When a cosmic impact strikes the sea bed

Self Cite

138

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

Section: General Description Of the Studied Cratersmentioning

confidence: 82%

Section: Stratigraphic and Environmental Setting Of The Brecciasmentioning

confidence: 99%

Sedimentological analysis of resurge deposits at the Lockne and Tvären craters: Clues to flow dynamics

Ormö

Sturkell

Lindström

2007

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138

“…Marine-target impact craters represent ∼20% of known craters, a number that is much too low given that more than two-thirds of the Earth's surface is covered by water. This is attributed mainly to plate tectonic destruction processes, sparsity of detailed seismic reflection data from deep-water regions, and the difficulty in identifying buried or exposed submarine craters (e.g., Ormö and Lindström 2000;Dypvik and Jansa 2003). The sparcity of marine craters can be also ascribed to the burial of marine craters by post-impact sediments.…”

Section: Introductionmentioning

confidence: 99%

Post‐impact structural crater modification due to sediment loading: An overlooked process

Tsikalas

Faleide

2007

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

“…Conversely, if γ > 1, the water layer does not affect the cratering process significantly; these impacts are called shallow water impacts (SWIs). However, the final crater morphology may still differ from crater structures on land (Shuvalov and Trubestkaya 2002;Ormö and Lindström 2000). In between these two extreme cases (0.1 < γ < 1), the water column has a significant influence on crater formation (Shuvalov and Trubestkaya 2002;Weiss et al 2006).…”

Section: Cratering Mechanics and Wave Generationmentioning

confidence: 99%

“…Ormö and Lindström (2000) showed that there are unique geological features of impact craters generated at the ocean bottom. To form such craters, the impactor must penetrate through the water column and modify the solid strata underneath.…”

Section: Introductionmentioning

confidence: 99%

Characteristics of oceanic impact‐induced large water waves—Re‐evaluation of the tsunami hazard

Wünnemann

Weiss

Hofmann

2007

available online at http://meteoritics.org Characteristics of oceanic impact-induced large water waves-Re-evaluation of the tsunami hazard Abstract-The potential hazard of a meteorite impact in the ocean is controversial with respect to the destructive power of generated large ocean waves (tsunamis). We used numerical modeling of hypervelocity impact to investigate the generation mechanism and the characteristics of the resulting waves up to a distance of 100-150 projectile radii. The wave signal is primarily controlled by the ratio between projectile diameter and water depth, and can be roughly classified into deep-water and shallow-water impacts. In the latter, the collapse of the crater rim results in a wave signal similar to solitary waves, which propagate and decay in agreement with shallow-water wave theory. The much more likely scenario for an asteroid impact on Earth is a relatively small body (much smaller than the water depth) striking the deep sea. In this case, the collapse of the transient crater results in a significantly different and much more complex wave signal that is characterized by strong nonlinear behavior. We found that such waves decay much more rapidly than previously assumed and cannot be treated as long waves. For this reason, the shallow-water theory is not applicable for the computation of wave propagation, and more complex models (full solution of the Boussinesq equations) are required.