impact melt rocks from the ries structure germany an origin as impact melt flows abstract the production of impact melt rocks and glasses is a characteristic feature of hypervelocity impact events on earth and other planetary bodies this investigation represents the first detailed study of an unusual series of coherent impact melt rocks intermittently exposed around the periphery of the 24 km diameter 14 5 ma ries impact structure germany optical and analytical scanning electron microscopy sem reveals that the groundmass comprises sanidine plagioclase quartz and ilmenite decreasing order of abundance with the interstices filled by either fresh or devitrified glassy mesostasis primary crystallites display skeletal dendritic and or spherulitic textures indicating rapid crystallization from a melt the mesostasis is characterized by extreme chemical heterogeneity e g feo and al 2 o 3 contents from 1 to 62 80 wt this is likely due to a combination of crystal liquid fractionation during rapid cooling and crystallization of an originally incompletely homogenized melt vapor phase crystallization of sanidine and cristobalite occurred in miarolitic cavities during late stage cooling of the impact melts the most likely protolith for the impact melt rocks are granitic rocks present in the crystalline basement target the high volatile content of the mesostasis suggests that a large volatile component was retained from this protolith field observations together with analytical data and micro textures indicate that the ries impact melt rocks were molten at the time of and after deposition field relations with other impactites also suggest that these rocks were emplaced subsequent to the excavation stage of crater formation and that they are not therefore ballistic ejecta thus it is proposed that the ries impact melt rocks were emplaced as ground hugging impact melt flows that emanated from different regions of the evolving transient cavity during the modification stage of crater formation this is consistent with and in fact predicted by field relations which indicate that these rocks overlie unequivocal ejecta this represents the first recognition of impact melt flows near the rim of a terrestrial impact structure and is consistent with observations of such features on the moon and venus

post impact alteration of surficial suevites in ries crater germany hydrothermal modification or weathering processes alteration of surficial suevites at ries crater germany was studied by means of x ray diffraction and scanning electron microscopy here we discuss the origin of hydrous silicate clay phases in these suevites that have been previously interpreted as resulting from post impact hydrothermal processes the results of this study indicate that the dominant alteration phases are dioctahedral al fe montmorillonite and halloysite which are typical low temperature clay minerals we suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes if the surficial suevites were indeed hydrothermally modified during the early stages of post impact cooling then the alteration was of limited character and is completely masked by later weathering

numerical modelling of impact melt production in porous rocks abstract melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure shock melted material is found in many terrestrial craters however most impact craters in sedimentary targets which are typically porous have been reported as containing significantly less melt than craters in nonporous crystalline targets to investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component to demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with hugoniot data for porous quartzite coconino sandstone and calcite our results show that the presence of porosity significantly reduces the critical pressure required for melting which enhances melt production we show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials so that all other things being equal impact melt production is greater when target porosity is higher we conducted a series of simulations of impacts into targets of different material composition quartzite calcite dunite and porosity of up to 50 and varied the impact velocity between 6 and 50 km s we found a significant increase in the production of impact melt with increasing velocity in agreement with several previous studies and quantify the net increase in impact melt due to target porosity for the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size 7 90 km in diameter in porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters however our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary or mixed sedimentary and crystalline targets this further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks

40ar 39ar laser probe age determination confirms the ries impact crater as the source of glass particles in graupensand sediments grimmelfingen formation north alpine foreland basin mineralogical and chemical characteristics of reworked suevite material recovered from graupensand deposits grimmelfingen formation north alpine foreland basin middle miocene are identical to those recorded in ries suevite 40ar 39ar laser probe analyses of two glass particles from graupensand components and of a glass fragment from the ries suevite yielded ages ranging between 14 3 and 14 4 ma identical within analytical uncertainties hitherto the ries impact event was dated at 15 ma conventional k ar and 40ar 39ar methods whereas the graupensande have been placed into the ottnangian around 18 5 ma on the basis of mammal biostratigraphy small sample masses involved in laser probe technique allowed rigorous preselection of highly pure glass particles thus reducing possible age determination errors through partly molten or even unmolten mineral fragments from the variscan crystalline basement our results reconfirm previous statements considering the graupensande as reworked products of the ries impact ejecta blanket thus placing the age of the grimmelfingen formation into the badenian of middle miocene rather than into the ottnangian early miocene

petrography and geochemistry of impactites and volcanic bedrock in the icdp drill core d1c from lake el gygytgyn ne russia the 3 6 ma old and 18 km diameter el gygytgyn impact structure in ne siberia was drilled in 2008 09 by icdp international continental scientific drilling program a 517 m long core hole d1c was drilled into the outer flank of the central uplift structure with an overall core recovery of approximately 63 thereby approximately 315 m lake sediments and approximately 202 m impactites were recovered here we present a detailed petrographic and geochemical assessment of the impact breccia and bedrock sections in this core the 97 m long lower bedrock unit 517 420 m below lake floor blf consists of an ignimbrite in the overlying upper bedrock unit 420 390 mblf the core recovered a sequence of similar ignimbrite and several decimeters of mafic rocks we interpret these units as rocks that are located close to their former preimpact position but have been somewhat rotated due to collapse of the central uplift i e it represents parautochthonous basement from about 390 to 328 mblf occurs a suevite package with an impact melt poor clast dominated matrix and lithic and mineral clasts that cover the entire range of volcanic target rocks known from the el gygytgyn region all stages of shock metamorphism unshocked to melted were observed in clasts and in microclasts of the matrix of suevite from different depths immediately below this package at the contact to the underlying bedrock occurs a 1 m wide sheared zone within vitrophyric ignimbrite which we consider the actual crater floor the uppermost approximately 12 m from 328 316 mblf depth seem to comprise reworked suevite consisting of a mixture of sediments and suevite with more and on average stronger shocked minerals than found in the main suevite unit this includes a small component of glassy spherules and impact melt fragments toward the top of this unit lake sediments progressively become the dominant material in this section we assume that this unit contains a fallback component from the ejecta plume that was mixed with the first sediments of the postimpact crater lake and possibly some rocks that slumped off the inner crater wall similar to a thin layer at the base of the sediment section of borehole lb 5a recovered in lake bosumtwi ghana

