary rocks mafic ultramafic dykes and monzodioritic porphyr

a possible impact crater for the 1908 tunguska event the so called tunguska event refers to a major explosion that occurred on 30 june 1908 in the tunguska region of siberia causing the destruction of over 2000 km2 of taiga globally detected pressure and seismic waves and bright luminescence in the night skies of europe and central asia combined with other unusual phenomena the tunguska event may be related to the impact with the earth of a cosmic body that exploded about 5 10 km above ground releasing in the atmosphere 10 15 mton of energy fragments of the impacting body have never been found and its nature comet or asteroid is still a matter of debate we report results from the investigation of lake cheko located 8 km nnw of the inferred explosion epicenter its funnel like bottom morphology and the structure of its sedimentary deposits revealed by acoustic imagery and direct sampling all suggest that the lake fills an impact crater lake cheko may have formed due to a secondary impact onto alluvial swampy ground the size and shape of the crater may have been affected by the nature of the ground and by impact related melting and degassing of a permafrost layer

sr and nd isotope compositions of atmospheric mineral dust at the summit of mt sefuri north kyushu southwest japan a marker of the dust provenance and seasonal variability abstract we analyzed rb sr nd isotope ratios of mineral dust in total aerosol load collected with rainwater continuously from 1998 to 2006 at the summit of mt sefuri northern kyushu southwestern japan during this period the total mass of the dust generally increased in late winter peaked in early spring and then decreased 87sr 86sr in atmospheric mineral dust varied from 0 7096 to 0 7180 and end 0 chur from 19 9 to 3 5 during heavy deposition periods the dust had high 87sr 86sr isotope ratios and low to middle end 0 chur values respectively these compositions are comparable to those of sand and loess in arid areas of northeast china takla makan and western beijing such particles were transported by westerlies from those areas to northern kyushu in winter and spring in summer and autumn the isotopic compositions of the dust varied greatly however during light deposition periods the sr isotope composition was low in these seasons the contributions to the dust from japanese soils and volcanic ash transported by southern winds were relatively larger than in winter and spring because of decreased mineral dust particle transport from the continent nevertheless fine sandy desert particles and loess in general accounted for most mineral dust deposition in northern kyushu year round even in summer local soils derived from weathered granite and volcanic ash were minor components only the net mass of water insoluble inorganic matter in the collected mineral dust fluctuated from year to year deposition on mt sefuri was relatively large before 2001 decreased from 2002 to 2005 and increased greatly in spring 2006 these year to year differences probably reflected changes in the strength of the westerlies and in climate conditions in the arid source areas

40ar 39ar age of the lake saint martin impact structure canada unchaining the late triassic terrestrial impact craters abstract new 40 ar 39 ar dating of impact melted k feldspars and impact melt rock from the 40 km lake saint martin impact structure in manitoba canada yielded three plateau ages and one mini plateau age in agreement with inverse isochron ages for the k feldspar melt aliquots and a minimum age for a whole rock impact melt sample a combination of two plateau ages and one isochron age with a weighted mean of 227 8 0 9 ma 1 1 ma including all sources of uncertainty 2 mswd 0 52 p 0 59 is considered to represent the best estimate age for the impact the concordant 40 ar 39 ar ages for the melted k feldspars derived from impact melt rocks in the eastern crater moat domain and the partially melted proterozoic central uplift granite suggest that the new dates accurately reflect the lake saint martin impact event in the carnian stage of the late triassic with a relative error of 0 4 on the 40 ar 39 ar age the lake saint martin impact structure counts among the most precisely dated impact structures on earth the new isotopic age for lake saint martin significantly improves upon earlier rb sr and u th he results for this impact structure and contradicts the hypothesis that planet earth experienced the formation of a giant impact crater chain during a major late triassic multiple impact event

asian continental nature of87sr 86sr ratios in north central pacific sediments strontium isotopic compositions were determined for silicate detritus in 152 deep sea and continental shelf surface sediment samples from the pacific ocean to trace their provenance the major characteristics of the spatial variation of the sr isotopes and the interpretation of this variation may be summarized as follows 1 sediments in the pacific have a lower average87sr 86sr ratio 0 7112 with a narrower range 0 703 0 724 than in the atlantic 0 7166 0 704 0 743 many western pacific volcanoes together with young continental crust supply material with low87sr 86sr ratios to the pacific in contrast to the large old cratonic crust that supplies material with high sr ratio to the atlantic 2 the87sr 86sr ratio is high in the north central pacific 0 711 0 719 the izu ogasawara and mariana arcs supply low87sr 86sr material to the western pacific but farther to east the abundant supply of loess carried by middle latitude westerlies from the asian continent 87sr 86sr ca 0 720 dominates the north central pacific

the use of heavy mineral correlation for determining the source of impact ejecta a manicouagan distal ejecta case study due to the nature of distal impact ejecta layers on earth preserved deposits are rare and are commonly altered by diagenetic weathering processes to establish the source of ejecta trace element signatures of impact glasses have typically been used to establish the source crater of a deposit however in many cases impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition this is the case for the late triassic ejecta deposit of sw britain where impact melt spherules have been completely altered to clay this late triassic ejecta deposit was originally believed to be derived from two possible sources the rochechouart or manicouagan impact structures to accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion ar ar dating of shocked biotite u pb dating of zircon was also used but proved inconclusive radiogenic dating of shocked biotites observed exclusively in the ejecta deposit yielded ages consistent with the grenvillian target rocks at manicouagan and excluded rochechouart as a potential source garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the manicouagan target rocks and impactite rocks a strong garnet composition signature correlation between the samples from manicouagan and the ejecta deposit provides convincing evidence that manicouagan is the source of the sw britain ejecta deposit furthermore we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta

geophysical evaluation of the enigmatic bedout basement high offshore northwestern australia abstract the bedout high in the roebuck basin formerly offshore canning basin on the northwest shelf of australia is an unusual structure which has been controversially interpreted as an end permian impact structure similar in size to the k t boundary chicxulub crater we present a geophysical perspective of the associated debate based on deep seismic reflection refraction and well data the basement and crust in the roebuck basin display a number of features that distinguish them from other basins along the northwest australian margin including major crustal thinning and the presence of a thick layer of interpreted magmatic underplating the bedout high consists of two separate highs separated by a paleozoic fault and is associated with a moho uplift of 7 8 km and is about 40 50 km wide the normal fault separating the two highs trends nnw sse roughly paralleling a paleozoic fault system associated with rifting in the canning basin and terminating below the interpreted top permian reflection there are no circular symmetric fault zones bounding the proposed annular trough and the distinct difference in seismic character normally associated with impact breccias versus layered sediments above are not expressed in deep multichannel seismic data the end permian horizon exhibits little topography with well layered units both below and above the area around the bedout high stands out as an area of low velocity basement 5400 5600 m s compared to 5800 6000 m s for other nearby basement areas located in a similar depth range but known complex impact sites are not characterized by a unique seismic basement velocity signature both seismic velocity analysis revealing a thick underplated layer in the lower crust and thermal modelling based on data from well la grange 1 and basalts drilled on top of the bedout high are consistent with rifting above anomalously hot mantle the available geophysical and geological data are compatible with an interpretation of the bedout structure as a basement high formed by two consecutive paleozoic and mesozoic episodes of rifting roughly orthogonal to each other associated with basin formation east and west of the bedout high but fail nearly all unequivocal criteria for impact crater recognition

mineralogical variation of sr nd isotopic and elemental compositions in loess and desert sand from the central loess plateau in china as a provenance tracer of wet and dry deposition in the northwestern pacific we determined sr and nd isotopic compositions and major trace and rare earth element ree compositions of evaporite and detrital minerals in loess and desert sand from the central loess plateau clp where 80 of the loess deposits in china are found the loess exhibited a narrow range of variation in 87sr 86sr 0 715 0 001 a sequential extraction experiment on the loess however showed that water and acetic acid hoac soluble minerals mainly halite and calcite hydrochloric acid hcl soluble minerals chlorite and phosphate and hcl insoluble minerals illite kaolinite smectite vermiculite feldspar and quartz have distinct 87sr 86sr ratios approximately 0 7111 0 0004 0 7141 0 0004 and 0 7195 0 0010 respectively the water and hoac soluble ratios were regionally uniform and were close to the 87sr 86sr value of river water from the huanghe yellow river and the maximum 87sr 86sr value of spring rainwater in japan suggesting selective solution of eolian halite and calcite into the chinese river water and into the japanese rainwater the mineralogical elemental and sr nd isotopic compositions of hcl soluble and insoluble minerals in the clp loess were different from those of the northern desert sand but were indistinguishable from those of its fine grained fraction supporting the view that the loess is composed of the fine grained fraction of desert sand that was transported to the southeast in the atmosphere the 87sr 86sr end and gd yb values of hcl insoluble minerals in the clp loess were almost identical to those of airborne dust soil minerals and deep sea sediments in the northwestern pacific considered to be of eolian origin suggesting dissolution of the loess evaporite minerals during atmospheric transportation these relationships demonstrate the usefulness of sr isotopes of water and hoac soluble minerals and sr and nd isotope ratios and ree compositions of acid insoluble minerals in loess as provenance markers for wet and dry deposition respectively

