Terrestrial Impact Craters: Their Spatial and Temporal Distribution and Impacting Bodies

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...

Section: Post-impact Modification Correction Factormentioning

confidence: 99%

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

Tsikalas

Faleide

2007

“…Based on the size criteria for terrestrial impact structures (e.g., Grieve and Pesonen, 1996), Bosumtwi should be a complex impact structure, but no evidence exists of a central uplift in lake bathymetric data (McGregor, 1937). It is possible that the central uplift has collapsed during the modification stage of the crater formation and is hidden underneath the lake sediments.…”

Section: Structure Of the Target And Cratermentioning

confidence: 99%

The Bosumtwi meteorite impact structure, Ghana: A magnetic model

Plado

Pesonen

Koeberl

et al. 2000

119

Abstract-A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (-1.07 Ma) structure with a diameter of -10.5 km is exposed within early Proterozoic Birimian-Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high-resolution, low altitude (-70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, -12 km in diameter. The central-north part of the lake reveals a central negative magnetic anomaly with smaller positive side-anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact-melt rock or melt-rich suevite bodies.Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.

“…Shoemaker and Shoemaker (1996) estimated a cratering rate for the Proterozoic in Australia of(3.8 ± 1.9) x 10-15 km-2 /year, and (6.3 ± 3.2) x 10-15 km-2 /year (by extrapolation from impact structures larger than 10 km) for the Phanerozoic in the USA. Grieve and Shoemaker (1994) estimated the cratering rate since 120 Ma to be (5.6 ± 2.8) x 10-15 km-2 /year, and Grieve and Pesonen (1996) estimated the cratering rate for craters larger than 20 km as (5.5 ± 2.7) x 10-15 km-2 /year. Finally, Hughes (2000) calculated a more conservative cratering rate estimate of(3.0 ± 0.3) x 10-15 k:nr 2 / year for craters larger than 22 Ian, based upon a re-examination using the mean areas of craters, together with the gradients of linear plots of crater numbers vs. crater ages.…”

Section: Cratering Rates and Crater Clustersmentioning

confidence: 99%

“…Finally, Hughes (2000) calculated a more conservative cratering rate estimate of(3.0 ± 0.3) x 10-15 k:nr 2 / year for craters larger than 22 Ian, based upon a re-examination using the mean areas of craters, together with the gradients of linear plots of crater numbers vs. crater ages. Taking the most recent estimates of cratering rate (Grieve and Pesonen, 1996;Hughes, 2000) and the power law relationship between the cumulative number ofcraters N, and crater diameter Dc to be N oc (Shoemaker and Shoemaker, 1996), it is possible to show that there are "on average" 1.5 to 2.8 impacts capable ofcreating craters larger than 20 km on the Earth's surface every million years. Neglecting oceanic impacts, since both Chixculub and Boltysh impacts are on continental crust, reduces this to one impact every 1.8 to 3.3 Ma.…”

Section: Cratering Rates and Crater Clustersmentioning

confidence: 99%

Boltysh, another end‐Cretaceous impact

Kelley

Gurov

2002

Abstract-The Chixculub impact occurred at the Cretaceous/Tertiary (KIT) boundary, and although several other Late Cretaceous and Paleogene impact craters have, at times, been linked with the KIT boundary, isotope geochronology has demonstrated that all have significantly different ages. The currently accepted age of the 24 km diameter Boltysh crater, a K-Ar whole-rock age, places it in the Coniacian at 88 ± 3 Ma. However, comprehensive Ar-Ar dating of a range of melt samples yields a mean age of 65.17 ± 0.64 Ma, within errors of the KIT boundary. Several of the fresh samples exhibit signs of excess argon but this seems to be concentrated in rapidly crystallized glass-rich samples.The Ar-Ar age confirms an earlier fission track measurement and thus two dating techniques have yielded an age within errors of the K/T boundary for this crater. Crucially, although the ages of Boltysh and Chixculub are within errors, they may not have formed synchronously. Craters of24 km diameter occur much more commonly than impacts of Chixculub dimensions, but their proximity does raise the important question of how many impacts there might have been close to the K/T boundary.