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Ejecta formation and crater development of the Mjolnir impact
Abstract
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.
... The energy released during the Mjølnir impact is directly related to the volume of ejected material and to tsunami generation. These processes affected both the Barents Sea region and adjacent areas in the Arctic Shuvalov & Dypvik 2004). ...
... The cross-sectional shape of the Ragnarok Formation can be described as consisting of two elements (Figs. 14 and 15): (1) a ~ 4 -6.5 km wide annular trough beneath the present annular basin with fairly steep and well defined flanks and a maximum depth of 1.0 -1.3 km, and (2) a thinner unit that fills in the block-faulted relief beneath the outer zone, ranging in thickness from 0.05 to 0.2 km (Tsikalas et al. 1998a). On the basis of seismic observations and numerical simulations an estimated 180 -230 km 3 excavated/ejected material volume was displaced from the impact-site and re-deposited in the near vicinity (Tsikalas et al. 1998a;Shuvalov et al. 2002;Shuvalov & Dypvik 2004). ...
... A feature that can be correlated over very long distances is the extreme Ir-enrichments published from time-equivalent beds in Nordvik, Siberia, about 2500 km to the northeast (Zakharov et al. 1993). Applying the estimate of an impact from the southwest at an angle of about 45 degrees (Tsikalas 2004), along with the ejecta distribution simulations of Shuvalov & Dypvik (2004), the most probable area to track the thicker developments of the Sindre Bed will be northeast of the Mjølnir crater, towards Siberia, e.g. Nordvik (Shuvalov & Dypvik 2004). ...
In this study we present the stratigraphic succession related to the Mjølnir impact in the Barents Sea, based on available cores, detailed sedimentological and palaeontological descriptions, as well as seismic reflection profiles. The Mjolnir impact took place in the palaeoBarents Sea, close to the Volgian - Ryazanian boundary. The epicontinental sea had a water depth of 300 - 500 m, and was characterized by anoxic to hypoxic deposition of organic rich clays, presently with kerogen of types II and I. The bolide, about 1.5 - 2.0 km in size, hit the sea/sea floor and created the 40 km wide Mjølnir crater. The Ragnarok Formation is defined as the locally derived allochthonous (mixture of re-deposited excavated material, fall back ejecta and back wash material) to parautochthonous (structurally uplifted, slumped and inverted target material from deeper levels) breccia deposits that were formed during and immediately after the Mjølnir impact. The uppermost part of the formation has been cored by a stratigraphic drilling (7329/03-U-01). It comprises siliciclastic sediments from claystones to conglomerates and consists of chaotic slump and avalanche deposits, along with different mass flow deposits. The formation is normally overlain by shales and siltstones of the uppermost part of the Hekkingen Formation (Oxfordian -Berriasian), and are succeeded by marls of the Klippfisk Formation (Berriasian-Hauterivian). On seismic reflection profiles the Ragnarok Formation can reach thicknesses of 1.3 km, including uplifted, reworked Lower Triassic fragments which originated about 3.5 km down in the crust and were structurally elevated during the impact cratering stage. The Ragnarok Formation reaches its maximum thickness in a wedge-shaped annular trough beneath the present annular crater basin, pinching out both towards the crater centre and towards the periphery. It can be recognised in seismic reflection profiles at the Mjølnir crater location and up to ∼55 km away from the crater centre, forming a wedge-shaped unit within the Mesozoic siliciclastic deposits of the Bjarmeland Platform. During the oblique Mjølnir impact event, large amounts of material were ejected and widely dispersed. Models of the impact process suggest that the ejecta were mainly spread in a north-easterly direction. The impact-related ejecta bed outside the crater boundaries varies from millimetres to a few metres in thickness and has been named the Sindre Bed. It has been recognized in core 7430/10-U-01, and in other wells of the Barents Sea. An Ir-enrichment in a time-equivalent formation at Nordvik, North Central Siberia, possibly represents a distal variety of the Sindre Bed.
... Most of marine impact craters discovered on the Earth were formed in very shallow water where projectile size considerably exceeds water depth (Ormoä nd Lindstro¨m, 2000). Typical examples are the recently intensively studied Mjølnir (Shuvalov and Dypvik, 2004) and Chesapeake Bay (Crawford and Barnouin-Jha, 2004) craters. Eltanin is the only known impact structure formed in deep water where water depth considerably exceeds projectile size (Gersonde et al., 1997). ...
... Eltanin is the only known impact structure formed in deep water where water depth considerably exceeds projectile size (Gersonde et al., 1997). In the Mjølnir case the water layer only slightly influences the crater-forming flow, although the structure and morphology of the resulting crater and ejecta distribution may differ considerably from the continental craters (Shuvalov and Dypvik, 2004). This is due to the different target material petrophysical properties and to the immediately after impact effects of water on the final crater morphology and ejecta deposition through resurge flow and tsunami waves. ...
... At this moment about 55% of the total ejecta mass (mainly from groups 3 and 4) has reached the sea floor. We should note that this is not a final distribution because sedimentation has not finished and resurge flow can disturb the ejecta blanket, but this distribution is very close to the final because the velocity of resurge flow is not high outside the crater (Shuvalov and Dypvik, 2004). Moreover, the resurge flow is believed to very slightly influence the brim structure. ...
The 455 Ma old Lockne crater in central Sweden is a well-preserved and accessible instance of marine impact crater. The process of formation of the over 7 km wide crater (referred to as inner crater) in crystalline Proterozoic basement is numerically modeled under the assumption of a 45∘ oblique impact of an asteroid-like impactor. The 3D version of the SOVA multi-material hydrocode is used to model the shock wave propagation through the target, transient crater growth, material ejection in water and basement target, and water and fragmented rock ejecta expansion. The model results in a crater formation with the greatest ejection and melting transferred in the downrange direction. The model reproduces the growth of the water crater accompanied by the growth of a “wall” of ejected water at its outer margin. The basement ejecta are mostly trapped in this transient “water wall”. Only the largest ejected rock fragments could break through this water wall and thus reach distances farther than about 6 km from the center of the target. The model predicts approximately of impact melt formation, less than 10% of which is ejected outside of the inner (basement) crater, whereas the rest is reckoned to have remained within the inner crater. We assume that most of the ejected melt occurs as sand-sized fragments in the resurge sediments that formed subsequent to the collapse of the water crater that resulted in the powerful backflow of water. The model results are in accordance with several important details of the known geology of the crater. The model also outlines the difference in the marine crater formation processes in contrast to a crater with similar size formed on land.
... Nonradial structural features possibly indicative of oblique incidence are: (1) dominance of a thrust direction within a central uplift, (2) bilateral symmetry of the central uplift, which may be divided into two parts, and (3) occurrence of anticlines and synclines parallel to the symmetry axis. Recent three-dimensional (3-D) numerical simulations of obliqueimpact events ( Shuvalov and Dypvik, 2004;Elbeshausen et al., 2007) also suggest that while the asymmetry of the crater fl ow fi eld decreases with time after impact, it persists during the crater modifi cation stage, and it is particularly visible in the center of the crater during formation of the central uplift. Thus, impact direction may infl uence the deep central structure of a crater without leading to measurable deviations from a circular crater shape. ...