geochemistry of impactites and basement lithologies from icdp borehole lb 07a bosumtwi impact structure ghana in 2004 a drilling project by the international continental scientific drilling program icdp at the bosumtwi impact crater ghana 1 07 myr old and 10 5 km in diameter obtained drill core lb 07a which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure the lb 07a core consists of three sequences 82 29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54 88 m of so called lower impactite of monomict impact breccia with several suevite intercalations and 74 53 m of meta graywacke and altered shale of the basement also containing a number of suevite intercalations major and trace element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein compositions of polymict impact breccias of the crater fill revealed by core lb 07a are compared with the compositions of the ivory coast tektites and the fallout suevites the impactites of the lb 07a borehole appear well homogenized with respect to the silicate component and little change in the ranges of many major and trace element differences is seen along the length of the borehole except for fe2o3 mgo and cao contents much scatter is observed for a number of elements and in many cases this increases with depth it is proposed that any variability in composition is likely the function of clast population differences i e also of relatively small sample sizes no systematic compositional difference between polymict lithic and suevitic impact breccias is evident an indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies the impactites of the borehole generally show intermediate compositions to previously defined target rocks the fallout suevites have comparable major element abundances except for relatively lower mgo contents the ivory coast tektites are generally similar in composition to the lb 07a suevites but broader ranges in mgo and cao contents are observed for the lb 07a suevites

geochemical studies of the subo 18 enkingen drill core and other impact breccias from the ries crater germany suevite and melt breccia compositions in the boreholes enkingen and polsingen are compared with compositions of suevites from other ries boreholes and surface locations and discussed in terms of implications for impact breccia genesis no significant differences in average chemical compositions for the various drill cores or surface samples are noted compositions of suevite and melt breccia from southern and northeastern sectors of the ries crater do not significantly differ this is in stark contrast to the published variations between within crater and out of crater suevites from northern and southern sectors of the bosumtwi impact structure ghana locally occurring alteration overprint on drill cores especially strong on the carbonate impregnated suevite specimens of the enkingen borehole does affect the average compositions overall the composition of the analyzed impact breccias from ries are characterized by very little macroscopically or microscopically recognized sediment clast component the clast populations of suevite and impact melt breccia are dominated consistently by granitic and intermediate granitoid components the polsingen breccia is significantly enriched in a dioritic clast component overall chemical compositions are of intermediate composition as well with dioritic granodioritic silica contents and relatively small contributions from mafic target components selected suevite samples from the enkingen core have elevated ni co cr and ir contents compared with previously analyzed suevites from the ries crater which suggest a small meteoritic component platinum group element pge concentrations for some of the enriched samples indicate somewhat elevated concentrations and near chondritic ratios of the most immobile pge consistent with an extraterrestrial contribution of 0 1 0 2 chondrite equivalent

distribution petrography and shock metamorphism of the ejecta of the ries crater in germany a review abstract the lithologies and distribution of the various ejecta formations from the ries crater germany are described they may be subdivided into 1 low shock level 10 gpa represented by polymict crystalline breccias fall out suevites crater suevite and tektites moldavites the ejecta from the mesozoic sedimentary column do not show effects of shock pressures exceeding 10 gpa and only basement gneisses were shock melted to yield suevite glasses it is therefore concluded that the maximum impact energy was transferred from the impactor to the target rocks at a depth of at least 1000 m below ground level where a complex of gneisses underlies shallower granitic intrusions the estimated depth of the transient crater was about 2700 m and its diameter was 12 13 km bunte breccia megablocks of sedimentary rocks and crystalline breccias and megablocks were ejected in ballistic trajectories and form the earliest ejecta deposits extending up to a radius of 22 km fall out suevites are understood to have been lofted from the transient crater by an expanding plume of vaporized rocks and deposited outside the crater as fluidized turbulent masses on top of the earlier ejecta in contrast the crater suevite never left the crater cavity it represents the main mass of suevite moldavites are understood to have been formed during the first encounter of the impactor with the ground by condensation from vaporized and ionizised tertiary sands covering the surface of the target area

a method of inferring changes in deep ocean currents from satellite measurements of time variable gravity 1 the nasa deutsches zentrum fur luft und raumfahrt dlr satellite gravity mission gravity recovery and climate experiment grace launched in march 2002 will map the earth s gravity field at scales of a few hundred kilometers and greater every 30 days we describe a method of using those gravity measurements to estimate temporal variations in deep ocean currents we examine the probable accuracy of the current estimates by constructing synthetic grace data based in part on output from an ocean general circulation model we ignore the possible contamination caused by short period gravity signals aliasing into the 30 day solutions we conclude that in the absence of aliasing grace should be able to recover the 30 day variability of midlatitude currents at a depth of 2 km with an error of about 6 15 in variance when smoothed with 500 700 km averaging radii

ries crater and suevite revisited observations and modeling part ii modeling we present the results of numerical modeling of the formation of the ries crater utilizing the two hydrocodes sova and isale these standard models allow us to reproduce crater shape size and morphology and composition and extension of the continuous ejecta blanket some of these results cannot however be readily reconciled with observations the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks containing very little material in comparison with the ejecta curtain at the end of the modification stage the crater floor is covered by a thick layer of impact melt with a total volume of 6 11 km3 the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters ejecta from all stratigraphic units of the target are transported ballistically no separation of sedimentary and crystalline rocks as observed between suevites and bunte breccia at ries is noted we also present numerical results quantifying the existing geological hypotheses of ries ejecta emplacement from an impact plume by melt flow or by a pyroclastic density current the results show that none of these mechanisms is consistent with physical constraints and or observations finally we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile rich sedimentary rocks