petrography geochemistry and argon 40 argon 39 ages of impact melt rocks and breccias from the ames impact structure oklahoma the nicor chestnut 18 4 drill core the 15 km diameter ames structure in northwestern oklahoma is located 2 75 km below surface in cambro ordovician arbuckle dolomite which is overlain by middle ordovician oil creek formation shale the feature is marked by two concentric ring structures with the inner ring of about 5 km diameter probably representing the collapsed remnant of a structural uplift composed of brecciated precambrian granite and arbuckle dolomite wells from both the crater rim and the central uplift are oil and gas producing making ames one of the economically important impact structures petrographic geochemical and age data were obtained on samples from the nicer chestnut 18 4 drill core off the northwest flank of the central uplift these samples represent the largest and best examples of impact melt breccia obtained so far from the ames structure they contain carbonate rocks which are derived from the target sequence the chemical composition of the impact melt breccias is similar to that of target granite with variable carbonate admixture some impact melt rocks are enriched in siderophile elements indicating the possible presence of a meteoritic component based on stratigraphic arguments the age of the crater was estimated at 470 ma previous ar 40 ar 39 dating attempts of impact melt breccias from the dorothy 1 19 core yielded plateau ages of about 285 ma which is in conflict with the stratigraphic age the new ar 40 ar 39 age data obtained on the melt breccias from the nicer chestnut core by ultraviolet uv laser spot analysis resulted in a range of ages with maxima around 300 ma these data could reflect processes related either the regional nemaha uplift or resetting due to hot brines active on a midcontinent wide scale perhaps related to the alleghenian and ouachita orogenies the age data indicate an extended burial phase associated with thermal overprint during late pennsylvanian permian

the lake st martin bolide has a big impact on groundwater fluoride concentrations the majority of residents of manitoba canada outside of the capital winnipeg rely on groundwater for their drinking water between lakes winnipeg and winnipegosis most aquifers occur in paleozoic carbonate lithologies proximal to the town of gypsumville however lithologies associated with the lake st martin impact structure and younger basin filling red bed and evaporite gypsum anhydrite sedimentary rocks complicate the hydrology and hydrochemistry here domestic wells have elevated salinities up to 8000 mg l total dissolved solids elevated sulfate up to 4000 mg l and elevated fluoride concentrations that are in excess of health limits f up to 15 2 mg l with 20 over 1 5 mg l groundwaters with elevated fluoride occur exclusively within the impact structure the impact melt rocks and younger red beds consistently have the highest fluoride abundances up to 2160 ppm groundwater ph values are alkaline ranging up to 10 7 with highest groundwater ph from wells in the impact melt rocks the spatial associations of impact melt rocks and red beds with elevated fluoride strong positive correlations between fluoride and ph sodium chloride sulfate boron and lithium greater fe 2 o 3 and al 2 o 3 concentrations of the host rocks and cation exchange capacity cec all indicate that fluoride concentrations in groundwaters are enhanced as a result of anion exchange wherein oh and co 3 2 displace f adsorbed onto fe and al oxyhydroxide surfaces thus the elevated fluoride contents of groundwaters at gypsumville are a consequence of the composition of the impact melt rocks and enhanced permeability and grain size reduction produced by bolide impact

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

the circular uneged uul structure east gobi basin mongolia geomorphic and structural evidence for meteorite impact into an unconsolidated coarse clastic target abstract the uneged uul structure is a 10 km circular complex multi ridged domal feature in the unegt subbasin of the east gobi basin southeastern mongolia as revealed by remote sensing and recent field reconnaissance the central part of the uneged uul structure comprises a complex central peak of outward radiating curved ridges composed of stratigraphically uplifted greenschist facies basement schists surrounded by an annular moat the most prominent feature of the structure is a central annular ridge 3 km in diameter composed of pebble boulder conglomerates and gravels of the upper jurassic sharilyn formation surrounded by three outer domal ridges composed of lower cretaceous conglomeratic sandstones and gypsum clays jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation the original population of pebbles cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated primary sedimentary features such as cross lamination or imbrication have been obliterated we explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent se nw trend of bedding plane alignment further outside the tilted and uplifted upper jurassic to lower cretaceous strata of the domal area are overlain by the flat lying upper cretaceous which stratigraphically constrains the timing of deformation at the uneged uul structure to most likely the early cretaceous endogenic formation models such as magmatism and salt gypsum or mud diapirism fail to explain the nature of the uneged uul structure the uneged uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on earth morphologically similar central peaks are observed at the spider and matt wilson impact structures in australia the central annular ridge reminds of that at gosses bluff in australia the outer domal ridges might correspond to ring like features as known from tin bider in algeria we therefore cautiously propose that an impact may have produced the uneged uul feature causing structural uplift 1000 m of basement rocks at its center so far no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses however it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress reducing deformation further studies will be in order to unravel the nature of the uneged uul structure which should be considered a promising possible impact structure

criteria for the retention of original seawater 87sr86sr in ancient shelf limestones abstract a suite of 40 pennsylvanian and permian shelf limestones from kansas and oklahoma has been analyzed for major and trace elements and for c o and sr isotopes the chemistry and c and o isotope ratios were compared to the difference between the measured 87 sr 86 sr and original seawater ratio concentrations of mn sr and fe varied most systematically with the change in the seawater sr ratio all samples containing 700 ppm mn yielded altered ratios fe and sr contents were also found to have a less well defined correlation with the retention of the original sr ratio all limestones with 900 ppm sr and 20 of 22 samples with a sr mn 2 0 yielded correct ratios the geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones mostly of pennsylvanian age from texas and oklahoma more than 92 of the samples that contain sr mn 2 0 yielded correct seawater ratios every sample with sr mn 2 0 retained the original seawater ratio

direct evidence of ancient shock metamorphism at the site of the 1908 tunguska event abstract shock metamorphism is rarely found at the surface of the earth the most used structures to identify shock metamorphism are true planar deformation features pdfs in quartz now accepted as diagnostic indicators of a meteorite impact here we present several lines of evidence for shock metamorphism and pdfs developed in quartz occurring on samples centered on a circular geological structure on mount stojkovic 60 54 06 n 101 55 40 e which lies within southern surface exposures of the siberian traps the shock event appears to have occurred during the eruption of the surface siberian traps basalts that cover this region curiously mount stojkovic lies within 3 km of the tree fall epicenter of the 1908 tunguska event based on current estimates of the phanerozoic impact distribution there is at most a 1 in 17 000 chance that the 1908 bolide would randomly fall on the site of a previous impact structure capable of creating shocked quartz just as improbable would be an airbust event incapable of creating a small crater that could have produced shock metamorphism our preferred least implausible hypothesis is that the shock metamorphism here was associated with a terrestrial event a hyperexplosive volcanic gas eruption called verneshot

element concentration and partitioning of loess in the huanghe yellow river drainage basin north china abstract loess samples from the middle reaches of the huanghe yellow river are analysed to determine the concentration and partitioning of fe mn cu pb zn cr co ni na and k very similar element concentrations are observed between the different areas of the loess plateau the relatively low trace element levels of loess as compared with the mean value of world soils are related to their wind borne origin and intense erosion over the loess plateau most of the studied elements are associated with mineral fractions carbonate aluminosilicates which account for 70 95 of total element concentrations low concentrations in the labile fractions suggest the weak element bioavailability to crop growth in this region

the strangways impact structure northern territory australia geological setting and laser probe 40ar 39ar geochronology abstract the strangways impact structure occurs within the mcarthur sedimentary basin northern territory australia the target rocks are primarily quartzites and subordinate shale siltstone sequences of the mesoproterozoic roper group these are primarily underlain by granitoid gneiss which is exposed in a 9 11 km diameter central uplift exposure is relatively limited and the upper parts of the impact structure have been removed by erosion based on the extent of the disturbance of the regional geology the diameter of strangways is estimated to be 24 26 km it is thus a complex crater and probably a central peak basin stratigraphic constraints place the time of impact as post roper group 145 ma in order to better define the time of impact infrared laser spot fusion 40 ar 39 ar data have been obtained from two impact melt lithologies and a highly shocked breccia clast these yield a combined age of 646 42 ma biotite dated from the granitoid gneiss central uplift yields 1686 1237 ma with the younger age reflecting the effects of shock induced resetting and or hydrothermal alteration due to hot melt sheet water interaction following impact the new age of 646 42 ma for strangways places it firmly in the neoproterozoic rather than being ordovician or younger as previously proposed

