paleoclimatic interpretations of buried paleosols within the pre illinoian till sequence in northern missouri usa abstract northern missouri preserves shallow buried paleosols ranging in age from early to late pleistocene that are developed in six pre illinoian tills beneath illinoian mis 6 and wisconsinan mis 4 2 loess the morphology of these paleosols changes with age reflecting a changing climate during the pleistocene and cosmogenic nuclide burial dates of the respective sola provide age control on the timing of these changes the depth to secondary calcium carbonate nodules within the weathering profiles increases with younger age indicating a transition to moister conditions during the pleistocene and these nodules are absent entirely within the modern soils after approximately 0 4 ma the sola became distinctly redder even as the time available for pedogenesis became shorter culminating in the bright red sangamon geosol mis 5 this trend is consistent with increasing interglacial temperatures and or precipitation finally erosion rates determined from cosmogenic nuclide concentrations within the sola also increase systematically with younger age this increase may be due to some combination of changing climate more frequent glaciations and the deposition of a thick cover of unconsolidated glacial sediment above the stable residuum dominated preglacial landscape

deep crustal structure of the chicxulub impact crater we present the results of a wide angle seismic survey conducted over the chicxulub impact crater profile chicx a a1 is a chord across the offshore portion of the crater and chicx b f is an onshore offshore profile through the center of the crater we use travel times recorded by 32 ocean bottom seismograph and 19 land based receivers to model the crustal structure of the crater the primary feature in the shallow velocity structure is a low velocity tertiary basin the edge of which correlates with a steep gravity gradient in some regions of our survey directly beneath the tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet this unit has a thickness of 1 km and a diameter of 100 km central uplift with a diameter of 40 60 km is observed on chicx b f a lower limit on the vertical extent of uplift is 9 km shallow basement is also observed along the northwest portion of chicx b f associated with a prominent northwest trending high in the gravity field superimposed on a regional trend is moho uplift of 1 km near the center of the chicx a a1 profile with adjacent moho deepening of 1 25 1 5 km the moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity

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

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

the formation of peak ring basins working hypotheses and path forward in using observations to constrain models of impact basin formation abstract impact basins provide windows into the crustal structure and stratigraphy of planetary bodies however interpreting the stratigraphic origin of basin materials requires an understanding of the processes controlling basin formation and morphology peak ring basins exhibiting a rim crest and single interior ring of peaks provide important insight into the basin formation process as they are transitional between complex craters with central peaks and larger multi ring basins new image and altimetry data from the lunar reconnaissance orbiter as well as a suite of remote sensing datasets have permitted a reassessment of the origin of lunar peak ring basins we synthesize morphometric spectroscopic and gravity observations of lunar peak ring basins and describe two working hypotheses for the formation of peak rings that involve interactions between inward collapsing walls of the transient cavity and large central uplifts of the crust and mantle major facets of our observations are then compared and discussed in the context of numerical simulations of peak ring basin formation in order to plot a course for future model refinement and development

oblique impacts and peak ring position venus and chicxulub 1 the circular peak ring of the chicxulub impact crater is not concentric with the outer rim of the structure as defined by both gravity and seismic data although this has been attributed to an oblique impact and used to define the direction of impactor approach different groups have suggested radically different approach azimuths based on the same data sets we investigate the putative correlation between peak ring offset and impactor azimuth by examining a suite of 19 peak ring craters on venus for which the impactor s approach direction is known from their asymmetric external ejecta we find no statistically significant correlation between peak ring offset and the azimuth of the impactor s approach

cretaceous paleogene boundary exposed campeche escarpment gulf of mexico abstract we present the first multibeam bathymetric maps of the campeche escarpment a mesozoic carbonate platform in the gulf of mexico which represents the closest cretaceous paleogene k pg boundary outcrops to the chicxulub impact structure the impact of an extraterrestrial body 65 million years ago on top of this platform is implicated in the end of the cretaceous mass extinction and caused the largest debris flow yet described on earth which is found across the floor of the gulf of mexico and the caribbean sea the location of the k pg boundary has been identified in the escarpment face by combining the new multibeam data with existing information from boreholes the boundary is represented by an abrupt change in gradient on the escarpment face the morphology of the escarpment combined with seismic data reveals that a significant amount of material is missing from the face which failed catastrophically due to seismic shaking produced by the impact the escarpment face is inferred to be an important source for the extensive debris flows triggered by the impact whose deposits are found throughout much of the gulf of mexico

