precipitation of dolomite using sulphate reducing bacteria from the coorong region south australia significance and implications dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral hypersaline dolomitic lakes of the coorong region south australia analyses of lake and pore water samples document rapid geochemical changes with time and depth in both dolomitic and non dolomitic lakes extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate reduction zone whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water ultimately sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments most probable number calculations of lake sediment samples record the presence of large populations of sulphate reducing bacteria whereas sulphur isotope analyses of lake water samples indicate microbial fractionation in all the lakes studied viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory samples were then injected into vials containing sterilized clastic or carbonate grains or glass beads immersed in a solution that simulated the lake water falls in the levels of sulphate and rising ph in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate within 2 months sub spherical sub micron size crystals of dolomite identical to those of lake sediments were precipitated it is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs and by generating elevated carbonate concentrations in a hypersaline strongly electrolytic solution the results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments it is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue applicable to thick platformal dolostones of the past where benthic microbial communities were the sole or dominant colonizers of shallow marine environments

numerical modeling of asteroid survivability and possible scenarios for the morokweng crater forming impact the fate of the impactor is an important aspect of the impact cratering process defining impactor material as surviving if it remains solid i e does not melt or vaporize during crater formation previous numerical modeling and experiments have shown that survivability decreases with increasing impact velocity impact angle with respect to the horizontal and target density here we show that in addition to these impactor survivability depends on the porosity and shape of the impactor increasing impactor porosity decreases impactor survivability while prolate shaped polar axis equatorial axis impactors survive impact more so than spherical and oblate shaped polar axis equatorial axis impactors these results are used to produce a relatively simple equation which can be used to estimate the impactor fraction shocked to a given pressure as a function of these parameters by applying our findings to the morokweng crater forming impact we suggest impact scenarios that explain the high meteoritic content and presence of unmolten fossil meteorites within the morokweng crater in addition to previous suggestions of a low velocity and or high angled impact this work suggests that an elongated and or low porosity impactor may also help explain the anomalously high survivability of the morokweng impactor

melt in the impact breccias from the eyreville drill cores chesapeake bay impact structure usa the center of the 35 3 ma chesapeake bay impact structure 85 km diameter was drilled during 2005 2006 in an icdp usgs drilling project the eyreville drill cores include polymict impact breccias and associated rocks 1397 1551 m depth tens of melt particles from these impactites were studied by optical and electron microscopy electron microprobe and microraman spectroscopy and classified into six groups m1 clear or brownish melt m2 brownish melt altered to phyllosilicates m3 colorless silica melt m4 melt with pyroxene and plagioclase crystallites m5 dark brown melt and m6 melt with globular texture these melt types have partly overlapping major element abundances and large compositional variations due to the presence of schlieren poorly mixed melt phases partly digested clasts and variable crystallization and alteration the different melt types also vary in their abundance with depth in the drill core based on the chemical data mixing calculations were performed to determine possible precursors of these melt particles the calculations suggest that most melt types formed mainly from the thick sedimentary section of the target sequence mainly the potomac formation but an additional crystalline basement schist gneiss precursor is likely for the most abundant melt types m2 and m5 sedimentary rocks with compositions similar to those of the melt particles are present among the eyreville core samples therefore sedimentary target rocks were the main precursor of the eyreville melt particles however the composition of the melt particles is not only the result of the precursor composition but also the result of changes during melting and solidification as well as postimpact alteration which must also be considered the variability of the melt particle compositions reflects the variety of target rocks and indicates that there was no uniform melt source original heterogeneities resulting from melting of different target rocks may be preserved in impactites of some large impact structures that formed in volatile rich targets because no large melt body exists in which homogenization would have taken place

dynamic modeling suggests terrace zone asymmetry in the chicxulub crater is caused by target heterogeneity we investigate the cause of terrace zone asymmetry in the chicxulub impact crater using dynamic models of crater formation marine seismic data acquired across the crater show that the geometry of the crater s terrace zone a series of sedimentary megablocks that slumped into the crater from the crater rim varies significantly around the offshore half of the crater the seismic data also reveal that at the time of impact both the water depth and sediment thickness varied with azimuth around the impact site to test whether the observed heterogeneity in the pre impact target might have affected terrace zone geometry we constructed two end member models of upper target structure at chicxulub based on the seismic data at different azimuths one model representing the northwest sector had no water layer and a 3 km thick sediment layer the other model representing the northeast sector had a 2 km water layer above a 4 km sediment layer numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at chicxulub moreover the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the chicxulub crater we conclude that asymmetry in the pre impact target rocks at chicxulub is likely to be the primary cause of asymmetry in the terrace zone

the lomonosov crater impact event a possible mega tsunami source on mars recent research suggests that major meteorite impact events into a late hesperian early amazonian ocean likely produced a mega tsunami that would have resurfaced coastal areas in northwestern arabia terra the orientations of the associated lobate deposits a conspicuous type of landforms called thumbprint terrain suggests that if an impact event triggered the mega tsunami the most likely location of the source crater is within the northern plains regions situated north of arabia terra this study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami we selected 10 complex impact craters based on their diameters location and geomorphic characteristics of those the late hesperian 120 km diameter lomonosov crater exhibits a unique topographic plan view asymmetry compared to other similar sized and similar aged craters in the northern plains such as micoud korolev and milankovic we attribute its broad and shallow rim in part to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity the likely marine formation of the lomonosov crater and the apparent agreement in its age with that of the thumbprint terrain unit 3 ga strongly suggests that it was the source crater of the tsunami these results have implications for the stability of a late northern ocean on mars

discovery of high pressure zrsio4 polymorph in naturally occurring shock metamorphosed zircons shock metamorphosed zircons were recovered from an upper eocene impact ejecta layer probably from the chesapeake bay impact structure found at three deep sea drilling project ocean drilling program sites 612 903 and 904 on the upper continental slope off new jersey the shocked zircons exhibit granular textures and some have planar features x ray diffraction patterns show that some of the shocked zircons contain a high pressure zrsio4 polymorph with a scheelite type structure that was first produced in laboratory experiments in 1969 the presence of the high pressure zrsio4 polymorph in some of the zircons indicates that they have been shock metamorphosed at pressures between 20 and 90 gpa the discovery of this phase in naturally occurring shocked zircons confirms previous predictions that this phase would be found in shock metamorphosed rocks from impact structures and it provides a new tool for calibrating peak shock pressures in impact structures and ejecta in addition because the high pressure zrsio4 polymorph has now been found in naturally occurring samples it can be given a mineral name

