characteristics of large terrestrial impact structures as revealed by remote sensing studies abstract remote sensing data sets of a spectrum of impact structures on earth have been analyzed in an effort to determine if such structures have identifiable signatures which could be used as an aid in searching broad areas for the missing craters predicted by current estimates of the cratering flux meteor crater arizona has been used as a control site due to the extensive data available for this feature laboratory thermal infrared reflectance spectra of shocked sandstones indicate that shock processes alter the appearance of fundamental absorption features enough to permit such effects to be observed in airborne thermal infrared imagery collected with the nasa tims instrument analysis of this tims imagery suggests that shocked and displaced target rocks can be readily identified aircraft multi frequency imaging radar data collected with the nasa jpl sar polarimeter clearly reveals the textural pattern of the preserved components of the ejecta blanket spot multi spectral imagery of bosumtwi ghana illustrates the difficulties associated with remote sensing in highly vegetated terrains nevertheless the imagery reveals the subtle expression of an outer tectonic ring at the crater and may be sensitive to geobotanical variations associated with impact related suevite deposits in contrast the zhamanshin impact feature on the arid sedimentary plains of kazakhstan is ideal for multi spectral remote sensing although much of the expected morphologic expression of the complex crater is missing or subdued virtually all of the field mapped impact related deposits display a unique spectral pattern in decorrelated landsat thematic mapper imagery the extensive deposits of allogenic breccias which cap the hills defining the inner ring of the crater are very distinctive as are certain of the impact glass deposits a second or outer ring at 13 13 5 km is apparent in the decorrelated principal component tm imagery the suggestion is that spectral disruption patterns for large multi ring impact features in arid settings should be identifiable from orbit by examining a pristine 144 km diameter multi ring impact feature on the venus rolling plains an impression of the original morphology of a continental impact crater of the magnitude of the cretaceous tertiary k t boundary impact structure can be deduced the 35 ma popigai structure is the only known 100 km class impact crater on earth approximately equal in age or younger than the k t event and its morphology is poorly expressed in single band landsat imagery thus spectral anomalies associated with key impact deposits such as allogenic breccias suevite melt and glasses may have to form the basis for identifying candidate k t impact sites in the absence of preserved morphologic elements of the crater it is apparent that remote sensing is not a panacea for the discovery and validation of new impact craters but it can contribute by serving as a pathfinder both for the identification of promising structures and the possible locations of key deposits for detailed fieldwork and sampling

the impact of landscape change on the significance of political centres along the lower elbe river in the 10th century a d abstract the hohbeck an insular elevation in the middle of the vast elbe river floodplain was a strategically important place in the early and high middle ages this is reflected in an unusually high density of strongholds the meetschow site was of major importance for several centuries even when political power changed saxons franks slavs and germans all kept their main fortification at this place obviously because of its topographical position controlling the intersection of a water route and a land route the significance of meetschow ended with the end of the 10th c and was declining until the final abandonment lenzen on the other bank of the river took over the functions of meetschow and developed into the administrative and political centre of the region geoarchaeological research revealed evidence for a short phase of heavy natural flooding events sometime between 950 and ca 980 a d its impact on the topographical situation around the hohbeck is discussed in the light of the political situation the paper focuses on the coincidences in the history of strongholds and flood activity in the 8th 10th c at the hohbeck it is stressed that the documented large scale natural events should be possible to detect in a wider area of northern europe

sanidine spherules at the cretaceous tertiary boundary indicate a large impact event the hypothesis that a catastrophic impact of an extraterrestrial body caused the terminal cretaceous mass extinctions of dinosaurs planktonic foraminfera and other species is now accepted as respectable following the discovery of a worldwide iridium enrichment in the cretaceous tertiary k t boundary clay1 5 in the basal lamina of the k t boundary clay of caravaca spain 7 numerous spherules were discovered composed of finely crystallized almost pure k feldspar in the structural state of high sanidine it is concluded here that these spherules solidified from a melt and were probably derived from the impacting body this poses problems as high k values are not reported from bulk analyses of meteorites6 the k feldspar phenocrysts reported in some iron meteorites23 suggest the body may have been a metal sulphide silicate planetesimal a cometary body is suggested as an alternative

sandstone sample analysis and additional structural data from jabal rayah a possible impact structure in saudi arabia the 5 5 km sized jabal rayah ring structure located at 28 390 n 37 120 e in saudi arabia has been classified as a possible complex impact structure located in flat lying paleozoic clastic sediments previous detailed mapping showed that erosional processes led to a relief inversion with displaced folded and faulted blocks of silurian to early devonian strata interpreted to form a ring syncline now forming a topographically outstanding 150 m high ring crest the drainage toward the center of the structure seems controlled by a set of radial faults this central part is eroded to the level of the surrounding plateau and partially covered with gravel analysis of 28 qusaiba formation sandstones showed that at the present outcrop level the sediments seem devoid of shock features measurement of fold axes in the central part of the structure shows radially outward plunging fold axes becoming steeper toward the center and also fold axes of other orientation and folded folds this fold axis pattern is interpreted as an upward pointing kilometer sized sheath fold assuming an impact scenario and using the present size of the structure the minimum central structural uplift is estimated at 500 m which is consistent with qusaiba formation occupying the center of the ring structure

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

geochemical identification of impactor for lonar crater india the only well known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the lonar crater india for the first time evidence of the impactor that formed the crater has been identified within the impact spherules which are 0 3 to 1 mm in size and of different aerodynamic shapes including spheres teardrops cylinders dumbbells and spindles they were found in ejecta on the rim of the crater the spherules have high magnetic susceptibility from 0 31 to 0 02 si mass and natural remanent magnetization nrm intensity both nrm and saturation isothermal remanent magnetization sirm intensity are 2 am2 kg demagnetization response by the nrm suggests a complicated history of remanence acquisition the spherules show schlieren structure described by chains of tiny dendritic and octahedral shaped magnetite crystals indicating their quenching from liquid droplets microprobe analyses show that relative to the target basalt compositions the spherules have relatively high average fe2o3 by 1 5 wt mgo 1 wt mn 200 ppm cr 200 ppm co 50 ppm ni 1000 ppm and zn 70 ppm and low na2o 1 wt and p2o5 0 2 wt very high ni contents up to 14 times the average content of lonar basalt require the presence of a meteoritic component in these spherules we interpret the high ni cr and co abundances in these spherules to indicate that the impactor of the lonar crater was a chondrite which is present in abundances of 12 to 20 percent by weight in these impact spherules relatively high zn yet low na2o and p2o5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume

the impact cratering rate in recent time data on the recent cratering rate in the earth moon system is somewhat ambiguous with respect to whether the rate has been constant over the last 3 b y rate estimates based on the present population of earth crossing bodies are higher than estimates based on crater counts at the apollo 12 and 15 sites while some estimates based on terrestrial crater counts are intermediate in value the terrestrial data base for structures with diameter d 20 km on the north american and european cratons has been investigated for variations in the rate with time there appear to be two populations with a 0 120 m y age group suggesting a higher cratering rate than an older 200 360 m y age group reexamination of the preservation levels of these structures indicates that the topography of a 20 km sized structure may be removed by erosion in 34 to 273 m y and the geologic anomaly resulting from structural uplift in 79 to 630 m y this suggests that the relatively lower rate for the 200 360 m y group may reflect problems in crater recognition associated with crater retention if rate estimates are restricted to the possibly more complete sample of 0 120 m y craters on the cratons the recalculated rate is 5 4 2 7 0 15 km 2 y 1 for d 20 km this is comparable to that based on earth crossers and reinforces previous suggestions that the recent cratering rate may be a factor of 2 3 higher than the average rate over the last 3 0 b y this rate estimate when extrapolated back in time is a better fit to lunar data from tycho and copernicus than a constant rate based on apollo 12 and 15 data and may be interpreted to suggest that the relatively higher recent rate has been in effect for 1 b y given the uncertainties however this must be regarded as speculative