... The observed deformation inventory suggests a central uplift that is initiated up range and migrates down range as the central uplift and crater grow to their fi nal size. This is in agreement with fl ow fi elds inferred from numerical models of oblique-impact cratering ( Shuvalov and Dypvik, 2004;Elbeshausen et al., 2007). Sweet et al. (2005) derived up to 1 km of erosion and a possible Neoproterozoic age for the cratering event. ...
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.
... The equilibrium depth is denoted by h and the surface elevation by η. Even for oblique impacts, the later stages of the crater formation and the tsunami generation are nearly symmetric processes (Gisler et al. 2004;Shuvalov et al. 2004). Moreover, the pre-impact bathymetry in the target region probably had a low profile. ...
... We have assumed a constant sea depth of 400 m and a normal impact of a 1.6 km asteroid with a velocity of 20 km/s . However, according to Shuvalov et al. (2004) there is evidence that the Mjølnir impact was oblique, but the crater formation (and hence the generation of the tsunami) is not much affected by an oblique impact. Therefore, we base our work on a normal impact because this allows the application of radially symmetric equations, reducing the number of mathematical dimensions by one, which is highly desirable from a computational point of view. ...
Abstract— In the late Jurassic period, about 142 million years ago, an asteroid hit the shallow paleo-Barents Sea, north of present-day Norway. The geological structure resulting from the impact is today known as the Mjølnir crater. The present work attempts to model the generation and the propagation of the tsunami from the Mjølnir impact. A multi-material hydrocode SOVA is used to model the impact and the early stages of tsunami generation, while models based on shallow-water theories are used to study the subsequent wave propagation in the paleo-Barents Sea. We apply several wave models of varying computational complexity. This includes both three-dimensional and radially symmetric weakly dispersive and nonlinear Boussinesq equations, as well as equations based on nonlinear ray theory. These tsunami models require a reconstruction of the bathymetry of the paleo-Barents Sea.
The Mjølnir tsunami is characteristic of large bolides impacting in shallow sea; in this case the asteroid was about 1.6 km in diameter and the water depth was around 400 m. Contrary to earthquake- and slide-generated tsunamis, this tsunami featured crucial dispersive and nonlinear effects: a few minutes after the impact, the ocean surface was formed into an undular bore, which developed further into a train of solitary waves. Our simulations indicate wave amplitudes above 200 m, and during shoaling the waves break far from the coastlines in rather deep water. The tsunami induced strong bottom currents, in the range of 30–90 km/h, which presumably caused a strong reworking of bottom sediments with dramatic consequences for the marine environment.
... SOVA is a two-step Eulerian code that can model multi-dimensional, multi-material, large deformation, strong shock wave physics. It was successfully used to model the distribution of high-velocity ejecta, such as tektites and martian meteorites ( Artemieva and Ivanov, 2004) and low-velocity ejecta deposited just near the crater rim (Shuvalov and Dypvik, 2004). The equations of state in use allow the degree of ionization of the vapor under thermodynamic equilibrium conditions to be estimated. ...
... As high-velocity ejecta leave the crater in a rather short time interval during the excavation stage, a planar target (vertical gravity) is used to yield higher resolution near the impact site. Also, the rigid–plastic approximation for describing material strength (Dienes and Walsh, 1970; Shuvalov and Dypvik, 2004) is used for these runs, allowing correct definition of ejection angles . To model solid–molten ejecta deposition, ballistic continuation on a sphere is used for all tracers ejected from the growing crater with velocities >500 m/s and less than the lunar escape velocity (∼2.4 km/s). ...
3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor–melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5–25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations.
... The distinctive high-amplitude reflections outside of the crater would be consistent with the presence of an ejecta blanket, evacuated from the transient crater on impact. This seismic facies is more extensive than the recorded extent of relatively well-preserved ejecta of terrestrial craters (33), and it is noted that impacts into marine environments tend to lack extensive or continuous ejecta deposits when compared to "dry" impacts (34). To explain the extent of this high-amplitude layer, we interpret this deposit to be caused by a combination of an ejecta blanket and sediment reworking by the rim wave tsunami induced by the impact (6,8). ...
Evidence of marine target impacts, binary impact craters, or impact clusters are rare on Earth. Seismic reflection data from the Guinea Plateau, West Africa, reveal a ≥8.5-km-wide structure buried below ~300 to 400 m of Paleogene sediment with characteristics consistent with a complex impact crater. These include an elevated rim above a terraced crater floor, a pronounced central uplift, and extensive subsurface deformation. Numerical simulations of crater formation indicate a marine target (~800-m water depth) impact of a ≥400-m asteroid, resulting in a train of large tsunami waves and the potential release of substantial quantities of greenhouse gases from shallow buried black shale deposits. Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.
... The comparison of the appearance of the annular landform with confirmed meteorite craters in limestone plains is inconclusive given a lack of craters in similar geological and post-denudational settings and sizes. 40 km diameter Mjølnir impact crater was described from much older, Permian limestone of the Southern Norwegian Barents Sea with characteristic shock-metamorphosed quartz grains, breccia units and Ir-and Ni-rich correlative beds (Shuvalov and Dypvik, 2004). Outcrops of the Alamo breccia in Nevada (USA) contain quartz grains with traces of shock metamorphism from the Devonian impact event (Leroux et al., 1995). ...
The Nullarbor Plain is a ~200,000 km ² planar karst surface in southern Australia, underlain by Cenozoic shallow‐water limestones. During the Miocene the area was uplifted, and although the plain is generally considered extremely flat, locally, the geomorphology of the Nullarbor Plain retains evidence of earth surface processes across a long, middle Miocene‐to‐present time span. The accessibility of the recent 0.4 arc‐second TanDEM‐X digital elevation model (DEM) by the German Aerospace Centre motivated the search for other possible fine‐scale landforms that would previously have been unresolvable. The analysis of DEM images revealed an enigmatic annular landform with an outer diameter between 1200 m and 1300 m. It consists of a circular elevated rim and a central dome. Its morphology is distinct from other landforms observed on the plain, and cannot be readily explained as a part of known active or inferred/expected processes on the plain (e.g., fluvial, aeolian, karst, tectonic or extra‐terrestrial impact processes).
More recent karst processes (dayas formation) overprinting the landform indicates the presence of the annular structure prior to dayas formation. A unique microbial boundstone facies sampled from the bed‐rock of the annular structure supports an interpretation of long‐lived, at least partial retention of a primary depositional structure. Differential carbonate deposition, especially bioherm growth, was a likely origin of the annular topographical expression of the present‐day landform and is comparable to biogenic structures in the modern day deeper Great Barrier Reef.
... Note that such an event has never been observed, though traces of past events were observed, in particular in the Bellinghausen sea in Antarctica 2.15 million years ago (Kyte et al. 1981), as well as in the Barents sea ca. 142 million years ago (Shuvalov and Dypvik 2004). This kind of event occurs with return periods of at least 10 4 to 10 5 years, which explains the poor number of available studies. ...