hf zr and ree partition coefficients between ilmenite and liquid implications for lunar petrogenesis hf zr and ree partition coefficients between ilmenite and coexisting liquid were determined under near lunar conditions ilmenite was synthesized at 1125 c under low oxygen fugacity fo2 10 12 4 from glass of a high ti mare basalt composition doped with 50 100 g g of ree and hf and 500 1000 g g of zr the partition coefficients d of ilmenite for hf and zr obtained by isotope dilution analysis are 0 41 and 0 33 respectively these values are significantly lower than those of ilmenite 3 0 and 1 8 respectively from a kimberlite megacryst fujimaki et al 1984a dree for the synthesized ilmenite are slightly smaller than those of ilmenite from the kimberlite megacryst and the dlu is 0 056 these results suggest that ilmenite may have played an important role in the lu hf evolution of lunar mare basalts using newly determined dlu and dhf for ilmenite the lu hf evolution or lunar cumulates and the coexisting magma was examined for several different crystallization sequences by a model calculation the lu hf variation trend of most high ti mare basalts is well explained by a small degree less than 5 of partial melting of the cumulates however a higher degree of partial melting is required to explain the variation of very low ti basalts green glass and apollo 12 low ti basalts apollo 15 low ti basalts could not be produced by simple partial melting but may require chromite crystallization as well

target delamination by spallation and ejecta dragging an example from the ries crater s periphery abstract subhorizontal shear planes detachments are observed in bedded limestones in the periphery of the ries impact crater germany these detachments occur at 0 8 1 8 crater radii distance from the crater center beneath deposits of the continuous ejecta blanket striations on detachment planes and offsets of markers indicate top outward shearing with radial slip vectors detachments were found at depths between a few meters and more than 50 m beneath the target surface the displacements along these faults range from meters to decameters and decrease with increasing depth and distance from the crater center with increasing crater distance detachment horizons tend to climb to shallower levels cross cutting relationships to faults associated with the crater collapse indicate that detachment faulting started prior to the collapse but continued during crater modification numerical modeling of the cratering process shows that near surface deformation outside the transient crater is induced by two separate mechanisms i weak spallation by interference of shock and release waves near the target surface and ii subsequent dragging by the deposition of the ejecta curtain spallation causes an upward and outward directed motion of target material that increases in magnitude toward the target surface it leads to decoupling of the uppermost target layers in the early cratering stage without totally disintegrating the rock the subsequent arrival of the oblique impact shower of the ejecta curtain at the target surface delivers a horizontal momentum to the uppermost target area and results in a second horizontal displacement increment by dragging with increasing depth this effect vanishes rapidly spallation decoupling and subsequent ejecta dragging of near surface rocks is probably a general cratering mechanism around craters in layered targets with weak interbeds

the nature of the groundmass of surficial suevite from the ries impact structure germany and constraints on its origin surficial suevites from the ries impact structure have been investigated in the field and using optical and analytical scanning electron microscopy the groundmass of these suevites comprises calcite clay minerals impact melt glass crystallites plagioclase garnet and pyroxene francolite and ba phillipsite the latter zeolite is a secondary phase abundant textures have been observed intricate flow textures between the various groundmass phases globules of each phase in the other phases spheroids of pyrrhotite in calcite the budding off of clay globules into silicate glass and or calcite euhedral overgrowths of francolite on apatite clasts and quench textured crystallites in the groundmass groundmass forming calcite displays higher feo mno and sio2 contents than limestone target material the composition of suevite clay minerals is highly variable and not always consistent with montmorillonite three types of glasses are distinguished in the groundmass type 1 glasses are sio2 rich and are clearly derived from sandstones in the sedimentary cover while the protoliths of the other two glass types remains unclear analytical data and micro textures indicate that the calcite silicate glass francolite and clay minerals of the groundmass of the ries suevites represent a series of impact generated melts that were molten at the time of and after deposition on cooling plagioclase pyroxene and garnet crystallized from the groundmass these results are at variance with the current traditional descriptive definition of suevite given that ries is the original type occurrence of suevite some modification to the traditional definition may be in order as the results of this study are most consistent with the groundmass of ries surficial suevites representing a mix of several types of impact generated melts we suggest that a possible origin for these suevites is as some form of impact melt flow s that emanated from different regions of the evolving crater

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

chemical record of the projectile in the graded fall back sedimentary unit from the ries crater germany abstract the so called graded unit comprises 17 2 m of the core from the research borehole nordlingen 1973 and is thought to represent fall back material which has been sorted similar to tephra samples from six levels of these air borne impact debris sediments in the ries crater have been analyzed for ir ni co cr and fe the concentrations of these elements are negatively correlated with average grain size and accordingly the depth of the samples indicating the presence of a surface correlated component and are hence consistent with vapour deposition of meteoritic material on small dust grains the largest ir concentration found is 230 pg g a factor of 28 higher than the indigenous concentration enrichment factors in the uppermost layer of the graded unit are about 28 for ir approximately 2 for fe co ni and 1 5 for cr if the enrichment of all elements determined were of meteoritic origin then the net meteoritic composition is most closely matched by aubrites as suggested previously however it seems more likely that the observed concentration trends of cr fe co and ni are due to mineral sorting therefore the only as yet certain indication of meteoritic material is the presence of ir which is not sufficient to further constrain the nature of the projectile

fluidization and hydrothermal alteration of the suevite deposit at the ries crater west germany and implications for mars the formation cooling and alteration of the impact ejecta deposits at the ries crater are an analog for impact processes and the formation of regoliths and soils on mars we have studied the suevite deposit an impact melt bearing breccia found outside of the crater rim this suevite contains large numbers of chimney like degassing pipes apparently formed by high gas pressures in the hot 500 c fluidized suevite immediately after deposition of the suevite the likely sources of the gas are volatiles from the melted and shocked basement inclusions the suevite outside of the crater rim contains about 15 wt clay indicating a large amount of alteration of the suevite the clay is an fe rich montmorillonite with less than 5 interstratified illite indicating that the temperature at which most of the alteration occurred was less than 100 130 c the duration of this low temperature period may have been extended by additional heating from the exothermal alteration reactions the surfaces of the degassing pipes are not altered to clays however but have a thin fe rich oxide or hydroxide coating the absence of alteration in the pipes suggests that the main alteration event outside the crater occurred under unsaturated conditions such that the walls of the pipes were dry suevite deposits are likely to exist on mars because of the presence of volatile rich target rock martian suevites may have also been fluidized and hydrothermally altered contributing significantly to the formation of the clay like phases inferred to exist in the martian soil

