U21E-01 INVITED
Cause(s) of the elevated bombardment in the late Eocene
The late Eocene (38 - 34 Ma) is marked by a high concentration of impact craters, ejecta layers and an elevated flux of interplanetary dust particles (IDP). The two largest craters in the Cenozoic formed in this period, the 100-km in size Popigai in Siberia, dated at 35.7 ± 0.2 Ma, and the 85-km in diameter Chesapeake Bay, offshore Virginia, dated at 35.5±0.6 Ma. Their almost coeval formation was triggered by the impact of projectiles, in the range of 4 to 5 km in diameter. Several other smaller impact structures, some of which are not as precisely dated, are also known in this late Eocene. The IDP, which arrival is recorded by an 3He anomaly in marine sediments, range in size from 1 to 1000 μm. The flux of both large bodies and small particles on Earth increased significantly over 2 to 3 Myr compared to the background. This anomalous bombardment is attributed to a comet shower in the inner Solar System, triggered by a perturbation of the Oort Cloud. The projectiles that formed the Popigai and Wanapitei (Canada, 8 km, 37± 2 Ma) craters were both ordinary chondrites; most likely L-chondrites based on platinum group elemental ratios, determined in their impact-melt lithologies. A composition not compatible with a cometary origin. Such objects were most likely derived from S-type asteroids located in the belt between Mars and Jupiter. Consequently, a major collision in the asteroid belt is another possible cause of the late Eocene elevated bombardment. The cosmic ray exposure ages of L-chondrites also support a collision on the L-chondrite parent body ~ 40 Ma ago. However, no asteroid family of that age range has so far been discovered in the belt and a particularly efficient delivery mechanism, so far unclear, is required to send large and small bodies at roughly the same time on Earth crossing orbits. In particular, considering the small fraction of large (5 km) projectiles likely to hit Earth, a huge number of fragments in this size range have to be injected into resonance positions capable of producing terrestrial impactors. Another possibility is to generate the fragments (large and small) by an asteroid disruption taking place in the region occupied by the Near Earth Objects. In this location, the break up produces a series of objects on orbits likely to impact Earth, reducing significantly the number of large fragments required. A cascade of collisions between the produced fragments could perhaps at the same time generate the continuous dust production over 2 Myr?
U21E-02 INVITED
Identification of Meteoritic Components in Terrestrial Impact Craters and Their Ejecta
The geochemistry and cosmochemistry of impacts (i.e., of impact craters and impact processes) is a rapidly developing research area that encompasses such wide-ranging topics as the simple chemical characterization of the various rock types involved (target rocks, impact breccias, melt rocks, etc.), the identification of extraterrestrial components in impact ejecta, the determination of the impactor (projectile) composition, and the determination of the causes of environmental changes from chemolithostratigraphic analyses. The recognition of geological structures and ejecta layers on Earth as being of impact origin requires the detection of either shock metamorphic effects in minerals and rocks, and/or the presence of a meteoritic component in these rocks. In addition, ejecta layers that formed from meteorite impacts can be found and confirmed by geochemical studies aimed at confirming the presence of an extraterrestrial component. In the absence of actual meteorite fragments, it is necessary to chemically search for traces of meteoritic material that is mixed in with the target rocks in breccias and melt rocks. Meteoritic components have been identified for just about 45 impact structures, out of the more than 170 impact structures that have so far been identified on Earth. The presence of a meteoritic component can be verified by measuring abundances and interelement ratios of siderophile elements, especially the platinum group elements (PGE), which are much more abundant in meteorites than in terrestrial upper crustal rocks. Often the content of the element iridium is measured as a proxy for all PGEs, because it can be measured with the best detection limit of all PGEs by neutron activation analysis, but taken out of context, small Ir anomalies alone have little diagnostic power. More reliable results can be achieved by measuring whole suites of elements, for example, the PGEs, which also avoids some of the ambiguities that result if only moderately siderophile elements (e.g., Cr, Co, Ni) are used. It is difficult to distinguish among different chondrite types based on siderophile element (or even PGE) abundances, which has led to conflicting conclusions regarding the nature of the impactor at a number of structures. In such cases, the Os and Cr isotopic systems can be used to establish the presence of a meteoritic component in a number of impact melt rocks and breccias. Both of these methods are based on the observation that the isotopic compositions of the elements Os and Cr, respectively, are different in most meteorites compared to terrestrial rocks; the Cr isotopic method allows, in addition, the identification of the projectile type (meteorite type) of the impactor.