We address the question of cosmogenic tsunamis, i.e., tsunamis due to the fall of an asteroid in the ocean, in particular on the Atlantic coasts of Europe. We apply Ward and Asphaug's method (Deep-Sea Res II 49:1073–1079, 2002) for assessing this probabilistic return period of tsunami waves, with simplifying assumptions allowing writing their results in the form of analytic formulae, some of them involving simple integrals to be computed by numerical quadrature.
We also account for uncertainties, (still following Ward and Asphaug (Deep-Sea Res II 49:1073–1079, 2002) on a simplified line. An upper bound of continental shelf shoaling is estimated ad hoc. The influence of several parameters is also analyzed, in particular the effect of tide is briefly addressed.
... Previous numerical studies clearly demonstrated that the behavior of materials moving at high velocities comparable to the impact velocity, including their masses and absolute particle velocities, is significantly affected by the spatial resolution employed in a simulation (Johnson et al., 2014;Kurosawa et al., 2018). Other numerical studies of high-speed ejecta after oblique impacts have shown that there are orders of magnitude more massive high-speed ejecta in oblique impacts than in vertical impacts (e.g., Artemieva & Shuvalov, 2008;Shuvalov & Dypvik, 2004;Shuvalov et al., 2012), and the masses of the high-speed ejecta are less sensitive to the spatial resolution in 20-100 cells per projectile radius (CPPR) (Artemieva & Ivanov, 2004). We should thus examine the spatial resolution required to reproduce the ejection dynamics of the materials originating near the impact point prior to the use of numerical codes. ...
High‐speed impact ejecta at velocities comparable to the impact velocity are expected to contribute to material transport between planetary bodies and deposition of ejecta far from the impact crater. We investigated the behavior of high‐speed ejecta produced at angles of 45° and 90°, using both experimental and numerical methods. The experimental system developed at the Planetary Exploration Research Center of Chiba Institute of Technology (Japan) allowed us to observe the initial growth of the ejecta. We succeeded in imaging high‐speed ejecta at 0.2 μs intervals for impacts of polycarbonate projectiles of 4.8 mm diameter onto a polycarbonate plate at an impact velocity of ~4 km s⁻¹. Smoothed particle hydrodynamics (SPH) simulations of various numerical resolutions were conducted for the same impact conditions as pertaining to the experiments. We compared the morphology and velocities of the ejecta for the experiments and simulations, and we confirmed a close match for high‐resolution simulations (with ≥10⁶ SPH particles representing the projectile). According to the ejecta velocity distributions obtained from our high‐resolution simulations, the ejection velocities of the high‐speed ejecta for oblique impacts are much greater than those for vertical impacts. The translational motion of penetrating projectiles parallel to the target surface in oblique impacts could cause long‐term, sustained acceleration at the root of the ejecta.
... The Hiawatha impact crater is located farther north (78.72°N) than any other known impact crater, a position that increases the probability of a northward-directed oblique impact given the majority of Earth-crossing asteroids that move in or near the ecliptic plane. Such a scenario might be analogous to the late-Jurassic Mjølnir crater, which is also large (40 km diameter), is high latitude (73.8°N), and produced an asymmetric (northward focused) ejecta layer (23). ...
... Единственная скважина на центральном Е.П. Гуров поднятии кратера вскрыла брекчированные породы и брекчии осадочных пород [20,48]. В составе брекчий и зювитов установлена слабая аномалия иридия [55]. ...
... The Hiawatha impact crater is located farther north (78.72°N) than any other known impact crater, a position that increases the probability of a northward-directed oblique impact given the majority of Earth-crossing asteroids that move in or near the ecliptic plane. Such a scenario might be analogous to the late-Jurassic Mjølnir crater, which is also large (40 km diameter), is high latitude (73.8°N), and produced an asymmetric (northward focused) ejecta layer (23). ...
We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.
... In this case, the rim that acted as a dam may burst locally and form gullies. Full destruction of the uplifted rim is expected if the resurge is able to overfl ow the entire rim, as was most likely the case for the Chesapeake Bay impact structure (Fig. 5). Numerical modeling studies show that depth of the water column, impact angle, and direction control the rim structure, ejecta distribution, and resurge characteristics of marine impact structures (Shuvalov and Dypvik, 2004;Lindström et al., 2005a). ...
The combination of petrographic analysis of drill core from the recent International Continental Scientifi c Drilling Program (ICDP)-U.S Geological Survey (USGS) drilling project and results from numerical simulations provides new constraints for reconstructing the kinematic history and duration of different stages of the Chesa-peake Bay impact event. The numerical model, in good qualitative agreement with previous seismic data across the crater, is also roughly consistent with the stratigraphy of the new borehole. From drill core observations and modeling, the following conclusions can be drawn: (1) The lack of a shock metamorphic overprint of cored basement lithologies suggests that the drill core might not have reached the parautochthonous shocked crater fl oor but merely cored basement blocks that slumped off the rim of the original cavity into the crater during crater modifi cation. (2) The sequence of polymict lithic breccia, suevite, and impact melt rock (1397-1551 m) must have been deposited prior to the arrival of the 950-m-thick resurge and avalanche-delivered beds and blocks within 5-7 min after impact. (3) This short period for transportation and deposition of impactites may suggest that the majority of the impactites of the Eyreville core never left the transient crater and was emplaced by ground surge. This
... However, the costs for deep coring in the harsh environments of the Barents Sea makes it Step 1. Drilling of five to six, up to 300-m-deep core holes five to six, up to 300-m-deep core holes to six, up to 300-m-deep core holes six, up to 300-m-deep core holes , up to 300-m-deep core holes -m-deep core holes m-deep core holes -deep core holes deep core holes in 350-400 m water depth around the Mjølnir structure to map and understand ejecta formation and distribution, coupled with in situ disturbance of sediments due to seismic and shock waves, or erosion by displaced water near the crater. Analysis of the cored material will be accompanied by sophisticated simulation models (Shuvalov and Dypvik, 2004) of the formation and deposition of ejecta in a marine environment. ...
available.
doi:10.2204/iodp.sd.6.09.2008
... However, the costs for deep coring in the harsh environments of the Barents Sea makes it Step 1. Drilling of five to six, up to 300-m-deep core holes five to six, up to 300-m-deep core holes to six, up to 300-m-deep core holes six, up to 300-m-deep core holes , up to 300-m-deep core holes -m-deep core holes m-deep core holes -deep core holes deep core holes in 350-400 m water depth around the Mjølnir structure to map and understand ejecta formation and distribution, coupled with in situ disturbance of sediments due to seismic and shock waves, or erosion by displaced water near the crater. Analysis of the cored material will be accompanied by sophisticated simulation models (Shuvalov and Dypvik, 2004) of the formation and deposition of ejecta in a marine environment. ...
In September 2007, thirty‐three scientists attended an international workshop in Norway to discuss impacts into marine targets and prepare the drilling of the 142‐million‐year‐old Mjølnir impact structure in the Barents Sea. The workshop focused on (1) mechanisms of marine impact cratering, including ejecta distribution, geothermal reactions, and tsunami production, and (2) environmental effects of marine impacts and potential links to the Jurassic/Cretaceous boundary (about 145 to 140 million years ago). A field trip visited the ejecta layer in Svalbard's Janusfjellet mountain.