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

40ar 39ar laser dating of tektites from the cheb basin czech republic evidence for coevality with moldavites and influence of the dating standard on the age of the ries impact abstract moldavites central european tektites are genetically related to the impact event that produced the 24 km diameter ries crater in germany representing one of the youngest large impact structures on earth although several geochronological studies have been completed there is still no agreement among 40ar 39ar ages on both moldavites and glasses from ries suevites even recently published data yielded within sample mean ages with a nominal spread of more than 0 6 ma 14 24 14 88 ma this age spread which significantly exceeds current internal errors must be in part ascribed to geological and or analytical causes this study reports the results of a detailed geochronological investigation of moldavites from the cheb area czech republic which have never been dated before and for comparison of two samples from type localities one in southern bohemia and the other in western moravia we used 40ar 39ar laser step heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization in addition with the purpose of ascertaining the influence of the dating standards on the age of the ries impact and making data from this study and literature consistent with the now widely used fish canyon sanidine fcs standard we performed a direct calibration of multi grain splits of the fish canyon biotite fct 3 with fcs the intercalibration factors r fcs fct 3 determined for eight stack positions in one of the three performed irradiations were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1 0086 0 0031 2 in agreement with previous works suggesting that biotite from the fish canyon tuff is somewhat older 0 8 than the coexisting sanidine laser total fusion analysis of milligram to sub milligram splits of five tektite samples from the cheb area yielded mostly concordant intrasample 40ar 39ar ages and within sample weighted mean ages of 14 66 0 08 14 75 0 12 ma 2 internal errors ages relative to fcs that overlap within errors these ages match those obtained for samples from western moravia 14 66 0 08 ma and southern bohemia 14 68 0 11 ma supporting the genetic link between cheb basin tektites and moldavites and consequently between cheb basin tektites and the ries impact in contrast to samples from the cheb area and moravia 40ar 39ar ages from total fusion experiments on the bohemian specimen ranged widely from 14 6 to 17 0 ma older apparent ages however were systematically obtained from fragments characterized by visible surface alteration laser step heating experiments although displaying slightly disturbed age profiles were in line with total fusion analyses and yielded well defined plateau ages of 14 64 0 11 14 71 0 11 ma 2 internal errors ages relative to fcs a thorough comparison of our and previous 40ar 39ar ages on both moldavites and ries suevite glasses recalculated relative to the 40ar 40k ratio recently determined for fcs using intercalibration factors available in or derivable from the literature reveals some inconsistencies which may be ascribed to either geological or analytical causes based on our data decay constants in current use in geochronology and ages calculated relative to fcs we infer that the age of moldavites is 14 68 0 11 ma 2 neglecting uncertainties in the 40k decay constants

geochemistry and petrology of witwatersrand and dwyka diamictites from south africa search for an extraterrestrial component abstract diamictites are poorly sorted sediments characteristically carrying coarse grained clasts in a fine grained matrix they have generally been considered of glaciogenic or glaciomarine origin recently however it has been suggested that some massive tillite diamictite layers could represent impact breccias an earlier petrographic study of rock and mineral clasts from dwyka group diamictites revealed no evidence for shock metamorphism such as planar deformation features detailed geochemical studies of diamictite samples from the archean witwatersrand supergroup and the dwyka group of the mesozoic karoo supergroup from south africa are reported we studied the contents of the siderophile elements in these breccias as elevated abundances of such elements especially iridium could be indicative for an impact origin by use of coincidence spectrometry and other trace element analysis geochemical tracers of extraterrestrial components were sought however no enrichments of indicator elements for extraterrestrial components compared with ordinary continental crust were found thus neither geochemical nor petrographic evidence supports an impact origin of the diamictites from the dwyka group and the witwatersrand supergroup in south africa

40ar 39ar laser probe dating of the central european tektite producing impact event a new 40ar 39ar data set is presented for tektites from the central european strewn field moldavites this is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages 14 15 3 myr the main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission track dating the laser total fusion ages obtained on chips from 7 individual specimens from the southern bohemian and moravian subfields are restricted to a narrow interval of time with an average of 14 34 0 08 myr relative to the 27 95 0 09 myr of the fish canyon tuff biotite this result gives a more precise age not only for the tektite field but also for its producing impact if the genetic link between the moldavites and the nordlinger ries impact crater is maintained then this new age has to be considered a reliable estimate for the ries crater also this new value places the formation of central european tektites within the lower serravallian period in the latest geologic timescales evidence of their impact products such as glass spherules or shocked minerals can therefore be sought in sedimentary marine formations in a more precisely defined age interval

geochemistry of 2 63 2 49 ga impact spherule layers and implications for stratigraphic correlations and impact processes thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long distance stratigraphic correlation of precambrian successions seven formations in western australia and south africa clustered around the archean proterozoic boundary each contain a single spherule layer all of which were deposited between ca 2 49 and 2 63 ga analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component etc in all six layers based on pge ratios and or cr isotopic composition the amount of etc varies from marginally detectable to clear and obvious it generally amounts to ca a few percent both pge ratios and cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition in contrast all spherule layers older than ca 3 0 ga that have been analyzed were produced by carbonaceous chondritic impactors normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in pge ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation the geochemical data also provide a means to test proposed intra and intercontinental stratigraphic correlations cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2 63 ga in contrast clear geochemical differences between the middle layers both deposited ca 2 54 ga suggest they were not produced by the same impact event the youngest banded iron formation hosted layers on both continents have also been correlated but no geochemical data are available on the african layer to test this at present

