U23A-0854
Physical Evidence of a Late-Glacial (Younger Dryas?) Impact Event in Southwestern Nova Scotia
An oval structure in southwestern Nova Scotia measuring 350 x 420 m has been identified as a possible Late- Glacial age impact crater. Evidence for an impact origin is based on integrated analysis of geomorphic, magnetic, petrographic, ground penetrating radar and stratigraphic data. A magnetic survey of the site indicates that the regional linear magnetic pattern is interrupted and distorted within the raised crystalline crater rim, due either to shock remagnetization or reorientation of broken blocks. Probable shock-metamorphic features in rim rocks, not present in unaltered rocks outside the structure, include common single and multiple sets of closely spaced (4-15 μm) planar microstructures in quartz and feldspar, kink- banded feldspar and biotite, reduced mineral birefringence, rare diaplectic feldspar and rare melt veinlets with flow textures. Fresh grain comminution, grain mosaicism and other lattice distortion features are pervasive. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 10-m-high buried scarp. Heterogeneous material under the floor, interpreted as ejecta fallback or slumpback deposits, is overlain by stratified and faulted lacustrine sediment. A Late-Glacial age is inferred through similarities in sedimentation rates to nearby bogs with well-constrained ages and the lack of any evidence of ice-sheet reworking and associated glacial deposits. Strata within the structure appear to be wedge-shaped, indicating post-glacial differential subsidence and compaction. In addition to the main crater, a cluster of arcuate, rimmed scarps 1 km north of the structure may record additional smaller impact sites, suggesting the impactor fragmented upon entry into the atmosphere producing a crater field. The oval shape of the main crater may also indicate an oblique impact or impact doublet. Continuing research focuses on identifying ejecta material in lake sediments from southwestern Nova Scotia in order to elucidate any potential link to Late-Glacial environmental change.
U23A-0855
High-precision 40Ar/39Ar Age of the Janisjärvi Impact Structure (Russia)
The ~14 km Jänisjärvi impact structure occurs within the Svecofennian Proterozoic terrains, in the southeastern part of the Baltic shield, Karelia, Russia. Previous K/Ar and 40Ar/39Ar studies were interpreted to give ages of 700 ± 5 Ma and 698 ± 22 Ma respectively, both results being difficult to interpret. Recent paleomagnetic results challenged those ages and propose instead ages of either 500 Ma or 850-900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new 40Ar/39Ar data for melt rocks from the crater. We obtained five concordant isochron ages (based on a total decay constant of 5.543 x 10-10/y and an age of 28.03 Ma for the FCs standard) that yield a combined isochron age of 682 ± 4 Ma (2 sigma) with a MSWD of 1.2, P = 0.14 and 40Ar/36Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to refine the position of the Baltica paleocontinent at this time. Argon isotopic results imply that melt homogenization has been achieved at the hundred-micron scale certainly because of the low-silica content of the molten target rock that allows fast 40Ar* diffusion in the melt. However, the large range of F(40Ar*inherited) (3 to 8 percents) observed for seven grains show that complete isotopic homogenization was not reached at the centimeter and perhaps millimeter scale. This result is in good agreement with previous Rb and Sr isotopic data.
U23A-0856
The Mjoelnir impact crater – revisited
The Mjølnir impact crater was discovered in seismic reflection data in the 1990's, and the detailed structure is well established. The submarine crater, situated in the southwestern Barents Sea, has a crater diameter of about 40 km, and a shallow relief (about 50 m). The crater is buried under a layer of sediments. Stratigraphic relationships (drill core 7329/03-U01) indicate a Cretaceous formation age at around 140 Ma ago. Here, density and magnetic susceptibility measurements of core samples are presented and used to interpret a newly available aeromagnetic survey and gravity data, and to forward model the potential field anomaly data of the area, in order to better constrain the signature of the crater. We will also develop a detailed Cretaceous palaeogeographic reconstruction at the time of impact. This investigation is conducted to support future drilling at the Mjølnir impact site.
U23A-0857
The Lake Saint Martin Impact Structure and its Relation to a Multiple Impact Event.