Impacts, particularly in marine environments, can significantly affect Earth's geological and biological evolution. However, detailed knowledge of the marine impact cratering process is still limited. Among the 170 terrestrial craters, Mjølnir and its proximal ejecta deposits are unique: It is one of few marine impacts, and since the proximal ejecta always remained under water in calm conditions, it is most likely very well preserved. Today, the crater's central uplift is buried under ∼50 meters of postimpact sediments and 350 meters of water.
... The deformation features studied in the craters listed above suggest a downrange transport of rock and a central uplift that initiates uprange and migrates downrange as the central uplift and crater grows to its final size. This is in agreement with flow fields inferred from sophisticated three-dimensional numerical models of oblique impact cratering ( Shuvalov and Dypvik, 2004;Elbeshausen et al., 2009) (Fig. 20). Layered sedimentary rocks with much less resistance to horizontal movement than to vertical movement seem to be particularly susceptible to this type of deformation. ...
The formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms. Understanding and interpreting these styles of micro-, meso- and macroscale deformation has proved itself challenging for the field of structural geology. In this paper, we give an overview of the structural inventory found in craters of all size ranges on Earth, and look into the structures of craters on other planetary bodies. Structural features are discussed here that are caused by i) extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile, ii) the resulting flow field in the target that excavates and ejects rock materials, and iii) the gravitationally induced modification of the crater cavity into the final crater form. A special focus is put on the effects that low-angle impacting bodies have on crater formation. We hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact.
... It has also been suggested that the central peak or peak rings of complex craters may become shifted uprange and even breached along the trajectory (Schultz and Anderson 1996), but studies of natural Venusian craters do not support any correlation between central peak and peak ring location and impact angle (Ekholm and Melosh 2001;McDonald et al. 2008). Numerical simulation shows that although there is skewed development of the central peak generation during the early stages of crater modification with a downrange migration from an initial uprange position, continued crater modification leads to a central position of the peak in the final crater (Shuvalov and Dypvik 2004). However, despite its final central position, the development of the central peak is not axially symmetric (Scherler et al. 2006;Kenkmann and Poelchau 2009;Wulf et al. 2012), the extreme case being the elongated central ridge seen in some elliptical craters from highly oblique impacts (e.g., Bottke et al. 2000). ...
Most impacts occur at an angle with respect to the horizontal plane.
This is primarily reflected in the ejecta distribution, but at very low
angle structural asymmetries such as elongation of the crater and
nonradial development of the central peak become apparent.
Unfortunately, impact craters with pristine ejecta layers are rare on
Earth and also in areas with strong past or ongoing surface erosion on
other planetary bodies, and the structural analysis of central peaks
requires good exposures or even on-site access to outcrop. However,
target properties are known to greatly influence the shape of the
crater, especially the relatively common target configuration of a
weaker layer covering a more rigid basement. One such effect is the
formation of concentric craters, i.e., a nested, deeper, inner crater
surrounded by a shallow, outer crater. Here, we show that with
decreasing impact angle there is a downrange shift of the outer crater
with respect to the nested crater. We use a combination of (1) field
observation and published 3-D numerical simulation of one of the best
examples of a terrestrial, concentric impact crater formed in a layered
target with preserved ejecta layer: the Lockne crater, Sweden; (2)
remote sensing data for three pristine, concentric impact craters on
Mars with preserved ejecta layers further constraining the direction of
impact; as well as (3) laboratory impact experiments, to develop the
offset in crater concentricity into a complementary method to determine
the direction of impact for layered-target craters with poorly preserved
ejecta layers.
... It is based on the same principles utilized in the well-known CTH code (McGlaun et al. 1990). It includes a general treatment of viscosity for modeling viscous flow with Newtonian or Bingham rheology, while the implementation of the rigid-plastic model (Dienes and Walsh 1970) allows us to model material strength (Shuvalov and Dypvik 2004). The initial spatial resolution is usually 20 CPPR near the impact point. ...
We present the results of numerical modeling of the formation of the
Ries crater utilizing the two hydrocodes SOVA and iSALE. These standard
models allow us to reproduce crater shape, size, and morphology, and
composition and extension of the continuous ejecta blanket. Some of
these results cannot, however, be readily reconciled with observations:
the impact plume above the crater consists mainly of molten and
vaporized sedimentary rocks, containing very little material in
comparison with the ejecta curtain; at the end of the modification
stage, the crater floor is covered by a thick layer of impact melt with
a total volume of 6-11 km3; the thickness of true fallback
material from the plume inside the crater does not exceed a couple of
meters; ejecta from all stratigraphic units of the target are
transported ballistically; no separation of sedimentary and crystalline
rocks—as observed between suevites and Bunte Breccia at
Ries—is noted. We also present numerical results quantifying the
existing geological hypotheses of Ries ejecta emplacement from an impact
plume, by melt flow, or by a pyroclastic density current. The results
show that none of these mechanisms is consistent with physical
constraints and/or observations. Finally, we suggest a new hypothesis of
suevite formation and emplacement by postimpact interaction of hot
impact melt with water or volatile-rich sedimentary rocks.
... It is based on the same principles utilized in the well-known CTH code (McGlaun et al. 1990). It includes a general treatment of viscosity for modeling viscous flow with Newtonian or Bingham rheology, while the implementation of the rigid-plastic model (Dienes and Walsh 1970) allows us to model material strength (Shuvalov and Dypvik 2004). The initial spatial resolution is usually 20 CPPR near the impact point. ...
Presented numerical models cannot reproduce the previous hypotheses on suevite origin as plume-related non-ballistic ejecta. We suggest an alternative explanation.
... In this case, the rim that acted as a dam may burst locally and form gullies. Full destruction of the uplifted rim is expected if the resurge is able to overfl ow the entire rim, as was most likely the case for the Chesapeake Bay impact structure (Fig. 5). Numerical modeling studies show that depth of the water column, impact angle, and direction control the rim structure, ejecta distribution, and resurge characteristics of marine impact structures (Shuvalov and Dypvik, 2004;Lindström et al., 2005a). ...
The combination of numerical simulation results and petrographic analysis of drill core from the recent ICDP-USGS drilling project provides new insight into the formation of the Chesapeake Bay impact crater.
... It provides an important baseline to help future studies of potential impact horizons in similar rocks. Smelror and Dypvik (2003) and Shuvalov and Dypvik (2004) have recently suggested that the distribution of Mjølnir ejecta close to the crater in the Barents Sea indicates that the impact was oblique and that principal transport and deposition of ejecta took place downrange to the northeast of the crater. Their modelling predicts little or no ejecta south of the impact site. ...
... It is based on the same principles utilized in the well-known CTH code ( McGlaun et al. 1990). It includes a general treatment of viscosity for modeling viscous flow with Newtonian or Bingham rheology, while the implementation of the rigid-plastic model (Dienes and Walsh 1970) allows us to model material strength (Shuvalov and Dypvik 2004). The initial spatial resolution is usually 20 CPPR near the impact point. ...