drill core lb 08a bosumtwi impact structure ghana geochemistry of fallback breccia and basement samples from the central uplift the 1 07 myr old bosumtwi impact structure in ghana west africa which measures 10 5 km in diameter and is largely filled by lake bosumtwi is associated with one of four currently known tektite strewn fields two boreholes were drilled to acquire hard rock samples of the deep crater moat and from the flank of the central uplift lb 07a and lb 08a respectively during a recent icdp sponsored drilling project here we present results of major and trace element analysis of 112 samples from drill core lb 08a this core which was recovered between 235 6 and 451 33 m depth below lake level contains polymict lithic breccia intercalated with suevite which overlies fractured brecciated metasediment the basement is dominated by meta graywacke from fine grained to gritty but also includes some phyllite and slate as well as suevite dikelets and a few units of a distinct light greenish gray medium grained meta graywacke most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types as well as from aqueous alteration processes which have undeniably affected the different rocks suevite from core lb 08a fallback suevite and fallout suevite samples from outside the northern crater rim display some differences in major mainly in mgo cao and na2o contents and minor mainly cr and ni element abundances that could be related to the higher degree of alteration of fallback suevites but also result from differences in the clast populations of the two suevite populations for example granite clasts are present in fallout suevite but not in fallback breccia and calcite clasts are present in fallback breccia and not in fallout suevite chondrite normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other siderophile element contents in the impact breccias are not significantly different from those of the metasediments or compared to target rocks from outside the crater rim so far no evidence for a meteoritic component has been detected in polymict impact breccias during this study in agreement with previous work

the determination of platinum group elements pge in target rocks and fall back material of the nordlinger ries impact crater germany abstract the nature of the impactor for the nordlinger ries has so far remained elusive suggestions have been made for a chondritic or an aubritic proctile but all studies had difficulties to establish beyond doubt the meteoritic parental material the majority of the ries samples so far analyzed derived from the present surface which may have lead to some bias therefore we analyzed fifty five drill core samples from the research borehole nordlingen 1973 to characterize the indigenous ir concentrations of the target rocks and to search for any enrichments of pt group elements in the suevite and the so called graded unit we found that ultramafic rocks in the crystalline basement at a depth of 826 m far below the current crater bottom contain up to 0 5 ng g ir the highest concentrations ever measured in the target rocks these rocks comprise about 8 of the crystalline basement and thus form a substantial indigenous source of pt group elements taking this into account one has to consider ir concentrations of up to about 0 060 ng g as indigenous although the median concentration is 0 013 ng g close to earlier estimates our results show that there is no indication of unequivocal extraterrestrial contamination in the cored materials of the crater ejecta it is surprising that this also holds for the current crater bottom and the graded unit which have previously been suspected to contain meteoritic contamination it seems that a chondritic projectile can be ruled out but a clear geochemical association of any of the known meteorite types with the ries bolide is not possible

modeling the ries steinheim impact event and the formation of the moldavite strewn field using detailed geological petrographic geochemical and geographical constraints we have performed numerical modeling studies that relate the steinheim crater apparent diameter da 3 8 km the ries crater da 24 km in southern germany and the moldavite tektite strewn field in bohemia and moravia czech republic lusatia east germany and lower austria the moldavite strewn field extends from 200 to 450 km from the center of the ries to the east northeast forming a fan with an angle of 57 an oblique impact of a binary asteroid from a west southwest direction appears to explain the locations of the craters and the formation and distribution of the moldavites the impactor must have been a binary asteroid with two widely separated components some 1 5 and 0 15 km in diameter respectively we carried out a series of three dimensional hydrocode simulations of a ries type impact the results confirm previous results suggesting that impacts around 30 50 from the horizontal are the most favorable angles for near surface melting and consequently for the formation of tektites finally modeling of the motion of impact produced tektite particles through the atmosphere produces in the downrange direction a narrow angle distribution of the moldavites tektites in a fan like field with an angle of 75 an additional result of modeling the motion of melt inside and outside the crater is the preferred flow of melt from the main melt zone of the crystalline basement downrange towards the east northeast rim this explains perfectly the occurrence of coherent impact melt bodies some tens of meters in size in a restricted zone of the downrange rim of the ries crater the origin of these melt bodies which represent chemically a mixture of crystalline basement rocks similar to the main melt mass contained as melt particles 0 5 m in size in the suevite do not occur at any other portion of the ries crater rim and remained enigmatic until now although the calculated distribution of moldavites still deviates to some degree from the known distribution our results represent an important step toward a better understanding of the origin and distribution of the high velocity surface melts and the low velocity deep seated melt resulting from an oblique impact on a stratified target

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

integrated stratigraphy and 40ar 39ar chronology of the early to middle miocene upper freshwater molasse in eastern bavaria germany a detailed integrated stratigraphic study biostratigraphy and magnetostratigraphy was carried out on five sections from the western part of the bavarian upper freshwater molasse of the north alpine foreland basin nafb greatly improving the chronostratigraphy of these sediments the sections belong to the lithostratigraphic units limnische untere serie ul and fluviatile untere serie uf and contain 19 mostly new small mammal bearing levels significantly refining the local biostratigraphy radiometric ages obtained from glass shards from tuff horizons are used together with the biostratigraphic information for constructing and confirming the magnetostratigraphic correlation of the studied sections to the astronomical tuned time scale ants04 lourens et al in geologic time scale 2004 cambridge university press 2004 this correlation implies that the ul lithostratigraphic unit corresponds to the latest ottnangian and the early karpatian whereas the uf corresponds to the karpatian and the early badenian this indicates that the brackish to freshwater molasse transition already occurred during the late ottnangian the pre riesian hiatus occurred in the latest karpatian and lower early badenian in eastern bavaria and bohemia and in the late karpatian and earliest badenian in western bavaria the geochemical and ar ar data of volcanic ashes suggest that highly evolved silicic magmas from a single volcano or volcanic center characterized by a uniform nd isotopic composition erupted repetitively over the course of at least 1 6 myr three phases of eruptive activity were identified at 16 1 0 2 ma zahling 2 15 6 0 4 ma krumbad and 14 5 0 2 ma heilsberg hegau the correlation of the local biostratigraphic zonation to the ants04 enables further the characterization of both the ottnangian karpatian and karpatian badenian boundaries in the nafb by small mammal biostratigraphy according to these results the ottnangian karpatian boundary is contemporaneous with the first appearance datum of megacricetodonbavaricus in the size of the type population and the first common occurrence of keramidomys thaleri whereas ligerimys florancei melissiodon dominans and prodeinotherium aff bavaricum have been already disappeared during the late ottnangian the karpatian badenian boundary is characterized by a significant size increase of the large megacricetodon lineage and possibly a re immigration of prodeinotherium bavaricum