siderophile interelement variations in the cretaceous tertiary boundary sediments from caravaca spain a detailed profile of the siderophiles fe co ni pd re os ir pt and au from the cretaceous tertiary boundary at caravaca spain indicates that the major fractionation mechanisms are local redistribution reworking and diagenetic mobilization large fractions of the fe co and re and perhaps a small amount of pd and au appear to be terrestrially derived but nearly all of the ni os ir and pt must be derived from an extraterrestrial source only the basal layers at caravaca and stevns klint denmark are representative of the initial fallout and integrated ir fluences across entire sections can be misleading these suggest a concentration oft 120mg g projectile in the fallout and a global fluence of chondritic matter oft 140mg cm 2

a possible impact origin for the bangui magnetic anomaly central africa abstract bangui is one of the most impressive magnetic anomalies on earth and by far the largest over africa at magsat altitudes it has an amplitude of 28 nt and covers an area of 700 000 km 2 evidence presented suggests that the anomaly might be related to a large impact structure early in the history of the earth north south west east and vertical derivatives of the 5 min topographic data base all reveal a double ring structure with outer ring diameter 810 km and inner ring diameter 490 km the precambrian geology shows several small ancient basins within a large basin with a basement high in the middle such as found in large impact structures the basement complex is early precambrian and hence the impact must be of this age the magnetic anomaly is interpreted as being due to strong remanent magnetism in the ancient crater floor and surrounds the anomaly can be satisfactorily modelled by a 4 5 km thick disc 800 km in diameter beneath the ancient basins to model the amplitude and shape of the anomaly requires an intensity of 10 a m 1 with direction d n 18 w i 25 i e significantly different from the present dipole field the magnetisation is likely to include strong shock remanence srm acquired at the time of impact thermal remanence trm partial thermal remanence ptrm thermochemical remanence tcrm and chemical remanence crm acquired soon after the impact all depend on the size of the impact and all contribute to the high intensity required to explain the anomaly

the bow city structure southern alberta canada the deep roots of a complex impact structure geological and geophysical evidence is presented for a newly discovered probable remnant complex impact structure the structure located near bow city southern alberta has no obvious morphological expression at surface the geometry of the structure in the shallow subsurface mapped using downhole geophysical well logs is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure evidence of deformation along the eastern margin of the central uplift includes thrust faulting folding and steeply dipping bedding normal faults were mapped along the northern margin of the annular region isopach maps reveal that structural thickening and thinning were accommodated primarily within the belly river group evidence from legacy 2 d seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure including a series of listric normal faults in the annular region and complex faulting within the central uplift the absence of any ejecta blanket breccia suevite or melt sheet based on available data is consistent with the bow city structure being the remnant of a deeply eroded complex impact structure accordingly the bow city structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks

gravity inferred crustal attributes of visible and buried impact basins on mars 1 the impact basins of mars reveal important insights on martian tectonic evolution they involve strongly disrupted depressed regions of crust with likely enhanced porosity and permeability that may locally concentrate water and other crustal fluids hence the crustal properties of impact basins can also be important in the hunt for water and related clues for life on mars we assess the crustal details of impact basins by separating the mars global surveyor free air anomalies into terrain correlated and terrain decorrelated components the separation is based on the correlation spectrum between the free air anomalies and the gravity effects evaluated from the topography mapped by the mars orbital laser altimeter mola for topographically visible multiring basins like isidis striking circular patterns of alternating terrain correlated free air maxima and minima mark the uncompensated components of the central mantle plug and surrounding rings the first vertical derivatives of these anomalies effectively estimate the basin ring locations and a transient cavity depth to diameter ratio of 0 09 that is consistent with the ratio observed for lunar nearside multiring basins for the isidis basin we obtain an excavation depth of roughly 66 km that reflects considerably disrupted crust for concentrating local fluids since the noachian impact furthermore the related crustal terrain decorrelated free air anomalies suggest up to 2 km of high density basin fill may cap the central basin subtle quasi circular depressions in the relatively featureless mola terrain of the northern hemisphere have identified potentially buried impact basins frey et al 2002 an altimetry depression in acidalia planitia and another in utopia are also associated with ringed patterns of terrain decorrelated free air anomalies that may mark the uncompensated mass effects of buried impact basins the gravity derived transient excavation depths for these inferred basins are roughly 41 and 20 km respectively while the related ring diameters d follow the ubiquitous d rule of planetary impact basins the crust of these buried basins is likely to contain water at higher levels than the crust of the equatorial basins that was substantially dewatered with the development of the great northern basin hence analyzing surface fractures faults and other crustal discontinuities that can tap the more saturated crust of these inferred buried basins may facilitate the hunt for water on mars

cooling fractures in impact melt deposits on the moon and mercury implications for cooling solely by thermal radiation we study the distribution morphology and geometrical properties of fractures in several young impact melt deposits on the moon and mercury and the ways that these fractures may form from cooling by thermal radiation in each impact melt complex the topography of the underlying terrain determines the orientation of cooling fractures such that interior fractures that formed in the relatively thick interior areas of the melt unit are wider and have a larger spacing than marginal fractures that formed in the relatively thin areas near the unit s margins solid debris entrained in molten deposits provides prefracture flaws that can seed cooling fractures but too much solid debris prevents cooling fractures from growing to macroscopic sizes the appearance of subparallel fractures is mainly caused by subsidence of the deposits during the process of cooling and solidification tensile stresses caused by thermal radiation are large enough to initiate cooling fractures on both the moon and mercury which may represent the initial stage of columnar joints formation but the cooling rate caused solely by thermal radiation is not large enough to form well organized columnar joints that feature polygonal colonnades we therefore propose that thermal conduction and convection are the major contributors in the formation of columnar joints on planetary bodies

cometary impacts into ocean their recognition and the threshold constraint for biological extinctions the montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied the impact crater is 45 km in diameter and was formed at 50 8 ma by a fall of probably an old cometary nucleus 3 4 km in diameter into shallow 600 m ocean comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim instead the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent return water flow into the excavating cavity by this process most of the fall out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land at a microscopic scale the shock metamorphism features are about the same as those for land impacts geochemically impacts of comet nucleii may not leave a recognizable signature at the impact horizon except for a minor increase in iridium thus stratigraphic horizons associated with extinctions and or major changes in biota have to be closely examined for other impact indicators like the presence of tectites glass spherules and quartz grains with shock features occurrence of megatsunami wave deposits extensive erosion on continental margins margin failures and faunal mixing above erosional unconformities are other potential impact indicators there is no single indicator that can provide sufficient proof of an impact event such interpretations have to be based on multiparameter studies of global extent since many of the impact indicators are only of regional extent the lack of extinction of any marine plankton genera or of bottom dwellers at the montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus 4 km in diameter the calculated frequency for a cometary impact which could result in a 10 extinction of marine genera is about 6 10 7 yr 1 and for the k t boundary type of extinctions about 2 10 8 yr 1 even allowing for a large degree of uncertainty in these estimates it is unlikely that the biological extinction events for the last 250 ma identified by sepkoski 1990 could have been all caused by meteorite impacts

effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on mars impact cratering is an important geological process on mars and the nature of martian impact craters may provide important information as to the volatile content of the martian crust terrestrial impact structures currently provide the only ground truth data as to the role of volatiles and an atmosphere on the impact cratering process recent advancements based on studies of several well preserved terrestrial craters have been made regarding the role and effect of volatiles on the impact cratering process combined field and laboratory studies reveal that impact melting is much more common in volatile rich targets than previously thought so impact melt rocks melt bearing breccias and glasses should be common on mars consideration of the terrestrial impact cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on mars furthermore recent studies at the haughton and ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta it is apparent that the relative abundance of volatiles in the near surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of martian ejecta deposits this study shows the value of using terrestrial analogues in addition to observational data from robotic orbiters and landers laboratory experiments and numerical modeling to explore the martian impact cratering record

the evolution of impact basins cooling subsidence and thermal stress potentially important contributors to the topography and tectonics of multi ring impact basins are the thermal contraction and thermal stress that accompany the loss of heat emplaced during basin formation heat converted from impact kinetic energy and contributed from the uplift of isotherms during cavity collapse are important components in the energy budget of a newly formed basin that the subsequent cooling may have been an important factor in the tectonic evolution of the orientale basin is suggested by the deep central depression and by a surrounding region of extensive fissuring to test these concepts we develop models for the anomalous temperature distribution immediately following basin formation and we calculate the resulting elastic displacement and stress fields that then would accompany cooling of the basin region all models predict subsidence of the basin floor and a near surface stress field consistent with fissuring in addition the rates of cooling and of accumulation of thermal stress are in agreement with the inferred timing of fissure formation in orientale the sensitivity of the predicted displacements and stresses to the initial temperature field allows us to place bounds on the quantity and distribution of impact heat emplaced during basin formation in order to be consistent with the observed topography and the distribution of fissures in the orientale basin the buried heat deposited during the basin forming event was between 1032 and 1033 erg it is likely that most of this heat was concentrated within a distance of 100 200 km from the point of impact