We present the results of numerical modeling of the formation of the Ries crater utilizing the two hydrocodes SOVA and iSALE. These standard models allow us to reproduce crater shape, size, and morphology, and composition and extension of the continuous ejecta blanket. Some of these results cannot, however, be readily reconciled with observations: the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks, containing very little material in comparison with the ejecta curtain; at the end of the modification stage, the crater floor is covered by a thick layer of impact melt with a total volume of 6–11 km 3 ; the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters; ejecta from all stratigraphic units of the target are transported ballistically; no separation of sedimentary and crystalline rocks— as observed between suevites and Bunte Breccia at Ries—is noted. We also present numerical results quantifying the existing geological hypotheses of Ries ejecta emplacement from an impact plume, by melt flow, or by a pyroclastic density current. The results show that none of these mechanisms is consistent with physical constraints and/or observations. Finally, we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile-rich sedimentary rocks.
... Therefore, the ejecta blanket is formed by particles launched at velocity u ¼ ffiffiffiffiffiffi ffi Dg p ¼ U ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi n=1:61 p , and the ejecta blanket around larger craters is formed by faster ejecta fragments that are launched at earlier times (and smaller distances from the crater center). On the other hand, the very early stage of an oblique impact is very asymmetric, and the asymmetry decreases in time as the crater grows (Shuvalov and Dypvik 2004). Consequently, the ejecta blanket around larger craters (or at larger gravity) should be more asymmetric than that around smaller craters (or smaller gravity) at the same impact angle. ...
Abstract– Simple estimates suggest that ejecta blankets around larger craters should be more asymmetric than around smaller craters for the same oblique impact angle. Numerical simulations presented in the paper confirm that an increase in the scale of gravity-dominated craters (and in the size of the corresponding projectiles) increases the asymmetry of both impact craters and ejecta blankets around them.
... The component of initial horizontal momentum is present until the start of the crater floor uplift. 3D numerical simulations of the crater floor flow in oblique impacts by Shuvalov and Dypvik (2004) strongly support the assumption of an asymmetric crater flow structure indicative for the impact vector. They also state that "late" or proximal ejecta near the crater rim still shows slight asymmetry. ...
Abstract— In oblique impacts with an impact angle under 45°, the bilateral shape of the distal ejecta blanket is used as the strongest indicator for an impact vector. This bilateral symmetry is attenuated and is superimposed by radial symmetry towards the crater rim, which remains circular for impact angles down to 10–15°. The possibility that remnants of bilateral symmetry might still be present in the most proximal ejecta, the overturned flap and the crater rim was explored with the intention of deducing an impact vector. A model is presented that postulates bilateral patterns using proximal ejecta trajectories and predicts these patterns in the orientation of bedding planes in the crater rim. This model was successfully correlated to patterns described by radial grooves in the proximal ejecta blanket of the oblique Tooting crater on Mars. A new method was developed to detect structural asymmetries by converting bedding data into values that express the deviation from concentric strike orientation in the crater rim relative to the crater center, termed “concentric deviation.” The method was applied to field data from Wolfe Creek crater, Western Australia. Bedding in the overturned flap implies an impactor striking from the east, which refines earlier publications, while bedding from the inner rim shows a correlation with the crater rim morphology.
... The validation of these features as diagnostic tools to infer the impact direction was possible at the elliptical impact structure Matt Wilson, Northern Territories, Australia , as well as for Martin crater on Mars, where an asymmetric ejecta blanket provided independent proof of the impact trajectory ). These findings are in agreement with flow fields inferred from numerical models of oblique impact cratering (Shuvalov and Dypvik 2004;Elbeshausen et al. 2007). ...
Abstract– Serra da Cangalha is a complex impact structure with a crater diameter of 13,700 m and a central uplift diameter of 5800 m. New findings of shatter cones, planar fractures, feather features, and possible planar deformation features are presented. Several ring-like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata. The target at Serra da Cangalha is composed of Devonian to Permian sedimentary rocks, mainly sandstones that are interlayered with siltstone and claystones. NNE–SSW and WNW–ESE-striking joint sets were present prior to the impact and also overprinted the structure after its formation. As preferred zones of weakness, these joint sets partly controlled the shape of the outer perimeter of the structure and, in particular, affected the deformation within the central uplift. Joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift was formed. These planes were used as major displacement zones. The asymmetry of the central uplift, with preferred overturning of strata in the northern to western sector, may suggest a moderately oblique impact from a southerly direction. Buckle folding of tilted strata, as well as strata overturning, indicates that the central uplift became gravitationally unstable at the end of crater formation.
... Laboratory-scale oblique impact experiments and 3D numerical modeling studies suggest that in oblique impacts the transient cavity is very asymmetric in the earliest stages of formation and progressively loses its asymmetry as it grows, due to geometric spreading (Schultz and D'Hondt, 1996;Anderson et al., 2003;Shuvalov and Dypvik, 2004;Elbeshausen et al., 2007). The asymmetry of the final crater, therefore, depends not only on the angle of impact, but also on the size of the crater in relation to the size of the projectilecommonly referred to as the cratering efficiency. ...
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.
... SOVA is a two-step Eulerian code that can model multidimensional, multimaterial, large deformation, strong shock wave physics. It includes a general treatment of viscosity for modeling viscous flow with Newtonian or Bingham rheology, while the implementation of the Rigid-Plastic Model (RPM; Dienes and Walsh 1970;Shuvalov and Dypvik 2004) allows us to mimic plastic behavior of the projectile. In addition, SOVA can describe the motion of solid/melt particles in an evolving ejecta-gas-vapor plume and their momentum-energy exchange using two-phase hydrodynamics, which takes into account both individual particle characteristics (mass, density, shape) and their collective behavior (momentum and energy exchange with surrounding gas). ...
Abstract— Meteor Crater is one of the first impact structures systematically studied on Earth. Its location in arid northern Arizona has been ideal for the preservation of the structure and the surviving meteoric material. The recovery of a large amount of meteoritic material in and around the crater has allowed a rough reconstruction of the impact event: an iron object 50 m in diameter impacted the Earth's surface after breaking up in the atmosphere. The details of the disruption, however, are still debated. The final crater morphology (deep, bowl-shaped crater) rules out the formation of the crater by an open or dispersed swarm of fragments, in which the ratio of swarm radius to initial projectile radius Cd is larger than 3 (the final crater results from the sum of the craters formed by individual fragments). On the other hand, the lack of significant impact melt in the crater has been used to suggest that the impactor was slowed down to 12 km/s by the atmosphere, implying significant fragmentation and fragments' separation up to 4 initial radii. This paper focuses on the problem of entry and motion through the atmosphere for a possible Canyon Diablo impactor as a first but necessary step for constraining the initial conditions of the impact event which created Meteor Crater. After evaluating typical models used to investigate meteoroid disruption, such as the pancake and separated fragment models, we have carried out a series of hydrodynamic simulations using the 3D code SOVA to model the impactor flight through the atmosphere, both as a continuum object and a disrupted swarm.Our results indicate that the most probable pre-atmospheric mass of the Meteor Crater projectile was in the range of 4.108to 1.2.109kg (equivalent to a sphere 46–66 m in diameter). During the entry process the projectile lost probably 30% to 70% of its mass, mainly because of mechanical ablation and gross fragmentation. Even in the case of a tight swarm of particles (Cd < 3), small fragments can separate from the crater-forming swarm and land on the plains (tens of km away from the crater) as individual meteorites. Starting from an impactor pre-atmospheric velocity of ˜18 km/s, which represents an average value for Earth-crossing asteroids, we find that after disruption, the most probable impact velocity at the Earth's surface for a tight swarm is around 15 km/s or higher. A highly dispersed swarm would result in a much stronger deceleration of the fragments but would produce a final crater much shallower than observed at Meteor Crater.