Prior to the collision of Shoemaker Levy 9 with Jupiter in 1994 it had been uncertain if fragmented bodies had impacted planets in the past. Approximately two hundred and fifteen million years ago a bolide impacted into what is now the Interlake region of Manitoba, Canada forming what is known as the Lake St. Martin Impact Structure (LSMIS). Today, the crater is not a visible surface feature due to past glacial activity. It has been proposed by Spray et. al, (1998) that LSMIS is one of five impact structures with approximately the same temporal and spatial characteristics to allow for a collision between a fragmented body and the Earth. The remaining suggested craters are Rochechouart, Manicouagan, Obolon and Red Wing. Our hypothesis is to examine if the LSMIS was indeed a part of a multiple impact event.
U23A-0858
Physical Properties of Suevite Section of the Eyreville Core, Chesapeake Bay Impact Structure
Chesapeake is a 35 Ma old shallow marine, complex impact structure with a diameter of ca. 85 km. The structure has previously been mapped with shallow drillings. Recently, the deep drilling into inner crater zone near Cape Charles was carried out in order to provide constraints on cratering processes in multi-layered marine targets. The Eyreville-1 core includes three holes with total depth of 1766m (Gohn et al. 2006). We are analyzing the fragments of the Eyreville core including post-impact, impact and basement units of the structure. The sampling interval was chosen dense enough to allow high-resolution petrophysical, paleomagnetic and rock magnetic data to be extracted from the core. Hereby we report the preliminary petrophysical and rock-magnetic data from suevite section of Eyreville core B. Results obtained so far show large variations in magnetic susceptibility data of suevite section. Polymict lithic breccias and cataclasites in lower part of the section are characterized by low magnetic susceptibility (below 0.0003 SI). The upper part, however, consists of more magnetic (susceptibility up to 0.006 SI) suevites. The rock- magnetic measurements (including thermal behavior of magnetic susceptibility and magnetic hysteresis) show the presence of magnetites in lower part of the section. Upper part shows additionally a distinct change in the slope of the susceptibility curve also near 350C, which may indicate the presence of pyrrhotites or maghemites. More extensive studies will be applied in near future in order to clarify the magnetomineralogy and will be presented. References: G. S. Gohn, C. Koeberl, K. G. Miller, W. U. Reimold, C. S. Cockell, J. W. Horton, W. E. Sanford, M. A. Voytek, 2006. Chesapeake Bay Impact Structure Drilled. EOS, vol 87. nr 35
U23A-0859
Insight into Impact Crater Structure Using Gravity Data
Impact cratering is one of the most important, often the most important, process occurring on planetary surfaces. On Earth, impact craters are relatively rare (~175 impacts). Almost none are pristine and exposed at the surface; those that are, are simple craters. Complex craters that are exposed are often highly eroded. However, several complex craters, 10s of km diameter, are buried in the subsurface; having been buried shortly after formation preserving their structure in pristine condition. Geophysical techniques are often the only way to assess the structural elements of a buried impact; while drilling provides direct samples; it is expensive. Gravity is a rapid technique that can reveal the major elements (e.g., diameter, presence of a central uplift) of an impact crater and in some cases, provide a reasonable interpretation that a structure is not of impact origin. The Chesapeake Bay structure is ~85 km in diameter. It has an 8 mGal central positive anomaly and a surrounding 10 mGal negative (defining the 35 km inner basin); the block-faulted outer margin does not display an anomaly because it is composed only of rotated sedimentary blocks. The central high is caused by high-density crystalline basement brought up in the central peak; the annular low is caused by low-density breccias. The overall horizontal and vertical dimensions as well as the original recognition of the central uplift were made using gravity data. The Mjolnir structure in the Barents Sea north of Norway is ~40 km in diameter. The structure has a central high (+2.5 mGal) about 14 km in diameter, and a surrounding low (-0.5 to -1.25 mGal); in this case the central high is produced by sedimentary rock with a slightly higher density. The density contrasts within the structure apparently are the result of small density / porosity changes among the sedimentary units involved in the impact. Two features serve as examples of structures that gravity data suggest are not of impact origin: Hackberry Flat, OK and Merna, NE. Hackberry Flat, a ~7 km diameter circular depression, was suggested to be an impact. Gravity data indicate that no anomaly is present. Similarly, a closed depression near Merna was suggested to be a 1.5 km young crater. Both features would be expected to exhibit a significant negative gravity anomaly; the absence of such an anomaly indicates they are not impact craters.