complex craters relationship of stratigraphy and rings to impact conditions one of the key issues associated with the understanding of large scale impacts is how the observable complex crater structural features e g central peaks and pits flat floors ring shaped ridges and depressions stratigraphic modifications and faults relate to the impactor s parameters e g radius velocity and density and the nonobservable transient crater measures e g depth of penetration and diameter at maximum penetration we have numerically modeled large scale impacts on planets for a range of impactor parameters gravity and planetary material strengths from these we found that the collapse of the transient cavity results in the development of a tall transient central peak that oscillates and drives surface waves that are arrested by the balance between gravitational forces and planetary strength to produce a wide range of the observed surface features in addition we found that the underlying stratigraphy is inverted outside of the transient cavity diameter overturned flap region but not inside this change in stratigraphy is observable by remote sensing drilling seismic imaging and gravity mapping techniques we used the above results to develop scaling laws and to make estimates of the impact parameters for the chicxulub impact and also compared the calculated stratigraphic profile with the internal structure model developed by hildebrand et al 1998 using gravity seismic and other field data for a stratigraphy rotation diameter of 90 km the maximum depth of penetration is 43 km the impactor diameter was also calculated from the scaling relationships we get for a 2 7 g cm 3 asteroid impacting at 20 km s or a 1 0 g cm 3 comet impacting at 40 km s an impactor diameter of 13 km and for a comet impacting at 60 km s an impactor diameter of 10 km

three dimensional joint inversion of traveltime and gravity data across the chicxulub impact crater in 2005 an extensive new seismic refraction data set was acquired over the central part of the chicxulub impact crater allowing us to image its structure with much better resolution than before however models derived from traveltime data are limited by the available ray coverage and the nonuniqueness that is inherent to all geophysical methods therefore many different models can fit the data equally well to address these issues we have developed a new method to simultaneously invert traveltime and gravity data to obtain an integrated model to convert velocity to density we use a linear relationship derived from measurements on core from the chicxulub impact basin thus providing a reliable conversion equation that is typical for lithologies of the central part of this crater prior to utilizing the inversion on the observed data we have run a suite of tests to establish the optimum weighting between traveltime and gravity constraints using a synthetic model of central crater structure and the real experimental geometry these synthetic tests indicate which inversion parameters lead to the best recovery of subsurface structure as well as which parts of the model are well resolved we applied the method to all existing gravity data and to seismic refraction data acquired in 1996 and the new higher resolution seismic refraction data acquired in 2005 we favor the traveltime model wherever we have sufficient ray coverage and the joint model where we have no ray coverage

grail gravity observations of the transition from complex crater to peak ring basin on the moon implications for crustal structure and impact basin formation abstract high resolution gravity data from the gravity recovery and interior laboratory grail mission provide the opportunity to analyze the detailed gravity and crustal structure of impact features in the morphological transition from complex craters to peak ring basins on the moon we calculate average radial profiles of free air anomalies and bouguer anomalies for peak ring basins protobasins and the largest complex craters complex craters and protobasins have free air anomalies that are positively correlated with surface topography unlike the prominent lunar mascons positive free air anomalies in areas of low elevation associated with large basins the bouguer gravity anomaly profiles of complex craters are highly irregular with central positive anomalies that are generally absent or not clearly tied to interior morphology in contrast gravity profiles for peak ring basins 200 km to 580 km are much more regular and are highly correlated with surface morphology a central positive bouguer anomaly is confined within the peak ring and a negative bouguer anomaly annulus extends from the edge of the positive anomaly outward to about the rim crest a number of degraded basins lacking interior peak rings have diameters and gravity patterns similar to those of well preserved peak ring basins if these structures represent degraded peak ring basins the number of peak ring basins on the moon would increase by more than a factor of two to 34 the gravity anomalies within basins are interpreted to be due to uplift of the mantle confined within the peak ring and an annulus of thickened crust between the peak ring and rim crest we hypothesize that mantle uplift is influenced by interaction between the transient cavity and the mantle further mascon formation is generally disconnected from the number of basin rings formed and occurs over a wide range of basin sizes these observations have important implications for models of basin and mascon formation on the moon and other planetary bodies