... Согласно численным расчетам В. Шувалова и В. Светцова (Shuvalov V., Dypvik H. 2004;Svetsov V., 2006Svetsov V., , Светцов В.В., 2007Svetsov V., , 2008) испаренная масса верхней оболочки в единичном ударе составляет 0.07-0.1 от массы ударника для 50-км тел (для астероидных тел, имеющих скорости около 15-20 км/с). Испаренное породообразующее вещество быстро (сутки) остывает, конденсируется и выпадает на планету. ...
Knowledge of the physical and chemical conditions on the primeval Earth is important for the study of the origin of the biosphere.
This paper discusses the latest modification of the theory of the origin of the Earth and other planets. Possible consequences
of the formation of the Sun in the area of the star formation closely surrounded by neighboring young stars are considered.
The classical problem of the rate of accretion of Earth and other planets is generalized with new estimates allowing the correlation
of the results from long-lived (U-Pb) and short-lived (Hf-W) space-chronometers. A model of the early evolution of the Earth,
based on both dynamic estimates and the latest geochemical data (earliest Australian zircons, relict xenon pleiad) is discussed.
The problems of the theory of early Earth’s evolution, which so far cannot be adequately solved, are discussed.
Key wordsOrigin of Earth–primeval Earth–Earth’s early evolution
... However, the costs for deep coring in the harsh environments of the Barents Sea makes it Step 1. Drilling of five to six, up to 300-m-deep core holes five to six, up to 300-m-deep core holes to six, up to 300-m-deep core holes six, up to 300-m-deep core holes , up to 300-m-deep core holes -m-deep core holes m-deep core holes -deep core holes deep core holes in 350–400 m water depth around the Mjølnir structure to map and understand ejecta formation and distribution, coupled with in situ disturbance of sediments due to seismic and shock waves, or erosion by displaced water near the crater. Analysis of the cored material will be accompanied by sophisticated simulation models (Shuvalov and Dypvik, 2004) of the formation and deposition of ejecta in a marine environment. ...
In September 2007, thirty-three scientists attended an international workshop in Longyearbyen (Svalbard, Norway) to discuss impacts of extraterrestrial bodies into marine environment and to prepare for the drilling of the 142-Ma-old Mjølnir impact structure in the Barents Sea (Fig. 1; Gudlaugsson, 1993; Dypvik et al., 1996, Tsikalas et al., 1998). A field trip visited the ejecta layer in the Janusfjellet Mountain in Isfjorden, just outside Longyearbyen (Fig. 2). The workshop focused on two topics: 1) mechanisms of marine impact cratering including ejecta formation and distribution, geothermal reactions, and the formation of tsunami, and 2) environmental effects of marine impacts. Both topics are highly relevant to the Mjølnir event and the geological evolution of the Arctic, as well as to the biological changes at the Jurassic-Cretaceous boundary. Against thisbackground were a) concrete drilling targets formulated, b) plans outlined for compiling data from existing geological and geophysical surveys as the basis for Integrated Ocean Drilling Program (IODP) and International Continental Scientific Drilling Program (ICDP) drilling proposals, and c) a steering group and science teams established for compiling old and new material as a foundation for the developmentof drilling proposal.
Impacts into marine targets are known to create abnormal crater morphologies. We investigate the formation of the ~4 km diameter Flynn Creek marine target impact crater using the iSALE hydrocode. We compare simulation results to topographic profiles, mineral pressure indicators, and breccia sequencing from drill cores to determine the most likely sea depth at this location at the time of impact (~360 Ma, Tennessee, USA): 700–800 m. Both the peak shock pressure produced by the impact and the mechanism(s) of central peak formation differ with sea depth. The large central mound of Flynn Creek could have been produced in three distinct ways, all requiring the presence of an ocean: (1) a relatively cohesive rim collapse deposit that reached the crater center as part of a ground flow and came to rest on top of the existing crater stratigraphy; (2) chaotic resurge of ejecta with the returning ocean that deposited at the crater center; (3) large uplift facilitated by the removal of overburden pressure from a deep ocean. The first two of these mechanisms create “false peaks” in which high‐shock uplifted material and original crater floor are buried beneath > 200 m of relatively low shock material. Our simulations suggest that drilling of marine impact sites might require deeper than expected drill cores, so that any high‐pressure mineralogical indictors at depth can be accessed.
The diameter range of 15 to 20 km is within the transition from simple to complex impact craters located on the Moon. This range spans roughly a factor of 3 in impact energy for the same impactor speed, composition, and trajectory angle. We analyzed the global population of well‐preserved craters in this size range in order to assess the effects of target and impactor properties on crater shapes and morphologies. We observed that within this narrow diameter range, simple craters are confined to the highlands, and complex craters are more abundant in the mare. We found unusually deep craters around the highlands‐mare boundaries and favor the hypothesis that they form by impact cratering on high‐porosity terrain. We infer that target properties primarily contribute to the observed morphological variations in the craters. Simple crater formation is favored by a terrain that is more homogeneous in strength and topography, while transitional and complex crater formation is aided by heterogeneity in lithology, topography, or strength, or a combination of these parameters. Clearly visible impact melt deposits in a small percentage of simple craters, and two cases of craters differing in morphologies from their nearest neighbors in similar geologic settings, suggest that variation in impactor properties such as impact velocity and impactor size may have some role in causing morphological differences between similar‐sized craters.
We investigate the stratigraphic record of the Permian--Triassic intracratonic Paraná Basin of South America for evidence of the Araguainha impact event. Soft-sediment deformation features are widespread in the latest Permian, uppermost Passa Dois Group at distances 50-1000 km from the impact site. These features include recumbent folds and slumps, clastic dikes, thixotropic wedges, and autoclastic breccias that resulted from seismically-induced sediment liquefaction. The vertical compaction of clastic dykes indicates their formation prior to sediment lithification, and dyke formation is generally confined to intervals of heterolithic sediments of varying density and viscosity, i.e., interbedded sandstone and siltstone. The relative proximity of these features to the impact site (<1000 km) versus the >2500 km to the nearest active plate boundary, together with the restricted stratigraphic horizon where seismites occur, i.e., generally ≤100 m below the paleosurface at the time of impact, indicate that this seismicity is likely the result of the Araguainha impact event. The paleosurface coeval with the impact event lies above the seismite-rich interval, and this surface is extensively scoured and commonly overlain by a ≤4.5 m-thick debrite bed. This sedimentary unit is a matrix-supported, unsorted conglomeratic breccia of variable thickness with an irregular, scoured base and tractional structures including chaotic to loosely imbricated clasts. Angulose clasts 10-400 cm in size comprise altered chert, siltstone, and fine-grained sandstone that are derived from the underlying beds and are arrayed in a fining-upward pattern. In some localities, a second, clast-rich horizon is also observed with similar grading but smaller average clast size. This debrite bed has been identified in localities across the Paraná Basin ~50 – 1200 km from the impact site. The identification of zircon crystals with likely shock metamorphic planar microstructures in multiple samples obtained from this debritic layer links this stratigraphic horizon to the Araguainha impact event, and we interpret this bed as an ejecta-bearing tsunami deposit. The youngest population (n=12) of unshocked, idiomorphic detrital zircon crystals provides a maximum depositional age of 253.0 ± 3.0 Ma for this event horizon, contemporaneous within analytical error of current geochronological constraints on the impact event. These findings demonstrate that a catastrophic event around the P–T boundary in Brazil created one of the world’s most extensive seismite–tsunamite couplets.