endogenous and nonimpact origin of the arkenu circular structures al kufrah basin se libya the twin arkenu circular structures acs located in the al kufrah basin in southeastern libya were previously considered as double impact craters the arkenu craters the acs consist of a ne arkenu 1 and a sw structure arkenu 2 with approximate diameters of about 10 km they are characterized by two shallow depressions surrounded by concentric circular ridges and silica impregnated sedimentary dikes cut by local faults our field petrographic and textural observations exclude that the acs have an impact origin in fact we did not observe any evidence of shock metamorphism such as planar deformation features in the quartz grains of the collected samples and the previously reported shatter cones are wind erosion features in sandstones ventifacts conversely the acs should be regarded as a paired intrusion of porphyritic stocks of syenitic composition that inject the nubia formation and form a rather simple and eroded ring dike complex stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite hematite horizons typical of iron oxide copper gold deposits their origin was nearly coeval with the development of silicified dikes in the surroundings plugs of tephritic phonolitic rocks and lamprophyres monchiquites inject the nubian sandstone along conjugate fracture zones trending nnw sse and ne sw that crosscut the structural axis of the basin

natural and anthropogenic sources of east asian dust nd sr isotopic signatures of loess modern dust and their potential source materials are systematically investigated to discriminate the natural and anthropogenic sources of eastern asian dust chinese loess is an eolian deposit that has accumulated over the past few million years and is characterized by a negative nd sr isotopic correlation that implies binary sources for the natural background of eastern asian dust loess in northeastern china shows the highest end 0 value while loess in western china has the lowest end 0 considering the nd sr isotopic signatures of the potential sources we propose that the high end 0 end member is derived from the arid lands around the northern boundary of china while the low end 0 end member originates from the deserts on the northern margin of the tibetan plateau chinese loess plateau deposits are dominated by northern margin tibetan plateau dust suggesting a strong link between the thick eolian deposit in this region and the evolution of the tibetan plateau the long range transported spring dust in both nanjing south china and japan has an nd sr isotopic composition similar to that of loess however spring dust in beijing north china has a much lower end 0 value indicating an additional anthropogenic contribution of low end 0 material possibly from the sandy lands in the adjacent north and west

half a century of progress in research on terrestrial impact structures a review abstract the author who investigated the wolfe creek australia in 1962 and edited two benchmark sets of readings on meteorite craters and possible astroblemes in 1977 and 1979 reviews the state of knowledge at the present time the text is concerned with terrestrial impact structures geological features without any consideration of extraterrestrial analogues a handful of definitive publications are drawn on to present the story of terrestrial impact in a single article the text covers historical aspects briefly the effect of target variations the paucity of human observation of such large scale events distinction from volcanic endogenous structures modification by geological processes the transience of the crater initially formed on the target and its subsequent modifications the global geographic distribution of the 174 structures now listed of which a number are dubious attributions their distribution in geological time many ages being known only known to wide limits maximum or minimum values their size distribution calculations of impact frequencies shock effects processes on impact the stages of formation impact into shallow marine and deep sea targets impacts on ice about which little is known and finally the input of impact into biotic extinctions in this last lengthy section the summaries of the conclusions of scientists researching impact on earth and palaeontologists researching biotic impact are set side by side it is concluded that if the recent foraminiferal evidence obtained by gerta keller and associates is taken at its face value the case of impact as a sole agent in extinction is non existent biotic extinction is clearly a complex process involving a number of causes in some cases it was staggered in time and different sets of organisms responded quite differently and surprisingly even in the same extinction event extraterrestrial impact may have been one of the causes in some cases but it may have been regional rather than global in its effects we may never know how much input it had into the record of biotic extinction on earth an enormous amount of new knowledge has arisen from detailed studies of this new family of remarkable geological structures

the suavjarvi impact structure nw russia abstract we present the geology and interpreted shock features of the suavjarvi circular structure suavjarvi is a circular feature illustrated by satellite imagery topography and magnetic data located in the central part of the karelian craton lat 63 07 n long 33 23 e to date little information on the geologic and impact features of the suavjarvi structure is available in the literature the structure is characterized by gravity and magnetic lows and disruption of the regional magnetic fabric in the northeastern and southwestern parts of the structure several erosional remnants of highly disturbed rocks occur referred to as monomict and polymict megabreccia these comprise blocks of both basement granitoids and supracrustal greenstone rocks the impact origin of polymict megabreccia and therefore of the suavjarvi structure is confirmed by observations of closely spaced planar microstructures at angles consistent with planes that have miller indices indicative of impact shock effects mostly of 10 13 the suavjarvi is considered to be a remnant of a deeply eroded and metamorphosed impact structure which has a diameter of 16 km and was formed during the paleoproterozoic older than 2 2 ga this is inferred from the age of the overlying volcanic sedimentary jatulian sequence suavjarvi underwent regional metamorphism that resulted in obliteration or transformation of shock metamorphic effects massive sulfides occur within megabreccia originating probably from postimpact redeposition of pre existing mineralization

laser argon dating of melt breccias from the siljan impact structure sweden implications for a possible relationship to late devonian extinction events in earlier studies the 6575 km diameter siljan impact structure in sweden has been linked to the late devonian mass extinction event the siljan impact event has previously been dated by kar and ar ar chronology at 342368 ma with the commonly quoted age being 362 7 2 2 ma 2 recalculated using currently accepted decay constants until recently the accepted age for the frasnian famennian boundary and associated extinction event was 364 ma which is within error limits of this earlier siljan age here we report new ar ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from siljan interpreted to be impact melt breccia the analytical results show some scatter which is greater in samples with more extensive alteration these samples generally yield younger ages the two samples with the least alteration yield the most reproducible weighted mean ages one yielded a laser spot age of 377 2 2 5 ma 95 confidence limits and the other yielded both a laser spot age of 376 1 2 8 ma 95 confidence limits and a laser stepped heating plateau age over 70 6 39ar release of 377 5 2 4 ma 2 our conservative estimate for the age of siljan is 377 2 ma 95 confidence limits which is significantly different from both the previously accepted age for the frasnian famennian f f boundary and the previously quoted age of siljan however the age of the f f boundary has recently been revised to 374 5 2 6 ma by the international commission for stratigraphy which is within error the same as our new age however the currently available age data are not proof that there was a connection between the siljan impact event and the f f boundary extinction this new result highlights the dual problems of dating meteorite impacts where fine grained melt rocks are often all that can be isotopically dated and constraining the absolute age of biostratigraphic boundaries which can only be constrained by age extrapolation further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times perhaps resulting in major extinction events in earths biostratigraphic record

87sr 86sr record of permian seawater abstract a population of 169 permian articulate brachiopod shells was analysed for their 87 sr 86 sr ratios 51 of these characterised as well preserved and stratigraphically well defined are utilized for delineation of the permian seawater strontium isotope trend the 87 sr 86 sr curve shows values of about 0 7080 in the lowermost permian asselian followed by a gradual decline to 0 70685 in the capitanian and a minor rise in the dzhulfian and lower dorashamian to values of 0 70715 just below the permian triassic boundary the early permian decrease in the strontium isotope curve that commences in the early sakmarian is coincident with the advancing deglaciation of the gondwana and with the increased aridity in large parts of the pangaea these factors may have led to a reduced continental weathering of rb rich silicate rocks and thus to the decline in seawater 87 sr 86 sr starting with the artinskian the opening of the neotethys and the associated widespread basaltic volcanism supplied low radiogenic strontium to seawater from an enhanced hydrothermal flux in the capitanian basaltic volcanism in the entire palaeotethys ceased and this may have been the reason for a slightly more radiogenic seawater isotopic composition during the lopingian higher input of riverine sr due to expansion of humid areas may have been a contributory factor

structural complexity inferred from anisotropic resistivity example from airborne em and compilation of historical resistivity induced polarization data from the gold rich canadian malartic district quebec canadaresistivity and ip data at malartic

multistage sedimentary and metamorphic origin of pyrite and gold in the giant sukhoi log deposit lena gold province russia gold mineralization at sukhoi log in eastern siberia is hosted in a deformed neoproterozoic organic bearing and pyritic black shale and siltstone sequence that is folded into a tight overturned anticline the deposit contains about 30 million ounces of gold at an average grade of 2 0 g t au and is one of the largest known undeveloped gold resources the high grade gold zone forms a gently dipping tabular body in the core of the anticline the best gold grades occur in narrow bedding parallel pyrite quartz veinlets that have been folded during the main deformation event lower grade gold is associated with disseminated pyrite developed in and around the high grade core of the deposit detailed paragenetic studies of the mineralization and host rocks have defined six stages of pyrite development in the carbonaceous sediments the two earliest forms of pyrite termed py1 and py2 are commonly developed in stratiform layers of micron sized crystals framboids and fine euhedra which are interpreted as synsedimentary to early diagenetic in origin coarser grained bedding parallel aggregates of inclusion rich pyrite termed py3 contain inclusions of arsenopyrite native gold and gold tellurides and are interpreted to form during late diagenesis and earliest deformation coarse euhedral pyrite py4 overgrows the earlier pyrite py1 py2 and py3 and the slaty cleavage developed in the host rocks indicating a syndeformation timing late stage inclusion free pyrite py5 overgrows and replaces earlier sulfides and is considered to be syn to late deformation laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the various pyrite types indicate that the synsedimentary py1 contains the highest levels of invisible gold varying from 0 4 to 12 1 ppm with a mean of 3 22 ppm au and 1 900 ppm as py1 is also enriched in a suite of trace elements mo sb ni co se te ag cu pb zn mn ba cr u v which are similar to those concentrated by organic processes in euxinic sedimentary environments later generations of pyrite from py2 to py5 including pyrite in bedding parallel pyrite quartz veinlets contain progressively lower contents of invisible gold and most other trace elements however this metamorphic and postmetamorphic pyrite contains microinclusions of free gold arsenopyrite pyrrhotite sphalerite and chalcopyrite the paragenetic textural and chemical relationships at sukhoi log suggest that gold was clearly initially introduced prior to cleavage development accompanying sedimentation of the organic rich shales and fixed during diagenesis within the structure of diagenetic arsenian pyrite subsequently accompanying deformation gold was liberated from recrystallized diagenetic pyrite to become concentrated as free gold and gold tellurides within metamorphic pyrite and folded bedding parallel pyrite quartz veinlets two key processes are considered vital to the formation of the sukhoi log deposit original synsedimentary and early diagenetic concentration of gold dissolved within arsenian pyrite in organic rich black shales and metamorphic processes that liberated gold from the early forms of arsenian pyrite to be concentrated as free gold and gold tellurides within late diagenetic and metamorphic pyrite and associated pyrite quartz veinlets in the core of an overturned anticline these ore forming processes are unlikely to be unique to sukhoi log other black shale and turbidite hosted deposits that occur in rifted continental margin environments which have undergone collision and basin inversion may form by similar processes