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

highly siderophile elements re os ir ru rh pd au in impact melts from three european impact craters saaksjarvi mien and dellen clues to the nature of the impacting bodies twenty two large 10 g impact melt samples from three scandinavian craters i e saaksjarvi finland and mien and dellen sweden were analyzed for highly siderophile elements hse platinum group elements rh and au by the nickel sulfide technique in combination with neutron activation the ten impact melt samples from saaksjarvi are enriched in ir and other highly siderophile elements ir 2 48 0 73 ng g relative to average upper crust concentrations 0 03 0 02 ng g ir the twelve dellen and mien samples are marginally enriched in ir 0 48 0 23 ng g for dellen and 0 37 0 23 ng g for mien the amount of meteoritic component corresponds to 0 5 of a nominal ci component for saaksjarvi and about 0 1 for mien and dellen the saaksjarvi pattern is fractionated relative to ci chondrites normalized abundances increase from the refractory to the more volatile siderophile elements ir ni the trend is qualitatively similar to magmatic iron meteorites and corresponds to 0 4 of a nominal magmatic iron meteorite component tamarugal iiiab for mien and dellen no projectile assignment can be made all samples from the three impact craters have low osir ratios compared to chondritic ratios either the projectile had low osir ratios or os was lost during the impact as volatile oso4 based on the results of this study and on a compilation of literature data average upper crustal abundances of the highly siderophile elements in the target area baltic shield are estimated as 0 03 ng g for ir and os 1 1 ng g for ru 0 38 ng g for rh 2 0 ng g for pd 8 g g for co and 37 g g for cr these data are representative of the upper crust they allow for the first time reliable estimates of crustal abundances for rh and ru upper crustal abundances also show that ir and os are the most favourable elements for the identification of meteoritic signatures fractionated osir ratios in magmatic iron meteorites or potential losses of os during the impact leaves ir as the best indicator element for meteoritic contamination with ir extraterrestrial components as low as 2 104 ci may be identified

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

geological overview and cratering model for the haughton impact structure devon island canadian high arctic the haughton impact structure has been the focus of systematic multi disciplinary field and laboratory research activities over the past several years regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to 1880 m new 40ar 39ar dates place the impact event at 39 ma in the late eocene haughton has an apparent crater diameter of 23 km with an estimated rim final crater diameter of 16 km the structure lacks a central topographic peak or peak ring which is unusual for craters of this size geological mapping and sampling reveals that a series of different impactites are present at haughton the volumetrically dominant crater fill impact melt breccias contain a calcite anhydrite silicate glass groundmass all of which have been shown to represent impact generated melt phases these impactites are therefore stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets the crater fill impactites provided a heat source that drove a post impact hydrothermal system during this time haughton would have represented a transient warm wet microbial oasis a subsequent episode of erosion during which time substantial amounts of impactites were removed was followed by the deposition of intracrater lacustrine sediments of the haughton formation during the miocene present day intracrater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the high arctic modern modification of the landscape is dominated by seasonal regional glacial and niveal melting and local periglacial processes the impact processing of target materials improved the opportunities for colonization and has provided several present day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain

absence of shock metamorphic effects in the bushveld complex south africa results of an intensive search abstract an extensive search was made for distinctive shock metamorphic effects in order to test the theory that the bushveld complex bc was formed by the impact of large extraterrestrial objects field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock metamorphic effects as shatter cones unusual fragmental or melt breccias and shock lamellae in quartz these are 1 pre bc quartzite xenoliths in the rooiberg group a thick sequence of acid lavas rhyolites and rhyodacites and related rocks proposed by earlier workers as a possible crater filling impact melt and 2 the locally observable contact between the rooiberg group and the underlying pre bc basement pretoria group a contact possibly corresponding to the original crater floor in established impact structures more than 320 samples of pre bc quartzite were collected and examined petrographically for shock metamorphic effects no megascopic shock metamorphic effects shatter cones or unusual breccias were observed in any outcrop in thin section no petrographic shock effects e g shock lamellae hi quartz and kink bands in biotite were observed in any sample the quartzites from both the basement 80 samples and the rooiberg xenoliths 245 samples are virtually undeformed cataclastic textures are rare and individual quartz grains show only normal metamorphic deformation lamellae bohm lamellae and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures although some of these parallel sets superficially resemble so called decorated shock lamellae universal stage measurements of their orientation demonstrate that they are totally different the absence of both megascopic and microscopic shock metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the bc are not correct and that the bc is not an impact structure this conclusion also agrees with 1 the complex stratigraphy now recognized in the lower rooiberg group which is not consistent with an impact melt origin 2 the absence in the rooiberg group of any of the distinctive melt rich fragmentai breccias which dominate the crater filling deposits in established large impact structures and 3 the undeformed character of the basement rocks below the rooiberg group and the apparently concordant contact between them

the role of episodic bombardment in geophysics it is shown that cometary or asteroidal impacts strongly perturb the boundary conditions at the core outer mantle and atmosphere of the earth impact episodes occur following the capture or orbital disturbance of these bodies as the solarsystem passes through the spiral arms of the galaxy there is thus a tendency for magnetic field reversals principal plate movements ice ages and mass extinctions to be correlated and to recur on time scales dictated by galactic processes it follows on the basis of a purely astronomical theory that it may be possible to synthesise the major phenomena of geophysics knowledge of the continuous state of the galaxy then becomes a prerequisite for the understanding of terrestrial history

iridium anomaly in the upper devonian of the canning basin western australia a moderate iridium anomaly about 20 times the local background has been found in upper devonian rocks in the canning basin it occurs at or near the frasnian famennian boundary which is known to be associated with a major massextinction event of global extent the anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions its occurrence suggests a causal link with some form of meteoroid impact moreover carbon isotope data indicate that a large reduction in biomass could have occurred at this level however the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium frutexites iridium platinum iron manganese cobalt arsenic antimony and cerium are preferentially concentrated in filaments of this organism with concentrations ranging from two to five times that of the matrix it is possible that frutexites extracted these elements directly from seawater without the need for their derivation from an extraterrestrial source

multi stage dehydration decompression in the metagabbros from the lower crustal rocks of the serre southern calabria italy porphyroblastic garnet bearing metagabbros from the base of the lower crust section of the serre southern italy exhibit multi stage dehydration and decompression after the panafrican emplacement of their protoliths the first dehydration event produced am opx cpx pl grt as the peak assemblage two decompression stages are documented by 1 coronas of opx pl and opx am and symplectites of opx am pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and 2 symplectites of pl am opx grt having different textures around the porphyroblastic garnet during the second decompression stage a new local somewhat intense dehydration occurred and produced rims of opx pl around the porphyroblastic amphibole or lenses of pl opx am spl bt between layers of dominant amphibole a deformation stage separates older from younger reaction textures the porphyroblastic garnet its inclusions and the matrix are affected by fractures which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix preferred p t estimates are 900 c and 1 1 gpa at the metamorphic peak 850 c and 0 8 0 9 gpa during the formation of corona around clinopyroxene 750 650 c and 0 7 0 8 gpa during the formation of corona around garnet all these textures formed under granulite facies conditions the subsequent metamorphic evolution consists of rehydration under amphibolite facies conditions the p t t path agrees with the path shown by the uppermost migmatites of the serre section and the p t estimates at the top and the bottom of the section are consistent with the thickness 7 8 km of the lower crustal segment a contractional regime which caused a crustal thickening of about 35 km was followed by an extensional one producing significant crustal thinning the change of tectonic regime probably occurred about 300 ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place