The 40 km diameter Mjølnir Crater is located on the central
Barents Sea shelf, north of Norway. It was formed about 142 ± 2.6
Myr ago by the impact of a 12 km asteroid into the shallow shelf
clays of the Hekkingen Formation and the underlying Triassic to Jurassic
sedimentary strata. A core recovered from the central high within the
crater contains slump and avalanche deposits from the collapse of the
transient crater and central high. These beds are overlain by gravity
flow conglomerates, with laminated shales and marls on top. Here, impact
and post-impact deposits in this core are studied with focus on clay
mineralogy obtained from XRD decomposition and simulation analysis
methods. The clay-sized fractions are dominated by kaolinite, illite,
mixed-layered clay minerals and quartz. Detailed analyses showed rather
similar composition throughout the core, but some noticeable differences
were detected, including varying crystal size of kaolinite and different
types of illites and illite/smectite. These minerals may have been
formed by diagenetic changes in the more porous/fractured beds in the
crater compared to time-equivalent beds outside the crater rim.
Long-term post-impact changes in clay mineralogy are assumed to have
been minor, due to the shallow burial depth and minor thermal influence
from impact-heated target rocks. Instead, the clay mineral assemblages,
especially the abundance of chlorite, reflect the impact and post-impact
reworking of older material. Previously, an ejecta layer (the Sindre
Bed) was recognized in a nearby well outside the crater, represented by
an increase in smectite-rich clay minerals, genetically equivalent to
the smectite occurring in proximal ejecta deposits of the Chicxulub
crater. Such alteration products from impact glasses were not detected
in this study, indicating that little, if any, impact glass was
deposited within the upper part of the crater fill. Crater-fill deposits
inherited their mineral composition from Triassic and Jurassic sediments
underlying the impact site.
Abstract– Despite its centennial exploration history, there are still unresolved questions about Meteor Crater, the first recognized impact crater on Earth. This theoretical study addresses some of these questions by comparing model results with field and laboratory studies of Meteor Crater. Our results indicate that Meteor Crater was formed by a high-velocity impact of a fragmented projectile, ruling out a highly dispersed swarm as well as a very low impact velocity. Projectile fragmentation caused many fragments to fall separately from the main body of the impactor, making up the bulk of the Canyon Diablo meteorites; most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression, and were redistributed randomly around the crater. Thus, the distribution of Canyon Diablo meteorites is not representative of projectile trajectory, as is usual for impactor fragments in smaller strewn fields. At least 50% of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles (spheroids) and highly shocked solid fragments (shrapnel). When compared with the known distribution, model results suggest an impactor from the SW. Overall, every model case produced much higher amounts of pure projectile material than observed. The projectile-target mixing was not considered in the models; however, this process could be the main sink of projectile melt, as all analyzed melt particles have high concentrations of projectile material. The fate of the solid projectile fragments is still not completely resolved. Model results suggest that the depth of melting in the target can reach the Coconino sandstone formation. However, most of the ejected melt originates from 30–40 m depth and, thus, is limited to Moenkopi and upper Kaibab material. Some melt remains in the target; based on the estimated volume of the breccia lens at Meteor Crater, our models suggest at most a 2% content of melt in the breccia. Finally, a high water table at the time of impact could have aided strong dispersion of target and projectile melt.
During the Late Jurassic and earliest Cretaceous the Barents Shelf was dominated by fine-grained clay sedimentation, with
mostly anoxic to hypoxic depositional conditions. The stratified water-masses contained typically relatively rich, but low
diversity, nectonic faunas and marine microfloras above the pycnocline. In contrast the benthic faunas contained only a few
bivalve species and low diversity communities of foraminifera. At the time of the Volgian-Ryazanian boundary (142.2±2.6 Ma)
a 1.5–2 km-diameter bolide hit the paleo-Barents Sea and created the 40 km-diameter Mjølnir Crater. The central peak of the
crater formed an island, and the high standing crater rims and annular ridges further led to significant changes in the sea-bed
topography. The impact and crater formation led to significant disturbance and environmental changes, both at the crater site
and over large distances of the paleo-Barents Shelf. Tsunamis were formed and travelled back and forth across the seas for
a day or two after the impact. Continuing collapse of unstable, unconsolidated highs and rims formed avalanches, slumps and
slides that developed into gravity flows in the crater surroundings. Computer simulations of ejecta formation and distribution
indicate that major ejecta transportation occurred along the trajectory of the incoming bolide, i.e., toward the northeast.
No evidence exists of any major biotic extinction or changes in diversity related to the impact event, but the overall compositions
of the microfossil assemblages show a significant change within the impact-influenced strata. In the lowermost post-impact
deposits in the Mjølnir Crater, and in association with the ejecta-bearing strata on the adjacent shelf, a conspicuous acme
of the marine prasinophyte Leiosphaeridia combined with an influx of abundant juvenile freshwater algae of the genus Botryococcus occur. The prolific blooms of Leiosphaeridia suggest that these algae had a behavioral pattern typical for so-called disaster species. The recovery of the algal bloom
in deposits off Troms, 500 km to the south of the Mjølnir Crater, and on Svalbard, 450 km to the north, suggest that a regional
eutrophication event was induced in the impact-ocean. The duration of the environmental change and the biotic turnover is
currently difficult to estimate, but was most likely relatively short. Depositional conditions comparable to those found on
the shelf prior to the impact (i.e., stratified water-masses, with anoxic — hypoxic bottom conditions and low diversity marine
benthic faunas) were restored during the earliest Ryazanian (i.e., prior to the time corresponding to the Heteroceras kochi ammonite zone).