impact crater formation in icy layered terrains on mars we present numerical simulations of crater formation under martian conditions with a single near surface icy layer to investigate changes in crater morphology between glacial and interglacial periods the ice fraction thickness and depth to the icy layer are varied to understand the systematic effects on observable crater features to accurately model impact cratering into ice a new equation of state table and strength model parameters for h2o are fitted to laboratory data the presence of an icy layer significantly modifies the cratering mechanics observable features demonstrated by the modeling include variations in crater morphometry depth and rim height and icy infill of the crater floor during the late stages of crater formation in addition an icy layer modifies the velocities angles and volumes of ejecta leading to deviations of ejecta blanket thickness from the predicted power law the dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials our simulations suggest that many of the unusual features of martian craters may be explained by the presence of icy layers including shallow craters with well preserved ejecta blankets icy flow related features some layered ejecta structures and crater lakes therefore the cratering record implies that near surface icy layers are widespread on mars

calculations of asteroid impacts into deep and shallow water contrary to received opinion ocean impacts of small 500 m asteroids do not produce tsunamis that lead to world wide devastation in fact the most dangerous features of ocean impacts just as for land impacts are the atmospheric effects we present illustrative hydrodynamic calculations of impacts into both deep and shallow seas and draw conclusions from a parameter study in which the size of the impactor and the depth of the sea are varied independently for vertical impacts at 20 km s craters in the seafloor are produced when the water depth is less than about 5 7 times the asteroid diameter both the depth and the diameter of the transient crater scale with the asteroid diameter so the volume of water excavated scales with the asteroid volume about a third of the crater volume is vaporised because the kinetic energy per unit mass of the asteroid is much larger than the latent heat of vaporisation of water the vaporised water carries away a considerable fraction of the impact energy in an explosively expanding blast wave which is responsible for devastating local effects and may affect worldwide climate of the remaining energy a substantial portion is used in the crown splash and the rebound jet that forms as the transient crater collapses the collapse and rebound cycle leads to a propagating wave with a wavelength considerably shorter than classical tsunamis being only about twice the diameter of the transient crater propagation of this wave is hindered somewhat because its amplitude is so large that it breaks in deep water and is strongly affected by the blast wave s perturbation of the atmosphere even if propagation were perfect however the volume of water delivered per metre of shoreline is less than was delivered by the boxing day 2004 tsunami for any impactor smaller than 500 m diameter in an ocean of 5 km depth or less near field effects are dangerous for impactors of diameter 200 m or greater hurricane force winds can extend tens of kilometers from the impact point and fallout from the initial splash can be extremely violent there is some indication that near field effects are more severe if the impact occurs in shallow water

shock metamorphism investigations of quartz grains in clasts from impact breccia of the eyreville b drill core chesapeake bay impact structure usa abstract the chesapeake bay impact structure approximately 85 km in diameter has been drilled in 2005 2006 at eyreville virginia usa to a total depth of 1766 m in the drill cores the abundance of shock metamorphosed material is very variable with depth shocked mineral and lithic clasts as well as melt particles are most abundant in suevitic impact breccia section 1397 1451 m depth shocked quartz i e quartz grains with planar fractures and or planar deformation features and melt particles although rare are also dispersed in the exmore formation unit 444 867 m depth other lithologies in the eyreville drill cores show no clear evidence of shock metamorphism here we report on the investigations of 40 samples from the impact breccia section a total of more than 27 000 quartz grains were examined in about 200 clasts the abundance of highly shocked clasts tends to decrease with increasing depth crystalline clasts derived from the crystalline basement are commonly only slightly shocked contain generally 10 rel of shocked quartz grains the clasts of metamorphosed sediments show a low proportion of shocked quartz grains mostly 10 rel sedimentary clasts show a wide range of proportions of shocked quartz grains with several of them being highly shocked clasts most values between 0 and 40 rel conglomerates show the highest proportion of shocked quartz grains of all types of clasts up to 83 rel polycrystalline quartz clasts are also commonly highly shocked contain mostly between 10 and 40 rel of shocked quartz grains these hard nonporous clasts are possibly more liable to show evidence of shock the investigations suggest that the intensity of shock metamorphism is the result of several parameters such as original position in the target both horizontal and vertical and the properties of each lithology e g grain size porosity and amount of matrix according to the universal stage investigations the dominant orientations of planar deformation features in quartz are and also

magnetic fabric of araguainha complex impact structure central brazil implications for deformation mechanisms and central uplift formation article i nfo the weakening mechanisms involved in the collapse of complex impact craters are controversial the ara guainha impact crater in brazil exposes a complex structure of 40 km in diameter and is an excellent object to address this issue its core is dominated by granite in addition to microstructural observations magnetic studies reveal its internal fabric acquired during the collapse phase all granite samples exhibit impact related planar deformation features pdfs and planar fractures pfs which were overprinted by cataclasis cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure fracture planes are systematically decorated by tiny grains b10 m of magnetite and hematite and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility ams and anhysteretic remanence aar are perfectly coaxial in all studied sites therefore we could track the orientation of deformation features which are decorated by iron oxides using the ams and aar the magnetic fabrics show a regular pattern at the borders of the cen tral peak with orientations consistent with the fabric of sediments at the crater s inner collar and complex in the center of the structure both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex im pact structures the widespread occurrence of cataclasis in the central peak and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales including the mineral scales the cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core

scaling and reproducibility of craters produced at the experimental projectile impact chamber epic centro de astrobiologia spain the experimental projectile impact chamber epic is a specially designed facility for the study of processes related to wet target e g marine impacts it consists of a 7 m wide funnel shaped test bed and a 20 5 mm caliber compressed n2 gas gun the target can be unconsolidated or liquid the gas gun can launch 20 mm projectiles of various solid materials under ambient atmospheric pressure and at various angles from the horizontal to test the functionality and quality of obtained results by epic impacts were performed into dry beach sand targets with two different projectile materials ceramic al2o3 max velocity 290 m s 1 and delrin max velocity 410 m s 1 23 shots used a quarter space setting 19 normal 4 at 53 from horizontal and 14 were in a half space setting 13 normal 1 at 53 the experiments were compared with numerical simulations using the isale code differences were seen between the nondisruptive al2o3 ceramic and the disruptive delrin polymer projectiles in transient crater development all final crater dimensions when plotted in scaled form agree reasonably well with the results of other studies of impacts into granular materials we also successfully validated numerical models of vertical and oblique impacts in sand against the experimental results as well as demonstrated that the epic quarter space experiments are a reasonable approximation for half space experiments altogether the combined evaluation of experiments and numerical simulations support the usefulness of the epic in impact cratering studies

