• New records of Ediacaran Acraman ejecta in drillholes from the Stuart Shelf and Officer Basin, South Australia

      Hill, A. C.; Haines, P. W.; Grey, K.; Willman, S. (The Meteoritical Society, 2007-01-01)
      New occurrences of the Acraman impact ejecta layer were recently discovered in two South Australian drillholes, SCYW-79 1a (Stuart Shelf) and Munta 1 (Officer Basin) using lithostratigraphy, acritarch biostratigraphy, carbon isotope stratigraphy, and biomarker anomalies to predict the stratigraphic position. The ejecta layer is conspicuous because it consists of pink, sandsized, angular fragments of volcanic rock distributed along the bedding plane surface of green marine siltstone. In SCYW-79 1a it forms a layer 5 mm thick; in Munta 1 the ejecta layer is thin and discontinuous because of its distance (~550 km) from the impact structure. Palynological, biomarker, and carbon isotope anomalies can now be shown to coincide with the ejecta layer in SCYW-79 1a and Munta 1 suggesting the Acraman impact event may have had far reaching influences on the rapidly evolving Ediacaran biological and geochemical cycles.
    • Post-impact structural crater modification due to sediment loading: An overlooked process

      Tsikalas, F.; Faleide, J. I. (The Meteoritical Society, 2007-01-01)
      Post-impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well-preserved impact craters: Mjølnir, Chesapeake Bay, Chicxulub, Montagnais, and Bosumtwi. The analysis demonstrates that the geometry and the structural and stratigraphic relations of post-impact strata provide information about the amplitude, the spatial distribution, and the mode of post-impact deformation. Reconstruction of the original morphology and structure for the Mjølnir, Chicxulub, and Bosumtwi craters demonstrates the long-term subsidence and differential compaction that takes place between the crater and the outside platform region, and laterally within the crater structure. At Mjølnir, the central high developed as a prominent feature during post-impact burial, the height of the peak ring was enhanced, and the cumulative throw on the rim faults was increased. The original Chicxulub crater exhibited considerably less prominent peakring and inner-ring/crater-rim features than the present crater. The original relief of the peak ring was on the order of 420-570 m (currently 535-575 m); the relief on the inner ring/crater rim was 300 450 m (currently ~700 m). The original Bosumtwi crater exhibited a central uplift/high whose structural relief increased during burial (current height 101-110 m, in contrast to the original height of 85-110 m), whereas the surrounding western part of the annular trough was subdued more that the eastern part, exhibiting original depths of 43-68 m (currently 46 m) and 49-55 m (currently 50 m), respectively. Furthermore, a quantitative model for the porosity change caused by the Chesapeake Bay impact was developed utilizing the modeled density distribution. The model shows that, compared with the surrounding platform, the porosity increased immediately after impact up to 8.5% in the collapsed and brecciated crater center (currently +6% due to post-impact compaction). In contrast, porosity decreased by 2-3% (currently -3 to -4.5% due to post-impact compaction) in the peak-ring region. The lateral variations in porosity at Chesapeake Bay crater are compatible with similar porosity variations at Mjølnir crater, and are considered to be responsible for the moderate Chesapeake Bay gravity signature (annular low of -8 mGal instead of -15 mGal). The analysis shows that the reconstructions and the long-term alterations due to post-impact burial are closely related to the impact-disturbed target-rock volume and a brecciated region of laterally varying thickness and depth varying physical properties. The study further shows that several crater morphological and structural parameters are prone to post-impact burial modification and are either exaggerated or subdued during post-impact burial. Preliminary correction factors are established based on the integrated reconstruction and post-impact deformation analysis. The crater morphological and structural parameters, corrected from post-impact loading and modification effects, can be used to better constrain cratering scaling law estimates and impact-related consequences.
    • Proceedings of the Workshop on Impact Craters as Indicators for Planetary Environmental Evolution and Astrobiology

      Ormö, J.; Deutsch, A. (The Meteoritical Society, 2007-01-01)
      Proceedings of the Workshop on Impact Craters as Indicators for Planetary Environmental Evolution and Astrobiology
    • Review of the population of impactors and the impact cratering rate in the inner solar system

      Michel, P.; Morbidelli, A. (The Meteoritical Society, 2007-01-01)
      All terrestrial planets, the Moon, and small bodies of the inner solar system are subjected to impacts on their surface. The best witness of these events is the lunar surface, which kept the memory of the impacts that it underwent during the last 3.8 Gyr. In this paper, we review the recent studies at the origin of a reliable model of the impactor population in the inner solar system, namely the near-Earth object (NEO) population. Then we briefly expose the scaling laws used to relate a crater diameter to body size. The model of the NEO population and its impact frequency on terrestrial planets is consistent with the crater distribution on the lunar surface when appropriate scaling laws are used. Concerning the early phases of our solar systems history, a scenario has recently been proposed that explains the origin of the Late Heavy Bombardment (LHB) and some other properties of our solar system. In this scenario, the four giant planets had initially circular orbits, were much closer to each other, and were surrounded by a massive disk of planetesimals. Dynamical interactions with this disk destabilized the planetary system after 500-600 Myr. Consequently, a large portion of the planetesimal disk, as well as 95% of the Main Belt asteroids, were sent into the inner solar system, causing the LHB while the planets reached their current orbits. Our knowledge of solar system evolution has thus improved in the last decade despite our still-poor understanding of the complex cratering process.
    • Sedimentological analysis of resurge deposits at the Lockne and Tvären craters: Clues to flow dynamics