new links between the chicxulub impact structure and the cretaceous tertiary boundary the 200 km diameter chicxulub structure1 3 in northern yucatan mexico has emerged as the prime candidate for the cretaceous tertiary k t boundary impact crater3 6 concentric geophysical anomalies associated with enigmatic occurrences of upper cretaceous breccias and andesitic rocks led penfield and camargo1 to suspect that this structure was a buried impact basin more recently the discovery of shocked quartz grains in a chicxulub breccia3 and chemical similarities between chicxulub rocks and k t tektite like glasses3 6 have been advanced as evidence that the chicxulub structure is a k t impact site here we present evidence from core samples that chicxulub is indeed a k t source crater and can apparently account for all the evidence of impact distributed globally at the k t boundary without the need for simultaneous multiple impacts or comet showers shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the k t boundary ejecta layer7 8 the chicxulub melt rocks that we studied contain anomalously high levels of iridium up to 13 5 parts per 109 also consistent with the iridium enriched k t boundary layer9 our best estimate of the crystallization age of these melt rocks as determined by 40ar 39ar analyses is 65 2 0 4 1 myr in good agreement with the mean plateau age of 64 98 0 05 myr recently reported10 furthermore these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity the only such episode consistent with40ar 39ar constraints is chron 29r which includes the k t boundary

mantle deformation beneath the chicxulub impact crater abstract the surface expression of impact craters is well known from visual images of the moon venus and other planets and planetary bodies but constraints on deep structure of these craters is largely limited to interpretations of gravity data although the gravity models are non unique they do suggest that large impact craters are associated with structure at the base of the crust we use seismic data to map moho crust mantle interface topography beneath the chicxulub crater the youngest and best preserved of the three largest known terrestrial impact craters the moho is upwarped by 1 5 2 km near the center of the chicxulub crater and depressed by 0 5 1 0 km at a distance of 30 55 km from the crater center a comparison with numerical modeling results reveal that immediately following impact a transient crater reached a maximum depth of at least 30 km prior to collapse and that subsequent collapse of the transient crater uplifted target material from deep below the crater floor these results demonstrate that deformation from large terrestrial impacts can extend to the base of the continental crust a similar moho topography is also modeled for some large lunar and martian craters which suggests that mantle deformation may play a prominent role in large crater formation

structural uplift beneath the chicxulub impact structure models of the central structure of large impact craters are poorly constrained partly because of the lack of well preserved terrestrial examples and partly because of the extreme nature of impact events even large impact craters take only a few minutes to form during which time rocks from the deep crust move upward many kilometers interacting with impact melts and breccias before settling to their final position we construct a new model of central uplift beneath the chicxulub crater based upon a well constrained 3 d velocity model obtained by jointly inverting seismic traveltime and gravity data the input tomographic data set has good resolution and many rays cross the central uplift in many directions we use laboratory measurements to convert between velocity and density our velocity model possesses a high velocity zone near the crater center and velocity gradually decreases outside this zone we use regional refraction data to interpret these velocities in terms of a broad 80 km wide zone of structural uplift in which the central rocks originate from the lower crust and the surrounding rocks from the midcrust and upper crust this is in contrast with previous models in which the zone of central uplift is either 40 50 km or 150 km wide our interpretation is consistent with scaling laws yucatan basement lithology other velocity data observations at similar sized terrestrial craters and dynamic modeling of peak ring formation our model of the uplift at chicxulub can be used to help distinguish between competing models of effective target strength in numerical models of crater formation

ring faults and ring dikes around the orientale basin on the moon the orientale basin is the youngest and best preserved multiring impact basin on the moon having experienced only modest modification by subsequent impacts and volcanism orientale is often treated as the type example of a multiring basin with three prominent rings outside of the inner depression the inner rook montes the outer rook montes and the cordillera here we use gravity data from nasa s gravity recovery and interior laboratory grail mission to reveal the subsurface structure of orientale and its ring system gradients of the gravity data reveal a continuous ring dike intruded into the outer rook along the plane of the fault associated with the ring scarp the volume of this ring dike is 18 times greater than the volume of all extrusive mare deposits associated with the basin the gravity gradient signature of the cordillera ring indicates an offset along the fault across a shallow density interface interpreted to be the base of the low density ejecta blanket both gravity gradients and crustal thickness models indicate that the edge of the central cavity is shifted inward relative to the equivalent inner rook ring at the surface models of the deep basin structure show inflections along the crust mantle interface at both the outer rook and cordillera rings indicating that the basin ring faults extend from the surface to at least the base of the crust fault dips range from 13 22 for the cordillera fault in the northeastern quadrant to 90 for the outer rook in the northwestern quadrant the fault dips for both outer rings are lowest in the northeast possibly due to the effects of either the direction of projectile motion or regional gradients in pre impact crustal thickness similar ring dikes and ring faults are observed around the majority of lunar basins