kalkkop crater cape province south africa confirmation of impact origin using osmium isotope systematics the kalkkop structure a circular depression with a diameter of 640 m is situated in the eastern cape province south africa in 1992 a bore hole was drilled in the center of the crater to a depth of 151 8 m to assess a possible impact origin of this structure the abundances and isotopic ratios of osmium and rhenium were measured in breccias and in sandstones and shales derived from the basement the basement rocks show rhenium and osmium abundances and isotopic compositions of osmium and neodymium 187os188os 0 61 to 0 92 and nd 6 to 7 that are typical for continental crust one of the breccia samples shows a significantly elevated osmium content and a much lower 187os188os ratio of about 0 215 187os188os ratio 1 79 this ratio is much closer to meteoritic compositions than to crust indicating the presence of about 0 05 of an extraterrestrial component in the breccia this is the first time that osmium isotope systematics have been used to confirm the impact origin of a crater structure and demonstrates their sensitivity for impact studies

carbon sulfur oxygen and strontium isotope records organic geochemistry and biostratigraphy across the permian triassic boundary in abadeh iran pelagic deposits at abadeh represent a complete biostratigraphic record across the permian triassic boundary ptb the presumed water depth during deposition of these sediments was between 60 and 90 m similar to other permian triassic boundary sections the succession at abadeh is characterised by a negative carbon isotope shift of approximately 4 the values start to decrease in the lower c changxingensis c deflecta s l zone reach 0 12 v pdb in the uppermost permian just below the ptb remain low to the early i isarcica zone 0 32 and increase subsequently in the upper i isarcica zone for the time interval of the ptb negative carbon isotope excursion between the c iranica and the i isarcica zones no correlation exists between the 13ccarb and the 18ocarb the above observations argue against the conclusion of heydari et al 2001 that the carbon isotope event at the p t transition is an alteration artefact and not a global signal the decrease in 13ccarb is accompanied by a 5 and potentially up to 10 increase in 34ssss together these features are thought to reflect a complex global event notably the development of widespread anoxic oceans with anoxic bottom layers rising onto the shelves for the carbon isotope drop other factors such as the collapse of ocean primary productivity may also have played a role the 87sr 86sr ratios of dzhulfian seawater show only a minor increase from 0 70705 to 0 70710 reaching 0 70720 in the dorashamian the increase becomes steeper in the early triassic reaching 0 70754 in the n dieneri zone the rise of the strontium isotope values is thought to be related to enhanced continental weathering under humid climatic conditions in the uppermost permian c meishanensis h praeparvus zone and the lack of a dense land vegetation in the early triassic prior to the spathian upper olenekian

age and effects of the odessa meteorite impact western texas usa the odessa meteorite craters texas united states include a main crater 160 m diameter 30 m deep plus four smaller meteorite craters the main crater was sampled by coring to 22 m depth to better understand its origin and history dating by optically stimulated luminescence indicates that it was produced immediately prior to ca 63 5 4 5 ka sediment filling the crater includes impact breccias produced at the time of impact wind dominated silts with minor amounts of pond sediments deposited ca 63 5 ka probably just after the impact and ca 53 2 ka wind dominated silt ca 38 1 7 ka and playa muds with a wind blown silt component younger than 36 ka the environment was arid or semiarid at the time of impact based on characteristics of soils on the surrounding landscape the impact caused severe damage within 2 km and produced 1000 km hr winds and thermal pulse animals within a 1 1 5 km diameter area were probably killed this is only the second well dated pleistocene hypervelocity impact crater in north america

evidence that lake cheko is not an impact crater in a provocative paper gasperini et al 2007 suggest that lake cheko a 300 m wide lake situated a few kilometres downrange from the assumed epicentre of the 1908 tunguska event is an impact crater in this response we present several lines of observational evidence that contradicts the impact hypothesis for the lake s origin un crater like aspects of the lake morphology the lack of impactor material in and around the lake and the presence of apparently unaffected mature trees close to the lake we also show that a tensile strength of 10 40 mpa is required for an asteroid fragment to traverse the earth s atmosphere and reach the surface intact and with sufficient velocity to excavate a crater the size of lake cheko inferred tensile strengths of large stony meteorites during atmospheric disruption are 10 100 times lower we therefore conclude that lake cheko is highly unlikely to be an impact crater

boron isotopic composition of marine and nonmarine evaporite borates abstract here we present the results of the boron isotopic analysis of borates from some well known marine and nonmarine evaporite deposits combining new analyses with analyses from the literature yields a mean 11 b value for nine marine evaporite borate analyses of 25 4 permil 11 b 10 b 4 15 0 02 and a mean for twenty five nonmarine evaporite borates of 7 10 permil 11 b 10 b 4 02 0 04 previous studies have shown that the boron isotopic composition of seawater 11 b 40 permil 11 b 10 b 4 20 is about four percent heavier than the boron isotopic composition of most crustal rocks and minerals we suggest that our results for marine and nonmarine evaporite borates reflect this difference thus it should be possible to use boron isotopic composition as a tracer for boron of marine evaporitic origin applications include borate bearing evaporite deposits and perhaps borate and or borosilicate bearing metamorphic rocks

on elevated fluoride and boron concentrations in groundwaters associated with the lake saint martin impact structure manitoba abstract hydrogeological investigations conducted by the geological survey of canada in the lake saint martin region of manitoba have confirmed earlier reports of naturally elevated f and b concentrations in local groundwaters fluoride and b concentrations are highly correlated r 2 0 905 and reach 15 1 mg l and 8 5 mg l respectively virtually all groundwaters with f concentrations greater than the drinking water limit of 1 5 mg l are from wells within the lake saint martin impact structure a 208 ma complex crater 23 km in diameter underlying a large part of the study area the high f groundwaters can be classified into two groups according to their anionic and isotopic compositions group i samples consist of na mixed anion groundwaters with cl greater than 100 mg l and highly depleted 18 o compositions indicative of recharge under much cooler climatic conditions than at present samples belonging to this group exhibit a striking relationship to crater morphology and are found in an arcuate belt within the southern rim of the impact structure group ii high f samples consist of na hco 3 so 4 groundwaters with little cl and less depleted 18 o compositions samples belonging to this group are associated with groundwaters recharged locally on a low ridge within the impact structure this paper traces the probable source of high f groundwaters to phosphatic pellets in shales of the winnipeg formation a regional basal clastic unit which sub crops at shallow depth beneath the crater rim as a result of more than 200 m of structural uplift associated with the impact event this extensive aquifer is known elsewhere in southern manitoba for its naturally softened groundwaters and locally elevated f concentrations group i groundwaters are interpreted as discharge from the winnipeg formation where it abuts against crater fill deposits group ii high f groundwaters are interpreted as modern recharge from within the impact structure displacing group i groundwaters thus elevated f and b concentrations observed in groundwaters of the lake saint martin area represent the geochemical signature of upwelling from a deep regional aquifer the previously unsuspected discharge zone occurs within an isolated sub crop of the aquifer formed as a result of structural uplift caused by the impact event

geochemistry and stable isotopic composition of tufa waters and precipitates from the interlake region manitoba canada constraints on groundwater origin calcitization and tufa formation abstract new major trace and isotopic geochemical results from a regional study of springs discharging from the major carbonate rock aquifer in the interlake region of manitoba canada are used to understand water rock reactions timing of recharge discharge tufa formation processes and as baseline data spring waters are fresh with total dissolved solids tds concentrations ranging from 150 to 880 mg l waters discharging in the northern part of the study area have lower tds are dominantly ca mg hco3 waters with low so4 concentrations