      Ormö, J.; Sturkell, E.; Lindström, M. (The Meteoritical Society, 2007-01-01)
      The Lockne and Tvären craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement. The target water depth for Tvären (apparent basement crater diameter D = 2 km) was probably not over 150 m, and for Lockne (D = 7.5 km) recent best-fit numerical simulations suggest the target water depth of 500-700 m. Lockne has crystalline ejecta that partly cover an outer crater (14 km diameter) apparent in the target sediments. Tvären is eroded with only the crater infill preserved. We have line-logged cores through the resurge deposits within the craters in order to analyze the resurge flow. The focus was clast lithology, frequencies, and size sorting. We divide the resurge into resurge proper, with water and debris shooting into the crater and ultimately rising into a central water plume, anti-resurge, with flow outward from the collapsing plume, and oscillating resurge (not covered by the line-logging due to methodological reasons), with decreasing flow in diverse directions. At Lockne, the deposit of the resurge proper is coarse and moderately sorted, whereas the anti-resurge deposit is fining upwards and better sorted. The Tvären crater has a smoothly fining-up section deposited by the resurge proper and may lack anti-resurge deposits. At Lockne, the content of crystalline relative to limestone clasts generally decreases upwards, which is the opposite of Tvären. This may be a consequence of factors such as crater size (i.e., complex versus simple) and the relative target water depth. The mean grain size (i.e., the mean phi value per meter, phi) and standard deviation, i.e., size sorting (sigma) for both craters, can be expressed by the equation sigma = 0.60phi 1.25.
    • The effect of the oceans on the terrestrial crater size-frequency distribution: Insight from numerical modeling

      Davison, T.; Collins, G. S. (The Meteoritical Society, 2007-01-01)
      On Earth, oceanic impacts are twice as likely to occur as continental impacts, yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size-frequency distribution. Despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments, detailed numerical modeling, and interpretation of geological and geophysical data, no definitive relationship between impactor properties, water depth, and final crater diameter exists. In this paper, we determine the relationship between final (and transient) crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models. This relationship applies for normal incidence impacts of stoney asteroids into water-covered, crystalline oceanic crust at a velocity of 15 km s-1. We use these relationships to construct the first estimates of terrestrial crater size-frequency distributions (over the last 100 million years) that take into account the depth-area distribution of oceans on Earth. We find that the oceans reduce the number of craters smaller than 1 km in diameter by about two-thirds, the number of craters ~30 km in diameter by about one-third, and that for craters larger than ~100 km in diameter, the oceans have little effect. Above a diameter of ~12 km, more craters occur on the ocean floor than on land; below this diameter more craters form on land than in the oceans. We also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact-related resurge feature, or disturbance on the seafloor, instead of a crater.
    • Trace element concentrations in the Mexico-Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous-Paleogene boundary

      Wigforss-Lange, J.; Vajda, V.; Ocampo, A. (The Meteoritical Society, 2007-01-01)
      Four exposures of Chicxulub impact ejecta along the Mexico-Belize border have been sampled and analyzed for major and trace element abundances. The ejecta deposits consist of a lower spheroid bed, containing clay and dolomite spheroids, and an upper diamictite bed with boulders and clasts of limestone and dolomite. The matrix of both beds is composed of clay and micritic dolomite. The rare earth element (REE) compositions in the matrix of both units show strong similarities in concentrations and pattern. Furthermore, the Zr/TiO2 scatter plot shows a linear correlation indicating one source. These results indicate that the basal spheroid bed has the same source and was generated during the same event as the overlying diamictite bed, which lends support to a single-impact scenario for the Albion Formation ejecta deposits. The elevated concentrations of non-meteoritic elements such as Sb, As, U, and Zn in the matrix of the lower spheroid bed are regarded to have been derived from the sedimentary target rocks at the Chicxulub impact site. The positive Eu and Ce anomalies in clay concretion and in the matrix of the lower part of the spheroid bed in Albion Island quarry is probably related to processes involved in the impact, such as high temperature and oxidizing conditions. Analogous trace element anomalies have been reported from the distal Cretaceous-Paleogene (K/T) boundary clay layer at different sites. Thus, the trace element signals, reported herein, are regarded to support a genetic link between the Chicxulub impact, the ejecta deposits along the Mexico-Belize border, and the global K/T boundary layer.