U23A-0860
Gravity and Magnetic Surveys of the Weaubleau Quadrangle, South Central Missouri: Implications for an Impact Origin of the Weaubleau – Osceola Structure
In August, 2001 (gravity, magnetic) and May, 2005 (magnetic) high density, GPS-registered gravity and magnetic surveys were conducted throughout the Weaubleau (pronounced WAH-blow), MO quadrangle (located approximately 90 km north of Springfield). The 2001 survey occupied 110 sites, while the 2005 survey occupied 73 sites, with some re-occupation of the 2001 survey sites. This quadrangle was chosen for study in part because it forms the southeast quarter of a proposed impact crater. Evans et al. (2003, 2004) have proposed a 19 km structure, named the Weaubleau-Osceola structure, based on surface geology and topography. We present the results of the geophysical surveys here. In addition, perform a comparison of our reduced anomaly results with synthetic anomalies from impact structure forward models. Preliminary analysis shows little geophysical support for an impact origin for the structure.
U23A-0861
Numerical Analysis of Impactor Energies on Venus
The amount of energy that an impactor has when it hits a planetary surface is a function of the impactor's velocity, mass, and the amount of energy it dissipates in the planetary atmosphere. Venus presents an unusual situation where the amount of energy lost to the atmosphere is much more significant than any other terrestrial body in the solar system. We present numerical analysis of this unique situation and what it implies for the distribution of impact craters.
U23A-0862
X-Ray Powder Diffraction as a Tool for the Identification of Impact Deformed Rocks
Previous X-ray powder diffraction (XRD) studies indicated shock deformed minerals have broader XRD peaks when compared to those of unshocked samples. Entire XRD patterns, single peak profiles and Rietveld refined parameters of carbonate samples from the Sierra Madera impact crater, west Texas and the Mission Canyon Formation of southwest Montana and western Wyoming were used to evaluate the use of X-ray powder diffraction as a tool for distinguishing impact deformed rocks from tectonically deformed rocks. Both sample locations contain rocks subjected to varying degrees of deformation. At Sierra Madera dolostone and limestone samples were collected from the crater rim (lower shock intensity) and the central uplift (higher shock intensity). Carbonate rocks of the Mission Canyon Formation were sampled along a transect across the tectonic front of the Sevier and Laramide orogenies. Peaks in the XRD patterns of shocked calcite in samples from Sierra Madera are generally broader than those of calcite samples from the Mission Canyon Formation whereas peak broadening of shocked dolomite in samples from the outer central uplift of Sierra Madera is similar to those of Mission Canyon Formation samples. Single peak profile patterns of calcite and dolomite samples from both locations are complex and their full width half maxima (FWHM) show no relationship to shock intensity, especially above ~80 ° 2θ. Rietveld refinement of peak shape parameters yields a more precise measure of the 2θ angular dependence of peak FWHM and, therefore, the degree of shock deformation. FWHM values obtained from Rietveld crystal structure refinements increase with shock intensity for all Sierra Madera samples. Additionally, FWHM values of some tectonically deformed Mission Canyon Formation calcites overlap with those of weakly shocked calcite from the crater rim of Sierra Madera. FWHM values of shocked dolomite from the central uplift of Sierra Madera are distinctly higher than tectonically deformed dolomite from the Mission Canyon Formation. This research suggests that while calcite subjected to significant shock intensities at the Sierra Madera impact crater can be differentiated from tectonically deformed calcite from the Mission Canyon Formation using Rietveld refined peak profiles, weakly shocked calcite from the crater rim may be indistinguishable from the tectonically deformed calcite. In contrast, Rietveld analysis readily distinguishes shocked Sierra Madera dolomite from tectonically deformed Mission Canyon Formation dolomite.