testing the resolution of a 3d velocity tomogram across the chicxulub crater abstract an integrated offshore onshore reflection and refraction experiment was shot across the chicxulub impact crater in 1996 the refraction data were previously inverted in 3d using first arrival travel time tomography a regularized inversion in which both data misfit and model roughness are minimized simultaneously was used to determine a smooth velocity tomogram across the inner crater region however the experimental geometry for the refraction data was irregular causing concern that this velocity model might not be well resolved in this paper we present a suite of checkerboard tests to investigate the lateral resolution of our velocity model the chicxulub crater is located partly onshore and partly offshore with its centre close to the yucatan coastline in mexico the shallow water limited acquisition of marine reflection data to distances of not closer than 25 km from the crater centre and the centre of the structure is imaged with refraction data only intriguing velocity anomalies were observed across the central crater region providing constraints on the lithological and structural form of chicxulub a high velocity body within the central crater is most likely to represent lower crustal rocks that were stratigraphically uplifted during the formation of this complex crater the concave shape of this stratigraphic uplift suggests clues to the mechanics of large crater collapse the rocks appear to have moved upward and outward an inward dipping zone of lowered velocity has been interpreted as delimiting the outer edge of a central zone of melt rich rocks the resolution tests presented here indicate that these observed velocity anomalies are likely to be real

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

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

importance of pre impact crustal structure for the asymmetry of the chicxulub impact crater the final geometry of an impact crater can be significantly influenced by the geological and geomorphological structure of the impact site therefore it may not on its own provide accurate information regarding the direction and angle of impact

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

structural record of an oblique impact abstract most impactors strike their target at an oblique angle the common criterion for identifying craters formed by an oblique impact is the pattern of the ejecta blanket on earth however ejecta blankets are rarely preserved and other morphological structural or geophysical criteria are needed here we present structural details from the central uplift of the upheaval dome impact structure that are diagnostic of the kinematics during crater collapse and central uplift formation a characteristic imbrication of thrust slices towards the se the pattern of strata orientation within the central uplift dominant radial faults that accommodated nw se shortening and an elliptical bedding outline indicate that the displacement field during crater collapse has not been axial symmetric instead an additional lateral component roughly towards the se is preserved in the internal structure of the central uplift the structural asymmetries are largest in the core of the central uplift and disappear outwards thereby preserving the large scale circular shape of the main structural elements rim monocline ring syncline comparison with numerical models of oblique impacts suggests that the additional lateral displacement component reflects a downrange material transport during the initial stages of central uplift formation similar patterns are identified in other impact structures and may serve as general criteria for identifying the impact direction of deeply eroded impact structures in sedimentary targets

the cretaceous paleogene boundary deposit in the gulf of mexico large scale oceanic basin response to the chicxulub impact hydrocarbon exploration in the last decade has yielded sufficient data to evaluate the gulf of mexico basin response to the chicxulub asteroid impact given its passive marine setting and proximity to the impact structure on the yucatan peninsula the gulf is the premier locale in which to study the near field geologic effect of a bolide impact we mapped a thick decimeter to hectometer scale deposit of carbonate debris at the cretaceous paleogene boundary that is ubiquitous in the gulf and readily identifiable on borehole and seismic data we interpret deposits seen in seismic and borehole data in the deepwater gulf to be predominately muddy debrites with minor turbidites based on cores in the southeastern gulf mapping of the deposit in the northern gulf of mexico reveals that the impact redistributed roughly 1 05 105 km3 of sediment therein and over 1 98 105 km3 gulfwide deposit distribution suggests that the majority of sediment derived from coastal and shallow water environments throughout the gulf via seismic and megatsunamic processes initiated by the impact the texas shelf and northern margin of the florida platform were significant sources of sediment while the central and southern florida platform underwent more localized platform collapse the crustal structure of the ancestral gulf influenced postimpact deposition both directly and indirectly through its control on salt distribution in the louann salt basin nevertheless impact generated deposition overwhelmed virtually all topography and depositional systems at the start of the cenozoic blanketing the gulf with carbonate debris within days

geophysical characterization of the chicxulub impact crater 1 geophysical data indicate that the 65 5 million years ago chicxulub impact structure is a multi ring basin with three sets of semicontinuous arcuate ring faults and a topographic peak ring pr slump blocks define a terrace zone which steps down from the inner rim into the annular trough fault blocks underlie the pr which exhibits variable relief due to target asymmetries the central structural uplift is 10 km and the moho is displaced by 1 2 km the working hypothesis for the formation of chicxulub is a 50 km radius transient cavity lined with melt and impact breccia formed within 10 s of the impact and within minutes weakened rebounding crust rose kilometers above the surface the transient crater rim underwent localized deformation and collapsed into large slump blocks resulting in a inner rim at 70 85 km radius and outer ring faults at 70 130 km radius the overheightened structural uplift collapsed outward buried the inner slump blocks and formed the pr most of the impact melt was ultimately emplaced as a coherent 3 km thick melt sheet within the central basin that shallows within the inner regions of the pr smaller pockets of melt flowed into the annular trough subsequently slope collapse ejecta ground surge and tsunami waves infilled the annular trough and annular basin with sediments up to 3 km and 900 m thick respectively testing this working hypothesis requires direct observation of the impactites within and adjacent to the pr and central basin

