An unusually well-preserved 45-meter-diameter crater provides ground truth for small-scale meteorite impacts on Earth. We report on the detection in southern Egypt of an impact crater 45 meters in diameter with a pristine rayed structure. Such pristine structures are typically observed on atmosphereless rocky or icy planetary bodies in the solar system. This feature and the association with an iron meteorite impactor and shock metamorphism provides a unique picture of small-scale hypervelocity impacts on Earth’s crust. Contrary to current geophysical models, ground data indicate that iron meteorites with masses of the order of tens of tons can penetrate the atmosphere without substantial fragmentation.

The Kamil Crater, Egypt

Associations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 ± 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area. The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (<100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer-diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150-N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (~0.15 km2) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1–3 km. For typical erosion rates in the Atlas region (~0.08 cm yr−1), the period of time required for the erosion of such a structure (1.25–3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12661

The Agoudal (High Atlas Mountains, Morocco) shatter cone conundrum: A recent meteorite fall onto the remnant of an impact site

Several models of shatter cone formation require a heterogeneity at the cone apex of high impedance mismatch to the surrounding bulk rock. This heterogeneity is the source of spherically expanding waves that interact with the planar shock front or the following release wave. While these models are capable of explaining the overall conical shape of shatter cones, they are not capable of explaining the subcone structure and the diverging and branching striations that characterize the surface of shatter cones and lead to the so‐called horse‐tailing effect. Here, we use the hierarchical arrangement of subcone ridges of shatter cone surfaces as key for understanding their formation. Tracing a single subcone ridge from its apex downward reveals that each ridge branches after some distance into two symmetrically equivalent subcone ridges. This pattern is repeated to form new branches. We propose that subcone ridges represent convex‐curved fracture surfaces and their intersection corresponds to the bifurcation axis. The characteristic diverging striations are interpreted as the intersection lineations delimiting each subcone. Multiple symmetric crack branching is the result of rapid fracture propagation that may approach the Raleigh wave speed. We present a phenomenological model that fully constructs the shatter cone geometry to any order. The overall cone geometry including apex angle of the enveloping cone and the degree of concavity (horse‐tailing) is largely governed by the convexity of the subcone ridges. Straight cones of various apical angles, constant slope, and constant bifurcation angles form if the subcone convexity is low (30°). Increasing subcone convexity leads to a stronger horse‐tailing effect and the bifurcation angles increase with increasing distance from the enveloping cone apex. The model predicts possible triples of enveloping cone angle, bifurcation angle, and subcone angle. Measurements of these quantities on four shatter cones from different impact structures and lithologies agree well with model predictions.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12677

Formation of shatter cones by symmetric fracture bifurcation: Phenomenological modeling and validation

The recently discovered Agoudal impact site in Morocco is a small, eroded impact structure with well-developed shatter cones. A scanning electron microscopic study of a shatter cone surface has revealed the presence of schreibersite – a phosphide very rare on Earth but common in iron meteorites – and Fe–Ni oxides. This is the first reported evidence for primary meteoritic matter adherent to shatter cones and suggests that the Agoudal crater was formed by the impact of an iron meteorite, probably the Agoudal IIAB iron. Shatter cones from other terrestrial impact structures might also hold valuable information about the nature of the impacting projectiles.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756815000047

Meteorite traces on a shatter cone surface from the Agoudal impact site, Morocco

This chapter provides an up-to-date review about confirmed, proposed, and disproven meteorite impact structures in the Arab countries. Among the ~190 confirmed impact structures/sites on Earth, only 13 have been identified in the countries of the Arab world: Agoudal (Morocco), Amguid (Algeria), Aouelloul (Mauritania), BP (Libya), Kamil (Egypt), Oasis (Libya), Ouarkziz (Algeria), Jebel Waqf as Suwwan (Jordan), Saqqar (Saudi Arabia), Talemzane (Algeria), Tenoumer (Mauritania), Tin Bider (Algeria), and Wabar (Saudi Arabia). Thirty-three other structures of alleged impact origin and located in the Arab countries have been proposed in the literature. Based on the examination of satellite images and available field geologic, petrographic, and geochemical documentation, some of these proposed features are not of impact origin. Others are promising impact crater candidates. The record of impact structures in the Arab region is still incomplete considering the great size of the territory and the number of impact craters known in other regions where there have been active programs to study impact structures. Field investigations, including the collection and analyses of rock samples, are required for adding new candidates to the list of confirmed impact craters in the Arab countries. The only criteria that prove evidence for an impact origin of circular geological structures are the occurrence of shock-metamorphic effects in the target rocks (shatter cones and diagnostic shock-metamorphic mineral deformation and transformation phenomena), the discovery of preserved meteorite fragments, or detection of geochemical traces of meteoritic projectiles. Particular attention should be paid during fieldwork and subsequent petrographic studies, as some structures previously identified as impact craters had to be discounted. Nine of these cases of discredited structures are described here.

Meteorite Impact Structures in the Arab World: An Overview

A relic impact structure was recognized within the strewn field of the Agoudal iron meteorite. The heavily eroded structure has preserved shatter cones in a limestone basement, and remnants of autochthonous and allochthonous breccias. Fragments of iron incorporated into the allochthonous breccia have a chemical composition (Ni = 5.16 wt%, Ir = 0.019 ppm) similar to that of the Agoudal meteorite, supporting a syngenetic origin of the strewn field and the impact structure. The total recovered mass of Agoudal meteorite fragments is estimated at approximately 500 kg. The estimated size of the SE–NW-oriented strewn field is 6 × 2 km. Model calculations with minimal preatmospheric size show that a similar meteorite strewn field plus one small crater with observed shock effects could be formed by fragmentation of a meteoroid approximately 1.4 m in diameter with an impact angle of approximately 60° from the horizontal. However, the most probable is an impact of a larger, 3–4 m diameter meteoroid, resulting a strewn field with approximately 10 craters, 10–30 m in diameter each, plus numerous meteorite fragments. The calculated scattering area of meteorite shrapnel ejected from these impact craters could completely cover the observed strewn field of the Agoudal meteorite.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12406

Formation of a small impact structure discovered within the Agoudal meteorite strewn field, Morocco

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Formation of a small impact structure discovered within the Agoudal meteorite strewn field, Morocco