structural outer rim of chesapeake bay impact crater seismic and bore hole evidence nine seismic reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the chesapeake bay impact crater the rim is manifested as a 90 km diameter ring of terraced normal fault blocks which forms a 320 m 1200 m high rim escarpment the top of the rim escarpment is covered by a 20 m 30 m thick ejecta blanket the escarpment encircles a flat floored annular trough which is partly filled with an 250 m thick breccia lens exmore breccia the exmore breccia overlies a 200 m 800 m thick interval of slumped sedimentary megablocks which in turn rests on crystalline basement rocks all postimpact strata upper eocene to quaternary sag structurally into the annular trough and most units also thicken as they cross the rim into the crater postimpact compaction and subsidence of the exmore breccia have created extensive normal faulting in overlying strata

non destructive analysis of amber artefacts from the prehistoric cioclovina hoard romania abstract amber beads from the cioclovina hoard hunedoara county romania dating from the transitional period between the late bronze age and the iron age were analysed by fourier transform infrared spectroscopy variable angle reflectance ftir var and fourier transform raman spectroscopy ft raman the aim was to determine if the raw material came from the shores of the baltic sea on the amber route which crosses central europe or if it originated from the area of the main amber occurrences in romania in buz u county around the village of col i all applied analyses strongly suggest that a large part of the amber from the cioclovina deposit has a romanian origin and thus no connection with the amber route

impact of a cosmic body into earth s ocean and the generation of large tsunami waves insight from numerical modeling 1 the strike of a cosmic body into a marine environment differs in several respects from impact on land oceans cover approximately 70 of the earth s surface implying not only that oceanic impact is a very likely scenario for future impacts but also that most impacts in earth s history must have happened in marine environments therefore the study of oceanic impact is imperative in two respects 1 to quantify the hazard posed by future oceanic impacts including the potential threat of large impact generated tsunami like waves and 2 to reconstruct earth s impact record by accounting for the large number of potentially undiscovered crater structures in the ocean crust reconstruction of the impact record is of crucial importance both for assessing the frequency of collision events in the past and for better predicting the probability of future impact we summarize the advances in the study of oceanic impact over the last decades and focus in particular on how numerical models have improved our understanding of cratering in the oceanic environment and the generation of waves by impact we focus on insight gleaned from numerical modeling studies into the deceleration of the projectile by the water cratering of the ocean floor the late stage modification of the crater due to gravitational collapse and water resurge furthermore we discuss the generation and propagation of large tsunami like waves as a result of a strike of a cosmic body in marine environments

subsurface damage from oblique impacts into low impedance layers 1 layered planetary surfaces occur ubiquitously in the solar system where sedimentary sequences or icy layers overlay crystalline bedrock previous experimental studies investigated how the presence of weak layer overlying a strong basement affects crater morphology subsurface damage and soft sediment compression numerical studies generally focus on the final morphology as a function of thicknesses and burial depths of weak layers in field studies of impact craters the shock state of minerals is a key metric here we evaluate the effect of a surficial low impedance layer on peak pressure magnitudes and consequent damage extent in the competent substrate laboratory experiments coupled with 3d cth models of oblique 30 from horizontal hypervelocity impacts at laboratory and planetary scales show that surface layers with a thickness on the order of the projectile diameter shield the underlying surface and absorb scatter 70 of the impact energy numerical simulations reveal that surficial layers reduce peak pressure magnitudes within the subsurface by 60 70 while damage in the substrate is due to shear failure sedimentary layers are more efficient shields than icy layers but both reduce the extent of subsurface damage and the resulting shock levels recorded by minerals these results indicate that a thin surficial low impedance layer mitigates the expression of shocked minerals in the substrate even when a structural response is still observed

post impact structural crater modification due to sediment loading an overlooked process post impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well preserved impact craters mjolnir chesapeake bay chicxulub montagnais and bosumtwi the analysis demonstrates that the geometry and the structural and stratigraphic relations of post impact strata provide information about the amplitude the spatial distribution and the mode of post impact deformation reconstruction of the original morphology and structure for the mjolnir chicxulub and bosumtwi craters demonstrates the long term subsidence and differential compaction that takes place between the crater and the outside platform region and laterally within the crater structure at mjolnir the central high developed as a prominent feature during post impact burial the height of the peak ring was enhanced and the cumulative throw on the rim faults was increased the original chicxulub crater exhibited considerably less prominent peakring and inner ring crater rim features than the present crater the original relief of the peak ring was on the order of 420 570 m currently 535 575 m the relief on the inner ring crater rim was 300 450 m currently 700 m the original bosumtwi crater exhibited a central uplift high whose structural relief increased during burial current height 101 110 m in contrast to the original height of 85 110 m whereas the surrounding western part of the annular trough was subdued more that the eastern part exhibiting original depths of 43 68 m currently 46 m and 49 55 m currently 50 m respectively furthermore a quantitative model for the porosity change caused by the chesapeake bay impact was developed utilizing the modeled density distribution the model shows that compared with the surrounding platform the porosity increased immediately after impact up to 8 5 in the collapsed and brecciated crater center currently 6 due to post impact compaction in contrast porosity decreased by 2 3 currently 3 to 4 5 due to post impact compaction in the peak ring region the lateral variations in porosity at chesapeake bay crater are compatible with similar porosity variations at mjolnir crater and are considered to be responsible for the moderate chesapeake bay gravity signature annular low of 8 mgal instead of 15 mgal the analysis shows that the reconstructions and the long term alterations due to post impact burial are closely related to the impact disturbed target rock volume and a brecciated region of laterally varying thickness and depth varying physical properties the study further shows that several crater morphological and structural parameters are prone to post impact burial modification and are either exaggerated or subdued during post impact burial preliminary correction factors are established based on the integrated reconstruction and post impact deformation analysis the crater morphological and structural parameters corrected from post impact loading and modification effects can be used to better constrain cratering scaling law estimates and impact related consequences