impact tectonics in the core of the vredefort dome south africa implications for central uplift formation in very large impact structures the 80 km wide vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure exposure of progressively older strata in the collar of the dome and of progressively higher grade metamorphic rocks toward its center is consistent with differential uplift however the deepest levels exposed correspond to pre impact mid crust rather than lower crust as has been suggested previously pre impact archean gneissic fabrics in the core of the dome are differentially rotated with the angle of rotation increasing sharply at a distance of 1619 km from the center the present asymmetric dips of the collar strata with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector and the eccentric distribution of the pre impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely nw dipping target sequence during central uplift formation the rocks in the core of the dome lack distinctive megablocks or large slip magnitude faults such as have been described in other central uplifts we suggest that the large scale coherent response of these rocks to the central uplift formation could have been accommodated by small scale shear and or rotation along pervasive pseudotachylitic breccia vein fractures

a study of candidate marine target impact craters in arabia terra mars previous workers have proposed that a northern ocean existed early during martian geologic history and the shorelines of that ocean would coincide roughly with the crustal dichotomy that divides the smooth northern lowlands with the cratered southern highlands arabia terra is a region on mars that straddles the crustal dichotomy and several proposed shorelines are located in the area shallow marine impact craters on mars likely would exhibit features like those on earth including characteristic morphological features that are distinctly different from that of craters formed on land common attributes of terrestrial marine impact craters include features of wet mass movement such as gravity slumps and debris flows radial gullies leading into the crater depression resurge deposits and blocks of dislocated materials crater rim collapse or breaching of the crater wall a central peak terrace or peak ring terrace and subdued topography an indicator of both age and possible flood inundation immediately following impact in this article these features have been used to evaluate craters on mars as to a possible marine origin this study used a simple quantification system to approximately judge and rank shallow marine impact crater candidates based on features observed in terrestrial analogs based on the quantification system 77 potential shallow marine impact craters were found within an area bounded by 20 n and 40 n as well as 20 w and 20 e nine exemplary candidates were ranked with total scores of 70 or more in a second smaller study area impact craters of approximately similar size and age were evaluated as a comparison and average total scores are 35 indicating that there is some morphological difference between craters inside and outside the proposed shorelines results of this type of study are useful in helping to develop a general means of classification and characterization of potential marine craters

the azuara impact structure spain new insights from geophysical and geological investigations a progress report on geophysical and geological investigations in the 35 40 km diameter azuara probable impact structure northeast spain is given the target is a 10 km thick sequence of palaeozoic mesozoic and cenozoic sediments gravity measurements establish a negative residual anomaly of about 100 m s2 and a mass deficiency of 1 24 1014 kg they suggest the existence of a buried inner ring with half the size of the outer ring the result of preliminary model calculations is compatible with the assumption of a flat structure only a few kilometers deep measurements of the total intensity of the earth s magnetic field do not indicate causative bodies related to the structure geological mapping reveals a tectonic style which is characterized by folds and faults with radial and tangential elements and a strong horizontal component ejecta abundant monomict and polymict breccias dislocated megablocks inverted stratigraphy and a megabreccia up to 80 m thick in the outer ring are evidence for intense and violent deformation networks of typical breccia dikes well known from many impact structure cut through nearly all stratigraphical units various types and generations of breccia dikes occur in addition to previously described shock deformation in quartz and mica we present further evidence of high pressure and high temperature signatures in rocks the age of the impact is estimated to be upper eocene or oligocene we conclude that the formation of the azuara structure is difficult to explain by mechanisms other than impact and we discuss our observations within the scope of the contact compression excavation and modification stages of impact cratering with special attention directed to the peculiarities of the azuara sedimentary target

impact craters of venus analysis of venera 15 and 16 data from description of impact craters on venus and crater size and depth distributions models are developed that analyze the size distribution curve and radar bright haloes around fresh craters the dense venusian atmosphere has a strong influence on cratering the estimated age of the surface of the area under study is from 0 5 to 1 0 109 years the estimated rate of resurfacing during this time interval is about 0 04 to 0 8 cm 106 years the lithosphere of venus seems to be sufficiently rigid to sustain craters of 100 140 km diameter and 0 8 2 5 km depth for about 108 years

the chicxulub shiva extraterrestrial one two killer punches to earth 65 million years ago abstract two large asteroids struck earth at almost the same time 65 million years ago causing the major extinctions recognized as ending the mesozoic era although occurring close together in time the earth s magnetic pole had moved from the south pole to the north pole in between allowing a time difference between the impacts to be calculated the first strike produced a 180 km diameter crater named chicxulub on the yucatan shelf of southern mexico the second hit the shelf of the northward drifting indian continent in the southern indian ocean producing a crater 450 600 km named shiva hitherto the main obstacle to verifying this scenario has been the paucity of geological sections containing evidence of both impacts here we present such evidence and conclude that the two impacts were separated by about 40 000 years

the number of impact craters on earth any room for further discoveries abstract only 128 impact craters exposed at earth s surface have been found so far while new craters are discovered occasionally taking into account the permanent consumption of craters by erosion we present the first estimate on the number of impact craters that should be present at earth s surface our study yields no evidence for any systematic incompleteness in the available inventory of the craters larger than about 6 km in diameter exposed at the surface in contrast more than 90 craters in the diameter range from 1 km to 6 km should still be waiting to be discovered and even more than 250 between 0 25 km and 1 km diameter the transition from a probably complete inventory above 6 km to a strongly incomplete record at smaller sizes may be related to the difference between simple and complex craters beyond these results on the terrestrial crater record our findings tentatively suggest that erosion rates on the 10 to 100 million year scale may be closer to present day erosion rates than previously assumed

the preservation of the agoudal impact crater morocco under a landslide indication of a genetic link between shatter cones and meteorite fragments abstract geological studies and tomographic profiles of a locality nearby the agoudal village morocco showed the presence of a single impact crater 500 600 m diameter largely hidden by a limestone block 220 m long and 40 m deep the site was interpreted as a landslide that followed the fall of a cosmic body the agoudal impact crater was not affected by intense erosion the lack of an evident impact structure as well as the sporadic distribution of impactites and their limited occurrence can be explained by a complex geological framework and by recent tectonics the latter is the result of the sliding of limestone block which hides almost two thirds of the crater s depression and the oblique fall of the meteoroid on sloping ground in addition some impact breccia dikes sharply cut the host rock in the agoudal impact structure they do not show any genetic relationship with tectonics or hydrothermal activity nor are they related to any karst or calcrete formations altogether the overlapping of the meteorite strewn field 11 km long and 3 km wide with the area of occurrence of shatter cones and impact breccias together with the presence of meteorite fragments shrapnel ejected from the crater the presence of shatter cones contaminated by products of iron meteorites and the presence of impact breccias that contain meteorite fragments of the same chemical composition of the agoudal meteorite indicate that the fall of this meteorite can be responsible for the formation of the impact structure

the anatomy of a buried complex impact structure the mjolnir structure barents sea analysis of the prominent seismic disturbance at the 40 km diameter mjolnir impact structure based on an extensive seismic reflection database shows that the observed broad brimmed bowl shaped disturbance was formed as a result of the impact of an asteroid or comet during volgian berriasian time 149 141 m seismic mapping exhibits and visualizes a 850 1400 km3 disturbed volume and analysis of several structural features within the disturbance provides insight into major cratering processes such as brecciation and excavation melting gravitational collapse of the transient crater and structural uplift a transient crater of 16 km in diameter and 4 5 km in depth is determined from transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2 5 considerably larger than the average expected values for typical terrestrial craters the extensive collapse took place by low angle decollement surfaces at the periphery and by chaotic debris mass flows toward the center furthermore we estimate the mjolnir projectile to have been 0 9 3 km in diameter and that the physical impact released energy in the range of 2 4 53 1020 j corresponding to an earthquake of magnitude 8 3 the mjolnir impact is not associated with a significant mass extinction however dissipation of the energy released during the mjolnir impact was sufficient to have caused several short term near field perturbations such as large amplitude tsunami waves affecting the barents sea region and possibly adjacent areas in the arctic

mineralogic evidence for an impact event at the cretaceous tertiary boundary a thin claystone layer found in nonmarine rocks at the palynological cretaceous tertiary boundary in eastern montana contains an anomalously high value of iridium the nonclay fraction is mostly quartz with minor feldspar and some of these grains display planar features these planar features are related to specific crystallographic directions in the quartz lattice the shocked quartz grains also exhibit asterism and have lowered refractive indices all these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth the shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile

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

ash shutbah a possible impact structure in saudi arabia we have investigated the ash shutbah circular structure in central saudi arabia 21 degrees 37n 45 degrees 39e using satellite imagery field mapping thin section petrography and x ray diffraction of collected samples the approximately 2 1km sized structure located in flat lying jurassic tuwaiq mountain limestone has been nearly peneplained by erosional processes satellite and structural data show a central area consisting of dhruma formation sandstones with steep bedding and tight folds plunging radially outward open folding occurs in displaced younger tuwaiq mountain limestone formation blocks surrounding the central area but is absent outside the circular structure an approximately 60cm thick unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure with exception of a possible concave shatter cone found in the orthoquartzite of the central area other diagnostic shock features are lacking some quartz rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures this deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea or groundwater heating the combination of central stratigraphic uplift of 140m concussion features in discolored sandstone outward dipping concentric folds in the central area deformation restricted to the rocks of the ring structure a complex circular structure of 2 1km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets and a possible shatter cone all point to an impact origin of the ash shutbah structure in fact the ash shutbah structure appears to be a textbook example of an eroded complex impact crater located in flat lying sedimentary rocks where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure less

compositional heterogeneity of impact melt rocks at the haughton impact structure canada implications for planetary processes and remote sensing connecting the surface expression of impact crater related lithologies to planetary or regional subsurface compositions requires an understanding of material transport during crater formation here we use imaging spectroscopy of 6 clast rich impact melt rock outcrops within the well preserved 23 5 ma 23 km diameter haughton impact structure canada to determine melt rock composition and spatial heterogeneity we compare results from outcrop to outcrop using clasts groundmass and integrated clast groundmass compositions as tracers of transport during crater fill melt rock formation and cooling supporting laboratory imaging spectroscopy analyses of 91 melt bearing breccia and clast samples and microscopic x ray fluorescence elemental mapping of cut samples paired with spectroscopy of identical surfaces validate outcrop scale lithological determinations results show different clast rich impact melt rock compositions at three sites kilometers apart and an inverse correlation between silica rich sandstone gneiss and phyllosilicate rich shales and gypsum rich rocks that suggests differences in source depth with location in the target stratigraphy gypsum is primarily sourced from 1 km depth while gneiss is from 1 8 km depth sandstone from 1 3 km and shales from 1 6 1 7 km observed heterogeneities likely result from different excavation depths coupled with rapid quenching of the melt due to high content of cool clasts results provide quantitative constraints for numerical models of impact structure formation and give new details on melt rock heterogeneity important in interpreting mission data and planning sample return of impactites particularly for bodies with impacts into sedimentary and volatile bearing targets e g mars and ceres

the evolution of impact basins viscous relaxation of topographic relief we evaluate the hypothesis that viscous relaxation has been an important process for modifying the topographic profiles of ancient large impact basins on the moon we adopt a representative topographic profile of the orientale basin the youngest large impact basin on the moon as an estimate of the initial topography of older basins of similar horizontal dimensions and we predict the topographic profiles that would result from viscous relaxation according to a number of simple analytical representations of the rheological response of the moon to surface topography at wavelengths greater than the thickness of a high viscosity lithosphere both a decrease in viscosity with depth and the partial to complete isostatic compensation of topographic relief have pronounced effects on the wavelength dependent relaxation times and must be considered in modeling viscous relaxation for features as large as impact basins the effect of a decrease in viscosity with depth is to enhance the rate of viscous relaxation at long wavelengths while the rate of relaxation decreases substantially for the fraction of long wavelength topography which is isostatically compensated as an initial condition these models are applied to two pre nectarian basins on the moon the tranquillitatis basin on the lunar nearside and the much larger and older south pole aitken basin on the farside the topographic profile of tranquillitatis after correcting for the effect of mare basalt fill is consistent with significant viscous relaxation of relief prior to the oldest episode of mare volcanism preserved as a surface unit the large topographic relief of the larger farside basin on the contrary is not consistent with significant viscous relaxation and implies a mean crustal viscosity at least a factor of 10 higher than for the central nearside over the time interval during which substantial viscous relaxation of older nearside basins such as tranquillitatis probably occurred a difference in typical crustal temperature profiles between the farside and nearside is the most likely explanation of such a viscosity difference such a nearside farside asymmetry in near surface temperature may date from the time of crust mantle differentiation and may have persisted because of the different histories of late stage impact basin formation and mare volcanism on the two hemispheres

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

siderophile rich magnetic spheroids from the cretaceous tertiary boundary in umbria italy the magnetic fraction of some cretaceous tertiary k t boundary sediments sampled at two locations in italy contains a spheroidal component which has retained some relict mineral textures and possibly mineral chemistry from the k t event the spheroids contain small crystals 1 50 m of magnetite in a highly altered groundmass these crystals are commonly skeletal indicating formation by rapid growth from a high temperature liquid variable concentrations of ni mg and al differing by at least a factor of 5 in the magnetite may also reflect a relict chemistry these spheroids contain higher concentrations of ir and probably other siderophiles than the bulk sediment and may have been an important carrier for these elements at 65 myr this discovery supports evidence for a major accretionary event at the end of the cretaceous

new 40ar 39ar dating of the clearwater lake impact structures quebec canada not the binary asteroid impact it seems abstract the two clearwater lake impact structures quebec canada are generally interpreted as a crater doublet formed by the impact of a binary asteroid here arguments are presented that raise important questions about the proposed double impact scenario new 40 ar 39 ar dating of two virtually fresh impact melt rock samples from the 36 km west clearwater lake impact structure yielded two statistically robust early permian plateau ages with a weighted mean of 286 2 2 2 2 6 ma 2 mswd 0 33 p 0 57 in contrast 40 ar 39 ar results for two chloritized melt rocks from the 26 km east clearwater lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component although individually weakly robust age spectra corrected for the trapped argon component and inverse isochron plots for the east clearwater melt rocks consistently yielded apparent ages around 460 470 ma no permian signal was found in either of these melt aliquots our new 40 ar 39 ar results reproduce earlier 40 ar 39 ar plateau ages 283 ma and 465 ma respectively for the two impact structures by bottomley et al 1990 and are in conflict with a previous statistically non robust rb sr age of 287 293 26 ma for east clearwater the combined cluster of apparent ages of 460 470 ma derived from four different samples across the impact melt sheet is very unlikely to represent a false age effect due to the incorporation of extraneous argon into the melt instead it strongly favors a middle ordovician age for the east clearwater impact and impact induced hydrothermal chloritization moreover the clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories an effect unexpected in the case of a permian double impact whereas the west clearwater impact affected ordovician carbonates incorporated into the impact breccia drill core reports from the 1960s concluded that clasts of ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the east clearwater lake impact structure which is overlain by 100 m of post impact sandstones shales and carbonates no resolvable impactor contamination has so far been detected in the west clearwater impact melt rocks whereas east clearwater carries a distinct ordinary possibly l chondritic impactor signature in its melt rocks east clearwater lake might thus represent one among a long list of ordovician impact structures in north america and northern europe that were presumably formed in response to the l chondrite asteroid breakup event 470 ma ago paleogeographic reconstructions show that the ordovician east clearwater impact probably occurred in a near coastal to shallow marine setting while the permian west clearwater impact hit continental pangaea along with the new 40 ar 39 ar data the paleomagnetic sedimentologic and geochemical findings suggest that the close spatial arrangement of the two clearwater lakes is most likely pure coincidence the two impact structures seem to represent a false doublet struck by impacts separated by 180 million years in time the new results for the clearwater lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on earth and on other planetary bodies in the inner solar system