the sirente crater field italy revisited 1 a scientific debate has developed in the last few years as to whether a 130 m diameter sag pond surrounded by a saddle shaped rim and neighboring smaller sags from the sirente plain abruzzi italy represent the only known italian meteoritic crater field a mud volcano or an anthropogenic feature to decipher the nature of the sirente landforms we carried out geophysical and geochemical investigations geoelectric profiles document two karstified shelf carbonate ridges lying at 10 40 m depth below calcareous lacustrine silts and deeper more conductive sediments likely soils tephra filling the plain the smaller sags lie just above the ridges implying a karstic origin whereas the main sag also resting above a carbonate ridge shows no roots in excess of 10 20 m depth in contrast to the crater interpretation high resolution magnetic surveys reveal negative positive anomaly stripes in correspondence with the buried ridges valleys respectively the smaller sags as well as the main crater are located in the domain of negative residuals the positive long wavelength magnetic signature is likely due to the strongly susceptive soils tephra filling the buried valleys magnetic modeling shows that the field observed over the crater is incompatible with the field generated by a buried meteorite with realistic characteristics the smaller sags are characterized by small magnetic anomaly couplets perfectly reproducible considering the susceptibility contrast between the fill in soil and the surrounding silts our data show that the sirente crater and the minor depressions are simply the results of human activity and karstic processes respectively

isotope geochemistry of quaternary deposits from the arid lands in northern china abstract quaternary deposits of various types lacustrine desert sand river loess and loess like deposits from the southwestern tarim basin north tianshan northeast china and the central loess plateau exhibit an nd 0 range from 5 to 12 8 and a 147 sm 144 nd range from 0 110 to 0 140 their nd depleted mantle model ages are between 1 50 and 1 92 ga with a mean value of 1 7 ga th sc ratios vary over a wide range from 0 1 to 2 comparison of these geochemical signatures with those of the old australian shales and modern pacific ocean dust suggests a continuous input of mantle derived material into the crust over geological time the deposits from the north tianshan show clearly different chemical and isotopic features with significantly higher sm nd high 143 nd 144 nd low 87 sr 86 sr and low la lu and th sc ratios relative to those in the deposits from the other areas this indicates a geological control on the local source which has an overall young age and an enrichment of basic components the ce 0 values of two loess samples from the central loess plateau are 1 8 and 3 9 corresponding to depleted mantle model ages of 2 2 and 3 4 ga like the rb sr system the evolution of the la ce isotopic system in the crust may be to some extent decoupled from the sm nd system due to la ce fractionation by sedimentary processes the loess deposits and the desert sand are not distinguishable in isotopic composition but show somewhat different chemical compositions we conclude that the loess can have originated from desert processes since the difference in the chemistry of the desert sand and loess may result from mineral sorting during desert processes and aeolian transportation the loess deposits on the central loess plateau chemically and isotopically resemble the desert loess deposits in the southwestern tarim basin but do not resemble those of the north tianshan accordingly the loess on the plateau may have a source region in the tarim basin but clearly not in the north tianshan

agate as an indicator of impact structures an example from saaksjarvi finland mineralogical petrographical and chemical determinations were made for 743 agate banded variety of chalcedonic quartz nodules diameters from 5 mm to 5 cm formed during postimpact low temperature hydrothermal activity as vesicle fillings in the melt rocks of the saaksjarvi meteorite impact structure diameter 5 km in southwest finland other hydrothermal vesicle fillings in the impact melt rocks include chlorite mordenite smectite and kaolinite the agates were classified into two types whose mineralogical properties and chemical compositions fall within the range of volcanic agates basaltic and rhyolitic host rocks the relatively high age 510 ma of the saaksjarvi impact melt rocks however is reflected by the presence of recrystallization textures which are rare in younger volcanic agates the saaksjarvi structure was initially located after following the fortuitous discovery of agate path finders in the glacial overburden it is recommended that wherever volcanic type agates are found as float in precambrian shield areas devoid of younger volcanic rocks the possible presence of impact or volcanic craters in the vicinity should be considered

dating impact craters palaeogeographic versus isotopic and stratigraphic methods a brief case study isotopic and stratigraphic ages of the 80 km diameter puchezh katunki russia 220 10 to 167 3 ma and the 20 km diameter obolon ukraine 215 25 to 169 7 ma impact structures are associated with significant age uncertainties as a case study reconstructions of the palaeogeography at the time of crater formation late triassic to middle jurassic based on recent palaeogeographic maps help further to constrain impact ages palaeogeographic studies suggest that puchezh katunki is older than 170 ma and that obolon is younger than 185 ma this also rules out that obolon formed during a 214 ma late triassic multiple impact event as recently discussed

the dhala structure bundelkhand craton central india eroded remnant of a large paleoproterozoic impact structure the newly discovered dhala structure madhya pradesh state india is the eroded remnant of an impact structure with an estimated present day apparent diameter of about 11 km it is located in the northwestern part of the archean bundelkhand craton the pre impact country rocks are predominantly granitoids of 2 5 ga age with minor 2 0 2 15 ga mafic intrusive rocks and they are overlain by post impact sediments of the presumably 1 7 ga vindhyan supergroup thus the age for this impact event is currently bracketed by these two sequences the dhala structure is asymmetrically disposed with respect to a central elevated area cea of vindhyan sediments the cea is surrounded by two prominent morphological rings comprising pre vindhyan arenaceousargillaceous and partially rudaceous metasediments and monomict granitoid breccia respectively there are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone occurring over a nearly 6 km long trend and with a maximum outcrop width of 170 m many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features including multiple sets of planar deformation features pdfs in quartz and feldspar ballen textured quartz occurrences of coesite and feldspar with checkerboard texture in addition various thermal alteration textures have been found in clasts of initially superheated impact melt the impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts the chemical compositions of the impact melt rock and the regionally occurring granitoids are similar the ir contents of various impact melt breccia samples are close to the detection limit 1 1 5 ppb and do not provide evidence for the presence of a meteoritic component in the melt breccia the presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm dhala as an impact structure at 11 km dhala is the largest impact structure currently known in the region between the mediterranean and southeast asia

using strontium isotopes to determine the age and origin of gypsum and anhydrite beds the variation of 87sr 86sr in seawater with time can be used to determine the age of calcium sulfate beds precipitated from brine derived from seawater the determination of a marine origin can be established by the consistency of strontium isotope results from samples at different stratigraphic levels in a single gypsum anhydrite bed collected at separated sites the scatter of strontium isotope results in the gypsum anhydrite samples examined here is interpreted to result from the contribution of meteoric strontium to the salina strontium isotope results from three evaporite settings the jurassic todilto formation from new mexico the permian blaine formation from blaine county oklahoma and the permian salado formation from new mexico are used to determine the age and origin of the parent brine for these gypsum anhydrite beds the calcium sulfate beds from each of these formations precipitated from salinas that originated with a marine flooding but show at different times and localities volume do

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

first observation of 2s1 and study of the splitting of the football mode 0s2 after the june 2001 peru earthquake of magnitude 8 4 the recent peru earthquake with a moment magnitude mw 8 4 on june 23 2001 presents a good opportunity for the observation of the seismic gravest normal modes for the longest periods below 1 mhz superconducting gravimeters sgs have been shown to be less noisy than long period seismometers a stack based on the spatial and temporal properties of the modes of degree one in a spherical harmonic development applied to 164 h of common records at the cantley canberra strasbourg sutherland and vienna sg stations has led to the first clear observation of the 2s1 long period seismic modes with a central frequency close to 0 4 mhz moreover the spectrum of the sg record at the strasbourg station confirms the benefit sgs can provide to normal mode seismology with the clear observation of the splitting of the fundamental mode 0s2 into 5 fairly well resolved singlets with only one record

structural setting for canadian malartic style of gold mineralization in the pontiac subprovince south of the cadillac larder lake deformation zone quebec canada abstract the structural setting of the pontiac subprovince in the vicinity of the world class canadian malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations felsic intermediate intrusive bodies are a key component of this deposit defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta sedimentary rocks of the pontiac group south of the cadillac larder lake deformation zone may further assist gold exploration in similar tectonic environment three structural domains are interpreted in the area based on the geometry of the bedding folds and structural fabrics related to the three major phases of deformation during these events four phases of magmatism and one metamorphic episode occurred the north domain which hosts the canadian malartic deposit is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal f 1 folds overprinted by open to tight steeply dipping f 2 folds the bedding in this domain is cut by a penetrative s 2 biotite foliation which possibly built on rheological changes enhanced by metasomatism in the footprint of the canadian malartic deposit by contrast the central and south domains display homogeneous bedding orientations cross cut by a discrete s 2 biotite foliation and syn to late d 2 metamorphic porphyroblasts in all domains the l 2 stretching lineation consistently plunges at about 60 degrees toward the east d 3 is a minor deformation event in the pontiac subprovince which possibly correlates with late dextral transcurrent movement along the cadillac larder lake deformation zone intrusive bodies were emplaced throughout the first and second deformation events phase 1 monzonite quartz monzodiorite and granodiorite plutons ca 2683 2680 ma intruded into consolidated pontiac sedimentary rocks during d 1 phase 2 quartz monzodiorite bodies ca 2679 2676 ma predominantly formed in the north domain along f 1 fold axial surfaces prior to or at the onset of d 2 phase 3 basic dykes ca 2675 2673 ma cross cut earlier felsic intermediate intrusions across all domains and subsequently developed an s 2 metamorphic hornblende foliation phase 4 magmatism ca 2672 2662 ma is related to the decelles batholith s type granite and pegmatite which is interpreted to be contemporaneous with the peak of regional metamorphism the decelles batholith may project at depth underneath the canadian malartic deposit and could be associated with magmatic hydrothermal mineralizing fluids in the canadian malartic deposit gold mineralization at canadian malartic is spatially located on the contact of phase 2 quartz monzodiorite bodies the proximity to the cadillac larder lake deformation zone combined with the rheological contrast between steeply dipping clastic meta sedimentary rocks and quartz monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses thus forming favorable conduits for hydrothermal fluids e g the sladen fault zone such specific rheological behavior is demonstrated by domains of structural complexity emphasized by the variance of the bedding dip these structurally complex zones systematically host gold mineralization in the canadian malartic district of the pontiac subprovince proximal to the cadillac larder lake deformation zone