40ar 39ar step heating ages of north american tektites and of impact melt rock samples from the chesapeake bay impact structure abstract this study presents 40ar 39ar step heating ages of four north american tektites three bediasites and one georgiaite and of two groundmass samples extracted at different depths from clast rich impact melt rocks cb w61 and cb w84 recovered by the usgs icdp eyreville b drill core about 9 km from the centre of the chesapeake bay impact structure radiometric age determination on both north american tektites and impact melt rocks from within chesapeake bay crater offers the first possibility to confirm the origin of these tektites for this aim argon isotopic data from 13 samples aliquots of tektite rims cores and bulk and 4 samples aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions age spectra of all tektite samples show plateaux comprising 62 98 of the 39ar release over consecutive intermediate and high temperature heating steps few low temperature extractions indicate excess 40ar inverse isochron 40ar 36ar intercepts of tektite samples are indistinguishable from air 295 5 however impact melt rock spectra presented complex ar release affecting primarily the low temperature heating steps inverse isochrones indicate excess argon from which the 40ar 36ar intercept was used to correct the age calculation cb w61 and cb w61 2 40ar 36ar intercepts are 354 5 2 5 and 327 2 6 3 respectively and those for cb w84 and cb w84 2 are 332 0 7 3 and 329 6 5 6 respectively the inverse isochron weighted mean age according to currently suggested k decay constants revisions by schwarz et al 2011 and renne et al 2011 for all four tektites is 34 86 0 25 ma mswd 0 96 p 0 41 n 4 and for the two impact melt rocks is 37 16 3 65 ma mswd 0 83 p 0 36 the combined tektite and impact melt rocks isochron mean age of 34 86 0 23 0 32 ma mswd 0 87 p 0 43 is slightly though not significantly higher than the plateau mean age of 34 55 0 27 0 36 ma mswd 0 66 p 0 62 placing this age in the global stratotype section and point gssp marine section exposed at massignano italy it falls below the eocene oligocene e o boundary overlapping with the 10 28 m ir anomaly these results agree within errors with previously reported ages of 35 20 0 54 ma especially those derived from k ar and ar ar total fusion analysis an age of 34 86 0 32 ma sets the chesapeake bay impact event close to the youngest of the three ir anomalies at 35 0 ma in the case the impactor was ir rich e g a chondrite primitive achondrite stony iron or iron meteorite the concordance with the e o boundary at 33 9 ma seems only marginally possible and only if the ir contribution from the ejecta were potentially due either to its small amount becoming diluted in the geologic record or the impactor being ir poor e g of differentiated achondritic composition this study also brings to front the need to re establish the stratigraphic and palaeo magnetic correlations across the globe for the ir anomalies and the magneto stratigraphy during the mid to late eocene and early oligocene and the need to re evaluate the markers for the eocene oligocene boundary

hydrocode simulations of chicxulub crater collapse and peak ring formation we use hydrocode modeling to investigate dynamic models for the collapse of the chicxulub impact crater our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide angle velocity data to further our understanding of the formation of large impact craters in our simulations we model the collapse of a 100 km diameter bowlshaped cavity formed in comprehensively fractured crustal material to facilitate wholesale collapse we require that the strength of the target be significantly weakened in the present model we achieve this using acoustic fluidization where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target at times and positions where the overburden pressure is sufficiently counteracted the frictional resistance is reduced enabling the rock debris to flow our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at chicxulub in particular we observe a topographic peak ring formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim this model for peak ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned c 2002 elsevier science usa

mid sized complex crater formation in mixed crystalline sedimentary targets insight from modeling and observation large impact crater formation is an important geologic process that is not fully understood the current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets real targets are rarely uniform for example the majority of earth s surface is covered by sedimentary rocks and or a water layer the ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process to advance understanding of the mechanics of crater collapse and the effect of variations in target properties on crater formation the first bridging the gap workshop recommended that geological observation and numerical modeling focussed on mid sized 15 30 km diameter craters on earth these are large enough to be complex small enough to be mapped surveyed and modelled at high resolution and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables in this paper we compare observations and numerical models of three 18 26 km diameter craters formed in different target lithology ries germany haughton canada and el gygytgyn russia based on the first order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid sized complex crater formation in a subaerial mixed crystalline sedimentary target we compared our results with interpreted geological profiles of ries and haughton based on detailed new and published geological mapping and published geophysical surveys our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre impact sedimentary cover in each case we conclude that the presence of an inner ring at ries and not at haughton is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post impact erosion for constant impact energy transient and final crater diameters increase with increasing sediment thickness

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

establishing the link between the chesapeake bay impact structure and the north american tektite strewn field the sr nd isotopic evidence the chesapeake bay impact structure which is about 35 ma old has previously been proposed as the possible source crater of the north american tektites nat here we report major and trace element data as well as the first sr nd isotope data for drill core and outcrop samples of target lithologies crater fill breccias and post impact sediments of the chesapeake bay impact structure the unconsolidated sediments cretaceous to middle eocene in age have srt 35 7 ma of 54 to 272 and ndt 35 7 ma ranging from 6 5 to 10 8 one sample from the granitic basement with a tndchur model age of 1 36 ga yielded an srt 35 7 ma of 188 and an ndt 35 7 ma of 5 7 the exmore breccia crater fill can be explained as a mix of the measured target sediments and the granite plus an as yet undetermined component the post impact sediments of the chickahominy formation have slightly higher tndchur model ages of about 1 55 ga indicating a contribution of some older materials newly analyzed bediasites have the following isotope parameters 104 to 119 srt 35 7 ma 5 7 ndt 35 7 ma 0 47 ga tsrur and 1 15 ga tndchur which is in excellent agreement with previously published data for samples of the nat strewn field target rocks with highly radiogenic sr isotopic composition as required for explaining the isotopic characteristics of deep sea drilling project dsdp site 612 tektites were not among the analyzed sample suite based on the new isotope data we exclude any relation between the na tektites and the popigai impact crater although they have identical ages within 2 errors the chesapeake bay structure however is now clearly constrained as the source crater for the north american tektites although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites

crater topography on titan implications for landscape evolution abstract we present a comprehensive review of available crater topography measurements for saturn s moon titan in general the depths of titan s craters are within the range of depths observed for similarly sized fresh craters on ganymede but several hundreds of meters shallower than ganymede s average depth vs diameter trend depth to diameter ratios are between 0 0012 0 0003 for the largest crater studied menrva d 425 km and 0 017 0 004 for the smallest crater studied ksa d 39 km when we evaluate the anderson darling goodness of fit parameter we find that there is less than a 10 probability that titan s craters have a current depth distribution that is consistent with the depth distribution of fresh craters on ganymede there is however a much higher probability that the relative depths are uniformly distributed between 0 fresh and 1 completely infilled this distribution is consistent with an infilling process that is relatively constant with time such as aeolian deposition assuming that ganymede represents a close airless analogue to titan the difference in depths represents the first quantitative measure of the amount of modification that has shaped titan s surface the only body in the outer solar system with extensive surface atmosphere exchange

meteoroid mayhem in ole virginny source of the north american tektite strewn field new seismic reflection data from chesapeake bay reveal a buried 85 km wide 1 5 2 0 km deep peak ring impact crater carved through upper eocene to lower cretaceous sedimentary strata and into underlying pre mesozoic crystalline basement rocks a polymictic late eocene impact breccia composed mainly of locally derived sedimentary debris determined from four continuous cores surrounds and partly fills the crater structural and sedimentary characteristics of the chesapeake bay crater closely resemble those of the miocene ries peakring crater in southern germany we speculate that the chesapeake bay crater is the source of the north american tektite strewn field

constraining the size of the south pole aitken basin impact abstract the south pole aitken spa basin is the largest and oldest definitive impact structure on the moon to understand how this immense basin formed we conducted a suite of spa scale numerical impact simulations varying impactor size impact velocity and lithospheric thermal gradient we compared our model results to observational spa basin data to constrain a best fit scenario for the spa basin forming impact our results show that the excavation depth to diameter ratio for spa scale impacts is constant for all impact scenarios and is consistent with analytical and geological estimates of excavation depth in smaller craters suggesting that spa scale impacts follow proportional scaling steep near surface thermal gradients and high internal temperatures greatly affected the basin forming process basin structure and impact generated melt volume in agreement with previous numerical studies of spa scale impacts crustal material is entirely removed from the basin center which is instead occupied by a large melt pool of predominantly mantle composition differentiation of the melt pool is needed to be consistent with observational data assuming differentiation of the thick impact generated melt sheet occurred and using observational basin data as constraints we find the best fit impact scenario for the formation of the south pole aitken basin to be an impact with an energy of 4 1026 j our specific model considered an impactor 170 km in diameter striking at 10 km s

titan s impact crater population after cassini abstract titan is saturn s largest moon and has a dynamic surface with methane rivers carved by an active hydrological cycle and sand seas shaped by aeolian processes one way to study the rates of these processes is by examining titan s impact craters because they provide quantitative constraints on the level of degradation with the end of the cassini mission in september 2017 we have reassessed the crater population using the entire cassini synthetic aperture radar sar dataset including 30 additional craters since the last assessment in 2012 for a total of 90 certain to possible impact craters on titan we adjust for incomplete coverage 69 of the moon by sar imaging using a monte carlo approach and find no major change in titan s inferred surface age from prior studies we then used the sartopo and stereo topography data sets to measure crater depths and diameters for the first time rim heights for twelve craters were also reported on average titan s craters are shown to be shallower with lower rims than those observed on similarly sized icy moons e g ganymede this suggests that the observed modification is due to a combination of sand and sediment infilling onto the crater floor and fluvial erosion of the rims with fluvial erosion playing a larger role than previously thought in crater degradation on titan

how big was the chesapeake bay impact insight from numerical modeling the chesapeake bay impact structure virginia is the largest impact crater in the united states the important question of how large the impactor was that formed the crater remains unanswered this is primarily due to peculiarities of the crater structure revealed by seismic exploration that complicate comparisons with other terrestrial and extraterrestrial craters one potential explanation for the unusual structure of the chesapeake bay crater is that the cratering process was affected by rheologic variations in the target at the time of impact using numerical modeling we find that for a sufficient contrast in material strength between the sedimentary and crystalline units at the chesapeake bay impact site we can produce a model of the final crater that is consistent with observational constraints and hence we can infer the energy released during the impact 1 75 3 10 6 mt in the absence of target strength variations the final crater diameter is likely to have been 40 km rather than the observed 80 90 km

the meteorite impact induced tsunami hazard when a cosmic object strikes the earth it most probably falls into an ocean depending on the impact energy and the depth of the ocean a large amount of water is displaced forming a temporary crater in the water column large tsunami like waves originate from the collapse of the cavity in the water and the ejecta splash because of the far reaching destructive consequences of such waves an oceanic impact has been suggested to be more severe than a similar sized impact on land in other words oceanic impacts may punch over their weight this review paper summarizes the process of impact induced wave generation and subsequent propagation whether the wave characteristic differs from tsunamis generated by other classical mechanisms and what methods have been applied to quantify the consequences of an oceanic impact finally the impact induced tsunami hazard will be evaluated by means of the eltanin impact event

deep drilling into the chesapeake bay impact structure samples from a 1 76 kilometer deep corehole drilled near the center of the late eocene chesapeake bay impact structure virginia usa reveal its geologic hydrologic and biologic history we conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact melt creation and distribution transient cavity collapse and ocean water resurge comparison of post impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long term sedimentation patterns in the mid atlantic region salty connate water of the target remains in the crater fill today where it poses a potential threat to the regional groundwater resource observed depth variations in microbial abundance indicate a complex history of impact related thermal sterilization and habitat modification and subsequent post impact repopulation

impact origin of the chesapeake bay structure and the source of the north american tektites seismic profiles drill core samples and gravity data suggest that a complex impact crater 35 5 million years old and 90 kilometers in diameter is buried beneath the lower chesapeake bay the breccia that fills the structure contains evidence of shock metamorphism including impact melt breccias and multiple sets of planar deformation features shock lamellae in quartz and feldspar the age of the crater and the composition of some breccia clasts are consistent with the chesapeake bay impact structure being the source of the north american tektites