The Mjølnir bolide created the 40-km diameter Mjølnir crater, when it impacted the black, mostly anoxic clays of the Hekkingen Formation in the paleo-Barents Sea about 142±2.6 million years ago. The normally calm, 300–500 m deep epicontinental depositional environment was suddenly disrupted by the dramatic effect of the impact, resulting in a brief period of extreme sediment reworking and redeposition. The hypoxic to anoxic depositional conditions characteristic of the Hekkingen Formation returned to the impact site soon after the collapse, when the major modification phases of the Mjølnir crater were completed.We have studied a shallow core (121 m long) retrieved from the flanks of the central high in the Mjølnir crater. The core shows a complex depositional succession of the Ragnarok Formation, which is related to both the uplift and the subsequent collapse and drowning of the central high. The basal part of the core consists of chaotically organised, large folded slabs of pre-impact substrate, which we infer to be related to the rapid steepening of the slope of the central high during its rising shortly after the impact. The slump deposits are overlain by a diamict, which is interpreted to originate from debris flows that originate by liquefaction and subsequent remoulding and remobilisation of sediment from the collapsing central high. The diamict is in turn covered by a brecciated, graded mudstone that records the action of impact-related tsunami and the subsequent submergence of the impact crater. A sequence of mainly debris flow and turbidite deposits separates the impact-related deposit from the overlying shelf sediments of the Hekkingen Formation and forms the last post-impact sedimentary recorder of the presence of a central high in the crater.
The detailed morphology of impact craters is now believed to be mainly caused by the collapse of a geometrically simple, bowl-shaped "transient crater." The transient crater forms immediately after the impact. In small craters, those less than approximately 15 km diameter on the Moon, the steepest part of the rim collapses into the crater bowl to produce a lens of broken rock in an otherwise unmodified transient crater. Such craters are called "simple" and have a depth-to-diameter ratio near 1:5. Large craters collapse more spectacularly, giving rise to central peaks, wall terraces, and internal rings in still larger craters. These are called "complex" craters. The transition between simple and complex craters depends on 1/g, suggesting that the collapse occurs when a strength threshold is exceeded. The apparent strength, however, is very low: only a few bars, and with little or no internal friction. This behavior requires a mechanism for tem-porary strength degradation in the rocks surrounding the impact site. Several models for this process, including acoustic fluidization and shock weakening, have been considered by recent investigations. Acoustic fluidization, in partic-ular, appears to produce results in good agreement with observations, although better understanding is still needed.
Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters' rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.
Analysis of the prominent seismic disturbance at the 40-km-diameter Mjølnir impact structure, based on an extensive seismic reflection database, shows that the observed broad-brimmed bowl-shaped disturbance was formed as a result of the impact of an asteroid or comet during Volgian-Berriasian time (149–141 M). Seismic mapping exhibits and visualizes a 850–1400 km3 disturbed volume and analysis of several structural features within the disturbance provides insight into major cratering processes, such as brecciation and excavation, melting, gravitational collapse of the transient crater, and structural uplift. A transient crater of 16 km in diameter and 4.5 km in depth is determined. From transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2.5, considerably larger than the average expected values for typical terrestrial craters. The extensive collapse took place by low-angle décollement surfaces at the periphery and by chaotic debris mass flows toward the center. Furthermore, we estimate the Mjølnir projectile to have been 0.9–3 km in diameter and that the physical impact released energy in the range of 2.4–53×1020 J corresponding to an earthquake of magnitude 8.3. The Mjølnir impact is not associated with a significant mass extinction. However, dissipation of the energy released during the Mjølnir impact was sufficient to have caused several short-term, near-field perturbations, such as large-amplitude tsunami waves, affecting the Barents Sea region and possibly adjacent areas in the Arctic.
Using Magellan data, we investigated two crater characteristics that have been cited as diagnostic of oblique impacts: an uprange offset of the central peak in complex craters, and an increasing central peak diameter relative to crater diameter with decreasing impact angle. We find that the offset distribution is random and very similar to that for high-angle impacts, and that there is no correlation between central peak diameter and impact angle. Accordingly, these two crater characteristics cannot be used to infer the impact angle or direction.
A systematic search for impact indicators was conducted on a core of Late Jurassic Early Cretaceous sedimentary strata from the vicinity of the proposed Mjølnir impact structure, Barents Sea. A 0.8-m-thick section of the core was found to contain unequivocal indicators of meteoritic impact: shocked quartz grains and a strong enrichment in iridium. The ejecta-bearing strata were discovered only 30 km north-northeast of the structure, within a stratigraphic interval corresponding to the seismically defined deformation event at Mjølnir. Further study of this unusually well preserved impact-crater-ejecta-layer pair may help constrain poorly understood aspects of large-magnitude meteorite impacts into the oceans.
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/cm3 asteroid impacting at 20 km/s, or a 1.0 g/cm3 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.
Gravity, magnetic and seismic traveltime anomalies observed at the 40-km-diameter Mjølnir impact structure reveal a distinct spatial correspondence with the radially zoned seismic structure. The gravity anomaly is dominated by a +2.5 mGal, 14-km-wide, centrally located high superimposed on a 45-km-diameter low that attains minimum values of −1.5 mGal. The magnetic anomaly field exhibits several local, low-amplitude anomalies within the ±100 nT range located towards the periphery, while seismic mapping of a prominent, originally planar reflector beneath the structure brings out a central, pull-up traveltime anomaly on the order of 80 ms. In terms of impact origin, the integrated geophysical modelling based on the characteristic bowl-shaped seismic disturbance beneath the structure supports the differentiation into a central uplift and a peripheral region. Interaction of several impact cratering processes, such as impact-induced porosity increase due to brecciation, mass transport during collapse, and structural uplift, explains the modelled physical properties associated with the disturbance. The modelling further substantiates the interpretation of the Mjølnir structure as an impact crater and demonstrates the incompatibility of alternative geological origins, such as salt or clay diapirism and igneous intrusions.
Seismic profiles in the central Barents Sea record a conspicuous structure exhibiting gross morphological and structural characteristics closely resembling those of large impact craters. This feature, named the Mjølnir structure, is 39 km in diameter and of Jurassic-Early Cretaceous age. It probably formed by the impact of an asteroid or comet, 0.7-2.5 km in diameter, into a shallow sea underlain by >5 km of sedimentary section. If the impact interpretation is correct, Mjølnir is one of the largest well-preserved craters in the errestrial record.
Eulerian, three-dimensional, numerical code, which conserves mass, momentum and energy simultaneously both in the Lagrangian
and remap steps, is developed. The use of special form of linear viscosity provides a weaker time step restriction as compared
with the Courant condition. The code is designed to investigate the multi-material problems, including dusty flows. The performance
of the code is illustrated by the modeling of shock wave interaction with a dusty thermal layer.
This paper deals with the strength of rock-like materials under triaxial stress conditions. The most common theories of brittle fracture are reviewed. Some methods of triaxial strength determination are described and the results compared. The strength dependence of lateral pressure, pore pressure and size of the test sample is described. Some standard tests are suggested. A method for determining the shear strength at high normal pressure has been used as a measure of the strength of a number of Swedish rocks and ores.
Cratering flow calculations for a series of oblique to normal (10 degrees to 90 degrees ) impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock-vaporizing both projectile and target material would form a downstream jet that could entrain and propel SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the Martian escape velocity (more than 5 kilometers per second). Two-dimensional finite difference calculations were performed that show that at highly probable impact velocities (7.5 kilometers per second), vapor plume jets are produced at oblique impact angles of 25 degrees to 60 degrees and have speeds as great as 20 kilometers per second. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 kilometer, the resulting vapor jets have densities of 0.1 to 1 gram per cubic centimeter. These jets can entrain Martian surface rocks and accelerate them to velocities greater than 5 kilometers per second. This mechanism may launch SNC meteorites to earth.