roter kamm impact crater namibia geochemistry of basement rocks and breccias abstract the roter kamm crater in the southern namib desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts to better define the variety of target rocks and breccias we studied the petrography and chemical composition of a new suite of twenty eight basement and breccia samples from the roter kamm crater based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two layer target region an upper layer of gariep metasediments schist marble quartzite and sandstone overlying the crystalline basement of the namaqualand metamorphic complex the basement was also heavily intruded by coarse grained quartz veins and quartz and quartz feldspar pegmatites the clast population in the melt breccias indicates that impact induced melting involved mainly metasedimentary target rocks with rarely detected contributions from pegmatite and granite granodiorite three varieties of melt breccias can be defined 1 schistose 2 quartzitic melt breccias 3 true impact melt breccias these melt breccia types are chemically heterogeneous and even the impact melt breccias may have been produced in situ and not from a coherent melt body the shapes of the schistose melt breccias previously thought to be ejected impact breccias are most likely caused by erosion and these breccias are now interpreted to be locally derived the crater basement as exposed at the rim was structurally severely affected and at least locally considerable thermal energy was generated during formation of large volumes of cataclastic mylonitic and pseudotachylitic breccias analyses of mylonite and pseudotachylites from the crater rim as well as their respective host rocks show that these breccias were mainly formed from local material analyses of pseudotachylite like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar similar to their tectonically produced counterparts although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated several breccias compared to their host rocks do show evidence of severe chemical alteration chloritisation and sericitisation the presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact induced hydrothermal event in the crater area

petrography of the impact breccias of the enkingen subo 18 drill core southern ries crater germany new estimate of impact melt volume a new drill core subo 18 that intersects a continuous sequence of impact breccias has been obtained at the town of enkingen at the southern edge of the ries inner crater detailed petrographic analysis of this core provides important new information regarding the nature and origin of within crater suevite which is not only relevant for the understanding of the formation of the ries impact structure but has strong implications for impact breccia deposition in general in the enkingen core below 21 m of crater sediment 90 m of impact breccia have been sampled most of this material is what in the past has been referred to as suevite defined as groundmass of clastic material entraining cogenetic melt fragments and target rock clasts only the bottommost 10 m of core are melt dominated impact breccia any relatively thick melt rock intersection along this core is revealed to be composed of individual centimeter to decimeter sized melt bodies best described as agglomerate due to its close resemblance to volcaniclastics of that type detailed macro meso and microscopic petrographic analysis has established that the subo 18 impact breccias cannot be divided into distinct units the groundmass is extensively altered to phyllosilicate smectite or chlorite or carbonate minerals results of a detailed scanning electron microscopic investigation of the groundmasses of samples from all parts of this sequence emphatically support the traditional definition of suevite irrespective of scale modal analysis at macroscopic to submicroscopic scales has shown that the overall average melt content of enkingen suevite is on the order of 48 vol this exceeds by far previous estimates and brings the ries crater melt volume in line with values traditionally suggested from crater and melt volume scaling for similar sized impact structures in crystalline and mixed targets

platinum group elements in the morokweng impact structure south africa evidence for the impact of a large ordinary chondrite projectile at the jurassic cretaceous boundary radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in earth history e g the cretaceous tertiary k t and jurassic cretaceous j k boundaries the morokweng impact crater in south africa is coincident in age with the j k boundary however the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks meteorites are highly enriched in the platinum group elements pge which have been widely used as a tool for identifying the presence of a meteorite signature here we present new pge analyses from the morokweng impact melt sheet our data reveal high pge concentrations and high degree of pge correlation through the melt sheet regression analysis was used to determine the projectile pge signature and constrain input from the terrestrial target rocks the closest match to morokweng is the pge signature of ordinary l or ll chondrite meteorites which is broadly in agreement with the results of an earlier cr isotope study the results of these independent studies provide strong evidence that a large ordinary chondrite projectile struck the area of morokweng in the late jurassic

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

chemistry small scale inhomogeneity and formation of moldavites as condensates from sands vaporized by the ries impact the formation of the ries impact and the moldavites have identical radiogenic ages of 14 3 ma according to this conformity in age moldavites are generally regarded as products of the ries impact this paper which is divided into two sections deals with two aspects of the moldavite forming process the formation of moldavite bodies by accretion of small precursors and the physical and chemical conditions under which these primary units originated from sands which covered the ries impact site first the chemical inhomogeneity of moldavite glasses is investigated in sections of 11 moldavites using back scattered electron bse images and electron microprobe analyses on 0 4 2 7 mm long traverses schlieren and lechatelierite particles are interpreted as relics of small chemically different precursors which accumulated to larger moldavite bodies at temperatures too low to be efficient for mixing the patterns of schlieren and lechatelierite inclusions represent two successive rheological regimes small agglomerating primary melt units were extended into thin lamellae and threads under conditions of laminar flow as evidenced by folded textures these fluidal arrays were later plastically deformed under conditions of compressional stress to elucidate the production of the primary melt units by the ries impact in the second section the geologic situation of the ries area is considered with regard to possible source materials the site of the ries impact situated at the northern border of the pre alpine molasse basin was covered at the time of the impact by fluviatile sediments of the lower sequence of the obere suswasser molasse osm formation of middle miocene age consisting of sands marly sands and clays chemical analyses for major and trace elements of 38 moldavites from bohemia and moravia and of 28 samples of the osm formation collected from outcrops and drill holes immediately south of the ries crater and south of the danube show chemical conformity of moldavites and sand samples with regard to the major parameters sio2 al2o3 feo and mgo cao indicating that these sands were the essential source material of moldavites but differences in contents of individual major and trace elements between moldavites and sands show that the formation of moldavites from sands involved a specific chemical differentiation which can not be explained by selective vaporization or melting nor by selective condensation from melt or vapeur because large ions were enriched and small ones depleted in moldavites the ionic radius has controlled the distribution of elements between sands and moldavites we assume that moldavites originated from a plasma that the impact produced at its encounter with the surficial sand formation at the impact site and that the primary units formed as early condensates in which large ions were preferentially trapped copyright 2005 elsevier ltd