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

the jebel hadid structure al kufrah basin se libya a possible impact structure and potential hydrocarbon trap abstract the jebel hadid structure that formed in the nubian sandstone of the southern al kufrah basin is a 4 7 km circular feature with a set of multiple concentric annular ridges the age of the structure is probably post early cretaceous to pre pliocene as the prevalent geological processes that usually account for the formation of circular structures magmatism diapirism sand volcanism karst dissolution glacial erosion can be widely excluded in this region we suggest that the jebel hadid structure might represent an eroded complex impact structure in particular the jebel hadid structure bears a strong morphological resemblance to the impact structure of tin bider in algeria general host rock properties may be compared to those at the upheaval dome impact structure usa numerous terrestrial impact structures are hosts to hydrocarbon reservoirs this investigation provides a first prospective approach based on remote sensing discussing the economic potential of a circular structure of suspected impact origin the still underexplored al kufrah basin is considered as a high potential area for future hydrocarbon exploration if we consider a possible impact origin of the jebel hadid structure this feature might provide local secondary pathways and traps for hydrocarbon migration accumulation in the nubian series and thus ought to be considered as a preferential site for oil and gas exploration

geochemistry of the melt and country rocks of the lake st martin impact structure manitoba canada impact melt and country rocks from the 23 km diameter lake st martin structure in manitoba were analysed for major and trace element abundances and for rb sr isotopic compositions our aim was to better understand the formation of the melt rocks in this probable impact structure to search for traces of the meteoritic projectile to date the melt sheet in order to improve the chronological record of impact events on the earth s surface and to constrain the target from which the melt rocks could have formed major element analyses are consistent with the observation by simonds and mcgee 1979 that many of the melt rocks from lake st martin are chemically homogenized to a degree similar to that of melt bodies from other impact structures however some samples are of slightly different composition this can be explained by post impact hydrothermal alteration particularly near the present erosion surface chemical exchange between melt bearing breccia and basement rock near the bottom of the melt sheet or limited mixing of different target lithologies so called pseudotachylites from the central uplift of the structure are chemically diverse and do not always represent the composition of their host rock we conclude that such samples are not pseudotachylite but probably represent injected impact breccia rees and other lithophile elements are well homogenized in the melt in comparison with the country rock variation there is a slight enrichment of the siderophile elements co and au and of cr in the melt rocks but further analyses are required to verify whether this is caused by a small contribution from the meteoritic projectile the melt rocks are also homogenized isotopically when compared with the spread of isotopic ratios determined for the country rocks the melt rock isotopic composition however can only be achieved by mixing of various granitoid country rock types with another component of high 87rb86sr ratio furthermore the melt rocks are isotopically reequilibrated whole rock rb sr isotope data yield an age of 212 4 43 ma isr 7132 3 at 2 level whole rock and mineral separate data yield an age of t 219 32 ma isr 7131 3 this age consistent with previously reported k ar ages of 200 25 and 250 25 ma is thought to be the best approximation to the age of the melt rocks and thus of the impact event however the 2 error of 32 ma is too high to allow this age to become a significant addition to the data base of well constrained impact ages for analysis of periodicity in the cratering record an age of t 2785 3 157 ma 2 for isr 6997 11 was obtained for the basement rocks of the lake st martin structure but the true meaning of this age is still unclear while the age obtained is comparable to other superior province data it appears that the lake st martin country rock data define a mixing line the new chemical and isotopic results are consistent with an impact origin for the lake st martin structure harmonic least squares mixing calculations suggest that the melt rocks were formed from ca 80 monzonitic and 15 granitic basement plus a small addition of ca 6 carbonate rock this is in good agreement with the results by simonds and mcgee 1979 and favors their suggestion that carbonaterich target rocks even when they form the upper portion of the target stratigraphy do not significantly take part in impact melt formation but are largely removed in a co2 rich vapor phase

87sr 86sr variation in north pacific sediments a record of the milankovitch cycle in the past 3 million years abstract temporal variations of sr isotopic compositions in the detrital component of north pacific sediments have been precisely examined the sr isotopic composition is controlled by the flux of eolian material from the asian continent with a high 87 sr 86 sr ratio 0 724 0 726 relative to volcanic material with a low ratio 0 703 0 705 from the izu ogasawara mariana arc and oceanic islands such as the hawaiian islands assuming that the volcanic flux is constant the temporal variation in sr isotopic ratio reflects the amount of eolian input from the continent the major characteristics of the temporal variations and the interpretation of them are summarized as follows 1 the 87 sr 86 sr records show cyclic fluctuations of 400 ky and 100 ky periodicities which are associated with the eccentricity of the earth s orbit milankovitch cycle 2 between 3 and 0 8 ma the 87 sr 86 sr ratio in the north central pacific sediment increases gradually this reflects an increased eolian input from the arid region in east asia the increase must be related to aridification of the asian continent 3 the decreased 87 sr 86 sr ratio during the past 0 8 m y implies a decreased eolian input the age of 0 8 ma may relate to the climatic event known as the middle pleistocene shift all of these phenomena reflect a fluctuation of the eolian flux corresponding to the paleoclimatic cycle of aridity in the asian continent the sr isotopic composition of pelagic sediment in the north pacific is sensitive to changes in the eolian input reflecting to the aridity of the asian continent

the younger dryas impact hypothesis a requiem abstract the younger dryas yd impact hypothesis is a recent theory that suggests that a cometary or meteoritic body or bodies hit and or exploded over north america 12 900 years ago causing the yd climate episode extinction of pleistocene megafauna demise of the clovis archeological culture and a range of other effects since gaining widespread attention in 2007 substantial research has focused on testing the 12 main signatures presented as evidence of a catastrophic extraterrestrial event 12 900 years ago here we present a review of the impact hypothesis including its evolution and current variants and of efforts to test and corroborate the hypothesis the physical evidence interpreted as signatures of an impact event can be separated into two groups the first group consists of evidence that has been largely rejected by the scientific community and is no longer in widespread discussion including particle tracks in archeological chert magnetic nodules in pleistocene bones impact origin of the carolina bays and elevated concentrations of radioactivity iridium and fullerenes enriched in 3 he the second group consists of evidence that has been active in recent research and discussions carbon spheres and elongates magnetic grains and magnetic spherules byproducts of catastrophic wildfire and nanodiamonds over time however these signatures have also seen contrary evidence rather than support recent studies have shown that carbon spheres and elongates do not represent extraterrestrial carbon nor impact induced megafires but are indistinguishable from fungal sclerotia and arthropod fecal material that are a small but common component of many terrestrial deposits magnetic grains and spherules are heterogeneously distributed in sediments but reported measurements of unique peaks in concentrations at the yd onset have yet to be reproduced the magnetic grains are certainly just iron rich detrital grains whereas reported yd magnetic spherules are consistent with the diffuse non catastrophic input of micrometeorite ablation fallout probably augmented by anthropogenic and other terrestrial spherular grains results here also show considerable subjectivity in the reported sampling methods that may explain the purported yd spherule concentration peaks fire is a pervasive earth surface process and reanalyses of the original yd sites and of coeval records show episodic fire on the landscape through the latest pleistocene with no unique fire event at the onset of the yd lastly with yd impact proponents increasingly retreating to nanodiamonds cubic hexagonal lonsdaleite and the proposed n diamond as evidence of impact those data have been called into question the presence of lonsdaleite was reported as proof of impact related shock processes but the evidence presented was inconsistent with lonsdaleite and consistent instead with polycrystalline aggregates of graphene and graphane mixtures that are ubiquitous in carbon forms isolated from sediments ranging from modern to pre yd age important questions remain regarding the origins and distribution of other diamond forms e g cubic nanodiamonds in summary none of the original yd impact signatures have been subsequently corroborated by independent tests of the 12 original lines of evidence seven have so far proven to be non reproducible the remaining signatures instead seem to represent either 1 non catastrophic mechanisms and or 2 terrestrial rather than extraterrestrial or impact related sources in all of these cases sparse but ubiquitous materials seem to have been misreported and misinterpreted as singular peaks at the onset of the yd throughout the arc of this hypothesis recognized and expected impact markers were not found leading to proposed yd impactors and impact processes that were novel self contradictory rapidly changing and sometimes defying the laws of physics the yd impact hypothesis provides a cautionary tale for researchers the scientific community the press and the broader public