origin of two verwey transitions in different generations of magnetite from the chesapeake bay impact structure usa 1 we observed two different verwey transition temperatures in fragments of crystalline basement rocks and impact sediments from the chesapeake bay impact structure usa our study aims to the question if this feature can be used as shock indicator in impact craters we distinguished three generations of magnetite 1 primary magnetite in crystalline basement rocks has average grain sizes up to several hundreds of micrometers and shows a regular tv at 121 k 2 shocked magnetite occurs in fragments of crystalline basement rocks and also in the suevite and impact breccia these magnetites show two verwey transitions a regular one and a low temperature transition ltv at around 89 k ltv is related to a small grain size fraction whereas a larger grain size fraction some hundreds of micrometers causes the regular tv the small grain size fraction contains a distinctly higher amount of superficially oxidized material due to the high surface volume ratio which causes a decrease of the verwey transition temperature ltv 3 a secondary magnetite generation shows also two verwey transition temperatures one at 121 k and a ltv range between 91 and 105 k the ltv in this generation is also linked to thin oxidized surface layers this study shows that especially the verwey transition temperature of small magnetite grains reacts very sensitively to surface oxidation and can therefore not be used as a reliable pressure indicator for impact structures on earth

numerical modelling of generation propagation and run up of tsunamis caused by oceanic impacts model strategy and technical solutions summary hypervelocity impacts of asteroids in marine environments produce tsunami waves independent of the water depth and the diameter of the projectile however the characteristics of the induced waves are affected by these parameters we present a model consisting of the wellknown sale impact model and a non linear wave propagation model to study the generation and subsequent spread out of the initial wave pattern caused by the strike of an asteroid or comet in the ocean the numerical simulation of oceanic impacts requires some changes and extensions to the original sale code especially the handling of different materials water and solid rocks is crucial as they are involved in the cratering process for the simulation of the propagation of tsunami waves that are generated by the impact process we use a newly developed wave propagation model which is based on the non linear shallow water theory with boundary conditions derived from the impact model the run up of the tsunami wave on the coastline is implemented as a special case of reflection and is realized by the wellestablished most code besides the model description we exemplify the capability of our modelling scheme by the simulation of the strike of an asteroid 800 m in diameter on a 5000 m deep ocean at 10 2 km s 1 the subsequent propagation of the induced tsunami waves over an artificial bathymetry and the run up of the wave on the coast

insights into the morphology geometry and post impact erosion of the araguainha peak ring structure central brazil the 40 km wide araguainha structure is the largest and best exposed complex impact crater in south america it was excavated in flat lying sediments of the intracratonic parana basin where target rocks are well exposed and have remained undeformed since the impact event ca 245 ma despite the excellent state of preservation and exposure information available on the target rock stratigraphy post impact erosion and morphology of the structure is limited our combined field observations and remote sensing analysis demonstrate that araguainha preserves all features of a shallowly eroded peak ring crater the interior of the structure exposes a central peak surrounded by a 5 km wide annular basin and two main ring features 10 12 and 14 18 km from the center analysis of the pre impact stratigraphy present morphology and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin but minimal to nonexistent beyond the inner ring feature the formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls regional stratigraphic data combined with field observations indicate two periods of post impact erosion associated with exhumation of the parana basin we estimate that 250 350 m of fallback deposits and target rocks have been removed by erosion erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure in contrast to previous suggestions that the impact took place in a shallow marine environment our observations are more consistent with an extremely shallow brackish to freshwater lacustrine environment with little effect on the developing impact crater

determining chondritic impactor size from the marine osmium isotope record decreases in the seawater 187os 188os ratio caused by the impact of a chondritic meteorite are indicative of projectile size if the soluble fraction of osmium carried by the impacting body is known resulting diameter estimates of the late eocene and cretaceous paleogene projectiles are within 50 of independent estimates derived from iridium data assuming total vaporization and dissolution of osmium in seawater the variations of 187os 188os and os ir across the late eocene impact event horizon support the main assumptions required to estimate the projectile diameter chondritic impacts as small as 2 kilometers in diameter should produce observable excursions in the marine osmium isotope record suggesting that previously unrecognized impact events can be identified by this method

a chemostratigraphic method to determine the end of impact related sedimentation at marine target impact craters chesapeake bay lockne tvaren abstract to better understand the impact cratering process and its environmental consequences at the local to global scale it is important to know when in the geological record of an impact crater the impact related processes cease in many instances this occurs with the end of early crater modification leaving an obvious sedimentological boundary between impactites and secular sediments however in marine target craters the transition from early crater collapse i e water resurge to postimpact sedimentation can appear gradual with the a priori assumption that the reworked target materials of the resurge deposits have a different chemical composition to the secular sediments we use chemostratigraphy 13ccarb corg major elements of sediments from the chesapeake bay lockne and tvaren craters to define this boundary we show that the end of impact related sedimentation in these cases is fairly rapid and does not necessarily coincide with a visual boundary e g grain size shift therefore in some cases the boundary is more precisely determined by chemostratigraphy especially carbonate carbon isotope variations rather than by visual inspection it is also shown how chemostratigraphy can confirm the age of marine target craters that were previously determined by biostratigraphy by comparing postimpact carbon isotope trends with established regional trends

deep sea drilling project site 612 bolide event new evidence of a late eocene impact wave deposit and a possible impact site u s east coast a remarkable 60 m thick upward fining polymictic marine boulder bed is distributed over 15 000 km sup 2 beneath chesapeake bay and the surrounding middle atlantic coastal plain and inner continental shelf the wide varieties of clast lithologies and microfossil assemblages were derived from at least seven known cretaceous paleocene and eocene stratigraphic units the supporting pebbly matrix contains variably mixed assemblages of microfossils from the same seven stratigraphic units along with trace quantities of impact ejecta tektite glass and shocked quartz the youngest microfossils in the boulder bed are of early late eocene age on the basis of its unusual characteristics and its stratigraphic equivalence to a layer of impact ejecta at deep sea drilling project dsdp site 612 new jersey continental slope the authors postulate that this boulder bed was formed by a powerful bolide generated wave train that scoured the ancient inner shelf and coastal plain of southeastern virginia the most promising candidate for the bolide impact site identified on seismic reflection profiles is 40 km north northwest of dsdp site 612 on the new jersey outer continental shelf

widespread erosion and mass failure from the 51 ma montagnais marine bolide impact off southwestern nova scotia canada12 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120241 the 51 ma montagnais impact crater on the outer scotian shelf is well known but the potential effects from the impact event on the slope and rise seaward of the crater have until now remained poorly understood through detailed seismic stratigraphic correlation and ties to available wells we define a threefold seismic stratigraphic subdivision for upper cretaceous to eocene strata on the shelf and slope calibrated to the most recent biostratigraphic results using this framework we identify a number of depositional and erosional products that are temporally consistent with a late ypresian impact event within the limits of seismic and biostratigraphic resolution we link a series of prominent failure scarps on the outer shelf and upper slope to a single widespread mass transport deposit mtd on the lower continental slope rise and abyssal plain failed material amassed in a large debris field referred to here as the montagnais mtd it covers an area of 93 000 km2 and travelled up to 580 km f