U21E-03 INVITED
Computational Modeling of Low-Altitude Airbursts
New simulations of airbursts in the Earth's lower atmosphere from hypervelocity asteroid impacts suggest that a re-evaluation of the impact hazard is necessary to properly account for the enhanced damage potential relative to point-source approximations. The intent of these simulations was to explore the phenomenology associated with low-altitude airbursts and to determine whether the altitude of maximum energy deposition can be used as a reasonable estimate of the equivalent height of a point explosion. The simulations suggest that this not a good approximation, because the center of mass of an exploding projectile is transported downward in the form of a high-temperature jet of expanding gas. The jet descends by a significant fraction of the burst altitude before its velocity becomes subsonic. The time scale of this descent is similar to the time scale of the explosion itself, so the jet simultaneously couples both its translational and its radial kinetic energy to the atmosphere. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are felt at the surface than would be predicted for a nuclear explosion of the same yield at the same height. For impacts with a kinetic energy above some threshold, the hot jet of vaporized projectile (the descending "fireball") makes contact with the Earth's surface, where it expands radially. During the time of radial expansion, the fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. Surface materials can ablate by radiative/convective melting under these conditions, and then quench rapidly to form glass after the fireball cools and recedes. One possible example of an airburst glass is the Libyan Desert Glass of western Egypt. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.
U21E-04
Craters Clusters on Mars: Atmospheric Dispersion of Small Impactors
The HiRISE imager aboard the Mars Reconnaissance Orbiter has imaged 18 of the 20 small dark areas that correspond to fresh impact scars on the surface of Mars, reported by Malin et al in 2006 to have appeared after 1999. Operating at a resolution of approximately 30 cm/pixel, HiRISE revealed that the largest of these craters contains eolian ripples and is probably more than just a few years old. The other dark halos often contain clusters of small impact craters with diameters ranging from 1 to 30 m in diameter. About half of these are clusters of 3-10 craters, with one cluster of more than 1,000 craters near 10 m diameter. The dispersion of the clusters is typically less than 100 m, although the largest cluster, whose craters exhibit clear signs of oblique impact, spreads almost 500 m. The size and dispersion of these clusters is consistent with dispersion of incoming meteoroids by atmospheric fragmentation at an altitude of about 20 km. Modeling of the atmospheric fragmentation and impact process shows that the size of the incoming meteoroids is in the range of 0.3 to 1 m (15 m for the largest, older crater), bulk density near 2000 kg/m3 and strength 0.5 to 1 bar, consistent with the properties of terrestrial or Venusian stony impactors. Small crater clusters on Mars are thus consistent with the theory for atmospheric dispersion of impactors observed on Earth and Venus, whose mean dispersions are typically 1 km and 10 km, proportional to the square root of surface atmospheric density.
U21E-05
Investigating of the effects of target heterogeneities on terrestrial crater formation.