the formation of peak rings in large impact craters large impacts provide a mechanism for resurfacing planets through mixing near surface rocks with deeper material central peaks are formed from the dynamic uplift of rocks during crater formation as crater size increases central peaks transition to peak rings without samples debate surrounds the mechanics of peak ring formation and their depth of origin chicxulub is the only known impact structure on earth with an unequivocal peak ring but it is buried and only accessible through drilling expedition 364 sampled the chicxulub peak ring which we found was formed from uplifted fractured shocked felsic basement rocks the peak ring rocks are cross cut by dikes and shear zones and have an unusually low density and seismic velocity large impacts therefore generate vertical fluxes and increase porosity in planetary crust

quantifying the attenuation of structural uplift beneath large lunar craters 1 terrestrial crater observations and laboratory experiments demonstrate that target material beneath complex impact craters is uplifted relative to its preimpact position current estimates suggest maximum uplift is one tenth of the final crater diameter for terrestrial complex craters and one tenth to one fifth for lunar central peak craters these latter values are derived from an analytical model constrained by observations from small craters and may not be applicable to larger complex craters and basins here using numerical modeling we produce a set of relatively simple analytical equations that estimate the maximum amount of structural uplift and quantify the attenuation of uplift with depth beneath large lunar craters

quantitative aspects of carbonate leaching of soils with differing ages and climates abstract carbonate leaching rates in several soils in switzerland and in the lake constance region were calculated and related to landscape development the leached depth of the soil the carbonate content and the soil density profile are sensitive to soil age a first database of carbonate leaching and its relationship to soil age is presented a simple relationship between the amount of precipitation and carbonate losses was found water draining from calcareous soils from subarctic to temperate areas was usually close to equilibrium with calcite and dolomite thus the annual amount of rainfall and consequently the percolation are the most important factors in carbonate leaching the influence of temperature is comparatively low carbonate dissolution rates amounted to approximately 1 7 3 0 mol m 2 year 1 if they are referred normalised to 1000 mm soil water fluxes soils on coarse parent material may have slightly lower carbonate dissolution rates in the order of approximately 1 7 2 2 mol m 2 year 1 normalised to a 1000 mm output while other sites with smaller grain sizes had slightly higher dissolution rates 2 2 3 0 mol m 2 year 1

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

a quantitative comparison of soil development in four climatic regimes abstract a new quantitative soil development index based on field data has been applied to chronosequences formed under different climatic regimes the four soil chronosequences developed primarily on sandy deposits have some numeric age control and are located in xeric inland merced calif xeric coastal ventura calif aridic las cruces n mex and udic susquehanna valley pa soil moisture regimes to quantify field properties points are assigned for developmental increases in soil properties in comparison to the parent material currently ten soil field properties are quantified and normalized for each horizon in a given chronosequence including two new properties for carbonate rich soils in addition to the eight properties previously defined when individual properties or the combined indexes are plotted as a function of numeric age rates of soil development can be compared in different climates the results demonstrate that 1 the soil development index can be applied to very different soil types 2 many field properties develop systematically in different climatic regimes 3 certain properties appear to have similar rates of development in different climates and 4 the profile index that combines different field properties increases significantly with age and appears to develop at similar rates in different climates the soil development index can serve as a preliminary guide to soil age where other age control is lacking and can be used to correlate deposits of different geographical and climatic regions

potential anomalies on a sphere applications to the thickness of the lunar crust a new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed we show that by raising the topography to the nth power and expanding this field into spherical harmonics potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision using a filter for downward continuing the bouguer anomaly we have computed a variety of crustal thickness maps for the moon assuming both a homogeneous as well as a dual layered crust the crustal thickness maps for the homogeneous model give plausible results but this model is not consistent with the seismic data petrologic evidence and geoid to topography ratios all of which suggest some form of crustal stratification several dual layered models were investigated and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data these models predict that the entire upper crust has been excavated beneath the major nearside multiring basins additionally significant amounts of lower crustal material was excavated from these basins especially beneath crisium this model also predicts that mantle material should not have been excavated during the south pole aitken basin forming event and that lower crustal material should be exposed at the surface in this basin