complex crater formation insights from combining observations of shock pressure distribution with numerical models at the west clearwater lake impact structure large impact structures have complex morphologies with zones of structural uplift that can be expressed topographically as central peaks and or peak rings internal to the crater rim the formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization here samples of shock metamorphosed quartz bearing lithologies at the west clearwater lake impact structure canada are used to estimate the maximum recorded shock pressures in three dimensions across the crater these measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift the distribution of peak shock pressures together with apparent crater morphology and geological observations is compared with numerical impact simulations to constrain parameters used in the block model implementation of acoustic fluidization the numerical simulations produce craters that are consistent with morphological and geological observations the results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse and should be included in future implementations based on the comparison between observational data and impact simulations we conclude that the west clearwater lake structure had an original rim final crater diameter of 35 40 km and has since experienced up to 2 km of differential erosion

chesapeake bay impact structure morphology crater fill and relevance for impact structures on mars the late eocene chesapeake bay impact structure cbis on the atlantic margin of virginia is one of the largest and best preserved wet target craters on earth it provides an accessible analog for studying impact processes in layered and wet targets on volatile rich planets the cbis formed in a layered target of water weak clastic sediments and hard crystalline rock the buried structure consists of a deep filled central crater 38 km in width surrounded by a shallower brim known as the annular trough the annular trough formed partly by collapse of weak sediments which expanded the structure to 85 km in diameter such extensive collapse in addition to excavation processes can explain the inverted sombrero morphology observed at some craters in layered targets the distribution of crater fill materials in the cbis is related to the morphology suevitic breccia including pre resurge fallback deposits is found in the central crater impact modified sediments formed by fluidization and collapse of water saturated sand and silt clay occur in the annular trough allogenic sediment clast breccia interpreted as ocean resurge deposits overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments the formation of chaotic terrains on mars is attributed to collapse due to the release of volatiles from thick layered deposits some flat floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar sized complex craters in solid targets studies of crater materials in the cbis provide insights into processes of crater expansion on mars and their links to volatiles

elevation distribution of titan s craters suggests extensive wetlands abstract using a new global topographic map of titan we find that craters on titan preferentially lie at higher than average elevations we explore several explanations for this observed behavior and judge the most reasonable explanation to be the presence of widespread wetlands of liquid hydrocarbons at low elevations over much of geologic time impacts into a shallow marine environment or a saturated layer of sediments more than several hundred meters thick would produce crater morphologies similar to terrestrial submarine impacts these are known to lack significant topographic expression and would thus be difficult to observe with the cassini spacecraft since titan s near surface methane inventory likely fluctuated over geologic time with episodic delivery and continuous depletion a few craters at low elevations can nonetheless be expected

new surveys of the chesapeake bay impact structure suggest melt pockets and target structure effect we present high resolution gravity and magnetic field survey results over the 85 km diameter chesapeake bay impact structure whereas a continuous melt sheet is antici pated at a crater this size shallow source magnetic field anomalies of 100 nt instead suggest that impact melt pooled in kilometer scaled pockets surrounding the base of a central peak a central anomaly of 300 nt may represent additional melt or rock that underwent shock induced remagnetization models predict that the total volume of the melt ranges from 0 4 to 10 km 3 a quantity that is several orders of magnitude smaller than expected for an impact structure this size however this volume is within predictions given a transient crater of diameter of 20 40 km for a target covered with water and sedimentary deposits such that melt fragments were widely dispersed at the time of impact gravity data delineate a gently sloping inner basin and a central peak via a contrast between crystalline and sedimentary rock both features are ovoid oriented parallel to larger preimpact basement structures conceptual models suggest how lateral differences in rock strength due to these preimpact structures helped to shape the crater s morphology during transient crater modification

petrography mineralogy and geochemistry of deep gravelly sands in the eyreville b core chesapeake bay impact structure the icdp usgs eyreville drill cores in the chesapeake bay impact structure reached a total depth of 1766 m and comprise from the bottom upwards basement derived schists and granites pegmatites impact breccias mostly poorly lithified gravelly sand and crystalline blocks a granitic slab sedimentary breccias and postimpact sediments the gravelly sand and crystalline block section forms an approximately 26 m thick interval that includes an amphibolite block and boulders of cataclastic gneiss and suevite three gravelly sands basal middle and upper are distinguished within this interval the gravelly sands are poorly sorted clast supported and generally massive but crude size sorting and subtle discontinuous layers occur locally quartz and k feldspar are the main sand size minerals and smectite and kaolinite are the principal clay minerals other mineral grains occur only in accessory amounts and lithic clasts are sparse only a few vol the gravelly sands are silica rich 80 wt sio2 trends with depth include a slight decrease in sio2 and slight increase in fe2o3 the basal gravelly sand below the cataclasite boulder has a lower sio2 content less k feldspar and more mica than the higher sands and it contains more lithic clasts and melt particles that are probably reworked from the underlying suevite the middle gravelly sand below the amphibolite block is finer grained contains more abundant clay minerals and displays more variable chemical compositions than upper gravelly sand above the block our mineralogical and geochemical results suggest that the gravelly sands are avalanche deposits derived probably from the nonmarine potomac formation in the lower part of the target sediment layer in contrast to polymict diamictons higher in the core that have been interpreted as ocean resurge debris flows which is in agreement with previous interpretations the mineralogy and geochemistry of the gravelly sands are typical for a passive continental margin source there is no discernible mixing with marine sediments no glauconite or paleogene marine microfossils noted during the impact remobilization and redeposition the unshocked amphibolite block and cataclasite boulder might have originated from the outer parts of the transient crater

numerical simulations of the mjolnir marine impact crater 1 the 40 km diameter mjolnir crater in the barents sea was created by a meteoroid impact into a 400 m shallow sea about 142 ma we have used a multimaterial hydrocode with depth depending target strength to model numerically the cratering and early modification stages of the mjolnir impact our models are constrained by the observed crater morphology and structure the best results were obtained for a composite target strength structure very low strength for the upper 3 km of sedimentary rocks and gradual increase from 3 to 6 5 km depth before using strength values typical for granitic rocks at greater depths the low strength of the target sediments at shallow depths led to exceptionally intensive slumping and gravitational collapse of the order of 2 0 2 5 considerably larger than the average expected values for typical terrestrial craters as a result we obtain a crater collapse in between simple and complex where slumping and gravitational collapse counteract the crater floor uplift and lead to suppression and burial of the central high the mjolnir impact in a shallow sea was responsible for a major disturbance of the water column following the impact induced outward water surge large amplitude tsunamis were formed and the rapid resurge of seawater into the excavated crater transported large amounts of ejecta and crater wall material back into the crater accounting for the redestribution of ejecta material and extensive infilling