U23A-0863
Deposition of Distal K/T Ejecta via Density Currents
While the K/T boundary ejecta layer is well known, both the mechanics of deposition and the environmental effects of this deposition are less established. KFIX-LPL, a two-phase fluid flow code, allows us to model the interactions between the atmosphere and ejecta spherules. KFIX-LPL accommodates a range of flow regimes and includes a complete treatment of thermal radiation. We modeled a distal Chicxulub scenario (impact plume ejecta only) by injecting 250-μm spherules into the atmosphere at 8 km/s with an inflow density consistent with observed spherule volumes. The spherules fall through the thin upper atmosphere, compressing the atmosphere until the spherules decelerate due to drag and increasing atmospheric pressure. The particles accumulate in dense layers at ~50-km altitude. At intermediate distances from Chicxulub, such as North America where a dual-layer is observed at the K/T boundary, ejecta curtain material must also be considered. For these models we include an initial brief injection 500-μm terrestrial ejecta at 4.5 km/s in addition to the more prolonged flux of fireball material. The ejecta curtain material compresses the atmosphere to below 40 km in altitude. As this brief pulse ends, the atmosphere rebounds upwards and ejecta from the fireball pulse accumulates at a higher level and the two types of ejecta are deposited separately. In both the distal and North American models, the spherules initially settle through the atmosphere as individual particles, but as the ejecta near the ground, density currents form. The modeled instabilities are real density currents and not numerical artifacts, as confirmed by KFIX-LPL simulations of a series of tephra fall experiments in water (Carey 1997). We modeled these experiments by dropping spherical particles at various mass fluxes into water. Instability formation was evaluated using a criterion yielded by the ratio between turbulent instability growth rate and Stokes velocity of individual particles. Instabilities in our tephra fall models agree with both the instability criterion and the experimental results. Thus the modeled density currents are real and both the single ejecta layer observed in distal localities and the double layer observed in North American localities were deposited on a scale of hours rather than settling out slowly via Stokes flow. The K/T boundary layer truly represents a blink in geologic time.
U23A-0864
The Incomplete Impact Record and Implications for Ice Core Studies
The impact risk is extremely uncertain for objects of order 0.1-1 km diameter, with kinetic energies in the range 100 to 1 million Mt (megaton TNT ~ 4×1015 J) and recurrence times estimated in thousands to many tens of thousands of years. Millennial timescales are especially interesting, since the character of explosions (e.g. impacts, large volcanic eruptions) that only occur every 103 to 104 years lies just beyond the reckoning of modern cultural history. The impact rate predicted for the Earth based on observing nearby objects is much higher than the endemic rate estimated by counting known craters on Earth's surface. We have examined the latest account of confirmed craters from the Earth Impact Database (http://www.unb.ca/passc/ImpactDatabase/) over the last 100 Ma. The cratering record contains a large gap between 35 and 5 Ma, during which the apparent impact rate drops by an order of magnitude. The gap occurs during a period of substantial climate change, notably the initiation of large scale permanent glaciers, based on climate proxies from deep-sea sediment cores. A likely partial explanation is that climate change eroded or precluded crater formation in the recent geologic past. Taken together with constraints from inner solar system cratering and observations of near earth objects, the apparent gap in crater formation suggests that the terrestrial impact record is grossly incomplete over timescales much shorter than 100 Ma. If the true impact rate is more commensurate with the higher rates inferred from the local planetary environment, then some of the explosive fallout layers now observed in ice cores may actually be the result of recent impacts rather than volcanic eruptions. Like very large eruptions, impact ejecta are likely to be widely distributed, since impactors disrupt all levels of the atmosphere and generate ballistic debris and vapor plumes that can rise above the stratosphere. Polar ice core records of the last ~50-100 ka have become sufficiently extensive and synchronized to suggest an emerging pattern between explosive volcanic layers and abrupt climate change, although atmospheric focusing and preservation of deposits complicate analysis. By comparing ice core ash and acid signals of explosive volcanic fallout in Antarctica and Greenland, we have identified a number of common horizons which likely resulted from "eruptions" that dispersed material world-wide. These events appear to have preferentially occurred during onsets or intensifications of cooling phases, contributing to long-standing debates over possible feedbacks between climatic and geologic phenomena. The correlation may be evidence that abrupt climate change and associated changes in sea level and ice sheets can trigger volcanism. An intriguing and conspicuous alternative possibility is that catastrophic explosions force long term cooling. A relatively high rate for small impact events (objects hundreds of meters in size) could imply that impacts (instead of volcanism) are a causal factor in some of the climate variations observed during the last million years. Our method of optically probing outward from the boreholes created by ice coring missions is capable of detecting particulate layers which are invisible or largely missing in the cores, identifying candidate horizons for chemical analyses. Deliberate and discriminative analysis of the "ash" layers in ice cores could shed light on the recent impact rate. This research was supported by grant NSF ANT-0440609.