The shape of terrestrial impact structures such as the Chesapeake or Ries crater indicate how important pre- existing target heterogeneities are even for fairly large impact structures. Both these craters possess an inverted sombrero structure as a result of a weaker sedimentary surface layer overlying a stronger crystalline basement. But beyond such horizontal layering, closer analyzes of the subsurface geology present in these and other terrestrial craters indicate that vertical heterogeneities in the strength and geochemistry of a target are also often present. These may influence the formation and subsequent modification of terrestrial craters. In deed, evidence indicates that at Meteor crater, for example, pre-existing vertical jointing of the target gives this crater its square appearance either by confining and re-directing the shock and subsequent rarefaction waves, or by allowing preferential weathering along zones of weakness at the joints. In this study, we present a series of 2 and 3 dimensional numerical investigations of crater formation in a conceptually simple but physical complex targets: a box of randomly distributed quartz spheres of identical size. These investigation should provide some constraints on how all types of target heterogeneities influence the cratering process at broad terrestrial scales. In this particular study, we analyze the formation of craters using a range of impact velocities, projectile sizes and strength of target components. We also consider a case where we immerse the spheres in a basaltic matrix. Our approach is to use the CTH code, that solves the equations of motion, while conserving mass, energy and momentum using a second order multi-material Eulerian methodology. The adaptive mesh refinement, a fairly new capability of CTH, is paramount to these studies, and allows investigating the effects of fine-scale target heterogeneity on the cratering process, through the use of a simple microscopic models with complex, but resolvable heterogeneous geometries, rather than a complex macroscopic model. Thus, the behavior of the impact shock traversing individual spheres in the target can be modeled, although slip between sphere is difficult for individual spheres separated by void; instead the material of each sphere merge and flow together. These calculations are not intended to exactly reproduce any given observations, but rather provide insights into what factors influence observed trends in shock propagation and excavation. Initial coupling between the projectile, initial distribution of the materials in the target and the presence of the matrix all influence the shape of resulting transient crater cavities. It is the subsequent collapse and modification of such transient craters that produce the structures seen on Earth.
U21E-06
Seismic Evidence for Complex, Multi-layered, Deep Structure of the Chesapeake Bay Impact Structure, Delmarva Peninsula, Virginia, USA
Seismic velocity and reflection images from a series of seismic reflection and refraction profiles across the Chesapeake Bay impact structure (CBIS), Delmarva Peninsula, Virginia, show that the buried, ~35-m.y.-old CBIS is a complex, central-peak crater with well-defined structural features. These features include (with increasing radial distance) a flat-topped central uplift (peak), a multi-layered moat, a collapsed transient-crater margin (present central-crater margin or inner rim), and a shallowly deformed annular trough. The radial widths of the central uplift, moat, and collapsed transient-crater margin are about 5 km, 13 km, and 2 km, respectively. These widths indicate a final central-crater diameter (inner-rim diameter or collapsed transient-crater diameter) of about 40 km. We use tomographic P-wave velocity images and low-fold seismic reflection images to infer the structure and composition of rocks within the upper 5 km of the crust along a northern radius of the impact structure. By combining the seismic images with published data from two boreholes located along the seismic profile, we interpret the structure to include (from about 3.5 km depth upward): a complex of variably reflective and fractured rocks of varying compositions (units 1 through 4); poorly reflective crystalline-rock breccias of the central uplift (unit 5), strongly reflective rocks of varying compositions (unit 6); mixed crystalline-rock and sediment breccias (unit 7), and a strongly reflective section of slumped sediment megablocks and ocean-resurge sedimentary breccia (unit 8). The entire impact structure is covered by about 350 to 450 m of essentially undeformed post- impact sediments (unit 9). Units 8 and 9 thicken and dip into the moat while thinning over the central uplift and over the raised inner rim. Unit 7 and the lower part of unit 8 are interpreted as debris from the collapsed transient-crater wall.
