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

Abstract

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.

Geological and geophysical studies of the Agoudal impact structure (Central High Atlas, Morocco): New evidence for crater size and age

“…Vertical to overturned N150–N160 trending strata, not observed elsewhere in the surrounding area, were reported by Chennaoui Aoudjehane et al. (). The breccias within the area of SC occurrence were considered to be impact breccias, based on the presence of cryptocrystalline inclusions, and were interpreted as possible relics of weathered impact melt rocks (Lorenz et al.…”

“…Agoudal is the only known impact structure in Morocco (Chennaoui Aoudjehane et al. ). It was confirmed by the observation of well‐preserved shatter cones (SCs) in a mid‐Jurassic marly limestone formation (Sadilenko et al.…”

“…These breccias may have been alternatively formed as a result of erosion, weathering, and cementation processes (Chennaoui Aoudjehane et al. ). Erosion of the Agoudal impact structure may have wiped out the circular morphological expression of the crater.…”

“…), SC‐bearing outcrops of ~500 × 300 m in Agoudal, and the fact that the area of SC occurrence is commensurable with the size of the crater, Chennaoui Aoudjehane et al. () proposed an age of the Agoudal impact event older than 1 Ma with a size of 1–3 km in diameter.…”

Shock‐thermal history of the Agoudal (IIAB) iron meteorite from microstructural studies

“…However, based on the terrestrial age of the Agoudal find, Chennaoui Aoudjehane et al. () concluded that the meteorite fall on the Earth postdates the formation of shatter cones and it is a mere coincidence that the meteorite fell on a previously formed impact structure. In Meteorite Database (Meteoritical Bulletin 102, 2013), Agoudal has been classified as IIAB based on its bulk composition and structural similarity with Ainsworth (Buchwald ).…”