U23A-0865
Micrometeorite Impacts in Beringian Mammoth Tusks and a Bison Skull
We have discovered what appear to be micrometeorites imbedded in seven Alaskan Mammoth tusks and a Siberian bison skull. The micrometeorites apparently shattered on impact leaving 2-5 mm hemispherical debris patterns surrounded by carbonized rings. Multiple impacts are observed on only one side of the tusks and skull consistent with the micrometeorites having come from a single direction. The impact sites are strongly magnetic indicating significant iron content. We analyzed several imbedded micrometeorite fragments from both tusks and skull with Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS) and X-ray Fluorescence (XRF). These analyses confirmed the high iron content and a uniform composition highly enriched in nickel and depleted in titanium. The Fe/Ni and Fe/Ti ratios are comparable to urelite meteorites and are unlike any terrestrial sources. Prompt Gamma-ray Activation Analysis (PGAA) of a micrometeorite extracted from the bison skull indicated it contained ~0.4 mg of iron, in agreement with a micrometeorite ~1 mm in diameter. Several tusks have an average radiocarbon age of ~33 ka. This age coincides with sudden increases in global radiocarbon ~35 ka agoa and 10Be ~32 ka agob, the Mono Lake geomagnetic excursion ~34 ka agoc, and significant declines in Beringian bison, horse, brown bear, and mammoth populations and genetic diversity <36 ka agod. The bison skull shows evidence of new bone growth over the micrometeorite impact sites indicating the animal survived the bombardment and is dated at ~26 ka which is younger than the tusks. This age is consistent with exposure of the bison to an enriched source of radiocarbon following the impact. It appears likely that the impacts, cosmogenic isotope increases, magnetic excursion, and population declines are related events (Occam's razor), although their precise nature remains to be determined. aK. Hughen, et al., Science 303, 202-207 (2004). bL.R. McHargue, P.E. Damon, & D.J. Donahue, Geophys. Res. Lett. 22, 659-662 (1995). cJ.E.T. Channell, Earth Planet. Sci. Lett. 244, 379-393 (2006). dI. Barnes, et al, Current Biology 17, 1-4 (2007). http://ie.lbl.gov/Mammoth/Impact.html
U23A-0866
The Permo-Triassic Araguainha impact structure (central Brazil): impact products and collapse history in the central peak-ring
A meteorite impact hit central Brazil ca. 250 Ma ago, just prior to the Permo-Triassic boundary, leaving a 40 km wide complex impact structure (the largest impact structure identified in South America). It is a well-preserved and shallowly eroded crater, developed on sediments of the intracratonic Parana basin. The impact has excavated more than 2 km of the original sedimentary pile, bringing to the surface granites and host metamorphic rocks of the crystalline basement after collapse. It comprises a central peak ring, annular basin, two main ring features and deformed rims. Field observations and petrographic studies in impact-related materials inside the central peak ring allow recognition of six lithofacies (porphiritic-granite, foliated-granite, partially-molten granite, melt, polymitic breccias and pseudotachylytes) that record different magnetic fabrics. Porphiritic granite preserved the original igneous fabric orientation, whereas the other rocks record crater collapse, comprising foliated granites bellow a lid of strongly deformed partially molten rocks covered by breccias and melts. Comparison of these observations with available models of crater collapse will be discussed.