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

the effect of the oceans on the terrestrial crater size frequency distribution insight from numerical modeling on earth oceanic impacts are twice as likely to occur as continental impacts yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size frequency distribution despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments detailed numerical modeling and interpretation of geological and geophysical data no definitive relationship between impactor properties water depth and final crater diameter exists in this paper we determine the relationship between final and transient crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models this relationship applies for normal incidence impacts of stoney asteroids into water covered crystalline oceanic crust at a velocity of 15 km s 1 we use these relationships to construct the first estimates of terrestrial crater size frequency distributions over the last 100 million years that take into account the depth area distribution of oceans on earth we find that the oceans reduce the number of craters smaller than 1 km in diameter by about two thirds the number of craters 30 km in diameter by about one third and that for craters larger than 100 km in diameter the oceans have little effect above a diameter of 12 km more craters occur on the ocean floor than on land below this diameter more craters form on land than in the oceans we also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact related resurge feature or disturbance on the seafloor instead of a crater

combining shock barometry with numerical modeling insights into complex crater formation the example of the siljan impact structure sweden siljan central sweden is the largest known impact structure in europe it was formed at about 380 ma in the late devonian period the structure has been heavily eroded to a level originally located underneath the crater floor and to date important questions about the original size and morphology of siljan remain unanswered here we present the results of a shock barometry study of quartz bearing surface and drill core samples combined with numerical modeling using isale the investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 gpa near the present surface to 10 15 gpa at 600 m depth a best fit model that is consistent with observational constraints relating to the present size of the structure the location of the downfaulted sediments and the observed surface and vertical shock barometry profiles is presented the best fit model results in a final crater rim to rim diameter of 65 km according to our simulations the original siljan impact structure would have been a peak ring crater siljan was formed in a mixed target of paleozoic sedimentary rocks overlaying crystalline basement our modeling suggests that at the time of impact the sedimentary sequence was approximately 3 km thick since then there has been around 4 km of erosion of the structure

chronostratigraphy composition and origin of ni rich spinel from the late eocene fuente caldera section in spain one impact or more here we report on the stratigraphic distribution and chemical composition of ni rich spinel a specific mineral tracer of meteorite impacts in the fuente caldera section in spain a major peak in spinel abundance is observed in a biostratigraphic interval defined by the last occurrence of the planktic foraminifera porticulasphaera semiinvoluta and the first occurrence of the planktic foraminifera turborotalia cunialensis two other peaks of lower abundances are observed higher up in the same biostratigraphic interval but geochemical considerations suggest that they likely originate from redeposition by turbiditic currents biostratigraphic correlations with the global stratotype section and point for the eocene oligocene boundary of massignano in italy give an age of 35 4 0 2 ma 1 for the major peak this age is indistinguishable from the age of the impact horizon at massignano 35 5 0 2 ma and within the age uncertainties for the popigai 35 7 0 2 ma and chesapeake bay 35 5 0 5 ma craters the fuente caldera spinel as the massignano spinel is assumed to be a relic mineral of microkrystites which are believed to derive from a unique source related to the popigai impact crater the morphologies and cr compositions of the fuente caldera and massignano spinel crystals are markedly different however the fuente caldera spinel occurs mostly as octahedral and skeletal crystals with 85 of the grains belonging to the cr rich magnetite series and 15 to the fe rich chromite series whereas the massignano spinel occurs mostly as dendritic crystals with 90 of the grains belonging to the cr poor magnetite series it is unlikely that these differences are the result of post depositional alteration processes because the compositions of the crystals as well as their morphologies are in general very similar to those reported for primary spinel crystals i e spinel crystals present in meteorite fusion crust or synthetized from meteoritic material in addition spinel crystals have quite homogeneous compositions except for a few grains 10 showing cr zonations but these are assigned to primary crystallization processes one possible explanation that is consistent with a single impact event producing spatial variations in spinel compositions and morphologies is that microkrystites are locally generated by the ablation in the atmosphere of impact debris an alternative explanation is that fuente caldera and massignano microkrystites derive from two closely spaced impact events which however requires another so far unknown source crater for microkrystites

u th he zircon dating of chesapeake bay distal impact ejecta from odp site 1073 single crystal u th he dating has been undertaken on 21 detrital zircon grains extracted from a core sample from ocean drilling project odp site 1073 which is located 390 km northeast of the center of the chesapeake bay impact structure optical and electron imaging in combination with energy dispersive x ray microanalysis eds of zircon grains from this late eocene sediment shows clear evidence of shock metamorphism in some zircon grains which suggests that these shocked zircon crystals are distal ejecta from the formation of the 40 km diameter chesapeake bay impact structure u th he dates for zircon crystals from this sediment range from 33 49 0 94 to 305 1 8 6 ma 2 implying crystal to crystal variability in the degree of impact related resetting of u th he systematics and a range of different possible sources the two youngest zircon grains yield an inverse variance weighted mean u th he age of 33 99 0 71 ma 2 uncertainties n 2 mean square weighted deviation 2 6 probability p 11 which is interpreted to be the u th he age of formation of the chesapeake bay impact structure this age is in agreement with k ar 40ar 39ar and fission track dates for tektites from the north american strewn field which have been interpreted as associated with the chesapeake bay impact event

impact effects and regional tectonic insights backstripping the chesapeake bay impact structure the chesapeake bay impact structure is a ca 35 4 ma crater located on the eastern sea board of north america deposition returned to normal shortly after impact resulting in a unique record of both impact related and subsequent passive margin sedimentation we use backstripping to show that the impact strongly affected sedimentation for 7 m y through impact derived crustal scale tectonics dominated by the effects of sediment compaction and the introduction and subsequent removal of a negative thermal anomaly instead of the expected positive thermal anomaly after this the area was dominated by passive margin thermal sub sidence overprinted by periods of regional scale vertical tectonic events on the order of tens of meters loading due to prograding sediment bodies may have generated these events