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

coeval 40ar 39ar ages of 65 0 million years ago from chicxulub crater melt rock and cretaceous tertiary boundary tektites 40 ar 39 ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180 kilometer subsurface chicxulub crater in yucatan mexico has yielded well behaved incremental heating spectra with a mean plateau age of 64 98 0 05 million years ago ma the glassy melt rock of andesitic composition was obtained from core 9 1390 to 1393 meters in the chicxulub 1 well the age of the melt rock is virtually indistinguishable from 40 ar 39 ar ages obtained on tektite glass from beloc haiti and arroyo el mimbral northeastern mexico of 65 01 0 08 ma mean plateau age for beloc and 65 07 0 10 ma mean total fusion age for both sites the 40 ar 39 ar ages in conjunction with geochemical and petrological similarities strengthen the recent suggestion that the chicxulub structure is the source for the haitian and mexican tektites and is a viable candidate for the cretaceous tertiary boundary impact site

chicxulub the third dimension of a multi ring impact basin the buried 65 ma chicxulub impact structure in mexico is the largest well preserved impact crater known on earth seismic reflection data have revealed chicxulub to be a multi ring basin it has the morphology of the largest impact craters in the solar system we use these seismic data to relate surface morphology and near surface structure to deeper deformation within the crust to provide the first high resolution look into the third dimension of a multi ring impact basin we observe three distinct topographic ring types crater rims peak rings and outer rings each is associated with a different style of deep deformation crater rims are the head scarp of the terrace zone formed during the collapse of the transient cavity the peak ring at chicxulub appears to have formed when the central uplift collapsed overthrusting and overriding the terrace zone the impact has affected the whole crust the outer rings at chicxulub are linked to whole crustal deformation in which the middle crust and lower crust have moved inward and downward strong reflections that dip craterward at 35 cut the entire crust and connect normal faulting in the sedimentary section with zones of downthrown moho at a crater radius of 35 55 km weakly developed exterior rings appear as thrust faults with small offsets these appear to be the progenitors of the more significant normally faulted outer rings in multi ring basins

the lunar crust global structure and signature of major basins new lunar gravity and topography data from the clementine mission provide a global bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar moho and therefore crustal thickness the large range of global crustal thickness 20 120 km is indicative of major spatial variations in melting of the lunar exterior and or significant impact related redistribution the 61 km average crustal thickness constrained by a depth to moho measured during the apollo 12 and 14 missions is preferentially distributed toward the farside accounting for much of the offset in center of figure from the center of mass while the average farside thickness is 12 km greater than the nearside the distribution is nonuniform with dramatic thinning beneath the farside south pole aitken basin with the global crustal thickness map as a constraint regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km in order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data the downward continuation of the bouguer gravity is stabilized by a three dimensional minimum slope and curvature algorithm both mare and non mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims the inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins for large 300 km diameter basins moho relief appears uncorrelated with diameter but is negatively correlated with basin age in several cases it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement suggesting that the lithosphere was strong enough to maintain their state of stress to the present

numerical modeling of the formation and structure of the orientale impact basin 1 the orientale impact basin is the youngest and best preserved lunar multi ring basin and has thus been the focus of studies investigating basin forming processes and final structures a consensus about how multi ring basins form however remains elusive here we numerically model the orientale basin forming impact with the aim of resolving some of the uncertainties associated with this basin by using two thermal profiles estimating lunar conditions at the time of orientale s formation and constraining the numerical models with crustal structures inferred from gravity data we provide estimates for orientale s impact energy 2 9 1025 j impactor size 50 80 km diameter transient crater size 320 480 km excavation depth 40 55 km and impact melt volume 106 km3 we also analyze the distribution and deformation of target material and compare our model results and orientale observations with the chicxulub crater to investigate similarities between these two impact structures

low angle collision with earth the elliptical impact crater matt wilson northern territory australia nearly all meteorite impact craters on earth are circular however 4 of craters should be formed by impacts at angles lower than 12 from the horizontal which should result in elongated crater structures the crater forming process that produces elliptical shapes is poorly understood we document the first elliptical crater on earth that contains a central uplift and that provides insights into the mechanisms of crater formation at a critical threshold angle of 10 15 the dimensions of the proterozoic matt wilson impact structure northern territory australia are 7 5 by 6 3 km corresponding to an aspect ratio of 1 2 with its long axis trending northeast southwest the exposed crater floor shows a preferred stacking of thrust sheets within the central uplift and in the surrounding syncline indicating northeast southwest shortening and a material transport top to the sw this is consistent with an up range to down range motion of rock caused by remnant horizontal momentum transferred from the impacting projectile to the target this preferential deformation interferes with a radially oriented convergent material flow characteristic for crater collapse the matt wilson crater provides evidence for the usefulness of structural asymmetries as a diagnostic tool to infer impact vectors the new impact crater is confirmed by the presence of planar deformation features planar fractures in quartz grains and its structural inventory