the metamorphic history of the vredefort dome at approximately 2 ga as revealed by coesite stishovite bearing pseudotachylites the vredefort dome 2 0 ga represents the central uplift of a very large impact structure this uplift exposed a nearly complete cross section through the continental crust in the region which is 25 30 km thick two metamorphic events took place at about the same time as the impact the first event so called static metamorphism is pre impact and produced lithologies varying from low grade shale to high grade hornfels it resembles contact metamorphism by its lack of schistosity but is more regional as it extends over a large area and is not associated with large intrusions the second event the post shock metamorphism is responsible for the recrystallization of the shock features the investigation of this event has been focused on the degree of alteration of the coesite stishovite bearing pseudotachylite veins that formed during the transit of the shock wave these high pressure silica polymorphs are only present in the upper part of the stratigraphic sequence downward they have been converted to fibrous quartz at the highest grade the fibrous quartz is in turn replaced by triple junctioned mosaic quartz the post shock metamorphism was generated by the heat of the rock before shock plus the heat released by the shock wave the isograds plotted on a map can be translated into depth of burial and therefore provide valuable information regarding the geological setting immediately before impact at the time of impact the rocks were relatively cool and the static metamorphism had ceased with several tens of millions of years separating the two metamorphic events the static metamorphism was probably caused by continental crustal extension in a stress free environment and the lack of deformation is probably due to rapid uplift during the later stages of the impact event

post impact deposits in tvaren a marine middle ordovician crater south of stockholm sweden the well preserved tvaren crater is noteworthy for being one of a small number of early and middle ordovician impact structures formed in a marine environment it is demonstrated to be an impact structure by the presence of a breccia lens consisting of crystalline basement rocks and shocked quartz the breccia lens formed under dry hot conditions after expulsion of sea water by the impact resurging sea water thereupon deposited a positively graded 60 m thick turbidite like unit this graded resurge deposit is a previously unknown feature to be expected in open sea impacts breccia in the lower part of this graded deposit contains fragments of a remarkably complete orthoceratite limestone succession that existed at the site of impact resting on non lithified sand of probably early to earliest middle cambrian age a sedimentary succession was deposited inside the crater at depths decreasing from more than 200 m in the initial stages to some 100 m at the time of deposition of the youngest preserved beds the environment within the crater thus favoured deposition of an essentially complete stratigraphic succession with depth controlled palaeoecology for a significant time interval after the impact whereas planktonic members like graptolites and chitinozoa are present throughout the post impact succession and asaphids almost as persistent became established at an early date burrowers were somewhat reluctant to enter and remopleuridids and small strophomenids came in at a late stage we suggest as a result of this study that structures formed by impact may offer unique information about the palaeogeology and palaeoenvironment of the region hit by the impact

palaeomagnetism and the siljan impact structure central sweden summary the siljan impact structure diameter c 52 km located in central sweden is considered the largest meteorite crater in western europe the time of impact has been estimated to be 368 ma from ar ar dating of an impact melt a palaeomagnetic study of rocks from inside and outside the impact structure has been carried out with the aim of identifying natural remanent magnetizations nrms related to the impact event primary magnetizations are partly preserved within the impact area however the directions of these original magnetizations are disturbed due to tectonic movements related to the impact three different directions of magnetizations not recognized elsewhere were identified within the central part of the structure these three magnetizations are defined in order of increasing coercivity hc and blocking temperature tb the origin of a high hc chemical remanent magnetization crm c of late palaeozoic time carried by haematite can be interpreted as due to oxidation caused by circulating meteoric water in cracks and fractures caused by the impact another component b identified in a lower temperature range 40d 580dc is probably carried by magnetite in pseudotachylite this magnetization is considered as a thermal remanent magnetization while in other rocks its pre tectonic character and occurrence also in the low coercivity range implies a shock remanent origin the palaeomagnetic pole for this remanence corresponds to a silurian devonian age which gives an age of impact which is not very diffirent from that estimated by radiometric data a third low hc and tb component a is interpreted to be a crm carried by slowly growing goethite the direction of this crm implies it to be a recent post impact magnetization

deep drilling at the siljan ring impact structure oxygen isotope geochemistry of granite the siljan ring is a 362 ma old impact structure formed in 1700 ma old i type granites a 6 8 km deep borehole provides a vertical profile through granites and isolated horizontal diabase sills fluid inclusion thermometry and oxygen isotope compositions of vein quartz granite diabase impact melt and pseudotachylite reveal a complex history of fluid activity in the siljan ring much of which can be related to the meteorite impact in granites from the deep borehole 18o values of matrix quartz increase with depth from near 8 0 at the surface to 9 5 at 5760 m depth in contrast feldspar 18o values decrease with depth from near 10 at the surface to 7 1 at 5760 m forming a pattern opposite to the one defined by quartz isotopic compositions values of 18o for surface granites outside the impact structure are distinct from those in near surface samples from the deep borehole in the deep borehole feldspar coloration varies from brick red at the surface to white at 5760 m and the abundances of crack healing calcite and other secondary minerals decrease over the same interval superimposed on the overall decrease in alteration intensity with depth are localized fracture zones at 4662 5415 and 6044 m depth that contain altered granites and which provided pathways for deep penetration of surface water the antithetic variation of quartz and feldspar 18o values which can be correlated with mineralogical evidence of alteration provides evidence for interaction between rocks and impact heated fluids 100 300 c in the upper 2 km of the pluton penetration of water to depths below 2 km was restricted by a general decrease in impact fracturing with depth and by a 60 m thick diabase sill at 1500 m depth that may have been an aquitard at depths below 4 km in the pluton where water rock ratios were low oxygen isotopic compositions preserve evidence for limited high temperature 500 c exchange between alkali feldspar and fluids the high temperature exchange may have been a post impact event involving impact heated fluids or a post magmatic event

the temperature of formation of carbonate in martian meteorite alh84001 constraints from cation diffusion we have measured the rates of chemical diffusion of mg in calcite and ca in magnesite and used these new data to constrain the formation temperature and thermal history of carbonates in the martian meteorite alh84001 our data have been collected at lower temperatures than in previous studies and provide improved constraints on carbonate formation during relatively low temperature processes 400 c mea sured log d0 values for chemical diffusion of mg in calcite and ca in magnesite are 216 0 6 1 1 and 27 8 6 4 3 m 2 s and the activation energies ea are 76 6 16 and 214 6 60 kj mol respectively measured diffusion rates of mg in calcite at temperatures between 400 and 550 c are substantially faster than expected from extrapolation of existing higher temperature data suggesting that different mechanisms may govern diffusion of mg at temperatures above and below 550 c we have used these data to constrain thermal histories which will allow the 1 mm variations in ca mg composition in alh84001 carbonates to survive homogenization by diffusion our results are generally consistent with models for formation of carbonates in alh84001 at low temperatures for initial cooling rates of between 10 21 and 10 3 ma our results demonstrate that carbonates formed at temperatures no higher than 400 c and most probably less than 200 c this range of cooling rates is similar to those observed within the terrestrial crust and suggests that formation of the carbonates by igneous metamorphic or hydrothermal or other processes in the martian crust most plausibly occurred at temperatures below 200 to 400 c models that suggest alh84001 carbonates formed during a martian impact event are also constrained by our data the thermal histories of terrestrial impact structures suggest that cooling rates sufficiently rapid to allow preser vation of the observed carbonate zoning at formation temperatures in excess of 600 c 10 7 c ma occur only within the uppermost melt rich portions of an impact structure material deeper within the impact structure where cooling is dominated by uplifted crustal material and where much of the formation of hydrothermal minerals is concentrated cools much slower typically at rates of 10 2 to 10 3 ma carbonates formed within this region would also only preserve 1 mm compositional zoning at formation temperatures of less than 200 to 400 c copyright 2001 elsevier science ltd

impacts tillites and the breakup of gondwanaland a discussion oberbeck et al 1993 have proposed that some ancient sedimentary rocks such as diamictites and associated deposits widely interpreted as glacigenic should be reassessed in the light of recent advances in the understanding of impact related phenomena such radical departures from accepted interpretations cf deitz 1964 can stimulate review of long accepted more traditional ideas and thus provide a catalyst for new avenues of research the proposed reinterpretations have potentially far reaching consequences for example the possible connections between long term climatic change and tectonic history of the earth fischer 1984 worsley et al 1984 young 1991 would be brought into question some aspects of atmospheric evolution hart 1978 interpretation of stable isotope distribution curves and ideas on the evolution of life knoll 1991 have been linked to inferences about cold climatic episodes glaciations based primarily on study of the rock record in the last two decades following publication of the benchmark paper by schermerhorn 1974 glacial specialists have been particularly circumspect in their interpretations and have developed what they consider to be a reliable set of criteria for the recognition of glacial deposits the following remarks are offered in an effort to reaffirm the validity of some of these criteria and to suggest that the proposed impact theory for the origin of many diamictite bearing sequences is correspondingly