suevite breccia of the ries impact crater germany petrography chemistry and shock metamorphism of crystalline rock clasts clasts of deep seated crystalline basement rocks in suevites of the ries crater germany were catalogued lithologically and classified with regard to their degree of shock metamorphism the sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior these clasts can be grouped into seven types of metamorphic and nine types of igneous rocks one hundred forty three clasts representing these lithologies were analyzed for major element bulk composition the fallout suevite contains on average 4 vol of crystalline basement clasts 0 4 vol of sedimentary rocks 16 vol of glass bodies some of them aerodynamically shaped and 79 vol of groundmass on average 52 of all crystalline clasts are from metamorphic sources and 42 are of igneous origin using the shock classification of stoffler 1974 8 of all crystalline clasts appear unshocked 10 gpa and 34 30 and 27 of clasts are shocked to stages i 10 35 gpa ii 35 45 gpa and iii 45 60 gpa respectively the bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations this indicates that the glasses originate by shock fusion of a similarly composed basement the crater suevite contains the same crystalline rock types that occur in the fallout suevites the bore hole nordlingen 1973 yields an average of 62 vol metamorphic and 38 vol igneous rocks the crater suevite differs from fallout suevites by a higher clast glass ratio by preponderance 65 95 of clasts shocked to stage i only and by the absence of aerodynamically shaped glass bodies the source of crystalline clasts and melt particles of suevites is a volume of rocks located deep in the crystalline basement to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 gpa deep burst impact model fallout suevites were ejected propelled by an expanding plume of vaporized rock and withdrew preferentially from this volume melt and highly shocked clasts leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt

chemical and ecological evolution of the miocene ries impact crater lake germany a reinterpretation based on the enkingen subo 18 drill core scientific drilling conducted at the inner slope of the miocene central ries impact crater recovered a partial section of crater lake sediments four sequences were recovered composed of suevite derived sandstones thin lignite seams bituminous shales and marlstones to claystones these flooding evaporation sequences reflect the impact of short term climatic fluctuations on a hydrologically closed basin the superimposed trend from sequences rich in bituminous shales in the lower parts of the section to sequences dominated by organic poor claystones and intercalated lignites in the upper parts of the section resembles that of the 300 m thick central crater basin succession which has previously been considered to reflect a climate controlled development from an alkaline saline lake to a freshwater lake with temporary coal swamps in the sediment core of enkingen however the change from bituminous shales to organic poor claystones with intercalated lignites is associated with a general increase in salinity as indicated by 1 palynomorphs 2 increase in 13c of the lipid biomarker archaeol bis o phytanylglycerol and 3 the occurrence of 13c enriched c20 c25 archaeol o phytanyl o sesterterpanylglycerol specific to halophilic archaea in addition the unidirectional trend in 87sr 86sr of carbonates declining from ratios of variscan basement rocks toward marine ratios indicates a change from 1 weathering of crystalline rocks and suevite to 2 ejected jurassic sediments bunte breccia in the catchment area as the major source of ion influx to the lake from that trend a change in lake water composition and a general increase in ion concentrations are inferred these new results can be applied to a reassessment of major parts of the lacustrine succession of the ries crater we use these data to propose a new hypothetical model for the chemical and ecological evolution of the ries crater lake 1 after the establishment of a stratified brackish eutrophic soda lake due to silicate weathering and evaporation the increasing influx of waters from the bunte breccia carbonate and authigenic silicate precipitation led to a mesotrophic halite lake with marine like ion ratios and concentrations 2 further increase in ions among them mg2 and sr2 resulted in hypersaline conditions with gypsum precipitation low primary production and phreatic sr rich dolomitization in marginal carbonates 3 the final sudden change to oligotrophic freshwater conditions is explained by the formation of an outlet late in the lake history we conclude that the chemical and ecological evolution of the ries lake therefore appears to have been mainly controlled by the weathering history of the catchment area with climate fluctuations causing superimposed cycles similarly changes in terrestrial palynomorph associations may at least partly reflect a change in soil types in the catchment area from fertile moist soils on suevite to dry karst soils and soils on bunte breccia these interpretations imply that the initial suevite blanket of the ries crater was much more continuous and widespread than previously assumed

the effect of target lithology on the products of impact melting impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system impact events generate pressures and temperatures that can melt a substantial volume of the target however there remains considerable discussion as to the effect of target lithology on the generation of impact melts early studies showed that for impacts into crystalline targets coherent impact melt rocks or sheets are formed with these rocks often displaying classic igneous structures e g columnar jointing and textures for impact structures containing some amount of sedimentary rocks in the target sequence a wide range of impactgenerated lithologies have been described although it has generally been suggested that impact melt is either lacking or is volumetrically minor this is surprising given theoretical constraints which show that as much melt should be produced during impacts into sedimentary targets the question then arises where has all the melt gone the goal of this synthesis is to explore the effect of target lithology on the products of impact melting a comparative study of the similarly sized haughton mistastin and ries impact structures suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets regardless of target properties furthermore using advanced microbeam analytical techniques it is apparent that for the structures under consideration here a large proportion of the melt is retained within the crater as crater fill impactites for impacts into sedimentary bearing target rocks thus it is suggested that the basic products are genetically equivalent but they just appear different that is it is the textural chemical and physical properties of the products that vary