impact induced porosity and microfracturing at the chicxulub impact structure porosity and its distribution in impact craters has an important effect on the petrophysical properties of impactites seismic wave speeds and reflectivity rock permeability strength and density these properties are important for the identification of potential craters and the understanding of the process and consequences of cratering the chicxulub impact structure recently drilled by the joint international ocean discovery program and international continental scientific drilling program expedition 364 provides a unique opportunity to compare direct observations of impactites with geophysical observations and models here we combine small scale petrographic and petrophysical measurements with larger scale geophysical measurements and numerical simulations of the chicxulub impact structure our aim is to assess the cause of unusually high porosities within the chicxulub peak ring and the capability of numerical impact simulations to predict the gravity signature and the distribution and texture of porosity within craters we show that high porosities within the chicxulub peak ring are primarily caused by shock induced microfracturing these fractures have preferred orientations which can be predicted by considering the orientations of principal stresses during shock and subsequent deformation during peak ring formation our results demonstrate that numerical impact simulations implementing the dynamic collapse model of peak ring formation can accurately predict the distribution and orientation of impact induced microfractures in large craters which plays an important role in the geophysical signature of impact structures plain language summary the chicxulub crater mexico is widely known for its association with the extinction of the nonavian dinosaurs at the end of the cretaceous period the crater was first identified due to its gravitational and magnetic anomalies potential impact structures are often identified in part on the basis of geophysical anomalies most commonly including a circular gravity low gravity is slightly weaker at craters because the impact cratering process removes mass from the impact site in this study we examine the cause of the chicxulub gravity anomaly by combining observations from recent drilling of the crater geophysical data measured across the crater and numerical impact simulations we demonstrate that porosity in rocks beneath the crater floor is primarily accommodated by fracturing during the impact cratering process that the orientation of those fractures are consistent with predictions from numerical impact simulations and that impact induced porosity is one of the primary causes of gravity anomalies in large impact craters

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

cretaceous tertiary chicxulub impact angle and its consequences the chicxulub impact structure exhibits asymmetries in its geophysical signatures that closely resemble asymmetries produced by oblique impacts in laboratory experiments and recognized on planetary surfaces these asymmetric signatures suggest a trajectory for the chicxulub bolide from the southeast to the northwest at a 20 30 angle from the horizontal as a result biotic extinctions may have been most severe and catastrophic in the northern hemisphere geographic variation in the magnitude of the cretaceous tertiary k t fern spike and palynofloral extinctions are consistent with the proposed trajectory

age geological setting and paragenesis of heavy rare earth element mineralization of the tanami region western australia metasedimentary rock hosted heavy rare earth element hree mineralization occurs as numerous orebodies distributed across a large district of the tanami region of central australia close to a regional unconformity between archean metasedimentary rocks of the browns range metamorphics brm and overlying proterozoic birrindudu group sandstones the orebodies consist predominantly of quartz xenotime and minor florencite and occur along steeply dipping structures within a stockwork of hydrothermal veins and breccias paragenetic stages of the mineralization include 1 a pre ore stage of a greenschist facies overprint of detrital minerals including quartz alkali feldspar plagioclase and muscovite aligned in the premineralization foliation 2 syn ore quartz and white mica alteration associated with a multistage mineralization of the ore minerals primarily in breccias and veins and 3 a post ore stage of veining and brecciation forming several generations of quartz plus hematite barite anhydrite and pyrite in situ u pb dating of xenotime from several deposits prospects yielded an age range for mineralization of 1 65 to 1 60 ga this timeframe lacks local magmatism or orogeny and is significantly younger than the ca 1 72 ga 40ar 39ar age of the pre ore muscovite far field stresses associated with the distal isan and liebig orogenies are invoked as drivers of large scale fluid flow and fault re activation in the region we propose that ore formation was achieved via fluid leaching of ree from the brm followed by fluid mixing in fault zones especially in the vicinity of the unconformity between the brm and overlying birrindudu group sandstones this mineralization style shares many features with unconformity related u deposits and there is significant potential for discovery of further ree orebodies of this style especially in the vicinity of regional unconformities in intercontinental sedimentary basins