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

polygonal impact craters on dione evidence for tectonic structures outside the wispy terrain abstract plan view impact crater geometries can be indicative of pre impact structures within the target material impact events that occur on a pre fractured surface generate craters exhibiting one or more straight rim segments termed polygonal impact craters pics impact craters that appear to be pics are abundant on the surface of saturn s icy satellite dione both within the wispy terrain a region with large visible fractures and also outside the wispy terrain the non wispy terrain where less evidence for fracturing has been observed in the non wispy terrain subtle lineaments are hypothesized to be ne sw nw se and e w trending fractures suggesting that tectonism may have been an important process in this terrain results of previous studies have shown that pic straight rim segment azimuths pic azimuths commonly parallel pre impact fracture azimuths although disagreements about this relationship exist in the literature we investigated the hypothesis that fractures either subtle or nonvisible with available spacecraft images are present within dione s non wispy terrain our first step was to assess the relationship between pics and pre existing fracture azimuths in the wispy terrain our results from this initial assessment show a parallel relationship between pic azimuths and fracture azimuths based on this correlation in the wispy terrain we find it likely that this relationship would hold true in the non wispy terrain if pics are present we tested for pics using crater rim azimuth data collected from randomly distributed study locations throughout the non wispy terrain from these data we identified widespread pics in this terrain which supports the hypothesis that subtle fractures are also present analysis of the pic azimuth data yield a pattern for these inferred fractures across dione s surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion our results provide evidence for these previously hypothesized events in dione s history and demonstrate that mapping pics and their azimuths is a useful tool for investigating subtle fractures on solar system bodies

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

apollo 16 site geology and impact melts implications for the geologic history of the lunar highlands the geology of the apollo 16 site is reconsidered on the basis of data from photogeology geochemical remote sensing and lunar samples the site possesses an upper surface of anorthositic gabbro and related rocks mafic components were deposited as basin ejecta the events involved in its geological evolution were the nectaris impact and the imbrium impact the role of large local craters in the history of the region was to serve as topographic depressions to accumulate basin ejecta the most abundant melt composition at apollo 16 is an aluminous variety of lkfm basalt supplied by the nectaris impact as ejected basin impact melt the mafic lkfm melt may have been supplied by the imbrium impact more aluminous melt groups are probably derived from local small craters the remainder of the deposits in the region are composed of anorthositic clastic debris derived from the nectaris basin the local crustal substrate and imbrium and other basins

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

accretion rates of meteorites and cosmic dust in the early ordovician abundant fossil meteorites in marine condensed lower ordovician limestones from kinnekulle sweden indicate that accretion rates of meteorites were one to two orders of magnitude higher during an interval of the early ordovician than at present osmium isotope and iridium analyses of whole rock limestone indicate a coeval enhancement of one order of magnitude in the influx rate of cosmic dust enhanced accretion of cosmic matter may be related to the disruption of the l chondrite parent body around 500 million years ago

the distribution of megablocks in the ries crater germany remote sensing field investigation and statistical analyses the ries crater is a well preserved complex impact crater that has been extensively used in the study of impact crater formation processes across the solar system however its geologic structure especially the megablock zone still poses questions regarding crater formation mechanics the megablock zone located between the inner crystalline ring and outer morphologic crater rim consists of allochthonous crystalline and sedimentary blocks bunte breccia deposits patches of suevite and parautochthonous sedimentary blocks that slumped into the crater during crater modification our remote sensing detection method in combination with a shallow drilling campaign and geoelectric measurements at two selected megablocks proved successful in finding new megablock structures 25 m mean diameter within the upper approximately 1 5 m of the subsurface in the megablock zone we analyzed 1777 megablocks of the megablock zone 81 of which are new discoveries in our statistical analysis we also included 2318 ejecta blocks 25 m beyond the crater rim parautochthonous megablocks show an increase in total area and size toward the final crater rim the sizes of allochthonous megablocks generally decrease with increasing radial range but inside the megablock zone the coverage with postimpact sediments obscures this trend the size frequency distribution of all megablocks obeys a power law distribution with an exponent between approximately 1 7 and 2 3 we estimated a total volume of 95 km3 of bunte breccia and 47 km3 of megablocks ejecta volume calculations and a palinspastic restoration of the extension within the megablock zone indicate that the transient cavity diameter was probably 14 15 km

rb sr sm nd k ca o and h isotopic study of cretaceous tertiary boundary sediments caravaca spain evidence for an oceanic impact site isotopic ratios and trace element abundances were measured on samples of ir enriched clay at the cretaceous tertiary boundary and in carbonate and marl from 5 cm below and 3 cm above the boundary samples were leached with acetic acid to remove carbonate and with hydrochloric acid leachates and residues were measured the sr nd o and h isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials the data indicate that most of the clay material was derived from a terrestrial source with relatively low87sr 86sr and high143nd 144nd ratios the 18o data suggest that the detritus has been modified by submarine weathering k ca and rb sr systematics as well as o isotope ratios of k feldspar spherules within the boundary clay suggest that they are predominantly authigenic and may have formed after the time of deposition however sm nd and rb sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus the isotopic evidence for foreign terrestrial detritus in the boundary clay the low rare earth element concentrations and high ni concentration support the hypothesis of a terminal cretaceous asteroidal impact that produced a global layer of fallout the data are most easily explained if the impact site was on oceanic crust rather than continental crust and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere 3 km this would probably require a single large impactor

late eocene microtektites and radiolarian extinctions on barbados a late eocene microtektite layer has been found in three exposed sections on barbados west indies these microtektites and associated tektite fragments are compositionally similar to the north american tektites and thus like the caribbean and gulf of mexico microtektites may belong to the strewn field of north american microtektites which have been found in deep sea deposits from the caribbean sea across the pacific and into the northeastern indian ocean1 2 the north american layer has been reported to be approximately synchronous with several radiolarian extinctions3 4 and with an anomaly in iridium concentrations5 7 and it has been hypothesized that the microtektites the extinctions and the iridium anomaly are all related to an asteroid impact we report here that the radiolarian extinctions predate the microtektite layer and that the highest ir concentrations apparently coincide with the extinctions thus although the extinctions may be causally related to the ir anomaly the microtektites appear not to be

cosmic impact versus terrestrial origin of the azuara structure spain a review the azuara structure is the largest one proposed so far in spain as possibly related to a cosmic impact event a review of the evidence set forward in favor of and against its cosmic origin indicates that the discussion is not yet finished some megascopic features inverted stratigraphy megabreccia negative gravity anomalies and shock metamorphic effects planar deformational features have been described in relation with the structure although their real significance has been questioned and is still being debated comparison with other similar sized verified impacts suggests that unequivocal impactogenic features are yet to be found before the azuara structure can be related to a cosmic impact until then the azuara structure should be considered as an unverified impact structure and should not be included in global comprehensive maps of terrestrial impact structures

new evidence on the size and possible effects of a late pliocene oceanic asteroid impact debris from a late pliocene asteroid impact is spread across at least 600 kilometers of the ocean floor in the southeast pacific on the basis of iridium concentrations in sediments from six deep sea cores the asteroid diameter was at least 0 5 kilometer the impacting projectile may have been one of the largest in the last few million years the stratigraphic age of this impact is the same as that inferred for the onset of northern hemisphere glaciation