asian provenance of glacial dust stage 2 in the greenland ice sheet project 2 ice core summit greenland samples of dust from the greenland ice sheet project 2 gisp2 ice core summit greenland dated within marine isotope stage 2 between 23 340 and 26 180 calendar years b p around the time of the coldest local last glacial temperatures have been analyzed to determine their provenance to accomplish this we have compared them with approximately coeval aeolian sediments mostly loesses sampled in possible source areas psas from around the northern hemisphere the 5 m grain size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale which was sufficiently fine grained to be transported over long distances on the basis of comparison of the clay mineralogy and sr nd and pb isotope composition with ice dust and psas and assuming that we have sampled the most important psas we have determined that the probable source area of these gisp2 dusts was in eastern asia the dust was not derived from either the midcontinental united states or the sahara two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling except for a brief period during an interstadial when dust transport was exceptionally low for glacial times and had a mineralogical composition indicative of a slightly more southern provenance the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size this includes the high dust period that correlates with the heinrich 2 period of major iceberg discharge into the north atlantic variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size

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

a 20 km diameter multi ringed impact structure in the north sea most craters found on earth are highly eroded poorly preserved and only exposed on land here we describe a multi ringed impact structure discovered in the north sea from the analysis of three dimensional seismic reflection data the structure is 20 km in diameter and has at least ten distinctive concentric rings located between 2 and 10 km from the crater centre the structure affects cretaceous chalk and jurassic shales and is well preserved below several hundred metres of post impact tertiary strata which constrains its age to be 60 65 myr old the formation of concentric ringed impact structures at this relatively small scale had not previously been thought possible especially on the terrestrial planets we have mapped the ring structures at a resolution of tens of metres both laterally and in depth and show that the rings are fault bounded graben structures similar to fault arrays formed in low strain rate detachment tectonic settings strata deeper than 500 m palaeodepth appear unfaulted and we infer that the concentric ring structures may have accommodated post impact extension towards the excavated crater through detachment on weak layers within the chalk

nd and sr isotopic characteristics of chinese deserts implications for the provenances of asian dust silicate nd sr isotopes of the fine grained fractions of the 10 major deserts and sandy lands in north china and the loess in chinese loess plateau were systematically investigated wide ranges in nd sr isotopic compositions have been observed the results of the 7 0 b the deserts on the northern margin of tibetan plateau with end 0 ranging from 11 9 to 7 4 and c the deserts on the ordos plateau with the lowest end 0 11 5 the distribution of the threes isotopic regions is controlled by the tectonic setting in north china which implies that the materials of the deserts are derived from the locally eroded rocks from the surrounding mountains and the nd sr isotopic signatures of these deserts could be quit stable over the past million years on the sub tectonic time scales if there is any desert at those times the nd sr isotopic compositions of the loess are mostly close to those of the deserts in isotopic region b suggesting that the main source regions of the last glacial loess in the chinese loess plateau are badain jaran desert tengger desert and qaidam desert also the comparison between the nd sr isotopes of the 5 lm silicate fractions of the deserts and the ancient dust falls in the north pacific and greenland show that the asian end members of these dust falls are derived most from the deserts in the isotopic region b and less from those in the isotopic region c

synthesis of the structural evolution and associated gold mineralization of the cadillac fault abitibi canada abstract the cadillac fault zone cfz is a 250 km long east trending crustal discontinuity associated with numerous world class gold deposits in the archean abitibi subprovince in quebec the fault follows the boundary between the abitibi and the pontiac subprovinces and can be divided into four segments 1 rouyn 2 joannes 3 malartic and 4 val d or each displays different attitudes and structural styles the malartic segment strikes se whereas the other segments are e trending abrupt changes in strike or smaller scale faults represent limits between the segments structural styles include four generations of foliation with numerous shear sense indicators and folding specifically related to each foliation structural features can be correlated from one segment to another but display clear variations in attitude or expression a stretching lineation signature is definitely associated with each segment in the malartic segment the predominant e trending schistosity displays an angular relationship with the contact between groups and bedding suggesting that the se trending architecture of this segment was already emplaced prior to ductile deformation the evidence suggests a model where the cfz originates from several independent structures inherited from volcanism prior to timiskaming deformation and abitibi pontiac accretion the proto cfz was then linked up during the timiskaming deposition forming high strain zones inherited structures form the frame on which each deformation event was overprinted with different responses depending on segmentation the structural styles of each segment can be integrated in a structural evolution model for the cfz the n s shortening led to the development of a dominantly e striking penetrative foliation and folding during the subsequent extensional stage the cfz was reactivated with normal kinematics during the pontiac uplift a third deformation event is represented by the local development of nw trending cleavage axial planar to the s shaped minor folds finally a late nw se shortening event produced a dextral strike slip movement with ne striking vertical foliation and development of z shaped folds on bedding surfaces and anterior fabrics linkage of the cfz had a major impact on hydrothermal activity in the southern abitibi as it channeled fluids along large scale pathways distribution of gold bearing intrusions can also be related to segmentation and inherited structures of the cfz this association is expressed by gold mineralization styles present in several longitudinal fields reflecting fault segmentation

nd sr isotope geochemistry of fine grained sands in the basin type deserts west china implications for the source mechanism and atmospheric transport abstract the basin type deserts of west china are among the greatest dust emission areas in the world mineral dust that is emitted from the deserts is transported and deposited in east central china and even in far east regions this study investigates the nd sr isotopic compositions of fine grained surface sands nd 0 10 9 to 15 and 87 sr 86 sr 0 714 to 0 718 while those of the dune sands from the gurbantunggut desert range from e nd 0 4 5 to 6 and 87 sr 86 sr 0 711 to 0 713 the isotopic compositions of the fine grained surface sands are not spatially uniform within each desert and are controlled by the lithological characteristics of the tectonic units in which the deserts are located the isotopic comparison of the dune sands with other sediments indicates that tectonic denudation and fluvial processes are the main mechanisms of fine particle production of the modern desert sands in terms of isotope analyses and forward trajectory results it is further found that the mineral dust is deposited not only in proximal areas such as the hexi corridor the chinese loess plateau and the tibet qinghai plateau but also in distal regions such as japan the pacific ocean and greenland once it is blown out of the taklimakan desert however the transport and sinks of mineral dust vary with the atmospheric currents

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

shocked quartz grains in the polymict breccia of the granby structure sweden verification of an impact the middle ordovician granby structure in sweden is generally considered the result of an asteroidal or cometary collision with earth although no hard evidence i e shock metamorphic features or traces of the impactor have been presented to date in this study drill core samples of a sedimentary breccia from the granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure the finding of multiple sets of decorated planar deformation features pdfs in quartz grains in these samples provides unambiguous evidence that the structure is impact derived furthermore the orientation of the pdfs e g 1013 1012 and r z 1011 is characteristic for impact deformation the fact that a majority of the pdfs are decorated implies a water bearing target the shocked quartz grains can be divided into two groups rounded grains found in the breccia matrix likely originated from mature sandstone and angular grains in fragments from crystalline target rocks the absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15 20 gpa and the frequency distribution of pdf orientations in the bedrock quartz implies pressures of the order of 10 gpa

orogenic disseminated gold in phanerozoic fold belts examples from victoria australia and elsewhere abstract orogenic gold mineralisation in the lachlan orogen of southeastern australia and in other phanerozoic fold belts has generally been assumed to be confined entirely to quartz veins however an improved understanding of the tectonic structural and geochronological framework in which these occurrences form and recent advances towards the extent of wallrock alteration around these veins cast doubt on the validity of this deeply entrenched lode gold paradigm ore grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant orogenic lode gold deposits lachlan orogen australia buller terrane and otago schist new zealand meguma terrane canada tien shan mountains in uzbekistan and kyrgyzstan yana kolyma province baikal verhoyansk and allakh yun fold belts in north central mongolia and far east russia disseminated mineralisation in these collisional belts is invariably hosted by pervasively altered greenschist to subgreenschist facies rocks these occurrences also share a number of characteristics with some carlin type gold deposits in the great basin of western north america e g getchell regarding structural relationships fluid composition mineral paragenesis and a common association of carbonaceous matter with mineralisation features of alteration physico chemical characteristics of the ore bearing fluids and strong structural control in all of the studied occurrences point to a close genetic association with lode gold mineralisation the two styles are considered to represent end members of a crustal continuum of orogenic gold emplacement in phanerozoic fold belts with disseminated mineralisation more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation these conditions facilitate ground preparation via brittle fracturing and the development of intricate stockwork systems efficient gold segregation from bisulphide complexes is largely controlled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids and accelerated by the presence of carbonaceous matter at the site of deposition