peak ring formation in large impact craters geophysical constraints from chicxulub a seismic reflection and three dimensional wide angle tomographic study of the buried v200 km diameter chicxulub impact crater in mexico reveals the kinematics of central structural uplift and peak ring formation during large crater collapse the seismic data show downward and inward radial collapse of the transient cavity in the outer crater and upward and outward collapse within the central structurally uplifted region peak rings are formed by the interference between these two flow regimes and involve significant radial transport of material hydrocode modeling replicates the observed collapse features impact generated melt rocks lie mostly inside the peak ring the melt appears to be clast rich and undifferentiated with a maximum thickness of 3 5 km in the center fl 2000 elsevier science b v all rights reserved

ejecta formation and crater development of the mjolnir impact crater ejecta correlation is an important element in the analysis of crater formation and its influence on the geological evolution in this study both the ejecta distribution and the internal crater development of the jurassic cretaceous mjolnir crater 40 km in diameter located in the barents sea are investigated through numerical simulations the simulations show a highly asymmetrical ejecta distribution and underscore the importance of a layer of surface water in ejecta distribution as expected the ejecta asymmetry increases as the angle of impact decreases the simulation also displays an uneven aerial distribution of ejecta the generation of the central high is a crucial part of crater formation in this study peak generation is shown to have a skewed development from approximately 50 90 sec after impact when the peak reaches its maximum height of 1 1 5 km during this stage the peak crest is moved about 5 km from an uprange to a downrange position ending with a final central position which has a symmetrical appearance that contrasts with its asymmetrical development

pliocene pleistocene incision of the green river kentucky determined from radioactive decay of cosmogenic 26al and 10be in mammoth cave sediments cosmogenic 26al and 10be in sediments washed into mammoth cave kentucky record the history of 3 5 m y of water table position governed by incision and aggradation of the green river a tributary of the ohio river upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2 3 2 4 ma a brief surge of river incision ca 2 ma was followed by river stability and cave passage formation at a lower level rapid incision through 15 m of bedrock ca 1 5 ma was prompted by repositioning of the ohio river to its present course along an ice sheet margin renewed incision ca 1 2 ma and aggradation at 0 7 0 8 ma correlate with major ice advances in the ohio river basin measurements of 26al and 10be also indicate that sandstone capped uplands have maintained slow erosion rates of 2 7 m m y for the past 3 5 m y despite accelerated pleistocene river incision rates of 30 m m y

the shape and appearance of craters formed by oblique impact on the moon and venus we surveyed the impact crater populations of venus and the moon dry targets with and without an atmosphere to characterize how the 3 dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle an empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits the results of the surveys were mostly consistent with predictions from experimental work assuming a sin2 dependence for the cumulative fraction of craters forming below angle on the moon the following transitions occur 45 degrees the ejecta blanket becomes asymmetric 25 degrees a forbidden zone develops in the uprange portion of the ejecta blanket and the crater rim is depressed in that direction 15 degrees the rim becomes saddle shaped 10 degrees the rim becomes elongated in the direction of impact and the ejecta forms a butterfly pattern on venus the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles the transitions on venus are 55 degrees the ejecta becomes heavily concentrated downrange 40 degrees a notch in the ejecta that extends to the rim appears and as impact angle decreases the notch develops into a larger forbidden zone 10 degrees a fly wing pattern develops where material is ejected in the crossrange direction but gets swept downrange no relationship between location or shape of the central structure and impact angle was observed on either planet no uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the moon for both planets as the impact angle decreases from vertical first the uprange and then the downrange rim decreases in elevation while the remainder of the rim stays at a constant elevation for craters on venus 15 km in diameter a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter the longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects one consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters

heat flow pattern at the chicxulub impact crater northern yucatan mexico abstract along an east west profile crossing the chicxulub impact structure in northern yucatan mexico curie depths were obtained from statistical spectral analysis of a grid of aeromagnetic data 256 km wide and 600 km long these depths were corrected for flight height and depth to the sea floor to determine the geothermal gradient assuming a temperature of 580 c for the curie temperature heat flow was then calculated from the geothermal gradients using a value of 2 67 w m k for the mean crustal thermal conductivity the results show a conspicuous heat flow high above on the impact basin in this location the heat flow is 80 mw m 2 approximately available offshore estimates of the depth to the crustal magnetic source bases on the northern yucatan platform and onshore heat flow determination on 8 shallow bore holes and in a 1511 m deep one support the existence of this major high heat flow anomaly associated with the impact crater this high heat flow might be related to the impact through 1 an uplift of the crystalline basement rocks in the center of the crater and 2 impact induced radioactive element concentration into the crust below the impact structure higher thermal conductivities at the lower crust might also play a key role available seismological and thermal property data are compatible with these mechanisms