geochemistry of drill core samples from yaxcopoil 1 chicxulub impact crater mexico the chemical composition of suevites displaced cretaceous target rocks and impactgenerated dikes within these rocks from the yaxcopoil 1 yax 1 drill core chicxulub impact crater mexico is reported and compared with the data from the yucatn 6 y6 samples within the six suevite subunits of yax 1 four units with different chemical compositions can be distinguished a upper lower sorted and upper suevite depth of 795846 m b middle suevite depth of 846861 m c brecciated impact melt rock depth of 861885 m and d lower suevite depth of 885895 m the suevite sequence a b and d display an increase of the cao content and a decrease of the silicate basement component from top to bottom in contrast the suevite of y6 shows an inverse trend the different distances of the yax 1 and y6 drilling sites from the crater center 60 and 47 km respectively lead to different suevite sequences within the cretaceous rocks of yax 1 a suevitic dike depth of 916 m does not display chemical differences when compared with the suevite while an impact melt rock dike depth of 1348 m is significantly enriched in immobile elements a clastic breccia dike depth of 1316 m is dominated by material derived locally from the host rock while the silicate rich component is similar to that found in the suevite significant enrichments of the k2o content were observed in the yax 1 suevite and the impact generated dikes all impactites of yax 1 and y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover an anhydrite component in the impactites is missing yax 1 or negligible y6

suevite of the ries crater germany source rocks and implications for cratering mechanics suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement glass inclusions and a small amount of sedimentary clasts data are given of the modal composition of fall out suevites deposited at isolated points around the crater and crater suevite forming a layer below post impact lake sediments in the crater cavity fall out suevites contain aerodynamically shaped bombs which are absent in crater suevite taking into account not only large glass fragments and bombs but also the finer fractions the glass content of fall out and crater suevites amounts to 47 and 29 vol respectively crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks mainly granites and a lower series of gneisses and amphibolite based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined suevites contain on the average 46 igneous and 54 metamorphic clasts in constrast weakly shocked and unshocked crystalline ejecta of the ries structure consist of 82 igneous and 18 metamorphic rocks

carbonate silicate liquid immiscibility upon impact melting ries crater germany the 24 km diameter ries impact crater in southern germany is one of the most studied impact structures on earth the ries impactor struck a triassic to upper jurassic sedimentary sequence overlying hercynian crystalline basement at the time of impact 14 87 0 36 ma storzer et al 1995 the 350 m thick malm limestone was present only to the south and east of the impact site to the north and west the malm had been eroded away exposing the underlying dogger and lias the largest proportion of shocked target material is in the impact melt bearing breccia suevite the suevite had been believed to be derived entirely from the crystalline basement calcite in the suevite has been interpreted as a postimpact hydrothermal deposit from optical inspection of 540 thin sections of suevite from 32 sites i find that calcite in the suevite shows textural evidence of liquid immiscibility with the silicate impact melt textural evidence of liquid immiscibility between silicate and carbonate melt in the ries suevite includes carbonate globules within silicate glass silicate globules embedded in carbonate deformable and coalescing carbonate spheres within silicate glass sharp menisci or cusps and budding between silicate and carbonate melt fluidal textures and gas vesicles in carbonate schlieren a quench crystallization sequence of the carbonate spinifex textured quenched carbonate separate carbonate spherules in the suevite mineral fragment matrix and inclusions of mineral fragments suspended in carbonate blebs given this evidence of liquid immiscibility the carbonate in the suevite therefore has like the silicate melt a primary origin by impact shock melting evidence of carbonate silicate liquid immiscibility is abundant in the suevites from the southwest to east of the ries crater the rarer suevites to the west to northeast of the crater are nearly devoid of carbonate melts this correspondence between the occurrence of outcropping limestones at the target surface and the formation of carbonate melt indicates that the malm limestones are the source rocks of the carbonate impact melt this correspondence shows that the suevites preserve a compositional memory of their source rocks from the regional distribution of suevites with or without immiscible carbonate melts it is inferred that the ries impactor hit the steep albtrauf escarpment at its toe in an oblique impact from the north

evidence of a microbial community associated with rock varnish at yungay atacama desert chile 1 rock varnish is a very slow growing nanostratigraphic coating consisting of approximately 70 fine grained clay and 30 iron and manganese oxides that forms on the surfaces of rocks in arid and semiarid climates the microbial diversity associated with rock varnish collected from the hyperarid yungay region of the atacama desert was investigated using culture independent biomolecular methods and an adenosine triphosphate atp assay the extraction of dna from rock varnish collected at yungay a region in which little to no dna has been extracted from the surface soil 1 cm to date indicates that rock varnish may provide a niche habitat for microbial life where water is essentially absent the clone library constructed suggests the presence of numerous phylogenetically distinct microorganisms ranging in diversity from cyanobacterial to a proteobacteria lineages the findings also show that only a few micrometers of varnish material are enough to shelter microbes like chroococcidiopsis spp from the intense ultraviolet radiation present in the atacama desert whether or not microorganisms are involved in its nucleation and or growth rock varnish appears to provide a microhabitat resembling cryptoendolithic communities seen on a larger scale

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the

impact glasses in fallout suevites from the ries impact structure germany an analytical sem study t impact generated glasses from fallout suevite deposits at the ries impact structure have been investigated using analytical scanning electron microscopy approximately 320 analyses of glass clasts were obtained four glass types are distinguished on the basis of composition and microtextures type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers major oxide concentrations indicate the involvement of granitic rocks amphibolites and minor al rich gneisses during melting type 2 glasses are chemically heterogeneous even within individual clasts with variations of several wt in most of the major oxides e g 57 70 wt sio2 this suggests incomplete mixing of 1 mineral derived melts or 2 whole rock melts from a wide range of lithologies aluminium rich clinopyroxene and fe mg rich plagioclase quench crystals are present in type 1 and 2 glasses respectively type 3 glasses contain substantial amounts of h2o 12 17 wt low sio2 50 53 wt high al2o3 17 21 wt and high cao 5 7 wt contents this suggests an origin due to shock melting of part of the sedimentary cover type 4 glasses form a ubiquitous component of the suevites based on their high sio2 content 85 100 wt the only possible protolith are sandstones in the lowermost part of the sedimentary succession calcite forms globules within type 1 glasses with which it develops microtextures indicative of liquid immiscibility unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1 2 and 4 glasses indicating that montmorillonite was originally an impact melt glass clearly the melt zone at the ries must have incorporated a substantial fraction of the sedimentary cover as well as the underlying crystalline basement rocks impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases type 2 3 and 4 glasses and carbonate melts