identification of the bloody creek structure a possible impact crater in southwestern nova scotia canada an approximately 0 4 km diameter elliptical structure formed in devonian granite in southwestern nova scotia herein named the bloody creek structure bcs is identified as a possible impact crater evidence for an impact origin is based on integrated geomorphic geophysical and petrographic data a near continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the bcs ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp heterogeneous material under the floor interpreted as deposits from collapse of the transient cavity walls is overlain by stratified and faulted lacustrine and wetland sediments alteration features found only in rim rocks include common grain comminution polymict lithic microbreccias kink banded feldspar and biotite single and multiple sets of closely spaced planar microstructures pms in quartz and feldspar and quartz mosaicism rare reduced mineral birefringence and chlorite showing plastic deformation and flow microtextures based on their form and crystallographic orientations the quartz pms consist of planar deformation features that document shock metamorphic pressures 25 gpa the age of the bcs is not determined the low diameter to depth ratio of the crater coupled with anomalously high shock metamorphic pressures recorded at its exposed rim may be a result of significant post impact erosion alternatively impact onto glacier ice during the waning stages of wisconsinian deglaciation about 12 ka bp may have resulted in dissipation of much impact energy into the ice resulting in the present morphology of the bcs

examining the controls on gold deposit distribution in the swayze greenstone belt ontario canada using multi scale methods of spatial data analysis abstract understanding the spatial controls on mineral deposits is crucial in mineral exploration targeting this paper uses a combination of spatial data analysis techniques such as the distance distribution method fry nearest neighbor index ripley s k function multi distance fractal and kernel density analysis to better define controls on the distribution of gold deposits in the swayze greenstone belt ontario canada according to results obtained with the distance distribution method d2 structures were critical in controlling the spatial distribution of the gold mineralization with 97 of the gold deposits occurring within 7 km of these structures and 65 within 4 km fry analysis confirmed this spatial relationship with a close agreement between the orientations of the d2 structures and gold deposit alignment the outcome of ripley s k function cluster analysis suggests that gold deposits occur within 4 km distances of other gold deposits these findings are consistent with the results from the fractal analysis showing that gold deposit distribution is bifractal and that different geological features occurred at local 8 km scales we conclude that higher order d2 structures and lower order faults connected to the d2 structures acted as pathways to focus gold bearing hydrothermal fluids thus leading to the observed concentration and distribution of gold deposits at the regional to local scales

boron isotope geochemistry of australian salt lakes boron isotope geochemistry has been investigated in brines groundwaters and sediments from the modern australian salt lakes of victoria south australia and western australia by applying negative thermal ionization mass spectrometry techniques the geochemical history of the brines has been reconstructed by using 11b bcl and nacl ratios the victorian volcanic crater lakes of southeastern australia have water salinities of up to 60 g l molar nacl ratios 0 87 similar to the marine ratio molar bcl ratios of 2 9 10 4 to 4 9 10 4 and 11b values of 54 to 59 relative to nbs 951 the depletion of total b and the high positive 11b values relative to seawater bclratio 7 9 10 4 11b 39 are attributed to a marine cyclic salt origin together with adsorption processes in closed systems with low water sediment wr ratios in contrast salt lakes from south australia and western australia which are large shallow playas associated with halite gypsum and detrital clay minerals have interstitial and surface brines characterized by salinities of 80 to 280 g l molar nacl ratios of 0 85 to 1 molar bcl ratios of 4 10 6 to 4 10 4 and 11 values of 25 to 48 the 11 values of these brines are different from those of groundwaters from the great artesian basin 11 15 9 to 2 2 with high molar bcl ratios of 1 10 3 to 3 8 10 2 country rocks 11b 8 7 to 6 8 and modern detrital sediments present in these salt lakes 11b 3 2 to 12 3 the 11b values of these salt lakes overlap with those of surface and brackish waters 11b 28 to 35 and with the b isotopic composition of seawater 11b 39 both low molar nacl ratios 39 of some brines indicate interaction of the brines with detrital sediments within the salt lake systems 11 values 39 suggest mixing of brines of marine origin from which b was partly removed by adsorption with waters of terrestrial origin with low 11 values nacl ratios are used as indicators of the origin of the salts as well as of halite dissolution precipitation the 11 values and bcl ratios are sensitive to a marine or non marine origin adsorption of boron onto clays and the effective water sediment ratio at low wr ratios the preferential removal of 10b from the solution affects the bulk solution whereas at high wr ratios the 11 value of a solution is not affected by adsorption although the 11 value of borate minerals may be a discriminant of marine or non marine origin boron isotopes are less distinctive in evaporative environments where boron is not an abundant component and where water sediment interaction occurs

predicting rock type and detecting hydrothermal alteration using machine learning and petrophysical properties of the canadian malartic ore and host rocks pontiac subprovince quebec canada abstract the canadian malartic deposit is a world class intrusion related archean gold deposit hosted in the pontiac subprovince superior province in quebec canada laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks the various rock types present in the malartic district mainly meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities using support vector machines it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89 within the meta sedimentary rocks class variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration these are caused by the destruction of iron bearing silicate minerals and magnetite in unaltered rocks 10 4 to 10 3 si to form pyrite carbonates k feldspar and fe depleted hydrothermal biotite in altered rocks 10 5 to 10 4 si within the felsic intermediate intrusive rocks grain densities below 2 7 g cm3 and magnetic susceptibilities in the 10 6 to 10 4 si range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration however magnetic susceptibility and grain density of these intrusive rocks are also dependent on their fe2o3 al2o3 and tio2 al2o3 ratios which are not related to the hydrothermal alteration footprint but are rather due to distinct protolith compositions within the mafic dykes grain density is the best indicator of hydrothermal alteration unaltered mafic dykes 2 95 to 3 10 g cm3 are mostly composed of amphibole whereas altered mafic dykes 2 70 to 2 95 g cm3 have reduced amphibole contents and higher abundances of carbonates pyrite quartz and biotite alteration the support vector machine classifier is extended to predict if meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average f1 scores of 73 69 and 93 respectively in altered meta sedimentary rocks the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples the classifier is further extended to predict if the gold content of meta sedimentary rocks is above or below threshold values of 0 01 0 1 and 1 ppm with average f1 scores of 83 80 and 76 respectively using conceptual models of the rock physical properties at the malartic district scale it is shown that ground magnetic surveys are the most promising geophysical tool for early stage greenfield exploration of this type of deposit however depending on the scale at which the surveys are conducted magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks this in part explains why past airborne geophysical exploration campaigns for this type of deposit in the malartic district were inconclusive finally the machine learning process used in this case study can be applied in advanced exploration stages during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models

rietveld analysis of x ray powder diffraction patterns as a potential tool for the identification of impact deformed carbonate rocks previous x ray powder diffraction xrd studies revealed that shock deformed carbonates and quartz have broader xrd patterns than those of unshocked samples entire xrd patterns single peak profiles and rietveld refined parameters of carbonate samples from the sierra madera impact crater west texas unshocked equivalent samples from 95 miles north of the crater and the mission canyon formation of southwest montana and western wyoming were used to evaluate the use of x ray powder diffraction as a potential tool for distinguishing impact deformed rocks from unshocked and tectonically deformed rocks at sierra madera dolostone and limestone samples were collected from the crater rim lower shock intensity and the central uplift higher shock intensity unshocked equivalent dolostone samples were collected from well cores drilled outside of the impact crater carbonate rocks of the mission canyon formation were sampled along a transect across the tectonic front of the sevier and laramide orogenic belts whereas calcite subjected to significant shock intensities at the sierra madera impact crater can be differentiated from tectonically deformed calcite from the mission canyon formation using rietveld refined peak profiles weakly shocked calcite from the crater rim appears to be indistinguishable from the tectonically deformed calcite in contrast rietveld analysis readily distinguishes shocked sierra madera dolomite from unshocked equivalent dolostone samples from outside the crater and tectonically deformed mission canyon formation dolomite

search for evidence of impact at the permian triassic boundary in antarctica and australia life on earth was almost destroyed some 250 m y ago in the most profound of all known mass extinction events we investigated the possible role of impact by an extraterrestrial bolide through chemical and mineralogical characterization of boundary breccias search for shocked quartz and analysis for iridium in permian triassic boundary sections at graphite peak and mount crean antarctica and wybung head australia thin claystone breccias at the isotopically and paleobotanically defined boundary at all three locations are interpreted as redeposited soil rather than impact ejecta the breccias at all three locations also yielded shocked quartz but it is an order of magnitude less abundant 0 2 vol and smaller only as much as 176 micrometers m diameter than shocked quartz at some cretaceous tertiary boundary sites faint iridium anomalies were detected up to 134 pg g 1 these values are an order of magnitude less than iridium anomalies at some cretaceous tertiary boundary sites furthermore peak iridium values are as much as 1 m below the isotopically and paleobotanically defined boundary the idea that impact caused the extinctions thus remains to be demonstrated convincingly