U21E-07
Multi-directional Block and Breccia Emplacement in the Moat of the Chesapeake Bay Impact Structure Revealed by High-Resolution Seismic Reflection Surveys near the 1.76-km-deep, ICDP-USGS Eyreville Corehole
In 2004–2006, the USGS acquired four 1- to 2-km-long, high-resolution seismic reflection and refraction profiles (5-m source and geophone spacing) across the northern half of the central crater of the Chesapeake Bay impact structure (CBIS). The resulting seismic images provide details of the upper 1.0 km of the moat that surrounds the central uplift within the central crater and serve locally as a complement to 38 km of deep seismic data acquired across the CBIS by the USGS and Virginia Tech in 2004. Two profiles that cross at the 1.76-km-deep, ICDP- USGS Eyreville corehole (EC) provide semi-3D views of the stratigraphy and structures. Preliminary data processing and analysis indicate excellent correlation of the images with the EC, including individual block and impact-debris patterns in the upper 300 to 400 m of the CBIS. Variations in reflection patterns correspond to variations in conditions and processes within the central crater during impact. The seismic images show strong, mostly continuous, subhorizontal reflections typical of marine strata for the post-impact sediment units identified in the EC above 444 m depth. These strata dip toward, and thicken above, the center of the moat with relatively minor disruption; underlying synimpact units are highly disrupted. The upper 50-75 m of synimpact deposits have subhorizontal, overlapping-to-shingled reflections on all profiles that dip and pinch-out in various directions and correlate with multiple fining-upwards sequences in matrix-supported breccia of the EC (527-444 m depth). These shingled units are interpreted as deposits formed by multiple tsunami wave reverberations. Seismic signatures vary with location in the underlying 200- to 300-m-thick section of the moat. In the central part, discontinuous, chaotic reflections that bound relatively continuous lenses correlate with resurge breccias in the EC (618-527 m depth). These units are underlain by relatively continuous reflections that pinch-out in various directions as well as chaotic, discontinuous reflections (some imbricated); this interval correlates with clast- supported, sediment-clast breccias assigned to the lower resurge section in the EC (867-618 m depth). Seismic images of the inner part of the moat show more continuous seismic reflections, suggesting more uniform, uni- directional movement of breccias from the central uplift. Collectively, the seismic images suggest that the upper impactites in the central crater are highly variable laterally, with similar units at different depths deposited from multiple directions at multiple times.
U21E-08
Post-Impact Sedimentation of the Chesapeake Bay Impact Structure: Local and Regional Effects
The late Eocene (~34.5 Ma) Chesapeake Bay Impact Structure (CBIS) is one of the largest and best preserved impact sites in the world. The impact location on a shallow marine shelf preserved a record of the impact effects; 1) the initial crater excavation from the impactor; 2) collapse, mega block slumping, and immediate catastrophic sedimentation due to resurge; 3) rapid creation of accommodation due to compaction of impact generated sediments; and 4) a return to "normal" passive margin sedimentation dominated by sea-level and regional tectonic changes. We evaluate the post-impact effects of CBIS on local and regional sedimentation using USGS coreholes drilled on the annular trough (Exmore and Langley) and the recently drilled ICDP-USGS corehole at Eyreville, VA that recorded a 444 m section of post-impact sediments in the central crater. Lithostratigraphic, biostratigraphic, and Sr-isotopic studies of the Eyreville corehole identified 11 Pliocene to mid-Miocene and at least 6 lower Miocene to Oligocene depositional sequences. High rates of accommodation immediately following the impact event resulted in one upper Eocene sequence. Results from CBIS coreholes were compared to the greater mid-Atlantic Margin to examine local and regional tectonic effects. We use backstripping to show that CBIS annular trough sedimentation was strongly affected by the impact for 7 myr by crustal scale tectonics and the introduction and subsequent removal of a negative thermal anomaly. The area was subsequently dominated by passive margin thermal subsidence overprinted by intervals of regional scale uplift (10's m) and excess subsidence. The Oligocene-lower Miocene is thin regionally, both within and outside of the CBIS, indicating relative uplift compared to NJ and DE unrelated to impact. Miocene crater sequences from ca. 18-8 Ma correlate with DE and NJ, indicating a dominant control by eustatic change overprinted by excess regional subsidence. The ca. 8-2 Ma marine section is well represented in VA, especially in the Eyreville C corehole, versus NJ and DE, where it is largely nonmarine or absent. Though cited as evidence for high sea level (>25 meters), preservation of Pliocene sediments in VA was likely due to excess regional subsidence; even assuming no excess subsidence, maximum eustatic estimates for the early Pliocene are less than 20 m above present. The mechanisms behind intra-marginal changes in subsidence are unclear, but could be related to: 1) differential sediment loading on the shelf from thick, prograding clastic wedges; or 2) the response of fault-bounded Precambrian terranes to variations in the intensity and direction of far-field intraplate stress. Future backstripping of Eyreville and additional mid-Atlantic coreholes will further constrain the rates and timing of changes in subsidence, allowing for greater resolution of the processes at play.