kardla hiiumaa island estonia the buried and well preserved ordovician marine impact structure abstract the kardla marine impact structure estonia 58 58 n 22 46 e was formed at 455 ma upper ordovician in a shallow epicontinental sea some tens of kilometres from the land and erosion area the iron rich projectile about 200 m in diameter approached from the west at an angle of 30 45 the impactor penetrated about 50 m thick water layer and the sedimentary cover and exploded in the uppermost part of the crystalline basement a complex crater 4 km wide and about 500 m deep with a central uplift rising 130 m from the crater floor was formed the highest point of the rimwall is 110 m above the target level the rimwall is cut by at least two resurge excavated gullies the variable height of the rimwall obviously results from the obliqueness of the impact outside the crater an elliptical area was revealed 12 15 km in diameter with deformed sedimentary rocks below the target level the elliptical shape of this area may also be due to the oblique impact because the crater and its surroundings were buried directly after the impact the whole complex of impact related sediments is preserved there they are recovered by 160 wells six of which penetrate the entire complex of impact breccias inside the crater

a perspective on the evidence for periodic cometary impacts on earth abstract recent suggestions of periodic cometary showers coincident with mass extinctions have prompted a review of the terrestrial cratering record from the point of view of completeness of search and crater retention the record used to argue for statistical periodicities is incomplete and may not be representative estimates of crater ages are of variable accuracy and precision and in some cases different dating methods yield discordant results it is possible to define through time series analysis a number of periodicities of different magnitude 29 m y 21 m y 18 5 m y 13 5 m y and phase depending on which craters are considered the most representative sample of the record arguing against periodic cometary showers is the observation that estimates of the average cratering rate in recent time and observations on apollo bodies are equivalent in addition siderophile element data from impact melt rocks suggest a variely of projectile types were responsible for relatively large terrestrial craters given these observations it is concluded that statements regarding periodic cometary showers are not categorical and require evidence beyond arguments based on time series analyses of an incomplete cratering record

on the significance of crater ages new ages for dellen sweden and araguainha brazil samples of a few grams in weight from the impact melt at the dellen crater sweden and the araguainha dome brazil were analyzed by rb sr and sm nd techniques for dellen pure labradorite orthopyroxene and glass separates from a clast free impact melt rock with holohyaline ground mass yield an internal rb sr isochron with 89 0 2 7 ma isr 0 83571 0 00007 2 compared to ar ar plateau ages of 102 3 2 ma and 109 6 2 0 ma published for total melt rocks various whole rock values not plotting on this rb sr isochron indicate primary variations in 87sr86sr over the melt sheet the age of the araguainha dome brazil is bracketed by the rb sr model age of 243 19 ma isr 0 7226 0 0005 for altered cordierite in the impact melt and by the internal rb sr isochron age of 449 9 ma isr 0 7140 0 0007 for an uplifted granite shock stage ib in melt rocks from both impact structures 143nd144nd was not homogenized during total rock melting sm nd data either scatter around the primary isochron araguainha or point to a partial resetting dellen this data array may be caused by tiny refractory ree rich inclusions in the main mineral phases which dominate the sm nd system of the impact melt rocks the significant difference between the new isochron age for dellen and ar ar ages show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy

an in depth look at the lunar crater copernicus exposed mineralogy by high resolution near infrared spectroscopy abstract newly acquired sequentially spaced high resolution near infrared spectra across the central section of crater copernicus interior have been analyzed using a range of complementary techniques and indexes we have developed a new interpretative method based on a multiple stage normalization process that appears to both confirm and expand on previous mineralogical estimations and mapping in broad terms the interpreted distribution of the principle mafic species suggests an overall composition of surface materials dominated by calcium poor pyroxenes and minor olivine but with notable exceptions the southern rim displays strong ca rich pyroxene absorption features and five other locations the uppermost northern crater wall opposite rim sections facing the crater floor and the central peak pk1 and at the foot of pk3 show instead strong olivine signatures we also propose impact glass an alternative interpretation to the source of the weak but widespread olivine like spectral signature found in low reflectance samples since it probably represents a major regolith constituent and component in large craters such as copernicus the high quality and performance of the sir 2 data allows for the detection of diagnostic key mineral species even when investigating spectral samples with very subdued absorption features confirming the intrinsic high quality value of the returned data

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

geology of the early palaeozoic lockne impact structure central sweden abstract the diameter of the crater at lockne as seen on topographic maps is 7 8 km the tandsbyn breccia the principal evidence of impact consists of strongly crushed local basement its distribution follows the crater margin on the north margin basement granite rests on an inverted sub cambrian erosion surface on the lowermost middle cambrian tangential faults occur at the periphery of the structure after the impact occurred under the sea during the middle ordovician there was a resurgence of ejecta loaded sea water which deposited a chaotic breccia lockne breccia which has the appearance of a debris flow the lockne breccia together with a less coarsely grained turbidite loftarsten which lies immediately above it predominantly consist of clasts of lower to middle ordovician limestone and mainly subordinate inclusions of tandsbyn breccia and isolated clasts of crystalline basement fragments of impact melt are important components of the turbidite a protective cover of sediments formed after the impact is in turn overlain by an outlier of a caledonian overthrust nappe which occupies the central and topographically deepest part of the crater shocked quartz has been identified in the loftarsten

sudden death at the end of the mesozoic abstract a paleoecological analysis of the fossil record before and after the cretaceous tertiary boundary indicates that the widespread extinctions and biological stresses around the boundary are best explained in terms of a sudden significant but short temperature rise l alvarez and co authors having found an enrichment in iridium at the same boundary postulated that it was associated with the impact of an extraterrestrial body if this body struck the ocean the water injected into the atmosphere may have led to a transient increase in the global surface temperature this temperature pulse may have been primarily responsible for the effects observed in the biosphere the pattern of extinction of higher plant species suggests that splash down occurred in the northern pacific bering sea area

are impact generated lower crustal faults observable abstract theoretical calculations coupled with present day knowledge of the dynamic tensile behaviour of rock indicate that the propagating shock wave and accompanying rarefaction wave from a large meteoritic impact event may fracture the crust down to the moho diapiric action by trapped upper mantle fluids could lead to annealing of these sub vertical faults by material of high carbon content are these faults observable on the surface using geophysical methods one possible means proposed is to try to monitor current flowing down them by an appropriate natural source electromagnetic experiment an example is discussed for which such current flow may be occurring

a geometric model for excavation and modification at terrestrial simple impact craters a geometric model for the excavation and modification of simple crater development is presented modification is modelled by considering the relative dimensions and geometries of the final crater and the transient cavity formed by excavation and displacement the input parameters required are the rim crest diameter of the final crater the depth to autochthonous basement the rim crest height and the outer rim slope the model calculates the diameter of the final crater at the original ground plane the dimensions of the transient cavity and the rim crest volume difference between the final and transient cavities this volume is considered to be the material which slumped off the unstable overheightened transient cavity wall during modification and corresponds to the observed volume of the breccia lens within the final crater excavation is approximated by a steady state z model with the diameter of excavation equivalent to the previously modelled diameter of the transient cavity at the original ground plane the center of flow taken as 1 projectile diameter and z 2 5 3 0 the z model predicts such parameters as the depth and volume of excavation which can be compared with observational data the applicability of the model has been tested with data from meteor and brent craters in the case of meteor the modelled final crater diameter at the original ground plane is within 26 m of the observed value and the modelled breccia lens and the rim crest volumes of the final true crater are within 9 5 and 2 5 respectively of observed values the results of the z model for meteor crater with z 2 8 0 1 give results consistent with observational data on the depth and volume of excavation the correspondence for the more degraded brent crater is less precise as there is greater uncertainty in the input parameters it is possible however to match the model parameters with the breccia lens volume and published estimates of original dimensions the modelled excavated volumes for meteor and brent are 58 and 55 respectively of the transient cavity volume below the original ground plane in the absence of additional observational data for further testing it is concluded that the model may provide first order spatial information on the formation of simple craters